ESTIMATING MOLECULAR COLLISION DIAMETERS USING COMPUTATIONAL METHODS

The collision diameters of a series of 34 atoms and both nonpolar and polar molecules were obtained from ab initio calculations. Molecular v olumes, V-M, were determined from a Monte Carlo integration of the ele ctron density distribution of the optimized HF/6-31G structures. The isotropic collision diameter, d(vol), defined as (6V(M)/pi)(1/3) was c ompared in all cases with the respective Lennard-Jones potential minim a positions, sigma(m). An excellent linear correlation between d(vol) and sigma(m) for the 34 species was found, with slope and intercept of 1.025 and -0.07 Angstrom, respectively. It is thus suggested that d(v ol) may be used directly as an estimate of the isotropic collision dia meter of a species. d(vol) values for several atoms and radicals, as w ell as for the electronically excited states of NH3 and acetone, are i ncluded. The results show that d(vol) values for the Rydberg excited s tates are larger than those of the respective ground states.