甲酸與氟化甲酸（順、反式）組合二聚物之分子間氫鍵理論研究

Inter-molecular Hydrogen Bonds in a Set of Dimers Constructed with HC02H, Syn-FCO_2H and Anti-FCO_2H

陳世忠(Shyh-Jong Chen) ； 陳成(Cheng Chen) ； 劉權文(Chuan-Wen Liu)

《化學》 61卷3期 (2003/09) Pp. 463-474

https://doi.org/10.6623/chem.2003048

摘要

由Gaussian 98程式群中選用的B3LYP/6-311+G (2d,p)為基底之密度泛函方法，研究甲酸(HCO_2H)、順式氣化甲酸(syn-FCO_2H )及反式氟化甲酸(anti-FCO_2H)所組合形成之二聚物(dimers)分子間雙氫鍵；並將計算之結果以我等多年研讀成功之半經驗「區域化鍵分佈分析法」，進一步分析此六種二聚物組合分子所形成之氫鍵，作區域化之氫鍵強弱比對，發現甲酸分子之C=O鍵與順式氟化甲酸分子之-OH鍵所形成之氫鍵( C=O……H-O型式)最強；二個反式氟化甲酸分子組合之-CF鍵與-OH鍵所形成之氫鍵(C-F……H-O型式)最弱。經核對所有氫鏈強弱之次序與二聚物組合分子之幾何結構，發現氫鍵強弱之順序與氫鍵距離( d_H…o 或d_H…F )縮短、O-H鍵與C=O鏈之鍵距(d_(O-H)或d_(C=O))拉長及振動頻率(ν_(C=O)及ν_(O-H))減少，所產生之紅位移(red shift)等現象之變化次序完全吻合。以能差法計算兩氫鍵總能量及相關氫鍵自由能差，並作BSSE (basis set superposition error)之修訂後，經能差之計算結果比較，我們發現氫鍵之總能及自由能差次序與由「區域化鍵分佈分析法」所得之結果一致。另由熱力學觀點我們更發現此六種二聚物中，與反式氟化甲酸分子組合形成之氫鍵(C-F…H-O型式)，在室溫下其自由能差(ΔG)值很小且為負值(negative values)，依此可推斷該型氫鏈在室溫以上是不易形成的。

關鍵字

區域化鍵分佈分析法 ； 二聚物組合分子 ； 紅位移 ； 能差 ； 自由能

並列摘要

A theoretical study of the two intermolecular H-bonds (hydrogen bonds) of various dimers of HCO_2H, s(syn)-FCO_2H and a(anti)-FCO_2H by the DFT type B3LYP/6-311+G(2d,p) calculations: to compare localized bonding strengths of each H-bond in all of the six dimer constructed by our newly developed ＂localized hydrogen bonding analysis methods＂, the strongest H-bond is connected by -C=O of HCO_2H and HO- of S-FCO_2H, while the weakest H-bonds are the two O-H... .F bonds in the (a-FCO_2H)_2 dimer. All orders of H-bonding strengths are checked well with the order of shortness of H-bond distances (dH....o or dH....F), elongation of the bond distances (d_(O-H) and d(C=O)), and the red shift of characteristic frequencies of both ) ν_(O-H) and ν_(C=O). With BSSE (basis set superposition error) correction, the energy differences of dimers and their related monomer are calculated for comparison. Results of the energy differences are checked well with results of the above mentioned local analysis. Comparing the differences of the Gibbs energy in the six dimers, we have discovered that H-bonds of the O-H.....F types are very weak from the thermodynamic point of view. Under room temperature, ΔG of the dimers formed with a-FCO_2H are of negative values, so that H-bonds of this kind cannot easily be constructed under ordinary conditions.

並列關鍵字

localized hydrogen bonding analysis methods ； dimer constructs ； red shift ； energy difference ； Gibbs energy