Indian Journal of Chemistry Vo1 .39A, Ian-March 2000, pp. 92-99

Structure, stability and reactivity parameters of (CH)g isomers and their cation and anion radical counterparts : A theoretical study

U Deva Priyakumar & G Narahari Sastry*

Department of Chemistry, Pondicherry University, Pondicherry 605 0 1 4, India Email: sastry@pu .pon.nic.in

Received 4 October 1 999 ; accepted 9 November 1 99 9

Among the hydrocarbons with (CH\k (k = 1 ,2,3,4 . . . . ) structural formula. (CH), where k=4. possesses nineteen local minima at semi­empirical level with comparable heats of formation. AM I procedure is found to be better than MNDO and PM3 in evaluating the heats of formation and geometries based on comparison with available experimental data of the neutral isomers. Vertical and adiabatic electron affinities and ionization potentials have been calculated for all the available isomers. The relative energy ordering is mainly controlled by the electronic factors rather than the strain . Koopman's theorem is not expected to yield reliable answers for ionization potentials. as the orbital relaxation seems to be very high for some isomers. The major geometric distortions are due to either Jahn­Teller distortions or strong vibronic interactions and the consequent reordering of the skeleton. Relaxation energies from vertical to adiabatic states are roughly proportional to the geometric deformations that occur upon ionization or addition of an electron.

Introduction The significance and beauty of members belonging

to the (CH)2k fami ly of hydrocarbons . such as cyclobutadiene, tetrahedrane, benzene, prismane, cubane, dodecahedrane, etc. cannot be overstated 1 .2

. These compounds played pivotal role in understanding the structure, bonding and reactivity and are partly re­sponsible for evolving the paradigms of general appli­cability in physical organic chemistry. Angular strain is a major factor determining the relative stabilities of vari­ous isomers for k S3 . The smallest group in the series is represented by acetylene, which is the only stable iso­mer for k = I . Annulenes constitute a very important class of compounds, which belongs to this family for values k �2. The annulenes, cyclobutadiene and ben­zene are the most stable isomers for k =2 and 3 respec­tively, while the other possible isomers for the same struc­tural formulae are heavily strained. For k �4, the planar annulenes encounter large angular strain, as k increases. Thus, for k = 4, (CH)x ' the planar structure of the cyclooctatetraene(COT) is not an energy minimum and quite a few isomers with comparable energy exist. Thus, for higher k values, three-dimensional structures become increasingly competitive energetically with the annulenes and other isomers in the series as against tetrahedrane and prismane, which are highly unstable with respect to

cyclobutadiene and benzene respectively. Thus, cubane (k=4) and dodecahedrane (k=1 0), whose synthesis is ac­complished, are not only very stable species on their re­spective potential energy surface, but also competitive with other isomers in the series. The aim of the present paper is to identify and characterize all the minima on the hypersurface of (CH)x isomers in the neutral as well as cationic and anionic forms. Individual members of th i s fami ly, such as cubane ( 16 ) , COT( 1 ) , semibul l valene(S) , barre lene(2) , cuneane( 12) , octavalene(7), tricyclooctadienes(13, 14), etc . , and the interconversions among the various isomers (see Fig. I later) are subjects of many theoretical and experimental studies,· ' 2 . The last comprehensive review on (CH)x i so­mers appeared almost a decade back, concentrating on thermal, photochemical, and catalytic behaviour of se­lected isomers .' Many interesting rearrangements of cubane and tricyclooctadienes are studied both experi­mentally and theoretically6.7. The chemistry of the cor­responding anion and cation radicals of (CH)x isomers is also interesting in its own right. For example, the ani­onic counterparts of cyclooctatetraene have been exten­sively studied, in terms of their structure, automerization and rearrangements 1 2 . The rearrangements between the cation radicals of cyclooctatetraene and semibullvalene have been studied both experimentally and theoreti-

PRIYAKUMAR e/ at. : STRUCTURE. STABILITY AND REACTIVITY PARAMETERS OF (CH)K ISOMERS 93

cally I4, 15, Theoretical treatments of the members of this family require very high levels of sophistication, similar to the coupled cluster and multi-reference configuration interaction, and the routine levels of calculations such as Hartree-Fock, MP2 and the currently popular hybrid density functional levels may not provide quantitative

, accuracyl6, However, it is important in a study of this sort, which involves comparison among a large number of isomers and aims to predict their relative energy or­derings and other trends qualitatively. In this regard, not only the ab initio SCF calculations but also the semi­empirical methods are expected to do the needful .

The qualitative and comparative evaluation of geo­metrical and electronic structures of the various isomers helps in predicting the relative stabilities and reactivities of these isomers, The purpose of the present paper is to identify all the possible isomers of (CH)x family, and obtain the trends in their heats of formation, rather than to calculate the geometry, energy and related properties of each species at the highest possible level. This is fol­lowed by studying the effects of adding and removing a single electron from the neutral species to understand the charge transfer reactivity in these isomers, The re­sultant cation and anion radicals are studied both in their vertical and adiabatic states.

Computational Details All the geometries were fully optimized without any

symmetry constraints using AMI 17, MND01x and PM3 1Y methods, and stationary points were obtained on the potential energy surfaces. The semi-empirical SCF pro­cedures in general do not make use of symmetry iil SCF cycles in economizing the CPU time, Therefore, we re­sorted to full geometry optimizations without imposing any symmetry c0!lstraints, At all these levels, frequency analysis was performed on each of these gradient optimized stationary points to confirm their nature, An exhaustive search was performed by optimiz ing geometries of several trial structures, followed by fre­quency calculations, All the real frequencies for a given structure represent a local minimum and one or more imaginary frequencies were characterized as transition state and higher order saddle points respectively, Only the stationary points which are characterized as local minima on potential energy surface are included here, and others which have imaginary frequencies are ignored except for D 4h -COT, 3. The vertical electron affinities and ionization energies were obtained by doing single point calculations on their corresponding anion and

cation radicals with the corresponding neutral geometries. Unrestricted formalism, where a and B elec­trons are treated independently, was adopted for the open shell calculations , These open shell calculations were done only at AMI level, after adjudging it as the best choice based on comparison with the available experi­mental data on the neutral species, Thus, unless other­wise stated, all the values obtained by AMI method are used in the discussion, The geometries of all the anion and cation radicals are also fully optimized without im­posing any symmetry constraints and further character­ized by frequency analysis, Although the l imitations of semi-empirical AM I and MNDO methods are well known, especially in dealing with strained hydrocarbons including three- and four-membered rings, and their ionic counterparts20, the size of the systems precludes the quan­titative study of all the aspects considered in the paper due to prohibitively high computational costs, The semi­empirical SCF methods are expected to provide trends of the geometries and energetics as reliable as those ob­tained from ab initio SCF, MP2 and hybrid density func­tional methods l 6. Strain energies of all the species are evaluated using Allinger's molecular mechanics II force field as implemented in PCMODEL 5 , 1 3 of Serena soft­ware2 1 .

Results and Discussion

The relative stabilities oj (CH)2k (k = 1 -5) isomers Figure 1 depicts the relative stabilities of (CH)2k

(k =1 -5), The criterion of heats of formation (Hf) per (CH) unit is used as it allows one to compare the rela­tive stabilities of isomers with different k values, Obvi­ously, as the k increases the number of isomers increases and for k =4 , which is the subject of the present paper and for k =5 , a large number of isomers exist which are energetically comparable, In Fig. 1 , only representative seven of all the possible isomers of (CH)IO are taken from previous studies22 , Benzene occupies the unique posi­tion energetically among all the (CH)2k isomers as the Hr per (CH) unit is probably the lowest for it. For benzene and for lower members of this family, i ,e. , for k :::;3, the energy difference between the most stable isomer and other isomers, whenever they exist, is very large and thus the isomers of comparable stability do not exist for k ::;3. In contrast when k =4 , (CH)x' there are twenty isomers existing which lie very close to each other energetically. Thus, (CH)x becomes the lowest member of the (CH)2k series, which provides a large number of isomers with

94 INDIAN J CHEM, SEC. A, JAN - MARCH 2000

' 5

'0

35 "0 e

I 30

'§ 2 5

i' 20 S � 15 :1t

--�

10 --

Fig. I -Hf per (CH) unit VS. number of (CH) units.

comparab le s tabi l i t i es and promi ses in teres t ing rearrangements among them. These series of isomers provide an excellent platform for performing theoreti­cal studies at various levels of sophistication, which help in realising the strengths and weaknesses of each of the methods employed and provide a way for benchmarking these calculations for future applications on simi lar class of isomers.

Optimized structures and their structural parameters Exploratory search for locating all the local minima

on the hypersurface of (CH)H resulted in nineteen minima, 1 , 2 and 4-20, corresponding to the isomeric formulae, with the D2d-COT,1, as the global minimum. Several other isomers, which were considered initially, either collapsed to one of the above structures on optimization or led to first or higher order saddle points and are not reported here. The D4h-cyclooctatetraene (3) is the only saddle point (second order) included for discussion due to its strategic significance as a member of planar annulene family, and for its comparison with the anion radical isomers of COT(later). All the considered struc­tures, including 3, are placed in the increasing order of energy in Fig. 2. Although the structures are optimized at all the three semi-empirical levels, only AM I skeletal parameters are given, as there is no significant differ­ence in the trends as well as in the absolute geometric parameters obtained using these methods. However, it is observed that MNDO uniformly overestimates the bond lengths by about 0.01 A and with virtually same trends as in AMI . The D2d-cyclooctatetrane (1) is the global minimum, which is in conformity with the earlier theoretical and experimental results. The cor responding

planar form, 3, is at a second order saddle point and the C2h structure is still higher in energy. The origin of pseudo J ahn-Teller distortion in going from planar D 4h -COT (3) to D2d-COT(1), is detailed in a recent theoretical study23. The optimized geometrical parameters appear to be close to the experimental as well as h igh-level theoretical cal­culations. For example, modelling the double-bond re­quires post SCF treatments since, SCF methods invari­ably underestimate the double bond lengths24. AM I level, which has been employed here is capable of identifying the local minima and so it is unlikely that any other genu­ine minima on the (CH)H potential energy surface is missed in the present study.

Energetics of the isomers on neutral hypersurface Table I gives the absolute heats of formation at the

three semi-empirical levels, the strain energies obtained using molecular mechanics calculations, and the experi­mental values wherever available. After a quick com­parison of the experimental heats of formation with those obtained at the three theoretical levels, one can see that in general MNDO grossly overestimates the stabilities. AM I seems to be closer to the experimental values, al­beit slight underestimation is seen here. The heats of formation calculated by the PM3 method are in be­tween those obtained by AM 1 and MNDO in all cases, except for the cyclooctatetraene isomers, 1 , 3, and 6. PM3 predicts barrelene, 2, to be of lower in energy than D2d-COT, 1, which is against the experimental predic­tion. In addition, the stability of cubane is grossly over­estimated by both MNDO and PM3 methods, as against the AM I result, which is in very good agreement with the experimental value. Based on the above observations and comparing the relative energies, AMI is adjudged as the best method of choice among the three methods tested here. The relative energies of all the isomers are plotted in Fig. 3, normalized to D2d-COT, 1 . The plot also includes the strain energies obtained by molecular mechanics procedure. In most cases, the MNDO and PM3 lines overlap with each other indicating their simi­larity, whereas, AM I is always higher. This is to do with the selecti ve overestimation of stability of COT by AM I method. Table 1 clearly indicates that only the stabilities of D2d-COT(1) and barrelene, 2, are overestimated com­pared to the experimental values at AM 1 level . In the other cases (4, 5, and 16), computed heats of formation are found to be higher than the experimental values. This leads to a situation where AMI selectively overestimates the stability of the reference structure in Fig.3 . There-

PRIYAKUMAR et at. : STRUCTURE, STABILITY AND REACTIVITY PARAMETERS OF (CH)x ISOMERS 95

1 . 3�8 137 . 6 1 . 4 5 2

\oW

, '3 . 0 6 3

1, D,d, 0.0(0)

3, D •• , 12.1 (2)

1 . 4 8ID1 . 535 1 . 52 4

1 . 558 1 . 35 2

1 . 52 0

5,C" 20.9(0)

1 . 4 5 9 1 . 51G�

1 . 4 9 1 1 . 4 4 2

#1 . 3 4 0

7 , Cz", 52.2(0) 1 . 358 9, c.. 55.1(0)

2 . 35 1 1I,e" 65.9(0)

L "�'"

#1 . 357

13, Clh, 74.6(0)

1 . 53�1 ' 50 7 1 1 7 . 2 1 . 47 1 _ 1 1 . 3 5 5

1 . 0 2 3

1 . 6 1 4

15, 4 , 86.1(0)

2, DJb, 3.8(0)

1 . 587 4, C .. 18.3(0)

6, ClO, 47.2(0)

1 . 54[) 1 . 3 4 4

1 . 5 3 5 1 . 4 6 3

1 . 5 2 2

8,Cz .. 54.5(0)

�4 �Vl.358

10,Dz.t, 61.8(0)

1 . 50 9 .

1 . 5 2 9�0 7 ' 3 1 . 5 3 0

1 . 5 4 7 1 . 6 2 7

l2,C, . . 70.4(0)

1 . 35'(jjd:8 . 11 . 7 1 . 5 18 1 . 6 1 9 1 . 535

14, C," 78.2(0)

16, Db, 87.7(0)

1 . 4 87

17,C,,,, 103.4(0) 18,0, .. 108.9(0)

1 . 5 4 3

1 . 3 1 7

19,c.. 1l�.I(O) lO,C" J48.4(O)

Fig. 2-0ptimized skeletal parameters of the stationary points on the neutral hypersurface of (CH\, indicated at AM I level. All bond lengths are given in A, bond angles are in degrees, the relative heats of formation are given in kcal mol') and the number of imaginary frequencies is given in the parenthesis.

fore, the relative energy curve normalized to 1 i s higher for AM I level compared to MNDO and PM3. The sta­bility values of the molecules with four membered rings are grossly overestimated by MNDO and PM3 methods, as can be seen by the comparison of Fig. 2 and Table I . Thus, the isomer w ith the h igher number of four membered rings is not expected to yield reliable answers at MNDO and PM3 levels. AM I appears to be better than either PM3 or MNDO in predicting the energetics of the isomers with four membered rings. ,

Strain energy in general and angular strain in par-ticular are the most important factors deciding the rela­tive stabilities of the hydrocarbon isomers. Therefore, all the geometries are optimized using molecular me­chanics and the strain energies are evaluated. Octavalene, 7, seems to be the compound with least strain in the se­ries and expectedly, cubane, 16, bears the maximum strain in the series. Even though the strain energy is a very important factor in deciding the relative energy or­dering, the electronic factors are dominant and are more important in deciding the relative stabilities in this group of isomers. It is to be noted that the most-strained iso­mer in the series, is cubane, 16, which is more strained than octabisvalene, 18, by about - 1 30 kcallmol;' it is en­ergetically more stable than the latter by about 2 I kcall mol (At MNDO and PM3 levels, the effect is even more dramatic). Thus, the strain energy is definitely not the deciding factor for predicting' the relative stabil ities.

96 INDIAN J CHEM, SEC. A, JAN - MARCH 2000

Table 1 - Heats of formation of each of the isomers obtained at AM I , MNDO and PM3 levels as well as the available experimental numbers. All the values are given in kcal mol· l .

Structure AM I MNDO HI Hr

I 63.4 S6.2 2 67.2 67.8 3 7S.S 64.4 4 8 1 .7 64.3 S 84.3 72.S 6 1 1 0.6 83.6 7 I I S .6 96.0 8 1 1 7 .9 92.7 9 1 1 8.S 1 08.S 1 0 1 2S .2 1 03.6 I I 1 29.3 98.8 1 2 1 33.8 1 00.8 1 3 1 3 8.0 1 02.8 1 4 1 4 1 .6 I OS.3 I S 1 49.S 1 1 8 .8 1 6 l S I . I 99. 1 1 7 1 66.8 I SS.8 1 8 1 72.3 1 42.9 1 9 1 78.S I S3.0 20 2 1 1 . 8 1 7S.7

a) Strain energies obtained from MM2 calculations.

Structures and energetics of the ionized counterparts The neutral isomers upon adding or removing a sin­

gle electron lead to the corresponding anion or cation radicals respectively. The ionization or addition of elec­tron may lead to the generation of some novel species, which have nof·counterparts on the corresponding neu­tral potential'e�ergy surfaces25 ., Firstly, calculating the energies of the cationic and anionic species using neu­tral geometries gives the vertical ionization energies and electron affinities, which in tum gauge the tendency of the individual isomers to ionize as well as accept the electron. Table 2 depicts the heats of formation, ioniza­tion energies and electron affinities of all the isomers in their vertical as well as adiabatic states. The appearance of vertical negative electron affinities of COT(I) , bicyclo-octa-triene(4), and octavalene(7) show that these molecules can exist as stable gas phase ions. It should be noted that the most stable isomer COT, 1, has the lowest ionization energy among all the isomers and also has negative electron affinity which make it an excel­lent electron donor and acceptor. Cubane, 16, the most strained compound in the series has the highest ioniza­tion energy, making it that much difficult to ionize; this arguably is the most stable kinetic isomer of the series.

PM3 HI

66.7 63.3 82.9 73.9 74.4 9 1 .7 1 09.2 1 03.6 1 06.2 I OS.7 I OS.9 1 08.7 1 1 4.0 1 1 7.6 1 26.6 1 1 3 .8 I S4.0 I S I .4 I S8.7 1 84.7

1 6 0

1 4 0

:t 1 2 0 "0 E 1 0 0 ] � B O .!l ff 60 G ... . � 4 0

� 2 0

SE'

2S.6 33. 1 38. 1 37.4 29.9 76.S 24.6 47.4 46.8 S2.0 79.S 9S.7 94.3 98.0 1 02.0 1 69.8 69.2 40.3 67.7 8S.8

-o- AMI __ MNOO -I!r- PM3 __ SE

1 0 Structures

Expt. HI

7 1 . 1 72.S

76.6 73.6

1 48.8

15 20

Fig. 3- Relative energies of AM I , MNDO and PM3 levels. Relative strain energy is obtained from molecular me­chanics calculations. Du-COT is used as the reference point.

Figure 4 gives the plot of vertical ionization energy and the negative of highest occupied molecular orbital en­ergy (which should represent the ionization energy ac­cording to Koopman's theorem) for each isomer, indi­cating that there are significant differences between the

PRIYAKUMAR et at. : STRUCTURE, STABILITY AND REACTIVITY PARAMETERS OF (CH)x ISOMERS 97

Table 2-Heats of formation of the anion and cation radicals in their vertical as well as adiabatic states. The vertical and adiabatic first ionization energies and electron affinities of neutral counterparts are also given.

Structure H[ (kcal mol-I)

2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 20

'"' :; � u .;i

12 . 0 1 1 . 5 1 1 . 0 10 . 5 10 . 0

9 . 5 9 . 0 8 . 5 8 . 0 7 . 5 7 . 0 6 . 5 6 . 0

cation adiabatic vertical

247.9 257. 1 272.0 276.6 248.5 254.0 270.5 275.6 252.3 292.0 247.9 309.4 303.5 308.4 3 1 1 .9 326.2 308.8 3 1 4.8 324.3 329.9 338.9 352.5 292.3 356.8 343.7 349.2 247.9 353.2 353.9 359.7 360.4 384.6 359. 1 366. 1 375.3 388.5 308.8 378. 1 422. 1 428.0

--+-Vertical Ionization Fncsgy. ---*--EHOMO of neutral.

10 Slructurcs

anion adiabatic vertical 42.8 58.0 79.3 8 1 .6 42.8 48.0 73.8 79.4 93.5 96.6 42.8 1 09.0 1 04.8 1 08.5 1 26.8 1 29.4 1 1 5 . 1 1 1 8 .7 1 29.2 1 32.6 1 40. 1 1 43.0 1 06.6 1 75.6 1 46.5 1 49.4 . 1 52. 1 1 55 .3 1 62.0 1 65.0 1 80. 1 208.2 1 72.5 1 76.2 1 94.7 208.7 1 84.8 1 88.0 221 .0 223.8

1 5 2 0

Fig. 4 - Plot comparing the vertical ionization energies and the negative HOMO energy for all twenty isomers.

two values. Therefore, orbital relaxation is significant and the Koopaman's ionizations are unreliable. While we find apparently no deviation from Koopman's theo­rem for isomer 11, the difference is as high as 0.7 eV for some other isomers (Fig. 4).

Structural deformations upon relaxation of ionized species

The extent of deviation of geometry upon ionization varies for different isomers. Thus, isomeric forms 1, 2,

Ionization energy(eV) Electron affinity(eV)

adiabatic vertical adiabatic vertical 8.00 8.40 -0.89 -0.23 8.88 9.08 0.53 0.62 7.50 7.74 - 1 .42 - 1 . 1 9 8. 1 9 8.4 1 -0.34 -0. 1 0 7.29 9.0 1 0.40 0.53 5.95 8.62 -2.94 -0.07 8. 1 5 8.36 -0.47 -0.3 1 8.4 1 9.03 0.39 0.50 8.25 8.5 1 -0. 1 5 0.01 8.63 8.88 0. 1 7 0.32 9.09 9.68 0.47 0.59 6.87 9.67 - 1 . 1 8 1 .8 1 8.92 9. 1 6 0.37 0.49 4.61 9. 1 8 0.46 0.59 8.86 9. 1 2 0.54 0.67 9.08 1 0. 1 3 1 .26 2.48 8.34 8.65 0.25 0.4 1 8.80 9.38 0.97 1 .58 5.65 8.66 0.27 0.4 1 9. 1 2 9.38 0.40 0.52

5, 8, 10, 12, 16, 18 and 19 (ref. Fig. 1 ) undergo signifi­cant distortion upon ionization, and the geometries of distorted forms are depicted in Fig. 5 . The COT isomers, 1, 3, and 6, with D2d, D4h and C2h symmetries respec­tively upon adding an electron, collapse to a planar form, with equalized bond lengths and with a minute differ­ence in the bond angle, which otherwise would have led to a perfectly symmetric DSh structure. Therefore, it seems that addition of one electron is sufficient to make COT planar and second electron just reinforces the pla­nar structure, as it is well known that the COT dianion has a perfectly symmetric planar structurel4 • The strain in C

2h structure of COT, 6, is reflected in the huge re­

laxation energies (>60kcal/mol) for the corresponding cation and anion radicals. Among the other isomers, only cubane (16), barrelene (2) and tricyclo-octa-3,7-diene (10) possess degenerate energy levels and thus can un­dergo first or second order Jahn-Teller d istortion, whereas the other isomers distort owing to the breaking of bonds upon ionization. Barrelene, 2, which has a non­degenerate HOMO and doubly degenerate LUMO, does not undergo J ahn-Teller distortion for going to its corre­sponding cation radical. In contrast, the extra electron in the anion radical will be placed in the doubly degen­erate level and hence it undergoes Jahn-Teller distor-

98 INDIAN J CHEM, SEC. A, JAN - MARCH 2000

tion. Cubane(16) which has triply degenerate HOMO as well as triply degenerate LUMO should undergo Jahn­Teller distortion either by losing or accepting an elec­tron. The effect of J ahn-Teller interaction for both cation and anion radicals for cubane leads to the breaking of C­C bonds which results in huge relaxation energies in both the cases. The D2dJricycIo-octa-3,7-diene(lO) has a dou­bly degenerate HOMO and singly degenerate LUMO. Thus the corresponding cation radical (lOC) should only expected to encounter Jahn-Teller distortions. However, both lOC and lOA are distorted to lower C2v isomers, with the former distortion being due to the first order J ahn-Teller effect and the later due to the vibronic inter­action. The lowering of symmetry is followed by locali­zation of both charge and spin on one double bond with no major skeletal distortions, and therefore only small relaxation energies are observed on both the ion radical surfaces.

Unlike most neutral molecules, the ion radicals in general and cation radicals in particular have very low lying exited states causing strong vibronic interactions. Generally, the vibronic interactions, or second order Jahn-Teller interactions lead to structural distortions in ion radical chemistry26.27 . For example, in cuneane(12), the breaking ofC-C central bond which removes the four membered rings, results in high relaxation energies on both anion and cation radical surfaces. Similarly, 19C where a three membered ring is broken experiences a huge relaxation energy whereas the corresponding anion radical, 19A has a very small relaxation energy since no bond-breaking takes place. S ignificantly, the cation radi­cals corresponding to C2h COT, 6, and syn-tricycIo­octadiene, 14, collapse to the D2d COT. The spontane­ous decay of syn-tricyclooctadiene radical cation , l4C, to D2d COT radical cation, lC, is also predicted by ab initio self-consistent field calculations using both re­stricted and unrestricted Hartree-Fock formalisms2x . S imilarly, the D2d and C2h forms of COT, 1 and 6 respec­tively, collapse to the planar D4h form, 3, on the anion radical hypersurface. Naturally, major structural devia­tions such as bond breaking, etc. , lead to larger relaxa­tion energies. The possibility of rearrangement reactions, such as hydrogen abstraction, etc. , is certainly an order of magnitude higher for the ion radicals compared to that for the neutral counterparts29. However, this is out­side the purview of this study.

In summary, the (CH\ group of isomers have rich chemistry and the characterization of nineteen local minima on the neutral potential surface, with only mi-

1 . 520.

:le, c". -, +.

sc. c.

. 1 . 72LD' -,.;. 1 . 34 3 1 . 4 7

1 . 4 7 7 1 . 5 1 6

1 . 51 4 -, .+

1 . �1 ' 355 . 1 . 631

IOC, C ••

-, .+ �'" 1 . 535 1 526 nc, C,• . -, .+ ,':::@ , . m

1 . 523 16C, C,y

1 . 5 2 2 -, .+

1 . 7 5 7Q1 . 4 8 7 . 1 . 5 1 L 52 1

18C, C,y 1 . 418 . -, .:.

1 . 3 8 6 -, .:.

""'-0,.,,, 1 35 . 9

-, .:.

1 . 52 1

:lA, c" •.

1 . 52 3

1 . 5�3 -, .:.

1 . 3 9 6 1 . 53

1 . 4 3 9 1 . 5 3 1

-, : 1 . 4 B 8

1�

4 7 7 1 . 3 7 5 1 . 4 0 1

1 . 633

lOA, C ••

�'" 1 . 5 1 7 � 5 3 4 t2A, C'v .

1 . 59�\ 2 . 24�: 1 . 545� 1 . 50 8

16A, c,.

1 . 4 9 1 -, .: 1 . 47501.592 1 . 53 1 . 514

t8A, ClY

Fig. 5-Geometries of significantly distorted ionized coun­terparts. All bond lengths are given in A and bond angles are in degrees.

· PRIYAKUMAR et al. : STRUCTURE, STABILITY AND REACTIVITY PARAMETERS OF (CH)s [SOMERS 99

nor differences between the stabilities of all these iso­mers forms an interesting area for pursuing further work at theoretical level. The qualitative predictions made by the present study about the stabilities and reactivities are to be further augmented with higher levels of theo­retical studies and experimental probes wherever possi­ble. Caution should be exercised in interpreting these studies quantitatively. However, we feel that the general qualitative picture of the potential energy surface should form a reliable basis for understanding the reactivity trends of the (CH)8 isomers.

Acknowledgement GNS thanks AICTE (8017 IRDIIIR&DffAP (868)/98-

99) for financial assistance. UDP thanks UGC, New Delhi for a JRF fel lowship . We thank Prof. 1 . Subramanian for critically going through the manuscript.

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