Ab initio calculation of carbon clusters. II. Relative stabilities of fullerene and nonfullerene C24

Chemical stabilities of six low-energy isomers of C24 derived from global-minimum search are investigated. The six isomers include one classical fullerene (isomer 1) whose cage is composed of only five- and six-membered rings (56-MRs), three nonclassical fullerene structures whose cages contain at least one four-membered ring (4-MR), one plate, and one monocyclic ring. Chemical and electronic properties of the six C24 isomers are calculated based on a density-functional theory method (hybrid PBE1PBE functional and cc-pVTZ basis set). The properties include the nucleus-independent chemical shifts (NICS), singlet-triplet splitting, electron affinity, ionization potential, and gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) gap. The calculation suggests that the neutral isomer 2, a nonclassical fullerene with two 4-MRs, may be more chemically stable than the classical fullerene (isomer 1). Analyses of molecular orbital NICS show that the incorporations of 4-MRs into the cage considerably reduce paratropic contributions from HOMO, HOMO-1, and HOMO-2, which are mainly responsible for the sign change in NICS from positive for isomer 1 (42) to negative (-19) for isomer 2, although C24 clusters satisfy neither 4N+2 nor 2(N+1)2 aromaticity rule. Anion photoelectron spectra of four cage isomers, one plate, one monocyclic ring, and one tadpole isomer, as well as three bicyclic ring isomers are calculated. The simulated photoelectron spectra of mono- and bicyclic rings (with C1 symmetry) appear to match the measured HOMO-LUMO gap (between the first and second band in the experimental spectra) [S. Yang et al., Chem. Phys. Lett. 144, 431 (1988)]. Nevertheless, the nonclassical fullerene isomers 3 and 4 apparently also match the measured vertical detachment energy (2.90 eV) reasonably well. These results suggest possible coexistence of nonclassical fullerene isomers with the mono- and bicyclic ring isomers of C24(-) under the experimental conditions.     

Si2CN分子结构的理论研究

在B3LYP／6－311G（d）水平上对Si2CN的各种可能异构体进行了研究，得到了其几何构型，结果表明：Si2CN有11个稳定的异构体，能量最低的是直线型异构体SiCNSi1,其次是四元环构型具有SiC桥键，电子态为^2A″的cSiSiCN6，第三稳定的是具有CSiSi三元环和环外NC键的N－cCSiSi10^2A1，第四稳定的是四元环具有SiN桥键^2A″电子态异构体cSiSiCN7。     

CCSD calculations on C(14), C(18), and C(22) carbon clusters

The structure and energetics of the ring isomers of C(4n+2) (n=3-5) carbon clusters were studied by using coupled-cluster singles and doubles excitation theory to overcome the vast differences existing in the literature. The results obtained in the present study clearly indicate that C(14), C(18), and C(22) carbon rings have bond-length and bond-angle alternated acetylenic minimum energy structures. Contrarily, density functional theory calculations were unable to predict these acetylenic-type structures and they ended up with the cumulenic structures. It is found from the coupled-cluster studies that the lowest-energy ring isomer for the first two members of C(4n+2) series is a bond-angle alternated cumulenic D((2n+1)h) symmetry structure while the same for the remaining members is a bond-length and bond-angle alternated C((2n+1)h) symmetry structure. In C(4n+2) carbon rings, Peierls-type distortion, transformation from bond-angle alternated to bond-length alternated minimum energy structures, occurs at C(14) carbon ring.     

Hole-burning spectra of phenol-Arn (n = 1, 2) clusters: resolution of the isomer issue

The hole-burning (HB) spectra of phenol-Arn (PhOH-Arn) clusters with n = 1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1-S0 spectrum recorded for the n = 1 cluster; however, one of the species was found to originate from dissociation of an n = 2 cluster. Similarly, three species were observed in the spectrum of the n = 2 cluster, and two of them were assigned to n = 3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n = 1 and the n = 2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n = 2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n = 2 cluster has Cs or lower symmetry.     

Vibrational spectra of small silicon monoxide cluster cations measured by infrared multiple photon dissociation spectroscopy

The first gas-phase infrared spectra of silicon monoxide cations (SiO)(n)(+), n = 3-5, using multiple photon dissociation in the 550-1250 cm(-1) frequency range, are reported. All clusters studied here fragment via loss of a neutral SiO unit. The experimental spectra are compared to simulated linear absorption spectra from calculated low energy isomers for each cluster. This analysis indicates that a "ring" isomer is the primary contributor to the (SiO)(3)(+) spectrum, that the (SiO)(4)(+) spectrum results from two close-lying bicyclic ring isomers, and that the (SiO)(5)(+) spectrum is from a bicyclic ring with a central, fourfold-coordinated Si atom. Experiment and theory indicate that the energies and energetic orderings of (SiO)(n)(+) isomers differ from those for neutral (SiO)(n) clusters.     

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.     

Photoelectron spectroscopy of cluster anions of naphthalene and related aromatic hydrocarbons

The electronic structures and structural morphologies of naphthalene cluster anions, (naphthalene)(n)(-) (n=3-150), and its related aromatic cluster anions, (acenaphthene)(n)(-) (n=4-100) and (azulene)(n)(-) (n=1-100), are studied using anion photoelectron spectroscopy. For (naphthalene)(n) (-) clusters, two isomers coexist over a wide size range: isomers I and II-1 (28 < or = n < or =60) or isomers I and II-2 (n > or = ~60). Their contributions to the photoelectron spectra can be separated using an anion beam hole-burning technique. In contrast, such an isomer coexistence is not observed for (acenaphthene)(n) (-) and (azulene)(n) (-) clusters, where isomer I is exclusively formed throughout the whole size range. The vertical detachment energies (VDEs) of isomer I (7 < or = n < or = 100) in all the anionic clusters depend linearly on n(-13) and their size-dependent energetics are quite similar to one another. On the other hand, the VDEs of isomers II-1 and II-2 produced in (naphthalene)(n)(-) clusters with n > or = approximately 30 remain constant at 0.84 and 0.99 eV, respectively, 0.4-0.6 eV lower than those of isomer I. Based upon the ion source condition dependence and the hole-burning photoelectron spectra experiments for each isomer, the energetics and characteristics of isomers I, II-1, and II-2 are discussed: isomer I is an internalized anion state accompanied by a large change in its cluster geometry after electron attachment, while isomers II-1 and II-2 are crystal-like states with little structural relaxation. The nonappearance of isomers II-1 and II-2 for (acenaphthene)(n)(-) and (azulene)(n)(-) and a comparison with other aromatic cluster anions indicate that a highly anisotropic and symmetric pi-conjugated molecular framework, such as found in the linear oligoacenes, is an essential factor for the formation of the crystal-like ordered forms (isomers II-1 and II-2). On the other hand, lowering the molecular symmetry makes their production unfavorable.     

DFT方法理论研究星际分子C3S+结构和光谱

在B3LYP／6—31lG(d)水平上对可能的星际分子C3S^ 的各种异构体进行了理论计算研究，得到其几何构型、红外光谱和精确能量以利于实验室和星际观测，讨论了其星际含义，并与其中性分子C3S做了比较．结果表明：C3S^ 有3个稳定的异构体，包括线形、三元环和四元环几何构型．按热力学稳定的异构体依次是直线型具有C∞v对称性的CCCS^ (1)，其次是具有CC桥键四元环构型的cC3S^ (2)，能量最高是三元环构型具有CC环外键的C—cCCS^ (3)。     

A photoelectron spectroscopic and computational study of sodium auride clusters, NanAun- (n = 1-3)

Negatively charged sodium auride clusters, NanAun- (n = 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun- (n = 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2- is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3- has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.     

An isomeric pair of fluoride-bridged cyclic dimolybdenum triads

A pair of isomeric cyclic triads containing three quadruply bonded [Mo2] units, [Mo2(cis-DAniF)2]2+ (DAniF = N,N'-di-p-anisylformamidinate), bridged by six fluoride anions, has been synthesized and crystallographically characterized. For the alpha isomer, the three [Mo2] units are oriented in two orthogonal directions. Two of them are structurally equivalent and parallel to each other, but oriented perpendicular to the third one. The beta isomer is a triangle with three geometrically identical [Mo2] units, parallel to each other, as the vertices. Thus, the beta isomer possesses idealized D3h symmetry while the alpha isomer only has C2v symmetry. These two isomers do not interconvert in boiling THF or toluene or under irradiation with ultraviolet light, but oxidation of the alpha isomer first generates an alpha+ species that changes to beta+. The two isomers have very similar electrochemical behavior, both showing three reversible one-electron redox processes for the [Mo2] centers and similar potential separations (DeltaE(1/2)). The first and second redox couples are well separated (ca. 390-410 mV), while the second and third ones are separated by only about 150 mV.     

Structures and electron affinities of the di-arsenic fluorides As2Fn/As2Fn- (n=1-8)

Developments in the preparation of new materials for microelectronics are focusing new attention on molecular systems incorporating several arsenic atoms. A systematic investigation of the As2Fn/As2Fn- systems was carried out using Density Functional Theory methods and a DZP++ quality basis set. Global and low-lying local geometric minima and relative energies are discussed and compared. The three types of neutral-anion separations reported in this work are: the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). Harmonic vibrational frequencies pertaining to the global minimum for each compound are reported. From the first four studied species (As2Fn, n=1-4), all neutral molecules and their anions are shown to be stable with respect to As-As bond breaking. The neutral As2F molecule and its anion are predicted to have Cs symmetry. We find the trans F-As-As-F isomer of C2h symmetry and a pyramidalized vinylidene-like As-As-F2- isomer of Cs symmetry to be the global minima for the As2F2 and As2F2- species, respectively. The lowest lying minima of As2F3 and As2F3- are vinyl radical-like structures F-As-As-F2 of Cs symmetry. The neutral As2F4 global minimum is a trans-bent (like Si2H4) F2-As-As-F2 isomer of C2 symmetry, while its anion is predicted to have an unusual fluorine-bridged (C(1)) structure. The global minima of the neutral As2Fn species, n=5-8, are weakly bound complexes, held together by dipole-dipole interactions. All such structures have the AsFm-AsFn form, where (m,n) is (2,3) for As2F5, (3,3) for As2F6, (4,3) for As2F7), and (5,3) for As2F8. For As2F8 the beautiful pentavalent F4As-AsF4 structure (analogous to the stable AsF5 molecule) lies about 30 kcal/mol above the AsF3 . . . AsF5 complex. The stability of AsF(5) depends crucially on the strong As-F bonds, and replacing one of these with an As-As bond (in F4As-AsF4) has a very negative impact on the molecule's stability. The anions As2Fn-, n=5-8, are shown to be stable with respect to the As-As bond breaking, and we predict that all of them have fluorine-bridged or fluorine-linked structures. The zero-point vibrational energy corrected adiabatic electron affinities are predicted to be 2.28 eV (As2F), 1.95 eV (As2F2), 2.39 eV (As2F3), 1.71 eV (As2F4), 2.72 eV (As2F5), 1.79 eV (As2F6), 5.26 eV (As2F7), and 3.40 eV (As2F8) from the BHLYP method. Vertical detachment energies are rather large, especially for species with fluorine-bridged global minima, having values up to 6.45 eV (As2F7, BHLYP).     

A quantum chemical study of three isomers of N20

Ab initio molecular orbital (MP2) and density functional theory (B3LYP) calculations using different basis sets have been employed to study the structures, energetics and vibrational frequencies of the large homonuclear polynitrogen compound, N₂₀. In the present study, three distinct forms were found to represent local minima on the potential energy surface. They are the fullerene-type cage form of I_h symmetry, a corannulene-like bowl form of C_5v symmetry, and a ring isomer with D₅ symmetry of which the cage form turns out to be the highest energy form. Both the bowl and ring forms are calculated to be more stable than the cage form by about 200 kcal/mol. The molecular properties calculated for these isomers may serve as valuable predictions for future experimental searches for new high energy density materials (HEDM).     

Lowest singlet and triplet potential energy surfaces of S2N2

Forty four stationary points have been located on the lowest singlet and triplet potential energy surfaces of S(2)N(2). Ten minima and ten saddle points on the lowest singlet surface and eleven minima and thirteen saddle points on the lowest triplet surface were found. All saddle points were connected to minima or lower-order saddle points by following the intrinsic reaction coordinate. Renner-Teller effects in the linear isomers were studied by examining their bending curves. The S(2)N(2) polymerization mechanism was investigated by first locating the transition state corresponding to ring opening and then considering all species connected to it that are close in energy. The commonly accepted mechanism is problematic due to the number of species that would lead to dissociation to SN + SN. Other possible isomers that are consistent with the experimental evidence but do not connect to SN radicals in the dissociation limit were examined. A mechanism of polymerization to (SN)(x)() is proposed that involves excitation of the square planar singlet molecule to the triplet surface. The triplet species then undergoes a puckering, and polymerization occurs in a direction approximately perpendicular to the S(2)N(2) plane. Consideration of the predicted vibrational frequencies suggests the structure of the second isomer of S(2)N(2). This isomer has a trans-NSSN structure with a long SS bond. The energetics of trans-NSSN are consistent with the observed temperature effects in the dimerization of SN. Analysis of the bending curves of linear NSSN and NSNS indicates that trans-NSSN is the only isomer which has a small yet significant barrier to that dimerization.     

Boron rings enclosing planar hypercoordinate group 14 elements

Sets of boron rings enclosing planar hypercoordinate group 14 elements (ABn(n-8); A = group 14 element; n = 6-10) are designed systematically based on geometrical and electronic fit principles: the size of a boron ring must accommodate the central atom comfortably. The electronic structures of the planar minima with hypercoordinate group 14 elements are doubly aromatic with six pi and six in-plane radial MO systems (radial MOs are comprised of boron p orbitals pointing toward the ring center). This is confirmed by induced magnetic field and nucleus-independent chemical shift (NICS) computations. The weakness of the "partial" A-B bonds is compensated by their unusually large number. Although a C7v pyramidal SiB8 structure is more stable than the D8h isomer, Born-Oppenheimer molecular dynamics simulations show the resistance of the D8h local minimum against deformation and isomerization. Such evidence of the viability of the boron ring minima with group 14 elements encourages experimental realization.     

Crosslinking investigation of polybutadiene thermal degradation by carbon-13 nuclear magnetic resonance

Thermal degradation of polybutadiene (PBD) in anaerobic atmosphere at 250 °C had been studied by carbon-13 nuclear magnetic resonance spectroscopy (¹³C NMR) before complete crosslinking. In this investigation four types of low molecular weight PBD with different 1,2-vinyl isomer content had been chosen, then pure and mixed samples of PBD were heated in different time periods. ¹³C NMR spectra showed that two kinds of crosslinking mechanisms occur that both of them produce methyl groups. The first mechanism is a reaction between 1,2-vinyl isomers of two PBD chains, and the second one occurs between 1,2-vinyl isomer of one chain via methylene carbon of cis or trans isomer in another chain. Also ¹³C NMR results showed that the presence of 1,2-vinyl isomer in the PBD structure is necessary and without it none of the mentioned reactions will occur. Furthermore isomers sequence is another important parameter which affects crosslinking. Results show that cis or trans isomer which is not adjacent to 1,2-vinyl isomer does not take part in crosslinking reaction. Moreover such cis or trans isomer can take part in second mechanism of crosslinking that 1,2-vinyl isomer was attached from head to cis or trans isomer, thus in this arrangement of isomers second mechanism of crosslinking will become dominant rather than first mechanism of crosslinking.     

Theoretical Study on the Electronic Structures of Small TinNm （n ＋ m = 5, 6） Clusters

45 isomers of TinNm （n ＋ m = 5, 6） clusters, including linear, some planar and some stero configurations, have been predicted by density functional theory method. For five-atom clusters Ti3N2 and Ti2N3, the most stable structures are trigonal bipyramid in D3h symmetry, and for TiaN cluster, the isomer with one nitrogen atom occupying the center of quasi-tetrahedron is the most stable. In the isomers of Ti4N2 and Ti3N3, the planar networks are more stable, but for Ti2N4, the six-membered ring configuration is the most favorable. Most linear structures can form weak-strong bonds alternately with higher energy. As regards to planar structures, the more Ti-N bonds are formed, the more stable they will be; for stero closed polyhedral isomers, their energies are lower.     

Efficiency of random search procedures along the silicon cluster series: Si(n) (n=5-10, 15, and 20)

The efficiency of the simplest isomeric search procedure consisting in random generation of sets of atomic coordinates followed by density functional theory geometry optimization is tested on the silicon cluster series (Si(5-10, 15, 20)). Criteria such as yield, isomer distributions and recurrences are used to clearly establish the performance of the approach with respect to increasing cluster size. The elimination of unphysical candidate structures and the use of distinct box shapes and theoretical levels are also investigated. For the smaller Si(n) (n=5-10) clusters, the generation of random coordinates within a spherical box is found to offer a reasonable alternative to more complex algorithms by allowing straightforward identification of every known low-lying local minima. The simple stochastic search of larger clusters (i.e. Si(15) and Si(20)) is however complicated by the exponentially increasing number of both low- and high-lying minima leading to rather arbitrary and non-comprehensive results.     

富勒烯C_(36)等电子体C_(34)BN异构体的结构及其相对稳定性理论研究

用半经验的AM1和MNDO方法优化了富勒烯C_(36)的等电子体C_(34)BN所有可能 异构体的构型,分析了各异构体相对稳定性与杂原子取代位置间的关系。另外,比 较了C_(36)碳笼上同位置地取代杂原子形成的C_34BN,C_(34)B_2和C_(34)N_2间的 电子结构,并分析了C_(34)BN最稳定异构体的振动模型。结果表明以C_(36):A (D_(6h))为母体形成的最稳定C_(34)BN异构体对应于碳笼赤道位置六元环中1,4- 取代产物,而以C_(36):B(D_(2d))为母体形成的最稳定C_(34)BN异构体对应于碳笼 近赤道位置的1,2-取代产物.C_(34)BN各异构体的稳定性可能主要由体系的共轭性 质决定。前线轨道能级表明B,N原子取代所得异构体的氧化-还原活性按以下顺序 递增：C_(34)B_2相似文献   

Clusters containing open-shell molecules: minimum-energy structures and low-lying isomers of ArnCH (X 2 pi), n = 1 to 15

The size evolution of the equilibrium structures of open-shell ArnCH (X 2 pi) Van der Waals clusters is investigated for n = 1 to 15. We describe a method for combining pair potentials for Ar-CH and Ar-Ar interactions to obtain potential energy surfaces for ArnCH clusters. For each cluster size considered, the global and a few energetically close local minima are calculated using simulated annealing followed by a direct minimization scheme. Ar2CH is found to have an unusually stable planar structure, which persists as a motif in larger ArnCH clusters and has a strong effect on their optimal geometries. The lowest-energy isomers of ArnCH with n = 3 to 11 have all Ar atoms in a shell around CH. The only exception is Ar4CH, where the fully solvated isomer is 3 cm-1 higher in energy than the optimal isomer with CH bound to the surface of the Ar4 tetrahedron. For n = 7 to 11, the minimum-energy structure of ArnCH derives from the global minimum of the Arn + 1 cluster, by replacing the Ar atom at the bottom of the pentagonal bipyramid with CH. The lowest-energy structure of Ar12CH is that of the optimal icosahedral Ar13 cluster, with CH replacing one of the Ar atoms on the cluster surface. This structure supports the proposition based on the spectroscopic data, that for ArnCH clusters with about 10 to 50 Ar atoms CH resides on the surface of Arn.     

Regiochemistry of [70]fullerene: preparation of C70(OOtBu)n (n = 2, 4, 6, 8, 10) through both equatorial and cyclopentadienyl addition modes

[reaction: see text] tert-Butylperoxy radicals add to [70]fullerene to form a mixture of adducts C(70)(OO(t)()Bu)(n)() (n = 2, 4, 6, 8, 10). Four isomers were isolated for the bis-adduct with the two tert-butylperoxo groups attached at 1,2-, 5,6-, 7,23-, and 2,5-positions, respectively. Two isomers were isolated for the tetrakis-adduct with the tert-butylperoxo groups located along the equator in C(s)() symmetry and on the side in C(1) symmetry, respectively. Similarly, two isomers were isolated for the hexakis-adducts with a structure related to the tetrakis-adducts, one of which has the cyclopentadienyl substructure. No isomer was detected for the octakis- and decakis-adducts. The C(s)()-symmetric octakis- and C(2)-symmetric decakis-adducts have all the tert-butylperoxo groups located along the equator. The decakis-adduct is the major product under optimized conditions. The compounds were characterized by their spectroscopic data. Chemical correlation through further addition of tert-butylperoxy radicals to isolated pure derivatives confirmed the structure assignment. Mechanisms of the tert-butylperoxy radical addition to C(70) follow two pathways: equatorial addition along the belt and cyclopentadienyl addition on the side.