Theoretical investigation of hydrogen bonds between CO and HNF2, H2NF, and HNO

Ab initio quantum mechanics methods were applied to investigate the hydrogen bonds between CO and HNF2, H2NF, and HNO. We use the Hartree-Fock, MP2, and MP4( SDQ) theories with three basis sets 6-311++ G(d,p), 6-311++G(2df, 2p), and AUG-cc-pVDZ, and both the standard gradient and counterpoise-corrected gradient techniques to optimize the geometries in order to explore the effects of the theories, basis sets, and different optimization methods on this type of H bond. Eight complexes are obtained, including the two types of (CH)-H-center dot center dot center dot-N and (OH)-H-center dot center dot center dot-N hydrogen bonds: (OCHNF2)-H-center dot center dot center dot(C-s), (OCH2NF)-H-center dot center dot center dot(C-s and C-1), and (OCHNO)-H-center dot center dot center dot(C-s), and (COHNF2)-H-center dot center dot center dot(C-s), (COH2NF)-H-center dot center dot center dot(C-s and C-1), and (COHNO)-H-center dot center dot center dot(C-s). The vibrational analysis shows that they have no imaginary frequencies and are minima in potential energy surfaces. The N-H bonds exhibit a small decrease with a concomitant blue shift of the N-H stretch frequency on complexation, except for (OCHNF2)-H-center dot center dot center dot and (OCH2NF)-H-center dot center dot center dot(C-1), which are red-shifting at high levels of theory and with large basis sets. The (OH)-H-center dot center dot center dot-N hydrogen bonds are very weak, with 0 K dissociation energies of only 0.2-2.5 kJ/mol, but the (CH)-H-center dot center dot center dot-N hydrogen bonds are stronger with dissociation energies of 2.7-7.0 kJ/mol at the MP2/AUG-ccpVDZ level. It is notable that the IR intensity of the N-H stretch vibration decreases on complexation for the proton donor HNO but increases for HNF2 and H2NF. A calculation investigation of the dipole moment derivative leads to the conclusion that a negative permanent dipole moment derivative of the proton donor is not a necessary condition for the formation of the blue-shifting hydrogen bond. Natural bond orbital analysis shows that for the (CH)-H-center dot center dot center dot-N hydrogen bonds a large electron density is transferred from CO to the donors, but for the (OH)-H-center dot center dot center dot-N hydrogen bonds a small electron density transfer exists from the proton donor to the acceptor CO, which is unusual except for (COH2NF)-H-center dot center dot center dot(C-s). From the fact that the bent hydrogen bonds in OC(CO)(H2NF)-H-center dot center dot center dot(C-s) are quite different from those in the others, we conclude that a greatly bent H-bond configuration shall inhibit both hyperconjugation and rehybridization.