The significance of the alkene size and the nature of the metal ion in metal-alkene complexes: a theoretical study

Cation interactions with π-systems are a problem of outstanding contemporary interest and the nature of these interactions seems to be quite different for transition and main group metal ions. In this paper, we have systematically analyzed the contrast in the bonding of Cu(+) and main group metal ions. The molecular structures and energetics of the complexes formed by various alkenes (A = C(n)H(2n), n = 2-6; C(n)H(2n- 2), n = 3-8 and C(n)H(2n + 2), n = 5-10) and metal ions (M = Li(+), Na(+), K(+), Ca(2+), Mg(2+), Cu(+) and Zn(2+)) are investigated by employing ab initio post Hartree-Fock (MP2/6-311++G**) calculations and are reported in the current study. The study, which also aims to evaluate the effect of the size of the alkyl portion attached to the π-system on the complexation energy, indicates a linear relationship between the two. The decreasing order of complexation energy with various metal ion-alkene complexes follows the order Zn(2+)-A > Mg(2+)-A > Ca(2+)-A > Cu(+)-A > Li(+)-A > Na(+)-A > K(+)-A. The increased charge transfer and the electron density at (3,-1) intermolecular bond critical point corroborates well with the size of the π-system and the complexation energy. The observed deviation from the linear dependency of the Cu(+)-A complexes is attributed to the dπ→π* back bonding interaction. An energy decomposition analysis via the reduced variational space (RVS) procedure was also carried out to analyze which component among polarization, charge transfer, coulomb and exchange repulsion contributes to the increase in the complexation energy. The RVS results suggest that the polarization component significantly contributes to the increase in the complexation energy when the alkene size increases.     

A squaraine-based colorimetric and "turn on" fluorescent sensor for selective detection of Hg2+ in an aqueous medium

A novel squaraine-based chemosensor SQ-1 has been synthesized, and its sensing behavior toward various metal ions was investigated by UV-vis and fluorescence spectroscopies. In AcOH-H(2)O (40:60, v/v) solution, Hg(2+) ions coordinate with SQ-1 causing a deaggregation which induces a visual color and absorption spectral changes as well as strong fluorescence. In contrast, the addition of other metals (e.g., Pb(2+), Cd(2+), Cu(2+), Zn(2+), Al(3+), Ni(2+), Co(2+), Fe(3+), Ca(2+), K(+), Mg(2+), Na(+), and Ag(+)) does not induce these changes at all. Thus SQ-1 is a specific Hg(2+) sensing agent due to the inducing deaggregation of the dye molecule by Hg(2+).     

Metal ion modulated organization and function of the Langmuir-Blodgett films of amphiphilic diacetylene: photopolymerization, thermochromism, and supramolecular chirality

Some novel properties of organized molecular films of 10,12-tricosadiynoic acid (TDA), which were modulated by transition metal ions, were investigated. It was found that metal ions such as Cu(2+), Zn(2+), Ni(2+), Cd(2+), and Ag(+) in the subphase can greatly affect the monolayer formation of TDA and the properties of the subsequently deposited Langmuir-Blodgett (LB) films, particularly in the case of Ag(+), Zn(2+), and Cu(2+) ions. TDA LB film from the subphase containing Ag(+) ion could not be photopolymerized. It was suggested that both the strong chelating property to the carboxylate and the easy reduction of Ag(+) in the film disrupted the topochemical sequence of TDA and resulted in no polymerization in the film. Zinc ion coordinated TDA film could be photopolymerized into a blue polydiacetylene (PDA) film, which showed a reversible thermochromism between blue and purple color upon thermal stimulation. Fourier transform infrared spectra revealed the difference of the Zn(2+)-PDA film from those of the other ions, and the mechanism of the thermochromism was discussed. Copper ion coordinated TDA film could only be photopolymerized to a red PDA film, which showed supramolecular chirality although TDA itself was achiral. Atomic force microscopic measurements revealed the nanofiber structure in the Cu(2+)-PDA film. The supramolecular chirality of the Cu(2+)-PDA film was suggested to be due to the arrangement of the polymer backbone in a helical sense. Furthermore, it was found that the chiral assemblies from the achiral TDA molecules were very stable and the chirality could be kept even upon heating or treating with alkaline solution. While many synthetic efforts have been devoted to the functionalization of PDA films, we provided a simple method of modulating the organization and function of PDA films through metal ions.     

Dramatic effect of the metal cation in dealkylation reactions of phosphinic esters promoted by complexes of polyether ligands with metal iodides

Metal ion electrophilic catalysis has been revealed in dealkylation reactions of phosphinic esters 1-4 promoted by complexes of polyether ligands 5-7 with metal iodides MI(n) (M[n+] = Li(+), Na(+), K(+), Rb(+), Ca(2+), Sr(2+), Ba(2+)) in low polarity solvents (chlorobenzene, 1,2-dichlorobenzene, and toluene) at 60 degrees C. The catalytic effect increases with increasing the Lewis acid character of the cation, in the order Rb(+)< K(+)< Na(+)< Li(+) and Ba(2+)< Sr(2+)< Ca(2+). The results are interpreted in terms of a transition state where the complexed cation (M[n+] subset Lig) assists the departure of the leaving group Ph(2)P(O)O(-) and, at the same time, favors the attack at carbon of the nucleophile I(-) ("push-pull" mechanism). The rate sequence found for 1-4 (Me > Et > i-Pr and t-Bu) shows that this reaction can be utilized for the selective dealkylation of these substrates.     

Competitive cesium-133 NMR spectroscopic study of complexation of different metal ions with dibenzo-21-crown-7 in acetonitrile-dimethylsulfoxide and nitromethane-dimethylsulfoxide mixtures

(133)Cs NMR spectroscopy was used to determine the stoichiometry and stability of the Cs(+) ion complex with dibenzo-21-crown-7 (DB21C7) in acetonitrile-dimethylsulfoxide (96.5:3.5, w/w) and nitromethane-dimethylsulfoxide (96.5:3.5, w/w) mixtures. A competitive (133)Cs NMR technique was also employed to probe the complexation of Na(+), K(+), Rb(+), Ag(+), Tl(+), NH(4)(+), Mg(2+), Ba(2+), Hg(2+), Pb(2+) and UO(2)(2+) ions with DB21C7 in the same solvent systems. All the resulting 1:1 complexes in nitromethane-dimethylsulfoxide were more stable than those in acetonitrile-dimethylsulfoxide solution. In both solvent systems, the stability of the resulting complexes was found to vary in the order Rb(+)>K(+) approximately Ba(2+)>Tl(+)>Cs(+)>NH(4)(+) approximately Pb(2+)>Ag(+)>UO(2)(2+)>Hg(2+)>Mg(2+)>Na(+).     

Gas-phase oxidation of Cm+ and Cm2+ --thermodynamics of neutral and ionized CmO

Fourier transform ion cyclotron resonance mass spectrometry was employed to study the products and kinetics of gas-phase reactions of Cm (+) and Cm (2+); parallel studies were carried out with La (+/2+), Gd (+/2+) and Lu (+/2+). Reactions with oxygen-donor molecules provided estimates for the bond dissociation energies, D[M (+)-O] (M = Cm, Gd, Lu). The first ionization energy, IE[CmO], was obtained from the reactivity of CmO (+) with dienes, and the second ionization energies, IE[MO (+)] (M = Cm, La, Gd, Lu), from the rates of electron-transfer reactions from neutrals to the MO (2+) ions. The following thermodynamic quantities for curium oxide molecules were obtained: IE[CmO] = 6.4 +/- 0.2 eV; IE[CmO (+)] = 15.8 +/- 0.4 eV; D[Cm-O] = 710 +/- 45 kJ mol (-1); D[Cm (+)-O] = 670 +/- 40 kJ mol (-1); and D[Cm (2+)-O] = 342 +/- 55 kJ mol (-1). Estimates for the M (2+)-O bond energies for M = Cm, La, Gd, and Lu are all intermediate between D[N 2-O] and D[OC-O] - that is, 167 kJ mol (-1) < D[M (2+)-O] < 532 kJ mol (-1) - such that the four MO (2+) ions fulfill the thermodynamic requirement for catalytic oxygen-atom transport from N2O to CO. It was demonstrated that the kinetics are also favorable and that the CmO (2+), LaO (2+), GdO (2+), and LuO (2+) dipositive ions each catalyze the gas-phase oxidation of CO to CO2 by N2O. The CmO 2 (+) ion appeared during the reaction of Cm (+) with O 2 when the intermediate, CmO (+), was not collisionally cooled - although its formation is kinetically and/or thermodynamically unfavorable, CmO 2 (+) is a stable species.     

Synthesis and Properties of 5-Chloro-8-hydroxyquinoline-Substituted Azacrown Ethers: A New Family of Highly Metal Ion-Selective Lariat Ethers

New 5-chloro-8-hydroxyquinoline (CHQ)-substituted aza-18-crown-6 (4), diaza-18-crown-6 (1), diaza-21-crown-7 (2), and diaza-24-crown-8 (3) ligands, where CHQ was attached through the 7-position, and aza-18-crown-6 (11) and diaza-18-crown-6 (10) macrocycles, where CHQ was attached through the 2-position, were prepared. Thermodynamic quantities for complexation of these CHQ-substituted macrocycles with alkali, alkaline earth, and transition metal ions were determined in absolute methanol at 25.0 degrees C by calorimetric titration. Two isomers, 1 and 10, which are different only in the attachment positions of the CHQ to the parent macroring, exhibit remarkable differences in their affinities toward the metal ions. Compound 1 forms very stable complexes with Mg(2+), Ca(2+), Cu(2+), and Ni(2+) (log K = 6.82, 5.31, 10.1, and 11.4, respectively), but not with the alkali metal ions. Ligand 10 displays strong complexation with K(+) and Ba(2+) (log K = 6.61 and 12.2, respectively) but not with Mg(2+) or Cu(2+). The new macrocycles and their complexes have been characterized by means of UV-visible and (1)H NMR spectra and X-ray crystallography. New peaks in the UV spectrum of the Mg(2+)-1 complex could allow an analytical determination of Mg(2+) in very dilute solutions in the presence of other alkali and alkaline earth metal cations. (1)H NMR spectral and X-ray crystallographic studies indicate that ligand 10 forms a cryptate-like structure when coordinated with K(+) and Ba(2+), which induces an efficient overlap of the two hydroxyquinoline rings. Such overlapping forms a pseudo second macroring that results in a significant increase in both complex stability and cation selectivity.     

Simultaneous detection of ultratrace lead and copper with gold nanoparticles patterned on carbon nanotube thin film

Highly sensitive detection of a Pb(2+)-Cu(2+) mixture using gold nanoparticles patterned on single-walled carbon nanotube (AuNP-SWCNT) film is reported. The gold nanoparticles were deposited electrochemically on carbon nanotube film using a cyclic voltammetry technique. The film showed a homogeneous size and density that could be easily controlled by the potential scanning cycle and gold precursor concentration. Square wave stripping voltammetry (SWSV) was applied to the simultaneous detection of Pb(2+) and Cu(2+) under optimized conditions. The AuNP-SWCNT electrode exhibited a high increase in sensitivity with a limit of detection of 0.546 ppb (R(2) = 0.984) and 0.613 ppb (R(2) = 0.991) for Pb(2+) and Cu(2+) ions, respectively, in a mixture of Pb(2+)-Cu(2+) solution (S/N = 3, n = 5), and a good linear response in the range from 3.31 ppb to 22.29 ppb. The electrode exhibited high reproducibility in repetitive measurements with a relative standard deviation as low as 4.2% and 2.6% for Pb(2+) and Cu(2+) ions, respectively. An interference study showed that Sb(3+), As(3+), Zn(2+), Ca(2+), and Na(+) ions did not have a significant effect. This study demonstrated an alternative approach to the rapid and reliable detection of heavy metals of environmental interest.     

Lower rim 1,3-di{4-antipyrine}amide conjugate of calix[4]arene: synthesis, characterization, and selective recognition of Hg2+ and its sensitivity toward pyrimidine bases

The structurally characterized lower rim 1,3-di{4-antipyrine}amide conjugate of calix[4]arene (L) exhibits high selectivity toward Hg(2+) among other biologically important metal ions, viz., Na(+), K(+), Ca(2+), Mg(2+), Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), Hg(2+), Pb(2+), and Ag(+) as studied by fluorescence, absorption, and ESI MS. L acts as a sensor for Hg(2+) by switch-off fluorescence and exhibits a lowest detectable concentration of 1.87 ± 0.1 ppm. The complex formed between L and Hg(2+) is found to be 1:1 on the basis of absorption and fluorescence titrations and was confirmed by ESI MS. The coordination features of the mercury complex of L were derived on the basis of DFT computations and found that the Hg(2+) is bound through an N(2)O(2) extending from both the arms to result in a distorted octahedral geometry with two vacant sites. The nanostructural features such as shape and size obtained using AFM and TEM distinguishes L from its Hg(2+) complex and were different from those of the simple mercuric perchlorate. L is also suited to sense pyrimidine bases by fluorescence quenching with a minimum detection limit of 1.15 ± 0.1 ppm in the case of cytosine. The nature of interaction of pyrimidine bases with L has been further studied by DFT computational calculations and found to have interactions through a hydrogen bonding and NH-π interaction between the host and the guest.     

Theoretical study of interaction of urate with li(+), na(+), k(+), be(2+), mg(2+), and ca(2+) metal cations

The geometries and energetics of complexes of Li(+), Na(+), K(+), Be(2+), Mg(2+), and Ca(2+)metal cations with different possible uric acid anions (urate) were studied. The complexes were optimized at the B3LYP level and the 6-311++G(d,p) basis set. Complexes of urate with Mg(2+), and Ca(2+)metal cations were also optimized at the MP2/6-31+G(d) level. Single point energy calculations were performed at the MP2/6-311++G(d,p) level. The interactions of the metal cations at different nucleophilic sites of various possible urate were considered. It was revealed that metal cations would interact with urate in a bi-coordinate manner. In the gas phase, the most preferred position for the interaction of Li(+), Na(+), and K(+) cations is between the N(3) and O(2) sites, while all divalent cations Be(2+), Mg(2+), and Ca(2+) prefer binding between the N(7) and O(6) sites of the corresponding urate. The influence of aqueous solvent on the relative stability of different complexes has been examined using the Tomasi's polarized continuum model. The basis set superposition error (BSSE) corrected interaction energy was also computed for complexes. The AIM theory has been applied to analyze the properties of the bond critical points (electron densities and their Laplacians) involved in the coordination between urate and the metal cations. It was revealed that aqueous solvation would have significant effect on the relative stability of complexes obtained by the interaction of urate with Mg(2+) and Ca(2+)cations. Consequently, several complexes were found to exist in the water solution. The effect of metal cations on different NH and CO stretching vibrational modes of uric acid has also been discussed.     

Terphenyl-phenanthroline conjugate as a Zn(2+) sensor: H(2)PO(4)(-) induced tuning of emission wavelength

A new terphenyl-based macrocycle 5 incorporating phenanthroline as a fluorophore has been designed, synthesized and examined for its recognition ability toward various cations (Pb(2+), Hg(2+), Ba(2+), Cd(2+), Ag(+), Zn(2+), Cu(2+), Ni(2+), Co(2+), K(+), Mg(2+), Na(+) and Li(+)) by UV-vis, fluorescence and NMR spectroscopy. The receptor 5 showed highly selective 'Off-On' fluorescence signaling behavior for Zn(2+) ions in THF. Interestingly, the addition of H(2)PO(4)(-) ions to the [5-Zn] complex regulates the binding site for additional Zn(2+) ions and hence leads to a blue-shifted emission band.     

CO2 adsorption in mono-, di- and trivalent cation-exchanged metal-organic frameworks: a molecular simulation study

A molecular simulation study is reported for CO(2) adsorption in rho zeolite-like metal-organic framework (rho-ZMOF) exchanged with a series of cations (Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), and Al(3+)). The isosteric heat and Henry's constant at infinite dilution increase monotonically with increasing charge-to-diameter ratio of cation (Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) < Al(3+)). At low pressures, cations act as preferential adsorption sites for CO(2) and the capacity follows the charge-to-diameter ratio. However, the free volume of framework becomes predominant with increasing pressure and Mg-rho-ZMOF appears to possess the highest saturation capacity. The equilibrium locations of cations are observed to shift slightly upon CO(2) adsorption. Furthermore, the adsorption selectivity of CO(2)/H(2) mixture increases as Cs(+) < Rb(+) < K(+) < Na(+) < Ca(2+) < Mg(2+) ≈ Al(3+). At ambient conditions, the selectivity is in the range of 800-3000 and significantly higher than in other nanoporous materials. In the presence of 0.1% H(2)O, the selectivity decreases drastically because of the competitive adsorption between H(2)O and CO(2), and shows a similar value in all of the cation-exchanged rho-ZMOFs. This simulation study provides microscopic insight into the important role of cations in governing gas adsorption and separation, and suggests that the performance of ionic rho-ZMOF can be tailored by cations.     

Synthesis and photochromism of spirobenzopyrans and spirobenzothiapyran derivatives bearing monoazathiacrown ethers and noncyclic analogues in the presence of metal ions

Spirobenzopyrans bearing monoazathiacrown ethers and noncyclic analogues were synthesized, and their ion-responsive photochromism depending on the dual metal ion interaction with the crown ether and the phenolate anion moieties was examined using alkali and alkaline-earth metal ions, Ag(+), Tl(+), Pb(2+), Hg(2+), and Zn(2+). The prepared spirobenzopyrans showed a selective binding ability to Mg(2+) and Ag(+) with negative and positive photochromism, respectively. Among the metal ions, only Ag(+) facilitated photoisomerization to the corresponding merocyanine form. Depending on the ring size of the monoazathiacrown ether moieties, soft metal ions such as Hg(2+) and Ag(+) showed significant shifts in the UV-vis absorption spectra, while hard metal ions such as Mg(2+), Zn(2+), and Pb(2+) did not afford any meaningful shift. This result reflects that the monoazathiacrown ether and phenolate anion moieties prefer soft and hard metal ions, respectively. Therefore, the Mg(2+) and Ag(+) selectivities are mainly derived from the phenolate anion and monoazathiacrown ether moieties, respectively. On the other hand, a spirobenzothiapyran bearing 3,9-dithia-6-monoazaundecane showed a remarkable selectivity to Ag(+).     

Modulating affinities of di-2-picolylamine (DPA)-substituted quinoline sensors for zinc ions by varying pendant ligands

We have developed a series of di-2-picolylamine (DPA)-substituted quinoline sensors, HQ1- 4, bearing a pendant ligand at the 8 position of quinoline. UV-vis spectra of HQ1- 4 showed similar variations to that of HQ5 but with different varying extents upon the titration of zinc ions. Fluorescence intensities of HQ1, HQ3, and HQ4 were enhanced 4-6 times upon the addition of 1 equiv of zinc ions under an aqueous buffer. Somewhat unexpectedly, HQ2 is nonfluorescent in the presence of metal ions, including zinc ions. The affinities of HQ sensors are distributed in a broad range from nanomolarity to femtomolarity by varying the pendant ligands near the coordination unit. More importantly, these new sensors exhibited very high selectivity for Zn(2+) over Na(+), K(+), Mg(2+), and Ca(2+) at the millimolar level and over other transition metal ions at the micromolar level, except for Cd(2+). These findings indicated that the incorporations of the pendant groups exerted no effect on the spectroscopic properties and selectivity of the parent fluorescent sensor, with the exception of HQ2. Finally, X-ray crystal structures of ZnHQ's revealed that the auxiliary pendant groups at the 8 position participated in zinc coordination and were able to tune the affinities of HQ sensors.     

Energetics and mechanisms of C-H bond activation by a doubly charged metal ion: guided ion beam and theoretical studies of Ta2+ + CH4

A guided ion beam tandem mass spectrometer is used to study the kinetic-energy dependence of doubly charged atomic tantalum cations (Ta(2+)) reacting with CH4 and CD4. As for the analogous singly charged system, the dehydrogenation reaction to form TaCH2(2+) + H2 is exothermic. The charge-transfer reaction to form Ta(+) + CH4(+) and the charge-separation reaction to form TaH(+) + CH3(+) are also observed at low energies in exothermic processes, as is a secondary reaction of TaCH2(2+) to form TaCH3(+) + CH3(+). At higher energies, other doubly charged products, TaC(2+) and TaCH3(2+), are observed, although no formation of TaH(2+) was observed. Modeling of the endothermic cross sections provides 0 K bond dissociation energies (in electronvolts) of D0(Ta(2+)-C) = 5.42 +/- 0.19 and D0(Ta(2+)-CH3) = 3.40 +/- 0.16. These experimental bond energies are in poor agreement with density functional calculations at the B3LYP/HW+/6-311++G(3df,3p) level of theory. However, the Ta(2+)-C bond energy is in good agreement with calculations at the QCISD(T) level of theory, and the Ta(2+)-CH3 bond energy is in good agreement with density functional calculations at the BHLYP level of theory. Theoretical calculations reveal the geometric and electronic structures of all product ions and are used to map the potential energy surface, which describes the mechanism of the reaction and key intermediates. Both experimental and theoretical results suggest that TaH(+), TaCH2(2+), and TaCH3(2+) are formed through a H-Ta(2+)-CH3 intermediate.     

Facile synthesis of rhodamine-based highly sensitive and fast responsive colorimetric and off-on fluorescent reversible chemosensors for Hg2+: preparation of a fluorescent thin film sensor

Fluorescence-active chemosensors (L1-L4), comprising a rhodamine scaffold and a pseudo azacrown cation-binding subunit, have been proposed and characterized as a fluorescent chemosensor for Hg(2+). An on-off type fluorescent enhancement was observed by the formation of the ring-opened amide form of the rhodamine moiety, which was induced by the interactions between Hg(2+) and the chemosensor. Upon the addition of Hg(2+), an overall emission change of 350-fold was observed, and the selectivity was calculated to be 300 times higher than Cu(2+) for receptors L2-L4. A polymeric thin film can be obtained by doping poly(methyl methacrylate) or PMMA with chemosensor L2. Such a thin film sensor can be used to detect Hg(2+) with high sensitivity and can be recovered using diluted NaOH.     

Simulation of Ca2+ and Mg2+ solvation using polarizable atomic multipole potential

The alkaline earth metals calcium and magnesium are critically involved in many biomolecular processes. To understand the hydration thermodynamics of these ions, we have performed molecular dynamics simulations using a polarizable potential. Particle-mesh Ewald for point multipoles has been applied to the calculation of electrostatic interactions. The parameters in this model have been determined from an ab initio quantum mechanical calculation of dimer interactions between ions and water. Two methods for ion solvation free energy calculation, free energy perturbation, and the Bennett acceptance ratio have been compared. Both predict results consistent with other theoretical estimations while the Bennett approach leads to a much smaller statistical error. Based on the Born theory and the ion-oxygen radial distribution functions, we estimate the effective size of the ions in solution, concluding that K(+) > Na(+) congruent with Ca(2+) > Mg(2+). There appears to be much stronger perturbation in water structure, dynamics, and dipole moment around the divalent cations than the monovalent K(+) and Na(+). The average water coordination numbers for Ca(2+) and Mg(2+) are 7.3 and 6, respectively. The lifetime of water molecules in the first solvation shell of Mg(2+) is on the order of hundreds of picoseconds, in contrast to only few picoseconds for Ca(2+), K(+), or Na(+).     

A new ion sensor based on fiber optics

A fluorimetric ion sensor based on fiber optics has been developed that employs Rhodamine 6G hydrophobically and electrostatically "trapped" on a Nafion film. The sensor is based on the measurement of quenching or enhancement of the Rhodamine 6G fluorescence by various ions. It was found that ions such as Co(2+), Cr(3+), Fe(2+), Fe(3+), Cu(2+), Ni(2+) and NH(+)(4) rapidly quench the Rhodamine 6G fluorescence at an initial rate that depends on the concentration of the ion. This quenching is then readily reversed by the addition of "reverser" ions such as H(+), Li(+), Na(+), K(+), Ba(2+), Ca(2+), Mn(2+), Zn(2+) and Mg(2+). Again, the initial rate for the attainment of the original fluorescence was found to depend on the concentration of the reverser ion. Therefore, by monitoring the quenching directly the concentration of quencher ions can be determined. In addition, by loading the film with quencher and monitoring the initial rate of return towards the original baseline signal, it is possible to determine non-quenching ions.     

Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water coordination on the structure and properties of L-arginine and zwitterionic L-arginine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-arginine is examined. The effects of metal ions (Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+)) and water on structures of Arg x M(H2O)m , m = 0, 1 complexes have been determined theoretically by employing the density functional theories (DFT) and using extended basis sets. Of the three stable complexes investigated, the relative stability of the gas-phase complexes computed with DFT methods (with the exception of K(+) systems) suggests metallic complexes of the neutral L-arginine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of arginine in the presence of the metal cations Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+) were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to arginine is exhibited by the Cu(2+) cation. The computed Gibbs energies DeltaG(o) are negative, span a rather broad energy interval (from -150 to -1500 kJ/mol), and are appreciably lowered upon hydration.     

Gas-phase diatomic trications of Se2(3+), Te2(3+), and LaF3+

Three gas-phase diatomic trications Se(2) (3+), Te(2) (3+), and LaF(3+) have been produced by Ar(+) ion beam sputtering of Se, Te, and LaF(3) surfaces, respectively. These exotic molecular ions were detected at noninteger m/z values in a magnetic sector mass spectrometer for ion flight times of >/=13 micros that correspond to lower limits of their respective lifetimes. Se(2) (3+) and Te(2) (3+) were unambiguously identified by their characteristic isotopic abundances. Ab initio calculations of the electronic structures of Se(2) (3+), Te(2) (3+), and LaF(3+) show that these molecular trications are metastable with respect to dissociation into fragment ions of Se(2+)+Se(+), Te(2+)+Te(+), and La(2+)+F(+), respectively. Their barrier heights are about 0.49, 0.29, and 0.53 eV, and the equilibrium internuclear distances (bond lengths) are about 0.23, 0.27, and 0.26 nm, respectively. The gas-phase diatomic dications Se(2) (2+) and Te(2) (2+) were also observed and unambiguously identified. They were found to be long-lived metastable molecules as well, whereas LaF(2+) is thermochemically stable.