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One of the most fascinating questions in chemistry is why nature chose CGAT as the alphabet of life. Very likely, such selection was the result of multiple factors and a long period of refinement. Here, we explore how the intermolecular interactions influenced such process, by characterizing the formation of dimers between adenine, theobromine and 4-aminopyrimidine. Using a combination of mass-resolved excitation spectroscopy and DFT calculations, we determined the structure of adenine-theobromine and 4-aminopyrimidine-theobromine dimers. The binding energy of these dimers is very close to the canonical adenine-thymine nucleobases. Likewise, the dimers are able to adopt Watson-Crick conformations. These findings seem to indicate that there were many options available to build the first versions of the informational polymers, which also had to compete with other molecules, such as 4-aminopyrimidine, which does not have a valid attaching point for a saccharide. For some reason, nature did not select the most strongly-bonded partners or if it did, such proto-bases were later replaced by the nowadays canonical CGAT., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The role of non-covalent interactions (NCIs) has broadened with the inclusion of new types of interactions and a plethora of weak donor/acceptor partners. This work illustrates the potential of chirped-pulse Fourier transform microwave technique, which has revolutionized the field of rotational spectroscopy. In particular, it has been exploited to reveal the role of NCIs' in the molecular self-aggregation of difluoromethane where a pentamer, two hexamers and a heptamer were detected. The development of a new automated assignment program and a sophisticated computational screening protocol was essential for identifying the homoclusters in conditions of spectral congestion. The major role of dispersion forces leads to less directional interactions and more distorted structures than those found in polar clusters, although a detailed analysis demonstrates that the dominant interaction energy is the pairwise interaction. The tetramer cluster is identified as a structural unit in larger clusters, representing the maximum expression of bond between dimers., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Intermolecular interactions are difficult to model, especially in systems formed by multiple interactions. Such is the case of caffeine-phenol. Structural data has been extracted by using mass-resolved excitation spectroscopy and double resonance techniques. Then the predictions of seven different computational methods have been explored to discover structural and energetic discrepancies between them that may even result in different assignments of the system. The results presented herein highlight the difficulty of constructing functionals to model systems with several competing interactions, and raise awareness of problems with assignments of complex systems with limited experimental information that rely exclusively on energetic data., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Bond length alternation is a chemical phenomenon in benzene rings fused to other rings, which has been mainly predicted theoretically. Its physical origin is still not clear and has generated discussion. Here, by using a strategy that combines microwave spectroscopy, custom-made synthesis and high-level ab initio calculations, we demonstrate that this phenomenon is clearly observed in the prototype indazole molecule isolated in the gas phase. The 1H-indazole conformer was detected by rotational spectroscopy, and its 17 isotopologues resulting from single and double heavy atom substitution ( 13 C and 15 N) were also unambiguously observed. Several experimental structures were determined and, in particular, the most useful semi-experimental equilibrium structure (r e SE ), allowed determination of the heavy atom bond lengths to milli-Ångstrom precision. The experimentally determined bond length alternation is estimated to correspond to 60:40 contributions from the two resonant forms of 1H-indazole., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Rotational spectra of several difluoromethane-water adducts have been observed using two broadband chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometers. The experimental structures of (CH 2 F 2 )⋅⋅⋅(H 2 O) 2 , (CH 2 F 2 ) 2 ⋅⋅⋅(H 2 O), (CH 2 F 2 )⋅⋅⋅(H 2 O) 3 , and (CH 2 F 2 ) 2 ⋅⋅⋅(H 2 O) 2 were unambiguously identified with the aid of 18 isotopic substituted species. A subtle competition between hydrogen, halogen, and carbon bonds is observed and a detailed analysis was performed on the complex network of non-covalent interactions which stabilize each cluster. The study shows that the combination of stabilizing contact networks is able to reinforce the interaction strength through a cooperative effect, which can lead to large stable oligomers., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The effects of halogenation on the tautomeric and conformational equilibria of the model system 2-hydroxypyridine/2-pyridone have been investigated through chlorine substitution at positions 3, 4, 5, and 6. In the gas phase, the lactim syn-periplanar tautomer (OH s ) was the predominant species for all compounds over the lactam form (C=O) and the less abundant anti-periplanar lactim (OH a ). However, the population of the three species was shown to be dependent on the position of the chlorine substitution. Chlorination in position 5 or 6 strongly stabilizes the OH s tautomer, whereas the C=O form has a significant population when the ring is chlorinated in positions 3 or 4. Overall, the OH a form is the least favourable form, although the 3-substitution favours the population of this tautomer. In addition, the C=O tautomer is strongly stabilized in the solvent, which makes it the dominant form in some substituted species. This study has been performed by means of rotational spectroscopy in the gas phase and/or theoretical calculations in the isolated phase and in solution. Both the OH s and C=O forms of 5-chloro-2-hydroxypyridine and the OH s form of 6-chloro-2-hydroxypyridine were experimentally observed. All transitions displayed a complex nuclear hyperfine structure owing to the presence of the chlorine and nitrogen nuclei. For all species, a full quadrupolar hyperfine analysis has been performed. This has provided crucial information for the unambiguous identification of tautomers., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)