ACCURATE COUPLED-CLUSTER REACTION ENTHALPIES AND ACTIVATION-ENERGIES FOR X-2-]XH+H (X=F, OH, NH2, AND CH3)(H)

Coupled cluster calculations at the CCSD(T)1[5s4p3d/4s3p] and CCSD(T)/ [5s4p3d2 f1g/4s3p2d] level of theory are reported for reactions X + H- 2 --> XH + H [X = F (1a), OH (1b), NH2 (1c), and CH3 (1d)] utilizing a nalytical energy gradients for geometry, frequency, charge distributio n, and dipole moment calculations of reactants, transition states, and products. A careful analysis of vibrational corrections leads to reac tion enthalpies at 300 K, which are within 0.04, 0.15, 0.62, and 0.89 kcal/mol of experimental values. For reaction (1a) a bent transition s tate and for reactions (1b) and (1c) transition states with a cis arra ngement of the reactants are calculated. The cis forms of transition s tates (1b) and (1c) are energetically favored because of electrostatic interactions, in particular dipole-dipole attraction as is revealed b y calculated charge distributions. For reactions (1a)-(1d), the CCSD ( T) 1[5s4p3d2 f1g/4s3p2d] activation energies at 300 K are 1.1, 5.4, 10 .8, and 12.7 kcal/mol which differ by just 0.1, 1.4, 2.3, and 1.8 kcal /mol, respectively, from the corresponding experimental values of 1 +/ -0.1, 4 +/- 0.5, 8.5 +/- 0.5, and 10.9 +/- 0.5 kcal/mol. For reactions (1), this is the best agreement between experiment and theory that ha s been obtained from ab initio calculations not including any empirica lly based corrections. Agreement is achieved after considering basis s et effects, basis set superposition errors, spin contamination, tunnel ing effect and, in particular, zero-point energies as well as temperat ure corrections. Net corrections for the four activation energies are -1.05, -0.2, 1.25, and 0.89 kcal/mol, which shows that for high accura cy calculations a direct comparison of classical barriers and activati on energies is misleading.