Search

Kinetic-energy-dependent collision-induced dissociation (CID) of complexes of a variety of N-donor ligands (N-L) with Ni+, Ni+(N-L)x, is studied using guided ion beam tandem mass spectrometry. The N-donor ligands investigated include: pyridine, 4,4′-dipyridyl, 2,2′-dipyridyl, and 1,10-phenanthroline. For most of the Ni+(N-L)xcomplexes, CID results in endothermic loss of a single neutral N-L ligand as the primary dissociation pathway. Sequential dissociation of additional N-L ligands is observed at elevated energies for the pyridine and 4,4′-dipyridyl complexes containing more than one ligand. The cross-section thresholds for the primary dissociation pathways are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) of the Ni+(N-L)xcomplexes after accounting for the effects of multiple ion−neutral collisions, the kinetic and internal energy distributions of the reactants, and their lifetimes for dissociation. Density functional theory calculations at the B3LYP/6-311+G(2d,2p)//B3LYP/6-31G* level are performed to obtain model structures, molecular parameters, and energetics for the neutral N-L ligands and the Ni+(N-L)xcomplexes. In general, theory is found to overestimate the strength of binding to the first N-L ligand, and underestimate the strength of binding to additional ligands. Trends in the sequential BDEs of the Ni+(N-L)xcomplexes are examined and compared to complexes of Ni+, to several other ligands previously investigated. The trends in the sequential BDEs are primarily determined by the valence electronic configuration and the effects of sd-hybridization of Ni+but are also influenced by repulsive ligand−ligand interactions. Natural bond orbital analyses indicate that the binding in these complexes is primarily noncovalent. [ABSTRACT FROM AUTHOR]

The gas-phase structures of alkali-metal cation complexes of threonine (Thr) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser in conjunction with quantum chemical calculations. Spectra of Li(Thr) and Na(Thr) are similar and relatively simple, whereas K(Thr), Rb(Thr), and Cs(Thr) include distinctive new IR bands. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311G(d,p) level to identify the structures present in the experimental studies. For the smaller metal cations, the spectra match those predicted for charge-solvated structures in which the ligand exhibits tridentate coordination, M1N,CO,OH, binding to the amide and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain. K(Thr), Rb(Thr), and Cs(Thr) exhibit evidence of the charge-solvated complex, M3COOH, in which the metal cation binds to the carboxylic acid group. Evidence for a small population of the zwitterionic analogue of this structure, ZWCO2-, is also present, particularly for the Cscomplex. Calculations indicate that the relative stability of the M3COOH structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. The present results are similar to those obtained previously for the analogous M(Ser) complexes, although there are subtle distinctions that are discussed. [ABSTRACT FROM AUTHOR]

The gas-phase structures of alkali-metal cation complexes of serine (Ser) are examined using infrared multiple photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. Spectra of Li(Ser) and Na(Ser) are similar and relatively simple, whereas Cs(Ser) includes distinctive new IR bands, and K(Ser) and Rb(Ser) exhibit intermediate behavior. Measured IRMPD spectra are compared to spectra calculated at a B3LYP/6-311G(d,p) level to identify the structures present in the experimental studies. On the basis of these experiments and calculations, the only conformations accessed for the complexes to the smaller alkali-metal cations, Liand Na, are charge-solvated structures involving tridentate coordination to the amine and carbonyl groups of the amino acid backbone and to the hydroxyl group of the side chain, M1N,CO,OH. For the cesiated complex, a band corresponding to a zwitterionic structure, ZWCO2-, is clearly visible. K(Ser) and Rb(Ser) exhibit evidence of the charge-solvated analogue of the zwitterions, M3COOH, in which the metal cation binds to the carboxylic acid group. Calculations indicate that the relative stability of the M3COOH structure is very strongly dependent on the size of the metal cation, consistent with the range of conformations observed experimentally. [ABSTRACT FROM AUTHOR]

Threshold collision-induced dissociation of K(xBA) complexes with xenon is studied using guided ion beam mass spectrometry. The xBA ligands studied include benzoic acid and all of the mono- and dihydroxy-substituted benzoic acids:  2-, 3-, and 4-hydroxybenzoic acid and 2,3-, 2,4-, 2,5-, 2,6-, 3,4-, and 3,5-dihydroxybenzoic acid. In all cases, the primary product corresponds to endothermic loss of the intact xBA ligand. The cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for K−xBA after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G level of theory are used to determine the structures of the xBA ligands and their complexes with K. Theoretical BDEs are determined from single-point energy calculations at the B3LYP/6-311G(2d,2p) and MP2(full)/6-311G(2d,2p) levels using B3LYP/6-31G optimized geometries. Four favorable binding modes for the K(xBA) complexes are found. In all complexes to an xBA ligand that does not have a 2-hydroxyl substituent, the most favorable binding mode corresponds to a single interaction with the carbonyl oxygen atom. Formation of a 4-membered ring via chelation interactions with both oxygen atoms of the carboxylic acid group is found to be the most favorable binding mode for all of the 2-hydroxy-substituted systems except K(2,3-dihydroxybenzoic acid). In these complexes, a hydrogen-bonding interaction between the hydrogen atom of the carboxylic acid moiety and the oxygen atom of the 2-hydroxy substituent provides additional stabilization. Formation of a 5-membered chelation ring via interaction of Kwith the oxygen atoms of adjacent hydroxyl substituents is also favorable and corresponds to the ground-state geometry for the K(23DHBA) complex. Formation of a 6-membered chelation ring via interaction of Kwith the carbonyl and 2-hydroxyl oxygen atoms is also quite favorable but does not correspond to the ground-state geometry for any of the systems examined here. The experimental BDEs determined here are in very good agreement with the calculated values. [ABSTRACT FROM AUTHOR]

Collision-induced dissociation of complexes of Cubound to a variety of N-donor ligands (N-L) with Xe is studied using guided ion beam tandem mass spectrometry. The N-L ligands examined include pyridine, 4,4-dipyridyl, 2,2-dipyridyl, and 1,10-phenanthroline. In all cases, the primary and lowest-energy dissociation channel observed corresponds to the endothermic loss of a single intact N-L ligand. Sequential dissociation of additional N-L ligands is observed at elevated energies for the pyridine and 4,4-dipyridyl complexes containing more than one ligand. Ligand exchange processes to produce CuXe are also observed as minor reaction pathways in several systems. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G level are performed to obtain model structures, vibrational frequencies, and rotational constants for the neutral N-L ligands and the Cu(N-L)xcomplexes. The relative stabilities of the various conformations of these N-L ligands and Cu(N-L)xcomplexes as well as theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311G(2d,2p) level of theory using B3LYP/6-31G optimized geometries. Excellent agreement between theory and experiment is observed for all complexes containing one or two N-L ligands, while theory systematically underestimates the strength of binding for complexes containing more than two N-L ligands. The ground-state structures of these complexes and the trends in the sequential BDEs are explained in terms of stabilization gained from sd-hybridization and repulsive ligand−ligand interactions. The nature of the binding interactions in the Cu(N-L)xcomplexes are examined via natural bond orbital analyses. [ABSTRACT FROM AUTHOR]

Experimental and theoretical studies are carried out to determine the influence of thioketo substitution on the properties of uracil and its noncovalent interactions with alkali metal ions. Bond dissociation energies of alkali metal ion−thiouracil complexes, M+(SU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M+ = Li+, Na+, and K+ and SU = 2-thiouracil, 4-thiouracil, 2,4-dithiouracil, 5-methyl-2-thiouracil, and 6-methyl-2-thiouracil. Ab initio electronic structure calculations are performed to determine the structures and theoretical bond dissociation energies of these complexes and provide molecular constants necessary for thermodynamic analysis of the experimental data. Theoretical calculations are also performed to examine the influence of thioketo substitution on the acidities, proton affinities, and A::SU Watson−Crick base pairing energies. In general, thioketo substitution leads to an increase in both the proton affinity and the acidity of uracil. 2-Thio substitution generally results in an increase in the alkali metal ion binding affinities but has almost no affect on the stability of the A::SU base pair. In contrast, 4-thio substitution results in a decrease in the alkali metal ion binding affinities and a significant decrease in the stability of the A::SU base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of both single-stranded and double-stranded nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases. [ABSTRACT FROM AUTHOR]