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.

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.

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.

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.

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.

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.

Vibrational spectra, quantum chemical calculations and spectral assignments of 1,1-difluoro-1-silacyclohexane.

Raman spectra of 1,1-difluoro-1-silacyclohexane as a liquid, and as a solid at 78 K were recorded and depolarization data obtained. The infrared spectra of the vapour, liquid and amorphous and crystalline solids have been studied. In the low temperature IR and Raman spectra eight and three bands, respectively, were shifted a few cm(-1) when the sample crystallized. No bands vanished after crystallization in agreement with the assumption that only one conformer (chair) was present in all the states of aggregation. The compound exists in the stable chair conformation, whereas in the parent silacyclohexane a possible twist form should have more than 15 kJ mol(-1) higher energies than the chair, as derived from various calculations. The wavenumbers of the vibrational modes were calculated in the harmonic and anharmonic approximation employing B3LYP/cc-pVTZ calculations. The 27 A' and 21 A″ fundamentals were assigned on the basis of the calculations, infrared vapour contours, Raman depolarization measurements and infrared and Raman band intensities. An average, relative deviation of 1.5% was found between the observed and the anharmonic wavenumbers for the 48 modes.

Structure and conformation studies from temperature dependent infrared spectra of xenon solutions and ab initio calculations of cyclobutylgermane.

The infrared spectra (3500-220 cm(-1)) of cyclobutylgermane, c-C(4)H(7)GeH(3) have been recorded of the gas. Also variable temperature (-65 to -100 °C) studies of the infrared spectra (3500-400 cm(-1)) of the sample dissolved in liquid xenon were recorded and both the equatorial and axial conformers were identified. The enthalpy difference has been determined from 10 band pairs 8 temperatures to give 112 ± 11 cm(-1) (1.34 ± 0.13 kJ mol(-1)) with the equatorial conformer the more stable form. The percentage of the axial conformer present at ambient temperature is estimated to be 37 ± 1%. From ab initio calculations conformational stabilities have been predicted from both MP2(full) and density functional theory calculations from a variety of basic sets. The r(0) structure parameters have been obtained for both conformers from the previously reported rotational constants from the three isotopologues. The determined heavy atom distances for the equatorial [axial] form are (Å) Ge-C(α)=1.952(3) [1.950(3)], [Formula: see text] , [Formula: see text] [1.551(3)] and angles in degrees (°) ∠GeC(α)C(ß)=118.6(5) [113.4(5)], [Formula: see text] , ∠C(α)C(ß)C(γ)=87.8(5) [88.8(5)], [Formula: see text] and a puckering angle of 29.1(5) [25.1(5)]. Data from ab initio calculations were used to predict vibrational harmonic force constants, fundamental wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are compared to the corresponding properties of some related molecules.

Molecular structure of methyldifluoroisocyanato silane: a combined microwave spectral and theoretical study.

The structure of methyldifluoroisocyanato silane, MeF2SiNCO (2), has been studied by molecular rotational spectroscopy. The rotational spectrum has a complicated structure from (14)N nuclear quadrupole coupling and internal rotation of the methyl group. Cavity Fourier-transform microwave spectroscopy measurements were important for providing high spectral resolution to analyze the quadrupole and internal rotation fine structure. Broadband chirped-pulse Fourier-transform microwave spectroscopy was used to achieve high measurement sensitivity making it possible to observe the lower abundance C, N, O, and Si isotopologues in natural abundance for structure determination. Analysis of the microwave spectrum of the most abundant isotopomer of MeF2SiNCO (2) yields the rotational constants: A = 3827.347(7), B = 1264.5067(14), and C = 1240.6182(11) MHz. The spectrum has been analyzed in the I(r) representation for Cs symmetry, with inclusion of the 3-fold rotor (V3 = 446(50) cm(-1)). A partial substitution structure was obtained for the C, Si, N, and O atoms. The analysis was assisted by calculations of the equilibrium structure, using a 6-311++G (3df, 3pd) basis set, with calculations at each of the B3LYP, MP2, and CCSD(T) levels. The calculated and experimental rotational constants are only consistent with a trans-orientation at each of the HCSiN, CSiNC, and SiNCO centers; there is relatively close agreement between the experimental and the theoretical structures, especially at the CCSD(T) level. In addition, the observed low value for the (14)N quadrupole coupling term (χbb - χcc) implies a wide SiNC angle, which is consistent with the calculated values: 165.3° (B3LYP), 157.6° (MP2), and 157.4° (CCSD(T)). The skeletal bending potential is discussed.

Microwave, infrared and Raman spectra, r0 structural parameters, ab initio calculations and vibrational assignment of 1-fluoro-1-silacyclopentane.

The microwave spectrum (6500-18 ,500 MHz) of 1-fluoro-1-silacyclopentane, c-C(4)H(8)SiHF has been recorded and 87 transitions for the (28)Si, (29)Si, (30)Si, and (13)C isotopomers have been assigned for a single conformer. Infrared spectra (3050-350 cm(-1)) of the gas and solid and Raman spectrum (3100-40 cm(-1)) of the liquid have also been recorded. The vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twist form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but much lower energy than the planar conformer. By utilizing the microwave rotational constants for seven isotopomers ((28)Si, (29)Si, (30)Si, and four (13)C) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r(0) structural parameters have been obtained for the twist conformer. The heavy atom distances in Å are: r(0)(SiC(2)) = 1.875(3); r(0)(SiC(3)) = 1.872(3); r(0)(C(2)C(4)) = 1.549(3); r(0)(C(3)C(5)) = 1.547(3); r(0)(C(4)C(5)) = 1.542(3); r(0)(SiF) = 1.598(3) and the angles in degrees are: [angle]CSiC = 96.7(5); [angle]SiC(2)C(4) = 103.6(5); [angle]SiC(3)C(5) = 102.9(5); [angle]C(2)C(4)C(5) = 108.4(5); [angle]C(3)C(5)C(4) = 108.1(5); [angle]F(6)Si(1)C(2) = 110.7(5); [angle]F(6)Si(1)C(3) = 111.6(5). The heavy atom ring parameters are compared to the corresponding r(s) parameters. Normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values, and infrared band contours. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.

Infrared spectra, vibrational assignment, and ab initio calculations for N-bromo-hexafluoro-2-propanimine.

The infrared spectra of gaseous and solid N-bromo-hexafluoro-2-propanimine, (CF(3))(2)CNBr, have been obtained from 2000 to 50 cm(-1). The vibrational assignment for the normal modes is proposed based on infrared band contours, group frequencies and normal coordinate calculations utilizing C(s) symmetry. The structural parameters have been obtained from ab initio MP2(full)/6-311+G(d,p) calculations employing the Gaussian-03 program. Additionally, the frequencies and potential energy distributions for the normal modes have been calculated with the MP2(full)/6-31G(d). All of these results are compared to the corresponding data for some similar molecules.

Infrared and Raman spectra, ab initio calculations, conformational stability and vibrational assignment of 1-bromo-1-silacyclopentane.

Infrared and Raman spectra (3500-60 cm(-1)) of gas and/or liquid and solid 1-bromo-1-silacyclopentane (c-C4H8SiBrH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/6-311+G(2df,2pd) predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but approximately 900 cm(-1) (5.98 kJ/mol) lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predict slightly lower energies for the two envelope forms and considerably lower energy for the planar form compared to the MP2 predictions. By utilizing the MP2(full)/6-31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.

Conformational stability from variable temperature infrared spectra of xenon solutions, r0 structural parameters, and ab initio calculations of cyclopropylisocyanate.

Infrared spectra (4000 to 400 cm(-1)) of the gas and variable temperature xenon solutions, and the Raman spectrum of the liquid have been recorded for cyclopropylisocyanate. The enthalpy difference has been determined to be 77 ± 8 cm(-1) (0.92 ± 0.10 kJ/mol) with the trans form more stable than the cis conformer with 59 ± 2% present at ambient temperature. By utilizing three rotational constants for each conformer, combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, the adjusted r(0) parameters have been obtained. Heavy atom structural parameters for the trans [cis] conformers are the following: distances (Å) (C-C(2,3)) = 1.509(3) [1.509(3)], (C(2)-C(3)) = 1.523(3) [1.521(3)], (C-N) = 1.412(3) [1.411(3)], (N═C) =1.214(3) [1.212(3)], (C═O) = 1.163(3) [1.164(3)]; angles (°) ∠CCN = 116.7(5) [120.1(5)], ∠CNC = 136.3(5) [137.6(5)]. The centrifugal distortion constants have been predicted from ab initio and DFT calculations and are compared to the experimentally determined values.

Conformational stability, r0 structural parameters, and vibrational assignments of mono-substituted cyclobutanes: fluorocyclobutane.

Variable temperature (-55 to -100°C) studies of the infrared spectra (3500-400 cm(-1)) of fluorocyclobutane, c-C(4)H(7)F, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 496±40 cm(-1) (5.93±0.48 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 8±1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 732±47 cm(-1) (9.04±0.44 kJ/mol) and the average value of 602±20 cm(-1) from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained. The determined heavy atom structural parameters for the equatorial [axial] conformer are: distances (Å) C-F=1.383(3) [1.407(3)], C(α)-C(ß)=1.543(3) [1.546(3)], C(ß)-C(γ)=1.554(3) [1.554(3)] and angles (°) ∠C(α)C(ß)C(γ)=85.0(5) [89.2(5)], ∠C(ß)C(α)C(ß)=89.3(5) [89.2(5)], ∠F-(C(ß)C(α)C(ß))=117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudo-halogen substituents.

Microwave, Raman, and infrared spectra; adjusted r(0) structural parameters; conformational stability; and vibrational assignment of germylcyclohexane.

The FT-microwave spectrum of germylcyclohexane, c-C(6)H(11)GeH(3), has been recorded, and more than 40 transitions for the (70)Ge, (72)Ge, and (74)Ge isotopomers (isotopologues) have been assigned for the chair-equatorial conformer. The heavy atom adjusted r(0) structural parameters have been determined [distances, C(gamma)-C(delta) = 1.533(3) A, C(gamma)-C(beta) = 1.532(3) A, C(alpha)-C(beta) = 1.540(3) A, C(alpha)-Ge = 1.957(3) A; angles, angleC(gamma)C(delta)C(beta) = 111.2(5) degrees , angleGeC(alpha)C(beta) = 111.1(5) degrees , with the dihedral angle angleC(gamma)C(delta)C(beta)C(alpha) = 55.6(10) degrees ]. Raman and/or infrared spectra of gas, liquid, and solid germylcyclohexane have been recorded. The temperature dependency of the Raman spectrum of the conformer pair 712 (equatorial)/683 (axial) cm(-1) gives an enthalpy difference of 453 +/- 38 cm(-1) (1.30 +/- 0.11 kcal/mol) with the chair-equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 11 +/- 1%. Substituent effects on the enthalpy difference and structure of monosubstituted cyclohexanes are discussed for a number of molecules.

Microwave and quantum chemical study of allyldifluorosilane (H(2)C=CHCH(2)SiF(2)H).

The microwave spectrum of allyldifluorosilane (H(2)C=CHCH(2)SiF(2)H) has been investigated for the first time in the 28-80 GHz spectral interval at a temperature of -30 degrees C. The spectrum of the ground vibrational state of one conformer characterized by an anticlinal orientation for the C=C-C-Si chain of atoms and a synclinal conformation for the C-C-Si-H link has been assigned. This rotamer was found to be at least 2 kJ/mol more stable than further rotameric forms. The spectroscopic investigation has been augmented with quantum chemical calculations employing the MP2 and B3LYP methods using the 6-311++G(3df,3pd) basis set. The theoretical predictions are generally in good agreement with the experimental results.