Torsional Transitions Research Articles

Vibronic analyses of the Rydberg and lower intravalence electronic transitions in thioacetone

The absorption spectrum of thioacetone, (CH3)2CS, has been surveyed over the region 700-190 nm, and five distinct absorptions have been identified. These are the spin-allowed and spin-forbidden n → π∗, Ã(1A2) ← X̃(1A1), and ã(3A2) ← X̃(1A1) transitions; the orbitally allowed π → π∗, B̃(1A1) ← X̃(1A1) transition; and the Rydberg transitions n → 4s, C̃(1B2) ← X̃(1A1), n → 4pz, D̃(1B2) ← X̃(1A1), and n → 4py, Ẽ(1A1) ← X̃(1A1). In addition, the laser phosphorescence excitation spectrum has been obtained for the ã ← X̃ transition. Partial vibrational assignments have been obtained for all of the excited electronic states. The analyses showed that the ã, Ã, and C̃ states are all planar or pseudo-planar. Of interest is the vibrational activity of the methyl torsional modes. The presence of torsional progressions in the visible absorption spectrum led to the conclusion that the CH3 groups are rotated in the ã state by 60° from the X̃ state. Observation of torsional sequence transitions rather than progressions in the C̃ ← X̃ spectrum indicates that the methyl groups do not change their configuration on excitation into the C̃ state.

Neutron and Raman scattering studies of the methyl dynamics in solid toluene and nitromethane

Solid toluene has been studied in its α phase at 5K by high resolution Inelastic Neutron Scattering (I.N.S.). The tunnel splitting of the methyl torsion group state has been found to be 0.196 cm −1 (24.3 μeV) for C 6 D 5 CH 3 and 0.0088 cm −1 (1.1 μeV) for C 6 D 5 CD 3 . Simple cosine (V 3 , V 6 ) or square well potentials are able to reproduce this splitting but the calculated first torsional transitions (∼56 cm −1 ) are then slightly higher than the value of 46.8 cm −1 deduced from the analysis of the Raman and I.N.S. spectra obtained for several isotopic derivatives in a large temperature range. The possibility of coupling of the first torsional excited states with some phonons is invoked to explain these observations. A similar Raman and I.N.S. study of solid nitromethane allows the ―CH 3 torsion to be situated at about 53 cm −1 and leads to similar conclusions Etude du toluene solide dans sa phase α a 5K par diffusion inelastique des neutrons. Eclatement de l'etat fondamental de torsion du methyle a 0,196 cm −1 pour C 6 H 5 CH 3 et 0,0088cm −1 pour C 6 D 5 CD 3 . Interpretation des resultats en supposant que les premiers niveaux excites de torsion se couplent avec des phonons. Etude similaire sur le nitromethane

A Raman spectroscopic study of the low-frequency vibrations of o-dimethoxybenzene and its methyl deuterated analogues

The low-frequency (100–400 cm−1) Raman spectra of liquid (at 300 K) and solid (at 130 K) veratrole (o-dimethoxybenzene), and its methyl deuterated analogues, have been measured. The methyl and methoxyl torsional transitions have been identified, and the corresponding rotational barriers have been determined. The interpretation of the spectra points to a conformationally mixed situation for solid veratrole, in which both planar and non-planar conformers may co-exist.

Observation and assignment of torsional transitions in FIR emission from optically pumped CH3OH

Using output coupling by reststrahlen reflection from NaCl we have observed 14 new FIR lines in the wavelength range between 30 μm and 60 μm. Two of them have been identified as torsional n=1→0 transitions in the CO stretch vibration. Together with lines previously reported in the literature they are the basis for an identification of 41 FIR lines associated with torsionally excited states.

ChemInform Abstract: VIBRATIONAL SPECTRA AND TORSIONAL ANALYSES OF CIS‐2,3‐DIMETHYLOXIRANE AND TRANS‐2,3‐DIMETHYLOXIRANE

Abstract The Raman spectra of cis -2,3-dimethyloxirane and trans -2,3-dimethyloxirane in the vapor, liquid, and polycrystalline solid phases are reported for the region between 25 and 3100 cm −1 . The IR spectra of these two compounds between 80 and 4000 cm −1 in the vapor and polycrystalline solid phases are also reported. In the IR and Raman spectra of gaseous trans -2,3-dimethyloxirane a total of eight torsional transitions have been observed. In the Raman spectrum of the cis compound in the vapor phase, four torsional transitions have been observed. From these experimental data, periodic barriers to the methyl torsional motions have been calculated to be 905 ± 7 cm −1 (2.5 kcal mol −1 ) for the trans molecule and 617 ±5 cm −1 (1.76 kcal mol −1 ) for the cis molecule. Additionally, complete vibrational assignments based on band contours, depolarization values, and group frequencies are proposed for both molecules and gas-phase thermodynamic functions have been calculated. These results are compared to the corresponding quantities for some similar molecules.

Vibrational spectra and torsional analyses of cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane

The Raman spectra of cis-2,3-dimethyloxirane and trans-2,3-dimethyloxirane in the vapor, liquid, and polycrystalline solid phases are reported for the region between 25 and 3100 cm−1. The IR spectra of these two compounds between 80 and 4000 cm−1 in the vapor and polycrystalline solid phases are also reported. In the IR and Raman spectra of gaseous trans-2,3-dimethyloxirane a total of eight torsional transitions have been observed. In the Raman spectrum of the cis compound in the vapor phase, four torsional transitions have been observed. From these experimental data, periodic barriers to the methyl torsional motions have been calculated to be 905 ± 7 cm−1 (2.5 kcal mol−1) for the trans molecule and 617 ±5 cm−1 (1.76 kcal mol−1) for the cis molecule. Additionally, complete vibrational assignments based on band contours, depolarization values, and group frequencies are proposed for both molecules and gas-phase thermodynamic functions have been calculated. These results are compared to the corresponding quantities for some similar molecules.

A Raman spectroscopic temperature study of NH3+ torsional motion as related to hydrogen bonding in the L‐alanine crystal

AbstractA Raman study of the NH3+ torsional mode in the amino acid L‐alanine has been performed. The temperature dependence of this mode has been followed in the temperature region 110–350 K. The activation energy for NH3+ reorientation obtained from a linewidth vs temperature investigation, Ea = 3100±400 cm−1, is in excellent agreement with corresponding NMR results, thus demonstrating that Raman spectroscopy can serve as a complementary method to NMR in obtaining torsional barrier heights. Based on an X‐ray investigation, crystal and hydrogen bond parameters have been calculated as a function of temperature. The increase of the NH3+ torsional barrier on going to lower temperatures is essentially attributed to the shortening of one specific hydrogen bond. The detection of the NH3+ torsional hot transition is important for the determination of the shape of the lower portion of the NH3+ torsional potential. The sixfold term V6 of the potential used, three‐plus sixfold, is thus very sensitive to the frequency difference between the NH3+ torsional fundamental and hot band. The optimum frequency fit gives, however, too large a value for the barrier height V3. The reason for this is interpreted as arising from the severe distortion of the hydrogen bonds at large torsional angles. The results presented here support recent findings that a revision of the dynamic picture of the hydrogen bonds in L‐alanine crystal, as reported in two earlier papers, is needed.

Calculation of Raman intensities for the torsional vibrations of ethyl halides

Raman intensities are calculated for the torsional vibrations of CH3CH2Cl, CH3CH2Br, CH3CH2I, CH3CHCl2, and CH3CHBr2 using an anisotropic atom-point dipole interaction model to calculate the elements of the molecular polarizability tensor. The calculated relative intensities for the members of the Δv = 2 torsional overtone progression of each of the ethyl halides are in good agreement with experiment. It is predicted that electrically anharmonic terms contribute substantially to the Raman intensities of these transitions. The Δv = 1 torsional transitions of the five molecules are predicted to be 20–30 times more intense than the overtones (although these transitions are not observed because of broadband contours and interference from other vibrational modes). Electrically anharmonic terms in the polarizability expansions also contribute substantially to the intensity of the fundamentals.

Conformational studies by far infrared and Raman spectra of gases

One of the major goals of conformational analysis is the calculation of the energy difference between two or more conformers, ΔE, as well as the energy necessary for interconversion. The calculation of these energy quantities is facilitated by using a potential function which describes the vibrational motion, or internal rotation (torsion), as a function of the dihedral angle, α. The potential function is called asymmetric because both the frame and top portions of the molecule have no symmetry element higher than a plane. The most common type of potential function where at least one of the minima coincides with the plane of symmetry is of the type: V(α) = 1 2 ΣV i(1 - cos iα). The kinetic energy term, F(α), is extremely complicated. In general, if the only data being used to calculate the potential function are torsional transitions, and if one continues within the boundary conditions of a one-dimensional problem, then a cosine expansion of F(α) should be adequate: F(α) = F 0 + ΣF i cos iα. For those systems where there is an equilibrium between a planar form and two non-planar forms, V 3 is usually the predominant term. This is because V 3 represents a three maxima/three minima potential per 2π (360°) internal rotation. In a similar fashion, V 2 is found to be the predominant term in the potential function for a system consisting of two equivalent non-planar conformers. Several examples of our most recent studies are given where the potential function for interconversion of two conformers has been determined.

Far i.r. absorption spectrum of methyl nitrite

Measurements of the far i.r. absorption spectra (between 10 and 300 cm −1) of methyl nitrite (CH 3ONO) by FTS are presented. For analysis of the data an approximate perturbation type theory of the rotation-internal rotation spectrum for the case of a very low barrier to internal rotation is developed. Application to the low frequency part (12–100 cm −1) of the far i.r. spectrum leads to proposition of an assignment of a number of transitions of the rotation internal rotation mode ν 15( a″) of s-trans methyl nitrite. The higher frequency region is shown to originate from a complicated superposition of the torsional transition of cis methyl nitrite [ν 15( a″)] and the rotation internal rotation structure of hot transitions of trans CH 3ONO.

Analysis of torsional spectra of molecules with two internal C3v rotors

Abstract The low frequency vibrational spectra of gaseous dimethylselenide and dimethylselenide-d 3 have been investigated from 50 to 500 cm −1 . A number of torsional transitions have been resolved in the far infrared spectra for both molecules and their analyses have given a torsional potential function based on a semirigid model. For the light molecule the effective barrier to internal rotation was found to be 507 ± 17 cm −1 (1450 cal mol −1 ) and the cosine-cosine coupling term, V 33 was found to be 54 ± 12 cm −1 (154 cal mol −1 ), whereas the sine-sine coupling term, V 33 , has a value of 10 10 ± 4 cm −1 (29 cal mol −1 ). An interaction between the low frequency CSeC bending mode and the (2,0) - (1,0) torsional transition is proposed as a possible explanation for the assignment of the torsional transitions for dimethylselenide-d 3 . The effective barrier for the -d 3 molecule was found to have a value of 520 ± 6 cm −1 (1487 cal mol −1 ) with a sine-sine coupling term value of −35 ± 7 cm −1 (−100 cal mol −1 ). A complete vibrational assignment is given for the dimethylselenide-d 3 molecule and a normal coordinate calculation has been carried out using a modified valence force field. These results are compared to the similar quantities in some corresponding molecules.

Analysis of torsional spectra of molecules with two internal C3v rotors. XXI. Analysis of the torsional spectra of dimethylsulfide-d0, -d3, and -d6

The Raman spectrum of gaseous dimethylsulfide-d6 has been recorded between 100–300 cm−1 with a resolution of 2 cm−1. The far infrared spectrum has been recorded from 100–250 cm−1 with a resolution of 0.25 cm−1. Considerable torsional data are reported and used to characterize the torsional potential function of (CD3)2S based on the semirigid model. Previous assignments of the torsional transitions in dimethylsulfide-d0 and -d3 have been revised. The average effective V3 for the dimethylsulfides was found to be 748 cm−1 (2139 cal/mol). A strong correlation was found between the V03 and the cosine–cosine coupling term (V33). The effective barriers obtained for all three isotopic species are comparable to the results obtained from the most recent microwave study.

An experimental and theoretical investigation of the internal rotation in anisole

The methyl torsional transitions in solid anisole (methoxybenzene) and its deuterated analogs have been studied by Raman spectroscopy at 130 K. The doublet structure of the transitions is interpreted as arising from torsional transitions in two different stable conformers. The methoxyl torsional potential is investigated with careful partial relaxation of chosen internal coordinates on the STO-3G level of accuracy. Evidence supporting the existence of two stable conformers, the locally stable orthogonal and the globally stable planar conformer, is found. The nature of the methoxyl torsional barrier is discussed taking into account the simultaneous motion of the methyl group.

Microwave, infrared, and Raman spectra, structures, quadrupole coupling constants, barrier to internal rotation, and vibrational assignment of 1-chloro-2-methylpropene

The microwave spectra of (CH3)2CCH35Cl and (CH3)2CCH37Cl have been recorded from 12.5 and 39.0 GHz. a-type transitions were observed and R-branch assignments have been made for the ground vibrational states for both isotopes. The rotational constants were found to have the following values: for (CH3)2CCH35Cl, A = 8400.77±1.18, B = 2258.18±0.02, C = 1818.72±0.02 MHz; for (CH3)2CCH37Cl, A = 8399.31±0.66, B = 2199.93±0.02, C = 1780.73±0.02 MHz. From a diagnostic least-squares adjustment to fit the six rotational constants and with reasonable assumed carbon–hydrogen distances and angles, the following heavy atom structural parameters were obtained: r(C–C) = 1.507±0.013 Å, r(C = C) = 1.355±0.019 Å, r(C–C1) = 1.750±0.019 Å, ∢C–C = C(cis) = 122.20°±4.50°, ∢C–C = C(trans) = 119.39°±1.84°, and ∢CCCl = 124.24°±2.79°. The quadrupole coupling constants for the (CH3)2CCH35Cl molecule were found to have the following values: χaa = −49.2, χbb = 19.5, and χcc = 29.7 MHz. From an analysis of the internal rotational splittings, the threefold barrier for the cis methyl group was found to be 288±5 cm−1 (0.824 kcal/mole). The infrared (3500–50 cm−1) and Raman spectra (3500–10 cm−1) have been recorded for both the gas and solid states. Additionally, the Raman spectrum of the liquid was recorded and qualitative depolarization values were obtained. All of the normal modes have been assigned based on band contours, depolarization values, and group frequencies. Several torsional transitions for the trans methyl group have been observed in the far infrared spectrum of the vapor, and from these transitions a barrier of 745±60 cm−1 (2.14 kcal/mole) has been calculated for the internal rotation of the trans methyl group. These results are compared to the corresponding quantities in similar molecules.

Depolarized light scattering from macromolecules: effects of torsional oscillations, conformational transitions, and overall rotations

Depolarized light scattering from macromolecules: effects of torsional oscillations, conformational transitions, and overall rotations

A kinetic theory of the temperature dependence of methyl tunnelling spectra

The spectral density function associated with methyl group reorientation at low temperatures is relatively complicated as a result of quantum tunnelling motion, and due to the existence of three proton spin symmetry species. To account for different aspects of the observed temperature dependence of this spectral density function, two models have been proposed (by J. Haupt, 1971 and by P.S. Allen, 1974) both of which assume that the spectral density is modified by thermally excited transitions between different methyl group torsional levels. The models are incompatible in that the torsional transitions of different spin symmetry species are uncorrelated in one case (Haupt) and correlated in the other (Allen). Both models fail in that each explains only one of the two main features of the spectrum. In the present discussion it is shown that the torsional states are mixed by thermal fluctuations in a way which cannot be described adequately by torsional transitions. A description of the mixing is given which yields an explanation similar to that of Allen for the temperature dependence of the tunnel frequency, but since the evolution of mixed torsional states belonging to different spin symmetry species may be essentially uncorrelated, a broadening similar to that invoked by Haupt is also obtained.

The vibrational analysis and torsional study of trans-1,2-dimethylcyclopropane and cis-1,2-dimethylcyclopropane

The infrared spectra of cis-1,2-dimethylcyclopropane and trans-1,2-dimethylcyclopropane have been recorded between 4000 and 200 cm −1 in the polycrystalline solid phase, and 4000 to 80 cm −1 in the gas phase. The Raman spectra of these two compounds in the gaseous and liquid phases were also recorded between 3100 and 10 cm −1. An assignment of the thirty-nine fundamental vibrations for both cis- and trans-1,2-dimethylcyclopropane is proposed, and comparisons are made with the vibrations of other similar molecules. Additionally, ten torsional transitions were observed in the far infrared and Raman spectra of cis-1,2-dimethylcyclopropane, and four transitions were observed in the spectra of the trans compound. From these spectral data, torsional barriers were determined. The effective barriers to methyl torsion are 2.92 kcal mol −1 (12.20 kJ mol −1) for cis-1,2-dimethylcyclopropane and 2.61 kcal mol −1 (11.14 kJ mol −1) for trans-1,2-dimethylcyclopronane.

Vibrational spectra of crystalline disilane and disilane-d6, barrier to internal rotation and some normal coordinate calculations on H3SiSiH3, H3SiNCO, and H3SiNCS

The Raman spectra of gaseous disilane and disilane-d6 as well as the Raman and infrared spectra of their crystalline solids at 20 K have been recorded from 25 to 3500 cm−1. Two quantum torsional transitions of the SiH3 moiety were observed at 233 and 251 cm−1 in the Raman spectrum of gaseous disilane. A barrier to internal rotation of 439±10 cm−1 (1.26 kcal/mol) has been calculated based on these frequencies. The crystalline modification of disilane at 20 K was shown to belong to the monoclinic space group P21/c≡C2h5. There are two molecules per unit cell each of which sits on identical Ci sites. The results of normal coordiante analyses of H3SiSiH3, H3SiNCO, and H3SiNCS are presented and the force fields obtained for the H3Si moieties are compared.

Laser-Stark spectrum of methyl alcohol at 890.761 GHz with the HCN laser

The laser-Stark spectrum of methanol has been studied up to Stark fields of about 55 kV/cm with the 890.761-GHz HCN laser, in both parallel and perpendicular polarizations. Extensive series of absorption lines have been assigned to the J = 6 to 15 members of the k = 6 ← 5 Q branch of the v = 0 E torsional state of CH 3OH. Also, 21 M components of the J k = 7 1 ← 6 2 A ± doublet in the v = 1 excited torsional state have been identified. Efforts to analyze the Q-branch spectra in terms of dipolar parameters and zero-field frequencies were hampered by very high statistical correlations in the least-squares fits. However, it proved necessary to permit the upper-state value of μ a to exceed that of the lower state, by 0.0014 D, and to introduce J dependence of μ a . Analysis of the Stark shifts of the excited torsional state transition yielded a μ a of 0.93 D, about 0.04 D greater than the mean value for the ground state.

Low frequency vibrational spectra and internal rotation of 2-chlorobuta-1,3-diene, propenoyl fluoride, and propenoyl chloride

The infrared and Raman spectra of gaseous 2-chlorobuta-1,3-diene, propenoyl fluoride, and propenoyl chloride have been recorded below 300 cm−1. It was observed that the low frequency spectra of these three molecules were quite similar. The infrared spectrum of 2-chlorobuta-1,3-diene exhibited a series of sharp torsional transitions whereas in the Raman spectrum a B-type band was observed. These torsional frequencies have been assigned to the s-trans conformer and no evidence for a higher energy conformer was observed. In the infrared and Raman spectra of the propenoyl halides, bands assigned to the torsional series of both conformers have been observed. The analyses of these spectra were simplified by comparison with that of the 2-chlorobuta-1,3-diene. Asymmetric potential functions and values for ΔH for propenoyl fluoride and propenoyl chloride were calculated. The enthalpy differences of 36 and 215 cm−1 between the lowest energy levels of the s-trans and s-cis wells for the propenoyl fluoride and chloride, respectively, were obtained.