Strong Proper C-H Hydrogen Bonds: Experimental Evidence across Spectral Ranges.

Area of Science:

• Physical Chemistry
• Spectroscopy
• Supramolecular Chemistry

Background:

• Carbon-hydrogen (C-H) bonds are generally weak hydrogen bond (H-bond) donors, forming improper H-bonds with distinct spectral characteristics.
• These improper H-bonds exhibit C-H bond shortening, blueshifted vibrational frequencies, and decreased intensity, contrasting with proper H-bonds.

Purpose of the Study:

• To demonstrate that sp-hybridized terminal alkynes ( une5fcC-H) are potent proper hydrogen bond donors.
• To characterize the H-bond formation of alkynyl C-H across diverse solvation environments.
• To investigate the alkynyl C-H as a sensor for molecular interactions.

Main Methods:

• Utilized over 50 solvation environments to study alkynyl C-H interactions.
• Employed infrared (IR) absorption, 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopies, and broadband time-domain terahertz spectroscopies.
• Interpreted experimental data using ab initio molecular dynamics simulations.

Main Results:

• Alkynyl C-H bonds form proper H-bonds with heteroatoms, aromatic π-systems, and even single π-bonds.
• Identified both electric field effects and nonelectrostatic interactions influencing the hydrogen-bonded une5fcC-H stretch frequency.
• Developed a method to experimentally disentangle and quantify these contributions.
• Demonstrated that non-hydrogen-bonded une5fcC-H acts as a sensitive probe of dispersion interactions via its IR shift.

Conclusions:

• The alkynyl une5fcC-H moiety functions as a proper hydrogen bond donor, similar to an O-H group, rather than a typical aliphatic C-H.
• Its spectral isolation, high electric field sensitivity, and ease of incorporation make it a valuable vibrational marker for sensing applications.
• This study highlights the overlooked potential of alkynyl C-H bonds in molecular recognition and sensing.