X is selected from the group consisting of O , S , and >N R5, wherein {HEAD ~ R5} is hydrogen or {TAIL ~ 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 hydrogen or { TAIL ~ 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 OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH