NMR Studies of Solvent Effect on Internal Rotation in Some Non-Symmetrical 1,2-Disubstituted Ethanes

The A₂B₂ system of PMR spectra of 3-bromopropionic acid, 3-chloropropionic acid, 3-bromopropionitrile, and 3-chloropropionitrile exhibit appreciable solvent effect at room temperature. NMR spectroscopic parameters of A₂B₂ spectrum as well as physical parameters related to internal rotation, i. e. the highest energy barrier and the energy difference between rotamers, were determined for these compounds in the medium of various solvents. It was found that in the case of 3-bromopropionic acid, the trans rotamer is more stable than the gauche rotamers, and the energy difference decreases with increasing dielectric constant of solvent. While in the case of 3-bromopropionitrile, the gauche rotamers were found to be more stable than the trans rotamer and the energy difference increases with increasing dielectric constant of solvent. In the remaining two compounds, 3-chloropropionic acid and 3-chloropropionitrile, both trans and gauche rotamers are equally stable in a solvent of low dielectric constant, however in a solvent of higher dielectric constant, the gauche rotamers become more stable than the trans rotamer and the energy difference becomes more pronounced with increasing dielectric constant of solvent.     

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.     

Aggregation behavior of imidazolium-based chiral surfactant in aqueous solution

A novel imidazolium-based chiral surfactant with a Y-type hydrophobic chain, (S)-(+)-1-(2,3-bis(octanoyloxy)propyl)-3-methylimidazolium chloride ([Bopmim]Cl), was synthesized. The aggregation behavior of [Bopmim]Cl in aqueous solution was then investigated by surface tension, electrical conductivity, ¹H NMR, and fluorescence measurements. Compared with [C₁₂mim]Cl, the critical micelle concentration for [Bopmim]Cl is lower, indicating that the novel chiral surfactant has superior capacity to form micelles. A larger value of pC ₂₀, a greater minimum area per surfactant molecule (A _min), a smaller degree of counterion binding (β), and a looser aggregate are caused by the relatively larger Y-type hydrophobic chain of [Bopmim]Cl. Furthermore, analysis of the ¹H NMR spectra revealed that the introduced Y-type hydrophobic chain may prevent the hydrophobic group from forming an extended chain configuration and cause a changeover from trans to gauche conformations upon micellization. The micelles of the novel chiral surfactant may provide some potential applications in the stereochemical recognition of surfaces or of biological structures.     

Étude par spectroscopie infrarouge de la conformation de quelques composés contenant le groupement —ochro—(région 2800–3000 cm−1

The specific effect of oxygen lone pairs on the trans CH vibrator, previously noted in dimethyl ether, has been observed in 1,3-dioxolane(D₅) and 1,3-dioxane(D₇). This interaction allows us to determine the conformation of some other compounds with the —OCHRO— group. In 2-methyl and 2-trichloromethyl-1,3-dioxolane, the R group is in an essentially pseudoequatorial position; in 2-ethynyl-1,3-dioxolane, an equilibrium between the two isomers is observed, the isomer with the pseudoaxial R group is more favored in solvent CCl₄ than in solvent CD₃CN. In dimethoxymethane, the gauche—gauche isomer predominates; although other forms appear in 1,1-dimethoxyethane the gauche—gauche isomer is still preferred. In trimethoxymethane, the existence of the two isomers TGG and TGG' has been confirmed. These results complete those previously obtained, essentially from the analysis of NMR spectra of the non-deuterated compounds.     

Synthesis of spiro ?,ζ-D-CNA in xylo configuration featuring noncanonical δ/?/ζ torsion angle combination

The synthesis of diastereoisomers of spiro xylo-?,ζ-D-CNA dinucleotide building units of nucleic acids, in which the ? and ζ torsional angles are stereocontrolled by a dioxaphosphorinane ring structure (D-CNA family), is described from uridine with a Mukaïyama's aldol condensation as key reaction. The NMR structure analysis showed that δ, ? and ζ torsional angles of the (S_C3′,R_P)- and (S_C3′,S_P)-configured xylo-?,ζ-D-CNA TT dimer are restricted to the {δ, ?, ζ}={cis, trans, gauche(+)} or {cis, trans, gauche(−)} conformation, respectively, that are significantly different from those typically observed in A- or B-type duplexes.     

Vibrational spectra and conformations of chloromethyldimethyl- and tris(chloromethyl)phosphine oxides

A study was carried out on the IR spectra of (CH₃)₂P(O)CH₂Cl and (CH₂Cl)₂PO. The frequencies and forms of the normal modes were calculated. The molecular mechanics method was used to calculate the energy of the various conformations of these molecules. (CH₃)₂P(O)CH₂Cl in the liquid and solution is a mixture of trans and gauche conformations, while (CH₂Cl)₃PO is a mixture of trans,gauche,gauche, trans,gauche,gauche, and gauche,gauche,gauche conformations.S. M. Kirov Kazan Chemical Engineering Institute and A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 6, pp. 38–46, November–December, 1989.     

Molecular structure of n-butane: calculation of vibrational shrinkages and an electron diffraction re-investigation

A normal coordinate analysis was carried out based on the force field of Schachtschneider and Snyder in order to calculate all amplitudes of vibration and shrinkage corrections for n-butane. The results are tabulated to aid diffraction analyses of related substances. A vapor-phase electron diffraction reinvestigation of n-butane led to experimental measurements of the principal amplitudes of vibration and to the following molecular parameters (± 3σ ): r_g(C-C) = 1.531(2)Å, r_g(C-H)= 1.117(5)Å, ∠CCC (trans. gauche average) = 113.8(4)°, ∠CCH (ave) = 111.0(5)° , gauche CCCC dihedral angle 65(6)°, % trans conformer = 54 ± 9%, and ΔG° (gauche— trans) = 497 ± 220 cal mol^?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.     

Conformational stability and rotational energy barrier of RC60–C60R dimers: hyperconjugation versus steric effect

The conformational stability of 4, 4?? disubstituted HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers were calculated at ONIOM approach (AM1:B3LYP/6-31+G**) and density functional theory (B3LYP/6-31G**). The new evidences for stability and rotational energy barriers of these dimers were obtained by natural bond orbital, natural steric and molecular orbital analyses. Based on B3LYP/6-31G** calculations, except for RC₆₀?CC₆₀R (R?=?hydrogen, tert-butyl and trimethylsilyl) where gauche is the most stable conformer, trans is a global energy minimum. The greater stability of the gauche conformer of HC₆₀?CC₆₀H over trans is the result of hyperconjugation, which dominates the instability caused by the steric effect. By increasing the size of the substituent of HC₆₀?CC₆₀R dimers, the trans becomes sterically unstable but the hyperconjugation of bulky substituents dominates (the trans is global energy minimum). The hyperconjugation stability of RC₆₀?CC₆₀R dimers dominates until R?=?iso-propyl (higher stability of trans). In the case of bulky tert-butyl and trimethylsilyl substituents, the steric energy of trans is large and overweighs the hyperconjugation effect. This favors gauche as the most stable conformer. The calculated rotational energy barrier for HC₆₀?CC₆₀R and RC₆₀?CC₆₀R dimers is less than 7.3 and more than 10?kcal/mol, respectively (depending on substitution).     

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.     

Fourier Transform Raman spectroscopy with polarization modulation

Fourier Transform Raman spectroscopy can be performed with either a Martin—Puplett interferometer (MPI) or a Michelson interferometer (MI). With linearly polarized exciting light, the MPI provides information on the Stokes parameters S₁, and S₃ of the scattered Raman light, from a single measurement. The parameter S₁ represents the difference in the scattered intensities with parallel and perpendicular polarizations while S₃ represents the difference in the scattered intensities with right and left circular polarizations. A procedure to extract these parameters with a Michelson interferometer is developed in this paper. A comparison of both approaches indicates that the use of the MPI is favored if the objective is to extract the Stokes parameters. Both approaches are equally favored if the objective is to measure the depolarization ratios. For measuring total Raman scattering, the use of a Michelson interferometer appears preferable.     

The harmonic force field and absolute infrared intensities of butyne-2

The frequencies, harmonic force field and absolute IR intensities for butyne-2-d₀ and butyne-2-d₆ are reported. The final set of “harmonized” fundamental frequencies for butyne-2-d₀ and butyne-2-d₆ obeys the Teller—Redlich product rule very well. Starting values for the force constants were obtained from the harmonic force field of propyne, and diagonal force constants were adjusted in order to reproduce the experimental “harmonized” frequencies for the d₀ and d₆ compounds.The integrated IR intensities were measured according to the Wilson—Wells—Penner—Weber method, using nitrogen as a broadening gas. Thirteen sets of ?μ/?S values were obtained from the experimental intensities, using an iterative least-squares fitting procedure. This number could be reduced to one by use of several selection criteria. The signs of the remaining set appeared to be in complete agreement with the best set for propyne as reported both by Kondo and Koga and by Bode et al. The final ?μ/?S parameters were transformed into atomic polar tensors. Both kinds of intensity parameters are discussed and compared with corresponding parameters for related molecules.     

Asymmetric synthesis of the allocolchicinoid natural product N-acetylcolchinol methyl ether (suhailamine), solid state and solution phase conformational analysis

An asymmetric synthesis of the allocolchicinoid N-acetylcolchinol methyl ether (NCME) from 3-methoxybenzaldehyde is reported. Comparison of ¹H and ¹³C NMR spectroscopic data obtained for this sample of NCME provide further evidence for the assertion that this compound is congruous with the natural product that has been dubbed suhailamine, establishing NCME as a naturally-occurring allocolchicinoid. The single crystal X-ray diffraction structure of NCME is also reported for the first time, revealing a preference for adoption of the (7S,R_a,Z) form—i.e., describing the orientation of the biaryl axis and the amide N–CO bond as well as the configuration of the stereocenic centre—in the solid state. A preference for the same form in DMSO-d₆ solution is revealed upon analysis by a range of NMR spectroscopic techniques, whilst an interconverting 69:24:7 mixture of the (7S,R_a,Z), (7S,S_a,Z) and (7S,R_a,E) forms is observed in CDCl₃.     

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.     

Molecular mechanics assessment of the configurational statistics of polyoxyethylene

Previous investigators have shown that statistical mechanical averages for configuration-dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche–trans energy difference for rotation about C? C bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM 2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH₃OCH₂CH₂O? CH₂? CH₂? OCH₂CH₂OCH₃. These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean-square unperturbed end-to-end distance, mean-square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM 2 is found to provide an estimate of the gauche–trans energy difference for rotation about C? C which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM 2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche–trans energy difference for rotation about the C? O bond. This energy difference is overestimated by MM 2.     

Orientational order of rod- and disk-like solutes in the nematic liquid crystal 5CB

Proton and deuteron nuclear magnetic resonance has been used to investigate orientational order in binary liquid crystalline mixtures. Results obtained from small rigid molecules (2-butyne, 2,4-hexadiyne, propyne, 1,2-propadiene, acetylene, 1,3,5-trichlorobenzene and 1,3,5-tribromobenzene) dissolved in the nematic phase of selectively deuterated 4-n-pentyl-4'-cyanobiphenyl (5CB-d^αβ₄), indicate that the solute and liquid-crystal molecules have different order parameters. Such observations are consistent with the predictions of mean-field theory, which, under certain assumptions, indicates that for a given mixture a plot of one order parameter, S⁽¹⁾, against that of the other parameter, S⁽²⁾, should yield a universal curve independent of temperature and composition. Fitting the experimental NMR results to these theoretical curves provides a virtual nematic-isotropic transition temperature, T^*_NI, for the pure solute.     

Picosecond spectroscopy of phenylnaphthylethylenes

S_n←S₁ transitions in the trans isomers of 1-phenyl-2-(1-naphthyl)ethylene (Ph-1N) and 1-phenyl-2-(2-naphthyl)ethylene (Ph-2N) peaked at 510 nm are assigned by picosecond absorption spectroscope. The S₁ state lifetimes of both conformers of Ph-2N are shown to depend upon solvent viscosity, demonstrating the singlet mechanism of trans—cis photoisomerization and the stilbene-like behavior of Ph-2N.     

Studies on the configuration of nitrogenous stereogenic centres in adducts of rhodium(II) tetraacylates with chiral amines: the application of 1H and 13C NMR spectroscopy

The ¹H and ¹³C NMR spectra of enantiomerically pure amines (S)-N,N-dimethyl-1-phenylethylamine, (S)-N-methyl-1-phenylethylamine, (S)-N-ethyl-1-phenylethylamine and (S)-N-ethyl-N-methyl-1-phenylethylamine in the presence of a twofold molar excess of dirhodium(II) tetratrifluoroacetate and dirhodium(II) Mosher’s acid derivatives [(4S) and (4R)] were measured in CDCl₃ as a solvent. The amines having various substituents at the nitrogen atom (H, CH₃ and CH₂CH₃) formed in such conditions as an equilibrium mixture of C_SN_R and C_SN_S 1:1 adducts. The signals of both diastereoisomers were observed in NMR spectra at either room temperature (303 K) or moderately decreased temperatures (263–273 K). The rates of mutual diastereoisomer conversion were estimated by selective inversion recovery experiments and varied from less than 0.1 to ca. 10 s^?1, depending on the ligand and temperature. Analysis of ¹³C NMR data and NOE experimental data resulted in the unambiguous determination of the configuration at the nitrogen atom with respect to the carbon stereogenic centre.Modelling of adduct structures and calculations of molecular energy and NMR parameters (GIAO) using Density Functional Theory (DFT) were performed in order to support the experimental findings. The calculations were carried out using 3-21G//B3LYP (structure optimizing) and 311G(2d,p)/LanL2DZ//B3LYP theory levels (molecular energy and NMR shielding).     

Modulation effects in the electron spin echo of trans bis-(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals

Electron spin echo modulation effects have been investigated in trans bis(2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl-4-il) ethylene biradicals involving ¹⁴N or ¹⁵N nuclei in different positions. The modulation frequencies are shown to be the nitrogen nuclear transition frequencies in radical fragments at m_S = 0 of a triplet electronic spin. Quadrupole splitting parameters for these nitrogen nuclei have been determined.     

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