Structural increments for 11-vertex nido-phospha- and aza(carba)boranes and -borates; dependence of energy penalties on the extent of Electron Localization

Relevant structural features and corresponding energy penalties were determined that allow to easily estimate the relative stabilities of 11-vertex nido-phospha- and aza-substituted boranes, borates, carbaboranes, and carbaborates. For this purpose, density functional theory computations at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d)+ZPE level were carried out to determine the relative energies of 95 phospha- and 46 aza(carba)boranes and -borates. Energy penalties assigned to disfavoring structural features show additive behavior and excellent precision with respect to the computed results, as in the case of 6- and 11-vertex nido-carboranes and -borates. An unsubstituted phosphorus atom was found to possess energy penalties quite similar to those of the three-electron-donating H-C group. A bare nitrogen atom has energy penalties much larger than those of a bare phosphorus atom. Four-electron-donating RP and RN moieties, however, have even more adverse energy penalties. The disfavoring effects of heteroatoms in a borane cluster are determined by the amount of electron localization, that is, primarily by the number of skeletal electrons that formally originate from the heterogroup and secondarily by the electronegativity. Heteroatom energy penalties are independent of the type of the other heteroatoms present in the same cluster. Some novel phospha(carba)borane geometries with bare and exo-substituted phosphorus atoms in the same cluster have favorable thermodynamic stabilities competitive with those of known isomers.     

The relative stabilities of 11-vertex nido- and 12-vertex closo-heteroboranes and -borates: facile estimation by structural or connection increments

The relative thermodynamic stabilities of ortho-, meta- and para-isomers of 12-vertex closo-heteroboranes and -borates with different p-block heteroatoms were determined using density functional theory. More electronegative (smaller) heteroatoms tend to occupy non-adjacent, whereas less electronegative (larger) heteroatoms tend to occupy adjacent vertices in the thermodynamically most stable closo-diheterododecaborane isomers. The computed relative stabilities agree perfectly with experimental observations. The energy differences of para- and meta- relative to ortho-isomers of 12-vertex closo-heteroboranes generally depend on the extent of electron localization by a given heteroatom and show highly periodic trends, i.e. increase along the period and decrease down the group. The energy penalties for the HetHet structural feature (two heteroatoms adjacent to each other) for the 12-vertex closo-cluster are apparently significantly different from those for the 11-vertex nido-cluster. Reformulating two 11-vertex nido-structural features, i.e. Het(5k)(2) and HetHet, in terms of connection increments along with the additional structural feature HetHet(m) give the relative stabilities of various isomeric 11-vertex nido- as well as 12-vertex closo-heteroboranes and -borates, using one unique set of increments.     

Which nido:nido-macropolyhedral boranes are most stable?

A comparison of the relative stabilities computed at RB3LYP/6-311+G(d,p)//RB3LYP/6-31G(d)+ZPE of the neutral nido-single clusters and two vertex-sharing macropolyhedral nido:nido-clusters shows single-cluster nido-boranes with up to 11 vertexes to be energetically more favorable than isomeric macropolyhedral boranes. Extra hydrogen atoms at the open face have a significant influence on the relative stabilities of the single cluster nido-boranes vs nido:nido-macropolyhedral boranes. For anionic species, a clear-cut turning point for macropolyhedral preference is shifted to no less than 17 vertexes. Thermodynamically most stable neutral and anionic nido:nido-macropolyhedral boranes usually consist of a nido-10-vertex and a nido-11-vertex unit, respectively. The relative stabilities of isomeric neutral macropolyhedra reflect the patterns exhibited by the sum of thermodynamic stabilities of the individual clusters.     

Geometry and stability of BenCm (n=1-10; m=1, 2, ..., to 11-n) clusters

Density functional theory B3PW91/6-31+G* calculations on BenCm (n=1-10; m=1, 2, ..., to 11-n) clusters have been carried out to examine the effect of cluster size, relative composition, binding energy per atom, HOMO-LUMO gap, vertical ionization potential, and electron affinity on their relative stabilities. The most stable planar cyclic conformations of these clusters always show at least a set of two carbon atoms between two beryllium atoms, while structures where beryllium atoms cluster together, or allow the intercalation of one carbon atom between two of them, generally seem to be the least stable ones. Clusters containing 1, 2, and 3 beryllium atoms (Be2C8, Be3C6, Be2C6, BeC6, Be2C4, BeC4, Be2C2, and BeC2) are identified as clusters of "magic numbers" in terms of their high binding energy per atom, high HOMO-LUMO gap, vertical ionization potential, and second difference in energy per beryllium atom.     

一种新型包含平面四配位碳二硼有机化合物的理论研究

采用B3LYP/6-311+G**方法, 研究了一种新型的包含平面四配位碳(ptC)二硼有机化合物C9B2H6的结构、稳定性和振动频率. 计算结果表明, C9B2H6结构的稳定性和两个硼原子的位置有很大关系, 硼原子起给予σ电子和接受π电子的作用. 在C9B2H6的15个异构体中, 最稳定的结构是具有C2v对称性的异构体(1,5), 在异构体(1,5)中, 两个硼原子位于同一个六元环中且与ptC相邻. 而且占据的π轨道说明异构体(1,5)具有10个π电子, 满足4n+2规则. 计算的核独立化学位移(NICS)值显示异构体(1,5)强的芳香性位于C9B2H6的两个三元环而不是两个六元环上.     

硼碳团簇BnC2 (n＝1～6)的理论研究

应用密度泛函理论在B3LYP/6-311＋G(d)水平上研究了硼碳团簇B_nC₂ (n＝1～6)的几何结构、生长机制和相对稳定性. 计算结果表明, 对于n＝2～6的簇, 平面多环状构型为最稳定的结构, 其中C原子分布于环的顶点、有尽可能多的三配位硼原子和尽可能多的B—C键. 碳原子作为杂原子倾向掺杂于团簇的顶点位置, 它的掺杂不改变硼团簇的主体结构. 与平面多环状结构相比, 随着簇尺寸的增大, 三维结构和线性链结构更不稳定. 在低能线性结构中, C原子位于链两侧的第二个位置. 计算的碎片分裂能、递增键能以及HOMO-LUMO能隙表明, B₄C₂为幻数簇.     

Hydrogen atom mediated Stone-Wales rearrangement of pyracyclene: a model for annealing in fullerene formation

We have investigated the Stone-Wales (SW) rearrangement of pyracyclene (C(14)H(12)) using quantum mechanical molecular modeling. Of particular interest in this study is the effect of an added hydrogen atom on the barriers to SW rearrangement. Hydrogen atoms are found in high abundance during combustion, and their effect upon isomerization of aromatic compounds to more stable species may play an important role in the combustion synthesis of fullerenes. We have calculated the barriers for the SW rearrangement in pyracyclene using density functional theory B3LYP/6-31G(d) and B3LYP/6-311G(d,p). Two mechanisms have been investigated: (i) a mechanism with two identical transition states of C(1) symmetry and a cyclobutyl intermediate and (ii) a mechanism with one transition state containing an sp(3) carbon (J. Am. Chem. Soc. 2003, 125, 5572-5580; Nature 1993, 366, 665-667). We find that the barriers for these mechanisms are 120.0 kcal mol(-1) for the cyclobutyl mechanism and 130.1 kcal mol(-1) for the sp(3) mechanism. Adding a hydrogen atom to the internal bridge carbon atoms of pyracyclene reduces the barrier of the cyclobutyl mechanisms to 67.0 kcal mol(-1) and the sp(3) mechanism to 73.1 kcal mol(-1). The bonding of carbon atoms in pyracyclene is similar to those found in isomers of C(60), and the barriers are low enough so that these reactions can become significant during fullerene synthesis in flames. Adding hydrogen atoms to the external bridge atoms on pyracyclene produces a smaller reduction in the SW barrier and adding hydrogen atoms to nonbridge external carbon atoms results in no reduction of the barrier.     

The prevalence of isocloso deltahedra in low-energy hypoelectronic metalladicarbaboranes with a single metal vertex: manganese and rhenium derivatives

Theoretical studies on the hypoelectronic metalladicarbaboranes CpMC(2)B(n-3)H(n-1) (M = Mn, Re; n = 9, 10, 11) having 2n skeletal electrons indicate that true isocloso MC(2)B(n-3) deltahedra are highly energetically favored in which the metal atom occupies the single degree 6 vertex. This contrasts with the previously studied isoelectronic diferradicarbaboranes Cp(2)Fe(2)C(2)B(n-3)H(n-1) for which the isocloso structure is clearly favored only for the 10-vertex system. For the 12-vertex hypoelectronic manganadicarbaborane CpMnC(2)B(9)H(11) with 2n (= 24) skeletal electrons the lowest energy structures have central MnC(2)B(9) icosahedra. However, for the corresponding rhenadicarbaborane CpReC(2)B(9)H(11) the lowest energy structures have central non-icosahedral ReC(2)B(9) deltahedra with two degree 6 vertices, one of which is occupied by the rhenium atom. The low-energy structures for the metalladicarbaboranes studied in this work relate to the preferences of transition metal atoms for degree 6 vertices but those of boron and carbon for degree 5 and 4 vertices, respectively.     

14顶点闭合型碳硼烷异构体的结构和稳定性的密度泛函理论研究

采用密度泛函理论(DFT) B3LYP/6-31G(d)方法对14顶点闭合型碳硼烷异构体的几何结构进行优化, 分析了它们的稳定性、电荷分布以及前线分子轨道能级. 结果表明, C₂B₁₂H₁₄碳硼烷的9个异构体都有对应的稳定构型, 并基本保持了B₁₄H₁₄^2－的骨架构型; 除两个C原子取代轴顶点位置B原子的1,14-C₂B₁₂H₁₄外, 其稳定性均随着两个C原子之间距离的增大而增加, 但C原子取代高配位数的B原子不利于其构型的稳定性. 各异构体的负电荷主要分布在C原子上, 同时处于轴向位置的B原子也有部分负电荷, 它们可能成为反应的亲核活性中心. 异构体的HOMO能级的高低与其稳定性相对应, HOMO能级低的异构体稳定性好.     

Prediction of pKa values of nido-carboranes by density functional theory methods

A great parallel exists between metal complexes of cyclopentadienyl and arene ligands on one side and metal complexes of the nido derivatives of the icosahedral o-carborane clusters. With few exceptions, the metal complexation in the cluster can be viewed as the substitution of one or more bridging hydrogen atoms by the metal. Therefore, a necessary requirement for the complexation is the deprotonation of the nido cluster to generate a coordination site for that metal. The reaction to remove these protons, which most probably is one of the most commonly done processes in boron and metallaborane chemistry, is barely known, and no quantitative data are available on the magnitude of their pKa values. With the purpose of determining the acidity of nido-carboranes, a procedure to calculate the pKa values of nido boron clusters is presented in this paper for the first time. To this objective, some nido clusters have been selected and their geometry and NMR-spectroscopic properties have been studied, giving a good correlation between the theoretical and experimental data in both geometry distances and 11B NMR spectroscopy. Of notice is the result that proves that the singular carbon atom in the thermodynamic isomer of [C2B10H13]- is definitely part of the cluster and that its connection with the C2B3 face would be better defined by adding additional interactions with the two boron atoms nearest to the second cluster carbon. The pKa values of the nido species have been calculated by correlating experimental pK(a) values and calculated reaction Gibbs energies DeltaG(s). Some pKa values of importance are -4.6 and +13.5 for 7,8-[C2B9H13] (1) and 7,8-[C2B9H12]- (2), respectively.     

苯乙烯桥联的三苯烯的电子结构和荧光光谱

以苯乙烯桥联的三苯烯分子为模型化合物, 自行设计一系列分子, 用半经验AM1方法和密度泛函理论在B3LYP/3-21G水平上对其进行结构优化. 在B3LYP/3-21G优化构型基础上, 用INDO/CIS和B3LYP/3-21G方法计算其电子吸收光谱和碳原子化学位移. 结果表明, 母体化合物中取代基的数目、供电子能力和立体效应将影响其LUMO-HOMO(最低空轨道-最高占据轨道)能隙、空穴注入性能及热稳定性. —CN和—OH的存在将使化合物电子吸收光谱的第一吸收峰及主要吸收峰与母体相比发生红移. —CN上的C原子受N原子影响化学位移出现在低场, 与—CN相连的C原子由于具有较大的电子云密度化学位移向高场移动, 骨架上的C原子化学位移基本保持不变.     

胸腺嘧啶-BH3构型与性质的DFT和MP2研究

Geometries and combination energies are predicated at B3LYP / 6-31G（d）and MP2 / 6-31G（d）level for thymine-BH3 complexes and 5 geometries have been obtained. Then single point energy calculations using larger basis sets（6-311 + G（2df）and aug-cc-pVDZ）and vibrational analysis and natural bond orbital analysis are carried out on the 5 optimized conformers. The outcome indicates that the conformers with the boron atom combined with O directly are relatively stable ones,（a）and（b）,with the combination energies of 90. 4 and 88. 0 kJ / mol （B3LYP / 6-31G（d）,BSSE corrected）. The fact is that the nitrogen atom offers electron to the empty atomic orbital of boron which produces the conformers（c）and（d）. Only one conformer is found which is formed because two carbon atoms offer π electron to the empty orbital of boron. The charge transference exists in all the conformers. The combination energies have a good line relation with their charge transference. The calculated results show that when the complex forms their IR spectrum moved to the red side and the frequency shifts are relative to the stabilities of the complexes.     

Bicyclo[3.2.1]octa-3,6-dien-2-yl cation: a bishomoantiaromate

Antiaromatic compounds with a closed loop of 4n p-electrons are relatively unstable and often difficult to study. We report in this article the synthesis of alcohols 2-(4'-fluorophenyl)bicyclo[3.2.1]octan-2-ol 11, 2-(4'-fluorophenyl)bicyclo[3.2.1]oct-3-en-2-ol 12, and 2-(4'-fluorophenyl)bicyclo[3.2.1]octa-3,6-dien-2-ol 13 and their transformations into corresponding carbocations 14-16, respectively, in a superacidic medium (FSO3H/SO2ClF) at -120 degrees C. Cations 14-16 are characterized by NMR analysis (1H, 13C, 19F), and 15 and 16 are further characterized by quenching in NaOCH3/H3COH at -120 degrees C. The relative stabilities of 14-16 are determined experimentally by 19F NMR spectroscopy. Cation 16 is found to be experimentally less stable than cation 15 by 3.7 kcal/mol. DFT calculations (structure and energy: B3LYP/6-31G(d); NMR: B3LYP/6-311+G(2d,p)) are performed for alcohols 11-13 and bicyclo[3.2.1]octyl cations 6, 7, 9, 14-16, 26, 28, and 30. In the case of 11-16, data from DFT calculations is in good agreement with experimental data. Because 6,7-dimethylenebicyclo[3.2.1]oct-3-en-2-yl cation 26 is more stable than cation 7 by 1.69 kcal/mol, the inductive effect of sp(2)-hybridized carbon atoms C6 and C7 in carbocations 6 and 16 cannot be the reason for the destabilization of 6 relative to 7 and 16 relative to 15. Destabilization of 6 relative to 7 and 16 relative to 15 and the calculated NICS of 6 (+4.17 ppm) and 16(+3.3 ppm) document that 6 and 16 are bishomoantiaromates.     

Hypercoordinated carbon in C-doped boron fullerenes: a quantum chemical study

The structures and stability of C-doped boron fullerenes with the three-dimensional arrangement of non-classical pentacoordinated quasi-flat carbon centers were studied using the density functional theory (DFT) B3LYP/6-311+G(d,p) method. The doping with carbon atoms in apical positions above the five-membered rings stabilizes the spherical boron fullerene forms due to multicenter interactions of p_z-orbitals of the carbons and adjacent boron atoms. Increasing in the size of the fullerene cluster is accompanied by change in the bonding pattern and by flattening of the hypercoordinated carbon centers. Endohedral metal atoms significantly affect on the structure and stability of the fullerene systems with hypercoordinated carbon centers.     

Electronic Structures and Fluorescent Spectra of 1,4-divinyphenyl-bridged Triphenylenes

A series of derivatives were designed based on the model compound 1,4-divinyphenyl-bridged triphenylene, and the structures were studied using the AM1 method and density functional theory (DFT) at the B3LYP/3-21G level. On the basis of the B3LYP/3-21G optimized geometries, the electronic spectra and ¹³C NMR spectra of the derivatives were calculated using the INDO/CIS and B3LYP/3-21G methods, respectively. The energy gaps, abilities of accepting holes, and thermal stabilities were affected by the number and electron-donating capability as well as steric effect of the substituents. The red shifts of the first and main absorptions in the electronic spectra of the derivatives relative to those of the parent compound were predicted in the presence of the groups -CN and -OH. The chemical shift of the carbon atom on -CN was transferred to the low field under the effect of the nitrogen atom. The chemical shifts of the carbon atoms associated with -CN were transformed into the high field owing to the high electron density. The chemical shifts of the carbon atoms on the conjugation skeleton were almost unchanged.     

Synthesis and rearrangements of aminosubstituted ferra- and ruthenatricarbaboranes

A room-temperature reaction between the [7-tBuNH-nido-7,8,9-C3B8H10]- anion (1a) and [Cp*RuCl]4 leads to the ruthenatricarbollide [1-Cp*-12-tBuNH-1,2,4,12-RuC3B8H10] (2) (yield 85%). Analogously, the room-temperature photochemical reaction of 1a with [CpFe(C6H6)]PF6 gives the previously reported iron complex [1-Cp-12-tBuNH-1,2,4,12-FeC3B8H10] (3) (yield 82%). Both reactions are associated with extensive polyhedral rearrangement, which occurs under very mild conditions and brings the carbon atoms to positions of maximum separation within the framework. Compounds 2 and 3 were also surprisingly obtained via complexation of the isomeric [8-tBuNH-nido-7,8,9-C3B8H10]- (1b) anion. Complex 2 rearranges further to [1-Cp*-10-tBuNH-1,2,4,10-RuC3B8H10] (4) upon refluxing in xylene (145 degrees C). Density functional theory calculations at the B3LYP/SDD level were used to estimate relative stabilities of these metallacarborane isomers. Compounds 2 and 4, along with the 11-vertex closo compounds [1-Cp*-1,2,3,10-RuC3B7H10] (5) and [1-Cp*-10-tBuNH-1,2,3,10-RuC3B7H9] (6), were also isolated from the reaction between [Cp*RuCl2]2 and 1a in boiling xylene. The structure of 2 was established by an X-ray diffraction study, and the constitution of all compounds was determined unambiguously by multinuclear NMR spectroscopy, mass spectrometry, and elemental analyses.     

Decaborane thiols as building blocks for self-assembled monolayers on metal surfaces

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B(10)H(13)] 1, [2-(HS)-nido-B(10)H(13)] 2, and [1,2-(HS)(2)-nido-B(10)H(12)] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B(18)H(22)·C(6)H(6) solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B(10)H(13)(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol (1)H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe(2)(PMe(2)Ph)(2)] reacts with the exposed and acidic B-H-B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe(2)Ph)(2)-nido-7-PtB(10)H(11)] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.     

14顶点双取代碳硼烷和金属硼烷极化率和二阶超极化率的DFT研究

采用量子化学密度泛函理论(DFT) B3LYP/6-31G(d)方法对14顶点双取代碳硼烷和金属硼烷几何构型进行优化, 结合有限场(FF)方法计算了各体系的极化率和二阶超极化率. 同时金属硼烷中金属原子采用赝势基组进行计算, 讨论基组对计算结果的影响. 结果表明, 14顶点碳硼烷和金属硼烷中碳和金属元素的成键方式不同, 金属硼烷中各原子间距离比碳硼烷中大, 平面偏移角增大. 金属原子的引入有效增加分子的NLO系数, 同时金属硼烷的前线分子轨道能级差比碳硼烷小很多, 金属硼烷材料有可能表现出半导体甚至导体特性, 金属原子采用不同基组对计算结果影响不大.     

Novel class of heterometallic cubane and boride clusters containing heavier group 16 elements

Thermolysis of an in situ generated intermediate, produced from the reaction of [Cp*MoCl(4)] (Cp* = η(5)-C(5)Me(5)) and [LiBH(4).THF], with excess Te powder yielded isomeric [(Cp*Mo)(2)B(4)TeH(5)Cl] (2 and 3), [(Cp*Mo)(2)B(4)(μ(3)-OEt)TeH(3)Cl] (4), and [(Cp*Mo)(4)B(4)H(4)(μ(4)-BH)(3)] (5). Cluster 4 is a notable example of a dimolybdaoxatelluraborane cluster where both oxygen and tellurium are contiguously bound to molybdenum and boron. Cluster 5 represents an unprecedented metal-rich metallaborane cluster with a cubane core. The dimolybdaheteroborane 2 was found to be very reactive toward metal carbonyl compounds, and as a result, mild pyrolysis of 2 with [Fe(2)(CO)(9)] yielded distorted cubane cluster [(Cp*Mo)(2)(BH)(4)(μ(3)-Te){Fe(CO)(3)}] (6) and with [Co(2)(CO)(8)] produced the bicapped pentagonal bipyramid [(Cp*MoCo)(2)B(3)H(2)(μ(3)-Te)(μ-CO){Co(3)(CO)(6)}] (7) and pentacapped trigonal prism [(Cp*MoCo)(2)B(3)H(2)(μ(3)-Te)(μ-CO)(4){Co(6)(CO)(8)}] (8). The geometry of 8 is an example of a heterometallic boride cluster in which five Co and one Mo atom define a trigonal prismatic framework. The resultant trigonal prism core is in turn capped by two boron, one Te, and one Co atom. In the pentacapped trigonal prism unit of 8, one of the boron atoms is completely encapsulated and bonded to one molybdenum, one boron, and five cobalt atoms. All the new compounds have been characterized in solution by IR, (1)H, (11)B, and (13)C NMR spectroscopy, and the structural types were unambiguously established by crystallographic analysis of 2 and 4-8.     

胞嘧啶-BH3复合物构型与性质的量子化学研究

对胞嘧啶-BH3复合物分别用B3LYP/6-31G和MP2/6-31G进行理论计算以预测该复合物的构型及稳定化能, 得到了5种构型。对各构型进行了振动频率分析和自然键轨道分析。结果表明, B与N和O直接相连的构型比较稳定, 其稳定化能为137.4和120.0 kJ/mol (MP2/6-31G)。其余的构型靠π-p作用而形成, 其稳定性较前两者弱。各构型的稳定化能与电荷转移量有良好的相关性。复合物的形成, 使其红外光谱均有不同程度的红移, 幅度与复合物的稳定性相关。