Polar solvents often exert a dramatic influence on reactions in solution. Equilibrium aspects of this influence involve differential solvation of reactants compared to the transition state that lead to alteration of the free-energy barrier to reaction. Such effects are well known, and often give rise changes in reaction rates of many orders of magnitude. Less well understood are effects arising from non-equilibrium, dynamical aspects of solvation. During the course of reaction, charge is rapidly redistributed among reactants. How the reaction couples to its solvent environment depends critically on how fast the solvent can respond to these changes in reactant charge distribution. In this article the dynamics of solvation in polar liquids and the influence of this dynamics on electron-transfer reactions are discussed. A molecular picture suggests that polar solvation occurs on multiple time scales as a result of the involvement of different types of solvent motion. A hierarchy of models from a homogeneous continuum model to one incorporating molecular aspects of solvation, combined with computer simulations, gives insight into the underlying dynamics. Experimental measures of solvation dynamics from picosecond and subpicosecond time-dependent Stokes shift studies are compared with the predictions of theoretical models. The implication of these results for electron-transfer reactions in solution are then briefly considered.

Recent efforts in these laboratories have been directed toward understanding the factors governing long distance intramolecular electron transfer (ET).2 In the models chosen for study the electronic coupling between donor and acceptor is sufficiently weak to assure nonadiabatic reactions. It occurred to us that long distance triplet energy transfer by the Dexter mechanism3 (TT) of which there are several examples in the literature4 should exhibit features similar to nonadiabatic electron transfer because both reactions are governed by the same theory of radiationless transitions. We therefore have started a program aimed at finding quantitative similarities and differences in these two processes when studied on directly comparable systems. Also, with one exception,5 the absolute rates of intramolecular triplet energy transfer have never been measured directly in liquid solution. In this communication we report our first results and conclusions.One of the series studied in ET, 1, involves compounds in which a 4-biphenylyl group (D) is connected via a rigid spacer (Sp) with a 2-naphthyl group (A).28 The spacers used were cyclohexane 1: A = 2-naphthyl; D = 4-biphenylyl 2: A = 2-naphthyl; D = 4-benzophenoneyl A-Sp-D and decalin ring systems. Simply by replacing the 4-biphenylyl group with 4-benzophenonyl, the series can by converted to an almost ideal series 2 for triplet energy transfer. The spacers are

A comparative analysis of the discriminating power of laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), each coupled with refractive index (RI) measurements, is presented for a study of 23 samples of automobile float glass. Elemental emission intensity ratios (LIBS) and elemental concentration ratios (LA-ICP-MS) and their associated confidence intervals were calculated for each float glass sample. The ratios and confidence intervals were used to determine the discrimination power of each analytical method. It was possible to discriminate 83% of the glass samples with 99% confidence based on LIBS spectra alone, and 96-99% of the samples could be discriminated based on LIBS spectra taken in conjunction with RI data at the same confidence level. LA-ICP-MS data allowed for 100% discrimination of the samples without the need for RI data. The results provide evidence to support the use of LIBS combined with RI for forensic analysis of float glass in laboratories that do not have access to LA-ICP-MS.

A theory for long time random coil peptide dynamics is developed based on a generalization of the optimized Rouse-Zimm model of Peri co et of. [J. Chem. Phys. 87, 3677 (1987)] and Perico [J. Chem. Phys. 88, 3996 (1988) and Biopolymers 28,1527Biopolymers 28, (1989]. The generalized model employs the rotational potential energy for specific amino acid residues and amino acid friction coefficients to compute all input parameters in the model. Calculations of the fluorescence depolarization correlation function P2(t ) and of the local persistence length are found to be sensitive to the amino acid sequence, the length of the polypeptide chain, and the location of the probe. Model computations of P 2 (t ) are compared with new experimentally determined rotational correlation times (of the order of nanoseconds) from fluorescence depolarization measurements of three different synthetic 17-residue peptides, each containing a single tryptophan (TRP) residue as a probe. In addition, the previous anisotropy measurements on ACTH, glucagon, and their fragments are discussed and compared with the model calculations. Our results indicate that the theory gives a reasonable prediction for the fluorescence depolarization correlation times of random coil polypeptides, but the calculated rotational correlation function predicts a much faster initial decay and a slower final decay than is observed. Possible theoretical improvements are discussed. 822