This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/jms.3848 This article is protected by copyright. All rights reserved. Vitamin C: An Experimental and Theoretical Study on the Gas Phase Structure and Ion Energetics of Protonated Ascorbic Acid TITLE RUNNING HEAD Ion energetics and gas-phase structures of protonated ascorbic acid Andreina Ricci a *, Federico Pepi b , Paola Cimino c , Anna Troiani b , Stefania Garzoli b , Chiara Salvitti b , Brunella Di Rienzo a , and Vincenzo Barone d a Department of Math and Phys, Second University of Naples, Viale Lincoln,5, 81100 Caserta, Italy b Department of Chemistry and Drug Technologies, „Sapienza‟ University of Rome, P.le A. Moro 5, 00185 Rome, Italy. c Department of Pharmaceutical Science University of Salerno, Via Giovanni Paolo II-84084-Fisciano (Sa) Italy. d Scuola Normale Superiore, Piazza Cavalieri 7, I-56126 Pisa, Italy. * Correspondence to: Andreina Ricci, Department of Mathematics and Physics, Second University of Naples, Caserta, Italy. Tel: 00 39 0823 274445; Fax: 00 39 0823 274571. E-mail: andreina.ricci@unina2.it ABSTRACT In order to investigate the gas-phase mechanisms of the acid catalyzed degradation of ascorbic acid (AA) to furan we undertook a mass spectrometric (ESI/TQ/MS) and theoretical investigation at the B3LYP/6-31+G(d,p) level of theory. The gaseous reactant species, the protonated ascorbic acid, [C6H8O6]H + , were generated by electrospray ionization (ESI) of a 10 -3 M H2O/CH3OH (1:1) ascorbic acid solution . In order to structurally characterize the gaseous [C6H8O6]H + ionic reactants, we estimated the proton affinity and the gas-phase basicity of ascorbic acid by the extended Cooks‟s kinetic method and by computational methods at the B3LYP/6-31+G(d,p) level of theory. As expected, computational results identify the carbonyl oxygen atom (O2) of AA as the preferred protonation site. From the experimental proton affinity (PA) of 875.0 ± 12 kJ mol -1 and protonation entropy ∆Sp (108.9  2 J mol -1 ) a GB value of AA of 842.5 ± 12 kJ mol -1 at 298 K was obtained, which is in agreement with the value issuing from quantum mechanical computations.