NOTES
2731
crystallographiczlly different chlorine atoms of the iodobenzene dichloride molecule; one intermolecular chlorine to iodine distance is shorter.’ For interpretation within the Townes-Dailey approximation6 we may take the mean of the two observed frequencies and double this to obtain an average value of the chlorine quadrupole coupling constant eQq/h = 46.666 Mc/sec in the crystalline solid. Here, q is the principal component of the electric field tensor; the principal axis lies along the I-Cl bond and the reasonable assumption that the other two components are equal is made; i.e., we assume that the 3pz and 3p, chlorine orbitals which contain two lone pairs do not participate in the bonding. Then
q = qow,
- ‘/2(Nz
+ Nu)]
where eQqo/h = 109.74 Mc/sec is the contribution made by one p electron and N , is the electron population of the pi valence orbital of the chlorine. N , is then 1.58 electrons. If the chlorine orbitals are assumed to be sp hybrids with 15% character this value is reduced to 1.51. Since each chlorine atom contributes one valence p electron to the bonding, the net formal charge at each chlorine is approximately -0.5. This is reminiscent of the chlorine electron populations previously obtained for IC12- and ICl4-S and is considerably greater than found for those iodinechlorine molecules, such as ICP and IC12+,6in which the bonds are of the two-center, two-electron type. The delocalized MO description is favored. The distributior, of electrons within the bonding MO and the nonbonding orbital is readily obtained. The nonbonding orbital is, by symmetry, $n = l/2/2 [&I’ +CC~”]while the bonding orbital is $b = a(&!,’ qh”)- b4I where h’, 4~1”and 41 are the colinear 3p orbitals of C1 and the 5p orbital of I. With N , -1.5, a is -0.5 and b is -
