Multireference coupled cluster and multireference configuration interaction studies of the potential surfaces for deprotonation of NH4 +

Uzi Kaldor*, Szczepan Roszak, P. C. Hariharan, Joyce J. Kaufman

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

Protonation/deprotonation reactions are represented by H+ + B⇄HB+. The ionization potential of H (13.6 eV) is higher than that of B for organic and most inorganic molecules (it is 10.166 eV for NH 3), and the separated pair H + B+ will be lower in energy than the closed-shell pair H+ + B. The reaction path involves, therefore, an avoided crossing, and its theoretical study requires multideterminant methods. The reaction with B = NH3 (or R 1R2R3N) is of interest in several fields, and its study is described here. The multireference coupled-cluster method (MR-CCM) and multireference double-excitation configuration interaction (MRD-CI) were used. At each (H3N⋯H)+ separation, from 1 to 11 bohr, the ground state MRD-CI energy was optimized with respect to the angle θ between the NH bond in the NH3 group and the C3 axis; MR-CCM and MRD-CI calculations were performed for the two lowest 1A1 states and the lowest 3A1. Two different reference determinants had to be used for the MR-CCM calculations at different regions, but this created no difficulties and the transition was smooth. Close agreement (a few mhartree) was obtained between MRD-CI and MR-CCM results. The avoided crossing, near R(H3N⋯H)+ = 4 bohr, is manifested by the rapid change in the CI and coupled-cluster method (CCM) mixing coefficients and by the transition of the NH3 group from pyramidal at small R to planar at large R. The lowest 1A1 state dissociates adiabatically to NH3+( 2A1) + H(2S), whereas the single determinant self-consistent field (SCF) function dissociates to NH3( 1A1) + H+.

Original languageEnglish
Pages (from-to)6395-6400
Number of pages6
JournalThe Journal of Chemical Physics
Volume90
Issue number11
DOIs
StatePublished - 1989

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