Isocyanurates are cyclic trimers of isocyanate molecules. They are generally known as highly thermostable compounds. However, it is interesting how the thermal stabilities of the isocyanurate molecules will be altered depending on the substituents of their three nitrogen atoms. We performed computational investigations on the thermochemical behaviors of isocyanurate molecules with various alkyl and phenyl substituents. The cyclotrimerization processes of isocyanates are highly exothermic. Our best estimate of the enthalpy change for the cyclotrimerization of methyl isocyanate into trimethyl isocyanurate was À66.4 kcal mol À1 . Additional negative cyclotrimerization enthalpy changes were observed for n-alkylsubstituted isocyanates. This trend was enhanced with an extension of n-alkyl chains. Conversely, low negative cyclotrimerization enthalpy changes were shown for secondary and tertiary alkyl-substituted isocyanates. The n-alkyl-substituted isocyanurates were shown to be stabilized due to attractive dispersion interactions between the substituents. Meanwhile, the branched alkyl-substituted isocyanurates were destabilized due to the deformation of their isocyanurate rings. For various alkylsubstituted isocyanates, the sum of the deformation energy of the isocyanurate ring and the intramolecular inter-substituent nonbonding interaction energies was found to be linearly correlated with their cyclotrimerization energies. The cyclotrimerization energy for phenyl isocyanate was shown to have significantly deviated from the linear relationship observed for the alkyl-substituted isocyanurates. This is probably attributable to a remarkable change in the orbital resonance interactions during the cyclotrimerization of phenyl isocyanate to triphenyl isocyanurate.Scheme 1 Isocyanurate framework resulting from the cyclotrimerization of isocyanate molecules.15956 | RSC Adv., 2020, 10, 15955-15965This journal is a The energy value is provided in kcal mol À1 . The cyclotrimerization enthalpy at 298 K can be calculated by adding the enthalpy correction term to the electronic energy changes provided in the Table. The enthalpy correction terms are +5.1, +5.1, and +5.2 kcal mol À1 , respectively, for the B3LYP/ cc-pVTZ, B3LYP-GD3/cc-pVTZ, and MP2/cc-pVTZ optimized geometries.
This journal isView Article Online a The intermolecular interaction energies were calculated with taking the corrections for the BSSEs into account. The calculated energy values are provided in kcal mol À1 . b The basis set cc-pVTZ was employed.This journal is