MM-WAVE SPECTROSCOPY FOR THE MASSES: COMBINING COMMERCIAL SOLID-STATE SOURCES WITH MOLECULAR BEAMS

Author Institution: Department of Chemistry \& Biochemistry, University of North Carolina GreensboroIn recent years, new broadband solid-state devices for generating mm-waves have been developed by the telecommunications industry for network analysis. Their use in molecular spectroscopy offers several advantages over traditional techniques. These advantages include their ease of use, flexibility, relatively low cost (\$15k), high power (1 mW), broadband tunability (e.g. 75-110 GHz), and high resolution (1 part in$10^{10}$). In my lab at UNC Greensboro I have developed a pulsed molecular beam instrument to measure the direct absorption of mm-waves by molecules in the beam. For mm-wave source powers as high as 1 mW, the sensitivity of the instrument is limited by the NEP$(2\times 10^{-13}{WH_{z}}^{-1/2})$of the InSb hot-electron bolometer rather than by the shot noise of the source, resulting in a sensitivity on the order of$10^{-10} H_{z}^{-1/2}$. The high sensitivity of the instrument rivals that of optothermal detection methods and should allow the technique to be used in a wide variety of molecular beam experiments. The instrument is capable of operating in both the frequency and time domain. In the frequency domain the source may be either stepped or swept as the molecules fly by. While in the time domain, coherent effects may be probed using double resonance techniques or by pulse modulating the source. Preliminary results on the UV photodissociation of HOCl will be presented. In these studies, the A-doublet states of the OH radical fragments will be probed by Doppler spectroscopy

Improved Computational Prediction of the Electrochemical Reduction Potential of Twenty 3-Aryl-Quinoxaline-2-Carbonitrile 1,4-Di-N-Oxide Derivatives

The ability of density functional theory (DFT) using the functional B3LYP with the cc-pVTZ basis set to accurately predict the electrochemical properties of 20 3-aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide derivatives in dimethylformamide (DMF) was investigated and compared to previous predictions from B3LYP/6-31G and B3LYP/lanl2dz. The B3LYP/cc-pVTZ method was an improvement over the B3LYP/6-31G and B3LYP/lanl2dz methods as it was able to predict the first reduction potential of the diazine ring (wave 1) for all of the 3-aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide derivatives accurately. The B3LYP/cc-pVTZ predicted electrochemical potentials had a strong correlation to experimental values for wave 1. None of the methods demonstrated the ability to predict the nitro wave reduction potential for derivatives containing a nitro group. B3LYP/cc-pVTZ predicted electrochemical potentials for the second reduction of the diazine ring (wave 2) had a low correlation to the experimental values for the derivatives without a nitro group and no correlation of the derivatives when the nitro group was included in the analysis

Reduction Potential Predictions for Some 3-Aryl-Quinoxaline-2-Carbonitrile 1,4-Di-N-Oxide Derivatives with Known Anti-Tumor Properties

The ability for DFT: B3LYP calculations using the 6-31g and lanl2dz basis sets to predict the electrochemical properties of twenty (20) 3-aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide derivatives with varying degrees of cytotoxic activity in dimethylformamide (DMF) was investigated. There was a strong correlation for the first reduction and moderate-to-low correlation of the second reduction of the diazine ring between the computational and the experimental data, with the exception of the derivative containing the nitro functionality. The four (4) nitro group derivatives are clear outliers in the overall data sets and the derivative E4 is ill-behaved. The remaining three (3) derivatives containing the nitro groups had a strong correlation between the computational and experimental data; however, the computational data falls substantially outside of the expected range

Voltammetric Study of Some 3-Aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide Derivatives with Anti-Tumor Activities

The electrochemical properties of twenty 3-aryl-quinoxaline-2-carbonitrile 1,4-di-N-oxide derivatives with varying degrees of cytotoxic activity were investigated in dimethylformamide (DMF) using cyclic voltammetry and first derivative cyclic voltammetry. With one exception, the first reduction of these compounds was found to be reversible or quasireversible and is attributed to reduction of the N-oxide moiety to form a radical anion. The second reduction of the diazine ring was found to be irreversible. Compounds containing a nitro group on the 3-phenyl ring also exhibited a reduction process that may be attributed to that group. There was good correlation between molecular structure and reduction potential, with reduction being facilitated by an enhanced net positive charge at the electroactive site created by electron withdrawing substituents. Additionally, the reduction potential was calculated using two common basis sets, 6-31g and lanl2dz, for five of the test molecules. There was a strong correlation between the computational data and the experimental data, with the exception of the derivative containing the nitro functionality. No relationship between the experimentally measured reduction potentials and reported cytotoxic activities was evident upon comparison of the data

MoD-QM/MM Structural Refinement Method: Characterization of Hydrogen Bonding in the <i>Oxytricha nova</i> G‑Quadruplex

A generalization of the Moving-Domain Quantum Mechanics/Molecular Mechanics (MoD-QM/MM) hybrid method [Gascon, J. A.; Leung, S. S. F.; Batista, E. R.; Batista, V. S. <i>J. Chem. Theory Comput.</i> <b>2006</b>, <i>2</i>, 175–186] is introduced to provide a self-consistent computational protocol for structural refinement of extended systems. The method partitions the system into molecular domains that are iteratively optimized as quantum mechanical (QM) layers embedded in their surrounding molecular environment to obtain an ab initio quality description of the geometry and the molecular electrostatic potential of the extended system composed of those constituent fragments. The resulting methodology is benchmarked as applied to model systems that allow for full QM optimization as well as through refinement of the hydrogen bonding geometry in <i>Oxytricha nova</i> guanine quadruplex for which several studies have been reported, including the X-ray structure and NMR data. Calculations of ¹H NMR chemical shifts based on the gauge independent atomic orbital (GIAO) method and direct comparisons with experiments show that solvated MoD-QM/MM structures, sampled from explicit solvent molecular dynamics simulations, allow for NMR simulations in much improved agreement with experimental data than models based on the X-ray structure or those optimized using classical molecular mechanics force fields