Inverse Sodium Hydride: Density Functional Theory Study of the Large Nonlinear Optical Properties

“Inverse sodium hydride” (AdzH+Na-) is an alkalide compound synthesized in recent experiments containing
the unusual charge distribution H+ and Na- (inverse charge state). The new class of compounds interests
scientists to investigate their especial structures and properties. In this paper, the structures of three alkalides
compounds, (Me)3NH+Na-, AdzH+Na-, and AdzLi+Na-, have been obtained in theory. Especially, the structure
of AdzLi+Na- is still researched by experimental scientists. We investigated the NLO properties of the alkalides
complexes for the first time and found that inverse sodium hydride AdzH+Na- has a considerably large NLO
response β0 = 5.7675 × 104 au by density functional theory (DFT). To understand the essential features of
the large NLO properties, four related systems have been also calculated. Their first hyperpolarizabilities are
β0 = 7.357 × 103 au for (Me)3NH+Na-, β0 = 3.9 au for (Me)3NH+, β0 = 1.10 × 102 au for (Me)3NH+Cl-,
and β0 = 6.20681 × 105 au for AdzLi+Na-, respectively. By comparing, we found that, first, the Na- anion
plays a crucial role in the considerably large first hyperpolarizability of inverse sodium hydride and, second,
the first hyperpolarizability of inverse sodium hydride increases with the charge value of the sodium anion.
The above results are useful for designing potential NLO materials.