Theoretical Investigations On The Structures And Stability For Nitrogen-rich And Planar Molecules

Detailed theoretical studies on the structures and stabilities of the nitrogen-rich andplanar molecules based on the doping stratege are performed by using the quantumchemical methods. The present work not only provides a theoretical basis for furtherprobing of the future laboratory, but demonstrates that the doping at the X-site wouldhave significant influence on the stability of the nitrogen-rich and planartetra-coordinate structure.The main results are summarized as follows:1. In this paper, we for the first time theoretically explored the spectroscopy andstability of CNN3employing the CCSD and CCSD(T) methods with the aug-cc-pVTZbasis set. In light of the potential multireference character, the involved isomers andtransition state structures on the reaction pathways were re-optimized using themulti-configurational CASSCF method in conjunction with the cc-pVTZ basis set. Itwas shown that CNN3should be intrinsically stable both against isomerization andfragmentation. The structural and spectroscopic properties obtained in this papershould provide useful information for future laboratory identification of CNN3. Forcomparison, we also calculated the aug-cc-pVTZ-CCSD(T) and cc-pVTZ-CASSCFproperties of NCN3, whose levels are hitherto the highest.2. In this paper, we report in detail the structures and stabilities of a tetra-atomiccluster CN3 that is isoelectronic to the well-known N4. A series of higher-levelenergetic calculations were carried out at the CCSD(T)/6-311+G(d)//B3LYP/6-311+G(d), CASPT2(12,12)/6-311+G(d)//CASSCF (12,12)/6-311+G(d)and CASPT2(12,12)/aug-cc-pVTZ//CASSCF(12,12)/ aug-cc-pVTZ levels. Thekinetic stability of CN3 was evaluated for the first time by studying the dissociation, isomerization and intersystem crossing barriers as well as the Born-Oppenheimermolecular dynamic (BOMD) simulations. We found that the ground state isomer, i.e.,chainlike triplet NCNN (Cs), and a higher-energy isomer, i.e., tetrahedral-like singletCN3 (C3v),are both kinetically stable even when the intersystem crossing (ISC) isincluded. Therefore, the two isomers3NCNN and1CN3 (C3v) could be observable infuture laboratory studies. By comparing with the isoelectronic N4system, weconclude that the carbon-doping for XN3(X=N to C ) greatly enhances the kineticstability of the chainlike triplet and the tetrahedral-like singlet structures. Finally, formore practical use, our studies on the related salts X+[CN3] (X=Li,Na,K) showed thatboth the chain-like3NCNN and tetrahedral-like1CN3 could act as promisingbuilding blocks for novel HEDMs.3. The literature has reported that the Si/Ge single-doped Al4 clusters have a groundstate with planar tetra-coordinate Si/Ge structure. At the CCSD(T)/aug-cc-pVTZ&CEP-121G//B3LYP/6-311+G(d)&LANL2DZ level, we for the first timeinvestigated the structures and stabilities of XAl4(X=Sn, Pb). Our results show thatdifferent with XAl4(X=Si, Ge), the ground state structures of SnAl4and PbAl4areplanar tetra-coordinate Al in stead of a planar tetra-coordinate Sn/Pb structure. Inaddition, the stabilities of non-planar three-coordinate structure of SnAl4/PbAl4arealso better than that of ptX(X=Sn,Pb)4. With the"un-traditional"electronic and topological structures, thethermodynamically stabilized planar tetra-/hyper-coordinate molecules usually needthe perfect matching between the"electronic"and"mechanical"factors, which hasposed great challenges for their design. Moreover, due to the higher metallicity andweaker pi-delocalization ability, the planar metal-centered molecules have to faceparticular difficulty. In this paper, by means of the global isomeric search method, weconstructed for the first time the isomerization potential energy surface of XAl4 (X=Sn,Pb) at the CCSD(T)/aug-cc-pVTZ&CEP-121G//B3LYP/6-311+G(d)&LANL2DZ level (CEP-121G and LANL2DZ for Sn, Pb only). The results show thatfor X=Si/Ge, the planar tetra-coordinate X structure is the global isomer. Yet, forX=Sn/Pb, in addition to the planar tetra-coordinate X isomer, a previously unfocused "pyramidal"structure lies almost isoenergetic. Morevoer, from X=Si/Ge to X=Sn/Pb,the potential energy surface becomes smoother. The present work not only providesnew clues to the 17 valence electron rule in the planar chemistry, but demonstratesthat the doping at the X-site would have significant influence on the stability of theplanar tetra-coordinate structure.