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.

Infrared Spectra and Density Functional Calculations for Singlet CH2═SiX2 and Triplet HC–SiX3 and XC–SiX3 Intermediates in Reactions of Laser-Ablated Silicon Atoms with Di-, Tri-, and Tetrahalomethanes

Abstract: Reactions of laser-ablated silicon atoms with di-, tri-, and tetrahalomethanes in excess argon were investigated, and the products were identified from the matrix infrared spectra, isotopic shifts, and density functional theory energy, bond length, and frequency calculations. Dihalomethanes produce planar singlet silenes (CH2═SiX2), and tri- and tetrahalomethanes form triplet halosilyl carbenes (HC–SiX3 and XC–SiX3). The Si-bearing molecules identified are the most stable, lowest-energy product in the reaction systems. While the C–Si bond in the silene is a true double bond, the C–Si bond in the carbene is a shortened single bond enhanced by hyperconjugation of the two unpaired electrons on C to σ*(Si–X) orbitals, which contributes stabilization through a small amount of π-bonding and reduction of the HCSi or XCSi angles. The C–Si bond lengths in these carbenes (1.782 A for HC–SiF3) are between the single-bond length in the unobserved first insertion intermediate (1.975 A for CHF2–SiF) and the double-bond...

Reactions of Laser-Ablated Rhodium and Iridium Atoms with Nitric Oxide in Neon and Argon. Matrix Infrared Spectra and Density Functional Calculations of Rh(NO)1-3, Ir(NO)1-3, NRhO, NIrO, RhNO+, and IrNO+

Abstract: Laser-ablated rhodium and iridium atoms are reacted with nitric oxide and the products are isolated in solid neon and argon. Neutral nitrosyl complexes Rh(NO)1-3 and Ir(NO)1-3 are the main products, and weaker bands due to RhNO+, IrNO+, NRhO, and NIrO are also observed. DFT calculations for these products using the BPW91 and B3LYP functionals determine frequencies with the same accuracy found for other metal nitrosyls and are a useful predictive tool. The stretching frequencies observed for the isolated nitrosyls are close to those for analogous nitrosyl species on rhodium and iridium surfaces, which will aid in interpreting vibrational data for surface species.

Assignment of Raman spectra for trifluoride anions in solid argon

Abstract: Cesium fluoride evaporated from a small stainless steel Knudsen cell at 495 °C was codeposited with F2 in excess argon onto a copper wedge at 15 K and examined by focused blue argon ion laser lines scattered from the matrix sample. An 892 cm−1 Raman shifted signal was observed for F2, and stronger 461 cm−1 and weaker 389 cm−1 Raman shifted signals were observed for reaction products. All of these signals decreased in intensity upon prolonged exposure to laser light, but the 389 cm−1 signal decreased more rapidly than the 461 cm−1 signal. Temperature cycling to 40 K destroyed the 389 cm−1 signal, but the 461 cm−1 band remained intense, which was the same behavior for the very intense 550 cm−1 infrared absorption in similar experiments using a different instrument. The two strong 461 and 550 cm−1 bands related by annealing behavior were assigned to the symmetric and antisymmetric (F–F–F)− stretching modes for the trifluoride anion, respectively, in the Cs+F3− ion pair species with mutual exclusion. The weaker more vulnerable 389 cm−1 signal was attributed to an unknown, less stable (i.e. more reactive) secondary reaction product (Ault and Andrews, J. Am. Chem. Soc., 1976, 98, 1591). Nevertheless, Redeker, Beckers and Riedel (RSC Adv., 2015, 5, 106568), on the basis of the detection of a very weak assigned combination band and CCSD(T) frequency calculations, reassigned the two clearly unrelated strong 550 cm−1 IR and weak 389 cm−1 Raman bands to the same stable Cs+F3− ion pair species. We can now identify the weaker 389 cm−1 Raman band for a more photosensitive and reactive carrier as the isolated F3− anion. The required Raman blue laser photo-ionization of Cs and Rb atoms in the system and the case for this new assignment are considered in the following paper. The antisymmetric stretching frequency for this isolated anion has been assigned at 510.6 cm−1 in other IR work. We again assign with confidence the two strong and related 550 cm−1 IR and 461 cm−1 Raman bands to the same stable Cs+F3− ion pair species in solid argon.

Infrared Spectra and DFT Calcula­tions of Planar and Bridged Methyl­idene Intermediates in Reactions of Laser‐Ablated Yttrium and Lanthanum Atoms with Di‐, Tri‐, and Tetrahalomethanes

Abstract: Reactions of laser-ablated Y and La atoms with di-, tri-, and tetrahalomethanes in excess argon were investigated and the products were identified from their infrared matrix spectra, isotopic shifts, and comparison with frequencies computed by density functional theory. These DFT calculations also show that the primary products are planar and bridged methylidenes depending on the number of halogen atoms with no trace of insertion and products, parallel to the previous Sc results. While the C–M bond in the planar configuration has a considerable amount of -character, the observed bridged Y and La methylidene structures are indicative of strong electron donation to the empty d-orbtals on the metal center. These identified products reconfirm that the electronic structures of group 3 metals (d1s2) do not allow formation of the higher oxidation-state product in reaction with halomethanes. The preference of a group 3 metal product with more M–F bonds over those with more M–Cl bonds is also investigated.

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