Asymmetric Torsional Mode Research Articles

Spectra and structure of organophosphorus compounds: Part XXVII. Infrared and Raman spectra and conformational stability of ethyldimethylphosphine sulfide

The Raman (10 to 3500 cm −1 and infrared (50 to 3500 cm −1) spectra have been recorded for solid ethyldimethylphosphine sulfide, CH 3CH 2P(S)(CH 3) 2. Additionally, the Raman spectrum of the liquid phase has been recorded and qualitative depolarization values have been obtained. From the fact that several distinct lines appear on going from the solid phase to the fluid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the liquid phase. A study of solutions of ethyldimethylphosphine sulfide in solvents of varying dielectric constants indicates that it is the gauche conformer which is more stable and the only one which remains in the solid state. Relying on group frequencies as well as relative depolarization values and relative intensities of the infrared and Raman bands, a complete vibrational assignment is proposed for the gauche conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The asymmetric torsional mode for the gauche conformer has been assigned to the broad band at 110 cm −1 in the infrared spectrum of the solid phase. All of these results are compared with corresponding quantities for several other organophosphines.

Spectra and structure of organophosphorus compounds. XXV—Raman and infrared spectra and conformational stability of ethyldimethylphosphine

AbstractThe Raman (10–3500 cm−1) and infrared (50–3500 cm−1) spectra have been recorded for gaseous and solid ethyldimethylphosphine, CH3CH2P(CH3)2. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. From the fact that several distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phase with the gauche conformer being more stable and the only one present in the spectrum of the solid. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be 134 ± 32 cm−1 (383 ± 92 cal mol−1). Relying on group frequencies and relative intensities of the infrared and Raman bands, and in some cases infrared band contours, a complete vibrational assignment is proposed for the gauche conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The CH3‐P torsions have been observed at 208 and 185 cm−1 in the gas phase and from these frequencies the periodic barriers to internal rotation have been calculated to be 905 ± 95 cm−1 (2.56 kcal mol−1). The CH3‐C torsion was also observed in the gas phase at 217 cm−1 from which a periodic barrier of 1134 cm−1 (3.22 kcal mol−1) was calculated. The asymmetric torsional mode has been observed for the gauche conformer in both the infrared and Raman spectra of the gas at 91 cm−1 with evidence of ‘hot bands’ at lower frequencies. All of these results are compared with corresponding quantities for several other organophosphines.

Far infrared and low frequency gas phase Raman spectra and conformational stability of the 1-halopropanes

The far infrared (375–50 cm−1) and low frequency Raman (400–70 cm−1) spectra of the gaseous 1-halopropanes CH3CH2CH2F, CH3CH2CH2Cl, and CH3CH2CH2Br have been recorded and both the methyl and asymmetric torsional modes have been observed and assigned for both the gauche and trans conformers for all of these molecules. The asymmetric torsions for each molecule have several observed excited states which fall on the low frequency side of the fundamental. The asymmetric torsional potential functions have been calculated and, from these potential functions, the enthalpy differences between the high energy trans and low energy gauche conformers have been determined to be 122±10 cm−1 for the fluoride, 127±10 cm−1 for the chloride, and 37±10 cm−1 for the bromide. The trans and gauche methyl torsions have also been observed and assigned for the three 1-halopropanes. The resulting barriers in cm−1 are: 936±4 (trans), 986±9 (gauche) for 1-fluoropropane; 929±2 (trans), 1080±3 (gauche) for 1-chloropropane; and 841 (trans), 1016±8 (gauche) for 1-bromopropane. A complete vibrational assignment has also been made for the 1-fluoropropane molecule and, from the spectral data for the solid, it appears that there are two or more molecules per primitive cell. Attempts to obtain experimental values for the enthalpy differences in the gas phase were made and these results, as well as the determined potential functions, are discussed in relation to previous studies.

Far infrared spectra, conformational potential function, and barrier to methyl rotation of propionyl fluoride

The far infrared spectra of propionyl fluoride CH3CH2CFO in the gaseous and solid states have been recorded from 500 to 40 cm−1. A rather complex spectrum of the gas was observed and a substantial number of bands have been assigned to the asymmetric torsional modes for both the s-cis (oxygen atom eclipsing the methyl group) and the high energy gauche conformers. Analysis of these bands permitted the calculation of the torsional potential function present in this molecule. The potential coefficients for the asymmetric torsional mode were calculated to be: V1=341±24, V2=236±20, V3=390±3, and V4=21±6 cm−1, with an enthalpy difference between the more stable s-cis and the high energy gauche conformers of 434±20 cm−1 (1.24±0.06 kcal/mol). This function gives a potential barrier of 692 cm−1 (1.98 kcal/mol) separating the s-cis from the gauche form and 283 cm−1 (0.81 kcal/mol) separating the two equivalent gauche forms. From a temperature study of the Raman spectrum of the gas, the enthalpy difference between the s-cis and gauche conformers was determined to be 486±45 cm−1 (1.39±0.13 kcal/mol) which is in excellent agreement with the value obtained from the potential function. The barrier to the methyl rotation for the s-cis conformer was calculated from the far infrared data to be 935±2 cm−1 (2.67 kcal/mol). Both the methyl torsion and CFO rock were observed as doublets in the spectrum of the solid which indicates that there are at least two molecules per primitive cell. These results are compared to the corresponding quantities in some related molecules.

ChemInform Abstract: CONFORMATIONAL ANALYSIS, BARRIERS TO INTERNAL ROTATION AND VIBRATIONAL ASSIGNMENT FOR DIMETHYLETHYLAMINE

Abstract The IR (50–3500 cm −1 ) and Raman (20–3500 cm −1 ) spectra have been recorded for gaseous and solid dimethylethylamine. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Due to the fact that three distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phases with the gauche conformer being more stable and the only one present in the spectra of the unannealed solid. From the temperature study of the Raman spectrum of the liquid a rough estimate of 3.9 kcal mol −1 has been obtained for ΔH . Relying mainly on group frequencies and relative intensities of the IR and Raman lines, a complete vibrational assignment is proposed for the gauche conformer. The potential functions for the three methyl rotors have been obtained, and the barriers to internal rotation for the two CH 3 rotors attached to the nitrogen atom have been calculated to be 3.51 and 3.43 kcal mol −1 , whereas the barrier for the CH 3 rotor of the ethyl group has been calculated to be 3.71 kcal mol −1 . The asymmetric torsional mode for the gauche conformer has been observed in both the IR and Raman spectra of the gas at 105 cm −1 with at least one hot band at a lower frequency. Since the corresponding mode has not been observed for the trans conformer, it is not possible to obtain the potential function for the asymmetric rotation although estimates on the magnitudes of some of the terms have been made. Significant changes occur in the low-frequency IR and Raman spectra of the solid with repeated annealing; several possible reasons for these changes are discussed and one possible explanation is that a conformational change is taking place in the solid where the trans form is stabilized by crystal packing forces. These results are compared to the corresponding quantities for some similar amines.

Conformational analysis, barriers to internal rotation and vibrational assignment for dimethylethylamine

The IR (50–3500 cm−1) and Raman (20–3500 cm−1) spectra have been recorded for gaseous and solid dimethylethylamine. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Due to the fact that three distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phases with the gauche conformer being more stable and the only one present in the spectra of the unannealed solid. From the temperature study of the Raman spectrum of the liquid a rough estimate of 3.9 kcal mol−1 has been obtained for ΔH. Relying mainly on group frequencies and relative intensities of the IR and Raman lines, a complete vibrational assignment is proposed for the gauche conformer. The potential functions for the three methyl rotors have been obtained, and the barriers to internal rotation for the two CH3 rotors attached to the nitrogen atom have been calculated to be 3.51 and 3.43 kcal mol−1, whereas the barrier for the CH3 rotor of the ethyl group has been calculated to be 3.71 kcal mol−1. The asymmetric torsional mode for the gauche conformer has been observed in both the IR and Raman spectra of the gas at 105 cm−1 with at least one hot band at a lower frequency. Since the corresponding mode has not been observed for the trans conformer, it is not possible to obtain the potential function for the asymmetric rotation although estimates on the magnitudes of some of the terms have been made. Significant changes occur in the low-frequency IR and Raman spectra of the solid with repeated annealing; several possible reasons for these changes are discussed and one possible explanation is that a conformational change is taking place in the solid where the trans form is stabilized by crystal packing forces. These results are compared to the corresponding quantities for some similar amines.

ChemInform Abstract: LOW‐FREQUENCY VIBRATIONAL SPECTRA, METHYL TORSIONAL POTENTIAL FUNCTIONS, AND INTERNAL ROTATIONAL POTENTIAL OF PROPANAL

The low frequency Raman and infrared spectra of gaseous propanal have been recorded in the region 300–50 cm−1. A complex far infrared spectrum was observed, and a substantial number of bands are assigned to the asymmetric and methyl torsional modes of both the s‐cis and gauche conformers. Analysis of these bands allowed calculation of the torsional potential functions present in this molecule. The results indicated that the potential functions calculated previously for this molecule are inadequate, and that an unusual interaction exists in the s‐cis conformer between an in‐plane bending mode and the asymmetric torsion. The potential function coefficients for the asymmetric torsion of propanal were calculated (in cm−1) to be: V2=559±7, V3=651±8, V4=−105±4, and V6=−46±6. This potential function has an enthalpy difference between the more stable s‐cis and the higher energy gauche conformers of 241±10 cm−1, with the gauche dihedral angles being about ±135 °.

Low frequency vibrational spectra, methyl torsional potential functions, and internal rotational potential of propanal

The low frequency Raman and infrared spectra of gaseous propanal have been recorded in the region 300–50 cm−1. A complex far infrared spectrum was observed, and a substantial number of bands are assigned to the asymmetric and methyl torsional modes of both the s-cis and gauche conformers. Analysis of these bands allowed calculation of the torsional potential functions present in this molecule. The results indicated that the potential functions calculated previously for this molecule are inadequate, and that an unusual interaction exists in the s-cis conformer between an in-plane bending mode and the asymmetric torsion. The potential function coefficients for the asymmetric torsion of propanal were calculated (in cm−1) to be: V2=559±7, V3=651±8, V4=−105±4, and V6=−46±6. This potential function has an enthalpy difference between the more stable s-cis and the higher energy gauche conformers of 241±10 cm−1, with the gauche dihedral angles being about ±135 °.

Low frequency vibrational spectra, methyl torsional potential functions, and internal rotational potential of methyl vinyl ether and methyl-d3 vinyl ether

The vibrational spectra (infrared and Raman) of gaseous methyl vinyl ether and methyl-d3 vinyl ether have been recorded below 500 cm−1. A large amount of torsional data are reported and the observed bands are assigned on the basis of an equilibrium between an s-cis conformer and a high energy conformer which is shown to be the gauche form. For methyl-d3 vinyl ether, the torsions are well separated in frequency and they are not greatly coupled, whereas for the ’’light’’ molecule, the torsional spectra are quite complicated, presumably due to coupling and/or a perturbation between the methyl and asymmetric torsional modes. Both series of transitions for the methyl and methoxy torsional modes were assigned for the methyl-d3 vinyl ether molecule and the assignments were confirmed by the observation of a large number of double jumps for both torsions. The potential function governing the asymmetric torsional mode was found to be V1=−413±6 cm−1 (1.18 kcal/mole), V2=1434±21 cm−1 (4.10 kcal/mole), V3=1177±10 cm−1 (3.36 kcal/mole), and V6=−96±3 cm−1 (0.27 kcal/mole) with the s-cis conformer being more stable than the gauche conformer (torsional angle 114°) by approximately 402 cm−1 (1.15 kcal/mole). The barriers to s-cis–gauche, gauche–gauche, and gauche-s-cis interconversion were found to be 2215 cm−1 (6.33 kcal/mole), 290 cm−1 (0.83 kcal/mole), and 1774 cm−1 (5.07 kcal/mole), respectively. The barriers to methyl rotation for the s-cis and gauche conformers were found to be slightly higher than 1610 cm−1 (4.60 kcal/mole) and 514 cm−1 (1.47 kcal/mole), respectively. Possible reasons for this large variation are discussed. Tentative assignments of the torsional modes for the light compound are also given and discussed.

Torsional frequencies and barriers to methyl rotation in isobutylene,O-xylene, and durene

The A2 and B2 torsional frequencies for liquid and solid isobutylene, solid O -xylene, and solid durene have been obtained at 173 206, 206 239, 146 182, and 144 180 cm−1, respectively, by the ``small kappa'' method of inelastic neutron scattering. Using a truncated, two-dimensional Fourier series to describe the potential energy interactions of the methyl groups, both the dominant threefold potential energy term and a methyl-methyl coupling term have been calculated from the torsional frequencies. The splittings of the symmetric and asymmetric torsional modes are explained as due entirely to the potential coupling of the two methyl groups. Although not resolved, the scattering data on gaseous isobutylene yields an ``average'' torsional mode of 170 cm−1. The total barrier heights for liquid and solid isobutylene of 2390 and 3070 cal/mole, which are much higher than gas phase barriers obtained by other methods, indicate rather large intermolecular forces affect the barriers in the condensed states. Due to large methyl-methyl interactions, the total barrier of 1860 and 1820 cal/mole in O -xylene and durene is found to be lower than results obtained by other methods which neglect methyl-methyl interactions.

Golden Gate Bridge Vibration Studies

The records of bridge movement obtained over a period of years by means of instruments at ten locations on the main and side spans of the Golden Gate Bridge are analyzed. They are plotted against wind velocity and correlated with theoretical analyses of wave form and against model test indications of the response to winds of different velocities. It is shown that the oscillations are almost always in a symmetric mode with the amplitude inhibited by the sharply downward deflection of the prevailing west wind as it passes over a high hill before striking the north side span. It is shown that during one forty minute period the oscillation was in an asymmetric torsional mode, uninhibited, and reached amplitudes predicted from the model tests. A bottom lateral system has been installed. Tests indicate that it will prevent a reoccurrence of the objectionable torsional motion.