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The parasitic nematode Angiostrongylus cantonensis (rat lungworm) is the leading cause of human eosinophilic meningitis worldwide. Most human infections occur through the accidental consumption of A. cantonensis hidden within produce as infectious third-stage larvae (L3), yet little research has been published addressing possible methods to mitigate this means of transmission. Here, we describe our tests of ozone gas-an oxidizing agent-and ultrasound, both used for disinfection of food and municipal water supplies and in industrial cleaning. We found that exposure to ozone, produced using two different commercially available ozone generators over varying durations of time and concentrations, was capable of achieving 100% larval mortality. In addition, we evaluated the impact of different sound frequencies on A. cantonensis L3 survival using two different commercially available ultrasonic cleaners, and found that 60 s of 40 kHz produced 46% mortality within 2 h. The combined use of ultrasound and ozone gas simultaneously resulted in a minimum of 89% normalized mean percent mortality within 2 h of treatment. Our study suggests that both ozone and ultrasound show high larvicidal efficacy, both independently and together, and thus show promise as methods for reducing the risk of rat lungworm infection via accidental consumption.

A perspective on the development of mechanistic carbene chemistry is presented. The author will point out questions that have been answered, and a next generation of questions will be proposed.

Reinvestigation of the thermolysis of azido-meta-hemipinate (I) yielded, in addition to known II, unusual products III and IV. These products are formed via a rare intramolecular nitrene insertion into an adjacent methoxy C-H bond followed by an intermolecular reaction during a ring-expansion and a ring-extrusion reaction followed by a carbene insertion. The structures of the new compounds were confirmed using a battery of techniques, including HRMS (ESI-QTOF) and 2D NMR as well as X-ray crystallography for compound IV. Density functional theory methods were used to support the proposed mechanism of formation of the products.

The photochemistry of 4,5-carbomethoxy-1,2,3-thiadiazole in solution was studied at room temperature with use of UV-vis and IR transient absorption spectroscopies (λ(ex) = 266 nm). Ultrafast time-resolved techniques demonstrate that there is a very fast rise (<0.4 ps) of bis(carbomethoxy)thiirene in acetonitrile, and that it is the only intermediate formed. The lifetime of the thiirene is limited by dimerization to eventually form tetra(carbomethoxy)thiophene.

The photochemistry of diazocyclohexadienone (1), o-phenylene thioxocarbonate (2), and 2-chlorophenol (3) in solution was studied using time-resolved UV-vis and IR transient absorption spectroscopies. In these three cases, the same product cyclopentadienyl ketene (5) is formed, and two different mechanistic pathways leading to this product are discussed: (a) rearrangement in the excited state (RIES) and (b) a stepwise route involving the intermediacy of vibrationally excited or relaxed carbene. Femtosecond UV-vis detection allows observation of an absorption band assigned to singlet 2-oxocyclohexa-3,5-dienylidene (4), and this absorption feature decays with an ∼30 ps time constant in hexane and acetonitrile. The excess vibrational energy present in nascent carbenes results in the ultrafast Wolff rearrangement of the hot species. IR detection shows that photoexcited o-phenylene thioxocarbonate (2) and 2-chlorophenol (3) efficiently form the carbene species while diazocyclohexadienone (1) photochemistry proceeds mainly by a concerted process.

The photochemistry of 4-methoxycarbonylphenyl azide (2a), 2-methoxycarbonylphenyl azide (3a), and 2-methoxy-6-methoxycarbonylphenyl azide (4a) were studied by ultrafast time-resolved infrared (IR) and UV-vis spectroscopies in solution. Singlet nitrenes and ketenimines were observed and characterized for all three azides. Isoxazole species 3g and 4g are generated after photolysis of 3a and 4a, respectively, in acetonitrile. Triplet nitrene 4e formation correlated with the decay of singlet nitrene 4b. The presence of water does not change the chemistry or kinetics of singlet nitrenes 2b and 3b, but leads to protonation of 4b to produce nitrenium ion 4f. Singlet nitrenes 2b and 3b have lifetimes of 2 ns and 400 ps, respectively, in solution at ambient temperature. The singlet nitrene 4b in acetonitrile has a lifetime of about 800 ps, and reacts with water with a rate constant of 1.9 × 10(8) L·mol(-1)·s(-1) at room temperature. These results indicate that a methoxycarbonyl group at either the para or ortho positions has little influence on the ISC rate, but that the presence of a 2-methoxy group dramatically accelerates the ISC rate relative to the unsubstituted phenylnitrene. An ortho-methoxy group highly stabilizes the corresponding nitrenium ion and favors its formation in aqueous solvents. This substituent has little influence on the ring-expansion rate. These results are consistent with theoretical calculations for the various intermediates and their transition states. Cyclization from the nitrene to the azirine intermediate is favored to proceed toward the electron-deficient ester group; however, the higher energy barrier is the ring-opening process, that is, azirine to ketenimine formation, rendering the formation of the ester-ketenimine (4d') to be less favorable than the isomeric MeO-ketenimine (4d).

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO(2)N(3)) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO(2)N(3) with 330 nm light promotes 2-NpSO(2)N(3) to its S(1) state. The S(1) excited state has a prominent azide vibrational band. This is the first direct observation of the S(1) state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S(1) state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO(2)) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO(2)N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl(4)) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO(2)N). Internal conversion of the S(1) excited state to the ground state S(0) is an efficient deactivation process. Intersystem crossing of the S(1) excited state to the azide triplet state contributes only modestly to deactivation of the S(1) state of 2-NpSO(2)N(3).

The photochemistry of tetrasulfur tetranitride (1) was studied in hexane solution by the laser flash photolysis technique (LFP). The experimental findings were interpreted using the results of previous matrix isolation studies (Pritchina, E.A.; Gritsan, N.P.; Bally, T.; Zibarev, A.V. Inorg. Chem. 2009, 48, 4075) and high-level quantum chemical calculations. LFP produces two primary intermediates, one of which is the boat-shaped 8-membered cyclic compound (2) and the other is the 6-membered S(3)N(3) cyclic compound carrying an exocyclic (S)-N═S group (3). The primary products 2 and 3 absorb a second photon and undergo transformation to the 6-membered S(3)N(3) cycle carrying an exocyclic (N)-S≡N group (4), which is very unstable and converts back to intermediate 3. The quantum yield of the primary product formation is close to unity even though the quantum yield of photodegradation of 1 is low (~0.01). Thus, 1 is a photochromic compound undergoing in solution the thermally reversible photochemical isomerization. The mechanism of the photochromic process was established, and the rate constants of the elementary reactions were measured., (© 2012 American Chemical Society)

A time-resolved resonance Raman (TR(3)) and computational investigation of the photochemistry of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide in acetonitrile is presented. Photolysis of 4-acetamidophenyl azide appears to initially produce singlet 4-acetamidophenylnitrene which undergoes fast intersystem crossing (ISC) to form triplet 4-acetamidophenylnitrene. The latter species formally produces 4,4'-bisacetamidoazobenzene. RI-CC2/TZVP and TD-B3LYP/TZVP calculations predict the formation of the singlet nitrene from the photogenerated S(1) surface of the azide excited state. The triplet 4-acetamidophenylnitrene and 4,4'-bisacetamidoazobenzene species are both clearly observed on the nanosecond to microsecond time-scale in TR(3) experiments. In contrast, only one species can be observed in analogous TR(3) experiments after photolysis of 4-N-methylacetamidophenyl azide in acetonitrile, and this species is tentatively assigned to the compound resulting from dimerization of a 1,2-didehydroazepine. The different photochemical reaction outcomes for the photolysis of 4-acetamidophenyl azide and 4-N-methylacetamidophenyl azide molecules indicate that the 4-acetamido group has a substantial influence on the ISC rate of the corresponding substituted singlet phenylnitrene, but the 4-N-methylacetamido group does not. CASSCF analyses predict that both singlet nitrenes have open-shell electronic configurations and concluded that the dissimilarity in the photochemistry is probably due to differential geometrical distortions between the states. We briefly discuss the probable implications of this intriguing substitution effect on the photochemistry of phenyl azides and the chemistry of the related nitrenes.

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S(1) state. Nitrene formation correlates with the decay of the relaxed S(1) state only upon 350 nm excitation (S(0) → S(1) excitation). When S(n) (n ≥ 2) states are populated upon excitation (λ(ex) = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S(1) and higher singlet excited states (S(n)) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S(1) state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.

Carboethoxycarbene reacts with methanol-OD to form an ylide. The formation and decay of this ylide was monitored by ultrafast time-resolved IR spectroscopy. The formation and decay of the ylide is linearly dependent on the concentration of methanol-OD in acetonitrile with second-order rate constants of ylide formation (8.4 × 10(9) M(-1) s(-1)) and decay (1.4 × 10(9) M(-1) s(-1)). Similar results were obtained with 1-butanol.

The photochemistry of two simple acyclic diazo carbonyl compounds, azibenzil and diazoacetone, were studied using the tools of ultrafast time-resolved spectroscopy. In the former case, UV-vis detection allows observation of an absorption band of singlet benzoylphenylcarbene, decaying with a 740 ± 150 ps time-constant in acetonitrile. IR detection shows that the ketene product of Wolff rearrangement (∼2100 cm(-1)) is formed by two parallel pathways: a stepwise mechanism with carbene intermediacy with a slow rise time-constant of 660 ± 100 ps, and directly in the diazo excited state as confirmed by the immediate formation of an IR band of a nascent hot ketene species. Photolysis (270 nm) of diazoacetone in chloroform leads mainly to the ketene species through a concerted process, consistent with the predominance of the syn conformation in the diazoacetone electronic ground state and a zero quantum yield of the internal conversion process.

The photochemistry of diphenylphosphoryl azide was studied by femtosecond transient absorption spectroscopy, by chemical analysis of light-induced reaction products, and by RI-CC2/TZVP and TD-B3LYP/TZVP computational methods. Theoretical methods predicted two possible mechanisms for singlet diphenylphosphorylnitrene formation from the photoexcited phosphoryl azide. (i) Energy transfer from the (π,π*) singlet excited state, localized on a phenyl ring, to the azide moiety, thereby leading to the formation of the singlet excited azide, which subsequently loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. (ii) Direct irradiation of the azide moiety to form an excited singlet state of the azide, which in turn loses molecular nitrogen to form the singlet diphenylphosphorylnitrene. Two transient species were observed upon ultrafast photolysis (260 nm) of diphenylphosphoryl azide. The first transient absorption, centered at 430 nm (lifetime (τ) ∼ 28 ps), was assigned to a (π,π*) singlet S(1) excited state localized on a phenyl ring, and the second transient observed at 525 nm (τ ∼ 480 ps) was assigned to singlet diphenylphosphorylnitrene. Experimental and computational results obtained from the study of diphenyl phosphoramidate, along with the results obtained with diphenylphosphoryl azide, supported the mechanism of energy transfer from the singlet excited phenyl ring to the azide moiety, followed by nitrogen extrusion to form the singlet phosphorylnitrene. Ultrafast time-resolved studies performed on diphenylphosphoryl azide with the singlet nitrene quencher, tris(trimethylsilyl)silane, confirmed the spectroscopic assignment of singlet diphenylphosphorylnitrene to the 525 nm absorption band.

Ultrafast photolysis (350 nm) of alkylphenyldiazirines promotes the diazirine to the S(1) excited state. Solvent and substituent effects on the excited state lifetimes indicate that the S(1) state is highly polarized and undergoes a [1,2]-H shift in concert with nitrogen extrusion in cyclohexane.

Phenyldiazirine and phenyldiazomethane were studied at the B3LYP/6-31+G(d) and RI-CC2/TZVP levels of theory, and the three lowest singlet excited states of both compounds were optimized at the RI-CC2/TZVP level. The calculations predict that the S(1) state of phenyldiazirine is a sigma --> pi* state, with a quinoidal structure, and the C-N bonds of the diazirine group are slightly deformed from the C(s) symmetry of the ground state's geometry. Both the S(2) and S(3) states are predicted to be pi --> pi* states localized primarily on the phenyl group. The S(1) state was predicted to have an exceedingly large dipole moment with a strong and distinct aromatic C=C vibrational mode around approximately 1600 cm(-1), which is not present in any of the other electronic states examined in this study. The calculations are consistent with the assignment of the S(1) state of phenyldiazirine to the polar intermediate recently observed by ultrafast time-resolved UV-vis and IR spectroscopic studies of arylhalo- and arylalkyldiazirines. The excited states of phenyldiazomethane were also studied, and the implications of interconversion between phenyldiazirine and phenyldiazomethane are discussed. The calculations predict that the chemistry of the ground state and the S(1) excited state of phenyldiazirine are very different. Formation of phenylcarbene is favored on the ground state surface of phenyldiazirine, whereas the S(1) excited state favors isomerization to the first excited state of phenyldiazomethane, which rapidly extrudes nitrogen and forms carbene.

The mechanism of 1,2-H migration process in photoexcited alkyl diazo esters was studied by femtosecond IR transient absorption spectroscopy and theoretical calculations. Two possible processes are discussed: rearrangement in the diazo excited state (RIES) and the intermediacy of singlet carbene species. In chloroform solvent ulrafast IR data show a subpicosecond rise of alkene species indicating the occurrence of the RIES process. Failure in carbene species detection is explained by their absence or very low quantum yield of formation.

Experimental and computational studies suggest that 375 nm excitation of arylchlorodiazirines furnishes their S(1) excited states with lengthened C-N bonds, a positive charge at the para and diazirine carbon atoms, and a negative charge at the nitrogen atoms. These structures rationalize the observed solvent and substituent effects on the excited state lifetimes.

2-(4-Aminophenyl)benzothiazoles related to 1 are potentially important pharmaceuticals. Metabolism apparently involves oxidation and esterification to 3. In water, hydrolysis and photolysis of 3 generates the nitrenium ion 4 that can be detected indirectly by N(3)(-) trapping and directly by UV-vis spectroscopy following laser flash photolysis. The transient, with lambda(max) 570 nm, and a lifetime of 530 ns, reacts with N(3)(-) at a diffusion-controlled rate and generates the quinol 6 by reaction with water.

Ultrafast laser flash photolysis (lambda(ex) = 375 nm) of para-methoxy-3-phenyl-3-methyl diazirine (p-CH(3)OC(6)H(4)CN(2)CH(3)) produced a transient absorption band in the 400-700 nm region. The carrier of the transient absorption is assigned to the S(1) electronic excited state of this compound based on quantum chemical calculations. The strongest vibrational mode of this S(1) excited state, predicted by RI-CC2/TZVP calculations, was directly observed in the mid-infrared region and had the same lifetime as the transient absorption band detected in the visible region, confirming that the same species is responsible for both transient spectra. The S(1) state undergoes solvation within 20 ps after its formation in polar solvents. Decay of the S(1) state leads to the formation of the isomeric diazo compound and singlet carbene. With 270 nm excitation, both singlet carbene and diazo compound are formed in a much more rapid process from the initially populated diazirine S(2) state (<4 ps), in competition with internal conversion to the S(1) state. The ultrafast spectroscopy and quantum calculations presented in this study provide a rather complete and consistent understanding of the structures and the decay kinetics of the excited states of an aryldiazirine and provide some conclusive answers to the pending general mechanistic questions concerning the photoisomerization of diazirine into diazo compound and the denitrogenation into carbenes.

Ultrafast UV-vis/IR spectroscopies were used to study the photochemistry of a vinyl diazo ester PhCH=CHCN2CO2CH3 (1) in solution. The results indicate that singlet styrylcarbomethoxy carbene ((1)2) is produced from the excited state of diazo precursor (1*). It is concluded that vinyl singlet carbene ((1)2) undergoes an intramolecular cyclopropenation reaction to produce the cyclopropene product (3), and undergoes intersystem crossing to ground triplet carbene ((3)2). The predictions of DFT calculations are consistent with the observations.

Ultrafast infrared spectroscopy shows that in chloroform, beta-lactam is formed immediately after the laser pulse but gamma-lactam is formed from both slow and fast processes. It is concluded that beta-lactam is formed from the diazoamide excited state via the rearrangement in the excited state (RIES) mechanism and that gamma-lactam is formed from both RIES and carbene. In methanol, both carbene decay and the rise of amide ether product are observed directly. Predictions from density functional theory calculations are consistent with these observations.

The photochemistry of three carbonyl azides was studied by ultrafast time-resolved IR spectroscopy. Benzoyl, 2-naphthoyl, and pivavoyl azides are promoted to upper excited states S(n) with 270 nm excitation in chloroform. The S(n) states decay in 300 fs to form both the carbonylnitrenes and the S(1) excited states. The decay of the S(1) states of the carbonyl azides correlates with the growth of isocyanates. Formation of carbonylnitrene from S(1) is at most a minor process if it happens at all. The quantum yields of azide decomposition of these azides with 270 nm light are close to unity in chloroform.

Ultrafast laser flash photolysis (lambda(ex) = 270 nm) of phenyldiazirine produces transient infrared absorptions at 2040 and 1582 cm(-1). The first band is assigned to phenyldiazomethane, and the second is assigned to singlet phenylcarbene. This assignment is consistent with DFT calculations. Diazo band integration reveals that photoisomerization from diazirine to diazo occurs within a few picoseconds of the laser pulse. The majority of carbene produced is also formed instantaneously.

Aryloxenium ions 1 are reactive intermediates that are isoelectronic with the better known arylcarbenium and arylnitrenium ions. They are proposed to be involved in synthetically and industrially useful oxidation reactions of phenols. However, mechanistic studies of these intermediates are limited. Until recently, the lifetimes of these intermediates in solution and their reactivity patterns were unknown. Previously, the quinol esters 2 have been used to generate 1, which were indirectly detected by azide ion trapping to generate azide adducts 4 at the expense of quinols 3, during hydrolysis reactions in the dark. Laser flash photolysis (LFP) of 2b in the presence of O(2) in aqueous solution leads to two reactive intermediates with lambda(max) 360 and 460 nm, respectively, while in pure CH(3)CN only one species with lambda(max) 350 nm is produced. The intermediate with lambda(max) 460 nm was previously identified as 1b based on direct observation of its decomposition kinetics in the presence of N(3)(-), comparison to azide ion trapping results from the hydrolysis reactions, and photolysis reaction products (3b). The agreement between the calculated (B3LYP/6-31G(d)) and observed time-resolved resonance Raman (TR(3)) spectra of 1b further confirms its identity. The second intermediate with lambda(max) 360 nm (350 nm in CH(3)CN) has been characterized as the radical 5b, based on its photolytic generation in the less polar CH(3)CN and on isolated photolysis reaction products (6b and 7b). Only the radical intermediate 5b is generated by photolysis in CH(3)CN, so its UV-vis spectrum, reaction products, and decay kinetics can be investigated in this solvent without interference from 1b. In addition, the radical 5a was generated by LFP of 2a and was identified by comparison to a published UV-vis spectrum of authentic 5a obtained under similar conditions. The similarity of the UV-vis spectra of 5a and 5b, their reaction products, and the kinetics of their decay confirm the assigned structures. The lifetime of 1b in aqueous solution at room temperature is 170 ns. This intermediate decays with first-order kinetics. The radical intermediate 5b decomposes in a biphasic manner, with lifetimes of 12 and 75 mus. The decay processes of 5a and 5b were successfully modeled with a kinetic scheme that included reversible formation of a dimer. The scheme is similar to the kinetic models applied to describe the decay of other aryloxy radicals.

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN2CO2CH3) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (tau = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene ((1)NpCCO2CH3). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by approximately 50% of their original intensity in cyclohexane in 3 ns. It is concluded that (1)NpCCO2CH3 has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only (1)NpCCO2CH3 is formed within 50 ps of the laser pulse rather than a singlet-triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.

The photochemistry of Diazo Meldrum's acid (DM) was investigated by fs time-resolved UV-vis and IR spectroscopic methods. UV (266 nm) excitation of DM pumps the molecule to the S 5 and S 7 excited states. After fast internal conversion (IC), the S 2 state is formed, which will undergo Wolff rearrangement to form vibrationally excited ketene, which relaxes in 9 ps. The S 2 state will also relax to the S 1 state, which isomerizes to diazirine, fragments to form carbene, and relaxes further to the ground state of DM. The singlet carbene absorbs at 305 nm, is formed within 300 fs of the laser pulse, and has a lifetime of 2.3 ps in acetonitrile. The lifetime of DM in the S 2 and S 1 states is less than 300 fs. The quantum efficiency of DM decomposition is approximately 50% in chloroform with 266 nm excitation.

The photochemistry of a p-biphenylyl diazo ester (BpCN2CO2CH3) and diazo ketone (BpCN2COCH3) were studied by ultrafast time-resolved UV-vis and IR spectroscopies. The excited states of these diazo compounds were detected and found to decay with lifetimes of less than 300 fs. The diazo ester produces singlet carbene with greater quantum efficiency than the ketone analogue due to competing Wolff rearrangement (WR) in the excited state of the diazo ketone. Carbene BpCCO2CH3 has a singlet-triplet gap that is close to zero in cyclohexane, but the triplet is the ground state. The two spin states are in rapid equilibrium in this solvent relative to reaction with cyclohexane. There is (for a carbene) a slow rate of singlet to triplet intersystem crossing (isc) in this solvent because the orthogonal singlet must rotate to a higher energy orientation prior to isc. In acetonitrile and in dichloromethane BpCCO2CH3 has a singlet ground state. Ketocarbene BpCCOCH3 has a singlet ground state in cyclohexane, in dichloromethane, and in acetonitrile and decays by WR to form a ketene detected by ultrafast IR spectroscopy in these solvents. Ketocarbenes have more stable singlet states, relative to carbene esters, because of the superior conjugation of the filled hybrid orbital of the carbene with the pi system of the carbonyl group, the same factor that makes methyl ketones more acidic than the analogous esters. The rate of WR of BpCCOCH3 is faster in cyclohexane than in dichloromethane and acetonitrile because of intimate solute-solvent interactions between the empty p orbital of the carbene and nonbonding electron pairs of heteroatoms of the solvent. These interactions stabilize the carbene and retard the rate of WR.

The influence of coordinating solvents on singlet-to-triplet carbene intersystem crossing (ISC) rates has been studied with diphenylcarbene (DPC) and para-biphenyltrifluoromethylcarbene (BpCCF 3) by using ultrafast time-resolved spectroscopy. DPC has a triplet ground state in all of the solvents considered, and the concentration of singlet carbene at equilibrium is too small to be measured. It is found that the lifetime of (1)DPC is extended in acetonitrile, benzene, tetrahydrofuran, dichloromethane, and halobenzene solvents relative to cyclohexane. The solvent effect does not well correlate with bulk measures of solvent polarity. The singlet-triplet energy separation of BpCCF 3 is close to zero. The data demonstrates that BpCCF 3 has a triplet ground state in benzene, fluorobenzene, and hexafluorobenzene. Halogenated solvents are found to dramatically retard the rate of ISC in (1)BpCCF 3. We postulate that the empty p orbital of a singlet carbene coordinates with a nonbonding pair of electrons of a halogen atom of the solvent to form a pseudoylide solvent complex, stabilize the singlet carbene, and decrease the singlet-triplet (S-T) energy gap. The "golden rule" of radiationless transitions posits that the smaller the energy gap between the two states, the faster their rate of interconversion. To explain the apparent violation of the golden rule of radiationless transitions for the carbene ISC processes monitored in this study, we propose that the significantly different specific solvation of the singlet and triplet carbenes imposes a Franck-Condon-like factor on the ISC process. Those solvents that most solvate the singlet carbene will also have the greatest structural difference between singlet carbene-solvent complex and their triplet spin isomer-solvent complex, the smallest S-T gap, and the slowest ISC rate. Alternatively, one can propose that a highly solvated singlet carbene must desolvate prior to ISC, and that this requirement decelerates the radiationless transition.

The photochemistry of 5-diazo-10,11-dihydrodibenzo[a,d]diazocycloheptene (DDBC) and 9-diazoanthrone (DAN) were studied by ultrafast time resolved techniques. The excited states of these diazo compounds were observed by UV-Vis spectroscopy and were found to decay in 300 fs. The diazo excited state decays led to the appearance and first direct observation of singlet 5-diazo-10,11-dihydrodibenzo[a,d]cycloheptenylidene (DBC) and 9-anthronylidene (AN). The dynamics of DBC and AN were studied in acetonitrile, cyclohexane and methanol. The lifetimes of (1)DBC are 83 ps and 72 ps in acetonitrile and cyclohexane, respectively. The lifetime of (1)DBC shortens to 9 ps in methanol due to rapid reaction with the solvent. The lifetime of (1)AN is 87 ps and 66 ps in acetonitrile and cyclohexane, respectively. In methanol, the lifetime of (1)AN cannot be determined due to spectral overlap of (1)AN and cation ANH+. The decays of (1)DBC and (1)AN are controlled by intersystem crossing (ISC) in acetonitrile and cyclohexane and the rates of ISC of (1)DBC or (1)AN are similar in these two solvents. This differs from the solvent dependence of other diarylcarbene intersystem crossing rates. This is attributed to the relatively large singlet-triplet (S-T) gaps of these carbenes and this factor dominates the influence of solvent.

The photochemistry of two isomeric aryl diazo ketones was investigated by fs time-resolved UV-vis and IR spectroscopies. Both diazo ketone excited states decompose in less than 300 fs by multiple pathways. One pathway involves concerted Wolff rearrangement and nitrogen extrusion, most likely in the syn rotomer. In the anti rotomer of one isomer, oxygen migration proceeds in concert with nitrogen extrusion to form rearranged keto carbene. This rotomer excited state also decomposes to form unrearranged carbene, which isomerizes in 5 ps.

Photoinduced Wolff rearrangements were studied by femtosecond time-resolved UV-vis and IR transient absorption spectroscopy. For BpCN2COCH3 in acetonitrile the IR data indicate the presence of at least two mechanisms of ketene formation. The first process is fast proceeding in either 1BpCN2COCH3*, or in a hot carbene, or in both species, while the second is slow proceeding through the intermediacy of a relaxed carbene. The slow time constant of the ketene formation dynamics obtained by ultrafast IR (700 ps) spectroscopy agrees with the relaxed carbene decay of 800 +/- 100 ps obtained by UV-vis absorption spectroscopy.

Time-resolved resonance Raman (TR3) studies of the photochemistry of phenyl azide, 3-hyroxyphenyl azide, 3-methoxyphenyl azide and 3-nitrophenyl azide in acetonitrile:water solutions is reported. After photolysis of these four aryl azides in room temperature solutions, only one species was observed in the TR3 spectra for each azide, respectively at the probe wavelengths employed in the TR3 experiments. The species observed after photolysis of 3-nitrophenyl azide was assigned to 3,3'-dinitroazobenzene, an azo compound formed from the dimerization reaction of triplet 3-nitrophenylnitrene. In contrast, the species observed after photolysis of phenyl azide, 3-hydroxyphenyl azide and 3-methoxyphenyl azide were tentatively assigned to intermediates formed from the dimerization of didehydroazepines that are produced from the ring expansion reaction of the respective singlet arylnitrene. To our knowledge, this is the first time-resolved vibrational spectroscopic observation of the dimerization reaction of didehydroazepines in solution. In addition, these are the first resonance Raman spectra reported for dimers formed from didehydroazepines. We briefly discuss the structures, properties and chemical reactivity of the dimer species observed in the TR3 spectra and possible implications for the photochemistry of aryl azides.

Ultrafast photolysis (lambda(ex) = 308 nm) of p-biphenylyltrifluoromethyl diazomethane (BpCN2CF3) releases singlet p-biphenylyltrifluoromethylcarbene (BpCCF3) which absorbs strongly at 385 nm in cyclohexane, immediately after the 300 fs laser pulse. The initial absorption maximum shifts to longer wavelengths in coordinating solvents (nitrile, ether, and alcohol). In low viscosity coordinating solvents, the initial absorption maximum further red shifts between 2 and 10 ps after the laser pulse. Similar effects are observed upon ultrafast photolysis of 2-fluorenyltrifluoromethyl diazomethane (FlCN2CF3) and therefore cannot be associated with torsional motion around the two phenyl rings of the biphenyl compound. Instead, the effect is attributed to the dynamics of solvation of the singlet carbene. The time constant of solvation in normal alcohols lengthens with solvent viscosity in a linear manner. Furthermore, the time constants of the red shift in methanol-O-d (16 ps), ethanol-O-d (26 ps), 2-propanol-OD (40 ps), and 2,2,2-trifluoroethanol-O-d (14 ps) are longer than those recorded in methanol (9.6 ps, KIE = 1.7), ethanol (14.3 ps, KIE = 1.8), 2-propanol (28 ps, KIE = 1.4), and 2,2,2-trifluoroethanol (4.4 ps, KIE = 3.2), which indicates that the solvent reorganization involves formation of hydrogen bonds. The kinetic data are consistent with motion of the solvent to achieve a specific interaction with the carbene, with the creation of a new hydrogen bond. The solvated carbene reacts with the solvent over tens, hundreds, and thousands of ps, depending upon the solvent.

The free energies of reaction (DeltaG) and activation (DeltaG) were determined for the Curtius-like rearrangement of dimethylphosphinoyl, dimethylphosphinyl, and dimethylphosphoryl azides as well as the corresponding singlet and triplet nitrenes by CBS-QB3 and B3LYP computational methods. From CASSCF calculations, it was established that the closed-shell configuration was the lower energy singlet state for each of these nitrenes. The triplet states of dimethylphosphinyl- and dimethylphosphorylnitrene are the preferred ground states. However, the closed-shell singlet state is the ground state for dimethylphosphinoylnitrene. The CBS-QB3 DeltaG values for the Curtius-like rearrangements of dimethylphosphinyl and dimethylphosphoryl azides were 45.4 and 47.0 kcal mol-1, respectively. For the closed-shell singlet dimethylphosphinyl- and dimethylphosphorylnitrene, the CBS-QB3 DeltaG values for the rate-limiting step of the Curtius-like rearrangement were 22.9 and 18.0 kcal mol-1, respectively. It is unlikely that the nitrenes will undergo a Curtius-like rearrangement because of competing bimolecular reactions that have lower activation barriers. The pharmacology of weaponized organophosphorus compounds can be investigated using phosphorylnitrenes as photoaffinity labels. Dominant bimolecular reactivity is a desirable quality for a photoaffinity label to possess, and thus, the resistance of phosphorylnitrenes to intramolecular Curtius-like rearrangements increases their usefulness as photoaffinity labels.

The rate of protonation of singlet 2-fluorenylnitrene was studied in eight protic solvents. The protonation rates vary between 10(11) and 10(9) s(-1) and are well-correlated with Swain's acity parameters. This demonstrates that Swain acity parameters and nitrene protonation rates can be used to evaluate bulk solvent acidity.

Laser flash photolysis of the quinol ester 2b in O2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O2, but reacts rapidly with N3- with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s-1. Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N3- completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N3--trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N3-/solvent selectivity ratio, kaz/ks, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by O2 or N3-. The lifetimes of the two intermediates of ca. 12 and 75 mus are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (

Ultrafast photolysis of 9-diazofluorene (DAF) produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the diazo compound. The diazo excited state fragments to form fluorenylidene (Fl) in both its lowest energy singlet state (1Fl, 405-430 nm, depending on the solvent) and a higher energy singlet state (370 nm, 1Fl*). The excited singlet carbene has a lifetime of 20.9 ps in acetonitrile and decays to the lower energy singlet state (1Fl), which relaxes to the triplet ground state (3Fl) in acetonitrile, cyclohexane, benzene, and hexafluorobenzene. The equilibrium mixture of singlet and triplet fluorenylidene reacts with these solvents. Singlet fluorenylidene reacts with methanol and cyclohexene in competition with relaxation to 3Fl. One of the reaction products in methanol is the 9-fluorenyl cation. The rate of intersystem crossing (ISC) in hexafluorobenzene and other halogenated solvents is remarkably slow given that carbene ISC rates are generally fastest in nonpolar solvents. An explanation of this effect is advanced.

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.

Exposure of 2-naphthyl azide in acetonitrile at ambient temperature to femtosecond pulses of 266 nm light produces a transient absorption with maxima at 350 and 420 nm. The carrier of the 350 nm band decays more rapidly than that of the 420 nm band which has a lifetime of 1.8 ps. Analogous experiments with 1-chloro-2-naphthyl azide in methanol allow the assignment of the 350 nm band to a singlet excited state of 2-naphthyl azide and the carrier of the 420 nm band to singlet 2-naphthylnitrene. This reactive intermediate has the shortest lifetime of any singlet nitrene observed to date and is a true reactive intermediate. Computational studies at the RI-CC2 level of theory support these conclusions and suggest that initial excitation populates the S2 state of 2-naphthyl azide. The S2 state, best characterized as a pi --> (pi*, aryl) transition, has a geometry similar to S0. S2 of 2-naphthyl azide can then populate the S1 state, a pi --> (in-plane, pi*, azide) excitation, and in the S1 state, electron density is depleted along the proximal N-N bond. S1 is dissociative along that N-N coordinate to form the singlet nitrene, and with a barrier of only approximately 5 kcal/mol for N2 extrusion.

Ultrafast photolysis (lambda(ex) = 308 nm) of phenyl azide in 100% formic acid produces a broadly absorbing transient within the instrument time resolution (300 fs), which is assigned to an excited state of the azide. The azide excited state fragments within 300 fs to form singlet phenylnitrene. The decay of the nitrene (tau = 12.0 ps) produces a new species with absorption centered at 500 nm, which is assigned to phenylnitrenium ion. The lifetime of phenylnitrenium ion is 110 ps in 100% formic acid. This is the first spectroscopic observation of phenylnitrenium ion.

p-biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of p- and o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (taugrowth = 11.5 ps), o-biphenylyl (taugrowth = 7.7 ps), and 1-naphthylnitrenium cations (taugrowth = 8.4 ps). The decay lifetimes of p- and o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.

Ketyl radicals produced by photolysis of ketones or di-tert-butyl peroxide (DTBP) in alcohol solvents react rapidly with tirapazamine (TPZ). The acetone ketyl radical (ACOH) reacts with TPZ with an absolute second-order rate constant of (9.7 +/- 0.4) x 108 M-1 s-1. The reaction kinetics can be followed by monitoring the bleaching of TPZ absorption at 475 nm or the formation of a reaction product which absorbs at 320 and 410 nm. The ACOD radical reacts with TPZ in 2-propanol-OD with an absolute rate constant of (6.7 +/- 0.5) x 108 M-1 s-1, corresponding to a kinetic isotope effect (KIE) of 1.4. Deuteration of the radical on carbon (ACOH-d6) retards the reaction of the radical with TPZ even further (absolute rate constant = (4.8 +/- 0.04) x 108 M-1 s-1). This result corresponds to a KIE of 2.0. Radicals derived from dioxane and diisopropyl ether by flash photolysis of DTBP in ethereal solvent react with TPZ more slowly than do ketyl radicals. It is concluded that ketyl radicals react, in part, with TPZ in organic solvents by transfer of a hydrogen atom from the OH and CH3 groups of the ketyl radical to the oxygen atom at the N4 position of TPZ to form acetone or acetone enol and a radical derivative of TPZ (TPZH). The latter species absorbs at 320 and 405 nm, has a lifetime of hundreds of microseconds in alcohol solvents, and decays by disproportionation to form TPZ and a reduced heterocycle. The reduced heterocycle eventually forms a desoxytirapazamine by a polar mechanism. The results are supported by density functional theory calculations. It is proposed that dioxanyl radical will also react, in part, with TPZ by transfer of a hydrogen atom from the carbon adjacent to the radical center to the oxygen atom at the N4 position of TPZ. This produces the enol ether and the previously mentioned TPZH radical. It is further posited that ether radicals react a bit more slowly than ketyl radicals because they lack the second mode of hydrogen transfer (from the OH group) that is present in the ACOH radical. Our data are permissive of the possibility that ether radicals add to TPZ at a rate that is competitive with beta-hydrogen atom transfer.

Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.

Photolysis of p-azidoacetophenone (1a) or 2,3,5,6-tetrafluoro-p-azidoacetophenone (1b) releases the corresponding singlet nitrenes 2a and 2b. In aqueous solutions singlet nitrenes relax (1.1 ps and 43 ns, respectively) to the lower energy triplet nitrenes 3a and 3b, intermediates which do not react to form cross-links or adducts with typical amino acids and nucleic acids. In a hydrophobic environment singlet nitrene 2a partitions between forming triplet nitrene 3a and an acyl-substituted didehydroazepine 4a, which can be detected by LFP and time-resolved IR spectroscopy. The absolute rate constant of reaction of didehydroazepine 4a with water, in acetonitrile, was determined (3.5 x 10(4) M-1 s-1) by laser flash photolysis (LFP) techniques with IR detection at ambient temperature. Photolysis of tetrafluoro azide 1b releases singlet nitrene 2b, which has a lifetime of 172 ns in benzene and can readily be intercepted by pyridine to form ylide 10b (lambdamax = 415 nm). Singlet nitrene 2b reacts with the unactivated CH bonds of cyclohexane to form adduct 8b in 46% yield. Absolute rate constants of reaction of 1b with N-methylimidazole, phenol, dibutyl sulfide, indole, methanol, and dimethyl sulfoxide were determined using the pyridine ylide probe method. It is concluded that photolysis of p-azidoacetophenone (1a) will not lead to cross-link formation but that tetrafluorinated azide 1b can form useful singlet nitrene derived adducts upon photolysis.

Ultrafast photolysis (lambdaex = 270, 350, or 360 nm) of bromophenyl, chlorophenyl, fluorophenyl, and fluoro-para-trifluoromethylphenyl diazirines produces transient species which absorb broadly in the UV and visible regions. Transient decay can be fit to either mono- or biexponential functions (tau1 approximately 0.3-10 ps, tau2 approximately 10-350 ps; dependent on solvent and halogen). Fluoro- and chlorophenylcarbene are formed within the time resolution of the spectrometer (300 fs, 270 nm excitation). Bromophenyl diazirine decay (270 nm excitation) correlates with the growth of bromophenylcarbene. Solvent and substituent effects on the slower decays of the transient absorptions are consistent with assigning the carriers of transient absorption in the visible region to ring-opened zwitterionic species.

Photolysis of phenyl and o-biphenylyl azide (at 270 nm) releases vibrationally excited singlet nitrene which isomerizes to the corresponding hot 1,2-didehydroazepine at a rate competitive with thermal relaxation. Using ultrafast vibrational spectroscopy we observe the formation of vibrationally excited 1,2-4,6-azacycloheptatetraene (1,2-didehydroazepine) in picoseconds following photolysis of phenyl azide in chloroform and o-biphenylyl azide in acetonitrile at ambient temperature.

The photochemistry of para- and ortho-biphenylyl azides and 1-naphthyl azide was studied by ultrafast spectroscopy. In every case, the singlet azide second excited states were observed by transient absorption spectroscopy and were found to have lifetimes of hundreds of femtoseconds. The decay of the S(2) states of the azides was accompanied by the growth of transient absorption of the corresponding singlet nitrenes. The intermediate S(1) state of the azides could not be observed due to its low instantaneous concentration resulting from fast fragmentation and nitrene formation. Quantum chemical calculations predict that the S(2) state of the azide is bound and that there is a much lower barrier toward arylnitrene formation from the S(1) state of the azide. Vibrational cooling of para-biphenylnitrene (11 ps) was experimentally observed. The lifetime of singlet ortho-biphenylnitrene was 16 ps in acetonitrile and was not affected by perdeuteration of the aryl ring. The lifetime of singlet 1-naphthylnitrene is 12 ps in acetonitrile at ambient temperature.

p-Biphenylyldiazomethane was excited by femtosecond pulses of UV light in acetonitrile, in cyclohexane, and in methanol. Ultrafast photolysis produces a singlet excited state of p-biphenylyldiazomethane with lambdamax = 490 nm, and lifetimes of less than 300 fs in acetonitrile, in cyclohexane, and in methanol. The decay of the excited state is accompanied by the growth of transient absorption with lambdamax = 360 nm. The carrier of this transient absorption is attributed to singlet p-biphenylylcarbene, a result that is consistent with the predictions of TD-DFT calculations. The singlet carbene lifetimes are 200 and 77 ps in acetonitrile and cyclohexane, respectively, and are controlled by intersystem crossing to the lower energy triplet state. The transient absorption does not decay to baseline in acetonitrile, because of the formation of nitrile ylide. The equilibrium mixture of singlet and triplet p-biphenylylcarbene reacts with acetonitrile to form a nitrile ylide (lambdamax = 370 nm), and with cyclohexane by C-H insertion 1-20 ns after the laser pulse. The singlet carbene lifetime is only 7.9 ps in methanol, owing to a rapid reaction with the solvent. Reaction with the solvent gives rise, in part, to a p-biphenylylbenzyl cation (lambdamax = 450 nm, tau = 6.3 ps) in methanol.

[reaction: see text] Ultrafast time-resolved studies of the photochemistry of 2-azidonitrobenzene demonstrate that the lifetimes of excited singlet 2-azidonitrobenzene and 2-nitrophenylnitrene are 500 fs and 8.3 ps, respectively, in solution at ambient temperature. The singlet nitrene is a discrete intermediate but is too short lived to undergo bimolecular reactions.