Estimation of equilibrium structure and anharmonicity by use of changes in the rz structure by isotopic substitution and vibrational excitation: I. General formulation and application to OCl2

A general formulation is presented on the differences in the average (r_z) structure caused by isotopic substitution and vibrational excitation; the differences in the average structure for both bonded and nonbonded atomic pairs are represented as linear combinations of quadratic average values of the Cartesian displacement coordinates. This method is applicable to a joint analysis of spectroscopic and electron diffraction data so that the equilibrium geometrical parameters and effective anharmonic constants for polyatomic molecules can be estimated even when only a limited number of rotational constants for isotopic species and vibrationally excited states are available. Its application to OCl₂ molecule as an example is discussed.