Microwave and Raman spectra,dipole moment,barrier to internal rotation,and vibrational assignment of silylphosphine-d2

The microwave spectra of SiH₃PD₂ have been recorded in the range 26.5–40.0 GHz. Both a- and c-type transitions were observed and assigned. The rigid rotor rotational constants were determined to be A = 37589.06 ± 0.11, B = 5315.70 ± 0.02, and C = 5258.70 ± 0.02 MHz. The barrier to internal rotation has been calculated from the A-E splittings to be 1512 ± 26 cal/mole. The dipole moment components of |μ_a| = 0.22 ± 0.01, |μ_c| = 0.56 ± 0.01, and |μ_t| = 0.60 ± 0.01 D were determined from the Stark effect. By using previously determined microwave data for SiH₃PH₂, several structural parameters have been calculated and their values are compared to similar ones in other compounds. The Raman (0–2500 cm^?1) spectra of gaseous, liquid, and solid SiH₃PH₂ and gaseous SiH₃PD₂ have been recorded and interpreted in detail on the basis of C_s molecular symmetry.     

Methyl group tilt and structure from rotational spectra of the CH2DNHCl monodeuterated species

The structure and tilt of the methyl group of CH₃NHCl have been determined by analysis of the three different ground state rotational spectra of the methyl monodeuterated species. The tilt was found to be ?3.5° which is of the same order of magnitude and towards the unshared pair of electrons at the nitrogen atom as in methylamine and its derivatives.The barrier to internal rotation was determined form the A-E splittings of the previously measured transitions and found to be 3781 ± 14 cal/mole for CH₃NHCl and 3784 ± 15 cal/mole for CH₃NDCl, while the values obtained under the assumptions of no tilt and of asymmetric methyl group as in methylamine were 3707 ± 30 cal/mole for CH₃NHCl and 3726 ± 25 cal/mole for CH₃NDCl. Some information has also been deduced on the orientation of the z axis of the principal quadrupole coupling tensor.     

Reinvestigation of Microwave Spectrum of Ethylsilane: II. Vibrationally Excited States and Internal Rotations

Microwave spectra of ethylsilane and its three deuterated species in vibrationally excited states have been measured. A least-squares analysis of the observed frequencies gave rotational constants and three of the quartic centrifugal distortion constants. The barriers to the internal rotation of both the methyl and silyl groups and the coefficients of the potential cross terms were solved from splittings of the multiplets.The averages of CH₃ and SiH₃ barriers were determined to be 2634±16 and 1992±21 cal/mol and the potential cross terms, V₁₂ and V′₁₂, were estimated to be −55 and −111 cal/mol, respectively, for the four species.     

Microwave spectrum,barrier for methyl rotation,methyl conformation,and dipole moment of ortho-xylene

Nine microwave ground-state spectra of seven isotopes of ortho-xylene have been measured between 9 and 29 GHz. From the rotational constants a partial substitution structure could be calculated. The dipole moment was determined from Stark-lobe shifts, μ_a = 0.640 ± 0.005 D. The high-J transitions were found split into multiplets due to the interaction of methyl top internal rotation with the overall molecular rotation; doublets through quintets with the correct nuclear spin weight dependence could be observed according to group-theoretical expectations. A weighted average, V₃ = 1490 ± 50 cal/mole, was derived for the internal rotation barrier neglecting top-top coupling and presumably small, higher than threefold barrier terms. The methyl groups both stagger the bond between the two benzene carbon atoms which carry them.     

The microwave spectrum,substitution structure,internal rotation barrier,and dipole moment of thioacetaldehyde,CH3CHS

The microwave rotational spectrum of the unstable species thioacetaldehyde, CH₃CHS, has been studied in a flow pyrolysis system. Eight isotopic variants have been studied allowing an accurate substitution structure to be derived. Most of the spectral lines show splittings due to internal rotation, analysis of which has allowed a barrier study to be made. For the torsional ground state of the most abundant species, V₃ = 1572 ± 30 cal/mole or 375.7 ± 7 J/mole. The dipole moment is μ = 2.33 ± 0.02 D with components μ_A = 2.26 ± 0.02 and μ_B = 0.56 ± 0.01 D.     

Microwave spectrum,structure, dipole moment,and internal rotation of methylsilylsulfide

Microwave spectra of methylsilylsulfide and its three isotopically substituted species were measured and their b-type transitions were assigned. The spectra of all the species exhibit doublet structures due to the internal rotation of the methyl group. Using the internal axis method, the potential barriers were determined from the observed A- and E-component frequencies to be 1081.0 ± 3.3, 1073.9 ± 2.0, 1065.1 ± 11.4, and 1076.0 ± 1.9 cal/mol for the normal, CH₃SSiD₃, CD₃SSiH₃, and ¹³CH₃SSiH₃ species, respectively. The analysis also yielded 3°49′ as the tilt angle of the methyl top. From the rotational constants obtained, a plausible structure was estimated. The molecular electric dipole moments were determined from the second-order Stark effect of some A-component transitions with low- J quantum numbers for the normal and SiD₃ species. A comparison of the obtained parameters was made with analogous molecules.     

The microwave spectra of ortho-fluorotoluene with the asymmetric internal rotors CH2D and CD2H

The rotational spectra of αd₁- and αd₂-ortho-fluorotoluene in the ground state of the methyl group torsion have been measured. The evaluation of the spectra has been based on the theory for the internal rotation of an asymmetric internal top formulated earlier by several authors. The barrier potential being threefold symmetric (V₃), each torsional level consists of three nondegenerate substates, designated as sy and ±asy. The sy-state is assigned to the conformation with the unique methyl hydrogen isotope within the molecular heavy-atom plane (sy-rotamer), while the ±asy-states belong to the respective out-of-plane conformation (asy-rotamer). In the torsional ground state the level spacing between the ±asy substates is very small and numerous accidental close degeneracies are present between the rotational level systems based on these torsional substates. The rotational levels involved are strongly perturbed by the coupling between molecular overall rotation and internal rotation. Large deviations from a rigid rotor spectrum and (+) ? (?) intersystem (“tunneling”) transitions are observed. The spectrum of the asy-rotamer can be well reproduced by a “two-dimensional” Hamiltonian containing 11 “rotational constants,” 9 of which are determined by a fit to the spectrum. Several are sufficiently barrier-dependent to derive V₃. We obtain (in cal/mole) 567 ± 48 for αd₁-ortho-fluorotoluene, 711 ± 40 for the αd₂-isotope. The deviations from 649 cal/mole for the normal isotope are appreciable, probably indicating shortcomings of the semirigid model. The sy-rotamer presents a rigid rotor spectrum.     

Microwave spectrum of chloromethyl methyl ether

The microwave spectrum of chloromethyl methyl ether has been studied in the region 12.4–40 GHz. For ³⁵Cl species, a- and c-type transitions have been assigned for the ground state, the first excited state of the chloromethyl torsional mode, and the first excited state of the methyl torsional mode. Assignments were also made for the ground state of ³⁷Cl species. The assigned transitions are due to the gauche conformer. The nuclear quadrupole coupling constants were determined for the ground state of ³⁵Cl and ³⁷Cl species. The observed A-E splittings of the rotational transitions arising from the three vibrational states indicate a strong coupling between the two torsional vibrations. A model calculation based on the Hamiltonian previously used by Butcher and Wilson (J. Chem. Phys.40, 1671 (1964)), was carried out to account for the splittings and the vibrational frequencies of the two torsional modes. The barrier to internal rotation of the methyl group is estimated to be V₃ = 647 ± 17 cm^?1 (1.84 ± 0.05 kcal/mole).     

Microwave spectrum of two top molecules in the excited states dimethyl ether, dimethyl sulfide, and dimethyl silane

Microwave spectra of dimethyl ether, dimethyl sulfide, and dimethyl silane in the torsionally excited states have been measured. The methyl internal rotations of these molecules were analyzed from the observed multiplets and from the reported multiplets of transitions. The method developed for ethyl silane in the previous paper was extended to equivalent two top molecules. For equivalent two top molecules, apparent barriers of methyl internal rotations obtained from the experiments were corrected by the kinetic and potential cross terms. They are V₃=2645.8±10.0, 2632.4 ± 42.9, 2146.0 ± 13.8, 1651.5 ± 10.1, 1648.0 ± 13.7, and 1649.9 ± 11.8 cal/mol for (CH₃)₂O, (CD₃)₂O, (CH₃)₂S, (CH₃)₂SiH₂, (CH₃)₂SiD₂, and (CH₃)₂SiHD, respectively. The potential cross terms, V₁₂(1−cos3α₁)(1−cos3α₂) terms are negligible for the three molecules, while V^′₁₂sin3α₁sin3α₂ terms are also very close to zero except those for (CH₃)₂O and (CD₃)₂O which are small but not negligible (V^′₁₂=−124.4,−158.0 cal/mol). The investigations were extended to those of non-equivalent two top species and the corrected barriers of the methyl tops, V₃, are obtained to be 2615.6 ± 8.6 and 2155.0 ± 15.2 cal/mol for CH₃OCD₃ and CH₃SCD₃. The corrected barrier, V₃(CD₃) of CH₃OCD₃, is obtained to be 2634.4 ± 7.1 cal/mol, while that of CH₃SCD₃ cannot be solved due to the lack of the data available.     

Rotational spectra of isotopic species of silyl fluoride. Part II: Theoretical and semi-experimental equilibrium structure

The equilibrium structure of silyl fluoride, SiH₃F, has been reinvestigated using both theoretical and experimental data. With respect to the former, quantum-chemical calculations at the coupled-cluster level have been employed together with extrapolation to the basis set limit, consideration of higher excitations in the cluster operator, and inclusion of core correlation as well as relativistic corrections (r(Si-F) = 1.5911 Å, r(Si-H) = 1.4695 Å, and ∠FSiH = 108.30°). A semi-experimental equilibrium structure has been determined based on the available rotational constants for the various isotopic species of silyl fluoride (²⁸SiH₃F, ²⁸SiD₃F, ²⁹SiH₃F, ²⁹SiD₃F, ³⁰SiH₃F, ³⁰SiD₃F, ²⁸SiH₂DF, and ²⁸SiHD₂F) together with computed vibrational corrections to the rotational constants (r(Si-F) = 1.59048(6) Å, r(Si-H) = 1.46948(9) Å, and ∠FSiH = 108.304(9)°).     

The ground-state rotational spectrum of cyclopropyl silane

The rotational spectrum of cyclopropyl silane has been recorded in the region 9.0–35.0 GHz. Eighty-eight transitions of the ground vibrational state were measured and analyzed to give rotational constants and centrifugal distortion constants. The dipole moment was determined to be μ_a = 0.847(17)D, μ_c = 0.273(10)D, and μ_tot = 0.890(18)D. The rotational constants are consistent with a shortening of the CC bond length opposite to the silyl group. Since no splittings due to internal rotation were observed, a lower limit for the hindering potential of the internal rotation of the silyl group is V₃ ≥ 1950cal/mole.     

Microwave spectrum,structure, dipole moment,and internal rotation of the GT isomer of fluoromethylethylether

Microwave spectra of fluoromethylethylether and its 13 isotopically substituted species have been measured. The r_s structure of the GT isomer of this molecule was determined from the observed moments of inertia. The structural parameters obtained are roughly close to those of fluoromethylmethylether and the GT isomer of chloromethylethylether. The dipole moments and their directions in the molecule were determined from the Stark effect measurements of several low-J transitions for the normal and two deuterated species. The dipole moment of the normal species was found to be 1.806 ± 0.012 D, making angles of 136°50′ and 107°40′ with the CF and FCH₂O bonds, respectively. From the A-E splittings of the spectra in the first excited methyl torsional state, the barrier to internal rotation of the methyl group was calculated to be 3150 ± 50 cal/mole in the one-top approximation.     

Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules ΔJ = 0, K = +1 /a3 ?1. For these “barrier” anticrossings, the values of the crossing fields E_c yield directly the internal rotation splittings; the E_c are independent of the difference (A-B) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several “rotational” anticrossings (J, K) = (1, ±1) /a3 (2, 0) for which E_c does depend on (A-B). The normal rotational transition (J, K) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH₃²⁸SiH₃ were combined with Hirota's microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I_α = 3.165(5) amu-Ų, the effective rotational constant A^eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V₃^eff = 592.3359(73) cm^?1. The correlations leading to these two effective constants are discussed and the true values of A and V₃ are determined within certain approximations. For the isotopic species CH₃³⁰SiH₃, barrier and rotational anticrossings were observed. The isotopic changes in A and V₃ were determined, as well as an upper limit to the corresponding change in I_α.     

The microwave spectrum of trans 1-nitropropene

Rotational transitions of 1-nitropropene arising from the ground vibrational state and from three excited states of the nitro torsional vibration have been assigned. The values of the rotational constants in MHz are: A₀=10 650B₀=2028.56C₀1722.16A₁10 615 B₁=2028.47 C₁=1725.11 A₂10 570 B₂ 2028.31 C₂= 1727.32 A₃= 10 512 B₃2028.11 C₃=1729.37The dipole moment components are μ_a = 4.52 D, μ_b = 0.42 D and μ_total = 4.54 D. From the lack of observable internal rotation splittings the barrier to internal rotation of the methyl group is shown to be greater than 2250 cal/mole.     

Microwave spectrum and structure of methyl phosphonic difluoride

The rotational spectrum of methyl phosphonic difluoride has been reinvestigated using a pulsed-molecular-beam Fabry-Perot cavity microwave spectrometer. The enhanced resolution of the Fourier transform microwave (FTMW) spectrometer (compared to the original work done in a conventional Stark spectrometer) has allowed the measurement of small A-E splittings of many of the rotational transitions caused by the internal rotation of the methyl top. The barrier to internal rotation, V₃ = 676 (25) cm⁻¹, has been determined experimentally from the A-E splittings of the rotational transitions in the ground vibrational state. This barrier height is substantially lower than the previously determined value for the barrier, which was 1252 (14) cm⁻¹. High-level ab initio calculations at the MP2/aug-cc-pVTZ level predict a barrier to internal rotation of 638 cm⁻¹, in agreement with the experimentally determined value found here. The high sensitivity of the FTMW spectrometer has also permitted the measurement of the ¹³C and ¹⁸O isotopomers in natural abundance. The addition of these two isotopomers has allowed an improved structural determination.     

Microwave spectrum of dimethylallene excited states and strong Coriolis coupling

The rotational spectra of six excited vibrational states of dimethylallene were measured and assigned to the corresponding vibrational levels, and for three more excited state spectra at least the rotational constants could be determined. Between the two lowest excited levels of symmetry species b₂ and b₁ of group C_2v a strong a-type Coriolis coupling was found to exist. The evaluation of the resulting perturbation by a diagonalization of the energy matrix yielded ζ^(a) = 0.36 and a precise value for the vibrational energy difference 48.761 GHz (1.6 cm^?1). The state b₂ is believed to be the first excited torsional substate (01, 10)₁ of methyl internal rotation, and the rotational transitions of this state as well as those of the strongly coupled state b₁ presented very irregular multiplet splittings. On the other hand, the splittings of the next-higher excited state of species a₂ which could be identified as the partner torsional substate (01, 10)₂, followed the regular pattern, yielding an internal rotation barrier V₃ (2079 cal/mole) not unlike that derived earlier from ground state splittings.     

Barrier to internal rotation of the silyl group in cyclopropyl silane from the MW spectrum in the first excited torsional state

The MW spectrum of the first excited state of the internal rotation of the silyl group in cyclopropyl silane has been recorded and analyzed in the frequency region 6.9–38.0 GHz. Recordings were made with conventional Stark spectroscopy as well as with MW-MW double resonance. From the analysis of the torsional splittings the following parameters were derived: V₃=1917.6(44)cal/mole, I_α=5.888(14)u^*A², V_a=21.56(21)°     

Microwave spectrum,barrier to internal methyl rotation,dipole moment,and partial structure of dimethylketene

From the microwave spectrum of dimethylketene which has been recorded from 8 to 37 GHz, the following rotational constants were derived: A = 8 267.832 ± 0.8, B = 3 884.101 ± 0.03, C = 2 728.826 + 0.03 MHz. The dipole moment is μ_a = 1.94 ± 0.01 D. Substitution coordinates for all methyl group atoms have been obtained by investigating the spectra of six isotopic species of the molecule. The potential barrier V₃ hindering internal rotation of the methyl tops has been fitted to the multiplet width of a number of high-J ground state ^aQ-transitions which were observed as triplets. V₃ is 2065 cal/mole, keeping fixed I_α = 3.132 amu Ų and angle (methyl-top to a-axis) = 58.94° as obtained from the partial substitution studies.     

The microwave spectrum of cis-2-pentene

The microwave spectrum of cis-2-pentene has been shown to originate from molecules in the skew-conformation (dihedral angle φ = 119 ± 3°). From A-E doublet splittings the barrier to internal rotation about the C₁C₂ axis was found to be 280 ± 4 cm^?1; furthermore the dipole moment components and centrifugal distortion constants are reported. In excited states of the C₃C₄ torsion the spectra exhibit further splittings; these are due to tunneling between the two equivalent skew-conformations through a barrier of 210 ± 20 cm^?1.     

Microwave spectrum and selenol internal rotation of 2-propaneselenol

Microwave spectra of 2-propaneselenol and its deuterated species were measured and assigned for the gauche and trans isomers. The double minimum splittings of the gauche isomers were directly observed from b-type transitions, which were assigned with the aid of a double resonance technique. Rotational constants and torsional splitting of the gauche isomer of the parent species were determined to be A = 7802.50 ± 0.75, B = 2847.68 ± 0.04, C = 2242.03 ± 0.03, ΔA = ?2.52 ± 0.74, ΔB = 0.02 ± 0.05, ΔC = ?0.34 ± 0.03, and Δν = 368.91 ± 0.94 MHz, where ΔA, and ΔB, and ΔC are the differences of the rotational constants between the (+) and (?) states. From the torsional splittings and the energy differences of the two isomers of the parent and SeD species, Fourier coefficients of the selenol internal rotation potential function were determined to be V₂ = ?88 ± 15, V₃ = 1543 ± 29 cal/mole on the assumption of V₁ = 0. Dipole moments and their components were also obtained for the two isomers.