Competing Dispersive Interactions: From Small Energy Differences to Large Structural Effects in Methyl Jasmonate and Zingerone

Low barriers to internal rotation in the microwave spectrum of 2,5-dimethylfluorobenzene

We investigated the rotational spectrum of 2,5-dimethylfluorobenzene containing coupled large amplitude motions of two methyl groups in the frequency range from 2 to 26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The internal rotation of two inequivalent methyl groups with low torsional barriers (around 16 and 226 cm-1) causes splittings of all rotational transitions into quintets with separations of up to hundreds of MHz between the torsional components. Spectral analysis and modeling of the observed splittings were performed using the programs XIAM and BELGI-Cs-2Tops, whereby the latter achieved measurement accuracy. The methyl internal rotation can be used to examine the electronic and steric environments around the methyl group because they affect the methyl torsional barrier. Electronic properties play a particularly important role in aromatic molecules in the presence of a π-conjugated double bond system. The experimental results were compared with those of quantum chemistry. Benchmark calculations resulted in the conclusion that the B3LYP-D3BJ/6-311++G(d,p) level of theory can be recommended for predicting rotational constants to guide the microwave spectral assignment of dimethylfluorobenzenes in particular and toluene derivatives in general.

Benchmarking quantum chemical methods for accurate gas-phase structure predictions of carbonyl compounds: the case of ethyl butyrate

High-resolution spectroscopy techniques play a pivotal role to validate and efficiently benchmark available methods from quantum chemistry. In this work, we analyzed the microwave spectrum of ethyl butyrate within the scope of a systematic investigation to benchmark state-of-the-art exchange-correlation functionals and ab initio methods, to accurately predict the lowest energy conformers of carbonyl compounds in their isolated state. Under experimental conditions, we observed two distinct conformers, one of C_s and one of C₁ symmetry. As reported earlier in the cases of some ethyl and methyl alkynoates, structural optimizations of the most abundant conformer that exhibits a C₁ symmetry proved extremely challenging for several quantum chemical levels. To probe the sensitivity of different methods and basis sets, we use the identified soft-degree of freedom in proximity to the carbonyl group as an order parameter. The results of our study provide useful insight for spectroscopists to select an adapted method for structure prediction of carbonyl compounds based on their available computational resources, suggesting a reasonable trade-off between accuracy and CPU cost. At the same time, our observations and the resulting sets of highly accurate experimental constants from high-resolution spectroscopy experiments give an appeal to theoretical groups to look further into this seemingly simple family of chemical compounds, which may prove useful for the further development and parametrization of theoretical methods in computational chemistry.

Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry-Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and ¹⁴N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm^-1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

A Competition between Relative Stability and Binding Energy in Caffeine Phenyl-Glucose Aggregates: Implications in Biological Mechanisms

Hydrogen bonds and stacking interactions are pivotal in biological mechanisms, although their proper characterisation within a molecular complex remains a difficult task. We used quantum mechanical calculations to characterise the complex between caffeine and phenyl-β-D-glucopyranoside, in which several functional groups of the sugar derivative compete with each other to attract caffeine. Calculations at different levels of theory (M06-2X/6-311++G(d,p) and B3LYP-ED=GD3BJ/def2TZVP) agree to predict several structures similar in stability (relative energy) but with different affinity (binding energy). These computational results were experimentally verified by laser infrared spectroscopy, through which the caffeine·phenyl-β-D-glucopyranoside complex was identified in an isolated environment, produced under supersonic expansion conditions. The experimental observations correlate with the computational results. Caffeine shows intermolecular interaction preferences that combine both hydrogen bonding and stacking interactions. This dual behaviour had already been observed with phenol, and now with phenyl-β-D-glucopyranoside, it is confirmed and maximised. In fact, the size of the complex's counterparts affects the maximisation of the intermolecular bond strength because of the conformational adaptability given by the stacking interaction. Comparison with the binding of caffeine within the orthosteric site of the A2A adenosine receptor shows that the more strongly bound caffeine·phenyl-β-D-glucopyranoside conformer mimics the interactions occurring within the receptor.

Competition between In-Plane vs Above-Plane Configurations of Water with Aromatic Molecules: Non-Covalent Interactions in 1,4-Naphthoquinone-(H2O)1-3 Complexes

Non-covalent interactions between aromatic molecules and water are fundamental in many chemical and biological processes, and their accurate description is essential to understand molecular relative configurations. Here we present the rotational spectroscopy study of the water complexes of the polycyclic aromatic hydrocarbon 1,4-naphthoquinone (1,4-NQ). In 1,4-NQ-(H₂O)_1,2, water molecules bind through O-H···O and C-H···O hydrogen bonds and are located on the plane of 1,4-NQ. For 1,4-NQ-(H₂O)₃, in-plane and above-plane water configurations are observed exhibiting O-H···O, C-H···O, and lone pair···π-hole interactions. The observation of different water arrangements for 1,4-NQ-(H₂O)₃ allows benchmarking theoretical methods and shows that they have great difficulty in predicting energy orderings due to the strong competition of C-H···O binding with π and π-hole interactions. This study provides important insight into water interactions with aromatic systems and the challenges in their modeling.

Determination of the semiexperimental equilibrium structure of 2-acetylthiophene in the presence of methyl internal rotation and substituent effects compared to thiophene

The microwave spectra of thiophene and 2-acetylthiophene were recorded in the frequency range from 2 to 40 GHz using two molecular jet Fourier transform microwave spectrometers. For 2-acetylthiophene, two conformers with a syn and an anti orientation of the S1-C2 and C6O bonds (with respect to the C2-C6 bond) were identified, and the syn-conformer was more stable. The spectra of the ³⁴S- and ¹³C-isotopologues of syn-2-acetylthiophene were also assigned, and the semiexperimental equilibrium structure could be determined. Compared to thiophene, at the substitution position, the S1-C2 and C2C3 bond lengths both increase by about 0.007 Å, and the bond angle S1-C2C3 decreases by 0.06°, noticeably larger than the experimental uncertainties. A-E torsional splittings were observed due to internal rotation of the methyl group hindered by a barrier height of 330.187(35) and 295.957(17) cm^-1 for the syn-conformer and the anti-conformer, respectively. Geometry and internal rotation parameters are compared with those of related thiophene derivatives, as well as those of furan and 2-acetylthiophene to gain a better understanding of structure determination in the presence of methyl internal rotation.

New Insights into Secondary Organic Aerosol Formation: Water Binding to Limonene

Limonene is an abundant monoterpene in the atmosphere and one of the main precursors of secondary organic aerosol. Understanding its interactions with atmospheric molecules is crucial to explain aerosol formation and the various products obtained from competing reaction pathways. Here, using broadband rotational spectroscopy in combination with computational calculations, we show that limonene effectively interacts with water, forming a variety of complexes. Seven different isomers of limonene-H₂O, where water and limonene are connected by O-H···π and C-H···O interactions, have been unambiguously identified. Water has been found to preferentially bind to the endocyclic double bond of limonene. Our findings demonstrate a striking ability of water to attach to limonene and enrich our knowledge on the possible interactions of limonene in the atmosphere.

Quantifying soft degrees of freedom in volatile organic compounds: insight from quantum chemistry and focused single molecule experiments

Sampling of the vast conformational landscape of organic compounds remains a challenging task in computational chemistry, especially when it comes to the characterization of soft-degrees of freedom and relatively small energy barriers between different local minima. Therefore, studying the intrinsic properties of isolated molecules using focused experiments such as high-resolution molecular spectroscopy provides a powerful approach to validate and improve available quantum chemical methods. Here, we report on the most abundant gas-phase structure of ethyl 2-methyl pentanoate under molecular jet conditions, which we used to benchmark several exchange-correlation functionals and ab initio methods at the quantum chemical level. The observed conformer of ethyl 2-methyl pentanoate in the gas-phase is of C1 symmetry and exhibits a large amplitude motion around the C-C bond in proximity to the carbonyl moiety, which, unlike in the case of its structural isomer ethyl 2-ethyl butyrate, is very sensitive to the applied quantum chemical method and basis set. Depending on the applied quantum chemical method, the dihedral angle of the lowest energy conformer is optimized to absolute values of ±20°. This is far above the usual convergence error of the theoretical methods and has a tremendous impact on the rotational constants of this conformer, which complicates the prediction of rotational spectra and the assignment of experimental data. We show that the loss of symmetry in the aliphatic chain bound to the carboxylic moiety of ethyl esters results in a shift of the dihedral angle value due to a flat potential well around the corresponding C-C bond. Our benchmark calculations further indicate the potential relevance of the wB97X-D functional for this ethyl pentanoate and other related ethyl esters.

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

Binding Site Switch by Dispersion Interactions: Rotational Signatures of Fenchone-Phenol and Fenchone-Benzene Complexes

Non-covalent interactions between molecules determine molecular recognition and the outcome of chemical and biological processes. Characterising how non-covalent interactions influence binding preferences is of crucial importance in advancing our understanding of these events. Here, we analyse the interactions involved in smell and specifically the effect of changing the balance between hydrogen-bonding and dispersion interactions by examining the complexes of the common odorant fenchone with phenol and benzene, mimics of tyrosine and phenylalanine residues, respectively. Using rotational spectroscopy and quantum chemistry, two isomers of each complex have been identified. Our results show that the increased weight of dispersion interactions in these complexes changes the preferred binding site in fenchone and sets the basis for a better understanding of the effect of different residues in molecular recognition and binding events.

Conformers of Allyl Isothiocyanate: A Combined Microwave Spectroscopy and Computational Study

The pure rotational spectrum of allyl isothiocyanate (CH₂=CHCH₂-NCS) was collected from 4 to 26 GHz using Fourier transform microwave (FTMW) spectroscopy. Its analysis revealed the presence of two conformers that arise due to variation in the CCCN and CCNC dihedral angles. The observed spectrum is consistent with the accompanying potential energy surfaces derived using quantum chemical calculations at the B3LYP-D3(BJ) and MP2 levels of theory. Together, this experimental and theoretical study unequivocally identifies a new conformer (I) as the global minimum geometry. The spectral assignment of this new conformer is verified by the observation of transitions consistent with its ³⁴S, ¹³C, and ¹⁵N isotopologues and with the characteristic ¹⁴N quadrupole hyperfine patterns. For conformer I, the substitution (r_s) and effective ground state (r₀) structures were derived and reveal contributions from a large amplitude motion in the CCNC angle. The remaining geometric parameters compare well with the equilibrium structure (r_e) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3)° for conformer I of allyl-NCS is found to be ∼15° larger than that of allyl-NCO (137.5(4)°), which is in line with a change in the hybridization at nitrogen from an orbital with more ∼sp character in allyl-NCS to ∼sp^1.5 in allyl-NCO as determined via natural bond orbital analyses.

Dispersion-driven conformational preference in the gas phase: Microwave spectroscopic and theoretical study of allyl isocyanate

The conformations of allyl isocyanate (CH₂=CHCH₂N=C=O) were explored in the gas phase by combining theoretical calculations and Fourier transform microwave spectroscopy, including the chirped pulse and Balle-Flygare types. Three conformers (I, II, and III) were predicted using D3(BJ) dispersion-corrected B3LYP and MP2 methods; however, the lowest energy conformer (conf. I) was absent at the standard B3LYP level. The observed microwave spectra are consistent with the presence of both conf. I and III in the supersonic jet, and surprisingly, this is the first report of the global minimum conf. I both experimentally and theoretically. Rotational transitions from the parent species of both conformers as well as their minor isotopologues (¹³C, ¹⁵N, and ¹⁸O) in natural abundance were assigned allowing experimental geometries to be derived. For conf. I, in addition to the typical splitting pattern due to the ¹⁴N quadrupole nucleus, the transitions show a tunneling splitting which arises from the interconversion motion between its two mirror images. The experimental observation of conf. I and the absence of conf. II in the jet are rationalized using quantum-chemical calculations to explore the importance of electron correlation and in particular, demonstrate the necessity of including dispersion effects in density functional theory calculations even for seemingly small molecules.

The role of dispersion and anharmonic corrections in conformational analysis of flexible molecules: the allyl group rotamerization of matrix isolated safrole

Conformational changes of the monomeric safrole (5-(2-propenyl)-1,3-benzodioxole) isolated in low temperature xenon matrices were induced thermally or using narrow-band UV radiation. The rotation of the allyl group taking place in the studied matrices was followed by FTIR spectroscopy. Safrole represents a challenging example of a flexible molecule highlighting the importance of dispersion interactions and anharmonic effects in the structural, spectroscopic and energetic analysis. Structures of the safrole conformers, their energetics and infrared spectra have been calculated using various computational methods ranging from density functional theory (DFT) to coupled cluster (CC). The best theoretical results were obtained by integrating CCSD(T) energies including complete basis set extrapolation and core-valence corrections with B2PLYP-D3 equilibrium structures and hybrid B2PLYP-D3/B3LYP-D3 anharmonic computations for IR spectra and thermodynamics.