Simpler Molecules of Theoretical Interest

Note In view of the uncertainties introduced into the calculation of heats of formation from heats of combustion of all but the simplest organic molecules, it has been found simpler and more reliable to measure directly the heats of certain types of reactions of theoretical interest, instead of... 

The theoretical treatment of vibration-vibration transfer was outlined in Section 3. Sufficient data for a priori theoretical calculations are only available for the simpler molecules. It is interesting first to discuss the general pattern revealed by the collision numbers in Tables 5 and 6 in terms of equations (18) and (19). 

When structural and dynamical information about the solvent molecules themselves is not of primary interest, the solute-solvent system may be made simpler by modeling the secondary subsystem as an infinite (usually isotropic) medium characterized by the same dielecttic constant as the bulk solvent, that is, a dielectric continuum. Theoretical interpretation of chemical reaction rates has a long history already. Until recently, however, only the chemical reactions of systems containing a few atoms in the gas phase could be studied using molecular quantum mechanics due to computational expense. Fortunately, very important advances have been made in the power of computer-simulation techniques for chemical reactions in the condensed phase, accompanied by an impressive progress in computer speed (Gonzalez-Lafont et al., 1996). 

Next to them it is of interest to consider the phenomena of thermal dissociation in the light of the information regarding molecular structure which we have obtained from band spectra. Particularly the theoretical determination of the degree of dissociation, not only in its dependence upon temperature and pressure which is predicted by the laws of mass action, but also as regards its absolute value, is a problem which may be faced, at least for the simpler molecules, on the basis of band spectroscopical data. 

The determination of molecular weight by ultracentrifugal means is of paramount importance in the study of proteins. The subject is treated in great detail in Svedberg s book and we shall go no further than to discuss a few points of general interest. Two methods have been applied in this field, the determination of the sedimentation velocity and the study of the sedimentation equilibrium. In both methods we must distinguish between molecules with a randomly kinked loosely built structure and those which form compact particles, whether spherical or otherwise. This distinction must be made for practical reasons, because the theoretical interpretation of experimental results is much simpler and can be carried out on a safer basis in the case of compact particles than in that of loose structures. 

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