{HEAD ~ L1} is ##STR24## where {TAIL ~ R5} and R6 are independently a hydrogen, alkyl or an aryl group 
{ HEAD ~ L1 } is # # STR24 # # where { TAIL ~ R5 } and R6 are independently a hydrogen , alkyl or an aryl group
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH