Electric nuclear quadrupole coupling reveals dissociation of HCl with a few water molecules.

Investigating the dissociation of acids in the presence of a limited number of water molecules is crucial for understanding various elementary chemical processes. In our study focusing on HCl(H2O)n clusters formed in a cold and isolated jet expansion, we utilized the nuclear quadrupole coupling tensor obtained via rotational spectroscopy to decipher the nature of the H-Cl chemical bond in a micro-aqueous environment. For n = 1 to 4, the H-Cl bond is covalent. At n = 5 and 7, the contact ion pair of H3O+Cl- is spontaneously formed within the hydrogen bond networks of "book" and "cube" acid-water clusters, respectively.

Evolution of the ionisation energy with the stepwise growth of chiral clusters of [4]helicene.

Polycyclic aromatic hydrocarbons (PAHs) are widely established as ubiquitous in the interstellar medium (ISM), but considering their prevalence in harsh vacuum environments, the role of ionisation in the formation of PAH clusters is poorly understood, particularly if a chirality-dependent aggregation route is considered. Here we report on photoelectron spectroscopy experiments on [4]helicene clusters performed with a vacuum ultraviolet synchrotron beamline. Aggregates (up to the heptamer) of [4]helicene, the smallest PAH with helical chirality, were produced and investigated with a combined experimental and theoretical approach using several state-of-the-art quantum-chemical methodologies. The ionisation onsets are extracted for each cluster size from the mass-selected photoelectron spectra and compared with calculations of vertical ionisation energies. We explore the complex aggregation topologies emerging from the multitude of isomers formed through clustering of P and M, the two enantiomers of [4]helicene. The very satisfactory benchmarking between experimental ionisation onsets vs. predicted ionisation energies allows the identification of theoretically predicted potential aggregation motifs and corresponding energetic ordering of chiral clusters. Our structural models suggest that a homochiral aggregation route is energetically favoured over heterochiral arrangements with increasing cluster size, hinting at potential symmetry breaking in PAH cluster formation at the scale of small grains.

Weak hydrogen bonding to halogens and chirality communication in propanols: Raman and microwave spectroscopy benchmark theory.

Constitutional and conformational isomers of bromopropanol are vibrationally and rotationally characterised with parallels drawn to the structural chlorine analogues. A previous microwave spectroscopic study of the chloropropanols is re-examined and all systems are explored by Raman jet spectroscopy. For bromine, the entire nuclear quadrupole coupling tensors are accurately determined and compared to their chlorine counterparts. Tensor asymmetry parameters are determined and linked with the hydrogen bond strength as indicated by the downshift of the OH-stretching frequency. The spectroscopic constants derived from the observed transitions are used as benchmarks for a large variety of electronic structure methods followed by harmonic and anharmonic rovibrational treatments. The CCSD(T) electronic structure calculations provide the best performance, in particular once anharmonic and relativistic corrections are applied or implied. Standard DFT approaches vary substantially with respect to their systematic error cancellation across the investigated species, and cost-effective compromises for the different observables are proposed.

Gas-Phase Characterization of Adipic Acid, 6-Hydroxycaproic Acid, and Their Thermal Decomposition Products by Rotational Spectroscopy.

We report the spectroscopic investigation of two bifunctional aliphatic carboxylic acids, namely, adipic acid and 6-hydroxycaproic acid, in the gas phase by combining high-resolution rotational spectroscopy and supersonic expansions. Their pure rotational spectra were successfully identified and characterized. However, due to the low thermal stability of these two chemicals, the measured rotational spectra were significantly congested with transitions corresponding to their decomposition products upon heating. We observed cyclopentanone and adipic anhydride in the spectrum of adipic acid and ε-caprolactone and its monohydrate in the spectrum of 6-hydroxycaproic acid. On the basis of the distinct fingerprints of both carboxylic acids and a series of their decomposition products, the spectra were analyzed in a time-segmented manner. This provides valuable insights into the thermal decomposition mechanisms of these two samples over time, which highlights the robustness of microwave spectroscopy as a potent tool for analyzing complex chemical mixtures in a species-, isomer-, and conformer-selective way.

Conformations of borneol and isoborneol in the gas phase: Their monomers and microsolvation clusters.

Borneol is a natural monoterpene with significant applications in various industries, including medicine and perfumery. It presents several diastereomers with different physical and chemical properties, influenced by their unique structures and interactions with molecular receptors. However, a complete description of its inherent structure and solvent interactions remains elusive. Here, we report a detailed investigation of the gas-phase experimental structures of borneol and isoborneol, along with the description of their microsolvation complexes with the common solvents water and dimethyl sulfoxide. The molecules and complexes were studied using chirped-pulse Fourier transform microwave spectroscopy coupled to a supersonic expansion source. Although three rotamers are potentially populated under the conditions of the supersonic expansion, only one of them was observed for each monomer. The examination of the monohydrated complexes revealed structures stabilized by hydrogen bonds and non-covalent C-H⋯O interactions, with water as the hydrogen bond donor. Interestingly, in the clusters with dimethyl sulfoxide, borneol and isoborneol change their roles acting as donors. We further identified a higher-energy rotamer of the borneol monomer in one of the complexes with dimethyl sulfoxide, while that rotamer was missing in the experiment for the monomer. This observation is not common and highlights a specific position in borneol especially favorable for forming stable complexes, which could have implications in the understanding of the unique physical and chemical properties of the diastereomers.

Wetting of a Hydrophobic Surface: Far-IR Action Spectroscopy and Dynamics of Microhydrated Naphthalene.

The interaction of water and polycyclic aromatic hydrocarbons is of fundamental importance in areas as diverse as materials science and atmospheric and interstellar chemistry. The interplay between hydrogen bonding and dipole-π interactions results in subtle dynamics that are challenging to describe from first principles. Here, we employ far-IR action vibrational spectroscopy with the infrared free-electron laser FELIX to investigate naphthalene with one to three water molecules. We observe diffuse bands associated with intermolecular vibrational modes that serve as direct probes of the loose binding of water to the naphthalene surface. These signatures are poorly reproduced by static DFT or Møller-Plesset computations. Instead, a rationalization is achieved through Born-Oppenheimer Molecular Dynamics simulations, revealing the active mobility of water over the surface, even at low temperatures. Therefore, our work provides direct insights into the wetting interactions associated with shallow potential energy surfaces while simultaneously demonstrating a solid experimental-computational framework for their investigation.

Increasing Complexity in Adamantyl Thioethers Characterized by Rotational Spectroscopy.

We report on the synthesis and characterization using high-resolution rotational spectroscopy of three bulky thioethers that feature an adamantyl group connected to a sulfur atom. Detailed experimental and theoretical structures are provided and compared with the 1,1'-diadamantyl ether. In addition, we expand on previous findings concerning microsolvation of adamantyl derivatives by investigating the cluster formation between these thioethers and a water molecule. The investigation of such clusters provides valuable insights into the sulfur-centered hydrogen bonding in thioethers with increasing size and steric repulsion.

Microwave Three-Wave Mixing Spectroscopy of Chiral Molecules in Weakly Bound Complexes.

Since the first experimental implementation in 2013, microwave three-wave mixing has emerged as a robust spectroscopic approach for analyzing and controlling chiral molecules in the gas phase. This resonant, coherent, and nonlinear technique is based on the three-dimensional light-matter interaction in the electric dipole approximation, allowing for isomer- and conformer-selective chiral analysis with high resolution. Here we demonstrate the utility of microwave three-wave mixing for analyzing a molecular complex, limonene-H2O, which serves as a compelling example of addressing its potential to improve the chiral sensitivity for only weakly polar chiral molecules. The use of molecular complexes can also extend the applicability of microwave three-wave mixing to chiral systems that are not in the C1 point group.

Hydrogen-Atom Tunneling in a Homochiral Environment.

The role-exchanging concerted torsional motion of two hydrogen atoms in the homochiral dimer of trans-1,2-cyclohexanediol was characterized through a combination of broadband rotational spectroscopy and theoretical modeling. The results reveal that the concerted tunneling motion of the hydrogen atoms leads to the inversion of the sign of the dipole moment components along the a and b principal axes, due to the interchange motion that cooperatively breaks and reforms one intermolecular hydrogen bond. This motion is also coupled with two acceptor switching motions. The energy difference between the two ground vibrational states arising from this tunneling motion was determined to be 29.003(2) MHz. The corresponding wavefunctions suggest that the two hydrogen atoms are evenly delocalized on two equivalent potential wells, which differs from the heterochiral case where the hydrogen atoms are confined in separate wells, as the permutation-inversion symmetry breaks down. This intriguing contrast in hydrogen-atom behavior between homochiral and heterochiral environments could further illuminate our understanding of the role of chirality in intermolecular interactions and dynamics.

Quantum Tunneling Facilitates Water Motion across the Surface of Phenanthrene.

Quantum tunneling is a fundamental phenomenon that plays a pivotal role in the motion and interaction of atoms and molecules. In particular, its influence in the interaction between water molecules and carbon surfaces can have significant implications for a multitude of fields ranging from atmospheric chemistry to separation technologies. Here, we unveil at the molecular level the complex motion dynamics of a single water molecule on the planar surface of the polycyclic aromatic hydrocarbon phenanthrene, which was used as a small-scale carbon surface-like model. In this system, the water molecule interacts with the substrate through weak O-H···π hydrogen bonds, in which phenanthrene acts as the hydrogen-bond acceptor via the high electron density of its aromatic cloud. The rotational spectrum, which was recorded using chirped-pulse Fourier transform microwave spectroscopy, exhibits characteristic line splittings as dynamical features. The nature of the internal dynamics was elucidated in great detail with the investigation of the isotope-substitution effect on the line splittings in the rotational spectra of the H218O, D2O, and HDO isotopologues of the phenanthrene-H2O complex. The spectral analysis revealed a complex internal dynamic showing a concerted tunneling motion of water involving its internal rotation and its translation between the two equivalent peripheral rings of phenanthrene. This high-resolution spectroscopy study presents the observation of a tunneling motion exhibited by the water monomer when interacting with a planar carbon surface with an unprecedented level of detail. This can serve as a small-scale analogue for water motions on large aromatic surfaces, i.e., large polycyclic aromatic hydrocarbons and graphene.

Chiral Control of Gas-Phase Molecules using Microwave Pulses.

Microwave three-wave mixing has emerged as a novel approach for studying chiral molecules in the gas phase. This technique employs resonant microwave pulses and is a non-linear and coherent approach. It serves as a robust method to differentiate between the enantiomers of chiral molecules and to determine the enantiomeric excess, even in complex chiral mixtures. Besides such analytical applications, the use of tailored microwave pulses allows us to control and manipulate chirality at the molecular level. Here, an overview of some recent developments in the area of microwave three-wave mixing and its extension to enantiomer-selective population transfer is provided. The latter is an important step towards enantiomer separation-in energy and finally in space. In the last section, we present new experimental results on how to improve enantiomer-selective population transfer to achieve an enantiomeric excess of about 40 % in the rotational level of interest using microwave pulses alone.

Inducing transient enantiomeric excess in a molecular quantum racemic mixture with microwave fields.

Chiral molecules with low enantiomer interconversion barriers racemize even at cryogenic temperatures due to quantum tunneling, forming a racemic mixture that is impossible to separate using conventional chemical methods. Here we both experimentally and theoretically demonstrate a method to create and probe a state-specific enantiomeric enrichment for such molecular systems. The coherent, non-linear, and resonant approach is based on a microwave six-wave mixing scheme and consists of five phase-controlled microwave pulses. The first three pulses induce a chiral wavepacket in a chosen rotational state, while the consecutive two pulses induce a polarization for a particular rotational transition (listen transition) with a magnitude proportional to the enantiomeric excess created. The experiments are performed with the transiently chiral molecule benzyl alcohol, where a chiral molecular response was successfully obtained. This signal demonstrates that enantiomeric excess can be induced in a quantum racemic mixture of a transiently chiral molecule using the developed microwave six-wave mixing scheme, which is an important step towards controlling non-rigid chiral molecular systems.

Rotational dive into the water clusters on a simple sugar substrate.

Most biomolecular activity takes place in aqueous environments, and it is strongly influenced by the surrounding water molecules. The hydrogen bond networks that these water molecules form are likewise influenced by their interactions with the solutes, and thus, it is crucial to understand this reciprocal process. Glycoaldehyde (Gly), often considered the smallest sugar, represents a good template to explore the steps of solvation and determine how the organic molecule shapes the structure and hydrogen bond network of the solvating water cluster. Here, we report a broadband rotational spectroscopy study on the stepwise hydration of Gly with up to six water molecules. We reveal the preferred hydrogen bond networks formed when water molecules start to form three-dimensional (3D) topologies around an organic molecule. We observe that water self-aggregation prevails even in these early stages of microsolvation. These hydrogen bond networks manifest themselves through the insertion of the small sugar monomer in the pure water cluster in a way in which the oxygen atom framework and hydrogen bond network resemble those of the smallest three-dimensional pure water clusters. Of particular interest is the identification, in both the pentahydrate and hexahydrate, of the previously observed prismatic pure water heptamer motif. Our results show that some specific hydrogen bond networks are preferred and survive the solvation of a small organic molecule, mimicking those of pure water clusters. A many-body decomposition analysis of the interaction energy is also performed to rationalize the strength of a particular hydrogen bond, and it successfully confirms the experimental findings.

Evolution of Solute-Water Interactions in the Benzaldehyde-(H2O)1-6 Clusters by Rotational Spectroscopy.

The investigation on the preferred arrangement and intermolecular interactions of gas phase solute-water clusters gives insights into the intermolecular potentials that govern the structure and dynamics of the aqueous solutions. Here, we report the investigation of hydrated coordination networks of benzaldehyde-(water)n (n = 1-6) clusters in a pulsed supersonic expansion using broadband rotational spectroscopy. Benzaldehyde (PhCHO) is the simplest aromatic aldehyde that involves both hydrophilic (CHO) and hydrophobic (phenyl ring) functional groups, which can mimic molecules of biological significance. For the n = 1-3 clusters, the water molecules are connected around the hydrophilic CHO moiety of benzaldehyde through a strong CO···HO hydrogen bond and weak CH···OH hydrogen bond(s). For the larger clusters, the spectra are consistent with the structures in which the water clusters are coordinated on the surface of PhCHO with both the hydrophilic CHO and hydrophobic phenyl ring groups being involved in the bonding interactions. The presence of benzaldehyde does not strongly interfere with the cyclic water tetramer and pentamer, which retain the same structure as in the pure water cluster. The book isomer instead of cage or prism isomers of the water hexamer is incorporated into the microsolvated cluster. The PhCHO molecule deviates from the planar structure upon sequential addition of water molecules. The PhCHO-(H2O)1-6 clusters may serve as a simple model system in understanding the solute-water interactions of biologically relevant molecules in an aqueous environment.

CO2 Aggregation on Monoethanolamine: Observations from Rotational Spectroscopy.

The initial stages of the gas-phase nucleation between CO2 and monoethanolamine were investigated via broadband rotational spectroscopy with the aid of extensive theoretical structure sampling. Sub-nanometer-scale aggregation patterns of monoethanolamine-(CO2 )n , n=1-4, were identified. An interesting competition between the monoethanolamine intramolecular hydrogen bond and the intermolecular interactions between monoethanolamine and CO2 upon cluster growth was discovered, revealing an intriguing CO2 binding priority to the hydroxyl group over the amine group. These findings are in sharp contrast to the general results for aqueous solutions. In the quinary complex, a cap-like CO2 tetramer was observed cooperatively surrounding the monoethanolamine. As the cluster approaches the critical size of new particle formation, the contribution of CO2 self-assembly to the overall stability increases.

Pump-probe spectroscopy of chiral vibrational dynamics.

A planar molecule may become chiral upon excitation of an out-of-plane vibration, changing its handedness during half a vibrational period. When exciting such a vibration in an ensemble of randomly oriented molecules with an infrared laser, half of the molecules will undergo the vibration phase-shifted by π compared to the other half, and no net chiral signal is observed. This symmetry can be broken by exciting the vibrational motion with a Raman transition in the presence of a static electric field. Subsequent ionization of the vibrating molecules by an extreme ultraviolet pulse probes the time-dependent net handedness via the photoelectron circular dichroism. Our proposal for pump-probe spectroscopy of molecular chirality, based on quantum-chemical theory and discussed for the example of the carbonyl chlorofluoride molecule, is feasible with current experimental technology.

Steroid Hormone Androsterone Observed in the Gas Phase by Rotational Spectroscopy.

We report the investigation of the steroid hormone androsterone in the gas phase. Androsterone is a male sex steroid hormone, being the first steroid hormone from this category isolated and discovered 90 years ago. Despite the chemical compositions of steroids being well-known since long ago, studying their structures in the gas phase is still a challenging task, and to date, just a handful of detailed experimental structures for steroids have been reported. The rotational spectrum of androsterone was recorded in the 2-8 GHz frequency region with a broadband chirped-pulse Fourier transform microwave spectrometer coupled with supersonic expansion. From the weak rotational spectrum, one conformer has been detected in the isolated and cold conditions of the molecular jet. The combination of the experimental results with quantum chemical calculations allowed us to unambiguously identify the conformation of androsterone in the gas phase.

The many forms of alpha-methoxy phenylacetic acid in the gas phase: flexibility, internal dynamics, and their intramolecular interactions.

We present a rotational spectroscopy study of alpha-methoxy phenylacetic acid in the gas phase. This acid is a derivative of mandelic acid and is used in various organic reactions. The conformational landscape of alpha-methoxy phenylacetic acid was explored to gain insight into its intramolecular dynamics. A rich rotational spectrum was obtained using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz range. Five conformers out of six calculated low-energy forms were identified in the spectrum, and the assignment of the 13C singly substituted isotopologues for the lowest-energy conformer led to its accurate structure determination. Splitting patterns were analyzed and attributed to the internal rotation of a methyl top. The analysis of the non-covalent interactions within the molecule highlights the subtle balance in the stabilization of the different conformers. We thus provide high-level structural and intramolecular dynamics information that is also used to benchmark the performance of quantum-chemical calculations.

Examining the gas-phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane using quantum chemistry and microwave spectroscopy.

Gas phase homodimers of 3,3,3-trifluoro-1,2-epoxypropane (TFO), a molecule which has shown promise as an effective chiral tag for determining the absolute stereochemistry and the enantiomeric composition of chiral analytes, are explored using a variety of quantum chemistry models and rotational spectroscopy. The potential surface governing the interaction of the two molecules is rapidly explored using the artificial bee colony algorithm for homodimer candidates that are subsequently optimized by quantum chemistry methods. Although all model chemistries employed agree that the lowest energy form of the heterochiral homodimer of TFO (RS or SR) is lower in energy than that of the homochiral dimer (RR or SS), the energy spacings among the lower energy isomers of each and indeed the absolute energy ordering of the isomers of each are very model dependent. The experimental results suggest that the B3LYP-D3BJ/def2-TZVP model chemistry is the most reliable and provides excellent estimates of spectroscopic constants. In accord with theoretical predictions the non-polar lowest energy form of the heterochiral homodimer is not observed, while two isomers of the homochiral dimer are observed and spectroscopically characterized. Observation and assignment of the spectra for all three unique singly-substituted 13C isotopologues, in addition to that of the most abundant isotopologue for the lowest energy isomer of the homochiral homodimer of TFO, provide structural information that compares very favorably with theoretical predictions, most notably that the presence of three fluorine atoms on the trifluoromethyl group removes their direct participation in the intermolecular interactions, which instead comprise two equivalent pairs of CH⋯O hydrogen bonds between the two epoxide rings augmented by favorable dispersion interactions between the rings themselves.

Microhydrated 3-Methyl-3-oxetanemethanol: Evolution of the Hydrogen-Bonding Network from Chains to Cubes.

Broadband rotational spectroscopy is used to investigate the geometries of 3-methyl-3-oxetanemethanol and its complexes with up to six water molecules, which are produced in supersonic jets. The main low-energy isomers of these clusters are unambiguously identified in the spectra with the support of quantum-chemical calculations. The conformation of the 3-methyl-3-oxetanemethanol geometry is found to be influenced by the microsolvation effects. The hydrogen-bond arrangements in the hydrate complexes, which are governed by the water-water and water-solute interactions, exhibit characteristic configurations with increasing number of water molecules and resemble the main isomers of the corresponding pure water clusters. Evolution of the hydrogen-bonding structures from one-dimensional chains to two-dimensional rings and further to multicyclic three-dimensional networks is observed, which provides information about the build-up process.