Presentation on theme: "Probing Halogen Bonds with NMR One- and Two-Bond Spin-Spin Coupling Constants Halogen bonds: 19 F- 35 Cl… 15 N 1 J(F-Cl) 1X J(Cl-N) 2X J(F-N) Janet E."— Presentation transcript:
Probing Halogen Bonds with NMR One- and Two-Bond Spin-Spin Coupling Constants Halogen bonds: 19 F- 35 Cl… 15 N 1 J(F-Cl) 1X J(Cl-N) 2X J(F-N) Janet E. Del Bene
Methods for structure optimization and the calculation of spin-spin coupling constants 1.Structure optimization: (Gaussian 03) F-Cl…N: MP2/aug’-cc-pVTZ 2.Coupling constants: a. Method: Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) (ACES II) b. Basis set: Ahlrichs (qzp,qz2p) c. J Terms: paramagnetic spin-orbit (PSO), diamagnetic spin- orbit (DSO), Fermi contact (FC), and spin dipole (SD)
Cl-N distances (Å) and binding energies (kcal/mol) for complexes with F-Cl…N halogen bonds Legon (gas-phase) a MP2/aug’-cc-pVTZ ComplexR o (Cl-N), ÅR e (Cl-N) ΔE e FCl…NCH FCl…NC-CH FCl…NH (“little ion-pair”) FCl…N(CH 3 ) (“70% ion-pair”) a) A. C. Legon, “The interaction of dihalogens and hydrogen halides with Lewis bases in the gas-phase” Struct. Bond. (2008) 126, 17.
Binding energies for complexes with halogen bonds vs. binding energies for complexes with hydrogen bonds ΔE X (kcal/mol) ΔE H (kcal/mol)
Nitrogen bases sp sp 2 sp 3 N ≡ N Z- HN=NHNH 2 F F-C≡NH 2 C=NHNH 3 H-C≡NNH(CH 2 ) 2 cyc H 3 C-C≡NNH 2 (CH 3 ) Li-C≡N F-Cl…N halogen-bonded complexes
R(Cl-N), Å Å Å R(F-N) and R(F-Cl) vs. R(Cl-N) (Å) in complexes with F-Cl…N halogen bonds ← R(F-N)R(F-Cl) → Strong base Weak base
2h J(F-N) vs. R(F-N) for complexes with F-H…N hydrogen bonds R(F-N), Å 2h J(F-N), Hz R 2 = 0.97
2X J(F-N) and 1X J(Cl-N) vs. the F-N distance for F-Cl…N halogen bonds Hz R(F-N), Å 2X J(F-N); R 2 = X J(Cl-N); R 2 = 0.95 _________________ ││││││││││
1 J(F-Cl), Hz 1 J(F-Cl) as a function of the F-Cl distance (Å) for complexes with F-Cl…N halogen bonds R(F-Cl), Å NCLi Monomer NH 2 F Z-N 2 H 2 R 2 = 0.93
Across the proton-transfer coordinate for F-H…NH 3 2h J(F-N) → 1h J(H-N) → ← 1 J(F-H) Hz R(F-H), Å
Across the Cl-transfer coordinate for F-Cl…NH 3 Hz R(Cl-N), Å 2X J(F-N) → ← 1 J(F-Cl) 1X J(Cl-N) →
Why doesn’t 1X J(Cl-N) in the complex approach 1 J(Cl-N) in the isolated ion? F - …… + Cl-NH 3 + Cl-NH Hz+9 Hz Complex: F(-0.41e) Cl(+0.25e) N(-0.49e) Ion: Cl(+0.32e) N(-0.04e)
Conclusions 1.Coupling constants across halogen bonds can be used to characterize complexes with F-Cl…N halogen bonds. Both 2h J(F-N) for hydrogen bonds and 2X J(F-N) for halogen bonds increase in absolute value with decreasing F-N distance. 2. Changes in coupling constants along the Cl transfer are quite different from changes in corresponding coupling constants across the proton-transfer coordinate. 3. Complexes of F-Cl with the stronger N bases are more stable than corresponding complexes of F-H, and should be amenable to experimental investigations of coupling constants at low temperatures.
Acknowledgments José Elguero and Ibon Alkorta, CSIC, Madrid US National Science Foundation CHE Ohio Supercomputer Center
Congratulations, Russ Pitzer, on a long and productive scientific career as a University Professor - teacher and researcher in quantum chemistry. Wishing you continued success and enjoyment!
J(exp) = 1.007J(EOM) – R 2 = J(exp) = 1.201J(SOP) – R 2 = Del Bene, Alkorta, Elguero, JCTC, 2008, 4, 967. EOM-CCSD and SOPPA (Second-order Polarization Propagator Approximation) vs. Experimental 1 J(X-Y), Hz H m X-YH n and F-substituted derivatives