Reply to the 'Comment on "Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2"' by C. J. Nielsen and Y. Tang, Phys. Chem. Chem. Phys., 2022, 24, DOI: 10.1039/D2CP03013F.

In this Reply, we answer the main argument raised in the Comment about the energy of the NO3 radical and its influence in the reaction profiles of the reaction of the NO3 radical with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2 by C. J. Nielsen and Y. Tang. The optimized geometry of the NO3 radical has been obtained using 49 DFT functionals: 26 functionals predict a minimum with D3h symmetry and 23 with C2v symmetry. The former functionals have been used to calculate the thermodynamic values of three reactions (X + HNO3 → XH + NO3, X= OH, CH3 and CCl3) and compared with experimental data. Those functionals with smaller errors have been used to recalculate the barriers of the reaction of NO3 with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2. The results show differences of 10.5 kJ mol-1 when compared to those obtained with the M08HX functional.

Photochemistry of oxidized Hg(I) and Hg(II) species suggests missing mercury oxidation in the troposphere.

Mercury (Hg), a global contaminant, is emitted mainly in its elemental form Hg0 to the atmosphere where it is oxidized to reactive HgII compounds, which efficiently deposit to surface ecosystems. Therefore, the chemical cycling between the elemental and oxidized Hg forms in the atmosphere determines the scale and geographical pattern of global Hg deposition. Recent advances in the photochemistry of gas-phase oxidized HgI and HgII species postulate their photodissociation back to Hg0 as a crucial step in the atmospheric Hg redox cycle. However, the significance of these photodissociation mechanisms on atmospheric Hg chemistry, lifetime, and surface deposition remains uncertain. Here we implement a comprehensive and quantitative mechanism of the photochemical and thermal atmospheric reactions between Hg0, HgI, and HgII species in a global model and evaluate the results against atmospheric Hg observations. We find that the photochemistry of HgI and HgII leads to insufficient Hg oxidation globally. The combined efficient photoreduction of HgI and HgII to Hg0 competes with thermal oxidation of Hg0, resulting in a large model overestimation of 99% of measured Hg0 and underestimation of 51% of oxidized Hg and ∼66% of HgII wet deposition. This in turn leads to a significant increase in the calculated global atmospheric Hg lifetime of 20 mo, which is unrealistically longer than the 3-6-mo range based on observed atmospheric Hg variability. These results show that the HgI and HgII photoreduction processes largely offset the efficiency of bromine-initiated Hg0 oxidation and reveal missing Hg oxidation processes in the troposphere.

The Chemistry of Mercury in the Stratosphere.

Mercury, a global contaminant, enters the stratosphere through convective uplift, but its chemical cycling in the stratosphere is unknown. We report the first model of stratospheric mercury chemistry based on a novel photosensitized oxidation mechanism. We find two very distinct Hg chemical regimes in the stratosphere: in the upper stratosphere, above the ozone maximum concentration, Hg0 oxidation is initiated by photosensitized reactions, followed by second-step chlorine chemistry. In the lower stratosphere, ground-state Hg0 is oxidized by thermal reactions at much slower rates. This dichotomy arises due to the coincidence of the mercury absorption at 253.7 nm with the ozone Hartley band maximum at 254 nm. We also find that stratospheric Hg oxidation, controlled by chlorine and hydroxyl radicals, is much faster than previously assumed, but moderated by efficient photo-reduction of mercury compounds. Mercury lifetime shows a steep increase from hours in the upper-middle stratosphere to years in the lower stratosphere.

Theoretical study of the NO3 radical reaction with CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2.

The potential reaction of the nitrate radical (NO3), the main nighttime atmospheric oxidant, with five alkyl halides, halons (CH2ClBr, CH2ICl, CH2BrI, CHCl2Br, and CHClBr2) has been studied theoretically. The most favorable reaction corresponds to a hydrogen atom transfer. The stationary points on the potential energy surfaces of these reactions have been characterized. The reactions can be classified into two groups based on the number of hydrogen atoms in the halon molecules (1 or 2). The reactions with halons with only one hydrogen atom show more exothermic profiles than those with two hydrogen atoms. In addition, the kinetics of the reaction of NO3 + CH2BrI was studied in much higher detail using a multi-well Master Equation solver as a representative example of the nitrate radical reactivity against these halocarbons. These results indicate that the chemical lifetime of the alkyl halides would not be substantially affected by nitrate radical reactions, even in the case of NO3-polluted atmospheric conditions.

Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury.

Mercury is a contaminant of global concern that is transported throughout the atmosphere as elemental mercury Hg0 and its oxidized forms HgI and HgII . The efficient gas-phase photolysis of HgII and HgI has recently been reported. However, whether the photolysis of HgII leads to other stable HgII species, to HgI , or to Hg0 and its competition with thermal reactivity remain unknown. Herein, we show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg0 by photolysis. Results are based on non-adiabatic dynamics simulations, in which the photoproduct ratios were determined with maximum errors of 3%. We construct for the first time a complete quantitative mechanism of the photochemical and thermal conversion between atmospheric HgII , HgI , and Hg0 compounds. These results reveal new fundamental chemistry that has broad implications for the global atmospheric Hg cycle. Thus, photoreduction clearly competes with thermal oxidation, with Hg0 being the main photoproduct of HgII photolysis in the atmosphere, which significantly increases the lifetime of this metal in the environment.

Gas-Phase Photolysis of Hg(I) Radical Species: A New Atmospheric Mercury Reduction Process.

The efficient gas-phase photoreduction of Hg(II) has recently been shown to change mercury cycling significantly in the atmosphere and its deposition to the Earth's surface. However, the photolysis of key Hg(I) species within that cycle is currently not considered. Here we present ultraviolet-visible absorption spectra and cross-sections of HgCl, HgBr, HgI, and HgOH radicals, computed by high-level quantum-chemical methods, and show for the first time that gas-phase Hg(I) photoreduction can occur at time scales that eventually would influence the mercury chemistry in the atmosphere. These results provide new fundamental understanding of the photobehavior of Hg(I) radicals and show that the photolysis of HgBr increases atmospheric mercury lifetime, contributing to its global distribution in a significant way.

Gas-Phase Acidities and Basicities of Alanines and N-Benzylalanines by the Extended Kinetic Method.

This paper reports an experimental determination of the gas-phase acidities and basicities of N-benzylalanines, in both their α and ß forms, by means of the extended kinetic method (EKM). The experimental gas-phase acidity of ß-alanine was also determined. Standard ab initio molecular orbital calculations at the G3 level were performed for alanines, and at the G3(MP2)//B3LYP level for N-benzylalanines. There is a very good agreement between the experimental and the calculated values. The more branched α-amino acids are more acidic and less basic than the linear ß-amino acids.

Energetic and Structural Properties of Two Phenolic Antioxidants: Tyrosol and Hydroxytyrosol.

Theoretical and experimental studies on the energetic, structural and some other relevant physicochemical properties of the antioxidant tyrosol (1), hydroxytyrosol (1OH) molecules and the corresponding radicals 1rad• and 1Orad• are reported in this work. The experimental values of the gas-phase enthalpy of formation, Δf Hm0(g), in kJ·mol-1, of 1 (-302.4 ± 3.4) and 1OH (-486.3 ± 4.1) have been determined. Quantum chemical calculations, at DFT (M05-2X) and composite ab initio G3 and G4 levels of theory, provided results that served to (i) confirm the excellent consistency of the experimental measurements performed, (ii) establish that the stabilizing effect of H-bond of hydroxyethyl chain and aromatic ring (OH···π interaction) is smaller in radicals than in parent molecules, (iii) deduce-combining experimental data in isodesmic reactions-Δf Hm0(g) of radicals 1rad• (-152.3 ± 4.4 kJ·mol-1) and 1Orad• (-370.6 ± 3.8 kJ·mol-1), (iv) estimate a reliable O-H bond dissociation enthalpy, BDE of 1 (368.1 ± 5.6 kJ·mol-1) and of 1OH (333.7 ± 5.6 kJ·mol-1), and (v) corroborate-using "BDE criteria"-than 1OH is a more effective antioxidant than 1.

Energy resolved mass spectrometry of chlorogenic acids and its application to isomer quantification by direct infusion tandem mass spectrometry.

INTRODUCTION: With the advent of high-perfomance liquid chromatography (HPLC)-tandem mass spectrometry (MS) using ion trap mass analysers it is possible to acquire unambigious structural information in particular with respect to aspects of regiochemistry and stereochemistry of organic compounds present in complex mixtures such as coffee extracts. However, HPLC-MS methods are resource extensive, laborious and lacking user friendliness. OBJECTIVE: To introduce a simple parameter - the energy threshhold for fragmentation - determined using energy resolved MS and demonstrate its value for the complete structural characterisation and even relative quantification of individual isomeric chlrogenic acids in direct infusion experiments. METHODOLOGY: Monocaffeoyl and dicaffeoyl quinic acids were investigated by direct infusion energy resolved mass spectrometry (ER-MS) in negative in mode, using a quadrupole ion trap and quadrupole time-of-flight (Q-TOF) mass spectrometer. Methanolic coffee extracts were quantitatively investigated by HPLC-MS and direct infusion ER-MS. RESULTS: Fragmentation occurs with retention of regiochemistry and regiochemistry of fragment ions can be determined using ER-MS. Analysis of breakdown graphs allows extraction of a single numerical parameter that allows assignment of regiochemistry. Analysis of monocaffeoyl and dicaffeoyl quinic acids revealed that regiosiomers could be distinguished and assigned based on their dissociation energies in collisional induced activation. Furthermore relative quantification of regioisomers by direct infusion ER-MS is possible within an error range of ±10% if compared with a conventional quantitative LC-MS method. CONCLUSION: ER-MS can be exploited in determining relative isomers quantities of chlorogenic acids (CGAs) in crude plant extracts by direct infusion tandem MS omitting time and resource intensive chromatographic separation.

Binary twinned-icosahedral [B21H18]- interacts with cyclodextrins as a precedent for its complexation with other organic motifs.

The weakly coordinating binary macropolyhedral anion closo,closo-[B21H18]- (B21; D3h symmetry) has been synthesized using a simplified strategy compared to that in the literature. While gas-phase complexes of B21 with ß- and γ-cyclodextrin (CD) were detected using ESI FT-ICR spectrometric measurements, α-CD did not bind to the B21 guest. This spectroscopic evidence has been interpreted using quantum-chemical computations, showing that ß- and γ-CD are able to interact with B21 due to their larger cavities, in contrast to the smaller α-CD. The hydridic B-H vectors of the B21 anion interact with K+ counterions and, via dihydrogen bonding, also with the partially positively charged hydrogens of the CD sugar units in the modeled ß- and γ-CD complexes. In summary, it has been shown by combined spectrometric/computational analysis that macropolyhedral boron hydride anions with two counterions can form stable complexes with ß- and γ-CD in the gas phase, offering a new perspective for the future investigation of this remarkable anion in the areas of supramolecular and medicinal chemistries.

Investigation of the molecular structure and VUV-induced ion dissociation dynamics of 2-azetidinone (C3 H5 NO).

RATIONALE: The class of active components of the group of ß-lactam antibiotics is very important for several fields and applications, although their stability and radiation reactivity properties are not yet well understood. We have studied the interaction of an important building block species, the 2-azetidinone (C3 H5 NO) molecule, with monochromatic VUV (synchrotron radiation) photons in the 9.5-21.5 eV range, using time-of-flight mass spectrometry (TOF-MS), electron-ion coincidence (PEPICO), and high-level theory methods. METHODS: A 2-azetidinine sample was introduced into the UH-vacuum chamber, without purification, through an inlet system for the gas-phase experiments with monochromatic light in the VUV range from the TGM beamline at the Brazilian Synchrotron Facility. A Wiley-McLaren type mass spectrometer in the PEPICO mode was employed to detect and characterize the photoionization and photodissociation products of the 2-azetidinone. The analysis and discussion of the results were supported by high-level density functional theory (DFT) and ab initio methods. RESULTS: The adiabatic ionization energy was determined experimentally in this work as 9.745 ± 0.020 eV, and this was supported by the high level of theory result with good agreement. The heat of formation for the 2-azetininone cation has been derived for the first time as 844.2 ± 1.9 kJ/mol. The dominant ion dissociation channel in the VUV energy range up to 21.5 eV is associated with the cation species at m/z 28. CONCLUSIONS: The structural properties, VUV-induced photoionization, and photodissociation dynamics of the 2-azetidinone molecule in the gas phase have been successfully investigated in the energy range of 9.5-21.5 eV. PEPICO mass spectra have been determined for the first time for this molecule at several selected photon energies from which the partial ion yields were determined for all cation species produced from this molecule.

Ab initio quantum-chemical computations of the electronic states in HgBr2 and IBr: Molecules of interest on the Earth's atmosphere.

The electronic states of atmospheric relevant molecules IBr and HgBr2 are reported, within the UV-Vis spectrum range (170nm≤λphoton≤600 nm) by means of the complete-active-space self-consistent field/multi-state complete-active-space second-order perturbation theory/spin-orbit restricted-active-space state-interaction (CASSCF/MS-CASPT2/SO-RASSI) quantum-chemical approach and atomic-natural-orbital relativistic-correlation-consistent (ANO-RCC) basis sets. Several analyses of the methodology were carried out in order to reach converged results and therefore to establish a highly accurate level of theory. Good agreement is found with the experimental data with errors not higher than around 0.1 eV. The presented analyses shall allow upcoming studies aimed to accurately determine the absorption cross sections of interhalogen molecules and compounds with Hg that are relevant to better comprehend the photochemical processes taking place in the atmosphere.

Static and dynamic properties of 1,1'-bi-2-naphthol and its conjugated acids and bases.

Several convergent techniques were used to characterize 1,1'-bi-2-naphthol (BINOL) and some of its properties. Its acidity in the gas-phase, from neutral species to monoanion, was measured by mass spectrometry. The conformation and structure of BINOL in the gas phase was determined by microwave rotational spectroscopy. NMR experiments in fluorosulfonic acid established that BINOL was monoprotonated on one of the hydroxyl oxygen atoms. The enantiomerization barriers reported in the literature for BINOL under neutral, basic, and acid conditions were analyzed with regard to the species involved. Finally, DFT calculations allowed all of these results to be gathered in a coherent picture of the BINOL structure.

Phase transition thermodynamics of bisphenols.

Herein we have studied, presented, and analyzed the phase equilibria thermodynamics of a bisphenols (BP-A, BP-E, BP-F, BP-AP, and BP-S) series. In particular, the heat capacities, melting temperatures, and vapor pressures at different temperatures as well as the standard enthalpies, entropies, and Gibbs energies of phase transition (fusion and sublimation) were experimentally determined. Also, we have presented the phase diagrams of each bisphenol derivative and investigated the key parameters related to the thermodynamic stability of the condensed phases. When all the bisphenol derivatives are compared at the same conditions, solids BP-AP and BP-S present lower volatilities (higher Gibbs energy of sublimation) and high melting temperatures due to the higher stability of their solid phases. Solids BP-A and BP-F present similar stabilities, whereas BP-E is more volatile. The introduction of -CH3 groups in BP-F (giving BP-E and BP-A) leads an entropic differentiation in the solid phase, whereas in the isotropic liquids the enthalpic and entropic differentiations are negligible.

Acidities of closo-1-COOH-1,7-C2B10H11 and amino acids based on icosahedral carbaboranes.

Carborane clusters are not found in Nature and are exclusively man-made. In this work we study, both experimentally and computationally, the gas-phase acidity (measured GA = 1325 kJ·mol(-1), computed GA = 1321 kJ·mol(-1)) and liquid-phase acidity (measured pKa = 2.00, computed pKa = 1.88) of the carborane acid closo-1-COOH-1,7-C2B10H11. The experimental gas-phase acidity was determined with electrospray tandem mass spectrometry (ESI/MS), by using the extended Cooks kinetic method (EKM). Given the similar spatial requirements of the title icosahedral cage and benzene and the known importance of aminoacids as a whole, such a study is extended, within an acid-base context, to corresponding ortho, meta, and para amino acids derived from icosahedral carborane cages, 1-COOH-n-NH2-1, n-R with {R = C2B10H10, n = 2, 7, 12}, and from benzene {R = C6H4, n = 2, 3, 4}. A remarkable difference is found between the proportion of neutral versus zwitterion structures in water for glycine and the carborane derived amino acids.

Energetic and structural study of bisphenols.

We have studied thermochemical, thermophysical and structural properties of bisphenols A, E, F, and AP. In particular, the standard enthalpies of sublimation and the standard enthalpies of formation in the gas phase at 298.15 K for all these species were experimentally determined. A computational study, through M05-2X density functional theory, of the various species shed light on structural effects and further confirmed, by means of the isodesmic reaction scheme, the excellent consistency of the experimental results. Our results reflect also the fact that energetic substituent effects are transferable from diphenylalkanes to bisphenols.

Gas phase acidity measurement of local acidic groups in multifunctional species: controlling the binding sites in hydroxycinnamic acids.

The applicability of the extended kinetic method (EKM) to determine the gas phase acidities (GA) of different deprotonable groups within the same molecule was tested by measuring the acidities of cinnamic, coumaric, and caffeic acids. These molecules differ not only in the number of acidic groups but in their nature, intramolecular distances, and calculated GAs. In order to determine independently the GA of groups within the same molecule using the EKM, it is necessary to selectively prepare pure forms of the hydrogen-bound heterodimer. In this work, the selectivity was achieved by the use of solvents of different vapor pressure (water and acetonitrile), as well as by variation of the drying temperature in the ESI source, which affected the production of heterodimers with different solvation energies and gas-phase dissociation energies. A particularly surprising finding is that the calculated solvation enthalpies of water and the aprotic acetonitrile are essentially identical, and that the different gas-phase products generated are apparently the result of their different vapor pressures, which affects the drying mechanism. This approach for the selective preparation of heterodimers, which is based on the energetics, appears to be quite general and should prove useful for other studies that require the selective production of heterodimers in ESI sources. The experimental results were supported by density functional theory (DFT) calculations of both gas-phase and solvated species. The experimental thermochemical parameters (deprotonation ΔG, ΔH, and ΔS) are in good agreement with the calculated values for the monofunctional cinnamic acid, as well as the multifunctional coumaric and caffeic acids. The measured GA for cinnamic acid is 334.5 ± 2.0 kcal/mol. The measured acidities for the COOH and OH groups of coumaric and caffeic acids are 332.7 ± 2.0, 318.7 ± 2.1, 332.2 ± 2.0, and 317.3 ± 2.2 kcal/mol, respectively.

Understanding the fragmentation of glucose in mass spectrometry.

The fragmentation mechanism of D-glucose was investigated in detail by two different fragmentation techniques, namely, collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) using all six 13 C-labeled isotopomers and 2 H-labeled isotopomers. For both CID and IRMPD energy-resolved measurements were carried out. Individual fragmentation pathways were studied at MS2 and MS3 levels. Additionally, we have developed an HPLC-tandem MS method to separate the anomers of D-glucose using a HILIC column and investigated their fragmentation patterns individually. We propose a complete fragmentation landscape of D-glucose, demonstrating that a rather simple multifunctional molecule displays extreme complexity in gas phase dissociation, following multiple parallel fragmentation routes yielding a total of 23 distinct fragment ions. The results allowed a detailed formulation of the complex fragmentation mechanism of D-glucose. The results have immediate consequences for the full structure analysis of complex carbohydrates.

Can an amine be a stronger acid than a carboxylic acid? The surprisingly high acidity of amine-borane complexes.

The gas-phase acidity of a series of amine-borane complexes has been investigated through the use of electrospray mass spectrometry (ESI-MS), with the application of the extended Cooks kinetic method, and high-level G4 ab initio calculations. The most significant finding is that typical nitrogen bases, such as aniline, react with BH(3) to give amine-borane complexes, which, in the gas phase, have acidities as high as those of either phosphoric, oxalic, or salicylic acid; their acidity is higher than many carboxylic acids, such as formic, acetic, and propanoic acid. Indeed the complexation of different amines with BH(3) leads to a substantial increase (from 167 to 195 kJ mol(-1)) in the intrinsic acidity of the system; in terms of ionization constants, this increase implies an increase as large as fifteen orders of magnitude. Interestingly, this increase in acidity is almost twice as large as that observed for the corresponding phosphine-borane analogues. The agreement between the experimental and the G4-based calculated values is excellent. The analysis of the electron-density rearrangements of the amine and the borane moieties indicates that the dative bond is significantly stronger in the N-deprotonated anion than in the corresponding neutral amine-borane complex, because the deprotonated amine is a much better electron donor than the neutral amine. On the top of that, the newly created lone pair on the nitrogen atom in the deprotonated species, conjugates with the BN bonding pair. The dispersion of the extra electron density into the BH(3) group also contributes to the increased stability of the deprotonated species.

Energetics and structural properties, in the gas phase, of trans-hydroxycinnamic acids.

We have studied the energetics and structural properties of trans-cinnamic acid (CA), o-, m-, and p-coumaric acids (2-, 3-, and 4-hydroxycinnamic acids), caffeic acid (3,4-dihydroxycinnamic acid), ferulic acid (4-hydroxy-3-methoxycinnamic acid), iso-ferulic acid (3-hydroxy-4-methoxycinnamic acid), and sinapic acid (3,5-dimethoxy-4-hydroxycinnamic acid). The experimental values of Δ(f)H(m)°(g), determined (in kJ·mol(-1)) for CA (-229.8 ± 1.9), p-coumaric acid (-408.0 ± 4.4), caffeic acid (-580.0 ± 5.9), and ferulic acid (-566.4 ± 5.7), allowed us to derive Δ(f)H(m)°(g) of o-coumaric acid (-405.6 ± 4.4), m-coumaric acid (-406.4 ± 4.4), iso-ferulic acid (-565.2 ± 5.7), and sinapic acid (-698.8 ± 4.1). From these values and by use of isodesmic/homodesmotic reactions, we studied the energetic effects of π-donor substituents (-OH and -OCH(3)) in cinnamic acid derivatives and in the respective benzene analogues. Our results indicate that the interaction between -OCH(3) and/or -OH groups in hydroxycinnamic acids takes place without significant influence of the propenoic fragment.