磺胺甲恶唑在羟基自由基作用下降解机理的密度泛函研究

Density functional studies of the reaction mechanism of sulfamethoxazole with hydroxyl radical

在分子水平上研究抗生素降解对减少环境中抗生素污染有重要意义。对磺胺甲恶唑(SMX)与羟基自由基(HO·)发生反应而降解的机理进行了密度泛函(B3LYP)研究,从理论上不仅得到了试验中检测到的产物,还发现了试验中尚未检测到且易生成的产物。结果表明,HO·较易进攻SMX的C3位,试验难以确定的HO·取代位置可能为C3位。HO·进攻C1位使S—C键裂解,可得到对氨基苯酚;HO·进攻C4位使N—C键裂解,可得到3-羟基-5-甲基异恶唑,这2种产物尚未被试验检测到。HO·进攻S或N原子使S—N键裂解的反应能垒分别为139.45 kJ/mol和124.56 kJ/mol,而三重态的SMX自身发生S—N键裂解的反应能垒仅为16.90 kJ/mol,这表明S—N键的裂解可能是SMX受光激发所致,并通过NBO分析揭示了裂解的原因。

This paper intends to bring about our study results of the density function of the reaction mechanism of sulfamethoxazole with hydroxyl radical （HO·）. As is known, density functional theory （B3LYP） can be applied to study the degradation mechanism of sulfamethoxazole （SMX） by hydroxyl radicals （HO·） attacking different sites of SMX. However, all the electronic structures are to be fully optimized with the basic set of 6-31＋G（d,p） for H atom and 6-31G（d） for the remaining atoms. In order to meet this need, we have worked out the solvation effect and NBO made analysis at the B3LYP/6-311＋G（3df,3p） level. In so doing, we have gained primary degradation products including the experimentally detected and undetected ones. The calculation results show that it would be easier for HO· to attack C3 site than C2 site in six-membered aromatic ring of SMX. Thus, we have determined the hydroxy-substituted site, which could not be identified in the previous experiments. The C1 site has been found attacked by HO· due to the barriers of 30.59 kJ/mol and 30.88 kJ/mol for two different interactive manners. Both of the processes lead to the breach of S-C bond, resulting in the product of p-amino-phenol, though it had not been found in the experiments. HO· attacking C4 site tends to lead to the cleavage of N-C bond, the reaction can result in the product of 3-hydroxyl-5-methylisoxazole, which hadn＇t been detected in the previous experiments, either. However, although the attack of S or N atoms is by HO· may lead to the S-N bond cleavage, the reaction processes can also form a high energy barrier of c.a. 125.00 kJ/mol and make it hard to come about. However, it would be quite easy for the triplet SMX to directly break the S-N bond with an energy barrier of 16.90 kJ/mol, which suggests that the cleavage of S-N bond might have resulted from the direct photodegradation of SMX. When making a careful comparison with the singlet SMX, it would be found very easy for the triplet SMX decomposed via S-N bond cleavage. Hence, all the above results indicate that the cleavage of S-N bond does not result from HO· attacking but likely from photoexitation. The NBO analysis can thus be used for the researchers to gain the insight into the degradation of the triplet SMX.