Conformational adaptation and large amplitude motions of 1-phenyl-2,2,2-trifluoroethanol with two water molecules: a rotational spectroscopic and ab initio investigation.

The 1 : 2 adduct of 1-phenyl-2,2,2-trifluoroethanol (PhTFE), a chiral fluoroalcohol, with two water molecules (PhTFE⋯2H2O) was investigated via chirped pulse Fourier-transform microwave (CP-FTMW) spectroscopy and theoretical calculations. A systematic search of the PhTFE⋯2H2O conformational landscape identified 38 stable minima at the B3LYP-D3BJ/def2-TZVPPD level of theory, 27 of which are within an energy window of 10 kJ mol-1 after applying zero-point energy corrections. Rotational spectra of a single PhTFE⋯2H2O conformer along with eight deuterated and three oxygen-18 isotopologues were assigned. Interestingly, the observed PhTFE⋯2H2O conformer contains PhTFE II, the second most stable monomer conformer, and the most stable PhTFE I dihydrate is ca. 4 kJ mol-1 higher in energy. In contrast, PhTFE I⋯H2O was identified experimentally and theoretically as the most stable 1 : 1 conformer. Furthermore, the observed dihydrate structure experiences large amplitude motions connecting three theoretical minima which differ only in which water oxygen lone pairs are involved in the hydrogen-bonds, i.e., the free OH pointing directions. Additionally, the ortho and para-H2O tunnelling splittings were detected and attributed to the interchange water hydrogen atoms which interact with the aromatic part of PhTFE but not for the water interacting with PhTFE hydroxy group. Extensive theoretical modelling was carried out to gain insight into the associated large amplitude motions including tunnelling, supported by the experimental isotopic and tunnelling splitting data.

Unlocking a new hydrogen-bonding marker: C-O bond shortening in vicinal diols revealed by rotational spectroscopy.

The conformational space of cis-1,2-cyclohexanediol, a model molecule for cyclic vicinal diols, was investigated using rotational spectroscopy and density functional theory calculations. Four low energy conformers within an energy window of 5 kJ mol-1 were identified computationally. A rotational spectrum of jet-cooled cis-1,2-cyclohexanediol was recorded with a chirped pulse Fourier transform microwave spectrometer. Two sets of rotational transitions were observed and could be assigned to conformers of cis-1,2-cyclohexanediol. The non-observation of other low energy conformers was explained by conformational conversion barrier height calculations and results from experimental spectra recorded with different carrier gases. Eight isotopologues, including those with 13C and 18O, of the lowest energy conformer were observed, allowing the determination of the semi-experimental equilibrium structure, reSE. Interestingly, the structural analysis revealed that the C-O bond length of the intramolecular hydrogen-bond donor is shorter than that of the acceptor. This appears to be a general characteristic of vicinal diols and can be used as a novel hydrogen-bond marker in such compounds.

Wetting vs Droplet Aggregation: A Broadband Rotational Spectroscopic Study of 3-Methylcatechol⋠⋠⋠Water Clusters.

Two competing solvation pathways of 3-methylcatechol (MC), an atmospherically relevant aromatic molecule, with up to five water molecules were explored in detail by using a combination of broadband rotational spectroscopy and computational chemistry. Theoretically, two different pathways of solvation emerge: the commonly observed droplet pathway which involves preferential binding among the water molecules while the solute serves as an anchor point for the formation of a water cluster, and an unexpected wetting pathway which involves interactions between the water molecules and the aromatic face of MC, i.e., a wetting of the π-surface. Conclusive identification of the MC hydrate structures, and therefore the wetting pathway, was facilitated by rotational spectra of the parent MC hydrates and several H2 18 O and 13 C isotopologues which exhibit splittings associated with methyl internal rotation and/or water tunneling motions. Theoretical modelling and analyses offer insights into the tunneling and conversion barriers associated with the observed hydrate conformers and the nature of the non-covalent interactions involved in choosing the unusual wetting pathway.

Conformers of Vanillic Acid and Its Monohydrate: A Rotational Spectroscopic and Theoretical Study.

Organic acids are released during wildfire combustion and can influence aerosol formation and growth. Conformational flexibility is thought to be advantageous in stabilizing the precritical nucleus in the process of aerosol particle formation and allowing for further complexation with other atmospheric constituents. We describe here a study of the conformational flexibility of vanillic acid and its monohydrate using electronic structure calculations and Fourier transform microwave spectroscopy. Computationally, 12 and 28 conformers were found for the monomer and monohydrate, respectively. The two lowest energy conformers of both the vanillic acid monomer and the vanillic acid-water complex could be experimentally identified. The deviation between experimental and theoretical rotational constants determined at the MP2/aug-cc-pVTZ and DFT B3LYP-D3(BJ)/def2-TZVP levels of theory is less than 1%. No tunneling splittings were observed, which suggests a relatively high barrier to methyl internal rotation, in agreement with other, previously studied vanillin derivatives. Furthermore, no c-type transitions could be observed for the vanillic acid monomer, in agreement with the computed zero c-dipole moment component of the two lowest energy structures. For the monohydrate the absence of c-type transitions is rationalized by averaging over a large amplitude motion involving the free H atom of the water unit. From the theoretical structures, it is apparent that intramolecular hydrogen bonds play a significant role in stabilizing the lowest energy conformers. To further characterize the intramolecular interactions in the monomer and intra- and intermolecular interactions in the monohydrate, quantum theory of atoms-in-molecules (QTAIM), noncovalent interactions (NCI), and intrinsic bond strength index (IBSI) analyses were performed. The atmospheric abundance of the vanillic acid monohydrate relative to the monomer was evaluated to assess its atmospheric significance.

Vibrational Spectroscopy of Homo- and Heterochiral Amino Acid Dimers: Conformational Landscapes.

The homo- and heterochiral protonated dimers of asparagine with serine and with valine were investigated using infrared multiple-photon dissociation (IRMPD) spectroscopy. Extensive quantum-chemical calculations were used in a three-tiered strategy to screen the conformational spaces of all four dimer species. The resulting binary structures were further grouped into five different types based on their intermolecular binding topologies and subunit configurations. For each dimer species, there are eight to fourteen final conformational geometries within a 10 kJ mol-1 window of the global minimum structure for each species. The comparison between the experimental IRMPD spectra and the simulated harmonic IR features allowed us to clearly identify the types of structures responsible for the observation. The monomeric subunits of the observed homo- and heterochiral dimers are compared to the corresponding protonated/neutral amino acid monomers observed experimentally in previous IRMDP/rotational spectroscopic studies. Possible chirality and kinetic influences on the experimental IRMPD spectra are discussed.