Experimental reference data dipole moments

If we take as reference the dipole moments calculated for the predicted most stable tautomers of the lactam forms of the hydroxypurines (as discussed in Section VII) and which are generally the N(7)H tautomers, we come out with a rather high dipole moment ( 9 D) for 2-hydroxypurine and relatively low dipole moments for 6-hydroxypurine (2.6 D) and 8-hydroxypurine (1.4 D) (the numbers quoted come from CNDO calculations). Under these conditions the lactimization should correspond to a decrease in the dipole moment in 2-hydroxypurine (3-7 D) and to its increase in 6-hydroxypurine (3.6-5.7 D) and 8-hydroxypurine (3.2-4.6 D). Unfortunately no experimental data are available in this field. Calculations are also available for the directions of the moments (as well as for the transition moments and the HOMO s and LEMO s of all these compounds). 

As in the case of the MM2 force field, parameterization of MM3 for amines was based mainly on experimental data with occasional references to ab initio calculations, mainly to evaluate relative conformational energies and derive appropriate torsional parameters. As mentioned above, one notable difference between the two force fields is the removal of lp on sp3 nitrogens from MM3. This simplifies the treatment of vibrational spectra and allows for a realistic treatment of nitrogen inversion which could not be handled by MM2. As usual with MM3, parameterization was aimed at reproducing a variety of molecular properties such as structure, steric energy, dipole moments, moments of inertia, heat of formation and vibrational spectra. A complete list of MM3 parameters for amines is provided in Reference 6. 

TABLE 3. Dipole moment of phenol. Experimental data are partly reproduced from Reference 128 ... 

The same procedure is, in principle, employed in the case of water. Only the degree of complexity is far larger than in the case of simple fluids. Again, essentially two sources of information can be used. One is to compute the interaction energy of a pair of water molecules at some few hundreds of configurations and then fit these results to an analytical function. The second is to guess an analytical form and then determine the parameters of this function (often referred to as a model function) that best fit to some experimental quantities, e.g., second virial coefficient, dipole moment, spectroscopic data, etc. 

XPS technique allows to study various processes on a level of the electronic shell structure. Such approach breaks the familiar sight of many concepts, so we can see the "virtual" reference point, which relates to the energy of a pure isolated atom. We seldom realize that in experiments we only can observe reliably transitions between various chemical states of materials. In such transitions, for example, at the boundary between two stoichiometric monocrystals, such a macro-level characteristic as composition is determined by matching dipole moments, i.e. by a condition that has to be met on a micro-level. Application of XPS method also brings to the conclusion that obtaining any experimental data is unavoidably accompanied by an interaction with the analyzed object, which changes its energy state. 

In addition, equation (15) suggests that the a parameter should have a temperature dependence proportional to 1/T in the case of dipole moments (it can also be shown to hold for quadrupole moments). The Redlich-Kwong equation was modified by de Santis, et al. to take the above factors into account for H2O, CO2 and mixtures of them with non-polar molecules. The modified equation will be referred to as the MRK in this chapter. For H2O and C02r de Santis, et al. (1974) calculated the temperature dependence of a from experimental data up to 800 C. They write the expression for a as ... 

The types of reference data used should reflect the properties that the method is designed to model. Among the more important of these properties are the molecular geometry, heat of formation (A/ff), dipole moment, and ionization potential. Heats of formation and molecular geometry data provide essential information on the energetics involved. No ideal experimental resource exists that can be used in defining the electron density distribution. X-ray structures provide almost complete information on where the electrons are, but the data are not in a form suitable For use in parameterization. The only... 

Experimental intensity data, reference Cartesian system, internal coordinates, force fields and L matrices for methyl chloride are the same as given in section m.C. Bond displacement coordinates are defined in Fig. 4.6. Rotational corrections to the dipole moment derivatives with respect to symmetry coordinates are evaluated using the heavy isotope method [34]. The rotational correction terms are given in Table 4.8. To illustrate the calculations in more detail the entire V matrix of methyl chloride is presented in Table 4.9. To remove die rotational terms fi om the sets of linear equations for symmetiy... 

Yurchenko et al. [80] reported flrst full-dimensional description of the electric-dipole moments of NH in their theoretical study of electric-dipole and Raman spectra of this ion. The ab initio DMSs were computed at the RCCSD(T)/aug-cc-pVTZ level of theory in the frozen-core approximation, referred to as ATZfc. Dipole moment values were computed in a numerical finite-difference procedure with an added external dipole field of 0.001 a.u. Relative to previously published theoretical results [84, 85] for NH, a noticeable improvement in the reproduction of the extant experimental data was achieved, with the electronic-property surfaces being more accurate than those previously available and therefore more useful for the prediction of spectroscopic properties, especially for transitions involving highly excited states. 

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