Spin echo experiments on 13C, 2H, 1H, and 19F in some small molecules in the liquid phase

Abstract

The dependence on the π pulse repetition rate (2 τ) −1 of the Carr-Purcell (CP) spin echo decay constant R is studied for four nuclei in C 6H 6, C 6D 6, C 6F 6, C 6H 12, C 6H 5CH 3, CH 3I, H 2O, D 2O, and CS 2. Both deuteron resonances, the proton resonances of CH 3I, of extremely pure H 2O and C 6H 6 and the 13C resonance of CS 2 yield straight lines when R is plotted vs. (2 τ) 2, i.e., R is governed by transverse relaxation and diffusion. However, in some unexpected cases, T 2 is found to be smaller than T 1. The H and F resonances of C 6H 6, C 6F 6, and H 2O with traces of impurities do not give straight-line plots of R vs. (2 τ) 2. An oscillatory dependence of R on the pulse repetition rate is found for the 13C resonances of C 6H 6 and C 6H 5CH 3. It can be shown to be due to the J coupling of the 13C spins to the directly bonded protons. The theory developed for exchange of chemically shifted spins can be applied and is extended for the slow exchange limit to an AX 3 system in an effort to explain the results of methyl 13C quantitatively. Because of the sensitivity of CP measurements on instrumental effects a detailed description is given of the measurement procedures and of the equipment of which a superconducting solenoid is an essential part. A connection between Carr-Purcell measurements, of the Gill-Meiboom version, and spin-locking experiments is pointed out.