Resonance Raman spectroscopy study of protonated porphyrin

Area of Science:

• Molecular Spectroscopy
• Computational Chemistry
• Materials Science

Background:

• Porphyrins are crucial macrocyclic compounds with diverse applications.
• Understanding their vibrational properties is key to characterizing their structure and function.
• Protonation significantly alters porphyrin electronic and structural characteristics.

Purpose of the Study:

• To investigate the resonance Raman scattering of diprotonated free-base porphyrin.
• To identify and assign spectral features related to out-of-plane vibrational modes.
• To elucidate the structural distortions responsible for changes in vibrational spectra upon protonation.

Main Methods:

• Resonance Raman microscopy was employed to acquire spectra from porphyrin samples.
• Density Functional Theory (DFT) methods were used for computational simulation of Raman spectra.
• Experimental data was rigorously compared with theoretical predictions.

Main Results:

• Intensive spectral lines were detected in the 100–1000 cm^-1 range, attributed to out-of-plane modes.
• DFT simulations successfully reproduced experimental observations.
• A direct correlation was established between spectral features and porphyrin macrocycle distortion.

Conclusions:

• The diprotonated form of porphyrin exhibits activated out-of-plane vibrational modes.
• Saddle distortion of the porphyrin macrocycle is the primary cause for the activation of these modes.
• Combined experimental and theoretical approaches provide a comprehensive understanding of porphyrin vibrational dynamics.