Pulsed jet discharge matrix isolation and computational study of bromine atom complexes: Br···BrXCH(2) (X = H, Cl, Br)

Halogen atoms are important reactive radicals in the atmosphere. In this work, pulsed jet discharge matrix isolation spectroscopy and computational methods were used to characterize prereactive complexes of halogen atoms with simple halons. Our experiments combined matrix isolation techniques with a pulsed DC discharge nozzle, where a dilute CH(2)XBr (X = H, Cl, Br)/rare gas sample was gently discharged and the products were deposited onto a cold KBr window. The Br···BrCH(2)X (X = H, Cl, Br) complexes were characterized by infrared and electronic spectroscopy, supported by ab initio and density functional theory (DFT) calculations, which shed light on the structure of, bonding in, and binding energy of the complexes. The correlation of charge-transfer energy with donor ionization potential (Mulliken correlation) was examined, and the charge-transfer photochemistry of the complexes was explored.     

Formation and relaxation dynamics of iso-CH2Cl-I in cryogenic matrices

Photolysis of chloroiodomethane (CH(2)ClI) in cryogenic matrices followed by recombination of the nascent radical pair produces an isomer (CH(2)Cl-I) that features a halogen-halogen (Cl-I) bond. Using ultrafast laser pulses, it is possible to follow the formation of this isomer by transient electronic absorption in low-temperature matrices of N(2), CH(4), and Ar. Frequency-domain measurements provide vibrational and electronic spectra, and electronic structure calculations give the structures of the isomers and the minimum energy path that connects them. The ultrafast experiments cleave the C-I bond with a 267-nm photolysis pulse and probe the formation of the isomer at wavelengths between 435 nm and 510 nm. The longest wavelengths preferentially interrogate vibrationally excited molecules, and their transient absorption shows that the highly vibrationally excited isomer appears within 1 to 2 ps, depending on the matrix, likely reflecting the loss of 2000 cm(-1) or more of energy in a strong, inelastic collision of the fragments with the matrix. The subsequent relaxation of the vibrationally excited isomer occurs in 20 to 40 ps, a time that is comparable to those observed for halomethane molecules and their isomers in liquids and in supercritical CO(2). These observations suggest that the formation and initial relaxation of the isomer in dense media do not depend strongly on the identity of the surroundings.     

Structural Investigations of Novel Indoloacridines

Abstract  5,6-Dihydro-9-chloro-7-phenylindolo[3,2-c]acridinium sulfates (3a-c) were formed from 2,3,4,9-tetrahydro-1H-carbazol-1-ones (1a-c) on reaction with 2-amino-5-chlorobenzophenone. 3b and 3c are isostructural and all three sulfates crystallize in the triclinic system with the space group P each with two crystallographically independent organic cations and one sulfate anion in the asymmetric unit. 3a exhibits the parameters a = 9.8896(17), b = 14.165(2) and c = 14.583(2) ?, α = 83.184(3)°, β = 78.985(3)° and γ = 75.903(3)°; 3b a = 10.5713(9), b = 14.5667(12) and c = 14.9947(13) ?, α = 90.916(1)°, β = 97.153(1)° and γ = 97.208(1)°; and 3c a = 10.5505(10), b = 14.6090(13) and c = 15.2960(14) ?, and α = 91.4310(10)°, β = 98.8630(10)° and γ = 98.9100(10)°. The structure of 3c exhibits voids of 286 ?³ of ill defined and disordered solvate molecules. The structure of 3a is both non-merohedrally twinned and also exhibits an interesting type of pseudosymmetry. All but the sulfate ion and the ethylene moieties also fit with a unit cell half the size with only one independent cation, and disordered ethylene moieties and sulfate anions. The choice of unit cell was made based on the diffraction pattern and the absence of disorder in the larger cell, pointing towards alternating ordered ethylene and sulfate units rather than randomly disordered ones. Index Abstract  [The structures of three indoloacridines 3a, 3b, and 3c with similar packing motifs are described. The structure of 3a is both non-merohedrally twinned and also exhibits an interesting type of pseudosymmetry by being a double volume ordered supercell of an alternative cell with disordered ethylene and sulfate anion moieties.]      

Spectroscopic and computational studies of matrix-isolated iso-CHBr3: structure, properties, and photochemistry of iso-bromoform

Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.     

Investigation of tyrosine nitration in proteins by mass spectrometry.

In vivo nitration of tyrosine residues is a post-translational modification mediated by peroxynitrite that may be involved in a number of diseases. The aim of this study was to evaluate possibilities for site-specific detection of tyrosine nitration by mass spectrometry. Angiotensin II and bovine serum albumin (BSA) nitrated with tetranitromethane (TNM) were used as model compounds. Three strategies were investigated: (i) analysis of single peptides and protein digests by matrix-assisted laser desorption/ionization (MALDI) peptide mass mapping, (ii) peptide mass mapping by electrospray ionization (ESI) mass spectrometry and (iii) screening for nitration by selective detection of the immonium ion of nitrotyrosine by precursor ion scanning with subsequent sequencing of the modified peptides. The MALDI time-of-flight mass spectrum of nitrated angiotensin II showed an unexpected prompt fragmentation involving the nitro group, in contrast to ESI-MS, where no fragmentation of nitrated angiotensin II was observed. The ESI mass spectra showed that mono- and dinitrated angiotensin II were obtained after treatment with TNM. ESI-MS/MS revealed that the mononitrated angiotensin II was nitrated on the side-chain of tyrosine. The dinitrated angiotensin II contained two nitro groups on the tyrosine residue. Nitration of BSA was confirmed by Western blotting with an antibody against nitrotyrosine and the sites for nitration were investigated by peptide mass mapping after in-gel digestion. Direct mass mapping by ESI revealed that two peptides were nitrated. Precursor ion scanning for the immonium ion for nitrotyrosine revealed two additional partially nitrated peptides. Based on the studies with the two model compounds, we suggest that the investigation of in vivo nitration of tyrosine and identification of nitrated peptides might be performed by precursor ion scanning for the specific immonium ion at m/z 181.06 combined with ESI-MS/MS for identification of the specific nitration sites.     

Structure determination of two conotoxins from Conus textile by a combination of matrix-assisted laser desorption/ionization time-of-flight and electrospray ionization mass spectrometry and biochemical methods

Two highly modified conotoxins from the mollusc Conus textile, epsilon-TxIX and Gla(1)-TxVI, were characterized by matrix-assisted laser desorption/ionization and electrospray mass spectrometry and also by electrospray ionization tandem and triple mass spectrometry in combination with enzymatic cleavage and chemical modification reactions. The mass spectrometric studies allowed the confirmation of the sequence determined by Edman degradation and assignment of unidentified amino acid residues, among which bromotryptophan residues and an O-glycosylated threonine residue were observed. Methyl esterification was found necessary for the site-specific assignment of the Gla residues in the peptides.     

Continuous hydrothermal synthesis of nanometric BaZrO3 in supercritical water

Nanocrystalline barium zirconate (BaZrO₃) was synthesized using a hydrothermal synthesis process working in supercritical conditions and in a continuous way. By this method, we succeeded in the continuous and rapid production of nanopowders. As a preliminary work three barium precursors have been investigated: barium hydroxide (Ba(OH)₂), barium acetate (Ba(CH₃COO)₂) and barium nitrate (Ba(NO₃)₂). Two of them (Ba(CH₃COO)₂ and Ba(NO₃)₂) led to the pure perovskite phase. Then an experimental design has been conducted in order to determine the influence of the experimental parameters on the crystallinity and the grain size of the final product.     

Characteristics of LiFePO4 obtained through a one step continuous hydrothermal synthesis process working in supercritical water

The olivine-like material LiFePO₄ was prepared via a continuous hydrothermal synthesis process working from subcritical to supercritical water conditions. The influence of some processing parameters–temperature and reaction time–was investigated in terms of material purity, grain size and morphology. Supercritical conditions were found to be attractive to synthesize in one step a well-crystallized material without impurities. The primary particles size was in the nanometric range. They showed a natural tendency to form micron size agglomerates, which were supposed to be the cause of the limited capacity, as demonstrated through a cross study using laser particle size distribution analysis, electrochemical measurements and XRD at different Li contents.