R' of the thiol R'SH can be a straight or branched chain alkyl group containing 1 to 22 carbon atoms, preferably 4 to 18 carbon atoms; an aralkyl group, an alkylated aryl group, a heterocyclic alkyl group, a cycloalkyl group, the last four containing 4 to 40 carbon atoms, preferably 6 to 24 carbon atoms, and the same groups containing substituents such as halogen, nitro or alkoxy groups.