Anne B. McCoy

The Ohio State University, Columbus, Ohio, United States

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Publications (172)538.06 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Ozonolysis of alkenes, an important nonphotolytic source of hydroxyl (OH) radicals in the troposphere, proceeds through energized Criegee intermediates that undergo unimolecular decay to produce OH radicals. Here, we used infrared (IR) activation of cold CH3CHOO Criegee intermediates to drive hydrogen transfer from the methyl group to the terminal oxygen, followed by dissociation to OH radicals. State-selective excitation of CH3CHOO in the CH stretch overtone region combined with sensitive OH detection revealed the IR spectrum of CH3CHOO, effective barrier height for the critical hydrogen transfer step, and rapid decay dynamics to OH products. Complementary theory provides insights on the IR overtone spectrum, as well as vibrational excitations, structural changes, and energy required to move from the minimum-energy configuration of CH3CHOO to the transition state for the hydrogen transfer reaction.
    Science (New York, N.Y.). 09/2014; 345(6204):1596-8.
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    ABSTRACT: The photofragmentation dynamics of BrCN(-) in the 270-355 nm and the 430-600 nm wavelength regions is explored both experimentally and theoretically. In the case of excitation between 430 nm and 600 nm, it is found that the molecular ion accesses two dissociation channels with a measured 60:40 branching ratio that is nearly constant over this range of photon energies. The dominant product channel corresponds to Br(-) + CN, while the second channel correlates to spin-orbit excited Br(*) with CN(-). A larger wavelength dependence of the branching ratio is observed at shorter wavelengths, where the fraction of Br(-) based products ranges from 80% to 95% at 355 nm and 270 nm, respectively. These branching ratios are reproduced and the mechanisms are explored by quantum dynamics calculations based on ground and excited state potential energy surfaces for BrCN(-), evaluated at the SO-MRCISD level of theory. It is found that the electronic states that correlate to the two observed product channels are coupled through the spin-orbit terms in the electronic Hamiltonian. The strength of this coupling displays a strong dependence on the Br-CN angle. Specifically, after promotion to the excited state that is energetically accessible with 430-600 nm photons, it is found that when the wave packet accesses Br-CN separations of between 4 Å and 6 Å, predominantly the Br(-) + CN products are formed when the Br-CN angle is smaller than 120°. For larger values of the Br-CN angle, the Br(*) + CN(-) channel dominates. At the shorter wavelength excitation, the dynamics is complicated by a pair of states that correlate to electronically excited CN(*) + Br(-) products that borrow oscillator strength from the bright state, leading to an increase in the amount of Br(-) relative to CN(-). The implications of these findings are discussed and compared to the experimentally measured product branching ratios for the photodissociation of BrCN(-).
    The Journal of chemical physics. 08/2014; 141(8):084305.
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    ABSTRACT: Vibrational predissociation spectra of D2 "tagged" Mg(2+)OH(-)(H2O)n=1-6 and Ca(2+)OH(-)(H2O)n=1-5 clusters are reported to explore how the M(2+)OH(-) contact ion pairs respond to stepwise formation of the first hydration shell. In both cases, the hydroxide stretching frequency is found to red-shift strongly starting with addition of the third water molecule, quickly becoming indistinguishable from nonbonded OH groups associated with solvent water molecules by n=5. A remarkably broad feature centered around 3200 cm(-1) and spanning up to ~1000 cm(-1) appears for the n≥4 clusters that we assign to a single-donor ionic hydrogen bond between a proximal first solvent shell water molecule and the embedded hydroxide ion. The extreme broadening is rationalized with a theoretical model that evaluates the range of local OH stretching frequencies predicted for the heavy particle configurations available in the zero-point vibrational wavefunction describing the low frequency modes. The implication of this treatment is that extreme broadening in the vibrational spectrum need not arise from thermal fluctuations in the ion ensemble, but can rather reflect combination bands based on the OH stretching fundamental that involve many quanta of low frequency modes whose displacements strongly modulate the OH stretching frequency.
    The journal of physical chemistry. A. 05/2014;
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    Zhou Lin, Anne B. McCoy
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    Zhou Lin, Anne B McCoy
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    ABSTRACT: The vibrational spectroscopy of the nitrate-water isotopologues is studied in the O-H and O-D stretching regions using temperature-dependent infrared multiple photon dissociation spectroscopy combined with calculations of the anharmonic spectra. At a temperature of 15 K a series of discrete peaks is observed in the IRMPD spectra of NO3(-)·H2O, NO3(-)·HDO, and NO3(-)·D2O. This structure is considerably more complex than predicted by harmonic calculations. A signal is only observed in the hydrogen-bonded O-H (O-D) stretching region, characteristic of a double hydrogen-bond donor binding motif. With increasing temperature, the peaks broaden, leading to a quasi-continuous absorption from 3150 to 3600 cm(-1) (2300-2700 cm(-1)) for NO3(-)·H2O (NO3(-)·D2O) and, above 100 K, an additional band in the free O-H (O-D) stretching region, suggesting the population of complexes containing only a single hydrogen bond at higher internal energies. Vibrational configuration interaction calculations confirm that much of the structure observed in the IRMPD spectra derives from progressions in the water rocking mode resulting from strong cubic coupling between the O-H (O-D) stretch and water rock degrees of freedom. The spectra of both NO3(-)·H2O and NO3(-)·D2O display a strong peak that does not derive from the water rock progression but results instead from a Fermi resonance between the O-H (O-D) stretch and H-O-H (D-O-D) bend overtone. Additional insight into the nature of the O-H (O-D) stretch and water rocking coupling in these complexes is provided by an effective Hamiltonian that allows for the cubic coupling between the O-H stretch and water rock degrees of freedom.
    The Journal of Physical Chemistry A 04/2014; · 2.77 Impact Factor
  • ANNE B. MCCOY, RON W. DARBEAU
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    ABSTRACT: The Committee on Professional Training (CPT) develops and administers the American Chemical Society’s Guidelines for Bachelor’s Degree Programs. The guidelines set benchmarks for the development of excellent undergraduate programs from which capable, competent, and versatile chemistry professionals emerge and in which chemistry faculty flourish professionally (see page 36). This committee function appears simple, yet challenges abound in defining excellence, identifying suitable metrics for student competence, determining the knowledge and skills required for new graduates to contribute effectively to the chemical enterprise, and defining program characteristics that encourage professional development of faculty. The wide range in size, resources, administrative structure, and institutional goals of approved programs and those seeking approval makes the task even more complex. Further, with chemistry’s central role in careers in industry, government, medicine, academia, law enforcement, and a host of other fields, ensuring that graduates are capable of meeting diverse challenges with competence and confidence makes guideline ...
    Chemical & Engineering News Archive. 03/2014; 91(24).
  • ANNE B. MCCOY, RON W. DARBEAU
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    ABSTRACT: One charge for the Committee on Professional Training (CPT) is to develop and administer “living” guidelines for bachelor’s degree programs in chemistry. These guidelines are intended to promote program excellence, stimulate the growth of enriching and supportive environments for chemistry academics, and provide standards for preparing the next generation of professionals to make positive and meaningful contributions to the chemistry enterprise. Programs that meet the guidelines are approved by the American Chemical Society. Change is a necessary component of the guidelines, as it is for the discipline they advance. Every few years, CPT shoulders the task of retooling the language and content of the guidelines to enhance clarity and promote the excellence and flexibility that they intend to foster. The revisions are also meant to ensure consideration of, and sensitivity to, trends in the chemistry discipline and in chemistry employment. The 2008 version of the guidelines represented a marked paradigm ...
    Chemical & Engineering News Archive. 03/2014; 91(46).
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    Zhou Lin, Anne B McCoy
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    ABSTRACT: Protonated hydrogen dimer, [Formula: see text], is the intermediate in the astrochemically important proton transfer reaction between [Formula: see text] and H2. To understand the mechanism for this process, we focus on how large amplitude motions in [Formula: see text] result in scrambling of the five hydrogen atoms in the collision complex. To this end, the one-dimensional zero-point corrected potential surfaces were mapped out as functions of reaction coordinates for the [Formula: see text] + H2 collision using minimized energy path diffusion Monte Carlo [C. E. Hinkle and A. B. McCoy, J. Phys. Chem. Lett. 1, 562 (2010)]. In this study, the previously developed approach was extended to allow for the investigation of selected excited states that are expected to be involved in the proton scrambling dynamics. Specifically, excited states in the shared proton motion between the two H2 groups, and in the outer H2 bending motions were investigated. Of particular interest is the minimum distance between [Formula: see text] and H2 at which all five hydrogen atoms become free to exchange. In addition, this diffusion Monte Carlo-based approach was used to determine the zero-point energy E0, the dissociation energy D0, and excitation energies associated with the vibrational motions that were investigated. The evolution of the wave functions was also studied, with a focus on how the intramolecular vibrations in [Formula: see text] evolve into motions of [Formula: see text] or H2. In the case of the proton scrambling, we find that the relevant transition states become fully accessible at separations between [Formula: see text] and H2 of approximately 2.15 Å, a distance that is accessed by the excited states of [Formula: see text] with two or more quanta in the shared proton stretch. The implications of this finding on the vibrational spectroscopy of [Formula: see text] are also discussed.
    The Journal of Chemical Physics 03/2014; 140(11):114305. · 3.12 Impact Factor
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    ABSTRACT: Survey vibrational predissociation spectra of several representative protonated peptides and model compounds reveal very diffuse absorptions near 2500 cm–1 that are traced to pentagonal cyclic ionic hydrogen bonds (C5 interactions) involving the excess charge centers. This broadening occurs despite the fact that the ions are cooled close to their vibrational zero-point levels and their spectra are obtained by predissociation of weakly bound adducts (H2, N2, CO2) prepared in a cryogenic ion trap. The C5 band assignments are based on H/D isotopic substitution, chemical derivatization, solvation behavior, and calculated spectra. We evaluate the extent to which this broadening is caused by anharmonic coupling in the isolated molecules by including cubic coupling terms in the normal mode expansion of the potential energy surface. This analysis indicates that the harmonic H-bonded stretching vibration is mixed with dark background states over much of the energy range covered by the observed features. The difficulty with identifying these features in earlier studies of dipeptides is traced to both the breadth and the fact they are calculated to be intrinsically weaker than cases involving linear variations of the N···H+···O motif.
    Journal of Physical Chemistry Letters 10/2013; 4(20):3450–3457. · 6.59 Impact Factor
  • Andrew S Petit, Jason Ford, Anne B McCoy
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    ABSTRACT: An extension to diffusion Monte Carlo (DMC) is proposed for simultaneous evaluation of multiple rotationally excited states of fluxional molecules. The method employs an expansion of the rotational dependence of the wave function in terms of the eigenstates of the symmetric top Hamiltonian. Within this DMC approach, each walker has a separate rotational state vector for each rotational state of interest. The values of the coefficients in the expansion of the rotational state associated with each walker as well as the the locations the walkers evolve in imaginary time under the action of the imaginary-time time-dependent Schr\"{o}dinger equation. The approach is first applied to H$_3^+$, H$_2$D$^+$ and H$_3$O$^+$ for which the calculated energies can be compared to benchmark values. For low to moderate values of J the DMC results are found to be accurate to within the evaluated statistical uncertainty. The rotational dependence of the vibrational part of the wave function is also investigated, and significant rotation-vibration interaction is observed. Based on the successful application of this approach to H3(+), H2D(+) and H3O(+), the method was applied to calculations of the rotational energies and wave functions for CH5(+) with v=0 and J≦ 10. Based on these calculations, the rotational energy progression is shown to be consistent with that for a nearly spherical top molecule, and little evidence of rotation-vibration interaction is found in the vibrational wave function.
    The Journal of Physical Chemistry A 09/2013; · 2.77 Impact Factor
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    ABSTRACT: Photodissociation of ICN(-)(CO2)n, n = 0-18, with 500-nm excitation is investigated using a dual time-of-flight mass spectrometer. Photoabsorption to the (2)Π1∕2 state is detected using ionic-photoproduct action spectroscopy; the maximum absorption occurs around 490 nm. Ionic-photoproduct distributions were determined for ICN(-)(CO2)n at 500 nm. Following photodissociation of bare ICN(-) via 430-650 nm excitation, a small fraction of CN(-) is produced, suggesting that nonadiabatic effects play a role in the photodissociation of this simple anion. Electronic structure calculations, carried out at the MR-SO-CISD level of theory, were used to evaluate the potential-energy surfaces for the ground and excited states of ICN(-). Analysis of the electronic structure supports the presence of nonadiabatic effects in the photodissociation dynamics. For n ≥ 2, the major ionic photoproduct has a mass corresponding to either partially solvated CN(-) or partially solvated [NCCO2](-).
    The Journal of Chemical Physics 08/2013; 139(6):064315. · 3.12 Impact Factor
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    ABSTRACT: We analyze the structures and spectral signatures of the cyclic intramolecular proton bond, N-H(+)-A, A = O and F, formed when an excess proton is added to derivatives of the 1,8-disubstituted naphthalene scaffold. These compounds provide a quasi-rigid framework with which to study the spectral complexity often associated with the N-H(+)-A entity. Vibrational spectra were obtained by monitoring photodissociation of weakly bound H2 adducts of the mass-selected ions cooled close to 10 K. Several bands across the 900-3500 cm(-1) spectral range were traced to involvement of the bridging proton by their telltale shifts upon selective H∕D isotopic substitution at that position. We account for the complex patterns that occur near the expected locations of the NH stretching fundamentals in the context of background levels mixing with a "bright" zero-order state through cubic terms in the potential energy expansion. Thus, this system provides a detailed picture of one of the mechanisms behind the line broadening often displayed by embedded excess protons. It does so in a sufficiently sparse density of states regime that many discrete transitions are observed in the vicinity of the harmonic stretching transition involving displacement of the trapped proton.
    The Journal of Chemical Physics 07/2013; 139(2):024301. · 3.12 Impact Factor
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    ABSTRACT: The ultraviolet (UV) photodissociation of mass-selected ICN(-)Arn and ICN(-)(CO2)n clusters (n = 0 - 5) is studied using a secondary reflectron mass spectrometer. Relative photodissociation cross sections of bare ICN(-) show the dominance of the I(-) photoproduct from 270 to 355 nm, the entire wavelength range studied. UV excitation populates both the (2)Σ(+) state that produces I* + CN(-) and the (2)Π states that produce I(-) + CN*. While the excited (2)Π states directly produce I(-), excitation to the (2)Σ(+) state also produces some I(-) product via nonadiabatic transitions to the (2)Π1/2 state, which produces I(-) + CN. Partial solvation of the anion by Ar atoms or CO2 molecules alters the UV-branching percentages between the various dissociation channels: I* + CN(-) and I(-) + CN or I(-) + CN*. In addition, solvation by two or more Ar atoms or three or more CO2 molecules results in recombination, re-forming ICN(-). Examination of the potential surfaces and transition moments in combination with the results of quantum dynamics calculations performed on the relevant excited states assist in the analysis of the experimental results.
    The Journal of Physical Chemistry A 07/2013; · 2.77 Impact Factor
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    ABSTRACT: We report vibrationally induced charge transfer from nitromethane anion to methyliodide in a molecular complex. Excitation of a CH stretching vibrational transition in either of the molecular constituents results in dissociative electron transfer to the CH3I molecule, resulting in I(-) product anions. Solvation of the pre-reactive complex with more than two Ar atoms leads to complete quenching of the reaction and can be used to estimate the barrier for this reaction. We discuss the results in the framework of electronic structure calculations and compare the intra-complex electron transfer with vibrationally mediated electron emission in bare nitromethane anion.
    The Journal of Chemical Physics 06/2013; 138(22):224301. · 3.12 Impact Factor
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    ABSTRACT: We report vibrational and electronic spectra of the hydroxyl-methyl-peroxy radical (HOCH2OO, or HMP), the primary product of the reaction of the hydroperoxy radical, HO2, and formaldehyde, HCHO. The ν1 vibrational (OH stretch) spectrum and the Ã-X electronic spectrum of HMP were detected by Infrared Cavity Ringdown Spectroscopy (IR-CRDS), and assignments were verified with density functional calculations. The HMP radical was generated in reactions of HCHO with HO2. Free radical reactions were initiated by pulsed laser photolysis (PLP) of Cl2 in the presence of HCHO and O2 in a flow reactor at 300-330 Torr and 295K. IR-CRDS spectra were measured in mid-IR and near-IR regions over the ranges 3525-3700 cm(-1) (ν1) and 7250-7800 cm(-1) (Ã-X) (respectively, at a delay time 100 µs after photolysis. The ν1 spectrum had an origin at 3622 cm(-1) and exhibited partially resolved P- and R-branch contours and a small Q branch. At these short delay times, spectral interference from HOOH and HCOOH was minimal, and could be subtracted. From B3LYP/6-31G+(d,p) calculations, we found that the anharmonic vibrational frequency and band contour predicted for the lowest energy conformer, HMP-A, The calculated anharmonic vibrational frequency and band contour computed using B3LYP/63-1G(d,p) level were in good agreement with the observed spectrum. In the near-IR, we observed four well spaced vibronic bands, each with partially resolved rotational contours. We assigned the apparent origin of the electronic spectrum of HMP at 7392 cm(-1) and two bands to the blue to a progression in ν15', the lowest torsional mode of the state (ν15'= 171 cm(-1)). The band furthest to the red was assigned as a hot band in ν15", leading to a ground state torsional frequency of (ν15"= 122 cm(-1)). We simulated the spectrum using second order vibrational perturbation theory (VPT2) with B3LYP/6-31+G(d,p) calculations at the minimum energy geometries of the HMP-A conformer on the X ̃ and A ̃ states. The predictions of the electronic origin frequency, torsional frequencies, anharmonicities and rotational band contours matched the observed spectrum. We investigated the torsional modes more explicitly by computing potential energy surfaces of HMP as a function of the two dihedral angles τOCOH and τOOCO. Wave functions and energy levels were calculated based on this potential surface; these results were used to calculate the Franck-Condon factors, which reproduced the vibronic band intensities in the observed electronic spectrum. The transitions that we observed all involved states with wave functions localized on the minimum energy conformer, HMP-A. Our calculations indicated that the observed near-IR spectrum was that of the minimum energy conformer HMP-A, but that this conformer is not the lowest energy conformer in the state, which remains unobserved. We estimated that the energy of this lowest conformer (HMP-B) of the à state to be T0 (Ã) ≈ 7200 cm(-1), based on the energy difference E0(HMP-B) - E0(HMP-A) on the à state computed at the B3LYP/6-31+G(d,p) level.
    The Journal of Physical Chemistry A 05/2013; · 2.77 Impact Factor
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    Zhou Lin, Anne B McCoy
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    ABSTRACT: The results of diffusion Monte Carlo (DMC) calculations of the ground and selected excited states of H5(+) and its deuterated analogues are presented. Comparisons are made between the results obtained from two recently reported potential surfaces. Both of these surfaces are based on CCSD(T) electronic energies, but the fits display substantial differences in the energies of low-lying stationary points. Little sensitivity to these features is found in the DMC results, which yield zero-point energies based on the two surfaces that differ by between 20 and 30 cm(-1) for all twelve isotopologues of H5(+). Likewise projections of the ground state probability amplitudes, evaluated for the two surfaces, are virtually identical. Using the ground state probability amplitudes, vibrationally averaged rotational constants and dipole moments were calculated. Based on these calculations, all isotopologues are shown to be near-prolate symmetric tops. Further, in cases where the ion had a non-zero dipole moment, the magnitude of the vibrationally averaged dipole moment was found to range from 0.33 to 1.15 D, which is comparable to the dipole moments of H2D(+) and HD2(+). Excited states with up to three quanta in the shared proton stretch and one quantum in the in-phase stretch of the outer H2 groups were also investigated. Trends in the energies and the properties of these states are discussed.
    The Journal of Physical Chemistry A 04/2013; · 2.77 Impact Factor
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    Anne B. McCoy, Ron W. Darbeau
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    ABSTRACT: The Committee on Professional Training (CPT) of the American Chemical Society (ACS) is revising the ACS Guidelines for Bachelor’s Degree Programs. It is expected that the revised version will be adopted in 2014. In this commentary, the history of the ACS Guidelines is summarized. Areas in which there are expected to be changes to the 2008 guidelines are highlighted. The committee is soliciting input from the community about these proposed changes and about the guidelines and the approval processes more generally.
    Journal of chemical education 04/2013; 90(4):398-400. · 0.82 Impact Factor
  • Andrew S Petit, Anne B McCoy
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    ABSTRACT: An internal coordinate extension of diffusion Monte Carlo (DMC) is described as a first step towards a generalized reduced-dimensional DMC approach. The method places no constraints on the choice of internal coordinates other than the requirement that they all be independent. Using H(3)(+) and its isotopologues as model systems, the methodology is shown to be capable of successfully describing the ground state properties of molecules that undergo large amplitude, zero-point vibrational motions. Combining the approach developed here with the fixed-node approximation allows vibrationally excited states to be treated. Analysis of the ground state probability distribution is shown to provide important insights into the set of internal coordinates that are less strongly coupled and therefore more suitable for use as the nodal coordinates for the fixed-node DMC calculations. In particular, the curvilinear normal mode coordinates are found to provide reasonable nodal surfaces for the fundamentals of H(2)D(+) and D(2)H(+) despite both molecules being highly fluxional.
    The Journal of Physical Chemistry A 02/2013; · 2.77 Impact Factor
  • Anne B. McCoy
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    ABSTRACT: ICN and ICN− have been studied at the MR-SO-CISD level of theory with triple-zeta basis sets for all three atoms. The potential surfaces for the ground states of ICN and ICN− as well as for the first five excited states of ICN− have been generated from the electronic energies, and properties of these states are described. The minimum energy geometry of ICN− is linear, with a local minimum in the INC− geometry. The zero-point corrected energy difference between these two isomers is 0.38 eV, and they are separated by a 0.5 eV barrier. The I···CN(COM) equilibrium distances are 3.27 and 3.14 Å in the ICN− and INC− geometries, respectively. These values are 0.6 Å larger than the I···CN distances in the corresponding minima in the ICN potential. Likewise, the zero-point amplitude of both the I···CN stretch and bend are much larger in ICN− than in ICN. This is captured by the calculated anharmonic fundamental frequencies for ICN− of 70 and 235 cm−1 for the bend and stretch, compared to anharmonic frequencies of 302 and 488 cm−1 for the bend and stretch fundamentals in ICN. The frequencies are lowered further in INC− where the bend and stretch fundamentals have frequencies of 59 and 220 cm−1. © 2012 Wiley Periodicals, Inc.
    International Journal of Quantum Chemistry 02/2013; 113(3). · 1.17 Impact Factor

Publication Stats

1k Citations
538.06 Total Impact Points

Institutions

  • 2–2014
    • The Ohio State University
      • Department of Chemistry and Biochemistry
      Columbus, Ohio, United States
  • 2005–2013
    • Yale University
      • Department of Chemistry
      New Haven, CT, United States
  • 2012
    • University of Pittsburgh
      • Department of Chemistry
      Pittsburgh, Pennsylvania, United States
  • 2004–2012
    • University of Colorado at Boulder
      • Department of Chemistry and Biochemistry
      Boulder, CO, United States
  • 2005–2011
    • Washington University in St. Louis
      • Department of Chemistry
      Saint Louis, MO, United States
  • 2006–2009
    • University of Pennsylvania
      • Department of Chemistry
      Philadelphia, PA, United States
  • 2004–2006
    • Emory University
      • Department of Chemistry
      Atlanta, GA, United States
  • 1994–1995
    • Hebrew University of Jerusalem
      • Fritz Haber Center for Molecular Dynamics Research
      Jerusalem, Jerusalem District, Israel
  • 1991–1992
    • University of Wisconsin, Madison
      • Department of Chemistry
      Madison, Wisconsin, United States