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

Effect of Temperature on the OH-Stretching Bands of the Methanol Dimer

We present a conceptually simple model for understanding the significant spectral changes that occur with the temperature in the infrared spectra of hydrogen-bound complexes. We have measured room-temperature spectra of the methanol dimer and two deuterated isotopologues in the OH(D)-stretching region. We correctly predict spectral changes observed in the gas phase for the bound OH stretch in the methanol dimer from jet-cooled to room temperature and corroborate this with experimental and theoretical results for deuterated isotopologues. The origin of the observed spectral features is explained based on a reduced-dimensional vibrational model, which includes the two high-frequency OH stretches, the two methyl torsions, and the six intermolecular low-frequency vibrations. Key to the success of the model is a new coordinate definition to describe the intrinsic large-amplitude curvilinear motion of low-frequency vibrations. Despite the deceivingly simple appearance of the room temperature bound OH-stretching fundamental band, it consists of ∼107 vibrational transitions.

Microhydration of Tertiary Amines: Robust Resonances in Red-Shifted Water

Tertiary amines are strong hydrogen bond acceptors. When a water molecule donates one of the OH groups, its in-phase stretching vibration wavenumber is decreased to such an extent that it comes close to the water bending overtone. This gives rise to anharmonic phenomena such as classical Fermi resonance, resonance with multiple-quantum dark states, or combination transitions with low-frequency intermolecular modes. These effects, which contribute to the spectral breadth of room-temperature hydrogen-bonded amine complexes, are disentangled by Fourier transform infrared spectroscopy in pulsed supersonic slit jet expansions. Monohydrates of the amines quinuclidine, N-methylpyrrolidine, N-methylpiperidine, and dimethylcyclohexylamine exhibit systematic mode coupling signatures. These suggest relatively fast energy flow out of the excited OH stretching fundamental into intra- and intermolecular degrees of freedom of the hydrogen-bonded water molecule. Trimeric complexes are spectroscopically separated from the amine monohydrates.

Quantum Chemical Investigation of the Cold Water Dimer Spectrum in the First OH-Stretching Overtone Region Provides a New Interpretation

Intramolecular vibrational transition wavenumbers and intensities were calculated in the fundamental HOH-bending, fundamental OH-stretching, first OH-stretching-HOH-bending combination, and first OH-stretching overtone (ΔvOH = 2) regions of the water dimer's spectrum. Furthermore, the rotational-vibrational spectrum was calculated in the ΔvOH = 2 region at 10 K, corresponding to the temperature of the existing jet-expansion experiments. The calculated spectrum was obtained by combining results from a full-dimensional (12D) vibrational and a reduced-dimensional vibrational-rotational-tunneling model. The ΔvOH = 2 spectral region is rich in features due to contributions from multiple vibrational-rotational-tunneling sub-bands. Origins of the experimental vibrational bands depend on the assignment of the observed sub-bands. Based on our calculations, we assign the observed sub-bands, and our reassignment leads to new values for the vibrational band origins of the free donor and antisymmetric acceptor OH-stretching first overtones of ∼7227 and ∼7238 cm-1, respectively. The observed bands with origins at 7192.34 and ∼7366 cm-1 are assigned to the symmetric acceptor OH-stretching first overtone and the OH-stretching combination of the donor, respectively.

Vibrational Spectroscopy of the Water Dimer at Jet-Cooled and Atmospheric Temperatures

The vibrational spectroscopy of the water dimer provides an understanding of basic hydrogen bonding in water clusters, and with about one water dimer for every 1,000 water molecules, it plays a critical role in atmospheric science. Here, we review how the experimental and theoretical progress of the past decades has improved our understanding of water dimer vibrational spectroscopy under both cold and warm conditions. We focus on the intramolecular OH-stretching transitions of the donor unit, because these are the ones mostly affected by dimer formation and because their assignment has proven a challenge. We review cold experimental results from early matrix isolation to recent mass-selected jet expansion techniques and, in parallel, the improvements in the theoretical anharmonic models. We discuss and illustrate changes in the vibrational spectra of complexes upon increasing temperature, and the difficulties in recording and calculating these spectra. In the atmosphere, water dimer spectra at ambient temperature are crucial. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Setting up the HyDRA blind challenge for the microhydration of organic molecules

The procedure leading to the first HyDRA blind challenge for the prediction of water donor stretching vibrations in monohydrates of organic molecules is described. A training set of 10 monohydrates...

Proton Transfer in Nitromethane-Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations

It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane-ammonia (CH₃NO₂-NH₃). It is found that in the neutral cluster, CH₃NO₂ and NH₃ are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH₃NO₂-NH₃, the positive charge redistributes from the ionized nitrogen atom of NH₃ to the CH₃NO₂ molecule immediately. Then, the proton of CH₃NO₂ transfers to NH₃ to form a proton-transferred type structure CH₂NO₂-NH₄⁺, without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH₃NO₂-NH₃ cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH₃NO₂-NH₃ cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.

Vibrational Signature of Dynamic Coupling of a Strong Hydrogen Bond

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 size-selected 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.

A Rather Universal Vibrational Resonance in 1:1 Hydrates of Carbonyl Compounds

When the lower frequency OH stretching fundamental of a water molecule is shifted to the 3500 cm^-1 spectral range by the solvation of a carbonyl compound, in this case a ketone, its infrared intensity is shared with a dark state. It is shown by chemical and isotope substitution for more than a dozen systems that the location of this resonance is remarkably substitution-independent. Harmonic and anharmonic model calculations support its assignment to a combination of the water bending overtone and in-plane water libration. This previously unrecognized intramolecular-intermolecular coupling in single solvent water has a strength of 7-10 cm^-1. It may have been sporadically observed before in a few other carbonyl compounds such as amides, without any previous exploration of its potential universality. The resulting generic picosecond energy redistribution channel for aqueous solutions may represent a slow counterpart and doorway model of what happens on a subpicosecond time scale when the hydrogen bonds become stronger, such as in carboxylic acid dimers or protonated water clusters.

Exploring the non-covalent interactions behind the formation of amine–water complexes: the case of N-allylmethylamine monohydrate

Rotational spectroscopy and quantum chemical studies reveal the effects of hydrogen bonding with water on the conformer equilibrium of N-allylmethylamine.