Showing papers by "Bruce S. Ault published in 2003"

Photolysis of alpha-azidoacetophenones: direct detection of triplet alkyl nitrenes in solution.

Abstract: We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of alpha-azido acetophenone derivatives, 1. Alphazides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. (13)C and (15)N isotope labeling studies allowed us to characterize the C-N stretch of the nitrene intermediate at 1201 cm(-)(1).

Theoretical Study of the Reaction Mechanism and Role of Water Clusters in the Gas-Phase Hydrolysis of SiCl4

Abstract: The energies and thermodynamic parameters of the elementary reactions involved in the gas-phase hydrolysis of silicon tetrachloride were studied using ab initio quantum chemical methods (up to MP4//MP2/6-311G(2d,2p)), density functional (B3LYP/6-311++G(2d,2p)), and G2(MP2) theories. The proposed mechanism of hydrolysis consists of the formation of SiCl 4-x(OH)x (x ) 1-4), disiloxanes Cl4-x(OH)x-1Si-O-SiCl4-x(OH)x-1, chainlike and cyclic siloxane polymers [-SiCl2sO-]n, dichlorosilanone Cl2SidO, and silicic acid (HO)2SidO. Thermodynamic parameters were estimated, and the transition states were located for all of the elementary reactions. It was demonstrated that the experimentally observed kinetic features for the hightemperature hydrolysis are well described by a regular bimolecular reaction occurring through a four-membered cyclic transition state. In contrast, the low-temperature hydrolysis reaction cannot be described by the traditionally accepted bimolecular pathway for SisCl bond hydrolysis because of high activation barrier (Ea ) 107.0 kJ/mol, ¢G q ) 142.5 kJ/mol) nor by reactions occurring through three- or four-molecular transition states proposed earlier for reactions occurring in aqueous solution. The transition states of SiCl 4 with oneand two-coordinated water molecules were located; these significantly decrease the free energy of activation ¢G q (to 121.3 and 111.5 kJ/mol, correspondingly). However, this decrease in ¢G q is not sufficient to account for the high value of the hydrolysis rate observed experimentally under low-temperature conditions.

Matrix isolation and theoretical study of the photochemical reaction of PH3 with OVCl3 and CrCl2O2

Abstract: The matrix isolation technique has been combined with theoretical calculations to identify and characterize the photoproducts in the reaction of PH3 with OVCl3 and CrCl2O2. In contrast to previous studies, HCl elimination from an initial complex is not observed. Instead, a number of product bands, some quite intense, were observed and have been assigned to phosphine oxide, H3PO, through the direct O atom transfer from the transition metal oxo compound. This identification was supported by extensive isotopic labeling, and by comparison to theoretical calculations. H3PO formed in this manner is cage-paired and interacts with the transition metal fragment (VCl3 or Cl2CrO), leading to a 113 cm-1 red-shift of the PO stretching mode relative to isolated monomeric H3PO.

Infrared Matrix Isolation and Theoretical Study of the Reactions of MoCl4O and MoCl2O2 with CH3OH: Characterization of Cl3Mo(O)OCH3

Abstract: The matrix isolation technique has been employed to investigate the reactions of Cl4MoO and MoCl2O2 with CH3OH and C2H5OH. Using twin jet deposition, the initial intermediate in the mechanism was identified as a weakly bound molecular complex, characterized by perturbations to the MoO and O−H stretching modes. The complex between Cl4MoO and CH3OH was destroyed by near-UV irradiation, producing the novel Cl3Mo(O)OCH3 species and cage-paired HCl. The complex between CH3OH and MoCl2O2 was not photosensitive. Merged jet deposition of CH3OH and Cl4MoO, with a 70 °C reaction zone, led to complete conversion to Cl3Mo(O)OCH3, the same product observed in after irradiation of the 1:1 complex. This species was identified by use of extensive isotopic labeling, by the observation of HCl (or DCl) as an additional reaction product, and by comparison to density functional calculations. Most of the fundamental vibrational modes of this species that lie above 400 cm-1 were observed, some with very high intensities. C2H5OH...

Matrix Isolation and Theoretical Study of the Photochemical Reaction of PH3 with OVCl3 and CrCl2O2.

Abstract: The matrix isolation technique has been combined with theoretical calculations to identify and characterize the photoproducts in the reaction of PH3 with OVCl3 and CrCl2O2. In contrast to previous studies, HCl elimination from an initial complex is not observed. Instead, a number of product bands, some quite intense, were observed and have been assigned to phosphine oxide, H3PO, through the direct O atom transfer from the transition metal oxo compound. This identification was supported by extensive isotopic labeling, and by comparison to theoretical calculations. H3PO formed in this manner is cage-paired and interacts with the transition metal fragment (VCl3 or Cl2CrO), leading to a 113 cm-1 red-shift of the PO stretching mode relative to isolated monomeric H3PO.