Molecular conformers in gas-phase ethanol: A temperature study of vibrational overtones

Overtone absorption spectra are reported for ethanol vapor (10150–19900 cm^?1) measured by intracavity photoacoustic spectroscopy. The OH overtones are composed of two sub-bands which are assigned as the transitions of two conformers of the OH bond in the trans or gauche position with respect to the methyl group. From the temperature dependence of the OH overtone intensity we determine the enthalpy difference between the conformers to be 0.7 ± 0.1 kcal/mole.     

New directions of matrix ir-spectroscopy

By use of thermal molecular beam IR matrix spectroscopy and IR irradiation in selected frequency ranges the vibrational spectra of conformers of 1,2-difluoroethane and methylvinylether will be discussed. The former molecule affords a case, where by comparison of CO₂ laser and 3000 cm^?1 irradiation the barriers to interconversion gauche?trans may be estimated. Methylvinylether on the other hand is a molecule, whose conformers may be interconverted by a 1-photon process in the 3000 cm^?1 region, but not by photons with frequency lower than 1400 cm^?1.     

Theoretical Gas Phase Study of the Gauche and Trans Conformers of 1-Bromo-2-Iodoethane and Solvent Effects

This is a gas-phase study of the gauche and trans conformers of 1-bromo-2-iodoethane. The methods used are the second-order Møller-Plesset theory (MP2) and density functional theory (DFT). The functional used for the DFT method is B3LYP and the basis sets used are 6-311++G(d,p) for all atoms except that different basis sets, namely 3-21G, LANECP, CRENBL ECP, Stuttgart RLC ECP and 6-311G(d,p), have been explored for the iodine atom. The results indicate that the trans conformer is preferred. The energy difference between the gauche and trans conformers (ΔE _g?t) and related thermodynamic parameters are reported. The ΔE _g?t values are 12.50 kJ?mol^?1 (B3LYP) and 10.00 kJ?mol^?1 (MP2) with the basis sets being 6-311++G(d,p)[C,H,Br]/6-311G(d,p)[I]. The conformers of 1-bromo-2-iodoethane have also been subjected to vibrational analysis. The results from the two theoretical levels are in good agreement but they are not much affected by the basis set of the iodine atom. The study has been extended to explore solvent effects using Self-Consistent Reaction Field methods. The structural parameters of the conformers are little affected by the polarity of the solvent but ΔE _g?t decreases and the solvation Gibbs energy increases with increasing polarity of the solvent.     

Gauche- and trans-n-butane and their IR-induced interconversion in solid neon

Both conformers of n-butane are trapped from the gas phase in solid Ne at 4 K. Broad band infrared irradiation (mainly the CH-stretching region) induces interconversions, the quantum yield for the gauche — trans isomerization being approximately eight times higher than that for the reverse process. The energy difference between the conformers was found to be ?3.05 kJ mol^?1. The vibrational spectra of both conformers are given.     

Simulation of orientation of uniaxially stretched poly(vinyl phenol) by molecular dynamics

Molecular dynamics were performed for the simulation of the uniaxial deformation of poly(vinyl phenol) under periodic boundary conditions with the Parrinello–Rahman scheme followed by relaxation under NVT conditions (constant number of atoms, volume, and temperature). Changes in the orientation of the main chain, benzene segments, and hydrogen bonds were analyzed with the second‐order Legendre polynomial, 〈P₂(cos θ)〉. Conformational changes were also followed. During deformation, backbone and phenyl rings both initially orient parallel to the draw direction. After relaxation, the chain is oriented parallel to the deformation direction, and side groups orient approximately perpendicular to this direction, in agreement with experimental data reported in the literature. Orientation values are higher than experimental values, as expected from the limited relaxation time range attainable in the simulations. Deformation proceeds by changes from gauche conformers to nontrans, nongauche, and trans conformers, whereas relaxation mainly allows high‐energy nontrans, nongauche conformers to convert into lower energy trans conformers. Values of the α angle for ring and bonded O? H segments agree with those in the literature. Differences observed for free hydroxyl moieties are discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1601–1625, 2002     

Stereochemistry of 1,2-dichloroethane adsorbed on Pt(111)

The rotational isomerism of 1,2-dichloroethane (DCE, CH2ClCH2Cl) adsorbed on Pt(111) was studied in the temperature range of 35-100 K using high-resolution electron energy loss spectroscopy and metastable atom electron spectroscopy. In the coverage below monolayer the physisorbed and chemisorbed species coexist at 35 K in the gauche and slightly distorted trans form, respectively. Owing to the direct Pt-Cl interactions, the nonbonding Cl 3p states of the chemisorbed DCE are split off, giving rise to degradation in symmetry from the pure trans form (C2h). The physisorbed gauche conformers are arranged with the C2 axis parallel (or heavily tilted) to the substrate and converted irreversibly to the pseudo-trans form by heating at 70 K. In the multilayer, the trans and gauche conformers exist at 35 K, where the former population is increased with increasing layer thickness. Upon annealing the bilayer at 80 K, the irreversible conversion takes place to yield a higher population of the gauche conformer in the topmost layer. The conformational stabilities and mutual changes of DCE adsorbed on a metal surface are discussed in terms of intramolecular rotational potential.     

The anomeric effect: Ab-initio studies on molecules of the type X?CH2?O?CH3

The anomeric effect of the functional groups X = C?N, C?CH, COOH, COO^?, O? CH₃, NH₂, and NH⁺₃ has been studied with ab initio techniques. Geometry effects upon rotation around the central C? O bond in X? CH₂? O? CH₃ have been compared in the various compounds. The energy differences between the conformers with a gauche and trans (X? C? O? C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non-sp³ hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO^?, O? CH₃, and NH₂(g⁺) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, C?N, and C?CH (ca. 6 kJ/mol). For X = NH₂(t) and NH⁺₃ a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X? C? O? C follows roughly the same trend.     

Aziridinyl ketones and their heteroanalogs. 5. Synthesis and structures of 2-aryl-3-aroylaziridines

A number of trans- and cis-isomeric 1-R-2-aryl-3-aroylaziridines were synthesized, and their IR spectra were studied. Intramolecular hydrogen bonding is realized in the trans isomers when R = H, and they exist in the only possible conformation (intermediate between a gauche and a cisoid conformation). cis-Isomers II (R = alkyl) exist in solutions in the form of two conformers, viz., gauche and cisoid conformers, and the gauche conformer is thermodynamically preferable.See [1] for Communication A.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No, 11, pp. 1489–1494, November, 1980.     

Modelization of vibrational spectra beyond the harmonic approximation from an iterative variation–perturbation scheme: the four conformers of the glycolaldehyde

This paper presents the computed anharmonic frequencies and IR intensities in the mid-infrared region for the four conformers of glycolaldehyde (Cis cis, Trans trans, Trans gauche and Cis trans forms). The fundamental transitions and their connected overtones and combination bands through strong anharmonic couplings (Fermi resonances) are provided. The results are stemmed from an iterative variational–perturbational resolution of the vibrational problem implemented in the VCI-P code. The four potential electronic surfaces are built as a Taylor series truncated to the fourth order around each minimum geometry. The second derivatives with respect to the normal coordinates were computed at the CCSD(T)/cc-pVTZ level, while the third and fourth derivatives were estimated with the B3LYP/6-31 + G(d,p) model chemistry. For the most stable Cc form, an average deviation of about 10 cm⁻¹ is obtained with respect to the unambiguous experimental values. Furthermore, some of the transitions observed in the CH stretchings region were reassigned. The theoretical values calculated for the Tt and Tg forms are compared to the experimental data obtained from the irradiation of the Cc conformer isolated in Ar matrix with an IR source.     

Vibrational and conformational analysis of chloromethyldifluorophosphine,ClCH2PF2

The vibrational spectra of chloromethyldifluorophosphine have been recorded in the different physical states. Both gauche and trans conformers are identified in the spectra. Temperature dependent studies and calculated dipole moments (CNDO/2) indicate that the gauche conformer is more stable in the gas-phase while the trans is more stable in the liquid phase. In the solid phase, both conformers are present, although the trans strongly dominates.     

FOURIER TRANSFORM INFRARED STUDIES OF POLY (ETHYLENE TEREPHTHALATE) FILM IN THE GLASS TRANSITION REGION

Three specimens from a solution-cast poly (ethylene terephthalate) (PET) film, one being liquid-N_2 quenched from 92℃(Q), one being slowly cooled from 92℃(SC) and one being quenched and sub-T_g annealed at 67℃(AN), have been studied by specimen difference spectra Q-SC and AN-Q and temperature difference spectra T-70 and T_2-T_1 for every 2℃steps on heating to 90℃at 2℃/min. SC and AN showed more gauche conformers than Q. That means that the PET chain has more trans conformers at higher temperatures and some of these are frozen during quenching through T_g. A band at 1340 cm~(-1) has been found to be complex containing overlapping bands reflecting trans in crystalline regions and trans in amorphous regions. The temperature difference spectra on heating through T_g showed that the spectral changes in Q are gradual while a rather abrupt change occurs in AN at 80—82℃for the bands at 1340, 1042 and 1020 cm~(-1). No new conformational structure or new vibrational mode is involved. A kind of locking mechanism is suggested which hinders the molecular vibrational changes in AN below T_g until a sudden release occurs at T_g. These locking sites can be nothing else than sites of tighter local packing of chain segments. Consequently it is believed that inter-chain van der Waals attraction energy plays a dominating role in the volume relaxation and sub-T_g annealing of quenched amorphous polymers.     

Structural effects of Zn+2/Mg+2 ratios on crystallization characteristics and microstructure of fluorophlogopite mica-containing glass-ceramics

We report the effect of Mg⁺² substitution (by Zn⁺²) on crystallization kinetics, microstructure, thermal and mechanical properties of boroaluminosilicate glass. Zn²⁺ was selected for Mg²⁺ on the basis of similar ionic radius in six coordination system (Mg²⁺∼0.72 Å, Zn²⁺∼0.75 Å). The melt-quenched glasses with SiO₂–(1 − x) MgO–Al₂O₃–K₂O–B₂O₃–MgF₂ (BPAS)/x ZnO system, have been investigated to establish the effect of Zn⁺²/Mg⁺² ratios. It is found that the density of BPAS glass without zinc content is 2.52 g/cm³ and increased linearly on substitution of Mg²⁺ by 5–32 mol% ZnO. T_g and T_d of BPAS glass initially increased on adding 5 mol% ZnO and then decreased on further addition. From DSC study, it is found that the crystallization exotherm changes significantly in the temperature range 750–1000 °C, where different crystalline phases are formed, and the activation energy of crystallization (E_C) varies in the range of 254–388 kJ/mol. The crystalline phases formed in opaque BPAS glass-ceramic, derived by controlled heat treatment at 800 and 1050 °C (4 h), are identified as fluorophlogopite [KMg₃(AlSi₃O₁₀)F₂] mica and willemite (Zn₂SiO₄) by XRD technique, and confirmed by FTIR spectroscopy. The change of crystallization phenomena varying Zn⁺²/Mg⁺² ratios correspond to significant microstructural change. A wide range of thermal expansion (CTE) values are obtained for the BPAS glasses and corresponding glass-ceramics. CTE (50–500 °C) of BPAS glass without zinc content is 7.76 × 10⁻⁶/K, and decreased sequentially on increasing Zn⁺²/Mg⁺² ratio. The density of glass-ceramics after heating at 800 and 1050 °C increased linearly with increasing Zn⁺² substitution for Mg⁺². Microhardness of the BPAS glasses is in the range of 4.26–6.15 GPa and found to be increased to 4.58–6.78 GPa after crystallized at 1050 °C.     

Structural and conformational changes during thermally‐induced crystallization of poly(trimethylene terephthalate) by infrared spectroscopy

Conformational changes occurring during thermally‐induced crystallization of poly(trimethylene terephthalate) (PTT) by annealing have been studied using density measurement, differential scanning calorimetry (DSC), and mid‐infrared spectroscopy (MIR). Infrared spectra of amorphous and semicrystalline PTT were obtained, and digital subtraction of the amorphous contribution from the semicrystalline PTT spectra provided characteristic MIR spectra of amorphous and crystalline PTT. The normalized absorbance of 1577, 1173, and 976 cm^?1 were plotted against the crystallinity showing that these bands can be used unambiguously to represent the trans conformation while the band at 1358 cm^?1 can be used to represent gauche conformation of methylene segment. The presence of a weak band at 1358 cm^?1 in the amorphous spectrum suggested that a small amount of gauche conformation is present in the amorphous phase. Infrared spectroscopy has been used for the first time as a means to estimate the trans and gauche conformations of methylene segments in PTT as a function of T_a. The amount of gauche conformation was plotted against the crystalline fraction and the extrapolation of this plot to zero crystalline fraction provided a value of 0.07, suggested that the pure amorphous phase consist of ～ 7% gauche conformation. It was found that the amorphous and crystalline gauche conformation increases at the expense of amorphous trans conformation during thermally induced crystallization of PTT. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1497–1504, 2008     

Conformational studies of 1,2,3-trichloro- and 1,2,3-tribromopropane by nuclear magnetic resonance spectroscopy: attractive parallel (1:3) halogen-hydr

Analysis of the NMR spectra of 1,2,3-trichloropropane and 1,2,3-tribromopropane in various media shows the most stable conformer to be AG_-. The populations of several conformera have been estimated by using pure trans and gauche coupling constants obtained from closely similar molecules. The calculated populations found in non-polar solvents agree well with those obtained by electron diffraction studies in the gas phase. It is suggested that the AG_- form is stabilised relative to AG₊ by the former having two parallel (1:3) halogen-hydrogen attractions against one in the AG₊ form. Comparison is made to related molecules where the most stable conformers also have the greatest number of parallel (1:3) halogen-hydrogen interactions.     

Rotation about the C?N bond in 2-aza-1,3-butadiene and the N?N bond in 2,3-diaza-1,3 butadiene: A molecular orbital study

The geometry and energy of 2-aza-1,3-butadiene and 2,3-diaza-1,3-butadiene have been calculated using the 6-31G* basis set as a function of the CNCC and CNNC dihedral angles, respectively. With the 2-aza derivative potential minima are located at 0° (trans) and at about 130° for a gauche structure approximately 9.5 kJ mol^?1 less stable than the trans. Potential maxima are at about 75° giving a gauche barrier height of approximately 19 kJ mol^?1 relative to the trans structure, and at 180° (cis) giving a barrier height of approximately 14.5 kJ mol^?1 relative to the 130° gauche structure. With the 2,3-diaza derivative the gauche barrier has disappeared and there are a series of gauche structures in the region 70°–100° of almost equal energy 12.5-15 kJ mol^?1 less stable than the trans. In addition the cis barrier is much greater, nearly 70 kJ mol^?1 relative to the trans structure. Inclusion of electron correlation, accounting for about 50% of the correlation energy, produces no significant changes in the shape of the potential energy curves. There are systematic and progressive changes in almost all the geometrical parameters as the ?CH? groups in butadiene are replaced by ?N? . The outward tilt and compression within the methylene groups show adverse steric interactions to be operative in the cis structures. The values of V_nn indicate that gauche structures of both the 2-aza and the 2,3-diaza derivatives near the cis structure are more compact (as with butadiene), and gauche structures of the 2-aza derivative near the trans structure are less compact (as with butadiene). Originating in the changes in bond lengths and bond angles, rotation-independent nuclear–nuclear interactions again play an important role.     

Conformational stability and vibrations of aminopropylsilanol molecule

Density functional theory (DFT), using the B3-LYP/6-31G(d,p) method have been used to investigate the conformation and vibrational spectra of aminopropylsilanetriol (APST) NH2CH2CH2CH2Si(OH)3. The potential function for CCCSi torsion gives rise to two distinct conformers trans and gauche. The predicted energy of the more stable trans conformer is 337 cm-1 less than the energy of gauche conformer. The calculated barriers to the conformation interchange are: 1095, 2845 and 438 cm-1 for the trans to gauche, gauche to gauche and gauche to trans conformers, respectively. For the trans conformer the potential energy curve for the Si(OH)3 groups torsion in APST has been calculated changing the HOSiC dihedral angle. The barrier for the internal rotation of 3065 cm-1 has been obtained. The optimized molecular structure of APST dimer calculated for trans conformer has a SiOSi angle of 143.2 degrees, and a SiOSi bond length of 0.164 nm. A complete vibrational assignment for both conformers as well as for trans-dimer is supported by the normal coordinate analysis, calculated IR intensities as well as Raman activities. On the basis of the results, the vibrational spectra of APST aqueous solution and APST polymer have been analyzed. The average error between the observed and calculated frequencies is 14 cm-1.     

Retracted: Acid‐Catalyzed Aquation of Ni(II)‐Hydrazone Complexes: Kinetics and Solvent Effect

Complexes of the type [Ni(L)(H₂O)]Cl₂·nH₂O, where L = 2‐pyridyl‐3‐isatinbishydrazone ligands, have been synthesized and characterized on the bases of elemental analysis, molar conductance, IR, electronic spectra, and thermal analysis (TGA and DTA). Acid‐catalyzed aquation of the Ni(II) isatin‐bishydrazone complexes was followed spectrophotometrically in various water–methanol and water–acetone mixtures at temperature 298 K. Kinetic behavior of the acid aquation is a linear rate law, indicating that the acid‐catalyzed aquation of these complexes in water–methanol and water–acetone mixtures follows a rate law with k_obs = k₂[H⁺]. The effect of the mole fraction of the ganic solvent, i.e., methanol and acetone, on the acid aquation has been analyzed; the decrease in the rate constant values with increasing of the methanol or acetone ratios is attributable to the effect of the co‐organic solvent on the initial states of the acid aquation by the destabilization of the H⁺ ion.     

Effects of electron irradiation on poly(vinylidene fluoride–trifluoroethylene) copolymers studied by solid‐state nuclear magnetic resonance spectroscopy

The effects of electron irradiation on the molecular chemical structure, conformation, mobility, and phase transition of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer have been investigated with variable‐temperature, solid‐state ¹⁹F nuclear magnetic resonance (NMR). It has been found that electron irradiation converts all‐trans conformations of both VDF‐rich and TrFE‐containing segments into dynamically mixed trans–gauche conformations accompanied by a simultaneous ferroelectric‐to‐paraelectric (or amorphous) transition. The variable‐temperature ¹⁹F magic‐angle‐spinning spectra results show that the paraelectric phase melts at much lower temperatures in irradiated films than in an unirradiated sample. Moreover, ¹⁹F NMR relaxation data (spin–lattice relaxation times in both the laboratory and rotating frames) reveal that electron irradiation enhances the molecular motion in paraelectric regions, whereas the molecular motion in a high‐temperature amorphous melt (>100 °C) is more constrained in irradiated films. Besides these physical changes, electron irradiation also induces the formation of several CF₃ groups. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1714–1724, 2006     

High‐resolution solid‐state NMR study of the domain structure and molecular motion in drawn films of a vinylidene fluoride and trifluoroethylene copolymer with 73 mol % vinylidene fluoride

The heterogeneous higher order structure and molecular motion in a single crystalline film of a vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer with 73 mol % VDF was investigated with the ¹H–¹³C cross‐polarization/magic‐angle spinning NMR technique. A transient oscillation was observed in plots of the ¹³C peak intensity versus the contact time for the CH₂, CHF, and CF₂ groups. On the basis of the extended cross‐relaxation theory of spin diffusion, we determined that the oscillation behavior was caused by the TrFE‐rich segments in the chain and that the crystal consisted of VDF‐rich and TrFE‐rich domains. The former had TrFE‐rich segments in VDF and TrFE fractions of 0.24 and 0.27, respectively, and the latter had VDF‐rich segments in a VDF fraction of 0.49. The spin–lattice relaxation time T_1ρH in the rotating frame for each group was minimal in the three temperature regions of β, α_b, and α_c (↑) on heating and in the two temperature regions of α_1D and α_c (↓) on cooling. The α_c (↑) and α_c (↓) processes depended on the first‐order ferroelectric phase‐transition regions on heating and cooling, respectively. The motional modes for the other processes were confirmed by the T_1ρH minimum behavior of the VDF and TrFE groups in the TrFE‐rich domain and the VDF‐rich segments in the VDF‐rich domain. The β and α_b processes were attributed to the flip–flop motion of the TrFE‐rich segments and the competitive motion of the TrFE‐ and VDF‐rich segments in the ferroelectric phase, respectively. The α_1D process was due to the one‐dimensional diffusion motion of the conformational defects along the chain in the paraelectric phase, accompanied by the trans and gauche transformation of the VDF conformers of ttg⁺tg^? and g⁺tg^?tt. The effect of the competitive motion of the TrFE‐rich segment on the thermal stability of the VDF‐rich segment in the chain near the Curie temperature was examined. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1026–1037, 2002