Has anyone heard of chemical shifts of methine carbons and protons that are
affected by different sets of neighboring residues?
I have a polylactide, (-O-CHCH3-CO-)n, which is a homopolymer except that
the methine carbon of each residue can have either R or S
stereochemistries. The chemical shift of the methine carbon of residue
"i" is determined by the relative stereochemistry of residue i-2, i-1, and
i+1. Conversely, the chemical shift of the methine proton of residue "i"
is determined by the relative stereochemistry of residue i-1, i+1, and
i+2. The chemical shift difference of the 8 types of carbons and protons
is only ~0.3ppm, but the effect is obvious at 750 MHz, and is also
apparent at 500 MHz.
I can't justify why the methine carbon and proton chemical shifts would be
affected be different sets of residues. Have there been other NMR studies
of polymers with residues that differ only in stereochemistry?
Thanks,
Marty Pagel mpagel@indiana.edu --- __o
Department of Chemistry Phone: (812)-855-4478 --- \<,
Indiana University Fax: (812)-855-8300 --- ()/ ()
Bloomington, IN 47405-4001 http://nmr.chem.indiana.edu/~mpagel