Biomedical subjects

Lester Andrews

Publications and source records attributed to Lester Andrews.

At least 19 recordsLinked to original sources

PubMed2006

Infrared spectra and density functional calculations of CH2 = MHX and CH[triple bond]MH2X complexes prepared in reactions of methyl halides with Mo and W atoms.

The simple methylidene and methylidyne complexes (CH2=MHX and CH[triple bond]MH2X; X = F, Cl, Br, and I) are prepared in reactions of laser-ablated Mo and W atoms with the methyl halides and investigated by matrix infrared spectroscopy and density functional theory calculations. These complex structures are photoreversible: visible irradiation converts the methylidene complex to the methylidyne complex, and UV irradiation reverses this effect via alpha-hydrogen migration. While the higher oxidation state complexes are readily formed regardless of halogen size, the Mo methylidyne complex is relatively less favored with increasing halogen size, and the W complex shows the opposite tendency. The group 6 metal methylidenes are predicted to have the most agostically distorted structures among the early transition-metal methylidenes. The computed carbon-metal bond shortens with increasing halogen size for both the methylidene and methylidyne complexes. Harmonic and anharmonic frequencies computed by DFT converge on the experimental values and thus provide support for the identification of these new Mo and W complexes.

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PubMed2006

Group 4 transition-metal atom reactions with CS2 and OCS: infrared spectra and density functional calculations of SMCS, SM-(eta2-CS), SMCO, and OMCS in solid argon.

Laser-ablated titanium, zirconium, and hafnium atoms were reacted with CS2 and OCS molecules during condensation in excess argon. With CS2, the SMCS and S-M(eta2-CS) products were formed on sample deposition. Photolysis increased both complexes, while annealing favored the lower energy S-M(eta2-CS) side-bound isomer. The OCS reactions produced SMCO, OMCS, and the simple M(eta2-CO)S adduct. Product absorptions are identified by comparison with density functional theory frequency calculations and isotopic substitutions. Investigations with OCS emphasized differences in the CS and CO bond insertion products.

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PubMed2006

Electron deficient carbon-titanium triple bonds: formation of triplet XC/TiX3 methylidyne complexes.

Laser-ablated titanium atoms react with CX4 (X = F and Cl) to produce triplet state XC/TiX3 complexes trapped in an argon matrix. Products are identified by their infrared spectra and comparison to theoretically predicted vibrations. Density functional theory calculations converge to C(3v) symmetry structures for these lowest-energy products. The two unpaired electrons in the carbon 2p orbitals are shared with empty titanium d orbitals leading to degenerate singly occupied pi molecular orbitals and an electron-deficient triple bond between the carbon and titanium centers, on the basis of DFT bonding analysis and spin density calculations. The carbon-titanium distances are near typical C=Ti double bond lengths, and the C-X bonds in the resulting products are shorter than in the CX4 precursors. It appears that X lone-pair conjugation contributes to the C-X bond strength and somewhat to the C-Ti bond, and Cl does better in this regard than F.

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PubMed2006

Reactions of Th and U atoms with C2H2: infrared spectra and relativistic calculations of the metallacyclopropene, actinide insertion, and ethynyl products.

Reactions of laser-ablated Th and U atoms with C(2)H(2) during condensation with excess argon at 7 K give several new product species. The metallacyclopropene, inserted hydride, and actinide ethynyl are identified from isotopic frequencies and relativistic DFT calculations. The higher-energy vinylidine isomer was not observed. These actinide metallacyclopropenes exhibit substantially stronger bonding interactions than found recently for the Pd and Pt metals. In the case of Th(C(2)H(2)) the argon matrix interaction is strong enough to reverse the computed order of states (MR-CISD) in favor of a triplet ground state for the (Ar)(n)(Th(C(2)H(2))) complex. The nature of the electronic interactions between various metal atoms and acetylene is compared and the origin of the particularly strong interaction for U and Th is traced to the higher energy of their 6d orbitals. The ThCCH and UCCH actinide ethynyl products are also observed and characterized by C[triple bond]C stretching modes 38+/-2 cm(-1) lower than acetylene itself.

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PubMed2006

Contrasting products in the reactions of Cr, Mo, and W atoms with H2O2: Argon matrix infrared spectra and theoretical calculations.

Products in the reactions of H2O2 and H2, O2 mixtures have been observed by matrix infrared absorptions and identified through comparisons with vibrational frequencies calculated for these molecules. The chromium reactions are dominated by lower oxidation state products, whereas molybdenum and tungsten chemistry favors higher oxidation state products. For example chromium dihydroxide, Cr(OH)2, molybdenum hydride oxide, H2MoO2, and tungsten hydride oxide, H2WO2, were observed in laser-ablated metal atom reactions with H2O2, and calculations show that these are the most stable molecules for this stoichiometry. Chromium monohydroxide, CrOH, was identified through O-H and Cr-O stretching modes, while HWO was observed by W-H and W=O stretching modes. The metal oxyhydroxides, HMO(OH), were observed for all metals. However, reactions with two H2O2 molecules give OCr(OH)2, MoO2(OH)2, and WO2(OH)2. The relative stabilities of different structures for Cr, Mo, and W are due to different participations of occupied d orbitals. The reactivity of the cold metal atoms with H2O2 on annealing the solid argon matrix increases on going down the group.

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PubMed2006

Reactions of group 3 transition metal atoms with CS2 and OCS: Matrix isolation infrared spectra and density-functional calculations of SMCS, SM-(eta2-CS), SMCO, and SM-(eta2-CO) in solid argon.

Laser-ablated scandium, yttrium, and lanthanum atoms were reacted with CS2 and OCS molecules in an argon matrix. Products of the type SMCX and S-M(eta2-CX) (X = S or O) were formed on sample deposition. Photolysis favored the S-M(eta(2)-CX) complex, while annealing increased the more stable SMCX isomer. Product absorptions are identified by density-functional frequency calculations and isotopic substitutions. This work reports the first vibrational spectroscopic characterization of Sc, Y, and La reaction products with CS2 and OCS and the subsequent interconversion between SMCX and S-M(eta2-CX) structural isomers.

Argon↗

PubMed2006

Infrared spectra of M(OH)(1,2,3) (M = Mn, Fe, Co, Ni) molecules in solid argon and the character of first row transition metal hydroxide bonding.

Reactions of laser-ablated Mn, Fe, Co, and Ni atoms with H(2)O(2) and with H(2) + O(2) mixtures in excess argon give new absorptions in the O-H and M-O stretching regions, which are assigned to metal dihydroxide and trihydroxide molecules, M(OH)(2) and M(OH)(3). Isotopic substitutions (D(2)O(2), (18)O(2), (16,18)O(2), D(2)) confirmed the assignments and DFT calculations reproduced the experimental results. The O-H stretching frequencies decreased in the dihydroxides from Sc to Zn. Mulliken and natural charge distributions indicate significant electron transfer from metal d orbitals to OH ligands that decreases from Sc to Zn, suggesting that the early transition metal hydroxides are more ionic and that the later transition metal hydroxides are more covalent.

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PubMed2006

Infrared spectra of the CH3-MX, CH2=MHX, and CH[triple bond]MH2X- complexes formed by reaction of methyl halides with laser-ablated group 5 metal atoms.

Reactions of group 5 metal atoms and methyl halides give carbon-metal single, double, and triple bonded complexes that are identified from matrix IR spectra and vibrational frequencies computed by DFT. Two different pairs of complexes are prepared in reactions of methyl fluoride with laser-ablated vanadium and tantalum atoms. The two vanadium complexes (CH(3)-VF and CH(2)=VHF) are persistently photoreversible and show a kinetic isotope effect on the yield of CD(2)=VDF. Identification of CH(2)=TaHF and CH[triple bond]TaH(2)F(-), along with the similar anionic Nb complex, suggests that the anionic methylidyne complex is a general property of the heavy group 5 metals. Reactions of Nb and Ta with CH(3)Cl and CH(3)Br have also been carried out to understand the ligand effects on the calculated structures and the vibrational characteristics. The methylidene complexes become more distorted with increasing halogen size, while the calculated C=M bond lengths and stretching frequencies decrease and increase, respectively. The anionic methylidyne complexes are less favored with increasing halogen size. Infrared spectra show a dramatic increase of the Ta methylidenes upon annealing, suggesting that the formation of CH(3)-TaX and its conversion to CH(2)=TaHX require essentially no activation energy.

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PubMed2006

Group 4 transition metal-benzene adducts: carbon ring deformation upon complexation.

Benzene is reacted with titanium, zirconium, and hafnium metal atoms, which are produced by laser-ablation. The M(C(6)H(6)), M(C(6)H(6))(2), and M(2)(C(6)H(6))(3) complexes are formed, isolated in solid argon, and identified by infrared spectroscopy using isotopic substitution of the benzene precursor. Density functional theory (DFT) calculations are used to confirm molecular assignments. Based on computed energies and the observed vibrational spectra and isotopic shifts, electronic ground states and geometries are predicted. Observed splitting of formerly degenerate modes provides the first experimental evidence for deformation of the planar carbon skeleton of benzene upon complexation with early transition metal atoms.

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PubMed2006

Experimental and theoretical investigations of IR spectra and electronic structures of the U(OH)2, UO2(OH), and UO2(OH)2 molecules.

Reactions of laser-ablated U atoms and H2O2 molecules produce UO2, H2UO2, and UO2(OH)2 as major products and U(OH)2 and HU(O)OH as minor products. Complementary information is obtained from similar reactions of U atoms with D2O2, with H2 + O2 mixtures, and with H2O in excess Ar. Through extensive relativistic density functional theory calculations, we have determined the geometry structures and ground states of these U species with a variety of oxidation states U(II), U(IV), U(V), and U(VI). The calculated vibrational frequencies, IR intensities, and isotopic frequency ratios are in good agreement with the experimental values, thus supporting assignments of the observed matrix IR spectra. We propose that the reactions proceed by forming an energized [U(OH)4] intermediate from reactions of the excited U atom with two H2O2 molecules. Because of the special stability of the U(VI) oxidation state, this intermediate decomposes to the UO2(OH)2 molecule, which reveals a distinctive difference between the chemistries of U and Th, where the major product in analogous Th reactions is the tetrahedral Th(OH)4 molecule owing to the stable Th(IV) oxidation state.

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PubMed2006

Infrared spectra and density functional calculations for M(OH)(2,3) and HOMO molecules and M(OH)2+ cations (M = Y, La).

Reactions of laser-ablated Y and La atoms with H2O2 gives the M(OH)2 and M(OH)3 molecules and the HOMO dehydration product, and the cation M(OH)2+ in solid argon. Density functional calculations show that the dihydroxide molecules and cations are bent at the metal center, and the symmetric and antisymmetric O-H stretching modes are both observed in the infrared spectra. The trihydroxide molecules have calculated C(3h) structures characterized by strong antisymmetric O-H and M-O stretching modes. Mulliken charges increase for all product molecules going down the Group 3 family and increase as one, two, and three OH ligands are bonded to the metal center. Evidence is also presented for the Y(OH)4- anion.

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PubMed2006

Methane activation by laser-ablated V, Nb, and Ta atoms: Formation of CH3-MH, CH2=MH2, CHMH3-, and (CH3)2MH2.

Methane activation by group 5 transition-metal atoms in excess argon and the matrix infrared spectra of reaction products have been investigated. Vanadium forms only the monohydrido methyl complex (CH3-VH) in reaction with CH4 and upon irradiation. On the other hand, the heavier metals form methyl hydride and methylidene dihydride complexes (CH3-MH and CH2=MH2) along with the methylidyne trihydride anion complexes (CHMH3-). The neutral products, particularly the methylidene complex, increase markedly on irradiation whereas the anionic product depletes upon UV irradiation or addition of a trace of CCl4 or CBr4 to trap electrons. Other absorptions that emerge on irradiation and annealing increase markedly at higher precursor concentration and are attributed to a higher-order product ((CH3)2MH2)). Spectroscopic evidence suggests that the agostic Nb and Ta methylidene dihydride complexes have two identical metal-hydrogen bonds.

Argon↗

PubMed2006

Formation and characterization of the uranium methylidene complexes CH2 = UHX (X = F, Cl, and Br).

The reactions between uranium atoms and CH3X (X = F, Cl, and Br) molecules are investigated in a solid argon matrix. The major products formed on ultraviolet irradiation are the CH2=UHX methylidene complexes. DFT calculations predict these triplet ground-state structures to be stable and to have significant agostic interactions. Parallels between the uranium and analogous thorium methylidene complexes are discussed.

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PubMed2006

Infrared spectra and density functional calculations for the Sc(OH)2,3 and HOScO molecules and the Sc(OH)2+ cation in solid argon.

Reactions of laser-ablated Sc atoms with H2O2 molecules or H2 and O2 mixtures in excess solid argon gives four major new products, which are identified from concentration dependence, isotopic substitution, the effect of electron trap doping, and comparison to frequencies calculated by the B3LYP density functional. These are the Sc(OH)3 trihydroxide, the Sc(OH)2 dihydroxide, the Sc(OH)2+ cation, and the trihydroxide anhydride HOScO molecule. The Sc(OH)2+ cation forms a complex in solid argon that is effectively modeled by calculations for the [(Ar)4Sc(OH)2]+ cation including frequency shifts between the neutral and cation dihydroxides. Finally, the Sc(OH)4- anion is detected in H2O2 experiments.

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PubMed2005

Infrared spectra and structures for group 4 dihydroxide and tetrahydroxide molecules.

Hafnium and zirconium atoms react with H(2)O(2) molecules and with H(2) + O(2) mixtures to form M(OH)(2) and M(OH)(4) molecules, which are trapped in solid argon and identified from isotopic shifts in the infrared spectra. Electronic structure calculations at the MP2 level converge to almost linear M(OH)(2) and tetrahedral M(OH)(4) molecules and predict vibrational frequencies for mixed isotopic molecules of lower symmetry that are in excellent agreement with experimental measurements, thus substantiating the identification of hafnium and zirconium dihydroxide and tetrahydroxide molecules. Titanium atoms react to give the same product molecules, but Ti(OH)(4) has an S(4) structure with bent Ti-O-H bonds, Ti(OH)(2) appears to be nearly linear, and the more stable tetravalent HM(O)OH isomer is more prominent for Ti. The Group 4 tetrahydroxides reported here are the first examples of pure metal tetrahydroxide molecules.

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PubMed2005

Infrared spectra and structures of the coinage metal dihydroxide molecules.

Laser-ablated Cu, Ag, and Au atoms react with H2O2 and with H2 + O2 molecules during condensation in excess argon to give four new IR absorptions in each system (O-H stretch, M-O-H bend, O-M-O stretch, and M-O-H deformation modes) that are due to the coinage metal M(OH)2 dihydroxide molecules. Isotopic substitution (D2O2, 18O2, 16O18O, D2, and HD) and comparison with frequencies computed by DFT verify these assignments. The calculations converge to 2B(g) ground electronic state structures with C2h symmetry, 111-117 degrees M-O-H bond angles, and substantial covalent character for these new metal dihydroxide molecules, particularly for Au(OH)2. This is probably due to the high electron affinity of gold owing to the effect of relativity.

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PubMed2005

Absorption spectra of ground-state and low-lying electronic States of copper nitrosyl: a rare gas matrix isolation study.

The reaction of ground-state Cu atoms with NO during condensation in solid argon, neon, and binary argon/neon mixtures has been reinvestigated. In addition to the ground-state already characterized in rare gas matrixes by its nu1 mode in reactions of laser-ablated Cu with nitric oxide, another very low lying electronic state is observed for CuNO in solid argon. Photoconversion and equilibrium processes are observed between the two lowest lying electronic states following photoexcitations to second and third excited states in the visible and near-infrared. The electronic spectrum of the CuNO complex was also recorded to understand the photoconversion processes. In solid neon, only the ground state (probably 1A') and the second and third excited states are observed. This suggests that interaction with the argon cage stabilizes the triplet state to make 1A' and 3A' ' states almost isoenergetic in solid argon. On the basis of previous predictions founded on DFT calculations on the very low lying 1A' and 3A' ', a mechanism is proposed, involving the singlet-triplet state manifolds. For these two lower and one higher electronic states, 14N/15N, 16O/18O, and 63Cu/65Cu isotopic data on nu1, nu2, and nu3 have been measured. On the basis of harmonic force-field calculations and relative intensities in the vibronic progressions, some structural parameters are estimated. The molecule is bent in all electronic states, with Cu-N-O bond angles varying slightly around 130 +/- 10 degrees , but the Cu-N bond force constants are substantially different, denoting larger differences in bond lengths.

Argon↗

PubMed2005

Formation and characterization of thorium methylidene CH2=ThHX complexes.

Laser-ablated thorium atoms react with methyl fluoride to give the CH2=ThHF molecule as the major product observed and trapped in solid argon. Infrared spectroscopy, isotopic substitution, and density functional theoretical frequency calculations confirm the identification of this methylidene complex. The four strongest computed absorptions (Th-H stretch, Th=C stretch, CH2 wag, and Th-F stretch) are the four vibrational modes observed. The CH2=ThHCl and CH2=ThHBr species formed from methyl chloride and methyl bromide exhibit the first three of these modes in the infrared spectra. The computed structures (B3LYP and CCSD) show considerable agostic interaction, similar to that observed for the Group 4 CH2=MHX (M = Ti, Zr, Hf) methylidene complexes, and the agostic angle and C=Th bond length decrease slightly in the CH2=ThHX series (X = F, Cl, Br).

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