Vibrational Spectra, DFT Calculations, Unusual Structure, Anomalous CH₂ Wagging and Twisting Modes, and Phase-Dependent Conformation of 1,3-Disilacyclobutane

Abstract

Our previously published infrared and Raman spectra of 1,3-disilacyclobutane (13DSCB) and its 1,1,3,3-d4 isotopomer have been reexamined and partially reassigned on the basis of DFT and ab initio calculations. The calculations confirm previous microwave work that the CSiC angles in the ring are unexpectedly larger than the SiCSi angles. This may arise from the partial charges on the ring atoms. The calculations are in excellent agreement with the observed spectra in both frequency and intensity. They also demonstrate that this molecule has CH2 wagging and twisting vibrations with frequencies below 1000 cm-1, about 200 cm-1 lower than expected. These unprecedented low values can be explained by the decreased slope in the potential energy curves for these vibrations as the sideways motions of the CH2 groups result in attractive forces between the positively charged hydrogens on the carbon atoms and the negatively charged hydrogens on the silicon atoms. The theoretical calculations also confirm the previous conclusions that the individual molecules (vapor) have C2v symmetry whereas in the solid the molecules become planar with D2h symmetry. The vibrational coupling between the ring-angle bending mode and the SiH2 in-phase rocking, which is present for the C2v structure, is forbidden for D2h and hence disappears.

Raman and infrared spectra and theoretical calculations of dipicolinic acid, dinicotinic acid, and their dianions

“…the presence of the hydrated water in these samples. For molecular systems which we have studied previously [17][18][19][20][21][22][23][24][25][26][27] we typically have found remarkably good agreement between the observed and calculated spectra. Here, however, there are significant differences which can be attributed to the intermolecular hydrogen bonding.…”

Theoretical Study on the Ring-Opening of 1,3-Disilacyclobutane and H2 Elimination

“…24,35 In this work, the geometry optimization and vibration frequencies of the reactants, products, and intermediates for each reaction were calculated using MP2 with the 6-311++G(d,p) basis set. The frozen core approximation was employed in all geometry optimizations.…”

On the geometric structure, puckering potential and electronic transitions of monosilacyclobutanes and disilacyclobutanes – A theoretical study

“…Previous studies [19,25,32,33] have shown that calculations using the second-order Möller-Plesset (MP2) perturbation theory does an excellent job in predicting the structures of silacylcobutane molecules. Therefore, the geometric structures of the six molecules mentioned above were optimized at the MP2 level of theory with the 6-311++G(d,p) basis set.…”

“…Various experimental methods including microwave spectroscopy [17][18][19], infrared/ Raman spectroscopy [20][21][22][23][24][25], and gas electron diffraction [26][27][28][29][30] have been employed to elucidate the geometric structures of silacyclobutane and its substituted derivatives. At the same time, theoretical calculations of a variety of such molecules as silacyclobutane (SCB) [30][31][32], 1-methyl-1-silacyclobutane (MSCB) [25], 1,1-dimethyl-1-silacycolbutane (DMSCB) [27], 1,3-disilacyclobutane (DSCB) [33], and 1,1,3,3-tetramethyl-1,3-disilacyclobutane (TMDSCB) [28] have been reported in the literatures. In a recent study of the structures of silacyclobutane and its 1,1-dicholoro and 1,1-difluoro derivatives using ab initio calculations [32], AlSaadi and Laane have shown that the dihedral angles of puckering decreased in the two 1,1-disubstituted derivatives of SCB as a result of increasing electronegativity of the substituent.…”