中文摘要：

为了解决D-和L-丙氨酸在约270K相变的分岐和机理,对其单晶、多晶粉末及原料利用微分扫描量热仪测定比热.用三线法以蓝宝石作校正,并与手册的D-和L-丙氨酸标准比热值比较.在单晶中,实验观察到吸热相变峰最高处时的温度及热焓为：D-丙氨酸,Tc=272.02K,△H=1.87J·mol^-1;L-丙氨酸,Tc=271.85K,△H=1.46J·mol^-1;热焓差为0.41J·mol^-1.参比晶体D-缬氨酸,Tc=273.59K,△H=1.75J·mol^-1;L-缬氨酸,Tc=273.76K,△H=1.57J·mol^-1;热焓差为0.18J·mol^-1.实验发现已测量过的单晶磨成多晶粉末后再测,相变峰消失.说明相变与晶格有关.变温中子衍射排除了D→L的构型相变,但发现N＋H…O-氢键沿D-和L-丙氨酸单晶的c轴反向变化.变温偏振拉曼散射反映相变机制与N＋H…O-中电子的轨道磁偶极矩相关,观察到偏振光的不对称散射.在外加磁场强度H为＋1T和-1T下,变温测定D-和L-丙氨酸晶体的直流磁化率,证明在270K有电子自旋翻转的相变.电子自旋的向上或向下,取决于晶格中NH＋3的扭曲振动及N＋H…O-氢键沿晶体c轴的方向.由于自旋的定轴性,可以解释单晶和多晶粉末比热结果的分岐.

英文摘要：

With a view to understanding the argument of phase-transition mechanisms of D-and L-alanine at around 270 K,the temperature dependence of heat capacity measurements was investigated, for single crystals,ground powders,and polycrystalline products,using differential scanning calorimetry （DSC）.The Cp （heat capacity under constant pressure）values of D-and L-alanine were calibrated with standard sapphire by the triple-curve method;these values coincide with the standard C p values in the literature.Endothermic transition peaks were observed at Tc =272.02 K,△H=1.87 J·mol^-1 and Tc =271.85 K, △H=1.46 J·mol^-1 for D-and L-alanine,respectively,and Tc =273.59 K,△H=1.75 J·mol^-1 and Tc =273.76 K, △H=1.57 J·mol^-1 for the reference crystals D-and L-valine,respectively.The energy differences of 0.41 J·mol^-1 for D-and L-alanine and 0.18 J·mol^-1 for D-and L-valine,which were observed from pre-aligned molecules in the single crystals and vanished in the ground powders and polycrystalline products,show that the phase transition is related to the crystal lattice.Neutron diffraction results exclude the possibility of a D→L configuration change,and show that the hydrogen bonds run antiparallel to the c-axis in the D-and L-crystals.Polarized Raman vibrational spectroscopy shows that the transition mechanism may be related to the electronic orbital angular momentum and magnetic dipole moments of N ＋ H···O - in the crystals.External magnetic fields,H=＋1,-1 T,were applied parallel to the c（z）-axis of the D-and L-alanine crystalline lattices, respectively.The DC-magnetic susceptibilities show electron spin-flip transitions at around 270 K in D-and L-alanine.The spin is＂up＂or＂down＂relative to the direction of N＋ H···O- bond along the c（z）-axis.Based on spin rigidity and magnetic anisotropy,the results help to explain the discrepancies among heat capacity and magnetic susceptibility data for single crystals and polycrystalline powders of D-and L-alanine.