Nitrogen theoretical calculations

Azaindole is structurally close to purine and provides a simplified model for possible proton-relay for purine-type nucleobases. The mobile proton can stay either on the pyrrole or on the pyridine nitrogen. Theoretical calculations using the 6-31G(d) basis set predicted that the tautomer with the pyrrole hydrogen is more stable by 13-16 kcal mol than the structure with a pyridine hydrogen [74,... 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Benzotriazole can exist in two tautomeric forms, l-//-benzotriazole (6.46, R = H) and 2-/f-benzotriazole. If the aromatic ring contains a substituent, the 1- and 3-nitrogen atoms of the triazole are not equivalent, and therefore a 3-//-benzotri-azole derivative can also exist. The equilibrium between the 1 -H and 2-H tautomers of benzotriazoles is strongly on the side of the 1 -H tautomer, in contrast to triazole where the 2-H tautomer is dominant. Tomas et al. (1989) compared experimental data (enthalpies of solution, vaporization, sublimation, and solvation in water, methanol, and dimethylsulfoxide) with the results of ab initio theoretical calculations at the 6-31G level. 

In accordance with the electropositive nature of the bridgehead atoms, all di(pyridyl) substituted anions behave like amides with the electron density accumulated at the ring nitrogen atoms rather than carbanions, phosphides or arsenides. The divalent bridging atoms (N, P, As) in the related complexes should in principle be able to coordinate either one or even two further Lewis acidic metals to form heterobimetallic derivatives. According to the mesomeric structures, (Scheme 7), it can act as a 2e- or even a 4e-donor. However, theoretical calculations, supported by experiments, have shown that while in the amides (E = N) the amido nitrogen does function as... 

Other studies performed on cytotoxic-Ex, like compound 126 (Eig. 20) [219-222], have revealed that the toxic effect is related to molecular LUMO energy, hpophilicity (theoretically calculated) and MulHken charge on Fx 2-nitrogen [236]. A tentative mechanism of cytotoxicity was proposed using this correlation. 

As kinetic control was ensured, this clear microwave effect could possibly be due to a difference in polarity of the transition states, with apparently a more polar TS being formed when n attack by the nitrogen atom in position 2 is concerned. Theoretical calculations are in progress at the present time to try to confirm this assumption. 

Nitrogen. Pyridine is one of the most important heterocycles. The aromaticity of pyridine was intensively connected to structural considerations and chemical behavior. The relative difference between the aromaticity of benzene and pyridine is controversial generally calculations give similar orders of magnitude and differences depend on the criterion of aromaticity considered and the mode of calculation used. A comprehensive review on the theoretical aspects in connection with the aromaticity of pyridine was published.191 Pyridine is about as aromatic as benzene according to theoretical calculations and to experimental data, while quinoline is about as aromatic as naphthalene and more aromatic than isoquinoline.192193 The degrees of aromaticity of pyridine derivatives strongly depend on their substituents. 

In the previous discussion of laminar and turbulent flames, the effects of the physical and chemical parameters on flame speeds were considered and the trends were compared with the experimental measurements. It is of interest here to recall that it was not possible to calculate these flame speeds explicitly but, as stressed throughout this chapter, it is possible to calculate the detonation velocity accurately. Indeed, the accuracy of the theoretical calculations, as well as the ability to measure the detonation velocity precisely, has permitted some investigators to calculate thermodynamic properties (such as the bond strength of nitrogen and heat of sublimation of carbon) from experimental measurements of the detonation velocity. 

Structural and spectroscopic studies of pyrazino[2,3- ]pyrazine derivates were discussed in CHEC(1984) <1984CHEC(3)329>, and more recent data were reported in CHEC-II(1996) <1996CHEC-II(7)737>. Theoretical studies of the parent molecule, as for pyridazino[4,5- ]pyridazine, lead to the description of the nonbonding orbitals on nitrogen as perturbed naphthalene cr-orbitals, with contributions from cr -orbitals <1999PCA3089>. Infrared and Raman data for the parent have been reviewed and reassigned based on theoretical calculations <2000PCA7386>. 

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