Configuration mono-excited

The symmetry of the quinolizinium ion is such that only four monosubstitution products are possible for any given substituent. On the basis of a computation using the Pariser-Parr-Pople method, including all singlet mono-excited electronic configurations, Galasso (68MI21000) concluded The carbon atoms at 3 and 7 are expected to be most reactive towards electrophilic substitution. ... 

Energy to second order (a) yii is a mono-excited configuration (i) Mono-excitation in a bond... 

The Slater rules predict that the one-electron term survives only for mono-excited configurations whereas the two-electron term survives also for biex-cited configurations. However, the perturbation formula for Ag b involves the product of the first-type (one electron only) and the second-type integrals so that the biexcited configurations cannot contribute. This allows to consider the whole second term as if it were constructed of an effective one-electron operator... 

To make progress, let us assume that the q- h configuration is a mono-excited configuration with respect to the //-th configuration, i.e. 

Chia and Simmons388 calculated the resonance energies (ER)20 of four mono- and dibenzotetraazapentalenes (Scheme 24). Values are comparable with those of o-condensed aromatic systems (naphthacene, ER = 110 kcal mol-1 chrysene, ER = 116.5 kcal mol-1), and, like these carbocyclic systems, angularly-shaped molecules are more stable than linear ones. HMO calculations of delocalization energies (DE) show that the tetraazapentalene structure 15 is more stable than the tetra-azacyclooctatetraene valence isomer 324 (Scheme 14, Section IV,B,2) whether 324 is planar or tub-shaped. Calculations of electrophilic reactivity (Section IV,C,4,d), electronic spectra (by the PPP method employing all singly excited configurations), and bond orders have been carried out, and they confirm the aromatic nature of these systems. 

The interest of the trinucleotides lies on the possibility of an interpretation of the genetic code. But before presenting the details for these systems, it is convenient to review the situation regarding dinucleotides and compare the results for some typical mono-, di-, and trinucleotides. Ladik and Appel (1966) have carried out Cl calculations for the dinucleotides GG, GC, CG, and CC. In these calculations the first sixteen singlet excited configurations have been used. (For comparison purposes, the calculations have been carried out for two sets of molecular orbitals those obtained after the first iteration and the true SCF orbitals). On the basis of these results, Ladik and Appel (1966) indicate the existence of a hypochromicity in the first absorption band for the four dinucleotides considered. 

Nieber and Doltsinis [64] calculated 15 nonadiabatic surface hopping trajectories starting from configurations and velocities sampled from a ground state simulation. The nonradiative excited state lifetime has been determined by fitting the time-dependent, decaying excited state ensemble population to a mono-exponential function subject to the boundary condition that all molecules were in the Sj state... 

Langer and Doltsinis [45] have calculated nonadiabatic surface hopping trajectories for 10 different initial configurations sampled from a ground state AIMD runs at 100 K. They later extended their study to a total of 16 trajectories [41, 42], From a mono-exponential fit to the 5) population a lifetime of 1.3 ps is obtained (see Table 10-1 the average transition probability and its standard deviation leads to the interval [0.6...1.1...3.5] ps. Thus methylation appears to result in a slightly longer excited state lifetime. 

Since the elementary reactions in the stoichiometric mechanism of Eq. (9.5) include participation of anionic particles, the basis set in the ab initio calculations [13] contained polarization functions as a supplement to the standard set STO-3G and for a correct description of the acts of bond rupture [Eq. (9.5b)], several hundreds of mono- and double-excited electron configurations were taken into account. Allowance for the solvent (nine water molecules in the first coordination shell) was made in terms of the point charge model, as described in Sect. 3.3.2. 

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