Specifically, they may be selected from amongst hydrogen, alkyl groups such as the methyl group and ethyl group, cycloalkyl groups such as cyclohexyl and norbomyl, aralkyl groups such as the benzyl group, alkenyl groups such as the vinyl group and allyl group, cycloalkenyl groups such as the cyclopentadienyl group and cyclohexenyl group, alkoxy groups such as the methoxy group, alkylthio groups wh