The gas-phase pyrolysis of cyclopropylamine. Quantum chemical characterisation of the intermediates involved

Abstract

A computational study on the molecular structure, harmonic vibrational frequencies and thermochemical parameters of the postulated intermediates and final products involved in the gas-phase pyrolysis of (c-C3H5)NH2 (CPA) has been carried out by ab-initio and DFT methods. Internal rotor potentials were determined employing CAS-MP2 calculations for the singlet ĊH2CH2ĊHNH2 biradical and DFT for singlet CH3CH2C̈NH2, cis- and trans-CH3CH=CHNH2, E- and Z-CH3CH2CH=NH, (c-C3H5)NHCH(NH2)CH2CH3, E- and Z-(c-C3H5)N=CHCH2CH3. The enthalpies of formation at 0 and 298 K for these species were derived from isodesmic reactions using the B3LYP and M06-2X functionals coupled to the 6-311++G(3df,3pd) basis set and ab-initio G3(MP2) and G3(MP2)//B3LYP/6-311++G(3df,3pd) composite models. At the last level, enthalpies of formation at 298 K of 69.94 and 40.37 kcal mol−1 were estimated for the first time for ĊH2CH2ĊHNH2 and CH3CH2C̈NH2, and values of 7.28, 8.12, 5.37 and 6.08 kcal mol−1 were derived for cis-CH3CH=CHNH2, trans-CH3CH=CHNH2, E-CH3CH2CH=NH and Z-CH3CH2CH=NH, respectively. Accordingly, the pyrolysis of CPA to give ĊH2CH2ĊHNH2 and CH3CH2C̈NH2 is predicted to be endothermic by 51.58 and 22.01 kcal mol−1, whereas all other reaction steps are exothermic. For the overall reaction, 2 CPA → (c-C3H5)N=CHCH2CH3 + NH3, a standard change in enthalpy of about −20 kcal mol−1 was obtained.