Torsional isomerization of biologically active bicyclic molecules

Abstract

Rotational isomerization of bipyridines C5H4N-C5H4N was studied by CNDO/Z, PPP/CI, and C:NDO/CI methods. It is shown that CNDO/Z overestimates the angle of rotation 6 between the pyridine rings ca. two times. The angle 6 was determined for 2,2'-bipyridine by means of correlation of the theoretical (CNDOIZ) and experimental dipole moment. It was also found from the correlation between the theoretical and experimental uv spectra. It is shown that there is an explicit dependence of the results upon the distance between heteroatoms (PPP/CI). It has been found that the CNDOICI method correctly predicts the value of the rotational angles and their sequence in bipyridines.

Ab Initio Computations of the Geometrical, Electronic, and Vibrational Properties of the Ground State, the Anion Radical, and the N,N‘-Dihydro Cation Radical of 4,4‘-Bipyridine Compared to Transient Raman Spectra

“…It reflects the relative contributions of the π-electronic interaction of the pyridyl fragments (favorable to a coplanar conformation) and the ortho nonbonded interactions of these fragments (favorable to a nonplanar structure). The 3-21G(+*) ab initio method used in our study correctly predicts a twisted configuration (A symmetry in the D 2 group), in good agreement with experiment 29 and with former theoretical studies. ,,, The torsional θ angle is slightly overestimated by our calculations (θ calc = 48.6°) compared to the experimental value. This value indicates that the ortho steric repulsions predominate with respect to the gain in energy resulting from the π-electron delocalization.…”

“…Some theoretical studies 11-15 have also been undertaken to determine the θ angle of 44BPY in the S 0 state, all other geometrical parameters being kept fixed. Ab initio STO-3G computations of the torsional potentials using the STO-3G optimized geometry of the pyridine ring and an inter-ring distance C 4 C 4‘ of 1.50 Å have given a θ angle of 40.8° and barrier heights V ∥ and V ⊥ of 2.6 and 2.5 kcal·mol -1 , respectively. , The semiempirical NDDO method, using the experimental structure of pyridine and an inter-ring bond of 1.50 Å, has led to an equilibrium twist angle θ of 40° and barrier heights V ∥ and V ⊥ equal to about 1.6 and 0.9 kcal· mol -1 , respectively, in qualitative agreement with the STO-3G results. , A correlation between the theoretical and experimental UV spectra in the framework of the semiempirical CNDO/CI method has led to a predicted torsional angle of 41° …”

“…Some theoretical studies [11][12][13][14][15] have also been undertaken to determine the θ angle of 44BPY in the S 0 state, all other geometrical parameters being kept fixed. Ab initio STO-3G computations of the torsional potentials using the STO-3G 11,12 The semiempirical NDDO method, using the experimental structure of pyridine and an inter-ring bond of 1.50 Å, has led to an equilibrium twist angle θ of 40°and barrier heights V | and V ⊥ equal to about 1.6 and 0.9 kcal‚ mol -1 , respectively, 14 in qualitative agreement with the STO-3G results.…”

“…Thus, there is a need for complementary methods, such as provided by quantum chemistry, to obtain a more precise and quantitative description of the phototransient geometries. Some scattered theoretical studies concerning S 0 44BPY − and the ion radicals ,, have been proposed. They have generally been limited to the determination of the rotational conformation of the pyridyl rings around the inter-ring bond.…”

Six‐Membered Heterocycles: Triazines, Tetrazines and Other Polyaza Systems

“…The UV absorption of the parent 3 shows two bands, at 272 and 222 nm, assigned as n-p à and p-p à transitions, respectively. The UV spectra of 1,3,5-triazine derivatives that contain p-electron donors have also been studied [467]. The presence of substituents that can conjugate with the triazine ring results in the expected bathochromic shift and the band may be very intense [468].…”