Showing papers on "Conformational isomerism published in 1974"

Linear Polyene Electronic Structure and Spectroscopy

Abstract: A linear polyene is a chain of conjugated carbon-carbon double bonds with an unbranched n-electron system. The conventional representation of alternating double and single bonds corresponds to the resonance structure of major impor­ tance in the ground state. Polyenes are distinguished from symmetrical cyanines and aromatic compounds by the fact that these other compounds have at least two resonance structures which contribute significantly to their ground state. A more quantitative distinction between linear polyenes and other classes of compounds concerns the presence of bond length

Activation Energy Spectrum of a Biomolecule: Photodissociation of Carbonmonoxy Myoglobin at Low Temperatures

Abstract: Carbon monoxide bound to myoglobin can be photodissociated with high quantum yield The subsequent rebinding can be followed optically In the temperature range between 40 and 200 K, rebinding can be described by a function of the form ${t}_{0}$, where $n$ and $H(t)={(1+\frac{t}{{t}_{0}})}^{\ensuremath{-}n}$ are temperature-dependent parameters This behavior can be explained by assuming the existence of an activation-energy spectrum; the form of this spectrum is determined The energy spectrum may be due to the existence of myoglobin conformers

Molecular Structure of N-Methylformamide as Studied by Gas Electron Diffraction

Abstract: The bond distances (rg) and angles (ra) in N-methylformamide HCONHCH3 have been determined by gas electron diffraction as follows: N–C (methyl)=1.459±0.006 A, C(carbonyl)–N=1.366±0.008 A, C=O=1.219±0.005 A, C–H(methyl)=1.114±0.025 A,

Conformational studies of ethylene glycol and its two methyl ether derivatives

Abstract: As a theoretical analysis of the conformational equilibria of ethylene glycol, methoxyethanol and dimethoxyethane, the energy of each stable conformational isomer (rotamer) of these molecules was calculated for various temperatures and solvent dielectric constants. Classical semi-empirical potential functions were used. Besides intrinsic potentials for rotation about single bonds, intramolecular dispersion and repulsive interaction, dipole-dipole interaction and hydrogen bonding energies were included. Interaction with the solvent was considered only in terms of a continuous dielectric medium interacting with the local dipoles and quadrupoles of the molecule. For each rotamer, the dihedral angles giving the lowest energy were determined. From the energies of each rotamer, Boltzmann distributions of populations were obtained, and total concentrations were calculated in various physically distinguishable states, e.g. those with and without internal hydrogen bonds, or those in which the central C-C bond take...

Rotational isomerism, barrier to internal rotation and electric dipole moment of acrylic acid by microwave spectroscopy

Abstract: Microwave spectra have been observed for the normal and acid deuterated isotopic species of the s-cis and s-trans rotamers of acrylic acid in the ground and a number of excited torsional states. Both rotameric forms are planar with the hydroxyl group adopting the cis conformation relative to the carbonyl group. The dipole moments determined from Stark effect measurements are [graphic omitted].The difference in zero point vibrational energy between the two rotamers and the 1 â†� 0 torsional frequencies have been determined from relative intensity measurements as E°s-trans—E°s-cis= 58 ± 20 cm–1, νs-cis= 105 ± 20 cm–1, νs-trans= 95 ± 20 cm–1. The potential function hindering the internal rotation is essentially two-fold with a barrier of 1340 ± 500 cm–1(16.0 ± 6.0 kJ mol–1) at an angle of 89 ± 4° from the s-cis configuration.

Conformational studies by nuclear magnetic resonance. VII—The direct observation of rotational isomers in some enamino carbonyl compounds and their thione derivatives by carbon magnetic resonance

Abstract: Carbon-13 magnetic resonance spectra of the s-cis and s-trans rotamers of enamino ketones and thiones of the general formula (where × = O or S and R-1, R-4 = alkyl) have been obtained With dimethylaminoacrolein and its thione analogue, restricted rotation could only be observed in the latter The chemical shift differences between rotamers and homologues are attributed to changes in conjugation and to induced bond polarisation, both these factors resulting from steric strain In particular, nonplanar deformations of the s-trans rotamers are deduced from their C-2 chemical shift values

Use of carbohydrate derivatives for studies of phosphorus stereochemistry. Part I. Stereochemistry of 1,3,2-dioxaphosphorinan-2-ones and synthesis of optically active phosphine oxides

Abstract: Pairs of 1,3,2-dioxaphosphorinan-2-ones, epimeric at phosphorus, have been prepared by treatment of methyl 2,3-di-O-methyl-α-D-glucopyranoside and -galactopyranoside with phosphonic or phosphoric dihalides. The configuration at phosphorus in these derivatives has been assigned on the basis of n.m.r. and i.r. data and of comparison of the rates of formation and relative stabilities of the epimers. For the formation of the 1,3,2-dioxaphosphorinan-2-ones it is suggested that the kinetic preference for the thermodynamically less stable isomers depends on steric interactions in twist-ring transition intermediates, and that the subsequent equilibration depends on the relative stabilities of the chair conformers. The conversions of methyl 2,3-di-O-methyl-α-D-glucopyranoside (R)- and (S)-4,6-methyl phosphonate into (S)- and (R)-ethylmethylphenylphosphine oxide, respectively, by sequential addition of phenylmagnesium bromide and ethylmagnesium bromide provide examples of the utility of carbohydrate 1,3,2-dioxaphosphorinan-2-ones for the stereospecific synthesis of optically active phosphine oxides.

Conformational Isomerism and Oriented Polycrystal Formation of Dimethyl Carbonate

Abstract: It has been found that dimethyl carbonate exists in the liquid phase as a conformational equilibrium mixture. The conformers differ by rotation about the C—O bonds. The enthalpy difference of the t...

Nonbonded interactions in membrane active cyclic biopolymers. I. valinomycin‐potassium ion complex

Abstract: A method for quantitative comparison of various conformers of cyclic macromolecules in terms of nonbonded interaction energy terms is proposed. The first step in the procedure is an analytical evaluation of a set of Cartesian atomic coordinates consistent with the ring closure condition and prescribed values for internal parameters. A variant of the same technique enables different conformations of the macromolecule to be generated. Nonbonded interactions of the electrostatic, polarization and dispersion type and van der Waals repulsion are calculated using the generated coordinates, empirical bond polarizability data and results of simple molecular orbital calculations. An application of the proposed techniques to the closed symmetric conformation of valinomycin indicates that the macromolecule by itself is somewhat constrained in this state but that its affinity for a potassium ion is very strong—much stronger than the hydration energy of the latter.

Localized orbitals and short‐range molecular interactions. III. Rotational barriers in C2H6 and H2O2

Abstract: Ab‐initio calculations for eclipsed and staggered C2H6 and for several conformers of H2O2 are performed using a single determinant of doubly occupied nonorthogonal localized bond orbitals constructed from a minimal set of Slater AO's. The energies calculated with optimized BO's closely follow the pattern of the full SCF calculations. A component analysis of the rotation barriers is given along the lines suggested in Paper I [V. Magnasco and G. F. Musso, J. Chem. Phys. 60, 3744 (1974)]. The barrier in ethane (2.67 kcal) is primarily due to the penetration of nonorthogonal electron pairs belonging to vicinal C–H bonds into the core provided by nuclei and unperturbed electrons of all other groups. Although the minimal set is not suited to account for the small trans barrier in H2O2 both in BO and full SCF calculations, the calculated cis barrier in H2O2 (13.79 kcal) is seen to have a nature similar to that found in ethane, except for an enhanced importance of the Coulombic contribution. Delocalization effects are found to be of little importance in determining the conformational behavior of both molecules.

Use of 13C-1H spin-coupling constants in the determination of side-chain conformations of amino-acids

Abstract: It is shown that by considering the 13C–C–C–H coupling constant between the α-CO2– and the β-protons in amino-acids it is possible to obtain side-chain conformational information concerning the rotamer populations which is not available from studies of H–H coupling constants alone.

Conformational analysis of poly(ethylene oxide) and model compounds by nuclear magnetic resonance spectroscopy

Abstract: In order to determine the conformation of poly(ethylene oxide) (PEO), 13C satellite spectra of PEO, 1,2-dimethoxyethane (DME), and dioxane were measured at various temperatures in various solvents, and analyzed. Relations between the coupling constants were derived from the linearity between the parameters N = JAB + JA′B and L = JA′B in AA′BB′ spectra of PEO and DME. The vicinal coupling constants for the individual rotational isomers were obtained from the above relations and the temperature dependences of N and L and the enthalpy differences were calculated in each solvent. The gauche rotamer is more stable than the trans isomer by 250–500 cal/mole in all cases examined.

All‐valence‐shell molecular orbital calculations on the chiroptical properties of substituted and unsubstituted 2,5‐diketopiperazine structures

Abstract: The chiroptical properties of L-3-methyl-2,5-diketopiperazine (L-alanylglycyl anhydride) are examined on a theoretical model in which the electronic wave functions are obtained from semi-empirical all-valence-shell molecular orbital calculations. The INDO molecular orbital model is used to perform SCF-MO calculations on the ground states of six conformation isomers of L-3-methyl-2,5-diketopiperazine and two chiral conformational isomers of unsubstituted 2,5-diketopiperazine. Excited-state wave functions are constructed in the virtual orbital-configuration interaction approximation. The rotatory strengths, dipole strengths, oscillator strengths, and dissymmetry factors of the first eight singlet–singlet transitions for each of the eight structures are calculated and reported. Additionally, ground-state dipole moments, net atomic charges, and the first four ionization potentials (calculated according to Koopman's theorem) are computed for each structure. The signs and the magnitudes of the rotatory strengths are found to be extremely sensitive to the conformation of the piperazine ring as well as to methyl substitution at the α carbon of the ring. Spectra–structure relationships based on the calculations reported here are discussed, and the available experimental CD data on dissymmetric 2,5-diketopiperazine are examined in terms of our theoretical results.