Least-squares mass-dependence molecular structures

The major part of the difference epsilon(0)(alpha) = I-0(alpha) - I-e(alpha ) (alpha = a, b, c) between the zero-point and equilibrium moment of inerti a of a molecule is a homogeneous function of degree 1/2 in the atomic masse s. The use of explicit mathematical models of epsilon(0)(alpha) is studied here as a means of determining near-equilibrium molecular structures from t he zero-point moments of inertia of a range of isotopomers. It is found tha t models with more than two parameters per axis generally give strongly cor related fits for the number of isotopomers typically available. The two-par ameter model epsilon(0)(alpha) = c(alpha)(I-e(alpha))(1/2) + d(alpha)(m(1)m (2) ... m(N)/M)(1/(2N-2)), where N is the number of atoms, m(i) are the ato mic masses, and M is the molecular mass, usually gives well-conditioned fit s with standard deviations in the parts-per-million range, and structures c alled r(m)((2)) that are often close to equilibrium structures. Laurie-type corrections for hydrogen atoms and parameters to describe isotopic rotatio ns of the epsilon tensor can also be included. (C) 1999 Academic Press.