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.

In CF2=CFX (X=H, Cl, Br, I) there are three probe nuclei for studying the magnitudes of intermolecular effects on nuclear shielding. The values of σ1=limρ→0 (∂σ/∂ρ)T, the change in the nuclear shielding due to interactions between pairs of molecules, have been obtained from the resonance frequencies in medium to low density gas samples. The nuclear site effect gives the most exposed one of the three probe nuclei the largest magnitude of σ1, i.e., the F trans to X=Cl, Br, I or the F cis to X=H. The gas-to-liquid shifts show exactly the same ordering as the σ1 in the dilute gas. A model is described which relates a calculated site factor to the observed σ1 values. With this model, the observed density dependence of the NMR isotope shift in D2/HD is calculated. This model provides a mechanism for nonspecific solvent effects on the NMR isotope shift.

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.

Articles you may be interested inVariation of nuclear magnetic shielding with intermolecular interactions and rovibrational motion. IX. 1 9F in F2C=CH2 and F2C=CF2 Variation of chemical shielding with intermolecular interaction and rovibrational motion. V. 15N in N2Variable density and temperature studies offtuorine-19 nuclear magnetic resonance spectra in the title molecules reveal the effects of intermolecular interactions, quantitatively expressed as a second virial coefficient of nuclear shielding 0'1 (T) and the effects of rovibrational averaging, expressed in terms of the temperature dependence of shielding in the molecules in the zeropressure limit O'o(T}. The quantities are reported as quadratic functions of temperature and compared with those of other molecules. In the four series of molecules: (a) CFH 2 CI, CFH 3 , IdO'oIdT I has been found to decrease with H substitution and increase with CI substitution. Gas-to-liquid shifts are also reported.

We have observed the temperature dependence of the 1H, 13C, and 15N NMR spectra in HCN in the liquid phase and also in the saturated vapor. Concentrations observed include 0.05 to 0.30 mol/l for the vapor and 21–27 mol/l for the liquid at temperatures ranging from 230 up to 380 K. The gas-to-liquid shifts and the chemical shift between the dimer and the monomer have been determined for 1H, 13C, and 15N nuclei. A dichotomy in the 15N shifts in nitrogen nuclear environments capable of hydrogen bonding is noted. 15N in HCN appears to be an archetype of nitrogen nuclear environment which is in the same class as pyridine and acetonitrile (and which behaves in a manner opposite to that of 15N in NH3 and saturated amines), such that the effect of intermolecular interactions is to lead to an unusual increase rather than the usual decrease in shielding observed for all other nuclei.