Indifference to Hydrogen Bonding in a Family of Secondary Amides

N−H stretch region IR spectroscopy has been used to probe the
hydrogen bond-donating properties of six
structurally related secondary amides in which the amide group is
flanked by sterically bulky groups. For three of
the compounds, solid state IR analysis reveals N−H stretch bands
>3400 cm-1, which suggests the absence
of
intermolecular N−H--OC hydrogen bonds. Crystallographic data
for these three amides confirm the absence of
a standard N−H--OC interaction. For the other three amides,
N−H stretch bands in the range 3370−3290
cm-1
suggest the occurrence of intermolecular N−H--OC hydrogen bonds in
the solid state, but high quality crystals
could not be grown. Steric hindrance to hydrogen bond donation by
these amides is manifested in IR data obtained
in DMSO:  although this solvent is a strong hydrogen bond acceptor,
N−H stretch bands are observed for both
solvent-hydrogen bonded and non-hydrogen bonded amide groups. The
amides described here and related compounds
should be useful for calibrating spectroscopic methods that are
intended to detect hydrogen bond formation.