Theoretical Studies On Bonding, Structures And Properties Of Molecules And Clusters

This paper shows systematically theoretical study on bonding, structures and properties of several molcules and clusters. The main contribution is as follows,(1) Theoretically predict the nonlinear optical (NLO) properties of the electron-solvated cluster (FH)2{e}(HF). In comparison with neutral (HF)2{}(HF), (FH)2{e}(HF) possesses exceptional large NLO properties. At the CISD level,μof (FH)2{e}(HF) is 1.58 au., almost the twice of that of (HF)2{}(HF). Theα0,Δαandβ0 values of anion are 38, 42 and 1.6×106 times those of (HF)2{}(HF), respectively. Obviously, the excess electron is the key factor in bringing to (FH)2{e}(HF) the significantly large NLO response. This work will give hints to experimentalists in designing of NLO materials.(2) A new kind of single-electron lithium bonding complexes H3C···LiY (Y = H, F, OH, CN, NC, CCH) was predicted and characterized in the present paper. For each C3v H3C···LiY complex, single-electron Li bond is formed between the unpaired electron of CH3 radical and positively charged Li atom of LiY molecule. Due to the formation of the single-electron Li bond, the C–H bonds of the CH3 radical bend opposite to the LiY molecule and the Li–Y bond elongates. It is found that for the three H3C···LiY (Y = H, F, OH) complexes withoutπelectrons, the Li–Y stretching frequencies are redshifted. Abnormally, the other three H3C···LiY (Y = CN, NC, CCH) complexes withπelectrons exhibit blueshifted Li–Y stretching frequencies along with the elongated Li–Y bonds. The single-electron Li bond energies are 5.20–6.94 kcal/mol for the H3C···LiY complexes at the CCSD(T)/aug-cc-pVDZ+BF level with counterpoise procedure. The LiY molecules which containπelectrons can form stronger single-electron Li bonds with the given electron donor CH3 radical. By comparisons with some related systems, it is concluded that the single-electron Li bonds are stronger than single-electron H bonds, and weaker than conventional Li bonds andπ–Li bonds. In this paper we suggested a possible way to experimentally obtain the single-electron lithium bonding complex and proposed the conditions for choice of proper electron donor to form single-electron Li bonding complex. This work will help the experimental study on such nontraditional Li bonds.(3) Theoretically predict a new kind of alkali-πcomplexes LiC4H4-nFn (n = 0-4). The 12 structures of the LiC4H4-nFn (n = 0-4) complexes with all real frequencies are got at the MP2/6-311++G** level. NBO analyses show that these complexes are all charge-separated species Li+(C4H4-nFn)-. Li acts as electron donor, and cyclic C4H4-nFn accepts the electron and becomes into (C4H4-nFn)-. These complexes show large interaction energies (-2.13~-5.34 eV) at the CCSD(T)/6-311++G** level. In LiC4H4, cyclic carbon framework of C4H4 subunit has undergone a'butterfly'-type distortion from rectangle-planar structure (D2h) to out-of-plane'butterfly'structure (C2v) upon the interaction with the Li atom.The LiC4H4-nFn complexes with F(2) substituent exhibit different structural characteristics from those without F(2) substituent. Thus the 11 F-substituted complexes plus LiC4H4 are clearly divided into A (without F(2)) and B (with F(2)) two groups. In the complex of group A, the Li atom generally locates above the center of C4 ring, and the C(2)–H(2) bond bends opposite to the Li atom. However, in the complex of group B, the Li atom deviates from above X towards C(3) and F(2) atoms, and the C(2)–F(2) bond shows an obvious diversion towards the Li atom. Therefore, the distances L between the Li atom and X in group B are larger by about 0.5 A°than those in group A. Interestingly, the electron correlation contributions are obviously different between the complexes in group A and those in group B for interaction energies and vertical ionization energies (VIE). The electron correlation contribution are negative for the complexes in group B, whereas are positive for most complexes in group A.(4) Theoretically study a threefold aromatic N33- ring and a new kind of aromatic trigonal bipyramidal MN3M (M = Be, B, Mg, Al, Ca) species. N33- ring and the N3 subunit in MN3M are all regular triangles; The nucleus-independent chemical shift values are -102.16 ppm for the N33- ring, and -74.09, -79.39, -65.06, -74.44,和-62.33 ppm (at the geometrical center of the trigonal bipyramid) for BeN3Be, BN3B, MgN3Mg, AlN3Al, and CaN3Ca, respectively, whereas the NICS value is only -7.8 ppm for benzene ring. It shows strong aromaticity in the N33- ring and the five MN3M species. From molecular orbital (MO) analyses, it is known that the N33- ring has three delocalizedπ, three delocalizedσp, and three delocalizedσs MOs. Each of the delocalized chemical bonding systems satisfies the 4n+2 electron-counting rule and therefore exhibits the characteristics of aromaticity. Similarly, the five MN3M species and the N33- ring have the same set of MOs and exhibit threefold aromaticity.From NBO analyses, the three MN3M (M=Be, Mg, Ca) species contain the N33- ring and an ionic bond between the M atom and N3 subunit. TheirΔE and VIE values are relatively small. For the BN3B and AlN3Al species with some covalent character between the M atom and N3 subunit, theΔE and VIE values are relatively larger compared to those of the former three species.As a result of the repulsion between the N33- ring trianion and the electron clouds of the two Ca cations, the CaN3Ca species has a very low VIE value of 3.64 eV, which is even lower than that of the Cs atom. A further study on the structure of the CaN3CaCl molecule has confirmed the superalkali characteristics of CaN3Ca. In the CaN3CaCl molecule, the CaN3Ca moiety retains the geometry of the isolated CaN3Ca species. In the [CaN3Ca]+ moiety, the two Ca atoms share alike the additional positive charges; the MOs of the CaN3CaCl molecule resemble those of a typical alkali halide (NaCl). Therefore, the CaN3Ca species is a new superalkali atom. In addition, the characteristic vibrational frequencies calculated for the MN3M species may be useful in the experimental identification of the MN3M (M= Be, B, Mg, Al, Ca) species.(5) Our systematical calculations using high-level ab initio and density functional theories on BLin (n = 1-7) reveal the lowest energy geometries for these B-doped Li clusters. Given the uncertainty in the accuracy of the energies for highly spin-contaminated BLi6 cluster, the multireference properties of the cluster were checked by CASSCF and CASPT2 methods, and it is found that the results agree very well with the CCSD(T) results.The BLi6 cluster shows very low vIP = 3.75 eV at the CCSD(T) level; besides, it has nine valence electrons and its electronic shell structure is 1s21p62s1, which results in significant resemblances between BLi6 and Na atom. Therefore, the BLi6 cluster can be viewed as a superalkali atom and used in the synthesis of a new class of charge-transfer salts in which the corresponding anions are formed by the species with low EA. Analogically, the BLi7 cluster with 10 valence electrons can be considered as a superalkaline earth metal atom.This paper suggests a new idea on designing NLO materials, provides new knowledges on structures and properties of molecules and clusters, and gives important theoretical basis to experimental investigations.