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

Theoretical Insights into Regulation of Red/Blue-Shifting Hydrogen Bonds Through Cooperativity with Regium Bonds

To deeply understand the characteristics and regulation of red/blue-shifting hydrogen bonds (HBs), a theoretical investigation was conducted to explore the cooperativity between regium bonds and HBs in the complexes of Y···MCN···HCX3 (M = Cu, Ag, Au; Y = H2O, HCN, NH3; X = F, Cl). When MCN formed a hydrogen bonding dimer with CHF3 or CHCl3, the blue shift of C-H vibration frequency v(C-H) decreases as the following sequence Au > Cu > Ag, and the red shift decreases following the order Ag > Cu > Au. Upon the formation of ternary complexes, the presence of regium bonding interactions exerts a positive synergistic effect, resulting in the strengthening of the HBs. This, in turn, leads to noticeable changes in the red and blue shifts of v(C-H). In CHF3 complexes, v(C-H) undergoes a decrease in the blue shift, whereas that in CHCl3 exhibits an increase in the red shift. Especially, a transition from blue to red shift is observed within the AuCN···HCCl3 complex. As the strength of the regium bond increases, the trend of shifting from blue to red becomes more pronounced. For a given MCN, the changes occur in the order of NH3 > HCN > H2O. The interplay between two interactions was revealed by the molecular electrostatic potentials (MEP), the atoms in the molecule (AIM), and natural bond orbitals (NBO) analysis. It is revealed that Δv(C-H) is linearly correlated with a series of configuration and energy parameters. We explain the red- and blue-shifting HBs and their changes from the perspective of hyperconjugation and rehybridization. The presence of the positive synergistic effect enhances the hyperconjugation effect, thereby leading to a reduction in the blue shift and an increase in the red shift of v(C-H) within the complexes. This study enriches previous mechanisms regarding red- and blue-shifting HBs and introduces a novel idea to manipulate the characteristics of HBs, with the potential to impact the functioning of intricate systems.

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.

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.

Fluorination effects probed in 4-fluoroacetophenone and its monohydrate

Rotational spectra of the 4-fluoroacetophenone monomer and its monohydrate were investigated by Fourier transform microwave spectroscopy complemented with quantum chemical calculations. One conformer of 4-fluoroacetophenone and two isomers of 4-fluoroacetophenone-H2O have been observed in the pulsed jets. The observation of all mono-substituted 13C isotopologues in natural abundance allows an accurate structural determination of the 4-fluoroacetophenone monomer. Both detected isomers of 4-fluoroacetophenone-H2O are stabilized by a dominant O-H⋯O and a secondary C-H⋯O hydrogen bond. The fluorination effects on the geometries, intermolecular non-covalent interactions and V3 barrier of the methyl internal rotation were analysed. The relative population ratio of the two observed isomers for 4-fluoroacetophenone-H2O was also estimated to be NI/NII ≈ 7/1.

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.

Interaction Types in C6H5(CH2)nOH-CO2 (n = 0-4) Determined by the Length of the Side Alkyl Chain

C₆H₅(CH₂)_nOH-CO₂ complexes have been investigated using rotational spectroscopy (n = 0-2) complemented by quantum chemical calculations (n = 0-4), which implies that the side alkyl chain length can determine the types of intermolecular interactions. Unlike the in-plane C···O tetrel bond in phenol-CO₂, the π*_CO2···π_aromatic interaction has been shown to link CO₂ to phenylmethanol and 2-phenylethanol, which is, to the best of our knowledge, the first time it has been demonstrated by rotational spectroscopy. Further elongations of the side alkyl chain gradually increase the energies of intramolecular hydrogen bonds in 3-phenylpropanol and 4-phenylbutanol so that CO₂ cannot break it. CO₂ will be pushed farther from the monomers and link with the -OH group through a dominating C···O tetrel bond. Our observations would allow, with the choice of the proper length of the side alkyl chain, new strategies for engineering C···π_aromatic-centered noncovalent bonding schemes for the capture, utilization, and storage of CO₂.

Competitive tetrel bond and hydrogen bond in benzaldehyde-CO2: characterization via rotational spectroscopy

The rotational spectrum of the 1 : 1 benzaldehyde-CO₂ complex has been investigated using pulsed-jet Fourier transform microwave spectroscopy complemented with quantum chemical calculations. Two isomers, both characterized by one C⋯O tetrel bond (n → π* interaction) and one C-H⋯O hydrogen bond (n → σ* interaction), have been observed in the pulsed jet. Competition between the tetrel bond and the hydrogen bond has been disclosed by natural bond orbital analysis: isomer I is characterized by one dominating OC_CO2⋯O tetrel bond (12.6 kJ mol^-1) and a secondary (C-H)_formyl⋯O hydrogen bond (2.2 kJ mol^-1); by contrast, in isomer II the (C-H)_phenyl⋯O hydrogen bond (7.6 kJ mol^-1) becomes the dominant bond, while the OC_CO2⋯O tetrel bond (5.8 kJ mol^-1) becomes much weaker with respect to that of isomer I. Using intensity measurements the relative population ratio of the two isomers was estimated to be N_I/N_II ≈ 2/1.

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.