ABSTRACT
Boron-centered radicals have received growing interest. Recently, two groups reported density functional theory investigations (GGA-PW91 and B3LYP) on a hexa-atomic boron-oxide radical, B(5)O, which has led to great discrepancies on the type of low-lying structures. In this article, we not only explore the energetics of doublet and quartet B(5)O isomers at high electron-correlated levels (CCSD(T)/6-311+G(2df), CCSD(T)/aug-cc-pVTZ, and G3B3) but also investigate the isomerization and fragmentation stability of the low-lying B(5)O isomers. All the high-level studies consistently show that the B(5)O radical possesses a belt-like ground structure (2)01 in doublet electronic state followed by isomer (2)02 with an exocyclic - BO moiety at around 3.0 kcal/mol. Kinetically, (2)01 and (2)02 are separated by a considerable barrier of about 20 kcal/mol. Thus, the two isomeric forms of B(5)O radical should be very promising for isolation in laboratory. However, the other four isomers reported recently are all kinetically unstable toward conversion to (2)01 and (2)02. The high thermodynamic and kinetic stability of (2)01 and (2)02 might make them as important building cores in the growth of boron-oxide clusters. This results would also help deeply understand the oxidation and doping mechanism of pure boron clusters.
Subject(s)
Boron/chemistry , Oxygen/chemistry , Thermodynamics , Free Radicals/chemistry , Isomerism , Kinetics , Molecular Structure , Quantum TheoryABSTRACT
Among the fascinating planar tetracoordinate carbon (ptC) species, pentaatomic molecules belong to the smallest class, well-known as "pptC". It has been generally accepted that the planarity of pptC structure is realized via the "delocalization" of the p(z) lone pair at the central carbon and the ligand-ligand bonding interaction. Although "localization" is as key driving force in organic chemistry as "delocalization", the "localization" concept has not been applied to the design of pptC molecules, to the best of our knowledge. In this paper, we apply the "localization" strategy to design computationally a series of new pptC. It is shown that the central carbon atom and one "electronegative" ligand atom X (compared to the Al ligand) effectively form a highly localized C-X multiple bond, converting the lone pair at the central carbon to a two-center two-electron π-bond. At the aug-cc-pVTZ-B3LYP, MP2 and CCSD(T) levels, the designed 18-valence-electron pptC species [XCAl(3)](q); [(X,q) = (B,-2), (C,-1), (N,0)] are found to each possess a stable ptC structure bearing a C-X double bond, indicated by the structural, molecular orbital, Wiberg bonding, potential energy surface and Born-Oppenheimer molecular dynamics (BOMD) analysis. Moreover, our OVGF calculations showed that the presently disclosed (yet previously unconsidered) pptC structure of [C(2)Al(3)](-) could well account for the observed photoelectron spectrum (previously only ascribed to a close-energy fan-like structure). Therefore, [C(2)Al(3)](-) could be the first pptC that bears the highly localized C-X double bond that has been experimentally generated. Notably, the pptC structure is the respective global minimum point for [BCAl(3)](2-) and [NCAl(3)], and the counterion(s) would further stabilize [BCAl(3)](2-) and [C(2)Al(3)](-). Thus, these newly designed pptC species with interesting bonding structure should be viable for future experimental characterization. The presently applied "localization" approach complements well the previous "delocalization" one, indicating that the general "localization vs. delocalization" concept in organic chemistry can be effectively transplanted to exotic pptC chemistry.
ABSTRACT
Molecules with hypercoordinate planar centers have continued to receive enthusiastic attention due to their violation of the traditional models of three-dimensional chemical bonding and maximum tetracoordination. These electronic exotic but structurally aesthetic species have been optimistically conceived as building blocks in cluster-assembly for bulky materials. Recently, the planar hexacoordinate nitrogen (phN) unit, NB(6)(-), has been theoretically incorporated into a series of sandwich-like transition-metal compounds. However, the intrinsic stability of NB(6)(-) in both gas-phase and assembly has not been tackled, though it is the key factor for predicting the viability of any molecules. In this paper, at the CCSD(T)/6-311+G(2df)//B3LYP/6-311+G(d)+ZPVE level, we investigate for the first time the thermodynamic and kinetic stability of the phN unit, NB(6)(-), in both free and assembled ([NB(6)](2)Fe) forms. The calculated least barrier height of phN towards conversion is 9.2 and 4.4 kcal mol(-1) in free and assembled forms, respectively. Most importantly, the phN structure is thermodynamically rather unstable, by 102.8 and 162.1 kcal mol(-1) higher than the respective lower-lying conversion isomers. Therefore, in view of the combined thermodynamic and kinetic consideration, we propose that isolation of the phN structure of NB(6)(-) in either gas phase or assembly is unlikely. The present results manifest that for predicting any viable molecule with exotic structures, investigation of its "intrinsic stability" is highly necessary. The maintenance of the phN-NB(6)(-) is discussed at the 6-311+G(d)-B3LYP, MP2, CCSD and CCSD(T) optimization levels in comparison with the isoelectronic and milestone phC-CB(6)(2-).
ABSTRACT
Contrasting the boranes B(n)H(n+4) with rich chemistry, the alanes Al(n)H(n+4) remain largely unknown in laboratory, except for the simplest Al(2)H(6). Though recent experimental and theoretical studies have proved Al(n)H(n+2) to be the borane analogues, whether or not the borane analogy can exist for the more complicated Al(n)H(n+4) is still unclear. In this paper, we find that at the B3PW91/TZVP level, Al(n)H(n+4) each has a nido-single cluster ground structure as B(n)H(n+4) for n < 12. For n >or= 12, the fusion cluster becomes energetically more competitive than the single cluster also as B(n)H(n+4). Thus, concerning the ground structures, the alanes Al(n)H(n+4) (n = 5-19) could be considered as the borane analogues. Remarkably, Al(8)H(12) has a novel closo(4)-closo(4) cluster fused by two T(d)-like subunits Al(4)H(6), lying only 0.49 kcal/mol above the single cluster. The Born-Oppenheimer molecular dynamic simulation shows that the closo(4)-closo(4) fusion cluster intrinsically has high kinetic stability, which can be ascribed to the rigidity of the T(d)-Al(4)H(6) subunit. Since T(d)-Al(4)H(6) has been experimentally characterized in a gas phase very recently, we strongly recommend that the unprecedented non-Wade-Mingos alane Al(8)H(12) can be effectively formed via the direct dimerization between two T(d)-Al(4)H(6), with the reaction energy (-39.65 kcal/mol) very similar to that of the known dialane (2AlH(3) --> Al(2)H(6), -35.27 kcal/mol).
ABSTRACT
The recent, experimentally-discovered, all-metal antiaromatic Li3Al4- has attracted great interest and extensive investigations due to its unique chemical bonds and exotic properties. Although a very recent theoretical study demonstrated that the all-metal species Li3Al4- can be effectively stabilized by complexation with 3d transition metals, unfortunately such stabilization is at the expense of losing antiaromaticity (rectangular Al4) to become aromatic (square Al4). Here, we predict theoretically a series of cluster-assembled compounds [DM(Li3Al4)]q- (D=Li3Al4-, Cp-; M=Li, Na, K, Be, Mg, Ca). The assembled species are ground states containing the all-metal antiaromatic Li3Al4- subunits. Many fusion isomers are energetically lower than the homo-decked cluster-assembled compounds, thus, the homo-decked assembly species [M(Li3Al4)2]q- are less likely due to their thermodynamic instability. In addition, the well-retained all-metal antiaromaticity is mainly ascribed to the ionic electrostatic interactions and the protections of rigid organic aromatic Cp-deck avoiding the fusion of Li3Al4-. Our results represent the first example that the all-metal antiaromaticity is well retained in assembled compounds as that in the free Li3Al4- cluster. Sufficiently large interaction energies make the realization of all-metal antiaromatic Li3Al4--incorporated compounds very promising.