J . Phys. Chem. 1987, 91, 4231-4235 They have also pointed out that the smaller C=O bond length in acetaldehyde could result in an increased a electron interaction with the out-of-plane C H bonds. Considering the situation in acetophenone, it is known that C=O bond length in aryl alkyl ketones is larger than in dialkyl ketones.28 This is clear from the decreased C=O group frequencies in aryl alkyl ketones. Thus our observation that the mechanical frequency and the anharmonicity of the out-of-plane CHb bonds show decrease as we pass from acetone to acetophenone is in line with the arguments of Fang et al.; that is, in acetophenone the large C=O bond length results in a decreased interaction of a electrons with CHb bonds. Another observation, as pointed out earlier, is that both the mechanical frequency and the anharmonicity of the in-plane CH, bond in acetophenone are much larger than in acetone and acetaldehyde. It is known28that dipolar structures with decreased C=O bond order (increased C=O bond length) contribute much to aryl alkyl ketone structures. These structures are such that the carbonyl oxygen atom is very rich in electron density. The interaction of this electron cloud seems to be the main reason for the very large mechanical frequency and anharmonicity of the in-plane C H bond. The in-plane phenyl ring may also affect the strength of the CH, bond. It can be seen from Table I that the dissociation energy of the CH, bond is less than that of CHb bonds. This is contrary to similar observations in other carbonyl compounds.22 The CH, potential curve in acetophenone is much shallower due to the large (28) Margreta Avram; Mateescu, GH.D. Infrared SpectroscopyApplications in Organic Chemistry; Wiley Interscience: New York, 1972; p 357.
4231
anharmonicity of this bond. As pointed out by Fang et a1.,22the exact bond strengthening mechanism in carbonyl compounds needs further investigations.
Conclusions Overtone spectroscopic studies of acetophenone and benzaldehyde by thermal lens spectroscopic and conventional methods are carried out. The aryl C H bonds of both the molecules show an increase of force constant over that in benzene. This is due to the electron-withdrawing property of the substituents. The methyl group of acetophenone shows two types C H bonds. Of the two, the in-plane C-H bond is assigned to the stronger force constant. The methyl C H bond parameters are compared to those of acetone and acetaldehyde. Possible factors influencing the mechanical frequency of C H bonds are discussed. A gas-phase study of selectively deuteriated compounds, though not studied here, will certainly yield more complete information on the overtone spectra of these compounds. Acknowledgment. T.M.A.R. thanks the Department of Atomic Energy (India) for providing a Senior Research Fellowship and K.P.B.M. thanks CSIR (India) for providing a Senior Research Fellowship. The financial assistance from Department of Science and Technology is also gratefully acknowledged. We are grateful to Dr. Joseph Francis, Head of PS & R T Department, Cochin University, for permission to use the spectrophotometer and to Mr. T. Ravindran for help in recording the NIR spectra. We also thank Mr. Thomas Baby for experimental assistance and other research students of the laser division for their interest in the work.
High-Resolution Photoelectron Spectra of BrCN and ICN: Vibronic Mixing Branka Kovai? Rudjer BoSkoviE Institute, Zagreb, Yugoslavia (Received: December 18, 1986; In Final Form: April 10, 1987)
High-resolution He I photoelectron (PE) spectra of BrCN and ICN have been measured and reexamined. Large spin-orbit splitting in the ground state of their molecular ions amounts to 1490 A 40 and-4320 A 40 cm-I, respectively, in agreement with the previously obtained results. In the higher energy degenerate state (B), the splitting is reduced to 880 f 40 and 1130 A 40 cm-l, respectively. Examination of the fine details observed in the three lowest systems reveals the activity of the v 2 ( r ) vibrational mode in addition to the totally symmetric ones, vl and v3. Nonvanishing coupling between (2Z+ and 211i) and within (*lTi) electronic states takes pla_ce_throughthe bending mode. Special attention is drawn to the complicated vibronic pattern in the overlapping region of A/B systems in ICN.
Introduction The electronic structure of cyanogen halide radical cations has received a great deal of attention,'-9 but the effects accompanying the ionization process are still not understood. The four electronic systems appearing in the He I region correspond to two a and two u ionizations, as expected on the basis of the molecular orbital (MO) configuration. The overall appearance of the photoelectron spectra (PE) of these 15 valence electron molecules is very similar, and the same types of vibrations afe expected to be excited upon ionization. In spite of that, the B systems in BrCN and ICN evaded e ~ p l a n a t i o n . ~Complicated ,~ vibrational structure, in ICN in particular, was attributed to spin-orbit (SO) interaction, yet the fine details could not be analyzed due to insufficient resolut i ~ n . ~Recent ,~ results obtained from the emission spectra of supersonically cooled halocyanide cations5 and their electronic absorption spectra in neon matrices6 have shed more light on the spin-orbit mixihg in the B systems. Since the selection rules for emission/absorption spectra are different from those governing 'No reprints available from the author.
0022-3654/81/2091-4231$01.50/0
photoionization processes, the information available from the PE spectra is still valuable. For small molecules such as triatomics, high-resolution spectra may give an insight into geometry changes,I0 vibrational and fine interactions which have been overlooked for many years until the observation of nontotally symmetric modesl3-I6 had provoked a group of theoreticians" to ( I ) Heilbronner, E.; Hornung, V.; Muszkat, K. A. Helu. Chim. Acta 1970, 53, 347.
(2) Hollas, J. M.; Sutherley, T. A. Mol. Phys. 1971, 22, 213. (3) Lake, R. F.; Thompson, H. Proc. R. SOC.London, Ser. A 1970, 317, 187. (4) Allan, M.; Maier, J. P. Chem. Phys. Lett. 1976, 41, 231. (5) Fulara, J.; Klapstein, D.; Kuhn, R.; Maier, J. P. J . Phys. Chem. 1985, 89. 4213. (6) Leutwyler, S.; Maier, J. P.; Spittel, U.J . Chem. Phys. 1985, 83, 506. (7) Bieri, G. Chem. Phys. Lett. 1977, 46, 107. (8) Dibeler, V. H.; Liston, S . K. J . Chem. Phys. 1967, 47, 4548. (9) Marthaler, 0. Ph.D. Thesis, University of Basel, 1980. (10) CvitaS, T.; Klasinc, L. J . Chem. SOC.,Faraday Trans. 2 1976, 72, 1240. (11) CvitaS, T.; Giisten, H.; Klasinc, L. J . Chem. Phys. 1979, 70, 57. (12) CvitaS, T.; Klasinc, L.; McDiarmid, R. Inr. J . Quantum Chem. Symp. 1984, 18, 537. ~
~~~
0 1987 American Chemical Society
4232 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 reexamine the selection rules for photoionization. It has been demonstrated in the PE spectra of several linear t r i a t o m i c ~ ' * , ~ ~ that substantial nonadiabatic effects are to be considered if two electronic states were coming close together. The structure of the electronic states can thus be explained only if the rigorous selection rules, rather than the restrictive ones derived on the basis of the Born-Oppenheimer (BO) approximation, are considered.20,21 The unsatisfactory interpretation of the complicated pattern of bands in Br and I cyanogens was one of the reasons for reexamination of their PE spectra. The influence of vibronic interactions was considered to be equally important and all the systems were analyzed under high-resolution trying to find out the importance of nonadiabatic effects.
KovaE
A&
12
BrCN I \
Experimental Section The He I spectra were recorded on a Vacuum Generators UVG spectrometer,22with the resolution (fwhm) of the Ar 2P3/2,2Pl12 lines being 12 meV for BrCN and ICN, respectively. Resolution remained constant throughout a scan, whereas the sensitivity of the instrument increased with decreasing kinetic energy of the photoelectrons. Each spectrum was recorded several times and with different amplitudes and count rates in order to check the reproducibility of the fine structure of electronic systems. The scanning times were 1000 s. BrCN and ICN are hazardous solids with sufficient vapor pressure. Both were of commercial origin and were used without further purification. Spectra were calibrated by admitting small amounts of Xe to the sample flow. Results and Discussion BrCN. x ( E J . The high-resolution P E spectrum of the outermost electronic system corresponding to the highest occupied molecular orbital is shown in Figure 1. The spectrum reveals a pronounced SO splitting of 185 meV (1490 cm-I), whereas the effective SO coupling parameter CBrt amounts to 389 meV.23 The effect of spin is indicated by the spin double-group notation (2n, X E I j 2= E312 E l / 2 ) ,hereafter used for doubly degenerate electronic systems.22 The intensity distribution in the vibrational progressions in both components of this state (E3,2, E l / 2 )appears to be very similar. Since in the molecular orbital (MO) picture the orbital is C N bonding and CBr antibonding, owing to the difference in electronegativity of the halogen and nitrogen atoms, the geometry change upon ionization is not considered to be substantial. Pronounced modes are the totally symmetric C N stretching, q ( u + ) , of 1920 cm-', and CBr stretching, u3(crf), of 640 cm-I, whereas the ground-state values of the neutral molecule are 2187 and 580 cm-', r e ~ p e c t i v e l y . ~Up ~ to this point the vibrational analysis agrees well with those previously reported (see Table I). However, one quantum of the bending mode (OI2O) of 260 cm-I seems to be excited too. Its observation indicates that a pattern of bands including the nontotally symmetric mode such as this should be described by vibronic mixing within a doubly degenerate state. The energy levels are thus characterized by a new quantum number K ( K = A + 1, with A and 1 being pro-
12 03
Ill 85
i i iJ
+
(13) Delwiche, J.; Hubin-Franskin, M.-J.; Capraace, G.: Natalis, P. J . Electron Spectrosc. Relat. Phenom. 1980, 21, 205. (14) Potts, A. W.; Fattahallah, G. H. J . Phys. B 1980, 13, 2545. (15) Dehmer, P. M.; Dehmer, J. L.: Chupka, W . A. J . Chem. Phys. 1980, 7 3 , 126. (16) Frost, D. C.; Lee, S.T.; McDowell, C. A . Chem. Phys. Lett. 1973, 23, 472. (17) Koppel, H.; Domcke, W.; Cederbaum, I. S.J. Chem. Phys. 1981, 74, 2945. (18) Koppel, H.; Domcke, W.; Cederbaum, L. S.A d r . Chem. Phys. 1984, 57, 59. (19) KovaE, B. J . Chem. Phys. 1983, 78, 1684. (20) Bunker, P. R. Molecular Symmetry and Spectroscopj>;Academic: New York, 1979. (21) Longuet-Higgins, H. C. Adv. Spectrosc. 1961, 2, 429. (22) Klasinc, L.; KovaE, B.; RuSEiE, B. Kem. Ind. 1974, 23, 569. (23) Rabalais, J. W. Principles of Ultraviolet Photoelectron Spectroscopy: Wiley-Interscience: New York, 1977. (24) Herzberg, G. Electronic Snectra of Polyatomic Molecules; Van Nostrand: New York, 1966. (25) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; Van Nostrand: Princeton, 1968.
12 6
12 L
12 2
12 0
11 8
€,/el
Figure 1. H e I PE spectrum showing the first electronic system (%, E i ) of BrCN, with resolved SO splitting. (Transitions corresponding to i = 1/2 are marked by the horizontal bar.) The vibrational structure is , K = I + 1. assigned by four quantum numbers u , c . , ~ o , with
jections of the angular momenta along the internuclear a ~ i s ~ ~ . ~ ~ ) . Since the coupling is small compared to the bending frequency, this is described as the Renner-Teller case a.24,27The molecule is still linear but the electronic and nuclear motions cannot be handled separately. Similar effects have been observed in the ground states of molecular ions of N2O,ISOCS,I9 CS2,I9 and C02.19,28 A(2Z+). The effect of ejection of a 4u electron is shown in Figure 2. In the MO picture the orbital is highly localized on the nitrogen atom, which is inconsistent with the change in the CBr bond length as indicated by the activity of the v3 stretching mode of 550 cm-I. Apart from this short progression, which has been observed p r e v i ~ u s l y ,one ~ , ~quantum of a bending vibration, v 2 ( r ) of -370 cm-I is observed too. It coulfi gain intensity through vibronic coupling with the closely lying B(EJ system, Evi(A) Eai(B) being approximately 0.5 eV. The energy separation is assumed to be large compared to the vibrational spacings, allowing one to discuss the vibronic structure within the adiabatic appro_ximation. B(Ei). The bonding characteristics of the r l molecular orbital is evident from the broad envelope of the system (Figure 3), which is the general characteristic of this series of molecules, as well as of some other t r i a t o m i c ~ . 'Although ~ the MO picture would (26) Hougen, J. T. J . Chem. Phys. 1962, 36, 519. (27) Dixon, R. N. Phil. Trans. R . Sot. London, Ser. A 1960, 252, 165. (28) Reineck, I.: Nohre, C.; Maripuu, R.; Lodin, P.; AI-Shamma, S. H.; Veenhuizen, H.; Karlsson, L.; Siegbahn, K . Chem. Phys. 1983, 78, 311
The Journal of Physical Chemistry, Vol. 91, No. 16, 1987 4233
Photoelectron Spectra of BrCN and ICN
A (*Z*) 1
1
I
13.7
3.5 Ei/eV
BrCN
x3
Figure 3. H e I PE spectrum showing the third electronic system (&Ei) of BrCN. (Transitions corresponding to i = 1/2 are marked by the horizontal bar.) The vibrational structure is assigned by four quantum with K = A + 1. numbers
J
1
11 e
11.2
0011
1095
0
q/cm-'
Figure 2. H e I PE spectrum showing the second electronic system (A, 2Z+)of BrCN. The vibrational structure is assigned by four quantum numbers vIv2Ku3,with K = A + 1.
suggest that both the CN and CBr stretchings will be excited upon ionization, the CBr stretching appears to be dominant, as observed p r e v i ~ u s l y . But ~ ~ ~the vibrational pattern cannot be properly understood just by assuming the excitation of the stretching vibration. The effect of SO coupling cannot be neglected. Therefore the adiabatic ionization energies of the two split components are crucial for the "deconvolution" of the band pattern. In order to estimate_ the expecJed values, the results from emission spectra for the_B2113i2 _X2113j2transitionS or matrix absorption study of the B2r13/2 X2113/2 transition6 can be used in conjunction with the results obtained from photoelectron spectra. Since the values for the 0; band amount to 19 234 (2.385 eV)S and 18 586 cm-l (2.30 eV),6 respectively, while Ea,(X211312)(this work) amounts to 11.85 eV, the OOIO band of the inverted doublet E 3 / 2 should appear either at 14.245or at 14.15 eV.6 Thus the 14.17-eV band is assigned to the adiabatic ionization of the *II3/2 state. A reasonable estimate for the 00'0 of the upper component can be made by assuming the validity of the sum rule for the SO ~ p l i t t i n g s . ~A~value of 880 cm-' is suggested by the vibrational
--
(29) Heilbronner, E.; Hornung, V.; Maier, J. P.; Kloster-Jensen, E. J. A m . Chem. SOC.1974, 96,4253.
I1
'
11' 0 p
e
"
11
6000 4000 2000 0 v/crr.' Figure 4. H e I PE spectrum showing the first ( z , E , )electronic system of ICN, with resolved SO splitting. (Transitions corresponding to i = 1/2 are marked by the horizontal bar.) The vibrational structure is , K = A + 1. assigned by four quantum numbers u , u ~ ~ u ,with
spacings, and the shoulder at 14.28 eV is assigned to the fi band. Since the SO splitting is small enough so that the geometry in the two component states are similar, the intensity distribution of two progressions associated with them should be similar too. This is evident from the expanded high-resolution spectrum sho.wn at the top of Figure 3. The prominent progression revealed in both component systems is that of -480 cm-l, as already observed in ref 1 and 3 and is ascribed to the reduced stretching mode 25. Whereas one quantum of vI(u+) CN appears to be excited in the higher energy component system ( E l l 2 ) ,one quantum of the bending vibration v 2 ( r ) is observed as a shoulder in the lower energy system (two quanta of the same vibration are more pronounced). That would indicate that vibronic mixing within the B system took pla_ce. The same type of vibration was revealed in the necr-lying A system. ICN. X ( E , ) . The most pronounced difference between the He I spectra of the two cyanogen halides is evident from Figu_res 1 and 4 and arises from much stronger SO coupling in the X ( E , ) system of the ICN molecule which amounts to ~ 4 3 2 cm-'. 0 The intensity distribution of the vibrational pattern (Figure 4) indicates that the corresponding geometries are alike. Two quanta of ul(o+)
4234 The Journal of Physical Chemistry, Vol. 91, No. 16, 1987
KovaE
TABLE I: Vibrational Wave Numbers and SO Splitting (both in em-') for Three Systems of BrCN and ICN ICN BrCN molecular ion state c m" cm "
ic
E, 1490 f 40" 1450 1480',d 1477 f 2'
so
v,(u+) CN
1940 f 40' 1 9OOb 1850'
E, 4320 f 40" 42706 4400C.d 4343 f 2'
SO
Vi(.+)
,
183jd 1906 f l e 312
C N 2050 f 40" 20006 2050 f 30' 312 1950 f 3OC 112 1 940d 2082 f l e 312
u 2 ( r ) def
260 f 40" 288 f l e 3/2
u 2 ( r ) def
280 f 40" 239 f I' 312 253 f 1' 112
v 3 ( u + ) CBr
640 f 40"
v 3 ( u + )CI
500 f 40" 3 / 2 530 f 40" 112 500 f 30' 3/2 520 f 30' 112 420* 535 f l e 312 559 f 1' 112
610 f 30' 3 / 2 555 f 30' 1/2 580d 650 f l e 312
A
B
2z+
u 2 ( r ) def
-370" 421 f 1'
u3(u+) CBr
550 f 40° 555 f 30' 580d 584 f l e
E, 880 f 40" 720b 1200d 900 f 200"g
so
u,(u+)
CN
u 2 ( r ) def
u z ( r ) def
22+ -300' 214 f l e 13 9
137
13 5
13 3
131 E , / e V
Figure 5. H e I PE spectrum showing the second ( A 2 P ) and the third (BE,) electronic systems of ICN. (Transitions corresponding to i = 1 / 2 are marked by the horizontal bar.) The vibrational structure is assigned by four quantum numbers oiC2Kc3,with K = A + 1.
SO
E, 1 I30 f 40" 12106
890 f 160e,g
1980 f 40" ui(u+) C N 2100 f 40" 112 1939 f l e 312 -400" 3 / 2 u 2 ( r ) def 300 f 40" 394 f l e 312
u3(u+) CI u,(a+) CBr 480 f 40" 5006 516d 471 f 1' 312 478 f 1 6 3 / 2
480 f 40a 5006 400d 473 f l e 112 400 f 2 6 3 / 2
"This work. bReference I . 'Reference 2. dReference 3. 'Reference 5. fReference 6. gDeduced from the spin-orbit sum rule. hGround-state values (in cm-' from ref 25) for u , . u 2 , and u3 are 2187. 368, and 580 for BrCN and 2158, 321, and 410 for ICN.
C N stretching vibration amounting to 2050 cm-l indicate some decrease upon ionization (ground-state value 21 88 cm-I) whereas the v3(u+)C I stretching is left almost unchanged (500 cm-') in comparison to its ground-state value of 486 cm-1.25 Here again t h e bending vibration of 280 cm-l (01%) has gained some intensity through vibronic coupling and is ascribed to the Renner-Teller case a, as in some previous examples.17Js It is evident that the splitting increases with the atomic number of the constituent atom, whereas such an increase is not expected in the case of RennerTeller splitting, A ( 2 2 + )and B(E,). The second and the third electronic systems of ICN represent the most interesting case in this series. Details of the vibrational structure are shown in Figure 5. First, the systems fall within a 1-eV region, which means that mixing of the states might become important if symmetry conditions are fulfilled. Second, SO splitting is expected in the doubly degenerate system, causing additional complication. Some results concerning SO coupling have been published alreadyi,5s6but an unambiguous assignment has not been reached yet. From the emission spectrum
-
(B2IIIj2 j(1211112)a difference in energy between the two states ( i = 1/2) of 2.005 eV is obtained. This result, in conjunction_with the data available from photoelectron spectra (Figure 4) (XE,,, = 11.47 eV), gives an E, value of 13.47 eV for the 00'0 band of E I l 2system. Thus the band at 13.44 eV (Figure 5) corresponds to an adiabatic ionization energy of the ElI2state. The adiabatic transition to OOi_O of E312 state should therefore lie somewhere between 13.17 (A) and 13.44 eV. But the intensity distribution appears rather irregular, thus preventing exact assignment. Electronjc absorptizn measurements in neon matrices at 4.5 K for the B2r13,2 e X2113,2 transition provide us with an energy separation value between the two states of 2.48 eV.6 Thus the obtained value of 13.4 eV for 00'0 of E3/2 state is probably somewhat too high, since the observed progression in v l ( o + )CN of 2100 cm-' would suggest a value of 13.3 eV, assuming little anharmonicity (Figure 5). The value of 140 meV (1 130 cm-') for SO splitting in this system is also in accord with the spin-orbit sum (0.54 eV 0.14 eV = 0.68 eV). The structure of 22;'(A) is only partially resolved. The expanded spectrum shown at the top of Figure 5 reveals two quanta of a bending mode, 01'0 and 02O0, amounting to -300 and -600 cm-', respectively. (Tw? quanta of v 2 ( r ) ( n 3 0 ) (Figure 5) are observed in the coupled B system (E3/2) too.) This is another indication of vibronic coupling between two states that has take? place through v 2 ( r ) . A similar effect was observed in the A/B systems of 0CS+l9 and j(l
