A CURIOUS ARTIFACT IN THE POTENTIAL-ENERGY SURFACE FOR THE F+HF ABSTRACTION REACTION

The reaction F + HF --> HF + F has been studied using ab initio molecular orbital theory at UHF, MP2, MP3, MP4, CCD, CCSD, CID, CISD, QCISD, QCISD(T), and CASSCF levels of theory with the 3-21G, 6-31G**, 6-311++G**, and D95++(3df,2p) basis sets. The UHF, MP4, CIS, CISD, CCSD, QCISD, QCISD(T), and CASSCF levels of theory predict the transition state to be a highly bent C2v symmetry structure, whereas MP2, MP3, and CCD levels predict the C2v symmetry structure to be a shallow minimum bound by up to 0.4 kcal/mol. The MP2/6-31G** C2v structure is bracketed by two C(s) symmetry transition states with bond lengths of 1.1411 and 1.0541 angstrom and a bond angle of 127.0-degrees. The C2v minimum on the MP2, MP3, and CCD surfaces is caused by inadequately treated avoided crossings between the bond making/breaking state and two broken symmetry F+HF hydrogen bonding states. Dynamic correlation methods that include single excitations in an interative or self consistent manner, such as CASSCF, CISD, CCSD, and QCISD, appear to treat the avoided crossings properly and predict a suitable transition state. At the QCISD(T)/D95++(3df,2p) level, the C2v transition state geometry is R(H-F) = 1.102 angstrom and angle F-H-F = 134.6-degrees and the reaction barrier is 17.5 kcal/mol.