Theoretical Investigation of the Cooperativity in CH3CHO·2H2O, CH2FCHO·2H2O, and CH3CFO·2H2O Systems

The hydrogen bond interaction between CH3CHO, CH2FCHO, and CH3CFO and two water molecules is investigated at the B3LYP/6-311++G(d,p) level. The results are compared with the complexes involving the same carbonyl derivatives and one water molecule. The calculations involve the optimization of the structure, the harmonic vibrational frequencies, and relevant NBO (natural bond orbital) parameters such as the NBO charges, the occupation of antibonding orbitals, and intra- and intermolecular hyperconjugation energies. Two stable cyclic structures are predicted. The two structures are stabilized by C = O？HO hydrogen bond. The A structures are further stabilized by CH？O bond involving the CH3 (CH2F) group. This bond results in an elongation of the CH bond and red shift of the ν(CH) vibration. The B structures are stabilized by CH？O interaction involving the aldehydic CH bond. The formation of this bond results in a marked contraction of the CH bond and blue shift of the ν(CH) vibration indicating the predominance of the lone pair effect in determining the CH distances. The total interaction energies range from ？12.40 to ？13.50？kcal mol？1. The cooperative energies calculated at the trimer geometry are comprised between ？2.30 and ？2.60？kcal mol？1. 1. Introduction Cooperative interactions involving three or more molecules are an important component of intermolecular interactions, particularly those involving hydrogen bonds. The cooperativity of hydrogen bonds plays an important role in controlling and regulating the processes in living materials. This has been recognized since a long time and quantitative aspects of cooperativity have been discussed [1–7]. Cooperativity can be positive or negative. It has been shown that a first hydrogen bond involving a given site of a molecule weakens the reactivity of the neighboring sites of the same nature whereas it enhances the electron donor power of the adjacent sites of opposite nature [4]. Cooperativity between water molecules is particularly important, because in liquid water at room temperature, the great majority of the molecules are hydrogen-bonded to each other, the concentration of “free” OH groups being very low [8]. For this reason, extensive theoretical calculations have been carried out on the cooperativity in water [9–13]. In recent works, the interaction between proton acceptors (or donors) and two (or more) water molecules has been discussed [14–21]. In a recent work [22], the complexes between acetaldehyde and some of its monofluorinated derivatives and one water molecule have been investigated by