Ethynes, dipole moments

These interactions (dd, di, ii) are a function of dipole moment and polarizability. It has been shown that the dipole moment cannot be replaced entirely by the use of electrical effect substituent constants as parameters52. This is because the dipole moment has no sign. Either an overall electron donor group or an overall electron acceptor group may have the same value of /x. It has also been shown that the bond moment rather than the molecular dipole moment is the parameter of choice. The dipole moments of MeX and PhX were taken as measures of the bond moments of substituents bonded to sp3- and sp2-hybridized carbon atoms, respectively, of a skeletal group. Application to substituents bonded to sp-hybridized carbon atoms should require a set of dipole moments for substituted ethynes. 

As with diatomic molecules, the principal selection rule is that a permanent dipole moment is required for a molecule to produce a microwave spectrum. Linear polyatomic molecules have rotational wave functions exactly like those of diatomic molecules, so their rotational selection rules and spectra are the same as those of diatomic molecules. A symmetric linear molecule such as acetylene (ethyne) has no permanent dipole moment, and does not have a microwave spectrum. The fact that N2O has a microwave spectrum establishes the fact that it is NNO, not NON. Spherical top molecules such as CCI4 and SFe are so symmetrical that they cannot have a nonzero permanent dipole moment, and they have no microwave spectrum. A symmetric top molecule with a permanent dipole moment will have a microwave spectrum. A microwave spectrum is always observed for an asymmetric top molecule, because it has so little symmetry that it must have a nonzero permanent dipole moment. 

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