The effect of spatial confinement on the properties of isoelectronic molecules HF, H2O, NH3, and CH4 has been studied by encapsulating them in a C60 fullerene cage. Second-order Møller-Plesset perturbation theoretical (MP2) calculations suggest that all the guest species are stable inside the fullerene cage. This stabilization arises from the dispersion interaction between the guest and the host. It is shown that the excitation energy (Esigma*-Esigma) for the X-H bond increases and that there is a blue shift in the stretching frequencies due to confinement.

The absorption spectra of nitroxides in the visible region are highly solvatochromic. In this paper we report some data on solvent effects on the band energy, ET, of 2,2,6,6-tetramethylpiperidinyl-1 -oxy (TEMPO) and 4-oxo-2,2,6,6-tetramethylpiperidinyl-l-oxy (OTEMPO). Special attention has been given to hydroxylic solvents and particularly to a series of reference solvents of graded hydrophilicity, dodecane, primary alcohols, ethanol-water and methanol-water mixtures, and water. The polarity index H, defined as the ratio of molar concentration of OH dipoles in these solvents to that in water (55.4 M), has been shown to exhibit a good linear correlation with their bulk dielectric constant, D. This relation has been examined in terms of theories of dielectric constants of associated liquids. ET values of both TEMPO and OTEMPO show excellent linear correlations with D and H in hydroxylic solvents including methanol-water and ethanol-water mixtures. The values also show good linear correlations with the nitrogen hyperfine splitting constants (AN) of di-tert-butyl nitroxide (DTBNO) and other similar molecules for both aprotic and hydroxylic solvents. A general theoretical approach for describing aprotic solvent effects has been explored. It is found that a reaction field treatment for estimating the effects of the solvent dielectric constant on the energy of the nitroxide dipole gives a reasonable description of An and ET data in many aprotic solvents. The role of the effective polarizability of the dipole has been investigated. It has been shown that for such systems the refractive index of the solvent plays a relatively minor role, and reasons have been adduced for this in terms of a general theory which indicates that in some other cases refractive index effects can be significant. For hydroxylic solvents equations applicable to aprotic solvents are unsatisfactory because of very pronounced hydrogen-bonding effects on ET and AN. It has been shown that corrections for general medium effects do not materially alter the linear relation of hydrogen-bonding effects to D and H. The hydrogen-bonding effects can be expressed in terms of proportionality to H. The general expressions combining equations for aprotic solvents with a term , where is a proportionality constant, are found to give good descriptions of AN and ET data in both aprotic and hydroxylic solvents. This approach to solvatochromic effects using nonempirical solvent parameters gives as good correlations as empirical polarity scales in representing such data. Preliminary studies indicate the usefulness of such treatments for other systems as well. The relationship of hydrogen-bonding effects to H and D have been analyzed by focusing attention on the electrostatic component of hydrogen-bonding interactions which is expected to be of primary importance. Such analyses have been shown to be consistent with some formal theoretical treatments. Their physical significance has been examined. A possibly important role of a solute-solvent correlation parameter similar ...

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as spectroscopic "probes" of the hydrocarbon interiors of micelles and other lipid assemblies. Many common simplifying assumptions about the behavior of such spectroscopic probes in lipid assemblies can be grossly misleading. The implications of some of the results obtained for spectroscopic probe studies are pointed out.

Hartree-Fock (HF) calculations using 6-31G*, 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets show that hydrogen peroxide molecular clusters tend to form hydrogen-bonded cyclic and cage structures along the lines expected of a molecule which can act as a proton donor as well as an acceptor. These results are reiterated by density functional theoretic (DFT) calculations with B3LYP parametrization and also by second-order Møller-Plesset perturbation (MP2) theory using 6-31G* and 6-311++G(d,p) basis sets. Trends in stabilization energies and geometrical parameters obtained at the HF level using 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ basis sets are similar to those obtained from HF/6-31G* calculation. In addition, the HF calculations suggest the formation of stable helical structures for larger clusters, provided the neighbors form an open book structure.

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