Temperature dependence of hydrogen bond structure by ^2D NMR.

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Many kinds of "hydrogen salt" make short hydrogen bonds with acidic hydrogen that is not substituted by metal ions. In such short hydrogen bonds, bond length and nuclear quadrupole coupling constant (QCC) have various values and their relation is not simple. This fact has been related to the change of potential energy surface that dominates the behavior of hydrogen bond. It becomes symmteric from asymetric, and its shape changes from double to single minimum shape with decreasing hydrogen bond length. However these distinctions is often difficult to account for both static and dynamic properties of hydrogen bond. In some very short hydrogen bond QCC increases with increasing temperature contrary to normal decreasing temperature dependence of QCC.Temperature dependence of QCC could be a good indicator of the nature of hydrogen bond because it varies not only its absolute value but also its sign. But a few observations was only exist. To proceed more detailed discussion, weUsing apparatus made in previous year, temperature dependences of 2D NMR spectra of acetylenedicarboxylic acid mono potassium salt (ADCAKH) with R_∞=2.445 Å and mono rubidium salt (ADCARbH) with R_∞=2.449 Å was observed. In both hydrogen salts, QCC increases linearly with increasing temperature. The temperature coefficients of both two salts are almost same, but the absolute value of QCC of ADCARbH is more than of ADCAKH in same order as their hydrogen bond length. If longer hydrogen bond length causes more QCC value, increase of QCC with increasing temperature would be explainable as an effect of thermal expansion of hydrogen bond.On the other hand, in potassium hydrogen carbonate with r_∞〜2.61 Å ^2D NMR spectra show slight decrease of QCC, and its temperature coefficient is very small. In KDCO_3, OH bond has a single bond character, therefore averaging effect due to anharmonic vibrational excitation of OH bond dominates the temperature dependence of QCC.