Feeling and investigating blue: On the enthalpy of formation of indigo

•Enthalpies of formation and sublimation of solid indigo at 298 K are presented.•B3LYP/6-311++G(d, p) DFT calculations are reported for the enthalpy of formation of indigo.•A consistent value is reported for the standard gas phase enthalpy of formation of indigo.

The enthalpy of formation of indigo, as a solid, was reported in 1893. The enthalpy of sublimation at a mean temperature of 577 K (ca. 298 °C) was reported some 90 years later, and corrected herein to 298 K. These values were summed to result in a standard gas phase enthalpy of formation. We have also performed quantum chemical calculations at the B3LYP/6-311++G(d, p) level and analyzed them using (not quite) isodesmic and isomerisation reactions. From these disparate calorimetric and computational approaches, we hereby present a recommended enthalpy of formation of gaseous indigo of (35 ± 16) kJ · mol−1.

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