An ab initio gauge-invariant molecular orbital theory is developed for nuclear magnetic shielding. The molecular orbitals are written as linear combinations of gauge-invariant atomic orbitals, the wavefunctions in the presence of a uniform external magnetic field being determined by self-consistent field perturbation theory. The final magnetic shielding result is broken up into contributions which can be related to various features of electronic structure. Calculated magnetic shielding constants are presented using three sets of atomic orbitals, all of which are taken as contracted gaussian-type functions. The first two sets are minimal and the third is slightly extended. All three levels of theory give good descriptions of shielding at first row and hydrogen atoms. Carbon and hydrogen chemical shifts calculated at the extended level are in excellent agreement with experimental values.

Self-consistent perturbation theory of diamagnetism

A new family of high-affinity buffers and optical indicators for Ca2+ is rationally designed and synthesized. The parent compound is 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a relative of the well-known chelator EGTA [ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] in which methylene links between oxygen and nitrogen are replaced by benzene rings. BAPTA and its derivatives share the high (greater than 10(5)) selectivity for Ca2+ over Mg2+ of EGTA but are very much less affected by pH changes and are faster at taking up and releasing Ca2+. The affinity of the parent compound for Ca2+ (dissociation constant 1.1 x 10(-7) M in 0.1 M KCl) may be strengthened or weakened by electron-releasing or -withdrawing substituents on the aromatic rings. The Ca2+ and Mg2+ affinities may further be altered by replacing the ether oxygens by heterocyclic nitrogen atoms. The compounds described are fluorescent Ca2+ indicators absorbing in the ultraviolet region; the very large spectral shifts observed on binding Ca2+ fit the prediction that complexation should hinder the conjugation of the nitrogen lone-pair electrons with the aromatic rings. Derivatives with quinoline nuclei are notable for their high sensitivity of fluorescent quantum yield to the binding of Ca2+ but not of Mg2+. Preliminary biological tests have so far revealed little or no binding to membranes or toxic effects following intracellular microinjection.

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New calcium indicators and buffers with high selectivity against magnesium and protons: design, synthesis, and properties of prototype structures.

Ab Initio Methods for the Calculation of NMR Shielding and Indirect Spin-Spin Coupling Constants

Chelation of the rare-earth element gadolinium (Gd) with diethylenetriaminepentaacetic acid (DTPA) results in a strongly paramagnetic, stable complex that is well tolerated in animals. The strongly paramagnetic gadolinium complex reduces hydrogen-proton relaxation times even in low concentrations (less than 0.01 mmol/L). The pharmacokinetic behavior of intravenously delivered Gd-DTPA is similar to the well known iodinated contrast agents used in urography and angiography; excretion is predominantly through the kidneys with greater than 90% recovery in 24 hr. The intravenous LD50 of the meglumine salt of Gd-DTPA is 10 mmol/kg for the rat; in vivo there is no evidence of dissociation of the gadolinium ion from the DTPA ligand. The combination of strong proton relaxation, in-vivo stability, rapid urinary excretion, and high tolerance favors the further development and the potential clinical application of gadolinium-DTPA as a contrast enhancer in magnetic resonance imaging.

https://www.ajronline.org/doi/pdfplus/10.2214/ajr.142.3.619

Characteristics of gadolinium-DTPA complex: a potential NMR contrast agent.

A nuclear magnetic double‐resonance method for the determination of chemical exchange rates has been developed. The method is applicable to systems in which a nuclear spin is reversibly transferred between two nonequivalent sites, A and B. The lifetime (τA) and spin—lattice relaxation time (T1A) in Site A are obtained through the study of the decay to a new equilibrium value of Signal A upon the sudden saturation of Signal B. The converse experiment permits the determination of τB and T1B. A number of data for cross checks are furthermore obtained through the study of the recovery of the signals upon the release of various combinations of saturating rf fields.A simple theory based on the Bloch equations as modified by McConnell to incorporate the effects of chemical exchange is given. Experimental results on the hydroxyl proton exchange in the system salicylaldehyde and 2‐hydroxyacetophenone are well described by this simple theory.The present method, which can readily be extended to systems with several ...

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Study of Moderately Rapid Chemical Exchange Reactions by Means of Nuclear Magnetic Double Resonance

High Resolution Nuclear Magnetic Resonance

In order to study the factors giving rise to proton resonance chemical shifts of free molecules, as well as the association shifts due to hydrogen bonding, proton resonance measurements were carried out for a variety of simple hydride molecules in both the liquid and gaseous states. It is found that the proton shifts in the gaseous state can be interpreted in terms of the combined effect of the electronegativity and magnetic anisotropy of the atom to which the proton is bonded. The proton signals measured in the liquid state near the melting point, which correspond to maximum association, show large shifts to lower magnetic field relative to the corresponding gas signals. It is suggested that the association, or hydrogen bond shifts, in a system Y ——— H — X can be interpreted largely in terms of the reduction of the diamagnetic circulation in the H — X bond by the electrostatic field of the Y donor. Further possible contributions in certain anomalous cases are also considered.

Proton Magnetic Resonance Chemical Shift of Free (Gaseous) and Associated (Liquid) Hydride Molecules

This paper deals with the analysis of the nuclear magnetic resonance spectrum of two pairs of two equivalent nuclei (of spin )whose relative chemical shift is of the same order as the spin-coupling...

The analysis of nuclear magnetic resonance spectra: ii. two pairs of two equivalent nuclei

This paper is concerned with the general problem of the interpretation and analysis of nuclear magnetic resonance spectra of systems in which chemical shift and spin-coupling constants are of the s...

The analysis of nuclear magnetic resonance spectra: i. systems of two and three nuclei

The proton resonance spectra of a number of alternant aromatic hydrocarbons are reported. With the exception of benzene and pyrene, two or more signals were obtained for each hydrocarbon. A probable assignment of the spectrum for each compound is given, and this was confirmed for anthracene and phenanthrene by deuteration. The spectrum of the non-alternant hydrocarbon azulene is also presented. The observed nuclear magnetic resonance spectra of the aromatic hydrocarbons cannot be interpreted on the basis of symmetry considerations or $\pi $-electron density, but are readily explained in terms of a simple model based on induced diamagnetic currents in the conjugated aromatic rings. The spectra calculated for each hydrocarbon on the basis of this model satisfactorily reproduce the main features of the observed spectra.

H. J. Bernstein

Papers

High Resolution Nuclear Magnetic Resonance

Proton Magnetic Resonance Chemical Shift of Free (Gaseous) and Associated (Liquid) Hydride Molecules

The analysis of nuclear magnetic resonance spectra: ii. two pairs of two equivalent nuclei

The analysis of nuclear magnetic resonance spectra: i. systems of two and three nuclei

The proton magnetic resonance spectra of conjugated aromatic hydrocarbons