Observation of Solvent-Dependence in the Mechanism of Neutral-Catalyzed Isomerization of para-Aminobenzoic Acid Protomers.

Proton-transfer reactions are commonplace during electrospray ionization (ESI) mass spectrometry experiments and are often responsible for imparting charge to analyte molecules. Multiple protonation-site isomers (protomers) can arise for polyfunctional molecules and these isomers can interconvert via solvent-mediated proton transfer reactions during various stages of the ESI process. Studying the populations and interconversion of protonation isomers provides key insight into the ESI process, ion-molecule interactions, and ion dissociation mechanisms. An archetype molecule to study protomer interconversion fundamentals in this context is para-aminobenzoic acid (pABA), where both the amino and carboxylic acid protomers are typically formed under ESI and the mechanisms for interconversion are still under refinement. Using ion-trap mass spectrometry reaction kinetics (2.5 mTorr, 300 K), this study examines gas-phase interconversion catalysis of pABA protomers by seven neutral species, which are commen solvents and additives used for ESI: water, formic acid, methanol, ethanol, propanol, ammonia, and acetonitrile. Three distinct reaction cases are reported: (i) formic acid, methanol, ethanol, propanol, and ammonia each catalyze the interconversion between the amino and carboxylic acid protomers via a n = 1 solvent-molecule vehicle mechanism; (ii) for water, however, a n = 6 adduct complex is detected and this suggests that the observed protomer interconversion occurs through a Grotthuss mechanism, in accord with literature reports; (iii) acetonitrile inhibits proton transfer by the formation of particularly stable n = 1 and 2 adduct complexes. The second-order rate constants for the protomer interconversion are observed to increase in the following order: H2O < HCO2H < MeOH < EtOH < PrOH < NH3. Potential energy schemes are reported for all neutral-catalyzed proton transfer reactions using the DSD-PBEP86-D3(BJ)/aug-cc-pVDZ level of theory. A central transition state, which connects the protonation site adducts, is shown to be the key rate-limiting step. The energy of this transition state is sensitive to the proton affinity of the neutral solvent, and this is supported by the correlation between the reaction rate and the solvent proton affinity.

Gas-Phase Phenyl Radical + O2 Reacts via a Submerged Transition State.

In the gas-phase chemistry of the atmosphere and automotive fuel combustion, peroxyl radical intermediates are formed following O2 addition to carbon-centered radicals which then initiate a complex network of radical reactions that govern the oxidative processing of hydrocarbons. The rapid association of the phenyl radical-a fundamental radical related to benzene-with O2 has hitherto been modeled as a barrierless process, a common assumption for peroxyl radical formation. Here, we provide an alternate explanation for the kinetics of this reaction by deploying double-hybrid density functional theory (DFT), at the DSD-PBEP86-D3(BJ)/aug-cc-pVTZ level of theory, and locate a submerged adiabatic transition state connected to a prereaction complex along the reaction entrance pathway. Using this potential energy scheme, experimental rate coefficients k(T) for the addition of O2 to the phenyl radical are accurately reproduced within a microcanonical kinetic model. This work highlights that purportedly barrierless radical oxidation reactions may instead be modeled using stationary points, which in turn provides insight into pressure and temperature dependence.

Divergent Reactivity of 1,2,3-Benzotriazin-4(3H)-ones: Photocatalytic Synthesis of 3-Substituted Isoindolinones Achieved through a Nitrogen-Mediated Hydrogen Atom Shift.

A regioselective visible-light-mediated denitrogenative alkene insertion of 1,2,3-benzotriazin-4(3H)-ones was developed to access 3-substituted isoindolinones, an important structural motif present in many biologically active molecules and natural products. Notably, divergent reactivity was achieved by switching from reported nickel catalysis (where C3-substituted 3,4-dihydroisoquinolin-1(2H)-ones form) to photocatalysis, where photocatalytic denitrogenation and a subsequent nitrogen-mediated hydrogen atom shift lead to exclusive 3-substituted isoindolinone formation. The developed photocatalytic reaction is compatible with activated terminal alkenes and cyclic α,ß-unsaturated esters and ketones, with wide functional group tolerance for N-substitution of the 1,2,3-benzotriazin-4(3H)-ones. The utility of this procedure is highlighted by a gram-scale synthesis and postsynthetic amidation. To understand the origin of this unique product selectivity, experimental and computational mechanistic studies were performed.

A Photoreactor-Interfaced Mass Spectrometer: An Online Platform to Monitor Photochemical Reactions.

An experimental platform is reported that allows for the online characterization of photochemical reactions by coupling a continuous flow photoreactor, equipped with LED light irradiation and a dual-tipped ESI source, directly to a mass spectrometer with electrospray ionization. The capabilities of this platform are demonstrated with two classes of photoreactions: (1) the photopolymerization of methyl methacrylate and (2) photocatalyzed alkyne insertion into a 1,2,3-benzotriazinone. The online technique provides rapid information to inform the underlying photochemical mechanism and evaluate the overall photochemistry.

Gas-Phase Internal Proton-Transfer of Protonated para-Aminobenzoic Acid Catalyzed by One Methanol Molecule.

Electrospray ionization (ESI) is used to deliver analytes for mass analysis across a huge range of mass spectrometry applications. Despite its ubiquitous application and many mechanistic investigations, it remains that a fundamental understanding of ESI processes is not complete. In particular, all the factors that influence the populations of protonation isomers are elusive such that it remains a challenge to optimize experimental conditions to favor one isomer over another. The molecule para-aminobenzoic acid has emerged as an archetype for the study of protonation isomers, with both amino and carboxylic acid protonation site isomers (protomers) typically formed upon ESI, with the isomer ratio shown to be sensitive to several physical and chemical parameters. Here we report an ion-trap mass spectrometry study of the time-resolved methanol-catalyzed proton transfer between the amine and carboxylic acid moieties of para-aminobenzoic acid. The experimental and computational results presented are consistent with a bimolecular mechanism where isomerization is mediated by a single methanol rather than a multimolecular Grotthuss proton transfer process. Pseudo-first-order rate constants for protomer specific product ions are reported and confirm the depletion of the amino protomer is correlated to the growth of the carboxylic acid protomer. Under the controlled conditions of a low-pressure ion-trap mass spectrometer (2.5 mTorr, 300 K), the number of methanol molecules required to isomerize para-aminobenzoic acid is determined to be one, and the second-order rate constant for methanol-catalyzed isomerization is (1.9 ± 0.1) × 10-11 cm3 molecule-1 s-1. The para-aminobenzoic acid vehicle mechanism is explored computationally at the DSD-PBEP86-D3BJ/aug-cc-pVDZ level of theory and reveals that the transition state for proton transfer is submerged (-10 kJ mol-1) relative to the separated reactant energies. The findings from this paper show that single-solvent catalyzed intramolecular proton transfer reactions are possible and must be considered during the late stages of ESI to predict the site(s) of protonation and the ion's stability in the presence of solvent molecules.

Protonation Isomer Specific Ion-Molecule Radical Reactions.

Through a combination of ion-mobility filtering and laser-equipped quadrupole ion-trap mass spectrometry, the gas-phase reaction kinetics of two protonation isomers of the distonic-radical quinazoline cation are independently measured with ethylene. For these radical addition reactions, protonation site variations drive significant changes in nearby radical reactivity, and this is primarily due to through-space electrostatic effects. Furthermore, quantum chemical methods specifically designed for calculating long-range interactions, such as double-hybrid density functional theory, are required to rationalize the experimentally measured difference in reactivity.

Modelling reaction kinetics of distonic radical ions: a systematic investigation of phenyl-type radical addition to unsaturated hydrocarbons.

Gas phase ion-molecule reactions are central to chemical processes across many environments. A feature of many of these reactions is an inverse relationship between temperature and reaction rate arising from a submerged barrier (an early reaction barrier that is below the energy of the separated reactants), which often arises due to a stable pre-reactive complex. While the thermodynamics and kinetics of many ion-molecule reactions have been extensively modelled, the reaction kinetics of ion-molecule reactions involving radical ions are less explored. In this investigation, the target reactions involve distonic radical ions, where the charge and radical moieties are separated within the molecular structure. Experimental rate coefficients for the reaction of either C2H2 or C2H4 with a suite of eighteen distonic radical ions are reported. Rate coefficients are modelled using potential energy schemes combined with a statistical reaction-rate (RRKM-ME) model. Second-order rate coefficients are in good agreement with experimental values with an average RMS deviation of 37% across three orders of magnitude. These predictions are generally sensitive to the relative energetics of the pre-reactive complex forward transition state but are relatively insensitive to the overall exothermicity of the covalent-addition product.

Picosecond excited-state lifetimes of protonated indazole and benzimidazole: The role of the N-N bond.

Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here, we target the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump-probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0-5.4 eV). The 1ππ* lifetimes of protonated indazole range from 390 ± 70 ps using 4.0 eV pump energy to ≤18 ps using 4.6 eV pump energy. The 1ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 ± 1100 ps at 4.6 eV pump energy to 400 ± 200 ps at 5.4 eV. Based on these experimental results and accompanying quantum chemical calculations and potential energy surfaces, the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N-N bond.

Five vs. six membered-ring PAH products from reaction of o-methylphenyl radical and two C3H4 isomers.

Gas-phase reactions of the o-methylphenyl (o-CH3C6H4) radical with the C3H4 isomers allene (H2C[double bond, length as m-dash]C[double bond, length as m-dash]CH2) and propyne (HC[triple bond, length as m-dash]C-CH3) are studied at 600 K and 4 Torr (533 Pa) using VUV synchrotron photoionisation mass spectrometry, quantum chemical calculations and RRKM modelling. Two major dissociation product ions arise following C3H4 addition: m/z 116 (CH3 loss) and 130 (H loss). These products correspond to small polycyclic aromatic hydrocarbons (PAHs). The m/z 116 signal for both reactions is conclusively assigned to indene (C9H8) and is the dominant product for the propyne reaction. Signal at m/z 130 for the propyne case is attributed to isomers of bicyclic methylindene (C10H10) + H, which contains a newly-formed methylated five-membered ring. The m/z 130 signal for allene, however, is dominated by the 1,2-dihydronaphthalene isomer arising from a newly created six-membered ring. Our results show that new ring formation from C3H4 addition to the methylphenyl radical requires an ortho-CH3 group - similar to o-methylphenyl radical oxidation. These reactions characteristically lead to bicyclic aromatic products, but the structure of the C3H4 co-reactant dictates the structure of the PAH product, with allene preferentially leading to the formation of two six-membered ring bicyclics and propyne resulting in the formation of six and five-membered bicyclic structures.

Reactivity Trends in the Gas-Phase Addition of Acetylene to the N-Protonated Aryl Radical Cations of Pyridine, Aniline, and Benzonitrile.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10-12 cm3 molecule-1 s-1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10-12 cm3 molecule-1 s-1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10-11 cm3 molecule-1 s-1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.