Binary conformers of a flexible, long-chain fluoroalcohol: dispersion controlled selectivity and relative abundances in a jet

The complex conformational panorama of binary 4,4,4-trifluoro-1-butanol (TFB) aggregates was investigated using chirped-pulse Fourier transform microwave spectroscopy, aided by conformational searches using CREST (Conformer-Rotamer Ensemble Sampling Tool) and quantum chemistry calculations. From nearly 1500 initial dimer geometries, 16 most stable binary candidates were obtained within a relative energy window of ∼4 kJ mol-1. Rotational spectra of five binary conformers were experimentally observed in supersonic expansion and assigned. Interestingly, three out of the five observed binary conformers are composed solely of monomer conformers, which were not observed in their isolated gas phase forms in jet expansion. In addition, an observed dimer that is made exclusively of the most stable TFB monomer subunits does not correspond to the global minimum. The intricate kinetically and thermodynamically controlled dimer formation mechanisms are discussed, and a modified kinetic-thermodynamic model was developed, providing conformational abundances that are in good agreement with the experiment. Subsequent non-covalent interaction analyses reveal that the observed conformers are held together by one primary O-H⋯O hydrogen bond and secondary intermolecular C-H⋯O, C-H⋯F, and/or O-H⋯F interactions, as well as C-H⋯H-C London dispersion interactions between the methylene groups. Further symmetry-adapted perturbation theory analyses of the TFB dimer conformers and related alcohol dimers reveal a considerable rise in dispersion contributions with increasing n-alkyl carbon chain length and highlight the role of dispersion interactions in preferentially stabilizing the global minimum of the TFB dimer.

Assignment of the absolute configuration of molecules that are chiral by virtue of deuterium substitution using chiral tag molecular rotational resonance spectroscopy

Chiral tag molecular rotational resonance (MRR) spectroscopy is used to assign the absolute configuration of molecules that are chiral by virtue of deuterium substitution. Interest in the improved performance of deuterated active pharmaceutical ingredients has led to the development of precision deuteration reactions. These reactions often generate enantioisotopomer reaction products that pose challenges for chiral analysis. Chiral tag rotational spectroscopy uses noncovalent derivatization of the enantioisotopomer to create the diastereomers of the 1:1 molecular complexes of the analyte and a small, chiral molecule. Assignment of the absolute configuration requires high-confidence determinations of the structures of these weakly bound complexes. A general search method, CREST, is used to identify candidate geometries. Subsequent geometry optimization using dispersion corrected density functional theory gives equilibrium geometries with sufficient accuracy to identify the isomers of the chiral tag complexes produced in the pulsed jet expansion used to introduce the sample into the MRR spectrometer. Rotational constant scaling based on the fact that the diastereomers have the same equilibrium geometry gives accurate predictions allowing identification of the homochiral and heterochiral tag complexes and, therefore, assignment of absolute configuration. The method is successfully applied to three oxygenated substrates from enantioselective Cu-catalyzed alkene transfer hydrodeuteration reaction chemistry.

Effects of aggregation on the structures and excited-state absorption for zinc phthalocyanine

In the present paper, the aggregated structures of zinc phthalocyanine (ZnPc) have been investigated by considering its dimers and trimers. Based on the density functional theory calculations, two stable conformations are obtained for the ZnPc dimer and trimer, respectively. The IGMH (independent gradient model based on the Hirshfeld partition of molecular density) analysis reveals that the π-π interaction between the ZnPc molecules causes the aggregation. Normally, stacked structures with a slight displacement are favorable for aggregation. In addition, the planar structure of the ZnPc monomer is largely maintained in the aggregated conformations. For the presently obtained structures, the first singlet excited state absorption (ESA) spectra of these aggregated conformations of ZnPc were calculated based on the linear-response time-dependent density functional theory (LR-TDDFT), which has been well applied by our group. The results of the excited state absorption spectra reveal that the aggregation causes the ESA band to blue shift compared to the ZnPc monomer. By using the conventional description of the interaction between monomer transition dipoles, this blue shift is elucidated by the side-by-side transition dipole moments in the constituted monomers. The present results for the ESA combined with the previously reported results for ground state absorption (GSA) will provide guidelines to tune the window of the optical-limiting effect for the ZnPc based materials.

Rotational Spectroscopy of the 2,2,3,3,3-Pentafluoropropanol⋅⋅⋅Water Complex: Conformations and Large Amplitude Motions

The 2,2,3,3,3-pentafluoropropanol (PFP) monomer can exist in five conformations defined by the CCCO and CCOH dihedral angles: four mirror-imaged pairs (G+g+/G-g-, G+g-/G-g+, G+t/G-t, Tg+/Tg-) and an achiral Tt form. We examined the conformational landscape of the PFP⋅⋅⋅water complex using chirped pulsed Fourier transform microwave spectroscopy and theoretical calculations. Rotational spectra of two PFP⋅⋅⋅water conformers, PFPG+g+⋅⋅⋅W_H and PFPTg+⋅⋅⋅W_H , and seven deuterated isotopologues of each, were assigned. Tunneling splittings were observed for both conformers and are attributed to the exchange of the bonded and non-bonded hydrogen atoms of water. On the other hand, the tunneling splitting associated with the OH flipping motion in PFPTg+/Tg- appears to be quenched upon hydrogen bonding with water. The large amplitude motions associated with the water subunits were examined in detail to explain the very different magnitudes of the experimental and theoretical permanent electric dipole moment components. The study highlights the challenge in correctly identifying the conformers observed when large amplitude motions are involved. Quantum theory of atoms in molecules (QTAIM) and non-covalent interaction (NCI) analyses, as well as electrostatic potential (ESP) calculations were carried out to explore the nature of the non-covalent interactions and to appreciate the effects of fluorination.