Chemical Bond Energy Research Articles

Reference Test Method for Calculating the Thermal Effect of Coal Spontaneous Combustion

During a heating experiment, there are two sources of heat that increase the temperature of a coal sample: the heat released by the oxidation reaction of the coal itself, and the heat provided by the experimental system. Here, we propose a method for measuring the thermal effect of oxidation and self-ignition through a reference experiment conducted with a material that is physically similar to coal but does not combust. The reference material used was an aggregate of alumina, fly ash, and concrete, and experiments were conducted on both materials simultaneously. The temperature of the coal sample was obtained under self-heating conditions, and compared with that of the non-combusting material. The relationship of temperature as a function of time for both materials was determined from the data, the comparison of which allowed for the thermal effect of oxidation and coal spontaneous combustion (CSC) to be calculated. The reliability of the thermal effect data obtained by the experiment was verified by chemical bond energy estimation. These results provide theoretical guidance for on-site fire prevention and extinguishing in coal mines, and are important for the further development of the understanding of CSC.

On the presence of oxyphosphorus bonds in TOPCon solar cell polycrystalline silicon films

The presence of silicon oxyphosphorus bonds in the heavily phosphorus (P)-doped n+-type polycrystalline silicon film (n+-poly Si) of TOPCon solar cell has been mainly revealed through a depth-profile X-ray photoelectron spectroscopy (XPS) characterization of chemical components. X-ray diffraction (XRD) pattern is applied to the verification of recrystallized phases in the n+-poly Si film, while high-resolution transmission electron microscopy (HR-TEM) with energy dispersive X-ray (EDX) pattern is used for distinguishing the crystallinity and the elemental segregation. The test sample is of a typical rear passivation contact stacks of c-Si/SiOx/n+-poly Si/SiO2, in which the n+-poly Si layer is heavily phosphorus-doped by ion implantation of as-deposited intrinsic thin film through low-pressure chemical vapor deposition (LPCVD) and recrystallization at 900 °C afterward. In the whole n+-poly Si layer, XPS P 2p electronic states at 129.8, 133.4 and 136.8 eV have been detected and assigned to be Si–P (or P–P), O–P and P–O–P bonds, respectively. In addition, the thermodynamic functions, such as the Gibbs free energy and mole formation energy of likely chemical bonds under the lowest energy as well as temperature-composition fraction (T-CF) phase diagram, are also implemented in the verification of the derivatives of phosphorus oxides.

Rhenium Carbonyl Molecular Catalysts for CO2 Electroreduction: Effects on Catalysis of Bipyridine Substituents Mimicking Anchorage Functions to Modify Electrodes.

Heterogenization of molecular catalysts on (photo)electrode surfaces is required to design devices performing processes enabling to store renewable energy in chemical bonds. Among the various strategies to immobilize molecular catalysts, direct chemical bonding to conductive surfaces presents some advantages because of the robustness of the linkage. When the catalyst is, as it is often the case, a transition metal complex, the anchoring group has to be connected to the complex through the ligands, and an important question is thus raised on the influence of this function on the redox and on the catalytic properties of the complex. Herein, we analyze the effect of conjugated and non conjugated substituents, structurally close to anchoring functions previously used to immobilize a rhenium carbonyl bipyridyl molecular catalyst for supported CO2 electroreduction. We show that carboxylic ester groups, mimicking anchoring the catalyst via carboxylate binding to the surface, have a drastic effect on the catalytic activity of the complex toward CO2 electroreduction. The reasons for such an effect are revealed via a combined spectro-electrochemical analysis showing that the reducing equivalents are mainly accumulated on the electron-withdrawing ester on the bipyridine ligand preventing the formation of the rhenium(0) center and its interaction with CO2. Alternatively, alkyl-phosphonic ester substituents, not conjugated with the bpy ligand, mimicking anchoring the catalyst via phosphonate binding to the surface, allow preserving the catalytic activity of the complex.

Molecular Engineering of Metal Complexes for Electrocatalytic Carbon Dioxide Reduction: From Adjustment of Intrinsic Activity to Molecular Immobilization.

The electrocatalytic CO2 reduction reaction (ECO2 RR) is one promising method for storing intermittent clean energy in chemical bonds and producing fuels. Among various kinds of catalysts for ECO2 RR, molecular metal complexes with well-defined structures are convenient for studies of their rational design, structure-reactivity relationships, and mechanisms. In this Review, we summarize the molecular engineering of several N-based metal complexes including Re/Mn bipyridine compounds and metal macrocycles, concluding with general modification strategies to devise novel molecular catalysts with high intrinsic activity. Through physical adsorption, covalent linking, and formation of a periodic backbone, these active molecules can be heterogenized into immobilized catalysts with more practical prospects. Finally, significant challenges and opportunities based on molecular catalysts are discussed.

Molecular Engineering of Metal Complexes for Electrocatalytic Carbon Dioxide Reduction: From Adjustment of Intrinsic Activity to Molecular Immobilization

AbstractThe electrocatalytic CO2 reduction reaction (ECO2RR) is one promising method for storing intermittent clean energy in chemical bonds and producing fuels. Among various kinds of catalysts for ECO2RR, molecular metal complexes with well‐defined structures are convenient for studies of their rational design, structure–reactivity relationships, and mechanisms. In this Review, we summarize the molecular engineering of several N‐based metal complexes including Re/Mn bipyridine compounds and metal macrocycles, concluding with general modification strategies to devise novel molecular catalysts with high intrinsic activity. Through physical adsorption, covalent linking, and formation of a periodic backbone, these active molecules can be heterogenized into immobilized catalysts with more practical prospects. Finally, significant challenges and opportunities based on molecular catalysts are discussed.

Effects of wet grinding combined with chemical activation on the activity of iron tailings powder

Utilizing iron tailings powder (ITP) as supplementary cementitious materials (SCMs) has significant economic and environmental benefits, while the low pozzolanic activity of ITP limits its incorporation to some extent. This paper aims to study the effect of compound activation (wet grinding with a chemical activator) on the activity of ITP. Two kinds of sulfate (CaSO4·2H2O and Na2SO4) were used as chemical activators, and the optimal dosages of CaSO4·2H2O and Na2SO4 were determined by the mechanical strength tests of cement-ITP pastes. Results demonstrate that adding 3 % CaSO4·2H2O (wet grinding with ITP) caused a slight increase in the early and later strength of cement pastes containing 30 % ITP, while adding 3 % Na2SO4 (wet grinding with ITP) resulted in a significant improvement in the early strength and a slight increase in the later strength. Compared to a single wet grinding activation, compound activation could produce more amorphous substances in ITP and cause a more prominent decrease in the surface binding energy of internal chemical bonds of ITP, thereby further improving the activity of ITP. Moreover, the hydration products of cement pastes containing 30 % compound activated ITP were compared with those of cement pastes containing 30 % wet ground ITP, and the activation mechanism of two sulfates on ITP was studied.

A high-temperature-resistant and metallic-crosslinker-free fracturing fluid based on guar gum/montmorillonite nanocomposite

A combination of moderate apparent viscosity, temperature resistance, and low damage is highly desired for fracturing fluids used in high-temperature reservoirs. The aim of this study is to achieve this combination by introducing inorganic nanolayered montmorillonite (MMT) to form a high-temperature-resistant and metallic-crosslinker-free intercalated nanocomposite fracturing fluid (GMN). The exfoliated MMT was uniformly dispersed and connected with polymer chains of guar gum to form a highly three-dimensional cross-linked network, which resulted in increased apparent viscosity and viscoelasticity of GMN compared with the guar gum fracturing fluid. Temperature resistance tests and thermogravimetric analyses demonstrated that the nanofiller enhanced the thermal stabilities of the nanocomposites, and this was attributed to three effects, including high chemical bond energies, heat barriers and minimization of chain free volumes in MMT. In the absence of a metallic crosslinker and temperature stabilizer, the resistance temperature of the GMN fracturing fluid reached 160 °C, a 40 °C enhancement compared to that of the guar gum fracturing fluid. Moreover, the GMN fracturing fluid were easily broken and exhibited a 10.34% damage rate to provide the desired fracture conductivity. It was concluded that layered MMT effectively improved the temperature resistance of guar gum fracturing fluid and exhibited great potential for use in high-temperature reservoirs.

(Digital Presentation) Photo-Thermo-Electrochemical Cells for on-Demand Solar Power and Hydrogen Generation

Converting solar energy into power and hydrogen provides a promising pathway to fulfilling instantaneous electricity demand (power generation) as well as continuous demand via storing energy in chemical bonds (hydrogen generation). Co-generation of power and hydrogen is of great interest due to its potential to overcome expensive electricity storage in conventional PV plus battery systems. Both solar thermochemistry processes and photo-electrochemical cells (PECs) are extensively explored technologies to produce solar hydrogen. The key challenges for solar thermochemistry processes are extremely high operating temperature (~ 1500 oC) and low demonstrated efficiency (< 1% for hydrogen generation). For PECs, the limited solar absorption together with sluggish electrochemical reactions, especially for OER, leads to limited theoretical solar fuel generation. Operating PECs at high temperature will lead to decreased photovoltage and interface stability. Inspired by the thermally regenerative batteries, we propose a photo-thermo-electrochemical (PTEC) device that uses the solid oxide-based moderate high temperature cell (~1000 ℃) as the photo-absorber for simultaneously converting concentrated solar radiation into heat and generating fuel or power electrochemically driven by the discharging power from the low temperature cell (~700 ℃). PTEC device enables full solar spectrum utilization, highly favorable thermodynamics and kinetics, and cost-effectiveness. A continuous PTEC device has two working modes, which are voltage differential (VD) mode and current differential (CD) mode. The current-voltage characteristics of a PTEC device are shown in Figure 1. It mainly consists of five parts. A high temperature cell (HTC) serves as a solar absorber and a low temperature cell (LTC) serves as heat recovery. Besides, the opposite electrochemical reactions take place in two cells meaning that HTC and LTC can also function as a hydrogen production as well as an electricity generator component, respectively. Heat exchanger(s) is placed between the HTC and LTC and hot fluids pass through a heat exchanger before entering LTC to reduce heat losses to environment as well as reducing input solar energy. The VD mode and CD mode can be realized in PTECs via controlling of DC-DC converter. In order to identify the main parameters, we develop a multi-physics model based on finite element method, including mass, heat and charge transfer, and electrochemical reactions. In addition, heat exchange is modeled by solving energy balance equation, DC-DC convertor is assumed by constant efficiency, and a lumped parameter model is used to describe solar receiver including energy losses of conduction and reradiation. This framework also allows us to provide design guidelines for PTEC devices with high solar-to-electricity (STE) efficiency and solar-to-hydrogen (STH) efficiency. The maximum STE and STH efficiency under reference conditions of PTEC device was found to be 4 % and 2 %. A further improved performance in terms of STE and STH efficiency are about 19 % and 16 %, respectively, via optimizing temperature configuration between HTC and LTC and material properties. It is also interesting to note that STH can reach higher than 80 % of STE at a large temperature difference, which shows a promising energy storage device by storing excessive electrical power in form of hydrogen. The main results show that the temperature of HTC and efficiency of heat exchange are key parameters to optimize PTEC efficiency. The performance of DC-DC convertor dominates STH efficiency. Besides, ionic conductivity of electrolyte can contribute to significantly expanding the operating current density range. The PTEC is a promising technology for solar energy conversion and storage as it is able to produce electricity and hydrogen in a single device. The solar conversion efficiency predicted with our numerical model supports that by optimizing the design and operational conditions, this technology can compete with existing solar fuel pathways. Figure 1

Hydrogen produced at low temperatures by electrochemically assisted pyrolysis of cellulose in molten carbonate

Cellulose was used as feedstock to investigate the effect of electrolysis on the molten salt pyrolysis process. The mechanism of cellulose transformation in the electrochemically assisted molten carbonate pyrolysis (EMCP) system was evaluated by comparison with the molten carbonate pyrolysis (MCP) system. The gas product yield from the EMCP system reached 70.5 wt%, at the expense of the liquid product and tar. The maximum H2 yields up to 8.31 mol/kg of cellulose at low temperatures without the catalyst in the EMCP system. Cellulose decomposition was catalyzed by the molten salt to produce a large number of volatiles, where the aldehyde groups in the volatiles were electrochemically oxidized to carboxyl groups. Subsequently, the carboxyl compounds combined with alkali metal cations to release hydrogen radicals and then underwent secondary cracking, along with the volatiles having low chemical bond energies. The EMCP system follows the Shrink Cylinder model, which is dominated by the two-dimensional reaction on the surface of the graphite electrode, where the electrolysis process is the determining step.

Carbon Catalysts for Electrochemical CO2 Reduction toward Multicarbon Products

AbstractElectrochemical CO2 reduction offers a compelling route to mitigate atmospheric CO2 concentration and store intermittent renewable energy in chemical bonds. Beyond C1, C2+ feedstocks are more desirable due to their higher energy density and more significant market need. However, the CO2‐to‐C2+ reduction suffers from significant barriers of CC coupling and complex reaction pathways. Due to remarkable tunability over morphology/pore architecture along with great feasibility of functionalization to modify the electronic and geometric structures, carbon materials, serving as active components, supports, and promoters, provide exciting opportunities to tune both the adsorption properties of intermediates and the local reaction environment for the CO2 reduction, offering effective solutions to enable CC coupling and steer C2+ evolution. However, general design principles remain ambiguous, causing an impediment to rational catalyst refinement and application thrusts. This review clarifies insightful design principles for advancing carbon materials. First, the current performance status and challenges are discussed and effective strategies are outlined to promote C2+ evolution. Further, the correlation between the composition, structure, and morphology of carbon catalysts and their catalytic behavior is elucidated to establish catalytic mechanisms and critical factors determining C2+ performance. Finally, future research directions and strategies are envisioned to inspire revolutionary advancements.

Interactions b/w Collagen and Polylactic-Acid Molecular Models Due to DFT Calculations

Collagen and Polylactic Acid (PLA) as biomaterials in the tissue engineering have been investigated for the structure and function in considerably large progress. From a viewpoint of its electronic structures in the subnano-meter range, we investigate the bonding nature for five models of the type I collagen, PLA models and the bimolecular interaction between the models by using Density Functional Theory (DFT) calculations with Frontier MO theory. In order to confirm the accuracy of the models, we compare the simulated IR and C1s X-ray photoelectron spectra with experimental results. Especially, the interaction between PLA and (AspHypGly)-collagen models was obtained as the chemical bond energy (1540 kJ/mol), and the results of PLA-other four collagen bimolecular models were given as intermolecular bond energies of 30 ∼ 81 kJ/mol. The intermolecular interaction is due to inter-H-bond of –OH---O=, -NH---O=, and –CH---O= functional groups, respectively, using MO calculations.

SnO2 doping effects on the phase constitution and microwave dielectric properties of a Co0.5Ti0.5NbO4 system

The effects of Sn4+ substitution on the structure–property relationship of Co0.5(Ti1−xSnx)0.5NbO4 ceramics were studied systemically using the dielectric theory of complex crystals, and the intrinsic dielectric properties were examined. A rutile Co0.5(Ti1−xSnx)0.5NbO4 solid solution was formed, and the properties were examined via X-ray diffraction and transmission electron microscopy. According to Rietveld refinement analysis, Sn4+ increased the axis length and unit-cell volume. The dielectric theory of complex crystals showed that the introduction of Sn4+ leads to a consistent variation between the average bond covalency value and dielectric constant, indicating a decline in dielectric polarizability. However, an evaluation of the lattice energy of chemical bonds revealed a decrease in the structural stability, with a concomitant increase in the dielectric loss. Moreover, far-infrared reflectivity analysis, complex dielectric function analysis, and Terahertz time-domain spectroscopy of the intrinsic dielectric properties of Co0.5(Ti1−xSnx)0.5NbO4 (x = 0.2) ceramic revealed similar results between the theoretical fitting and experimental test.

Study on Curing Kinetics and the Mechanism of Ultrasonic Curing of an Epoxy Adhesive.

Ultrasonic curing is an effective way to enhance the curing extent of composite material bonding in the aerospace industry. The non-thermal effect of ultrasonic has been revealed to improve curing efficiency. However, the mechanism of the ultrasonic non-thermal effect is still not clear. In this work, a variable activation energy model of ultrasonic curing was established by utilizing the iso-conversional method, including the activation energy of the thermal effect and activation energy of the non-thermal effect. The thermal effect caused by ultrasonic was accurately peeled off. An obvious decrease in activation energy was found from 54 kJ/mol in thermal curing to 38 kJ/mol in ultrasonic curing. The activation energy of the reaction system in ultrasonic curing was substituted into the modified Kamal autocatalytic equation, and the parameters of the ultrasonic curing kinetic model were estimated by means of an ALO algorithm. Further discussion based on in situ FTIR showed that the non-thermal effect of ultrasonic can affect the vibration strength, stability, and chemical bond energy of internal groups, but cannot cause the fracture of chemical bonds. Moreover, frontier molecular orbital analysis showed that the chemical reactivity of epoxy/amine molecules increased and the HOMO–LUMO energy gap decreased from 6.511 eV to 5.617 eV under the effect of ultrasonic.

Experimental and Theoretical Study of Gamma Radiolysis and Dose Rate Effect of o-Cresol Formaldehyde Epoxy Composites.

Gamma radiolysis behaviors and mechanisms of silica-filled o-cresol formaldehyde epoxy are studied at 2.20 × 10-5 to 1.95 × 10-1 Gy/s. The radiolysis-induced changes in chemical structures do not severely affect its thermostability. The slightly deteriorated mechanical strength at temperature exceeding 100 °C is accompanied by the declining glass transition temperature (from 185.9 to 172.2 °C) and enhanced damping ability. The gas yields of hydrogen, methane, and carbon dioxide manifest a remarkable dose rate effect. Based on the Schwarzschild law, their yields at an extremely low dose rate are accurately predicted by the established master curves. Besides, the latent radiolysis of gas products and postradiation effect are found with caution. The radiation-caused residual spin species are proved to be composed of silica defects and a phenoxy-type free radical with a tert-butyl group, according to the experimental results, theoretical calculations, and spectra simulations. The lower vertical ionization potential (7.6 eV) and adiabatic ionization potential (7.1 eV) are primarily due to the ionization of the benzene ring moiety with the tert-butyl group, which is likely to suffer from radiolysis. The calculated bond dissociation energy (260.8-563.5 kJ/mol) of the typical chemical bonds of epoxy is consistent with its radiolytic vulnerability and degradation mechanisms.

Switching Catalyst Selectivity via the Introduction of a Pendant Nitrophenyl Group.

The conversion of abundant small molecules to value-added products serves as an attractive method to store renewable energy in chemical bonds. A family of macrocyclic cobalt aminopyridine complexes was previously reported to reduce CO2 to CO with 98% faradaic efficiency through the formation of hydrogen-bonding networks and with the number of secondary amines affecting catalyst performance. One of these aminopyridine macrocycles, (NH)1(NMe)3-bridged calix[4]pyridine (L5), was modified with a nitrophenyl group to form LNO2 and metalated with a cobalt(II) precursor to generate CoLNO2, which would allow for probing the positioning and steric effects on catalysis. The addition of a nitrophenyl moiety to the ligand backbone results in a drastic shift in selectivity. Large current increases in the presence of added protons and CoLNO2 are observed under both N2 and CO2. The current increases under N2 are ∼30 times larger than the ones under CO2, suggesting a change in the selectivity of CoLNO2 to favor H2 production versus CO2 reduction. H2 is determined to be the dominant reduction product by gas chromatography, reaching faradaic efficiencies up to 76% under N2 with TFE and 71% under CO2 with H2O, in addition to small amounts of formate. X-ray photoelectron spectroscopy (XPS) reveals the presence of a cobalt-containing heterogeneous deposit on the working electrode surface, indicating the addition of the nitrophenyl group reduces the electrochemical stability of the catalyst. These observed catalytic behaviors are demonstrably different relative to the tetra-NH bridged macrocycle, which shows 98% faradaic efficiency for CO2-to-CO conversion with TFE, highlighting the importance of pendant hydrogen bond donors and electrochemically robust functional groups for selective CO2 conversion.

Photoelectrochemical water oxidation improved by pyridine N-oxide as a mimic of tyrosine-Z in photosystem II.

Artificial photosynthesis provides a way to store solar energy in chemical bonds with water oxidation as a major challenge for creating highly efficient and robust photoanodes that mimic photosystem II. We report here an easily available pyridine N-oxide (PNO) derivative as an efficient electron transfer relay between an organic light absorber and molecular water oxidation catalyst on a nanoparticle TiO2 photoanode. Spectroscopic and kinetic studies revealed that the PNO/PNO+˙ couple closely mimics the redox behavior of the tyrosine/tyrosyl radical pair in PSII in improving light-driven charge separation via multi-step electron transfer. The integrated photoanode exhibited a 1 sun current density of 3 mA cm−2 in the presence of Na2SO3 and a highly stable photocurrent density of >0.5 mA cm−2 at 0.4 V vs. NHE over a period of 1 h for water oxidation at pH 7. The performance shown here is superior to those of previously reported organic dye-based photoanodes in terms of photocurrent and stability.

An azobenzene-based photothermal energy storage system for co-harvesting photon energy and low-grade ambient heat via a photoinduced crystal-to-liquid transition

Ambient heat, slightly above or at room temperature, is a ubiquitous and inexhaustible energy source that has typically been ignored due to difficulties in its utilization. Recent evidence suggests that a class of azobenzene (Azo) photoswitches featuring a reversible photoinduced crystal-to-liquid transition could co-harvest photon energy and ambient heat. Thus, a new horizon has been opened for recovering and regenerating low-grade ambient heat. Here, a series of unilateral para-functionalized photoinduced liquefiable Azo derivatives is presented that can co-harvest and convert photon energy and ambient heat into chemical bond energy and latent heat in molecules and eventually release them in the form of high-temperature utilizable heat. A straightforward crystalline-to-liquid phase transition achieved with ultraviolet light irradiation (365 nm) is enabled by appending a halogen/alkoxy group on the para-position of the Azo photoswitches, and the release of thermal energy is triggered by short-wavelength visible-light irradiation (420 nm). The phase transition properties of the trans- and cis-isomers and the energy density, storage lifetime and heat release performance of the cis-liquid are investigated with differential scanning calorimetry, ultraviolet-visible absorption spectroscopy, and an infrared (IR) thermal camera. The experimental results indicate a high energy density of 335 J/g, a long lifetime of 5 d and a heat release of up to 6.3 °C due to the coupled photochemical-thermophysical mechanism. This work presents a new model for utilizing renewable energy, i.e., the photoinduced conversion of ambient thermal energy.

The mechanism for the enhanced mechanical and piezoelectricity properties of La2O3 doped BaTiO3 ceramic coatings prepared by plasma spray

Lanthanum oxide (La2O3) doped Barium titanate (BaTiO3) ceramic coatings were obtained by plasma spray. The detail discussion was conducted on the characteristic morphologies (microstructure and cross section of BaTiO3 +xwt% La2O3 composite powders) and phase. In order to further study the reasons for the improvement of its mechanical properties. Using first-principles calculation methods, theoretically analyze the mechanism of its mechanical performance improvement. The piezoelectric coefficient observed in the BaTiO3 + xwt%La2O3 coatings in this study. The reason for the increase in relative microhardness is the elimination of microcracks and pores, which increases the tendency of covalent bonds between atoms to form bonds, shortens the bond length of chemical bonds, and improves the bond energy and strength of chemical bonds. The piezoelectric performance simulation result is consistent with the experimental result. Adding La2O3 to the BaTiO3 ceramic coating can significantly improve the piezoelectric performance.

BSE49, a diverse, high-quality benchmark dataset of separation energies of chemical bonds

We present an extensive and diverse dataset of bond separation energies associated with the homolytic cleavage of covalently bonded molecules (A-B) into their corresponding radical fragments (A. and B.). Our dataset contains two different classifications of model structures referred to as “Existing” (molecules with associated experimental data) and “Hypothetical” (molecules with no associated experimental data). In total, the dataset consists of 4502 datapoints (1969 datapoints from the Existing and 2533 datapoints from the Hypothetical classes). The dataset covers 49 unique X-Y type single bonds (except H-H, H-F, and H-Cl), where X and Y are H, B, C, N, O, F, Si, P, S, and Cl atoms. All the reference data was calculated at the (RO)CBS-QB3 level of theory. The reference bond separation energies are non-relativistic ground-state energy differences and contain no zero-point energy corrections. This new dataset of bond separation energies (BSE49) is presented as a high-quality reference dataset for assessing and developing computational chemistry methods.

Machine-Learning Photodynamics Simulations Uncover the Role of Substituent Effects on the Photochemical Formation of Cubanes.

Photochemical [2 + 2]-cycloadditions store solar energy in chemical bonds and efficiently access strained organic molecular architectures. Functionalized [3]-ladderdienes undergo [2 + 2]-photocycloadditions to afford cubanes, a class of strained organic molecules. The substituents (e.g., methyl, trifluoromethyl, and cyclopropyl) affect the overall reactivities of these cubane precursors; the yields range from 1 to 48%. However, the origin of these substituent effects on the reactivities and chemoselectivities is not understood. We now integrate single and multireference calculations and machine-learning-accelerated nonadiabatic molecular dynamics (ML-NAMD) to understand how substituents affect the ultrafast dynamics and mechanism of [2 + 2]-photocycloadditions. Steric clashes between substituent groups destabilize the 4π-electrocyclic ring-opening pathway and minimum energy conical intersections by 0.72-1.15 eV and reaction energies by 0.68-2.34 eV. Noncovalent dispersive interactions stabilize the [2 + 2]-photocycloaddition pathway; the conical intersection energies are lower by 0.31-0.85 eV, and the reaction energies are lower by 0.03-0.82 eV. The 2 ps ML-NAMD trajectories reveal that closed-shell repulsions block a 6π-conrotatory electrocyclic ring-opening pathway with increasing steric bulk. Thirty-eight percent of the methyl-substituted [3]-ladderdiene trajectories proceed through the 6π-conrotatory electrocyclic ring-opening, whereas the trifluoromethyl- and cyclopropyl-substituted [3]-ladderdienes prefer the [2 + 2]-photocycloaddition pathways. The predicted cubane yields (H: 0.4% < CH3: 1% < CF3: 14% < cPr: 15%) match the experimental trend; these substituents predistort the reactants to resemble the conical intersection geometries leading to cubanes.