S. Szymański

External neat nitromethane scale for nitrogen chemical shifts

Abstract Precise NMR measurements of 14N screening constants relative to those of neat nitromethane as external standard in concentric spherical sample containers show that external and internal referencing systems which have so far been used are not suitable for nitrogen shift calibration with an accuracy of the order of 0.1 ppm. Accurate data are reported for recalculating nitrogen shifts referred to various external and internal standards. Neat nitromethane is recommended as a primary reference substance. The role of solvent and concentration effects on nitrogen chemical shifts is indicated for a few selected molecules.

14N NMR studies on some azolopyridines using a differential saturation method

Abstract The chemical shifts and signal half-height widths of the nitrogen nuclei in some azolopyridines are reported. By means of simple molecular orbital arguments the nitrogen nuclei at the junctions of the fused five- and six-membered conjugated rings (indolizine-type nitrogen atoms) are shown to reflect the electronic charge distribution within the azolopyridines studied. As indicated by the nitrogen NMR spectra the electronic structures of the five-membered rings, of the azolopyridines, are very similar to those of the parent azoles. A complete analysis of the complex 14 N NMR spectra, including severe signal overlap regions, has been performed by means of differential saturation effects for signals with different widths.

Analysis of DNMR lineshape by an iterative least-squares method

Abstract A general treatment of lineshape analysis by an iterative least-squares method for coupled spin systems undergoing intramolecular exchange is presented. The method is effective because of a general expression for derivatives of sum of squares with respect to shape parameters. The ASESIT computer-program (written in Algol) has been tested on simulated (with addition of noise) AB ⇌ CD and A2B ⇌ C2 D spectra.

A 14N NMR study of the isomeric structures of urea and its analogues

Abstract In urea and its structural analogues differences in excess of 100 ppm are found in the nitrogen screening constants for the amide- and isoamide-type structures. The nitrogen chemical shifts are interpreted within the framework of Poples gauge dependent atomic orbital approximation using INDO molecular orbitals. It is demonstrated that this method provides reasonable values for nitrogen absolute screening constants. The experimental data reported should be useful in examinations of tautomeric equilibria in amides and related structures.

Theory of magnetic equivalence breaking in dynamic NMR

Abstract Possible spectroscopic relevance is discussed of scalar spin-spin couplings between magnetically equivalent nuclei in the exchanging spin systems where the magnetic equivalence symmetry is not conserved in the course of exchange. Test calculations of dynamic NMR spectra indicate that the equivalence-breaking effects may be nonnegligible. This implies that the problem of dynamic NMR lineshape analysis can in some cases be more complicated than thought previously. On the other side, in favorable circumstances effects of this type might allow one to differentiate between certain exchange mechanisms which according to the standard theory are regarded as NMR-nondifferentiable.

The Symmetrization Postulate of Quantum Mechanics in NMR Spectra

Abstract In NMR spectroscopy, consequences of the symmetrization postulate of quantum mechanics for groupings of identical atomic nuclei are usually ignored. A common view is that only in rare instances this general practice must be suspended. However, according to recent experimental and theoretical findings, such instances are not so exotic as it is generally believed. Occurrence of the so-called quantum exchange couplings, stemming from the spin-space correlations imposed by the symmetrization postulate, is now routinely oberved in liquid-phase NMR of some transition metal tri-and dihydrides. Moreover, most recent studies on the dynamics of methyl-type molecular rotors reveal that even stochastic motions of such nuclear systems, generally treated as classical par excellence, can in fact be shaped up by the symmetrization postulate. Interestingly, the quantum nature of such motions can be visible not only at cryogenic but also at ambient temperatures. The main issues that are addressed in the present review include manifestations of the symmetrization postulate in NMR spectra of the metal hydrides and methyl-type rotors. Emphasis is laid on the new developments in the field as well as on the relevant problems that await further analysis.

Lineshape function for absorption-mode DNMR spectrum

Abstract A new way of calculating NMR absorption for intramolecularly exchanging systems is proposed. Symmetrization of the exchange matrix and extraction of the absorption part from the complex lineshape function implies that the calculation of the absorption spectrum consists in a decomposition, at every point ω, of the real, symmetric, and positive definite matrix into a product of triangular matrices. It seems that the computer program ASES based on this algorithm, written in Algol, may in some cases successfully compete with programs in which the method of diagonalization of the complex matrix is used.

NMR lineshape equations for hindered methyl group: a comparison of the semi-classical and quantum mechanical models.

The semiclassical and quantum mechanical NMR lineshape equations for a hindered methyl group are compared. In both the approaches, the stochastic dynamics can be interpreted in terms of a progressive symmetrization of the spin density matrix. However, the respective ways of achieving the same limiting symmetry can be remarkably different. From numerical lineshape simulations it is inferred that in the regime of intermediate exchange, where the conventional theory predicts occurrence of a single Lorentzian, the actual spectrum can have nontrivial features. This observation may open new perspectives in the search for nonclassical effects in the stochastic behavior of methyl groups in liquid-phase NMR.

Permutation Symmetry in NMR Relaxation and Exchange

Publisher Summary The chapter discusses the permutation symmetry in nuclear magnetic resonance (NMR) relaxation and exchange. The issue of permutation symmetry entered NMR spectroscopy soon after the unraveling of the origin of the fine structure observed in the high-resolution spectra of liquids. Implemented on a computer, permutation symmetry option has become an inherent part of the numerical procedures used to analyze static spectra, affording substantial simplification of the problem when the spin system in question exhibits relatively high symmetry. The chapter explores a general theory of permutation symmetry, encompassing both relaxation and exchange. The basic idea of the theory works, described first for intramolecular exchange and then extended to relaxation and intermolecular exchange, is that the problem must be considered at two levels— that is, macroscopic and microscopic. The macroscopic symmetry properties, having a general character, are essentially identical for all these phenomena. The microscopic symmetry properties, however, are contingent features of certain specific systems, in which the underlying random processes have intramolecular character. The theory offers a method for exploiting in full the computational simplifications that are implied by the permutation symmetry present in an exchanging or relaxing system.