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
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++GÂ(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
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
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