The conformation and vibrational spectra of trans-1,4-dihalocyclohexanes: Part III. trans-1,4-Diiodo-, trans-1,4-bromoiodo- and trans-1,4-dibromocyclohexane

The IR spectra of trans-1,4-diiodo- and trans-1,4-bromoiodocyclohexane as solutes in various solvents, as pellets and as solids under high pressure are recorded in the region 4000–30 cm^?1. Additional spectra of the melts, amorphous and annealed crystalline solids at 90 K and dichroic spectra of oriented crystals are recorded above 200 cm^?1. Raman spectra of the amorphous and annealed solids at 90 K and as solutes in various solvents, are obtained, including polarization measurements. IR and Raman spectra of trans-1,4-di-bromocyclohexane in the temperature range 90–250 K are recorded. Equilibrium mixtures of ee and aa conformers of the title compounds are observed in solution, in the melts and in the amorphous solid at 90 K. The ee conformer only is present in the stable crystal, while the aa conformer predominates in apparently metastable crystals annealed to ca. 205 K. The concentration of the aa conformer increases under high pressure (50 kbar). Fundamental frequencies for both ee and aa conformers are assigned. A normal coordinate analysis is carried out, and the force Fields adjusted to nine halogenated cyclohexanes using the overlay technique.     

The conformation and vibrational spectra of trans-1,4-dicyanocyclohexane

The IR spectra of trans-1,4-dicyanocyclohexane as a melt, as a solute in various solvents, as KI and polyethylene pellets and as amorphous and annealed crystalline solids at 90 K have been recorded in the region 4000-50 cm^?1. Additional spectra at high pressures (1–50 kbar) have been recorded and the dichroic ratios of oriented polycrystalline films are obtained above 200 cm^?1. Raman spectra of the compound as a melt, as an amorphous and crystalline solid at 90 K and dissolved in acetone, chloroform and benzene have also been recorded.The compound exists as an equilibrium mixture of ee and aa conformers in solution, in the melt and in the amorphous solid at 90 K, but as one conformer only, apparently the ee form, in the crystalline state. Unlike the corresponding dihalocyclohexanes, trans-1,4-dicyanocyclohexane cannot be converted to an “aa crystal” either by exposure to high pressure or by annealing to a metastable crystal.The fundamental frequencies of both conformers have been interpreted in terms of C_2h molecular symmetry and the assignments supported with a force constant calculation by the overlay technique transferring force constants from various related molecules.     

The conformation and vibrational spectra of isocyanato and isothiocyanatocyclohexane

The IR spectra of isocyanato and isothiocyanatocyclohexane (C₆H₁₁NCX) as liquids and as amorphous and crystalline solids at low temperatures have been recorded in the region 4000-50 cm^?1. High pressure (0–30 kbar) IR spectra of the neat samples were obtained in a diamond anvil cell and various high-pressure solid phases were studied. Raman spectra of the compounds as liquids and as low-temperature solids were obtained.Isocyanatocyclohexane crystallized directly as anisotropic solids containing equatorial molecules at low temperature and axial molecules at high pressure. Isothiocyanatocyclohexane formed a possibly plastic phase between 225 and 260 K where both equatorial and axial conformers are present. A solid high-pressure phase (1–3 kbar) at ambient temperature appeared anisotropic and contained both the e and a conformers. Below 225 K (atmospheric pressure) and above 10 kbar (ambient temperature) anisotropic crystals were formed which both contained equatorial conformers only.Normal coordinate analyses were carried out for the equatorial and axial conformers of the two molecules with different orientations (C_s and C_l symmetries) of the side chain. Force constants were transferred from various halo and pseudohalocyclohexanes. Tentative assignments of the fundamentals belonging to both the e and a-conformers are presented.     

The conformation and vibrational spectra of trans-1,4-dihalocyclohexanes: Part IV. Force constant calculations and spectral correlations

A force field is derived by the overlay technique, iterating on the 48 assigned fundamentals for the ee and the aa conformers of six trans-1,4-dihalocyclohexanes. Some additional fundamentals of three monohalocyclohexanes (chloro-, bromo- and iodocyclohexane) are used in the iterations. The final force field has 49 independent parameters; it is compatible with the Snyder and Schachtschneider valence force field on n-alkyl chlorides, and is adjusted to more than 600 observed frequencies. Practically all the force constants are common to the ee and aa conformers and the force field can be easily transferred to other substituted cyclohexanes.The assigned fundamentals for the six trans-1,4-dihalocyclohexanes (dichloro-, dibromo-, diiodo-, chlorobromo-, chloroiodo- and bromoiodocyclohexane) are correlated and the spectral features discussed. A few new assignments are made for trans-1,4-dichlorocyclohexane.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid state.     

Vibrational analysis of 1,4-dibromopentane

Infrared and Raman spectra have been obtained for 1,4-dibromopentane and interpreted with the aid of normal coordinate calculations. It is concluded that all six conformers resulting from rotation about the C₁-C₂ and C₃-C₄ bonds probably exist in the neat liquid and amorphous solid, and that the compound crystallizes in the P_HS_HH conformer that has the two bromine atoms on opposite sides of the plane of carbon atoms. The compound therefore behaves like 1,3-dibromopropane and 1,4-dibromobutane.     

Quantitative estimation of trans-1,4 and cis-1,4 isomers in polychloroprene by high-resolution NMR

In the ¹H-NMR spectrum of polychloroprene dissolved in C₆D₆, the ?CH proton signal was separated into two triplet peaks. These triplet signals were assigned to the ?CH proton in the trans-1,4 and cis-1,4 isomers by measurement of ¹H-NMR spectra of 3-chloro-1-butene and a mixture of trans- and cis-2-chloro-2-butene as model compounds for the 1,2, trans-1,4 and cis-1,4 isomers. In ¹H-NMR spectra (220 Mcps) of polychloroprene dissolved in C₆D₆, two triplet signals were separated completely from which the relative concentrations of trans-1,4 and cis-1,4 isomers could be obtained quantitatively.     

Spectroscopic studies of 1,4-dibromobutyne-2

Infrared spectra of 1,4-dibromobutyne-2 have been recorded over the 4000-200 cm^?1 region in the vapour, liquid, amorphous and crystalline states Raman spectra were extended to ca. 20 cm^?1 in the same states of aggregation, except for the non-recorded vapour phase spectrum. The temperature was varied between ?190 and 160 °C, and the pressure up to 10 kbar.A high proportion of the molecules exhibited free, internal rotation in the vapour and liquid phases, but to a smaller extent in the amorphous state at ?190 °C. For those molecules not being excited beyond the potential barrier, an unsymmetric conformation was preferred, whereas in the crystalline state the molecules possessed the anti conformation (C_2h) both at low temperature and at high pressure at ambient temperature.A vibrational analysis based upon force field calculations was carried out and the mean amplitudes of vibration computed. The data have been related to preliminary results from dipole moment and electron diffraction investigations.     

The conformation and vibrational spectra of 1,5-hexadiene-3-yne (divinylacetylene) and perchloro-1,5-hexadiene-3-yne (perchlorodivinylacetylene)

Infrared spectra of 1,5-hexadiene-S-yne (divinylacetylene) have been recorded in the vapour phase, in solution and in the amorphous and crystalline solid states at 90 K in the region 4000–4020 cm^?1. Correspondingly, IR spectra ofperchloro-1,5-hexadiene-3-yne (perchlorodivinylacetylene) as a melt, as a solute in various solvents and as a solid at 90 K have been obtained. Raman spectra of the two compounds were recorded in the liquid (molten) state including polarization measurements, and as crystalline solids at 90 K.The spectral data indicate that each compound exists as one conformer only in the various states of aggregation. In divinylacetylene the molecular symmetry appears to be anti (C_2h) while for perchlorodivinylacetylene the symmetry is either C_2v (syn) or C₂ (gauche). Vibrational assignments for the spectra of both molecules are presented and the values are compared with the results of normal coordinate analyses.     

Spectra and structure of organophosphorus compounds—XXXVII. Infrared and Raman spectra,conformational stability,vibrational assignment and normal coordinate analysis of ethyldichlorophosphine-d0 and -d5

The infrared (3500 to 40 cm⁻¹) and Raman (3500 to 10 cm⁻¹) spectra have been recorded for the gaseous and solid phases of ethyldichlorophosphine, CH₃CH₂PCl₂, and CD₃CD₂PCl₂. Additionally, the Raman spectra of the liquids were recorded and qualitative depolarization values were obtained. In the spectrum of the gas the gauche conformer is predominant with about 65% abundance whereas in the spectrum of the liquid at ambient temperature the amount of gauche conformer is reduced compared to the gas phase and at −100°C the trans conformer predominates. The trans conformer is the more stable species in the solid. A variable temperature study was carried out on the Raman spectrum of the liquid and ΔH and ΔS values of 190 ± 30 cm⁻¹ (543 ± 87 cal/mol) and 2.86 ± 0.3 eu were determined, respectively, with the trans conformer being more stable. Similar variable temperature studies have been carried out on a number of conformer peaks in the infrared spectrum of the gas and a ΔH value of 53 ± 38 cm⁻¹ (152 ± 110 cal/mol) was obtained, again with the trans conformer being more stable. All the fundamental modes of both conformers have been assigned utilizing band contours, depolarization values, isotopic shift factors and group frequencies. A normal coordinate calculation has been carried out utilizing a modified valence force field to calculate the frequencies and potential energy distribution for both conformers. The barriers to methyl rotation of the trans and gauche conformers are 2.2 ± 0.1 and 2.3 ± 0.1 kcal/mol, respectively. These results are compared to similar quantities for some corresponding molecules.     

Spectra and structure of phosphorus—boron compounds—XXVI. Infrared and Raman spectra,conformational stability and normal coordinate analysis of ethyldichlorophosphine-borane

The Raman (3500-10 cm⁻¹) and infrared (3500-50 cm⁻¹) spectra of solid ethyldichlorophosphine-borane, CH₃CH₂P(BH₃)Cl₂ and its deuterated analog, CH₃CH₂P(BD₃)Cl₂ have been recorded. Additionally, the infrared spectra of the gases and the Raman spectra of the liquids have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several distinct Raman lines disappear on going from the liquid to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers, with the trans conformer being more stable in the liquid phase, and the only one present in the solid phase. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be nearly zero. Based on Raman depolarization data, group frequencies, isotopic shift factors and infrared band contours, a complete vibrational assignment has been proposed for the trans 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 BH₃ torsion has been observed at 188 cm⁻¹, while the BD₃ torsion was not observed. The methyl torsions in the spectra of the solids have been observed at 209 and 202 cm⁻¹ for the “light” and deuterated species, respectively. From the torsional data, barriers to internal rotation have been calculated. The asymmetric torsional mode has been observed for the trans conformer in the infrared spectra of the gas phase at 108 and 104 cm⁻¹ for the BH₃ and BD₃ species, respectively. These results are compared with similar quantities for some corresponding organophosphine—borane compounds.     

Vibrational spectra of 1-butanol

Infrared and Raman spectra were obtained for 1-butanol that showed the presence of trans and gauche conformers in the liquid, vapor, and amorphous solid, but only the trans conformer is present in the annealed solid.     

Vibrational analysis of alkyl iodides: 1-iodo-2- methylpropane and 1-iodo-3-methylbutane

Liquid and solid-state infrared spectra were obtained for 1-iodo-2-methylpropane and 1-iodo-3-methylbutane. The C-I stretching bands of the P_C and P_H' conformers of the propane were observed at 601 and 582 cm^?1, respectively, and those of the P_C and P_H conformers of the butane were observed at 595 and 512 cm^?1. Both conformers of each compound are present in the amorphous solid. Only the more sterically hindered P_H' conformer is present in the crystalline solid of the propane, and only the P_H conformer is present in the crystals of the butane. Vibrational assignments were made for both conformers of each compound with the aid of normal coordinate calculations. The increase in C-I stretching frequency of the P_H' conformer of the propane from the normal value in alkyi iodides (from ca. 500 to 582 cm^?1) is attributed to the increased contribution of C-C stretch and decreased contribution of C-I stretch.     

Vibrational spectra and normal coordinate analysis of trans-1,4-poly(2,3-dimethylbutadiene) and the two polymorphic forms of trans-1,4-poly(isoprene)

The infrared and Raman spectra of the two polymorphic forms of trans-1,4-poly(isoprene) (TPI) and trans-1,4-poly(2,3-dimethyl butadiene) (TPDMB) are presented. Infrared spectra of the alpha and beta forms of TPI were obtained by digitally subtracting the amorphous contribution from the corresponding spectra of semicrystalline TPI polymers. Normal coordinate calculations are presented for both polymorphic forms of TPI and TPDMB, and the assignments of the normal modes are discussed.     

Preparation and properties of bromocarbonyl isocyanate,BrC(O)NCO

Starting from chlorocarbonyl isocyanate and BBr₃, the compound bromocarbonyl isocyanate, BrC(O)NCO, has been prepared. The IR, Raman, mass and ¹³C NMR spectra have been obtained and interpreted. The vibrational data point to the existence of planar trans and cis conformers (with respect to the double bonds), the trans conformer being by far the more abundant at room temperature.     

Conformations and vibrational spectra of 2-chloro-3-fluoro-1-propene and 2-bromo-3-fluoro-1-propene

The infrared spectra of 2-chloro-3-fluoro- and 2-bromo-3-fluoro-1-propene as vapours and liquids were recorded in the region 4000–4050 cm^?1. Additional spectra of the amorphous and crystalline solids at ?170 °C and of the liquids in polar and non-polar solvents were recorded between 4000 and 200 cm^?1.Raman spectra, including semi-quantitative polarization measurements of the liquids were obtained. Spectra were also recorded with the samples dissolved in polar and non-polar solvents, and unannealed as well as annealed crystalline solids were studied at ?180 °C.Approximately 14 vibrational bands present in the spectra of the liquids, solutions and the glassy solids vanished in the infrared and Raman spectra of the crystals. From various criteria it can be concluded with certainty that the more polar (gauche) and less polar (cis) conformers were present in the crystalline chloro- and bromo- compounds, respectively. From infrared and Raman band intensities it was estimated that the conformational equilibrium in chlorofluoro-propene was highly displaced towards cis in the vapour, with both conformers approximately equally abundant in the liquid state (30 °C). For bromofluoro- propene the equilibrium was still further displaced towards the cis conformer.A striking similarity between the spectra of the two compounds was ob- served. The fundamental frequencies have been tentatively assigned and checked by force constant calculations. Dipole moments and relative stabilities of the conformers were estimated by a CNDO calculation.     

Pressure Effect on Conformational Equilibria of Analog Peptides in Aqueous Solution by Raman Spectroscopy

We investigated the pressure effect on the conformational equilibria of glycinamide (GA) and 2-chloroacetamide (MCA) in aqueous solution by Raman spectroscopy. Scattering intensities in the CH₂ scissoring mode of GA and the NH₂ rocking mode of MCA in aqueous solution were decomposed into two component bands by ab initio MO calculations at the HF/6-31G(d,p) level. From the pressure dependence of the Raman band intensities, we determined the difference in the partial molar volume (PMV) between the cis and trans conformers of each for GA and MCA. The volume changes for the isomerization of the cis to trans conformer are ?(1.9 ± 0.3) and ?(1.5 ± 0.3) cm³-mol^?1 for GA and MCA, respectively. The volume difference between the cis and trans conformers is due to the hydration effect, which seems to be mainly the result of local effects of solute–solvent interactions in both cases. This contribution is due to the influence of the solute–solvent interaction with water molecules on the PMV of the cis conformer being less than that of the trans conformer.     

Microwave and vibrational spectra, ab initio calculations, conformational stabilities and assignments of the fundamentals of the Cs conformer of n-butylgermane

The FT-microwave spectrum of n-butylgermane, CH₃CH₂CH₂CH₂GeH₃ has been investigated from 4000 to 18,000 MHz and the microwave spectra have been observed for all of the five naturally occurring germanium isotopologues for the anti-anti (aa) conformer. The dipole moment for the ⁷⁴Ge containing species has been measured, giving a total dipole moment of 0.881 (26) D. In addition, the vibrational spectrum of n-butylgermane is described. Modestly complete assignments are made for the aa conformer. The relative stabilities of the five conformers are calculated, and the anti-anti (aa) conformer is found to be the most stable in all calculations done. This conclusion is confirmed by the infrared and Raman spectrum of the annealed crystal. The dipole moments of all conformers are calculated to be approximately equal and less than 1 D, ranging from approximately 0.8 to 0.9 D.     

Conformations and vibrational spectra of 2,3-dichloro and 2,3-dibromo-1-propene

The infrared spectra of 2,3-dichloro- and 2,3-dibromo-1-propene in the region 4000-200 cm^?1 were recorded of the liquids and of the crystalline solids at ?180 °C. Raman spectra, including semiquantitative polarization measurements were obtained of the liquids. Additional spectra were recorded of the samples dissolved in polar and non-polar solvents and of unannealed as well as of annealed crystalline solids at ?180 °C.Approximately 13 vibrational bands present in the spectra of the liquids and solutions as well as of the amorphous solid vanished in the crystal spectra. From the intensity variations with changing solvent polarity it was concluded that the conformer present in the crystal was more polar (gauche) than the other, simultaneously present in the liquid (cis). A striking similarity between the spectra of the two compounds was observed. All the fundamentals for the gauche conformers have tentatively been assigned.     

Vibrational spectra,conformational equilibria,ab initio calculations and vibrational assignments of ethylmethylfluorosilane

The infrared spectra of ethylmethylfluorosilane (CH₃SiHFCH₂CH₃) have been recorded as a vapour, liquid and solid at 78 K in the 4000–50 cm⁻¹ range and isolated in an argon matrix at ca. 5 K. Infrared spectra of two different solid phases were obtained after annealing to temperatures of 120 and 130 K, and recooling to 78 K. Although the IR spectra were quite similar in the MIR region, certain differences were noted in the FIR region below 400 cm⁻¹. The most stable conformer MeMe was present after annealing to 130 K, but three bands belonging to MeH were detected after annealing to 120 K. Various infrared bands changed intensity when the argon matrix was annealed to temperatures between 20 and 35 K, and some of these were related to changes in the conformational abundance.Raman spectra of the liquid were recorded at room temperature and at various temperatures between 295 and 153 K. Spectra of an amorphous and annealed solid were recorded at 78 K. In the variable temperature Raman spectra, various bands changed in intensity and were interpreted in terms of conformational equilibria between the three possible conformers. Complete assignments were made for all the bands of the most stable conformer MeMe. From various bands assigned to the three conformers, the conformational enthalpy difference ΔH from MeMe to the intermediate energy conformer MeH was found to be 0.5 kJ mol⁻¹ and to the highest conformer MeF was 0.7 kJ mol⁻¹. At ambient temperature this leads to 39% MeMe, 32% MeH and 29% of the MeF conformer in the liquid.Ab initio calculations in the RHF, MP2, DFT approximations and very accurate G2 calculations were carried out. With one exception, the MeMe conformer had the lowest enthalpy in all these calculations, the MeH had the intermediate and the MeF the highest enthalpy, and the calculations were in good agreement with the measurements.