The interaction between C28 (Td) and X (X=H and F)

Hydrogen and fluorine addition reactions with C₂₈(T_d) have been investigated by the density function theory method at B3LYP/6-31G level. The interaction potential between C₂₈(T_d) and atom X (X=H and F) shows that there are three possible stable isomers of C₂₈(T_d)X (X=H and F) and the average binding energy calculations suggest that C₂₈(T_d)H₄ is the most stable hydrogen adduct among C₂₈(T_d)H_n (n=1–28). Furthermore, by comparisons of the energy between C₂₈(T_d)H and C₂₈(C_s)H we found that the former are more stable than the later, and the structural and energy analysis further indicate that C₂₈(C_s)H is only with a small distortion of C₂₈(T_d)H symmetry. In addition, the transition states, as well as reaction pathways of X transfer reactions between different key points on C₂₈(T_d) representative patch are given to explore the possible reaction mechanism.     

Theoretical study of the N20 molecule

In the study of the N₂₀ molecule with an I_h symmetry group, the following methods were applied: 6-31G, 3-21G and STO-3G ab initio and PM3 semiempirical MO methods. Both geometrical optimization and frequency calculations are reported. Results of optimized bond distances (d_N---N), first ionization potential, ΔH_a, ΔG_a and bond energy, for the cases of 6-31G, 3-21G, and PM3 showed that the N₂₀ molecule is a highly stable compound with a delocalized N---N single bonded cluster structure.     

Semiempirical and ab initio transition state calculations for the transformation of N-acetyltetrazoles into corresponding 1,3,4-oxadiazoles

The transformation of 2-acetyl-5-substituted-tetrazoles into the corresponding 1,3,4-oxadiazoles was studied with the semiempirical and ab initio methods. Two mechanisms, one with two transition states and the other with three, were elucidated by . The first mechanism supported by PM3 and MNDO has a two-step, almost concerted, mechanism for the elimination of a nitrogen molecule from the tetrazole ring and formation of the oxadiazole product from an open-chain intermediate through carbon C₅ and acetyl oxygen bond formation. The second mechanism supported by AM1 and MINDO/3 breaks the elimination of the nitrogen molecule into two steps: first breaking the N₄-C₅ and then the N₂-N₃ bonds. Even when the AM1 and MINDO/3 transition state structures were optimized by PM3 and MNDO, the obtained transition states present only one bond breaking. The HF/STO-3G and HF/3-21G ab initio methods agree with the first mechanism where two bonds are breaking almost simultaneously. Despite the disagreement in the mechanism of the nitrogen elimination, the transition state that presents the product formation from open-chain intermediates is quite similar for all methods studied. The semiempirical calculation of this transition state is possible only if it is assumed that it has biradical character. The activation energies calculated by PM3 seem to be insensitive to the nature of the substituents.     

Comparison studies of fullerenes C72, C74, C76 and C78 by tight-binding Monte Carlo and quantum chemical methods

From C₇₂ to C₇₈, the top 20 low-energy isomers screened out from all isomers of each fullerene are optimized and computed by tight-binding Monte Carlo (TBMC), semi-empirical PM3, and ab initio B3LYP/6-31G*//HF/3-21G methods. The comparison results show that the TBMC method can efficiently optimize the structures and correctly predicate the low energy isomers. The relative energies computed by TBMC are in good agreement with the high-lever B3LYP calculation results. Our TBMC and B3LYP results show that the most energetically favorable structure of C₇₂ is not an isomer satisfying the isolated pentagon rule (IPR), which is different with the result by PM3. The symmetry of the most stable IPR isomer tends to low as the fullerene becomes large and several non-isolated-pentagon structures are found to have low symmetries and low energies close to the most stable isomer.     

Relative energies of isomers of C36F2

The symmetry unrestricted C₃₆F₂ isomers formed from fullerene C₃₆, the initial symmetry of which is C_6v, C_6h, or D_2d, have been extensively studied with semi-empirical (AM1 and PM3) calculations. Based on the relationship between the isomer's stability and the adding positions, three patterns of the adding sites of F₂ moiety in the additive reactions have been deducted. The results of the π-orbital axis vector (POAV) analysis indicate that the chemical reactivity of C₃₆ is the result of the high strain in the C₃₆ cage. But, in order to form stable compounds, the effects, which guide the F₂ moiety to select carbon atoms in the C₃₆ cage, are dominated by the conjugate effect in C₃₆F₂ system rather than the strain release in the C₃₆ cage.     

C94 IPR isomeric set: large-scale computations of relative stabilities based on the Gibbs function

At present C₉₄ is one of the highest sets of isomeric fullerenes that has been characterized by ¹³C NMR spectra. This contribution reports quantum-chemical computations on the C₉₄ system. The complete set of 134 isolated-pentagon-rule isomers of C₉₄ is described by four semiempirical quantum-chemical methods (MNDO, AM1, PM3, and SAM1). The C₉₄ energetics is also checked with Hartree–Fock SCF calculations in the standard 4-31G basis set (HF/4-31G). The calculations point out a C₂ structure as the system ground state. As energetics itself cannot produce reliable relative stabilities at high temperatures, entropy terms are also computed and the relative-stability problem is entirely treated in terms of the Gibbs function. The lowest-energy structure remains the most populated isomer at higher temperatures. However, several other structures show significant populations at higher temperatures. The six most populated species at the AM1 computational level read: C₂, C₂, C₁, C₁, C_s, and C₂. This selected six-membered isomeric set indeed contains the four symmetries observed in the available experiment (C₂, C_s, C₂, and C₂). This incidence represents a good agreement with the experiment and can be viewed as another evidence that the supposed inter-isomeric thermodynamic equilibrium does exist in experiments.     

Formation of [C60C5H5] adduct cations and theoretical study of the isomers

We study here the reactions between C₆₀ and planar C₅H₅⁺ cations that lead to the formation of [C₆₀C₅H₅]⁺ adduct cations in the chemical ionization source of the mass spectrometer. The structures, stabilities and charge locations of some possible isomers of [C₆₀C₅H₅]⁺: σ-adduct, π-complex, [1,4]- and [l,2]-addition cations, are studied by AM1 semiempirical molecular orbital calculations. We find that the most stable is the σ-addition cation. Another interesting and stable structure is the π-complex cation which is bonded by the electrostatic interaction at the inter-ring distance of 1.589 Å with the C_5v symmetry. The C₅H₅⁺ cyclopentadienium cation seems to be an “inverted umbrella” sitting on a five-membered ring of the C₆₀ cage.     

Semiempirical molecular dynamics studies of C60/C70 fullerene oxides: C60O, C60O2 and C70O

The spectral analysis indicates that all isomers of C₆₀O, C₇₀O and C₆₀O₂ have an epoxide-like structure (an oxygen atom bridging across a C–C bond). According to the geometrical structure analysis, there are two isomers of fullerene monoxide C₆₀O (the 5,6 bond and the 6,6 bond), eight isomers of fullerene monoxide C₇₀O and eight isomers of fullerene dioxide C₆₀O₂. In order to simulate the real reaction conditions at 300 K, the calculation of the different isomers of C₆₀O, C₆₀O₂ and C₇₀O fullerene oxides was carried out using the semiempirical molecular dynamics method with two different approaches: (a) consideration of the geometries and thermodynamic stabilities, and (b) consideration of the ozonolysis mechanism. According to the semiempirical molecular dynamic calculation analysis, the probable product of this ozonolysis reaction is C₆₀O with oxygen bridging over the 6–6 bond (C_2v). The most probable product in this reaction contains oxygen bridging across in the upper part of C₇₀ (6–6 bond in C₇₀O-2 or C₇₀O-4) an epoxide-like structure. C₆₀O₂-1, C₆₀O₂-3 and C₆₀O₂-5 are the most probable products for the fullerene dioxides. All of these reaction products are consistent with the experimental results. It is confirmed that the calculation results with the semiempirical molecular dynamics method are close to the experimental work. The semiempirical molecular dynamics method can offer both the reaction temperature effect by molecular dynamics and electronic structure, dipole moment by quantum chemistry calculation.     

Theoretical study of tricyclo[3,3,1,1]decane, tricyclo[3,3,1,1]decsilane molecules and their halogen derivatives, C10H12X4 and Si10H12X4 (X=F, Cl, Br, I)

The B3LYP/3-21G* ab initio molecular orbital method from the program package was applied to study tricyclo[3,3,1,1^3,7]decane and tricyclo[3,3,1,1^3,7]decsilane molecules and their halogen derivatives (1,3,5,7-tetrahalotricyclo[3,3,1,1^3,7]decane and 1,3,5,7-tetrahalotricyclo[3,3,1,1^3,7]decsilane, C₁₀H₁₂X₄ and Si₁₀H₁₂X₄ respectively). The optimized structures of these compounds were obtained. Ionization potentials, HOMO and LUMO energies, energy gaps, heats of formation, atomization energies and vibration frequencies were calculated. The calculations indicate that these molecules are stable and have T_d symmetry. Tricyclo[3,3,1,1^3,7]decsilane and its halogen derivatives (Si₁₀H₁₂X₄) are found to have higher conductivity than tricyclo[3,3,1,1^3,7]decane and its halogen derivatives (C₁₀H₁₂X₄). 1,3,5,7-Tetrafluorotricyclo[3,3,1,1^3,7]decane (C₁₀H₁₂F₄) and 1,3,5,7-tetrafluorotricyclo[3,3,1,1^3,7]decsilane (Si₁₀H₁₂F₄) were found to be the easiest compounds to form and the most difficult to dissociate of all 1,3,5,7-tetrahalotricyclo[3,3,1,1^3,7]decane and 1,3,5,7-tetrahalotricyclo[3,3,1,1^3,7]decsilane compounds, respectively.     

Thermal studies of novel molecular perovskite energetic material (C6H14N2)[NH4(ClO4)3]

(C6H(14)N2)[NH4(ClO4)3] is a newly developed porous hybrid inorganic-organic framework material with easy access and excellent detonation performances,however,its thermal properties is still unclear and severely hampered further applications.In this study,thermal behaviors and non-isothermal decomposition reaction kinetics of(C6H(14)N2)[NH4(ClO4)3] were investigated systematically by the combination of differential scanning calorimetry(DSC) and simultaneous thermal analysis methods.In-situ FTIR spectroscopy technology was applied for investigation of the structure changes of(C6H(14)N2) NH4(ClO4)3]and some selected referents for better understanding of interactions between different components during the heating process.Experiment results indicated that the novel molecular perovskite structure renders(C6H(14)N2)[NH4(ClO4)3] better thermal stability than most of currently used energetic materials.Underhigh temperature s,the stability of the cage skeleton constructed by NH4^+and ClO4^-ions determined the decomposition process rather than organic moiety confined in the skeleton.The simple synthetic method,good detonation performances and excellent thermal properties make(C6H(14)N2)[NH4(ClO4)3] an ideal candidate for the preparation of advanced explosives and propellants.     

Ab initio molecular orbital study of Fe2Cl6 and FeAlCl6

Ab initio molecular orbital theory was used to determine the equilibrium structure and vibrational frequencies of Fe₂Cl₆ and FeAlCl₆. The equilibrium structure the Fe₂Cl₆ dimer has D_2h symmetry with a planar arrangement of the four membered {FeCl_brFeCl_br} ring, similar to the Al₂Cl₆ dimer. The calculated bond distances and vibrational frequencies are in good agreement with experiment. The potential energy surface for the puckering of the {FeCl_brFeCl_br} ring is extremely flat. This prevents an unambiguous assignment of either D_2h or C_2v symmetry to the Fe₂Cl₆ structure in electron diffraction measurements. The FeAlCl₆ molecule is found to have a C_2v structure similar to Fe₂Cl₆ with vibrational frequencies in good agreement with experiment.     

Formation and structure of B24N24 clusters

Boron nitride (BN) nanocage clusters of B₂₄N₂₄ were synthesized and detected by laser desorption time-of-flight mass spectrometry. The B₂₄N₂₄ clusters consisted of 4-, 6- and 8-membered BN rings satisfying the isolated tetragonal rule, which was optimized by molecular orbital calculations. The electronic structure showed a bandgap energy of 4.9 eV, which is smaller than that of B₃₆N₃₆ cluster.     

Ammonia deposition in fullerene: (NH3)n@C60

Deposition of ammonia molecules in fullerene has been investigated theoretically by performing semi-empirical molecular orbital calculation at PM3 level within RHF formalism. C₆₀ cluster has been doped endohedrally by ammonia molecules. Structural and electronic properties of the systems considered have been studied. It has been found that C₆₀ cluster can store at most six ammonia molecules. The ammoniacal endofullerenes, (NH₃)_n@C₆₀, have been found stable but endothermic.     

A theoretical study on a fullerene derivative C54N4 and some of its protonated forms

Semiempirical quantum chemical calculations at the level of AM1 (restricted Hartree–Fock) have been performed on a fullerene derivative, C₅₄N₄, theoretically obtained from C₆₀ and its mono and diprotonated forms. All these structures are stable, but endothermic in nature. Some calculated geometrical and physicochemical properties of these have also been reported.     

Fourier transform IR spectrum and photoreactivity of the C3O2:HCl complex isolated in solid argon

IR spectroscopy was coupled with the matrix isolation technique to study the molecular complex formed between C₃O₂ and HCl and its photodissociation. The vibrational frequencies of the complex were compared with those of HCl and C₃O₂ monomers. For C₃O₂, a bent structure was characterized in the solid environment.

The vibrational frequencies were calculated in the 4000–400 cm⁻¹ range using an ab initio method at the MP2/6-31G** level for the most stable complex; these frequencies describe the hydrogen interaction with the central carbon atom of C₃O₂ (T complex). The measured shifts between the vibrational mode frequencies of the complex and monomers were in good agreement with the calculated values.

Broad-band UV irradiation ( > 230 nm) of the T complex leads preferentially to ketene chloride and carbon monoxide. Ketene chloride formation can be explained by the reaction between HCl and the carbene C₂O, which results from photo-dissociation at the C=CO bond of C₃O₂.     

Studies of N-heterocyclic compounds with triosmium clusters: Formation of the heterocyclic-linked dicluster compound [Os 6(μ-H)2(μ3-C6H4N3)2(CO)18]

The cluster [Os₃(CO)₁₀(MeCN)₂] reacts with indazole (C₇H₆N₂) to give two isomeric products [0s₃(μ-H)(μ-C₇H₅N₂)(CO)₁₀] in which the five-membered ring has been metallated with N-H cleavage to give an N,N-bonded isomer or with C-H cleavage to give a C,N-bonded isomer. These two isomers have very similar X-ray structures but can be clearly distinguished by ¹H NMR methods. They are shown to correspond to related clusters derived from pyrazole. Benzotriazole (C₆H₅N₃) also reacts (as shown earlier by others) to give two isomers: an N,N-bonded species [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₁₀] coordinated only through the five-membered ring and a minor C,N-bonded isomer [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₁₀], metallated at the C₆ ring and coordinated through both rings. The former isomer reacts with Me₃NO in acetonitrile to give [Os₃(μ-H)(μ-C₆H₄N₃)(CO)₉(MeCN)] which thermally looses MeCN to produce the coupled product [Os₆(μ-H)₂(μ₃-C₆H₄N₃)₂(CO)₁₈] which was shown by X-ray structure determination to have all six nitrogen atoms coordinated to osmium, a novel situation for coordinated benzotriazole. The two Os₃ units are linked together by an OsNNOsNN ring in a boat conformation with the whole cluster adopting C₂ symmetry.     

Ab initio computation of the potential energy surfaces of the water·hydrocarbon complexes H2O·C2H2, H2O·C2H4 and H2O·CH4: minimum energy structures, vibrational frequencies and hydrogen bond energies

On the basis of ab initio MP2/6–31 + + G(2d,2p) calculations, we examined the potential energy surfaces of the water·hydrocarbon complexes H₂O·CH₄, H₂O·C₂H₂ and H₂O·C₂H₂ to locate all the minimum energy structures and estimate the hydrogen bond energies and vibrational frequencies associated with the C(spⁿ)---H·O and the O---H·C(spⁿ) bonds (n = 1−3). Our calculations show that H₂O·C₂H₂, H₂O·C₂H₄ and H₂O·CH₄ have two minimum energy structures (i.e., the C---H·O and O---H·C hydrogen bond forms), but H₂O·C₂H₄ has only one when the vibrational motion is taken into account, the O---H·C hydrogen bond form. We have also computed the barrier for the interconversion from one minimum to the other. The fully optimized geometries of H₂O·CH₄, H₂O·C₂H₄ and H₂O·C₂H₂ as well as the vibrational shifts of the C---H stretching frequencies in their C---H·O hydrogen-bonded forms are in good agreement with the available experimental data. The calculated hydrogen bond energies show that the C(spⁿ---H·O bond strengths decrease in the order C(sp)---H·O>C(sp²)---H·O>C(sp³)---O>C(sp³---H·O, which is also consistent with the available experimental data.     

Structural features of 4,4′-diaminooctafluorobiphenyl

Molecules of C₁₂H₄F₈N₂ crystallize in the orthorhombic space group P2₁2₁2₁ with cell constants a=9.200(1), b=10.896(1), c=23.178(3) Å and V=2323.4(5) ų. There are two molecules in the asymmetric unit which have D₂ symmetry. However these two molecules have C₂ symmetry in central C–C bonds, separately. Intramolecular steric repulsions between F atoms and N–HF hydrogen bonds have very much affected the molecular conformation. The mean dihedral angle between intramolecular phenyl rings is 119.2(1)°. The N–C bonds have lengths 1.363(4)–1.407(4) Å with a mean of 1.388 Å. This is shorter than the conventional C–N (1.47(1) Å) bond length due to π-electron delocalizations (F.H. Allen, O. Kennard, D.G. Watson, L. Brammer, A.G. Orpen, R. Taylor, J. Chem. Soc. Perkin Trans. II (1987) S1–S19).

The molecular structure of the title compound was also investigated by IR spectroscopy. It was shown that the IR spectra are in agreement with the crystal structure. On the other hand, theoretical and semi-emprical molecular mechanic calculations were carried out to obtain the most probable low-energy conformations by using MM3, PM3 and AM1 programs.     

Theoretical studies of third-order nonlinear optical response for B12N12, B24N24 and B36N36 clusters

In this paper, we have calculated the third-order nonlinear optical polarizabilities corresponding to three optical processes: third-harmonic generation (THG), electric-field-induced second-harmonic generation (EFISHG) and degenerate four-wave mixing (DFWM) for B₁₂N₁₂, B₂₄N₂₄ and B₃₆N₃₆ clusters. The calculations have been performed by employing ab initio time-dependent density functional theory combined with sum-over-states method (SOS//TDDFT). We obtained the similar dynamic behavior of third-order NLO polarizabilities for three BN clusters. At input photon energy below 1.25 eV, the resonance enhancements of response haven't occurred. This is due to the fact that the calculated BN clusters have the large transition energy. B₂₄N₂₄ cluster has the larger transition dipole moments and the third-order polarizabilities of B₂₄N₂₄ are much larger than those of B₁₂N₁₂ and B₃₆N₃₆. We also estimate the static third-order optical susceptibility χ⁽³⁾ for BN fullerene materials from the average static third-order polarizability <γ>. The static χ⁽³⁾ of B₂₄N₂₄ fullerene materials are 1.36×10⁻¹⁴ esu for three NLO processes.     

Prediction of geometries and interaction energies of complexes formed by small molecules using semiempirical and ab initio methods

The accuracy of the semiempirical quantum mechanics methods (AM1 and PM3), and the ab initio methods (6-31G^** and MP2/6-31G^**) in predicting intermolecular geometries and interaction energies have been evaluated by detailed studies of 17 bimolecular complexes formed by small molecules. Comparisons between calculated and experimental geometries for 12 complexes are presented. It was found that AM1 gave reasonably good predictions of the geometries of complexes such as CH₄ · CH₄, which have very weak interactions, but it is not as good as other methods in predicting intermolecular geometry for complexes where hydrogen bonding interactions play an important role. This is consistent with its inability to reproduce the charge transfer in the formation of hydrogen bonds in these complexes.

PM3 is able to predict intermolecular geometries for most complexes, including those with hydrogen bonding; its major flaw is its tendency to overestimate the strength of the interactions between hydrogen atoms. Care should be taken therefore in using PM3 to study complicated molecular systems with multiple hydrogen atom interactions and the method's weakness in handling complexes in which electrostatic forces are important should also be noted.

Among ab initio methods, both the 6-31G^** and the MP2/6-31G^** were found to outperform AM1 and PM3 in prediction of intermolecular geometry. Both of these ab initio methods showed excellent consistency in geometry prediction for most of the complexes studied, although MP2/6-31G^** is better than 6-31G^**. It is noted that the MP2/6-31G^** did not produce the correct geometry for the CO₂· HF complex.

For 12 complexes for which experimental geometry data are available, AM1, PM3, 6-31G^**, and MP2/6-31G^** successfully predicted the geometry in 10, 12, 12, and 11 cases, respectively. The average errors given by AM1 in the predicted intermolecular distances were 0.264, 0.272, 0.091, and 0.061 Å, respectively. In comparison to the ab initio methods, AM1 and PM3 commonly underestimated the molecular interaction energy in such complexes by ˜ 1–2 kcal mol⁻¹.