A combined zero electronic kinetic energy spectroscopy and ion-pair dissociation imaging study of the F2 + (X 2Pi g) structure

Rotationally resolved pulsed field ionization and zero electronic kinetic energy photoelectron spectra for the transition F(2) (+)(X (2)Pi(g))<--F(2)(X (1)Sigma(g) (+)) have been recorded using the extreme ultraviolet coherence radiation. The vibrational energy spacings, rotational constants, and spin orbit coupling constants for the first three vibrational states of F(2) (+)(X (2)Pi(g)) have been determined accurately. The first adiabatic ionization potential (IP) of F(2) is determined as IP(F(2))=126 585.7+/-0.5 cm(-1). To determine the threshold E(tipp) for ion-pair production of F(2), the images of F(-)((1)S(0)) in the velocity mapping conditions have also been recorded at the photon energy of 126 751 cm(-1). Taking the Stark effect into account, the E(tipp) is determined as E(tipp)(F(2))=126 045+/-8 cm(-1) (15.628+/-0.001 eV). By combing the IP(F(2)) and the E(tipp)(F(2)) determined in this work and together with the reported ionization potential and electronic affinity of the F atom, the bond dissociation energies of F(2) and F(2) (+) are determined as D(0)(F(2))=1.606+/-0.001 eV and D(0)(F(2) (+))=3.334+/-0.001 eV, respectively.     

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.     

Dihydrophenanthrene-type intermediates during photoreaction of trans-4'-benzyl-5-styrylfuran

[structure: see text] Photoreaction of trans-4'-benzyl-5-styrylfuran (trans-BSF) has been studied by the 355-nm laser flash photolysis (LFP) in CH2Cl2 using a Nd3+:YAG laser (30 ps, 5 mJ pulse(-1) or 5 ns, 30 mJ pulse(-1)). Transient fluorescence and absorption spectra assigned to the singlet excited trans-BSF were observed during the 30-ps LFP, whereas a transient absorption spectrum with two peaks at 400 and 510 nm, assigned to the trans-fused dihydrophenanthrene (DHP)-type intermediate (DP1), was observed during the 5-ns LFP. It is clearly suggested that a two-photon absorption process is involved in the formation of DP1. The first photoreaction is the photoisomerization of trans-BSF, which occurs to give cis-BSF. The second photoreaction process is photocyclization of cis-BSF, which occurs to give DP1 decaying with the half lifetime (tau1/2) of 2.8-4.0 micros to produce another DHP-type intermediate (DP2) with an absorption peak at 400 nm in the absence of O2, through [1,9]-hydrogen shift. DP2 decayed with tau1/2 > 500 micros to give the product through aromatization. In O2-saturated CH2Cl2, DP1 decayed with tau1/2 = 250 ns to give a radical intermediate (X) with two peaks at 410 and 510 nm, through hydrogen abstraction of DP1 by O2. X decayed with tau1/2 = 150 micros to give the product through successive hydrogen abstraction.     

The lowest excited triplet (T1) energies of p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors estimated from the temperature dependence of the T2(n,pi*) phosphorescence and the S1(n,pi*) fluorescence spectra

Invisible energy levels of the T1(pi, pi*) state of p-methoxybenzaldehyde (anisaldehyde) and p-cyanobenzaldehyde vapors have been estimated through the temperature dependence of the T2(n, pi*) --> S0 phosphorescence and the S1(n, pi*) --> S0 delayed fluorescence spectra. It is shown that the T1(pi, pi*) levels are located at 900 +/- 100 and 300 +/- 100 cm(-1) below the T2(n, pi*) levels, respectively, for p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors. The estimated T1 energy levels are in good agreement with the phosphorescence origins in rigid glass at 77 K.     

Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands

Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) mol(-1), respectively. For [Ni(II)(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, DeltaH(double dagger) = 22 +/- 4 kJ mol(-1), and DeltaS(double dagger) = -59 +/- 5 J K(-1) mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.     

Inner-valence photoionization of O((1)D): experimental evidence for the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition

Photoionization and autoionization of electronically excited atomic oxygen O((1)D) are investigated in the energy range between 12 and 26 eV using tunable laser-produced plasma radiation in combination with time-of-flight mass spectrometry. A broad, asymmetric, and intense feature is observed that is peaking at 20.53+/-0.05 eV. It is assigned to the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition, which subsequently autoionizes by a Coster-Kronig transition, as predicted by the previous theoretical work [K. L. Bell et al., J. Phys. B 22, 3197 (1989)]. Specifically, the energy of the unperturbed transition occurs at 20.35+/-0.07 eV. Its shape is described by a Fano profile revealing a q parameter of 4.25+/-0.8 and a width of gamma=2.2+/-0.15 eV. Absolute photoionization cross section sigma is derived, yielding sigma=22.5+/-2.3 Mb at the maximum of the resonance. In addition, weak contributions to the O((1)D) yield from dissociative ionization originating from molecular singlet oxygen [O(2)((1)Delta(g))] are identified as well. Possible applications of the 2s(2)2p(4)((1)D)-->2s(1)2p(5)((1)P) transition as a state-selective and sensitive probe of excited oxygen in combination with photoionization mass spectrometry are briefly discussed.     

Self-assembly of luminescent Sn(IV)/Cu/S clusters using metal thiolates as metalloligands

Through the use of (Bu4N)2[Sn3S4(edt)3] (edt=SCH2CH2S(2-)) and Sn(SPh)4 as metalloligands, three neutral compounds have been obtained: [(Ph3P) 2Cu] 2SnS(edt)(2).2CH2Cl2.H2O (1a), [(Ph3P) 2Cu]2SnS(edt)2.2DMF.H2O (1b), and [(Ph3P)Cu] 2Sn(SPh)(6).3H 2O (2). Single-crystal X-ray diffraction studies revealed that compounds 1a and 1b contain the same neutral butterfly-like [(Ph3P)2Cu]2SnS(edt)2 cluster, which consists of one central SnS 5 dreich trigonal bipyramid sharing one vertex and two sides with two slightly distorted CuS 2P2 tetrahedrons. Compound 2 has a linear [(Ph3P)Cu]2Sn(SPh)6 cluster that is composed of a central distorted SnS 6 octahedron sharing two opposite planes with two slightly distorted CuS 3P tetrahedrons. Compound 1a exhibited an emission at 568 nm (tau=12.86 micros) in the solid state, while in CH 2Cl 2 solution, 1a exhibited a green emission at 534 nm (tau=4.75 micros). Compound 2 showed an intense red emission at 696 nm (tau=3.64 micros) upon excitation at 307 nm in the solid state.     

Photodissociation of methane: exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.     

Interpretation of light-scattering spectra in terms of particle displacements

Quasielastic light-scattering spectroscopy is regularly used to examine the dynamics of dilute solutions of diffusing mesoscopic probe particles in fluids. For probes in a simple liquid, the light-scattering spectrum is a simple exponential; the field correlation function g(1)(q,tau) of the scattering particles is related to their mean-square displacements X2 identical with [(delta x(tau))2] during tau via g(1)(q,tau) = exp(-1/2 q2X2). However, demonstrations of this expression refer only to identical Brownian particles in simple liquids and show that if the form is correct then it is also true for all tau that g(1)(q,tau) = exp(-gamma tau), a pure exponential in tau. In general, g(1)(q,tau) is not a single exponential in time. A correct general form for g(1)(q,tau) in terms of the X(2n), replacing the incorrect exp(-1/2 q2X2), is obtained. A simple experimental diagnostic determining when the field correlation function gives the mean-square displacement is identified, namely, g(1)(q,tau) only reveals X2 if g(1)(q,tau) is a single exponential in tau. Contrariwise, if g(1)(q,tau) is not a single exponential, then g(1)(q,tau) depends not only on X2 but on all higher moments X(2n). Corrections to the crude approximation g(1)(q,tau) = exp(-1/2 q2X2) closely resemble the higher spectral cumulants from a cumulant expansion of g(1)(q,tau).     

The triplet-state lifetime of indole derivatives in aqueous solution

An important feature of tryptophan phosphorescence, crucial for probing protein structure and dynamics, is the drastic reduction of the lifetime (tau) in fluid solutions. Initial reports of indole and derivatives showed that tau decreases from 6 s in rigid glasses to about 1 ms in aqueous solutions at ambient temperature. Recently a report by Fischer et al. questioned the validity of the millisecond lifetime, claiming that in millimolar electrolyte solutions tau is about 40 micros, similar to the 12-30 micros of earlier determinations based on flash photolysis. Longer lived phosphorescence was detected in pure water but because it exhibited an initial growing phase and an anomalously large triplet yield, the emission was attributed to an artifact arising from the slow, first-order, geminate recombination of the radical cation and electron generated by photochemistry. In this study, we reexamine both the phosphorescence lifetime and the triplet quantum yield of indole, N-acetyl tryptophanamide (NATA), N-methyl tryptophan and the tryptophan-glycine-glycine tripeptide under the same conditions adopted by Fischer et al. as well as over a wider range of electrolyte and buffering salts concentrations, pH, solvent and temperature. Throughout, the results show that the phosphorescence decay is slow and uniform down to the 12 micros resolution of the instrument, with no evidence of short-lived, 40 micros-like components. Most compelling was the similarity between the fluorescence-normalized triplet yield of indole derivatives in water and that of W59 in the protein ribonuclease T1 or of NATA in rigid glasses. Its invariance over experimental conditions that varied the production of photoproducts several fold and the characteristic susceptibility of the triplet lifetime to O2, proton and ground state quenching demonstrated that the triplet state was formed predominantly through normal intersystem crossing and that its unquenched lifetime was at least 9 ms.     

Molecular probe for a fluorous medium: long-lived phosphorescence of alpha-diketones in perfluoromethylcyclohexane at room temperature.

We found that alpha-diketones (2,3-butanedione (BD) and 1-phenyl-1,2-propanedione (PPD)) were very suitable luminescence probes for studying the properties of a perfluorinated solvent (i.e., fluorous solvent; perfluoromethylcyclohexane (PFMC)), since these compounds were soluble in PFMC and showed long-lived phosphorescence even at room temperature. The phosphorescence lifetime (tau(p)) of BD in PFMC (650 micros) was much longer than that in cyclohexane (CH, 270 micros). The longer tau(p) value of BD in PFMC was ascribed to the variation of the intersystem crossing rate constant (k(isc)) from the excited triplet state (T1) to the ground state (S0) with the solvent. Some possible reasons for the change in k(isc) were discussed in terms of solute-solvent interactions. Furthermore, by utilizing phosphorescence quenching of BD by pyrene, we, determined a rate constant of the diffusion-controlled reaction in PFMC. Characteristic behaviors of mixing/separation processes between PFMC and a common organic solvent observed by Schlieren photographs were also reported.     

Velocity map imaging study of the O2 ion-pair production at 17.499 eV: simultaneous parallel and perpendicular transitions

The fine structure resolved photofragment O(-)((2)P(j)) image from the O(2) ion-pair production at 17.499 eV has been recorded. The branching ratio for producing the low energy spin-orbit O(-)((2)P(3/2)) component to the high energy spin-orbit O(-)((2)P(1/2)) component is 1:0.78 and the optical transitions for them correspond to perpendicular and parallel transitions, respectively. The anisotropy parameters, 1.64 for channel producing O(-)((2)P(1/2)) and -0.35 for O(-)((2)P(3/2)), suggest that the dissociation proceeds via the states with symmetry (3)Sigma(u)(-) and (3)Pi(u), respectively. Although the main mechanisms for the O(2) ion-pair production are the predissociation via the intermediate Rydberg states, the direct dissociation mechanism for the channel producing O(-)((2)P(1/2)) may also be involved.     

Theoretical studies of the photochemical dynamics of acetylacetone: isomerzation, dissociation, and dehydration reactions

The potential energy surfaces of the C-O cleavage, rotational isomerization, keto-enolic tautomerization, and dehydration reactions of acetylacetone in the lowest triplet and ground states have been determined using the complete active space self-consistent field and density functional theory methods. The main photochemical mechanism obtained indicates that the acetylacetone molecule in the S(2)((1)pipi*) state can relax to the T(1)((3)pipi*) state via the S(2)-S(1) vibronic interaction and an S(1)/T(1)/T(2) intersection. The C-O fission pathway is the predominant dissociation process in the T(1)((3)pipi) state. Rotational isomerization reactions proceed difficultly in the ground state but very easily in the T(1)((3)pipi*) state. Keto-enolic tautomerization takes place with little probability for acetylacetone in the gas phase.     

Gadolinium (III) ion in liquid water: structure, dynamics, and magnetic interactions from first principles

We applied first principles molecular dynamics (MD) technique to study structure, dynamics, and magnetic interactions of the Gd(3+) aqua ion dissolved in liquid water, a prototypical system for Gd-based complexes used as contrast agents for magnetic resonance imaging. The first coordination sphere contains eight water molecules with an average Gd-O distance of 2.37 A and an average geometric arrangement close to a square antiprism. The mean tilt angle of the electric dipole vector of these water molecules is theta=145 degrees . In our picosecond time scale simulation we observe no exchange event from the first coordination sphere but only fast "wagging" motions. The second coordination sphere is well pronounced though water molecules in this sphere are subjected to large amplitude dynamic motions. The isotropic hyperfine coupling constants for the inner sphere water molecules [A(iso)((17)O(I))=0.65+/-0.03 MHz, A(iso)((1)H(I))=0.085+/-0.005 MHz] are in good agreement with experimental data and with an earlier study using classical MD. Second sphere Fermi contact hyperfine coupling constants calculated are more than one order of magnitude smaller and of opposite sign as those of the first coordination sphere. The effect of spin polarization induced by the paramagnetic Gd(3+) ion on the dipolar hyperfine interaction was found to be sizable only for the (17)O nuclei of inner sphere water molecules and has a screening character.     

Oxidation of di- and tripeptides of tyrosine and valine mediated by singlet molecular oxygen, phosphate radicals and sulfate radicals.

Kinetics and mechanism of the oxidation of tyrosine (Tyr) and valine (Val) di- and tripeptides (Tyr-Val, Val-Tyr and Val-Tyr-Val) mediated by singlet molecular oxygen [O(2)((1)Delta(g))], phosphate (HPO(4)(*-) and PO(4)(*2-)) and sulfate (SO(4)(*-)) radicals was studied, employing time-resolved O(2)((1)Delta(g)) phosphorescence detection, polarographic determination of dissolved oxygen and flash photolysis. All the substrates were highly photooxidizable through a O(2)((1)Delta(g))-mediated mechanism. Calculated quotients between the overall and reactive rate constants for the quenching of O(2)((1)Delta(g)) by Tyr-derivatives (k(t)/k(r) values, accounting for the efficiency of the effective photooxidation) were 1.3 for Tyr, 1 for Tyr-Val, 2.8 for Val-Tyr and 1.5 for Val-Tyr-Val. The effect of pH on the kinetics of the photooxidative process confirms that the presence of the dissociated phenolate group of Tyr clearly dominates the O(2)((1)Delta(g)) quenching process. Products analysis by LC-MS indicates that the photooxidation of Tyr di- and tripeptides proceeds with the breakage of peptide bonds. The information obtained from the evolution of primary amino groups upon photosensitized irradiation is in concordance with these results. Absolute rate constants for the reactions of phosphate radicals (HPO(4)(*-) and PO(4)(*2-), generated by photolysis of the P(2)O(8)(4-) at different pH) and sulfate radicals (SO(4)(*-), produced by photolysis of the S(2)O(8)(2-)) with Tyr peptides indicate that for all the substrates, the observed tendency in the rate constants is: SO(4)(*-) > or = HPO(4)(*-) > or = PO(4)(*2-). Formation of the phenoxyl radical of tyrosine was detected as an intermediate involved in the oxidation of tyrosine by HPO(4)(*-).     

1H and (17)O NMR detection of a lanthanide-bound water molecule at ambient temperatures in pure water as solvent

Lanthanide complexes of a tetra-amide derivative of DOTA (structure 4 in text) with four extended carboxymethyl esters have been characterized by X-ray crystallography and multinuclear NMR spectroscopy. [Eu(4)(H(2)O)](triflate)(3) crystallized from water in the monoclinic, P(21/)(c) space group (a = 10.366 A, b = 22.504 A, c = 23.975 A, and beta = 97.05 degrees ). The Eu(3+) cation is bound to four macrocyclic nitrogen atoms (mean Eu-N = 2.627 A) and four amide oxygen atoms (mean Eu-O(amide) = 2.335 A) in a square antiprismatic geometry with a twist angle of 38.5 degrees between the N4 and O4 planes. A single bound water molecule (Eu-O(W) = 2.414 A) occupies a typical monocapped position on the O4 surface. In pure water, resonances corresponding to a single Eu(3+)-bound water molecule were observed in the (1)H (53 ppm) and (17)O (-897 ppm) NMR spectra of [Eu(4)(H(2)O)](triflate)(3) at 25 degrees C. A fit of the temperature-dependent Eu(3+)-bound (1)H and (17)O water resonance line widths in acetonitrile-d(3) (containing 4% v/v (17)O enriched water) gave identical lifetimes (tau(m)(298)) of 789 +/- 50 micros (in water as solvent; a line shape analysis of the Eu(3+)-bound water resonance gave a tau(m)(298) = 382 +/- 5 micros). Slow water exchange was also evidenced by the water proton relaxivity of Gd(4) (R(1) = 2.2 mM(-1) s(-1), a value characteristic of pure outer-sphere relaxation at 25 degrees C). With increasing temperature, the inner-sphere contribution gradually increased due to accelerated chemical exchange between bound water and bulk water protons. A fitting of the relaxation data (T(1)) to standard SBM theory gave a water proton lifetime (tau(m)(298)) of 159 micros, somewhat shorter than the value determined by high-resolution (1)H and (17)O NMR of Eu(4). Exchange of the bound water protons in Gd(4) with bulk water protons was catalyzed by addition of exogenous phosphate at 25 degrees C (R(1) increased to 10.0 mM(-1) s(-1) in the presence of 1500-fold excess HPO(4)(2-)).     

Intramolecular energy transfer involving heisenberg spin-coupled dinuclear iron-oxo complexes

The synthesis, structure, and physical properties of a series of oxo-bridged dinuclear Fe(III) complexes containing pendant naphthalene groups are described. The compounds [Fe(2)O(O(2)CCH(2)-C(10)H(7))(tren)(2)](BPh(4))(NO(3))(2) (8), [Fe(2)O(O(2)CCH(2)-C(10)H(7))(TPA)(2)](ClO(4))(3) (9), Fe(2)O(O(2)CCH(2)-C(10)H(7))(2)(Tp)(2) (10), and Fe(2)O((O(2)CCH(2)CH(2))(2)-C(10)H(6))(Tp)(2) (11) (where tren is tris(2-aminoethyl)amine, TPA is tris(2-pyridyl)amine, and Tp is hydrotrispyrazolylborate) have been characterized in terms of their structural, spectroscopic, magnetic, and photophysical properties. All four complexes exhibit moderately strong intramolecular antiferromagnetic exchange between the high-spin ferric ions (ca. -130 cm(-)(1) for H = -2JS(1).S(2)). Room-temperature steady-state emission spectra for compounds 8-11 in deoxygenated CH(3)CN solution reveal spectral profiles similar to methyl-2-naphthyl acetate and [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) (13, where TACN-Me(3) is N,N,N-1,4,7-trimethyltriazacyclononane) but are significantly weaker in intensity relative to these latter two compounds. Time-resolved emission data for the iron complexes following excitation at 280 nm can be fit to simple exponential decay models with tau(obs)(S)()1 = 36 +/- 2, 32 +/- 4, 30 +/- 5, and 39 +/- 3 ns for compounds 8-11, respectively. The decays are assigned to the S(1) --> S(0) fluorescence of naphthalene; all of the lifetimes are less than that of the zinc model complex (tau(obs)(S)()1 = 45 +/- 2 ns), indicating quenching of the S(1) state by the iron-oxo core. Nanosecond time-resolved absorption data on [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4)) reveal a feature at lambda(max) = 420 nm that can be assigned as the T(1) --> T(n) absorption of the naphthalene triplet; the rise time of 50 +/- 10 ns corresponds to an intersystem crossing rate of 2 x 10(7) s(-1). A similar feature (though much weaker in intensity) is also observed for compound 8. The order-of-magnitude reduction in the T(1) lifetime of the pendant naphthalene for all of the iron-oxo complexes (tau(obs)(T)1 = 5 +/- 2 micros vs 90 +/- 10 micros for [Zn(2)(OH)(O(2)CCH(2)-C(10)H(7))(2)(TACN-Me(3))(2)](ClO(4))) indicates quenching of the naphthalene triplet with an efficiency of >90%. Neither the naphthalene radical cation nor the reduced Fe(II)Fe(III) species were observed by transient absorption spectroscopy, implying that energy transfer is the most likely origin for the quenching of both the S(1) and T(1) states. Spectral overlap considerations strongly support a F?rster (i.e., dipolar) mechanism for energy transfer from the S(1) state, whereas the lack of phosphorescence from either the free naphthyl ester or the Zn model complex suggests Dexter transfer to the diiron(III) core as the principal mechanism of triplet quenching. The notion of whether spin exchange within the diiron(III) core is in part responsible for the unusual ability of the iron-oxo core to engage in energy transfer from both the singlet and triplet manifolds of naphthalene is discussed.     

Kinetics of liposome adhesion on a mercury electrode

The adhesion of liposomes on a mercury electrode leads to capacitive signals due to the formation of islands of lecithin monolayers. Integration of the current-time transients gives charge-time transients that can be fitted by the empirical equation Q(t) = Q(0) + Q(1)(1 - exp(-t/tau(1))) + Q(2)(1 - exp(-t/tau(2))), where the first term on the right side is caused by the docking of the liposome on the mercury surface, the second term is caused by the opening of the liposome, and the third term is caused by the spreading of the lecithin island on the mercury surface. The temperature dependence of the two time constants tau(1) and tau(2) and the temperature dependence of the overall adhesion rate allow determination of the activation energies of the opening, the spreading, and the overall adhesion process both for gel-phase 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and for liquid-crystalline-phase DMPC liposomes. In all cases, the spreading is the rate-determining process. Negative apparent activation energies for the spreading and overall adhesion process of liquid-crystalline-phase DMPC liposomes can be explained by taking into account the weak adsorption equilibria of the intact liposomes and the opened but not yet spread liposomes. A formal kinetic analysis of the reaction scheme supports the empirical equation used for fitting the charge-time transients. The developed kinetic model of liposome adhesion on mercury is similar to kinetic models published earlier to describe the fusion of liposomes. The new approach can be used to probe the stability of liposome membranes.     

Binding of polyanions by biogenic amines. III. Formation and stability of protonated spermidine and spermine complexes with carboxylic ligands

The formation and stability of protonated spermidine and spermine-carboxylic ligand complexes (seven systems) were studied potentiometrically (H(+)-glass electrode). ALH(r) species are formed (A=spermidine, spermine, L(z-)=acetate, malonate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate; r=1...m+n-1, where m and n are the maximum degree of protonation of the amine and of the carboxylic ligand, respectively), and their stability is a function of charges involved in the formation reaction. For the equilibrium H(i)A(i+)+H(j)L((j-z))=ALH(i+j)((i+j-z)) the linear relationship logK(ij)=0.46 zeta (zeta=mid R:i(z-j)mid R:) was found. By means of this simple equation a mean free energy value can be obtained per salt bridge (n), namely -DeltaG degrees =5.25+/-0.15 kJ mol(-1)n(-1). Species formed by the highest charged ligands are quite stable (K>10(3) mol(-1) dm(3)) and potentially play an important role in the speciation of biofluids, as shown by speciation diagrams and simulated experiments.     

Oxidations of NADH analogues by cis-[RuIV(bpy)2(py)(O)]2+ occur by hydrogen-atom transfer rather than by hydride transfer

Oxidations of the NADH analogues 10-methyl-9,10-dihydroacridine (AcrH2) and N-benzyl 1,4-dihydronicotinamide (BNAH) by cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+) have been studied to probe the preferences for hydrogen-atom transfer vs hydride transfer mechanisms for the C-H bond oxidation. 1H NMR spectra of completed reactions of AcrH2 and RuIVO2+, after more than approximately 20 min, reveal the predominant products to be 10-methylacridone (AcrO) and cis-[RuII(bpy)2(py)(MeCN)]2+. Over the first few seconds of the reaction, however, as monitored by stopped-flow optical spectroscopy, the 10-methylacridinium cation (AcrH+) is observed. AcrH+ is the product of net hydride removal from AcrH2, but hydride transfer cannot be the dominant pathway because AcrH+ is formed in only 40-50% yield and its subsequent oxidation to AcrO is relatively slow. Kinetic studies show that the reaction is first order in both RuIVO2+ and AcrH2, with k = (5.7 +/- 0.3) x 10(3) M(-1) s(-1) at 25 degrees C, DeltaH(double dagger) = 5.3 +/- 0.3 kcal mol(-1) and DeltaS(double dagger) = -23 +/- 1 cal mol(-1) K(-1). A large kinetic isotope effect is observed, kAcrH2/kAcrD2 = 12 +/- 1. The kinetics of this reaction are significantly affected by O2. The rate constants for the oxidations of AcrH2 and BNAH correlate well with those for a series of hydrocarbon C-H bond oxidations by RuIVO2+. The data indicate a mechanism of initial hydrogen-atom abstraction. The acridinyl radical, AcrH*, then rapidly reacts by electron transfer (to give AcrH+) or by C-O bond formation (leading to AcrO). Thermochemical analyses show that H* and H- transfer from AcrH2 to RuIVO2+ are comparably exoergic: DeltaG degrees = -10 +/- 2 kcal mol(-1) (H*) and -6 +/- 5 kcal mol(-1) (H-). That a hydrogen-atom transfer is preferred kinetically suggests that this mechanism has an equal or lower intrinsic barrier than a hydride transfer pathway.