Existence of both blue-shifting hydrogen bond and Lewis acid–base interaction in the complexes of carbonyls and thiocarbonyls with carbon dioxide

Abstract

In this study, 16 gas phase complexes of the pairs of XCHZ and CO₂ (X = F, Cl, Br; Z = O, S) have been identified. Interaction energies calculated at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ level including both BSSE and ZPE corrections range from −5.6 to −10.5 kJ mol⁻¹ for XCHO⋯CO₂ and from −5.7 to −9.1 kJ mol⁻¹ for XCHS⋯CO₂. Substitution of one H atom by one halogen in formaldehyde and thioformaldehyde reduces the interaction energy of XCHZ⋯CO₂, while a CH₃ substitution increases the interaction energy of both CH₃CHO⋯CO₂ and CH₃CHS⋯CO₂. NBO and AIM analyses also point out that the strength of Lewis acid–base interactions decreases going from >C1S3⋯C6 to >C1O3⋯C6 and to >C1–X4⋯C6. This result suggests the higher capacity of solubility of thiocarbonyl compounds in scCO₂, providing an enormous potential application for designing CO₂-philic materials based on the >CS functional group in competition with >CO. The Lewis acid–base interaction of the types >CS⋯C, >C–Cl⋯C and >C–Br⋯C is demonstrated for the first time. The contribution of the hydrogen bonding interaction to the total interaction energy is larger for XCHS⋯CO₂ than for XCHO⋯CO₂. Upon complexation, a contraction of the C1–H2 bond length and a blue shift of its stretching frequency have been observed, as compared to the isolated monomer, indicating the existence of a blue-shifting hydrogen bond in all complexes examined. Calculated results also lend further support for the viewpoint that when acting as proton donor, a C–H bond having a weaker polarization will induce a stronger distance contraction and frequency blue shift upon complexation, and vice versa.