Abstract
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