The conformational behavior of methyl 2-methoxybenzoate has been explored using rotational spectroscopy. The two conformers identified show for each rotational transition a quadruplet attributable to two sets of splittings due to two tunneling motions, the internal rotation of the ester methyl group and the internal rotation of the entire ester group with respect to the benzene ring, which in addition interconverts both observed conformers. The analysis of such splittings allowed the determination of the potential energy barriers for the tunneling vibrations. The spectra of the complexes of methyl 2-methoxybenzoate with water and formic acid have also been analyzed. In both cases, only the global minimum structure could be identified in the spectra. In contrast with the observations in many other systems, the conformational landscape of the monomer is altered upon complexation. In the complexes the relative stability of the conformers observed in the monomer turn out to be inverted. Ester group torsion and methyl group internal rotation splittings were observed in the monohydrated complex, but not in the complex with formic acid.

Noncovalent interactions of aromatic complexes are highly significant in biological systems such as DNA, in materials science and in supramolecular chemistry. Gas phase studies of small aromatic complexes allow the determination of their preferred structural arrangements and of the relative contributions of various intermolecular forces to the interaction energies, laying the foundation for understanding the properties and interactions of larger systems. Azulene is one of the smallest aromatic hydrocarbons with a dipole moment. Here we present the investigation of azulene aggregation and its interactions with water using chirped-pulse Fourier transform microwave spectroscopy. Experimental observations are compared with predictions by various theoretical methods to evaluate their performance.

Solvation of aromatic molecules is a fundamental chemical process. Stability and function of almost all biomolecules are governed by the surface water molecules, typically known as biological water. Despite a plethora of reports available for the prototype aromatic molecule, benzene,¹ studies related to the solvation of larger aromatic moieties are rather scarce. Herein, we probe the microsolvated structures of the simplest polycyclic aromatic hydrocarbon, naphthalene (Np), by high-resolution infrared (IR) spectroscopy inside helium nanodroplets and quantum chemical calculations. Our results show that in the monohydrated Np-water dimer, solvent water preferentially binds to the π electron cloud of the two fused phenyl rings via OH-π hydrogen bonds (H-bond) and acts as a double H-bond donor. This binding motif is strikingly different from the corresponding cationic Np-water complex, in which water lies in the Np-ring plane and acts as a double H-bond acceptor by the simultaneous formation of two CH-O H-bonds.^2,3 Therefore, we see the presence or absence of charge causes a substantial modification of solvent binding motif. Further stepwise water-addition to neutral Np-water complex leads to evolution of H-bond network as reflected from the IR spectra. In larger Np-(water)_2,3 clusters, hydration motifs mimic the bare water network and thus leads to the formation of linear H-bonded water dimer and cyclic water trimer, which simultaneously interact with Np π electrons. Changing the solvent from water to methanol does not change the qualitative π bonding motif. However, in this case, the solvent only interacts with a single phenyl ring via a OH-π H-bond due to the presence of a single proton donor. The addition of the second solvent molecule leads to the formation of H-bonded methanol dimer that interacts with Np. References: 1. R. N. Pribble and T. S. Zwier, Science, 1994, 265, 75 2. K. Chatterjee and O. Dopfer, Chem. Sci., 2018, 9, 2301. 3. K. Chatterjee and O. Dopfer, Phys. Chem. Chem. Phys., 2017, 19, 32262

The rotational spectrum of the 2-methylaminoethanol-water (2MAE-water) complex has been observed using a cavity based Fourier-transform microwave spectrometer in the range of 10-19 GHz. 2MAE exists in trans and gauche conformationsR. E. Penn and L. W. Buxton, J. Mol. Spectrosc. 56, 229-238 (1975).^,C. Calabrese, A. Maris, L. Evangelisti, A. Piras, V. Parravicini and S. Melandri, Front. Chem. 6:25 (2018).^,Y. Liu, C.A. Rice and M.A. Suhm, Can. J. Chem. 82, 1006-1012 (2004). We modeled 10 possible conformers of the 2MAE-water complex, five trans and five gauche conformers, using ab initio calculations (MP2/6-311++G(d,p)). 14 rotational transitions were fit to Watson's A-reduced Hamiltonian: A=3368.02 MHz, B=2282.60 MHz, and C=1538.00 MHz. ¹⁴N nuclear quadrupole hyperfine splittings were resolved, and the 38 hyperfine components were fit to χ_aa = 1.543(7) MHz and χ_bb = -1.083(25) MHz. The measured spectrum is assigned to the lowest energy model structure of the complex, which has two intermolecular hydrogen bonds: from hydroxyl group to water and from water to the methylamino group. The structure of 2MAE-water is compared with 2-aminoethanol-water (2AE-water) and 2-methoxyethylamine-water (2MEA-water) complexes.

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Footnotes:

R. E. Penn and L. W. Buxton, J. Mol. Spectrosc. 56, 229-238 (1975).\end C. Calabrese, A. Maris, L. Evangelisti, A. Piras, V. Parravicini and S. Melandri, Front. Chem. 6:25 (2018). Y. Liu, C.A. Rice and M.A. Suhm, Can. J. Chem. 82, 1006-1012 (2004)..

Microwave spectra have been recorded for methanesulfonic acid (CH₃SO₃H) and its –OD isotopologue. No internal rotation of the methyl group was observed, consistent with the calculated high barrier of 2.7 kcal/mol. A pair of tunneling states has been observed for both species, however, and is attributed to large amplitude wagging of the hydroxyl hydrogen from one side of the molecule to the other. The predicted barrier to this motion, obtained from M06-2X/6-311++G(3df,3pd) calculations, is 0.7 kcal/mol. For CH₃SO₃D, the tunneling energy was directly determined to be ∆E = 6471.9269(17) MHz from the measurement of c-type spectra. In the case of the parent species, however, transitions displaced by the tunneling energy have not been located and are likely above the maximum frequency accessible by the spectrometer (20 GHz). Thus, the value of ∆E could not be experimentally determined. Nonetheless, a satisfactory fit was obtained for transitions involving J"= 0 and J" = 1 (nine frequencies for each state). Suggestions for further work at higher frequencies will be presented.

The methanesulfonic acid water - complex (CH₃SO₃H-H₂O) has been observed using pulse-nozzle Fourier transform microwave spectroscopy. The rotational spectra for the CH₃SO₃H-D₂O and CH₃SO₃D-D₂O isotopologues have also been obtained and analyzed. DFT calculations predict the two lowest energy conformers of CH₃SO₃H-H₂O to form a strong hydrogen bond between the water molecule and the acidic proton and a second, longer hydrogen bond with one of the S=O oxygens to form the 6-membered ring-like structure that is typical of oxyacid monohydrates. The observed rotational constants and isotope shifts are in best agreement with those predicted for the global minimum structure of CH₃SO₃H-H₂O, where the unbound H₂O hydrogen atom is oriented away from the methyl group. In contrast to the triflic acid monohydrate (CF₃SO₃H-H₂O) spectrum, there was no evidence of a pair of tunneling states arising from internal motion of the water. A and E internal rotor states were not resolvable in the observed spectrum, consistent with the predicted high barrier for methyl group internal rotation (V₃=1000 cm⁻¹).

r0.33 Figure We have recorded gas-phase Fourier transform Infrared spectra at room temperature of several hydrogen (H) and deuterium (D) bound bimolecular complexes. These complexes were formed using methanol and ethanol as OH donors, and methanol-d₁ and ethanol-d₁ as OD donors, to compare isotopic effects in H/D-bond stability. The stability of these complexes are governed by the Gibbs energy of complex formation. By combining experimental and calculated intensities, the pressure of the complex can be determined, and from that and monomer pressures the Gibbs energy of complex formation. At room temperature, we find similar Gibbs energies for corresponding H/D-bound complexes. For the pair of H/D-complexes, methanol·dimethylether (OH·O) and methanol-d₁·dimethylether (OD·O), Gibbs energies of 8.3 and 7.7 kJ/mol were determined, and for methanol·trimethylamine (OH·N) and methanol-d₁·trimethylamine (OD·N) were determined Gibbs energies of 3.2 and 2.7 kJ/mol. Our findings of similar Gibbs energies for H/D-bound complexes at room temperature, is in contrast to what has been observed in spectra recorded in cold conditions. The deuterium bound complex is heavily favored, at low temperatures due to a lower zero-point vibrational energy. From calculations, we find that the difference in stability of the H/D-bound complexes become smaller with increasing temperature. The change in stability with temperature, arise from changes in both the rotational and vibrational entropic contributions.[1]

[1] A. Kjaersgaard, E. Vogt, N. F. Christensen, H. G. Kjaergaard, J. Phys. Chem. A., 2020,
       DOI: 10.1021/acs.jpca.9b11762.

The CH₃CN-CO₂ complex was investigated using a pulsed nozzle Fourier transform microwave spectrometer. Nitrogen containing compounds generally form a T-shaped complex with CO₂ having a N-CO₂ ‘tetrel bond’. For example, microwave spectrum confirmed this structure for HCN-CO₂ back in 1984, though specific names of intermolecular bonds were not used those daysLeopold, K. R.; Fraser, G. T.; Klemperer, W. J. Chem. Phys. 1984, 80, 1039–1046. We have observed two structures for the CH₃CN-CO₂ complex, the T-shaped and a π-stacked (the CO₂ is parallel to the CH₃CN). The ab initio calculations show that the two structures have similar binding energies. The T-shaped structure has a nearly prolate ‘a’–type spectra with the K=1 lines missing, which is consistent with the C_2v symmetry of the T-shaped structure. All rotational transitions observed show hyperfine splitting due to nuclear quadrupole coupling of the nitrogen atom. Measurements with isotopic substitutions have been carried out to ascertain the assignment of the rotational transitions. Leopold, K. R.; Fraser, G. T.; Klemperer, W. J. Chem. Phys. 1984, 80, 1039–1046..

The microwave spectrum of the NH₃−Ne van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Together with the well-known lines related to the (ortho)-NH₃−Ne spectrumJ. van Wijngaarden and W. Jäger, J. Chem. Phys. 115, 6504 (2001). new transitions were detected and attributed to the missing (para)-NH₃−Ne nuclear spin isomer. The assignments were guided by the rovibrational bound state (J = 0 ... 10) calculations based on the recent ab initio NH₃−Ne intermolecular potential surfaceJ. Loreau and A. van der Avoird, J. Chem. Phys. 143, 184303 (2015). The hyperfine structure arising from quadrupole ¹⁴N nucleus of NH₃−Ne was resolved, and the quadrupole coupling constant associated with the (para)-NH₃ subunit was precisely determined. This constant provided the dynamical information about the angular orientation of ammonia indicating that the average angle between the C₃ axis of NH₃ and intermolecular axis is about 68^°.

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Footnotes:

J. van Wijngaarden and W. Jäger, J. Chem. Phys. 115, 6504 (2001)., J. Loreau and A. van der Avoird, J. Chem. Phys. 143, 184303 (2015)..

The study of hydrogen-bonded complexes in the gas phase aims to advance our knowledge of the intermolecular forces at play in these systems. To this end, ethanol clusters are of particular interest as archetypes of the interplay between hydrogen bonding and London dispersion forces. Here we present an investigation into the preferred isomers of ethanol trimer using computational methods in combination with broadband chirped-pulse Fourier transform microwave spectroscopy. The potential energy surface of ethanol trimer has been investigated through DFT and ab initio calculations, showing that the –OH groups forms a hydrogen bonded cyclic structure in the lower-energy isomers. From the analysis of the rotational spectrum, four distinct rotamers of ethanol trimer have been detected. All rotamers display O-H…O hydrogen bonds between the three hydroxyl groups, where each hydroxyl group simultaneously acts as hydrogen bond donor and acceptor.

2-furoic acid (FA), a heterocyclic carboxylic acid, is a significant precursor of esters, acrylic chloride, and acid anhydride, and is often used for medical purpose, for example in an orally active antidiabetic vanadyl complex.1 FA monomer may exist in a number of possible conformations based on the orientation of the COOH group with the ring O atom and on the relative orientation of the C=O and OH in the COOH group. Some of these possible conformations were investigated before.2 In this work, we studied conformational landscapes of the FA monomer and dimer by using a chirped pulse Fourier transform microwave (FTMW) spectrometer and DFT calculations. Conformational searches were performed to identify possible conformations using a conformer-rotamer ensemble sampling tool (CREST).3 The resulting candidates were further optimized at the B3LYP-D3(BJ)/def2-TZVP level of theory. Three FA conformers and four (FA)2 conformers 2-furoic acid dimers were predicted. Experimentally, rotational spectra of three monomeric conformers and two binary conformers were assigned. Two other FA dimers predicted have zero electric dipole moments. Detailed analyses of the double proton tunneling motion and conformational conversion barriers will also be presented. 1 M. Yin, X. Lei, M. Li, L. Yuan, J. Sun, J. Phys. Chem. Solids, 2006, 67, 1372-1378. 2 A. Halasa, L. Lapinski, I. Reva, H. Rostkowska, R. Fausto, M. J. Nowak, J. Phys. Chem. A, 2015, 119, 1037-1047; H. Ghalla, N. Issaoui, M. V. Castillo, S. A. Brandán, H. T. Flakus, Spectrochim. Acta A Mol. Biomol. Spectrosc, 2014, 121, 623-631. 3 S. Grimme, J. Chem. Theory Comput., 2019, 15, 2847-2862.

r0.40 Figure A hydrogen bound bimolecular complex consists of a hydrogen bond donor and acceptor unit. The OH-stretching fundamental transition of the hydrogen bond donor is typically redshifted and its infrared intensity enhanced upon complexation.^[1] This facilitates detection of weak complexes even though the equilibrium is strongly shifted towards the monomers at room temperature. The ratio of a measured and calculated intensity of a vibrational band is proportional to the complex abundance, which with known monomer pressures gives the equilibrium constant.^[2] We calculate absolute transition intensities with a reduced dimensionality variational local mode model that also includes low-frequency vibrations. Calculated and experimental intensities of multiple bands are combined to give the equilibrium constant of complex formation for the water·dimethylamine and water·trimethylamine complex.^[3] The equilibrium constant obtained from different bands should be equivalent, and the detection of multiple bands therefore allows us to validate the accuracy of our combined experimental and theoretical approach.

Carbenes are classified as an important reactive intermediate in chemical reactions owing to their multiple spin states. Recent studies in matrix isolation technique (3 K) demonstrated that a single water molecule is able to switch the spin state of diphenylcarbene¹ (triplet to singlet) and furthermore confirmed the formation of the carbenium ion in the LDA ice matrix². In this line helium nanodroplet spectroscopy provides an ideal environment to study such solvation processes at ultracold temperature (0.37 K) by successive addition of water molecules. Here, we aim to determine the minimum number of water molecules required for the formation of the carbenium ion from its parent carbene molecule. To understand the solvation process of diphenylcarbene, it is crucial to investigate the initial hydration framework of the precursor molecule, diazodiphenylmethane. Therefore, the present study elucidated the IR spectroscopic investigation of diazodiphenylmethane (C₁₃H₈N₂)-water (D₂O) complexes in helium nanodroplets. To assign the observed bands corresponding to C₁₃H₈N₂-D₂O complex, we also performed ab-initio calculation at B3LYP-D3/def2-TZVP level. References: 1. Costa, P.; Sander, Angew. Chem. 2014, 126, 5222– 5225. 2. Costa, P.; Fernandez-Oliva, M; Sanchez-Gracia, E;Sander, W., J.Am. Chem. Soc. 2014, 136, 15625-30.