7 research outputs found
Modeling Coherent Anti-Stokes Raman Scattering with Time-Dependent Density Functional Theory: Vacuum and Surface Enhancement.
We present the first density functional simulations of coherent anti-Stokes Raman scattering (CARS) and an analysis of the chemical effects upon binding to a metal surface. Spectra are obtained from first-principles electronic structure calculations and are compared with available experiments and previously available theoretical results following from HartreeâFock polarizability derivatives. A first approximation to the nonresonant portion of the CARS signal is also explored. We examine the silver pyridine cluster models of the surface chemical signal enhancement, previously introduced for surface-enhanced Raman scattering. Chemical resonant intensity enhancements of roughly 10<sup>2</sup> are found for several model clusters. The prospects of realizing further enhancement of CARS signal with metal surfaces is discussed in light of the predicted chemical enhancements
Complex Chemical Reaction Networks from Heuristics-Aided Quantum Chemistry
While
structures and reactivities of many small molecules can be
computed efficiently and accurately using quantum chemical methods,
heuristic approaches remain essential for modeling complex structures
and large-scale chemical systems. Here, we present a heuristics-aided
quantum chemical methodology applicable to complex chemical reaction
networks such as those arising in cell metabolism and prebiotic chemistry.
Chemical heuristics offer an expedient way of traversing high-dimensional
reactive potential energy surfaces and are combined here with quantum
chemical structure optimizations, which yield the structures and energies
of the reaction intermediates and products. Application of heuristics-aided
quantum chemical methodology to the formose reaction reproduces the
experimentally observed reaction products, major reaction pathways,
and autocatalytic cycles
Excitonics: A Set of Gates for Molecular Exciton Processing and Signaling
Regulating energy transfer pathways
through materials is a central goal of nanotechnology, as a greater
degree of control is crucial for developing sensing, spectroscopy,
microscopy, and computing applications. Such control necessitates
a toolbox of actuation methods that can direct energy transfer based
on user input. Here we introduce a proposal for a molecular exciton
gate, analogous to a traditional transistor, for regulating exciton
flow in chromophoric systems. The gate may be activated with an input
of light or an input flow of excitons. Our proposal relies on excitation
migration <i>via</i> the second excited singlet (S<sub>2</sub>) state of the gate molecule. It exhibits the following features,
only a subset of which are present in previous exciton switching schemes:
picosecond time scale actuation, amplification/gain behavior, and
a lack of molecular rearrangement. We demonstrate that the device
can be used to produce universal binary logic or amplification of
an exciton current, providing an excitonic platform with several potential
uses, including signal processing for microscopy and spectroscopy
methods that implement tunable exciton flux
Separation of Electromagnetic and Chemical Contributions to Surface-Enhanced Raman Spectra on Nanoengineered Plasmonic Substrates
Raman signals from molecules adsorbed on a noble metal surface are enhanced by many orders of magnitude due to the plasmon resonances of the substrate. Additionally, the enhanced spectra are modified compared to the spectra of neat molecules; many vibrational frequencies are shifted, and relative intensities undergo significant changes upon attachment to the metal. With the goal of devising an effective scheme for separating the electromagnetic and chemical effects, we explore the origin of the Raman spectra modification of benzenethiol adsorbed on nanostructured gold surfaces. The spectral modifications are attributed to the frequency dependence of the electromagnetic enhancement and to the effect of chemical binding. The latter contribution can be reproduced computationally using moleculeâmetal cluster models. We present evidence that the effect of chemical binding is mostly due to changes in the electronic structure of the molecule rather than to the fixed orientation of molecules relative to the substrate
Solid-State End-On to Side-On Isomerization of (NN)<sup>2â</sup> in {[(R<sub>2</sub>N)<sub>3</sub>Nd]<sub>2</sub>N<sub>2</sub>}<sup>2â</sup> (R = SiMe<sub>3</sub>) Connects In Situ Ln<sup>III</sup>(NR<sub>2</sub>)<sub>3</sub>/K and Isolated [Ln<sup>II</sup>(NR<sub>2</sub>)<sub>3</sub>]<sup>1â</sup> Dinitrogen Reduction
Examination
of the reduction chemistry of Nd(NR2)3 (R =
SiMe3) under N2 has provided
connections
between the in situ Ln(III)-based LnIII(NR2)3/K reductions of N2 that form side-on bound neutral
(N=N)2â complexes, [(R2N)2(THF)Ln]2[ÎŒ-η2:η2-N2], and the Ln(II)-based [LnII(NR2)3]1â reductions by Sc, Gd, and Tb that
form end-on bound (N=N)2â complexes, {[(R2N)3Ln]2[ÎŒ-η1:η1-N2]}2â, which are dianions.
The reduction of Nd(NR2)3 by KC8 under
dinitrogen in Et2O in the presence of 18-crown-6 (18-c-6)
forms dark yellow solutions of [K2(18-c-6)3]{[(R2N)3Nd]2N2} at low temperatures
that become green as they warm up to â35 °C in a glovebox
freezer. Green crystals obtained from the solution turn yellow-brown
when cooled below â100 °C, and the yellow-brown compound
has an end-on Nd2(Ό-η1:η1-N2) structure. The yellow-brown crystals isomerize
in the solid state on the diffractometer upon warming, and at â25
°C, the crystals are green and have a side-on Nd2(Ό-η2:η2-N2) structure. Collection
of X-ray diffraction data at 10 °C intervals from â50
to â90 °C revealed that the isomerization occurs at temperatures
below â100 °C. In the presence of tetrahydrofuran (THF),
the dianionic {[(R2N)3Nd]2N2}2â system can lose an amide ligand to provide
the monoanionic [(R2N)3NdIII(ÎŒ-η2:η2-N2)NdIII(NR2)2(THF)]1â, characterized by
X-ray crystallography. These data suggest a connection between the
in situ Ln(III)/K reductions and Ln(II) reductions that depends on
solvent, temperature, the presence of a chelate, and the specific
rare-earth metal
Can Mixed-Metal Surfaces Provide an Additional Enhancement to SERS?
We explore the chemical contribution to surface-enhanced
Raman
scattering (SERS) in mixed-metal substrates, both experimentally and
by computer simulation. These substrates are composed of a chemically
active, transition-metal overlayer deposited on an effective SERS
substrate. We report improved analytical enhancement factors obtained
by using a small surface coverage of palladium or platinum over nanostructured
silver substrates. Theoretical predictions of the chemical contribution
to the surface enhancement using density functional theory support
the experimental results. In addition, these approaches show that
the increased enhancement is due not only to an increase in surface
coverage of the analyte but also to a higher Raman scattering cross
section per molecule. The additional chemical enhancement in mixed-metal
SERS substrates correlates with the binding energy of the analyte
on the surface and includes both static and dynamical effects. SERS
using mixed-metal substrates has the potential to improve sensing
for a large group of analyte molecules and to aid the development
of chemically specific SERS-based sensors
Introducing Ionic and/or Hydrogen Bonds into the SAM//Ga<sub>2</sub>O<sub>3</sub> Top-Interface of Ag<sup>TS</sup>/S(CH<sub>2</sub>)<sub><i>n</i></sub>T//Ga<sub>2</sub>O<sub>3</sub>/EGaIn Junctions
Junctions
with the structure Ag<sup>TS</sup>/SÂ(CH<sub>2</sub>)<sub><i>n</i></sub>T//Ga<sub>2</sub>O<sub>3</sub>/EGaIn (where
SÂ(CH<sub>2</sub>)<sub><i>n</i></sub>T is a self-assembled
monolayer, SAM, of n-alkanethiolate bearing a terminal functional
group T) make it possible to examine the response of rates of charge
transport by tunneling to changes in the strength of the interaction
between T and Ga<sub>2</sub>O<sub>3</sub>. Introducing a series of
Lewis acidic/basic functional groups (T = âOH, âSH,
âCO<sub>2</sub>H, âCONH<sub>2</sub>, and âPO<sub>3</sub>H) at the terminus of the SAM gave values for the tunneling
current density, <i>J</i>(<i>V</i>) in A/cm<sup>2</sup>, that were indistinguishable (i.e., differed by less than
a factor of 3) from the values observed with n-alkanethiolates of
equivalent length. The insensitivity of the rate of tunneling to changes
in the terminal functional group implies that replacing weak van der
Waals contact interactions with stronger hydrogen- or ionic bonds
at the T//Ga<sub>2</sub>O<sub>3</sub> interface does not change the
shape (i.e., the height or width) of the tunneling barrier enough
to affect rates of charge transport. A comparison of the injection
current, <i>J</i><sub>0</sub>, for T = âCO<sub>2</sub>H, and T = âCH<sub>2</sub>CH<sub>3</sub>âtwo groups
having similar extended lengths (in Ă
, or in numbers of non-hydrogen
atoms)âsuggests that both groups make indistinguishable contributions
to the height of the tunneling barrier