The X˜1A1, a˜3B1, a˜1B˜1, and B˜1A1 electronic states of SiH2
Four electronically low-lying states of silylene (SiH2) have been studied systematically using high level ab initio electronic structure theory. Self-consistent field (SCF), two-configuration (TC) SCF, complete active space (CAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground (X˜1A1 or 1 1A1) and B˜1A1 (or 2 1A1) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the B˜1A1 (or 2 1A1) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P(2f,2d)+2diff], the energy separation (T0) between the ground (X˜1A1) and first excited (a˜3B1) states is determined to be 20.5 kcal/mol (0.890eV,7180cm−1), which is in excellent agreement with the experimental T0 value of 21.0 kcal/mol (0.910eV,7340cm−1). With the same method the T0 value for the a˜1B1−X˜1A1 separation is predicted to be 45.1 kcal/mol (1.957 eV,15780 cm−1), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV,15530 cm−1). The T0 value for the B˜1A1−X˜1A1 separation is determined to be 79.6 kcal/mol (3.452 eV,27 840 cm−1). After comparison of theoretical and experimental T0 values for the a˜3B1 and a˜1B1 states and previous studies, error bars for the B˜1A1 state are estimated to be ±1.5 kcal/mol (±525 cm−1). The predicted geometry of the B˜1A1 state is re(SiH)=1.458 and θe=162.3∘. The physical properties including harmonic vibrational frequencies of the B˜1A1 state are newly determined.
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