7 research outputs found
Characterization of Parallel Ī²-Sheets at Interfaces by Chiral Sum Frequency Generation Spectroscopy
Characterization of protein secondary
structures at interfaces
is still challenging due to the limitations of surface-selective optical
techniques. Here, we address the challenge of characterizing parallel
Ī²-sheets by combining chiral sum frequency generation (SFG)
spectroscopy and computational modeling. We focus on human islet amyloid
polypeptide aggregates and a de novo designed short polypeptide at
lipid/water and air/glass interfaces. We find that parallel Ī²-sheets
adopt distinct orientations at various interfaces and exhibit characteristic
chiroptical responses in the amide I and NāH stretch regions.
Theoretical analysis indicates that the characteristic chiroptical
responses provide valuable information on the symmetry, orientation,
and vibrational couplings of parallel Ī²-sheet at interfaces
Beyond Local Group Modes in Vibrational Sum Frequency Generation
We combine deuterium labeling, density
functional theory calculations,
and experimental vibrational sum frequency generation spectroscopy
into a form of ācounterfactual-enabled molecular spectroscopyā
for producing reliable vibrational mode assignments in situations
where local group mode approximations are insufficient for spectral
interpretation and vibrational mode assignments. We demonstrate the
method using <i>trans</i>-Ī²-isoprene epoxydiol (<i>trans</i>-Ī²-IEPOX), a first-generation product of isoprene
relevant to atmospheric aerosol formation, and one of its deuterium-labeled
isotopologues at the vapor/silica interface. We use our method to
determine that the SFG responses that we obtain from <i>trans</i>-Ī²-IEPOX are almost exclusively due to nonlocal modes involving
multiple CāH groups oscillating at the same frequency as one
vibrational mode. We verify our assignments using deuterium labeling
and use DFT calculations to predict SFG spectra of additional isotopologues
that have not yet been synthesized. Finally, we use our new insight
to provide a viable alternative to molecular orientation analysis
methods that rely on local mode approximations in cases where the
local mode approximation is not applicable
Surface-Induced Anisotropic Binding of a Rhenium CO<sub>2</sub>āReduction Catalyst on Rutile TiO<sub>2</sub>(110) Surfaces
Vibrational
sum frequency generation (SFG) spectroscopy has been
utilized to study the spatial orientation and alignment of ReĀ(CO)<sub>3</sub>ClĀ(dcbpy) (dcbpy = 4,4ā²-dicarboxy-2,2ā²-bipyridine)
(or ReC0A) on the (001) and (110) surfaces of rutile single-crystalline
TiO<sub>2</sub>. The SFG intensity of the CO stretching modes shows
an isotropic distribution on the (001) surface and an anisotropic
distribution on the (110) surfaces with respect to the in-plane rotation
of the crystal relative to the surface normal (or the incident laser
beam plane). By combining these results with ab initio SFG simulations
and with modeling of ReC0AāTiO<sub>2</sub> cluster binding
structures at the density functional theory level, we reveal that
the origin of the optical anisotropy for ReC0A on the TiO<sub>2</sub>(110) surface is associated with the binding preference of ReC0A
along the [ā110] axis. Along this direction, the binding structure
is energetically favorable, because of the formation of proper hydrogen
bonding between the carboxylate group and passivating water molecules
adsorbed on the TiO<sub>2</sub>(110) surface. Simulations of dimers
of ReC0A molecules binding close together with full nearest-neighbor
effects give a structure that reproduces the experimental SFG polar
plot. The tilt angle, defined by the bpy ring angle relative to the
surface normal, of the catalyst is found to be 26Ā° for one monomer
and 18Ā° for the other, which corresponds to an aggregate at high
surface coverage
Assessment of DFT for Computing Sum Frequency Generation Spectra of an Epoxydiol and a Deuterated Isotopologue at Fused Silica/Vapor Interfaces
We
assess the capabilities of eight popular density functional
theory (DFT) functionals, in combination with several basis sets,
as applied to calculations of vibrational sum frequency generation
(SFG) spectra of the atmospherically relevant isoprene oxidation product <i>trans</i>-Ī²-isoprene epoxydiol (IEPOX) and one of its
deuterated isotopologues at the fused silica/vapor interface. We use
sum of squared differences (SSD) and total absolute error (TAE) calculations
to estimate the performance of each functional/basis set combination
in producing SFG spectra that match experimentally obtained spectra
from <i>trans</i>-Ī²-IEPOX and one of its isotopologues.
Our joined SSD/TAE analysis shows that while the twist angle of the
methyl <i>C</i><sub>3<i>v</i></sub> symmetry axis
of <i>trans</i>-Ī²-IEPOX relative to the surface is
sensitive to the choice of DFT functional, the calculated tilt angle
relative to the surface normal is largely independent of the functional
and basis set. Moreover, we report that hybrid functionals such as
B3LYP, ĻB97X-D, PBE0, and B97-1 in combination with a modest
basis set, such as 6-311GĀ(d,p), provides good agreement with experimental
data and much better performance than pure functionals such as PBE
and BP86. However, improving the quality of the basis set only improves
agreement with experimental data for calculations based on pure functionals.
A conformational analysis, based on comparisons of calculated and
experimental SFG spectra, suggests that <i>trans</i>-Ī²-IEPOX
points all of its oxygen atoms toward the silica/vapor interface
Sum Frequency Generation Spectroscopy and Molecular Dynamics Simulations Reveal a Rotationally Fluid Adsorption State of Ī±āPinene on Silica
A rotationally
fluid state of Ī±-pinene at fused silica/vapor
interfaces is revealed by computational and experimental vibrational
sum frequency generation (SFG) studies. We report the first assignment
of the vibrational modes in the notoriously congested CāH stretching
region of Ī±-pinene and identify its bridge methylene group on
the four-membered ring (ā<sup>Ī²</sup>CH<sub>2</sub>ā)
as the origin of its dominant spectral feature. We find that the spectra
are perfused with Fermi resonances that need to be accounted for explicitly
in the computation of vibrational spectra of strained hydrocarbons
such as Ī±-pinene. The preferred orientations of Ī±-pinene
are consistent with optimization of van der Waals contacts with the
silica surface that results in a bimodal distribution of highly fluxional
orientations in which the <sup>Ī²</sup>CH<sub>2</sub> group points
ātowardsā or āaway fromā the surface.
Classical molecular dynamics simulations further provide rotational
diffusion constants of 49 Ā± 1 ps and 2580 Ā± 60 ps, which
are attributed to two broad types of adsorption modes on silica. The
reported findings are particularly relevant to the exposure of Ī±-pinene
to primary oxidants in heterogeneous catalytic pathways that exploit
Ī±-pinene as a sustainable feedstock for fine chemicals and polymers
Orientation of Cyano-Substituted Bipyridine Re(I) <i>fac</i>-Tricarbonyl Electrocatalysts Bound to Conducting Au Surfaces
Sum frequency generation spectroscopy
(SFG) and calculations of
SFG spectra based on density functional theory are combined to elucidate
the orientation of two ReĀ(R-2,2ā²-bipyridine)Ā(CO)<sub>3</sub>Cl (R = 4-cyano or 4,4ā²-dicyano) electrocatalysts when adsorbed
on conductive gold surfaces. We find that the electrocatalysts lean
on the Au surface to orient the plane of the bipyridine ligand at
63Ā° relative to the surface normal. While the weak binding of
the complexes to the gold surface precluded the ability to perform
surface immobilized catalysis, homogeneous electrochemical experiments
show that the molecular catalysts are active toward the reduction
of CO<sub>2</sub> to CO and carbonate in the triply reduced state
(TOF of 13.3 and 7.2 s<sup>ā1</sup> for the doubly and singly
substituted complexes, respectively). These findings demonstrate the
capabilities of the approach of including rigorous spectroscopic and
theoretical methods for revealing the conformation and orientation
of CO<sub>2</sub> reduction catalysts bound to electrode surfaces,
which are critical considerations for redox state transitions and
catalytic turnover
Accurate Line Shapes from Subā1 cm<sup>ā1</sup> Resolution Sum Frequency Generation Vibrational Spectroscopy of Ī±āPinene at Room Temperature
Despite the importance of terpenes
in biology, the environment,
and catalysis, their vibrational spectra remain unassigned. Here,
we present subwavenumber high-resolution broad-band sum frequency
generation (HR-BB-SFG) spectra of the common terpene (+)-Ī±-pinene
that reveal 10 peaks in the CāH stretching region at room temperature.
The high spectral resolution resulted in spectra with more and better
resolved spectral features than those of the Fourier transform infrared,
femtosecond stimulated Raman spectra in the bulk condensed phase and
those of the conventional BB-SFG and scanning SFG spectroscopy of
the same molecule on a surface. Experiment and simulation show the
spectral line shapes with HR-BB-SFG to be accurate. Homogeneous vibrational
decoherence lifetimes of up to 1.7 ps are assigned to specific oscillators
and compare favorably to lifetimes computed from density functional
tight binding molecular dynamics calculations. Phase-resolved spectra
provided their orientational information. We propose the new spectroscopy
as an attractive alternative to time domain vibrational spectroscopy
or heterodyne detection schemes for studying vibrational energy relaxation
and vibrational coherences in molecules at molecular surfaces or interfaces