1816
J. R. HOYLAND AND LIONELGOODMAN
Vol. 64
BONDING I N CONJUGATED HALOGEN COMPOUNDS’ BY J. R. HOYLAND~ AND LIONELGOODMAN W h i t m r e Chemical Laboratory, T h e Pennsylvania State University, University Park, Pa. Received March 84, 1960
The effect of inclusion of d n orbitals in halogen conjugation is considered. The resulting d hybridization yields a considerable increase, -2 e.v., in bonding energy (greater than that found in Clz through p r - d r Rybridization). For conjugated C-C1 bonds the valence state of the chlorine atom is estimated a t pn1.90dn.l0. The greatly increased C-C1 bond order indicates considerable t,riple bond character. The factors determining the conditions for dn-pn hybridization are analyzed, and V.B. structures are proposed. In particular, Y + = R = X-, where Y is a strong electron donating group, seems to be important. It is suggested that dipole moments, resonance energies and intensities in the ultraviolet spectra of halogen compounds (in particular chloro-, bromo- and iodobenzenes) need reinterpretation in the light of the above.
Introduction The halogen compounds have been an enigma in the theory of bonding of simply conjugated derivatives of olefinic and aromatic systems. In the interpretation of reactivity data, a mesomeric release of electrons in the order3 F > C1> Br > I usually has been utilized for conjugated halogen compounds. However, in the ultraviolet spectra of aromatic halogen compounds, red shifts of the 2600 A. benzene analog transition are in the order4: I > Br > C1 > F, indicating a mesomeric order exactly opposite to that obtained from reactivity considerations. Other sources of electron density information, such as asymmetry parameters and coupling constants5a obtained from nuclear quadrupole resonance spectra, solvent effects on reactivities3 and ultraviolet spectralSband intensities of halobenzene spectfra,6cyield one or the other of these mesomeric orderings, seemingly dependent upon the nature of the system bonded to the halogen. Several authors, notably Mulliken, have pointed out that the halogen atoms, C1, Br and I, have d-orbitals available in the valence shell for acceptor action6suggesting the possibility that the net charge release in conjugated systems is controlled by competition between the donor action of pn and the accepter action of d n orbitals. The purpose of this paper is to investigate whether halogen d-orbital part,icipation in conjugation in olefinic and aromatic halogen derivatives is feasible, and whether the apparent paradox in the mesomeric behavior of the halogens can be reasonably understood through d-p hybridization. Mulliken6 has demonstrated that t’he greater strength of the C1-C1 bond in Clz compared to the F-F bond in Fz is due largely to the participation of the 3da orbitals of chlorine, which are normally unoccupied in the ground state of atomic chlorine, and are usually not taken into account in the LCAOMO scheme. The inclusion of 3da orbitals in the bonding scheme of the Clz molecule was shown by Mulliken to lea,d to pronounced multiple-bond (1) Presented a t the Symposium on Electronic Distributions in Organic Molecules a t the Atlantic City Meeting of the American Chemical Society, September, 1959. (2) National Science Foundation, Predoctoral Fellow, 1959-1960. (3) R. W. T a f t , Jr., THIS JOURNAL, 64, 1805 (19HO), and references therein. (4) F. 9. Matsen, J . A m . Chem. Soe., 7 2 , 5248 (1950). (5) (a) W. G. Laurita and W. S. Xoski, zbid., 81, 3179 (1959); (b) W. M. Schubert and J. M. Craven, ibid., 82, 1353 (1960); (e) L. Goodman and I,. J. Frolen, J . Chem. Phgs., S O , 1361 (1959). (6) R. S. Mulliken, J . A m . Chem. Soe., 7 7 , 884 (1955); R. W. T a f t , Jr., J . Chena. Phys., 2 6 , 93 (1957) and references therein; also private communication with Professor J. G. h t o n .
character because of the increased overlap of the a-orbitals between the two C1 atomic centers. (Presumably the same result should be true for Brz and Iz.) Mulliken further concluded that only a small amount of d n character is needed for a pronounced increase in double bond character. It was estimated that only 5% d n character leads to 207, double bond character in Clz. The participation of d-orbitals in carbon-halogen n-conjugation differs from that in X? in four major ways. (1) The “size” of the carbon 2pn atomic orbital is considerably smaller than that of a 3 p r chlorine orbital. This causes the overlap between the “natural”’ C13dn A 0 and a ‘batural” C 2pn A 0 in the C-C1 bond to be very much smaller than the overlap between 3pa and 3dn C1 orbitals on adjacent centers. The net effect of the small “size” of the carbon 2 p r orbital is to require greater contraction* of the 3d C1 orbital for effective overlap and thus the promotion energy is increased relative to that required in Clz. Thus this factor will decrease d-orbital participation in the carbon-chlorine resonance relative to that in Clz,the same presumably being true for bromine and iodine. ( 2 ) Because of the equivalence degeneracy of the A 0 on each center, a resonance condition exists in Clz regardless of hybridization. This is not true in the case of the C-C1 bond where this ordering of orbital term values holds: Cl,, (-13 e.v.) < Cm (-11 e.v.) < C1a (-1 e.v.). The term value for C13d is for the natural orbital (2 = 1.1). Formation of a pn-dn hybrid orbital of the form no1
= cu3pr
+ P3dr
(1)
causes the resonance condition to be approached, thus making the energy factor more favorable for pn-dn hybridization in the C-C1 bond than in Clz. (3) In any parent conjugated hydrocarbon, the individual orbitals in the II-140scheme ((ai) differ in their mixing with the C1 dn-orbital. Because of energetic considerations, the frontier and antibonding MO’s in the parent hydrocarbon will in general interact more strongly with the halogen dn orbital than the lowest-lying hydrocarbon bonding MO’s. In addition to the energy factor the charge order a t the substituted carbon atom (c2ip) in the (7) A natural A 0 here refers t o an A 0 with an effective nuclear charge appropriate t o the free atom ( Z d . The term is not t o be confused with t h a t of Lowdin and Shull pertaining to spin density considerations (€’. 0. Lliwdin and H. Shull, Phgs. Rev.,101, 1730 (1956)). (8) The smaller C-Cl internuclear distance (1.69 A,) compared to the C1-C1 internuclear distancr (1.99 A.) only partially mitigates this effect.
Dec., 1960
BONDING IN HALOGEN COMPOUNDS
1817
parent hydrocarbon governs the mixing of C1 d n with any given MO Q’il
N
*i
+CipP
Ei
4ds
(2)
E(d)
Since both the charge orders and energies of frontier MO’s undergo a variation in a series of parent hydrocarbons, considerable variation in the import,ance of dn interaction is expected. In addition to the above factors, because of orthogonality considerations, a process of forced mixing takes place in which some of the C1 d a character in the antibonding orbitals is transmitted into the bonding (and occupied) orbitals. In Clz, mixing of the nu and n, MO’s is prevented by symmetry considerations, so that the forced mixing referred to above, along with its resulting stabilization, is absent. This latter factor isznot as important as the charge-order and energy considerations. (4) The II-310 configuration of R-C1, where R, is an alternate liydroca&on, may be written as
.. .
a112@1’2. , an)2aC1J2
(3)
where the electron configuration of the parent hydrocarbon is @12iPa2 --- (Pn2, (PI --- @= being bonding or non-bonding for alternate hydrocarbons. The a-electron configuration of Clz is, in Mulliken’s notation
...
.(2nu)4 (2rg)4
where au is bonding and a, antibonding. The effect of d r - p r hybridization on ng is to reduce its antibonding nature, whereas no such possibility exists for molecules having configuration (3). Thus one is led to the conclusion that the key factors which cause da-pa hybridization to yield a large net bonding effect in Clz differ sufficiently in R-C1 to prevent an analogous argument from being used in these latter molecules. If dn-pn hybridization is to be significant in R-C1, factor (2) and a favorable factor (3) must overcome the unfavorable factors (1) and (4). Mention should be made of two additional differences in the interaction of a halogen with a hydrocarbon from the interactions in XZ. (5) The dn A 0 may interact with 2pa orbitals centered on non-nearest neighbor atoms. This effect may be considered as quite small. (6) Donation of charge from the C1 3pn orbital into the IT-system of the hydrocarbon leaves the chlorine atom with a formal positive charge, reducing the promotion energy required for hybridization. Keither factor ( 5 ) nor (6) was investigated in any detail in this study. rI-Bonding in Vinyl Chloride.-The lI-electron wave function for the ground state of vinyl chloride may be written as9 (4)
a’,2GrC12
To determine @I’ and ~PcI’, along with the corresponding eigenvalues, a secular equation of rank 4 was solved. The evaluation of the matrix elements was carried out in the following manner. , a C-X bond (1) The exchange integral, ~ C X for was assumed proportional to the overlap integral (9) QCI, is the delocalized C1 function.
Zalb,
+ baci.
2.
Fig. 1.-Variation of E(nd) in e.v. with 2 for the halogens as given by the Virial Theorem: - C1; ---, Rr; _ . _ > 1.
XCX (all Pst for s,t not nearest neighbors are taken to be zero). Thus pcx is given by the expression Bcx = Pccscx/scc
(5)
( 2 ) The coulomb integral for a halogen orbital was set equal to the expression ax = a0
+ E(C) - E ( X )
(6)
where cyo is the coulomb integral for a carbon 2 pa orbital and E(C) amd E ( X ) are the energies before interaction of a carbon 2pz orbital and an orbital (d or p) of the halogen, respectively. For carbon 2p, E(C) was taken to be -10 e.v. while for the halogen n p orbital, E ( X ) was set equal to the negative of the observed ionization potential. A numerical value of - 3 e.v. mas assigned to Bee. This procedure should give reasonable values to the parameters in question. The energy values E(d), for the d-orbital before interaction with the hydrocarbon were obtained by applying the Virial Theorem as follows. The Virial Theorem states that in a conservative central field system = -2T
(7)
Here is the average potential energy and T the average kinetic energy. The kinetic energy of an electron in an orbital of effective nuclear charge Z depends on Z 2 whereas the potential energy is proportional to 2. Thus we may write E(d) = aZ2
+ bZ
(8)
But by the Virial Theorem 2aZ2 natural =
So that
- bZ natural
(9)
1818
J. R. HOYLAND AND LIONELGOODMAN
Vol. 64
of Slater orbitals for these higher shells, however, the Br and I overlap integrals should be regarded as schematic only. In accord with the Variation Theorem, the value of ZM was varied until the minimum ground state energy for the molecule was obtained. The problem was programmed for calculation on a digital computor (Pennstac) in the Pennsylvania State University Computing Center." The variation of the ground state energy of vinyl chloride with Z is shown in Fig. 3, the magnitude of the depression of the energy is about 2 e.v. which is somewhat larger than the gain in bonding through da-pn hybridization in CL8 The major portion of the depression in ground state energy (Table I) with pn-dn hybridization occurs because of an increase in the C-C1 bond strength. The remaining and smaller part is Fig. 2.-Variation of SCX (2pa, n d r ) with Z for the halo- due to increased delocalization of the R n-elecgens: -, S C C l (2pn, 3da); - - -, S C B(2pa, ~ 4da); trons. Some notion of the degree of dn-ps hy_ . _ ,Scr (2pr, 5d77). bridization may be obtained from the computed atomic population (Table I). The atomic population C1: 3p1.g23d0J1corresponds to 5.4% pn-dn hybridization or 4% promotion (p2nd p r d n ) . The excess C-C1 bond order (Table I) corresponds to a 90yo.increase in double bond character (or to 20% triple bond character,)12 largely a result of the contribution of the VB structure
+c-CkCl-
(borne out by the computed charge density changes engendered by d r p n hybridization). At Z = Zmin 3 these are (sign convention: excess electron density is negative)
-
W
7' -
C-4 +0.33 -0.20
OJ3
The effect of the inclusion of the originally unoccupied C13dn orbital in the conjugation scheme is to compete with the chlorine mesomeric release of the 3pn electrons. Thus there is considerably less net release to the vinyl group. Since the charge denaities are governed by the competition of the vinyl group orbitals and the C1 3dn orbital for the C1 3pn-electronsl accurate vinyl chloride charge densities will be governed by accurate values for the chlorine coulomb integrals, CY^^" and LY3dm. In the extremely simplified treatment carried out here these are only roughly obtained and hence no reliance whatsoever should be placed on absolute charge densities calculated by this scheme 11-Bonding in Chlorobenzene.-The ground state II-electron configuration of chlorobenzene is OO'*01'2*,l'~0C1'2
rlo)
where @o', @TI, all, are perturbed benzene orbitals in order of increasing energy.13 Orbitals of B2 (C,) symmetry (@",%, plus excited MO's bz,i p 3 ) may interact with C1 3dn which is also of Bz sym(11) The authors gratefully acknowledge the help of Mr. Franklin P. Prosser of this Laboratory and the members of the computer group at the Pennsylvania State University for their assistance and guidance in carrying out the computations. (12) This corresponds t o the contribution of PC.CI ( Z p r , 3 d r ) to the total bond order. (13) M. Goeppert-Mayer and A. L. Sklar, J . Chem. Phya., 6, 645 (1938).
Dec., 1960
1819
BONDING IN HALOGEN COMPOUNDS
metry. Orbital @ I is of Az symmetry and hence does not interact with C1 3dn but is of proper symmetry to mix with C1 3dn'. However the leading term in this latter interaction involves overlap with the 2p, functions on the ortho carbon atoms of the ring, and hence in practice @I (and h)are nearly unperturbed. The variation of the ground state energy with Z of chlorobenzene is shown in Fig. 3 and of bromobenzene is shown in Fig. 4. The C1 orbital population, hybridization values and excess C-C1 bond order are given in Table I. Charge density changes at various atomic centers due to dn-pr hybridization are -0.16 +O.O,(-J51 0 +0.10
-0.11
_-
The stabilization energy due to dn-p-hybridization is slightly less for chlorobenzene than for vinyl chloride reflecting the smaller dn-pn hybridization shown in Table I. The important contributing V.B. structures in chlorobenzene in addition to the usual ones are
8
8
oc3 I1
II
Fig. 4.-Variation of the ground state energy (in units of p = -3.0 e.v.) with Z for bromobenzene ( - - - - ) and p-bromoanisole (-).
A
&Bonding in p-Chloroaniso1e.-In conjugated chlorine compounds where there is a strongly electron-donating substituent there is the possibility of the C1-3dn orbital acting as an electron acceptor so that cross conjugation can take place. A favorable case should be p-chloroanisole, since the -OCH3 group is strongly electron releasing. Structure C, and also less strongly structure D, should be the principal contributing st.ructures for such cross conjugation. ClGI II
Ii \
II
-t -
\
14
0)
U
12
The change in charge density brought about by the inclusion of C1 3dn orbitals in the bonding scheme is +0.06
H
8
t 'r0.02
I1
-0.16
C 0
O
~ -0.12 ~
I
$0.10
Since the oxygen charge density decreases and the chlorine charge density is greater than 2, we conclude that structure C and possibly structure D are important in the ground state. Other important structures are of course the analogs of A and B (preceding section). The increased stabilization energy of > 2 kcal.
9
I
I
GI
Br
Fig. 5.-Ionization energies of the free halogens and the methyl halides. The upper plot is that for the free halogens.
(Table I) compared to chlorobenzene, bears out the importance of these additional structures. Remarks on Electronic Structure of Conjugated Fluorine Compounds.-Account must be taken of Factor (6) in applying the preceding calculations
J. R. HOYLAND AND LIONELGOODMAN
1820
VOl. 64
to R-F because of the extremely large electrone- essential 3pa-3da orthogonality is thus an imporgativity of F. Figure 5 shows a plot of the ioniza- tant prerequisite to carrying out the naive calculation potentials of atomic halogens and also of the tions reported here. methyl halides. In all cases except F, a fairly A more d a c u l t question to answer is whether a constant interval is maintained between the ioniza- flexibility has been introduced in the C13da orbital tion potentials of the atomic halogen and that of which is, in part, compensating for the deficiencies the methyl halide. Maintenance of this constant of the C1 3pn and C 2pa orbitals. Orthogonality interval would require the ionization potential of F restrictions preclude strong interdependence beto be 14.25 e.v. rather than the observed value of tween the scale factors of the C13pa and 3da orbi17.52 e.v. It is reasonable to assume that this dis- tals. Therefore variation of Z e B for C13p may only crepancy of over 3 e.v. occurs in CHIF due to the change the C1 3p energy level, thus affecting the extreme inductive effect of F pulling charge from promotion energy in a fairly minor way. Although the carbon atom through the u framework. The the overlap integral Sccl (2ps, 3da) will be affected resulting increased shielding lowers the ionization by deficiencies in the C 2pa orbitals, it is highly potential. This effect will be important in estimat- unlikely that any essential changes in the concluing the effect of d-orbitals on a-bonding in con- sions can be obtained by variation of Zea for C 2p. jugated fluorine compounds, inasmuch as a major It should be emphasized that the magnitudes listed increase in shielding of the d-orbitals will result in a in Table I will be dependent upon S C C ~(2pa, large increase in promotion energy. One might be 3dn) and must be regarded as schematic only. tempted to make a crude estimation. Using Slater Examination of Table I shows that C1 dn-pn rules, the postulated decrease in the ionization hybridization for the three parent conjugated syspotential of over 3 e.v. in CHIF corresponds to an tems considered causes considerably greater inincreased F-electronic charge of approximately creased stabilization energy than in Claa (- 2 e.v. 0.6e assuming that the sole effect is an increase in us. 1 e.v.). This appears to be due mainly to the the F 2p population. Therefore a “Natural” 3dn favorable factor (2) mentioned in the Introduction, F orbital is acted upon by a much smaller effective since the atomic population and degree of hybridinuclear charge ( Z e g 0.4) than if there were no F zation in every case is approximately constant and inductive effect. Promotion of 2pn F to natural comparable to that predicted for Clz. (We again 3da F ( Z e ~= 0.4) costs only about 13-14 e.v.; mention that no reliance should be placed on the however, the overlap is vanishingly small. Pro- actual magnitudes of the atomic population (see motion t o 3dn F with Zetr increased t o maximize Vinyl Chloride Section), but only in their relative SCF (2pa, 3dn) (Zee -3) costs well over 50 e.v. values.) The considerable variation in stabilization energy This high promotion energy is prohibitive as far as obtaining any appreciable gain in bond energy arises from the variation in the energy level and through pn-dn hybridization. (For comparison charge order scheme of the parent part of the molepromotion of 3pn C1 to 3da C1 ( Z m i n = 3) costs cule (factor 3 of the Introduction). The additional roughly 15 e.v. and 4pa Br to 4dn Br (Ze8 = 3) depression of over 0.5 e.v. for the ground state perhaps 10 e.v.) In the case where no shielding energy listed in Table I of the 3a-electron system CHzCl (chloromethyl radical) provides verification can be brought about by the F inductive effect (e.g., F a ) promotion of 2pa F to natural 3dn (Zeg of this statement since in this case the energy of = 1.0) costs about 16 e.v., and promotion with TABLE I Zee = 3 costs only about 30 e.v. It is thus clear that the large inductive effect of F acting on a car- ATOMICPOPULATIONS, BONDORDERSASD STABILIZATION bon skeleton14 is strongly inimical to dn-pn ENERGIES OF SOMECOKJUGATED CHLOROCOMPOUNDS % hybridization. da-px hyC1 orbital AE,b bridiza(c) Conclusions Molecule population e.v. tion APcci On the basis of the preceding results da-pa Vinyl chloride 3p1.923do.11 - 2.07 5.4 0.29 hybridization appears to be important in conjugated Chlorobenzene 3p1.933d0J0 - 1.86 4.9 .29 C1 compounds. Presumably this also is true in p-Chloroanisole 3p1.943do.11 - 2.00 5.4 .29 conjugated Br and I compounds. Some additional Chloromethyl points should be mentioned, however. radical” 3p’*’33d0.16 -2.64 8.4 ,510 The C1 4pa orbital has approximately the same Depression of the total The planar species CH2CI. energy as C1 3dn and the overlap integral SCX ground state energy as solved by the variation theorem as (2pa, 4pn) varies with 2 in much the same way as a function of d-orbital interaction (see Fig. 3): Indoes Scx (2pn, 3dn). Thus in the naive framework crease in C-Cl bond order due t o C1 p~ - d r hybridmition. the 4pa orbital produces effects similar to those the siiigle parent MO is much higher than in any brought about by the 3d7r orbitals. However, no other case considered. In addition the charge essential orthogonality exists between the 3pa and order is unity, so that this according to factor 3 4pa orbitals. With increasing 2, causing contrac- should be an unusually favorable case.16 The intion of the 4pa orbital, strong forced mixing takes creased stabilization energy of p-chloroanisole is place opposing any stabilization gained by spon- tied up with factor 3 since the introduction of the taneous 3pa-4pa mixing. This effect precludes (15) R. S. Mulliken, “Laboratory of Molecular Structure and Specany appreciable stabilization from mixing of 2p and 3p orbitals in the second-row diatomics.16 The tra,” Technical Report 1957-1959, P a r t I1 (p. 25-27).
-
(14) The above estimates would yield a small amount of pn-dn hybridization in Fn however.
(16) Thus the C-C1 bond in the unprepared species C1-CrH (chloropropargylene) 17 is predicted t o show unusual stability. (17) P. S. Skell and K. Klohe, J . Am. Chem. Sac., 89, 247 (1960).
BOLUDING IN HALOGEN COMPOUNDS
Dec., 1960
OCH3 group onto the benzene ring both raises the frontier level and increases its charge order. This allows the somewhat tentative prediction that the effect of d-orbital mixing on the stabilization energy should be most pronounced when there is a charge donor (Y) present Le., when the structure of type C + Y = R = X T o paraphrase Mulliken, a little of
is important.'* structure C goes a long way! Because of the lower energy differences between the d-orbital and that of ((excited" n-electron orbitals, dn-pa hybridization effects should be exhibited much more strongly in excited states. This is evidenced by considering the excited state wave function for the 2600 8. benzene analog transition ( ~ L+ B lA) to a first-order CI approximation previously described . 2 0 Second-order CI with doubly excited configurations is neglected here. While not precisely correctz1 the first-order approximation gives the leading terms. Charge density changes are shown for bromobenzene in the excited state upon inclusion of dn-pa hybridization.22 -0.19 -0.02C)iBr +OJl +0.11
-0.23
For the excited states of bromoanisole, the charge density changes upon inclusion of da-pa hybridization are $0.34
+0.16
+0.01 I 1 8 c o D L B r-0.11 -0.15 -0.05
The excited state of wave functions for bromoanisole for these systems are (see ref. 18 for terminology) qLb
= cos (n/4
- AB) XIZ - sin (n/4 - AB)
The transition moment integral for the band may be written as M L b = COS
xis
('Lb +
'A)
+
A B M ' B ~ ~sin ABM'E~"
The larger the value of the El, mixing parameter AB, the greater should be the intensity of the band, and conversely. In Table I1 the values of AB for bromobenzene and p-bromoanisole are given. In these molecules two opposing effects occur. The mixing of the d a halogen orbital tends to lower the value of AB, thereby decreasing the intensity. However, mixing with the occupied p n orbital tends to raise the value of AB, and hence the intensity of (18) Factor (6) wouM tend to reduce the degree of d s - p a hybridization because of the negative charge induced by structure C. However for C1 we estimate t h a t a n increase of O.le in 3d would tend t o increase the promotion energy 3pz-t 3p3d by -0.4 e.v. (at Znat) which correStructures of type A and B involve sponds t o only ~1 e.v. a t &in. a decrease in the 3p population and if important may lead t o a major decrease in the promotion energy.'!' (19) Craig, Maccoll, Nyholm, Orgel and Sutton, J . Chem. Soc., 332 (1954). For a decrease of O.le in the 3p population we estimate a decrease of -2 e.v. in the promotion energy 3pz 3p3d. ( 2 0 ) L. Goodman and H. Shull, abid., 27, 1388 (1957). (21) J. N. Murre11 and K. L. McEwen, ibid., 26, 1143 (1956). (22) These values given above for charge density changes in the excited state are wbject to even greater error than for the ground state. These values are not only sensitive to the coulomb integral both for X n p s and X ndn, b u t aliio t o the approximations in finding the excited state wave funotion,20 8 0 that these values must be regarded with extreme caution.
-
1821
the transition. The crude method of calculation employed cannot be expected to yield an accurate net result of these two effects. Three conclusions may be drawn, however. TABLE I1
VALUESOF
Elu MIXINGPARAMETER AB"
Molecule
AB*
AB b
Bromobenzene - 19" - 5" p-Bromoanisole - 2.3" +26" 4d orbital included in conjugation. 4d orbital negSee footnote 25 in text for discussion of magnitude lected. and sign of AB.
(1) The lowered intensities found in p-bromoand iodoanisoles by Goodman and Frolens can be explained reasonably by means of pa-dn hydridization causing a change in the excited state wave function.23 (2) The low intensity of the 2600 k . band in chloro- and bromobenzenes (and presumably iodobenzene) is due, a t least in part, to d orbital effects (Table 11). (3) The mixing of the halogen d n orbitals in the excited state causes a red shift of the 2600 8. transition due to additional stabilization of the excited state relative to the ground state. Hence the red shift order: Br > C1 > F arises a t least partially from dn-pa mixing. These effects indicate that halogen parameters obtained from spectroscopic data, intensities or frequencies, neglecting d-orbital effects, are likely to be in error. The same holds true for information taken from quadrupole moment and coupling data24 since the d-orbital population should in general contribute to the moment in a different fashion (smaller) from the p-orbital populations. Finally, it would appear that in the light of Table I, reinterpretation of dipole moments of conjugated C1 compouiids may be needed, since the n-electron moment apparently is reduced strongly by d n-p n hybridization, Acknowledgment.-L. G. thanks Professor J. G. hston for pointing out evidence of d-orbital participation in reactions involving the lower halogens. The authors gratefully acknowledge correspondence with Professor R. S. Mulliken, who pointed out that 4p mixing in C1 is mitigated by the non-orthogonality to 3p: and very helpful conversations with Professors C. A. Coulson and R. W. Taft, Jr. ( 2 3 ) The effect of dz-pn hybridization on M ' B z and ~ M'F.~,,is minor compared to the effect on AB, hence the reduction in intensity arises almost entirely from the change in E l u mixing. (24) J. H. Goldstein, J . Chem. Phys., 24, 106 (1956). (25) The enhancement in intensity is dependent upon sin2AB, AB in turn, is dependent upon the magnitude of [(a'-61') (e2'-ed)]. Including d-orbitals in the conjugation scheme causes tzl to be depressed so that ( d - e z ' ) < 0. The halogen np-orbital forces ( c I L c I '> ) ~0 if a x !> lai: iSi,but (cl'-ti') < 0 for laxl < la0 81. Thus AB decreases f o r the case lax/> l~~ @ . Our method of choosing parameters yields ax (np) N aC 8. Therefore the magnitude and sign of A B is strongly dependent upon the initial parameter ax (np), and a small variation in the magnitude of u x (np) can reverse the direction of the effect of d-orbitals upon AB. The values listed in Table I1 should in no way be regarded as correct. They have been included only for the purpose of showing that d-orbital effects will in general be large in considering the excited states of conjugated halogen compounds.
+
+ +
+
