Hydrogen bond breaking kinetics

Dynamics of water molecular motions and hydrogen-bond-breaking kinetics... 

Finally, we note that ID DQ-filtered MAS experiments (corresponding to setting tj = 0) can also provide insight into dynamic processes. The principle, in this case, is that signal is only observed for pairs of protons which remain dipolar coupled for the timescale of the experiment, which in this case is the time required to excite and reconvert the DQC. For example, in [126], the kinetics of hydrogen bond breaking and formation is quantitatively analysed for a carboxylic acid dimer on the basis of the fall off in the DQ intensity with increasing temperature. 

Figures 7-9 show the fractional conversion of methanol in the pulse as a function of temperature for the three catalysts and the three methanol feeds. Evidently the kinetic isotope effect is present on all three catalysts and over the complete temperature range, indicating that the rate limiting step is the breaking of a carbon-hydrogen bond under all conditions. From these experiments, the effect cannot be determined quantitatively as in the case of the continuous flow experiments, but to obtain the same conversion of CD,0D, the temperature needs to be 50-60° higher. This corresponds to a factor of about three in reaction rate. The difference in activity between PfoCL and Fe.(MoO.), is larger in the pulse experiments compared to tHe steady stateJ results.

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