Theory vs. experiment for molecular clusters: Spectra of OCS trimers and tetramers

A Rotational Study of 2-tert-Butylphenol and Its 1 : 1 Argon Complex

The chirped-pulse Fourier Transform microwave spectrum of 2-tert-butylphenol, an industrial intermediate for the production of antioxidants, has been investigated in the 2-8 GHz frequency range. The spectral analysis has allowed obtaining precise structural information on the most stable conformer and its complex with argon. The conformation of the monomer reveals that the hydroxyl group is coplanar with the ring but points in the opposite direction to the tert-butyl group, reducing steric interactions. In the tert-butyl group one methyl group is coplanar and the other two are symmetrically staggered respect to the ring. The complex shows the rare gas sitting above the aromatic ring. Interestingly, neither the monomer nor the complex exhibit large-amplitude hydroxyl torsion motions, previously observed in 2,6-disubstituted phenols such as 2,6-di-tert-butylphenol or propofol. The experimental results are supported by computational calculations, validating the molecular structure. Additionally, symmetry-adapted perturbation theory has allowed determining the van der Waals intermolecular interaction energy of the complex.

The Structure of 2,6-Di-tert-butylphenol-Argon by Rotational Spectroscopy

The molecular structure of a van der Waals-bonded complex involving 2,6-di-tert-butylphenol and a single argon atom has been determined through rotational spectroscopy. The experimentally derived structural parameters were compared to the outcomes of quantum chemical calculations that can accurately account for dispersive interactions in the cluster. The findings revealed a π-bound configuration for the complex, with the argon atom engaging the aromatic ring. The microwave spectrum reveals both fine and hyperfine tunneling components. The main spectral doubling is evident as two distinct clusters of lines, with an approximate separation of 179 MHz, attributed to the torsional motion associated with the hydroxyl group. Additionally, each component of this doublet further splits into three components, each with separations measuring less than 1 MHz. Investigation into intramolecular dynamics using a one-dimensional flexible model suggests that the main tunneling phenomenon originates from equivalent positions of the hydroxyl group. A double-minimum potential function with a barrier of 1000 (100) cm-1 effectively describes this extensive amplitude motion. However, the three-fold fine structure, potentially linked to internal motions within the tert-butyl group, requires additional scrutiny for a comprehensive understanding.

Characterizing the Interactions of Dimethyl Sulfoxide with Water: A Rotational Spectroscopy Study

The interaction of dimethyl sulfoxide with water has been investigated by Fourier-transform microwave spectroscopy of the 1:1 complex and its isotopologues, complemented with quantum chemical calculations. The rotational spectra of ³⁴S and ¹³C isotopologues in natural abundance and the H₂¹⁸O and deuterated water enriched isotopologues have been measured, allowing a partial structure determination and establishing the position of water in the complex. In the most stable conformation water was found to be the donor of a primary OH···OS bond to the oxygen atom of dimethyl sulfoxide and acceptor of two weak CH···OH bonds with the methyl hydrogen atoms of dimethyl sulfoxide. From the structural determination confirmed by quantum chemical calculations, the water molecule lies in the symmetry plane of dimethyl sulfoxide, and the complex has an overall C_s symmetry. The experimental findings are supported by atoms in molecules and symmetry-adapted perturbation theories, which allowed for determining the hydrogen bond and intermolecular interaction energies, respectively.

A rotational study of the 1:1 adduct of ethanol and 1,4-dioxane

The pure rotational spectra of the 1:1 ethanol - 1,4-dioxane complex and its OD mono-deuterated species have been measured using pulsed-jet Fourier transform microwave spectroscopy. Conformational predictions for the plausible isomers of ethanol - 1,4-dioxane have been carried out considering the spatial orientation of gauche/trans ethanol with respect to the chair/boat and twisted conformations of 1,4-dioxane. Using Helium for the supersonic expansion, the microwave spectrum has been observed for the most stable structure. In the observed isomer, the two subunits are linked together by an OH⋯O hydrogen bond with gauche ethanol acting as proton donor to dioxane in the chair conformation. The non-covalent interactions have been characterized using different computational approaches. A small inverse Ubbelohde effect was observed after H → D isotopic substitution in the OH⋯O hydrogen bond.

Computational optimal transport for molecular spectra: The fully discrete case

The use of computational optimal transport is investigated as a tool for comparing two molecular spectra. Unlike other techniques for comparing molecular spectra in a pattern-recognition framework, transport distances simultaneously encode information about line positions and intensities. In addition, it is shown that transport distances are a useful alternative to Euclidean distances as Euclidean distances are based on line-by-line comparisons, while transport distances reflect broader features of molecular spectra and adequately compare spectra with different resolutions. This paper includes a tutorial on the use of optimal transport and investigates several well-chosen examples to illustrate the utility of computational optimal transport for comparing molecular spectra.

The Role of Non-Covalent Interactions on Cluster Formation: Pentamer, Hexamers and Heptamer of Difluoromethane

The role of non-covalent interactions (NCIs) has broadened with the inclusion of new types of interactions and a plethora of weak donor/acceptor partners. This work illustrates the potential of chirped-pulse Fourier transform microwave technique, which has revolutionized the field of rotational spectroscopy. In particular, it has been exploited to reveal the role of NCIs' in the molecular self-aggregation of difluoromethane where a pentamer, two hexamers and a heptamer were detected. The development of a new automated assignment program and a sophisticated computational screening protocol was essential for identifying the homoclusters in conditions of spectral congestion. The major role of dispersion forces leads to less directional interactions and more distorted structures than those found in polar clusters, although a detailed analysis demonstrates that the dominant interaction energy is the pairwise interaction. The tetramer cluster is identified as a structural unit in larger clusters, representing the maximum expression of bond between dimers.

Observation of the elusive "oxygen-in" OCS dimer

The carbonyl sulfide (OCS) dimer serves as a prototype system for studying intermolecular forces between nonsymmetrical linear polyatomic molecules. Here, we performed a laser spectroscopic investigation of OCS dimers embedded in helium nanodroplets and found rovibrational bands corresponding to the non-polar "sulfur-in" and parallel polar dimers that have been extensively characterized in the gas phase, as well as a new non-polar "oxygen-in" dimer that has long been predicted by theory. Frequency alternations in the rotational branches along with the absence of a Stark effect provided important clues as to its assignment.

Atmospherically relevant acrolein–water complexes: spectroscopic evidence of aldehyde hydration and oxygen atom exchange

Rotational spectroscopy and isotopic studies evidence oxygen exchange in water complexes of atmospherically important acrolein.

Weak Hydrogen Bond Network: A Rotational Study of 1,1,1,2-Tetrafluoroethane Dimer

1,1,1,2-Tetrafluoroethane dimer was investigated by pulsed jet Fourier transform microwave spectroscopy. One conformer, stabilized through a network of four C-H···F-C interactions, was observed, although several almost isoenergetic configurations were suggested by ab initio calculations. The measurements, extended to four ¹³C species in natural abundance, allowed determination of the carbon skeleton structures and evaluation of the weak hydrogen bond parameters. Information on the dissociation energy is also provided.

VMS-ROT: A New Module of the Virtual Multifrequency Spectrometer for Simulation, Interpretation, and Fitting of Rotational Spectra

The Virtual Multifrequency Spectrometer (VMS) is a tool that aims at integrating a wide range of computational and experimental spectroscopic techniques with the final goal of disclosing the static and dynamic physical-chemical properties "hidden" in molecular spectra. VMS is composed of two parts, namely, VMS-Comp, which provides access to the latest developments in the field of computational spectroscopy, and VMS-Draw, which provides a powerful graphical user interface (GUI) for an intuitive interpretation of theoretical outcomes and a direct comparison to experiment. In the present work, we introduce VMS-ROT, a new module of VMS that has been specifically designed to deal with rotational spectroscopy. This module offers an integrated environment for the analysis of rotational spectra: from the assignment of spectral transitions to the refinement of spectroscopic parameters and the simulation of the spectrum. While bridging theoretical and experimental rotational spectroscopy, VMS-ROT is strongly integrated with quantum-chemical calculations, and it is composed of four independent, yet interacting units: (1) the computational engine for the calculation of the spectroscopic parameters that are employed as a starting point for guiding experiments and for the spectral interpretation, (2) the fitting-prediction engine for the refinement of the molecular parameters on the basis of the assigned transitions and the prediction of the rotational spectrum of the target molecule, (3) the GUI module that offers a powerful set of tools for a vis-à-vis comparison between experimental and simulated spectra, and (4) the new assignment tool for the assignment of experimental transitions in terms of quantum numbers upon comparison with the simulated ones. The implementation and the main features of VMS-ROT are presented, and the software is validated by means of selected test cases ranging from isolated molecules of different sizes to molecular complexes. VMS-ROT therefore offers an integrated environment for the analysis of the rotational spectra, with the innovative perspective of an intimate connection to quantum-chemical calculations that can be exploited at different levels of refinement, as an invaluable support and complement for experimental studies.

Infrared spectra reveal box-like structures for a pentamer and hexamer of mixed carbon dioxide-acetylene clusters

Except for a few cases like water and carbon dioxide, identification and structural characterization of clusters with more than four monomers is rare. Here, we provide experimental and theoretical evidence for existence of box-like structures for a pentamer and a hexamer of mixed carbon dioxide-acetylene clusters. Two mid-infrared cluster absorption bands are observed in the CO2ν3 band region using a tunable diode laser to probe a pulsed supersonic jet. Each requires the presence of both carbon dioxide and acetylene in the jet, and (from observed rotational spacings) involves clusters containing about 4 to 7 molecules. Structures are predicted for mixed CO2 + C2H2 clusters using a distributed multipole model, and the bands are assigned to a specific pentamer, (CO2)3-(C2H2)2, and hexamer, (CO2)4-(C2H2)2. The hexamer has a box-like structure whose D2d symmetry is supported by observed intensity alternation in the spectrum. The pentamer has a closely related structure which is obtained by removing one CO2 molecule from the hexamer. These are among the largest mixed molecular clusters to be assigned by high-resolution spectroscopy.