溴代蔗糖的密度泛函理论研究

Density Functional Theory Studies on Bromo Substituted Sucroses

采用密度泛函B3LYP/6-31G**方法,计算研究了45个溴代蔗糖的结构和性能,重点探讨了取代基效应.计算发现,C6和C3位取代物能量较低,而C3’和C2位取代物能量较高,8个一溴蔗糖和28个二溴蔗糖的能量差异分别达到42.46和72.20kJ.溴取代对远离取代位的键长、键角和原子电荷影响较小,但对蔗糖分子的立体构型、偶极矩和取代位附近的电荷分布影响较大.C3’和C6位取代可使偶极矩增大,而C2和C4’位取代使偶极矩减小.溴取代物的前线轨道能隙△E均低于蔗糖,在C3’位取代△E较低,而在C6位取代△E较高,与C3’位取代能量较高稳定性较低而C6位取代能量较低稳定性较高相一致.溴代蔗糖的红外光谱与蔗糖很相近,在1000～1200cm-1和3500cm-1处分别有很强的C-O和O-H伸缩振动吸收峰,且峰的位置变化不大.在振动分析基础上,通过统计热力学方法计算得到了热力学函数及其与温度的关系.

The structures and properties of 45 bromo substituted derivatives have been studied at the B3LYP/6- 31G^＊＊ level of density functional theory. The calculated results show that the isomers with Br substitution at C6 and C3 positions have lower energy, while at C3＇ and C2 higher energy. The largest energy differences among mono- and di- bromo sucroses are 42.46 and 72.20 kJ respectively. Bromo substitution has little effect on the bond lengths, bond angles, and charge distributions on the atoms far from the substitution positions, while greatly affecting the steric structure, dipole moment and charge distributions near the substitution positions. Substitutions at C3＇ and C6 positions lead to larger while at C2 and C4＇ to smaller dipole moment. Bromo substitution decreases the energy gap AE between the frontier molecular orbitals of sucrose. The AEs of the derivatives are relatively lower with Br substitution at C3＇ and higher at C6. The IR spectra of all sucroses are quite similar. There are strong absorption bands around 1000- 1200 cm^-1 and 3500 cm^-1, which correspond to the stretch vibration of C-O and O-H, respectively. Moreover, the positions of these peaks are essentially fixed in different sucrose derivatives. Thermodynamic functions have been calculated on the basis of vibrational analysis with statistical thermodynamic method and the relationships between the thermodynamic functions and temperature are provided.