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When a nuclear spin system is subjected to a repetitive sequence of strong radiofrequency pulses, a steady state is established where there is a dynamic balance between the effect of the pulses and spin relaxation. Under certain readily satisfied pulse conditions, the deviation of the intensity of the free induction signal from its thermal equilibrium value is an exponential function of the pulse interval with time constant equal to the spin-lattice relaxation time. The determination is unaffected by spin-spin relaxation provided that the interval between pulses is long enough to permit all transverse components of magnetization to be eliminated, and provided precautions are taken to inhibit spin-echo formation. Through Fourier transformation of the transient response, high resolution spectra with many component resonances may be studied, and the spin-lattice relaxation times of the individual lines determined. The technique lends itself particularly well to repeated accumulation of the transient signal for the purpose of improving sensitivity. It has been applied to the problem of determining the spin-lattice relaxation rates of the eight different carbon-13 resonances in 3, 5-dimethylcyclohex-2-ene-1-one. The results span a range from 2.6 to 39 sec, and are in good agreement with those obtained by applying 180°-1-90° sequences to the same sample. INTRODUCTIONwith the spin-lattice relaxation time, and the free precession signal reaches its thermal equilibrium value, In conventional nuclear magnetic resonance spec-S",. When the signal intensity is studied as a function of troscopy one of the simplest ways to study relaxation the pulse interval, the difference (S",-St) follows an effects is the "progressive saturation" technique, where exponential with time constant equal to the spin-lattice the signal peak height is monitored as a function of the relaxation time, provided certain operating conditions applied radiofrequency field HI. Since the experiment are satisfied. The experiment thus consists of a series of excites a finite transverse component of magnetization, measurements of signal amplitude at different settings spin-spin as well as spin-lattice relaxation is involved, of the pulse repetition rate, with at least one measureand the measurement is commonly used to derive the ment at a pulse separation that is large compared with spin-lattice relaxation time in situations where the all spin-lattice relaxation times, to give the thermal spin-spin relaxation time is known from other experi-equilibrium value. ments, or is known to be equal to the spin-lattice Fourier transformation of the transient free prerelaxation time. The normal procedure is to determine cession signal gives the conventional slow-passage the radiofrequency level which yields the maximum spectrum 2 ,3 so that it is particularly interesting to apply signal peak height; to derive the relaxation information this technique under high resolution conditions to this radiofrequency level must be calibrated in fre-molecules containing several nonequ...

Equilibrium constants are reported for several ligand replacement reactions of cobalamins and cobinamides (see Figure ) and are interpreted in terms of the thermodynamic trans-effect of the ligand which is not replaced. It is shown t h a t as this ligand is changed in the order H,O, NC-, HCEC-, H,C=CH-, CH,-, so the trans cobalt-cyanide bond becomes less stable relative t o the cobalt-benziminazole bond which in turn becomes less stable relative to the cobalt-water bond. The work involved the identification of certain cobalamins and cobinamides.* DBC is cr-(5,6-diniethylbenziminazolyl)cobinamide coenzyme.

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Dipole-dipole interactions between protons in a glucopyranose derivative have been studied by means of the nuclear Overhauser effect and by spin-lattice relaxation experiments. The Overhauser enhancement measurements utilize a new pulse technique which permits the saturating field to have any specified spectral density function so that finite frequency bands in the NMR spectrum can be irradiated. By this method, the dipolar interactions between the various pairs of protons are evaluated. In contrast, the dipolar relaxation of a chosen proton by all other protons in the sample has been determined by comparing the inversion-recovery rates after selective and nonselective 180· pulses. The relaxation measurements indicate that the relaxation in this molecule is purely dipolar in origin. The Overhauser enhancement measurements of one proton are used to determine the contribution to its relaxation from two neighboring protons and, from these results, information about the geometry of the pyranose ring is evaluated.4466