Diffuse Vibrational Signature of a Single Proton Embedded in the Oxalate Scaffold, HO₂CCO₂^–

Abstract

To understand how the D2d oxalate scaffold (C2O4)(2-) distorts upon capture of a proton, we report the vibrational spectra of the cryogenically cooled HO2CCO2(-) anion and its deuterated isotopologue DO2CCO2(-). The transitions associated with the skeletal vibrations and OH bending modes are sharp and are well described by inclusion of cubic terms in the normal mode expansion of the potential surface through an extended Fermi resonance analysis. The ground state structure features a five-membered ring with an asymmetric intramolecular proton bond. The spectral signatures of the hydrogen stretches, on the contrary, are surprisingly diffuse, and this behavior is not anticipated by the extended Fermi scheme. We trace the diffuse bands to very strong couplings between the high-frequency OH-stretch and the low-frequency COH bends as well as heavy particle skeletal deformations. A simple vibrationally adiabatic model recovers this breadth of oscillator strength as a 0 K analogue of the motional broadening commonly used to explain the diffuse spectra of H-bonded systems at elevated temperatures, but where these displacements arise from the configurations present at the vibrational zero-point level.

Hidden role of intermolecular proton transfer in the anomalously diffuse vibrational spectrum of a trapped hydronium ion

“…Note that the OD stretching bands in the perdeuterated Eigen cation (Fig. 4 + complexes suggests that the underlying cause is anharmonic coupling between the OH stretching frequency and the soft modes of the cluster (21,22). The relevant calculated soft modes at the harmonic level are included in SI Appendix, Table S2B, and the two likely candidates for anharmonic coupling are the frustrated translation of the ion in the cage and the ] and are included as inverted traces in C, E, G, and I. ]…”

“…In effect, the zero-point displacement plays a similar role in distorting the scaffold as does thermal excitation in a classical system, leading the system to explore a range of OH stretching frequencies (21,22). Like the wellknown "reflection principle" in electronic spectroscopy, when the displacements in VA curves are large, the envelope of the vibrational excitation profile can be viewed as a mirror of the groundstate vibrational wavefunction (33,34).…”

Ultrafast dynamics induced by the interaction of molecules with electromagnetic fields: Several quantum, semiclassical, and classical approaches

“…This broadening reflects strong coupling between the X–H stretch and other framework modes of the environment and structural heterogeneity 199 . The broadening even persists down to low temperatures and cooling the species does not lead to sharper bands 258 …”

Infrared Spectra of Deprotonated Dicarboxylic Acids: IRMPD Spectroscopy and Empirical Valence‐Bond Modeling

“…[27,28] Focusing on proton sharing between two carboxylate groups in the gas phase, the low temperature IR spectrum of oxalate (deprotonated dicarboxylic acid with n = 0) has recently become an interesting test bed for a number of theoretical approaches, as it displays a highly diffuse feature in the high frequency portion (ca. 2500-3200 cm À 1 ) corresponding to the stretching motion of the shared proton, even at 30 K. [13] While variational configuration interaction in an extended Fermi resonance formulation was found to account for the low frequency skeletal and OH bending motions, it was found necessary to rely on an adiabatic treatment of the variational wavefunction to describe the couplings of such motions with the high-frequency OH stretch. [13] In another theoretical study of the same spectrum, [28] MD was used in a number of flavors to account for both parts of the spectrum.…”

“…2500-3200 cm À 1 ) corresponding to the stretching motion of the shared proton, even at 30 K. [13] While variational configuration interaction in an extended Fermi resonance formulation was found to account for the low frequency skeletal and OH bending motions, it was found necessary to rely on an adiabatic treatment of the variational wavefunction to describe the couplings of such motions with the high-frequency OH stretch. [13] In another theoretical study of the same spectrum, [28] MD was used in a number of flavors to account for both parts of the spectrum. While semi-empirical MD using a density-functional based tight-binding approach did not reproduce the IR spectrum successfully, using the DFT B3LYP/6-31G PES yielded much better results, especially near room temperature.…”