Gas-Phase Retinal Spectroscopy: Temperature Effects Are But a Mirage

Area of Science:

• Computational chemistry
• Spectroscopy
• Photochemistry

Background:

• Protonated Schiff base retinal exhibits an unusually broad and flat spectrum in photodissociation spectroscopy.
• Recent proposals suggest temperature effects may be responsible for this spectral anomaly.

Purpose of the Study:

• To investigate the role of temperature effects in the observed spectral properties of protonated Schiff base retinal.
• To determine if temperature is the primary cause of the broad and flat spectrum in photodissociation experiments.

Main Methods:

• State-of-the-art first-principle calculations were employed.
• Realistic geometrical models of retinal were constructed and calibrated.
• The M06-2X exchange-correlation functional was used for accurate electronic structure description.
• Modern multiconfigurational perturbative methods (NEVPT2) computed electronic excitations.
• Room-temperature molecular dynamics simulations identified active degrees of freedom.

Main Results:

• The M06-2X functional provided an accurate description, outperforming the commonly used CASSCF method.
• Conformations with varying β-ionone ring orientations showed similar excitation energies.
• Other identified active degrees of freedom did not account for the anomalous spectral shape.
• Temperature effects were found to be insufficient to explain the broad and flat spectrum.

Conclusions:

• Temperature effects are not the primary cause of the broad and flat spectrum of protonated Schiff base retinal in photodissociation spectroscopy.
• The study indicates that photodissociation experiments may not accurately represent the gas-phase optical spectrum of retinal.
• Further experimental characterization of retinal dissociation spectra is recommended.