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

Abstract

The enthalpies of formation of the NH x ( 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 NH 3 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 NH 2 radical is ΔH f,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 ΔH f,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 NH 2 radical.