Examples of the hydrocarbon groups of 2 to 6 carbon atoms include an alkyl group such as ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, neopentyl and n-hexyl; cycloalkyl group such as cyclohexyl; and an alkenyl group such as vinyl and propenyl.