Quantum Tunneling Facilitates Water Motion across the Surface of Phenanthrene

Formation of Water Networks on Anionic Perylene

We present infrared photodissociation spectra of hydrated perylene anion clusters with up to four water molecules, as well as electronic structure calculations based on density functional theory. Water molecules form weak hydrogen bonds to the π system of the perylene anion. For clusters with more than one water molecule, water-water hydrogen bonds are formed, which generally appear to be stronger than water-π hydrogen bonds, especially for the trihydrate and tetrahydrate. The resulting water networks exist as water subclusters on the surface of the carbon frame of perylene. We observe temperature-dependent dynamic effects, which highlight large amplitude motions of the water network and the shallowness of the potential energy surfaces governing the structures of these clusters.

Quantum Tunneling Fingerprints of Chirality-Induced Symmetry Preferences in Methyl Lactate Dimer

Methyl lactate, a chiral molecule with multiple functional groups, has played a pivotal role in advancing experimental and theoretical chiroptical methods. Leveraging conformer-specific jet-cooled rotational spectroscopy in tandem with extensive conformational searches and quantum chemical calculations, we investigated chirality self-recognition in the methyl lactate dimer. The experimental fingerprint-like spectral patterns, including methyl rotor tunneling splittings, allowed the definite identification of one heterochiral and two homochiral binary conformers from a large number of low-energy candidates. Nuclear spin statistics analyses and methyl internal rotor parameters reveal different nuclear tunneling dynamics in the homochiral versus heterochiral environments and highlight the associated chirality-driven symmetry preference in the observed conformers. The results provide comprehensive experimental data for benchmarking quantum chemical calculations of chiral properties and pave the way for the exploration of this prototypical dimer across different frequency ranges using other spectroscopic tools.

Quantum mechanical and machine learning prediction of rotational energy barriers in halogenated aromatic alcohols

CONTEXT

Rotation about a chemical bond is important in many chemical processes and can be influenced by neighboring substituents on a molecule. Rotational energy barriers can be predicted by density functional theory (DFT) calculations. Here, we specifically explore how substituents influence the barrier to rotation about the C-O bond in symmetrically halogenated aromatic alcohols. A machine learning model was trained on the DFT-calculated rotational energies and was found to do a good job predicting rotational energy barriers from the electronegativity, atomic radius, and Hammett constant for each substituent. The machine learning model was found to perform better when it was trained separately on pyrenols, anthranols, or phenols than when it was trained on all classes of compounds together. Even though the models were trained on compounds containing only one kind of substituent, they were found to perform similarly well on compounds containing mixed substituents. Machine learning was able to predict the rotational energy barrier heights better than correlations among parameters that would be expected to be relevant based on chemical intuition.

METHODS

DFT calculations were done with Gaussian 16 software at the B3LYP/6-311 + G(d.p) level of theory. Machine learning was done using the classification and regression training (caret) package in R version 4.4.0.

Scaling of Rotational Constants

This manuscript introduces the concept of scaling factors for rotational constants. These factors are designed to bring computed equilibrium rotational constants closer to experimentally fitted ground-state-averaged rotational constants. The parameterization of the scaling factors was performed for several levels of theory, namely DF-Dn/def2-mVP (DF=B3LYP,PBE0, n=3(BJ),4, m=S,TZ), PBEh-3c, and r⁢2SCAN-3c. The obtained scaling factors systematically improved the consistency between the theoretical and experimental rotational constants.

Complex Dance of Molecules in the Sky: Choreography of Intermolecular Structure and Dynamics in the Cyclopentene-CO2-H2O Hetero Ternary Cluster

This study delves into driving forces behind the formation of a hetero ternary cluster consisting of volatile organic compounds from industrial or combustion sources, specifically cyclopentene, alongside greenhouse gases like carbon dioxide, and water vapor. While substantial progress has been made in understanding binary complexes, the structural intricacies of hetero ternary clusters remain largely uncharted. Our research characterized the cyclopentene-CO2-H2O hetero ternary cluster utilizing Fourier transform microwave spectroscopy. The observed isomer in the pulsed jet has CO2 and H2O aligning above the cyclopentene ring, with water undergoing an internal rotation approximately about its C2 symmetry axis. Theoretical analyses support these observations, identifying an O-H⋅⋅⋅π hydrogen bond and a secondary C⋅⋅⋅O tetrel bond within this cluster. This study marks a critical step towards comprehending the molecular dynamics and interactions of VOCs, greenhouse gases, and water in the atmosphere, paving the way for further investigations into their roles in climate dynamics and air quality.

The complexities of ligand/receptor interactions: Exploring the role of molecular vibrations and quantum tunnelling

Molecular vibrations and quantum tunneling may link ligand binding to the function of pharmacological receptors. The well-established lock-and-key model explains a ligand's binding and recognition by a receptor; however, a general mechanism by which receptors translate binding into activation, inactivation, or modulation remains elusive. The Vibration Theory of Olfaction was proposed in the 1930s to explain this subset of receptor-mediated phenomena by correlating odorant molecular vibrations to smell, but a mechanism was lacking. In the 1990s, inelastic electron tunneling was proposed as a plausible mechanism for translating molecular vibration to odorant physiology. More recently, studies of ligands' vibrational spectra and the use of deuterated ligand analogs have provided helpful information to study this admittedly controversial hypothesis in metabotropic receptors other than olfactory receptors. In the present work, based in part on published experiments from our laboratory using planarians as an experimental organism, I will present a rationale and possible experimental approach for extending this idea to ligand-gated ion channels.

Competition between Water-Water Hydrogen Bonds and Water-π Bonds in Pyrene-Water Cluster Anions

We present infrared spectra and density functional theory calculations of hydrated pyrene anion clusters with up to four water molecules. The experimental spectra were acquired by using infrared Ar messenger photodissociation spectroscopy. Water molecules form clusters on the surface of the pyrene, forming hydrogen bonds with the π-system. The structures of the water clusters and their interaction with the π-system are encoded in OH stretching vibrational modes. We find that the interactions between water molecules are stronger than the interactions between water molecules and the π-system. While all clusters show multiple conformers, three- and four-membered rings are the lowest energy structures in the larger hydrates.

Water-Hydrocarbon Interactions in Anionic Pyrene Monohydrate

Interactions between water and polycyclic aromatic hydrocarbons are essential in many aspects of chemistry, from interstellar and atmospheric processes to interfacial hydrophobicity and wetting phenomena. Despite their growing importance, the intermolecular potentials of the water-hydrocarbon interactions are underdeveloped compared to the water-water potentials, and there are similarly few experimental probes that are sensitive to the details of the water-hydrocarbon potential. We present a combined experimental and computational study of anionic pyrene monohydrate, one of the simplest water/hydrocarbon clusters. The action spectrum in the OH region of the mass-selected cluster ion provides a rigorous benchmark for intermolecular potentials and computational methodologies. We identify missing intermolecular interactions and shortcomings in conventional dynamics calculations by comparing experimental data to density functional theory and classical molecular dynamics calculations. Kinetic trapping is prevalent, even for one water molecule and one pyrene molecule, leading to slow equilibration in conventional molecular dynamics calculations, even on nanosecond time scales and at low temperatures (50 K). At constant energy, temperature fluctuations for the pair of molecules are substantial. Immersing the system in a bath of soft spheres and employing parallel tempering alleviates kinetic trapping and dampens temperature fluctuations, bringing the system closer to the thermodynamic limit. With such augmented sampling, a simple, flexible water model reproduces the line width and the asymmetric broadening of the symmetric OH stretching mode, which we assign to spectral diffusion. In the OH stretching region, dynamics calculations predict a more intense antisymmetric peak than experiments observe but do not predict the bimodal split symmetric peak that the experiments show. Our work suggests that electronic polarization, missing in the empirical force field, is responsible for the first discrepancy and that quantum nuclear effects, captured neither in density functional theory nor in classical dynamics, may be responsible for the second.

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.