Matrix isolation infrared spectroscopic study of 4-Pyridinecarboxaldehyde and of its UV-induced photochemistry

• 4-Pyridinecarboxaldehyde (4PCA) is studied by matrix isolation IR spectroscopy and quantum chemical calculations.
• Equilibrium geometry of the molecule and the internal rotation barrier of the aldehyde group are reported.
• The IR spectrum of the matrix isolated compound (Ar, 10 K) is described and assigned in the 4000-400 cm–1 spectral range.
• The narrowband UV-induced photochemistry of PCA in Ar matrix is investigated and the bands due to photoproducts are assigned.

The structure, infrared spectrum, barrier to internal rotation, and photochemistry of 4-pyridinecarboxaldehyde (4PCA) were studied by low-temperature (10 K) matrix isolation infrared spectroscopy and quantum chemical calculations undertaken at both Moller-Plesset to second order (MP2) and density functional theory (DFT/B3LYP) levels of approximation. The molecule has a planar structure (Cs point group), with MP2/6-311 ++G(d,p) predicted internal rotation barrier of 26.6 kJ mol− 1, which is slightly smaller than that of benzaldehyde (~ 30 kJ mol− 1), thus indicating a less important electron charge delocalization from the aromatic ring to the aldehyde moiety in 4PCA than in benzaldehyde. A complete assignment of the infrared spectrum of 4PCA isolated in an argon matrix has been done for the whole 4000–400 cm− 1 spectral range, improving over previously reported data. Both the geometric parameters and vibrational frequencies of the aldehyde group reveal the relevance in this molecule of the electronic charge back-donation effect from the oxygen trans lone electron pair to the aldehyde CH anti-bonding orbital. Upon in situ UV irradiation of the matrix-isolated compound, prompt decarbonylation was observed, leading to formation of pyridine.

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