Equivalent Protons Research Articles

Excited-State Dynamics of 1-Piperidino- or 1-Pyrrolidino-anthraquinone for Formation of N−Ylide and Reduced Compounds

By means of subpicosecond laser photolysis of the title compound at room temperature, time-dependent shift of an absorption band has been observed at short delay times which is ascribed to solvent reorganization around the lowest excited singlet state of the solute molecule with an intramolecular charge-transfer character. In comparison with the results obtained by nanosecond laser photolysis and steady-state photolysis at 77 K, it is concluded that the lowest excited singlet state undergoes rapid intramolecular proton transfer forming an N-ylide compound. For formation of the N-ylide compound from the lowest excited singlet state of 1-piperidino-anthraquinone, it is considered that there exists an intermediate which can be ascribed to transfer of the equatorial proton to the carbonyl oxygen anion. At room temperature, no formation of N-ylide compound as a stable compound can be observed due to its rapid reversion to 1-piperidino- or 1-pyrrolidino-anthraquinone. Steady-state photolysis in ethanol at room ...

An ESR study on Jahn–Teller effect in corannulene monoanion

Solution ESR spectra of alkali-metal-doped corannulene monoanion, C 20H 10 −, have been observed from RT to 183 K. Eleven peaks have been observed from RT to 273 K. As the decrease of temperature, the 11 signals due to ten equivalent protons split into complicated ones. This behavior has been interpreted in terms of pseudorotation about the C 5 v Jahn–Teller crossing. The potential barriers between five equivalent C s wells originating from the fifth-order anharmonic term have been estimated to be ca. 2.2×10 −3 eV.

Quantum beats in recombination of spin-correlated radical ion pairs with equivalent protons

Quantum beats caused by the hyperfine coupling (hfc) with magnetically equivalent protons were observed in the recombination fluorescence of the singlet-correlated pairs of secondary radical ions generated in cyclohexane solutions by ionizing radiation. In agreement with theoretical predictions, the ratio I H( t)/ I 0( t) of fluorescence decay kinetics in high and zero magnetic fields showed narrow peaks for the radical cations with an even number of the equivalent protons. To generate radical pairs, p-terphenyl- d 14 was used as an electron acceptor while the solvent holes were trapped by octamethyl-1,4-cyclohexadiene, tetramethylethylene, 9,10-octalin, cis-decalin, hexamethylbenzene, durene, p-diethylbenzene, p-xylene, or benzene. When diisopropylamine was used as a hole acceptor, a trough was observed instead of the strongest peak. The position of the strongest peak (trough) is determined by the value of hfc constant only and is independent of the number of protons.

Heteronuclear editing method for detecting NOE between chemically equivalent protons: application to polycyclic aromatic hydrocarbons

The complete characterization of polycyclic aromatic hydrocarbons (PAHs) and tetrasubstituted ethylenes is critical to an understanding of their reactivity, for which NMR is an important tool. Chemical shifts can provide a direct measure of charge distribution and aromaticity. Unfortunately, COSY, NOESY and heteronuclear correlation cannot provide a complete assignment of the NMR spectra for some carbon-rich PAHs with symmetrical bay regions. The protons in the bay regions would yield NOE signals if they were not symmetrical. Natural substitution of 12C with 13C can be used to break the symmetry and yield these useful NOE signals. Using gradient-assisted editing, unequivocal assignments have been achieved for some previously problematic molecules. Copyright © 2000 John Wiley & Sons, Ltd.

Heteronuclear editing method for detecting NOE between chemically equivalent protons: application to polycyclic aromatic hydrocarbons

Heteronuclear editing method for detecting NOE between chemically equivalent protons: application to polycyclic aromatic hydrocarbons

Structure of dimeric radical cations of benzene and toluene in halocarbon matrices: an EPR, ENDOR and MO study

The structure of dimeric cations of benzene and toluene formed in X-irradiated halocarbon matrices containing relatively high concentration of the solutes was investigated. EPR and ENDOR (electron nuclear double resonance) spectra of these dimer cations were observed and accurate values of the hf coupling constants were obtained. The ENDOR spectrum of the dimeric radical cation of benzene, (C6H6)2+, exhibited hf couplings due to twelve equivalent protons and the isotropic coupling (aiso) was almost one-half of that in the monomer cation. ENDOR transitions with a rhombic symmetry were observed in a CFCl3 matrix, whereas clear axially symmetric transitions with ∣A‖∣<∣A⊥∣ were obtained in CF3CCl3 even at 50 K. The rhombic dipolar coupling tensor was used as a parameter to evaluate the distance between the two partner rings. As regards the toluene dimer cation, (CH3C6H5)2+, ENDOR transitions of the CH3 and H(4,4′) protons were observed. The isotropic hf coupling of the CH3 protons deviated even more strongly from the half value, being rather close to one-third of the value of the monomer. The hf coupling of the H(4,4′) protons was almost half the coupling of the monomer. It was suggested that the anomalous hf couplings of the CH3 protons were due to the interaction between the two rings through the σ-bond. Density functional theory (DFT) calculations were employed to obtain the optimized geometries and hf coupling tensors and suggested sandwich structures in both dimers. Furthermore, the distances between the two rings and the anomalous CH3 protons hf coupling in the (CH3C6H5)2+ were successfully evaluated.

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H 4]L, and N, N′-dimethylated tetrahydrosalen type complexes, Cu[H 2Me 2]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H 2salphen= N, N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H 4]L and Cu[H 2Me 2]L are air-stable at ambient temperature, except for the complex Cu( t Bu, Me)[H 4]salphen [H 2( t Bu, Me)[H 4]salphen= N, N′-bis(2-hydroxy-3- tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu( t Bu, Me)[H 4]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH 2–NH–→–CN–) to form Cu( t Bu, Me)[H 2]salphen and finally, in the presence of base, Cu( t Bu, Me)salphen. X-ray structure analysis of Cu( t Bu, Me)[H 2Me 2]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN 2O 2 coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H 4]L and Cu[H 2Me 2]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, 14N donor nuclei and to a distant interaction with two equivalent protons [∣ A Cu(iso)∣≈253 MHz, ∣ A N(iso)∣≈43 MHz, ∣ A N(iso)∣≈20 MHz]. These protons are attached to the carbon atoms adjacent to the 14N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H 4]L and Cu[H 2Me 2]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [∣ A N(iso)∣≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the CN bond hydrogenation, the coordination polyhedra of the complexes Cu[H 4]L and Cu[H 2Me 2]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu( t Bu, Me)[H 4]salphen and Cu( t Bu, Me)[H 2Me 2]salphen complexes [∣ A Cu(iso)∣≈215 MHz].

Photorearrangement in a Semiquinone to Yield an Unusual 1,3-Carbon/Carbon Interaction and Spin Delocalization

Solutions of 1,4-bis(dihydroquininyl)-anthraquinone {(DHQ)2AQN} in either hexamethylphosphoramide (HMPA) or liquid ammonia undergo a photoinduced electron transfer from the solvent to the (DHQ)2AQN. The resulting anion radical {(DHQ)2AQN•-} yields coupling to the six (3 sets of 2 equivalent) protons on the anthraquinone moiety upon EPR analysis. The same anion radical can be generated via alkali metal reduction in tetrahydrofuran. Further exposure of the HMPA solution to visible light causes the anion radical to undergo a conformational change. The photorearranged product allows delocalization of spin into the γ proton on the sp3 hybridized alkoxy carbon. In NH3(liq) and THF, hydrogen bonding and ion association prevents this photorearrangement, respectively.

Photoinduced Isomerization of trans-Acetylene Radical Anion to Vinylidene Radical Anion in 2-Methyltetrahydrofuran

Photoinduced reactions of trans-HC⋮CH- in a glassy 2-MTHF matrix were studied by ESR and electron absorption (EA) spectroscopy. ESR and EA spectra were interpreted with the aid of the results of molecular orbital (MO) calculations. The trans-HC⋮CH- was generated and stabilized in the 2-MTHF matrix at 77 K by ionizing irradiation. The ESR parameters observed for the radical anion are A1 = 5.3, A2 = 4.6, A3 = 4.5 mT for two equivalent 1H atoms, A⊥ = 1.5, A∥ = 4.2 mT for two equivalent 13C atoms, and g1 = 2.0009, g2 = 2.0024, g3 = 2.0029 for the g tensor. By illuminating with light of λ ≥ 430 nm, the trans-HC⋮CH- was found to be irreversibly isomerized into the vinylidene radical anion, H2CC-, in the matrix. The experimental ESR parameters of H2CC- [A1 = 5.4, A2 = 6.0, A3 = 5.7 mT for two equivalent protons, g1 = 2.0026, g2 = 2.0008, g3 = 2.0026] were fully consistent with the results obtained by semiempirical MO methods, (1) INDO and Analytical Dipolar Calculation method (ANADIP) for isotropic and anisotropic 1H hyperfine (hf) couplings, respectively, and (2) AM1 for the g tensor, for a geometrical structure optimized by ab initio method (Gaussian90/uhf/6-31++G**). The structure of the anions was in agreement with the results obtained from the EA spectroscopic study. The photoinduced isomerization was initiated by the light absorption at λ ≥ 430 nm of trans-HC⋮CH-. The EA band at λmax = 374 nm observed for H2CC- was attributed to the electron transition from the 2B2 ground state and to the 2A1 excited state.

Deuteration effects on the in vivo EPR spectrum of the reduced secondary photosystem I electron acceptor A1 in cyanobacteria.

The photoreduction of the secondary PSI electron acceptor A1 in vivo has recently been detected via X-band EPR spectroscopy in intact spinach chloroplasts and in marine cyanobacteria Synechococcus PCC 7002 [Klughammer, C., and Pace, R. J. (1997) Biochim. Biophys. Acta 1318, 133-144]. A further study of the A1- EPR spectrum of Synechococcus PCC 7002 at room temperature with higher-field resolution revealed partially resolved hyperfine structure which was dominated by 0.4 mT splittings of three equivalent protons. The hyperfine splitting was not significantly affected by incubation of the cyanobacteria in 2H2O medium for 20 h, but was absent in fully deuterated cyanobacteria that were grown in 2H2O medium. Anisotropic g-factors consistent with a phylloquinone radical were derived by spectra simulation. Biosynthetic protonation of quinones via the CH3 donor L-methionine in deuterated cells maintained hyperfine structure in the A1- spectrum, indicating the incorporation of CH3 groups in 60% of the deuterated, photoactive A1 molecules. Conversely, biosynthetic quinone deuteration via L-[methyl-d3]methionine in protonated cells led to the loss of the 0. 4 mT splittings in 54% of the A1 molecules. These observations confirm the conclusion of Heathcote et al. [(1996) Biochemistry 35, 6644-6650] of the identity of EPR-detected, photoreduced A1- in vivo with a phylloquinone (vitamin K1) radical in PSI. The partially resolved hyperfine structure of the A1- spectrum indicates an altered spin distribution in the bound vitamin K1- radical in vivo compared to that of unbound vitamin K1- in vitro.

The radical cation of trimethylamine–boron hydride: An EPR study

Exposure of dilute solutions of trimethylamine–boron hydride (Me3N–BH3) in freon (CFCl3) at 77 K to ionizing radiation gave a radical cation whose EPR spectrum was that expected for the parent radical cation. This resembled those obtained for the BH4 radical previously studied, having a nearly isotropic hyperfine coupling to boron, and a large coupling to two equivalent protons. The possibility that this might be a σ1-radical the SOMO being the N–B σ-bond is dismissed: the favoured structure places the hole on two hydrogens and on boron only.

Synthesis, redox properties, EPR and ENDOR spectroscopy of the 17-electron cycloheptatrienylmolybdenum complexes [MoX(Ph2PCH2CH2PPh2)(η-C7H7)]z+ (z = 1, X = I, Br, Cl, F, NCO, NCS, CN, 13CN, Me, CCPh; z = 2, X = NCMe, CNMe, 13CNMe, oxacyclopentylidene)

The synthesis of a series of new 18-electron cycloheptatrienylmolybdenum complexes [MoX(dppe)(η-C7H7)]z+ (dppe = Ph2PCH2CH2PPh2; z = 0, X = NCO, NCS, CN, 13CN; z = 1, X = CNMe, 13CNMe, oxacyclopentylidene) and [Mo(triphos)(η-C7H7)]+ (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) is reported. Cyclic voltammetric investigations reveal that each of these complexes undergoes a reversible one-electron oxidation to the corresponding 17-electron radicals which have been generated by chemical oxidation, isolated and characterised. Additional examples of 17-electron radicals [MoX(dppe)(η-C7H7)]z+ (z = 1, X = I, Br, Cl, F, Me, CCPh; z = 2, X = NCMe) were either isolated directly or reinvestigated in this work. Fluid solution, X-band EPR studies at 243 K on the 17-electron radicals [MoX(dppe)(η-C7H7)]z+ (z = 1 or 2) afford well resolved spectra characteristic of molybdenum species split by ligand hyperfine interactions with two equivalent phosphorus atoms, seven equivalent protons of the cycloheptatrienyl ring and an additional splitting from the ligand X (where this contains nuclei with non-zero spin). Calculations to determine contributions to the ground state molecular orbital from (i) the ns orbital of the ligand X (based on the EPR work) and (ii) the dppe phosphorus s and p orbitals (based on ENDOR work) suggest that the unpaired electron occupies an essentially metal-based molecular orbital.

First detection of a terrestrial MeV X‐ray burst

We report the first detection of a terrestrial X‐ray burst extending up to MeV energies, made by a liquid‐nitrogen‐cooled germanium detector (∼2 keV FWHM resolution) on a high‐altitude balloon at 65.5° magnetic latitude (L=5.7) in the late afternoon (1815 MLT) during low geomagnetic activity. The burst occurred at 1532–1554 UT on August 20, 1996, and consisted of seven peaks of ∼60–90 s duration, spaced by ∼100–200 s, with quasi‐periodic (∼10–20 s) modulation of the peak count rates. The very hard X‐ray spectrum extends to the instrumental limit of 1.4 MeV, and is consistent with bremsstrahlung emission from monoenergetic, ∼1.7 MeV, precipitating electrons. Since the trapped relativistic electrons showed a steeply falling energy spectrum from 0.6 to 4 MeV (at L=6.6), the precipitation mechanism appears to be highly energy selective. The modulation frequencies suggest scattering of the MeV electrons due to gyro‐resonance with Doppler‐shifted electromagnetic ion cyclotron waves, but either equatorial proton densities a factor of ∼10² higher than typical for the plasmasphere or significant O+ densities would be required.

Dehydroabietic acid, a major effluent component of paper and pulp industry, decreases erythrocyte pH in lamprey ( Lampetra fluviatilis)

Resin acids, forming a major component of wood industry, cause numerous toxic effects on liver and red blood cells of fish. Effect of dehydroabietic acid (DHAA) on erythrocyte pH of river lamprey was investigated using [ 14C]DMO distribution to determine pH i. Exposure of the cells to more than 200 μM DHAA caused hemolysis within 10 min of incubation. Treatment with 50–200 μM DHAA resulted in a rapid dose-dependent decrease in erythrocyte pH. DHAA-induced reduction of pH i was dependent on pH of the extracellular medium. Application of 100 μM DHAA caused a 0.13 units drop of intracellular pH at pH o 8.0, 0.20 units at pH o 7.6, and 0.35 units at pH o 6.5. Cell pH was a saturable function of concentration of the uncharged form of DHAA (different concentrations of the uncharged form reached by varying of DHAA (total) concentrations and extracellular pH) with a rapid decline in pHi from 8.0 to 7.2 at 3–4 μM concentration of the resin acid followed by modest pH i change at concentrations exceeding 10 μM. DHAA-induced acidification was reversible with pH i returning to control values in 15 min after removal of DHAA o. The DHAA effect on cell pH was not due to inhibition of Na +/H + exchanger. Two alternative mechanisms of DHAA action suggested are: alteration of membrane permeability to proton equivalents due to incorporation of DHAA into the lipid bilayer or carrier-like mode of penetration of the uncharged form of DHAA into the cell with the following dissociation.

Acid secretion, calcification, and photosynthetic carbon concentrating mechanisms

Bicarbonate-based photosynthesis requires equal numbers of proton equivalents. Aquatic plants and photosynthetic symbioses can scavenge protons from ambient solutions, or they can manufacture protons through calcification. Both mechanisms are widely employed. This review examines how aquatic photoautotrophs couple calcification to photosynthesis, and the advantages and limitations of this physiology.Key words: calcification, calcium, carbonate, bicarbonate, photosynthesis, aquatic.

Inframolecular acid–base properties of myo-inositol 1,2,6-trisphosphate analogues: influence of the hydroxyl groups, phosphate configuration and intracyclic atom substitution

Abstract The microscopic acid–base properties of analogues of d - myo -inositol 1,2,6-trisphosphate 1 are reported. These analogues differ by the hydroxyl configuration ( l - chiro -inositol 1,2,3-trisphosphate 2 ) or the replacement of the inositol ring by a ring containing an atom of oxygen (1,5-anhydro- d -arabinitol 2,3,4-trisphosphate 3 and 1,5-anhydroxylitol 2,3,4-trisphosphate 4 ) or an atom of nitrogen (1,5-anhydro-1,5-imino- d -arabinitol 2,3,4-trisphosphate 5 ). In addition, the discussion includes the analogue of 1 where all three hydroxyls were removed ( 6 ). The studies, performed by potentiometric and 31 P-NMR titrations in a 0.2 M KCl medium at 37 °C, allowed the resolution without approximation of the microprotonation scheme of the trifunctional molecules. The constants as well as the chemical shifts of the phosphorus nuclei that characterize each individual phosphate group are analyzed in terms of hydration and hydrogen bonding. Thus, for 2 , the loss of a hydrogen bonding possibility between P-1 and OH-6 leads to an increase of the P-1 basicity with regard to 1 . Also, by comparing 3 and 4 it appears that the lower basicity of P-2 for 4 with regard to 3 may be attributed to a more favourable interaction between P-1 and P-2 in 3 when the two possible chair forms are allowed. Moreover, for 5 , the addition of the first equivalent of protons will protonate only about 75% of the amine, thus indicating that for polyfunctional molecules a given macroscopic protonation constant can hardly be attributed to a specific protonation site.

On the unexpected isotope effect on the exchange coupling constant in partially deuterated transition‐metal trihydride complexes

AbstractIn this paper we attempt to explain the unusual isotope effect displayed by the 1H‐NMR scalar H‐H coupling of the hydrides in complexes [CpIrL(H)3]+ (L=various phosphine and phosphite ligands) when one of the equivalent protons is turned into a deuteron (D.M. Heinekey, J.M. Millar, T.F. Koetzle, N.G. Payne, and K.W. Zilm, J. Am. Chem. Soc. 112, 909 (1990)]. It is found that in the case of the perprotio species the system can be described as a three‐coupled well system, but when a deuterium is introduced the system is better described as a double‐coupled well. Finally, it is shown that the coupling in the case of a double‐well system is greater than in an equivalent triple‐well system, in this way giving a plausible explanation to the experimental facts reported.

Distributed torsion angle grid search in high dimensions: a systematic approach to NMR structure determination.

Two complementary approaches for systematic search in torsion angle space are described for the generation of all conformations of polypeptides which satisfy experimental NMR restraints, hard-sphere van der Waals radii, and rigid covalent geometry. The first procedure is based on a recursive, tree search algorithm for the examination of linear chains of torsion angles, and uses a novel treatment to propagate the search results to neighboring regions so that the structural consequences of the restraints are fully realized. The second procedure is based on a binary combination of torsion vector spaces for connected submolecules, and produces intermediate results in Cartesian space for a more robust restraint analysis. Restraints for NMR applications include bounds on torsion angles and internuclear distances, including relational and degenerate restraints involving equivalent and nonstereoassigned protons. To illustrate these methods, conformation search results are given for the tetrapeptide APGA restrained to an idealized beta-turn conformation, an alanine octapeptide restrained to a right-handled helical conformation, and the structured region of the peptide SYPFDV.

Structure and dynamics of radicalized hydrocarbon derivatives included in perhydrotriphenylene single crystals

The radical obtained by γ-irradiation of the nonadecan-10-one/perhydrotriphenylene (10-NDO/PHTP) inclusion compound has been studied by EPR spectroscopy. The radical is formed by removal of one of the protons in the α position to the carbonyl group and the unpaired electron spin is coupled to the α proton and to two equivalent β protons. The hyperfine (hf) coupling constants vary slowly on lowering the temperature until an abrupt change occurs at T = 150 K. The radical reorients quickly on the EPR timescale inside the PHTP channels. The reorientational correlation time τr is obtained from the linewidth values and it is found to be shorter than the τr value measured previously for the same radical included in urea. The EPR spectrum changes in the timescale of weeks and after two months displays the presence of an additional hf coupling with a pair of equivalent protons, the other spectral parameters being unchanged with respect to those of the freshly irradiated radical. This modification is probably due to a folding of the radicalized guest molecule inside the host channel.

Electron Addition to Trimethylphosphine Sulfide

Exposure of dilute solutions of trimethylphosphine sulfide (Me3PS) in methanol (CD3OD)−water glasses to ionizing radiation at 77 K gave the parent radical anion Me3PS-, identified by its EPR spectrum. This is shown to have a trigonal bipyramidal structure characteristic of phosphoranyl radicals. Similar features were obtained for the pure compound at 77 K, with resolved proton splitting on the parallel features from one CH3 group. Doublet features assigned to the parent cations, Me3P−Ṡ+ were observed for solutions in freon (CFCl3) and also in the pure compound. The latter also gave features assigned to H2ĊP(Me2)S and/or H2ĊP(Me)2SH+ radicals. On annealing to ca. 140 K, doublet features with a considerably reduced 31P hyperfine splitting, assigned to the phosphoryl radical Me2ṖS, were revealed for the pure compound. At 77 K, these features were further split into 1:2:1 triplets, apparently from only two equivalent protons. However, at ca. 140 K, septets from all six protons were obtained with almost the same separation between the outmost features. Also, the original 31P parallel and perpendicular features gave way to features that had converged, although some anisotropy was still seen. At still higher temperatures, an isotropic doublet of septets was observed, prior to loss of the features. This is assigned to freely rotating Me2ṖS radicals. Possible mechanisms for the formation of these Me2ṖS radicals are discussed. The only reasonable one that we can discover is electron return to give electronically excited (Me3PS)* molecules, which dissociate to give ĊH3 and Me2ṖS radicals. The former radicals (ĊH3) were not detected, but they may well react to give methane and the H2ĊP(Me)2S radical. The reversible change from 3 to 7 lines from the two methyl groups in Me2ṖS radicals is discussed in terms of hindered rotation for the two methyl groups.