The Near Ultraviolet Absorption Spectrum of Toluene Vapor

Abstract

The experimental work of Savard and the theoretical work of Sponer on the absorption spectrum of toluene vapor is extended. A spectrum of 209 bands was observed with partial resolution of the fine structure. A tentative analysis is given based on ground state frequencies 514, 620, 785, 1003, 1012, 1176, and 1209, which correlate completely with Raman data; on excited state frequencies, 456, 528, 751, 932, 964, and 1186; and on difference frequencies 59 and 178 cm−1. Assuming C2v symmetry, all frequencies below 1300 cm−1 of symmetry class A1 appeared in 1–0 transitions and with one exception in 0–1 transitions. In addition, a vibrational frequency of symmetry class B1, deriving from an εg+ vibration in benzene, appeared strongly both in the ground and excited state.

Vibrations of the low energy states of toluene ($\tilde X$X̃ 1A1 and $\tilde A$Ã 1B2) and the toluene cation ($\tilde X$X̃ 2B1)

“…The 371.0 cm -1 feature in the Tol-h 8 spectrum was also seen in the fluorescence study by Hickman et al 3 who dispersed the fluorescence, finding a spectrum displaced from that of the origin by 604 cm -1 ; they assigned this band as M 14 M 20 . This feature was also seen by Murakami et al 27 and is evident in the spectrum of Doyle et al 24 Persual of Table II shows that both this and the 289.9 cm -1 feature are likely associated with the two lowest-wavenumber fundamentals, M 14 and M 20 . Clearly neither fundamental can be attributed to this feature, on the grounds of both wavenumber and symmetry; however, combinations and overtones are possible.…”

“…They only report a single band below 200 cm -1 for Tol-h 8 , at 148 cm -1 , and none for Tol-d 3 , and this band was assigned as a hot band therein. Perusal of the papers reporting earlier absorption spectra of Tolh 8 reveals that Ginsburg et al 14 observe a very weak band at 172 cm -1 , but this was unassigned. Krogh-Jesperson et al 20 assign a band at 157 cm -1 above the origin to M 20 in their two-photon REMPI spectra using visible light; they also observe the M 20 0-1 hot band at −224 cm -1 , and the M 20 1-1 sequence band at −61 cm -1 .…”

“…18 The M 2 14 overtone would be expected to be >400 cm -1 and so it seems most likely that the 290 cm -1 band is due to M 2 20 . These assignments then suggest values of 226 and 145 cm -1 for M 14 and M 20 , respectively. The corresponding bands in the Tol-d 3 spectrum lie at 268.9 and 359.2 cm -1 , with the same rationale leading to values of 225 and 134 cm -1 for M 14 and M 20 , respectively.…”

“…These assignments then suggest values of 226 and 145 cm -1 for M 14 and M 20 , respectively. The corresponding bands in the Tol-d 3 spectrum lie at 268.9 and 359.2 cm -1 , with the same rationale leading to values of 225 and 134 cm -1 for M 14 and M 20 , respectively. We note that this suggests that the wavenumber of M 14 is largely unchanged by deuteration, in line with the calculated values, and the form of the vibrational mode; on the other hand, the M 20 band is decreased significantly upon deuteration, again in line with the calculated values, and the form of the vibrational mode.…”

“…The majority of the observed absorption features were identified with the aid of dispersed fluorescence, making use of the known S 0 frequencies to support these assignments, although it was noted that a few of those assignments were not wholly certain. These measurements yielded frequencies for 13 normal modes in the S 1 excited state, which were compared with the early UV absorption measurements of Ginsburg et al, 14 which in turn were based on earlier experimental work by Savard 15 and Masaki 16 and theoretical work by Sponer,17 with slightly later work by Kahane-Paillous and Leach. 18 There are also two-photon resonant spectra recorded via fluorescence by Vasudev and Brand 19 and via multiphoton ionization by Krogh-Jespersen et al 20 Independent confirmation of some of the vibrational assignments were provided by Balfour and co-workers, who obtained infrared 21 and UV 22 absorption spectra for liquid toluene and seven deuterated isomers thereof.…”