Scheme 1. Tautomer and Ionization Equilibria of Studied Benzothiophene Analogues (Table 1).

Species, present in at least 0.01% fraction under experimental conditions, are labeled S1–S12 and their parent tautomers are labeled as T1–T5. The neutral forms of the T3, T4, and T5 neutral-zwitterion pairs, marked with a prime, are present in negligible fractions and are listed only as intermediates in the formation of more abundant zwitterionic counterparts. The protonated form of T2 is shown as T2+. The charges of substituent groups are given in brackets. The R3–R4 substituent group for S2/T1 and S6/T2 refers to the cyclic derivatives (20–33, Table 1). Species S3 and S4 are not present in this series.