Nadezhda A. Popova

A tricotage-like failure of nanographene

The response of a nanographene sheet to external stresses was considered in terms of a mechanochemical reaction. The quantum chemical realization of the approach was based on the coordinate-of-reaction concept for the purpose of introducing a mechanochemical internal coordinate (MIC) that specifies a deformational mode. The related force of response is calculated as the energy gradient along the MIC, while the atomic configuration is optimized over all of the other coordinates under the MIC constant-pitch elongation. The approach is applied to the benzene molecule and (5,5) nanographene. A drastic anisotropy in the microscopic behavior of both objects under elongation along a MIC was observed when the MIC was oriented either along or normally to the C–C bond chain. Both the anisotropy and the high stiffness of the nanographene originate from the response of the benzenoid unit to stress.

Odd-electron molecular theory of graphene hydrogenation

This paper highlights the molecular essence of graphene and presents its hydrogenation from the viewpoint of the odd-electron molecular theory. This chemical transformation was performed computationally, using a particular algorithm, through the stepwise addition of either hydrogen molecules or hydrogen atoms to a pristine graphene molecule. The graphene was considered to be a membrane, such that either both sides or just one side of the membrane was accessible to adsorbate, and the atoms on the perimeter of the membrane were either fixed (fixed membrane) or free to move (free-standing membrane). The algorithm explored the spatial distribution of the number of effectively unpaired electrons NDA over the carbon skeleton of the molecule. The highest ranked NDA values were considered to indicate the target atoms at each reaction step. The dependence of the hydrogenation itself and the final graphene hydrides on external factors such as whether the membrane was fixed, if both sides or only one side of the membrane were accessible to hydrogen, and whether the hydrogen was in the molecular or atomic state. Complete hydrogenation followed by the formation of a regular chairlike graphane structure (CH)n was only found to be possible for a fixed pristine graphene membrane for which the basal plane is accessible to hydrogen atoms from both sides.

Structure-sensitive mechanism of nanographene failure

The quantum-mechanochemical-reaction-coordinate approach has disclosed atomically matched peculiarities that accompany the deformation-failure-rupture process occurring in nanographenes. The high stiffness of the graphene body is provided by the benzenoid unit. The anisotropy of the unit mechanical behavior in combination with different configurations of the unit packing with respect to the body C-C bond chains forms the ground for the structure-sensitive mechanism of the mechanical behavior that is drastically different for two different deformation modes. The zig-zag deformation mode is particularly manifested with the formation of one-atom chains. The approach allows tracing a deformation-stimulated change in the chemical reactivity of both the nanographene body and its individual atoms.

Topological Mechanochemistry of Graphene

In view of the formal topology, two common terms, namely, the connectivity and adjacency, determine the ‘quality’ of the C–C bonds of sp 2 nanocarbons. The feature is the most sensitive point of the inherent topology of the species so that such external action as the mechanical deformation should obviously change it and result in particular topological effects. The current chapter describes the effects caused by uniaxial tension of a graphene molecule in due course of the mechanochemical reaction. Basing on the molecular theory of graphene, the effects are attributed to both mechanical loading and chemical modification of the edge atoms of the molecule. The mechanical behavior is shown to be not only highly anisotropic with respect to the direction of the load application, but greatly dependent on the chemical modification of the molecule edge atoms thus revealing the topological character of the graphene deformation.