Quantum chemical calculations bond rotations

Fig. 3. Calculated potential energy curve for the rotation about the O-C-C-N bond in the muscimol zwitterion using HF/6-31+G ab initio quantum chemical calculations. Dihedral angles corresponding to the global energy minimum of muscimol and the solid-state conformations of the muscimol zwitterion and the THIP cation are denoted by arrows.

The bond angles given in parentheses were taken from quantum-chemical calculations and were used in calculations of the bond lengths from the rotational constants. 

The rotational and conformational isomerism in dimeric proanthocyanidines 101 was studied by NMR spectroscopy. It was found that the geometry of these important polyfla-vanoids depends on the nature of the solvent (in organic solvents and water). The effect of the Y atom and the substituents X on the planarity and the barrier to internal rotation about the aryl-Y bond were estimated by semiempirical quantum-chemical calculations... 

Fig. 5.5 Models of the green fluorescent protein chromophore in the neutral, anionic, and zwitterionic forms used in the quantum chemical calculations, shown in those resonance structures that best represent the calculated bond orders. Rotation by 180° around (p leaves the structure unchanged. The configurations displayed represent r = 0° and are referred to as cis configurations. The upper panels show energy profiles for rotation around the dihedral angles r and (p and for...

Quantum chemical calculations indicate that the barriers to internal rotation in FOOF are much higher than in HOOH the energies of the planar syn and anti forms are both calculated to be about 100 kJ moP above that of the equilibrium structure[6]. Interestingly the 0-0 bond distances in the planar forms are similar to the 0-0 bond distance in HOOH while the O-F bond distances are similar to the O-F bond distance in OF2. 

The bond between adjacent thienylene units in PT can adopt the trans or c/s configuration by virtue of adjacent units being syn or anti. X-ray diffraction reveals an anti conformation (planar) leading to straight chains with an orthorhombic unit cell in the solid state [71,97,114,115]. The syn conformation which can occur in alkyl substituted oligo- and poly(thiophene)s represents a deviation from the ideal all-anti conformation and introduces rotational defects into the polymer which interrupt or weaken the extent of conjugation [115]. Theoretical information on the gas-phase conformations in PMT in the neutral state can be obtained by the ab initio quantum-chemical calculations on bithiophene and methyl substituted bithiophenes [116]. 

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