Energetic landscapes and spectroscopic signatures of micro-solvated Isopropyl Cyanide (i-PrCN) in astrochemical environments

The recent detection of isopropyl cyanide (i-PrCN) in giant molecular clouds of interstellar space represents a major milestone in astrochemical and prebiotic research. As the first branched organic molecule identified in the interstellar medium (ISM), i-PrCN provides a key structural link between simple linear species and the branched frameworks of biomolecules such as amino acids. Because hydrogen bonding governs many chemical and biochemical processes relevant to molecular assembly and stability, understanding its role in i-PrCN aggregation is essential. In this work, we investigate the intermolecular hydrogen-bonding interactions within ternary clusters formed through sequential two-body association of binary i-PrCN–(methanol and/or water) complexes with additional methanol and/or water molecules, Particular emphasis is placed on the structural, energetic, electronic, vibrational, and rotational spectroscopic properties of the gas-phase clusters. Thermochemical properties and temperature-dependent quantities were evaluated at 10 and 75 K to characterize their stability under cryogenic conditions relevant to astrochemical environments. Complementary PCM–water calculations at 10 K were additionally performed only to qualitatively assess the influence of environmental polarization on the intermolecular interactions and electronic properties of the complexes. The energetic and vibrational analyses show that all investigated clusters are thermodynamically stable, with methanol-containing systems exhibiting larger dissociation energies and more favorable Gibbs free energy changes for cluster formation than the corresponding water-rich complexes. Water acts as a stronger local hydrogen-bond donor toward the nitrile nitrogen atom, whereas methanol contributes more effectively to global stabilization through enhanced dispersive and induction interactions. The rotational spectroscopic parameters, including rotational constants, asymmetry parameters, centrifugal distortion constants, indicate substantial structural and dynamical changes induced by microsolvation. Overall, the present results provide new insight into the role of weak intermolecular interactions in the stabilization and spectroscopic behaviour of branched nitrile molecules in interstellar environments.