An electrostatic model for the prediction of heats of formation and dipole moments of fluorine- and chlorine- substituted alkanes

Abstract

An electrostatic point charge model is proposed which is successful in predicting the heats of formation and dipole moments of fluorine- and chlorine- substituted straight chain alkanes. Atoms are assigned intrinsic point charges at infinite distance. As these point charges are allowed to approach to their respective bond distances, the electric fields generated at the midpoint of and parallel to each bond cause charge transfer across the chemical bonds, resulting in the final charges. The heat of formation is then formulated as the sum of three contributions: (1) a contribution to account for chemical bond formation; (2) the electrostatic work of assembling these charges to molecular dimension; and (3) the polarization work required to change the charges from their intrinsic to their final values. Parameters selected for the model include initial charges y(,H), y(,F), and y(,Cl) of 0.31, -1.18, and -0.87 x 10('-10) esu, respectively; polarizability parameters (alpha)(,CH), (alpha)(,CF), (alpha)(,CCl), and (alpha)(,CC) of 0.0053, 0.030, 0.190, and 0.041 x 10('-24) A('3), respectively; and bond contributions B(C-H), B(C-F), B(C-Cl), and B(C-C) of -1.71, -48.10, -12.57, and -1.17 kcal mol('-1), respectively. The model is applied to several areas with some rather interesting results. The increment to the (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) in the series CX(,3)(CH(,2))(,n)CX(,3) with X = F or Cl is found to be dependent on the value of n up to about n = 3, after which the increment is essentially constant. This is in contrast to the series of straight chain alkanes which shows an essentially constant increment after ethane. Electrostatic contributions to rotational barriers and conformational energies in substituted ethanes are calculated and found to be small compared to the total values. The difference in barriers between CH(,3)CX(,2)Y and CF(,3)CX(,2)Y with X,Y = H,F is suggested to be due in part to small electrostatic effects. The stability of g-CH(,2)FCH(,2)F relative to its anti conformer is not explained by electrostatic effects, since the conformational electrostatic energy is the same as that in the CHF(,2)CHF(,2) and CHF(,2)CH(,2)F cases which are both more stable in the anti conformation. A linear correlation is found between the calculated carbon charges and both the ('1)H NMR shift of several fluorine- and chlorine- substituted ethanes and methanes, and the ESCA shift of fluoro- and chloro- methanes.