Jean-Claude Boubel

Self-diffusion measurements using a radiofrequency field gradient

Abstract Self-diffusion coefficient measurements have been carried out by means of a novel and efficient NMR technique, which employs the gradient of an RF field produced by a single-turn coil. The sequence involves also pulses of a homogeneous RF field and has the form δ(g) x −( Δ 2 )−180°(h) y −( Δ 2 )−δ(g) x −90°(h) x -Acq., where h refers to the homogeneous RF field whereas g denotes the application of the RF field gradient. The theory of the classical PGSE (pulsed field gradient spin echo) technique (which makes use of a static field gradient) is reconsidered according to a simplified approach. This shows that similar equations prevail for the sequence using RF field gradient pulses. A probe has been designed in order to obtain a linear RF gradient across a 10 mm o.d. sample. A conventional saddle-shaped coil orthogonal to the single-turn coil produces the homogeneous RF field. It is doubly tuned at proton and deuterium frequencies and therefore allows stabilization of the spectrometer in the course of the experiment. The use of an RF field makes it possible to determine directly the magnitude of the gradient by a capillary whose position is varied. The method has been checked on water, benzene, and acetone samples at room temperature, with self-consistent results, in agreement with literature data. The proposed method permits measurement of self-diffusion coefficients with good accuracy (error less than 5%) in a much simpler way than with pulsed B0 gradients.

Automatic intensity, phase, and baseline corrections in quantitative carbon-13 spectroscopy

Abstract Calibration curves are shown for intensity and phase corrections in FT NMR. Algebraic expressions are proposed for a theoretical simulation of these curves and for digital intensity, phase, and baseline corrections of the spectrum, as well as automatic procedures to recognize signals free from any dispersion component and thus to determine convenient phase angle values. Digital corrections are shown to be necessary for obtaining a high level of accuracy in measuring integrated signals areas; e.g., the uncertainty range in determining the aromaticity factor of oil samples was reduced from ca. 4 to 1% in this way.

Un exemple de relaxation nucléaire controlée par un échange chimique: la solvatation du cation Mn2+ dans le diméthylsulfoxyde par RMN du proton

Proton relaxation times of dimethylsulfoxide containing 10−2–10−1 M manganese (II) perchlorate are measured at two frequencies, 8 and 60 MHz, and sixteen temperatures (20–165 °C). These data allow us to compute: the contact shift ΔωM = 748 ± 13 Hz (25 °C and 60 MHz); the hyperfine coupling constant; the kinetic parameters for the exchange: bound DMSO bulk DMSO: kM = (6.3 ± 2.3) × 106 s−1; ΔH≠ = 7.4 ± 0.5 kcal mol−1; ΔS≠ = −2.4 ± 0.5 u.e (25 °C), the rotational correlation time: τC = (9.65 ± 0.63) × 10−11 s and the distance between protons and Mn2+ cation: r = 4.37 ± 0.05 A (25 °C); the electronic relaxation time: 1/T1e = (5.0 ± 1.0) × 107 s−1. These results describe the geometry and mobility of the first solvation shell and are compared with other examples from the literature.

Determination of the rotation-diffusion tensor orientation from NMR 13C-1H cross-relaxation rates

Taking advantage of the fact that α,α,2,6 tetrachlorotoluene possesses only one symmetry element (the aromatic ring plane), it proved possible to measure seven different 13C-1H cross-relaxation rates which enable one to determine the three rotation-diffusion coefficients (Dxx , Dyy , Dzz ), in addition to the orientation of the relevant principal axis system (PAS) with respect to a chosen molecular axis system. It turns out that molecular reorientation is strongly anisotropic and that the rotation-diffusion PAS cannot be directly correlated with electrical molecular properties.

Slow motions undergone by surfactant molecules in micelles. A 1H and 14N nuclear magnetic relaxation study

Abstract Surfactant motion in spherical micelles of the system cetyltrimethylammonium bromide/D 2 O has been investigated by 1 H and 14 N longitudinal relaxation at different frequencies. Such measurements allow extraction of the correlation time characterizing the overall reorientation of the surfactant molecule, which includes micelle tumbling and lateral diffusion around the micelle. The proton data, which reflect the alkyl chain mobility, require the definition of a local director, distinct from the normal to the aggregate surface, thus making possible the occurrence of an additional slow motion. Conversely 14 N data can be analyzed accord- ing to the classical two-step model; this yields a correlation time associated with the slow motion of as ≈5 ns leading to a value of 4× 10 −7 cm 2 s −1 for the lateral diffusion coefficient.

Paramagnetic ion binding to amino acids, structural and dynamical information on a Mn(II) complex of L-proline from 13C relaxation data

Abstract The 13C relaxation times (T1 and T2) and isotropic contact shift (Δω) of 1.28 molar aqueous solutions of L-Proline at pH = 11 (or pD = 11.4) containing 10−4 - 10−5 M manganese perchlorate are measured at 62.86 MHz over a temperature range of 28–80°C. Under these conditions, the Mn2+ cation is bound to three L-Proline molecules in their dibasic form, and a fast exchange is occurring between bound and bulk L-Proline molecules. The longitudinal relaxation of carbons α, β, γ, δ of L-Proline molecules in this complex is shown to be purely dipolar, and is controlled by the rotational reorientation of the complex. The transverse relaxation of bound L-Proline molecules is mainly scalar and is controlled by the electronic relaxation. Overall relaxation rates and paramagnetic shifts also depend on the ligand exchange rate kM (from bound to free sites) at lower temperatures. The measurement of these quantities allow us to determine (i) the structure of the complex: the Mn(II) cation may be positioned with respect to each proline ligand, the sites of coordination are the unchanged nitrogen and one carboxylic atom, the distance to the Mn2+ cation are respectively 2.08 and 1.97 A; (ii) Hyperfine coupling constants: A= + 0.16; 0.08; 0.25 and 0.22 MHz for carbons α, β, γ, δ, respectively. (iii) Electronic relaxation parameters: assuming that T1e ( = 2.18 x 10−8 s at 25°C) is controlled by the modulation of the quadratic crystalline zero-field splitting interaction allows us to estimate the trace of the corresponding tensor: Δ = 0.0305 cm−1, and a correlation time τν(25°C) = 1.32 ps for the impact of solvent molecules against the Mn2+-L-Proline complex (iv) Kinetic parameters for ligand exchange: kM(25°C) = 7.41 x 104s−1; ΔH‡ = 15.6 kcal.mol.−1; ΔS‡ = 16.1 e.u.

The structure and dynamics of the copper(II)-L-proline 1:2 complex in solution

Abstract The 13 C relaxation times ( T 1 and T 2 ) and isotropic contact shifts (Δω) of a one molar aqueous solution of l -proline at pH = 11 (or p D = 11.4) containing ca 10 −4 M copper(II) perchlorate are measured at 62.86 MHz over a temperature range of 26–70°C. The purely dipolar longitudinal relaxation of carbon-13 nuclei contrasting with purely scalar transverse relaxation allowed us to extract carbon-to-metal distances (through T 1 measurements) and hyperfine coupling constants and dynamic parameters (from T 2 and Δω measurements). The structure of the complex in solution is found closely similar to that in the solid state. Curve-fitting procedures allowed us to derive the hyperfine electron—carbon coupling constants A c = −1.95, + 0.42, + 1.90 and −1.70 MHz for carbons α, β, γ, δ, of the pyrrolidinic ring, the reorientation correlation time of the complex, τ R (25°C) = 1.15 × 10 −10 sec, the l -proline exchange rate, k M (25°C) = 4.0 × 10 5 sec −1 (and the corresponding activation parameters Δ H ≠ = 9.0 kcal mol −1 and Δ S ≠ = −0.7 e.u.), and the electronic relaxation time, T 1 e = 1.13 × 10 −8 sec (at 25°C). The latter value was found in agreement with the one computed from ESR data and the above τ R value, showing the predominant contributions of spin—rotation interaction and, to a lesser extent, of the effect of g -tensor anisotropy to the electronic relaxation rate.

Paramagnetic ion binding to amino acids : the structure of the manganese (II)-L-proline complex from carbon-13 relaxation data

Abstract Carbon-13 longitudinal relaxation times T 1 of aqueous solutions of proline at pH=11 containing 10 −4 –10 −5 M manganese(II) perchlorate are measured at 62.86 MHz and 60°C. Under these conditions, the Mn 2+ cation is bound to three proline molecules in their dibasic form to form the complex [Mn(L-PRO − ) 3 ] − . The relaxation of carbons α, β, γ, δ in this complex is shown to be dipolar. The relevant correlation time is rotational τ r =4.3×10 −11 s (at 60°C). A method is given to compute the Mn 2+ - 13 C distances in the complex from the paramagnetic relaxation rates 1/T 1M of carbons α to δ and an assumed geometry of the proline molecule. The manganese (II) cation may be positioned with respect to each proline ligand, thus determining the structure of the hexacoordinated complex. The sites of coordination are the uncharged nitrogen and one carboxylic oxygen atom of the proline molecules, their distance to the Mn 2+ cation are respectively 2.22 and 1.97 a.

Solvent-exchange kinetics in nickel(II) solutions of aqueous tris(dimethylamino)phosphine oxide studied by pulsed phosphorus-31 nuclear magnetic resonance spectroscopy

When small quantities of water are progressively added to solutions of nickel(II) perchlorate in anhydrous P(NMe2)3O electronic and n.m.r. spectroscopy provide evidence for the presence of the six-co-ordinate mixed complex [Ni{P(NMe2)3O}2(OH2)4]2+ at a [H2O] : [Ni2+] mol ratio of ca. 8 : 1. Phosphorus-31 relaxation times of such solutions have been measured at two frequencies (8 and 14 MHz) and five temperatures (8–45 °C). These data yield the 31P hyperfine coupling constant (A)= 17.5 MHz, the electronic relaxation times (T1e and T2e)= 3.91 × 10–12 and 1.86 × 10–12 s (at 25 °C and 14 MHz), the trace of the square of the zero-field splitting tensor (Δ)= 1.96 cm–1, the relevant correlation time (τv)= 6.3 × 10–12 s, and the kinetic parameters for the exchange bound P(NMe2)3O [graphic omitted] bulk P(NMe2)3O, i.e. kM(25 °C)=(2.8 ± 0.4)× 106 s–1, ΔH‡= 25.5 ± 6.3 kJ mol–1, and ΔS‡=–35.5 ± 16.7 J K–1 mol–1. The mechanism of ligand substitution is dissociative. Rates are higher than for identically substituted nickel(II) complexes by about three orders of magnitude. This result is compared with literature data, and accounted for by a lower activation enthalpy, by ca. 25 kJ mol–1, as qualitatively expected from crystal-field theory.

Ligand Substitution Processes II — Solvation Shells of Paramagnetic Cations Ni2+ and Co2+

Nuclear relaxation rates are measured on the following Systems: nickel (II) and cobalt (II) Perchlorates in dimethyl-sulfoxide (DMSO); nickel (II) Perchlorate in hexamethylphosphoro-triamide (HMPA). Application of pulsed nmr to chemical exchange in paramagnetic systems is discussed on these examples. Kinetic parameters are obtained for the solvent substitution reaction around the Ni2+ or Co2+ cations.