Mössbauer study of the three-coordinate planar Fe(II) thiolate complex [Fe(SR)(3)](-) (R = C(6)H(2)-2,4,6-tBu(3)): model for the trigonal iron sites of the MoFe(7)S(9):homocitrate cofactor of nitrogenase

The cofactor (M-center) of the MoFe protein of nitrogenase, a MoFe(7)S(9):homocitrate cluster, contains six Fe sites with a (distorted) trigonal sulfido coordination. These sites exhibit unusually small quadrupole splittings, Delta E(Q) approximately 0.7 mm/s, and isomer shifts, delta approximately 0.41 mm/s. M?ssbauer and ENDOR studies have provided the magnetic hyperfine tensors of all iron sites in the S = 3/2 state M(N). To assess the intrinsic zero-field splittings and hyperfine parameters of the cofactor sites, we have studied with M?ssbauer spectroscopy two salts of the three-coordinated Fe(II) thiolate complex [Fe(SR)(3)](-) (R = C(6)H(2)-2,4,6-tBu(3)). One of the salts, [Ph(4)P][Fe(SR)(3)] x 2MeCN x C(7)H(8), 1, has a planar geometry with idealized C(3h) symmetry. This S = 2 complex has an axial zero-field splitting with D = +10.2 cm(-1). The magnetic hyperfine tensor components A(x) = A(y) = -7.5 MHz and A(z) = -29.5 MHz reflect an orbital ground state with d(z(2)) symmetry. A(iso) = (A(x) +A(y) +A(z))/3 = -14.9 MHz, which includes the contact interaction (kappa P = -21.9 MHz) and an orbital contribution (+7 MHz), which is substantially smaller than A(iso) approximately -22 MHz of the tetrahedral Fe(II)(S-R)(4) sites of both rubredoxin and [PPh(4)](2)[Fe(II)(SPh)(4)]. The largest component of the electric field gradient (EFG) tensor is negative, as expected for a d(z(2)) orbital. However, Delta E(Q) = -0.83 mm/s, which is smaller than expected for a high-spin ferrous site. This reduction can be attributed to a ligand contribution, which in planar complexes provides a large positive EFG component perpendicular to the ligand plane. The isomer shift of 1, delta = 0.56 mm/s, approaches the delta-values reported for the six trigonal cofactor sites. The parameters of 1 and their importance for the cofactor cluster of nitrogenase are discussed.     

An ab initio study of some structural features and the rotational barriers in performic acid,formic acid and hydrogen peroxide

Calculations on performic acid at the 4-31G level, with and without bond functions with complete geometry optimization, and at the (9, 5) level, with and without polarization functions and rigid rotation, all give no sign of a well in the potential energy curve for rotation about the O/O bond axis in the region of 50° – 90° ; and all but the unaugmented 4-31G basis set find the cis-cis planar conformer to be the most stable form. Calculations at the (9,5) level with rigid rotation find the energies of the other planar conformers, relative to the cis-cis conformer, to be 0.94, 1.50 and 14.80 kcal mol^?1 for the trans-trans, cis-trans and trans-cis structures respectively. These energies and also that for the barrier separating the cis-cis and cis-trans conformers, 1–2 kcal mol^?1, are discussed in relation to corresponding data for formic acid, hydrogen peroxide and several planar four heavy-atom molecules. Dipole moment calculations using the same basis sets would seem to favor a skew conformation as the most stable for performic acid, but comparisons between calculated and experimental values for formic acid and for hydrogen peroxide cast doubt on the validity of such results.     

Radium-224 in natural waters measured by γ-ray spectrometry

A method based on Ge(Li) γ-ray spectrométry is applied to the determination of ²²⁴Ra (t_{$\text{1}\text{2}$}= 3.64 days) in natural waters. The ²²⁴Ra is first removed from several hundred liters of water by preconcentration onto manganese dioxide-impregnated acrylic fibers. The fibers are leached, radium is coprecipitated with barium sulfate, and the γ-ray activity is counted so that activity ratios among ²²⁴Ra, ²²⁵Ra and ²²⁶Ra can be calculated. Concentrations are determined by using the ²²⁶Ra concentration determined on a small separate sample. Results from samples collected from ground water, estuarine, and continental shelf environments are presented.     

Acoustical measurement of solid state non-linearity: Application to CsCdF3 and KZnF3

Combination of the results of two sets of measurements on the same crystalline samples of CsCdF₃ and KZnF₃ has made possible the evaluation of the third-order elastic (TOE) constants of these two fluoroperovskites. In the first technique the hydrostatic pressure dependence of the velocity of ultrasonic waves of different propagation and polarization directions has been measured to determine three linear combinations of TOE constants. In the second technique the fundamental and the second harmonic amplitudes of an initially sinusoidal longitudinal ultrasonic wave of finite amplitude propagating along the principal directions have been measured to determine three other linear combinations. Combination of the two sets of data leads to the following room temperature values of the TOE constants (in units of 10¹² dynes/cm²):

     

26.

A molecular orbital study of the rotation about the C-C bond in 1,3-butadiene     

Charles W. Bock  Philip George  Mendel Trachtman 《Theoretical chemistry accounts》1984,64(4):293-311

The geometry and energy of 1,3-butadiene have been calculated using the 6-311G** basis set as a function of the CCCC dihedral angle-0 ° (trans), 30 °, 60 °, 75 °, 90 °, 120 °, 135 °, 150 °, 165 ° and 180 ° (cis)-assuming that the vinyl groups remain planar. Potential minima are located at 0 ° and 141.4 °, with the trans structure more stable than the gauche by 13.2 kJ mol^–1. Potential maxima are located at 76.7 °, giving a barrier height of 25.4 kJ mol^–1 relative to the trans structure, and at 180 ° giving a barrier height of 3.0 kJ mol^–1 relative to the 141.4 °-gauche structure. Using the 6-31G* basis set the inclusion of electron correlation, accounting for about 52% of the correlation energy, was found to produce no significant change in the shape of the potential energy curve. The magnitude of the expectation energy differences is such that both barriers with respect to the 14l.4 °-gauche maximum structure can be categorized unequivocally as attractive-dominant, whereas the values for the energy barrier with respect to the trans structure, although characteristic of a repulsive-dominant barrier at the 6–311G** level, are sufficiently small that higher level calculations might give the opposite result. Analysis of V _nn for the conversion reactions cis 150 °-gauche, trans 60 °-gauche, and trans 90 °-gauche in terms of the individual contributions from the various internuclear interactions shows that nonbonded interactions are important, not only in initiating the destabilization of the crowded cis structure, but also through-out the entire range of CCCC dihedral angles, 0 ° to 180 °.     

27.

Antibody properties for chemically reversible biosensor applications     

W.Gregg Miller  F.Philip Anderson 《Analytica chimica acta》1989

A recently reported fiber-optic sensor based on a homogeneous fluorescence energy-transfer immunoassay operates in a continuous, reversible manner to quantify the anticonvulsant drug phenytoin (5,5-diphenylhydantoin). The chemical kinetics of the two simultaneous antibody-hapten (analyte) and antibody-hapten (labeled indicator) reactions in the sensor are now modeled mathematically. Simulation shows that the chemical response time is controlled by the dissociation rate constant and is independent of the association rate constant, and that an equalibrium chemical response can be achieved in minutes. The sensitivity and dynamic range of the analyte concentration which can be measured depends on the ratio of dissociation rate constants for the labeled and unlabeled hapten reactions, and on the total concentration of reactants in the sensor. The relative concentration ratios of antibody to labeled hapten has little impact on the sensitivity or dynamic range of the system, but can be optimized to provide the maximum amount of labeled hapten availble for instrumental measurement.     

28.

Evidence for facile atropisomerism and simple (non-nucleophilic) biradical-forming cycloaromatization within kedarcidin chromophore aglycon     

Myers AG  Hurd AR  Hogan PC 《Journal of the American Chemical Society》2002,124(17):4583-4585

The kinetics and thermodynamics of atropisomerism within the protected kedarcidin chromophore aglycon 8, as well as a series of ansa-bridged synthetic intermediates leading to 8, were determined by 1H NMR spectroscopy. The data show that the ratio of atropisomeric forms of chloropyridine-bridged ansa intermediates is subject to wide variation with seemingly subtle structural variation. The vinyl bromide 4, for example, in the first X-ray structure determination of a kedarcidin ansa-bridged system, was found to exist as a single atropisomer in the solid state, but a nearly equal mixture (K = 0.70) of isomers in solution (t1/2 for isomer interconversion approximately 0.2 s at 20 degrees C). The aglycon 8, a 2.2:1 mixture of atropisomers, was found to undergo direct unimolecular biradical-forming cycloaromatization at ambient temperature in a mixture of 1,4-cyclohexadiene-benzene, without nucleophilic activation. The product 9 was formed as a single atropisomer (k = 2 x 10-4 s-1, t1/2 = 58 min, 81% yield), suggesting that the rate of atropisomerism within 8 is rapid with respect to cycloaromatization. The rate of cycloaromatization of 8 was found to be highly solvent-dependent, being more rapid in the presence of a good hydrogen-atom donor, consistent with the earlier model studies of Hirama et al. that showed that certain nine-membered cyclic (Z)-enediynes may equilibrate with their biradical cycloaromatization products. Incubation of 8 with beta-mercaptoethanol, under conditions mimicking experiments leading to DNA cleavage with kedarcidin, showed no evidence for nucleophilic activation. The product of direct cycloaromatization (9) was isolated instead. The evidence suggests that kedarcidin, like the enediyne agent C-1027, is capable of spontaneous thermal biradical formation without prior chemical activation.     

29.

A short synthetic route to the calystegine alkaloids     

Skaanderup PR  Madsen R 《The Journal of organic chemistry》2003,68(6):2115-2122

An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4).     

30.

Structural studies in main group chemistry : XIX. Organotin(IV) derivatives of tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrachlorobenzoquinone. The isolation of stable organotin-substituted radicals     

Anthony B. Cornwell  Philip G. Harrison  John A. Richards 《Journal of organometallic chemistry》1977,140(3):273-287

The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R₃SnTCNQ^. (R = Me, n-Pr, n-Bu) and Me₂Sn(TCNQ^.)₂. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph₃SnSnPh₃)·TCNQ, whilst [Me₂SnCl(terpyridyl)⁺](TCNQ^-·) was isolated from the reaction of [Me₂SnCl(terpyridlyl)⁺][Me₂SnCl₃^-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R₃SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R₃SnOC₆Cl₄O·-p (R = Me) or R₃SnOC₆Cl₄OSnR₃-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R₃SnTCNQ· and R₃TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC₃] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me₃SnOC₆Cl₄O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me₂Sn(TCNQ·)₂ has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me₃SnTCNQ· may exist in both forms depending upon the mode of preparation.     

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Sample	C111	C112	C114	C166	C123	C456
CsCdF3	?13·2	?4·55	?3·12	?0·69	+2·6	?3·8
KZnF3	?16·6	?4·75	?0·52	?1·79	+3·2	?6·87

A molecular orbital study of the rotation about the C-C bond in 1,3-butadiene

The geometry and energy of 1,3-butadiene have been calculated using the 6-311G** basis set as a function of the CCCC dihedral angle-0 ° (trans), 30 °, 60 °, 75 °, 90 °, 120 °, 135 °, 150 °, 165 ° and 180 ° (cis)-assuming that the vinyl groups remain planar. Potential minima are located at 0 ° and 141.4 °, with the trans structure more stable than the gauche by 13.2 kJ mol^–1. Potential maxima are located at 76.7 °, giving a barrier height of 25.4 kJ mol^–1 relative to the trans structure, and at 180 ° giving a barrier height of 3.0 kJ mol^–1 relative to the 141.4 °-gauche structure. Using the 6-31G* basis set the inclusion of electron correlation, accounting for about 52% of the correlation energy, was found to produce no significant change in the shape of the potential energy curve. The magnitude of the expectation energy differences is such that both barriers with respect to the 14l.4 °-gauche maximum structure can be categorized unequivocally as attractive-dominant, whereas the values for the energy barrier with respect to the trans structure, although characteristic of a repulsive-dominant barrier at the 6–311G** level, are sufficiently small that higher level calculations might give the opposite result. Analysis of V _nn for the conversion reactions cis 150 °-gauche, trans 60 °-gauche, and trans 90 °-gauche in terms of the individual contributions from the various internuclear interactions shows that nonbonded interactions are important, not only in initiating the destabilization of the crowded cis structure, but also through-out the entire range of CCCC dihedral angles, 0 ° to 180 °.     

Antibody properties for chemically reversible biosensor applications

A recently reported fiber-optic sensor based on a homogeneous fluorescence energy-transfer immunoassay operates in a continuous, reversible manner to quantify the anticonvulsant drug phenytoin (5,5-diphenylhydantoin). The chemical kinetics of the two simultaneous antibody-hapten (analyte) and antibody-hapten (labeled indicator) reactions in the sensor are now modeled mathematically. Simulation shows that the chemical response time is controlled by the dissociation rate constant and is independent of the association rate constant, and that an equalibrium chemical response can be achieved in minutes. The sensitivity and dynamic range of the analyte concentration which can be measured depends on the ratio of dissociation rate constants for the labeled and unlabeled hapten reactions, and on the total concentration of reactants in the sensor. The relative concentration ratios of antibody to labeled hapten has little impact on the sensitivity or dynamic range of the system, but can be optimized to provide the maximum amount of labeled hapten availble for instrumental measurement.     

Evidence for facile atropisomerism and simple (non-nucleophilic) biradical-forming cycloaromatization within kedarcidin chromophore aglycon

The kinetics and thermodynamics of atropisomerism within the protected kedarcidin chromophore aglycon 8, as well as a series of ansa-bridged synthetic intermediates leading to 8, were determined by 1H NMR spectroscopy. The data show that the ratio of atropisomeric forms of chloropyridine-bridged ansa intermediates is subject to wide variation with seemingly subtle structural variation. The vinyl bromide 4, for example, in the first X-ray structure determination of a kedarcidin ansa-bridged system, was found to exist as a single atropisomer in the solid state, but a nearly equal mixture (K = 0.70) of isomers in solution (t1/2 for isomer interconversion approximately 0.2 s at 20 degrees C). The aglycon 8, a 2.2:1 mixture of atropisomers, was found to undergo direct unimolecular biradical-forming cycloaromatization at ambient temperature in a mixture of 1,4-cyclohexadiene-benzene, without nucleophilic activation. The product 9 was formed as a single atropisomer (k = 2 x 10-4 s-1, t1/2 = 58 min, 81% yield), suggesting that the rate of atropisomerism within 8 is rapid with respect to cycloaromatization. The rate of cycloaromatization of 8 was found to be highly solvent-dependent, being more rapid in the presence of a good hydrogen-atom donor, consistent with the earlier model studies of Hirama et al. that showed that certain nine-membered cyclic (Z)-enediynes may equilibrate with their biradical cycloaromatization products. Incubation of 8 with beta-mercaptoethanol, under conditions mimicking experiments leading to DNA cleavage with kedarcidin, showed no evidence for nucleophilic activation. The product of direct cycloaromatization (9) was isolated instead. The evidence suggests that kedarcidin, like the enediyne agent C-1027, is capable of spontaneous thermal biradical formation without prior chemical activation.     

A short synthetic route to the calystegine alkaloids

An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4).     

Structural studies in main group chemistry : XIX. Organotin(IV) derivatives of tetracyanoethylene, 7,7,8,8-tetracyanoquinodimethane and 2,3,5,6-tetrachlorobenzoquinone. The isolation of stable organotin-substituted radicals

The reaction of organotin chlorides with the lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) or hexaalkylditins with TCNQ yield stable organotin-substituted free radicals of the types R₃SnTCNQ^. (R = Me, n-Pr, n-Bu) and Me₂Sn(TCNQ^.)₂. The reaction of hexaphenylditin with TCNQ yields a (σ → π) charge transfer complex of stoichiometry (Ph₃SnSnPh₃)·TCNQ, whilst [Me₂SnCl(terpyridyl)⁺](TCNQ^-·) was isolated from the reaction of [Me₂SnCl(terpyridlyl)⁺][Me₂SnCl₃^-] and LiTCNQ. The oxidation of hexaalkylditins by tetracyanoethylene (TCNE) yields stable free radicals of the type R₃SnTCNE·, but treatment with 2,3,5,6-tetrachlorobenzoquinone yields either R₃SnOC₆Cl₄O·-p (R = Me) or R₃SnOC₆Cl₄OSnR₃-p (R = n-Bu, Ph). Tin-119 Mössbauer spectroscopy shows that the derivatives R₃SnTCNQ· and R₃TCNE· have trigonally-bipyramidally coordinated tin with planar [SnC₃] skeletons and bridging [TCNQ·] and [TCNE·] groups forming infinite one-dimensional chain structures. Me₃SnOC₆Cl₄O·-p was inferred to possess a similar structure but with oxy bridges forming chains with a Sn---O---Sn---O backbone. Me₂Sn(TCNQ·)₂ has a structure intermediate between tetrahedral and octahedral with a non-linear MeSnMe unit and anisobidentate chelation by two TCNQ groups. The TCNQ derivatives were of two types: (i) “green” or “brown”, indicative of delocalisation of the Ione electron over the cyanoquinone ligand, and (ii) a “blue” form in which spin-pairing of the Ione electron between adjacent organic groups takes place. Me₃SnTCNQ· may exist in both forms depending upon the mode of preparation.