The Lowest-Energy Isomer of C2Si2H4 Is a Bridged Ring: Reinterpretation of the Spectroscopic Data Based on DFT and Coupled-Cluster Calculations

Jesse J Lutz,Larry W Burggraf

doi:10.3390/inorganics7040051

Abstract

The lowest-energy isomer of C 2 Si 2 H 4 is determined by high-accuracy ab initio calculations to be the bridged four-membered ring 1,2-didehydro-1,3-disilabicyclo[1.1.0]butane (1), contrary to prior theoretical and experimental studies favoring the three-member ring silylsilacyclopropenylidene (2). These and eight other low-lying minima on the potential energy surface are characterized and ordered by energy using the CCSD(T) method with complete basis set extrapolation, and the resulting benchmark-quality set of relative isomer energies is used to evaluate the performance of several comparatively inexpensive approaches based on many-body perturbation theory and density functional theory (DFT). Double-hybrid DFT methods are found to provide an exceptional balance of accuracy and efficiency for energy-ordering isomers. Free energy profiles are developed to reason the relatively large abundance of isomer 2 observed in previous measurements. Infrared spectra and photolysis reaction mechanisms are modeled for isomers 1 and 2, providing additional insight about previously reported spectra and photoisomerization channels.

Highlights

The C2 Si2 H4 molecule has been the subject of very few literature studies, despite being a bicyclobutane/butadiene analog of fundamental interest
Dozens of unique cyclic species can be imagined, with some resembling cyclobutane or methylcyclopropane, but, unlike their hydrocarbon analogs, C2 Si2 H4 isomers differing by a transannular hydrogen migration are often separated by only a few kcal/mol
Due to the diversity of possible bonding configurations, ranking such structures by energy demands a high-level treatment of the correlation energy and a large basis set

Summary

Introduction

The C2 Si2 H4 molecule has been the subject of very few literature studies, despite being a bicyclobutane/butadiene analog of fundamental interest. Holme et al [1] predicted the existence of over a dozen cyclic C2 Si2 H4 stationary points at the Hartree-Fock (HF) level, assigning the lowest-energy isomer as the three-membered ring silylsilacyclopropenylidene. Structural compression is energetically favorable when electron-correlation stabilization dominates over strain destabilization. If this is the case for C2 Si2 H4 , one might expect that the maximally compressed isomer, a bridged bicyclobutane analog, should be the global minimum. One such isomer was considered by Holme et al, but it was predicted to be over 10 kcal/mol higher in energy than silylsilacyclopropenylidene

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