Torben Lund

On Electron Transfer in Aliphatic Nucleophilic Substitution

Since the classical studies by Ingold in the nineteen thirties the mechanism of the aliphatic nucleophilic substitution reaction has been described as a polar two-electron mechanism in which the nucleophile transfers two electrons to the new bond which is established to the electrophilic center. Depending on the reaction order the abbreviations SN1 and SN2 were used for first-order and second-order reactions, respectively.

Sensitive and selective analysis of coenzyme Q10 in human serum by negative APCI LC-MS

Two new sensitive and selective LC-MS methods have been developed for the quantification of the total coenzyme Q(10) concentration in human blood serum. The sensitivity of the methods is based on the very efficient formation of the radical anions of CoQ(10)[M(-)[radical dot]] by negative atmospheric pressure ionisation, APCI(-). The mass detection of the [M(-)[radical dot]] ions, m/z= 862.6, was performed either in selective ion monitoring (SIM) or in MS(2) mode (m/z= 862.6 [rightward arrow]m/z= 847.6) using an LCQ-deca ion-trap mass spectrometer. Two standard serum samples with medium (0.73 [micro sign]g ml(-1)) and high (1.96 [micro sign]g ml(-1)) total CoQ(10) concentrations were analysed by LC-APCI(-)-SIM and LC-APCI(-)-MS(2) and the results compared with a HPLC literature procedure with electrochemical detection (ECD). Both the LC-MS methods were shown to be more selective and with comparable or better sensitivity than the HPLC-ECD method. The LC-MS-SIM and LC-MS(2) chromatograms of the medium concentration sample showed CoQ(10) signal to noise ratios of 25 and 625, respectively. In addition, a simple and fast serum pre-treatment procedure was developed, in which the serum CoQ(10)H(2) content was quantitatively oxidised quantitatively to CoQ(10) in less than 15 min by 1,4-benzoquinone.

Long-term effect of Sea-Nine on natural coastal phytoplankton communities assessed by pollution induced community tolerance

Sea-Nine211 has been introduced as a new biocide in antifouling paints with an immediate degradation when it is released from ship hulls. The active component of Sea-Nine211 is 4,5-dichloro-2-n-octyl-isothiazoline-3-one (DCOI). In the present study, the toxicity of DCOI and the occurrence of Pollution Induced Community Tolerance (PICT) were tested in microcosms containing eutrophic coastal water with its natural composition of phytoplankton. The experiment was performed in closed systems with a single addition of the nominal concentrations 0, 3.2, 10, 32 and 100 nM DCOI, for a period of 16 days. Pollution induced community tolerance (PICT) was observed in the phytoplankton communities exposed to the nominal concentrations 32 and 100 nM DCOI. Chemical analysis of DCOI in the coastal water utilised in the toxicity and PICT experiment was performed by GC-MS using a solid- phase extraction method. Half-life was calculated to be 2.5 days for the nominal concentrations 32 and 100 nM DCOI. The results of the present study show that nominal concentrations of 32 and 100 nM DCOI significantly increased the community tolerance already after 2 days of exposure and that the tolerance was maintained for a period of 16 days even when DCOI was degraded during this period. The causes for the persistent tolerance are discussed in relation to the degradation of DCOI and structural changes in the phytoplankton communities.

Indirect Electrochemical Reduction of Some Benzyl Chlorides.

Etude par voltammetrie cyclique de la reduction des chlorures de benzyle, methoxy-4 benzyle α-methylbenzyle et (α-ethyl α-methyl) benzyle en presence de mediateurs chimiques (arenes, arenes condenses, cetones aromatiques)

Regioselectivity in the reductive bond cleavage of diarylalkylsulfonium salts: variation with driving force and structure of sulfuranyl radical intermediates.

This investigation was stimulated by reports that one-electron reductions of monoaryldialkylsulfonium salts never give aryl bond cleavage whereas reductions of diarylmonoalkylsulfonium salts preferentially give aryl bond cleavage. We studied the product ratios from the reductive cleavage of di-4-tolylethylsulfonium and di-4-tolyl-2-phenylethylsulfonium salts by a variety of one-electron reducing agents ranging in potential from -0.77 to +2.5 eV (vs SCE) and including thermal reductants, indirect electrolyses mediated by a series of cyanoaromatics, and excited singlet states. We report that the cleavage products vary from regiospecific alkyl cleavage to predominant aryl cleavage as a function of the potential of the reducing agent. We conclude that differences between the reductive cleavages of mono- and diarylsulfonium salts are direct consequences of the structures of the sulfuranyl radical intermediates and the bond dissociation energies of the alkyl and aryl bonds. Competitions between the rates of cleavage and oxidation of the intermediate sulfuranyl radicals and between concerted and stepwise mechanisms are discussed to explain the variations in bond cleavage products as a function of the driving forces for the reductions. Density functional theory investigations of the nature of the antibonding S-alkyl and S-aryl orbitals of the starting sulfonium salts provide additional insight.

Determination of D/L‐amino acids by zero needle voltage electrospray ionisation

Ion formation may be made more efficient than in normal electrospray ionization (ESI) for certain classes of compounds, such as the polar amino acids Glu, Asn, His, Ser, Asp, Arg, Tyr and Lys, by adjusting the voltage of a normal ESI interface needle to zero voltage. For aspartic acid (Asp) the gain in signal-to-noise (S/N) ratio of the liquid chromatography/mass spectrometry (LC/MS) chromatograms obtained in the selective ion monitoring (SIM) mode (m/z 134) with zero needle potential was 40-50 times higher than detection at 4 kV. Ion formation at zero potential is likely to follow a mechanism related to sonic spray ionization. The utility of the zero needle voltage ESI was illustrated by determining the age of a human tooth by the aspartic acid epimerization method. The procedure involved separating the D- and L-aspartic acid of a tooth extract on a chiral HPLC column and detection by zero voltage ESI-MS3.

Measurements of standard potentials for nucleophiles by fast cyclic voltammetry. I: 9-substituted fluorenide ions in dimethyl sulphoxide

Abstract Standard potentials E Fl . /Fl − for 12 different 9-substituted fluorenide ions were measured in dimethyl sulphoxide (DMSO) by fast cyclic voltammetry. 9-Amino-fluorenide ions are found to exhibit two reversible single-electron waves in cyclic voltammetry showing that the corresponding 9-aminofluorenyl radicals and fluorenium cations are stable. Standard potentials for the 9-aminofluorenide ion and for the 9-aminofluorenyl radical were measured. For 9-alkyl-, phenyl-, methoxy-, phenylthiofluorenide ions, only one single-electron irreversible wave was found and linear sweep voltammetric results indicate a DIM1 mechanism for less sterically hindered fluorenyl radicals. For severely sterically hindered fluorenyl radicals a more complicated mechanism was found to occur. The rate constants for the dimerization of the fluorenyl radicals were determined by comparison of experimental and simulated voltammograms, from the potential shift of oxidation potentials relative to standard potentials or by double potential step chronoamperometry. 9-aminofluorenyl radicals are found to be stable owing to spin delocalization to the nitrogen present and it was possible to measure the standard potentials for the 9-aminofluorenide ion and the 9-aminofluorenyl radical. The former potentials are quite close to the oxidation potentials published by Bordwell. For the other group of 9-substituted fluorenide ions we find that the corresponding fluorenyl radicals dimerize. If the steric hindrance is not too severe, two fluorenyl radicals will dimerize and 9,9′-disubstituted-9,9′-bifluorene is obtained. When steric hindrance becomes more important the dimerization in the 9-position becomes slower and an alternative dimerization of one fluorenyl radical and one fluorenide ion results in the anion radical of the bifluorene. This anion radical is finally oxidized by a fluorenyl radical to the bifluorene in a rate-determining solution electron transfer.

Direct determination of rate constants for coupling between aromatic radical anions and alkyl and benzyl radicals by laser-flash photolysis

Coupling rates between the radicals methyl, n-, sec-, tert-butyl and benzyl (R.) and the aromatic radical anions of 1,4-dicyanonaphthalene, 9,10-dicyanoanthracene and fluorenone (A-.) have been obtained using a new laser-flash photolysis method. The radicals R. and the radical anions A-. were generated by a photoinduced electron transfer reaction between the aromatic compound A and the alkyl or benzyl triphenylborate anion RB(Ph)3-. For the first time the rate constants of the coupling reaction between methyl and benzyl radicals with aromatic radical anions have been obtained. For all the measured coupling rate constants an average value of k1 = 1.9 x 10(9) M-1 s-1 was found with a relatively small variation in the coupling rates (0.8-2.9 x 10(9) M-1 s-1). The results demonstrate that the coupling rate k1 is insensitive to changes in the steric and electronic properties of the radicals and the structure and standard potentials of the aromatic radical anions.

Dye-sensitized solar cells and complexes between pyridines and iodines. A NMR, IR and DFT study.

Interactions between triiodide (I(3)(-)) and 4-tert-butylpyridine (4TBP) as postulated in dye-sensitized solar cells (DSC) are investigated by means of (13)C NMR and IR spectroscopy supported by DFT calculations. The charge transfer (CT) complex 4TBP·I(2) and potential salts such as (4TBP)(2)I(+), I(3)(-) were synthesized and characterized by IR and (13)C NMR spectroscopy. However, mixing (butyl)(4)N(+), I(3)(-) and 4TBP at concentrations comparable to those of the DSC solar cell did not lead to any reaction. Neither CT complexes nor cationic species like (4TBP)(2)I(+) were observed, judging from the (13)C NMR spectroscopic evidence. This questions the previously proposed formation of (4TBP)(2)I(+) in DSC cells.

Are Reactions Between Metal Cyanides and Aryl Diazonium Ions Really Outer-Sphere Electron Transfer Processes?

Substitution-inert complexes such as Fe(CN)(6)(4-) are usually considered to react by outer-sphere electron transfer (ET) with most electron acceptors, including aryl diazonium ions (ZC(6)H(4)N(2)(+), where Z denotes a substituent on the benzene ring). However, in contrast to the conclusion drawn in a previous report ( J. Am. Chem. Soc. 1987 , 109 , 1536 - 1540 ), kinetic studies and identification of products from the reactions of 4-nitro- and of 4-methoxybenzenediazonium with an excess of Fe(CN)(6)(4-) show that this is not the case and that the reactions actually go via the formation of an adduct, a diazoisocyanide complex [ZC(6)H(4)N(2)(+) + Fe(CN)(6)(4-) --> ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-)]. The adduct decomposes heterolytically by expulsion of nitrogen either to form an isocyanide complex [ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) --> ZC(6)H(4)(NC)Fe(CN)(5)(3-) + N(2)] or the 4-substituted benzonitrile via a ligand exchange [ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) --> ZC(6)H(4)CN + Fe(CN)(5)(3-) + N(2)]. A competing homolytic decomposition resulting in an overall ET reaction occurs only to a minor extent, giving small amounts of Fe(CN)(6)(3-), ZC(6)H(5), and various organic compounds. In oxygenated solutions ZC(6)H(4)N(2)(NC)Fe(CN)(5)(3-) decomposes to Fe(CN)(6)(3-) and ZC(6)H(4)OH. The measurements with Fe(CN)(6)(4-) were supplemented by the study of the analogous reactions of Os(CN)(6)(4-), Mo(CN)(8)(4-), and W(CN)(8)(4-). The observation that isocyanide and even short-lived diazoisocyanide complexes are formed is in accordance with an inner-sphere mechanism. Further support of this conclusion comes from the observation that the slope of the activation-free energy plots for the reactions of NO(2)C(6)H(4)N(2)(+) and MeOC(6)H(4)N(2)(+) with the four metal cyanides is higher than that expected for an outer-sphere ET mechanism. The implication of these results are discussed in the context of the previous report (vide supra) on the extraction of the self-exchange reorganization energies for substituted benzenediazonium salts from their reactions with Fe(CN)(6)(4-) and decamethylferrocene. Our conclusion is that Marcus theory is not applicable in the interpretation of the measured rate constants, thereby also precluding a determination of such energies.