Fast Exchange Limit Research Articles

Nuclear magnetic resonance study of ligand exchange on hexakis(1,1,3,3-tetramethylurea)yttrium(III)

The first reported direct study of monodentate ligand exchange on yttrium(III) shows the rate of 1,1,3,3-tetramethylurea exchange on [Y {O=C(NMe2)2}6]3+ to be independent of free ligand concentration consistent with the operation of either a dissociative mechanism or an interchange mechanism where the encounter complex stability constant is ≥ 300. Typical data from this 270-MHz 1H n.m.r. study, where rate of ligand exchange = kex 6[Y {O=C(NMe2)2}63+] are as follows: kex(250 K) = 25�1 s-1, ΔH‡ = 27.1 � 0.5 kJ mol-1 and ΔS‡ = -108 � 2 J K-1 mol-1 for a CD3CN solution in which [Y{O=C(NMe2)2}63+] and free [O=C(NMe2)2] are 0.0039 and 0.028 mol dm-3 respectively. The preparations of [Y(ligand)6] (ClO4)3 where the ligand is O=C(NMe2)2, O=C(Me)(NHMe), O=C(Me)(NMe2), or O=C(Me)(NEt2) are also reported. Solutions of the latter three species and their respective ligands exhibit spectra consistent with ligand exchange being in the fast exchange limit of the n.m.r. time scale down to the lowest accessible temperatures.

A phosphorus-31 magnetic resonance study of ligand exchange on Hexakis[methyl methyl(phenyl)phosphinate]yttrium(III) ion and the solution chemistry of related yttrium(III) species

A 31P n.m.r, study shows that the species [YLn3+ where n = 4,5,6 for L = OP(NMe2), or OPPh3 n = 5,6 for L = OPMe(0Me)Ph are the major yttrium(III) species formed with these ligands in CD2CI2 solution. A detailed ligand exchange study of the latter species shows the rate law to be: exchange rate = 6(kl+k2[OPMe(OMe)Ph])[Y(OPMe(OMe)Ph)63+] where k1(215 K) = 312�13 s-1, ΔH1‡ = 3l·4 � 1.4 kJmol-1, ΔS1‡ = -48·4�6.6 J K-1 mol-1, k2(215 K) = 455 � 31 dm3 mol-1 s-1, ΔH2‡ = 35·2�2.8 kJmol-1 and ΔS2‡ = -27·6 � 12·7 JK-1 mol-1, where the k1 and k2 terms are assigned to D and A mechanisms respectively. The rate of OP(OMe)3 and OPMe(OMe)2 exchange on yttrium(III) is found to be in the fast exchange limit of the n.m.r. time scale and thus the coordination numbers of the yttrium(III) species formed with these ligands are not directly determined in solution. Qualitatively it appears that the coordination number and the lability of the yttrium(III) species increases as the size of the ligand decreases. The isolation of the crystalline species [Y(OP(NMe2)3)5ClO4](ClO4)2, [Y(OPPh3)4](ClO4)3, [Y(OPMe(OMe)Ph)6] (ClO4)3, [Y(OP(OMe)3)6](ClO4)3 and [Y(OPMe(OMe)2)6](ClO4)3 is reported.

Novel bonding modes of .alpha.-diimines. Synthesis and characterization of [MCl2L(.alpha.-diimine)] and [MCl2(.alpha.-diimine)n] (M = palladium, platinum; L = phosphine, arsine; n = 1, 2) containing .sigma.,.sigma.-N,N', .sigma.-N, or .sigma.-N .tautm. .sigma.-N' bonded .alpha.-diimines

Compounds of the type [PdX2(RN=CHCH=NR),] (n = 1,2; X = C1, Br, I) and [MCl,L(RN=CHCH=NR)] (L = monodentate phosphine or arsine; M = Pd, Pt; R = alkyl) have been synthesized starting from the a-diimine (R-dim) and [PdCI,(PhCN),] or [(MCl,L),], respectively. Three types of metal-a-diimine interactions have been established: (i) up-N,N' chelate bonded R-dim in [PdXz(R-dim)]; (ii) u-N monodentate bonded Rdim in monomeric tr~ns-[PdX~(R-dim)~] ('H and C NMR spectroscopy reveals that the R-dim ligand has the anti conformation with the C-C and Pd-N bonds of the coordinated part of the ligand in the cis configuration. This brings the @-imine proton (S(CH) 9.70) in the proximity of the palladium atom.); (iii) u-N - u-N' bonded R-dim in the monomeric complexes, tr~ns-[M~~Cl,L(R-dim)], which contain monodentate (u-N) bonded R-dim in the slow-exchange limit (-55 C). At 120 OC (fast-exchange limit) the complexes are fluxional by an intramolecular process which involves a pentacoordinate intermediate containing up-N,N' bonded R-dim. These novel bonding modes for the R-dim ligand in these palladium and platinum complexes are a clear indication of the versatile coordination properties of the R-dim ligand compared with ligands such as bpy and phen, which also contain the N=CC=N skeleton.

A proton magnetic resonance study of ligand exchange on the hexakis(1,1,3,3-tetramethylurea)scandium(III) ion and its NN-dimethylacetamide analogue

Proton n.m.r. spectroscopic studies show that the rate law for tetramethylurea (tmu) exchange on the hexakis-(tetramethylurea)scandium(III) ion is: rate = 6k1[Sc(tmu)63+] where, typically, k1(300 K)= 0.26 ± 0.03 s–1, ΔH‡1= 91.2 ± 2.3 kJ mol–1 and ΔS‡1= 47.8 ± 6.7 J K–1 mol–1 in CD3NO2 diluent, while k1(300 K)= 1.08 ± 0.05 s–1, ΔH‡1= 68.6 ± 1.3 kJ mol–1, and ΔS‡1=–15.7 ± 3.8 J K–1 mol–1 in CD3CN diluent. In contrast, the rate law for ligand exchange on the hexakis (NN-dimethylacetamide)scandium(III) ion is: rate = 6(k1+k2[dma])[Sc(dma)63+] in CD3NO2 and CD3CN diluents. For the nitromethane system, k1(300 K)= 4.6 ± 0.3 s–1, ΔH‡1= 30.3 ± 2.0 kJ mol–1, and ΔS‡1=–132 ± 6 J K–1 mol–1 and k2(300 K)= 112 ± 3 dm3 mol–1 s–1, ΔH‡2= 26.0 ±0.6 kJ mol–1, and ΔS‡2=–119 ± 2 J K–1 mol–1. Similar parameters characterize the acetonitrile solutions. Ligand exchange in solutions of either [Sc(dmf)6][ClO4]3 or [Sc(dmso)6][ClO4]3 lies in the fast-exchange limit of the n.m.r. time scale. The mechanistic implications of these data are discussed and comparisons are made with other tripositively charged metal complex systems.

A phosphorus-31 magnetic resonance study of ligand exchange on tetrakis(hexamethylphosphoramide)zinc(II) ion

The rate of the associative ligand exchange process on tetrakis (hexamethylphosphoramide)zinc(II) ion in CD2Cl2 solution, determined by 31P n.m.r. spectroscopy, is given by: ������������������� rate = 4k2[O=P(Nme2)3][Zn(O=P(NMe2)3)42+] The characteristic kinetic parameters are: k2(240 K) 304�17 dm3 mol-1 s-1, ΔH2‡ 22.7�0.9 kJ mol-1, and ΔS2‡ -101�4 J K-1 mol-1. ��� The preparations of [Zn(O=P(NMe2)3)4](ClO4)2, [Zn(O=P(OMe)3)5] (ClO4)2 and [Zn {O=P(Me)-(OMe)2}5](ClO4)2 are reported. The rate of ligand exchange upon the last two species was found to be in the fast-exchange limit of the n.m.r. time scale.

The effect of cytochrome P-450cam on the NMR relaxation rate of water protons.

Cytochrome P-450cam in the native, substrate-free state (Fe3+, S = 1/2) substantially reduces the NMR relaxation times, T1 and T2, of water protons. Temperature and frequency dependences of T1 and T2 were measured; they are consistent with a model of one or two protons exchanging between a binding site on a heme ligand and bulk water. The relevant parameters of this model have been deduced from the data. The spin relaxation time of the heme iron, tau S similar to 0.5 ns at 25 degrees C, is unusually long for a low spin ferric heme protein but is compatible with the line widths measured for paramagnetically shifted heme resonances. The proton residence time on the ligand, tau M similar to 1 microsecond at 25 degrees C, follows an Arrhenius law with activation energy EM similar to 15 kcal/mol. A scalar hyperfine interaction A/h = 2.2 MHz (3.1 MHz for one-proton exchange) of the found proton(s) with the heme iron is deduced from the difference between T1 and T2 observed in the fast exchange limit. The iron-proton distance is found to be 2.9 A (2.6 A for one-proton exchange). Variation of pH between pH 6.4 and 8.6 does not affect T1. The bearing of these results on the question of the axial heme ligand is discussed.

Nuclear magnetic relaxation of spin 5/2 and spin 7/2 nuclei including the effects of chemical exchange

An equation describing the multiexponential longitudinal and transverse relaxation of quadrupolar nuclei, including the effects of chemical exchange in the limit of fast exchange, is derived. It is difficult to produce analytical solutions for nuclei with spins greater than 3/2, but the equation is in a form convenient for numerical solution, and such solutions are presented for both the longitudinal and transverse relaxation of nuclei with spins 5/2 and 7/2 in the absence of chemical exchange, and for the case of two-site exchange where one site is in the extreme narrowing limit. Longitudinal relaxation may be described approximately by a single exponential in all cases. However, the transverse relaxation generally must be described as the sum of three and four exponentials for the spin 5/2 and spin 7/2 cases, respectively, the amplitudes of which vary with the correlation time in the absence of exchange, and also with the ratio of the lifetimes of the nucleus in each state when two-site exchange is considered.

A Proton Magnetic Resonance Study of Ligand Exchange on Tetrakis(tetramethylthiourea)zinc(II) Ion

The rate of ligand exchange on the tetrakis(tetramethylthiourea) zinc(II) ion, determined by 1H N.M.R. methods, in CD2Cl2 solution is found to be independent of free-tetramethylthiourea concentration. This is consistent with a dissociative ligand-exchange mechanism being operative. Typically for a solution in which [Zn{S=C(NMe2)2}42+] and [S=C(NMe2)2] were 0�00101 and 0�00376 respectively, kex(200 K) = 16�0�0.6 s-1, ΔH‡ = 64�9�O.8 kJ mol-1 and ΔS‡ = 106�4 J K-1 mol-1 where the observed ligand-exchange rate is given by rate = 4kex[Zn {S=C(NMe2)2}42+] The preparations of [Zn(O=CHNMe2)6](ClO4)2, [Zn{O=CMe(NMe2)}6](Cl04)2, and [Zn{O=C- (NMe2)2}4](ClO4)2 are also reported. The rate of ligand exchange upon these three species in CD2Cl2 solution was found to be in the fast-exchange limit of the N.M.R. time scale.

McConnell's Q in metalloporphyrin radical cations

Electron transfer NMR line broadening in the fast exchange limit gives Q CH: Q CCH 3 : Q CCH 2 = 1.0:1.28:0.54 for the radical cation of magnesium deuteroporphyrin(IX) dimethyl ester. For protons β to the π system, the relation Q = −15 + 224 cos 2 φMHz is proposed, giving Q CCH 3 = 97 MHz.

Hyperfine coupling in chlorophyll radical cations. A nuclear magnetic resonance approach

Most of the relative proton hyperfine coupling constants in the radical cations of chlorophyll a, chlorophyll b, and bacteriochlorophyll a have been determined using electron transfer NMR line broadening in the fast exchange limit. The results have been used to confirm ENDOR assignments for some methyl groups and to give a detailed spin density distribution for many positions exhibiting no ENDOR signals. Agreement with ENDOR is qualitatively good for Chl a and Bchl a, but is poor for Chl b.

An NMR study of the exchange rates for protons involved in the secondary and tertiary structure of yeast tRNAphe

Solvent exchange rates of all the protons of yeast tRNAphe resonating in the lowfield NMR region (-11 to-15 ppm from DSS) have been measured by saturation-recovery long-pulse Fourier transform NMR. All these protons in yeast tRNAphe are in the fast exchange limit with H2O relative to their intrinsic longitudinal relaxation processes. Most rates show very little temperature dependence; however, tertiary base pair protons are preferentially destabilized in the absence of Mg++ at higher temperatures. The measured exchange rates are between 2 and 125 sec-1 for a temperature range from 10 degrees C to 45 degrees C and MgCl2 concentrations between 0 and 15 mM.

Investigation of the structure of yeast tRNAphe by nuclear magnetic resonance: paramagnetic rare earth ion probes of structure.

The binding of paramagnetic rare earth ions to yeast tRNA(Phe) shifts some resonances in the low-field nuclear magnetic resonance spectrum that have been assigned to ring nitrogen protons of specific Watson-Crick base pairs. The changes in the nuclear magnetic resonance spectrum as the tRNA is titrated with Eu(3+) indicate that 4 (or 5) Eu(3+) ions are tightly bound, that the metal binding is in the fast exchange limit, and that the binding to different sites in the molecule is sequential rather than cooperative. The first metal bound simultaneously shifts resonances associated with the dihydrouridine and the -C-C-A stem. This permits us to conclude that the folding of the tRNA(Phe) in solution brings the phosphate backbone of the -C-C-A and the dihydrouridine stems into close proximity. A model of the three-dimensional structure of tRNA(Phe) incorporating this new information appears to be compatible with the results obtained from x-ray diffraction.

NMR line shapes for exchanging systems at high rf field

A method is described by which NMR line shapes at high rf may be calculated for chemically exchanging systems. First order exchange results in a set of manifestly linear density matrix equations that are solved with standard methods. Second order exchanging systems, a model example of which is considered here, give rise to second order density matrix equations. Inspection of sample values of density matrix elements reveals that products of off-diagonal elements are small enough to drop from the equations, while products of off-diagonal and diagonal elements, respectively, may be linearized as ρi jAρk,kB=ρi jA/n B. This selective neglect of bilinear terms (SNOB) leads to a set of first order density matrix equations. Inspection of the calculated line shapes reveals that NMR at high rf in exchanging systems allows more precise determination of rate constants for exchange (compared to low power) in the slow, and, in a limited sense, in the fast exchange limits.

A Deuteron Magnetic Resonance Study of the Motion of Ligands in [Co(ND3)5(D2O)](ClO4)3

Abstract The deuteron magnetic resonance of single crystals of aquopentamminecobalt(III) perchlorate was measured at various temperatures. The observed quadrupole coupling shows that [Co(ND3)5(D2O)]3+ ions execute a rapid isotropic reorientation at high temperatures (above 320°K), while only a C3 axis reorientation of the ligand ND3groups is present at low temperatures (below 290°K). No anisotropic uniaxial reorientation was observed about the C4 axis parallel to the Co–(D2O) direction. At temperatures between 310°K and 320°K, the spectrum of a fast exchange limit was observed, together with the spectrum of a slow exchange limit. This anomaly may be explained by the classical rotation of the D2O ligand.

An Electron Paramagnetic Resonance Study of a fast Lewis Acid–Base Reaction between Copper (II) bis(Diethyldithiocarbamate) and Pyridine in Benzene

The fast Lewis acid–base reaction between copper(II) bis(diethyldithiocarbamate) and pyridine[Formula: see text]has been studied from 280 to 320 °K in benzene. The method employed is the analysis of electron paramagnetic resonance line positions and line widths using the solutions of the modified Bloch equations in the fast exchange limit. Arrhenius plots, over the temperature range employed, gave values for the thermodynamic data of the process.

Multi-site chemical exchange by nmr

The application of a recent multi-site chemical exchange formulation of the Bloch equations is made to an intramolecular and an intermolecular process. The experimental precautions to achieve a good complete line shape lit are discussed in some detail. The rates of internal rotation about the C—N bond in N,N-dimethylcarbamyl fluoride have been measured, and the relative signs of the long range couplings H—C—N—C—F 4J proton to fluorine are obtained from the exchange modified spectra as well as from the comparison of the 4J measured in the fast and slow exchange limits. A 4-site rate matrix is defined with one adjustable parameter for the spectral fit. The measurements are especially difficult because of the small chemical shift varying between 2.2 Hz and 1.4 Hz over the range of interest. The long range coupling methyl to fluorine are 0.8 ± 0.05 Hz trans and 0.3 ± 0.05 Hz cis and do not vary with temperature within 0.05 Hz for a 16.5 % solution in carbon tetrachioride. The intermolecular exchange of halogens in the dimethyltin dihalide mixtures is formulated in detail. A 15 site exchange process breaks down into 5 x 3 exclusive exchange groups and providing line positions and intensities are known with the line width in the absence of exchange, the whole spectrum can be reproduced using only 2-site rate matrix elements. Representative experimental and computed spectra are presented. The relationship of the matrix elements to chemical rate constants is derived but determinations of these constants are not yet complete. INTRODUCTION The study of chemical exchange rates through the modified transverse magnetization is well understood and the theory is highly developed (for example references 1—8). Some simple aspects of chemical exchange theory in nmr spectra for first order cases have been emphasized in a recent paper by Reeves and Shaw9. For intramolecular exchange in the first order limit, the determination of relative signs of coupling constants to non-exchanging nuclei influences the line shapes in a readily recognizable manner'°' Exchanges among many sites (Larmor frequencies) of arbitrary population, saturation effects and site-dependent relaxation times have been accommodated in this recent theory9. A single matrix diagonalization enables sub.tL.W.R. thanks the NRCC for financial support in the form of operating and capital equipment grants.

Homogeneous and heterogeneous electron exchange in the reduction of aromatic hydrocarbons

Homogeneous electron exchange rates, determined by a modified e.s.r. method in both the slow and fast exchange limits, are compared with published heterogeneous rates at mercury for aromatic hydrocarbon reduction in dimethylformamide; the ratio is approximately constant. This constant can be derived from theories of Marcus and Hush on a model in which image forces are negligible in the electrode process and homogeneous exchange occurs only on contact. Potential-step methods are used to study the source of electrons transferred to radicals formed by protonation of the hydrocarbon radical anions (e.c.e. mechanism) : electron transfer occurs from other radical anions for anthracene and from the electrode for naphthalene. This result is interpreted in terms of relative rates of cross-exchange reactions as predicted by Marcus theory from the “isotopic” exchange rates and redox potentials of the systems. General considerations imply that cross-exchange reactions will predominate in many aromatic systems. Proton addition from phenol to the radical anions appears to involve direct contact of acid and base rather than the free (solvated) proton.

Errors in the Measurement of Electron-Transfer Rates by Means of Electron Spin Resonance

Rates of electron-transfer reactions can be conveniently determined from linewidths of electron spin resonance spectra in the limit of fast exchange by using an equation derived by Piette and Anderson (PA). Here the range of validity of this equation is investigated by a comparison of its predictions with exact linewidths obtained by numerical calculations. A correction term, depending on the second and fourth moments of the uncollapsed spectrum, is also derived which extends the range of validity of the PA fast-exchange equation. The ratio of the experimental linewidth to the square root of the second moment is a good indicator of the fast-exchange limit. This ratio must be less than 0.2 for the uncorrected fast-exchange equation to hold within 5%.

ESR studies of electron transfer: trans-stilbene-stilbene-system.

The rate of electron transfer between the stilbene anion radical and trans-stilbene has been measured in dimethoxyethane and tetrahydrofuran. The second-order rate constants are approximately 109 liters mole−1·sec−1, and the activation energies fall between 2 and 3 kcal mole−1. In all cases the rates were measured from the ESR linewidths at the fast-exchange limit. Also, it is shown that the required second moment of the unbroadened spectrum can be determined with sufficient accuracy directly from the unresolved experimental spectra.

Linewith variations in the electron resonance spectra of transition metal ions undergoing ligand exchange

The electron spin resonance spectra of solutions containing two differently coordinated complexes of a transition metal ion show line-broadening if interconversion of the two species occurs at the critical rate. The relaxation matrix formulation is used to discuss the lineshapes for spin ½ systems. The result, for the exchange broadening in the fast exchange limit, is the same as that obtained from the modified Bloch equations, provided that the rotational correlation times are much shorter than the lifetimes of the individual complexes.