Semiempirical Treatment of Hindered Rotation in Simple Hydrides and Halosubstituted Ethanelike Molecules

Abstract

A semiempirical electrostatic model for internal rotation in certain molecules is developed from the integral Hellmann—Feynman theorem. The model is shown to be capable of rationalizing and predicting barriers to internal rotation in terms of well‐established physical and chemical concepts. For simple hydrides containing only atoms from the first two rows of the periodic table (CH₃NH₂, CH₃SH, etc.), the barriers to methyl rotation are shown to be correlated with the electronegativity of the heavy atom (N,S, etc.). Effects due to d and f character in bond hybrids appear to be important only for molecules containing atoms in Rows III and IV of the periodic table. The remarkably low internal‐rotation barriers for acetylene derivatives are attributed to the polarizability of the triple bond. The model is consistent with observed changes of the barrier with fluorosubstitution in ethane and methyl silane when inductive effects are included. The sets CH₃CH₂X and CF₃CH₂X are examined, and barriers are predicted for SiH₃CH₂X molecules giving 1.99, 2.44, 2.45, and 2.32 kcal/mole for X=F, Cl, Br, I, respectively. Barriers are also predicted for SiH₃CHF₂ (2.00), SiH₃CF₃ (2.42), SiH₃SiHF₂ (0.85), and SiH₃SiF₃ (0.81). It is argued that disilane should have a barrier of 1.08 kcal/mole—lower than the barriers for ethane or digermane. The relationship between the shape and the physical source of the barrier is discussed. The model predicts V₆≈−0.005V₃ for molecules of the type considered. For ethyl fluoride, V₆ is predicted to be −16 cal/mole compared to the experimental value of −15 cal/mole. The relations between this model and other theories of the barrier are briefly discussed.