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Journal of the American Chemical Society
In processes where reactant and product species have similar structures entropy changes are negligible. Hence, the enthalpy difference associated with the HX exchange should reasonably reflect the free-energy change for such reactions. However, when cyclic and acyclic onium ions are compared, the entropy change associated with ring formation is generally in the neighborhood of 10 eu. This will cause A@ to favor the acyclic structure by 3 kcal/mol over Ah" at 298 K. (30) G. A. Olah, D. A. Beal, and P. W. Westerman, J. Am. Chem. Soc., 95,3387 (29)
(1973). (31) G. A. Olah and J. M. Bollinger, J. Am. Chem. SOC., 89, 4744 (1967). (32) G. A. Olah, J. M. Bolinger, and J. Brinich, J. Am. Chem. SOC.,90, 2587 (1968). (33) G. A. Olah, J. M.Boilinger, Y. K. Mo, and J. M.Brinich, J. Am. Chem. SOC., 94, 1164 (1972).
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(34) (35) (36) (37) (38)
G. A. Olah, Y. K . Mo, and V. Haipern, J. Org. Chem., 37, 1169 (1972). G. A. Olah and A. M. White, J. Am. Chem. SOC.,91,5801 (1969). G. A. Olah and R. D. Porter, J. Am. Chem. SOC., 93,6877 (1971). B. H. Solka and M. E. Russell, J. Phys. Chem., 78, 1268 (1974). R. H. Staley, R. R. Corderman, M. S.Foster, and J. L. Beauchamp, J. Am.
(39) (40)
W. L. Jolly and C. Gin. Int. J. Mass Specbom. /onPhys., 25, 27 (1977). See, for example, F. A. Cotton and G.Wilkinson, "Advances in Inorganic Chemistry", 3rd ed., Wiley-interscience, New York, N.Y., 1972, pp 115,
Chem. SOC.,96, 1260 (1974).
72-125,531 (41) D. H. Aue and M. Bowers, to be published. (42) R. R. Corderman and J . L. Beauchamp. unpublished results. (43) J. F.Wolf, R. H. Staley, I. Koppel, M. Taagepera. R. T. Mclver, J. L. Beauchamp, and R. W. Taft, J. Am. Chem. SOC.,99,5417 (1977).
Matrix Laser Fluorescence Spectra of Several Fluorobenzene Radical Cations V. E. Bondybey,* Terry A. Miller, and J. H. English Contribution front Bell Laboratories, Murray Hill, New Jersey 07974. Receiced September 7 , I978
Abstract: Several fluorobenzene radical cations were generated by vacuum UV photolysis of the parent fluorobenzenes in solid Ar matrix, and their laser fluorescence excitation spectra were studied. Their spectra show a well-resolved vibronic structure, which appears to be essentially unperturbed by the solid. Vibrational analysis provides strong evidence for the occurrence of a Jahn-Teller distortion in the degenerate 2El, ground state of C6F6+
1. Introduction Until fairly recently, the number of spectroscopic studies of polyatomic ions was rather In the last few years there has been a surge of activity in this Most of the reported ions were generated either in discharges or by electron impact and identified by analysis of their emission spectra. Although immensely useful, studies of this nature have several limitations. In the first place, the emission spectra provide mostly information about the ground electronic state, and only limited data are generally obtained about the vibrational structure of the excited electronic state. Furthermore, particularly in larger molecules, the dense rotational structure and the large number of levels populated a t high or even ambient temperatures usually result in poorly resolved spectra with broad and overlapping bands and make the interpretation difficult. The first limitation can be overcome by the study of laser fluorescence excitation spectra, which can provide extensive information about the excited electronic states. Very recently we have indeed reported such a study of the C6H3F3+ cation.9 The difficulties inherent in the complexity of the spectra can be eliminated by reducing the sample temperature. A convenient means for the study of low temperature spectra is provided by the matrix isolation technique. Numerous studies of the matrix spectra of a variety of polyatomic anions and cations have appeared in the last few years, most of them employing infrared spectroscopy. Jacox and MilliganIo have observed the matrix spectrum of CC13+. Since then a number of other halogenated methylene and methyl radical ions have been reported,' ' - I 3 mainly by Jacox and by Andrews and co-workers. Also a variety of other small inorganic ions have been observed.I4-l6 In contrast to the large number of infrared studies, very little is known about the electronic spectra of matrix isolated ions, and only the diatomic C2- was studied in some detai1.'7~18 Studies of electronic spectra would clearly be desirable because of the inherent, much higher sensitivity. Furthermore, if viOOO2-7863/79/l5Ol-l2485Ol.OO/O
brationally resolved electronic spectra can be obtained, they should provide mainly the symmetric vibrational modes and thus complement conveniently the infrared spectroscopy. In the present manuscript we report the observation of vibrationally resolved emission and laser excitation spectra of several fluorobenzene radical cations. In a preliminary communicationI9 we have already reported the matrix spectrum of C6F6+.Such studies are desirable for two reasons. In the first place, the low-temperature spectra with their well-resolved vibrational structure should provide useful information about these interesting species or, when available, assist in the interpretation of the more congested gas-phase spectra.20In the second place, they provide the possibility of comparison of the molecular constants of these ionic species with their gas-phase values and give an indication of the extent to which the ions are perturbed by the solvent. This is of particular value, since, for most of the ions whose matrix IR spectra have been reported, such comparisons are not available.
2. Experimental Section The parent fluorobenzenes (Aldrich) were purified by several freeze-pump-thaw cycles and mixed with Ar. Typically dilutions of 1:2000 to 15000 were employed. The samples were then deposited on a sapphire substrate at 0 5 K, mostly with simultaneous photolysis. In several experiments the samples were deposited without photolysis and subsequently photolyzed in situ. The photolysis was accomplished using an atomic resonance lamp excited by a microwave discharge. The hydrogen Lyman a 1216-A line was usually employed, although the Xe 1470-Aradiation was used in several experiments. The sample fluorescence was excited using a tunable dye laser pumped by an N2 laser. To avoid saturating the individual vibronic transitions in the matrix isolated molecules, the laser power was typically attenuated by inserting an O.D. 2-3 neutral density filter into the laser beam. The sample emission was resolved in a SPEX 14018 monochromator. The PMT signal was time resolved and averaged in a Nicolet signal averager. The data acquisition as well as the scanning of the laser and the monochromator were controlled by a minicomputer.
0 1979 American Chemical Society
Bondybey, Miller, English f Fluorescence Spectra of Fluorobenzene Radical Cations EXCITATION SPECTRUM IN SOLID A i
EXCITATION SPECTRUM SOLID Ar
F&
1
24200
24400
F
2 70
24600
200
24800
25000
25400
256000
25800
ii ( c rn.9
Table I. 1,2,4,5-ChH>Fd+ Vibrational Frequenciesa Symmetry)
(&h
B X, solid Ar
solid Ar
gas
VI
u3 u4 u5 U6
480 289
1536 1392 122 470 219
23600
23800
24000
”2
Figure 1. Two sections of the 1.2,4,5-C6HzF4+excitation spectrum in solid Ar. Spectrum \*as not corrected for laser power variation. The signal in the 0-0 band a t 24 072 cm-’ was monitored with =2-cm-I band pass.
a, species
23400
v(cm-’1 Figure 2. Part of the excitation spectrum of I ,2.3.5-ChHzF4+. The intensity of the 0-0 band at 22 903 cm-’ was monitored as a function of laser wavelength.
.;1
2%
1249
460 270
parent,
ChH2F4
3097 1643 1374 748 487 280
a I n reciprocal centimeters. Parent data were taken from ref 21 and 22; gas-phase data were taken from ref 23. Note that in all of the fluorobenzenes studied we label the emitting, excited electronic state “B”. The label “X” refers to the ground state which is, in the case of CsFh+, doubly degenerate.
3. Results 1. 1,2,4,5-Tetrafluorobenzene. Two sections of the excitation spectrum of the photolyzed 1,2,4,5-tetrafluorobenzenesample are shown in Figure 1. This spectrum was obtained by tuning the laser and monitoring the intensity of the 0-0 emission band a t 24 072 cm-I as a function of the excitation wavelength. The monochromator band pass was -2 cm-I, and selected thus only a narrow range out of the inhomogeneously broadened line width. The origin band with a maximum at 24 072 cm-l and an inhomogeneous linewidth of =25 cm-’ is slightly red shifted from its gas-phase position a t 24 440 cm-I. The spectrum consists of a large number of rather sharp bands, which clearly correspond to the upper state vibrational structure. The strongest bands in this spectrum form an array characterized by vibrational intervals of 470 and 279 cm-I, which are readily assigned to the two lowest frequency ag fundamentals, v5, the C-F bending, and 4 , the C-C-C ring deformation. These frequencies agree extremely well with the parent 487- and 280-cm-’ values2’.22 and provide strong support for the reassignment22of the weak Raman band a t 280 cm-I as the lowest ag fundamental. Vibrational frequencies of 1536 and 1392 cm-’ and their combinations with the v5 and V 6 vibrations also appear moderately strongly in the spectrum. These are assigned to the v2 and u3 ag modes-the C-F and C-C stretching frequencies, probably in that order. These occur in
the parent fluorobenzenes at 1643 and 1514 cm-I, respectively. Finally a weak band at 24 794 cm-I does not fit the pattern of the other frequencies and must therefore involve an additional vibrational mode of 722 cm-I. This is probably the ring “breathing” C-C stretching frequency, which was assigned at 748 cm-l in the parent compound. Emission spectra obtained by excitation in any of the sharp vibronic bands in Figure 1 are identical, indicating that vibrational relaxation is considerably faster than the radiation and that all the emission originates from the vibrationless level. The vibrational frequencies derived from both the emission and excitation spectra are with their assignments summarized in Table I . For comparison, the corresponding ground-state frequencies of the parent compound are also given. Where data are available, we also give excited-state vibrational frequencies derived from the gas-phase laser excitation spectra.23 2. 1,2,3,5-Tetrafluorobenzene. The spectrum of this cation has an origin a t 22 903 cm-I in solid Ar. Its excitation spectrum is shown in Figure 2. While the 1,2,3,5-substituted benzene has a lower Cz, symmetry, and therefore all totally symmetric al vibrations are allowed, its spectra are qualitatively very similar to those of the more symmetric isomer. Two low-frequency vibrations of 43 1 and 307 cm-I, respectively, occur strongly in the spectrum, both as simple progressions and in combinations with other vibrations. These agree remarkably well with the lowest al modes of the parent,?4 V I O of 443 and V I ) of 305 cm-I. Like the 279- and 470-cm-‘ vibrations of 1,2,4,5-C6HzF4+, they are probably due to the C-F and C-C-C bending vibrations. Also frequencies of 576,783, 1263, and 1536 cm-l occur in the spectrum, both singly and in combinations and are readily assigned to vg, us, v5, and u 2 based on the close agreement with the parent a1 vibrations a t 580, 789, 1249, and 1531 cm-l. Finally, a relatively strong band a t 24 486 cm-l implies a frequency of 1582 cm-I; this may be due to V I which occurs at 1642 cm-’ in the parent. Vibrational frequencies of 586,429, and 307 cm-I appear in the emission spectrum. These are clearly due to the corresponding v3, v10, and V I 1 ground-state frequencies. The vibrational data derived from our spectra are summarized in Table 11. 3. Pentafluorobenzene. Part of the excitation spectrum of CbHFs+ is presented in Figure 3. As in the other species, three
low-frequency vibrations appear repeatedly in the spectra. Their 567-, 456-, and 273-cm-’ values are again remarkably close to the parent V8, vg, and V I 1 vibrations (578,470, and 272 cm-1)22 and are therefore assigned accordingly. Two bands in the C-F stretching region occur at 1450 and 1545 cm-I and are assigned to the u4 and v3 modes (1 41 0 and 1514 cm-’ in the parent). A 678-cm-I frequency appearing weakly in the spectrum
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Journal of the American Chemical Society
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Table 111. ChHF
