Exchangeability measurements

For saturated hydrocarbons, exchange is too slow and reference points are so uncertain that direct determination of pAT values by exchange measurements is not feasible. The most useful proach to obtain pK data for such hydrocarbons involves making a measurement of the electrochemical potential for the reaction... 

Wokaun s group (Lippert et al., 1992) showed, on the basis of thermostability data (differential scanning calorimetry) and H NMR exchange measurements that the n-electron distribution in l-aryl-3,3-dialkyltriazenes (13.10a) indicates that there is a contribution from the 1,3-dipolar mesomeric structure 13.10b. 

Fig. 1. Rates of CO2 assimilation, A (/miol s ) leaf conductance, g (mol m s ) intercellular partial pressure of CO2, Pi (Pa) soil water potential and leaf water potential, xp (MPa) during gas-exchange measurements of a 30-day-old cotton plant, plotted against day after watering was withheld. Measurements were made with 2 mmol m sec" photon flux density, 30 °C leaf temperature, and 2.0 kPa vapour pressure difference between leaf and air (S.C. Wong, unpublished data).

The capacity of an ion-exchanger measures the amount of material that can be exchanged by a given amount of the resin, and is normally expressed in milliequivalents per g of dry resin. 

Fig. 11.7 a Pulse sequence for rotational-resonance recoupling of homonuclear spin pairs, b The spinning frequency is matched to the isotropic chemical-shift difference, and one of the resonances is selectively inverted and the polarization exchange measured as a function of the mixing time, c The difference polarization as a function of the mixing can be evaluated to give the dipolar coupling constant. 

Either the information obtained during the data-dependent acquisition is sufficient or a fraction of interest can be re-analyzed by chip-based infusion at a flow rate ca. 200 nl min. Due to the miniaturization sample consumption is very low (typically 1-3 pi) and acquisition time is no longer critical. Therefore various MS experiments can be performed on various instruments, including MS and accurate mass measurements. An additional advantage is that the eluent can be removed and the infusion solvent can be optimized for positive or negative ion detection or for deuterium exchange measurements. 

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