Theoretical study on the structures and stabilities of the isomers of N6H6

Forty-eight possible isomers of N₆H₆ have been investigated at the B3LYP/6-311++G^** and G3B3 levels. The results show that heats of formation of all isomers are positive and A2-3 is the lowest and C2 is the highest. In addition, the energies and the heats of formation of the ring-shaped isomers are higher than those of the chain-shaped isomers. Among the ring-shaped isomers, the four-membered ring isomers have the highest heat of formation and the five-membered ring isomers have the lowest. For six-membered ring isomers, only the chair-shaped isomers are stable. Among all those chain-shaped isomers, the ones which have N–N bond and its length is in the range of NN double bond can improve the stability of the compounds.     

Pathways for the thermally induced dehydrogenation of C60H2

The thermolysis of C₆₀H₂ to yield C₆₀ and H₂ was studied by hybrid density functional theory (B3LYP/6-311G**//B3LYP/3-21G). The concerted loss of dihydrogen requires an activation energy of 92 kcalmol⁻¹ atT=452 K. An alternative radical mechanism, which is first order in the C₆₀H₂ concentration, has an activation energy at 452 K of only 61 kcalmol⁻¹. Monitoring of the C₆₀H₂ decomposition in 1,2-dichloro-[D₄]-benzene solution by NMR spectroscopy indicates a pseudo first-order reaction with an activation energy of 61.38±2.35 kcalmol⁻¹.     

Synthesis, structure and theoretical study of two rhodium(I) complexes [Rh(TropNMe)(CO)(PPh3)] and [Rh(Trop)(CO)(PPh3)] · Acetone

Rhodium(I) complexes [Rh(TropNMe)(CO)(PPh₃)] (TropNMe = 2-(N-methylamino)tropone, ONC₈H₉) (1) and [Rh(Trop)(CO)(PPh₃)] · Acetone (Trop = Tropolone, O₂C₇H₆) (2) have been synthesized and characterized by single-crystal X-ray diffraction analysis. A distorted square planar geometry about the rhodium(I) metal centre is observed in both compounds 1 and 2. Substitution of an oxygen atom with a methyl functionalized nitrogen atom does not significantly alter the bond distances and angles in the rhodium(I) complex. A theoretical study at B3LYP/6-31G(d) (main group) and LANL2DZ (Rh) level is presented to clarify the solid state behaviour of these complexes.     

Theoretical study on the molecule C30H20: five carbon–carbon single bonds linking a dodecahedrane cage and a pentaprismane cage

Using geometry optimization and DFT method at the B3LYP/6-31G* level of theory for C₃₀H₂₀, an equilibrium geometry is identified that has the form of polyhedral hydrocarbon with five carbon–carbon single bonds linking a dodecahedrane cage and a pentaprismane cage. Thus, this molecule is a tri-cage molecule with two pentaprismane cages and one dodecahedrane cage. Vibrational frequencies and the infrared spectrum are computed at the same level of theory. The heat of formation for C₃₀H₂₀ has been estimated in this paper. The heat of formation of C₃₀H₂₀ as well as the vibrational analysis indicates that this molecule enjoys sufficient stability to allow for its experimental preparation.     

双环金属铱(Ⅲ)异腈配合物的二阶非线性光学性质

采用密度泛函理论(DFT)方法对双环金属Ir(III)异腈配合物的非线性光学(NLO)性质进行计算研究。用B3PW91(UB3PW91)(金属原子采用LANL2DZ基组,非金属原子采用6-31G*基组)方法对配合物进行几何结构优化。在优化构型基础上,采用B3PW91(UB3PW91)和B3LYP(UB3LYP)方法计算了配合物的第一超极化率(βtot),并用CAM-B3LYP(UCAM-B3LYP)(金属原子采用LANL2DZ基组,非金属原子采用6-31G**基组)方法模拟配合物的吸收光谱。结果表明,主配体的取代基(R1)和副配体的取代基(R2)对第一超极化率值贡献不大。配合物发生氧化还原反应,电荷转移方式增多,电荷转移程度增大,使βtot值显著增加,其中1a+([(C∧N)2Ir(CNR)2]+(R=CH3))发生氧化反应和还原反应的βtot值分别增大了75倍和144倍。因此,这类双环金属铱(III)异腈配合物的氧化还原反应可以有效地调节其二阶NLO性质。     

Thermochemistry of the higher chlorine oxides ClOx (x=3, 4) and Cl2Ox (x=3–7)†

Heats of formation for ClO₃, ClO₄, Cl₂O₃, Cl₂O₄, Cl₂O₅, Cl₂O₆ and Cl₂O₇ molecules are determined at the B3LYP, B3PW91, mPW1PW91 and B1LYP levels of the density functional theory employing a series of extended basis sets, and using Gaussian-3 model chemistries. Modified Gaussian-3 calculations, which employ accurate B3LYP/6-311+G(3d2f) molecular geometries and vibrational frequencies, were also performed. Heats of formation were calculated from both total atomization energies and isodesmic reaction schemes. The latter method in conjunction with Gaussian-3 models leads to the most reliable results. The best values at 298 K for ClO₃, ClO₄, Cl₂O₃ and Cl₂O₄ as derived from an average of G3//B3LYP and G3//B3LYP/6-311+G(3d2f) calculations are 43.1, 54.8, 31.7 and 37.4 kcal mol⁻¹. From calculations carried out at the G3(MP2)//B3LYP and G3(MP2)//B3LYP/6-311+G(3d2f) levels, heats of formation for Cl₂O₅, Cl₂O₆ and Cl₂O₇ are predicted to be 53.2, 52.2 and 61.5 kcal mol⁻¹. All best values are reproduced within 1 kcal mol⁻¹ by using mPW1PW91/6-311+G(3d2f) isodesmic energies. Enthalpy changes for relevant Cl–O bond fission reactions are reported. Comparisons with previous thermodynamics data are made.     

Exit transition state of endohedral X@Si20H20 complexes (X=Li, Na, K, Be, Mg)

The kinetic stability of endohedral X@Si₂₀H₂₀ complexes (X=Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺) has been studied at the B3LYP/6-31G* level of density functional theory (DFT). The transition states (TS) are investigated by the QST3 method of Gaussian 98 package and demonstrated with Intrinsic reaction coordinate (IRC). It is found that K⁺@Si₂₀H₂₀ cluster has the most stable structure kinetically, that the exit barrier heights (H_exit) for K⁺ expulsion from or insertion in the cage are 187.38 and 205.58 kcal/mol, respectively. However, the smallest Be²⁺ dication is not endohedrally encapsulated and the Be²⁺@Si₂₀H₂₀ cluster is expected to be the least stable structure kinetically. By comparison, other endohedral complexes have a moderate kinetic stability.     

Peculiar basis set dependence of the energetics of C2S2H2 isomers. In search of adequate and affordable basis set for routine calculations

Quantum mechanical calculations using the Hartree-Fock (HF), post-Hartree-Fock {MP2 and CCSD(T)}and gradient-corrected hybrid density functional variant, B3LYP methodology in conjugation with basis sets like 6-31G*, 6-311G** and cc-pVTZ, were employed to critically probe the right quality of basis set and the appropriate level of theory that can be applied in modeling the organo-sulfur compounds. The relative stability ordering of 1,2-dithiete and dithioglyoxal reveal the delicate basis set dependence especially on ‘S’ atom. Single point calculations at the B3LYP, MP2 and CCSD(T) levels using a series of basis set ranging from double ζ quality all the way up to the quintuple ζ quality were done to illustrate the effect of electron correlation and the basis set dependency for these compounds. Basis set requirements are much higher than affordable for medium sized molecules as very slow convergence is seen even when the calculations are carried out with basis set as high as cc-pV5Z at the B3LYP and MP2 level of theories.     

Energetic analysis of 24 C20 isomers

Twenty-four individual geometric structures of the 20-carbon system are investigated using hybrid Hartree–Fock/density functional theory (DFT-B3LYP) in combination with the 6-31G and 6-311G* basis sets. These computations are carried out in order to evaluate the relative energies of the various C₂₀ isomers utilizing both geometry optimization and single-point energy calculations. The relative energies of the three most widely studied C₂₀ isomers (ring, bowl, and cage) are compared to the energies of other cyclic, bowl-like, and miscellaneous isomers. Although the ring and the bowl isomers are predicted to be the two most stable species, we have identified three to four additional cyclic structures that are predicted to be energetically competitive with the fullerene structure.     

An ab initio and matrix isolation infrared study of the 1:1 C2H2–CHCl3 adduct

The details of weak C–Hπ interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C₂H₂–CHCl₃ adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C₂H₂ and CHCl₃ species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C₂H₂–CHCl₃ complex has a weak hydrogen bond involving a C–Hπ interaction, where the C₂H₂ acts as a proton acceptor and the CHCl₃ as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl₃ and a hydrogen in C₂H₂. The combination of the C–Hπ interaction and the secondary ClH interaction determines the structure and the energetics of the C₂H₂–CHCl₃ complex. In addition to the vibrational assignments for the C₂H₂–CHCl₃ complex we have also observed and assigned features owing to the proton accepting C₂H₂ submolecule in the acetylene dimer.     

Comparative studies on the structures,infrared spectrum,and thermodynamic properties of phthalocyanine using ab initio Hartree–Fock and density functional theory methods

The molecular structure, vibrational spectrum, standard thermodynamic functions, and enthalpy of formation of free base phthalocyanine (Pc) have been studied using the density functional theory B3LYP procedure, as well as the ab initio Hartree–Fock method. Various basis sets 3‐21G, 6‐31G*, and LANL2DZ have been employed. The results obtained at various levels are discussed and compared with each other and with the available experimental data. It is shown that calculations performed at the Hartree–Fock level cannot produce a reliable geometry and related properties such as the dipole moment of Pc and similar porphyrin‐based systems. Electron correlation must be included in the calculations. The basis set has comparatively less effect on the calculated results. The results derived at the B3LYP level using the smaller 3‐21G and LANL2DZ basis sets are very close to those produced using the medium 6‐31G* basis set. The geometry of Pc obtained at the B3LYP level has D_2h symmetry and the diameter of the central macrocycle is about 4 Å. The enthalpy of formation of Pc in the gas phase has been predicted to be 1518.50 kJ/mol at the B3LYP/6‐311G(2d,2p)//B3LYP/6‐31G* level via an isodesmic reaction. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Structure and stability of closo-BnHn−2(CO)2 (n = 5–12)

Closo-B_nH_n−2(CO)₂ (n = 5–12), isolobal analogues of closo-C₂B_n−2H_n, have been investigated at the B3LYP/6-311+G^**density functional level of theory. The most stable isomers of closo-B_nH_n−2(CO)₂ are similar to those of closo-C₂B_n−2H_n in geometric patterns apart from closo-B₆H₄(CO)₂, and closo-B_nH_n−2(CO)₂ is much less strained than closo-C₂B_n−2H_n. Energetic analysis identifies closo-B₆H₄(CO)₂, closo-B₁₂H₁₀(CO)₂ and closo-B₁₀H₈(CO)₂ to be most stable, of which the latter two cages have been prepared experimentally. On the basis of the negative and rather large nucleus independent chemical shifts (NICS), closo-B_nH_n−2(CO)₂ are aromatic. To aid further experimental study, the CO stretching frequencies have been computed.     

Structure and stability of endohedral X@Si20H20 complexes (X = Li, Na, K, Be, Mg, Ca)

The structure and stability of endohedral X@Si₂₀H₂₀ complexes (X = Li^0/+, Na^0/+, K^0/+, Be^0/2+, Mg^0/2+, Ca^0/2+) have been studied at the B3LYP/6-31G* level of density functional theory. It is found that complexes with X = Na^0/+, K^0/+, Mg and Ca^0/2+ are energy minimum structures with X at the cage center in I_h symmetry, while those with X = Li^0/+, Be^0/2+, Mg²⁺ have off-centered structures with X towards one pentagon face in C_5v symmetry. Large electron or charge transfer between the Si₂₀H₂₀ cage and the encapsulated X has been observed.     

Theoretical investigation of adenine–BX3 (X=F,Cl) complex

Geometries and binding energies are predicted at B3LYP/6-311+G* level for the adenine–BX₃ (X=F,Cl) systems and four conformers with no imaginary frequencies have been obtained for both adenine–BF₃ and adenine–BCl₃, respectively, and single energy calculations using much larger basis sets (6-311+G(2df,p)) and aug-cc-pVDZ were carried out as well. The most stable conformer is BF₃ or BCl₃ connected to N₃ of adenine and with the stabilization energy of 22.55 or 20.59 kcal/mol at B3LYP/6-311+G* level (BSSE corrected). The analyses for the combining interaction between BX₃ and adenine with natural bond orbital method (NBO) and the atom-in-molecules theory (AIM) have been performed. The results indicate that all the conformers were formed with σ–p type interactions between adenine and BX₃, in which pyridine-type nitrogen or nitrogen atom of amino group offers its lone pair electron to the empty p orbital of boron atom and the concomitances of charge transference from adenine to BX₃ were occurred. Frequency analysis suggested that the stretching vibration of BX₃ underwent a red shift in complexes. Adenine–BF₃ complex was more stable than adenine–BCl₃ although the distance of B–N is shorter in the later.     

A comparative study on the nature and strength of O–O, S–S, and Se–Se bond

A computational study on dichalcogenide molecules (R₂X₂; X = O, S, Se; R = H, CH₃, NH₂) has been carried out employing B3LYP and MP2 levels using 6-31+G*, 6-311+G*, 6-311++G**, and PVDZ basis sets. The relative energies have been evaluated at G2MP2 also. The rotational barriers and bond dissociation energies indicate that S–S bond is stronger than Se–Se and O–O bond. NBO analysis at MP2/6-31+G* suggest the presence of partial π character between X–X bond that decreases in the order S–S > Se–Se > O–O. Fuki functions for nucleophilic and electrophilic attack fail to distinguish the reactivity of S and Se. The proton affinities of the O₂H₂, S₂H₂, Se₂H₂ decrease in the order Se > S > O.     

An ab initio and density functional theory study of the dimethylzinc-hydrogen selenide adduct: (CH3)2Zn:SeH2

The molecular structure (equilibrium geometry) and binding energy of the dimethylzinc (DMZn)-hydrogen selenide (H₂Se) adduct, (CH₃)₂Zn:SeH₂, have been computed with ab initio molecular orbital and density functional theory (DFT) methods and, where possible, compared with experimental results. The structure of the precursors DMZn and H₂Se are perturbed to only a small extent upon adduct formation. (CH₃)₂Zn:SeH₂ was found to be 3 kcal mol⁻¹ less stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation, indicating that the (CH₃)₂Zn:SeH₂ adduct is unlikely to be a stable gas-phase species under chemical vapour deposition conditions. Further calculations at the B3LYP/6-311 + G(2d,p)//B3LYP/6-311 + G(2d,p) level of computation suggest that the 1:2 adduct species, (CH₃)₂Zn:(SeH₂)₂, is much less stable than the 1:1 adduct and consequently the precursors by 19 kcal mol⁻¹.     

Molecular structure of the azidoalane [Me2N(CH2)3]AltBu(N3). X-ray structural determination and ab initio calculations

A single crystal of the azidoalane [Me₂N(CH₂)₃]AltBu(N₃) (1a), grown in a capillary using a miniature zone melting procedure, was investigated by X-ray analysis. Compound 1a (C₉H₂₁AlN₄) is a monomeric species in the solid state, which crystallizes in the monoclinic space group P2₁ with a=6.8560(14) Å, b=12.251(3) Å, c=7.786(2) Å, β=108.51(3)° and Z=2. The results of the X-ray structural determination are compared with the calculated structure of 1a (HF/6-31G(d) and B3LYP/6-31G(d) level of theory). Whereas the overall agreement between the measured and calculated structure is good, the Al–N donor-bond length differs by 11 and 12 pm at the HF and B3LYP level, respectively. To evaluate the effects of a polar environment on the molecular structure of 1a self-consistent reaction field (SCRF) calculations at the HF and B3LYP level with the 6-31G(d) basis set were performed.     

Geometry prediction of hafnocenes by quantum chemical methods

The performance of quantum chemical methods for geometry prediction of hafnocenes was evaluated. HF, B3LYP and MP2 in combination with nonrelativistic (MHF) and relativistic (MWB and LANL2DZ) basis sets for hafnium together with standard basis sets 3-21G*, 6-31G* and 6-311G** for other elements were applied. Five basic structural parameters of the optimized structures of the hafnocenes were compared with experimental crystal structures obtained from the Cambridge structural database. Altogether 80 hafnocenes were included in the analysis. The results show that relativistic corrections are necessary for Hf atom. However, even the Hartree–Fock (HF) method, when combined with relativistic pseudopotentials, reproduces the experimental crystal structures with significant accuracy. The good performance of the HF method can be understood to originate from the absence of significant near-degeneracy correlations for hafnium. On average, the B3LYP and MP2 methods provide structural parameters somewhat closer to the experimental ones.     

Ab initio studies of methanol, methanethiol and methylamine dimer

Quantum chemical calculations are used to provide structural, vibrational and energetical information on the dimers of the methanol, methylamine and methanethiol systems. These systems were studied employing the DFT(B3LYP) and MP2 methods together with the 6-31+G^** and 6-311+G^** basis sets. We found two distinct potential minima for methylamine (one of them is a transition structure) and methanethiol, and one for the methanol dimer. The properties of these dimers are compared with those of the dimers (H₂O)₂, (NH₃)₂ and (CH₃SH)₂. The interactions in these dimers were analyzed using electron density properties at the bond critical point.