Showing papers on "Conformational isomerism published in 1970"

Detection of Hindered Rotation and Inversion by NMR Spectroscopy

Abstract: Rotations and inversions in organic molecules are readily recognizable from the temperature dependence of the NMR spectra. The study of the effects of substituents on the activation barriers of such processes permits the elucidation of reaction mechanisms, and the stability limits of isomers can also be determined. The knowledge of the stability limits is necessary for the specific synthesis of stable rotamers and invertomers.

Interpretation of Open‐Shell SCF Calculations on the T and V States of Ethylene

Abstract: Self-consistent field calculations are reported on (planar and perpendicular geometric conformers of) the lowest energy triplet and singlet (pi,pi*) orbital configurations of ethylene, which are usually identified with the spectroscopic T and V states, respectively. For the planar conformation the calculation predicts a V state with the characteristics of a Rydberg state, but a T state of expected valence-shell character. The pi* orbital is much too large and the internuclear distance too small for the calculated V state, but are as expected for the T state. It is concluded that the calculated result for the supposed V state in the planar conformation is spurious due to the inadequacy of the Hartree–Fock single-configuration theory. The supposed V state here calculated may be identified with a Rydberg state which has been observed spectroscopically at ~ 1.4 eV higher energy than the V state in the optical spectrum. For the perpendicular conformation, the calculations yield resonable results for both the T and V states. Calculations also have been carried out on a singlet (sigma,pi*) state for planar ethylene; here the size of the pi* orbital is reasonable.

Infrared spectra and the molecular conformations of some aliphatic amines

Abstract: The presence of two CD stretching bands for (CH3)2CDNH2 in dilute CCl4 solution is interpreted in terms of two conformations differing in enthalpy by 0.12 ± 0.02 kcal/mole. The low frequency component is assigned to the thermodynamically more stable conformer with the CD bond trans to the N lone pair. Similar rotational isomerism is observed in CH3CD2NH2 and CD3CH2NH2. Band contours for the NH stretching vibration in some N-alkyl anilines reported previously are also interpreted in terms of rotational isomerism and a change in hybridization of the N atom when the lone pair is trans to an α-CH bond, which results in an increase in vNH and a decrease in vCH.

Conformation of poly-L-tyrosine in aqueous solution.

Abstract: The conformational transition of poly-L-tyrosine in 0.1M KCl was investigated by ORD and infrared spectroscopy, potentiometric titration, and sedimentation velocity experiments. It is shown that the fully ordered conformer is obtained by slow titration of the random coil with 0.1N HCl at 25°C. The charge-induced transition, at variance with other poly-α-amino acids, is completed in a narrow range of α. An aggregation process was detected both by potentiometric titration and sedimentation velocity. The polyamino acid aggregates around α = 0.7 at 25°C when the conformational transition is almost complete. Infrared spectra, in the region of the amide I band (1650 cm−1) showed that the transition is a random coil antiparallel β one. Evidence exists that the form is of the intramolecular type. The foregoing interpretations of ORD and CD spectra in terms of the α-helix conformation are discussed.

Rotational isomerism. Part VIII. A calculation of the rotational barriers and rotamer energies of some halogenated compounds

Abstract: A method is suggested for calculating internal rotation barriers and rotamer energy differences in halogenated ethane-like molecules. The total interaction energy in a Particular orientation is given by the summation of all steric and electrostatic interactions between non-bonded atoms plus a torsional term which is directly related to the intrinsic barrier to rotation in ethane. The electrostatic term includes all hydrogen–halogen attractive interactions besides the halogen–halogen repulsions. The derivation of parameters to describe the various interactions together with the selection of a consistent set of molecular geometries is discussed. The model reproduces barrier heights and rotamer energy differences of a large number of molecules satisfactorily. The consequences and limitations of the model are critically assessed.

Rotational isomerism: Part IX. The N.M.R. spectrum and rotational isomerism of 1,1,2,2-tetrafluoroethane [1]

Abstract: The 1H and 19F spectra of 1,1,2,2-tetrafluoroethane are completely analysed as an A4X2 system, both in the pure liquid and in a number of solvents, to obtain all the 1H and 19F chemical shifts and couplings. The solvent dependence of the coupling constants enables the couplings in the individual rotamers to be evaluated.

Assignment and conformational properties of the sugar-ring hydroxyl protons of the common nucleosides

Abstract: A 60 MHz proton magnetic resonance study has been made of the hydroxyl protons of the ribose and deoxyribose rings for the common nucleosides dissolved in mixtures of dry DMSO-d6 and C6D6. Use of this solvent system permitted the detection and assignment of the exchange-free spin-coupled multiplets for the hydroxyl protons. The assignment was confirmed by spin-decoupling experiments.Based on the magnitudes of the observed coupling constants, it is concluded that the O—H5′, group is freely rotating around the C5′—O5′ bond. It is calculated that the O—H3′ bond favors a gauche conformation relative to the C3′—H3′ bond, consistent with the preferred conformation found in the solid state for the H3′—C3′—O3′—P bonds of the nucleoside 3′-phosphates. The conformation of the O—H2′ bond relative to the C2′—H2′- bond demonstrates no preference for the gauche rotamer. Such a rotamer might be expected if a hydrogen-bond between the 2′-OH and the purine base exists in this solvent system, as well as in aqueous solution...

Neuartige Isomeriefälle bei 1:2‐CrIII‐ und CoIII‐Komplexen von o, o'‐Dihydroxyazoverbindungen: Pyramidal gebundener Stickstoff mit hoher Inversionsbarriere?

Abstract: From the dimensions described in previously known X-ray structure analyses and from NMR.-spectroscopical datas of a 1:2 CoIII complex it is concluded that metal chelates of two and tri-dentated azo compounds are present in the diketo or quinone mono hydrazone form, provided that such a form is possible. As the metal atoms replaces the hydrazone proton the coordinating nitrogen atom can be predetermined. For the first time in the case of 1:2 Cr complexes from o,o'-dihydroxy azo compounds small amounts of byproducts have been observed which have the empirical composition of the main complexes. As the coordinated nitrogen atom is sp3-hybridized, its three bonds should be arranged in a pyramidal form. Therefore the ligands should be slightly bent. In the 1:2 Cr complexes where two of these ligands are coordinated to the metal atom perpendicularly to each other (Drew-Pfitzner-arrangement) 3 conformers (+ 3 mirror images) are possible. These conformers are ascribed to one group of the observed isomeric complexes. For another group it is supposed that at least one ligand is coordinated in the plane azo form. The postulated thrice bonded nitrogen atom possesses the requirements for a high inversion barrier: it is a member of two ortho condensed rings; moreover, it is bound to a Lewis acid. No such system seems to have been described, but in some published X-ray structure analyses, there is evidence to be found of its existence.

Conformation of non-aromatic ring compounds. Part 64. NMR spectra and dipole moments of 2-alkylthiotetrahydropyrans†

Abstract: The conformational equilibrium in six 2-alkylthiotetrahydropyrans is studied by means of dipole moment measurements and NMR spectroscopy. In these compounds the anomeric alkylthio group has, in general, a slight preference for the axial position. From the occurrence of a linear relationship between the squares of the dipole moments and vicinal coupling constants the conclusion follows that only one axial and one or two equatorial conformers are present in the equilibrium. A comparison is made with the conformation of the anomeric alkoxy group.

Konformationsinduzierter Circulardichroismus in Hämoglobinen von Chironomus th. thummi

Abstract: The conformational isomers of Chironomus haemoglobin first postulated on the strength of the O2-binding properties [24] and demonstrated by electron spin resonance spectroscopy [23] could now be confirmed by measurement of the circular dichroism. All the circular dichroism bands of the oxidized haemoglobin from Chironomus in the visible and Soret region are induced or influenced conformationally. The pH-dependency of the extinction and the ellipticity of the individual bands follows a circuit process with four reaction steps, two of which describe the allosteric transformation and two the protolytic reaction of water at the 6th coordination point of the haem iron. With the aid of fit calculations we get the protolytic constants pK′2= 8.1 and pK”2= 8.3 and the allosteric constants pKa1= 7.3 and pKa2= 7.1. The existence of conformationally induced circular dichroism bands becomes obvious, if one compares the separated haemoglobins showing different degrees of allosterism. Thus for instance the individual haemoglobin IV, showing the smallest amplitude of the Bohr effect, is characterized by a minimal conformationally-induced change of ellipticity. The individual haemoglobin VIII with the highest amplitude of Bohr effect shows the strongest conformationally-induced circular dichroism bands. The pH-dependency of the absorption spectra in the visible range displays further characteristics of conformational isomerism. The 4-component system to be expected from the circuit process reveals at sufficient distance between the allosteric and protolytic pK values a system of two 2-component mixtures, which can be transformed into each other depending on pH and thus produce “isosbestic triangles”. The helix content of the Chironomus haemoglobins is not influenced by the described change of conformation and amounts to 60 and 70% according to the individual haemoglobins. The results are discussed under the aspect of isomerism of conformations and allosterism controlled by the trans-effect of the 5th ligand of the haem iron.

Conformers and Intramolecular Hydrogen Bond of trans-2-Halocyclohexanols

Abstract: By infrared studies it has been ascertained that trans-2-halocyclohexanols possess two conformers, namely, the ee-isomer and the aa-isomer, both in the liquid and in a solution, while only the eeisomer exists in the solid. In a dilute solution the ee-isomer is more stable and exists exclusively in the form with an intramolecular hydrogen bond. The values of the energy difference between the two isomers are 1.3, 1.9, and 2.2 kcal/mol for the chloro-, the bromo-, and the iodo-derivative respectively. The magnitude of the frequency shifts of the O–H stretching bands increases in the order: Cl Br>I, while the delocalization energy due to the chargetransfer increases in the opposite order. Therefore, it can be concluded that the energy due to the charge-transfer contributes dominantly to the energy o...

Molecular acoustics. Part 4.—Volume change between conformational isomers

Abstract: By comparing ultrasonic with nmr and equilibration data on conformational equilibria it has been shown that the conformers present in each of the substances 1,1,2-tribromoethane, meso-2,3-dibromobutane and 4-methyl-1,3-dioxan differ in volume by 1–3 % This result is also consistent with density data on some cyclic compounds This finite volume change is sufficient in many cases to invalidate an assumption used in evaluating enthalpy difference from molecular acoustic data

Conformational analysis of 14- and 16-membered rings

Abstract: Cyclotetradecane has in all phases the diamond-lattice conformation, whereas cyclohexadecane uses the diamond-lattice conformation in the low-temperature solid phase, but exists as a conformer mixture above the transition point and in solution; the carbonyl and ether oxygen groups prefer the position β to two “corner” atoms.

Conformation of non‐aromatic ring compounds. Part 61 Conformational equilibrium in trans‐2‐halogenocyclohexanols

Abstract: The conformational equilibrium in trans-2-bromo- and trans-2-chloro-cyclohexanol has been studied with the aid of dipole moments, NMR, IR and Raman spectroscopy. The physical properties of the individual conformers are obtained from the suitably biased 4- or 5-t-butyl derivatives. The diequatorial form appears to predominate strongly in solution. A comparison is made with the conformational features of the trans-1,2-dihalogenocyclohexanes.