The broadband rotational spectrum of jet-cooled laser-ablated thioproline was recorded. Two conformers of this system were observed and identified with the help of DFT and ab initio computations by comparison of the observed and calculated rotational constants and 14N quadrupole coupling constants as well as the predicted energies compared to the observed relative populations. These conformers showed a mixed bent/twisted arrangement of the five-membered ring similar to that of the related compound thiazolidine with the N–H bond in axial configuration. The most stable form had the COOH group in an equatorial position on the same side of the ring as N-H. The arrangement of the C=O group close to the N-H bond led to a weak interaction between them (classified as type I) characterized by a noncovalent interaction analysis. The second form had a trans-COOH arrangement showing a type II O–H···N hydrogen bond. In thioproline, the stability of conformers of type I and type II was reversed with respect to proline. We show how the conformation of the ring depends on the function associated with the endocyclic N atom when comparing the structures of isolated thioproline with its zwitterion observed in condensed phases and with peptide forms.

The combination of atropisomerism and chirality in flurbiprofen is shown to be relevant concerning its pharmacological activity. The two most stable conformers of a total of eight theoretically predicted for each R-or S-flurbiprofen enantiomers have been isolated in the cooling conditions of a supersonic jet and structurally characterized by laser ablation Fourier transform microwave spectroscopy. The detected conformers, whose structure is mainly defined by three dihedral angles, only differ in the sign of the phenyl torsion angle giving rise to S a and R a atropisomers. A comparison with the structures available for the R-and S-enantiomers complexed to COX isoforms reveals that the enzymes select only the S a atropisomers, resulting in a diastereoisomerspecific recognition. The most stable gas phase conformer is exclusively selected when using the S-enantiomer while the second is recognized only for the R-enantiomer. These experimental results highlight the importance of atropisomerism in drug design.

The rotational spectra of m-anisaldehyde and its microsolvated complexes generated in a supersonic jet have been studied by chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) in the 2-8 GHz region. Four conformers have been detected for the monomer. The three most intense rotamers have line intensities high enough to allow the observation of the monosubstituted 13 C isotopologues in natural abundance allowing the determination of their r e , r s and r m structures. When allowing water vapor to expand in the supersonic jet, the spectra of ten new species assigned to microsolvated complexes appear. Seven monohydrated species have been identified reflecting the two possible interactions of water and the aldehyde group. The two dihydrated species observed are related to the most stable m-anisaldehyde conformer. The most abundant dihydrated complex presents a structure with water dimer closing a cycle with the aldehyde and methoxy groups which confers high stability. In the less abundant dihydrate, water dimer closes a cycle with the aldehyde group, a structure of great interest to better understand the solvation of aldehydes. Additionally, one complex of m-anisaldehyde with four molecules of water has been detected. In this species, the most stable conformer of manisaldehyde captures the tetramer of water adopting a stacked configuration. Work is in progress.

The rotational spectrum of laser-ablated gallic acid has been recorded using CP-FTMW spectroscopy. Two rotamers have been detected, and their rotational spectra have been assigned and analyzed to obtain the molecular spectroscopic parameters. The observed rotamers have been unambiguously identified in the light of theoretical computations, based on the comparison of the experimental line intensities and rotational parameters with the rotational constants and electric dipole moments predicted from theoretical calculations. The values of the planar inertial moments confirm that the observed conformers are planar, and their relative stability and population have been determined from relative intensity measurements. The B3LYP-D3/6-311++G(2d,p) level has been shown to be the best method among a series of levels normally used to predict the rotational parameters in rotational spectroscopy. In the observed conformers, the three adjacent OH groups are arranged in a sequential form, and the only difference between them lies in the orientation of the COOH group. Although weak attractive OH···O interactions seem to exist, the analysis of the electron density topology does not show the existence of any critical point corresponding to these interactions.

In ferrocene (Fe(C 5 H 5 ) 2 ), the first discovered metallocene, iron is sandwiched between two cyclopentadienyl rings in an eclipsed configuration. Ferrocene is an orange solid that sublimates easily and is stable at high temperatures. Due to its symmetry (D 5h ) this compound does not have a dipole moment, so it is not active in the microwave region and consequently, its gas phase structure is not accessible through microwave spectroscopy. However, as it has been shown for triacetone triperoxide [1], its complexation with water makes it possible. In this work, we have done a combined theoretical and experimental work to observe and analyze the microwave spectrum of ferrocene -H 2 O. The theoretical computations predict two possible low-energy structures of the complex. In one form, water lies in the σ h plane of ferrocene. In the other form, water is close to the C 5 axis of ferrocene on top of one of the cyclopentadienyl rings. Both forms have been observed. The most intense spectrum is that of a symmetric top with satellite patterns consistent with the effects of the free rotation of water. The rotational constant B determined for this spectrum is close to that predicted for the second axial form so that we can conclude that water is located along the C 5 axis and freely rotating around it. This motion averages the ferrocene -H 2 O structure to that of a symmetric top. Different isotopic species have been detected, including 54 Fe and 13 C in their natural abundances, which have made it possible to determine the structure of the heavy atom skeleton of ferrocene and the axial location of water. A second weaker rotamer with an asymmetric top spectrum has rotational constants very close to those predicted for the other ferrocene-water conformer. Experimental and theoretical work is still in progress.1.