3 research outputs found

    Local Mode Approach to OH Stretch Spectra of Benzene–(H<sub>2</sub>O)<sub><i>n</i></sub> Clusters, <i>n</i> = 2–7

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    Isomer-specific resonant ion-dip infrared spectra are presented for benzene–water (Bz–(H<sub>2</sub>O)<sub><i>n</i></sub>) clusters with two to seven water molecules. Local mode Hamiltonians based on scaled M06-2X/6-311++G­(2d,p) density functional calculations are presented that accurately model the spectra across the entire OH stretch region (3000–3750 cm<sup>–1</sup>). The model Hamiltonians include the contribution from the water bend overtone and an empirical parameter for the local OH stretch–bend Fermi coupling. The inclusion of this coupling is necessary for accurate modeling of the infrared spectra of clusters with more than three water molecules. For the cyclic water clusters (<i>n</i> = 3–5), the benzene molecule perturbs the system in a characteristic way, distorting the cycle, splitting degeneracies, and turning on previously forbidden transitions. The local OH stretch site frequencies and H···OH hydrogen bond lengths follow a pattern based on the each water monomer’s proximity to benzene. The patterns observed for these cyclic water clusters provide insight into benzene’s effects on the three-dimensional hydrogen-bonded networks present in water hexamer and heptamer structures, which also have their spectra dramatically altered from their pure water counterparts

    Infrared-Enhanced Fluorescence-Gain Spectroscopy: Conformation-Specific Excited-State Infrared Spectra of Alkylbenzenes

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    An ultraviolet–infrared (UV-IR) double-resonance method for recording conformation-specific excited-state infrared spectra is described. The method takes advantage of an increase in fluorescence signal in phenylalkanes produced by infrared excitation of the S<sub>1</sub> origin levels of different conformational isomers. The shorter lifetimes of these IR-excited molecules, combined with their red-shifted emission, provides a way to discriminate the fluorescence due to the infrared-excited molecules from the S<sub>1</sub> origin fluorescence, resulting in spectra with high signal-to-noise ratios. Spectra for a series of phenylalkanes and a capped phenylalanine derivative (Ac-Phe-NHMe) demonstrate the potential of the method. The excited-state spectrum in the alkyl CH stretch region of ethylbenzene is well-fit by an anharmonic model developed for the ground electronic state, which explicitly takes into account stretch–bend Fermi resonance

    Isomer-Specific Spectroscopy of Benzene–(H<sub>2</sub>O)<sub><i>n</i></sub>, <i>n</i> = 6,7: Benzene’s Role in Reshaping Water’s Three-Dimensional Networks

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    The water hexamer and heptamer are the smallest sized water clusters that support three-dimensional hydrogen-bonded networks, with several competing structures that could be altered by interactions with a solute. Using infrared–ultraviolet double resonance spectroscopy, we record isomer-specific OH stretch infrared spectra of gas-phase benzene-(H<sub>2</sub>O)<sub>6,7</sub> clusters that demonstrate benzene’s surprising role in reshaping (H<sub>2</sub>O)<sub>6,7</sub>. The single observed isomer of benzene-(H<sub>2</sub>O)<sub>6</sub> incorporates an inverted book structure rather than the cage or prism. The main conformer of benzene-(H<sub>2</sub>O)<sub>7</sub> is an inserted-cubic structure in which benzene replaces one water molecule in the <i>S</i><sub>4</sub>-symmetry cube of the water octamer, inserting itself into the water cluster by engaging as a π H-bond acceptor with one water and via CH···O donor interactions with two others. The corresponding <i>D</i><sub>2d</sub>-symmetry inserted-cube structure is not observed, consistent with the calculated energetic preference for the <i>S</i><sub>4</sub> over the <i>D</i><sub>2d</sub> inserted cube. A reduced-dimension model that incorporates stretch–bend Fermi resonance accounts for the spectra in detail and sheds light on the hydrogen-bonding networks themselves and on the perturbations imposed on them by benzene
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