The Challenging Conformational Landscape of Cysteamine···H2O Revealed by the Strong Interplay of Rotational Spectroscopy and Quantum Chemical Calculations

A 1:1 molecular complex of cysteamine with water is shown to adopt a cage-like structure where cysteamine accepts a relatively strong hydrogen bond from water while also engaging in two additional weaker interactions (SH···Ow and CH···Ow). Experimental and theoretical approaches confirm this conformer as the global minimum on the potential energy surface. Fitting of key structural parameters to experimentally determined moments of inertia yields consistent and accurate results for rotational and 14N nuclear quadrupole coupling constants which are shown to be challenging to calculate using ab initio methods. Comprehensive analysis of the intermolecular interactions and a thorough comparison with the properties of aminoethanol-water is presented, utilizing independent gradient models based on Hirshfeld partition, quantum theory of atoms-in-molecules, and symmetry-adapted perturbation theory approaches. As expected, the OH group of aminoethanol is a stronger hydrogen bond donor than the SH group in cysteamine, while the CH···Ow interaction is a key determining factor of the conformational landscape in both cysteamine-water and aminoethanol-water complexes. The results show very clearly that the synergy between theoretical calculations and experimental results is not only desirable but mandatory to get the right answers in such complex conformational surfaces. The results are also clear benchmarks for the accuracy of different theoretical methods in assessing the structures and energy order of the conformations.

Intermolecular Interactions between Aldehydes and Alcohols: Conformational Equilibrium and Rotational Spectra of Acrolein-Methanol Complex

The rotational spectra of the 1:1 complex formed by acrolein and methanol and its deuterated isotopologues have been analyzed. Two stable conformations in which two hydrogen bonds between the two moieties are formed were detected. The rotational lines show a hyperfine structure due to the methyl group internal rotation in the complex and the V3 barriers hindering the motion were determined as 2.629(5) kJ mol-1 and 2.722(5) kJ mol-1 for the two conformations, respectively. Quantum mechanical calculations at the MP2/aug-cc-pVTZ level and comprehensive analysis of the intermolecular interactions, utilizing NCI and SAPT approaches, highlight the driving forces of the interactions and allow the determination of the binding energies of complex formation.

How Water Interacts with the NOH Group: The Rotational Spectrum of the 1:1 N,N-diethylhydroxylamine·Water Complex

The rotational spectrum of the 1:1 N,N-diethylhydroxylamine-water complex has been investigated using pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency region. The most stable conformer has been detected as well as the 13C monosubstituted isotopologues in natural abundance and the 18O enriched water species, allowing to determine the nitrogen nuclear quadrupole coupling constants and the molecular structure in the vibrational ground state. The molecule has a Cs symmetry and the water lies in the bc symmetry plane forming two hydrogen bonds with the NOH frame with length: dHOH·NOH = 1.974 Å and dH2O·HON = 2.096 Å. From symmetry-adapted perturbation theory calculations coupled to atoms in molecule approach, the corresponding interaction energy values are estimated to be 24 and 13 kJ·mol-1, respectively. The great strength of the intermolecular interaction involving the nitrogen atom is in agreement with the high reactivity of hydroxylamine compounds at the nitrogen site.

Spectroscopic and quantum mechanical study of a scavenger molecule: N,N-diethylhydroxylamine

We report a combination of quantum mechanical calculations and a range of spectroscopic measurements in the gas phase of N,N-diethylhydroxylamine, an important scavenger compound. Three conformers were observed by pulsed jet Fourier transform microwave spectroscopy in the 6.5-18.5 GHz frequency range. They are characterized by the hydroxyl hydrogen atom being in trans orientation with respect to the bisector of the CNC angle while the side alkyl chains can be both trans (global minimum, C_s symmetry, A = 7608.1078(4), B = 2020.2988(2) and C = 1760.5423(2) MHz) or one trans and the other gauche (second energy minimum, A = 5302.896(1), B = 2395.9822(4) and C = 1804.8567(3) MHz) or gauche' (third energy minimum, A = 5960.8025(6), B = 2273.6627(4) and C = 1975.8074(4) MHz). For the global minimum, the ¹³C_α,¹³C_β and ¹⁵N isotopologues were observed in natural abundance, allowing for an accurate partial structure determination. Moreover, several lines were detected by free jet absorption millimeter wave spectroscopy in the 59.6-74.4 GHz spectral range. The electron binding energies of the highest occupied molecular orbital and the next-to-highest occupied molecular orbital, determined by photoelectron spectroscopy, are 8.95 and 10.76 eV, respectively. Supporting calculations evidence that, (i) upon ionization of the HOMO, the molecular structure changes from an amine to an N-oxoammonium arrangement and (ii) the 0-0 of the HOMO-1 photoionization is 10.46 eV. The K-shell binding energies, determined by X-ray photoelectron spectroscopy, are 290.42 eV (C_β), 291.45 eV (C_α), 405.98 eV (N) and 538.75 eV (O). The Fourier transform near infrared spectrum is reported and a tentative assignment is proposed. The equilibrium wavenumber (ω̃ = 3811 cm^-1) and the anharmonicity constant (ω̃χ = -87.5 cm^-1) of the hydroxyl stretching mode were estimated using a quadratic model.

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.

Testing the Scalability of the HS-AUTOFIT Tool in a High-Performance Computing Environment

In the last years, the development of broadband chirped-pulse Fourier transform microwave spectrometers has revolutionized the field of rotational spectroscopy. Currently, it is possible to experimentally obtain a large quantity of spectra that would be difficult to analyze manually due to two main reasons. First, recent instruments allow obtaining a considerable amount of data in very short times, and second, it is possible to analyze complex mixtures of molecules that all contribute to the density of the spectra. AUTOFIT is a spectral assignment software application that was developed in 2013 to support and facilitate the analysis. Notwithstanding the benefits AUTOFIT brings in terms of automation of the analysis of the accumulated data, it still does not guarantee a good performance in terms of execution time because it leverages the computing power of a single computing machine. To cater to this requirement, we developed a parallel version of AUTOFIT, called HS-AUTOFIT, capable of running on high-performance computing (HPC) clusters to shorten the time to explore and analyze spectral big data. In this paper, we report some tests conducted on a real HPC cluster aimed at providing a quantitative assessment of HS-AUTOFIT’s scaling capabilities in a multi-node computing context. The collected results demonstrate the benefits of the proposed approach in terms of a significant reduction in computing time.

σ-Hole activation and structural changes upon perfluorination of aryl halides: direct evidence from gas phase rotational spectroscopy

Enhancement of the σ-hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes-Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C₆F₅Cl and C₆F₅Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C₆F₅Cl and C₆F₅Br and their ¹³C, ³⁷Cl or ⁸¹Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved.

Characterizing hydrogen and tetrel bonds in clusters of CO2 with carboxylic acids

The interaction between carbon dioxide and planar carboxylic acids has been investigated through the analysis of the microwave spectrum of the acrylic acid·CO2 complex and quantum chemical modeling of the R-COOH·(CO2)1,16 clusters, where R = H, CH2CH. As regards the 1 : 1 compounds, two species, involving the s-cis and s-trans conformers of acrylic acid were observed. For both of them, a similar bidentate interaction arises between the carbonyl group of CO2 and the carboxylic group of the organic acid, leading to the formation of a planar six-membered ring. The binding energy is estimated to be De ≃ 21 kJ mol-1, 1/3 being the energy contributions of the tetrel to hydrogen bonds, respectively. In the 1 : 16 clusters, the ring arrangement is broken, allowing for the interaction of the acid with several CO2 molecules. The CO2 molecules completely surround formic acid, whereas, in the case of acrylic acid, they tend to avoid the allyl chain.

Chlorine "Equatorial Belt" Activation of CF3Cl by CO2: The C···Cl Tetrel Bond Dominance in CF3Cl-CO2

Besides its typical halogen donor behavior (exhibiting a Cl σ-hole) in forming Cl···B halogen bonds (B is an electron-rich region), CF₃Cl reveals a new interaction site in its complex with CO₂ when explored by rotational spectroscopy. Experimental evidence and theoretical analyses point out irrefutably that CF₃Cl prefers to link to CO₂ through its Cl "equatorial belt" consisting of the lone pairs of the Cl atom, resulting in a C···Cl tetrel bond. In addition, a secondary plausible C···O tetrel bond and a F···O halogen bond might contribute to the relative orientation of the moieties forming the complex. The effects of the Cl "equatorial belt" present in perhalogenated molecules, such as CF₃Cl, have been hitherto overlooked in describing the origin of noncovalent interactions. That left a significant void that the present study tries to fill by outlining its importance.

Relative Strengths of a Pnicogen and a Tetrel Bond and Their Mutual Effects upon One Another

The ability of the T and Z atoms of TR₃ZR₂ to engage in a noncovalent interaction with NH₃ is assessed by DFT calculations, where the T atom refers to C, Si, and Ge; Z = As, Sb, and P; and substituents R = H and F. In most instances, the tetrel bond (TB) is both stronger and shorter than the pnicogen bond (ZB). These two bond strengths can be equalized, or preference shifted to the ZB, if F substituents are placed on the Z and H on the T atoms. Employing C as the T atom results in a very weak TB, with the ZB clearly favored energetically. The simultaneous formation of both TB and ZB weakens both, particularly the latter, but both bonds survive intact. Geometric and spectroscopic perturbations of the subunits reflect the two types of noncovalent bonds.

Characterizing the lone pair⋯π–hole interaction in complexes of ammonia with perfluorinated arenes

Stronger and more flexible lone pair⋯π–hole interaction of ammonia with respect to water in complexes with perfluorinated arenes.

The ditetrel bond: noncovalent bond between neutral tetrel atoms

The ability of a tetrel atom to serve in the capacity of electron donor in a σ-hole noncovalent bond is tested by quantum calculations.

Evaluation of the aggregation process in a mixture of propofol and benzocaine

We report on a mass-resolved IR spectrosopic study on propofol-benzocaine aggregates. This is a complex system due to the several conformational isomers that both monomers may adopt and to the combination of functional groups they present, which allow the molecules to interact in many possible ways. However, our results demonstrate that a single conformation is favored for each stoichiometry. In the heterodimer, propofol acts as a proton donor to the ester group of benzocaine, while the whole cluster is stabilized by dispersive forces. These dispersive forces account for an important part of the system's stabilization energy as the calculations suggest. Propofol does not show any affinity for the amino group of benzocaine, even when a second molecule of propofol is introduced. These results demonstrate the difficulty in anticipating the aggregation preferences of even small organic molecules.

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.

Structure, Conformational Equilibria, and Weak Hydrogen Bonding in the CH2 F2 -CF3 CH2 F Dimer

To probe the multiple configurations of a weakly bound intermolecular complex, the difluoromethane and 1,1,1,2-tetrafluoroethane dimer was investigated by using pulsed jet Fourier transform microwave spectroscopy coupled with quantum chemical calculations. Three isomers were detected in the supersonic jet. Spectroscopic assignments, ab initio calculations and quantum theory of atoms in molecules (QTAIM) analyses prove that all the observed isomers are stabilized through a net of three weak C-H⋅⋅⋅F-C interactions. The interaction energies are estimated to be within 12-13 kJ mol^-1 , dominated by electrostatic and dispersion according to Symmetry-Adapted Perturbation Theory (SAPT) analysis. The spectroscopic measurements were also extended to three and two ¹³ C isotopologues in natural abundance for the isomers II and III, respectively, which lead to precisely structural determinations of these two isomers. The comparison of the relative intensity of these isomers measured in the carrier gases of argon and helium evidenced that isomer II is the global minimum, while the intensities measured in helium suggested that the population of the three isomers in the jet to be N_I /N_II /N_III ≈1/11/4.

Rotational Spectrum and Conformational Analysis of N-Methyl-2-Aminoethanol: Insights into the Shape of Adrenergic Neurotransmitters

We describe an experimental and quantum chemical study for the accurate determination of the conformational space of small molecular systems governed by intramolecular non-covalent interactions. The model systems investigated belong to the biological relevant aminoalcohol's family, and include 2-amino-1-phenylethanol, 2-methylamino-1-phenylethanol, noradrenaline, adrenaline 2-aminoethanol, and N-methyl-2-aminoethanol. For the latter molecule, the rotational spectrum in the 6–18 and 59.6–74.4 GHz ranges was recorded in the isolated conditions of a free jet expansion. Based on the analysis of the rotational spectra, two different conformational species and 11 isotopologues were observed and their spectroscopic constants, including ¹⁴N-nuclear hyperfine coupling constants and methyl internal rotation barriers, were determined. From the experimental data a structural determination was performed, which was also used to benchmark accurate quantum chemical calculations on the whole conformational space. Atom in molecules and non-covalent interactions theories allowed the characterization of the position of the intramolecular non-covalent interactions and the energies involved, highlighting the subtle balance responsible of the stabilization of all the molecular systems.

Cooperative role of halogen and hydrogen bonding in the stabilization of water adducts with apolar molecules

Conversely to the H₂O–CF₄ adduct, an appreciable intermolecular bond stabilization by charge transfer is operative in the H₂O–CCl₄ system.

Halogen Bonds Formed between Substituted Imidazoliums and N Bases of Varying N-Hybridization

Heterodimers are constructed containing imidazolium and its halogen-substituted derivatives as Lewis acid. N in its sp³, sp² and sp hybridizations is taken as the electron-donating base. The halogen bond is strengthened in the Cl < Br < I order, with the H-bond generally similar in magnitude to the Br-bond. Methyl substitution on the N electron donor enhances the binding energy. Very little perturbation arises if the imidazolium is attached to a phenyl ring. The energetics are not sensitive to the hybridization of the N atom. More regular patterns appear in the individual phenomena. Charge transfer diminishes uniformly on going from amine to imine to nitrile, a pattern that is echoed by the elongation of the C-Z (Z=H, Cl, Br, I) bond in the Lewis acid. These trends are also evident in the Atoms in Molecules topography of the electron density. Molecular electrostatic potentials are not entirely consistent with energetics. Although I of the Lewis acid engages in a stronger bond than does H, it is the potential of the latter which is much more positive. The minimum on the potential of the base is most negative for the nitrile even though acetonitrile does not form the strongest bonds. Placing the systems in dichloromethane solvent reduces the binding energies but leaves intact most of the trends observed in vacuo; the same can be said of ∆G in solution.

The interaction of CCl4 with Ng (Ng = He, Ne, Ar), O2, D2O and ND3: rovibrational energies, spectroscopic constants and theoretical calculations

This investigation generated rovibrational energies and spectroscopic constants for systems of CCl₄ with Ng (Ng = He, Ne, Ar), O₂, D₂O and ND₃ from scattering experimental data, and the results presented are of interest for microwave spectroscopy studies of small halogenated molecules. The rovibrational spectra were obtained through two different approaches (Dunham and DVR) within the improved Lennard Jones (ILJ) model. Spectra were also generated within ordinary Lennard Jones and deviations suggest that the ILJ model should be preferred due to interactions beyond dispersion forces presented in these systems. Data from the literature and additional high level quantum mechanical calculations presented in this work show that these systems should not be considered as van der Waals complexes due to halogen bonding (HB) interactions, and this is especially true for the CCl₄-D₂O and CCl₄-ND₃ complexes. The charge displacement from the latter systems are one order of magnitude higher than the values from literature for CCl₄ and He, Ne, Ar and O₂ systems, and show significant deviations between DFT and Hartree-Fock values not previously reported in the literature.

Competition and cooperativity of σ-hole and π-hole intermolecular interactions between carbon monoxide and bromopentafluorobenzene

The electronic complementary relationship between C₆F₅Br and CO enables them to interact with each other via σ-hole and π-hole intermolecular interactions.