| Abstract
| - Strain effects have been quantitatively evaluated for a set of 22compounds including ketones (R2CO), carboxamides (RCONH2), andN,N-dimethylcarboxamides (RCONMe2),where R = Me, Et,i-Pr, t-Bu, 1-adamantyl (1-Ad), in their neutraland protonated forms. To this end, use was madeof the gas-phase proton affinities and standard enthalpies of formationof these compounds in thegas phase, as determined by Fourier transform ion cyclotron resonancemass spectrometry (FTICR) and thermochemical techniques, respectively. The structuresof 1-AdCOMe and (1-Ad)2COwere determined by X-ray crystallography. Quantum-mechanicalcalculations, at levels rangingfrom AM1 to MP2/6-311+G(d,p)//6-31G(d), were performed onthe various neutral and protonatedspecies. Constrained space orbital variation (CSOV) calculationswere carried out on selectedprotonated species to further assess the contributions of the variousstabilizing factors. Takingneutral and protonated methyl ketones as references, we constructedisodesmic reactions thatprovided, seemingly for the first time, quantitative measures of strainin the protonated species.A combination of these data with the results of theoreticalcalculations (which also included several“computational experiments”) lead to a unified, conceptuallysatisfactory, quantitative descriptionof these effects and their physical link to structural properties ofthe neutral and protonated species.
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