The isomerization of nitrobenzene to phenylnitrite

Ultraviolet Excitation Dynamics of Nitrobenzenes

Time-resolved photoelectron imaging was used to investigate nonadiabatic processes operating in the excited electronic states of nitrobenzene and three methyl-substituted derivatives: 3,5-, 2,6-, and 2,4-dimethylnitrobenzene. The primary goal was evaluating the dynamical impact of the torsional angle between the NO₂ group and the benzene ring plane-something previously implicated in mediating the propensity for branching into different photodissociation pathways (NO vs NO₂ elimination). Targeted, photoinitiated release of NO radicals is of interest for clinical medicine applications, and there is a need to establish basic structure-dynamics-function principles in systematically varied model systems following photoexcitation. Within our 200 ps experimental detection window, we observed no significant differences in the excited-state lifetimes exhibited by all species under study using a 267 nm pump and ionization with an intense 400 nm probe. In agreement with previous theoretical predictions, this suggests that the initial energy redistribution dynamics within the singlet and triplet manifolds are driven by motions localized predominantly on the NO₂ group. Our findings also imply that both NO and NO₂ elimination occur from a vibrationally hot ground state on extended (nanosecond) timescales, and any variations in NO vs NO₂ branching upon site-selective methylation are due to steric effects influencing isomerization prior to dissociation.

Photocontrol of NO, H2S, and HNO Release in Biological Systems by Using Specific Caged Compounds

Nitric oxide (NO) is a gas that plays various roles in physiological signal transduction, for example, in vasodilation, neural transmission, and biodefence. Recently, other gaseous signal mediators such as carbon monoxide (CO) and hydrogen sulfide (H2S) have also been found to have important biological activities. Since experimental studies with gaseous mediators are difficult, chemicals that enable controlled release of these gases are indispensable. We have developed a range of photocontrollable releasers that generate NO, H2S, and related species with fine spatiotemporal control, and we have also employed these caged compounds in various applications. This paper briefly reviews our work on photocontrollable NO, H2S, and HNO releasers, and presents some typical applications illustrating the suitability of our compounds for controlled release of these biologically active species in cellular and tissue systems. These compounds also appear to have potential for future therapeutic applications.

Visible Light-Controlled Nitric Oxide Release from Hindered Nitrobenzene Derivatives for Specific Modulation of Mitochondrial Dynamics

Nitric oxide (NO) is a physiological signaling molecule, whose biological production is precisely regulated at the subcellular level. Here, we describe the design, synthesis, and evaluation of novel mitochondria-targeted NO releasers, Rol-DNB-mor and Rol-DNB-pyr, that are photocontrollable not only in the UV wavelength range but also in the biologically favorable visible wavelength range (530-590 nm). These caged NO compounds consist of a hindered nitrobenzene as the NO-releasing moiety and a rhodamine chromophore. Their NO-release properties were characterized by an electron spin resonance (ESR) spin trapping method and fluorometric analysis using NO probes, and their mitochondrial localization in live cells was confirmed by costaining. Furthermore, we demonstrated visible light control of mitochondrial fragmentation via activation of dynamin-related protein 1 (Drp1) by means of precisely controlled NO delivery into mitochondria of cultured HEK293 cells, utilizing Rol-DNB-pyr.

[Caged Gaseous Mediators and Their Control of Cellular Functions]

  Nitric oxide (NO) is historically well known as a toxic gas but now recognized as a physiological cellular mediator acting at very low concentrations. It is biosynthesized within the body, and modulates many signal transduction processes. For investigation of the functions of this gaseous mediator, it is necessary to use chemical donors that release NO specifically, and it is highly advantageous if the release can be made with precise spatiotemporal control. For this purpose, we have developed caged NO (photocontrollable NO-releasing compounds) with unique releasing mechanisms. One employs the photoinduced rearrangement of an arylnitro group and subsequent release of NO, and another uses photoinduced electron transfer to release NO. One of our caged NO was confirmed to induce a NO-dependent cellular response in vivo under photocontrol. Photocontrollable NO releasers are expected to become indispensable tools for physiological experiments, and are also potential therapeutic agents for photodynamic therapy.

Identification of the free radical produced in the photolysis of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB)

1,3,5-Triamino-2,4,6-trinitrobenzene (TATB) is a typical insensitive high explosive (IHE) that possesses excellent heat, impact, and shock stability. However, it is sensitive to light irradiation, which can produce a long-lived free radical. In this study, (time-dependent) density functional theory is employed to study the features of the ground state (S0), the first singlet excited state (S1), and the first triplet excited state (T1). Results indicate the progress of photolysis, which involves an intersystem crossing from S1 to T1 followed by NO2-ONO isomerization. The long-lived radical produced in TATB photolysis is identified as the phenoxyl radical R-NO by investigating the formation feasibility and the stability. The experimental observation of the changes in the ultraviolet absorption spectra of TATB supports the identification.

Does addition of NO2 to carbon-centered radicals yield RONO or RNO2? An investigation using distonic radical ions

Nitrogen dioxide is used as a "radical scavenger" to probe the position of carbon-centered radicals within complex radical ions in the gas phase. As with analogous neutral radical reactions, this addition results in formation of an [M + NO2](+) adduct, but the structural identity of this species remains ambiguous. Specifically, the question remains: do such adducts have a nitro- (RNO2) or nitrosoxy- (RONO) moiety, or are both isomers present in the adduct population? In order to elucidate the products of such reactions, we have prepared and isolated three distonic phenyl radical cations and observed their reactions with nitrogen dioxide in the gas phase by ion-trap mass spectrometry. In each case, stabilized [M + NO2](+) adduct ions are observed and isolated. The structure of these adducts is probed by collision-induced dissociation and ultraviolet photodissociation action spectroscopy and a comparison made to the analogous spectra of authentic nitro- and nitrosoxy-benzenes. We demonstrate unequivocally that for the phenyl radical cations studied here, all stabilized [M + NO2](+) adducts are exclusively nitrobenzenes. Electronic structure calculations support these mass spectrometric observations and suggest that, under low-pressure conditions, the nitrosoxy-isomer is unlikely to be isolated from the reaction of an alkyl or aryl radical with NO2. The combined experimental and theoretical results lead to the prediction that stabilization of the nitrosoxy-isomer will only be possible for systems wherein the energy required for dissociation of the RO-NO bond (or other low energy fragmentation channels) rises close to, or above, the energy of the separated reactants.

Fragmentation reactions of phenoxide anions: deprotonated Dinoterb and related structures

Dinoterb (6-t-butyl-2,4-dinitrophenol), 1, Dinoseb (6-secbutyl-2,4-dinitrophenol), 2, TBP (2-t-butylphenol), 3, and DNP (2,4-dinitrophenol), 4, have been analyzed by electrospray ionization in the negative mode (ESI-N) - tandem mass spectrometry. Nominal laboratory collision energy was varied from zero to 60 eV during the experiments. Apparent fragmentation energies were estimated from a parametric fitting of the collision efficiency curves. In parallel, fragmentation mechanisms of the deprotonated molecules [M-H](-) were explored using quantum chemistry modeling at the B3LYP/6-31 + G(d,p) level. A major fragmentation of the [M-H](-) ions of Dinoterb and Dinoseb is elimination of an alcohol molecule. This reaction is shown to involve one oxygen atom originating from a nitro group rather than the phenoxide moiety. Eliminations of NO, C(4) and CH(2) = C(CH(3))(2), i.e. reactions involving significant rearrangements, constitute the major part of the other fragmentation pathways observed from [3-H](-) and [4-H](-) ions.

Resonance Raman and density functional theory investigation of the photodissociation dynamics of the A-band absorption of (E)-β-nitrostyrene in cyclohexane solution

Resonance Raman spectra were obtained for (E)-beta-nitrostyrene in cyclohexane solution with excitation wavelengths in resonance with the charge transfer (CT)-band absorption spectrum. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion predominantly along the nominal NO(2) symmetric stretch mode (nu(14)), the nominal C=C stretch mode (nu(8)), the nominal benzene ring stretch mode (nu(9)), accompanied by a smaller amount of motion along the nominal ONO symmetric bend/benzene ring stretch mode (nu(34)), the nominal CCH in-plane bending mode (nu(20)), the nominal HC=CH in-plane bending mode (nu(18)), the nominal NO(2) asymmetric stretch mode (nu(11)), the nominal C-N stretch/benzene ring breathing mode (nu(27)), and the nominal CCC trigonal bending mode (nu(25)). A preliminary resonance Raman intensity analysis was done and these results for (E)-beta-nitrostyrene were compared to results previously reported for several nitrobenzene and trans-stilbene compounds. The differences and similarities between the CT-band resonance Raman spectra and vibrational reorganizational energies for (E)-beta-nitrostyrene relative to those for nitrobenzene and trans-stilbene were briefly discussed.

Resonance Raman intensity analysis of the excited state proton transfer dynamics of 2-nitrophenol in the charge-transfer band absorption

Resonance Raman spectra were obtained for 2-nitrophenol in cyclohexane solution with excitation wavelengths in resonance with the charge-transfer (CT) proton transfer band absorption. These spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion along more than 15 normal modes: the nominal CCH bend+CC stretch nu(12) (1326 cm(-1)), the nominal CCC bend nu(23) (564 cm(-1)), the nominal CO stretch+NO stretch+CC stretch nu(14) (1250 cm(-1)), the nominal CCH bend+CC stretch+COH bend nu(15) (1190 cm(-1)); the nominal CCH bend+CC stretch nu(17) (1134 cm(-1)), the nominal CCC bend+CC stretch nu(22) (669 cm(-1)), the nominal CCN bend nu(27) (290 cm(-1)), the nominal NO(2) bend+CC stretch nu(21) (820 cm(-1)), the nominal CCO bend+CNO bend nu(25) (428 cm(-1)), the nominal CC stretch nu(7) (1590 cm(-1)), the nominal NO stretch nu(8) (1538 cm(-1)), the nominal CCC bend+NO(2) bend nu(20) (870 cm(-1)), the nominal CC stretch nu(6) (1617 cm(-1)), the nominal COH bend+CC stretch nu(11) (1382 cm(-1)), nominal CCH bend+CC stretch nu(9) (1472 cm(-1)). A preliminary resonance Raman intensity analysis was done and the results for 2-nitrophenol were compared to previously reported results for nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone. The authors briefly discuss the differences and similarities in the CT-band absorption excitation of 2-nitrophenol relative to those of nitrobenzene, p-nitroaniline, and 2-hydroxyacetophenone.

Ultrafast Electron Diffraction: Structural Dynamics of Molecular Rearrangement in the NO Release from Nitrobenzene

Nitro compounds release NO, NO2, and other species, but neither the structures during the reactions nor the time scales are known. Ultrafast electron diffraction (UED) allowed the study of the NO release from nitrobenzene, and the molecular pathways and the structures of the transient species were identified. It was observed, in contrast to previous inferences, that nitric oxide and phenoxyl radicals are formed dominantly and that the time scale of formation is 8.8+/-2.2 ps. The structure of the phenoxyl radical was determined for the first time, and found to be quinoid-like. The mechanism proposed involves a repulsive triplet state, following intramolecular rearrangement. This efficient generation of NO may have important implications for the control of by-products in drug delivery and other applications.

Water solvent effect on the first hyperpolarizability of p-nitrophenol and p-nitrophenylphosphate: A time-dependent density functional study

The first hyperpolarizabilities of p-nitrophenol and p-nitrophenylphosphate have been investigated in vacuum and in neutral aqueous solution by means of time-dependent density functional theory. The calculated excited states and hyperpolarizabilities obtained for these systems and for the molecules of phenol, nitrobenzene, and p-nitroaniline in vacuum match well with the experimental trends. The water solvent has been described by the conductorlike screening model and has been completed by water molecules interacting by hydrogen bonds with the solute. The results show a significant effect of the solvent on the first hyperpolarizability. In particular, the hyperpolarizability of p-nitrophenylphosphate (6.78 x 10(-30) esu) in vacuum is only 1.2 times larger than p-nitrophenol (5.63 x 10(-30) esu), whereas it is almost twice higher in aqueous environment, 12.6 x 10(-30) and 6.5 x 10(-30) esu, respectively. This difference in the nonlinear response in neutral water makes the p-nitrophenylphosphate substrate a suitable probe for measuring the activity of alkaline phosphatase enzymes.

Photoinduced nitric oxide release from nitrobenzene derivatives

A new type of photoinduced nitric oxide (NO) donors was designed from nitrobenzene derivatives. Visible-light irradiation of 2,6-dimethylnitrobenzenes bearing extended pi-electron systems at the 4-position revealed efficient NO release using ESR analysis and the Griess assay. Computational study and ultraviolet spectrum analysis suggested that the NO-releasing activity was closely related to the conformation of the nitro group, the absorption intensity, and the length of the conjugated pi-electron system. Employing the photodependent cytotoxicity of compound 14 against HCT116 human colon cancer cells, it was demonstrated that 4-substituted-2,6-dimethylnitrobenzene analogues are useful NO donors for the time- and site-controlled NO treatment.

Infrared spectra of phenyl nitrite and phenoxyl radical-nitric oxide complex in solid argon

Infrared spectra and frequency assignment of two isomers of nitrobenzene, namely the phenyl nitrite C6H(5-)ONO molecule and the phenoxyl radical-nitric oxide complex C6H5O-NO, in solid argon are presented. The phenoxyl radical-nitric oxide complex was produced through UV light irradiation of nitrobenzene in low-temperature solid argon matrix. The complex rearranged to the more stable phenyl nitrite molecule on sample annealing. The aforementioned species were identified on the basis of isotopic IR studies with C6H(5-)(15)NO2 and C6D5NO2, as well as density functional theory calculations.

Computational Study on the Kinetics and Mechanism for the Unimolecular Decomposition of C6H5NO2and the Related C6H5+ NO2and C6H5O + NO Reactions†

The kinetics and mechanisms for the unimolecular dissociation of nitrobenzene and related association reactions C(6)H(5) + NO(2) and C(6)H(5)O + NO have been studied computationally at the G2M(RCC, MP2) level of theory in conjunction with rate constant prediction with multichannel RRKM calculations. Formation of C(6)H(5) + NO(2) was found to be dominant above 850 K with its branching ratio > 0.78, whereas the formation of C(6)H(5)O + NO via the C(6)H(5)ONO intermediate was found to be competitive at lower temperatures, with its branching ratio increasing from 0.22 at 850 K to 0.97 at 500 K. The third energetically accessible channel producing C(6)H(4) + HONO was found to be uncompetitive throughout the temperature range investigated, 500-2000 K. The predicted rate constants for C(6)H(5)NO(2) --> C(6)H(5) + NO(2) and C(6)H(5)O + NO --> C(6)H(5)ONO under varying experimental conditions were found to be in good agreement with all existing experimental data. For C(6)H(5) + NO(2), the combination processes producing C(6)H(5)ONO and C(6)H(5)NO(2) are dominant at low temperature and high pressure, while the disproportionation process giving C(6)H(5)O + NO via C(6)H(5)ONO becomes competitive at low pressure and dominant at temperatures above 1000 K.

Theoretical study of the internal rotational barriers in nitrotoluenes, nitrophenols, and nitroanilines

The molecular geometries and internal rotational barriers of the nitro group of 3-nitrotoluene (3-NT), 4-nitrotoluene (4-NT), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP), 3-nitroaniline (3-NA), and 4-nitroaniline (4-NA) were calculated by five different types of density functional theory (DFT) methods with three different levels of basis sets. Analyses of the torsional angles of the nitro, methyl, amino, and hydroxyl groups indicate that 3-NP, and 4-NP are planar molecules, but 3-NT, 4-NT, 3-NA, and 4-NA are not planar molecules. Internal rotational barriers of the nitro group were calculated as the V2 barrier, and the NO2 torsional potentials for each molecule were given. The heights of the V2 barrier vary with the DFT methods, the basis sets, and the kinds and positions of substituents. The average values of the V2 barriers for 3-NT, 4-NT, 3-NP, 4-NP, 3-NA, and 4-NA are 6.44, 6.92, 6.64, 7.93, 6.38, and 9.13 kcal/mol, respectively. Torsional potentials of the OH and NH2 groups of nitrophenol and nitroaniline derivatives were also studied by a B3LYP/6-31G* approach. Except for the OH group in 2-NP, these derivatives have the V2 barrier.