首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 468 毫秒
1.
Reference completely ab initio 6–3G and nonempirical 3G/MODPOT (ab initio effective core model potential) LCAO -MO -SCF calculations (using the same valence atomic orbital basis) were performed for a series of boron hydrides (B4H10, B5H9, B5H11, and B6H10) and a test 3G/MODPOT + VRDDO (variable retention of diatomic differential overlap for charge conserving integral prescreening) calculation were also performed for B5H9, B6H10, and B10H14. The agreement between the ab initio 6–3G and the corresponding 3G/MODPOT calculations was excellent for valence orbital energies, gross atomic populations, and dipole moments. The results also compared favorably to previous ab initio minimum STO basis results of Lipscomb and coworkers. The 3G/MODPOT + VRDDO calculations verified that for such spatially compact molecules (such as boron hydrides, which are fragments of polyhedra), the VRDDO procedure does not result in a noticeable savings in computer time for molecules of the size and shape of B5H9 and B6H10, in contrast to the savings previously realized for organic molecules of comparable atomic size. However, the agreement in calculational results between the 3G/MODPOT and the 3G/MODPOT +VRDDO results was still as extremely close as it had been for the organic molecules. 3G/MODPOT calculations were also carried out for B8H12, B9H15, B10H14, B10H14?2, 1,2-C2B4H6, and 1,6-C2B4H6 and the results compared to the previous minimum STO basis results. For B10H14, the 3G/MODPOT +VRDDO method led to savings in computer time of 28% over the 3G/MODPOT method itself. The agreement of the 3G/MODPOT results with available experimental photoelectron spectral data for B5H9 and 1,6-C2B4H6 was as good as that of the previous ab initio minimum STO basis calculations.  相似文献   

2.
We have explored two areas of approximately rigorous calculations for computing nonempirical wave functions for heavy and/or large molecules orders of magnitude faster than with conventional ab-initio methods but with the same chemical accuracy. First, we have developed and used a series of programs (starting from our new fast sets of ab-initio Gaussian SCF and SCF -CI programs) incorporating ab-initio effective core model potentials (MOD -POT ) which allow one to treat only the valence electrons explicitly, plus a charge conserving integral prescreening, which cuts down significantly on the number of integrals that have to be calculated, stored, or processed for a large molecule. We have named this latter procedure VRDDO (variable retention of diatomic differential overlap). With these MODPOT and MODPOT /VRDDO methods we have explored a variety of small, medium, and large systems ranging from electron affinities of atoms through to molecules of biological interest and large boron hydrides. The results compared to ab-initio SCF or SCF /CI calcuations are very good, usually within 0.001 to 0.002 a.u. for orbital energies and gross atomic populations (GAPS ) and even better along potential energy curves. Secondly, we have explored the use of the MS -Xα method for less conventional molecules and properties than those for which it is customarily employed.  相似文献   

3.
Ab initio MODPOT /VRDDO calculations have been carried out on carcinogenic benzo(a)pyrene and its metabolites. The MODPOT /VRDDO method incorporates two very desirable options into our fast ab initio Gaussian programs: MODPOT —ab initio effective core model potentials—and a charge-conserving integral prescreening approximation which we named VRDDO (variable retention of diatomic differential overlap). For orbital energies and population analyses the MODPOT /VRDDO results agree to essentially three decimal places with completely ab initio calculations using the same valence atomic basis set. For this series of very closely related congeners a new MERGE technique was implemented that allows reuse of integrals of a common skeletal fragment. Since our program computes integrals efficiently by blocks, reusing information common to the block, it was more difficult to implement a MERGE technique than for integral programs which calculate the integrals one-byone. The MODPOT /VRDDO calculations were performed for benzo(a)pyrene (BP), BP oxides, BP dihydrodiols, and BP dihydrodiol epoxides. The metabolites investigated were BP-7,8-oxide, BP-4,5-oxide, BP-7,8-dihydrodiol [cis(e, a), cis(a, e), trans(e, e), and trans(a, a)], and BP-7,8-dihydrodiol-9,10-epoxide [β,β,β (the most stable), β,β,α; α,α,β, and α,α,α all derived from cis-BP-7,8-dihydrodiol and β,α,β; α,β,β and α,β,β derived from trans-BP-7,8-dihydrodiol]. Several different conformations were calculated for each of the BP dihydrodiols and BP dihydrodiol epoxides. Calculations were carried out for the opening of the C9—O—C10 epoxide ring both toward C9 and C10 for the, most stable β,β,β isomer of BP-7,8-dihydrodiol-9,10-epoxide. Opening the epoxide ring between C10 and O leads to a more stable intermediate than opening the epoxide ring between C9 and C10. However, there is no buildup of positive charge in C10 as has been postulated by some cancer researchers, but rather the C10 becomes slightly more negative. Nor is there a buildup of negative charge on the O atom. rather it becomes slightly less negative. As the epoxide ring is opened further than 90° for the O—C9—C10 or O—C10—C9 angles, there appears to be a possible mixing of configurations that is being investigated further.  相似文献   

4.
Ab initio MODPOT/VRDDO/MERGE calculations were carried out on carcinogenic 3-methylcholanthrene (3-MCA) and its metabolites. The results for 3-MCA were compared to our earlier similar calculations for carcinogenic benzo(a)pyrene (BP). Both compounds 3-MCA and BP are carcinogenic and are metabolically activated by similar mechanisms but in different positions. Both the calculated wave functions for 3-MCA and BP and the electrostatic molecular potential contour maps generated from these wave functions correctly reflect the similarity of mechanisms of metabolic activation and the differences in position. Our calculated results both for BP and for 3-MCA reflect accurately their experimentally observed behavior. Thus this combination of theoretical techniques can be used with confidence to describe the behavior of the polycyclic aromatic hydrocarbons (PAH's) and their metabolites. The ab initio MODPOT/VRDDO method incorporates two very desirable options into our fast ab initio Gaussian programs: MODPOT –ab initio effective core model potentials—and a charge-conserving integral prescreening approximation which we named VRDDO (variable retention of diatomic differential overlap). For orbital energies and population analysis the MODPOT/VRDDO results agree to essentially three decimal places with completely ab initio calculations using the same valence atomic basis set. For this series of very closely related congeners our recent MERGE technique which allows reuse of integrals from a common skeletal fragment was used. The ab initio MODPOT/VRDDO/MERGE calculations were carried out for 3-MCA, 3-MCA oxides, 3-MCA dihydrodiols, and 3-MCA dihydrodiolepoxides. The metabolites investigated were 3-MCA 9,10-oxide; 3-MCA 7,8-oxide; 3-MCA 9,10-dihydrodiol [trans(axial, axial); trans(equatorial, equatorial); cis(axial, equatorial); cis(equatorial, axial)]; and 3-MCA 9,10-dihydrodiol–7,8-epoxide [for both conformations A and B of the dihydrodiol and for all stereoisomers of the dihydrodiolepoxides relative to below and above the plane: ααα, and ααβ αβα αββ βαα βαβ ββα and βββ (most stable)]. Calculations were also carried out for opening of the C7? O? C8 epoxide ring both towards C7 and C8 for the most stable isomer Aβββ (above the ring). Opening the epoxide ring between C7 and O leads to a more stable intermediate than opening the epoxide ring between C8 and O. Again, however, as with opening the epoxide ring in BP 7,8-dihydrodiol–9,10-epoxide there is no buildup of positive charge on C7 in the 3-MCA metabolites as postulated by some cancer researchers, but rather the C7 becomes slightly more negative. Nor is there a buildup of negative charge on the O atom, but rather it becomes slightly more positive. As the epoxide ring is opened further than 90° for the O? C7? C8 or O? C8? C7 angles, there appears to be a possible mixing of configurations that is being investigated further.  相似文献   

5.
Quantum chemical ab initio MODPOT /VRDDO calculations have been carried out on the following aminonitrobenzenes for which crystal structures had been determined experimentally: 4-nitroaniline; N,N-dimethyl-p-nitroaniline; 2,4,6-trinitroaniline; 1,3-diamino-2,4,6-trinitrobenzene (DATB—Form I); 1,3,5-triamino-2,4,6-trinitrobenzene (TATB); 2,3,4,6-tetranitroaniline; N-methyl-N,2,4,6-tetranitroaniline (Tetryl); and N-(β,β,β-trifluoroethyl)-N,2,4,6-tetranitroaniline. These quantum chemical calculations were performed on the molecules in their conformations as found in their crystal structures. The calculations were carried out with our own ab initio programs which also incorporate as options several desirable features for calculations on large molecules: ab initio effective core model potentials (MODPOT) which enable calculations of valence electrons only explicitly, yet accurately, and a charge conserving integral prescreening evaluation (which we named VRDDO-variable retention of diatomic differential overlap) especially effective for spatially extended molecules. Aminonitrobenzenes are especially interesting since there are inherent intramolecular ring distortions and deviations from planarity and intramolecular hydrogen bonds as well as intermolecular hydrogen bonds causing further deviations from planarity. The theoretical indices resulting from the quantum chemical calculations are relevant to a number of properties and behavioral characteristics of these molecules, both intramolecular and intermolecular. The charges on the atoms [from the gross atomic populations (GAP 's)] are needed for calculation of the atomic multipole–atomic multipole electrostatic contributions (a dominant factor) to the intermolecular interaction energies. These electrostatic interaction energies are part of the input necessary for calculations on the crystal packing and densities of these molecules. These GAP 's are also of value in interpreting the experimental photoelectron and ESCA spectra of these molecules. The total overlap populations (TOP 's) between atoms are related to the inherent bond strengths and can serve as a quantitative replacement for the old empirical bond length-bond order-bond energy relationship still used by explosives chemists to identify the “target bonds” (the weakest bonds). The TOP 's are of considerable value in predicting and tracing initiation and subsequent steps of explosive phenomena. The molecular orbital energies of the lowest unoccupied orbitals are of interest since nitroexplosives have been implicated in testicular toxicity and the initial metabolic activation appears to proceed through a one-electron reduction of the nitroexplosive.  相似文献   

6.
Ab initio MODPOT /VRDDO /MERGE calculations have been carried out for all the different position isomers of nitrocubane from mononitrocubane through octanitrocubane for a perfect symmetrical cubic cubane skeleton and for mononitrocubane through septanitrocubane for the almost cubic experimentally determined cubane skeleton. These calculations were carried out with our own rapid efficient ab initio programs which also incorporate a number of desirable computational strategies for calculations on large molecules. The skeletal total overlap population of the cubane skeleton (a theoretical index we showed years ago to be sensitive and predictive of stability of energetic molecular frameworks) indicates that successive nitration seems to increase the stability of the cubane skeleton. Successive nitration also seems to increase the total overlap population of the C? NO2 bond. There are subtle differences depending on the exact positional isomer for a constant number of nitro groups—but the overall trend is definite. We have also generated electrostatic molecular potential contour (EMPC ) maps around these nitrocubanes. These maps are indicative of preferred positions of electrophilic and nucleophilic attack as a function of the number of nitro groups or their positions. These EMPC maps can also indicate, to a first approximation, a limit on how close these molecules may be able to approach each other in a crystal.  相似文献   

7.
The transport of C6H5O? (or similarly charged moieties) through a lipoidal membrane may possibly be facilitated by forming complexes with the neutral compound. Thus, theoretical studies were performed on the model [C6H5OH ?OC6H5]? molecular complex to obtain some information concerning the possible molecular and electronic structure of such complexes. Ab initio MODPOT /VRDDO SCF calculations were carried out on the neutral-anion dimer [C6H5OH ?OC6H5] to optimize the equilibrium geometry. Electrostatic molecular potential contour maps have been generated from the ab initio MODPOT /VRDDO results in the molecular plane and in the plane perpendicular to the molecular plane and intersecting the hydrogen bond O ?H? O. Difference maps have also been generated showing the change of potential on complex formation. There is a decrease of electrostatic interactions of the phenoxide anion upon complex formation with the neutral phenol. Counterpoise corrections for basis set size could not be made since calculation of the phenoxide anions in the basis set of the phenol plus the phenoxide anion led to an excited state for the phenoxide anion. This behavior is somewhat similar to that occurring in the stabilization method for excited states of negative ions as the size of the basis set is increased.  相似文献   

8.
The electrostatic molecular potential contour maps were calculated for carcinogenic 3-methylcholanthrene (3-MCA) and a number of its metabolites {3-MCA 7,8-oxide and 3-MCA 9,10-oxide; 3-MCA 7,8-dihydrodiols[several stereoisomers: A trans(equatorial, equatorial) and A cis(equatorial, axial)]; 3-MCA 9,10-dihydrodiol–7,8-epoxide A βββ and 3-MCA 9,10-dihydrodiol–7,8-epoxide A αβα}. The maps were generated from our ab initio MODPOT/VRDDO/MERGE wave functions calculated for these species. The results of these maps for 3-MCA [similarly to our results for the maps we generated for benzo(a)pyrene (BP)] show that these electrostatic molecular potential contour maps can be used to indicate favored positions of attack for electrophilic species, such as “electrophilic” oxygen to form an epoxide as well as for positive ion attack. The 3-MCA maps indicate the favored site for attack and the pathways. The maps around 3-MCA 9,10-oxide and around 3-MCA 9,10-dihydrodiol-7,8-epoxide indicate the directional preferences for proton assisted epoxide ring opening. The maps around the 3-MCA dihydrodiols indicate that while for certain stereoisomers the “electrophilic” oxygen will prefer to attack from below, for other isomers it will prefer to attack from above. This gives great insight into the stereochemical preference for formation of different 3-MCA 9,10-dihydrodiol–7,8-epoxides.  相似文献   

9.
A new method is proposed for the evaluation of numerical similarity measures for large molecules, defined in terms of their electron density (ED) distributions. The technique is based on the Molecular Electron Density Lego Assembler (MEDLA) approach, proposed earlier for the generation of ab initio quality electron densities for proteins and other macromolecules. The reliability of the approach is tested using a family of 13 substituted aromatic systems for which both standard ab initio electron density computations and the MEDLA technique are applicable. These tests also provide additional examples for evaluating the accuracy of the MEDLA technique. Electron densities for a series of 13 substituted benzenes were calculated using the standard ab initio method with STO-3G, 3-21G, and 6-31G** basis sets as well as the MEDLA approach with a 6-31G** database of electron density fragments. For each type of calculation, pairwise similarity measures of these compounds were calculated using a point-by-point numerical comparison of the EDs. From these results, 2D similarity maps were constructed, serving as an aid for quick visual comparisons for the entire molecular family. The MEDLA approach is shown to give virtually equivalent numerical similarity measures and similarity maps as the standard ab initio method using a 6-31G** basis set. By contrast, significant differences are found between the standard ab initio 6-31G** results and the standard ab initio results obtained with smaller STO-3G and 3-21G basis sets. These tests indicate that the MEDLA-based similarity measures faithfully mimic the actual, standard ab initio 6-31G** similarity measures, suggesting the MEDLA method as a reliable technique to assess the shape similarities of proteins and other macromolecules. The speed of the MEDLA computations allows rapid, pairwise comparisons of the actual EDs for a series of molecules, requiring no more computer time than other simplified, less detailed representations of molecular shape. The MEDLA method also reduces the need to store large volumes of numerical density data on disk, as these densities can be quickly recomputed when needed. For these reasons, the proposed MEDLA similarity analysis technique is likely to become a useful tool in computational drug design. © 1995 John Wiley & Sons, Inc.  相似文献   

10.
The focal point of our discussion is the examination of truncated basis sets used in obtaining an accurate first principles clculation of the effective valence shell Hamiltonian by the canonical transformation-cluster expansion approasch. Subsequent diagonalization of this effecitve valence shell hamiltonian yields the valence shell transition energies. A detailed analysis of numerical results obtained using a number of different basis sets of hydrogen-like orbitals together with rigorous symmetry arguments celarly demonstrates the special role played by d orbitals in computing the 3P1D transition energy in carbon. The failure of early attempts to calculate the effective Hamiltonian for ethylene from first principles is examined in the light of recent ab initio calculations on ethylene involving d orbitals and the computations reported in this paper. We conclude that accurate calculations of the effective valence shell Hamiltonian for molecules must consider d orbitals in the excited orbital basis set.  相似文献   

11.
The ab initio valence bond method has been used to study the ground and the lowest vertical valence ionized states of pyridine. On the basis of our calculations the first two ionization potentials are assigned to π and n electron removals, respectively. The final wave functions have been interpreted in terms of valence bond structures by means of an appropriate population analysis.  相似文献   

12.
To promote accuracy of the atom‐bond electronegativity equalization method (ABEEMσπ) fluctuating charge polarizable force fields, and extend it to include all transition metal atoms, a new parameter, the reference charge is set up in the expression of the total energy potential function. We select over 700 model molecules most of which model metalloprotein molecules that come from Protein Data Bank. We set reference charges for different apparent valence states of transition metals and calibrate the parameters of reference charges, valence state electronegativities, and valence state hardnesses for ABEEMσπ through linear regression and least square method. These parameters can be used to calculate charge distributions of metalloproteins containing transition metal atoms (Sc‐Zn, Y‐Cd, and Lu‐Hg). Compared the results of ABEEMσπ charge distributions with those obtained by ab initio method, the quite good linear correlations of the two kinds of charge distributions are shown. The reason why the STO‐3G basis set in Mulliken population analysis for the parameter calibration is specially explained in detail. Furthermore, ABEEMσπ method can also quickly and quite accurately calculate dipole moments of molecules. Molecular dynamics optimizations of five metalloproteins as the examples show that their structures obtained by ABEEMσπ fluctuating charge polarizable force field are very close to the structures optimized by the ab initio MP2/6–311G method. This means that the ABEEMσπ/MM can now be applied to molecular dynamics simulations of systems that contain metalloproteins with good accuracy. © 2014 Wiley Periodicals, Inc.  相似文献   

13.
14.
A modified scheme of a previously developed concept of atomic valence numbers in molecules is presented. The relation to population analysis is demonstrated. The scheme is suitable for ab initio wave functions with extended basis sets. The procedure involved a combination of symmetric orthogonalization and contraction of the basis set. The method is used for a systematic investigation of a series of compounds with first-row atoms from Li to F. The predicted atomic charges and valence numbers are in line with results from infrared spectroscopy and multipole moment analysis.  相似文献   

15.
A theoretical study of homocyclic sulfur species S6, S7, and S8 was carried out using a molecular valence method involving stepwise approximations for orthogonality and core-valence interactions. The valence shell orbitals are described at the minimal basis level. The geometries of the molecules are predicted well as compared with other theoretical studies and the experimental values. The slight overestimation of the SS bond length is typical to the nonpolarized basis sets. The energies of the valence orbitals are well in accord with the conventional all-electron ab initio results. The trend in the stabilities of the three molecules is discussed. The present method provides an attractive possibility to study homocyclic and heterocyclic systems involving heavier chalcogens with no increase of the computing time.  相似文献   

16.
The initiation step in the cationic polymerization of cyclic ethers is influenced by basicity and ring strain. We carried out ab-initio MODPOT/VRDDO/MERGE calculations on a variety of substituted oxetanes and generated electrostatic molecular potential contour (EMPC) maps in three-dimensions around the molecules. The size of the negative EMPC map region around the oxygen enabled us to predict the propensity to polymerize prior to the syntheses of the actual monomers themselves. We carried out ab-initio MODPOT/VRDDO/MERGE MRD-CI calculations for the propagation step of oxetane reacting with protonated oxetanes to cause ring opening of protonated oxetane. Similar MRD-CI calculations on variously substituted oxetanes will shed insight into relative copolymerization preferences.  相似文献   

17.
Ab initio valence bond method is employed to quantitatively study the concepts of ionic resonance energy and ionicity of a chemical bond in the cases of hydrides XH(X=Li,Be,B,C,N,O,F) and fluorides XF(X=Li,Be,B),By establishing the relationship between resonance and stability,and comparing the calculated ionicities with Pauling‘s earlier estimations in the above diatomic molecules,the merits of Pauling‘s classical resonance theory were demonstrated at the ab initio level.  相似文献   

18.
The ultraviolet photoelectron spectrum of the N-chlorothionylimide molecule (Cl-NSO) has been obtained and analyzed. The spectrum is interpreted by comparison with the known spectra of the parent compounds H-NSO, SO2 and the related molecules Cl-NCO and H-NCO as well as with the aid ofab initio SCF-calculations in terms of ionization from the eight highest filled valence molecular orbitals in this molecule.
  相似文献   

19.
A new approach for the calculation of electrostatic potential derived atomic charges is presented. Based on molecular orbital calculations in the PRDDO/M approximation, the new parametrized electrostatic potential (PESP) method is parametrized against ab initio MP2/6-31G** calculations. For a data set of 820 atoms in 145 molecules containing H, C, N. O, F, P, S, Cl, and Br (including hypervalent species), the PESP method achieves a mean absolute error of 0.037 e with a correlation coefficient of 0.990. Unlike other approximate approaches, no scaling factor is required to improve the agreement between PESP charges and the underlying ab initio results. PESP calculations are an order of magnitude faster than the simplest ab initio calculation (STO-3G) on large molecules while achieving a level of accuracy that rivals much more elaborate ab initio methods. © 1997 by John Wiley & Sons, Inc. J Comput Chem 18: 955–969, 1997  相似文献   

20.
Compact contracted Gaussian basis sets introduced in the preceding article are tested for ab initio molecular calculations on molecules containing third-row atoms (Na through Cl). It is found that the effect of splitting valence orbitals is essential for these molecules and addition of polarization functions to split basis sets can yield computed geometries, spectroscopic constants, and atomization energies in close agreement with the result of near Hartree–Fock calculations.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号