Nascent OH Research Articles

Experimental and Theoretical Kinetic Studies of the Ozonolysis of Selected Allyl Sulfides (H2C═CHCH2SR, R = CH3, CH3CH2): The Effect of Nascent OH Radicals.

Rate coefficients of the O3-initiated oxidation of allyl methyl sulfide (H2C═CHCH2SCH3, AMS) and allyl ethyl sulfide (H2C═CHCH2SCH2CH3, AES) were determined at atmospheric conditions by "in situ" FTIR. The relative kinetic experiments were performed using methylcyclohexane (McH) and carbon monoxide (CO) as nascent OH radical scavengers and in the absence of any scavenger, to determine the impact that the formation of OH radicals has on the rate coefficients. In the absence of scavengers, values of kAMS+O3 = (5.23 ± 3.57) × 10-18 and kAES+O3 = (5.76 ± 1.80) × 10-18 cm3 molecule-1 s-1 were obtained. In the presence of the scavengers, however, the rates decreased to kAMS+O3+McH = (3.92 ± 1.92) × 10-18 and kAMS+O3+CO = (2.63 ± 0.47) × 10-18 cm3 molecule-1 s-1 for AMS, and kAES+O3+McH = (4.78 ± 1.38) × 10-18 and kAES+O3+CO = (3.50 ± 0.27) × 10-18 cm3 molecule-1 s-1 for AES. On the basis of these results, we have decided to recommend the values obtained using CO as scavenger as those best representing the rate coefficient for the reactions of O3 with AMS and AES. The reaction mechanism was explored using DFT and post-Hartree-Fock computational methods. It is shown that the barrier for the common cyclization to primary ozonide (-3.7 ± 0.1 kcal mol-1) followed by other reactions, as well as that for the reaction with the sulfur atom (-5.1 ± 0.1 kcal mol-1), is small and quite close, meaning that both reaction paths should contribute significantly to the global reaction rate.

OH mid-infrared emission as a diagnostic of H2O UV photodissociation

Context. Water is an important molecule in interstellar and circumstellar environments. Previous observations of mid-infrared (IR) rotational lines of OH toward star-forming regions suggest that OH emission may be used to probe the photodissociation of water. Aims. Our goal is to propose a method to quantify H2O photodissociation and measure the local ultraviolet (UV) flux from observations of mid-IR OH lines. Methods. Cross sections for the photodissociation of H2O resolving individual electronic, vibrational, and rotational states of the OH fragment are collected. The state distribution of nascent OH following H2O photodissociation is computed for various astrophysically relevant UV radiation fields (e.g., a single Lyα line or a broadband spectrum). These distributions are incorporated in a new molecular excitation code called GROSBETA, which includes radiative pumping, collisional (de)excitation, and prompt emission (i.e., following the production of OH in excited states). The influence of the photodissociation rate of H2O, the spectral shape of the UV radiation field, the density, the temperature of the gas, and the strength of the IR background radiation field on the integrated line intensities are studied in detail. As a test case, our model is compared to Spitzer-IRS observations at the tip of the HH 211 bow-shock. Results. The OH rotational line intensities in the range 9–16 μm, covering rotational transitions with Nup = 18–45, are proportional to the column density of H2O photodissociated per second by photons in the range 114–143 nm (denoted as ΦB̃) and do not depend on other local properties such as the IR radiation field, the density, or the kinetic temperature. Provided an independent measurement of the column density of water is available, the strength of the local UV radiation field can be deduced with good accuracy, regardless of the exact shape of the UV field. In contrast, OH lines at longer far-IR wavelengths are primarily produced by IR radiative pumping and collisions, depending on the chemical pumping rate defined as 𝒟B̃ = ΦB̃/N(OH) and on the local physical conditions (nH, TK, IR radiation field). Our model successfully reproduces the OH mid-IR lines in the 10–16 μm range observed toward the tip of the HH 211 bow-shock and shows that the jet shock irradiates its surroundings, exposing H2O to a UV photon flux that is about 5 × 103 times larger than the standard interstellar radiation field. We also find that chemical pumping by the reaction H2 + O may supplement the excitation of lines in the range 16–30 μm, suggesting that these lines could also be used to measure the two-body formation rates of OH. Conclusions. The mid-IR lines of OH constitute a powerful diagnostic for inferring the photodissociation rate of water and thus the UV field that water is exposed to. Future JWST-MIRI observations will be able to map the photodestruction rate of H2O in various dense (nH ≳ 106 cm−3) and irradiated environments and provide robust estimates of the local UV radiation field.

Photodissociation Dynamics of H2O via the Ẽ' (1B2) Electronic State.

Photodissociation dynamics of H2O via the Ẽ'1B2 state were studied using the high-resolution H atom photofragment translational spectroscopy method, in combination with the tunable vacuum ultraviolet free electron laser (VUV FEL). The measured translational energy spectra allow us to determine the respective quantum state population distributions for the nascent OH(X2Π) and OH(A2Σ+) photofragments. Analyses of the quantum state population distributions show both the ground and electronically excited OH fragments to be formed with moderate vibrational excitation but with highly rotational excitation. Unlike the dissociation via the lower-lying electronic states, where OH(X) is the major fragment, the OH(A) products are predominant via the Ẽ' state. These products are mainly ascribed to a fast dissociation on the B̃1A1 state surface after nonadiabatic transitions from the initial excited Ẽ' state to the B̃ state. Meanwhile, another dissociation pathway from the Ẽ' state to the 1B2 3pb2 state, followed by coupling to the 1A2 3pb2 state, is also observed, which yields the OH(X) + H and O(3P) + 2H products.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.

Reaction Mechanism of Nerve-Agent Hydrolysis with the Cs8Nb6O19 Lindqvist Hexaniobate Catalyst

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs8Nb6O19 obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH– to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH– to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blue...

Investigation of photochemical effects in flame diagnostics with picosecond photofragmentation laser-induced fluorescence

Photofragmentation laser-induced fluorescence (PFLIF) is for the first time performed based on picosecond laser pulses for detection of hydroperoxyl radicals (HO2) in a stoichiometric laminar methane/air flame. Photofragmentation is performed with a pump laser pulse of 80ps duration and a wavelength of 266nm, whereupon the produced OH photofragments are detected by a second picosecond probe laser pulse, inducing fluorescence via excitation in the A2Σ+(v=1) ← X2Π(v=0) band of OH near 283nm. Excitation spectra of the OH photofragments formed in the reaction zone were recorded for pump-probe delays ranging from 0 to 5ns. The spectra suggest that the population distribution of the nascent OH fragments is rotationally cold and that it takes on the order of 5ns for the nascent non-equilibrium rotational distribution to relax into a thermal distribution. The radial OH-fragment distribution was extracted from spectral images (radial position versus emission wavelength) recorded at six different pump-probe delays. Photochemical OH production was observed both in the reaction zone and the product zone. Comparison with a kinetic model for OH production suggests that more than 20% of the oxygen fragments produced by photolysis in the reaction zone are formed in the excited 1D state, explaining a very rapid initial signal growth. The OH-production model was also compared with previous reaction-zone data, acquired with nanosecond laser pulses in the same flame, indicating that no O(1D), but only O(3P), is formed. A plausible explanation of the discrepancy between the two results is that the picosecond pulses, having more than two-orders of magnitude higher irradiance than the nanosecond pulses used in the previous study, might cause 2-photon photodissociation, allowing production of O(1D). In terms of flame diagnostics with PFLIF, it is concluded that a setup based on nanosecond laser pulses, rather than picosecond pulses, appears preferable since photochemical OH production in the reaction zone can be avoided while for short delay times the ratio between the photofragment signal and the photochemical interference in the product zone, stemming from CO2 photolysis, is sufficiently large to clearly visualize the photofragments.

Dynamics of OH formation in the photodissociation of enolic-acetonylacetone at 193nm

Abstract The photodissociation dynamics of enolic-acetonylacetone (H3C C(O) CH2 CH C(OH) CH3), in gas phase, is studied using Laser Photolysis–Laser Induced Fluorescence (LP–LIF) technique at room temperature. The nascent state distribution of the OH radical is probed formed after photo-excitation of the parent molecule to its π C = C − π C = C * state using 193 nm laser light. The ro-vibrational distribution of the nascent OH photofragment is measured using LIF in collision free conditions. The rotational distribution is Boltzmann-like, and characterized by a single rotational temperature of 650 ± 50 K. The spin-orbit and Λ-doublet ratios of OH fragment formed in the dissociation process are also determined. For OH producing channel, the average translational energy partitioned into the photofragment pairs in the center-of-mass co-ordinate is found to be 25.0 ± 4.0 kcal/mol, corresponding to a fT value of 0.43. The experimental studies along with theoretical calculations employing time-dependent density functional theory (TD-DFT) along with ab initio molecular orbital (MO) theory suggest that the initially prepared π C = C − π C = C * state crosses over to a nearby σ* repulsive state along the C O bond, from where the dissociation takes place. The dynamics of OH formation is compared with that of acetylacetone photodissociation and effect of H-bonding is discussed.

Reactivity of Dimeric Tetrazirconium(IV) Wells-Dawson Polyoxometalate toward Dipeptide Hydrolysis Studied by a Combined Experimental and Density Functional Theory Approach.

Detailed kinetic studies on the hydrolysis of glycylglycine (Gly-Gly) in the presence of the dimeric tetrazirconium(IV)-substituted Wells-Dawson-type polyoxometalate Na14[Zr4(P2W16O59)2(μ3-O)2(OH)2(H2O)4] · 57H2O (1) were performed by a combination of (1)H, (13)C, and (31)P NMR spectroscopies. The catalyst was shown to be stable under a broad range of reaction conditions. The effect of pD on the hydrolysis of Gly-Gly showed a bell-shaped profile with the fastest hydrolysis observed at pD 7.4. The observed rate constant for the hydrolysis of Gly-Gly at pD 7.4 and 60 °C was 4.67 × 10(-7) s(-1), representing a significant acceleration as compared to the uncatalyzed reaction. (13)C NMR data were indicative for coordination of Gly-Gly to 1 via its amide oxygen and amine nitrogen atoms, resulting in a hydrolytically active complex. Importantly, the effective hydrolysis of a series of Gly-X dipeptides with different X side chain amino acids in the presence of 1 was achieved, and the observed rate constant was shown to be dependent on the volume, chemical nature, and charge of the X amino acid side chain. To give a mechanistic explanation of the observed catalytic hydrolysis of Gly-Gly, a detailed quantum-chemical study was performed. The theoretical results confirmed the nature of the experimentally suggested binding mode in the hydrolytically active complex formed between Gly-Gly and 1. To elucidate the role of 1 in the hydrolytic process, both the uncatalyzed and the polyoxometalate-catalyzed reactions were examined. In the rate-determining step of the uncatalyzed Gly-Gly hydrolysis, a carboxylic oxygen atom abstracts a proton from a solvent water molecule and the nascent OH nucleophile attacks the peptide carbon atom. Analogous general-base activity of the free carboxylic group was found to take place also in the case of polyoxometalate-catalyzed hydrolysis as the main catalytic effect originates from the -C═O···Zr(IV) binding.

Resonant four-wave mixing spectra: A fresh look at photodissociation dynamics

Frequency- and polarization-resolved degenerate resonant four-wave mixing is used for the first time to study the rotational anisotropy of the OH radicals produced by photolysis of H2O2 by a plane-polarized laser radiation at 266 nm. For that, a flexible polarization setup is designed to directly isolate the rotationally anisotropic signals in the A2Σ+ – X2Π(0, 0) OH vibronic band. The results are quantitatively interpreted by using the line-space approach and accounting for all possible correlations between the electric field ED of the photolysis laser, the recoil velocity v and the angular momentum J of nascent particles. In so doing, the resonance amplitudes are decoupled into products of the polarization factors, the Doppler-broadened profiles and the Einstein B-coefficients. The anisotropic signals have characteristic shapes of well resolved Doppler doublets providing a direct evidence of translational helicity of the nascent OH radicals, a hitherto exprementally inaccessible key characteristic of the photodissociation path.

Photodissociation dynamics of 2-chloro-6-nitrotoluene and nitrocyclopentane in gas phase: Laser-induced fluorescence detection of OH

Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266nm and nitrocyclopentane (NCP) at 193nm leads to the formation of OH, as detected by laser-induced fluorescence (LIF). The nascent OH produced from the photolysis of ClNT at all the wavelengths is vibrationally cold, with the Boltzmann type rotational state distributions. However, the nascent OH product from NCP is in the ground and vibrationally excited states with the measured average relative population in ν″=1 to that in ν″=0 of 0.12±0.03, and these levels are characterized by rotational temperatures of 650±180K and 1570±90K, respectively. The translational energy partitioned in the OH fragment has been measured for photodissociation of both ClNT and NCP. On the basis of both the experimental results and the ground state molecular orbital (MO) calculations, a plausible mechanism for the OH formation has been proposed.

UV photochemistry of peroxyformic acid (HC(O)OOH): an experimental and computational study investigating 355 nm photolysis.

The photochemistry of peroxyformic acid (PFA), a molecule of atmospheric interest exhibiting internal hydrogen bonding, is examined by exciting the molecule at 355 nm and detecting the nascent OH fragments using laser-induced fluorescence. The OH radicals are found to be formed in their ground electronic state with the vast majority of available energy appearing in fragment translation. The OH fragments are vibrationally cold (v" = 0) with only modest rotational excitation. The average rotational energy is determined to be 0.35 kcal/mol. Further, the degree of OH rotational excitation from PFA is found to be significantly less than that arising from the dissociation of H2O2 as well as other hydroperoxides over the same wavelength. Ab initio calculation at the EOM-CCSD level is used to investigate the first few electronic excited states of PFA. Differences in the computed torsional potential between PFA and H2O2 help rationalize the observed variation in their respective OH fragment rotational excitation. The calculations also establish that the electronic excited state of PFA accessed in the near UV is of (1)A" symmetry and involves a σ*(O-O) ← n(O) excitation. Additionally, the UV absorption cross section of PFA at 355 and 282 nm is estimated by comparing the yield of OH from PFA at these wavelengths to that from hydrogen peroxide for which the absorption cross sections is known.

Site and bond-specific dynamics of reactions at the gas–liquid interface

The dynamics of the interfacial reactions of O((3)P) with the hydrocarbon liquids squalane (C30H62, 2,6,10,15,19,23-hexamethyltetracosane) and squalene (C30H50, trans-2,6,10,15,19,23-hexamethyltetracosa-2,6,10,14,18,22-hexaene) have been studied experimentally. Laser-induced fluorescence (LIF) was used to detect the nascent gas-phase OH products. The O((3)P) atoms are acutely sensitive to the chemical differences of the squalane and squalene surfaces. The larger exothermicity of abstraction from allylic C-H sites in squalene is reflected in markedly hotter OH rotational and vibrational distributions. There is a more modest increase in translational energy release. A larger fraction of the available energy is deposited in the liquid for squalene than for squalane, consistent with a more extensive geometry change on formation of the allylic radical co-product. Although the dominant reaction mechanism is direct, impulsive scattering, there is some evidence for OH being accommodated at both liquid surfaces, resulting in thermalised translation and rotational distributions. Despite the H-abstraction reaction being strongly favoured energetically for squalene, the yield of OH is substantially lower than for squalane. This is very likely due to competitive addition of O((3)P) to the unsaturated sites in squalene, implying that double bonds are extensively exposed at the liquid surface.

Trends in organic hydroperoxide photodissociation and absorption cross sections between 266 and 377 nm

Organic hydroperoxides have atmospheric roles in secondary organic aerosol formation and maintaining OH radical concentrations. Monitoring OH from CH3CH2OOH and (CH3)3COOH at 266, 291, 331, 362, and 377nm yields trends in energy disposal and absorption cross sections. Nascent OH radicals emerge similarly, but with less translational energy from the heavier parent. Cross sections are new for CH3CH2OOH and extend the range for (CH3)3COOH, which has implications for modeling atmospheric OH concentrations. Lower cross sections and higher OH rotation for (CH3)3COOH <331nm could be due to a higher trans barrier for torsion about the O–O bond, compared to CH3OOH and CH3CH2OOH.

Polarization‐ and time‐resolved DFWM spectroscopy of the A 2Σ+− X 2Π (0,0) band transitions of nascent OH radicals generated by 266 nm laser photolysis of H2O2

The potential of a non‐linear spectroscopic technique – degenerate four‐wave mixing (DFWM) spectroscopy – for photo‐dissociation studies is investigated. By applying a carefully considered control of the input beam polarizations, the technique is used to measure the line shapes of specific rovibrational transitions in the A 2Σ+ − X 2Π (0,0) band of nascent OH radicals upon pulsed photo‐dissociation of H2O2. The photofragmentation is performed by linearly polarized radiation at 266 nm, and DFWM spectra are observed at various time delays. In contrast to linear spectroscopic methods, isotropic and anisotropic components of the non‐linear susceptibility tensor are separately accessible by using appropriate polarization geometries. In particular, four‐wave mixing signals can be generated that are exclusively due to the occurrence of the transient anisotropy of the recoil velocity and angular momentum vector distributions of the fragments emerging from the dissociation reaction. The observed line shapes in isotropic and anisotropic DFWM spectra are governed by the Doppler effect and vectorial correlations between the parent H2O2 molecule's transition dipole moment, the OH fragments’ recoil velocity and angular momentum, as well as by the processes of collision‐induced rotational and translational relaxation. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Photodissociation Dynamics of Enolic 1,2-Cyclohexanedione at 266, 248, and 193 nm: Mechanism and Nascent State Product Distribution of OH

The photodissociation dynamics of 1,2-cyclohexanedione (CHD), which exists in enolic form in gas phase, is studied using pulsed laser photolysis (LP)-laser induced fluorescence (LIF) "pump-and-probe" technique at room temperature. The nascent state distribution of the OH radical, formed after initial photoexcitation of the molecule to it is (π, π*) and Rydberg states, is determined. The initial (π, π*) and Rydberg states are prepared by excitation with the fourth harmonic output of Nd:YAG (266 nm)/KrF (248 nm) and ArF (193 nm) lasers, respectively. The ro-vibrational distribution of the nascent OH photofragment is measured in collision-free conditions using LIF. The OH fragments are formed in the vibrationally cold state at all the above wavelengths of excitation but differ in rotational state distributions. At 266 nm photolysis, the rotational population of OH shows a curvature in Boltzmann plot, which is fairly described by two types of Boltzmann-like distributions characterized by rotational temperatures of 3100 ± 100 and 900 ± 80 K. However, at 248 nm photolysis, the rotational distribution is described by a single rotational temperature of 950 ± 80 K. The spin-orbit and Λ-doublets ratios of OH fragments formed in the dissociation process are also measured. The average translational energy in the center-of-mass coordinate, partitioned into the photofragment pairs of the OH formation channels, is determined to be 12.5 ± 3.0, 12.7 ± 3.0, and 12.0 ± 3.0 kcal/mol at 266, 248, and 193 nm excitation, respectively. The energy partitioning into various degrees of freedom of products is interpreted with the help of different models, namely, statistical, impulsive, and hybrid models. To understand the nature of the dissociative potential energy surface involved in the OH formation channel, detailed ab initio calculations are performed using configuration interaction-singles (CIS) method. It is proposed that at 266 nm photolysis, the OH fragment is formed from two different excited state structures, one with a strong H bonding, similar to that in the ground state, and another without effective H bonding, whereas, at 248 nm photodissociation, it seems that the OH formation occurs mainly from the excited state, which lacks effective H-bonding. At 193 nm excitation, the initially prepared population in the Rydberg state crosses over to a nearby σ* repulsive state along the C-O bond, from where the dissociation takes place. The exit barrier for the OH dissociation channel is estimated to be 14 kcal/mol. The existence of dynamical constraint due to strong hydrogen bond in the ground state is effectively present in the dissociation process at 266 and somewhat deficient at 248 nm photolysis.

A combined crossed-beam and theoretical study of the reaction dynamics of O(3P) + C2H3 → C2H2 + OH: Analysis of the nascent OH products with the preferential population of the Π(A′) component

The gas-phase reaction dynamics of ground-state atomic oxygen [O((3)P) from the photo-dissociation of NO(2)] with vinyl radicals [C(2)H(3) from the supersonic flash pyrolysis of vinyl iodide, C(2)H(3)I] has been investigated using a combination of high-resolution laser-induced fluorescence spectroscopy in a crossed-beam configuration and ab initio calculations. Unlike the previous gas-phase bulk kinetic experiments by Baulch et al. [J. Phys. Chem. Ref. Data 34, 757 (2005)], a new exothermic channel of O((3)P) + C(2)H(3) → C(2)H(2) + OH (X (2)Π: υ" = 0) has been identified for the first time, and the population analysis shows bimodal nascent rotational distributions of OH products with low- and high-N" components with a ratio of 2.4:1. No spin-orbit propensities were observed, and the averaged ratios of Π(A('))∕Π(A") were determined to be 1.66 ± 0.27. On the basis of computations at the CBS-QB3 theory level and comparison with prior theory, the microscopic mechanisms responsible for the nascent populations can be understood in terms of two competing dynamical pathways: a direct abstraction process in the low-N" regime as the major pathway and an addition-complex forming process in the high-N" regime as the minor pathway. Particularly, during the bond cleavage process of the weakly bound van der Waals complex C(2)H(2)-OH, the characteristic pathway from the low dihedral-angle geometry was consistent with the observed preferential population of the Π(A') component in the nascent OH products. A molecular-level discussion of the reactivity, mechanism, and dynamical features of the title reaction are presented together with a comparison to gas-phase oxidation reactions of a series of prototypical hydrocarbon radicals.

Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

First results are reported on overtone (v(OH) = 2 ← 0) spectroscopy of weakly bound H(2)-H(2)O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H(2)-H(2)O, followed by (ii) UV photodissociation of the resulting H(2)O, and (iii) UV laser induced fluorescence on the nascent OH radical. In addition, intermolecular dynamical calculations are performed in full 5D on the recent ab initio intermolecular potential of Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] in order to further elucidate the identity of the infrared transitions detected. Excellent agreement is achieved between experimental and theoretical spectral predictions for the most strongly bound van der Waals complex consisting of ortho (I = 1) H(2) and ortho (I = 1) H(2)O (oH(2)-oH(2)O). Specifically, two distinct bands are seen in the oH(2)-oH(2)O spectrum, corresponding to internal rotor states in the upper vibrational manifold of Σ and Π rotational character. However, none of the three other possible nuclear spin modifications (pH(2)-oH(2)O, pH(2)-pH(2)O, or oH(2)-pH(2)O) are observed above current signal to noise level, which for the pH(2) complexes is argued to arise from displacement by oH(2) in the expansion mixture to preferentially form the more strongly bound species. Direct measurement of oH(2)-oH(2)O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and <5(2) ns for the Σ and Π states, respectively. Theoretical calculations permit the results to be interpreted in terms of near resonant energy levels and intermolecular alignment of the H(2) and H(2)O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.

Why Is Cobalt the Best Transition Metal in Transition-Metal Hangman Corroles for O-O Bond Formation during Water Oxidation?

O-O bond formation catalyzed by a variety of β-octafluoro hangman corrole metal complexes was investigated using density functional theory methods. Five transition metal elements, Co, Fe, Mn, Ru, and Ir, that are known to lead to water oxidation were examined. Our calculations clearly show that the formal Co(V) catalyst has a Co(IV)-corrole(•+) character and is the most efficient water oxidant among all eight transition-metal complexes. The O-O bond formation barriers were found to change in the following order: Co(V) ≪ Fe(V) < Mn(V) < Ir(V) < Co(IV) < Ru(V) < Ir(IV) < Mn(IV). The efficiency of water oxidation is discussed by analysis of the O-O bond formation step. Thus, the global trend is determined by the ability of the ligand d-block to accept two electrons from the nascent OH(-), as well as by the OH(•) affinity of the TM(IV)═O species of the corresponding TM(V)═O·H2O complex. Exchange-enhanced reactivity (EER) is responsible for the high catalytic activity of the Co(V) species in its S = 1 state.

A Single-Site Mutation (F429H) Converts the Enzyme CYP 2B4 into a Heme Oxygenase: A QM/MM Study

The intriguing deactivation of the cytochrome P450 (CYP) 2B4 enzyme induced by mutation of a single residue, Phe429 to His, is explored by quantum mechanical/molecular mechanical calculations of the O-OH bond activation of the (Fe(3+)OOH)(-) intermediate. It is found that the F429H mutant of CYP 2B4 undergoes homolytic instead of heterolytic O-OH bond cleavage. Thus, the mutant acquires the following characteristics of a heme oxygenase enzyme: (a) donation by His429 of an additional NH---S H-bond to the cysteine ligand combined with the presence of the substrate retards the heterolytic cleavage and gives rise to homolytic O-OH cleavage, and (b) the Thr302/water cluster orients nascent OH(•) and ensures efficient meso hydroxylation.

The nascent OH detection in photodissociation of 2-(bromomethyl)hexafluoro-2-propanol at 193 nm: Laser-induced fluorescence study

Photodissociation of 2-(bromomethyl)hexafluoro-2-propanol (BMHFP) and 3-bromo-1-propanol (BP), involving σ C – Br * ← n Br transition at 193 nm, has been investigated by measuring laser-induced fluorescence spectra of the expected OH product. The OH channel is a minor dissociation pathway with a quantum yield of 0.17 ± 0.05 in BMHFP, whereas it was not observed in BP. Partitioning of the available energy into translation, rotation, and vibration of the photoproducts has been measured by state selective detection of the nascent OH product in BMHFP. OH is produced mostly in the ground vibrational level ( v″ = 0), with a rotational distribution being characterized by a temperature of 465 ± 25 K. But, a significant fraction of the available energy of 30.2 kcal mol −1 is partitioned into translation of OH (14.6 kcal mol −1). The OH( v″ = 0, J″) populations in the spin-orbit states as well as in the Λ-doublet states are statistical. A plausible mechanism of OH formation on excitation of BMHFP at 193 nm is suggested, with the primary reaction channel being elimination of Br atom by direct C–Br bond dissociation from a repulsive surface. The Br radical is detected using (2 + 1) resonance-enhanced multiphoton ionization (REMPI) at ∼234 nm. It is produced in both the ground ( 2P 3/2) and the excited ( 2P 1/2) spin-orbit states with the relative quantum yield of the latter to be 0.36. The co-fragment of Br undergoes secondary C–O bond dissociation to produce OH and F 3C–C( CH 2)–CF 3, with the reaction having a barrier located in the exit channel. In this two-step three-body dissociation process, a major fraction of the available energy is released into translation (〈 f T〉 ∼ 0.75), resulting from an impulsive C–Br bond dissociation in the primary step and presence of an exit barrier in the secondary process. Experimental results combined with theoretical calculations provide a clear picture of the dynamics of OH formation from BMHFP at 193 nm. In addition, the energetics of another channel, competing with OH, have been calculated from the primary product F 3C–C(CH 2)(OH)–CF 3. In contrast to BMHFP, the OH product could not be observed from the photolysis of 3-bromo-1-propanol (another saturated halogenated propanol) at 193 nm under the detection limit of the present experimental condition, although it has a higher absorption cross-section at 193 nm.