Separating σ-inductive and π-resonance effects of substituents on modulating resonance-assisted hydrogen bonds

In this work, we proposed an efficient and accurate approach to separate the σ-inductive and π-resonance effects of substituents on modulating the strength of resonance-assisted hydrogen bonding (RAHB) in malonaldehyde. This is achieved using the block-localized wavefunction (BLW) method, which effectively quenches the resonance effect by localizing the π electrons solely on the substituent at the density functional theory (DFT) level. By comparing the BLW-DFT results for substituted systems and the DFT results for unsubstituted malonaldehyde, we were able to isolate the σ-inductive effect. Subsequent analysis of the structural, energetic, and spectral differences between the BLW-DFT and DFT results for substituted systems reveals the π-resonance effect. Our computational results indicate that the σ-inductive effect overwhelms the π-resonance effects on tuning the RAHB in malonaldehyde, suggesting that the σ-inductive effect should not be neglected when analyzing the overall behavior of substituents in conjugated systems.