Torsional Structure Research Articles

Deuteron NMR spectra of ND 4ClO 4 single crystal at low temperatures

2H NMR spectra of ND 4ClO 4 single crystal were obtained at ν 0 = 44 MHz. Orientation and temperature (1.9–75 K) dependences were measured. Fitting the spectra gives the effective quadrupole coupling constants for all deuterons and the ground torsional level structure. The isotope reduction of the (A-T) and (A-E) tunnelling splittings, i.e., the ratios of the respective splittings for NH 4 + and ND 4 +, were found to be different. The splittings at T = 24 K are about 60% of the helium temperature values. The spectrum undergoes intermediate narrowing by reorientations between 26 and 34 K and tunnelling related features in the spectra are eradicated. After reaching the extreme narrowing limit, a doublet with gradually decreasing separation was observed, what was attributed to averaging by torsional oscillations of increasing amplitude. At high temperatures ( T > 75 K), the narrow spectrum reflects fast multiaxial reorientation of the ammonium ion.

CF 3 torsional potentials in the S 0 and S 1 electronic states of 3-aminobenzotrifluoride

Torsional structure in the supersonic jet spectra of 3-aminobenzotrifluoride has been interpreted. The CF 3 torsional potentials are surprisingly similar to the CH 3 potentials in 4-toluidine: there is a low barrier with a minimum near φ = 25° in S 0, and a higher barrier with an eclipsed conformation in S 1. However, because of the greater mass of the CF 3 group, the spectra are quite different. Potential constants, in cm −1, are: V″ 3 = 9 ± 5, V″ 6 = −1 10 ± 2, V′ 3 = 155 ± 5, and V′ 6 = −40 ± 2, for F″ = F′ = 0.29 ± 0.01 cm −1.

Methyl torsional fine structure in the high-resolution S→T(nπ*) excitation spectrum of acetophenone and its three methyl-deuterated isotopomers

The cold-beam excitation spectrum of the S0→T(nπ*) origin band of acetophenone shows two maxima separated by about 1.2 cm−1, interpreted as due to methyl tunneling. Consecutive deuteration of the methyl group leads to three maxima in acetophenone-d1, two maxima in acetophenone-d2 and a single broad maximum in acetophenone-d3, with maximum separations of about 7 cm−1 in -d1 and about 3.5 cm−1 in -d2. These separations are due to the occurrence of two rotamers for isotopomers without threefold permutation symmetry. The spectra are analyzed in terms of hindered rotor potentials. It is shown that S0 and T(nπ*) share the same torsional equilibrium configuration, relative to which the S1(nπ*) equilibrium configuration is rotated by 60°. However, both nπ* states have very shallow torsional potentials with barrier heights of 70 and 90 cm−1 for T and S, respectively, compared with a barrier of about 800 cm−1 in the ground state. Hence, relative to S0 the two potentials are quite similar despite the shift. The potential terms entering upon partial deuteration are quite similar for S0 and T(nπ*) and hence depend only weakly on the barrier height. They are ascribed to kinetic coupling of the asymmetric rotor of the CH2D or CHD2 group with other modes in the molecule.

CF 3 torsional potentials and bending—torsion interaction in the S 0 and S 1 electronic states of 4-aminobenzotrifluoride

Torsional structure in supersonic-jet fluorescence spectra of 4-aminobenzotrifluoride has been interpreted. The sixfold CF 3 torsional barriers are ⩽ 5 cm −1 in S 0 and 33±4 cm −1 in S 1, similar to those in 4-substituted toluenes, with the same equilibrium conformation, probably staggered, in both states. However, because of the larger mass of the CF 3 rotor, there are significant differences between the spectra. Activity of CCF 3 bending modes is explained by a novel mechanism involving cross-sequence and combination bands which combine bending and torsional intervals of appropriate symmetry, and provides evidence for significant interaction between CCF 3 bending and torsional motions.

Torsional band assignment and intramolecular twist potential of 9,9′-bianthryl and its 10-cyano derivative in a free jet

Using a pulsed supersonic beam and laser-induced fluorescence spectroscopy the torsional structure of the S0 and S1 states of 9,9′-bianthryl (BA) and its derivative 10-(cyano)-9,9′-bianthryl (CBA) was investigated. Taking into account the very different equilibrium positions of the S0 and S1 potentials which result in a nonobservable 0–0 transition within the jet spectra, a new and straightforward procedure of torsional band assignment is carried out. This is based on a characteristic pattern of Franck–Condon factors within the dispersed fluorescence spectra. The torsional potentials were determined by a fit procedure of a one-dimensional model to the experimental data. The results show that the S1 double minimum potential for BA is shallower than for CBA indicating a stronger interaction between the molecular halfs of the latter compound. The observed rotational contours of torsional bands recorded for CBA reflect the change from a symmetric top molecule (for states above the S1 torsional barrier) to an asymmetric top (for states below the barrier) and manifest the tunneling splitting of the level just below the barrier. The dispersed fluorescence spectra of CBA are discussed in terms of intramolecular vibrational redistribution (IVR) processes. The measured fluorescence decay rates as a function of excess vibrational energy of CBA reflect a saturation behavior already within the origin region in contrast to BA (saturation near 380 cm−1). This is tentatively ascribed to a low lying dark background state possibly of charge transfer character.

Methyl Internal Rotation Fine Structure in the Ground State Rotational Spectrum of Gauche Butane

Abstract The methyl torsional fine structure in the rotational spectrum of gauche butane in the vibrational ground state was investigated in the frequency range between 10 and 141 GHz. Using the internal axis method (IAM) in the formulation of Woods, all internal rotation parameters were determined with high accuracy. The barrier height of the methyl internal rotation was determined to 11.34 (29) kJ/mol (2.710 (69) kcal/mol)

The spectroscopy, photophysics and photodissociation dynamics of jet-cooled CF3NO[Ã(n,π*)

The LIF excitation spectrum of jet-cooled CF3NO in the visible has been reinterpreted and the torsional structure has been analysed in detail yielding a V′3 barrier to internal rotation of 601.5 cm–1. The predissociation dynamics of jet-cooled CF3NO(A1A″) have been re-examined using single vibranic level excitation with a pulsed dye laser and fluorescence decay measurements. Quantum beats and biexponential fluorescence decay behaviour indicate that all levels of the S1(n,π*) state are coupled to levels of the T1 state. The NO photofragment yield spectrum indicates that vibrations with CNO bending character act as promoting modes for the radiationless transition out of S1. Rotational distributions of the nascent NO photoproduct have been characterised in detail and are found to deviate little from statistical distributions, but the F2/F1 spin-orbit branching ratio is very non-statistical, favouring the lower 2Π1/2 state of NO. The threshold for the dissociation of CF3NO(D0) and the electronic origin of the Aâ†�X transition (T00) have been redetermined and are found to coincide (D0= 13 980 ± 60 cm–1, T00= 13 929.7 cm–1). Intersystem crossing and the T1 state are found to be important in the predissociation dynamics of all but the lowest A-state levels.

Fourier-transform electronic spectroscopy in the near-infrared: Torsional barrier of the Ā 1A″ state of nitrosobenzene

The Fourier-transform absorption spectrum of nitrosobenzene vapour in the near-infrared has been recorded, and the vibrational bands near the 869 nm electronic origin of the Ā 1A″ ← X̄ 1A′ (n,π *) transition have been re-analysed. The prominent torsional structure observed is shown to represent sequence transitions in the torsional mode. Electronic excitation leads to an increase of the V 2 barrier hindering internal rotation from roughly 1300 cm −1 in the ground to 2500 cm −1 in the (n,π *) excited state.

Determination of a High Barrier Hindering Internal Rotation from the Ground State Spectrum. The Methylbarrier of Propane

The internal rotation barrier V3, the moment of inertia Iα of the methyl tops and the angle between the two internal rotation axes were determined from the torsional fine structure of the rotational spectrum in the torsional ground state. A tilt angle of 1.4° of the methyl groups toward each other results.

Centrifugal Distortion and Ground State Internal Rotation Analysis of the Microwave Spectrum of Dimethylsulfoxide

The torsional Fine structure of dimethylsulfoxide has been reinvestigated with MWFT-spectroscopy. The higher resolution results in a more accurate determination of the internal rotation barrier V3. The centrifugal distortion analysis is extended to sixth order. The prediction of lines is excellent, but some centrifugal distortion constants are poorly determined

Deuteron NMR lineshape computations for tunnelling ND4+ions. II. Exact treatment of quadrupolar interactions

For pt.I see ibid., vol.11, p.4187 (1978). Effects of tunnelling rotation of ND4+ ions on 2H-NMR lines are analysed. Detailed lineshape calculations are supplied. All terms in the quadrupolar Hamiltonian and secular dipolar interaction between deuterons are taken into account. Asymmetry and level crossing are characteristic features observed in the lineshapes. The latter effect makes measurement of the tunnelling frequency possible. Sets of theoretical lineshapes are presented showing their evolution with tunnelling splitting and with magnetic field. The main conclusion is that complete interpretation of a given ammonium compound (ground torsional level structure and tunnelling splitting value) requires measurement of the field dependence of the 2H-NMR lineshape.

Proton magnetic resonance study of classical reorientations and tunneling in LiNH4SO4

Nuclear magnetic resonance studies have been made of the protons and lithiums in LiNH4SO4 at temperatures below 285 K. The proton line shape at 4.2 K has shown that tunneling rotation of the NH+4 ion around one of its C2 axes is favored over the others. Additional tunneling around threefold axes is also present. The ground torsional level structure of the NH+4 ion has been determined. The classical reorientations of the two nonequivalent NH+4 groups can be described as simultaneous two and threefold reorientations with estimated activation energies ranging from 1.0 to 1.6 kcal/mol. Slight differences in activation energies have been observed during cooling and heating cycles below 133 K. The 7Li relaxation time is determined to a large extent by the reorientations of the NH+4 ions. Evidence has been found that the phase transition at 133 K is accompanied by a slight rearrangement of the Li–O–S skeleton.

Torsional Splittings and Assignments of the Doppler-Limited Spectrum of Ethane in the C-H Stretching Region.

The Doppler-limited absorption spectrum of the C-H stretching region of ethane has been recorded at T≃ 119 K with a tunable difference-frequency laser spectrometer. The strong torsional hot band structure at room temperature is eliminated at 119 K, and the enhanced resolution from the Doppler width reduction allows us to observe small torsional splittings. The two fundamentals in the region, v 7, a perpendicular band and, v 5, a parallel band have been essentially completely assigned as have a large number of transitions in the parallel component of the v 8 + v 11 combination band. A number of perturbations of both global and local nature have been observed. The complete spectrum and a listing of transition wavenumbers, intensities and assignments are presented here to facilitate identification and quantitative analysis of ethane in a variety of monitoring applications. Precise ground state rotational constants have been determined from combination differences.

The rotation-torsion contour of the 2668 Å origin band of toluene. High-resolution 295 K absorption spectrum

The 2668 Å vibrational origin band of toluene was measured and analyzed using an asymmetric-top/free-internal-rotor model. The change in the methyl group geometry between the ground ( e A′ 1) and the excited e A″ 1 ( ππ ∗ ) state was found to be slight. Rough values found for the changes in the phenyl frame rotational constants are similar to those for other monosubstituted benzenes with substituents lacking lone-pair electrons. The torsional structure in the toluene origin band is blue-degraded whereas the rotational structure in each Δ m = 0 torsional manifold of transitions is red-degraded. Deconvolution of the torsional structure in the calculated contour shows that strong R-branch heads for transitions emanating from each torsional level of the ground state persist in the experimental contour for 0 ≤ | m| ≤ 12.

Torsional structure in i.r. bands of propenoyl chloride

Torsional structure in i.r. bands of propenoyl chloride

Torsional Effect in Soil-Structure Interaction

A simplified method to evaluate the dynamic response of a building, including the wave propagation effects of ground motion is described. This method is based on a computer code that can calculate the dynamic response of buildings subject to multisupport excitation given impedance data calculated by other methods. Only linear systems and symmetrical structure are considered. The soil-structure interaction effect is accounted for by including in the dynamic model, the compliance functions of the soil medium. The computer capability required is widely available within the nuclear industry. The study concluded that within the assumptions made and analytical models considered, the practical solution scheme adopted in this study by applying earthquake input motion sequentially at nodal points on the foundations seems to be simple to use in the investigation of translational and torsional structure responses subjected to the traveling wave effect of seismic ground motion. Also, the soil-structure interaction effect may be important in calculating translational and torsional responses due to the traveling wave effect of earthquake excitation.

PMR spectroscopy of the tunneling ammonium ion: Generalized line shape theory and some experimental examples

The site symmetry of the NH4+ ion is low in many crystal structures of ammonium compounds. This renders inadequate the commonly considered theoretical models of the tunnel-split ground librational state of NH4+, which are based on a perfectly tetrahedral crystal field due to assumption of equivalent threefold, and neglect of twofold, axes of symmetry. A general treatment of the NMR line shape theory is therefore considered and Hamiltonian matrix elements for tunneling are explicitly calculated in terms of the seven overlap-integral parameters related to the four C3– and three C2–symmetry axes of a tetrahedron. The model allows us to discuss all possible ground torsional level structure of the tunneling ammonium ion. The proton NMR line shapes are calculated for all representative cases, and the results are used in comparison with experimental data to elucidate the ground torsional level structures of NH4VO3, (NH4)2ZrF6, (HN4)2TiF6, (NH4)2TeO4, and (NH4) IO4.

Microwave Spectra of Dimethylether (CH3)2O and (CD3)2O in Excited Torsional States

Abstract The rotational spectra of the torsional excited states v̅n = 11,12,21, 22, 23 have been measured and assigned. The torsional fine structure was used to determine the coefficients V3 and V' 12 of the hindering potential. For (CH3)2O : V3 = 2618 ± 4 cal/mole, V'12 = 18 ± 8 cal/mole, for (CD3)2O : V3 = 2572 ± 6 cal/mole, V'12 = 29 ± 12 cal/mole, V12 was correlated to V3 .

Proton magnetic resonance study of ammonium ion tunnelling and its ground torsional level structure in NH 4ReO 4

The ground torsional level structure of NH + 4 in ammonium perrhenate (NH 4ReO 4) has been analysed by a study of its proton magnetic resonance lineshape in the polycrystalline state at 4 K. It is shown that, in this structure, tunneling rotation of NH + 4 around one of its C 2 axes is greatly favoured over the others.

Torsion-Torsion Interaction in the Microwave Spectrum of Dimethylether

Abstract We report an analysis of the torsional fine structure of rotational lines of (CH3)2O in excited torsional states leading to the potential parameters V3 and V′12 of a Hamiltonian for the overall-and internal rotation.