Strain Effects in Protonated Carbonyl Compounds. An Experimental and ab Initio Treatment of Acyclic Carboxamides and Ketones

• Published in: Journal of organic chemistry Vol. 62; no. 24; pp. 8503 - 8512
• Main Authors: Homan, H, Herreros, M, Notario, R, Abboud, J.-L. M, Esseffar, M, Mó, O, Yáñez, M, Foces-Foces, C, Ramos-Gallardo, A, Martínez-Ripoll, M, Vegas, A, Molina, M. T, Casanovas, J, Jiménez, P, Roux, M. V, Turrión, C
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.11.1997
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo971330d

Strain effects have been quantitatively evaluated for a set of 22 compounds including ketones (R2CO), carboxamides (RCONH2), and N,N-dimethylcarboxamides (RCONMe2), where R = Me, Et, i-Pr, t-Bu, 1-adamantyl (1-Ad), in their neutral and protonated forms. To this end, use was made of the gas-phase proton affinities and standard enthalpies of formation of these compounds in the gas phase, as determined by Fourier transform ion cyclotron resonance mass spectrometry (FT ICR) and thermochemical techniques, respectively. The structures of 1-AdCOMe and (1-Ad)2CO were determined by X-ray crystallography. Quantum-mechanical calculations, at levels ranging from AM1 to MP2/6-311+G(d,p)//6-31G(d), were performed on the various neutral and protonated species. Constrained space orbital variation (CSOV) calculations were carried out on selected protonated species to further assess the contributions of the various stabilizing factors. Taking neutral and protonated methyl ketones as references, we constructed isodesmic reactions that provided, seemingly for the first time, quantitative measures of strain in the protonated species. A combination of these data with the results of theoretical calculations (which also included several “computational experiments”) lead to a unified, conceptually satisfactory, quantitative description of these effects and their physical link to structural properties of the neutral and protonated species.