How to estimate the intramolecular hydrogen-bond energy of complex RAHB systems? A theoretical study

Abstract

In the present study, new estimations of the intramolecular hydrogen-bond (IMHB) energy in complex RAHB systems were represented. In this regard the malonaldehyde and its 16 halogenated (F, Cl, Br) derivatives, as a set of model compounds, were used. Various enol forms of these compounds at MP2/6-311++G(3df,3pd) level were optimized. Theoretically, enol forms has three degree of freedom correspond to the rotation around C–C, C=C, and C–O bonds. Same as original related rotamers method (RRM1), which is based on rotation about C–C/C=C bonds, by considering rotations about C–C/C–O and C=C/C–O bonds, we can obtain two different method, RRM2 and RRM3, respectively. These methods were used to estimate the IMHB energies in RAHB systems. Exploring and comparing the correlations between these RRM’s energies with various descriptors of hydrogen-bond strength, such as geometrical, topological, molecular orbital, and spectroscopic properties were carried out. Moreover, we found that the RRM2, as original RRM1, has excellent linear correlations while RRM3 has weak dependence with hydrogen bond descriptors. Consequently, according to the regression coefficients, the order of accuracy of these methods is as follows:

$${\text{RRM1}} \approx {\text{RRM2}} > {\text{RRM3}}$$