Ab Initio Characterization of C2H4N2 Isomers: Structures, electronic energies, spectroscopic parameters and formation pathways

TL;DR

Using high-accuracy CCSD(T)-F12/cc-pVTZ-F12 and related methods, the paper comprehensively characterizes the C2H4N2 isomer family, detailing structures, energies, vibrational frequencies, dipole moments, and plausible gas-phase formation pathways. It finds six low-energy isomers within $1\, \text{eV}$ of the global minimum, with methylcyanamide (MCA) emerging as a strong detector candidate due to a large dipole moment of about $5.04\,\text{D}$; MCA is also among the lowest-energy species. The authors map a reaction network (R1–R22) including barrierless channels and proton-transfer steps, supplying activation energies and enthalpies to support astrochemical and atmospheric modeling. They provide detailed spectroscopic parameters, UV absorption tendencies in the $115$–$230\,\text{nm}$ range, and discuss implications for ISM, planetary atmospheres, and exoplanet photochemistry, offering data ready for incorporation into chemical networks such as UMIST and KIDA. Overall, the work establishes a robust spectral–kinetic foundation for identifying and modeling C2H4N2 isomers in space and on planets.

Abstract

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 (IRC) calculations were performed at the B3LYP/6-31G(d,p) level to explore reaction pathways. Zero-point energy corrections were determined for all 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 gas-phase detection. The rotational constants for MCA, computed at the CCSD(T)-F12/cc-pVTZ-F12 level, are Ae = 34932.44 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 methylcyanamide and related C2H4N2 isomers via radioastronomy, infrared, and microwave 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 provide a robust foundation for future observational and modeling efforts.

Figures (8)

• Figure 1: Isomers of the formula C2H4N2, ranked by increasing relative energy $E_{\rm rel}$ (in $\text{cm}^{-1}$), referred to the most stable isomer (A1). Their electronic ground states (g.s.), point groups (P.G.), zero-point vibrational energies ZPVE (in $\text{cm}^{-1}$), relative energies including the ZPVE corrections $E_{\text{rel}}^{+ZPVE}$ (in $\text{cm}^{-1}$), and dipole moments $\mu$ (in Debye) are also shown. These calculations were carried out with the level of theory CCSD(T)-F12/cc-pVTZ-F12. The pairs A2 & A3, C6 & C12, C7 & C8, and C10 & C13 are conformers of the same isomer. Transition state structures are highlighted in dotted frames as C12 and C13.
• Figure 2: Proton transfer energy diagram of the two most stable isomers of C2H4N2 with their relative energies obtained from CCSD(T)-F12/cc-pVTZ-F12. Aminoacetonitrile derivatives are labeled as An while cyanomethylamine derivatives as Cn.
• Figure 3: Comparison of the relative energies $E_{\text{rel}}$ (in eV) calculated with MP2/aug-cc-pVTZ, CCSD(T)-F12/cc-pVTZ-F12, and B3LYP/6-31G(d,p) methods across 18 isomeric species. The relative energies are given with respect to the value of the most stable isomer A1.
• Figure 4: Optimised equilibrium geometry of MCA (bond lengths (in Å, red) and bond angles (in degrees, black)) calculated using CCSD(T)-F12/cc-pVTZ-F12
• Figure 5: Torsional potential energy surface of CH3 top of the species MCA computed at the MP2/aug-cc-PVTZ level of theory with respect to the dihedral angle H4C1N5H6.