C-H···O Hydrogen Bonding. The Prototypical Methane-Formaldehyde System: A Critical Assessment

Area of Science:

• Computational Chemistry
• Supramolecular Chemistry
• Chemical Physics

Background:

• Distinguishing and quantifying C-H···O hydrogen bond (HB) functionality is challenging due to inherent difficulties in reliable property measurement.
• Previous studies have explored C-H···O interactions, but a comprehensive understanding of their characteristics, especially in model systems, remains incomplete.

Purpose of the Study:

• To investigate and characterize the hydrogen bonding properties of the model methane-formaldehyde complex (MFC).
• To compare the HB characteristics of the MFC with those of established hydrogen bonds, such as the water dimer.
• To elucidate the influence of potential energy surface (PES) flatness on HB property determination.

Main Methods:

• High-level ab initio calculations, including CCSD(T)/ANO2 and CCSDT(Q)/CBS levels, were employed to obtain stationary points on the MFC PES.
• Analysis of electron density, Bader's method, noncovalent interaction (NCI) method, and Natural Bond Orbital (NBO) computations were used to characterize HB interactions.
• Nuclear Magnetic Resonance (NMR) chemical shifts and coupling constants were calculated to assess HB-induced effects.

Main Results:

• Six stationary points were identified on the MFC PES, with very small interaction energy differences, indicating a flat PES.
• The lowest energy conformer (MFC1) exhibits prominent HB properties, including elongated C-H bonds, attractive C-H···O interactions, and characteristic electronic and spectroscopic signatures.
• These HB properties in MFC1 closely resemble those of the prototypical water dimer O-H···O HB, despite the challenges posed by the flat PES.

Conclusions:

• The methane-formaldehyde complex (MFC) is predominantly bound by a C-H···O hydrogen bond, as evidenced by the clear HB properties observed in the MFC1 conformer.
• The study highlights the relevance of charge transfer in C-H···O HB geometries, supported by Broyden–Fletcher–Goldfarb–Shanno (BFGS) and NBO computations.
• Despite experimental challenges in determining HB-induced shifts for C-H stretches due to PES flatness, the computational findings provide strong evidence for significant C-H···O HB interactions.