The X̃ 2Π and à 2Σ+ electronic states of the HCSi radical: Characterization of the Renner–Teller effect in the ground state

Abstract

The electronic structures of the ground and lowest lying excited state of the silicon methylidyne radical (HCSi) have been investigated at the self-consistent field, configuration interaction with single and double excitations, coupled cluster with single and double excitations (CCSD), and CCSD including a perturbative expansion for connected triples CCSD(T) levels of theory with a wide range of basis sets. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and Renner–Teller splitting are reported. At our highest level of theory [CCSD(T)/cc-pVQZ], the ground electronic state (X̃ 2Π) has a linear geometry with re(CH)=1.0781 Å and re(CSi)=1.6956 Å. This is in good agreement with the experimental values of r0(CH)=1.0677 Å and r0(CSi)=1.6925 Å, respectively. In the à 2Σ+ state, HCSi is also found to have a linear geometry with re(CH)=1.0737 Å and re(CSi)=1.6130 Å at the [CCSD(T)/cc-pVQZ] level, confirming experimental values of r0(CH)=1.0625 Å, r0(CSi)=1.6118 Å, and the observation of C–Si triple bond character. With the same method, the X̃ 2Π and à 2Σ+ state C–H stretching vibrational frequencies are predicted to be 3271 and 3319 cm−1, respectively, for which experimental values are not available. The classical X̃–à splitting (Te value) was determined to be 32.6 kcal/mol (1.41 eV, 11 730 cm−1) and quantum mechanical splitting (T0 value) to be 33.5 kcal/mol (1.45 eV, 11726 cm−1), which are in excellent agreement with the experimental T0 value of 11 766.721 cm−1 (33.64 kcal/mol, 1.459 eV). The linear excited à 2Σ+ state of the molecule has a real degenerate bending vibrational frequency, whereas the ground state (X̃ 2Π) is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies. The Renner parameter (ε) and average harmonic bending frequency (ω2) of the X̃ 2Π state are predicted to be ε=−0.114 and ω2=518 cm−1 at the CCSD(T)/cc-pVQZ level of theory. The electronic structure analysis of the ground state showed that the HCSi radical is an A-type Renner–Teller molecule.