Value of the Rings Plus Double Bonds

The value of the rings plus double bonds (R + DB value) is a measure of the degree of unsaturation in a molecule. It is also a convenient means of ascertaining whether an ion is an OE or an EE species. The R + DB value will be an integer number for OE ions and an odd multiple of 0.5 for EE ions. If the elemental composition of an ion is known, the value of R + DB can be calculated. For a molecule of the general formula QHyN On, R + DB is given by 

This formula is based on the fact that each ring or double bond in a hydrocarbon decreases the number of hydrogen atoms by two units, and each nitrogen atom increases the number of hydrogen atoms by one unit. If elements other than C, H, N, and O are present, they are counted with one of these elements by matching their valences. For example, all tetravalent elements are counted with C, the trivalent elements with N, the bivalent elements with O, and the monovalent elements with H. Also, remember that a triple bond is treated as two double bonds. 

Because only an OE+ ion can be a molecular ion, the formula C7H6O2 is a possible molecular ion. 

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