부산대학교 도서관

The crystal structure of squaric acid [3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4, M = 114.1] has been determined from neutron diffraction data at 15 K, Tc − 21 K, Tc − 13 K and Tc + 5 K below and above the phase transition at 374 K. The space group of the material is monoclinic P21/m from 15 K and up to Tc and tetragonal I4/m above. After corrections for absorption and thermal diffuse scattering, least-squares refinements were carried out with 69 variable parameters in the model below Tc and 25 above Tc. The analysis of difference vibrational parameters along the C–O internuclear direction strongly suggest that the high-temperature structure has a disordered C4O4 unit. In agreement with previous findings, the protons are distributed on two principal positions 0.53 Å apart along the O … O hydrogen bond above Tc. In the low-temperature phase, an increasing amount of disorder with increasing temperature is found, particularly in the protonic positions. However, the protons in the two different hydrogen bonds show distinctly different behavior, and hence immediately below Tc different degrees of order are found along the x- and z-directions. These structural findings are discussed in the light of the 1-D correlations observed in the material.