Title Reaction Research Articles

Small molecules (CO2 , iPrNCO, and iPrNCNiPr) activation by the metallomimetics (μ-Hydrido) diborane anion: A DFT investigation on mechanism and chemoselectivity controlling.

Main-group metallomimetics provide a new way to replace transition metal complexes to activate inert small molecules under mild conditions. In this work, the activation mechanisms of CO2 , iPrNCO, and iPrNCNiPr by (μ-Hydrido) diborane anion ([1H]- ) have been investigated by density functional theory (DFT) calculations. Two different activation sites, BB versus BH bond of [1H]- , are investigated and compared. The results show that these inert molecules can be activated by [1H]- through cycloadditions under mild conditions. The reactions with iPrNCO and iPrNCNiPr are dynamic and thermodynamic controlling, the obtained products are related not only to the energy barrier but also to the stability of the products. Moreover, the competition for BB/BH bond site activation is directly related to the steric effect of small molecules. CO2 , which is without steric hindrance, can only be activated by the BB bond, whereas iPrNCNiPr can only be activated by the BH bond due to the large steric effect. The medium iPrNCO can be activated not only by the BB bond but also by the BH bond. Our study provides theoretical explanations for the reaction activity and chemoselectivity controlling of the title reaction, and displays the potential applications for compounds containing boron-boron bonds and inert small molecule activation.

Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect.

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt = -kobs[CH2OO]; kobs = k0 + k1[water] + k2[water]2 + k3[water]3; k1 = (4.22 ± 0.48) × 10-16 cm3 s-1, k2 = (10.66 ± 0.83) × 10-33 cm6 s-1, k3 = (1.48 ± 0.17) × 10-50 cm9 s-1 at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1 = 1.8 ± 1.1 kcal mol-1, Ea2 = -11.1 ± 2.1 kcal mol-1, Ea3 = -17.4 ± 3.9 kcal mol-1. The contribution of the k3[water]3 term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction.

Alternating Stereospecificity upon Central-Atom Change: Dynamics of the F- +PH2 Cl SN 2 Reaction Compared to its C- and N-Centered Analogues.

Central-atom effects on bimolecular nucleophilic substitution (SN 2) reactions are well-known in chemistry, however, the atomic-level SN 2 dynamics at phosphorous (P) centers has never been studied. We investigate the dynamics of the F- +PH2 Cl reaction with the quasi-classical trajectory method on a novel full-dimensional analytical potential energy surface fitted on high-level ab initio data. Our computations reveal intermediate dynamics compared to the F- +CH3 Cl and the F- +NH2 Cl SN 2 reactions: phosphorus as central atom leads to a more indirect SN 2 reaction with extensive complex-formation with respect to the carbon-centered one, however, the title reaction is more direct than its N-centered pair. Stereospecificity, characteristic at C-center, does not appear here either, due to the submerged front-side-attack retention path and the repeated entrance-channel inversional motion, whereas the multi-inversion mechanism discovered at nitrogen center is also undermined by the deep Walden-well. At low collision energies, 6 % of the PH2 F products form with retained configuration, mostly through complex-mediated mechanisms, while this ratio reaches 24 % at the highest energy due to the increasing dominance of the direct front-side mechanism and the smaller chance for hitting the deep Walden-inversion minimum. Our results suggest pronounced central-atom effects in SN 2 reactions, which can fundamentally change their (stereo)dynamics.

Cracking the puzzle of CO2 formation on interstellar ices

Context. Carbon dioxide (CO2) is one of the dominant components of interstellar ices. Recent observations show CO2 exists more abundantly in polar (H2O-dominated) ice than in apolar (H2O-poor) ice. Formation of CO2 ice is primarily attributed to the reaction between CO and OH, which has a barrier. Aims. We investigate the title reaction in H2O ice and CO ice to quantify the efficiency of the reaction in polar ice and apolar ice. Methods. Highly accurate quantum chemical calculations were employed to analyze the stationary points of the potential energy surfaces of the title reaction in the gas phase on H2O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under interstellar medium conditions. We simulated the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation. Results. The CO + OH reaction proceeds through the remarkably stable intermediate HOCO radical. On the H2O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO2 ice cannot be formed by the title reaction alone either on an H2O cluster or a CO cluster. Conclusions. In the polar ice, CO2 ice formation is possible via CO + OH → HOCO followed by HOCO + H → CO2 + H2, as demonstrated by abundant experimental literature. In apolar ice, CO2 formation is less efficient because HOCO formation requires external energy. Our finding is consistent with the JWST observations. Further experimental work using low-temperature OH radicals is encouraged.

Electrocatalytic, Homogeneous Ammonia Oxidation in Water to Nitrate and Nitrite with a Copper Complex

Electrocatalytic ammonia oxidation (AO) at room temperature and pressure allows energy-economical and environmentally-friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]+, a homogenous, earth-abundant metal electrocatalyst that realizes the title reaction in water. The complex selectively catalyzes the ammonia oxidation electrochemically at 94% Faradaic efficiency without generating the less-valuable N2 product commonly seen in molecular AO catalysis. Moreover, our complex is designed to not mediate water oxidation (WO), which is beneficial for high selectivity in AO in aqueous media. Initial mechanistic studies by electrochemistry propose monometallic catalysis and water or ammonia-nucleophilic attack for N–O bond formation. Figure 1

SiS formation in the interstellar medium via SiH + S collisions

ABSTRACT One of the most important Si-bearing species in the intersellar medium is the SiS molecule. Thermal rate coefficients and other collisional properties are calculated for its formation via the title reaction using the quasi-classical trajectory method. An accurate representation of the HSiS potential energy surface is employed, which has been modelled from high-level ab initio calculations and a reliable description of long-range interactions as implied by the underlying double many-body expansion method. The calculated rate coefficients for the $\rm SiH + S \rightarrow SiS + H$ reaction can be modelled with k(T) = α(T/300)βe−γ/T where $\alpha =0.63\times 10^{-10}\, \rm cm^3\, s^{-1}$, β = -0.11, and $\gamma = 11.6\, \rm K$. This result is only slightly lower than that for SiS formation via Si + SH collisions. The contribution of each reaction mechanism and the rovibrational energy distributions of the nascent SiS molecule are also calculated. The title collision can also yield SH ($\rm SiH + S \rightarrow SH + Si$), but the corresponding rate coefficient is 20 to 27 times smaller than for SiS formation ($\alpha =0.025\times 10^{-10}\rm cm^3\, s^{-1}$, β =-0.13, and $\gamma = 9.38\, \rm K$). The role of intersections between excited electronic states is also discussed, based on novel calculations including eight electronic states.

The complex-formation dynamics of O + H(D)2+ (v = 0,1; j = 0) reactions on 12A'' and 12A′

The reactions O + H(D)2+ are studied based on the ground state potential energy surface (PES) 12A″ and the first excited PES 12A′. Both two potential energy surfaces comprise deep wells during the minimum energy reaction path. The average lifetimes of the complex-formation of the title reactions as a function of translational energy are calculated. The results indicated that the average lifetime of the complex-formation decreases with the increasing of collision energy for the two potential energy surfaces. There are two kinds of reaction mechanisms, one is direct reaction mechanism; another is indirect one. The indirect mechanism is dominated for all the collision energies studied. However, with the increasing of the collision energy, the direct reaction mechanism becomes more and more pronounced. Besides, the influences of reagent vibrational excitation and isotopic effect on the scalar properties and vector properties are studied in detail respectively in this work. The reagent vibrational excitation enhances the number of the direct reactive trajectories remarkably on 12A″, though it has nearly no influence on the average lifetime of the complex formation. In strong contrast, the reagent vibrational excitation suppresses the number of indirect reactive trajectories and reduces the average lifetime of the complex formation on the 12A′. The isotopic substitution has no influence on the scalar properties; however, it affects the vector properties strongly.

Laser induced fluorescence and computational studies on the tropospheric photooxidation reactions of methyl secondary butyl ether initiated by OH radicals.

The kinetics of the reaction of methyl secondary butyl ether with OH radicals was investigated experimentally using the pulsed laser photolysis-laser induced fluorescence technique (PLP-LIF) over temperatures ranging from 268 to 363K. The rate coefficient value at 298K was measured to be (1.09 ± 0.02) × 10-11 cm3 molecule-1s-1 and the deduced Arrhenius expression is [Formula: see text]= (2.21 ± 0.29) × 10-12 exp ((471.71 ± 38.50)/T) cm3 molecule-1s-1. To complement the experimental data, the kinetic study of the title reaction was performed computationally at CCSD(T)/cc-pVTZ//M06-2X/6-311 + G(d,p) level of theory with the incorporation of tunnelling correction from 200 to 400K. The end products formed were qualitatively analyzed by using gas chromatography equipped with mass spectrometry (GC-MS) as detection technique and the mechanism for degradation was proposed. Thermochemical parameters were evaluated to determine the feasibility of individual reaction pathways. Atmospheric implications were evaluated and discussed in this manuscript.

Vibrational Mode-Specific Dynamics of the OH + C2H6 Reaction.

We investigate the effects of the initial vibrational excitations on the dynamics of the OH + C2H6 → H2O + C2H5 reaction using the quasi-classical trajectory method and a full-dimensional analytical ab initio potential energy surface. Excitation of the initial CH, CC, and OH stretching modes enhances, slightly inhibits, and does not affect the reactivity, respectively. Translational energy activates the early-barrier title reaction more efficiently than OH and CC stretching excitations, in accord with the Polanyi rules whereas CH stretching modes have similar or higher efficacy than translation, showing that these rules are not always valid in polyatomic processes. Scattering angle, initial attack angle, and product translational energy distributions show the dominance of direct stripping with increasing collision energy, side-on OH and isotropic C2H6 attack preferences, and substantial reactant-product translational energy transfer without any significant mode specificity. The reactant vibrational excitation energy of OH and C2H6 flows into the H2O and C2H5 product vibrations, respectively, whereas product rotations are not affected. The computed mode-specific H2O vibrational distributions show that initial OH excitation appears in the asymmetric stretching vibration of the H2O product and allow comparison with experiments.

Kinetics for the reaction of Criegee intermediate CH2OO with n-butyraldehyde and its atmospheric implications

The Criegee intermediates (CIs), formed through ozone oxidation of unsaturated hydrocarbons, can undergo reactions with various carbonyl compounds in the atmosphere. In this study, we have investigated the 1,3-cycloaddition reaction of n-butyraldehyde with the simplest Criegee intermediate, CH2OO, using the OH laser-induced fluorescence (LIF) method. The experiments were conducted over a temperature and pressure range of 283–318 K and 5–100 Torr, respectively. Our results reveal that the bimolecular rate constant of the reaction is dependent on both temperature and pressure. Specifically, the measured rate constants at 100 Torr are (3.51 ± 0.63), (2.97 ± 0.53), (2.70 ± 0.49), and (2.52 ± 0.45) × 10−12 cm3 molecule−1 s−1 at temperatures of 283, 298, 308, and 318 K, respectively, with a total error of 18%. By fitting the pressure-dependent rate constants according to the Lindemann mechanism, we obtained a high-pressure limit rate constant of (3.05 ± 0.55) × 10−12 cm3 molecule−1 s−1 at 298 K. The Arrhenius plot of the temperature-dependent rate constants yields an activation energy of (−1.71 ± 0.30) kcal mol−1 and a pre-exponential factor of (1.66 ± 0.24) × 10−13 cm−3 s−1 (total error). Furthermore, our findings indicate that the title reaction exhibits a more pronounced temperature-dependent effect compared to that of CH2OO with water dimer and SO2, as well as that of OH with n-butyraldehyde. This observation suggests that the significance of title reaction in the atmosphere increases at lower temperature.

Cl atoms-initiated degradation of 1-Chlorobutane and 2-Chlorobutane: Kinetics, product analysis and atmospheric implications

The relative rate method was employed to investigate the kinetics of the Cl-initiated reactions of 1-chlorobutane (1-CB) and 2-chlorobutane (2-CB) over 263–363 K, and the measured rate coefficients at room temperature are (1.04 ± 0.24) × 10−10 and (5.84 ± 0.27) × 10−11 cm3 molecule−1 s−1, respectively. The Arrhenius equations for the title reactions were derived to be k1-CB + Cl (T = 263–363 K) = (2.77 ± 0.72) × 10−11 exp [(422 ± 79)/T] and k2-CB + Cl (T = 263–363 K) = (1.40 ± 0.32) × 10−11 exp [(415 ± 70)/T] cm3 molecule−1 s−1, respectively. The products were analysed qualitatively using gas chromatography-mass spectrometry (GC-MS), and the reaction mechanism was proposed for the reactions. The rate coefficients for the title reactions were calculated computationally over the temperature range of 200–400 K using canonical variational transition state theory with appropriate tunnelling corrections at CCSD(T)/6–311++G(2d,2p)//BHandHLYP/6–311++G(2d,2p) level of theory to complement our experimentally measured kinetic parameters. The experimental and theoretical data obtained were used to evaluate the impact of the studied molecules in the troposphere.

Coupled three-dimensional quantum mechanical wave packet study of proton transfer in H2+ + He collisions on accurate abinitio two-state diabatic potential energy surfaces.

We have carried out fully close-coupled three dimensional quantum mechanical wave packet dynamical calculations for the reaction He+H2+→HeH++H on the ground electronic adiabatic potential energy surface and on the lowest two electronic states of newly constructed abinitio calculated diabatic potential energy surfaces for the system [Naskar et al., J. Phys. Chem. A 127, 3832 (2023)]. With the reactant diatom (H2+) in its roto-vibrational ground state (v = 0, j = 0), the calculations have been carried out in hyperspherical coordinates to obtain the reaction attributes. Convergence profiles of the reaction probability with respect to the total angular momentum quantum number at different collision energies are presented for the title reaction. State-to-state as well as initial state selected integral reaction cross sections are calculated from the fully converged reaction probabilities over a range of collision energies. The integral cross section values computed using the two-state diabatic potential energy surfaces are significantly lower than those obtained using the ground electronic state adiabatic potential energy surface and are in much better agreement with the available experimental results than the latter for total energy greater than 1.1eV. Therefore, it becomes clear that it is important to include the nonadiabatic coupling terms for a quantitative prediction of the dynamical observables.

Mechanistic Insights into Oxazolone Synthesis by Bimetallic Au-Pd-Catalyzed Catalysis and Catalyst Design: DFT Investigations.

Bimetallic synergistic catalysis is one of the most effective and powerful strategies for the synthesis of oxazolones, an important species in organic synthesis. In this work, the mechanism of AuCl(PMe3)/AgOTf-Pd(0) ([Au-Pd]) bimetallic catalyst-catalyzed oxazolone synthesis using N-alkynyl carbamates as precursors was studied in detail by DFT calculations and the catalytic performances of a series of bimetallic catalysts were evaluated. The results show that the reaction begins from the [Au]-catalyzed cycloisomerization of N-alkynyl carbamates. After the five-membered intermediate is formed, the [Pd(0)]-catalyzed cycle starts, which contains three steps: oxidation addition, transmetalation, and reductive elimination. The whole reaction belongs to a catalyzed catalysis, and the reductive elimination is the rate-determining step. In the transmetalation process, both the [Pd(0)] catalyst and the ionic bridge are necessary. For the [Au-Pd]-catalyzed process, it is Cl- as the bridge, not OTf-. The cheaper metal compound, AgCl(PMe3), can serve as the alternative of AuCl(PMe3) to co-catalyze with the [Pd(0)] catalyst for the title reaction.

Effects of C-H stretching excitation on the dynamics of the O(1D) + CHD3 → OH/OD + CD3/CHD2 reaction.

The vibrationally excited reaction O(1D) + CHD3(ν1 = 1) has been investigated by crossed-molecular-beam experiments with a time-sliced velocity map imaging technique. Detailed and quantitative information is extracted on the C-H stretching excitation effects on the reactivity and dynamics of the title reaction, with the help of preparation of C-H stretching excited CHD3 molecules by direct infrared excitation. Experimental results show that the vibrational stretching excitation of the C-H bond almost does not affect the relative contributions between different dynamical pathways for all product channels. For the OH + CD3 product channel, the vibrational energy of the C-H stretching excited CHD3 reagent is channeled exclusively into the vibrational energy of the OH products. The vibrational excitation of the CHD3 reactant changes the reactivities for the ground-state and umbrella-mode-excited CD3 channels very modestly, while it significantly suppresses the corresponding CHD2 channels. For the CHD2(ν1 = 1) channel, the stretching excited C-H bond of the CHD3molecule acts almost as a pure spectator.

Impact of double headed geminis on leucine and ninhydrin reaction in buffer solvent

Herein, the rates of the study on title reaction were determined in double headed micellar medium varying various constituents by applying UV–vis single beam spectroscopy. The rate constant, kψ, is more in geminis compared to CTAB and follows the first- and fractional order of paths in Leu and ninhydrin (nin), respectively. The impact of double headed geminis on the reaction of Leu and nin study was elucidated in a buffer system. An excellent development in rate led by gemini surfactants is noted and discussed in the text. This observation is interpreted successfully by employing a pseudo-phase model of micellar activity. To acquire the data of cmc, conductivities of surfactant and their system with added reactants were measured on a Systronics conductivity meter at 303 K and 343 K. In our present case, the evaluated cmc values are in excellent matching with the data observed, previously. Based on our current observations and cognizance, an appropriate reaction mechanism is provided.

Tandem Cycloaddition of Azides to 3,3-Diaminoacrylonitriles (2-Cyanoacetamidines) and Cornforth-Type Rearrangement as an Approach to 5-Amino-1,2,3-triazole-4-N-substituted Imidamides.

An efficient base-catalyzed, metal-free method for the synthesis of 5-amino-1,2,3-triazole-4-N-sulfonyl- and arylimidamides, directed by the structure of the amidine group, has been developed. It is based on a previously unknown tandem process involving cycloaddition reaction to 3,3-diaminoacrylonitriles (2-cyanoacetamidines) with aryl(alkyl)sulfonyl or aryl azides and Cornforth-type rearrangement. During the reaction optimization, different factors were found to facilitate the title reaction, which include the use of a strong base and N-mono- or N,N'-disubstituted 3,3-diaminoacrylonitriles. The reaction is tolerant to variously N-monosubstituted and N,N'-disubstituted 3,3-diaminoacrylonitriles and to various aryl- and aryl/alkyl sulfonyl azides. The developed method has a broad scope and can be applied to obtain a variety of 5-amino-1,2,3-triazole-4-carbimidamides bearing at the N1 position alkyl, allyl, propargyl, benzyl, cycloalkyl, and heteroaryl substituents and sulfonyl and aryl substituents at the amidine group. Post-cyclization reactions of prepared 5-amino-1,2,3-triazoles with DMF-DMA DMA-DMF leads to 1,2,3-triazolo[4,5-d]pyrimidines, 8-aza purine analogues demonstrating the applicability of the prepared compounds in organic synthesis.

Reaction dynamics for the Cl(2P) + XCl → XCl + Cl(2P) (X = H, D, Mu) reaction on a high-fidelity ground state potential energy surface.

In this work, the dynamics of a prototypical heavy-light-heavy abstract reaction, Cl(2P) + HCl → HCl + Cl(2P), is investigated both by constructing a new potential energy surface (PES) and by rate coefficient calculations. Both the permutation invariant polynomial neural network method and the embedded atom neural network (EANN) method, based on abinitio MRCI-F12+Q/AVTZ level points, are used for obtaining globally accurate full-dimensional ground state PES, with the corresponding total root mean square error being only 0.043 and 0.056 kcal/mol, respectively. In addition, this is also the first application of the EANN in a gas-phase bimolecular reaction. The saddle point of this reaction system is confirmed to be nonlinear. In comparison with both the energetics and rate coefficients obtained on both PESs, we find that the EANN is reliable in dynamic calculations. A full-dimensional approximate quantum mechanical method, ring-polymer molecular dynamics with a Cayley propagator, is employed to obtain the thermal rate coefficients and kinetic isotopic effects of the title reaction Cl(2P) + XCl→ XCl + Cl(2P) (H, D, Mu) on both new PESs, and the kinetic isotope effect (KIE) is also obtained. The rate coefficients reproduce the experimental results at high temperatures perfectly but with moderate accuracy at lower temperatures, but the KIE is with high accuracy. The similar kinetic behavior is supported by quantum dynamics using wave packet calculations as well.

Mechanism determination on [LCu-Fp] heterobimetallic-catalyzed C-H borylation of benzene and regioselectivity regulation: A DFT investigation

The heterobimetallic complex [(NHC)Cu-FeCp(CO)2] ([LCu-Fp]) is an efficient and sustainable catalyst for C-H borylation under UV light irradiation, whereas the catalytic mechanism is still debating. One opinion is the thermal induced mechanism, in which Cu-Fe bond in [LCu-Fp] firstly dissociated by B-H activation, and then induced the decarbonylation of FpBpin by UV irradiation. The other is the UV-induced mechanism, the decarbonylation of [LCu-Fp] occurred before the Cu-Fe broken. In this work, detailed mechanism of UV-induced has been investigated by DFT calculations and compared with the thermal induced mechanism. Our calculated results show that both of them can occur under UV light irradiation at room temperature. The UV-induced mechanism contains four steps: CO dissociation, Cu-Fe bond breakage by B-H activation, C-H borylation, and H-H reductive elimination. In the last step, it is the re-coordination of CO to the Fe occurs before the releasing of H2. The strong election-withdrawing substituent of benzene would increase the yield of para-product, whereas the election-donating substituent has little influence on the regioselectivity. Our calculation determines the detail mechanism of the title reaction and provides reasonable explanation for the regioselectivity of C-H borylation of substituted benzene, which would help for the widely usages of the heterobimetallic catalysts.

HO2•$_{2}^{\bullet }$+O3 → OH•+2O2 reaction: A potential source of vibrationally hot OH radicals in the atmosphere

AbstractIn the present work, using on‐the‐fly classical trajectory calculations along with quantum chemical computation, we have shown that HO+O3→ OH•+2O2 reaction can be a potential source of the vibrationally excited OH radical. The investigation suggests that OH radical will be majorly produced in ν=1 and ν=2 states. We have also shown that the vibrationally hot OH radical is key in interpreting the observed branching fraction of 16OH• in the gas phase experiment of Nelson and Zahniser (J. Phys. Chem. 1994, 98, 2101–2104). Lastly, we have discussed the atmospheric implications of title reaction by comparing it with other chemical reactions known to produce hot OH radical in the atmosphere.

New Measurements and Calculations on the Kinetics of an Old Reaction: OH + HO2 → H2O + O2.

Literature rate coefficients for the prototypical radical-radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO2 radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H2O2 reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k1,298K ∼1 × 10-11 cm3 molecule-1 s-1, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k1,H2O,298K = (2.17 ± 0.09) × 10-28 cm6 molecule-2 s-1, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k1,298K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k1,298K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO2 → HCl + O2. The implications of these results in atmospheric models are discussed.