Ab initio study of acetyl salicylic acid: NBO & AIM analyze

It is believed that a subset of pharmaceutical agents including acetyl salicylic acid (Aspirin) have low biodegradability in the sewage system. Several approaches that have been applied for the removal of this drug had little or no success. We believe that more detailed crystallographical studies on Aspirin may enhance our knowledge about the properties of this drug and hence may help to create new methods to overcome this problem. Using Density functional theory (DFT), we aimed to understand the crystal morphology of Aspirin from a new perspective. DFT calculations on conformers of aspirin were carried out for geometrical optimization and stability predictions at the B3LYP/6-311G (d,p) and B3LYP/6-311+G (3df,2p) levels of theory. These conformers are related and they differ by only the position of the carboxylic acid proton and they are rotational isomers with respect to the Ph–COOH bond. The aim of this study is to investigate the effect of the carboxylic acid group's orientations on the intra-molecular interactions and the conformational stability of these conformers. The structural parameters of intra-molecular hydrogen bonds were analyzed, while the nature of these bonds was considered using the atoms in molecules (AIM) approach. Natural bond orbital (NBO) analysis was used to determine bond orders and the effective nonbonding interactions. These results showed that there are two conformers for aspirin which both possess only one intra-molecular hydrogen bond. One of them is more stable in the gas phase, because of stronger intra-molecular hydrogen bond.

کلیدواژه ها:

Aspirin ، Density functional theory ، Atoms in molecules theory ، Natural bond orbital analysis