The weakly-bound CO–(D₂O)₂ and CO–(D₂O)₃ complexes have been studied in the C-O stretching fundamental of the CO monomer. The van der Waals complexes are generated in a supersonic slit-jet apparatus and probed using a quantum cascade laser. One band was observed and analysed for each complex. The trimer, CO–(D₂O)₂, band is composed of a/b-type transitions establishing that the CO monomer lies nearly in the a-b inertial plane. The observed rotational constants lead to a small value of the inertial defect indicating that the heavy atoms in the trimer are co-planar. We observe no evidence of tunneling splitting and conclude that the large amplitude tunneling that exists in the free D₂O dimer is quenched by the presence of the CO monomer. The CO–(D₂O)₃ band is also composed of a/b-type transitions establishing that the CO monomer lies nearly in the a-b inertial plane.
Theoretical calculations were performed to find minima on the potential energy surfaces for both complexes at B2PLYP-D3BJ level of theory and applying counterpoise correction for the basis set superposition error. Further optimisations were then carried out at different coupled cluster levels of theory and extrapolating to the complete basis set limit. The rotational parameters at CCSD(T*)-F12c level of theory give results in very good agreement with those obtained from the observed spectra. In both complexes, the experimental structure corresponds to the lowest energy isomer.
The corresponding bands for CO–(H₂O)₂ and CO–(H₂O)₃ are significantly predissociated which hampers their detailed rovibrational analysis.

Tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, exhibits a complex conformational landscape in its monomeric form.1 Detailed analyses of its conformational distribution and conversion barriers using rotational spectroscopy and theoretical modeling were reported recently.1 In the current study, we focus on investigated how intermolecular interactions with water and THFA itself influence the aforementioned conformational preference. A large number of initial conformational geometries were generated using a semiempirical tight-binding (TB) quantum chemistry code, GFN-xTB, designed for prediction of equilibrium structural properties, such as geometries, vibrational frequencies, and non-covalent interactions.2 The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Very interestingly, in the THFA monohydrate, the two monohydrates observed contain the most stable THFA monomeric conformer and are ranked the ninth and tenth in terms of their relative energy ordering. On the other hand, the THFA dimer observed contains the third most stable monomeric conformers. We interpret the observed phenomena in terms of the conformational conversion barriers and conformational cooling effects.

The structures of 3-mercaptopropionic acid (mpa) [HS-CH₂-CH₂-C(O)-OH] and methyl 3-mercaptopropionate (mp) [HS-CH₂-CH₂-C(O)-O-CH₃] were investigated in the range of 8-18 GHz using both chirped pulse and cavity-based Fourier transform microwave spectrometers. One (mpa1) and two (mp1 and mp2) stable conformers were assigned in the rotational spectra for the acid and the ester compound, respectively. Despite our efforts, no other energy minima predicted by theory was experimentally observed in the spectra which could be a result of their high relative energies and/or small barriers to re-arrangement (relaxation to lower energy conformers). Splitting due to the presence of the methyl internal rotor in the ester species were observed in the rotational transitions of both mp1 and mp2. The latter also presents an additional splitting related to the torsional motion of the SH group around the C-C-S-H dihedral angle that allows the interconversion between two enantiomers of mp2. The observed rotational transitions for the acid and the ester were fit using the Pickett’s SPFIT and the XIAM programs, respectively. The derived rotational parameters are well-determined and consistent with the quantum mechanical calculations performed at both DFT B3LYP-D3BJ and ab initio MP2 methods with the aug-cc-pVTZ basis set. The higher stability of conformers mpa1 and mp1 is explained by the presence of an intramolecular SH^...O=C interaction, investigated on the basis of the quantum theory of atoms in molecules (QTAIM) and the non-covalent interaction (NCI) analyses.

Non-covalent molecular aggregate formation is dictated by inter- and intramolecular forces. The role of these interactions in stabilizing biological molecules is of great interest to many scientific communities. Characterizing these forces has also gained a lot of importance for understanding grain formation in the interstellar medium, especially for aromatic systems such as polycyclic aromatic hydrocarbons (PAHs).^a Broadband rotational spectroscopy studies of weakly bound complexes are able to accurately reveal the structures and internal dynamics of molecular clusters isolated in the gas phase. To understand the weak interactions in biological and astrochemical relevant molecules, we report here our studies on the homodimers of fluorene (C₁₃H₁₀), dibenzofuran ((C₆H₄)₂O), and diphenylether ((C₆H₅)₂O). While their structures show overall similarities, they differ in structural flexibility, planarity, and dipole moment. In order to determine the structure of the corresponding homodimers, we targeted transitions in the 2-8 GHz range using broadband rotational spectroscopy. Our experimental results show that all the observed homodimers are dominated by dispersion interactions such as CH-π or π-π, but the dibenzofuran dimer is also influenced by repulsion between the free electron pairs of the oxygen atoms and the π clouds.^{b a} A. L. Steber, et al., J. Phys. Chem. Lett., 8 (2017), 5744-5750. ^b M. Fatima et al., Angew. Chem. Int. Edit., 131 (2019), 3140.

Current quantum chemical methods, such as DFT and coupled-cluster excel at describing the physical properties of small molecular systems. However, they perform quite poorly when used to describe large molecules. We recently demonstrated the generality of this problem when benchmarking different theoretical methods against experimental gas-phase bond dissociation energies values. While theory and experiment agree for small systems, bigger molecules show a large discrepancy. This indicates that certain phenomena are size-intensive and others size-extensive, and that the currently available techniques do not treat all of the phenomena uniformly, which calls into question their transferability to large systems. To overcome this, we must first identify the origin of these difficulties. This will then inform the formulation of better, more accurate computation strategies. To shed light on the source of these discrepancies, we have designed an independent experiment, one which is not affected by the common mass spectrometric “suspects” such as kinetic shifts. We have measured the infrared spectra of pyridines, quinolones, and pyridinium dimers using a home-made, newly constructed cryogenic FT-ICR mass spectrometer. Our results reveal that the N-H stretching frequency is an excellent probe to test the accuracy of the optimized geometry—and hence dispersion interactions—as it is very sensitive to the spatial proximity of substituents. The pyridinium cations with pendant substituents become exquisitely sensitive molecular torsion balances for the measurement of non-covalent interactions in the gas phase. In addition, we computed the differences in N-H-N frequency for the proton-bound pyridinium dimers and obtained shifts within a range of 500 cm⁻¹, depending on the method/basis-set combination used. IR spectroscopy in the gas phase hence can be used to determine which combination of method and basis-set gives the right answer and for what reasons.

The triflic acid monomer (CF₃SO₂OH) and its 1:1 complex with water (CF₃SO₂OH-H₂O) have been observed by chirped-pulse and cavity Fourier transform microwave spectroscopy. A pair of tunneling states was identified in the rotational spectra for both species. For CF₃SO₂OH, the assignment of b-type spectra led to the direct measurement and experimental determination of the tunneling energy (∆E = 52.96704(90) MHz). M06-2X/6-311++G(3df,3pd) calculations predict a 2.8 kcal/mol barrier for the tunneling motion of the hydroxyl proton rotating around the S-O bond through a transition state in which the O-H is oriented anti with respect to the CF₃ group. A complete 360^° scan of the hydroxyl proton around the S-O bond shows an additional transition state in the syn orientation resulting in a 6.2 kcal/mol barrier. In the CF₃SO₂OH-H₂O complex, spectral doublets were observed with an approximate 3:1 intensity ratio indicative of an equivalent exchange of the protons in H₂O resulting from rotation about its C₂ axis. The proposed water motion is further supported by the quenching of the spectral pairs for the asymmetrically deuterated CF₃SO₂OH-DOH complex. Despite the strong acidity of triflic acid, no evidence of protonation of the water was observed.

Laser-induced fluorescence excitation (LIFE) and dispersed fluorescence spectra of a binary complex between p-fluorophenol (pFP) and ammonia (NH₃) have been measured in a supersonic jet expansion. The measured spectra reveal that para fluorine substitution, which is remote from the binding phenolic site, has a very pronounced effect on the intermolecular vibrational features that appear in the LIFE spectrum. These features also differ significantly compared to those of the 1:1 pFP-H₂O complex [1]. The most intense low-frequency feature is the fundamental of the intermolecular stretching mode (σ), which displays a long progression in the DF spectrum implying occurrence of significant distortion of geometry upon electronic excitation of the complex. In addition, the DF spectra display features of very low threshold for vibrational mixing in the excited state [2]. However, no direct evidence for proton/hydrogen transfer is revealed for vibronic excitation of the complex up to 1¹ level (822 cm⁻¹) of the p-FP moiety. [1] D.P. Mukhopadhyay, S. Biswas, T. Chakraborty, J. Phys. Chem. A 120 (2016) 9159. [2] D.P. Mukhopadhyay, S. Biswas, T. Chakraborty, Chem. Phys. Lett. 674 (2017) 71.