Formamide, molecular models

PI-TPMs are soluble in dimethyl formamide (DNff), so they are amenable to be studied by solution NMR in that solvent, hi this paper we rqxirt a study of the solution spectra of a model compound and PtTPMs obtamed by the one-step route, in DMF-d7. NOE measurements sided by molecular modeling proved to be a very valuable tool to understand the conformational details of dns class of polymers in solution. It was found th a pentamer model satisfactorily explains the NOE values measured experimentally. The structure of the model ccmipound and the polymers studied are shown in Figure 1. 

In addition to heterocycles, other molecular systems have attracted theoretical attention with respect to prediction of tautomeric equilibria and solvation effects thereon. The most commonly studied example in this class is the equilibrium between formamide and formamidic acid, discussed in the next section. In addition, some continuum modeling of solvation effects on keto/enol equilibria have appeared these are presented in section 4.2.2.2. We note that the equilibrium... 

Arad et al. (1990) simulated the reaction sequence of papain by constructing several enzyme-substrate models with molecular mechanics and following reaction paths with semiempirical quantum mechanics. AMBER force field (Weiner et al., 1986a) was employed for the construction. AMI (Dewar et al, 1985) results for proton affinities of the modeled molecules were compared to 4-31G and to experiments. AMI underestimates the proton affinities of methanethiol and of imidazole but overestimates the proton affinity of methanol. However, the proton transfer reactions from methanol to imidazole and from methanethiol to imidazole are overestimated by only 6 and 11 kcal/mol, respectively, and PT from imidazolium to formamide is underestimated by 6 kcal/mol. 

Gel Permeation Chromatography. A Waters (model 150-C ALC/GPC) liquid chromatograph with a refractive index detector was used for molecular weight characterization. The unit was equipped with two sets of silanized Dupont blmodcLl columns (Zorbax PSM 60S/100S), with molecular weight range 2 x 10z to 10° and total retention volume of 24 ml. Samples were cut across the plaque thickness as with DSC. Polymer solutions were prepared 1n N,N -dimethyl -formamide, DMF, at 0.1%... 

Proton NMR studies of N-methyl formamide (NMF) and NMA at high dilution in deuterated solvents have shown that the level of cis isomer of NMF is 8% in water, 10.3% in chloroform, 8.8% in benzene, and 9.2% in cyclohexane, while the level of cis-NMA (a model for the secondary peptide bond) is 1.5% in water and does not change very much in nonpolar solvents [18]. Ab initio molecular calculations suggest that the small difference in dipole moments in cis and trans forms explain the relative insensitivity of amides to solvent change, unlike esters [22,41], This may be explained by nearly identical free energies of solvation for the two isomers [18]. The energy difference between cis and trans isomers in aqueous solution (AG° = 2.5 kcal mol-1) accounts for the preferential trans conformation adopted by most peptide bonds. Similar results were obtained with nonproline tertiary amides [22]. 

Dinur and Hagler propose a novel method to determine atomic point charges and point dipoles from derivatives of the molecular dipole moment and second moments. The method is limited to planar molecules and has been applied to hydrogen fluoride, water, formaldehyde, formamide, ethylene, benzene, and pyridine. As was also noted by Williams, they found that atomic dipoles do not necessarily point along the bond directions. Price proposed a distributed multipole model for several aromatic hydrocarbons using carbon sites only. 

Molecular Weight Determination. For polyurethanes, the number average molecular weight was estimated by vapor phase osomometry (Perkin-Elmer-Coleman Model 115) on a 2% polymer solution in dimethyl formamide. For polyurethaneureas, number average molecular weight was estimated by GPC with a dilute polymer solution (0.25% in DMF, which contains 0.05M LiBr to prevent aggregation). 

K. P. Sagarik and R. Ahlrichs, /. Chem. Phys., 86, 5117 (1987). A Test Particle Model Potential for Formamide and Molecular Dynamics Simulations of the Liquid. 

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