Richard A. Caldwell

CALCULATION OF THE AQUEOUS SOLVATION FREE ENERGY OF THE PROTON

The value of the proton hydration free energy, ΔGhyd(H+), has been quoted in the literature to be from −252.6 to −262.5 kcal/mol. In this article, we present a theoretical model for calculating the hydration free energy of ions in aqueous solvent and use this model to calculate the proton hydration free energy, ΔGhyd(H+), in an effort to resolve the uncertainty concerning its exact value. In the model we define ΔGhyd(H+) as the free energy change associated with the following process: ΔG[H+(gas)+H2nOn(aq)→H+(H2nOn)(aq)], where the solvent is represented by a neutral n-water cluster embedded in a dielectric continuum and the solvated proton is represented by a protonated n-water cluster also in the continuum. All solvated species are treated as quantum mechanical solutes coupled to a dielectric continuum using a self consistent reaction field cycle. We investigated the behavior of ΔGhyd(H+) as the number of explicit waters of hydration is increased from n=1 to n=6. As n increases from 1 to 3, the hydration...

Recent advances in photoacoustic calorimetry: Theoretical basis and improvements in experimental design

Recent developments in time‐resolved photoacoustic calorimetry (PAC) are discussed. An equation is derived relating the amplitude of the maximum photoacoustic signal to parameters of a photoacoustic cell, to physical properties of the solvents, and to the energy deposited as heat. The equation is validated by a series of scaling experiments: it correctly correlates the dependence of the PAC signal on cell thickness, on energy deposition, and on solvent properties. The dominant sources of background signal have been determined and the background signal reduced substantially relative to previous work. It is now possible to measure energies and lifetimes of transients in solutions with optical densities as low as ∼5×10−4. Data are presented for the energy of 2‐cyclopentenone triplet, a transient for which interpretable PAC measurements were previously not possible. Its triplet energy is 73.1±1.1 kcal/mole. With reduced background and a faster digital storage oscilloscope, a quite short transient lifetime, 7....

Intersystem crossing in organic photochemical intermediates

The lifetimes of a number of olefin triplets and triplet biradicals have been determined by nanosecond laser flash spectroscopy. Examples were chosen in an attempt to segregate structural features which affect intersystem crossing rates and thus determine the lifetimes. For olefin triplets, the effects of constraint to planarity, of substitution at the twisted bond for twisted triplets, of deuterium, and of polar substituents are presented. For biradi— cals, the main examples are derived from the Norrish II photoreaction of alkanophenones, and effects of substituent and conforma— tional constraint are discussed. Including Norrish I biradicals of cyclohexanones, examples from zero to four carbons between the termini have been studied.

PHOTOCHEMICAL ELECTRON TRANSFER REACTIONS and EXCIPLEXES

This review covers the literature on photochemical electron transfer (p.e.t.) reactions from January 1983 to the end of June 1984. Because of space considerations, certain topics have been excluded: reactions of excited semiconductors and at other solid surfaces, electron transfer initiated photooxidations unless the work provided insights into p.e.t. per se, and in vivo processes including studies of photosynthetic reaction centers. Of the many published papers, several are especially noteworthy. Pulse radiolysis has been used to obtain the first evidence for the Marcus inverted region (Miller et al., 1984) in electron transfer processes and for long range intramolecular electron transfer across rigid saturated hydrocarbon spacers (Calcaterra etal., 1983). In contrast, Indelli et al. (1984) were unable to observe the Marcus inverted region for outer sphere electron transfer reactions exergonic by up to 58 kcal/mol. Strauch et al. (1983) have demonstrated exponential dependence of p.e.t. rates on distance leading to the observation (Guarr et al., 1983) of collisionless p.e.t. Other noteworthy reports are those of Moore et al. (1984) on photoinduced charge separation in a model for photosynthetic reaction centers, a series of studies by Hub et d. (1983, 1984a,b,c) of ion pairs and exciplexes in the stilbene-amine system, and direct observation by picosecond flash photolysis of dissociation of geminate ion pairs and multiple exciplexes and ion pairs (Mataga, 1983; Hirata et al., 1983a). Several reviews of general interest have appeared. Most noteworthy are those of Juillard and Chanon (1983,1985) in which the connections between p.e.t. and thermal electron and proton transfer are imaginatively explored with reference to a wide range of organic and inorganic examples, and that of Davidson (1983) on reactions of excited state complexes. Meyer (1983a,b) and Endicott et al. (1983) have also reviewed p.e.t. processes with emphasis on theory and examples mainly from inorganic and organometallic chemistry. Finally, Balzani et al. (1983) and Sutin and Creutz (1983) have written useful introductory level reviews. Less general reviews are mentioned in the appropriate sections below.

Determination of triplet excitation energies of cyclic enones by time-resolved photoacoustic calorimetry.

Abstract— –Energies and lifetimes of triplet states of a series of 2‐cyclohexenones have been measured in solution under ambient conditions by time‐resolved photoacoustic calorimetry (PAC). The PAC triplet lifetimes are in excellent agreement with lifetimes measured by kinetic absorption spectrophotometry (KAS) using nanosecond flash techniques, indicating that the present data indeed pertain to the enone triplets previously studied using KAS. The data demonstrate that, as previously proposed, the π,π* triplet energies in these systems are indeed correlated with triplet lifetimes, and vary sensibly with the anticipated extent of conformational flexibility of the C=C bond. No evidence supporting formation of metastable frans‐cyclohexenones was obtained. The data also demonstrate greater precision and, for lifetimes, greater accuracy than has hitherto been claimed for the PAC technique for study of transients with lifetimes in the 25 ns range.

The heat of formation of the benzophenone ketyl radical by time-resolved photoacoustic calorimetry

Abstract Photoacoustic calorimetry is used to determine the heat of hydrogen abstraction from benzhydrol by the triplet state of benzophenone, which yields a pair of ketyl radicals. A thermodynamic cycle using this value, experimentally determined solution heats of benzophenone and benzhydrol crystals in benzene, and tabulated formation heats of these crystals, gives the formation heat of the ketyl radical, the dissociation energy of the C(Ph)2(OH)H bond, and other thermochemical parameters. While the thermochemistry of producing radicals R1 and R2 form even-electron precursors cannot yield information about R1 or R2 separately, in the present case R1 = R2 = Ph2COH and consequently no (potentially erroneous) assumption about another radical need be made.

Photoacoustic calorimetry of cycloheptene derivatives. Relaxed triplet energies of 1-phenylcycloheptene and 1,3-cycloheptadiene and the energetics of ground-state cis-trans isomerization

Abstract— The heat of isomerization Ect of cis‐1‐phenylcycloheptene (c‐PC7) to its highly strained trans isomer has been determined by a combination of a quantitative study of acid‐catalyzed trapping of the trans isomer with time‐resolved photoacoustic calorimetry. The relaxed triplet energy has been determined for PC7 and for cis, cis‐1, 3‐cycloheptadiene. The triplet energies are in line with previous studies of other alkenes but the Ect value is significantly higher than that calculated by molecular mechanics calculations for cycloheptene. The significance of the results for cis‐trans thermal isomerization is discussed.

An olefin triplet lifetime measurement by electron transfer interception

Abstract The lifetime of triplet anethole ( p -methoxy-⨿-methylstyrene) and its rate of electron transfer to paraquat dication in methanol hae been determined by laser flush photolysis. The triplet is substantially shorter-lived than cyclic analogs or the more conjugated stilbenes.

Stereospecific (2π+2π) photocycloaddition of arylalkenes to pyrene via exciplex: formation of 1:1- and 2:1-cycloadducts

Abstract Irradiation of benzene solutions containing pyrene and electron-deficient arylalkenes such as E - and Z -methyl cinnamates afforded (2π+2π) photocycloadducts including 1:2-cycloadduct in high yields in a stereospecific and endo -selective manner. Sandwich-type singlet exciplexes between pyrene and arylalkenes were proposed as reactive intermediates.

Stereospecific photocycloaddition of electron-deficient arylalkenes to 9-cyanophenanthrene via exciplex

Abstract Irradiation of benzene solutions containing 9-cyanophenanthrene and electron-deficient arylalkenes such as E- and Z-methyl cinnamates afforded (2π + 2π) photocycloadducts between the two electron-deficient components in a stereospecific manner.