Spin-rotation Mechanism Research Articles

Relaxation mechanism of 125Te and 123Te and the temperature effect on chemical shift

Relaxation times T 1 and T 2 were measured in detail for 125Te and 123Te in Te(CH 3) 2 at different temperatures and different fields. The results lead unambiguously to a spin-rotation mechanism. Relaxation is also measured in other different species. A value of about 4 Hz/°C or 0.14 ppm/°C has been found for the chemical shift variation with temperature on different compounds.

Phosphorus relaxation mechanisms in trimethyl phosphite, 2′,3′-cAMP, and 5′-AMP

31P NMR relaxation measurements ( T 1, T 2, and nuclear Overhauser enhancement (NOE)) have been carried out on trimethyl phosphite ((CH 3O) 3P), 2′,3′-cyclic adenosine monophosphate (2′,3′-cAMP), and 5′-adenosine monophosphate (5′-AMP) as a function of temperature and field strength in order to evaluate the relative contributions of various relaxation mechanisms to the total relaxation. In (CH 3O) 3P, the dominant relaxation arises from the spin-rotation mechanism (95%) and a small amount (5%) from the dipole-dipole mechanisms. In 2′,3′-cAMP, chemical shift anisotropy accounts for 60% of the total relaxation with dipolar interactions representing the remainder. In 5′-AMP, the relaxation is accounted for by a combination of spin-rotation and dipolar interactions, influenced by internal motion at the phosphate group. The results suggest that symmetry (or lack of) at the phosphate group plays a dominant role in determining the extent to which the chemical shift anisotropy mechanism contributes to relaxation of phosphorous-containing molecules.

77Se NMR on an XL-100 spectrometer. An investigation of isotope effects and spin-lattice relaxation in H 2Se

The design of a voltage-controlled oscillator (VCO), phase-locked to the 1-MHz output of a commercial frequency synthesizer (General Radio 1061), is described. The output frequency of the VCO, capable of producing either 15.400000 or 15.300000 MHz, has been injected into the lock channel of a Varian XL-100 spectrometer as the deuterium field/frequency lock transmitter frequency. This modification allows “normal” proton-decoupled 77Se FT NMR spectra to be obtained on this spectrometer. Secondary deuterium-induced isotope shifts of more than 7 ppm have been observed in the 77Se NMR spectrum of a solution of H 2−nD nSe ( n = 0, 1, and 2) in deuteriochloroform. Primary and secondary isotope effects on the 77Se- 1H coupling constants are also observed. Also, it is shown that 77Se spin-lattice relaxation in hydrogen selenide is dominated completely by the spin-rotation mechanism.

Nuclear spin relaxation in trimethylphosphine

The 31P and 13C relaxation times of trimethylphosphine have been measured between 166 K and 353 K. The 31P nucleus relaxes through the dipolar mechanism at low temperatures and through a spin-rotation mechanism at high temperatures. The 13C relaxation is almost exclusively dipolar except at high temperatures. Combining of the 13C and 31P relaxation time measurements shows the motion of P(CH 3) 3 to be highly anisotropic at low temperatures, the rotation being faster around the C 3 symmetry axis of the molecule. The variation of the 13C relaxation time with temperature suggests that the molecular motion changes from diffusional to inertial as the temperature increases.

Modular IC tester is easily optimized : William F. Arnold. Electronics p. 155 (5 July 1979)

The design of a voltage-controlled oscillator (VCO), phase-locked to the 1-MHz output of a commercial frequency synthesizer (General Radio 1061), is described. The output frequency of the VCO, capable of producing either 15.400000 or 15.300000 MHz, has been injected into the lock channel of a Varian XL-100 spectrometer as the deuterium field/frequency lock transmitter frequency. This modification allows “normal” proton-decoupled 77Se FT NMR spectra to be obtained on this spectrometer. Secondary deuterium-induced isotope shifts of more than 7 ppm have been observed in the 77Se NMR spectrum of a solution of H2−nDnSe (n = 0, 1, and 2) in deuteriochloroform. Primary and secondary isotope effects on the 77Se-1H coupling constants are also observed. Also, it is shown that 77Se spin-lattice relaxation in hydrogen selenide is dominated completely by the spin-rotation mechanism.

Carbon and phosphorus relaxation in some phenylphosphorus compounds

Carbon and phosphorus relaxation times have been determined for several phenylphosphorus compounds. The relaxation time for the α carbon is dominated by the dipole-dipole mechanism at lower temperature and the spin-rotation mechanism at high temperature. All other carbons in these molecules are relaxed predominantly by the dipole-dipole mechanism, except that at the highest temperatures a spin-overall rotation mechanism becomes important for some. The importance of the spin-rotation mechanism is related to chemical shielding. The chemical shift anisotropy mechanism makes a significant contribution to phosphorus relaxation in most of these compounds. Spin-rotation is also a significant mechanism, while the dipole-dipole mechanism is apparently relatively unimportant in phenylphosphorus compounds.

EPR and optical absorption studies on bis(ethylenediamine) copper(II) perchlorate and bis(1,2-diaminopropane) copper(II) perchlorate

Electron paramagnetic resonance and optical absorption studies have been made on the title complexes in different solvents to study the metal—ligand bond nature and analyse the hyperfine line widths of copper(II). The 4s character in the ground state is found to influence the spin-Hamiltonian constants and bond parameters. The contribution of the spin-rotational relaxation mechanism to the residual line width is found to be considerably less.

Carbon‐13 NMR. The analysis of the relaxation times T1 of benzofuran and of a series of its methyl derivatives. Correlations between molecular motions and structural properties

AbstractCarbon‐13 relaxation times (T1) and nuclear Overhauser enhancements (η) have been measured for benzofuran and a series of its methyl derivatives. The contributions of dipolar (T1 DD) and spin rotation (T1SR) mechanisms have both been determined. The temperature dependence of T1 has been studied. The relationships between molecular motions and structural properties have been emphasized. The overall motional anisotropy of the benzofuran molecule is increased by substitution in positions 2 and 5. The internal rotation of a methyl group may change depending on its position in the molecule and on the influence of other methyl groups in its close neighbourhood.

Methyl NMR relaxation: The effects of spin rotation and chemical shift anisotropy mechanisms

The formalism developed by the author in a previous publication for the calculation of methyl NMR relaxation by intramolecular dipolar interactions is applied to the spin rotation and chemical shift anisotropy mechanisms. The spin rotation and chemical shielding tensors are put into spherical form to allow the Hamiltonians for the above mechanisms to be developed in terms of spherical tensors. Because the calculations are directed towards elucidating the behavior of a methyl group attached to a macromolecule in solution, only the methyl internal rotation is assumed to contribute through the spin rotation mechanism to the overall relaxation. The importance of properly taking the spin rotation contribution into account, even when the dipolar mechanism is dominant, is demonstrated. A simple picture for the occurrence of correlations between the dipolar and chemical shift anisotropy mechanisms is presented, and the relaxation matrices, including the cross terms, are given for these two mechanisms. The forms of the relaxation matrices indicate that the effect due to the cross terms is small, and this contention is supported by calculations performed for a trifluoromethyl group.

Platinum-195 magnetic resonance spectra of some platinum(II) and platinum(IV) complexes

The 195Pt magnetic resonance spectra of fourteen symmetrical, substitutionally inert octahedral Pt(IV) and square-planar Pt(II) complexes have been measured at 9.75 MHz using Fourier transform techniques. Chemical shifts observed in aqueous solutions of the PtL 6 m complexes cover a range of 6033 ppm and the shielding of the 195Pt nucleus is in the order Cl > 1 2 en > Br > SCN > CN > I. For aqueous solutions of the PtL 4 m complexes, the chemical shifts cover a range of 3876 ppm and the order of shielding is CI > NO 2 > NH 3 > Br > 1 2 en > SCN > CN > I. These results are interpreted in terms of covalency effects in the σ p contribution to shielding. Relaxation times T 1 and T 2 have been measured in several cases where the signal to noise was favorable. In addition the temperature dependence of T 1 has been investigated for H 2PtCl 6 and H 2PtCl 4 in aqueous HCl. The relaxation process for these two complexes appears consistent with a spin-rotation mechanism. The solvent dependence of the 195Pt resonance in PtCl 6 2− and PtCl 4 2− has also been measured for solutions tetra- n-butylammonium salts in a variety of nonaqueous solvents. Some proton and 13C NMR spectra have also been obtained for Pt(en) 3 4+ and Pt(en) 2 2+ in aqueous solution and some 13C spectra for Pt(CN) 6 2− and Pt(CN) 4 2− in D 2O/H 2O. Platinum-195 spin-coupling constants have been determined and some of the factors affecting coupling are discussed.

Nitrogen-15 and carbon-13 spin-lattice relaxation in trans-azobenzene and n-butyl nitrite

Spin-lattice relaxation of 15N in trans-azobenzene dissolved in CDCl 3 occurs by a mixture of spin-rotation, dipole-dipole, and chemical shift anisotropy mechanisms. Relaxation of 15N in n-butyl nitrite occurs almost entirely by the spin-(internal rotation) mechanism. In both molecules, 15N relaxation is remarkably fast ( T 1 < 50 sec) for nuclei not bonded to other magnetic nuclei. Whereas the spin-(internal rotation) mechanism maintains its exclusivity throughout the temperature range 5–80°C in n-butyl nitrite, the mix of relaxation mechanisms changes considerably for trans-azobenzene in the same temperature range. Relaxation of the 13C nuclei in both molecules occurs predominantly by the dipole-dipole mechanism. Differences in the relaxation times of the aromatic carbons indicate that molecular rotation in trans-azobenzene occurs preferentially about the long axis of the molecule.

Proton spin relaxation in the halomethanes

T 1 proton relaxation times were measured for the halomethanes (CH 2Cl 2, CH 3,Cl, CH 2Br 2, CH 3,Br, CH 2I 2, CH 3I) at room temperature as a function of concentration in CS2. The data were extrapolated to infinite dilution. Experimental dipolar correlation times were computed from the extrapolated relaxation times assuming the intramolecular dipolar mechanism. In contrast to previous results for the chloroethanes, the experimental correlation times did not increase with increasing halogen substitution for corresponding halides. Temperature-dependent T 1 studies on CS 2 solutions of CH 3Br and CH 2Br 2 showed that the spin-rotation mechanism is an important relaxation mechanism at room temperature. After correctly accounting for the spin-rotation contribution to relaxation, the resulting dipolar correlation times increased with increasing halogen substitution. The experimental results indicated that the methylene halide reorientation can be described by a diffusional model, whereas the methyl halide reorientation can best be described by an inertial model.

31P nuclear magnetic relaxation in solid P 4S 3

P 4S 3 is found to provide an exceptionally clear solid-state example of nuclear spin-lattice relaxation by the anisotropic shift mechanism. At high temperatures the spin-rotation mechanism becomes dominant. The dipolar interaction plays only a minor role.

On the Role of Nuclear Spin Symmetry of Symmetric Top Molecules in Gas Phase Nuclear Magnetic Relaxation

For symmetric top molecules the role of nuclear spin symmetry is included in the derivation of the nuclear magnetic relaxation rate. It is assumed that this is dominated by a spin-rotation mechanism. Of the three nuclear spin species which exist for ZX3Y' symmetric tops, it is found that the "zero frequency" K = −1 to +1 term results in two of the species relaxing with a different rate from the third. The difference, which allows a direct NMR evaluation of the relative values of [Formula: see text] and [Formula: see text], is enhanced at low temperatures.

Cross correlation terms of the spin-rotational interaction in the methyl group

An expression is obtained for the spectral density, which is the Fourier transform of the cross-correlation function of the spin-rotational interactions operating on the different protons in the methyl group. Such a term will appear in the theory of the nonexponential 13C relaxation in the methyl group and might thus give additional information about the spin-rotational relaxation mechanism.

Internal rotation and nonexponential methyl nuclear relaxation for macromolecules

The transverse and longitudinal nuclear magnetizations for an internally rotating methyl group attached to a symmetric top molecule have been calculated; cross-correlation between the motion of two of the methyl protons and motion of the third has been taken into account, by extending an existing treatment by Hubbard to a range of rotational correlation times which are greater than or equal to the reciprocal nuclear Larmor frequency. For macromolecules in liquids, both the transverse and longitudinal magnetizations show a time-dependence which is more nonexponential than is possible for small molecules in liquids, but less nonexponential than for rigid solids. Immediately following a magnetic field pulse to prepare the spin system, the initial magnetization decay is approximated by the result obtained when cross-correlation is completely neglected. Transverse relaxation is more nonexponential than longitudinal, except in the extreme narrowing limit where their behavior is identical. Finally, nonexponential -CH 3 or -CF 3 relaxation in liquids will be difficult to observe experimentally for small molecules because of a competing spinrotation mechanism; the optimum system for experimental detection of nonexponential relaxation should be a hindered methyl group on a large molecule.

Interactions of Cu2+ and Mn2+ with Agarose and Calcium Alginate Gels by ESR

NITROXIDE radicals have been used1,2 as spin labels or probes for the study of biological systems for the past decade. Transition metal ions have received little attention as the spectra are often broad and difficult to interpret. The theory of electron spin resonance (ESR) for transition metal ions, such as Mn2+ and Cu2+, is, however, now sufficiently well understood that they can be used to probe biological systems. A discussion of the ESR theory for Mn2+ ions and its application to micelle and mesomorphic phase systems has recently been given3. Linewidths of Cu2+ ions are determined4,5 by a spin rotational mechanism and modulation of g− and hyperfine tensors by random Brownian motion. We have used Mn2+ and Cu2+ ions to probe the structures of agarose and calcium alginate gel systems and we demonstrate here the kind of information that can be obtained from such studies.

13C relaxation studies: dipolar versus spin rotation mechanisms for camphor

The dipolar and the spin-rotation mechanism are both involved in the relaxation of the 13C nuclei. Their respective contributions are evaluated from the nuclear Overhauser effect in the presence and absence of Cr(acac)3, and confirmed through the study of the temperature dependence of T 1. The molecular rotation appears to be isotropic, and to proceed according to the Debye diffusion mechanism at 296 K.

31P and 1H Spin–Lattice Relaxation in Several Substituted Phenylphosphines in the Liquid State

The temperature dependences of the 31P and 1H spin–lattice relaxation times of dichlorophenylphosphine, chlorodiphenylphosphine, and triphenylphosphine have been measured by the adiabatic fast passage method. Both dipolar and spin–rotation interactions contribute to the 31P relaxation mechanism in dichlorophenylphosphine and chlorodiphenylphosphine whereas only dipolar interactions provide the mechanism for the 1H relaxation in all three compounds studied. As the high-temperature limit of the measurements was 160°C, no spin–rotation interactions were observed for the 31P relaxation in triphenylphosphine. The calculations of the approximate potential functions for the internal rotation about the C–P bond in dichlorophenylphosphine and chlorodiphenylphosphine indicate hindered rotation and thus the spin–rotation mechanism arises mainly from spin–over-all-rotation coupling. Spin–rotation constants were estimated for 31P in dichlorophenylphosphine and chlorodiphenylphosphine from relaxation data and shielding constants. In addition to relaxation measurements, the temperature dependences of densities and viscosities of dichlorophenylphosphine and chlorodiphenylphosphine have also been measured.

Paramagnetic resonance of SO2-radical-ion in solution: Spin-rotational relaxation mechanism

The E.P.R. linewidth of the SO2 - radical-ion in water solution has been studied as a function of temperature and viscosity. A strict dependence of the linewidth on T/η has been observed over the range of temperature and viscosity examined. The analysis of all possible broadening mechanisms shows that the spin-rotational interaction is the only process effective on the linewidth. The theory extended by Atkins and Kivelson [3] to E.P.R. relaxation has been checked quantitatively. From the slope of ΔH versus T/η and the principal values of the g tensor of the SO2 - radical-ion, the value of the hydrodynamic radius is r = 0·91 a. Comparison with the linewidth of the ClO2 isoelectronic radical has been made.