Applications of the ETS-NOCV method in descriptions of chemical reactions

• Published in: Journal of molecular modeling Vol. 17; no. 9; pp. 2337 - 2352
• Main Authors: Mitoraj, Mariusz Paweł, Parafiniuk, Monika, Srebro, Monika, Handzlik, Michał, Buczek, Agnieszka, Michalak, Artur
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01-09-2011
• Subjects: Algorithms; Boranes - chemistry; Butadienes - chemistry; Catalysis; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Computer Appl. in Life Sciences; Computer Applications in Chemistry; Computer Simulation; Cyanates - chemical synthesis; Electrons; Ethylenes - chemistry; Hydrogen Cyanide - chemistry; Isomerism; Models, Chemical; Models, Molecular; Molecular Conformation; Molecular Medicine; Organometallic Compounds - chemistry; Original Paper; Quantum Theory; Static Electricity; Surface Properties; Theoretical and Computational Chemistry; Thermodynamics; Chemical bonding; Chemical reaction; Deformation density; ETS-NOCV scheme; Reference state
• ISSN: 1610-2940; 0948-5023
• DOI: 10.1007/s00894-011-1023-6

The present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state–natural orbitals for chemical valence). Decomposition of the activation barrier, Δ
E
#
, into stabilizing (orbital interaction, Δ
E
orb
, and electrostatic, Δ
E
elstat
) and destabilizing (Pauli repulsion, Δ
E
Pauli
, and geometry distortion energy, Δ
E
dist
) factors is discussed in detail for the following reactions: (
I
) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; (
II
) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., (
III
) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and (
IV
) B–H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δ
ρ
i
) and the corresponding energies Δ
E
orb
(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE
#
into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of Δ
E
#
for all processes (
I
,
II
,
III
and
IV
) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions
II
,
III
and
IV
, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of Δ
E
#
.
Figure
Contours of dominant deformation density contributions, Δρ
1
, together with the corresponding energies ΔE
orb
(1) obtained from the ETS-NOCV method for the transition state (
TS
) and the product (
cyclohexene
) of Diels-Alder cycloaddition of ethene to 1,3-butadiene.
Black
and
white
colors used in the representation of Δρ
1
indicate carbon and hydrogen atoms, respectively. The
blue
/
red
colors in the contours corresponds to accumulation/depletion of electron density upon bond formation