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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

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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)

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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…NCH2.69 2.54 5.8 FCl…NC-CH 3 2.56 2.47 7.1 FCl…NH 3 2.37 (“little ion-pair”) 2.24 11.7 FCl…N(CH 3 ) 3 2.09 (“70% ion-pair”) 2.04 23.9 a) A. C. Legon, “The interaction of dihalogens and hydrogen halides with Lewis bases in the gas-phase” Struct. Bond. (2008) 126, 17.

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Binding energies for complexes with halogen bonds vs. binding energies for complexes with hydrogen bonds ΔE X (kcal/mol) ΔE H (kcal/mol)

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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

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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

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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

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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 = 0.99 1X J(Cl-N); R 2 = 0.95 _________________ ││││││││││

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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

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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

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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) →

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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 3 -31 Hz+9 Hz Complex: F(-0.41e) Cl(+0.25e) N(-0.49e) Ion: Cl(+0.32e) N(-0.04e)

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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.

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Acknowledgments José Elguero and Ibon Alkorta, CSIC, Madrid US National Science Foundation CHE-9873815 Ohio Supercomputer Center

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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!

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J(exp) = 1.007J(EOM) – 2.123 R 2 = 0.997 J(exp) = 1.201J(SOP) – 17.48 R 2 = 0.962 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

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