Vibrations and hydrogen bonding in porphycene

Area of Science:

• Molecular Spectroscopy
• Quantum Chemistry
• Physical Chemistry

Background:

• Porphycene exhibits complex vibrational spectra.
• The N-H stretching band is unexpectedly absent in standard infrared spectra.
• Previous calculations predicted a strong N-H band.

Purpose of the Study:

• To assign the fundamental vibrations of porphycene.
• To explain the absence of the N-H stretching band in infrared spectra.
• To investigate the role of hydrogen bonding and molecular dynamics.

Main Methods:

• Combined use of Infrared (IR), Raman, neutron scattering, and fluorescence spectroscopy.
• Isolation of porphycene in helium nanodroplets, supersonic jets, cryogenic matrices, solid, and liquid solutions.
• Ab initio molecular dynamics simulations for theoretical modeling of IR spectra.

Main Results:

• Assignment of nearly all 108 fundamental vibrations of porphycene.
• Identification of the N-H stretching mode as a broad band (2250-3000 cm(-1)) in theoretical IR spectra.
• Demonstration of strong coupling of N-H stretching to other modes.

Conclusions:

• The apparent lack of the N-H stretching band is explained by its broad nature and coupling effects.
• Intramolecular double hydrogen transfer in porphycene has a multidimensional character.
• The findings are applicable to other strongly hydrogen-bonded systems.