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Thermochemistry of Simple Enols and Enol Cation-Radicals Revisited. A G2(MP2) ab Initio Study

Turecek, F.; Cramer, C. J.
J. Am. Chem. Soc. in press.

Simple enols, vinyl alcohol (1), propen-2-ol (2), (E)-propen-1-ol (3), (Z)-propen-1-ol (4), (E)-2-buten-2-ol (5), and (Z)-2-buten-2-ol (6), and their cation-radicals are investigated by ab initio calculations at the Gaussian 2 (MP2) level of theory. syn-Planar conformations are found to be thermodynamically more stable for gaseous 1, 2, 3, 5, and 6, which give Delta Gs(o,298)(anti-syn) 2.5, 4.0, 1.4, 5.6, and 3.3 kJ mol-1 respectively. The syn form of 4 is thermochemically favored by Delta Hs(o,298)(anti-syn) = 1.2 kJ mol-1, but disfavored by entropy, which leads to Delta Gs(o,298)(anti-syn) = -0.5 kJ mol-1. The enols are predicted to exist as mixtures of rotamers in the gas phase. Standard enthalpies of formation for 1, 2, 3, 4, 5, and 6 are calculated from isodesmic reactions as -123, -167, -147, -148, -190, -195 kJ mol-1, respectively. Enol cation radicals uniformly prefer anti conformations with Delta Gs(o,298)(anti-syn) = 3.5-8.1 kJ mol-1. Standard enthalpies of formation from isodesmic and isogyric reactions were calculated as 765, 677, 682, 682, 603, and 601 kJ mol-1 for 1+*, 2+*, 3+*, 4+*, 5+*, and 6+*, respectively. The calculated Delta Hs(o,f,298) show good agreement with experimental estimates for 1 and 1+*, but diverge for the higher enols. The divergence is significantly diminished when 298 K enthalpy corrections are included in the experimental threshold energies. The calculated adiabatic ionization energies are in excellent to fair agreement with experimental data for 1, 2, 3, and 4, but show 0.2-0.3 eV deviations for 5 and 6.

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