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.
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 [
The UV spectra of pyrimidines have been investigated [
A detailed series of MO studies at the HMO level have been carried out on purines, pyrazolopyrimidines, triazolopyrimidines, and pyrazolopyrimidines [
In part I of this work [
The electronic structure of molecules usually manifests itself in the electronic absorption and emission spectra. This manifestation enables the detailed understanding of the forces that govern the electronic structure of the studied Triazolopyrimidines. Although several investigations have been published that dealt with the electronic spectra of pyrimidines, yet there is no systematic study of substituents and solvent effects on the observed spectra. Such study is of critical importance in understanding their electronic structure that underlines their biological activity.
The aim of our paper is to explore (1) the type and extent of conjugative interaction between different subsystems of Triazolopyrimidines, (2) the effect of solvent polarity on the observed spectra and, hence, predict the relative stabilities, extent of charge transfer character, and assignment of the observed electronic transitions, and (3) the effect of para-phenyl substitution, using electron-donating and withdrawing groups on the observed electronic spectra of Triazolopyrimidines.
Compounds studied were prepared and purified by standard procedures cited in the literature [
Polar (methanol) and nonpolar (dioxane) solvents were obtained from Merck, AR-grade and were used without any further purification.
Spectra were scanned on a Perkin Elmer lambda 4B spectrophotometer using 1.0 cm fused silica cells. The machine records linearly in percent transmittance over the range 190–900 nm.
Molecular orbital calculations, employing full geometry optimization, were performed on the ground states of the molecules studied, at the DFT- B3LYP/6-31G [
The electronic absorption spectra in nonpolar (Dioxane) and polar (methanol) solvents are presented in Figure
Calculated and observed band maxima, intensities, and dipole transition of 3-Acetyl-1-phenyl-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Theoretical | Experimental | |||||
Configuration | Coefficient | Polar | Nonpolar | ||||
I | 56-57 | 0.5193 | 382 | 0.001 | 5.6759 | 375 | 375 |
56–58 | 0.8130 | ||||||
II | 56-57 | 0.870 | 350 | 0.558 | 0.9529 | 362 | 360 |
III | 56-57 | 0.912 | 320 | 0.7622 | 2.5338 | 320 | 325 |
56–59 | 0.646 | 300 | 0.1449 | 5.5869 | 300 | 301 | |
IV | 55–57 | 0.503 | |||||
54–57 | 0.520 | ||||||
V | 56–59 | 0.864 | 286 | 0.0437 | 13.6527 | 289 | 298 |
VI | 54–57 | 0.775 | 252 | 0.2082 | 8.5341 | 248 | 250 |
VII | 55–58 | 0.472 | 223 | 0.0466 | 5.0933 | 212 | 223 |
54–58 | 0.759 |
State energies, configurations, type of transition, and assignment of all transitions for 3-Acetyl-1-phenyl-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Configuration | Type | Assignment | ∆ | |
Obs. | Calc. | ||||
I | 56-57 | Delocalized band | 3.31 | 3.25 | |
56–58 | Delocalized band | ||||
II | 56-57 | Delocalized band | 3.44 | 3.54 | |
III | 56-57 | Delocalized band | 3.82 | 3.88 | |
56–59 | CT: N- aryl →Triazolo-pyrimidine | ||||
IV | 55–57 | CT: N- aryl →Triazolo-pyrimidine | 4.12 | 4.13 | |
54–57 | CT: N- aryl →Triazolo-pyrimidine | ||||
V | 56–59 | Delocalized band | 4.16 | 4.34 | |
VI | 54–57 | CT: N- aryl →Triazolo-pyrimidine | 4.96 | 4.92 | |
VII | 55–58 | CT: N- aryl →Triazolo-pyrimidine | 5.56 | 5.56 | |
54–58 | CT: N- aryl →Triazolo-pyrimidine |
Electronic absorption spectra of
The spectrum, in dioxane, is composed of three main band systems. The long wavelength system is composed of two bands centered at 375 and 360 nm and the medium envelope composed of three bands centered at 325, 301, and 298 nm. The third band system is composed of two bands centered at 250 and 223 nm. Increasing solvent polarity causes blue shift of all observed bands. Furthermore, increasing solvent polarity causes a marked decrease in the intensity of all bands. All the observed bands may be assigned to
Excited configurations, considered in this paper, are those which result from an electron excitation between the highest three occupied molecular orbitals
The long wave length absorption band has been computed theoretically at 382 nm and composed of two configurations, namely,
The second (
The third (
The fourth (
The fifth (
The last two states computed theoretically appear at 252 nm and 223 nm. Both states are
Figure
Calculated and observed band maxima, intensities, and dipole transition of 3-Acetyl-1-p-tolyl-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Theoretical | Experimental | |||||
Configuration | Coefficient | Polar | Nonpolar | ||||
I | 59-60 | 0.813 | 371 | 0.5612 | 5.2449 | 375 | 376 |
59–61 | 0.484 | ||||||
II | 59-60 | 0.897 | 326 | 0.8362 | 4.1182 | 340 | 325 |
III | 59–61 | 0.895 | 274 | 0.5813 | 8.5551 | 275 | 275 |
IV | 58–61 | 0.556 | 224 | 0.0273 | 4.9168 | 220 | 225 |
57–61 | 0.653 |
State energies, configurations, type of transition, and assignment of all transitions for 3-Acetyl-1-p-tolyl-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Configuration | Type | Assignment | ∆ | |
Obs. | Calc. | ||||
I | 59-60 | Delocalized band | 3.30 | 3.34 | |
59–61 | Delocalized band | ||||
II | 59-60 | Delocalized band | 3.82 | 3.80 | |
III | 59–61 | Delocalized band | 4.51 | 4.53 | |
IV | 58–61 | CT: N- aryl → Triazolo-pyrimidine, | 5.51 | 5.54 | |
57–61 | CT: N- aryl → Triazolo-pyrimidine |
Electronic absorption spectra of
The first (
The second (
The last two bands are computed theoretically at 274 nm and 224 nm which agree with the experimentally observed spectra in both polar and nonpolar solvents. The band at 274 nm may be assigned as a delocalized transition whereas the band at 224 is assigned to a CT transition from the N-aryl moiety to the triazolo pyrimidine subsystem (cf. Tables
Calculated and observed band maxima, intensities, and dipole transition of 3-Acetyl-1-(4-methoxy-phenyl)-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Theoretical | Experimental | |||||
Configuration | Coefficient | Polar | Nonpolar | ||||
I | 62-63 | 0.8505 | 361 | 0.5234 | 5.0213 | 368 | 355 |
II | 62–64 | 0.7953 | 326 | 0.3086 | 10.1508 | 312 | 313 |
III | 61–63 | 0.839 | 263.5 | 0.3584 | 10.6888 | 262 | 263 |
IV | 61–63 | 0.845 | 228 | 0.0543 | 15.9894 | 227 | 225 |
Figure
Electronic absorption spectra of
The long wavelength band computed theoretically centered at 361 nm is composed of one configuration,
State energies, configurations, type of transition, and assignment of all transitions for 3-Acetyl-1-(4-methoxy-phenyl)-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Configuration | Type | Assignment | ∆ | |
Obs. | Calc. | ||||
I | 62-63 | Delocalized band | 3.49 | 3.43 | |
II | 62–64 | Delocalized band | 3.96 | 3.80 | |
III | 61–63 | CT: N- aryl → Triazolo-pyrimidine | 4.71 | 4.71 | |
IV | 61–63 | CT: N- aryl → Triazolo-pyrimidine | 5.51 | 5.44 |
The methyl and methoxy groups are electron donor groups, whereas Cl-atom is an electron donor by its mesomeric effect and is an electron acceptor by its inductive effect. The electronic spectra of
Calculated and observed band maxima, intensities, and dipole transition of 3-Acetyl-1-(4-chloro-phenyl)-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Theoretical | Experimental | |||||
Configuration | Coefficient | Polar | Nonpolar | ||||
I | 59-60 | 0.447 | 377 | 0.001 | 7.3992 | — | 374 |
59–61 | 0.824 | ||||||
II | 59-60 | 0.894 | 355 | 0.6553 | 3.0741 | 358 | 355 |
III | 59-61 | 0.830 | 310 | 0.2924 | 7.0622 | 304 | 305 |
IV | 59–62 | 0.919 | 280 | 0.0792 | 4.2378 | 298 | 290 |
V | 56–60 | 0.767 | 243 | 0.3583 | 6.4620 | 250 | 250 |
VI | 57–60 | 0.688 | 224 | 0.3585 | 12.0667 | 227 | 228 |
57–62 | 0.697 |
State energies, configurations, type of transition, and assignment of all transitions of 3-Acetyl-1-(4-chloro-phenyl)-1H-[1, 2, 4] triazolo [3, 4-b] quinazolin-5-one.
State | Configuration | Type | Assignment | ∆ | |
Obs. | Calc. | ||||
I | 59-60 | Delocalized band | 3.32 | 3.29 | |
59–61 | Localized band | ||||
II | 59-60 | Delocalized band | 3.49 | 3.49 | |
III | 59–61 | Localized band | 4.07 | 4.00 | |
IV | 59–62 | CT: N- aryl → Triazolo-pyrimidine | 4.28 | 4.43 | |
V | 56–60 | CT: N- aryl → Triazolo-pyrimidine | 4.96 | 5.10 | |
VI | 57–60 | CT: N- aryl → Triazolo-pyrimidine | 5.44 | 5.54 | |
57–62 | CT: N- aryl → Triazolo-pyrimidine |
Electronic absorption spectra of
The charge density maps of the occupied and vacant MOs for
The spectrum in dioxane show six bands centered at 374 nm, 355 nm, 305 nm, 290 nm, 250 nm, and 228 nm, whereas the spectrum in methanol shows bands centered at 358 nm, 304 nm, 298 nm, 250 nm, and 227 nm. The band at 374 nm in dioxane does not appear in methanol due to the low solubility of the compound. Increasing solvent polarity, on going from dioxane to methanol, decreases the intensities of all bands with a red shift of the band at 290 in dioxane to 298 nm in methanol (
The long wavelength bands, corresponding to states I, II, and III, are localized and delocalized bands, whereas states IV, V, and VI show considerable charge transfer character from N-aryl to triazolopyrimidine.
The present analysis of the electronic absorption spectra of the studied Triazolopyrimidines indicates clearly the following. The spectra are diffuse and reflect the complexity of the molecular forces in this series of molecules. Conjugation extents all over the entire molecular frame work. The spectra are both solvent and substituent sensitive. Spectra in methanol are, in general, diffuse and less resolved. In case of the parent compound The methyl derivative Increasing solvent polarity has thus the general effect of diffusing the observed band envelopes and reducing their intensity. This may be attributed to marked solvent-solute interaction rather than just dipolar effect. Electron-donating substituents, –CH3 and –OCH3, show the one and same trend. The long wavelength envelope is diffuse and can only be resolved into one peak. It is interesting to note that the –CH3 derivative shows its long wavelength peak at 376 nm whereas that for the –OCH3 derivative is at 355 nm. The second band system shows almost the same trend. Only one peak is observed whose position is blue shifted by 12 nm in case of the –OCH3 as compared to the –CH3 derivative.