Absolute Shielding Scale Research Articles

Cobalt-59 Nuclear Magnetic Relaxation Studies of Aqueous Octahedral Cobalt(III) Complexes

Variable-temperature 59Co nuclear magnetic longitudinal relaxation and line width measurements have been performed on dilute aqueous solutions of 12 octahedral cobalt(III) complexes spanning a large range of isotropic chemical shifts (−1−12 570 ppm) at two fields (4.7 and 11.7 T). A variable-concentration 59Co longitudinal relaxation study of Na[Co(ethylenediaminetetraacetate)] was performed, and the results of this study indicated a need for the use of dilute solutions. The quadrupolar mechanism of relaxation was found to be dominant for 59Co nuclei in diamagnetic octahedral complexes in aqueous solution, and values of the quadrupolar coupling constants in all 12 complexes have been determined. At temperatures above 323 K, the spin−rotation relaxation mechanism contributes to the overall rate of relaxation for 59Co in cobalt(III) complexes with high local symmetry or large moments of inertia. Previous claims that the chemical shielding anisotropy mechanism of relaxation is also a major contributor to the total 59Co relaxation in cobalt(III) complexes are apparently unjustified. We believe that the previous incorrect conclusions were due to concentration-dependent relaxation behavior that we and others have recently observed. An approximate absolute chemical shielding scale for cobalt is also presented.

Absolute shielding scale for 29Si

Simultaneous T 1 measurements for 29Si and 1H or 19F in SiH 4 and SiF 4 gas lead to spin-rotation constants for 29Si which provide shielding values σ 0( 29Si, SiH 4) = 475.3 ± 10 ppm and σ 0( 29Si, SiF 4) = 482 ± 10 ppm. The absolute shielding in Me 4Si liquid reference is 368.5 ± 10 ppm, with which 29Sichemical shifts can be converted to absolute shielding. We find that the 1H absolute shielding scale, the 19F absolute shielding scale and the two completely independent determinations of the 29Si shielding scale are all in agreement.

Gas-phase 13C chemical shifts in the zero-pressure limit: refinements to the absolute shielding scale for 13C

13C chemical shifts have been measured relative to 13CO in the zero-pressure limit for over twenty molecules for which theoretical calculations of 13C nuclear shielding have recently been reported. Rovibrational averaging effects on the spin-rotation constant in 13C 16O have been used to find σ e( 13C in 13C 16O) = 3.0 ± 1.2 ppm and σ 0( 13C in 13C 16O) = 1.0 ± 1.2 ppm. With the latter, the σ 0 values for the 13C nuclei in this work have been determined absolutely and compared with calculated values. Agreement is generally good in most cases except where low-lying n → π ★ transitions contribute significantly to the paramagnetic shielding.

A more reliable oxygen-17 absolute chemical shielding scale

For the first time, oxygen-17 chemical shifts together with experimental details are reported for CO, CO2, OCS, N2O, and OF2. An absolute 17O shielding scale, based on a recent redetermination of the 17O spin-rotation constant in C 17O is proposed: σ (C 17O)=−42.3±17.2 ppm. Absolute shielding constants derived from 17O NMR chemical shifts are compared with those calculated from recent molecular orbital studies. Also scalar coupling constants involving 17O are reported for OF2 and CO. In OF2, J(F,O)=300±30 Hz; in carbon monoxide, J(O,C) is less than 5 Hz.

An approximate absolute 33S nuclear magnetic shielding scale

Sulfur-33 chemical shifts have been measured for a number of simple compounds, e.g. SF6, HFSO3, X2SO2, SO2, Cl2SO, CH3SH, H2S, and OCS, which have not been previously reported. Our results indicate that the range of 33S chemical shifts is approximately 1000 ppm, slightly less than that observed for 17O and approximately one half the range reported for 77Se. Using the 33S nuclear spin-rotation constant of OCS measured by molecular-beam electric resonance and Flygare's procedure, an absolute 33S shielding constant of 843 ± 12 ppm is calculated for OCS, permitting an approximate absolute shielding scale to be proposed for this nucleus.

15N nuclear magnetic shielding scale from gas phase studies

We have measured the 15N nuclear magnetic resonance frequencies in 15N-labeled molecules (NNO, NNO, NH3, N2, and HCN) in gas phase samples and also in CH3NO2 as neat liquid. By using the previously determined temperature dependence of samples of the these gases at various densities, we are able to reduce the measured frequencies to the zero-density limit at 300 K, and obtain shielding differences between rovibrationally averaged isolated molecules at this shielding measurements from molecular beam studies to provide an 15N absolute shielding scale based on 15NH3.

Natural abundance nitrogen-15 nuclear magnetic resonance. Liquid nitrogen

Following the estimation by Ramsey of the absolute value for the paramagnetic contribution (σ_p) to the absolute shielding of ^(15)N_2 relative to the bare nucleus ^(15)N, considerable interest has been expressed3 in establishing an absolute shielding scale for ^(14)N (and ^(15)N) nuclear magnetic resonance. This requires ^(15)N chemical shifts referenced to the ^(15)N_2 resonance via some secondary standard, usually ^(15)NH_4^+ or ^(15)NO_3^- in aqueous solution. To date, two measurement of the resonance position of ^(14)N_2 have been published as 14 and 70 ppm upfield from ^(14)NO_3^-, but doubts have been expressed about the accuracy of these values. One source of uncertainty in the reported chemical shifts arises from the line widths of the ^(14)N resonances. From the relaxation time data for liquid ^(14)N_2 at 77oK of Armstrong and Speight, we estimate the width at half height of the ^(14)N resonance to be ca. 30 Hz (9.5 and 7 ppm at 10 and 13 kG, respectively.

Rotational Diffusion and Magnetic Relaxation of 119Sn in Liquid SnCl4 and SnI4

Longitudinal and transverse relaxation times of 119Sn have been measured as a function of temperature in liquid SnCl4 and SnI4. For SnCl4, T1 is a monotonically decreasing function of temperature and is dominated mechanistically by spin-rotation over the entire liquid range. In SnI4, T1 passes through a maximum near 190°C. Spin-rotation and scalar coupling to 127I, respectively govern the relaxation above and below this temperature. T2≪ T1 in both liquids; scalar coupling, modulated by relaxation of the halogen isotopes, dominates the transverse relaxation. Dipolar coupling does not contribute appreciably to relaxation in either compound. Knowledge of the scalar contributions to T1 and T2 in SnI4 permit calculation of the 127I relaxation time [τ127=1.5(10−7) sec at 150°C] and the angular correlation time [τθ=3.67(10−12) sec at 150°C]. These values, and the published τ35 for SnCl4, give the tin-halogen scalar coupling constants: J(119Sn–35Cl)=470 Hz and J(119Sn–127I)=940 Hz. Spin-rotation constants are obtained using Steele's rotational diffusion theory and are used to calculate an absolute shielding scale for 119Sn. From the shielding constants and the relative resonance frequencies of 119Sn and 1H, the 119Sn magnetic moment is calculated to be (−1.04347±0.00036μN). Rotational correlation times in SnCl4 and SnI4 have been compared with theoretical predictions of the J diffusion and damped diffusion models. SnCl4 shows significant deviations from Hubbard's relation although the motion is diffusive according to both theories. Inertial effects are important for reorientation in SnI4(τJ*∼ 2 at 220°C), and damped diffusion appears to describe reorientation more accurately at these temperatures than does J diffusion. The relative roles of spin-rotation, scalar coupling, and dipolar coupling as relaxation pathways in other tin-containing systems are considered. It is concluded that the first two interactions usually dominate dipolar coupling for tin and for other heavy metals.

Comparison of 31P Spin—Rotation Interaction Constants Derived from Chemical Shift Data and from NMR Relaxation and Viscosity Data

The complete rotational diffusion tensors of the three symmetric top molecules PCl3, PBr3, and POCl3 are obtained from viscosity and halogen NMR relaxation time data. These tensors allow us to obtain estimates of the average correlation times for each molecule, which in turn are used in Hubbard's isotropic spin—rotation equation to derive NMR values of their spin—rotation interaction constants Ceff. These and similarly derived values of Ceff for a number of other 31P compounds are compared to values of the spin—rotation interaction constant Cσ deduced from an absolute shielding scale based on a recent molecular beam determination of the 31P spin—rotation interaction tensor in PH3. In contrast to previous work the values of Cσ are in reasonable agreement with the values of Ceff, therefore offering evidence for the validity of Hubbard's relationship. Earlier discrepancies between Cσ and Ceff appear to be mainly due to poor estimates of the average correlation times, although effects of anisotropy appear to contribute slightly. The results of this paper offer evidence that the so-called microviscosity approach should be the favored method for estimating average reorientational correlation times in complex rigid molecules and correlation times for reorientation perpendicular to the symmetry axis in symmetric top molecules whenever direct quadrupolar or dielectric relaxation times are not available. The derived rotational diffusion tensors for PCl3, PBr3, and POCl3 are used to discuss a recent extension to Hubbard's isotropic spin—rotation equation which takes account of the anisotropic rotational motions in symmetric top molecules. In favorable circumstances this approach can be used to calculate the entire spin—rotation interaction tensor for a nucleus in a symmetric top molecule.

Nuclear Magnetic Shielding in F2

The 19F chemical shift between gaseous F2 and SiF4 has been measured to be − 596.0 ± 0.2 ppm at 30°C. With a recently established absolute shielding scale for 19F this value yields an absolute shielding in F2 of − 232.6 ppm. Combination of this with the measured spin–rotation constant leads to a separate determination of experimental values for σFd and σFp, with arbitrary choice of gauge origin, which are then compared with available theory. An estimate of the vibrational effects is given.