Lai Xu

Bifurcations on Potential Energy Surfaces of Organic Reactions

A single transition state may lead to multiple intermediates or products if there is a post-transition-state reaction pathway bifurcation. These bifurcations arise when there are sequential transition states with no intervening energy minimum. For such systems, the shape of the potential energy surface and dynamic effects, rather than transition-state energetics, control selectivity. This Minireview covers recent investigations of organic reactions exhibiting reaction pathway bifurcations. Such phenomena are surprisingly general and affect experimental observables such as kinetic isotope effects and product distributions.

Dynamics of 1,3-Dipolar Cycloadditions: Energy Partitioning of Reactants and Quantitation of Synchronicity

The dynamics of 1,3-dipolar cycloadditions of nine 1,3-dipoles with ethylene and acetylene have been explored by quasiclassical trajectory and single trajectory calculations in the retro-cycloaddition direction to compute energy partitioning of reactants among relative translation, vibration, and rotation. The results are interpreted with an expanded version of Polanyis Rules for bimolecular reactions, and three trends are evident. (1) Relative translation of reactants is the main contributor to surmounting the barrier, since all transition states (TSs) are early with respect to sigma bond formation. (2) Vibrational excitation in the 1,3-dipole bending modes required for reaction is related to the lateness of the TS with respect to dipole bending: diazonium betaines (late TS, dipole bending required) > nitrilium betaines > azomethine betaines (early TS, dipole bending least important). This is also the order of the activation barriers (high --> low). (3) The previously reported linear correlation between activation barriers and the energy required to distort reactants to their TS geometries are understandable in terms of the requirements for vibrational excitation computed here. For the 1,3-dipolar cycloadditions, single trajectory calculations, which contain no zero point vibrational energy, give reasonable estimates of the mean energy partitioning of reactants derived from potential energy barrier release. The timing of bond formation and relative reactivities of different 1,3-dipoles are discussed.

Dynamics, transition states, and timing of bond formation in Diels–Alder reactions

The time-resolved mechanisms for eight Diels–Alder reactions have been studied by quasiclassical trajectories at 298 K, with energies and derivatives computed by UB3LYP/6-31G(d). Three of these reactions were also simulated at high temperature to compare with experimental results. The reaction trajectories require 50–150 fs on average to transverse the region near the saddle point where bonding changes occur. Even with symmetrical reactants, the trajectories invariably involve unequal bond formation in the transition state. Nevertheless, the time gap between formation of the two new bonds is shorter than a C─C vibrational period. At 298 K, most Diels–Alder reactions are concerted and stereospecific, but at high temperatures (approximately 1,000 K) a small fraction of trajectories lead to diradicals. The simulations illustrate and affirm the bottleneck property of the transition state and the close connection between dynamics and the conventional analysis based on saddle point structure.

Dynamics of 1,3‐Dipolar Cycloaddition Reactions of Diazonium Betaines to Acetylene and Ethylene: Bending Vibrations Facilitate Reaction

Getting the bends: The dynamics of 1,3-dipolar cycloaddition reactions have been explored by decomposing transition vector, quasi-classical trajectories, and single trajectories. Dipole bending (see picture) makes the largest contribution to the TS distortion energy and constitutes the major part of transition-state distortion energy in the favored concerted pathway.

CuxCo1−xO Nanoparticles on Graphene Oxide as A Synergistic Catalyst for High‐Efficiency Hydrolysis of Ammonia–Borane

Ammonia-borane (AB) is an excellent material for chemical storage of hydrogen. However, the practical utilization of AB for production of hydrogen is hindered by the need of expensive noble metal-based catalysts. Here, we report Cux Co1-x O nanoparticles (NPs) facilely deposited on graphene oxide (GO) as a low-cost and high-performance catalyst for the hydrolysis of AB. This hybrid catalyst exhibits an initial total turnover frequency (TOF) value of 70.0 (H2 ) mol/(Cat-metal) mol⋅min, which is the highest TOF ever reported for noble metal-free catalysts, and a good stability keeping 94 % activity after 5 cycles. Synchrotron radiation-based X-ray absorption spectroscopy (XAS) investigations suggested that the high catalytic performance could be attributed to the interfacial interaction between Cux Co1-x O NPs and GO. Moreover, the catalytic hydrolysis mechanism was studied by in situ XAS experiments for the first time, which reveal a significant water adsorption on the catalyst and clearly confirm the interaction between AB and the catalyst during hydrolysis.

Dynamics of Carbene Cycloadditions

Quasiclassical trajectory calculations using quantum mechanical energies and forces generated by the Venus and Gaussian programs provide for the first time a detailed dynamical picture of singlet carbene, CCl(2) and CF(2), cycloadditions to alkenes on the B3LYP/6-31G* surface. For CF(2), B3LYP/6-31G* with exact exchange reduced to 12% HF was also employed to better mimic the high accuracy surface. The range of geometries sampled in reactive trajectories and the timing of bond formation were explored. All trajectories follow the nonlinear approach proposed by Moore and Hoffmann. The reaction of CCl(2) with ethylene is a dynamically concerted reaction, with an average time gap between formation of the two bonds of 50 fs. The reaction of CF(2) with ethylene is dynamically complex with biexponential decay of the diradical species formed from the first bond formation. A general quantitative dynamical classification of cycloaddition mechanisms is proposed, based on the timing of bond formation.

1D Coordination Polymer Nanofibers for Low-Temperature Photothermal Therapy

Near-infrared (NIR)-light-triggered photothermal therapy (PTT) usually requires hyperthermia to >50 °C for effective tumor ablation, which can potentially induce inflammatory disease and heating damage of normal organs nearby, while tumor lesions without sufficient heating (e.g., the internal part) may survive after treatment. Achieving effective tumor killing under relatively low temperatures is thus critical toward successful clinical use of PTT. Herein, we design a simple strategy to fabricate poly(ethylene glycol) (PEG)-modified one-dimensional nanoscale coordination polymers (1D-NCPs) with intrinsic biodegradability, large surface area, pH-responsive behaviors, and versatile theranostic functions. With NCPs consisting of Mn2+/indocyanine green (ICG) as the example, Mn-ICG@pHis-PEG display efficient pH-responsive tumor retention after systemic administration and then load Gambogic acid (GA), a natural inhibitor of heat-shock protein 90 (Hsp90) that plays an essential role for cells to resist heating-induced damage. Such Mn-ICG@pHis-PEG/GA under a mild NIR-triggered heating is able to induce effective apoptosis of tumor cells, realizing low-temperature PTT (~43 °C) with excellent tumor destruction efficacy. This work not only develops a facile approach to fabricate PEGylated 1D-NCPs with tumor-specific pH responsiveness and theranostic functionalities, but also presents a unique low-temperature PTT strategy to kill cancer in a highly effective and minimally invasive manner.

Photooxidation of n-octanal in air: experimental and theoretical study.

Dilute mixtures of n-octanal in synthetic air (up to 100 ppm) were photolyzed with fluorescent UV lamps (275-380 nm) at 298 K. The main photooxidation products were 1-hexene, CO, vinyl alcohol, and acetaldehyde. The photolysis rates and the absolute quantum yields were found to be slightly dependent on the total pressure. At 100 Torr, Φ(100) = 0.41 ± 0.06, whereas at 700 Torr the total quantum yield was Φ(700) = 0.32 ± 0.02. Two decomposition channels were identified: the radical channel C(7)H(15)CHO → C(7)H(15) + HCO and the molecular channel C(7)H(15)CHO → C(6)H(12) + CH(2)═CHOH, having absolute quantum yields of 0.022 and 0.108 at 700 Torr. The product CH(2)═CHOH tautomerizes to acetaldehyde. Carbon balance data lower than unities suggest the existence of unidentified decomposition channel(s) which substantially contributes to the photolysis. On the basis of experimental and theoretical evidence, n-octanal photolysis predominantly proceeds to form Norrish type II products as the major ones.

Responsive Guest Encapsulation of Dynamic Conjugated Microporous Polymers

The host-guest complexes of conjugated microporous polymers encapsulating C60 and dye molecules have been investigated systematically. The orientation of guest molecules inside the cavities, have different terms: inside the open cavities of the polymer, or inside the cavities formed by packing different polymers. The host backbone shows responsive dynamic behavior in order to accommodate the size and shape of incoming guest molecule or guest aggregates. Simulations show that the host-guest binding of conjugated polymers is stronger than that of non-conjugated polymers. This detailed study could provide a clear picture for the host-guest interaction for dynamic conjugated microporous polymers. The mechanism obtained could guide designing new conjugated microporous polymers.

Ab initio and density functional theory study of keto-enol equilibria of deltic acid in gas and aqueous solution phase: a bimolecular proton transfer mechanism.

Keto-enol tautomerism in deltic acid (2,3-dihydroxycycloprop-2-en-1-one) has been studied using ab initio methods and the B3LYP functional of density functional theory, as well as complete basis set (CBS-QB3 and CBS-APNO) and G4 methods. Relative and absolute energies were calculated with each of the methods, whereas computations of geometries and harmonic frequencies for dihydroxycyclopropenone and hydroxycyclopropanedione were computed in the gas phase but were limited to HF, MP2, and the B3LYP functional, in combination with the 6-31++G(3df,3pd) basis set. Using the MP2/6-31++G(3df,3pd) gas phase optimized structure, each species was then optimized fully in aqueous solution by using the polarizable continuum model (PCM) self-consistent reaction field approach, in which HF, MP2, and B3LYP levels of theory were utilized, with the same 6-31++G(3df,3pd) basis set. In both gas and aqueous solution phases, the keto form is higher in energy for all of the model chemistries considered. From the B3LYP/6-31++G(3df,3pd) Gibbs free energy, the keto-enol tautomeric equilibrium constant for 2,3-dihydroxycycloprop-2-en-1-one/3-hydroxy-1,2-cyclopropanedione is computed to be K(T)(gas) = 2.768 × 10(-12) and K(T)(aq) = 5.469 × 10(-14). It is concluded that the enol form is overwhelmingly predominant in both environments.