10 Negative I o n States o f Three- a n d F o u r - M e m b e r e d R i n g Hydrocarbons Studied by Electron Transmission Spectroscopy ALLISON E. HOWARD and STUART W. STALEY Resonances Downloaded from pubs.acs.org by YORK UNIV on 12/02/18. For personal use only.
Department of Chemistry, University of Nebraska, Lincoln, NB 68588-0304 Electron transmission spectra have been obtained for the C -C cis-cycloalkenes (3-6) and cycloalkanes (7-10). The somewhat low value for the first negative ion state of 3 (1.73 eV) can be understood on the basis of compen sating effects due to the shortC=Cbond and the "reversed" polarity of 3 relative to 4-6. The lowest * resonances observed in 3, 4, 7, and 8 have been assigned on the basis of ab initio 6-31G molecular orbital calcu lations and by consideration of the components of the angular momentum associated with each resonance. 3
6
Three- and four-membered ring compounds have been of great interest to chemists for over a half century. This is due primarily to the fact that the high degree of ring strain in these compounds, partic ularly in cyclopropane and cyclopropene and their derivatives, con fers upon them certain properties usually associated with "unsat urated or olefinic compounds. Such properties originate from the character of the "frontier orbitals", i.e., the higher occupied and lower unoccupied molecular orbitals. Although the occupied orbitals have been extensively investigated by photoelectron spectroscopy, essentially no research has been reported on the unoccupied orbitals of small-ring compounds. This paper summarizes the first investigations of three- and four-membered ring compounds by the technique of electron transmis sion spectroscopy (ETS). We will briefly discuss two general areas associated with the negative ion states of small ring hydrocarbons: a) TT* states in cyclopropene and related molecules, and b) negative ion states associated with the system in cycloalkenes and cyclo alkanes. Of special interest is an analysis which we believe allows us to assign orbital symmetries to the cr* negative ion states of cyclopropane, cyclopropene, cyclobutane and cyclobutene. 11
0097-6156/ 84/0263-0183$06.00/ 0 © 1984 American Chemical Society
184
RESONANCES
Experimental Method Electron transmission spectroscopy (ETS) i s an invaluable technique for obtaining gas-phase electron a f f i n i t i e s (J.,2). In this experi ment, a monoenergetic beam of electrons (FWHM = 20-30 meV) i s a l lowed to interact with molecules i n a s t a t i c c e l l . The electrons then pass through a retarding region and those electrons which possess an a x i a l velocity s u f f i c i e n t to overcome the retarding po t e n t i a l are c o l l e c t e d . The spectra discussed i n t h i s paper were obtained under high rejection conditions (£); that i s , they approx imate the t o t a l electron scattering cross-sections for the molecules of i n t e r e s t . A sharp decrease i n the transmitted current i s seen at energies which correspond to the energies of the negative ion states of these molecules. In the context of Koopmans theorem (k), these reso nances are associated with the negative of the SCF molecular o r b i t a l energies of the neutral molecule. In order to accentuate the variations i n the transmitted current, a small (20-50 meV) AC v o l t age O) i s applied to the c o l l i s i o n chamber. Thus, the derivative of the transmitted current with respect to energy i s obtained. The ?o/2 resonance i n argon (5) was used to c a l i b r a t e the spectra. We have found t h i s to be a useful c a l i b r a t i o n gas due to the narrow width, symmetrical p r o f i l e , and r e l a t i v e l y low energy of the resonance. The r e l a t i v e errors are estimated to be + 0.8$ of the attach ment energy. The actual errors associated with these energies are probably much larger, on the order of + 0.05 eV below 2 eV and +0.10 eV above 2 eV. 1
2 The TT* ( A ) States of the Temporary Negative Ions of and Cyclobutene.
Cyclopropene
The f i r s t negative ion states for the cis-cycloalkenes (3-6), as well as for the ethylene (1) and cis-2-butene (2), are associated with
=
w
1
2
P O O
A 3
4
5
6
temporary capture of an electron into the TT* o r b i t a l . The electron transmission spectra of 2-6 are presented i n Figure 1 and the values for the resonances are given i n Table I. Note that the TT* o r b i t a l i s destablized on substitution of 1 with two methyl groups, as i n 2, or with two or more methylene groups i n a ring, as i n 4-6. This i s i n accord with the commonly (2,7,12) (but not universally ob served d e s t a b i l i z a t i o n of TT* o r b i t a l s by a l k y l substituents. The e f f e c t of hyperconjugation on spectroscopic parameters as sociated with the TT and TT* states of 1-6 i s i l l u s t r a t e d by the data i n Table I I . In p a r t i c u l a r , the attention of the reader i s directed to the r e l a t i v e values of the Coulomb i n t e g r a l (J) which i s equal to , the mutual repulsion of two electrons (1 and 2) i n two o r b i t a l s (TT and TT*). This i n t e g r a l can be calculated from 12
H O W A R D A N D STALEY
Negative Ion States
Ref.
( J ! ) . —2.07 eV; Ref. (2).
Ref. (8).
- 8.81
- 8.71
- 8.63 -16.02,-16.15
- 8.81 -13.70
-5.10
-5.07
-4.94 -7.89,-8.11
-4.71 -7.53
-4.72
Calcd (6-31G)-
-Structure: Ref. (£). ^Structure: Ref. (J_0).
-Structure:
*-2.22 eV,
a (3,2) b*(2,1) b](2,1)
a (3,2) b|(2,1)
6-31G O r b i t a l Symmetry (&)
-Structure: Ref. (6). *-1.78 eV; Ref. (2,7).
-2.13-4.84 -7.70
-2.14 -6.32
-2.00 -6.27
Theorem approximation.
Ref. (2,2). -Structure:
-Koopmans
1
71* a* a*
cyclohexene (6)—
71 * a*
71* a*
f (4)—
cyclopentene ( 5 ) ^
cyclobutene
-1.73 -5.50
TT*
cyclopropene (3)—
a*
-2.16*
71*
cis-2-butene(2)
ethylene - 8.64
Electron A f f i n i t y (eV) Experimental Calcd. (ST0-3G)--
-1.74*
(1)-
Orbital
71*
Compound
Table I. Observed and calculated electron a f f i n i t i e s of small and medium ring cycloalkenes and related compounds
10.
HOWARD AND STALEY
Table I I .
187
Spectroscopic parameters^ associated with the ir and TT* o r b i t a l s of 1-6.
IP*
EA-
«,*
10.51 9.29 9.86 9.43 9.18 9.12
-1.74 -2.16 -1.73 -2.00 -2.14 -2.13
4.25 4.20 4.16 4.23 4.15 4.24
Compound ethylene (1) cis-2-butene (2) cyclopropene (3) cyclobutene (4) cyclopentene (5) cyclohexene (6) —In eV; see
Negative Ion States
text. *Ref. (J4).
- J = IP - EA - T „ .
- T h i s work. -Ref.
2K* 7.65 7.12 7.19 7.03 7.00 6.81
3.40 2.92 3.03 2.80 2.85 2.57
8.00 7.25 7.43 7.20 7.17 7.01
Q 5 ) . -Ref.
(J4b).
fi
2K = S, - T
the relationship J = IP - EA - T , where IP i s the *rr i o n i z a t i o n p o t e n t i a l , EA i s the TT* electron a f f i n i t y , and T^ i s the energy of the v e r t i c a l IT •+ TT* singlet triplet transition. Note that the value of J decreases i n the order 1 >2, 4, 5 > 6, i . e . , as the region i n space over which TT and TT* are delocalized through hyperconjugat i o n increases. Interestingly, the TT* o r b i t a l i n 3 has a node bisecting the C=C bond and therefore cannot interact with the CH group through hyperconjugation, whereas the corresponding TT o r b i t a l can so i n t e r a c t . This i s i n accord with the value of J for 3, which i s about 0.2 eV greater than those for 2, 4, and 5* The value of 2K, where K i s the "exchange" i n t e g r a l ( ) , which c o r r e s ponds to the e l e c t r o s t a t i c repulsion between equal overlap charge densities due to electrons (1) and (2), decreases i n the same order as does that of J . Note that the values of J and 2K i n Table II probably have uncertainties of +0.1 eV or more associated with them. Since the TT* o r b i t a l i n 3 does not interact with the CH o r b i t a l s , i t might at f i r s t seem not very surprising that the EA of 3 i s about equal to that of 1. However, t h i s observation assumes con siderable interest when i t i s recognized that the C=C bond i n 3 (1.296 8) (8) i s 0.04 8 shorter than that i n 1 (1.339 8) (6). In fact, the TT* o r b i t a l of ethylene i s predicted by ab i n i t i o c a l c u l a tions (6-31G basis set) to be destabilized by 0.29 eV on shortening the C=C bond from 1.339 to 1.296 8. In addition, the r e l a t i v e order of the TT* o r b i t a l energies for 1, 3, and 4 (Table I) i s not reproduced at the ST0-3G basis set l e v e l , but i s at the 6-31G l e v e l . This can be traced to the effect of the hyperconjugative donation of electron density from the TT o r b i t a l to the pseudo-rr* CH. o r b i t a l which leads to a "reversed" p o l a r i t y i n 3 ( u = 0.45 D with the positive end toward the double bond) (_16) compared with 4-6. The resultant de creased screening of the TT* o r b i t a l by the bonding electrons leads to a lower electron a f f i n i t y than would be expected on the basis of the 12
2
188
RESONANCES
C=C bond length alone. This represents the f i r s t time that a "TTinductive" or screening e f f e c t has been demonstrated as a factor i n determining electron a f f i n i t i e s .
P * Resonances i n Three- and Four-Membered Rings In t h i s section, we s h a l l refer to resonances which are associated with v i r t u a l o r b i t a l s which are antibonding at a bonds as a* reso nances, even though these o r b i t a l s may have o v e r a l l TT* symmetry. The spectra of 7-10 are given i n Figure 2 and the energies of the ob served a* resonances are l i s t e d i n Tables I and I I I along with t h e i r
o 7
8
o 9
10
symmetry designations and the angular momentum quantum number U) associated with the "leading waves" (lowest I values) (JJ_) i n the resonances. The resonances were assigned to s p e c i f i c o r b i t a l s on the basis of a) the c o r r e l a t i o n (r = 0.98) between resonance energies and ab i n i t i o 6-31G o r b i t a l energies, as shown i n Figure 3, and b) consideration of the J£ values associated with each o r b i t a l . Orbi t a l s with an I = 0 contribution were excluded because the corres ponding resonances are expected (but not necessarily required) to have too short of a l i f e t i m e for observation. Drawings of the 6-31G o r b i t a l s for cyclopropane (7) are given i n Figure 4. The f i r s t resonance i s not associated with the lowest (a*) a* o r b i t a l calculated at 6.64 eV which has I values of 2 and 0, but with the a" o r b i t a l at 6.97 eV (£ = 3,1). The second resonance i n 7 appears to be associated with the e" o r b i t a l s (I = 3,2) calculated to be at 10.35 eV (on the basis of the c o r r e l a t i o n i n Figure 3), rather than with the e o r b i t a l s U = 2,1) at 8.21 eV or the a' o r b i t a l (i = 5,3) at 8.62 eV. Whether t h i s assignment i s correct and, i f so, for what reasons, awaits further investigation. I f our interpretation of the a* data i n Tables I and I I I i s correct, t h i s suggests a d i r e c t i o n for future research. Thus, i f one wants to gain insight into the conjugating a b i l i t y of three-membered rings, a study of the f i r s t a* resonance i n substituted cyclopropanes may prove unrewarding, but a study of the corresponding reso nance i n cyclopropenes substituted at C may be very worthwhile. 1
H O W A R D A N D STALEY
Negative Ion States
ELECTRON ENERGY (eV) Figure 2. Electron transmission spectra of
7-10.
(10)
cyclohexane
—Koopmans
-6.97
-7.75
-4.11
-9.69
2
2
e , b , e ( a l l 2,1)
b (2,1),e(2,1)
e«(2,1),a£(5,3)
a^(3,D
6-31G O r b i t a l Symmetry (£)
—Structure: Ref. (.17). —Structure: Ref. (18).
-9.34 -6.14
-6.44,-7.64 -9.44,-9.47,-9.68
-14.68
-8.21,-8.62
-17.43,-18.34
-15.57
-5.80
-13-34
-8.78
Electron A f f i n i t y (eV) Calcd. (ST0-3G)Calcd (6-31G)-
-5.29
Experimental
Theorem approximation.
(9)
cyclopentane
1
(8)—
(7)—
cyclobutane
cyclopropane
Compound
Table I I I . Observed and calculated electron a f f i n i t i e s of small and medium ring cycloalkanes.
10.
HOWARD A N D STALEY
i
> 0
191
Negative Ion States i
1
8.00
•
>
o UJ —I
A
/->
/
7.00
