On the Factors Affecting H-Bonding : A CCSD(T) // B3LYP Study on Malonaldehyde Cation-Radical

Among the factors that may affect H-bond strength, the effect of the molecular ionization has not received any considerable attention. Here, malonaldehyde and its cation-radical as models were studied using DFT (B3LYP) and ab-initio theories (HF, MP2, and CCSD(T)). Based on establishing a comparison with the CCSD(T)/CBS energy barrier (from literature), we have found that the CCSD(T)/D95(d,p)//B3LYP/6-31G(d,p) computational protocol can be used as a reference for evaluating performance of the B3LYP and the MP2 methods on this model. In this computational organic chemistry study several aspects were investigated, they are: structures, energies, natural population analysis (NPA) charges, intramolecular charge transfer (delocalization) energies, and atoms in molecule (AIM) electron densities with regard to the Hbonding. The results indicate that loss of an electron can cause considerable effects on the various electronic structure aspects and the details are presented and discussed. All the aspects confirm that ionizing the molecule disturbs the electron density distribution in the HOMO orbital and makes the two oxygen atoms, relatively, electron deficient centers which consequently weakens both the H1-O1 bond and the O2/H1-O1 H-bond. The details of this investigation are new addition to the hydrogen bond theory. Other types of molecular models and computational protocols are of interest for future investigations. Keyword: Malonaldehyde cation-radical; DFT; MP2; CCSD(T); NBO; AIM; H-bond.