Superalkali-doped borazine and lithiated borazine complexes: diffuse excess electron and large first-hyperpolarizability.

A number of superalkali (M3O / M3S; M = Li, Na, K)-doped borazine and hexalithio borazine complexes are considered for the theoretical study of their electronic structure and quadratic polarizability. Electron-rich O/S atom of superalkali species remains very close to one boron atom of the ring through non-covalent interaction. The first-hyperpolarizability increases rather significantly upon superalkali doping. The chosen complexes possess diffuse excess electron which is located on the superpalkali moiety of borazine complexes and at the ring site of lithiated borazines. First-hyperpolarizability of M3O(S)@B3N3Li6 complexes are significantly larger than that of the corresponding M3O(S)@B3N3H6 complexes. The magnitude of first-hyperpolarizability of Li3S@B3N3Li6 is larger than that of Li3S@B3N3H6 by about three orders of magnitude.

Second hyperpolarizability of diffuse-electron compounds M-NH3, M-NLi3 and M-NLi3-M of alkaline earth metals: Effect of lithiation.

Present investigation demonstrates that push electron effect of nitrogen in M … NH3is reduced rather than enhanced upon lithiation which has been explained by natural bond orbital analysis showing that nitrogen atom in NLi3 changes it's state of hybridization from sp3 to sp2 leaving the 2pz orbital partially occupied. This makes NLi3 a weak acceptor of electrons from alkaline earth metals. When the second alkaline earth metal is introduced the 2pz 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 (106 au) in M … NLi3 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.

Static second hyperpolarizability of inverse sandwich compounds (M1-C5H5-M2) of alkali (M1 = Li, Na, K) and alkaline earth metals (M2 = Be, Mg, Ca).

In the investigated inverse sandwich complexes, charge transfer from alkali metal (M1) led to aromatically stabilized Cp ring, which prevented further charge transfer from the alkaline earth metal (M2). This electron push effect resulted in diffusion of electron density from the outermost "ns" subshell of alkaline earth metal. The alkaline earth metal is weakly bound to the alkali metal-C5H5 complex. The vertical ionization energy of the chosen M1-Cp-M2 complexes was smaller than that of the corresponding alkaline earth metals. Large second hyperpolarizability (106-108 a.u.) was obtained for the studied molecules. The correction due to the basis set superposition error (BSSE) in the calculated second hyperpolarizability was found to be small for larger systems, while it was rather significant for small systems. The MP4SDQ and CCSD results were in fair agreement, which indicates the necessity of higher order electron correlation treatment in the accurate description of second hyperpolarizability. Calculated dynamic second hyperpolarizabilities at 1064 nm were found to increase considerably for most of the chosen metal complexes.

Successive lithiation of acetylene, ethylene and benzene: a comprehensive computational study of large static second hyperpolarizability.

This work is a revisit of the study of the electron correlation effect of lithium substitution on the second hyperpolarizability (106 a.u.) of acetylene, ethylene and benzene. The large quenching of mean second hyperpolarizability has been addressed by CCSD calculations. The inclusion of triple excitation in the MP4 method generally overestimates second hyperpolarizability in comparison to the MP4SDQ method. The present CCSD γav value of C6Li6: 405 × 104 a.u. obtained with a relatively larger basis set established the earlier prediction of Sadlej et al. [Phys. Chem. Phys. Chem., 2000, 2, 3393-3399] where degenerate non-dipolar transitions in low lying excited states play the crucial role. The successive lithiation results in gradual red shifting of transition energy leading to significant enhancement of second hyperpolarizability. Most of the chosen DFT functionals predict the correct qualitative trend of second hyperpolarizability. The quantitatively different results may be attributed to the case when the ground state wave function cannot be approximated by a single SD.

Second hyperpolarizability of delta shaped disubstituted acetylene complexes of beryllium, magnesium, and calcium.

Present theoretical study involves the delta shape complexes of beryllium, magnesium, and calcium where the metal atom interacts perpendicularly with disubstituted acetylene. Most of the complexes are found to be fairly stable. The dependence of second-hyperpolarizability on the basis set with increasing polarization and diffuse functions has been examined which showed the importance of 'f-type' type polarization function for heavy metal (Mg, Ca) and 'd-type' polarization function for beryllium. Larger second hyperpolarizability has been predicted for complexes having significant ground state polarization and low lying excited states favoring strong electronic coupling. Transition energy plays the most significant role in modulating the second hyperpolarizability.

Double coned inverse sandwich complexes [M-(η4-C 4H 4)-M'] of Gr-IA and Gr-IIA metals: theoretical study of electronic of structure and second hyperpolarizability.

Double coned inverse sandwich complexes of Gr-IA and Gr-IIA metals have been considered for the theoretical study of electronic structure and second hyperpolarizability by employing density functional theory methods for different exchange and correlation functionals. For the investigated metal complexes the 6-311++G(d,p) basis set can give reliable values of NLO property. The chosen complexes are found to be thermally stable with respect to the dissociation into neutral fragments. Interesting charge transfer interaction has been noted. The replacement of a lighter metal atom with a heavier one leads to the significant enhancement of the longitudinal component of second hyperpolarizability. The relatively much stronger enhancement of second hyperpolarizability has been noted for the alkaline earth metal complexes compared to that of alkali metals. The emergence of second hyperpolarizability of alkali/alkaline earth metal inverse sandwich complexes can be explained qualitatively in terms of the two-state model. The largest magnitude of second hyperpolarizability (~10(7) au) has been predicted for the Ca-C(4)H(4) -Ca complex which may be ascribed to the smallest transition energy and the highest transition moment associated with the most intense linear transition.

Beryllium-cyclobutadiene multidecker inverse sandwiches: electronic structure and second-hyperpolarizability.

Beryllium forms stable sandwich and inverse sandwich complexes with the cyclobutadiene molecule. Two types of multidecker complexes are designed. Multidecker inverse sandwiches are found to be thermally more stable than the corresponding sandwich complexes. The average distance between two consecutive metals and the two consecutive cyclobutadiene rings increase gradually on increasing size of the chosen inverse sandwich complexes. The density functional theory functionals B3LYP, BHHLYP, BLYP, M06, CAM-B3LYP, and B2PLYP in conjunction with the 6-311++G (d, p) basis set have been employed for calculating the third-order electric response properties of the chosen beryllium-cyclobutadiene complexes and the results obtained for each functional are found to have a consistent trend. Compared to the normal sandwich compounds the second-hyperpolarizability of inverse sandwiches is predicated to be larger, which fairly correlates with the extent of ground-state polarization. The significant enhancement of cubic polarizability of higher-order multidecker inverse sandwiches arises from the strong coupling between the ground and the low lying charge transfer excited states. The rather strong enhancement of second-hyperpolarizability on increasing size of the beryllium-multidecker inverse sandwiches may provide a new route to design efficient nonlinear optical materials.