Using TURBOMOLE,
I am studying a mechanism of Heterolytic vs Homolytic cleavage.
When the bond breaks, there are three possibilities
a) Heterolytic Cleavage (RHF, singlet)
b) Homolytic Cleavage (UHF, triplet, parallel diradical)
c) Homolytic Cleavage (UHF, singlet, anti diradical)
After calculation, I found the energy values of ion couple generated from heterolytic cleavage and singlet diradical couple generated from homolytic cleavage to be very close....
ΔG(heterolytic) = -1.0 ( ΔGts# = +7.6)
ΔG(Homolytic)Singlet= -0.8 ( ΔGts# = +9.0) ( = 0.000)
ΔG(Homolytic)Triplet= +9.0 ( ΔGts# = +36.3) ( = 2.015)
* Solvent=DCM, All values are in kcal/mol. [ = s(s+1)]
But, I am unable to understand properly, whether diradical couple whose spin might be anti to each other, can be treated as UHF singlet or not ?
If yes, how would I conclude which is the preferred mechanism? And whether this is a right approach?
Hallo, Soumya!
Diradical singlet (at HF and DFT levels of theory) can be found in literature as "broken-symmetry treatment" (el. structure has lower symmetry than the molecular one). In such case is nonzero and the singlet - triplet energy separation cannot be evaluated simply as the energy difference of both states (several formulas can be found in literature).
According to me such systems are real (see e.g. the works of Neese group) despite many authors are of opposite meaning (only mathematical construction for better desription of singlet state energies).
Best wishes
mb
Hello there,
Check this recent review by Roald Hoffmann Article Do Diradicals Behave Like Radicals?
Martin Breza Hello Sir,
What is your opinion on the way this article has handled diradicals?
(calculated both fragments separately)
Article The Grignard Reaction-Unraveling a Chemical Puzzle
JACS- (JAN 2020)
Dear friends!
The 'broken symmetry' treatment works at DFT level of theory using hybrid functionals (the pure ones produce the same spin-less results as for the restricted formalism). Nevertheless, frequently you may obtain the same (spin-less) results as for the restricted formalism (mainly for non-transition metal complexes - such as the Mg ones used in the Grignard reaction). In the 1st step I always use the unrestricted treatment (for singlet states - at least a single point calculation) and if the spin-less solution is obtained then I continue by using the restricted formalism only (it is less time-consuming).
Concerning the calculations of the reactions fragment separately to evaluate reaction energy data - this treatment suffers by neglecting a basis set superposition error (BSSE - different number of basis functions used for the reactant and for each of the reaction product fragments). BSSE corrections are satisfactory for weak bonding interactions only. In the case of standard chemical bonds they are significantly overestimated (and so they are neglected). BSSE decreases with the increasing basis set size.
Coupled cluster methods ( such as CCSD) are able to describe singlet biradicals without any 'broken symmetry' treatment. Nevertheless, they are usable for small systems only (due to technical reasons).
Best wishes
mb
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