High Proton Affinity Research Articles

Minimum proton affinity for efficient ionization with atmospheric pressure desorption/ionization on silicon mass spectrometry

We performed a systematic study using a set of compounds with different proton affinities (PAs) on the ionization in atmospheric pressure desorption/ionization on silicon mass spectrometry (AP-DIOS-MS). The compounds studied included various aromatic molecules of different sizes. The PAs of these compounds were calculated using ab initio and hybrid density functional theory calculations at the B3LYP/6-31G(d) level of theory. We observed that only compounds with relatively high PAs above a threshold value of 920-950 kJ/mol were efficiently ionized as protonated molecules under AP-DIOS conditions and produced very clean mass spectra.

Correlation between H2 Gas Sensitivity and Structure of o-, m-, and p-Dipyridyldiketopyrrolopyrroles as Viewed from the Electron Delocalization within the Molecule and the Crystal Structure

We have previously developed a high-sensitive H gas sensor utilizing a high proton affinity of pdipyridyldiketopyrrolopyrrole (p-DPPP). The sensor exhibits a remarkable reduction of the electrical resisitivity by two orders of magnitude under0.05% H due to protonation at the parasite of the pyridyl ring. The present outstanding result motivated us to further investigate o-and m-derivatives in order to achieve an even better performance. However,the performance of these isomers was extremely poor. For this reason,the present investigation has been carried out in order to clarify the mechanism of the poor sensitivity from the standpoint of the electron delocalization (i.e.electron conduction)within the molecule as well as the electron hopping from one molecule to another (i.e. structural problem). As for the electron delocalization in p-DPPP, the change in electron density at the para-site(due to e.g.protonation)is found to be well propagated throughout the molecule,while those at the o-and m-sites are ineffective. This explains why p-DPPP is much superior for H gas sensors to o-and m-DPPPs. Another support is also given by the structure analysis of o-,m-,and p-derivatives. The N atom of the pyridyl ring (that serves as the antenna for protonation) remains unbonded (i.e. free) in p-DPPP and is capable of accepting protons. On the other hand,the N atoms are totally blocked by the formation of NH...N hydrogen bonds in o-and m-DPPPs. The above molecular and crystallographic considerations lead us to conclude that p-DPPP is,by far,advantageous to H sensors over o-and m-DPPPs.

New Proton‐Sponge‐Like Macrocyclic Compound: Synergistic Hydrogen Bonds of Aminopyridine

AbstractN,N′,N″‐Tris(p‐tolyl)azacalix[3](2,6)pyridine (1) that shows a high proton affinity (pKBH+ = 23.1 in CD3CN) has been classified as a new type of proton‐sponge‐like compound. X‐ray crystallography reveals that 1 is ideal for accommodating a single proton in its cavity. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Electron Capture Dissociation of Peptides Metalated with Alkaline-Earth Metal Ions

The possible use of divalent alkaline-earth metal ions, including Mg2+, Ca2+, Sr2+, and Ba2+, as charge carrier for electron capture dissociation of peptides was investigated. Model peptides of RGGGVGGGR and NGGGWGGGN were used to simplify the interpretation of spectral information. It was demonstrated that useful electron capture dissociation (ECD) tandem mass spectra of these metalated peptides could be generated. Interestingly, peptides metalated with different alkaline-earth metal ions generated very similar ECD tandem mass spectra. Metalated c-ions and z-ions were the predominant fragment ions. Only Mg2+-metalated peptides gave somewhat different results. Some nonmetalated c-ions were observed from ECD of [RGGGVGGGR + Mg]2+ but not from [NGGGWGGGN + Mg]2+. Together with some ab initio calculations, it was established that the bound metal ions might activate the acidity of the amide hydrogen. With the presence of high proton affinity moiety, such as N-terminal amino group and/or side chain of the arginine residues, the metalated peptide ions could exist predominantly in their zwitterion forms, in which one or two backbone amide group(s) was deprotonated and the high proton affinity functional group(s) was protonated. It was believed that electron capture leads primarily to the reduction of the mobile proton rather than the metal ions. With this zwitterion model, the formation of nonmetalated c-fragments and the generation of similar ECD spectra for peptides metalated with various alkaline-earth metal ions could readily to be explained. Another interesting observation in the ECD mass spectra of metalated peptides is related to the enhanced formation of the minor ECD products, i.e., (c - 1)(+*) and (z + 1)+ ions. Together with ab initio calculations using a truncated peptide model, various possible reaction mechanisms for the formation of these minor ECD products were evaluated. It was concluded that hydrogen transfer between the initiated formed c and z(.) species plays an important role in the formation (c - 1)(+*) and (z + 1)+ ions. Although peptides metalated with these metal ions do not have better ECD efficiency compared to the multiply-protonated peptides, it provides practical accessibility of ECD methods to analyze small peptides with no basic amino acid residues.

Structures of Heterogeneous Proton-Bond Dimers with a High Dipole Moment Monomer: Covalent vs Electrostatic Interactions

A number of calculated structures of heterogeneous proton-bound dimers containing monomers such as acetonitrile, cyanamide, vinylene carbonate, and propiolactone, which have high dipole moments, are presented. These proton-bound dimers are predicted to have a structural anomaly pertaining to the bond distances between the central proton and the basic sites on each of the monomers. The monomers with the high dipole moments also have the larger proton affinity and, on the basis of difference in proton affinities, it would be expected that the proton would be closer to this monomer than the one with the lower proton affinity. However, the proton is found to lie substantially closer to the monomer with the lower proton affinity in most cases, unless the difference in proton affinity is too large. Simply stated, the difference in proton affinities is smaller than the difference in the affinity to form an ion-dipole complex for the two monomers and it is the larger affinity for the high dipole moment monomer (which also has the higher proton affinity) to form an ion-dipole complex that is responsible for the proton lying closer to the low proton affinity monomer. The bond distances between the central proton and the monomers are found to be related to the difference in proton affinity. It is found, though, that the proton-bound dimers can be grouped into two separate groups, one where the proton-bound dimer contains a high dipole moment monomer and one group where the proton-bound dimer does not contain a high dipole moment monomer. From these plots it has been determined that a high dipole moment monomer is one that has a dipole moment greater than 2.9 D.

Rational Design and First Principles Studies Toward the Discovery of a Small and Versatile Proton Sponge

Basic principles: The use of simple topological principles permits the design of small molecules with very high proton affinities. According to DFT calculations, 3,6,7,8-tetraazatricyclo[3.1.1.12,4]octane and its analogues behave as superbases, and show important structural deviations from known proton sponges. Their electrostatic potential maps show a characteristic very compact region at the protonation site (blue regions in the picture).

Ab Initio Study of Sorption on Pyrophyllite: Structure and Acidity of the Edge Sites

Pyrophyllite, Al2[Si4O10](OH)2, is the simplest structural prototype for 2:1 dioctahedral phyllosilicates. Since it does not possess permanent structural charge as other clay minerals do, it is used to investigate sorption properties of the clay surfaces not related to the permanent structural charge. The bulk structure and surface geometries of pyrophyllite have been modeled using an orthorhombic constrained supercell. The results of the calculations are in excellent agreement with available experimental data and earlier ab initio simulations. It is shown that the symmetry-constrained model is able to accurately reproduce the basic structural characteristic of pyrophyllite. The electrostatic potential near the (001) surface was used to analyze a possible scenario for the water sorption on the basal plane of pyrophyllite. The calculations predict a slightly hydrophobic behavior of the basal plane. The hydronium ion was found to form a strongly bonded conformation in the siloxane cavity. The relative stability and composition of lateral facets of pyrophyllite have been studied using the supercell approach. The crystals of pyrophyllite are predicted to have a prismatic habit dominated by (110) and (-110) edge facets and basal plane. On the basis of the Fukui functions and the relative protonation/deprotonation enthalpies, the relative acidity and density of the reactive surface sites have been predicted. The triple bond Al-O-Si triple bond sites have the highest proton affinity on the (100), (110), and (130) facets and three line sign Al-OH groups on the (010) edges. The deprotonation of the triple bond Al-OH2 sites is followed by triple bond Al-OH and triple bond Si-OH groups. The calculations suggest a new scale for acidities of edge sites in pyrophyllite that should facilitate the thermodynamic modeling of the sorption processes in compacted clays.

An Investigation of Chlorophenol Proton Affinities and Their Influence on the Biological Activity of Microorganisms

The proton affinities of 15 chlorophenols are calculated by ab initio methods. Straight correlation between proton affinities and changes in the electronic structure is observed. The proton affinities decrease linearly with the electronic density gain on the chlorine atoms, as the liberation of the proton increases. To confirm the importance of the proton affinities on the toxicity of chlorophenols, calorimetric responses of these molecules and related ones where the acid proton is changed to a methyl group (anisol and its chlorinated derivatives) were used to verify their effects on Chromobacterium violaceum. The results confirmed that the chlorophenols are more toxic than the respective chloroanisols and suggest that high proton affinities are associated with low toxic activity. The toxicity of the chlorophenols can be associated with the respiratory mechanism in some microorganisms.

Concerted Effects of Substituents in the Reaction of •OH Radicals with Aromatics: The Cresols

The concerted effects of hydroxyl and methyl substituents in controlling the site of .OH radical attack on aromatics in aqueous solutions are explored using the cresols as typical examples. The distributions of dihydroxytoluenes produced in the radiolysis of aqueous solutions of the cresols containing ferricyanide as a radical oxidant were examined by capillary electrophoretic and liquid chromatographic methods. Because .OH is a strong electrophile, it adds preferentially at the electron-rich sites of an aromatic ring. As a result, the observed distributions of dihydroxytoluenes reflect the charge distributions in the cresols. It is shown that in the case of m-cresol the hydroxyl substituent has a dominant ortho-para directing effect similar to that observed for phenol. In o- and p-cresol, this effect is modified, indicating that the methyl substituent has a significant effect on the electronic structure of those cresols. Correlation of the charge distribution in the cresols indicated by the observed distribution of dihydroxytoluenes with the unpaired spin distribution in the corresponding methylphenoxyl radicals demonstrates that the electronic structures of o- and p-cresol and their corresponding phenoxyl radicals are similarly affected by hydroxyl and methyl substitution. Addition of .OH at the methyl-substituted positions of o- and p-cresol to produce o- and p-dienone is also reported. The observation of these dienones demonstrates that addition of .OH at the ipso positions of alkylated aromatics can be of considerable importance. Mass spectrometric studies show that these dienones have relatively higher proton affinities than their isomeric analogues.

Proton Affinities of N-Heterocyclic Carbene Super Bases

The gas-phase proton affinity of the N-heterocyclic carbene, 1-ethyl-3-methylimidazol-2-ylidene, was determined to be 251.3 +/- 4 kcal/mol using the kinetic method, a value which makes the carbene one of the strongest bases reported thus far. Density functional theory calculations have been carried out at the B3LYP/6-31+G(d) level to compare the high experimental value with that estimated theoretically. Experimental results also show that two other N-heterocyclic carbenes with larger substituents have even higher proton affinities. [structure: see text]

Proton Binding to Humic Acids from Organic Amendments and Amended Soils by the NICA-Donnan Model

The acid-base properties of humic acids (HAs) are known to significantly affect the acid-base buffering capacity of soils, thus having a marked influence on the speciation of cations in the soil solid and liquid phases. Detailed information on the proton binding behavior of humic-like acids (HALs) from organic amendments and humic acids (HAs) from amended soils is, therefore, of intrinsic interest for the evaluation of the agronomic efficacy and environmental impact of soil amendment. In this work, the acid-base properties of HLAs isolated from sewage sludge (SS) and municipal solid waste compost (MSWC), and HAs isolated from soils amended with either SS or MSWC and the corresponding nonamended control soils were investigated by potentiometric titrations at various ionic strengths (0.01, 0.05, 0.1, and 0.3 M) over the pH range from 3.5 to 10.5. The nonideal competitive adsorption (NICA)-Donnan model that describes proton binding by two classes of binding sites with low and high proton affinity, i.e., carboxylic- and phenolic-type groups, was fit to titration data, and a set of fitting parameters was obtained for each HLA and HA sample. The NICA-Donnan model successfully described the shapes of the titration curves, and highlighted substantial differences in site density and proton-binding affinity between the HLAs and HAs examined. With respect to the nonamended control soil HAs, SS-HLA and MSWC-HLA were characterized by smaller carboxylic-type and phenolic-type group contents, larger affinities for proton binding by the carboxylic-type groups, and smaller affinities for proton binding by the phenolic-type groups. Amendment with SS and MSWC determined a number of modifications in soil HAs, including decrease of acidic functional group contents, slight increase of proton affinity of carboxylic-type groups, and slight decrease of the affinities for proton binding by phenolic-type groups. These effects were more evident in the HA fraction from the SS-amended soil than in the HA fraction from the MSWC-amended soil. Thus, both organic amendments examined can be a considered as a valuable source of organic matter for soil. However, MSWC appears to be an amendment of greater quality producing a smaller impact than SS on proton-binding behavior of soil HA.

Cooperative Intramolecular Hydrogen Bonding Effect and Basicity –An Ab Initio and DFT Study of the Superbasic Properties ofN‐[(Dimethylamino)alkyl]‐2,3‐diaminocycloprop‐2‐ene‐1‐imines

AbstractThe MP2(fc)/6‐311+G**//HF/6‐31G* and B3LYP/6‐311+G**//HF/6‐31G* calculations show that N‐[3‐(dimethylamino)propyl] and N‐[4‐(dimethylamino)butyl] derivatives of 2,3‐diamino‐cycloprop‐2‐ene‐1‐imine (CPI) possess high gas phase proton affinities (PA) and pronounced basicities in acetonitrile (MeCN). Tris[N‐dimethylaminopropyl(‐butyl)]‐CPI derivatives achieve superbasic PA and pKa values. The reason behind these remarkable features is the stability of the resulting conjugate acids. They are stabilized by aromatization of the three‐membered ring accompanied by the cationic resonance between the cyclopropyl ring and amino nitrogen atoms of the side chains, and, last but not least, by the cooperative multiple intramolecular hydrogen bonding effect. The latter is particularly enhanced in tris‐substituted CPIs. It was found that the N‐[4‐(dimethylamino)butyl] side chains contribute more to the stability of the conjugate acids than those of the corresponding compound with a propyl chain, because they undergo a somewhat larger relaxation upon protonation. The studied compounds might be helpful in building up a dense ladder of strong bases and superbases, which could contribute to the development of the chemistry of superacids and superbases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Sensitivity enhancement in liquid chromatography/atmospheric pressure ionization mass spectrometry using derivatization and mobile phase additives

High performance liquid chromatography with atmospheric pressure ionization (API) mass spectrometry has been essential to a large number of quantitative analytical applications for a variety of compounds. Poor detection sensitivity however is a problem observed for a number of analytes because detection sensitivity can be affected by many factors. The two most critical factors are the chemical and physical properties of the analyte and the composition of the mobile phase. In order to address these critical factors which may lead to poor sensitivity, either the structure of the analyte must be modified or the mobile phase composition optimized. The introduction of permanently charged moieties or readily ionized species may dramatically improve the ionization efficiency for electrospray ionization (ESI), and thus the sensitivity of detection. Detection sensitivity may also be enhanced via introduction of moieties with high proton affinity or electron affinity. Mobile phase component modification is an alternative way to enhance sensitivity by changing the form of the analytes in solution thereby improving ionization efficiency. pH adjustment and adduct formation have been commonly used to optimize detection conditions. The sensitivity of detection for analytes in bio-matrices could also be enhanced by decreasing ion-suppression from the matrix through derivatization or mobile phase addition. In this review, we will discuss detection-oriented derivatization as well as the application of mobile phase additives to enhance the sensitivity of detection in liquid chromatograph/atmospheric ionization/mass spectrometry (LC/API/MS), focusing in particular on the applications involving small molecules in bio-matrices.

FTIR Adsorption Studies of H2O and CH3OH in the Isostructural H-SSZ-13 and H-SAPO-34: Formation of H-Bonded Adducts and Protonated Clusters

The acidity of the isostructural H-SSZ-13 and H-SAPO-34 has been investigated by transmission FTIR spectroscopy using H2O and CH3OH as molecular probes. Interactions between the zeolitic samples and the probe molecules led to perturbations and proton transfers directly related to the acidity of the materials. The entire set of acidic sites in H-SSZ-13 interacts with H2O and CH3OH to give H-bonded adducts or protonated species. H3O+ is not formed in appreciable amounts upon H2O adsorption on H-SSZ-13, but at high coverages H2O generates clusters that have a proton affinity sufficiently high to abstract protons from the zeolite framework. Parallel experiments carried out for H-SAPO-34 showed that the H2O clusters abstract protons from Brønsted sites only to a minor extent. Moving to CH3OH, even if it has a higher proton affinity than H2O and should expectingly experience an easier protonation, proton transfer is totally absent in H-SAPO-34 under our set of conditions. The clear evidence of methanol protonation in H-SSZ-13 definitely states the strong acidic character of this material. When irreversibly adsorbed CH3OH is present in H-SSZ-13, an appreciable amount of (CH3)2O is formed upon heating to 573 K. Compared to its SAPO analogue, the present set of data indisputably points to H-SSZ-13 as the strongest Brønsted acidic material.

Capillary electrophoresis‐laser induced fluorescence‐electrospray ionization‐mass spectrometry: A case study

The simultaneous hyphenation of capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection and electrospray ionization-mass spectrometry (ESI-MS) as a novel combined detection system for CE is presented. beta-Carbolines were chosen as model analytes with a forensic background. Nonaqueous CE as well as conventional CE with an aqueous buffer system are compared concerning efficiency and obtainable detection limits. The distance between the optical detection window and the sprayer tip was minimized by placing the optical cell directly in front of the electrospray interface. Similar separation efficiencies for both detection modes could thus be obtained. No significant peak-broadening induced by the MS interface was observed. The high fluorescence quantum yield and the high proton affinity of the model analytes investigated resulted in limits of detection in the fg (nmol/L) range for both detection methods. The analysis of confiscated ayahuasca samples and ethanolic plant extracts revealed complementary selectivities for LIF and MS detection. Thus, it is possible to improve peak identification of the solutes investigated by the use of these two detection principles.

A sensitive negative-ion electrospray ionization mass spectrometry detection for metallothionein in tris(hydroxymethyl)aminomethane acetate buffer

Although positive-ion (PI) electrospray ionisation mass spectrometry (ESI-MS) has been usually applied for the analysis of native metallothioneins (MT) isoforms binding with metal ions, it suffers from the lack of insensitivity under neural conditions because of the low efficiency of protonation during ESI process. In this study, multiply deprotonated metallothionein (Zn 7-MT-2a), produced from tris(hydroxymethyl)aminomethane (TRIS) acetate solutions under near neutral condition, was analyzed by negative-ion (NI) ESI-MS. Compared with the ammonium acetate buffer system which has been normally used for ESI-MS experiments, the use of TRIS acetate buffer results in the formation of more abundant ions and higher charge states of MT-2a, and consequently higher intensity is attained. The sensitivity enhancement of the system could be explained by the high gas-phase proton affinity, small molecular volume of the anion (acetate), and the high hydrogen consumption by TRIS in the buffer system, which would all favor the deprotonation of the analyte during NI-ESI process. Factors that may affect the NI-ESI responses, such as acid added in buffer, pH and concentration of TRIS acetate had been evaluated, and the result showed that using acetic acid, lower pH under the pH range evaluated, higher concentration of TRIS acetate all favored the MS detection of MT-2a. Our finding sheds light on a buffer system that may offer substantial sensitivity advantages in the studying of weakly bound, non-covalent complexes such as metal binding MTs, which are usually analyzed under near neutral conditions.

Polymorph of 1,4-diketo-3,6-bis-(4′-dipyridyl)-pyrrolo-[3,4- c]pyrrole and their hydrogen bond network: A material for H 2 gas sensor

Diketodiphenylpyrrolopyrroles (DPP) are industrially important red pigments. The title compound (DPPP) is a dipyridyl derivative that possesses a high proton affinity because of the N atoms of the pyridyl ring. A drastic change in shade as well as electrical conductivities by several orders of magnitude is observed due to protonation at the N-atom. Therefore, DPPP has newly attracted attention as a material for H 2 gas sensors. Then, we noticed that there are two crystal phases: one is quite sensitive to protons (phase I grown from the vapor phase) while the other is rather resistant to protonation (phase II recrystallized from solution). Therefore, structure analysis has been carried out in the present investigation in order to clarify the correlation between the structure and proton affinity. A striking difference is recognized in intermolecular hydrogen bonds between two phases: phase I is characterized only by NH⋯O hydrogen bonds where two N atoms of the pyridyl rings remain free. On the other hand, phase II includes two types of the H-bond based on NH⋯O and NH⋯N where the one N atom of the pyridyl ring is used by NH⋯N hydrogen bond and the other remains unused (i.e. free). Therefore, phase I is found to correspond to the phase which exhibits a high proton affinity while phase II is the one which is rather inactive for protons.

Synthesis of chiral crown ethers and complexation with chiral protonated amine compounds: X-ray crystal and nuclear magnetic resonance studies of perchlorate salt of chiral benzo-monoaza-15-crown-5 and chiral monoaza-15-crown-5

The X-ray crystal structure of IX, perchlorate salt of R-(−-2-ethyl-N-benzyl-4,7,19,13-tetraoxa-8,9-benzo-1-azacyclopentadec-8-ene has been determined. In the molecule, the protonated nitrogen atom participates in two N-H⋯O hydrogen bonds. The unusually high proton affinity of aza crown ether leads to the formation of diastreomer instead of complex formation with chiral R-(+)-1-phenyl ethyl ammonium perchlorate and S-(−)-1-phenyl ethyl ammonium perchlorate. The complex ability of host ethers was evaluated in terms of structural modification.

Ultraviolet photolytic vapor generation from particulate ensembles for detection of malathion residues in aerosols

A method to selectively generate vapor signatures from malathion entrained within matrices of surface-impacted aerosol particles has been demonstrated. The method uses ultraviolet radiation (172 or 222 nm) from a continuous wave discharge lamp to photodissociate malathion molecules collected within and on surface-impacted particles, followed by detection via ion mobility spectrometry (IMS). Since surface heating does not occur, only those molecules whose photofragments exhibit high vapor pressure are introduced into the IMS instrument and then only those exhibiting high proton affinity are subsequently detected. This process produces less signal clutter than in pyrolysis-IMS, where the background aerosol is pyrolyzed along with the sample. Quantities of malathion as small as 50 ng could be detected when the malathion was entrained on a clean surface, and as small as 100 ng when co-entrained on a surface with much larger quantities of background aerosols such as diesel soot, road dust, Bacillus globigii, albumin, and cotton lint. This sensitivity indicates that, when combined with a particle collector as an effective pre-concentrator, detection of malathion aerosol concentrations of <0.01 mg/m 3 will be possible. Since malathion can be viewed as a model compound, this technique is also extendable to the detection of organophosphate war chemicals.

Structures and stabilities of CaC 3+ and CaC 3H + isomers

A theoretical study of the CaC 3 + and CaC 3H + systems has been carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. In addition, a topological analysis of the electron density and its associated Laplacian has also been carried out. The predicted global minimum for CaC 3 + is a rhombic four-membered ring 2d( 2A 1). Both doublet states of the other cyclic structure 3, 2B 2 and 2A 2, lie quite close in energy (about 4.0 and 9.6 kcal/mol, respectively, higher in energy at the G2(P) level). The linear isomer lies about 14.6 kcal/mol above the global minimum, at the G2(P) level. In the case of CaC 3H + the predicted global minimum is a bicyclic structure 2s( 1A 1). There are two isomers that lie close in energy: a linear species, 1t( 2Σ), and a four-membered ring, 3s( 1A 1); lie about 16.2 and 19.4 kcal/mol, respectively, at the G2(P) level, above the global minimum. Except in the case of linear isomer, protonation favours the singlet states over the triplet ones. Low ionization potential and high proton affinities are obtained for the most stable CaC 3 isomers. Therefore, if present in the interstellar medium, CaC 3 should be easily ionized and would react quite easily to give the protonated species.