Binding energies of proton-bound ether/alcohol mixed dimers determined by FTICR radiative association kinetics measurements

The reactions of protonated diethyl ether with ethanol and protonated di-n-propyl ether With n-propanol, producing the mixed proton-bound dimers, were studied at low pressures, in a FTICR cell. The pressure dependence of the apparent rate constant for proton-bound dimer formation was investigated and yielded unimolecular dissociation rate constants, k(b), and photon emission rate constants, k(ra), for the nascent proton-bound dimers at internal energies equal to the dissociation energies of the dimers. The experimental k(ra) values were found to be 17 +/- 3 and 6.3 +/- 0.6 s(-1), respectively, for the ethanol/diethyl ether and n-propanol/di-npropyl ether proton-bound dimers, RRKM modeling of the unimolecular dissociation rate constants as a function of the binding energies yielded the 0 K dissociation energies of the proton-bound dimers as 109 I and 105.1 +/- 0.6 kJ mol(-1) for the ethanol/diethyl ether and n-propanol/di-n-propyl ether proton-bound dimers, respectively. Using B3LYP/6-311G** thermal energies, the corresponding 298 K bond strengths were determined to be 103 +/- 1 and 99.5 +/- 0.6 kJ mol(-1). These agree quite well with those predicted by a simple relationship based upon the differences in proton affinities of the neutral monomer,,;. In addition, the association reaction Of n-propanol with protoriated di-n-propyl ether reaches what appears to be equilibrium at long react. ion times. The relative intensities of the product and reactant ions as well as the pressure of neutral n-propanol were used to obtain an equilibrium constant for the reaction of 9.8(+/-0.9) x 10(10) at 294 K. From this equilibrium constant and a calculated entropy of reaction of 128 J K-1 mol(-1), the 294 K bond strength was determined to be 100.3 +/- 6.0 kJ mol(-1), in excellent agreement with the values determined from the radiative association kinetics and B3LYP/6-311G** calculations.