The NMR isotope shift in polyatomic molecules. Estimation of the dynamic factors

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Abstract
In a continuing study of the factors which determine the isotope shifts in NMR we consider here substitution of end atoms in molecular types in which the bond angle deformation does not play a significant role. We show that in molecules of the type AXn the rovibrational correction to shielding of nucleus A can be obtained directly from the isotope shift without dynamical calculations. We propose a method for estimating 〈Δr〉, knowing only the equilibrium bond length, the masses, and the rows of the Periodic Table of the atoms in the bond. We test this on diatomic molecules for which we are able to calculate 〈Δr〉 directly from the spectroscopic constants and examine its dependence on bond order. We apply the estimation method to polyatomic molecules for which we have completed a full dynamical calculation using the best available force fields. We use the estimated 〈Δr〉 for other molecular systems and obtain estimates of the shielding derivatives from the observed isotope shifts. The results compare well with derivatives that are known for related electronic environments.