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1.
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 D2h symmetry with a planar arrangement of the four membered {FeClbrFeClbr} 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 {FeClbrFeClbr} ring is extremely flat. This prevents an unambiguous assignment of either D2h or C2v symmetry to the Fe2Cl6 structure in electron diffraction measurements. The FeAlCl6 molecule is found to have a C2v structure similar to Fe2Cl6 with vibrational frequencies in good agreement with experiment. 相似文献
2.
In order to assign the bands in the IR spectra of silicates to the appropriate normal vibrations, a vibrational model has been proposed. A complex silicooxygen ring is considered as a ‘unit cell' composed of the appropriate number of [SiO4]4− tetrahedra. According to this model, in the ring silicates spectra we have to observe bands due to internal vibrations of individual tetrahedra and bands corresponding exclusively to the ring structure. Change in the tetrahedra symmetry from Td (ideal tetrahedron) to C2v (tetrahedron in a ring) and then to the ring symmetry: D3h, D4h and D6h (ideal rings) with respect to reducible representations makes it possible to differentiate between the bands due to ring structure (pseudo-lattice vibrations) and internal modes of tetrahedra. It has been established that in the case of all ideal rings there is only one IR active vibrational mode, namely the one symmetric with respect to the axis of the highest fold, i.e. A2″ in the case of 3-membered rings and A2u in the case of 4- and 6-membered rings. The model proposed has been verified for different membered ring silicates. 相似文献
3.
Four neutral bimetallic clusters X2M2 (X=Si, Ge, M=Al, Ga) are investigated using density functional theory (DFT) and post-HF methods. The calculated results show that each of four X2M2 species has two energetically close stable isomers with rhombic structure (D2h symmetry) and trapezoidal structure (C2v symmetry) respectively. For the Ge2Al2 species the rhombic (D2h) isomer is the ground state, whereas for other three species Ge2Ga2, Si2Al2, and Si2Ga2, the trapezoidal (C2v) isomers are the ground states. The calculated magnetic susceptibility anisotropy (χanis) and nucleus-independent chemical shift (NICS) indicate that a strong diatropic ring current exists in the two heterocyclic planar isomers, suggesting they are highly aromatic. A detailed molecular orbital analysis further reveals that both heterocyclic isomers possess multiple aromaticity derived from one delocalized π MOs and two delocalized σ MOs. 相似文献
4.
Zuoyin Yang Guichang Wang Zhenfen Shang Zunsheng Cai Yinmin Pan Xuezhuang Zhao 《Journal of Molecular Structure》2001,549(3):8560-251
The symmetry unrestricted C36F2 isomers formed from fullerene C36, the initial symmetry of which is C6v, C6h, or D2d, 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. 相似文献
5.
Mark A. Fox Judith A. K. Howard J. A. Hugh MacBride Angus Mackinnon Kenneth Wade 《Journal of organometallic chemistry》2003,680(1-2):155-164
The macrocyclic compound, [1,2-C2B10H10-1,4-C6H4-1,7-C2B10H10-1,4-C6H4]2 (5)—a novel cyclooctaphane, was prepared by condensation of the C,C′-dicopper(I) derivative of meta-carborane with 1,2-bis(4-iodophenyl)-ortho-carborane. The X-ray crystal structure of 5·C6H6·6C6H12 was determined at 150 K, revealing an extremely loose packing mode. Molecule 5 has a crystallographic Cs and local C2v symmetry; the macrocycle adopts a butterfly (dihedral angle 143°) conformation with the ortho-carborane units at the wingtips and the phenylene ring planes roughly perpendicular to the wing planes. Multinuclear NMR spectra suggest that molecule 5 in solution inverts rapidly via the planar D2h geometry, which (from ab initio HF/6-31G* calculations) is only 1 kcal mol−1 higher in energy than the C2v one. An attempt to prepare an even larger macrocycle, comprising three para-carborane and three ortho-carborane units linked by six para-phenylene units, was unsuccessful. 相似文献
6.
In the study of the N20 molecule with an Ih 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 (dN---N), first ionization potential, ΔHa, ΔGa 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. 相似文献
7.
Kenzo Hiraoka Akitaka Shimizu Akihito Minamitsu Masayuki Nasu Susumu Fujimaki Shinichi Yamabe 《Journal of the American Society for Mass Spectrometry》1995,6(12):1137-1142
The gas-phase stabilities of cluster ions SF+m (SF6)n with m = 0−5 were determined by using a high pressure mass spectrometer. The bond energies of SF+m (SF6)1 were found to be less than 10 kcal/mol and to decrease with m = 0 → 5. There appear to be rather large gaps in the bond energies between n = 1 and 2 for the clusters SF+m (SF6)n with m = 0−4. The structures of SF+5, SF+ (SF6)1, SF+3 (SF6)1, and SF+5 (SF6)1 were investigated by ab initio molecular orbital calculations. For SF+5, the D3h geometry is found to be most stable andC4v is a transition state of the Berry pseudorotation. For the ion-molecule complexes, the “on-top hat” models were found to be the most stable structures. 相似文献
8.
The structure of cyclopentadienyl(duroquinone)cobalt dihydrate, (C5H5)Co-[(CH3)4C6O2]·2H2O, has been determined by three-dimensional X-ray analysis. The crystal structure consists of discrete cyclopentadienyl(duroquinone)cobalt molecules linked together by a complex network of hydrogen bonds between water molecules and duroquinone oxygen atoms. Each (C5H5)Co[(CH3)4C6O2] molecule consists of a cobalt atom sandwiched between a cyclopentadienyl ring and a duroquinone ring. A detailed comparison of the molecular parameters of this complex with those of closely related complexes is given. Crystallographic evidence that the metal---duroquinone interaction in cyclopentadienyl(duroquinone)cobalt dihydrate is considerably stronger than that in the electronically-equivalent 1,5-cyclooctadiene(duroquinone)nickel complex is given not only by the metal---C(olefin) distances being 0.12 Å (av) shorter in the duroquinone---cobalt complex [viz., 2.104(8) Å vs. 2.222(7) Å] but also by the much greater C2v-type distortion of the duroquinone ring from the planar D2h configuration in free duroquinone. The compound crystallizes with two formula species in a triclinic unit cell of symmetry P
and reduced cell dimensions á = 8.60 Å, b = 9.00 Å, c = 10.15 Å, = 87° 34′, β = 84° 10′, γ = 73° 44′. Least-squares refinement yielded final unweighted and weighted discrepancy factors of R1 = 10.8% and R2 = 12.0%, respectively, for 2481 independent diffraction maxima collected photographically. 相似文献
9.
Dorothy J. Gearhart Katharine L.C. Hunt James F. Harrison 《Journal of Molecular Structure》2008,858(1-3):31-38
The electronic structure of Na2C2 is studied using ab initio electronic structure methods and is compared to the companion molecule Li2C2. Both the linear D∞h and planar structures are minima on the ground state potential surface with the planar D2h conformation being the lowest energy form, similar to Li2C2. At the CCSD(t) level the planar form is more stable that the linear by 11.2 kcal/mol as compared with 7.34 kcal/mol for Li2C2. Both molecules are significantly ionic. The vibrational frequencies, atomization energy at 0 K, D0, and the standard enthalpy of formation, are calculated and compared to those of Li2C2 as well as HCCH, FCCF and ClCCCl. We find D0 and to be 331.1 and 84.92 kcal/mol for Li2C2 and 298.3 and 93.25 kcal/mol for Na2C2. We calibrate these by calculating the same quantities for HCCH, FCCF and ClCCCl. 相似文献
10.
The electron scattering pattern of gaseous dicyclopentadienylberyllium, Cp2Be, has been recorded from s = 2.00 to 39.00 Å−1 with a nozzle temperature of about 120°C. Molecular models of D5d symmetry or models containing one π-bonded and one σ-bonded Cp ring are not compatible with the data. The possibility the gaseous Cp2Be consists fo a mixture D5d and π-Cp, σ-Cp conformers is considered and rejected. A model of C5v symmetry can be brought into satisfactory agreement with the data. It is also found that a slip sandwich model obtained from the C5v model by moving sideways the ring which is at the greatest distance from Be, while keeping the two rings essentially parallel is compatible with the electron diffraction data. The best fit between experimental and calculated intensity curves is obtained with a model with a sideways slip of 0.8(1) Å. This model is similar to that indicated by the X-ray diffraction investigations by Wong and coworkers [4,5]. It is suggested that the potential energy of the molecule does not change much as the magnitude of the slip changes and that the molecule thus undergoes large amplitude vibration. 相似文献
11.
CaRgn+ (Rg=He, Ne, Ar) complexes with n=1–4, are investigated by performing using the B3LYP/6-311+G (3df) density functional theory calculations. The CaHen+ (n=1–4) complexes are found to be stable. In the case of CaNen+ and CaArn+, stable structures and stationary point were found only for n=1 and 2. For n=3 in the C3V and the D3h point group as well as for n=4 in the Td (tetrahedral) point group a saddle point (imaginary frequency) is observed and global minimum could be obtained along the potential energy surface. 相似文献
12.
Robin J. H. Clark Vincent B. Croud
Helen M. Dawes
Michael B. Hursthouse 《Polyhedron》1988,7(24):2611-2614The complex [H3NCH2CH2NH3][PdBr6] has been isolated as well-formed brown crystals. The Raman (single crystal) and FTIR (wax disc) spectra of the complex have been recorded but the band assignments are complicated by extensive factor group splitting and resonance effects. The crystal belongs to space group Pnnm, with Z = 2, each ion occupying sites of 2/m (C2h) symmetry. The [PdBr6]2− ion is very close to octahedral, the two unique PdBr distances, 2.466(3) and 2.470(3) Å, being equal within experimental error and the BrPdBr angles being 90 ± 0.8°. The diammonium cation has an extended, planar, trans structure. 相似文献
13.
Jingping Peng David L. Cedeo Carlos Manzanares I 《Journal of Molecular Structure》1998,440(1-3):265-288
The infrared spectra of cis-3-hexene and trans-3-hexene dissolved in liquid argon have been obtained at temperatures from 93 to 120 K. The absorptions were observed with a low-temperature cell and a Fourier transform infrared spectrophotometer. Ab initio molecular orbital calculations were performed to obtain the equilibrium geometry, vibrational frequencies, force fields, and infrared intensities. The calculations were done at the Hartree-Fock level using 6-31G basis set. The Cartesian force fields from ab initio calculations have been converted to the force field in symmetry coordinates. The scale factors of ab initio calculated force fields were determined. Normal coordinate calculations were performed using a scaled quantum mechanical (SQM) force field. Vibrational normal modes calculated for the lowest energy rotamers of cis- and trans-3-hexene have been assigned to infrared absorption bands observed in liquid argon solution. The assignments were based on calculated frequencies and potential energy distributions. The equilibrium geometries of the two lowest energy rotamers (symmetry C2 and Cs) of cis-3-hexene and of the three lowest energy rotamers (symmetry Ci, C2, and C1) of trans-3-hexene were calculated. Variable temperature studies of the infrared spectrum of cis- and trans-3-hexenes dissolved in liquid argon were done to obtain the ΔH of conversion between the rotamers C2 and Cs of cis-3-hexene and between the rotamers Ci, C2, and C1 of trans-3-hexene. 相似文献
14.
Point group in crystallography is one of the important subjects in structural chemistry.Some topics are very difficult to understand.To name a few, why does point group of D2d belong to tetragonal? Why are D4d and D6d not included in 32 kinds of crystallographic point groups? The two questions are easy to answer if we understand the following topic:for the Dnd point group, when n is odd, it contains an In symmetry axis; when n is even, it contains an I2n symmetry axis.In this work, graphic method and matrix method are adopted to clarify why the Dnd point group includes an S2n axis, and thus give explanations that D2d belongs to tetragonal as well as D4d and D6d are not included in 32 kinds of crystallographic point groups. 相似文献
15.
Fengling Liu 《大学化学》2020,35(9):168-172
A method for obtaining the delocalized π bonds πnm in a molecule has been discussed in this paper, and the delocalized π bonds πnm in linear, bent, planar trigonal, single cyclic conjugated, and polycyclic conjugated molecules have been studied. The reasons of π34 in NO2 and 2π32 in C3 molecules have been proposed. The delocalized π bonds 2π1818 in cyclo[18] carbon are analyzed. 相似文献
16.
The integral enthalpies of solution of glycylglycine and diglycylglycine in water–ethanol, water–n-propanol and water–i-propanol mixtures were measured at 298.15 K and alcohol mole fraction concentrations (x2) ranging up to 0.3 by calorimetry. The ΔsolH° and ΔtrH° vs. x2 were found to have extrema. Enthalpic coefficients of pairwise interactions (hxy) between peptide and alcohol molecules were positive and increased in the series ethanol, n-propanol, i-propanol. 相似文献
17.
The molecular structures of n-hexane were determined by RHF/4-21G ab initio geometry optimization at 30° grid points in its three-dimensional τ1(C11–C8–C5–C1), τ2(C14–C11–C8–C5), τ3(C17–C14–C11–C8) conformational space. Of the resulting 12×12×12=1728 grid structures, 468 are symmetrically non-equivalent and were optimized constraining the torsions τ1, τ2, and τ3 to the respective grid points, while all other structural parameters were relaxed without any constraints. From the results, complete parameter surfaces were constructed using natural cubic spline functions, which make it possible to calculate parameter gradients, |P|=[(∂P/∂τi)2+(∂P/∂τj)2]1/2, where P is a C–C bond length or C–C–C angle. The parameter gradients provide an effective measure of the torsional sensitivity of the system and indicate that dynamic activities in one part of the molecule can significantly affect the density of states, and thus the contributions to vibrational entropy, in another part. This opens the possibility of dynamic entropic conformational steering in complex molecules; i.e. the generation of free energy contributions from dynamic effects of one part of a molecule on another. When the conformational trends in the calculated C–C bond lengths and C–C–C angles are compared with average parameters taken from some 900 crystallographic structures containing n-hexyl fragments or longer C–C bond sequences, some correlation between calculated and experimental trends in angles is found, in contrast to the bond lengths for which the two sets of data are in complete disagreement. The results confirm experiences often made in crystallography. That is, effects of temperature, crystal structure and packing, and molecular volume effects are manifested more clearly in bond lengths than bond angles which depend mainly on intramolecular properties. Frequency analyses of the τ1, τ2 and τ3 torsional angles in the crystal structures show conformational steering in the sense that, if τ1 is trans peri-planar (170°≤τ1≤180°; −180°≤τ1≤−170°), the values of τ2 and τ3 are clustered closely around the ideal gauche (±60°) and trans (±180°) positions. In contrast, when τ1 is in the region (50°≤τ1≤70°), there is a definite increase in the populations of τ2 and τ3 at −90 and −150°. 相似文献
18.
Carl Nyeland 《Chemical physics》1981,60(3):359-367
The classical path sudden approximation has been used to calculate cross sections for rotational excitation in atom—molecule collisions. Three different decoupling approximations for the angular momentum of the molecule are used: one effective potential approach and two different coupled states approaches, all in their semiclassical versions. Numerical results are presented and discussed for the systems Ar---N2, Ar---Li2, Ar---LiH, He---CO and Ar---HCl assumming intermolecular potentials of P1 symmetry, P2 symmetry and combinations. 相似文献
19.
Propynyl isocyanide, CH3C2NC, has been prepared by vacuum pyrolysis of pentacarbonyl-(1,2-dichloropropenyl isocyanide) chromium, (CO)5Cr–CN–C(Cl)=C(Cl)CH3, and its ground state millimeter and microwave spectrum has been observed for the first time. rs structural parameters of this molecule with a C3v symmetry could be obtained from the rotational constants of several isotopomers: r(C1–C2)=1.456(2) Å, r(C2–C3)=1.206(2) Å, r(C3–N)= 1.316(2) Å, r(N–C4)= 1.175(2) Å, r(H–C1)= 1.090(1) Å, >HCC=110.7(4)°. The nitrogen quadrupole coupling constant has been determined to be 878(2) kHz and measurements of the Stark effect allowed to obtain an electric dipole moment of 4.19(3) Debye. The results fit well into a series of related compounds and are in good agreement with data from ab initio calculations. 相似文献
20.
Both ab initio 6-31G, 3-21G and STO-3G basis sets and semiempirical PM3 and AM1 molecular orbital calculations are carried out on the C24N4 molecule of the Td symmetry group. Results on the fully optimized structure which constrained Td symmetry, molecular orbitals and vibrational frequency were obtained by both ab initio and semiempirical methods. The binding energy and various thermodynamic properties were also calculated via the PM3 and AM1 semiempirical methods. All the evidence of this work proves that the C24N4 molecule is stable and that its four six-membered rings with a remarkable delocalized C…C bond are similar to the related rings in the C60 buckminsterfullerene structure. 相似文献