Slow Pulse Modulation in Nuclear Magnetic Double Resonance. Indirect Determination of the NMR Parameters of 13C

Abstract

A new experimental technique is described whereby the weak 13C satellite lines of high-resolution proton magnetic resonance spectra are selectively detected and the remaining strong proton lines effectively suppressed, making possible the retrieval of previously obscured information about spin coupling between 13C and 1H. The detection system is sensitive only to those features of the proton spectrum that are perturbed by the application of a second rf field w2 near the 13C resonance frequency. The NMR signal is periodically inverted in synchronism with a 0.25-cps pulse modulation of w2, and integrated over at least one full period, by the use of the addition and subtraction modes of a time-averaging computer or low-frequency synchronous detector. Experimental evidence is presented that the proton magnetic resonance response to the modulation is quite rapid (rise time of the order of 0.7 sec) and is probably determined by magnetic field inhomogeneities and not by the 13C relaxation times. An analogy is drawn with the pulse experiments which were performed by Torrey in 1949. The technique has been extended to investigate 13C resonance spectra while benefitting from the much higher inherent sensitivity of proton magnetic resonance. The proton resonance frequency of one of the 13C satellites is continuously monitored while a pulse-modulated weak rf field is swept through the 13C region. Modulation is detected whenever w2 passes through a 13C transition that shares an energy level with the monitored proton line. In this way the 13C precessional frequencies of the carbonyl group in acetic acid and in acetone have been measured and related to the proton resonance frequency of tetramethylsilane in the same magnetic field with an accuracy of about ±2 parts in 108. These methods have been applied to the study of the inner 13C satellite spectrum and the 13C spectrum of 1,3,5-trichlorobenzene, and indicate that J(HH, meta), J(13CH) and J(13CCCH) all have like signs, a determination that has some bearing on the question of absolute signs of proton—proton spin coupling constants.