High Proton Affinity Research Articles

Role of the amine and phosphine groups in oleylamine and trioctylphosphine in the synthesis of copper chalcogenide nanoparticles

The effect of the functional groups of capping agents was investigated in the synthesis of copper selenide, copper sulphide and copper oxide nanoparticles using oleylamine (OLA) and trioctylphosphine (TOP). These capping molecules have demonstrated their ability to act as reducing agents, surfactants, solvents and enhancement of colloidal stabilization. They also offer electron donating abilities from the two group 5A elements, P and N. Nitrogen atom in an amine group possess stronger surface interactions and higher basicity than P atom in the phosphines. Copper chalcogenide nanoparticles were prepared using Hot-injection method and characterized using UV/Vis spectroscopy, TEM and XRD. The optical and structural properties of the yielded nanoparticles showed dependence on the type of capping interactions from the two agents. Nanoparticles synthesized using TOP produced two phases whereas a single phase was observed from OLA as confirmed by XRD. Although TOP and OLA exhibit similar features, but their affinity to metals differs resulting to significant different morphology and crystallinity of the produced nanoparticles. Amine group has higher affinity for protons than phosphine due to the lone pair of electrons it possesses which it easily donates to H+ compared to phosphine. The high proton affinity of oleylamine makes it interact faster than trioctylphosphine. OLA in overall produced larger particle sizes compared to TOP but generated a wider variety of shapes.

Protonation of Planar and Nonplanar Porphyrins: A Calorimetric and Computational Study.

Herein, we report the first calorimetric study of the protonation of planar and nonplanar free-base porphyrins: H2OETPP (strongly saddled by its substituents), H2T(tBu)P (strongly ruffled by its substituents), and the nominally planar porphyrins (npPs) H2OEP, H2TPP, H2T(nPe)P, and H2T(iPr)P. The observed enthalpies of protonation in solution (ΔHprotsoln) for formation of the dications in 1,1,2,2-tetrachloroethane with 2% trifluoroacetic acid are -45 ± 1 kcal mol-1 for the npPs, -52.0 kcal mol-1 for H2T(tBu)P, and -70.9 kcal mol-1 for H2OETPP. The corresponding enthalpies of protonation (ΔHDFT) obtained from DFT calculations (-27 ± 5, -42, and -63 kcal mol-1, respectively) reproduce this trend. The much more negative enthalpy of protonation seen for H2OETPP is consistent with this molecule being pre-deformed into the saddle structure favored by porphyrin dications. Except for OETPP, the calculated enthalpies of the first protonations (ΔH1) are significantly more positive than the enthalpies of the second protonations (ΔH2). In addition, the structural strain energies for the first protonations (ΔEst(1)) are also significantly more positive than ΔEst(2). According to the calculations, the monocations thus have higher proton affinities than the corresponding free-base porphyrins due to a structural strain effect, which is consistent with the generally elusive nature of the porphyrin monocation. The recent observations of monocations for free-base porphyrins with a high degree of saddling can be rationalized in terms of ΔH1 and ΔH2 being similar; so, the monocation is no longer an unstable intermediate.

Discovery of HC3O+ in space: The chemistry of O-bearing species in TMC-1.

Using the Yebes 40m and IRAM 30m radio telescopes, we detected a series of harmonically related lines with a rotational constant B 0=4460.590±0.001 MHz and a distortion constant D 0=0.511 ±0.005 kHz towards the cold dense core TMC-1. High-level-of-theory ab initio calculations indicate that the best possible candidate is protonated tricarbon monoxide, HC3O+. We have succeeded in producing this species in the laboratory and observed its J u -J l = 2-1 and 3-2 rotational transitions. Hence, we report the discovery of HC3O+ in space based on our observations, theoretical calculations, and laboratory experiments. We derive an abundance ratio N(C3O)/N(HC3O+)~7. The high abundance of the protonated form of C3O is due to the high proton affinity of the neutral species. The chemistry of O-bearing species is modelled, and predictions are compared to the derived abundances from our data for the most prominent O-bearing species in TMC-1.

In situ monitoring of photocatalyzed isomerization reactions on a microchip flow reactor by IR-MALDI ion mobility spectrometry

The visible-light photocatalytic E/Z isomerization of olefins can be mediated by a wide spectrum of triplet sensitizers (photocatalysts). However, the search for the most efficient photocatalysts through screenings in photo batch reactors is material and time consuming. Capillary and microchip flow reactors can accelerate this screening process. Combined with a fast analytical technique for isomer differentiation, these reactors can enable high-throughput analyses. Ion mobility (IM) spectrometry is a cost-effective technique that allows simple isomer separation and detection on the millisecond timescale. This work introduces a hyphenation method consisting of a microchip reactor and an infrared matrix-assisted laser desorption ionization (IR-MALDI) ion mobility spectrometer that has the potential for high-throughput analysis. The photocatalyzed E/Z isomerization of ethyl-3-(pyridine-3-yl)but-2-enoate (E-1) as a model substrate was chosen to demonstrate the capability of this device. Classic organic triplet sensitizers as well as Ru-, Ir-, and Cu-based complexes were tested as catalysts. The ionization efficiency of the Z-isomer is much higher at atmospheric pressure which is due to a higher proton affinity. In order to suppress proton transfer reactions by limiting the number of collisions, an IM spectrometer working at reduced pressure (max. 100 mbar) was employed. This design reduced charge transfer reactions and allowed the quantitative determination of the reaction yield in real time. Among 14 catalysts tested, four catalysts could be determined as efficient sensitizers for the E/Z isomerization of ethyl cinnamate derivative E-1. Conversion rates of up to 80% were achieved in irradiation time sequences of 10 up to 180 s. With respect to current studies found in the literature, this reduces the acquisition times from several hours to only a few minutes per scan.

Catalytic hydrolysis of β-lactam antibiotics via MOF-derived MgO nanoparticles embedded on nanocast silica

The antibiotics abuse and the proliferation of antibiotic-resistant bacteria in the environment have a severe impact on both human health and ecosystem. In this study, a silica-nanocasting method was applied in Mg-MOF-74 template to generate a series of MgO/SiO2 catalysts for the hydrolysis of β-lactam antibiotics. The Mg-based subunits in MOF-74 were converted to highly dispersed MgO nanoparticles with controllable particle size. MgO/SiO2-80 with the smallest MgO particle size exhibits the best catalytic performance in the hydrolysis of four β-lactam antibiotics. The kinetics study reveals the higher degradation rate and lower activation energy of MgO/SiO2-80 than other benchmark solid base catalysts. The proposed mechanism suggests that small MgO particle size provides more accessible oxygen anions with high proton affinity for the cleavage of the β-lactam ring, so that all hydrolytic products lose antimicrobial activity. The MgO/SiO2-80 serves as the potential high-performance solid base catalyst for the real-world antibiotic wastewater treatment.

Isomeric Differentiation and Acidic Metabolite Identification by Piperidine-Based Tagging, LC-MS/MS, and Understanding of the Dissociation Chemistries.

We demonstrate a method for facile differentiation of acidic, isomeric metabolites by attaching high proton affinity, piperidine-based chemical tags to each carboxylic acid group. These tags attach with high efficiency to the analytes, increase the signal, and result in the formation of multiply-charged cations. We illustrate the present approach with citrate and isocitrate, which are isomeric metabolites each containing three carboxylic acid groups. We observe a 20-fold increase in signal-to-noise for citrate and an 8-fold increase for isocitrate as compared to detection of the untagged analytes in negative mode. Collision-induced dissociation of the triply tagged, triply charged analytes results in distinct tandem mass spectra. The phenylene spacer groups limit proton mobility and enable access to structurally informative C-C bond cleavage reactions. Modeling of the gas-phase structures and dissociation chemistry of these triply charged analyte ions highlights the importance of hydroxyl proton mobilization in this low proton mobility environment. Tandem mass spectrometric analyses of deuterated congeners and MS3 spectra are consistent with the proposed fragment ion structures and mechanisms of formation. Direct evidence that these chemistries are more generally applicable is provided by subsequent analyses of doubly tagged, doubly charged malate ions. Future work will focus on applying these methods to identify new metabolites and development of general rules for structural determination of tagged metabolites with multiple charges.

Mono‐Phosphazenyl Phosphines (R2N)3P=N–P(NR2)2 – Strong P‐Bases, P‐Donors, and P‐Nucleophiles for the Construction of Chelates

We present a convenient three‐step synthesis of amino substituted phosphazenyl phosphines of the general formula (R2N)3P=N–P(NR2)2 [NR2 = N(CH2)4, N(CH2)5, N(CH2)6]. These easily accessible mixed valent compounds display a surprisingly high proton affinity and basicity in the same range as the corresponding Schwesinger diphosphazene (Me2N)3P=N–P=NEt(NMe2)2 (Et‐P2) and Verkade's proazaphosphatrane superbases. Within the central [PIII–N=PV] scaffold, the phosphine PIII and not the phosphazene NIII atom is the center of highest proton affinity, basicity and donor strength. As P‐bases, the title compounds display calculated proton affinities between 265.8 (NR2 = NMe2) and 274.7 kcal·mol–1 [NR2 = N(CH2)4] and pKBH+ values between 26.4 (NR2 = NMe2) and 31.5 [NR2 = N(CH2)4] on the acetonitrile scale. As P‐nucleophiles, they are key intermediates in the synthesis of hyperbasic bis(diphosphazene) proton sponges, chiral bis(diphosphazene) proton pincers, bisphosphazides, and superbasic P2‐bisylides. Their Staudinger reactions as nucleophile towards 1,8‐diazidonaphthalene leading to 1,8‐naphthalene‐bisphosphazides is described in detail. The donor strength of the title compounds towards fragments [Se] and [Ni(CO)3] is in the same range as that of N‐heterocyclic carbenes.

Measurement of ammonia, amines and iodine compounds using protonated water cluster chemical ionization mass spectrometry

Abstract. Here we describe the design and performance of a new water cluster chemical ionization–atmospheric pressure interface time-of-flight mass spectrometer (CI-APi-TOF). The instrument selectively measures trace gases with high proton affinity such as ammonia and dimethylamine, which are important for atmospheric new particle formation and growth. Following the instrument description and characterization, we demonstrate successful measurements at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber where very low ammonia background levels of ∼4 pptv were achieved (at 278 K and 80 % RH). The limit of detection of the water cluster CI-APi-TOF is estimated to be ∼0.5 pptv for ammonia. Although no direct calibration was performed for dimethylamine (DMA), we estimate its detection limit is at least 3 times lower. Due to the short ion–molecule reaction time and high reagent ion concentrations, ammonia mixing ratios up to at least 10 ppbv can be measured with the instrument without significant reagent ion depletion. Besides the possibility to measure compounds like ammonia and amines (dimethylamine), we demonstrate that the ionization scheme is also suitable for the measurement of trace gases containing iodine. During CLOUD experiments to investigate the formation of new particles from I2, many different iodine-containing species were identified with the water cluster CI-APi-TOF. The compounds included iodic acid and neutral molecular clusters containing up to four iodine atoms. However, the molecular structures of the iodine-containing clusters are ambiguous due to the presence of an unknown number of water molecules. The quantification of iodic acid (HIO3) mixing ratios is performed from an intercomparison with a nitrate CI-APi-TOF. Using this method the detection limit for HIO3 can be estimated as 0.007 pptv. In addition to presenting our measurements obtained at the CLOUD chamber, we discuss the applicability of the water cluster Ci-APi-TOF for atmospheric measurements.

A Novel Application of Dopants in Ion Mobility Spectrometry: Suppression of Fragment Ions of Citric Acid.

Ion mobility spectra of citric acid (CA) are complex, and several peaks are observed for CA and its fragments in both the positive and negative modes. Using DFT calculations, we found that the fragments are both less acidic and less basic than CA in gas phase. Hence, we used a strong base, NH3, in positive mode to produce NH4+ as an alternative reactant ion (RI) and prevent protonation of the fragments. In the presence of NH4+, only one peak for CA was observed because of its higher proton affinity (873 kJ mol-1) compared to NH3 (854 kJ mol-1). In the negative mode, CHCl3, CHBr3, and CHI3 were used as dopant gases to produce Cl-, Br-, and I- as RIs. These halides have less basicity than the common RIs in negative mode (NO2-, NO3-, O2-) and selectively deprotonated CA in the presence of its fragments. Hence, using dopants with appropriate basicity, we could suppress the fragment peaks and obtain a plain IMS spectrum for CA containing only one peak in both the positive and negative modes. Using NH3 and CHCl3 dopants, the amount of CA in fresh lemon juice was determined as 39.5-42 g L-1 by direct injection without any purification. The effect of hydration of the reactant and product ions on the ionization mechanism in both negative and positive modes was investigated theoretically.

One electron less or one proton more: how do they differ?

From the NIST website and the literature, we have collected the Ionisation Energies (IE) of 3,052 and the Proton Affinities (PA) of 1,670 compounds. For 614 of these, both the IE and PA are known; this enables a study of the relationships between these quantities for a wide variety of molecules. From the IE and PA values, the hydrogen atom affinities (HA) of molecular ions M•+ may also be assessed. The PA may be equated to the heterolytic bond energy of [MH]+ and HA to the homolytic bond energy. Plots of PA versus IE for these substances show (in agreement with earlier studies) that, for many families of molecules, the slope of the ensuing line is less negative than −1, i.e. changes in the PA are significantly less than the concomitant opposite changes in IE. At one extreme (high PA, low IE) are the metals, their oxides and hydroxides, which show a slope of close to −1, at the other extreme (low PA, high IE) are the hydrogen halides, methyl halides and noble gases, which show a slope of ca. −0.3; other molecular categories show intermediate behaviour. One consequence of a slope less negative than −1 is that the changes in ionic enthalpies of the protonated species more closely follow the changes in the enthalpies of the neutral molecules compared with changes in the ion enthalpies of the corresponding radical cations. This is consistent with findings from ab initio calculations from the literature that the incoming proton, once attached to the molecule, may retain a significant amount of its charge. These collected data allow a comparison of the thermodynamic stability of protonated molecules in terms of their homolytic or heterolytic bond cleavages. Protonated nitriles are particularly stable by virtue of the very large hydrogen atom affinities of their radical cations.

Backbone Cleavages of Protonated Peptoids upon Collision-Induced Dissociation: Competitive and Consecutive B-Y and A1-YX Reactions.

Mass spectrometric techniques and more particularly collision-induced dissociation (CID) experiments represent a powerful method for the determination of the primary sequence of (bio)molecules. However, the knowledge of the ion fragmentation patterns say the dissociation reaction mechanisms is a prerequisite to reconstitute the sequence based on fragment ions. Previous papers proposed that protonated peptoids dissociate following an oxazolone-ring mechanism starting from the O-protonation species and leading to high mass Y sequence ions. Here we revisit this backbone cleavage mechanism by performing CID and ion mobility experiments, together with computational chemistry, on tailor-made peptoids. We demonstrated that the B/Y cleavages of collisionally activated O-protonated peptoids must involve the amide nitrogen protonated structures as the dissociating species, mimicking the CID behavior of protonated peptides. Upon the nucleophilic attack of the oxygen atom of the N-terminal adjacent carbonyl group on the carbonyl carbon atom of the protonated amide, the peptoid ions directly dissociate to form an ion-neutral complex associating an oxazolone ion to the neutral truncated peptoid residue. Dissociation of the ion/neutral complex predominantly produces Y ions due to the high proton affinity of the secondary amide function characteristic of truncated peptoids. Whereas the production of Yx ions from acetylated peptoids also involves the B/Y pathway, the observation of abundant Yx ions from non-acetylated peptoid ions is shown in the present study to arise from an A1-Yx mechanism. The consecutive and competitive characters of the A1-Yx and the B/Y mechanisms are also investigated by drift time-aligned CID experiments.

Glow Discharge Plasma Ionization Mass Spectrometry for Direct Detection of Oxygenated Organic Compounds in the Gas-phase.

This study describes a direct analysis of oxygenated organic compounds, such as ketones, esters and ethers, rapidly and easily using a soft plasma ionization (SPI) source combined with a Q-mass spectrometer. A related molecular ion, [2M+H]+, in which a sample molecule (M) can undergo protonation via water clusters, such as [(H2O)n+H]+ and [N2(H2O)n+H]+, in an ambient air glow discharge plasma, can be dominantly detected as a base peak with little or no fragmentation at a pressure of several kPa. Oxygenated organic compounds with high proton affinity were found to generate their dimers through the hydrogen bonding interaction at higher pressures. A deuterated solvent was used to examine whether or not the adduct ion of analyte was derived from the solvent. The formation of [2M+H]+ strongly depended on the time. A two-dimensional spectrometer was used to obtain the distribution of several excited species and then to confirm the ionization reactions of the analyte in the SPI source. The sample molecule would be readily ionized through Penning-type collisions with excited N2, which causes fragmentation for oxygenated compounds due to the lower pressures (approx. 1.0 kPa) while it is ionized by an attachment with protons from water clusters at higher pressures (several kPa). The SPI source can be a new and powerful tool for soft ionization in direct analysis of volatile organic compounds (VOCs).

TriQuinoline

The bottom-up synthesis of structurally well-defined motifs of graphitic materials is crucial to understanding their physicochemical properties and to elicit new functions. Herein, we report the design and synthesis of TriQuinoline (TQ) as a molecular model for pyridinic-nitrogen defects in graphene sheets. TQ is a trimer of quinoline units concatenated at the 2- and 8-positions in a head-to-tail fashion, whose structure leads to unusual aromatisation behaviour at the final stage of the synthesis. The central atomic-sized void endows TQ with high proton affinity, which was confirmed empirically and computationally. TQ•H+ is a two-dimensional cationic molecule that displays both π–π and CH–π contact modes, culminating in the formation of the ternary complex ([12]cycloparaphenylene(CPP) ⊃ (TQ•H+/coronene)) that consists of TQ•H+, coronene (flat), and [12]cycloparaphenylene ([12]CPP) (ring). The water-miscibility of TQ•H+ allows it to serve as an efficient DNA intercalator for e.g. the inhibition of topoisomerase I activity.

Characterization of protonated AcAlaDab and AcDabAla by IRMPD spectroscopy and molecular modeling

Alanine (Ala) based oligopeptides containing a residue of 2,4-diaminobutyric acid (Dab) serve as a unique model to examine the effect of the side-chain ionization on the conformational changes of the peptide backbone structure. In this work, two isomeric dipeptide ions, AcAlaDabH+ and AcDabAlaH+, are studied by infrared multiple photon dissociation (IRMPD) spectroscopy and quantum chemical modeling. The IRMPD spectra were recorded in a Fourier transform-ion cyclotron resonance mass spectrometer coupled to an infrared free electron laser. For each peptide ion, seven theoretical infrared (IR) spectra were obtained from a set of unique low-energy conformations calculated at the ωB97X-D/6-311 + G(d,p) level of theory. A manually weighted IR spectrum containing two conformations gives the best prediction for each IRMPD spectrum. An interesting spectral feature is that the frequency of the amide I band of AcAlaDabH+ is about 30 cm−1 lower compared to that of AcDabAlaH+. This redshift of the amide I band implies a stronger hydrogen-bonding interaction between the charged NH3+ group on the side-chain of Dab and the CO group of the backbone in the former ion. The stronger hydrogen-bonding interaction is likely due to a combination of a shorter H⋯O distance and a more favorable alignment of the N–H–O moiety in a unique conformation adopted by AcAlaDabH+, which results in an increased stability of the ion. The results explain an earlier observation that AcAlaDab has a higher proton affinity than AcDabAla.

Novel silicon super bases at DFT level of theory: effects of fused benzene rings on the basicity of 2,4,6-cycloheptatrienesilylene

Super bases are extremely important compounds with high proton affinities (PAs) and many applications in organic, inorganic, polymer, and photochemistry. Here, we have compared and contrasted the basicity of 2,4,6-cycloheptatrienesilylene (1), with its benzo-substituted derivatives including: 4,5-benzo-2,4,6-cycloheptatrienesilylene (2), 3,4-benzo-2,4,6-cycloheptatrienesilylene (3), 2,3-benzo-2,4,6-cycloheptatrienesilylene (4), dibenzo[a,c]-2,4,6-cycloheptatrienesilylene (5), dibenzo[a,d]-2,4,6-cycloheptatrienesilylene (6), dibenzo[a,e]-2,4,6-cycloheptatrienesilylene (7), dibenzo[a,b]-2,4,6-cycloheptatrienesilylene (8), and tribenzo[a,c,e]-2,4,6-cycloheptatrienesilylene (9), at B3LYP/6-311++G** level. All scrutinized silylenes (1–9) and their corresponding protonated forms (1H–9H) appear as minima on their energy surfaces. The conductor-like polarizable continuum model is applied to predict the pka values for nucleophilic silylenes (1–9) in dimethyl sulfoxide, using thermodynamic cycles of Gibbs free energies. In most cases, the scrutinized 1–9 show relatively high basicity, which qualify them for being categorized as super bases or proton sponges. The overall trend of basicity (7 > 6 > 3 > 1 > 2 > 4 > 5 > 8 > 9) appears consistent with both proton affinity in solution phase (PA2) and nucleophilicity (N). Among our scrutinized silylenes, 7 shows the highest basicity, Mulliken electronegativity ( $${\mathcal{X}}$$ ), pka, N, PA, the lowest singlet–triplet energy gap (ΔEs–t), absolute chemical hardness (ηabs), band gap (ΔEH–L), divalent angle (C–Si–C, $${\hat{\text{A}}}$$ ), and Si–C bond length ( $${\dot{\text{A}}}$$ ). The least basic silylene turns out to be 9, which is the most non-planar structure. It shows the lowest dipole moment (D), nucleus independent chemical shift value (NICS (1)), N, PA, the widest dihedral angle (C–Si–C–C, $${\hat{\text{D}}}$$ ), and the highest ΔEH–L. Our investigation introduces novel silylenic super bases with possible applications in organic chemistry. Following up on our previous report (Kassaee et al. [41]), we investigate the basicity of novel one-, two-, three-, and four-cyclic conjugated silylenes (1–9). Most of these silylenes turn out as a super base for showing high pKa and proton affinity.

Viability of half sandwich complexes of borole with group 14 (II) ions: Structure, stability and reactivity

Density functional theory (DFT) calculations have been carried out to investigate the viability of the formation of half sandwich complexes of borole dianion with group 14 dications. All these sandwich complexes have been found to be thermodynamically stable. The reactivity of these half sandwich complexes has also been studied. They have high proton affinity as well as hydride ion affinity indicating their ambiphilic character. The kinetics of their formation has also been studied which indicates that there is very low barrier for formation of most of the complexes. Topological analyses within the realm of quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) suggest that the borole dianion prefers η5 coordination modes. Energy decomposition analysis was performed to obtain a better insight of the interaction between the key fragments which indicates that all these molecules have significant covalency which increases down the group.

Identification of new interferences leached from plastic microcentrifuge tubes in electrospray ionization mass spectrometry.

The incredible sensitivity of the modern mass spectrometry instrument enables scientists to detect a large number of molecules ranging from small organic compounds to biological macromolecules. However, the same sensitivity often throws up challenges with respect to background interferences and contaminants. The identification and source of these contaminants is very important for reducing background contamination and ensuring the accuracy of the analysis results. The interfering compounds were analyzed by high-performance liquid chromatography coupled with a hybrid quadrupole-orbitrap mass spectrometer. The structural analysis was conducted by obtaining the accurate masses of precursors and their fragment ions. The retention time and MS/MS spectrum of one of the interfering compounds (N-lauryldiethanolamine) were compared with an authentic standard to reach an unequivocal structural assignment. The interferences (m/z 274 and 318 in positive mode) were observed during the analysis of herbicides in tea samples by electrospray ionization mass spectrometry (ESI-MS). Their structures were identified to be N-lauryldiethanolamine and N-(2-hydroxyethyl)-N-(2-(2-hydroxyethoxy)ethyl)dodecylamine by fragmentation interpretation and further confirmed by a standard compound. These interferences were found to be leached from the plastic microcentrifuge tubes used during sample pretreatment. The plastic tubes from two of the five suppliers tested were found to contain these two interferences. In this work, we presented an example about the observation, identification and source of interferences in ESI-MS. The N-lauryldiethanolamine and other ethoxylated aliphatic alkylamines are common plastic antistatic agents. They possess high proton affinity so that they show a strong response in ESI positive mode. In order to avoid their interference during mass spectrometric analysis we need to choose plastic tubes (or other plastic materials) that do not contain such antistatic agents.

Sulfuric acid and aromatic carboxylate clusters H2SO4·ArCOO−: Structures, properties, and their relevance to the initial aerosol nucleation

Sulfuric acid and aromatic compounds are abundant in the atmosphere and play an important role in aerosol nucleation and growth. The main objective of this work is to provide molecular level description of pre-nucleation clusters formed by these species. Our approach is based on size- and composition-selective generation of anionic clusters, consisting of sulfuric acid and series of aromatic carboxylates (ArCOO−; Ar = Phenyl, o-, m-, and p-CH3C6H4), and their subsequent characterization using photoelectron spectroscopy and quantum chemical calculations. The photoelectron spectra show that formation of these clusters is accompanied by a significant increase of electron binding energies, implying significant stabilization gained from electron delocalization upon complexation. Comparison between the experimental and calculated results suggests that under low temperature conditions these clusters behave more like a complex of sulfuric acid and aromatic carboxylate, (H2SO4)(ArCOO–), despite the fact that ArCOO– has much higher proton affinity than HSO4–. Similar result has been previously reported for sulfuric acid formate cluster [Hou, Wang, and Valiev, J. Am. Chem. Soc. 139 (2017) 11321–11324], and indicates that interaction energy gained though the formation of these clusters is sufficient to counteract proton affinity imbalance. The calculated intermolecular binding energies (BEs), Gibbs free energies (ΔG), and evaporation rates (γ) show that these (H2SO4)(ArCOO–) clusters are extremely stable, with BEs of ∼54 kcal/mol, ΔG of ∼‒40 kcal/mol, and γ of 10−20 s−1. The experimental and computational results reported in this work suggest that aromatic acids may play a significant role in facilitating the early stages of sulfuric acid nucleation.

Proton Affinities of N-Heterocyclic Olefins and Their Implications for Organocatalyst Design.

The proton affinity (PA) of a range of structurally different N-heterocycles with an exocyclic double bond (= N-heterocyclic olefins, NHOs) has been determined using DFT calculations on the BLYP/def2-TZVPP level. It was found that NHOs belong to the upper end of the superbasicity scale, covering PA values from 262 to 296 kcal/mol. Different types of NHOs are compared with each other and with frequently employed organocatalysts. To boost PA, (a) the ability to delocalize the positive charge and (b) steric pressure/ring strain which can be relieved after protonation were identified as key tuning parameters. Importantly, by analyzing PA alongside partial charges and molecular electrostatic potentials, it is shown that an increase of double bond polarization is not a necessary prerequisite for high PA. In contrast, the more basic, more sterically congested NHOs minimize unfavorable interactions by partly pyramidalyzing the nitrogen atoms, rendering the olefinic bond less electron rich and less polarized. These findings are in excellent agreement with experimental evidence on NHO catalysis, not only providing guidelines for a more rational design regarding PA/basicity but also suggesting that NHOs could be specifically tailored toward either nucleophilic or base-type reaction pathways.

Development of Novel Free Radical Initiated Peptide Sequencing Reagent: Application to Identification and Characterization of Peptides by Mass Spectrometry.

By incorporating a high proton affinity moiety to the charge localized free radical-initiated peptide sequencing (CL-FRIPS) reagent, FRIPS-MS technique has extended the applicability to hydrophobic peptides and peptides without basic amino acid residues (lysine, arginine, and histidine). Herein, the CL-FRIPS reagent has three moieties: (1) pyridine acting as the basic site to locate the proton, (2) 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, a stable free radical) acting as the free radical precursor to generate the nascent free radical in the gas phase, and (3) N-hydroxysuccinimide (NHS) activated carboxylic acid acting as the coupling site to derivatize the N-terminus of peptides. The CL-FRIPS reagent allows for the characterization of peptides by generating sequencing ions, enzymatic cleavage-like radical-induced side chain losses, and the loss of TEMPO simultaneously via one-step collisional activation. Further collisional activation of enzymatic cleavage-like radical-induced side chain loss ions provides more information for the structure determination of peptides. The application of CL-FRIPS reagent to characterize peptides is proved by employing bovine insulin as the model peptide. Both scaffold structure of bovine insulin and sequencing information of each chain are achieved. Graphical Abstract.