Electrochemically induced transformations of ruthenium(III) trichloride microcrystals in salt solutions

Ruthenium (III) trichlorid solid crystals have been mechanically attached to gold surfaces and studied by cyclic electrochemical quartz crystal microbalance measurements in the presence of aqueous solutions of different concentrations containing M⁺Cl⁻, where M⁺=H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺. The RuCl₃ and the complexes formed during the electrochemical transformations show two or more reduction and reoxidation pairs of waves, depending on the experimental conditions (concentration, scan rate, and potential range). The voltammetric peaks are shifted into the direction of higher potentials with increasing electrolyte concentrations except at very high concentrations when the peaks belong to the first reduction/reoxidation processes move oppositely. The mass change was reversible, during reduction mass increase, while during oxidation mass decrease occurred at medium electrolyte concentrations in two, more or less distinct steps. At high or low concentrations the mass excursions are more complex involving different mass increase/decrease regions as a function of potential which vary with the potential range of the measurements. The peak potentials and the electrochemical activity strongly depend on the nature of the cations and pH. It is related to the formation of complexes in different compositions. The mass change decreases with increasing electrolyte concentrations attesting the important role of the water activity and the transport of solvent molecules. It was concluded that in dilute solutions during the first reduction step M⁺ ions enter the surface layer. The strongly hydrated Li⁺ ions transfer water molecules into the microcrystals, while simultaneously with the incorporation of K⁺, Rb⁺, and Cs⁺ ions H₂O molecules leave the surface layer. The opposite transport of ions and solvent molecules occur during oxidation. In the course of further reduction the incorporation of all ions studied except that of Cs⁺ ions is accompanied with water sorption. The number of sorbed water molecules is proportional to the hydration number of these ions. A reaction scheme is proposed in which M⁺ _m-3[Ru^IIICl _m (H₂O) _n ]^3-m · xH₂O (m≥3) and [Ru^IIICl _m (H₂O) _n ]^3-m (Cl⁻)_3-m · xH₂O (m≤3) type complexes are reduced to the respective – or depending on the electrolyte concentration higher or lower – Ru(II)chloro complexes resulting in mixed valence compounds (phases). Taking into account the layered structure of RuCl₃ the electrochemical reduction can be explained as an intercalation reaction in that mixed valence intercalation phases with a general formula M _x ⁺(H₂O) _y [RuCl₃] ^x− are formed from RuCl₃·x H₂O. The reduction/reoxidation waves are related to the redox transformations of Ru(III) to Ru(II) sites, while the composition of the polynuclear complexes and the structure of microcrystals change. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13.−16., 2005.     

Neutralization–reionization efficiencies for gaseous [C3Hn]+ Ions

A neutralization-reionization study of [C₃H_n]⁺ ions, n = 6?0, shows that neutralization efficiencies are larger for hydrocarbon radical cations than they are for even-electron carbocations with one more H atom. Reionization efficiencies to positive ions do not depend appreciably on the electronic properties of the neutral species undergoing reionization. However, reionization efficiencies to negative ions increase dramatically with the degree of unsaturation, by nearly two orders of magnitude, from C₃H₆ to C₃. All C₃H_n neutrals studied survive intact neutralization to produce a considerable fraction of stable, non-dissociating [C₃H_n]⁺ upon cationization. All C₃H_n radicals (n = 5, 3, 1) can be transferred collisionally to stable even-electron anions. For the C₃H_n hydrocarbon molecules studied (n = 6, 4, 2, 0) the stability of the molecular radical anion increases substantially with the degree of unsaturation.     

On the structures of X- and Y-type zeolites. Structural changes within the cavities of Na-X during dehydration

Samples of partially dehydrated and dehydrated Na-X were examined structurally by X-ray diffraction methods, revealing the progressive structural changes which occur as water is removed. In general, the total number of Na atoms in the small pore region remains unchanged by dehydration (ca. 18 per unit cell), as does the total number (non-mobile) in the 12-ring and site III regions (ca. 39). The site II population, however, is more than doubled by dehydration, from about 12 to about 25 Na, accounting for most of the loss from the mobile phase. The 12-ring sites, which in hydrated samples appear to comprise pairs of centrosymmetrically related [Na(H₂O)₂]⁺ units, rearrange during dehydration, with site III becoming an important location of Na atoms. At intermediate levels of dehydration, the remaining localized water molecules in the 12-ring region are found in a variety of associations with Na atoms, including perhaps as [Na(H₂O)₅]⁺ units whose Na atom occupies site III. In a sample containing H₃O⁺ ions as well as Na⁺ as counter ions, site II was found to have a very low occupancy.     

Effect of method of ion preparation on low-energy collision-induced dissociation mass spectra

The collision-induced dissociation (CID) mass spectra of protonated cocaine and protonated heroin have been measured using a triple quadrupole mass spectrometer at 50 eV ion/neutral collision energy for protonated molecules prepared by different protonating agents. The CID mass spectra of protonated cocaine using H⁺(H₂O)_n, H⁺(NH₃)_n and H⁺((CH₃)₂NH)_n as protonating agents are essentially identical and it is concluded that, regardless of the initial site of protonation, the fragmentation reactions occurring on collisional activation are identical. By contrast, protonated heorin prepared with H⁺(H₂O)_n and H⁺(NH₃)_n as protonating agents show substantial differences. That formed by reaction of H⁺(H₂O)_n shows a much more abundant peak corresponding to loss of CH₃CO₂H. From a comparison with model compounds, and from a consideration of the three-dimensional structure of heroin, it is concluded that with H⁺(H₂O)_n as protonating agent significant protonation occurs at the acetate group attached to the alicyclic ring, leading to acetic acid loss on collisional activation, but that reaction of H⁺(NH₃)_n leads to protonation at the nitrogen function. The proton attached to nitrogen cannot interact with the acetate group and, consequently, the probability of loss of acetic acid on collislional activation is greatly reduced.     

Gas phase reactivity of cation radicals and protonated species from isomeric C4H8S thioethers

Four isomeric thioethers, 2,3-dimethylthiirane ( 1 ), 2-methylthietane ( 2 ), tetrahydrothiophene ( 3 ), and allyl methyl thioether ( 4 ), have been subjected to mass spectrometric analysis in the gas phase, under electron impact (El) and chemical ionization (CI) conditions. The metastable molecular ions M⁺′ generated from 1-4 under EI (70 eV) conditions give distinct patterns of unimolecular fragmentation, thus indicating that isomer interconversion reactions are slower than dissociation (a possible exception, to some extent, is the case of [M₂]⁺′ and [M₂]⁺′). The change of the relative intensities of some prominent peaks with increasing ion lifetime (decomposition within the ion source, the first, and the second field-free regions of the mass spectrometer) is pointed out. Metastable [MH]⁺ ions, generated from 1-4 in chemical ionization experiments with CH₄, all eliminate H₂ and H₂S, although in different relative proportions. In addition to these processes protonated 4 also undergoes loss of C₂H₄ and C₃H₆, likely from a C-protonated structure.     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

Cationic Complexes of Hydrogen with Helium

High‐resolution mass spectra of helium nanodroplets doped with hydrogen or deuterium reveal that copious amounts of helium can be bound to H⁺, H₂⁺, H₃⁺, and larger hydrogen‐cluster ions. All conceivable He_nH_x⁺ stoichiometries are identified if their mass is below the limit of ≈120 u set by the resolution of the spectrometer. Anomalies in the ion yields of He_nH_x⁺ for x=1, 2, or 3, and n≤30 reveal particularly stable cluster ions. Our results for He_nH₁⁺ are consistent with conclusions drawn from previous experimental and theoretical studies which were limited to smaller cluster ions. The He_nH₃⁺ series exhibits a pronounced anomaly at n=12 which was outside the reliable range of earlier experiments. Contrary to findings reported for other diatomic dopant molecules, the monomer ion (i.e. H₂⁺) retains helium with much greater efficiency than hydrogen‐cluster ions.     

Ammonium and caesium carbonate peroxosolvates: supramolecular networks formed by hydrogen bonds

Diammonium carbonate hydrogen peroxide monosolvate, 2NH₄⁺·CO₃²⁻·H₂O₂, (I), and dicaesium carbonate hydrogen peroxide trisolvate, 2Cs⁺·CO₃²⁻·3H₂O₂, (II), were crystallized from 98% hydrogen peroxide. In (I), the carbonate anions and peroxide solvent molecules are arranged on twofold axes. The peroxide molecules act as donors in only two hydrogen bonds with carbonate groups, forming chains along the a and c axes. In the structure of (II), there are three independent Cs⁺ ions, two of them residing on twofold axes, as are two of the four peroxide molecules, one of which is disordered. Both structures comprise complicated three‐dimensional hydrogen‐bonded networks.     

Six two‐ and three‐component ammonium carboxylate salt structures with a ladder‐type hydrogen‐bonding motif,three incorporating neutral carboxylic acid molecules

Six ammonium carboxylate salts are synthesized and reported, namely 2‐propylammonium benzoate, C₃H₁₀N⁺·C₇H₅O₂⁻, (I), benzylammonium (R)‐2‐phenylpropionate, C₆H₁₀N⁺·C₉H₉O₂⁻, (II), (RS)‐1‐phenylethylammonium naphthalene‐1‐carboxylate, C₈H₁₂N⁺·C₁₁H₇O₂⁻, (III), benzylammonium–benzoate–benzoic acid (1/1/1), C₆H₁₀N⁺·C₇H₅O₂⁻·C₇H₆O₂, (IV), cyclopropylammonium–benzoate–benzoic acid (1/1/1), C₃H₈N⁺·C₇H₅O₂⁻·C₇H₆O₂, (V), and cyclopropylammonium–ea‐cis‐cyclohexane‐1,4‐dicarboxylate–ee‐trans‐cyclohexane‐1,4‐dicarboxylic acid (2/1/1), 2C₃H₈N⁺·C₈H₁₀O₄²⁻·C₈H₁₂O₄, (VI). Salts (I)–(III) all have a 1:1 ratio of cation to anion and feature three N⁺—H...O⁻ hydrogen bonds which form one‐dimensional hydrogen‐bonded ladders. Salts (I) and (II) have type II ladders, consisting of repeating R₄³(10) rings, while (III) has type III ladders, in this case consisting of alternating R₄²(8) and R₄⁴(12) rings. Salts (IV) and (V) have a 1:1:1 ratio of cation to anion to benzoic acid. They have type III ladders formed by three N⁺—H...O⁻ hydrogen bonds, while the benzoic acid molecules are pendant to the ladders and hydrogen bond to them via O—H...O⁻ hydrogen bonds. Salt (VI) has a 2:1:1 ratio of cation to anion to acid and does not feature any hydrogen‐bonded ladders; instead, the ionized and un‐ionized components form a three‐dimensional network of hydrogen‐bonded rings. The two‐component 1:1 salts are formed from a 1:1 ratio of amine to acid. To create the three‐component salts (IV)–(VI), the ratio of amine to acid was reduced so as to deprotonate only half of the acid molecules, and then to observe how the un‐ionized acid molecules are incorporated into the ladder motif. For (IV) and (V), the ratio of amine to acid was reduced to 1:2, while for (VI) the ratio of amine to acid required to deprotonate only half the diacid molecules was 1:1.     

Peculiarities of interaction of H+, Me3C+, and Me3Si+ ions with multifunctional molecules in the gas phase

Peculiarities of interaction of H⁺, Me₃C⁺, and Me₃Si⁺ ions with functional groups of molecules in the gas phase have been studied. Proton tends to form chelates with virtually all of the functional groups studied, whereas Me₃Si⁺ ions exhibit no capacity for chelation. Using isomeric xylenes as examples it was shown that Me₃Si⁺ ions (unlike Me₃C⁺ ions) experience virtually no steric hindrance when they react with nucleophilic centers. Effects of functional groups present in molecules of nitriles on the generation of [M+Me₃C]⁺ adducts in the gas phase and the Ritter reaction in solution were estimated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1767–1773, September, 1995.This work was carried out with financial support from the International Science Foundation (Grant MA7 000) and the Russian Foundation for Basic Research (Project No. 93-03-18033).     

Organic cluster ions from continuous-flow fast atom bombardment

Cluster ions from fast atom bombardment of liquid alcohols and nitriles were examined using a continuous-flow technique. Protonated molecular M_nH⁺ species are the dominant cluster ions observed in molecules of formula M. The abundances of the M_nH⁺ cluster ions decrease monotonically with increasing n, and within a homologous series the M_nH⁺ abundance diminishes more rapidly for higher molecular mass compounds. Reaction products (ROH)_n(H₂O)H⁺ and (ROH)_n(ROR)H⁺ are observed also in the case of alcohols, and the ion abundances decrease with increasing n. Radiation damage yields fragment ions and ionic alkyl reaction products which are captured in solvent clusters. Semi-empirical molecular orbital methods were used to examine the energetics of cluster ion formation and decomposition pathways. Metastable decomposition processes exhibit only evaporative loss of monomers, with the probability of loss increasing sharply with n. The evaporative ensemble model of Klots was used to predict the cluster size-dependent trends of metastable dissociation processes observed for alcohol and nitrile cluster ions.     

Tetra‐ and hexahydrates of bis(adeninium) zoledronate

The present paper reports the structures of bis(adeninium) zoledronate tetrahydrate {systematic name: bis(6‐amino‐7H‐purin‐1‐ium) hydrogen [1‐hydroxy‐2‐(1H‐imidazol‐3‐ium‐1‐yl)‐1‐phosphonatoethyl]phosphonate tetrahydrate}, 2C₅H₆N₅⁺·C₅H₈N₂O₇P₂²⁻·4H₂O, (I), and bis(adeninium) zoledronate hexahydrate {systematic name: a 1:1 cocrystal of bis(6‐amino‐7H‐purin‐1‐ium) hydrogen [1‐hydroxy‐2‐(1H‐imidazol‐3‐ium‐1‐yl)‐1‐phosphonatoethyl]phosphonate hexahydrate and 6‐amino‐7H‐purin‐1‐ium 6‐amino‐7H‐purine dihydrogen [1‐hydroxy‐2‐(1H‐imidazol‐3‐ium‐1‐yl)ethane‐1,1‐diyl]diphosphonate hexahydrate}, 2C₅H₆N₅⁺·C₅H₈N₂O₇P₂²⁻·6H₂O, (II). One of the adenine molecules and one of the phosphonate groups of the zoledronate anion of (II) are protonated on a 50% basis. The zoledronate group displays its usual zwitterionic character, with a protonated imidazole ring; however, the ionization state of the phosphonate groups of the anion for (I) and (II) are different. In (I), the anion has both singly and doubly deprotonated phosphonate groups, while in (II), it has one singly deprotonated phosphonate group and a partially deprotonated phosphonate group. In (I), the cations form an R₂²(10) base pair, while in (II), they form R₂²(8) and R₂²(10) base pairs. Two water molecules in (I) and five water molecules in (II) are involved in water–water interactions. The presence of an additional two water molecules in the structure of (II) might influence the different ionization state of the anion as well as the different packing mode compared to (I).     

Aromaticity in Group 14 homologues of the cyclopropenylium cation

The nature of the bonding and the aromaticity of the heavy Group 14 homologues of cyclopropenylium cations E₃H₃⁺ and E₂H₂E′H⁺ (E, E′=C–Pb) have been investigated systematically at the BP86/TZ2P DFT level by using several methods. Aromatic stabilization energies (ASE) were evaluated from the values obtained from energy decomposition analysis (EDA) of charged acyclic reference molecules. The EDA‐ASE results compare well with the extra cyclic resonance energy (ECRE) values given by the block localized wavefunction (BLW) method. Although all compounds investigated are Hückel 4n+2 π electron species, their ASEs indicate that the inclusion of Group 14 elements heavier than carbon reduces the aromaticity; the parent C₃H₃⁺ ion and Si₂H₂CH⁺ are the most aromatic, and Pb₃H₃⁺ is the least so. The higher energies for the cyclopropenium analogues reported in 1995 employed an isodesmic scheme, and are reinterpreted by using the BLW method. The decrease in the strength of both the π cyclic conjugation and the aromaticity in the order C?Si>Ge>Sn>Pb agrees reasonably well with the trends given by the refined nucleus‐independent chemical shift NICS(0)_πzz index.     

Hydrogen bond in the H3O(Ph3PO) 3+ complex. Features of the electron density distribution

A quantum chemical study of the hydrogen bond in the H₃O(Ph₃PO)₃ ⁺ complex is carried out using topological methods: quantum theory of atoms in molecules (QTAIM) and of electron localization function (ELF) theory. In the H₃O(Ph₃PO)₃ ⁺ complex, three lone electron pairs on the oxygen atom in Ph₃PO are found to be combined in one ELF basin, and they all participate in the hydrogen bond formation. The observed topological features of the H₃O(Ph₃PO)₃ ⁺ complex are compared to the topological features of related complexes and the literature data.     

AM1 studies on the potential energy surface for the proton transfer in protonated water clusters,H+(H2O)n

The potential energy surfaces for the proton transfer processes in H⁺(H₂O)_n with n=2 ～ 11 have been studied using the semiempirical AM1 method. Two model systems were adopted: branched and linear systems. The branched system showed a tendency to form a bulk cluster, while the linear system showed a tendency toward a constant barrier height with increasing number of water molecules in the model system. The potential energy surfaces were discussed using Marcus theory. In the case of H⁺ (H₂O)_n with n=10 and 11, the intrinsic barrier to the proton transfer was found to be around 1.0 kcal/mol.     

Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions

The acid-catalyzed aquation of [Cr(pic)(H₂O)₄]₂ ²⁺ and [Cr(dpic)(H₂O)₃]⁺(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO₃ range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H⁺] have been established:k _obs=a+b[H⁺](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and k _obs=b[H⁺]+c[H⁺]²(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.     

Neutral 4‐phenyl‐1‐[phenyl(pyridin‐2‐yl)methylidene]semicarbazide and its salt forms with inorganic anions

Semicarbazones can exist in two tautomeric forms. In the solid state, they are found in the keto form. This work presents the synthesis, structures and spectroscopic characterization (IR and NMR spectroscopy) of four such compounds, namely the neutral molecule 4‐phenyl‐1‐[phenyl(pyridin‐2‐yl)methylidene]semicarbazide, C₁₉H₁₆N₄O, (I), abbreviated as HBzPyS, and three different hydrated salts, namely the chloride dihydrate, C₁₉H₁₇N₄O⁺·Cl^?·2H₂O, (II), the nitrate dihydrate, C₁₉H₁₇N₄O⁺·NO₃^?·2H₂O, (III), and the thiocyanate 2.5‐hydrate, C₁₉H₁₇N₄O⁺·SCN^?·2.5H₂O, (IV), of 2‐[phenyl({[(phenylcarbamoyl)amino]imino})methyl]pyridinium, abbreviated as [H₂BzPyS]⁺·X^?·nH₂O, with X = Cl^? and n = 2 for (II), X = NO₃^? and n = 2 for (III), and X = SCN^? and n = 2.5 for (IV), showing the influence of the anionic form in the intermolecular interactions. Water molecules and counter‐ions (chloride or nitrate) are involved in the formation of a two‐dimensional arrangement by the establishment of hydrogen bonds with the N—H groups of the cation, stabilizing the E isomers in the solid state. The neutral HBzPyS molecule crystallized as the E isomer due to the existence of weak π–π interactions between pairs of molecules. The calculated IR spectrum of the hydrated [H₂BzPyS]⁺ cation is in good agreement with the experimental results.     

Sequential bond energies of water to sodium glycine cation

Absolute bond dissociation energies of water to sodium glycine cations and glycine to hydrated sodium cations are determined experimentally by competitive collision-induced dissociation (CID) of Na⁺Gly(H₂O)_x, x = 1–4, with xenon in a guided ion beam tandem mass spectrometer. The cross sections for CID are analyzed to account for unimolecular decay rates, internal energy of reactant ions, multiple ion–molecule collisions, and competition between reaction channels. Experimental results show that the binding energies of water and glycine to the complexes decrease monotonically with increasing number of water molecules. Ab initio calculations at four different levels show good agreement with the experimental bond energies of water to Na⁺Gly(H₂O)_x, x = 0–3, and glycine to Na⁺(H₂O), whereas the bond energies of glycine to Na⁺(H₂O)_x, x = 2–4, are systematically higher than the experimental values. These discrepancies may provide some evidence that these Na⁺Gly(H₂O)_x complexes are trapped in excited state conformers. Both experimental and theoretical results indicate that the sodiated glycine complexes are in their nonzwitterionic forms when solvated by up to four water molecules. The primary binding site for Na⁺ changes from chelation at the amino nitrogen and carbonyl oxygen of glycine for x = 0 and 1 to binding at the C terminus of glycine for x = 2–4. The present characterization of the structures upon sequential hydration indicates that the stability of the zwitterionic form of amino acids in solution is a consequence of being able to solvate all charge centers.     

Intrinsic Acid–Base Properties of a Hexa‐2′‐deoxynucleoside Pentaphosphate,d(ApGpGpCpCpT): Neighboring Effects and Isomeric Equilibria

The intrinsic acid‐base properties of the hexa‐2′‐deoxynucleoside pentaphosphate, d(ApGpGpCpCpT) [=(A1?G2?G3?C4?C5?T6)=(HNPP)^5?] have been determined by ¹H NMR shift experiments. The pK_a values of the individual sites of the adenosine (A), guanosine (G), cytidine (C), and thymidine (T) residues were measured in water under single‐strand conditions (i.e., 10 % D₂O, 47 °C, I=0.1 M , NaClO₄). These results quantify the release of H⁺ from the two (N7)H⁺ (G?G), the two (N3)H⁺ (C?C), and the (N1)H⁺ (A) units, as well as from the two (N1)H (G?G) and the (N3)H (T) sites. Based on measurements with 2′‐deoxynucleosides at 25 °C and 47 °C, they were transferred to pK_a values valid in water at 25 °C and I=0.1 M . Intramolecular stacks between the nucleobases A1 and G2 as well as most likely also between G2 and G3 are formed. For HNPP three pK_a clusters occur, that is those encompassing the pK_a values of 2.44, 2.97, and 3.71 of G2(N7)H⁺, G3(N7)H⁺, and A1(N1)H⁺, respectively, with overlapping buffer regions. The tautomer populations were estimated, giving for the release of a single proton from five‐fold protonated H₅(HNPP)^±, the tautomers (G2)N7, (G3)N7, and (A1)N1 with formation degrees of about 74, 22, and 4 %, respectively. Tautomer distributions reveal pathways for proton‐donating as well as for proton‐accepting reactions both being expected to be fast and to occur practically at no “cost”. The eight pK_a values for H₅(HNPP)^± are compared with data for nucleosides and nucleotides, revealing that the nucleoside residues are in part affected very differently by their neighbors. In addition, the intrinsic acidity constants for the RNA derivative r(A1?G2?G3? C4?C5?U6), where U=uridine, were calculated. Finally, the effect of metal ions on the pK_a values of nucleobase sites is briefly discussed because in this way deprotonation reactions can easily be shifted to the physiological pH range.     

A photo-ionization study of organosulfur ring compounds: Thiirane,thietane and tetrahydrothiophene

The gas phase heats of formation of several organosulfur cations were determined from thiirane, thietane and tetrahydrothiophene precursor molecules by photoionization mass spectrometry. Heats of formation at 0 K and 298 K are reported for the following ions: [H₂CS]^+˙, [H₃CS]⁺, [C₂H₃S]⁺, [C₂H₄S]^+˙, [C₃H₅S]⁺, [C₃H₆S]^+˙, [C₄H₇S]⁺ and [C₄H₈S]^+˙. The [C₄H₇S]⁺ (m/z 87), [C₂H₄S]^+˙ (m/z 60), [C₂H₃S]⁺ (m/z 59), [C₄H₇]⁺ (m/z 55), [C₄H₆]^+˙ (m/z 54) and [CH₂S]^+˙ (m/z 46) ions are produced from metastable tetrahydrothiophene ions at photon energies between 10.2 and 10.7 eV. Decay rates of internal energy selected parent ions to the m/z 60, 59, 55 and 54 fragments were measured by threshold photoelectron-photoion coincidence, the results of which are compared to statistical theory (RRKM/QET) calculations. The [C₂H₄S]^+˙ ion from tetrahydrothiophene is found to have the thioacetaldehyde structure. From the measured [C₂H₄S]^+˙ onset a ΔH = 50±8 kJ mol^?1 was calculated for the thioacetaldehyde molecule.