Miguel Carvajal

The atmospheric degradation of volatile organic compounds (VOCs) begins through oxidation by hydroxyl or nitrate radicals, producing highly reactive dehydrogenated radicals. While the stability—and thus, the reactivity—of these radicals critically depends on their isomeric form, directly monitoring individual isomers under laboratory conditions remains a major analytical challenge. Here, we combine high-level ab initio calculations to unambiguously identify radicals via their structures, energies, and internal dynamics with a very sensitive high-resolution millimeter wave spectroscopy (150–500 GHz) setup to resolve and characterize the two isomeric forms of dehydrogenated methyl formate: the formyloxymethyl radical (CH2OCHO) and the methoxy carbonyl radical (CH3OCO). Our results demonstrate that millimeter wave spectroscopy enables the direct, isomer-specific detection of medium-sized dehydrogenated VOCs—a critical advancement for oxidation experiments and environmental monitoring. This integrated approach paves the way for real-time tracking of radical intermediates under diverse atmospheric conditions, with broad implications for understanding VOC degradation pathways.

Structural and spectroscopic properties of the CH3COO and CH2OH radicals and various monosubstituted isotopologues containing 13C, 18O, and deuterium in their electronic ground state, are determined by using UCCSD(T)-F12 and RCCSD(T)-F12 calculations. Rotational, centrifugal distortion and spin-rotation constants, frequencies and intensities, are computed. CASPT2 vertical excitations establish the first excited electronic state of CH3COO in the region of the C-H overtones, whereas the region of the fundamental transitions is free from rovibronic interactions. The most stable Cs geometry of CH3COO shows a very low methyl torsional barrier (V6=6.8 cm-1). CH2OH shows a C1 structure corresponding to a double minimum separated by a barrier of 116 cm-1 lying below the ground vibrational state. Isotopic displacements of the varieties containing 18O and deuterium, are significant. In CH2OH, the OH torsional barrier of 1602 cm-1 splits the rovibrational levels. The splitting of the ground vibrational state was determined to be 0.005 cm-1.

This work presents a comprehensive theoretical investigation of key isomers of C2H4N2 using state-of-the-art quantum chemical methods. The objective is to characterize their molecular structures, spectroscopic constants, and electronic energies and to elucidate plausible formation and destruction pathways, providing data critical for astrochemical and atmospheric detection. High-accuracy ab initio methods were employed, notably CCSD(T)-F12/cc-pVTZ-F12 for optimized geometries. Additional calculations were performed at the CCSD(T)/aug-cc-pVTZ, CCSD(T)/cc-pVTZ, MP2/aug-cc-pVTZ, and CIS levels. Intrinsic reaction coordinate calculations were performed at the B3LYP/6-31G(d,p) level to explore reaction pathways. The Zero-Point Energy (ZPE)-corrections were determined for all the isomers considered. Six low-energy C2H4N2 isomers were identified, all within 1 eV of the global minimum. Among them, methylcyanamide (MCA) exhibits the lowest relative energy (∼0.2 eV) and a significant electric dipole moment of 5.00 D, making it a strong candidate for detection in gas-phase environments. The rotational constants for MCA, computed at the level of CCSD(T)-F12/cc-pVTZ-F12, are Ae = 34 932.44 MHz, Be = 4995.31 MHz, and Ce = 4520.30 MHz. The V3 torsional barrier was found to be 631.19 cm−1. Centrifugal distortion constants were computed up to sextic order for all isomers. Formation pathways for MCA—such as CH3N + HCN → CH3NHCN—and related isomers were characterized. The combination of large dipole moments and distinct rotational signatures supports the detectability of MCA and related C2H4N2 isomers via radioastronomy, IR, and MW spectroscopy. Isomerization and reaction pathways involving radical-neutral and neutral-neutral processes were found to be key to their formation in gas-phase environments. These results offer a robust foundation for future observational and modeling efforts.

Signatures of excited-state quantum phase transitions in the bending degree of freedom of triatomic systems that undergo an isomerization reaction have been recently evinced. In this work, we study the carbonyl sulfide bending motion using an effective Hamiltonian within the two-dimensional limit of the vibron model framework, which has been shown to accurately describe critical phenomena in molecular bending spectra within experimental precision. To estimate the transition state energy barrier, we propose an improvement to a phenomenological formula proposed by Baraban et al. [Science 350, 1338–1342 (2015)], introducing a new term to capture the anharmonicity change that characterizes quasilinear molecules.

Signatures of excited-state quantum phase transitions in the bending degree of freedom of triatomic systems that undergo an isomerization reaction have been recently evinced. In this work, we study the carbonyl sulfide bending motion using an effective Hamiltonian within the two-dimensional limit of the vibron model framework, which has been shown to accurately describe critical phenomena in molecular bending spectra within experimental precision. To estimate the transition state energy barrier, we propose an improvement to a phenomenological formula proposed by Baraban et al.[1] , introducing a new term to capture the anharmonicity change that characterizes quasilinear molecules

Context. Physical and chemical properties, such as kinetic temperature, volume density, and molecular composition of interstellar clouds are inherent in their line spectra at submillimeter wavelengths. Therefore, the spectral line profiles could be used to estimate the physical conditions of a given source. Aims. We present a new bottom-up approach, based on machine learning (ML) algorithms, to extract the physical conditions in a straightforward way from the line profiles without using radiative transfer equations. Methods. We simulated, for the typical physical conditions of dense molecular clouds and star-forming regions, the emission in spectral lines of the two isomers HCN and HNC, from J = 1–0 to J = 5–4 between 30 and 500 GHz, which are commonly observed in dense molecular clouds and star forming regions. The generated data cloud distribution has been parametrised using the line intensities and widths to enable a new way to analyse the spectral line profiles and to infer the physical conditions of the region. The line profile parameters have been charted to the HNC/HCN ratio and the excitation temperature of the molecule(s). Three ML algorithms have been trained, tested, and compared aiming to unravel the excitation conditions of HCN and HNC and their abundance ratio. Results. Machine learning results obtained with two spectral lines, one for each isomer HCN and HNC, have been compared with the local thermodynamic equilibrium (LTE) analysis for the cold source R CrA IRS 7B. The estimate of the excitation temperature and of the abundance ratio, in this case considering the two spectral lines, is in agreement with our LTE analysis. The complete optimisation procedure of the algorithms (training, testing, and prediction of the target quantities) have the potential to predict interstellar cloud properties from line profile inputs at lower computational cost than before. Conclusions. It is the first time that the spectral line profiles are mapped according to the physical conditions charting the ratio of two isomers and the excitation temperature of the molecules. In addition, a bottom-up approach starting from a set of simulated spectral data at different physical conditions is proposed to interpret line observations of interstellar regions and to estimate their physical conditions. This new approach presents the potential relevance to unravel hidden interstellar conditions with the use of ML methods.

Context. Phosphorus plays an essential role in the chemistry of living organisms, being present in several fundamental biomolecules. The investigation of chemical reactions taking place in different astronomical environments involving phosphorus-containing molecules is essential for understanding how these species are produced and destroyed. Ultimately, it can help unravel the pathways that lead to important prebiotic molecules. Aims. Phosphorus monoxide (PO) and phosphorus nitride (PN) are key reservoirs of phosphorus in the Interstellar Medium (ISM). Understanding their reaction mechanisms and accurately predicting rate coefficients are crucial for modelling phosphorus chemistry in space. This work presents a computational study of the CPN system to identify viable reaction pathways involving atom-diatom collisions and to explore a potential destruction route for PN in the ISM. We also evaluate the role of several neutral-neutral reactions involving PO and PN in chemical models simulating interstellar environments. Methods. In this work we explore the potential energy landscape of the C(³P) + PN(¹Σ⁺), N(⁴S) + CP(²Σ⁺) and P(⁴S) + CN(²Σ⁺) reactions by performing high-accuracy ab initio calculations and provide their rate coefficients over a wide range of temperatures. The temperature-dependent rate coefficients were fitted to the modified Arrhenius equation: k(T) = α(T/300)^βexp(−γ/T). An updated chemical network for P-bearing species was used to model the time-dependent abundances and reaction contributions of P, PO, PN, and PH (phosphinidene) during the chemical evolution of diffuse and translucent clouds and dense clouds. Results. The only neutral-neutral reaction capable of destroying PN without an activation energy seems to be the PN + C one. We have also shown that reactions between CP and N can yield CN and PN barrierlessly. Chemical models indicate that PO is a crucial species driving the gas-phase formation of PN. Typically, PO/PN ratios exceed 1, though their chemistry is influenced by photon- and cosmic-ray-induced processes. Over time in simulated dense clouds, neutral-neutral reactions such as PO + N, PH + N, P + OH, and PH+O play a significant role in determining the relative abundances of PO and PN.

In this paper, we present a new multifrequency study of the giant star-forming complex RCW 122. We used molecular data obtained with the ASTE 10 m and the APEX 12 m telescopes, along with infrared observations spanning from 3.6 µm to 870 µm, obtained from available databases. We also incorporated a range of public datasets, including the radio continuum at 3 GHz, narrowband Ha images, and deep JHK photometry. Our analysis focuses mostly on cataloged ATLASGAL sources, showcasing a spectrum of evolutionary stages from infrared dark cloud (IRDC)/high-mass protostellar object (HMPO) to ultra-compact HII region (UCHII), as inferred from preliminary inspections of the public dataset. Based on ASTE HCO⁺(4−3) and CO(3−2) data, we identified five molecular clumps, designated A, B, C, D, and E, as molecular counterparts of the ATLASGAL sources. These clumps have radial velocities ranging from ~−15 km s⁻¹ to −10 km s⁻¹, confirming their association with RCW 122. In addition, we report the detection of 20 transitions from 11 distinct molecules in the APEX spectra in the frequency ranges from 258.38 GHz to 262.38 GHz, 228.538 GHz to 232.538 GHz, and 218.3 GHz to 222.3 GHz, unveiling a diverse chemical complexity among the clumps. Utilizing CO(2−1) and C¹⁸O(2−1) data taken from the observations with the APEX telescope, we estimated the total LTE molecular mass, ranging from 200 M_⊙ (clump A) to 4400 M_⊙ (clump B). Our mid- to far-infrared (MIR-FIR) flux density analysis yielded minimum dust temperatures of 23.7 K (clump A) to maximum temperatures of 33.9 K (clump B), indicating varying degrees of internal heating among the clumps. The bolometric luminosities span 1.7×10³ L_⊙ (clump A) to 2.4×10⁵ L_⊙ (clump B), while the total (dust+gas) mass ranges from 350 M_⊙ (clump A) to 3800 M_⊙ (clump B). Our analysis of the molecular line richness, L/M ratios, and CH₃CCH and dust temperatures reveals an evolutionary sequence of A/E→C→D/B, consistent with preliminary inferences of the ATLASGAL sources. In this context, clumps A and E exhibit early stages of collapse, with clump A likely in an early HMPO phase, which is supported by identifying a candidate molecular outflow. Clump E appears to be in an intermediate stage between IRDC and HMPO. Clumps D and B show evidence of being in the UCHII phase, with clump B likely more advanced. Clump C likely represents an intermediate stage between HMPO and HMC. Our findings suggest clump B is undergoing ionization and heating by multiple stellar and protostellar members of the stellar cluster DBS 119. Meanwhile, other cluster members may be responsible for ionizing other regions of RCW 122 that have evolved into fully developed HII regions, beyond the molecular dissociation stage.