A spectroscopic and theoretical analysis of the internal rotation bands appearing in the S1–S transition of phenylsilane

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Abstract
The laser‐induced fluorescence excitation spectra of phenylsilane near the 0–0 transition have been observed and analyzed in terms of the torsional (internal rotational) motion of the silyl group. The rotational constants of phenylsilane in the excited state were also determined. The high‐resolution spectrum of the 0–0 band shows a unique profile, which consists of an ordinary b‐type profile and a central peak. The rotational contour analysis showed that the band consists of two overlapping torsional bands of a b‐type transition. From the analysis including the rotational structure of the torsional bands, the potential barrier was determined to be 45.0 cm−1, which is much higher than that of toluene (27.0 cm−1). The large difference in the barrier height between toluene and phenylsilane reflects a difference in the electronically excited states of these compounds. An ab initio molecular orbital calculation was carried out to get an insight into the nature of the electronically excited phenylsilane. The calculated structures in both the S1 and S0 states are in good agreement with the observed ones. The interaction between the silyl group and the π system turned out to be due to hyperconjugation with the Si–H antibonding orbital, and the interaction with vacant d‐orbitals of the Si atom is not significant.