Nuclear Scalar Spin–Spin Coupling Reveals Novel Properties of Low-Barrier Hydrogen Bonds in a Polar Environment
✍ Scribed by Nikolai S. Golubev; Ilja G. Shenderovich; Sergei N. Smirnov; Gleb S. Denisov; Hans-Heinrich Limbach
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 164 KB
- Volume
- 5
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
The structure of the hydrogen bridge 19 F ´´´1H ´´´1 5 N in the acid ± base complex A ´´´H ´´´B formed by HF and [ 15 N]2,4,6-trimethylpyridine in CDF 3 / CDF 2 Cl has been studied between 112 K and 200 K by low-temperature, multinuclear NMR spectroscopy. For the first time scalar spin ± spin coupling between all three nuclei of a hydrogen bridge is observed. This bridge exhibits a two-bond coupling constant 2 J19 F 15 N of about 96 Hz, which is larger than the one-bond coupling constants 1 J1 H 15 N and 1 J19 F 1 H . The latter are strongly dependent on temperature. The function 1 J1 H 15 N f( 1 J19 F 1 H ) cannot be described in terms of a conventional equilibrium between the molecular and the zwitterionic form, but only with the intermediate forma-tion of very strongly hydrogen-bonded complexes of the type A dÀ ´´´H ´´´B d that exhibit a vanishing or very small barrier for the proton motion. Here, the difference between the covalent bond and the hydrogen bond disappears even in the case of a polar solvents, as indicated by the large value of 2 J19 F 15 N . Implications for the mechanism of proton transfer and of acid ± base catalyzed enzyme reactions in a locally aprotic but polar environment are discussed.