Conformational steering in dicarboxy acids: the native structure of succinic acid

Gas-Phase Characterization of Adipic Acid, 6-Hydroxycaproic Acid, and Their Thermal Decomposition Products by Rotational Spectroscopy

We report the spectroscopic investigation of two bifunctional aliphatic carboxylic acids, namely, adipic acid and 6-hydroxycaproic acid, in the gas phase by combining high-resolution rotational spectroscopy and supersonic expansions. Their pure rotational spectra were successfully identified and characterized. However, due to the low thermal stability of these two chemicals, the measured rotational spectra were significantly congested with transitions corresponding to their decomposition products upon heating. We observed cyclopentanone and adipic anhydride in the spectrum of adipic acid and ε-caprolactone and its monohydrate in the spectrum of 6-hydroxycaproic acid. On the basis of the distinct fingerprints of both carboxylic acids and a series of their decomposition products, the spectra were analyzed in a time-segmented manner. This provides valuable insights into the thermal decomposition mechanisms of these two samples over time, which highlights the robustness of microwave spectroscopy as a potent tool for analyzing complex chemical mixtures in a species-, isomer-, and conformer-selective way.

Unveiling Five Naked Structures of Tartaric Acid

The unbiased, naked structures of tartaric acid, one of the most important organic compounds existing in nature and a candidate to be present in the interstellar medium, has been revealed in this work for the first time. Solid samples of its naturally occurring (R,R) enantiomer have been vaporized by laser ablation, expanded in a supersonic jet, and characterized by Fourier transform microwave spectroscopy. In the isolation conditions of the jet, we have discovered up to five different structures stabilized by intramolecular hydrogen-bond networks dominated by O-H⋅⋅⋅O=C and O-H⋅⋅⋅O motifs extended along the entire molecule. These five forms, two with an extended (trans) disposition of the carbon chain and three with a bent (gauche) disposition, can serve as a basis to represent the shape of tartaric acid. This work also reports the first set of spectroscopy data that can be used to detect tartaric acid in the interstellar medium.

Interaction topologies of the SO chalcogen bond: the conformational equilibrium of the cyclohexanolSO2 cluster

The conformational landscape of the cyclohexanolSO2 cluster was revealed in the gas phase using chirped-pulsed broadband rotational spectroscopy and quantum chemical calculations. Four isomers stabilized by a dominant SO chalcogen bond and cooperative C-HO[double bond, length as m-dash]S and O-HO[double bond, length as m-dash]S secondary weak hydrogen bonds were observed, with a near-parallel orientation of the S[double bond, length as m-dash]O and O-H bonds. Isomers formed by equatorial-gauche cyclohexanol are more stable than the isomers containing axial cyclohexanol. The multiple conformations of cyclohexanol and the versatile binding properties of SO2, simultaneously operating as nucleophile and electrophile through its π-holes and non-bonding electrons lead to a complex conformational behavior when the cluster is formed. The long (2.64-2.85 Å) attractive SO interaction between SO2 and cyclohexanol is mainly electrostatic and the contribution of charge transfer is obvious, with an NBO analysis suggesting that the strength of the SO interaction is nearly two orders of magnitude larger than the hydrogen bonds. This study provides molecular insights into the structural and energetic characteristics that determine the formation of pre-nucleation clusters between SO2 and a volatile organic compound like cyclohexanol.

The puzzling hyper-fine structure and an accurate equilibrium geometry of succinic anhydride

An accurate semiexperimental equilibrium structure of succinic anhydride has been determined from a combination of experiment and theory. The cm-wave and mm-wave rotational spectra of succinic anhydride, 3,4-dihydrofuran-2,5-dione, were recorded in a pulsed supersonic jet using Fourier-transform microwave spectroscopy and in a free-jet using mm-wave absorption spectroscopy. Many lines in the cm-wave spectrum show fine structure and after eliminating all other possibilities the origin of this fine structure is determined to be from spin-spin interaction. Accurate rotational and quartic centrifugal distortion constants are determined. Assignments of 13C and 18O singly substituted isotopologues in natural abundance were used to obtain a substitution geometry for the heavy atoms of succinic anhydride. Theoretical approaches permitted the calculation of a Born-Oppenheimer ab initio structure and the determination of a semiexperimental equilibrium structure in which computed rovibrational corrections were utilized to convert vibrational ground state rotational constants into equilibrium constants. The agreement between the semiexperimental structure and the Born-Oppenheimer ab initio structure is excellent. Succinic anhydride has been shown to have a planar heavy atom equilibrium structure with the effects of a large amplitude vibration apparent in the resultant rotational constants.

Structure and Stability of Molecular Crystals with Many-Body Dispersion-Inclusive Density Functional Tight Binding

Accurate prediction of structure and stability of molecular crystals is crucial in materials science and requires reliable modeling of long-range dispersion interactions. Semiempirical electronic structure methods are computationally more efficient than their ab initio counterparts, allowing structure sampling with significant speedups. We combine the Tkatchenko-Scheffler van der Waals method (TS) and the many-body dispersion method (MBD) with third-order density functional tight-binding (DFTB3) via a charge population-based method. We find an overall good performance for the X23 benchmark database of molecular crystals, despite an underestimation of crystal volume that can be traced to the DFTB parametrization. We achieve accurate lattice energy predictions with DFT+MBD energetics on top of vdW-inclusive DFTB3 structures, resulting in a speedup of up to 3000 times compared with a full DFT treatment. This suggests that vdW-inclusive DFTB3 can serve as a viable structural prescreening tool in crystal structure prediction.