Zero Point Level Effects on Structure, Energy and Spectra of Molecules & Materials

Abstract

This review encompasses the considerations that must come into play when dealing with molecules or hydrogen bonded crystals where there are two or more equivalent potential energy minima and those cases where isotopic replacement influences the relative energy or spectral properties such as 13C NMR chemical shifts. In such cases the properties of the zeropoint level must be investigated. The specific cases discussed are changes in the relative energies of polymorphic crystal structures, the effect of deuterium substitution on the 13C NMR chemical shifts and the hyperfine coupling constant of a radical species involving addition of either muonium or hydrogen atom. We discuss first molecules with double minimum potentials: ammonia, the potentially aromatic [18] annulene and polyacetylene. Ammonia has minima in its potential energy at equivalent out of plane positions with a maximum at the D3h planar geometry. The vibrational energy levels of the outofplane bending mode of the ground electronic state of NH3 are shown in the lower part of Figure 1. The pattern of levels in the A1 ground state is due to tunneling splitting. The excited electronic state is D3h symmetric at its minimum. Raman transitions excited with 212.8 nm radiation are shown [1]. [18]annulene Figure 2. with N = 4n+2 for n = 4 is potentially aromatic. X-ray diffraction gives D6h symmetry for this material, but this structure does not agree with calculations of proton NMR.