Adsorption of neutral and protonated Lewis bases on $$\hbox {Na}_{8}$$ Na 8 -nanocluster: basicity enhancement of the Lewis bases

Abstract

Structural and energetic properties of $$\hbox {Na}_{\mathrm{n}}$$ ( $$\hbox {n}=1$$ –8) nanoclusters were studied employing the B3LYP method and using 6-31+(d,p) basis set. Comparison of the calculated binding energies per atom ( $$E_{\mathrm{b}})$$ of the cluster shows that the $$\hbox {Na}_{8}$$ cluster with $$\hbox {D}_{2}\hbox {d}$$ symmetry group is the most stable structure among the $$\hbox {Na}_{\mathrm{n}}$$ clusters. Adsorption of neutral and protonated forms of ammonia, methylamine, ethylamine, hydrazine, guanidine, pyridine, methanolamine, and methanimine on $$\hbox {Na}_{8}$$ cluster was studied. Interaction of the neutral Lewis bases with $$\hbox {Na}_{8}$$ was very weak such that the interaction energies were smaller than $$-\,50$$ kJ mol $$^{-1}$$ . However, the protonated bases interact with $$\hbox {Na}_{8}$$ more strongly with interaction energies generally in the range of $$-\,90$$ to $$-\,350$$ kJ mol $$^{-1}$$ . The protonated forms of methanolamine undergo dissociative adsorption on $$\hbox {Na}_{8}$$ with adsorption energies of about $$-\,800$$ kJ mol $$^{-1}$$ . Since the protonated bases are stabilized more than the corresponding neutral ones upon adsorption on $$\hbox {Na}_{8}$$ cluster, the basicity of the Lewis bases enhances in the presence of $$\hbox {Na}_{8}$$ . The calculated proton affinities of the isolated Lewis bases were smaller than 990 kJ mol $$^{-1}$$ while in the presence of the $$\hbox {Na}_{8}$$ cluster, the basicity increases so that the calculated proton affinities were higher than 1000 kJ mol $$^{-1}$$ . SYNOPSIS Adsorption of neutral amines on $$\hbox {Na}_{8}$$ is very weak while protonated amines are more strongly adsorbed on $$\hbox {Na}_{8}$$ . These results showed that the $$\hbox {Na}_{8}$$ cluster stabilizes the protonated amines better than the corresponding neutral ones. Therefore, basicity of the amines is enhanced in the presence of the $$\hbox {Na}_{8}$$ cluster.