Cynthia J. Jameson

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

Abstract 13 C chemical shifts have been measured relative to 13 CO in the zero-pressure limit for over twenty molecules for which theoretical calculations of 13 C nuclear shielding have recently been reported. Rovibrational averaging effects on the spin-rotation constant in 13 C 16 O have been used to find σ e ( 13 C in 13 C 16 O) = 3.0 ± 1.2 ppm and σ 0 ( 13 C in 13 C 16 O) = 1.0 ± 1.2 ppm. With the latter, the σ 0 values for the 13 C 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.

Calculation of Chemical Shifts. I. General Formulation and the Z Dependence

Explicit expressions for the paramagnetic contribution σ(2) to the nuclear magnetic shielding are derived in the valence bond and the LCAO—MO framework including d as well as p orbitals on the atom in question. A survey of published experimental data reveals a periodic dependence of the range of chemical shifts on atomic number, which is explained in terms of the paramagnetic contribution to the chemical shift and its dependence on 〈1/r3〉 for the bonding electrons.A brief discussion is given of related but more complex periodicities in the electron coupling of nuclear spins, using the M—H coupling in Group IV hydrides as an example. It is suggested that the anisotropy in the nuclear shielding and internuclear coupling tensors may combine to give observable linewidth differences in the high‐resolution NMR spectra of directly bonded nuclei of large Z.

Density Dependence of 129Xe Chemical Shifts in Mixtures of Xenon and Other Gases

Unlike other chemical shifts in gaseous systems which have been found to have strictly linear dependence on density, we have found the 129Xe chemical shift in pure xenon gas to have a quadratic and cubic dependence in addition to the dominant linear dependence on density. This implies the importance of three or more body interactions in xenon. In mixtures of xenon with another gas (Ar, Kr, CO2, HCl, CH4, CH3F, CH2F2, CHF3, CF4), the dependence of the 129Xe chemical shift on the density of the other gas is found to be linear within experimental error, and varying from 2300–11 700 ppm/mol cc−1. These shifts are orders of magnitude greater than the reported H and F shifts in gases. Analysis of the results show that the density dependence cannot adequately be reproduced by the contributions, Σ1 = Σb − B〈e2〉 − B〈F2〉, which had been adequate for H and F shifts. The general formulation for calculation of the A and B parameters, the coefficients of the linear and quadratic electric field terms in the theory of ch...

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.

Temperature and density dependence of 129Xe chemical shift in xenon gas

Studies of density dependence of 129Xe chemical shift in xenon gas at room temperature have shown that while the chemical shielding does have quadratic and cubic dependence on density over densities up to 250 amagat, σ(ρ, T) = σ0 + σ1(T)ρ + σ2(T)ρ2 + σ3(T)ρ3, the curve is essentially linear up to about 100 amagat. We have now obtained σ1(T), the linear density coefficient of chemical shielding, for pure xenon over the temperature range 240–440 °K. The experimental values of σ1(T) can be fitted by a fourth degree polynomial: σ1(τ) = 0.536 − 0.135 × 10−2τ + 0.132 × 10−4 τ2 − 0.598 × 10−7τ3 + 0.663 × 10−10τ4 (ppm/amagat), where τ = T − 300 °K. Comparison is made with σ1(T) for other nuclei and with σ1(T) predicted by various theoretical models.

Absolute shielding scale for 31P from gas-phase NMR studies

Abstract Differences in the 31 P nuclear shielding in the zero-pressure limit have been measured in seven compounds. An absolute 31 P shielding scale based on the PH 3 molecular beam data is established and the absolute shielding of the standard liquid reference (85% aqueous H 3 PO 4 ) is found to be 328.35 ppm, based on PH 3 being 594.45 ± 0.63 ppm. Comparisons with ab initio calculations show that calculations using local origins (the IGLO method) are in good agreement with experiment.

Nuclear spin relaxation by intermolecular magnetic dipole coupling in the gas phase. 129Xe in oxygen

The nuclear spin relaxation times (T1) of 129Xe in xenon–O2 gas mixtures have been measured as a function of temperature and density at different magnetic fields. This system is used to characterize the intermolecular dipolar relaxation of nuclear spins in the gas phase. An empirical Boltzmann‐averaged collision cross section associated with the collision‐induced transitions between 129Xe nuclear spin states is obtained as a function of temperature.

Theoretical Aspects of Isotope Effects on Nuclear Shielding

Publisher Summary This chapter presents the theoretical basis for the interpretation of isotope shifts in NMR. The interpretation of isotope shifts largely involves consideration of the vibrational and rotational averaging of nuclear shielding. Therefore, the isotope shift is intimately related to the observed temperature dependence of nuclear shielding in the gas phase in the zero-pressure limit. The theory considered essentially provides a good account of generally observed trends along with some of the more specific correlations with electronic structure and with other molecular properties. Isotope shifts are because of rovibrational effects and can be described by an electronic factor multiplied by a dynamic factor. The nearly uniform sign (negative) that is observed in one-bond isotope shifts comes from the nearly uniform sign (negative) of the electronic factor combined with the widely observed shortening of the average bond length upon heavy isotope substitution. The dynamic factor largely depends on the masses of the atoms involved in the bond. The mass factor confirms the observed dependence of isotope shifts on the fractional change in mass. It favors the observation of isotope shift upon substitution of light atoms while observing heavy nuclei.

The isotope shift in NMR

The isotope shift observed in NMR is formulated in general for the diatomic molecule. The results are extended to polyatomic molecules and to a linear triatomic molecule in particular. General empirical observations which have been made on isotope shifts are explained in terms of the theoretical formulation given here.

Systematic Trends in the Coupling Constants of Directly Bonded Nuclei

The indirect coupling constant JXN has been observed for the magnetic nuclei in 50 different pairs of directly bonded X–N atoms. A synopsis is given of the reported values along with the corresponding reduced constant KXN = (2π / ℏγXγN)JXN which depends only on the molecular electronic structure. There are three nuclei, N=1H, 13C, and 19F for which KXN is now known for 15 or more different nuclei X, enough that trends are visible in the dependence of KXN upon the position of X in the periodic table. The sign of KXN(positive for H2) changes across the table somewhat between Groups V and VI, the sense of the change for N=19F being the reverse of that for N=1H and 13C. Furthermore, there is a marked increase in the magnitude of KXN with increasing nuclear charge of atom X in each Group, for negative as well as positive coupling constants. The significance of these observed trends is considered. The Ramsey theory for the electron coupling of the nuclear spins includes orbital, spin‐dipolar, and contact contri...