Long-Range σ-π Interactions in Tetrahydro-4H-thiopyran End-Capped Oligo(cyclohexylidenes). Photo-Electron Spectroscopy, ab Initio SCF MO Calculations, and Natural Bond Orbital Analyses

Article abstract of DOI:10.1021/jo000199y

Long-range σ-π interactions in tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes) were identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G* calculations. The vertical ionization energies I_v,j of the highest occupied molecular orbitals (MO's) were assigned using Koopmans' theorem (I_v,j = -∈_j) and by correlation with the ionizations of related reference compounds. The experimental (PES) and theoretical (RHF/6-31G*) results are in good agreement. For tercyclohexylidene derivatives which contain two nonconjugated π-bonds splittings ΔI_v,j of the π-bands in the range from ～0.5 to 0.7 eV (Δ-∈ ～0.6 to 0.9 eV). For the bi- and tercyclohexylidene compounds containing two sulfur atoms at their α- and ω-end positions the π-type sulfur lone pair bands [Lp_π(S)] split significantly by ΔI_v,j ～0.3 to 0.4 eV (Δ-∈_j ～0.3 to 0.4 eV), i.e. σ-π interactions over distances of ca. 8 and 12 A, respectively, occur. The magnitude of the interactions and the observed splittings are independent of the anti and syn conformations of the oligo(cyclohexylidene) hydrocarbon skeletons. RHF/6-31G* Natural Bond Orbital analyses reveal that the H_ax-C-C-H_ax precanonical MO's (PCMO's) centered on the cyclohexyl-type rings are paramount for the relay of the through-bond σ-π interactions; no through-space σ-π interactions were identified.

Full text of DOI:10.1021/jo000199y

4584
J . Org. Chem. 2000, 65, 4584-4592
Lon g-Ra n ge σ-π In ter a ction s in Tetr a h yd r o-4H-th iop yr a n
En d -Ca p p ed Oligo(cycloh exylid en es). P h oto-Electr on
Sp ectr oscop y, a b In itio SCF MO Ca lcu la tion s, a n d Na tu r a l Bon d
Or bita l An a lyses
Albert W. Marsman,† Remco W. A. Havenith,†,‡ Sabine Bethke,§ Leonardus W. J enneskens,*,†
Rolf Gleiter,§ J oop H. van Lenthe,‡ Martin Lutz,| and Anthony L. Spek|
Debye Institute, Department of Physical Organic Chemistry, Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands, Debye Institute, Theoretical Chemistry Group, Utrecht University,
Padualaan 8, 3584 CH Utrecht, The Netherlands, Organisch-Chemisches Institut der Universita¨t
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany, and Bijvoet Center for Biomolecular
Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands
jennesk@chem.uu.nl.
Received February 12, 2000
Long-range σ-π interactions in tetrahydro-4H-thiopyran end-capped oligo(cyclohexylidenes) were
identified by He(I) photoelectron spectroscopy (PES) and ab initio RHF/6-31G* calculations. The
vertical ionization energies Iv,j of the highest occupied molecular orbitals (MO’s) were assigned
using Koopmans’ theorem (Iv,j ) -ꢀj) and by correlation with the ionizations of related reference
compounds. The experimental (PES) and theoretical (RHF/6-31G*) results are in good agreement.
For tercyclohexylidene derivatives which contain two nonconjugated π-bonds splittings ∆Iv,j of
the π-bands in the range from ∼0.5 to 0.7 eV (∆-ꢀj ∼0.6 to 0.9 eV). For the bi- and tercyclohexylidene
compounds containing two sulfur atoms at their R- and ω-end positions the π-type sulfur lone pair
bands [Lpπ(S)] split significantly by ∆Iv,j ∼0.3 to 0.4 eV (∆-ꢀj ∼0.3 to 0.4 eV), i.e. σ-π interactions
over distances of ca. 8 and 12 Å, respectively, occur. The magnitude of the interactions and
the observed splittings are independent of the anti and syn conformations of the oligo(cyclohexy-
lidene) hydrocarbon skeletons. RHF/6-31G* Natural Bond Orbital analyses reveal that the
Hax-C-C-Hax precanonical MO’s (PCMO’s) centered on the cyclohexyl-type rings are paramount
for the relay of the through-bond σ-π interactions; no through-space σ-π interactions were
identified.
In tr od u ction
Oligo(cyclohexylidenes) containing a functionality at
their R- and/or ω-termini were recently proposed as novel
molecular building blocks for supramolecular and func-
tional materials.4 Since their hydrocarbon skeleton con-
sists of cyclohexyl-type rings interconnected via a sp2
hybridized olefinic bond, an issue that remains to be
addressed is to what extent these frameworks can
mediate electronic interactions between functionalities
incorporated at their R- and ω-positions. It is noteworthy
that the He(I) photoelectron (PE) spectrum of the parent
compound 1,1′-bicyclohexylidene (5, Chart 1) indicated
that its olefinic unit interacts with σ-orbitals that are
centered on the cyclohexyl-type rings.5,6 Recently, ab
initio (RHF/6-31G) calculations confirmed this inter-
pretation and enabled the assignment of the orbitals
It is well established that ground-state electronic
interactions in nonconjugated π-systems can occur either
through-space (TS) or through-bond (TB).1 Whereas TS
interactions are predominantly governed by the orienta-
tion and spatial separation of the interacting function-
alities, in the case of TB interactions the number as well
as the conformation of intervening sp3 hybridized carbon-
carbon bonds play an important role. TB interactions
have been identified in compounds in which π-systems
are separated by, for example, either a single sp3 hybrid-
ized carbon atom (homoconjugation)2 or an odd number
of sp3 hybridized carbon-carbon bonds, viz. aliphatic
chains or bonds incorporated in ring and cage systems.3
* To whom correspondence should be addressed. Tel.: +31 302533128.
Fax.: +31 302534533.
(4) (a) Hoogesteger, F. J .; Havenith, R. W. A.; Zwikker, J . W.;
J enneskens, L. W.; Kooijman, H.; Veldman, N.; Spek, A. L. J . Org.
Chem. 1995, 60, 4375. (b) Hoogesteger, F. J .; Kroon, J . M.; J enneskens,
L. W.; Sudho¨lter, E. J . R.; de Bruin, T. J . M.; Zwikker, J . W.; ten
Grotenhuis, E.; Maree´, C. H. M.; Veldman, N.; Spek, A. L. Langmuir
1996, 12, 4760. (c) ten Grotenhuis, E.; Marsman, A. W.; Hoogesteger,
F. J .; van Miltenburg, J . C.; van der Eerden, J . P.; J enneskens, L. W.;
Smeets, W. J . J .; Spek, A. L. J . Cryst. Growth 1998, 191, 834 and
references cited. (d) Marsman, A. W.; Leusink, E. D.; Zwikker, J . W.;
J enneskens, L. W.; Smeets, W. J . J .; Spek, A. L. Chem. Mater. 1999,
11, 1484.
† Department of Physical Organic Chemistry, Utrecht University.
‡ Theoretical Chemistry Group, Utrecht University.
§ Organisch-Chemisches Institut der Universita¨t Heidelberg.
| Bijvoet Center for Biomolecular Research, Crystal and Structural
Chemistry, Utrecht University.
(1) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1. Hoffmann, R.; Ima-
mura, A.; Hehre, W. J . J . Am. Chem. Soc. 1968, 90, 1499.
(2) Winstein, S. Spec. Publ. Chem. Soc. 1967, 21, 5. Warner, P. M.
In Topics in Nonbenzenoid Aromatic Chemistry; Hirokawa Publishing
Co.: Tokyo, 1977; Vol. 283. Paquette, L. A. Angew. Chem., Int. Ed.
Engl. 1978, 17, 100.
(3) Gleiter, R.; Scha¨fer, W. Acc. Chem. Res. 1990, 23, 369. Gleiter,
R.; Kissleer, B.; Ganter, C. Angew. Chem., Int. Ed. Engl. 1987, 26,
1252.
(5) Allan, M.; Snyder, P. A.; Robin, M. B. J . Phys. Chem. 1985, 89,
4900.
(6) Heilbronner, E.; Honegger, E.; Zambach, W.; Schmitt, P.; Gu¨nther,
H. Helv. Chim. Acta 1984, 67, 1681.
10.1021/jo000199y CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/24/2000

σ-π Interactions in End-Capped Oligo(cyclohexylidenes)
J . Org. Chem., Vol. 65, No. 15, 2000 4585
Ch a r t 1. In vestiga ted Oligo(cycloh exylid en es)
Sch em e 1. Syn th esis of 2 via th e Mod ified
Ba r ton -Kellogg P r oced u r e11
Ch a r t 2. Refer en ce Com p ou n d s
Sch em e 2. Syn th esis of 4 via a
Deca r boxyla tive-Deh yd r a tion Rou te
involved.7 Furthermore, it was shown for 1,1′;4′,1′′-
tercyclohexylidene (6, Chart 1), viz. the next higher
homologue, and the reference compounds 1,4-dimeth-
ylidenecyclohexane (8, Chart 2), for which He(I) photo-
electron spectroscopy (PES) results are available,8 as well
as 1,4-diisopropylidenecyclohexane (9, Chart 2) that the
two π(CdC) units are nondegenerate. Due to TB interac-
tions the π-type HOMO and HOMO-1 are split by ca.0.6
eV. A similar splitting between remote olefinic substit-
uents was identified in the PE spectrum of stella-2,6-
diene.9
Resu lts a n d Discu ssion
Syn th eses. 4-(Cyclohexylidene)tetrahydro-4H-thiopy-
ran (1) and 4-(4-(cyclohexylidene-cyclohexylidene))tet-
rahydro-4H-thiopyran (3, Chart 1) were prepared using
previously reported procedures.4a 4,4′-(Tetrahydro-4H-
thiopyran-ylidene) (2) was obtained in four steps using
a 2-fold extrusion methodology.4a,11 Reaction of 2 equiv
of tetrahydro-4H-thiopyran-4-one (12) with H2NNH2‚H2O
gave azine 13, which was treated with H2S(g) to give
thiadiazolidine 14. The latter was converted into thia-
diazoline 15 by oxidation. Subsequently, extrusion of N2
by heat treatment and with the aid of triethyl phosphite
from 15 gave 2 (Scheme 1 and Experimental Section).
4-(Tetrahydro-4H-thiopyran-4-cyclohexylidene-4′-ylidene-
)tetrahydro-4H-thiopyran (4) was prepared using similar
procedures as for the syntheses of 1 and 3. Tetrahydro-
4H-thiopyran-4-carboxylic acid (16)12 was coupled with
4-(tetrahydro-4H-thiopyran-4-ylidene)cyclohexanone, viz.
an intermediate product in the synthesis of 3,4 furnishing
â-hydroxy acid 17. The latter was converted into 4 by
treatment with N,N-dimethylformamide dineopentyl
acetal (Scheme 2 and Experimental Section).
Here we report that in tetrahydro-4H-thiopyran end-
capped oligo(cyclohexylidenes) (1-4, Chart 1) σ-π inter-
actions occur between the 3p-sulfur lone pairs [Lpπ(S)]
positioned at the R and/or ω termini and both σ/π-orbitals
located on the oligo(cyclohexylidene) hydrocarbon skel-
eton; i.e., those involving the olefinic bonds and σ-MO’s
of appropriate symmetry centered on the cyclohexyl-type
rings. In the case of 2 and 4 these σ-π interactions are
mediated over distances of ca. 8 and 12 Å! The occurrence
of TB interactions was unequivocally established by
measurement of the He(I) PE spectra of oligo(cyclohexy-
lidenes) 1-7. Vertical ionization energies (Iv,j) of 1-7
were assigned by band correlation with He(I) PES data
of the reference compounds 8, 10, and 11 (Chart 2). The
empirical assignments for 1-7 were substantiated by
comparison of the Iv,j values with RHF/6-31G* MO
energies (ꢀj) of the canonical molecular orbitals (CMO’s)
of each compound applying Koopmans’ theorem (Iv,j
)
-ꢀj).10 Finally, RHF/6-31G* natural bond orbital (NBO)
analyses were executed to identify TS and TB contribu-
tions.
Gr ou n d -Sta te Con for m a tion a l P r op er ties of 1-6.
In line with the behavior of the reference compounds 8
(7) Havenith, R. W. A.; van Lenthe, J . H.; J enneskens, L. W.;
Hoogesteger, F. J . Chem. Phys. 1997, 225, 139.
(8) Heilbronner, E.; Schmelzer, A. Helv. Chim. Acta 1975, 58, 936.
(9) (a) Gleiter, R.; Lange, H.; Borzyk, O. J . Am. Chem. Soc. 1996,
118, 4889. (b) Lange, H.; Gleiter, R.; Fritzsche, G. J . Am. Chem. Soc.
1998, 120, 6563.
(11) (a) Barton, D. H. R.; Willis, B. J . J . Chem. Soc, Perkin Trans.
1 1972, 305. (b) Buter, J .; Wassenaar, S.; Kellogg, R. M. J . Org. Chem.
1972, 3, 7, 4045.
(12) Stra¨ssler, S.; Linden, A.; Heimgartner H. Helv. Chim. Acta
1997, 80, 1528.
(10) Koopmans, T. Physica 1934, 1, 104.

4586 J . Org. Chem., Vol. 65, No. 15, 2000
Marsman et al.
F igu r e 3. Displacement ellipsoid plot of 2 (50% probability
level; symmetry operation′: 1 - x, 1 - y, 1 - z).
F igu r e 1. Schematic representation of the different possible
conformers of 1-6.
F igu r e 2. He(I) photoelectron spectra of anti-7 and syn-7.
and 9 (Chart 2),13 temperature-dependent 1H NMR
(CDCl3) spectroscopy showed that the cyclohexyl-type
rings of 1-6 are conformationally mobile in solution (see
Experimental Section).4 The interconnected chairlike,
cyclohexyl-type rings can adopt either an anti or syn
conformation with respect to each other (Figure 1). Ab
initio calculations revealed that anti-5 and syn-5 are both
genuine minima and possess a nearly identical total
energy [RHF/6-31G*; ∆Etot ) Etot(syn) - Etot(anti) -0.007
kcal/mol]; both in the gas phase and in solution an
equilibrium mixture of the conformers (ratio ca. 1:1) is
expected.14 It is noteworthy that the ꢀj values as well as
the character of the highest occupied MO’s of anti-5 and
syn-5 are virtually identical. This was verified by a
comparison of the He(I) PE spectra of anti- (anti-7) and
syn-4,4′-tert-butyl-1,1′-bicyclohexylidene (syn-7), viz. con-
formationally stable derivatives of 5 [7; ∆Etot ) Etot(syn)
- Etot(anti) -0.088 kcal/mol, Chart 1 and Figure 2].15
Their PE spectra are similar and closely resemble that
of 5 (Figures 2 and 4 and Table 2).5 These results suggest
that also in the case of 1-4 and 6 the He(I) PE spectra
will originate from mixtures of conformers (1 and 2; anti-
and syn-3;16 anti/anti, syn/anti, anti/syn and syn/syn, and,
4 and 6; anti/anti, anti/syn and syn/syn, Figure 1). To
establish if a similar conformational independence holds
for the ꢀj values and the character of the highest occupied
MO’s of the different conformers of 1-4 and 6, their
conformers were studied using ab initio methods (see
Experimental Section). Since 1-4 contain sulfur, the use
F igu r e 4. He(I) photoelectron spectra of 5 and 6.
of polarization functions is required. Consequently, all
conformers were optimized with a RHF/6-31G* basis set.
The results obtained for the distinct conformers of each
compound, which were all shown to be genuine minima,
revealed that the ꢀj values and the character of the
highest occupied MO’s were virtually identical. This is
exemplified for 1 and 2 in Table 3 and 6 in Table 2 (for
3 and 4 see Table 1 of the Supporting Information). For
each compound, the ∆Etot values of each conformers with
respect to that with the lowest Etot are less than 0.15 kcal/
mol. Hence, in the gas phase, mixtures of conformers are
expected. Since the anti and anti/anti conformer of 5 and
6, respectively, were previously studied at the RHF/
6-31G level of theory,7 they were also re-optimized with
a 6-31G* basis set; similar results were obtained. In
addition, for 6 its anti/syn and syn/syn conformers were
also calculated at the 6-31G* level of theory. For all
conformers again nearly identical results were obtained
(Table 2).
(13) Lautenschla¨ger F.; Wright; G. F. Can. J . Chem. 1963, 41, 1972.
Lambert, J . B. J . Am. Chem. Soc. 1967, 89, 67.
(14) Havenith, R. W. A.; J enneskens, L. W.; van Lenthe, J . H., Chem.
Phys. Lett. 1998, 282, 39.
The effect of basis set size was assessed for anti-1, anti-
2, and syn-2, i.e. the calculations were repeated with a
6-311G** basis set. Although the RHF/6-311G** ꢀj
values are ca. 0.06 eV larger than the corresponding
(15) Due to the preference of the tert-butyl substituents to occupy
an equatorial position both anti-7 and syn-7 are conformationally
stable.11b
(16) The first designated bicyclohexylidene unit contains the sulfur
atom.

σ-π Interactions in End-Capped Oligo(cyclohexylidenes)
J . Org. Chem., Vol. 65, No. 15, 2000 4587
Ta ble 1. Sa lien t F ea tu r es of th e Sin gle-Cr ysta l X-r a y a n d RHF /6-31G* (6-311G**) Str u ctu r e of a n ti-2
single-crystal
X-ray structure
RHF/6-31G* (6-311G**)
structure
structural parameter
bond lengths (Å)
C4-C4′
1.343(3)
1.5167(19)/1.5123(18)
1.528(2)/1.526(2)
1.334 (1.333)
1.520 (1.520)
1.534 (1.533)
1.815 (1.820)
C2-C4/C5-C4
C2-C3/C5-C6
C3-S1/C6-S1
1.8067(15)/1.8071(15)
valence angles (deg)
C3-S1-C6
97.16(7)
111.48(11)
112.57(12)/113.28(12)
123.95(15)/124.57(15)
112.33(10)/112.40(10)
98.5 (98.6)
C2-C4-C5
110.9 (111.0)
112.1 (112.3)
124.6 (124.5)
113.0 (112.9)
C3-C2-C4/C4-C5-C6
C2-C4-C4′/C5-C4-C4′
C2-C3-S1/C5-C6-S1
dihedrals angles (deg)
C2-C4-C4′-C5′
-0.4(3)
180.0(7)
60.79(16)/-60.45(15)
-118.91(19)/119.2(2)
-62.76(14)/61.74(14)
55.11(11)/-54.42(11)
-0.34 (-1.20)
-180 (-180)
64.1 (64.0)
-114.5 (-114.9)
61.4 (61.3)
C2-C4-C4′-C2′
C3-C2-C4-C5/C6-C5-C4-C2
C3-C2-C4-C4′/C6-C5-C4-C4′
C4-C2-C3-S1/C4-C5-C6-S1
C6-S1-C3-C2/C5-C6-S1-C3
51.1 (51.0)
a
For atom numbering see figure 3.
Ta ble 2. P ES Ver tica l Ion iza tion En er gies Iv,j (eV),
RHF /6-31G* Or bita l En er gies - Ej (eV), a n d Th eir
6-31G* and RHF/6-311G** calculations satisfactorily
reproduce the single-crystal X-ray structural data (Table
1 and Figure 3).18,19
Assign m en ts of a n ti-5 (C2h ), a n ti/a n ti-6 (C2h ), a n ti-7 (C2h ),
a -c
a n d syn -7 (C2v
)
P h otoelectr on Sp ectr oscop y of 1-6. To establish
if in the case of 1-6 ground-state interactions are relayed
via the oligo(cyclohexylidene) σ-π hydrocarbon skeleton,
their He(I) PE spectra were measured (Figures 4-7). The
recorded vertical ionization energies (Iv,j) are collected in
Tables 2 and 3. Empirical assignment of the Iv,j values
below ca. 10 eV, which are separated from the high
energy σ-MO manifold, was achieved by band-shape
analysis as well as band correlation between structurally
related derivatives.
To discuss the PE spectra of 1-6 we first focus on those
of 8, 10, and 11 to estimate the ionization energies of
the unperturbed 3p-sulfur lone pair [Lpπ(S)] (11) and the
exo-cyclic olefinic bond (10) as well as the interaction of
two double bonds in 1,4-dimethylidenecyclohexane (8).
The PE spectrum of 10 shows one band at 9.13 eV with
vibrational fine structure (ν ≈ 1330 cm-1).20 It is assigned
to ionization from the π-orbital. This band is shifted
approximately 1 eV toward lower energy in the PE
spectrum of 5 (8.16 eV, Figure 4).5 The vibrational fine
structure (ν ≈ 1450 cm-1) is similar to that found for 10.
In the PE spectrum of 7 a further shift of the first band
toward lower energy was observed (Table 2). In the PE
spectrum of 8 two clearly separated bands at 9.0 and 9.4
eV have been reported.20 A similar gap (0.4-0.8 eV) was
also found for several bridged derivatives.21This splitting
between the two bands has been interpreted as being due
to a considerable TB interaction with the ribbon-type
orbitals of the six-membered ring.22
compd
anti-514
band j
Iv,j
-ꢀj
assignment
14bu
1
2
1
2
1
2
3
4
1
2
3
4
1
2
3
4
1
2
1
2
8.16
9.80
8.58
11.00
8.58
11.00
8.35
8.91
π
9bg
σ
syn-514
15a1 π
9a2 σ
anti/anti-6
7.88d
8.34
9.60
21ag π+
20bu π-
10.86
11.42
8.36
13bg
14au
σ
σ
10.2
anti/syn-6
syn/syn-6
41a′ π+
40a′ π-
27a” σ
8.91
10.86
11.42
8.37
26a” σ
21ag π+
8.91
20bu π-
10.87
11.42
8.56
13bg
14au
24bu
15bg
25a1
15a2
σ
σ
π
σ
π
σ
anti-7
syn-7
8.04
9.50
8.07
9.50
10.81
8.56
10.82
a
In addition, RHF/6-31G* -ꢀj (eV) values and their assignments
of syn-5 (C2v), anti/syn-6 (Cs), and syn/syn-6 (C2h) are presented.
Etot: anti-5 -466.064786 au, syn-5 -466.064797 au, anti/anti-6
-697.921586 au, anti/syn-6 -697.921597 au, syn/syn-6 -697.921608
au, anti-7 -778.304855 au, and syn-7 -778.304869 au (1 au )
627.52 kcal/mol). c ∆Etot ) Etot(syn) - Etot(anti): 5 -0.007 kcal/
mol and 7 -0.088 kcal/mol. For 6 ∆Etot ) Etot(syn/syn) - Etot(x):
anti/anti-6 -0.014 kcal/mol and anti/syn-6 -0.007 kcal/mol. Av-
erage value of the discernible peaks at 7.82 and 7.94 eV.
b
d
RHF/6-31G* values, the relative energies and the
character of the highest occupied MO’s do not change
(Table 3).
Sin gle-Cr yst a l X-r a y St r u ct u r e of 2. The RHF/
6-31G* geometries7 of anti-5 and anti-6 are in satisfactory
agreement with the single-crystal X-ray structural data
of 5 and trans-4,4′′-diheptyl-1,1′:4′,1′′-tercyclohexylidene,17
respectively. To verify that the structural features of 1-4
are adequately reproduced with both the 6-31G* and the
6-311G** basis set, the optimized geometries of anti-2
were compared with its single-crystal X-ray structure
(Ci; approximate C2h). As shown by the salient bond
lengths, valence angles and dihedral angles, the RHF/
(18) In the solid-state 2, like 5, possesses an anti-type structure with
crystallographic Ci and noncrystallographic, approximate C2h sym-
metry. Crystallographic details for 2 are presented in the Supporting
Information. Cf. also Veldman, N.; Spek, A. L.; Hoogesteger, F. J .;
Zwikker, J . W.; J enneskens, L. W. Acta Crystallogr. 1994, C50, 742.
(19) No bond length alternations are found in the sp3-sp3 carbon-
carbon bonds of the cyclohexyl-type rings of 2; cf. Baldridge, K. K.;
Battersby, T. R.; Vernonclark, R.; Siegel, J . S. J . Am. Chem. Soc. 1997,
119, 7048.
(20) Bieri, G.; Binger, F.; Heilbronner, E.; Maier, J . P. Helv. Chim.
Acta 1977, 60, 2213.
(21) Chou, T.-C.; Lange, H.; Gleiter, R.; Go¨gh, T.; Kriz, M.; Pfen-
ninger, R. H.; Valentiuy, M.; Ganter, C. Helv. Chim. Acta 1995, 28,
2011.
(17) Suitable single crystals for X-ray analysis could hitherto not
be obtained for 6.
(22) Hoffman, R.; Molle`re, P. D.; Heilbronner, E. J . Am. Chem. Soc.
1973, 95, 4860.

4588 J . Org. Chem., Vol. 65, No. 15, 2000
Marsman et al.
Ta ble 3. Ver tica l Ion iza tion En er gies Iv,j (eV), RHF /6-31G* Or bita l En er gies -Ej (eV) a n d Assign m en ts of a n ti-1 (Cs),
a -c
syn -1 (Cs), a n ti-2 (C2h ), syn -2 (C2v), a n ti/a n ti-3 (Cs), a n d a n ti/a n ti-4 (C2h
)
a
b
RHF/6-311G** values in parentheses. Etot: anti-1 -824.537362 au (RHF/6-311G** -824.658005 au), syn-1 -824.537353 au, anti-2
-1183.009415 au (RHF/6-311G** -1183.144416 au), syn-2 -1183.009285 au (RHF/6-311G** -1183.144179 au), anti/anti-3 -1056.394149
au and anti/anti-4-1414.866488 au (1 au ) 627.52 kcal/mol). c ∆Etot ) Etot(anti) - Etot(syn): 1 -0.006 kcal/mol and 2 -0.082 (-0.149)
kcal/mol.
In the PE spectrum of 6 we observe one broad feature
centered around 8 eV to which we assign ionizations out
of the two π-orbitals of 6 (Figure 4). Assuming a similar
vibrational fine structure as for 5 we ascribe the first two
peaks at 7.82 and 7.94 eV to the vibrational progression
of the first band, while the peak at 8.34 eV must
consequently be assigned to a second band. The splitting
of the two bands (0.46 eV) is in the same order of
magnitude as observed for 8 and its bridged congeners.21
In the PE spectrum of 1 two bands below 9 eV are
found at 8.16 and 8.63 eV (Figure 5). The second band is
sharper than the first one. We assign the first one to the
ionization from the π-orbital, the second one to the
ionization from the Lpπ(S) orbital. This assignment is
corroborated by comparing the PE spectrum of 1 with
that of 5 and of tetrahydro-4H-thiapyrane (11).23,24 In the
latter molecule the first ionization energy of the 3p-sulfur
lone pair was found at 8.45 eV. The three bands in the
PE spectrum of 2 at 8.27, 8.49, and 8.89 eV are assigned
to ionizations from the π-orbital and the two symmetry
adapted Lpπ(S) combinations bu and ag. This conclusion
is confirmed by comparing the PE data of 1, 2, and 5 as
shown in Figure 6.
The qualitative interpretation of the PE spectra of 3
and 4 (Figure 7) is less straightforward. The PE spectrum
of 3 shows two broad features centered around 8.3 and
8.65 eV. A comparison with the first ionization energies
of 6 and 1123,24 indicates that these two broad features
consists of three transitions (bands 1-3). In the PE
(23) Gleiter, R.; Spanget-Larsen, J . In Topics in Current Chemistry;
Springer-Verlag: Berlin Heidelberg, New York, 1979; 86, 139 and
references cited.
(24) Sweigart, D. A.; Turner, D. W. J . Am. Chem. Soc. 1972, 94,
5599.

σ-π Interactions in End-Capped Oligo(cyclohexylidenes)
J . Org. Chem., Vol. 65, No. 15, 2000 4589
F igu r e 5. He(I) photoelectron spectra of 1 and 2.
F igu r e 7. He(I) photoelectron spectra of 3 and 4.
F igu r e 8. Schematic correlation diagram of the first ioniza-
tion energies, Iv,j, of 3, 4, and 6. For detailed band assignments
(RHF/6-31G* calculations) see Tables 2 and 3.
F igu r e 6. Schematic correlation diagram of the first ioniza-
tion energies, Iv,j, of 1, 2, and 5. For detailed band assignments
(RHF/6-31G* calculations) see Tables 2 and 3.
PE spectrum of 1 (PES, 0.47 eV and RHF/6-31G*: anti-
1, 0.53 eV, syn-1, 0.52 eV). For 2 the measured energy
gaps between the first three bands (0.22 and 0.40 eV)
are reproduced fairly well by the calculations in both of
its conformations [anti-2 (C2h), 0.36 and 0.41 eV or syn-2
(C2v), 0.25 and 0.48 eV].
In the case of anti/ anti-3 (Cs) the calculation predicts
three bands positioned at 8.48, 8.88, and 9.29 eV in the
PE spectrum. Unfortunately, only a broad band is
observed. The occurrence of band overlap might be
rationalized by the presence of a mixture of conformers
in the gas phase at the recording temperature (125 °C).
For 4 the calculation predicts small energy differences
(ca. 0.3 eV) for the four highest occupied orbitals. Only
the very sharp peak (band 4; ∼8.6 eV) is clearly separated
from the first broad peak (Figure 7, Table 3 and see the
Supporting Information).
spectrum of 4 two peaks are observed below 9 eV; one
broad (8.0-8.5 eV) centered around 8.3 eV and a second
sharp one at ∼8.6 eV. A comparison with the PE data of
3 and 6 accordingly suggests assigning four transitions
to both peaks. The steep onset of the second peak (8.62
eV) suggests that it has to be ascribed to the ionization
from a Lpπ(S) orbital combination. In both the PE spectra
of 3 and 4 the bands of the individual transitions show
overlap, which is too strong to further discriminate. In
Figure 8 the first bands of 3, 4 and 6 are compared.
The assignment of the PE data of 1-6 presented in
Tables 2 and 3 was further supported by a comparison
of the experimental ionization energies (Iv,j) with calcu-
lated orbital energies by using the HF-SCF procedure
with a 6-31G* basis set25 and applying Koopmans’
theorem (Iv,j ) -ꢀj).10
An a lysis of σ-π In ter a ction s in 1-6. The electronic
interactions in 1-6 were further analyzed on the basis
of the concept of through-space (TS) and through-bond
The energy difference between the first two bands in
6 (0.46 eV) is reproduced quite well by the calculations
(0.56 eV). The same holds for the first two bands in the
(TB) interactions introduced by Hoffmann et al.;1
a

4590 J . Org. Chem., Vol. 65, No. 15, 2000
Marsman et al.
F igu r e 9. Through-space (TS) and through-bond (TB) inter-
action diagram of 6: NBO, self-energy of the π-NBO’s; TS,
energy after the TS interaction; TB1, energy after the TB
interaction with the σ-orbitals shown in Figure 11; CMO,
energy of the CMO’s.
methodology first suggested by Heilbronner and Schmelz-
er is used.8 For a quantitative treatment the procedure
of Imamura et al.26 is applied using Weinhold’s natural
bond orbitals27 (NBO’s) as a starting point. The analysis
is based on the Fock matrix in a localized basis of the
one-electron wave functions and comprises a transforma-
tion of the Hartree-Fock canonical molecular orbitals
(CMO’s) into a set of localized MO’s called natural bond
orbitals (NBO’s).27 Subsequently, diagonalization of a
sub-matrix of the nondiagonal NBO Fock matrix results
in a set of precanonical MO’s (PCMO’s), which possess
eigenvalues corresponding to the MO energies after
interaction of the NBO’s within that specific sub-matrix.
F igu r e 10. Through-space (TS) and through-bond (TB)
interaction diagram of 2 and 4: NBO, self-energy of the π-
and Lpπ(S) NBO’s; TS, energy after TS interaction; TB1, energy
after the interaction of either the π- or the Lpπ(S) NBO’s with
the σ-orbitals shown in Figure 11; TB2, energy after simul-
taneous interaction of the π- and Lpπ(S) NBO’s with the
σ-orbitals; CMO, energy of the CMO’s.
In the case of 5 the NBO analysis reveals a self-energy
for the π-orbital of -9.28 eV (not shown). This level is
significantly destabilized by interaction with a preca-
nonical MO (PCMO) comprising of four Hax-C-C-Hax
and the two C-Heq NBO’s of the cyclohexyl-type rings
(Figure 11). For the interaction of 6 the NBO analysis
reveals that TS interaction between both degenerate
π-NBO’s is minute; the TS splitting between the sym-
metry-adapted linear combinations of both π-NBO’s [π+
(ag) and π- (bu)]28 is negligible (∆-ꢀj 0.04 eV, Figure 9).
This is expected on the basis of their through-space
separation (3 Å)29 and is confirmed by ghost-atom cal-
culations.30 Like the π-NBO of 5, interaction with a
PCMO comprising six Hax-C-C-Hax and two C-Heq
NBO’s destabilizes the π+- and π- combinations of 6
(TB1) in such a way that π+ is placed on top of π- in
accordance with the occurrence of TB interactions over
an odd number of intervening sp3 hybridized carbon-
carbon bonds. A splitting of the resulting PCMO’s of ∆-ꢀj
0.54 eV is found, which is close to that of the related
CMO’s (∆Iv,j 0.46 eV and ∆-ꢀj 0.56 eV). Additional
admixture of other σ-NBO’s does not significantly affect
F igu r e 11. Schematic representation of the σ-orbitals con-
sidered in the through-bond interactions (TB1 and TB2).
this splitting.31 The NBO analyses show that TB σ-π
interactions indeed involve the Hax-C-C-Hax PCMO’s
(Figure 11). While the Hax-C-C-Hax PCMO responsible
for the splitting of the π-NBO’s of 6 is that positioned in
the middle cyclohexyl-type ring, those located in the outer
rings mainly destabilize their neighboring π-NBO. How-
ever in going from 5 and 6 to 1-4, the terminal (R and/
or ω) methylene units are replaced by sulfur atoms of
which the Lpπ(S) NBO’s possess a favorable energy and
(25) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213.
(26) Imamura, A.; Ohsaku, M. Tetrahedron 1981, 37, 2191.
(27) Reed, A. E.; Weinhold, F. J . Chem. Phys. 1983, 78, 4066. Reed,
A. E.; Weinstock, R. B.; Weinhold, F. J . Chem. Phys. 1985, 83, 735.
(28) The π+ combination is symmetrical with respect to the molec-
ular C2 axis.1
(29) Paddon-Row, M. N.; Wong, S. S.; J ordan, K. D. J . Chem. Soc.,
Perkin Trans. 2 1990, 425.
(30) RHF/6-31G calculations on a system consisting of two ethene
molecules located at the position of the π-bonds in the optimized
geometry of anti/ anti-6 augmented by ghost centers, viz. the proper
basis functions without nuclei on all other atomic positions discernible
for 6, show that under these conditions the π-bonds are nearly
degenerate.7
(31) Admixture of the anti-bonding PCMO’s of the Hax-C-C-Hax
units causes a stabilization of the PCMO’s towards the levels of the
CMO’s.

σ-π Interactions in End-Capped Oligo(cyclohexylidenes)
J . Org. Chem., Vol. 65, No. 15, 2000 4591
belong to the appropriate irreducible representation23 for
interaction with the outer ring Hax-C-C-Hax PCMO.
Hence, the Lpπ(S) NBO’s will perturb the MO topologies
of 5 and 6.
(C2h, Cs, or C2v) of the studied end-functionalized bi- and
tercyclohexylidenes.
Exp er im en ta l Section
The NBO analyses for 1-4 reveal different basis
orbitals for the localized π-unit and the Lpπ(S) NBO’s,
which is shown for 2 and 4 on the left side of Figure 10.
TS interactions between the Lpπ(S) and π-NBO’s is
insignificant in all cases (NBO/TS, Figure 10). In the case
of 2 and 4 no sizable splitting between the Lpπ(S) or the
π-orbitals in 4 is observed. This is in line with the TS
separation of the sulfur- and nearest π-bond carbon
atoms [d(S1‚‚‚C4) ) 3.2 Å, 2].32 The situation changes if
TB interaction occur (TB1 and TB2). In the case of 2 the
interaction of a PCMO comprising of four Hax-C-C-Hax
NBO’s of the σ-skeleton with either the Lpπ(S) NBO’s
or the π-NBO’s causes a considerable destabilization of
the 3p-sulfur lone pairs and the π-orbital (TB1 Figure
10). Subsequently, in the TB2 step, simultaneous inter-
action of the aforementioned units causes the Lpπ(S)
NBO’s to split. In the case of 4 interaction of either
the π- or Lpπ(S) NBO’s with a PCMO comprising of six
Hax-C-C-Hax NBO’s not only results in a destabilization
of the π- and 3p-sulfur lone pairs, but also in a splitting
of the symmetry adapted π-orbitals [ag(π+) and bu(π-)]
by 0.55 eV (Figure 10). In the next step (TB2) the
interaction between the lone pairs at sulfur and the
π-orbital(s) occurs. The splittings obtained after the TB2
step in 2 and 4 are about the same as found for the
canonical MOs (right side of Figure 10).
Gen er a l Meth od s. Melting points are uncorrected. NMR
spectra (1H 300.133 MHz and 13C 75.47 MHz) are recorded in
CDCl3 unless otherwise stated; chemical shifts (ppm) are
reported downfield from TMS. Raman spectra were recorded
for neat solids. Elemental analyses were performed by Dornis
U. Kolbe, Microanalytisches Laboratorium, Mu¨lheim a.d.
Ruhr, Germany. Dry THF and CH2Cl2 were prepared by
distillation from Na and CaCl2, respectively. CH3CN was dried
by storage on 3 Å molecular sieves.
Syn th eses. 4-(Cyclohexylidene)tetrahydro-4H-thiopyran (1)
and 4-(4-(cyclohexylidenecyclohexylidene)tetrahydro-4H-thio-
pyran (3)4a as well as anti- (anti-7) and syn-4,4′-tert-butyl-1,1′-
bicyclohexylidene (syn-7)11b were synthesized according to
literature procedures.
4,4′-Bis(tetr a h yd r o-4H-th iop yr a n ylid en e) (2). A solu-
tion of 15 (3.39 g, 13.0 mmol) and triethyl phosphite (12.0 mL,
70 mmol) in toluene (200 mL) was heated at reflux tempera-
ture for 24 h. After cooling of the reaction mixture, the solvent
was removed in vacuo and the solid residue triturated with
CH3OH (25 mL). The solid was filtered off and, subsequently,
sublimed (80 °C, 0.1 Torr) to give pure 2 (1.93 g, 0.63 mmol,
74%): mp 144.1 °C; 1H NMR δ 2.55-2.59 (m, 8H), 2.63-2.67
(m, 8H) ppm; 13C NMR δ 30.6, 31.9, 129.6 ppm; Raman ν 2958,
2945, 2891, 2838, 1661, 1652, 1643, 1453, 1424, 1420 cm-1
.
Anal. Calcd for C10H16S2: C, 59.98; H, 8.06; S, 31.96. Found:
C, 60.11; H, 8.01.
4-(Te t r a h yd r o-4H -t h iop yr a n -4-cycloh e xylid e n e -4′-
ylid en e)t et r a h yd r o-4H-t h iop yr a n (4). A suspension of
â-hydroxy acid 17 (0.280 g, 0.63 mmol) and N,N-dimethylfor-
mamide dineopentyl acetal (0.58 g, 2.5 mmol, 0.7 mL) in dry
CH3CN (10 mL) was heated at reflux temperature under an
inert N2 atmosphere overnight. The resulting white precipitate
was filtered off and recrystallized form hot CHCl3 (40 mL) to
afford pure 4 (0.150 g, 0.54 mmol, 62%): mp 213.9 °C; 1H NMR
The NBO analyses for the less symmetric cases 1 and
3 yield analogous results (not shown). The TB interaction
is mainly due to the PCMO’s shown in Figure 11. After
interaction of the semilocalized π- and Lpπ(S) orbitals the
energy differences are close to those calculated for the
canonical MO’s.
δ 2.25 (s, 8H), 2.53-2.58 (m, 8H), 2.62, 2.67 (m, 8H) ppm; 13
C
NMR δ 28.9, 30.6, 32.0, 127.7, 130.6 ppm. Anal. Calcd for
C
16H24S2: C, 68.52; H, 8.62; S, 22.86. Found: C, 68.40; H, 8.70.
Tetr a h yd r o-4H-th iop yr a n -4-on a zin e (13). To a solution
Con clu sion s
of tetrahydro-4H-thiopyran-4-one (12, 5.00 g, 43.0 mmol) in
CH3CH2OH (125 mL) heated at reflux temperature was added
dropwise to a solution of H2NNH2‚H2O (>99%, 105 mL, 21.6
mmol) in CH3CH2OH (50 mL). The reaction mixture was
heated at reflux temperature overnight. After cooling to room
temperature and removal of solvent in vacuo pure 13 was
obtained (4.90 g, 21.5 mmol, 100%): 1H NMR δ 2.66-2.87 (m,
16H) ppm; 13C NMR δ 29.1, 30.2, 30.3, 37.5, 163.5 ppm.
3,7,11-Tr it h ia -14,15-d ia za d isp ir o[5.1.5.2]p e n t a d e c-
a n e (14). A solution of 13 (4.90 g, 21.5 mmol) in CH3CN (150
mL) was stirred under a H2S(g) atmosphere (balloon) for 5
days. After evaporation of the solvent in vacuo pure 14 was
obtained as a light yellow solid (5.54 g, 21.1 mmol, 98%): 1H
NMR δ 2.01-2.13 (m, 8H), 2.65-2.82 (m, 8H), 3.99 (s, 2H)
ppm; 13C NMR δ 27.5, 40.5, 85.4 ppm.
3,7,11-Tr ith ia -14,15-d ia za d isp ir o[5.1.5.2]p en ta d ec-14-
en e (15). Compound 14 (5.54 g, 21.1 mmol) was oxidized with
a mixture of Pb(OAc)4 (13.0 g, 29.3 mmol) and CaCO3, (13.0 g,
0.13 mol) in dry CH2Cl2 (160 mL).4a After work up a brown
solid (5.87 g) was obtained, which was recrystallized from hot
ethyl acetate to yield pure 15 (3.39 g, 13.0 mmol, 62%):. 1H
NMR δ 1.88-1.95 (m, 4H), 2.64-2.93 (m, 12H) ppm; 13C NMR
δ 26.4, 40.3, 110.9 ppm.
The PES and RHF/6-31G* data show that ground-state
electronic interactions between the Lpπ(S)- and/or π-MO’s
of 1-6 occur. For 3, 4 and 6 this leads to splitting of the
π-MO’s 3: ∆Iv,j ∼ 0.7 (estimated from bandwidth) and
∆-ꢀj ) 0.81 eV, 4: ∆Iv,j ∼ 0.6 (estimated from bandwidth)
and ∆-ꢀj ) 0.89 eV and 6: ∆Iv,j ) 0.46 and ∆-ꢀj ) 0.56
eV). Moreover, in case of 2 and 4, which both contain
sulfur atoms at their termini, the Lpπ(S) MO’s are also
significantly split [2: ∆Iv,j ) 0.40 eV and ∆-ꢀj ) 0.41 eV
and 4: ∆Iv,j ∼ 0.3 eV (estimated using the two band
maxima) and ∆-ꢀj ) 0.32 eV]. Hence, the interactions
between Lpπ(S) are efficiently mediated over lengths of
8-12 Å.
RHF/6-31G*/NBO analyses on 1-6 show that TB
interactions occur and that the Hax-C-C-Hax PCMO’s
of the cyclohexyl-type rings play a dominant role. The
RHF/6-31G* results of the different conformers of 1-6 in
combination with the He(I) PES results of conformation-
ally locked anti- and syn-7 show that the conformation
of the hydrocarbon skeleton does not affect the magnitude
of the TB coupling, viz. the Hax-C-C-Hax PCMO’s can
be combined with π-type orbitals within all point groups
4-(4-(Tetr a h yd r o-4H-th iop yr a n -4-ylid en e)-1-h yd r oxy-
cycloh exyl)tetr a h yd r o-4H-th iop yr a n -4-ca r boxylic Acid
(17). Tetrahydro-4H-thiopyran-4-carboxylic acid (16,12 0.26 g,
1.80 mmol) and 4-(tetrahydro-4H-thiopyran-4-ylidene)cyclo-
hexanone4a (0.35 g, 1.80 mmol) were coupled to give the
â-hydroxy acid 17 employing a literature procedure.4a How-
ever, the workup procedure was slightly modified. After the
water layer was acidified to ca. pH 1 with concentrated HCl
(32) The calculated (RHF/6-31G*) Lpπ(S) MO’s of a model system
consisting of two dimethyl sulfide and one ethene molecule in the
geometry of 2 and ghost centers on the missing carbon atoms are nearly
degenerate (HOMO, -9.22 eV and HOMO-1, -9.29 eV).30

4592 J . Org. Chem., Vol. 65, No. 15, 2000
Marsman et al.
(37%), it was extracted with CHCl3 (3 × 50 mL). The combined
CHCl3 layers were dried (MgSO4), filtered, and concentrated
in vacuo. The residue was dried in a vacuum desiccator (P2O5)
overnight yielding 17 as an off-white solid (0.4 g, 0.87 mmol,
48%): 1H NMR (CDCl3/DMSO-d6, 1:1) δ 1.44 (m, 2H), 1.72 (m,
4H), 2.11 (m, 2H), 2.4-2.6 (m, 14H), 2.79 (m, 2H) ppm; the
COOH and OH protons were not observed; 13C NMR (CDCl3/
DMSO-d6, 1:1) δ 23.9, 25.2, 29.0, 31.1, 32.8, 53.9, 73.1, 125.7,
130.6, 175.9 ppm.
Ca lcu la tion s
The different conformers of 1-6 (Cs: anti- and syn-1, anti/
anti-, syn/anti- and anti/syn-3 and syn/syn-3, anti/syn-4 and
anti/syn-6, C2h: anti-2, anti-5, anti/anti-4, anti/anti-6, syn/syn-6
and anti-7 and C2v: syn-2, syn/syn-4, syn-5 and syn-7) were
optimized at the RHF/6-31G* level of theory using GAMESS-
UK.36 In addition, anti-1 (Cs), anti-2 (C2h), and syn-2 (C2v) were
also optimized at the RHF/6-311G** level of theory. Hessian
calculations revealed that all conformers represent genuine
minima (see Supporting Information).
Sin gle-Cr ysta l X-r a y Str u ctu r e Deter m in a tion of 2.
C
10H16S2, M ) 200.35, colorless needle, 0.50 × 0.08 × 0.08
The NBO analyses were performed using the NBO 3.0
mm3, triclinic, P 1h (no 2), a ) 5.1689(10) Å, b ) 6.2329(12) Å,
c ) 8.3528(7) Å, R ) 72.321(11)°, â ) 78.442(11)°, γ ) 76.591-
(15)°, V ) 246.94(7) Å3, Z ) 1, dx ) 1.347 g/cm3, 3682 measured
reflections, 1715 unique reflections (Rint ) 0.0574). µ ) 0.48
mm-1. 87 refined parameters, no restraints. R(I > 2σ(I)): R1
) 0.0387, wR2 ) 0.0858. R (all data): R1 ) 0.0509, wR2 )
0.0906, S ) 1.057. Intensities were measured on a Enraf-
Nonius CAD4T diffractometer with rotating anode (Mo KR, λ
program.37
Ack n ow led gm en t. We thank A. Flatow for record-
ing the PE spectra. R.G. is grateful to the Deutsche
Forschungsgemeinschaft, the Volkswagenstiftung, the
Fonds der Chemischen Industrie, and the BASF
Aktiengesellschaft for financial support. Partial finan-
cial support (M.L., A.L.S.) by the Council for Chemical
Sciences of The Netherlands Organization for Scientific
Research (NWO-CW) is also gratefully acknowledged.
) 0.71073 Å) at 150 K up to a resolution of (sin θ λ-1
)
)
max
0.74 Å-1. An absorption correction was not considered neces-
sary. The structure was solved with DIRDIF9733 and refined
with SHELXL9734 against all F2 of all reflections. Non
hydrogen atoms were refined freely with anisotropic displace-
ment parameters. Hydrogen atoms were located in the differ-
ence Fourier map and refined freely with isotropic displace-
ment parameters. The drawing, structure calculations and
checking for higher symmetry was performed with PLATON.35
He(I) P h otoelectr on Sp ectr oscop y. The He(I) photoelec-
tron (PE) spectra were recorded on a Perkin-Elmer PS 18
spectrometer at the following temperatures: 1, 60 °C; 2, 70
°C; 3, 125 °C; 4, 60 °C; 5, 25 °C; 6, 115 °C; anti- and syn-7 145
°C. The calibration was performed with Ar (15.76 and 15.94
eV) and Xe (12.13 and 13.44 eV). Resolution of 20 meV on the
2P3/2 Ar line was obtained.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates of the RHF/6-31G*-optimized geometries of all conform-
ers of 1-7 as well as Cartesian coordinates of the RHF/6-
311G**-optimized geometries of anti-1, anti-2, and syn-2.
Calculated RHF/6-31G* molecular orbital (MO) energies and
MO assignments of anti/ syn-3, syn/ anti-3, syn/ syn-3, anti/
syn-4, and syn/ syn-4 (Table 1). Single-crystal X-ray structural
data of 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O000199Y
(36) GAMESS-UK is a package of ab initio programs written by:
Guest, F.; van Lenthe, J . H.; Kendrick, J .; Schoffel, K.; Sherwood, P.;
Harrison, R. J . 1998 with contributions from: Amos, R. D.; Buenker,
R. J .; Dupuis, M.; Handy, N. C.; Hiller, I. H.; Knowles, P. J .; Bonacic-
Koutecky, V.; Niessen, W.; Saunders: V. R.; Stone, A. J . The package
is derived from the original GAMESS code due to: Dupuis, M.;
Spangler, D.; Wendolowski, J ., NRCC Software Catalog, Vol 1, Program
No. QG01 (GAMESS) 1980.
(37) NBO 3.0 program: Natural Bond Orbital/Natural Population
Analysis/Natural Localized Molecular Orbitals Program, Glendering,
E. D.; Reed, A. E.; Carpenter, J . E.; Weinhold, F. as implemented in
GAMESS-UK (see also ref 27).36
(33) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J . M. M.; Smykalla, C. The
DIRDIF97 Program System, Technical Report of the Crystallographic
Laboratory; University of Nijmegen: The Netherlands, 1997.
(34) Sheldrick, G. M. SHELXL97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Germany, 1997.
(35) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: The Netherlands, 1999.