Showing papers on "Pi interaction published in 1969"

Effects of Substitution on the Phosphorescence Process of Aromatic Carbonyl Compounds

Abstract: Phosphorescence and absorption spectra, phosphorescence- and excitation-polarization, spectra and phosphorescence lifetimes were measured at 77°K for benzaldehyde, acetophenone and their eight derivatives with a halogen atom, or a hydroxyl, methoxyl or amino group as a substituent at the para position. In chloro- and bromo-derivatives, the phosphorescent triplet state is of the (n,π*) type, and is mixed with the 1La(π,π*) state by direct spin-orbit interaction, as in the case of the parent carbonyls. In addition, owing to the heavy-atom effect, it gains its singlet character by the mechanism (Remark: Graphics omitted.) which accounts for the observation that the phosphorescence spectrum involves out-of-plane-polarized vibrational bands. Substitution of a more electron-donating hydroxyl, methoxyl, or amino group brings about a change of the phosphorescent state from the 3(n,π*) state to the; 3La(π,π*). The following mechanisms are shown to be important to the 3La(π,π*) state: (Remark: Graphics omitted.) Th...

Photochemische Reaktionen 49. Mitteilung [1] Spezifisch π → π*‐induzierte Keton‐Photoreaktionen: Die Doppelbindungsverschiebung von O‐Acetyl‐10α‐testosteron Protonisierung der Doppelbindung seines δ5‐Isomeren

Abstract: The α,β-unsatured ketone 10α-testosterone has been reported previously [6] to photoisomerize in t-butanol solution to the β,γ-unsaturated ketone. The irradiation had been carried out using a high-pressure mercury lamp in a quartz vessel. For structural reasons this double bond shift cannot proceed through a photoenolization mechanism involving an intramolecular hydrogen transfer from the γ-position to the enone oxygen as has been suggested to operate in several formally analogous cases of aliphatic enone isomerizations. In the present reinvestigation, O-acetyl 10α-testosterone (1) was used, employing selectively either excitation of its n π* (with wavelengths > 300 nm) or its π π* absorption band (with 253,7 nm). In t-butanol solution the doublebond shift 1 2 could be effected with π* excitation only. Experiments in deuterated solvent (t-BuOD) resulted in deuterium in corporation in both the δ5-ketone in the C(4)-position, cf.(3) and in the conjugated ketone. These results indicate that the reactions is initiated either in the, Sπ,π* state or in a high vibrational mode of the S0 or tππ*state. n π* Excitation of 1 in t-butanol gave essentially no over-all chemical change, while in benzene solution it resulted again in a double bond isomerization (1 2). In analogy to results with similar enones [28] under identical conditions the deconjugation in benzene may be the consequence of an intermolecular hydrogen abstraction of the Tn,π* excited state of the enone. Another specifically π π* induced photoreaction was observed on irradiation of the β, γ-unsaturated ketone 2 in t-BuOD with 253,7 nm. The olefinic hydrogen at C-6 of 2 was exchanged with deuterium and, to a small extent, isomerization to the conjugated ketone 1 with concomitant deuterium incorporation occurred. It is concluded that from the higher excited state of the β, γ-unsaturated ketone, but not from its Sn,π* state, an activation mode of the double bond is accessible to effect D+ addition at C-6 followed by deprotonation to 4 and to deuterated 1, respectively.

A simplified molecular orbital model for octahedral metal carbonyl compounds

Abstract: A simple model for the π-electron interaction between metal and CO groups in octahedral metal carbonyl systems has been developed. The basis set employed includes metal d orbitals of π symmetry with respect to the M—CO bond axes, and two orbitals of π symmetry on each CO, corresponding to the vacant π* orbitals of the CO groups. Each CO is assumed to donate 0·5 electrons to the metal via the σ bond. In compounds of the form M (CO)6–nLn, the ligands L are characterized by a certain degree of σ donor ability, and by two acceptor orbitals of π symmetry and variable energy. The model is sufficiently simple to permit exploration of a wide variety of ligand characteristics.With the aid of assumptions relating the charge distribution to bond orders, relative CO stretching force constants and CO,CO stretching interaction force constants can be calculated as a function of variable ligand characteristics. The results support the adequacy of the simple force field commonly employed in determining relative force constants in substituted octahedral metal carbonyl systems.