Constant dihedral angles

Interactive, online programs are available for the calculation of coupling constants, dihedral angles, and the H distance across thr a-linkages of some disaccharides, as well as a program for plotting up to five Karplus curves. 

J is almost always positive and its magnitude often exceeds that of T. It always depends in a predictable way on the dihedral angle ( ) between the outer two of the tluee bonds in die coupling patliway. Karplus first showed theoretically that T varies to a good approximation as A cos ( ) + B cos ( ), where A and B are constants, and also that A S>B [17]. Flis equation has received wide-ranging... 

Recommended values for the force constant are 7.0 kcal/mol A-for an in teratorn ic distari cc. 12,5 kcal/rn ol degree- for an angle, and 16.0 kcal/mol degree- for a dihedral angle. Use the recommended values and then, if the internal coordinate is not sufficiently restrained, try a larger force eonstant. 

In this representative dihedral potential, V is the dihedral force constant, n is the periodicity of the Fourier term, (jtg is the phase angle, and (]) is the dihedral angle. 

The stmcture of O2F2 is that of a nonlinear FOOF chain, having the foUowiag molecular constants (56,57) O—O distance, 0.122 nm OOF angle, 109°30 dihedral angle, 87°30 dipole moment, 4.8 x 10 ° C-m (1.44 D). Additional physical and spectral data are summarized ia References 4 and 58. 

Figure 1 The principal sources of structural data are the NOEs, which give information on the spatial proximity d of protons coupling constants, which give information on dihedral angles < i and residual dipolar couplings, which give information on the relative orientation 0 of a bond vector with respect to the molecule (to the magnetic anisotropy tensor or an alignment tensor). Protons are shown as spheres. The dashed line indicates a coordinate system rigidly attached to the molecule.

Figure 2.18. Vicinal HH coupling constants Jhh as a function of the dihedral angle 9 of the CH bonds concerned (Karplus-Conroy relationship)...

The //NMR spectrum (Fig. 2.19) displays anAB system for the protons adjacent to this bond the coupling constant = 72 Hz. From this can be deduced first that the dihedral angle 9 between the C7/bonds is about 180°, second that conformer 14b with minimised steric repulsion between the substituents predominates and third that there is restricted rotation around this CC bond. 

Vicinal CH coupling constants Hqh resemble vicinal HH coupling constants in the way that they depend on the cosine of the dihedral angle 9 between the CC bond to the coupled C atom and the C//bond to the coupled proton (cf Fig. 2.16), as illustrated by the Newman projections of the conformers 20a-c of a propane fragment. 

Current option 136, 214 density matrix 263 l,2-dichloro-l,2-difluoroethane 24 dichloroethane 239, 242 dielectric constant 239 diffuse functions 99 difluoroethylene 45 dihedral angles 290 valid range 288 n,n-dimethylformamide 105 dipole moment 20... 

The coupling constant, J, between vicinal protons varies with dihedral angle, 0. The relationship between J and 0 is given by the Karplus equation . 

For each molecule (isomer A and isomer B), obtain dihedral angles for the following pairs of vicinal hydrogens Hg-Hy, H Hg, Hy-Hn, and Hg-Hyaxiai- Use the Karplus equation to estimate coupling constants for each pair, and then compare your predictions to the experimental coupling constants (see above). Which molecule is artemisin acetate and which is 6-epiartemisin acetate ... 

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