Ab <i>initio</i> characterization of C2H4N2 isomers: Structures, electronic energies, spectroscopic parameters, and formation pathways

This work presents a comprehensive theoretical investigation of key isomers of C2H4N2 using state-of-the-art quantum chemical methods. The objective is to characterize their molecular structures, spectroscopic constants, and electronic energies and to elucidate plausible formation and destruction pathways, providing data critical for astrochemical and atmospheric detection. High-accuracy ab initio methods were employed, notably CCSD(T)-F12/cc-pVTZ-F12 for optimized geometries. Additional calculations were performed at the CCSD(T)/aug-cc-pVTZ, CCSD(T)/cc-pVTZ, MP2/aug-cc-pVTZ, and CIS levels. Intrinsic reaction coordinate calculations were performed at the B3LYP/6-31G(d,p) level to explore reaction pathways. The Zero-Point Energy (ZPE)-corrections were determined for all the isomers considered. Six low-energy C2H4N2 isomers were identified, all within 1 eV of the global minimum. Among them, methylcyanamide (MCA) exhibits the lowest relative energy (∼0.2 eV) and a significant electric dipole moment of 5.00 D, making it a strong candidate for detection in gas-phase environments. The rotational constants for MCA, computed at the level of CCSD(T)-F12/cc-pVTZ-F12, are Ae = 34 932.44 MHz, Be = 4995.31 MHz, and Ce = 4520.30 MHz. The V3 torsional barrier was found to be 631.19 cm-1. Centrifugal distortion constants were computed up to sextic order for all isomers. Formation pathways for MCA-such as CH3N + HCN → CH3NHCN-and related isomers were characterized. The combination of large dipole moments and distinct rotational signatures supports the detectability of MCA and related C2H4N2 isomers via radioastronomy, IR, and MW spectroscopy. Isomerization and reaction pathways involving radical-neutral and neutral-neutral processes were found to be key to their formation in gas-phase environments. These results offer a robust foundation for future observational and modeling efforts.