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1.
The structures of the three phenol–water minima are optimized with MP2 and the interaction-optimized DZPi basis set. Single point calculations are carried out using the slightly larger ESPB basis set, which contains a set of (s,p) bond functions at the midpoint of the hydrogen-bond. The binding energies and hydrogen-bond distances are corrected for basis set superposition error. For all minima, our binding energies De are larger than the previous theoretical estimates. Despite this, our best estimate of the binding energy D0 for the global minimum, 21.08 kJ/mol, is about 2 kJ/mol smaller than the experimental values (23.45±0.48 and 22.92±0.36 kJ/mol). 相似文献
2.
Errol Lewars 《Journal of Molecular Structure》1996,360(1-3):67-80
Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (“ketocarbene”), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G*//MP2(FULL)/6-1G* with MP2(FULL)/ 6-31G* zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and −17.8 kJ mol−1. Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G* geometry is C2v but the MP2(Fermi contact)/6-31G* geometry is Cs with a 6-/3-ring coplanarity deviation of about 6.9 °, although in the MP2(FULL)/6-31G* geometry this is reduced to about 3.1 °. 相似文献
3.
Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the 1A1(π → π*) state. The 1A1(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing. 相似文献
4.
Jan Almlf 《International journal of quantum chemistry》1974,8(6):915-924
Ab initio SCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH ? HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin-restricted SCF calculations. The lowest state is found to have B1u symmetry with an ionization potential of 8.0 eV. 相似文献
5.
A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through the rather than the-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.
Zusammenfassung Für die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems der-Elektronen als durch diejenige der-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.相似文献
6.
Extensive ab initio molecular-orbital calculations were carried out on trifluoromethylamine (TFM) to elucidate changes in geometry and electronic structure upon fluorination. The calculations show that the decomposition of CF3NH2 is slightly endoenergetic, and the heats of atomization of CF3NH2 and CH3NH2 show decreased stability of the species upon fluorination. Characteristic of CF3NH2 is a highly polar, strong, short CN bond. More limited calculations were carried out on CF3OH and CH3OH, and the electronic structure of CF3OH is found to be generally similar to that of CF3NH2. The reduced basicity of the fluorinated amine cannot be ascribed to the inductive effect; the enhanced acidity of the fluorinated alcohol reflects the weakening of the OH bond. No evidence leads to a confirmation of the existence of nitrogen–fluorine hyperconjugation in the fluorinated amine. 相似文献
7.
硫代甲酰胺双聚体的量子化学计算 总被引:1,自引:0,他引:1
在MP2/6 31G(d)和MP2(FC)/6 311 G(d,p)水平上,对硫代甲酰胺(HC-SNH2)及其3种构型双聚体进行几何全优化计算,经振动频率分析,确认为势能超曲面上的稳定驻点.然后在MP2/6 311 G(2df,2p)水平上进行单点能计算和基组重叠误差(BSSE)校正以获得相互作用能.并利用自然键轨道(NBO)理论和分子中的原子(AIM)理论探讨HCSNH2之间相互作用的本质. 相似文献
8.
A new ab initio effective two-body potential that aims at mimicking the average copper–water interaction energy of the first solvation shell was developed. This new potential, together with the MCY water–water potential and a three-body ion–water–water induction potential, is tested in simulations of gas-phase clusters [Cu2+? (H2O)20] and diluted solutions [Cu2+? (H2O)200] at T = 298 K. The results of simulations with conventional ab initio pair potentials, with and without three-body induction corrections, are also presented. The different types of copper–water interaction potentials are evaluated comparatively and the efficiency of the newly proposed effective pair potential is discussed. © 1993 John Wiley & Sons, Inc. 相似文献
9.
The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered. 相似文献
10.
Arndt Jenichen 《Monatshefte für Chemie / Chemical Monthly》1992,123(1-2):17-24
Summary Structure and stability of molecular clusters modelling halogen (F, Cl) double bridges between silicon atoms, H3SiF2SiH3 (1), H3SiF2SiF3 (2), H3SiCl2SiH3 (3), and H3SiCl2 SiCl3 (4), have been investigated by an ab initio pseudopotential method. Asymmetrical bridges Si-X...Si with one strong Si-X bond and one weak Si...X bonding interaction (X=F, Cl) result from the geometry optimization using the LP-31 G basis set. Dissociation energy calculations using the MP2/LP-31G*//LP-31G procedure and considering the basis set superposition error provide a decrease of stability of the structures in the order2>4>3>1. The results are discussed with respect to formation and decomposition of halogenated reaction overlayers formed during the etching of silicon by halogen atoms.
Ab-Initio-Berechnungen von Silizium-Halogen-Silizium-Doppelbrücken
Zusammenfassung Struktur und Stabilität von molekularen Clustern, die Halogen(F, Cl)-Doppelbrücken zwischen Siliziumatomen modellieren, H3SiF2SiH3 (1), H3SiF2SiF3 (2), H3SiCl2SiH3 (3) und H3SiCl2SiCl3 (4), werden mittels eines Ab-Initio-Pseudopotentialverfahrens untersucht. Bei der Geometrieoptimierung unter Verwendung des LP-31 G-Basissatzes ergeben sich asymmetrische Brücken Si-X...Si mit einer starken Si-X-Bindung und einer schwachen bindenden Si...X-Wechselwirkung (X=F, Cl). Dissoziationsenergieberechnungen durch das MP2/LP-31 G*//LP-31 G-Verfahren unter Berücksichtigung des Basissatzüberlagerungsfehlers liefert eine abnehmende Stabilität der Cluster in der Reihenfolge2>4>3>1. Die Resultate werden im Zusammenhang mit der Bildung und dem Zerfall von halogenierten Reaktionsschichen, welche während des reaktiven Ätzens von Silizium mit Halogenatomen gebildet werden, diskutiert.相似文献
11.
Multireference configuration interaction (MRD-CI) calculations are reported for a large series of electronic states of the HeNe quasimolecule up to 170000 cm(-1) excitation energy, including those that dissociate to the 3S1 and 2 1S0 excited states of the He atom. Spin-orbit coupling is included through the use of relativistic effective core potentials (RECPs). Good agreement is obtained with experimental spectroscopic data for the respective atomic levels, although there is a tendency to systematically underestimate the energies of the Ne atom by 1000-1500 cm(-1) because of differences in the correlation effects associated with its ground and Rydberg excited states. Potential curves are calculated for each of these states, and a number of relatively deep minima are found. The CI Omega-state wave functions are sufficiently diabatic until r = 4-5 a0 to allow for a clear identification of the He 1s-2s excited states. Electric dipole transition moments are computed between these states and the HeNe X 0+ ground state up to r = 4.0 a0, and it is found that the 2 (1)S0 - X maximum value is over an order of magnitude larger than that for the corresponding (3)S1 - X excitation process. 相似文献
12.
Ab initio calculations of both ClCH2OCH3 and ClCH2CH3 molecules and various ClCH2OCH3 structures with fixed angles of rotation of the methoxy group about the C−O bond were performed by the restricted Hartree-Fock
method in the valence-split 6–31 G* basis set with full optimization of the geometry. The populations of the valent p-orbitals of the chlorine atoms in these
molecules have been analyzed. The35Cl NQR frequencies and the asymmetry parameters of the electric field gradient (EFG) at the35Cl nuclei have been calculated. Good agreement with experimental NQR frequencies was obtained for the calculations where only
the populations of the less diffuse 3p-components of these orbitals were used. The35Cl NQR frequency in ClCH2OCH3 is lower than that in ClCH2CH3 due to the higher population of the less diffuse component of the pσ-orbital of the Cl atom in the former molecule.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 431–434, March, 1997. 相似文献
13.
Yuichi Yamamoto Takeshi Noro Kimio Ohno 《International journal of quantum chemistry》1992,42(5):1563-1575
Ab initio configuration interaction (CI ) calculations were carried out on low-lying singlet and triplet π–π* states and ionized states of free-base porphin. We take into account single and double excitations from σ and π electrons in the CI calculations. The composite natural orbitals were employed in order to reduce the size of orbital set to be used in the CI . The calculated excitation energies were in good agreement with experimental values. The use of split-valence-type basis and the inclusion of correlation effects of σ electrons were proved to be important to describe the low-lying π–π* states, especially the Soret band. 相似文献
14.
Kuznetsov AN Kloo L Lindsjö M Rosdahl J Stoll H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(13):2821-2828
Ab initio calculations on bismuth polycationic species of the types Bi(n(n-2))+, Bi(n(n-4))+, and Bi(n(n-6))+ (n = 3 - 12) were performed at the Hartree-Fock and density functional theory levels in order to investigate their general properties and the applicability of Wade's rules on bismuth polycations. Some exceptions to Wade's rules were encountered, and, moreover, several predicted and calculated minima show only meta-stable behavior. The bonding in bismuth polycations is characterized by a high degree of electron delocalization and "three-dimensional aromaticity". 相似文献
15.
The effect of the chirality of the amino acid at position i + 2 on a β-turn was investigated by a grid scan ab initio calculation on the Ac-
-Pro-
-Ala-NH2 and Ac-
-Pro-
-Ala-NH2 blocked dipeptides. Th6-31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the H-bond that closes the 10 membered ring which stabilizes the turn. The lowest energy of all 64 probed conformations of the
-Ala containing peptide corresponded to a good type II β-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Å. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the
-Ala-containing dipeptide for a type II β-turn, but does not seem to enhance the propensity of the
-Ala-containing dipeptide for a type I β-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated. 相似文献
16.
se Stg»rd 《Chemical physics letters》1976,40(3)
Ab initio calculations are presented for the molecules CICN and ONCI with optimization of all geometric parameters. Calculated equilibrium geometries for CICN are in good agreement with microwave data; however, the calculated N-Cl distance in ONCI is about 0.1 Å shorter than obtained by electron diffraction. Orbital energies are calculated by means of Koopmans' theorem and also by ΔSCF calculations. The importance of relaxation energy is shown by comparing the calculated orbital energies with experimental data from photoelectron spectra of the valence levels. 相似文献
17.
Jacques Breulet 《Journal of computational chemistry》1981,2(3):244-250
A set of programs has been developed to calculate molecular spin–orbit interaction with Gaussian-type wavefunctions in connection with the popular GAUSSIAN 76 program. The spin–orbit contributions to the fine structure of O2 (X3∑g?), NH (X3∑?), and CH2 (X3B1) are evaluated with the standard STO -3G and 6-31G basis sets; for NH the influence of bond functions added to the latter basis set is also investigated. The results are compared to values previously obtained with other types of basis sets. 相似文献
18.
19.
Vladimir Murashov 《Journal of Molecular Structure》2003,650(1-3):141-157
Silica surface sites, which can be formed in cleavage processes, and their hydrolyzed counterparts are investigated with ab initio cluster calculations. Natural Bond Orbital (NBO) theory is used to characterize bonding around silica surface sites. Higher energy lone pairs of electrons on oxygen atoms either hyperconjugate to vicinal silanol/siloxane antibonding orbitals or backdonate electron density via donor–acceptor π-type bonding with participation of pd or p hybrids on silicon atoms. Upon substitution of hydroxyl groups of orthosilicic acid with silica monomers the strength of siloxane and silanol Si–O bonding increases as energies of bonding orbitals and contributions from p-orbitals decrease. Silanone sites and a complementary pair of silyl/siloxy radical sites are found to be the most stable geminal and single non-hydrolyzed sites, respectively. Atomic charges based on natural wavefunctions and on fitting to electrostatic potential, and characteristic bands of IR spectra associated with siloxane and silanol stretching vibrations of silica surface sites are reported. 相似文献
20.
Lj. Paa-Toli L. Klasinc H. Spiegl J. V. Knop S. P. McGlynn 《International journal of quantum chemistry》1992,41(6):815-827
The electronic structure of 5α-androstane, the parent hydrocarbon of the hormonal steroids, has been computed by ab initio SCF methods in an STO -3G basis. The results are compared with existing MNDO computations and are used to discuss long-range electronic interactions between distant substituents that might be appended to rings A and D of 5α-androstane. It is thought that these interactions are mediated by the ribbonlike MO 'S of the parent molecule. 相似文献