Theoretical values of the enthalpies of formation of the NHx (x = 1,2,3) compounds. Importance of the core-correlation effects

The enthalpies of formation of the NHx (x = 1,2,3) compounds were theoretically estimated using the isogyric and hydrogenation reactions as working chemical reactions. Energy differences were computed at seven levels of calculation, using MP4 (with spin projection and post-PMP4 corrections), QCI, CC, and multireferential methods with two extended basis sets. Using NH3 as the test molecule, we found that accurate results can be obtained with theoretical methods using large basis sets, elaborate correlated wavefunctions, and, above all, with the core-correlation effects explicitly considered. The value obtained for the NH2 radical is ΔHf,298 K = 43.8 ± 0.6 kcal mol−1, which is smaller than the recommended JANAF value and the latest experimental values. For the NH species, the value obtained is ΔHf,298 K=86.3±0.8 kcal mol−1, in excellent agreement with other high-quality results. This last value confirms indirectly the accuracy of our proposed value for the NH2 radical.