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An indication of slowing down of hydrogen atom transfer in isotopically mixed hydrogen bonds of benzoic acid crystals
Author(s) -
Takeda Sadamu,
Tsuzumitani Akihiko
Publication year - 2001
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.944
Subject(s) - chemistry , deuterium , hydrogen bond , proton , benzoic acid , crystallography , relaxation (psychology) , protonation , mole fraction , hydrogen atom , hydrogen , molecule , analytical chemistry (journal) , atomic physics , organic chemistry , quantum mechanics , psychology , social psychology , ion , physics , alkyl
Abstract Benzoic acid molecules form a dimeric unit of carboxyl groups in the crystalline phase and two acid protons form a pair. In the case of a proton/deuteron mixed hydrogen‐bond system, proton–deuteron pairs (HD pairs) as dimers may exist in a stochastic distribution probability depending on the mole fraction of deuteron. Transfer rates of proton and deuteron in the HD pairs were investigated by spin–lattice relaxation rate T 1 −1 of proton and deuteron NMR, respectively, for a series of different mole fractions of deuteron in the hydrogen bonds. For the measurement of the spin–lattice relaxation rate of the proton, 17 O was enriched to provide an additional relaxation pathway and enhance the signature of the HD pairs in the spin–lattice relaxation. The rates of proton and deuteron transfer in the HD pairs were in between those of the proton transfer in fully protonated specimen (HH pairs) and the deuteron transfer in fully deuterated specimen (DD pairs). This result indicates that H and D in the HD pairs transfer in a concerted manner within the time‐scale of NMR (10 −6 –10 −10 s). Here we found an interesting result that the transfer rate of the HD pairs in the high‐temperature region decreased as the mole fraction of deuteron of hydrogen bonds in the crystal was increased from 0.13 to 0.96. This result is semiquantitative at present and the precise value of the transfer rate may depend on the method of analysis. However, an estimation of the decrease is almost one order of magnitude at 200 K. Although this effect should be strictly confirmed by other experiments, an indirect geometrical isotope effect for the hydrogen bond length and/or some dynamic correlation among dimers may be responsible for this phenomenon. Copyright © 2001 John Wiley & Sons, Ltd.