Structural insights into nitrile–methanol hydrogen-bonded complexes based on mass-selected infrared spectroscopy and theoretical calculations

Hydrogen bonds, crucial for the formation and stabilization of complex organic molecules, present intriguing possibilities for the existence of such molecular complexes in extraterrestrial environments. To investigate the microscopic hydrogen-bonding networks in nitrile-methanol clusters-potential candidates for interstellar organic complexes-we recorded infrared spectra of neutral XN-M (XN = nitriles; M = methanol) heterodimers in the 2600-3900 cm-1 range using infrared-vacuum ultraviolet (IR-VUV) spectroscopy. Quantum chemical calculations based on harmonic approximations are typically used to simulate such spectral peaks; however, they often fail to fully reproduce the experimental features. To overcome these limitations, we employed ab initio anharmonic algorithms and ab initio molecular dynamics (AIMD) simulations. Anharmonic algorithms successfully assigned elusive spectral features by accounting for vibrational couplings (e.g., Fermi resonances and combination bands), and AIMD proved essential for accurately assigning characteristically broad hydrogen-bonded OH bands unresolved by harmonic methods. Applying this integrated approach, we confirmed the distinct H-bonding preferences in the XN-M heterodimers: acetonitrile (ACN) and unsaturated aliphatic nitriles (e.g., 3-butene nitrile, 3BN; 4-pentene nitrile, 4PN) predominantly adopt linear N⋯H-O configurations, whereas acrylonitrile and benzonitrile favor cyclic double H-bonded structures. This study establishes a powerful framework that integrates advanced computational methodologies to decipher hydrogen-bonding motifs from complex spectral data, enabling the direct interpretation of both laboratory and astronomical infrared observations.