W. G. Schneider

Substituent Effects on the C13 and H1 Chemical Shifts in Monosubstituted Benzenes

The origin of the relative nuclear resonance shifts in monosubstituted benzenes has been investigated. In order to obtain more complete experimental information both C13 and H1 resonance shifts in a variety of aryl‐X compounds were measured. The H1 resonances were measured on 5 mole % solutions in cyclohexane to minimize solvent effects; the carbon shifts were obtained from natural abundance C13 resonance measurements in the neat liquid. Unambiguous assignments of both H1 and C13 resonance spectra were made possible with the aid of deuterated compounds. The largest resonance shifts were observed for the carbon atom directly bonded to X. As in the corresponding CH3X compounds, these shifts arise primarily from the inductive and magnetic anisotropy effects of X. Magnetic anisotropy effects of X are also observable in both the C13 and H1 resonances at the ortho position. A very close correspondence between C13 and H1 resonances is observed at the para position, where the primary contribution to the relative ...

Solvent Effects in Nuclear Magnetic Resonance Spectra

Contributions to nuclear screening (chemical shifts) arising from molecular interactions with solvent molecules (excluding hydrogen bonding) are discussed in terms of appropriate theoretical models. These include contributions from van der Waals interactions σw, from the magnetic anisotropy of the solvent molecule σa, and from polar effects σE. By a suitable choice of solute‐solvent systems it has been possible to demonstrate each of these effects experimentally for proton resonances. For CH4 as a solute, σw was in all cases negative, its magnitude varying with the nature of the solvent and amounting to as much as 0.6 ppm for high molecular weight solvents. In agreement with the theoretical models, σa was found to be positive for disk‐shaped solvent molecules and negative for cylindrically symmetrical rod‐shaped molecules, its magnitude in extreme cases reaching 0.75 ppm. For CH3CN as a solute, σE was negative and showed the expected dependence on the dielectric constant of the solvent.

Effect of Electronegativity and Magnetic Anisotropy of Substituents on C13 and H1 Chemical Shifts in CH3X and CH3CH2X Compounds

An investigation has been made of the major contributions which make up the relative chemical shifts in CH3X and CH3CH2X compounds. In order to obtain more detailed information, both the carbon and hydrogen chemical shifts were measured. The carbon shifts were obtained by measuring natural abundance C13 resonances in the pure liquids; the H1 resonances were measured on gaseous samples to avoid solvent effects. The results reveal surprisingly large contributions to both C13 and H1 shifts arising from magnetic anisotropy effects of the X substituent. In CH3X compounds, the contribution to the proton shifts is negative while that to the C13 shifts is positive. In CH3CH2X compounds, these effects contribute to the resonance shifts of carbon and hydrogen nuclei in both the methylene and methyl group. When such contributions are allowed for, an approximate correlation with the electronegativity of X can be obtained, indicating that inductive effects, together with anisotropy effects, account for the major part ...

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

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.

Laser Generation of Excitons and Fluorescence in Anthracene Crystals

Experimental and theoretical studies are reported of the short‐lived and delayed fluorescence of anthracene single crystals, excited by single‐ and double‐photon absorption. A giant‐pulse ruby laser provides the primary source of radiation of 14 400 cm−1 (up to 1027 photons/cm2·sec) and is also used to generate second‐harmonic radiation from ADP, as well as stimulated Raman radiation of 12 800 and 17 500 cm−1 from liquid oxygen. The time dependence of the fluorescence intensity is studied as a function of laser intensity, crystal temperature and excitation wavelength. The very intense fast fluorescence with a half‐life of 30 nsec at 300°K, characteristic of singlet exciton decay, and the relatively weak delayed fluorescence which involves intermediate triplet states, are separated using sectored disks. It is concluded that the triplet state at 14 750 cm−1 can be populated (i) by direct absorption of laser photons involving an activation energy of 350 cm−1; (ii) via two‐photon absorption, presumably leadin...

Transients of Volume‐Controlled Current and of Recombination Radiation in Anthracene

The transients of volume‐controlled doubly injected currents and of the accompanying recombination radiation were measured on anthracene crystals. By this method, which allows the trap‐free case to be approximated, the carrier recombination rate constant K was determined in a more reliable way than is possible with steady‐state currents. The result, K=(1.05±0.5)×10−6 cm3 sec−1, agrees well with the theoretical value predicted by a model in which the rate is a function of carrier mobility. The recombination fluorescence was found to consist of two components differing by their rise times. The fast component, Ifast, is due to singlet excitons generated directly by carrier recombination. The slow component, Islow, is proof that carrier recombination produces also triplet excitons (which can recombine to give singlets producing fluorescence). The ratio Islow/Ifast=0.65±0.1 indicates that two recombining triplet excitons do not always produce a singlet exciton but may also form an excited triplet exciton. In a...

Nuclear Spin—Spin Coupling Involving Heavy Nuclei. The Coupling between Hg199 and H1 Nuclei in CH3HgX and CH3CH2HgX Compounds

Nuclear spin coupling between protons and Hg199 nuclei in CH3HgX and CH3CH2HgX compounds has been investigated. As the X substituent is made more electronegative very large increases in the spin‐coupling constant up to 2½‐fold were observed. Smaller alterations occur in different solvent media, and alkyl exchange could be observed in several compounds. In the CH3CH2HgX compounds the changes in the spin‐coupling constants of the Hg nucleus with the protons in the methylene group and in the methyl group are comparable in magnitude. The results are interpreted in terms of a simplified model for contact interaction employing optical hyperfine structure (hfs) constants. Qualitative arguments are employed to assess the relative importance of other coupling mechanisms. It is concluded that spin coupling arising from electron orbital motion is negligible, and while dipole—dipole contributions may be important, the evidence indicates the Fermi contact interaction is the dominant coupling mechanism.

Intermolecular Potentials of Argon, Krypton, and Xenon

The intermolecular potentials of argon, krypton, and xenon are investigated using the recently measured second virial coefficients and the crystal properties at 0°K. The potential functions considered are the 9:6, 12:6, and exp:6. The methods of determination of parameters are reviewed and the method of least squares used in this paper described. All the potentials fitted the second virial coefficient very well, but with the same parameters fitted the crystal properties only moderately well.

On the Nature of Solvent Effects in the Proton Resonance Spectra of Unsaturated Ring Compounds. I. Substituted Benzenes

The relative chemical shift between the resonances of ortho and meta hydrogens in certain para‐disubstituted benzenes are markedly dependent on solvent and concentration. This behavior, which is observed also in other substituted benzenes, is characterized by a pronounced shift of some proton signals to low field in acetone solution and to high field in benzene solution. In compounds of the type [Complex chemical formula] where X is an electron‐withdrawing substituent, the resonance signals of protons meta to the X substituent are displaced selectively in donor solvents. When the CH3 group is replaced by F, both ortho and meta hydrogens show pronounced shifts. The observations can be interpreted in terms of a specific molecular association involving hydrogen bonding. Although generally these bonds appear relatively weak, for some compounds the hydrogen‐bond strength is estimated to be comparable to those formed by chloroform in donor solvents. These effects offer a valuable practical aid in proton resonance measurements since in such compounds, within limits, it is possible to alter the relative chemical shifts by a suitable adjustment of solvent conditions.The relative chemical shift between the resonances of ortho and meta hydrogens in certain para‐disubstituted benzenes are markedly dependent on solvent and concentration. This behavior, which is observed also in other substituted benzenes, is characterized by a pronounced shift of some proton signals to low field in acetone solution and to high field in benzene solution. In compounds of the type [Complex chemical formula] where X is an electron‐withdrawing substituent, the resonance signals of protons meta to the X substituent are displaced selectively in donor solvents. When the CH3 group is replaced by F, both ortho and meta hydrogens show pronounced shifts. The observations can be interpreted in terms of a specific molecular association involving hydrogen bonding. Although generally these bonds appear relatively weak, for some compounds the hydrogen‐bond strength is estimated to be comparable to those formed by chloroform in donor solvents. These effects offer a valuable practical aid in proton resonan...

Thermally Stimulated Currents and Carrier Trapping in Anthracene Crystals

Thermally stimulated currents in anthracene single crystals have been investigated. A fairly large current peak observed at a temperature near 0°C, is shown to be characteristic of anthracene and corresponds to a carrier trap level of depth 0.76±0.05 eV. The effects of temperature, applied voltage, and the time of illumination with uv light on trap filling were investigated. Illumination at low temperatures, with or without an applied field, generates charge carriers which are localized in the crystal. Below —60°C the rate of carrier release and/or recombination appears negligibly small, permitting charge storage for indefinitely long periods of time. In addition to thermal detrapping, it was found that carrier detrapping could also be brought about by optical radiation with wavelengths in the visible and near infrared region. The resulting optically stimulated currents depended linearly on the intensity of the long wavelength light. The results suggest that on uv illumination at low temperatures ion pair...