Experimental quantification of electrostatics in X-H···π hydrogen bonds

Area of Science:

• Physical Chemistry
• Chemical Physics
• Spectroscopy

Background:

• Hydrogen bonds are fundamental to chemistry and biology, with ongoing debate regarding the balance of electrostatic and quantum mechanical forces.
• The vibrational Stark effect offers a method to experimentally dissect these forces into electrostatic and nonelectrostatic components.

Purpose of the Study:

• To extend the application of the vibrational Stark effect to various X-H···π hydrogen bonds beyond O-H···π.
• To investigate the relationship between the nonlinearity of the X-H vibrational response and the nature of the X-H group.
• To quantitatively assess the electrostatic binding energies of X-H···π interactions using vibrational spectroscopy.

Main Methods:

• Utilizing the vibrational Stark effect to probe the response of X-H vibrational modes to applied electric fields.
• Comparing the linearity and nonlinearity of the response for O-H, N-H, and S-H bonds.
• Employing model compounds like indole and thiophenol complexed with aromatic acceptors.

Main Results:

• The nonlinearity of the X-H vibrational response to electric fields increases in the order O-H < N-H < S-H, correlating with atomic polarizabilities.
• X-H stretching vibrations in model compounds accurately report on the electric fields experienced in hydrogen-bonded complexes.
• Estimated electrostatic binding energies closely match results from computational energy calculations.

Conclusions:

• Vibrational probes, when carefully calibrated, can provide direct quantitative measurements of the electrostatic component of hydrogen bonds.
• The study confirms the dominance of electrostatics in many X-H···π interactions.
• Differences in atomic polarizabilities influence the nonlinear response of X-H vibrational modes in electric fields.