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

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 OCCO2⋯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 OCCO2⋯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 NI/NII ≈ 2/1.