Allan R. Gregory

Vibrational progressions in vibronically induced transitions

It is shown that progressions of totally symmetric vibrations in vibronically induced spectra do not show the same intensity distribution as comparable progressions in allowed spectra. This is due to cross terms, involving both totally symmetric and inducing mode coordinates, in the Herzberg–Teller expansion. These cross terms express the modulation of the vibronic coupling resulting from differences in interatomic distances between the borrowing and lending states. The effect is universal, as opposed to effects based on normal‐coordinate rotation or totally symmetric inducing modes, and enters even in the absence of anharmonic coupling. It leads to anomalous intensity distributions which, moreover, are different for emission and absorption spectra. Quantitative estimates are presented which indicate that it should be readily observable in many molecules.

Resonance Raman scattering in molecular dimers

A systematic study is initiated of resonance Raman scattering in molecular systems with overlapping electronic absorption bands. Resonance Raman spectra, excitation profiles, and depolarization ratios are calculated for a simple molecular dimer in which the two overlapping Franck–Condon progressions are related through the permutational symmetry of the dimer. The model used is highly idealized, but can be solved exactly for a complete range of intermolecular coupling strengths. In the weak and strong intermolecular coupling regions, the exact results are compared with the physically more transparent results obtained from a low‐order perturbation treatment. The spectra and profiles show properties characteristic of both totally symmetric and nontotally symmetric modes. The profiles and polarization dispersion curves are subject to both electronic and vibronic interference effects. Anomalous extrema of the depolarization ratio are often found to coincide with minima rather than maxima in the excitation prof...

Exact solution of a vibronic coupling model and its implications for the herzberg—teller expansion method

Abstract The nonadiabatic coupling between two electronic manifolds through a single harmonic oscillator is calculated exactly and compared with the first-order Rayleigh—Schrodinger and Tanaka—Fukuda perturbation results. The treatment leads to a physical interpretation of the Herzberg—Teller expansion for adiabatic coupling.

A new perturbation approach to coupling between adiabatic states

Abstract A new perturbation expansion, formally related to Tanaka-Fukuda perturbation theory, is derived and applied to the general problem of nonadiabatic coupling. The coupling between adiabatic Born-Oppenheimer states in poly-atomic molecules is treated in detail in both the one- and multi-phonon limits, for which previous methods are often unreliable, even for order-of-magnitude estimates.

On the structure of the radical SF5

Abstract Unrestricted SCF MO calculations in the INDO approximation lead to a satisfactory explanation for the absence of a detectable hyperfine interaction between the unpaired electron and the unique. 19 F nucleus of the radical SF 5 . They thus support the re-assignment of an EPR spectrum that was first attributed to the radical cation SF 4 + to SF 5 .

Vibrational frequencies of the ã (1A2) and ã(3a2) nπ states of formaldehyde. scaled ab initio force fields - A new application

Abstract A new method based on scaled ab initio force fields is introduced for calculating excited-state vibrational frequencies. Application to formaldehyde confirms the need to revise the value of v 3 (A, H 2 CO), but not the value of v 3 (a, D 2 CO).