IR spectrum of the O–H⋯O hydrogen bond of phthalic acid monomethylester in gas phase and in CCl4 solution

The absorption spectrum of the title compound in the spectral range of the hydrogen-bonded OH-stretching vibration has been investigated using a five-dimensional gas phase model as well as a QM/MM classical molecular dynamics simulation in solution. The gas phase model predicts a Fermi-resonance between the OH-stretching fundamental and the first OH-bending overtone transition with considerable oscillator strength redistribution. The anharmonic coupling to a low-frequency vibration of the hydrogen bond leading to a vibrational progression is studied within a diabatic potential energy curve model. The condensed phase simulation of the dipole–dipole correlation function results in a broad band in the 3000 cm−1 region in good agreement with experimental data. Further, weaker absorption features around 2600 cm−1 have been identified as being due to motion of the hydrogen within the hydrogen bond.