Radiation Damping Induced Half-Frequency-Spaced Harmonic Peaks in Two-Dimensional J-Resolved NMR Spectrum on Liquid Water

Overview & Synopsis

In two-dimensional J-resolved NMR spectrum of radiation damped water with the simple pulse sequence of π/2-t1/2-π-t1/2-FID, harmonic peaks spaced by half frequency can either appear or disappear in the indirectly detected dimension, depending on the number of t1 increments and on the sensitivity of probe head. When the experiments are performed with a less sensitive probe having a poor Q factor, the harmonic peaks are hardly observable. The half frequency harmonic peaks observed with a tuned sensitive probe must be due to radiation damping. Theoretically, an inverted, strongly damped magnetization needs an infinitely long time to recover across the xy plane following the radiation damping pathway; while in experiments, it takes less than tens of milliseconds. Therefore, when t1 is long enough compared to the radiation damping time T r, the “echo" is not symmetric about the π pulse with respect to the π/2 pulse, modulating the FID by a pair of angles θ and φ that are complicated functions of both t1 and the frequency offset Δω, and producing the harmonic peaks. If tmax1 is short, the radiation damping effects during t1 are wiped out by the spin echo. Consequently, the harmonic peaks disappear. Experimental results have been exactly simulated based on the radiation damping line shape theory.