Variation of chemical shielding with intermolecular interactions and rovibrational motion. VI. 19F in SF4 and COF2

Abstract

The 19F nuclear resonance frequency in SF4 and COF2 molecules were measured as a function of density ρ and temperature T in the gas phase. In the virial expansion σ(T, ρ)=σ0(T)+σ1(T)ρ+..., the effect of intermolecular interactions on nuclear magnetic shielding is given by σ1(T) for low densities, and the average of the shielding over vibration and rotation is observed as σ0(T)–σ0(300 K). For SF4, the measurements are made in the fast-exchange limit, so that the average over the Berry-pseudorotation type exchange is observed. The average shielding terms are as follows: [σ0av(T)−σ0av(300 K)] =−4.332×10−3(T−300)−1.3465×10−5(T−300)2+1.623×10− 8 (T−300)3 ppm for COF2 and −1.297×10−2(T−300)−7.047×10−6 (T−300)2 ppm for SF4. σ1(T)=−(1.987±0.14)×10−2 +8.907×10−5 (T−300) −7.983×10−7 (T−300)2 ppm amagat for COF2 and −(8.35±1.5)×10−3 ppm/amagat for SF4. These functions are defined over the temperature range 230–380 K for COF2 and 300–380 K for SF4.