Restricted bond dipole

When considering goodness-of-fits in Table 12 one should remember the number of variables in the model. In the absence of symmetry, there is one parameter per atom for the M model, three per atom for the D model, and five per atom for the Q model. Thus the M + D + Q model can have nine adjustable parameters per atom. The restricted bond dipole has one parameter per bond the other two parameters of a dipole model are eliminated by requiring the dipole to be parallel to the bond. 

Williams showed that an unrestricted bond dipole model gave erratic results for the bond dipole directions. To pursue the goals of chemical reasonableness and transferability, it seems appropriate to restrict the direction of the bond dipoles. The most natural direction is along the bond. Table 12 shows that restricted bond dipole models represent the electric potential about as well as monopole models. The two models also have nearly the same number of parameters. So the choice between these two models is a matter of convenience. If long distance interaction is considered, as in crystals, use may be made of the fact that dipole-dipole energy converges much faster than monopole-monopole energy. However, as mentioned above, if ions are considered, monopole interactions are still needed. Table 13 summarizes values of restricted bond dipole moments. 

The H —O dipole varies from 0.375 to 0.450 eA for this group of compounds, while the Me —O dipole varies from 0.242 to 0.311. The restricted bond dipoles of formyl groups attached to oxygen range from 0.189 to 0.231, while those for acetyl groups range from 0.265 to 0.308. 

Table 13 Values of Restricted Bond Dipole Moments (units of 10 eA) ...

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This intramolecular interaction restricts bond rotation and leads to a stiff chain. As a result, polyacrylonitrile has a very high crystalline melting point (317 C) and is soluble... 

In polyacrylonitrile appreciable electrostatic forces occur between the dipoles of adjacent nitrile groups on the same polymer molecule. This intramolecular interaction restricts bond rotation and leads to a stiff chain. As a result, polyacrylonitrile has a very high crystalline melting point (317°C) and is soluble in only a few solvents such as dimethylacetamide and dimethylformamide and in aqueous solutions of inorganic salts. Polyacrylonitrile cannot be melt processed since extensive decomposition occurs before any appreciable flow occurs and fibres are therefore spun from solution. In one process, for example, a solution of the polymer in dimethylformamide is extruded into a coagulating bath of glycerol arid the fibre formed is drawn and wound. 

In the liquid state, the molecules are still free to move in three dimensions but stiU have to be confined in a container in the same manner as the gaseous state if we expect to be able to measure them. However, there are important differences. Since the molecules in the liquid state have had energy removed from them in order to get them to condense, the translational degrees of freedom are found to be restricted. This is due to the fact that the molecules are much closer together and can interact with one another. It is this interaction that gives the Uquid state its unique properties. Thus, the molecules of a liquid are not free to flow in any of the three directions, but are bound by intermolecular forces. These forces depend upon the electronic structure of the molecule. In the case of water, which has two electrons on the ojQ gen atom which do not participate in the bonding structure, the molecule has an electronic moment, i.e.- is a "dipole". 

Several other molecular orbital models have been applied to the analysis of VCD spectra, primarily using CNDO wave functions. The nonlocalized molecular orbital model (NMO) is the MO analog of the charge flow models, based on atomic contributions to the dipole moment derivative (38). Currents are restricted to lie along bonds. An additional electronic term is introduced in the MO model that corresponds to s-p rehybridization effects during vibrational motion. 

The stereochemical outcome of the reaction can be reconciled by assuming that the generated yhde lies in such a way as to form an anti dipole of the type seen in Figure 3.17, which subsequently undergoes cycloaddition to either the a,p- 283, or y,8- 284 double bond via endo transition states. A j yn-dipole would suffer high steric restriction between the methylene a to the nitrogen and the protons of the iminium bond. 

---