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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