A. Keith 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.

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.

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.

Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. I. Distribution and 129Xe chemical shifts

The equilibrium distribution of the Xe atoms among the alpha cages of the zeolite NaA have been measured directly by nuclear magnetic resonance (NMR) in ten samples ranging from very low xenon loading up to saturation. These distributions are simulated by a grand canonical Monte Carlo (GCMC) method which reproduces the experimental data quantitatively for all ten samples at 296 K and also at 360 K. The adsorption isotherm of the high loading samples has been determined directly from the chemical shift of the gas in equilibrium with the adsorbed xenon. The data compare favorably with the adsorption isotherms resulting from the simulations. The previously reported 129Xe chemical shifts of the individual Xen clusters and their temperature dependences in the range 188–420 K are reproduced quantitatively by the GCMC simulation which makes use of pairwise additive ab initio intermolecular shielding functions. These cluster shifts and their temperature dependence encode the distribution of configurations for a g...

Nuclear magnetic resonance studies of xenon clusters in zeolite NaA

We have observed the equilibrium distribution of Xe atoms trapped in the alpha cages of zeolite NaA at 300 and at 360 K for low to high xenon loadings. The experimental distributions obtained by nuclear magnetic resonance (NMR) spectroscopy differ from two previously proposed statistical distributions. The experimental deviations from these statistical models can be explained by the attractive Xe–Xe interactions which favor clustering at low to medium loading, and the higher energies associated with the overcrowded cage disfavoring clusters of eight Xe atoms at high loadings. The temperature dependence of the 129Xe NMR chemical shift of each cluster has been measured in the range 188–421 K, except that for Xe8, which was determined only up to 300 K. The observed shifts and their temperature dependence are interpreted by using the results of ab initio calculations of the intermolecular shielding function in the 39Ar system as a model for the 129Xe system.

Temperature and density dependence of 129Xe chemical shift in rare gas mixtures

Pulsed Fourier transform NMR spectroscopy was used to obtain 129Xe chemical shifts in gases with densities of xenon much lower than ever before observed. At densities of 3–28 amagat, the contamination of the desired linear term in chemical shielding by contributions due to three‐body or higher order interactions is completely eliminated. This allows the determination of very precise values of σ1(T) for 129Xe in xenon, krypton, and argon gas.

The 31P shielding in phosphine

The temperature dependence of the phosphorus shielding in phosphine has been remeasured in the range 300–400 K in samples with densities in the range 7–30 amagat. The shielding surfaces are calculated using the localized orbital–local origin (LORG) method of Hansen and Bouman in terms of the symmetry coordinates for the molecule. These surfaces are used to calculate the rovibrationally averaged 31P shielding. The calculated temperature dependence and the deuterium‐induced isotope shift for phosphine are in agreement with experiment. The shapes of the 31P in PH3 and the 15N in NH3 shielding surfaces are very similar. With the exception of the inversion coordinate, the remarkable similarity of the surfaces becomes obvious when the shielding functions are scaled by the values of 〈r−3〉np for the ground states of the neutral P and N atoms.

Nuclear magnetic shielding of nitrogen in ammonia

The nitrogen shielding surface in ammonia is calculated using the localized orbital‐local origin (LORG) method of Hansen and Bouman, in terms of the symmetry coordinates for the molecule. With respect to the inversion coordinate, the N shielding surface has a shape similar to the potential surface. Rovibrational averaging of the N shielding in NH3 and ND3 molecules is carried out using numerical wave functions which are solutions to the inversion potential which best fits the spectra of all isotopomers. The other coordinates are vibrationally averaged in the usual way, assuming small amplitude motions. The calculated temperature dependence of the N shielding due to inversion is in the opposite sense to that observed for a large number of molecules, and is nearly canceling the contributions from all the other coordinates. The temperature dependence of the nitrogen shielding in ammonia has been measured in the range 300–400 K in samples with densities in a hundredfold range (0.37–33 amagat). When the temper...