The UV spectra of 15 2-N-substituted 4-nitropicolines and their N-oxides . UV spectroscopy, in particular, has been prominent [3][4][5][6][7][8][9].The importance of the overlapping between the highest occupied molecular orbital (HOMO) of an electron donor and the lowest unoccupied molecular orbital (LUMO) of an electron acceptor in determining the favorable position and spatial direction of a chemical reaction is emphasized by setting up the following auxiliary principles: the principle of positional parallelism between the charge-transfer and the bond-interchange; the principle of narrowing of the interfrontier energy-level separation; and the principle of growing frontier density along the reaction path. These subprinciples work in a cooperative manner to enable us to arrive at this general governing principle: most of the chemical interactions are liable to occur at the positions (at the direction) where the overlapping of the HOMO and LUMO of the respective reactants is at its maximum in an electron-donating reactant, the HOMO predominates in the overlapping interaction with the MO's of the other reactant, whereas the LUMO does in an electron-accepting reactant; in the reactants which possess singly-occupied MO's, these play the part of HOMO or of LUMO, or of both. These particular MO's, HOMO, LUMO, and singly-occupied, are "generalized frontier orbitals" in chemical reactions [10]. Fukui [11] found some parallelism between the LUMO energy level and the fungicidal activity. The properties of 4-nitropyridine N-oxide and its methyl derivatives are especially interesting because of their antifungal activity [12, 13] and optical nonlinearity on the molecular as well as the macroscopic level [14, 15].An indispensable prequisite for achieving large second-order nonlinear optical response in an organic molecule is the existence of intramolecular charge transfer (ICT), resulting from the electron-donor and electron-acceptor groups communicating through a ~'-conjugated molecular framework.In the literature there are no data on the UV spectra of 2-N-substituted 4-nitropyridine N-oxide and their methyl derivatives; there are few data on the spectra of 2-nitraminopyridine [9, 16, 17]. In recent papers the UV spectra of 2-alkylamino-, 2-phenylamino-, 2-morpholyl-3-or -5-nitro-4-, or -6-methylpyridines are reported [8, 9, 18]. The aim of this paper is to determine the mutual communication of NO and NO 2 groups and its modification by the steric effects of the methyl and 2-methylamino group.
EXPERIMENTALThe title compounds were prepared as described previously [19]. The UV spectra of 0.1 mM solutions in ethanol were recorded by means of a Specord UV-vis spectrophotometer equipped with a thermostated cell compartment keeping the temperature at 25~ the quartz cell being of 0.097 cm thickness.
