Singlet oxygen INDEX

J. H. Clennan Castro, Increasing the index of covalent oxygen bonding at nitrogen attached carbon, Chem. Rev., 80 (1980) 495-561 see H. H. Wasserman and R. W. Murray (Eds.), Singlet Oxygen, Academic Press, New York, 1979. 

Compound A (C4H602) has an index of hydrogen deficiency of 2. With two oxygen atoms and a peak in the infrared at 1760 cm-1, it is likely that one of the elements of unsaturation is the carbon-oxygen double bond of an ester. The H NMR spectrum contains a three-proton singlet at 8 2.1 ppm, which is consistent with a CH3C unit. It is likely that compound A is an acetate ester. 

The index pd corresponds to a Zhang-Rice singlet formation with one hole placed on the copper site whereas the second hole is distributed on the neighboring oxygen sites [12], Namely, T/ " " = -2= is the copper-oxygen creation operator in terms of copper (X) and oxygen... 

---