Electronic spectroscopy of the cyanogen halides - American Chemical

639

J. Phys. Chem. 1991, 95, 639-656 spectral assignments have been made. As with norbornane and norbomadiene, there exist many possibilities for Fermi resonance between these stretching modes and overtones of the skeletal vibrations. Norbomadiene has been shown to exhibit at least three cases of Fermi resonance.'J3J7 The use of high-pressure vibrational spectroscopy to vary the degree of coupling between the two transitions has been widely documented.'" In our study of norbornadiene, the previously assigned Fermi resonant pairs in the C H stretching region did not conform to the general pressure behavior observed in the cases cited in the literature. It was concluded that the peaks in this region exhibit more complicated

behavior due to the C H bonds being on the outside of the molecule. It was also noted that, in both pairs, the pressure dependence of the lower frequency component was less than half that of the higher frequency component of the pair. Furthermore, no exchange of peak intensities was observed over the 30 kbar range of the measurements. By examining the pressure dependences of the peaks in the C H stretching region of norbornylene, while taking into account the overtone possibilities, there is only one pair that may be considered a Fermi resonance doublet. The overtone of the CH2 scissoring mode at 1466 cm-I may be coupled with a methylene C H stretch to produce the two bands at 2915 and 2976 cm-I.

(17) (a) Butler, 1. S.;Barna, G. J . Raman Spectrarc. 1973,828,1645. (b) Adams, D. M.; Fernando, W. S. Inorg. Chim. Acta 1973, 7 , 277. (18) Sec, for example: (a) Sherman, W. F.; Lewis, S. Spectrochim. Acia 1979,354 613. (b) Wong, P. T. T.; Chagwedera. T. E.; Mantsch, H. H. J . Chem. Phys. 1987, 87, 4487.

Acknowledgment. This research was supported by grants from NSERC and CANMET (Canada) and FCAR (Quebec). N.T.K. also acknowledges the award of a postgraduate scholarship from NSERC.

Electronic Spectroscopy of the Cyanogen Halides W.S. Felps, K.Rupnik, and S. P.McClynn* Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803 (Received: June 25, 1990; In Final Form: August 6, 1990)

The electronic absorption spectra of the cyanogen halides, XCN where X = CI, Br, and I, have been investigated in the range 3100-1050 A. The spectra are analyzed in terms of vibronic structure, oscillator strengths, and effective quantum numbers. The spectra of the cyanogen halides exhibit no regular Rydberg structure. The absence of regularity is shown to be a direct consequence of the presence of intravalence excitations and Rydberglvalence interactions. In confirmation of the above, the intravalence transitions arising from the 2a (5a, 37r, and 6a) configurational excitations are observed and assigned. In addition, we presume to extract "term values" for the antibonding 5u, 3r, and 64 MOs and then use these to predict the energies of the remaining nine low-energy intravalence excitations of the cyanogen halides, {4u; 1r; 3u) {Sa;3a; 64. There is a danger in this, in that we seem, too blithely perhaps, to make use of a simple one-electron MO model in situations where it is known that many-electron effects may dominate. We believe that the use of one-electron considerations is moderated by the extensive use of a vast amount of empirical, correlative experimental data. Finally, all excited states that arise from the 12 low-energy intravalence excitations are correlated with the states of the separated halogen atom and CN radical such that hotoprocesses in the cyanogen halides may be rationalized. The A-band and a-band continua are assigned as 2a 5a,'3 ll and 2a 3r;',3A,'*32+,'93Z-configurational excitations, respectively. The 4a 5a;'2+ transition is associated with the discrete structure atop the a continuum; the intense, discrete band systems that lie to the blue of the B and C Rydberg band systems are associated with the 2a 6a;'*'ll intravalence transitions. The states that arise from the remaining eight configurational excitations are shown to be mostly dissociative in nature. The correlation scheme predicts (i) CN (X22+)to be the primary product of photolysis within the A continua; (ii) CN a211 toi) be the primary product of photolysis from the onset of the a continua up to photon energies of 80000 cm-' for each of the cyanogen halides, and (iii) CN m22+) to be a primary product for photon energies greater than 105000,94000, and 80000 cm-'in ClCN, BrCN, and ICN, respectively.

-

-

P

-

-

-

-

-

Introduction The advent of UV and VUV lasers has generated considerable interest in the high-energy, high-resolution photochemistry of the cyanogen halides. However, most of the new photochemical investigations are predicted on absorption spectra obtained in 1966 using a 1-m Cary Model 14 spectrophotometer for X > 1340 AIo and a 21-ft vacuum spectrograph for 1250 A < X < 1840 A.Ib It is known that some of these spectra are inaccurate or incomplete and that the assignments need reconsideration. We demonstrate these problems by citing a few seemingly trivial, but important examples: (i) The more recent UV2 and VUVZaspectra of ICN suggest that the two lowest energy continuous absorption systems (Le., ( I ) (a) King, G. W.; Richardson, A. W. J . Mol. Specirosc. 1966,21,339. (b) King, 0 . W.; Richardson, A. W. J . Mol. Specrrosc. 1966. 21, 353. (2) (a) Myer, J. A.; Samson, J. A. R. J . Chem. Phys. 1970,52,266. (b) Holdy, K. E.; Klotz, L. C.; Wilson. K. R. 1.Chem. Phys. 1970 52,4588. (c) Ling, J. H.; Wilson, K. R. J . Chem. Phys. 1975. 63, 101.

0022.365419 112095-0639$02.50/0

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the so called A and a bands) are actually centered 100 A to the red and blue, respectively, of the maxima reported in 1966.'* (ii) Some shorter wavelength studies, Xmin 1050 A, dealt',' with photodissociation and predissociation of the a band and with the discrete Rydberg band systems at shorter wavelengths. These studies indicate that the maxima of the a band lie at considerably shorter wavelengths in ClCN, BrCN, and ICN than was reported in 1966.'* (iii) It has recently been shownSathat the A and a bands of ClCN are actually two separate and distinct features and that

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(3) West, G. A.; Berry, M. J. Chem. Phys. L r t . 1978. 56, 423. (4) (a) Macpherson, M. T.; Simons, J. P. J . Chem. Soc., Faraday Trans. I1 1979, 75, 1572. (b) Ashfold, M. N. R.; Simons, J. P. J . Chem. Soc., Faraday Trans. I1 1978. 74, 280. (c) Ashfold, M. N. R.; Macpherson, M. T.; Simons, J. P. Top. Curr. Chem. 1979,86, 1 . (5) (a) Felps, W. S.; McGlynn, S. P.; Findley, G. L. J . Mol. Spectrosc. 1981,86, 71. (b) Rabalais, J. W.; McDonald, J. M.; Scherr, V.; McGlynn. S.P. Chem. Rev. 1971, 71, 73.

0 1991 American Chemical Society

Felps et al.

640 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 40

,

I

,

CH3CI

60

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C

...........

100

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CI-CN

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........-

.......... Br-CN

....

A

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Br-CN

CH3CI 0

.......... A I-CN

3u

A ...........A..

A

0

A

I-CN

HI 0

....

CH38r 0

........ I-CN 3 u

.......A ............ 2U

H CI

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.........

....................

.. 1

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2r

1

1

1

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ENERGY fcm-lx103)

1

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Figure 1. Schematic of the low energy electronic absorption and photoelectron spectra of the methyl, cyanogen, and hydrogen halides.

the a band, unlike the A band, exhibits extensive vibrational structure. (iv) The new assignmentsSa of the A and a bands of item iii were the same as those given by King et al.la (i.e., the A band excitation, the was associated with the intravalence 27r u*;~JII (Y band with the intravalence 2a 7r*;1*3A,1*32+,1*32excitation). This assignment is at odds with Rabalais et al.,5bwho associated the A and a bands with absorptions to the l2- and IA states, respectively ( Le., to an intravalence 27r A* excitation), and who computed the intravalence 27r a* excitation to lie at much higher energies. In view of these illustrative examples, there is an obvious need for accurate, detailed VUV absorption spectra of cyanogen halides. The major purpose of the present work is to present a detailed atlas of the VUV absorption spectra of ClCN, BrCN, and ICN down to 1050 A. In view of the variety of the assignments, one of which was illustrated in item iv, there is also a need for analysis/assignment of the electronic molecular spectra. Such assignments constitute a secondary purpose of this work. There is also a need to try to achieve congruence of the spectroscopic and photochemical findings, which is a tertiary purpose of the present work. Thus, we present the VUV absorption spectra of CICN, BrCN, and ICN in the 3100-1050 A region. We also provide the spectra of the methyl halides for the purpose of correlative comparison. Oscillator strength data are presented for the A band and a band of the cyanogen halides. The vibrational structure of the a-band system is tabulated and the nature of the vibrational activity is discussed. We also assign a third valence band system in the spectra of the cyanogen halides. We refer to this, intense, absorption band system as the B band. The vibrational structure and oscillator strength data for the fl band are tabulated and a vibrational analysis is given. Oscillator strength data are also given for the B and C Rydberg band systems and the intense, narrow Rydberg bands that lie to the red and blue, respectively, of the &band system of the cyanogen halides. The spectra of cyanogen halides, unlike the methyl halides, lack "regular" Rydberg structure. We attribute this "irregularity", in part anyway, to the prescnce of higher energy valence transitions and to the Rydberg/valence interactions they induce. We propose a M O scheme for the cyanogen halides that encompasses the four usual lowest occupied MOs (27r, 4a, IT, and 3a) and the three usual virtual MOs (Sa, 3a,and 6a). This MO

- - -

14

16

VERTlCAL

..... ............ Jlbll, ...............

..... 1

.........

Ir

C

I

4 L 1 . I ~ l " ' ~ . l . ~ " ' I

2!-

I. P. ( e V )

18

20

Figure 2. Plot of the vertical 4a, (0)and 2a ( 0 )ionization potential versus the C-X and H-X bond energy in the methyl and hydrogen halides, respectively. Plot of the vertical 4a (O), 30 (A),and 4a/3o average (A) ionization potential versus the C-X bond energy in the cyanogen halides. Various linearities should be noted.

-

scheme is frankly empirical and is constructed by fitting the 2a, 4a, IT, and 3a filled MOs to known ionizations; the 2u 5a and 27r 37r configurational excitations to the continuous A and a bands, respectively; and the 27r 6a configurational excitation to the discrete &band systems that lie immediately to higher energy of the B and C Rydberg band systems. This fitting procedure is supposed to approximate binding energies for the Sa, 3a, and 6a virtual MOs. We then treat these binding energies as term values, T, and presume to obtain "predicted" transition energies for the remaining nine intravalence excitations, namely, 4a; 1 a;3a Sa: 3 r ; 6a, using the expression B = I - T, where I is an ionization energy. Obviously, the success of such a very simple approach can only be gauged by extensive cross-correlation with experiments. Parent-to-fragment (electronic) state correlation diagrams are constructed for each of the three cyanogen halides assuming linear symmetry, the Born-Oppenheimer approximation, and Hund's case (c) coupling. These correlation diagrams are used to discuss the photoprocesses that occur in the cyanogen halides. The assumption of adiabaticity is questionable and, consequently, many parent-fragment correlations are nonunique. Again, the validity of these procedures can only be vindicated by extensive cross-talk between experiment and prediction. Vibrational analyses are presented for 27r n s R ; W and the 4a nsR;IZ+ Rydberg band systems and for a number of other structured band systems, ones that correspond to the lowest energy 2a npR and ndR transitions.

-

-

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-

-

Experimental Section Absorption spectra were obtained on a McPherson Model 225 I-m VUV scanning monochromator with a 1200-line/mm grating Slit widths varied from and reciprocal dispersion of 8.3 A/". 90 pm at the 1050 A limit to 30 pm at the 3000 A limit. Sodium salicylate phosphors were used as converters in the 2200-1050 A region. Tabulated wavelengths refer to band maxima and are accurate to hO.5 A for appropriately narrow ands. The ClCN sample was obtained from Scienti IC Gas Products, BrCN from Eastman Kodak, ICN from Matheson, Coleman and Bell, CH3CI and CH,Br from Matheson Gas Products, and CHJ from J. T. Baker. The major impurities in the ClCN sample were CO and HCN, while the major impurity in both the BrCN and ICN samples was 12. The BrCN sample also contained minor traces of ICN. The minor impurity in the ICN sample was CHJ. The methyl halide samples contained no major impurities. Dissolved gases were removed from all samples by a freezepump-thaw cycle.

bz

Electronic Spectroscopy of the Cyanogen Halides 90 1.0

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'

The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 641

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ENERGY (cm-lx103) 50 I

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32 I

1

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CYANOGEN CHLORIDE

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A(d)Figure 3. Electronic absorption spectra of the cyanogen halides in the 3100-1050 A spectral region. For CICN, the pressures in the absorption cell were (a) 23.2 pm, (b) 175 pm, (c) 2.7 mm, and (d) 7.9 mmHg. For BrCN, the pressures were (a) 18.7 pm, (b) 1.9 mm, and (c) 20.9 mmHg. For ICN, the pressures were (a) 37 pm, (b) 330 pm, (c) 1.84 mm, 306 K cell temperature; (d) 5.6 mm, 324 K cell temperature; and (e) 13.1 mmHg, 334 K cell temperature. The dashed line in each spectrum is the base line. Unless otherwise stated, the spectra refer to room-temperature (295 K) gaseous samples.

The spectra of ClCN, BrCN, the methyl halides, and ICN (but only for X < 2000 A in this last instance) were measured under continuous sample flow conditions by using a IO-cm path length, stainless steel, sample cell held at 22 OC. Continuous sample flow was required to remove photodissociation products, most notably I2 in the case of ICN. However, these flow conditions inhibit accurate sample pressure measurements; therefore, our sample pressures and oscillator strengths are accurate to only &lo%. Vapor pressures were measured with a Datametrics Model 1173 Barocel capacitance manometer. Spectra of BrCN, ICN, CH3Br, and CH31for X > 1600 A were also measured with the sodium salicylate phosphors removed. The absorption spectra of ICN in the 3100-1700 A spectral region were obtained by using a l k m path length, stainless steel, sample cell that was resistively heated to obtain suitable vapor pressures. The heated sample cell contained several grams of the solid ICN sample and was sealed during absorption scans in order to maintain constant sample vapor pressure. The temperature of the heated sample cell was controlled to f 2 OC with an Omega Model 4002 controller. The vapor pressures of ICN reported here for the heated sample cell measurement were deduced from temperature-vapor pressure relationship.6

cation states"J2 and differs from that of King and Richardsonta in that the order of the 4~ and 1r MOs is reversed. (i) r MOs. The PES studies also show that the cyanogen halides are linear in at least the three lowest energy ionic states. They also indicate that the r MOs may be regarded as linear combinations of the nonbonding halogen np, AOs with the Kbonding group orbitals (GOs) of the C=N group. This view is substantiated by Frank-Condon analysis of PES vibrational intensities in CICN, BrCN, and ICNa9 King and Richardson,' on the other hand, had concluded that the 2r MO is largely localized on the halogen atom, their conclusion being based on a comparison of the lowest energy s-Rydberg transitions (the B and C band systems) of the cyanogen halides with those of the corresponding hydrogen and methyl halides. Lake and Thompson7 also inferred that the degree of mixing of the 2r and 1 s MOs of the cyanogen halides decreases from ClCN to ICN, basing their conclusion on the observed activity of the v3 vibrational mode (C-N stretch) within the 2r band system and on a comparison of the doublet splitting within the 2.n band system with that observed within the 1r band system of the corresponding hydrogen halide. (ii) 4a MO. Four simple descriptions are proposed for the 40. and 30. MOs: nitrogen lone pair, uN; halogen lone pair, ox;

Electronic Structure (7) Lake, R. F.; Thompson, A. Proc. R. Soc. London A 1970,317, 187. ( A ) Filled MO's. The ground-state outer-electron configuration ( 8 ) Heilbronner, E.; Homung, V.; Muszkat, K. A. Helu. Chlm. Acta 1970, ) ~-341. of the cyanogen halides is found t o be ( 1 ~ ) ~ ( 2 ~ ) ~ ( 3 a ) ~ ( l r53. --.(9) Hollas, J. M.; Sutherley, T.A. Mol. Phys. 1971. 22, 213. by photoelectron spectroscopy, PES.7-'o This ( 4 ~ )2r)';x12+ ~( (10) Bieri, G. Chem. Phys. t r r r . 1977, 46, 107. M O order is confirmed by emission studies of the XCN+ radical ( I 1) Fulara, J.; Klapstein, D.; Kuhn, R.; Maier, J. P. J . Phys. Chem. 1985, (6) Yost, D. M.; Stone, W. E. J . Am. Chem. SOC.1933,55, 1889. Also see: Ketelaar, J . A. A.; Kruvyer, S.Rccl. Trao. Chim. 1943, 62, 550.

--

89. - - , 421 . .1.

(12) Grieman, F. J.; Mahan, B. H.; OKeefe, A. J . Chem. Phys. 1981,74, 857.

Felps et al.

642 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991

-

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Figure 4. Electronic absorption spectra of the methyl halides in the 3 100-1050A spectral region. For CH$I the pressures in the absorption cell were (a) 35 pm, (b) 515 pm, and (c) 5.76 mmHg. For CH,Br the pressures were (a) 27 pm and (b) 8.3 mmHg. For CHJ the pressures were (a) 5.3 pm, (b) 16.5 pm, and (c) 5.0 mmHg. The dashed line in each spectrum is the base line.

carbon-nitrogen bonding pair, ucN; and the carbon-halogen bonding pair, Lake and Thompson7 concluded that the 4 u MO is best described as a nitrogen lone pair because of the adiabaticity of the associated PES band structure. For FCN, CICN, and BrCN, however, this same PES band does exhibit9J0 a weak vibrational progression that consists of three members in the carbon-halogen stretching mode, vl. The vl frequency exhibits a percentage increase of 14,8, and 2% in the A2Z+ ionic state relative to its value in the X'Z+ ground state of the neutral molecule for FCN, CICN, and BrCN, respectively."' In the case of ICN, any weak vibrational structure that may be associated with the second PES band system (Le., the presumed 4 u ionization event) is obscured by the onset of the 1~ ionization event. Hollas and Sutherleyg concluded from the results of their geometry calculations that the 4u M O could not be viewed simply as a nitrogen lone pair and that it is very dangerous, especially in the case of u MOs, to draw even semiquantitative conclusions concerning the bonding/nonbonding character of polyatomic molecular orbitals from such geometry calculations. The second PES band system of HCN has also been associated with the removal of an electron from a nitrogen lone pair u-orbital.I3-l5 Since the ionization potentials of the A22+state of the cyanogen halides (14.5,13.8, 13.6, and 13.2 eV for FCN, CICN, BrCN, and ICN, respectively) show only a slight dependence on the halogen substituent and also compare closely with that for the A2Z+state of HCN (14.9eV), BieriIoconcluded that a nitrogen lone pair description was only reasonable for the 5a

MO of HCN and 4u MO of the cyanogen halides. Bieri, however, also noted that the "perfluoro-effect" should stabilize the A2Z+ state of FCN by 2 to 4 eV relative to the A2Z+ state of HCN, whereas the observed stabilization was less than 0.5 eV. Recent ab initio calculationsI6of the ionic states of HCN predict that the 5u MO will contain nitrogen lone pair as well as carbon-nitrogen bonding characteristics. The near-energetic equivalence of the Sa MO of H C N and 4u MOs of the cyanogen halides would appear to require an equivalent description for these MOs. The 44 MOs of the cyanogen halides, however, must possess some carbon-halogen antibonding character if they are to account for the observed increases in the uI frequency in the second PES band system of FCN, CICN, and BrCN. (iii) 3u MO. The fourth ionization feature in the PES spectra of the cyanogen halides is broad and apparently unstructured!*'0 Lake and Thompson' associated this 3u ionization event with removal of an electron from a carbon-halogen bonding MO. Other investigators, however, have preferred a halogen lone pair description for the 3u M0.8-'o On the basis of the following arguments, the 3u MO of the cyanogen halides cannot be viewed as a simple localized MO and is, rather, heavily mixed with the 4u MO. In Figure 1, we present a schematic of the lower energy absorption and photoelectron spectrum of the methyl, cyanogen, and hydrogen halides. The ground-state outer-electron configuration of the hydrogen halides is usually writtenI7 (lu)*(2~)~(lr)';X'r Photoelectron in~estigation'~J~ and a b initio calculationsI6 have

(13) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Speciroscopy; Wiley: New York, 1970. (14) Hollas, J. M.; Sutherley, T. A. Mol. Phys. 1972, 24, 1123. ( I S ) Frost, D. C.; Lee, S.T.; Mchwell, C. A. Chem. Phys. Lett. 1973, 23, 472.

(16) Kimura, K.;Katsumata, S.;Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HrI Photoeleciron Spectra of Fundamental &gamic Molecules, Halsted Press: New York, 1981. (17) (a) Mulliken, R.S.Rev. Mod.Phys. 1932, I , 3. (b) Mulliken, R.S. Phys. Rev. 1936, 50, 1017.

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Electronic Spectroscopy of the Cyanogen Halides I

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TABLE I: Absorbance Data for the Cy.agen .ad Methyl Halides" 1

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A. A-Band Absorption System ClCN BfCN 49 600 56 300 band max, cm-l A 1776 2016 FWHM, cm-' 7250 6900 75 189 6.0 x 10-3 2.6 x 10-3 5.9 x 10-3 2.2 x 10-3 f,:

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CHyCI 58 400 1712 5550 278 9.4 x 10-3 6.7 x 10-3

band max, cm-'

A

FWHM, cm-l

;msl fa:

CH3Br 49 600 2016 7500 189 6.6 X IO-' 6.1 x 10-3

B. a-Band Absorption System ClCN BrCN band max, cm-l 68 650 63 250 A 1457 1581 FWHM, cm-' 8000 6000 555 695 1.8 X 2.5 X 1.9 X 1.8 X fat

;mix

ENERGY ( ~ m ' ~ x I 0 ~ ) Figure 5. Electronic absorption spectra of the cyanogen halides in the A-band region. The dashed line in each spectrum is the base line.

shown that the 1~ MO is mainly a nonbonding np, A 0 on the halogen and that the 2u MO is mainly H-X bonding, consisting largely of the np, halogen AO. The hydrogen-halogen stretching frequency exhibits an average percentage decrease relative to its value in the ground state of the neutral H X molecule of 7% in thefirst PES band system as opposed to 45% in the second PES band system." The 1u MO is viewed as a nonbonding ns halogen A 0 and, as such, would be expected to lie above 20 eV since the ns-np level separation in the C1 and Br atom is 12 eve2@ The maximum of the third PES band of HCl,ZObHBr, and HI2&occurs at 25.8, 24.3, and 21.2 eV, respectively, as expected for such descriptions of the 1~ and 1u MOs. The ground-state outer-electron configuration of the methyl le)4(4a1)2(2e)4;X'AI. halides2' is written as ( la,)2(2al)2(3a1)2( Although there is some debate on the nature of the high-energy 3al and 4al MOs, the 2e and the 4a1 MOs are usually taken to relate closely to the 1r and 2u MO,respectively, of the hydrogen halide^.'^*^'-^^ Photoelectron in~estigation~~.~' and a b initio calculationsI6 have shown the 2e MO to be mainly a nonbonding np, A 0 On the halogen and the 4al MO to be largely carbon-halogen bonding. Turner et a]." have shown that the energy required to remove an electron from the 4al MO of the CH3X species (X = Cl, Br, and I) is directly related to the energy of the carbon-

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(18)Lempka, H.; Passmore, T. R.; Price, W. C. Proc. R. Soc. London A 1968,301, 53.

(19)Eland, J. H. D. Photoelectron Spectroscopy; Buttenvorths: London,

1974.

(20)(a) Slater, J. C. Quanrum Theory 01Atomic Structure, Vol. 1; McGraw-Hill: New York, 1960 p 206. (b) Brion, C. E.;Hood, S.T.; Suzuki, 1. H.; Weigold, E.;Williams. G. R. J. J . Electron Spectrosc. Refat. fhenona. 1980.21,71. (c) Brion, C. E.;McCarthy, I. E.;Suzuki, I. H.;Weigold, E.; Williams, G. R. J.; Bedford, K. L.; Kunz, A. B.;Weidman, R. J . Electron Spectrosc. Relat. Phenom. 1982,27, 83. (21)Mulliken, R. S.Phys. Reo. 1935,17, 413. (22)Potts, A. W.; Lcmpka, H.J.; Street, D. G.;Price,W. C. Phil. Tram. R. Soc. London A 1970, 268, 59. (23)Karlwon, L.; Jadrny, R.; Mattsson, L.; Chau, F. T.; Siegbahn, K. Phys. Scrip. 1911. 16, 225.

C. B and C Rydberg ClCN B band; band origin, cm-' 70075 A 1427.0 FWHM, cm-' 100 tfMX 65 /(origin) 9.27 X IO" flband system) 1.25 X lo4 C band: 1111 band origin, cm-'

A

FWHM, cm-' CtMX

florigin) flband system)

/(e + C)

ICN 39 800 2513 8200 125 4.7 x 10-3 4.4 x 10-3 CHJ 38 800 2577 4250 315 6.2 x 10-3

5.8 x 10-3

ICN 57 000 1754 4000 940 2.3 X 1.6 X

Band Systemsb BrCN ICN 66 280 58 900 1508.7 1697.8 175 210 4560 20155 4.48 X 2.07 X 9.53 X IO" 3.62 X

ClCN 71 395 1400.7 30 7400 1.56 X lo-' 5.28 X IO-'

BrCN 68 265 1464.9 85 20 490 1.10 X IF2 3.38 X IF2

ICN 63 485 1575.2 150 33 785 2.79 X 5.68 X

5.40 X

4.33 X

9.30 X

D. &Band Absorption System component band max, cm-l

ClCN 72 305 1383.0 180(0!) 206 2.8 X

BrCN 72 985 1370.I 400(0!) 4620 2.03 X

ICN 67 970 1471.2 1700 24 845 1.99 X IO-'

BrCN 74 735 1338.1

flband system)

ClCN 74 420 1343.7 235(0!) 5545 4.06 X

21 235 9.41 X

ICN 72 620 1377.0 1950 15900 1.01 X IO-'

/(3n + In)

4.07 X

1.14 X IO-'

3.00 X IO-'

A

FWHM, cm-' Cmax

/(band system)

'Ill component band max, cm-'

A

FWHM, cm-' tmax

1000

E. Intense np Rydbcrg Band ClCN BrCN 79745 76 080 band ori in, cm-' 1254.0 1314.4 FWHM, cm-l 265 205 67910 81 895 1.03 X IO-' 1.11 X IO-'

1

;msl

ICN 71 325 1402.0 215 44 965 7.52 X

OThe oscillator strength is defined as j = 4.315 X I04Jrdu. The estimated oscillator strength is defined as/-, = 4.315 X 10-9[e,.x X AB] where Ai, is the band width (in cm-I) at the half maximum (FWHM) and emx is the extinction coefficient at the band maximum. The units of c are dm3 mol-' cm-'. Also see ref I . bThe reported FWHM, c, andjvalues do not include contributions of the underlying continuum.

halogen bond: the vertical 1P associated with removal of an electron from the 4al MO was found to correlate directly with the corresponding C-X bond dissociation energy. The carbonhalogen bonding character of the 4al MO is further supported

The Journal of Physical Chemistry, Vol. 95, No. 2, I991

644

I BAND

MAXIMUM

TABLE II: Vibrational Analysis of the 40

I

Systema

-

Felps et al. 5u; '2' a-Band

A. CICN

LL

STRENGTH

L

8r I Ci 8r I Figure 6. Plots of absorption data for the A-band and a-band systems: (A) band maximum of the A band of XCN (A), CH3X (0),HX ( 0 )and of the a band of XCN (0);(B) oscillator strength; (C) band width; and (D) extinction coefficient at the band maximum, e, of the A band of XCN (A)and CH3X (0)and of the a band of XCN (0). CI

1

'Or 0 8-

1

1

~

1

1

1

1

1

1

1

1

1

~

1

1

1

1

1

~

1

~

1

'

1

CI-CN

r

..

48

50

52

54

ENERGY (cm-l,/03)

56

-

58

60

Figure 7. Electronic absorption spectra of the cyanogen halides in the spectral region of the a band. For CICN, the pressures in the absorption cell were (a) 175 pm, (b) 2.8 mm, and (c) 8.0 mmHg. For BrCN, the p r w u m were (a) 310 pm, (b) 1.7 mm, (c) 3.0mm, (d) 6.2 mm, (e) 11.2 mm, and (f) 24.8 mmHg. For ICN, the pressures were (a) 42 pm, (b) 330 pm, (c) 1.84 mm, 306 K cell temperature; (d) 5.6 mm, 324 K cell temperature; and (e) 13.1 mmHg, 334 K cell temperature. Possible vibrational progressions are indicated for uj (long pips), ut (short pips), and uI (intermediate-length pips). The dashed line in each spectrum is the base line. Unless otherwise stated, the spectra refer to room-temperature (295 K) gaseous samples.

by the extensive vibrational structure exhibited by the second PES

band system of CH31.23Under high resolution, a progression of

1612.2 1594.1 1577.0 1570.7 1559.7 1554.2 1543.3 1537.0 1527.7 1521.5 1512.2 1505.5 1497.8 1483.8 1471.0 1457.5 1443.5 1428.5 1416.5

0 705 1385 1640 2090 2315 2770 3035 3430 3700 4100 4395 4740 5370 5955 6585 7250 7975 8670

56625 56731 56944 57 163 57200 57355 57770 58040 58185 58 895 59700 59900 60315 60535 60875 61 130 61480 62075 62655 63310 64000 64560 65200

1766.0 1762.7 1756.1 1749.4 1748.3 1743.5 1731.0 1722.9 1718.7 1698.0 1675.0 1669.5 1658.0 1652.0 1642.7 1635.8 1626.5 1611.0 1596.0 1579.5 1562.5 1549.0 1533.8

B. BrCN 0 00 106 1' impurity 3 19 I2 impurity 538 1' impurity 575 1' 730 1'2' 1145 l2 1415 1'2' 1560 3' 2270 ICN impurity 3075 '3 3275 2132 3690 1'3' 3910 1'2132 4250 1232 4505 122'3' 4855 1332 5450 1432 6030 1'3 6685 1632 7375 '31 1832 7935 8575 1932

52210 52355 52825 53305 53820 54115 54630 55095 55650 56195 56705 56 731 56 944 57 163 57 343 57534 57835

1915.3 1910.0 1893.0 1876.0 1858.0 1848.0 1830.5 1815.0 1797.0 1779.5 1763.5 1762.7 1756.1 1749.4 1743.9 1738.1 1729.0

C. ICN 0 0: 145 It 615 1' 1095 l2 1610 I' 1905 3' 2420 1'3' 2885 1'3' 3440 1'3' 3985 1'3' 4495 1'3' 4521 l2 impurity 4734 I2 impurity 4953 I2 impurity 51 33 l2 impurity 5324 I impurity 5625 133'

62025 62730 63410 63665 64115 64340 64795 65060 65460 65725 66130 66425 66765 67395 67980 68610 69275 70005 70595

'

1

~

1

00

I' 1'

122' I' 1'2' I' 1'2' I' 1'2' l6 162' I' l8 19 1'O

I" 1'2

lI3

0 705 680 255 705 225 680 265 665 265 670 295 635 630 585 630 665 730 590 0

575

155 570 270 1560

1515 200 615 220 560 255 605 595 580 655 690 560 640

615 480 515 1905 515 465

555 545 510

565

OThe quantity B, is the observed frequency interval for indicated progressions; ground-state frequencies are enclosed in parentheses.

38 vibrational bands with an average spacing of 275 cm-' is observed and corresponds to a 4846 average percentage decrease of the carbon-halogen stretching frequency relative to its value in the ground state of the neutral CH3X molecule. This finding is clearly indicative of a major weakening of the C-I bond upon removal of an electron from the 4al MO and, in this manner, the related bonding characters of the 4al and 2a MOs of the methyl

Electronic Spectroscopy of the Cyanogen Halides TABLE 111: vibrational Analysis of the 2r System0

-


nsR;'JII Band

TABLE I System'

V Vibrational Analysis of the 2 1

energy, cm-' 70075 1427.0 -

A. CICNb

o

72040 71020 71 395 71 555 71695 72040 72305 72650 73330 73585 73660 73960 75205 75860 76505 77100 77750

1388.1 1408.1 1400.7 1397.5 1394.8 1388.1 1383.0 1376.5 1363.7 1359.0 1357.6 1352.1 1329.7 1318.2 1307.1 1297.0 1286.2

65730

1521.4 1508.7 1491.2 1477.7 1477.7 1464.9 1448.9 1436.2 1434.6 1425.0 1421.7 1410.4 1397.5 1388.0 1374.4 1361.5 1353.3

780 1390 -590 0;1980 755 1365 1440 1910 2075 2640 3290 3780 4495 5185 5630

1711.7

-480

- -

067060 67675 67675 68265 69020 69630 69705 70175 70340 70900 71555 72045 72760 73450 73895

- -

1965 -375

0;1320 160 300 645 915 1255 1935 2190 2265 2565 3810 4465

5110 5705 6355

WnI)

1965 (375) 0 -

2'

300 645 265 610 1935 (375) 330 630 1875 655 645 1895 650

1'

1'2' 12

3' 1'213' 2'3' 1 '3' 32 1132 1232 33 1133

B. BrCN -555 I'

o

0 -

3' 29 Oo,('II')

- 1697.8 1682.4 1668.8 1641.6 1627.5 1587.4 1575.2 1562.5 1549.5 1540.2 1537.0 1528.5 1516.3

- -

o

540 1025 2545 2545 -490

0;4585 515 1055 1455 1580 1940 2465

A

72 305 1383.0 -

1' 12

I' Oo,(Q,) 1'

113' 12

3' I3;113l 1'3' 123' 32 1'32 1232 33

0 -

I'

oO,(~~,) 1' 12

3' 1'3' 1'

oo,('n,)

0;2115

75 100 76 290 76 970 78 065 78 760

331.6 310.8 299.2 28 1.O 269.7

675 1870 2550 3645 4340

72985

370.1

73 580 74 625 74 735 74 790 74 895 75 155 75 265 75 345 75 490 75 580

359.1 340.0

-

0;1750 55 160 420 530 610 755 845

13

3' 1'3a

(480) 0 540 48 5 1015 530 (490) 0

71 830

1392.2 1387.7 1383.9 1380.5 1377.0 1374.0 1371.5 1369.1 1368.1

OThe quantity P, is the observed frequency interval for indicated progressions; ground-state frequencies are enclosed in parentheses. Also see refs Ib, 4a, 5a, and 57. *The 72305 cm-' band of ClCN is also tentatively assigned as the 0; band of 2n 6u;)II1 transition; see text and Table IVA.

-

and hydrogen halides, respectively, are confirmed. The third PES band of the CH3X species, which is broad and energetically independent of the halogen substituent, suggests that the l e MO of the CHpX species is largely localized on the methyl group.2' *~~ This view has been confirmed by other PES s t ~ d i e s ~as* well as by ab initio calculations.I6 As noted above, the 3a MO of the XCN species has been described both as carbon-halogen bonding' and as halogen nonbonding.*-IO In Figure 2, we present a plot of the vertical IP associated with the 4al, 2a, and the 3a and 4 a MOs against the bond dissociation energy of the respective methyl, hydrogen, and

assignment

07)

P),

cm-l 0 -

0 -

675 1870 680 1775 695 0 595 1640 0 -

55

105 260 1IO

610 145 90

C. ICN

480.5 - 11477.4

535 (495) 525 1940 525

113'

0 595 1640

337.1 1335.2 1330.6 1328.6 1327.2 1324.7 1323.1

780 610 (590) 0 755 (545) 690 1910 635, (565) 725 655 1870 715 690 1845

515

6 ~ , &Band ~ ~ ~ n

B. BrCN

338.1

1486.5

12

0 -

1343.7

67 270 67 545 67 685 67815 67 970 68 105 68215 68 375 68 565 68 660 68 835 68 925 69 065 69 235 69 365 69 530 69 845 70075

I'

AP,cm-' A. ClCN

74420

(555)

~ n , )

C. ICN 58420 58900 59440 59925 60915 61445 62995 63485 64000 64535 64925 65060 65425 65950

A,

-

72 060 72 260 72435 72 620 72 780 72915 73 040 73 095

1474.6 1471.2 1468.3 1466.0 1462.5 1458.5 1456.5 1452.7 1450.8 1447.9 1444.4 1441.7 1438.2 1431.7 1427.0

'The quantity progressions.

Dj is

-275

0

140 270 425 560 670 830 1020

Ill5 1290 1380 1520 1690 1820 1985 2300 2530

0;4285 230 430 605 790 960 1085 1210 1265

-275 0 140 130 425 135 110

405 190 285 175 265 140 170 440 165 480 230

0 230 430 175 360 160 295 125 55

the observed frequency interval for indicated

cyanogen halide. A linear behavior is obtained for both the 3a and 4a MO of the cyanogen halides, thereby invalidating the supposition that the 3a MO of the cyanogen halides is solely carbon-halogen bonding. Only when the bond dissociation energies of the cyanogen halides are plotted against the average of the vertical IP's of the 4u and 3a MOs (solid triangles of Figure 2) do we obtain even fair agreement with the CH3Xt3and HX species. This result implies that the 4 a and 3a MOs of the cyanogen halides are strongly "mixed" and that neither should be viewed as a simple localized MO. It should be noted that Lake and Thompson had earlier reached similar conclusions for the low-energy a MOs of chlorinated methyl cyanides and alkyl cyanides.' ( B ) Virtual MO's. The lower energy virtual MO's of the cyanogen halides are usually described as antibonding with respect to the carbon-halogen bond, 5a, and the carbon-nitrogen bond, 6a and 37r.la In view of our previous discussions, such simple descriptions are surely inadequate. However, we will accept these descriptions tentatively for the purpose of facilitating initial

646 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 TABLE VI: V i h H o a r l Analysis of tbe lo

energy, cm-I

n'(2r) A,

A

77 535 79 340

-

1289.7 1260.4

78 065 -

1281.0 1251.0

79 935 79 745 81 585

energy, cm-'

1254.0

81 620 81 665 81 755 81 800 83485

1225.7 1225.2 1224.5 1223.2 1222.5 1197.8

84 090 -

1189.2

74235

1347.1

74 980

1333.7

76080

1314.4

76 475 77 035 77 520 77 620 78 090 '-78 525 79 000 79095 79 200 79 470 80065 80 160 80 400 80 790 80 865 80 900 80 965 81 050 81 200 81 275 81 320 81 420 81 865 82 170

1307.6 1298.1 1290.0 1288.3 1280.6 1273.5 1265.8 1264.3 1262.6 1258.3 1249.0 1247.5 1243.8 1237.8 1236.6 1236.1 1235.1 1233.8 1231.5 1230.4 1229.7 1228.2 1221.5 1217.0

70 860 71 325 71 530 71 830 72 065 73410 73 600

1411.2

AI,cm-l

assignment

E)

A. ClCN 0 ox 1805 3' and/or 0:

0 1870

0

1840 1875 1920 2010 2055 3740

0

I 2.23 -

I1

2.33

2.32

2.25

3' and/or 0:

2.37

2.35

ox

2.35 -

2.34 -

32

2.47 2.47 2.48 2.48 2.49 2.61

ox

2.67 -

3' and/or 0:

2.46 2.46 2.46 2.47 2.47 2.60

1402.0

74 020 74 975 75 120 75 325 75 575 75 965 76215

1351.0 1333.8 1331.2 1327.6 1323.2 1316.4 1312.1

-

"n'(2r)

1398.0 1392.2 1387.6 1362.2 1358.7

78435 79790 80300

0

0 395

2010 2445

4080

ox

2.25 -

2.18 -

1'

2.29

2.21

ox

2.26

2.27 -

2'

2.38 2.42 2.45 2.46 2.49 2.52

2.29 2.32 2.35 2.36 2.39 2.41

2.56

2.45

2.59 2.63 2.65 2.67 2.70 2.71 2.71 2.72 2.73 2.74 2.75 2.75 2.76 2.80 2.83

2.48 2.52 2.53 2.54 2.57 2.58 2.58 2.59 2.59 2.61 2.61 2.62 2.62 2.66 2.69

3' 2'3'

32

-465 0 205 505 740 2085 2275

1:

2.53

2.26

0; 2' I' 1'2' 3' 2'3'

2.57 -

2.28

2.58 2.6 1 2.62 2.74 2.76

2.30 2.31 2.33 2.41 2.42

2.80 2.90 2.92 2.94 2.97 3.02 3.02

2.45 2.51 2.52 2.54 2.56 2.58 2.58

-1 100

-145

0

205 455 845 I095

0:

= [ R / I - I)]'? also see refs la, 4a, and 5a of text.

discussions. We shall also assume that (i) the lower energy valence transitions arise from the promotion of an electron from one of the four filled MOs into one of the three virtual MOs, (ii) as is generally accepted for extravalence Rydberg configurational excitations, we will extrapolate and assume that the energies of the 12 intravalence configurational excitations may be predicted, with a proper choice of term value for the three virtual MOs, by means of the expression B = I - T,where I is the energy of the intravalence configurational excitation, I is the ionization energy of

AI, cm-l A. ClCN

1246.5 1242.7 1239.8 1234.6 1217.4 1214.0 1211.3 1207.2

1274.9 1253.3 1245.3

76465 1307.8

B. BrCN 745

A

77875 1284.1

2.65

C. ICN

-

80 470 80 660 81 000 82 145 82 370 82 555 82 835

2.22 -

2.26

ox

80 225 -

A,

76965 77415 78450 78925

1299.3 1291.7 1274.7 1267.0

0

-

m R I FBand S y s W assignment E) vj, cm-l

ox 2' 22

245 435 775 1920 2145 2330 2610

B. BrCN

0

Felps et al.

I' 3' 2'3' 2231 1'3'

ox

560 1915 2425

1'

3' 1'3'

C. ICN

ox

0 500 950 1985 2460

I' 12

3' 1'3'

0 245 190 775 II920 225 185 690 0 560 1915 510 0 500 450 1985 475

OThe quantity vj is the observed frequency interval for indicated progressions. Also see ref 4a. the filled MO, 277,4a, 17, or 3a, and T i s the chosen term value of the virtual MO, 5u, 37r,or 6u (all quantities expressed in cm-' units), and (iii) the energies of the excited states that derive from a given intravalence configurational excitation are centered about the 'predicted" energy of the corresponding intravalence configurational excitation (Le., we assert the existence of a direct state energy-configuration energy correspondence).

Results and Assignments A . A-Band System. King and Richardsonla convincingly relatedSathe lowest energy absorption feature of the cyanogen halides to the so-called A band of the hydrogen and alkyl hali d e ~ . " ~ * *The ~ A band in the hydrogen and methyl halides ~ I I2e 5a,;193Econfigurational corresponds to the lx ~ s , ' ~and excitation, respectively. Both of these are usually referred to as n us excitation. The lower MO is supposed to be a nonbonding np, A 0 on the halogen, and the 30 and 5al virtual MOs are thought to be strongly antibonding with respect to the halogen and the corresponding hydrogen or carbon atom.24 In Figures 3 and 4, we present electronic absorption spectra of cyanogen and methyl halides, respectively. The absorbance data are collected in Table I. As can be seen from Figures 3 and 4, the A bands of the corresponding cyanogen and methyl halides are very similar in (i) absorption energies and (ii) band breadths and profiles. Furthermore, the A bands of the different cyanogen halides (Figure 5) are so similar that even their band widths (FWHM E 7500 f 700 cm-I) are essentially identical. Indeed, the A bands (see Figure 1) of the cyanogen and hydrogen halides are also virtually identical. A graphic presentation of the absorbance data of Table IA is given in Figure 6, whence it is clear that the A-band maxima exhibit similar linear dependencies on the halogen substituent for each species, XCN, CH3X, and HX. The oscillator strengths (Figure 6B) and extinction coefficients, in dm3 mol-' cm-' (Figure 6D) of the A bands of the respective CH3X and XCN species compare very favorably except in the case of CH3Br and BrCN, which is somewhat surprising in view of the fact that the band widths of the bromide species are quite similar while those of the respective chloride and iodide species are quite different (Figure 6C). We conclude that the A bands of the oyanogen halides are a 277 5u;'Q configurational excitation just as in the methyl halides.

-

-

-

-

(24) (a) Mulliken, R. S . Phys. Reo. 1937. 51, 310. (b) Mulliken. R. S . J . Chem. Phys. 1940,8, 382. ( c ) Mulliken, R. S.Phys. Reu. 1942,61,277.

Electronic Spectroscopy of the Cyanogen Halides


TABLE VII: Tramition b m i e s md Vibrational Freauencksl Br I

-

intravalence 4u 5u;12+

-

Rydberg 2r n~R;~n, 2r

-

-

nsR;'n,

2~

-

6u;'nl

CI Br CI Br 1

6~;'nl

c1 Br

I CI

Rydberg 2r

I CI Br I I

intravalence 2r

CI Br

np or ndR

Br I

-

Rydberg nsR;IZ+

CI

cationic (2*)-';EZn3/2

CI Br

4u

Br I

(2r)-"1/2

I CI Br I

cationic

CI

(444

+

cationic (1 r)-'@n3/2 (1

r)-';EnI/2

Br I CI

Br I

CI Br I

0 0 62 025 56 625 52 210 70075 66 280 58 900 71 395 68 265 63 485 72 305 72985 67 545 74 420 74 735 71 830 77 535 78 065 79 745 74235 76 080 71 325 75 120 80 225 77 875 76 465 99 535 95 825 88 000 99 775 97 355 92 355 111310 IO9 535 106 230 121 775 114455 107 680 122 I40 1 I5 355 108 570

575 486 660 (-8) 605 ( + 5 ) 530 (+9) 695 (+21) 520 (+7) 640 (-10) 705 (+23) 525 (+8)

379 342 305 260 (-31) 220 (-36)

315 (-17)

595 (+4) 385 (-21) 685 (-4) 610 (+a) 360 (-26)

2216 2198 2188 1540 (-30) 1905 (-13) 1965 (-1 1) 2015 (-8) 1900 (-14) 1875 (-15) 1940 (-1 1) 1640 (-25) 1870 (-16) 1805 (-19) 1870 (-16) 1920 (-13)

745 (+30) 505 (+4)

735 (+3) 560 (-3) 475 (-2) 823 (+15) 650 (+13) 535 (+IO) 827 ( + l a ) 5 5 5 30 (-3) 559 (+15) 744 (+8) 5 5 5 30 (-3)

*

525 471 440 526 516 473

(-26) (-18) (-9) (-26) (-10) (-3)

415 (+21) 210 (-31)

2040 (-7) 2085 (-5)

210 (-45)

1920 (-13) 1915 (-13) 1985 (-9) 1916 (-14) 1906 (-1 1) 2082 (-5) 1914 (-14) 1850 f 30 (-16) 1950 30 (-1 1)

288 (-16) 239 (-22) 253 (-17)

1850

30 (-16)

274 (-10) 394 (+15)

1939 (-12)

'The ground-state vibrational frequencies are taken from Wang, V. K.; Overend, J. Spectrochim. Acra 1973 29A, 1623. The vibrational frequencies listed for the intravalence and Rydberg states are, in most instances, average values (see Tables 11-VI) and are from this work; also see. refs I , 4a, and 5a. The cationic values are taken from refs 7,9, and 11 . The percentage change of the excited-state value relative the ground-state value is shown in parentheses.

B. a-Band System. The a-band absorption feature of the cyanogen halides has no counterpart in the spectra of the methyl halides (see Figures 3 and 4). The a-and A-band systems differ in that the former exhibits (i) a lesser energy dependence on the halogen substituent and (ii) a significantly larger oscillator strength (Figure 6 and Table I). Furthermore, as is evident in Figures 3 and 7, the a band, unlike the A band, exhibits a distinct, weak vibrational structure atop a broad continuum. In ClCN, this structure is regular in both intensity and frequency intervals. In CICN, the major progression consists of the symmetric C-CI stretching made, u l , upon which is built a much weaker progression in the asymmetric Cl-C-N bending mode, u~.~' A vibrational analysis is given in Table 11. In BrCN, the vibrational structure is similar but the uI progression is less pronounced. (It should be noted that the sharp absorption features centered about 57 000 cm-I in the spectra of BrCN and ICN (Figure 7) are due to an I2 impurity and that the weak feature in the spectrum of BrCN at 58 900 cm-I is due to an ICN impurity.) In the spectrum of BrCN, an apparent discontinuity occurs in the 58 200-59 700 cm-' region. However, as is indicated in Table 11, the structure above 59 700 cm-' may be associated with an origin (or pseudoorigin) at 56 625 cm-I by means of a 38 assignment of the 59 700 cm-' absorption feature. The a-band system of ICN differs from those of ClCN or BrCN in that the origin band of the former discrete system is relatively intense and because activity in the u2 bend is absent. As in BrCN,

activity in the symmetric C-N stretching mode, u3, must be invoked in order to associate the entire band system with a single origin. In ICN, the origin band and the first vibrational band exhibit a doublet structure (- 145 cm-I) that is tentatively attributed to difference bands in the uI mode. The a-band oscillator strengths of Table I, obtained by doubling the area from onset of absorption to absorption maximum, are of similar size, f = 2.2 X lo-*, and are indicative of an electric dipole allowed transition. The allowedness of the discrete system is also supported by the dominance of progressions in the symmetric C-X stretch and the absence (in ICN) or near-absence (in BrCN and CICN) of activity in the asymmetric X-C-N bend. An alternative interpretation of the vibrational structure for ClCN and BrCN would associate the major progression with the asymmetric X-C-N bending mode and the weaker features with members of a corresponding hot-band progression in the u2 mode. This interpretation is rejected because the u2 frequency would have had to increase, on the average, by -75% relative to its ground-state values for both ClCN and BrCN. In our preferred interpretation, the u, mode exhibits an average frequency change of -8%, +5%, and +9% relative to its ground state value for CICN, BrCN, and ICN, respectively; the u2 mode exhibits a 31% and 36% average frequency reduction for ClCN and BrCN, respectively; and the u3 mode exhibits a 30% and a 13% frequency reduction for BrCN and ICN, respectively; relative to its ground-state value.

Felps et al.


,

I

2 n*f2rJ

, , ,,,,,

I

4

naf4eJ 3

2

1

1

1

.

62

58

I

r

I

l

1

8

1

I 1 1 L

?

n*frrJ

2 $ f i ' l * l * f 1i 1

oo

-

;

1

1

.

70 74 78 ENERGY (cm-lx IO3)

4

I

8

1

,

I

I

82

I

I

.

3

_ , . I

3 8 wm

.

l

a

1

I

90

86

>

1

94

Figure 8. Electronic absorption spectra of the cyanogen halides in the 57 000-95 000 cm-l spectral region. The dashed line in each spectrum is the base line. See text and Figure 19 for alternative correlation scheme. 72 1

--

'

74

1

~

76 1

~

78 1

'

82

80 1

'

1

84 '

1

1 !

'

-

I

I

,

, n'(2rJ

2

l 72

2.6 ,

l 74

,

l 76

,

l 78

ENERGY(cm-fxf03)

,

l

,

l 82

80

,

l 84

,

.

Figure 9. Electronic absorption spectrum of ClCN in the 71 000-85 OOO cm-' spectral region. Possible vibrational progressions are indicated for u3 (long pi?), v2 (short pips), and uI (intermediate-length pips). The dashed line in each spectrum is the base line.

We associate the continuum and the discrete absorption of the cr band with different intravalence configurational excitations. The continuum is associated with transitions to the (dissociative) IlA, 133Z+,Iv3Z- states that arise from the 2r 3 r configurational excitation and the discrete structure with the transition to the IZ+ state that arises from the 4a SU;'*~Z+ configurational excitation.

-

-

1

'

72 1

'

74 I

'

76 I

78 I

'

80 I

82 '

1

1

1

-

i:j w

1

4 2

2.6

2

'

0

Flgure 10. Electronic absorption spectrum of BrCN in the 71 000-83 OOO cm-' spectral region. Possible vibrational progressions are indicated for ~3 (long pi?), Y Z (short pips), and uI (intermediate-length pips). The dashed line in each spectrum is the base line.

C. @-BandSysrem. As is the case for the intravalence A- and a-band systems, the @-bandsystem is common to the spectra of the three cyanogen halide species. Whereas in ClCN only one


66

68


72

70

ENERGY

74

-

78

76

I

80

(d, lo3)

Figure 11. Electronic absorption spectrum of ICN in the 66000-81 500 cm-' spectral region. Possible vibrational progressions are indicated for uj (long pips), u2 (short pips), and uI (intermediate-length pips). The dashed line in each spectrum is the base line.

74

76

78

80

82

86

88

90 4

5 4

3

6."

2

9 X 1 7E 0

r . m __________________ _______--------

0

-

ENERGY ( ~ r n - ~ x 1 0 ~ )

Figure 12. Electronic absorption spectra of CHJ and ICN in the spectral region ofthe lowest energy ionization potential, 2e and 2r, respectively. The dashed line in each spectrum is the base line.

discrete band system is clearly observed, in BrCN and ICN the j3 band is composed of two discrete absorption band systems having an energy separation (1750 and 4285 cm-I, respectively) corresponding closely to the spin-orbit-induced separation of the (2*)-';X2II3/2,1/2 states" of the ion (1477 and 4343 cm-', respectively). Absorbance data is presented for the @-band system in Table 1. For comparison purposes, we also include absorbance data for I l , lie at the B and C Rydberg band systems, 2~ n ~ R ; l - ~ that

-

longer wavelength and for the intense Rydberg band that lies at shorter wavelength relative to the &band system. In Tables 111 and IV we present vibrational analysis for these bands as well as some other Rydberg band systems that lie in this same energy region. The excited state vibrational frequencies are tabulated and compared with the ground and cationic state vibrational frequencies in Table VI1. In CICN, we associated the discrete 8-band system with electronic origin a t 74420 cm-l 5a with the Inl state that arises

650 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 82

84

88

86

90 I

I

92 I

I

I

,

TABLE VIII: Hydrogen Hdide Data of Figure 14

MO

lr-3r

4u+6u

8

94 I

I

Felps et al.

6I '3' 1

fa""

A.

cis,

HCI HBr HI Molecular Orbital Binding Energy, cm-' (I

-38 145 -102825 -103470 -133930

30

I* 2a In

2a

--

-40 275 -93 930 -96 625 -125895

-41 450 -83 775 -89 125 -122925

B. Transition Energy, cm-l 65 OOO 55 000 95 785 85 620

3a 3a

45 OOO 71 475

C. Dissociation Energy, cm-' fragment states

HCI

(QY.0")

molecule HBr

HI

corresponding parent states

X(2P)+ H(X2S) X-('S)+ H+

1

I

1

nC(2nil

-

4

I

3

I

I

I

,

I

I

, < I

1

9

1

5

I

4

I

.

1

5

I

I

1 1 1

1 10

I

6

,

IJZ+

(om

116290

112830

109650

'8+

(090)

162570

147650

135570

'2+

X+(lS) + H-('S)

3

'*'ll

35 730b 30 250b 24650b 36610 33935 32255

(3/2,1/2) ( 1 /2,1/2)

D. Values Used To Obtain Dissociation Energies confinuration CI Br I 1. Electronic Energies of the Halogens, cm-' nPS 2p3,2

0 880 104990 132 890

2Pl,* np' 'P2 I

so

0 3685 95 550 [ 123 4501

~

0 7605 84 340 1 I6 970

2. Ionization PotentialCof H: IO9 678 cm-' 3. Electron Affinitiesd

atom H CI Br I

TRANSITION ENERGY

BINDING ENERGY

140

3c7I20

-.., ..-

nBr 100

i lL < Y

X-tHt

HI

-80

DISSOCIATION ENERGY

-100

nci

I20

60 HBI

-120

-160

1 1

J

OL

t

1

X P P ) t Hf2S,,pl

I2O

-lo

Figure 14. Diagram of M O binding energies (left), transition energies (center), and dissociation energies (right) of the hydrogen halides.

-.

from the 2 s ~ u ; ' * ~configurational II excitation (Table IVA and Figures 8 and 9). We also tentatively assign the weak feature at 7 2 3 0 5 cm-' as the origin band for the corresponding I l l , component (Table IVA) based on the vibrational analysis of the B and C band systems of ClCN (Table IIIA). In BrCN (Table IVB and Figures 8 and IO), we associate the discrete 8-band systems, origins a t 7 2 9 8 5 and 7 4 7 3 5 cm-', with the 3rIl and In, states, respectively, that arise from this same configurational excitation. The discrete @-bandsystems in ICN, origins at 6 7 545 and 71 830 cm-I, are assigned similarly (Table IVC, Figures 8 and 11). The experimental evidence for this assignment is as follows: ( I ) the observed vibronic interferences between the C Rydberg band system, 2 n 4sR;IrIl, and the 7 4 4 2 0 cm-'band in ClCN;5a (2) the diffuseness and vibrational structure of these band systems (Figures 8-1 I , Table 1V); and ( 3 ) the alternative assignment of

-

6050 29 120 27 100 24 680

a Binding energies for the In and 20 MOs of HCI, HBr, and HI are taken from refs 32,33, and 19, respectively. Vertical values are listed for the 20 MO. "eference 34. cReference 35. Value in brackets is estimated. Reference 36.

-

100

80

cm-'

these band systems as the first member of a 27r npR series is unsatisfactory because of (a) the large oscillator strengths of the band systems (Table ID), (b) the absence of a correspondingly intense band system in the spectra of methyl halides. (The effective quantum number scales, n*, for the respective 2 r , 4u, 17r ionization potentials of the cyanogen halides, as obtained from the expression n* = [ R / ( I - i j ) ] ' / * , are shown below the spectra of Figure 8; the n* scale for the 27r ionization potential is also shown on the spectra of Figures 9-13) [On the basis of effective quantum numbers magnitudes, this band should lie at 1517 A (n* = 2.08) in the spectrum of CH3CI,should be centered at 1609 A (n* = 2.19) and 1535 A (n* = 2.21) in CH3Br,and at 1752 A and 1609 A (n* = 2.35) in CHJ. Examination of the spectra of the methyl h a l i d e ~ ~reveals ~ q ~ ~no such band systems.], (c) the effective quantum number (n*) of the lowest energy 17r npR transition in HC1,27HBr,28and HIz9 is 2.27, 2.31, and 2.35, respectively [On the basis of similar effective quantum numbers, the lowest energy 27r npR transition should lie at - 7 8 240 cm-' (n* = 2.27) and - 7 5 260 cm-l (n* = 2.31) in ClCN and BrCN, respectively. An assignment of the 7 4 4 2 0 cm-' and 7 2 9 8 5 cm-I feature in the spectra of ClCN and BrCN (Figures 9 and 10) as the lowest energy 27r npR transitions is unreasonable since this necessitates a spectral red shift of 3820 cm-I and 2275 cm-',

-

-

(25)Felps, W.S.;Hochmann, P.;Brint, P.; McGlynn, S . P. J . Mol.

Specrrosc. 1976, 59, 355.

(26)Scott, J . D.;Felps, W. S.; Findley, G.L.;McGlynn, S.P.J . Chcm. Phys. 1978, 68,4618. (27)Tilford, S.G.;Ginter, M. L. J . Mol. Specrrosc. 1971, 40, 568. (28)Ginter, M. L.;Tilford, S . G . J . Mol. Specrrosc. 1971, 37, 159. (29)Ginter, M. L.;Tilford, S.G.;Bass, A. M. J . Mol. Specrrosc. 1975, 57, 271.


Electronic Spectroscopy of the Cyanogen Halides O

TABLE IX: Cynnogen Hnlide Dpta of Figure I5

[ ,.:p]I8O[

A. Molecular Orbital Binding Energy, cm-l "

-2040

-

m - -60-

P

E'

.

40-

2

-

6

.

& -100-

2 -120-

-160 -

-

-140

1

TRANSITION ENERGY -180-

lo

DISSOCIATION ENERGY

Figure 15. Diagram of MO binding energies (left), transition energies

(center), and dissociation energies (right) of the cyanogen halides.

-

respectively, a t least compared to the corresponding HX species. For ICN, however, the lowest energy 2 r npR transition is predicted to occur at 68 130 cm-'(n* = 2.35). In the case of ICN, the 68 000 and 7 2 250 cm-I band systems (Figure 1 1 ) can then be s u p p e d to arise from both a 27r 6a intravalence excitation and the lowest energy 2 r npR excitations. This dual assignment for ICN is in complete accord with the observed structural complexity of both band systems.], and (d) in the case of ClCN and BrCN, an apparent absence of any higher series members with similar quantum defects (Figure 8). Based upon the criteria listed under item 3, it is clear that few if any of the discrete features in the spectra of the cyanogen halides, the B and C bands excepted, can be assigned as 27r nlR excitations. Indeed, examination of Figure 8 reveals no regular Rydberg series converging to the 2 r ionization limits in ClCN, BrCN, or ICN. A comparison of the spectra of CHJ and ICN in the spectral region of the 2e and 2 r ionization limits, respectively, reveals many convergent series for CH31but none for ICN (Figure 12). Similarly, examination of the spectra of ClCN and BrCN (Figure 1 3 ) reveals no obvious convergent Rydberg series such as have been reported for CH3ClMand CHJBr.31 Although the apparent absence of "regular" Rydberg structure in the cyanogen halides may be ascribed, in part anyway, to interaction among the 2r, 4a, and 1%Rydberg manifolds, this observation lends considerable credence to the introduction here of the virtual 6a M O and to its imputed importance in producing intravalence excitations.

- -

-

MO

ClCN

BrCN

ICN

60 3r 50 2r 40 Ir

-26 155 -31 005 -43 355 -99 535 -99 775 -111310 -123 815

30

-1 53 495

-22 580 -33 330 -46 980 -95 825 -97 355 -109 535 -114455 -115665 -145 750

-19680 -33 180 -50 380 -88 OOO -92 355 -106 230 -107 680 -108 890 -134780

----

B. Transition Enerav. -.. cm-'

excitation

ClCN

BrCN

2r 2r 2r

56 300 68 650 73 500 67 955 80 305 85 155 80 460 92810 97 660 1 IO 140 122 490 127 340

49 600 63 250 74010 62 555 76 205 86 955 68 080 81 730 92 480 98 770 122 420 I23 I70

40

50 3r 60

50

40 3r 40 -. 60 Ir 50 Ir -. 3 r Ir 60 30 50 30 3r 30 60 -+

---. -

ICN 39 800 57 OOO 70 500 55 850 73 050 86 550 57 905 75 105 88 605 84 400 101 600 115 100

C. Dissociation Energy, cm-' fragment states (QY,Q"

X(2P) + CN(x2Z+) (3/2,1/2) ( 1 12.1/2)

Molecular Orbital Scheme

ClCN 34925b 35805

molecule BrCN

ICN

corresponding parent states

304106 34095

26215c 1*3n 33820 'q3Z+

39 655 39 705 43 340 43 390

35 460 35510 43 065 43115

56 160 59 845

51 965 59 570

84 895 84 900 88 580 88 585

80 700 80 705 88 305 88310

117040

115265

123065

112390

A. Hydrogen Halides. In order to discuss the configurational excitations that produce the intravalence band systems of the cyanogen halides, we must discuss an M O scheme that includes the virtual MOs (Le., Sa, 357, and 6 4 . Precedence for such a scheme for the hydrogen halides is found in Mulliken.2" Figure 14 is a graphical representatiton of Mulliken's HX MO scheme. The binding energies for the 1r and 2a MOs are taken from PES investigations. The binding energy of the 3a M O is chosen such that the difference between the binding energy of the 3a M O and the "average" binding energy of the lr M O equals the observed transition energy of the 1r 3a;'q'II configurational excitation (Le., the maximum of the A-band continuum (see Figure 1 ) ) . A prediction of the transition energy of the 2a 3a;'*'2+configurational excitation, for example, may then be obtained by taking the difference between the binding energies of the 3 0 and 2a MOs. Transition energies for the 2a 3a configurational excitation obtained in this manner are plotted in Figure 14 (center) together with the observed 1r 3a excitation energies for HCI,HBr, and

CReference42. dSee Table IllD for halogen atom data. Data for CN taken from ref 43.

(30) Truch, D. T.;Salomon, D. R.;Armstrong, D. A. J . Mol. Specrrosc. 1979, 78, 3 1. (31) Causley. G. C.; Russell, B. R. J. Chem. Phys. 1975, 62, 848. (32) Weiss, M. J.; Lawrence, G. M.;Young, R. A. J. Chem. Phys. 1970, 52, 2867. Natalis, P.;Pennetreau, P.; Longton, L.; Collin, J. E. Chem. Phys. 1982. 73. 191. (33) hlwiche, J.; Natalis, P.; Momigny. J.; Collin, J. E. J . Elecrron Specrrosc. Relat. Phenom. 1972/13, 1. 219.

Hi. The energies of the states of the separated halogen and hydrogen atom species that are pertinent to discussion of the low-energy, excited valence states of the hydrogen halides are plotted on the right-hand side of Figure 14. The data of Figure 14 are tabulated and Dresented in Table VIII. Under case (c) couiling, the X(2P3/2) + H(2S1/2)atom fragstates and the X(*P1,,) + H(2S1/2) ments connect with 'II,3112,1,0*

-

-

-

D. Values Used To Obtain Dissociation Energiesd state CN I . Electronic Energy of CN, cm-' 0 9245 9295 25 750 54 485 54 490

-

2. Ionization Potential of CN: I 1 3 730 cm-' 3. Electron Affinity of CN: 30795 cm-'

"Binding energies of the 2 r , 40, IT, and 30 MO's are taken from Vertical values are listed for the 30 MO. bReference 41.

refs 7-9.


Yo

CYANOGEN CHLORIDE

I6Or

CYANOGEN IODIDE

1 In-37

100

-...-a

-160

160-

0*

-

- 140

140-

(b)

fa)

120-

fI'+CN-

loo:

30-3n-

-

I77-60-

--

40-fz-

- I20

I-+CNt

3u-s

(c)

- 100

-

80

'I

X'Z+ -

ZIP+_ Figure 16. Parent-to-fragment state correlation diagram for CICN. 0

CYANOGEN BROMIDE

n-

100-

0

-

N

To {loo

-$E -

7

80-

60-

e t : Lu

5

40-

2oL

X'z+-

0

Figure 17. Parent-to-fragment state correlation diagram for BrCN.

atom fragments connect with l F 1 3 Z +states of the parent HX m o l e c ~ l e .The ~ ~ ionic ~ ~ ~fragments, X-(lSo) H+ and X+('So) H-('So) connect with IZ+ states of HX. According to the potential energy curve constructed by M ~ l l i k e nfor~ ~HI, ~ the l(2P3/2) H(2Sl/2)atom fragments connect with the I F ground state of HI and with the Ill,3112,1,0states that arise from the In 3u configurational excitation; the I(2P,/2)+ H(2Sl/2)atom fragments connect with the remaining 'I&,+ component of the In 3u configurational excitation and with the 32+component that arises from the 2u 3u configurational excitation; and the remaining IZ+component of the 2u 3u configurational excitation connects with the I-(IS0) + H+ atom fragments. The transition energy and dissociation energy diagrams of Figure 14 for the hydrogen halides suggest that the 'E+state that arises from the 2u 3u configurational excitation is a bound state, whereas the 32+component is dissociative, as also are all states that arise from the 17r 3u configurational excitation. Recent investigations of the hydrogen halides verify the accuracy of these predictions. The bound IZ+ state arising from the 2u 3u configurational excitation has been identified in the absorption spectrum of HCI, DCI," HBr, DBr,'* and HI,D139and lies near the predicted energy of 95 785 cm-', 85 620 cm-I, and 71 475 cm-' for HCI, HBr, and HI, respectively (see Figure l ) , and recent

+

+

-+

+

-

-

-

-

04) Herzberg, G. Molecular Spectra a d Molecular Structure I. Spectra of Diatomic Molecules, Van Nostrand Reinhold: New York, 1950. (35) Moore, C. E.Atomic Energy Levels, US. Narl. Bur. Srd. Circ. No. 467, 1949;Vol. I; 1952;Vol. 11; 1958;Vol. 111. (36)Handbook of Chemistry and Physics, S2nd ed.; Weast, R. C., The Chemical Rubber Co.: Cleveland, OH, I97I , 37) Douglas, A. E.;Greening, F.R. Can. J. Phys. 1979, 57, 1650. Ginter, D. .; Ginter, M.L. J . Mol. Sprcrrosc. 1981, 90,177. (38)Ginter, D. S.;Ginter, M. L.; Tilford, S.0.J . Mol. Spectrosc. 1981, 90, 152. (39) Ginter, D. S.; Ginter, M. L.; Tilford, S. G.; Bass, A. M. J . Mol. Spectrosc. 1982,92, 55.

L

-

i,

Figure 18. Parent-to-fragmentstate correlation diagram for ICN.

photofragmentation studies of HI within the A-band absorption continuum confirm the presence of the 1*311and 3Z+states, which arise from the 1 7 3 0 and 20. 3u configurational excitations, respe~tively.~~ (B) Cyanogen Halides. The proposed MO scheme for the cyanogen halides is presented in Figure 15. We shall show that this M O scheme reliably predicts the major absorption features of the cyanogen halides and permits the construction of parent state/fragment state correlation diagrams that, for the most part, support all reported photochemical investigations. The data used to construct Figure 15 are tabulated in Table IX. The binding energies of the occupied 27r, 4u, IT, and 3u MOs used in constructing the binding energy diagram for the cyanogen halides are of PES origin. The binding energy of the 50 M O is chosen such that the difference between the binding energy of the Sa MO and the "average" binding energy of the 27r M O equals the transition energy at the maximum of the A-band continuum. Similarly, the binding energy of the 37r MO is chosen such that the difference between the binding energy of the 37r MO and the "average" binding energy of the 27r MO equals the transition energy at the maximum of the a-band system. Finally, the binding energy of the 60 MO is chosen such that the difference between the binding energy of the 60 M O and the "average" binding energy of the 237 MO equals the transition energy a t the barycenter of the discrete @band systems that lie immediately to higher energy of the B and C Rydberg transitions (see Figures 3 and 8-1 1). The @-bands stems are centered at 1470 A and 1380 A in ICN and at 1370 and 1335 A in BrCN. In CICN only one intense band system is identified and the origin band lies at 1344 A. The predicted transition energies for the remaining nine intravalence excitations are shown in Figure 15 and listed in Table IX by taking the appropriate differences of MO binding energies. The "fitted" transition energies of the 27r 50, 27r 37r, and 27r 6 u intravalence excitations are also shown in Figure 15. The anergies of the states of the separated halogen and cyanogen fragments pertinent to a discussion of the lower energy excited valence states of the cyanogen halides are shown on the right-hand side of Figure 15 and are listed in Table IXC. The parent state/fragment state correlation diagrams for CICN, BrCN and ICN, constructed using the data of Figure 15, are shown in Figures 16-18. The data of Figures 16-18 are tabulated in detail in Table X. An alternate correlation diagram to that shown in Figures 16b, 17b, and 18b for C I Q I , BrCN, and ICN, respectively, is given in Figure 19 for the purpose of later discussions. This alternate correlation diagram is also included in Table X. where it is identified as "case 11".

-

-

1

-

- -

(40)Van Veen, G. N. A.; Mohamed, K. A.; Baller, T.; De Vries, A. E. Chem. Phys. 1983,80, 113. Brewer, P.; Dss, P.; Ondrey, G.; Bersohn, R. J .

Chem. Phys. 1983,79, 720. (41) Davis, D.D.;Okabe, H.1. Chem. Phys. 1986,49, 5526. (42)Baronavski, A. P. Chem. Phys. 1982,66, 217. (43)Huber, K. P.;Herzberg, G. Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979.



TABLE X Parent State to Fragment State Correlation

enernvb 0 56 300 -I

67 955

68 650 73 500 80 305 80 460 85 155 92810 97 660 1 IO I40 122 490 I27 340

energyb 0 49 600

62 555

63 250 68 080 74010 76 205 81 730 86 955

-

92 480 98 770 1 I2420 123 170

energyb 0 39 800

55 850

57 OOO 57 905 70 500 73 050 75 105 84 400 86 550 88 605 101 600 115100

A. ClCN parent state' state (comments) ( 3 0 ) ~l(r ) 4 ( 4 ~ ) 2 ( 2 r ) 4 ; g Z (ground + state) 2r ~U;~II~,~,~,'II~ (A-band continuum) 2r 5u:3&+ 40 5uf3Zr (lies within A band) 4u 5u;'Z+ (a-band system; origin 62025 cm-I) 3r;'*3A,Zt,Z- (a-band continuum) 2r 2r 6u;Is3II @-band system; origin: 72 305, 74420 cm-I) 3 r ; W I (dissociative) 4u Ir 5u;'JII (dissociative) 1. 40 6u;%+ (see text and Figure 16b) 6u;'2+ (origin: 80 225 cm-I) 4u 11. 4u ~ u ; ~ - (dissociative; ~Z+ see text and Figure 19) lr 3 ~ ; ' * ~ A , 2 + ,(dissociative) 2In 6a;lq'II (dissociative) 5 u ; ' W (dissociative) 3u 3u 3r;ls311 (dissociative) I. 3u ~ u ; ~ Z(see + text and Figure 16b) 3u 6u;IZ' (bound) 11. 3u 6u;'*%+ (dissociative; see text and Figure 19)

-----------

B. BrCN parent state" state (comments) ( ~ I J )1~r )( ' ( 4 ~ ) ~ ( 2 r ) ' ; X Z +(ground state) 2r ~U;~II~,~,,-.~II~ (A-band continuum) 2r 5u;'&+ 40 50;)2+ (lies within A band) 40 ~ u ; ~ Z(a-band + system; origin: 56625 cm-I) 2r 3r;'~3A,Et,Z- (a-band continuum) Ir 5u;l.'II (dissociative) 277 6u;'*'II @-band system; origin: 72 985, 74 735 cm-I) 3 r ; l 4 I (dissociative) 44 Ir 3r;1*3A,Zt,2- (bound) 1. 40 6u;%+ (see text and Figure 17b) 4u 6u;I2+ (origin: 77 875 cm-I) 11. 4u 617;'*~2+ (dissociative; see text and Figure 19) 1r 6u;'s311 (dissociative) 5u;l%+ (dissociative) 3u 30 3r;'v311 (dissociative) 1. 3u 6u;%+ (see text and Figure 17b) 3u 6u;lZ+ (dissociative) 11. 3u ~u;'*~Z (dissociative; + see text and Figure 19)

--

----------

C. ICN parent state' state (comments) ( 3 ~ )lr)4(4u)2(2r)4;gZ+ ~( (ground state) 2r ~U;'II~,~,~-,'II~ (A-band continuum) 50;3&+ 2r 40 5u;'Zt (lies within A band) 5u;'Zt (a-band system; origin: 52210 cm-I) 4u 2r 3r;'*3A,Zt,Z- (a-band continuum) In 5u;I"II (dissociative) 2r 6u;lv3II (,%band system: origin: 67 545, 71 830 cm-I) 4u 3r;lJII (dissociative) lr 3r;'+3A,Z+,Z-(bound) I. 3u 5u;'Z+ (dissociative; see text and Figure 18b) 30 5u;'Z+ 11. 30 50;'9~Z+(dissociative; see text and Figure 19) I. 4u 6a;'Zt (see text and Figure 18b) 4u 6u;'Z' (origin: 76 465 cm-I) 11. 40 6 ~ ; ' + ~(dissociative; 2+ see text and Figure 19) 1r 6u;'*'II (dissociative) 30 3r;'v'II (dissociative) I. 3u 6u;'Z' (dissociative; see text and Figure 18b) 3u 6u;'Z' 11. 3u 60;'8~Z+(dissociative; see text and Figure 19)

-------------

CI 2p3/2 2p3/2

2PI I ?

Br

fragment state CN X2Z+ X22+

".

ener& 34 925 34 925 35 805 35 805 119535 44 I70 89415 44 220 45 050 45 100 90 295 45 100 89410 90 290 61 555 60 675 90 295 137020 90 295

RE+

fragment state CN

energy 30410 30410 34 095 34 095 117040 39 655 39 705 84 900 43 340 84 895 43 390 88 585 43 390 88 580 59 845 56 160 88 585 123065 88 585

X9+ ZZ+

2p3/2 2p3/2 %/2

X22+

2pl/2

Br-( ISo)

ZZ+ CN+(I Z+)

2p3/2

gn3/2

2p3/2

A2&/2

2p3/2

2pl/2

m / 2 gn3/2

2p3/2

En312

2pl/2

A2&/2

2pl/2

$&/2

2pl/2 2pl/2

m / 2 D2n3/2

2pl/z

EZ+

2p3/2

BZZ+

2Plj2

m

Br

CN-(IZ+)

('so)

2pl/2

I 2p3/2 2p3/2 2pl/2

/

2

D 2 q 2

fragment state CN X2Z+

$Z+

2pl 2

X%+ ZZ+

I-({So)

CN'('2')

2p.3/2

2p3/2 2p3/2

2pl/2 2p3/2

2pl/2

2pl/2 2pl/2 2Pl/2

2pl/2

m / 2 D2q2 En3/2 D2n3/2

@P

+p

2pl/2

$;)I2

2pl/2

$;:I2

2p3/2

Bn,

%(2

I+( So) 2pl/2

,

2

CN-4 E+)

D2n,/2

-

energy 26215 26215 33 820 33 820 115265 35 460 35510 80705 43 065 80 700 43 115 59 570 43 115 59 570 88 310 59 570 88 305 51 965 88 310 112390 88310

'See Figures 16b-18b for "case I" correlations and Figure 19 for *case 11" correlations of the I%+ states arising from u u* type excitations; also see text. bEnergy values are in cm-I. "Fitted" energy values are underlined to distinguish these from "predicted" energy values. CEnergyvalues are in cm-1.


,

"I

160 CI-CN

Or-CN

140

Felps et al.

- -

-

F

u

-. i

i

19. Alternative parent-to-fragment state correlation diagrams for

us configurational excitations of

the cyanogen halides.

Fragment Correlations A. Parent States Connecting with the X(2P3/21/2) + CN(&P) and X(lS0) CW('Z+) Fragmenr States. According to Figures 16a-I8a, the lowest energy configurational excitation, 217 Sa, yields both X(2P3/2) and X(2Pi/2)fragments, which agrees with the photochemical products obtained by excitation within the A-band continuum of ICN.44*45Photochemical investigations have also shown that only CN(X22+) is produced by photolysis in the A continuum of ClCN,BrCN,"-5i and ICN.42,5+56 We connect the 32+state that arises from the lowest energy u u* configurational excitation, namely, 4u ~ u ; ~ Z with +, the X(2Pi/2)+ CN(X2Z+) fragment state (Figures 16b-18b). According to our correlation diagrams, the transition to the 4u ~ u ; ~ Zstate + must lie within the A continuum because we associate the discrete structure of the a-band system with the transition to the corresponding 4u 5u;'Z' component. This lowest energy 'Z+ state is connected with the X-(l&) CN+('Z+) fragment states by direct analogy with the hydrogen halides. As in HX, the IZ+ state is predicted to be a bound state. The predicted energy for the 4u 5u;lZ+ excitation of the cyanogen halides is greater than that observed by only 5930 cm-I in ClCN and BrCN and 3640 cm-' in ICN. Furthermore, this transition in the cyanogen halides resembles its counterpart in the hydrogen halides in that the associated band systems exhibit long progressions in the C-X and H-X stretching modes, respectively (see Figure I ) . B. Parent States Connecting with the X(2P3/2,i/2)+ CN(A2113/2,1,2) Fragment States. As noted previously, we have fitted ~JZthe transition energy of the 27r 3 ~ ; ~ 3 4 ~ 3 Z + ,configurational excitation to the maximum of the a continuum. We connect the parent states that arise from this 2~ 37r configurational excitation (Figures 16c-18c) with the X(2P3/2) + CN(A2113/2) fragment states and, thereby, account for the continuum underlying the a band. The X(2P3/2)+ CN(A2111/2)fragment states connect with the Iv3II parent states arising from the 4u 37r configurational excitation of ClCN (Figure 16a) while, for BrCN

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+

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-

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+

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__

(44) Amimoto, S.T.; Wiesenfeld, J. R.; Young, R. H. Chem. ffiys.Leu.

__.

1979. _ _69. , 4112

(45) Pitts, W. M.; Baronavski, A. P. Chem. Phys. Leu. 1980, 71, 395. (46) Sabety-Dnonik, M.; Cody, R. J . Chem. Phys. 1976, 64, 4794. (47) Halpern, J. B.; Jackson, W. M. J . Phys. Chem. 1982, 86, 3528. (48) Lu, R.; Halpern, J. B.; Jackson, W. M. J . Chem. Phys. 1984, 88, 3419; Erratum, ibid. 1984.88, 6460. (49) Heaven, M.; Miller, T. A.; Bondybey, V. E. Chem. Phys. Lett. 1981, 84, 1.

(50) Nadler, J.; Reister, H.; Wittig, C. Chem. Phys. Lett. 1984, 103, 451. (51) Fisher, W. H.; Eng, R.; Carrington, T.; Dugan, C. H.; Filscth, S.V.; Sadowski, C. M. Chem. Phys. 1984,89, 457. (52) Sabety-Dzvonik, M. J.; Cody, R. J. J . Cfiem. Phys. 1977, 66, 125. (53) Baronavski,A. P.; McDonald, J. R. Chem. Phys. Letr. 1977,45, 172. (54) Krieger, W.; Hager, J.; Phab, J. Chem. Phys. Lett. 1982, 85, 69. (55) Fisher, W. H.; Carrington,T.; Filscth, S.V.; Sadowski, C. M.; Dugan, C. H. Chem. Phys. 1983,82,443. (56) Marinclli, W. J.; Sivakumar, N.; Houston, P. L. J . Phys. Chem. 1984, 88, 6685.

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and ICN, the I J l I parent states arise from the 17r 50 configurational excitation (Figures 17a and 18a). The predicted transition energy, 17r Su, is 57 905 cm-' for ICN and 68 080 cm-I for BrCN, whereas that, 4a 37r, for ClCN is 80 305 cm-I (Table X). On the basis of these predictions, the continuous absorption that undergirds the a band should arise primarily from the 27r 37r configurational excitation in CICN, whereas in BrCN and to a greater extent in ICN part of this continuous absorption should be associated with the 17r 5u configurational excitation. This conclusion follows from Figure 8 in which is plotted the "fitted" and predicted transitions of Table X together with the state of the connecting CN fragment (in brackets). The "no prime" and "primed" notations refer to the 2P3/2and 2Pi states of the associated halogen atom fragment, respectively. fn those cases where configurationally identical triplet and singlet states of the parent molecule connect with different states of the halogen and C N fragments, the fragment state that connects with the parent triplet state is listed first. The thermochemical threshold energies for the photodissociation channels are also shown in Figure 8 and are listed in Table IX. A similar notation (without brackets) is used to identify the fragment states. According to our correlation diagrams (Figures 16a-I8a), the X(2P1/2)+ CN(A2113/2)fragment states connect with the i*311 parent states arising from the I T 5u configurational excitation 37r configurational excitation in in ClCN and from the 4u BrCN and ICN. The predicted transition energy of the l x 5u configurational excitation in ClCN is 80460 cm-I and this, together with the 4u 37r;I3II configurational excitation discussed in the previous paragraph, should account for the continuum in the 80000 cm-I region of ClCN (Figures 3, 8, and 9). The predicted transition energy of the 40 37r;i.311configurational excitation is 76 205 cm-I in BrCN and 73 050 cm-I in ICN. Examination of the spectra of BrCN and ICN (Figures 8, 10, and 1 1) reveals an underlying continuum throughout the 76 000 and 73 000 cm-I regions, respectively, but no obvious maximum on which to fixate the 4u 37r transition energies. The predicted energy of the 40 60;'9~Z+configurational excitation (Table IX) is essentially independent of halogen substituent as shown in Figure IS. An examination of the spectra of the cyanogen halides (Figure 8) reveals a vibronic structure with origin at 80225 cm-', 77 875 cm-l, and 76465 cm-' in ClCN, BrCN. and ICN, respectively. These bands are labeled 40 nsR in Figures 8-1 1. However, one could also associate this band system with the 40 6a;IZ' intravalence excitation because of its energy independence. In this latter case, the correlation diagrams of Figures 16b, 17b, and 18b for CICN, BrCN, and ICN, respectively, become appropriate and do rationalize the discrete nature of these band systems. However, there exist two good reasons for rejecting this interpretation. First, there is a significant difference between the energy of the observed band system and the predicted energy for the 4u 6u;'v3Z configurational excitation (Table IX). This difference is 4930 cm-' for CICN, 9080cm-I for BrCN, and 10085 cm-' for ICN. Second, this discrete band system may be assigned as a first s-Rydberg series member with respect to the 40 ionization potential on the basis of its effective quantum numbers, n*, in ClCN (n* = 1.88), BrCN (n* = 1.86), and ICN (n* = 1.92). As is evident in Figure 8,these effective quantum numbers agree nsR closely with those of the B and C bands for which a 2 x assignment is valid.i~b*3~"~5~57 A vibrational analysis of this band system is presented in Table VI. In view of these arguments, we identify the 80 225 cm-I, 77 875 cm-I and 76 465 cm-l band systems in CICN, BrCN, and ICN (Figures 8-1 1) as 4u nsR;'Z+. Examination of Figure 8 does not reveal any other discrete band system at a relevant energy 6u;'Z+ assignment could be appropriate. for which the 4u Therefore, in Figure 19 we present an alternate correlation diagram to that shown in Figures 16b, 17b, and 18b for CICN, BrCN, and ICN, respectively. Based on this latter correlation diagram

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(57) McGlynn, S.P.; Felps, W. S.;Findley, G.L. Chem. Phys. Leu. 1981,

78, 89.

Electronic Spectroscopy of the Cyanogen Halides (Figure 19), one may conclude (i) that the underlying continuum in the 85 OOO cm-' and 87 OOO cm-' spectral regions of ClCN and BrCN, respectively, is associated mainly with the dissociative 4a 6a;'JZ+intravalence excitation; (ii) the 40 6~;'9~Z+ parent states connect with the X(2Pl/2)+ CN(/JII,/~) fragment states in ClCN and BrCN and, together with the parent states arising from the 27r 3x, 4a 3x, and 1~ 5u configurational excitations (Figures 16 and 17), constitute the total number of parent states that may connect with the X('P3/2.1/2) + CN(A2113/2,1/2) fragment states; (iii) the underlying continuum in the 85 500 cm-I spectra of ICN is associated with the dissociative 3a 50;'*~2+ and 40 60;l*~Z+ intravalence excitations; and (iv) the 3u 5u;'9'2+ parent states connect with the I(2Pl/2) + CN(A2111/2)fragment states in ICN and, together with the parent states arising from the 27r 37r, 17r 5a, and 4a 37r configurational excitation (Figure 18), constitute the total number of arent states that may connect with the I('P3/2,1/2) + CN(A II3/2,1/2) fragment states. Finally, based on items ii and iv, one predicts an upper limit of 85 OOO to 90000 cm-I for the direct production of CN(A211312,112) by means of photolysis of CICN, BrCN, or ICN; and, similarly, lower limits of -61 500 cm-I, 56OOO cm-I, and -48 OOO cm-'are predicted for CICN, BrCN, and ICN, respectively. These lower limits correspond to the onsets of the continuum a-band systems. The threshold wavelength for CN(A211i)formation is 58 140 cm-I), 55 555 cm-I), and 50OOO cm-l) in ClCN, BrCN, and ICN, respectivelys8-in very close agreement with the a-band continuum onsets (Figures 3 and 7). The reported CN(A211i)fluorescence excitation spectrum of ICN' onsets at 50000 cm-l, reaches a maximum at -58 500 cm-l, and then decreases gradually to zero at -90000 cm-', in full agreement with prediction (Figure 8, 18a,c, and 19). Additional support for this correlative scheme for ICN is provided by the observation that CN(A211i) is a primary product in the VUV photolysis (A > 1450 A) of ICNSZS8*59 and that the quantum yield of the A state of C N may be of comparable magnitude to or even greater than that of the B2Z+ state.52 C. Parenl States Connecting with the X(2P3/2,1/z) + CN(D2II3/2,i/2) Fragment States. According to the correlation diagrams (Figures 16a-18a), the I.311 parent states arising from the 27 6a configurational excitation must connect with the X(2P3/2) + CN(D2111/2)fragment states in order to account for the discrete nature of the band (Figures 8-1 1). A vibrational analysis of the @-bandsystem is present in Table IV. In the case of CICN, we expect the 27r 6a;'III component to be weak (as is the observed '111 ~ o m p o n e n t ~of* the ~ ~ ~27r 4sR excitation) and tentatively associate the weak feature at 72 305 cm-I with this excitation (Table IVA and Figures 8 and 9). As noted previously, much of the detailed structure in the 2u 6~;lIIlcomponent of BrCN (Figure 10) may arise from excitations to the lowest energy 27r npR manifold, at least based on analogy with HBr (n* > 2.31). Similarly for ICN (Figure 1 l), the detailed structure in the 27r ~ U ; ~ Jcomponents II~ may arise from excitations to the lowest energy 27r npR manifold, based this time on analogy with HI (n* > 2.35). The parent states arising from the 17r 37r configurational excitation are connected with the X(2P3/2)+ CN(D2113/2)fragment states (Figures 16-1 8) and are predicted to be dissociative in ClCN and bound in BrCN and ICN. The predicted energies of the 17r 37r configurational excitation in BrCN (81 730 cm-I) and ICN (75 105 cm-I) correspond well with discrete absorption features centered at 80900 and 75 120 cm-l, respectively (Figures 8, 10, and 1 1 ) . In both instances, however, these absorption features also could represent the lowest energy 2r ndR transition because the effective quantum numbers in BrCN (n* = 2.71) and ICN (n* = 2.92) are of an appropriate magnitude. In both BrCN and ICN, it is worth emphasizing that the band structure is irregular and cannot be analyzed in terms of the three funda-

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(58) West, G. A.; Berry, M. J. J . Chcm. Phys. 1974, bl, 4700. (59) Jackson, W. M.; Faris, J. L. J. Chem. Phys. 1972, 56.95.

The Journal of Physical Chemistry, Vol. 95, NO. 2, 1991 655

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mental vibrational frequencies (see Table V). The I J I l parent states arising from the 17r 6a configurational excitation connect with the X(2P1/2) + CN(D2113/2) fragment states and are predicted to be dissociative in ClCN, BrCN, and ICN (Figures 16-18). The predicted transition energy is 92480 cm-I and 88 605 cm-' in BrCN and ICN, respectively, and corresponds to regions of significant, continuous absorption intensity (Figures 8, 12, and 13). The predicted transition energy is 97 600 cm-' in ClCN and lies outside the spectral cut-off of the LiF optics (- 1050 A). The 1*3Z+ parent states arising from the 3a 6a configurational excitation are connected with the X(2Pl/2) + CN(D211,/,) f r a g ment states (Figure 19) and, together with the parent states arising from the 17r 3n, 27r 6a, and 17r 6a configurational excitations (Figures 16-18), constitute the total number of parent states that may connect with the X(2P3/~,1/2)+ CN(D2113/2,1/2) fragment states. The 3u 6a;IJZ+configurational excitation is predicted at 127340 cm-l, 123170 cm-I, and 1 1 5100 cm-l in ClCN, BrCN, and ICN, respect'ively, and should be dissociative in each case (Figure 19). D. Parent States Connecting with the X(2P3/21/2)+ C N B 2 Z + ) Fragment States. The parent states arising from the 3a 5a configurational excitation in CICN and BrCN and the 4a 6a configurational excitation in ICN are connected with the X(*P1/2)CN(B2Z+) fragment states (Figure 19). The predicted transition energy for this dissociative excitation is 1 1 0140 cm-I, 98770 cm-l, and 86550 cm-I in ClCN, BrCN, and ICN, respectively (Table IX). The 1*311 parent states arising from the 3u 37r configurational excitation are connected with the X(2P3/2) CN(B22+) fragment states (Figures 16a-18a). The predicted transition energy for this dissociative excitation is 122490 cm-', 1 12 420 cm-I, and 101 600 cm-I in CICN, BrCN, and ICN, respectively. Investigations of the cyanogen halides4~41~58*s9~6W3 using flash photolysis techniques have indicated that CN(B22+) is a significant primary product for photolysis at all VUV wavelengths. The reported threshold wavelength for the production of CN(B2Z+) is X = 1645 A (60790 cm-I), 1765 A (56655 cm-I), and 1965 A (50890 cm-') for C1CN,34 BrCN,5' and ICN,"*51 respectively, and, thus, lies near the onset of the discrete structure within the a-band system (Figures 3 and 7). (It should be noted that, according to the current JANAF value of @(ICN) = 75 f 2 kcal/m01,6~the calculated threshold wavelength for CN(B2Z+) production in ICN is 1924 A (51 965 cm-I).) The photodissociation results of Simons et al.4*62*63 indicate that collisionally induced processes are primarily responsible for the observed CN(S2Z+) population and the corresponding absence or near absence of a primary CN(A211i)p o p u l a t i ~ n . ~ ~ , This ~ ~ . ~in-~ , ~ , ~ ~ terpretation agrees with the photolysis investigations of ICN at low sample ressures?' the results of which (see also ref 3) indicate that CN(A IIi) (with u ' = 0,l primarily) is the major (and possibly sole) product of photodissociation of ICN in the region 2000 A > X > 1450 A. Therefore, we can interpret all photolysis investigations of the cyanogen halides in a way that accords with our correlation schemes (Figures 16-19). The results of recent laser photolysis investigations of C1CN6s*66 may also be interpreted such that they accord with our correlation scheme. Craig and F a u s P have reported the observation of two distinct emissive populations of C N Q 2 Z + ) upon photolysis of a static ClCN sample using 266 nm, 25 ps, Nd:YAG laser pulses. The authors attributed the prompt CN(B2Z+) component to a 2hv

- 'l3Z+

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+

+

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P

(60) Mele, A.; Okabe, H. J . Chem. Phys. 1%9, 51, 4798. (61) Luk, C. K.; Bersohn, R. J. Chcm. Phys. 1973.58, 2153. (62) Ashfold, M. N. R.; Georgiou, A. S.;Quinton, A. M.; Simons, J. P. J . Chem. Soc., Forodoy Trans. I1 1981, 77, 259. (63) Ashfold, M. N. R.; Simons, J. P. Chem. Phys. Lerr. 1977, 17,65. (64) JANAF Thermochemical Tobles. 2nd ed.;Stull, D. R.. Prophet, H., Eds.; NSRDS-NBS 37 US. Govt. Printing Office: Washington, DC, 1971. (65) Craig, B. B.; Faust, W. L. J. Phys. Chem. 1983, 87, 4568. (66) Guest, J. A.; OHalloran, M. A.; Zan, R. N . Chcm. Phys. Lerr. 1984, 103, 261.

J . Phys. Chem. 1991, 95,656-660

656

(75 190 cm-I) absorption process and subsequent unimolecular dissociation, while the delayed CN(B2Z+) component was attributed to a 3hv (1 12 780 cm-') absorption process that yielded long-lived superexcited parents. On the basis of our vibrational analysis of ClCN (Table IVA), a 2hu absorption process (75 190 cm-) in ClCN populates the 1 ; vibrational component of the 2 i ~ 60;lIIl valence band system that lies at 75 205 cm-' (Figures 8 and 9 ) . Photoabsorption at 2hu should also produce a CN(A2113/2,1/2)population directly by means of any (or of all) the 2?r 3a, 4a 3a, or la 5 0 configurational excitations that are predicted to lie at 68 650 cm-l, 80 305 cm-I, or 80460 cm-I, respectively (see Table XA and Figure 8 ) . We prefer, therefore, to attribute the reported delayed CN(B22+)component to either a 2hv absorption process whereby the CN(B2Z+) population arises because of collisionally induced intersystem crossing from the primary CN(A211i)population, or as originally proposed,65to a 3hu absorption process. According to our correlation diagrams, a 3hv absorption process could yield CN(D2113/2,1/2)either through the la 60 (Figure 16a) or 3u 60 (Figure 19) configurational excitations (predicted transition energies of 97 660 cm-I and 127 340 cm-I, respectively). If a primary CN(D211i) population does result from the 3hv absorption process, then the delayed CN(B2Z+) population may arise either from radiative relaxation or collisional quenching of the CN(D211i)population. The prompt CN(S2Z+) component is readily associated with a 3hu absorption process ( 1 12780 cm-I) according to our alternative correlation diagram for ClCN (Figure 19). The CN(B2Z+) population is produced directly by the 3 a 5 ~ r ; l - ~ configuraZ+

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tional excitation that is predicted to lie at 110 140 cm-l in ClCN (Tables IXB and XA). have reported the observation of a primary Guest et CN(B2Z+) population upon photolysis of a low-pressure, flowing ClCN sample using the 157.6 nm (and 157.5 nm) output of an F2 excimer laser. On the basis of our vibrational analysis of ClCN (Table HA), a single photoabso tion process at 157.6 nm (63450 cm-I) in ClCN populates the I, vibrational component (63 410 cm-I) of the 4u 5a;lZ+ valence band system (Figure 7). Photodissociation of ClCN at this energy (Figure 8) yields only a primary CN(A2Z3/2) population by means of the 2a 37r configurational excitation based upon our correlation diagram for ClCN (Figures 16 and 19). The observed primary CN(B2Z+) population results from a 2hu (126905 cm-I) absorption process, according to our correlation scheme (Figure 16a), and is associated ~ I I excitation that is predicted with the 3a ~ T ; I ~ configurational to lie at 122 490 cm-' in ClCN (Tables IXB and XA).

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Summary and Conclusions

We have attempted to analyze the electronic absorption spectra of the cyanogen halides in a way that accounts for their electronic molecular spectroscopy and photochemistry. We have shown that intravalence excitations may not be ignored in the analysis of the electronic spectra of those polyatomic molecules in the VUV. We have discussed the relevance of our analysis to the photochemistry of the cyanogen halides. Specific predictions are presented for the production, as primaries, of various states of the C N radical and halogen atom fragments. These predictions, in most instances, are verifiable by further photochemical investigations.

Dilute-Solution Fieid Gradient Induced Birefringence and Molecular Quadrupole Moment of Benzene Gary R. Dennis1' and Geoffrey L. D. Ritchie*,lb School of Science and Technology, University of Western Sydney, Nepean. New South Wales 2750, Australia, and Department of Chemistry, University of New England, Armidale, New South Wales 2351, Australia (Received: April 30, 1990)

Improved equipment for measurements of dilute-solution field gradient induced birefringence, specifically a novel four-pole cell characterized by simplicity of design and applicability to reactive compounds under inert-atmosphere conditions, has been constructed and tested. A comparativestudy of the field gradient birefringence of benzene in three different cells (four-pole, monopole, and two-wire) and in two solvents (carbon tetrachloride and cyclohexane) is described, and a better value of the dilute-solution quadrupole moment of benzene (IO%/C m2 = -28.3 f 1.2) is reported.

Introduction

The electric quadrupole moment is a fundamental descriptor of the molecular charge distribution and it is therefore of great relevance in considerations of intermolecular forces.* Some years ago,3 we extended the electric field gradient induced birefringence (1) (a) University of Western Sydney. (b) University of New England. The experimental work described here was performed in the School of Chemistry, University of Sydney, New South Wales 2006, Australia. (2) (a) Buckingham, A. D. Electric Moments of Molecules. In Physical Chemistry, An Advanced Treatise; Eyring, H., Henderson, D., Yost, W., Us.; Academic Press: New York, 1970; Vol. 4, pp 349-386. (b) Buckingham, A. D. Basic Theory of Intermolecular Forces: Applications to Small Molecules. In Perspectives in Quantum Chemistry and Biochemistry; Pullman, B., Ed.; Wiley-Interscience: Chichester, 1978; Vol. 2, pp 1-67.

route to this property (originally developed and applied to a range of simple gases by Buckingham and his collaborators4) to the study (3) (a) Vrbancich, J.; Ritchie, G. L. D. J . Chem. Soc., Faraday Trans. 2 1980, 76, 648-659. (b) Ritchie, G. L. D.; Vrbancich, J. J . Chem. SOC.. Faraday Trans. 2 1980,76, 1245-1248. (c) Calvert, R. L.; Ritchie, G. L. D. J . Chem. SOC.,Faraday Trans. 2 1980, 76, 1249-1253. (d) Vrbancich, J.; Bogaard, M. P.; Ritchie, G. L. D. J . Phys. E 1981, 14, 166-169. (e) Brereton, M. P.; Cooper, M. K.; Dennis, G. R.; Ritchie, G. L. D. Ausr. J . Chem. 1981,34.2253-2261. (f) Ritchie, G. L. D. Chem. Phys. &ti. 1982, 93,410-414. (g) Dennis, G. R.; Gentle, I. R.; Ritchie, G. L. D. J. Chem. Soc., Furaday Trans. 2 1983, 79,529-538. (h) Dennis, G. R.; Gentle, I. R.; Ritchie, G. L. D.; Andrieu, C. G. J. Chem. Soc., Faraday Trans. 2 1983,79,539-545. (i) Ritchie, G. L. D.; Cooper, M. K.; Calvert, R. L.; Dennis, G. R.; Phillips, L.; Vrbancich, J. J . Am. Chem. SOC.1983, 105, 5215-5219. (j)Craven, I. E.; Hesling, M. R.; Laver, D. R.; Lukins, P. B.; Ritchie, G. L. D.; Vrbancich. J. J . Phys. Chem. 1989, 93, 627-631.

0022-3654191 12095-0656%02.50/0 0 1991 American Chemical Society

Electronic spectroscopy of the cyanogen halides - American Chemical

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