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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