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 AHf,298K = 43.8 + 0.6 kcal mol -l, which is smaller than the recommended JANAF value and the latest experimental values. For the NH species, the value obtained is AHf,298K = 86.3 + 0.8 kcal mol-~, in excellent agreement with other high-quality results. This last value confirms indirectly the accuracy of our proposed value for the NH 2 radical.
* Corresponding author.
species present a lack of coincidence among proposed experimental values. Thus, for the enthalpy of formation of the NH 2 radical, the proposed experimental values vary between AHf,0K = 44.7 + 2.4 kcal mol -t (AHf,298K = 44.0 _+ 2.3) based on a bracketing of the proton affinity of NH 2 [3] and AHf,0K = 48.8 __+ 3.5 kcal mol 1 (AHf,29s K = 48.1 + 3.5) based on the enthalpy of formation of benzylamine [4]. However, the latest compilations diminish this range. The Russian compilation [2] proposed a value of AHf, oK = 46.1 _+ 2.4 kcal mo1-1, the most recent JANAF tables [1] recommend AHf, oK = 46.2 __ 1.5 kcal mo1-1 (AHf,298K = 45.5 + 1.5), and Anderson [5] after critical examination of all pertinent