X is selected from the group consisting of O , S , and >N R5, wherein {HEAD ~ R5} is {TAIL ~ hydrogen} or an alkyl group having from 1 to 9 carbon atoms 
is
X is selected from the group consisting of O , S , and > N R5 , wherein { HEAD ~ R5 } is { TAIL ~ hydrogen } or an alkyl group having from 1 to 9 carbon atoms
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH