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 (theta(COH) and theta(NHO)), and the dihedral between the two amides (Phi(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)parallel to HF/6-31+G(d,p)) at given values of Phi(CNCN) and theta(COH) predict the same change in interaction energies with changes in theta(NHO) as the semiempirical calculations. With formamide, hydrogen-bond interaction energies are independent of the dihedral angle Phi(CNCN) when theta(COH) and theta(NHO) deviateless than 40 degrees from 180 degrees. Most importantly, the increased interaction energies at theta(COH) and theta(NHO) below 140 degrees and above 220 degrees 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 (theta(COH), theta(NHO), and Phi(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 MP2/6-3+G(d,p)parallel to 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.