Different aspects of an interesting glassy crystal: 1-cyanoadamantane

Abstract

All the different crystalline (brittle (II), glassy (I_g), plastic stable (I) and metastable (I')) phases of 1-cyanoadamantane: C₁₀H₁₅CN are characterised by differential scanning calorimetry studies. The transition temperatures and their respective enthalpies are determined I_g-I' (170K), II to I (280K, 5.5 kJ mol^-1), I-liquid, (458K, 15 kJ mol^-1). A difference of heat capacity between the I_g and I' phases is observed: Delta C_p=(16.0+or-1.3) J mol^-1K^-1. The structure of the ordered phase is analysed at two temperatures: 135 and 420K. The space groups is C₂/m with Z=4 and A_M=11.278 AA, B_M=6.874 AA, C_M=12.092 AA and beta =101 degrees 37' (at 240K). In this ordered phase the antiparallel order of the dipole moments is pointed out. From the positions of the molecular mass centres, a pseudo-cubic lattice is built which tends to the cubic plastic lattice at the transition II to I. The orientations of the dipolar axes are compared with those found in the plastic phase. The longitudinal relaxation time T_1Z in ¹H-NMR gives a very good description with temperature of the molecular motions in all the different crystalline phases. The residence times deduced from this NMR study agree very well with the corresponding ones obtained with dielectric relaxation and incoherent neutron diffusion. In the glassy state a change in the uniaxial rotation occurs which may be attributed to a large distribution of residence times.