Acta Phys. -Chim. Sin. ›› 2012, Vol. 28 ›› Issue (04): 773-780.doi: 10.3866/PKU.WHXB201202132


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
  • 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).

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


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