Studies of Molecular Recognition by Solid NMR

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Determination of a molecular structure in a powder sample without no long range order has been one of the long-pending problems in chemistry, and most of recent ingenuity has been devoted to develop solid-state NMR as a tool to exploit structural informations in solids. The aim of this work is to apply solid NMR techniques for structural determination developed so far by ourselves to the long-standing and important problem of microscopic mechanisms of molecular recognition. We relate structure and molecular recognition mechanisms. For a molecular recognition system, we take up one-dimensionalguest alignment of optical resolved 7-valerolactone (VAL), (S)-(-)-enantiomer (SVAL) and R-(+) enantiomer (RVAL), enclathrated in chiral cholic acid host (CA). It has been known that CA recognizes the S and R enantiomer and preferably enclathraes SVAL. To study this recognition process, we prepared SVAL whose methyl protons are deuterated and its carbon is replaced by 13C, and enclathrate it to CA. … More We then undertook one-dimensional spin-diffusion experiments among the methyl carbons of SAVL and RVAL in the one-dimensional channel of CA. The spin diffusion can be successfully simulated by the molecular alignment of VAL in the one-dimensional CA chanel calculated by assuming the inclusion process to be the 1st order Malkov process. We can show that there exists attractive force between same enantiomers, which is the key mechanism of optical resolution. Further, in an effort to achieve better efficiency for spin diffusion, we developed a new ^1H-^<13>C dipolar-assisted rotational resonance method. This is a method for determining distances among spins roughly by one experiment. This kind of high throuput distance determination method has been long desired and was published in Chem. Phys. Lett., 344, 631-637 (2001). Since this method does not require rfiradiation to spins to be observed, the spin-diffusion time is limited by a generally long spin-lattice relxation time. In other words, we can conduct a long-range distance measurement. This method allows us to fully enjoy high-esolution observation, and has another desirable features suitable for application to multiply labeled molecule. Less