Experimental and theoretical structure and vibrational analysis of ethyl trifluoroacetate,CF3CO2CH2CH3

The molecular structure and conformational properties of ethyl trifluoroacetate, CF₃CO₂CH₂CH₃, were determined in the gas phase by electron diffraction, and vibrational spectroscopy (IR and Raman). The experimental investigations were supplemented by ab initio (MP2) and DFT quantum chemical calculations at different levels of theory. Experimental and theoretical methods result in two structures with C_s (anti–anti) and C₁ (anti–gauche) symmetries, the former being slightly more stable than the latter. The electron‐diffraction data are best fitted with a mixture of 56% anti–gauche and 44% anti–anti conformers. The conformational preference was also studied using the total energy scheme, and the natural bond orbital scheme. Also, the infrared spectra of CF₃CO₂CH₂CH₃ are reported for the gas, liquid and solid states, as is the Raman spectrum of the liquid. The comparison of experimental averaged IR spectra of C_s and C₁ conformers provides evidence for the predicted conformations in the IR spectra. Harmonic vibrational wavenumbers and scaled force fields have been calculated for both conformers. Copyright © 2009 John Wiley & Sons, Ltd.     

In situ analysis of chiral components of pichtae essential oil by means of ROA spectroscopy: experimental and theoretical Raman and ROA spectra of bornyl acetate

In this paper, a novel approach to analyze in situ (−)‐bornyl acetate (BA) in pichtae essential oil (Siberian fir needle oil, Abies sibirica oil) by means of Raman optical activity (ROA) is reported. As part of this approach, a conformational study in the gas phase of (+)‐ and (−)‐BA has been carried out, predicting the presence of three conformers for each enantiomer at 298.15 K. The structures of these conformers were optimized with density functional theory with the Becke 3LYP functional and 6–311 + + g** basis set. Subsequently, the Raman and ROA spectra were simulated in order to compare them with the experimentally measured spectra of the neat enantiomers of BA. Finally, the combination of Raman and ROA spectroscopy as well as DFT calculations was successfully applied not only for the detection of BA but also for the determination of the specific enantiomer of BA present in the investigated pichtae essential oil samples. Thus, the ROA technique described here has the potential to be used as a fast and easy commercial method to control the quality of essential oils. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman and ab initio study of the conformational isomerism in the 1‐ethyl‐3‐methyl‐imidazolium bis(trifluoromethanesulfonyl)imide ionic liquid

The calculated and experimental Raman spectra of the (EMI⁺)TFSI⁻ ionic liquid, where EMI⁺ is the 1‐ethyl‐3‐methylimidazolium cation and TFSI⁻ the bis(trifluoromethanesulfonyl)imide anion, have been investigated for a better understanding of the EMI⁺ and TFSI⁻ conformational isomerism as a function of temperature. Characteristic Raman lines of the planar (p) and non‐planar (np) EMI⁺ conformers are identified using the reference (EMI⁺)Br⁻ salt. The anion conformer of C₂ symmetry is confirmed to be more stable than the cis (C₁) one by 4.5 ± 0.2 kJ mol⁻¹. At room temperature, the population of trans (C₂) anions and np cations is 75 ± 2% and 87 ± 4%, respectively. Fast cooling quenches a metastable glassy phase composed of mainly C₂ anion conformers and p cation conformers, whereas slow cooling gives a crystalline phase composed of C₁ anion conformers and of np cation conformers. Copyright © 2006 John Wiley & Sons, Ltd.     

乙醇分子的高分辨电子动量谱学研究:单个构象异构体的轨道电子动量分布

利用高分辨电子动量谱仪测量了乙醇分子外价轨道的电离能谱,通过对一系列角度关联的电离能谱进行解谱,获得了各个电离能峰对应的分子轨道电子动量分布.利用密度泛函理论方法计算了乙醇分子两种构象异构体的轨道电子动量分布,通过与实验结果进行比较,发现实验测量的电离能为14.5和15.2 eV能峰对应的电子动量分布分别与理论计算的单个构象异构体trans 8a''和gauche 9a轨道电子动量分布符合较好.     

Substituent effects on the conformational stability of allyl fluorides

By the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCH?CHCH₂F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for Y?H, Cl, Me, Vinyl, CF₃, CN, CHO, and NO₂, whereas the gauche is favored for Y?OMe, OH. A correlation of ΔH with the substituent constants σ⁺(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH₂?CHCH₂F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C₂–C₃ vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C₂–C₃ vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive π_C?C→σ*_{C? F} interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.     

Conformational,vibrational, and structural studies of 2,2,3,3,3‐pentafluoropropylamine from Raman and infrared spectra of gas,liquid, xenon solutions,and solid supported by ab initio calculations

The Raman and infrared spectra (3500–50 cm⁻¹) of the gas, liquid or solution, and solid were recorded of 2,2,3,3,3‐pentafluoropropylamine (CF₃CF₂CH₂NH₂) and the ND₂ isotopomer. Variable temperature (−55 to − 100 °C) studies of the infrared spectra (3600–400 cm⁻¹) of samples dissolved in liquid xenon have been carried out. From these data, two of the five possible conformers have been identified and their relative stabilities obtained. The enthalpy difference was determined between the more stable Tt conformer and the less stable Tg form to be 280 ± 14 cm⁻¹ (3.35 ± 0.17 kJ/mol). The first indicator is the NCCC dihedral angle (G = gauche or T = trans), and the second one (g = gauche or t = trans) is the relative position of the lone pair of electrons on nitrogen with respect to the β‐carbon. The percentage of the Tg conformer at ambient temperature is estimated to be 34 ± 2%. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for most of the observed bands for both isotopomers, supported by MP2(full)/6‐31G(d) ab initio calculations to predict the harmonic force fields, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311 + G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2011 John Wiley & Sons, Ltd.     

Computational study on the conformational preferences in nateglinide

Ab initio and semi‐empirical calculations were performed on the monomers, dimers and tetramers of the antidiabetic drug nateglinide to understand the conformational preferences and to explore their possible relation with polymorphism. The reported crystal structure of bis(nateglinide) hydronium chloride shows one asymmetric unit consisting of four different conformations of the drug nateglinide. The Becke, three‐parameter, Lee–Yang–Parr /6‐31+G(d,p) optimizations indicate that these conformers are energetically quite comparable and the differences disappear in gas phase. Our analysis shows that Φ (phi) torsion angle of this phenylalanine derivative is responsible for the observed differences in stability among the nateglinide conformations. Four different polymorphs of nateglinide (B, H, S and X2) were reported but the structural differences are not available. This quantum chemical study on the dimers of nateglinide helps in proposing the structures of polymorphs. As per the quantum chemical analysis, the dimer N‐44 is the structure of the stable polymorph, whereas, the dimers N‐AA, N‐CC and N‐AC are almost isoenergetic, thus proposed to be the structures of metastable state. The dimerization and tetramerization energies are estimated to be about ?9.0 and ?38.67 kcal/mol, respectively. The extra stability in tetrameric state compared with the dimeric form is attributed to additional hydrophobic and van der Waals interactions. Copyright © 2011 John Wiley & Sons, Ltd.     

DFT and MP2 based quantum mechanical calculations and a theoretical vibrational spectroscopic investigation on roscovitine,a potential drug to treat cancers

Theoretically possible stable conformers of free roscovitine molecule in its electronic ground state were searched by means of molecular dynamics and energy minimization calculations performed using the MM2 force field. Afterwards, geometry optimization and thermochemistry calculations were carried out at room temperature for each of the found minimum‐energy conformers using the MP2 and DFT based electronic structure methods and different Pople‐style basis sets. The results obtained from these calculations confirmed that the strong intramolecular hydrogen bonding between the purine‐nitrogen and hydroxyl‐hydrogen atoms plays an important role on the rigidity of roscovitine molecule and causes a dramatic reduction in the number of the possible stable conformers of this molecule at room temperature. Furthermore, the same calculation results also revealed that two of the found seven stable conformers are considerably more favorable in energy than the others and thus dominate the experimental room‐temperature spectra of the molecule. In the light of the theoretical vibrational spectral data obtained for these two conformers, a successful assignment of the fundamental bands observed in the experimental IR and Raman spectra recorded at room temperature for solid roscovitine and for its ethanol solution is given, and the effects of the substitution and intramolecular hydrogen bonding on the fundamental bands associated with purine and phenyl group vibrations are discussed in detail. In the fitting of the calculated harmonic wavenumbers to the corresponding experimental wavenumbers, two different scaling procedures, called ‘dual scale factors’ and ‘Scaled Quantum Mechanical Force Field (SQM FF) methodology’, were applied independently. Both procedures yielded results generally in good agreement with the experiment; however, the SQM FF methodology proved its superiority over the other. Copyright © 2010 John Wiley & Sons, Ltd.     

The pure rotational spectra of the two lowest energy conformers of the asymmetric ether C4H9OC2H5

An experimental study has been performed shedding light on the conformational energies of the asymmetric ether n-butyl ethyl ether. Rotational spectroscopy between 7.8 GHz and 16.2 GHz has identified two conformers of n-butyl ethyl ether, C₄H₉OC₂H₅. In these experiments spectra were observed as the target compound participated in an argon expansion from high to low pressure causing molecular rotational temperatures to be below 4 K. For one conformer, 95 pure rotational transitions have been recorded, for the second conformer, 20 pure rotational transitions were recorded. Rotational constants and centrifugal distortion constants are presented for both butyl ethyl ether conformers. The structures of both conformers have been identified by exploring the multi-dimensional molecular potential energy surface using ab initio calculations. From the numerous low energy conformers identified using ab initio methods, the three lowest conformers were pursued at increasingly higher levels of theory, i.e. complete basis set extrapolations, coupled cluster methods, and also taking into consideration zero point vibrational energies. The two conformers observed experimentally are only revealed to be the two lowest energy conformers when high levels of quantum chemical methodologies are employed.     

Raman spectroscopic characterization of a thiophene‐based active material for resistive organic nonvolatile memories

A combined theoretical and experimental Raman study is presented on a diphenyl bithiophene molecule known as a good candidate for the development of organic nonvolatile memory devices. Spectroscopic markers suitable to distinguish the different stable conformers of the molecule have been predicted and detected. The combined analysis of theoretical and experimental Raman spectra recorded in solution indicates that at room temperature a dynamical equilibrium, characterized by interconversion between the two more stable conformers (namely trans and cis), takes place and that the more populated species is the cis form. Referring to the solid phase instead, Raman spectra of single‐crystal samples show the presence of the only trans conformer, as confirmed by X‐ray measurements. Finally, Raman spectra of thin films, as those used for the memory device, were collected; samples just deposited from solution and after few hours from the deposition were analyzed. Following the evolution of selective spectroscopic Raman markers, an isomerization process from the abundant cis (as‐deposited) to the totally trans (after few hours) conformer in the solid phase was detected. These results open the way to the identification of the molecular isomers present in the thin film of the memory cell and finally of the active molecular species involved in the switching mechanism of the operating device. Copyright © 2009 John Wiley & Sons, Ltd.     

Toward ab initio potential energy surface for paclitaxel: a baccatin III conformational study

The ab initio conformational energy profile of baccatin III, the diterpene part of paclitaxel, a natural, taxoid anticancer agent, has been investigated using medium‐size basis sets including an electron correlation method (MP2/DFT). By exploiting the internal coordinates using random search methods, 39 conformers of baccatin III were found. The most stable conformer, competing with its 13‐C rotamers, was shown to have three intramolecular hydrogen bonds of 1, 7 and 13 hydroxyls to the carbonyl oxygen atoms of 2‐benzoate and 10‐ and 4‐acetate ester groups, respectively. The results of the polarizable continuum model (PCM) used to account for the dielectric effect of the environment show that in water, the delicate balances between intramolecular hydrogen bridges and general conformational preferences are conserved, at least in the absence of explicit solvent molecules. Copyright © 2016 John Wiley & Sons, Ltd.     

High-resolution spectroscopy near the continuum limit: the microwave spectrum of trans-3-bromo-1,1,1,2,2-pentafluoropropane

ABSTRACT

The microwave spectrum of 3-bromo-1,1,1,2,2-pentafluoropropane has been observed using CP-FTMW spectroscopy. Potential energy scans have been performed and confirm the existence of two conformers – trans and gauche – for which further structural optimisations and electric field gradient calculations have been performed in order to get highly accurate nuclear quadrupole coupling constants for assignment purposes. The combination of multiple conformers and large nuclear quadrupole coupling constants produce a very dense spectrum at an estimated 1?transition/MHz, near the continuum limit. This spectral density makes it necessary to have very sophisticated computational approaches in order to get geometric and electronic structures that are very close to experimental observation. Analysis of the spectrum allowed for the assignment of the trans conformer, but the gauche proved to be prohibitive, although it is believed to be present in the current spectrum. Full analysis of the rotational spectroscopic parameters of two isotopologues – the⁷⁹Br and⁸¹Br – have been observed and are reported. Geometric analysis of the experimentally observed conformer is also reported using Kraitchman coordinate and second moments arguments. Further analysis of the spectrum reveals the occurrence of dipole-forbidden, nuclear quadrupole allowed transitions with one forbidden transition possessing the first known x-type forbidden transition linkage pathway.     

Conformational stability and vibrational study of phenylacetyl chloride

The Fourier transform infrared spectroscopy and Fourier transform Raman spectra of phenylacetyl chloride were recorded and analyzed in the range 3500–400 and 3500–200 cm^?1 at room temperature, respectively. In order to obtain the structural information and conformational stabilities, a potential energy surface scan for internal rotation was carried out at the B3LYP/6‐31G(d) level. The potential energy surface reveals that the title compound has two minimal conformers (A and B). The optimized geometries, structural parameters, stabilities, energies, thermodynamic parameters, vibrational wavenumbers, infrared intensities, and Raman activities for the two conformers (A and B) have been obtained by employing B3LYP and MP2 calculations with 6‐311++G (d, p) basis sets. The conformational energy difference between A and B is very small, indicating that the B conformer coexists with the A conformer. The detailed vibrational assignments of vibrational spectra of each conformer have been made on the basis of the potential energy distributions analysis. The highest occupied molecular orbital –lowest unoccupied molecular orbital energy gap and molecular electrostatic potential of the two conformers have been also calculated for comparison of their chemical activities. Copyright © 2015 John Wiley & Sons, Ltd.     

Conformational landscape of small organosilicon compounds from the combined use of gas electron diffraction,IR and Raman spectroscopies and quantum chemical calculations: diethyldichlorosilane

A conformational study of diethyldichlorosilane and the elucidation of the gas‐phase molecular structures of its four conformers have been performed using the combined approach of gas‐phase electron diffraction and computational techniques. Moreover, the Raman spectrum of the liquid and the IR spectra of the gas and liquid phases have been recorded and thoroughly analyzed on the basis of the scaled quantum‐mechanical force field methodology. The results of the vibrational assignment have given spectroscopic evidence of the presence of the different conformers in the samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Conformation effects on the molecular orbitals of serine

This paper calculates the five most stable conformers of serine with Hartree--Fock theory, density functional theory (B3LYP), Moller--Plesset perturbation theory (MP4(SDQ)) and electron propagation theory with the 6-311++G(2d,2p) basis set. The calculated vertical ionization energies for the valence molecular orbitals of each conformer are in agreement with the experimental data, indicating that a range of molecular conformations would coexist in an equilibrium sample. Information of the five outer valence molecular orbitals for each conformer is explored in coordinate and momentum spaces using dual space analysis to investigate the conformational processes, which are generated from the global minimum conformer Ser1 by rotation of C₂--C₃ (Ser4), C₁--C₂ (Ser5) and C₁--O₂ (Ser2 and Ser3). Orbitals 28a, 27a and 26a are identified as the fingerprint orbitals for all the conformational processes.     

Concentration‐ and temperature‐dependent conformation change of cyclohexyl isocyanide on gold nanoparticle surfaces

The conformational changes of cyclohexyl isocyanide (CHNC) on gold nanoparticle surfaces were investigated by means of concentration‐ and temperature‐dependent surface‐enhanced Raman scattering (SERS). The equatorial chair conformer appeared to be dominant at high‐bulk concentrations or low temperatures, whereas both the equatorial and axial chair conformers of CHNC were found to exist at low‐bulk concentrations or high temperatures as in the previous reports of cyclohexanethiol (CHT). Depending on concentrations and temperatures, the spectral changes of the NC stretching vibration on gold nanoparticles appeared to be more conspicuous than those of the cyclohexyl ring modes. A density functional theory (DFT) calculation was performed at the level of B3LYP/6‐31G + + (d,p) to compare the energetic stability of the various conformers of CHT and CHNC. The energy differences between the equatorial and axial chair conformers were predicted to be smaller for CHNC than for CHT by ∼3 kJ mol⁻¹ from the DFT calculation. Copyright © 2008 John Wiley & Sons, Ltd.     

Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

The Raman and infrared spectra (4000 to 50 cm^–1) of the gas, liquid or solution, and solid have been recorded of n‐propylamine, CH₃CH₂CH₂NH₂. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm^–1) and far infrared (600 to 10 cm^–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm^–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm^–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm^–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm^–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ from both second‐order Møller–Plesset (MP2, full) and density functional theory calculations by the Becke, three‐parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r₀ structural parameters were obtained from adjusted MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.     

四氯化碳和二硫化碳中乙醇构象异构体布居的羟基伸缩振动拉曼光谱

综合利用拉曼光谱和密度泛函理论研究了乙醇在四氯化碳、二硫化碳溶液中乙醇构象异构体的布居. 首先确定了trans和gauche构象乙醇在OH伸缩振动拉曼光谱中的归属,然后结合理论计算的拉曼散射截面估计了两个异构体的能级差。 可以看出在四氯化碳和二硫化碳中trans乙醇更稳定。通过分析不同温度的拉曼光谱,发现范霍夫方程在这里并不适用. 利用玻尔兹曼分布律和理论拉曼散射截面,发现了两个异构体的能级差随着温度升高而增大,这反映了溶剂与乙醇之间越弱的分子间相互作用更有利于trans构象乙醇的布居。     

Infrared and Raman Spectra,conformations, ab initio calculations and spectral assignments of 1,3‐disilabutane (SiH3CH2SiH2CH3)

Raman spectra of 1,3‐disilabutane (SiH₃CH₂SiH₂CH₃) as a liquid were recorded at 293 K and as a solid at 78 K. In the Raman cryostat at 78 K an amorphous phase was first formed, giving a spectrum similar to that of the liquid. After annealing to 120 K, the sample crystallized and large changes occurred in the spectra since more than 20 bands present in the amorphous solid phase vanished. These spectral changes made it possible to assign Raman bands to the anti or gauche conformers with confidence. Additional Raman spectra were recorded of the liquid at 14 temperatures between 293 and 137 K. Some Raman bands changed their peak heights with temperature but were countered by changes in linewidths, and from three band pairs assigned to the anti and gauche conformers, the conformational enthalpy difference Δ_confH(gauche–anti) was found to be 0 ± 0.3 kJ mol⁻¹ in the liquid. Infrared spectra were obtained in the vapor and in the liquid phases at ambient temperature and in the solid phases at 78 K in the range 4000–400 cm⁻¹. The sample crystallized immediately when deposited on the CsI window at 78 K, and many bands present in the vapor and liquid disappeared. Additional infrared spectra in argon matrixes at 5 K were recorded before and after annealing to temperatures 20–34 K. Quantum chemical calculations were carried out at the HF, MP2 and B3LYP levels with a variety of basis sets. The HF and DFT calculations suggested the anti conformer as the more stable one by ca 1 kJ mol⁻¹, while the MP2 results favored gauche by up to 0.4 kJ mol⁻¹. The Complete Basis Set method CBS‐QB3 gave an energy difference of 0.1 kJ mol⁻¹, with anti as the more stable one. Scaled force fields from B3LYP/cc‐pVQZ calculations gave vibrational wavenumbers and band intensities for the two conformers. Copyright © 2007 John Wiley & Sons, Ltd.     

N-acetyl-L-aspartic acid-N'-methylamide with side-chain orientation capable of external hydrogen bonding

In this study, we generated and analyzed the side-chain conformational potential energy hypersurfaces for each of the nine possible backbone conformers for N-acetyl-L-aspartic acid-N' methylamide. We found a total of 27 out of the 81 possible conformers optimized at the B3LYP/6-31G(d) level of theory. The relative energies, as well as the stabilization energies exerted by the side-chain on the backbone, have been calculated for each of the 27 optimized conformers at this level of theory. Various backbone-backbone (N-H ^{. . .} O=C) and backbone-side-chain (N-H ^{. . .} O=C; N-H ^{. . .} OH) hydrogen bonds were analyzed. The appearance of the notoriously absent backbone conformer may be attributed to such side-chain-backbone (SC/BB) and backbone-backbone (BB/BB) hydrogen bonds. Received 10 February 2002 Published online 13 September 2002