Results and DiscussionSuch a phase shift makes it difficult to measure J from the coupled spectrum. We report here a simple means for removing this phase shift, so that all lines in the multiplet have identical phase. The modification, called by the palindrome PCJCP (phase corrected J-cross polarization), consists of the conventional JCP sequence1'5 6 in which either the I or S spin locking rf field remains on an additional time Í90 given by 7s5|st9o = 7i-S| [Í90 = tt/2. As usual, 713 is the magnetogyric ratio and Bn.s is the rf field strength for the nuclear species 1,S. To the extent that it is instantaneous, the S extension pulse flips the transverse S magnetization into the direction of the static field, where it is unobservable. An I extension pulse, either in phase or in quadrature with the cross-polarization rf, can also be used; the explanation in this case is more involved and will appear elsewhere.4 The operation of this sequence is shown in Figure 1. During cross polarization the multiplet lines grow at different rates4 and the conventional multiplet intensity ratios are not preserved either in JCP or PCJCP.The ease of operation suggests that, for a weak resonance of unknown J, one should use a reasonable guess for the cross-polarization time to produce a coupled cross-polarization spectrum by PCJCP. The value of J determined is then 4059 used to select an optimal r for maximum cross-polarization signal.The PCJCP sequence is best used near resonance (| -7¿?oI « 761). Operating significantly off resonance introduces amplitude asymmetries in the S multiplet which the tt/2 extension pulse will convert back into phase shifts. In a reciprocal manner, any phase shifts initially present will, under off-resonance operation, be converted by the pulse extension into an asymmetry in the multiplet. References and Notes(1) P.

SummaryThe products of decomposition of solutions of p-chlorpbenzenediazonium tetrafluoroborate in aqueous buffer solutions (pH 9.0-10.3; ionic strength 0.1-0.5) at 20.0" have been analyzed quantitatively. Up to eleven low molecular weight compounds could be identified besides the major product, the complex polymeric diazo tar. The distribution of products is influenced by trace amounts of oxygen as well as by p-chlorophenol and the radical trapping reagent iodoacetic acid. Mechanisms of formation of the products are discussed.1. Introduction. -The reaction products of arenediazonium ions in aqueous solutions depend on acidity, and three main areas of interest with respect to pH can be defined. In the case of the unsubstituted benzenediazonium ion heterolytic hydroxydediazoniation is dominant below about pH 4, and phenol is formed. At pH-values higher than 12, cis-benzenediazotate (originally called syn-diazotate) is formed; it is rearranged slowly into the trans-diazotate (anti-diazotate). In the intermediate pH-range a number of products can be detected, in particular the so-called diazo tars, i.e. macromolecular compounds of a complex structure which has not yet been investigated in detail2). For substituted benzenediazonium ions the above pH ranges are shifted to higher and lower values, depending on the basifying and acidifying character, respectively, of the substituents.The mechanisms of heterolytic hydroxydediazoniation and hydroxyaddition have been investigated in detail. In contrast, only a few mechanistic studies have been made of reactions of diazonium ions in the medium range of pH values (pH 6-11) in spite of the fact that solutions of diazonium salts are often used for the synthesis of azo dyes in precisely this pH range. The lack of data.on the fate of diazonium salts in slightly alkaline solutions is due to the complex mixture of products and to the difficulty in reproducing the kinetic data.

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SummaryThe kinetics of reactions of p-chlorobenzenediazonium ions in aqueous buffer solutions (pH 9.0-10.6) under N, (< 5 ppb of 0,) have been measured between 20 and 50°C. The formation of trans-diazotate is first-order with respect to the concentration of hydroxyl ions and to the equilibrium concentration of diazonium ions, if the diazonium ion+cis-diazotate equilibrium is considered as a fast prior equilibrium. This indicates that the p-chlorobenzenediazonium ion, in contrast to all previous investigations with the p-nitrobenzenediazonium ion and benzenediazonium ions carrying similar substituents with a -M effect, rearranges from the cis-to the trans-configuration as diazohydroxide and not as diazotate. The formation of trans-diazotate is catalyzed by carbonate and inhibited by hydrogen carbonate ions; mechanisms of these catalyses are discussed, and the solvent isotope effect KH20/KD20 measured by an 'H-NMR. technique reported. The kmetics of the dediazoniations can be analyzed as a mixture of two reactions, a relatively fast first reaction, reaction A, which is responsible for about 5% of the total reaction, and a second reaction F. Both are first-order with respect to diazonium ion; reaction A is also first-order in hydroxyl ions. There are some indications that reaction A corresponds to the hydrolysis of the diazonium ion to give eventually amine and nitrite ions. Reaction F shows a complex dependence on hydroxyl ions; it is related to the homolytic dediazoniation.

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