| 1. |
- Al-Abdalla, A, et al.
(författare)
-
Ab initio model potential embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6
- 1998
-
Ingår i: Journal of Chemical Physics. - : Springer Science and Business Media LLC. - 0021-9606 .- 1089-7690. ; 108, s. 2005-2014
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper we present the results of an ab initio model potential (AIMP) embedded-cluster study of the ground and lowest excited states of Cr3+ defects in the elpasolites Cs2NaYCl6 and Cs2NaYBr6; complete active space SCF (CASSCF) and averaged coupled-pair functional (ACPF) calculations are performed on CrCl63- and CrBr63- clusters embedded in ab initio model potential representations of the surrounding lattices Cs2NaYCl6 and Cs2NaYBr6. The experimental structural data are revisited and some new results are found which differ significantly from those available in the literature. The calculated local structure parameters and electronic transition energies which can be compared to experiments are found to be very good; new structural and spectroscopic results are produced which have been neither measured nor calculated, which are complementary to the available ones, and whose quality is expected to be high as well. In particular, the question of the competition of the excited-state absorptions with the potential vibronic laser emission has been adressed: A considerable overlap between the broad E-2(g) –> (2)A(1g) excited-state absorption and (4)A(2g) <– T-4(2g) emission bands is predicted in both materials, which must result in a reduction in the emission efficiency. Finally, it is shown that the quantum mechanical embedding effects due to the fact that the external Cs+, Na+, Y3+, Cl-, and Br- ions are not point charges, are non-negligible; lacking of these effects must be one of the reasons which make previous Density Functional Theory calculations show significantly larger discrepancies with the available experiments.
|
|
| 2. |
- Jonsson, D., et al.
(författare)
-
Magnetic hyperpolarizabilities in a cubic response formulation
- 1996
-
Ingår i: Theoretical Chemistry accounts. - 1432-881X .- 1432-2234. ; 93:4, s. 235-241
-
Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the applicability of a recently derived cubic response function formalism for performing fully analytical calculations of hypermagnetizabilities, their anisotropy and their dispersion. The calculations involve the noble gas atoms, He, Ne, and Ar, and the corresponding isoelectronic halogen anions and alkali cations.
|
|
| 3. |
- Lindh, Roland, et al.
(författare)
-
On the significance of the trigger reaction in the action of the calicheamicin γI 1 anti-cancer drug
- 1997
-
Ingår i: Theoretical Chemistry Accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 97:1, s. 203-210
-
Tidskriftsartikel (refereegranskat)abstract
- The significance of the so-called trigger reaction in the reaction mechanism of the calicheamicin γI 1 anti-cancer drug has been studied with ab initio quantum chemical methods. The structures of four fragments of calicheamicin γI 1, consisting of either 39 or 41 atoms, have been fully optimized using the Becke-Perdew86 density functional method and the 6-31G* basis sets. The four structures constitute members of an isodesmic reaction for which the reaction energy is a direct measure of the change in activation energy of the Bergman reaction, caused by the structural rearrangements of the preceding trigger reaction. This difference in activation energy has been calculated with density functional theory, using the exchange-correlation functional mentioned above, and with second-order Møller-Plesset perturbation theory (MP2), employing an ANO-type basis set. In both cases a value of 12 kcal/ mol is obtained. The study firmly supports the hypothesis that the significance of the trigger reaction is to saturate a double bond in the vicinity of the enediyne group, which counteracts the formation of the biradical state of the drug. The MP2 computations became feasible by a novel implementation of an integral-direct, distributed-data, parallel MP2 algorithm.
|
|
| 4. |
- Salhi-Benachenhou, Nessima, et al.
(författare)
-
Formation of 2-hexene by cationic dimerization of propene : an ab initio and density functional theory study
- 1997
-
Ingår i: Theoretical Chemistry accounts. - : Springer Science and Business Media LLC. - 1432-881X .- 1432-2234. ; 97:1-4, s. 277-282
-
Tidskriftsartikel (refereegranskat)abstract
- The formation of a 2-hexene radical cation from a propene radical cation and a neutral propene molecule is investigated by means of ab initio UHF and spin projected MP2 calculations, as well as the SVWN and B3LYP levels of density functional theory. A stable addition complex, with loose CC bonds, is found. To proceed from the addition complex to the product. a locally planar transition state must be passed, with a migrating hydrogen located half-way between the donating and the accepting carbon atoms. At the highest computational levels considered, PMP2/6-31G(d,p)//MP2/3-21G and B3LYP/6-31G(d,p), this transition state lies approximately 11 and 13 kcal/mol, respectively above the addition complex. The high barrier is believed to be one reason why radical cation oligomerization of propene has not been detected experimentally, in contrast to the case of ethene, where the corresponding barrier is only a few kcal/mol.
|
|
| 5. |
- Snis, Anders, et al.
(författare)
-
Effective dynamic correlation in multiconfigurational wave-function calculations on atoms and molecules
- 1997
-
Ingår i: Theoretical Chemistry Accounts. - 1432-881X .- 1432-2234. ; 97:1-4, s. 232-239
-
Tidskriftsartikel (refereegranskat)abstract
- An intuitive understanding of dynamic correlation in terms of a regularized electron repulsion expression is outlined. Expressions for cusp kinetic energy corrected regularized electron repulsion integrals are deduced and implemented in a multiconfigurational wave-function framework. A regularized complete active space self-consistent field (reg-CASSCF) technique is suggested and tested on atomic total energies, molecular structures and binding energies.
|
|
| 6. |
- Andres, J, et al.
(författare)
-
Transition-state structures for describing the enzyme-catalyzed mechanisms of rubisco
- 1999
-
Ingår i: THEORETICAL CHEMISTRY ACCOUNTS. - : SPRINGER VERLAG. - 1432-881X. ; 101:1-3, s. 234-240
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The carboxylation and oxygenation processes of a model substrate, 3,4-dihydroxy-2-pentanone, have been theoreticaly characterized as a set of steps? mimicking the corresponding reactions of D-ribulose-1,5-bisphosphate catalyzed by rubisco. A theoretical c
|
|
| 7. |
- Contreras, R, et al.
(författare)
-
Theory of non-local (pair site) reactivity from model static-density response functions
- 1998
-
Ingår i: THEORETICAL CHEMISTRY ACCOUNTS. - : SPRINGER VERLAG. - 1432-881X. ; 99:3, s. 183-191
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Activation is a fundamental and well-known concept in chemistry, It may be qualitatively, defined as an increase in the chemical reactivity pattern of a molecule at a given site k when the system is locally perturbed at a different site l, say. This exter
|
|
| 8. |
|
|
