A molecule may be homonuclear, that is, it consists of atoms of one chemical element, as with oxygen ( O 2 ); or it may be heteronuclear, a chemical compound composed of more than one element, as with water ( H 2 O ).
32.
Time-dependent interactions, such as homonuclear dipolar couplings, are more difficult to average to their isotropic values by magic angle spinning; a network of strongly coupled spins will produce a mixing of spin states during the course of the sample rotation, interfering with the averaging process.
33.
In the special case of a homonuclear diatomic molecule, there is an extra symmetry since in addition to the axis of symmetry provided by the internuclear axis, there is a centre of symmetry at the midpoint of the distance between the two nuclei ( the symmetry discussed in this paragraph only depends on the two nuclear charges being the same.
34.
For homonuclear A A bonds, Linus Pauling took the covalent radius to be half the single-bond length in the element, e . g . " R " ( H & ndash; H, in H 2 ) = 74.14 pm so " r " cov ( H ) = 37.07 pm : in practice, it is usual to obtain an average value from a variety of covalent compounds, although the difference is usually small.
