Competitive tetrel bond and hydrogen bond in benzaldehyde–CO2: characterization via rotational spectroscopy

Abstract

The rotational spectrum of the 1 : 1 benzaldehyde–CO₂ complex has been investigated using pulsed-jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Two isomers, both characterized by one C⋯O tetrel bond (n → π* interaction) and one C–H⋯O hydrogen bond (n → σ* interaction), have been observed in the pulsed jet. Competition between the tetrel bond and the hydrogen bond has been disclosed by natural bond orbital analysis: isomer I is characterized by one dominating OC_CO2⋯O tetrel bond (12.6 kJ mol⁻¹) and a secondary (C–H)_formyl⋯O hydrogen bond (2.2 kJ mol⁻¹); by contrast, in isomer II the (C–H)_phenyl⋯O hydrogen bond (7.6 kJ mol⁻¹) becomes the dominant bond, while the OC_CO2⋯O tetrel bond (5.8 kJ mol⁻¹) becomes much weaker with respect to that of isomer I. Using intensity measurements the relative population ratio of the two isomers was estimated to be N_I/N_II ≈ 2/1.