AB-INITIO AND EXPERIMENTAL STUDIES ON THE PROTONATION OF GLUCOSE IN THE GAS PHASE

Protonations of alpha- and beta-D-glucopyranose in the gas phase were investigated using the ab initio molecular orbital approach at the HF/ 6-31G level with full geometry optimization. Minimum-energy structure s of three neutral and six protonated species for each anomer were cal culated, Geometries, energies, and intramolecular hydrogen bonding in these structures are discussed. For the neutral species at 298 K the o rder of stability for the hydroxymethyl conformers is calculated to be GT > CG > TG for the a anomer and GG > CT > TG fur the beta anomer. P rotonated species that least disrupt the internal hydrogen bonding net work in the neutral species are considered: these include protonations on the oxygen sites labeled as the hydroxymethyl O6, the ring O5, and the exocylic hydroxyl O4. The O6 protonation in the TG conformation i s electronically most favored. Energy corrections fur basis-set defici ency and electron-correlation omission in the adopted theoretical proc edure were estimated from high-level calculations on ethanol, 2-propan ol, and dimethyl ether. In addition, the gas-phase basicity (GB) of gl ucose was measured by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer. The experimental GB values for both anomers were determined to be 188 +/- 3 kcal/mol. The experi mental values are compared with the ab initio estimates of 178-190 and 177-189 kcal/mol for the respective alpha and beta anomers. Theoretic al structures for the lowest-electronic-energy protonated species in t he three hydroxymethyl conformations of each anomer are also presented to serve as reference data for postulating various kinetic pathways.