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.

Silylidyne, HSi≡MoH3 and HSi≡WH3, and Silyl Metal Hydride, SiH3—CrH, Products in Silane Reactions

Abstract: Laser-ablated group 6 metal atoms react with silane to form inserted SiH3−MH hydride intermediates, which are identified from M−H and Si−H stretching modes. Following two successive α-H-transfers, the HSi≡MH3 (M = Mo, W) silylidyne molecules are produced. These silicon−metal triple-bonded species are identified as major products from the strong M−H stretching modes through deuterium substitution and comparison with frequencies and intensities from density functional calculations and from the analogous methylidynes. The silylidynes have calculated C3v structures and longer Si−H bonds than silane, but the C3v methylidyne analogues have shorter C−H bonds than methane. The Si≡Mo and Si≡W bonds are polarized differently and have slightly lower effective bond orders than their carbon analogues. In addition, calculations for the group 6 silylidene molecules reveal Cs structures with no evidence of agostic distortion, in contrast to the corresponding methylidene molecules.

Matrix infrared spectroscopic and computational investigations of the lanthanide-methylene complexes CH2LnF2 with single Ln-C bonds.

Abstract: Laser-ablated lanthanide metal atoms were condensed with CH2F2 in excess argon at 6 K or neon at 4 K. New infrared absorption bands are assigned to the oxidative addition product methylene lanthanide difluorides on the basis of deuterium substitution and vibrational frequency calculations with density functional theory (DFT). Two dominant absorptions in the 500 cm−1 region are identified as lanthanide−fluoride stretching modes for this very strong infrared absorption. The predominantly lanthanide−carbon stretching modes follow a similar trend of increasing with metal size and have characteristic 30 cm−1 deuterium and 14 cm−1 13C isotopic shifts. The electronic structure calculations show that these CH2LnF2 complexes are not analogous to the simple transition and actinide metal methylidenes with metal−carbon double bonds that have been investigated previously, because the lanthanide metals (in the +2 or +3 oxidation state) do not appear to form a π-type bond with the CH2 group. The DFT and ab initio correl...

Matrix reactions of alkaline earth metal atoms with oxygen molecules: Infrared spectra of the metal superoxide and metal dioxide species

Abstract: Infrared spectra of the reaction products of oxygen molecules and alkaline earth metal atoms in argon and nitrogen matrices have been recorded. The ν2 interionic modes of barium, strontium, and calcium superoxides, MO2, have been located at 541, 474, and 498 cm−1, respectively, in nitrogen matrices. The metal oxide, MO, was isolated for each metal, and several bands of the metal oxide dimer were measured. Frequencies observed for the antisymmetric modes of (BaO)2 at 487 and 393 cm−1 indicate a D2h structure with an O−Ba−O angle of approximately 102°. Also, a band attributable to ν3 of a bent O−M−O metal dioxide species has been identified near 752 cm−1 for barium, 730 cm−1 for strontium, and 743 cm−1 for calcium, leading to a bond angle near 140° for all three species. A mechanism is proposed for the formation of the metal oxide from molecular oxygen, which includes MO2M and (MO)2 intermediates. It was also determined that nitrogen is much more effective as a matrix material in stabilizing ionic compounds...

Proton radiolysis of CHCl3 and CHBr3 at high dilution in argon during condensation at 15 °K. Infrared spectra of the CHX+2, CHX−2, and CHX−3 molecular ions

Abstract: Matrix samples of chloroform and its deuterium and bromine analogs have been subjected to irradiation by a 2.0 keV proton beam during deposition, and the products trapped in an argon matrix at 15 °K. Several infrared absorptions have been observed and assigned to a number of new molecular species. Extremely photosensitive bands observed at 652, 621, 593, and 570 cm−1 have been assigned to the CHCl−3, CHCl2Br−, CHClBr−2, and CHBr−3 molecular anions, respectively. Absorptions at 998, 907, and 839 cm−1, which were decreased by operating an exposed bulb filament, have been attributed to the isolated cations CHCl+2, CHClBr+, and CHBr+2. Bands stable to the bulb filament at 1044, 988, and 898 cm−1 were assigned to the CHX+2 (X−) species and similar bands at 838, 795, and 735 cm−1 were assigned to the CHX2− anions. In addition, bands attributable to the species CX3, CX2CX3+(X−), CHX2, and HX−2, which have been identified previously, were observed here as well.

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