Substituent Effect in the Reactions between Criegee Intermediates and 3-Aminopropanol.

Via intramolecular H atom transfer, 3-aminopropanol is more reactive toward Criegee intermediates, in comparison with amines or alcohols. Here we accessed the substituent effect of Criegee intermediates in their reactions with 3-aminopropanol. Through real-time monitoring the concentrations of two Criegee intermediates with their strong UV absorption at 340 nm, the experimental rate coefficients at 298 K (100-300 Torr) were determined to be (1.52 ± 0.08) × 10-11 and (1.44 ± 0.22) × 10-13 cm3 s-1 for the reactions of 3-aminopropanol with (CH3)2COO (acetone oxide) and CH2CHC(CH3)OO (methyl vinyl ketone oxide), respectively. Compared to our previous experimental value for the reaction with syn-CH3CHOO, (1.24 ± 0.13) × 10-11 cm3 s-1, we can see that the methyl substitution at the anti position has little effect on the reactivity while the vinyl substitution causes a drastic decrease in the reactivity. Our theoretical calculations based on CCSD(T)-F12 energies reproduce this 2-order-of-magnitude decrease in the rate coefficient caused by the vinyl substitution. Using the activation strain model, we found that the interaction of Criegee intermediates with 3-aminopropanol is weaker for the case of vinyl substitution. This effect can be further rationalized by the delocalization of the lowest unoccupied molecular orbital for the vinyl-substituted Criegee intermediates. These results would help us better estimate the impact of similar reactions like the reactions of Criegee intermediates with water vapor, some of which could be difficult to measure experimentally but can be important in the atmosphere. We also found that the B2PLYP-D3BJ/aug-cc-pVTZ calculation can reproduce the CCSD(T)-F12 reaction barrier energies within ca. 1 kcal mol-1, indicating that the use of the B2PLYP-D3BJ method is promising for future predictions of the reactions of larger Criegee intermediates.

Reactions of Criegee Intermediates are Enhanced by Hydrogen-Atom Relay Through Molecular Design.

We report a type of highly efficient double hydrogen atom transfer (DHAT) reaction. The reactivities of 3-aminopropanol and 2-aminoethanol towards Criegee intermediates (syn- and anti-CH3 CHOO) were found to be much higher than those of n-propanol and propylamine. Quantum chemistry calculation has confirmed that the main mechanism of these very rapid reactions is DHAT, in which the nucleophilic attack of the NH2 group is catalyzed by the OH group which acts as a bridge of HAT. Typical gas-phase DHAT reactions are termolecular reactions involving two hydrogen bonding molecules; these reactions are typically slow due to the substantial entropy reduction of bringing three molecules together. Putting the reactive and catalytic groups in one molecule circumvents the problem of entropy reduction and allows us to observe the DHAT reactions even at low reactant concentrations. This idea can be applied to improve theoretical predictions for atmospherically relevant DHAT reactions.

Screening Enzymatic Degradation of Polyester Polyurethane with Fluorescent Single-walled Carbon Nanotube and Polymer Nanoparticle Conjugates.

Enzymatic biodegradation is a promising method to reclaim plastic materials. However, to date, a high-throughput method for screening potential enzyme candidates for biodegradation is still lacking. Here, we propose a single-walled carbon nanotube (SWCNT) fluorescence sensor for screening the enzymatic degradation of polyester polyurethane nanoparticles. Through wrapping the SWCNT with cationic chitosan, an electrostatic bond is formed between the SWCNT and Impranil, a widely applied model substrate of polyester polyurethane. As Impranil is being degraded by the enzymes, a characteristic quenching at a short reaction time followed by a brightening at a longer reaction time in the fluorescence signal is observed. The time-dependent fluorescence response is compared with turbidity measurement, and we conclude that the brightening in fluorescence results from the binding of the degradation product with the SWCNT. The proposed SWCNT sensor design has the potential to screen enzyme candidates for selective degradation of other plastic particles.

Unimolecular decomposition rates of a methyl-substituted Criegee intermediate syn-CH3CHOO.

Criegee intermediates play important roles in atmospheric chemistry. Methyl Criegee intermediate, CH3CHOO, has two conformers, syn- and anti-conformers. Syn-CH3CHOO would undergo fast unimolecular decomposition to form OH radical via 1,4 H-atom transfer. In this work, unimolecular decomposition of syn-CH3CHOO was probed in real time with UV absorption spectroscopy at 278-318 K and 100-700 torr. We used water vapor as the scavenger of anti-CH3CHOO to distinguish the absorption signals of the two conformers. After removing the contributions from reactions with radical byproducts, reaction with water vapor and wall loss, we obtained the unimolecular reaction rate coefficient of syn-CH3CHOO (at 300 torr), which increases from (67 ± 15) s-1 at 278 K, (146 ± 31) s-1 at 298 K, to (288 ± 81) s-1 at 318 K with an Arrhenius activation energy of ca. 6.4 kcal mol-1 and a weak pressure dependence for 100-700 torr. Compared to previous studies, this work provides temperature dependent unimolecular rates of syn-CH3CHOO at higher pressures, which are more relevant to atmospheric conditions.