Article Cite This: J. Phys. Chem. A 2019, 123, 5537−5541
pubs.acs.org/JPCA
Tautomeric Hydrogen Bond in Dimers of Ibuprofen Artem G. Demkin† and Boris A. Kolesov*,†,‡ †
A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk 630090, Russia Faculty of Physics, Novosibirsk National Research State University, Novosibirsk 630090, Russia
‡
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ABSTRACT: The Raman spectra of polycrystalline samples of (RS)-2-(4-isobutylphenyl)propionic acid of the common name ibuprofen have been measured in the temperature range 5−300 K. In the low-frequency spectrum of the normal C12H17(COOH) and deuterated C12H17(COOD) species, modes with ∼103 and ∼95 cm−1 wavenumbers were detected, which corresponded to translational vibrations of O−H(D)···O hydrogen bonds of two different tautomers: left L and right R, respectively. At temperatures below 150 K, only the Ltautomer is found, and at T ≥ 150 K, both tautomers are observed. The energy difference ΔE of the ground vibrational state of potential minima for L- and R-tautomers is ∼80 meV for COOH and ∼70 meV for COOD. At T ≥ 150 K, the vibrational frequency of the CO bond in the COOH moiety exhibits an unusual temperature dependence.
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INTRODUCTION In the formation of 2-, 3-, and 4-membered rings from hydrogen bonds of the same type, protons can coordinatively shift along the bond from the donor to the acceptor, which are structurally indistinguishable in this case and produce identical tautomers,1 commonly named in dimers as left (L) and right (R) tautomers. This phenomenon is called proton tautomerism. The potential function of protons on the tautomeric bond (further, the τ-bond) consists of two potential wells of the same depth. They are separated by a barrier with a height U0, and the minima of the wells are located near the donor and the acceptor. It is shown by the example of terephthalic acid2 that when the O−H···O τ-bond is symmetric, both coordinated jumps of protons over the potential barrier assisted by lattice vibrations (phonon-assisted jumping) and tunneling through the barrier are observed in the Raman spectra. The tunneling process influences the temperature behavior of the translational O···O mode frequency, and proton jumps over the barrier affect presumably the temperature dependences of the half-width of this mode. It is found that when the temperature decreases, the tunneling intensity increases due to the freezing-out of translational vibrations of the O···O hydrogen bond, whereas jumps over the barrier fade out. The characteristic temperatures of both processes are different and formally not related to each other. Ibuprofen (C13H18O2) molecules form dimers of Ci point symmetry (Figure 1) with an O···O distance of 2.664 Å at ambient temperature.3 The structure of O−H···O hydrogen bonds of the dimer is very similar to that observed in the crystals of terephthalic and benzoic acids. However, as seen from Figure 1, the carbon atom adjacent to the α-carbon atom has two different substituents (a proton and a methyl group), which may formally generate different energy conditions for the tautomers, and the potential function of the τ-bond protons can theoretically be presented by a curve with two minima of © 2019 American Chemical Society
Figure 1. Dimer of the ibuprofen molecule in the crystal.
different depths (Figure 2). In other words, ibuprofen dimers do not refer to systems with a symmetric τ-bond. However, we
Figure 2. Expected proton potential energy of the tautomeric hydrogen bond in ibuprofen dimers. x1 and x2 are the coordinates of minima.
would like to explore the real energy difference of the minima and whether this difference affects the proton tautomeric transitions. In this work, we have attempted to reveal whether the tautomeric transitions are possible in ibuprofen dimers or not and how minor asymmetry of the τ-bond affects the proton tunneling and jumps. As in the previous work,2 to this end, we Received: March 25, 2019 Revised: May 23, 2019 Published: May 31, 2019 5537
DOI: 10.1021/acs.jpca.9b02813 J. Phys. Chem. A 2019, 123, 5537−5541
The Journal of Physical Chemistry A
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Article
RESULTS The low-frequency (
