Formamide structural parameters

Fig. 7.8 Structural parameters for formamide determined from gas-electron diffraction (values with uncertainty estimates in parentheses taken from M. Kitano and K. Kuchitsu, Bull. Chem. Soc. Japan, 47 (1974) 67) and HF/4-21G calculations (values taken from H. L. Sellers, V. J. Klimkowski, and L. Schafer, Chem. Phys. Lett. 58 (1978) 541).

A new parameter space for the synthesis of silsesquioxane precursors was defined by six different trichlorosilanes (R=cyclohexyl, cyclopentyl, phenyl, methyl, ethyl and tert-butyl) and three highly polar solvents [dimethyl sulfoxide (DMSO), water and formamide]. This parameter space was screened as a function of the activity in the epoxidation of 1-octene with tert-butyl hydroperoxide (TBHP) [26] displayed by the catalysts obtained after coordination of Ti(OBu)4 to the silsesquioxane structures. Fig. 9.4 shows the relative activities of the titanium silsesquioxanes together with those of the titanium silsesquioxanes obtained from silsesquioxanes synthesised in acetonitrile. The values are normalised to the activity of the complex obtained by reacting Ti(OBu)4 with the pure cyclopentyl silsesquioxane o7b3 [(c-C5H9)7Si7012Ti0C4H9]. 

From these data it is suspected that the molecules of the solvate structure of lithium ion might be largely effected by the solvent molecules. Since the solubility of some lithium salts is relatively high in MN-dimethyl formamide (DMF), concentrated solutions can also be examined. In a previous study the solvate structure of lithium has been described in an 1.5 mol dm LiNCS solution in DMF [38]. A new XD measurement has been carried out for a LiCl solution of the same concentration. Table 1 hows the structural parameters for the lithium solvates in both solutions. The structural parameters were determined by a least-squares fitting method (LSQ). After the subtraction of the contributions ascribed to the intramolecular stmcture of the DMF molecules and to the assumed structure around the anions from the total structure function of the solution, the resulted difference curve was approximated by calculated model curves. The result is shown in Figure 1. 

Structural parameters are also accurate with this approach, most often independently of whether the Coulomb potentials are included or not. A special case is, however, formamide, for which the bonds have both ionic and covalent characters. Therefore, the results of Table 2 show a clear difference between the results obtained with the inclusion of the Coulomb potentials and those obtained without these potentials. This example is, however, atypieal. 

In addition to Trouton s rule, some other parameters for measuring the structuredness of solvents have been recommended, for example a solvent dipole orientation correlation parameter [175, 200], the solvent s heat capacity density [175, 200], and a so-called Ap parameter derived from the solvent s enthalpy of vapourization minus EPD/ EPA and van der Waals interactions [201], According to these parameters, solvents can be classified as highly structured e.g. water, formamide), weakly structured e.g. DMSO, DMF), and practically non-structured e.g. -hexane and other hydrocarbons) [200, 201]. 

N.Q.R. The C1 n.q.r. spectra of cyclic 1,3,2-/i -diazaphosphorines correlate with parameters estimated by CNDO/2 calculations. The structures of the intermediates produced in the reactions of formamides and acetamides with phosphorus pentachloride and phosphoryl trichloride have been studied by n.q.r. spectroscopy. The stereochemistry of dichlorodiazadiphosphetane has been studied, and evidence on the polarity of radial chloride atoms in chlorophosphoranes discussed. ... 

Proper parametrization of proteins requires the selection of appropriate model compounds for which adequate target data exist. As the peptide backbone C, O, N, H and C atoms are common to all amino acids selection of the appropriate model compounds for optimization of the peptide backbone parameters is central to the success of any protein force field. The most often used model compounds are NMA and ALAD, shown in Figure 1. Both structures contain the peptide bond capped by methyl groups. Earlier studies often employed formamide or acetamide as model compounds however, the free amino or aldehyde groups make them poor models for the peptide bond in proteins. Data available on NMA range from structural and vibrational data in both the gas and conden.sed pha.ses to crystal structures, pure solvent properties and heats... 

We used a very large data set for the optimization of the in-termolecular energy parameters for the amide crystals. It included the structural data (unit cell parameters and the orientation of the molecules in the unit cell) for 10 amide crystals, the heat of sublimation of 6 amide crystals, and the dipole moments of urea, formamide aind N-methyl-acetamide. The experimental data on the geometry of the hydrogen bond were rather varied, including 0...H dis-... 

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