Everything about Vibronic Coupling totally explained
In
theoretical chemistry, the
vibronic coupling terms, (which are neglected within the
Born-Oppenheimer approximation), are proportional to the interaction between electronic and nuclear motions of molecules. The term "vibronic" originates from the concatenation of the terms "vibrational" and "electronic". The word coupling denotes the idea that in a molecule, vibrational and electronic interactions are interrelated and influence each other.
Vibronic coupling is large in the case two
adiabatic potential energy surfaces come close to each other, for example when the energy gap between them is of the order of magnitude of one oscillation quantum. This usually happens in the neighbourhood of an
avoided crossing of
potential energy surfaces corresponding to distinct electronic states of the same spatial and spin symmetry. However, vibronic coupling also exists at real crossings. In this case the
adiabatic or
Born-Oppenheimer approximation fails and non-adiabatic terms (the so-called vibronic coupling terms) have to be taken into account. The vibronic coupling terms are usually difficult to evaluate. This is because they're proportional to the first and second derivatives of the electronic wave function with respect to the
molecular coordinates. A simpler way to solve this problem is to switch from the
adiabatic to the
diabatic representation of the
potential energy surfaces. The vibronic terms are responsible for example for surface hopping or the
Berry phase. The
Berry phase has been discovered by
Longuet-Higgins in this context. The vibronic coupling becomes infinite in the neighbourhood of a
conical intersection. This singularity in the potential energy landscape is the origin of the
Berry phase.
Perhaps the earliest examples of the importance of vibronic coupling were found during the 1930s. In 1934
Renner wrote about the vibronic coupling in an electronically excited Π-state in CO
2. Calculations of the lower
excited levels of
benzene by Sklar in 1937 (with the valence bond method) and later in 1938 by
Goeppert-Mayer and Sklar (with the
molecular orbital method) demonstrated a correspondence between the theoretical predictions and experimental results of the benzene
spectrum. The benzene spectrum was the first qualitative computation of the efficiencies of various vibrations at inducing intensity absorption.
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