Sweet truth: The search for sugars in interstellar space is hampered by a lack of spectroscopic information. D‐Ribose is now the first C5 sugar observed in the gas phase using microwave spectroscopy. The rotational spectrum revealed six conformations of free ribose, adopting preferentially β‐pyranose rings and higher‐energy α‐pyranose forms. No evidence of α‐/β‐furanoses or linear forms was found, unlike biological systems, where β‐furanoses are found in RNA.

Fructose has been examined under isolation conditions using a combination of UV ultrafast laser vaporization and Fourier-transform microwave (FT-MW) spectroscopy. The rotational spectra for the parent, all (six) monosubstituted (13)C species, and two single D species reveal unambiguously that the free hexoketose is conformationally locked in a single dominant β-pyranose structure. This six-membered-chair skeleton adopts a (2)C(5) configuration (equivalent to (1)C(4) in aldoses). The free-molecule structure sharply contrasts with the furanose form observed in biochemically relevant polysaccharides, like sucrose. The structure of free fructose has been determined experimentally using substitution and effective structures. The enhanced stability of the observed conformation is primarily attributed to a cooperative network of five intramolecular O-H···O hydrogen bonds and stabilization of both endo and exo anomeric effects. Breaking a single intramolecular hydrogen bond destabilizes the free molecule by more than 10 kJ mol(-1). The structural results are compared to ribose, recently examined with rotational resolution, where six different conformations coexist with similar conformational energies. In addition, several DFT and ab initio methods and basis sets are benchmarked with the experimental data.

The conformational equilibria of vanillin and ethylvanillin have been investigated in a supersonic jet expansion using rotational spectroscopy. Two conformers have been detected for each molecule, with a dominant O-H···O intramolecular hydrogen bond locking the local conformation of the hydroxyl and methoxy/ethoxy groups. As a consequence, the observed conformers of vanillin differ only in the orientation of the aldehyde group, either cis or trans with respect to the methoxy group. For ethylvanillin the ethoxy group would plausibly generate additional trans (in-plane) or gauche (out-of-plane) orientations. However, the two detected conformations exhibit only planar ethoxy trans arrangements, with the gauche forms most probably depopulated by collisional relaxation in the jet. Torsional tunneling effects due to internal rotation of the terminal methyl groups were not detectable, indicating internal rotation barriers above 12.3 kJ mol(-1). The conformational population ratios in the jet have been estimated from relative intensity measurements. Ab initio (MP2) and DFT calculations using B3LYP and the recent M05-2X empirical functional supplemented the experimental work, describing the rotational parameters, conformational landscape and the aldehyde and methyl internal rotation barriers in these molecules.

Tunneling effects have been measured in the pulsed jet Fourier transform microwave spectra of two isotopologues of the benzoic acid-formic acid bimolecule. The tunneling splittings are originated by the concerted proton transfer of the two carboxylic hydrogens. From the values of these splittings for the OH-OH and OD-OD species, it has been possible to model/size the barrier to the concerted double proton transfer.

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