Calculation of the Potential Energy Surface for Intermolecular Amide Hydrogen Bonds Using Semiempirical and Ab Initio Methods

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 (θ COH and θ NHO ), and the dihedral between the two amides (Φ CNCN ) 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 Φ CNCN and θ COH predict the same change in interaction energies with changes in θ NHO as the semiempirical calculations. With formamide, hydrogen-bond interaction energies are independent of the dihedral angle Φ CNCN when θ COH and θ NHO deviate less than 40° from 180°. Most importantly, the increased interaction energies at θ COH and θ NHO 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 (θ COH, θ NHO, and Φ CNCN ) 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.