蓝移氢键起因的理论计算研究

【作者】 纪爱萍；

【导师】 张桂秋；

【作者基本信息】 山东师范大学 ， 物理化学， 2008， 硕士

【摘要】 氢键在化学和生物科学中具有特别重要的作用,长期以来一直是科学家们关注的重点。蓝移氢键的发现,更是引起了许多研究者的兴趣。近十年来,许多理论化学工作者对蓝移氢键的本质进行了报道。蓝移氢键与正常的红移氢键有着不同的红外光谱性质,它的主要特征是电子供体X-H键收缩,伸缩振动频率增大,发生蓝移。目前,对蓝移氢键的起因主要有两种流行的理论解释,一是Hobza提出的电子密度重新排布,另一种是Weinhold提出的轨道重新杂化解释。值得注意的是前面所研究的蓝移氢键主要集中在直接参与氢键形成的X-H键中,但有些实验发现在不参与氢键形成的一些键中也存在蓝移。能否用Hobza及Weinhold的理论来解释不参与氢键形成的一些键的蓝移呢?最近Hobbza小组对HCOOH···HF和HCOO-···HF/H2O/NH3/CH4复合物中的C-H的蓝移进行了研究,他们认为从电子供体到电子受体的电子密度转移会导致电子供体内电子密度的重新排布,产生一个新的电子结构,从而使得不参与氢键的C-H键蓝移。为了进一步研究不直接参与氢键形成的C-H键蓝移的起因,我们对甲酰胺和HF等小分子形成的氢键复合物进行了理论计算研究。本论文的主要结论如下:(1)对NH2CHO···HF/H2O/ H2S/NH3复合物中不参与氢键的C-H和C-N键,不管他们的位移是红移还是蓝移,其主要原因是电子密度重新排布和轨道重新杂化两种作用共同作用导致的。(2)同样在甲酰胺和甲醇形成的复合物中,再次证明不参与氢键的键的红移和蓝移是由电子密度重新排布和轨道重新杂化两种作用共同作用的结果。(3)对formic acid—HF/H2O/NH3和formate anion—HF/H2O/NH3/CH4复合物中的C-H键,不管其位移是红移还是蓝移,同样是由电子密度重新排布和轨道重新杂化两种作用共同引起的,对formic acid―H2O(1)复合物中不参与氢键的C-H键的红移,仅用电子密度重新排布是不能解释的。在氢键复合物中,由于质子受体和质子供体间发生电荷转移,从而会导致一个新的电子结构的产生并且引起单体几何结构的变化。Hobza的电子密度重新排布理论和Weinhold的轨道重新杂化理论不仅可以解释氢键复合物中一些蓝移氢键的起因,而且还可以解释不直接参与氢键形成的一些键的红移和蓝移的起因。更多还原

【Abstract】 Hydrogen bond has been one of the most interesting areas mainly due to its vital role in many chemical and biological processes. The improper blue-shifted hydrogen bond has attracted great interest to many scientist since it was detected. In the last 10-odd years, numerous theoretical studies on the nature of improper blue-shifted hydrogen bond have been reported.Improper blue-shifted hydrogen bond is characterized by contraction of the X-H bond in the electron donor and a concomitant increase (blue shift) of the X-H stretch frequency. It’s IR spectrum character differs from that of the red-shifted hydrogen bond. Now, two prevailing theories are applied in rationalizing the origin of blue-shifted H-bond. One theory, suggested by Hobza and his co-workers, is based on reorganization of electronic density in electron acceptor. The other, developed by Weinhold et al., is based on rehybridization. It must be emphasized that previous studies on the origin of blue shifts completely focused on the C-H bond directly participating in H-bond formation. However, some experiments indicated that blue shifts also exist in some bonds not directly participating in H-bond formation. In a very recent paper, Parreira et al. have studied the blue shift of C-H in formic acid―HF and formate anion―HF/H2O/NH3/CH4 complexes. Their results showed that pronounced electron-density transfer (EDT) from the electron donor to the electron acceptor led to reorganization of the electron density (ED) in electron donor, and resulted in formation of a new electronic structure and the blue shift of C-H bond not directly participating in H-bond formation. To further explore the origin of the blue shift for C-H bond not directly participating in H-bond formation, we present a theoretical investigation on the complexes formed by formamide and small molecules such as HF et al.The most important results are as follows:i. Whether C-H and C-N bonds not directly participating in H-bond formation in NH2CHO···HF/H2O/H2S/NH3 complexes is red-shifted or blue-shifted depends on the electron density redistribution and rehybridization.ii. For formamide-methanol complexes, the result again proved that the origin of the red and blue shifts of some bonds not directly participating in H-bond formation was due to the result of the electron density redistribution and rehybridization effect .iii. The red and blue shifts of C-H bond in formic acid—HF/H2O/NH3 and formate anion—HF/H2O/NH3/CH4 complexes can be also explained by redistribution and rehybridization. The red and blue shifts of some bonds not directly participating in H-bond formation in some complexes can’t be explained only by electron density redistribution.Electron-density transfer (EDT) from the proton acceptor to the proton donor in hydrogen bond complexes leads to formation of a new electronic structure and changes of geometry of monomer. Hobza’s electron density redistribution theory and Weinhold’s rehybridization theory not only can explain the origin of blue-shifted H-bond for some hydrogen bond complexes, but also can explain the origin of the red and blue shifts of some bonds not directly participating in H-bond formation.更多还原