Gas Phase H/D Exchange of Sodiated Amino Acids: Why Do We See Zwitterions?

The gas-phase interaction of sodiated amino acids and sodiated amino acid methyl esters with various deuterium donors is investigated by combining results of H/D exchange reactions with those from density functional theory and molecular dynamics calculations. Discrepancy between experimentally and theoretically obtained structures for sodium cationized amino acids is explained by deuterium donor caused perturbation of the most stable amino acid conformation. Detailed study of H/D exchange mechanism on sodiated amino acids shows that the H/D exchange reaction is preceded by a multistep quasi-isoenergetic transition (perturbation) from a charge solvated to zwitterionic structure in the amino acid. Although the computation refers to the system AlaNa(+) and D(2)O, these mechanisms apply to all amino acids, except those where a functional side-chain group takes part in the perturbation process. The suggested perturbation mechanism applies also for other deuterium donors such as CD(3)OD or even ND(3) and indicates that a single water molecule suffices to convert the sodiated amino acid from charge solvated to zwitterionic form.     

Mechanisms for the proton mobility-dependent gas-phase fragmentation reactions of S-alkyl cysteine sulfoxide-containing peptide ions

Mechanisms for the gas-phase fragmentation reactions of singly and multiply protonated precursor ions of the model S-alkyl cysteine sulfoxide-containing peptides GAILCGAILK, GAILCGAILR, and VTMGHFCNFGK prepared by reaction with iodomethane, iodoacetamide, iodoacetic acid, acrylamide, or 4-vinylpyridine, followed by oxidation with hydrogen peroxide, as well as peptides obtained from an S-carboxyamidomethylated and oxidized tryptic digest of bovine serum albumin, have been examined using multistage tandem mass spectrometry, hydrogen/deuterium exchange and molecular orbital calculations (at the B3LYP/6-31 + G(d,p) level of theory). Consistent with previous reports, CID-MS/MS of the S-alkyl cysteine sulfoxide-containing peptide ions resulted in the dominant "non-sequence" neutral loss of an alkyl sulfenic acid (XSOH) from the modified cysteine side chains under conditions of low proton mobility, irrespective of the alkylating reagent employed. Dissociation of uniformly deuterated precursor ions of these model peptides determined that the loss of alkyl sulfenic acid in each case occurred via a "charge-remote" five-centered cis-1,2 elimination reaction to yield a dehydroalanine-containing product ion. Similarly, the charge state dependence to the mechanisms and product ion structures for the losses of CO(2), CO(2) + H(2)O and CO(2) + CH(2)O from S-carboxymethyl cysteine sulfoxide-containing peptides, and for the losses of CH(2)CHCONH(2) and CH(2)CHC(5)H(4)N, respectively, from S-amidoethyl and S-pyridylethyl cysteine sulfoxide-containing peptide ions have also been determined. The results from these studies indicate that both the proton mobility of the peptide precursor ion and the nature of the S-alkyl substituent have a significant influence on the abundances and charge states of the product ions resulting from the various competing fragmentation pathways.     

New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from 11B NMR measurements

To gain insight on the mechanistic aspects of the palladium-catalyzed hydrolysis of NaBH(4) in alkaline media, the kinetics of the reaction has been investigated by (11)B NMR (nuclear magnetic resonance) measurements taken at different times during the reaction course. Working with BH(4)(-) concentration in the range 0.05-0.1 M and with a [substrate]/[catalyst] molar ratio of 0.03-0.11, hydrolysis has been found to follow a first-order kinetic dependence from concentration of both the substrate and the catalyst (Pd/C 10 wt %). We followed the reaction of NaBH(4) and its perdeuterated analogue NaBD(4) in H(2)O, in D(2)O and H(2)O/D(2)O mixtures. When the process was carried out in D(2)O, deuterium incorporation in BH(4)(-) afforded BH(4)(-)(n)D(n)(-) (n = 1, 2, 3, 4) species, and a competition between hydrolysis and hydrogen/deuterium exchange processes was observed. By fitting the kinetics NMR data by nonlinear least-squares regression techniques, the rate constants of the elementary steps involved in the palladium-catalyzed borohydride hydrolysis have been evaluated. Such a regression analysis was performed on a reaction scheme wherein the starting reactant BH(4)(-) is allowed both to reversibly exchange hydrogen with deuterium atoms of D(2)O and to irreversibly hydrolyze into borohydroxy species B(OD)(4)(-). In contrast to acid-catalyzed hydrolysis of sodium borohydride, our results indicate that in the palladium-catalyzed process the rate constants of the exchange processes are higher than those of the corresponding hydrolysis reactions.     

peri-Dimethylamino substituent effects on proton transfer at carbon in α-naphthylacetate esters: a model for mandelate racemase

The rate constants for exchange of hydrogen for deuterium at the α-CH(2) positions of 8-(N,N-dimethylaminonaphthalen-1-yl)acetic acid tert-butyl ester 1 and naphthalen-1-ylacetic acid tert-butyl ester 2 have been determined in potassium deuteroxide solutions in 1 : 1 D(2)O : CD(3)CN, in order to quantify the effect of the neighbouring peri-dimethylamino substituent on α-deprotonation. Intramolecular general base catalysis by the (weakly basic) neighbouring group was not detected. Second-order rate constants, k(DO), for the deuterium exchange reactions of esters 1 and 2 have been determined as 1.35 × 10(-4) M(-1) s(-1) and 3.95 × 10(-3) M(-1) s(-1), respectively. The unexpected 29-fold decrease in the k(DO) value upon the introduction of a peri-dimethylamino group is attributed to an unfavourable steric and/or electronic substituent effect on intermolecular deprotonation by deuteroxide ion. From the experimental k(DO) values, carbon acid pK(a) values of 26.8 and 23.1 have been calculated for esters 1 and 2.     

Hydrogen–Deuterium Exchange Reactions of Aromatic Compounds and Heterocycles by NaBD4‐Activated Rhodium,Platinum and Palladium Catalysts

Conventional thermal and microwave conditions were compared for hydrogen–deuterium (H/D) exchange reactions of aminobenzoic acids catalysed by NaBD₄‐activated Pd/C or RhCl₃ with D₂O as the deuterium source. We also investigated different NaBD₄‐activated metal catalysts (including Pd/C, RhCl₃ and Pt/C) under microwave conditions for an efficient H/D exchange of aromatic and heterocyclic compounds. Even higher deuterium incorporations were obtained for Pd/C and Pt/C catalyst mixtures due to the previously observed synergistic effect. Finally, we have applied these optimised conditions for one‐step syntheses of the MS standards of several pharmaceutically active compounds.     

Characterization of alpha- and gamma-glutamyl dipeptides by negative ion collision-induced dissociation

The low-energy CID mass spectra of the [M-H](-) ions of a variety of dipeptides containing glutamic acid have been obtained using cone-voltage collisional activation. Dipeptides with the gamma-linkage, H-Glu(Xxx-OH)-OH, are readily distinguished from those with the alpha-linkage, H-Glu-Xxx-OH, by the much more prominent elimination of H-Xxx-OH from the [M-H](-) ions of the former isomers, resulting in formation of m/z 128, presumably deprotonated pyroglutamic acid. Dipeptides with the reverse linkage, H-Xxx-Glu-OH, show distinctive fragmentation reactions of the [M-H](-) ions including enhanced elimination of CO(2) and formation of deprotonated glutamic acid. Exchange of the labile hydrogens for deuterium has shown that there is considerable interchange of C-bonded hydrogens with labile (N- and O-bonded) hydrogens prior to most fragmentation reactions. All dipeptides show loss of H(2)O from [M-H](-). MS(3) studies show that the [M-H-H(2)O](-) ion derived from H-Glu-Gly-OH has the structure of deprotonated pyroglutamylglycine while the [M-H-H(2)O](-) ions derived from H-Glu(Gly-OH)-OH and H-Gly-Glu-OH show a different fragmentation behaviour indicating distinct structures for the fragment ions.     

Full-dimensional quantum dynamics study of exchange processes for the D + H2O and D + HOD reactions

The exchange processes of D + H(2)O and D + HOD reactions are studied using initial state-selected time-dependent wave packet approach in full dimension. The total reaction probabilities for different partial waves, together with the integral cross sections, are obtained both by the centrifugal sudden (CS) approximation and exact coupled-channel (CC) calculations, for the H(2)O(HOD) reactant initially in the ground rovibrational state. In the CC calculations, small resonance peaks in the reaction probabilities and quick diminishing of the resonance peaks with the increase of total angular momenta J do not lead to clear step-like features just above the threshold in the cross sections for the title reactions, which are different in other isotopically substituted reactions where the hydrogen atom was included as the reactant instead of the deuterium atom [B. Fu, Y. Zhou, and D. H. Zhang, Chem. Sci. 3, 270 (2012); B. Fu and D. H. Zhang, J. Phys. Chem. A 116, 820 (2012)]. It is interesting that the shape resonance-induced features resulting from the reaction tunneling are significantly diminished accordingly in the reactions of the deuterium atom and H(2)O or HOD, owing to the weaker tunneling capability of the reagent deuterium atom in the title reactions than the reagent hydrogen atom in other reactions. In the CS calculations, the resonance peaks persist in many partial waves but cannot survive the partial-wave summations. The cross sections for the D(') + H(2)O → D(')OH + H and D(') + HOD → D(')OD + H reactions are substantially larger than those for the D(') + HOD → HOD(') + D reaction, indicating that the D(')/H exchange reactions are much more favored than the D(')/D exchange.     

Experimental and theoretical studies of charge transfer and deuterium ion transfer between D2O+ and C2H4

The charge transfer and deuterium ion transfer reactions between D(2)O(+) and C(2)H(4) have been studied using the crossed beam technique at relative collision energies below one electron volt and by density functional theory (DFT) calculations. Both direct and rearrangement charge transfer processes are observed, forming C(2)H(4) (+) and C(2)H(3)D(+), respectively. Independent of collision energy, deuterium ion transfer accounts for approximately 20% of the reactive collisions. Between 22 and 36 % of charge transfer collisions occur with rearrangement. In both charge transfer processes, comparison of the internal energy distributions of products with the photoelectron spectrum of C(2)H(4) shows that Franck-Condon factors determine energy disposal in these channels. DFT calculations provide evidence for transient intermediates that undergo H/D migration with rearrangement, but with minimal modification of the product energy distributions determined by long range electron transfer. The cross section for charge transfer with rearrangement is approximately 10(3) larger than predicted from the Rice-Ramsperger-Kassel-Marcus isomerization rate in transient complexes, suggesting a nonstatistical mechanism for H/D exchange. DFT calculations suggest that reactive trajectories for deuterium ion transfer follow a pathway in which a deuterium atom from D(2)O(+) approaches the pi-cloud of ethylene along the perpendicular bisector of the C-C bond. The product kinetic energy distributions exhibit structure consistent with vibrational motion of the D-atom in the bridged C(2)H(4)D(+) product perpendicular to the C-C bond. The reaction quantitatively transforms the reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the deuterium ion is transferred in a configuration in which both the breaking and the forming bonds are extended.     

Gas-phase hydrogen/deuterium exchange of 5′- and 3′-mononucleotides in a quadrupole ion trap: Exploring the role of conformation and system energy

Gas-phase hydrogen/deuterium (H/D) exchange reactions for deprotonated 2'-deoxy-5'-monophosphate and 2'-deoxy-3'-monophosphate nucleotides with D(2)O were performed in a quadrupole ion trap mass spectrometer. To augment these experiments, molecular modeling was also conducted to identify likely deprotonation sites and potential gas-phase conformations of the anions. A majority of the 5'-monophosphates exchanged extensively with several of the compounds completely incorporating deuterium in place of their labile hydrogen atoms. In contrast, most of the 3'-monophosphate isomers exchanged relatively few hydrogen atoms, even though the rate of the first two exchanges was greater than observed for the 5'-monophosphates. Mononucleotides that failed to incorporate more than two deuterium atoms under default reaction conditions were often found to exchange more extensively when reactions were performed under higher energy conditions. Integration of the experimental and theoretical results supports the use of a relay exchange mechanism and suggests that the exchange behavior depends highly on the identity and orientation of the nucleobase and the position and flexibility of the deprotonated phosphate moiety. These observations also highlight the importance of the distance between the various participating groups in addition to their gas-phase acidity and basicity.     

Direct evidence of singlet molecular oxygen [O2(1Deltag)] production in the reaction of linoleic acid hydroperoxide with peroxynitrite

Peroxynitrite (ONOO-), a biologically active species, can induce lipid peroxidation in biological membranes, thereby leading to the formation of various hydroperoxides. We report herein on the formation of singlet molecular oxygen [O(2) ((1)Delta(g))] in the reaction of peroxynitrite with linoleic acid hydroperoxide (LAOOH) or (18)O-labeled LAOOH. The formation of O(2) ((1)Delta(g)) was characterized by (i) dimol light emission in the red spectral region (lambda > 570 nm) using a red-sensitive photomultiplier; (ii) monomol light emission in the near-infrared region (lambda = 1270 nm) with a liquid nitrogen-cooled germanium diode or a photomultiplier coupled to a monochromator; (iii) the enhacing effect of deuterium oxide on chemiluminescence intensity, as well as the quenching effect of sodium azide; and (iv) chemical trapping of O(2) ((1)Delta(g)) or (18)O-labeled O(2) ((1)Delta(g)) with the 9,10-diphenylanthracene (DPA) and detection of the corresponding DPAO(2) or (18)O-labeled DPA endoperoxide by HPLC coupled to tandem mass spectrometry. Moreover, the presence of O(2) ((1)Delta(g)) was unequivocally demonstrated by a direct spectral characterization of the near-infrared light emission attributed to the transition of O(2) ((1)Delta(g)) to the triplet ground state. For the sake of comparison, O(2) ((1)Delta(g)) deriving from the thermolysis of the endoperoxide of 1,4-dimethylnaphthalene or from the H(2)O(2)/hypochlorite and H(2)O(2)/molybdate systems were also monitored. These novel observations identified the generation of O(2) ((1)Delta(g)) in the reaction of LAOOH with peroxynitrite, suggesting a potential O(2) ((1)Delta(g))-dependent mechanism that contributes to cytotoxicity mediated by lipid hydroperoxides and peroxynitrite reactions in biological systems.     

Zinc and cadmium dihydroxide molecules: matrix infrared spectra and theoretical calculations

Laser-ablated zinc and cadmium atoms were mixed uniformly with H2 and O2 in excess argon or neon and with O2 in pure hydrogen or deuterium during deposition at 8 or 4 K. UV irradiation excites metal atoms to insert into O2 producing OMO molecules (M = Zn, Cd), which react further with H2 to give the metal hydroxides M(OH)2 and HMOH. The M(OH)2 molecules were identified through O-H and M-O stretching modes with appropriate HD, D2, (16,18)O2, and (18)O2 isotopic shifts. The HMOH molecules were characterized by O-H, M-H, and M-O stretching modes and an M-O-H bending mode, which were particularly strong in pure H2/D2. Analogous Zn and Cd atom reactions with H2O2 in excess argon produced the same M(OH)2 absorptions. Density functional theory and MP2 calculations reproduce the IR spectra of these molecules. The bonding of Group 12 metal dihydroxides and comparison to Group 2 dihydroxides are discussed. Although the Group 12 dihydroxide O-H stretching frequencies are lower, calculated charges show that the Group 2 dihydroxide molecules are more ionic.     

Isotope exchange and structural rearrangements in reactions between size-selected ionic water clusters, H3O(+)(H2O)(n) and NH4(+)(H2O)(n), and D2O

Hydrogen/deuterium exchange in reactions of H3O(+)(H2O)n and NH4(+)(H2O)n (1 < or = n < or = 30) with D2O has been studied experimentally at center-of-mass collisions energies of < or = 0.2 eV. For a given cluster size, the cross-sections for H3O(+)(H2O)n and NH4(+)(H2O)n are similar, indicating a structural resemblance and energetics of binding. For protonated pure water clusters, H3O(+)(H2O)n, reacting with D2O the main H/D exchange mechanism is found to be proton catalyzed. In addition the H/D scrambling becomes close to statistically randomized for the larger clusters. For NH4(+)(H2O)n clusters reacting with D2O, the main mechanism is a D2O/H2O swap reaction. The lifetimes of H3O(+)(H2O)n clusters have been estimated using RRKM theory and a plateau in lifetime vs. cluster size is found already at n = 10.     

Hydrogen atom transfer reactions of C2-, C4-, and C6-: bond dissociation energies of linear H-C2n- and H-C2n (n = 1, 2, 3)

The reactions of C2-, C4-, and C6- with D2O and ND3 and of C4- with CH3OH, CH4, and C2H6 have been investigated using guided ion beam tandem mass spectrometry. Hydrogen (or deuterium) atom transfer is the major product channel for each of the reactions. The reaction threshold energies for collisional activation are reported. Several of the reactions exhibit threshold energies in excess of the reaction endothermicity. Potential energy calculations using density functional theory show energy barriers for some of the reactions. Dynamic restrictions related to multiple wells along the reaction path may also contribute to elevated threshold energies. The results indicate that the reactions with D2O have the smallest excess threshold energies, which may therefore be used to derive lower limits on the C-H bond dissociation energies of the C2nH- and C2nH (n = 1-3) linear species. The experimental lower limits for the bond dissociation energies of the neutral radicals to linear products are D0(C2-H) >or= 460 +/- 15 kJ/mol, D0(C4-H) >or= 427 +/- 12 kJ/mol, and D0(C6-H) >or= 405 +/- 11 kJ/mol.     

Synthesis and structure of nickel-containing cuboidal clusters derived from [W3Se4(H2O)9]4+. Site-differentiated substitution at the nickel site in the series [W3NiQ4(H2O)10]4+ (Q = S, Se)

New Ni-containing heterometallic cuboidal cluster aqua complex [W3(NiCl)Se4(H2O)9]3+, the missing link in the family of the M3NiQ4 clusters (M = Mo, W; Q = S, Se), has been prepared by the reaction of [W3Se4(H2O)9]4+ with Ni in 2 M HCl. Single crystals of edge-linked double-cuboidal cluster [{W3NiSe4(H2O)9}2](pts)8.18H2O (pts = p-toluenesulfonate) were grown from the solution of the aqua complex in 3 M Hpts, and their structures were determined. The Ni site in the clusters [W3(NiCl)Q4(H2O)9]3+ selectively coordinates typical pi-acceptor ligands such as CO, olefins, acetylenes, phosphines, arsines, or SnCl3-. This allows stabilization by coordination of such elusive species as HP(OH)2 and As(OH)3. The stability constants for coordination of HP(OH)2, As(OH)3, and SnCl3- were determined. The Se for S substitution increases the stability by 1-2 orders of magnitude. Supramolecular adducts with cucurbit[6]uril (Cuc), [W3(Ni(HP(OH)2))Q4(H2O)9]Cl4.Cuc.11H2O and [W3(NiAs(OH)3)S4(H2O)8Cl]Cl3.Cuc.13H2O, were isolated and structurally characterized.     

OMP decarboxylase: phosphodianion binding energy is used to stabilize a vinyl carbanion intermediate

Orotidine 5'-monophosphate decarboxylase (OMPDC) catalyzes the exchange for deuterium from solvent D(2)O of the C-6 proton of 1-(β-d-erythrofuranosyl)-5-fluorouracil (FEU), a phosphodianion truncated product analog. The deuterium exchange reaction of FEU is accelerated 1.8 × 10(4)-fold by 1 M phosphite dianion (HPO(3)(2-)). This corresponds to a 5.8 kcal/mol stabilization of the vinyl carbanion-like transition state, which is similar to the 7.8 kcal/mol stabilization of the transition state for OMPDC-catalyzed decarboxylation of a truncated substrate analog by bound HPO(3)(2-). These results show that the intrinsic binding energy of phosphite dianion is used in the stabilization of the vinyl carbanion-like transition state common to the decarboxylation and deuterium exchange reactions.     

Identification of non-covalent structure in apocytochrome c by hydrogen exchange and mass spectrometry

Apocytochrome c, the in vivo precursor to active cytochrome c, was analyzed by amide hydrogen exchange and mass spectrometry to search for fixed, non-covalent structure. The protein was incubated in H(2)O at pH 3.3 or 6.7 for various times, then exposed to D(2)O to initiate isotope labeling of unfolded regions. Following acid quenching of hydrogen exchange, the labeled apocytochrome c was digested with pepsin into fragments that were analyzed by directly coupled high-performance liquid chromatography/electrospray ionization mass spectrometry. The intermolecular distribution of deuterium and the deuterium levels in structurally distinctive populations were determined from the mass spectra of the peptic fragments. Spectra of peptic fragments derived from apocytochrome c incubated at pH 3.3 had single envelopes of isotope peaks with masses indicating that all of the amide hydrogens had been replaced with deuterium. These results showed that apocytochrome c at pH 3.3 offered little resistance to hydrogen exchange, indicating that it was unfolded with little fixed structure. However, mass spectra of peptic fragments including residues 81-94 of apocytochrome c incubated at pH 6.7 had two envelopes of isotope peaks, indicating that one population was unfolded and the other population was highly structured in this region. Mass spectra of peptic fragments including residues N-terminal to residue 81 indicated that this region of the protein remained unfolded with little fixed structure at pH 6.7.     

Gas-phase reactions and rearrangements of alkyl esters with H3O+, NO+, and O2*+: a selected ion flow tube study

Selected ion flow tube mass spectrometry (SIFT-MS) has been employed to study the ion-molecule reactions of 17 alkyl esters reacting with the common SIFT-MS reagent ions, H3O+, H3O+.nH2O (n = 1, 2, 3), NO+, and O2+. The majority of reactions were observed to proceed at or near collision rate, with the exception of H3O+.3H2O, which was found to be slow for 8 of 17 alkyl esters. Unexpected product ions in the form of the parent carboxylic acid cation were observed to arise from the H3O+ and NO+ reactions of some alkyl esters. The observed reactions have been probed by the ab initio CBS-4M and G2(MP2,SVP) methods. The postulated reaction pathway involves a 1,5 H atom migration from a beta-carbon onto the carbonyl oxygen.     

Atmospheric chemistry of dimethyl phosphonate, dimethyl methylphosphonate, and dimethyl ethylphosphonate

Rate constants for the reactions of OH radicals and NO3 radicals with dimethyl phosphonate [DMHP, (CH3O)2P(O)H], dimethyl methylphosphonate [DMMP, (CH3O)2P(O)CH3], and dimethyl ethylphosphonate [DMEP, (CH3O)2P(O)C2H5] have been measured at 296 +/- 2 K and atmospheric pressure using relative rate methods. The rate constants obtained for the OH radical reactions (in units of 10(-12) cm3 molecule(-1) s(-1)) were as follows: DMHP, 4.83 +/- 0.25; DMMP, 10.4 +/- 0.6; and DMEP, 17.0 +/- 1.0, with a deuterium isotope effect of k(OH + DMMP)/k(OH + DMMP-d9) = 4.8 +/- 1.2. The rate constants obtained for the NO3 radical reactions (in units of 10(-16) cm3 molecule(-1) s(-1)) were as follows: DMHP, < 1.4; DMMP, 2.0 +/- 1.0; and DMEP, 3.4 +/- 1.4. Upper limits to the rate constants for the O3 reactions of < 8 x 10(-20) cm3 molecule(-1) s(-1) for DMHP and < 6 x 10(-20) cm3 molecule(-1) s(-1) for DMMP and DMEP were determined. Products of the reactions of OH radicals with DMHP, DMMP, and DMEP were investigated in situ using atmospheric pressure ionization mass spectrometry (API-MS) and, for the DMMP and DMEP reactions, Fourier transform infrared (FT-IR) spectroscopy. API-MS analyses showed the formation of products of molecular weight 96 and 126, attributed to CH3OP(O)(H)OH and (CH3O)2P(O)OH, respectively, from DMHP; of molecular weight 110, attributed to CH3OP(O)(CH3)OH, from DMMP; and of molecular weight 124 and 126, attributed to CH3OP(O)(C2H5)OH and (CH3O)2P(O)OH, respectively, from DMEP. FT-IR analyses showed formation (values given are % molar yields) of the following: from DMMP, CO, 54 +/- 6; CO2, 5 +/- 1 in dry air; HCHO, 3.9 +/- 0.7; HC(O)OH, < 1.4 in dry air; RONO2, approximately 4; and formate ester, approximately 8; and from DMEP, CO, 50 +/- 7; CO2, 11 +/- 4; CH3CHO, 18 +/- 8; HCHO, < 7; HC(O)OH, < 6; RONO2, < or = 5; and formate ester, 5.0 +/- 1.5. Possible reaction mechanisms are discussed.     

From a Dy(III) Single Molecule Magnet (SMM) to a Ferromagnetic [Mn(II)Dy(III)Mn(II)] Trinuclear Complex

The Schiff base compound 2,2'-{[(2-aminoethyl)imino]bis[2,1-ethanediyl-nitriloethylidyne]}bis-2-hydroxy-benzoic acid (H(4)L) as a proligand was prepared in situ. This proligand has three potential coordination pockets which make it possible to accommodate from one to three metal ions allowing for the possible formation of mono-, di-, and trinuclear complexes. Reaction of in situ prepared H(4)L with Dy(NO(3))(3)·5H(2)O resulted in the formation of a mononuclear complex [Dy(H(3)L)(2)](NO(3))·(EtOH)·8(H(2)O) (1), which shows SMM behavior. In contrast, reaction of in situ prepared H(4)L with Mn(ClO(4))(2)·6H(2)O and Dy(NO(3))(3)·5H(2)O in the presence of a base resulted in a trinuclear mixed 3d-4f complex (NHEt(3))(2)[Dy{Mn(L)}(2)](ClO(4))·2(H(2)O) (2). At low temperatures, compound 2 is a weak ferromagnet. Thus, the SMM behavior of compound 1 can be switched off by incorporating two Mn(II) ions in close proximity either side of the Dy(III). This quenching behavior is ascribed to the presence of the weak ferromagnetic interactions between the Mn(II) and Dy(III) ions, which at T > 2 K act as a fluctuating field causing the reversal of magnetization on the dysprosium ion. Mass spectrometric ion signals related to compounds 1 and 2 were both detected in positive and negative ion modes via electrospray ionization mass spectrometry. Hydrogen/deuterium exchange (HDX) reactions with ND(3) were performed in a FT-ICR Penning-trap mass spectrometer.     

V2O5-WO3/TiO2-ZrO2脱硝催化剂中 ZrO2和 WO3的促进作用：催化性能、形态及反应机理

商业选择性催化还原(SCR)催化剂成分主要有 V2O5, WO3和 TiO2,但适用温度窗口较窄(300?400℃),使得实际操作过程中活性较低.目前,过渡金属广泛应用于催化剂制备中以提高其催化活性.相比于纯 TiO2和 ZrO2载体, TiO2-ZrO2具有较高的热稳定性以及较多的酸位,虽然有关 TiO2-ZrO2为载体的催化剂研究较多,但未与商业催化剂进行对比研究.而针对 NH3-SCR脱硝机理的实验研究也存在一些争议,主要原因归为以下两方面：(1)多数催化剂不同会直接导致催化剂的活性酸位不同;(2)不同 NH3-SCR脱硝催化剂的起活温度不同.同时, NH3和 NO在反应温度的吸附情况仍需要进一步研究.因此,有必要深入探究 NH3-SCR脱硝机理,以解决现行研究中存在的问题.本文首先采用共沉淀法制备摩尔比为1：1的 TiO2-ZrO2固溶体,并分步浸渍不同质量比的 WO3和1%V2O5,最终得到一系列1%V2O5-x%WO3/TiO2-ZrO2.然后通过 X射线衍射(XRD)和比表面积测试(BET)、程序升温还原(TPR)、原位漫反射红外光谱(in situ DRIFTS)研究了 WO3和 ZrO2对催化性能的影响以及 V2O5-WO3/TiO2-ZrO2催化剂的反应机理. N2物理吸附结果表明, WO3的添加使得催化剂孔结构的热稳定性有所提高,同时随着 WO3含量增加催化剂的比表面积逐渐减小,但仍高于 V2O5/TiO2-ZrO2催化剂; ZrO2对催化剂比表面积增大效果比较明显.结合 XRD结果表明, WO3能促进金属氧化物在载体上的分散;相比于 V2O5-WO3/TiO2催化剂, ZrO2有利于活性组分的分散负载.比较系列 V2O5-x%WO3/TiO2-ZrO2的氨吸附情况,发现 WO3的添加增加了 Br?nsted酸的稳定性,其中以9%WO3的效果最显著.催化剂氨吸附中间物种(–NH2)的发现,证实了 WO3添加促进了 NH3的活化,有利于脱硝反应的进行. SCR反应结果显示, V2O5-9%WO3/TiO2-ZrO2催化剂在300–450oC时 NOx转化效率最优,并发现 O2的存在促进了 NOx的转化.采用in situ DRIFTS研究了 V2O5-x%WO3/TiO2-ZrO2催化剂脱硝机理,300和350oC时 NH3, NO, NO + O2吸附情况表明,在真实的反应温度下,脱硝过程中的活性中心为 Lewis酸中心, Br?nsted酸中心的 NH4+极易从催化剂表面脱附,无法吸附在催化剂表面,且与 NH3相比, NO只能以 NO2的形式弱吸附在催化剂表面.因此,该催化剂遵循 Eley-Ridel脱硝机理.而 V2O5-9%WO3/TiO2-ZrO2催化剂具有相对较高的脱硝效率,因此用来着重研究 NH3-SCR机理.在 NH3吸附过程中, NH3(1204,1602,3156,3264,3347 cm?1)和活性中产物 NH2(1550 cm?1)在催化剂表面的吸附(恒温300oC)是稳定的;随后通入 NO + O2时, NH3吸附过程中的所有吸收峰(包括 NH2)均逐渐减小(NH3吸附态与 NO结合后分解为 N2和 H2O),同时出现 H2O的振动峰,这证明了 V2O5-x%WO3/TiO2-ZrO2催化剂的脱硝反应过程.各类气体吸附情况表明, NO在商业催化剂的吸附状态与 V2O5-x%WO3/TiO2-ZrO2催化剂相同;但 NH3吸附结果表明, Br?nsted酸中心和 Lewis酸中心都是催化剂的活性中心; NO + O2的通入使得催化剂表面的 NH3和 NH4+都逐渐消失.这两种催化剂脱硝反应过程差异主要在于催化剂表面活性中心的不同,导致了不同的 NOx脱除路径.通过in situ DRIFTS比较 O2的存在对脱硝反应产生的不同影响来确定 O2的作用.两类催化剂上 O2均参与了 H2O的形成,促进了催化反应的完成;当 O2不存在时, NO的还原受到了极大地抑制,同时也未出现 H2O;两者的脱硝效率大大降低. H2-TPR和 NH3-TPR结果进一步证实 O2的作用主要是氧化 NO及参与催化过程 H2O的形成.