Energy Transfer Steps Research Articles

Average collisional vibrational energy transfer quantities and the inversion temperature

Abstract : Collisional efficiencies for vibrational energy transfer have previously been related to certain quantities, which have been examined here as a function of molecular complexity, critical reaction threshold, E sub O temperature and energy transfer step size. Model calculations were made for the decomposition of nitryl chloride and for the isomerizations of methyl isocyanide, cyclopropane and cycloheptatriene. Additional keywords: high temperature; inversion temperature; surfaces; tin; vibrational relaxation.

Room-temperature phosphorescence, sensitized phosphorescence and fluorescence of licit and illicit drugs enhanced by organized media

The ability of microscopically organized media, in the form of surfactant micelles and α- and β-cyclodextrins, to enhance the luminescence phenomena of several licit and illicit drugs is discussed. Because physiological samples are not often amenable to direct spectrometric measurements without pretreatment, the applicability of these organized media to liquid chromatography is also considered. Fluorescence enhancements for certain hallucinogenic drugs such as N,N-dimethyltryptamine, mescaline and ibogaine are seen in cyclodextrin media compared to conventional, homogeneous solutions. Heavy-atom substituted sodium dodecyl sulfate micelles induce phosphorescence from cationic and/or hydrophobic drugs at room temperature in fluid solution; drugs such as propranolol, diflunisal, naphalozine, and selected quinoline derivatives can be determined conveniently. Sensitized phosphorescence is observed for several drugs including brethine, cocaine, didrate, estradiol, meprobarbital, methaqualone, phenobarbital, and sulfanilamide; it can be enhanced markedly when micellar solutions are used as the solvent. The energy-transfer step is facilitated by the organizing ability of the micelle; limits of detection can be decreased by over two orders of magnitude compared to homogeneous solvents. Sensitized phosphorescence can also be measured in cyclodextrin solutions, but the detectability is inferior to that in micellar media. Which form of organized medium is superior for determination of drugs is discussed.

Vibration–vibration energy-transfer kinetics monitored by thermal lens

The time development of the thermal-lens signal from CO2 at various pressures has been measured and interpreted. The waveforms obtained are found to be very sensitive measures of the rate constants for individual energy-transfer steps and comparison of theory with experiment has allowed the determination of rate constants for the processes CO2(001)→ CO2(030), k=(10.22 ± 0.7)× 10–15 molecule–1 cm3 s–1 and CO2(010)→ CO2(000), k= 7.9 × 10–15 molecule–1 cm3 s–1. The process CO2(001)→ CO2(040) is shown to be relatively insignificant and it is also shown that it is not necessary to postulate a specifically endothermic step in the mechanism to explain the transient cooling of pure CO2.

Cage effect in the dissociation of van der Waals complexes RgI2 (Rg=Ar, Kr, Xe): A quasiclassical trajectory study

The dissociation dynamics of the RgI2 (Rg=Ar,Kr,Xe) van der Waals complexes have been studied using three-dimensional quasiclassical trajectories. Specifically, the unimolecular dissociation of RgI2(B 3π) with initial I2 vibrational excitation above the Rg+I+I dissociation limit of the B 3π state was studied. In addition to complete dissociation to atoms, iodine ‘‘recombination’’ was observed to be a major reaction channel. This result is interpreted as a cage-like effect due to the inert gas atom, which also carries away a large fraction of the energy when the complex dissociates to form I2. The reaction mechanism leading to the formation of molecular products has been found to involve both direct and long-lived, complex trajectories. Dissociation of the complex RgI2 is favored by near collinear orientations. The decomposition kinetics of the complex are found to be complex and non-RRKM in character. A four-step reaction mechanism involving an explicit intramolecular energy transfer step is proposed to explain the calculated time dependence of the product concentrations. The calculated product vibrational distributions are in qualitative agreement with the experimental results.

Interactive modeling of scintillation pulses by visual overlay of computed pulse shapes with the raw data: J.M. Flournoy, S.S. Lutz, L.A. Franks, C.B. Ashford, and P.B. Lyons ∗∗, EG & G, Santa Barbara Operations, Goleta, California 93117, University of California, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

The modeling technique described was developed to aid in interpretation of the effects of various changes in scintillator formulations on the shape of scintillation pulses. Theoretical pulse shapes were synthesized, the system response function is folded in, and the result is overlaid on the raw data arrays. It has thus been possible to distinguish relatively unambiguously between quenching of the solvent and the solute when various heavy-atom quenchers were added to solutions of a series of substituted terphenyls. The method is found to be valuable in providing basic information about energy transfer steps in a multicomponent scintillator. (LEW)

Atom and Radical Recombination Reactions

Radical reactions are rarely reviewed as a separate field. They are either included in general surveys of reaction kinetics, or they are considered in dis­ cussions of the reverse dissociation processes. It is therefore attractive to present the subject from the less common recombination point of view instead of the normal unimolecular reaction side. The intimate relationship between the and the reverse dissociation processes, nevertheless, remains a most important fact. The present review considers the of atoms and the combination or association of atoms or poly atomic species with molecules in the gas phase. Many experimental examples have already been included either in earlier reviews in this series (1-4) or in other earlier reviews and compilations (e.g. 5-18). Various aspects of the theoretical analysis have been described for the cases of atom (19) and combination of poly atomic species in reviews of modern unimolecular rate theory ( 16, 17, 20). Since these references include fairly exhaustive collections of experimental data, only a few are cited in the following pages. Instead we intend to present characteristic experiments for the reactions under a wide variety of conditions and to compare these with theoretical models. Only for a qualitative orientation do we first consider the simple energy transfer mechanism with association, dissociation, and collisional energy transfer steps. Two species, A and B, associate and form highly excited unstable molecules AB* that then either redissociate or are collisionally stabilized in collisions with arbitrary particles M:

Dynamical treatment of unimolecular decomposition reactions. II. Short-range interfragment coupling and incomplete randomization

We investigate the dynamics of an isolated polyatomic molecule undergoing unimolecular dissociation. The intramolecular vibrational energy transfer step is treated by applying a theory of vibrational relaxation previously developed for permanently bound molecules. In contrast to an earlier study, the decomposition step is treated here subject to the assumption that, when sufficient energy is concentrated in the reaction coordinate, transitions between internal states of the molecule occur only while the incipient fragments are near their minimum classically allowed separation. Under such circumstances, it is generally not possible to divide configuration space cleanly into a randomized and a nonrandomized region, as required by the RRKM theory of unimolecular reactions. In spite of this, it is shown that, under certain conditions, the RRKM specific rate constant expression may remain valid. More generally, it is shown that the accurate specific rate constant assumes a form predicted by the transition state treatment of unimolecular reactions. The explicit rate expression derived here reveals factors which determine the accuracy of the transition state approximation for unimolecular reactions—the basic, simplifying assumption that a transition state exists which coincides with a ’’configuration of no return’’ for both decomposition and association reactions. Estimates based on our rate formulas suggest that the transition state approximation may often be sufficiently accurate to justify use of the RRKM rate expression. As part of our analysis of randomization and decomposition dynamics, we examine the time-dependent behavior of a molecule which has fragment–fragment interactions as described above but which is constrained to remain bound by the presence of an artificial ’’wall’’ which prevents the fragments from separating completely. It is found that statistical equilibrium may be achieved in this system—a significant result because the system violates some rather stringent assumptions previously invoked to prove randomization in bound molecules. This motivates a future search for a more general theory of intramolecular relaxation.

Physical versus chemical quenching of electronically excited benzene by Me 3SnH

Theory and e.p.r. experiments show that upon electronic excitation, benzene loses its D 6h symmetry; the description of the 1B 2u state by a 1,3-diradical (with zwitterionic character) and that of the 3B 1u state by a 1,4- or a 1,2-diradical has been proposed and found viable. However, it would be interesting to know whether these diradicals can be considered as such with respect to the chemical reactivity of the states. The chemical quenching of excited benzene by Me 3SnH, a species highly reactive towards free radicals, has thus been attempted. We obtained no evidence for any reaction of the triplet benzene whilst the singlet did react but only scarcely; instead the tin hydride acted as a moderate intersystem crossing catalyst as evidenced by the fluorescence quenching and by the enhanced Φ P/Φ F ratio observed for benzene. The decay to the ground state in fluid solutions ultimately involves a step of energy transfer to the hydride which partly decomposes. Similar results were obtained for the intramolecular quenching studied on β-phenethyldimethyltin hydride.

Physical versus chemical quenching of electronically excited benzene by Me3SnH

Theory and e.p.r. experiments show that upon electronic excitation, benzene loses its D6h symmetry; the description of the 1B2u state by a 1,3-diradical (with zwitterionic character) and that of the 3B1u state by a 1,4- or a 1,2-diradical has been proposed and found viable. However, it would be interesting to know whether these diradicals can be considered as such with respect to the chemical reactivity of the states. The chemical quenching of excited benzene by Me3SnH, a species highly reactive towards free radicals, has thus been attempted. We obtained no evidence for any reaction of the triplet benzene whilst the singlet did react but only scarcely; instead the tin hydride acted as a moderate intersystem crossing catalyst as evidenced by the fluorescence quenching and by the enhanced ΦP/ΦF ratio observed for benzene. The decay to the ground state in fluid solutions ultimately involves a step of energy transfer to the hydride which partly decomposes. Similar results were obtained for the intramolecular quenching studied on β-phenethyldimethyltin hydride.

Polymer photoprotection by copolymerized triplet quenchers

AbstractFrom a model of delocalized triplet energy in an aromatic vinyl polymer, the relative efficiency of triplet‐energy transfer to either an added or copolymerized quenching molecule is determined. It is found that copolymerization can increase the photoprotective efficiency only if the quenching rate is determined by the final energy transfer step. A study of the quenching of polystyrene phosphorescence by added or copolymerized naphthalene indicates that there is no enhancement of quenching efficiency by copolymerization.

Model systems for photosynthesis II. Concentration quenching of chlorophyll b fluorescence in solid solutions

The three parameters, fluorescence polarization ( P ), relative quantum yield ( ϕ ), and lifetime ( T ) have been reinvestigated for solutions of chlorophyll b in lecithin as a function of concentration. All three show different concentration dependence, of which the depolarization only, may be described by a simple single step energy transfer. It is proposed that the self-quenching occurs when nonfluorescent pairs of molecules are excited directly, and that the reduction in lifetime is a result of energy transfer through the bulk solution to such 'pairs’ or other quencher. The origin of these ‘pairs’ of molecules may be shown to be purely statistical. The apparatus used for measurement of fluorescence lifetimes is described.

Inhibition of energy transfer reactions in mitochondria by hydroxylamine

The effect of hydroxylamine on electron and energy transfer reactions has been investigated in rat liver mitochondria with the following results: 1. 1. Succinate oxidation is slightly inhibited by hydroxylamine in the absence of phosphate acceptor (state 4) and in the presence of uncoupler, but is strongly inhibited in the presence of a phosphate acceptor (state 3). Similar effect is observed with other substrates with the extent of inhibition increasing in the order of succinate > ascorbate > NAD-linked substrates. Hydroxylamine strongly inhibits soluble cytochrome oxidase activity but only partially retards terminal electron transfer in mitochondria. 2. 2. Hydroxylamine decreases the rate of energy transfer reactions such as reversed electron transfer, ion translocation, and ATP formation with Ki of 2 mM during succinate oxidation. ATP- 32Pi exchange reaction and DNP-stimulated ATPase activity are not affected by hydroxylamine. 3. 3. Hydroxylamine does not inhibit the oxidation of fatty acids in the presence of carnitine, ATP, and uncoupler, suggesting that the reagent is not reacting with acyl compounds in mitochondria. It is concluded that hydroxylamine interacts at an early, as yet unidentified, energy transfer step in mitochondria.

Molecular-Beam Scattering from the (111) Plane of Silver

The spatial distributions of molecular beams of CH4, NH3, He, Ne, Ar, and Xe scattered from the (111) plane of an epitaxially grown Ag single crystal have been examined both during growth of the Ag crystal and immediately afterward, and no distinction between these modes was observable. The scattering of Ne, CH4, and NH3 has been compared to illustrate the important role of the gas—metal interaction energy; NH3, the most strongly interacting gas, gives rise to nearly diffuse scattering, while Ne is nearly specularly directed. Although the scattering of He is essentially specularly directed at all incident angles and temperatures, the heavier gases (Ar and Xe) exhibit scattering maxima lying between the specular direction and the tangent to the surface (supraspecular scattering), with the location of the maxima depending in a complex manner on the angle of incidence of the beam and the beam energy. Since a necessary condition for supraspecularity is that the beam temperature be greater than the surface temperature, an energy-transfer step seems to be involved in determining the particle trajectory. Similar complexities in the intensity of the scattered-beam maxima are observed; however, no simple explanation seems to suffice in this case.