Rovibrational effects on nuclear shielding of apex nuclei in bent molecules

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Abstract
The isotope shifts of non-end nuclei with lone pairs are generally larger than those of nuclei without lone pairs. Using the bent triatomic molecule as a prototype, we examine the mass and temperature dependence of the mean bond angle deformation and the mean displacement along a bond due to centrifugal stretching and anharmonic vibration. We find that the temperature dependence of 〈Δα〉 determines whether the temperature dependence of the shielding of the apex nucleus will be normal [(dσ0/dT)<0] or abnormal. The dominant contribution of the rotation to the temperature dependence of 〈Δα〉 in the hydrides can lead to opposing temperature effects on shielding while the mass effects lead to normal isotope shifts. We performed similar calculations for the trigonal pyramidal molecules in order to explain the observed abnormal temperature dependence of the 15N and 31P nuclear shielding in NH3 and PH3.