Beppo Hartwig

Rotational spectroscopy was used to systematically tune and quantify halogen bond donor ability, i.e. changing the extent of the sigma hole, by applying the extended Townes–Dailey model to a total of 19 newly measured fluorinated iodobenzenes.

Rotational spectra for hexafluoroacetone imine, the singly substituted ¹³C isotopologues, and the ¹⁵N isotopologue, have been recorded using both cavity and chirped pulse Fourier transform microwave spectrometers. The spectra observed present as being doubled with separations between each pair of transitions being on the order of a few tens of kilohertz which is consistent with a large amplitude motion producing two torsional substates. The observed splitting is most likely due to the combined motions of the CF3 groups, for which the calculated barrier is small. However, no transitions between states could be observed and, similarly, no Coriolis coupling parameters were required to achieve a satisfactory fit for the transition frequencies. Hence, and somewhat curiously, the two states have been fit independently of each other such that the two states may simply be considered near-equivalent conformers. The structural properties of hexafluoroacetone imine are compared with two isoelectronic molecules hexafluoroisobutene and hexafluoroacetone. Rotational constants, quartic centrifugal distortion constants, and the ¹⁴N nuclear electric quadrupole coupling tensor have been determined and are presented together with supporting quantum chemical calculations.

Microwave spectra of citraconic anhydride and its tautomer itaconic anhydride have been recorded in a frequency range of 6–18 GHz. Both a- and b-type transitions were observed for both tautomers, while c-type transitions could only be observed for the E torsional symmetry state of citraconic anhydride. For both molecules, a molecular substitution structure, r_S, was obtained by measurements of mono-substituted ¹³C isotopologues in natural abundance. For citraconic anhydride, ¹⁸O isotopologues were also observed and the V₃ barrier to internal rotation has been determined at 326.5153(61) cm⁻¹. In addition to the microwave spectra, a gas-phase study of isomerisation between the tautomers was carried out, which was assisted by theoretical transition state calculations, employing a variety of different density functionals as well as the wavefunction based Møller–Plesset perturbation theory, MP2, and coupled cluster methods, CCSD(T)-F12c and DCSD-F12b. These were also used to benchmark the experimentally determined r_S structures and V₃ barrier of rotation in citraconic anhydride. Via theoretical ground state vibrational calculations, semi-experimental equilibrium structure, r^SE _0→e, were derived for each theoretical method and were compared to the coupled cluster equilibrium structures, r_e. In addition, mass dependent r_m ⁽¹⁾ and r_m ⁽²⁾ fits were conducted to obtain approximate r_e structures. Using the determined structures we can revise a previous study that misidentified citraconic anhydride as itaconic anhydride.

Rotational spectroscopy is an excellent tool for structure determination, which can provide additional insights into local electronic structure by investigating the hyperfine pattern due to nuclear quadrupole coupling. Jet-cooled molecules are good experimental benchmark targets for electronic structure calculations, as they are free of environmental effects. We report the rotational spectra of 2-chlorobenzaldehyde, 3-chlorobenzaldehyde, and 4-chlorobenzaldehyde, including a complete experimental description of the nuclear quadrupole coupling constants, which were previously not experimentally determined. We identified two conformers for 3-chlorobenzaldehyde and one conformer each for 2-chlorobenzaldehyde and 4-chlorobenzaldehyde. Rigorous structure fitting of 4-chlorobenzaldehyde was performed to determine bond lengths for r₀, r_s, r_e^se, and r_m⁽¹⁾ structures. Comparing experimental nuclear quadrupole coupling constants to computational results showed agreement in the nuclear axis system, but the accuracy of the projection into the principal axis system decreases in near-oblate 2-chlorobenzaldehyde. The experimental angle Θ_az = 19.16° between the principal a-axis and nuclear z-axis is larger than predicted by multiple computational methods by ≥4°. It is attributed to the high sensitivity of 2-chlorobenzaldehyde to low-energy vibrational contributions.

Substitution of the transiently chiral cis-1,2-cyclohexanediol (cis-CD) by a phenyl results in (S,S)-(+)-1-phenylcyclohexane-cis-1,2-diol (cis-PCD) and modifies the hydrogen bond pattern in its dimer. The heterochiral preference observed in cis-CD changes for homochiral in cis-PCD.

Raman and rotational spectroscopy allow us to benchmark different properties with theory and indirectly helps understanding chirality recognition. A possible correlation between OH stretching frequencies and the asymmetry parameter η is explored.

The theoretical description of spectral signatures for weakly bound hydrogen contacts between alcohol groups is challenging and remains poorly characterised. By combining Raman jet spectroscopy with appropriately scaled harmonic DFT predictions and relaxation path analyses for 16 vicinal diols (ethylene glycol (ethane-1,2-diol), propane-1,2-diol, 3,3,3-trifluoro-propane-1,2-diol, rac-butane-2,3-diol, 2-methyl-propane-1,2-diol, 2-methyl-butane-2,3-diol, pinacol (2,3-dimethyl-butane-2,3-diol), 3-butene-1,2-diol, 1-phenyl-ethane-1,2-diol, trans-cyclobutane-1,2-diol, trans-cyclopentane-1,2-diol, trans-cyclohexane-1,2-diol, trans-cycloheptane-1,2-diol, cis-cyclohexane-1,2-diol, 1-(1-hydroxy-1-methylethyl)-cyclopentanol and [1,1′-bicyclopentyl]-1,1′-diol), 69 conformational assignments become possible in a two-tier approach with a 5 diol training and an 11 diol test set. The latter reveals systematic deviations for ring strain and secondary π interactions, but otherwise a remarkably robust correction and correlation model based on hybrid DFT with a minimally augmented triple-zeta basis set is obtained, whereas GGA functionals perform significantly worse. Raw experimental data in the 3560–3700 cm⁻¹ wavenumber range as well as computed geometries of all conformations invite further vibrational and structural benchmarking at the onset of hydrogen bonding. Beyond this diol-probed threshold, the accurate prediction of hydrogen bond induced shifts of different magnitudes remains one of the challenges for DFT functionals.

We analyse how dispersion interactions impact chirality recognition, both in the structure and energy of diol clusters.

By a combination of linear FTIR and Raman jet spectroscopy, racemic trans-1,2-cyclohexanediol is shown to form an energetically unrivalled S₄-symmetric heterochiral dimer in close analogy to 1,2-ethanediol. Analogous experiments with enantiopure trans-1,2-cyclohexanediol reveal the spectral signature of at least three unsymmetric homochiral dimers. A comparison to signal-enhanced spectra of 1,2-ethanediol and to calculations uncovers at least three transiently homochiral dimer contributions as well. In few of these dimer structures, the intramolecular OH⋯O contact present in monomeric 1,2-diols survives, despite the kinetic control in supersonic jet expansions. This provides further insights into the dimerisation mechanism of conformationally semi-flexible molecules in supersonic jets. Racemisation upon dimerisation is shown to be largely quenched under jet cooling conditions, whereas it should be strongly energy-driven at higher temperatures. The pronounced energetic preference for heterochiral aggregation of vicinal diols is also discussed in the context of chirality-induced spin selectivity.