Role of central core and methyl substitutions in XH4-x(CH3)x (X = N, P, As; x=0-4) superalkalis: an ab initio study

Superalkalis are known for their lower ionization energy than alkali atoms. In the last four decades, a lot of exploration has been done to enrich the superalkali series. In this work, we report the superalkali behavior of -XH4-x(CH3)(x) series of molecules for X = N, P, As, and x = 0.4. We have systematically studied neutral and cationic -NH4-x(CH3)(x) and -PH4-x(CH3) (x) as well as -AsH4-x(CH3) (x) species using MP2/6.311 ++ G(d, p) level by successive substitutions of methyl -(CH3) groups. We have also analyzed the effect of substitutions as well as the role of central atom on the ionization energy (IE) of -XH4-x(CH3)(x). As we move from x = 1 to x = 4, the IE decreases in -NH4-x(CH3)(x), -PH4-x(CH3)(x), and -AsH4-x(CH3)(x), being in the range 3.97-2.77 eV, 4.10-2.76 eV, and 4.40-2.78 eV, respectively. Consequently, these molecules go to superalkalis. Their superalkali behavior has been explained on the basis of charge (de)localization in these species. These findings will provide new way to design superalkali molecules with even lower ionization energies.