Representations of molecular force fields. IV. Vibrational stress, isotope effects, and structure of ethane

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Abstract
The transformation of molecular force fields from natural curvilinear coordinates S to rectilinear coordinates S (yielding simpler equations of motion) is discussed. Rectilinear cubic constants include mass dependent terms arising from the nonlinear coordinate transformation as well as contributions from the cubic constants for the S displacements. These contributors to anharmonicity lead to intramolecular ’’vibrational stresses’’ resolvable into components FC, FT, and FF which induce rectilinear molecular strain components ⟨S⟩C, ⟨S⟩T, and ⟨S⟩F, each of which has its own physical interpretation. More significant than the rectilinear quantities are the curvilinear stresses and strains which, it is shown, are readily expressed in terms of the above components. Each of the foregoing quantities is easily decomposed into contributions from the various types of vibration. Magnitudes are illustrated by computations of cubic force constants and selected stress and strain components in C2H6 and C2D6