Thermochemical Determinations by the Kinetic Method with Direct Entropy Correction

Abstract

The kinetic method, which is based on the competitive dissociations of mass-selected cluster ions, has been increasingly applied in thermochemical determination mainly due to its simplicity and sensitivity. Its use, however, is limited by the requirement that the reference and unknown compounds be similar in structure. To address this limitation, entropic contributions are considered explicitly here. This allows the method to be applied in cases where neither the assumption of zero protonation entropy difference of the standard form nor the precondition of constant protonation entropy difference of the extended version of the method is satisfied. The validity of the proposed procedure for entropy correction is examined using urea as the test compound, and amides, ketones, ethylene glycol dimethyl ether, and polyols as the reference bases. Afters but not beforesappropriate entropy correction, the results show a linear correlation of the logarithm of the branching ratio versus proton affinity. Using data obtained as a function of collision energy, a second plot of the intercepts versus the slopes of the initial graph yields a value for the urea proton affinity (PA) of 868.4 ( 2.5 kJ/mol, a gas-phase basicity (GB) of 838.7 ( 3.0 kJ/mol, and a protonation entropy (¢Sp )o f 9.4( 0.5 J/mol‚K. Experiments using different functionalized reference bases give similar results. The reported procedure extends the capabilities of the kinetic method in making thermochemical measurements and can be applied to the measurement of reaction entropies, provided enthalpies are known.