Aromatic isolated molecule

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

For aromatic hydrocarbon molecules, in particular, the main acceptor modes are strongly anharmonic C-H vibrations which pick up the main part of the electronic energy in ST conversion. Inactive modes are stretching and bending vibrations of the carbon skeleton. The value of Pf provided by these intramolecular vibrations is so large that they act practically as a continuous bath even without intermolecular vibrations. This is confirmed by the similarity of RLT rates for isolated molecules and the same molecules imbedded in crystals. 

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

Calculations with one isolated molecule in vacuum often result in overestimation of intramolecular contacts, however, because competing interactions are absent. We started our investigation with the semi-rigid guanidinothiazole ICI27032, a potent competitive H2-antagonist which adopts solely extended conformations, because of its aromatic ring system (Fig. 9.8). 

M.o. theory and the isolated molecule treatment In this model, reaction is considered to occur preferentially at that position in the aromatic molecule to which the approach of the electrophile causes the smallest increase in zero energy. In molecules possessing polar or dipolar groups, long range electrostatic forces will initially be the most important. 

If these shift data really do represent the onset of an intermolecular electron transfer reaction in DABCO, ABCO, HMT clustered with amine, either, and aromatic solvents, one ought to be able to observe the reaction kinetics or dynamics. Consider the specific instance of DABCO. The singlet Rydberg state lifetime for DABCO (and all the other Rydberg molecules studied for this determination (Shang et al. 1993c, 1994a) is ca. 2 ps for the isolated molecule and ca. 1.2 ps for the nonpolar rare gas, hydrocarbon, and fluorocarbon solvents. This... 

Independent of the precision of the diffraction experiment, if a symmetrical molecule is placed in the crystal lattice in a nonspecific position (i.e., it is not in a position of symmetry required by the space group to which the crystal belongs), then the determined molecular geometry need not follow the symmetry properties obseiwed for the isolated molecule. This is due to an obvioirs experimental condition which holds that each bond length has its own, in principle independent, error of estimation. Therefore, for instance three outer rings in triphenylene (cf Chart 1) may have different (geometry-based) indices of aromaticity, despite C3 symmetry of the isolated molecule. 

---