NF5 - VIABLE OR NOT

The structure, bonding, harmonic vibrational frequencies, and decompos ition reactions of NF5 were studied employing complete active space SC F (CASSCF), multireference configuration interaction (MRCI), hybrid Ha rtree-Fock/density functional theory (B3LYP), second-order perturbatio n theory (MP2), and the coupled-cluster method with single, double (CC SD), and perturbative triple excitations [CCSD(T)] in conjunction with basis sets of up to triple-zeta quality. The overall NF5 --> NF3 + F- 2 reaction is exothermic by 42 kcal mol(-1) at the CCSD(T)/cc-pVTZ//CC SD/DZP level. Nevertheless, the trigonal-bipyramidal form of NF5 (D-3h ) is found to be a minimum at all levels of theory employed for harmon ic vibrational frequency analysis [up to CCSD(T)/DZP]. The C-4v NF5 st ationary point is a 4 kcal mol(-1) [CCSD(T)/cc-PVTZ// CCSD/DZP] higher lying transition structure for Berry rotation. No C-3v NF4+F- ion pai r minimum could be found at correlated levels of theory. A natural bon d orbital comparison of NF5 and PF5 revealed the much greater polarity of the PF than the NF bonds. NF5 has one well-developed three center- two electron and three two center-two electron bonds. The weak ionic c haracter and the relatively short FF separations, within the sum of th e van der Waals radii, are responsible for the metastable nature of NF 5. The lowest energy transition state found, 16 to 23 kcal mol(-1) abo ve NF5 at MRCI/cc-pVTZ//CASSCF(4,3)/DZP, MRCI/cc-pVTZ//UB3LYP/cc-pVTZ, or CCSDT-1/TZ2P//UHF-CCSD/DZP, corresponds to the 8.5 kcal mol(-1) ex othermic [CCSD(T)/cc-pVTZ//CCSD/DZP + ZPVE] decomposition into C-3v sy mmetric NF4 and F radicals.