IRMPD and DFT study of the loss of water from protonated 2-hydroxynicotinic acid

Collision-induced dissociation (CID) of protonated 2-hydroxynicotinic acid (2-OHNic) generates a dominant product ion through loss of 18 mass units, presumably the elimination of water. Subsequent isolation and storage of this product ion in the gas-phase environment of an ion trap mass spectrometer, without imposed collisional activation, shows that the species undergoes addition reactions to furnish new products that are higher in mass by 18 and 32 units. Density functional theory (DFT) calculations suggest that an acylium ion (i.e., loss of H2O from the acid group) is energetically more favored than is a species generated by elimination of H2O from the hydroxypyridine ring. Formation of the acylium product is confirmed by comparing the infrared multiple photon dissociation (IRMPD) spectrum to theoretical spectra from (DFT) harmonic calculations for several possible isomers. A thorough DFT study of the reaction dynamics suggests that the acylium ion is generated from the global minimum for the protonated precursor along a pathway that involves proton transfer from the hydroxypyridine ring and elimination of OH from the acid group.

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► Infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) calculations used to study the elimination of water from protonated 2-hydroxynicotinic acid.
► IRMPD shows that the product generated by loss of water is an acylium ion with 2-pyridone like structure.
► DFT calculations suggest that loss of water occurs through concerted proton transfer and COH bond cleavage through 6 membered ring transition state.