Infrared Intensification and Hydrogen Bond Stabilization: Beyond Point Charges

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Infrared (IR) band intensification of A-H stretching modes in A-H···B acid-base systems is a prominent spectral signature of hydrogen bonding.
• Understanding the electronic origins of this intensification is crucial for correlating spectral data with thermodynamic properties.

Purpose of the Study:

• To quantitatively explain the electronic structure origins of infrared hydrogen bond stretching intensifications.
• To investigate the correlation between experimental enthalpies of formation and observed IR spectral changes.

Main Methods:

• Utilized a Quantum Theory of Atoms in Molecules (QTAIM) / Charge and Current Density Perturbation Theory (CCTDP) model.
• Performed augmented correlation-consistent polarized triple-zeta quantum calculations at the MP2 level.
• Calculated A-H stretching band intensities for various hydrogen-bonded complexes.

Main Results:

• Calculated A-H stretching band intensities were 3 to 40 times larger than monomer values.
• Charge transfer was identified as the primary factor driving intensification, with dipolar polarization effects also significant.
• Sums of intermolecular electron transfer and acid dipolar polarization contributions strongly correlated with enthalpies of formation and H-bond intensifications.

Conclusions:

• A mechanism involving intra-acid/intermolecular electron transfers and atomic polarizations explains IR band intensifications.
• The findings establish a quantitative link between electronic structure changes, spectral properties, and hydrogen bonding energies.