Sheila M. Cohen

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.

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.

Variation of chemical shielding with intermolecular interactions and rovibrational motion. I. 19F nuclei in BF3, CF4, SiF4, and SF6

The chemical shifts due to rotational and vibrational motion in CF4, SiF4, BF3, and SF6 are obtained by extrapolation of 19F resonance frequencies to zero density. The second virial coefficients of chemical shielding, σ1(T), are obtained at the same time from the slopes. It is found that the σ1 values for these molecules do not depend on temperature nearly as strongly as was previously reported.

Temperature dependence of the 15N and 1H nuclear magnetic shielding in NH3

The temperature and density dependence of the 15N and the 1H nuclear resonance in 15NH3 gas have been observed. The density dependence which is a measure of the effect of intermolecular interactions on the nuclear shielding is linear, with slopes of −0.041±0.002 ppm/amagat for the 15N nucleus and −0.0032±0.0001 ppm/amagat for the 1H nucleus. The shielding in the limit of zero pressure σ0 varies with temperature due to rovibrational motion. This is of special interest for the 15N shielding in NH3 because of the previously reported exceptional temperature dependence of 31P in PH3. It is found that 15N in NH3 is also an exceptional case and, in fact, near 300 K 15N in NH3 and 31P in PH3 have nearly identical values of dσ0/dT(+0.00651±0.00082 ppm/deg for 15N in NH3 from 320 to 380 K). All other cases, involving a variety of nuclei in a wide variety of molecular types, exhibit dσ0/dT<0.

The temperature dependence of chemical shielding in diatomic molecules: CO, F2, ClF, HBr, and HCl

The temperature dependence of chemical shielding in CO, F2, ClF, HBr, and HCl gas has been measured. The functions σ0(T) and σ1(T) in the virial expansion of chemical shielding σ =σ0(T)+σ1(T) ρ+⋅⋅⋅ are reported here for 13C, 19F, and 1H nuclei. The largest shifts with temperature are observed for F2.

Variation of chemical shielding with intermolecular interactions and rovibrational motion. II. 15N and 13C nuclei in NNO and CO2

In the gas phase, the chemical shielding of a nucleus in terms of temperature and density can be written in a virial expansion, σ (T,ρ) =σ0(T)+σ1(T) ρ+σ2(T) ρ2+⋅⋅⋅. σ0(T) and σ1(T) functions have been determined for 13C in CO2 and both 15N nuclei in NNO. As expected, the terminal 15N has the largest chemical shift. The results are compared with data for other systems such as 1H, 19F, 31P, and 129Xe.

Second virial coefficient of 129Xe chemical shielding in mixtures of Xe with spherical top molecules CH4, CF4, and SiF4

129Xe chemical shifts in mixtures of Xe with CH4, CF4, and SiF4 as a function of temperature and density have been obtained by pulsed Fourier transform NMR spectroscopy. The low densities of gases used and the previously determined 129Xe shifts in pure xenon enable the determination of the chemical shielding contribution which is linear in density for Xe–CH4, Xe–CF4, and Xe–SiF4 molecular pairs, with a high degree of precision. An approximate reduced second virial coefficient of chemical shielding for 129Xe in Xe interacting with other molecules can be defined, which leads to superposition of Xe–Xe and Xe–Kr curves over a large part of the reduced temperature range for which data is available. Estimates of well depths for Xe–Ar, Xe–CH4, Xe–CF4, Xe–SiF4, and Xe–HCl have been obtained by this method.

Effect of interactions with nonspherical molecules on 129Xe magnetic shielding

Second virial coefficients of chemical shielding are reported for Xe interacting with HBr, HCl, CO2, N2O, C2H2, C2H4, C2H6, and BF3 from 230 to 440 K. Values range from 0.15 to 0.52 ppm amagat−1. There is some evidence for the anisotropy of the potential function for Xe and these molecules.

Contact interaction between 129Xe and nitric oxide

The chemical shift of 129Xe in nitric oxide gas has been observed as a function of temperature and density of NO. The derivative of the frequency with respect to density gives the second virial coefficient of chemical shielding of 129Xe in nitric oxide, σ1(Xe–NO) = −0.6840 + 1.5516 × 10−3τ − 1.3142 × 10−5τ2 +7.2769 × 10−8τ3 ppm amagat−1, in which τ=T−300, and T goes from 220 to 380 K. The results are compared with the Xe in oxygen gas and the contact contribution to σ1(Xe–NO) is determined. By using Ar as the diamagnetic counterpart of an NO or O2 molecule, empirical values of the integral Fρspine−V/kTdx3 are obtained for the Xe–NO and the Xe–O2 interaction. The empirical values of this integral are greater for the Xe–NO pair than the Xe–O2 pair. Both exhibit a slight temperature dependence.

(Methylenecyclopropane)iron tetracarbonyl complexes: Optical activity, epimerization, and carbon-13 NMR spectra

The synthesis of (diethyl 1-methylene-trans-2,3-cyclopropanedicarboxylate)iron tetracarbonyl in optically active form is described. This complex was oxidized with cupric bromide, and yielded its original trans-Feists ester precursor with full retention of optical activity. The iron tetracarbonyl complex of the cis isomer was epimerized to the trans complex by sodium ethoxide in ethanol. A description of the nature of the bonding between the olefin and the metal in these complexes was derived from carbon-13 magnetic resonance experiments.