A reformulated aromaticity index equation under consideration for non-aromatic and non-condensed aromatic cyclic carbonyl compounds

Abstract The extent of unsaturation level in a compound, commonly measured in terms of its number of double bond equivalents (DBEs), reflects the total number of π bonds plus rings and can be calculated if the molecular formula is known. The extent of unsaturation can also be calculated via DBE/C, where C is the number of carbon atoms. For hydrocarbons, if DBE/C is > 0.67, the structure corresponds unequivocally to a condensed aromatic (CA) compound, but this criterion cannot be applied directly to heteroatom containing compounds. A new parameter, the aromaticity index (AI) was recently proposed to solve this problem, and has been applied to establish a region of CA compounds in high resolution mass spectrometry (MS) van Krevelen diagrams to characterize natural organic matter (NOM), such as humic acids (HAs). In this study, we report a reformulated AI (rAI) that also considers formulae that accommodate non-aromatic (NA) and non-condensed aromatic (NCA) cyclic carbonyl compounds. We also propose a correction for AI with respect to N and P containing formulae and show an application of the rAI index for the analysis and data treatment of a certified reference material (CRM) of NOM.