Isobaric Molecular Formulae of C, H, and O: A View from the Negative Quadrants of van Krevelen Space

When compositions of an isobaric series of molecular formulae consisting of C, H, and O are displayed in a van Krevelen plot, a remarkable number of nonparallel lines of compositions can be observed. Each group of related lines converge at a point in a negative quadrant of van Krevelen space (e.g., H/C = 4, O/C = −1). These points of convergence have H/C and O/C ratios that correspond to “molecular formulae” in which the stoichiometric coefficients of some of the isotopes are negative. In this manner, a group of related low-mass moieties have been identified (CH4O–1, C4O–3, C2H–8O–1, CH–12, etc.). Each of these moieties has a nominal mass of zero and a very small exact mass. Furthermore, all of these low-mass moieties have compositions that fall on the line H/C = −12 – 16­(O/C), which lies entirely in the negative quadrants of van Krevelen space. This paper demonstrates that all low-mass moieties consisting of C, H, and O can be expressed formally as linear combinations of any two moieties. Likewise, all molecular formulae that fall on a line that passes through a given low-mass moiety must differ compositionally by a multiple of the composition of that moiety, and their exact masses must differ by a multiple of the exact mass of the moiety. This latter relationship has been invoked for very rapid assignment of a molecular formula to an exact mass. Finally, a more comprehensive and theoretically based understanding of family scores has been developed around the concept of low-mass moieties.