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Microwave spectrum and conformation of 6-thiabicyclo[3.1.0]hexane
JOURNAL
OF MOLECULAR
Microwave
SPECTROSCOPY
Spectrum
60, 179-187
(1976)
and Conformation
of 6-Thiabicyclo[3.1.O]hexane
P. J. MJ~~BERG,W. M. RALOWSKI, AND S. 0. LJ~JNGGREN Department
ofPhysical
Chemistry, The Royal Institute of Technology, S-100 44 Stockkolm 70, Sweden
AND J. E. B;~CKVALL Department
of Organic Chemistry, The Royal Institute of Tecknology, S-100 4# Stockholm 70, Sweden
The microwave spectrum of 6-thiabicyclo[3.1.0]hexane (cyclopentene sulfide) has been measured in the region 26,50&40,000 MHz. The experimental data are consistent with a single stable conformation. Furthermore, these data can only be satisfactorily explained by assuming that this conformation is the boat form. Rotational constants were obtained, both for the ground state and two excited vibrational states, while centrifugal distortion coefficients were obtained for the ground state and one excited vibrational state. The ground state rotational constants found were Ao = 5026.243 f 0.003 MHz, Bo = 2833.813 f 0.003 MHz, and CO = 2411.679 f 0.03 MHz. For the ground state of the molecule, the electric dipole moment components were found to be Ib[ = 1.800 f 0.012 D and 1~~1 = 1.155 f 0.024 D, yielding a total dipole moment p = 2.139 f 0.027 D. INTRODUCTION
In the last ten years much interest has been focused on the conformations and lowfrequency vibrational modes of ring molecules with four, five and six members (I-3,and references cited therein). It is considered that the conformation of these compounds is determined by a delicate balance between Baeyer or ring strain and torsional forces between adjacent hydrogen atoms. Research activity in small ring compounds has partly been concerned with some analogs of bicyclo[3.1.O]hexane. Using far infrared, Raman, and microwave spectroscopy, extensive investigations have been made of the following compounds: bicyclo[3.l.O]hexane (#-6), 3-oxabicyclo[3.1.O$exane (4, 5, 7), 6-oxabicyclo[3.1.O]hexane (5, 8, 9), and 3,6-dioxabicyclo[3.l.O]hexane (4, 5, 10). All these molecules have been found to be stable in a boatlike conformation. Furthermore, NMR (11-23) and ultraviolet (24) spectroscopic investigations as well as X-ray crystallographic structure determinations (25-27) also indicate that in bicyclo[3.1.O]hexane systems the most stable conformations are boatlike forms. Additional support of the boat form is also given by dipole moment studies (28). This study of the microwave spectrum of 6-thiabicycloC3.1.0Jhexane (cyclopentene sulfide, CFS, cf. Fig. 1) was undertaken in order to obtain direct evidence about the conformation of this molecule, and to determine its electric dipole moment. 179 Copyright
0
1976 by Academic
411 rights of reproduction
Press, Inc.
in any form reserved.
MJCiBERG EZ- ;lL.
1so
FIG. 1. The 6-thiabicyclo[3.1.O]hexane molecule. The numbering of the atoms is shown, as well as the orientation of the principal axes of the molecule.
EXPERIMENTAL First,
2-chlorocyclopentyl
Warasch
and Buess
2’,4’-dinitrophenyl
(29)by reacting
sulfide
cyclopentene
chloride in glacial acetic acid. The m.p. was found to be Then, 2-chlorocyclopentyl2’,4’-dinitrophenyl in methanol.
was prepared
according
to
and 2,4-dinitrobenzenesulfenyl
7%79°C [Ref. (30), 76..5-78"C].
sulfide was treated with sodium methoxide
This yielded CPS, as described in Ref.
(31). The substance was purified
by vacuum distillation. The microwave spectrum was obtained with a Hewlett-Packard wave spectrometer modulation
equipped with a phase-stabilized
at 33.33 kHz. The measurements
40,000 MHz) at room temperature The Stark
effect was measured
Model 8460A micro-
source oscillator
and with Stark
were made in the R-band region (26,500-
and at pressures ranging from 20 to 60 mTorr. using the J =
2 ---f 3 ground state line of carbonyl
sulfide as a standard to calibrate the waveguide spacing of the Stark cell. However, the ground state J = 1 -+ 2 transition was used when checking the consistency of the calculated
values of the dipole moment components
in the K-band
region as described
below. The dipole moment of OCS was assumed to be 0.71521 D (32). These calibrations were done at each voltage setting used in the experimental measurements. In order to minimize drift, they were repeated
each day that measurements
with as little delay as possible between calibration
were made,
and measurement.
MICROWAVE SPECTRUM AND ROTATIONAL CONSTANTS CPS is an asymmetric
rotor with the asymmetry
parameter
K
=
-
0.677. Due to the
low absolute value of K, no band structures were obtained in the spectrum. Owing to the symmetry of the molecule the dipole moment components must lie along the a and c principal axes. At room temperature, the spectrum shows the ground state transitions accompanied by two vibrational satellites. These are presumably due to the ring-bending (ringpuckering) motion. As anticipated, the spectrum is dominated by strong cc,, R-branch transitions together with quite strong pc, Q-branch transitions. However, some quite
6THIABICYCLO
[3.1 .O] HEXANE
181
weak cc,, R-branch and pa, Q-branch transitions are also present. The P,, R-branch transitions show strong Stark-lobes and they could be straightforwardly assigned from preliminary calculations assuming a boatlike conformation. Then, by assigning some isolated Q-branch lines the rotational constant A could be more accurately determined. After this, all rotational transitions could be readily identified, including transitions with high quantum numbers (up to J = 37). Table I shows the measured frequencies and assignments of the lines used in fitting by the method of least squares. The rotational constants were determined from the data by using a Watson first-order centrifugal distortion analysis (33). Table II lists the resulting rotational constants and centrifugal distortion coefficients. Table I and II also lists the lines and rotational constants of two vibrational satellites. In the case of the second vibrational satellite, it was not possible to find a strong enough Q-branch line. The rotational constants of this satellite are accordingly much less accurate than those of the other. The centrifugal distortion coefficients of the second satellite were poorly determined too, so they are not recorded here. It is characteristic of the spectrum that the u-type R-branch lines of the satellites are located on the lowfrequency side of the corresponding ground state lines while all c-type Q-branch lines are on the high-frequency side. The vibrational satellites have two notable features. First, there is a slight deviation from a linear dependence of the rotational constants on the vibrational quantum number. This indicates an anharmonic ring-puckering vibrational potential. Second, the dependence of the rotational constants on the vibrational quantum number of this low-frequency mode is very similar in the four analogous molecules studied so far (7,9,10). [N o vr‘b ra t’ronal satellite was observed in the case of bicyclo[3.l.O]hexane (6)]. The change in the constant A is positive and relatively small, while B and C exhibit negative changes of the order 7-14 MHz for each vibrational state. These facts indicate that the vibrational motion is similar in these molecules. The assumption that the motion is of a ring-puckering nature is supported by the far infrared (4, 8) and Raman (5) spectroscopic investigations. An estimate was made of the intensity ratio of the first vibrational satellite to the ground state. It yielded a value of 0.31. This corresponds to an energy difference slightly above 240 cm-l. Although the spectrum of CPS was searched carefully, no transitions were identified that could be assigned to another conformation. Both the chair and boat forms of the molecule would be expected to have nearly the same total dipole moment (although the relative magnitude of the a and c components would differ, of course), and calculations for both forms predict a rich spectrum in the R-band region. If two ring conformations do indeed exist, it thus appears likely that the energy separation between them must be greater than approximately 400 cm-‘. (Because of the Boltzmann factor, conformations with higher energy would hardly be detectable by microwave spectroscopy). The remaining unassigned lines are probably high J transitions arising either from the ground or from excited vibrational states. STARK EFFECT AND DIPOLE MOMENT
Of all R-branch transitions with low J-values in the R-band region none exhibited pure second-order Stark shifts for fields up to 2000 V/cm. Furthermore, many of these
M JijBERG
182
Rotational Constants
Transition for
the
Frequencies
Ground
State,
(in the
ET AL.
MHz)
1) =
I
Used State
in
Der1vlng
and
the
Rotationai
il =
2
State
cf
6-Thiabicyclo~3.1.Olhexane
Ground obs
43,2 43.1 51,4 5
+ +
23'413
52:3 60,6 '1.6
'2,2 32,l 41.3
26990.81
42,2
+
50,5
+
51,5
61,5
+. 51.4 +
52,4
+
63,4
+
53.3
+
53.2
64,3
+
=4,2
65,2
+
55,1
70.7
+
60,6
71.7
+
61,6
'2,6
+
62,5
157,9
+
156,9
167,10
+
166,10
"6,ll
+
"5,13
v=2 obs-talc
33118.12
-0.05
32878.32
0.02
32944.64
0
27002.98
0.02
30118.52
0.04
29873.42
-0.05
32170.46
-0.02
32076.24
31226.75
0.01
52,3
63.3
a=1 obs
-0.02
,
+
62,5 62,4
State obs-talc
30014.77
-0.03
26835.68
0.12
26847.25
-0.10
29910.28
-0.09
29663.04
0.07
-0.01
31982.11
-0.05
31128.25
-0.02
31029.29
-0.01
32477.65
0.01
32385.40
0.03
0.02 31710.85
0.01
31418.14
0.01
34496.88
0.01
0.03
31569.72
31901.60
-0.0,
31806.49
0.05
31560.77
-0.01
34777.32
0.00
-0.03
0bs-ca1c
02
31666.09
34899.31
obs
34747.35
0.02
34624.69
0.00
36300.05
0.01
36184.53
-0.01
30033.10
0.08
29395.13
-0.02
29522.06
-0.07
26808.50
0.02
lines occur in rather crowded regions of the spectrum and are not suitable for Stark effect measurements. However, the lines 62,G+ 52,4 and 72,6 +- 62.6 show very well-shaped Stark lobes. The / M 1 = 2 and 3 lobes of the first of these lines were measured and used in the calculations, and SO were the 1M 1 = 3,4, and 5 lobes of the second line. Deviations from secondorder theory arise due to coupling between the nearly degenerate levels .52,4 and 52,s (by pa). The same applies to 72.6 and 71,s (by cl=). Perturbation calculations were performed in the usual manner (34) and the matrix elements of p.E were calculated according to the method described by Schwendeman (35) using a computer program originally written by M. Ribeaud of the Swiss Federal Institute of Technology for calculating the necessary coefficients for the degenerate Stark effect. The dipolemoment components were determined by a least-squares fit to the measured lobes. This yielded Ip,[ = 1.800 f 0.012 D and (~~1 = 1.155 f 0.024 D. From these components
6-THIABICYCLO TABLE
I
Ground obs
"',I1
+
"6,ll
186,12
+
185,14
'67.12
+
'86,12
'87,11
+
'*6,13
"7,12
+
"6,14
I'S,12
+
"',I2
206,13
+
207,13
+
214,18
2'7,14
+
2'6,16
2'8.14
-
217,14
227,1S
-
226,17
228,15
-
227,15
238,16
+
237,16
238,15
+
237,17
248,17
+
247,17
269,18
+
266,16
"9,19
+
278,19
289,20
+
286,20
3'10,22
+
3210,23
+
3310,24 369,27 3711,27
183
(Contd.)
Transition
214,17
[3.1 .O] HEXAN’E
+ +
3'9,22
339,24
3710,27
v=2
v=1 ohs
obs-talc
obs
obs-talc
-0.02
28114.18
0.01
27619.73
-0.01
29814.32
0.00
33877.53
-0.02
33180.54
0.01
29084.89
0.00
31361.10
-0.02
32210.24
0.0,
32930.63
0.01
30872.93
0.00
29086.01
-0.02
26812.08
-0.0,
31776.87
0.03
29116.68
0.00
34267.53
0.03
329,23
369,28 +
28663.21
State obs-talc
27703.88
-0.01
27818.61
-0.01
29236.85
-0.01
30095.03
0.01
32347.84
-0.03
34299.48
0.00
34059.47
0.00
34407.34
0.00
31345.24
0.02
27811.08
0.01
29348.00
-0.02
the total dipole moment of the molecule is p = 2.139 f 0.027 D. This value is in good agreement with the dipole moment of ‘I-thiabicyclo[4.1.O]heptane (cyclohexene sulfide) which has been determined to be 2.2 f 0.4 D (36). It was desired to make a further check of the consistency of the calculated values of the dipole moment components. The Stark patterns of three additional lines in the Kband region (18,00&26,500 MHz) were therefore investigated at a Stark voltage of 1013 V/cm. These measurements were performed on the [MI = 1 and 2 lobes of the line 41,3+ 31,2, an the (Ml = 2 and 3 lobes of 40,4t 30,3 and on the /MI = 1, 2, 3, and 4 lobes of 51.6 +- 41,4. Calculations were made using the determined values of the dipole moment components. The results fit the measured frequencies of all these Stark components within the limits of experimental error. CONFORMATION
OF THE MOLECULE
It is obvious that the three measured rotational constants of the ground vibrational state of CPS are insufficient for a detailed determination of the molecular structure.
MJiiBERG ET .4L
184
Rotational
Constants
the Ground
State
and
v=2 mng-Fuckerlng
given
for the
rrlncipal
(MHz),
hloments cf
of 6-Thia~icyclo(3.1.Ol"exane, Centrifugal
States.
Ground
State
and
the
Distortion
(a.m.u.
A*)
ds for the
Coefficients
for
c=l are
o=l State
state
Ground
Inertia as well
0=2
“=I
A
5026.243
+
5028.605
+ 0.005
5029.13
r 1.09
B
2833.813
f 0.003
2826.677
r 0.006
2818.981
* 0.076
c
2411.679
+ 0.003
2402.598
k 0.008
2394.374
c 0.075
I:
-
0.003
- 0.677016
0.677090
- 0.677687
100.54747
!. 0.00006
100.50024
t 0.00010
100.490
t 0.022
178.33781
+ 0.00019
178.7880
2 0.0004
179.276
+ 0.005
209.55359
+ 0.00026
210.3456
r 0.0007
211.068
t 0.007
465
c 36
-363
f
1081
+ 38
26.8 260
6.Y
6
442 +
83
- 384 r
66
960 t 230
t
0.3
k
9
27?
4
288 + 108
Molecular
Parameters
Assumed
Constants
of 6-Thiabicyclo[3
in Calculating
Cl - C2
1.513
A
1.530
A
c1 - c5
1.47,
A
Cl - s
1.819
A
c, - H
1.082
A
1.092
A
=
c3-11
104.6O
L C2-C3‘Cq L H-C2-H
=
a The
groups
CH2
L H-C3-H
rrng
spectively).
The
the vector
109.47O
at C2 and
the adjacent
unit
Rotational
.l.O lhexane
C2 - C3
c2 - H
the
angle C,-H
C3 share (i C,-C2-C3
bond
a
a comr~on bisector and i C2-C3-C4
is selected
with re-
so as to lie along
(;, _c + zC _c + zC _S), where 12 15 1 vector along sow particular bond.
; denotes
a
GTHIABICYCLO
[3.1 .O] HEXANE
TABLE
Dependence
of Various
Calculated C,-S-C5
Dipole
Angle
on the Moment
Bisector
T
<
Calculated
Dihedral
is Assumed
for 6-Thia-
Angles to
lie
w and along
60°
MHz
(too large)
T = 600
- 7o"
Neither
of
!l,/u, toO
A. B or
C
large
If *.
B and
c fit
fits
A > 5280 a = 25O
- 35O
7. The the
in the Episulfide
A > 5380 u, < 2S"
I”
constants
bicyclo[3.l.Olhexane
18.5
MHZ
(too large!
A fits quite well,
B and
, > 7o"
> 0.92 uo'u* (too large)
C fit well
A > 5140 a$ > 35O
MHz
(too large)
A fits well,
B > 2950
MHZ
B and
C > 2500
MHz
not
C do
fit
(too large)
However, starting from reasonable molecular parameters some significant qualitative results can be deduced. Table III shows the molecular parameters that were initially assumed. The remaining parameters consist of the angle cp = lSO”-i$ (ClC&,C,)-(C&&J with positive values corresponding to a boat conformation and the angle T = ISO’-& (C1C&Y4C6)-(C1SC6). As has been previously pointed out (lo), the quantity Ab = I, f I, - I& depends only on the atomic masses and the perpendicular distances of the atoms from the symmetry plane containing the a and c axes of the molecule. Thus, Ab is almost independent of the angles cpand T. The C-C&., angle and the distance C1<6 were chosen to reproduce this value of da. A shorter C1-Cs distance than 1.471 I%was considered unlikeIy, and a larger value resulted in a questionably small value of the C&-Cp angle. For this reason the Cr-Cb distance was kept at 1.471 A, even though a slightly larger value is possible. The values of the C&C-C4 angle and the C+Zs distance shown in Table III are the actual values obtained from this kind of first fit. It was found that the calculated rotational constants are quite insensitive to reasonable variations of the parameters listed in Table III. On the other hand, they are much more sensitive to the angles (o and 7. An estimate of the conformation of the molecule is thus possible. The dependence of the rotational constants on the dihedral angles cp and 7 was then investigated. The results of the calculations are best presented in a table. Table IV shows the results obtained for different intervals of the angles cp and 7. Almost irrespective of the value of ‘p, we find that T < 60” yields A-values which are too high and 7 > 70” along yields (P~/LC&,I~ too large if the dipole moment is assumed to lie approximately the ClSC, angle bisector in the episulfide. Finally, for T = 60”-70” the constants B and C fit very well and the constants A fits the measured values fairly well for cp = 30°.
186
M JiiBERG
ET a.1L.
5200
!
. -60'
-30"
0"
30"
60"
SO"
-30'
0'
30'
60'
SO'
>
'p
2700
4
!
-60'
+
‘+'
FIG. 2. Calculated rotational constants in MHz plotted versus the dihedral angle q. The horizontal dashed lines correspond to the values of the observed rotational constants. Negative dihedral angles correspond to a chair conformation, while positive angles correspond to a boat conformation.
The derivative of A with respect to cpis rather small in this range of cp, so this constant is less suitable for a determination of cp. Figure 2 shows how A, B, and C depend on cp for r = 67.4”. This value of T was chosen by analogy with similar systems. In the region of r = 60”-70”, the value of (pc/&aic is 0.74-0.93 for cp = 2.5”-3.5” The deviation from the observed value (0.642) may be explained by assuming a deviation between the direction of the dipole moment and the direction of the Cr.SC6 angle bisector in the episulfide. Such a deviation is actually to be expected from the fact that the dipole moment of bicyclo[3.l.O]h exane has been found to be 0.192 D (6). Furthermore, this contribution should almost coincide with the c axis of CPS. For r > 70”, however, the value of (&~(&i~ deviates so strongly from the observed value that it cannot be explained by such small contributions. We may thus conclude that the boat form of the molecule is the most stable conformation of the ring. The angle cp will be around 30” and the angle r around 60” to 70”. As pointed out by various authors (e.g., 4, 7, 9) the low energy of the boat conformation is obviously due to the fact that the hydrogen atom on carbon-l (or 5) is staggered with respect to the hydrogen atoms on carbon-2 (or 4). In a chair conformation, the latter hydrogen atoms would assume eclipsed positions.
6-THIABICYCLO
[3.1.0]
HEXANE
187
ACKNOWLEDGMENTS This work was supported by NFR, the Swedish Natural Science Research Council. Part of the equipment was donated by the Knut and Alice Wallenberg Foundation. Two of the authors (P. J. M. and J. E. B.) would like to express their appreciation to the W. Roos foundation for scholarship grants.
RECEIVED: October 2. 1975 REFERENCES 1. V. W. LAURIE, Accounts Chem. Res. 3,331 (1970). 2. C. S. BLACKWELLANDR. C. LORD, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 1, Marcel Dekker, New York, 1972. 3. J. LAANE, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 1, Marcel Dekker, New York, 1972. 4. R. C. LORD ANDT. B. MALLOY, JR., J. ikfol. Spec~rosc. 46, 358 (1973). 5. J. D. LEWIS, J. LAANE, ANDT. B. MALLOY, JR., J. Chem. Phys. 61, 2342 (1974). 6. R. L. COOKANDT. B. MALLOY, JR., J. Amer. Che-m.Sot. 96, 1703 (1974). 7. T. B. MALLOY, JR., J. Mol. Spectrosc. 49, 432 (1974). 8. L. A. CARREIRAAND R. C. LORD, J. Chew Phys. 51, 2735 (1969). 9. W. J. LAFFERTY,J. Mol. Spectrosc. 36, 84 (1970). 10. R. A. CRESWELLANDW. J. LAFFERTY,J. Mol. Spectrosc. 46, 371 (1973). 11. M. S. BERGQ~ISTANDT. NORIN, Ark. Kemi 22, 137 (1964). 12. K. TORI, Chem. Pharm. Bull. Tokyo 12, 1439 (1964). 13. P. K. FREEMAN,F. A. RAYMOND,AND M. F. GROSTIC,J. Org. Chem. 30, 771 (1965). 14. S. WINSTEIN, E. C. FRIEDRICH,R. BAKER, AND Y. LIN, Tetrahedron Suppl. 8, Part II, 621 (1966). 15. H. E. S~IITH,J. C. D. BRAND, E. H. MASSEY, ANDL. J. DURHAM,J. Org. Chew. 31, 690 (1966). 16. A. DIEFFENBACHER ANDW. VONPHILIPSBORN,Helv. Chim. Acta 49,897 (1966). 17. Y. LIN, Thesis, University of California, 1967. 18. W. G. DAUBENAND W. T. WIPKE, J. Org. Chem. 32, 2977 (1967). 19. S. P. ACHARYA,H. C. BROWN,A. SUZUKI,S. NOZAWA,ANJIM. ITOH, J. Org. Chem. 34,301s (1969). 20. W. VON E. DOERINGANDE. K. G. SMITH, T&ah&cm 27, 200.5 (1971). 21. T. NORIN, S. STRBYBERG,ANDM. WEBER, Acta Chem. Stand. 27, 1579 (1973). 22. M. A. COOPER,C. M. HOLDEN,P. LOFTUS,ANDD. WHITTAKER,J. C. S. Perkin II, 665 (1973). 23. R. J. ABRAHAM,C. M. HOLDEN,P. LOFTIJS,ANDD. WHITTAKER,Org. Magn. Resonance 6,184 (1974). 24. R. T. GRAY ANDH. E. SMITH, Tetrahedron 23,4229 (1967). 2.5. A. F. CA-RON, G. FERGUSON,AND J. M. ROBERTSON,J. Chem. Sot. B, 692 (1969). 26. M. F. GROSTIC,D. J. DUCHAMP,AND C. G. CHIDESTER,J. Org. Chem. 36,2929 (1971). 27. F. H. HERBSTEINANDH. REGEV, J. Chem. Sot. B, 1696 (1971). 28. J. J. MCCULLOUGH,H. B. HENBEST,R. J. BISHOP,G. M. GLOVER,AM) L. E. SUTTON,J. Chem. SOC., 5496 (1965). 29. N. KHARASCHAND C. M. BUESS, J. Amer. Chem. Sot. 71,2724 (1949). 30. F. W. BOLLINGER,F. N. HAYES, AND S. SIEGEL,J. Amer. Chem. Sot. 75, 1729 (1953). 31. D. R. DOGG ANDN. C. DANN, Int. J. Sulfur Chem. 1, 117 (1971). 32. J. S. MUENTER,J. Chem. Phys. 48,4X4 (1968). 33. J. K. G. WATSON,J. Chem. Phys. 45, 1360 (1966). 34. S. GOLDENAND E. B. WILSON, JR., J. Chem. Phys. 16, 669 (1948). 35. R. H. SCHWENDEMAN, J. Mol. Spectrosc. 7, 280 (1961). 36. R. KEWLEY, Can. J. Chem. 51, 529 (1973).
OF MOLECULAR
Microwave
SPECTROSCOPY
Spectrum
60, 179-187
(1976)
and Conformation
of 6-Thiabicyclo[3.1.O]hexane
P. J. MJ~~BERG,W. M. RALOWSKI, AND S. 0. LJ~JNGGREN Department
ofPhysical
Chemistry, The Royal Institute of Technology, S-100 44 Stockkolm 70, Sweden
AND J. E. B;~CKVALL Department
of Organic Chemistry, The Royal Institute of Tecknology, S-100 4# Stockholm 70, Sweden
The microwave spectrum of 6-thiabicyclo[3.1.0]hexane (cyclopentene sulfide) has been measured in the region 26,50&40,000 MHz. The experimental data are consistent with a single stable conformation. Furthermore, these data can only be satisfactorily explained by assuming that this conformation is the boat form. Rotational constants were obtained, both for the ground state and two excited vibrational states, while centrifugal distortion coefficients were obtained for the ground state and one excited vibrational state. The ground state rotational constants found were Ao = 5026.243 f 0.003 MHz, Bo = 2833.813 f 0.003 MHz, and CO = 2411.679 f 0.03 MHz. For the ground state of the molecule, the electric dipole moment components were found to be Ib[ = 1.800 f 0.012 D and 1~~1 = 1.155 f 0.024 D, yielding a total dipole moment p = 2.139 f 0.027 D. INTRODUCTION
In the last ten years much interest has been focused on the conformations and lowfrequency vibrational modes of ring molecules with four, five and six members (I-3,and references cited therein). It is considered that the conformation of these compounds is determined by a delicate balance between Baeyer or ring strain and torsional forces between adjacent hydrogen atoms. Research activity in small ring compounds has partly been concerned with some analogs of bicyclo[3.1.O]hexane. Using far infrared, Raman, and microwave spectroscopy, extensive investigations have been made of the following compounds: bicyclo[3.l.O]hexane (#-6), 3-oxabicyclo[3.1.O$exane (4, 5, 7), 6-oxabicyclo[3.1.O]hexane (5, 8, 9), and 3,6-dioxabicyclo[3.l.O]hexane (4, 5, 10). All these molecules have been found to be stable in a boatlike conformation. Furthermore, NMR (11-23) and ultraviolet (24) spectroscopic investigations as well as X-ray crystallographic structure determinations (25-27) also indicate that in bicyclo[3.1.O]hexane systems the most stable conformations are boatlike forms. Additional support of the boat form is also given by dipole moment studies (28). This study of the microwave spectrum of 6-thiabicycloC3.1.0Jhexane (cyclopentene sulfide, CFS, cf. Fig. 1) was undertaken in order to obtain direct evidence about the conformation of this molecule, and to determine its electric dipole moment. 179 Copyright
0
1976 by Academic
411 rights of reproduction
Press, Inc.
in any form reserved.
MJCiBERG EZ- ;lL.
1so
FIG. 1. The 6-thiabicyclo[3.1.O]hexane molecule. The numbering of the atoms is shown, as well as the orientation of the principal axes of the molecule.
EXPERIMENTAL First,
2-chlorocyclopentyl
Warasch
and Buess
2’,4’-dinitrophenyl
(29)by reacting
sulfide
cyclopentene
chloride in glacial acetic acid. The m.p. was found to be Then, 2-chlorocyclopentyl2’,4’-dinitrophenyl in methanol.
was prepared
according
to
and 2,4-dinitrobenzenesulfenyl
7%79°C [Ref. (30), 76..5-78"C].
sulfide was treated with sodium methoxide
This yielded CPS, as described in Ref.
(31). The substance was purified
by vacuum distillation. The microwave spectrum was obtained with a Hewlett-Packard wave spectrometer modulation
equipped with a phase-stabilized
at 33.33 kHz. The measurements
40,000 MHz) at room temperature The Stark
effect was measured
Model 8460A micro-
source oscillator
and with Stark
were made in the R-band region (26,500-
and at pressures ranging from 20 to 60 mTorr. using the J =
2 ---f 3 ground state line of carbonyl
sulfide as a standard to calibrate the waveguide spacing of the Stark cell. However, the ground state J = 1 -+ 2 transition was used when checking the consistency of the calculated
values of the dipole moment components
in the K-band
region as described
below. The dipole moment of OCS was assumed to be 0.71521 D (32). These calibrations were done at each voltage setting used in the experimental measurements. In order to minimize drift, they were repeated
each day that measurements
with as little delay as possible between calibration
were made,
and measurement.
MICROWAVE SPECTRUM AND ROTATIONAL CONSTANTS CPS is an asymmetric
rotor with the asymmetry
parameter
K
=
-
0.677. Due to the
low absolute value of K, no band structures were obtained in the spectrum. Owing to the symmetry of the molecule the dipole moment components must lie along the a and c principal axes. At room temperature, the spectrum shows the ground state transitions accompanied by two vibrational satellites. These are presumably due to the ring-bending (ringpuckering) motion. As anticipated, the spectrum is dominated by strong cc,, R-branch transitions together with quite strong pc, Q-branch transitions. However, some quite
6THIABICYCLO
[3.1 .O] HEXANE
181
weak cc,, R-branch and pa, Q-branch transitions are also present. The P,, R-branch transitions show strong Stark-lobes and they could be straightforwardly assigned from preliminary calculations assuming a boatlike conformation. Then, by assigning some isolated Q-branch lines the rotational constant A could be more accurately determined. After this, all rotational transitions could be readily identified, including transitions with high quantum numbers (up to J = 37). Table I shows the measured frequencies and assignments of the lines used in fitting by the method of least squares. The rotational constants were determined from the data by using a Watson first-order centrifugal distortion analysis (33). Table II lists the resulting rotational constants and centrifugal distortion coefficients. Table I and II also lists the lines and rotational constants of two vibrational satellites. In the case of the second vibrational satellite, it was not possible to find a strong enough Q-branch line. The rotational constants of this satellite are accordingly much less accurate than those of the other. The centrifugal distortion coefficients of the second satellite were poorly determined too, so they are not recorded here. It is characteristic of the spectrum that the u-type R-branch lines of the satellites are located on the lowfrequency side of the corresponding ground state lines while all c-type Q-branch lines are on the high-frequency side. The vibrational satellites have two notable features. First, there is a slight deviation from a linear dependence of the rotational constants on the vibrational quantum number. This indicates an anharmonic ring-puckering vibrational potential. Second, the dependence of the rotational constants on the vibrational quantum number of this low-frequency mode is very similar in the four analogous molecules studied so far (7,9,10). [N o vr‘b ra t’ronal satellite was observed in the case of bicyclo[3.l.O]hexane (6)]. The change in the constant A is positive and relatively small, while B and C exhibit negative changes of the order 7-14 MHz for each vibrational state. These facts indicate that the vibrational motion is similar in these molecules. The assumption that the motion is of a ring-puckering nature is supported by the far infrared (4, 8) and Raman (5) spectroscopic investigations. An estimate was made of the intensity ratio of the first vibrational satellite to the ground state. It yielded a value of 0.31. This corresponds to an energy difference slightly above 240 cm-l. Although the spectrum of CPS was searched carefully, no transitions were identified that could be assigned to another conformation. Both the chair and boat forms of the molecule would be expected to have nearly the same total dipole moment (although the relative magnitude of the a and c components would differ, of course), and calculations for both forms predict a rich spectrum in the R-band region. If two ring conformations do indeed exist, it thus appears likely that the energy separation between them must be greater than approximately 400 cm-‘. (Because of the Boltzmann factor, conformations with higher energy would hardly be detectable by microwave spectroscopy). The remaining unassigned lines are probably high J transitions arising either from the ground or from excited vibrational states. STARK EFFECT AND DIPOLE MOMENT
Of all R-branch transitions with low J-values in the R-band region none exhibited pure second-order Stark shifts for fields up to 2000 V/cm. Furthermore, many of these
M JijBERG
182
Rotational Constants
Transition for
the
Frequencies
Ground
State,
(in the
ET AL.
MHz)
1) =
I
Used State
in
Der1vlng
and
the
Rotationai
il =
2
State
cf
6-Thiabicyclo~3.1.Olhexane
Ground obs
43,2 43.1 51,4 5
+ +
23'413
52:3 60,6 '1.6
'2,2 32,l 41.3
26990.81
42,2
+
50,5
+
51,5
61,5
+. 51.4 +
52,4
+
63,4
+
53.3
+
53.2
64,3
+
=4,2
65,2
+
55,1
70.7
+
60,6
71.7
+
61,6
'2,6
+
62,5
157,9
+
156,9
167,10
+
166,10
"6,ll
+
"5,13
v=2 obs-talc
33118.12
-0.05
32878.32
0.02
32944.64
0
27002.98
0.02
30118.52
0.04
29873.42
-0.05
32170.46
-0.02
32076.24
31226.75
0.01
52,3
63.3
a=1 obs
-0.02
,
+
62,5 62,4
State obs-talc
30014.77
-0.03
26835.68
0.12
26847.25
-0.10
29910.28
-0.09
29663.04
0.07
-0.01
31982.11
-0.05
31128.25
-0.02
31029.29
-0.01
32477.65
0.01
32385.40
0.03
0.02 31710.85
0.01
31418.14
0.01
34496.88
0.01
0.03
31569.72
31901.60
-0.0,
31806.49
0.05
31560.77
-0.01
34777.32
0.00
-0.03
0bs-ca1c
02
31666.09
34899.31
obs
34747.35
0.02
34624.69
0.00
36300.05
0.01
36184.53
-0.01
30033.10
0.08
29395.13
-0.02
29522.06
-0.07
26808.50
0.02
lines occur in rather crowded regions of the spectrum and are not suitable for Stark effect measurements. However, the lines 62,G+ 52,4 and 72,6 +- 62.6 show very well-shaped Stark lobes. The / M 1 = 2 and 3 lobes of the first of these lines were measured and used in the calculations, and SO were the 1M 1 = 3,4, and 5 lobes of the second line. Deviations from secondorder theory arise due to coupling between the nearly degenerate levels .52,4 and 52,s (by pa). The same applies to 72.6 and 71,s (by cl=). Perturbation calculations were performed in the usual manner (34) and the matrix elements of p.E were calculated according to the method described by Schwendeman (35) using a computer program originally written by M. Ribeaud of the Swiss Federal Institute of Technology for calculating the necessary coefficients for the degenerate Stark effect. The dipolemoment components were determined by a least-squares fit to the measured lobes. This yielded Ip,[ = 1.800 f 0.012 D and (~~1 = 1.155 f 0.024 D. From these components
6-THIABICYCLO TABLE
I
Ground obs
"',I1
+
"6,ll
186,12
+
185,14
'67.12
+
'86,12
'87,11
+
'*6,13
"7,12
+
"6,14
I'S,12
+
"',I2
206,13
+
207,13
+
214,18
2'7,14
+
2'6,16
2'8.14
-
217,14
227,1S
-
226,17
228,15
-
227,15
238,16
+
237,16
238,15
+
237,17
248,17
+
247,17
269,18
+
266,16
"9,19
+
278,19
289,20
+
286,20
3'10,22
+
3210,23
+
3310,24 369,27 3711,27
183
(Contd.)
Transition
214,17
[3.1 .O] HEXAN’E
+ +
3'9,22
339,24
3710,27
v=2
v=1 ohs
obs-talc
obs
obs-talc
-0.02
28114.18
0.01
27619.73
-0.01
29814.32
0.00
33877.53
-0.02
33180.54
0.01
29084.89
0.00
31361.10
-0.02
32210.24
0.0,
32930.63
0.01
30872.93
0.00
29086.01
-0.02
26812.08
-0.0,
31776.87
0.03
29116.68
0.00
34267.53
0.03
329,23
369,28 +
28663.21
State obs-talc
27703.88
-0.01
27818.61
-0.01
29236.85
-0.01
30095.03
0.01
32347.84
-0.03
34299.48
0.00
34059.47
0.00
34407.34
0.00
31345.24
0.02
27811.08
0.01
29348.00
-0.02
the total dipole moment of the molecule is p = 2.139 f 0.027 D. This value is in good agreement with the dipole moment of ‘I-thiabicyclo[4.1.O]heptane (cyclohexene sulfide) which has been determined to be 2.2 f 0.4 D (36). It was desired to make a further check of the consistency of the calculated values of the dipole moment components. The Stark patterns of three additional lines in the Kband region (18,00&26,500 MHz) were therefore investigated at a Stark voltage of 1013 V/cm. These measurements were performed on the [MI = 1 and 2 lobes of the line 41,3+ 31,2, an the (Ml = 2 and 3 lobes of 40,4t 30,3 and on the /MI = 1, 2, 3, and 4 lobes of 51.6 +- 41,4. Calculations were made using the determined values of the dipole moment components. The results fit the measured frequencies of all these Stark components within the limits of experimental error. CONFORMATION
OF THE MOLECULE
It is obvious that the three measured rotational constants of the ground vibrational state of CPS are insufficient for a detailed determination of the molecular structure.
MJiiBERG ET .4L
184
Rotational
Constants
the Ground
State
and
v=2 mng-Fuckerlng
given
for the
rrlncipal
(MHz),
hloments cf
of 6-Thia~icyclo(3.1.Ol"exane, Centrifugal
States.
Ground
State
and
the
Distortion
(a.m.u.
A*)
ds for the
Coefficients
for
c=l are
o=l State
state
Ground
Inertia as well
0=2
“=I
A
5026.243
+
5028.605
+ 0.005
5029.13
r 1.09
B
2833.813
f 0.003
2826.677
r 0.006
2818.981
* 0.076
c
2411.679
+ 0.003
2402.598
k 0.008
2394.374
c 0.075
I:
-
0.003
- 0.677016
0.677090
- 0.677687
100.54747
!. 0.00006
100.50024
t 0.00010
100.490
t 0.022
178.33781
+ 0.00019
178.7880
2 0.0004
179.276
+ 0.005
209.55359
+ 0.00026
210.3456
r 0.0007
211.068
t 0.007
465
c 36
-363
f
1081
+ 38
26.8 260
6.Y
6
442 +
83
- 384 r
66
960 t 230
t
0.3
k
9
27?
4
288 + 108
Molecular
Parameters
Assumed
Constants
of 6-Thiabicyclo[3
in Calculating
Cl - C2
1.513
A
1.530
A
c1 - c5
1.47,
A
Cl - s
1.819
A
c, - H
1.082
A
1.092
A
=
c3-11
104.6O
L C2-C3‘Cq L H-C2-H
=
a The
groups
CH2
L H-C3-H
rrng
spectively).
The
the vector
109.47O
at C2 and
the adjacent
unit
Rotational
.l.O lhexane
C2 - C3
c2 - H
the
angle C,-H
C3 share (i C,-C2-C3
bond
a
a comr~on bisector and i C2-C3-C4
is selected
with re-
so as to lie along
(;, _c + zC _c + zC _S), where 12 15 1 vector along sow particular bond.
; denotes
a
GTHIABICYCLO
[3.1 .O] HEXANE
TABLE
Dependence
of Various
Calculated C,-S-C5
Dipole
Angle
on the Moment
Bisector
T
<
Calculated
Dihedral
is Assumed
for 6-Thia-
Angles to
lie
w and along
60°
MHz
(too large)
T = 600
- 7o"
Neither
of
!l,/u, toO
A. B or
C
large
If *.
B and
c fit
fits
A > 5280 a = 25O
- 35O
7. The the
in the Episulfide
A > 5380 u, < 2S"
I”
constants
bicyclo[3.l.Olhexane
18.5
MHZ
(too large!
A fits quite well,
B and
, > 7o"
> 0.92 uo'u* (too large)
C fit well
A > 5140 a$ > 35O
MHz
(too large)
A fits well,
B > 2950
MHZ
B and
C > 2500
MHz
not
C do
fit
(too large)
However, starting from reasonable molecular parameters some significant qualitative results can be deduced. Table III shows the molecular parameters that were initially assumed. The remaining parameters consist of the angle cp = lSO”-i$ (ClC&,C,)-(C&&J with positive values corresponding to a boat conformation and the angle T = ISO’-& (C1C&Y4C6)-(C1SC6). As has been previously pointed out (lo), the quantity Ab = I, f I, - I& depends only on the atomic masses and the perpendicular distances of the atoms from the symmetry plane containing the a and c axes of the molecule. Thus, Ab is almost independent of the angles cpand T. The C-C&., angle and the distance C1<6 were chosen to reproduce this value of da. A shorter C1-Cs distance than 1.471 I%was considered unlikeIy, and a larger value resulted in a questionably small value of the C&-Cp angle. For this reason the Cr-Cb distance was kept at 1.471 A, even though a slightly larger value is possible. The values of the C&C-C4 angle and the C+Zs distance shown in Table III are the actual values obtained from this kind of first fit. It was found that the calculated rotational constants are quite insensitive to reasonable variations of the parameters listed in Table III. On the other hand, they are much more sensitive to the angles (o and 7. An estimate of the conformation of the molecule is thus possible. The dependence of the rotational constants on the dihedral angles cp and 7 was then investigated. The results of the calculations are best presented in a table. Table IV shows the results obtained for different intervals of the angles cp and 7. Almost irrespective of the value of ‘p, we find that T < 60” yields A-values which are too high and 7 > 70” along yields (P~/LC&,I~ too large if the dipole moment is assumed to lie approximately the ClSC, angle bisector in the episulfide. Finally, for T = 60”-70” the constants B and C fit very well and the constants A fits the measured values fairly well for cp = 30°.
186
M JiiBERG
ET a.1L.
5200
!
. -60'
-30"
0"
30"
60"
SO"
-30'
0'
30'
60'
SO'
>
'p
2700
4
!
-60'
+
‘+'
FIG. 2. Calculated rotational constants in MHz plotted versus the dihedral angle q. The horizontal dashed lines correspond to the values of the observed rotational constants. Negative dihedral angles correspond to a chair conformation, while positive angles correspond to a boat conformation.
The derivative of A with respect to cpis rather small in this range of cp, so this constant is less suitable for a determination of cp. Figure 2 shows how A, B, and C depend on cp for r = 67.4”. This value of T was chosen by analogy with similar systems. In the region of r = 60”-70”, the value of (pc/&aic is 0.74-0.93 for cp = 2.5”-3.5” The deviation from the observed value (0.642) may be explained by assuming a deviation between the direction of the dipole moment and the direction of the Cr.SC6 angle bisector in the episulfide. Such a deviation is actually to be expected from the fact that the dipole moment of bicyclo[3.l.O]h exane has been found to be 0.192 D (6). Furthermore, this contribution should almost coincide with the c axis of CPS. For r > 70”, however, the value of (&~(&i~ deviates so strongly from the observed value that it cannot be explained by such small contributions. We may thus conclude that the boat form of the molecule is the most stable conformation of the ring. The angle cp will be around 30” and the angle r around 60” to 70”. As pointed out by various authors (e.g., 4, 7, 9) the low energy of the boat conformation is obviously due to the fact that the hydrogen atom on carbon-l (or 5) is staggered with respect to the hydrogen atoms on carbon-2 (or 4). In a chair conformation, the latter hydrogen atoms would assume eclipsed positions.
6-THIABICYCLO
[3.1.0]
HEXANE
187
ACKNOWLEDGMENTS This work was supported by NFR, the Swedish Natural Science Research Council. Part of the equipment was donated by the Knut and Alice Wallenberg Foundation. Two of the authors (P. J. M. and J. E. B.) would like to express their appreciation to the W. Roos foundation for scholarship grants.
RECEIVED: October 2. 1975 REFERENCES 1. V. W. LAURIE, Accounts Chem. Res. 3,331 (1970). 2. C. S. BLACKWELLANDR. C. LORD, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 1, Marcel Dekker, New York, 1972. 3. J. LAANE, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 1, Marcel Dekker, New York, 1972. 4. R. C. LORD ANDT. B. MALLOY, JR., J. ikfol. Spec~rosc. 46, 358 (1973). 5. J. D. LEWIS, J. LAANE, ANDT. B. MALLOY, JR., J. Chem. Phys. 61, 2342 (1974). 6. R. L. COOKANDT. B. MALLOY, JR., J. Amer. Che-m.Sot. 96, 1703 (1974). 7. T. B. MALLOY, JR., J. Mol. Spectrosc. 49, 432 (1974). 8. L. A. CARREIRAAND R. C. LORD, J. Chew Phys. 51, 2735 (1969). 9. W. J. LAFFERTY,J. Mol. Spectrosc. 36, 84 (1970). 10. R. A. CRESWELLANDW. J. LAFFERTY,J. Mol. Spectrosc. 46, 371 (1973). 11. M. S. BERGQ~ISTANDT. NORIN, Ark. Kemi 22, 137 (1964). 12. K. TORI, Chem. Pharm. Bull. Tokyo 12, 1439 (1964). 13. P. K. FREEMAN,F. A. RAYMOND,AND M. F. GROSTIC,J. Org. Chem. 30, 771 (1965). 14. S. WINSTEIN, E. C. FRIEDRICH,R. BAKER, AND Y. LIN, Tetrahedron Suppl. 8, Part II, 621 (1966). 15. H. E. S~IITH,J. C. D. BRAND, E. H. MASSEY, ANDL. J. DURHAM,J. Org. Chew. 31, 690 (1966). 16. A. DIEFFENBACHER ANDW. VONPHILIPSBORN,Helv. Chim. Acta 49,897 (1966). 17. Y. LIN, Thesis, University of California, 1967. 18. W. G. DAUBENAND W. T. WIPKE, J. Org. Chem. 32, 2977 (1967). 19. S. P. ACHARYA,H. C. BROWN,A. SUZUKI,S. NOZAWA,ANJIM. ITOH, J. Org. Chem. 34,301s (1969). 20. W. VON E. DOERINGANDE. K. G. SMITH, T&ah&cm 27, 200.5 (1971). 21. T. NORIN, S. STRBYBERG,ANDM. WEBER, Acta Chem. Stand. 27, 1579 (1973). 22. M. A. COOPER,C. M. HOLDEN,P. LOFTUS,ANDD. WHITTAKER,J. C. S. Perkin II, 665 (1973). 23. R. J. ABRAHAM,C. M. HOLDEN,P. LOFTIJS,ANDD. WHITTAKER,Org. Magn. Resonance 6,184 (1974). 24. R. T. GRAY ANDH. E. SMITH, Tetrahedron 23,4229 (1967). 2.5. A. F. CA-RON, G. FERGUSON,AND J. M. ROBERTSON,J. Chem. Sot. B, 692 (1969). 26. M. F. GROSTIC,D. J. DUCHAMP,AND C. G. CHIDESTER,J. Org. Chem. 36,2929 (1971). 27. F. H. HERBSTEINANDH. REGEV, J. Chem. Sot. B, 1696 (1971). 28. J. J. MCCULLOUGH,H. B. HENBEST,R. J. BISHOP,G. M. GLOVER,AM) L. E. SUTTON,J. Chem. SOC., 5496 (1965). 29. N. KHARASCHAND C. M. BUESS, J. Amer. Chem. Sot. 71,2724 (1949). 30. F. W. BOLLINGER,F. N. HAYES, AND S. SIEGEL,J. Amer. Chem. Sot. 75, 1729 (1953). 31. D. R. DOGG ANDN. C. DANN, Int. J. Sulfur Chem. 1, 117 (1971). 32. J. S. MUENTER,J. Chem. Phys. 48,4X4 (1968). 33. J. K. G. WATSON,J. Chem. Phys. 45, 1360 (1966). 34. S. GOLDENAND E. B. WILSON, JR., J. Chem. Phys. 16, 669 (1948). 35. R. H. SCHWENDEMAN, J. Mol. Spectrosc. 7, 280 (1961). 36. R. KEWLEY, Can. J. Chem. 51, 529 (1973).