Theoretical Study On The Effects Of Water Molecules On The Properties Of Several Carbon-based Nanomaterials Via Hydrated Cation-π Interactions

Cations andπelectron-rich structures widely exist in various carbon-based nanomaterials and in nature,cation-πinteractions is strong non-covalent interactions between cations andπelectron-rich structures.Although the cation-πinteractions in most cases refer to the interactions between the cations and the aromatic ring structures,a molecule that does not contain aromatic rings can also produces the cation-πinteractions with the cations if it containsπelectrons.Thus,the cation-πinteractions are very important in various physical,chemical,and biological processes involving carbon-based nanomaterials.These processes usually involve aqueous environment rich in salt ions,and thus water molecules have great effects on the properties and applications of carbon-based nanomaterials via hydrated cation-πinteractions.In this paper,the effects of water molecules on the specific adsorption of cations,magnetism and photocatalysis of carbon-based nanomaterials via hydrated cation-πinteractions are theoretically studied by first-principles calculations.Theπelectron-rich structures in the corresponding carbon-based nanomaterials are polyaromatic rings,monoaromatic rings and functional groups containingπbonds,respectively.（1）The effects of water molecules on the specific adsorption of cations on graphene through the hydrated cation-πinteractions.Firstly,by using the first principles calculations combined with the structure search algorithm,we obtained the most stable structures of the hydrated cations and the hydrated cations adsorbed on the graphene surface when increasing the number of water molecules n from 0 to 9.Further analyses on structures and interaction energies revealed that the graphene sheet can distort the hydration shell of the hydrated K⁺to interact with K⁺directly,which is hereafter called the water-cation-πinteractions;As a contrast,the hydration shell of the hydrated Li⁺is quite stable and the graphene sheet interacts with Li⁺indirectly,mediated by water molecules,which we hereafter called it the cation-water-πinteractions.The behavior of hydrated cations adsorbed on a graphene surface is mainly attributed to the competition between the cation-πinteractions and hydration effects.In addition,we found that under the influence of water molecules（n≥7）,the strengths of the adsorption energies of these three hydrated cations follow an anomalous order as hydrated Li⁺<hydrated Na⁺<hydrated K⁺,which is opposed to the order of cation-πinteraction in the gas phase（n=0）.These findings provide valuable details of the structures and the adsorption energy of hydrated cations adsorbed onto the graphene surface,as well as provide references for people to design components or devices in the fields of seawater desalination,wastewater treatment and ion sieving.（2）The effects of water molecules on the magnetic properties of aromatic polypeptides through the hydrated cation-πinteractions.Firstly,by using the first principles calculations combined with the structure search algorithm,we obtained the most stable structures of the magnesium and calcium chlorides and the magnesium and calcium chlorides adsorbed on the surfaces of monoaromatic rings when increasing the number of water molecules n from 0 to 6.Further analyses on structures and interaction energies revealed that the adsorption stability of hydrated monochloride of magnesium and calcium is comparable to that of hydrated dichloride due to the strong hydrated cation-πinteractions with the help of water molecules（n>2）,which is opposed to the results of the gas phase（n=0）.This enables Mg⁺and Ca⁺ions in the solution to be stably adsorbed on the monoaromatic ring of aromatic peptides AYFFF,which is confirmed by electrochemical experiments and spectroscopy experiments（XPS and XANES）.Therefore,we found that divalent cations,including Mg²⁺,and Ca²⁺,which are not traditional magnetic materials,could abruptly switch the aromatic peptides AYFFF self-assemblies from diamagnetic to super strong paramagnetic.The unpaired electrons on Mg⁺and Ca⁺ions are the fundamental electronic structure of super strong paramagnetism.The findings not only provide new insights into the origin of magnetoreception and the bio-effects of magnetic fields,but also support the development of novel magnetic-control techniques and drugs that can be monitored.（3）The effects of hydrated cation-πinteractions on the photocatalytic activity of amidoxime groups containingπbonds.By using the first principles DFT calculations,we studied the coordination structures and binding energy of hydrated Fe（Ⅲ）ions and amidoxime groups containingπbonds,and we found that the hydrated cation-πinteractions in solution can promote the stable adsorption of Fe（Ⅲ）by amidoxime groups containingπelectrons.Further frontier molecular orbital analysis revealed that the complex formed by Fe（Ⅲ）and amidoxime group has a lower HOMO-LUMO energy gap,and the electrons on HOMO mainly occupy the C=N double bond of the oxime group.These results indicate that the hydration cation-πinteractions between the hydrated Fe（Ⅲ）cations andπelectrons in amidoxime group can significantly reduce the energy barrier of electrons excited from the complex to Fe（Ⅲ）ions under light irradiation,which can improves the efficiency of photoreduction of Fe（Ⅲ）to Fe（Ⅱ）,and promotes Photo-Fenton catalytic performance.These findings not only reveal the important role of the hydrated cation-πinteractions in stabilizing the ligands of transition metal cations and the mechanism of promoting Photo-Fenton catalytic performance,but also provide a theoretical foundation for exploring new photocatalytic materials composed ofπelectron-rich ligands and transition metal cations for the rapid degradation of organic pollutants.The findings of this paper can help people understand the effects of water molecules through hydrated cation-πinteractions on the adsorption and kinetic behavior of cations on theπelectron-rich carbon-based nanomaterials in solution,as well as these effects results in the changes in the structures,functions,and applications of carbon-based nanomaterials.It also provides the theoretical foundations for the design of devices or catalytic materials in the fields of seawater desalination,ion sieving,magnetic control technology and wastewater treatment.