Application of carbon-13 nuclear magnetic resonance spectrometry to the study of isomer and conformer ratios of dichlorocyclohexanes in their mixtures

Abstract

A procedure less complex than that of Russel et al. [J. Am. Chem. Soc., 85, 2988(1963] for the determination of the isomer content of mixtures formed by chlorination of chlorocyclohexane with molecular chlorine is described. The method utilizes analysis of 13C NMR data. Information about the conformer content is also obtained.

A reoptimized GROMOS force field for hexopyranose‐based carbohydrates accounting for the relative free energies of ring conformers, anomers, epimers, hydroxymethyl rotamers, and glycosidic linkage conformers

“…Fourth, a Hassel–Ottar repulsion (N5) enhances the fourth‐neighbor repulsive interaction between alkyl and hydroxyl ring substituents in 1,3‐relationship, accounting for the so‐called Hassel–Ottar effect 117, 118, 208, 224–229. Taken together, the terms N3–N5 describe various forms of 1,3‐ syn ‐diaxial repulsions 117, 118, 208, 224–244. They were implemented in the form of Lennard–Jones exceptions rather than in the form of torsional interaction terms to account for the direct dependence of these steric effects on the distance between the interacting groups, i.e., these repulsive terms will only significantly affect the stability of the conformations where the substituents in 1,3‐relationship actually adopt a syn ‐diaxial relative orientation. The covalent torsional dihedral parameters, listed in Table 6, are entirely reoptimized and differ nearly systematically from those of the 53A6 (45A4) force field (see ref 73.…”

“…A set of six torsions (T6–T11) account (in balance with Lennard–Jones interactions) for the rotational preferences of exocyclic methoxyl (T6 and T7), ethyl (T8 and T9), and oxymethyl (T10 and T11; e.g., hydroxymethyl or methoxymethyl) groups. Finally, two oxygen–oxygen torsions (T12 and T13; gauche and intracyclic, respectively) determine (in balance with Lennard–Jones interactions) the relative orientational preferences of oxygen atoms (ring or exocyclic; within alkyl chains or around six‐membered rings) in third‐neighbor relationship, encompassing the gauche effect,232, 248, 263–270 the Δ2 effect,208, 229, 262, 271 other vicinal‐ gauche repulsions,117, 118, 225, 227, 229, 233, 234, 239, 262, 271 and other effects 241, 260, 266, 272–276 …”

A GROMOS Parameter Set for Vicinal Diether Functions: Properties of Polyethyleneoxide and Polyethyleneglycol

“…The main classical force fields for (bio)molecular simulation such as CHARMM, − AMBER, − OPLS, − or GROMOS − include parameters for the ether function and are typically able to reproduce the main thermodynamic and dynamic properties of simple monoethers such as DME or DEE (Figure ) with a reasonable accuracy, in the liquid phase as well as in aqueous solution. However, the direct transfer of these parameters to compounds involving vicinal diether functions, such as DXE, PEO, and PEG (Figure ), or carbohydrates, leads to a relatively inaccurate description. ,− The main reason for this lack of transferability is related to the so-called gauche -effect, ,− namely, a stereoelectronically induced preference of the OCCO dihedral angle of a vicinal diether for the two gauche over the trans configuration, although the latter would a priori appear to be favored in terms of bond-dipole and steric interactions. In aqueous solution, high dielectric screening as well as specific hydration (water bridging) may also add up to the purely stereoelectronic gauche -effect. , As a result, the appropriate calibration of the vicinal diether function within a classical force field requires an additional parametrization step subsequent to that of the monoether function.…”

Conformation, dynamics, solvation and relative stabilities of selected β-hexopyranoses in water: a molecular dynamics study with the gromos 45A4 force field

“…In the context of aldohexopyranoses, these include (i) the Hassel-Ottar effect 18,23,41,66,67,71,72,83,116 and other 1,3-syn-diaxial repulsions; 18,41,50,[67][68][69]71,72,102,[116][117][118][119][120][121][122][123][124][125][126][127] (ii) the D2 effect 23,41,128,129 and other vicinal-gauche repulsions. 18,23,50,67,69,71,72,128,129 Note, however, that the above effects may not be entirely steric in nature, that is, they may also involve a significant dipolar 126,130,131 (electrostatic) or hyperconjugative 124,[132][133]…”

“…These effects may also involve a dipolar component 165 and thus be somewhat solvent sensitive, 166,167 but this dependence should be limited compared to that of electrostatic effects. Stereoelectronic effects of relevance for aldohexopyranoses include (i) the endo-anomeric effect; 18,67,120,127,129,132,[165][166][167][168][169][170][171][172][173][174][175][176][177] (ii) the exo-anomeric effect; 166,[171][172][173]175,176,[178][179][180] (iii) the gauche effect; 119,172,[181][182][183][184][185][186][187][188] and (iv) other effects. 124,132,135,137,184,[189][1...…”