Bridging Proton Research Articles

Hydrogen bonding in systems containing imidazoline nitroxide radicals

Hydrogen bonding between imidazoline nitroxide free radicals and hydrogen donors of varying strength [acetone, chloroform, trifluoroacetic acid (TFA)] has been examined using magnetic resonance techniques. Low-field dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. Dipolar coupling was found to increase in the order acetone <CHCl3<TFA, while only TFA systems showed substantial scalar coupling. For acetone, observed dipolar relaxation rates are consistent with translational modulation and little, if any, complexation. With CHCl3, both translational and rotational modulation are required; the data suggest that CHCl3 forms weak hydrogen bonds with the radical chromophore with little spin density transferred to the bridge proton. In contrast, the protons of TFA show strong scalar coupling with the various radicals which results in positive proton NMR enhancements or strong hyperfine coupling (AH?5 G). The 1H scalar relaxation rates are consistent with strong hydrogen bond formation at the imine site and complex lifetimes of ∼10−9–10−8 sec. The 19F nuclei of TFA show much weaker, radical-independent scalar coupling and little rotational dipolar modulation consistent with, at best, weak, nonstereospecific complex formation.

Catalytic proton bridge in acetyl imidazolium ion hydrolysis implicated by a proton inventory

The origin of the solvent isotope effect, k/sub H/sub 2/O/k/sub D/sub 2/O/ = 2.58, for the water-promoted hydrolysis of acetylimidazolium ion has been probed using the proton inventory technique. The proton inventory suggests that the observed effect is comprised of three transition state contributions. A chemical model is proposed for the hydrolysis transition state which contains a catalytic proton bridge between the reorganizing substrate and a water molecule serving as a general base catalyst.

The synthesis, structure, and chemical reactions of metalloboranes

Recent work on the synthesis, structure, and chemical reactions of metalloboranes is reviewed. In the decaborane series, single-crystal X-ray studies indicate that the metal atom can be (a) fully incorporated within the borane cluster, (b) joined to the cluster by an exopolyhedral B-M-B bond in which the metal replaces a bridging proton, or (c) joined by a normal o B-M bond by replacement of a terminal hydrogen atom. Complexes of B 10 H 12 2- with Fe; Zn, Cd, Hg; Mg; Al, Ga, In, Tl; and Sn are specifically discussed. In the pentaborane series numerous complexes have been prepared for a wide variety of transition metals and main group metals (Ti; Fe; Co, Rh, Ir; Ni, Pd, Pt; Cu, Ag, Au; Cd, Hg; and Si) and details of the bonding are elucidated. Kinetic control of products frequently appears to be more important than the achievement of greatest thermodynamic stability. Replacement of bridge protons or terminal hydrogen atoms in the basal borons are the prevalent modes of attachment for B 5 and B 6 systems and no examples of B-H-M bonding have been detected. By contrast, B 3 H 8 is found to bond exclusively via B-H-M bonds in its non-ionic derivatives (examples include complexes of Mn; Fe; Ir; Zn; Al, Ga, In; Si, Ge, Sn, and Pb). NMR spectroscopy proves to be valuable in elucidating the structure and in defining which complexes are fluxional and which have a rigid ‘tetraborane-type’ structure in which a coordinated metal centre formally replaces a BH 2 group in tetraborane (10). The results, taken as whole, establish the general ability of boron hydrides and their anions to act as electron-donating ligands to most, if not all, metals in the Periodic Table.

Molecular orbital studies of hydrogen bonds

The ground state, the lowest singlet and triplet n-π* states, and the lowest triplet π-π* state of the formic acid monomer and dimer are studied with the ab initio molecular orbital theory. The two-configuration electron-hole potential method is used for calculations of excited states of dimers. The potential energy curves for the symmetrical simultaneous movement of two bridging protons are studied for all of the states. The barrier of the proton transfer in the ground state is found to be the smallest of the states studied. The association energy is analyzed in terms of various components.

Studies of the smaller boron hydrides and their derivatives

Abstract A survey of systematic studies of the smaller boron hydrides is presented. Relative Bronsted acidities have been determined and the Lewis base properties of anions derived by removing a bridge proton have been investigated. Addition of BH 3 to these anions results in expanded borane anion structures. These expanded structures can be converted on a large scale to B 5 H ll, B 6 H 10 , and B 6 H 12 in high yield. Principles are described for preparing certain specifically methyl substituted derivatives of the smaller boron hydrides and boron hydride anions. The nature of the basic boron-boron site in B 6 H 10 and in boron hydride anions is discussed with respect to fluxional properties and the formation of selected metalloboranes. The structures and properties of some bis-ligand adducts of B 5 H 9 and B 6 H 10 (B 5 H 9 L 2 and B 6 H 10 L 2 ) and their relationships to the hypho class of cluster systems is discussed.

ChemInform Abstract: COMPLEX HYDROGEN BONDED CATIONS. THE BENZIMIDAZOLE BENZIMIDAZOLIUM CATION

The complex salts (Bz2H)+Y−, Bz = benzimidazole, Y = PF6− or BF4− have been identified in solution, and the molecularity of the fluoroborate, which has been crystallised, confirmed gravimetrically. Infrared and Raman spectra, together with those of the deuterated systems, indicate a similar structure in solution and the crystal, viz., an asymmetric single minimum bridging proton potential, in conformity with the X-ray structure, proving that the pronounced doublet observed in the vs band of the H bridged system is not due to proton tunnelling. The origin of the asymmetric structure is discussed.

Complex hydrogen bonded cations. The benzimidazole benzimidazolium cation

The complex salts (Bz2H)+Y−, Bz = benzimidazole, Y = PF6− or BF4− have been identified in solution, and the molecularity of the fluoroborate, which has been crystallised, confirmed gravimetrically. Infrared and Raman spectra, together with those of the deuterated systems, indicate a similar structure in solution and the crystal, viz., an asymmetric single minimum bridging proton potential, in conformity with the X-ray structure, proving that the pronounced doublet observed in the vs band of the H bridged system is not due to proton tunnelling. The origin of the asymmetric structure is discussed.

Diastereotope Methylenprotonen und Methylgruppen in 2-Substituierten 1,6-Methano [10] Annulenen

Abstract The double magnetic non equivalence of the diastereotopic methylene protons resp. geminal methyl groups of 2-substituted 1,6-methano [10]annulenes were observed in five solvents. It was found that ΔνAB for one of the two methylene groups were much smaller than ΔνA ‘B’ for the other and that ΔνA‘B’ was much more solvent dependent than ΔνAB. ΔνA “B” for the bridge protons varied very little with the solvent used.

Complex hydrogen bonded cations-V further asymmetric ions

Solutions have been prepared containing the complex cations ( 1BH 2B) +, where the differing bases may be pyridine, 4-Me pyridine, NMe imidazole, thiazole, isooxazole, trimethylamine or NMe piperidine. The infrared spectra usually show a double maximum in the v s band, the intensity of the lower component decreasing as the difference in the p Ka's of the bases increases. In the D-bridged analogues only a single v sband occurs. Other features of the spectra indicate that the bridging proton potential is asymmetric. The lower frequency component of the v s band of the H-bonded system is attributed to Fermi resonance, not proton tunnelling. The proton potential in these and the related symmetric complex cations is reviewed.

ChemInform Abstract: RECENT ADVANCES IN THE CHEMISTRY OF BORON HYDRIDES

AbstractDas Lewisbase‐Verhalten von B5H10 gegenüber den Lewissäuren Bortrichlorid, Bortribromid, Aluminiumtrialkylen und den Borwasserstoffen B3H7, B4H3, B3H12 und i‐B9H13 wird untersucht.

Proton bridges in enzyme catalysis

Chains of two hydrogen bonds (charge-relay systems) exist in the active sites of serine proteases and may be catalytic entities. It is shown theoretically that the coupling of the motions of the two protons in such arrays (and thus the efficiency with which they can relay charge) is a critical function of the distances across the hydrogen bonds; long distances favour uncoupled motion, short distances coupled motion. Rate measurements in mixtures of light and heavy water show that the serine proteases chymotrypsin, trypsin and elastase function as one-proton catalysts, one proton of the chain presumably bridging as in ordinary general catalysis. On the other hand, three enzymes of the amidohydrolase class, a glutaminase and two asparaginases, show two-proton catalysis. This may arise from a charge-relay chain, although evidence for such a structure has not yet been advanced for these enzymes, or from some other catalytic entity involving two coupled proton bridges.

Pseudoaromatic compounds. Part XXII. Reactions of 2-functionalized tropones with sodium toluene-p-thiolate

Relative rates for replacement, without rearrangement, of the X group of some 2-X-tropones by sodium toulene-p-thiolate in dimethyl sulphoxide are in the order 82,2.6,3.5,1.0,and ca. 29 000 for X = F,Cl,Br,I, and N[CH2CH2]3-CHI–, respectively, whilst with X = OCH3 slow demethylation occurs instead. The rate of replacement of the quinuclidinium group has been computed for the free reagents present in equilibrium with an adduct which lies off the reaction co-ordinate. These data give support to the idea that ready replacement of strongly bound X groups, such as fluorine or methoxy, of 2-X-tropones by primary or secondary amines is due to stabilisation of the transition state by proton bridging among the protonated nitrogen, the tropone carbonyl oxygen, and for fluorine, the leaving-group.

A Fermi-resonance window effect

The broad ν s (NH stretching) band in the i.r. spectra of the complex cations (TMAH B) +, where TMA is Trimethylamine, and B is Isooxazole, Thiazole, Pyridine, 3-Me Pyridine, 4-Me Pyridine, or methyl Imidazole, moves to lower frequency as the pK of B increases. A ‘window’ remains fixed at ∼ 2600 cm −1. This is ascribed to Fermi resonance of ν s with a TMA combination mode, and implies the bridging proton is closer to the TMA.

The triethylammonium-pyridine complex cation

The infrared spectra of the hydrogen bonded complex cation (triethylamine H + pyridine) and of the three similar cations resulting from deuterium substitution in pyridine and at the bridge, have been examined in solution. The bridging proton occupies a potential with a single minimum closer to the triethylamine moiety. The V s band structure is ascribed to Fermi resonance with triethylamine mode combinations. The N · N stretching mode of this complex, and also of the (trimethylamine H + pyridine) complex have been observed in the far-infrared range. The present complex is weaker than the corresponding trimethylamine one.

Stereochemical effects in the chemical ionization mass spectra of cyclic diols

The chemical ionization mass spectra of configurational isomers of many cyclic diols give substantial differences which are characteristic of their stereochemistry. For the cis-isomer of 1,3- and 1,4-cyclohexanediols, formation of a stable intramolecular proton bridge involving the OH groups gives rise to dominant MH + peaks, suppressing the (M-H) + peaks found in the spectra of the trans-isomers and monoalcohols. The stability of the proton bridge in cis-1,3-cyclohexanediol structures is decreased by a sterically interfering cis-5-methyl substituent, but increased by a cis-5-OH group due to additional proton solvation. cis-Stereochemistry also gives increased formation of the dimeric M 2H + ions, but decreased formation of trimers, at higher diol concentrations, for the 1,3- and 1,4-diols. The similarity of the CI spectra of cis- and trans-1,2-cyclohexanediol are explicable in terms of the similarities of the most stable proton-bridged conformers; the reduced ring flexibility in cis- and trans-1,2-cyclopentanediols makes such similar structures unfavorable, as shown by the substantial differences between their CI spectra. The substantial, but expected, variations in behavior with temperature and reagent gas are useful for maximizing stereochemical effects on CI spectra; conditions of lowest energy are usually, but not always, the most useful.

Recent advances in the chemistry of boron hydrides

Abstract

Complex hydrogen bonded cations: I. The pyridinium-pyridine ion

The occurrence of the (py 2H) + ion in solutions containing pyridinium ion and pyridine has been established spectroscopically. The infrared and Raman spectra of the four species ▪, ▪, ▪, and ▪ have been examined in the range 80–6000 cm −1. The spectra are independent of concentration, solvent, or counterion if this is inert. The NH stretching band is a strong, broad doublet with maxima at 2080 and 2530 cm −1. In the bridge deuterated species there is a single strong, broad, ND stretching band, maximum at 1920 cm −1. The N-N stretching is infrared active, at ~138 cm −1. The internal modes of the base are all singlets in both the infrared and Raman spectra. The proton bridge is between the σ orbitais on the N atoms.

The trimethylammonium-pyridine complex cation

The infrared spectra of the hydrogen-bonded complex cation (trimethyl-amine/H/pyridine) +, and of the seven other cations resulting from all combinations of deuterium substitution in the bases and at the bridge, have been examined in solution. The bridging proton is inferred to occupy a single potential minimum closer to the trimethylamine moiety, the v s band structure is accounted for by Fermi resonance with trimethylamine mode combinations, and several other complex bands are assigned to particular normal modes.

Synthesis and nuclear magnetic resonance spectra of some bridged polyhydromethanonaphthalenes. Bridge proton assignments

Synthesis and nuclear magnetic resonance spectra of some bridged polyhydromethanonaphthalenes. Bridge proton assignments

Bridge proton exchange in B6H10 and 2-CH3B6H9; low temperature nuclear magnetic resonance spectra of static structures

Low temperature 1H and 11B n.m.r. spectra of B6H10 and 2-MeB6H9 demonstrate: (1) the bridge proton exchange rate is slow on the n.m.r. time scale; (2) the spectra of B6H10 thus obtained are consistent with the known solid state structure; and (3) the static structure of 2-MeB6H9 is without a mirror plane of symmetry.