Udo-Burckhard Richter

Pulse radiolysis of pyrene in aprotic polar organic solvents: simultaneous formation of pyrene radical cations and radical anions

Abstract Simultaneous formation of pyrene radical cations Py · + and radical anions Py · − was observed by pulse radiolysis of pyrene in DMSO or sulfolane solutions containing 10 vol % toluene. Using 5×10 −5 mol dm −3 pyrene in the oxygen-free solvent mixture of 90 vol % DMSO and 10 vol % toluene, the radical cation Py · + (λ max =450 nm ;e 450 =2.29×10 4 dm 3 mol −1 cm −1 ) was formed with the rate constant k=4.0×10 9 dm 3 mol −1 s −1 and decayed by a first-order reaction with k 1 =1.2×10 4 s −1 , forming the dimer radical cation ( Py ) 2 · + . Simultaneously, the radical anion Py · − (λ max =495 nm ; e 495 =2.17×10 4 dm 3 mol −1 cm −1 ) was produced with k=6.0×10 10 dm 3 mol −1 s −1 and disappeared by formation of the dimer radical anion ( Py ) 2 · − with k 1 =4.6×10 4 s −1 . The first-order rate constants (k1) for the decay of the species Py · + and Py · − with formation of the dimer radical ions ( Py ) 2 · + and ( Py ) 2 · − increased with raising the substrate concentration. By scavenging all radiolytically generated electrons with oxygen (k≈8×10 8 dm 3 mol −1 s −1 in 90 vol % DMSO and 10 vol % toluene), the formation of the Py · − transients was suppressed without interfering with the generation of the radical cations Py · + . In oxygen-free solutions of pyrene (5×10 −5 mol dm −3 ) in 90 vol % sulfolane and 10 vol % toluene the corresponding rate constants of the transient formation and the decay were for Py · + :k=1.5×10 10 dm 3 mol −1 s −1 and k 1 =0.9×10 4 s −1 , and for Py · − :k=2.3×10 10 dm 3 mol −1 s −1 and k 1 =4,1×10 4 s −1 .

Radiation-induced C—C bond cleavage in 1,2-diarylethanes as model compounds of coal. Part 1.—Pulse and steady-state radiolysis of 1,2-di(1-naphthyl)ethane

1,2-Di(1-naphthyl)ethane (1,2-DNE) has been used to study the radiation-induced C—C bond cleavage of the ethane linkage. Pulse radiolysis was applied both in the absence and presence of NaAlH4 and NaAlH2(OR)2 used as scavengers for solvent cations. In the presence of the sodium aluminium hydrides the radical anions of 1,2-DNE are stabilized by the metal cation, resulting in the formation of radical anion/sodium cation pairs (Na+, 1,2-DNE˙–); these species have a rather long lifetime (τ > 2 ms) in DME as well as in THF, and show three absorption bands at 330, 370 and 750 nm. In THF Iµ370= 14000 dm3 mol–1 cm–1 and Iµ750= 5000 dm3 mol–1 cm–1 were established. Cleavage of the ethano bridge of 1,2-DNE was achieved by means of steady-state as well as multi-pulse radiolysis in the presence of alkali-metal aluminium hydrides. The efficiency of this process was studied by using NaAlH4, NaAlH2(OR)2, NaAlH2Et2, NaAlEt4 and LiAlH4 in THF as well as in DME. The most efficient C—C bond cleavage was found to occur with NaAlH4 in DME. Based on all the experimental results it is concluded that the C—C bond cleavage results from the successive attack of two sodium cation/electron ion pairs (Na+, e–), forming the unstable dianions (2 Na+, 1,2-DNE2–). These decompose under scission of the ethano bridge, producing naphthylmethyl carbanionic fragments C10H7CH–2Na+ which supposedly form aluminate salts, e.g. C10H7CH2AlH–3Na+, with AlH3 generated in the scavenging processes. From these aluminate salts 1-methylnaphthalene (1-MN) is obtained through hydrolysis.

Radiation-induced C—C bond cleavage in 1,2-diarylethanes as model compounds of coal. Part 2.—Pulse and steady-state radiolysis of 1,2-di(pyren-1-yl)ethane in tetrahydrofuran, dimethoxyethane and toluene in the presence of sodium dihydridobis(2-methoxyethoxy)aluminate

1,2-Di(pyren-1-yl)ethane (1,2-DPE) has been used as a model for coal to study the C—C bond cleavage of the ethano linkage in the radiolysis of solutions containing NaAlH2(OCH2CH2OCH3)2[NaAlH2(OR)2]. Transient species were investigated by pulse radiolysis of solutions of 1,2-DPE, 1-methylpyrene (1-MP) and pyrene (P) in THF, DME and toluene in the absence and the presence of NaAlH2(OR)2. In the presence of the latter stabilized and long-lived radical anion/sodium cation pairs of 1,2-DPE, 1-MP and P were generated even in the non-polar solvent toluene. The reaction mechanisms differ substantially for the ether solvents (THF or DME) and solutions in toluene. In the case of the ether solvents (SH) NaAlH2(OR)2 acts as a scavenger for solvent radical cations SH˙+ and solvent cations SH(H+) generated through ionization. This produces stabilized sodium cation/electron pairs (Na+, e–s) which reduce the aromatic substrates to their radical anion/sodium cation pairs. The prevailing species generated in the radiolysis of solutions in toluene are electronically excited molecules of the solvent and the substrate from which the excited triplet molecules are preferentially reduced to radical anions by NaAlH2(OR)2 in an electron-transfer process. Steady-state radiolysis (60Co γ-rays) of solutions of 1,2-DPE in THF, DME or toluene containing NaAlH2(OR)2 resulted in the C—C bond cleavage of the ethano linkage. This is attributed to the unstable dianion (1,2-DPE2–, 2Na+) formed in two successive reductions via(1,2-DPE˙–, Na+). UV–VIS spectroscopy and product analysis showed that the radiation-induced reduction of 1,2-DPE, 1-MP and P requires lower doses in THF and DME than in toluene. However, reactions of (1,2-DPE˙–, Na+) with THF and DME to form substrate–solvent addition products limit C—C bond cleavage (16% in THF and 32% in DME). The inertness of toluene towards such side reactions makes the radiation-induced cleavage process of 1,2-DPE in this solvent (despite of the higher dose required) much more efficient than in THF and DME, yielding after hydrolysis up to 52% 1-MP. Radiation-induced reduction of polycyclic arenes in the NaAlH2(OR)2–toluene system is very useful for investigating the radical anions of these arenes by UV–VIS and EPR spectroscopy, since essentially no other interfering species are formed.

Radiation-Induced C -C Bond Cleavage in 1,2-Diarylethanes as Model Compounds of Coal, Part 3. Pulse and Steady-State Radiolysis of 1,2-Di(9-anthryl)ethane in Organic Solvents

1,2-Di(9-anthryl)ethane (1,2-DAE) was used as a model for coal to study the C -C bond cleavage of the ethano linkage in the radiolysis of solutions containing NaAlH2(OCH2CH2OCH3)2 [NaAlH2(OR)2]. Transient species were investigated by pulse radiolysis of solutions of 1,2-DAE in THF, DME and toluene in the absence and the presence of NaAlH2(OR)2. In the presence of NaAlH2(OR)2 stabilized, long-lived radical anion/sodium cation pairs of 1,2-DAE were generated even in the non-polar solvent toluene. The reaction mechanisms differ substantially for solutions in the ether solvents (THF or DME) and for solutions in toluene. Steady-state radiolysis (60Co γ-rays) of solutions of 1,2-DAE in toluene containing NaAlH2(OR)2 resulted in the C -C bond cleavage of the ethano linkage. This is attributed to the unstable dianion (1,2-DAE2-, 2Na+) formed in two successive radiation- induced reductions via (1,2-DAE∙- , Na+). The resulting (9-anthryl)methyl carbanionic fragments C14H9CH2-, Na+ react with AlH(OR)2 generated in the process, to form the aluminate salts [C14H9CH2AlH(OR)2-, Na+]. From the aluminate salts, 9-methylanthracene (9- MA) was obtained by hydrolysis in yields up to 65 wt.%.

Stabile Radikalanionen von Aromaten: Strahlenchemische Erzeugung in Gegenwart von Natrium-dihydridobis( 2-methoxyethoxy) aluminat

Abstract Arenes are reduced at room temperature to their very long-lived radical anions by exposing the solution of the arene and NaAlH2(OCH2CH2OCH3 )2 (1) in dimethoxyethane (DME) to an ionizing radiation (electrons, gamma- or X-rays) for few minutes: Thereby 1 is acting as a scavenger for the solvent counter ions which are produced concomitantly to solvated electrons by the radiolysis. The method is very convenient for the ESR and UV-VIS spectroscopy of aromatic radical anions.