Attenuated Deuterium Stabilization of Hydrogen-Bound Complexes at Room Temperature

Abstract

We have observed nine bimolecular hydrogen-or deuterium-bound complexes at room temperature using Fourier transform infrared (FTIR) spectroscopy. The complexes were formed using methanol or ethanol as hydrogen bond donors, as well as deuterated isotopologues of these, in order to study isotopic effects on hydrogen bonds. The complexes were formed using either a dimethylether-(O) or trimethylamine (N) acceptor, to facilitate comparison of two different types of hydrogen or deuterium bonds, OH(D)•O and OH(D)•N. For each complex, the characteristic OH-or OD-stretching fundamental band in the bimolecular complex was observed. The Gibbs energy of complex formation was determined at room temperature for each complex to compare the relative stability of hydrogen-and deuterium-bound bimolecular complexes. It is well known that deuterium-bound complexes are more stable at low temperatures because of the lower frequency of its intermolecular modes and thus a lower zero point vibrational energy. However, at room temperature, entropic contributions to the stability should also be considered. At room temperature, we find the Gibbs energy of complex formation for each pair of corresponding hydrogen-and deuterium-bound complex to be similar. The similar values of the Gibbs energies at room temperature is explained from a difference in the entropy, upon complexation, which favors the formation of the hydrogen-bound complex more than the deuterium-bound complex at higher temperatures.

Room Temperature Gas-Phase Detection and Gibbs Energies of Water Amine Bimolecular Complex Formation

“…In comparison, the equilibrium constants of MeOH•DMA and MeOH•TMA have previously been determined with the combined theoretical and experimental approach used here. 24,33,61,62 For MeOH•DMA, a dual determination of K was obtained from the OH bstretching fundamental band and from the NH-stretching second overtone of the free NH stretch in the DMA acceptor molecule. For both bands, similar K values were obtained.…”

“…The asymmetric band profile, with a tail extending toward higher wavenumbers, is typical for bands related to the OH b stretch in hydrogen bound complexes. ,,, Furthermore, ν OH b is typically red shifted and its intensity is significantly enhanced, compared with the corresponding OH-stretching fundamental band in the monomer . H 2 O has two highly coupled OH stretches, which become partly decoupled upon complex formation, and the decoupled OH stretch of H 2 O can be used as a reference with ν̃ OH = (ν̃ 1 + ν̃ 3 )/2 = (3657.1 cm –1 + 3755.9 cm –1 )/2 = 3706.5 cm –1 .…”

“…After ∼15 min, the pressure of the H 2 O, DMA, and TMA added to the cell had stabilized following an initial decrease due to condensation. We have previously found that condensation is easier to account for if a pressure gauge is attached directly to the gas cell. , However, here we used a 19 cm path length cell to which a pressure gauge could not be attached. Comparison with pressures recorded for a 10 cm path length cell with a pressure gauge attached indicated an error in the determined amine pressure of less than 3% (see Supporting Information, section S2.1).…”

“…This challenge may be dealt with by either using a variational LM model or applying van Vleck perturbation theory. , The latter approach has the advantage of reducing the number of basis functions needed for a converged solution of the associated Schrödinger equation. , In this work, we have used the more computationally demanding variational LM model. We have recently shown, for OH·O and OH·N bound complexes, in which intramolecular resonances are not present, that OH b -stretching intensities calculated with a variational LM model and the LMPT model are within 5% …”

“…We have recently shown, for OH•O and OH•N bound complexes, in which intramolecular resonances are not present, that OH bstretching intensities calculated with a variational LM model and the LMPT model are within 5% 33. …”