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  1. Article ; Online: Nuclear quantum effects in gas-phase 2-fluoroethanol.

    Arandhara, Mrinal / Ramesh, Sai G

    Physical chemistry chemical physics : PCCP

    2024  Volume 26, Issue 8, Page(s) 6885–6902

    Abstract: Torsional motions along the FCCO and HOCC dihedrals lead to the five unique conformations of 2-fluoroethanol, of which the conformer that ... ...

    Abstract Torsional motions along the FCCO and HOCC dihedrals lead to the five unique conformations of 2-fluoroethanol, of which the conformer that is
    Language English
    Publishing date 2024-02-22
    Publishing country England
    Document type Journal Article
    ZDB-ID 1476244-4
    ISSN 1463-9084 ; 1463-9076
    ISSN (online) 1463-9084
    ISSN 1463-9076
    DOI 10.1039/d3cp05657k
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  2. Article ; Online: Nuclear Quantum Effects in Hydroxide Hydrate Along the H-Bond Bifurcation Pathway.

    Arandhara, Mrinal / Ramesh, Sai G

    The journal of physical chemistry. A

    2024  Volume 128, Issue 9, Page(s) 1600–1610

    Abstract: Path integral (PI) simulations are used to explore nuclear quantum effects (NQEs) in hydroxide hydrate and its perdeuterated isotopomer along the H-bond bifurcation pathway. Toward this, a new potential energy surface using the symmetric gradient domain ... ...

    Abstract Path integral (PI) simulations are used to explore nuclear quantum effects (NQEs) in hydroxide hydrate and its perdeuterated isotopomer along the H-bond bifurcation pathway. Toward this, a new potential energy surface using the symmetric gradient domain machine learning method with ab initio data at the CCSD(T)/aug-cc-pVTZ level is built. From PI umbrella sampling (US) simulations, free energy profiles along the bifurcation coordinate are explored as a function of temperature. At ambient temperature, the bifurcation barrier is increased upon inclusion of NQEs. At low temperatures in the deep tunneling regime, the barrier is strongly decreased and flattened. These trends are examined, and the role of the O-O distance is also investigated through two-dimensional US simulations.
    Language English
    Publishing date 2024-02-23
    Publishing country United States
    Document type Journal Article
    ISSN 1520-5215
    ISSN (online) 1520-5215
    DOI 10.1021/acs.jpca.3c08027
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  3. Article ; Online: Multidimensional H-atom tunneling in the catecholate monoanion.

    Bhattacharyya, Debabrata / Ramesh, Sai G

    Physical chemistry chemical physics : PCCP

    2022  Volume 24, Issue 18, Page(s) 10887–10905

    Abstract: We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with ... ...

    Abstract We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with different strengths. With a view to studying mode-specific tunneling in the catecholate monoanion, we have developed a full (33) dimensional potential energy surface in transition state (TS) normal modes using a Distributed Gaussian Empirical Valence Bond (DGEVB) based approach. We have computed eigenstates in different subspaces using both unrelaxed and relaxed potentials based on the DGEVB model. With unrelaxed potentials, we present results up to 7D subspaces that include the imaginary frequency mode and six modes coupled to it. With relaxed potentials, we focus on the two most important coupling modes. The structures of the ground and vibrationally excited eigenstates are discussed for both approaches and mode-specific tunneling splitting and their trends are presented.
    Language English
    Publishing date 2022-05-11
    Publishing country England
    Document type Journal Article
    ZDB-ID 1476244-4
    ISSN 1463-9084 ; 1463-9076
    ISSN (online) 1463-9084
    ISSN 1463-9076
    DOI 10.1039/d1cp04590c
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  4. Article ; Online: Excited-state proton transfer in a 2-aminopyridine dimer: a surface hopping study.

    Ray, Jyotirmoy / Ramesh, Sai G

    Physical chemistry chemical physics : PCCP

    2022  Volume 24, Issue 12, Page(s) 7274–7292

    Abstract: We present a theoretical investigation of the excited-state intermolecular proton transfer process in a 2-aminopyridine dimer. Previous experimental and theoretical studies on this doubly hydrogen bonded system have attributed an ultrafast 50 fs ... ...

    Abstract We present a theoretical investigation of the excited-state intermolecular proton transfer process in a 2-aminopyridine dimer. Previous experimental and theoretical studies on this doubly hydrogen bonded system have attributed an ultrafast 50 fs timescale to the process at low excitation wavelengths and have shown that it involves access to the charge transfer (CT) states of the dimer. We have carried out a trajectory-based surface hopping study of the proton transfer process. To this end, we have further studied the key intersections between locally excited (LE) and CT states that facilitate the proton transfer as well as the eventual ground state return at the XMS-CASPT2 level of theory. The dynamical simulations to investigate the charge transfer-driven event are performed at both the XMS-CASPT2 and TDDFT levels of theory. Trajectories are initiated from the excited states that are either already of CT character or become so upon a short extension of the NH bond. This kind of dynamics is found to be ultrafast with a timescale of about 100 fs, where the dimer rapidly accesses the LE/CT intersection regions en route to single proton transfer. After the transfer, some trajectories are also able to reach the ground state as well through a non-adiabatic transition. In contrast, trajectories that are initiated on an LE state remain on states of that character and do not show proton transfer. However, additionally providing 3 or 4 quanta of initial excitation to the NH stretch was found to promote CT state-driven proton transfer in the dimer.
    Language English
    Publishing date 2022-03-23
    Publishing country England
    Document type Journal Article
    ZDB-ID 1476244-4
    ISSN 1463-9084 ; 1463-9076
    ISSN (online) 1463-9084
    ISSN 1463-9076
    DOI 10.1039/d1cp05517h
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  5. Article ; Online: N-H photodissociation dynamics of electronically excited aniline: a three dimensional time-dependent quantum wavepacket study.

    Ray, Jyotirmoy / Ramesh, Sai G

    Physical chemistry chemical physics : PCCP

    2021  

    Abstract: We have simulated the dynamics of 1πσ* state-mediated nonadiabatic N-H bond dissociation in photo-excited aniline (C6H5NH2). A three electronic state diabatic model potential, involving the ground, 1ππ*, and 1πσ* diabatic states, and focussing on the NH2 ...

    Abstract We have simulated the dynamics of 1πσ* state-mediated nonadiabatic N-H bond dissociation in photo-excited aniline (C6H5NH2). A three electronic state diabatic model potential, involving the ground, 1ππ*, and 1πσ* diabatic states, and focussing on the NH2 degrees of freedom alone is constructed using XMS-CASPT2 energies. Using a kinetic energy operator in the polyspherical framework, wavepacket dynamics in three vibrational modes, viz. NH stretch, NH2 out-of-plane wag and torsion, is carried out using the Chebyshev propagation scheme. For optically bright 1ππ* excitation, the wavepacket can access the 1πσ*/1ππ* and 1ππ/1πσ* conical intersections that lie en route to dissociation. For both intersections, NH2 out-of-plane wag and torsional motions are the most dominant coupling coordinates. Carrying out dynamics with initial wavepackets varying in excitation in the three degrees of freedom, we probe their roles in the evolution of the state populations, probability densities, and product branching for the NH dissociation process.
    Language English
    Publishing date 2021-07-19
    Publishing country England
    Document type Journal Article
    ZDB-ID 1476244-4
    ISSN 1463-9084 ; 1463-9076
    ISSN (online) 1463-9084
    ISSN 1463-9076
    DOI 10.1039/d1cp01990b
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  6. Article ; Online: Assessing Permutationally Invariant Polynomial and Symmetric Gradient Domain Machine Learning Potential Energy Surfaces for H

    Pandey, Priyanka / Arandhara, Mrinal / Houston, Paul L / Qu, Chen / Conte, Riccardo / Bowman, Joel M / Ramesh, Sai G

    The journal of physical chemistry. A

    2024  Volume 128, Issue 16, Page(s) 3212–3219

    Abstract: The singly hydrated hydroxide anion ... ...

    Abstract The singly hydrated hydroxide anion OH
    Language English
    Publishing date 2024-04-16
    Publishing country United States
    Document type Journal Article
    ISSN 1520-5215
    ISSN (online) 1520-5215
    DOI 10.1021/acs.jpca.4c01044
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  7. Article ; Online: Alkyl hydrogen atom abstraction reactions of the CN radical with ethanol.

    Athokpam, Bijyalaxmi / Ramesh, Sai G

    The Journal of chemical physics

    2018  Volume 148, Issue 13, Page(s) 134503

    Abstract: We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the ... ...

    Abstract We present a study of the abstraction of alkyl hydrogen atoms from the β and α positions of ethanol by the CN radical in solution using the Empirical Valence Bond (EVB) method. We have built separate 2 × 2 EVB models for the H
    Language English
    Publishing date 2018-04-07
    Publishing country United States
    Document type Journal Article
    ZDB-ID 3113-6
    ISSN 1089-7690 ; 0021-9606
    ISSN (online) 1089-7690
    ISSN 0021-9606
    DOI 10.1063/1.5021634
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  8. Article: A multi-sheeted three-dimensional potential-energy surface for the H-atom photodissociation of phenol.

    Ramesh, Sai G / Domcke, Wolfgang

    Faraday discussions

    2013  Volume 163, Page(s) 73–94; discussion 117–38

    Abstract: Extending the previous work of Lan et al. [J. Chem. Phys., 122, 224315 (2005)], a multi-state potential model for the H atom photodissociation is presented. All three "disappearing coordinates" of the departing H atom have been considered. Ab initio ... ...

    Abstract Extending the previous work of Lan et al. [J. Chem. Phys., 122, 224315 (2005)], a multi-state potential model for the H atom photodissociation is presented. All three "disappearing coordinates" of the departing H atom have been considered. Ab initio CASSCF computations have been carried out for the linear COH geometry of C(2v) symmetry, and for several COH angles with the OH group in the ring plane and also perpendicular to the ring plane. By keeping the C6H5O fragment frozen in a C(2v)-constrained geometry throughout, we have been able to apply symmetry-based simplifications in the constructions of adiabatic model. This model is able to capture the overall trends of twelve adiabats at both torsional limits for a wide range of COH bend angles.
    Language English
    Publishing date 2013-08-27
    Publishing country England
    Document type Journal Article
    ISSN 1359-6640
    ISSN 1359-6640
    DOI 10.1039/c3fd00006k
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  9. Article ; Online: Isotopic fractionation in proteins as a measure of hydrogen bond length.

    McKenzie, Ross H / Athokpam, Bijyalaxmi / Ramesh, Sai G

    The Journal of chemical physics

    2015  Volume 143, Issue 4, Page(s) 44309

    Abstract: If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for ... ...

    Abstract If a deuterated molecule containing strong intramolecular hydrogen bonds is placed in a hydrogenated solvent, it may preferentially exchange deuterium for hydrogen. This preference is due to the difference between the vibrational zero-point energy for hydrogen and deuterium. It is found that the associated fractionation factor Φ is correlated with the strength of the intramolecular hydrogen bonds. This correlation has been used to determine the length of the H-bonds (donor-acceptor separation) in a diverse range of enzymes and has been argued to support the existence of short low-barrier H-bonds. Starting with a potential energy surface based on a simple diabatic state model for H-bonds, we calculate Φ as a function of the proton donor-acceptor distance R. For numerical results, we use a parameterization of the model for symmetric O-H⋯O bonds [R. H. McKenzie, Chem. Phys. Lett. 535, 196 (2012)]. We consider the relative contributions of the O-H stretch vibration, O-H bend vibrations (both in plane and out of plane), tunneling splitting effects at finite temperature, and the secondary geometric isotope effect. We compare our total Φ as a function of R with NMR experimental results for enzymes, and in particular with an earlier model parametrization Φ(R), used previously to determine bond lengths.
    MeSH term(s) Deuterium/chemistry ; Hydrogen/chemistry ; Hydrogen Bonding ; Magnetic Resonance Spectroscopy ; Proteins/chemistry ; Solvents/chemistry ; Thermodynamics
    Chemical Substances Proteins ; Solvents ; Hydrogen (7YNJ3PO35Z) ; Deuterium (AR09D82C7G)
    Language English
    Publishing date 2015-07-28
    Publishing country United States
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 3113-6
    ISSN 1089-7690 ; 0021-9606
    ISSN (online) 1089-7690
    ISSN 0021-9606
    DOI 10.1063/1.4927391
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  10. Article ; Online: Effect of quantum nuclear motion on hydrogen bonding.

    McKenzie, Ross H / Bekker, Christiaan / Athokpam, Bijyalaxmi / Ramesh, Sai G

    The Journal of chemical physics

    2014  Volume 140, Issue 17, Page(s) 174508

    Abstract: This work considers how the properties of hydrogen bonded complexes, X-H⋯Y, are modified by the quantum motion of the shared proton. Using a simple two-diabatic state model Hamiltonian, the analysis of the symmetric case, where the donor (X) and acceptor ...

    Abstract This work considers how the properties of hydrogen bonded complexes, X-H⋯Y, are modified by the quantum motion of the shared proton. Using a simple two-diabatic state model Hamiltonian, the analysis of the symmetric case, where the donor (X) and acceptor (Y) have the same proton affinity, is carried out. For quantitative comparisons, a parametrization specific to the O-H⋯O complexes is used. The vibrational energy levels of the one-dimensional ground state adiabatic potential of the model are used to make quantitative comparisons with a vast body of condensed phase data, spanning a donor-acceptor separation (R) range of about 2.4-3.0 Å, i.e., from strong to weak hydrogen bonds. The position of the proton (which determines the X-H bond length) and its longitudinal vibrational frequency, along with the isotope effects in both are described quantitatively. An analysis of the secondary geometric isotope effect, using a simple extension of the two-state model, yields an improved agreement of the predicted variation with R of frequency isotope effects. The role of bending modes is also considered: their quantum effects compete with those of the stretching mode for weak to moderate H-bond strengths. In spite of the economy in the parametrization of the model used, it offers key insights into the defining features of H-bonds, and semi-quantitatively captures several trends.
    Language English
    Publishing date 2014-05-07
    Publishing country United States
    Document type Journal Article
    ZDB-ID 3113-6
    ISSN 1089-7690 ; 0021-9606
    ISSN (online) 1089-7690
    ISSN 0021-9606
    DOI 10.1063/1.4873352
    Database MEDical Literature Analysis and Retrieval System OnLINE

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