Spectroscopic characterization of the rotational conformations in the disordered phase of poly(ethylene terephthalate)

Recently developed data processing techniques have been applied to determine the complete spectra of the trans and gauche isomers of PET and to use these spectra to determine the conformational composition of PET samples isothermally crystallized from the glassy state between 75 and 240°C. A linear correlation was found between normalized absorbance of conformational bands and density measurements, with the trans bands showing a positive slope and the gauche bands a negative slope. Extrapolation of the results from trans bands to zero percent of trans yields the density of the pure gauche isomer, 1.326 ± 0.002 g/cm³. Similarly, from the gauche bands the density of 100% crystalline trans is found to be 1.510 ± 0.014 g/cm³. Meanwhile, the density of the amorphous trans is calculated to be 1.430 ± 0.003 g/cm³. From these values and the combined density and infrared measurements, the content of trans isomers in the amorphous phase is calculated. It is found that the content of amorphous trans is a function of both annealing temperature and time. It is observed that the amorphous trans content decreases as the crystalline trans content increases. At high crystallinity, the amorphous trans content approaches zero. These amorphous trans isomers are associated with the extended units making up the interlamellar links. The loss of these extended trans interlamellar links is reflected in the mechanical properties of PET.     

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.     

Infrared and raman spectra,conformational stability,barriers to internal rotation,ab initio calculations and vibrational assignment of difluoroacetyl chloride

The Raman (3100–10 cm⁻¹) and infrared (3100–30 cm⁻¹) spectra of difluoroacetyl chloride, CHF₂CClO, in the gas and solid phases have been recorded. Additionally, the Raman spectrum of the liquid with qualitative depolarization ratios has been obtained. From these data, a trans/gauche equilibrium is proposed in the gas and liquid phases, with the trans conformer (hydrogen atom eclipsing the oxygen atom and trans to the chlorine atom) the more stable form in the gas, but the gauche rotamer is more stable in the liquid and is the only form present in the annealed solid. From the study of the Raman spectrum of the gas at different temperatures, a value of 272 ± 115 cm⁻¹ (778 ± 329 cal mol⁻¹) was determined for ΔH, with the trans conformer the more stable form. Similar studies were carried out on the liquid and a value of 109 ± 9 cm⁻¹ (312 ± 26 cal mol⁻¹) was obtained for ΔH, but now the gauche conformer is the more stable form. A potential function for the conformational interchange has been determined with the following potential constants: V₁ = 397 ± 23, V₂ = −101 ± 5, V₃ = 474 ± 3, V₄ = −50 ± 3, and V₆ = 10 ± 2 cm⁻¹. This potential has the trans rotamer more stable by 179 ± 31 cm⁻¹ (512 ± 89 cal mol⁻¹) than the gauche conformer. A complete vibrational assignment is proposed for both conformers based on infrared band contours, Raman depolarization data, group frequencies and normal coordinate calculations. The experimental conformational stability, barriers to internal rotation, and fundamental vibrational frequencies are compared with those obtained from ab initio Hartree-Fock gradient calculations employing both the RHF/3-21G^* and RHF/6-31G^* basis sets, and to the corresponding quantities obtained for some similar molecules.     

Pressure-Induced Solidification of 1-Butyl-3-methylimidazolium Tetrafluoroborate

We have used Raman spectroscopy to investigate the high-pressure phase behavior of 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), a representative ionic liquid, at pressures up to ~7.5 GPa. We have also studied how increasing pressure leads to conformational changes in the [bmim]⁺ cation. We have found that liquid [bmim][BF₄] undergoes pressure-induced solidification (freezing) into a superpressed (metastable) state at 2.5 GPa; another structural change probably occurs at ~6 GPa. Remarkably, conformational changes in the [bmim]⁺ cation between trans and gauche conformers are concordant with the metastable structural changes of [bmim][BF₄]. As the pressure is increased from ambient, the fraction of gauche conformers increases, but the gauche fraction decreases above the solidification pressure (2.5 GPa), and slope of the gauche/trans ratio changes again above 6 GPa. We interpret these results in terms of the fragility of the ionic liquid.     

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.     

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.     

Far infrared spectrum and conformational potential function of n-butane

The far i.r. spectrum of gaseous n-butane obtained at 0.06 cm⁻¹ resolution is reported between 80 and 230 cm⁻¹. Several transitions for the asymmetric torsion of the trans conformer have been identified. Utilizing these data along with the previously reported asymmetric torsional transitions of the gauche conformer from Raman spectroscopic data, the potential function for the conformational change has been obtained. The determined potential parameters were found to be: V₁ = 181, V₂ = 148, V₃ = 1154 and V₆ = −33 cm⁻¹. The s-trans to gauche, gauche to gauche, and gauche to s-trans barriers in cm⁻¹ were found to be: 1315 (3.76 kcal/mol), 1090 (3.12 kcal/mol) and 1070 (3.06 kcal/mol), respectively. The potential functions obtained from these spectroscopic data are consistent with the trans to gauche energy difference, but not with the high trans/cis potential barrier suggested by recent ab initio calculations.     

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 and molecular conformations of unbranched dialkyl selenides

The IR and Raman spectra of six unbranched dialkyl selenides, namely ethylmethyl selenide, methylpropyl selenide, butylmethyl selenide, diethyl selenide, ethylpropyl selenide and dipropyl selenide, have been systematically investigated. The spectra are analysed on the basis of the spectral examination and the normal coordinate treatment. The CH₂-Se stretching wavenumbers of the dialkyl selenides correlate well with the conformation about the CSe-C-C bond axes. The molecular conformation in the solid and liquid states is studied by utilizing this correlation in conjunction with the wave-numbers of the CH₂ rocking and skeletal deformation vibrations. The conformational stability in the liquid state is also examined and the enthalpy difference between the trans and gauche forms of ethylmethyl selenide is obtained as 0.22 ± 0.05 kcal mol^?1, the gauche form being more stable. The conformational stability of dialkyl ethers, sulphides and selenides are comparatively discussed.     

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

The infrared spectra of 1,5-hexadiyne (bipropargyl) as a vapour, liquid and crystalline solid and in several solvents have been recorded. Raman spectra were obtained for the liquid (including polarization measurements), the crystal and for several solutions.The data were interpreted in terms of two conformers, trans and gauche, in the vapour and liquid states and one, the trans, in the crystalline form. Interpretation of the spectra in terms of the conformational equilibria was not straightforward, and firm conclusions could not be drawn from the spectra alone. Vibrational assignments were made, supported by normal coordinate calculations.     

Vibrational analysis of molten poly(ethylene glycol)

The infrared absorption of poly(ethylene glycol) was measured in the molten state. Characteristic bands of the molten state were identified. Normal vibrations and frequency distributions were treated for various conformation models with CH₂CH₂O repeat units. The infrared absorption peaks of the molten state closely correspond to the frequency distribution peaks of the TGT conformation with gauche O? CH₂? CH₂? O groups, although infrared bands due to trans O? CH₂? CH₂? O groups are also observed. Vibrational assignments of the infrared bands and Raman lines were made on the basis of potential energy distributions.     

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.     

Far infrared and low frequency gas phase Raman spectra,vibrational assignment,and conformational stability of 1-chloro-2-methylpropane and 1-bromo-2-methylpropane

The Raman (3200—10cm⁻¹) and infrared (3200—50 cm⁻¹) spectra of gaseous and solid 1-chloro-2-methylpropane and 1-bromo-methylpropane, as well as the Raman spectra of the liquids, have been recorded and assigned. The gauche asymmetric torsion of the 1-chloro-2-methylpropane molecules has been observed at 110 cm⁻¹ in the Raman spectrum of the gas. For the 1-bromo-2-methylpropane molecule, both the trans and gauche asymmetric torsions have been observed at 106.70 and 103.94 cm⁻¹, respectively, along with three additional transitions for the gauche conformer. From these data, the asymmetric potential function for the bromide molecules to V₁ = —493 ±16, V₂ = 595 ± 18, and V₃ = 2006 ± 6 cm⁻¹ with the trans conformer being more stable than the gauche conformer by 44 ± 20 cm⁻¹. The trans form is found experimentally to be more stable in the liquid phase by 30 ± 14 cm⁻¹ (83 ± 40 cal mol⁻¹). From the relative intensities, in the Raman spectra, of the CCl stretches measured as a function of temperature, the gauche conformer of the chloride molecules to be 167 ± 71 cm⁻¹ (479 ± 203 cal mol⁻¹) more stable than the trans conformer in the gas phase, and 73 ± 10 cm⁻¹ (208 ± 29 cal mol⁻¹) more stable in the liquid phase. The methyl torsions for the gauche and trans conformers of both molecules are tentatively assigned in the gas phase and the barriers have been calculated. The results of this study are compared with previous studies on these molecules.     

Conformational studies of n-propylamine by combined ab initio MO calculations and Raman spectroscopy

The results of ab initio SCF-MO calculations performed with a 3-21G(N*) basis set, for fully optimized geometries of five conformations of n-propylamine, are presented. The calculated relative order of total energies for these conformers is TT≈GG′>TG>GT>GG. At 300 K, the Boltzmann distribution of populations is 18, 37, 20, 19 and 7%, respectively.Raman spectra of n-propylamine and n-propylamine-N-d₂ in the liquid phase exhibit a number of bands whose temperature-dependent intensities clearly suggest the occurrence of different conformers in simultaneous equilibria. Deuteration of the amine group originates pairs of Raman bands at 428 and 440 cm⁻¹ and at 863 and 885 cm⁻¹. The bands at 428 and 885 cm⁻¹ are favoured by reduction of temperature. Normal coordinate calculations permit the assignment of the Raman and i.r. spectra in good agreement with experimental evidence. Among the five possible conformers of n-propylamine, it is possible to detect the presence of at least three conformations in the liquid phase, corresponding to the skeletal trans (TT and GT) and at least one of the skeletal gauche (TG, GG or GG′) forms. In the solid phase, only the bands ascribed to the TT form were observed.The ab initio results for the isolated molecule show that the all-trans conformation, TT, and the conformation GG′ have the smallest energies. On the other hand, the vibrational results for the liquid and solid phases indicate that the all-trans conformation, TT, is the more populated form. In addition, this conformer presents the highest calculated dipole moment, in good agreement with the liquid phase Raman spectroscopic results which point out that this conformation is favoured by polar solvents. Intermolecular interactions operating in the liquid n-propylamine, possibly of the hydrogen bonding type, are responsible for altering the relative order of conformational stability as predicted by the ab initio SCF-MO results for the isolated molecule.     

Assignment and anharmonicity analysis of overtone and combination bands observed in the resonance Raman spectra of carotenoids

The resonance Raman spectra of all-trans carotenoids have been observed in the region of 5000-500 cm⁻¹ for samples in glassy solution at 77 K and in the in vivo state at room temperature. Prominent bands in the wavenumber region higher than 2000 cm⁻¹ are assigned to either overtones or combinations of three modes due to skeletal stretches and the CH₃ in-plane rock. From the wavenumbers of the observed Raman bands, anharmonicity constants for these three modes (including cross-term constants) are obtained. It is found that, for each carotenoid studied, the cross-term anharmonicity constant between the CC and CC stretches is significantly larger than the other anharmonicity constants.     

The infrared and Raman spectra, ab initio calculations and spectral assignments of cyclopropylmethyl dichlorosilane (c-C3H5)SiCl2CH3

Raman spectra of cyclopropylmethyl dichlorosilane (c-C₃H₅)SiCl₂CH₃ as a liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163 K, and intensity changes of certain bands with temperature were detected. No crystallization was ever obtained in the Raman cryostat in spite of extensive annealing. The infrared spectra have been studied as a vapour, as an amorphous solid at 78 K and as a liquid in the range 600-100 cm⁻¹. No infrared bands present in the vapour or liquid seemed to vanish upon cooling, and the sample never formed crystals on the CsI window of an infrared cryostat.The compound exists a priori in two conformers, syn and gauche, and the experimental results suggest an equilibrium in which the gauche conformer has 1.64 kJ mol⁻¹ lower enthalpy than syn in the liquid, leading to 20% syn at ambient temperature. Most of the syn bands were situated close to the corresponding gauche bands and it was difficult to obtain reliable ΔH values.B3LYP calculations with various basis sets and the CBS-QB3 and G2 and G3 models were employed, yielding the conformational enthalpy difference ΔH (syn-gauche) between 2.6 and 3.4 kJ mol⁻¹. Infrared and Raman intensities, polarization ratios and vibrational frequencies for the syn and gauche conformers were calculated. Instead of scaling the calculated wavenumbers in the harmonic approximation, calculations from B3LYP/cc-pVTZ were derived in the anharmonic approximation. In most cases these values were in good agreement with the experimental results for 38 observed modes of the gauche and 8 modes of the syn conformer with a deviation of ca. 1%.     

Spectra and structure of small ring compounds: Part XXXVIII. Cyclobutanol

The Raman spectrum of gaseous cyclobutanol has been recorded and the far infrared spectrum of the gas has been obtained at a resolution of 0.5 cm^?1. At least six Q-branches arising from the low frequency ring-puckering motion have been observed and assigned on the basis of a potential of the form V(X) = (6.32 ± 0.21) × 10⁵X⁴?(4.18 ± 0.04) × 10⁴X²+ (8.81 ± 1.20) × 10³X³ with a reduced mass of 170 amu. An energy difference between the equatorial and axial forms was found to be 50–150 cm^?1 with the equatorial being more stable and a barrier of 700–900 cm^?1 was found for the interconversion. Three O-H stretching modes were observed in the Raman spectrum. It is concluded that the O-H moiety has both the gauche and trans conformations present in the equatorial form but only the gauche conformer is present in the axial form of the ring. Three O-H torsional modes were observed at 244 (trans conformer), 226.5 and 181.5 cm^?1 (gauche conformer) for the equatorial form and one O-H torsion at 237.5 cm^?1 (gauche conformer) for the axial form. The potential function governing the O-H torsional motion for the equatorial form was found to be V₁ = 280 ± 7 cm^?1 (800 cal mole^?1) and V₃ = 425 ± 3 cm^? (121.5 cal mole^?1) with the trans conformer being more stable than the gauche by approximately 206 cm^?1 (589 cal mole^?). The barriers to trans-gauche and gauche-gauche interconversion have essentially the same values, 500 cm^?1 (1430 cal mole^?1).     

The study of Turnip Mosaic virus coat protein by surface enhanced Raman spectroscopy

Using Turnip Mosaic virus (TuMV) coat protein as material, the secondary structure has been studied by both normal Raman spectroscopy (NRS) and surface enhanced Raman spectroscopy (SERS). The NRS of TuMV coat protein under certain conditions showed the α-helix, β-sheet and random coil structure. The CSSC comformations are trans—gauche—gauche and gauche—gauche—gauche. The SERS spectrum of TuMV coat protein under certain conditions reveals the α-helix structure. By studying SERS at different adsorbing times, we have observed the amide III vibration of α-helix, β-sheet and random coil structure. The CSSC conformations drawn from the SERS spectra are trans—gauche—gauche and trans—gauche—trans. Besides the amide I, amide III and CSSC bands, the CαCN band, aromatic amino acid bands and some other bands can also be seen in the SERS spectra.     

Cryochemistry : edited by M. Moskovits and G.A. Ozin,John Wiley & Sons,New York,London, Sydney and Toronto, 1976, pp. xi + 532, price £25.85

The IR spectra of N₂F₄ solutions in liquid argon and nitrogen have been recorded and reanalyzed in the 3300–3550 cm^?1 region. The fundamental absorption bands of NF-stretching modes of both the gauche and trans isomers and the bands of the second order transition have been observed. These new IR data on the spectra of cryosystems and published Raman data allowed us to make an unambiguous interpretation of the NF₂-stretching vibrational bands for both the N₂F₄ conformers.     

Conformational analysis,barriers to internal rotation,vibrational assignment and ab initio calculations of chloroacetone

The infrared (3500-20 cm⁻¹) and Raman (3200-10 cm⁻¹) spectra have been recorded for gaseous and solid chloroacetone (1-chloro-2-propanone), CH₂ClC(O)CH₃. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. These data have been interpreted on the basis that the molecule exists predominantly in a gauche conformation having a “near cis” structure of C₁ symmetry (dih ClCCO=142°C) in the vapor but for the liquid a second conformer having a trans structure (chlorine atom oriented trans to the methyl group) with C_s point group symmetry is present. From a study of the Raman spectrum of the liquid at variable temperatures, the trans conformation has been determined to be more stable than the gauche form by 1042±203 cm⁻¹ (2.98±0.6 kcal mol⁻¹ and is the only conformer present in the spectrum of the annealed solid. From ab initio calculations at the 3-21G* and 6-31G* basis set levels optimized geometries for both the gauche and trans conformers have been obtained and the potential surfaces governing internal rotation of the symmetric and asymmetric rotors have been obtained. The observed vibrational frequencies and assignments to the fundamental vibrations for both the gauche and trans conformers are compared to those calculated with the 3-21G* basis set. The results are discussed and compared with the corresponding quantities obtained for some similar molecules.