Peculiar Concentration Dependence of H/D Exchange Reaction in 1-Butyl-3-methylimidazolium Tetrafluoroborate-D2O Mixtures

Souichi Ohta1, Akio Shimizu1, Yusuke Imai2, Hiroshi Abe2, Naohiro Hatano3, Yukihiro Yoshimura3


1Department of Environmental Engineering for Symbiosis, Soka University, Tokyo, Japan
2Department of Materials Science and Engineering, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan
 3Department of Applied Chemistry, National Defense Academy, Yokosuka, Kanagawa 239-8686, Japan

Open Journal of Physical Chemistry 1 (2011) 70-76.

Abstract
We have investigated the H/D exchange reaction between heavy water and an ionic liquid, 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim][BF4]), throughout the whole concentration region as a function of D2O mol% at room temperature. We expected that the extent of the H/D reaction would increase linearly with increasing content of D2O, but the results show an extended N-shaped behavior having a small maxi-mum at around 40 mol% and the reaction becomes very slow at a specific concentration around 80 mol%. We found that this non-linear concentration dependence correlates with the pD dependence of the solutions.

Figure 1.
Representative 1H NMR spectrum of [bmim] [BF4]-D2O mixtures (x = 90 mol%) at 500 MHz obtained after a deuterium exchange for the C(2)-H proton at 23.3°C. 1H NMR chemical shifts δ relative to TMS internal stan-dard are shown. The exchange for deuterium of the C(2)-H of imidazolium cation in D2O were determined by monitor-ing the decay of the C(2)-H proton (②) and the deuterium exchange results in the disappearance of the signal due to the C(2)-H proton and the appearance of that due to HDO. The inset shows a chemical structure and the numbering of the skeleton atoms for the [bmim] cation.


Figure 2.
The intensity ratio of the IHDO/(IC(2)-H + IHDO) for [bmim][BF4]-D2O mixed solution against the D2O concen-tration x. The extent of a deuterium exchange is examined with time evolution from 0 - 42 days. The symbols show the data depicted. ○: 0 day, △: 14 days, □: 23 days, ▽: 29 days, ●: 42 days, ◎: heated for 2 hours at 75°C immedi-ately after the sample preparations.
Figure 3.
Chemical shifts of respective protons in [bmim] cation and HDO as a function of water concentration x. The chemical shifts correspond to the deviations from the position of the reference (DSS).
Figure 4.
Concentration dependence for the pD of [bmim] [BF4]-D2O mixed solutions. The values for the mixed solu-tions after 42 days from the sample preparation are plotted.
Figure 5.
The intensity ratio (IHDO/(IC(2)-H + IHDO)) of the H/D exchange reaction in buffered solutions at a basic con-dition (adjusted to pD = 11 - 12) against the D2O concentration x. For a comparison, the values for the mixed solutions without the addition of the base (open circle symbols) after 42 days left from the sample preparation are plotted in the same figure.
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