Understanding the abundances of molecules observed in dense interstellar clouds requires knowledge of the rates of gas phase reactions between two neutral species. However, reactions possessing an activation barrier were considered too slow to play any important role at the low temperatures in these clouds. Here we show that despite the presence of a barrier the rate coefficient for the reaction between the hydroxyl radical (OH) and methanol, one of the most abundant organic molecules in space, is almost two orders of magnitude larger at 63K than previously measured at ~200K. We also observe formation of the methoxy radical product that was recently detected in space. These results are interpreted through the formation of a hydrogen-bonded complex that is sufficiently long-lived to undergo quantum mechanical tunnelling to form products. We postulate that this tunnelling

The kinetics of the reactions of the hydroxyl radical (OH) with acetone and dimethyl ether (DME) have been studied between 63-148 K and at a range of pressures using laser-flash photolysis coupled with laser induced fluorescence detection of OH in a pulsed Laval nozzle apparatus. For acetone, a large negative temperature dependence was observed, with the rate coefficient increasing from k1 = (1.6 ± 0.8) × 10(-12) cm(3) molecule(-1) s(-1) at 148 K to (1.0 ± 0.1) × 10(-10) cm(3) molecule(-1) s(-1) at 79 K, and also increasing with pressure. For DME, a similar behaviour was found, with the rate coefficient increasing from k2 = (3.1 ± 0.5) × 10(-12) cm(3) molecule(-1) s(-1) at 138 K to (1.7 ± 0.1) × 10(-11) cm(3) molecule(-1) s(-1) at 63 K, and also increasing with pressure. The temperature and pressure dependence of the experimental rate coefficients are rationalised for both reactions by the formation and subsequent stabilisation of a hydrogen bonded complex, with a non-zero rate coefficient extrapolated to zero pressure supportive of quantum mechanical tunnelling on the timescale of the experiments leading to products. In the case of DME, experiments performed in the presence of O2 provide additional evidence that the yield of the CH3OCH2 abstraction product, which can recycle OH in the presence of O2, is ≥50%. The experimental data are modelled using the MESMER (Master Equation Solver for Multi Energy Well Reactions) code which includes a treatment of quantum mechanical tunnelling, and uses energies and structures of transition states and complexes calculated by ab initio methods. Good agreement is seen between experiment and theory, with MESMER being able to reproduce for both reactions the temperature behaviour between ~70-800 K and the pressure dependence observed at ~80 K. At the limit of zero pressure, the model predicts a rate coefficient of ~10(-11) cm(3) molecule(-1) s(-1) for the reaction of OH with acetone at 20 K, providing evidence that the reaction can proceed quickly in those regions of space where both species have been observed. The results and modelling build considerably on our previous experimental study performed under a much more limited range of conditions (Shannon et al., Phys. Chem. Chem. Phys., 2010, 12, 13511-13514).

As molecular scientists have made progress in their ability to engineer nano-scale molecular structure, we are facing new challenges in our ability to engineer molecular dynamics (MD) and flexibility. Dynamics at the molecular scale differs from the familiar mechanics of everyday objects, because it involves a complicated, highly correlated, and threedimensional many-body dynamical choreography which is often non-intuitive even for highly trained researchers. We recently described how interactive molecular dynamics in virtual reality (iMD-VR) can help to meet this challenge, enabling researchers to manipulate real-time MD simulations of flexible structures in 3D. In this article, we outline various efforts to extend immersive technologies to the molecular sciences, and we introduce 'Narupa', a flexible, opensource, multi-person iMD-VR software framework which enables groups of researchers to simultaneously cohabit realtime simulation environments to interactively visualize and manipulate the dynamics of molecular structures with atomic-level precision. We outline several application domains where iMD-VR is facilitating research, communication, and creative approaches within the molecular sciences, including training machines to learn reactive potential energy surfaces (PESs), biomolecular conformational sampling, protein-ligand binding, reaction discovery using 'on-the-fly' quantum chemistry, and transport dynamics in materials. We touch on iMD-VR's various cognitive and perceptual affordances, and how these provide research insight for molecular systems. By synergistically combining human spatial reasoning and design insight with computational automation, technologies like iMD-VR have the potential to improve our ability to understand, engineer, and communicate microscopic dynamical behavior, offering the potential to usher in a new paradigm for engineering molecules and nano-architectures.

Rate constants for the C( 3 P) + CH 3 OH reaction have been measured in a continuous supersonic flow reactor over the range 50 K  T  296 K. C( 3 P) was created by the in-situ pulsed laser photolysis of CBr 4 , a multiphoton process which also produced some C( 1 D), allowing us to investigate simultaneously the low temperature kinetics of the C( 1 D) + CH 3 OH reaction. C( 1 D) atoms were followed by an indirect chemiluminescent tracer method in the presence of excess CH 3 OH. C( 3 P) atoms were detected by the same chemiluminescence technique and also by direct vacuum ultra-violet laser induced fluorescence (VUV LIF). Secondary measurements of product H( 2 S) atom formation have been undertaken allowing absolute H atom yields to be obtained by comparison with a suitable reference reaction. In parallel, statistical calculations have been performed based on ab-initio calculations of the complexes, adducts and transition states (TSs) relevant to the title reaction. By comparison with the experimental H atom yields, the preferred reaction pathways could be determined, placing important constraints on the statistical calculations. The experimental and theoretical work are in excellent agreement, predicting a negative temperature dependence of the rate constant increasing from 2.2  10 -11 cm 3 molecule -1 s -1 at 296 K to 20.0  10 -11 cm 3 molecule -1 s -1 at 50

ReuseUnless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. have been measured over the temperature and pressure ranges of 195 -650 K and 5 -500 Torr by detecting the hydroxyl radical using laser induced fluorescence following the excimer laser photolysis (248 nm) of CH3OCH2Br. The reaction proceeds via the formation of an energised CH3OCH2O2 adduct, which either dissociates to OH + 2 H2CO or is collisionally stabilised by the buffer gas. At temperatures above 550 K, a secondary source of OH was observed consistent with thermal decomposition of stabilized CH3OCH2O2 radicals. In order to quantify OH production from the CH3OCH2 + O2 reaction, extensive relative and absolute OH yield measurements were performed over the same (T, P) conditions as the kinetic experiments. The reaction was studied at sufficiently low radical concentrations (~10 11 cm -3 ) that secondary (radical + radical) reactions were unimportant and the rate coefficients could be extracted from simple bi-or tri-exponential analysis. Ab initio (CBS-GB3) / master equation calculations (using the program MESMER) of the CH3OCH2 + O2 system were also performed to better understand this combustion-related reaction as well as be able to extrapolate experimental results to higher temperatures and pressures. To obtain agreement with experimental results (both kinetics and yield data), energies of the key transition states were substantially reduced (by 20 -40 kJ mol -1 ) from their ab initio values and the effect of hindered rotations in the CH3OCH2 and CH3OCH2OO intermediates were taken into account.The optimised master equation model was used to generate a set of pressure and temperature dependent rate coefficients for the component nine phenomenological reactions that describe the CH3OCH2 + O2 system, including four well-skipping reactions. The rate coefficients were fitted to Chebyshev polynomials over the temperature and density ranges 200 to 1000 K and 1×10 17 to 1×10 23 molecule cm -3respectively for both an N2 and He bath gas. Comparisons with an existing autoignition mechanism show that the well-skipping reactions are important at a pressure of 1 bar but are not significant at 10 bar. The 3 main differences derive from the calculated rate coefficient for the CH3OCH2OO CH2OCH2OOH reaction, wh...

The rate coefficients (k) for reactions of OH with acetone, methyl ethyl ketone (MEK) and dimethyl ether (DME) have been measured in the temperature range 86-112 K using a pulsed Laval nozzle apparatus. Large increases in k at lower temperatures were observed, with k(86K)/k(295K) = 334 for acetone, and k(93K)/k(295K) = 72 and 3, for MEK and DME respectively. A mechanism involving the formation of a hydrogen bonded complex prior to an overall barrier on the potential energy surface is proposed to explain this behaviour.

The low temperature kinetics of the reactions of OH with ethanol and propan-2-ol have been studied using a pulsed Laval nozzle apparatus coupled with pulsed laser photolysis -laser induced fluorescence (PLP-LIF) spectroscopy. The rate coefficients for both reactions have been found to increase significantly as the temperature is lowered, by~a factor of 18 between 293 and 54 K for ethanol, and by~10 between 298 and 88 K for OH + propan-2-ol. The pressure dependence of the rate coefficients provide evidence for two reaction channels; a zero pressure bimolecular abstraction channel leading to products and collisional stabilization of a weakly bound OH-alcohol complex. The presence of the abstraction channel at low temperatures is rationalized by a quantum mechanical tunneling mechanism, most likely through the barrier to hydrogen abstraction from the OH moiety on the alcohol.

The significance of removal of atmospheric ammonia and amines by reaction with Criegee intermediates is assessed by kinetic studies.