The coordination of Sr2+ by hydroxide: a density functional theoretical study

The molecular structures of gas-phase strontium hydroxide complexes are quantum chemically calculated using density functional theory, and the effects of hydroxyl groups on strontium coordination are studied. It is found that the presence of a single hydroxyl group results in the near-degeneracy of complexes with a coordination number (CN) of 5, 6 and 7. The presence of a second hydroxyl group destabilises the heptacoordinated complexes, and marks the onset of a weakening of the Sr-O(H(2)O) bonds, as evidenced by analysis via the quantum theory of atoms in molecules (QTAIM) and measurements of the average angle between the Sr-O(H(2)O) bond and the H(2)O dipole moment. A third hydroxyl group strongly destabilises both CN = 6 and 7 complexes through significant weakening of the Sr-O(H(2)O) interaction; here, hydrogen bonding interactions between hydroxyl groups and water molecules begin to dominate. The tetrahydroxide complex is found to be electronically unstable in the gas phase, but can be stabilised by coordination of explicit water molecules. Replacement of the explicit water molecules by a continuum solvation model poorly reproduces the polarisation of the wavefunction by the explicit solvent, suggesting that a combined approach incorporating both explicit solvation and a continuum model is required for the accurate modelling of this dianionic complex.     

Two hydrogen ligands on tetrairidium clusters: a relativistic density functional study

Structural and energetic properties of Ir(4)H(2) have been determined by applying a relativistic density functional method. As previously obtained for Ir(4)H, terminal coordination of H ligands is preferred, in contrast to some other transition metals. Square-planar Ir(4) isomers with an H binding energy of up to 318 kJ mol(-1) per atom were determined as the most stable structures of Ir(4)H(2). Isomers with a tetrahedral or butterfly structure of the metal framework exhibit average H atom binding energies of up to approximately 300 kJ mol(-1). For all three types of isomers, a surprisingly large number of stable minima was identified. Unexpectedly, structural as well as energetic properties of Ir(4)H(2) complexes are very similar to Ir(4)H. Thus binding of an H atom to Ir(4) is only slightly affected by the presence of a second H ligand. In all cases examined, the reaction H(2)+ Ir(4)--> H(2)Ir(4) was found to be exothermic with reaction energies of up to 170 kJ mol(-1).     

Signature of nanodiamond in Raman spectra: a density functional theoretical study

Nanocrystalline diamond (NCD) has attracted great attention both experimentally and theoretically in the past few years. The identification of the presence and the amount of NCD in amorphous carbon film has been a challenging issue. Although Raman spectroscopy has become a standard tool for the characterization of various carbon phases, a simple criterion for identifying NCD has not yet been well-established. In the present work, a theoretical study of the Raman spectra of the model compounds of NCD is presented on the basis of the density functional calculations. The reliability of the computational approach has been tested by comparing the predicated Raman spectra of several reference molecules to those obtained experimentally. To show the unique Raman spectrum of a NCD phase, a series of the model compounds of various carbon materials including tetrahedral and hexagonal clusters, and trans-polyacetylene fragments, were considered, and subsequently their Raman spectra below 2000 cm(-1) were calculated and compared with each other. The calculated results indicate that the relatively stronger broad peak at about 480 cm(-1) could be used as the signature of a NCD phase in the sample.     

Growth pattern and electronic properties of acetonitrile clusters: a density functional study

We report a systematic theoretical study on the growth pattern and electronic properties of acetonitrile clusters [(CH(3)CN)(n) (n = 1, 9, 12)] using density functional approach at the B3LYP6-31++G(d,p) level. Although we have considered a large number of configurations for each cluster, the stability of the lowest energy isomer was verified from the Hessian calculation. It is found that the lowest energy isomer of the dimer adopts an antiparallel configuration. For trimer and tetramer, cyclic ring structures were found to be favored over the dipole stabilized structure. In general, it is found that the intermolecular CH...N interactions play a significant role in the stabilization of the cyclic layered geometry of acetonitrile clusters. A critical comparison between trimer and tetramer clusters suggests that the three member cyclic ring is more stable than four member rings. The growth motif for larger clusters (n = 5-9, 12) follows a layered pattern consisting of three or four membered rings, which, in fact, is used as the building block. Based on the stability analysis, it is found that clusters with an even number of molecular entities are more stable than the odd clusters, except trimer and nonamer. The exceptional stability of these two clusters is attributed to the formation of trimembered cyclic rings, which have been found to form the building blocks for larger clusters.     

BFW: a density functional for transition metal clusters

Ionization potentials (IPs) or electron affinities (EAs) for transition metal clusters are an important property that can be used to identify and differentiate between clusters. Accurate calculation of these values is therefore vital. Previous attempts using a variety of DFT models have correctly predicted trends, but have relied on the use of scaling factors to compare to experimental IPs. In this paper, we introduce a new density functional (BFW) that is explicitly designed to yield accurate, absolute IPs for transition metal clusters. This paper presents the numerical results for a selection of transition metal clusters and their carbides, nitrides, and oxides for which experimental IPs are known. When tested on transition metal clusters, the BFW functional is found to be significantly more accurate than B3LYP and B3PW91.     

Impurity effects on small Pd clusters: a relativistic density functional study of Pd4X, X = H, C, O

We carried out relativistic density functional calculations to investigate systematically the effect of main group element impurities H, C, and O on a Pd4 cluster. We determined a bridging coordination for Pd4H as most stable, whereas several other local minima are energetically close. The interaction of C with Pd4 is strong enough to restructure the cluster, resulting in two Pd2 units bridged by 4-fold coordinated C, but other isomers are again almost degenerate. Nearly degenerate isomers of Pd4O exhibit 2- and 3-fold coordination of O. In the most stable structures, the binding energies of the impurities, 295 kJ/mol for Pd4H, 655 kJ/mol for Pd4C, and 367 kJ/mol for Pd4O, are large enough to allow bond breaking of common small molecules when they interact with an ensemble of Pd4 clusters. Interestingly, the noteworthy relativistic effect on the properties of Pd4 also affects the interaction with impurity atoms. Comparison with other metals reveals similarities with Ni4X and differences from Ir4H, Ir4C, and Pt4H.     

Small ScCn cyclic clusters: a density functional study of their structure and stability

A theoretical study of the ScCn, ScCn+, and ScCn- (n = 1-10) cyclic clusters has been carried out employing the B3LYP density functional method. Predictions for several molecular properties that could help in their possible experimental characterization, such as equilibrium geometries, electronic structures, dipole moments, and vibrational frequencies, are reported. All ScCn cyclic clusters are predicted to have doublet ground states. For cationic clusters the ground state is alternate between singlets (n-even species) and triplets (n-odd members). In the case of anionic clusters the singlet-triplet separation is relatively small, with the singlets being favored in most cases. In general, even-odd parity effects are also observed for different properties, such as incremental binding energies, ionization energies, and electron affinities. For all neutral, cationic, and anionic clusters it is found that cyclic species are more stable than their open-chain counterparts. Therefore, cyclic structures are the most interesting possible targets for an experimental search of scandium-doped carbon clusters.     

Structure and energy of Mo27SxCy clusters: a density functional theory study

For understanding the carburization processes of MoSx catalysts, the structures and energies of Mo27SxCy cluster models have been computed at the level of density functional theory. The surface sulfur atoms on the Mo edge and S edge as well as bulky sulfur atoms have been replaced by atomic carbon, and the corresponding structures have S/C ratios in the range of 0.8-55. The formation of all Mo27SxCy structures is favored thermodynamically. It is also found that the formation of CS and C2 bridging units is more favored than the individual or separated replacements and that the formation of C2 bridging units is more favored than that of CS units. In contrast, the replacement of sulfur on the Mo edge is least favored. Furthermore, the replacement of the bulky sulfur on the Mo edge is equally favored as those of sulfur on the S edge. For aiding further experimental studies, the C=S and C=C stretching frequencies have been computed.     

Time-dependent density functional theoretical study of the absorption properties of BN-substituted C60 fullerenes

Time-dependent density functional theoretical calculations using the B3LYP functional and 6-31G* basis set for a series of BN-substituted C60 fullerenes reveal that, unlike C60, these molecules would absorb in the visible region and that the optical and electronic properties of fullerenes can be fine-tuned with proper BN substitution.     

NO adsorption on MoS(x) clusters: a density functional theory study

The density functional theory (DFT) method has been used to investigate NO probe molecule adsorption on the stoichiometric (Mo(16)S(32)) and nonstoichiometric (Mo(16)S(34) and Mo(16)S(29)) clusters. The calculated adsorption energies indicate that the stoichiometric cluster has stronger NO affinity than the nonstoichiometric surfaces. It is also found that mononitrosyl adsorption is favored at low NO coverage, while dinitrosyl (germinal) and (NO)(2) dimer adsorption at high NO coverage are possible. Strong repulsive interaction has been found for the adsorbed dinitrosyl and (NO)(2) dimer species. In addition, the computed NO stretching frequencies for the mononitrosyl and dinitrosyl species agree well with the experimental data, while those of the dimer species are much lower than the suggested experimental data.     

A density functional study of propylene glycol conversion to propanal and propanone of various acid-catalyzed reaction models: a water-addition effect

The acid-catalyzed models on reaction mechanisms of pinacol rearrangement of propylene glycol conversion to propanal and propanone have been investigated using the density functional method at 298.15 K. Thermodynamic quantities of activation steps of four water-addition models were obtained. The number of added water interacting with the transition states of three concerted pathways has obviously affected the product ratio. The relative energetic profiles of the conversion reactions of all solvation models have been comparatively displayed. Estimation of the percent ratio of product composition computed from activation free energies of each acid-catalyzed reaction model was carried out. The percent ratios of propanal and propanone were decreased as the number of added water increased.     

CO coordination at XNi4 clusters with impurities X = H, C, O. A density functional study

We report a computational investigation of CO adsorption on small nickel clusters that contain single impurity atoms H, C, or O. At bare Ni 4 and clusters with H or O impurity, the most stable coordination of the probe molecule is on top of a Ni atom which interacts with the impurity. The CNi 4 cluster is an exception where 3-fold coordination of CO was determined to be more stable than that on top, however, by 4 kJ/mol only. Our results suggest that the heteroatoms X (X = H, C, O) affect only weakly the reactivity of the cluster with respect to CO; the binding energy of CO in the most stable complexes (CO)XNi 4 increases at most by 10% compared to the value for bare Ni 4, 194 kJ/mol. The impurity induces a small decrease of the CO infrared frequency shift for on-top coordinated CO, compared to Ni 4, because of partial oxidation of the metal moiety. A notable difference is predicted for clusters that contain a C impurity because of the different preferred coordination mode which results in a strong CO frequency red shift of approximately 300 cm (-1). The calculated characteristic CO frequency shifts may be helpful in identifying experimentally clusters with impurity atoms.     

五味子素A、B和五味子丙素的密度泛函研究

采用密度泛函B3LYP方法在6-31G基组水平上对五味子素A、B及五味子丙素3种五味子提取物进行了优化计算,并从平衡几何构型、前线分子轨道、净电荷分布等方面对计算结果做了比较.计算结果表明分子中的二氧五环对分子的药物活性具有较大影响.随着分子中二氧五环数目的增加,分子中联苯环扭转角减小,前线轨道能级和能级差都减小,联苯环上正电荷增加,由此可判断3种分子活性顺序应为五味子丙素>五味子素B>五味子素A.     

Partial oxidation of propylene catalyzed by VO3 clusters: a density functional theory study

Density functional theory (DFT) calculations are carried out to investigate partial oxidation of propylene over neutral VO 3 clusters. C=C bond cleavage products CH 3CHO + VO 2CH 2 and HCHO + VO 2CHCH 3 can be formed overall barrierlessly from the reaction of propylene with VO 3 at room temperature. Formation of hydrogen transfer products H 2O + VO 2C 3H 4, CH 2=CHCHO + VO 2H 2, CH 3CH 2CHO + VO 2, and (CH 3) 2CO + VO 2 is subject to tiny (0.01 eV) or small (0.06 eV, 0.19 eV) overall free energy barriers, although their formation is thermodynamically more favorable than the formation of C=C bond cleavage products. These DFT results are in agreement with recent experimental observations. VO 3 regeneration processes at room temperature are also investigated through reaction of O 2 with the CC bond cleavage products VO 2CH 2 and VO 2CHCH 3. The following barrierless reaction channels are identified: VO 2CH 2 + O 2 --> VO 3 + CH 2O; VO 2CH 2 + O 2 --> VO 3C + H 2O, VO 3C + O 2 --> VO 3 + CO 2; VO 2CHCH 3 + O 2 --> VO 3 + CH 3CHO; and VO 2CHCH 3 + O 2 --> VO 3C + CH 3OH, VO 3C + O 2 --> VO 3 + CO 2. The kinetically most favorable reaction products are CH 3CHO, H 2O, and CO 2 in the gas phase model catalytic cycles. The results parallel similar behavior in the selective oxidation of propylene over condensed phase V 2O 5/SiO 2 catalysts.     

Comparative study of the interaction of O2 and C2H4 with small vanadium clusters from density functional theory

We have studied C(2)H(4) and O(2) molecules separately or simultaneously for adsorption on V(n) (n = 2-8) clusters, and V(n) clusters catalyzed ethylene oxidation to acetaldehyde using spin-polarized density functional theory calculations. Molecular adsorption and clear size-dependent adsorption energy are predicted for C(2)H(4). O(2) is dissociately adsorbed with nearly constant adsorption energy. In the case of coadsorption, O(2) and C(2)H(4) adsorb on the V(n) surface simultaneously. Each keeps the same adsorption form, molecular or dissociative, as in separate adsorption. The noted cooperative effect is noted in C(2)H(4) and O(2) coadsorption, which activates the C-C double bond of C(2)H(4) and favors its oxidization. Furthermore, both the separate and coadsorptions result in magnetic enhancement or reduction of V(n), which is found to be dependent on the cluster size and the adsorbates. In addition, we reveal the reaction mechanism of V(2) (V(6))-catalyzed ethylene oxidation to acetaldehyde and find the overall reaction is exothermic and barrierless.     

A density functional study of bare and hydrogenated platinum clusters

We perform density functional theory calculations using Gaussian atomic-orbital methods within the generalized gradient approximation for the exchange and correlation to study the interactions in the bare and hydrogenated platinum clusters. The minimum-energy structures, binding energies, relative stabilities, vibrational frequencies and the highest occupied and lowest unoccupied molecular-orbital gaps of Pt_nH_m (n = 1–5, m = 0–2) clusters are calculated and compared with previously studied pure platinum and hydrogenated platinum clusters. We investigate any magic behavior in hydrogenated platinum clusters and find that Pt₄H₂ is more stable than its neighboring sizes. The lowest energy structure of Pt₄ is found to be a distorted tetrahedron and that of Pt₅ is found to be a bridge site capped tetrahedron which is a new global minimum for Pt₅ cluster. The successive addition of H atoms to Pt_n clusters leads to an oscillatory change in the magnetic moment of Pt₃–Pt₅ clusters.     

A density functional study on the aggregation of alumina clusters

A calculation has been performed to explore the mechanism of aggregation reaction between two small molecular clusters [(Al₂O₃) _n1 and (Al₂O₃) _n2] by the density functional theory method. Five pathways of aggregation reactions between two different (Al₂O₃)₁ clusters isomers were identified. The detailed process of (Al₂O₃)₁ interaction with (Al₂O₃)₂ were also obtained. All the products of the aggregation reactions between two cage structures are cage-dimer structures. We calculated the thermodynamic properties of all the aggregate clusters. The Gibbs free energy changes of aggregation reactions in 0–1000 K were negative and increased with the temperature increase. The energy changes of enthalpy and entropy were also obtained.     

Raman spectral study of metal-cytosine complexes: a density functional theoretical (DFT) approach

The fluctuation of surface-enhanced Raman scattering (SERS) spectra has been an obstacle to the analysis of the adsorbate on the metal surface. In this paper, we aim at using the density functional theory (DFT) to study the fluctuant Raman spectra of the cytosine molecule which interacts with a coinage metal atom or cation via N1 and N3 sites. The results show that the adsorption site strongly influences the Raman spectral property of cytosine molecule, especially the relative intensity of some bands. In addition, the SERS spectra of cytosine which is adsorbed on the gold, silver, and copper electrodes are measured, and the possible orientation and adsorption site of the cytosine molecule adsorbed on metal electrodes surface are proposed with the help of DFT simulations.     

p-benzoquinone-benzene clusters as potential nanomechanical devices: a theoretical study

The equilibrium structures and binding energies of the benzene complexes of p-benzoquinones (PBQ) and its negatively charged anionic species (PBQ- and PBQ2-) have been investigated theoretically using second-order M?ller-Plesset calculations. While neutral p-benzoquinone-benzene clusters (PBQ-Bz) prefer to have a parallel displaced geometry (P-c), CH...pi interactions (T-shaped geometries) prevail in the di-anionic PBQ-benzene (PBQ2- -Bz) complexes (T-e2-). Studies on dianionic p-benzoquinone-benzene clusters showed that two nonbonded intermolecular interactions compete in the most stable conformation. One is H-bonding interaction (C-H...O type) between carbonyl oxygen of p-benzoquinone and one of the hydrogen atoms of benzene, and the other is a pi-H interaction between pi-electron cloud of PBQ2- and another hydrogen atom of benzene. Blueshifted H-bonds were observed in T-shaped clusters. The changes in the geometrical preference of PBQ-Bz complex upon addition of electrons would be useful in designing optimized molecular mechanical devices based on the edge-to-face and face-to-face aromatic interactions.     

Stability and structure of rare-gas ionic clusters using density functional methods: A study of helium clusters

Several modelings of exchange and correlation forces which can be carried out using density functional theory (DFT) methods have been analyzed to study their efficiency and reliability when evaluating possible competing structures of helium ionic clusters of increasing size. This study examines He_n⁺systems with n from 1 to 7 and compares the present calculations with earlier evaluations that used more conventional, and more computationally intensive, methods with configuration interaction (CI) approaches. The present results indicate that it is indeed possible to strike a fruitful balance between reduction of computational times and quality of the ensuing structural information. © 1996 John Wiley & Sons, Inc.