Showing papers by "Lester Andrews published in 2010"

Polyfluoride Anions, a Matrix-Isolation and Quantum-Chemical Investigation

Abstract: Laser-ablation experiments with metals provide a source of electrons for capture processes, which are codeposited with solid argon and neon containing molecular fluorine. New argon and neon matrix absorptions at 510.6 and 524.7 cm−1, respectively, are photosensitive upon irradiation at >290 nm, which is consistent with their assignment to an isolated anion. These bands are below the [M]+[F3]− antisymmetric trifluoride stretching frequency of 550 cm−1 in an argon matrix, which is the typical relationship for cation−anion complexes and matrix-isolated anions. Thus, we report the isolated [F3]− anion in solid argon and neon environments. Moreover, we have carried out quantum-chemical calculations up to and including the CCSD(T) method to investigate the stabilities of polyfluoride anions higher than the [F3]− anion.

Infrared spectra and quantum chemical calculations of the uranium carbide molecules UC and CUC with triple bonds.

Abstract: Laser evaporation of carbon-rich uranium/carbon alloys followed by atom reactions in a solid argon matrix and trapping at 8 K gives weak infrared absorptions for CUO at 852 and 804 cm−1. A new band at 827 cm−1 becomes a doublet with mixed carbon 12 and 13 isotopes and exhibits the 1.0381 isotopic frequency ratio, which is appropriate for the UC diatomic molecule, and another new band at 891 cm−1 gives a three-band mixed isotopic spectrum with the 1.0366 isotopic frequency ratio, which is characteristic of the linear CUC molecule. CASPT2 calculations with dynamical correlation find the C≡U≡C ground state as linear 3Σu+ with 1.840 A bond length and molecular orbital occupancies for an effective bond order of 2.83. Similar calculations with spin-orbit coupling show that the U≡C diatomic molecule has a quintet (Λ = 5, Ω = 3) ground state, a similar 1.855 A bond length, and a fully developed triple bond of 2.82 effective bond order.

Noble Gas Matrices May Change the Electronic Structure of Trapped Molecules: The UO2(Ng)4 [Ng=Ne, Ar] Case

Abstract: In the last two decades matrix isolation techniques have achieved a notable success for several types of spectroscopy, ranging from infrared, optical absorption, laser-induced fluorescence and electron-spin resonance spectroscopy. Their major accomplishment lies on the possibility of embedding a guest molecule inside a host that acts as spectator and is inert towards any type of chemical interaction at very low temperatures (4–12 K). The most successful isolations occur when the gas used to trap the guest molecule is a light noble gas, known for its reluctance to form any type of chemical bond. The recent discoveries of stable noble gas complexes, such as HArF and NgAuF (Ng=Ar, Kr, Xe), have spurred new questions regarding the actual inertness of Ng hosts in matrix isolation, especially when actinide molecules, capable of forming bonds possibly involving the close-lying 5f, 6d, and 7s orbitals, are trapped. In particular, the molecules CUO and UO2 have challenged this paradigm, having shown in recent years a very probable noble-gas-induced ground state reversal going from Ne to Ar as the isolating gases. On investigating the UO2 molecule in more detail, Andrews et al. in 2000 noticed a large frequency shift for the asymmetric stretching mode at n=776 cm!1 in solid Ar, as compared to solid Ne at n=915 cm!1.[8] Such a significant shift can only be explained with the help of highly accurate calculations by exploring several electronic states and matching the measured asymmetric U!O stretch vibration with the computed ones. Following this procedure, the two possible candidates to the ground-state reversal have been recognized in the F2u (ground state in gas phase) and the H4g states, with their values being computed at n=919 and 824 cm!1, respectively, by using density functional theory (DFT). Subsequently, Heaven et al. have recorded fluorescence spectra attributed to UO2 in solid Ar and concluded that the pattern of low-lying electronic excited states is consistent with their previous gas-phase measurements, where they established the F2u as the ground state and stated that reversal of the ground state in solid Ar is unlikely. As a consequence of this contradicting situation, theoretical chemists carried out state-of-the-art calculations pinpointing the F2u state as the ground state and describing with great precision the excitation spectrum of the UO2 molecule. However, the calculations have been done for the bare molecule, certainly a good approximation for the gas phase, but not for the matrix. Now with improvement of computer architecture, we can use computationally intensive methods to study the electronic excited states of the UO2 molecule with a shell of noble gas atoms. In this work, we decided to run CASSCF/ CASPT2 calculations on the UO2(Ng)4 [Ng=Ne, Ar] complex, where the four noble gas atoms are displaced along the equatorial plane at a fixed equal U!Ng bond length, thus enforcing a D4h symmetry (D2h in the calculations). We decided to keep the inversion center to distinguish between the ungerade and gerade states and to allow maximum computational advantage. See the Supporting Information for more details. In Figure 1, the SO-CASPT2 potential energy surface (PES) computed with MOLCAS is depicted for the most [a] Prof. Dr. L. Gagliardi Department of Chemistry University of Minnesota and Supercomputing Institute 207 Pleasant St. SE, Minneapolis, MN 55455 (USA) Fax: (+1)6126 267 541 E-mail : gagliard@umn.edu [b] Dr. I. Infante Kimika Fakultatea Euskal Herriko Unibertsitatea and Donostia International Physics Center (DIPC) P.K. 1072, 20080 Donostia, Euskadi (Spain) [c] Prof. Dr. L. Andrews Department of Chemistry, University of Virginia Charlottesville, Virginia 22904-4319 (USA) [d] Dr. X. Wang Department of Chemistry, Tongji University 200092 Shanghai (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002549.

Infrared spectra of CH2=M(H)NC, CH3-MNC, and eta2-M(NC)-CH3 produced by reactions of laser-ablated group 5 metal atoms with acetonitrile.

Abstract: Methylidene isocyanides, methyl isocyanides, and η2-nitrile-π-complexes are observed in the matrix IR spectra from reactions of Group 5 metals with acetonitrile isotopomers. The primary isocyanide products with no trace of cyanide complexes are consistent with the reaction path proposed in the analogous Zr study. The major products (CH2═Ta(H)NC, CH3−NbNC, η2-Nb(NC)−CH3, and η2-V(NC)−CH3) after codeposition and reaction of metal with CH3CN clearly show the increasing preference for the higher oxidation-state complex on going down the group column, and the subsequent photochemistry provides further information for molecular rearrangements. The Group 5 metal methylidene isocyanides exhibit more agostic distortion than the Zr counterparts and are comparable to the previously studied Group 5 metal methylidene hydrides and halides. The computed structures and observed frequencies indicate that the effects of metal conjugation (C═Ta−N═C:) are minor.

Infrared spectra of HC≡C-MH and M-η2-(C2H2) produced in reactions of laser-ablated group 5 transition-metal atoms with acetylene.

Abstract: The ethynyl metal hydride (HC≡C−MH) and metallacycle complexes (M-η2-(C2H2)) are identified in the matrix infrared spectra from reactions of laser-ablated group 5 metal atoms with acetylene. The observed intensity variations reveal spontaneous formation of the cyclic complex upon annealing and its conversion to the insertion product via oxidative C−H insertion reaction and also during subsequent photolysis. The less stable vinylidene complex is not identified. The high binding energies, low C−C stretching frequencies, and long C−C bonds of the group 5 metal M-η2-(C2H2) species all indicate strong bonding relative to the late-transition-metal and light-metal analogues, but they are somewhat weaker than the group 4 metal counterparts.

Infrared Spectra of CH2=Zr(H)NC, CH3-ZrNC, and η2-Zr(NC)-CH3 Produced by Reactions of Laser-Ablated Zr Atoms with Acetonitrile

Abstract: The zirconium methylidene isocyanide, methyl isocyanide, and η2-nitrile-π-complexes are observed in the matrix IR spectra from reactions of laser-ablated Zr atoms and acetonitrile isotopomers. The methylidene CH2═Zr(H)NC has a C1 agostic structure in line with simple early transition-metal methylidenes recently produced from reactions with small alkanes and methyl halides, and the extent of agostic distortion is also comparable. Formation of the isocyanide complexes from acetonitrile is interesting but not surprising according to previous studies of metal reactions with nitrile-containing compounds, and their stabilities over the cyanide species are reproduced by DFT calculations. Observation of the relatively rare nitrile π-complex and its photodissociation suggests that the reaction proceeds in the order of Zr←NCCH3, η2-Zr(NC)—CH3, CH3—ZrNC, and CH2═Zr(H)NC. The intermediate transition-state structures are also examined.

Infrared spectra of XCIrX3 and CX2IrX2 prepared by reactions of laser-ablated iridium atoms with halomethanes

Abstract: Small iridium high oxidation-state complexes with carbon–iridium multiple bonds are identified in the product matrix infrared spectra from reactions of laser-ablated Ir atoms with tetra-, tri- and dihalomethanes. In contrast to the previously studied Rh case, Ir carbyne complexes (XCIrX3) are generated in reactions of tetrahalomethanes, and their short Ir–C bond lengths of 1.725–1.736 A are appropriate for the carbon–metal triple bonds. DFT calculations also show that the Ir carbynes with an Ir–F bond have unusual square planar structures, similar to the recently discovered Ru planar complexes. Diminishing preference for the carbyne complexes leads to methylidene product absorptions in the tri- and dihalomethane spectra, marking a limit for generation of small metal carbynes. The insertion complexes, on the other hand, are not observed in this study, suggesting that X migration from C to Ir following initial C–X insertion is swift.

Infrared spectra of MF2, MF2+, MF4-, MF3, and M2F6 molecules (M = Sc, Y, La) in solid argon.

Abstract: Reactions of laser-ablated Sc, Y and La atoms with F2 in excess argon gave new absorptions in the M−F stretching region, which are assigned to metal fluoride neutral species MF2 and MF3 and ions MF2+ and MF4−. Dibridged MF3 dimers, M2F6, were also identified through terminal M−F and bridge M−F−M stretching modes. Density functional theory (DFT) calculations substantiated the experimental assignments. Mulliken and natural charge distributions indicate significant electron transfer from metal d orbitals to F ligands that increase from Sc to La, suggesting that strong participation of La 5d orbital hybridization drives the F−La−F bond angle below 120°.

Infrared spectra of CX2=CoX2 and CX3-CoX complexes from reactions of laser-ablated cobalt atoms with halomethanes.

Abstract: Simple cobalt complexes with substantial carbon−cobalt double bond character are produced in Co atom reactions with tetra-, tri-, and dihalomethanes, whereas insertion complexes are identified only in the dihalomethane matrix infrared spectra. These complexes are identified from matrix infrared spectra and comparison with frequencies computed by density functional theory. Exclusive generation of carbenes in the tetrahalomethane systems is consistent with the computational results that the staggered allene-type conformer is the only meaningful energy minimum in the reaction path. Their short C−Co bondlengths of 1.732−1.764 A and CASSCF computed bond orders near 1.7 are also appropriate for carbon−cobalt double bonds. Hence, reactions of laser-ablated Co atoms are effective means to generate rarely reported high oxidation-state Co complexes with carbon−cobalt double bonds. Unlike the Rh and Ir cases, Co carbynes (with C−Co triple bonds) are not formed. The observation of CH2Cl−CoF with photoreversible inten...

Polyfluoride Anions, a Matrix-Isolation and Quantum-Chemical Investigation.

Abstract: Isolated [F3]- anions are identidied by IR spectroscopy to be formed by electron capture processes from laser-ablated metals (Cu, Ag, Au, Ti, Zr, Sc, Y) with excess F2 in Ne and Ar during condensation at 4 K.

Reaction of Pulsed Laser Evaporated Magnesium Atoms with Oxygen. Infrared Spectra of Linear OMgO and MgOMgO in Solid Argon.

Abstract: Pulsed laser evaporated magnesium atoms were codeposited with O 2 in excess argon on a 10 K substrate. The sharp dominant product band a 767.7 cm -1 showed magnesium and oxygen isotopic splittings and shifts in excellent agreement with shifts predicted for linear OMgO. Sharp 971.7- and 591.7 cm -1 bands exhibited isotopic shifts appropriate for the linear MgOMgO species. The pulsed laser evaporation process imparts sufficient kinetic energy to Mg atoms to provide the activation energy for insertion into the O 2 molecule