Bond lengths and rotational

Table I. Calculated and Experimental W—H and H—H Bond Lengths and Rotational Barriers for W(C0)3(PR3)2(H2) Species...

The derived system of equations of motion describes simultaneously a periodic variation of the H-bond length and rotation of this bond. We obtain dielectric response both to small translational oscillations of charges and to rigid-dipole reorientations. Each response (for charges and for dipoles) is characterized by two Lorentz lines. 

Also shown for comparison in Table XXXIX are the values of calculated assuming free rotation about each single bond of the chain (see Chap. X). Appropriate bond lengths and angles have... 

At that point, it should also be kept in mind that the values of bond lengths and angles are not directly accessible from experiments but are indirectly determined so as to reproduce the rotational constants which are themselves deduced from microwave experiments. Thus, comparison are always subject to some controversy since there is no biunivoque correspondence between the geometry and the rotational parameters. 

Polysiloxane chains have a longer bond length and a larger bond angle, giving rise to an even lower barrier to rotation. 

If one rotates about a C-C single bond in a compound of type X-C-C-Y, at each step of this torsional motion there are electron redistributions, and the bond lengths and angles will change. This phenomenon is the basis of the local nature of molecular structure. Several examples are illustrated in Figs. 7.6 and 7.7. 

Fig. 7.10 When the H-O-C-O torsions in dihydroxymethane, CH2(OH)2, are 180° and form a planar backbone (structure on the left), the H-C-0 angles (110.9°), C-H (1.086 A), O-H (0.963 A), and C-0 bond lengths (1.425 A) are identical, the molecule has a mirror plane, and the carbon atom is symmetric. In contrast, when one of the O-H bonds is rotated out of the plane, equivalent bond lengths and angles are different (1.078 A and 1.085 A, for C-H 1.438 A and 1.417 A for C-O 0.963 A and 0.965 A for O-H and 111.9° and 109.4° for H-O-C). Thus, in the structure on the right, the carbon atom is asymmetric with four different substituents. (All values from Schafer et al. 1984G).

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