Dynamics of water and aqueous protons studied using ultrafast multi-dimensional infrared spectroscopy

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.Vita. Cataloged from PDF version of thesis.Includes bibliographical references.Liquid water consists of a highly dynamic network of hydrogen bonds, which evolves on timescales ranging from tens of femtoseconds to a few picoseconds. The fast structural evolution of water's hydrogen bond network is at the heart of numerous fundamental aqueous processes, such as proton transport, solvation, the hydrophobic effect and protein folding. In this thesis, I present our efforts in understanding the dynamics governing hydrogen bond switching and vibrational energy dissipation in water, and the transport of excess protons in strong acid solutions. We use ultrafast nonlinear infrared spectroscopy to study hydrogen bond and proton transfer dynamics in water and acids since vibrational frequencies, intensities and line shapes are closely associated with chemical structure and dynamics. We employed and characterized a new source of ultrafast broadband infrared pulses that span the entire mid-infrared region from 4000 cm-1 down to hundreds of cm-I, with <70 fs pulse duration. We have demonstrated the use of these pulses in studying ultrafast vibrational dynamics in water and aqueous proton transfer dynamics in acids, where broad and feature-less vibrational transitions are present across the mid-infrared. Rearrangements of the hydrogen bond network in liquid water involve rapid switching of hydrogen bonds, which is believed to be a concerted process where a water molecule undergoes large angle molecular reorientation as it exchanges hydrogen-bonding partners. To test this picture of hydrogen bond dynamics, we performed ultrafast 2D IR spectral anisotropy on the OH stretching vibration of HOD in D₂0 to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment within the framework of restricted rotation models. Our results show evidence for concerted motions involving large angular deviations in the molecular dipole when a water molecule evolves from a strained configuration to a stable hydrogen bonded geometry. In addition to its rapidly evolving hydrogen bonding network, the ability of liquid water to efficiently dissipate energy through ultrafast vibrational relaxation processes plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The ability of liquid water to efficiently dissipate energy through ultrafast vibrational relaxation plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The vibrational couplings that govern energy relaxation in liquid H₂0 remain difficult to characterize due to the complex interplay of inter- and intramolecular forces present in the liquid and the limitations of current methods to visualize these ultrafast motions simultaneously. Using ultrafast broadband infrared pulses, we performed 2D IR spectroscopy, pump-probe spectroscopy, and polarization anisotropy of H₂0 by exciting the OH stretching transition in water and characterizing the response of the liquid from 1350-4000 cm-¹ with <70 femtosecond time resolution. These spectra reveal vibrational transitions at all frequencies simultaneous to the excitation, including cross peaks to the bend vibration and a continuum of induced absorptions to previously unobserved combination bands that are not present in linear spectra. These observations provide evidence for strong mixing of inter- and intra-molecular character and delocalization of the vibrations in liquid H20. Unlike traditional weak coupling models, it appears that excitation of OH stretch motion simultaneously drives bending and intermolecular motions as a result of strong anharmonic mixing. These delocalized vibrations, or excitons, have mixed stretch and bend character and evolve over several hundred femtoseconds, giving rise to a complex network of vibrational energy relaxation processes. Protons are known to exhibit very high mobility in water compared to other ions due to the Grotthuss proton hopping mechanism, which describes proton transfer as a process where displacement of charge takes place through breaking and forming of O-H covalent bonds in water. Infrared spectra of strong acids are marked by broad and featureless transitions that span the entire mid-infrared due to a large distribution of rapidly exchanging solvated proton configurations. While previous research efforts have assigned different regions of the mid-infrared spectrum of acids to vibrations of the Eigen (H₉O₄+) and the Zundel (H₅O₂+) limiting structures of the solvated proton, experimental evidence for these assignments in solution and the dynamics that underlie the broad transitions have been largely absent. Using ultrafast broadband infrared pulses in pump-probe and 2D IR spectroscopy, we studied the evolution of frequency correlations across the entire mid-infrared spectrum of concentrated HCl in H₂O. Our results provide experimental evidence for the existence of Eigen and Zundel configurations of the solvated proton in HCl/H₂O with persistence times of at least 0.8 to 1 ps and 250 fs, respectively, and show that fleeting excursions of the proton from the Eigen configuration towards shared configurations are stabilized in less than 100 fs by electric field fluctuations of the solvating water molecules.by Krupa Ramasesha.Ph.D

Ultrafast 2D IR anisotropy of water reveals reorientation during hydrogen-bond switching

Rearrangements of the hydrogen bond network of liquid water are believed to involve rapid and concerted hydrogen bond switching events, during which a hydrogen bond donor molecule undergoes large angle molecular reorientation as it exchanges hydrogen bonding partners. To test this picture of hydrogen bond dynamics, we have performed ultrafast 2D IR spectral anisotropy measurements on the OH stretching vibration of HOD in D[subscript 2]O to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment using restricted rotation models. From the inertial 2D anisotropy decay, we find that water molecules initiating from a strained configuration and relaxing to a stable configuration are characterized by a distribution of angles, with an average reorientation half-angle of 10°, implying an average reorientation for a full switch of ≥20°. These results provide evidence that water hydrogen bond network connectivity switches through concerted motions involving large angle molecular reorientation.United States. Dept. of Energy (Grant DE-FG02-99ER14988)Petroleum Research Fund (Grant 46098-AC6)National Science Foundation (U.S.). Graduate Research Fellowship Progra

Real-Time Probing of Electron Dynamics Using Attosecond Time-Resolved Spectroscopy.

Attosecond science has paved the way for direct probing of electron dynamics in gases and solids. This review provides an overview of recent attosecond measurements, focusing on the wealth of knowledge obtained by the application of isolated attosecond pulses in studying dynamics in gases and solid-state systems. Attosecond photoelectron and photoion measurements in atoms reveal strong-field tunneling ionization and a delay in the photoemission from different electronic states. These measurements applied to molecules have shed light on ultrafast intramolecular charge migration. Similar approaches are used to understand photoemission processes from core and delocalized electronic states in metal surfaces. Attosecond transient absorption spectroscopy is used to follow the real-time motion of valence electrons and to measure the lifetimes of autoionizing channels in atoms. In solids, it provides the first measurements of bulk electron dynamics, revealing important phenomena such as the timescales governing the switching from an insulator to a metallic state and carrier-carrier interactions

Excited State Electronic Structure of Dimethyl Disulfide Involved in Photodissociation at ∼200 nm

Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, serves as a model system for understanding photofragmentation in polypeptides and pro- teins. Prior studies of DMDS photodissociation excited at ∼266 nm and ∼248 nm have elucidated the mechanisms of S-S and C-S bond cleavage, which involve the lowest excited electronic states S1 and S2. Far less is known about the dissociation mechanisms and elec- tronic structure of relevant excited states of DMDS excited at ∼200 nm. Herein we present calculations of the electronic structure and properties of excited states S1-S6 accessed when DMDS is excited at ∼200 nm. Our analysis includes a comparison of theoretical and ex- perimental UV spectra, as well as theoretically predicted one-dimensional cuts through the singlet and triplet potential energy surfaces along the S-S and C-S bond dissociation coordinates. Finally, we present calculations of spin-orbit coupling constants at the Franck- Condon geometry to assess the likelihood of ultrafast intersystem crossing. We show that choosing an accurate yet computationally efficient electronic structure method for calcu- lating the S0-S6 potential energy surfaces along relevant dissociation coordinates is chal- lenging due to excited states with doubly excited character and/or mixed Rydberg-valence character. Our findings demonstrate that the extended multi-state complete active space second-order perturbation theory (XMS-CASPT2) balances this computational efficiency and accuracy, as it captures both the Rydberg character of states in the Franck-Condon region and multiconfigurational character toward the bond-dissociation limits. We com- pare the performance of XMS-CASPT2 to a new variant of equation of motion coupled cluster theory with single, double, and perturbative triple corrections, EOM-CCSD(T)(a)*, finding that EOM-CCSD(T)(a)* significantly improves the treatment of doubly excited states compared to EOM-CCSD, but struggles to quantitatively capture asymptotic ener- gies along bond dissociation coordinates for these states

Mode-Selective Vibrational Energy Transfer Dynamics in 1,3,5-Trinitroperhydro-1,3,5-Triazine (RDX) Thin Films

Herein, we report on the sub-picosecond to sub-nanosecond vibrational energy transfer (VET) dynamics of the solid energetic material 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) using broadband, ultrafast infrared transient absorption spectroscopy. Experiments reveal VET occurring on three distinct timescales: sub-picosecond, 5 ps, and 200 ps. The ultrafast appearance of signal at all probed modes in the mid-infrared suggests strong anharmonic coupling of all vibrations in the solid whereas the long-lived evolution demonstrates that VET is incomplete, and thus thermal equilibrium is not attained, even on the hundred picosecond timescale. Mode-selectivity of the longest dynamics suggests coupling of the N–N and axial NO2 stretching modes with the long-lived, excited phonon bath

Unveiling the UV photofragmentation pathways of dimethyl disulfide initiated by a Rydberg excitation

Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, can serve as a model system for understanding photofragmentation in polypeptides and proteins. Prior studies using ~266 nm and ~248 nm excitation of DMDS have shed light on dissociation pathways involving the lowest excited electronic states (S1), but far less is understood about photodissociation at higher excitation energies. In this work, we characterize the excited states of DMDS with equation of motion coupled cluster theory (EOM-CCSD) and compare computed and experimental UV spectra. Through Natural Transition Orbital analysis of the excited states, we find significant Rydberg character in numerous excited states that are accessed with ~200 nm excitation. One-dimensional potential energy scans along the C-S and S-S bond coordinates reveal novel photodissociation routes resulting from ~200 nm excitation, involving excited state potential energy surfaces S1-S6. Our high-level ab-initio investigation validates and rationalizes previous experimental conclusions, including prompt S-S cleavage observed at ~266 nm, presence of competing C-S and S-S cleavage pathways, and production of excited thiomethoxy radicals after excitation at ~200 nm. Comparative benchmarking of a low cost time-dependent density function theory (TDDFT) method reveals that the CAM-B3LYP-D3 functional with diffuse aug-cc-pVDZ basis reproduces the UV spectrum and one-dimensional potential energy scans computed with EOM-CCSD, enabling its use in future non-adiabatic dynamics calculations. Calculations of spin-orbit coupling constants reveal a high likelihood of ultrafast intersystem crossing, which has not been predicted or reported to date

Proton Transfer in Concentrated Aqueous Hydroxide Visualized using Ultrafast Infrared Spectroscopy

While it is generally recognized that the hydroxide ion can rapidly diffuse through aqueous solution due to its ability to accept a proton from a neighboring water molecule, a description of the OH− solvation structure and mechanism of proton transfer to the ion remains controversial. In this report, we present the results of femtosecond infrared spectroscopy measurements of the O−H stretching transition of dilute HOD dissolved in NaOD/D2O. Pump−probe, photon echo peak shift, and two-dimensional infrared spectroscopy experiments performed as a function of deuteroxide concentration are used to assign spectral signatures that arise from the OH− ion and its solvation shell. A spectral feature that decays on a 110 fs time scale is assigned to the relaxation of transiently formed configurations wherein a proton is equally shared between a HOD molecule and an OD− ion. Over picosecond waiting times, features appear in 2D IR spectra that are indicative of the exchange of population between OH− ions and HOD molecules due to deuteron transfer. The construction of a spectral model that includes spectral relaxation, chemical exchange, and thermalization processes, and self-consistently treats all of our data, allows us to qualitatively explain the results of our experiments and gives a lower bound of 3 ps for the deuteron transfer kinetics.United States. Dept. of Energy (Contract No. DE-FG02-99ER14988)American Chemical Society (Petroleum Research Fund)Henry & Camille Dreyfus FoundationCarlsberg Foundatio

Ultrafast N-H Vibrational Dynamics of Cyclic Doubly Hydrogen-Bonded Homo- and Heterodimers

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen bonded donor and acceptor species (i.e. NH---O → N---HO. Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton transfer dynamics. Here we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low frequency inter-dimer stretch and twist modes and not to interference between multiple Fermi-resonances that dominate the substructure of the linear spectra.Chemistry and Chemical Biolog

A phenomenological approach to modeling chemical dynamics in nonlinear and two-dimensional spectroscopy

We present an approach for calculating nonlinear spectroscopic observables, which overcomes the approximations inherent to current phenomenological models without requiring the computational cost of performing molecular dynamics simulations. The trajectory mapping method uses the semi-classical approximation to linear and nonlinear response functions, and calculates spectra from trajectories of the system's transition frequencies and transition dipole moments. It rests on identifying dynamical variables important to the problem, treating the dynamics of these variables stochastically, and then generating correlated trajectories of spectroscopic quantities by mapping from the dynamical variables. This approach allows one to describe non-Gaussian dynamics, correlated dynamics between variables of the system, and nonlinear relationships between spectroscopic variables of the system and the bath such as non-Condon effects. We illustrate the approach by applying it to three examples that are often not adequately treated by existing analytical models – the non-Condon effect in the nonlinear infrared spectra of water, non-Gaussian dynamics inherent to strongly hydrogen bonded systems, and chemical exchange processes in barrier crossing reactions. The methods described are generally applicable to nonlinear spectroscopy throughout the optical, infrared and terahertz regions.United States. Dept. of Energy (Grant DE-FG02-99ER14988