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Effects of alkylation upon the proton affinities of nitrogen and oxygen bases
Author(s) -
Redfern Paul,
Scheiner Steve
Publication year - 1985
Publication title -
journal of computational chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.907
H-Index - 188
eISSN - 1096-987X
pISSN - 0192-8651
DOI - 10.1002/jcc.540060303
Subject(s) - protonation , chemistry , substituent , proton affinity , affinities , molecule , proton , valence (chemistry) , methyl group , alkyl , oxygen atom , oxygen , atom (system on chip) , computational chemistry , methanol , medicinal chemistry , stereochemistry , ion , organic chemistry , physics , quantum mechanics , computer science , embedded system
The protonation energies of alkylated derivatives of NH 3 and OH 2 are calculated at the Hartree–Fock level with the split‐valence 4‐31G basis set. The methyl, dimethyl, and ethyl amines are studied; oxygen bases include methanol, dimethylether, and ethanol. The geometries of each molecule and its protonated analog are fully optimized. It is found that protonation leads to significant changes in the molecular structures. In particular, the bonds to the N and O atoms are substantially elongated, especially when the other atom involved is C rather than H. The calculated absolute proton affinities are somewhat larger than the experimental values. However, the differences in protonation energies of the various molecules relative to one another agree quantitatively with experiment. Replacement of one H atom of the base by a methyl group induces an increase in proton affinity of some 10 kcal/mol. If a second methyl group is added to the N or O atom, a further increment of about 70% this amount is noted. On the other hand, placement of the second C atom on the first methyl group (to form an ethyl substituent) leads to a smaller increase (∼30%). The magnitudes of these alkyl substituent effects are somewhat larger for the oxygen bases than for the amines.