Intramolecular H-Bond Dynamics of Catechol Investigated by THz High-Resolution Spectroscopy of Its Low-Frequency Modes

Aminocyclopropenium as a novel hydrogen bonding organocatalyst for cycloaddition of carbon disulfide and epoxide to prepare cyclic dithiocarbonate

The smallest Hückel aromatic ring cyclopropenium substituted by electron-donating C-amino groups produced a aminocyclopropenium electron-rich cation. A bifunctional aminocyclopropenium halide catalyst installed with bis-(hydroxyethyl) functions on the amino group was then designed. A typical (diethanolamino)cyclopropenium halide catalyst C5·I was screened optimally for the cycloaddition of carbon disulfide into an epoxide to produce cyclic dithiocarbonate with an excellent conversion (95%) and high selectivity (92%). The electrostatic enhancement of alkyl C-H HBD capability was implemented via vicinal positive charges on the cyclopropenium core, and the acidity of the terminal O-H hydrogen proton increased by intramolecular H-bonding between the two hydroxy groups on the diethanolamino group (O-H⋯O-H). Then, a hybrid H-bond donor comprising enhanced alkyl C-H and hydroxy O-H was formed. The hybrid HBD offered by aminocyclopropenium was vital in activating the epoxide and stabilizing the intermediate, resulting in reduced O/S scrambling. Moreover, weakly coordinated iodide anion served as a nucleophilic reagent to open the ring of the epoxide. The cooperative catalytic mechanism of the HBD cation and halide anion was supported by NMR titrations and control experiments. Eleven epoxides with various substituents were converted into the corresponding cyclic thiocarbonate with high conversion and selectivity under mild conditions (25 °C, 6 h) without a solvent. The cycloaddition of carbon disulfide with epoxides catalyzed by aminocyclopropenium developed a new working model for hydrogen bonding organocatalysis.

Solvent effects on catechol's binding affinity: investigating the role of the intra-molecular hydrogen bond through a multi-level computational approach

The subtle interplay between the inter-molecular interactions established by catechol with the surrounding solvent and the intra-molecular hydrogen bond (HB) characterizing its conformational dynamics is investigated through a multi-level computational approach. First, quantum mechanical (QM) calculations are employed to accurately characterize both large portions of the catechol's potential energy surface and the interaction energy with neighboring solvent molecules. The acquired information is thereafter exploited to develop a QM derived force-field (QMD-FF), in turn employed in molecular dynamics (MD) simulations based on classical mechanics. The reliability of the QMD-FF is further validated through a comparison with the outcomes of ab initio molecular dynamics, also purposely carried out in this work. In agreement with recent experimental findings, the MD results reveal remarkable differences in the conformational behavior of isolated and solvated catechol, as well as among the investigated solvents, namely water, acetonitrile or cyclohexane. The rather strong intramolecular HB, settled between the vicinal phenolic groups and maintained in the gas phase, loses stability when catechol is solvated in polar solvents, and is definitively lost in protic solvents such as water. In fact, the internal energy increase associated with the rotation of one hydroxyl group and the breaking of the internal HB is well compensated by the intermolecular HB network available when both phenolic hydrogens point toward the surrounding solvent. In such a case, catechol is stabilized in a chelating conformation, which in turn could be very effective in water removal and surface anchoring. Besides unraveling the role of the different contributors that govern catechol's conformational dynamics, the QMD-FF developed in this work could be in future employed to model larger catechol containing molecules, due to its accuracy to reliably model both internal flexibility and solvent effects, while exploiting MD computational benefits to include more complex players as for instance surfaces, ions or biomolecules.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

Large amplitude motions (LAMs) form a fundamental phenomenon that demands the development of specific theoretical and Hamiltonian models. In recent years, along with the strong progress in instrumental techniques on high-resolution microwave spectroscopy and computational capacity in quantum chemistry, studies on LAMs have become very diverse. Larger and more complex molecular systems have been taken under investigation, ranging from series of heteroaromatic molecules from five- and six-membered rings to polycyclic-aromatic-hydrocarbon derivatives. Such systems are ideally suited to create families of molecules in which the positions and the number of LAMs can be varied, while the heteroatoms often provide a sufficient dipole moment to the systems to warrant the observation of their rotational spectra. This review will summarize three types of LAMs: internal rotation, inversion tunneling, and ring puckering, which are frequently observed in aromatic five-membered rings such as furan, thiophene, pyrrole, thiazole, and oxazole derivatives, in aromatic six-membered rings such as benzene, pyridine, and pyrimidine derivatives, and larger combined rings such as naphthalene, indole, and indan derivatives. For each molecular class, we will present the representatives and summarize the recent insights on the molecular structure and internal dynamics and how they help to advance the field of quantum mechanics.

Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation

Methylfurans are methylated aromatic heterocyclic volatile organic compounds and primary or secondary pollutants in the atmosphere due to their capability to form secondary organic aerosols in presence of atmospheric oxidants. There is therefore a significant interest to monitor these molecules in the gas phase. High resolution spectroscopic studies of methylated furan compounds are generally limited to pure rotational spectroscopy in the vibrational ground state. This lack of results might be explained by the difficulties arisen from the internal rotation of the methyl group inducing non-trivial patterns in the rotational spectra. In this study, we discuss the benefits to assign the mm-wave rotational-torsional spectra of methylfuran with the global approach of the BELGI-Cs code compared to local approaches such as XIAM and ERHAM. The global approach reproduces the observed rotational lines of 2-methylfuran and 3-methylfuran in the mm-wave region at the experimental accuracy for the ground vt=0 and the first torsional vt=1 states with a unique set of molecular parameters. In addition, the V3 and V6 parameters describing the internal rotation potential barrier may be determined with a high degree of accuracy with the global approach. Finally, a discussion with other heterocyclic compounds enables the study of the influence of the electronic environment on the hindered rotation of the methyl group.

Multidimensional H-atom tunneling in the catecholate monoanion

We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with different strengths. With a view to studying mode-specific tunneling in the catecholate monoanion, we have developed a full (33) dimensional potential energy surface in transition state (TS) normal modes using a Distributed Gaussian Empirical Valence Bond (DGEVB) based approach. We have computed eigenstates in different subspaces using both unrelaxed and relaxed potentials based on the DGEVB model. With unrelaxed potentials, we present results up to 7D subspaces that include the imaginary frequency mode and six modes coupled to it. With relaxed potentials, we focus on the two most important coupling modes. The structures of the ground and vibrationally excited eigenstates are discussed for both approaches and mode-specific tunneling splitting and their trends are presented.

Characterization of the Coriolis Coupled Far-Infrared Bands of syn-Vinyl Alcohol

Rotational emission from vibrationally excited molecules are responsible for a large fraction of lines in the spectra of interstellar molecular clouds. Vinyl alcohol (VA) has two rotamers that differ in energy by 6.4 kJ/mol, both of which have been observed toward the molecular cloud, Sagittarius B2(N) [Turner and Apponi, Astrophys. J. 2001, 561, 207]. Previously, we reported an analysis of the far-infrared spectrum of the higher energy rotamer, anti-VA [Bunn et al. Astrophys. J. 2017, 847, 67], yielding rotational and higher order distortion constants in the first excited vibrational state, and here, we report an analysis of the far-infrared spectrum of the lower energy rotamer, syn-VA, whose spectrum is significantly more complicated on account of Coriolis interactions that result in perturbations to the rovibrational spectrum. We account for those perturbations with the inclusion of Coriolis coupling constants in the fit, which couples the first excited OH torsional (ν₁₅) and CCO bending (ν₁₁) states. Inclusion of them resulted in more physically meaningful rotational and centrifugal distortion constants, and allows for accurate pure rotational line predictions to be made up to high energies. These will be particularly useful in searches for vibrationally excited syn-VA toward warm regions of interstellar molecular clouds, where we predict that it may be significantly abundant.