Entropy, related thermodynamic properties, and structure of methylisocyanate

The entropy and related thermodynamic properties of methylisocyanate, CH3NCO, have been determined by isothermal calorimetry. The entropy in the ideal gas state at 298.15 K and 1 atmosphere is Smo = 284.3 ± 0.6 J/K · mol. Other thermodynamic properties determined include: the heat capacity from 15 to 300 K, the temperature of fusion (Tfus = 178.461 ± 0.024 K), the enthalpy of fusion (ΔHfus = 7455.2 ± 14.0 J/mol), the enthalpy of vaporization at 298.15 K (ΔHvap = 28768 ± 54 J/mol), and the vapor pressure from fusion to 300 K. Using statistical thermodynamics, the entropy in this same state has been calculated for various assumed structures for methylisocyante which have been proposed based on several spectroscopic and ab initio results. Comparisons between the experimental and calculated entropy have led to the following conclusions concerning historical differences among problematic structural properties: (1) The CNC/CNO angles can have the paired values of 140/180° or 135/173° respectively. It is not possible to distinguish between the two by this thermodynamic analysis. (2) The methyl group functions as a free rotor or near free rotor against the NCO rigid frame. The barrier to internal rotation is less than 2100 J/mol. (3) The CNC vibrational bending frequency is consistent with the more recently observed assignments at 165 and 172 cm−1 with some degree of anharmonicity or with a pure harmonic at about 158 cm−1.

► The thermodynamic properties of methylisocyanate have been determined by isothermal calorimetry from 15 to 298.15 K.
► The third law entropy has been compared with the entropy calculated by statistical thermodynamics.
► The comparisons are consistent with selected proposed molecular structures and vibrational frequencies.