Analysis of competing bonding parameters. Part 1. The structure of halomethanes (CH 4− nX n, n=1–4, X=F, Cl, Br)

Abstract

A qualitative rationalization of the structural anomalies in the series of halomethanes, (CH 4− n X n n=1–4, X=F, Cl, Br), is presented. Factors influencing the geometry of these molecules, which have not been considered hitherto, are discussed on the basis of ab initio molecular orbital calculations using natural bond orbital analysis. The electrostatic attraction between the central carbon and the terminal sites and the repulsion between the terminal sites compete in their influence on the molecular geometry. Intra-molecular interactions between the H and F sites are proposed as one of the reasons for the large H–C–H bond angle in CH 2F 2. We find a correlation between the hybridization on the central atom and the terminal atoms. In this context, we have decoupled the influence of a variation in n and changes in the bond angles on charge distribution at the C and F sites. Decoupling these two influence parameters is the prerequisite to elucidate their role in directing rehybridization on the central carbon and the terminal halides. The maximum in the experimental dipole moment of the fluoromethanes at n=2 is explained by a point-charge approximation. A constant charge density at the F sites and a monotonous decrease in the C–F bond distance as n increases account for this variation.