Excimer laser photolysis studies of translational-to-vibrational energy transfer

A new technique is presented to study translation-to-vibrational (T—V) energy transfer and translationally enhanced reactive events. The method utilizes excimer laser photolysis to produce translationally “hot” atoms and time-resolved infrared detection of specific excited states.     

Triplet-doublet electronic energy transfer from benzophenone to the ketyl radical: viscosity and magnetic field effects

The influence of solvent viscosity and an external magnetic field on the rate constant of electronic energy transfer from triplet bonzophenone to the ketyl radical is studied; it is concluded that the transfer is mediated by electron exchange.     

Intermode energy transfer in vibrationally excited O3

The laser excited fluorescence method has been employed to determine the rate constants for vibrational relaxation of the O₃ (010), O₃ (100) and O₃ (001) levels at 298 K. The fluorescence observations from the O₃ (010) level provide direct measurements of the rate for intermode vibrational energy transfer from the coupled ν₁ and ν₃ modes to the ν₂ mode. The slowness of this process indicates the likelihood that the ν₁ and/or ν₃ modes (rather than the ν₂ mode) play a predominant role in the laser enhanced reaction between O₃² and NO at 298 K.     

Contrasting picosecond photokinetics of CoII and CoIII porphyrins verify excited-state predictions

Using picosecond transient absorption techniques, we have different low-lying excited states for several Co^II and Co^III porphyrins. These differences are interpreted in terms of theoretical calculations that predict the lowest-energy excited states to be ²(π·d) in Co^II and ³(d, d) in Co^III.     

Studies of UV photochemistry of psoralen and angelicin by infrared spectroscopy

The UV homodimerization reaction of psoralen and angelicin in crystalline thin layers has been investigated by means of transmission infrared and infrared ATR (attenuated total reflection) spectroscopy.

In the case of psoralen layers isoorientation was found. Dichroic ratios for several vibrational bands have been obtained. After irradiation, cis-syn photodimers were found for both psoralen and angelicin.     

Phosphorescence of 1,1,1-trifluoroacetone using pulsed nitrogen laser excitation

Phosphorescence decay times of 1,1,1-trifluoroacetone in the vapour phase have been measured as a function of pressure of ketone with low-power excitation from a pulsed N₂ laser. A strong pressure dependence is observed, yielding a rate constant for self-quenching of (4.6 ± 0.5) × 10⁶ ? mol^?1 s^?1, and an extrapolated zero-pressure lifetime of 60 μs, considerably shorter than that measured earlier in a higher-energy flash photolysis experiment. Reasons for these differences are discussed.     

Rotation of molecules around specific axes: Axes reorientation under rotational excitation

Rotational energy levels of nearly spherical molecules in an isolated vibrational state are studied. It is shown that, in the limit of high rotational quantum number J, the rotational states may be interpreted as those of a stable rotation around axes properly oriented in the molecular frame. The orientation of the axes depends on J. Simple analytical solutions are given for the problem considered in the asymptotic and harmonic approximations. The results obtained possess a clear quantitative interpretation of the phenomena considered and, at the same time, agree quantitatively with the results of numerical diagonalization. The analogy between the effects of rearrangement of the rotational levels under the variation of J and the critical phenomena in macroscopic systems is discussed. The intensities of rovibrational transitions between totally symmetric vibrational states are calculated. A new selection rule is introduced which is due to a overlap of the rotational functions corresponding to the rotation around differently oriented axes.     

Phase transfer catalyzed ortho-metalation

The first examples of the application of phase transfer catalysis to orthometalation are described.     

Analysis of the decay of picosecond fluorescence in semiconductors: Criteria for the presumption of electroneutrality during the decay of an exponential electron-hole profile

When an exponential profile of electron-hole pairs is photogenerated (in a semiconductor) with a delta-function light pulse, unequal diffusion coefficients of holes and electrons (i.e. D_e≠ D_h) effect deviations from electroneutrality as electrons and holes diffuse into the bulk semiconductor. These deviations will in turn effect errors in the analysis of data (e.g. time resolved fluorescence) when using theory based on the presumption of electroneutrality. We deduce here the experimental conditions required for an effective electroneutrality to be maintained during the course of an experiment. Analyses were carried out using computer simulations without the presumption of electroneutrality and the analytic solution with the presumption of electroneutrality. The differences in the measured fluorescences predicted by the two computations are characterized as a function of a variety of experimental parameters and physical properties: intensity (of the excitation pulse), the absorption of the exciting and emitted light, the the ratio D_h/D_e, bulk dielectric constant of the semiconductor, bulk and surface recombination kinetics. It is shown that a conditon of adequate electroneutrality can be effectively attained when a well defined a minimum number of electron-hole pairs is generated; an upper limit of the number of e^?–k⁺ pairs is also established in order to avoid an intolerable temperature pulse.     

Ab initio calculation of distortion and bonding energy components of the cis-trans conversion energy for formic,thiolformic and thionformic acids

The energy change in the formation of the intramolecular hydrogen-bonded ring conformers of formic, thiolformic and thionformic acids from the chain conformers has been divided up into distortion and bonding energy components, following the treatment of Smit, Derissen, and van Duijneveldt for the formation of the formic acid dimer.     

Liberation of excess energy and retransformation of mechanically activated cadmium sulphide

On heating vibro-milled CdS containing the strained high-pressure rock salt phase, the excess energy was gradually liberated accompanied by the retransformation into more stable phases and the restoration of their crystallinity. Profiles of DSC curves, inter-relationships between thermal and crystallographical properties, the stability of each phase and the mechanism of the mechanochemical activation are discussed. A direct retransformation of the strained high-pressure phase was found to accompany the liberation of the excess energy.     

Dimethylsilylene transfer from hexamethylsilirane to olefins

The thermolysis of hexamethylsilirane in the presence of cis- and trans-4-octene, cyclooctene, propenyltrimethylsilane and trimethylethylethylene resulted in dimethylsilylene transfer and formation of the respective silacyclopropanes. In contrast, silacyclopentane derivatives were formed when such thermolysis was carried out in the presence of styrene and α-methylstyrene. This is believed to be a result of the interception of the intermediate diradical from hexamethylsilirane ring opening by the styrene.     

The effects of sample preparation,pyrolysis and pyrolyzate transfer conditions on pyrolysis mass spectra

Two different biopolymers, bovine serum albumin and glycogen, were examined by pyrolysis-mass spectrometry under various experimental conditions. The parameters tested include the method of cleaning the pyrolysis wire, the choice of the sample solvent, the sample size, the temperature-time profile and the temperature of the mass spectrometric inlet system.In order to determine quantitatively the effects of changes in experimental conditions on reproducibility of the spectra, the differences observed between the pyrolysis mass spectra represented as numerical dissimilarity values.It is concluded that the long-term differences observed are only partly explained by fluctuations in the conditions studied. However, it appears feasible to define a set of standard conditions with respect to the production of the pyrolyzate which should afford a reasonable degree of interlaboratory reproducibility.     

Some hydrodynamic boundary layer influences on mass transfer coefficients

As an initial approach to the quantitative understanding of the importance of various components of intestinal' contents as factors in mass transfer resistance, studies have been performed in diffusion cells containing polydimethylsiloxane membranes under circumstances sensitive to the boundary layers. (1) The effect of temperature change on permeability was measured. Under the conditions of the study, demonstration of a clear transition from membrane control to boundary control with rising temperature proved elusive. (2) When membrane thickness was systematically decreased, a gradual transition from membrane to boundary control was demonstrated, yet practical restrictions of the experimental conditions and system limited the degree to which either membrane or diffusion layers could be manipulated into a rate controlling role. (3) The effect of solution viscosity on mass transfer was also studied. Methylcellulose and sucrose were used as polymeric and low molecular weight viscosity inducing agents respectively. It was readily shown that viscosity has a much greater influence on the flux of a permeant through decreased diffusion coefficients than through increased boundary layer thicknesses, as is predicted by diffusional theories.     

Thermal energy storage materials — a DSC study

A DSC investigation of a thermal energy storage, first-order phase change system has been carried out. The material is a borax—common salt modified sodium sulfate decahydrate system, distributed throughout a fumed silica supporting matrix. The qualitative features of the thermal behavior of the material can be explained in terms of the phase diagram of the sodium sulfate—water peritectic system, modified by the presence of a low-temperature eutectic. Fusion and recrystallization enthalpies have been measured as a function of cycle number. The thermal conversion efficiency of the system is high. Combining these data with specific heat measurements, the calculated storage capacity is in good agreement with published engineering information.     

Submicrosecond spectroelectrochemistry applied to chlorpromazine cation radical charge transfer reactions

A beam of light reflected from a 50–400 μm diameter disk electrode was used to monitor the concentration of an electrogenerated absorber in the submicrosecond time scale. The electrode potential was established in less than 1 μs after a potential step, and charge injection further reduced this response time to less than 40 ns. Absorbance measurements agreed with theory for planar diffusion from 150 ns to 100 ms, at which point nonlinear diffusion caused a negative deviation from planar behavior. The technique was used to monitor the kinetics of the oxidation of dopamine by electrogenerated chlorpromazine cation radical. The method was used to monitor this process in the physiological pH region, where the reaction is too fast for stopped-flow techniques.