The electronic absorption spectra of triazolo pyrimidine and some of its derivatives were measured in polar as well as nonpolar solvents. Assignment of the observed transitions is facilitated via molecular orbital calculations. Charge density distributions, dipole moments, and the extent of delocalization of the MOS were used to interpret the observed solvent effects. The observed transitions are assigned as charge transfer (CT), localized, and delocalized according to the contribution of the various configurations in the CI-states. The correspondence between the calculated and experimental transition energies is satisfactory. 1. Introduction Pyrimidine and fused heterocyclic pyrimidine derivatives have attracted a great deal of interest due to their biochemical and pharmaceutical activities. Triazolopyrimidines, in particular, were tested for their medicinal, bactericidal, and fungicidal activity [1–3]. Wide variety of interesting biological activities were observed for those compounds, such as anticancer [4], antiviral [5], anti-H1V-1 activity [6], anti-inflammatory [7] and antimicrobial activities [8]. In addition, several substituted dihydronaphthothieno pyrimidine derivatives showed antimicrobial activities against Bacillus subtilis, Escherichia coli, Aspergillus niger, and Candida albicans [9], and their ester-containing derivatives demonstrated more antimicrobial activities than the corresponding cyano-containing analogs. The UV spectra of pyrimidines have been investigated [10]. The spectrum, in general, depends on the state of the nucleus, whether it is fully aromatic, partly reduced, or fully reduced and on the position, number, and nature of any chromophoric substituent attached to it. Furthermore, it also depends greatly on presence of ionic species in solution being measured. The UV absorption spectra of pyrimidines show two main band systems centered at 243 and 298？nm in cyclohexane. The second band is ascribed to an - * transition on account of the hypsochromic shift observed as changing solvent from cyclohexane to water. The lone pair become engaged in H-bonding in water so that the absorbed radiation must be of higher energy, that is, of lower wavelength. Loakes et al. [11, 12] studied the spectra of some triazolopyrimidines and discussed the effect of varying the acidity of the medium on the observed spectra. Effect of alkyl substitution on the spectra of some triazolopyrimidines was recently investigated [13] qualitatively. A detailed series of MO studies at the HMO level have been carried out on purines, pyrazolopyrimidines,
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