Formamide electrostatic parameters

In this section we discuss the cases of CO, H2O, and formamide. Their atomic electrostatic parameters are shown in... 

Figure 3 The force exerted on the oxygen nucleus in formamide by an in-plane external point charge of -1-0.1 e that approaches the oxygen along the CO bond. Shown are the components of the force along and perpendicular to the line connecting the external charge with the nucleus. A positive force points toward the external point charge. The lines are interpolations between ab initio results (Hartree-Fock 6-31G ) whereas the full circles denote calculations with atomic electrostatic parameters (charges, dipoles, and their fluxes). (From Ref. 4, reproduced by permission of the American Chemical Society)...

It can be seen in Figure 5.6 that even an almost 10% inaccuracy in the density introduces only a <2 unit (about 7%) change in the solubility parameters. Formamide was chosen for this test because it has both large van der Waals and electrostatic contributions and it is expected that the other molecular solvents are even less sensitive to changes in the density. 

Further, for studying the role of pH and salt concentrations on bulk-electrostatic and non-bulk electrostatic contributions the same approach was made to experiments on the influence of the alcohols mentioned above on the oxygen affinity at various KC1 concentrations and pH-values 144,146). The results obtained indicate that at a low alcohol concentration the bulk-electrostatic contributions are dominant and that with increasing size of the alkyl group, alcohol and KC1 concentration, the nonbulk electrostatic, hydrophobic contributions increase. Recent results of kinetic measurements of 02 release show that cosolvents such as alcohols and formamide influence mainly the allosteric parameter L, i.e. -the equilibrium between T and R conformation and that the separation of the alcohol effects into bulk-electrostatic and hydrophobic (non-bulk electrostatic) contributions is justified. 

A priori, one might guess that the partial atomic charges of the oxygens in the carbonyl group would be more similar to each other than to the water oxygen. One would be hard put, however, to decide whether the oxygens in formaldehyde and formamide can be described by the same parameters for electrostatic and nonbonded interactions. The values obtained from the second derivatives (reported in Table 4) allow a direct determination of the answer to... 

The parameters of Williams and Cox (1984), derived from fitting the azahydro-carbons, were used for C, N and all H atoms bonded to C atoms. The O parameters were taken from fits to oxohydrocarbons (Cox et al., 1981). The parameters of Williams et al. were the most suitable for this simple comparison, as they had been carefully derived in conjunction with an electrostatic model, whereas those of Gavezzotti and Filippini (1994) had absorbed the electrostatic contribution. However, it should be noted that they were not derived for any molecule containing both N and O atoms, nor any markedly polar or hydrogen bonded system. The parameters for polar hydrogen atoms Hp(-N) were derived (Coombes, 1996) from a fit to an ab initio surface for formamide/formaldehyde (Mitchell and Price, 1990). 

Two Components of the Solubility Parameter of Formamide as Determined in Molecular Dynamics Simulations Using Charges Assigned by the COMPASS Force Field and Electrostatic Potential-Derived Charges along with the Experimental Values... 

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