Infrared Intensities, Atomic Charges, and Dipole Moments in the Fluoroethane Series Using Atomic Polar Tensor Analysis

Abstract

To gain insight into the interpretation of the atomic polar tensors (APT) in the series of fluoroethanes, the fluorine, carbon, and hydrogen APT components were calculated, in the inertial-axes coordinate system, and examined by determining the mean dipole derivatives p̄ α, the King effective atomic charges χα, and APT anisotropies βα. From this, relationships between chemical bonding environment and p̄ α 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 terms, and (iii) as the first moments of the p̄ α atomic charges at the nuclear equilibrium positions, and a detailed comparison was made.

Characteristic Substituent-Shift Models for Carbon 1s Ionization Energies and Mean Dipole-Moment Derivatives

“…In this equation, p̄ C refers to the carbon atom for which n F* fluorine atoms and n Cl* chlorine atoms substitute hydrogen atoms and n F fluorine atoms and n Cl chlorine atoms substitute hydrogens on its neighboring carbon atom. The coefficients for the fluorine terms are the same, after roundoff, as those found by Illinger and co-workers, +0.52 and −0.05 for n F* and n F , respectively (see their eq 13), for MP2/6-31G(d,p) results on only the fluoroethanes. This model indicates that substitution of hydrogen by fluorine raises the mean dipole-moment derivative on the substituted carbon atom by +0.52 e. Chlorine substitution has a smaller effect, as expected, owing to its smaller electronegativity relative to fluorine raising the carbon atom charge by about half that amount, +0.29 e. These values are almost exactly the same as those found in eq 11 for the halomethanes.…”

Quantum-Chemical Studies of Fluoroethanes: Vibrational Assignments, Isolated CH Stretching Frequencies, Valence Force Constants, and Bond Length Relationships

“…The environmental interest in the properties of hydrofluorocarbons has prompted a number of recent studies of the vibrational properties of fluoroethanes, experimentally, [1][2][3][4][5][6][7][8][9][10][11] by the ab initio approach [12][13][14][15][16][17][18] or by both methods. [19][20][21][22] These have led to improvements in the assignment of observed vibration frequencies and to better predictions of those which remain as yet unobserved.…”