共查询到20条相似文献,搜索用时 11 毫秒
1.
Yurii N. Panchenko Jean Vander Auwera Yahia Moussaoui George R. De Maré 《Structural chemistry》2003,14(4):337-348
The positions of some IR bands of the s-trans-1,3-butadiene-h
6 and -1,1,2-d
3 isotopomers in the gas phase have been measured using a Brucker IFS 120 HR spectrometer with a resolution of 2 cm–1. The structural parameters of the s-trans- and s-gauche-1,3-butadiene conformers were optimized completely at the MP2/6-31G* theoretical level and their MP2/6-31G*//MP2/6-31G* quantum mechanical force fields (QMFFs) were calculated. Using only the experimental vibrational frequencies of s-trans-1,3-butadiene-h
6 the QMFF of the s-trans conformer was corrected by Pulay's scaling method (eight scale factors were involved). The scaled QMFF was used to calculate the mean vibrational amplitudes and the Coriolis coupling constants of s-trans-1,3-butadiene-h
6 and the vibrational frequencies of 12 of its deuterated isotopomers. The set of scale factors obtained for correction of the s-trans QMFF was transferred to the QMFF of the s-gauche conformer. Its theoretical vibrational spectrum and those of some deuterated and C13 isotopomers were calculated. The ability of this scaling approach (transferring of scale factors) to predict the vibrational frequencies of rotational conformers and their isotopomers, as well as other molecular characteristics, and to permit detection of perturbations of the experimental bands are discussed. 相似文献
2.
Ab initio calculations of the structure, atomic charges, natural bond orbital, and thermodynamic functions have been performed at HF/6-311G∗∗ and B3LYP/6-311G∗∗ levels of theory for the title compound of 4-phenyl-1-(propan-2-ylidene)thiosemicarbazide. The calculated results show that
the sulfur atom and all of the nitrogen atoms have bigger negative charges and they are the potential sites to react with
the metallic ions, which make the title compound become a multidentate ligand. The coordination ability of the sulfur atom
and the nitrogen atom of C=N double bond will increase with the increase of the polarity of the solvent. Electronic absorption
spectra have been calculated by time-dependent density functional theory (TD-DFT) method. The calculation of the second-order
optical nonlinearity also has been carried out, and the molecular hyperpolarizability is 3.068×10−30 esu. 相似文献
3.
Ab initio molecular orbital theory and density functional theory have been employed to study N14 cluster with low spin at the HF/6-31G*, B3LYP/6-31G*, B3PW91/6-31G*, BP86/6-31G*, and BHLYP/6-31G* levels of theory. Twelve isomers were studied, including one previously investigated cage molecule. The most stable isomer of N14 is a C
2h
-symmetric molecule that contains two separated five-membered nitrogen rings connected by a —N=N—N=N— bridge. The second, third, and fifth most stable isomers each have one five-membered nitrogen ring. The theoretical results suggest that the five-membered nitrogen ring gives rise to a particularly stable structural unit, and the more side chains that the five-membered nitrogen ring links with, the less stable the structure will become. 相似文献
4.
Roberto Gangemi Giovanna Longhi France Lebon Sergio Abbate Leo Laux 《Monatshefte für Chemie / Chemical Monthly》2005,136(3):325-345
Summary. A set of vibrational circular dichroism (VCD) spectra in the CH-stretching fundamental region for about twenty compounds belonging to the class of essential oils was empirically analyzed by the use of a sort of vibrational exciton mechanism, involving three centers. Through a general formula applicable to many coupled dipole oscillators, the rotational strengths of the previously identified vibrational excitons are evaluated. The results are then critically reviewed by the use of recent ab initio methodology, as applied to selected molecules of the original set. Further insight is gained by model calculations adding up the contribution of the coupling between electric dipole moments associated with normal mode behavior and that of the polarizability from polarizable groups. The former part is responsible for the excitonic behavior of the VCD spectra. For the same selected molecules we have also investigated whether some excitonic behavior is taking place in the second overtone region, and have concluded that this is not the case. 相似文献
5.
Cornelia Kozmutza 《Theoretical chemistry accounts》1981,60(1):53-63
Anab initio integral program is described. It utilizes the local symmetries to avoid the redundant computation of integrals over spatially equivalent subsets of the basis. The integrals are grouped in a particular way to facilitate their transfer. The program is very suitable for the treatment of related systems with model geometries. The computing times of different programs are compared and the efficiency of the presented one is demonstrated. 相似文献
6.
Self-consistent-field variational approach to the interaction between a polymer and a small molecule
A variational SCF treatment based on a perturbational concept is developed and applied to the interaction between trans-polyacetylene and a small molecule. The validity of the present method is examined by comparing the results with those from the conventional tight-binding SCF crystal orbital method. The interaction energies and charge distributions obtained are in good agreement between the two methods. This result suggests that the present variational approach is promising for application to complicated interactions between a polymer and impurities. 相似文献
7.
E. B. Burgina V. P. Baltakhinov E. V. Boldyreva T. P. Shakhtschneider 《Journal of Structural Chemistry》2004,45(1):64-73
IR spectra of paracetamol and phenacetin have been measured for powder crystals of these compounds and for their solutions in chloroform and dimethylsulfoxide. Ab initio calculations of their equilibrium geometry and vibrational spectra were carried out for spectrum interpretation. Differences between the experimental IR spectra of solutions and crystalline samples have been analyzed. Variations of molecular structure from the isolated state to molecular crystal were estimated based on the difference between the optimized molecular parameters of free molecules and the experimental bond lengths and angles evaluated for the crystal forms of the title compounds. The role of hydrogen bonds in the structure of molecular crystals of paracetamol and phenacetin is investigated, and spectral ranges with maximal intermolecular interactions are determined. 相似文献
8.
Ab initio calculations of the structure, atomic charges, natural bond orbital and thermodynamic functions have been performed at HF/6-311G** and B3LYP/6-311G** levels of theory for the title compound of 4-phenyl-1-(1-phenylethylidene) thiosemicarbazide. The calculated results show
that the sulfur atom and all of the nitrogen atoms have bigger negative charges and that they are the potential sites to react
with the metallic ions, which make the title compound a multidentate ligand. The coordination ability of the sulfur atom and
the nitrogen atom of C=N double bond increases with the increase of polarity of the solvent. Electronic absorption spectra
have been calculated by time-dependent density functional theory (TD-DFT) method. The calculation of the second-order optical
nonlinearity has also been carried out with the PM3 semi-empirical method, resulting in the molecular hyperpolarisability
is 5.477×10−30 esu. 相似文献
9.
Summary. Conformational analysis and frequency calculation were achieved for 1-phenyl-1,2-propandione 1-oxime and its four tautomers:
1-nitroso-1-phenyl-1-propen-2-ol, 1-nitroso-1-phenyl-2-propanone, 2-hydroxy-1-phenyl-propenone oxime, and 3-nitroso-3-phenyl-propen-2-ol.
Calculations were carried out at the Hartree–Fock (HF), Density Functional Theory (B3LYP), and the second-order M?ller–Plesset perturbation (MP2) levels of theory using 6-31G* and 6-311G** basis sets. Five conformers with no imaginary vibrational frequency were obtained by free rotations around three single bonds
of 1-phenyl-1,2-propandione-1-oxime: Ph–C(NOH)C(O)CH3, PhC(NOH)–C(O)CH3, and PhC(N–OH)C(O)CH3. Similarly, eight structures with no imaginary vibrational frequency were encountered upon rotations around three single
bonds of 1-nitroso-1-phenyl-1-propen-2-ol: Ph–C(NO)C(OH)CH3, PhC(N–O)C(OH)CH3, and PhC(NO)C(–OH)CH3. In the same manner, six minima were found through rotations around three single bonds of 1-nitroso-1-phenyl-2-propanone:
Ph–CH(NO)C(O)CH3, PhCH(–NO)C(O)CH3, and PhCH(NO)–C(O)CH3. Also, two minima were found through rotations around four single bonds of 2-hydroxy-1-phenyl-propenone oxime: Ph–C(NOH)C(OH)CH2, PhC(N–OH)C(OH)CH2, PhC(NOH)–C(OH)CH2, and Ph-C(NOH)C(–OH)CH2. Finally, two minima were found through rotations around four single bonds of 3-nitroso-3-phenyl-propen-2-ol: Ph–CH(NO)C(OH)CH2, PhCH(–NO)C(OH)CH2, PhCH(NO)–C(OH)CH2, and PhCH(NO)C(–OH)CH2. Interconversions within the above sets of conformers were probed through scanning (one and/or two dimensional), and/or QST3
techniques. The order of the stability of global minima encountered was: 1,2-propandione-1-oxime > 1-nitroso-1-phenyl-2-propanone
> 1-nitroso-1-phenyl-1-propen-2-ol > 2-hydroxy-1-phenyl-propenone oxime > 3-nitroso-3-phenyl-propen-2-ol. Hydrogen bonding
appears significant in tautomers of 1-nitroso-1-phenyl-1-propen-2-ol and 2-hydroxy-1-phenyl-propenone oxime. The CIS simulated
λmax for the first excited singlet state (S1) of 1-phenyl-1,2-propandione 1-oxime is 300.4 nm, which was comparable to its experimental λmax of 312.0 nm. The calculated IR spectra of 1-phenyl-1,2-propandione 1-oxime and its tautomers were compared to the experimental
spectra. 相似文献
10.
Summary The geometries of three hydrogen-bonded dimers of hydroxylamine have been optimized, at the MP2 level of theory, using the 6-31G** basis set. These calculations yielded three separate local minima on the dimer potential energy surface. The interaction energies of these three species have been calculated, and corrected for basis set superposition error. The infrared band wavenumbers and intensities have been computed, and the monomer-dimer wavenumber shifts and intensity enhancements rationalized in terms of the types and strengths of hydrogen bonds present. The predicted wavenumbers have been correlated with those measured in a recent matrix isolation spectroscopic study, and an argument for the structure of the preferred dimer has been presented. 相似文献
11.
Kristof Van Hecke Vu Thi Ngan Peter Nockemann Ben Thijs Minh Tho Nguyen Koen Binnemans Luc Van Meervelt 《Journal of Molecular Structure》2008,885(1-3):97-103
The structure of one tautomer (amine form) of cyano-carbamimidic acid ethyl ester or (amino-ethoxy-methylidene)aminoformonitrile (CAS: 13947-84-7) was determined by single crystal X-ray diffraction. Ab initio quantum chemical calculations at the B3LYP, MP2 and G3 levels were performed to investigate the stability and the formation of the different tautomers and conformers. The calculations indicate that the amine form is the more stable tautomer, showing a high degree of electron conjugation. The most stable amine conformer located by the calculations corresponds to the crystallized structure. On the contrary, in the less stable imine form, the conjugation is separated by a N2–C2 single bond. 相似文献
12.
An accurate full-potential density-functional method is used to study the magnetic and half-metallic properties in the pure organic materials: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl. The total and partial density of states and atomic spin magnetic moments are calculated and discussed. It is found that the unpaired electrons in this compound are localized in a molecular orbital constituted primarily of π*(NO) orbital, and the main contribution of the spin magnetic moment comes from the NO free radicals. It is predicted that this compound is half-metallic magnet. It is also found that there exists ferromagnetic intermolecular interaction in the compound. 相似文献
13.
Accurate Hartree-Fock LCAO calculations for moderately complex crystalline systems are now feasible; a number of important applications may be envisaged in the areas of material science and technology. Some critical aspects of the corresponding computer schemes are discussed which are of fundamental importance in determining the cost of the computation. Data are provided concerning actual computations which are indicative of the kind of periodic systems that can (or cannot) be treated at present. The result of a perfect-crystal ab initio HF study can be used as an input for treating with the same approximation local-defect problems, by use of suitable embedding techniques. A scheme of this kind is presented, and its computational implications are discussed: due to the intrinsic complexity of this problem, it may be foreseen that the study of defects in crystals will be a typical application of supercomputers in the area of quantum chemistry.This paper was presented at the International Conference on The Impact of Supercomputers on Chemistry, held at the University of London, London, UK, 13–16 April 1987 相似文献
14.
According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer. 相似文献
15.
Summary. The conformational preferences of the lignin guaiacyl structural unit were studied at the MP2/6-311G(d,p) level of theory using guaiacol (2-methoxyphenol) as model compound. The potential energy surface of guaiacol
was investigated by the ab initio method with full geometry optimization by varying the torsion angles of the guaiacol functional groups (hydroxyl and methoxy).
An overall of nine stationary points were located, four of which were found to be minima and the other five transition structures
between them. The energy minima of guaiacol can adopt one cisoid and three transoid conformations for the hydroxyl and methoxy groups. The transoid structures differ by the orientation of the methoxy group inside and outside of the aromatic plane. The most stable cisoid conformer has an intramolecular hydrogen bond between phenolic hydrogen and methoxy oxygen with a binding energy of 18.09–18.51 kJ/mol
as calculated with the second-order (MP2) and fourth-order (MP4SDQ) M?ller-Plesset methods and with larger polarized basis sets including diffuse functions. When comparing the geometrical parameters of the
global energy structure with relevant experimental data from crystallographic structures good agreement between the data was
found. The saddle points, the effect of calculation level on the energy relative stability, the rotational barrier heights,
and the relative concentrations of the conformers are also discussed. 相似文献
16.
Junyong Wu Hua Yan Yanxian Jin Hao Chen Guoliang Dai Aiguo Zhong Fuyou Pan 《Journal of Molecular Structure》2009,911(1-3):132-136
The nature of interactions of thiophene with various hydrides (Y) (Y = HF, HCl, H2O, H2S, NH3, PH3) is investigated using ab initio calculations. In contrast with the previous results on similar furan complexes, only the π-type is observed for the thiophene complexes. Variations in complexes geometry can be accounted for by the differences in the electrostatic potential on the aromatic ring. To further study the nature of the intermolecular interactions, an SAPT (the symmetry-adapted perturbation theory) energy decomposition analysis was carried out and the results indicate that the dispersion and electrostatic interactions dominate the thiophene complexes. 相似文献
17.
18.
Summary
Ab initio molecular orbital calculations of electron coupled nuclear spin-spin coupling constants are performed for CH4, SiH4, AlH
4
–
and GeH4 systems using the SCF perturbation theory. Basis set dependence of the major contributing terms such as orbital diamagnetic, orbital paramagnetic, spin dipolar and Fermi contact terms are studied. The study also illustrates the relative importance of bond centred functions and nuclear centred polarization functions in predicting the directly bonded and geminal couplings in the systems selected. Basis sets having uncontracted cores functions and augmented with bond functions seem to predict most of these couplings fairly satisfactorily when compared to the experimental values. 相似文献
19.
The dynamic properties of the hydrogen abstraction reactions of CF2H2 and CF3H with F atom are investigated in the temperature range of 182–2000 K. The minimum-energy path (MEP) is optimized at MP2/6-311 G(d, p) level, then the energy profiles are refined at the CCSD(T)/6-311++G(3df, 2pd) level (single-point). The theoretical rate constants, which are calculated by the variational transition state theory (VTST) including the small curvature tunneling (SCT) correction, are in good agreement with the experimental ones. It is found that the rate constant of the CF2H2 + F reaction are larger than that of the CF3H + F reaction and the activation energies exhibit in the just opposite order. This phenomenon can be rationalized by the hardness η of the halomethane molecules. The comparison of the two reactions with the CFH3 + F reaction is made. It is found that the rate constants decrease in the order of CFH3 + F > CF2H2 + F > CF3H + F. The effect of fluorine substitution leads to a dramatic increase in the activation energy and a decrease in the preexponential factor. We hope that present theoretical studies for these compounds can give further information concerning how fluorine substitution affects the rate constants of hydrogen abstraction reactions. 相似文献
20.
Dipankar Datta 《Journal of Chemical Sciences》1988,100(6):549-557
A correspondence betweenab initio calculations, the principle of electronegativity equalisation and group electronegativity has been established within the
framework of Mulliken population analysis. Using this we have calculated electronegativities of some 37 groups/atoms. These
electronegativities show excellent linear correlation with1
J
CC coupling constants in monosubstituted benzenes and Inamoto’si scale and a satisfactory one with Wells’ group electronegativity data. The correspondence however required a scaling of charge
(obtained byab initio calculations) and a proportionality between the electronegativity of the neutral group and its hardness. It is shown that
using these electronegativity values it is possible to calculate group charges in molecules where groups under consideration
interact with each other through σ bond only. 相似文献