The vibrational spectra and rotational isomerism of 1,1,1-Trimethoxyethane

The liquid, vapour and solid-state infrared spectra of 1,1,1-trimethoxy-ethane were recorded in the region 250–4000 cm⁻¹. The laser-Raman spectrum with qualitative depolarization data was obtained for liquid only. The spectra show that there are at least two rotational isomers present in the liquid phase. The solution spectra reveal that the less polar form is the more stable in the liquid phase. The form playing an important role has GGG configuration. A vibrational assignment has been attempted for the observed infrared and Raman bands.     

The vibrational spectra and rotational isomerism of monochloromethyl acetate and methyl oxalyl chloride

The IR spectra of monochloromethyl acetate and methyl oxalyl chloride have been studied for the vapour, liquid and solid states. For the first ester only one conformation is present and it has a planar arrangement of heavy atoms with the chlorine directed away from the carbonyl group and with a s-cis arrangement for the basic ester framework. For methyl oxalyl chloride, evidence is presented for the presence of two rotamers, the more stable of which is probably planar with a trans arrangement of carbonyl groups.     

Vibrational spectra and rotational isomerism of fluoroacetone

Infrared spectra have been redetermined for fluoroacetone in the vapor, liquid, and solid states, and Raman spectra have been obtained for the liquid. There are two rotational isomers present in the liquid, but only the more polar form is present in the crystalline solid and only the less polar form is present in the vapor. Vibrational assignments were made for the two rotamers with the aid of normal coordinate calculations that utilized a twenty-five parameter valence force field.     

Structure and rotational isomerism of chloroacetyl chloride molecules

The structure of chloroacetyl chloroide (CH₂ClCOCl) molecule in different conformations arising from rotation of the CH₂Cl group about the C-C bond was determined by the Hartree-Fock RHF/6-31G(d) quantum-chemical calculations. The energy difference between the two stable rotamers was estimated at 5.9 kJ mol^?1, and barriers to intramolecular reorientations of the CH₂Cl group were calculated.     

Vibrational spectra and rotational isomerism of ethyl trichloroacetate

The IR and Raman spectra of ethyl trichloroacetate (E-TCA) and its deuterate (E-TCA-d₅) have been measured in the liquid, glassy and crystalline states. Vibrational assignment was made by referring to isotopic wavenumber-shift, characteristic group frequencies of related esters and with the aid of a normal coordinate calculation on E-TCA and E-TCA-d₅. It is suggested that in the liquid and glassy states there exist two molecular forms (trans—trans—trans and trans—trans—gauche) with regard to the internal rotations about the ClC---C---O---CH₂CH₃ axis, and that the former persists in the crystalline state. The band pairs of E-TCA and the other ethyl esters are discussed in relation to the nature and number of rotational axes, effect of the heavy trichloromethyl group, and enhancement of band intensity by vibrational coupling.     

Vibrational spectra and rotational isomerism of triallyl amine

The infrared spectra of triallyl amine in the vapour and liquid phases (as solutions in CS₂, CCl₄, CH₃Cl and CH₃CN), and in the solid state at low temperature were measured from 250 to 4000 cm⁻¹. The Raman spectrum of the liquid was recorded and qualitative depolarization measurements were made. It is shown that in the liquid and vapour phases the molecule exists as a mixture of at least two rotational isomers, while in the crystalline phase it assumes a single configuration having point-group symmetry C₃. A vibrational assignment for the observed bands in the infrared and Raman spectra is proposed on the basis of the C₃ point group symmetry for the more stable form of the molecule.     

Vibrational spectra and rotational isomerism of 1,2-dimethyldisilane

The i.r. and Raman spectra of 1,2-dimethyldisilane and its Si-perdeuterated species have been measured for the liquid and solid states. Existence of rotational isomers about the SiSi bond was shown on the basis of the spectral examination and the normal coordinate treatment. The fundamental vibrations were assigned for the two possible rotational isomers.     

The vibrational spectra and rotational isomerism of methyl ethyl sulphone-d0, -d3 and -d5

The infrared and Raman spectra of methyl ethyl sulphone, methyl ethyl sulphone-1,1,1-d₃ and methyl ethyl sulphone- 1,1,1,3,3-d₅ have been presented with vibrational assignments. Two solid states have been identified in which the molecules are either the trans or the gauche rotational isomers: one solid, the A form, occurs in cooled KBr disks used for infrared spectra while the other was found in CsI disks under ambient conditions. A mixed crystal form has also been found. In the liquid state the enthalpy of the B rotational isomer is slightly less than that of the A isomer by 90 (± 140) J mol⁻¹     

Vibrational spectra and rotational isomerism of methyl methylsilyl sulphide

Infrared and Raman spectra of methyl methylsilyl sulphide are measured for the liquid and solid states. The fundamental vibrations are assigned and the normal vibrations calculated for two possible rotational isomers about the Si-S bond. Two different solid spectra are obtained, one corresponding to the trans form and the other to the gauche form; the liquid spectrum shows the presence of both forms. The gauche form is more stable than the trans form in the liquid state by 80 ± 50 cal mol^?1. The crystallization process indicates that the freezing and melting points of the trans form are slightly higher than those of the gauche form.     

Vibrational spectra and rotational isomerism of 2,2-difluoropropyl chloride

Infrared spectra were obtained for 2,2-difluoropropyl chloride in the neat liquid, vapor, and solid states, and Raman spectra were obtained for the liquid. This compound exists as a mixture of trans and gauche rotamers in the vapor and liquid states, but only the trans form is present in the solid. Vibrational assignments were made for the two rotamers with the aid of zero-order normal coordinate calculations using force constants transferred from 2,2-difluoropropane and 1,1,1-trifluoroethyl chloride.     

Infrared spectra and rotational isomerism of certain crystalline vinyl ethers

Conclusions In contrast to conformationally inhomogeneous liquid vinyl ethers, crystalline trialkylvinyloxyalkylammonium salts, according to the data of IR spectroscopy, exist practically only in the form of a flat (conjugated) conformation. An exception is trimethylvinyloxyethylammonium bromide, which crystallizes almost exclusively in a nonplanar form.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1457–1460, July, 1971.     

Infrared spectra and rotational isomerism of ethyl fluoroformate and ethyl propiolate

The infrared absorption spectra of ethyl fluoroformate and ethyl propiolate have been examined in the vapour, solution and solid states. For both esters evidence is found of rotational isomerism arising from internal rotation about the O-C₂H₅ bond, in close agreement with previous work on other ethyl esters.     

Calculation of force constants and vibrational frequencies of octahedral MX6 molecules

The vibrational frequencies of some octahedral species ([SbX₆]⁻, [SbX₆]³⁻, [NbX₆]⁻, [NbX₆]²⁻, [TaX₆]⁻, [TaX₆]²⁻; X = F, Cl, Br, I) have been calculated by means of six extrapolated molecular force constants using some linear relations between the force constants and the reciprocal radii of the ligands. A statistical treatment of these correlations allowed the calculation of error limits for a probability of 90%. The computations of the force constants and vibrational frequencies were based on the GF-matrix method.     

The dynamics of rotational isomerism in crystals as studied by vibrational spectroscopy

Vibrational spectroscopy is uniquely capable of determining the structure and dynamics arising from the rotational degrees of freedom in molecular solids. Vibrational spectroscopy is sensitive to phenomena occurring on a time scale between the slow scale of magnetic, resonance methods and fast scale of diffraction methods; a time scale appropriate for both internal and overall rotation. Rotational motion of molecules in crystals provide examples of very simple reactions. Our understanding of the spectra of reacting molecules can thus be tested on these systems, and we conclude that Redfield equations can describe such spectra.A rich variety of motional effects are described: (1) The libration of the water of hydration in sodium perchlorate which illustrates a simple reacting system. (2) The libration of the adamantane molecule in both its ordered and disordered crystal phases which illustrates intermolecular interactions in organic crystals and the consequences of disorder. (3) The libration of the ammonium ion in crystals of ammonium salts which illustrated both change of orientational position by tunneling and the subtle orientating effects of isotopic substitution. (4) The internal rotation in n-alkane crystals which illustrates the ability to determine conformers and the relationship between the occurrence of disordered conformers and the occurrence of phase transitions.     

Infrared spectra and rotational isomerism of epifluorohydrin and other propene oxide derivatives

The infrared spectrum for the vapour, liquid and solid states of epifluorohydrin has been studied, and evidence has been obtained for the presence of rotational isomers. The dominant conformer in all phases corresponds to a gauche structure, which was the only form identified in the vapour by microwave spectroscopy [1]. Similar evidence for conformational equilibria has been found for the chloro, bromo and iodo derivatives, and the enthalpy differences between the rotational isomers of these compounds were measured respectively as 4580 ± 970, 3950 ± 400, and 2350 ± 170 J mol^?1 respectively. At low temperatures, epiiodohydrin was found to crystallize in one of two possible crystalline phases. These two phases correspond to the two rotational isomeric conformations of the molecule.     

Calculation of vibrational spectra of linear tetrapyrroles. 4. Methine bridge C-H out-of-plane modes

Quantum chemical force fields obtained by density functional theory (DFT) calculations systematically overestimate the frequencies of normal modes including ethylenic C-H out-of-plane (HOOP) coordinates. Compensation of this deviation requires a specific scaling factor for this type of coordinate that is distinctly lower than those applicable to out-of-plane coordinates in general. Such a specific scaling factor (0.900) has been optimized for the DFT(B3LYP) level of theory on the basis of vibrational analyses of training molecules including the HOOP coordinate. Thus, the root-mean-square deviation for the calculated frequencies of these modes is reduced from 16 to 8 cm(-1). Although Raman intensities are yet not reproduced in a satisfactory manner, implementation of the HOOP scaling factor into the set of global scaling factors determined previously (Magdo et al. J. Phys. Chem. A 1999, 103, 289-303) allows for a substantially improved reproduction of the experimental (resonance) Raman spectra of test molecules including linear methine-bridged tetrapyrroles. A very good agreement between calculated and experimental spectra is noted for the phycocyanobilin dimethylester dimer as well as for the protein-bound phycocyanobilin in the antenna pigment alpha-CPC. However, for the phycocyanobilin chromophore in the P(r) state of the plant photoreceptor phytochrome phyA, considerable deviations remain in the spectral range between 800 and 500 cm(-1), which are attributed to the effect of specific protein-chromophore interactions. The influence of the protein environment is not considered in the present calculations that refer to the molecule in vacuo.     

Calculation of the vibrational frequencies of all isotopic WOCl4 molecules

The vibrational frequencies of the isotopic WOCl₄ molecules have been calculated. Their correlations, the isotopic shifts and splittings, respectively, have been presented and the results compared with those of VOCl₃.