Exploring the correlation between the π-electron delocalization and intramolecular hydrogen bond in malonaldehyde derivatives; a quantum chemical study

Abstract

The resonance-assisted hydrogen bond (RAHB) theory claims that the intramolecular hydrogen bond and π-electron delocalization (π-ED) are interrelated. In the present study, for the first time, the relation between the π-ED and intramolecular hydrogen bond strength were examined. Along this line, the π-ED of the cis enol forms of malonaldehyde and its halogenated derivatives using the various indicators, such as geometrical factor of Gilli (λ), the harmonic oscillator model of aromaticity (HOMA), the nucleus-independent chemical shift (NICS), the para delocalization index (PDI), the average two center index (ATI), the aromatic fluctuation index (FLU), and the π-fluctuation aromatic index (FLUπ) were evaluated. For checking the relation between the π-ED and hydrogen bond strength, we explored and compared all of the correlations between these indices with various descriptors of hydrogen bond, such as geometrical, spectroscopic, topological, and molecular orbital parameters. Our theoretical results show that the geometrical-based indices (λ and HOMA) have the best linear correlations with all of hydrogen bond descriptors (R ≈ 0.90), while the results of other indices (ATI, PDI, FLU, FLUπ, NICS(0), and NICS(1)) are not appropriate (R ≤ 0.80). Surprisingly, we found that the NICS(0) has the weakest linear dependence with all of the HB descriptors. Consequently, the linear dependence of π-ED indices and HB descriptors are in line with the RAHB theory. Finally, according to the absolute correlation coefficients, the approximate order of linearity is as follows: $$\lambda > {\text{HOMA}} > {\text{FLU}} > {\text{ATI}} \approx {\text{FLU}}\pi > {\text{NICS}}\left( 1\right) > {\text{PDI}} > {\text{NICS}}\left( 0 \right) .$$