Browsing by Author "Montgomery, Jason M."
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Item Enantioenrichment of racemic BINOL by way of excited state proton transfer(Royal Society of Chemistry, 2019) Ayad, Suliman; Posey, Victoria; Das, Anjan; Montgomery, Jason M.; Hanson, KennethHere 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 enantio-enriched BINOL is easily recovered by cleaving the auxiliary group in mild conditions.Item Maple's Quantum Chemistry Package in the Chemistry Classroom(ACS Publications, 2020) Montgomery, Jason M.; Mazziotti, David A.An introduction to the Quantum Chemistry Package (QCP), implemented in the computer algebra system Maple, is presented. The QCP combines sophisticated electronic structure methods and Maple's easy-to-use graphical interface to enable computation and visualization of the electronic energies and properties of molecules. Here we describe how the QCP can be used in the chemistry classroom using lessons provided within the package. In particular, the calculation and visualization of molecular orbitals of hydrogen fluoride, the application of the particle in a box to conjugated dyes, the use of geometry optimization and normal mode analysis for hypochlorous acid, and the thermodynamics of combustion of methane.Item Mosaic-like Silver Nanobowl Plasmonic Crystals as Highly Active Surface-Enhanced Raman Scattering Substrates(The Journal of Physical Chemistry, 2015) Baca, Alfred J.; Baca, Joshua; Montgomery, Jason M.; Cambrea, Lee R.; Funcheon, Peter; Johnson, Linda; Moran, Mark; Connor, DanWe present a simple approach to creating a type of surface-enhanced Raman scattering (SERS) substrate composed of a mosaic-like structured Ag metal surface on nanobowl plasmonic crystals (NBPCs) formed by combining soft nanoimprinting and substrate (in situ) heating during metal deposition. This new type of sensor exploits the electromagnetic enhancement of localized surface plasmon resonances (LSPR) produced by a template nanostructured metal surface and surface plasmons (SP) in-between the gaps of the mosaic surface to create a highly SERS-active substrate. Our approach is simple, in that it implements low processing temperatures (200 °C) and does not require any postdeposition annealing or exposure to high temperature environments, enabling the use of mechanically flexible substrates. These SERS substrates exhibit higher SERS intensities in comparison to those obtained with the corresponding square array of smooth (room temperature metal deposition) nanobowl structures with similar spatial layouts. As an example toward an application, we demonstrate polychlorinated biphenyl (PCB-77) SERS detection using Ag mosaic NBPC substrates. Three-dimensional finite-difference time-domain (3D FDTD) simulations qualitatively capture the key features of these systems and suggest a route to the fabrication of optimized, highly efficient SERS substrates in silico.Item Optimization of nanopost plasmonic crystals for surface enhanced Raman scattering(The Journal of Physical Chemistry, 2011) Baca, Alfred J.; Montgomery, Jason M.; Cambrea, Lee R.; Moran, Mark; Johnson, Linda; Yacoub, Jeanine; Truong, Tu T.We present experimental and theoretical studies of a type of Surface Enhanced Raman Scattering (SERS) substrate composed of a metal coated square array of nanopost structures formed via soft nanoimprinting. These SERS substrates exhibit higher SERS intensities in comparison to those obtained with the corresponding square array of nanowell structures with similar spatial layouts and demonstrate multiple analyte detection using SERS. Three-dimensional finite-difference time-domain (3D FDTD) simulations qualitatively capture the key features of these systems and suggest a route to the fabrication of optimized, highly efficient SERS substrates in silico. Collectively, the ease of fabrication, high sensitivities, and predictable responses suggest an attractive route to SERS substrates for portable chemical warfare agent detection, environmental monitors, and other applications.Item Prediction of the Existence of LiCH: A Carbene-like Organometallic Molecule(ACS Publications, 2020) Montgomery, Jason M.; Alexander, Ezra; Mazziotti, David A.Carbenes comprise a well-known class of organometallic compounds 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, CH$_2$, 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.Item Strong Electron Correlation in Nitrogenase Cofactor, FeMoco(ACS Publications, 2018) Montgomery, Jason M.; Mazziotti, David A.FeMoco, MoFe$_7$S$_9$C, 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 wavefunction approach for a singlet state using a [54,54] active space would require 10$^{29}$ variables. In this work, we systematically explore the active-space size necessary to qualitatively capture strong correlation in FeMoco and two related moieties, MoFe$_3$S$_7$ and Fe$_4$S$_7$. 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 multi-reference solutions to the electronic Schr\"{o}dinger equation for smaller active spaces and a consistent multi-reference solution for larger active spaces.Item The design and optimization of plasmonic crystals for surface enhanced Raman spectroscopy using the finite difference time domain method(MDPI AG, 2018) Bigness, Alec; Montgomery, Jason M.We present computational studies of quasi three-dimensional nanowell (NW) and nanopost (NP) plasmonic crystals for applications in surface enhanced Raman spectroscopy (SERS). The NW and NP plasmonic crystals are metal coated arrays of cylindrical voids or posts, respectively, in a dielectric substrate characterized by a well/post diameter (D), relief depth (R D), periodicity (P), and metal thickness (M T). Each plasmonic crystal is modeled using the three-dimensional finite-difference time-domain (FDTD) method with periodic boundary conditions in the x- and y-directions applied to a computational unit cell to simulate the effect of a periodic array. Relative SERS responses are calculated from time-averaged electric field intensity enhancements at λ exc and λ scat or at λ mid via G SERS 4 = g 2 ( λ exc ) × g 2 ( λ scat ) or G mid 4 = g 4 ( λ mid ) , respectively, where g 2 = | E | 2 / | E 0 | 2 . Comparisons of G SERS 4 and G mid 4 are made to previously reported experimental SERS measurements for NW and NP geometries. Optimized NW and NP configurations based on variations of D, P, R D, and M T using G SERS 4 are presented, with 6× and 2× predicted increases in SERS, respectively. A novel plasmonic crystal based on square NP geometries are considered with an additional 3× increase over the optimized cylindrical NP geometry. NW geometries with imbedded spherical gold nanoparticles are considered, with 10× to 10 3 × increases in SERS responses over the NW geometry alone. The results promote the use of FDTD as a viable in silico route to the design and optimization of SERS active devices.