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.     

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.     

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.     

Infrared and Raman spectra,conformational stability,barriers to internal rotation,ab initio calculations and vibrational assignment of ethyl methyl selenide

The Raman spectra (3200–10 cm⁻¹) of ethyl methyl selenide in the gas, liquid and solid phases and the infrared spectra (3200–30 cm⁻¹) of the gas and solid have been recorded. Qualitative depolarization ratios have been obtained for the lines in the Raman spectrum of the liquid. By a variable temperature Raman study of the liquid, it has been determined that the gauche conformer is more stable than the trans rotamer by 158±16 cm⁻¹ (452±46 cal mol⁻¹), and the gauche conformer is the rotamer present in the solid. A complete vibrational assignment for the gauche conformer is presented. All of these data are compared to the corresponding quantities obtained from ab initio Hartree—Fock gradient calculations employing the STO-3G* and 4–31G*/MIDI-4* basis sets. Complete equilibrium geometries have been calculated for both rotamers and the results are discussed and compared with the corresponding quantities for some similar molecules.     

Vibrational spectra of the trans-trans and trans-cis conformers of dimethyl fumarate

Infrared and Raman spectra of dimethyl fumarate (DMFU) have been recorded in the temperature range 12–390 K. The solid state spectra are consistent with the trans-trans conformation. However, in the liquid phase (melt or solution in CCl₄) and in the vapour phase, additional infrared and Raman bands were observed due to the presence of the trans-cis conformer. For the trans-trans conformer the observed spectra have been assigned on the basis of a C_2h molecular symmetry. A normal coordinate analysis of trans-trans DMFU has been carried out using a modified Urey- Bradley force field to assist in the assignment. Observed frequencies for the trans-cis conformer have been assigned on the basis of a structure of C_s symmetry.     

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.     

Conformational and vibrational analysis of butyronitrile

IR and Raman spectra have been obtained for butyronitrile and are interpreted with the aid of normal coordinate calculations. This compound exists as trans and gauche conformers in the liquid and amorphous solid states, but only the gauche conformer is present in the crystalline solid. Simultaneous calculations for propionitrile and the two conformers of butyronitrile result in an average error of 4.7 cm⁻¹, or 0.42%. Vibrational assignments are made for all three molecules in terms of ordinary symmetry coordinates.     

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.     

The role of internal H-bonding in the conformational preferences of some molecules of the formula CH3CHYCH2X

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti. The other H-bonded conformer, where the methyl and amino groups are gauche, is predicted to be ca. 1.2 kcal mol^?1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol^?1, with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol^?1) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol^?1should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.     

Conformational Stability of Propenoyl Bromide from Temperature-Dependent Infrared Spectra of Krypton Solutions,Ab Initio Calculations,and r 0 Structural Parameters of the Propenoyl Halides (F,Cl, Br)

Variable temperature (–100 to –150°C) studies of the infrared spectra (3500–400 cm^–1) of propenoyl bromide, CH₂=CHCBrO, dissolved in liquid krypton, have been carried out. Utilizing six different conformer pairs, an enthalpy difference of 204 ± 20 cm^–1 (2.44 ± 0.24 kJ/mol) was obtained, with the anti conformer (carbonyl bond trans to C=C bond) the more stable form. At ambient temperature, there is approximately 28 ± 2% of the syn conformer present. The anti conformer also remains in the infrared and Raman spectra of the polycrystalline solid. The optimal geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies, are reported for both conformers from MP2/6-31G(d) ab initio calculations. The potential function governing the conformational interchange has been obtained from the MP2/6-31G(d) ab initio calculations. The conformational stabilities were calculated from a variety of basis sets and at the highest level of calculations, MP2/6-311 + (2df,2pd), the anti conformer is predicted to be more stable by 178 cm^–1, which is in excellent agreement with the experimental results. The r ₀ adjusted structural parameters have been obtained for propenoyl fluoride and chloride from a combination of the previously reported microwave rotational constants and ab initio predicted parameters. Several of the parameters for the chloride are significantly different than those proposed from an electron diffraction investigation. The results of these spectroscopic, structural, and theoretical studies are discussed and compared to the corresponding results for some similar molecules.     

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.     

Conformation and vibrational spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6-d2

The infrared spectra of 1,5-hexadiyne (bipropargyl) and 1,5-hexadiyne-1,6-d₂ as vapours, liquids, as solutes in various solvents and as crystalline solids at low temperatures and at high pressures have been recorded. Raman spectra were obtained for the liquids, including semiquantitative polarization measurements, and for the low temperature crystals.The data were interpreted in terms of two conformers, anti and gauche, in the vapour and liquid state and one, the anti, in the crystalline forms. A phase transition for 1,5-hexadiyne was observed at ca. 240 K. Both the high and low temperature crystals had molecules in the anti conformer.Interpretation of the spectra in terms of conformational equilibria was facilitated by a thorough vapour phase band contour analysis. With a few exceptions, all the vibrational fundamentals for both conformers were assigned and found to be in good agreement with results from normal coordinate calculations.     

The vibrational spectra,molecular structure and conformation of organic azides—IX. Azidoethane

A safe method for the synthesis of azidoethane from ethylbromide is given and ¹H and ¹³C NMR data are reported.The i.r. and Raman spectra of azidoethane have been recorded in the region 4000-40 cm⁻¹ and interpreted in terms of two conformers, anti and gauche, present in the vapour and in the liquid and of the gauche conformer in the crystalline solid. Matrix isolation studies reveal the gauche conformation to be the more stable in argon and in nitrogen matrices and probably also more stable in the vapour. The enthalpy difference between the conformers is calculated to be ΔH⁰_a→g (N₂ matrix) ≈ ΔH⁰_a→g (vap.) = −0.56(10) kJ mol⁻¹, and the barrier to rotational isomerism (anti → gauche) as 9.0(10) kJ mol⁻¹ in the nitrogen matrix and less than 6 kJ mol⁻¹ in the argon matrix. Careful Raman studies of the liquid at 140–290 K reveal the gauche conformation to be the more stable in the liquid phase as well, the enthalpy difference being ΔH⁰_a→g (liq.) = − 1.15 (5) kJ mol⁻¹.The majority of the fundamentals for both conformers have been assigned with the aid of normal coordinate calculations using previously developed scaled quantum mechanical force fields which are also presented.     

Spectra and structure of silicon containing compounds: XXIX. Conformational studies of methyl vinyl difluorosilane from temperature dependent FT-IR spectra of xenon and krypton solutions

Variable temperature (−55 to −150°C) studies of the infrared spectra (3500–400 cm⁻¹) of methyl vinyl difluorosilane, CH₂CHSiF₂CH₃, dissolved in liquid xenon and krypton have been recorded. Utilizing three sets of conformer doublets due to the cis and gauche rotamers from the krypton solution and two pairs from the xenon solution, the enthalpy difference has been determined to be 67±7 cm⁻¹ (0.80±0.09 kJ/mol) and 83±11 cm⁻¹ (0.99±0.14 kJ/mol), respectively, with the gauche conformer the more stable form. However, in the crystalline solid only the cis conformer is present. Ab initio calculations have been carried out with several different basis sets up to MP2/6-311+G(2d,2p) with full electron correlation by the perturbation method from which the conformational stabilities have been determined. With the largest basis set MP2/6-311+G(2d,2p), the cis conformer is predicted to be the more stable conformer by 10 cm⁻¹ which is inconsistent with the experimental results; however, this value is so small that the ab initio prediction cannot be relied on to give the correct conformer stability. The spectroscopic and theoretical results are compared to the corresponding quantities for some similar molecules.     

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.     

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.     

Raman and infrared spectra, conformational stability, barriers to internal rotation, vibrational assignment, and ab initio calculations of 3-methyl-3-butenenitrile

The Raman (3500-30 cm⁻¹) spectra of liquid and solid and the infrared (3500-40 cm⁻¹) spectra of gaseous and solid 3-methyl-3-butenenitrile, CH₂C(CH₃)CH₂CN, have been recorded. Both cis and gauche conformers have been identified in the fluid phases but only the cis form remains in the solid. Variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 163±16 cm⁻¹ (1.20±0.19 kJ mol⁻¹), with the cis conformer the more stable rotamer. It is estimated that there is 48±2% of the gauche conformer present at  25°C. A complete vibrational assignment is proposed for the cis conformer based on infrared band contours, relative intensities, depolarization ratios and group frequencies. Several of the fundamentals for the gauche conformer have also been identified. The vibrational assignments are supported by normal coordinate calculations utilizing ab initio force constants. Complete equilibrium geometries have been obtained for both rotamers by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree-Fock (HF) and/or Møller-Plesset perturbation theory to the second order (MP2). Only with the 6-311G(2d,2p) and 6-311G(2df,2pd) basis sets with or without diffuse functions is the cis conformer predicted to be more stable than the gauche form. The potential energy terms for the conformational interchange have been obtained at the MP2(full)/6-311+G(2d,2p) level, and compared to those obtained from the experimental data. The results are discussed and compared to the corresponding quantities obtained for some similar molecules.     

Spectra and structure of organophosphorus compounds—XXX. Vibrational and conformational study of methoxy difluorophosphinoxide-d0 and -d3

The i.r. (4000-40 cm⁻¹) and Raman (4000-10 cm⁻¹) spectra of gaseous, liquid and solid methoxy difluorophosphinoxide, CH₃OP(O)F₂, and the deuterated analog have been recorded. Results obtained from variable solvent and matrix isolation studies are consistent with the existence of both trans (CO bond trans to the PO bond) and gauche (dihedral angle approximately 120° from the trans form) conformers in the fluid phases. From simulations of observed gas phase i.r. band profiles, it was possible for assignments to be made to the individual conformers for a number of the fundamentals. Variable temperature studies carried out for the gaseous and liquid phases give energy differences between the gauche and trans conformers of 451 ± 100 cm⁻¹ (1.29 ± 0.3 kcal/mol) and 69 ± 20 cm⁻¹ (197 ± 57 cal/mol), respectively. Furthermore, these data are consistent with the gauche form being the thermodynamically preferred conformer for the gas phase whereas the trans conformer is preferred in the liquid phase and the only conformer present in the annealed solid. The methoxy torsional mode of the gauche conformer has been assigned to a very strong band observed in the far i.r. spectrum of the gas phase at 42 cm⁻¹. The matrix isolation spectra of the normal compound in Ar, CO and N₂ matrices indicated no changes in the conformational equilibrium among these different matrices and this equilibrium remains unchanged upon annealing the matrices.     

The vibrational spectra of maleimide and N-D maleimide

The infrared spectra of maleimide as a vapour (160°C), melt (100°C), oriented polycrystalline film, pellet and when dissolved in various solvents were recorded between 4000 and 400 cm^?1. Also certain spectra in the far infrared region 400-40 cm^?1 were obtained. Raman spectra of the crystalline solid, melt and as a saturated solution in acetonitrile were recorded and semiquantitative polarization measurements carried out. For N-D maleimide infrared and Raman spectra of the solid compound were recorded.The fundamental frequencies have been assigned in terms of C_2v, symmetry on the basis of infrared vapour contours and dichroism of the oriented film as well as on Raman polarization data. A force field was derived for maleimide, by initially transferring force constants from maleic anhydride and subsequent refinement of the force constants. The agreement between observed and calculated frequencies for the in-plane modes was satisfactory whereas certain large discrepancies remained for the out-of-plane vibrations.     

A suggested revised conformation of methylthionitrite as seen by its spectra and preliminary ab initio calculations

A proof has recently been given that gaseous methylthionitrite, CH₃SNO, occurs exclusively (or mainly) in its anti conformation [1]. The present paper claims that existing spectral evidence and ab initio calculations now performed suggest that gaseous methylthionitrite is mainly the syn conformer, the extra stability being of the order 1–2 kcal mol^?1. Methyl group rotation in the syn conformer is hindered by a three-fold barrier of height 689 cm^?1 while the methyl group rotation in the anti conformer is hardly hindered. The syn/anti energy difference and the barriers hindering methyl group rotation closely parallel the corresponding measured quantities in methylnitrite, CH₃ONO.