The nitrogen-15 shifts of a series of enriched benzamides and benzonitriles have been measured utilizing the Fourier pulse method. A correlation of the nitrogen shifts with Hammett substituent constants has been observed. n-Density changes are shown to contribute to the observed changes in nitrogen-15 shifts within each series.Road, London E.1
In addition to appreciable gains in sensitivity resulting from reduction of long spin-lattice relaxation times, the addition of small amounts of paramagnetic material eliminates the possibility of low or zero intensities arising from partial quenching of the negative nuclear Overhauser effect when relaxation mechanisms other than dipole-dipole interactions are present.VARIOUS workers have recently demonstrated the possibility of observing n.m.r. spectra of the nitrogen-15 isotope in its natural abundance.l Apart from the obvious limitations imposed by the low sensitivity of this nucleus, the results have shown that there are two major problems inhibiting its routine detection. One arises as a result of the long nitrogen-15 spin-lattice relaxation times, particularly notable in cases where there are no protons directly bonded to the 15N nucleus.2 The problem is equally severe when the pulsed-mode is employed, since the small flip angles and/or long delays between successive pulses needed to obtain the free induction decay reduce the TABLE 1. Variation in l5N signal intensities as a function of added paramagnetic for benzonitrile (1) and 4-bromobenzonitrile (2)
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