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FTIR spectroscopy accompanied by quantum chemical simulations can reveal important information about molecular structure and intermolecular interactions in the condensed phase. Simulations typically account for the solvent either through cluster quantum mechanical (QM) models, polarizable continuum models (PCM), or hybrid quantum mechanical/molecular mechanical (QM/MM) models. Recently, we studied the effect of aqueous solvent interactions on the vibrational frequencies of lumiflavin, a minimal flavin model, using cluster QM and PCM models. Those models successfully reproduced the relative frequencies of four prominent stretching modes of flavin’s isoalloxazine ring in the diagnostic 1450–1750 cm−1 range but poorly reproduced the relative band intensities. Here, we extend our studies on this system and account for solvation through a series of increasingly sophisticated models. Only by combining elements of QM clusters, QM/MM, and PCM approaches do we obtain an improved agreement with the experiment. The study sheds light more generally on factors that can impact the computed frequencies and intensities of IR bands in solution. 

For more than half a century, pericyclic reactions have played an important role in advancing our fundamental understanding of cycloadditions, sigmatropic shifts, group transfer reactions, and electrocyclization reactions. However, the fundamental mechanisms of photochemically activated cheletropic reactions have remained contentious. Here we report on the simplest cheletropic reaction: the [2+1] addition of ground state 18 O-carbon monoxide (C 18 O, X 1 Σ + ) to D2-acetylene (C 2 D 2 ) photochemically excited to the first excited triplet (T1), second excited triplet (T2), and first excited singlet state (S1) at 5 K, leading to the formation of D2- 18 O-cyclopropenone (c-C 3 D 2 18 O). Supported by quantum-chemical calculations, our investigation provides persuasive testimony on stepwise cheletropic reaction pathways to cyclopropenone via excited state dynamics involving the T2 (non-adiabatic) and S1 state (adiabatic) of acetylene at 5 K, while the T1 state energetically favors an intermediate structure that directly dissociates after relaxing to the ground state. The agreement between experiments in low temperature ices and the excited state calculations signifies how photolysis experiments coupled with theoretical calculations can untangle polyatomic reactions with relevance to fundamental physical organic chemistry at the molecular level, thus affording a versatile strategy to unravel exotic non-equilibrium chemistries in cyclic, aromatic organics. Distinct from traditional radical–radical pathways leading to organic molecules on ice-coated interstellar nanoparticles (interstellar grains) in cold molecular clouds and star-forming regions, the photolytic formation of cyclopropenone as presented changes the perception of how we explain the formation of complex organics in the interstellar medium eventually leading to the molecular precursors of biorelevant molecules.