Azabenzene molecules

Mulder F, Van Dijk G, Huiszoon C (1979) Ab initio calculations of multipole moments, polarizabilities and long-range interaction coefficients for the azabenzene molecules. Mol Phys 38 577-603... 

S. L. Price and A. J. Stone, Chem. Phys. Lett., 98, 419 (1983). A Distributed Multipole Analysis of the Charge Densities of the Azabenzene Molecules. 

The objective of MPA is to calculate the static equilibrium crystal structure using the observed molecular structure and the observed space group symmetry. The molecule in the crystal was assumed to be rigid, a requirement that is well satisfied by the rigid azabenzene molecules. The success of the MPA model is measured against how well it predicts the observed lattice constants and the molecular translational and rotational orientation in the cell. 

Price, S. L. and Stone A. J., A six-site intermolecular potential scheme for the azabenzene molecules derived by crystal structure analysis. Mol. Phys., 51, 569, 1984. 

Since trivial names are so well established, it is now very difficult to abandon them in favour of a logical nomenclature system that provides structural information. Nevertheless, a predictive method of this type is very desirable, especially for molecules where there may be two or more heteroatoms present in the ring. One approach is to use a prefix, which is indicative of the heteroatom [aza (N), oxa (O), thia (S), bora (B), phos-pha (P), sila (Si), etc.], to the name of the corresponding carbocycle. Thus, pyridine becomes azabenzene and piperidine is azacyclohexane. 

The benzene and azabenzenes form iso-electronic series of molecules, as naphthalene and the azanaphthalenes also do. The ground state electronic and geometric structures are therefore quite similar within one series. The substitution of CH groups with nitrogens introduces lone-pair to 7r transitions, and lowers the benzene and naphthalene symmetries. Small and systematic trends are found for linear response properties of the azabenzenes [189]. Each molecule is, however, very specific with respect to phosphorescence due to the delicate nature of the SOC and electric dipole activity interactions. 

An interestixig subject we may comment upon is the interaction between (planar) molecules with conjugated K-electron system (e.g. aromatic molecules). It is sometimes argued that these motocules show a particularly strong di )ersion attraction because of the large in-plane polarizability of the 71-elcctrons. In a series of ab initio calculations for benzene and several azabenzenes, Mulder et al. 

The contribution to the predicted electrostatic potential of the anisotropic atomic multipoles (Q , / > 0), which represent the lone pair and n-electron density, rapidly become less important as the distance between the molecules increases. This not only results from the inverse power of R increasing with I, but also from the cancellation between the contributions from different multipoles and different atoms. For example, there is generally an atomic dipole component along a bond that opposes the polarity implied by the atomic charges, as shown in the results of distributed multipole analyses (DMAs) of the azabenzenes. ° Thus, the accuracy gained by using a distributed multipole model is very dependent on the relative separation and orientation of the molecules, as well as the actual distribution of charge in the molecule. 

Barone, V., Accurate vibration spectra of large molecules by density functional computations beyond the harmonic approximation the case of azabenzenes, J. Phys. Chem. A, 108, 4146 150(2004). 

Perturbation theory has been applied to anharmonic calculations of spectroscopy from ab initio potentials in a large number of studies [19-25,115-121]. In nearly all cases so far, second-order perturbation theory was employed. The representation of the anharmonic potential generally used in these studies is a polynomial in the normal modes, most often a quartic force field. A code implementing this vibrational method was recently incorporated by V. Barone in gaussian [24]. Calculations were carried out for relatively large molecules, such as pyrrole and furan [25], uracil and thiouracil [118], and azabenzenes [119]. We note that in addition to spectroscopy, the ab initio perturbation theoretic algorithms were also applied to the calculation of thermodynamic properties... 

Fig. 15b shows that in all-trans-hex -l,3,5-tnene there are two distinct kinked Si/So conical intersections, one with the -(CH)3- kink located at the C2-C3-C4 position mediating simultaneous Z/E and s-ds/s-trans iso-merizations, and the other with the kink located at the C1-C2-C3 position, mediating simultaneous s-cis/s-trans isomerization and methylene twisting. The conical intersection of benzene,displaying the typical -(CH)s-kink arrangement, has already been reported in Fig. 8(a) of Sec. 2.2. The -(CH)3- kink intersection has been recognized in other aromatic systems such annulene, styrene,stilbene and azabenzenes ° and pyrrole, and antiaromatic molecules such as cyclo-l,3,5,7-octatetraene (see below). 

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