The H218O Solvent-Induced Isotope Shift in 19F NMR

• Published in: Journal of magnetic resonance (1997) Vol. 142; no. 1; pp. 1 - 10
• Main Authors: Arnold, John R.P, Fisher, Julie
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.01.2000
• Subjects: 18O; 19F; isotope shift; solvent; 18O; solvent; isotope shift; 19F
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1006/jmre.1999.1931

The H216O/H218O solvent-induced isotope shifts (18O SIIS) of the 19F NMR signals of a number of fluorine compounds have been measured. These isotope shifts are observed to be upfield, downfield, or zero, depending on the specific compound and the precise solution conditions. At 25°C and with an 18O enrichment of 86%, the 18O SIIS of several fluorinated amino acids were in the range of 0.0014–0.0018 ppm downfield. 5-Fluorouridine displays a significantly wider range of 18O SIIS values. A 5-fluorouridine-labeled 16-mer RNA also displayed observable 18O SIIS values, but the characteristics of these were significantly modified from those of free 5-fluorouridine. The experimental observations are consistent with the 18O SIIS being composed of upfield and downfield components, with the relative contributions of these determining the size and direction of the overall isotope shift. This is discussed in terms of a combination of van der Waals interactions between the fluorine atom and the solvent, electrical and hydrogen bonding effects, and the perturbations to these due to 18O substitution in the solvent water. This isotope effect promises to be a highly useful tool in a range of 19F NMR studies.