An ab initio investigation on (CO₂)_n and CO₂(Ar)_m clusters: geometries and IR spectra

Abstract

An ab initio investigation on CO₂ homoclusters is done at MPWB1K/6-31++G(2d) level of theory. Electrostatic guidelines are found to be useful for generating initial structures of (CO₂)_n clusters. The ab initio minimum energy geometries of (CO₂)_n with n = 2-8 are T shaped, cyclic, trigonal pyramidal, tetragonal pyramidal, tetragonal bipyramidal, pentagonal bipyramidal, and pentagonal bipyramid with one CO₂ molecule attached to it. A test calculation on (CO₂)₂₀ cluster is also reported. The geometric parameters of the energetically most favored (CO₂)_nclusters match quite well their experimental counterparts (wherever available) as well as those derived from molecular dynamics studies. The effect of clustering is quantified through the asymmetric C-O stretching frequency shift relative to the single CO₂ molecule. (CO₂)_n clusters show an increasing blueshift from 1.8 to 9.6 cm-1 on increasing number of CO₂ molecules from n = 2 to 8. The energetics and geometries of CO₂(Ar)_m clusters have also been explored at the same level of theory. The geometries for m = 1-6 show a predominant T type of the argon-CO₂ molecule interaction. Higher clusters with m = 7-12 show that the argon atoms cluster around the oxygen atom after the saturation of the central carbon atom. The CO₂(Ar)_m clusters exhibit an increasing redshift in the C-O asymmetric stretch relative to CO₂ molecule of 0.7-5.6 cm^-1 with increasing number of argon atoms through m = 1-8.