Showing papers on "Conformational isomerism published in 1973"
TL;DR: In this article, the authors determined the bond distances (rg) and angles (rα) in N-methylacetamide by gas electron diffraction, and the two methyl groups are in the trans conformation about the N-C (carbonyl) bond, no other conformer being observed.
Abstract: The bond distances (rg) and angles (rα) in N-methylacetamide have been determined by gas electron diffraction as follows: C–C=1.520±0.005 A, N–C (methyl)=1.469±0.006 A, N–C (carbonyl)=1.386±0.004 A C=O=1.225±0.003 A, C–H (average)=1.107±0.005 A, ∠N–C=O=121.8±0.4°, ∠C–N–C=119.7±O.8°, ∠C–C–N=114.1±1.5°, ∠H–C–H(average)=110.4±2° and ∠H–N–C(carbonyl)=110±5°. In comparison with the molecular structure in the crystal, the N–C(carbonyl) bond is about 0.10 A longer, whereas the C–C and C=O bonds are about 0.02 and 0.01 A shorter, respectively. The two methyl groups are in the trans conformation about the N–C (carbonyl) bond, no other conformer being observed.
129 citations
TL;DR: In this paper, the infrared and Raman spectra of 1,3-dichloro-, 1, 3-bromochloro, 1 3-dibromo-chloropropane and 1 3 diiodopropanes were recorded as liquids and in the crystalline state at low temperatures.
55 citations
TL;DR: Circular dichroism, optical rotatory dispersion, and absorption studies allowed direct observation of the functioning enzyme protein and confirmed the major transformations of rhodanese indicated by study of the thermodynamic parameters.
52 citations
TL;DR: In this article, the structural and dynamical properties of [(h5-C5H5)Mo(CO)3]2 have been studied by infrared and nmr spectroscopy in various solvents.
51 citations
TL;DR: In this article, a computer method to simulate the observed Lanthanide Induced Shifts (LIS) is described and its applications to the study of the conformational equilibria in solution for simple systems are reported.
41 citations
39 citations
TL;DR: In this article, the potential curve for rotation around the central bond in 1,3-butadiene has been estimated by ab initio calculations using Gaussian-type basis functions, which suggest that the second stable form of the molecule is a gauche conformation rather than a cis.
36 citations
TL;DR: The results obtained predict that guanine containing nucleotide tend to favor the syn glycosyl conformation in sharp contrast to the anti conformation found for adenine and the common pyrimidine nucleotides.
Abstract: Potential energy calculations have been carried out on guanosine 5'-monophosphate and guanosine 3',5'-diphosphate by simultaneous variation of the rotation angles X and Ψ to assess the influence of the base on the glycosyl and the sugar-phosphate backbone conformations and their possible interdependence. The results obtained predict that guanine containing nucleotides tend to favor the syn glycosyl conformation in sharp contrast to the anti conformation found for adenine and the common pyrimidine nucleotides. This characteristic behavior of guanine nucleotides is largely a result of increased non-bonded van der Waals' and electrostatic attractive interactions between the 5'-phosphate and the amino group of the base. As seen previously [N. Yathindra and M. Sundaralingam, Biopolymers, 12, 297 (1973)] changes in the glycosyl conformation mainly affects the C(4')-C(5') bond rotations of the backbone. The most favored (χ ,ψ ) conformer corresponding to the global minimum both in 5'-GMP and 3',5'-GDP is syn-gg, (190°,80°) for C(2')-endo rings and (180°,50°) for C(3')-endo rings. It is noteworthy that the syn angle is different from the usually observed range of 210-260°. The next significant minimum in the (χ ,ψ ) energy surface corresponds to the syn-gt conformational combination (the syn angle now in the normal range), indicating that the occurrence of the normal syn conformer induces a distortion in the backbone c(4')-c(5') bond conformation from the preferred gg. These studies reveal the significant influence of the base not only on the glycosyl conformation but also on the backbone conformations of polynucleotides.
35 citations
TL;DR: This finding is regarded as additional evidence for a newly conceived conformational structure of bilirUBin and bilirubin dimethyl ester that is maximally stabilized by intramolecular hydrogen bonds.
Abstract: Dimethoxybilirubin dimethyl ester and monomethoxybilirubin dimethyl ester were prepared by treating bilirubin with diazomethane, and the correctness of the assigned structures was proved by elemental analysis as well as by i.r. and n.m.r. spectroscopy. The phenylazo compounds derived from monomethoxybilirubin dimethyl ester were also prepared and characterized spectroscopically. Dimethoxybilirubin dimethyl ester occurs in solution as a single molecular species, unlike bilirubin dimethyl ester, which in non-polar solvents exists as an equilibrium mixture of conformational isomers. This difference in the behaviour of the two compounds is explained by the absence of intramolecular hydrogen bonds in dimethoxybilirubin dimethyl ester, a situation that allows free rotation about the central methylene bridge, whereas in bilirubin dimethyl ester an internally hydrogen-bonded conformation can be distinguished by n.m.r. spectroscopy from a non-bonded family of rotamers. This finding is regarded as additional evidence for a newly conceived conformational structure of bilirubin and bilirubin dimethyl ester that is maximally stabilized by intramolecular hydrogen bonds. This is discussed in detail in the Appendix (Kuenzle et al., 1973), which also includes a description of the molecular mechanism pertaining to the reaction of bilirubin with diazomethane.
30 citations
26 citations
TL;DR: In this paper, the free energy difference between chair conformations of trans-1,2-dichloro- and -1, 2-dibromo-cyclohexane (II) and trans- 1, 4-dihalogenocyclohexanes (III) has been determined by peak area n.m. measurements in a number of solvents at low temperature.
Abstract: The free energy difference between the two chair conformations of trans-1,2-dichloro- and -1,2-dibromo-cyclohexane (II) and trans-1,4-dichloro- and -1,4-dibromo-cyclohexane (III) has been determined by peak area n.m.r. measurements in a number of solvents at low temperature. Both equilibria show a strong solvent dependence [despite the absence of a dipole moment for either of the interconverting species in (III)] and this is given a quantitative explanation in terms of a known theory of solvation. From this the vapour phase free energy differences [ΔG(aa→ee)] have been estimated as 0·9 and 1·5 kcal mol–1(II; X = Cl and Br) and 0·8 and 0·9 kcal mol–1(III; X = Cl and Br), i.e. in all cases the diaxial conformer is preferred in the vapour phase. In the trans-1,2-dihalogenocyclohexanes this is interpreted on the basis of steric (repulsive) X ⋯ X interactions in the diequatorial conformer. The extra stability of the diaxial conformer of (III), over that of the corresponding cyclohexyl halides is shown by calculations of both dipole–dipole and charge–charge interactions between the C–X bonds not to arise from this mechanism. Attractive 1,3-polar interactions between the positively charged axial hydrogen atoms and the axial halogens are shown to account for the stabilisation of the axial conformers in the 1,4-compounds.
TL;DR: The possibility that the barrier to rotation of the trans double bond within the nine-membered ring of caryophyllene might be high enough to prevent the interconversion of conformers 2 and 3 near room temperature has been examined as mentioned in this paper.
Abstract: The possibility that the barrier to rotation of the trans double bond within the nine-membered ring of caryophyllene might be high enough to prevent the interconversion of conformers 2 and 3 near room temperature has been examined. Contrary to earlier reports, it has been found that two trans epoxides are formed on peracid epoxidation of caryophyllene and hence both conformers are present. Attempts to determine whether 2 and 3 are interconverting near room temperature were inconclusive.
TL;DR: In this article, the local interaction model was used for NMR conformational analysis of 2,4-disubstituted pentanes, CH3CHRCH2CHRCH3, which are model compounds of vinyl polymers, and the relative energies of all possible conformers were expressed as sums of the five kinds of intramolecular interaction energy parameters, which were determined to reproduce the observed temperature dependence of the vicinal spin spin coupling constants between the methine and methylene protons.
Abstract: The practical usefulness of the local interaction model in the NMR conformational analysis of chain molecules is demonstrated for 2,4‐disubstituted pentanes, CH3CHRCH2CHRCH3, which are model compounds of vinyl polymers. In the present treatment based on this model, the relative energies of all the possible conformers are expressed as sums of the five kinds of intramolecular interaction energy parameters, which are determined to reproduce the observed temperature dependence of the vicinal spin‐spin coupling constants between the methine and methylene protons. The method was applied to the analyses of pentane‐2,4‐diol, 2,4‐dichloropentane and 2,4‐diphenylpentane on the basis of the NMR data in the literature. The energy parameters, the conformer energies, and the populations were determined. The observed solvent effect in pentane‐2,4‐diol can be interpreted directly as the variation in the intramolecular interaction energy between the two hydroxyl groups, −1.0 to 0.8 kcal mol−1. In the analysis of this solv...
TL;DR: In this article, a series of alkyl substituted β-chlorovinyl ketones have been determined using the IR and UV spectroscopic data of the NMR aromatic solvent induced shifts of the β-alkyl group.
TL;DR: The intramolecular potential energy of c‐(Pro‐Gly)3 has been computed over all of conformational space and conformers with all peptide bonds trans have the lowest total potential energy.
Abstract: An analytic cyclization procedure has allowed the complete specification of c-(Pro-Gly)3 conformation by three dihedral angles. Utilizing this simplification, the intramolecular potential energy of c-(Pro-Gly)3 has been computed over all of conformational space. Conformers with all peptide bonds trans have the lowest total potential energy. Circular dichroism spectra calculated for the low-energy regions provide the basis for the interpretation of experimental CD spectra.
TL;DR: In this article, the complete AA′BB′X analysis of the 1H and 19F NMR spectrum of CH2F is presented and the coupling constants obtained show that the compound exists almost entirely in the gauche rotamer in all solvents.
Abstract: The complete AA′BB′X analysis of the 1H and 19F NMR spectrum of CH2F.CH2OCO·CCl3 (1) as a neat liquid and in various solvents is given and considered in detail. The differentiation of the two conjugate solutions is obtained both from the X spectrum and by analysis at two field strengths. The coupling constants obtained show that the compound exists almost entirely in the gauche rotamer in all solvents.
TL;DR: In this article, all conformers of ortho-hydroxybenzoic acid and diortho-hoyl-hydroxynzoic acids were studied using the CNDO/2 method.
TL;DR: In this paper, the nuclear pure quadrupole resonance of the chlorine atom bound to carbon atom C1 in several hexo-and pentopyranosyl derivatives has been detected.
Abstract: Nuclear pure quadrupole resonance of the chlorine atom bound to carbon atom C1 in several hexo‐ and pentopyranosyl derivatives has been detected. Observed resonance frequencies are distributed over two groups, centered, respectively, at 35.535 and 35.123 MHz. The two groups can be related to the two possible positions, axial and equatorial, of the chlorine atom bound to the anomeric carbon. The interpretation is supported by general rules of conformational analysis and by conclusions derived from the study of the equilibrium between conformers in solution. An all electron, ``ab initio'' calculation of molecular wavefunction, carried out on a simplified model (2‐chlorotetrahydropyran) similar to the molecules under study, yields electronic densities on each atomic orbital, especially those of chlorine, as well as bond indexes for both axial and equatorial conformations. Values of the chlorine electric field gradient, as deduced from the computed populations with the help of the Townes and Dailey theory, ar...
TL;DR: In this paper, the dominant conformer in the liquid and vapour states, the cis, has been shown to exist exclusively in the crystal of cyclopropyl methyl ketone.
TL;DR: The quantum mechanical PCILO method has been used for the determination of the conformational properties of 2′,3′- and 3′,5′-cyclic nucleotides and the available experimental data from crystallographic and solution studies agree with these principal conclusions.
TL;DR: In this paper, the chiroptical properties of S-alanine (zwitterionic and non-zwittersionic), Salanine cation, and S-anion structural isomers are calculated from LCAO-MO-SCF-CI wave functions constructed on the semi-empirical CNDO MO model.
TL;DR: In this article, a scheme of structure identification by the dipole moment method with determination of conformer concentration from IR-spectra has been proposed, based on which rotational isomerism about an ordinary P-X bond in compounds have been discussed on the basis of spectroscopic data.
TL;DR: Stereochemical equilibria and conformer populations calculated for poly(vinyl chloride)(PVC) from the same parameters show significant departures from those of the oligomers of TCH.
Abstract: Stereochemical equilibration of 2,4,6-trichloro-n-heptane (TCH) in dimethyl sulphoxide at 70°C in the presence of LiCl yields a mixture containing 111 ± 02 % of the isotactic isomer and 427 ± 02 % of the syndiotactic isomer These results, in conjunction with the ratio 0362/0638 for the meso and racemic 2,4-dichloro-n-pentanes (DCP) at equilibrium at 70°C are interpreted according to the theory of stereochemical equilibriumThe statistical weight parameters thus evaluated are used for theoretical calculation of the proportions of various conformers in the three diastereoisomers of TCH These calculations are compared with estimates of Doskocilova and coworkers from nmr coupling constants It is confirmed that the less-favoured conformations contribute appreciably to the conformer populations in the isomers of TCH, owing to the small size of the chlorine substituent Stereochemical equilibria and conformer populations calculated for poly(vinyl chloride)(PVC) from the same parameters show significant departures from those of the oligomers The calculated average length of sequences of the preferred conformation (tgtg, etc) for isotactic PVC is much smaller than for an isotactic vinyl polymer having a larger substituent
TL;DR: In this article, the infra-red absorption spectra of liquid and solid methyl cyanoacetate and ECC were examined and rotational isomerism arising from internal rotation about the NCH2C-C bond was found.
Abstract: The infra-red absorption spectra of liquid and solid methyl cyanoacetate and ethyl cyanoacetate have been examined. For the methyl ester, evidence is found of rotational isomerism arising from internal rotation about the NCH2C—C bond. The enthalpy difference between the two stable rotameric forms (with the CN group cis and gauche to the CO bond) was found to be 4060 ± 300 J mol–1.For ethyl cyanoacetate, the infra-red spectrum suggests the presence of several rotamers brought about by rotation about both the NCH2C—C and the O—C2H5 bonds.
TL;DR: In this paper, Kupchik et al. showed that 10,10-dimethylphenoxastannin is a twelve-membered organotin heterocycle, and showed that it is present in solution as an equilibrium mixture of conformational isomers.
TL;DR: In this article, the infrared and Raman spectra for cyclopropanecarboxylic acid and cyclophosphamide were obtained in the liquid and crystalline states.
TL;DR: In this article, the i.r. spectra of 3-aminocrotonic esters with primary amino-groups indicate that these substances exist, in liquid state and in solution, almost exclusively as the isomers with the chelated (Z)-configuration.
Abstract: The i.r. and 1H n.m.r. spectra of 3-aminocrotonic esters with primary amino-groups indicate that these substances exist, in the liquid state and in solution, almost exclusively as the isomers with the chelated (Z)-configuration. Some complexities of the i.r. spectra are interpreted in terms of the existence of conformational isomers arising from restriction of rotation around the C–CO2R single bond. The strength of the hydrogen bond in these compounds is less than that in 3-(alkylamino)crotonates. The i.r. spectra of N-monodeuteriated 3-aminocrotonates also reveal the existence of conformers arising from restriction of rotation around the C–N single bond. The spectra of the N-dideuteriated derivatives show the existence of strong mechanical couplings which affect mainly the ν(CC), ν(C–N). and δ(NH2) vibration modes. Some very weak bands observed in the ν(N–H) region in certain solvents are attributed to a small concentration of non-chelated molecules.
TL;DR: In this article, the rotational assignment of two rotamers with the aid of normal coordinate calculations that utilized a twenty-five parameter valence force field was made for fluoroacetone.