Gauche Structures Research Articles

Gas Phase Structure of Methyl Trifluoromethanesulfonate, CH3OSO2CF3, and Conformational Properties of Covalent Sulfonates

The molecular structure of methyl trifluoromethanesulfonate (CH3OSO2CF3, methyl triflate) was investigated by gas electron diffraction (GED) and quantumchemical calculations (HF/3-21G* and B3LYP/6-31G*). The GED analysis revealed a gauche conformation (methyl group gauche with respect to CF3 group) with a dihedral angle φ(C−O−S−C) = 89(7)°. This value is reproduced correctly by the calculations. Some calculated bond lengths and bond angles, however, differ from experimental values by up to ±0.07 Å and ±6°. Additional B3LYP/6-31G* calculations have been performed for some covalent sulfonates. The potential functions for internal rotation around the S−O single bond in FOSO2F and ClOSO2F possess minima for gauche and trans orientation of the O−X (X = F or Cl) bond. The trans minima are 1.6 and 1.3 kcal mol-1 higher in energy. The potential functions for triflic acid, HOSO2CF3, for CH3OSO2Cl and CH3OSO2F possess minima only for gauche structures and maxima for trans orientation. These results are in partial contrast to previous experimental studies.

A Statistical Model for the Cooperative Thermochromic Transition of Polysilanes

A statistical model for the cooperative thermochromic transition of polysilanes is proposed. The basic assumption of this treatment is that the origin of the thermochromism is the conformational transition between two ordered states, i.e., all-trans and helix structures. We have calculated the partition function of a single polysilane chain by assuming that the total conformational energy of the chain can be described by a simple sum of the nearest-neighbor interaction energies and the energies of trans, gauche(+), and gauche(−) structures. A comparison of the theoretical results with the experimental intensity of absorption maxima has enabled us to estimate the free energy of stabilization of the ordered sequences. The calculation suggests that the transition-type thermochromism is observable only when the conformational stabilization energies of the ordered sequences are large.

Analysis of internal rotations in ethyl nitrite using molecular beam Fourier transform microwave spectroscopy

The microwave spectrum of ethyl nitrite (C2H5ONO) shows the presence of three rotational isomers, corresponding to cis–trans, cis–gauche, and trans–gauche structures. We have performed an investigation of the internal rotation of the methyl group in the cis–trans and cis–gauche rotamers and the motion of the ethyl group in the trans–gauche structure by using molecular beam-Fourier transform microwave (MB-FTMW) spectroscopy. Rotational spectra of these three isomers were recorded in the frequency regions 1–4 GHz and 7–26 GHz with two different spectrometers. For cis–trans ethyl nitrite we observed µa- and µb-type and for the cis–gauche form we detected µa-, µb-, and µc-type spectra. Due to the internal rotation of the methyl group, some lines were split and the torsional barriers could be determined, V3(CH3)=1082(2) cm-1 for the cis–trans and V3(CH3)=918(10) cm-1 for the cis–gauche form. For the trans–gauche form we found µa-, µb-, and µc-type spectra. The µb- and µc-type lines showed a doubling due to the interconversion between the two equivalent gauche conformers through rotation about the C–O axis. We determined the difference between the two lowest energy levels of the torsion around the C–O bond axis to be 68(3) kHz. Based on the observed splittings and the rotational constants, the Fourier coefficients of the potential hindering this internal rotation were determined to be V1=-1322 cm-1, V2=-1048 cm-1, V3=-412 cm-1, and V4=-109 cm-1 with a low barrier of 183(3) cm-1 in the trans position and a higher barrier of 1916(100) cm-1 in the cis position. Rotational, centrifugal distortion, and quadrupole coupling constants and some structural parameters have also been obtained from the analysis of spectra.

Theoretical investigation of the structure and conformational behaviour of small selenophene oligomers

The molecular structure and the conformational behaviour of selenophene, 2,2′-biselenophene (α-2Se), 2,2′:5′,2″-terselenophene (α-3Se) and 2,2′:5′,2″:5″,2″′-quaterselenophene (α-4Se) were determined through conventional ab initio and density functional calculations using a polarized valence double zeta basis set. Hartree—Fock (HF) calculations predict very flat four-fold torsional potentials where the minimum energy conformations correspond to anti—gauche structures and the less stable conformations to the syn form. The planar and perpendicular conformations are transition states. Zero point vibrational energy corrections have negligible effects. B3LYP calculations favour π-electron interactions suggesting that the planar anti form is the ground state of α-oligoselenophenes. Torsional potentials were analysed by a Fourier procedure in terms of non-bonding and conjugative interactions and were compared with corresponding data in sulfur analogues.

C(4)-h and c(5)-h Tautomers of Amthamine and Histamine Monocations

Tautomers and conformers (T/C's) involving the C4 and C5 atoms of the thiazole or imidazole rings were characterized in amthamine (2-amino-5-(2'-aminoethyl)-4-methylthiazole) and histamine (4(5)-2'-aminoethylimidazole), respectively. Critical points of the potential energy surfaces were calculated at the RHF/6-31G** level, using analytical gradients. Conformers were also determined at the MP2/6-31G**//6-31G** and MP2/6-31G**//MP2/6-31G** levels. In amthamine, a gauche conformer of C(5)-H tautomer, I+, turns out to be the second most stable T/C in the gas phase in the RHF approach. MP2 calculations generally yield larger potential energy. Topology of the charge density shows internal electrostatic interactions between S1 ... Nα for some of the gauche conformers (I+ and L+). Transition state structures for conformational changes and internal proton transfer reactions have also been characterized. In histamine, the C(4)-H tautomers are more stable than the C(5) -H ones, and they have a much higher potential energy than the most stable tautomers of amthamine. N(1) C(i)-H tautomers present similar electronic features to those of amthamine. Internal hydrogen bonds (HyB) and the ring formed in the gauche structures with internal HyB's have been characterized by using the charge density topology procedures. MP2 results are approximately similar to the RHF results, especially with respect to the conformational angles, the potential energy being larger than that from the RHF wavefunction. MP2 distances of HyB's reveal stronger HyB's than the RHF ones.

Aspects of the reaction mechanism of ethane combustion. Conformations of the ethylperoxy radical

The detailed molecular understanding of hydrocarbon combustion processes is recognized as an important goal. The specific system studied-via ab initio theoretical methods - in this work is the reaction between the ethyl radical (C{sub 2}H{sub 5}{degrees}) and molecular oxygen (O{sub 2}). It may be argued that further experimental work on this system will yield limited new insights until theoretical investigations of the potential energy surface are made. Theoretical methods used include self-consistent-field (SCF) and configuration interaction including all single and double excitations (CISD) with up to double zeta plus polarization (DZP) quality basis sets. A total of 55 distinct stationary points on the C{sub 2}H{sub 5}(O82){degrees} potential energy surface were considered here. Two excited states of the ethylperoxy radical, six conformers of the ground {sup 2}A inch state, and four conformers of the excited {sup 2}A (foot), state were studied. For the ground-state surface, the barrier between the staggered and gauche structures is only 1.0 kcal mol{sup {minus}1}. The barrier to internal rotation of the methyl group is between 2.5 and 3.0 kcal mol{sup {minus}1} depending on the level of theory. 74 refs., 3 figs., 7 tabs.

Characterization of molecular orientation in polyethylene by Raman spectroscopy

The polarization dependences of several Raman bands of roll-drawn high-density polyethylene of uniaxial symmetry were investigated at different draw ratios between 7.0 and 11.7. When the right-angle scattering geometry was used, it was possible to determine quantitatively the principal components of the Raman tensors of the 1080-, 1130-, and 1170-cm-' vibrations. The orientation coefficients (P*(cos e)) and (P4(cos e)) of the C-C bonds with trans conformation and gauche defects were also calculated by using these bands. The results obtained from the 1080-cm-' band show that the orientation of gauche structures does not vary much with the draw ratio (between 7.0 and 11.7). On the other hand, the polarization behavior of the 1130-cm-' band indicates that trans conformers in the amorphous phase orient more readily in the draw direction. When the components of the Raman tensors obtained from right-angle scattering were used, it was found that backscattering measurements could yield good estimates of (P&os e) ) and (P~(cos e)) orientation averages. Finally, a method is proposed for the rapid evaluation of these parameters from calibration curves obtained from backscattering spectra measured with the polarization direction of both the incident and scattered light parallel to the draw direction. This method should be particularly useful for the determi- nation of the distribution of the orientation of trans bonds in thick processed samples of polyethylene.

Role of Proteins in Protection against Radiation-Induced Damage in Membranes

The effect of gamma irradiation on liposomes in the presence of a large number of commercially available proteins has been studied. Experiments were designed to demonstrate that the configuration of both acyl chain and cis C = C bonds created by lipid-protein associations are crucial in autocatalyzed radiation-induced lipid peroxidation. Raman spectroscopy was used to characterize these states. Raman spectra in the C-C stretching region show three prominent bands at 1064, 1090, and 1125 cm-1, assigned to trans, gauche, and trans C-C bonds, respectively. A single symmetrical C = C stretching band assigned to the cis isomer occurs at 1660 cm-1. The intensity ratios (I1064/I1090) and (I1660/I1440) are used as Raman probes to define the conformational states of acyl chains and C = C bonds, respectively. Our data show that the ratio (I1064/I1090) decreases in the presence of proteins, indicating that these proteins induce more gauche structures. Upon irradiation, the ratio (I1064/I1090) increases by about 30% in the absence of proteins and by about 15% in the presence of proteins. This shows that proteins retain the gauche structures in irradiated samples. The ratio (I1660/I1440) decreases in liposomes containing proteins, showing that proteins modify the configuration of cis C = C bonds. Upon irradiation, this ratio decreases by about 45-50% in samples without proteins and by about 10% in samples with proteins. These data show that proteins inhibit the radiation-induced configurational changes in the cis C = C bonds. The determination of radiation-induced peroxides (as malondialdehyde equivalents) in liposomes reveals that proteins inhibit the formation of peroxide products at low molar ratio and that the preventive capacity of different proteins is different. We conclude that proteins alter the conformation of both acyl chains and cis C = C bonds in liposomes and that these altered states are less sensitive to radiation-induced peroxidation.

The structure of cis-butadiene

We have optimized the s-cis and gauche structures of butadiene with methods that include the effects of dynamical electron correlation. On the basis of the structural parameters and vibrational frequencies we conclude that the conjugation along the carbon chain is little affected by torsion of up to ≈40°, but that the steric hindrance between the terminal hydrogen atoms is considerably reduced. Our best results indicate that the gauche conformer is the minimum on the Born-Oppenheimer potential energy surface by 0.7 kcal/mol, and this value is remarkably insensitive to improvements in the n-particle and one-particle bases. It is shown that dynamical effects, as estimated by the zero-point vibrational correction, reduce this energy difference to 0.4 kcal/mol. Harmonic vibrational frequency analyses of the s-cis and gauche structures are reported and contrasted with experiment.

Rotational isomerism and charge density properties of vinylcyclopropane (VCP)

Ab initio SCF calculations of more than double zeta quality have been carried out for nearly fully optimizing the s-trans, gauche and s-cis structures of VCP. The potential curve of internal rotation, which is obtained under (partial) consideration of the structural parameter relaxation, confirms the three minimum curve of preceding experimental and theoretical investigations. As was conjectured in a microwave study,] the dipole moment of gauche VCP (0.29 D) proves to be considerably smaller than the s-trans value (0.48 D). The different CC bonds (the ring bonds, the double bond, and the bond between ring and vinyl group) are mutually compared on the basis of bond lengths, force constants, population analyses (Mulliken and Davidson—Roby analyses), and bond path considerations. Especially, a π-conjugative interaction between the cyclopropyl ring and the vinyl group is thoroughly studied as a function of internal rotation. The occurrence of two gauche structures is interpreted as a compromise between the attempt to achieve maximal π-Conjugative interaction (which favours the s-cis structure) and to avoid steric interaction of the endstanding hydrogen atoms (which disfavours the s-cis structure).

A Molecular orbital study of the rotation about the CC bond in styrene

Abstract The geometry and energy of styrene have been calculated using the 6-31G basis set as a function of the C β C 2 C 1 C 2 dihedral angle-Φ = 0°(cis), 15°, 30°, 60° and 90° — assuming that the vinyl and phenyl groups remain planar, but otherwise with full geometry optimization. Similar calculations have been carried out for 1,3-butadiene and 3-methylene-1,4-pentadiene (MPD) where rotation about 180° generates a different and not the same conformer. The torsional potential energy curve for styrene has a very flat minimum Φ = 0, i.e. the cis structure is the most stable, whereas butadiene and MPD have minima in the region Φ = 37° to 40°, indicative of more stable gauche structures. For styrene the barrier height Φ = 90° is 131.1 KJ mol −1 . These results provide strong support for the potential function obtained by Hollas and Ridley from single level vibronic fluorescence and other spectroscopic data. The distortion of the benzene ring brought about the vinyl group substitution is discussed, also the variation of the C/C and H/C bond lenghts with Φ and the change in charge on the vinyl group and the polarity of the various bonds in the conversion of the cis into the 90° gauche conformer. The stabilization energy for styrene relative to that for benzene has been evaluated according to various criteria, and, in addition, the energy associated with the distortion of the ring.

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

AbstractThe 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 Vnn 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.

Dipole Moments and Conformational Equilibrium in Some Substituted 1 -Phenyl- 1 -fluoro-2-halogenoethanes

Experimental dipole moments of substituted 1-phenyl-1-fluoro-2-halogenoethanes are compared with the vectorially and the theoretically calculated values using the CNDO/2 method. Results support the existence of a conformational mixture of these compounds as solutes; gauche structures are the prevailing conformations as in the related catecholamines.

Thermotropic state transition in isolated platelet membranes studied by Raman spectroscopy.

The thermotropic state transition of the lipids in isolated platelet membranes has been studied by Raman spectroscopy in the temperature range from -3 degrees C to +45 degrees C using the (C-H) stretching vibrations, carotenoid(s) vibrations at 1530 cm-1 and 1160 cm-1, as well as the skeletal optical vibrations. 1. The increase of temperature causes a decrease in intensity of the 2885 cm-1 band relative to the 2855 cm-1 one. The evaluation of the ratio I2885/I2855 as a function of temperature indicates a double thermotropic state transition of platelet membrane lipids: the first one near 5 degrees C and the second near 17.5 degrees C. 2. The ratio I1530/I1160 shows that the intensity variations in the carotenoid(s) peaks follow the second lipid transition. Thus, it seems that the platelet membrane carotenoid(s) might be linked to the lipids which undergo transition near 17.5 degrees C. 3. The spectral changes in the skeletal optical range suggest a considerable proportion of all trans chains in the membrane lipids at lower temperatures whereas gauche structures seem to be introduced at higher temperatures.

Spectroscopic determination of the average content of gauche structures in organic molecules containing polymethylene chains

From the analysis of Fermi resonances in the CH-stretching and HCH-bending regions of the Raman spectra of trans-polymethylene chains and CD 3CH 3CD 2, it is proposed that when gauche structures are present additional spectral features occur. A method for the determination of the relative concentration of gauche structures is proposed.

ChemInform Abstract: CONFORMATIONAL STUDY OF ETHYL NITRITE BY MICROWAVE SPECTROSCOPY

The microwave spectrum of ethyl nitrite shows the presence of three rotational isomers. Two rotamers correspond to text-decoration:underlinecis, trans[τ1(ON—OC)= 0°, τ2(CC—ON)= 180°] and text-decoration:underlinecis, gauche(τ1= 0°, τ2= 80°) structures. Relative intensities show the text-decoration:underlinecis, gauche to be less stable by 238 ± 50 cm–1. A third rotamer, less stable than the text-decoration:underlinecis, trans by 81 ± 20 cm–1, has been assigned a text-decoration:underlinetrans, gauche structure (τ1= 180°, τ2= 90°). Internal rotation about the C—O bond in the text-decoration:underlinecis isomer has been studied through relative intensities of spectra in excited torsional states. For the text-decoration:underlinetrans isomer, a precise barrier height of 137 ± 7 cm–1 separating the two equivalent gauche forms has been derived from vibration–rotation interactions in the v= 1, C—O torsional state spectra of both text-decoration:underlinetrans, gauche CH3CH2ONO and CD3CD2ONO. Dipole moments for the three rotamers are also reported.

Conformational study of ethyl nitrite by microwave spectroscopy

The microwave spectrum of ethyl nitrite shows the presence of three rotational isomers. Two rotamers correspond to text-decoration:underlinecis, trans[τ1(ON—OC)= 0°, τ2(CC—ON)= 180°] and text-decoration:underlinecis, gauche(τ1= 0°, τ2= 80°) structures. Relative intensities show the text-decoration:underlinecis, gauche to be less stable by 238 ± 50 cm–1. A third rotamer, less stable than the text-decoration:underlinecis, trans by 81 ± 20 cm–1, has been assigned a text-decoration:underlinetrans, gauche structure (τ1= 180°, τ2= 90°). Internal rotation about the C—O bond in the text-decoration:underlinecis isomer has been studied through relative intensities of spectra in excited torsional states. For the text-decoration:underlinetrans isomer, a precise barrier height of 137 ± 7 cm–1 separating the two equivalent gauche forms has been derived from vibration–rotation interactions in the v= 1, C—O torsional state spectra of both text-decoration:underlinetrans, gauche CH3CH2ONO and CD3CD2ONO. Dipole moments for the three rotamers are also reported.

Infrared and Raman group frequency correlations: Substituted oxamides

Infrared and Raman spectra of N,N′-dialkyl oxamides and N,N′-diaryl oxamides or ( N,N′-oxanilides) were recorded, and characteristic group frequencies useful in spectra-structure identification are presented. The i.r. and Raman data recorded for N,N′-dialkyl or N,N′-diaryl oxamides in the solid phase can be explained on the basis that they exist in an intermolecularly hydrogen bonded trans configuration where each oxamide group can be viewed as having C 2 h symmetry. In the vapor phase, N,N′-dimethyloxamide also exists in the trans configuration. Evidence is presented that suggest N,N,N′,N′-tetraethyloxamide exists as rotational isomers in solution, and that these are the trans and gauche structures.

Effect of Chain Conformations on the Frequency Distribution of an Idealized Chain

The frequency distribution of a simplified carbon skeleton chain is calculated for several conformations, given by successions of trans- and gauche structures in various orders. It is found that the phonon frequency distribution is sensitive to these orders. Validity of the model and use of the results are discussed.

Structure and Rotational Isomerization of Free Hydrocarbon Chains

An electron diffraction investigation has been carried out for the molecules n-butane, n-pentane, n-hexane, and n-heptane in the gaseous phase. Mean bond lengths and bond angles were reported in earlier papers. The present paper is concerned with information derivable from the observed spectrum of nonbonded distances. A scheme is presented for determining both trans and gauche bond angles despite the fact that the 2.5-Å nonbonded C···C trans and gauche radial distribution peaks are too badly overlapped to be resolved. Formulas are given for calculating torsional ``shrinkage effect'' corrections for nonbonded distances. It was found that a given gauche conformation has a higher free energy than its corresponding trans counterpart by about 610 cal/mole. If the source of this free-energy difference is steric, arising from repulsions between hydrogens on gauche carbons, there should be structural as well as thermodynamic manifestations. The positions of a total of 21 independent nonbonded C···C radial distribution peaks were interpreted in terms of a model of steric deformations in gauche structures. The results, including estimates of standard errors, were φ(gauche)=61.0±3° dihedral angle, α(trans)=112.65±0.3° C–C–C angle, α(gauche)—α(trans)=0.1°±1°, and the distortion from planarity of trans conformations adjacent to gauche was η=7±4°. The C–C bond length and C–C–C bond angle, averaged over all conformations, were 1.5328±0.002 Å and 112.71±0.15°, respectively.