Second hyperpolarizability of diffuse-electron compounds M-NH 3 , M-NLi 3 and M-NLi 3 -M of alkaline earth metals: Effect of lithiation

Present investigation demonstrates that push electron effect of nitrogen in M … NH is reduced rather than enhanced upon lithiation which has been explained by natural bond orbital analysis showing that nitrogen atom in NLi changes it's state of hybridization from sp to sp leaving the 2p orbital partially occupied. This makes NLi a weak acceptor of electrons from alkaline earth metals. When the second alkaline earth metal is introduced the 2p orbital of nitrogen attains the maximum occupation as a result of cumulative charge transfer from two metal atoms. However, the push electron effect still operates resulting in the diffuse electron localized over the metals. Large enhancement of second hyperpolarizability (10 au) in M … NLi has been rationalized by its low lying intense transitions. Effect of basis set on the second hyperpolarizability has been studied at the CCSD level which showed that Sadlej's pol and 6-311++G(3df,3pd) basis sets give comparable results that are obtained for the aug-cc-pVTZ basis set. The results of second hyperpolarizability calculated by using different DFT functionals are found to be consistent with the CCSD results obtained for the aug-cc-pVTZ basis set.