Inelastic neutron scattering study of electron reduction in Mn12 derivatives

We report inelastic neutron scattering (INS) studies on a series of Mn(12) derivatives, [Mn(12)O(12)(O2CC6F5)16(H2O)4]z, in which the number of unpaired electrons in the cluster is varied. We investigated three oxidation levels: z = 0 for the neutral complex, z = -1 for the one-electron reduced species and z = -2 for the two-electron reduced complex. For z = 0, the ground state is S = 10 as in the prototypical Mn12-acetate. For z = -1, we have S = 19/2, and for z = - 2, an S = 10 ground state is retrieved. INS studies show that the axial zero-field splitting parameter D is strongly suppressed upon successive electron reduction: D = -0.45 cm(-1) (z = 0), D = -0.35 cm(-1) (z = -1), and D approximately -0.26 cm(-1) (z = -2). Each electron reduction step is directly correlated to the conversion of one anisotropic (Jahn-Teller distorted) Mn3+ (S = 2) to one nearly isotropic Mn2+ (S = 5/2).     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

Electrospray ionization mass spectrometry monitoring of indigo carmine degradation by advanced oxidative processes

The degradation of the dye indigo carmine in aqueous solution induced by two oxidative processes (H(2)O(2)/iodide and O(3)) was investigated. The reactions were monitored by electrospray ionization mass spectrometry in the negative ion mode, ESI(-)-MS, and the intermediates and oxidation products characterized by ESI(-)-MS/MS. Both oxidative systems showed to be highly efficient in removing the color of the dye aqueous solutions. In the ESI(-)-MS of the indigo carmine solution treated with H(2)O(2) and H(2)O(2)/iodide, the presence of the ions of m/z 210 (indigo carmine in its anionic form, 1), 216, 226, 235, and 244 was noticeable. The anion of m/z 235 was proposed to be the unprecedented hydroperoxide intermediate 2 formed in solution via an electrophilic attack by hydroxyl and hydroperoxyl radicals of the exocyclic C=C bond of 1. This intermediate was suggested to be rapidly converted into the anionic forms of 2,3-dioxo-1H-indole-5-sulfonic acid (3, m/z 226), 2-amino-alpha-oxo-5-sulfo-benzeneacetic acid (4, m/z 244), and 2-amino-5-sulfo-benzoic acid (5, m/z 216). In the ESI(-)-MS of the indigo carmine solution treated with O(3), two main anions were detected: m/z 216 (5) and 244 (4). Both products were proposed to be produced via an unstable ozonide intermediate. Other anions in this ESI(-) mass spectrum were attributed to be [4 - H + Na](-) of m/z 266, [4 - H](2-) of m/z 121.5, and [5 - H](2-) of m/z 107.5. ESI-MS/MS data were consistent with the proposed structures for the anionic products 2-5.     

New forced degradation products of vildagliptin: Identification and structural elucidation using LC-MS,with proposed formation mechanisms

Abstract

A new and simple RP-HPLC-UV method was developed for well-separation of vildagliptin raw material and its degradation products at different conditions; it uses of ammonium acetate buffer at pH= 7.5 and methanol with Athena C18 -WP (250?mm) column. Results show that six degradants have been identified using LC–MS technique, in addition to the NMR approach in some cases. One degradant at relative retention time (RRT) 1.3 was formed under acidic condition and designated as 2-((1R, 3S, 5R, 7S)-3-hydroxyadamantan-1-yl) hexahydropyrrolo[1,2-a]pyrazine-1,4-dione at m/z = 304. Three degradants were formed under various conditions of basic hydrolysis at RRTs 1.2, 0.6 and 0.4 with following names and molar masses (m/z), respectively: 1-(((1S, 3S, 5S, 7S)-1,3-dihydroxyadamantan-2-yl)glycyl)pyrrolidine-2-carboxamide at m/z = 337.2, 1-(((1R, 3S, 5R, 7S)-3-hydroxyadamantan-1-yl)glycyl)pyrrolidine-2-carboxamide at m/z = 321.1 and (1,4-dioxo-1,4,6,7,8,8a-hexahydropyrrolo[1,2-a]pyrazin-3-yl)glycylproline at m/z = 322.6. Another three degradants were also formed under oxidative oxidations of vildagliptin, one at RRT 0.38 and designated as N-hydroxy-N-((1R, 3S, 5R, 7S)-3-hydroxyadamantan-1-yl) glycinate with m/z 241.1, the second one was identical to that formed under basic hydrolysis at RRT 0.6 and the last one has RRT 0.8 and was identified as (1S, 3R, 5R, 7S)-3-(hydroxyamino)adamantan-1-ol at m/z 183.1. Formation mechanisms for the degradation products were described.     

Electron impact mass spectra of 6-substituted 12H-tetrachlorodibenzo

Bis(2-hydroxy-3,5-dichlorophenyl)methane and its cyclized derivatives (6-substituted 12H-tetrachlorodibenzo[d,g] [1,3, 2]dioxaphosphorin-6-oxide and sulfides) were analyzed by EI-MS, and the mass spectra are discussed. It was shown that the mass spectra of the cyclized derivatives are rather different from that of the parent compound. The relative roles of the S and O atoms were very important, and led to special ions which could indicate certain moieties within the molecular structure. Thus, the fragment ions at m/z 63 and 333 indicate a cyclic structure with a (P=S) group, while m/z 397 indicates P(=S)S or P(=S)-Cl) moieties, and m/z 380 with high intensity indicates P(=O)O. Copyright 2000 John Wiley & Sons, Ltd.     

Metastable decompositions of gem-dialkoxyalkanes upon electron impact. III. Diethoxymethane (CH(2)(OCH(2)CH(3))(2))

Unimolecular metastable decomposition of diethoxymethane (CH(2)(OCH(2)CH(3))(2), 1) upon electron impact has been investigated by means of mass-analyzed ion kinetic energy (MIKE) spectrometry and theD-labeling technique in conjunction with thermochemistry. The m/z 103 ion ([M - H](+) : CH(OCH(2)CH(3)) = O(+)CH(2)CH(3)) decomposes into the m/z 47 ion (protonated formic acid, CH(OH) = O(+)H) by consecutive losses of two C(2)H(4) molecules via an m/z 75 ion. The resulting product ion at m/z 47 further decomposes into the m/z 29 and 19 ions by losses of H(2)O and CO, respectively, via an 1,3-hydroxyl hydrogen transfer, accompanied by small kinetic energy release (KER) values of 1.3 and 18.8 meV, respectively. When these two elimination reactions are suppressed by a large isotope effect, however, another 1,1-H(2)O elimination with a large KER value (518 meV) is revealed. The m/z 89 ion ([M - CH(3)](+) : CH(2)(OCH(2)CH(3))O(+) = CH(2)) decomposes into the m/z 59 ion (CH(3)CH(2)O(+) = CH(2)) by losing CH(2)O in the metastable time window. The source-generated m/z 59 ion ([M - OCH(2)CH(3)](+) : CH(2) = O(+)CH(2)CH(3)) decomposes into the m/z 41 (CH(2) = CH(+)CH(2)) and m/z 31 (CH(2) = O(+)H) ions by losses of H(2)O and C(2)H(4), respectively, with considerable hydrogen scrambling prior to decomposition. Copyright 2000 John Wiley & Sons, Ltd.     

Liquid chromatography with electrospray ion-trap mass spectrometry for the determination of anatoxins in cyanobacteria and drinking water

Anatoxin-a (AN) and homoanatoxin-a (HMAN) are potent neurotoxins produced by a number of cyanobacterial species. A new, sensitive liquid chromatography/multiple tandem mass spectrometry (LC/MS(n)) method has been developed for the determination of these neurotoxins. The LC system was coupled, via an electrospray ionisation (ESI) source, to an ion-trap mass spectrometer in positive ion mode. The [M+H](+) ions at m/z 166 (anatoxin-a) and m/z 180 (homoanatoxin-a) were used as the precursor ions for multiple MS experiments. MS(2)bond;MS(4) spectra displayed major fragment ions at m/z 149 (AN), 163 (HMAN), assigned to [Mbond;NH(3)+H](+); m/z 131 (AN), 145 (HMAN), assigned to [Mbond;NH(3)bond;H(2)O+H](+), and m/z 91 [C(7)H(7)](+). Although the chromatographic separation of these neurotoxins is problematic, reversed-phase LC, using a C(18) Luna column, proved successful. Calibration data for anatoxin-a using spiked water samples (10 mL) in LC/MS(n) modes were: LC/MS (25-1000 microg/L), r(2) = 0.998; LC/MS(2) (5-1000(microg/L), r(2) = 0.9993; LC/MS(3) (2.5-1000 microg/L), r(2) = 0.9997. Reproducibility data (% RSD, N = 3) for each LC/MS(n) mode ranged between 2.0 at 500 microg/L and 7.0 at 10 microg/L. The detection limit (S/N = 3) for AN was better than 0.03 ng (on-column) for LC/MS(3) which corresponded to 0.6 microg/L.     

Unusual syntheses, structures, and electronic properties of compounds containing ternary, T3-type supertetrahedral M/Sn/S anions [M5Sn(mu3-S)4(SnS4)4](10- (M = Zn, Co)

A recently discovered series of quaternary compounds of the general type [K(m)(ROH)(n)()][M(x)Sn(y)()Se(z)] (R = H, Me), containing ternary anions with [SnSe(4)](4-)-coordinated transition metal centers (M = Co, Mn, Zn, Cd, Hg) has now been extended by the synthesis and characterization of the two ortho-thiostannate-coordinated species, [Na(10)(H(2)O)(32)][M(5)Sn(mu(3)-S)(4)(SnS(4))(4)].2H(2)O (M = Zn (1), Co (2)). The central structural motifs of compounds 1 and 2 are highly charged [M(5)Sn(mu(3)-S)(4)(SnS(4))(4)](10-) anions, being the first T3-type supertetrahedral ternary anions reported to date. The exposure of single crystals of 2 to a dynamic vacuum for several hours resulted in the reversible formation of a partially dehydrated, but still monocrystalline material of the composition [Na(10)(H(2)O)(6)][Co(5)Sn(mu(3)-S)(4)(SnS(4))(4)] (3). The loss of 28 of the 34 water molecules only slightly affects the internal structure of the ternary anion in 3 and leads to a significant compacting of the crystal structure with closer linkage of the [Co(5)Sn(5)S(20)](10-) cluster units via the Na(+) cations. Magnetic measurements on 3 show that the ground state of the Co/Sn/S cluster is S = 1/2, indicating a significant antiferromagnetic coupling between the Co centers, which has also been rationalized by DFT investigations of the electronic situation in the ternary subunits of 1-3.     

Inelastic neutron scattering on three mixed-valence dodecanuclear polyoxovanadate clusters

The magnetic exchange interactions in the mixed-valence dodecanuclear polyoxovanadate compounds Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].23H(2)O, Na(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(D(2)O)].16.5D(2)O, and (NHEt(3))(4)[V(IV)(8)V(V)(4)As(III)(8)O(40)(H(2)O)].H(2)O were investigated by an inelastic neutron scattering (INS) study using cold neutrons. In addition, the synthesis procedures and the single-crystal X-ray structures of these compounds have been investigated together with the temperature dependence of their magnetic susceptibilities. The magnetic properties below 100 K can be described by simply taking into account an antiferromagnetically exchange coupled tetramer, consisting of four vanadium(IV) ions. Up to four magnetic transitions between the cluster S = 0 ground state and excited states could be observed by INS. The transition energies and the relative INS intensities could be modeled on the basis of the following exchange Hamiltonian: H(ex) = -2J(12)(xy)[S(1x)S(2x)+ S(3x)S(4x)+ S(1y)S(2y)+ S(3y)S(4y)] - 2J(12)(z)[(S(1z)S(2z)+ S(3z)S(4z)] - 2J(23)(xy)[(S(2x)S(3x)+ S(1x)S(4x)+ S(2y)S(3y)+ S(1y)S(4y)] - 2J(23)(z)[(S(2z)S(3z)+ S(1z)S(4z)]. The following sets of parameters were derived: for Na(4)[V(12)As(8)O(40)(H(2)O)].23H(2)O, J(12)(xy)() = J(12)(z)= -0.80 meV, J(23)(xy) = J(23)(z) = -0.72 meV; for Na(4)[V(12)As(8)O(40)(D(2)O)].16.5D(2)O, J(12)(xy) = J(12)(z) = J(23)(xy) = J(23)(z = -0.78 meV; for (NHEt(3))(4)[V(12)As(8)O(40)(H(2)O)].H(2)O, J(12)(xy) = -0.80 meV, J(12)(z) = -0.82 meV, J(23)(xy)() = -0.67 meV, J(23)(z) = -0.69 meV. This study of the same [V(12)As(8)]-type cluster in three different crystal environments allows us to draw some conclusions concerning the applicability on INS in the area of nondeuterated molecular spin clusters. In addition, the effects of using nondeuterated samples and different sample container shapes for INS were evaluated.     

Electronic structures of tris(dioxolene)chromium and tris(dithiolene)chromium complexes of the electron-transfer series [Cr(dioxolene)(3)](z) and [Cr(dithiolene)(3)](z) (z = 0, 1-, 2-, 3-). A combined experimental and density functional theoretical study

From the reaction mixture of 3,6-di-tert-butylcatechol, H2[3,6L(cat)], [CrCl3(thf)3], and NEt3 in CH3CN in the presence of air, the neutral complex [CrIII(3,6L*(sq))3] (S = 0) (1) was isolated. Reduction of 1 with [Co(Cp)2] in CH2Cl2 yielded microcrystals of [Co(Cp)2][CrIII(3,6L*(sq))2(3,6L(cat))] (S = 1/2) (2) where (3,6L*(sq)(1-) is the pi-radical monoanionic o-semiquinonate of the catecholate dianion (3,6Lcat)(2-). Electrochemistry demonstrated that both species are members of the electron-transfer series [Cr(3,6LO,O)]z (z = 0, 1-, 2-, 3-). The corresponding tris(benzo-1,2-dithiolato)chromium complex [N(n-Bu)4][CrIII(3,5L*S,S)2(3,5LS,S)] (S = 1/2) (3) has also been isolated; (3,5LS,S)(2-) represents the closed-shell dianion 3,5-di-tert-butylbenzene-1,2-dithiolate(2-), and (3,5L*S,S)(1-) is its monoanionic pi radical. Complex 3 is a member of the electron-transfer series [Cr(3,5L(S,S))3]z (z = 0, 1-, 2-, 3-). It is shown by Cr K-edge and S K-edge X-ray absorption, UV-vis, and EPR spectroscopies, as well as X-ray crystallography, of 1 and 3 that the oxidation state of the central Cr ion in each member of both electron-transfer series remains the same (+III) and that all redox processes are ligand-based. These experimental results have been corroborated by broken symmetry density functional theoretical calculations by using the B3LYP functional.     

Further insights into the spectroscopic properties, electronic structure, and kinetics of formation of the heme-peroxo-copper complex [(F8TPP)FeIII-(O2(2-)-CuII(TMPA)]+

In the further development and understanding of heme-copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII-(O22-)-CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate-iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(MeCN)](ClO4) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t1/2 (RT; MeCN) approximately 20 min; lambda max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O2 = 1:1:1}, MALDI-TOF mass spectrometry {16O2, m/z 1239 [(3 + MeCN)+]; 18O2, m/z 1243}, and resonance Raman spectroscopy {nu(O-O) = 808 cm-1; Delta16O2/18O2 = 46 cm-1; Delta16O2/16/18O2 = 23 cm-1}. Consistent with a mu-eta2:eta1 bridging peroxide ligand, two metal-O stretching frequencies are observed {nu(Fe-O) = 533 cm-1, nu(Fe-O-Cu) = 511 cm-1}, and supporting normal coordinate analysis is presented. 2H and 19F NMR spectroscopies reveal that 3 is high-spin {also muB = 5.1 +/- 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized mu-oxo complex [(F8TPP)FeIII-O-CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). M?ssbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; delta = 0.57 mm/s, |DeltaEQ| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and M?ssbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII-(O2-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of mu-peroxo complex 3 yields mu-oxo complex 4 with concomitant release of approximately 0.5 mol O2 per mol 3. Characterization of the reaction 1a/1b + O2 --> 2 --> 3 --> 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.     

Determination of iodine and bromine compounds by ion chromatography/dynamic reaction cell inductively coupled plasma mass spectrometry.

An inductively coupled plasma mass spectrometer (ICP-MS) was used as an ion chromatographic detector for the speciation of iodine and bromine. Gradient elution using NH4NO3 at pH 10 allowed the chromatographic separation of ionic iodine (I- and IO3-) and bromine (Br- and BrO3-) species in less than 8 min. Effluents from the ion-exchange column were delivered to the nebulization system of ICP-MS for the determination of I and Br. The potentially interfering 38Ar40ArH+ and 40Ar40ArH+ at the bromine masses m/z 79 and 81 were significantly reduced in intensity (by approximately two orders of magnitude) by using 0.6 mL min(-1) O2 as a reactive cell gas in the dynamic reaction cell (DRC). Moreover, the signal-to-background ratio at iodine mass m/z 127 increased significantly when O2 was used as the reaction gas. The detection limits were in the range of 0.001-0.002 and 0.03-0.04 ng mL(-1) for various I and Br compounds, respectively, based on the peak height. The relative standard deviation of the peak areas for five injections of a 2 ng mL(-1) I-, IO3- and 20 ng mL(-1) Br-, BrO3- mixture was in the range of 3-4%. The concentrations of I and Br compounds have been determined in selected water and urine samples. The spike recoveries were in the range of 94-102% for all of the determinations. This method has also been applied to determine various I and Br compounds in an NIST RM 8435 whole-milk powder reference material and a seaweed sample obtained locally. A microwave-assisted extraction method was used to extract these compounds, which were quantitatively leached with a 10% mass/volume (m/v) tetramethylammonium hydroxide (TMAH) solution in a focused microwave field within a period of 6 min. The major components of I and Br in milk powder and seaweed were I- and Br-.     

Liquid chromatography with electrospray ion-trap mass spectrometry for the determination of yessotoxins in shellfish

Yessotoxins are a group of large polyether toxins, produced by marine dinoflagellates, which cause widespread contamination of filter-feeding shellfish. A new, sensitive liquid chromatography-mass spectrometry (LC-MS) method has been developed for the determination of yessotoxin (YTX) and 45-hydroxy-yessotoxin (45-OHYTX), a major metabolite in shellfish. The LC system was coupled, via an electrospray ionisation (ESI) source, to an ion-trap MS in negative mode. The molecular related ion species at m/z 1141 [M-2Na+H]- was used as the parent ion for multiple MS experiments. MS-MS and MS3 gave major fragment ions at m/z 1061 [1141-SO3H]- and m/z 945 [1061-C9H12O]-. Predominant ions, that are due to the fragmentation of the backbone structure of YTXs, were observed at the MS4 stage. Reversed-phase LC using a C16 amide column was preferable to C18 phases for the separation of YTX and 45-OHYTX. Optimum calibration and reproducibility data were obtained for YTX using LC-MS-MS; r 2=0.9960, RSD < or = 6.3% at 0.25 microg YTX/g (n=5). The detection limit (S/N=3) was 30 pg YTX on-column which corresponded to 3 ng/g shellfish tissue.     

Analysis of triacetone triperoxide (TATP) and TATP synthetic intermediates by electrospray ionization mass spectrometry

The explosive triacetone triperoxide (TATP) has been analyzed by electrospray ionization mass spectrometry (ESI-MS) on a linear quadrupole instrument, giving a 62.5 ng limit of detection in full scan positive ion mode. In the ESI interface with no applied fragmentor voltage the m/z 245 [TATP + Na](+) ion was observed along with m/z 215 [TATP + Na - C(2)H(6)](+) and 81 [(CH(3))(2)CO + Na](+). When TATP was ionized by ESI with an applied fragmentor voltage of 75 V, ions at m/z 141 [C(4)H(6)O(4) + Na](+) and 172 [C(5)H(9)O(5) + Na](+) were also observed. When the precipitates formed in the synthesis of TATP were analyzed before the reaction was complete, a new series of ions was observed in which the ions were separated by 74 m/z units, with ions occurring at m/z 205, 279, 353, 427, 501, 575, 649 and 723. The series of evenly spaced ions is accounted for as oligomeric acetone carbonyl oxides terminated as hydroperoxides, [HOOC(CH(3))(2){OOC(CH(3))(2)}(n)OOH + Na](+) (n = 1, 2 ... 8). The ESI-MS spectra for this homologous series of oligoperoxides have previously been observed from the ozonolysis of tetramethylethylene at low temperatures. Precipitates from the incomplete reaction mixture, under an applied fragmentor voltage of 100 V in ESI, produced an additional ion observed at m/z 99 [C(2)H(4)O(3) + Na](+), and a set of ions separated by 74 m/z units occurring at m/z 173, 247, 321, 395, 469 and 543, proposed to correspond to [CH(3)CO{OOC(CH(3))(2)}(n)OOH + Na](+) (n = 1,2 ... 5). Support for the assigned structures was obtained through the analysis of both protiated and perdeuterated TATP samples.     

Electronic structures of five-coordinate iron(III) porphyrin complexes with highly ruffled porphyrin ring

The spin states of the iron(III) complexes with a highly ruffled porphyrin ring, [Fe(TEtPrP)X] where X = F-, Cl-, Br-, I-, and ClO4(-), have been examined by 1H NMR, 13C NMR, EPR, and M?ssbauer spectroscopy. While the F-, Cl-, and Br- complexes adopt a high-spin (S = 5/2) state, the I- complex exhibits an admixed intermediate-spin (S = 5/2, 3/2) state in CD2Cl2 solution. The I- complex shows, however, a quite pure high-spin state in toluene solution as well as in the solid. The results contrast those of highly saddled [Fe(OETPP)X] where the I- complex exhibits an essentially pure intermediate-spin state both in solution and in the solid. In contrast to the halide-ligated complexes, the ClO4(-) complex shows a quite pure intermediate-spin state. The 13C NMR spectra of [Fe(TEtPrP)ClO4] are characterized by the downfield and upfield shifts of the meso and pyrrole-alpha carbon signals, respectively: delta(meso) = +342 and delta(alpha-py) = -287 ppm at 298 K. The data indicate that the meso carbon atoms of [Fe(TEtPrP)ClO4] have considerable amounts of positive spin, which in turn indicate that the iron has an unpaired electron in the d(xy) orbital; the unpaired electron in the d(xy) orbital is delocalized to the meso positions due to the iron(d(xy))-porphyrin(a(2u)) interaction. Similar results have been obtained in analogous [Fe(TiPrP)X] though the intermediate-spin character of [Fe(TiPrP)X] is much larger than that of the corresponding [Fe(TEtPrP)X]. On the basis of these results, we have concluded that the highly ruffled intermediate-spin complexes such as [Fe(TEtPrP)ClO4] and [Fe(TiPrP)ClO4] adopt a novel (d(xz), d(yz))3(d(xy))1(d(z)(2)1 electron configuration; the electron configuration of the intermediate-spin complexes reported previously is believed to be (d(xy))2(d(xz)), d(yz))2(d(z)(2))1.     

Recognition of topological isomerism: synthesis,structure, and magnetic properties of two pentanuclear high-spin molecules of the type [NiI(N-N)2]3[FeIII(CN)6]2

The syntheses, structures, and magnetic properties of two pentanuclear cyanide-bridged compounds are reported. The trigonal bipyramidal molecule [[Ni(tmphen)(2)](3)[Fe(CN)(6)](2)].14H(2)O, (1).14H(2)O (tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) crystallizes in the space group P2(1)/c (No. 14) with unit cell parameters a = 19.531(4) A, b = 24.895(5) A, c = 24.522(5) A, beta = 98.68(3) degrees, V = 11787(4) A(3), and Z = 4. The pi-pi interactions between the tmphen ligands provide the closest intermolecular contacts of 3.37 A leading to large intermolecular M...M distances (> 8.68 A). The dc magnetic susceptibility of 1 indicates a ferromagnetically coupled S = 4 ground state best fit to the parameters g = 2.23, J = +4.3 cm(-1), and D(Ni) = +8.8 cm(-1) for the Hamiltonian H = -2J [(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)) + S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. The extended square molecule [Ni(bpy)(2)(H(2)O)][[Ni(bpy)(2)](2)[Fe(CN)(6)](2)].12H(2)O, (2).12H(2)O (bpy = 2,2'-bipyridine) crystallizes in the space group P1 (No. 2) with unit cell parameters a = 13.264(3) A, b = 17.607(4) A, c = 18.057(4) A, alpha = 94.58(3) degrees, beta = 103.29(3) degrees, gamma = 95.18(3) degrees, V = 4065(2) A(3), and Z = 2. The pi-pi interactions of 3.29 A between the bpy ligands are the closest intermolecular contacts, and the intermolecular M...M separations are greater than 7.76 A. The dc magnetic susceptibility data for 2 are also in accord with an S = 4 ground state arising from intramolecular ferromagnetic coupling. The data were best fit to the parameters g = 2.25, J = J' = +3.3 cm(-1), and D(Ni) = +5.8 cm(-1) for the Hamiltonian H = -2J[(S(Fe(1)) + S(Fe(2))).(S(Ni(1)) + S(Ni(2)))] - 2J'[(S(Fe(2)).S(Ni(3)))] + D[S(Ni(1))(z)(2) + S(Ni(2))(z)(2) + S(Ni(3))(z)(2)]. No evidence for long-range magnetic ordering was observed for crystalline samples of 1 or 2.     

Temporal evolution of an electron-free afterglow in the pulsed plasma polymerisation of acrylic acid

By use of time and energy-resolved mass spectrometry, negative ions with masses ranging from m/z = 1-287 amu have been observed in the afterglow of a low-pressure (10 mTorr) pulsed acrylic acid polymerizing plasma. The most intense peaks, seen at m/z = 71, 143, 215, and 287, are assigned to the dehydrogenated oligomer of the form [nM-H](-) for n = 1, 2, 3, and 4, respectively. The results strongly suggest that both m/z = 71 and 143 ions are produced in the on period of the pulse cycle (0.1 ms duration), with higher masses m/z = 215 and 287 being produced by neutral ion chemistry in the off period (up to 40 ms in duration). The increase in the intensity of the [3M-H](-) and [4M-H](-) peaks in the off period is accompanied by a rapid fall in the concentration of [M-H]- ions and electrons, the latter decreasing from approximately 10(15) m(-3) to zero within 150 micros. Deep into the afterglow, Langmuir probe measurements show that the charge species only consist of positive and negative ions, present at equal concentrations in excess of approximately 10(14) m(-3) even after 10 ms that is, the plasma is wholly electron free. To describe the growth of large negative ions a number of possible ion-neutral chemical pathways have been postulated, and a calculation of the ambipolar diffusion rates to the walls suggests that, in the off period, the positive and negative ion contribution to the deposition rate is small ( approximately 1%) compared to the net total deposition rate. However, the observations do indicate that it may be necessary to update models of film growth in the pulsed plasma polymerization of acrylic acid to account for negative ions.     

Precursors to clusters with the topology of the P(N) cluster of nitrogenase: edge-bridged double cubane clusters [(Tp)2Mo2Fe6S8L4]z: synthesis, structures, and electron transfer series

Members of the cluster set [(Tp)2Mo2Fe6S8L4]z contain the core unit M2Fe6(mu3-S)6(mu4-S)2 in which two MoFe3S4 cubanes are coupled by two Fe-(mu4-S) interactions to form a centrosymmetric edge-bridged double cubane cluster. Some of these clusters are synthetic precursors to [(Tp)2Mo2Fe6S9L2]3-, which possess the same core topology as the P(N) cluster of nitrogenase. In this work, the existence of a three-member electron-transfer series of single cubanes [(Tp)MoFe3S4L3](z) (z = 3-, 2-, 1-) and a four-member series of double cubanes [(Tp)2Mo2Fe6S8L4]z (z = 4-, 3-, 2-, 1-) with L = F-, Cl-, N3, PhS- is demonstrated by electrochemical methods, cluster synthesis, and X-ray structure determinations. The potential of the [4-/3-] couple is extremely low (<-1.5 V vs SCE in acetonitrile) such that the 4- state cannot be maintained in solution under normal anaerobic conditions. The chloride double cubane redox series was examined in detail. The members [(Tp)2Mo2Fe6S8Cl4]4-,3-,2- were isolated and structurally characterized. The redox series includes the reversible steps [4-/3-] and [3-/2-]. Under oxidizing conditions, [(Tp)2Mo2Fe6S8Cl4]2- cleaves with the formation of single cubane [(Tp)MoFe3S4Cl3]1-. The quasireversible [2-/1-] couple is observed at more positive potentials than those of the single cubane redox step. Structure comparison of nine double cubanes suggests that significant dimensional changes pursuant to redox reactions are mainly confined to the Fe2(mu4-S)2 bridge rhomb. The synthesis and structure of [(Tp)2Mo2Fe6S9F2.H2O]3-, a new topological analogue of the P(N) cluster of nitrogenase, is described. (Tp = hydrotris(pyrazolyl)borate(1-)).     

Multinuclear and dynamic NMR study of trans-[Pt(Cl)(PHCy2)2(PCy2)], [Pt(Cl)(PHCy2)3][BF4], [Pt(Cl)(PHCy2)3][Cl], trans-[Pt(Cl)(PHCy2)2[P(S)Cy(2)]], and trans-[Pt(Cl)(PHCy2)2[P(O)Cy2]]. Influence of intramolecular P=O...H-P and Cl...H-P interactions on restricted rotation about Pt-P bond. X-ray structure of trans-[Pt(Cl)(PHCy2)2[P(O)Cy2]

Dynamic NMR experiments on trans-[Pt(Cl)(PHCy2)2[P(X)Cy2]]z where X is a lone pair (1, z = 0), H (2, z = +1), S (3, z = 0), or O (4, z = 0) show that the rotation around the P(X)-Pt bond is hindered for all molecules studied, with deltaG++ ranging from 8.2 to 11.0 kcal/mol. The highest value of the series was calculated for trans-[Pt(Cl)(PHCy2)2[P(O)Cy2]] (4) where intramolecular P=O...H-P interactions act as a molecular brake at room temperature. Single-crystal X-ray diffraction confirms the presence of both intra and intermolecular P=O...H interactions in solid 4. In the case of [Pt(Cl)(PHCy2)3]Cl, multinuclear NMR analysis indicates the presence of a P-H...Cl- interaction in aromatic or halogenated solvents which could have also a minor effect on the rotational barrier around the P(X)-Pt bond.     

Ozonolysis of uric acid at the air/water interface

Uric acid (UA) epoxide, peroxide, and ozonide species produced in aqueous UA microdroplets exposed to O(3)(g) are detected by online mass spectrometry within approximately 1 ms. UA conversions are independent of its initial concentration below approximately 0.1 mM and are unaffected by addition of excess H(2)O(2) or t-butanol. UA reactivity increases approximately 380 times from pH 4 to 7, which is at variance with the pH-independent rates reported for the UA + O(3)(aq) reaction in bulk water. At pH approximately 7, UA and ascorbic acid (AH(2)) microdroplets react with O(3)(g) at similar rates, although UA is approximately 40 times more reactive than AH(2) toward O(3)(aq) in bulk water. Only the UA epoxide, plus traces of UA peroxide, are formed upon mixing UA(aq) and O(3)(aq) solutions. We infer that the gas-liquid ozonolysis of UA proceeds in an interfacial aqueous medium quite distinct from bulk water. Thus, UA, a component of the pulmonary epithelial lining fluid that scavenges atmospheric O(3)(g) into less deleterious species (similar to AH(2)), is rendered inactive below pH approximately 5. The potential implications of these findings on synergistic health effects between tropospheric ozone and acidic particulates are briefly analyzed.