Determination of Electronic Structure of Molecules from Nuclear Quadrupole Effects

Abstract

Nuclear quadrupole coupling constants in molecules depend on the nuclear quadrupole moments and the variation in electrostatic field at the nucleus. It is shown that this variation of electric field is usually simply related to the molecular electronic structure, being primarily dependent on the way in which valence electrons fill the lowest-energy p-type orbits. Structural information which can consequently be obtained from known quadrupole coupling constants is discussed. Hybridization of the normal covalent bonds of N, Cl, and As with at least 15 percent s character is clearly shown. The alkali halides appear to be almost purely ionic; the quadrupole coupling data allow no more than 3 percent covalent character. In addition to molecular structure, some nuclear quadrupole moments are approximately evaluated by use of the theory developed here.

The Microwave Spectrum of Oxazole

“…Though first formulated by Townes and Dailey [8], the quantum chemical concepts for correlating observed ^-values with the occupation of particular atomic orbitals have been elucidated and discussed by Lücken [9], whose treatment is now widely adopted.…”

Direct Investigation of Covalently Bound Chlorine in Organic Compounds by Solid‐State 35Cl NMR Spectroscopy and Exact Spectral Line‐Shape Simulations

“…All chlorine atoms bound to CH 2 carbon atoms have isotropic chemical shift (d iso ) values ranging from 150 to 200 ppm, whereas those bound to aromatic rings had d iso values on the order of 300 to 350 ppm (Table 1). Moreover, as has been explained using Townes-Dailey theory, [11] the back donation of p-electron density from the chlorine atom into the p system of the aromatic rings creates an EFG different from zero that is perpendicular to the plane of the ring and that differs from the An asterisk is used to indicate a trace NaCl or NH 4 Cl impurity, whereas a cross marks a singularity from the satellite transition of the other chlorine isotope. The sharp lines on the high-frequency ends of the spectra are caused by radio interference.…”

Theory of Nuclear Quadrupole Interactions in Solid Fluoromethanes with Implanted 19F* Nuclei. Coupling of HF* and Host Molecule

“…Considering CH 3 F and CHF 3 , the theoretical values are nearly the same, while the experimental values show a small decrease of 1 MHz in going from CH 3 F to CHF 3 . From the consideration that there is increasing competition for electron acquisition between the three fluorines in the case of the latter molecule, one would expect a decrease in ionic character of the C-F bond relative to CH 3 F and therefore an increase in the 19 F* coupling constant, according to the Townes and Dailey relation [16]. The opposite trend is observed experimentally.…”

“…Other factors include, for instance, differences in the radial characters of the atomic orbitals for the fluorine and carbon in CHF 3 and CH 3 F. These differences could lead to both different bonding strengths between carbon and fluorine atoms in the two molecules and in the expectation value <1 /r 3 > for the atomic p-orbitals of the fluorine atoms in these molecules. These effects could counteract the influence of the difference in the ionic characters of the two molecules, the latter effect being thought to cause the main variations in the fieldgradient in Townes and Dailey theory [16].…”