An ab initio molecular orbital study of hydrogen bonding and ion‐molecule association in model systems for DNA bases

Ab initio calculations have been performed to investigate hydrogen bonding and ion‐molecule association in complexes of H 2 O with the neutral, protonated, and Li + complexes of N ‐formylformaldehyde and N ‐formylformamidine. In the complexes with the neutral bases, H 2 O assumes an in‐plane bridging position in the amide and amidine regions. The most stable complex is the bridging N ‐formylformamidine–H 2 O complex in the amidine region, which has an MP 2/6–31 + G (d,p) binding energy of −9 kcal/mol. Hydrogen bonded complexes of H 2 O with the oxygen‐protonated bases have open structures with the protonated bases as proton donors, and binding energies ranging from −16 to −24 kcal/mol. Nitrogen protonation of N ‐formylformamidine leads to an equilibrium chelated hydrogen bonded structure with a stabilization energy of –21 kcal/mol. When Li + associates with these bases at a carbonyl oxygen, hydrogen‐bonded bridging structures with H 2 O reappear, and wobble complexes exist in the amide and amidine regions of N ‐formylformaldehyde and N ‐formylformamidine. These complexes have binding energies of –13 to –14 kcal/mol. However, the most stable comples has H 2 O directly bonded to Li + , with an MP 2 binding energy of –30 kcal/mol. No hydrogen bonded structures of H 2 O with N ‐formylformamidine exist in the amide region when Li + associates with this base at the CN group. Hydrogen bond energies computed at the single‐determinant Hartree–Fock level with the 6−31 G (d) basis set approximate correlated MP 2/6–31 + G (d, p) energies to within 1 kcal/mol for all of the neutral and charged complexes. However, when H 2 O is bonded to Li + , HF 6–31 G (d) association energies overestimate MP 2/6–31 + G (d, p) energies by 3 kcal/mol.