Stability and three-dimensional aromaticity of closo-NB(n-1)H n azaboranes,n = 5-12

Computations on all the possible positional isomers of the closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) reveal substantial differences in the relative energies. Data at the B3LYP/6-311+G level of density functional theory (DFT) agree well with expectations based on the topological charge stabilization, with the qualitative connectivity preferences of Williams, and with the Jemmis-Schleyer six interstitial electron rules. The energetic relationship involving each of the most stable positional isomers, 1-NB(4)H(5), NB(5)H(6), 2-NB(6)H(7), 1-NB(7)H(8), 4-NB(8)H(9), 1-NB(9)H(10), 2-NB(10)H(11), NB(11)H(12), was based on the energies (DeltaH) of the model reaction: NBH(2) + (n-1)BH(increment) --> NB(n)()H(n)()(+1) (n = 4-11). This evaluation shows that the stabilities of closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) increase with increasing cluster size from 5 to 12 vertexes. The "three-dimensional aromaticity" of these closo-azaboranes NB(n)()(-)(1)H(n)() (n = 5-12) is demonstrated by their the nucleus-independent chemical shifts (NICS) and their magnetic susceptibilities (chi), which match one another well. However, there is no direct relationship between these magnetic properties and the relative stabilities of the positional isomers of each cluster. As expected, other energy contributions such as topological charge stabilization and connectivity can be equally important.     

Local aromaticity of [n]acenes, [n]phenacenes, and [n]helicenes (n = 1-9)

The local aromaticity of the six-membered rings in three series of benzenoid compounds, namely, the [n]acenes, [n]phenacenes, and [n]helicenes for n = 1-9, has been assessed by means of three probes of local aromaticity based on structural, magnetic, and electron delocalization properties. For [n]acenes our analysis shows that the more reactive inner rings are more aromatic than the outer rings. For [n]phenacenes, all indicators of aromaticity show that the external rings are the most aromatic. From the external to the central ring, the local aromaticity varies in a damped alternate way. The trends for the [n]helicene series are the same as those found for [n]phenacenes. Despite the departure from planarity in [n]helicenes, only a very slight loss of aromaticity is detected in [n]helicenes as compared to the corresponding [n]phenacenes. Finally, because of magnetic couplings between superimposed six-membered rings in the higher members of the [n]helicenes series, we have demonstrated that the NICS indicator of aromaticity artificially increases the local aromaticity of their most external rings.     

Electron affinities of Al(n) clusters and multiple-fold aromaticity of the square Al4(2-) structure

The concept of aromaticity was first invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized pi electrons. Recent photoelectron spectroscopy experiments on all-metal MAl(4)(-) systems with an approximate square planar Al(4)(2-) unit and an alkali metal led to the suggestion that Al(4)(2-) is aromatic. The square Al(4)(2-) structure was recognized as the prototype of a new family of aromatic molecules. High-level ab initio calculations based on extrapolating CCSD(T)/aug-cc-pVxZ (x = D, T, and Q) to the complete basis set limit were used to calculate the first electron affinities of Al(n)(), n = 0-4. The calculated electron affinities, 0.41 eV (n = 0), 1.51 eV (n = 1), 1.89 eV (n = 3), and 2.18 eV (n = 4), are all in excellent agreement with available experimental data. On the basis of the high-level ab initio quantum chemical calculations, we can estimate the resonance energy and show that it is quite large, large enough to stabilize Al(4)(2-) with respect to Al(4). Analysis of the calculated results shows that the aromaticity of Al(4)(2-) is unusual and different from that of C(6)H(6). Particularly, compared to the usual (1-fold) pi aromaticity in C(6)H(6), which may be represented by two Kekulé structures sharing a common sigma bond framework, the square Al(4)(2-) structure has an unusual "multiple-fold" aromaticity determined by three independent delocalized (pi and sigma) bonding systems, each of which satisfies the 4n + 2 electron counting rule, leading to a total of 4 x 4 x 4 = 64 potential resonating Kekulé-like structures without a common sigma frame. We also discuss the 2-fold aromaticity (pi plus sigma) of the Al(3)(-) anion, which can be represented by 3 x 3 = 9 potential resonating Kekulé-like structures, each with two localized chemical bonds. These results lead us to suggest a general approach (applicable to both organic and inorganic molecules) for examining delocalized chemical bonding. The possible electronic contribution to the aromaticity of a molecule should not be limited to only one particular delocalized bonding system satisfying a certain electron counting rule of aromaticity. More than one independent delocalized bonding system can simultaneously satisfy the electron counting rule of aromaticity, and therefore, a molecular structure could have multiple-fold aromaticity.     

Consistent aromaticity evaluations of methylenecyclopropene analogues

Quantitative evaluations of the aromaticity (antiaromaticity) of neutral exocyclic substituted cyclopropenes (HC)(2)C=X (X = BH to InH (group 13), CH(2) to SnH(2) (group 14), NH to SbH (group 15), O to Te (group 16)) by their computed extra cyclic resonance energies (ECRE, via the block-localized wave function method) and by their aromatic stabilization energies (ASEs, via energy decomposition analyses) correlate satisfactorily (R(2) = 0.974). Electronegative X-based substituents increase the aromaticity of the cyclopropene rings, whereas electropositive substituents have the opposite effect. For example, (HC)(2)C=O is the most aromatic (ECRE = 10.3 kcal/mol), and (HC)(2)C=InH is the most antiaromatic (ECRE = -15.0 kcal/mol). The most refined dissected nucleus-independent chemical shift magnetic aromaticity index, NICS(0)(πzz), also agrees with both energetic indexes (R(2) = 0.968, for ECRE; R(2) = 0.974, for ASE), as do anisotropy of the induced current density plots.     

The structure and energetics of (GaAs)n, (GaAs)n(-), and (GaAs)n+ (n=2-15)

Electronic and geometrical structures of neutral, negatively, and positively charged (GaAs)n clusters are computed using density functional theory with generalized gradient approximation. All-electron computations are performed on (GaAs)2-(GaAs)9 while effective core potentials (ECPs) are used for (GaAs)9-(GaAs)15. Calibration calculations on GaAs and (GaAs)9 species support the use of the ECP for the larger clusters. The ground-state geometries of (GaAs)n(-) and/or (GaAs)n+ are different from the corresponding neutral ground-state geometry, except for n=7, 9, 12, 14, and 15, where the neutral and ions have similar structures. Beginning with n=6, all atoms are three coordinate, except for (GaAs)10+ and (GaAs)13+. For the larger species, there is a competition between fullerenes built from hexagons and rhombi and geometrical configurations where Ga-Ga and As-As bonds are formed, which results in the formation of pentagons. As expected, the static polarizability varies in the order of anion>neutral>cation, but the values are rather similar for all three charge states. The thermodynamic stability for the loss of GaAs is reported.     

The closo-Pb10(2-) Zintl ion in the [Ni@Pb10]2- cluster

The cluster [Ni@Pb10](2-), prepared from Pb9(4-) and Ni(COD)2, contains a new Zintl ion subunit, closo-Pb10(2-), centered by a Ni atom.     

Homoleptic mononuclear and binuclear osmium carbonyls Os(CO)n(n = 3-5) and Os2(CO)n (n = 8, 9): comparison with the iron analogues

The structures and energetics of the experimentally known Os(CO)n ( n = 3-5), Os2(CO)9, and Os2(CO)8 have been investigated using density functional theory. For Os(CO)5, the lowest-energy structure is the singlet D(3h) trigonal bipyramid. However, the C(4v) square pyramid for Os(CO)5 lies only approximately 1.5 kcal/mol higher in energy, suggesting extraordinary fluxionality. For the coordinatively unsaturated Os(CO)4 and Os(CO)3, a D(2d) strongly distorted tetrahedral structure and a Cs bent T-shaped structure are the lowest-energy structures, respectively. For Os2(CO)9, the experimentally observed singly bridged Os2(CO)8(mu-CO) structure is the lowest-energy structure. A triply bridged Os2(CO)6(mu-CO)3 structure analogous to the known Fe2(CO)9 structure is a transition state rather than a true minimum and collapses to the singly bridged global minimum structure upon following the corresponding normal mode. An unbridged (OC)5Os --> Os(CO)4 structure with a formal Os --> Os dative bond analogous to known stable complexes of the type (R3P)2(OC)3Os --> W(CO)5 is also found for Os2(CO)9 within 8 kcal/mol of the global minimum. The global minimum for the coordinatively unsaturated Os2(CO)8 is a singly bridged (OC)4Os(mu-CO)Os(CO)3 structure derived from the Os2(CO)9 global minimum by loss of a terminal carbonyl group. However, the unbridged structure for Os2(CO)8 observed in low-temperature matrix experiments lies only approximately 1 kcal/mol above this global minimum. In all cases, the triplet structures for these osmium carbonyls have significantly higher energies than the corresponding singlet structures.     

Low-lying isomers of Si(n)(+) and Si(n)(-) (n=31-50) clusters

We carry out a systematic search for the atomic structures of silicon cluster cations and anions in the size range n=31-50 using density functional theory in the generalized-gradient approximation. The obtained lowest-energy candidates feature cagelike structures. We find that the computed binding energies and the dissociation pathways as well as the mobilities of our lowest-energy isomers of the cations are all in good agreement with the measured data from experiments. Furthermore, based on these isomers, we reveal that the steplike feature appearing in the measured high-resolution mobilities can be correlated with the corresponding fullerenes explicitly, which strongly support the notion that endohedral silicon fullerenelike structures are the most favored growth pattern for silicon clusters in the range n=31-50. Our calculation and analysis suggest that the proposed isomers are probably very close to the major-abundance isomers observed in experiments.     

Structures and energetics of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters

The structures and energies of Be(n)Si(n) and Be(2n)Si(n) (n = 1-4) clusters have been examined in ab initio theoretical electronic structure calculations. Cluster geometries have been established in B3LYP/6-31G(2df) calculations and accurate relative energies determined by the G3XMP2 method. The two atoms readily bond to each other and to other atoms of their own kind. The result is a great variety of low-energy clusters in a variety of structural types.     

Structure and vibrations of Ga(n)N(n) (n = 3-10) clusters

The structure and harmonic vibrations of Ga(n)N(n) (n = 3-10) clusters have been investigated using the B3LYP (Becke 3-parameter-Lee-Yang-Parr) density functional theory. All structures are found to be cumulenic D(nh) rings (equal bonds, alternating angles), with one intense out of plane mode and three infrared-active degenerate modes, of which the highest one is extremely intense and asymptotically increases to 1029 cm(-1) for n = 10. Comparisons with C2n, B(n)N(n), and Al(n)N(n) clusters, the structure and bonding type for the Ga(n)N(n) (n=3-10) clusters are consistent with those of the C2n (n = 3, 5, 7, ...) clusters, the B(n)N(n) (n = 3-10), and Al(n)N(n) (n = 3-9) clusters.     

Structure and properties of Mn(n), Mn(n)-, and Mn(n)+ clusters (n = 3-10)

Electronic and geometrical structures of Mn(3)-Mn(10) together with their singly negatively and positively charged ions are computed using density functional theory with generalized gradient approximation. The ground-state spin multiplicities in the neutral series are 16, 21, 4, 9, 6, 5, 2, and 5, for Mn(3)-Mn(10), respectively. Thus, there is a transition from a ferromagnetic ground state to a ferrimagnetic ground state at Mn(5). The energy difference between ferrimagnetic and ferromagnetic states in Mn(n) grows rapidly with increasing n and exceeds 2 eV in Mn(10). The corresponding change from ferro- to ferrimagnetic ground state occurs at Mn(6)(-) and Mn(3)(+) in the anionic and cationic series, respectively. Beginning with Mn(6), the ion spin multiplicities differ from that of the neutral by +/-1 (i.e., they obey the empirical "+/-1 rule"). We found that the energy required to remove an Mn atom is nearly independent of the charge state of an Mn(n) cluster and the number of atoms in the cluster, except for Mn(3). The results of our calculations are in reasonable agreement with experiment, except for the experimental data on the magnetic moments per atom, where, in general, we predict smaller values than the experiment.     

Topological relationships and building blocks in Zintl phases of the composition Ba(n+1)(Mg,Li)2nSi2(n+1)

The structures of two novel Zintl phases, Ba6Mg5.2Li2.8Si12 and BaMg0.1Li0.9Si2, are presented. Both compounds contain chains in cis-trans conformation. The silicon partial structure of Ba6Mg5.2Li2.8Si12 (C2/m; a = 1212.0(1), b = 459.78(4), c = 1129.10(9) pm; beta = 91.77(2) degrees; Z = 1) is built of unbranched, planar Si6 chains while BaMg0.1Li0.9Si2 (Pnma; a = 725.92(5), b = 461.36(3), c = 1169.08(8) pm; Z = 4) consists of infinite Si(n) chains. The compounds show all electronic and structural characteristics that are typical for the special subset of Zintl phases with highly charged planar anions. The structures of the new compounds, as well as that of Ba2Mg3Si4, can be derived from the common parent type BaMg2Si2. It is shown that a comprehensive picture of a chemical twinning based on BaMg2Si2 can be derived.     

Stability of cyclic (H2O)n clusters within molecular solids: Role of aromaticity

Cyclic water clusters (H₂O)_n (n = 3–12) trapped inside organic/inorganic hosts do not correspond to the global energy minimal structures. Their closed loop connections through the H‐bonds, although weakly interacting, result in diamagnetic ring currents leading to what we term “H‐bonded aromaticity.” Such H‐bonded aromaticity in supramolecular structures generalizes the formation of such stable (H₂O)_n molecules confined within various host systems. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Global optimization of ionic Mg(n)F(2n) (n=1-30) clusters

The global optimization basin-hopping (BH) method has been used to locate the global minima (GM) of Mg(n)F(2n) (n=1-30) clusters using a Born-Mayer-type potential. Some of the GM were particularly difficult to find, requiring more than 1.5 x 10(4) BH steps. We have found that both the binding energy per MgF2 unit and the effective volume of the GM isomers increase almost linearly with n, and that cluster symmetry decreases with cluster size. The data derived from the BH runs reveal a growing density of local minima just above the GM as n increases. Despite this, the attraction basin around each GM is relatively large, since after all their atomic coordinates are randomly displaced by values as high as 2.0 bohrs, the perturbed structures, upon reoptimization, relax back to the GM in more than 50% of the cases (except for n=10 and 11). The relative stabilities derived from energy second differences suggest that n=8,10,13,15, and 20 are probably the magic numbers for these systems. Mass spectrum experiments would be very useful to clarify this issue.     

Theoretical study of Al(n) and Al(n)O (n = 2-10) clusters

The stable structures, energies, and electronic properties of neutral, cationic, and anionic clusters of Al(n) (n = 2-10) are studied systematically at the B3LYP/6-311G(2d) level. We find that our optimized structures of Al5(+), Al9(+), Al9(-), Al10, Al10(+), and Al10(-) clusters are more stable than the corresponding ones proposed in previous literature reports. For the studied neutral aluminum clusters, our results show that the stability has an odd/even alternation phenomenon. We also find that the Al3, Al7, Al7(+), and Al7(-) structures are more stable than their neighbors according to their binding energies. For Al7(+) with a special stability, the nucleus-independent chemical shifts and resonance energies are calculated to evaluate its aromaticity. In addition, we present results on hardness, ionization potential, and electron detachment energy. On the basis of the stable structures of the neutral Al(n) (n = 2-10) clusters, the Al(n)O (n = 2-10) clusters are further investigated at the B3LYP/6-311G(2d), and the lowest-energy structures are searched. The structures show that oxygen tends to either be absorbed at the surface of the aluminum clusters or be inserted between Al atoms to form an Al(n-1)OAl motif, of which the Al(n-1) part retains the stable structure of pure aluminum clusters.     

Parity alternation of ground-state P(n)(-) and P(n)(+) (n = 3-15) phosphorus clusters

The ground-state structures of neutral, cationic, and anionic phosphorus clusters P(n), P(n)(+), and P(n)(-) (n = 3-15) have been calculated using the B3LYP/6-311+G* density functional method. The P(n)(+) and P(n)(-) (n = 3-15) clusters with odd n were found to be more stable than those with even n, and we provide a satisfactory explanation for such trends based on concepts of energy difference, ionization potential, electron affinity, and incremental binding energy. The result of odd/even alternations is in good accord with the relative intensities of cationic and anionic phosphorus clusters observed in mass spectrometric studies.     

The structural and electronic properties of In(n)N(n = 1-13) clusters

The structural and electronic properties of In(n)N(n=1-13) clusters have been investigated by density-functional theory with the generalized gradient approximation. The results indicate that the equilibrium structures of In(n)N are linear for n=1,2, planar for n=3-5, and three dimensional for n=6-13. Maximum peaks were observed for In(n)N clusters at n=3,7,9 on the size dependence for second-order energy difference. These imply that these clusters possess relatively higher stability, which is consistent with the case of binding energy per atom. Moreover, the results show that the bonding in small In(n)N clusters has a little ionic character by Mulliken population analysis. The energy gap between the highest occupied and lowest unoccupied molecular orbitals, the vertical ionization potential and electron vertical affinity (VIP and VEA) form an even-odd alternating pattern with increasing cluster size. In general, the VIP tends to lower as the cluster size increases, while the VEA tends to increase as the cluster size increases.     

Microwave-enhanced synthesis of new (-)-steganacin and (-)-steganone aza analogues

[reaction: see text]. A novel, microwave-enhanced, highly efficient protocol for the synthesis of hitherto unknown (-)-steganacin and (-)-steganone 7-aza analogues containing a 1,2,3-triazole ring has been presented. Microwave irradiation was found to be highly beneficial in promoting the Suzuki reaction and the 1,3-dipolar cycloaddition reaction to generate the highly strained medium-sized ring system of the title molecules.