Showing papers by "Lester Andrews published in 2007"
TL;DR: Multiconfigurational quantum chemical methods have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U.
Abstract: Multiconfigurational quantum chemical methods (complete active space self-consistent field (CASSCF)/second-order perturbation theory (CASPT2)) have been used to study the agostic interaction between the metal atom and H(C) in the methylidene metal dihydride complexes H2MCH2, where M is a second row transition metal or the actinide atoms Th or U. The geometry of some of these complexes is highly irregular due to the formation of a three center bond CH center dot center dot center dot M, where the electrons in the CH bond are delocalized onto empty or half empty orbitals of d- or f-type on the metal. No agostic interaction is expected when M = Y, where only a single bond with methylene can be formed, or when M = Ru, because of the lack of empty electron accepting metal valence orbitals. The largest agostic interaction is found in the Zr and U complexes.
73 citations
TL;DR: In this article, it was shown that triple-bonded actinide-alkylidynes have relatively strong UC triple bonds between the U 6d-5f hybrid orbitals and carbon 2s-2p orbitals.
Abstract: Chemistry of the actinide elements represents a challenging yet vital scientific frontier. Development of actinide chemistry requires fundamental understanding of the relative roles of actinide valence-region orbitals and the nature of their chemical bonding. We report here an experimental and theoretical investigation of the uranium methylidyne molecules X3UCH (X = F, Cl, Br), F2ClUCH, and F3UCF formed through reactions of laser-ablated uranium atoms and trihalomethanes or carbon tetrafluoride in excess argon. By using matrix infrared spectroscopy and relativistic quantum chemistry calculations, we have shown that these actinide complexes possess relatively strong UC triple bonds between the U 6d-5f hybrid orbitals and carbon 2s-2p orbitals. Electron-withdrawing ligands are critical in stabilizing the U(VI) oxidation state and sustaining the formation of uranium multiple bonds. These unique UC-bearing molecules are examples of the long-sought actinide-alkylidynes. This discovery opens the door to the rational synthesis of triple-bonded actinidecarbon compounds.
actinide multiple bond
heavy element
laser ablation
matrix isolation
relativistic quantum chemistry
69 citations
TL;DR: Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2, which is found to be less reactive in methane activation than thorium atoms.
Abstract: Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U + CH4 reaction products are identified through isotopic substitution ((CH4)-C-13, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C-1 ground-state structure for the triplet CH2UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2UH2 complex is distorted more than CH2ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes. (Less)
62 citations
TL;DR: In this paper, the number of valence electrons of the metal is evidently an important factor in the magnitude of the agostic interaction in simple methylidene dihydride systems, and two primary products, CH3−MH and CH2−MH2, are identified for each metal from the matrix IR spectra.
45 citations
TL;DR: In this paper, laser-ablated Cr, Mo, and W atoms react with di-, tri-, and tetrahalomethanes to form XC⋮MX3 (M = Mo, W; X = H, F, Cl) methylidyne molecules as major products.
41 citations
38 citations
TL;DR: In this article, a triplet state electron-deficient FC÷MF3 methylidyne complexes are identified by their infrared spectra and comparison to density functional vibrational frequency calculations of stable possible products.
37 citations
TL;DR: Two interconvertible iron dioxide-dioxygen complexes were prepared and characterized by matrix isolation infrared absorption spectroscopy as well as theoretical calculations.
Abstract: Two interconvertible iron dioxide−dioxygen complexes were prepared and characterized by matrix isolation infrared absorption spectroscopy as well as theoretical calculations. Iron atoms react with O2 to form the inserted FeO2 molecule in solid argon only upon UV−visible light irradiation. Annealing allows the dioxygen molecules to diffuse and to react with FeO2 and form the side-on and end-on bonded dioxygen−iron dioxide complexes, (η2-O2)FeO2 and (η1-O2)FeO2. The side-on bonded structure is a peroxide complex having a singlet ground state with a nonplanar C2v symmetry. The end-on bonded isomer is characterized to be a superoxide complex with a planar 3A‘ ‘ ground state. These two isomers are photoreversible, that is, near-infrared light (λ > 850 nm) induces the conversion of the side-on bonded (η2-O2)FeO2 complex to the end-on bonded (η1-O2)FeO2 isomer and vice versa with red light irradiation (λ > 600 nm).
34 citations
TL;DR: In this article, the reactivity of thorium atoms for N-H bond activation was revealed, revealing a new type of multiple bond to actinide atoms and showing that this bond is strong for thorium as a result of an important contribution from the f orbitals.
Abstract: Laser-ablated thorium atoms react with ammonia to form thorimine (NH=ThH 2 ), the first actinide imine to be reported. This work underscores the high reactivity of thorium atoms, particularly for N-H bond activation, reveals a new type of multiple bond to actinide atoms, and shows that this bond is strong for thorium as a result of an important contribution from the f orbitals.
31 citations
TL;DR: The vibrational spectra of UBz and ThBz have been measured in solid argon and according to the calculations, AcBz are stable molecules, as well as other species like BzAcBZ and Bz Ac2Bz.
Abstract: The vibrational spectra of UBz and ThBz have been measured in solid argon. Complementary quantum chemical calculations have allowed the assignments of the vibrational spectra. According to the calculations, AcBz are stable molecules, as well as other species like BzAcBz and BzAc2Bz. Experimentally, there is no evidence for the sandwich compounds BzAcBz and BzAc2Bz due to the limitations in the reagent concentrations.
31 citations
TL;DR: Aluminum atom reactions with the H2 + O2 mixtures favor the HAl(OH)2 product, suggesting that AlH3 generated by UV irradiation combines with O2 to form HAl[OH]2.
Abstract: Reactions of laser-ablated Al, Ga, In, and Tl atoms with H2O2 and with H2 + O2 mixtures diluted in argon give new absorptions in the O−H and M−O stretching and O−H bending regions, which are assigned to the metal mono-, di-, and trihydroxide molecules. Isotopic substitutions (D2O2, 18O2, 16,18O2, HD, and D2) confirm the assignments, and DFT calculations reproduce the experimental results. Infrared spectra for the Al(OH)(OD) molecule verify the calculated C2v structure. The trihydroxide molecules increase on annealing from the spontaneous reaction with a second H2O2 molecule. Aluminum atom reactions with the H2 + O2 mixtures favor the HAl(OH)2 product, suggesting that AlH3 generated by UV irradiation combines with O2 to form HAl(OH)2.
TL;DR: The trihydrido methylidyne complex (HC⋮ReH3) as discussed by the authors is formed in the reaction of Re atoms with CH4 in excess argon.
TL;DR: The simplest possible chemical reduction of dihydrogen using two lithium valence electrons to form the rhombic (LiH)(2) dimer is observed.
Abstract: A matrix isolation IR study of laser-ablated lithium atom reactions with H2 has been performed in solid para-hydrogen, normal hydrogen, neon, and argon. The LiH molecule and (LiH)(2,3,4) clusters were identified by IR spectra with isotopic substitution (HD, D(2), and H(2) + D(2)) and comparison to frequencies calculated by density functional theory and the MP2 method. The LiH diatomic molecule is highly polarized and associates additional H(2) to form primary (H(2))(2)LiH chemical complexes surrounded by a physical cage of solid hydrogen where the ortho and para spin states form three different primary complexes and play a role in the identification of the bis-dihydrogen complex and in characterization of the matrix cage. The highly ionic rhombic (LiH)(2) dimer, which is trapped in solid matrices, is calculated to be 22 kcal/mol more stable than the inverse hydrogen bonded linear LiH-LiH dimer, which is not observed here. The cyclic lithium hydride trimer and tetramer clusters were also observed. Although the spontaneous reaction of 2 Li and H(2) to form (LiH)(2) occurs on annealing in solid H(2), the formation of higher clusters requires visible irradiation. We observed the simplest possible chemical reduction of dihydrogen using two lithium valence electrons to form the rhombic (LiH)(2) dimer.
TL;DR: The present results reconfirm that the number of valence electrons on the metal is important for agostic interaction in simple methylidene complexes.
Abstract: Reactions of laser-ablated group 3 metal atoms with methyl halides have been carried out in excess of Ar during condensation and the matrix infrared spectra studied. The metals are as effective as other early transition metals in providing insertion products (CH3-MX) and higher oxidation state methylidene complexes (CH2-MHX) (X = F, Cl, Br) following alpha-hydrogen migration. Unlike the cases of the group 4-6 metals, the calculated methylidene complex structures show little evidence for agostic distortion, consistent with the previously studied group 3 metal methylidene hydrides, and the C-M bond lengths of the insertion and methylidene complexes are comparable to each other. However, the C-Sc bond lengths are 0.013, 0.025, and 0.029 A shorter for the CH2-ScHX complexes, respectively, and the spin densities are consistent with weak C(2p)-Sc(3d) pi bonding. The present results reconfirm that the number of valence electrons on the metal is important for agostic interaction in simple methylidene complexes.
TL;DR: Calculations indicate that the CH2=MF2 complexes have near C2v symmetry and are considerably more stable than other possible products (CH2(mu-F)MF and CHF=MHF).
Abstract: Group 4 transition metal methylidene difluoride complexes (CH2MF2) are formed by the reaction of methylene fluoride with laser-ablated metal atoms and are isolated in an argon matrix. Isotopic substitution of the CH2F2 precursor and theoretical computations (B3LYP and CCSD) confirm product identifications and assignments. Our calculations indicate that the CH2MF2 complexes have near C2v symmetry and are considerably more stable than other possible products (CH2(μ-F)MF and CHFMHF). The primary reaction exothermicity provides more than enough energy to activate the initial bridge-bonded CH2(μ-F)MF products on the triplet potential energy surface to complete an α-F transfer to form the very stable CH2MF2 products. Analogous experiments with CHF3 produce CHFTiF2, which is not distorted at the C−H bond, whereas the heavier group 4 metals form lower-energy triplet HC÷MF3 complexes, which contain weak degenerate C(p)−M(d) π-bonding interactions. Comparisons are made with the CH2MHF methylidene species, which sho...
TL;DR: The XC[triple bond]ReX3 complexes (X = F, Cl) were identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory as discussed by the authors.
Abstract: The XC[triple bond]ReX3 complexes (X = F, Cl) are produced by CX(4) reaction with laser-ablated Re atoms, following oxidative C-X insertion and alpha-halogen migration in favor of the carbon-metal triple bond and are identified through the observation of characteristic absorptions in the argon matrix infrared spectra and comparison with vibrational frequencies calculated by density functional theory. The methylidyne C-F and C-Cl stretching absorptions are observed near 1584 and 1328 cm-1, and the C-H stretching modes for HC[triple bond]ReX3 at 3104 and 3097 cm(-1), respectively, which are substantially higher than the precursor stretching modes and in agreement with the general trend that higher s-orbital character in carbon hybridization leads to a higher stretching frequency. The Jahn-Teller effect in the doublet-state XC[triple bond]ReX3 and HC[triple bond]ReX3 complexes gives rise to distorted structures with Cs symmetry and two equivalent longer Re-X bonds and one slightly shorter Re-X bond.
TL;DR: The transmittance peak for down may be significant in the conservation of radiation heat: the 1700 cm −1 radiation transmitted by the down is scattered and reflected by the pennaceous region of the contour feather, thereby containing the heat radiation within the feather coat.
TL;DR: In this paper, the authors used infrared spectra, isotopic substitution of the reactant precursors, and comparison to density functional theoretically predicted vibrational modes for the lowest energy structures.
TL;DR: The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies.
Abstract: Reactions of ethylene with laser-ablated group 5 metal atoms in excess argon have been carried out during codeposition at 8 K, and the matrix infrared spectra of intermediate products have been investigated. Oxidative C-H insertion of the transition metal into a C-H bond occurs and beta-hydrogen transfer follows to form the dihydrido complexes (MH2-C2H2). In the Ta spectra, the dihydrido complex is the primary product, whereas the Nb and V spectra reveal absorptions from both the insertion (MH-C2H3) and dihydrido complexes. The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies. Calculations also show that the higher oxidation-state complex becomes more stable relative to the insertion product going down the group 5 family.
TL;DR: In this article, a triplet state X1C÷MX23 and X2C ÷MX1X22 complexes are identified by their infrared spectra, carbon-13 isotopic shifts, and comparison to density functional vibrational frequency calculations.
TL;DR: Evidence is provided for a (H2)nNaH subcomplex surrounded by the solid hydrogen matrix cage in solid molecular hydrogens and neon, and for the spontaneous reaction of two Li and H2 to form (LiH)2 occurs on annealing in solid H2.
Abstract: Laser-ablated sodium atom reactions with H2 have been investigated in solid molecular hydrogens and neon. The NaH molecule and (NaH)2,3,4 clusters were identified by IR spectra with isotopic substitution (HD and D2) and comparison to frequencies calculated by density functional theory and the MP2 method. The use of para-hydrogen enriched samples provides evidence for a (H2)nNaH subcomplex surrounded by the solid hydrogen matrix cage. The ionic rhombic (NaH)2 dimer is characterized by strong absorptions at 761.7, 759.1, and 757.0 cm-1, respectively, in solid neon, para-hydrogen, and normal hydrogen matrices. The cyclic sodium hydride trimer and tetramer clusters are also observed. Although the spontaneous reaction of two Li and H2 to form (LiH)2 occurs on annealing in solid H2, the formation of (NaH)2 requires near uv photoexcitation.
TL;DR: This work underscores the high reactivity of thorium atoms, particularly for N-H bond activation, reveals a new type of multiple bond to actinide atoms, and shows that this bond is strong for thorium as a result of an important contribution from the f orbitals.
Abstract: Laser-ablated thorium atoms react with ammonia to form thorimine (NH=ThH(2)), the first actinide imine to be reported. This work underscores the high reactivity of thorium atoms, particularly for N-H bond activation, reveals a new type of multiple bond to actinide atoms, and shows that this bond is strong for thorium as a result of an important contribution from the f orbitals.