Structural scaffold of 18-crown-6 tetracarboxylic acid for optical resolution of chiral amino acid: X-ray crystal analyses and energy calculations of complexes of D- and L-isomers of tyrosine, isoleucine, methionine and phenylglycine

To clarify the structural scaffold of (+)-18-crown-6 tetracarboxylic acid ((+)-18C6H4) for the optical resolution of a chiral amino acid, the crystal structures of its equimolar complexes with L- and D-isomers of tyrosine (Tyr), isoleucine (Ile), methionine (Met) and phenylglycine (PheG) were analysed by X-ray diffraction methods. (+)-18C6H4 took very similar conformations for all complexes. Although the chemical structure of (+)-18C6H4 is C2-symmetric, it took a similar asymmetric ring conformation of radius ca. 6.0 A. In all complexes, the amino group of chiral amino acids was located near the center of the ring and formed three hydrogen bonds and five electrostatic interactions with eight oxygen atoms of the ether ring and carboxyl groups. Also, the Calpha atom of chiral amino acids participated in Calpha-H...O interaction with the oxygen atom of (+)-18C6H4. In contrast, the carboxyl group of chiral amino acids did not directly interact with (+)-18C6H4. These results indicate that the structural scaffold of (+)-18C6H4 for the optical resolution of chiral amino acids is mainly based on the mode of interaction of (+)-18C6H4 with the amino and Calpha-H groups of chiral amino acids. The differences in interaction pattern and binding energy between the L- and D-isomers of each amino acid are discussed in relation to the chiral recognition of (+)-18C6H4.     

Side arm participation in lariat ether carboxylate-alkali metal cation complexes in solution

Lariat ether carboxylic acids of structure CECH₂OCH₂C₆H₄-2-CO₂H with crown ether (CE) ring sizes of 12-crown-4, 15-crown-5 and 18-crown-6 are prepared and converted into alkali metal-lariat ether carboxylate complexes. Absorptions for the diastereotopic benzylic protons in the ¹H NMR spectra of the complexes in CDCl₃ are utilized to probe the extent of side arm interaction with the crown ether-complexed metal ion as a function of the crown ether ring size and identity of the alkali metal cation.     

Crown ether complexes with H₃O⁺ and NH₄⁺: proton localization and proton bridge formation

The complexes formed by crown ethers with hydronium and ammonium cations are of key relevance for the understanding of their supramolecular behavior in protic solvents. In this work, the complexes of the 15-crown-5 (15c5) and 18-crown-6 (18c6) ethers with H?O? and NH?? and their deuterated variants are investigated under isolated conditions. The study employs infrared multiple photon dissociation (IRMPD) vibrational spectroscopy and DFT B3LYP/6-31++G(d,p) calculations for conformational assignment. The 18c6 ether provides two energetically nearby C(3v) conformations with commensurate linear O-H···O and N-H···O bonds. The 15c5 ether ring adopts partially folded asymmetric pyramidal geometries, yielding one shorter linear H bond and two longer non-linear H bonds. Remarkably, an appreciable broadening of the IRMPD vibrational bands is observed for the 15c5-H?O?/D?O? complexes. This can be interpreted as a signature for partial sharing of the proton (or deuteron) between the water and the crown ether along the linear O-H···O intermolecular H bond, which is indeed particularly short for this complex.     

Gas-phase chemistry of ionized and protonated GeF4: a joint experimental and theoretical study

The gas-phase ion chemistry of GeF(4) and of its mixtures with water, ammonia and hydrocarbons was investigated by ion trap mass spectrometry (ITMS) and ab initio calculations. Under ITMS conditions, the only fragment detected from ionized GeF(4) is GeF(3)(+). This cation is a strong Lewis acid, able to react with H(2)O, NH(3) and the unsaturated C(2)H(2), C(2)H(4) and C(6)H(6) by addition-HF elimination reactions to form F(2)Ge(XH)(+), FGe(XH)(2)(+), Ge(XH)(3)(+) (X = OH or NH(2)), F(2)GeC(2)H(+), F(2)GeC(2)H(3)(+) and F(2)GeC(6)H(5)(+). The structure, stability and thermochemistry of these products and the mechanistic aspects of the exemplary reactions of GeF(3)(+) with H(2)O, NH(3) and C(6)H(6) were investigated by MP2 and coupled cluster calculations. The experimental proton affinity (PA) and gas basicity (GB) of GeF(4) were estimated as 121.5 ± 6.0 and 117.1 ± 6.0 kcal mol(-1), respectively, and GeF(4)H(+) was theoretically characterized as an ion-dipole complex between GeF(3)(+) and HF. Consistently, it reacts with simple inorganic and organic molecules to form GeF(3)(+)-L complexes (L = H(2)O, NH(3), C(2)H(2), C(2)H(4), C(6)H(6), CO(2), SO(2) and GeF(4)). The theoretical investigation of the stability of these ions with respect to GeF(3)(+) and L disclosed nearly linear correlations between their dissociation enthalpies and free energies and the PA and GB of L. Comparing the behavior of GeF(3)(+) with the previously investigated CF(3)(+) and SiF(3)(+) revealed a periodically reversed order of reactivity CF(3)(+) < GeF(3)(+) < SiF(3)(+). This parallels the order of the Lewis acidities of the three cations.     

Infrared spectroscopy of divalent zinc and cadmium crown ether systems

The gas-phase structures of transition-metal dication (Zn(2+) and Cd(2+)) complexes with varying sized crown ethers, 12-crown-4 (12c4), 15-crown-5 (15c5), and 18-crown-6 (18c6), are investigated using infrared multiple photon dissociation (IRMPD) spectroscopy and quantum mechanical calculations. The measured spectra span the 750-1600 cm(-1) infrared range, utilizing light generated by a free electron laser, and are compared to predicted spectra calculated at the B3LYP/6-311+G(d,p) or B3LYP/Def2TZVP levels of theory. Spectra with the largest and most flexible crown ether, 18c6, indicate that the crown is highly distorted, wrapping in a tight cage-like structure around both dications studied. The 15c5 adopts a folded orientation for the Zn(2+) complex yet is almost planar when complexed with the larger Cd(2+) ion. The Zn(2+)(12c4) spectrum has bands appearing at lower frequencies than the other systems, consistent with an open conformation such that the metal is exposed, lying above the center of mass of the crown ether ring. The open structures of the Zn(2+)(12c4) and Cd(2+)(15c5) complexes have implications for solvent interactions in the condensed phase. The conformation of each metal-crown complex is highly dependent on metal size, charge, and crown ether flexibility, such that a delicate balance of minimizing the metal-oxygen bond lengths but maximizing the oxygen-oxygen distances arises. These competing influences are reflected in both the spectra and lowest-energy conformations.     

Cation [M = H+, Li+, Na+, K+, Ca2+, Mg2+, NH4+, and NMe4+] interactions with the aromatic motifs of naturally occurring amino acids: a theoretical study

Ab initio (HF, MP2, and CCSD(T)) and DFT (B3LYP) calculations were done in modeling the cation (H(+), Li(+), Na(+), K(+), Ca(2+), Mg(2+), NH(4)(+), and NMe(4)(+)) interaction with aromatic side chain motifs of four amino acids (viz., phenylalanine, tyrosine, tryptophan and histidine). As the metal ion approaches the pi-framework of the model systems, they form strongly bound cation-pi complexes, where the metal ion is symmetrically disposed with respect to all ring atoms. In contrast, proton prefers to bind covalently to one of the ring carbons. The NH(4)(+) and NMe(4)(+) ions have shown N-H...pi interaction and C-H...pi interaction with the aromatic motifs. The interaction energies of N-H...pi and C-H...pi complexes are higher than hydrogen bonding interactions; thus, the orientation of aromatic side chains in protein is effected in the presence of ammonium ions. However, the regioselectivity of metal ion complexation is controlled by the affinity of the site of attack. In the imidazole unit of histidine the ring nitrogen has much higher metal ion (as well as proton) affinity as compared to the pi-face, facilitating the in-plane complexation of the metal ions. The interaction energies increase in the order of 1-M < 2-M < 3-M < 4-M < 5-M for all the metal ion considered. Similarly, the complexation energies with the model systems decrease in the following order: Mg(2+) > Ca(2+) > Li(+) > Na(+) > K(+) congruent with NH(4)(+) > NMe(4)(+). The variation of the bond lengths and the extent of charge transfer upon complexation correlate well with the computed interaction energies.     

Inorganic crown: the host-guest chemistry of a high nuclearity 'Celtic-ring' isopolyoxotungstate [H12W36O120]12-

A range of complexes based on the high-nuclearity [W36] isopolyoxotungstate cluster, [H12W36O120]12-, with a triangular topology has been isolated by using the organic cation, protonated triethanolamine. In analogy to an 18-crown-6 crown ether with six oxygen donors on a ring, the cluster can form alkali and alkaline earth metal complexes [M within W36] (M = K+, Rb+, Cs+, NH4+, Sr2+ and Ba2+, 1-6, respectively). Compounds 1-6 were characterized by single-crystal X-ray diffraction, elemental analysis, IR spectroscopy. Comparisons between the structures of 1-6 and 18-crown-6 as well as the symmetry directing influence of the organo-cations in the isolation of the overall cluster architecture are discussed.     

UV and IR spectroscopic studies of cold alkali metal ion-crown ether complexes in the gas phase

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li(+), Na(+), K(+), Rb(+), and Cs(+)) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved UV bands in the 36,000-38,000 cm(-1) region. The Li(+) and Na(+) complexes each exhibit two stable conformations in the cold ion trap (as verified by IR-UV double resonance), whereas the K(+), Rb(+), and Cs(+) complexes exist in a single conformation. We analyze the structure of the conformers with the aid of density functional theory (DFT) calculations. In the Li(+) and Na(+) complexes, DB18C6 distorts the ether ring to fit the cavity size to the small diameter of Li(+) and Na(+). In the complexes with K(+), Rb(+), and Cs(+), DB18C6 adopts a boat-type (C(2v)) open conformation. The K(+) ion is captured in the cavity of the open conformer thanks to the optimum matching between the cavity size and the ion diameter. The Rb(+) and Cs(+) ions sit on top of the ether ring because they are too large to enter the cavity of the open conformer. According to time-dependent DFT calculations, complexes that are highly distorted to hold metal ions open the ether ring upon S(1)-S(0) excitation, and this is confirmed by extensive low-frequency progressions in the UVPD spectra.     

Organometallic ruthenium(II) diamine anticancer complexes: arene-nucleobase stacking and stereospecific hydrogen-bonding in guanine adducts

Organometallic ruthenium(II) arene anticancer complexes of the type [(eta(6)-arene)Ru(II)(en)Cl][PF(6)] (en = ethylenediamine) specifically target guanine bases of DNA oligomers and form monofunctional adducts (Morris, R., et al. J. Med. Chem. 2001). We have determined the structures of monofunctional adducts of the "piano-stool" complexes [(eta(6)-Bip)Ru(II)(en)Cl][PF(6)] (1, Bip = biphenyl), [(eta(6)-THA)Ru(II)(en)Cl][PF(6)] (2, THA = 5,8,9,10-tetrahydroanthracene), and [(eta(6)-DHA)Ru(II)(en)Cl][PF(6)] (3, DHA = 9,10-dihydroanthracene) with guanine derivatives, in the solid state by X-ray crystallography, and in solution using 2D [(1)H,(1)H] NOESY and [(1)H,(15)N] HSQC NMR methods. Strong pi-pi arene-nucleobase stacking is present in the crystal structures of [(eta(6)-C(14)H(14))Ru(en)(9EtG-N7)][PF(6)](2).(MeOH) (6) and [(eta(6)-C(14)H(12))Ru(en)(9EtG-N7)][PF(6)](2).2(MeOH) (7) (9EtG = 9-ethylguanine). The anthracene outer ring (C) stacks over the purine base at distances of 3.45 A for 6 and 3.31 A for 7, with dihedral angles of 3.3 degrees and 3.1 degrees, respectively. In the crystal structure of [(eta(6)-biphenyl)Ru(en)(9EtG-N7)][PF(6)](2).(MeOH) (4), there is intermolecular stacking between the pendant phenyl ring and the purine six-membered ring at a distance of 4.0 A (dihedral angle 4.5 degrees). This stacking stabilizes a cyclic tetramer structure in the unit cell. The guanosine (Guo) adduct [(eta(6)-biphenyl)Ru(en)(Guo-N7)][PF(6)](2).3.75(H(2)O) (5) exhibits intramolecular stacking of the pendant phenyl ring with the purine five-membered ring (3.8 A, 23.8 degrees) and intermolecular stacking of the purine six-membered ring with an adjacent pendant phenyl ring (4.2 A, 23.0 degrees). These occur alternately giving a columnar-type structure. A syn orientation of arene and purine is present in the crystal structures 5, 6, and 7, while the orientation is anti for 4. However, in solution, a syn orientation predominates for all the biphenyl adducts 4, 5, and the guanosine 5'-monophosphate (5'-GMP) adduct 8 [(eta(6)-biphenyl)Ru(II)(en)(5'-GMP-N7)], as revealed by NMR NOE studies. The predominance of the syn orientation both in the solid state and in solution can be attributed to hydrophobic interactions between the arene and purine rings. There are significant reorientations and conformational changes of the arene ligands in [(eta(6)-arene)Ru(II)(en)(G-N7)] complexes in the solid state, with respect to those of the parent chloro-complexes [(eta(6)-arene)Ru(II)(en)Cl](+). The arene ligands have flexibility through rotation around the arene-Ru pi-bonds, propeller twisting for Bip, and hinge-bending for THA and DHA. Thus propeller twisting of Bip decreases by ca. 10 degrees so as to maximize intra- or intermolecular stacking with the purine ring, and stacking of THA and DHA with the purine is optimized when their tricyclic ring systems are bent by ca. 30 degrees, which involves increased bending of THA and a flattening of DHA. This flexibility makes simultaneous arene-base stacking and N7-covalent binding compatible. Strong stereospecific intramolecular H-bonding between an en NH proton oriented away from the arene (en NH(d)) and the C6 carbonyl of G (G O6) is present in the crystal structures of 4, 5, 6, and 7 (average N...O distance 2.8 A, N-H...O angle 163 degrees ). NMR studies of the 5'-GMP adduct 8 provided evidence that en NH(d) protons are involved in strong H-bonding with the 5'-phosphate and O6 of 5'-GMP. The strong H-bonding from G O6 to en NH(d) protons partly accounts for the high preference for binding of [(eta(6)-arene)Ru(II)en](2+) to G versus A (adenine). These studies suggest that simultaneous covalent coordination, intercalation, and stereospecific H-bonding can be incorporated into Ru(II) arene complexes to optimize their DNA recognition behavior, and as potential drug design features.     

Solution and solid-state preparation of 18-crown[6] complexes with M[HSO4]n salts (M = NH4+, K+, Sr2+ and n = 1, 2) and an investigation of solvation/desolvation processes and crystal polymorphism

18-Crown[6] ether has been used to prepare a new class of organic-inorganic complexes of general formula 18-crown[6]M[HSO(4)](n) (where M = NH(4) (+), K(+), Sr(2+) and n = 1, 2) by reacting directly in solution or in the solid state the crown ether 18-crown[6] with inorganic salts such as [NH(4)][HSO(4)], K[HSO(4)], and Sr[HSO(4)](2). The structures of 18-crown[6][NH(4)][HSO(4)]2 H(2)O (12 H(2)O), 18-crown[6][NH(4)][HSO(4)] (1), 18-crown[6]K[HSO(4)]2 H(2)O (22 H(2)O), 18-crown[6]K[HSO(4)] (2), and 18-crown[6]Sr[HSO(4)](2) (3) have been characterized by single-crystal X-ray diffraction. The reversible water loss in compounds 12 H(2)O and 22 H(2)O leads to formation of the corresponding anhydrous phases 18-crown[6][NH(4)][HSO(4)] (1), and 18-crown[6]K[HSO(4)] (2), which undergo, on further heating, enantiotropic solid-solid transitions very likely associated with the on-set of a solid state dynamical process. Similar high-temperature behavior is shown by 18-crown[6]Sr[HSO(4)](2) (3). The dehydration and phase-transition processes have been investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and variable temperature X-ray powder diffraction.     

一个新的二氮六元杂环化合物的合成、结构及表征

二氮杂环化合物及其衍生物在配合物的自组装和生物无机等领域中具有诱人的研究前景,因而引起各国化学家的广泛关注．这类化合物的合成可以追溯到60年前,Buhle等首次合成了1,5-二氮环辛烷(DACO)．随后,其一系列衍生物也被合成出来,并应用于配位化学及相关领域的研     

Synthesis of 6-mono-6-deoxy-beta-cyclodextrins substituted with isomeric aminobenzoic acids. Structural characterization, conformational preferences, and self-inclusion as studied by NMR spectroscopy in aqueous solution and by X-ray crystallography in the solid state

The synthesis, purification, and characterization of mono-6-modified-beta-cyclodextrins bearing N-attached o-, m-, and p-aminobenzoic acids (2, 3, and 4, respectively) are presented. The structures in aqueous solution were investigated using one- and two-dimensional NMR spectroscopy. Detailed assignment of the spectra together with intramolecular NOE correlations revealed the way each of the isomeric appendages is positioned relative to the macrocyclic cavity. No self-inclusion is observed. The o-isomer 2 turns inward over the top of the primary side and interacts with specific protons of the substituted glucopyranose unit A and those of a neighboring unit. The m-isomer 3 displays two conformations, where the substituent resides above the primary side in a tilted manner and interacts either with the previous or the next unit. We propose that the carboxyl groups in both 2 and 3 are localized through H-bonding with one or two, respectively, primary hydroxyl groups of the neighboring glucopyranose units. In a similar positioning of the aromatic ring of the p-isomer 4, the hydrophilic carboxyl end is fully exposed to the aqueous environment. The X-ray structure of 4 shows that the solution conformation has evolved such that in the crystalline state, the aromatic moiety is inserted through its carboxyl part inside another CD where it establishes intermolecular H-bonds with inward-turned primary OH groups. Besides this stabilization, 4 forms parallel and antiparallel supramolecular chains in the crystal that are additionally stabilized by direct H-bonds.     

Transformation of ammonium dicyanamide into dicyandiamide in the solid

Ammonium dicyanamide NH(4)[N(CN)(2)] was synthesized through aqueous ion exchange. The crystal structure was investigated by single-crystal X-ray diffraction (P2(1)/c, a = 378.67(6) pm, b = 1240.9(3) pm, c = 911.84(14) pm, beta = 91.488(18) degrees, Z = 4). It derives from the CsCl structure type. Medium strong hydrogen bonds between NH(4)(+) and [N(CN)(2)](-) ions are indicative of the observed formation of dicyandiamide H(4)C(2)N(4) during heating. According to DSC and temperature-dependent X-ray powder diffractometry, this isomerization is exothermic and occurs between 102 and 106 degrees C in the solid. The reaction represents the isolobal analogue to the classical synthesis of urea by heating NH(4)OCN. While other alkali and alkaline earth dicyanamides undergo trimerization or polymerization of their anions during heating, ammonium dicyanamide thus shows a different reactivity.     

The Structure of Ammonium,D,L—Tartrate

1 INTRODUCTION The enantiomers separated by a co- crystallization method using an appropriate chiral as separating reagent has attracted many attentions in the past years[1]. Recently we are interested in enantiomeric discrimination by chiral amino acid. When racemic D,L-tartaric acid was tried to be separated by the L-glutamine in a hot aqueous solution, the well shaped crystals were obtained. The determination of melting point (220℃) and density (1.66 g穋m－3) showed it is…     

A 15-crown-5-functionalized carbosilane dendrimer as ionophore for ammonium selective electrodes

The synthesis of CH(2)CHCH(2)OCH(2)[15-crown-5] (III) is achieved by the treatment of HOCH(2)- [15-crown-5] (I) with equimolar amounts of CH(2)CHCH(2)Br (II) in the presence of KOH. The hydrosilylation of III with Si(CH(2)CH(2)CH(2)SiMe(2)H)(4) (IV) in the presence of the Karstedt catalyst affords the crown ether end-capped carbosilane dendrimer Si(CH(2)CH(2)CH(2)Si-Me(2)CH(2)CH(2)CH(2)OCH(2)[15-crown-5])(4) (V). PVC-based membranes of V as ionophore with sodium tetraphenyl borate (NaTPB) as anion excluder and dioctyl phthalate (DOP), diphenyl ether (DPE), dibutyl amine (DBA) and dibutyl phthalate (DBP) as plasticizing solvent mediators were prepared and investigated as NH(4)(+)-selective electrode. The response of the electrode was linear with a Nernstian slope of 53.3mV/decade over an NH(4)(+) ion concentration range of 7.60x10(-6) to 1.0x10(-1)M and a detection limit of 3.9x10(-6)M. The response time to achieve a steady potential for NH(4)(+) ions was between 6 and 10s, and the electrode is suitable for use within the pH range of 2.2-8.5. The selectivity relative to alkali, alkaline earth, and transition heavy metal ions is good. The newly developed ionophore showed higher NH(4)(+) selectivity over K(+) ( [Formula: see text] ) and Na(+) ( [Formula: see text] ). The electrode could be used for at least 45 days without considerable alteration in its potential. The electrode also shows a better working concentration range and slope in comparison to other NH(4)(+)-selective electrodes reported in literature.     

Theoretical conformational analysis for neurotransmitters in the gas phase and in aqueous solution. Norepinephrine

The natural neurotransmitter (R)-norepinephrine takes the monocationic form in 93% abundance at the physiological tissue pH of 7.4. Ab initio and DFT/B3LYP calculations were performed for 12 protonated conformers of (R)-norepinephrine in the gas phase with geometry optimizations up to the MP2/6-311++G level, and with single-point calculations up to the QCISD(T) level at the HF/6-31G-optimized geometries. Four monohydrates were studied at the MP2/6-31G//HF/6-31G level. In the gas phase, the G1 conformer is the most stable with phenyl.NH(3)(+) gauche and HO(alc).NH(3)(+) gauche arrangements. A strained intramolecular hydrogen bond was found for conformers (G1 and T) with close NH(3)(+) and OH groups. Upon rotation of the NH(3)(+) group as a whole unit about the C(beta)-C(alpha) axis, a 3-fold potential was calculated with free energies for barriers of 3-12 kcal/mol at the HF/6-31G level. Only small deviations were found in MP2/6-311++G single-point calculations. A 2-fold potential was calculated for the phenyl rotation with free energies of 11-13 kcal/mol for the barriers at T = 310 K and p = 1 atm. A molecular mechanics docking study of (R)-norepinephrine in a model binding pocket of the beta-adrenergic receptor shows that the ligand takes a conformation close to the T(3) arrangement. The effect of aqueous solvation was considered by the free energy perturbation method implemented in Monte Carlo simulations. There are 4-5 strongly bound water molecules in hydrogen bonds to the conformers. Although hydration stabilizes mostly the G2 form with gauche phenyl.NH(3)(+) arrangement and a water-exposed NH(3)(+) group, the conformer population becomes T > G1 > G2, in agreement with the PMR spectroscopy measurements by Solmajer et al. (Z. Naturforsch. 1983, 38c, 758). Solvent effects reduce the free energies for barriers to 3-6 and 9-12 kcal/mol for rotations about the C(beta)-C(alpha) and the C(1)(ring)-C(beta) axes, respectively.     

三水合硝酸钪18-冠-6配合物的合成及其晶体结构

报道了[Sc(NO3)3(OH2)3].(18-冠-6)的合成及其晶体结构.Sc(III)离子同三个双齿配体硝酸根与三个水分子氧配位,构成九配位的配合物.配位多面体是稍有扭曲的单帽四方反棱柱.配位水分子的六个氢原子分别与上下两层冠醚环上的氧原子生成氢键,形成多层夹心分子缔合物.     

FT-IR, FT-Raman and FT-SERS spectra of 4-aminosalicylic acid sodium salt dihydrate.

FT-IR, FT-Raman and FT-SERS spectra of 4-aminosalicylic acid sodium salt dihydrate (4ASAS) have been recorded and analysed. The vibrational bands due to NH2, OH, carboxyl group, and the benzene ring are identified. The CX ipb(17a, 17b), CC ipb(6,18a) and CH ipb (3,14a, 14b) bands are more enhanced in SERS. Broadening of the inplane carboxyl bend indicates interaction with the silver surface. Further the vC=O, v(C-O)c and v(C-O)h are intense in the SERS spectrum. The rocking and wagging modes of NH2 also show up in SERS. The molecule (O, N donor ligand) is thought to adsorb through the carboxyl oxygen atom with the benzene ring in a 'perpendicular side on orientation' with respect to the silver surface.     

Water adducts of BX(3) and CX(3)(+): implications for structure, bonding, and reactivity

Good quality ab initio calculations (MP2) show that the water adducts of BX(3) and CX(3)(+) have totally different structures (X = F-I). While all H(2)O-BX(3) complexes have classical C(s) symmetric structures with strong B-O bonds and additional H-bonding, the heavier CX(3)(+) cations (X = Cl-I) form weakly bonded "non-classical" water adducts that maximize C-X pi-bonding rather than C-O sigma-bonding. The delocalization of the positive charge as the driving force for pi-bond formation is absent in BX(3), and therefore, pi-bonding is only weak and not structure determining in H(2)O-BX(3). Since the PES of all H(2)O --> EX(3)(0/+1) particles (E = B, C) is very flat, flexible basis sets (like TZVPP) are required to rigorously characterize the adducts. In earlier calculations (J. Am. Chem. Soc. 1997, 119, 6648), classical structures were reported for all H(2)O --> EX(3)(0/+1) (E = B, C) complexes, likely resulting from the insufficient quality of the basis sets employed. By introducing a positive charge to three coordinate boron-halogen cations Do --> BX(2)(+) (Do = NH(3), OH(2), X-H), also the B-X bonds shrink due to the stronger pi-bonding induced by the positive charge delocalization and if compared to the respective neutral compounds like H(2)N-BX(2) or BX(3). The "non-classical" water adducts also suggest that the mechanism of organic reactions involving carbenium ion intermediates with alpha-bromine or -iodine substituents and a nucleophile may proceed through halogen- rather than carbon coordination.     

Analysis of leachable and total trace metals in air particulate matters by capillary electrophoresis

A new analytical procedure was developed for simultaneous determination of ammonium, leachable and total metals in fine and coarse air particulate matters using a new capillary electrophoresis (CE) procedure, with a new buffer system containing 10 mM histidine, 2 mM 18-crown-6 and 8 mM lactic acids with pH adjusted to 4.0. A two complexes system, 18-crown-6 ether and lactic acid, was developed to solve the co-migration problem of NH(4)(+) and K(+) and to give satisfactory separation of transition metals. Satisfactory separation and quantitation of NH(4)(+), K(+), Ca(2+), Na(+) , Mg(2+) and Zn(2+) were obtained using the CE procedure developed for both leachable and total metals in coarse (10-3 mum) and fine (<3 mum) air particulate matters. Wide working ranges (ppb to ppm range) and sensitive detection limits (ppb) were obtained for the cations investigated. The reliability was established by parallel method comparison with the ICP-AES method. The analytical procedure developed is shown to provide a quick, sensitive, precise and economic method for simultaneous determination of ammonium, leachable and total metals in air particulate matters.