Journal of Molecular Structure, 76 (1981) 285-297

THEOCHEM Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

ELECTRONIC STRUCTURE, UV SPECTRA AND REACTIVITY OF NON-BENZENOID SYSTEMS BY THE INDO/S-CI METHOD

Part II. Methylenecyclopropene and its derivatives

GIUSEPPE BUEMI, ANTONIO RAUDINO and FELICE ZUCCARELLO

Zstituto Dipartimentale di Chimica e Chimica Zndustriale, Universith di Catania, Viale A. Doria 8, 95125 Catania (Italy)

(Received 28 July 1980)

ABSTRACT

The electronic spectra, frontier molecular orbitals, charge distribution and dipole

moments of nine compounds containing the cyclopropenyl ring have been studied by the

all-valenceelectrons INDO/S-CI method. The theoretical results agree fairly well with

the experimental indications, where available. The o-contribution to the net charge density on the ring as well as the p0 and sp polarization contributions to the dipole

moments are higher than in the fulvene derivatives, indicating greater participation of the

smaller-ring system. The UV spectra of compounds containing two coupled rings correlate with those of the component frameworks, in agreement with previous semi-empirical calculations.

INTRODUCTION

Several non-benzenoid hydrocarbons (fulvenes, heptafulvenes, benzo- fulvenes and fulvalenes) were studied in a previous paper [ 11 by the all- valenceelectrons INDO/S-CI method to investigate their UV spectra and frontier molecular orbit&. Good agreement was found between the theo- retical predictions about the more reactive sites and the experimental behaviour of the compounds examined. Moreover the electronic spectra and the available lowest-energy ionization potentials were well reproduced by the calculations.

The same method is applied here to methylenecyclopropene (triafulvene) and other molecules (see Fig. 1) containing the cyclopropenyl ring. Previous investigations utilized semi-empirical procedures limited to the n-electron system [ 2-71, although in some cases a CNDO/S study has also been reported [ 81 as well as a CNDO/Z study for cyclopropenone [ 91. The geometries of the latter and of the parent triafulvene optimized by ab initio calculations [ 10,111 show only a small variation in the bond lengths compared to those obtained more recently [ 61 by the p-variable method, and adopted here. A systematic study of the frontier molecular orbit&, and reactivity and

0166-1280/81/0000-0000/$02.50 0 1981 Elsevier Scientific Publishing Company

286

Fig. 1. Topology of the molecules studied.

electronic spectra of the cyclopropenyl derivatives should prove as interesting as the work on fulvene derivatives [ 11. The particularly limited triatomic ring-size forces the bond angles to assume anomalous small values compared to the standard bond angle value of sp2- hybridized C atoms. Consequently for me thylenecyclopropene and its derivatives the 07 separation might be too drastic an approximation. Moreover zwitter-ionic structures can affect the overall molecular electron system to such an extent that relevant dif- ferences can be observed in related compounds with a fulvene ring [l] .

The calculations were performed on a CDC 7600 computer using the INDO/S-CI program parameterized as described by Galasso and Pappalardo [ 121. Forty monoexcited configurations for molecules I-V and seventy five for molecules VI-IX were taken into account in the CI treatment.

RESULTS AND DISCUSSION

Frontier molecular orbitals

The topology of the molecules studied is shown in Fig. 1. The frontier molecular orbitals are correlated schematically in Fig. 2. The HOMO of methylenecyclopropene (I), on passing to the 0 or N analogue, is strongly stabilized (for II by = 2.7 eV but to a lesser extent in III, - 1.3 eV); the LUMO and NLUMO (next to the lowest unoccupied MO) become slightly stabilized and remain localized on the ethylenic or C-X fragment. The n orbit& are less stable than the n-HOMO and differ very little from each other in energy. This is contrary to what happens in cyclopentadienone, iminocyclopentadiene, tropone and troponeimine [ 11 where these orbitals are intermediate between two n-MOs (HOMO and NHOMO).

When the exocyclic H atoms are substituted by CN groups (compound IV) the HOMO as well as the nearly degenerate LUMO and NLUMO become

287

Fig. 2. Correlation diagram of the lowest vacant and highest occupied molecular orbitals of the compounds examined.

more stable than the corresponding orbitals of I; however the HOMO is stabilized less than in II and III. In triafulvalene (V) the LUMO and NLUMO are nearly degenerate and originate from a combination of the LUMOs of the two triatomic rings, since they are located on the opposite ethylenic systems of the two cyclopropenyl rings. Their energy is intermediate between the energies of the LUMO and NLUMO of I, while the HOMO is destabilized by about 1.5 eV.

The HOMO and NHOMO of calicene (VI) are also degenerate and have nearly the same energy as the HOMO of I. The one with a, symmetry corres- ponds to the HOMO of fulvene and cis-butadiene; that of b, symmetry, in contrast, closely resembles the HOMO of triafulvene. The LUMO is delocal- ized over the whole n-system of calicene, while the NLUMO is an ethylenic orbital localized on the C7Cs fragment which constitutes the basis of the triatomic ring.

On passing to benzocalicene (VII) and dibenzocalicene (VIII) this ethyl- enic orbital remains unchanged, while a great or lesser effect from perturba- tion of the benzene ring(s) is noted on the other MOs. The HOMO of VII particularly (7.27 eV), like the HOMO (b, symmetry) of calicene (7.83 eV), is extended principally on the vinyltriafulvene (C, . . . . C,) and cis-butadiene

(C, . . * . Cl*) frameworks. The same orbital in VIII (7.21 eV) involves both the phenyl rings, but to a greater extent on the methylenecyclopropene atoms. This orbital can be considered for both VII and VIII as the HOMO of I perturbed by the phenyl n-system(s). The NHOMO of VII (7.97 eV) is localized on the benzofulvene framework while in VIII (7.89 eV) it is de- localized on both benzene rings. In both cases it seems to correlate with that MO of calicene (VI) which has a2 symmetry (7.85 eV), i.e. it is the buta- dienic MO of fulvene now extended to benzene. The LUMO of VII and VIII does not show appreciable variation with respect to the corresponding de- localized orbital of VI, but in dibenzocalicene a new MO is found between the LUMO and the ethylenic orbital (NLUMO of VII), with bl symmetry and delocalized over the whole n-system except for the C7 and C8 atoms.

288

Molecular orbitals with prevailing benzenic character appear at 9.47 eV and -0.28 eV in VII and at 9 eV and -0.34 in VIII.

In heptatriafulvalene, IX, the HOMO is delocalized and at an energy com- parable with that of the corresponding HOMO of triafulvalene (V). The NHOMO is localized on opposite ethylenic systems (C3C4 and C,C,,) of the two rings, and close to it there is an ethylenic orbital which corresponds almost completely to the NHOMO of heptafulvene [l] . Similarly the LUMO of IX corresponds to the LUMO of heptafulvene [l] and is localized on the hexatriene fragment C 1CZC3C4C5C6. The NLUMO is delocalized over the whole molecule while the NNLUMO (az) at -0.32 eV is localized on the ethylenic fragment of the triatomic ring. This latter is noted at nearly the same energy in I, VI, VII and VIII, and shifted to a great or lesser extent in II, III, IV and V.

The experimental ionization potentials are available only for II. The first two of these (9.57 eV for n and 11.19 eV for 71 [ 191) agree well with the energies of the HOMO (9.52 eV for n) and NHOMO (10.73 eV for n).

Charge distribution and dipole moments

For a better visualization, the charge distribution and the dipole moments of the polar compounds have been resolved into u- and n-contributions. Moreover the component due to the cyclopropenyl ring and that due to the remaining molecular portion are reported separately, see Table 1. It is im- mediately evident that the u-component on the triatomic ring is always negative while the total net charge on the same system is always positive. Moreover the net charge is much higher than that found in heptafulvene [l] , indicating the greater electron-donating power of the cyclopropenyl ring. This is also evident from the higher calculated dipole moment of I compared to that of heptafulvene, as previously noted on the basis of PPP calculations [ 21. Of course the largest positive charges on the ring are found for II, III and IV (where very electronegative atoms are present at the exocyclic group), and this change then decreases on going toward calicene (VI) and its benzo and dibenzo derivatives and to triaheptafulvalene. The fulvene ring in VI has a negative charge higher than that found in the free molecule while the seven-membered ring in IX has opposite polarity (negative) to that in hepta- fulvene.

If the charge density on the atoms is taken into account the exocyclic C4 atom of triafulvene, I, is the most negative (-0.167) and therefore the most appropriate for electrophilic substitution. C, and Cz are also negative but about ten times less. These positions however become positively charged in all the remaining compounds considered, except triafulvalene V. It is interest- ing to note that both the calculated r-atomic charge densities and dipole moment of I are very close to those obtained by ab initio calculations at the 4-31G level [ 121.

All the carbon atoms of the fulvene ring in calicene are negative in the

TA

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ty

dis

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290

order: C, (-0.062) > Cq (-0.060) = C, > C3 (-0.050) = CZ. Fusion of a benzene ring onto the five-membered ring of calicene increases the charge density on C5 (-0.085) and C3 (-0.056) but decreases that on C, (-0.037), making C, and CZ slightly positive. The same trend is apparent for C5 (-0.097) in dibenzocalicene where the next negatively charged atoms are those of the most external of the two benzene rings. Finally the heptatomic ring in IX has the most negative charge on CZ, C5 and CT. On the whole one may conclude that nucleophilic attacks are facilitated on the triatomic ring, while electrophilic reactions must occur at the penta-atomic or heptatomic ring or at the heteroatom (in II and III). Experience agrees with the theoretical predictions [ 161. A notable situation occurs in V, which has all the C atoms negatively charged (-0.028 on C, and -0.044 on C,) and should be greatly reactive towards electrophihc reagents.

If cyclopropenone (II) and dicyanotriafulvene (IV) are excluded, no experimental dipole moment is available for the molecules examined. However the calculated and experimental values for II and IV agree fairly well, as shown in Table 1. Similarly the calculated moment of VI agrees well with that estimated from the experimental moment of hexaphenylcalicene [ 2, 8, 171. The high value of the dipole moments as well as the high boiling point of II [ 13, 171 demonstrate the large contribution from the highly polar zwitterionic limit structure to the ground state of the molecule. The dipole moment of VI decreases with the increased conjugation on going from VII to VIII. As noted in previously [l] , the u-contribution to the total moment is lower than the n-contribution. However the u-moment as well as the s-p polarization terms appear generally greater than those found in fulvene and its derivatives. This is certainly a consequence of the greater influence of the u-electron system of the cyclopropenyl ring owing to the strain in this very small ring.

Electronic spectra

The calculated electronic spectra of the molecules studied are shown in Table 2 together with a description of the molecular orbitals involved. In methylenecyclopropene analogues II and III, two 7~* + n transitions are calculated at about 4 eV, and that at lowest energy in II is symmetry- forbidden. The experimental spectrum of III is not available; in II the n* + n transition has been observed at 4.49 eV [ 181, in good agreement with the theoretical value. In acetone (heptane solution) the same transition occurs at 4.48 eV [ 241. The nearly equal energy values in the two compounds indicates that no appreciable conjugation is present in cyclopropenone, supported also by the nature of the MOs involved which are essentially those of the CO (or CN in III) group and of the ethylenic system C 1C2.

For r* + 7r transitions, in the parent compound, I, these are predicted at 4.54 eV (‘B,) and 6.22 eV (‘Al), in agreement with previous calculations [ 2, 4,6,8] , both having noticeable charge transfer character [ 41. In II and

291

III they appear shifted to lower wavelengths since in these two compounds the n-HOMO is much more stable than the HOMO of I, while the LUMO and NLUMO show negligible energy variation.

When methylenic H atoms are substituted by CN groups (compound IV) the strong ‘A 1 + ‘A I transition (calculated in I at 6.22 eV) is predicted to be bathochromically shifted to 4.82 eV; it probably correlates with the absorp- tion band at 3.4 eV which was observed in the dioxane-solution spectrum of the more conjugated derivative l,l’-diphenyldicyanotriafulvene [ 151. Close to this a weak ‘& +- ‘A 1 transition is predicted (5.0 eV). These two transi- tions appeared much more spaced and reversed in energy in the parent mol- ecule I. Moreover a n* + 71 transition appears at 5.75 eV owing to charge transfer from the triatomic ring towards the CN groups.

The first two transitions of V are predicted to be degenerate at 3.56 eV. The one of ‘Bjg symmetry is forbidden while that with ‘B,, symmetry is weak. Both start from the HOMO and end at the ethylenic MOs LUMO and NLUMO. The next transitions up to about 8.5 eV are 7~* +- u or u* + 71, except that at 5.26 eV, with ‘B,, symmetry, which is the strongest and involves the NNLUMO localized on the central ethylenic fragment joining the two cyclopropenyl rings.

For compounds I-VI, only for II is there available an experimental spec- trum [ 181 which agrees well with the theoretical one. On increasing the molecular dimension the number of n* + 71 transitions in the low-energy range increases. In the range 3.8-4.9 eV, four 7r* + 71 transitions are calculated in calicene VI. A direct comparison with the experimental data is not possible since calicene has not yet been isolated as the free molecule. However the theoretical results reproduce well the spectrum estimated [ 20, 211 for this compound from the available experimental spectrum of its derivative tetra- chlorodi-n-propylcalicene. The tail below 4.13 eV is associated with the weak n* + 77, ‘Al + ‘Al transition calculated at 3.87 eV, which originates from electronic transfer between a butadienic orbital of the pentatomic ring and a completely delocalized MO, i.e. it shows the same character as the transition found at about 3.5 eV in fulvene [l] . The strong absorption which, accord- ing to Kende et al. [ 211, should lie very close to 4.13 eV, appears to origi- nate from two transitions: the stronger, calculated at 4.18 eV (‘A J, corresponds to the second n* + 71 transition of fulvene, bathochromically shifted owing to the greater delocalization of the LUMO which is now extended also on the C& ethylenic fragment; the other transition, at 4.41 eV (‘B,), shows charge transfer character and corresponds both in energy and oscillator strength to the first n * + 71 transition of methylenecyclo- propene. The fourth (very weak) X* + rr transition, predicted at 4.92 eV, originates from electron transfers from the pentatomic ring (butadienic NHOMO) towards the triatomic ring (NLUMO, localized on the C& ethyl- enic system).

The next transition at 6.30 eV, which agrees well with the estimated absorption band at about 6.02 eV, has a butadienic nature. From the

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(4.6

4)

‘6.0

2 3.L

ige

(3.6

4)

4.64

(3

.92)

3.51

f (4

.24)

3.69

(4

.27)

3.81

(4

.27)

4.20

(4

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4.37

sh

(4.2

4)

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(4

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4.

79

(4.1

0)

4.9s

l-5.

Osb

(4

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5.14

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5.27

(4

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20

21

22

23

1,

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18

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1 b

,,

19

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,,

20

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0 b

sg

17

11.0

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,

18

7.85

a

,

19

7.83

b

,

20

0.62

b

,

21

0.26

a

z

22

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5 b

,

23

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1

24

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,

25

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,

28

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24

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25

10.6

6 -

26

9.47

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27

7.97

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28

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29

0.57

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30

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31

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32

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33

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34

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35

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35

9.00

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2

36

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2

37

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,

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41

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43

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44

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x-

CJ*

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a*

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n lT*

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R* a*

L+J4

D

elo

cali

zed

c,c,

+

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all c

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n D

elo

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zed

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hy

len

ecy

clo

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pen

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cis-

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tad

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clo

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pen

e +

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oca

lize

d

CA

T

ria

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ic

rin

g

cis-

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tad

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m.

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hy

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ecy

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pen

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C,C

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C,

C,C

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Del

oca

lize

d

Del

oca

lize

d

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zofu

lven

e sy

stem

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zofu

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stem

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, +

C

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,*

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oca

lize

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Ben

zen

e,

less

o

n

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zofu

lven

e sy

stem

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ato

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ri

ng

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,C,C

,, le

ss

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b

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hy

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zen

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s

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d

elo

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ole

n

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stem

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cep

t C

,C,

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zen

e ri

ng

s

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zen

e ri

ng

s

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zen

e ri

ng

s

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zen

e ri

ng

s +

tr

iato

mic

ri

ng

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ato

mic

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ng

TABL

E 2

(con

tinue

d)

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mp

ou

nd

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heo

reti

cal

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per

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tal

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eore

tica

l

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f p

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a

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ture

Sym.

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mp

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(%jb

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2.23

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01

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0

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03

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0.

01

6.11

0.

01

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6.19

0

6.32

0.

52

6.35

0.

44

V$(

87)

--

v::w

)

V::(

S6)

V::(

83)

q(7

9)

v;;(

41);

V

i”,(

51)

V::(

84)

21

11.0

1 b

, u

22

10.2

6 a

2 n

23

9.93

b

, R

24

6.35

b

, n

25

0.63

o

2 n

*

26

-0.1

5 b

, R

*

27

4.32

o

2 n

*

28

-2.2

6 o

2 n

*

29

-2.3

7 b

, n

*

30

-2.4

4 o

, a

*

31

-2.4

6 b

, a

*

32

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5 b

, a

*

33

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8 a

, o

*

34

-3.9

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, a

*

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ato

mic

ri

ng

C,C

* +

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, +

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elo

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zed

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atr

ien

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ag

m.

Del

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lize

d

C&

10

Hex

atr

ien

e fr

ag

m.

C,C

,C,C

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*

C,C

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* T

ria

tom

ic

rin

g

C,C

,C,C

, D

elo

cali

zed

Tri

ato

mic

ri

ng

aThe

re

port

ed

angl

es

for

asym

met

rica

l m

olec

ules

ar

e th

ose

betw

een

the

tran

sitio

n m

omen

t ve

ctor

an

d th

e y-

axis

. bC

ontr

ibut

ions

lo

wer

th

an

10%

ar

e no

t re

port

ed.

CV

alue

s in

par

enth

eses

ar

e lo

g E.

dEs

timat

ed

from

th

e sp

ectr

um

of

tetr

achl

orod

i-n-p

ropy

lcal

icen

e in

met

hano

l [ 2

0,

211.

eD

ata

rela

tive

to

5,6-

dim

ethy

lben

zoca

licen

e in

eth

er

[ 141

. fD

ata

rela

tive

to

5,6-

dim

ethy

ldib

enzo

calic

ene

in c

yclo

hexa

ne

(sh

= sh

ould

er).

295

composition of the associated wavefunction it appears to be the butadienic transition found at 6.84 eV in fulvene [l] strongly perturbed by the neigh- bouring ring. If a comparison is made with the results previously obtained for the same compound by the MIM and PPP approximations [ 251, agree- ment is good, in particular for the composition of the wavefunctions. For the fourth and fifth transitions (for which disagreement was found between the MIM and PPP methods), the results predicted by the MIM method are confirmed here.

On passing to benzocalicene and dibenzocalicene, the fusion of one or two benzene rings onto the pentatomic fragment of calicene give rise to numerous 7~* + 71 transitions very close to each other, and it is difficult to correlate the transitions of these molecules with those of the component systems. The absorption band observed at 3.69 eV in the spectrum of VII [ 221 originates from at least two n* + 7~ transitions calculated at 3.75 eV (strong) and 4.18 eV (weak), similar to the first two transitions of calicene but reversed in energy. The next four transitions at 4.27 eV, 4.57 eV, 4.61 eV and 5.35 eV are associated with the experimental band lying at about 4.64 eV. The first three are due to configurations which involve principally the benzofulvene 7rr-system, while the weak transition at 5.35 eV originates from electron transfer from the benzofulvene system to the C-,Cs ethylenic fragment. It is similar to the z-polarized transition found at 4.92 eV in calicene. The perturbation induced by the benzene ring is also evident in the composition of the 7~* + 71 transitions calculated at 5.62 eV (mainly inside the benzofulvene framework) and at 6.04 eV and 6.15 eV, the latter two being nearly degenerate.

The experimental spectrum of VIII [ 231 both in cyclohexane and in ethanol solution shows numerous absorption bands and shoulders, which agree well with the numerous calculated n* + ‘II transitions in the low-energy range. The lowest-energy transition, of conjugative character, as well as the second one (corresponding to the first transition of calicene) lie at the same energy as in VII. The degenerate transitions at 4.25 eV and 4.39 eV corres- pond to the third, fourth and fifth transitions of VII. The strong transitions at 4.70 eV, 5.08 eV and 5.42 eV appear to be principally benzenic transitions perturbed by the fulvene or methylenecyclopropene systems. The charge transfer transition involving electronic transfer from the benzene rings to the C& ethylenic system, analogous to that noted at 5.35 eV in VII and at 4.92 eV in VI, occurs at 5.54 eV in dibenzocalicene and is associated with the band observed at 5.14 eV. The three transitions at about 6 eV again appear to have benzenic nature, and are perturbed to a great or lesser extent.

According to the present INDO/S calculations, the first two rr* + 71 transi- tions of triaheptafulvalene (IX) are predicted to lie at 2.23 eV and 3.57 eV, in full agreement with CNDO/CI results [ 81 and the PPP approximation [ 21, while on the MIM model they are calculated at higher energies [ 261. In both cases, however, they are estimated to be very weak ‘B, + ‘A 1 transitions. As previously noted [ 261, the former is mainly localized inside the heptatomic ring and the latter inside the methylenecyclopropene system.

296

The strongest ‘A 1 + ‘A 1, z-polarized transition is computed at 3.92 eV and involves two delocalized orbitals. In the range 4.6-5.4 eV only u* + 71 transitions appear, while two degenerate, weak n* +- 71 transitions are pre- dicted at 5.46 eV. The one assigned to the ‘A, +- ‘Al transition originates from electron transfer from the delocalized HOMO to an orbital localized on the framework constituted by the three coupled ethylenic systems C 1CZC5C6C7Cs; that of ‘Bz symmetry seems mainly to be a transition characteristic of the heptafulvene fragment.

Two strong 7r* + 71 transitions, perpendicular to each other and again de- generate, are calculated at 6.3 eV. The former is due to electron transfer from the two opposite ethylene systems C3C4 (in the seven-membered ring) and C&i0 (triatomic ring) to a delocalized orbital; the latter is inside the heptafulvene system with some charge transfer from the C9C10 fragment to the hexatriene framework C1 . . . Cg.

The experimental spectrum of heptatriafulvalene is not available so that a comparison between the theoretical and experimental results is not possible. However the calculated spectrum of IX shows transitions which are charac- teristic of the component rings, so this can be correlated with the spectra of heptafulvene and triafulvene. The transitions at 2.23 eV, 3.92 eV, 5.46 eV (degenerate) and 6.3 eV ( ‘BZ) correspond to the transitions of heptafulvene at 3.17 eV, 4.4 eV, 5.99-6.11 eV and 6.7 eV, respectively, perturbed by the cyclopropenyl ring. Similarly the transition at 3.57 eV closely resembles that at 4.54 in I. The resemblance of the transition energies calculated from the different approximations and the similarity of many with the transitions of the component rings (where fair agreement with experience is found) suggest that the experimental spectral trend might not be very different from that predicted.

ACKNOWLEDGEMENTS

We thank Professor V. Galasso, University of Trieste, who kindly furn- ished the copy of the INDO/S program utilized here.

REFERENCES

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