X-ray diffraction and inelastic neutron scattering study of 1:1 tetramethylpyrazine chloranilic acid complex: temperature, isotope, and pressure effects

Abstract

The x-ray diffraction studies of the title complex were carried out at room temperature and 14 K for H/D (in hydrogen bridge) isotopomers. At 82 K a phase transition takes place leading to a doubling of unit cells and alternation of the hydrogen bond lengths linking tetramethylpyrazine (TMP) and chloranilic acid molecules. A marked H/D isotope effect on these lengths was found at room temperature. The elongation is much smaller at 14 K. The infrared isotopic ratio for O-H(D)...N bands equals to 1.33. The four tunnel splittings of methyl librational ground states of the protonated complex required by the structure are determined at a temperature T=4.2 K up to pressures P=4.7 kbars by high resolution neutron spectroscopy. The tunnel mode at 20.6 microeV at ambient pressure shifts smoothly to 12.2 microeV at P=3.4 kbars. This is attributed to an increase of the strength of the rotational potential proportional to r(-5.6). The three other tunnel peaks show no or weak shifts only. The increasing interaction with diminishing intermolecular distances is assumed to be compensated by a charge transfer between the constituents of deltae/e approximately 0.02 kbar(-1). The phase transition observed between 3.4 and 4.7 kbars leads to increased symmetry with only two more intense tunneling bands. In the isotopomer with deuterated hydrogen bonds and P=1 bar all tunnel intensities become equal in consistency with the low temperature crystal structure. The effect of charge transfer is confirmed by a weakening of rotational potentials for those methyl groups whose tunnel splittings were independent of pressure. Density functional theory calculations for the model TMP.(HF)2 complex and fully ionized molecule TMP+ point out that the intramolecular rotational potential of methyl groups is weaker in the charged species. They do not allow for the unequivocal conclusions about the role of the intermolecular charge transfer effect on the torsional frequencies.

High-temperature protonic motion and low-temperature lattice deformation in one-dimensional hydrogen-bonded molecular chain in tetramethylpyrazine–chloranilic acid (1:1)

“…[11]. This agrees well with the result of X-ray analysis [8,9]. Besides a T 1 dip observed at T c , a steep decrease of T 1 with increasing temperature was observed above 250 K. The slope of the Arrhenius plot corresponds to an activation energy of 35 kJ mol −1 .…”

“…The sample was identified by comparing the observed X-ray powder pattern with the expected one from the crystal data [8,9] …”

“…Similarly to Phz, tetramethylpyrazine (TMP) forms with H 2 ca a 1:1 molecular compound, TMP-H 2 ca, with a supramolecular chain [8,9]. Since the pK a = 2.8 of the conjugate acid of TMP, TMPH + [10], is greater than that of PhzH + , the TMP-H 2 ca is also likely to show a similar neutral-to-ionic phase transition with decreasing temperature.…”

“…Since the pK a = 2.8 of the conjugate acid of TMP, TMPH + [10], is greater than that of PhzH + , the TMP-H 2 ca is also likely to show a similar neutral-to-ionic phase transition with decreasing temperature. Indeed, recent structural analysis showed the existence of a phase transition at T c = 82 ± 3 K [9]. However, both the high-and low-temperature phases were shown to consist of neutral molecular components without proton transfer.…”

Pressure effect on hydrogen tunneling and vibrational spectrum inα-Mn

“…For example, compressing methyl iodide CH 3 I results in a strong (almost exponential) decrease of tunneling splitting of the methyl librational ground state from about 2.5 µeV at ambient pressure to 0.8 µeV at 3.6 kbar [8]. Application of a 3.4 kbar pressure on the tetramethylpyrazine and chloranilic acid complex at a temperature of 4.2 K results in a shift of one (out of four total) methyl rotational tunneling peak from 20.6 µeV to 12.2 µeV, while the other three tunneling peaks shift little or not at all [9], and the softening of the tunneling peak was attributed to an increase of the strength of the rotational potential. Another example is methyl tunneling in α-toluene, where two tunneling peaks were observed in the INS spectra corresponding to the two inequivalent methyl groups of this crystalline structure [10].…”