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1. Trapped Fragments Reshape Ultrafast Photoinduced Surface Reaction Dynamics and Pathways on TiO2(110)

   https://doi.org/10.26434/chemrxiv.10001684/v1  

   Gupta, Aakash; Wang, Tian; Blackman, Keith; Smith, Corine M; Li, Xiaosong; Vaida, Mihai E (January 2026, ChemRxiv)

   Understanding how intermediate species form, evolve, and interact with semiconductor surfaces is essential for controlling heterogeneous photoinduced reactions. Here, we employ a combined time-of-flight mass spectrometry and femtosecond laser spectroscopy approach, coupled with density functional theory, to unravel the photoinduced reaction mechanism of CH3I, used as a model system, on TiO₂(110). Direct detection of intermediates and final products reveals that on a freshly prepared surface, a fraction of the photogenerated fragments becomes trapped, passivating reactive sites. On this fragment-decorated surface, CH₃radicals react with undissociated CH₃I to form surface-bound CH₃ICH₃species, an intermediate stabilized exclusively by the modified TiO₂(110) interface. The presence of CH₃ICH₃profoundly alters the ultrafast surface dynamics, giving rise to coherent oscillations of the CH₃⁺transient signal with a ~100 fs periodicity, attributed to the dissociation of CH₃ICH₃through an intermediate vibrational exited bound electronic state. The concurrent reformation dynamics of the CH₃I⁺signal indicates two parallel reaction channels: recombination of CH₃and I fragments, and dissociation of CH₃ICH₃. Density functional theory calculations support these observations by describing the formation and vibrational modes of CH₃ICH₃at the TiO₂(110) surface. This study demonstrates that fragment-induced surface modification can dynamically reshape potential energy landscapes, opening new reaction pathways and altering ultrafast surface reactivity, a principle broadly relevant to photocatalysis, surface photochemistry, and semiconductor interfaces. 

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2. Direct detection of reactive intermediates in photocatalytic water splitting with small carbon species on TiO2(100)

   https://doi.org/10.26434/chemrxiv.15005763/v1  

   Gupta, Aakash; Sherazi, Syeda Faiza; Le, Duy; Sweatman, Christopher; Putz, Mihai V; Rahman, Talat S; Vaida, Mihai E (July 2026, ChemRxiv)

   Direct detection of elusive intermediates is crucial for deciphering the mechanisms of photocatalytic reactions at semiconductor interfaces. Time-of-flight mass spectrometry under ultrahigh vacuum conditions in conjunction with ultraviolet laser irradiation is employed to investigate photocatalytic water (D₂O) splitting and its coupling with methyl iodide (CH₃I) fragments on TiO₂(100). Direct detection of elusive intermediates, such as D, OD, and OOD species, unequivocally provides experimental evidence for the elementary steps of photocatalytic water splitting under ultrahigh vacuum conditions. In the presence of CH₃I, additional pathways emerge leading to methane (CH₃D) and methanol (CH₃OD) formation via reactions between photogenerated water and methyl iodide fragments. Temperature-dependent measurements indicate that photocatalytic water splitting proceeds most efficiently at room temperature, whereas density functional theory calculations elucidate the underlying mechanisms that govern the surface reactions. These results reveal molecular-level pathways of light-driven reactivity and provide a framework for controlling selective photocatalytic transformations at semiconductor interfaces. 

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3. Can DNA‐Protein Crosslinking Be Induced by One‐Electron Oxidation of 8‐Oxoguanosine? A Model Study of 9‐Methyl‐8‐Oxoguanine Radical Cation With Methylamine

   https://doi.org/10.1002/chem.202503540  

   Zhou, Wenjing; Feng, Liang; Moe, May Myat; Ceballos, Johnny; Tsai, Midas; Liu, Jianbo (February 2026, Chemistry – A European Journal)

   ABSTRACT 8‐oxoguanosine (OG) is a common form of DNA damage and is itself more susceptible to further oxidative transformations. The oxidized OG derivatives can covalently bond with nucleophilic amino acid residues, contributing to DNA–protein crosslinks (DPCs). Previous research in this context focused on the reactivity of the two‐electron oxidized OG intermediate, 2‐amino‐7,9‐dihydro‐purine‐6,8‐dione (OG^OX), toward nucleophilic addition. In contrast, the role of one‐electron oxidized OG radical cations (OG^•+) remains unexplored. Herein, we investigate a model system using 9‐methyl‐8‐oxoguanine radical cation (9MOG^•+) as an analogue of OG nucleoside and CH₃NH₂as a mimic for the lysineε‐amine. The reaction of 9MOG^•+with methylamine was measured across a range of kinetic energy in the gas phase by guided‐ion‐beam mass spectrometry. Density functional theory (DFT) and DLPNO‐CCSD(T) quantum chemistry computations were performed to elucidate reaction pathways and structures. Our results reveal that at low reaction energies, DPCs occur through direct nucleophilic addition, yielding C2‐⁺NH₂CH₃[9MOG]^•and C4‐⁺NH₂CH₃[9MOG]^•. At high energies, DPCs become possible between protonated [9MOG+H]⁺and^•CH₂NH₂/^•NHCH₃generated from hydrogen abstraction. This work provides insights into the distinct roles of one‐ versus two‐electron oxidized OG species in DPC formation, expanding knowledge of OG lesions and their biological consequences. 

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4. A chemical dynamics study of the reaction of the methylidyne radical (CH, X ² Π) with dimethylacetylene (CH ₃ CCCH ₃ , X ¹ A _1g )

   https://doi.org/10.1039/D1CP04443E  

   He, Chao; Fujioka, Kazuumi; Nikolayev, Anatoliy A.; Zhao, Long; Doddipatla, Srinivas; Azyazov, Valeriy N.; Mebel, Alexander M.; Sun, Rui; Kaiser, Ralf I. (December 2021, Physical Chemistry Chemical Physics)

   The gas-phase reaction of the methylidyne (CH; X 2 Π) radical with dimethylacetylene (CH 3 CCCH 3 ; X 1 A 1g ) was studied at a collision energy of 20.6 kJ mol −1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C 5 H 7 reaction intermediate(s) ultimately dissociating to C 5 H 6 isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon–carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon–hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively ‘screen’ the carbon–carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C 2 H 6 ) forming propylene (CH 3 C 2 H 3 ). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH 3 CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne – ethane system. 

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5. Imaging transient molecular configurations in UV-excited diiodomethane

   https://doi.org/10.1063/5.0284410  

   Venkatachalam, Anbu Selvam; Lam, Huynh_Van Sa; Bhattacharyya, Surjendu; Kaderiya, Balram; Wang, Enliang; Ding, Yijue; Greenman, Loren; Rudenko, Artem; Rolles, Daniel (October 2025, The Journal of Chemical Physics)

   Femtosecond structural dynamics of diiodomethane (CH2I2) triggered by ultraviolet (UV) photoabsorption at 290 and 330 nm are studied using time-resolved coincident Coulomb explosion imaging driven by a near-infrared probe pulse. We map the dominant single-photon process, the cleavage of the carbon–iodine bond producing rotationally excited CH2I radical, identify the contributions of the three-body (CH2 + I + I) dissociation and molecular iodine formation channels, which are primarily driven by the absorption of more than one UV photon, and demonstrate the existence of a weak reaction pathway involving the formation of short-lived transient species resembling iso-CH2I2-like geometries with a slightly shorter I–I separation compared to the ground-state CH2I2. These transient molecular configurations, which can be separated from the other channels by applying a set of conditions on the correlated momenta of three ionic fragments, are formed within ∼100 fs after the initial photoexcitation and decay within the next 100 fs. 

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