Fig. 1. ' H N M R spectrum of the thiaselanes from reaction (b). The labels on the signals indicate tlie satellites due to "Se.'H coupling. Additionally. the number of bonds is given through which the nuclei are coupled: this number is repeated in the structural formulae; frequency 300.13 MHs. temperature 295 K. tant relationships between the structure and N M R parameters are summarized here.1. The number of Se atoms in a molecule can be derived from the number of 77Se satellite pairs"] in the 'H N M R spectrum (Fig. 1).Within a molecule, the value of the coupling constant decreases with the number of bonds separating the coupling nuclei. Table I ) .3. The environment of the "Se nuclei has. as expected, a very strong influence on their magnetic shielding. According to the equation in Scheme l , 6(Se) of a given Se atom is dependant on the number of Se and S atoms in the u-, p-, and ;-positions. As is found for alkanes"] and phosphanes,"'] the neighboring atoms in the uand fl-positions cause a downfield shift and those in ?,-positions an upfield shift. Consistent with the greater electronegativity, the constants for sulfur are always more positive (stronger downfield shifts).The constitution of the thiaselanes 1-9 follows unequivocally from the structural dependence of the N M R parameters (points --S ( S e ) = -227 + i x x i j & + k ; x i 5: X = S, Se I . j. X = number of S o r Se atoms in z , /l, a n d 7 positions, respectively zs = 377, /Is = 264. ys= -34. zSc = 311, /IsL. = 254. ySc= -36 Scheme 1 . Calculation of n'(Se) with chemical shift increment5
Magnetic field effects I Paramagnetic relaxation I Magnetokinetics I Photo electron transfer I Spin chemistryA regularity previously observed in the ligand dependence of the magnetokinetic effect on the photoelectron transfer efficiency between photoexcited bileptic Ru-trisdiimine complexes and methylviologen has been substantiated by investigating, in addition, the bipyridine/phenanthroline series of ligands. Our theoretical model of the mechanism has been improved by taking into account non-rotational mechanisms of spin relaxation in the photooxidized Ru 3 + -complex. The field dependences observed for the two bileptic series of complexes can be theoretically reproduced in a very consistent way. Paramagnetic relaxation times found to range around 10 -11 s, reveal a striking regularity in the course of ligand variation through the bileptic series.Eine kiirzlich beobachtete RegelmiiJ3igkeit in der Ligandenabhiingigkeit des magnetokinetischen Effekts auf die Effizienz des Photoelektronentransfers zwischen photoangeregten bileptischen Ru-trisdiimin Komplexen und Methylviologen wurde an einem weiteren Beispiel, Komplexen mit der Bipyridin/Phenanthrolin Ligandenreihe, bestiitigt. Unser Modell zur theoretischen Beschreibung der Magnetfeldabhiingigkeit wurde verbessert, indem es nun auch rotationsunabhiingige Mechanismen der Spinrelaxation im photooxidierten Ru 3 + -Komplex beriicksichtigt. Die fUr die beiden bileptischen Komplexserien beobachteten Feldabhiingigkeiten k6nnen theoretisch in sehr konsistenter Weise reproduziert werden. Die paramagnetischen Relaxationszeiten, die sich dabei ergeben, liegen urn 10 -11 S und weisen einen sehr regelmiiJ3igen Gang mit der Ligandenvariation innerhalb der bileptischen Reihen auf.
Photo electron transfer / Magnetic field effects / Spin-orbit couplingThe heteroleptic series of Ru(bpY)n(dce)~:::n,with bpy = 2,2'-bipyridine, and dce = 4,4'-diethoxycarbonyl-2,2'-bipyridine, has been investigated for the kinetic parameters of photo electron transfer, and for the magnetic field dependence of the net electron transfer efficiency (rIce) in photooxidations with methylviologen and propylviologen sulfonate (the latter only in the case n = 3) using ns-laser-flash spectroscopy and stationary illumination methods. Whereas the electric charge of the electron acceptor had no influence on the magnetic field effect on !Jceo the variation of ligands was found to result in a strictly linear (with n) increase of the magnetic field effect on k bet , the effective rate constant of backward electron transfer in the primary redox pair. The physical implications of these observations with respect to a previously developed magnetokinetic model are discussed.Die heteroleptische Reihe der Komplexe Ru(bpY)n(dceH:::n mit bpy = 2,2'-bipyridin. und dce = 4,4'-diethoxycarbonyl-2.2'-bipyridin, wurde im Hinblick auf die kinetischen Parameter der photochemischen Elektroneniibertragung und auf die Magnetfeldabhiingigkeit der Nettoausbeute der Elektroneniibertragung (tTce) fUr die Photoreaktion mit Methylviologen und Propylviologensulfonat (letzteres nur fUr n = 3) mittels ns-Laserblitzspektroskopie und kontinuierlicher Belichtungsmethoden untersucht. Wiihrend die Ladung des Elektronenakzeptors keinen EinfluJ3 auf den Magnetfeldeffekt auf !Jce hatte. bewirkte die Variation der Liganden eine streng lineare Zunahme (beziiglich n) des Magnetfeldeffektes auf k belo die effektive Geschwindigkeitskonstante der Elektronen-Riickiibertragung im primiiren Redox-Paar. Die physikalische Bedeutung dieser Beobachtungen wird im Rahmen eines kiirzlich entwickelten magnetokinetischen Modells diskutiert.
In this paper we present two examples demonstrating how magnetic field effects can be utilized for a quantitative exploration of mechanistic details of the behaviour of short-lived redox intermediates in photoelectron transfer reactions with excited triplet states. This magnetokinetic approach takes advantage of the spin memory present in the intermediates (exciplexes, correlated radical pairs) which controls the competition between spin-allowed formation of free redox products and spin-forbidden backward electron transfer regenerating the ground state reactants. In particular we demonstrate that the strong spin-orbit coupling exhibited by a heavy atom substituent or by heavy transition metal centers can be very useful for inducing and exploiting such diagnostic magnetic field effects.
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