Steric and stereoelectronic effects in saturated heterocycles I. Small molecular fragment constituents. Theory vs. experiment

Probing the Influence of Solvent Effects on the Conformational Behavior of 1,3-Diazacyclohexane Systems

The conformational behavior of a 1,3-diazacyclohexane system has been investigated using the DFT B3LYP/6-311+G** level of theory. The structural parameters and relative energies predicted that anomeric effects are operative in the conformations of 1,3-diazacyclohexane. The stability of conformers predicted in the solvent continuum model (water and acetonitrile) is similar to the gas-phase results. The explicit water molecules stabilized the least-stable conformer, and the predictive trend is opposite to that of the gas-phase results. The stability of the conformers in the gas phase is a compromise between avoiding repulsions and maximizing hyperconjugative stabilization. The NBO analysis suggests that the interactions of explicit solvent molecules with 1,3-diazacyclohexane conformers attenuate the anomeric stabilization. The hydrogen-bonding interactions of explicit solvent molecules with 1,3-diazacyclohexane swamped the anomeric effects to alter the conformational stability compared to the gas-phase and solvent continuum model studies.

Probing Molecular Shape. 1. Conformational Studies of 5-Hydroxyhexahydropyrimidine and Related Compounds

Understanding the factors that determine molecular shape enables scientists to begin to understand and tailor molecular properties and reactivity. Many biomolecules and bioactive compounds contain aliphatic heterocyclic rings whose conformations play a major role in their biological activity. The interplay of a number of factors, both steric and electronic, is examined for 5-hydroxyhexahydropyrimidine (1) and related compounds with use of spectroscopy and molecular modeling.

Conformations of dimethoxydimethylsilane: matrix isolation infrared and ab initio studies

Conformations of dimethoxydimethylsilane (DMDMS) were studied using matrix isolation infrared spectroscopy, by trapping the silane in argon and nitrogen matrixes. The matrix was deposited using both an effusive and a supersonic jet source. The effusive source was maintained at two different temperatures, viz. 298 and 433 K, during deposition to alter the conformational population of the silane. The experimental results were supported by computations performed at both the HF and B3LYP levels, using 6-31++G** basis set. Vibrational frequency calculations were carried out to assign the experimental features and also to ensure that the computed structures did indeed correspond to minima. A conformer with a G+/-G-/+ structure was found to be the ground state, while G+/-T and G+/-G+/- structures were the next higher energy conformers with energies of 1.32 and 1.48 kcal/mol, respectively. Natural bond orbital analysis was carried out at both HF/6-31++G** and B3LYP/6-31++G** level which indicated that the charge-transfer hyperconjugative interactions largely determine the conformational preferences in this molecule. This interaction appears to be smaller in DMDMS than in the corresponding carbon analogue, dimethoxypropane (DMP).

Infrared spectroscopy and theoretical calculations as tools for the conformational analysis of 2-methoxycyclohexanone

The conformational equilibrium of 2-methoxycyclohexanone has been analyzed through infrared spectroscopy and theoretical calculations. These calculations indicate that six conformations may be present in the vapor phase, due to the rotation of the methoxy group around the O-C(2) bond, leading to axial (g(+), g(-) and anti) and equatorial (g(+), g(-) and anti) conformers. However, the infrared spectrum in CCl(4) solution shows just three carbonyl stretching bands, corresponding to the conformers of lower energies. An additional band is observed for the CH(3)CN solution, attributed to a fourth conformer stabilized by a dipole-dipole interaction between this conformer, having a high dipole moment, and the very polar solvent. An interpretation of the governing factors of 2-methoxycyclohexanone equilibria is also given.

Substituent influences on the stability of the ring and chain tautomers in 1,3-O,N-heterocyclic systems: characterization by 13C NMR chemical shifts, PM3 charge densities, and isodesmic reactions

Substituent effects on the stabilities of the ring and chain forms in a tautomeric equilibrium of five series of 2-phenyloxazolidines or -perhydro-1,3-oxazines possessing nine different substitutions at the phenyl moiety have been studied with the aid of 13C NMR spectroscopy and PM3 charge density and energy calculations. Reaction energies of the isodesmic reactions, obtained from the calculated energies of formation, show that electron-donating substituents stabilize both the chain and ring tautomers but the effect is stronger on the stability of the chain form than on that of the ring form. The 13C chemical shift changes induced by the phenyl substituents (SCS) were analyzed by several different single and dual substituent parameter approaches. The best correlations were obtained by equation SCS = rhoFsigmaF + rhoRsigmaR. In all cases the rhoF values and in most cases also the rhoR values were negative at both the C=N and C-2 carbons, indicating a reverse behavior of the electron density. This concept could be verified by the charge density calculations. The 13C chemical shifts of the C=N and C-2 carbons show a normal dependence on the charge density (q(tot)), but the charge density shows a reverse dependence on substitution. Correlation analysis of the 13C chemical shifts, solvent effect (CDCl3 vs DMSO-d6) on the NMR behavior as well as the effect of substituents on the electron densities and on the stabilities of the ring and chain tautomers show that the substituent dependence of the relative stability of the ring and chain tautomers in equilibrium is governed by several different electronic effects. At least intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group as well as polarization of the C=N bond seem to contribute in the chain form. Stereoelectronic and electrostatic effects are possible to explain the increase in stability of the ring form by electron-donating substituents.

Stereoelectronic effects in negatively and positively (Protonated) charged species. Ab initio studies of the gauche effect in 1,4-dioxa systems

A computational study ab initio of the conformational dependence of proton affinities of 2-methoxyethoxide (MEO), dimethoxyethane (DME), and 1,4-dioxane has been carried out at the MP2/6-31+G level of theory. The results were discussed in comparison with reference systems, from simple alkoxides and ethers to anomeric moieties, in open, cyclic, and bicyclic molecules. The COCCOC species are stronger bases than the COCOC (anomeric) ones and approach regular ethers in their strength. The gauche forms in MEO and DME are altogether stronger bases than the anti forms, and anti (equatorial) protonation is preferred over gauche (axial), unless ditopic protonation is possible, like in aga-DME or cis-tetraoxadecalin. The gauche effect plays a significant role in the formation, relative stability, and reactivity of the charged species.