Methyl Torsional Modes Research Articles

A Raman spectroscopic study of the low-frequency vibrations in guaiacol and its deuterated analogues

Low-frequency Raman spectra of solid guaiacol (2-methoxyphenol) and its hydroxyl and methoxyl deuterated analogs have been measured at 130 K. Two crystalline phases, α and β, were detected. The methoxyl, methyl and hydroxyl torsions in both phases are assigned and the corresponding barriers calculated. A coupling between the methyl torsional mode and the low-frequency ring mode δCC is postulated both from spectral data and calculated barrier values. The spectra of both phases are discussed in terms of different conformational properties of the methoxyl group. The strength of the internal hydrogen bond is discussed.

ChemInform Abstract: MICROWAVE, RAMAN, AND FAR INFRARED SPECTRA, BARRIER TO INTERNAL ROTATION, AND DIPOLE MOMENT OF 2,2‐DIFLUOROPROPANE

The microwave spectrum of 2,2‐difluoropropane has been investigated in the region from 26.5 to 40.0 GHz. Only b‐type transitions were observed and these were assigned on the basis of the rigid rotor model. The rotational constants in the ground vibrational state were found to be A = 5149.79±0.04, B = 4840.11±0.02, and C = 4805.62±0.02 MHz. Two excited states were also identified and assigned as the excited states of the two methyl torsional modes. From relative intensity measurements the frequencies for these two methyl torsional modes were determined to be ν(A2) = 217±15 and ν(B2) = 250±30 cm−1. From the Stark effect the dipole moment was determined to be ‖μb‖ = ‖μt‖ = 2.40±0.02 D. No splitting due to the internal rotations of the methyl groups were observed in the ground or first excited states. From a diagnostic least‐squares adjustment to fit the three rotational constants, the following heavy atom structural parameters were obtained: r(C–F) = 1.352±0.013 A; r(C–C) = 1.532±0.013 A; ∢FCF = 106.09±1.44°...

Spectra and structure of gallium compounds: Part VI. Infrared and Raman spectra of trimethylgalliumtrimethylphosphine

The infrared (50–4000 cm −1 ) and Raman (50–3500 cm −1) spectra of (CH 3) 3GaP(CH 3) 3 have been recorded for the solid state at the temperature of boiling liquid nitrogen. The spectra have been interpreted on the basis of C 3v molecular symmetry and a complete vibrational assignment except for the methyl torsional modes is presented. A modified valence force field model has been utilized in calculating the frequencies and potential energy distribution. The calculated potential constants for the adducts are compared to those previously reported for the Lewis acid and the Lewis base moieties and the differences are shown to be consistent with structural changes upon adduct formation. Extensive coupling has been observed between the Ga-P stretching mode and the PtC 3 and GaC 3 deformational modes. Substantial coupling is also observed between the PC 3 and the GaC 3 rocking motions. The magnitude of the Ga-P stretching force constant is found to be much smaller (0.88 mdyn Å −1) than that reported for (CH 3) 3PGaH 3 and the difference possibly reflects the relative stabilities of the donor-acceptor bond in the two complex species. The fact that none of the A, modes appear as doublets in the spectra, nor are any of the E modes split except for the GaC 3 antisymmetric stretch, which is believed to be due to the two isotopes of gallium, indicates that there is only one molecule per primitive cell sitting on a C 3v or C 3 site. A rhombohedral space group such as R3m( C 5 3v) is consistent with these observations.

Microwave, Raman, and far infrared spectra, barrier to internal rotation, and dipole moment of 2,2-difluoropropane

The microwave spectrum of 2,2-difluoropropane has been investigated in the region from 26.5 to 40.0 GHz. Only b-type transitions were observed and these were assigned on the basis of the rigid rotor model. The rotational constants in the ground vibrational state were found to be A = 5149.79±0.04, B = 4840.11±0.02, and C = 4805.62±0.02 MHz. Two excited states were also identified and assigned as the excited states of the two methyl torsional modes. From relative intensity measurements the frequencies for these two methyl torsional modes were determined to be ν(A2) = 217±15 and ν(B2) = 250±30 cm−1. From the Stark effect the dipole moment was determined to be ‖μb‖ = ‖μt‖ = 2.40±0.02 D. No splitting due to the internal rotations of the methyl groups were observed in the ground or first excited states. From a diagnostic least-squares adjustment to fit the three rotational constants, the following heavy atom structural parameters were obtained: r(C–F) = 1.352±0.013 Å; r(C–C) = 1.532±0.013 Å; ∢FCF = 106.09±1.44°; ∢CCC = 112.54±0.79°. These parameters are compard to the corresponding ones in some related molecules. The Raman spectra (0–3500 cm−1) were recorded for the gaseous, liquid, and solid states of 2,2-difluoropropane. The far infrared spectra (40–520 cm−1) of the gas and the solid were also investigated. A revised assignment is presented for the skeletal bending modes. The methyl torsions were observed in the Raman spectrum of the solid at 255 and 285 cm−1 from which a periodic barrier of 1675 cm−1 (4.79 kcal/mole) for the methyl internal rotation was calculated for the solid state. From combination and overtone bands of the methyl torsions, the barrier for the gaseous state has been determined to be 1207 cm−1 (3.45 kcal/mole). These results are compared to the corresponding quantities in related molecules.

Low-temperature vibrational spectra of cis-dichloro-( meso-2,3-diaminobutane)palladium(II) and platinum(II)

IR and Raman spectra of MCl 2( meso-2,3-diaminobutane), (M = Pd, Pt), have been recorded down to liquid nitrogen temperature or lower. It is shown that correlation coupling occurs between closely spaced hydrogen-bonded pairs of molecules, which form a “super molecule,” but not between all eight molecules in the unit cell. The lattice mode region is also understood in outline on the basis of motions of the “super molecules” Methyl torsional modes appear to be near 140 cm −1.

The vibrational Raman optical activity of deuterated 1-methylindanes

High quality VROA spectra disprove the idea of a methyl torsional mode leading to sizable optical activity above 220 cm −1 in arylethanes. The analysis of the 1450 cm −1 region shows that VROA. provides unique information on the coupling of vibrations in large molecules.

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 °.

The inertial contribution to vibrational optical activity in methyl torsion modes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThe inertial contribution to vibrational optical activity in methyl torsion modesL. D. Barron and A. D. BuckinghamCite this: J. Am. Chem. Soc. 1979, 101, 8, 1979–1987Publication Date (Print):April 1, 1979Publication History Published online1 May 2002Published inissue 1 April 1979https://doi.org/10.1021/ja00502a009RIGHTS & PERMISSIONSArticle Views172Altmetric-Citations32LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (975 KB) Get e-Alerts Get e-Alerts

A Raman spectroscopic study of the low-frequency vibrations in anisole- d5 and anisole- d8

Low-frequency Raman spectra of solid anisole- d 5 and anisole- d 8 have been measured at 130 K. The phenyl- d 5 torsion, which is observed at 140 cm −1 in the spectrum of anisole- d 5, is shifted to lower frequency and hidden under lattice bands in the case of anisole- d 8. The twofold barrier calculated from the observed torsional frequency is 3892 ± 85 cm −1. The methyl torsional mode was observed at 280 cm −1 in the spectrum of solid anisole- d 5 and at 177 cm −1 in the spectrum of solid anisole- d 8. The threefold barriers calculated from these frequencies are 1820 ± 20 cm −1 and 1422 ± 18 cm −1, respectively. The barrier values indicate that the methyl torsion is coupled to the low-frequency ring torsion mode (δ CC).

Microwave Spectrum, Barrier to Internal Rotation, and Dipole Moment of Methyl Selenocyanate

Abstract Microwave rotational spectra of five isotopic species of methyl selenocyanate (CH3SeCN with 82Se, 80Se, 78Se, 77Se, and 76Se) in the ground vibrational state, and two isotopic species of methyl selenocyanate-d3 (CD3SeCN with 80Se and 78Se) in the ground and first excited states of the methyl torsional mode have been assigned in the 8.0 to 35.5 GHz frequency range. For the most abundant species in the ground vibrational state, the rotational constants determined are AA=10147±2, BA=3483.57±0.04, and C=2631.58±0.04 MHz for CH380SeCN and A=8322.57±0.81, B=3308.06±0.02, and C=2436.92±0.01 MHz for CD380SeCN. The A-E splittings due to the methyl internal rotation were observed for the rotational transitions in the ground vibrational state for the normal species and in the first excited state of the methyl torsional mode for the deuterated species. Analysis of the splittings gave a barrier height to the methyl internal rotation of 1241±50 and 1228±50 cal/mol for the CH3 and CD3 species, respectively. Stark measurements yielded the principal axis dipole moments of μa=4.35±0.04, μb=0.76±0.10, and the total dipole moment of μ=4.42±0.05 D for CH380SeCN, and μa=4.31±0.04, μb=0.67±0.07, and μ=4.36±0.04 D for CD380SeCN. The direction of the dipole moment in the molecule is discussed.

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.

A Raman spectroscopic study of the low-frequency vibrations in anisole and anisole- d3

Low-frequency Raman spectra of solid anisole and of solid anisole- d 3 have been recorded at 130 K. The phenyl torsion observed at 148 cm −1 is shifted to 133 cm −1 upon deuteration of the methyl group. The twofold torsional barriers calculated from these frequencies are 4033 ± 110 cm −1 and 4094 ± 123 cm −1 indicating that coupling to other low-frequency modes in both cases is of the same order of magnitude. The methyl torsional mode was observed at 285 cm −1 in the spectrum of solid anisole and at 183 cm −1 in the spectrum of anisole- d 3. The threefold barriers calculated using these frequencies are 1847 ± 20 cm −1 and 1465 ± 18 cm −1 respectively. These barrier values indicate that the methyl torsion is coupled to another low-frequency mode. A doublet centered at 230 cm −1 in anisole is shifted to 245 cm −1 in anisole- d 3; it is proposed that this is due to a ring mode coupled to the methyl torsion. The splitting is interpreted as an example of Davydov splitting.

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the 35Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the 37Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm −1. The CH 3 internal rotation frequency was found to be 197 cm −1. Nuclear quadrupole coupling constants were determined for the ground state of the 35Cl and 37Cl species. From observed A-E splittings of bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V 3 = 2480 ± 40 cal mol −1 (867 ± 14 cm −1).

ChemInform Abstract: RAMAN SPECTRA OF GASES. XVIII. INTERNAL ROTATIONAL MOTIONS IN ETHYLAMINE AND ETHYLAMINE‐D2

The Raman spectra of gaseous CH3CH2NH2 and CH3CH2ND2 have been recorded at wavenumbers from 100 to 600. Both ground and excited states of the methyl and amino torsional modes for the gauche conformer have been observed. The overtone of the methyl torsion has also been observed for the gauche conformer. The methyl barriers in the gauche conformers of the CH3CH2NH2 and CH3CH2ND2 molecules were found to be 3.71 + or - 0.05 and 3.59 + or - 0.05 kcal/mole, respectively. The apparent difference in the barrier heights for these two isotopic species is attributed to a difference in the coupling between the methyl and amino torsions in these two molecules. The potential function for internal rotation around the C-N bond in CH3CH2ND2 was determined. The energy difference between the potential energy minima of the gauche and trans conformations is 0.592 kcal/mole with the trans being the more stable.

Vibrational spectra and structure of nitrogen containing molecules—III. Acetone azine and acetone azine- d12

The i.r. spectra of (CH 3) 2CNNC(CH 3) 2 and (CD 3) 2CNNC(CD 3) 2 have been studied from 4000 to 300 cm −1 in all three physical states. Absorption spectra for the polycrystalline samples were also obtained down to 33 cm −1. Raman spectra for liquid and crystalline acetone azine and acetone azine- d 12 were recorded between 4000 and 100 cm −1. Since mutual exclusion was observed for the i.r. and Raman bands assigned to the skeletal modes of acetone azine, the spectral data have been interpreted in terms of C 2 h molecular symmetry. An assignment of the fundamental vibrations is based upon the observed isotopic shift ratios, depolarization measurements, relative band intensities and positions. The methyl torsional modes were assigned to bands at 235 and 215 cm −1, respectively, in the Raman spectrum of crystalline acetone azine. The frequencies associated with the CN and NN stretching modes of acetone azine suggest the π electrons may not be localized and this observation is consistent with the bond orders for the CN (1.9) and NN (1.5) groups calculated from simple molecular orbital theory. The results of the present study are compared with the data recently reported for tetramethylhydrazine and tetramethyltetrazine.

Raman spectra of gases. XVIII. Internal rotational motions in ethylamine and ethylamine-d2

The Raman spectra of gaseous CH3CH2NH2 and CH3CH2ND2 have been recorded from 100 to 600 cm−1. Both ground and excited states of the methyl and amino torsional modes for the gauche conformer have been observed. The overtone of the methyl torsion has also been observed for the gauche conformer. The methyl barriers in the gauche conformers of the CH3CH2NH2 and CH3CH2ND2 molecules were found to be 3.71±0.05 and 3.59±0.05 kcal/mole, respectively. The apparent difference in the barrier heights for these two isotopic species is attributed to a difference in the coupling between the methyl and amino torsions in these two molecules. The potential function for internal rotation around the C–N bond in CH3CH2ND2 was determined and the following potential constants found: V1=279±16 cm−1, V3=707±4 cm−1, V6=−12±3 cm−1. The energy difference between the potential energy minima of the gauche and trans conformations is 207 cm−1 (0.592 kcal/mole) with the trans being the more stable. The calculated gauche–gauche barrier is 779 cm−1 (2.23 kcal/mole) and the trans–gauche barrier is 572 cm−1 (1.64 kcal/mole). A similar potential function was determined for the CH3CH2NH2 isomer but the fit to this one-dimensional model was not as good as that obtained for the deuterium compound probably because of the greater coupling in the light molecule.

Vibrational spectra and structure of nitrogen containing molecules—I. Formaldehyde dimethylhydrazone

The i.r. spectrum of formaldehyde dimethylhydrazone, (CH 3) 2NNCH 2, has been studied from 3400 to 200 cm −1 in the gaseous state and between 3400 and 140 cm −1 at liquid nitrogen temperatures. A comparison of the vibrational spectra in the solid and fluid states indicates the molecule exists in only one conformer in all three physical states. The Raman spectra of the gas, liquid and solid have also been recorded and depolarization values measured. In view of the number of polarized Raman lines, the vibrational spectra have been interpreted in terms of C 1 symmetry. A vibrational assignment is presented based upon the observed band positions, intensities and group frequency considerations. Whereas the methyl torsional modes were assigned to bands at 271 and 241 cm −1 in the absorption spectrum of solid (CH 3) 2NNCH 2, the counterparts to these transitions were centered at 258 and 239 cm −1 in the Raman effect. A similar difference in the i.r. and Raman frequencies of other fundamentals was ago observed and these splittings have been attributed to the correlation field. A low frequency i.r. spectrum of (CH 3) 2NNCH 2 isolated in an argon matrix was also studied and is compared to the spectra obtained for the crystalline state.

Microwave spectrum of chloromethyl methyl ether

The microwave spectrum of chloromethyl methyl ether has been studied in the region 12.4–40 GHz. For 35Cl species, a- and c-type transitions have been assigned for the ground state, the first excited state of the chloromethyl torsional mode, and the first excited state of the methyl torsional mode. Assignments were also made for the ground state of 37Cl species. The assigned transitions are due to the gauche conformer. The nuclear quadrupole coupling constants were determined for the ground state of 35Cl and 37Cl species. The observed A- E splittings of the rotational transitions arising from the three vibrational states indicate a strong coupling between the two torsional vibrations. A model calculation based on the Hamiltonian previously used by Butcher and Wilson ( J. Chem. Phys. 40, 1671 (1964)), was carried out to account for the splittings and the vibrational frequencies of the two torsional modes. The barrier to internal rotation of the methyl group is estimated to be V 3 = 647 ± 17 cm −1 (1.84 ± 0.05 kcal/mole).

Vibrational spectra and structure of esters—I. Infrared and Raman spectra of CH 3COOCH 3, CH 3COOCD 3, CD 3COOCH 3 and CD 3COOCD 3

The infrared spectra of gaseous, liquid and solid methyl acetate and its deuterium derivatives have been investigated from 4000 to 200 cm −1. The absorption spectrum of the crystalline state of each isotope has also been measured down to 33 cm −1, but the methyl torsional modes were not resolved. Raman spectra of the gaseous, liquid and solid states were obtained between 100 and 4000 cm −1. It is concluded that methyl acetate is present as a single molecular conformation in the temperature range studied since there is a one to one correspondence between the infrared and Raman transitions measured at ambient and liquid nitrogen temperatures. An assignment of the 27 normal modes, based upon relative band intensities, isotopic shift ratios and group frequency correlations is presented.