On the Structure of Some Pentaerythritol-Related Molecular Crystals with Pseudotetrahedral Molecular Structure. Ordered Structure of 2, 2-Bis(bromomethyl)-1, 3-propanediol and Orientationally Disordered Structure of 2-Chloromethyl-2-methyl-1, 3

摘要

C(CH_2Cl)_3(CH_3), melting point T_m = 291.3 K, shows in the DTA/DSC experiment a phase transition from an orientationally disordered plastic phase Ⅰ into an ordered phase Ⅱ at T_(Ⅰ→Ⅱ) = 235.9 K. From 77 K up to T_(Ⅱ→Ⅰ) the ~(35)Cl NQR spectrum is a triplet with (ν in MHz, T = 77 K) ν_1 =34.213, ν_2 = 34.183, ν_3 = 33.786. The ~(35)Cl NQR fade-out temperature T_f = 247 K coincides with T_(Ⅱ→Ⅰ) found from the DSC experiment. The plastic phase of 2-chloromethyl-2-methyl-1, 3-propanediol is cubic bcc, Im3m, Z = 2 and the lattice constant increases linearly 755 pm at 235 K to 768 pm at 283 K. In the heating cycle we found (ΔH in kJ/mol): ΔH_(Ⅱ→Ⅰ) = 12.0, ΔS_m/R = 1.0. The compound belongs to the group of plastic molecules formed by ellipsoid like distorted tetrahedra and is derived by substitution from methane. The crystal structure of 2, 2-bis(bromomethyl)-1, 3-propanediol was determined. DTA-DSC show that an orientationally disordered plastic phase does not exist. At room temperature C(CH_2Br)_2(CH_2OH)_2 crystallizes monoclinic, space group Cc, Z = 4, a = 628.2 pm, b = 2015.5 pm, c = 659.6 pm, β = 94.27°. The molecules interact by intermolecular hydrogen bonds between the OH-groups and by van der Waals forces Br • • • Br.