Theoretical Study Of The Impact Factors On AA-DD/AAAA-DDDD Multiple Hydrogen-bonding Dimers Stability

As we all know multiple hydrogen-bonds arrays have been widely used in supramolecular chemistry, and a larger number of chemists are devoted to the study of polymers based on hydrogen-bonding with strong stability and high polymerization degree in recent years. The efforts of Leigh group in experiment certify that AAAA and DDDD monomers (A:hydrogen-bonding acceptor; D:hydrogen-bonding donor) can form strongly stable dimers in any conditions. To further understand the nature of H-bonds and the influencing factors of polymerization, we accurately calculate the contribution of electronegativity of X atom in X-H…Y, hydrogen bonds number and solvent polarity to the stability of dimers by using DFT method. Detailed results are as follows:Our theoretical calculation of dimer unit is based on hydrogen-bonding donor with N-H and hydrogen-bonding acceptor with N atom. We have performed detailed DFT study about the effect of secondary electrostatic interaction, hydrogen-bonding number and the solvent polarity on the stability of dimers. Adding two N-H bonds in hydrogen-bonds donor unit can improve the binding energy by 5.98 kcal/mol and reduce free energy of dimerization by 7.07 kcal/mol. The Frequency and NBO results of dimers 1a,2b,3d certify that all intermolecular H-bonds show a clearly red shift when monomers form dimers. The calculations of energies indicate that adding two intermolecular hydrogen bonds to dimers will increase the binding energy of 19.78 kcal/mol and will decrease the free energy of 12.17 kcal/mol. Meanwhile, the obvious difference of λmax and absorption peaks number are found between monomers and dimers from UV-visible spectra, and it is can be regarded as a significant evidence to identify the formation of dimers and dimerization degree.DFT simulation is carried out on three dimers formed an AAAA accepter and three DDDD donors. The purpose of this study is to reveal contribution of X atom in X-H--Y to the stability of hydrogen-bond arrays. Energy calculation and AIM analysis indicate that the dimers stabilities have a great decrease when the intermolecular N-H bonds are replaced by C-H bonds in any conditions (vacuum, dichlomathane, acetonitrile, water), respectively, the binding energy of N-H…N is about 5.41 kcal/mol larger than C-H…N, and the dimers stabilities in order of 2b> 2e> 2f. Meanwhile, the polarity of sovent has a clear influence on the polymerization process. Geometries and NBO analyses prove that intramolecular and intermolecular hydrogen bonds exist within the monomers and dimers.1H NMR and UV-visible spectra show an evident difference of between monomer and dimer, it may be used a powerful information to distinguish the formation of polymers.