Chlorine, orbital population

The nuclear quadrupole coupling constant, e Qq, of a singly-bonded chlorine atom, R—Cl, is related to the halogen atom valence p and p orbital populations, a and a , by the equation (7,2)... 

If the terminal atom is multiply bonded to its neighbor, slightly different equations result. The jt bonding results in a reduction of population(s) of the chlorine orbitals below 2.00 electrons, to a value closer to the population of the chlorine p orbital. Therefore the quadrupole coupling constant and the NQR frequency of the chlorine atom are reduced. 

The NQR frequencies of halogenated compounds are related in a very simple manner to the valency p orbital populations on the halogen atom by the Townes and Dailey formula (4 ). For a carbon-chlorine bond, the Cl nucleus NQR frequency can be set equal to ... 

Let us now come back to equation (1). So far we have used it to calculate 35ci NQR frequencies from the computed 3pz orbital populations of two simplified models. Then we compared the calculated figures with the observed resonances of the more complicated pyranosyl chlorides. Conversely, we may start from these observed resonances, and use formula (1) to derive experimental values of the 3pz orbital populations of pyranosyl chlorides, so as to gain direct insight into their electronic structure. If no double bonding is involved, and if a constant value of 2 is adopted bor b9 we can accept as an experimental observation that on all pyranosyl chlorides examined, the 3pz orbital population on axial chlorine is higher than the 3pz orbital population on equatorial chlorine by an amount of about 5% (up to 7% in the case of mannose).Some people may consider that a statement in terms of ionicity i is more suggestive. Equation (1) may be written ... 

Fig. 4.3 b. The changes in the LU partial population of pt orbital at -carbon ethylene-chlorine cation system... 

Turning to the expression of orbitals and solvent effect on CN and CC1 bond polarization. Because of the simplicity of Equations (24) and (25), it is convenient to perform a Mulliken population analysis (see for example [26]) on the above orbitals at different CC1 distances in vacuo and in solution. In terms of contributions coming only from and (f>z, the Mulliken charges on nitrogen and chlorine turn out to be as follows... 

Quantum-chemical calculations of MH4 and MCI4 (M = C, Si, Ge, Sn, Pb) demonstrated that it is necessary to take into account the relativistic effects, which are proportional to the square of the atomic number of M and therefore essential when M = Ge, Sn, Pb . This was considered in calculations of the Me4- SnCl (n = 0-4) series . The mixing of orbitals of the unshared electron pairs of the chlorine atoms with the a(Sn—C) orbitals (a-n conjugation) intensifies with the rise of the number of methyl groups. On the contrary, increase in the number of chlorine atoms is accompanied by an increase in the population of the 5d-orbitals of the tin atom due to the d-n conjugation . The calculated HOMO energies and NMR chemical shifts of Me4- SnCl conform satisfactorily with experimental values. 

This implies a quasi-parallelism between the axes of the two orbitals. An angle, 6 = 15°, cos 6 = 0.97, is observed. In the anri-conformation, the two orbitals would be orthogonal with no interaction. The introduction of electrons furnished by oxygen makes the orbital more populated and hence lowers the resonance frequency, but since it involves antibonding electrons, the carbon-chlorine bond is weakened and lengthened. On the other hand, the carbon-oxygen bond, in which two p orbitals with parallel axes participate, takes on a certain tt character which shortens it. The delocalization hypothesis explains satisfactorily all the behaviour of methyl chloromethyl ether. 

On account of the loss of symmetry, the chlorine atom has a larger share of the total electron population. In other words, the coefficient on chlorine for the bonding orbital, crCci is larger than that on carbon. It follows from the requirement that the sum of the squares of all the c-values on any one atom in all the molecular orbitals must equal one, that the coefficients in the corresponding antibonding orbital, [Pg.52]

The previously unreported ClFg radical has now been produced by y-irradiation of CIF5 in an SF matrix, and has been identified by means of its e.s.r. spectrum. The unpaired electron in this radical evidently populates the chlorine 3s orbital (spin density 0.46). It is interesting to note that the lines in the e.s.r. spectrum are broader than those of the CIF radical it has been suggested that this broadening arises from the geometry of CIF (which should be non-octahedral, for reasons similar to those discussed elsewhere for XeF ). 

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