物理化学学报 >> 2012, Vol. 28 >> Issue (04): 773-780.doi: 10.3866/PKU.WHXB201202132

热力学，动力学和结构化学 上一篇    下一篇

比热和直流磁化率证明N+H…O-氢键的电子自旋翻转在D-和L-丙氨酸单晶中的不对称相变

王文清1, 沈新春1,2, 吴季兰1, 龚?1, 申国华1, 赵洪凯1   

  1. 1. 北京大学化学与分子工程学院应用化学系, 北京分子科学国家实验室, 北京 100871;
    2. 山东大学化学与化工学院, 济南 250061
  • 收稿日期:2011-10-31 修回日期:2012-01-19 发布日期:2012-03-21
  • 通讯作者: 王文清 E-mail:wangwqchem@pku.edu.cn
  • 基金资助:

    国家自然科学基金(21002006, 20452002)和国家科技部基础研究重大项目(2004-973-36)资助

Heat Capacity and DC-Magnetic Susceptibility Evidence for the Asymmetry of Electron Spin-Flip Phase Transition of N+H…O- Bond in Chiral Alanine Crystal

WANG Wen-Qing1, SHEN Xin-Chun1,2, WU Ji-Lan1, GONG Yan1, SHEN Guo-Hua1, ZHAO Hong-Kai1   

  1. 1. Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China;
    2. School of Chemistry and Chemical Engineering, Shandong University, Jinan 250061, P. R. China
  • Received:2011-10-31 Revised:2012-01-19 Published:2012-03-21
  • Contact: WANG Wen-Qing E-mail:wangwqchem@pku.edu.cn
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (21002006, 20452002) and Special Program for Key Basic Research of the Ministry of Science and Technology, China (2004-973-36).

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摘要： 为了解决D-和L-丙氨酸在约270 K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=272.02 K, ΔH=1.87 J·mol-1; L-丙氨酸, Tc=271.85 K, ΔH=1.46 J·mol-1; 热焓差为0.41 J·mol-1. 参比晶体D-缬氨酸, Tc=273.59 K, ΔH=1.75 J·mol-1; L-缬氨酸, Tc=273.76 K, ΔH=1.57 J·mol-1; 热焓差为0.18 J·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+1 T和-1 T下, 变温测定D-和L-丙氨酸晶体的直流磁化率, 证明在270 K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NH3+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.

关键词: 比热, 直流磁化率, N+H…O-氢键, 电子自旋翻转, 不对称相变, D-和L-丙氨酸单晶

Abstract: 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 Cp values in the literature. Endothermic transition peaks were observed at Tc=272.02 K, ΔH=1.87 J·mol-1 and Tc=271.85 K, ΔH=1.46 J·mol-1 for D- and L- alanine, respectively, and Tc=273.59 K, ΔH=1.75 J·mol-1 and Tc=273.76 K, ΔH=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 Lcrystals. 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.

Key words: Heat capacity, DC-magnetic susceptibility, N+H…O- bonding, Electron spin-flip, Asymmetry transition, D- and L-alanine crystals