Rotational Spectrum and Ring Structures of Silacyclohex-2-ene and 1,1-Difluorosilacyclohex-2-ene.

Silacyclohex-2-ene and 1,1-difluorosilacyclohex-2-ene have been synthesized, and the chirped-pulse, Fourier-transform microwave spectra of each have been observed and analyzed from 4.9 to 23.1 GHz. Quantum chemical calculations have been performed at the B3LYP-D3BJ/Def2TZVP level of theory and predict µa to be the largest dipole moment component with a significantly larger value in this component for 1,1-difluorosilacyclohex-2-ene. In accordance with this prediction, the spectra were predominantly a-type with the observation of a few b- and c-type transitions. The signal-to-noise ratio was adequate in both spectra to observe 29Si, 30Si, and all singly substituted 13C isotopologues in natural abundance. All spectra have been fit to a semirigid rotational Hamiltonian and are presented. Analysis of the physical meaning of the fitted parameters is explored and determined to hold for the rotational constants while being more empirical for the centrifugal distortion terms. Experimental structures of both molecules indicate that the quantum chemically calculated structures for the atoms in the ring are a very close depiction of the experimentally determined structures. The structures of each molecule are compared to similar molecules for context, where it is shown that both molecules possess a similar "half-chair" conformation to that of the all-carbon analogue, cyclohexene.

Rotational spectrum, structure, and quadrupole coupling of cyclopropylchloromethyldifluorosilane.

Cyclopropylchloromethyldifluorosilane, c-C3H5SiF2CH2Cl, has been synthesized, and its rotational spectrum has been recorded by chirped-pulse Fourier transform microwave spectroscopy. The spectral analysis of several isotopologues indicates the presence of two distinct conformations in the free-jet expansion, which are interconvertible through a rotation of the chloromethyl group. A partial substitution structure is presented for the lower energy conformation and is compared to the equilibrium structure obtained from quantum chemical calculations. Additionally, the presence of the chlorine nucleus leads to the rotational transitions splitting into multiple hyperfine components and χaa, a measure of the electric field gradient along the a axis, is unusually small at merely +0.1393(73) MHz. Various common ab initio and density functional theory methods fail to predict good quadrupole coupling constants (in the principal axis system) that adequately reproduce the observed hyperfine splitting, although diagonalizing the quadrupole coupling tensor from the principal axis system into a nucleus-centered axis system reveals that, overall, these methods calculate reasonably the electric field gradient about the chlorine nucleus. Finally, a total of nine electric dipole forbidden, quadrupole allowed transitions are observed in the rotational spectra of the parent species of the higher energy conformation and the 37Cl isotopologue of the lower energy conformation. These include those of x-type (no change in parity of Ka or Kc), which, to our knowledge, is the first time such transitions have been observed in a chlorine-containing molecule.

The molecular structure and curious motions in 1,1-difluorosilacyclopent-3-ene and silacyclopent-3-ene as determined by microwave spectroscopy and quantum chemical calculations.

The molecules 1,1-difluorosilacyclopent-3-ene (3SiCPF2) and silacyclopent-3-ene (3SiCP) have been synthesized and studied using chirped pulse, Fourier transform microwave (CP-FTMW) spectroscopy. For 3SiCP this is the first ever microwave study of the molecule and, for 3SiCPF2, the spectra reported in this work have been combined with that of previous work in a global fit. The spectra of each contain splitting which has been fit using a Hamiltonian consisting of semirigid and Coriolis coupling parameters. A refit of the original 3SiCPF2 work was also carried out. All fits and approaches are reported. Analyses of the spectra provide evidence that the molecule is planar which is in agreement with the high-level calculations, but the source of the splitting in the spectra has not been determined.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes.

High-level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) were obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

The Conformational Landscape, Internal Rotation, and Structure of 1,3,5-Trisilapentane using Broadband Rotational Spectroscopy and Quantum Chemical Calculations.

The rotational spectrum of 1,3,5-trisilapentane was observed on a chirped-pulse Fourier transform microwave spectrometer and is reported. During assignment, multiple conformations of the molecule were identified in the molecular beam. Prior quantum-chemical calculations performed on the molecule show that the identified spectra correspond to the lowest three calculated energetic structures. These structures are of C2 (Conf.1), C2v (Conf.2), and C1 (Conf.3) symmetry, with relative energy ordering of Conf.1 < Conf.3 < Conf.2, which is in stark contrast to n-pentane and all known silicon-substituted n-pentane derivatives. This is found to most likely arise from the elongation of the Si-C bond and the size of the silicon atoms providing for the C2 and C1 structures relieving steric hindrance in comparison to that of the C2v. In the C2v and C1 conformers, splitting in the spectra due to internal rotation of the -SiH3 end groups of 1,3,5-trisilapentane was observed and determined. The C2v equivalent V3 values are 368.46(33) cm-1, and the C1V3 values are 347.78(21) and 360.18(88) cm-1, respectively. These barriers are compared to similar species in order to help verify their veracity and are determined to be accurate based on similar molecular silyl rotors.

Molecular Structure of 1-Isocyano-1-silacyclopent-3-ene: A Combined Microwave Spectral and Theoretical Study.

The microwave spectrum of 1-isocyano-1-silacyclopent-3-ene has been obtained from broad-band chirped-pulse Fourier transform microwave spectroscopy. The rotational constants (RCs) for the standard abundant isotopic species are A = 3328.4182(23), B = 1017.69404(53), and C = 1012.33297(58) MHz. The symmetric quartic centrifugal distortion constants, using the Ir representation in CS symmetry for ΔJ,ΔJK, ΔK, and δJ, have been evaluated; similarly, the 14N nuclear quadrupole coupling has been determined. Several singly substituted isotopologues observed in natural abundance enabled most of the heavy atom substructure to be determined. The five-membered ring is close to planar, but the orientation of the isocyanate unit, derived from the N13CO spectrum, unexpectedly lies above the ring center in a cis C2,5-Si-N═C conformation. Our initial equilibrium structural searches led to a trans orientation of the C2,5-Si-N═C unit, i.e., bending away from the ring. When the cis conformation was applied, the final equilibrium structure, assuming CS symmetry, gave RC values of 3221.3 ( A), 1037.0 ( B), and 1031.3 ( C) MHz, very close to the MW values. This enabled the full-equilibrium structure to be determined with confidence. The principal bond lengths were 1.7157 (Si-N), 1.8696 (Si-C), 1.1998 (N═C), and 1.1737 (C═O) Å, with angles 163.3 (Si-N═C), 178.1 (N═C═O), 96.5 (C-Si-C), and 118.7° (C-C═C), respectively. The extensive widening of the SiNC angle is particularly notable; the SiNCO unit has a trans dihedral angle. The cis orientation implies a (weak) attractive force between the ring and isocyanate groups by a through-space interaction. An atoms in molecule study, where the local minima of electron density are determined, fails to disclose the exact nature of the interaction; however, a highly polarized skeleton was obtained. A systematic theoretical study of the Si-N═C angle potential energy surface (PES) relative to the ring gave a very shallow double minimum with the barrier being less than 1 cm-1; a polynomial fit to the surface shows major contributions of both harmonic and quartic components. A similar study of the XSiN angle, where X is at the ring center, also gave a PES with considerable quartic character.

Molecular Structure of Cyclopropyl (Isocyanato) Silane: A Combined Microwave Spectral and Theoretical Study.

The molecular equilibrium structures of two conformers (cis and gauche) of C3H5-SiH2-NCO have been deduced by a combination of microwave (MW) spectra at natural abundance including data from (13)C and (29,30)Si isotopomers and ab initio calculations. The MW rotational constants (RCs) for the most abundant isotopes are cis: A = 4216.3617(64), B = 1225.76654(91), and C = 1037.31468(77) MHz and gauche: A = 4955.55(79), B = 1094.9276(81), and C = 942.7031(80) MHz. The symmetric quartic centrifugal distortion constants have been evaluated for the cis conformer, using the I(r) representation for CS symmetry. Only partial substitution structures (PSSs) could be derived from the spectra after inclusion of the above isotopic combinations at each center. Using the PSSs, the full structures were determined by ab initio calculation of the equilibrium structures using coupled-cluster singles and doubles with selected triples configuration calculations (CCSD(T)); the two conformers have an energy difference of 228 cm(-1) (cis lower than gauche). The similarity of the calculated and MW RC results confirms the identities of the two compounds. The more interesting cis conformer has bond lengths C2-Si3, 1.9072(73), C2-C9 1.464(22), and C9-C10 1.4944(33) Å and angles Si3-C2-C10 119.4(12)° and C9-C2-C10 57.1(12)°, with similar results for the gauche conformer. The Si3N4C5 angle is wide in the cis conformer (145°) and nearly linear in the gauche conformer (179°). New physical insights into the bonding of cis conformers of this type have led the identification of an attractive force between the relatively crowded cyclopropyl and isocyanato groups in the cis conformation. This is demonstrated by three methods: Comparing electronic charges (both AIMALL and Mulliken analyses) in the pair of conformers shows a relative shift of density between these groups in the cis compound. Comparison of the highest occupied molecular orbitals (HOMOs) shows major mixing of density, exemplified by HOMO-1 in these structural units for the cis conformer but which is absent for the gauche conformer. Finally, the nearly linear isocyanate moiety (and the molecular dipole moment) of the cis conformer points closely toward the connected C atom of the cyclopropyl ring, while the gauche conformer dipole moment is significantly different in direction and points toward the midpoint of the C2Si3 bond. Both the HCSiN torsional and Si-N═C bending surfaces connecting these conformers were explored at the Møller-Plesset second-order perturbation theory level (MP2), which led to the exclusion of other conformers. The bending surface shows a very high amount of quartic potential function.

Structural studies of 1,1-dimethyl-2-oxy-1-silacyclohexane by means of matrix isolation infrared absorption spectroscopy.

Infrared spectra of gaseous, liquid, and matrix-isolated samples of newly synthesized 1,1-dimethyl-2-oxy-1-silacyclohexane were recorded. Raman spectra of 1,1-dimethyl-2-oxy-1-silacyclohexane in liquid and solid states were obtained in the temperature range from 170 to 340 K. Ab initio HF and DFT B3LYP calculations were performed in order to determine the possible conformations of 1,1-dimethyl-2-oxy-1-silacyclohexane and to make accurate assignment of the vibrational spectral bands. The study confirms the existence of only one chair-type conformer of 1,1-dimethyl-2-oxy-1-silacyclohexane.

The molecular structure of methylfluoroisocyanato silane: a combined microwave spectral and theoretical study.

The structure of methylfluoroisocyanato silane (Me-SiHF-NCO) has been deduced by a combination of microwave (MW) spectra including data from (12,13)C, (14,15)N, and (28,29,30)Si isotopomers, and ab initio calculations. The rotational constants (RC) for the most abundant isotopes are A = 6301.415(45), B = 1535.078(39), and C = 1310.485(39) MHz. The symmetric quartic centrifugal distortion constants have been identified, using the I(r) representation for C1 symmetry, which includes the 3-fold rotor. The spectra of the isotopomer combinations gave a partial substitution structure where the C2Si3, Si3N4, and N4C9 bond lengths are 1.8427(70), 1.7086(77), and 1.2120(90)Å; although the C2Si3N4 angle is close to tetrahedral (109.71° (52)), the Si3N4C9 angle is wide (157.69° (18)). The rotational constants are only consistent with a trans-orientation for each of the dihedral angles (HC2Si3N4, C2Si3N4C9, and Si3N4C9O10). The structural analysis was completed by calculations of the equilibrium structure, using MP3 in conjunction with an aug-cc-pVTZ basis set (434 Cartesian basis functions). This gave A = 6240.324, B = 1518.489, and C = 1297.819 MHz. The equilibrium structure bond lengths for C2Si3, Si3N4, and N4C9 were 1.8485, 1.7147, and 1.1947 Å, with the C2Si3N4 and Si3N4C9 angles 109.55 and 156.67°, respectively. Although the SiNC polynomial bending surface is complex, the data points can be fit to the simple form V(x) = 50.36(91)x(4) - 7.53(44)x(5), with comparatively little loss of accuracy. The A-rotational constant is strongly influenced by the Si3N4C9 angle, and smaller bases lead to this angle being nearly linear. The theoretical results suggest a very heavily polarized molecule, which is supported by the positions of the local electron density minima within the bonds and electron density calculations.

Microwave, r0 Structural Parameters, Conformational Stability, and Vibrational Assignment of (Chloromethyl)fluorosilane.

The FT-microwave spectrum (6.5-26 GHz) of (chloromethyl)fluorosilane (ClCH2-SiH2F) has been recorded and 250 transitions for the parent species along with (13)C, (37)Cl, (29)Si, and (30)Si isotopologues have been assigned for trans conformer. Infrared spectra (3100 to 400 cm(-1)) of gas, solid, and the variable temperature (-100 to -60 °C) studies of the infrared spectra of the sample dissolved in xenon have been recorded. Additionally, the variable temperature (-153 to -133 °C) studies of the Raman spectra of the sample dissolved in krypton have been recorded. The enthalpy difference between the trans and gauche conformers in xenon solutions has been determined to be 109 ± 15 cm(-1) (1.47 ± 0.16 kJ mol(-1)), and in krypton solution, the enthalpy difference has been determined to be 97 ± 16 cm(-1) (1.16 ± 0.19 kJ mol(-1)) with the trans conformer as the more stable form. Approximately 46 ± 2% of the trans form is present at ambient temperature. By utilizing the microwave rotational constants of five isotopologues for trans and the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained for trans conformer. The r0 structural parameter values for the trans form are for the heavy atom distances (Å): Si-F = 1.608 (3); C-Cl = 1.771 (3); Si-C = 1.884 (3); and angles (deg): ∠FSiC = 108.9 (5); ∠ClCSi = 104.9 (5). The results are discussed and compared to some related molecules.