Coupled Unimolecular Dissociation Kinetics of Bromotoluene Radical Cations

The unimolecular dissociations of <i>o</i>-, <i>m</i>-, and <i>p</i>-bromotoluene radical cations to C₇H₇⁺ (benzylium and tropylium) are examined by considering the coupling of the three isomers in the dissociation pathways. The potential energy surface obtained from ab initio calculations suggests the interconversion of isomers through methylene and hydrogen migrations on the ring. The rate equations for each isomer are combined together to form a rate matrix for coupled reactions. The rate matrix contains the microcanonical rate constants for all elementary steps, which are calculated using Rice–Ramsperger–Kassel–Marcus theory based on the molecular parameters obtained from density functional theory. The unimolecular dissociation rates for coupled reactions are determined by numerically solving the matrix equation. As a result of reaction coupling, the product branching ratio becomes time-dependent and the reaction rates of three isomers become parallel to one another as the energy increases, although their initial rates differently vary with energy. The calculated rate–energy curves fall below the time-resolved photodissociation data in the energy range 2.2–2.7 eV but are in line with the photoelectron photoion coincidence data in the energy range 2.7–3.5 eV. The discrepancy between experiment and theory in the low-energy region is ascribed to the uncertainties of the potential energy surface as well as the contribution of the radiative relaxation rate that has not been taken into account in the theoretical calculations. The rate–energy curves are then used to calculate the thermal reaction rate constants, and the Arrhenius parameters are determined in the temperature range 700–1300 K. Comparison of the activation energy and entropy obtained from the Arrhenius plot with the calculated enthalpy and entropy changes between the reactant and the highest-lying transition state suggests that a series of [1,2] H-atom migrations occurring near the entrance comprise the rate-determining steps and the subsequent [1,2] H-atom migrations play an important role in increasing the activation energy and decreasing the entropy by reducing the net flux to the exit

Surface-Dependent, Ligand-Mediated Photochemical Etching of CdSe Nanoplatelets

Photochemical etching of CdSe nanoplatelets was studied to establish a relationship between the nanocrystal surface and the photochemical activity of an exciton. Nanoplatelets were synthesized in a mixture of octylamine and oleylamine for the wurtzite (W) lattice or in octadecene containing oleic acid for the zinc-blende (ZB) lattice. For photochemical etching, nanoplatelets were dispersed in chloroform containing oleylamine and tributylphosphine in the absence or presence of oleic acid and then irradiated with light at the band-edge absorption maxima. Etching phenomena were characterized using UV–vis absorption spectroscopy and transmission electron microscopy. The absorption spectra of both W and ZB CdSe nanoplatelets showed that the exciton was confined in one dimension along the thickness. However, the two nanoplatelets presented different etching kinetics and erosion patterns. The rate of etching for W CdSe nanoplatelets was much faster than that for ZB nanoplatelets. Small holes were uniformly perforated on the planar surface of W nanoplatelets, whereas the corners and edges of ZB nanoplatelets were massively eroded without a significant perforation on the planar surface. This suggests that the amine-passivated surface of trivalent cadmium atoms on CdSe nanoplatelets is photochemically active, but the carboxylate-passivated surface of divalent cadmium atoms is not. Hence, the ligand, which induces the growth of W or ZB CdSe nanoplatelets, mediates the surface-dependent photochemical etching. This result implies that an electron–hole pair can be extracted from the planar surface of amine-passivated W nanoplatelets but from the corners and edges of carboxylate-passivated ZB nanoplatelets

N‑Acylated Dipeptide Tags Enable Precise Measurement of Ion Temperature in Peptide Fragmentation

Peptide fragmentations into b- and y-type ions are useful for the identification of proteins. The b ion, having the structure of a N-protonated oxazolone, dissociates to the a-type ion with loss of CO. This CO-loss process affords the possibility of characterizing the temperature of the b ion. Herein, we used N-acylated dipeptide tags, isobaric tags originally developed for protein quantification, as internal standards for the measurement of the ion temperature in peptide fragmentation. Amine-reactive dipeptide tags were attached to the N-termini of sample peptides. Collision-induced dissociation (CID) of the tagged peptides yielded a b-type quantitation signal (b_S) from the tag, which subsequently dissociated into the a_S ion with CO-loss. As the length of alkyl side chain on the dipeptide tag was extended from C₁ to C₈, the yield of a_S ion gradually increased for the 4-alkyl-substituted oxazolone ion but decreased for the 2-alkyl-substituted one. To gain insights into the unimolecular dissociation kinetics, we obtained the potential energy surface from ab initio calculations. Theoretical study suggested that the 4-alkyl substitution on N-protonated oxazolone decreased the enthalpy of activation by stabilizing the productlike transition state, whereas the 2-alkyl substitution increased it by stabilizing the reactant. Resulting potential energy surfaces were used to calculate the microcanonical and canonical rate constants as well as the a_S-ion yield. Arrhenius plots of canonical rate constants provided activation energies and pre-exponential factors for the CO-loss processes in the 600–800 K range. Comparison of experimental a_S-ion yields with theoretical values led to precise determination of the temperature of b_S ion. Thus, the b_S-ion temperature of tagged peptide can be measured simply by combining kinetic parameters provided here and a_S-ion yields obtained experimentally. Although the b-type fragment patterns varied with the chain length and position of alkyl substituent on the N-protonated oxazolone, the y-type fragment patterns were almost identical under these conditions. Furthermore, b_S-ion temperatures were nearly the same with only a few degrees K difference. Our results demonstrate a novel use of N-acylated dipeptide tags as internal temperature standards, which enables the reproducible acquisition of peptide fragment spectra

Halogen−π Interactions between Benzene and X₂/CX₄ (X = Cl, Br): Assessment of Various Density Functionals with Respect to CCSD(T)

Various types of interactions between halogen (X) and π moiety (X−π interaction) including halogen bonding play important roles in forming the structures of biological, supramolecular, and nanomaterial systems containing halogens and aromatic rings. Furthermore, halogen molecules such as X₂ and CX₄ (X = Cl/Br) can be intercalated in graphite and bilayer graphene for doping and graphene functionalization/modification. Due to the X−π interactions, though recently highly studied, their structures are still hardly predictable. Here, using the coupled-cluster with single, double, and noniterative triple excitations (CCSD­(T)), the Møller–Plesset second-order perturbation theory (MP2), and various flavors of density functional theory (DFT) methods, we study complexes of benzene (Bz) with halogen-containing molecules X₂ and CX₄ (X = Cl/Br) and analyze various components of the interaction energy using symmetry adapted perturbation theory (SAPT). As for the lowest energy conformers (S1), X₂–Bz is found to have the T-shaped structure where the electropositive X atom-end of X₂ is pointing to the electronegative midpoint of CC bond of the Bz ring, and CX₄–Bz has the stacked structure. In addition to this CX₄–Bz (S1), other low energy conformers of X₂–Bz (S2/S3) and CX₄–Bz (S2) are stabilized primarily by the dispersion interaction, whereas the electrostatic interaction is substantial. Most of the density functionals show noticeable deviations from the CCSD­(T) complete basis set (CBS) limit binding energies, especially in the case of strongly halogen-bonded conformers of X₂–Bz (S1), whereas the deviations are relatively small for CX₄–Bz where the dispersion is more important. The halogen bond shows highly anisotropic electron density around halogen atoms and the DFT results are very sensitive to basis set. The unsatisfactory performance of many density functionals could be mainly due to less accurate exchange. This is evidenced from the good performance by the dispersion corrected hybrid and double hybrid functionals. B2GP-PLYP-D3 and PBE0-TS­(Tkatchenko-Scheffler)/D3 are well suited to describe the X−π interactions adequately, close to the CCSD­(T)/CBS binding energies (within ∼1 kJ/mol). This understanding would be useful to study diverse X−π interaction driven structures such as halogen containing compounds intercalated between 2-dimensional layers