Showing papers by "Lester Andrews published in 2016"

Matrix Infrared Spectra and Quantum Chemical Calculations of Ti, Zr, and Hf Dihydride Phosphinidene and Arsinidene Molecules.

Abstract: Laser ablated Ti, Zr, and Hf atoms react with phosphine during condensation in excess argon or neon at 4 K to form metal hydride insertion phosphides (H2P-MH) and metal dihydride phosphinidenes (HP═MH2) with metal phosphorus double bonds, which are characterized by their intense metal–hydride stretching frequencies. Both products are formed spontaneously on annealing the solid matrix samples, which suggests that both products are relaxed from the initial higher energy M-PH3 intermediate complex, which is not observed. B3LYP (DFT) calculations show that these phosphinidenes are strongly agostic with acute H–P═M angles in the 60° range, even smaller than those for the analogous methylidenes (carbenes) (CH2═MH2) and in contrast to the almost linear H-N═Ti subunit in the imines (H-N═TiH2). Comparison of calculated agostic and terminal bond lengths and covalent bond radii for HP═TiH2 with computed bond lengths for Al2H6 finds that these strong agostic Ti–H bonds are 18% longer than single covalent bonds, and t...

Properties of Cerium Hydroxides from Matrix Infrared Spectra and Electronic Structure Calculations

Abstract: Reactions of laser ablated cerium atoms with hydrogen peroxide or hydrogen and oxygen mixtures diluted in argon and condensed at 4 K produced the Ce(OH)3 and Ce(OH)2 molecules and Ce(OH)2+ cation as major products. Additional minor products were identified as the Ce(OH)4, HCeO, and OCeOH molecules. These new species were identified from their matrix infrared spectra with D2O2, D2, and 18O2 isotopic substitution and correlating observed frequencies with values calculated by density functional theory. We find that the amounts of Ce(OH)3 and of the Ce(OH)2+ cation increase on UV (λ > 220 nm) photolysis, while Ce(OH)2, Ce(OH)4, and HCeO are photosensitive. The observed major species for Ce are in the +III or +II oxidation state, and the minor product, Ce(OH)4, is in the +IV oxidation state. The calculations for the vibrational frequencies with the B3LYP functional agree well with the experiment. The NBO analysis shows significant backbonding to the metal 4f and 5d orbitals for the closed shell species. Most o...

Structures and Properties of the Products of the Reaction of Lanthanide Atoms with H2O: Dominance of the +II Oxidation State.

Abstract: The reactions of lanthanides with H2O have been studied using density functional theory with the B3LYP functional. H2O forms an initial Lewis acid-base complex with the lanthanides exothermically with interaction energies from -2 to -20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H2LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from -25 to -75 kcal/mol. The formation of LnO + H2 for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H2LnO is mostly in the +III formal oxidation state with either Ln-O(-)/Ln-H(-) or Ln-(H2)(-)/Ln=O(2-) bonding interactions. A few of the H2LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with Ln=O(2-)/Ln-H(-) bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger Ln-O(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies.

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...

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.