Our primary focus in this work has been upon the relationship between atomic polarizability and volume, although we also looked at the role of ionization energy. For approximating volumes in this context, we tried ten different measures of atomic radii, based upon both empirical and theoretical criteria. Our results confirm that the polarizability can be expressed, to good accuracy, as directly proportional to the volume alone, provided that an appropriate set of radii is used. Most effective for the present purpose are (a) the distances to the outermost maxima of the orbital radial densities and (b) outermost orbital 〈r〉 values. Our data also support an earlier prediction that the correlation would be enhanced by the inclusion of a slowly varying periodic function of the nuclear charge.

ABSTRACT:We investigate relationships between molecular polarizability ␣, volume V, ionization energy I, and average local ionization energy I (r ៝). The last of these is a measure of the energy required to remove an electron from any point r ៝ in the space of an atom or molecule. Molecular polarizability shows a good direct correlation with volume, which can be improved to a minor extent by including an inverse dependence on I and more so by using I (r ៝) averaged over the surface of the molecule, I S,ave . The pattern of I (r ជ) on a molecular surface, I S (r ៝), exhibits significant variability, and it is shown through a series of examples that this can be related to the electronic structure of the molecule and to local polarizability. Thus, I S (r ៝) will provide a means of assessing the transferability of regional (e.g., group/bond) polarizabilities from one molecule to another.

We have examined the effects of substituents on the computed electrostatic potentials V(S)(r) and average local ionization energies I(S)(r) on the surfaces of model carbon nanotubes of the types (5,5), (6,1) and (6,0). For the (5,5) and the (6,1), the effects upon both V(S)(r) and I(S)(r) of substituting a hydroxyl group at one end are primarily localized to that part of the system. For the (6,0) tube, however, a remarkable change is observed over its entire length, with V(S)(r) showing a marked gradation from strongly positive at the substituted end to strongly negative at the other; I(S)(r) correspondingly goes from higher to lower values. Replacing OH by another resonance- donor, NH2, produces similar results in the (6,0) system, while the resonance withdrawing NO2 does the opposite, but in equally striking fashion. We explain these observations by noting that the arrangement of the C-C bonds in the (6,0) tube facilitates charge delocalization over the full length and entire surface of the tube. Substituting NH2 and NO2 at opposite ends of the (6,0) tube greatly strengthens the gradations in both V(S)(r) and I(S)(r). The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications. [figure: see text]. HF/STO-5G electrostatic potential on outer surface of open (6,0) C72H10NH2NO2. The nitro group is at the right end of the tube, the amino group at the left. In eV: purple is less than 14, blue is between 14 and 15, green is between 15 and 16.5, yellow is between 16.5 and 17.5, and red is more than 17.5.

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We have recently shown that the anti-HIV activities of reverse transcriptase inhibitors can be related quantitatively to properties of the electrostatic potentials on their molecular surfaces. We now introduce the technique of using only segments of the drug molecules in developing such expressions. If an improved correlation is obtained for a given family of compounds, it would suggest that the segment being used plays a key role in the interaction. We demonstrate the procedure for three groups of drugs, two acting on reverse transcriptase and one on HIV protease. Segmental analysis is found to be definitely beneficial in one case, less markedly so in another, and to have a negative effect in the third. The last result indicates that major portions of the molecular surfaces are involved in the interactions and that the entire molecules need to be considered, in contrast to the first two examples, in which certain segments appear to be of primary importance. This initial exploratory study shows that segmental analysis can provide insight into the nature of the process being investigated, as well as possibly enhancing the predictive capability.