Spectroscopic and Quantum Chemical Evidence of Amine–CO[sub.2] and Alcohol–CO[sub.2] Interactions: Confirming an Intriguing Affinity of CO[sub.2] to Monoethanolamine

A recent broadband rotational spectroscopic investigation of the cross-association mechanisms of CO[sub.2] with monoethanolamine (MEA) in molecular beams [F. Xie et al., Angew. Chem. Int. Ed., 2023, 62, e202218539] revealed an intriguing affinity of CO[sub.2] to the hydroxy group. These findings have triggered the present systematic vibrational spectroscopic exploration of weakly bound amine··CO[sub.2] and alcohol··CO[sub.2] van der Waals cluster molecules embedded in inert “quantum” matrices of neon at 4.2 K complemented by high-level quantum chemical conformational analyses. The non-covalent interactions formed between the amino and hydroxy groups and the electron-deficient carbon atom of CO[sub.2] are demonstrated to lift the degeneracy of the doubly degenerate intramolecular CO[sub.2]-bending fundamental significantly with characteristic observed spectral splittings for the amine··CO[sub.2] (≈35–45 cm[sup.−1]) and alcohol··CO[sub.2] (≈20–25 cm[sup.−1]) interactions, respectively, despite the almost identically predicted total association energies (≈12–14 kJ·mol[sup.−1]) for these van der Waals contacts, as revealed by benchmark Domain-based Local Pair Natural Orbital Coupled Cluster DLPNO-CCSD(T) theory. These high-level theoretical predictions reveal significantly higher “geometry preparation energies” for the amine··CO[sub.2] systems leading to a more severe distortion of the CO[sub.2] linearity upon complexation in agreement with the infrared spectroscopic findings. The systematic combined spectroscopic and quantum chemical evidences for cross-association between CO[sub.2] and amines/alcohols in the present work unambiguously confirm an intriguing binding preference of CO[sub.2] to the hydroxy group of the important carbon capture agent MEA, with an accurate vibrational zero-point energy corrected association energy (D [sub.0]) of 13.5 kJ·mol[sup.−1] at the benchmark DLPNO-CCSD(T)/aug-cc-pV5Z level of theory.