Coupled-cluster characterization of the ground and excited states of the CH2N and CH2P radicals

High-level coupled-cluster theory with large basis sets was used to determine the optimized geometries and harmonic vibrational frequencies for the ground and low-lying excited electronic states of the CH2N and CH2P radicals. Additionally, isotropic hyperfine splitting constants were determined for the C-2v CH2X, trans-HCXH and cis-HCXH (where X=N and P) isomers as a gauge of the delocalization of the unpaired electron. The geometrical parameters of (X) over tilde B-2(2) CH2X, the trans-HCXH and cis-HCXH conformers, and the first three excited states are reported at the coupled-cluster level with single, double, and perturbatively applied triple excitations [CCSD(T)] using Dunning's correlation consistent polarized valence quadruple-zeta basis set (cc-pVQZ). The C-2v structures on the ground state surface are predicted to lie 9.3 and 13.5 kcal/mol lower than the trans- and cis-isomers, respectively, for CH2N and 28.1 and 30.0 kcal/mol, respectively, for CH2P. There are many other important properties of CH2N and CH2P which are not known from experiment. The geometrical parameters of the CH2N ground state [r(C-N)=1.2462 Angstrom, r(C-H)=1.0921 Angstrom, and theta (HCH)=119.4 degrees] and the CH2P ground state [r(C-P)=1.6583 Angstrom, r(C-H)=1.0842 Angstrom, and theta (HCH)=118.9 degrees] agree well with the C-N and C-P bond distances of the r(0) structures derived from microwave data, although notable differences were observed in the C-H bond distance and HCH bond angle. This research resolves an earlier discrepancy between theory and experiment for the ground state C-P distance in CH2P. The Fermi contact terms for H-1, C-13, N-14, and P-31 were determined at CCSD(T) level of theory with the cc-pVTZ and cc-pVQZ basis sets and are in reasonable agreement with the experimental values with a maximum deviation of 26 MHz for CH2N and 11 MHz for CH2P. The excited states of CH2N are predicted to lie 33 000 cm(-1) ((A) over tilde B-2(1)), 36 000 cm(-1) ((B) over tilde (2)A'), and 38 000 cm(-1) ((C) over tilde (2)A(1)) above the ground state, and the excited states of CH2P to lie approximately 21 000 cm(-1) ((A) over tilde (2)A'), 26 000 cm(-1) ((B) over tilde B-2(1)), and 33 000 cm(-1) ((C) over tilde (2)A(1)) above the ground state. (C) 2001 American Institute of Physics.