J. Steven Ogden

Synthesis, Structures and DFT Calculations on Alkaline‐Earth Metal Azide‐Crown Ether Complexes

The first examples of azide complexes of calcium, strontium or barium with crown ethers have been prepared and fully characterised, notably [Ba([18]crown-6)(N3)2(MeOH)], [Sr([15]crown-5)(N3)2(H2O)], [Ca([15]crown-5)(N3)2(H2O)] and [Sr([15]crown-5)(N3(NO3)]. Crystal structures reveal the presence of a variety of coordination modes for the azide groups including kappa 1-, mu-1,3- and linkages via H-bonded water molecules, in addition to azide ions. The [Ba([18]crown-6)(N3)2(MeOH)].1/3 MeOH contains dinuclear cations with three mu-1,3-NNN bridges, the first example of this type in main group chemistry. The structures obtained have been compared with molecular structures computed by density functional theory (DFT). This has allowed the effects of the crystal lattice to be investigated. A study of the M--N terminal metal-azide bond length and charge densities on the metal (M) and terminal nitrogen centre (N terminal) in these complexes has allowed the nature of the metal-azide bond to be investigated in each case. As in our earlier work on alkali metal azide-crown ether complexes, the bonding in the alkaline-earth complexes is believed to be predominantly ionic or ion-dipole in character, with the differences in geometries reflecting the balance between maximising the coordination number of the metal centre, and minimising ligand-ligand repulsions.

The X̃ 2B1, 2B2, 2A1, and 2A2 states of oxygen difluoride cation (F2O+): High-level ab initio calculations and simulation of the ultraviolet photoelectron spectrum of F2O

The ultraviolet photoelectron spectrum of F2O was recorded with a higher resolution than previously published. New vibrational structure was observed in the second and third bands. Near state-of-the-art molecular orbital calculations were performed on the X 1A1 state of F2O and the X 2B1, 2B2, 2A1, and 2A2 state of F2O+, and their potential energy functions were computed. Spectral simulations based on Franck–Condon factor calculations including the Duchinsky effect were carried out within the harmonic oscillator model and also with the inclusion of anharmonicity, in order to assist spectral assignment. Based on the computed ionization energies obtained with the coupled cluster and multireference configuration interaction methods with basis sets of up to quintuple zeta quality, the order of the low-lying cationic states of F2O+ has been firmly established. However, the detailed assignment of the overlapping second and third photoelectron bands was only achieved with the aid of spectral simulation. The it...

A mechanistic study of cyclic siloxane pyrolyses at low pressures

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethyl cyclopentasiloxane (D5), and the results interpreted in the context of various kinetic models. In particular, it is shown that the significant pyrolysis products--which include CH3, CH4, C2H2, C2H4, C2H6 and SiO--may be satisfactorily accounted for by radical reactions involving dimethylsiloxane (D1), and estimates are made of the various chain lengths for the proposed reactions based on a range of ambient conditions.

Osmium LIII edge extended X-ray absorption fine-structure studies on the osmium(VIII) oxide-fluorides, OsO3F2, K[OsO3F3] and Cs2[OsO4F2]

Abstract Osmium LIII edge extended X-ray absorption fine-structure data have been obtained for the title compounds and refined to give, for OsO3F2, d(OsO)1.70 A and d(OsF)  1.89 and 2.09 A, for K[OsO3F3], d(OsO)1.70 A and d(OsF)1.92 A and, for Cs2[OsO4F2], d(OsO)  1.70 A and d(OsF)  2.05 A (bond lengths accurate to ±0.02 A).

A mechanistic study of the low pressure pyrolysis of linear siloxanes

Matrix isolation IR spectroscopy has been used to study the vacuum pyrolysis of 1,1,3,3-tetramethyldisiloxane (L1), 1,1,3,3,5,5-hexamethyltrisiloxane (L2) and 3H,5H-octamethyltetrasiloxane (L3) at ca. 1000 K in a flow reactor at low pressures. The hydrocarbons CH3, CH4, C2H2, C2H4, and C2H6 were observed as prominent pyrolysis products in all three systems, and amongst the weaker features are bands arising from the methylsilanes Me2SiH2 (for L1 and L2) and Me3SiH (for L3). The fundamental of SiO was also observed very weakly. By use of quantum chemical calculations combined with earlier kinetic models, mechanisms have been proposed involving the intermediacy of silanones Me2Si=O and MeSiH=O. Model calculations on the decomposition pathways of H3SiOSiH3 and H3SiOSiH2OSiH3 show that silanone elimination is favoured over silylene extrusion.

Matrix-isolation studies on the vaporisation of arsenic(V) oxide: the characterisation of molecular species As4On(n= 7–10) by infrared spectroscopy

The vapour species produced on heating arsenic(V) oxide in vacuo have been isolated in low-temperature matrices, and examined by i.r. spectroscopy. Detailed studies reveal the existence of As4O6 and four higher oxides As4O7, As4O8, As4O9, and As4O10 which are characterised for the first time as stable molecular entities. Band assignments are proposed for several of the more intense absorptions, and the effects of 18O enrichment in the terminal AsO stretching region are interpreted on the basis of a restricted force-field analysis.

The characterisation of molecular boric acid by mass spectrometry and matrix isolation infrared spectroscopy

When crystalline orthoboric acid is heated in vacuo to ca. 40 °C, it vaporises to yield molecular H3BO3. The i.r. spectrum of this species isolated in low-temperature nitrogen matrices shows characteristic absorptions at 3 668.5 (E′), 1 426.2 (E′), 1 009.9 (E′), 675.0 (A″), 513.8 (A″), and 448.9 (E′) cm–1 consistent with C3h symmetry. These spectral assignments are supported by extensive isotope labelling, and by a partial normal co-ordinate analysis.

A spectroscopic study of the reaction between Br2 and dimethyl sulphide, and comparison with a parallel study made on Cl2+DMS

The reaction between molecular bromine and dimethyl sulfide (DMS) has been studied both as a co-condensation reaction in low temperature matrices by infrared (IR) matrix isolation spectroscopy and in the gas-phase at low pressures by UV photoelectron spectroscopy (PES). The co-condensation reaction leads to the formation of the molecular van der Waals adduct DMS-Br(2). This was identified by IR spectroscopy supported by results of electronic structure calculations. Calculation of the minimum energy structures in important regions of the reaction surface and computed IR spectra of these structures, which could be compared with the experimental spectra, allowed the structure of the adduct (C(s)) to be determined. The low pressure (ca. 10(-5) mbar) gas-phase reaction was studied by UV-PES, but did not yield any observable products, indicating that a third body is necessary for the adduct to be stabilised. These results are compared with parallel co-condensation and gas-phase reactions between DMS and Cl(2). For this reaction, a similar van der Waals adduct DMS-Cl(2) is observed by IR spectroscopy in the co-condensation reactions, but in the gas-phase, this adduct converts to a covalently bound structure Me(2)SCl(2), observed in PES studies, which ultimately decomposes to monochlorodimethylsulfide and HCl. For these DMS + X(2) reactions, computed relative energies of minima and transition states on the potential energy surfaces are presented which provide an interpretation for the products observed from the two reactions studied. The implications of the results obtained to atmospheric chemistry are discussed.

Interaction of diatomic ligands with Fe(II) meso-mono-4-pyridyl-tri-phenyl-porphyrinate. Spectral evidence of self-assembly in sublimed layers

The interaction of CO, NO and O2 gases with thin layers of meso-mono-4-pyridyl-tri-phenylporphyrinatoiron(II) (FeMPyTPP) obtained by sublimation onto low-temperature (T = 77 K) substrates has been investigated by means of IR and UV-visible spectroscopy. In contrast to the closely-related meso-tetraphenylporphyrinatoiron(II) (FeTPP), the formation of two types of axial complexes has been observed. In one of these the 5th coordination site is occupied by the pyridyl group of an adjacent molecule indicating the self-assembly of Fe(MPyTPP) in layers with formation of coordinatively linked oligomers. The degree of oligomerisation depends on the nature of the interacting gas. Due to the specificity of the supramolecular structure, the layers are fairly stable in ambient conditions and conserve their microporosity to bind reversibly with the aforementioned ligands.

Characterisation of the oxo-anions of bromine BrOx–(x= 1–4) by infrared, Raman, nuclear magnetic resonance, and bromine K-edge extended X-ray absorption fine structure techniques

The bromine oxo-anions BrOx–(x= 1–4) have been studied by vibrational and multinuclear n.m.r. spectroscopy and bromine K-edge extended X-ray absorption fine structure (EXAFS) measurements. Raman and/or i.r. data for BrO– and BrO2– are presented, and conflicts in the literature for the latter resolved. Oxygen-17 n.m.r. data for the halogen oxo-anions have been recorded, and an increase in δ(17O) with formal oxidation state of the halogen noted. Coupling to the halogen nucleus has been observed for ClO4–, 1J(35/37Cl–17O)= 85, and for BrO4–, 1J(79/81Br–17O)= 420Hz, but could not be resolved for IO4–. The 81Br n.m.r. resonance of BrO4–(δ= 2 476) and the 127I resonance of IO4–(δ= 4 090 p.p.m.) are reported. Bromine K-edge EXAFS data are presented for all four ions in solution, and for all but BrO– in the solid state, and differences in bond lengths between solid state and solution found to be within experimental error. Bond lengths obtained are: BrO4– 1.61, BrO3– 1.65, BrO2– 1.72, and BrO– 1.81 A. This is the first bond length reported for the hypobromite ion.