Coupled-cluster calculations of C2H2Si and CNHSi structural isomers

Abstract

Results of large-scale coupled-cluster calculations of selected C(2)H(2)Si and CNHSi structural isomers are reported. Equilibrium molecular structures of a total of 12 molecules in their singlet electronic states have been calculated systematically employing the coupled-cluster singles and doubles model augmented by a perturbative correction for triple excitations (CCSD(T)) in combination with Dunning's hierarchy of correlation consistent basis sets. In addition, anharmonic force fields were calculated to yield fundamental vibrational frequencies and rotation-vibration interaction constants alpha(i) (A,B,C). The latter were used to determine empirical equilibrium structures r(e) (emp) of two molecules - silacyclopropenylidene, c-C(2)H(2)Si, and silapropadienylidene, H(2)CCSi - for which sufficient isotopic data are available from literature. Very good agreement with theoretical equilibrium structures from CCSD(T) calculations employing core-valence basis sets of quadruple and quintuple-zeta quality - i.e., cc-pwCVQZ (337 basis functions), cc-pCV5Z, and cc-pwCV5Z (581 basis functions) is found - to within 0.001 A for bond lengths and 0.1 degrees for bond angles. Theoretical ground state rotational constants of HSiCN and HSiNC compare very favorably with experimental microwave data from literature, to within 0.15% (HSiCN) and 0.1% (HSiNC) for the B(0) and C(0) rotational constants. In the case of c-C(2)H(2)Si and H(2)CCSi this agreement is even better than 0.1%. For the latter two molecules effects of higher-level electron-correlation and relativity to the equilibrium geometry as well as the electronic contributions to the rotational constants are investigated. For eight molecules not yet studied at high spectral resolution in the gas-phase theoretical molecular parameters are provided to support future laboratory investigations. Theoretical vibrational fundamentals compare well with data of eight species studied previously with infrared matrix isolation spectroscopy.

High-resolution rotational spectroscopy of iminosilylene, HNSi

“…Statistical uncertainties in the new empirical parameters are in the order of 10 −4Å for r(H-N) and much smaller for r(N-Si). As has been observed in earlier studies, for molecules containing second-row elements, the balance obtained in CCSD(T) calculations using the cc-pwCVQZ basis sets yields equilibrium structures of very high quality [45,46], and this is also found here. Further expansion of the basis set in a CCSD(T)/cc-pwCV5Z calculation has only a very small effect.…”

Carbon–sulfur chains: A high‐resolution infrared and quantum‐chemical study of C3S and SC7S

“…[35] Theoretical best-estimate structures were generally calculated at the CCSD(T)/cc-pwCVQZ level of theory (considering all electrons in the correlation treatment), an approach shown previously to provide very accurate equilibrium structural parameters. [36][37][38] Vibrational effects were treated using second-order vibrational perturbation theory (VPT2) based on the formulas given in Ref. [39].…”

Rotational spectra and equilibrium structures of H2SiS and Si2S

“…14). In a recent computational study of selected small silicon-carbon molecules, 43 neglect of quadruple excitations was identified as a cause of the pronounced shortening of multiple bonds involving second-row elements in the CCSD(T)/cc-pwCV5Z calculations, and hence an intrinsic shortcoming of the CCSD(T) method.…”