Showing papers by "Nicholas J. Turro published in 1989"

Dynamics of flexible triplet biradicals

Abstract: A large number of thermal and photochemical reactions proceed via biradical intermediates. Typically, a biradical generated in the triplet state must first undergo intersystem crossing (ISC) to the singlet biradical before forming products. This presents one of the major challenges of photochemistry; to understand a reaction in which the molecule must hop from one potential energy surface to another. In this Account we are concerned with triplet-derived flexible chain biradicals, and Scheme I shows the important processes. A triplet precursor (e.g., an {sup 3}n, {pi}* ketone) cleaves with conservation of spin to produce a triplet biradical in a set of conformations where the radical centers are close together. Two processes ensue: ISC in a given conformation produces a singlet biradical in the same conformation, and internal rotation gives rise to chain dynamics, which changes the end-to-end distance.

Picosecond laser studies on photochemical reactions in restricted environments: the photoisomerization of trans-stilbene complexed to cyclodextrins

Abstract: The effect on picosecond reaction dynamics due to complexation of a reactive molecule with cyclodextrins was examined, with the trans-cis photoisomerization of stilbene as an example. In the presence of a-cyclodextrin, a single, slow exponential decay of trans-stilbene fluorescence is observed, consistent with the formation of a single complex. In contrast, in the presence of 8-cyclodextrin, the fluorescence of trans-stilbene showed a clear double-exponential decay, consistent with the formation of two complexes. In the latter case, a dynamic equilibrium between a loose and a more tightly bound conformation of the complex is proposed, resulting in different photoisomerization reaction rates of trans-stilbene.

Excited-State Resonance Raman Spectroscopy as a Probe of Alumina-Sodium Dodecyl Sulfate Hemimicelles

Abstract: Excited-state resonance Raman spectroscopy has been shown to be a sensitive technique to monitor the formation of hemimicelles. The alumina-sodium dodecyl sulfate hemimicelles are examined by excited-state Raman spectroscopy, for the first time, by observing the Raman spectrum of tris(2,2'-bipyridyl)ruthenium(II)* incorporated in the solid-liquid interface under in situ equilibrium conditions. The study clearly shows several transitions are sensitive to the evolution and structure of hemimicelles.

Negative temperature dependence in the decay of triplet biradicals

Abstract: The lifetimes of the 1 ,n-diphenylalkane1 ,n-diyl triplet biradicals 3, derived from type I photolyses of cup'-diphenylcycloalkanones were studied by nanosecond transient absorption as a function of biradical chain length n and temperature. Intermediate-length biradicals 3s,6,9,10 have longer lifetimes than smaller (33,4) or larger (311,14) biradicals. The temperature dependence of the biradical lifetimes was measured in methanol and nonane solvents for biradicals 3s,9,11. For 35, the Arrhenius activation energy for decay is normal. However for 39,11 a novel negatiue activation energy has been observed in the range 0-100 "C. The results are interpreted in terms of an intrinsic relation between the sign of the high-temperature Arrhenius slope and the mechanism of intersystem crossing in the biradical. The temperature dependence of the lifetimes of a number of nonconjugated biradicals have been reported,Is2 many of which come from nanosecond transient UV absorption studies of flexible-chain biradicals.1a*b~2 The Arrhenius activation energies E, are very small-only a few kcal/mol, and in many cases E, is 1 kcal or less. In flexible triplet biradicals, the Arrhenius plot reveals the nature of the rate-limiting step for biradical decay.2 Scheme I shows the important spinand chain-dynamics processes of reactive polymethylene biradicals. Nanosecond experiments are able to probe intersystem crossing (ISC) and chain dynamics; product formation is subnanosecond.l,2 A t low enough temperature, chain motions are slowed to the point where they become the rate-limiting step for biradical decay. A t high temperature, chain motions a re much faster and ISC becomes rate limiting. The intrinsically different temperature dependences of chain dynamics and ISC gives rise to a large curvature in the Arrhenius plot for biradical decay over a range of ca. -80 to 30 In this paper we are particularly interested in the high temperature region (>0 "C) where rapid chain motions guarantee rotameric equilibrium of the triplet biradical prior to rate limiting ISC2 W e report a study of the lifetimes of the triplet biradicals 33, derived from type I cleavage and subsequent decarbonylation of a,a'-diphenylcycloalkanones 1,+1 (Scheme 11)3 as a function of biradical chain length n and temperature. A novel negative actiuation energy for triplet biradical decay has been observed a t high temperature. Our results suggest a n intrinsic relation between the sign of the high-temperature Arrhenius slope and the mechanism of ISC in the biradical. Experimental Section Biradical lifetimes, T , were measured by monitoring the transient absorption of the benzylic moiety at 320 nm following excitation by an excimer laser at 308 nm (fwhm ca. 18 ns) or a Nd-YAG laser at 266 nm (fwhm 6 ns), with a previously described apparatus.' Assignment of the transients was based on the transient absorption spectrum (A,, = 320 & 2 nm) and the invariance of T to addition of diene quencher. From -80 to 70 "C, the transient absorption spectrum was invariant and the product distribution was in accord with previous observation^.^ The transient decays for 344 were previously reportedsaq6 and fit well to single-exponential decay. The decays for 3,, n t 9, contained a small additional fast-decay component with an amplitude which increased as (1 ) (a) Scaiano, J. C. Acc. Chem. Res. 1982, 15, 252. (b) Caldwell, R. A. Pure Appl. Chem. 1984, 56, 1167. (c) Buchwalter, S.; Closs, G. L. J . Am. Chem. SOC. 1975, 97, 3857; 1979, 101, 4688. (d) Jain, J.; Snyder, G.; Dougherty, D. A. J . Am. Chem. SOC. 1984, 106, 7294. (e) Sponsler, M.; Jain, R.; Coms, F.; Dougherty, D. A. J . Am. Chem. SOC. 1989, I l l , 2240. (2) Zimmt, M.; Doubleday, C.; Turro, N. J. J . Am. Chem. SOC. 1986,108, 3618. (3) Syntheses of 3,: Lei, X.; Doubleday, C.; Turro, N. J. Tefrahedron Left. 1986. 27. 467 1, (4) Lei, X.; Doubleday, C.; Turro, N. J. Terrahedron Lett. 1986, 27, 4675. (5) (a) Previous report on 34,5: Zimmt, M.; Doubleday, C.; Turro, N. J. Chem. Phys. Left. 1987, 134, 549. (b) Report on p-acetyl-substituted 33: Mizuno, K.; Ichinose, N.; Otsuji, Y.; Caldwell, R. A. J . Am. Chem. SOC. 1985, 107, 5797. (6) Zimmt, M.; Doubleday, C.; Turro, N. J. J . Am. Chem. Soc. 1985,107, 6724. 0002-7863/89/1511-3962$01.50/0 Scheme I

Emission properties of dioxorhenium(V) complexes in aqueous solutions of anionic and nonionic surfactants: a sensitive probe of hydrophobic binding regions

Abstract: The emission properties of ReO 2 (py) 4 + (ReO 2 + ) in aqueous solutions of anionic and nonionic surfactants have been investigated. The optical properties of this complex are extremely sensitive to water and therefore it is an excellent probe of hydrophobic as well as hydrophilic regions. The emission and absorption maxima of ReO 2 + are dependent on the water content of its environment. Emission lifetimes vary over four orders of magnitude upon shifting from aqueous to nonaqueous environments

Large magnetic field effect on the decay rates of triplet hydrocarbon diradicals

Abstract: Intersystem crossing rate constants, k{sub ISC}, of triplet 1,n-diphenyl 1,n-diyl diradicals (n = number of carbons in diradical chain) generated by type I photolyses of {alpha},{alpha}{prime}-diphenylcycloalkanones were measured in an external magnetic field H variable up to 2 kG. No magnetic field effect was observed for n = 4 and 5, but for n = 9, 11, and 14, k{sub ISC} first increased then decreased as H was increased. For n = 11 and 14, the value of k{sub ISC} decreased to an apparent asymptotic value at high field equal to 0.17 and 0.07, respectively, of the value of k{sub ISC} at H = O. For diradicals with n {ge} 9, it is proposed that, at H = O, electron-nuclear hyperfine coupling is the only important ISC mechanism, that at H = 2 kG electronic spin-lattice relaxation is the major ISC mechanism, that the three triplet magnetic sublevels do not equilibrate during the diradical lifetime, and that one of the outer two triplet magnetic sublevels is preferentially populated initially.

Pressure-Induced Diastereoselectivity in Photoinduced Diels-Alder Reactions

Abstract: The product distributions for the photochemical electron transfer and triplet-triplet energy-transfer-induced dimerization of 1,3-cyclohexadiene (CHD) were determined as a function of pressure and solvent in the range of 0.1-203 MPa. In the case of the photochemically (dicyanonaphthalene sensitized) induced electron-transfer dimerization, differences in activation volumes for the formation of endo-1 and exo-2 adduct are small and positive in acetonitrile (ca. +1 to +2 cm3/mol) but are negative and unusually large (ca. -9 to -12 cm3/mol) in benzene. The results are consistent with the involvement of different types of solvated ion pain in the two solvents. Although the product distribution does not change for any of the sensitizers and pressures used in the triplet-sensitized reaction, the efficiency of dimerization was surprisingly both pressure and solvent dependent. The activation volumes for efficiency of dimerization of CHD by triplet-triplet energy transfer fall in the range -22 to +2.1 cm3/mol and depend on both the triplet energy of the sensitizer and the solvent used.

Investigations of the influence of molecular geometry on the spectroscopic and photochemical properties of .alpha.-oxo[1.n]paracyclophanes (cyclophanobenzophenones)

Abstract: The spectroscopic and photochemical properties of a family of (Y-OXO( l.n)paracyclophanes (8-12) for n = 8-12, respectively) have been investigated. Compared to a model structure possessing a nominal T,(na*) state (4,4'-dimethyl- benzophenone, DM-BZ), photochemical and photophysical evidence is presented that the na* character of TI decreases as the cyclophane ring size decreases. For example, the rate constants for hydrogen atom abstraction from 1,4-cyclohexadiene in acetonitrile are 1.6 X lo8, 0.93 X lo8, 0.30 X lo8, 0.06 X lo8, and 0.04 X lo8 M-' s-) for triplet 12-8, respectively, a result consistent with decreasing nr* character and decreasing reactivity toward hydrogen atom abstraction with decreasing value of n. The spectroscopic properties (vibrational structure of the phosphorescence spectrum, phosphorescence lifetime, phos- phorescence excitation spectra, triplet-triplet absorption spectra) also vary in a manner consistent with decreasing na* character as the cyclophane ring size decreases. Since the early 1960s it has been known that aryl-alkyl ketones and diary1 ketones may possess nearly isoenergetic 3na* and %a* The relative energies of the two states, each with its own distinctive photophysical and photochemical behavior, can and have been manipulated by chemical substitution and solvent polarity. Substantial efforts have been devoted to understanding these effects. Some ketones, such as acetophenone, have their two triplets so close in energy that they may be inverted simply by adjusting the polarity of the solvent. When the states are very close, interesting consequences may arise due to the proximity effect, state mixing, or dynamic equilibrium between the states.'" In spite of extensive research on the role of conformation on photochemical and photophysical beha~ior,~ there has been rel- atively little work reported on the influence of molecular geometry on the photochemical and photophysical properties of a family of structurally related compounds.

Adsorption of nitroxide spin probes at the alumina/cyclohexane interface in the presence of Aerosol OT

Abstract: Etude de l'influence de l'addition d'un agent de surface non ionique (Aerosol OT) sur le taux d'adsorption de radicaux nitroxyles sur l'alumine

Chemically induced dynamic nuclear polarization in systems containing large hyperfine coupling constants

Abstract: Nuclear spin polarization effects induced in radical pairs with one or more strong ({sup 13}C) hyperfine coupling constants have been evaluated. The pairs were generated by photoinduced {alpha}-cleavage or hydrogen abstraction reactions of carbonyl compounds. Several examples illustrate how changes in the magnetic field strength (H{sub 0}) and the g-factor difference ({Delta}g) affect the general appearance of the resulting CIDNP multiplets. The results bear out an earlier caveat concerning the qualitative interpretation of CIDNP effects observed for multiplets.

Photochemistry of Dibenzyl Ketone Adsorbed on Size/Shape Selective Faujasite Zeolites. Steric Effects on Product Distributions

Abstract: Publisher Summary This chapter describes the structure of zeolites in general and then surveys the structure of an important class of zeolites, the faujasites The chapter describes how a photochemical probe, the photochemistry of dibenzyl ketone, can yield information on how intracrystalline dynamics can be influenced by cation type, cation number density and coadsorbed guests and, in turn, how intracrystalline dynamics can determine the products of photoreactions Zeolites are robust, crystalline, porous aluminosilicates possessing an enormous internal surface area that is capable of adsorbing large quantities of guest molecules, the size and shape of whose structures allow them to pass from the external to the internal zeolitic surface and to diffuse on the internal surface The framework composition, the presence of cations associated with the framework, and the topology of the void space internal to the zeolite all contrive to imbue these materials with special properties that contribute to their widespread use as catalysts, ion exchange materials, and molecular sieves Photochemical probes have been developed to explore the structure of zeolites near the sites of adsorption and to examine the dynamics of reactions of molecules adsorbed on the internal zeolitic surface

Terminal chlorination of normal alkane hydrocarbons

Abstract: A terminally chlorinated normal alkane hydrocarbon is produced at high terminal selectivity, by adsorbing a normal alkane into a select zeolite and chlorinating the adsorbed alkane in the zeolite The zeolite may be of a type having internal channels of diameter slightly larger than that of the alkane, and substantially no internal chambers, and may be selected from the pentasil family of zeolites The chlorination reaction may be conducted under heating to enhance terminal selectivity even further, in many cases to levels approaching 100%

Photochlorination of n-Alkanes Adsorbed on Pentasil Zeolites.

Abstract: The photochlorination of n-alkanes adsorbed on pentasil zeolites proceeds with up to a 20-fold greater selectivity for the monochlorination of terminal methyl groups compared to the selectivity observed when the reaction is carried out in a homogeneous solution. This enhanced selectivity, which provides a novel means of synthesizing terminally functionalized linear alkanes, was found to be a function of the percent loading of the alkane on the zeolite, the zeolite's silicon to aluminum ratio, the percent conversion of the starting material, and the water content of the zeolite.

Hyperconjugation as a Factor in Face Selectivity During Cycloaddition.

Abstract: oxidizer20) to be endothermic.26 It is likely that the endothermic oxidations of many azoalkanes are driven to nitrogen evolution by the instability of R-N=N-R'+.7 These cation radicals become less stable when the hydrocarbon cation radicals formed by deazatation are more heavily substituted. In the bridgehead dimethyl derivative of DBO, for example, we find that reaction with 2 equiv of TBPA" causes quantitative nitrogen evolution and no formation of red material. The pattern of behavior of azoalkanes which begins to emerge from this and other studies may, perhaps, be fitted to the general concept of nucleophile/cation radical reactions popularized by Parker.20 If an initial encounter between an azoalkane and a cation radical leads to a complex, one can imagine that this complex either breaks up to produce the azoalkane cation radical or undergoes further oxidation by the reactant cation radical. In the case of DBO, further oxidation would end in covalent bonding as e'lown in products 1 and 2. In summary, we find that DBO oxidatively forms an adduct with stable cation radical salts, in contrast to previously studied azoalkanes that undergo oxidative deazatation.