TRIGGERING INTRA-CLUSTER ELECTRON CAPTURE WITH VIBRATIONAL EXCITATION: AN IR STUDY OF THE CH3_3NO2_2(H2_2O)6_6 ANION

Author Institution: Sterling Chemistry Laboratory, Yale Universtiy, PO Box 208107, New Haven, CT 06520Nitromethane (NM), the simplest of the nitro-containing organic molecules, possesses a large dipole moment of 3.46 D nderline{\textbf{105}} (9), September 1996.}. Nagata and co-workers nderline{\textbf{130}} (22), June 2009.} have demonstrated that Ar-mediated condensation of NM can trap a significant fraction of the collision complexes in a situation where the electron is retained in a diffuse ??hydrated electron ? configuration. This raises the possibility of triggering the electron capture onto NM to form the NM$^-$ radical anion, releasing the substantial exothermicity of the reaction by evaporation of water molecules and allows the barrier to evaporation to be probed. We report vibrational predissociation spectra of both the product NM$^-$(H$_2$O)$_n$ anions as well as the high energy species that features a diffuse electron cloud. Interestingly, the spectra indicate that the high energy isomer has a neutral NM moiety and occurs with the same spectral signature of the excess electron binding site as that in the isolated water hexamer anion, indicating that it is attached in a position remote from the charge. Detailed comparison of the C-H and N-O stretching regions suggests that the reactive isomer occurs with the NM molecule attached to the backside of the water network via accepting H-bonds rather than attachment of the methyl group to the electron cloud, where the NM and water network would share the electron cloud

SPECTROSCOPIC INTERROGATION OF PHOTOINDUCED, SITE-TO-SITE MIGRATION OF SOLVENT MOLECULES IN WATER CLUSTER ANIONS

Author Institution: Sterling Chemistry Laboratory, Yale University, PO Box 208107, New Haven, CT 06520We present results of a new trace isotope variation of our recently developed experimental approach in which we use IR-IR pump-probe methods to measure the transition states and relative energies of isomers associated with the negatively charged water clusters. This modification, in which clusters of the form [(D$_2$O)$_6$\cdot\textrm{H}$_2$O]$^-$\cdot\textrm{Ar}$_{n}$ are produced, allows us the powerful ability to monitor discrete, site-to-site migration of a lone water molecule as cluster melting and refreezing occurs. This technique works by first systematically disentangling the vibrational spectra of various isotopomers using hole-burning Ar predissociation spectroscopy in a triple-stage time-of-flight mass spectrometer and then monitoring the spectra of fragment ions that are created by photoevaporation of Ar atoms through the various vibrational levels identified in the spectroscopic step

USING AN ORGANIC SCAFFOLD TO MODULATE THE QUANTUM STRUCTURE OF AN INTRAMOLECULAR PROTON BOND: CRYOGENIC VIBRATIONAL PREDISSOCIATION SPECTROSCOPY OF H2_{2} ON PROTONATED 8-NAPHTHALENE-1-AMINE

Author Institution: Sterling Chemistry, Yale University, New Haven, Ct, 06520; DEPARTMENT OF CHEMISTRY, JOHNS HOPKINS UNIVERSITY, 3400 NORTH CHARLES STREET, BALTIMORE, MD, 21218The quantum structure of the intermolecular proton bond is a key aspect in understanding proton transfer events that govern the efficiency of fuel cells and various biological membranes. Previously, we have constructed a soft binding motif, that consists of a "point contact" between the lone pairs of two small molecules (combinations of ethers, alcohols, ammonia, and water) that are linked by a shared proton [\textit{Science} 2007, \textit{613}, 249]. Although the frequency of the shared proton vibration has been correlated with effects of acid and base structure, such as proton affinities and dipole moments, the spatial arrangement of the proton donor and acceptor remains unexplored. Towards this aim, we have obtained a molecule of rigid topology that contains a proton donor and acceptor capable of intramolecular proton-bonding (protonated 8-flouronaphthalene-1-amine). Using electrospray ionization coupled with a novel cryogenic mass spectrometry scheme, we employ vibrational predissociation spectroscopy of H$_{2}$ tagged ions to elucidate how a forced spatial configuration of the acid and base perturbs the energetics of the proton bond

Isolating the Spectral Signatures of Individual Sites in Water Networks Using Vibrational Double-Resonance Spectroscopy of Cluster Isotopomers

We report the spectral signatures of water molecules occupying individual sites in an extended H-bonding network using mass-selective, double-resonance vibrational spectroscopy of isotopomers. The scheme is demonstrated on the water heptamer anion, (H₂O)₇¯, where we first randomly incorporate a single, intact D₂O molecule to create an ensemble of isotopomers. The correlation between the two OD stretching frequencies and that of the intramolecular DOD bending transition is then revealed by photochemical modulation of the isotopomer population responsible for particular features in the vibrational spectrum. The observed patterns confirm the assignment of the dominant doublet, appearing most red-shifted from the free OD stretch, to a single water molecule attached to the network in a double H-bond acceptor (AA) arrangement. The data also reveal the unanticipated role of accidentally overlapping transitions, where the highest-energy OD stretch, for example, occurs with its companion OD stretch obscured by the much stronger AA feature

EXPLORING THE VIBRATIONAL STRUCTURE OF THE VINYLIDENE ANION USING ARGON PREDISSOCIATION SPECTROSCOPY

Author Institution: Sterling Chemistry Laboratory, Yale Universtiy, PO Box 208107, New Haven, CT 06520We report Ar-mediated vibrational spectra of the vinylidene anion, a relevant intermediate in various chemical processes, and its fully deuterated form in order to characterize the vibrational energy levels present in this species. Identification of the C-H asymmetric and symmetric stretching frequencies was made and confirmed by the deuterium isotope shift. This information could then be used to clarify the origin of two higher energy peaks around 4000 and 4200 cm^-^1 in the light isotope, which occur quite close to the photodetachment threshold. Preliminary analysis indicates their assignment to combination bands involving excitation of the C=C stretch along with the C-H fundamentals. The work was then extended to include the NNO molecule as a messenger species

Bottom-Up View of Water Network-Mediated CO₂ Reduction Using Cryogenic Cluster Ion Spectroscopy and Direct Dynamics Simulations

The transition states of a chemical reaction in solution are generally accessed through exchange of thermal energy between the solvent and the reactants. As such, an ensemble of reacting systems approaches the transition state configuration of reactant and surrounding solvent in an incoherent manner that does not lend itself to direct experimental observation. Here we describe how gas-phase cluster chemistry can provide a detailed picture of the microscopic mechanics at play when a network of six water molecules mediates the trapping of a highly reactive “hydrated electron” onto a neutral CO₂ molecule to form a radical anion. The exothermic reaction is triggered from a metastable intermediate by selective excitation of either the reactant CO₂ or the water network, which is evidenced by the evaporative decomposition of the product cluster. Ab initio molecular dynamics simulations of energized CO₂·(H₂O)₆^– clusters are used to elucidate the nature of the network deformations that mediate intracluster electron capture, thus revealing the detailed solvent fluctuations implicit in the Marcus theory for electron-transfer kinetics in solution

Transition Metal Associations with Primary Biological Particles in Sea Spray Aerosol Generated in a Wave Channel

In the ocean, breaking waves generate airbubbles which burst at the surface and eject sea spray aerosol(SSA), consisting of sea salt, biogenic organic species, andprimary biological aerosol particles (PBAP). Our overallunderstanding of atmospheric biological particles of marineorigin remains poor. Here, we perform a control experiment,using an aerosol time-of-flight mass spectrometer to measurethe mass spectral signatures of individual particles generated bybubbling a salt solution before and after addition ofheterotrophic marine bacteria. Upon addition of bacteria, animmediate increase occurs in the fraction of individual particlemass spectra containing magnesium, organic nitrogen, andphosphate marker ions. These biological signatures areconsistent with 21% of the supermicrometer SSA particles generated in a previous study using breaking waves in an oceanatmospherewave channel. Interestingly, the wave flume mass spectral signatures also contain metal ions including silver, iron,and chromium. The nascent SSA bioparticles produced in the wave channel are hypothesized to be as follows: (1) whole orfragmented bacterial cells which bioaccumulated metals and/or (2) bacteria-derived colloids or biofilms which adhered to themetals. This study highlights the potential for transition metals, in combination with specific biomarkers, to serve as uniqueindicators for the presence of marine PBAP, especially in metal-impacted coastal regions