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81.
Janet E. Del Bene 《Journal of computational chemistry》1981,2(3):251-260
Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH3, NH2, OH, F, and C2H3 and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C2H3, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine. 相似文献
82.
83.
Janet E. Del Bene Guy T. Worth Francis T. Marchese Michael E. Conrad 《Theoretical chemistry accounts》1975,36(3):195-206
Ab initio SCF calculations with a minimal STO-3G basis set have been performed to determine the equilibrium geometries of two series of carbonyl compounds, RCHO and R2 CO. For the mono-substituted compounds, R may be CH3, NH2, OH, F, CHO, and C2 H3. In the disubstituted compounds, R has been restricted to the isoelectronic saturated groups. The computed equilibrium geometries of these compounds are in satisfactory agreement with the experimentally-determined geometries, with an average difference of 0.021 Å between computed and experimental bond lengths, and 1.9 ° in corresponding bond angles. An analysis of the effect of the substituent on the electronic structure of the carbonyl group has also been made. The saturated groups are found to be electron-withdrawing groups relative to H, with the electron withdrawing ability increasing in the order CH3 2 2H2 are also electron withdrawing relative to H, and comparable to CH3 in acetaldehyde. Vertical ionization potentials andn* transition energies have also been calculated for these molecules at their optimized geometries, experimental geometries, and geometries given by a standard model. The effect of changes in molecular geometry on these computed properties has been analyzed. 相似文献
84.
M. Benešová-Pacltová M. Kyrš J. Rais 《Journal of Radioanalytical and Nuclear Chemistry》1974,21(1):107-118
A study has been made of the extraction of microamounts of caesium from the aqueous phase into 28 organic solvents in the
presence of sodium dipicrylaminate and nitrate, and the extraction of macroamounts of ceasium and rubidium dipicrylaminates
from nitrobenzene and nitromethane into some other organic solvents. The equilibrium constants of the reaction Cs
a
+
+Na
O
+
=Cs
O
+
+Na
a
+
were determined and a correlation with the water content of the organic phase has been discussed. 相似文献
85.
86.
Kalenda Petr Koudelka Ladislav Mošner Petr Beneš Ludvík Černošek Zdeněk 《Journal of Thermal Analysis and Calorimetry》2018,131(3):2303-2310
Journal of Thermal Analysis and Calorimetry - The thermal behavior and crystallization of barium molybdate-phosphate glasses were studied in two compositional series, namely A:... 相似文献
87.
T. Tomiyama H. Suetomi W. Skorecki A. J. J. van de Velde J. Grant K. W. Brown P. A. Villaruz E. L. Breazeale R. A. Greene P. Hamer H. E. Evans L. Blanquet P. P. Tully N. M. Carter L. A. Kulski G. M. Mitilino M. P. Babkin J. C. Harral T v. Fellenberg W. Ohle R. E. Larson V. Bene E. Nolte H. J. Bandt G. Gad D. M. Taylor L. Urbányi und Käte Naumann 《Fresenius' Journal of Analytical Chemistry》1939,118(1-2):41-48
Ohne Zusammenfassung 相似文献
88.
High-level ab initio quantum chemical calculations (G4(MP2)//MP2/6-311+G(2df,p)) have been used to examine homodimers of second-row
bases, and to compare the results with those obtained previously for the first-row analogs. The relationship between the binding
energies of the dimers and the proton affinities (PAs) of the bases follows the same pattern as that for the first-row systems,
with the binding energies initially increasing with increasing proton affinity but subsequently decreasing. This may be attributed
to the opposing effects of increased PA on the hydrogen-bond donor and hydrogen-bond acceptor. The binding energies are generally
smaller for the second-row dimers than for the corresponding first-row dimers. There is an increased tendency for asymmetrical
hydrogen bonds in homodimers of the second-row compared with first-row dimers. This may be attributed to the lower electronegativities
of second-row atoms relative to their first-row counterparts, and to the longer internuclear separation between the hydrogen-bonded
second-row atoms. 相似文献
89.
Prof. Ibon Alkorta Prof. José Elguero Prof. Janet E. Del Bene Prof. Otilia Mó Prof. Manuel Yáñez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(39):11897-11905
Understanding the bonding in complexes X:BH3?nFn and X:BH3?nCln, for X=N2, HCN, LiCN, H2CNH, NF3, NH3 with n=0–3, is a challenging task. The trends in calculated binding energies cannot be explained in terms of any of the usual indexes, including π donation from the halogen lone pairs to the p(π) empty orbital on B, deformation energies, charge capacities, or LUMO energies, which are normally invoked to explain the higher Lewis acidity of BCl3 relative to BF3. The results of the high‐level G3B3 ab initio calculations reported in this study suggest that the interaction energies of these complexes are determined by a combination of at least three factors. These include the decrease in the electron‐accepting ability of B as a result of π donation by the halogen atom, the increase in the electron‐acceptor capacity of B due to deformation of the acid, and the large increase in the deformation energy of the acid with increasing halogen substitution. The dominant effects are those derived from the electronic effects of acid deformation. Deformation not only has direct energetic consequences, which are reflected in the large differences between dissociation (D0) and interaction (Eint) energies, but also leads to an enhancement of the intrinsic acidities of BH3?nFn and BH3?nCln moieties by lowering the LUMO energies to very different extents, consistent with the frontier orbital model of chemical reactivity. Although this lowering depends on both the number and the nature of the halogen substituents, binding energies do not systematically increase or decrease as the number of halogen atoms increases. 相似文献
90.
We report the first complete leading-twist next-to-leading-order perturbative-QCD predictions for the two-photon exclusive channels gammagamma-->M(+)M(-) (M=pi,K) at large momentum transfer. The asymptotic distribution amplitude is utilized as a candidate form for the nonperturbative dynamical input. Comparison of the obtained results with the existing experimental data does not provide sufficiently clear evidence to support the applicability of the hard-scattering approach at currently accessible energies. 相似文献