Internal Rotation Splittings Research Articles

Internal rotation effects in the pulsed jet rotational spectrum of the trifluoromethane–carbon dioxide dimer

The rotational spectrum of the normal isotopic species of the HCF 3–CO 2 weakly bound complex has been measured by Fourier-transform microwave (FTMW) spectroscopy. All transitions are split into A and E states by internal rotation of the trifluoromethane subunit. A global fit of these states gives rotational constants that are consistent with a structure predicted by an MP2/6-311++G(2d,2p) ab initio calculation in which the axes of the monomers are coplanar, with the hydrogen atom of the trifluoromethane angled toward one of the oxygen atoms of the CO 2. Measured dipole moment components ( μ a = 0.431(6) D, μ b = 0, μ c = 1.436(6) D, μ total = 1.499(6) D) confirm the ab initio prediction of an ac plane of symmetry; however, the very near-prolate nature of the complex ( κ = −0.997), combined with the relatively high barrier to internal rotation (∼30 cm −1) leads to asymmetry splittings and internal rotation splittings of similar magnitude, resulting in the observation of dipole forbidden b-type E-state transitions in addition to the expected a- and c-type lines. Although this effect has been observed previously in several monomer spectra, this appears to be one of few examples for a weakly bound complex.

Rotational spectrum of a chiral α-hydroxyester: conformation stability and internal rotation barrier heights of methyl lactate

High resolution spectrum of methyl lactate, a chiral alpha-hydroxyester, has been investigated using a molecular jet Fourier transform microwave spectrometer. High level ab initio calculations were employed to study the conformational isomerism of methyl lactate. The observed rotational spectrum confirms that the most stable conformer has an intramolecular hydrogen bond of OH...O==C type, as predicted by the ab initio calculations. The internal rotation barrier heights of the ester methyl group and the alpha-carbon methyl group were calculated to be 5.4 and 14.5 kJ mol(-1) at the MP2/aug-cc-pVDZ level of theory for the most stable conformer. The internal rotation splittings due to the ester methyl group were observed and analyzed and the ester methyl group tunneling barrier height was determined experimentally to be 4.762 (3) kJ mol(-1).

Millimeterwave rotational spectrum and internal rotation in o-chlorotoluene

The millimeterwave rotational spectrum of o-chlorotoluene is investigated in the frequency region 150–250 GHz. Many rotational lines show splitting due to internal rotation of the methyl group. The analysis of the internal rotation splitting allows us to determine with precision the potential barrier to internal rotation of the methyl group. However, it is found that the moment of inertia of the methyl top is probably much smaller than usually assumed, which significantly affects the value of the barrier. Accurate centrifugal distortion constants are obtained for the ground states of 35Cl and 37Cl isotopologues as well as for an excited vibrational state.

Microwave structural studies of organoselenium compounds 1. Microwave spectra, molecular structure, and methyl barrier to internal rotation of dimethyl diselenide

The rotational spectra of nine isotopomers of dimethyl diselenide, CH 3SeSeCH 3, have been measured with a molecular-beam Fourier transform microwave spectrometer. The spectra were complex due to the presence of many isotopomers in natural abundance and the splitting caused by the interactions with two methyl internal rotors. The spectra were assigned and fit to experimental precision to an effective rotational Hamiltonian for molecules with two periodic internal motions. The spectra of the symmetric isotopomers are consistent with a C 2 equilibrium structure. The rotational constants were used to determine the r s structure of the C–Se–Se–C frame with the results r(SeSe)=2.306(3) Å, r(SeC)=1.954(6) Å, ϑ(CSeSe)=99.8(2)°, ϕ(CSeSeC)=85.2(1)°. A barrier to internal rotation of the methyl groups of 395 ± 2 cm −1 was derived from the internal rotation splittings.

Rotational Spectrum, Hyperfine Structure, and Internal Rotation of Methyl Carbamate

Methyl carbamate, an isomer of glycine, is a possible candidate for interstellar detection. It might be more abundant than glycine and its dipole moment is much larger. Furthermore, using high-level quantum chemical calculations, it is shown that syn methyl carbamate has a lower energy than glycine. The quadrupole hyperfine structure due to 14N has been measured using microwave Fourier transform spectroscopy. The rotational spectrum of the ground vibrational state has been measured from 8 to 240 GHz and accurate spectroscopic constants have been determined for the A internal rotation components of the rotational lines. Finally, the internal rotation splittings have been analyzed.

Rotational Spectrum, Hyperfine Structure, and Internal Rotation of Methyl Carbamate

Methyl carbamate, an isomer of glycine, is a possible candidate for interstellar detection. It might be more abundant than glycine and its dipole moment is much larger. Furthermore, using high-level quantum chemical calculations, it is shown that syn methyl carbamate has a lower energy than glycine. The quadrupole hyperfine structure due to 14N has been measured using microwave Fourier transform spectroscopy. The rotational spectrum of the ground vibrational state has been measured from 8 to 240 GHz and accurate spectroscopic constants have been determined for the A internal rotation components of the rotational lines. Finally, the internal rotation splittings have been analyzed.

Measurements of microwave spectra and structural parameters for methylferrocene

Rotational transitions for methylferrocene were measured, in the 4–12 GHz range, using a Flygare–Balle type, pulsed-beam Fourier transform spectrometer. Mono-substituted ferrocenes are nearprolate asymmetric tops with a and b dipole moment components, providing numerous possible transitions in this frequency range. Eighteen rotational constants were calculated from the data for six isotopomers. 59 transitions were measured for the normal isotopomer, and much smaller data sets were obtained for the Fe54, and four C13 isotopomers. Despite the small data sets for C13 and Fe54 transitions, good fits were obtained with small standard deviations ranging from 2 to 5 kHz. The eighteen A, B, and C rotational constants were used to determine the structural parameters for methylferrocene. Measured rotational constants for the normal isotopomer are: A=1592.6050(6), B=957.2565(4), and C=825.9892(4) MHz. The values for the averaged structural parameters are: r1(Fe–C)=2.050(7) Å, r2(C–C)(cyclopentadienyl ligands)=1.433(7) Å and r3(C–CH3)=1.52(2) Å (for the methyl cyclopentadienyl ligand). Small splittings (20–30 kHz) were observed for some transitions, and these could be due to proton hyperfine structure or small internal rotation splittings. The structural parameters for this complex were calculated using density functional theory (DFT) methods, and the results are in very good agreement with the measured values. Results are compared with earlier measurements on substituted ferrocenes. Distortions of the substituted cyclopentadienyl ligand are significantly smaller than those observed for chloroferrocene.

Millimeter-Wave Absorption Free Jet Spectrum, Barriers to Internal Rotation, and Torsional Relaxation inp-Anisaldehyde

The rotational spectrum ofp-anisaldehyde in a supersonic expansion has been investigated in the frequency range 60–78 GHz. Transitions up toJ= 57 measured for theantiandsynconformers have been used to determine complete sets of fourth-order centrifugal distortion constants. Methyl group internal rotation splittings have also been observed for some of the lines and have yielded the respective barriers for both conformers. A vibrational satellite observed for each of the two conformers has been assigned to the respective first excited methoxy torsional state. The jet conditions, in particular the stagnation pressure of the carrier gas, can be adjusted to control the degree of cooling achieved in the expansion. This possibility has been exploited in the present work to enhance the intensities of the observed vibrational satellites. Applying a two-dimensional flexible model to the methoxyl and aldehydic groups torsions, the potential energy and structural deformation parameters transferred from anisole and benzaldehyde have been found to be suitable to describe these motions inp-anisaldehyde. Mixing ofantiandsynconformations has been found to be insignificant within the first 82 calculated torsional eigenstates and therefore less likely to explain the intermediate bands observed in the low-resolution microwave spectrum than the internal vibrational relaxation suggested by R. K. Bohn, M. S. Farag, C. M. Ott, J. Radhakrishnan, S. A. Sorenson, and N. S. True (1992,J. Mol. Struct.268,107–121). A qualitative discussion of relaxation among the torsional states and its effect on the rotational spectrum is given.

The Microwave Spectrum of trans-2,3-Dimethyloxirane in Torsional Excited States

Abstract The microwave spectrum of trans-2,3-dimethyloxirane (CH3CHOCHCH3) in the excited tor-sional states υ17 = 1 and υ33 = 1 has been measured in the range from 8 to 26 GHz and assigned. An analysis of internal rotation splittings of the observed rotational transitions was performed using the internal axis method (or "combined axis method") with a newly developed program accounting for the top-top coupling. The threefold hindering potential V3 and the direction cosines λ g,i of the internal rotation axes i with respect to the principal inertia axes g are in a good agreement with the ground state values. Additionally, the sixfold hindering parameter V6 was found to be - 0.2600(12) kJ/mol. The value of the parameter V′12 describing the top-top coupling in the potential function (via V′12 sin 3 τ1 sin 3 τ2), was determined to -0.4240(6) kJ/mol.

Rotational spectrum, internal rotation barrier andabinitiocalculations on 1‐chloro‐1‐fluoroethane

The microwave spectrum of 1‐chloro‐1‐fluoroethane (CFC 151‐a) has been studied in the frequency region 8–250 GHz using waveguide Fourier transform, Stark, and source modulation spectrometers. Accurate rotational, quartic, and sextic centrifugal distortion and quadrupole coupling constants have been obtained from a global fit for the ground, v17=1 (Cl–F skeletal bending mode), and v18=1 (CH3 torsional) vibrational states of the 35Cl isotopomer and for the ground state of the 37Cl isotopomer. The larger off‐diagonal element of the χ tensor was also determined for the 35Cl isotopomer. Assignment of the v17=1 and v18=1 states was confirmed by the presence of small A–E internal rotation splittings in the v18=1 state, in agreement with ab initio calculations, but in contradiction with previous assignment of the microwave spectrum by Thomas et al. [J. Chem. Phys. 61, 5072 (1974)]. The barrier height for the internal rotation of the methyl group was determined to be 3814(11) cal/mol, and compared with the result of ab initio calculations made for 1‐chloro‐1‐fluoroethane and other related chlorine or fluorine substituted ethanes.

Microwave Investigation of (Z)- and (E)-Ethanethial S-Oxide

Pulsed-beam Fourier transform microwave spectroscopy was used to study ethanethial S-oxide, an unstable species generated in a (Z)/(E) ratio of 97/3 by pyrolysis of 2-methyl-2-propylvinyl sulfoxide at 350 °C in Ar or Ne/He gas flows. Rotational transitions of the (Z) normal isotopomer exhibited A−E state splittings due to internal rotation of the methyl group. A global fit of the observed A and E state lines of the (Z) isomer to an internal rotor Hamiltonian gave rotational constants and centrifugal distortion constants of A = 14237.0861 (5) MHz, B = 4678.4488 (3) MHz, C = 3594.8008 (2) MHz, ΔJK = −9.43 (55) kHz, ΔJ = 3.48 (11) kHz, and δJ = 1.11 (2) kHz and a 3-fold internal rotor barrier of 285.6 (3) cm-1. No internal rotation splitting was observed for the (E) isomer and a pseudorigid rotor fit of the observed transitions of the normal isotopomer gave A = 31 128 (27) MHz, B = 3475.8521 (16) MHz, C = 3188.4429 (23) MHz, and ΔJ = 0.49 (12) kHz. Microwave spectra of eight (Z) isotopomers were assigned, and a partial substitution structure was derived: r(CS) = 1.618 (3) Å, r(SO) = 1.477 (4) Å, r(CC) = 1.493 (3) Å, θ(CSO) = 113.9 (2)°, and θ(CCS) = 125.4 (2)°. The electric dipole moment components of the (Z) isomer along the a and b principal axes were measured to be μa = 2.714 (5) D and μb = 1.869 (35)D, respectively.

Rotational Spectrum of 1,1-Difluoroethane: Internal Rotation Analysis and Structure

The rotational spectrum of CH3CHF2 in its ground state was measured up to 653 GHz. Accurate rotational and centrifugal distortion constants were determined. The internal rotation splittings were analyzed using the internal axis method. An ab initio structure has been calculated and a near-equilibrium structure has been estimated using offsets derived empirically. This structure was compared to an experimental r0 structure. The four lowest excited states (including the methyl torsion) have also been assigned.

On the Apparent Methyl Internal-Rotation Barrier Decrease in Weakly Bound Methanol Complexes

The microwave spectra of the van der Waals complexes acetone-20Ne and acetone-22Ne were measured using a cavity-based supersonic jet Fourier-transform microwave spectrometer in the region from 5 to 18 GHz. For these two isotopologues, both c- and weaker a-type transitions were observed. The transitions are split into multiplets due to the internal rotation of the two methyl groups in acetone. Initial electronic structure calculations were performed at the MP2/6-311++g (2d, p) level of theory and the internal rotation barrier height of the methyl groups was calculated to be ∼2.8 kJ/mol. The ab initio rotational constants were the basis for the spectroscopic searches, but the multiplet structures and floppiness of the complex made the quantum number assignment very difficult. The assignment was finally achieved with the aid of constructing closed frequency loops and predicting internal rotation splittings using the XIAM internal rotation program. The acetone methyl group tunneling barrier height was determined experimentally to be 3.10(6) kJ mol−1 [259(5) cm−1] in the acetone-Ne complex, which is lower than in the acetone monomer but comparable to the acetone-Ar complex (Kang et al., 2002). Experimental data and high-level CCSD(T)/aug-cc-pVTZ calculations suggest that the Ne atom lies directly above the plane formed by the carbonyl group and the two carbon-carbon bonds, which is different than the slightly offset position found previously in the acetone-Ar complex. Additionally, ab initio calculations and Quantum Theory of Atoms in Molecules analyses were used to analyze the methyl internal rotation motions in acetone and acetone-Ne.

The submillimeter-wave spectrum of propionitrile (C2H5CN)

The laboratory millimeter-wave and submillimeter-wave rotational spectrum of propionitrile (ethyl cyanide, C2H5CN) in its ground vibrational state has been extended through J = 70 and K(sub a) = 36. Transitions measured in this study are in the frequency range 250-610 GHz. A fit of 952 transitions including 732 new measurements to a Watson A-reduced Hamiltonian has yielded a complete set of quartic and sextic distortion constants, in addition to six octic distortion constants. Internal rotation and hyperfine splittings have not been resolved, so they are not included in the analysis. Predicted transition frequencies for a large number of additional transitions of interest to radioastronomers are provided.

Cross-conjugated compounds: microwave spectrum of 4,4-dimethyl-2,5-cyclohexadien-1-one

4,4-Dimethyl-2,5-cyclohexadien-1-one has been prepared by a three-step reaction from 4,4-dimethyl-2-cyclohexen-1-one. Microwave transitions of the cross-conjugated compound have been measured over extended regions from 11 to 40 GHz. The spectrum is a classical a-type spectrum of a near-prolate top with many interspersed lines presumably originating from excited vibrational states. No internal rotation splitting could be observed. We were able to assign 108 rotational transitions with J-quantum numbers up to 40. The least-squares fit showed a standard deviation of 30 kHz and yielded the three rotational constants, A = 3332.158(13) MHz, B = 1193.07386(52) MHz, C = 1082.21280(45) MHz, as well as all five quartic centrifugal distortion constants from Watson's A-reduction: Δ J = 0.02149(73) kHz, Δ JK = 1.1008(16) kHz, Δ K = −68.42(32) kHz, δ J = 0.00332(16) kHz, δ K = −2.7513(99) kHz (representation I R used). The dipole moment was determined from the Stark shift of the M-components of two transitions and was found to be 4.4522(83) D from 93 measurements.

Millimeter-wave spectra and revised spectroscopic parameters of methylsilane

Rotational spectra of methylsilane in the ground and several torsionally excited states were measured in the region from 87 to 350 GHz with Doppler-limited resolution. For some of the transitions in the ground vibrational state, the internal rotation splitting has been recorded with sub-Doppler resolution via the saturation technique. Transitions in the torsional states with v t ≤ 2 have been assigned and analyzed with a least-squares procedure, employing a direct diagonalization method for calculating the torsion-rotation energy levels. A phenomenological Hamiltonian has been developed to describe on the free-internal rotor basis the internal and overall rotation of symmetric-top molecules. The potential barrier of methylsilane to internal rotation was determined to be V 3 ≈ 590.5 cm −1.

ChemInform Abstract: The Rotational Spectrum of Methacrylonitrile: Dipole Moment, Internal Rotation, and Centrifugal Distortion

The rotational spectrum of methacrylonitrile has been investigated in the vibrational ground state between 8 and 200 GHz. High-J transitions have been measured and fitted to a semirigid rotor Hamiltonian including some sextic constants. The internal rotation splittings have been analyzed using the internal axis method. The derived barrier to internal rotation of the methyl group is V3 = 1986 cal/mole. The components of the molecular dipole moment were calculated from Stark-effect measurements as μa = 3.940(2) D, μb = 0.26(5) D, and μt = 3.948(4) D. The first two excited states of the lowest-frequency in-plane CCN bending vibrational mode, the first excited state of the out-of-plane CCN bending mode, and the first excited state of the methyl torsion have also been assigned.

The rotational spectrum of methacrylonitrile: Dipole moment, internal rotation, and centrifugal distortion

The rotational spectrum of methacrylonitrile has been investigated in the vibrational ground state between 8 and 200 GHz. High- J transitions have been measured and fitted to a semirigid rotor Hamiltonian including some sextic constants. The internal rotation splittings have been analyzed using the internal axis method. The derived barrier to internal rotation of the methyl group is V 3 = 1986 cal/mole. The components of the molecular dipole moment were calculated from Stark-effect measurements as μ a = 3.940(2) D, μ b = 0.26(5) D, and μ t = 3.948(4) D. The first two excited states of the lowest-frequency in-plane CCN bending vibrational mode, the first excited state of the out-of-plane CCN bending mode, and the first excited state of the methyl torsion have also been assigned.

Microwave spectrum, internal rotation, structure and dipole moment of trifluoromethyl mercaptan, CF 3SH

The microwave spectra of CF 3SH and CF 3SD have been studied in the frequency range 12–40 GHz in the ground vibrational state. the spectra are heavily complicated by internal-rotation splittings which have been analysed using an IAM treatment of Valenzuela and Woods. The torsional parameters determined for CF 3SH are s=22.7454(3), F=293,364(15) MHz, ϱ=0.980674(3) and β=0.025472(14); and for CF 3SD, s=41.724(2), F=156,314(65) MHz, ϱ=0.96343(2) and β=0.00651(17). The ground-state torsional splitting for CF 3SH, Δ 0=−3021.1(2) MHz, has been combined with early infrared data for tosionally excited states to obtain V 3=448(4) cm −1. Stark effects in the microwave spectrum are significantly affected by internal rotation enabling the orientation of the dipole moment to be determined. The total dipole moment is found to be 1.52(2) D making an angle of 36.0° with respect to the CS bond direction. The zero-point average structure of CF 3SH has been estimated from isotopic data for CF 3SD and CF 34 3SH plus internal-rotation information. The parameters for an assumed staggered conformation are: r(CF)=1.337(5), r(CS)=1.801(10), r(SH)=1.347(4) Å, ∠ CHS=92.0(1)°, ∠ SCF 1=108.4(8)° and ∠ SCF 2,3=112.9(4)°. The CF 3 top is determined to be asymmetric and titled 3.0° away from the SH bond, in keeping with C s overall symmetry of the molecule but in contrast to the C 3v behaviour of the top indicated by the internal rotation.

The Millimeter and Submillimeter-Wave Spectrum of Dimethylether

Abstract We report the analysis of the rotational spectrum of dimethylether measured between 60 and 400 GHz. Rotational and quartic centrifugal distortion constants are given. Internal rotation splittings are analysed with the I AM method. The value of is compared to the values obtained for similar molecules