Interplay of intermolecular interactions and flexibility to mediate glass forming ability and fragility: A study of chemical analogs
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We have investigated the enthalpic and dielectric relaxations of four groups of quinoline analogs having similar structural properties (i.e., rigidity, stiffness, and bulkiness) but a different steric character and the nature of intermolecular interactions and flexibility. The dielectric fragility index (md) and the enthalpic one (mH), determined by the Tool-Narayanaswamy-Moynihan-Hodge formalism, are comparable. Generally, for the four sets of molecules of similar structures, both the interactions and flexibility are found to be critical in making the large span of fragility (i.e., from 59 to 131) and glass forming ability. By contrast, individual impacts of the interaction and flexibility can only explain fragility partly among each group of isomers. We found that the molecules with high fragility are of relatively low liquid density, reflecting the joint impact of the interactions and flexibility. An interesting result is observed among the isomers that the molecules which are fragile have enhanced glass forming ability. The results are unveiling the joint impacts of molecular structure (flexibility) and intermolecular interaction on the molecular dynamics.Keywords:
Rigidity (electromagnetism)
Intermolecular interaction
This paper introduces a new approach to probing intermolecular interactions based on a framework of two-dimensional (2D) synchronous spectroscopy. Mathematical analysis performed on 2D synchronous spectra using variable concentration as an external perturbation shows that the cross-peaks are composed of two parts. The first part reflects intermolecular interactions that manifest in the form of deviation from the Beer–Lambert law. The second part is related simply to the concentration variations of the solutes and is responsible for the generation of interfering cross-peaks not related to the intermolecular interactions in the system. It is the second part that prevents the reliable identification of intermolecular interactions. We propose a way of selecting the concentrations of solutes so that the resultant dynamic concentration vectors of different solutes become orthogonal to one another. Therefore, the contribution of the second part to the cross-peaks can be effectively removed by the dot product of orthogonal vectors. Our new approach has been tested on a simulated chemical system and a real chemical system. The results demonstrate that interfering cross-peaks can be successfully removed from a 2D synchronous spectrum so that the cross-peaks can be used as a reliable tool to characterize or probe intermolecular interactions.
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One of the most familiar carbon-centered noncovalent interactions (NCIs) involving an antibonding π*-orbital situated at the Bürgi-Dunitz angle from the electron donor, mostly lone pairs of electrons, is known as n → π* interactions, and if it involves a σ* orbital in a linear fashion, then it is known as the carbon bond. These NCIs can be intra- or inter-molecular and are usually weak in strength but have a paramount effect on the structure and function of small-molecular crystals and proteins. Surprisingly, the experimental evidence of such interactions in the solution phase is scarce. It is even difficult to determine the interaction energy in the solution. Using NMR spectroscopy aided with molecular dynamics (MD) simulation and high-level quantum mechanical calculations, herein we provide the experimental evidence of intermolecular carbon-centered NCIs in solution. The challenge was to find appropriate heterodimers that could sustain room temperature thermal energy and collisions from the solvent molecules. However, after several trial model compounds, the pyridine-N-oxide:dimethyltetracyanocyclopropane (PNO-DMTCCP) complex was found to be a good candidate for the investigation. NBO analyses show that the PNO:DMTCCP complex is stabilized mainly by intermolecular n → π* interaction when a weaker carbon bond gives extra stability to the complex. From the NMR study, it is observed that the NCIs between DMTCCP and PNO are enthalpy driven with an enthalpy change of -28.12 kJ mol-1 and dimerization energy of ∼-38 kJ mol-1 is comparable to the binding energies of a conventional hydrogen-bonded dimer. This study opens up a new strategy to investigate weak intermolecular interactions such as n → π* interaction and carbon bonds in the solution phase.
Intermolecular interaction
Non-covalent interactions
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Carbon fibers
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The systematic ab initio studies of intermolecular interaction energy components in the model unsaturated hydrocarbon complexes have been performed. The influence of the weak intermolecular interactions on the optical properties has been analyzed within the supermolecular approach. The estimated interaction energy components and electric properties of the studied systems indicate the substantial influence of the intermolecular forces on the optical response of the studied systems. The obtained results could be important for understanding the properties of the organic materials exploited for the purposes of the nonlinear optics.
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Intermolecular interaction
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