Conformational equilibria in N,N-dialkyltthioureas

¹H NMR spectra at low temperature of the amide protons in the symmetrical dimethyl-, dibutyl-, and dicyclohexylthiourea in CDCl₃ solutions reve     

Conformational equilibria for 2-substituted cyclohexylidenemalononitriles

The conformational effect of predominance for the axial conformer has been observed for the title compounds and is discussed in terms of steric and orbital interactions.     

Conformational equilibria of ethanolamine and its hydrochloride in solution

The conformational preferences of ethanolamine and its hydrochloride in solution were estimated by comparing experimental NMR vicinal proton-proton coupling constants to semiemprical coupling constants for each staggered rotamer, derived by the Haasnoot-Altona method. Strong gauche preferences are observed for both ethanolamine and its hydrochloride over a wide range of solvent polarities. Concentration was not observed to significantly affect the position of the conformer equilibria.     

Conformational equilibria and nitrogen inversion in tetrahydro-1,2,5-oxadiazines

Conformational equilibria and nitrogen inversion barriers in some tetrahydro-1,2,5-oxadiazines have been investigated by ¹H NMR spectroscopy and by chemical equilibration. The conformational free energy differences obtained are: 4-methyl, 1.2±0.2; 6-p-nitrophenyl, 1.4±0.2kcal mole^?1. Barriers to inversion of the N atom at position 2 are in the region 14.1–15.6 kcal mole^?1. The conformational behaviour of the tetrahydro-1,2,5-oxadiazine ring is shown to be composed of aspects of the behaviour of the tetrahydro-1,2-and 1,3-oxazine rings.     

Conformational equilibria in 7-membered ring metal chelate complexes

Bis(diphosphine)metal and (diphosphine)(diene)metal (M = Rh, Ir) cationic complexes containing 7-membered chelate rings have been studied by low temperature ³¹P and ¹H NMR spectroscopy. For cyclooctadiene-1,4-bis(diphenylphosphino)butane- and -DIOP-rhodium, and cyclooctadiene-1,4-bis(diphenylphosphino)butane-iridium complexes, boat and chair conformations may be distinguished at low temperature. ¹H NMR spectra of these and analogous complexes suggest that both boat and chair conformers are significantly populated at room temperature. Bis(diphosphine) complexes of rhodium and iridium show very complex dynamic behaviour.     

Conformational equilibria in halocyclohexanones,an NMR and solvation study

The conformational equilibrium in 2-chloro cyclohexanone is measured in thirteen solvents from the 220 MHz¹H NMR spectrum using the C₂-H couplings and chemical shifts and the cis and trans 4-t-butyl-2-chlorocyclohexanones as reference compounds. ΔG_ea varies from 1.04 $\text{kcal}\text{mole}$ in n-pentane to ?0.58 $\text{kcal}\text{mole}$ in DMSO. The large concentration dependence of the NMR parameters in non-polar solvents noted previously is confirmed.Generalised reaction-field theory is used to calculate this solvent dependence, using a refined model of the geometry and dipole moments of the conformers.The cyclohexanone ring is considerably flatter than that of cyclohexane and this has an appreciable effect on the resultant dipole moments. Use of this geometry and CO and C-Cl bond moments which reproduce the observed dipole moments of the t-butyl compounds together with the generalised reaction field theory gives calculated solvation energies in good agreement with the observed data and hence allows the prediction of the vapour state energy difference.The model is applied to a variety of halo-ketones and gives both a reasonable explanation of the observed solvent dependencies and also the vapour state energy differences.The vapour state conformer energies are compared with the corresponding values for the halocyclohexanes and illustrate the large polar and steric effects due to the introduction of the CO group.     

Conformational equilibria in isomeric methylcyclohexanols studied by 13C NMR spectroscopy

The conformational equilibrium constants for isomeric methylcyclohexanols (cis- and trans-1,2-, 1,3- and 1,4-methylcyclohexanols) have been determined from peak area measurements in the completely proton decoupled low temperature ¹³C NMR spectra of the individual conformers. The ¹³C chemical shifts are discussed in terms of the additive model.     

Conformational equilibria in diols: the rotational spectrum of chiral 1,3-butandiol

The rotational spectra of five conformers of 1,3-butandiol have been measured by pulsed jet Fourier transform microwave spectroscopy. All of them are stabilized by an internal hydrogen bond and all of them have a GG' or a G'G arrangement of the two hydroxyl oxygens, which means that both oxygen atoms are on the same side with respect to the C1C2C3 plane. Apart from the spectroscopic constants, the relative abundances in the supersonic expansion are provided.     

Molecular mechanics of organic halides. V. Conformational equilibria in solution

With a view of using data on solutions and liquids for parameter fitting in molecular mechanical force fields, Abraham's theory of solvation is incorporated in the force field procedure. Geometries and bond moments are estimated internally, partial account being taken of bond–bond induction, and used to calculate the intramolecular electrostatic energy, dipole moment, and the dipole and quadrupole terms in the solvation energy. Three dielectric constants are used, one for the solute in the vapor, one for the solution, and one for the intramolecular space through which dipole–dipole interactions take place. Examples are given, including such where computation differs with measurement, to illustrate the performance of the scheme.     

Stereochemical studies—XXV: Conformational equilibria of 2-substituted 1,1-dialkylcyclohexanes

The positions of the conformational equilibria in a series of 1,1,2-trisubstituted cyclohexanes have been determined by ¹H NMR. The gauche-interaction of substituents in the 1,1-dimethyl series are, in general, very close to those of monosubstituted cyclohexanes. However, in the spiro-compounds 5 and 6 the equatorial conformers are somewhat destabilized by gauche-interactions though still enthalpy preferred. In contrast, the enthalpy preferred conformer in the 1,1-diethyl series is the axial one.     

1,2-Oxazine chemistry—III : Conformational equilibria in methyl derivatives of tetrahydro-1,2-oxazine

The conformational equilibria of methyl groups on the four carbon atoms of the tetrahydro-1,2-oxazine ring are investigated. Correlation of these equilibria with the 1,3-transannular distances in this system shows that oxygen is a “softer” atom than nitrogen.     

Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.     

Conformational equilibria of α-substituted carbonyl compounds. Study of solvent effects

PM3 computations have been performed to study confonnational equilibria of a series of α-substituted carbonyl compounds in solution. The solvent is taken into account in the calculations by using cavity models. The compounds studied are a series of substituted acetaldehydes CH₂R-CHO and cyclohexanones C₆H₉OR (R = F, Cl, Br, CN, NO;). The predicted solvent effects are in agreement with available experimental data. The discussion emphasizes the role played by the multipole moments and the necessity of going beyond the ellipsoidal cavity shape approximation in the case of substituted cyclohexanones.     

Conformational analysis—XXVIII : Four-component equilibria in 1,3-dioxanes. Ring deformations and the chair-twist free energy difference

Four-component equilibria in substituted 1,3-dioxanes were applied to the determination of conformational energies not accessible by conventional equilibration, with the following conclusions: 1. The difference in free energy between the chair and twist forms of 2,2,trans - 4,6 - tetramethyl - 1,3 -dioxane is 7·4 kcal/mol. 2. Equatorial Me substituents at C-4,6 exert a palpable buttressing effect on the corresponding axial substituents. 3. Equatorial substituents at C-2 exert a similar buttressing effect on the geminal axial substituent. 4. The effect of equatorial t-Bu substitution or gem-dimethyl substitution at C-5 on conformational energy seems to be of minor importance. The more complex effects of equatorial 4-t-Bu substitution are discussed.     

Conformational equilibria of terminally blocked single amino acids at the water-hexane interface. A molecular dynamics study

The conformational equilibria of the acetyl and methyl amide terminally blocked L-alanine, L-leucine and L-glutamine amino acids are examined in vacuum, in bulk water, and at the water-hexane interface, using multi-nanosecond molecular dynamics simulations. The two-dimensional probability distribution functions of finding the peptides at different dihedral angles of the backbone, phi and psi, are calculated, and free energy differences between different conformational states are determined. All three peptides are interfacially active, i.e. tend to accumulate at the interface even though they are not amphiphilic. Conformational states stable in both gas phase and water are also stable in the interfacial environment. Their populations, however, cannot be simply predicted from the knowledge of conformational equilibria in the bulk phases, indicating that the interface exerts a unique effect on the peptides. Conformational preferences in the interfacial environment arise from the interplay between electrostatic and hydrophobic effects. As in an aqueous solution, electrostatic solute-solvent interactions lead to the stabilization of more polar peptide conformations. The hydrophobic effect is manifested at the interface by a tendency to segregate polar and nonpolar moieties of the solute into the aqueous and the hexane phases, respectively. For the terminally blocked glutamine, this favors conformations for which such a segregation is compatible with the formation of strong, backbone-side chain intramolecular hydrogen bonds on the hexane side of the interface. The influence of the hydrophobic effect can be also noted in the orientational preferences of the peptides at the interface. The terminally blocked leucine is oriented such that its nonpolar side chain is buried in hexane, whereas the polar side chain of glutamine is immersed in water. The free energies of rotating the peptides along the axis parallel to the interface by more than 90 degrees are substantial. This indicates that peptide folding at interfaces is strong by driven by the tendency to adopt amphiphilic structures.