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

Abstract

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.

Close agreement between the orientation dependence of hydrogen bonds observed in protein structures and quantum mechanical calculations

“…3: the dimerization energies at 120°are Ϫ6.90 kcal͞mol for MP2 and Ϫ6.82 kcal͞mol for DFT, whereas the dimerization energies at 175°are Ϫ5.99 kcal͞mol for MP2 and Ϫ5.74 kcal͞mol for DFT. Because DFT and MP2 calculations use different treatments of electron correlation and exchange, the similarity in the results strongly suggests that the difference in energy between the two configurations is on the order of 1 kcal͞mol, considerably larger than that found in older studies using semiempirical methods (43) or HF calculations over the full range supplemented with limited Moller-Plesset results for near linear geometries (42), which suggested weak hydrogen bonding energy dependence on the acceptor angle or even preference for more linear hydrogen bonds.…”

Isomorphous Crystals from Diynes and Bromodiynes Involved in Hydrogen and Halogen Bonds

“…Without the crystal lattice constraint, calculations could obviously yield extremely different final structures, especially in the current cases. Indeed, the potential energy surface (PES) involving weak interactions is known to be shallow [73]. This means that important geometrical variations of distances and angles between H-bond and halogen bond partners may result in minute energy changes (e.g., <0.2 kcal¨mol´1).…”

High-Precision Measurement of Hydrogen Bond Lengths in Proteins by Nuclear Magnetic Resonance Methods

“…1A), little or no correlation of strength with hydrogen bond angle, over a range of 180 Ϯ 30°is found experimentally 6 or theoretically. 7 Angle bending beyond Ϯ30°can lead to weakening of strong hydrogen bonds. 8 In aqueous environments, due to competition with the solvent, weak hydrogen bonding is expected and is generally found.…”