Anne B. McCoy

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (196)653.51 Total impact

  • Jason E. Ford · Anne B. McCoy
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    ABSTRACT: In this work the efficacy of a combined approach for capturing rovibrational coupling is investigated. Specifically, the multi-state rotational DMC method is used in combination with fixed-node DMC in a study of the rotation vibration energy levels of H2D+ and HD2+. Analysis of the results of these calculations shows very good agreement between the calculated energies and previously reported values. Where differences are found, they can be attributed to Coriolis couplings, which are large in these ions and which are not fully accounted for in this approach.
    No preview · Article · Dec 2015 · Chemical Physics Letters
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    ABSTRACT: To understand how the D2d oxalate scaffold (C2O4)(2)¯ distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO2CCO2¯ anion and its deuterated isotopologue DO2CCO2¯. The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydron stretches, on the other hand, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high frequency OH-stretch and the low frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero point level.
    No preview · Article · Nov 2015 · The Journal of Physical Chemistry A
  • Zhou Lin · Anne B McCoy
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    ABSTRACT: Understanding the spectroscopy and dynamics of $\ce{H5+}$ is central in gaining insights into the $\ce{H3+ + H2 -> H5+ -> H2 + H3+}$ proton transfer reaction. This molecular ion exhibits large amplitude vibrations, which allow for the transfer of a proton between $\ce{H3+}$ and $\ce{H2}$ even in its ground vibrational state. With vibrational excitation, the number of open channels for permutations of protons increase. In this work, the minimized energy path variant of diffusion Monte Carlo is used to investigate how the energetically accessible proton permutations evolve as $\ce {H5+}$ is dissociated into $\ce {H3+} + \ce{H2}$. Two mechanisms for proton permutation are investigated. The first is the proton hop, which correlates to large amplitude vibrations of the central proton in $\ce {H5+}$. The second is the exchange of a pair of hydrogen atoms between $\ce{H3+}$ and $\ce{H2}$. This mechanism requires several proton hops along with a 120$^\circ$ rotation of $\ce {H3+}$ within the $\ce {H5+}$ molecular ion. This analysis shows that while there is a narrow region of configuration space over which both isomerization processes are energetically accessible, full permutation of the five protons in $\ce {H5+}$ more likely occurs through a step-wise mechanism. Such full permutation of the protons becomes accessible when the shared proton stretch is excited to the $v_{\rm pt}$ = 2 or 3 excited state. The effects of deuteration and rotational excitation of the $\ce {H2}$ and $\ce {H3+}$ products are also investigated. Deuteration inhibits permutation of protons, while rotational excitation has only a small impact on these processes.
    No preview · Article · Aug 2015 · The Journal of Physical Chemistry A
  • Melanie L Marlett · Zhou Lin · Anne B McCoy
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    ABSTRACT: Two methods for studying the rotation/torsion coupling in H5(+) are described. The first involves a fixed-node treatment in which the nodal surfaces are obtained from a reduced dimensional calculation in which only the rotations of the outer H2 groups are considered. In the second, the torsion and rotation dependence of the wave function is described in state space, while the other internal coordinates are described in configuration space. Such treatments are necessary for molecules, like H5(+), where there is a very low-energy barrier to internal rotation. The results of the two approaches are found to be in good agreement with previously reported energies for J=0. The DMC treatment allows us to extend the calculations to low J, and results are reported for the three lowest energy torsion excited states with J ≦ 3. For the level of rotational and vibrational excitation investigated, only modest changes in the vibrational wave functions are found. The effects of deuteration are also investigated, focusing on D5(+) and the symmetric variants of H4D(+) and HD4(+).
    No preview · Article · Jul 2015 · The Journal of Physical Chemistry A
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    Thomas J. Wenzel · Anne B. McCoy · Clark R. Landis
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    ABSTRACT: The Committee on Professional Training (CPT) of the American Chemical Society (ACS) has recently approved a new set of Guidelines and Evaluation Procedures for Bachelor’s Degree Programs. Input from the community was invaluable in informing the deliberations of CPT during the three-year process of Guidelines revisions. The Guidelines describe standards that are used to approve programs to offer ACS certified undergraduate degrees. The most significant changes in the Guidelines are described in this commentary.Keywords: Curriculum; Professional Development; First-Year Undergraduate/General; Administrative Issues; Standards National/State; Student/Career Counseling
    Preview · Article · May 2015 · Journal of chemical education
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    ABSTRACT: To explore the extent of molecular cation perturbation induced by complexation with He atoms required for the application of cryogenic ion vibrational predissociation (CIVP) spectroscopy, we compare the spectra of the bare NH4(+)(H2O) ion (obtained using infrared multiple photon dissociation (IRMPD)) with the one-photon CIVP spectra of the NH4(+)(H2O)·He1-3 clusters. Not only are the vibrational band origins minimally perturbed, the rotational fine structure on the NH and OH asymmetric stretching vibrations, which arise from free internal rotation of the -OH2 and -NH3 groups, also remains intact in the adducts. To establish the location and quantum mechanical delocalization of the He atoms, we carried out Diffusion Monte Carlo (DMC) calculations of the vibrational zero point wavefunction, which indicate that the barriers between the three equivalent minima for He attachment are so small that the He atom wavefunction is delocalized over the entire -NH3 rotor, effectively restoring C3 symmetry for the embedded -NH3 group.
    No preview · Article · Apr 2015 · The Journal of Physical Chemistry A
  • ANNE B. MCCOY · THOMAS J. WENZEL
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    ABSTRACT: Since January 2012, the American Chemical Society Committee on Professional Training (CPT) has been developing a revised edition of the ACS Guidelines for Bachelor’s Degree Programs. Last month, the committee released a draft of the revised guidelines to chemistry department chairs and invited them to provide feedback. The committee will review the responses and then release the finalized new guidelines early next year. The current draft guidelines can be found at www.acs.org/cpt; the final version will be posted there as well. The guidelines describe the attributes of excellence in chemistry departments, with an emphasis on the curriculum and the infrastructure needed to support the curriculum. As such, they provide departments a context for discussing their chemistry major and other degree tracks and for obtaining the necessary resources to maintain an excellent program. The guidelines also outline the experiences and knowledge that a student who is awarded an ACS-certified degree must ...
    No preview · Article · Mar 2015 · Chemical & Engineering News
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    J D Mosley · J W Young · M Huang · A B McCoy · M A Duncan
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    ABSTRACT: The carbenium ion with nominal formula [C,H4,O]+ is produced from methanol or ethylene glycol in a pulsed-discharge supersonic expansion source. The ion is mass selected, and its infrared spectrum is measured from 2000 to 4000 cm−1 using laser photodissociation spectroscopy and the method of rare gas atom tagging. Computational chemistry predicts two isomers, the methanol and methylene-oxonium cations. Predicted vibrational spectra based on scaled harmonic and reduced dimensional treatments are compared to the experimental spectra. The methanol cation is the only isomer produced when methanol is used as a precursor. When ethylene glycol is used as the precursor, methylene-oxonium is produced in addition to the methanol cation. Theoretical results at the CCSD(T)/cc-pVTZ level show that methylene-oxonium is lower in energy than methanol cation by 6.4 kcal/mol, and is in fact the global minimum isomer on the [C,H4,O]+ potential surface. Methanol cation is trapped behind an isomerization barrier in our source, providing a convenient method to produce and characterize this transient species. Analysis of the spectrum of the methanol cation provides evidence for strong CH stretch vibration/torsion coupling in this molecular ion.
    Full-text · Article · Mar 2015 · The Journal of Chemical Physics
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    ABSTRACT: One-dimensional (1D) and two-dimensional (2D) models are investigated, which help to understand the unusual rovibrational energy-level structure of the astronomically relevant and chemically interesting astructural molecular ion H+5. Due to the very low hindering barrier characterising the 1D torsion-only vibrational model of H+5, this model yields strongly divergent energy levels. The results obtained using a realistic model for the torsion potential, including the computed (near) degeneracies, can be rationalised in terms of the model with no barrier. Coupling of the torsional motion with a single rotational degree of freedom is also investigated in detail. It is shown how the embedding-dependent rovibrational models yield energy levels that can be rationalised via the 2D vibrational model containing two independent torsions. Insight into the complex rovibrational energy level structure of the models and of H+5 is gained via variational nuclear motion and diffusion Monte Carlo computations and by the analysis of the wavefunctions they provide. The modelling results describing the transition from the zero barrier limit to the large barrier limit should prove to be useful for the important class of molecules and molecular ions that contain two weakly coupled internal rotors.
    Full-text · Article · Mar 2015 · Molecular Physics
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    ABSTRACT: We clarify the role of the critical imidazolium C(2)H position (the central C between N atoms in the heterocycle) in the assembly motif of the [EMIM][BF4] ionic liquid by analyzing the vibrational spectra of the bare EMIM+ ion as well as that of the cationic [EMIM]2[BF4]+ (EMIM+ = 1-ethyl-3-methylimidazolium, C6H11N2 +) cluster. Vibrational spectra of the cold, mass-selected ions are obtained using cryogenic ion vibrational predissociation of weakly bound D2 molecules formed in a 10 K ion trap. The C(2)H behavior is isolated by following the evolution of key vibrational features when the C(2) hydrogen, the proposed binding location of the anion to the imidazolium ring, is replaced by either deuterium or a methyl group (i.e., in the EMMIM+ analogue). Strong features in the ring CH stretching region of the bare ion are traced to Fermi resonances with overtones of lower frequency modes. Upon incorporation into the EMIM+ ⋅ ⋅ ⋅ BF4 − ⋅ ⋅ ⋅ EMIM+ ternary complex, the C(2)H oscillator strength is dramatically increased, accounting for the much more complicated patterns derived from the EMIM+ ring CH stretches in the light isotopomer, which are strongly suppressed in the deuterated analogue. Further changes in the spectra that occur when the C(2)H is replaced by a methyl group are consistent with BF4 − attachment directly to the imidazolium ring in an arrangement that maximizes the electrostatic interaction between the molecular ions.
    No preview · Article · Feb 2015 · The Journal of Chemical Physics
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    ABSTRACT: The strong temperature dependence of the I⁻∙(H2O)2 vibrational predissociation spectrum is traced to the intracluster dissociation of the ion-bound water dimer into independent water monomers that remain tethered to the ion. The thermodynamics of this process are determined using van't Hoff analysis of key features that quantify the relative populations of H-bonded and independent water molecules. The dissociation enthalpy of the isolated water dimer is thus observed to be reduced by roughly a factor of three upon attachment to the ion. The cause of this reduction is explored with electronic structure calculations of the potential energy profile for dissociation of the dimer, which suggest that both reduction of the intrinsic binding energy and vibrational zero-point effects act to weaken the intermolecular interaction between the water molecules in the first hydration shell. Additional insights are obtained by analyzing how classical trajectories of the I⁻∙(H2O)2 system sample the extended potential energy surface with increasing temperature.
    No preview · Article · Feb 2015 · The Journal of Physical Chemistry A
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    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.
    No preview · Article · Sep 2014 · Science
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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(-).
    No preview · Article · Aug 2014 · The Journal of Chemical Physics
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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.
    No preview · Article · May 2014 · The Journal of Physical Chemistry A
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    Zhou Lin · Anne B McCoy

    Full-text · Dataset · May 2014
  • Source
    Zhou Lin · Anne B. McCoy

    Full-text · Dataset · May 2014
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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.
    No preview · Article · Apr 2014 · The Journal of Physical Chemistry A
  • Anne B McCoy
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    ABSTRACT: The origin of the intensity of the feature in the spectrum of liquid water near 2100 cm(-1) is investigated through calculations of the spectra of water clusters based on low-order expansions of the potential and dipole surfaces in internal and normal mode coordinates. The intensity near 2100 cm(-1) is attributed to combination bands involving the HOH bend and intermolecular vibrations that break the hydrogen bonding network. Further the leading contribution to the intensity reflects large second derivatives of the dipole moment with respect to the internal coordinates that are excited, or electrical anharmonicity. This picture changes if the derivatives of the potential and dipole surfaces are taken with respect to normal modes. In the normal mode representation, the second derivatives of the dipole moment are often vanishingly small, while the mixed third and fourth derivatives of the potential become quite large. Based on this result, mechanical anharmonicity appears to be responsible for the intensity in the 2100 cm(-1) region. This strong dependence of the interpretation of the origins of the intensity in the 2100 cm(-1) region of the water spectrum is investigated and discussed.
    No preview · Article · Apr 2014 · The Journal of Physical Chemistry B
  • 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 ...
    No preview · Article · Mar 2014
  • 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 ...
    No preview · Article · Mar 2014

Publication Stats

3k Citations
653.51 Total Impact Points

Institutions

  • 2015
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 2-2015
    • The Ohio State University
      • Department of Chemistry and Biochemistry
      Columbus, Ohio, United States
  • 2004-2005
    • University of Colorado at Boulder
      • Department of Chemistry and Biochemistry
      Boulder, CO, United States
  • 1996
    • University of Wisconsin–Madison
      Madison, Wisconsin, United States
  • 1993-1995
    • Hebrew University of Jerusalem
      • Fritz Haber Center for Molecular Dynamics Research
      Jerusalem, Jerusalem District, Israel