Laser

Reaction Dynamics in Liquids

Our ambition to extend the field of reaction dynamics to larger molecular systems, more closely linked with the wider discipline of Chemistry, is also reflected in our studies of the dynamics of chemical reactions in solution in liquids. Most chemical synthesis and almost all biological chemistry occur in liquid solutions, yet, with the exception of pioneering studies by Hochstrasser (Pennsylvania) in the 1990s, and more recent work by Crim (Wisconsin), the dynamics of bimolecular reactions in solution remain unexplored experimentally.

We have employed ultrafast (~50 fs time resolution) ultraviolet pump and infra-red probe laser methods to capture the transient dynamics of reactions such as CN + c-C6H12 → HCN + c-C6H11  in solution in organic solvents (e.g. CHCl3, CH2Cl2, CDCl3 and tetrahydrofuran). Our work has demonstrated that dynamical information of comparable quality and detail to experimental studies of gas phase reactions can be obtained for reactions in bulk liquids, despite the ~100 fs time interval between solvent collisions.  The chosen method of ultrafast IR absorption spectroscopy uses broadband pulses to capture vibrational spectra. The figure below shows schematically the experimental set-up, with liquid samples flowing through a Harrick cell, a UV laser pulse initiating reaction, and a broadband IR pulse (dispersed onto a 128-pixel array detector) probing the absorption spectra of reaction products.

Schematic diagram of the sample cell and ultrafast laser pulses used in our experiments

Schematic diagram of the sample cell and ultrafast laser pulses used in our experiments

The experiments are being carried out in Bristol and at the ULTRA Facility of the Rutherford Appleton Laboratory, with theoretical studies at the University of Bristol in collaboration with Prof J.N. Harvey and Dr D.R. Glowacki.

With sub-ps time resolution, nascent reaction products can be observed with excitation in specific vibrational modes (for HCN, the C=N stretch, bend and C-H stretch), prior to coupling of their internal motions to the solvent bath and thermalization on timescales that can be as long as ~250 ps (in CH2Cl2).  This work demonstrates that the Polanyi rules developed for interpretation of gas-phase collision dynamics can persist for reactions in solution, and detailed comparisons with gas-phase reaction data also allow the consequences of coupling of the reaction pathway to the solvent bath to be quantified.  We have shown that dynamical information of comparable quality and detail to experimental studies of gas phase reactions can be obtained for reactions in bulk liquids, despite the ~100 fs time interval between solvent collisions.

The top row of the figure below shows illustrative time-resolved IR absorption data.  Vibrationally excited HCN reaction products are observed  on the C-H stretching hot band at 3150 cm-1 at early times (left hand panel) following reaction of CN radicals with cyclohexane in solution in dichloromethane.  These vibrationally hot molecules relax over several hundred picoseconds (middle and right hand panels) by loss of energy to the solvent bath, resulting in a rise in intensity of the fundamental HCN absorption band at 3250 cm-1.  The bottom row shows the signals observed for CN reaction with the dichloromethane solvent in the absence of cyclohexane.

Time-resolved IR absorption spectra for HCN products of the reaction of CN radicals with cyclohexane in dichloromethane

Time-resolved IR absorption spectra for HCN products of the reaction of CN radicals with cyclohexane in dichloromethane

Our first paper on this work was published in Science, and has been followed by several further publications that illustrate the potential of this new field.  Systematic studies are examining the effects of solvents of different polarity and molecular character.  A theory-led collaborative study with colleague Dr Dave Glowacki and published in Nature Chemistry highlighted how the experiments can examine two different regimes of the reaction:  at early times in the “wake” of the reaction, the CN and organic radical co-product are confined closely together by a solvent cage and energy is coupled from the HCN to the organic radical, whereas on longer timescales the products diffuse apart and any subsequent thermalization of excess energy occurs by direct coupling to the solvent.  Insights such as these reveal reaction mechanisms in solution at a molecular level and are expected to have wide impact on the treatment of chemistry in liquids.

The reaction systems under study in solution have now been extended to those of Cl atoms with organic molecules, to complement the large body of prior work by the Bristol group on such reactions in the gas-phase.

Transient absorption bands seen in the near-UV and visible regions following 266-nm photoexcitation of CCl4. The structures show the species to which the bands are assigned.

Transient absorption bands seen in the near-UV and visible regions following 266-nm photoexcitation of CCl4. The structures show the species to which the bands are assigned.

The figure above shows transient absorption spectra of CCl4 photo-excited by a <100 fs duration UV pulse at a wavelength of 266 nm. Two bands are observed: a UV band assigned to complexes of Cl atoms with the solvent and a broad visible band caused by formation of the isomeric form of CCl4 shown. The arrows indicate how the band intensities change with time.

Most recently, we have studied F-atom reactions in liquid d3-acetonitrile and demonstrated that they form vibrationally hot DF, which will allow comparisons with the seminal studies of gas-phase F-atom reaction dynamics by Profs Y.T. Lee (formerly Berkeley, now Taipei), D.M. Neumark (Berkeley), K. Liu (Taipei) and others. This work was published in Science in 2015 and a separate webpage summarises the outcomes together with a link to our paper.

This research project is now supported by an ERC Advanced Grant which has allowed us to set up an ultrafast laser laboratory in the School of Chemistry at the University of Bristol (see picture below).  The laboratory is equipped with a Coherent Vitara and Legend Elite amplified Ti:S laser system (5 W, 1 kHz), two OPAs with difference and sum frequency generation, and two 128-pixel MCT array spectrometers for transient IR absorption spectroscopy.   The laser systems are also being used to study the dynamics of photochemical processes in solution.

The Ultrafast Laser Laboratory at the University of Bristol

The Ultrafast Laser Laboratory at the University of Bristol

Science and Nature Chemistry covers resulting from our work.

Science and Nature Chemistry covers resulting from our work.

Our ultrafast dynamics studies have been supported by EPSRC (e.g. via Programme Grant EP/G00224X/1), the ERC (Advanced Grant 290966 CAPRI) and STFC (through access to the ULTRA Facility at the Central Laser Facility, Rutherford Appleton Laboratory).

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Representative publications:

Vibrational relaxation and micro-solvation of DF after F-atom reactions in polar solvents, G.T. Dunning, D.R. Glowacki, T.J. Preston, S.J. Greaves, G.M. Greetham, I.P. Clark, M. Towrie, J.N. Harvey and A.J. Orr-Ewing, Science 347, 530-533 (2015).

Vibrationally quantum-state-specific reaction dynamics of H atom abstraction by CN radical in solution, S.J. Greaves, R.A. Rose, T.A.A. Oliver, D.R. Glowacki, M.N.R. Ashfold, J.N. Harvey, I.P. Clark, G.M. Greetham, A.W. Parker, M. Towrie and A.J. Orr-Ewing, Science, 331, 1423-1426 (2011).

Bimolecular chemical reaction dynamics in solution, A.J. Orr-Ewing, Perspective Article, J. Chem. Phys. 140, 090901 (2014).

Chemical reaction dynamics in liquid solutions, A.J. Orr-Ewing, D.R. Glowacki, S.J. Greaves and R.A. Rose, J. Phys. Chem. Lett. 2, 1139-1144 (2011).

Ultrafast energy flow in the wake of solution phase bimolecular reactions, D.R. Glowacki, R.A. Rose, S.J. Greaves, A.J. Orr-Ewing and J.N. HarveyNature Chemistry 3, 850-855 (2011).

Reaction dynamics of CN radicals with tetrahydrofuran in liquid solutions, R.A. Rose, S.J. Greaves, F. Abou-Chahine, D.R. Glowacki, T.A.A. Oliver, M.N.R. Ashfold, I.P. Clark, G.M. Greetham, M. Towrie and A.J. Orr-Ewing, PCCP 14, 10424-10437 (2012).

Vibrationally resolved dynamics of the reaction of Cl atoms with 2,3-dimethylbut-2-ene in chlorinated solvents, F. Abou-Chahine, S.J. Greaves, G.T. Dunning, A.J. Orr-Ewing, G.M. Greetham, I.P. Clark, and M. Towrie, Chemical Science 4, 226-237 (2013).

Photo-isomerization and photo-induced reactions in liquid CCl4 and CHCl3, F. Abou-Chahine, T.J. Preston, G.T. Dunning, A.J. Orr-Ewing, G.M. Greetham, I.P. Clark, M. Towrie and S.A. Reid, J. Phys. Chem. A 117, 13388-13398 (2013).