The dependence of hydrogen-bond interaction energies
between identical amides (two formamides and
two N-methylacetamides) on the hydrogen bond length
(r
O···H), the two hydrogen bond angles
(θCOH and θNHO), and
the dihedral between the two amides (ΦCNCN) has been
assessed by semiempirical calculations (SAM1 with single
point transfers to AM1/SM2.1 aqueous solvation calculations).
Ab initio calculations
(MP2/6-31+G(d,p)//HF/6-31+G(d,p)) at given values of ΦCNCN and
θCOH predict the same change in interaction energies with
changes in
θNHO as the semiempirical calculations. With
formamide, hydrogen-bond interaction energies are independent
of
the dihedral angle ΦCNCN when θCOH and
θNHO deviate less than 40° from 180°. Most
importantly, the increased
interaction energies at θCOH and θNHO below
140° and above 220° are found to be associated with steric
interference
between the carbonyl oxygen of the hydrogen-bond acceptor and the amide
nitrogen of the hydrogen-bond donor.
Comparing formamide and N-methylacetamide, the angle
requirements (θCOH, θNHO, and
ΦCNCN) of favorable hydrogen-bond interaction energies are much more stringent for the latter due to
the steric effects of the methyl substituents.
In summary, by both semiempirical SAM1 and ab initio
MP2/6-31+G(d,p)//HF/6-31+G(d,p) calculations, the
strength
of amide hydrogen bonding in the absence of steric hindrance is
essentially independent of the angles defining the
hydrogen bond.
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