Rotational spectrum and structure of the pyridine–CO2 van der Waals complex

Exploring Uncharted Territory: Spectroscopic Evidence on a Novel Tetrel Bond with -C≡C- Triple Bond

This study presents an investigation of the 2-butynyl alcohol···CO2 adduct, combining pulsed jet Fourier transform microwave spectroscopy with quantum chemical calculations. Two distinct isomers have been observed in the pulsed jet with a relative population ratio of 3/2. It marks the first instance of microwave spectroscopic evidence, to the best of our knowledge, suggesting the existence of a CCO2···π-C≡C- tetrel bond (π-C≡C-···π*CO2 interaction) in both observed isomers. This study highlights the importance of noncovalent interactions involving CO2 in reactant complexes, paving the way for more efficient applications of CO2 by understanding the physical basis of these noncovalent bonds.

Elucidating the non-covalent interactions in thiazole-carbon dioxide complexes through rotational spectroscopy and theoretical computations

The complexes formed between thiazole and carbon dioxide were studied to probe the non-covalent bonding properties between carbon dioxide and a heteroaromatic ring. The rotational spectra of the thiazole-CO2 complex were analyzed using a supersonic jet Fourier transform microwave spectrometer in conjunction with theoretical calculations. A rotational spectrum corresponding to the global minimum of the thiazole-CO2 complex was identified. The observed structure of the complex is stabilized by a C⋯N tetrel-bond, with additional stability provided by a C-H⋯O hydrogen bond. The computational analysis of the thiazole-(CO2)2 and thiazole-(CO2)3 complexes demonstrated the notable impact of C⋯N interactions on aggregation, with the significance of interactions between CO2 molecules increasing with the number of CO2 molecules present. NCI analysis, NBO analysis, and SAPT analysis were utilized to elucidate the properties of non-covalent interactions between thiazole and CO2, as well as those among CO2 molecules.

Electron-rich pyridines with para-N-heterocyclic imine substituents: ligand properties and coordination to CO2, SO2, BCl3 and PdII complexes

Electron-rich pyridines with π donor groups at the para position play an important role as nucleophiles in organocatalysis, but their ligand properties and utilization in coordination chemistry have received little attention. Herein, we report the synthesis of two electron-rich pyridines 1 and 2 bearing N-heterocyclic imine groups at the para position and explore their coordination chemistry. Experimental and computational methods were used to assess the donor ability of the new pyridines showing that they are stronger donors than aminopyridines and guanidinyl pyridines, and that the nature of the N-heterocyclic backbone has a strong influence on the pyridine donor strength. Coordination compounds with Lewis acids including the CO2, SO2, BCl3 and PdII ions were synthesized and characterized. Despite the ambident character of the new pyridines, coordination preferentially occurs at the pyridine-N atom. Methyl transfer experiments reveal that 1 and 2 can act as demethylation reagents.

Conformational equilibria in acrolein-CO2: the crucial contribution of n → π* interactions unveiled by rotational spectroscopy

Using gas phase Fourier-transform microwave spectroscopy complemented by theoretical analysis, this study delivers a comprehensive depiction of the physical origin of the 'n → π* interaction' between CO2 and acrolein, one of the most reactive aldehydes. Three distinct isomers of the acrolein-CO2 complex, linked through a C⋯O tetrel bond (or n → π* interaction) and a C-H⋯O hydrogen bond, have been unambiguously identified in the pulsed jet. Relative intensity measurements allowed estimation on the population ratio of the three isomers to be T1/T2/C1 ≈ 25/5/1. Advanced theoretical analyses were employed to elucidate the intricacies of the noncovalent interactions within the examined complex. This study not only sheds light on the molecular underpinnings of n → π* interactions but also paves the way for future exploration in carbon dioxide capture and utilization, leveraging the fundamental principles uncovered in the study of acrolein-carbon dioxide interactions.

Capturing CO2 in Quadrupolar Binding Pockets: Broadband Microwave Spectroscopy of Pyrimidine-(CO2)n, n = 1,2

Pyrimidine has two in-plane CH(δ+)/N̈(δ-)/CH(δ+) binding sites that are complementary to the (δ-/2δ+/δ-) quadrupole moment of CO2. We recorded broadband microwave spectra over the 7.5-17.5 GHz range for pyrimidine-(CO2)n with n = 1 and 2 formed in a supersonic expansion. Based on fits of the rotational transitions, including nuclear hyperfine splitting due to the two 14N nuclei, we have assigned 313 hyperfine components across 105 rotational transitions for the n = 1 complex and 208 hyperfine components across 105 rotational transitions for the n = 2 complex. The pyrimidine-CO2 complex is planar, with CO2 occupying one of the quadrupolar binding sites, forming a structure in which the CO2 is stabilized in the plane by interactions with the C-H hydrogens adjacent to the nitrogen atom. This structure is closely analogous to that of the pyridine-CO2 complex studied previously by (Doran, J. L. J. Mol. Struct. 2012, 1019, 191-195). The fit to the n = 2 cluster gives rotational constants consistent with a planar cluster of C2v symmetry in which the second CO2 molecule binds in the second quadrupolar binding pocket on the opposite side of the ring. The calculated total binding energy in pyrimidine-CO2 is -13.7 kJ mol-1, including corrections for basis set superposition error and zero-point energy, at the CCSD(T)/ 6-311++G(3df,2p) level, while that in pyrimidine-(CO2)2 is almost exactly double that size, indicating little interaction between the two CO2 molecules in the two binding sites. The enthalpy, entropy, and free energy of binding are also calculated at 300 K within the harmonic oscillator/rigid-rotor model. This model is shown to lack quantitative accuracy when it is applied to the formation of weakly bound complexes.

C···S Tetrel Bond Favored in the Phenyl Isothiocyanate-CO2 Complex: A Rotational Study

The rotational spectrum of the phenyl isothiocyanate-CO2 complex was investigated by pulsed-jet Fourier transform microwave spectroscopy complemented by quantum chemical calculations. Only one isomer, with CO2 almost in the plane of phenyl isothiocyanate, has been detected in the pulsed jet, of which the spectrum displays a quadrupole coupling hyperfine structure due to the presence of a 14N nucleus (I = 1). This structure is nearly equal to the lowest energy geometry obtained by B3LYP-D3(BJ)/6-311++G(d,p), which has been dominated by a C···S tetrel bond (n → π* interaction) and one secondary C-H···O hydrogen bond (n → σ* interaction). Molecular electrostatic potential and natural bond orbital analysis were used to characterize the noncovalent interactions of the complex. The results from this study would lay the groundwork for the design and advancement of materials that exhibit high efficiency in capturing CO2.

Rotational spectrum of anisole-CO2: Cooperative C···O tetrel bond and CH···O hydrogen bond

Rotational spectrum of the 1:1 anisole-CO₂ complex has been investigated using a pulsed jet Fourier transform microwave spectrometer supplemented with quantum chemical calculations. In the pulsed jet, only one isomer has been observed which is characterized by a dominant C···O tetrel bond and two CH···O_CO2 weak hydrogen bonds. Different theoretical methods predict different orders of relative energies of plausible conformations. The experimental observation is most consistent with the theoretical estimation at the B3LYP-D3(BJ)/6-311++G(d,p) level of theory. Johnson's non-covalent interaction, quantum theory of atoms in molecules and natural bond orbital analyses have been applied to better understand the nature of non-covalent interactions at play in the anisole-CO₂ complex.

Boron based intramolecular heterocyclic frustrated Lewis pairs as organocatalysts for CO2 adsorption and activation

The massive increase in the amount of carbon dioxide (CO₂ ) in the atmosphere has led to serious environmental problems. One of the best ways to tackle this problem is the CO₂ capture and its utilization as a C1 carbon source for the production of industrially valuable chemicals. But the thermodynamic stability of the CO₂ molecule poses a great challenge in its transformation. Since the last two decades, various metal-based and organic catalysts have been developed for the adsorption and activation of CO₂ . Among all the catalysts the Frustrated Lewis pairs (FLPs) have been shown great potential in CO₂ capture and conversion. Thus, in the present work, Intramolecular Frustrated Lewis pairs (IFLP) based on N-Heterocycles with boron group functionalization at the α-position to N has been theoretically investigated for CO₂ activation. Thorough orbital analysis has been carried out to investigate the reactivity of the proposed catalytic systems. The result shows that the considered IFLPs are capable of activating CO₂ with minimum energy requirements. The CO₂ activation energy range between 8 and 14 kcal/mol. The non-polar solvent was found to be the suitable medium for the reaction. Also, the reversibility of the adducts formed with the IFLPs can be controlled by appropriate substitution at B atom in the IFLPs.

Rotational Spectra of 2-Ethynylpyridine and Its Monohydrate: Influence of the Ortho-Substitution on Ring Geometry and Intermolecular Hydrogen Bonds

Rotational spectra of the 2-ethynylpyridine monomer and its monohydrate have been characterized by pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. The measurements of rotational transitions of the 2-ethynylpyridine monomer and its eight monosubstituted isotopologues (¹⁵N and ¹³C) in natural abundances allow an accurate structural description of the skeleton of 2-ethynylpyridine. For the monohydrate, only the most stable isomer, stabilized by an O-H···N and a secondary C-H···O hydrogen bonds, has been observed in the supersonic jet. Johnson's noncovalent interaction and quantum theory of atoms in molecules analyses have been performed and compared with results for several ortho-substituted pyridine derivatives to elucidate the general trend in their binding energies.

Sequestration of Carbon Dioxide with Frustrated Lewis Pairs Based on N-Heterocycles with Silane/Germane Groups

Frustrated Lewis pairs (FLPs) based on nitrogen heterocycles (pyridine, pyrazole, and imidazole) with a silane or germane group in the α-position of a nitrogen atom have been considered as potential molecules to sequestrate carbon dioxide. Three stationary points have been characterized in the reaction profile: a pre-reactive complex, an adduct minimum, and the transition state connecting them. The effect of external (solvent) or internal (hydroxyl group) electric fields in the reaction profile has been considered. In both cases, it is possible to improve the kinetics and thermodynamics of the complexation of CO2 by the FLP and favor the formation of adducts.

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.

Competitive and cooperative n →π* and n →σ* interactions in benzaldehyde-formaldehyde: rotational characterization

The rotational spectrum of the 1 : 1 benzaldehyde-formaldehyde complex has been investigated by pulsed jet Fourier transform microwave spectroscopy combined with ab initio calculations. The two most stable isomers were observed, with the relative abundance ratio N_I/N_II≈ 3/1 estimated with intensity measurements. Both observed isomers are stabilized by one dominating O[double bond, length as m-dash]CO tetrel bond (n →π* interaction) and one secondary C-HO hydrogen bond. Natural bond orbital analysis and electron localization function analysis were applied to characterize the nature of the noncovalent interactions in the target complex.

Rotational spectrum and internal dynamics of the hydrogen-bonded pyrrole-pyridine aromatic pair

Non-covalent interactions determine the three-dimensional structure and activity of biological molecules. In this work, the pyrrole-pyridine complex considered as a model of the NH⋯N hydrogen-bonded Watson-Crick base pairs has been generated in a supersonic expansion and characterized by chirped pulse Fourier transform microwave spectroscopy. The analysis of the unconventional spectral pattern of the 1:1 pyrrole-pyridine adduct and its ¹³C and ¹⁵N isotopologues reveal a non-planar complex, with a bent NH⋯N hydrogen bond and large amplitude motion of the pyrrole subunit. The bent structure is likely to arise from the stablishment of the secondary CH⋯N interaction between pyridine and pyrrole moieties.

Conformational Equilibria of 2-Methoxypyridine⋅⋅⋅CO2 : Cooperative and Competitive Tetrel and Weak Hydrogen Bonds

The rotational spectrum of 2-methoxypyridine⋅⋅⋅CO₂ was recorded and analysed employing a cavity-based Fourier transform microwave spectrometer, complemented with quantum chemical calculations which predicted three possible isomers within energies less than 1000 cm^-1 . The two most stable isomers were observed in the pulsed jet, which are stabilized by a network of C⋅⋅⋅N/O tetrel and C-H⋅⋅⋅O weak hydrogen bonds. The relative population ratio of the two detected isomers was estimated to be N_I /N_II ≈2.5. The competition and cooperation of the present non-covalent interactions in both isomers are discussed within the framework of Bader's quantum theory of atoms in molecules and Johnson's non-covalent interaction analyses. The study shows, that when looking for CO₂ adsorbents, one might prefer candidates with multiple interactions in one site over candidates with few but strong interactions.

Structure and C⋯N tetrel-bonding of the isopropylamine–CO2 complex studied by microwave spectroscopy and theoretical calculations

The structural and energetic features of C⋯N tetrel bond and C–H⋯O hydrogen bonds linking CO₂ and aliphatic amines were characterized with rotational spectroscopy and quantum chemical calculations.

Tetrel bonds and conformational equilibria in the formamide–CO2 complex: a rotational study

Rotational studies point out that two isomers of the formamide–CO₂ complex are stabilized by the dominated C⋯O tetrel bond.

Microwave spectroscopy of 2-(trifluoromethyl)pyridine⋯water complex: Molecular structure and hydrogen bond

In order to explore the -CF₃ substitution effect on the complexation of pyridine, we investigated the 2-(trifluoromethyl)pyridine⋯water complex by using pulsed jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Experimental assignment and ab initio calculations confirmed that the observed complex is stabilized through N⋯H-O and O⋯H-C hydrogen bonds forming a five-membered ring structure. The bonding distance in N⋯H-O is determined to be 2.027(2) Å, whilst that in O⋯H-C interaction is 2.728(2) Å. The quantum theory of atoms in molecules analysis indicates that the interaction energy of N⋯H-O hydrogen bond is ∼22 kJ mol^-1 and that for O⋯H-C hydrogen bond is ∼5 kJ mol^-1. The water molecule lies almost in the plane of the aromatic ring in the complex. The -CF₃ substitution to pyridine quenches the tunneling splitting path of the internal motion of water molecule.

Tetrel, pnictogen and chalcogen bonds identified in the gas phase before they had names: a systematic look at non-covalent interactions

Tetrel, pnictogen and chalcogen-bonded complexes: old bonds but new names.

Interactions between alkanes and aromatic molecules: a rotational study of pyridine-methane

The rotational spectrum of the adduct pyridine-methane shows that methane links to an aromatic molecule apparently through a C-H···π weak hydrogen bond. The shape and the internal dynamics behaviour of this complex are very similar to that of the van der Waals complexes involving aromatic molecules with rare gases.