arXiv subjects

María Mallo

Publications and source records attributed to María Mallo.

4 recordsLinked to original sources

arXiv2026

Thioacetaldehyde (CH3CHS) on interstellar ices: a key molecule to unravel two chemical dichotomies in the ISM

Thioacetaldehyde (CH3CHS), recently detected in TMC-1, has an abundance approximately 36 times lower than its oxygen analog, acetaldehyde (CH3CHO). This makes the CH3CHS/CH3CHO pair the one with the largest column density difference among the detected oxygen/sulfur analogue pairs in this cloud. We investigate the hydrogenation pathways of CH3CHS to address two chemical dichotomies in the ISM: (i) the differenciation between CH3CHS and CH3CHO, and (ii) the apparent absence of both CH3CHS in the G+0.693-0.027 molecular cloud and ethyl mercaptan (CH3CH2SH), in TMC-1. Our results reveal a complex scheme that involves multiple competing reactions, highlighting an efficient sequence of consecutive hydrogenations that can lead to CH3CH2SH. This finding suggests that the high S/O ratio observed for thioacetaldehyde in TMC-1 (~36), and even more pronounced in G+0.693-0.027 ($\geq$112), may result from its conversion via hydrogenation on the ice surface, contrary to the case of CH3CHO, which is more resistant to that chemical process. The straightforward hydrogenation of CH3CHS on ices, which can also take place even in the gas-phase at 150 K, provides a reliable explanation for its non-detection in G+0.693-0.027, where grain-surface chemistry is expected to play an important role, favoring the conversion of CH3CHS into CH3CH2SH, which is indeed detected in G+0.693-0.027. In contrast, TMC-1 represents a more pristine gas-phase environment, where grain-surface chemistry has a lower impact. Under these conditions, CH3CHS can persist, while CH3CH2SH remains undetected. Overall, our results show the entirely different reactivity that contributes to the chemical complexity of two of the largest interstellar sulfur factories.

astro-ph.GA↗

arXiv2026

Laboratory spectroscopy, theoretical characterization, and astronomical search for syn-propenethial (CH2CHCHS)

We report the laboratory characterization of the higher-energy isomer of propenethial, syn-CH2CHCHS. While the lower-energy isomer, anti-CH2CHCHS, was detected in the interstellar medium during the QUIJOTE line survey of TMC-1, we report the non-detection of the syn isomer in the same source, deriving an upper limit to its column density of 1.5 $\times$ 10$^{10}$ cm$^{-2}$. A subsequent theoretical investigation into the origin of this non-detection reveals that the most plausible formation pathway for CH2CHCHS is highly isomer-specific, producing nearly 95\% of the anti isomer. This predicted branching ratio allows us to infer an even lower effective upper limit for the abundance of the syn isomer. In addition, calculations of tunneling-mediated unimolecular isomerization in the gas phase show that syn-CH2CHCHS converts to the lower-energy anti isomer on timescales of the order of 10$^{3}$ years. Overall, we conclude that the detection of syn-CH2CHCHS is highly challenging in both cold and warm interstellar environments. These results underscore the importance of electronic-structure and kinetic effects in determining isomer abundances across diverse interstellar environments.

astro-ph.GA↗

arXiv2025

Ion-molecule routes towards cycles in TMC-1. An automated study of the C2H4 + CH2CCH+ reaction

Cyclopentadiene (c-C5H6) is considered a key molecule in the formation of polycyclic aromatic hydrocarbons (PAHs) in the interstellar medium (ISM). The synthesis of PAHs from simpler precursors is known as the "bottom-up" theory, which, so far, has been dominated by reactions between organic radicals. However, this mechanism struggles to account for the origin of the smallest cycles themselves. Ion-molecule reactions emerge as promising alternative pathways to explain the formation of these molecules. In the present work, we investigate the reaction network of the main ionic precursor of cyclopentadiene c-C5H7+ . To this end, we establish an integrated protocol that combines astrochemical modelling to identify viable formation routes under cold interstellar medium conditions, automated reaction path search and kinetic simulations to obtain accurate descriptions of the reaction pathways and reliable rate constants. In particular, we examine the reaction between ethylene (C2H4) and the linear propargyl cation (CH2CCH+). Our results reveal that the formation of c-C5H7+ by radiative association turns out to be inefficient, contrary to our initial expectations. Instead, the system predominantly evolves through bimolecular channels yielding c-C5H5+ and CH3CCH2+ with the formation of c-C5H5+ offering new insights into reactivity that supports molecular growth in the ISM.

astro-ph.GA↗

arXiv2025

Delayed photoisomerisation of the trans-PSB3 retinal toy model using on-the-fly quantum dynamics

We explore the trans-cis photoisomerisation process in a representative retinal protonated Schiff base known as trans-PSB3, employing the quantum dynamics method direct dynamics variational multiconfigurational gaussian -- DD-vMCG -- in full dimensionality, i.e., 36 degrees of freedom on potential energy surfaces computed on-the-fly using the SA(2)-CAS(6,6)SCF electronic structure method with the 6-31G basis set. Although the toy molecule has been extensively studied using trajectory methods such as Tully Surface Hopping and Ab Initio Multiple Spawning, the on-the-fly quantum dynamics method DD-vMCG shows a trans-cis isomerisation hundreds of femtoseconds slower using the same electronic structure method, which can be explained in terms of the accesibility to the conical intersection connecting the ground and the excited state.

physics.chem-ph↗