Low-energy Electronic Transitions Research Articles

Substituent and solvent effects on the 1π ∗ ← n, 3π ∗ ← n and 1π ∗ ← π transitions of benzaldehyde

The substituent effects ( o-CH 3, m-CH 3, p-CH 3, o-Cl, m-Cl, p-Cl, o-F, m-F, p-F) on the energy of the 1π ∗ ← n, 3π ∗ ← n and 1 L b ← 1 A transitions of benzaldehyde have been investigated. These effects are discussed in terms of the variations of electron distributions accompanying excitation. It is inferred that charge distributions are similar in the lowest 3( n, π ∗) and 1( n, π ∗ states. This conclusion is supported by an examination of the frequency shifts of the lowest energy electronic transitions of benzaldehyde in 17 solvents.

Semi-empirical LCAOMO theory for infinite systems. Part 1.—Application to polyethylene, polyacetylene and polyvinylchloride

Tight-binding molecular orbital theory for infinite systems developed in terms of the SCF CNDO formalism allows for the explicit inclusion of orbital rather than atomic core Hamiltonian matrix elements and electron repulsion integrals. It also seeks to establish a relationship, in terms of parametrisation, between the molecular and the solid-state tight-binding method. By applying this theory to a regular polyethylene polymer model, the band gap has been compared with that for an ab initio calculation and optimized with respect to the Mulliken-Wolfsberg A parameter. The results suggest that parametrisation of the semi-empirical solid-state tight-binding approach can be carried through in the same way as in molecular versions of this theory. The method, applied to regular polyacetylene, reveals that the energy bands on either side of the band gap have π character and the lowest energy electronic transitions are therefore πâ†�π* in nature. The highest filled band in polyvinylchloride has chlorine 3p character; the band gap is lower than that of polyethylene.The density-of-states plots for all three systems are sharply peaked around the energy gaps.

Near-Infrared Spectrophotometry

Abstract Near-infrared spectroscopy is usually considered to cover that part of the electromagnetic spectrum which falls between 0.71μ. and approximately 3 p. From the theoretical viewpoint, one might place the upper wavelength limit near 2.51μ. and define near-infrared spectroscopy as the study of low-energy electronic transitions and overtones and combinations of hydrogenic (C-H, N-H, 0-H, etc.) stretching and bending vibrations. This definition is favored by the fact that several commercial near-infrared instruments have an upper wavelength limit of about 2.65μ. Some instruments cover wavelengths up to 3.6μ) however, and the definition of near-infrared spectroscopy is sometimes broadened to include the study of fundamental hydrogenic stretching vibrations.

Einige gesetzmässigkeiten für die valenzkraftkonstanten tetraedrischer oxoanionen vom typ XO 4n− mit d0-konfiguration des zentralatoms

Recently calculated values of the valence force constants ⨍ MeO in transition metal oxoanions and tetroxides XO 4 n− with d 0 configuration of the central atom (VO 4 3−, CrO 4 2−, MnO 4 −, MoO 4 2−, TcO 4 −, WO 4 2−, ReO 4 −, OsO 4) are evaluated as empirical functions of n as well as of the two lowest energy electronic transitions. It is shown that there are nearly linear relationships between the force constants and each of these three parameters for isoelectronic species and species the central atoms of which belong to the same group of the periodic table.

Oxygen-17 nuclear magnetic resonance of inorganic compounds

The 17 O chemical shifts of a number of com pounds have been measured at 6.000 Mc/s in the liquid or solution state. The resonances observed for the ‘closed-shell’ oxy-anions of the transition metals indicate a high degree of paramagnetic shielding of the oxygen nuclei for these anions. A linear relationship has been observed between the chemical shifts for these anions and their lowest-energy electronic transitions as obtained from u.v. and visible spectra. Calculations in term s of a molecular orbital model for these anions have successfully accounted for this relationship and at the same time have produced a mechanism for the observed paramagnetic shielding. It has been shown that a similar linear relationship exists in the cases of organic carbonyl compounds for which nuclear resonance and optical spectral data are available.