Bridging Proton Research Articles

Proton-exchange mechanisms in phenol–tertiary amine–aprotic solvent systems

1 H NMR experiments at different temperatures exhibit perturbations of the phenol–amine 1:1 complexes by the phenol or amine molecules, in solutions with a molar excess of either the acid or base. In the former, (phenol)2–amine 2:1 complexes with OHO and OHN hydrogen bonds are formed. The exchange between these bridging protons require the rupture and reformation of the OHN bonds. In the latter, the phenol–amine 1:1 complexes weakly interact with the base molecules. The exchange between the complexing and surrounding base molecules is achieved by the rupture and reformation of the OHN bonds when the phenols are less acidic. For more acidic systems, phenol–(amine)2 transition complexes with bifurcated hydrogen bonds are formed. This phenomenon is accompanied by the reversible proton transfer inside the hydrogen bridges, O—H⋯N ⇄ O–⋯H—N+, of the complexes dissolved in halogenated solvents at low temperatures.

The conformational equilibrium of [2.2]paracyclophanes in solution

AbstractA conformational equilibrium exists in solution between two skew forms (s+ and s−) of [2.2]paracyclophane (1a). The vicinal coupling constants in the 1H‐NMR spectra of the bridge protons in the six ar‐monosubstituted derivatives 1b–1g (RCN, NO, Br, CH3, CHO, and NO2) indicate an increasing shift of the equilibrium towards one side in the order mentioned. The s+ form (positive CCCC torsional angles of the bridges) is favored in the (S) enantiomer. MM2(91) and MM3(89) molecular mechanics computations suggest that this is largely caused by increasing unfavorable nonbonded interactions in the s− conformer between the substituent and the syn proton on the ortho bridge carbon atom. Thus the „effective size”︁ of the substituent increases from 1b towards 1g.

Influence of different ionization conditions on the mass spectrometric behaviour of some fluorinated acyclic nucleoside analogs

AbstractThe behaviour of three fluorinated acyclic nucleoside analogs has been examined under different ionization conditions, i.e. by electron ionization, fast‐atom bombardment and positive‐ion chemical ionization using methane, isobutane and ammonia as reactant gases. The protonated molecules have been studied by collisional spectroscopy. In general, protonation takes place on the functional groups exhibiting a proton affinity higher than that of the reactant species but some discrepancies are present. They have been explained by the presence of quasi SHF structures implicating proton bridging between sulphur and fluorine atoms.

Reasons for the variability of isotopic ratios of stretching vibration frequencies of H-bonded complexes

The effect of random resonances and vibration anharmonicity on the isotopic ratios of frequencies for molecules containing hydrogen atoms has been studied. It was found that the “anomalous” frequency dependence of the v(XH)/v(XD) isotopic ratio of the stretching vibration frequencies observed for H-bonded systems can be explained in terms of the theory of harmonic vibrations of molecules. The substantial deviation of experimental v(XH)/v(XD) from the magnitude equal to the square root of the reciprocal ratio of adjusted masses results from random resonances between the stretching mode of the bridging proton and some other vibration of the system.

The role of short hydrogen bonds in mechanisms of enzymatic action

1H and 13C NMR spectra of trypsin and ribonuclease, stabilized by chemical modification with a hydrophilic polymer, have been obtained over a wide pH range (1–11). The spectral features, referred to some nuclei of the catalytic sites (the “catalytic triad” for trypsin and the His-12—His-119 pair for ribonuclease), have been identified using different NMR techniques as well as chemical modification with selective reagents. It is found that monoprotonation of these systems leads to symmetrical (or quasi-symmetrical) H-bonds formed between the basic groups. This allows us to explain the discrepancies between experimental data obtained by different authors on the protonation sites in these catalytic systems. The simulation of the catalytic triad by a 15N labeled low molecular weight model has led us to the conclusion that external agents do not cause any discrete proton transfers but do cause a smooth shift of the bridging protons from one basic atom to another, with the quasi-symmetrical H-bonds being formed in intermediate cases. On the basis of these experimental data, a new concept has been proposed for the mechanism of acid—base catalysis performed by the pairs of weak basic groups like His—Im and Asp(Glu)—COO − (p K a 3—7) which are not capable of proton abstraction from alcoholic or water OH groups (p K a > 13). This catalysis may consist on the one hand of changing the charge densities on reacting groups due to strong H-bonding and, on the other hand, of facilitating the free movement of a proton in the field of several basic atoms when going along the reaction coordinate. The energy of the very strong H-bonds thus formed diminishes the activation energy of the reaction.

Photoenolization of 4‐Naphthoyl[2.2]paracyclophanes

AbstractWhen ethanolic solutions of 4‐(1‐naphthoyl)[2.2]paracyclophane (3) and 4‐(2‐naphthoyl)[2.2]paracyclophane (4) are subjected to UV irradiation at low temperature, 1,5‐hydrogen migration of the 2‐H bridge proton to the carbonyl group takes place, leading to the enol of type 2. In the context of mechanistic considerations the triplet spectroscopic properties of 3 and 4 are discussed.

The structure and IR spectrum of the hydrogen-bonded adduct of 2,6-dichloro-4-nitrophenol with 3,4-dimethylpyridine

Abstract The adduct of 2,6-dichloro-4-nitrophenol with 3,4-dimethylpyridine crystallizes in the space group P 1 with a = 7.124(4), b = 13.469(3), c = 15.797(3) A, α = 103.80(2), β = 97.94(3), γ = 102.16(3)°, Z = 4. Two crystallographically non-equivalent molecules of the adduct occur in the lattice. The OHN intermolecular hydrogen bridges are very short, with values of 2.532(3)_and 2.529(3) A. The geometry of the molecules and the localization of the bridge proton indicate that the proton is transferred to the N atom although the IR spectrum is close to those of “critical” hydrogen bridges where the proton is shared between the O and N atoms.

Structural and magnetic consequences of protonation of the oxo bridge in bis(hydridotripyrazolylborato)(.mu.-oxo)bis(.mu.-carboxylato)divanadium complexes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructural and magnetic consequences of protonation of the oxo bridge in bis(hydridotripyrazolylborato)(.mu.-oxo)bis(.mu.-carboxylato)divanadium complexesCarl J. Carrano, Ruben Verastgue, and Marcus R. BondCite this: Inorg. Chem. 1993, 32, 17, 3589–3590Publication Date (Print):August 1, 1993Publication History Published online1 May 2002Published inissue 1 August 1993https://doi.org/10.1021/ic00069a007RIGHTS & PERMISSIONSArticle Views120Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (241 KB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information Get e-Alerts

Cobaltocenium metallacarboranide mixed-sandwich salts, Cp2Co+(Et2C2B4H4)2M- (M = CoIII, ironIII) and Cp2Co+(Et2C2B3H4-X)CoIII(Et2C2B4H4)- (X = H, Me)

The first metallocenium metallacarboranide salts have been synthesized and isolated. The title compounds were prepared by reaction of cobaltocene and CoCl 2 or FeCl 2 with the carborane anion Et 2 C 2 B 4 H 5 - in THF solution, in which the oxidation of two M(II) metal centers to M(III) is accompanied by reduction of two B-H-B bridging protons to H 2 .

1H and 13C NMR spectra of multibridged [2n]cyclophanes

AbstractThe 1H and 13C NMR spectra of all [2n] cyclophanes with the same substitution pattern in both benzene rings and of the skew [2.2.2](1,2,4)(1,2,5)cyclophane were assigned by one‐dimensional NOE difference and two‐dimensional correlation spectroscopy. The 1H NMR spectra of the bridge protons including the 13C satellites of the symmetrical compounds [2.2]paracyclophane, [2.2.2](1,3,5)cyclophane and superphane were analysed by iterative methods. Chemical shifts and H,H coupling constants are discussed with regard to molecular geometries.

Switching the mechanism of spin-exchange coupling in a (.mu.-oxo)bis(.mu.-acetato)chromium(III)vanadium(III) complex by protonation of the oxo bridge

Switching the mechanism of spin-exchange coupling in a (.mu.-oxo)bis(.mu.-acetato)chromium(III)vanadium(III) complex by protonation of the oxo bridge

A hydrogen atom in an organoplatinum-amine system. 1. Synthesis and spectroscopic and crystallographic characterization of novel zwitterionic complexes with a Pt(II)-.cntdot..cntdot..cntdot.H-N+ unit

The unusual bridging-hydrogen complexes PtX(L-C,N)(LH-C,H) containing a zwitterionic Pt(II)[sup [minus]][center dot][center dot][center dot]H-N[sup +] moiety have been obtained in a reaction of cis-Pt(L-C,N)[sub 2] with R[sub 2]SnX[sub 2] (R = Me, Ph; X = Cl, Br) in a mixture of CH[sub 2]Cl[sub 2] and MeOH. The chelating amine ligand systems L used in this reaction are [1-C[sub 10]H[sub 6]NMe[sub 2]-8][sup [minus]], [C[sub 6]H[sub 4]CH[sub 2]NMe[sub 2]-2][sup [minus]], and [C[sub 6]H[sub 4]CH[sub 2]NEt[sub 2]-2][sup [minus]]. The complex PtCl(1-C[sub 10]H[sub 6]NMe[sub 2]-8-C,N)(1-C[sub 10]H[sub 6]NHMe[sub 2]-8-C,H) can also be obtained by reaction of cis-Pt(1-C[sub 10]H[sub 6]NMe[sub 2]-8-C,N)[sub 2] with dry HCl. The bridging proton of the zwitterionic Pt(II)[sup [minus]][center dot][center dot][center dot]H-N[sup +] moiety shows a characteristic [sup 1]H NMR resonance at ca. 11-16 ppm with a [sup 1]J([sup 195]Pt,[sup 1]H) of ca. 65-180 Hz; the latter decreases with increasing ligand flexibility. In the Pt[center dot][center dot][center dot]H-N moiety of the PtX(L-C,N)(LH-C,H) complexes, an increase of the Pt[center dot][center dot][center dot]H interaction is associated with a simultaneous decrease of the H-N interaction. When L is chiral [C[sub 6]H[sub 4]CH(Me)NMe[sub 2]-(R)-2][sup [minus]], cis-Pt(L-C,N)[sub 2] affords in a reaction with Me[sub 2]SnBr[sub 2] the nonbridging complex PtBr(C[sub 6]H[sub 4]CH(Me)NMe[sub 2]-(R)-2-C,N)(C[sub 6]H[sub 4]CH(Me)NHMe[sub 2]-(R)-2-C) as themore » major product. The X-ray crystal structures of both a bridging complex and a nonbridging complex are described. Especially the latter shows good evidence that the bridging interaction is largely electrostatic. Internal steric influences only affect the stability of the bridging moiety when they arise from groups on the benzylic carbon atom. 48 refs., 5 figs., 4 tabs.« less

Multifield Saturation Magnetization Measurements of Oxidized and Reduced Ribonucleotide Reductase from Escherichia coli

A diiron(II) center doubly bridged by carboxylate groups (see sketch) is present in reduced ribonucleotide reductase according to magnetization measurements: the strong anti-ferromagnetic coupling of the oxidized diiron(III) site consistent with an oxo bridge is no longer observed. Protonation of the oxo bridge during the reduction is postulated. The loss of the resulting aqua bridge then leads to the structural unit shown here.

The proton position in amine-HX (X  Br, I) complexes

Proton transfer potential energy surfaces are computed by ab initio methods for complexes of HBr and HI with NH 3 and mono, di, and trimethylamine. The fundamental natures of the SCF and MP n surfaces are distinct in that two separate minima are present in the SCF surfaces, corresponding to a neutral and ion pair, whereas only one minimum is encountered in most of the correlated surfaces. The single minimum in the MP2 surfaces of most of the complexes represents an ion pair. The first exception is BrH⋯NH 3 which exists as a neutral pair. The bridging proton in BrH⋯NH 2CH 3 is located approximately midway between the two molecules in a “shared” arrangement. The last exception is IH⋯NH 3, the surface of which may be best characterized as a long shallow valley for proton motion. The binding energies of these complexes rise as the amine becomes more basic due to progressive methylation.

Simple general acid-base catalysis of physiological acetylcholinesterase reactions

Elements of transition-state stabilization by proton bridging have been characterized by measuring solvent isotope effects and proton inventories for hydrolyses of (acetylthio)choline (ATCh), (propionylthio)choline (PrTCh), and (butyrylthio)choline (BuTCh) catalyzed by acetylcholinesterases (AChEs) from Electrophorus electricus, fetal bovine serum, human erythrocytes, and Torpedo californica. For the Electrophorus enzyme, the acylation rate constant, k cat /Km {=(V/K)/[E] T } decreases in the order ATCh>PrTCh>>BuTCh

Chemical ionization mass spectra of the stereoisomeric 1,5‐anhydropentofuranoses, 1,6‐anhydrohexofuranoses and 1,6 : 3,5‐dianhydrohexofuranoses

AbstractThe isobutane and methane direct chemical ionization (DCI) mass spectra of mono‐ and dianhydrosugars are substantially influenced by their stereochemistry at temperatures of 150–250°C. The best results are obtained under isobutane CI at 200 or 250°C. l,6:3,5‐Dianhydrogulofuranose gives proton‐bridge stabilized MH+ ions and fewer fragment ions compared with its ido epimer. The four diastereomeric 1,5‐anhydropentofuranoses show increased MH+ and M2H+ abundances for the lyxo and ribo isomers with cis‐1,2‐diol sites capable of proton bridging. The xylo isomer gives a dominant [M  OH]+ ion. All isomers, but especially xylo and ribo, give abundant [C3H5O2]+ skeleton cleavage ions. They are attributed to an RDA fragmentation of pyranoglycosyl‐type MH+ ions. The eight diastereomeric 1,6‐anhydrohexofuranoses with endo/exo configured triol structures show very characteristic MH+ abundances (4–41% Σ). They parallel the amount of proton solvation in cis‐diol sites of the isomers (gluco ⩽ ido ⩽ altro < gulo < galacto < talo < allo < manno). The [M  OH]+ ion abundances (15–47% Σ) are also very characteristic (galacto ⩽ talo < manno < altro ≪ atto < gluco ⩽ ido ≪ gulo). They reflect the possible H2O or C4H,OH loss from the MH+ and (M · C4H9]+ ions, respectively, with formation of protonated 1,6:3,5‐and 1,6: 23‐dianhydrehexofuranose species; the slightly flattened C(5) site of M might favour the departure of the 5‐OH group. In addition, there are especially high abundances of skeleton fragment ions, 15% Σ [MH  HCOOH]+, 10% Σ [M  OH  H3O  CH3O]+ and 42% Σ [M  OH  C2H4O2)+, for the ido, galacto and gluco isomers, respectively.

Studies on bis(η-indenyl)niobium complexes

The reaction between [NbCl4(thf)2](thf = tetrahydrofuran) and Li[C9H7] or Na[C9H7]·thf gives [Nb(η-C9H7)2Cl2]1, which reacts with NaBH4 to give [Nb(η-C9H7)2(µ-H2BH2)]2. The bridging and terminal tetrahydroborate protons in 2 exchange on the NMR time-scale (ΔG‡= 55.2 ± 1.6 kJ mol–1). Compound 2 reacts with PMe2Ph or pyridine via BH3 extraction to give [NbH(η-C9H7)2(PMe2Ph)]3 and [NbH(η-C9H7)2(NC5H5)]4 respectively. The pyridine ligand in 4 undergoes restricted rotation about the Nb–N bond (ΔG‡= 52 ± 1 kJ mol–1). The reaction of 2 with NMe3 in the presence of CO, H2CCCH2, trans,trans-hexa-2,4-diene or styrene gives the new complexes [Nb(η-C9H7)(CO)4]5, [Nb(η-C9H7)2(η-CH2CHCH2)]6, [Nb(η-C9H7)2(η-MeCHCHCHCH2Me)]7 and endo-[NbH(η-C9H7)2(η-H2CCHPh)]9 respectively. The styrene hydride (complex 9) is fluxional and this process has been studied using both one- and two-dimensional NMR techniques. The activation barrier to olefin hydride exchange has been determined by coalescene methods to be ΔG‡= 70.0 ± 2 kJ mol–1 at 50 °C, this is significantly lower than that for the analogous η-C5H5 and η-C5Me5 complexes. A mechanism that involves a ring slipped intermediate is proposed.

Synthesis and conformational behaviour of ditosyldiaza[2.2]orthometacyclophanes

The N,N′-ditosyl-diaza[2.2]orthometacyclophanes5a, b were synthesized from N,N-ditosyl-metaphenylenediamine by reaction with (Z)-1,4-dichlorobutene and 1,2-bis-bromomethylbenzene, respectively. Low temperature NMR studies showed that the compound5b exists as a 1:1 mixture of chair and boat form of the strained (E,Z)-diazanonadiene ring. At room temperature all corresponding resonances are averaged on the NMR time scale (including all four ethylene bridge protons). Going to lower temperatures, in a first step the methylene bridge inversion is frozen (giving twoexo H and twoendo H, ΔG≠ ca. 52kJmol−1). In a second step the chair and boat form can be observed separately (ΔG≠ ca. 43.5 kJ mol−1 for the chair/boat flip). The assignments were confirmed by 2D NMR experiments.

Switching the mechanism of spin-exchange coupling in (.mu.-oxo)bis(.mu.-carboxylato)divanadium(III) complexes by protonation of the oxo bridge

Switching the mechanism of spin-exchange coupling in (.mu.-oxo)bis(.mu.-carboxylato)divanadium(III) complexes by protonation of the oxo bridge

Comparison of the spectroscopic properties of metalloporphyrins

Electron ionisation (EI) and positive NH 3 chemical ionisation (CI) mass spectra, 1H NMR and IR spectra were obtained for octaethylporphyrin (OEP) and some of its metal derivatives. In each case, the spectra displayed a strong metal-dependency. For EI mass spectra, there was no obvious metal-dependency for singly charged fragment ions; however, the ratio of doubly charged fragment ions to singly charged fragment ions was metal-dependent. For chelated metal ions with closed outer shell electron configurations, there was a correlation between this ratio, and the ratio of the electronegativity (Pauling scale) to the atomic radius of the metal ion. The correlation does not work for open shell transition metal complexes. In CI mass spectra there appears to be an inverse relationship between the relative abundance of the pyrrolic fragment ions and the electronegativity of the metal ion. The extent of the de-shielding of the meso (bridge) protons in the 1H NMR was directly related to both the ionic radius and the oxidation state of the chelated metal ion. The IR spectrum showed several metal-dependent peaks. The peak at ca. 920 cm −1 was dependent on both the metal-pyrrolic nitrogen bond distance and the charge of the complexed metal ion. It is hoped that these data will prove valuable both in determining the role of post-chelation factors in the predominance of Ni(II) and O = V(IV) porphyrins in the geosphere, and in the detection of any other geoporphyrin metal complexes.