Sublimation thermodynamics of indoline and benzoxazine based spiropyrans

Abstract

Sublimation of 1,3,3,6′-tetramethyl-8′-formyl-spiroindoline-2,2′-[2H] -chromene (SP-1) and 3,6′-dimethyl-8′-formyl-spiro (4-oxo-3,4-dihydro-2H-1,3-benzoxazine)-2,2′-[2H] -chromene (SP-2) was studied by Knudsen effusion mass spectrometry in the temperature range 309–363 K and 331–384 K, respectively. It has been shown that, in the temperature range under study, both spiropyrans are thermally stable and sublime congruently in the form of monomeric molecules. The dependences of the saturated vapor pressure (Pa) on temperature are described by the equations:lg p (SP-1) = – (6.239±0.093)∙103∙T–1+ (15.832±0.278),lg p (SP-2) = – (6.625±0.111)∙103∙T–1 + (15.459±0.312).The sublimation enthalpies have been evaluated by the second law method as ΔsH°(298.15 K) = 119±5 kJ∙mol–1 (SP-1) and ΔsH°(298.15 K) = 128±5 kJ∙mol–1 (SP-2). The enthalpies of formation of spiropyran molecules in the ideal gas state, ΔfH°g(298.15 K) = 199±9 kJ∙mol–1 (SP-1) and ΔfH°g(298.15 K) = –296±10 kJ∙mol–1 (SP-2), have been calculated using isodesmic reactions method implying quantum chemical calculations. Combining the latter values with the corresponding sublimation enthalpies the enthalpies of formation for crystalline state spiropyrans were determined: ΔfH°cr(298.15 K) = 80±10 kJ∙mol–1 (SP-1) and ΔfH°cr(298.15 K) = –424±11 kJ∙mol–1 (SP-2).