N-苄叉基-2-氨基噻唑及其类似物的构象和分子内的扭曲驱动力

确定了N-苄叉基-2-氨基噻唑(2a)和N-对硝基苄叉基-2-氨基噻唑(2b)的晶体结构.结合已报道的N-对硝基苄叉基-2-氨基嘧啶(1a),N-对硝基苄叉基-2-氨基吡啶(1b)和N-苄叉基-3-氨基吡啶(2c)的晶体结构,利用AM1,RHF,DFT方法和6-311G,6-311G^**基组,优化每个分子的22个旋转构象(θ=0°～90°).由DFT法所得到的最优构象的扭角θ(1a,22°;1b,0°;1c,42°;2a、2b:0°)与实验值(1a,26°;1b,20°;1c,46°;2a,8.8°;2b,3.8°)最接近.尽管分子最优构象扭角的差异很大,但总电子能最稳定的构象都在θ=±42°附近.在任一分子、任何电子态中,离域的π体系总是失稳定的,全平面构象不是π体系最稳定的构象.无论是离域的还是定域的π体系,它们均倾向于扭曲的几何构象.π电子的离域是分子扭曲的驱动力之一.与经典观点相反,非键原子间的核排斥作用是分子扭曲的阻力,而不是动力.

The Conformation of N-Phenylmethylene-2-thiazoleamine Species and the Driving Forces for Twisting Molecule

The crystal structures of N-phenylmethylene-2-thiazoleamine (2a) and N-[(4-nitrophenyl) methylene]-2-thiazoleamine (2b) were determined.Twenty-two rotational geometries (θ=0°～ 90°) of each of two molecules were optimized using AM1,RHF/6-311G**,B3LYP/6-311G and B3LYP/6-311G**.The twist angles of their preferential geometries obtained from DFT are both 0°,which are most close to the experimental values (2a,θ=8.9°; 2b,θ=3.9°) of all angles obtained from various optimized methods.Although there is a great difference in the twist angles of the five molecules,2a,2b,N-(4-nitrophrnyl)methylene-2-pyrimidineamine (1a),N-(4-nitrophenyl)methylene-2-pyridine-amine (1b) and N-phenyl) methylene-3-pyridineamine (1c),their lowest total electronic energies occur in the optimized geometries with the similar twist angles which are in the region of θ from 37 to 42° and almost have nothing to do with the optimized methods (HF and DFT) and Gaussian basis sets.The π system in the geometry with θ= 0° is destabilized no matter whether it is delocalized or not.It appears that the π system always prefers a twisted geometry.The driving force for out-of-plane twist of the molecules arises from the electronic interaction,and the nuclear repulsion is,practically,a resistance for distortion of molecule.