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Collision-induced dissociation (CID) of the peptide alanine-4-aminomethylbenzoic acid-glycine, A(AMBz)G generates a prominent b(2) ion despite a previous report [ER. Talaty, T.J. Cooper, S.M. Osburn, M.J. Van Stipdonk, Collision-induced dissociation of protonated tetrapeptides containing beta-alanine, gamma-aminobutyric acid, e-aminocaproic acid or 4-aminomethylbenzoic acid residues, Rapid Commun. Mass Spectrom. 20 (2006) 3443-3455] which showed that incorporation of the aromatic amino acid into a peptide sequence inhibits generation of b(n) ions formed by cleavage to the immediate C-terminal side of the residue. Infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations suggest that the b(2) ion generated from A(AMBz)G has an acylium structure. The b2 ion generated from (AMBz)AG, in which the aromatic residue is situated at the amino-terminus, is instead a conventional oxazolone. (c) 2012 Elsevier B.V. All rights reserved.

UO(2) (+)-solvent complexes having the general formula [UO(2)(ROH)](+) (R=H, CH(3), C(2)H(5), and n-C(3)H(7)) are formed using electrospray ionization and stored in a Fourier transform ion cyclotron resonance mass spectrometer, where they are isolated by mass-to-charge ratio, and then photofragmented using a free-electron laser scanning through the 10 mum region of the infrared spectrum. Asymmetric O=U=O stretching frequencies (nu(3)) are measured over a very small range [from approximately 953 cm(-1) for H(2)O to approximately 944 cm(-1) for n-propanol (n-PrOH)] for all four complexes, indicating that the nature of the alkyl group does not greatly affect the metal centre. The nu(3) values generally decrease with increasing nucleophilicity of the solvent, except for the methanol (MeOH)-containing complex, which has a measured nu(3) value equal to that of the n-PrOH-containing complex. The nu(3) frequency values for these U(V) complexes are about 20 cm(-1) lower than those measured for isoelectronic U(VI) ion-pair species containing analogous alkoxides. nu(3) values for the U(V) complexes are comparable to those for the anionic [UO(2)(NO(3))(3)](-) complex, and 40-70 cm(-1) lower than previously reported values for ligated uranyl(VI) dication complexes. The lower frequency is attributed to weakening of the O=U=O bonds by repulsion related to reduction of the U metal centre, which increases electron density in the antibonding pi* orbitals of the uranyl moiety. Computational modelling of the nu(3) frequencies using the B3LYP and PBE functionals is in good agreement with the IRMPD measurements, in that the calculated values fall in a very small range and are within a few cm(-1) of measurements. The values generated using the LDA functional are slightly higher and substantially overestimate the trends. Subtleties in the trend in nu(3) frequencies for the H(2)O-MeOH-EtOH-n-PrOH series are not reproduced by the calculations, specifically for the MeOH complex, which has a lower than expected value.

Supramolecular triads have been constructed by using covalently linked zinc porphyrin−ferrocene(s) dyads, self-assembled via axial coordination to either pyridine- or imidazole-appended fulleropyrrolidine. These triads were characterized by optical absorption, computational, and electrochemical methods. The calculated binding constants (K) revealed stable complexation and suggested the existence of intermolecular interactions between the ferrocene and fullerene entities. Accordingly, the optimized geometry obtained by ab initio B3LYP/3-21G(*) methods revealed closely spaced ferrocene and fullerene entities in the studied triads. Photoinduced charge-separation and charge-recombination processes were examined in the dyads and triads by means of time-resolved transient absorption and fluorescence lifetime measurements. In the case of zinc porphyrin−ferrocene(s) dyads, upon photoexcitation, efficient (ΦCS = 0.98) to moderate (ΦCS = 0.54) amounts of electron transfer from the ferrocene to the singlet excited z...

Collision-induced dissociation (CID) of protonated 2-hydroxynicotinic acid (2-OHNic) generates a dominant product ion through loss of 18 mass units, presumably the elimination of water. Subsequent isolation and storage of this product ion in the gas-phase environment of an ion trap mass spectrometer, without imposed collisional activation, shows that the species undergoes addition reactions to furnish new products that are higher in mass by 18 and 32 units. Density functional theory (DFT) calculations suggest that an acylium ion (i.e., loss of H2O from the acid group) is energetically more favored than is a species generated by elimination of H2O from the hydroxypyridine ring. Formation of the acylium product is confirmed by comparing the infrared multiple photon dissociation (IRMPD) spectrum to theoretical spectra from (DFT) harmonic calculations for several possible isomers. A thorough DFT study of the reaction dynamics suggests that the acylium ion is generated from the global minimum for the protonated precursor along a pathway that involves proton transfer from the hydroxypyridine ring and elimination of OH from the acid group.

Raman and infrared spectra of trimethylsulfonium dicyanamide [(CH(3))(3)SN(CN)(2)] are reported and accurately reproduced by DFT methods (B3LYP and B3PW91), MP2, and MP3, and to a lesser extent by the RHF method. The (CH(3))(3)SN(CN)(2) ionic liquid forms two isomeric dimers that are of cyclic structure, one of which is 13 kcal/mol lower in energy than the other. Both isomeric cyclic pairs (versions 1 and 2), [(CH(3))(3)SN(CN)(2)](2), have the potential to further combine and form a common structure containing four pairs of (CH(3))(3)SN(CN)(2). This structure can then conceivably undergo a stacking procedure to form extended ionic liquid nanotubes of eight ionic liquids, [(CH(3))(3)SN(CN)(2)](8). The possible formation of gas phase ionic liquid clusters of two, four, and eight trimethylsulfonium dicyanamide ionic liquids is supported by highly exergonic free energy changes obtained from B3LYP/(6-311+G(d,p)) density functional calculations.