THE (3)A(2), (1)A(2), B-3(2), AND B-1(2) ELECTRONIC STATES OF CH2 - SMALL BOND ANGLE STATES

摘要

Molecular structures with very small bond angles are a curiosity in chemistry. The two triplet ((3)A(2) and B-3(2)) and two singlet ((1)A(2) and B-1(2)) excited states of CH2 have been investigated systematically using ab initio electronic structure theory. For these four states total energies and physical properties including geometries, dipole moments, harmonic vibrational frequencies, and associated infrared intensities were determined with the single and double excitation configuration interaction (CISD) method using four different basis sets. It is confirmed in this study that the four states of CH2 all have bent structures with longer CH bond lengths and smaller bond angles than the four lower-lying ((X) over tilde, (a) over tilde (b) over tilde, and (c) over tilde) states of CH2. At the CISD optimized geometries single point energies were determined with complete active space self-consistent-held (CASSCF) and CASSCF second-order configuration interaction (SOCI) levels of theory. For the triplet excited states single point energies were also determined employing coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations methods. At the CISD level with the largest basis set, the triple zeta plus triple polarizations with two sets of higher angular momentum and two sets of diffuse functions basis set [TZ3P(2f,2d) f2diff], the bond angles were predicted to be 40.6 degrees ((3)A(2)), 46.1 degrees ((1)A(2)), 76.3 degrees (B-3(2)), and 81.3 degrees (B-1(2)), while the dipole moments were determined to be 2.35 ((3)A(2)), 2.26 ((1)A(2)), 1.69 (B-3(2)), and 1.60 debye (B-1(2)), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2 f, 2d) + 2diff basis set, the energy separations (T-e value) between the ground state ((X) over tilde B-3(1)) and the four excited states were predicted to be 73.7 kcal/mol (3.20 eV, 25 800 cm(-1)) for the (3)A(2) state, 96.8 kcal/mol (4.20 eV, 33 800 cm(-1)) for the (1)A(2) state, 151.0 kcal/mol (6.55 eV, 52 800 cm(-1)) for the B-3(2) state, and 182.5 kcal/mol (7.91 eV; 63 800 cm(-1)) for the B-1(2) state, respectively. (C) 1997 American Institute of Physics. [References: 43]