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A new assignment for the S0→S1 transition of N,N‐dimethylaniline (DMA) and related derivatives is presented. The low frequency bands and long Franck–Condon envelope observed in DMA‐h6 and DMA‐d6 are assigned to the coupled methyl torsion mode of the amino group, not to torsion of the amino substituent about the C–N bond. This new assignment is consistent with the change in frequency of the excitation bands upon deuteration of the methyl groups and the strong origin transitions observed in the excitation spectra of other alkyl anilines. The assignment was confirmed by simulations of the excitation spectra of DMA‐h6 and DMA‐d6, with parameters of the calculated potential energy surface determined to be V3=148.0±0.5 cm−1, V+=−31.6±0.5 cm−1, V−=8.5±0.5 cm−1, and V6=−15±0.5 cm−1. By Franck–Condon analysis, it was determined that the weak origin transition is due to the shifting of the S1 torsion minimum by 40° along the gearing coordinate relative to the corresponding minimum in the ground state.

Coupling between the methyl torsion (υ15) and inversion (υ14) modes is used to model the singlet π*←n spectrum of jet‐cooled acetaldehyde and its deuterated analogs. The anomalies observed in the two torsion progressions built onto the lowest inversion doublet are accurately reproduced by using a potential of the form 1/2V3[1−cos3(τ−kq)], where k indicates the proportionality of interaction between the torsion and inversion motions. The coupling also results in intensity to torsion–inversion combination levels that, in the absence of coupling, would not be observed. Consequently, many of the observed transitions are simultaneously Franck–Condon allowed and vibronically induced. The weak origin transition and long progression in the methyl torsion mode is also reproduced in the intensity calculation, indicating that the equilibrium position of the methyl rotor has changed from the S0 eclipsed position to a nearly staggered geometry in the S1 state. The calculated potential energy surface and Franck–Condon ...

In a study of the 281 nm S1–S0 electronic spectrum of 1,4‐benzodioxan, the high‐resolution vapor absorption spectrum, the fluorescence excitation spectrum in a supersonic jet, and the dispersed single‐vibronic‐level fluorescence spectra for excitation into several different S1 levels have been measured. Four out‐of‐plane ring vibrations have been characterized in each electronic state, and it is shown that the partially saturated six‐membered ring is rigidly nonplanar with a twisted C2 conformation. The twisting vibration is nearly harmonic and the barrier to planarity is high in both electronic states; however, there is some lowering of the barrier upon excitation. There is strong Duschinsky mixing in S1 between two of the out‐of‐plane ring bending modes, one of which is quite anharmonic with a flat‐bottomed potential.

The effects of rotation–torsion interactions on the rotational structure of torsional vibronic bands in an electronic spectrum are examined, and a computer program to calculate the relevant energies and transition intensities is described. Rotationally resolved type C bands in the 1nπ* spectrum of methylglyoxal are reported and interpreted. Since the torsional conformation of methylglyoxal differs in the two electronic states, a long progression of bands is observed, involving levels well below, near, and above the 190 cm−1 S1 torsional barrier. The rotation–torsion interaction increases rapidly towards the free rotor limit as the barrier is approached. Observed effects include energy shifts and the appearance of transitions forbidden by rigid rotor or torsional vibronic selection rules.

An S1 ring‐N(CH3)2 torsional potential function with V2≂3700 and V4≂−1450 cm−1 is deduced from the published spectrum [Grassian et al., J. Chem. Phys. 90, 3994 (1989)] of 4‐N,N‐dimethylaminobenzonitrile. The equilibrium conformation is twisted by 25°, and there are barriers of 185 and 3900 cm−1 at the planar and perpendicular conformations. Similar results are obtained for the (CD3)2 derivative, and for N,N‐dimethylaniline, 3‐N,N‐dimethylaminobenzonitrile, and 4‐(trifluoromethyl)‐N,N‐dimethylaniline.

Profiles of lines in the 3–1 and 2–0 bands of the B 3Π0+←X 1Σ+ system of I35Cl and I37Cl vapor, as well as in several fragmentary P and R branches associated with an adiabatic B′ state, have been recorded using a Fabry–Perot interferometer spectrometer. The potential curves of the B state, and of the adiabatic B′ state formed by an avoided crossing near 3.6 A and 18 140 cm−1 between the B state and a repulsive Y O+ state, have been determined using the intermediate coupling model of Child. Linewidths in v=2 of the B state are observed to be 0.034 cm−1, independent of J and of isotopic composition; however, linewidths in v=3 are broader, corresponding to predissociative lifetimes in the subnanosecond range, and are found to oscillate as a function of J. The major features of the observed linewidth pattern can be explained in terms of analytical approximations developed by Child and co‐workers, and provide evidence for a second curve crossing, near 2.94 A and 17 975 cm−1, of the B state and an Ω=1 state.

The conformation of acetaldehyde in its n→3s Ryberg state is stated to be s–cis (one methyl hydrogen eclipsed by oxygen). Heath et al.1 stated on the basis of their measurements that it was not possible to tell whether this excited state was a s–cis. The calculations presented here definitely establish the excited state as a s–cis.(AIP)