Structure, stability and interactions in the complexes of carbonyls with cyanides

Our theoretical study at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level for twenty-two stable structures of the interactions between aldehyde RCHO (R=H, CH 3 , NH 2 , F, Cl, Br) and cyanides XCN (X=H, F) found that the strength of these complexes is contributed by the C–H···O(N) and/or N–H···N hydrogen bonds and/or >C=O···C Lewis acid–base interaction. The stability of the RCHO···FCN complexes is mainly determined by the >C=O···C Lewis acid–base interaction, while the C sp –H···O hydrogen bond plays a decisive role in the RCHO···HCN complexes. For each complex of RCHO with either HCN or FCN, the substitution of R by CH 3 or NH 2 strengthens the complex, while the halogen substitution negligibly affects the complex’s stability, as compared to the non-substituted HCHO···XCN complexes. The decomposition analysis using SAPT2+ approach showed that the attractive electrostatic term plays the predominating role of up to 50% overcoming the induction and dispersion terms in stabilizing these complexes. Remarkably, the obtained results give further support to the observation that the change in C–H bond involved in the hydrogen bond and its stretching vibrational frequency depend both on the polarity of the C–H bond in the proton donor and on the gas-phase basicity of the proton acceptor.