Skeletal Torsion Research Articles

Skeletal Torsion Tunneling and Methyl Internal Rotation: The Coupled Large Amplitude Motions in Phenyl Acetate.

The rotational spectrum of phenyl acetate, CH3COOC6H5, is measured using a free jet absorption millimeter-wave spectrometer in the range from 60 to 78 GHz and two pulsed jet Fourier transform microwave spectrometers covering a total frequency range from 2 to 26.5 GHz. The features of two large amplitude motions, the methyl group internal rotation and the skeletal torsion of the CH3COO group with respect to the phenyl ring C6H5 (tilted at about 70°), characterize the spectrum. The vibrational ground state is split into four widely spaced sublevels, labeled as A0, E0, A1, and E1, each of them with its set of rotational transitions and with additional interstate transitions. A global fit of the line frequencies of the four sublevels leads to the determination of 51 spectroscopic parameters, including the ΔEA0/A1 and ΔEE0/E1 vibrational splittings of ~36.4 and ~33.5 GHz, respectively. The V3 barrier to methyl internal rotation (~136 cm−1) and the skeletal torsion B2 barrier to the orthogonality of the two planes (~68 cm−1) are deduced.

Large Amplitude Motions of Pyruvic Acid (CH3-CO-COOH).

Torsional and rotational spectroscopic properties of pyruvic acid are determined using highly correlated ab initio methods and combining two different theoretical approaches: Second order perturbation theory and a variational procedure in three-dimensions. Four equilibrium geometries of pyruvic acid, Tc, Tt, Ct, and CC, outcome from a search with CCSD(T)-F12. All of them can be classified in the Cs point group. The variational calculations are performed considering the three internal rotation modes responsible for the non-rigidity as independent coordinates. More than 50 torsional energy levels (including torsional subcomponents) are localized in the 406–986 cm−1 region and represent excitations of the ν24 (skeletal torsion) and the ν23 (methyl torsion) modes. The third independent variable, the OH torsion, interacts strongly with ν23. The A1/E splitting of the ground vibrational state has been evaluated to be 0.024 cm−1 as it was expected given the high of the methyl torsional barrier (338 cm−1). A very good agreement with respect to previous experimental data concerning fundamental frequencies (νCAL − νEXP ~ 1 cm−1), and rotational parameters (B0CAL − B0EXP < 5 MHz), is obtained.

The analysis of the microwave absorption spectrum of trans-ethyl methyl ether in the skeletal torsion v30 = 2 state

The microwave absorption spectrum of the trans-ethyl methyl ether in the skeletal torsional v30 = 2 state was analyzed for the first time. The trans-ethyl methyl ether molecule has two inequivalent methyl group internal rotors. Splitting due to the methyl group connected to the oxygen was dominant and observed in the v30 = 2 state. This result was supported by considering the torsional force field in the same way as in the ground and the v30 = 1 states. Over 800 lines up to J = 55 and Ka = 6 were assigned. The spectrum was analyzed using an IAM-like matrix formulation. Three rotational constants, eight centrifugal distortion constants, and four O-CH3 internal rotation tunneling parameters were determined for the v30 = 2 state. It was also made clear in the present analysis that the tunneling parameter dominant concerning the splitting due to the O-CH3 internal rotation has the same sign as that of the v30 = 1 state, and the sign opposite to that of the ground state.

Torsion-rotation-vibration effects in the ground and first excited states of methacrolein, a major atmospheric oxidation product of isoprene.

Methacrolein is a major oxidation product of isoprene emitted in the troposphere. New spectroscopy information is provided with the aim to allow unambiguous identification of this complex molecule, characterized by a large amplitude motion associated with the methyl top. State-of-the-art millimeter-wave spectroscopy experiments coupled to quantum chemical calculations have been performed. For the most stable s-trans conformer of atmospheric interest, the torsional and rotational structures have been characterized for the ground state, the first excited methyl torsional state (ν27), and the first excited skeletal torsional state (ν26). The inverse sequence of A and E tunneling sub-states as well as anomalous A-E splittings observed for the rotational lines of v26 = 1 state clearly indicates a coupling between methyl torsion and skeletal torsion. A comprehensive set of molecular parameters has been obtained. The far infrared spectrum of Durig et al. [Spectrochim. Acta, Part A 42, 89-103 (1986)] was reproduced, and a Fermi interaction between ν25 and 2ν27 was evidenced.

Analysis of the pure-rotational spectrum in the ν28 = 1 excited torsional state and torsional couplings of trans-ethyl methyl ether

The trans-ethyl methyl ether molecule has three low-lying torsional modes, that is, two inequivalent methyl internal rotations and an asymmetric skeletal torsion. The internal rotations of the CCH 3 and OCH 3 methyl rotors and the skeletal torsion correspond to the vibrational modes, ν 28, ν 29 and ν 30 respectively. In this study, the microwave absorption spectrum in the ν 28 = 1 CCH 3 torsional state was analyzed for the first time. Nine hundred fifty seven lines up to J = 48 and K = 4 were assigned, and the rotational, centrifugal distortion and internal rotational tunneling parameters were determined with the use of a tunneling matrix formalism. By combining the present results on the ν 28 = 1 torsional state with those for the ν 30 = 1 skeletal torsional state and the ν 29 = 1 OCH 3 torsional state, torsional couplings are estimated in order to understand quantitatively the inverted A/E sequence patterns observed for those three excited torsional states.

Prevention of severe contractures might replace multilevel surgery in cerebral palsy: results of a population-based health care programme and new techniques to reduce spasticity

During the 1990s three new techniques to reduce spasticity and dystonia in children with cerebral palsy (CP) were introduced in southern Sweden: selective dorsal rhizotomy, continuous intrathecal baclofen infusion and botulinum toxin treatment. In 1994 a CP register and a health care programme, aimed to prevent hip dislocation and severe contractures, were initiated in the area. The total population of children with CP born 1990-1991, 1992-1993 and 1994-1995 was evaluated and compared at 8 years of age. In non-ambulant children the passive range of motion in hip, knee and ankle improved significantly from the first to the later age groups. Ambulant children had similar range of motion in the three age groups, with almost no severe contractures. The proportion of children treated with orthopaedic surgery for contracture or skeletal torsion deformity decreased from 40 to 15% (P = 0.0019). One-fifth of the children with spastic diplegia had been treated with selective dorsal rhizotomy. One-third of the children born 1994-1995 had been treated with botulinum toxin before 8 years of age. With early treatment of spasticity, early non-operative treatment of contracture and prevention of hip dislocation, the need for orthopaedic surgery for contracture or torsion deformity is reduced, and the need for multilevel procedures seems to be eliminated.

Conformational equilibrium and potential energy surface of 1-fluorobutane by microwave spectroscopy and Ab initio calculations

The rotational spectra of four (GT, TT, TG, and GG) of the five possible conformers of 1-fluorobutane have been assigned by combining free jet and conventional microwave spectroscopy. The geometry optimization was performed at the MP2 (full) level of theory with the 6-31G (d) and 6-311G (d, p) basis sets and by using the B3LYP (3df, 3pd) density functional method. The relative stability of the five rotamers is calculated at the QCISD (T)/6-311G (d, p) level of theory. In spite of the fact that ab initio calculations indicated the unobserved GG' conformer to be more stable than at least one of the observed conformers it was not possible to detect its rotational spectrum. GT and TG are the most and the least stable species, respectively. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. The overall conformational equilibrium is governed by the two-dimensional potential energy surface of the skeletal torsions MeC-CC and FC-CC, which have been evaluated by a flexible model analysis, based on the experimental values of the relative conformational and vibrational energy spacings, and on the shifts of second moments of inertia upon conformational change and vibrational excitation. The relative energy of the fifth stable conformer (GG') was determined to be 333 cm(-1) from flexible model calculations, and to be 271 cm(-1) from the most accurate ab initio calculations.

Molecular Dynamics of Atactic Polypropylene Melts

A molecular model suitable for predicting thermodynamic and dynamical properties of polypropylene is developed. A systematic effort is undertaken to improve the representation of the potential. Molecular dynamics (MD) simulations of atactic polypropylene, based on the new potential, indicate good agreement of volumetric properties and of the solubility parameter with experimental data for a wide range of temperatures and pressures. To characterize the system dynamics at the segmental level, time autocorrelation functions have been computed for pendant (C−CH3) bonds and skeletal torsion angles. Predicted correlation times are in good agreement with NMR measurements. An analysis of conformational transitions reveals the development of spatial heterogeneity in the segmental mobility at constant observation time; this heterogeneity increases strongly with decreasing temperature. The rate of conformational isomerizations in the polymer has been analyzed by constructing a hazard plot of first passage times, whi...

Dynamics of Crystalline N-Methylacetamide: Temperature Dependence of Infrared and Inelastic Neutron Scattering Spectra

N-methylacetamide may be viewed as a simple model for polypeptides. Unusual intensities have previously been observed in its amide vibrational spectra and were attributed to contradictory causes, namely, Fermi resonance, vibrational polarons, and dynamic proton transfer along the H bond. We report the first inelastic neutron scattering study of three specifically deuterated derivatives at different temperatures along with new infrared spectra. The analysis of the librations in the INS spectra indicates a degeneracy between the N−CH3 methyl torsional mode and another degree of freedom assigned to one component of the skeletal torsion. No direct evidence for a methyl−methyl coupling is observed. However γ(CO) appears strongly coupled to the motions of the neighboring C−CH3 group. A new absorption band (“X”) is observed at approximately 1590 cm-1 in the IR spectrum of the amide-1/amide-2 region. This calls into question the previous assignment of a band at ∼1600 cm-1 as the overtone of γ(NH). In addition, all observed peaks in the amide-2 region shift to higher frequency on cooling so that none of them could be assigned to the ν(NH) mode, as previously proposed. Previous theoretical explanations of the anomalous amide spectra of N-methylacetamide based on either Fermi resonance or the proton-transfer model are therefore in disagreement with our new experimental data.

Conformational equilibrium and potential-energy surface of 1-chlorobutane by microwave spectroscopy and ab initio calculations

Ab initio calculations proved to be very useful in the assignment of the microwave spectrum of a fourth conformer (GG) of jet cooled 1-chlorobutane. Its energy has been calculated to be 140(50) cm -1 above the most stable conformer, TT. The rotational spectra of several vibrational satellites of the four conformers have been studied by conventional microwave spectroscopy. Using the relative conformational and vibrational energy spacings, and the shifts of second moments of inertia upon conformational change and vibrational excitation, the two-dimensional potential-energy surface of the skeletal torsions has been evaluated by a two-dimensional flexible model analysis. The relative energy of the fifth stable conformer (GG′) was found to be 475 cm -1 from flexible model calculations, and 730 cm -1 from the most accurate ab initio calculations. The high value of the energy probably prevented the observation of the rotational spectrum of this conformer.

Conformational characteristics of polystyrene

AbstractConformational energies, computed with a forcefield including coulombic interactions and a simple accounting for the effects of solvent, of meso and racemic 2,4‐diphenylpentane as model substances of polystyrene have been computed as functions of the skeletal torsion angles and the phenyl torsion angles. The relatively high energies of the ḡ conformations rendered these states negligible and no minimum was found in the meso‐tt domain. Three minima for the meso diad (gt, tg, gg) and four minima for the racemo diad (tt, tg, gt, gg) are relevant. A two‐state rotational isomeric state model is applicable with states at φt = 5°C and φg = 110° for both meso and racemo diads. Statistical weight matrices have been derived and values predicted for the characteristic ratio, the fractions of 2,4‐diphenylpentane and 2,4,6‐triphenylheptane at stereo‐chemical equilibrium and the vicinal NMR coupling constants are found to be consistent with experimental results.

Infrared and Raman spectra, conformational stability, vibrational assignment and normal coordinate analysis of methyltrifluoromethyldiselenide and bis(trifluoromethyl)diselenide

The IR and Raman (3500-35 cm −1) spectra of gaseous and solid methyltrifluoromethyldiselenide, CH 3SeSeCF 3, and bis (trifluoromethyl)diselenide, (CF 3) 2Se 2, have been recorded. Additionally, the Raman spectra of the neat liquids have been obtained and qualitative depolarization values have been measured. These data are interpreted, for both molecules, on the basis that the CSeSeC dihedral angle is approximately 90° and only a single conformer is present in all three physical states for both molecules. Vibrational assignments are proposed which nare supported by normal coordinate calculations utilizing modified valence force fields to obtain the frequencies for the normal modes and potential energy distributions. The SeSe skeletal torsions for methyltrifluoromethyldiselenide and bis (trifluoromethyl)diselenide have been observed at 69.5 and 49.5 cm −1, respectively, in both the Raman and far-IR spectra of the gas. These results are compared to the corresponding quantities for some related molecules.

ChemInform Abstract: Theoretical and Experimental Study of the Non‐s‐cis Form of Unsaturated Ethers: Molecular Structure and Vibrational Assignment of cis‐Methyl‐1‐propenylether

Abstract Ab initio calculations of the internal potential energy function of cis -methyl-1-propenylether, with complete optimization of the structural parameters, have been performed at the SCF level using the 6–31G and 6–31G* bases. By keeping the SCF-optimized geometries, electron correlation was accounted for using the MP2 Moller-Plesset perturbation theory. The most stable isomer was found to occur in a wide double-minimum potential well at a torsional angle o = 152° about the C( sp 2 )O bond; the interconversion barrier between the two skew enantiomers was predicted as low as 53 cm −1 . The s- cis form of the molecule (o=0°) appeared at a relative energy of 1286 cm −1 , in a shallow potential well, only 135 cm −1 deep. The Raman spectra of the solid and liquid phases and the complete IR spectrum of the vapour phase have been recorded. They yield evidence that the molecule occurs in a single, non-planar isomer. An assignment of the vibrational spectrum has been made from the harmonic force constants derived at the SCF 6–31G* level; the calculated frequencies have been corrected by transferring scale factors from propene, methylvinylether and methylisopropenylether, for which parallel calculations of the same accuracy have been made. The average difference between observed and predicted frequencies amounted to 5.7 cm −1 . Structureless gas-phase IR bands observed at 100 and 58 cm −1 are assigned to the skeletal torsion vibration. In a model calculation using the one-vibration approximation and the computed MP2 potential only two closely spaced levels, corresponding to a genuine skew conformation, appear below the interconversion barrier; the 1←0 and 2←0 torsional transitions have frequencies of 40 and 72 cm −1 , while single- and double-jump transitions between higher levels converge to frequencies of about 45 and 90 cm −1 , respectively.

ChemInform Abstract: Microwave Spectrum and Torsional Potential Energy Surfaces of cis‐Crotyl Alcohol

Abstract The microwave spectrum of cis-crotyl alcohol is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen-bonded conformation having the oxygen atom skew with respect to the CC and the hydroxylic hydrogen gauche with respect to the CC bond. The valence angles CCC and OCC are a few degrees larger than in trans-crotyl alcohol. From measurements of Stark lobes displacements the electric dipole moment has also been determined. Analysis of the A-E doublings in the ground and some torsionally excited states allowed us to infer the potential energy surfaces of the methyl and skeletal torsions and to determine kinetic and potential energy interaction between the two motions through the use of a two-dimensional flexible model.

Theoretical and experimental study of the non-S- cis form of unsaturated ethers: Molecular structure and vibrational assignment of cis-methyl-1-propenylether

Ab initio calculations of the internal potential energy function of cis-methyl-1-propenylether, with complete optimization of the structural parameters, have been performed at the SCF level using the 6–31G and 6–31G* bases. By keeping the SCF-optimized geometries, electron correlation was accounted for using the MP2 Møller-Plesset perturbation theory. The most stable isomer was found to occur in a wide double-minimum potential well at a torsional angle ø = 152° about the C( sp 2)O bond; the interconversion barrier between the two skew enantiomers was predicted as low as 53 cm −1. The s- cis form of the molecule (ø=0°) appeared at a relative energy of 1286 cm −1, in a shallow potential well, only 135 cm −1 deep. The Raman spectra of the solid and liquid phases and the complete IR spectrum of the vapour phase have been recorded. They yield evidence that the molecule occurs in a single, non-planar isomer. An assignment of the vibrational spectrum has been made from the harmonic force constants derived at the SCF 6–31G* level; the calculated frequencies have been corrected by transferring scale factors from propene, methylvinylether and methylisopropenylether, for which parallel calculations of the same accuracy have been made. The average difference between observed and predicted frequencies amounted to 5.7 cm −1. Structureless gas-phase IR bands observed at 100 and 58 cm −1 are assigned to the skeletal torsion vibration. In a model calculation using the one-vibration approximation and the computed MP2 potential only two closely spaced levels, corresponding to a genuine skew conformation, appear below the interconversion barrier; the 1←0 and 2←0 torsional transitions have frequencies of 40 and 72 cm −1, while single- and double-jump transitions between higher levels converge to frequencies of about 45 and 90 cm −1, respectively.

Microwave spectrum and torsional potential energy surfaces of cis-crotyl alcohol

The microwave spectrum of cis-crotyl alcohol is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen-bonded conformation having the oxygen atom skew with respect to the CC and the hydroxylic hydrogen gauche with respect to the CC bond. The valence angles CCC and OCC are a few degrees larger than in trans-crotyl alcohol. From measurements of Stark lobes displacements the electric dipole moment has also been determined. Analysis of the A- E doublings in the ground and some torsionally excited states allowed us to infer the potential energy surfaces of the methyl and skeletal torsions and to determine kinetic and potential energy interaction between the two motions through the use of a two-dimensional flexible model.

Methyl and skeletal torsion interaction in normal propyl fluoride

The analysis of the microwave spectra of some excited states of the skeletal torsion in both antiperiplanar and synclinal forms of normal propyl fluoride has been extended in order to obtain the splittings (Δ0=EE -EA ) due to the methyl group internal rotation. The results have been used, together with the experimental data already available, to obtain information on the structural relaxation upon torsional excitation and conformation exchange, and to evaluate a simple set of potential energy parameters for the two torsions by applying one- and two-dimensional flexible models.

Molecular flexibility of polymethylene molecules: A Raman spectroscopic study

The Raman spectrum in the CH stretching region of molecules containing long alkyl residues shows characteristic features which provide direct information on their molecular flexibility, as probed by the C–H stretching oscillator which vibrates on the picosecond time scale. From their Raman spectra, librational motions of the (CH2)n units can be studied for molecules in the solid and liquid phases. In the latter case, segmental motions must be considered. In this paper, the spectra of hydrocarbons in the solid, liquid and as clathrates in urea and perhydrotriphenylene are studied in terms of their overall mobility. Using molecular dynamical calculations we show the existence of a selective coupling between CH stretchings and skeletal torsions, which is modulated by the collective mobility of the carbon skeleton. In particular, we account for the frequency dependence of the antisymmetic CH stretching mode using a model which allows for selective coupling between this high frequency mode and the low frequency torsional oscillations about the C–C bonds in the chain.

High resolution microwave spectrum and torsional potential energy surfaces oftrans-crotyl alcohol

The high resolution microwave spectrum of trans crotyl alcohol, CH3-CH=CH-CH2-OH, is reported for the normal and -OD isotopic species. The assigned transitions are consistent with a hydrogen bonded conformation having the oxygen atom skew with respect to the C=C bond and the hydroxylic hydrogen gauche with respect to the C-C bond. The analysis of the A-E doublings in the ground and some torsionally excited states allowed the potential energy surfaces of the methyl and skeletal torsions to be inferred and the kinetic and potential energy interaction between the two motions, through the use of a two-dimensional flexible model, to the determined. The electric dipole moment has also been determined.

2-methoxyethylamine: flexible model analysis and partial vibrational reassignment of the low-lying torsionally excited states

The analysis of the microwave spectrum of the gauche form of 2-methoxyethylamine has been extended to high J high K -1 transitions. A-E splittings due to the methyl group internal rotation have been measured in the ground state and in some torsionally excited states due to the skeletal torsions. A flexible model treatment of the microwave data led to the reassignment of some torsional satellites and to the interpretation of the ordering of the A-E sublevels.