Crystal Packing Modulation of the Strength of Resonance-Assisted Hydrogen Bonds and the Role of Resonance-Assisted Pseudoring Stacking in Geminal Amido Esters: Study Based on Crystallography and Theoretical Calculations
Perumal Venkatesan, Subbiah Thamotharan, M. Judith Percino, Andivelu Ilangovan
Abstract
A detailed experimental and theoretical investigation of a series of substituted geminal amido esters (ethyl (2E)-3-(arylamino)-2-(arylcarbamoyl)prop-2-enoate, AME-1–8) leading to the identification of a unique angularly fused pseudotricyclic (S(6),S(6),S(6)) ring system stabilized by an intramolecular resonance-assisted hydrogen bond (RAHB) and a non-RAHB are presented in addition to weak intermolecular interactions. An analysis of X-ray and theoretical models reveals that the strength of the intramolecular RAHB (N1–H1N···O1) varies in a wide range (6.9–11.4 kcal mol–1) due to crystal-packing constraints arising from different aromatic ring substitutions. However, the effect is less significant and the strength differs only in a narrow range (8.2–9.9 kcal mol–1) in the case of non-RAHB. The downfield shift (δ ∼12.3) observed for the N–Haniline signal in 1H NMR spectra of AME-1–8 is due to the presence of intramolecular RAHB. A PIXEL energy analysis suggests that the molecular dimer formed by stacking of RAHB pseudorings is found to be strong (Etot = −14.4 to −17.9 kcal mol–1), and this dimer forms the basic motif in most of the structures reported herein. A detailed analysis of the isostructurality suggests that the basic motif exists in most of the structural combinations. The weak intermolecular C–H···O, C–H···Cl, and C–H···π interactions play a vital role in the stabilization of these crystal structures, as evaluated by PIXEL and Bader’s quantum theory of atoms in molecules approach (QTAIM). A lattice energy analysis suggests that the Coulombic contribution and total lattice energies are higher in the para-substituted compounds (AME-2, AME-5, and AME-8) in comparison to the other isomeric compounds. Further, the crystal packing of these compounds is analyzed on the basis of the energy frameworks. It shows that most of the crystals show similar 3D topologies, suggesting that these compounds may have similar mechanical behavior.