A DFT study of hydrogen bond formation into nucleophilic diamines in aprotic solvent media

F. BERGERO; C.E.S. ALVARO; N. NUDELMAN; S. RAMOS DE DEBIAGGI

Journal of the Argentine Chemical Society

ASOCIACIÓN QUÍMICA ARGENTINA

Lugar: Buenos Aires; Año: 2013 vol. 100 p. 35 - 47

0365-0375

A detailed theoretical study on 3-dimethylamino-l-propylamine (DMPA), 1-(2-aminoethyl)-piperidine (2-AEPip) and 3-(aminopropyl)-morpholine (3-APMo) is presented. These are special nucleophiles of appropriate flexible structure, regarding their inter- and intramolecular hydrogen bond (H-bond) self-aggregation states, that are of interest in connection with our last studies of Aromatic Nucleophilic Substitution (ANS) carried out in aprotic solvents.According to kinetic results, ANS reactions of 1-halo-2,4-dinitrobenzenes with DMPA and 2-AEPip in toluene are third-order in amine (overall fourth order kinetics), results that can be interpreted in terms of the "dimer nucleophile mechanism". By contrast, the reactions with 3-APMo shows second-order in amine, consistent with the classical ANS mechanism, suggesting that this diamine reacts in the monomeric state due to an internal H-bond formation ("intramolecular dimer").To provide valuable insight into the predominant type of H-bond formed we performed ab initio Density Funcional Theory calculations on the above mentioned amines determining the optimal geometry and its corresponding energy in vacuum for monomers and dimers at the B3LYP/6-31++G(d) level. We implemented a methodology to simultaneously evaluate Counterpoise corrections and solvent effects within the polarized continuum model (PCM). In all cases we found that solvation energies are favorable and H-bonded dimers are more stable than their monomers. Consistent with kinetic results, for 3-APMo the dimerization energy is much lower than for 2-AEPip and DMPA. These theoretical findings are significant and are in line with available experimental results that shows the nucleophile structure is crucial to determine the predominant type of H-bonds in ANS reactions of diamines in aprotic solvents.