Selective Reaction Pathways Research Articles

Material Engineering Solutions toward Selective Redox Catalysts for Chemical-Looping-Based Olefin Production Schemes: A Review.

Chemical looping (CL) has emerged as a promising approach in the oxidative dehydrogenation (ODH) of light alkanes, offering an opportunity for significant reductions in emissions and energy consumption in the ethylene and propylene production industry. While high olefin yields are achievable via CL, the material requirements (e.g., electronic and geometric structures) that prevent the total conversion of alkanes to CO x are not clearly understood. This review aims to give a concise understanding of the nucleophilic oxygen species involved in the selective reaction pathways for olefin production as well as of the electrophilic oxygen species that promote an overoxidation to CO x products. It further introduces advanced characterization techniques such as X-ray photoelectron spectroscopy, Raman spectroscopy, electron paramagnetic resonance spectroscopy, and resonant inelastic X-ray scattering, which have been employed successfully in identifying such reactive oxygen species. To mitigate CO x formation and enhance olefin selectivity, material engineering solutions are discussed. Common techniques include doping of the bulk or surface and the deposition of functional coatings. In the context of energy consumption and CO2 intensity, techno-economic assessments of CL-ODH systems have predicted energy savings of up to 80% compared to established olefin production processes such as steam cracking or dehydrogenation. Finally, although their practical application has been limited to date, the potential advantages of the use of fluidized bed reactors in CL-ODH are presented.

Understanding and Controlling Reactivity Patterns of Pd1@C3N4-Catalyzed Suzuki-Miyaura Couplings.

Using heterogeneous single-atom catalysts (SACs) in the Suzuki-Miyaura coupling (SMC) has promising economic and environmental benefits over traditionally applied palladium complexes. However, limited mechanistic understanding hinders progress in their design and implementation. Our study provides critical insights into the working principles of Pd1@C3N4, a promising SAC for the SMC. We demonstrate that the base, ligand, and solvent play pivotal roles in facilitating interface formation with reaction media, activating Pd centers, and modulating competing reaction pathways. Controlling the previously overlooked interplay between base strength, reagent solubility, and surface wetting is essential for mitigating mass transfer limitations in the triphasic reaction system and promoting catalyst reusability. Optimum conditions for Pd1@C3N4 require polar solvents, intermediate base strength, and increased water content. Our investigations reveal that high selectivity requires minimizing competitive coordination of bases and phosphine ligands to the Pd centers, to avoid homocoupling and alternative reductive elimination mechanisms giving rise to phosphonium side-products. Furthermore, in situ XAS investigations probing electronic structures and coordination environments of Pd sites further rationalize the base and ligand coordination, confirming and expanding upon previous computational hypotheses for Pd1@C3N4. This understanding allows for designing a more selective ligand-free reaction pathway using the solvent and base to modulate the chemical environment of the active sites. We highlight the importance of environment-compatible surface tension, the creation of coordinatively available active sites, and the stabilization of partially reduced Pd centers, emphasizing the importance of mechanistic studies to advance the design of SACs in organic liquid phase reactions.

Bifunctional Oxygen-Defect Bismuth Catalyst toward Concerted Production of H2O2 with over 150% Cell Faradaic Efficiency in Continuously Flowing Paired-Electrosynthesis System.

The electrosynthesis of hydrogen peroxide (H2O2) from O2 or H2O via the two-electron (2e-) oxygen reduction (2e- ORR) or water oxidation (2e- WOR) reaction provides a green and sustainable alternative to the traditional anthraquinone process. Herein, a paired-electrosynthesis tactic is reported for concerted H2O2 production at a high rate by coupling the 2e- ORR and 2e- WOR, in which the bifunctional oxygen-vacancy-enriched Bi2O3 nanorods (Ov-Bi2O3-EO), obtained through electrochemically oxidative reconstruction of Bi-based metal-organic framework (Bi-MOF) nanorod precursor, are used as both efficient anodic and cathodic electrocatalysts, achieving concurrent H2O2 production at both electrodes with high Faradaic efficiencies. Specifically, the coupled 2e- ORR//2e- WOR electrolysis system based on such distinctive oxygen-defect Bi catalyst displays excellent performance for the paired-electrosynthesis of H2O2, delivering a remarkable cell Faradaic efficiency of 154.8% and an ultrahigh H2O2 production rate of 4.3 mmol h-1 cm-2. Experiments combined with theoretical analysis reveal the crucial role of oxygen vacancies in optimizing the adsorption of intermediates associated with the selective two-electron reaction pathways, thereby improving the activity and selectivity of the 2e- reaction processes at both electrodes. This work establishes a new paradigm for developing advanced electrocatalysts and designing novel paired-electrolysis systems for scalable and sustainable H2O2 electrosynthesis.

Enhancing stability of mononuclear Pt atoms via chloride-to-bromide ligand exchange for hydrosilylation reaction

Single atom catalysts (SACs) offer high atom efficiency with highly selective reaction pathway. Various strategies have been developed to synthesize SACs, among which thermally stable metal centers are usually in high oxidation while they are susceptible to facile sintering in reduced state or under reducing conditions. One of the strategies developed very recently to achieve stable zero valent metal atoms is based on the selection of ligands with moderate coordination strength with the metal center. In this work, the crucial importance of ligand to the stability of single Pt0 atoms is investigated. While HCl and diether have been found to be able to stabilize single Pt0 atoms by forming Pt1@HCl adducts, HBr coordinated Pt single atom (Pt1@HBr) obtained by simply exchanging HCl with HBr shows increased thermal stability over the Pt1@HCl under H2 atmosphere at elevated temperature, as evidenced by DRFITs of CO chemisorption and HRTEM. This work offers a facile approach to tune the stability of Pt single atoms while keeping the structure of active sites of single atom center to achieve excellent catalytic performance, using hydrosilylation in the demonstrative studies.

Recent developments of single atom alloy catalysts for electrocatalytic hydrogenation reactions

This review discusses the recent developments of single-atom alloy (SAA) catalysts toward CO2, CO, N2, and NO3– hydrogenation reactions in the electrocatalysis context, a natural expansion from the thermal catalysis origin where the concept of SAA was born and validated. Electrocatalytic hydrogenations of CO2, CO, NO3–, and N2 into value-added chemicals offer an attractive general route to mitigate greenhouse gas emissions using renewable energy without H2 feed stream. These reactions typically involve multiple electron-transfer-coupled elementary reaction steps that are of broad relevance to many electrocatalytic applications, making them well-positioned for translational fundamental studies relevant to other emerging electrochemical reactions. While there has been exciting progress in SAA research for various thermal catalytic reactions in the last decade, the design, application, and characterization of SAA catalysts in electrocatalysis remain nascent with intriguing uncertainties and ample opportunities. We highlight the recent research findings in this growing area, where the SAAs, owing to their unique doped metal properties and atomically precise bimetallic synergies, promote selective reaction pathways by tuning binding energies and distributions of adsorbates/intermediates. We also provide our perspective on this growing research area's potential challenges and opportunities.

In-situ surface enhanced Raman spectroscopy revealing the role of metal–organic frameworks on photocatalytic reaction selectivity on highly sensitive and durable Cu-CuBr substrate

Photocatalytic reactions using copper-based nanomaterials have emerged as a new paradigm in green technology. Selective photocatalysis is very important for improving energy utilization efficiency, and in order to directional improve catalytic selectivity, it is necessary to understand the mechanism of interfacial reactions at the molecular level. Therefore, a unique bifunctional Cu-CuBr substrate is first fabricated via an electrochemical method, which overcomes the instability of traditional copper-based materials and endows high surface-enhanced Raman spectroscopy (SERS) sensitivity and photocatalytic performance and can be stored stably for more than a year. Further modification of the surface with Metal-Organic Frameworks (MOFs) containing carboxyl functional groups can significantly tune the surface properties of the substrate. This increases the adsorption of cationic dyes to improve the SERS effect, and 10−10 M methylene blue can easily be detected with this substrate. Surprisingly, in-situ SERS monitoring of the interfacial photocatalytic dehalogenation reaction of aromatic halides through its intrinsic SERS effect reveal two competing selective reaction pathways, self-coupling and hydrogenation. Typically, the SERS spectra reveal that the latter's selectivity was greatly enhanced after MOFs modification, and the yield rate of the hydrogenated product increased from 27.6 % to 46.9 % (selectivity increased from 32.7 % to 51.5 %). This proves that the surface properties of catalysts, especially the affinity for reaction intermediates, can effectively regulate catalytic selectivity.

Combining Renewable Electricity and Renewable Carbon: Understanding Reaction Mechanisms of Biomass-Derived Furanic Compounds for Design of Catalytic Nanomaterials.

ConspectusDespite the growing deployment of renewable energy conversion technologies, a number of large industrial sectors remain challenging to decarbonize. Aviation, heavy transport, and the production of steel, cement, and chemicals are heavily dependent on carbon-containing fuels and feedstocks. A hopeful avenue toward carbon neutrality is the implementation of renewable carbon for the synthesis of critical fuels, chemicals, and materials. Biomass provides an opportune source of renewable carbon, naturally capturing atmospheric CO2 and forming multicarbon linkages and useful chemical functional groups. The constituent molecules nonetheless require various chemical transformations, often best facilitated by catalytic nanomaterials, in order to access usable final products.Catalyzed transformations of renewable biomass compounds may intersect with renewable energy production by offering a means to utilize excess intermittent electricity and store it within chemical bonds. Electrochemical catalytic processes can often offer advantages in energy efficiency, product selectivity, and modular scalability compared to thermal-driven reactions. Electrocatalytic reactions with renewable carbon feedstocks can further enable related processes such as water splitting, where value-adding organic oxidation reactions may replace the evolution of oxygen. Organic electroreduction reactions may also allow desirable hydrogenations of bonds without intermediate formation of H2 and need for additional reactors.This Account highlights recent work aimed at gaining a fundamental understanding of transformations involving biomass-derived molecules in electrocatalytic nanomaterials. Particular emphasis is placed on the oxidation of biomass derived furanic compounds such as furfural and 5-hydroxymethylfurfural (HMF), which can yield value-added chemicals, including furoic acid (FA), maleic acid (MA), and 2,5-furandicarboxylic acid (FDCA) for renewable materials and other commodities. We highlight advanced implementations of online electrochemical mass spectrometry (OLEMS) and vibrational spectroscopies such as attenuated total reflectance surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS), combined with microkinetic models (MKMs) and quantum chemical calculations, to shed light on the elementary mechanistic pathways involved in electrochemical biomass conversion and how these paths are influenced by catalytic nanomaterials. Perspectives are given on the potential opportunities for materials development toward more efficient and selective carbon-mitigating reaction pathways.

Ultrabroadband plasmon driving selective photoreforming of methanol under ambient conditions

Liquid methanol has the potential to be the hydrogen energy carrier and storage medium for the future green economy. However, there are still many challenges before zero-emission, affordable molecular H2 can be extracted from methanol with high performance. Here, we present noble-metal-free Cu-WC/W plasmonic nanohybrids which exhibit unsurpassed solar H2 extraction efficiency from pure methanol of 2,176.7 µmol g-1h-1 at room temperature and normal pressure. Macro-to-micro experiments and simulations unveil that local reaction microenvironments are generated by the coperturbation of WC/W's lattice strain and infrared-plasmonic electric field. It enables spontaneous but selective zero-emission reaction pathways. Such microenvironments are found to be highly cooperative with solar-broadband-plasmon-excited charge carriers flowing from Cu to WC surfaces for efficient stable CH3OH plasmonic reforming with C3-dominated liquid products and 100% selective gaseous H2. Such high efficiency, without any COx emission, can be sustained for over a thousand-hour operation without obvious degradation.

Atomically Precise Integration of Multiple Functional Motifs in Catalytic Metal-Organic Frameworks for Highly Efficient Nitrate Electroreduction.

Ammonia production plays a central role in modern industry and agriculture with a continuous surge in its demand, yet the current industrial Haber-Bosch process suffers from low energy efficiency and accounts for high carbon emissions. Direct electrochemical conversion of nitrate to ammonia therefore emerges as an appealing approach with satisfactory sustainability while reducing the environmental impact from nitrate pollution. To this end, electrocatalysts for efficient conversion of eight-electron nitrate to ammonia require collective contributions at least from high-density reactive sites, selective reaction pathways, efficient multielectron transfer, and multiproton transport processes. Here, we report a catalytic metal-organic framework (two-dimensional (2D) In-MOF In8) catalyst integrated with multiple functional motifs with atomic precision, including uniformly dispersed, high-density, single-atom catalytic sites, high proton conductivity (efficient proton transport channel), high electron conductivity (promoted by the redox-active ligands), and confined microporous environments. These eventually lead to a direct and efficient electrochemical reduction of nitrate to ammonia and record high yield rate, FE, and selectivity for NH3 production. A novel "dynamic ligand dissociation" mechanism provides an unprecedented working principle that allows for the use of a high-quality MOF crystalline structure to function as highly ordered, high-density, single-atom catalyst (SAC)-like catalytic systems and ensures the maximum utilization of the metal centers within the MOF structure. Further, the atomically precise assembly of multiple functional motifs within a MOF catalyst offers an effective and facile strategy for the future development of framework-based enzyme-mimic systems.

Mechanistic Insights into the Electrochemical Oxidation of Cyclohexane

The detrimental effects of CO2 emissions from fossil fuels and the decreasing cost of electricity have accelerated interest in electrochemical synthesis. Electro-organic synthesis offers a sustainable and cost-effective pathway for chemical manufacturing. This method has the potential to minimize greenhouse gas emissions, enhance reaction selectivity, and replace hazardous chemical reagents with electric current.Electrochemistry enables alternative mild temperature and pressure pathways to traditional thermochemical reactions. An intermediate step to producing nylon 6,6 is synthesis of KA oil, a mixture of cyclohexanone and cyclohexanol, from cyclohexane. This reaction is limited by low conversion and high pressures. An electrochemical approach can introduce a more selective reaction pathway at more benign conditions. Although electrochemical cyclohexane oxidation to KA oil has been demonstrated in literature, its mechanism remains poorly understood. In this work, we elucidate the mechanism of electrochemical cyclohexane oxidation. We report the oxygen source (water vs. oxygen) and the role of the counter electrode reaction cyclohexane oxidation. Through analysis of side products and electroanalytical methods, we suggest a reaction pathway. In addition, we use chronoamperometry to demonstrate that electrode material affects cyclohexane conversion and recommend the optimal catalyst for this reaction.

Identification of Highly Selective Surface Pathways for Methane Dry Reforming Using Mechanochemical Synthesis of Pd-CeO2.

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO2 catalysts (PdAcCeO2M), which yield unique Pd–Ce interfaces, where PdAcCeO2M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO2 (PdCeO2IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO2M can be attributed to the presence of a carbon-modified Pd0 and Ce4+/3+ surface arrangement, where distinct Pd–CO intermediate species and strong Pd–CeO2 interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CHx to CO, identified on PdAcCeO2M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd–CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm–1] species as key DRM intermediates, stemming from associative CO2 reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO2IW) where the competing reverse water gas shift reaction predominates.

The fate of tetrathionate during the development of a biofilm in biogenic sulfuric acid attack on different cementitious materials

The biodeterioration of cement-based materials in sewer environments occurs because of the production of sulfuric acid from the biochemical oxidation of H2S by sulfur-oxidizing bacteria (SOB).In the perspective of determining the possible reaction pathways for the sulfur cycle in such conditions, hydrated cementitious binders were exposed to an accelerated laboratory test (BAC test) to reproduce a biochemical attack similar to the one occurring in the sewer networks. Tetrathionate was used as a reduced sulfur source to naturally develop sulfur-oxidizing activities on the surfaces of materials.The transformation of tetrathionate was investigated on materials made from different binders: Portland cement, calcium aluminate cement, calcium sulfoaluminate cement and alkali-activated slag. The pH and the concentration of the different sulfur species were monitored in the leached solutions during 3 months of exposure.The results showed that the formation of different polythionates was independent of the nature of the material. The main parameter controlling the phenomena was the evolution of the pH of the leached solutions. Moreover, tetrathionate disproportionation was detected with the formation of more reduced forms of sulfur compounds (pentathionate, hexathionate and elemental sulfur) along with thiosulfate and sulfate. The experimental findings allowed numerical models to be developed to estimate the amount of sulfur compounds as a function of the pH evolution. In addition, biomass samples were collected from the exposed surface and from the deteriorated layers to identify the microbial populations. No clear influence of the cementitious materials on the selected populations was detected, confirming the previous results concerning the impact of the materials on the selected reaction pathways for tetrathionate transformation.

Selective Phosphorylation of RNA‐ and DNA‐Nucleosides under Prebiotically Plausible Conditions

AbstractNucleotides play a fundamental role in organisms, from adenosine triphosphate (ATP), the body‘s main source of energy, to cofactors of enzymatic reactions (e. g. coenzyme A), to nucleoside monophosphates as essential building blocks of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Although nucleotides play such an elemental role, there is no pathway to date for the selective formation of nucleoside 5′‐monophosphates. Here, we demonstrate a selective reaction pathway for 5’ mono‐phosphorylation for all canonical purine and pyrimidine bases under exceptionally mild prebiotic relevant conditions in water and without using a condensing agent. The pivotal reaction step involves activated imidazolidine‐4‐thione phosphates. The selective formation of non‐cyclic mono‐phosphorylated nucleosides represents a novel and unique route to nucleotides and opens exciting perspectives in the study of the origins of life.

Catalytic activity of ethylbenzene with product selectivity by gold nanoparticles supported on zinc oxide.

The oxidation of ethylbenzene (EB) using tert-butyl hydroperoxide as the oxidizing agent was carried out in presence of gold nanoparticles (3 nm) supported on zinc oxide in acetonitrile solution. A higher selectivity towards acetophenone (ACP) as the major product, and a moderate selectivity towards other products such as 1-phenylethanol (PE), benzaldehyde (BZL), and benzoic acid (BzA) were observed using the prepared Au/ZnO nanocatalysts at 100 °C for 24 h. It is suggested the reaction produces an intermediate product, which is dimethylethyl-1-phenylethyl peroxide through a radical mechanism. A small amount of benzaldehyde was observed because benzaldehyde went autoxidation to form benzoic acid with the presence of oxidation agent of TBHP during reaction. The factors affecting the catalytic activity such as gold loading, calcination treatment at 300°C, type of solvent, reaction time, reaction temperature, oxidant to substrate molar ratio, catalyst weight, and solvent volume were studied. The gold nanoparticle catalyst synthesized by deposition precipitation method using urea was characterized by XRD, HRTEM, ATR-IR, XRF, and BET and offers a very selective reaction pathway for the oxidation of ethylbenzene.

Machine learning–assisted CO 2 utilization in the catalytic dry reforming of hydrocarbons: Reaction pathways and multicriteria optimization analyses

The catalytic dry reforming (DR) process is a clean approach to transform CO2 into H2 and CO-rich synthetic gas that can be used for various energy applications such as Fischer–Tropsch fuels production. A novel framework is proposed to determine the optimum reaction configurations and reaction pathways for DR of C1-C4 hydrocarbons via a reaction mechanism generator (RMG). With the aid of machine learning, the variation of thermodynamic and microkinetic parameters based on different reaction temperatures, pressures, CH4/CO2 ratios and catalytic surface, Pt(111), and Ni(111), were successfully elucidated. As a result, a promising multicriteria decision-making process, TOPSIS, was employed to identify the optimum reaction configuration with the trade-off between H2 yield and CO2 reduction. Notably, the optimum conditions for the DR of C1 and C2 hydrocarbons were 800°C at 3 atm on Pt(111); whereas C3 and C4 hydrocarbons found favor at 800°C and 2 atm on Ni(111) to attain the highest H2 yield and CO2 conversion. Based on the RMG-Cat (first-principle microkinetic database), the energy profile of the most selective reaction pathway network for the DR of CH4 on Pt(111) at 3 atm and 800°C was deducted. The activation energy (Ea) for CH bond dissociation via dehydrogenation on the Pt(111) was found to be 0.60 eV, lower than that reported previously for Ni(111), Cu(111), and Co(111) surfaces. The most endothermic reaction of the CH4 reforming process was found to be C3H3* + H2O* ↔ OH* + C3H4 (218.74 kJ/mol).

Mechanistically Guided Design of an Efficient and Enantioselective Aminocatalytic α-Chlorination of Aldehydes.

The enantioselective aminocatalytic α-chlorination of aldehydes is a challenging reaction because of its tendency to proceed through neutral intermediates in unselective pathways. Herein we report the rational shift to a highly selective reaction pathway involving charged intermediates using hexafluoroisopropanol as solvent. This change in mechanism has enabled us to match and improve upon the yields and enantioselectivities displayed by previous methods while using cheaper aminocatalysts and chlorinating agents, 80–95% less amount of catalyst, convenient temperatures, and shorter reaction times.

Electrochemically Exfoliating MoS2 into Atomically Thin Planar‐Stacking Through a Selective Lateral Reaction Pathway

AbstractThe production of atomically thin transition‐metal dichalcogenides (TMDs) has been investigated through various top‐to‐down exfoliation methods, such as mechanical and chemical exfoliation, while large‐scale chemical exfoliation is sluggish and needs over ten hours to achieve atomically thin TMDs. Herein, a new strategy is reported for exfoliating bulk MoS2 into two/three‐layer flakes within tens of seconds through a mild electrochemical treatment. This exfoliation method is driven by a lateral inward oxidation reaction starting from the typical layer edge with a rapid depth penetration, whereby a stacked few‐layer (two/three layers) structure is ultimately formed. This efficient reaction process is monitored based on an individual MoS2 on‐chip device combined with in situ Raman and cross‐sectional scanning transmission electron microscopy, and the uniformity of thickness is demonstrated. This preferentially initiated method can be also extended to produce few‐layer MoSe2 and the selective extraction mechanism is assumed to be related to intrinsic layer‐dependent energy band properties. Moreover, the special reassembled few‐layer MoS2 possesses great performance as functional materials in electrocatalysis (127 mV overpotential for hydrogen evolution reaction) and surface‐enhanced Raman spectroscopy (105 enhancement factor). These results illustrate the broad prospects of the reassembled few‐layer MoS2 for optics, catalysis, and sensors.

Selective Methanol‐to‐Formate Electrocatalytic Conversion on Branched Nickel Carbide

AbstractA methanol economy will be favored by the availability of low‐cost catalysts able to selectively oxidize methanol to formate. This selective oxidation would allow extraction of the largest part of the fuel energy while concurrently producing a chemical with even higher commercial value than the fuel itself. Herein, we present a highly active methanol electrooxidation catalyst based on abundant elements and with an optimized structure to simultaneously maximize interaction with the electrolyte and mobility of charge carriers. In situ infrared spectroscopy combined with nuclear magnetic resonance spectroscopy showed that branched nickel carbide particles are the first catalyst determined to have nearly 100 % electrochemical conversion of methanol to formate without generating detectable CO2 as a byproduct. Electrochemical kinetics analysis revealed the optimized reaction conditions and the electrode delivered excellent activities. This work provides a straightforward and cost‐efficient way for the conversion of organic small molecules and the first direct evidence of a selective formate reaction pathway.

Selective Methanol-to-Formate Electrocatalytic Conversion on Branched Nickel Carbide.

A methanol economy will be favored by the availability of low-cost catalysts able to selectively oxidize methanol to formate. This selective oxidation would allow extraction of the largest part of the fuel energy while concurrently producing a chemical with even higher commercial value than the fuel itself. Herein, we present a highly active methanol electrooxidation catalyst based on abundant elements and with an optimized structure to simultaneously maximize interaction with the electrolyte and mobility of charge carriers. In situ infrared spectroscopy combined with nuclear magnetic resonance spectroscopy showed that branched nickel carbide particles are the first catalyst determined to have nearly 100 % electrochemical conversion of methanol to formate without generating detectable CO2 as a byproduct. Electrochemical kinetics analysis revealed the optimized reaction conditions and the electrode delivered excellent activities. This work provides a straightforward and cost-efficient way for the conversion of organic small molecules and the first direct evidence of a selective formate reaction pathway.

Structure, Stability, and Spectroscopic Properties of Small Acetonitrile Cation Clusters.

Ionization and fragmentation pathways induced by ionizing agents are key to understanding the formation of complex molecules in astrophysical environments. Acetonitrile (CH3CN), the simplest organic nitrile, is an important molecule present in the interstellar medium. In this work, DFT and MP2 calculations were performed in order to obtain the low energy structures of the most relevant cations formed from electron-stimulated ion desorption of CH3CN ices. Selected reaction pathways and spectroscopic properties were also calculated. Our results indicate that the most stable acetonitrile cation structure is CH2CNH+ and that hydrogenation can occur successively without isomerization steps until its complete saturation. Moreover, the stability of distinct cluster families formed from the interaction of acetonitrile with small fragments, such as CHn+, C2Hn+, and CHnCNH+, is discussed in terms of their respective binding energies. Some of these molecular clusters are stabilized by hydrogen bonds, leading to species whose infrared features are characterized by a strong redshift of the N-H stretching mode. Finally, the rotational spectra of CH3CN and protonated acetonitrile, CH3CNH+, were simulated using distinct computational protocols based on DFT, MP2, and CCSD(T) considering centrifugal distortion, vibrational-rotational coupling, and vibrational anharmonicity corrections. By adopting an empirical scaling procedure for calculating spectroscopic parameters, we were able to estimate the rotational frequencies of CH3CNH+ with an expected average error below 1 MHz for J values up to 10.