Prediction of the Existence of LiCH: A Carbene-like Organometallic Molecule.

Carbenes comprise a well-known class of organometallic compounds each consisting of a neutral, divalent carbon and two unshared electrons. Carbenes can have singlet or triplet ground states, each giving rise to a distinct reactivity. Methylene (CH2), the parent hydride, is well-known to be bent in its triplet ground state. Here, we predict the existence of LiCH, a carbene-like organometallic molecule. Computationally, we treat the electronic structure with parametric and variational two-electron reduced density matrix (2-RDM) methods, which are capable of capturing multireference correlation typically associated with the singlet state of a diradical. Similar to methylene, LiCH is a triplet ground state with a predicted 15.8 kcal/mol singlet-triplet gap. However, unlike methylene, LiCH is linear in both the triplet state and the lowest excited singlet state. Furthermore, the singlet state is found to exhibit strong electron correlation as a diradical. In comparison to dissociation channels Li + CH and Li+ + CH-, the LiCH was found to be stable by approximately 77 kcal/mol.

Enantioenrichment of racemic BINOL by way of excited state proton transfer.

Here we report a method for enantioenriching BINOL using a chiral auxiliary and an excited state proton transfer (ESPT) event. Regardless of the starting enantiomeric excess (ee), after irradiation the solution reaches a photostationary state whose ee is dependent solely on the identity of the chiral auxiliary group. The enantioenriched BINOL is easily recovered by cleaving the auxiliary group in mild conditions.

Strong Electron Correlation in Nitrogenase Cofactor, FeMoco.

FeMoco, MoFe7S9C, has been shown to be the active catalytic site for the reduction of nitrogen to ammonia in the nitrogenase protein. An understanding of its electronic structure including strong electron correlation is key to designing mimic catalysts capable of ambient nitrogen fixation. Active spaces ranging from [54, 54] to [65, 57] have been predicted for a quantitative description of FeMoco's electronic structure. However, a wave function approach for a singlet state using a [54, 54] active space would require 1029 variables. In this work, we systematically explore the active-space size necessary to qualitatively capture strong correlation in FeMoco and two related moieties, MoFe3S7 and Fe4S7. Using CASSCF and 2-RDM methods, we consider active-space sizes up to [14, 14] and [30, 30], respectively, with STO-3G, 3-21G, and DZP basis sets and use fractional natural-orbital occupation numbers to assess the level of multireference electron correlation, an examination of which reveals a competition between single-reference and multireference solutions to the electronic Schrödinger equation for smaller active spaces and a consistent multireference solution for larger active spaces.

Controlling plasmonic wave packets in silver nanowires.

Three-dimensional finite-difference time-domain simulations were performed to explore the excitation of surface plasmon resonances in long silver (Ag) nanowires. In particular, we show that it is possible to generate plasmonic wave packets that can propagate along the nanowire by exciting superpositions of surface plasmon resonances. By using an appropriately chirped pulse, it is possible to transiently achieve localization of the excitation at the distal end of the nanowire. Such designed coherent superpositions will allow realizing spatiotemporal control of plasmonic excitations for enhancing nonlinear responses in plasmonic "circuits".

SERS enhancements via periodic arrays of gold nanoparticles on silver film structures.

We discuss surface enhanced Raman spectroscopy (SERS) structures aimed at providing robust and reproducible enhancements. The structures involve periodic arrays of gold nanospheres near silver film structures that may also be patterned. They enable one to excite Bloch wave surface plasmon polaritons (SPPs) that can also couple to local surface plasmons (LSPs) of the nanospheres, leading to the possibility of multiplicative enhancements. If the magnitude of the average electric field, /E/, between the particles is enhanced by g such that /E/ = g/E(0)/, /E(0)/ being the incident field, realistic finite-difference time-domain simulations show that under favorable circumstances g approximately equal 0.6 g(SPP) g(LSP), where g(SPP) and g(LSP) are enhancement factors associated with the individual components. SERS enhancements for the structures can be as high as O(g(4)) = 10(8).

Plasmonic electromagnetic hot spots temporally addressed by photoinduced molecular displacement.

We report the observation of temporally varying electromagnetic hot spots in plasmonic nanostructures. Changes in the field amplitude, position, and spatial features are induced by embedding plasmonic silver nanorods in the photoresponsive azo-polymer. This polymer undergoes cis-trans isomerization and wormlike transport within resonant optical fields, producing a time-varying local dielectric environment that alters the locations where electromagnetic hot spots are produced. Finite-difference time-domain and Monte Carlo simulations that model the induced field and corresponding material response are presented to aid in the interpretation of the experimental results. Evidence for propagating plasmons induced at the ends of the rods is also presented.

High Fidelity Nano-Hole Enhanced Raman Spectroscopy.

Surface Enhanced Raman Spectroscopy (SERS) is a sensitive technique that can even detect single molecules. However, in many SERS applications, the strongly inhomogeneous distribution of intense local fields makes it very difficult for a quantitive assessment of the fidelity, or reproducibility of the signal, which limits the application of SERS. Herein we report the development of exceptionally high fidelity Hole-Enhanced Raman Spectroscopy (HERS) from ordered, two-dimensional hexagonal nanohole arrays. We take the fidelity f to be a measure of the percent deviation of the Raman peaks from measurement to measurement. Overall, area averaged fidelities for 12 gold array samples ranged from f ~ 2% - 15% for HERS using aqueous R6G molecules. Furthermore, intensity modulations of the enhanced Raman spectra were measured for the first time as a function of polarization angle. The best of these measurements, which focus on static laser spots on the sample, could be consistent with even higher fidelities than the area-averaged results. Nanohole arrays in silver provided supporting polarization measurements and a more complete enhanced Raman fingerprint for phenylalanine molecules. We also carried out finite-difference time-domain calculations to assist in the interpretation of the experiments, identifying the polarization dependence as possibly arising from hole-hole interactions. Our results represent a step towards making quantitative and reproducible enhanced Raman measurements possible and also open new avenues for a large scale source of highly uniform hot spots.