Thermodynamic properties and third-law cycle for malononitrile

Abstract

The heat capacity of malononitrile, CH2( CN)2, has been determined by adiabatic calorimetry from 5' to 320OK. A gradual transition occurs in the stable form at 260.3OK. with an entropy increment of 1.15 cal. per mole O K . The compound melts at 304.9OK. with an entropy of melting of 8.46 cal. per mole ' K. Lowtemperature heat capacities were also made on the metastable crystal obtained by quenching the stable form through the 260.3OK. anomaly. This phase has a gradual transformation which takes place over the 137' to 151OK. with an excess entropy of 0.055 cal. per mole O K. The heat capacity (Cp), entropy ( S O ) , and Gibbs energy function [ -(GO -/ -/ : ) / T I at 298.15'K. are 26.36, 31.30, and 15.34 cal. per mole ' K., respectively. The approximate accord of the experimental entropy of gaseous malononitrile at 298.15'K. (68.5 =! z 0.4 cal. per mole O K . ) with the value (69.02) calculated from spectroscopic data indicates the absence of disorder in the crystal at Oo K.

Molecular motion in succinonitrile and malononitrile by nuclear magnetic resonance

“…(CNCH2CN) is a similar molecule to succinonitrile and the question arises as to whether it too may have a cubic rotator phase. Heat capacity measurements [25] indicate that there are two solid phases. Phase I exists from the melting point at 304-9~ to 260-3 ~ but this form is readily supercooled to any lower temperature.…”

“…The transformation from I to II, which is the true equilibrium phase below 260.3 ~ is very slow, the transformation being achieved for instance by holding at 190~ for several days. Girdhar et al [25] conclude, from the high value of the entropy of melting, that phase I is not a plastic crystal and presumably that there is no large scale molecular motion in either solid phase. However, the thermal measurements are not entirely conclusive and so we have sought to settle the question of whether malononitrile has a plastic phase by use of N.M.R.…”

Polymorphism of malononitrile. Crystallographic characterization of theL–Fphase transition

“…can be easily undercooled), whereas the reverse L --, F transition is very fast and shows a normal specific-heat anomaly at 260 K (Girdhar et al, 1968).…”

Fusion and solid-to-solid transitions of a homologous series of alkane-α,ω-dinitriles

“…those for maloninitrile [1,32,34] and succinonitrile [2]. The investigations of glutaronitrile only provided evidence of the existence of a metastable phase crystal II but the thermodynamic parameters of its transition to crystal I were not determined [4].…”

“…A first-order transition at T = 260 K observed by using calorimetry [32] and nuclear quadrupole resonance [36] was attributed to conversion of the undercooled (metastable) d phase to b phase [35]. Another first-order transition was observed at about 2 K below the fusion temperature [32]. We detected the solidto-solid transitions occurring in our measurement temperature range and found T trs = (260.9, 294.2, and 303.6) K in very good agreement with the literature data (table 3).…”