Pyrimidine resonance energy

Topological resonance energy studies of the three isomeric thienopyrimidines show that while all of the ring systems are aromatic, the thieno[3,4-, pyrimidine class of compounds is less aromatic by approximately 20%. Correspondingly this system is believed to be considerably more reactive than its isomers <1997CCA841>. 

Quinazoline is a benzo-fused pyrimidine with a cyclic conjugated tOrr-electron system. It is a bicyclic, n-electron-deficient/ aromatic heterocycle (hetarene) with a resonance energy value of 127.2 kJ mol, and an aromaticity index of 143 (cf. naphthalene 142,- quinoline 134," quinazolin-4(3/7)-one 110.4, 2-methylquinazoline 3-oxide 119.8 ). Unlike pyrimidine, quinazoline has no symmetry. 

Energy levels and charge densities have been calculated by Hiickel MO calculations,3 and the resonance energy of pyrimido[5,4-c/]pyrimidine (1) has been approximated using an empirical relation.107 The AM 1-calculated heats of formation for pyrimido[5,4-(/Jpyrimidine and its protonated forms amount to 87kcal-mol-1 and 251/252 kcal mol -1 (Nl—H/N3 —H), respectively.2... 

The MM3 force field has been extended by Allinger and co-workers to cover aromatic heterocycles of the pyridine and pyrrole types <93JA11906>. Structures (32 compounds), dipole moments (35 compounds), heats of formation (35 compounds), and vibrational spectra (11 compounds) were examined. The results are good for structure and fair for the other items resonance energies were reported for the series benzene (17.79 kcal mol ), pyridine (17.02 kcal mol ), pyridazine (14.35 kcal mol ), pyrazine (17.01 kcal mol ), pyrimidine (15.60 kcal mol ), 1,3,5-triazine (13.51 kcal mol ), and 1,2,4,5-tetrazine (17.72 kcal mol ). Finally, ab initio studies of the dipole polarizabilities of conjugated molecules have been reported in which monocyclic azines (pyridine, pyridazine, pyrimidine, pyrazine, 5-triazine, and 5-tetrazine) are compared <94JST(304)109>. 

We have mentioned above the theoretical results concerned with the values of the resonance energies. In such highly conjugated heteroaromatic systems as the biological purines and pyrimidines they are extremely important... 

Each molecule meets the Hiickel criteria for aromaticity Each is cyclic and planar, has one 2p orbital on each atom of the ring, and has six electrons in the pi system. In pyridine, nitrogen is sp hybridized, and its unshared pair of electrons occupies an sp orbital perpendicular to the 2p orbitals of the pi system and thus is not a part of the pi system. In pyrimidine, neither unshared pair of electrons of nitrogen is part of the pi system. The resonance energy of pyridine is 134 kj/mol (32.0 kcal/mol), slightly less than that of benzene. The resonance energy of pyrimidine is 109 kJ/mol (26.0 kcal/mol). 

In pyrimidine, neither unshared pair of electrons of nitrogen is part of the tt system. The resonance energy of pyridine is estimated to be 134 kJ (32 kcal)/mol, slightly less than that of benzene. The resonance energy of pyrimidine is estimated... 

The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser-Parr-Pople method, and by the introduction of a new empirical parameter to differentiate resonance integrals between a orbitals and tt orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyri-dazine, pyrimidine and pyrazine. The results obtained were refined by a limited Cl calculation, and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both the n TT and n tt singlet transitions. The relative energies of the tt and the lone pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the lone pairs in these compounds is discussed. 

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