On the fragmentation of haloaromatic radical anions and their orbital isomerism. A theoretical study

THEO CHEM ELSEVIER

Journal of Molecular Structure (Theochem) 311 (1994) 343-352

On the fragmentation of haloaromatic radical anions and their orbital isomerism. A theoretical study Adriana B. Pierini *, JOse S. Duca, Jr., Maria T. Baumgartner INFIQC. Departamento de Quimica Orginic a, Facultad de Ciencias Quimicas , Universidad Nacional de Cordoba, Su e. /6. C.c. 6/. (50/6) Cordoba. Argentina

(Received 20 July 1993; accepted 13 October 1993)

Abstract We present a semiempirical theoretical study on the electronic nature of the radical anions of halobenzenes, nitrobenzyl halides, o-halodiphenyl sulfides and halophenyl-substituted cyclohexadienones = CI, Br, I). Th e 7r' -u' rad ical anions (RAs) were localized as minima on the potential energy surface for the molecules under study with the exception of the nitro benzyl halides for which a different behavior was determined. Both RAs differ in their charge and unpaired spin distribution, which is mainly located on the carbon-halogen bond for the o" isomer. A good correlation between the experimental and theoretically predicted properties is obtained, Thi s is the first report about the localiz ation of both isomeric RAs.

ex

1. Introduction

has been formed:

A radical anion (RA) can be formed by a single electron transfer (SET) from a donor (D) to a neutral compound or acceptor (A):

A,-Sp-A 2 -----+(A,) -Sp-A2

A+D-~(A)-;- +D'

where Sp = spacer. According to an RHF open-shell orbital picture, each of these RAs has one single occupied molecular orbital (SOMO) whose basis coefficients are mainly centered on the acceptor moiety. For the lET to take place, an effective electronic coupling between both groups is required [2]. This does not necessarily mean covalent bonding between them, since lET is possible even for distances greater than loA (for examples, see Ref. 3). The lET process has been studied extensively and there are many different examples of systems that follow Eq. (2). Some of these, where the size of

(1)

This SET will take place only if an adequate relationship between the ionization potential of D and the electronic affinity of A is satisfied [1]. There is an important family of compounds for which there exist more than one functional group able to accept the extra electron. An intramolecular electron transfer (lET) can occur once the first RA ·Corresponding author.

-

lET

-----+ A 1-Sp-(A2)

0166-1280/94/507.00 © 1994 Elsevier Science B.V. All rights reserved SSDIOI66-1280(93)03534-E

SET-;-

-;-

(2)

344

A.B. Pierini et al.ll. Mol. Struct . (Theochem) 311 (/994) 343-352

Table I MNDO carbon-halogen bond distances a nd heats of formation for halobenzene RAs RA

RAr.

RAa

C-X(A)

(C6HsCI)~

(C 6H s Bd ~ (C6H sI) .

Dihedral an gle' (deg)

1.78 1.87 1.97

AH r

C-X (A)

(kcal mol")

167.5 166.3 175.4

-2.92 7.85 13.65

1.99 2.02 2.11

sn,

Dihedral angl e' (deg)

(kcal mol v]

179.9 179.9 179.9

3.23 10.70 20.36

'Dihedral angle of the C-X bond with respect to the aromatic ring.

the spacer may vary, are haloaromatic and haloaliphatic compounds [4], aryl-alkyl ethers [5,6], nitro-substituted compounds [7], organic glasses [3(b)], etc. When both RAs differ in geometry and energy, the two states may correspond to distinct local minima on the potential energy surface [8]. If, at the same time, both acceptor groups have different orbital symmetries they can be considered to be orbital isomers. RAs have been proposed as intermediates in numerous chemical reactions [1]. Nucleophilic substitution processes that take place via SET, such as radical chain (SRN 1) [4] or cage collapse mechanisms [9], are an example. The initial step for both mechanisms has been proposed to be the formation of the RA for haloaliphatic and haloaromatic compounds. These species have enhanced reactivity concerning the parent neutral molecule when a bond breaking reaction is involved. Following a thermodynamic cycle it is possible to demonstrate that the strength

Table 2 MNDO carbon-carbon bond distances for halobenzene RAs RA

Atom number

:~: 7

r(i,j) (A)

(i)

U)

RA r.

RA

2 3 4 5 6 7

3 4 7 2 5 6

1.43 lAO 1.42 1.43 1.38 1.43

1.41 1041 1.41 1.41 1.41 1.41


of the bond is significantly decreased in an RA intermediate [1OJ. Aromatic RAs have the possibility of presenting tt" - a' isomerism (henceforth a" and n' will be denoted by a and n respectively) depending on the orbital symmetry of the acceptors. Of the two possible isomers, it has been suggested that the n RA (henceforth RA n) is the most stable one, the a RA (RA u) being the only one capable of dissociating in the sense indicated by Eq. (3) to give the corresponding radical and halide ion as the leaving group: -;-

ArX + e" -t(Ar) -X -tAr" +X-

lET

-

Ar-'X

(3)

Thus, for aromatic RAs, the fragmentation reaction can be viewed as an lET step from the 7i- aromatic system to the a carbon-halogen bond [11]. Electron transfer reactions have attracted much attention particularly in relation to indirect DjA coupling mediated by intervening fragments as spacers [3,12,13]. However, not much theoretical work has been performed on the a-rt: orbital isomerism of haloaromatic RAs, where no spacers mediate the process. An MNDO [14] study on halobenzene RAs of the type (G.6HsX) . (X = CI, Br, I) and 0 -,111- and p(C6H4X2 ) • (X = Br, I) has been reported [15J while recently an AMI [16] study of the carbon-oxygen bond fragmentation in RAs derived from phenyl and nitrophenyl methyl ethers has been published [I 7]. In the present paper we report a theoretical study, using the semiempirical MNDO and AM 1 methods, of the potential surface of

A.B. Pierini et al.fJ. Mol. Struct , (Theochem} 311 (1994) 343-352

(ArX) . (X = CI, Br, I) in order to determine the feasibility of the a-rt: orbital isomerism of halobenzenes, nitrobenzyl halides, o-halodiphenyl sulfides and halophenyl-substituted cyclohexadienones. The feasibility of the lET reaction between the 7r and a system is discussed in terms of the energy differences between both local minima, which is related to the experimentally observed chemical reactivity.

2. Procedure The calculations were performed with the semiempirical MNDO and AM I methods as implemented in AM PAC [18] and the programs were adapted for running on a microcomputer. The RAs were calculated within the UHF formalism in order to account for the electronic nature of the intermediates (19]. All the equilibrium geometries were obtained with complete geometry optimization making no assumptions. The stationary points were characterized by calculating their hessian matrix, provided they have no negative eigenvalues [20]. Generally the most stable RA is the first one obtained in the calculation. The less stable one can be obtained by an adequate modification of the starting electronic density matrix [8]. It is known that MNDO and AM 1 as implemented in AMPAC do not include d orbitals for halogens, so the charge distribution for the species under study could be dilTerent from those calculated in this paper. However, the good correlation observed between experiments and certain theoretically determined properties may indicate that these differences are not significant for the system under study.

3. Results and discussion 3.1. H'alobenzenes The MNDO reported calculations for halobenzenes [15] indicated the existence of the RA a for bromobenzene and iodobenzene while only the 7r species was identified for chlorobenzene, The

345

possibility of an orbital isomerism was not taken into account. We reinvestigated this potential surface with the MNDO method localizing both isomers for the three halobenzenes. The main geometric parameters and heats of formation calculated are presented in Tables 1 and 2 while the negative charge and unpaired spin distribution are shown in Table 3. The a-rt: states for bromobenzene and iodobenzene are stationary points on the potential surface. The situation is slightly different for the RA a of the chloro derivative and will be discussed below. The most important geometric differences between both kinds of isomers is in the carbonhalogen (C-X) bond length. Even though a certain degree of bond alteration is observed in the aromatic ring, the bond distances do not change essentially from the neutral compound to the RA 7r. This is ascribed to a vertical transition as indi cated in the Franck-Condon theory, when one electron is transferred to the neutral molecule; the movement of the electron being much faster than the nuclei [1]. An energy profile for the process is shown in Fig. I. In contrast, the carbon-halogen bond lengthens in the RA a isomer (by about 0.14A for bromobenzene and iodobenzene, and by about 0.23 A for chlorobenzene). All the RAs a retain the C2v symmetry with almost the same aromatic C-C bond distances of the neutral species. The a states are planar, even when a certain deviation from planarity in the RAs 7r of chlorobenzene and bromobenzene, i.e. 15°, was present. The RA a can be considered as an excited state of the RA 7r at its equilibrium geometry (Fig. 1). As the lengthening of the C-X bond is taking place, the aromatic charge and the tt spin distribution are shifted to the a bond, resulting in a higher net charge and spin density on the halogen and a practically null spin distribution on the aromatic ring (Table 3). As shown in Table I the most stable RAs are of tt nature. However, at this MNDO level the differences between the 6.H r of the RA a and the RA 7r, here called 6.£(7'" do not

A.B. Pierini et al.jJ, Mol. Struct. (Theochem) 311 (1994) 343-352

346

Table 3 Charge and spin distribution calculated by MNDO for halobenzenes

RAO'

RA1I"

Atom number

RA

(I)

Spin density

Charge distr.

8(i)

q(i)

0.01

-0.28

Spin density 8(i)a 0'

0.34

Charge distr. q(i)

-0.63

0.01 2

·6· ·6· ·6· 6

3 4 5 6 . 7

..

7

6

2

0.64

-0.08

0.20 -0.20 0.18 -0.19 0.48

-0.09 -0.08 -0.08 -0.08 -0.23

0.43 -0.42 0.42 -0.43 0.42 -0.43

-0.06 -0.10 -0.06 -0.10 -0.12

0.01

-0.24

0'

0.46

-0.47

..

-0.26

0'

0.46

-0.16

0.20 -0.20 0.17 -0.17 0.53

-0.08 -0.08 -0.08 -0.08 -0.21

0.28 -0.21 0.20 -0.21 0.20 -0.22

-0.15 -0.16 -0.15 -0.16 -0.21

0.01

-0.02

0'

0.49

-0.44

0.45

3 4 5 6 7

0.02 2

0.41 0.26 -0.25 0.28 -0.26 0.54

3 4 5 6 7

..

7

a1l"

0'

0.01

7

6

-0.19

0.45

-0.50

0'

0.51

-0.28

-0.04 -0.10 -0.05 -0.10 -0.20

0.42 -0.43 0.42 -0.42 0.43 -0.44

-0.09 -0.10 -0.04 -0.10, -0.11

unpaired spin distribution, otherwise indicated.

>. ~

(l)

c:

W

RAlt

RAcr

R. C.

Fig. I. Energy profile for the lET reaction presented in Eq. (3).

correlate with the expected nucleofugacity order CI> Br > I. This probably arises from a failure in the MNDO parameterization for iodine compounds [21]. After a careful examination we could not localize the RA a as a minim_urn on the potential energy surface for (PhCI)' since there is one negative eigenvalue in the hessian matrix; however, this structure is not a maximum (transition state). This special behavior of the potential surface can be attributed to the nearness of the avoided crossing point between the o and 7f surfaces [22]. The fragmentation process for the RAs a are strongly endergonic since no stabilization for the halide anion is present in gas phase.

347

A.B. Pierini et a/.IJ. Mol . Struct, (Theochcm) 311 (1994) 343-352

Table 4 Comparison between MNDO and AM 1 calculated properties for the most stable RAs of nitrobenzyl halide derivatives RA

p-NO ZC 6H sCH1Cl p-N0 1C6H sCH1Br III-N01C6H sCH 1Cl m-N01C6H sCH 1Br p-NOzC6H sCHMeCI P·N01C6H sCHMeBr

c-x (A)

Dihedral angle' (deg)

MIr (kcal rnol ")

MNDO

AMI

MNDO

AMI

MNDO

AMI

1.84 1.92 1.83 1.92 1.85 1.93

1.79 1.97 1.78 1.95 1.80 1.99

91.66 90.10 96.27 96.84 61.36 69.66

91.69 90.90 94.81 95.04 63.89 73.55

-31.24 -19.27 -29.47 -17.38 -32.44 -20.78

-35.18 -23.45 -33.30 -21.18 -38.56 -26.29

' Dihedral angle of the C-X bond with respect to the nitroaryl ring.

3.2. Nitrobenzyl halides The RAs of these compounds belong to a structural group in which direct overlap between the IT system and the a C-X breaking bond is possible, as shown by ESR [23]. This overlap should increase as. the cleavage reaction progresses and could be responsible for the faster dissociation rate of nitrobenzyl halides versus nitroaryl halides as determined by Behar and co-workers [24,25]; furthermore, it is known that a benzylic C-X bond is approximately 20 kcal mol-I less stable than its analogous aromatic bond [24]. In order to investigate the effects of this structural difference in the cleavage reaction of the RAs of aryl and nitrobenzyl halides by adding an extra electron, we inspected the potential energy surface for p- and lIl-nitrobenzyl halides (X = CI, Br) and their a-methyl derivatives. The calculations were performed with the MNDO and AM I serniempirical methods. Previous studies on the system were based only on calculations of the neutral molecule for X = CI [26]. The inspection of the potential hypersurface was carried out by varying the dihedral angle of the C-X bond in relation to the nitrophenyl ring from 0 to 1800 and lengthening the C-X distance from near its equilibrium value to closer to the dissociation point into the nitrophenyl radical and the halide ion. No potential energy local minimum was observed other than the corresponding RA IT for all the compounds under study. Their main geometric parameters and the calculated heats of formation are presented in Table 4. According to both

methods the para RAs are more stable than the meta ones for both halogens, the existence of the a-IT overlap being responsible for the perpendicularity of the benzylic C-X bond concerning the nitrophenyl ring, even when the a-methyl substitution is present. The MNDO C-CI bond distances are 0.05 A longer and the C- Br are 0.05 A shorter than the AMI values. MNDO underestimates D.Hf by at least 4 kcal mol-I with reference to AMI. In Fig. 2(a) the complete hypersurface for the fragmentation process is illustrated for the RA of p-nitrobenzyl chloride. It can be noted that the dissociation can be considered complete when the C-X bond length is close to 3 A. The process is endergonic by around 20 kcal mol- t • The nonexistence of the RA a as a local minimum on the potential hypersurface can also be better seen from Fig. 2(b), where the topological cut of the amplified hypersurface only denoted the existence of the RA IT. A smooth redistribution of the charge and electron spin density takes place when the C-X bond dissociates. Even though this cleavage is assisted by the overlap between the orbitals of interest, it seems to happen slower than in the halobenzene case. Thus, near the C-X equilibrium bond distances for the RAs a of the halobenzenes (r(C-CI) = I.95A and r(C-Br) = 2.06A), the a-IT interchange took place in about 55%. This can probably be explained on the basis of the higher electron withdrawing ability of the nitrophenyl functional group. According to our calculations, the fact that no RA a was found in the potential hypersurface is indicative of a dissociative lET process for the

348

A.B. Pierini et al.ll, Mol. Struct, (Theochem) 311 (1994) 343-352

Fig. 2. (a) Energy potential hypersurface for the dissociation process of p-nitrobenzyl chloride RA. (b) Amplified energy potential hypersurface and energy counters for the p-nitrobenzyl chloride RA.

sou«. (Theochem)

A.B. Pierini et al.II. Mol.

ro

o

X~

~

f

~s

'X

VV 2

halonitrobenzyl assisted by the a-7r overlap. Similar types of overlap have been established by X-ray and NMR for RAs of related structural types [27]. 3.3. c-Halodiphenyl sulfides and halophenylsubstituted cyclohexadienones

The corresponding Ms of the following molecules, i.e. l' and 2', have been proposed as

311 (1994) 343-352

349

intermediates in the reactions of o-dihalobenzenes with 2-naphthoxide and 2-naphthyl sulfide anions respectively [28]. Their proposal as reaction intermediates is based on experimental evidence as well as on theoretical calculations. The reactivity of these intermediates depends on the ratio between the lET to the o-dihalobenzene present in the reaction media and the lET between the tt and the a C-X systems. In order to interpret the chemical reactivity of these RAs, their orbital isomerism was studied. Table 5 depicts the heats of formation and the c-x bond distances calculated by AMI for the RAs 3' and 4', where the naphthyl radical was replaced by a phenyl group in order to reduce the dimensions of the system. According to these calculations, the existence of a-rt: orbital isomerism was detected for both RAs. The RAs a were found for the bromo and iodo

Table 5 AMI calculated heats of formation and C-X bond distances for 3'" and 4'"

RA

Halogen

r(C-X) (A)

AHc (kcalmol")

(X=)

RAer

3

RAer

4

~"" ~7

1 X.

H

5._-

6

12~o,O

-15.3

CI Br I

1.71 1.89 2.03

2.07

2.17

-3.0 8.4

CI Br I

1.89 1.89 2.03

2.09

18.0 28.6

2.17

39.4

9.1" 11.8 19.3

24.4 14.8

36.7' 36.8 39.5

18.7 8.2 0.1

10.9

13V11 14

3

4 "Estimated from the potential surface for r(C-CI) ~ 1.96A.

350

A.B. Pierini et al.fJ. Mol. Struct, (Theochem} 3Il (/994) 343-352

Table 6 AMI sharge and unpaired spin distribution for the RAs and 4'

1r

3""

Atom number (i)

Charge density

q(i)

Spin density 6(i)

9 10 II 12 13 14

0.22 -0.48 -0.38 -0.27 -0.17 -0.17

0.05 O.oI -0.10 0.55 -0.26 0.31

2 3 6

-0.20 -0.40 -0.20

0.41 0.15 0.52

derivatives while it could not be placed for the chloro substitution. The difference in heat of formation between both RAs was estimated by interpolation to the C-CI bond distance for chlorobenzene (about 1.96 A). In addition, for this structural type of RA, the geometric modification of the dihedral angle between both rings is possible for the lET process. The dihedral angle is about 130° for the RA 7r 3' and is slightly modified when the RA a is formed (about 145°) [29]. The RA tt 4' has a dihedral angle of approximately 100°, but decreases to about 60° when the RA a is produced [30]. In all cases the most stable RAs are of tt nature, the dissociation process (in the sense of Eq. (3)) being endergonic in the gas phase. The differences in heats of formation between the isomers can be taken as an indication of the activation energy for the lET reaction, i.e. the fragmentation process. In this system no higher energy point than the RA a was found. The AM 1 D.Eq1r values, in contrast with the MNDO ones for halobenzenes, correlate very

well with the reduction potentials (leaving-group properties) of the halogens; CI < Br < I. The greater the D.Eq", the slower the lET process and the easier the intermolecular electron transfer (in the presence of good acceptors). For the s~e halogen substitution,_the D.Eq;r values for 3 are greater than for 4' (X = CI, Br, I). Experimental data indicate that no fragmentation takes place for RA 3'. It has also been determined that for 4-: the lET occurs partially on the bromo and totally on the iodo derivative. For X = CI, no experimental evidence of lET could be found. Tables 6 and 7 present thecharge and unpaired spin distribution of 3' and 4' . The RA 3' has the spin and charge distribution localized on the ketyl 7r system. On the contrary, 4-: has this distribution delocalized on the 7r system of the aromatic halogen-substituted ring. Based on this theoretical information, the lET for 4' can be visualized as a through-bond lET between the two orthogonal systems [12] with no spacer mediating the process. This is the same case as in the halobenzene RAs. In contrast for 3-: the • 3 " existence of one sp carbon (C s in Table 5) as a spacer between both 7r systems decreases the degree of interaction between them. Thus, it can be concluded that for the calculated preferred spatial distribution the lET for 3' is not favored. This correlates with the fact that the caIc~lated D.Eutf for this RA is greater than for 4-:. 4. Conclusions The existence of the orbital isomerism for haloaromatic RAs is theoretically evidenced here since both RAs (RA tt and RA a) were located as

Table 7 AM I charge and unpaired spin distribution for the RAs a 3""and 4"" RA a

Atom number (i)

Charge distribution q(i)

Spin distribution 6(/)

CI

Br

CI

Br

I 2

-0.53 -O.oI

-0.48 -0.17

-0.40 -0.26

0.27 0.64

0.44 0.87

0.49 0.78

I 2

-0.43 -0.02

-0.38 -0.17

-0.29 -0.27

0.16 0.64

0.30 0.84

0.48 0.94

A.B. Pierini et

our. Mo/. Struct.

rmruma on the potential energy surface. The behavior is distinct for RAs of compounds that belong to other structural families such as nitrobenzyl halides. In this type of compound the overlap between the a and 1r systems is responsible for a gradual electron transfer while the C-X bond dissociates. In this hypersurface the RA a could not be located as a real minimum. The main characteristics of orbital isomerism can be summarized as follows: (I) geometric changes are induced by the relaxation process on going from the RA 1r to the RA a, mainly at the C-X bond length level. These changes do not depend on the type of aromatic ring the halogen is bonded to. Dihedral angle . modification is another geometric change that can occur during the process; (2) for halo aromatic compounds the most stable RAs are, in general, of 1r nature; (3) the unpaired spin distribution on the halogen atom increases when the halogen is heavier (CI < Br < I); (4) the spin density and the charge distribution usually delocalize on 1r (pz-type) molecular orbitals, but are strongly restricted to the a (Px-, py-type) C-X bond orbital when the RA a is generated; (5) the charge density in the RA a follows the electronegativity order for the halogens, i.e. CI> Br > I. It should be noted that the orbital isomerism can take place even when molecules have fragments acting like molecular spacers, e.g. 1 .. Another interesting calculated property of the system is the 6.E(J1r which can give information about the enthalpy of the transition state of the lET, since no extra activation barriers of energy were found between the RA 1r and the RA a. The AMI calculated values for 6.E(J1r are in accordance with the reduction potential for halo aromatic compounds. The calculations reported here are supported by the experimentally obtained chemical reactivity for the systems under study.

Acknowledgments J.S.D., Jr., and M.T.B. gratefully acknowledge receipt of a fellowship from the Consejo Nacional

(Theochcm) 311 (1994) 343-352

351

de Investigaciones Cientificas y Tecnicas (CONICET) of Argentina. INFIQC is jointly sponsored by the CONICET and Universidad Nacional de Cordoba. This work was in part supported by the Consejo de Investigaciones de la Provincia de Cordoba (CONICOR), CONICET and Antorchas Foundation.

Supplementary material AMPAC files for the species reported here are available from the authors on or floppy disks in MS-DOS.

5!"

3!"

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Program Exchange, Program No. 506, QCPE Bull., 9 (1989) 123. [19] J.A. Pople and R.K. Nesbet, J. Chern. Phys., 22 (1954) 571. [20] (a) J.W. Mciver and A. Kornomicki, Chern. Phys. Lett., 10 (1971) 303. (b) J.W. Mciver and A. Kornomicki, J. Am. Chern. Soc., 94 (1972) 2625. [21] M.J.S. Dewar and E.G. Zoebisch, J. Mol. Struct, (Theochern), 180 (1988) I. [22] J.S. Duca, Jr., and A.B. Pierini, unpublished results. [23] M.C.R. Syrnrnons and W.R. Bowman, J. Chern. Soc., Chern. Cornrnun., (1984) 1445. [24] D. Behar and P. Neta, J. Phys. Chern., 85 (1981) 690. [25] J.P. Bays, S.T. Blumer, S. Baral-Tosh, D. Behar and P. Neta, J. Am. Chern. Soc., 105 (1983) 320. [26] M. Julliard, J.-P. Scagliarini, M. Rajzrnann and M. Chanon, Chirnia, 40 (1986) 16. [27] P. Maslak, J.N. Narvaez and D.S. Malinski, J. Org. Chern., 55 (1990) 4550. [28] M.T. Baumgartner, A.B. Pierini and R.A. Rossi, J. Org. Chern., 58 (1993) 2)93. [29] The dihedral angle for 3' is defined by LC2CSCS4 as in Table 5. The values for the RA 'If are: X = CI, 14r; Br, 125°; I, 139'. For the RA a they are: X = Br, 155°; I, 146°. 7 [30] The dihedral angles for 4 are defined by LC2C3C9C12• The values for the RA 'If are: X = CI, Br, 110°; I, 105°. For the RA a they are: X = Br, 59°; I, 57°.