Infrared intensities, atomic charges, and dipole moments in the fluoroethane series using atomic polar tensor analysis

To gain insight into the interpretation of the atomic polar tensors (APT) in the series of fluoroethanes, the fluorine, carbon, and hydrogen APT components P-alpha(ss') were calculated, in the inertial-axes coordinate system, and examined by determining the mean dipole derivatives (p) over bar(c), the King effective atomic charges chi(alpha), and APT anisotropies beta(alpha). From this, relationships between chemical bonding environment and (p) over bar(alpha) terms are clearly discernible, with the carbon-atom charges exhibiting a labile response to the degree and form of fluorine substitution. We examine the dependence of the total absolute infrared intensities, as predicted by the APT sum rules, on the degree of fluorine substitution and molecular structure for the entire set of fluoroethanes. Finally, dipole-moment components were calculated (i) directly from ab initio methods at the MP2/6-31G** level, (ii) from the P-alpha(ss') terms, and (iii) as the first moments of the (p) over bar(alpha) atomic charges at the nuclear equilibrium positions, and a detailed comparison was made.