2- pyrazine photoisomerization

The vibronic coupling model has been applied to a number of molecular systems, and used to evaluate the behavior of wavepackets over coupled surfaces [191]. Recent examples are the radical cation of allene [192,193], and benzene [194] (for further examples see references cited therein). It has also been used to explain the lack of structure in the S2 band of the pyrazine absoiption spectrum [109,173,174,195], and recently to study the photoisomerization of retina] [196],... 

Tetra-r-butylpyridazine (34) is converted into its Dewar isomer (35) when irradiated in pentane with UV light of wavelength > 300 nm. Irradiation of this product at shorter wavelengths, or thermolysis, gives rise to further reaction (91TL57). Irradiation of 4-amino-2,6-dimethylpyrimidine gives the acyclic amino imine via the Dewar pyrimidine as shown in Scheme 2a. The photoisomerization of perfluoropyridazines to pyrazines is considered also to involve Dewar diazine intermediates. 

These reactions can be interpreted as proceeding via an intermediate of the diazabenzvalene type, formed from the lowest singlet w, it excited state of the pyrazine, and are analogous to the results reported for the photoisomerization of alkylbenzenes.133 Support for this postulate comes from a study of 2,5-dimethylpyrazine which is isomerized to a mixture of 2,5-dimethylpyrimidine and 4,6-dimethyl-pyrimidine, presumably via the two diazabenzvalenes [Eq. (36)]. There is no evidence for products arising via bicyclohexadiene and prismane intermediates which are also involved in the photorearrangement of alkylbenzenes. 

Photoisomerization. Studies have been reported for the photoisomerization of cis- to tra y-2,3-diphenylpiperazine,250 cis- to trails-1,4-dimethyl -2,3-diphenylpiperazine,250 and between possible geometric forms of 2-[2-(naph-thalen-2-yl)vinyl]pyrazine (240)66 or related compounds.1236... 

Quaternary salts of l,2-bis(pyrazin-2-yl)ethylene (372) underwent (E — Z) photoisomerization the quantum yield for the chloride salt was better than that for the iodide salt.1165... 

The photoisomerization of pyrazine and its methyl derivatives to pyrimidines has been described (632). Thus photolysis at 254 nm of 2-methylpyrazine gave 4-and 5-methylpyrimidine, 2,6-dimethylpyrazine gave 4,5-dimethylpyrimidine, and... 

Peculiarly, the 3,6-bonded Dewar pyridazine appears to be more stable than pyridazine. This conclusion could originate from the large approximations and the difference in the structure between the aromatic and the Dewar forms. Thus, the order in the Dewar isomers of one diazine might be correct. The photochemical and thermal isomerization of perfluoroalkylated diazines have been investigated mainly by Chamber s group, and Dewar intermediates were proposed for photoisomerizations. The photoisomerization of tetrafluoropyridazine to the pyrazine was given a mechanism shown in Eq. (34).101... 

Examples of the photoisomerization of pyrazoles to imidazoles have been reported by Nishiwaki et al. In the photoisomerization of perfluoroalkyl-pyridazines to the corresponding pyrazines, Chambers et al. have shown that an intermediate diaza Dewar-benzene is involved rather than a diazaprismane. Ogata and Takagi have observed a novel photoisomerization of 2-pyridylaceto-nitrile to u-aminobenzonitrile, possibly via a Dewar-intermediate. 

The goal of this review is to critically compare — from both a concep-tional and a practical point of view — various MQC strategies to describe non-Born-Oppenheimer dynamics. Owing to personal preferences, we will focus on the modeling of ultrafast bound-state processes following photoexcitation such as internal-conversion and nonadiabatic photoisomerization. To this end, Sec. 2 introduces three model problems Model I represents a three-mode description of the Si — S2 conical intersection in pyrazine. Model II accounts for the ultrafast C B X internal-conversion process in the benzene cation, and Model III represents a three-mode description of ultrafast photoisomerization triggered by a conical intersection. Allowing for exact quantum-mechanical reference calculations, all models have been used as benchmark problems to study approximate descriptions. 

Photochemistry. The cw-isomer of [PtCl2(py)2] undergoes two primary photoprocesses, isomerization and dissociation, with quantum yields of ca. 0.04 and 0.025 respectively (313 nm irradiation). /ra 5-(PtCl2(py)2] also undergoes parallel photoisomerization and dissociation, but with very much lower quantum yields of ca. 10" . The effect of acidity on the photochemistry of cw-[Pt(gly)2] has been examined-the balance between photoisomerization and photodissociation is pH-dependent. Photochemical cis trans isomerization of [Pt(gly)2] is sensitized by pyrazine or by xanthone, but not by thioxanthone, quinoline, naphthalene, or biacetyl. Ni + quenches the photoisomerization, but Mn + does not. The sensitization process was shown to involve triplet-triplet intermolecular energy transfer. ... 

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