Conformational populations, calculated

Mixtures of diastereomers of 2,4,6-trlphenylheptane are epimerized. The mole fractions olisotactic, heterotactic, and syndiotactic isomers at equilibrium at 343 K are 0.217, 0.499, and 0.284, respectively. There results are interpreted according to the theory of stereochemical equilibrium. The theory of equilibria between isomers and the associated theory of the conformer populations for each isomer provide a mutually consistent interpretation of the two kinds of results, the same arbitrary parameters being used for both. Stereochemical equilibria and conformer population calculated for PS for the same parameters differ considerably from those for the oligomers. 

Some results on the molecular modeling of N,N -di-methylxYlaramide (1) and IV lV -dihexYlxylaramide (2) using MacroModel V.2 are presented. Nine minimized conformers were considered and their populations calculated. Average 2 3 3 4 were then calculated and those values... 

Conformer populations were calculated on the basis of the following relationship ... 

Table II. Calculated Energies and Conformation Populations of -Dimethylxylaraiiiide (1) Using UacrcModel V 2 ...

When a van der Waals cutoff radius of 3 A was used to calculate the conformational populations of JV,J -dihexylxYlaramide (2), similar results were obtained (Table III). Sickles-1 and 3 and the Extended conformation accounted for 94% of the total conformational population. Again, good agreement was obtained for calcd (3.66 Hz) and J2 3 xpt (3.32 Hz). 

Stereochemical equilibration of DCP in DMSO at 343 K in the presence of LiCI yields a mixture containing 36.4 I + 0.3) % of the meso isomer. The statistical weight parameters evaluated from this result are used for theoretical calculation of the proportions of various conformers in meso and racemic DCP, and also in the three diastereoisomers of TCH. Calculations for TCH are compared with estimates of others for NMR coupling constants. It is shown that the less-favoured conformations, often ignored, contribute appreciably to the conformer populations of... 

Inductive effects on dipole moments and the effects of intervening atoms on electrostatic interaction energies are represented by polarizability centers In conjunction with bond centered dipoles. Solvation energies are estimated by means of a continuum dlpole-quadrupole electrostatic model. Calculated energies of a number of conformations of meso and racemic 2,4-dichloropentane and the iso, syndio, and hetero forms of 2,4,6-triehloroheptane give satisfactory representations of isomer and conformer populations. Electrostatic effects are found to be quite important. 

Tvaroska showed in a model calculation of the solvation of these conformations of P-cellobiose that in water the conformer with

crystal structure conformation. The calculated populations of the five conformers change quite a bit in going from a polar solvent (water) to an unpolar one (dioxane). 

Figure 2.22 Calculated and experimental VCD spectra of 18. Spectra of conformations a and b are calculated at the B3LYP/TZ2P level for S-18. Lorentzian band shapes are used (y = 4.0 crrr1). The spectrum of the equilibrium mixture of a and b is obtained using populations calculated from the B3LYP/TZ2P energy difference of a and b. The numbers indicate fundamental vibrational modes. Where fundamentals of a and b are not resolved only the number is shown.

The PCM/DFT model failed to predict the intrinsic rotation (i.e. the specific rotation extrapolated to infinite dilution) of (R)-3-methylcyclopentanone dissolved in carbon tetrachloride, methanol and acetonitrile [68], This molecule has been investigated because it exists in both an equatorial and an axial form, allowing researchers to investigate the interplay of solvent and conformational effects. The conformer populations used in the Boltzmann averaging were derived from IR absorption and VCD spectra. The deviation of the calculated optical rotation from experiment was found actually to be larger when IEF-PCM was used to account for direct solvent effects (and geometry relaxation) on the optical rotation than when the gas-phase values were used. 

Agonists - The existence of two receptor populations for histamine raises the interesting question of whether the chemical mechanism of histamine interaction differs between the two receptor types. Some indications of the chemical properties which may differentiate receptor action come from studies of histamine chemistry and from structure-activity considerations of congeners. Histamine in aqueous solution is a mixture of equilibrating species, viz. ionic forms, tautomers and conformers nmr studies confirm earlier pK work indicating a N -H N -H (structures 1 and 2) tautomer ratio of approximately 4 1 for histamine monocation, and a comparable ratio for histamine base. The latter result contrasts with crystal structure data and molecular orbital predictions, and may indicate an influence of solvent on tautomer stability. Recent studies of properties pertinent to consideration of ligand-receptor interactions are conformation (MO calculations and infra-red comparison of solid state and chloroform solutions of histamine base ), electronic charge distribution, metal complexation, and phospholipid inter-... 

---