Lester Andrews

Bio: Lester Andrews is an academic researcher from University of Virginia. The author has contributed to research in topics: Infrared spectroscopy & Molecule. The author has an hindex of 68, co-authored 888 publications receiving 24613 citations. Previous affiliations of Lester Andrews include Ames Research Center & Environmental Molecular Sciences Laboratory.

Laser-Ablated U Atom Reactions with (CN)2 to Form UNC, U(NC)2, and U(NC)4: Matrix Infrared Spectra and Quantum Chemical Calculations.

Abstract: Laser-ablated U atoms react with (CN)2 in excess argon and neon during codeposition at 4 K to form UNC, U(NC)2, and U(NC)4 as the major uranium-bearing products, which are identified from their matrix infrared spectra using cyanogen substituted with 13C and 15N and from quantum chemical calculations. The 12/13CN and C14/15N isotopic frequency ratios computed for the U(NC)1,2,4 molecules agree better with the observed values than those calculated for the U(CN)1,2,4 isomers. Multiplets using mixed isotopic cyanogens reveal the stoichiometries of these products, and the band positions and quantum chemical calculations confirm the isocyanide bonding arrangements, which are 14 and 51 kJ/mol more stable than the cyanide isomers for UNC and U(NC)2, respectively, and 62 kJ/mol for U(NC)4 in the isolated gas phase at the CCSD(T)/CBS level. The studies further demonstrate that the isocyano nitrogen is a better π donor, so it interacts with U(VI) better than carbon. Although the higher isocyanides are more stable th...

Formation of metal oxyfluorides from specific metal reactions with oxygen difluoride: infrared spectroscopic and theoretical investigations of the OScF2 radical and OScF with terminal single and triple Sc-O bonds.

Abstract: The scandium oxydifluoride free radical, OScF(2), is produced by the spontaneous, specific reaction of laser ablated Sc atoms with OF(2) in solid argon and characterized by using matrix infrared spectroscopy and theoretical calculations. The OScF(2) molecule is predicted to have C(2v) symmetry and a (2)B(2) ground state with an unpaired electron located primarily on the terminal oxygen atom, which makes it a scandium difluoride molecule coordinated by a neutral oxygen atom radical in forming the Sc-O single bond. The closed shell singlet OScF molecule with an obtuse bent geometry has a much shorter Sc-O bond of 1.682 A than that of the OScF(2) radical (1.938 A) on the basis of B3LYP calculations. The Sc-O bond in OScF consists of two covalent bonds and a dative bond in which the oxygen 2p(π) lone pair donates electron density into an empty Sc 3d orbital thus forming a triple oxo bond. Density functional calculations suggest it is highly exothermic for fluorine transfer from OF(2) to scandium, which favors the formation of the OScF(2) radical species as well as the OScF molecule after fluorine loss.

Matrix Infrared Spectra and Density Functional Calculations of Three Al, N, O Isomers

Abstract: Laser-ablated aluminum atoms have been reacted with NO, 15NO, and 15N18O during condensation in excess argon using a variety of concentrations and laser energies/cm2. Four of the five major product absorptions with higher laser energy were also observed with lower laser energy. The isotopic frequency ratios characterize different normal modes, and these are uniquely matched to the stretching modes of triplet AlON (1282.1, 566.7 cm-1) and triplet AlNO (1644.3, 510.2 cm-1) by density functional theory (DFT) isotopic frequency calculations. The fifth band (1079.5 cm-1) is due to a terminal Al−O stretching mode that is consistent with triplet NAlO based on DFT isotopic frequency calculations. The total product yield depended on laser energy; both AlON and AlNO addition products were produced with lower energy, but the insertion product NAlO required higher energy. Anions were favored with lower laser energy, and a weak 1380.6 cm-1 band is assigned to AlNO- in accord with DFT isotopic frequency calculations.

Matrix Infrared Spectra of Dihydrido Cyclic and Trihydrido Ethynyl Products from Reactions of Th and U Atoms with Ethylene Molecules

Abstract: Reactions of laser-ablated thorium and uranium atoms with ethylene isotopomers have been carried out and the primary products identified in the matrix IR spectra. The dihydrido cyclic and trihydrido ethynyl products (MH2−C2H2 and MH3CCH) are identified from the matrix spectra of both Th and U, whereas the insertion product absorption is observed only in the U spectra. The present results indicate that C−H insertion and following H migration from C to M also occur in reaction of the actinides with ethylene. Formation of the trihydrido ethynyl products is parallel to the previous results of group 4 metals. Metal hydride (MHx) absorptions are not observed in the infrared spectra, suggesting that the H2 elimination from the dihydrido cyclic intermediate is relatively slow. Density functional theory calculations reproduce the vibrational characteristics of the identified products and the relative stabilities. The cyclic triangular ThH2−C2H2 system with two π electrons is aromatic, which contributes to the uniq...

Reactions of Laser-Ablated Mo and W Atoms, Cations, and Electrons with CO in Excess Neon: Infrared Spectra and Density Functional Calculations on Neutral and Charged Unsaturated Metal Carbonyls

Abstract: Laser-ablated Mo and W atoms react with CO in excess neon to give the same unsaturated M(CO) 3 , 4 , 5 carbonyls observed from photodissociation of M(CO) 6 . The observed neon matrix fundamentals for these M(CO) 3 , 4 , 5 species bridge previous argon matrix and gas phase values. The M(CO) 2 dicarbonyls absorb at 1895.2 and 1884.5 cm - 1 , and the MCO monocarbonyls absorb at 1881.2 and 1859.9 cm - 1 for Mo and W, respectively. The M(CO) 1 , 2 + cations and M(CO) 1 , 2 - anions are observed as the ablation process also provides cations and electrons. Density functional theory calculations support the product identifications and vibrational assignments.

A Chemist's Guide to Density Functional Theory

Abstract: "Chemists familiar with conventional quantum mechanics will applaud and benefit greatly from this particularly instructive, thorough and clearly written exposition of density functional theory: its basis, concepts, terms, implementation, and performance in diverse applications. Users of DFT for structure, energy, and molecular property computations, as well as reaction mechanism studies, are guided to the optimum choices of the most effective methods. Well done!" Paul von RaguE Schleyer "A conspicuous hole in the computational chemist's library is nicely filled by this book, which provides a wide-ranging and pragmatic view of the subject.[...It] should justifiably become the favorite text on the subject for practioneers who aim to use DFT to solve chemical problems." J. F. Stanton, J. Am. Chem. Soc. "The authors' aim is to guide the chemist through basic theoretical and related technical aspects of DFT at an easy-to-understand theoretical level. They succeed admirably." P. C. H. Mitchell, Appl. Organomet. Chem. "The authors have done an excellent service to the chemical community. [...] A Chemist's Guide to Density Functional Theory is exactly what the title suggests. It should be an invaluable source of insight and knowledge for many chemists using DFT approaches to solve chemical problems." M. Kaupp, Angew. Chem.

The density functional formalism, its applications and prospects

Abstract: A scheme that reduces the calculations of ground-state properties of systems of interacting particles exactly to the solution of single-particle Hartree-type equations has obvious advantages. It is not surprising, then, that the density functional formalism, which provides a way of doing this, has received much attention in the past two decades. The quality of the energy surfaces calculated using a simple local-density approximation for exchange and correlation exceeds by far the original expectations. In this work, the authors survey the formalism and some of its applications (in particular to atoms and small molecules) and discuss the reasons for the successes and failures of the local-density approximation and some of its modifications.

The Halogen Bond

Abstract: The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Generation and Detection of Reactive Oxygen Species in Photocatalysis

Abstract: The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2–), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption–desorption equilibrium at the TiO2–solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity...

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg