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1.
J Org Chem ; 84(6): 3260-3269, 2019 Mar 15.
Article in English | MEDLINE | ID: mdl-30779577

ABSTRACT

The regioselectivity of hydroxyl radical addition to arenes was studied using a novel analytical method capable of trapping radicals formed after the first elementary step of reaction, without alteration of the product distributions by secondary oxidation processes. Product analyses of these reactions indicate a preference for o- over p-substitution for electron donating groups, with both favored over m-addition. The observed distributions are qualitatively similar to those observed for the addition of other carbon-centered radicals, although the magnitude of the regioselectivity observed is greater for hydroxyl. The data, reproduced by high accuracy CBS-QB3 computational methods, indicate that both polar and radical stabilization effects play a role in the observed regioselectivities. The application and potential limitations of the analytical method used are discussed.

2.
J Phys Chem A ; 122(42): 8326-8335, 2018 Oct 25.
Article in English | MEDLINE | ID: mdl-30277773

ABSTRACT

The effect of ring substitution on the kinetics of reaction of arenes, heterocycles, and alkenes with hydroxyl radical is investigated in terms of reactivity and selectivity, using laser flash photolysis (LFP) in acetonitrile solution. The LFP data indicate that charge-transfer contributions in the transition state play an important role in dictating reactivity, and there is a correlation between the experimental and calculated ionization potentials of the arenes and alkenes and their respective reactivities. The reactivity observed for arenes in acetonitrile exhibits a much greater sensitivity toward substitution on the ring than in water, and therefore aqueous data cannot be used to predict reactivity in nonaqueous environments. Nonaqueous solution data may be predictable from gas phase data, and vice versa.

3.
J Phys Chem A ; 120(25): 4267-76, 2016 Jun 30.
Article in English | MEDLINE | ID: mdl-27301876

ABSTRACT

The species 4-nitrenopyridine 1-oxide is known to exhibit triplet nitrene dominated chemistry to yield azo-dimer products exclusively, even at room temperature. As such, this species, and its analogue 4-nitrenoquinoline 1-oxide, are useful models to probe the mechanism of formation of azo-dimers, which is postulated to proceed by self-reaction of the nitrene or reaction of nitrene with the parent azide. A laser flash photolysis study is described where the kinetics of formation of azo-dimer were found to be most adequately modeled by competition between both mechanisms, and rate coefficients for the competing reactions were determined.

4.
J Phys Chem A ; 115(51): 14687-96, 2011 Dec 29.
Article in English | MEDLINE | ID: mdl-22148587

ABSTRACT

The energetics and kinetics of the reaction of variously substituted benzyl radicals with a model alkene were calculated at the G3(MP2)-RAD//B3-LYP/6-31G(d) level of theory to determine whether such reactions are amenable to Hammett analysis. The reactions were studied both in the gas phase and in toluene solution in the temperature range 298-353 K; calculations include 1D-hindered rotor corrections for low frequency torsional modes, and the solvation energies were calculated using COSMO-RS at the BP/TZP level of theory. The addition reaction was found to be dominated by radical stabilization effects, but under circumstances where olefin substituent effects were decoupled from aryl substituent effects, a modest polar effect comes into play, which is enhanced by solvation. Reasonable correlations with empirical substituent parameters such as Hammett σ and σ(•) are observed for the enthalpy of activation, but additional entropic factors act to decrease the degree of correlation with respect to free energies and rate coefficients, confirming hypotheses from earlier experimental work. Substituent effects on the reverse ß-fragmentation reaction, and potential cyclization of the 3-phenylpropyl radicals formed by addition are also discussed.


Subject(s)
Benzene Derivatives/chemistry , Quantum Theory , Free Radicals/chemistry , Kinetics , Thermodynamics
5.
J Phys Chem A ; 115(11): 2431-41, 2011 Mar 24.
Article in English | MEDLINE | ID: mdl-21355533

ABSTRACT

Density functional theory (DFT) and G3-type (G3(MP2)-RAD) composite calculations were performed on a series of substituted 3-phenylpropyl radicals, to determine the relative importance of fragmentation and cyclization reactions in the chemistry of such species. Our studies indicate that cyclization is generally the more important of these reactions, with exceptions where fragmentation yields highly stabilized benzylic species. The energetic barriers for the cyclization reactions (enthalpies of activation) were found to be determined largely by the stability of the reactant radical and to a lesser but significant extent, by steric factors. Polarity effects in the transition state (modeled by SOMO-LUMO gaps of the products) appear to be less important. The data obtained indicated that the addition of benzyl radical to alkenes may be considered to be irreversible, but calculations for α-substituted styrenic systems indicate that reversibility of addition may become a factor in dilute polymerizing solutions for select systems.

6.
J Org Chem ; 73(9): 3441-51, 2008 May 02.
Article in English | MEDLINE | ID: mdl-18399657

ABSTRACT

The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.


Subject(s)
Azides/chemistry , Oxides/chemistry , Pyridines/chemistry , Models, Molecular , Molecular Structure , Photochemistry , Spectrophotometry
7.
J Org Chem ; 71(24): 9023-9, 2006 Nov 24.
Article in English | MEDLINE | ID: mdl-17109525

ABSTRACT

Laser flash photolysis of 4-azidopyridine-1-oxide at 266 or 308 nm yields triplet 4-nitrenopyridine-1-oxide as the dominant reactive intermediate species, with k(ISC) of approximately 2 x 10(7) s(-1). No evidence of products arising from the singlet nitrene was observed, indicating a slow rate of cyclization to the benzazirine and didehydroazepine species. The slow rate of cyclization is postulated to be due to the aminoxyl-like electronic configuration of this species, which withdraws spin density from sites for potential cyclization.


Subject(s)
Oxides/chemistry , Photochemistry , Pyridines/chemistry , Chromatography, High Pressure Liquid , Cyclization , Kinetics , Spectrophotometry, Infrared
8.
J Am Chem Soc ; 127(19): 7094-109, 2005 May 18.
Article in English | MEDLINE | ID: mdl-15884952

ABSTRACT

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.

9.
J Phys Chem A ; 109(8): 1491-6, 2005 Mar 03.
Article in English | MEDLINE | ID: mdl-16833470

ABSTRACT

Laser flash photolysis (LFP, 400 nm excitation) of the anti-cancer drug tirapazamine (TPZ) in acetonitrile produces the singlet excited-state S1 with lambda(max) = 544 nm. The lifetime of this state is 130 ps, in good agreement with the reported fluorescence lifetime. The excited state is reduced to the corresponding radical anion by KSCN or KI. The spectrum of the radical anion is in good agreement with previously reported pulse radiolysis studies and time-dependent density functional theory (TD-DFT) calculations. LFP of desoxytirapazamine (dTPZ) also produces the first excited singlet state, S1. The fluorescence quantum yield and lifetime (5.4 ns) of the dTPZ singlet excited state are both much greater than the corresponding values of TPZ. This is explained by DFT calculations that predict that cyclization of TPZ to form an oxaziridine is thermodynamically facile but that cyclization of dTPZ to form an oxadiaziridine is not. Thus, the S1 state of TPZ has a short lifetime and low fluorescence quantum yield due to ready cyclization whereas the cyclization of the S1 state of dTPZ is unimportant and does not limit either the fluorescence quantum yield or the fluorescence lifetime. This conclusion is confirmed by studies of dTPZ', an isomer of dTPZ containing the C=N-O moiety which has a low quantum yield and short fluorescence lifetime similar to that of TPZ.

10.
J Phys Chem A ; 109(11): 2547-51, 2005 Mar 24.
Article in English | MEDLINE | ID: mdl-16833557

ABSTRACT

Laser flash photolysis (LFP) of acetonitrile solutions of N-hydroxypyridin-2-thione in the presence of trans-stilbene generates a transient absorbance at 392 nm, attributed to the addition of hydroxyl radical to stilbene. The observed transient absorbance was used in competitive LFP experiments to determine relative rates of reaction for hydroxyl radical with a range of aromatic hydrocarbons in acetonitrile. Structure-reactivity relationships for the reaction of hydroxyl radical with arenes are derived. With these aromatic hydrocarbons, we observe a good correlation between the rates of hydroxyl-radical reaction and the ionization potential of the arene. Kinetic isotope effects are consistent with hydroxyl-radical addition being the dominant reaction pathway with the arene.


Subject(s)
Acetonitriles , Hydrocarbons, Aromatic/chemistry , Hydroxyl Radical/chemistry , Photolysis , Solutions , Structure-Activity Relationship
11.
J Am Chem Soc ; 124(32): 9489-97, 2002 Aug 14.
Article in English | MEDLINE | ID: mdl-12167045

ABSTRACT

The rate of coupling of alkyl radicals with the persistent aminoxyl radical 1,1,3,3-tetramethylisoindolin-N-oxyl (1) has been used as a kinetic probe to determine absolute rate coefficients for the addition of alkyl radicals to methyl acrylate. The results are discussed in terms of the role of the structure and functionalization of the attacking radical on the rates of addition, particularly as they affect steric, polar, and enthalpic factors. The aminoxyl method is assessed against other methods for determining free radical addition rate coefficients.

12.
Photochem Photobiol ; 75(4): 339-45, 2002 Apr.
Article in English | MEDLINE | ID: mdl-12003122

ABSTRACT

The absorption and fluorescence spectra of 3-aminobenzo-1,2,4-triazine di-N-oxide (tirapazamine) have been recorded and exhibit a dependence on solvent that correlates with the Dimroth ET30 parameter. Time-dependent density functional theory calculations reveal that the transition of tirapazamine in the visible region is pi-->pi* in nature. The fluorescence lifetime is 98+/-2 ps in water. The fluorescence quantum yield is approximately 0.002 in water. The fluorescence of tirapazamine is efficiently quenched by electron donors via an electron-transfer process. Linear Stern-Volmer fluorescence quenching plots are observed with sodium azide, potassium thiocyanate, guanosine monophosphate and tryptophan (Trp) methyl ester hydrochloride. Guanosine monophosphate, tyrosine (Tyr) methyl ester hydrochloride and Trp methyl ester hydrochloride appear to quench the fluorescence at a rate greater than diffusion control implying that these substrates complex with tirapazamine in its ground state. This complexation was detected by absorption spectroscopy.


Subject(s)
Antineoplastic Agents/chemistry , Triazines/chemistry , Electron Transport , Photochemistry , Spectrophotometry, Ultraviolet , Tirapazamine
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