J. Phys. Chem. 1991, 95, 2727-2732
2727
smaller rate constants than internally relaxed species. This could also manifest itself as a pressure dependence for an association reaction.6~~J I The rate constant for dissociation of a M(CO),(L) complex will depend on the binding energy of the ligand and the number of degrees of freedom of the complex.I2 A priori a more weakly bound ligand with fewer degrees of freedom will have a larger rate constant for dissociation. This is expected to characterize the H2Fe(CO)3system versus both Fe(C0)4 formed via addition of CO from Fe(CO)3 and Fe(CO),(ethyIene). Though relative bond energies for the addition of H2 to Fe(CO)3 versus H2 to Fe(CO)3(ethylene) are not known, the latter complex benefits from additional degrees of freedom versus the former. Thus, it is to be expected that if the addition step in one of the actual microscopic association reactions under investigation is not rate limiting for the processes investigated in these studies it would be the addition of H2 to Fe(CO),. Finally, it is interesting to compare the rate constants reported in this work to the ratios of rate constants for addition of CO, C2H4 and H2 to Fe(CO),(ethylene) reported by Grant et ale3 Though they do not actually measure these rate constants directly, they are able to report a ratio of 2166.2:l for these rate constants. We find that the ratio of the rate constants for addition of C2H4
versus H2 to Fe(CO)3(ethylene) is approximately 7 which, within experimental error, agrees well with Grant's prediction of 6.2. We did not directly measure the rate constant for addition of CO to the Fe(CO)3(ethylene) species. However, a rate constant 216 times that for the addition of H2 to Fe(CO),(ethylene) would yield a value of 10.8 ps-l Torr-I. Based on the lower limits of the reported experimental error brackets in our work and the lower limit in Grant's work3 one would expect a value of 8.5 p d Torr-I. Though this rate constant is larger than we would anticipate given the values that have been previously measured for what would be expected to be a spin-disallowed process, it is certainly within the range of values that have been observed for the addition of ligands to coordinatively unsaturated metal carbonyls in spinallowed processes. It is clear that a direct measurement of this rate constant is warranted to determine if it is an anomalously large rate constant for the addition of a ligand to an Fe(CO),(L) species. I
(1 2) Robinson, P. J.; Holbrook, K. A. Vnimolecular Reactions; Wiley: New York, 1972.
H,Fe(CO),, 86880-3 1- 1; Fe(CO),(C2H4),84520-95-6; Fe(CO),(C2H4),, 74278-01-6; H,Fe(CO),(C,H,), 84333-1 7-5; H4Fe(CO),, 132046-92-5.
Acknowledgment. We thank the National Science Foundation for support of this work under grant no. CHE 88-06020. We also acknowledge a Research Opportunities Award as a supplement to the aforementioned grant which was instrumental in support of this work. Registry No. Fe(CO),, 52491-41-5; H,, 1333-74-0; C2H4, 74-85-1;
High-Resolution Fourier Transform Studies of Molecular Ion Emission: Ground-State Degeneracy Splitting in the 1,2,3-Trifiuorobenzene Radical Cation Myeong H. Suh, Sang K. Lee,+ Brent D. Rehfuss,* Terry A. Miller,* Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210
and V. E. Bondybey Institut fur Physikalische Chemie, T. U. Munchen, 8046 Garching, Germany, and Laser Spectroscopy Facility, Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, Ohio 43210 (Received: September 10, 1990)
Molecular radical cations are generated in a jet by a high-voltage dc discharge. High-resolution fluorescence emission spectra of the supersonically cooled ions are measured on a Fourier transform spectrometer_. The asymmetric fluorine substitution splits the do_ublydegenerate benzene cation ground state into an X and low-lying A level. Transitions from the B state to each of the X and A states can be easily identified based on the analysis of the rotational contours. The results are compared with a previous matrix study, and it is shown that the ionic spectra are insignificantly perturbed in solid neon.
Introduction u p to some 15 years ago, the polyatomic molecular ions for be which spectroscopic data were available could on the fingers ofboth hands.'*2 Most availabie information about the electronic states and properties of molecular ions was obtained from photoelectron spectr~scopy.~The first breakthrough in this situation occurred in 1972, when Turner and co-workers observed fluorescent decay in the hexafluorobenzene radical cation! This observation motivated subsequent fluorescence studies by Maier and mworkerss and several other groups? as well as laser-induced fluorescence work in our laboratorie~.~-~ The transitions of nuyerous halogenated benzene cations from the ground electronic X state to the excited state arising from *-electron excitation, usually referred to as B state, have been
'
Prcsent address: Department of Chemistry, Tae Gu University, Tae Gu, South Korea. !Ohio State University Postdoctoral Fellow.
particularly thoroughly studied.+I' In the species possessing at least D3h symmetry, the ground state is doubly degenerate and as a consequence subject to a Jahn-Teller distortion.12J3 In (1) Herzberg, G. Reo. Chem. Soc. 1971, 25, 201. (2) Callomon, J. H. Proc. R. SOC.London 1958, A244, 220. (3) Turner, D. N.; Baker, C.; Baker, A. D.; Brindle, C. R. Molecular Phoroelecrron Specrroscopy; Wiley-Interscience: New York, 1970. (4) Daintih, J.; Dinsdale, R.; Maier, J. P.; Sweigart, D. A.; Turner, D. W. Molecular Specrroscopy; Institute of Petroleum: London, 1971; p 16. (5) Allan, M.; Maier, J. P.; Marthaler, 0. Chem. Phys. 1977, 26, 131. (6) Cossart-Magos, C.; Cossart, D.; Leach, S . J . Chem. Phys. 1978,69. 4313.
(7) Bondybey, V. E.; English, J. H.; Miller, T. A. J . Am. Chem. Soc. 1978, 100, 5251. (8) Miller, T. A.; Bondybey, V. E. Chem. Phys. Lerr. 1978, 58, 454. (9) Bondybey, V. E.; English, J. H.; Miller, T. A. J. Mol. Specrrosc. 1980, 81, 455. (IO) Klapstein, D.; Leutwyler, S.;Maier, J. P. Mol. Phys. 1984, 51, 413. (11) Tuckett, R. P. Chem. Phys. 1981, 58, 151. (12) Sears, T.; Miller, T. A.; Bondybey, V. E. J . Chem. Phys. 1980, 72, 6070.
0022-3654/91/2095-2727$02.50/00 1991 American Chemical Society
2128 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991
Suh et al. 5.0
-
!4m
3.75-
I
2.5-
1.25 nn -.22WO
p6w
p4w
p)#)
plw
22100
PWO
21000
2lMO
2l7M
a800
WAVENUMBER CM"
Figure 2. Broad scan of emission spectrum of 1,2,3-trifluorobenzene cation. The labels above many of the bands indicate whether the rotational contour is a- or b-type. A number sf very sharp lines in the spectrum arise from He and fragments of the sample.
Figure 1. Schematic diagram showing details of the supersonic jet,
high-voltage discharge and the Fourier transform spectrometer. molecules of lower symmetry, the degeneracy is lifted, giving ris, to a low-lying excited electronic state, usually denoted as the A state. Unlike the 2 state and state, which are well studied, relatively l!ttl,e is known about the A state, and even the magnitude of the X-A splitting is not well-known. Mast of the available information comes from low-resolution photoelectron spectroscopy or from_th_e observation of a broad, long-wavelength shoulder on the B-X fluorescence.14 Some time ago, we have demonstrated that by taking advantage-of the fact that the transition dipoles for the B X and B A transitions are orthogonal, one can distinguish the two transitions experimentally, by studying the fluorescence polarization in low-temperature matrices. In this way, the magnitude of the ground-state splitting can be measured.IsJ6 While our studies of numerous species have shown that in solid neon the vibrational structures of the ions are remarkably unperturbed, the possibility of medium effects upon the magnitude of degeneracy splitting remains. The same information that one gets from fluorescence polarization in a matrix "photoselection" experiment can, in principle, be obtained by studies of rotational band contours in high-resolution gas-phase emission spectra. In the present experiment we demonstrate, with the example of the 1,2,3-trifluorobenzene radical cation, that by producing the ions in a dc discharge one can generate high-resolution Fourier transform (FT)emission spectra of the supersonically cooled species. Bands involving transition moments along two orthogonal axes are-cleyly observable and distinguishable in the spectrum, and the X-A splitting can thus be accurately measured. Our work further shows, a t least for the present case, excellent agreement between the gas-phase and matrix results.
8
+. -
Experimental Section
The parent compound 1,2,3-trifluorobenzene was obtained commercially from Aldrich and was used without further purification. The vapors of the compound were entrained in a carrier gas, usually helium, although argon was also used in several experiments. The concentration of the organic compound in the carrier gas could be controlled by immersing the sample in a constant-temperature bath and by controlling the opening of the valve of the sample container. The experimental apparatus used is shown schematically in Figure 1. The gaseous mixture was expanded through a 0.20.4" nozzle into a vacuum chamber formed by a four-way glass cross, evacuated by a 1140 L/min roots blower pump, Edwards Model E2M80. Stagnation pressures of about 150 kPa (20 psi) (13) Cossart-Magos,C.; Cossart, D.;Leach, S.; Maier, J. P.;Miscv, L. J . Chem. Phys. 1903, 78, 3613. (14) Allan, M.; Maier, J. P.;Marthaler, 0. Chem. Phys. 1977, 26, 131. (15) Bondybey, V. E.; English, J. H.; Miller, T. A.; Shiley, R.H. J. Mol. Specrrosc. 1980, 84, 124. (16) Bondybey, V. E.; English, J. H.; Miller, T. A. J . Mol. Specrrosc. 1981, 90, 592.
were used, resulting in pressures in the range 0.5-1 Torr (60-120 Pa) in the vacuum chamber during operation. The nozzles were formed from a thick walled glass tube, narrowed at one end to a capillary to produce the desired size opening. The anode, formed by a 2-mm-diameter stainless steel wire, was inserted via an O-ring seal into the glass tube to within 1-3 mm from the orifice. The anode was biased by a 3000-V high-voltage power supply, and the discharge was stabilized by using a 500-kQ current-limiting balast resistor. Typically the power supply was operated at 3000 V. Under these operating conditions the discharge current was 4-5 mA, and the anode potential 500-1000 V. The light emanating from the discharge was collimated by using a combination of two quartz lenses and focused using a concave mirror onto the emission port of a Bruker IFS-120 HR Fourier transform spectrometer. The instrument was operated with a Quartz-vis beamsplitter, an EG&G 113 preamplifier, and a Hamamatsu R106UH photomultiplier for photon detection. Initially, survey scans were obtained at low, 2 cm-I, resolution. Subsequently, after the conditions were optimized, higher resolution scans were obtained a t or near the 0.05-cm-' Doppler limit. To improve the signal-to-noise ratio in the high-resolution studies, the spectral region of interest was isolated either by a combination of color glass filters or by -100-A bandpass interference filters. The signal-to-noise ratio was mainly limited by the source noise, Le., the fluctuations in the discharge intensity. A considerable amount of experimentation was done regarding the optimum position and/or material of the cathode. Ultimately it transpired that most stable discharge conditions were obtained without inserting any counter electrode at all into the vacuum chamber and by using simply the grounded metal bellows connecting the glass cross to the pump as the cathode. Typically 150 scans were averaged in several minutes to obtain good-quality survey spectra. For the final, high-resolution scans, usually 150 scans over 65 min were averaged.
Results and Discussion Observed Spectra. A typical spectrum of the 1,2,3-trifluorobenzene radical cat@ is shown in Figure 2. It consists of a strong origin of the B X transition at 22 466 cm-', followed to lower energies by a series of vibronic bands. Several weak features attributable to hot bands are observable to the blue of the origin. Overall, the observable spectrum extends from =23 OOO to 21 OOO cm-I. The bands identified as belonging to the radical cation are listed in Table I. Given here are the measured wavenumbers of the maximum of the gas-phase bands, their band type, and their vibronic energies relative to the origin band. On the basis of our contour analyses (see below), we estimate the band origin to be -2.47 cm-' for the b-type bands and -2.09 cm-l for the a-type bands from the maximum peak position. We estimate the accuracy of the gas-phase measurements, limited by the uncertainty in establishing the position of the origins of the rotationally unresolved bands at rtO.10 cm-I. The data can be compared with our previously reportedt6 matrix results also outlined in Table I. The reliability of the comparison
-
1,2,3-Trifluorobenzene Radical Cation
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2129
TABLE I: Frequencies and Assignments of 1,2&Trifluorobenzene Cation Emission Bands (cm-')
-
shic from transition"
intensityb
22 466.32 22 440.83 22 4 19.48 22 377.64 22 356.65 22 274.52 22 256.26 22 249.27 22 247.82 22 177.05 22 154.63 22 129.10 22 06 1 .OO 22 008.16 21 947.80 21 871.36 21 804.23 21 770.72 21 670.86 21 596.92
S
m W
vw vw S
vw vw m S
S
m m m W
S
m S S
S
band type b b ? ? ? a ? ? a b a a a b ? b a b a a
0; (B
X)
0.0 25.49 46.84 88.68 109.67 191.80 2 10.06 217.05 21 8.50 289.27 31 1.69 337.22 405.32 458.16 5 18.52 594.96 662.09 695.60 795.46 869.40
-
shift of corresponding matrix band from 0 : %) 0
band type
196
I
293 316
II
414 465 525 600
I
697 804 876
II I I
(n
II
I II
II II
Measured frequency corresponds to peak of band. On the basis of the contour analysis, the origin of the b-type bands should be shifted to the red b~ = strong, m = medium, w = weak, vw = very weak.
-2.47 cm-' and the a-type bands -2.09 cm-I.
is naturally limited by the matrix work, which was carried out with a conventional scanning spectrometer and whose relative accuracy is estimated at f l cm-I, with a probably slightly lower absolute frequency measurement accuracy. Except for an overall red shift of 176 cm-' (0; bands of B-R transition) of the matrix spectrum, the agreement between the two spectra is very good, although a few of the vibronic shifts appear to be outside the experimental accuracy. In particular, from Table I the Toofor the A state is 191.8 cm-' from the present gas studies, while it was measured to be 197 cm-I in a Ne matrix. Ground-State Degeneracy Splitting by Asymmetric Substitution. As noted above, the ground state of the benzene cation is formed by removal of an electron from the highest occupied molecular orbital, which happens to be a doubly degenerate ?r orbital. Any asymmetric substitution of the ring, of course, results in IiftingJhe ground-state degeneracy and splits the groucd state into the X state and a relatively low lying A state. The B state, on the other hand, removes an electron from the deeper lying nondegenerate T orbital and should be little affected qualitatively by the substitution. We have previously examined" numerous asymmetric cations isolated in a Ne matrix. In some cases, for instance in chloropentafluorobenzene or 1,3-dichloro-5-fluorobenzene,the ring is substituted in symmetric positions, but with unlike substituents, differing somewhat in their ele_ctronegativity. In such "quasisymmetric" compounds, the X and A electronic splitting is quite small and the vibrational manifolds of the two states overlap extensively. As a result, one observes a very irregular, perturbed vibrational structure. On the other hand, for more dramatically asymmetric substitution, e.g., in pentafluorobenzene and tetrafluorobenzene, the splitting is much larger, ca.3000-6000 cm+. On the face of it, one might expect a rather large 2-A splitting for the 1,2,3-trifluorobenzene cation. However, a more detailed examination of the relevant molecular orbitals shows this not to be the case. Figure 3 shows our symmetry and inertial axes conventions for the ion. Figure 4 shows schematically the ?r orbitals for a benzenoid system, and Figure 5 illustrates how the bonding of the two states differs. The individual T molecular orbitals, qm,are formed as linear combinations of the individual one-electron carbon p atomic orbitals, xi: 6
tcIm
=
C cixi i- 1
(17) Bondybey, V. E.; Vaughn, C. B.; Miller, T. A.; English, J. H.;Shiley, 1981, 74,6584.
R. H. J . Chem. Phys.
H
I' Figure 3. Schematic diagram of the 1,2,3-trifluorobenzene radical cation illustrating the symmetry axes (x,y,z) and the inertial axes (a,b,c) conventions. Both the z and x axes are into the plane of the figure. The C, axis coincides with the z(b) axis. Y
W
W
2.
1
(b2) 29
W
Figure 4. Schematic representation of the T orbitals of a benzenoid system, e.g., C6H6 or c6F6. The symmetry classification of the orbitals are given for D6,, and in parenthesis for C, symmetry (C,axis intersecting the topmost and bottommost atoms and the three F atoms occupying the three topmost carbon atoms). The size of the circles are proportional to the square of the atomic coefficients (c, of the text) and the shading distinguishes the phases. Normalization requires the values of lc,12 for all i to be ' / 6 for the a,,(b,) orbital. The value of lc,I2 for the larger two coefficients of the el,(b2) orbital is for the and it is remaining coefficients. For the nonzero coefficients of the elI(a2), it is I /4*
Suh et al.
2730 The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 (ab 4.6826 r 3.5127
.-
2.3428
..
1.1729
.-
'E* F
0.0031
22480
22470
22460
22450
WAVENUMBER CM"
c 2v COMPRESSED C 2 v Figure 5. Representation of benzene ring distortion expected for C6H3F3+ from the two quasidegenerate electronic configurations (lb#( la2)2(2b2)l and (lb2)2(la2)1(2b2)2. The pictured ordering of the b2 and a2orbitals is consistent with our experimental results. ELONGATED
T E
It should be a re_aso_nableapproximation to assume that the magnitude of the X-A splitting is proportional to the difference in the electron density of the two originally degenerate orbitals, +2a and +*by on the atoms bonded to the substituents, Le. Ccja2 j
- CC,*
lA!LLJ 20.0
0.0 20
15
0
5
10
-10
-5
-15
-20
WAVENUMBER CM-'
Figure 6. Comparison of experimental (top) and simulated (bottom)
1
wherej runs over the substituted atoms. For instance, as one can see from Figure 4 preserving C, symmetry for the 1,2,4,5 and 1,2,3,4 isomers of the tetrafluorobenzene and for the 1,3,5 trifluorobenzene cations, the corresponding sums have values of 2/3, and 0. The splittings observed in the matrix are 61 15, 2442, and 0 cm-l, respectively, in good agreement with our simple expectations. While for the seemingly highly asymmetric 1,2,3 isomer, one might intuitively expect a large ground-state splitting, the above treatment yields for the electron density difference sum a value of zero. In agreement with the simple theory, a splitting of less than 200 cm-' is observed, placing the compound clearly in the "quasisymmetric" category. Matrix Photoselection Studies. For quasisymmetric cations, one observes a single band in a photoelectron spectrum, with no quantitative information about the magnitude of the splitting. Fletcher and BondybeyI8 have, however, demonstrated that such information can be obtained from matrix photoselection experiments. Molecules isolated in rare-gas low-temperature matrices are generally trapped in fixed but random orientations. In a typical LIF experiment, the polarized laser light preferentially excites an oriented subset of the guest molecules, whose transition dipole is oriented parallel to the electric vector of the exciting light. If the reemission occurs on the same electronic transition, the dipole for the emission process has the same orientation. Under these circumstances it can be shown that the emission will contain a prevailing parallel polarization with a, ratio of 3: 1. If excitation occurs on the B X transitiy b t t emission terminates in the A state, then the dipole for the B A transition is orthogonal to that of the excitation process. Under those and was shown circumstances it can be derived the~retically'~ experimentallyla that perpendicularly polarized emission will prevail, and one will obtain a ratio of tll/Zl = 0.5. The sharp, well-resolved neon matrix spectra of the radical cations show very clearly the distinction between the bands of the two transitions, with the experimental polarization ratios very close to the theoretically predicted values of 3.0 and 02,respectively. This forms the basis for the II(8-X) and I(8-A) assignments in Table 1 and yields an experimental value for splitting. One drawback of a condensed medium study is, of course, that one always has to take into account the possibility of effects from the medium, whose magnitude is uncertain. An alternative
-
0: b type simulation
N
-
(18) Bondybey, V. E.;Fletcher, C. J . Chem. Phys. 1976, 64, 3615. (19) Albrccht, A. C. J . Mol Spectrosc. 1961, 6, 84.
b-type rotational contour of the band assigned as the 8-%origin. The rotational constants used in the simulation are given in Table 11, and the assumed temperature is 80 K. 3.9069 2.9604 2.012
It
I\
1.0636 0.1 152 22290
22280
22270
22260
WAVENUMBER CM'l
i
20.0
yo
-0.0
Y
22250
'
{ 20
15
10
5
I
0
'
-5
Y-10
'
-15
3
-20
'
WAVENUMBER CM'l
Figure 7. Comparison of experimental (top) and simulated (bottom)
a-type rotational contour of the band assigned as the 8-A origin. The rotational constants used in the simulation are given in Table 11, and the assumed temperature is 80 K.
gas-phase technique therefore is highly desirable to verify the matrix result. Band Contours in the High-Resolution Gas-Phase Fluorescence. Unlike in the rare-gas low-temperature matrices, where the molecular orientation is fixed, the excited molecules in the gas phase can rotate and possibly undergo collisions during their 4 8 - n s radiative lifetime, and these effects will tend to randomize the fluorescence polarization. However, the rovibronic selection rules differ depending on whether the transition dipole is oriented parallel or perpendicular to the molecular symmetry axis, and this will, in general, give rise to different rotational band contours. When these contours can be resolved and distinguished, one can obtain the same information as is obtainable from the matrix photoselection experiments. origin Figure 6a shows a high-resolution spectrum of the B band a t 22466 cm-I, while Figure 7a shows ajimilar scan of the band at 22 275 cm-I, identified as the B A origin. The dif-
-
-
The Journal of Physical Chemistry, Vol. 95, No. 7, 1991 2731
1,2,3-Trifluorobenzene Radical Cation TABLE II: Rotational Constants (cm-I) for the & A, and % States
A
B
B 28,
A lA2
2 2B2 exp“
Calcb
CalcC
expa
cakb
calcf
expa
calcb
calcC
0.07861 0.05716 0.03310
0.07755 0.05794 0.03316
0.07866 0.05715 0.03310
0.07826 0.05742 0.03312
0.07755 0.05794 0.03316
0.07824 0.05745 0.03313
0.07615 0.05696 0.03259
0.07755 0.05794 0.03316
0.07615 0.05696 0.03259
C a Constants used to produce simulated spectra in Figures 6 and 7. An accuracy of 0.000 05 is needed to reproduce experimental spectra. Values of the rotational cosntants of 1,2,3-trifluorobenzene cation assuming r c q = 1.395 A, fC-F = 1.330 A, ~ C - H= 1.081 A, and C C C and C-C-F angles are all 120O. CBestvalues of the rotational constants obtainable from the geometric distortions illustrated in Figure 8 with the numerical values of B = 0.022A, t = 0.003 A, and Aa = 2O. ference between the perpendicular (a-type) band (Figure 7) with a central gap and the band with a sharp central Q branch (Figure 6 ) is unmistakable. In this way one can easily identify the individual vibronic tLansitions and associate the lower level with either the X or the A state. This information is presented in Table I. It is clear that there is a one-to-one correspondence between the identification based on the matrix polarization and the present analysis of the gas-phase contours. Fitting of the Band Contours. Even in the absence of full rotational resolution, the gas-phase contours contain more information than the matrix polarization data. In particular, they can be used to estimate the rotational temperature, as well as the bonding changes occurring during the electronic transition. Our approach is then to try to simulate the observed rotational contours with a “reajon_able set” of rotational constants for each of the three states, X,A, and B and a variable temperature, T. The fittings were performed using a standard asymmetric top Hamiltonian for all states as embodied in the Fortran program ASYROT written by Judgem and adopted for use in our laboratory. As Figures 6 and 7 show, excellent agreement can be obtained between the observed and calculated contours. The fit, incorporating the appropriate rovibronic selection rules, establishes that the 8-X transition is a b-type band. Reference to Figure 3 and the C2, character table shows that for an electronic transition moment along the z ( b ) axis, the lower state must b_e 2B2if the u_pp_erstate is 2B2. Thus the transition is 8 2B2* X 2B2. The B-A transition illustrated in Figure 7 is clearly a-type, requiring the transition to be B 2B2 A 2A2. The rotational temperature for these simulations is 80 K. The values of the rotational constants used in the simulation are given in Table 11. Of course, we do not know whether this set of constants is unique; probably it is not. However, we do know that it gives an excellent spectral fit and corresponds to a “reasonable” geometry as defined below. We also see that they are rather different from those calculated for neutral 1,2,3-C6F3H,. Band shapes and contours are typically much more sensitive to the changes in the geometry (and rotational constants1 associated with the transition than to their absolute values. The B-state vibrational frequencies are very similar to those of the ground-state neutral molecule, and the 1,2,3-trifluorobenzene radical cation can be expected to have a geometry quite similar to the parent compound, which is assumed to be planar. We have therefore assumed for the excited state an undistorted geometry with all C-C bonds of equivalent length and all the bond angles 120°, appropriate for s$ hybridized C atoms. The C-C, C-H, and C-F bond lengths (see Table 11) were obtained from ref 21. Recent rotational analyses of the 1,3,5-trifluorobenzene and hexafluorobenzene cations have indicated that the most significant excited-state geometric change from neutral ground-state geometry is a slight lengthening of the C-C bond distance. Thus we take the B state geometry as identical with the neutral except that we treat rc4 = roc< + 6, where 6 is a variable parameter, rc4 is the B C-C bond distance, and roc< is that of neutral 1,2,3-trifluorobenzene. This geometric distortion is illustrated in Figure 8. The ground-state geometry is again predicated upon that of the neutral. However, there are now some additional effects
-
~~
~~
~~~
~~
R. H. Comput. Pbys. Commun. 1987.17, 361. (21)Boggs. J. E.; Pang, F.;Way, P. J. Comput. Cbem. 1982, 3, 344.
(20) Judge,
F
F
H
H
PaXNt. Figure 8. Schematic diagram showing ring distoEtion parameter, 6 for the B state (left) and Aa (see text) for the A and X states (right). Here @ = a + Aa.
introduced by the degeneracy of the A orbitals +& and +2b in unsubstituted benzene. Whether the odd electron resides in +2a or +2b results in significantly different bonding as shown in Figure 4. This is the fundamental cause of the Jahn-Teller effect in species such as the benzene and hexafluorobenzene cations.22 For the truly degenerate species, the molecule dynamically oscillates between all the equivalent distorted geometries of which Figure 5 illustrates two. Our studies of the Jahn-Teller effect in C6F6’ determined that the alternation of the C-C bond length and the C-C-C bond angles were the most significant distortions. We have determined that the effects of these distortions upon the rotational constants are almost indistinguishable and hence cannot be independently determined. Here we simply ascribe all of this ring distortion to the C-C bond length change and neglect the C-C-C bond angle distortion. While the separation of the A and state in 1,2,3-C6H3F3+ is not large, the degeneracy of the state is truly broken. Thus rather than the ion dynamically oscillating between the compressed and elongated geometriesof Figure 5, the X state should represent one geometry and the A state the other. As noted above, the expected geometric changes can be represented by a distortion of the C-C bond. The symmetry-allowed C-C bond length distortion for an unsubstituted benzene ring is shown-in Figure 8. Simply put, we note from Figures 5 and 8, that the X 2B2state should have two bonds shortened by an amount representable by 2t while the othecfour bonds lengthen by t . The opposite effect is expect for the A *A2state, Le., two C-C bonds lengthen by 2t while the other four decrease by t . In our attempts to find a geometry that reproduces the 2 and A state rotational constants, we introduced the variable t. Initial efforts to reproduce the observed rotational constants for all three states using only the variables b and t failed to achieve acceptable results. We then decided to introduce a second geometric distortion in the 1,2,3-trifluorobenzene cation geometry. Obviously, the fluorine atoms are much larger than the hydrogens, so some steric repulsion of the fluorines is not unreasonable. This could be modeled by the introduction of an angular distortion, Aa, in the nominally 120° C-C-F bond angles, (3, for the off-axis fluorine atoms in positions 1 and 3. This is also shown schematically in (22) Miller, T. A.; Bondybey, V. E. Molecular Ions: Spectroscopy, Structure, and Chemistry; Miller, T. A., Bondybey, V. E., Eds.; NorthHolland: Dordrecht, Holland, 1983.
2732
J . Phys. Chem. 1991, 95. 2732-2738
Figure 8. Moreover, since the C-C bond lengths change, a may well change between_the states. We assume for simplicity that 0 is the same in the X and A states but differs by an amount Aa in the B state. Using these three-variable parameters and the planarity condition, the nine rotational constants are well reproduced. These results along with the determined values of 6, t, and Act are given in Table 11. As noted above, to obtain the initial guess for the rotational constants, an undistorted geometry with all the angles 120°, all the C-C bonds equivalent, and the lengths of the C-H and C-F bonds equal to their values in the corresponding symmetrical D6* species was assumed. The necessity of including the parameter Pa simply indicates that this assumption is not quite correct. The bonding angles and lengths are somewhat distorted in the asymmetrically substituted ions, and more significantly the magnitude of this deviation of the average geometric parameters from those of C6H6+and c6F6+need not be identical in the ground and excited electronic states. While the exact nature of this distortion cannot be unambiguously determined from the available experimental data, a slight opening of the C C F angle for the 2 and 6 positions due to steric effects appears quite reasonable. The small, positive value of the parameter 6 implies a slight symmetrical lengthening of the C-C bonds in the excited electronic state, as compared with the ground state. This result is again quite reasonable and consistent with the promotion of an electron from the azuorbital to the weaker bonding, degenerate e, orbital. It also agrees with the experimental observations of reduced values for the C-C ring-stretching vibrations in the excited ionic B state, as well as with the recent analyses of the spectra of the symmetric C6F6+ and 1,3,5-C6H3F3+ ions. Perhaps most interesting is the need to invoke a nonzero value of the distortion t . This implies that the dynamic Jahn-Teller distortion of the symmetric species is stabilized by the asymmetric
substitution and becomes a static distortion of the two quasidegenerate components of the electronic ground state. The size of this distortion is of the same order of magnitude, but somewhat smaller than the value obtained from a Jahn-Teller analysis of the spectrum of the symmetric C6F6+ion, where a value o f t = 0.013 A was deduced,D compared to the present t = 0.003 A. One notes, moreover, that the present distortion is static, whereas in C6F6+ it is dynamic.
Summary A dc discharge in a rare-gas carrier gas is used to generate high concentrations of electronically excited molecular radical cations in a free jet. The technique is applied to the study of the fluorescence of supersonically cooled 1.2,btrifluorobenzene radical cation. High-resolution, Doppler limited electronic emission spectra are recorded by using a Bruker IFS 120HR Fourier transform spectrometer. Studies of the rotational band contours provide information about the rotational temperature-80 K for this study-and geometry changes occurring during the transition and identify clearly transitions involving the low-lying A state. The results are compared with our previous study of this compound in the solid neon matrix. It is shown that the magnitude of the splitting of the two components of the benzene ground state by the asymmetric substitution, as well as the vibrational structures in both states are insignificantly perturbed by the solid neon matrix. Acknowledgment. B.D.R.thanks The Ohio State University Graduate School for the University Postdoctoral Fellowship. The support of this work by the National Science Foundation under Grants C H E 8803169 and CHE 9005963 and by the Ohio State University Research Grant Program is gratefully acknowledged. Registry No. 1,2,3-Trifluorobenzeneradical cation, 74626-94-1.
Matrix Isolation Electron Spin Resonance Studies of 28i2*Si 2 9 28i29Si 2 9 29p29Si 2 9 Ge2 and 73Ge2+Produced by Pulsed Laser Vaporization. Comparison with Theoretical Calculations +
+
+
9
+
Lon B. Knight, Jr.,* J. 0. Herlong, Robert Babb, E. Earl, Devon W. Hill, and C. A. Arrington Chemistry Department, Furman University, Greenville, South Carolina 2961 3 (Received: September 1 7, 1990)
The Si2+and Ge2+cation radicals were generated by pulsed laser vaporization and isolated in rare-gas matrices at 4 K for electron spin resonance (ESR)investigations. The electronic ground states were established as X'Z for both cations with the three unpaired electrons occupying predominantly valence p-type orbitals. In the case of 29Si2+,the observed nuclear hyperfine interaction (A tensor) was compared with that computed in an ab initio configuration interaction type calculation. The dependence of the hyperfine parameters on internuclear distance was also investigated. For comparison purposes, all diatomic and triatomic cations studied in the gas phase and in rare-gas matrices at sufficiently high resolution to observe nuclear hyperfine interaction are listed. For ?Si2+ in neon, g, = 1.993 (I), lA,l = 52.4 ( 5 ) MHz, and D = 27.6 (8) GHz; for 73Ge2+,g, = 1.939 ( I ) and lAll = IO (3) MHz.
Introduction Laser vaporization combined with neon matrix isolation a t 4 K has been employed to generate the Si2+ and Ge2+ open-shell cation radicals for electron spin resonance (ESR) study. No previous spcctnwcopic measurements have been reported for these fundamentally important semimetallic ion molecules. The ESR results provide the first experimental evidence that Si2+and Ge2+ have X4Z ground electronic states, in agreement with earlier theoretical calculations for Si2+.1-3 Matrix isolation ESR and (1) Bruna, P. J.; Petrongolo, C.; Buenker, R. J.; Peyerimhoff, S. P. J . Chem. Phys. 1981, 74,461 1,
gas-phase electronic spectroscopic observztions have shown that C2+is also a X42radical i0n.49~ Electronic structure information on these simple diatomic ions should contribute to the advancement of theoretical methods that are vitally important for understanding the larger clusters of these species and their solid-state properties. (2) McLean, A. D.; Liu, B.; Chandler, G.S.J. Chem. Phys. 1984.80, 5 130. ( 3 ) Jocrg, H.; Rosch, N.; Sabin, J. R.;Dunlap, 8 . I. Chem. Phys. Lerf. 1985, 114, 529. (4) Knight, L. B., Jr.; Cobranchi, S.T.; Earl, E. J . Chem. Phys. 1988,88, 7348. ( 5 ) Forney,D.; Althaus, H.; Maier, J. P. J . Phys. Chem. 1987,91,6458. Maicr, J. P.; Rosslcin, M. J . Chem. Phys. 1988, 88, 4614.
0022-3654J9 1/2095-2732$02.50JO 0 1991 American Chemical Society
