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Thermodynamic stabilities of 92 carbenes, singlets and triplets, have been evaluated on the basis of hydrogenation enthalpies calculated at the G3MP2 level. The carbenes include alkyl-, aryl-, and heteroatom-substituted structures as well as cyclic 1,3-diheteroatom carbenes. Over a wide energy range, a good correlation is seen between the singlet-triplet gaps and the hydrogenation enthalpies of the singlets, but there are some clear outliers, which represent cases where the triplet has unusual stability or instability. By use of hydrogenation enthalpies, separate carbene stabilization enthalpy scales (CSEs) have been developed for singlets and triplets, and these highlight structural features that affect the stability of each. The treatment also allows estimates of aromaticity in cyclic carbenes. In this way, imidazol-2-ylidene is estimated to have an aromatic stabilization energy of about 20 kcal/mol.

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Calculations are reported showing that primary hydrogen isotope effects depend smoothly on transition-state geometry and yield consistently low values, kH/kD = 1-2. for sufficiently non-linear configurations. Specific calculations for 1.2-hydride shifts, borane hydrolyses, and E2C eliminations are compared with available measurements for these and for other hydrogen-transfer reactions proceeding through 3-. 4-, 5-, and 6-membered cyclic transition states. A unique temperature-independent kH/kD is noted. The low isotope effects are ascribed to the presence of a large isotopically sensitive vibration v,S (e.g., 3500 crn.-l) in the transition state. The results are fairly insensitive to reasonable force-constant changes. The usefulness of k,/k, in defining the geometry of transition states for hydride transfer reactions is discussed.