Skeletal Torsion Research Articles

Coupled torsional and bending motions in s-c i s methyl vinyl ether

New microwave measurements on s-cis methyl vinyl ether and a study of the interactions among skeletal and methyl torsions and COC bending are reported. Using pulsed microwave Fourier transform spectroscopy, the small methyl torsional A–E splitting (0.16 MHz) in the vibrational ground state has been resolved for the first time. Large splittings are observed in the first excited states of skeletal torsion and COC bending. Ab initio results on the torsional coupling, allowing for structural relaxation, are reported and used to specify, in part, a flexible model for the torsional and bending motions. The spectroscopic properties of this three-dimensional model sytem, as estimated from the results for the one-dimensional and two-dimensional subsystems, explain the relevant experimental data. The gearing type torsional interaction predicted ab initio is confirmed by this treatment. The adjusted potential function as well as the structural relaxations upon torsion suggest repulsive interaction between the methyl hydrogen atoms and the methylenic hydrogen atom next to the methyl group.

Torsional interactions in methyl vinyl ketone

Splittings between A and E sublevels in methyl vinyl ketone (arising from the internal rotation of the methyl group) in the ground and first excited states following methyl and skeletal torsions, and variations of the rotational constants with respect to the ground state, have been measured. A flexible model analysis of these and of previously available far infrared data yields information on the interaction between the two torsions, and partial information on structural relaxation.

Application of a flexible model to the analysis of the methyl group A-E doubling, as obtained from the microwave spectrum, in several torsionally excited states of 2-methoxyethanol

From the A-E doubling measured in several rotational transitions, the A-E splittings for J = 0 have been accurately determined in the ground and various torsionally excited states belonging to the methyl and to the two skeletal torsions. The inversion of the sequence of A and E type levels with respect to the ground state, in various excited states of the two skeletal torsions, proves their mixing with the first excited state of the methyl torsion. The observed splittings of the ground and 5 excited states have been accounted for by two-dimensional flexible models with simple potential functions. The variations of the rotational constants upon torsional excitation have been reproduced by allowing a partial structural relaxation to take place along with the torsional motions.

Hydrogen bond and torsion-torsion interaction in 2-methylallyl alcohol from the microwave spectrum

The microwave spectrum of 2-methylallyl alcohol, CH 2C(CH 3)CH 2OH, has been investigated 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. From the Stark effect measurements on several transitions the electric dipole moment has been determined. The V 3 barrier to the internal rotation of the methyl group has been obtained in first approximation from the ground state AE rotational splittings. The analysis of the AE doublings has been extended to the first excited state of the methyl group internal rotation and to the first excited state of the CC skeletal torsion. By using a two-dimensional flexible model the potential surface of the two motions has been described with the following parameters V 3 = 605(5) cm −1; V 6 = −13(6) cm −1 for the methyl torsion and k = 1.76(2) cm −1 deg −2; a = −0.0022(4) deg −1 for the skeletal torsion harmonic and cubic force constants.

The lower Rydberg states of trans-hexatriene

The one photon (optical), and two and three photon resonant multiphoton spectra of trans-hexatriene have been recorded and analyzed. Transitions to the 3s, 3px(Ag), 3pȳ(Bg), 3dȳz̄(Au), and 4s(3d) orbitals (states) have been observed. Vibrational modes ag: C=C stretch, CH2 rock, and CH and CCC deformations, au: CH and skeletal torsions, bu: C=C stretch have been observed in many of the studied Rydberg states. The assignments of the electronic states and vibrational modes are discussed.

Force fields of allylamine conformers as studied by an ab initio calculation and infrared spectroscopy

The quadratic force constants of the five possible conformers of allylamine were calculated by an ab initio MC method. The infrared spectra of this molecule in the gas phase were remeasured and analyzed on the basis of the calculated vibrational frequencies. A potential energy surface with respect to the skeletal torsions was also calculated. The relative abundances of the S +G +, ST, CT, CG, and S +G − conformers, predicted to be 44:32:16:7:1 at room temperature, are consistent with the experimental data.

Infrared and Raman spectra, vibrational assignment and normal coordinate calculations for ethylisothiocyanate- d0 and - d5

The i.r. spectra (3500-40 cm −1) of gaseous and solid ethylisothiocyanate, CH 3CH 2NCS, and ethylisothiocyanate- d 5, CD 3CD 2NCS, and the Raman spectra (3200-10 cm −1) of the liquids and solids have been recorded. Additionally, the i.r. spectrum of matrix (N 2) isolated CH 3CH 2NCS was recorded from 3500 to 200 cm −1. The vibrational spectra have been interpreted on the basis of a cis form of C s symmetry for all three physical states; however, in the spectum of the matrix isolated sample at 10 K, several of the fundamentals are doublets which may be arising from two conformers at this temperature. A vibrational assignment is given based on i.r. band contours, depolarization values, isotopic shifts and group frequencies. The assignment is supported by the Teller—Redlich product ratios. The methyl torsion was observed at 270 cm −1 in the solid phase which gives a periodic barrier of 4.7 kcal/mole. A normal coordinate calculation has been carried out utilizing a modified valence force field consisting of 15 diagonal force constants and seven interaction terms which reproduced the observed frequencies with an error of 1.45% for the - d 0 molecule and 3.40% for the - d 5 compound. Because of the large CNC angle (∼ 145°) and low barrier to internal rotation of the asymmetric rotor, the skeletal bend and torsion couple to give a very broad band with two maxima and apparent P band heads. These results are compared to corresponding studies of similar molecules.

Methyl group internal rotation A–E line splittings in several torsionally excited states of methyl glycolate and 2-methoxyethanol

The rotational spectra of several torsional satellites of methyl glycolate and 2-methoxyethanol have been investigated. The methyl barrier to internal rotation in methyl glycolate increases with the torsional quantum number of the C-C skeletal torsion, for which A–E line splittings have been measured up to v SK=3. The V 3 value determined from the A–E line splittings in the first excited state of the methyl internal rotation is nearly the same of that previously determined in the ground state. For 2-methoxyethanol the V 3 barrier as determined from the A–E line splittings in the ground state is about 20% lower than the value previously obtained from the splittings observed in the first excited state of the methyl internal rotation. The sequence of the A–E splittings in the first excited state of the O-C skeletal torsion (v SK=1) is probably reversed with respect to the ground state, while in the v SK=2 state the sequence is like that in the ground state.

ChemInform Abstract: TORSIONAL COUPLING IN PYRUVIC ACID

Abstract New assignments of rotational transitions in the first excited state of the skeletal torsion were made and showed an unexpected interchange in the sequence of energy levels for the internal rotation of the methyl group. This fact is traced back to a strong coupling between the skeletal torsion and the methyl torsion. A two dimensional flexible model for torsional motion was developed which could account for the observations. The model includes structural relaxation and torsional potential energy coupling and allowed successful prediction and further assignments of transitions in the second excited state of the skeletal torsion.

Torsional coupling in pyruvic acid

New assignments of rotational transitions in the first excited state of the skeletal torsion were made and showed an unexpected interchange in the sequence of energy levels for the internal rotation of the methyl group. This fact is traced back to a strong coupling between the skeletal torsion and the methyl torsion. A two dimensional flexible model for torsional motion was developed which could account for the observations. The model includes structural relaxation and torsional potential energy coupling and allowed successful prediction and further assignments of transitions in the second excited state of the skeletal torsion.

ChemInform Abstract: MICROWAVE SPECTRUM, STRUCTURE, DIPOLE MOMENT, AND INTERNAL ROTATION OF FLUOROMETHYL METHYL ETHER

Abstract Microwave spectra of fluoromethyl methyl ether and its 10 isotopically substituted species were measured. The r s structure of this molecule was determined from the observed moments of inertia. Structural parameters obtained for this molecule, which was in the gauche form, were compared with those of the analogous molecules. Dipole moments of the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment is 1.744 ± 0.029 D making an angle of 100°54′ with the OCH 2 bond toward the CF direction and lies in the plane whose dihedral angles with the FCO and COC planes are 114°9′ and 44°56′, respectively. The barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion using the observed splitting data of the spectra in the ground, first excited methyl torsional, and skeletal torsional states. The barrier, skeletal torsional frequency, and coupling term were determined to be V 3 = 1538 ± 40 cal/mole, ω t = 158 ± 4 cm −1 , and V s = 490 ± 500 cal/mole, respectively.

ChemInform Abstract: METHYL BARRIER TO INTERNAL ROTATION AND EVIDENCE OF TORSION‐TORSION INTERACTION IN METHYL THIOLCYANOFORMATE

Abstract The rotational spectrum of methyl thiolcyanoformate has been measured in the ground state and in four vibrationally excited states. The methyl group has the conformation syn with respect to the carbonyl group. The E component lines were assigned only in the ground state and the methyl group barrier to internal rotation has the value V 3 = 705 ± 20 cal/mole. The A species rotational transitions of the first excited state of the methyl and skeletal torsions do not follow a semirigid-rotor pattern probably because they strongly interact.

Methyl and skeletal torsion interaction in methyl thiolfluoroformate

The microwave spectrum of methyl thiolfluoroformate (FCOSCH 3) is reported for the ground state and seven vibrational satellites. The methyl group is in the syn conformation to the carbonyl group. The dipole moment components are μ a = 2.89(2) D, μ b = 0.30(8) D, and μ c = 0. Spacings of A and E levels due to methyl internal rotation are analyzed for the ground state, the first excited methyl torsional state, and the first excited skeletal torsional state. An anomalous sequence of A and E levels occurring in the latter satellite arises from torsional interaction, according to two-dimensional model calculations. Potential parameters consistent with the three observed level separations are V 3 = 304(5) cm −1, V 6 = 23(1) cm −1 for the methyl torsion and either k = 1.912 or k = 2.936 cm −1 deg −2 for the skeletal torsional force constant.

Microwave spectrum, structure, dipole moment, and internal rotation of fluoromethyl methyl ether

Microwave spectra of fluoromethyl methyl ether and its 10 isotopically substituted species were measured. The r s structure of this molecule was determined from the observed moments of inertia. Structural parameters obtained for this molecule, which was in the gauche form, were compared with those of the analogous molecules. Dipole moments of the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment is 1.744 ± 0.029 D making an angle of 100°54′ with the OCH 2 bond toward the CF direction and lies in the plane whose dihedral angles with the FCO and COC planes are 114°9′ and 44°56′, respectively. The barrier to internal rotation of the methyl group was calculated taking into account the coupling effect with the skeletal torsion using the observed splitting data of the spectra in the ground, first excited methyl torsional, and skeletal torsional states. The barrier, skeletal torsional frequency, and coupling term were determined to be V 3 = 1538 ± 40 cal/mole, ω t = 158 ± 4 cm −1, and V s = 490 ± 500 cal/mole, respectively.

Methyl barrier to internal rotation and evidence of torsion-torsion interaction in methyl thiolcyanoformate

The rotational spectrum of methyl thiolcyanoformate has been measured in the ground state and in four vibrationally excited states. The methyl group has the conformation syn with respect to the carbonyl group. The E component lines were assigned only in the ground state and the methyl group barrier to internal rotation has the value V 3 = 705 ± 20 cal/mole. The A species rotational transitions of the first excited state of the methyl and skeletal torsions do not follow a semirigid-rotor pattern probably because they strongly interact.

Low frequency skeletal vibrations and rotational isomers of solid 1,4-dibromobutane and 1,5-dibromopentane studied by neutron inelastic scattering

The neutron inelastic spectra of solid 1,4-dibromobutane and 1,5-dibromopentane have been taken at several temperatures. The observed neutron spectra are assigned to the skeletal bending and torsion modes on the basis of normal coordinate calculations, in which only intramolecular valence force fields are used. The spectral assignments are consistent with the GG conformer being the predominant species in solid 1,4-dibromobutane while the TT conformer is predominant in 1,5-dibromopentane; lesser amounts of the TT isomer in the former and the GG isomer in the latter have also been identified.

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

Microwave spectra of methylpropargylether and its nine isotopically substituted species were measured. The plausible structure of this molecule was determined from the observed moments of inertia. The r s structural parameters of the OCH 3 part of the molecule could be obtained and were compared with the corresponding parameters of the analogous molecules. The dipole moment and its direction in the molecule were determined by Stark-effect measurements. The barrier to internal rotation of the methyl group was determined from the A- E splittings of the spectra reported by K. M. Marstokk and H. Møllendal ( J. Mol. Struct. 32, 191–202 (1976) taking into account the coupling effect of the skeletal torsion.

Low frequency vibrations in solid n-butane and n-hexane by incoherent inelastic neutron scattering

Incoherent inelastic neutron scattering cross sections of solid n-butane and n-hexane have been measured at 10 K, from which the hydrogen-amplitude-weighted density of states functions, GH(ω), have been derived. A theoretical GH(ω) has been calculated for n-hexane using a model force field. Comparison of the experimental and theoretical GH(ω) leads to unambiguous assignments of the observed peaks due to methyl and skeletal torsions and a skeletal deformation vibration. For n-butane, of which the crystal structure is not known, vibrational amplitudes of hydrogen atoms calculated for a free molecule have been shown to be helpful to interpret the observed GH(ω).

ChemInform Abstract: A DOUBLE‐MINIMUM SKELETAL TORSION IN 1‐SILABICYCLO(2.2.2)OCTANE BY MICROWAVE SPECTROSCOPY

Abstract The 1-silabicyclo[2.2.2]octane molecule HSi(CH 2 CH 2 ) 3 CH was investigated by microwave spectroscopy. The observed spectra followed the symmetric-rotor pattern with the unresolved K structures, but were accompanied by many strong vibrational satellites. A series of the prominent satellites was assigned to the excited states of the skeletal torsion. The transition frequencies and the relative intensities which were measured for the satellites were used to determine the double-minimum potential function to the skeletal torsion. The height of the potential hump and the equilibrium torsional angle, which was defined as the SiCCC dihedral angle, were determined to be 606 ± 40 cal/mole and 21.3° ± 1.0°, respectively. The results that the double-minimum nature of the potential function is more pronounced in 1-silabicyclo[2.2.2]octane than in other bicyclo[2.2.2]octane derivatives studied previously are discussed in detail in terms of the internal-rotation potential around the CC and CSi bonds and the strains in the valence angles of the C and Si atoms.

Torsional interaction in 2-haloethanols: infrared data, ab initio results and treatment of a two-dimensional model

The splittings of the hydroxyl torsional absorption bands observed in the gas phase IR spectra of 2-haloethanols, XCH 2-CH 2-OH are reported for X = Cl, Br, I. The satellite absorptions, assigned to excited states of the skeletal torsion, appear on the low-frequency side of the fundamental, possibly because of weakening of the internal hydrogen bond. The torsional potential surface is investigated by calculations using a two-dimensional model based on the known experimental data for the compound with X = Cl, and by ab initio calculations for the haloethanols with X = F and X = Cl. Models based on the assumption of pure torsion and simple dipole-dipole or central force interaction terms failed to reproduce the observed data for 2-chloroethanol, but good agreement was obtained after local modification of the potential surface interpolated from the ab initio calculations. The results may help devise the phenomenological theory of potential energy needed for quantitative study of the internal hydrogen bond.