Deshielding Of Protons Research Articles

Dynamics, magnetic properties, and electron binding energies of H2O2 in water.

Results for the magnetic properties and electron binding energies of H2O2 in liquid water are presented. The adopted methodology relies on the combination of Born-Oppenheimer molecular dynamics and electronic structure calculations. The Keal-Tozer functional was applied for predicting magnetic shieldings and H2O2 intramolecular spin-spin coupling constants. Electron binding energies were calculated with electron propagator theory. In water, H2O2 is a better proton donor than proton acceptor, and the present results indicate that this feature is important for understanding magnetic properties in solution. In comparison with the gas-phase, H2O2 atoms are deshielded in water. For oxygen atoms, the deshielding is mainly determined by structural/conformational changes. Hydrogen-bond interactions explain the deshielding of protons in water. The predicted chemical shift for the H2O2 protons in water (δ∼11.8 ppm) is in good agreement with experimental information (δ=11.2 ppm). The two lowest electron binding energies of H2O2 in water (10.7±0.5 and 11.2±0.5 eV) are in reasonable agreement with experiment. In keeping with data from photoelectron spectroscopy, an ∼1.6 eV red-shift of the two first ionisation energies relative to the gas-phase is observed in water. The strong dependence of magnetic properties on changes of the electronic density in the nuclei environment is illustrated by a correlation between the σ(17O) magnetic shielding constant and the energy gap between the [2a] lowest valence and [1a] core orbitals of H2O2.

Interpretation of Spectroscopic Markers of Hydrogen Bonds.

Quantum calculations are used to examine whether an AH⋅⋅⋅D H-bond is unambiguously verified by a downfield shift of the bridging proton's NMR signal or a red (or blue) shift of the AH stretching frequency in the IR spectrum. It is found that such IR band shifts will occur even if the two groups experience weak or no attractive force, or if they are drawn in so close together that their interaction is heavily repulsive. The mere presence of a proton-acceptor molecule can affect the chemical shielding of a position occupied by a protondonor by virtue of its electron density, even if there is no H-bond present. This density-induced shielding is heavily dependent on position around the proton-acceptor atom, and varies from one group to another. Evidence of a hydrogen bond rests on the measurement of a proton deshielding in excess of what is caused purely by the presence of the proton acceptor species.

Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone-Pair and Bond Orbital Contributions.

NMR spectroscopic parameters of the proton involved in hydrogen bonding are studied theoretically. The set of molecules includes systems with internal resonance-assisted hydrogen bonds, internal hydrogen bonds but no resonance stabilization, the acetic acid dimer (AAD), a DNA base pair, and the hydrogen succinate anion (HSA). Ethanol and guanine represent reference molecules without hydrogen bonding. The calculations are based on zero-point vibrationally averaged molecular structures in order to include anharmonicity effects in the NMR parameters. An analysis of the calculated NMR shielding and J-coupling is performed in terms of "chemist's orbitals", that is, localized molecular orbitals (LMOs) representing lone-pairs, atomic cores, and bonds. The LMO analysis associates some of the strong de-shielding of the protons in resonance-assisted hydrogen bonds with delocalization involving the π-backbone. Resonance is also shown to be an important factor causing de-shielding of the OH protons for AAD and HSA, but not for the DNA base pair. Nitromalonamide (NMA) and HSA have particularly strong hydrogen bonds exhibiting signs of covalency in the associated J-couplings. The analysis results show how NMR spectroscopic parameters that are characteristic for hydrogen bonded protons are influenced by the geometry and degree of covalency of the hydrogen bond as well as intra- and intermolecular resonance.

Strong deshielding in aromatic isoxazolines.

Very strong proton deshielding was found in di/tri-aromatic isoxazoline regioisomers prepared from acridin-4-yl dipolarophiles and stable benzonitrile oxides (BNO). Three alkenes, (acridin-4-yl)-CH=CH-R (R = COOCH3, Ph, and CONH2), reacted with three BNO dipoles (2,4,6-trimethoxy, 2,4,6-trimethyl, 2,6-dichloro) to give pairs of target isoxazolines with acridine bound to C-4 or C-5 carbon of the isoxazoline (denoted as 4-Acr or 5-Acr). Regioselectivity was dependent on both the dipolarophile and dipole character. The ester and amide dipolarophile displayed variable regioselectivity in cycloadditions whereas the styrene one afforded prevailing 4-Acr regioisomers. 2,4,6-Trimethoxy-BNO was most prone to form 5-Acr isoxazolines while mesitonitrile oxide gave major 4-Acr isoxazolines. Basic hydrolysis of the amide cycloadduct led to an unexpected isoxazolone product. The structure of the target compounds was studied by NMR, MS, and X-ray crystallography.

Does hydrogen bonding to xenon affect its 129Xe NMR chemical shift? Computational study on selected Brønsted acid–xenon complexes

H-bonding-like interactions between Xe and twelve A–H acids is examined by analysis of calculated A–H⋯Xe geometry and the 129Xe shielding. It was shown that δ(129Xe) is proportional to the 129Xe shielding anisotropy in most cases, generally increases with the A–H proton deshielding and depends on the A–H acidity (PA) on two ways: for stronger and weaker acids δ(129Xe) increases and decreases with PA, respectively. The highest contribution to the δ(129Xe) tensor variations comes from its principal components perpendicular to the H⋯Xe bond. These components are also sensitive on side interactions in non-linear dimers.

Structural correlations for 1H, 13C and 15N NMR coordination shifts in Au(III), Pd(II) and Pt(II) chloride complexes with lutidines and collidine

(1)H, (13)C and (15)N NMR studies of gold(III), palladium(II) and platinum(II) chloride complexes with dimethylpyridines (lutidines: 2,3-lutidine, 2,3lut; 2,4-lutidine, 2,4lut; 3,5-lutidine, 3,5lut; 2,6-lutidine, 2,6lut) and 2,4,6-trimethylpyridine (2,4,6-collidine, 2,4,6col) having general formulae [AuLCl(3)], trans-[PdL(2)Cl(2)] and trans-/cis-[PtL(2)Cl(2)] were performed and the respective chemical shifts (delta(1H), delta(13C), delta(15N)) reported. The deshielding of protons and carbons, as well as the shielding of nitrogens was observed. The (1)H, (13)C and (15)N NMR coordination shifts (Delta(1H) (coord), Delta(13C) (coord), Delta(15N) (coord); Delta(coord) = delta(complex) - delta(ligand)) were discussed in relation to some structural features of the title complexes, such as the type of the central atom [Au(III), Pd(II), Pt(II)], geometry (trans- or cis-), metal-nitrogen bond lengths and the position of both methyl groups in the pyridine ring system.

Stable‐Ion NMR Spectroscopy and GIAO‐DFT Study of Carbocations Derived from Multibridged [3n]Cyclophanes

AbstractMono‐ and diprotonation of the tri‐, tetra‐, and pentabridged cyclophanes [33](1,3,5)cyclophane (4), [34](1,2,3,5)cyclophane (5), and [35](1,2,3,4,5)cyclophane (6) were realized, and NMR spectroscopic studies of the resulting carbocations are reported. Unlike [2.2]paracyclophane and its fluorinated analogs that are protonated at a position ipso to the ethano‐bridge, the monocations derived from 4, 5, and 6 are protonated at the unsubstituted ring positions. Protonation regioselectivity in the dications is pseudo‐meta for 4 and 5 at unsubstituted ring positions, but for more‐crowded 6 the second protonation occurs at a position that is ipso to the trimethylene bridge. Transannular π–π interactions in the monocations are manifested in the observed proton deshielding in the unprotonated π‐deck. DFT and GIAO‐DFT were employed to study the mono‐ and dications for comparison with the solution studies in superacids. GIAO‐derived ΔNICS(1)zz data for the [3n]cyclophane monocations imply decreased aromaticity in the cofacial unprotonated arene, consistent with transannular donor–acceptor interactions.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Intramolecular apical Metal–H–Csp3 interaction in molybdenum and silver complexes

The reaction of HTIMP3 (HTIMP3=tris[1-diphenylphosphino)-3-methyl-1H-indol-2-yl]methane) with AgBF4 and Mo(CO)3(NCCH3)3 leads to Ag(HTIMP3)BF4 and Mo(CO)3(HTIMP3), respectively. The metal centre is coordinated to the three phosphorus atoms of the HTIMP3 ligand, which adopts a facial coordination mode, placing a H-Csp3 hydrogen atom at the apical position close to the metal centre. The solid-state structure of Mo(CO)3(HTIMP3) has been determined by X-ray crystallography, and the data have been used as input parameters for obtaining the optimised geometry of the complex using the B3PW91 functional. The silver structure has been modelled from the X-ray parameters of the molybdenum structure. In addition, theoretical calculations on the H-Csp3 downfield shift upon metal coordination has also been performed. They reproduce the experimental H-Csp3 chemical shifts well and supports that proton deshielding is mainly due to the presence of the metal, since the hydrogen is already located in the cone created by the aromatic-phosphino arms in the free ligand.

Quantum vs. classical models of the nitro group for proton chemical shift calculations and conformational analysis

A model based on classical concepts is derived to describe the effect of the nitro group on proton chemical shifts. The calculated chemical shifts are then compared to ab initio (GIAO) calculated chemical shifts. The accuracy of the two models is assessed using proton chemical shifts of a set of rigid organic nitro compounds that are fully assigned in CDCl3 at 700 MHz. The two methods are then used to evaluate the accuracy of different popular post-SCF methods (B3LYP and MP2) and molecular mechanics methods (MMX and MMFF94) in calculating the molecular structure of a set of sterically crowded nitro aromatic compounds. Both models perform well on the rigid molecules used as a test set, although when using the GIAO method a general overestimation of the deshielding of protons near the nitro group is observed. The analysis of the sterically crowded molecules shows that the very popular B3LYP/6-31G(d,p) method produces very poor twist angles for these, and that using a larger basis set [6-311++G(2d,p)] gives much more reasonable results. The MP2 calculations, on the other hand, overestimate the twist angles, which for these compounds compensates for the deshielding effect generally observed for protons near electronegative atoms when using the GIAO method at the B3LYP/6-311++G(2d,p) level. The most accurate results are found when the structures are calculated using B3LYP/6-311++G(2d,p) level of theory, and the chemical shifts are calculated using the CHARGE program based on classical models.

Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. II. Furo-furan isomers.

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two furan units ([2,3-b], [3,2-b], [3,4-b], and [3,4-c] isomers) have been computed by several approximated techniques and a large Gaussian basis set to achieve near Hartree-Fock estimates. Ab initio models of the ring currents induced by a magnetic field normal to the molecular plane were obtained for the three isomeric systems of higher symmetry, showing that the pi electrons give rise to intense diamagnetic circulation. The pi currents are responsible for enhanced magnetic anisotropy and strong out-of-plane proton deshielding. The theoretical findings are used to build up a "diatropicity matrix" for two fused five-membered heterocyclic systems.

Helical zinc complexes of pyrazine–pyridine hybrids

The zinc(II) complexes 1aZnCl 2 and 1bZnCl 2 ( 1a: 2-(6 ′,2″-bipyrid-2 ′-yl)-3-(2-pyridyl)pyrazine; 1b: 2-(6 ′,2 ′-bipyrid-2 ′-yl)-5,6-dinitrilo-3-(2-pyridyl)pyrazine) were prepared by treatment of the ligands with ZnCl 2. The structures of both were investigated by X-ray crystallography and 1H NMR spectroscopy. Both complexes display proton deshielding phenomena that are attributed to a twisted solution-state molecular conformation. In the solid state, 1aZnCl 2 exhibits a high degree of torsion about the axis through the uncomplexed pyridine ring and the pendant chlorine atoms. The solid-state structure and solution-state self-associative behavior of 1bZnCl 2 are indicative of a partial self-assembly motif.

Current density maps, magnetizability, and nuclear magnetic shielding tensors of bis-heteropentalenes. I. Di-hydro-pyrrolo–pyrrole isomers

Magnetic susceptibility and nuclear magnetic shielding at the nuclei of bis-heteropentalenes formed by two pyrrole units ([2,3−b], [3,2−b], [3,4−b], and [3,4−c] isomers) have been evaluated by a series of different approximations and a large Gaussian basis set. An ab initio model of magnetic field induced current density was obtained for four isomeric systems, showing that strong diamagnetic flow takes place within the π electrons. The π currents are responsible for exalted magnetic anisotropy and proton deshielding. The theoretical findings are used to assess a diatropicity scale for these molecules.

Molecular complexes. Part 12.1 Dimeric toluene, torsional vibrations, dipoles and isomeric complexes in 1H NMR studies of weak arene complexes. Temperature dependence of CH signals

The refined (AUS concept, CCl4, external ref.) 1H NMR method provided association constants K and approximate complex shifts IK for stacking complexes of aromatic hydrocarbons D with A when A is 4-nitrobenzaldehyde (1) or is related to 1. D covers the benzene ring of A but torsionally vibrating substituents in A influence topology and K. In the absence of D, vibrations of A can be slowed by low temperatures making A signals go downfield in accord with an increased planarity of A. Vibration of CHO (a dipole) in complexes of 1 with benzene B or toluene T is made non-symmetric by interactions with the quadrupole of D. The large naphthalene (N) hinders vibrations thus enhancing the contact interface by a more planar 1 whose intramolecular deshielding of protons is increased providing small IK values. Aldehyde 1 forms complexes both with T and with its stacking dimer T2 since IK values for 1–T are significantly greater than for 1–B. Complexing with T2 is not found when the molecular dimensions of A allow a dipole–dipole interaction with T as in face-to-face complexes of 1-ethyltheobromine or of 1-chloro-2,4-dinitrobenzene (7). Different protons give slightly different values of K for 7–T and this points to an isomeric edge-on complex with both T and T2 where K for the two edge-on protons (shortest distance to D) is the sum of the binary K and the small K for the the ternary (7 + T2) complex. Compound 7 and 1,3,5-trimethylbenzene show some of the effect described for 1–N.

Probing the properties of C 60 through fulleroids ABC 61

We show that the fulleroid synthesis opens the opportunity to investigate the response of the fulleroid ‘π’ electrons to external magnetic fields. The response is expressed as a general deshielding of protons which are near the surface of the sphere. A more subtle effect is the ‘shielding’ effect experienced by protons which are over certain junctures of the fulleroid. A remarkable consequence of the various deshielding strengths of the spheroid is observable as a quasi shift reagent effect. We also show that the discovery of control over the electron acceptor and donor strength of fulleroids through periconjugation by the use of 9-fluorenyl-derived fulleroids opens further the already large arsenal for fullerene modification into the preparation of more electropositive (electron-donating) and more electronegative (electron-withdrawing) fulleroids by suitable substitution into fluorene and fluorene-like moieties.

Proton NMR investigation of the collective proton motion in hydrogen-bonded systems of monoaurates of 2,6-bis((diethylamino)methyl)-4-R-phenols

This paper discusses studies of monoaurates of 2,6-bis((diethylamino)methyl)-4-R-phenols in CD{sub 2}Cl{sub 2} solutions using {sup 1}H NMR. Proton deshielding increases with increasing phenolic group acidity, with the strongest deshielding and largest polarizability in the R=Cl system. Increasing acidity causes increasing proton shielding by N atoms. When H{sub 2}-bonded protons are preferentially present at the N atoms, Ph-Ch{sub 2} and N-CH{sub 2} protons signals broaden strongly with doublet and octet formation occurring when the protons are almost localized at these atoms; this is due to methylene proton-NH{sup +} proton spin-spin coupling. 9 refs., 1 fig., 2 tabs.

The use of inner projections of the polarization propagator to study different contributions to the magnetic shielding tensor. I. Proton deshielding by steric compression

AbstractA new method for studying different contributions to the magnetic shielding tensor is proposed. These contributions come from different molecular fragments, which are defined through a localization procedure. They are obtained by means of inner projections of the polarization propagator onto the subspace defined by the chosen molecular fragment. To illustrate the utility of this method, results are presented for the proton desheilding effect of steric crowding. Different degrees of approximation both in the polarization propagator formalism and in the ground state wave function can be used. Results presented in this paper were obtained at the INDO level, within the CHF scheme. Good agreement with experimental values is obtained.

Proton deshielding by spatial proximity: An experimental and theoretical study for ortho‐substituted anisoles

AbstractA quantitative estimation of the proton deshielding effect produced by a sterically crowded environment was obtained experimentally, based on departures from the chemical shift additivity rule of protons ortho to OCH3 and NO2 groups in benzene derivatives, where a conformational change from these side‐chain groups is expected. The influences of these conformational changes were also analysed from a theoretical point of view. Calculations of the magnetic shielding tensor were carried out at the INDO approximation with gauge invariant atomic orbitals, together with the uncoupled Hartree‐Fock approach. Theoretical results for these proton deshielding effects are in very good agreement with those determined experimentally.

The validity of Musher's model for the magnetic properties of aromatic molecules

Plots of proton magnetic shielding density in the benzene molecule, obtained via accurate coupled Hartree-Fock calculations, are reported. They unequivocally show that proton deshielding is due to local effects arising in the adjacent carbon atom and C H bond. Therefore downfield chemical shifts observed in proton magnetic resonance spectroscopy can be entirely explained in terms of magnetic anisotropy of the carbon environment. This shows the unphysical character of the ring current model and the reliability of Musher's model.

High-resolution proton magnetic resonance spectra of polynuclear heterocycles-II : Benzo [ b] thiophen and dibenzothiophen

Full iterative analyses have been carried out of 60 MHz spectra of benzo [ b] thiophen (I) and of 60 and 100 MHz spectra of dibenzothiophen (II) in acetone and carbon tetrachloride solutions at several concentrations. From the 33 peaks in the spectrum of I, analysed as a six-spin system with appreciable inter-ring coupling between H(3) and H(7) and between H(2) and H(6), 15 parameters were determined and 190 transitions assigned. For II, inter-ring coupling is absent but H(2) and H(3) of the ABCD system have closely similar shifts. Deshielding of protons of I and II in acetone solution relative to carbon tetrachloride is attributed to reaction-field effects; some ordering of the polar solute molecules is suggested. For the thiophen rings in I and II, currents calculated from chemical shifts remote from the CS bonds are comparable with those in benzene. In terms of partial bond fixation, the J 4,5, J 5,6 coupling constants observed in I lead to an aromaticity comparable with that in naphthalene, and J 2,3 of II implies a β-bond equivalence approaching that in fluorene.

Magnetic deshielding of protons due to intramolecular steric interactions with proximate hydrogens

Magnetic deshielding of protons due to intramolecular steric interactions with proximate hydrogens