Infrared spectrum of liquid and crystalline ammonia

Solvation Shell Structures of Ammonia in Reline and Ethaline Deep Eutectic Solvents

Because of increasing atmospheric anthropogenic ammonia (NH₃) emission, researchers are devising new techniques to capture NH₃. Deep eutectic solvents (DESs) are found as potential media for NH₃ mitigation. In the present study, we have carried out ab initio molecular dynamics (AIMD) simulations to decipher the solvation shell structures of an ammonia solute in reline (1:2 mixture of choline chloride and urea) and ethaline (1:2 mixture of choline chloride and ethylene glycol) DESs. We aim to resolve the fundamental interactions which help stabilize NH₃ in these DESs, focusing on the structural arrangement of the DES species in the nearest solvation shell around NH₃ solute. In reline, the hydrogen atoms of NH₃ are preferentially solvated by chloride anions and the carbonyl oxygen atoms of urea. The nitrogen atom of NH₃ renders hydrogen bonding with hydroxyl hydrogen of the choline cation. The positively charged head groups of the choline cations prefer to stay away from NH₃ solute. In ethaline, strong hydrogen bonding interaction exists between the nitrogen atom of NH₃ and hydroxyl hydrogen atoms of ethylene glycol. The hydrogen atoms of NH₃ are found to be solvated by hydroxyl oxygen atoms of ethylene glycol and choline cation. While ethylene glycol molecules play a crucial role in solvating NH₃, the chloride anions remain passive in deciding the first solvation shell. In both the DESs, choline cations approach NH₃ from their hydroxyl group side. We observe slightly stronger solute-solvent charge transfer and hydrogen bonding interaction in ethaline than those in reline.

Electrofreezing of Liquid Ammonia

Here we prove that, in addition to temperature and pressure, another important thermodynamic variable permits the exploration of the phase diagram of ammonia: the electric field. By means of (path integral) ab initio molecular dynamics simulations, we predict that, upon applying intense electric fields on ammonia, the electrofreezing phenomenon occurs, leading the liquid toward a novel ferroelectric solid phase. This study proves that electric fields can generally be exploited as the access key to otherwise-unreachable regions in phase diagrams, unveiling the existence of new condensed-phase structures. Furthermore, the reported findings have manifold practical implications, from the safe storage and transportation of ammonia to the understanding of the solid structures this compound forms in planetary contexts.

Crystal engineering of aurophilic supramolecular architectures and coordination polymers based on butterfly-like Copper-dicyanoaurate complexes: vapochromism, P-T behaviour and multi-metallic cocrystal formation

Using the equilibrium properties of CuII in the presence of the chelating ligand and the characteristics of the dicyanoaurate anion, we were able to obtain a family of 10 bimetallic...

Benzene Radical Anion in the Context of the Birch Reduction: When Solvation Is the Key

The benzene radical anion is an important intermediate in the Birch reduction of benzene by solvated electrons in liquid ammonia. Beyond organic chemistry, it is an intriguing subject of spectroscopic and theoretical studies due to its rich structural and dynamical behavior. In the gas phase, the species appears as a metastable shape resonance, while in the condensed phase, it remains stable. Here, we approach the system by ab initio molecular dynamics in liquid ammonia and demonstrate that the inclusion of solvent is crucial and indeed leads to stability. Beyond the mere existence of the radical anion species, our simulations explore its condensed-phase behavior at the molecular level and offer new insights into its properties. These include the dynamic Jahn-Teller distortions, vibrational spectra in liquid ammonia, and the structure of the solvent shell, including the motif of a π-hydrogen bond between ammonia molecules and the aromatic ring.

Aqueous Solvation of Ammonia and Ammonium: Probing Hydrogen Bond Motifs with FT-IR and Soft X-ray Spectroscopy

In a multifaceted investigation combining local soft X-ray and vibrational spectroscopic probes with ab initio molecular dynamics simulations, hydrogen-bonding interactions of two key principal amine compounds in aqueous solution, ammonia (NH₃) and ammonium ion (NH₄⁺), are quantitatively assessed in terms of electronic structure, solvation structure, and dynamics. From the X-ray measurements and complementary determination of the IR-active hydrogen stretching and bending modes of NH₃ and NH₄⁺ in aqueous solution, the picture emerges of a comparatively strongly hydrogen-bonded NH₄⁺ ion via N-H donating interactions, whereas NH₃ has a strongly accepting hydrogen bond with one water molecule at the nitrogen lone pair but only weakly N-H donating hydrogen bonds. In contrast to the case of hydrogen bonding among solvent water molecules, we find that energy mismatch between occupied orbitals of both the solutes NH₃ and NH₄⁺ and the surrounding water prevents strong mixing between orbitals upon hydrogen bonding and, thus, inhibits substantial charge transfer between solute and solvent. A close inspection of the calculated unoccupied molecular orbitals, in conjunction with experimentally measured N K-edge absorption spectra, reveals the different nature of the electronic structural effects of these two key principal amine compounds imposed by hydrogen bonding to water, where a pH-dependent excitation energy appears to be an intrinsic property. These results provide a benchmark for hydrogen bonding of other nitrogen-containing acids and bases.

In Situ Attenuated Total Reflection Infrared Spectroscopy on Electrochemical Ammonia Oxidation over Pt Electrode in Alkaline Aqueous Solutions

The electrochemical oxidation of ammonia over Pt electrode in alkaline aqueous solutions was studied by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. In 0.1 M NH3-1 M KOH, the band ascribable to the HNH bending mode of adsorbed NH3 was confirmed at 1662-1674 cm(-1) in the potential range of 0.1-1.1 V. The intensity of this band decreased continuously with a rise in potential, indicating the oxidative consumption of adsorbed ammonia. In response to this behavior, the band at 1269 cm(-1) appeared alternatively above 0.2 V, and its intensity reached the local maximal value at ca. 0.4 V. Note that this potential of ca. 0.4 V agreed well with the onset potential of ammonia oxidation, ca. 0.45 V, in the linear sweep voltammogram. This 1269 cm(-1) band was assigned to the NH2 wagging mode of N2H4, which was one of the active intermediates, N2H(x+y,ad) (x = 1 or 2, y = 1 or 2), according to the mechanism proposed by Gerischer and Mauere. To the best of our knowledge, this is the first report for the detection of N2H4 as a reaction intermediate over Pt electrode. Furthermore, the formation of bridged NO was also observed above the onset potential of ammonia oxidation, ca. 0.5 V. Such adsorbed NO species probably inhibit the electrochemical reaction due to the occupation of reaction sites at higher potential.

Infrared spectroscopy of large-sized neutral and protonated ammonia clusters

Size selective IR spectroscopy shows the nature of hydrogen bond networks in neutral and protonated ammonia clusters.

Infrared photodissociation spectroscopy of protonated ammonia cluster ions, NH4+(NH3)n (n=5–8), by using infrared free electron laser

Infrared photodissociation action spectra of protonated ammonia cluster ions, NH(4) (+)(NH(3))(n) (n=5-8), were measured in the range of 1020-1210 cm(-1) by using a tunable infrared free electron laser. Analyses by the density functional theory (DFT) show that the spectral features observed can be assigned to the nu(2) vibrational mode of the NH(3) molecules in NH(4) (+)(NH(3))(n). Size dependence of the spectra supports structural models obtained by the DFT calculations, in which the NH(4) (+) ion is solvated by the four nearest-neighbor NH(3) molecules. For NH(4) (+)(NH(3))(5), the spectrum between 1000 and 1700 cm(-1) was measured. The nu(4) bands of the NH(3) molecules and the NH(4) (+) ion were found in the range of 1420-1700 cm(-1).

Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes n = 29, 80, 212, 447, and 989 and by depletion spectroscopy for n=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu3 NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for n = 8 and 29, but only about 0.40 of the intensity of the nu3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2nu4 and the symmetric NH-stretch nu1 in the range from 3150 to 3450 cm(-1) is changed between n = 10 and 100, while the asymmetric NH-stretch nu3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.

Raman Frequency Shifts for the Rotatory Lattice Mode Close to the Melting Point in Ammonia Solid I

We correlate here the thermal expansivity alphap to the frequency shifts 1/nu(partial differential nu/partial differential P)T for the rotatory lattice (librational) mode in ammonia solid I close to the melting point. This is carried out for the pressures of 0, 1.93, and 3.07 kbar at various temperatures for this solid structure. By obtaining linear plots of alphap versus 1/nu(partial differential nu/partial differential P)T for the pressures studied, we extract the values of the slope dPm/dT according to our spectroscopic relation. Our calculated values of dPm/dT can be compared with the experimental ones for ammonia solid I close to the melting point.

Vibrational delocalization in ammonia aerosol particles

The present contribution investigates shape effects and surface effects in the infrared spectra of pure (NH(3)) and mixed (NH(3)-CO(2) and NH(3)-NH(2)D-NHD(2)-ND(3)) ammonia particles with sizes between about 1 and 50 nm. The particles investigated have been generated in a collisional cooling cell as aerosols at temperatures between 20 and 80 K. The contribution reveals that only the combination with a microscopic model leads to a comprehensive understanding of the various features observed in the experimental infrared spectra. As one of the major results, the corresponding exciton model explains why pronounced shape effects observed for pure particles only play a minor role in the case of mixed particles.

Infrared spectroscopy of the solid phases of ammonia

Thin films of solid ammonia (NH(3) and ND(3)) have been characterized using low temperature (25-110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the nu(2) and nu(3) regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time.