| 1. |
- BRINCK, T, et al.
(författare)
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A COMPUTATIONAL ANALYSIS OF THE BONDING IN BORON-TRIFLUORIDE AND BORON-TRICHLORIDE AND THEIR COMPLEXES WITH AMMONIA
- 1993
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 32:12, s. 2622-2625
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Tidskriftsartikel (refereegranskat)abstract
- Complexation energies for the interactions of BF3 and BCl3 with NH3 have been calculated at the ab initio Hartree-Fock and MP2 levels of theory, using large polarized basis sets. The formation of H3N.BCl3 is found to be favored by 4.27 kcal/mol over H3N.BF3 at the MP2 level. This is in agreement with the experimental observation that the Lewis acidities of the boron trihalides increase in the order BF3 < BCl3 < BBr3. Calculated atomic charges and molecular electrostatic potentials show the boron to be much more positive in BF3 than in BCl3, as would be expected from the respective electronegativities of fluorine and chlorine. These results and the relevant ppi-ppi overlap integrals do not support using the concept of back-bonding and consequent stabilization to explain the trend in Lewis acidities. As an alternative explanation, it is suggested that this trend reflects the importance of Lewis base --> BX3 charge transfer in these complexes and the fact that the ability to accept the charge, as indicated by charge capacities, increases in the order BF3 < BCl3 < BBr3.
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| 2. |
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| 3. |
- BRINCK, T, et al.
(författare)
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POLARIZABILITY AND VOLUME
- 1993
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Ingår i: Journal of Chemical Physics. - : AMER INST PHYSICS. - 0021-9606 .- 1089-7690. ; 98:5, s. 4305-4306
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Tidskriftsartikel (refereegranskat)
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| 4. |
- BRINCK, T, et al.
(författare)
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QUANTITATIVE-DETERMINATION OF THE TOTAL LOCAL POLARITY (CHARGE SEPARATION) IN MOLECULES
- 1992
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Ingår i: Molecular Physics. - : TAYLOR & FRANCIS LTD. - 0026-8976 .- 1362-3028. ; 76:3, s. 609-617
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a polarity index-PI that is proposed as a measure of the charge separation in a molecule. PI can be viewed as the average deviation of the electrostatic potential on the surface of the molecule. It permits a quantitative assessment of total local polarity, even in a molecule that has zero dipole moment. In support of our formulation of PI, we show that it is related to an empirical polarity-polarizability parameter and also to the dielectric constant.
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| 5. |
- BRINCK, T, et al.
(författare)
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RELATIONSHIPS BETWEEN THE AQUEOUS ACIDITIES OF SOME CARBON, OXYGEN, AND NITROGEN ACIDS AND THE CALCULATED SURFACE LOCAL IONIZATION ENERGIES OF THEIR CONJUGATE BASES
- 1991
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Ingår i: Journal of Organic Chemistry. - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : AMER CHEMICAL SOC. - 0022-3263 .- 1520-6904. ; 56:17, s. 5012-5015
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Tidskriftsartikel (refereegranskat)abstract
- Average local ionization energies (IBAR(r)) have been computed on the molecular surfaces of the conjugate bases for four different groups of carbon and oxygen acids using an ab initio self-consistent-field molecular orbital approach. The lowest surface IBAR(r) (IBAR(S,min)) are generally found on the atom from which the proton has been abstracted. Good linear relationships between aqueous acidities and IBAR(S,min) are found for the different groups. A single linear relationship between pK(a) and IBAR(S,min) that includes the four groups and three additional nitrogen acids also exists; the correlation coefficient is 0.97. This provides a means for predicting the pK(a)'s of a large variety of carbon, oxygen, and nitrogen acids.
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| 6. |
- HAEBERLEIN, M, et al.
(författare)
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CALCULATED ELECTROSTATIC POTENTIALS AND LOCAL SURFACE-IONIZATION ENERGIES OF PARA-SUBSTITUTED ANILINES AS MEASURES OF SUBSTITUENT EFFECTS
- 1992
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Ingår i: Canadian journal of chemistry (Print). - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : NATL RESEARCH COUNCIL CANADA. - 0008-4042 .- 1480-3291. ; 70:8, s. 2209-2214
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Tidskriftsartikel (refereegranskat)abstract
- Using an ab initio self-consistent-field molecular orbital approach, we computed 3-21G//STO-3G* and STO-5G//STO-3G* electrostatic potentials and average local ionization energies for 17 para-substituted anilines. Our results demonstrate that the most negative potentials (V(min)) and the local surface ionization energy minima (I(s,min)BAR) associated with the amine nitrogen lone pairs are highly sensitive indicators of the electron-donating and electron-attracting tendencies of the para substituents. We find excellent linear relationships between the 3-21G//STO-3G* amine nitrogen V(min) and I(S,min)BAR and the sigma(p)0 Hammett constants of the substituent X; the correlation coefficients are 0.99. Correlations of slightly lesser quality are shown to exist between V(min), I(S,min)BAR, and sigma(p)-, sigma(p), and pK(a). Estimates of previously unknown sigma(p)0 and pK(a) values are given. The presence of ring carbon I(S,min)BAR meta to the substituent X also provides a predictive capability for determining sigma(m) values.
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| 7. |
- MURRAY, JS, et al.
(författare)
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APPLICATIONS OF CALCULATED LOCAL SURFACE-IONIZATION ENERGIES TO CHEMICAL-REACTIVITY
- 1992
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Ingår i: Journal of Molecular Structure. - UNIV NEW ORLEANS, DEPT CHEM, NEW ORLEANS, LA 70148 USA. : ELSEVIER SCIENCE BV. - 0166-1280. ; 87, s. 271-281
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Tidskriftsartikel (refereegranskat)abstract
- A recently introduced property, the average local ionization energy IBAR(r), is discussed in relation to chemical reactivity. IBAR(r) is defined within the framework of self-consistent-field molecular-orbital (SCF-MO) theory and is interpreted as the average energy required to remove an electron from any point r in the space of a molecule. A survey of applications of ab initio SCF IBAR(r) computed on molecular surfaces defined by the 0.002 e bohr-3 contour of the electronic density is presented. The positions at which the surface IBAR(r) has its lowest values (IBAR(S,min)) are indicative of sites most reactive toward electrophiles. An excellent correlation was found between the ring carbon IBAR(S,min) of monosubstituted benzenes and the Hammett constants. for a large variety of carbon, oxygen and nitrogen acids, we have shown that good correlations exist between pK(a) and the conjugate base IBAR(S,min). Finally, a good relationship is demonstrated to exist between the conjugate base IBAR(S,min) of substituted acetic acids and the inductive substituent constants sigma(I) (which were derived from the pK(a) values). IBAR(S,min) found near the C-C bond midpoints of the three-membered ring portions of saturated strained hydrocarbons are interpreted as reflecting the sigma aromaticity of these rings.
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| 8. |
- MURRAY, JS, et al.
(författare)
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CORRELATIONS BETWEEN MOLECULAR ELECTROSTATIC POTENTIALS AND SOME EXPERIMENTALLY-BASED INDEXES OF REACTIVITY
- 1992
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Ingår i: Journal of Molecular Structure. - UNIV NEW ORLEANS, DEPT CHEM, NEW ORLEANS, LA 70148 USA. : ELSEVIER SCIENCE BV. - 0166-1280. ; 88, s. 29-45
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Tidskriftsartikel (refereegranskat)abstract
- The key role that electrostatics plays in molecular reactive behavior is demonstrated in this work, which surveys and further explores correlations that we found between the molecular electrostatic potential V(r) calculated by an ab initio SCF-MO approach (a gas-phase property) and experimentally-based indices of reactivity (derived from solution studies). In our relationships involving negative V(r), we find in all cases that spatial minima (V(min)) provide correlations of higher quality than surface minima (V(S,min)). Relationships between V(min) and the hydrogen-bond-acceptor parameter-beta, and V(S,max) and the hydrogen-bond-donor parameter-alpha, confirm the physical validity of the empirically-derived solvatochromic parameters-alpha and beta. Correlations between the V(min) of NH2-X molecules and the substituent constants sigma(I), and sigma(I) + sigma(R) (when sigma(R) > 0) for the substituents X show how electrostatic properties reflect the electron-attracting tendencies of substituent groups. Whereas good relationships exist between the pK(a) values of some limited groups of molecules and their conjugate base V(min), we find that the average local ionization energy IBAR-(r) is better suited as a general measure of aqueous acidity.
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| 9. |
- MURRAY, JS, et al.
(författare)
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Electrostatic potentials on the molecular-surfaces of cyclic ureides
- 1991
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 95:2, s. 844-848
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio SCF-MO electrostatic potentials have been computed at the STO-5G/STO-3G level on the molecular surfaces of a group of cyclic ureides, in order to assess their relative reactivities toward nucleophiles, as in hydrolysis. The surfaces were defined by the 0.002 electron/bohr3 contour of the molecular electronic density. The relative hydrolytic stabilities within a series of NO2 and NF2 derivatives are predicted, on the basis of the magnitudes of the potentials above the acyl carbons. The surface electrostatic potentials of the polycarbonyl systems parabanic acid and alloxan are shown to be fully consistent with unusually short intermolecular distances that have been observed in crystallographic studies of these compounds.
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| 10. |
- MURRAY, JS, et al.
(författare)
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RELATIONSHIPS BETWEEN COMPUTED MOLECULAR-PROPERTIES AND SOLUTE SOLVENT INTERACTIONS IN SUPERCRITICAL SOLUTIONS
- 1993
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY. - UNIV NEW ORLEANS,DEPT CHEM,NEW ORLEANS,LA 70148. : AMER CHEMICAL SOC. - 0022-3654 .- 1541-5740. ; 97:19, s. 5144-5148
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Tidskriftsartikel (refereegranskat)abstract
- We focus specifically on solute-solvent interactions in supercritical solutions, as reflected in the enhancement factor E = y2P/P2sat, where Y2 and P2sat are the solubility and vapor pressure of the solute and P is the pressure of the system. For a series of 12 organic solutes in supercritical CO2, We find that E can be represented well in terms of three key solute molecular properties: (a) surface area, (b) the sum of the variances of the positive and negative electrostatic potentials on the surface, and (c) a ''balance' parameter which indicates the degree to which the solute molecule can interact to a similar extent through both its positive and negative regions. These three solute properties were computed at an ab initio self-consistent field minimum basis set level. On the basis of limited data, the same type of relationship appears to be applicable for a given solute-cosolvent combination. When various solvents or cosolvents are used, the enhancement factor of a weakly polar solute correlates well with the solvent/cosolvent molecular sizes. For polar solutes, however, an explicit dependence upon electrostatic interaction tendencies must be included.
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