Anharmonic vibrational analyses for the 1-silacyclopropenylidene molecule and its three isomers

Abstract

The global minimum among possible structures of SiC₂H₂ has been experimentally and theoretically determined to be 1-silacyclopropenylidene (1S). In 1994 Maier and Reisenauer reported the generation of 1-silacyclopropenylidene and its three isomers (2S–4S) by pulsed-flash pyrolysis followed by matrix-spectroscopic identification. Reliable quartic force fields for 1-silacyclopropenylidene and its three isomers are determined employing ab initio coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)] and the correlation-consistent core-valence quadruple zeta (cc-pCVQZ) basis set. Second-order vibrational perturbation theory (VPT2) has been utilized to determine equilibrium and zero-point vibration corrected rotational constants, centrifugal distortion constants, and harmonic and anharmonic vibrational frequencies. The distances between the average nuclear positions (r _α ) are also determined. The predicted rotational constants, centrifugal distortion constants, and anharmonic frequencies for the four lowest-lying isomers (1S-4S) of SiC₂H₂, as well as their ¹³C and deuterated isotopologues, agree well with available experiments. Excluding the CH and CD stretching modes, the mean absolute deviation between theoretical anharmonic and experimental fundamental frequencies for isomer 1-silacyclopropenylidene (1S) is 4.1 cm⁻¹ (5 isotopologues, 25 modes). The corresponding deviation for ethynylsilanediyl (2 S) is 4.9 cm⁻¹ (7 isotopologues, 38 modes) without the SiH and SiD stretching modes, while it is 8.6 cm⁻¹ (5 isotopologues, 22 modes) for silacyclopropyne (4S) without the SiC s-stretching, SiH₂ a-stretching and SiD₂ wagging modes. By comparing the theoretical harmonic and anharmonic with the experimental fundamental vibrational frequencies for the four isomers (1S-4S), it is demonstrated that the anharmonic effects greatly improve the harmonic results. This theoretically derived spectroscopic data should aid in the experimental detection of the transitions that have yet to be observed, particularly for the vinylidensilanediyl isomer.

^†Present address: Department of Chemistry, Tsinghua University, Beijing, P. R. China 100084

Keywords:

• 1-silacyclopropenylidene
• second-order vibrational perturbation theory
• rotational constants
• centrifugal distortion constants
• anharmonic vibrational frequencies

Acknowledgements

This research was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Chemistry, Fundamental Interactions Branch (Grant No. DE-FG02-00ER14748) and used resources of the National Energy Research Scientific Computing Center (NERSC), which is supported by the Office of Science of the US Department of Energy under Contract No. DE-AC02-05CH11231. Q.W. and Q.H. gratefully acknowledge the support provided by the China Scholarship Council (CSC) [2008] 3019, and the University of Georgia Center for Computational Quantum Chemistry for hospitality during their one-year visit. We thank Dr Justin M. Turney for many helpful discussions. Q.W. is indebted to China Postdoctoral Science Foundation. We are also indebted to the National Natural Science Foundation of China (Grant No. 21103097) and the 111 Project (B07012) by Ministry of Education in China.

Notes

^†Present address: Department of Chemistry, Tsinghua University, Beijing, P. R. China 100084