Molecular orbital theory of the hydrogen bond. XII. Amide hydrogen bonding in formamide–water and formamide-formaldehyde systems

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Abstract
A b initio SCF calculations with a minimal STO−3G basis set have been performed to determine the equilibrium structures and energies of dimers having formamide as the proton donor molecule and either water or formaldehyde as proton acceptor molecules. The structures of dimers in which the N−H proton ’’s−trans’’ to the carbonyl group is hydrogen bonded (t dimers) are consistent with structures anticipated from the general hybridization model. In these dimers, there is essentially free rotation of the proton acceptor molecule about the intermolecular line. When hydrogen bond formation involves the ’’s−cis’’ proton (c dimers), a single equilibrium formamide−water and formamide−formadehyde dimer exists, the structure of which is strongly influenced by the secondary factors of dipole alignment and long−range interaction. These factors are also responsible for the increased stability of c dimers relative to t dimers. A set of 1:2 formamide:water trimers has been constructed from the equilibrium formamide−water dimers, in which the formamide molecule forms two hydrogen bonds. Only in three open−chain trimers are the hydrogen bonds stronger than those of the corresponding dimers. CI calculations have also been performed to determine the effect of hydrogen bond formation on n→π* transition energies in the dimers and trimers. The n→π* transition energy of the proton acceptor formaldehyde molecule increases in the formamide−formaldehyde dimers. In these dimers, the magnitude of the blue shift is determined by the dimer hydrogen bond energy. The formamide n→π* band is also blue shifted to some extent in the formamide−water and formamide−formaldehyde dimers, even though the n→π* transition originates in the proton donor molecule. The blue shift of the formamide n→π* band in the open−chain trimers having formamide as the central molecule is equal to the sum of the blue shifts in the corresponding dimers.