The 2750-Å band system of phenol: Part II. Extended vibrational assignments and band contour analysis

Abstract

Vibrational analysis of the electronic bands of phenol vapor at 2500–2900 Å has been extended to the a 1 modes of phenol- d 5 and the a 2, b 1, and b 2 modes of phenol- h 6, - d 1, and - d 5. The results give an essentially complete set of frequencies for the fundamentals lying below 2000 cm −1 in the excited state. Ground-state fundamentals to about 1000 cm −1 can be identified from “hot” bands in the electronic spectrum and are generally in good agreement with the infrared vapor absorption. The envelopes of A-, B-, and C-type bands have been calculated in the rigid rotor approximation for a range of values of the excited-state inertial constants. Features in the origin band contour are matched in the computed envelope when, and only when, the band is taken as B-type ( y-polarized); therefore the electronic transition is B 2 ← A 1, analogous to the B 2 u ← A 1 g transition of benzene. The changes in rotational constants accompanying the transition are Δ A = −0.0113 5 and Δ B = 0.00116 cm −1. Combined with micro-wave data for the ground state, these constants lead to approximate values for I a and I b in the electronically excited state. While the results are not sufficient for a structure determination, they show that the phenyl ring is almost certainly not a regular hexagon in the B 2 state, though the question whether the distortion is associated with unequal CC bond distances (“quininoid” structure) or unequal angles within the ring, or from a blend of both, cannot be decided. The picture that emerges is consistent qualitatively with the fact that torsional motion in the B 2 state is much more rigid than in the ground state, the torsional barrier being about 3.5 times greater. It therefore appears probable that the phenyl ring has appreciable quininoid character in the B 2 state.