Proton-bound dimers of aliphatic carboxamides: gas-phase basicity and dissociation energy

The gas phase basicity of butyramide (2a), caproamide (3a), isobutyramide (5a), pivalamide (6a), cyclopentanecarboxamide (7a), cyclohexanecarboxamide (8a), their N-methyl derivatives 2b-8b and their N,N-dimethyl derivatives 2c-8c have been determined by the 'deprotonation' method using FT-ICR spectrometry (2a-8a only) and by the 'kinetic' method observing the metastable decomposition of proton-bound heterodimers by tandem mass spectrometry. Both methods give identical results for the primary carboxamides studied within the limit of error of +/- 6 kJ/mol. The GB(2a-8a) and PA(2a-8a) correlate linearly with the GB and PA of other aliphatic carbonyl compounds (aldehydes, ketones, methyl carboxylates) as expected for a protonation of the carboxamides at the carbonyl-O atom. The GB of the N-methyl- and N,N-dimethylcarboxamides exceed the GB of the primary carboxamides by 22 +/- 3 kJ/mol and 46 +/- 4 kJ/mol, respectively. The bond dissociation energy BDE of the proton-bound homodimer of formamide, acetamide, propionamide and pivalamide and of their N,N-dimethyl derivatives have been derived from the appearance energy AE of the protonated carboxamide from the proton-bound homodimers using collision energy resolved mass spectrometry GERMS (collision energy resolved mass spectrometry) by FT-ICR spectrometry. The BDE of all proton-bound dimers of the primary carboxamides measured are virtually constant at 1.19 +/- 0.08 eV in spite of a considerable variation of the GB by 0.68 eV. The BDE of the proton-bound homodimers of the N,N-dimethylcarboxamides are consistently smaller than those of the primary carboxamides in spite of the increased GB of tertiary carboxamides and decreases from the proton-bound homodimer of N,N-dimethylformamide to N,N-dimethylpivalamide. As a consequence of this effect formamide of the lowest GB exhibits the highest BDE in its proton-bound homodimer while N,N-dimethylpivalamide with the highest GB forms the weakest ionic hydrogen bond. (C) 1997 Elsevier Science B.V.