Basic gel permeation chromatography studies. III. Mathematical analysis of peak spreading

Peak spreading in gel permeation chromatography has been studied with a range of gels including those whose permeation limit corresponded to about 10³, 10⁶, 10⁸, and 10⁹ molecular weight polystyrene. Peak spreading conformed to the equation Y_V ² = Y_M ² + Y_A ² + Y_I ² + Y_D ² + Y_S ², where Y_V is the peak width of a normal chromatogram, Y_M is the contribution due to the true molecular weight of the sample, Y_A is due to peak spreading in the apparatus, Y_I is spreading in the interstitial volume, Y_D is diffusional spreading due to time spent in the gel, and Y_S is due to sorption. Evaluating the appropriate parts of the equation leads to measures of the true molecular weight distribution and the contribution due to diffusion into and out of the gel. The data also allowed estimates as to the diffusional spreading with small molecules. With polystyrene having 100 000 molecular weight, diffusional spreading accounts for 80% of Y_V ,² but with small molecules the contribution due to diffusion was not detected.     

Nitrogen-15 n.m.r. studies of 13C, 15N labeled arginine

¹⁵N n.m.r. spectra of [¹³C-2, 3-¹⁵N₂-guanidino]arginine and [¹³C, ¹⁵N₂] urea were obtained in D₂O and H₂O at a variety of pH values both with and without proton decoupling. The effects of the proton exchange rate are readily observable in the proton coupled ¹⁵N spectra. When the guanidino group is deprotonated (pK = 12.5), the terminal nitrogens give a single resonance 6.6 ppm downfield of the protonated species, indicating a rapid tautomeric exchange. The observed NH and CN couplings are compared with calculated values, and good agreement is found for ¹J(CN) using a Blizzard–Santry type calculation. The ramifications of the proton exchange on ¹⁵N n.m.r. spectra of amino acids and peptides are discussed.     

Nuclear magnetic resonance studies: VII—13C and 15N n.m.r. of diazo compounds

The ¹³C and ¹⁵N n.m.r. results for a series of diazo compounds are reported. It is found that the diazo carbon is shielded by an extraordinary amount compared with normal sp² hybridized carbons. The ¹⁵N chemical shifts reveal that the terminal nitrogen is deshielded relative to the central one. This is contrary to that expected from charge effects but support is found for this phenomenon in other systems. One-bond ¹³C? ¹⁴N coupling in diazomethane is also reported for the first time. INDO MO calculations of the charges and finite perturbation calculations of ¹³C? ¹⁴N and C? H couplings are compared with the experimental results.     

Nuclear Magnetic Resonance Studies of Ion-Exchange Resins. I. Effect of Counter Ion

The nuclear magnetic resonance spectra of packed beds containing the cation exchanger Dowex 50W-X8 in water vary with counter ion. Chemical shifts for water protons internal to the resin bead are in the order H⁺ (lowest frequency) » Zn²⁺ ? Cd²⁺ ≤ Pb²⁺ ? Mg²⁺ ≤ Ag⁺ ≤ Ca²⁺ ? Tl⁺ ? NEt⁺ ₄ ? Ba²⁺ ? Li⁺ ? NMe₄ ⁺ ? Sr²⁺ ≤ NH₄ ⁺ ≤ Rb⁺ ? K⁺ ? Na⁺ ? NBu₄ ⁺. The concentrated electrolyte model of ion-exchange resins is substantiated by generally good agreement between 1) chemical shifts/g equiv./kg for cations in aqueous solutions referred to NH₄ ⁺ and 2) chemical shifts/g equiv./kg for resin forms referred to NH₄ ⁺ form resin.     

Phosphate Derivatives of Natural Lactones. I. Synthesis of Novel Isoalantolactone Dialkylphosphonates

Novel isoalantolactone dialkylphosphonates were synthesized in 70-87% yields by reacting this eudesmanolide with dialkylphosphites. Their structures were proved by spectral analysis using IR, PMR, ¹³C and ³¹P NMR, and two-dimensional ¹H-¹H (COSY) spectroscopy. The reaction of isoalantolactone with dialkylphosphites is highly stereoselective.     

Specialist Periodical Reports. Organometallic Chemistry,Volume 11. : E.W. Abel and F.G.A. Stone,senior reporters. The Royal Society of Chemistry, 1983, xviii + 549 pages, £ 77.

CpNi-vinyl, which is formed as an intermediate in the reaction of nickelocene with vinyllithium, in the presence of ethylene reacts in part via reductive coupling of two vinyl groups to give the binuclear (CpNi)₂(η⁴-butadiene) and in part via ethylene insertion into the vinylnickel bond to give the η¹,η²-3-butenyl complex, which isomerizes at elevated temperatures to the η³-1-methylallyl compound.     

Theoretical investigations of Fe porphyrins. V. Low-lying electronic states of bisammineporphinatoiron(III)

Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [Fe^III(P)(NH₃)₂]⁺. The low-spin ground state is calculated as (d_xy² (d_π)³ with d_xy(d_π)⁴ higher by 0.15 eV. The near-ir bands at ～1 eV observed in low-spin ferriheme proteins are attributed to (π → d_π) transitions. The lowest high-spin state is ⁶A_1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ～1.2 eV are attributed to higher ⁶[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on Fe^III(TPP)CI [TPP = tetrapbenylporphyrin] may be an ¹[tripsextet] state.     

B.A. Sastry,S.Md. Asadullah,G. Ponticelli and R. Pinna,EPR studies on copper(II) [2,5-diphenyleoxazole]2 (ClO4)2 · 4H2O,Chem. Phys. Letters 73 (1980) 118.

The potential curves tor the ^1,3σ⁺ and ^1,3Π states of BLi and for the ²σ⁺ and ²Π states of BLi⁺ were caleatated with the help of the CAS SCF method. The ³π state was found to be the ground state of BLi. The molecule is stable with dissociation energy 0.70 eV.     

Polysulfonylamine. VII. Aliphatische Trisulfonylamine

Polysulfonyl Amines. VII. Aliphatic Trisulfonyl Amines The compounds N(SO₂R¹)₂(SO₂R²) with R¹ = R² = CH₃ ( 2a ), R¹ = R² = C₂H₅ ( 2b ) and R¹ = CH₃, R² = C₂H₅ ( 2c ) are prepared by cleavage of aminostannanes (CH₃)₃SnN(SO₂R¹)₂ with sulfonyl chlorides R²SO₂Cl. A simple synthesis of 2a from AgN(SO₂CH₃)₂ and CH₃SO₂Cl is described. From the vibrational spectra of 2a , evidence is obtained for a planar NS₃ group in this compound. X-ray structure determinations of 2b and HN(SO₂C₂H₅)₂ ( 3 ) are reported. In 2b , the NS₃ group is approximately planar (S? N? S bond angles 119.0 ± 0.6°, sum of bond angles at N 356.9°); the S? N bond lengths of ca. 173 pm indicate a bond order of 1. In compound 3 , the nitrogen atom has a planar coordination (S? N? S angle 125.3°, sum of bond angles at N 359.3°), the S? N bond lengths of ca. 165 pm correlate with a bond order of 1.3? 1.4.     

The chemistry and kinetic behavior of Cu-Cr-As/B wood preservatives. I. Fixation of chromium on wood

The kinetic behavior of CrO₃ in its reaction with wood has been elucidated. Various reactions take place between CrO₃ and the lignin and cellulose in wood. CrO₃ reacts with cellulose in a two-step reaction: the first step is an adsorption of Cr^VI onto the cellulose to form Cr^VI/cellulose activated complexes. The second step is a Cr^VI → Cr^III reduction taking place on the cellulose surface. The Cr^III formed is only physically adsorbed to the cellulose or very weakly bound as small amounts of Cr^III can be released into the reaction medium. The Cr^VI adsorbed by cellulose appears mainly to be reduced to Cr^III. The reaction of Cr^VI with lignin has been shown to be the composition of the three successive reaction of Cr₂O₇^2?, HCrO₄^?, and CrO₄^2? with the guaiacyl units of lignin. Insoluble and stable Cr^VI/lignin complexes in which chromium maintains its hexavalent oxidation state are formed. Rate constants and energies of activation for all the reactions have been determined. The fixation of CrO₃-derived compounds on wood has been explained as the combination of the various reactions investigated. The results indicate that 60% of Cr is fixed irreversibly to the lignin of wood as Cr^VI and 40% is weakly bound, probably just precipitated, on the cellulose surface as Cr^III of which small amounts can be released in a water medium. The complex Cr^VI and Cr^III species forming complexes with the guaiacyl units have been identified.     

Anionic polymerization with alkaline-earth counterion. I. Preparation and characterization of dicarbanionic oligostyrylbarium and -strontium in tetrahydrofuran

Dicarbanionic oligostyrylbarium or -strontium, PS⁼Ba⁺⁺ and PS⁼Sr⁺⁺, were prepared by reacting styrene with finely divided barium or strontium in THF or THP. The effects of reaction temperature, monomer, and metal concentration were described. Difunctionality of these living oligomers is confirmed from calculated and observed molecular weight data and also from conductivity measurements. λ_max of carbanions is found to vary with DP . At 25°C it decreases from 368 nm for dimers to 359 nm for DP > 10. As observed for monocarbanionic polystyrylbarium (PS^?)₂Ba⁺⁺, λ_max of PS⁼Ba⁺⁺ of high DP decreases with temperature, while λ_max of PS⁼Ba⁺⁺ of low DP increases with decreasing temperature. At ?100°C the difference between the two extreme values is about 30 nm. Similar results are obtained with PS⁼Sr⁺⁺. Such unusual spectral properties are interpreted, on the basis of excitation molecular theory, in terms of ring structures for the living oligomers.     

Professor K.S.G. Doss in honour of his reaching his seventieth year

Recent studies have clearly demonstrated the importance of including the sphericity of the dropping mercury electrode in the theoretical analysis of certain polarographic phenomena. Experimental effects of sphericity on second harmonic a.c. polarograms¹ and on polarograms for reversible electrode reactions with amalgam formation^2.3 have been successfully described using a theoretical treatment augmented by computer simulation. The simulation procedure we present here allows one to simulate diffusion phenomena at the growing mercury drop (sphere) electrode without modification of other simulation operations, e.g. computation of surface boundary conditions⁴ or of homogeneous kinetics⁵. We treat two cases: “External” diffusion where the diffusing species, A, is in the solution phase with the boundary condition A_x=0=0 for t>0; and “internal” diffusion where the diffusing species is in the mercury phase with the same boundary condition.     

Kinetics and mechanism of the cationic polymerization of tetrahydrofuran in solution. I. THF–CCI4 system

Polymerization of THF in CCl₄ solvent was initiated with 1,3-dioxolan-2-ylium eations with AsF₆^?, PF₆^?, and SbF₆^? anions as well as with esters of fluorosulfonic and trifluoromethanesulfonic acids. With these esters polymerization proceeds with a marked acceleration period, due to slow initiation. The corresponding rate constants of initiation and their dependence on the polarity of the THF/CCl₄ mixture were determined. The rate constant of propagation on the macroion-pairs (k_p^±) of the polytetrahydrofurylium cation with AsF₆^?, PF₆^?, and SbF₆^? and CF₃SO₃^?, anions was found to be independent in CCl₄ solvent on the anion structure and given by the expression: k_p^± = 2.93 × 10^?2 exp {?4.7 × 10³/T} at [THF]₀ = 8.0M. This constant depends on the polarity of the polymerization mixture, and at 25°C for the THF-CCl₄ system, k_p^± = 1.78 × 10^?2 exp {?4.9/D}; thus, in CCl₄ at [THF]₀ = 8.0M, and at 25° k_p^± = 4.0 × 10^?21/mole-sec. In the polymerization with derivatives of CF₃SO₃H (able to form the corresponding macroester) the overall polymerization rate is much lower than that with complex anions because of the reversible conversion of the macroion-pairs into the macroester (internal return). The macroester is much less reactive than the macroionpair (10²–10³ times) in the monomer addition reaction. At [THF]₀ = 8.0M and at 25°C, 96.5% of the growing species exists in the macroester form. Polymerization of THF initiated with derivatives of CF₃SO₃H is a subject of a strong special salt-effect. At a sufficiently high ratio of [AgSbF₆] to [I]₀, where the initiator I is C₂H₅OSO₂CF₃, the overall polymerization rate is equal to that observed for the polymerization of THF on the macroion-pairs, since the internal return within the triflate ion-pair (the macroester formation) is eliminated and polymerization proceeds on the macroion-pairs with SbF₆- anions exclusively.     

Anionic polymerization with alkaline-earth counterion. II. Polymerization of styrene initiated by dicarbanionic oligostyrylbarium in THF

The propagation kinetics of anionic polymerization of styrene initiated by dicarbanionic oligostyrylbarium (PS⁼Ba⁺⁺) in THF are described. The apparent propagation rate constant k_p increases drastically with the degree of polymerization (DP) of living chains and tends at 20°C, for the highest molecular weight (DP ? 5000), to the value determined for monocarbanionic polystyrylbarium(PS^?)₂Ba⁺⁺. At given DP, the propagation step follows usual first-order kinetics with respect to monomer, and k_p is inversely proportional to carbanion concentration; as observed for (PS^?)₂Ba⁺⁺. Similar behavior is observed in the temperature range from ?60 to +20°C. The activation energy of the propagation is 4–5 kcal/mole (16.7–21 kJ/mole). It is shown that k_p may be considered as directly proportional to the dissociation constant K_d of ion pairs (～S^?Ba^++?S～ is considered as an ion pair ～(SBa)⁺S^?～). The striking variation of k_p with the DP living chains is interpreted in terms of cyclic living chains, in which both carbanionic ends are bound to the same cation. Values of the intramolecular dissociation constant K_d of ion pairs included in such a model are computed as a function of DP, and their variation is found to fit rather well with experimental data.     

Explanation of Ionic Sequences in Various Phenomena. I. Salting-Out of Uncharged Molecules

Abstract

Theoretical models for hydrated ions and their calculated effective dielectric constants obtained previously were used to explain the salting-in or salting-out of nonionic molecules. Three types of salting-out sequences were obtained: nonpolar (Na⁺ > K⁺ > Li⁺ Rb⁺ > Cs⁺), basic (K⁺ > Na⁺ > Rb⁺ > Cs⁺ > Li⁺), and acidic (Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺). The nonpolar sequence is not influenced by the A region of a cation, and therefore the ability to salt-out is great if the effective dielectric constant of the ion is small. The A region on hydrated Li⁺ ions (the tightly bound water) salts-in basic compounds because of the interaction of its positively charged hydrogen atoms with the negative dipolar charge of the base. Conversely, the A region of a cation salts-out acidic compounds because the hydroxyl group on carboxylic acids behaves as a similar cationic A region. A sulfonic polymer will cause the salting-in of the base p-nitroaniline because the addition of salts to an aqueous solution of the base and polymer destroys hydrogen bonds in the polymer and in so doing releases hydronium ions from the polymer. This release of H⁺, in turn, produces a positive charge on part of the p-nitroaniline molecules, which produces a salting-in effect.     

Electron transfer. 121. Oxidations by Rhodium-Bound Superoxide

The violet superoxo complex, [(H₂O)₄(OH)Rh^III(O₂)Rh^III(OH)(H₂O)₄]³⁺, formed by treatment of (Rh^II)₂⁴⁺ with O₂ in HClO₄, is converted to a le^? reduction product, the corresponding μ-peroxo complex, by the reductants I^?, IrCl₆^3?, and the trinuclear aquamolybdenum(III) cation, (Mo^III)₃. Each reaction is first-order in both redox partners, and the le^? reduction by IrCl₆^3? is followed by a much slower conversion to a peroxide-free complex. Among the rapid reductions of the superoxo derivative examined here and in a previous study, only that by IrCl₆^3? is accelerated by increases in acidity; the rate law for this reaction features both an acid-independent and a [H⁺]-proportional component, the latter stemming from partial conversion of the oxidant to its conjugate acid (pK_A < ?1.0). Rate laws for reductions by other metal-center reagents generally exhibit inverse-[H⁺] terms, reflecting deprotonation of the reductant. All reductions thus far observed involving this superoxo species appear to be outer-sphere. Treatment of acid-independent rate constants within the framework of the Marcus model, allows estimates of the self-exchange rate, k₁₁, for the (Rh^III)₂-bound superoxo-peroxo couple. Because values of k₁₁ calculated from the several reductions span a range of 10^4.5, reductions of the superoxo complex cannot be taken to conform satisfactorily to the Marcus treatment, being in this respect comparable to the systems VO(OH)^+/2+, Mn^2+/3+, Eu^2+/3+, and Ti(OH)^2+/3+, each of which exhibits similar divergences. The wide range of calculated self-exchange rates appears to invalidate an earlier suggestion that reduction of the superoxo complex by Fe²⁺ proceeds primarily through a bridged path. © 1994 John Wiley & Sons, Inc.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

Electron transfer. 138. Potentials involving chelated chromium(V)

The bis(chelated) complex of Cr^V(0) derived from the dianion (L^{2 −}) of 2-ethyl-2-hydroxybutanoic acid is readily reduced to a bis(chelate of Cr^III, featuring the monoanion (LH⁻) [Cr ^V(0)(L²⁻)₂]⁻+4H⁺+H₂O+2e⁻→[Cr^III(OH₂)₂(LH⁻ ₂]⁺ of this acid. Potentials estimated by Ghosh in 1993 for this 2e⁻ change, E⁰ (pH 0) 1.32 V, E^eff (pH 3.3) 0.93 V, are in accord with the nearly irreversible reductions of the Cr(V) species (in 1∶1 ligand buffer) by Fe²⁺, V0²⁺, IrCl₆ ^{3 −} and I⁻, whereas lower values reported by Bose in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.45 V, are potentiometrically inconsistent with these conversions. A similar discrepancy is noted for potentials for Cr(V,IV) estimated in 1996, E⁰ (pH 0) 0.84 V, E^eff (pH 3.3) 0.46 V, which, wholly contrary to observation, predict that the reductions of excess Cr(V) to CR(IV) by Fe²⁺, V0²⁺, and I⁻ are thermodynamically disfavored.     

Kinetics of nucleophilic attack on coordinated organic moieties. XVIII. Nucleophilicity of anions towards the tricarbonyl-(η-benzene) manganese (I) cation.

Kinetic results for the addition of OH^? to [Mn(CO)₃(η-C₆H₆)]⁺ (I) in water (eq. 1, X  OH) obey the expression k_obs  k_OH[OH^?], and give a k_OH value of 290 mol^?1 dm³ s^?1 at 20.0°C and ionic strength of 0.25 mol dm^?3. The analogous reaction of NaCN with I in water fits the two-term expression k_obs = k_OH[OH^?] + k_CN[CN^?], and leads to a k_CN value of 0.8 mol^?1 dm³ s^?1 at 20.0°C and ionic strength of 0.25 mol dm^?3. Interestingly, the related reaction (eq. 1, X  N₃) is too rapid to follow by stopped-flow spectrophotometry, indicating the overall rate trend N₃^? » OH^? » CN^?. This unusual nucleophilicity order, unexpected on the basis of both basicity and polarizability, is similar to that previously observed for anion addition to free carbonium ions.     

Electronic rearrangements during chemical reactions. II. Planar dissociation of ethylene

The direct dissociation of ethylene into two methylenes is studied along the least motion reaction path by means of an ab initio multiconfiguration self-consistent-field (MCSCF ) calculation. All eight configurations arising from those valence orbitals that form the CC bonds, seven of them singlet coupled and one triplet coupled, are taken into account. The HCH bond angle is optimized along the entire reaction path. Separate MCSCF optimizations are carried through for the lowest two states of ¹A_g symmetry. The (¹A_gσ²π²) ethylene ground state dissociates into two (³B₁σπ) ground-state methylenes. The (¹A_gσ²π*²) excited state of ethylene dissociates into two (¹A₁σ²) excited methylenes. It is established that both these dissociations proceed without any barrier in the energy curve. In the ground state, where orbital symmetry is conserved, the π-bond breaks before the σ-bond, and the calculated heat of reaction agrees within 6 kcal/mol with the experimental value. In the excited state, where orbital symmetry is not conserved, the nonbonded repulsion between methylene σ₂ lone pairs is found to blend into the antibonding character of the excited ethylene, yielding an energy curve that is everywhere repulsive. However, the variation of the HCH angle during the dissociation process is not simple, initially it expands and subsequently it contracts. Quantitative analytical approaches are developed which furnish conceptual interpretations of the orbital changes and configurational changes along the reaction path.