Viewpoint Cite This: J. Phys. Chem. A 2018, 122, 882−889
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Session Viewpoints on the 2017 Dynamics of Molecular Collisions Conference
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INTRODUCTION The first Dynamics of Molecular Collisions (DMC) conference was organized in 1965 by John Fenn (Yale University). Since 1981, the DMC conference has been held every two years by a vibrant and enduring community of scientists united by a common interest in mapping the fundamental series of motions that control complex chemical phenomena. Although this conference has a style and format similar to the popular Gordon Research Conference coordinated by a nonprofit with the same name, the DMC conference is independently run by members of the DMC community. DMC conference chairs alternate between experimentalists and theorists, a practice that highlights and supports a long history of productive collaboration between the two groups. Since 2007, the DMC conference has also functioned as a venue for awarding the Herschbach Prize to two or more scientists to celebrate outstanding experimental and theoretical contributions to the field. Many types of presentations are scheduled during the DMC conference including session chair introductions, posters, invited talks, contributed talks, hot topics, keynote, and Herschbach award presentations. In recent years, many of the presentations at the DMC conference address frontier questions in unimolecular, bimolecular, cluster, cold-collision, nonadiabatic, and interfacial dynamics for applications in atmospheric chemistry, astrochemistry, and combustion chemistry. The 26th DMC conference was organized by David Yarkony (Chair, Johns Hopkins University) and Timothy Minton (CoChair, Montana State University) and was held at the Granlibakken Conference Center in Tahoe City, California, from July 9th to 14th, 2017. Tucked away in the mountains above Lake Tahoe, this conference site combines excellent accommodations with mild weather, natural beauty, and many outdoor activities. A total of 114 participants were in attendance with more than 72 posters distributed across four evening poster sessions and 47 speakers distributed across 11 presentation sessions. Each presentation session was introduced and moderated by one of nine session chairs as listed in Table 1. This Viewpoint contains accounts of all participating speakers and is grouped according to session title (sessions with similar titles were combined).
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Table 1. Session Titles and the Session Chairs That Delivered Introductions, Held Presenters to the Necessary Time Limits, and Moderated Discussion session title Cold and Ultracold Collisions
David Chandler (Sandia National Laboratories) Keynote Address Hua Guo (University of New Mexico) Hot Topics David Yarkony Bimolecular Collision Dynamics I Hua Guo Bimolecular Collision Dynamics II Ryan Steele (University of Utah) Nonadiabatic Dynamics I David Yarkony Interfaces, Clusters and David Nesbitt (University of Colorado Condensed Phase Boulder, JILA) Unimolecular/Photochemical Curt Wittig (University of Southern Dynamics California) Atmospheric, Astrochemistry and Anna Krylov (University of Southern Combustion California) Herschbach Prize Session: Dudley Herschbach (Harvard Introduction University) Herschbach Prize Session: Hanna Curt Wittig Reisler Herschbach Prize Session: John David Yarkony Tully Nonadiabatic Dynamics II Marsha Lester (University of Pennsylvania)
reaction rates, which followed a Poisson distribution. Croft predicts that this result will be applicable to other atom + dimer reactions that involve heavy alkali atoms. Stephen Cotton (Miller Group, University of California, Berkeley) has further developed a symmetrical quasiclassical (SQC) protocol for quantizing the electronic degrees of freedom in the Meyer−Miller classical vibronic Hamiltonian. This method treats both the nuclear and electronic degrees of freedom classically, yielding an efficient, dynamically consistent framework for nonadiabatic dynamics. Additionally, the need to calculate second derivative couplings was eliminated by including a kinematic adiabatic representation of Meyer−Miller vibronic dynamics [J. Chem. Phys. 2017, 147, 064112. DOI: 10.1063/1.4995301]. When benchmarked against exact quantum results for spin-boson-type models of condensed-phase electronic dynamics, it is seen that the latest SQC windowing models well-describes very weak to very strong nonadiabatic transitions [J. Chem. Phys. 2016, 145, 144108. DOI: 10.1063/ 1.4963914]. Bin Jiang (University of Science and Technology of China) presented calculations detailing the vibrational-mode specificity of the dissociation of CH4 on Ni(111), which is the rate limiting step in the commercial production of hydrogen [Phys. Chem. Chem. Phys. 2017, 19, 30540. DOI: 10.1039/ C7CP05993K]. The calculations were performed on a 15dimensional potential energy surface (PES) constructed using the permutation invariant polynomial-neural network (PIP-
HOT TOPICS
James Croft (Naduvalath Group, University of Nevada, Las Vegas) presented findings regarding the benchmark reaction of K with KRb to form K2 and Rb at ultracold temperatures [Nat. Commun. 2017, 8, 15897. DOI: 10.1038/ncomms15897]. An accurate potential energy surface for the K−Rb−K reaction complex was developed and used to perform numerically exact quantum reactive-scattering calculations. The total reaction rate coefficient depended only on the long-range interaction properties whereas the chaotic nature of the short-range chemical interactions determined the rotationally resolved © 2018 American Chemical Society
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DOI: 10.1021/acs.jpca.8b00072 J. Phys. Chem. A 2018, 122, 882−889
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conversion to the ground state before fragmenting statistically [J. Chem. Phys. 2017, 147, 134304. DOI: 10.1063/1.4994713].
NN) method, which was developed for gas phase molecules and has been extended to gas-surface interactions. The calculated dissociative sticking coefficients were in good agreement with previous experimental and ab initio molecular dynamics (AIMD) results. This chemically accurate PES will aid in the study of the mode specificity of the dissociation of polyatomic molecules on metal surfaces. Alicia Hernandez-Castillo (Zwier Group, Purdue University) has studied the thermal decomposition pathways relevant for biofuel combustion. Using a high-temperature flash pyrolysis microreactor coupled with a supersonic source, the pyrolysis of 2-methoxyfuran and guaiacol were studied spectroscopically [J. Chem. Phys. 2016, 145, 114203. DOI: 10.1063/1.4962505]. The spectral assignments were facilitated by the newly developed protocol referred to as “strong field coherence breaking”, which differentiates conformer and isomer transitions by operating a broad-band chirped-pulse Fourier transform microwave spectrometer in the strong-field coupling regime. For the thermal decomposition of 2-methoxyfuran and guaiacol, loss of a methyl group led to resonance stabilized intermediates which were clearly identified and structurally characterized. Chris Hansen (Ashfold Group, University of Bristol) presented recent improvements to the velocity map imaging (VMI) apparatus at the University of Bristol, which has been modified to include a PImMS2 camera for multimass imaging and a vacuum ultraviolet laser for universal ionization [J. Chem. Phys. 2017, 147, 013914. DOI: 10.1063/1.4979559]. The ultraviolet photodissociation of 2-bromothiophene was studied on the revamped machine over the photolysis wavelength range 265−245 nm and was compared to previous experiments carried out on a conventional VMI setup [J. Chem. Phys. 2015, 142, 224303. DOI: 10.1063/1.4921315]. The results of the Brloss photoproduct (C4H3S) were found to agree well with prior findings and revealed addition information on a wavelengthdependent dissociative ionization channel for C4H3S. Nandini Mukherjee (Zare Group, Stanford University) discussed experiments which investigate the potential energy surfaces on which rotationally inelastic collisions of HD and D2 occur [Science 2017, 358, 356. DOI: 10.1126/science.aao3116]. A collision temperature of 1 K was achieved by coexpanding a mixture of D2 and HD in a supersonic molecular beam. Starkinduced adiabatic Raman passage was used to prepare HD (v = 1, j = 2) with its bond axis oriented either parallel or perpendicular to the direction of molecular beam propagation. It was determined that rotational relaxation of HD (v = 1, j = 2) to HD (v = 1, j = 1) was 3 times more likely to occur when the HD was initially aligned perpendicular to the beam relative velocity vector. Bethan Nichols (Neumark Group, University of California, Berkeley) delivered a talk on the photodissoication of tert-butyl peroxy radical at 248 nm. Fast-radical-beam coincidence translational spectroscopy was used to collect fragment mass distributions, translational energy distributions, and anisotropy information for two- and three-body dissociation events. Three photodissociation channels were observed, with 83% producing O + CH3 + OC(CH3)2, 10% producing O2 + C(CH3)3, and 7% producing HO2 + H2CC(CH3)2. The high-energy component of the dominant three-body channel was determined to proceed through an asynchronous concerted mechanism on the excited B̃ state, whereas all other photodissociation events were found to undergo internal
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COLD AND ULTRACOLD COLLISIONS Heather Lewandowski (University of Colorado Boulder, JILA) has studied ultracold reactions of molecular cations with neutral molecules and radicals to understand processes that form and destroy complex chemical species in the interstellar medium. Both the reaction kinetics and product species can be determined by combining a Stark decelerator with a linear Paul ion trap and a high-resolution time-of-flight mass spectrometer [Rev. Sci. Instrum. 2017, in press]. It has been demonstrated that this method can deliver quantum-state control by studying the reaction of NO with trapped Ca+. This technique has been extended to study more complicated systems, including the reaction of sympathetically cooled acetylene ions with propyne. Jeremy Hutson (Durham University) discussed the randomization of energy in ultracold collisions [Phys. Rev. A 2016, 93, 052713. DOI: 10.1103/PhysRevA.93.052713]. Near-threshold bound states of Li + CaH show strong signatures of quantum chaos for zero total angular momentum. In contrast, the Li + CaF system showed reduced levels of chaos despite its higher coupling strength when the angular momentum was equal to zero, suggesting the emergence of a nearly good quantum number. This result implies that the accessible density of states in ultracold collisions may be smaller than the actual density of states, which could shorten the lifetimes of collision complexes. This work demonstrated that the relationship between coupling strength and chaos is complex; consequently, the level of chaotic behavior that a system will display is difficult to predict. Stefan Willitsch (University of Basel) presented new experimental results that examine the mechanisms of charge transfer at very low energies. A dynamic hybrid trap was used to transfer a cloud of cold Rb atoms across a Ca+ Coulomb crystal which contained sympathetically cooled N 2 + and O 2 + [ChemPhysChem 2016, 17, 3769. DOI: 10.1002/ cphc.201600643]. This method allowed for precise tuning of the collision energy with the cold ions so subtle details in the charge transfer between the diatomic molecules and the Rb could be determined. Willitsch also briefly discussed progress toward studying the conformationally selected cycloaddition reactions of cis- and trans-dibromobutadiene with sympathetically cooled maleic anhydride and results on the reactivity of oand p-H2O with cold ions. Lorenzo Petralia (Softly Group, University of Oxford) discussed the charge transfer between cold ND3 and Xe+ that was sympathetically cooled by a Ca+ Coulomb crystal. The charge-transfer kinetics were determined by monitoring the shape of the Coulomb crystal while simultaneously recording the formation of reaction products using time-of-flight mass spectrometry. Additionally, Petralia reported on a new experimental setup that employs a buffer gas cell and an electrostatic guide to collisionally cool and orient ND3 molecules. Simulations indicate that this technique orients more than 95% of the molecules. [J. Mol. Spectrosc. 2017, 332, 94. DOI: 10.1016/j.jms.2016.11.003]. When combined with a linear Paul ion trap, this method can be used to study the effect of molecular orientation on cold reactions. Evardas Narevicius (Weizmann Institute of Science) has investigated the effect of anisotropy on shape resonances in the Penning ionization of H2 with metastable He at low collision energies by utilizing merged molecular beam techniques. High883
DOI: 10.1021/acs.jpca.8b00072 J. Phys. Chem. A 2018, 122, 882−889
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The Journal of Physical Chemistry A resolution velocity map imaging was used to determine the final quantum rotational states of the H2+ product. Experiments and ab initio theory have shown that anisotropy can be switched on and off by controlling the initial rotational state of the H2 [Nat. Phys. 2016, 13, 35. DOI: 10.1038/nphys3904]. New experimental techniques for trapping a decelerated molecular beam of O2 and Li in a permanent magnetic trap were also discussed. These methods have yielded the highest number of trapped molecules yet achieved.
mapped and time-sliced imaging techniques, referred to as FinA [J. Chem. Phys. 2017, 147, 074201. DOI: 10.1063/1.4986966]. This method uses radial basis functions to model the out-ofplane elements of the three-dimensional particle distribution, improving the overall resolution and signal-to-noise of the velocity mapped images. Xueming Yang (Dalian Institute of Chemical Physics) has combined high-resolution cross-molecular beam methods and quantum reaction dynamics simulations to study the impact of vibrational excitation on reaction resonances. For the Cl + HD reaction, resonances were observed when HD was in the v = 1 state but not when v = 0 [Science 2015, 347, 60. DOI: 10.1126/ science.1260527]. In the F + H2 reaction, vibrational excitation of the H2 molecule to v = 1 did not lead to the observation of resonances [J. Phys. Chem. A 2015, 119, 12284. DOI: 10.1021/ acs.jpca.5b06395]. These results indicate that vibrational excitation of the reactant molecule does not ensure that a reaction resonance will be observed. For a resonance to be detected, the energy of the reaction must be comparable to a resonance-state energy. The energy for the F + H2 (v = 1) reaction was not resonant with any resonance state, whereas the energy for the Cl + HD (v = 1) was resonant with the resonance states. Anne McCoy (University of Washington) introduced work aimed at understanding the vibrational spectra of protonated and deuterated water clusters. The vibrational spectra of different sized water clusters were modeled using a variety of approaches including vibrational perturbation theory and diffusion Monte Carlo. These methods are able to account for anharmonic effects that originate from large amplitude motions caused by proton transfer between water molecules. These calculations revealed that the stretching modes for OH groups involved in hydrogen bonding are highly coupled to other bending and stretching modes in the cluster. Consequently, assigning vibrational spectra to specific motions is not a straightforward process [J. Phys. Chem. Lett. 2017, 8, 3782. DOI: 10.1021/acs.jpclett.7b01599]. Amy Mullin (University of Maryland) has studied the collision dynamics of CO2 in ultrahigh rotational states [J. Chem. Phys. 2017, 147, 154309. DOI: 10.1063/1.4997701]. The molecules were driven to J ≈ 220 with unidirectional spin using an optical centrifuge (OC), which oriented the CO2 angular momentum along the OC propagation axis. The translational energy distributions and population changes in individual J states for CO2 were monitored using highresolution transient infrared (IR) adsorption with the IR polarization either parallel (in-plane) or perpendicular (out-ofplane) to the initial plane of CO2 rotation. In-plane measurements showed that inelastic collisions between super rotors resulted in near-resonant rotation-to-rotation energy transfer that yielded molecules with small amounts of translational energy. David Osborn (Sandia National Laboratories) has been continuing the hunt for elusive hydroperoxyalkyl radicals (QOOH), which are important reaction intermediates that are crucial for radical chain propagation in both combustion and atmospheric chemistry. Osborn and co-workers first detected QOOH intermediates by using photoionization mass spectrometry and kinetic measurements in the oxidation of 1,3cycloheptadiene [Science 2015, 347, 643. DOI: 10.1126/ science.aaa1495]. Osborn also presented new improvements to photoelectron photoion coincidence spectroscopy (PEPICO), which will facilitate the identification of QOOH
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BIMOLECULAR COLLISION DYNAMICS H. Floyd Davis (Cornell University) discussed new findings on bimolecular reaction dynamics studied using a crossed-beams apparatus. The Davis Group has designed a new method for generating intense pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) laser radiation, which replaces the electron-bombardment ionizer in a universal crossed-beams apparatus. This is done by resonance enhanced mixing of pulsed nanosecond lasers, providing light sources in the 9−12 eV range for “soft” universal and resonant photoionization detection of products [Rev. Sci. Instrum. 2016, 87, 063106. DOI: 10.1063/1.4952749]. The high sensitivity of this detection method allows detailed characterization of bimolecular interactions where multiple reaction channels are possible. As a demonstration, Davis discussed recent measurements on the photochemistry of OIO where two reaction pathways were unambiguously observed. Kopin Liu (Institute of Atomic and Molecular Sciences, Taiwan) described an ingenious method to measure angledependent reaction barriers which have eluded experimental determination. Liu and co-workers used polarized scattering experiments to examine the benchmark reaction of Cl + CHD3(v1 = 1). At the transition-state geometry, the Cl−H−C bend potential deduced from experiments is in agreement with ab initio results [Nat. Chem. 2017, 9, 1175. DOI: 10.1038/ nchem.2858]. His talk demonstrated the method to map similar anisotropic properties of the transition state, and it should be applicable to many other direct reactions with a collinear barrier. Such innovative experiments further demonstrate the firm footing of fundamental transition-state theory. Matthew L. Costen (Heriot-Watt University) used crossed molecular beam scattering, velocity map imaging, and statespecific detection to study the stereodynamics of rotational energy transfer of electronically excited NO(A2Σ+) with rare gases and molecules. Costen described findings from NO(A2Σ+) + Ne experiments [J. Chem. Phys. 2016, 145, 174304. DOI: 10.1063/1.4966688] and compared these findings to quantum scattering calculations, concluding that the best ab initio potential energy surfaces substantially underestimate the attractive interactions for this system. Collisions of NO(A2Σ+) with N2 display angle-dependent rotation-rotation correlations, with sideways scattering correlated with rotational excitation of both molecules [J. Chem. Phys. 2017, 147, 013912. DOI: 10.1063/1.4979487]. Arthur Suits (University of Missouri) presented a new experimental technique that combines a chirped-pulse microwave spectrometer with a uniform supersonic flow to identify the intermediates of photodissociation and bimolecular reactions [J. Phys. Chem. Lett. 2015, 6, 1599. DOI: 10.1021/ acs.jpclett.5b00519]. The quantitative branching ratios of these processes can be determined from the peak intensities of the collected microwave spectra. The Suits Group has also pioneered a new image reconstruction procedure for velocity884
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ab initio multiple spawning nonadiabatic molecular dynamics simulations were used to identify an avoided crossing that transfers population across the molecule to a dissociative σ* state on the HPALD OOH group. When these data were incorporated in nonadiabatic statistical mechanics calculations, the OH quantum yield at atmospherically relevant temperatures and pressures was determined to be near 100%. Benjamin G. Levine (Michigan State University) discussed a new theoretical method to overcome the discontinuities that arise at conical intersections, which play a significant role in facilitating nonradiative transitions in electronically excited molecules and materials. The discontinuities can be overcome by defining a quasidiabatic basis; however, such a definition is system dependent. He introduced a technique based on constructing diabatized Gaussians on adiabatic surfaces (DGAS) from a basis of vibronic functions ensuring that the wave function is continuous and the Hamiltonian matrix elements are finite. Although this method results in discontinuities in vibronic functions, the singularities that arise are integrable and can be handled through interpolation techniques. His approach combines the advantages of adiabatic and quasidiabatic representations while avoiding the pitfalls. Jingsong Zhang (University of California, Riverside) discussed the UV photodissociation dynamics of a series of alkyl radicals where the H-atom channels were studied with Rydberg atom tagging. Zhang found that the dissociation of the ethyl state (Ã 2A′(3s)) was mediated through a conical intersection. Several different channels were identified: a nonstatistical repulsive H + C2H4 product channel, a statistical H + C2H4 product channel, and a slow H + triplet C2H4 pathway. Similar statistical and nonstatistical dissociation channels were observed in the photodissociation of n-propyl and isopropyl radicals via the 3s and 3p Rydberg states [J. Chem. Phys. 2015, 142, 224306. DOI: 10.1063/1.4922311]. Nonstatistical pathways were also observed in unsaturated hydrocarbons. Arthur E. Bragg (John Hopkins University) is interested in understanding the impact of molecular structure on nonadiabatic photochemistry of excited states with the aim of controlling photochromic switches. He discussed his findings on a new class of photoswitches (4TCE) where the trans-to-cis isomerization occurs within the singlet manifold but cis-to-trans isomerization occurs via triplets populated by one of several ultrafast relaxation mechanisms [Angew. Chem. Int. Ed. 2015, 54, 4782. DOI: 10.1002/anie.201410945]. Bragg’s Group uses ultrafast pump−probe methods to explore these photochemical pathways and have recently expanded their efforts to study the photochemical dynamics of a variety of ortho-arenes and related diarylethenes in solution. Changjian Xie (Guo Group, University of New Mexico) discussed recent work on the tunneling-facilitated photodissociation of phenol on the S1 excited state with a reduceddimensional model. Model calculations that invoke the Born− Oppenheimer approximation yield photodissociation lifetimes 2 orders of magnitude shorter than the exact ones [J. Am. Chem. Soc. 2016, 138, 7828. DOI: 10.1021/jacs.6b03288]. Xie demonstrated that this was a failure to describe tunneling near a conical intersection and can be attributed to a neglect of geometric phase effects. Specifically, destructive interference between trajectories passing around the conical intersection on opposite sides lead to a nodal structure that can significantly suppress the tunneling rate.
intermediates from more complex reactions [J. Chem. Phys. 2017, 147, 013944. DOI: 10.1063/1.4984304]. The sensitivity of this technique has been improved by utilizing a positionsensitive ion detector in tandem with temporal ion deflection, which suppresses the false coincidence background inherent in PEPICO.
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NONADIABATIC DYNAMICS Michael Ashfold (University of Bristol) provided a lecture on photoinduced ring-opening reactions of heterocyclic molecules. These reactions are often the result of directly or indirectly populating repulsive (n/π)σ* excited states that sample avoided crossings or conical intersections as a ring bond is stretched toward dissociation. For example, photodissociation of 2bromothiophene in the gas phase yields slow products that can be rationalized with this paradigm. The (n/π) σ*-state driven dissociation paradigm remains useful in rationalizing the initial dynamics toward ring-opening in molecules like 2-thiophenone, 2-furanone, α-pyrone, and coumarin in a weakly interacting solvent. However, the fraction of molecules that undergo ring opening vs revert to the ring-closed structure in solution is highly molecule dependent [Phys. Chem. Chem. Phys. 2016, 18, 2629. DOI: 10.1039/C5CP06597F]. Nandini Ananth (Cornell University) has been focused on advancing theoretical descriptions of charge and energy transport in complex, condensed-phase systems. Her group is primarily interested in developing approximate methods that transform quantum systems to isomorphic classical systems that are more computationally tractable and successfully capture quantum tunneling, zero-point energy, and coherence. For example, the recently developed Mixed Quantum-Classical Initial-Value Representation (MCQ-IVR) can interpolate between classical and quantum limit semiclassical methods on a per-mode basis, recovering the efficiency of hybrid quantum mechanics/molecular mechanics methods without introducing unphysical boundary effects [J. Chem. Phys. 2017, 146, 234104. DOI: 10.1063/1.4986645]. The Ananth Group is also active in the development of simple and accurate approaches to characterize electron-transfer reactions using new variants of Ring-Polymer Molecular Dynamics [J. Chem. Phys. 2013, 139, 124102. DOI: 10.1063/1.4821590]. Oleg Vasyutinskii (Ioffe Institute) discussed a new study on femtosecond time-resolved vector correlations in CH3I photolysis. In this experiment, a molecular beam of CH3I was crossed with two laser beams within a femtosecond pump− probe scheme to excite the CH3I B-band transition at 201.2 nm and interrogate I(2P1/2) or CH3(v = 0) dissociation products with REMPI. The ionized products were detected using velocity map imaging, and the resulting time-resolved data were interpreted by fitting to a quasiclassical model. Electronic predissociation was found to be the dominant photodissociation mechanism. Parameters of the model were used to quantify the influence of predissociation lifetime, CH3I rotation and bending motion during the photolysis, and methyl fragment angular momentum alignment [J. Phys. Chem. Lett. 2016, 7, 4458. DOI: 10.1021/acs.jpclett.6b01874]. David Glowacki (University of Bristol) discussed recent work on the photochemistry of isoprene oxidation intermediates known as hydroperoxyaldehydes or HPALD’s. Isoprene is a major volatile organic hydrocarbon in the atmosphere and indirect evidence suggests that HPALD photolysis may be an important source of OH radicals. After the initial n → π* transition responsible for this photolysis channel was identified, 885
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The Journal of Physical Chemistry A Donald Truhlar (University of Minnesota) explained that the primary challenge in computational photochemistry is fitting adiabatic potential energy surfaces (PESs) that have (F − 2)dimensional seams of conical intersections, where F is the number of internal degrees of freedom. To expand the size of chemical systems that can be studied computationally, the Truhlar Group has developed the anchor points reactive potential (APRP) method for fitting multidimensional PESs. As a demonstration, the APRP method was recently used to develop a full-dimensional set of potential energy surfaces and couplings for the lowest three electronic states of thioanisole. Multistate semiclassical dynamics simulations of the photodissociation of thioanisole fail to reproduce an experimentally observed, mode-specific effect on the product energy distributions but identify mode-specific behavior in the dissociation lifetime and in the distribution of minimum-energy gaps along trajectories.
Eric Smoll (Minton Group, Montana State University) delivered a presentation on reactive scattering from the vacuum−liquid interface of a common ionic liquid. Existing methods for characterizing vacuum−liquid structure and composition at a molecular level are highly averaged and often in qualitative disagreement resulting from an inability to provide sufficient information on penetration depth. Though it is known that reactive scattering of a gas phase radical from the vacuum−liquid interface can proceed by one of several mechanistically distinct pathways, new observations from the Minton lab characterize the penetration depth of each pathway for a more complete description of vacuum-liquid surface structure. The increased detail afforded by these measurements permit the identification of two molecular orientations at the vacuum−liquid interface. Sharani Roy (University of Tennessee) described the kinetics of surface and subsurface oxygen atoms on Ag surfaces. On Ag(111), the binding energies of subsurface oxygen decreased rapidly with increasing oxygen coverage resulting in oxygen binding more strongly to the subsurface than to the surface at oxygen coverages above 0.5 monolayer. However, on Ag(110), surface adsorption remained stronger than subsurface adsorption at all studied oxygen coverages. The calculations indicate that kinetic barriers for formation of subsurface oxygen decrease significantly with increasing surface-oxygen coverage, suggesting that the participation of subsurface oxygen in catalysis might strongly depend on coverage. Such calculations performed using DFT enable a deeper understanding of heterogeneous catalysis, which would help in the design of next-generation industrial catalysts.
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INTERFACES, CLUSTERS, AND CONDENSED PHASE Steven J. Sibener (University of Chicago) presented on two different efforts in his group. He began by discussing the embedding of atoms and molecules in ice. Such interactions are of fundamental importance in both terrestrial and astrophysical chemistry. Complementary techniques reveal a rich palette of gas−surface dynamics with differing dynamics for crystalline ice and amorphous solid water. Studies of CO 2 −surface interactions indicate that energetic ballistic embedding in ice is a general phenomenon [J. Phys. Chem. A 2015, 119, 12238. DOI: 10.1021/acs.jpca.5b06287]. Sibener also discussed an innovative way to use a surface-scattering apparatus for isotopedependent scattering that can lead to new and efficacious routes for isotopic enrichment and separation [Phys. Rev. Lett. 2017, 119, 176001. DOI: 10.1103/PhysRevLett.119.176001]. Alec Wodtke (Georg-August University of Göttingen and the Max Planck Institute for Biophysical Chemistry) presented both experimental and theoretical results that provide a fundamental understanding of how H-atom adsorption proceeds on gold and graphene surfaces. High translational energy H atoms, formed by the photolysis of HI, were directed at gold or graphene surfaces and scattered atoms were detected by Rydberg tagging. It was determined that a large portion of the H-atom incidence energy was converted to electronic excitation during collisions with a gold surface [Science 2015, 350, 1346. DOI: 10.1126/science.aad4972]. In contrast, nonadiabatic electronic excitations did not have an impact on the scattering dynamics of H on graphene, which were dominated by the formation of short-lived C−H bonds. Swetlana Schauermann (Institute of Physical Chemistry, Kiel) presented experiments results detailing the complex mechanisms of surface reactions catalyzed by either Pd(111) single crystals or Pd nanoparticles [J. Am. Chem. Soc. 2015, 137, 13496. DOI: 10.1021/jacs.5b04363]. It was shown that the hydrogenation of the CO bond in acrolein to form propanol was possible on Pd single crystals; however, the reaction could only occur after an overlayer of oxopropyl spectator species made the surface selective to the propanol formation. In contrast, Pd nanoparticles promoted the hydrogenation of the CC bond in acrolein. The hydrogenation of the CO bond could not occur on the Pd nanoparticles because the surface became saturated with CO originating from the decarboxylation of acrolein.
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UNIMOLECULAR/PHOTOCHEMICAL DYNAMICS Daniel Neumark (University of California, Berkeley) summarized recent progress on the development and application of a high-resolution negative ion photoelectron spectroscopy called cryo-SEVI. The high resolution of this method (≥1 cm−1) has proven useful in resolving subtle vibrational differences in the isomers of large polycyclic aromatic hydrocarbon radicals important in combustion and identifying the structure of small metal oxide clusters. Additionally, because electron detachment from cold anions can produce neutral radical complexes near transition states, cryo-SEVI is a powerful tool to directly probe vibrational structure, metastable resonances, and state-dependent reactivity in this region of the PES [Science 2017, 358, 336. DOI: 10.1126/science.aao1905]. Victoria Barber (Lester Group, University of Pennsylvania) delivered a talk on an atmospherically relevant class of molecules known as Criegee intermediates. For syn-alkyl substituted Criegee intermediates, such as syn-CH3CHOO, unimolecular decay proceeds through a rate-limiting 1,4hydrogen transfer, followed by decomposition to a hydroxyl radical and a vinoxy radical. Barber and her colleagues use IR excitation to deposit well-defined amounts of energy in synCH3CHOO and syn-CD3CHOO and probe the resulting OH or OD radicals using time-resolved laser-induced fluorescence. The appearance of products at energies below the transitionstate barrier and a pronounced deuterium kinetic isotope effect indicate that tunneling is crucial to accurately describe unimolecular decay in these Criegee intermediates [Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 12372. DOI: 10.1073/ pnas.1715014114]. Helen Fielding (University College London) summarized several recent studies on the relaxation dynamics of photo886
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sulfate are aerosolized and dried in the gas phase before deposition on a surface and imaging with cryo-TEM. For certain organic compounds and aerosol compositions, morphology is observed to be dependent on particle size: smaller particles are homogeneous and larger particles form phase-separated structures. This phenomenon is not observed for particle compositions that favor spontaneous, spinoidal phase-separation suggesting that it may be a consequence of activated, nucleation-based demixing [Chem. Commun. 2016, 52, 9220. DOI: 10.1039/C6CC03826C]. Yanice Benitez (Continetti Group, University of California, San Diego) delivered a talk on photoelectron−photofragment coincidence spectroscopy of OH−(CH4). Photodetatchment from this anion is observed to produce both stable and dissociative neutral products, primarily by populating a well in the hydrogen-abstraction entrance channel between OH and CH4. For this prereaction complex, ab initio calculations predict a structure where the H atom of the OH radical is directed at one tetrahedral face of CH4. Additionally, the experimental data provide evidence for excitation of the antisymmetric stretch of CH4, suggesting that this mode may be important in accessing the transition state. Because OH + CH4 is a prototypical reaction in atmospheric and combustion chemistry, this experiment provides crucial benchmark information for theory. Jim Lin (IAMS, Academia Sinica) delivered a presentation on the reactivity and kinetics of Criegee intermediates. Recent experiments show that CH2OO and anti-CH3CHOO react very quickly with water vapor (primarily water dimer) and undergo slow unimolecular decomposition. However, for synCH 3 CHOO and (CH 3 ) 2 COO, the trend is reversed. Theoretical calculations suggest that the decreased reactivity of syn-CH3CHOO and (CH3)2COO with water may be the result of a steric effect that blocks access to the central carbon. Enhanced unimolecular decomposition of syn-CH3CHOO and (CH3)2COO occurs because the molecular structure of these Criegee intermediates permit the formation of a relatively lowenergy, 1,4-hydrogen shift transition state where tunneling has a significant contribution to the observed kinetics [Chem. Soc. Rev. 2017, 46, 7483. DOI: 10.1039/C7CS00336F].
excited, biorelevant molecules in the gas phase using timeresolved photoelectron spectroscopy and quantum calculations. For isolated pyrrole, photoexcitation of the B2(21ππ*) state results in an ultrafast population cascade to the A2(11 πσ*) state that is difficult to resolve with experiment alone. Additional electronic relaxation pathways emerge in pyrrole dimers, and Fielding has identified a new, long-lived intermolecular charge-transfer state mediated by NH bond stretching in this system [Nat. Commun. 2016, 7, 11357. DOI: 10.1038/ncomms11357]. Lastly, Fielding described how competition between internal conversion and electron detachment in the isolated chromophore anion of green fluorescent protein can be tuned by chemical modification. Robert Continetti (University of California, San Diego) summarized a study on the dissociative photodetatchment of [F(H2O)]−. Photoelectron−photofragment coincidence spectroscopy measurements were combined with theory to quantify the contribution of the lowest two electronic states on the F(H2O) dissociation dynamics and identify Feshbach resonances on the electronic ground state. Continetti also provided an update on the development of an aerosol impact spectrometer to study the collisional dynamics of charged nanoparticles with surfaces, membranes, and free-standing nanostructures. This instrument has been constructed to measure the absolute charge and mass of single nanoparticles before accelerating them toward a collision target and monitoring particle recoil velocity [EPJ Tech. Instrum. 2017, 4, 2. DOI: 10.1140/epjti/s40485-017-0037-6].
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ATMOSPHERIC, ASTROCHEMISTRY, AND COMBUSTION Ralf Kaiser (University of Hawaii at Manoa) described new crossed molecular beam experiments exploring the reactive chemistry of silylidyne (SiH) radicals with unsaturated hydrocarbons under single-collision conditions. Many of the resulting organosilicon compounds are difficult to synthesize by other means and may play a role in the formation of silicon carbide dust grains in the interstellar medium. Although silicon and carbon are isoelectronic, ab initio calculations and reactive scattering data demonstrate that corresponding SiCxHy and C(x+1)Hy species have different potential energy surfaces and asymptotic products. For example, the dominant product of SiH + C2H2 is cyclic SiC2H2 + H whereas the CH + C2H2 reaction primarily yields linear HCCCH + H [Chem. Phys. Lett. 2016, 654, 58. DOI: 10.1016/j.cplett.2016.05.003]. Mitchio Okumura (California Institute of Technology) described a recent study on the mechanism and kinetics of the gas phase OD + CO reaction. The protium variant of this reaction, OH + CO, is important in regulating the concentration of OH in the atmosphere and in combustion processes. The kinetics of both isotopic variants of this reaction are complicated by the formation of an energized and collisionstabilized HOCO or DOCO intermediate. By using an emerging method known as time-resolved cavity-enhanced frequency comb spectroscopy, Okumura and his colleagues have experimentally confirmed the reaction mechanism under thermal conditions and quantified the termolecular dependence of the reaction rate coefficients for CO and N2 bath gases [Science 2016, 354, 444. DOI: 10.1126/science.aag1862]. Miriam Freedman (Pennsylvania State University) described some of her recent work on the morphology and phase behavior of submicron aerosol particles. In these experiments, aqueous solutions of an organic compound with ammonium
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KEYNOTE ADDRESS The 26th DMC conference invited William H. Miller to deliver the keynote address. Miller is the Kenneth S. Pitzer Distinguished Professor at University of California, Berkeley, and is known for making seminal contributions in semiclassical methods to study chemical dynamics. He has won numerous awards, notably the Welch Award in Chemistry (2007) and the Herschbach medal (2007). He is a prominent member of the DMC community and holds membership in the International Academy of Quantum Molecular Sciences (1985), the National Academy of Sciences (1987), the American Academy of Arts and Sciences (1993), and the German National Academy of Sciences (2011). In 2015, Miller was elected as a Foreign Member of the Royal Society. In his talk, Miller discussed the development of a novel symmetrical quasi-classical (SQC) windowing method capable of describing systems with discrete electronic states [J. Phys. Chem. A 2013, 117, 7190. DOI: 10.1021/jp401078u]. By treating nuclear and electronic degrees of freedom classically, this approach retains the simplicity of classical molecular dynamics permitting the efficient simulation of electronically nonadiabatic processes. Miller also described a variant and general extension of the SQC windowing method that can be 887
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The Journal of Physical Chemistry A
product energy distributions in the unimolecular reaction of NO2. Reisler’s current work focuses on photoinitiated reactions of free radicals and other transient species [J. Phys. Chem. A 2014, 118, 11916. DOI: 10.1021/jp505108k] and the dynamics of hydrogen-bonded and weakly covalently bound complexes [Chem. Rev. 2016, 116, 4913. DOI: 10.1021/acs.chemrev.5b00506]. She concluded her presentation by detailing her involvement in encouraging women in science through the Women in Science and Engineering (WiSE) program and the success of these efforts toward a diverse inclusion of women in the scientific community. John Tully is the Sterling Professor of Chemistry at Yale University. Although Tully is known for his pioneering work in theoretical chemistry, he started his career as an experimentalist at University of Colorado Boulder, performing crossed molecular beams experiments on H+ + D2. His theoretical work started at Yale with a seminal paper applying the trajectory surface hopping method to the nonadiabatic molecular collisions between H+ and D2 [J. Chem. Phys. 1971, 55, 562. DOI: 10.1063/1.1675788]. Tully's surface hopping method is widely used in theoretical chemistry where nonadiabatic effects cannot be neglected. He explained that the initial discrepancies between experiment and early applications of the surface hopping method were later shown to be the result of low-quality potential energy surfaces. Also, in contrast to Ehrenfest dynamics, the surface hopping method satisfies detailed balance. Tully has also made important contributions to the theory and application of electronic friction interactions. For example, while computing nonadiabatic vibrational lifetimes of CO on Cu(100), his group found electronic-frictioninduced mode-coupling between neighboring CO adsorbates in a c(2×2) overlayer to be important to understand experimental results [Phys. Rev. B 2016, 94, 115432. DOI: 10.1103/ PhysRevB.94.115432]. Tully also discussed recent work comparing electronic friction models to benchmark data [Angew. Chem. Int. Ed. 2012, 124, 5038. DOI: 10.1002/ ange.201201168].
used to obtain the full electronic density matrix with no additional cost or complexity. It was demonstrated that the SQC windowing method could accurately capture the behavior of several important nonadiabatic systems including the Tully single avoided and dual avoided crossing models, symmetric and asymmetric boson hopping models, and Marcus theory models. In contrast to the Ehrenfest dynamics generated by the classical vibronic coupling Hamiltonian, the SQC windowing method satisfies the principle of detailed balance.
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HERSCHBACH PRIZE SESSION A special award session, instituted in 2007, is held at the biennial DMC meeting to honor Dudley R. Herschbach. He received the Nobel Prize in 1986 together with Yuan T. Lee and John C. Polanyi, for “their contributions concerning the dynamics of chemical elementary processes.” The Herschbach Prize recognizes one experimentalist and one theorist for outstanding advances in the field. This year, the awardees were Hanna Reisler for experiment and John Tully for theory. As a prelude to their talks, Herschbach joined the session via Skype and introduced the conference attendees to the song “Experiment”, by Cole Porter, from the famed musical Nymph Errant. Herschbach stressed that both in the song and in science “Experiment” is not limited to a laboratory apparatus but includes things that happen in the curious apparatus within our heads. He expressed his appreciation when asked a decade ago by David Chandler to design the dualsided Herschbach Medal, which represents theory on one side and experiment on the other. Both sides are shown on the cover of the special issue commemorating Fifty Years of Chemical Reaction Dynamics ([J. Phys. Chem. A 2015, 119, 11949. DOI: 10.1021/acs.jpca.5b08530]; organized by Hua Guo and Arthur Suits). Herschbach described the symbolism of the Medal: “The Theory side symbolizes, rather whimsically, our yearning to attain an exalted, exhilarating comprehension. The Experimental side, more realistically, symbolizes our grasping for incisive means to find out what actually happens in encounters of atoms, molecules, and/or photons.” While applauding Reisler and Tully for their inspiring work, Herschbach also noted that they both exemplify a striking phenomenon. For nearly 60 years he has observed that recruits attracted to dynamics of molecular collisions seem unusually excitable, zestful, friendly, and generous. Pursuing new insights in such a congenial field is a splendid prize. Hanna Reisler is the Lloyd Armstrong Jr. Chair in Science and Engineering and Professor of Chemistry at the University of Southern California. Reisler is also the first female scientist to win the Herschbach medal. Her award presentation was a medley of her scientific achievements over the years and the struggles she overcame in her professional life. She guided the conference participants on a journey through her career. Reisler began as an organic chemist before eventually switching to physical chemistry. Reisler followed several threads of research before she started her tenured position at the University of Southern California in 1987. At first, her work focused on the dissociation of gas phase molecules like NO2 following highvelocity collisions in crossed-molecular beam experiments and with surfaces. She also focused on the spectroscopy and dynamics of dissociative states and made successful strides in developing experimental probes for elucidating potential energy surfaces. Around 1992, her work moved toward studies of the dynamics on dissociative states, including interference effects. A central topic was elucidating fluctuations, resonances, and
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CONCLUSION There is no question that the DMC conference continues to draw some of the best and brightest scientists in physical chemistry. The longevity of the conference is a testament to the strong sense of community among its members and their commitment to high-quality, innovative research. The 2017 DMC was intellectually stimulating and professionally valuable for all participants from beginning to end. This is largely the result of an immense amount of work from the conference-site staff and the conference organizers, David Yarkony and Timothy Minton. It is also important to acknowledge generous contributions from The Journal of Chemical Physics, The Journal of Physical Chemistry, the journal Physical Chemistry Chemical Physics, the journal Molecular Physics, Montana State University, Johns Hopkins University, the U.S Air Force Office of Scientific Research, the U.S. Army Research Office, and the U.S. Department of Energy. We thank the session chairs, the participants who contributed to the exchange of ideas and lively discussion after every talk, Dudley Herschbach, and all 47 speakers for their fantastic presentations and for their assistance in preparing this Viewpoint. We also thank the poster presenters who contributed to a vigorous poster session and acknowledge the poster prize winners Vanessa Murray (Minton Group, Montana State University), Xinyou Ma (Hase Group, Texas Tech University), and Jessalyn Devine (Neumark Group, 888
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The Journal of Physical Chemistry A University of California, Berkeley). The 27th DMC will continue the tradition of excellence in 2019 where Timothy Minton and Anna Krylov will act as Chair and Co-Chair for the conference at Big Sky, Montana, from July 7th to 12th.
Eric J. Smoll, Jr.* Savio Poovathingal* Vanessa J. Murray*
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Department of Chemistry and Biochemistry, Montana State University, Bozeman, Montana 59717, United States
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (E.J.S.). *E-mail:
[email protected] (S.P.). *E-mail:
[email protected] (V.J.M.). Notes
The authors declare no competing financial interest.
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