Benzoyl peroxide (BPO) represents a structurally simple yet hazardous organic peroxide with widespread applications across industrial and pharmaceutical domains. Despite its extensive use, detailed molecular-level understanding of its thermal instability remains limited. Here, we present the first rotational spectroscopy characterization of isolated BPO in the gas phase, enabled by laser ablation and supersonic jet expansion techniques. Our analysis reveals a C2-symmetric structure with quasi-perpendicular aromatic rings, in excellent agreement with crystallographic data. Quantum chemical calculations and topological analysis identify a stabilizing reciprocal n→π* interaction between adjacent carbonyl groups that may contribute to BPO's thermal resilience compared to other organic peroxides. Furthermore, we detect several photofragmentation products, including benzoic acid, benzyne, benzaldehyde, and benzophenone, providing insights into potential decomposition pathways. This molecular-level investigation bridges the gap between macroscopic hazard assessments and fundamental understanding of peroxide reactivity, with implications for safer handling and rational design of peroxide-based systems.

The conformational landscape of 3-indoleacetamide, a key intermediate in plant hormone biosynthesis, has been comprehensively investigated using state-of-the-art laser-ablation chirped-pulse Fourier transform microwave (LA-CP-FTMW) and laser-ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy. Remarkably, 3-indoleacetamide exhibits unprecedented conformational rigidity within the tryptophan-derived molecule family, displaying only a single stable conformer characterized by distinctive a-, b-, and c-type rotational transitions. This singular conformational behavior contrasts dramatically with the structural flexibility observed in closely related tryptophan derivatives such as tryptophan, serotonin, tryptamine, and 3-indoleacetic acid. The unique structural constraint imposed by the acetamide functional group provides unprecedented insights into the molecular determinants governing the distinct biological roles of tryptophan-derived compounds. This work establishes a potential correlation between conformational flexibility and biological function, from neurotransmission to plant hormone regulation, offering new perspectives on structure-activity relationships in bioactive natural products.

We report the first detection in the interstellar medium (ISM) of a C 2 H 5 O 2 N isomer: syn -glycolamide (NH 2 C(O)CH 2 OH). The exquisite sensitivity at sub-mK levels of an ultradeep spectral survey carried out with the Yebes 40 m and IRAM 30 m telescopes toward the G+0.693–0.027 molecular cloud has allowed us to unambiguously identify multiple transitions of this species. We derived a column density of (7.4 ± 0.7) × 10 12 cm −2 , which implies a molecular abundance with respect to H 2 of 5.5 × 10 −11 . The other C 2 H 5 O 2 N isomers, including the higher-energy anti conformer of glycolamide and two conformers of glycine, were not detected. The upper limit derived for the abundance of glycine indicates that this amino acid is surely less abundant than its isomer glycolamide in the ISM. The abundances of the C 2 H 5 O 2 N isomers cannot be explained in terms of thermodynamic equilibrium; thus, chemical kinetics need to be invoked. While the low abundance of glycine might not be surprising, based on the relative low abundances of acids in the ISM compared to other compounds (e.g., alcohols, aldehydes, or amines), several chemical pathways can favor the formation of its isomer glycolamide. It can be formed through radical–radical reactions on the surface of dust grains. The abundances of these radicals can be significantly boosted in an environment affected by a strong ultraviolet field induced by cosmic rays, such as that expected in G+0.693–0.027. Therefore, as shown by several recent molecular detections toward this molecular cloud, it stands out as the best target to discover new species with carbon, oxygen, and nitrogen with increasing chemical complexity.

We report the first detection in the interstellar medium of a C_2H_5O_2N isomer: syn-glycolamide (NH_2C(O)CH_2OH). The exquisite sensitivity at sub-mK levels of an ultra-deep spectral survey carried out with the Yebes 40m and IRAM 30m telescopes towards the G+0.693-0.027 molecular cloud have allowed us to unambiguously identify multiple transitions of this species. We derived a column density of (7.4 ± 0.7)×10^12 cm^-2, which implies a molecular abundance with respect to H_2 of 5.5×10^-11. The other C_2H_5O_2N isomers, including the higher-energy anti conformer of glycolamide, and two conformers of glycine, were not detected. The upper limit derived for the abundance of glycine indicates that this amino acid is surely less abundant than its isomer glycolamide in the ISM. The abundances of the C_2H_5O_2N isomers cannot be explained in terms of thermodynamic equilibrium, and thus chemical kinetics need to be invoked. While the low abundance of glycine might not be surprising, based on the relative low abundances of acids in the ISM compared to other compounds (e.g. alcohols, aldehydes or amines), several chemical pathways can favour the formation of its isomer glycolamide. It can be formed through radical-radical reactions on the surface of dust grains. The abundances of these radicals can be significantly boosted in an environment affected by a strong ultraviolet field induced by cosmic rays, such as that expected in G+0.693-0.027. Therefore, as shown by several recent molecular detections towards this molecular cloud, it stands out as the best target to discover new species with carbon, oxygen and nitrogen with increasing chemical complexity.

A combined experimental and theoretical study has been carried out on 4-fluoro-threonine, the only naturally occurring fluorinated amino acid. Fluorination of the methyl group significantly increases the conformational complexity with respect to the parent amino acid threonine. The conformational landscape has been characterized in great detail, with special attention given to the inter-conversion pathways between different conformers. This led to the identification of 13 stable low-energy minima. The equilibrium population of so many conformers produces a very complicated and congested rotational spectrum that could be assigned through a strategy that combines several levels of quantum chemical calculations with the principles of machine learning. Twelve conformers out of 13 could be experimentally characterized. The results obtained from the analysis of the intra-molecular interactions can be exploited to accurately model fluorine-substitution effects in biomolecules.

In recent times, large organic molecules of exceptional complexity have been found in diverse regions of the interstellar medium. In this context, we aim to provide accurate frequencies of the ground vibrational state of two key aliphatic aldehydes, n-butanal and its branched-chain isomer, i-butanal, to enable their eventual detection in the interstellar medium. We employ a frequency modulation millimeter-wave absorption spectrometer to measure the rotational features of n- and i-butanal. We use the spectral line survey ReMoCA performed with the Atacama Large Millimeter/submillimeter Array to search for n- and i-butanal toward the star-forming region Sgr B2(N). We also search for both aldehydes toward the molecular cloud G+0.693-0.027 with IRAM 30 m and Yebes 40 m observations. Several thousand rotational transitions belonging to the lowest-energy conformers have been assigned in the laboratory spectra up to 325 GHz. A precise set of the relevant rotational spectroscopic constants has been determined for each structure. We report non-detections of n- and i-butanal toward both sources, Sgr B2(N1S) and G+0.693-0.027. We find that n- and i-butanal are at least 2-6 and 6-18 times less abundant than acetaldehyde toward Sgr B2(N1S), respectively, and that n-butanal is at least 63 times less abundant than acetaldehyde toward G+0.693-0.027. Comparison with astrochemical models indicates good agreement between observed and simulated abundances (where available). Grain-surface chemistry appears sufficient to reproduce aldehyde ratios in G+0.693-0.027; gas-phase production may play a more active role in Sgr B2(N1S). Our astronomical results indicate that the family of interstellar aldehydes in the Galactic center region is characterized by a drop of one order of magnitude in abundance at each incrementation in the level of molecular complexity.

We report a state-of-the-art spectroscopic study of an archetypical barbaralone, conclusively revealing the valence tautomerism phenomena for this bistable molecular system. The two distinct 1- and 5-substituted valence tautomers have been isolated in a supersonic expansion for the first time and successfully characterized by high-resolution rotational spectroscopy. This work provides irrefutable experimental evidence of the [3,3]-rearrangement in barbaralones and highlights the use of rotational spectroscopy to analyze shape-shifting mixtures. Moreover, this observation opens the window toward the characterization of new fluxional systems in the isolation conditions of the gas phase and should serve as a reference point in the general understanding of valence tautomerism.

We used high-resolution rotational spectroscopy coupled to a laser ablation source to study the conformational panorama of perillartine, a solid synthetic sweetener. Four conformers were identified under the isolation conditions of the supersonic expansion, showing that all of them present an E configuration of the C=N group with respect to the double bond of the ring. The observed structures were verified against Shallenberger–Acree–Kier’s sweetness theory to shed light on the structure–sweetness relationship for this particular oxime, highlighting a deluge of possibilities to bind the receptor.

Herein, we report the first rotational study of neutral L-DOPA, an extensively used supramolecular synthon and an amino acid precursor of the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline) using broadband and narrowband Fourier transform microwave spectroscopies coupled with a laser ablation vaporization system. The spectroscopic parameters derived from the analysis of the rotational spectrum conclusively identify the existence of four distinct conformers of L-DOPA in the supersonic jet, further rejecting the previously reported catechol ring-induced conformational restriction. The analysis of the 14N nuclear quadrupole coupling hyperfine structure further revealed the orientation of the N-bearing functional group, proving the existence of stabilizing N-H⋯π interactions for the observed structures.

We report a detailed structural study of cytisine, an alkaloid used to help with smoking cessation, looking forward to unveiling its role as a nicotinic agonist. High-resolution rotational spectroscopy has allowed us to characterize two different conformers exhibiting axial and equatorial arrangements of the piperidinic NH group. Unexpectedly, the axial form has been found as the predominant configuration, in contrast to that observed for related molecules, such as piperidine. This anomalous behavior has been justified in terms of an intramolecular NH···N hydrogen bond. Moreover, this interaction justifies the overstabilization of the axial conformer over the equatorial one and is crucial for the mechanism of action of cytisine over the nicotinic receptor, further rationalizing its behavior as a nicotinic agonist.

Sulfanilamide, a widely used antibacterial drug, has been brought into the gas phase using laser ablation techniques, and its structure has been characterized in the isolated conditions of a supersonic expansion using Fourier transform microwave techniques. A single conformer stabilized by an N-H⋯OS intramolecular interaction in an equatorial disposition has been unequivocally characterized. To emulate the microsolvation process, we studied its hydrated cluster. The results show that a single water molecule alters the conformational preference and forces sulfanilamide to switch from its initial eclipsed configuration to a staggered disposition. The observed hydrated cluster adopts a structure in which water forms three hydrogen bonds with sulfanilamide stabilizing the molecule.

In the present work, we report the first rotational study of N-acetylgalactosamine, a cancer-associated sugar derivative, by means of high-resolution rotational spectroscopy. Two different conformers have been conclusively characterized using broadband Fourier transform microwave spectroscopy coupled with a laser ablation vaporization system. Additionally, we performed a comprehensive analysis of the intramolecular interactions that govern these structures, which allowed us to both characterize the existence of intramolecular hydrogen bond networks that drive the intrinsic conformation panorama of N-acetylgalactosamine and further rationalize the biological role of this aminosugar derivative as part of the Tn antigen.

This work reports a complete conformational analysis of caffeic acid, an exceptionally versatile pharmacophore, using laser ablation chirped-pulse Fourier transform microwave spectroscopy. The whole conformational space consisting of eight distinct species has been fully deciphered based on the trend of the rotational constants supported by theoretical computations. We show how rotational spectroscopy can be confidently used to distinguish between conformers even when the structural differences are minimal, such as those involved in the conformational panorama of caffeic acid. Additionally, the structural information here provided, such as the planarity observed in all the conformers, could help to elucidate the mechanisms underlying the biological and pharmacological activity of hydroxycinnamic acids.

An integrated experimental-computational strategy for the accurate characterization of the conformational landscape of flexible biomolecule building blocks is proposed. This is based on the combination of rotational spectroscopy with quantum-chemical computations guided by artificial intelligence tools. The first step of the strategy is the conformer search and relative stability evaluation performed by means of an evolutionary algorithm. In this step, last generation semiempirical methods are exploited together with hybrid and double-hybrid density functionals. Next, the barriers ruling the interconversion between the low-lying conformers are evaluated in order to unravel the possible fast relaxation paths. The relative stabilities and spectroscopic parameters of the "surviving" conformers are then refined using state-of-the-art composite schemes. The reliability of the computational procedure is further improved by the inclusion of vibrational and thermal effects. The final step of the strategy is the comparison between experiment and theory without any ad hoc adjustment, which allows an unbiased assignment of the spectroscopic features in terms of different conformers and their spectroscopic parameters. The proposed approach has been tested and validated for homocysteine, a highly flexible non-proteinogenic α-amino acid. The synergism of the integrated strategy allowed for the characterization of five conformers stabilized by bifurcated N-H2⋯O=C hydrogen bonds, together with an additional conformer involving a more conventional HN⋯H-O hydrogen bond. The stability order estimated from the experimental intensities as well as the number and type of conformers observed in the gas phase are in full agreement with the theoretical predictions. Analogously, a good match has been found for the spectroscopic parameters.

Aims. In this work, we aim to achieve the first laboratory detection of acetohydroxamic acid (CH3CONHOH), a relevant glycine isomer, to enable its eventual identification in the ISM. Methods. We employed a battery of state-of-the-art rotational spectroscopic techniques in the time domain to measure the microwave spectrum of acetohydroxamic acid. We then used the spectral GOTHAM survey performed with the Green Bank Telescope (GBT) to search for the lowest-energy Z-conformer toward the cold and quiescent molecular cloud TMC-1. We also employed a sensitive spectral survey of the chemically rich Galactic Center molecular cloud G+0.693-0.027, based on IRAM 30 m and Yebes 40 m observations. Results. We report direct experimental frequencies of the ground state of acetohydroxamic acid (up to 40 GHz). The 14N nuclear quadrupole hyperfine structure and the A-E splittings due to the internal rotation were observed and analyzed. Hence, a precise set of the rotational spectroscopic parameters were determined for the two distinct conformers, Z- and E-acetohydroxamic acid, which is the initial and prerequisite step of their radio astronomical search in the ISM using low-frequency surveys. We report the nondetection of acetohydroxamic acid toward both astronomical sources. We derive an upper limit to the column density of this molecule very similar to that obtained for glycine. Its corresponding molecular abundance with respect to molecular hydrogen is found to be ≤1 × 10−9 and 2 × 10−10 in TMC-1 and G+0.693-0.027, respectively, which further constrain the abundance of this glycine isomer in the ISM.

The conformational landscape of β-D-allose, a rare sugar, was investigated using laser ablation in combination with high-resolution rotational spectroscopy. Altogether, three species are identified, exhibiting a counter-clockwise intramolecular hydrogen bond network. The effect of epimerization on the main aldohexose is also studied and, despite the main conformers being very similar, the position of the hydroxyl groups in allose allows the formation of considerably stronger intramolecular hydrogen bonds than in glucose, and this could explain the low abundance of β-D-allose in Nature.