Theoretical Studies of Heavy- Theoretical Studies of Heavy- Atom NMR Spin-spin Coupling Atom NMR Spin-spin Coupling Constants Constants With Applications to Solvent Effects in With Applications to Solvent Effects in Heavy Atom NMR Heavy Atom NMR Jochen Autschbach & Tom Ziegler, University of Calgary, Dept. of Chemistry University Drive 2500, Calgary, Canada, T2N-1N4 Email: [email protected]1
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Theoretical Studies of Heavy-Atom NMR Spin- spin Coupling Constants With Applications to Solvent Effects in Heavy Atom NMR Jochen Autschbach & Tom Ziegler,
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Theoretical Studies of Theoretical Studies of Heavy-Atom NMR Spin-Heavy-Atom NMR Spin-spin Coupling Constantsspin Coupling Constants
With Applications to Solvent With Applications to Solvent Effects in Heavy Atom NMREffects in Heavy Atom NMR
Jochen Autschbach & Tom Ziegler, University of Calgary, Dept. of Chemistry University Drive 2500, Calgary, Canada, T2N-1N4Email: [email protected]
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What is interesting about What is interesting about Heavy Metal Compounds ?Heavy Metal Compounds ?Spin-orbit coupling, scalar relativistic effectsRelativistic theoretical treatment: sizeable effects on bonding for 6th row elements (bond contractions, De,e,IP, …) are already textbook knowledge (e.g. “Au maximum”)Simple estimates propose absolute (!) scalar relativistic effects of 100% for 6th row elements for NMR spin-spin coupling constantsCoordination by solvent molecules possible
Spin-orbit (SO) coupling causes cross terms between the spin-dependent ope-
rators (FC,SD) and the orbital dependent ones (here: PSO). The differences
between Scalar and SO in the table above is mainly caused by these cross terms,
and by the SO effects on the PSO contribution itself. Tl-I is the first example where
SO coupling was demonstrated to cause the major contributions to heavy atom
spin-spin couplings. JCP 113 (2000), 9410.11
Results III : solvent effectsResults III : solvent effects
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Experimental results on pages 9 and 10 obtained fromsolution. The cases where results are unsatisfactory aremarked red (linear Hg and square planar Pt complexes)
SO coupling yields only minorcorrections in all these cases!
Is coordination of the heavy atoms by solvent moleculesimportant?
Some structures that were optimized, explicitly including a number of solvent molecules
Mercury compounds with solvents: K / 1020 kg/m-2C-2 *)
Results III : more solvent effectsResults III : more solvent effects
Two-bond coupling much larger than one-bond coupling
Four water molecules can coordinate toTl in aqueous solution (exp. confirmed)
Complex I
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Results III : more solvent effectsResults III : more solvent effects
Spin-spin couplings complex I, J / kHzCoupling nrel scalar Scalar
+ 4H2OSO+ 4H2O
Exp.(in H2O)
Pt-Tl 5.4 19.0 43.1 40.3 57.0
Tl-CB 1.2 5.7 3.1 3.0 2.4
Tl-CA 3.4 5.7 8.0 7.5 9.7
Tl-CC 0.2 0.5 0.4 0.4 0.5
The unintuitive experimental result 2J(Tl-CA) >> 1J(Tl-CB) questions the proposed structure with a direct Tl-Pt bond(page 15). However, our computations confirm the structureand the unusual coupling pattern. The solvent coordination effect on J(Pt-Tl) and the Tl-C cpouplings is remarkably large.
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Results III : more solvent effectsResults III : more solvent effects
JACS 123 (2001), in press.
free complex: both couplingsare comparably large in magni-tude but of opposite signinclusion of solvent moleculesshifts both couplings. The one-bond coupling is – as expected –influenced much stronger.As a result, the two-bond coup-ling is much larger than the one-bond couplingDelocalized bonds along theC-Pt-Tl-C axis are responsiblefor the large magnitude of thetwo-bond Tl-C coupling in thefree complex
SummarySummary NMR shieldings and spin-spin couplings with ADF now
available for light and heavy atom systems Based on the variationally stable two-component ZORA
method Relativistic effects on spin-spin couplings are
substantial and recovered by the ZORA method Spin-orbit effects are rather small for many cases, but
dominant for Tl-X Coordination by solvent molecules has to be explicitly
taken into account for coordinatively unsaturated systems. Saturating the first coordination shell yields satisfactory results in these cases.
Further solvent contributions within the DFT error bars18