J. Am. Chem. SOC.1985, 107, 8082-8086

8082
z(nN)

= cBzB(nN)+ cNzN(nN)

I(no) = C B ~ B ( ~+OC)N ~ N ( ~ O )where CBand CN are the relative populations for the bonded (B)and nonbonded (N) conformers, respectively, and they satisfy CB+ CN = 1. One must here note that the relative PIES intensity
for the no oribtal remains nearly the same for the bonded andnonbonded conformers (a, and a3 in Figure 4, a4 in Figure 3, andTable 11). M O calculations also showed that exterior electron
distributions for the n,orbitals are rather insensitive to the changes
in conformation. Thus one may assume thatzB(nO)= IN(n0)

and one obtains
z(nN)/z(nO)= cBzB(nN)/zB(nO) +cNzN(nN)/zN(nO)

One may then introduce a relationship with the enthalpy andentropy changes due to dissociation of the hydrogen bond
In K = In (CN/CB) = - A H / R T+ AS/RA least-squares fit to this equation with the observed ratios ofI(nN)/z(no) in Table 111 leads to the following results:

AH = 1.25 kJ mol-’ (3.0 kcal/mol)and A S = 29 J K-’ mo1-l
The relative populations of the bonded conformer CBwere also
obtained and listed in Table 111. The present result for AH is ingood agreement with the earlier value (2.8 kcal/mol) obtained
for a tetrachloroethylene solution by infrared spectroscopy.I6
The above analyses also yields the following ratios:zN(nN)/zN(nO) = 0.88 and zB(nN)/zB(nO) = 0.62
It is noted that even in the nonbonded conformation Z(nN)is
smaller than Z(no). This is apparently contrary to the observed
ratio for H2NCH2CH20H(Z(n,) > Z(no)) in Figure 3. However,an explanation can be made in terms of effective solid angles open
to incoming metastable atoms. In the dimethyl analogue, the(16) Krueger, P. J.; Mettee, H. D. Can. J . Chem. 1965, 43, 2970-2977.

effective solid angle for the nN orbital is decreased, as observed
for some substituted anilines with methyl groups.’ Such a stericeffect may occur to a larger extent than one would expect for asmall reagent like a proton, since the radius for He* (23S) is
estimated to be as large as 2.5 A. This estimate can be made fromthe van der Waals radius of Ne (1.5 A) together with the factthat the He* (23S)-Ner;,ential becomes repulsive even at a large
distance of about 4.0
5. Concluding Remarks. Formation of intramolecular hydrogenbonding was found to cause a considerable deactivation of thenonbonding orbitals in Penning ionization. Helium atoms inmetastable states attack easily the nonbonding orbitals when (i)the molecule has no hydroxyl group accessible to the n orbitalsor (ii) the molecule is in a nonbonded molecular conformation.
For an intramolecular H-bonded conformer, however, the nonbonding orbital involved in the hydrogen bond is geometricallyplaced in the “shadow” of the O H bond and shows a considerably
suppressed activity in electrophilic attacks by metastable helium
atoms. When the dissociation equilibrium is shifted at higher
temperatures and the steric shadow effect by the O H bond isdecreased, the reactivity of the n orbitals increases, since much
more electron density is then exposed to the exterior region accessible to the incoming species.
It is noted that Penning ionization is a typical process ofelectron-transfer reactions; an electron in an occupied molecularorbital of the sample molecule is extracted by an electrophile. The
Penning process may thus be used to study relative reactivities
of sample molecules. In this use, metastable rare gas atoms may
be considered to be microscopic probes of the stereochemical
properties of molecular orbitals in the sample molecule. The
present study demonstrates that Penning ionization electron
spectroscopy may find its applications not only to the comparative
study of relative reactivities of orbitals for various molecules but
also to the study of dynamic variations of reactivity due to conformational changes.

Acknowledgment. We thank T. Ishida and Y. Itoh for their
help in calculations.
(17) Chen, C. H.; Haberland, H.; Lee, Y. T. J . Chem. Phys. 1974, 61,
3095-3103.

Study of Stereochemical Properties of Molecular Orbitals by
Penning Ionization Electron Spectroscopy. Effects ofThrough-Space/Through-BondInteractions on Electron
Distributions
Koichi Ohno,* Toshimasa Ishida, Yukito Naitoh, and Yasuo Izumi
Contribution f r o m the Department of Chemistry, College of Arts and Sciences, The University ofTokyo, Komaba, Meguro-ku, Tokyo 153, Japan. Received July 19, 1985

Abstract: Experimental evidence for effects of through-space/through-bondinteractions on electron distributions of molecularorbitals was observed by Penning ionization electron spectroscopy. For norbornadiene, the next highest occupied molecularorbital, which has high electron densities in the endo face augmented by the through-space interactions between T electrons
on the double bonds, showed a large reactivity upon electrophilic attacks by metastable helium atoms. In the case of
1,4-diazabicyclo[2.2.2]octane, the highest occupied MO composed of a symmetric coupling of the lone-pairs on nitrogen atoms,
which has a smaller exterior electron density than the next HOMO because of the strong through-bond interactions, was foundto give the smaller reactivity in Penning ionization.

Molecular orbitals have profound scientific meanings as they
explain varieties of experimental results in systematic ways.
Recent progress in the chemical orbital theory has made it possible
to elucidate chemical reactions on the basis of M O calculations.
0002-7863/85/l507-8082$01.50/0

Even though such a relationship between theory and experimentis approximate, the relation cannot be limited to one way. One
may, therefore, expect that an experimental study is able toelucidate a theoretical construct. A concrete example of this0 1985 American Chemical SocietyJ . Am. Chem. Soc., Vol. 107, No. 26, 1985 8083

Penning Ionization Electron Spectroscopy
possibility one should note here is the observation of molecularorbitals by utilizing a kind of reaction involving probe particles
as well as sample molecules. One of the important reactions forthis purpose is the (e, 2e) reaction; e- + M
M + + e- + e-.
McCarthy, Weigold, and their collaborators have made extensive
studies on (e, 2e) electron spectroscopy and have obtained valuable
information on individual molecular orbitals.'.* Tossell et al. also
reported such applications of the (e, 2e) t e ~ h n i q u e . ~ In
- ~ the (e,2e) reaction, the incident electron plays the role of bringing theionization energy as well as the role of the catalyst which cancels
out in the apparent reaction of the molecule M, M -,M + + e-.
Another example is also an ionization reaction which involves raregas atoms in metastable states, such as He* (23S, 19.82 eV); He*
M He M+ + e-. This reaction is called Penning ionization,and the metastable atom plays the role of the electron receptor
as well as the role of the ionization energy source in the electron-exchange mechanism;6 an electron in an occupied orbital of
the sample molecule is extracted into an inner-shell hole of themetastable atom, and an electron in the outer-shell orbital of themetastable atom is ejected into a continuum state with a kinetic
energy nearly equal to the difference between the energy of themetastable state and the ionization energy. In our previous work,
Penning ionization electron spectroscopy was applied to the studyof electron distributions of molecular orbital^.',^ Some importantfeatures of Penning ionization are summarized as follows: (i) raregas atoms in metastable states behave as electrophiles, (ii) raregas atoms cannot penetrate through the repulsive surface of thetarget sample molecules, (iii) electron-transfer reactions are most
likely to occur when the electrophile (the metastable atom) andthe electron donor (the sample molecule) are in close contact tocause effective overlapping of both orbitals, (iv) orbitals exposedto the outside beyond the repulsive molecular surface react withelectrophiles more easily than orbitals localized inside the molecular surface, and (v) a bulky group in the target molecule canact as a shield protecting a target orbital against attacks ofelectrophiles. On these findings, one may be encouraged to study
stereochemical properties of molecular orbitals by the use ofmetastable atoms as a probe.
In spite of many successful applications of the molecular orbital
theory, there are some problems for experimentalists in order toexplain the results in terms of group orbitals or localized orbitals.
In principle, usual SCF molecular orbitals (canonical molecularorbitals) can be obtained from a transformation of localized
molecular orbitals9 which are easily connected with chemical
concepts, such as lone-pairs or functional groups. However, thetransformation is purely mathematical and usually not deduciblefrom some chemical insights. For example, the symmetry of afrontier orbital, which often plays an important role, is difficult
to be seen from the localized or semilocalized scheme. Notablesituations with such difficulties are encountered for molecules
having a couple of equivalent lone-pairs or a couple of nonconjugated K bonds. An important contribution to this problem wasmade by Hoffmann, Imamura, and Hehre.'o*'' They introduced
concepts of through-space/through-bondinteractions which madeit possible to bring a qualitative insight connecting the localized

-

- +

+

~~

~

~~

~

~

~

~

~~

(1) McCarthy, I. E.;Weigold, E. Endeavour, 1978, 2, 72-79.
(2) Brion, C. E.;H o d , S. T.; Suzuki, I. H.; Weigold, E.; Williams, G. R.
J. J . Electron Spectrosc. Relat. Phenom. 1980, 21, 81-87 and references citedtherein.
(3) Moore, J. H.; Tossell, J. A,; Coplan, M. A Ace. Chem. Res. 1982, 15,
192-198 and references cited therein.
(4) Tossell, J. A,; Moore, J. H.; Coplan, M. A,; Stefani, G.; Camilloni, R.
J . A m . Chem. SOC.1982, 104, 7416-7423.
(5) Hiser, S.;Chornay, D. J.; Coplan, M. A,; Moore, J. H.; Tossell, J. A.
J . Chem. Phys. 1985, 82, 5571-5576.
(6) Hotop, H.; Niehaus, A. Z . Phys. 1969, 228, 68-88.
(7) Ohno, K.; Mutoh, H.; Harada, Y. J . A m . Chem. SOC.1983, 105,4555-4561.
(8) Ohno, K.;Matsumoto, S.; Harada, Y . J . Chem. Phys. 1984, 81,4447-4454 and references cited therein.
(9) Edmiston, C.; Riidenberg, K. Reo. Mod. Phys. 1963, 35, 457-465.
(IO) Hoffmann, R.; Imamura, A,; Hehre, W. J. J . A m . Chem. SOC.1968,
90, 1499-1509.
(11) Hoffmann, R.Acc. Chem. Res. 1971, 4 , 1-9.

or group orbital representation to the MO representation.
Through-space/through-bond concepts have been successfully
applied to analyses of IP values observed by photoelectron
spectr~scopy'~-'~
as well as the study of spin-density distributions.16
The purpose of the present study is to observe the difference
between symmetric and antisymmetric molecular orbitals causedby through-space/through-bondinteractions of equivalent localized
orbitals. The stress is placed on observation of the difference inspatial electron distributions of the molecular orbitals as well as
their relative reactivities. The results obtained by Penning ionization electron spectroscopy demonstrate that the use of He* (23S)
reactions has a potential value to observe stereochemical propertiesof molecular orbitals.

Experimental Section
He* (Z3S) Penning ionization electron spectra (PIES) and He I ultraviolet photoelectron spectra (UPS) were measured for norbornadieneand 1,4-diazabicyclo[2.2.2]octane(DABCO) by means of a transmission-corrected electron spectrometer.'^'' The helium metastable atoms(23S, 19.82eV) were produced with an electron gun and a quench lamp.
The helium I resonance line (584 A, 21.22 eV) was produced by a dc
discharge. The electron energy spectra were obtained for electronsejected a t the angle of 90" with respect to the metastable helium beams
or the photon beams by means of a hemispherical analyzer equipped withscanning electrodes at the entrance.

Calculations
Molecular orbital calculations were performed at the computer
center of the University of Tokyo. A library program GSCF2I8 wasused for ab initio MO calculations. A split-valence type basis set(4-31G)I9 and the more extended basis set (HDD2G) were used
for the basis functions. The HDD2G set includes the 6-31 1G
functions20for hydrogen atoms and (1 ls7p)/[5s4p] functions forheavier atoms which were designed for calculations of Rydberg
states by Dunning and Hay.2' Exterior electron densities (EED)
for occupied molecular orbitals were calculated with our program.
The repulsive molecular surfaces were estimated from the conventional van der Waals radii of the atoms (rc = 1.70 A, rH =1.20 A, rN = 1.50 A). Electron densities outside the molecularsurface are counted for a cubic lattice with a lattice parameterd . The size of d was suitably chosen as indicated below. Exterior
regions were divided into several parts with a thickness of 0.5 8,(from the inside region I, 11, 111, IV, etc.) to see the convergencecharacter. Contributions from the regions farther than 2.0 A fromthe molecular surface were found to be negligible for the HDD2G
basis set even though it includes very diffuse primitive GTO's.
For the 4-31G basis set, contributions due to the region IV (13 2 . 0A) were also found to be negligible. The lattice parameter d waschosen as follows in order to save computation time without losing
accuracy; d = 0.05 8, for the region I, d = 0.10 8, for the region
11, d = 0.30 A for the regions I11 and IV. Electron density contour
plots were drawn for some molecular orbitals. The electron density
a ~ - where~ , kof the nth line from the outside is k X 2" X
is a parameter. In the electron density maps, thick solid curves(12) Hoffmann, R.; Heilbronner, E.; Gleiter, R. J . A m . Chem. SOC.1970,
92, 706-707.
(13) Broglie, F.;Heilbronner, E.; Ipaktschi, J. Helc. Chim. Acra 1972, 55,
2447-245 1.
(14) Heilbronner, E.;Martin, H. D. Helu. Chim. Acta 1972, 55,
1490-1502.
(15) Heilbronner, E.; Muszkat, K. J . A m . Chem. SOC. 1970, 92,
38 18-3821.
.
~
~
~
.
(16) Ohsaku, M.; Imamura, A,; Hirao, K.; Kawamura, T. Tetrahedron
1979, 35, 701-706.
(17) Harada, Y.; Ohno, K.; Mutoh, H. J . Chem. Phys. 1983, 79,~~

3251-3255.
.
-.
. -..

(18) Kosugi, N.Program GSCFZ, Program Library, The Computer Center,
The University of Tokyo, Tokyo, Japan 198 1.
(19) Ditchifield, R.; Hehre, W. J.; Pople, J. A. J . Chem. Phys. 1971, 54,
174-77R. - . . -- .
(20) Krishnan, R.;Binkley, J. S.; Seeger, R.; Pople, J. A. J . Chem. Phys.
1980, 72, 650-654.
(21) Dunning, T. H., Jr.; Hay, P. J. "Methods in Electronic Structure
Theory"; Schaefer, H. F., 111, Ed.; Plenum: New York, 1977; pp 1-27.

Ohno et al.

8084 J . Am. Chem. Soc., Vol. 107, No. 26, 1985
Table I. Data for Norbornadiene

4-31G
8.484
9.499
12.41712.65613.124
13.633

I PCSlCd/ eVd
HDD2G
8.725
9.729
12.62912.88813.34313.83914.251
14.17515.26714.48414.392
15.486

2.142
2.652

indicate repulsive molecular surface. In order to compare relative
PIES band intensities with calculated EED values, theoretical
spectra were synthesized from Gaussian-type bands with area
proportional to the respective EED values. Band positions andbandwidths were estimated from the observed spectra, since
theoretical calculations of these quantities were not of interestin the present study. Although 4-31G calculations were foundto be satisfactory for hydrocarbon molecules,* electron density
contour plots and theoretical spectra in the present study wereobtained from calculations with the HDD2G basis set which was
found to be more reliable for molecules containing l o n e - p a i r ~ . ~ ~ ~ ~ ~, l o ,band

IPobSd/eVb

MOc

1

8.71

6b2

2
3
4
5

9.56
11.34

loa,
6bl

11.79
12.14

5b2

6
7

12.37
12.70
13.15
14.18
15.68
16.44

8

3a~
sa,
5b1
4b2

EED" (%)
HDD2G
4.705
5.772
1.733
1.571
1.645
2.009
2.061
2.227
2.267

4-31G
2.943
3.842
1.571

I(PIES)P
1.000
1.334

2.21 1
2.742
2.260

2.307
17.65717.900
1.974
1.946
11
4b1
18.33318.529
1.945
1.903
12
17.14
7al
18.93819.1712.596
2.609"Exterior electron density. The EED is defined for each MO by the total electron density outside the repulsive molecular surface (see text).
bobserved vertical IP from UPS. CCorrespondingmolecular orbital. dTheoreticalIP from ab initio MO calculations with 4-31G and HDD2G basis
sets. Cobservedrelative PIES band intensity.
9

10

3b2

Results and Discussion
1. Through-Space Interactions in Norbornadiene. In Figure
1 He I UPS and He* (23S) PIES for norbornadiene are shown
together with a theoretical spectrum for PIES synthesized fromexterior electron densities (EED). Bands are labeled by numbers
from the left and assigned to relevant molecular orbitals fromwhich electrons are ejected upon ionization. Table I lists vertical
IP values obtained from the UPS and calculated IP values obtained
from M O calculations via Koopmans' approximation. Band 1 isassigned to the 6b2 orbital (ra)made of an antisymmetric couplingof P electrons on two separated double bonds. Band 2 is assigned
to the loa, orbital (a,)composed of a symmetric coupling of aelectrons. The through-space interaction involving in-phase
coupling of the p orbitals in the endo face causes a considerable
stabilization of the symmetric MO (P~).Heilbronner et al.lWl3
discussed the relative order of the two types of P orbitals fornorbornadiene and its analogues. They concluded from comparatixe studies of observed I P values that for norbornadiene thehighest occupied M O ( H O M 0 ) is of antisymmetric type. The
present study of PIES gives a confirmation of this ordering fromdifferent aspects of these molecular orbitals.
It is of note that in the PIES band 2 corresponding to the next
HOMO (P,)is enhanced with respect to band 1 which is due tothe integrated band area is muchionization from the HOMO (ra);larger for band 2 in PIES (Table I). This indicates that ionization
caused by electrophilic attacks of metastable helium atoms is more
likely to occur at the symmetric P orbital (loa.,) rather than theantisymmetric one (6bz). The relative reactivities for these orbitals
are qualitatively understood from the electron density contour plots
shown in Figure 2; exterior electron distributions outside the,repulsive molecular surface for the symmetric orbital ( P ~loa,)
are much larger than those for the antisymmetric orbital (pa,6b2).
Since metastable helium atoms attack exterior electrons outside
the molecular surface, electrons are much more likely to be extracted from the symmetric orbital. This nature of the reactive
electron distributions in the endo face for the next HOMO (rSloa,) may be connected with a formation of metal carbonylcompounds, such as n o r b ~ r n a d i e n e - F e ( C O ) ~ . ~ ~,I, P, / e, V ,

,

,

,

20

UPS
ELECTRON ENERGY / e V

1050ELECTRON ENERGY / eV
Figure 1. He* (2%) Penning ionization electron spectrum (PIES, bottom), theoretical Penning spectrum calculated from EED (calcd, middle),
and He I photoelectron spectrum (UPS, top) for norbornadiene.
The more quantitative analyses can be made for the PIES on
the exterior electron model;'-* the integrated band area or thePenning ionization probability has been supposed to be proportional to the exterior electron density (EED) integrated over theregion outside the molecular surface. Table I includes EED values
obtained for two types of basis sets. The EED values are shownor percent (%). In Figure 1, a synthesizedin the unit ofspectrum from the EED values (HDD2G) gave good agreementwith the observed PIES. This provides a confirmation of the~(22) Ohno, K.; Matsumoto, S . ; Harada, Y.J . Chem. Phys. 1984, 81,
2 183-2 184.
(23) Ohno, K.; Ishida, T. Int. J . Quantum Chem., to be published.

(24) King, R. B. "Organic Chemistry of Iron"; Koerner von Gustorf, E.
A,, Grevels, F.-W., Fishier, I. Eds.; Academic Press: New York, 1978; Vol.
I, pp 525-625.

J. x

Penning Ionization Electron Spectroscopy

Chem. Soc., Vol. 107, No. 26, 1985 8085

Table 11. Data for 1,4-Diazabicyclo[2.2.2]octane
EED“ (%)

IPc,,CdleVd
band

IPOM/eVb

MO‘

4-31G

HDD2G

1
2
3
4
5
6

7.60
9.64
11.10
11.53
12.66
13.29
13.88
14.37
15.90
16.33
17.10

6al’

8.225
11.33512.30312.85314.1 1214.78515.49215.76018.10718.58619.3898.343
11.37812.42512.941
14.24314.90215.60015.84218.18818.69419.455

I
8
9

IO
11

4e”
lal”
5e‘
4e‘
3e”
1a i
Sa,‘
3e’
4a/

4-31G
2.892
3.259
2.363
1.710
1.904
2.494
1.792
2.022
2.515
2.017
1.868

I(PIES)e

HDD2G
3.546
4.250
2.396
1.730
1.823
2.460
1.792
2.039
2.483
1.996
1.927

1.000
1.754

“Exterior electron density. As for degenerate orbitals (e‘ or e”) the EED value for a single moiety is listed. bobserved vertical I P from UPS.
CCorresponding molecular orbital. dTheoretical I P from ab initio MO calculations with 4-31G and HDD2G basis sets. ‘Observed relative PIES
band intensity.

I P/eV

10

iIU
n

I%

20

E
J

ELECTRON ENERGY / eV

Calcd.

Figure 2. He* PIES and electron density contour plots of the antisym6b2) and the symmetric r orbital (rs,loa,) for
metric r orbital (rarnorbornadiene. The contour plots were drawn for a plane perpendicular
to the double bonds and included two carbon atoms on the double bonds.
The electron density of the nth line from the outside is 0.5 X 2n X lo4
a ~ - ~Thick.
solid curves in the maps indicate the repulsive molecularsurface.

conclusions given above from qualitative discussions. The EED
calculations also suggest that the level ordering of the 8al andthe 5bl orbitals may be reversed as indicated in Table I.
In addition, one must note again that the through-space interactions occur at the endo face where p orbitals for A electrons
on two separated double bonds overlap effectively. A different
situation is found for 1,4-cyclohexadiene. It also involves acyclic-diene structure, but the geometry is nearly flat so that itis difficult for the p orbitals on separated double bonds to overlap
effectively. This is consistent with an observation in the previous
study;s EED values as well as PIES band intensities were foundto be nearly the same for symmetric and antisymmetric T MO’sin 1,4-~yclohexadiene.
2. Through-Bond Interactions in 1,4-Diazabicyclo[2.2.2]octane(DABCO). In Figure 3 H e I UPS, He* (23S) PIES, and a

10

5
ELECTRON ENERGY / eV

Figure 3. He* (2%) Penning ionization electron spectrum (PIES, bottom), theoretical Penning spectrum calcualted from EED (calcd, middle),
and H e I photoelectron spectrum (UPS, top) for 1,4-diazabicyclo[2.2.2]octane.

theoretical spectrum from EED/HDDZG are shown from DABCO. The observed bands are assigned to molecular orbitals. Table
I1 lists the observed vertical IP, calculated IP, EED, and observed
band area for PIES. The bands 1 and 2 are assigned to symmetricand antisymmetric couplings of nitrogen lone-pair electrons, respectively; the HOMO is a symmetric nonbonding MO (ns,6a,’)and the next H O M O is an antisymmetric one (na, 5a2”).I5 Assignments of other bands can also be made on the basis of calculated IP values (Table 11).
In the PIES, an emphasis must be placed on the remarkableenhancement of band 2 with respect to band 1. This indicatesthat the next HOMO (n,) composed of an antisymmetric coupling8086 J . Am. Chem. SOC.,Vol. 107, No. 26, 1985n,6a;Figure 4. He* PIES and electron density contour plots of the symmetricnonbonding orbital (ns,6al’) and the antisymmetric nonbonding orbital(na, 5 a F ) for 1,4-diazabicyclo[2.2.2]octane. The contour plots weredrawn for a plane including a couple of nitrogen atoms as well as a C-C
single bond. The electron density of the nth line from the outside is 0.25
X 2” Xa d . Thick solid curves in the maps indicate the repulsive
molecular surface.

of nitrogen lone-pair is much more reactive than the HOMO (n,)
of symmetric type. This observation is qualitatively understoodfrom Figure 4 where electron densities for the n, orbital are much
more exposed to the exterior region than those for the n, orbital.
It is also seen in Figure 4 that through-bond interactions involving
the C-C bonds for the n, orbital result in a considerable reductionof exterior electron distributions on the nitrogen atoms. On theother hand for the n, orbital, such a through-bond interaction isforbidden from its symmetry requirement with a nodal planebisecting the C-C bonds. The theoretical spectrum obtained fromthe EED values (Figure 3) also demonstrates that the n, orbitalis more reactive than the n, orbital in Penning reaction. It is inaddition noted that band 4, which is clearly seen in the UPS, isalmost missing in the PIES. This is explained from the small EED

Ohno et al.
value for the corresponding orbital (lal”) (Table 11).
3. Concluding Remarks. Effects of through-space/throughbond interactions on electron distributions of molecular orbitals
were observed by the use of metastable helium atoms. In norbornadiene, a symmetric and constructive coupling between
geometrically separated x electrons on two double bonds causes
an augmentation of electron densities in the endo face. This effectof the through-space interactions results in the higher reactivityof the symmetric x orbital ( x , , loa,) upon electrophilic attacks
by metastable helium atoms. In DABCO, a symmetric coupling
between a couple of lone-pairs having most of their electron
densities toward opposite directions involves bonding interactions
through the C-C bonds and yields a symmetric nitrogen-nonbonding orbital (ns, 6al’) with relatively small electron densities
on the nitrogen atoms. This leads to the observation that the next
HOMO (na,5aF) of antisymmetric type without the through-bondeffect shows a larger reactivity than the HOMO (n,, 6a,’) ofsymmetric type.
It is worthy to note that the orbital symmetry is not necessarily
coherent with the orbital activity in Penning reaction; in norbornadiene a, > x,, while in DABCO n, > n,. One may deduce
the following principles from the present results. (1) When athrough-space interaction causes a constructive superposition of
localized orbitals, it pumps up inner-electron densities to resultin an enhancement of exterior-electron densities for the symmetric
MO. This is highly likely because the spatial region, where thethrough-space interaction is most effective, is usually the reactive
region outside the repulsive molecular surface. (2) When athrough-bond interaction introduces a strong bonding character
between certain bonded atoms, an absorption of exterior-electron
densities into the inner inactive region occurs for the symmetric
MO. This is quite natural since the through-bond interaction is
only operative inside the repulsive molecular surface.
These findings carry two important messages: (i) the conceptof through-space/through-bondinteractionslOJ is of great importance to discuss spatial distributions of molecular orbitals,
especially for systems having lone-pairs, nonconjugated T electrons,or some functional groups and (ii) change in electron distributions
as well as reactivity can be probed for individual occupied molecular orbitals experimentally by the use of He* (23S)reactions.
Finally one may comment on the basis-set dependence of thecalculations. In a qalitative understanding, a middle size basis
set (4-31G) was found to be satisfactory. From a quantitative
point of view, theoretical spectra synthesized from the EED values
gave much better agreement with experiment when the more
extended basis set was used. This indicates that the PIES technique combined with the EED model may be used to study thequality of wave function tails which depend sensitively on the
choice of basis functions in the ab initio MO calculations.

Acknowledgment. We thank Professor Y . Harada for helpfulcomments and Dr. S. Masuda for his help in the experiments.