Vibrational Signature of Dynamic Coupling of a Strong Hydrogen Bond

Abstract

Elucidating the dynamic couplings of hydrogen bonds remains an important and challenging goal for spectroscopic studies of bulk systems, because their vibrational signatures are masked by the collective effects of the fluctuation of many hydrogen bonds. Here we utilize sizeselected infrared spectroscopy based on a tunable vacuum ultraviolet free electron laser to unmask the vibrational signatures for the dynamic couplings in neutral trimethylamine−water and trimethylamine−methanol complexes, as microscopic models with only one single hydrogen bond holding two molecules. Surprisingly broad progression of OH stretching peaks with distinct intensity modulation over ∼700 cm −1 is observed for trimethylamine−water, while the dramatic reduction of this progression in the trimethylamine−methanol spectrum offers direct experimental evidence for the dynamic couplings. State-of-the-art quantum mechanical calculations reveal that such dynamic couplings are originated from strong Fermi resonance between the stretches of hydrogen-bonded OH and several motions of the solvent water/ methanol, such as translation, rocking, and bending, which are significant in various solvated complexes commonly found in atmospheric and biological systems.

Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C

“…Others have shown that an increase in temperature induces a frequency mismatch between the stretch fundamental and the bending overtone of bulk water, leading to a decrease in the Fermi resonance intramolecular coupling at ∼3250 cm –1 . − Intramolecular coupling dominates the vibrational relaxation pathways in interfacial water. ,− Thus, intramolecular coupling is possibly driving the temperature-dependent spectral shifts observed in Figure a, but the exact mechanisms responsible for these spectral shifts are currently unknown. Vibrational coupling to octadecanol- d 37 vibrational modes is also possible. , Additionally, it is nontrivial to distinguish between peak intensity contributions caused by changes in hydration shell number density and structure versus changes in transition dipole moment strength as a function of temperature, further complicating the spectral interpretation. Consequently, calculations were performed that intrinsically include intra- and intermolecular coupling to study the contribution of transition dipole moments changes due to temperature for a single hydrogen-bonding configuration (Section ).…”

“…Vibrational coupling to octadecanol-d 37 vibrational modes is also possible. 96,97 Additionally, it is nontrivial to distinguish between peak intensity contributions caused by changes in hydration shell number density and structure versus changes in transition dipole moment strength as a function of temperature, further complicating the spectral interpretation. Consequently, calculations were performed that intrinsically include intra-and intermolecular coupling to study the contribution of transition dipole moments changes due to temperature for a single hydrogen-bonding configuration (Section 3.2.2).…”

Evolution of the linker in microhydrated hydrogen dinitrate anions: From H+ to H5O2+

“…Instead, dynamic simulations are essential to reproduce its vibrational profile. [42][43][44][45][46][47] In this study, AIMD simulations are conducted by CP2K/Quickstep software. 48,49 The potential energy and atomic forces are calculated with DFT, using the BLYP functional with D3 correction, in consistency with the Gaussian calculations.…”

“…With the temperature calculated from the average kinetic energy, it is employed as a parameter to adjust the extent of atomic motions. [42][43][44][45][46][47] 3 | RESULTS AND DISCUSSION 3.1 | Structure and dynamics of HNO 3 (NO 3 À )…”

“…involves quantum effects such as tunneling or Fermi resonance. Quantum treatment is needed for the related modes, while neither harmonic analysis nor AIMD is expected to provide an accurate estimate of its frequency 47,[57][58][59]. 3.3 | Structure and dynamics of HNO 3 (NO 3 À )(H 2 O) The addition of one H 2 O molecule can be considered as adding a perturbation to the proton-bound dimer HNO 3 (NO 3 À ).…”