Photoelectron spectrum pyridine

In the ultraviolet photoelectron spectrum, the most readily ionized level of pyridine is the nonbonding orbital (with contributions from the o -framework). The three diazines show two lone-pair levels, with the greatest splitting in the case of pyridazine but considerable also in pyrimidine and pyrazine. These long-distance splittings are attributed to both through-space and through-bond interactions, particularly the latter. 

Pyridinophane 2 has also been synthesized in a four-step procedure which includes a pyrolytic sulfur dioxide extrusion as illustrated in Scheme 15). The UV photoelectron spectrum of 2 has been studied on the basis of a pertubational molecular orbital analysis and indicates the presence of through-space and through-bond interactions between the two pyridine rings. This finding has also been substantiated by CNDO/2 calculations. In addition, there appears to be a strong interaction between the nitrogen lone pairs and the n-system of the rings 6). 

Other experimental evidence leads to essentially the same conclusion regarding the n ionization of pyridine. El Sayed and Kasha (1961) have detected Rydberg series in the absorption spectrum similar to those in benzene and ascribable to n orbitals (9-266 e.v., 02 11-56 e.v., 62) and, in addition, reported a fragmentary series leading to a third ionization potential of 10-3 e.v. which they ascribed to the nitrogen lone pair. Similar values are found by photoelectron spectroscopy which also indicated the 10-3 e.v. (10-54 e.v.) level to be only weakly bonding. 

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