The effect of hydrogen bonding on structural parameters

The effect of changing the ∠ CO … H on the energy of the linear hydrogen bond has been studied by ab initio calculations for one formamide-methane complex and two formamide-water complexes, which differ in the position of the second hydrogen (H2₀) of the water molecule (i.e. the one not involved in the hydrogen bond). When the hydrogen bond is cis to the CN bond (∠ CO … H  120°), this hydrogen is, respectively, trans (1) and cis (II) to the CO bond. It is found that the formamide-methane complex has two minima with hydrogen bond energies of ∼; −2.6 kcal mol⁻¹, at values of about 120° and 240° for the ∠ CO … H. The barrier height at 180° is ∼; 0.9 kcal mol⁻¹. A different situation is found for the formamide-water complexes, where the hydrogen bond energy is largely dependent on the position of H2₀. The formamide-water (I) complex has one minimum of −10.74 kcal mol⁻¹ for the hydrogen bond at a value of ∼; 110° for ∠ CO … H. The energy rises sharply with changes in the angle, and is −7.54 kcal mol⁻¹ at 180°. The other complex (II) has one minimum of −9.14 kcal mol⁻¹ at 240° and a hydrogen bond energy of − 7.12 kcal mol⁻¹ at 180°. It is also found that the hydrogen bond length increases in a regular manner when ∠ CO … H changes from 120° to 240°.