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Conformational Effects on Gas-Phase Acidities of Isomeric Alcohols: A Guided Ion Beam Tandem Mass Spectrometry Study
AuthorGhale, Surja B.
AdvisorErvin, Kent M.
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The conformational effects on gas phase acidities of isomeric alcohols are studied using the competitive threshold collision-induced dissociation (TCID) technique and quantum chemical calculations. A statistical RRKM model has been used to analyze the branching reaction cross-sections of the products formed by dissociating proton-bound heterodimer alcohol complexes. Torsional modes of all species involved in the dissociative reaction process are treated as hindered rotors, and the calculated hindered energy levels are included in the RRKM analysis by replacing harmonic oscillator frequencies. DFT calculations at the ωB97X-D/6-311++G(2df,2p) theory level consider the dispersion interactions of the long chain alky group affecting the deprotonation process in alcohols. A gas-phase thermochemical network is constructed using HF and H2O as reference acids, and the absolute gas-phase acidities of methanol, ethanol, n-propanol, iso-propanol, tert-butanol, n-pentanol and iso-pentanol are determined with uncertainties estimated at 95% confidence level. The differences in the intrinsic and experimentally measured gas-phase acidities for isomeric C3 and C5 alcohols are largely due to rotation about the α-β carbon-carbon bond in alkyl chain resulting in twisted geometries of the neutral and anionic species via formation of a moderate intramolecular H-bonding. Using the known ionization energy of hydrogen atom and the experimental electron affinities of alkoxy radicals from literature, the improved O-H bond dissociation energies are reported for methanol, ethanol, n-propanol, iso-propanol, and tert-butanol via the negative ion thermochemical cycle. These bond dissociation energies are important for modeling combustion and atmospheric reactions involving hydrocarbons.