Formation and Reactions of Heteroaromatic Anions in the Gas Phase

The gas-phase acidities of furan, thiophene,pyridine, 1-methylpyrrole, and a number of their methyl derivatives have been determined in a flowing afterglow apparatus. The △G_acid (kcal/mol) and site of deprotonation of the parent heterocycles are as follows: furan 380 ± 3 (2-position), thiophene 373 ± 3 (2-position), pyridine 384 ± 3 (3- and/or 4-position), 1-methylpyrrole 386 ± 3 (methyl group). For the methylated species studied the acidities are 2-methylfuran 377 ± 3, 2-methylthiophene 373 ± 3, 3-methylthiophene 373 ± 3, 3-methylpyridine 371 ± 3, 2-methylpyridine 370 ± 3, and 4-methylpyridine 368 ± 3. The relative acidities of the hydrogens in the other sites in these molecules have been estimated by hydrogen-deuterium exchange studies and by other chemical methods. Anions (M - 1) from these molecules have been allowed to react with N₂O, O₂, COS, and CS₂ and the ionic products determined. The gas-phase acidities have been compared with those calculated by the semiempirical methods AMI and MNDO.