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 th eoretically estimated using the isogyric and hydrogenation reactions a s working chemical reactions. Energy differences were computed at seve n 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 re sults can be obtained with theoretical methods using large basis sets, elaborate correlated wavefunctions, and, above all, with the core-cor relation effects explicitly considered. The value obtained for the NH2 radical is Delta H-f,H-298 K = 43.8 +/- 0.6 kcal mol(-1), which is sm aller than the recommended JANAF value and the latest experimental val ues. For the NH species, the value obtained is Delta H-f,H-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 prop osed value for the NH2 radical.