Pyrazine resonance energy

For pyridine, pyrazine, and related six-membered heterocyclic molecules Kekul6 resonance occurs as in benzene, causing the molecules to be planar and stabilizing them by about 40 kcal/mole. The interatomic distances observed in these molecules,106 C—C = 1.40 A, C—N = 1.33 A, and N—N 1.32 A, are compatible with this structure. The resonance energy found for quinoline, 69 kcal/mole, is about the same as that of naphthalene. 

Pyrazine may be represented as a resonance hybrid of the canonical structure illustrated (21a -<+ 21d). The molecule is planar, and Pauling (114) states that it is stabilized by about 40kcal/mol as in benzene and pyridine, but resonance energies derived by different methods show considerable variation. Some of these resonance energies together with values for benzene and related heterocycles are summarized in Table 1.2 (115-117). More recent measurements of heats of hydrogenation are given in Section IV. 1B. 

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>. 

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. 

These computations establish conclusively that there is a seam of low-energy conical intersections, primarily in the Gi. Gsfl. QiOa subspace of normal-coordinate space. Here, Vi and V6 are two of the five totally synunetric normal modes, while V[o<, is the (single) mode of appropriate symmetry (82, X Bou = Big) to couple the 5 and S2 excited states in first order. Two other totally symmetric modes, vga and i>9u, turn out to be of secondary importance, while the coupling strength of the remaining mode of this symmetry, V2, is found to be negligible. The existence of a conical intersection has already been inferred from lower-level ab initio data, as well as from a combination of semiempirical calculations and adjustments of parameters to reproduce the spectral profile. " This conical intersection dominates the short-time photophysics of 5 - and 52-excited pyrazine, as documented below for absorption and resonance Raman spectra. 

---