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Lester Andrews

Publications and source records attributed to Lester Andrews.

At least 55 records · Page 3Linked to original sources
PubMed2005

Infrared spectra of the group 2 metal dihydroxide molecules.

Group 2 metal atoms (Mg, Ca, Sr, and Ba) react on ultraviolet photoexcitation with O(2), H(2) mixtures in solid argon at 10 K to produce new absorptions in the O-H and O-M-O stretching regions. The effect of detailed isotopic substitution on these two absorptions identifies the M(OH)(2) molecules. The stepwise decrease in the O-H stretching modes in this chemical family demonstrates an increase in ionic character, which parallels the increase in base strength for the analogous solid compounds.

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PubMed2005

Infrared spectrum of Hg(OH)2 in solid neon and argon.

Mercury(II) hydroxide molecules have been prepared upon mercury arc lamp irradiation of Hg, H(2), and O(2) mixtures in solid neon and argon. The strongest three infrared absorptions are identified through isotopic substitution (D(2), HD, (18)O(2), (16)O(18)O) and comparison to frequencies from DFT calculations. The isolated Hg(OH)(2) molecule is stable and has a linear O-Hg-O linkage in a C(2) structure with an 86 degrees dihedral angle. However, in aqueous solution Hg(2+) and 2OH(-) may form an Hg(OH)(2) intermediate, which eliminates water and precipitates solid HgO: The solid Hg(OH)(2) compound is not known.

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PubMed2004

Solid mercury dihydride: mercurophilic bonding in molecular HgH(2) polymers.

Atomic mercury subjected to mercury arc irradiation reacts in solid hydrogen to give the linear HgH(2) molecule with strong IR absorptions at 1902 and 773 cm(-1). Annealing leads to HgH(2) dimer and trimer, and warming above 7 K allows the hydrogen matrix to sublime and solid HgH(2) to form. This covalent molecular solid is characterized by strong IR absorptions at 1802 and 673 cm(-1) and by decomposition at 150-170 K.

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PubMed2004

A discharge investigation of hydrogen and deuterium atom formation, and parahydrogen and orthodeuterium reconversion.

Hydrogen is flowed through a mild tesla-coil discharge and trapped at 3.8 K: New infrared absorptions of H2 are induced by interaction with trapped H atoms and H- anions. High purity parahydrogen and orthodeuterium samples are 1%-9% reconverted depending on the discharge pressure and recombination of atoms. Annealing the solid samples to 7 K reveals growth in p-H2 induced by o-H2, which shows that H atom recombination produces thermal nuclear spin populations. Similar results are found in discharge experiments with HD and on annealing solid HD. The observed increase in induced HD absorption by J = 1, H2 and D2 molecules formed on recombination gives approximately 1% for the H[D] atom concentration in our solid HD samples.

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PubMed2004

Persistent photo-reversible transition-metal methylidene system generated from reaction of methyl fluoride with laser-ablated zirconium atoms and isolated in a solid argon matrix.

A photoreversible transition-metal methylidene system has been formed for the first time by reaction of methyl fluoride and laser-ablated Zr atoms, isolated in solid argon, and investigated by means of infrared spectroscopy. Four different groups of absorptions are characterized on the basis of behaviors upon broad-band irradiation and sample annealing. Growth of Group I is accompanied by demise of Group II on irradiation with visible light (lambda > 530 nm) and vice versa with UV light (240 < lambda < 380 nm). The methylidene complex CH(2)=ZrHF is responsible for Groups I and II either in different singlet-triplet spin states or argon matrix packing configurations. The ground singlet state is stabilized by an agostic interaction. On the other hand, Group III, which arises from the Grignard type compound CH(3)-ZrF, disappears upon irradiation of UV light (lambda > 380 nm), increasing the concentration of CH(2)=ZrHF by alpha-H elimination. Fragments of methyl fluoride such as the CH(2)F radical comprise Group IV. Theoretical calculations are carried out for the alkylidene complex and other plausible products, and the results are compared with the experimental frequencies.

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PubMed2004

Preparation of CH3TiF and (CH3)2TiF2 from the reaction of CH(3)F with laser-ablated Ti atoms.

Laser-ablated Ti atoms react with CH(3)F upon condensation with excess argon to form primarily CH(3)TiF and (CH(3))(2)TiF(2). Irradiation in the UV region promotes alpha-hydrogen rearrangement of CH(3)TiF to CH(2)=TiHF and increases the yield of (CH(3))(2)TiF(2). Annealing to allow diffusion and reaction of more CH(3)F markedly increases the yield of (CH(3))(2)TiF(2). This shows that the CH(3)TiF + CH(3)F reaction is spontaneous and that triplet state CH(3)TiF is an extremely reactive molecule. B3LYP calculations are extremely effective in predicting vibrational frequencies and isotopic shifts for CH(3)TiF and (CH(3))(2)TiF(2) and thus in confirming their identification from matrix infrared spectroscopy.

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PubMed2004

On the electronic structure of molecular UO2 in the presence of Ar atoms: evidence for direct U-Ar bonding.

Calculations via scalar-relativistic density functional theory (DFT) and ab initio CCSD(T) methodologies are used to explore the possibility of direct interactions between molecular UO2 and Ar atoms. The 3Hg electronic state of UO2, which is an excited state of the isolated molecule, exhibits significant bonding to Ar in the model complexes UO2(Ar) and UO2(Ar)5. The calculated vibrational frequencies of ground-state 3Phiu UO2 and UO2(Ar)5 with an (fphi)1(fdelta)1 electron configuration agree well with the observed frequencies of UO2 in solid neon and solid argon, respectively. The results strongly suggest that the ground electron configuration of UO2 changes from 5f17s1 to 5f2 when the matrix host is changed from neon to argon.

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PubMed2004

On the noble-gas-induced intersystem crossing for the CUO molecule: experimental and theoretical investigations of CUO(Ng)n (Ng = Ar, Kr, Xe; n = 1, 2, 3, 4) complexes in solid neon.

Uranium atoms excited by laser ablation react with CO in excess neon to produce the novel CUO molecule, which forms distinct Ng complexes (Ng = Ar, Kr, Xe) when the heavier noble gases are added. The CUO(Ng) complexes are identified through CO isotopic and Ng substitution on the neon matrix infrared spectra and by comparison to DFT frequency calculations. The U-C and U-O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from frequencies for the (1)Sigma(+) CUO ground state, which identifies singlet ground state CUO(Ng) complexes. In solid neon the CUO molecule is also a complex CUO(Ne)(n), and the CUO(Ne)(n-1)(Ng) complexes are likewise specified. The next singlet CUO(Ne)(x)(Ng)(2) complexes in excess neon follow in like manner. However, the higher CUO(Ne)(x)(Ng)(n) complex (n = 3, 4) stretching modes approach pure argon matrix CUO(Ar)(n) values and isotopic behavior, which are characterized as triplet ground state complexes by DFT frequency calculations. This work suggests that the singlet-triplet crossing occurs with 3 Ar, 3 Kr, or 4 Xe and a balance of Ne atoms coordinated to CUO in the neon matrix host.

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PubMed2004

Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes.

Metal hydrides are of considerable importance in chemical synthesis as intermediates in catalytic hydrogenation reactions. Transition metal atoms react with dihydrogen to produce metal dihydrides or dihydrogen complexes and these may be trapped in solid matrix samples for infrared spectroscopic study. The MH(2) or M(H(2)) molecules so formed react further to form higher MH(4), (H(2))MH(2), or M(H(2))(2), and MH(6), (H(2))(2)MH(2), or M(H(2))(3) hydrides or complexes depending on the metal. In this critical review these transition metal and dihydrogen reaction products are surveyed for Groups 3 though 12 and the contrasting behaviour in Groups 6 and 10 is discussed. Minimum energy structures and vibrational frequencies predicted by Density Functional Theory agree with the experimental results, strongly supporting the identification of novel binary transition metal hydride species, which the matrix-isolation method is well-suited to investigate. 104 references are cited.

Hydrogen↗
PubMed2003

Bonding of multiple noble-gas atoms to CUO in solid neon: CUO(Ng)n (Ng=Ar, Kr, Xe; n=1, 2, 3, 4) complexes and the singlet-triplet crossover point.

Laser-ablated U atoms co-deposited with CO in excess neon produce the novel CUO molecule, which forms distinct Ng complexes (Ng=Ar, Kr, Xe) with the heavier noble gases. The CUO(Ng) complexes are identified through CO isotopic and Ng reagent substitution and comparison to results of DFT frequency calculations. The U[bond]C and U[bond]O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from neon matrix (1)Sigma(+) CUO values, which indicates a (1)A' ground state for the CUO(Ng) complexes. The CUO(Ng)(2) complexes in excess neon are likewise singlet molecules. However, the CUO(Ng)(3) and CUO(Ng)(4) complexes exhibit very different stretching frequencies and isotopic behaviors that are similar to those of CUO(Ar)(n) in a pure argon matrix, which has a (3)A" ground state based on DFT vibrational frequency calculations. This work suggests a coordination sphere model in which CUO in solid neon is initially solvated by four or more Ne atoms. Up to four heavier Ng atoms successively displace the Ne atoms leading ultimately to CUO(Ng)(4) complexes. The major changes in the CUO stretching frequencies from CUO(Ng)(2) to CUO(Ng)(3) provides evidence for the crossover from a singlet ground state to a triplet ground state.

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PubMed2003

Infrared spectra and structures of the stable CuH(2)(-), AgH(2)(-), AuH(2)(-), and AuH(4)(-) anions and the AuH(2) molecule.

Gold is noble, but excited gold is reactive. Reactions of laser-ablated copper, silver, and gold with H(2) in excess argon, neon, and pure hydrogen during condensation at 3.5 K give the MH molecules and the (H(2))MH complexes as major products and the MH(2)(-) and AuH(4)(-) anions as minor products. These new molecular anions are identified from matrix infrared spectra with isotopic substitution (HD, D(2), and H(2) + D(2)) and comparison to frequencies calculated by density functional theory. The stable linear MH(2)(-) anions are unique in that their corresponding neutral MH(2) molecules are higher in energy than M + H(2) and thus unstable to M + H(2) decomposition. Infrared spectra are observed for the bending modes of AuH(2), AuHD, and AuD(2) in solid H(2), HD, and D(2), respectively. The observation of square-planar AuH(4)(-) attests the stability of the higher Au(III) oxidation state for gold. The synthesis of CuH(2)(-) in solid compounds has potential for use in hydrogen storage.

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PubMed2003

Reactions of B atoms and clusters with NO: experimental and theoretical characterization of novel molecules containing B, N, and O.

Reactions of boron atoms and clusters with NO molecules in solid argon have been studied using matrix isolation infrared absorption spectroscopy. The reaction products were identified by isotopic substitution ((10)B, (11)B, (15)N(16)O, (14)N(18)O, and mixtures) and comparison with density functional calculations of isotopic frequencies. In solid argon, boron atoms spontaneously reacted with NO to form the insertion molecule NBO. The BNBO and OBNNO molecules were formed by the B and NO addition reactions to NBO. The linear BBNO and BBBNO nitrosyls also were formed spontaneously on annealing. These molecules photochemically rearranged to the more stable BNBO and BNBBO isomers, which have linear polyyne-like structures. The photosensitive OBNNO molecule decomposed to form the NNBO(2) van der Waals complex. In addition, the novel OBON diradical was also formed on photolysis in high-concentration NO experiments.

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PubMed2003

Infrared spectra of aluminum hydrides in solid hydrogen: Al2H4 and Al2H6.

The reaction of laser-ablated Al atoms and normal-H(2) during co-deposition at 3.5 K produces AlH, AlH(2), and AlH(3) based on infrared spectra and the results of isotopic substitution (D(2), H(2) + D(2) mixtures, HD). Four new bands are assigned to Al(2)H(4) from annealing, photochemistry, and agreement with frequencies calculated using density functional theory. Ultraviolet photolysis markedly increases the yield of AlH(3) and seven new absorptions for Al(2)H(6) in the infrared spectrum of the solid hydrogen sample. These frequencies include terminal Al-H(2) and bridge Al-H-Al stretching and AlH(2) bending modes, which are accurately predicted by quantum chemical calculations for dibridged Al(2)H(6), a molecule isostructural with diborane. Annealing these samples to remove the H(2) matrix decreases the sharp AlH(3) and Al(2)H(6) absorptions and forms broad 1720 +/- 20 and 720 +/- 20 cm(-1) bands, which are due to solid (AlH(3))(n). Complementary experiments with thermal Al atoms and para-H(2) at 2.4 K give similar spectra and most product frequencies within 2 cm(-1). Although many volatile binary boron hydride compounds are known, binary aluminum hydride chemistry is limited to the polymeric (AlH(3))( solid. Our experimental characterization of the dibridged Al(2)H(6) molecule provides an important link between the chemistries of boron and aluminum.

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