Weak hydrogen bonding in uracil and 4-cyano-4'-ethynylbi
phenyl, for which single-crystaldiffraction structures reveal close CH···O=C and C
![](/images/entities/tbd1.gif)
CH···N
![](/images/entities/tbd1.gif)
C distances, is investigated in a study thatcombines the ex
perimental determination of
1H,
13C, and
15N chemical shifts by magic-angle s
pinning (MAS)solid-state NMR with first-
princi
ples calculations using
plane-wave basis sets. An o
ptimized synthetic route,including the isolation and characterization of intermediates, to 4-cyano-4'-ethynylbi
phenyl at naturalabundance and with
13C
13CH and
15N
![](/images/entities/tbd1.gif)
C labeling is described. The difference in chemical shifts calculated,on the one hand, for the full crystal structure and, on the other hand, for an isolated molecule de
pends onboth intermolecular hydrogen bonding interactions and aromatic ring current effects. In this study, the twoeffects are se
parated com
putationally by, first, determining the difference in chemical shift between thatcalculated for a
plane (uracil) or an isolated chain (4-cyano-4'-ethynylbi
phenyl) and that calculated for anisolated molecule and by, second, calculating intra
plane or intrachain nucleus-inde
pendent chemical shiftsthat quantify the ring current effects caused by neighboring molecules. For uracil, isolated molecule to
plane changes in the
1H chemical shift of 2.0 and 2.2
ppm are determined for the CH
protons involved inCH···O weak hydrogen bonding; this com
pares to changes of 5.1 and 5.4
ppm for the NH
protons involvedin conventional NH···O hydrogen bonding. A com
parison of CH bond lengths for geometrically relaxeduracil molecules in the crystal structure and for geometrically relaxed isolated molecules reveals differencesof no more than 0.002 Å, which corres
ponds to changes in the calculated
1H chemical shifts of at most 0.1
ppm. For the C
![](/images/entities/tbd1.gif)
CH···N
![](/images/entities/tbd1.gif)
C weak hydrogen bonds in 4-cyano-4'-ethynylbi
phenyl, the calculated moleculeto chain changes are of similar magnitude but o
pposite sign for the donor
13C and acce
ptor
15N nuclei. Inuracil and 4-cyano-4'-ethynylbi
phenyl, the CH hydrogen-bonding donors are
sp2 and
sp hybridized,res
pectively; a com
parison of the calculated changes in
1H chemical shift with those for the
sp3 hybridizedCH donors in maltose (Yates et al.
J. Am. Chem. Soc. 2005,
127, 10216) reveals no marked de
pendenceon hybridization for weak hydrogen-bonding strength.