Enthalpies of formation of monoderivatives of hydrocarbons: Interaction of polar groups with an alkyl group

Energies of hydrocarbon monoderivatives CH 3 X, C 2 H 5 X, n‐C 4 H 9 X, and n‐C 5 H 11 X with 16 different substituents X were calculated at the levels B3LYP/6‐311+G(d,p) and B3LYP/AUG‐cc‐pVTZ//B3LYP/6‐311+G(d,p). The results were used to test the validity of the additive rule that has served commonly for estimating the enthalpies of formation Δ f H ( T ). The exact additivity corresponds to zero reaction energy Δ E of the isodesmic reaction, in which the substituent X is transferred from one alkyl group R to another. Additivity is approximately fulfilled for butyl and pentyl derivatives with the differences less than 0.3 kJ mol −1 (except charged groups X). Methyl derivatives deviated from the additive rule up to 22 kJ mol −1 for dipolar groups X and 45 kJ mol −1 for charged group, in agreement with the available experiments and with the anticipation of all suggested empirical schemes. In addition, smaller deviations of ethyl derivatives (3 or 20 kJ mol −1 , respectively) were observed here for the first time. There is no correlation between the deviations of methyl and ethyl derivatives; they are also not related to steric effects, and only partly to polarization. Deviations of methyl derivatives are proportional to the electronegativity of the first atom of the substituent; even when the definition of electronegativity is somewhat questionable, one can say in any case that it is controlled by the first atom. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1979–1986, 2004