Estimation of Equilibrium Structure by Use of Isotopic Differences in the rz Structure Application to Several Symmetric Hydrides

Abstract

The equilibrium (r_e) structures of H₂O, NH₃, CH, , H₂S, PH₃, and SiH₄ have been estimated from isotopic differences in their zero-point average (r_z) structures by application of the r_e method, which has recently been developed in our laboratory. This method is a modification of Watson's mass-dependence method (r_m) for extending its applicability to molecules including hydrides; the second-order correction terms, which depend on isotopic masses, in Watson's expression of the moment of inertia are compensated for and only a limited number of the isotopic rotational constants for the ground vibrational state are needed. Its general form is
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where δr_z(i) represents an isotopic difference for the r_z distance in question when i-th atom, m_i, is substituted by one of its isotopic species, m_e. The sum is taken over al l the constituent atoms. The r_e structures thus obtained are in good consistency with the reported re structures derived using rotational constants for excited vibrational states to within a 002Å and O.1°, except for the H-N-H angle in NH₃, where the influence of higher -order interactions is significant, as shown in Table 7.