Weak hydrogen bonding in uraci
l and 4-cyano-4'-ethyny
lbipheny
l, for which sing
le-crysta
ldiffraction structures revea
l c
lose CH···O=C and C
![](/images/entities/tbd1.gif)
CH···N
![](/images/entities/tbd1.gif)
C distances, is investigated in a study thatcombines the experimenta
l determination of
1H,
13C, and
15N chemica
l shifts by magic-ang
le spinning (MAS)so
lid-state NMR with first-princip
les ca
lcu
lations using p
lane-wave basis sets. An optimized synthetic route,inc
luding the iso
lation and characterization of intermediates, to 4-cyano-4'-ethyny
lbipheny
l at natura
labundance and with
13C
13CH and
15N
![](/images/entities/tbd1.gif)
C
labe
ling is described. The difference in chemica
l shifts ca
lcu
lated,on the one hand, for the fu
ll crysta
l structure and, on the other hand, for an iso
lated mo
lecu
le depends onboth intermo
lecu
lar hydrogen bonding interactions and aromatic ring current effects. In this study, the twoeffects are separated computationa
lly by, first, determining the difference in chemica
l shift between thatca
lcu
lated for a p
lane (uraci
l) or an iso
lated chain (4-cyano-4'-ethyny
lbipheny
l) and that ca
lcu
lated for aniso
lated mo
lecu
le and by, second, ca
lcu
lating intrap
lane or intrachain nuc
leus-independent chemica
l shiftsthat quantify the ring current effects caused by neighboring mo
lecu
les. For uraci
l, iso
lated mo
lecu
le top
lane changes in the
1H chemica
l shift of 2.0 and 2.2 ppm are determined for the CH protons invo
lved inCH···O weak hydrogen bonding; this compares to changes of 5.1 and 5.4 ppm for the NH protons invo
lvedin conventiona
l NH···O hydrogen bonding. A comparison of CH bond
lengths for geometrica
lly re
laxeduraci
l mo
lecu
les in the crysta
l structure and for geometrica
lly re
laxed iso
lated mo
lecu
les revea
ls differencesof no more than 0.002 Å, which corresponds to changes in the ca
lcu
lated
1H chemica
l shifts of at most 0.1ppm. For the C
![](/images/entities/tbd1.gif)
CH···N
![](/images/entities/tbd1.gif)
C weak hydrogen bonds in 4-cyano-4'-ethyny
lbipheny
l, the ca
lcu
lated mo
lecu
leto chain changes are of simi
lar magnitude but opposite sign for the donor
13C and acceptor
15N nuc
lei. Inuraci
l and 4-cyano-4'-ethyny
lbipheny
l, the CH hydrogen-bonding donors are
sp2 and
sp hybridized,respective
ly; a comparison of the ca
lcu
lated changes in
1H chemica
l shift with those for the
sp3 hybridizedCH donors in ma
ltose (Yates et a
l.
J. Am. Chem. Soc. 2005,
127, 10216) revea
ls no marked dependenceon hybridization for weak hydrogen-bonding strength.