NF5Viable or Not?

Abstract

The structure, bonding, harmonic vibrational frequencies, and decomposition reactions of NF5 were studied employing complete active space SCF (CASSCF), multireference configuration interaction (MRCI), hybrid Hartree−Fock/density functional theory (B3LYP), second-order perturbation theory (MP2), and the coupled-cluster method with single, double (CCSD), and perturbative triple excitations [CCSD(T)] in conjunction with basis sets of up to triple-ζ quality. The overall NF5 → NF3 + F2 reaction is exothermic by 42 kcal mol-1 at the CCSD(T)/cc-pVTZ//CCSD/DZP level. Nevertheless, the trigonal-bipyramidal form of NF5 (D3h) is found to be a minimum at all levels of theory employed for harmonic vibrational frequency analysis [up to CCSD(T)/DZP]. The C4v NF5 stationary point is a 4 kcal mol-1 [CCSD(T)/cc-pVTZ// CCSD/DZP] higher lying transition structure for Berry rotation. No C3v NF4+F- ion pair minimum could be found at correlated levels of theory. A natural bond 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 character and the relatively short FF separations, within the sum of the van der Waals radii, are responsible for the metastable nature of NF5. The lowest energy transition state found, 16 to 23 kcal mol-1 above 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 exothermic [CCSD(T)/cc-pVTZ//CCSD/DZP + ZPVE] decomposition into C3v symmetric NF4 and F radicals.