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The microwave spectrum, dipole moment, and molecular conformation of 2-cycloheptene-1-one
JOURNAL
OF
MOLECULAR
SPECTROSCOPY
75,429-439
(1979)
The microwave Spectrum, Dipole Moment, and ~olecuiar Conformation of Z-Cycloheptene-I -one R. W. KITCHIN, Ueparfment
THOMAS B. MALLOY,
JR., AND
ROBERT L. COOK
ofPhysics and Department of Chemistry, Mississippi State Wniversity, Mississippi State, Mississippi 39762
The microwave spectrum of 2-cycloheptene-l-one, an unsaturated cyclic ketone, has been studied in the regions 26.5-40 and 7.0-12.4 GHz. An analysis of the ground-state “a”-type transitions yielded the rotational constants (in MHz): A = 2997.27, B = 2049.24, C = 1399.76. The “a’‘-type transitions of an excited vibrational state were also assigned, giving A = 3000.51, B = 2046.65, C = 1398.88. The centrifugal distortion constants, R, and DJx, were needed to fit the data adequately. A study of the Stark effect yielded the dipole moment components (in debye) p0 = 3.63 + 0.023 and pr = 0.882 r?r0.040. The /+, component could not be determined from the Stark effect data. These data are used to discuss the molecular conformation of cycloheptene-l-one. I. INTRODUCTION
The molecular structures of cyclic compounds have been of theoretical and experimental interest for many years. Many compounds have been studied by various spectroscopic techniques-infrared, microwave, and electron diffraction -as well as by calculational methods (Z-3). However, little microwave experimental work has been done on seven-membered rings primarily because of the multiple structures these compounds might assume, especially if the compound is completely saturated. But if some rigidity is introduced into the ring skeleton, these structure studies can be made with much less difficulty. Compounds of this type already studied by microwave spectroscopy include 1,3,5cycloheptatriene (4), 2,4,6-cycloheptatrien-I-one (tropone) (.5), 1,3,-cycloheptadiene (61, and the bicyclics: 7-oxabicyclo[2.2.l]heptane (7), A6-bicyclo[3.2.0]heptene (8), and 7-oxabicyclo[4.l.O]hept-3-ene (9). A microwave study of 2-cycloheptene-l-one, an unsaturated cyclic ketone, was undertaken to provide more information on the structure of seven-membered rings. II. EXPERIMENTAL
DETAILS
The sample of 2-cycloheptenone (C,H,,O) used in this investigation was obtained commercially from Aldrich Chemical Company and was used without any extended purification other than pumping under vacuum to remove any highly volatile impurities. Original measurements were made at room temperature with sample pressures of 50 ym or less. Under further study, it was found that much better sensitivity for ground-state measurements was obtained at 429
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0 1979 by Academic
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in any form reserved.
430
KITCHIN,
MALLOY,
AND COOK
near dry ice temperature where the sample pressure did not exceed 12 pm. The Stark effect measurements were also made at near dry ice temperature, The microwave spectra were obtained with a stabilized Stark-mod~ated microwave spectrometer employing phase-sensitive detection. Both R-band and X-band regions were used in this study. Frequency measurements are considered accurate to better than 0.1 MHz. All calculations were made with a Univac 1106 computer at the Mississippi State tfniversity computing Center.
The Eve-membered ring of 2-cyclopentenone is planar f&J). conjugation of the double bond and the carbonyl group favors planarity, as well as angle strain. In the case 2-cyclohexene-l-one, microwave data obtained by Manley and Tyler (II) have been interpreted as indicating that the ring atom C5 is out of the plane which contains the oxygen atom and the remaining carbon atoms. No serious angle strain, torsional strain, or Van der Waals strain would seem to be present for this conformation, and maximum delocalization of the g-bonds is possible, For the seven-membered ring 2-~y~loheptene-l-one angle strain favors a nonplanar ring structure. The average ring angle of 128.5” for a planar sevenmembered ring may be compared with 114”, the average of three sp2 and four sp3 bond angles. By analogy with 2-cyclohexenone, a very appealing model for 2-cycloheptenone is one in which all the atoms are in a plane except for C5 and Cg, as illustrated in Fig. 1. For model calculations using this configuration typical values for bond distances and bond angles were assumed, and these are given in Fig. 2. Rotational constant calculations indicated a highly asymmetric top (K= -0.2) with ‘*a”-type transitions expected to be dominant. These initial structure calculations were sufficient to enable the assignment of the spectrum. A low-resolution A-band spectrum of 2-~yclohepte~e-l-one is shown in Fig. 3.
SPECTRUM OF 2-CYCLOHEPTENE-l-one
431
HALF-CHAIR
FIG. 2. Molecular parameters used for the half-chair form of 2-cycloheptene-l-one. Typical hydrogen parameters have been employed. Here, as elsewhere, the angle between the perpendiculars to the planes (3,2,1) and (2.1.8) is denoted as 4(3,2,1,8) = 0”. Similarly,4(7,6,5,4) = 1..5”, X(3,4,7,6) = 111.7”.
R-branch transitions of a highly asymmetric In general, for “a”-type rotor, the pair of transitions (J + I),,.,,, +-.I,, and (1 + l),,J+, +J1,., turn out to be coincident or nearly coincident in frequency and are expected to be comparatively strong. These transitions are also expected to possess mirror-image Stark
200 V/cm
GHz FIG. 3. Low-resolution spectrum of 2-cycloheptene-l-one. Sweep rate about 5 MHzkec. The promment JO,J+ (.I + I),,,.,+, and J,,, -+ (J + l)l,J+l ground-state transitions which are coincident or nearly coincident in frequency are clearly apparent. The lines at slightly lower frequency are from an excited vibrational state.
432
KITCHIN,
MALLOY, AND COOK TABLE I
Assigned Ground-State Rotational Transitions, Rotat~oa~ Constants, and Cent~fugal Distortion Constants of Z-Cycloheptene-l-one (in MHz) -_- _--_._l___ -.-_.-. Transition
2 + ll,O 1,1 30,3 + 20,2
Obs. Freq.
D~v.~
TIWk3itiOIl Obs. Freq.
7547.55
0.09
94,5 f a4,4
9551.41
0.12
%,5 + a5,4
Dl?V.a
34177.16
-0.09
32336.46
-0.03
31,3 * 21,2
9243.75
0.04
P&4 + 85,3
33245.32
-0.02
31,2 * 21,1
11133.30
-0.04
96,4 * ab,3
32268.97
0.25
32,2 + 22,l
10347.05
0.09
96,3 + 86,2
32379.28
0.03
32,1 + 22,0
11142.65
0.02
l"o,lo+9o.9
29013.50
-0.15
40,4 * 30.3
12296.57
0.17
""l,lo+g1,9
29013.50
-0.04
41,4 + 31,3
12148.50
0.22
31062,83
-0.22
8 + 72,5 2,6
27897.24
0.02
31056.83
-0.08
30032.31
-0.03
102,a+ 92,7
33208.40
0.04
28431.22
0.05
lo3, 8 + 93>7
33085.24
0.03
a4,4 * 74,3
30125.44
0.07
103,?* 93 6 I)
35780.20
0.04
a5,4 + 75,3
28692.11
0.21
104,7+ 94,6
34861.82
-0.13
a7,1
28379.52
0.02
lo4,6 + 94,5
37831.33
0.14
a3,5 + ?3,4 a4,5 * 74,4
*
7?,o
0,,2 + 7,,1 92,a + a2,7
"1,9 + %,8 l"2,9+ '2,a
26378.57
-0.10
lo&6 + 95,5
35871.02
-0.19
28254.72
-0.03
105,5 + %,4
37192.53
0.13 -0.11
9 + *2,6 2,7 93,? - a3,6
30522.36
0.03
106,5 + 96,4
36010.37
30223.26
0.00
IO&,4+ g6,3
36363.63
-0.19
9 3*6 * a3,5 94,6 + %,5
33093.90
-0.11
107,3* g7,2
35819.33
-0.12
31729.97
-0.14
35583.60
0.04
loa,2+ 9a,1
effects. The Stark effect will be particularly fast when ,uu,is nonzero due to the near degeneracy of the Jo,J and J,, levels. These lines are noted in Fig. 3 and provide the key to the assignment. At first glance, these lines might appear to be doublets, but under higher resolution the lower-frequency lines turn out to have less intensity and were subsequently assigned to an excited vibrational state. After precise measurements of K-, = 0 and 1 lines, the derived ro~tional constants gave reasonably good predictions, and once the other “#“-type K_% = 1 and also K1 = 2 lines had been assigned, the rotational constants became quite well determined. The large concentration of lines near 35-37 GHz in Fig. 3 comes from the higher K_, lines associated with the J = 9 + 10 transitions. The 63 “a’‘-type transitions measured and assigned to the ground state are given in Table I, and the 36 transitions assigned to an excited vibrational state are given in Table II. Only transitions up to K = 3 were measured for the excited state. The rotational constants derived from these data are also given in these tables. In addition, the distortion constants which could be evaluated from the spectro-
433
11o,rl*lOo,lo 31812.56
-0,07
117,4 +107.3
39669.22
-0.12
ll~,llflol,10 31812.56
-0.04
"8,3 f108,2
19306.55
O.L6
346LL.62
0.01
34611.62
0.02
36654.W
O.QQ
LLL,LQ*LQL,$ LL2,1QCL02,9 LL2,9 'L??,% 113.9 fiQ3,%
-0.22
33857.x 33856.11 35x51.51 35906.38
0.23 -0.m 0.26
12L L.,+ILLLL * L2L:iILLL,Lo L22,Li*L12,LQ
36654.0%
0.02
122,1Qf11?,9
38725,OS
-0.12
L23,10*"3,9
38709.4%
-0.01
0.07
L30,13'L20,L2
37b10.66
0.10
0.01
L3i,L3+LZL,L2
37410.66
0.10
LL3,8 +lQ3,7
38341.11
0.12
LL4,8 *LO4,7
37850.63
-0.09
Li5,7 *LO5 ,6
39247.40
LL6,6 ?,5
34714.73 39559.09
0.m
LL7,s +L07,4
i20,L2*LLo,lL
L31,1ZfL2L,1~ L32,L2"L22,LL
A = 2947.2673Q.Q3% 3 = 2049.235t0.009 C = 1399.760t0.009
39452.m 39452.30
-0.03 Q.f?
x = -Q.L%6%%96b DJ = <2.58%+Q.B21 x la-' DJK
= (-2.925+1.561)x LO-4
--
scopic; data are listed in the tables. The statistical errors quoted in the constants are two standard deviations as obtained from the least-squares analysis and represent essentially 95% confidence limits. IV. DIPOLE MOMENT
Considerable difficulty was experienced in trying to find transitions suitable for a Stark effect study+ None of the R-band transitions had resolvable Stark lobes, and o&y two ~s~t~u~s were found in the X-band region with resoivabte Stark lobes In addition, at higher electric fields, other strong fines appeared which interfered with the measurements of the Stark displacements. Because of this, Stark fields larger than about 450 V/cm could nat be used. The Stark effect was found ta be quite fast for both transitions due to neardegeaeracies and the large a component of the dipole moment. Because of the low voltages required in these measurements, neither methyl acetylene nor carbonyj sulfide was a satisfactory standard for calibrating the electric field. Therefore, the Stark fields were calibrated with the secondary standard tri~uoromethy~ acetylene &! = 2 +- If, which in turn was calibrated against the 3 = 1 + 0 line of oes (12)_
434
KITCHIN,
MALLOY, AND COOK TABLE II
Assigned Excited Vibrational State Rotational Transitions, Rotational Constants, Centrifugal Distortion Constants of 2-Cycloheptene-l-one (in MHz) ---
T?CaIK3itiClIl
21,1 30,3
+
11,o
+ 20,2
31,3
+ %,2
31,2
+ 21,1
32,l 41,4 82,6 83,5
+2
2,0
+ 31,3 + 72,5 *
73,4
‘2,8
+ 82,7
92,7
+ 82,6
‘3,7 g3,6 10o,loc 10
1,10+
+ ‘3,6 + 83,5 90,9 g1,9
lo1 , 9 + g1,8 102,9 l”2,8 103,s
+ g2,8 + ‘2,7 + 93,7
A = 3000.510~0.046 B = 2046.652*0.007
Frequency
De”.”
Transition
Frequency
De”.a
35770.58
-0.13
7538.88
0.06
9546.42
0.08
llo,ll+lOo,lo
31793.66
-0.07
9237.02
0.12
lll,llflO1,10
31793.66
-0.04
33838.83
-0.05
11122.32
-0.01
11126.62
-0.16
12140.29 27888.32 30007.50 28238.83
0.04 -0.05 0.01 -0.06
30511.37
0.06
30205.02
0.17
33078.28
-0.13
28996.29
-0.09
103,7
+ ‘3,6
111,10*101,9 112,10+102,9 112,9
+102,8
113,9
+l”3,8
“3,8
+103,7
33836.95
0.02
35934.11
0.00
35886.78
0.00 0.05
38330.52
120,12f110,11
34591.11
-0.02
121,12+111,11
34591.11
-0.01
121,11+111,10
36634.05
-0.04
i22,11*112,10
36633.52
0.00
38705.31
0.04
=2
, lo*l~z,Y
28996.29
0.03
38688.72
-0.16
31045.99
0.05
13,,13+120,12
37388.44
-0.08
31039.58
0.03
131,13+121,12
37388.44
-0.07
33193.80
0.02
‘31,12+121,1r
39430.29
0.07
33066.85
0.17
132,12*122,11
39430.29
0.24
123,10’“3,9
and
k = -0.1911056b DJ =
(1.417+0.299)
x
10
-4
C = 1398.87510.007 =Dev. = Observed - calculated frequency. The from the rotational and centrifugal constants deviation of the fit is 0.093 MHz.
calculated frequencies are listed here. The standard
bDimensionless.
The 3,,3 +- 2x,2 transition was studied first. The levels of this transition each interact with a nearby level via pC, and the data were analyzed by the usual near-degenerate perturbation theory (12). The iA41 = 1 and 2 lobes were measured at fields up to 454 V/cm. However, the Stark displacements could not be measured at high enough fields to provide sufficient data to specify both pa and pC uniquely; the correlation coefficients for these quantities were very high. To provide additional data the 3,,Z c 22,1 transition was studied. This transition could be studied to fields of only 100 V/cm. With larger fields, interference precluded further measurements and only the 1A4 1 = 1 lobe could be studied. This
435
SPECTRUM OF 2-CYCLOHEPTENE-l-one
transition was found to be extremely sensitive to pa-the lower level interacting with a nearby level via pa. Several attempts were made with both transitions to study the M = 0 lobes, but nearby lines were modulated at the higher fields, thus preventing these studies. At extremely low voltages, the [M( = 2 lobe for the 3,,, + 2,,, transition was located but could not be followed. However, this did further confirm the M assignment. A least-squares analysis of the splitting data of both transitions was carried out to evaluate pa and J.L,.The Stark effect measurements and the resulting dipole moment components are given in Table III. The data were not sensitive to ,!&,.
TABLE III Stark Shifts and Dipole Moment of 2-Cycloheptene-l-one Field (Volts/cm)
Ohs.*
Dev.b
ww
miz)
31,3
Field mlts/cm)
Dev .
(MHz)
(MHz)
31,3
+ 21,2=
+ 21,2c
jM/
/MI = 2
= 1
43.9
0.49
96.4
0.64
0.08
50.7
0.71
0.04
120.5
0.90
0.03
64.4
1.19
0.11
153.0
1.51
0.10
77.6
1.50
86.4
2.00
96.4
2.41
120.5
3.77
130.7
4.18 3
-0.01
-0.07 0.05 -0.01 0.00 -0.25 d
2,2
-
[M/
22,1
= 1
36.3
1.84
48.3
3.44
0.10
61.9
5.69
0.26
73.2
7.66
0.14
84.1
9.82
-0.04
107.3
15.71
Stark -
174.9
1.97
0.13
195.8
2.39
0.09
216.1
2.67
239.3
3.69
0.26
292.1
5.40
0.32
349.3
7.20
402.0
9.59
454.3
11.78
pa = 3.626D
-0.13
-0.02 0.09 -0.26
0.023De
L
“b
-
uc
= 0.8820
-0.24
12.54
96.4
aObserved b Observed
-0.06
b
Obs.a
0.040De
0.08
shift.
Calculated
shift.
‘The 3 level interacts inten?&& with 2. 2, both d The 22 1 level interacts f No transition ‘In Deb&. ference restricted Stark
with the nearby level interactions via uc. with
the
nearby
found with measurements
3.
22 o via
ubdependence. to very low
’
3 and
21 2 Ieve
II,. In addition, voltages.
inter-
436
KITCHIN, ?&ALLOY, AND COOK TABLE IV Comparison of Calculated to Observed Values for the Structures of 2-Cycloheptene-l-ones B NW
C ww
Half-chair 3243
2077
1451
3.58
0.68
0.63
Chair
3007
2017
1366
3.25
0.28
1.74
Twist
2983
2085
1424
3.35
0.49
1.49
Observed
2997
2049
1400
3.63
."-
0.83
Avg. Dav. Half-ChairChair of A, B, C 108 25
UC (D)
Twist 25
%he dipolemouientcomponentswere calculatadasmmin~ the total dipole mmwit (3.7D)lies along the C=O bond,
The results compare favorably with a microwave study made on the six-membered analog, 2-cyclohexene-l-one (If), for which pa = 3.75 2 0.02 D, JL~ = 0.31 + 0.01 D, and the ,ob component was also undetermined. The “t&al” dipole moment, excluding ,u&,for each molecule is faund to be very nearly the same-3.76 D for 2-cyclohexene-l-one and 3.73 D for %cycloheptene-l-one. However, the “total” dipole moment of 2-cycloheptene-l-one measured from the Stark effect is considerably larger than the total dipole moment found by dielectric absorption measurements, where p = 3.45 + 0.05 D (13). In contrast, the dielectric absorption measurement for 2-cyciohexene-l-one is p = 352 ? 0.05 D (13). Another similar molecule, 2-~yc~opentene-~-one, has a dipole moment of J.45 I?: .05 D as determined by a dielectric absorption measurement (M). The reported microwave study of this compound did not include a dipole moment study (IO).
CHAIR
FIG. 4. Chair co~tio~
of 2-cycioheptene-l-one.
SPECTRUM OF 2-CYCLUHEPTENE-l-one
437
CHAIR
I%. 5. MoIecular parameters used for the chair form of 2-cycloheptene-l-one. = 2.2”, *(1,4,3,2) = 3.1”, 4(7,1,4,3) = 128.8”. and Q(I,7,5,6) = 119.5”. V. ~OL~~U~R
4(1,7,5,4)
STRUCTURE
There are a number of possible conjurations that one might envisage for 2cycloheptenone. Two of the more reasonable forms would be a chair and a halfchair configuration. The molecule can quite easily be arranged into a chair form as illustrated in Fig. 4. Furthermore, reasonable structural parameters can be assumed (see Fig. 5), and little angle strain or torsion strain would be present. Alternatively, the half-chair form used previously in the initial calculations of the rotational constants may be considered. Here again reasonable parameters as given in Fig. 2 can be selected for the bond distances and angles, and little angle strain is expected. A comparison of the rotational constants and dipole components of these structures with the observed quantities is given in Table IV. The dipole components were calculated assuming that the total dipole moment lies along the C=O bond. It is seen that the agreement, in the case of the rotational constants, is fair, and it is not possible to choose between the half-chair and the chair form solely on the basis of the rotational constants. A comparison
FIG. 6. projection of the haIf&air form of kycioheptene-f-one in the ac principaf axis plane. The closeness of the two inner hydrogen atoms on C, and Ci is easily seen. The eclipsing of the out-of-p&me metfiyiene groups is also apparent.
KITCHIN,MALLOY,ANDCOOK
FIG. 7. Twist configuration of 2-cycloheptene-l-one. found in the half-chair form are relieved in this form.
The nonbonded
interaction and the eclipsing
of dipole components would favor the half-chair form. However, it is found that the pu, component is quite sensitive to the exact chair form assumed. In any case, comparison of the dipole components is not particularly significant if delocalization is important. A typical C=O bond moment is about 2.9 D. The large dipole moment of 3.7 D would indicate substantial delocalization of the m-electrons of the C=C and C=O bonds. This would not be possible for the chair form. However, there are serious problems in the case of the halfchair form, as shown in Fig. 6. Considerable delocalization is possible, but there are significant torsional and Van der Waals strains. The hydrogens on C, and C, are clearly eclipsed, and there is severe cross-ring interaction between the inner hydrogens of C, and Cr. Calculations indicate that it would be difficult to distort this structure sufficiently to relieve these interactions and maintain reasonable bond angles and bond distances. One could relieve these interactions by appropriate rotation of the CH, groups, or by relaxing the planarity of five-
1.349
c
1400
talc. 22888~ 1424
FIG. 8. Molecular parameters for the twist configuration of 2-cycloheptene-l-one. This form is consistent with the microwave data and has no serious angle or torsional strain. 4(4,3,2,1) = X7”, X(7,6,5,4) = 42.W, and 4(8,1,2,3) = 157”.
439
SPECTRUM OF 2-CYCLOHE~ENE-l-one
ring carbon atoms and the oxygen atom. If one takes a model of the half-chair form and twists about an axis perpendicular to the C,--C, bond without keeping the oxygen atom in the plane, the twisted form shown in Fig. 7 is obtained. In this form, the torsional and Van der Waals strains have been removed, and this can be accomplished without moving the carbonyl system too far out of the plane, thereby maintaining substantial delocalization. Just how much the carbony1 group is kicked out of the plane becomes a delicate balance between minimization of torsional and Van der Waals strains while one is trying to maint~n as much delo~~ization as possible. We have recently made a molecular mechanics calculation for this molecule using a program kindly provided by Professor N. L. Allinger (15). Starting with our initial half-chair structure, the program converges to the type of twisted configuration shown in Fig. 7. The details of the structure are shown in Fig. 8. It is apparent that most of the molecular parameters are reasonable, and the observed and calculated rotational constants, given in Table IV, are in quite good agreement. This twisted configuration is therefore consistent with the microwave data and would seem to have no serious drawbacks with regard to angle strain, torsional strain, etc. ACKNOWLEDGMENT We wouId like to thank the Langley Research Center, Hampton, Virginia, for an eq~pment loan which has aided us greatly in our studies. One of us (T.B.M.) would like to acknowledge the Research Corporation for a Cottrell Grant. RECEIVED:
August 17, 1978 REFERENCES
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89, 7036-7061
(1%7).
2. D. F. BOCIAN, H. M. PICKETT,T. C. ROUNDS, AND H. L. STRAUSS,J. Amer. Chem. Sot. 97, 687-695 (1975). 3. N. L. ALLINGERAND J. T. SPRAGUE,J. Amer. Chem. Sot. 94, 5734-f747 (1972). 4. S. S. BUTCHER,f. Chem. Phys. 42, 1830-1832 (1965). 5. C. L. NORRIS, R. C. BENSON, P. BECK, AND W. H. FLYCARE, J. Amer. Chem. SM. 95, 2766-2112 (1973). 6. T. K. AVIRAH, T. B. MALLOY, JR., AND R. L. COOK, J. Chem. Phys., to be published. 7. R. A. CRESSWELL,1. Mol. Spectrosc. 56, 133- 137 (1975). 8. T. K. AVIRAH, R. L. COOK, AND T. B, MALLOY, JR., “Symposium on Molecular Structure
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Chem.
Sot.
95, 3893-3907
(1973).
OF
MOLECULAR
SPECTROSCOPY
75,429-439
(1979)
The microwave Spectrum, Dipole Moment, and ~olecuiar Conformation of Z-Cycloheptene-I -one R. W. KITCHIN, Ueparfment
THOMAS B. MALLOY,
JR., AND
ROBERT L. COOK
ofPhysics and Department of Chemistry, Mississippi State Wniversity, Mississippi State, Mississippi 39762
The microwave spectrum of 2-cycloheptene-l-one, an unsaturated cyclic ketone, has been studied in the regions 26.5-40 and 7.0-12.4 GHz. An analysis of the ground-state “a”-type transitions yielded the rotational constants (in MHz): A = 2997.27, B = 2049.24, C = 1399.76. The “a’‘-type transitions of an excited vibrational state were also assigned, giving A = 3000.51, B = 2046.65, C = 1398.88. The centrifugal distortion constants, R, and DJx, were needed to fit the data adequately. A study of the Stark effect yielded the dipole moment components (in debye) p0 = 3.63 + 0.023 and pr = 0.882 r?r0.040. The /+, component could not be determined from the Stark effect data. These data are used to discuss the molecular conformation of cycloheptene-l-one. I. INTRODUCTION
The molecular structures of cyclic compounds have been of theoretical and experimental interest for many years. Many compounds have been studied by various spectroscopic techniques-infrared, microwave, and electron diffraction -as well as by calculational methods (Z-3). However, little microwave experimental work has been done on seven-membered rings primarily because of the multiple structures these compounds might assume, especially if the compound is completely saturated. But if some rigidity is introduced into the ring skeleton, these structure studies can be made with much less difficulty. Compounds of this type already studied by microwave spectroscopy include 1,3,5cycloheptatriene (4), 2,4,6-cycloheptatrien-I-one (tropone) (.5), 1,3,-cycloheptadiene (61, and the bicyclics: 7-oxabicyclo[2.2.l]heptane (7), A6-bicyclo[3.2.0]heptene (8), and 7-oxabicyclo[4.l.O]hept-3-ene (9). A microwave study of 2-cycloheptene-l-one, an unsaturated cyclic ketone, was undertaken to provide more information on the structure of seven-membered rings. II. EXPERIMENTAL
DETAILS
The sample of 2-cycloheptenone (C,H,,O) used in this investigation was obtained commercially from Aldrich Chemical Company and was used without any extended purification other than pumping under vacuum to remove any highly volatile impurities. Original measurements were made at room temperature with sample pressures of 50 ym or less. Under further study, it was found that much better sensitivity for ground-state measurements was obtained at 429
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430
KITCHIN,
MALLOY,
AND COOK
near dry ice temperature where the sample pressure did not exceed 12 pm. The Stark effect measurements were also made at near dry ice temperature, The microwave spectra were obtained with a stabilized Stark-mod~ated microwave spectrometer employing phase-sensitive detection. Both R-band and X-band regions were used in this study. Frequency measurements are considered accurate to better than 0.1 MHz. All calculations were made with a Univac 1106 computer at the Mississippi State tfniversity computing Center.
The Eve-membered ring of 2-cyclopentenone is planar f&J). conjugation of the double bond and the carbonyl group favors planarity, as well as angle strain. In the case 2-cyclohexene-l-one, microwave data obtained by Manley and Tyler (II) have been interpreted as indicating that the ring atom C5 is out of the plane which contains the oxygen atom and the remaining carbon atoms. No serious angle strain, torsional strain, or Van der Waals strain would seem to be present for this conformation, and maximum delocalization of the g-bonds is possible, For the seven-membered ring 2-~y~loheptene-l-one angle strain favors a nonplanar ring structure. The average ring angle of 128.5” for a planar sevenmembered ring may be compared with 114”, the average of three sp2 and four sp3 bond angles. By analogy with 2-cyclohexenone, a very appealing model for 2-cycloheptenone is one in which all the atoms are in a plane except for C5 and Cg, as illustrated in Fig. 1. For model calculations using this configuration typical values for bond distances and bond angles were assumed, and these are given in Fig. 2. Rotational constant calculations indicated a highly asymmetric top (K= -0.2) with ‘*a”-type transitions expected to be dominant. These initial structure calculations were sufficient to enable the assignment of the spectrum. A low-resolution A-band spectrum of 2-~yclohepte~e-l-one is shown in Fig. 3.
SPECTRUM OF 2-CYCLOHEPTENE-l-one
431
HALF-CHAIR
FIG. 2. Molecular parameters used for the half-chair form of 2-cycloheptene-l-one. Typical hydrogen parameters have been employed. Here, as elsewhere, the angle between the perpendiculars to the planes (3,2,1) and (2.1.8) is denoted as 4(3,2,1,8) = 0”. Similarly,4(7,6,5,4) = 1..5”, X(3,4,7,6) = 111.7”.
R-branch transitions of a highly asymmetric In general, for “a”-type rotor, the pair of transitions (J + I),,.,,, +-.I,, and (1 + l),,J+, +J1,., turn out to be coincident or nearly coincident in frequency and are expected to be comparatively strong. These transitions are also expected to possess mirror-image Stark
200 V/cm
GHz FIG. 3. Low-resolution spectrum of 2-cycloheptene-l-one. Sweep rate about 5 MHzkec. The promment JO,J+ (.I + I),,,.,+, and J,,, -+ (J + l)l,J+l ground-state transitions which are coincident or nearly coincident in frequency are clearly apparent. The lines at slightly lower frequency are from an excited vibrational state.
432
KITCHIN,
MALLOY, AND COOK TABLE I
Assigned Ground-State Rotational Transitions, Rotat~oa~ Constants, and Cent~fugal Distortion Constants of Z-Cycloheptene-l-one (in MHz) -_- _--_._l___ -.-_.-. Transition
2 + ll,O 1,1 30,3 + 20,2
Obs. Freq.
D~v.~
TIWk3itiOIl Obs. Freq.
7547.55
0.09
94,5 f a4,4
9551.41
0.12
%,5 + a5,4
Dl?V.a
34177.16
-0.09
32336.46
-0.03
31,3 * 21,2
9243.75
0.04
P&4 + 85,3
33245.32
-0.02
31,2 * 21,1
11133.30
-0.04
96,4 * ab,3
32268.97
0.25
32,2 + 22,l
10347.05
0.09
96,3 + 86,2
32379.28
0.03
32,1 + 22,0
11142.65
0.02
l"o,lo+9o.9
29013.50
-0.15
40,4 * 30.3
12296.57
0.17
""l,lo+g1,9
29013.50
-0.04
41,4 + 31,3
12148.50
0.22
31062,83
-0.22
8 + 72,5 2,6
27897.24
0.02
31056.83
-0.08
30032.31
-0.03
102,a+ 92,7
33208.40
0.04
28431.22
0.05
lo3, 8 + 93>7
33085.24
0.03
a4,4 * 74,3
30125.44
0.07
103,?* 93 6 I)
35780.20
0.04
a5,4 + 75,3
28692.11
0.21
104,7+ 94,6
34861.82
-0.13
a7,1
28379.52
0.02
lo4,6 + 94,5
37831.33
0.14
a3,5 + ?3,4 a4,5 * 74,4
*
7?,o
0,,2 + 7,,1 92,a + a2,7
"1,9 + %,8 l"2,9+ '2,a
26378.57
-0.10
lo&6 + 95,5
35871.02
-0.19
28254.72
-0.03
105,5 + %,4
37192.53
0.13 -0.11
9 + *2,6 2,7 93,? - a3,6
30522.36
0.03
106,5 + 96,4
36010.37
30223.26
0.00
IO&,4+ g6,3
36363.63
-0.19
9 3*6 * a3,5 94,6 + %,5
33093.90
-0.11
107,3* g7,2
35819.33
-0.12
31729.97
-0.14
35583.60
0.04
loa,2+ 9a,1
effects. The Stark effect will be particularly fast when ,uu,is nonzero due to the near degeneracy of the Jo,J and J,, levels. These lines are noted in Fig. 3 and provide the key to the assignment. At first glance, these lines might appear to be doublets, but under higher resolution the lower-frequency lines turn out to have less intensity and were subsequently assigned to an excited vibrational state. After precise measurements of K-, = 0 and 1 lines, the derived ro~tional constants gave reasonably good predictions, and once the other “#“-type K_% = 1 and also K1 = 2 lines had been assigned, the rotational constants became quite well determined. The large concentration of lines near 35-37 GHz in Fig. 3 comes from the higher K_, lines associated with the J = 9 + 10 transitions. The 63 “a’‘-type transitions measured and assigned to the ground state are given in Table I, and the 36 transitions assigned to an excited vibrational state are given in Table II. Only transitions up to K = 3 were measured for the excited state. The rotational constants derived from these data are also given in these tables. In addition, the distortion constants which could be evaluated from the spectro-
433
11o,rl*lOo,lo 31812.56
-0,07
117,4 +107.3
39669.22
-0.12
ll~,llflol,10 31812.56
-0.04
"8,3 f108,2
19306.55
O.L6
346LL.62
0.01
34611.62
0.02
36654.W
O.QQ
LLL,LQ*LQL,$ LL2,1QCL02,9 LL2,9 'L??,% 113.9 fiQ3,%
-0.22
33857.x 33856.11 35x51.51 35906.38
0.23 -0.m 0.26
12L L.,+ILLLL * L2L:iILLL,Lo L22,Li*L12,LQ
36654.0%
0.02
122,1Qf11?,9
38725,OS
-0.12
L23,10*"3,9
38709.4%
-0.01
0.07
L30,13'L20,L2
37b10.66
0.10
0.01
L3i,L3+LZL,L2
37410.66
0.10
LL3,8 +lQ3,7
38341.11
0.12
LL4,8 *LO4,7
37850.63
-0.09
Li5,7 *LO5 ,6
39247.40
LL6,6 ?,5
34714.73 39559.09
0.m
LL7,s +L07,4
i20,L2*LLo,lL
L31,1ZfL2L,1~ L32,L2"L22,LL
A = 2947.2673Q.Q3% 3 = 2049.235t0.009 C = 1399.760t0.009
39452.m 39452.30
-0.03 Q.f?
x = -Q.L%6%%96b DJ = <2.58%+Q.B21 x la-' DJK
= (-2.925+1.561)x LO-4
--
scopic; data are listed in the tables. The statistical errors quoted in the constants are two standard deviations as obtained from the least-squares analysis and represent essentially 95% confidence limits. IV. DIPOLE MOMENT
Considerable difficulty was experienced in trying to find transitions suitable for a Stark effect study+ None of the R-band transitions had resolvable Stark lobes, and o&y two ~s~t~u~s were found in the X-band region with resoivabte Stark lobes In addition, at higher electric fields, other strong fines appeared which interfered with the measurements of the Stark displacements. Because of this, Stark fields larger than about 450 V/cm could nat be used. The Stark effect was found ta be quite fast for both transitions due to neardegeaeracies and the large a component of the dipole moment. Because of the low voltages required in these measurements, neither methyl acetylene nor carbonyj sulfide was a satisfactory standard for calibrating the electric field. Therefore, the Stark fields were calibrated with the secondary standard tri~uoromethy~ acetylene &! = 2 +- If, which in turn was calibrated against the 3 = 1 + 0 line of oes (12)_
434
KITCHIN,
MALLOY, AND COOK TABLE II
Assigned Excited Vibrational State Rotational Transitions, Rotational Constants, Centrifugal Distortion Constants of 2-Cycloheptene-l-one (in MHz) ---
T?CaIK3itiClIl
21,1 30,3
+
11,o
+ 20,2
31,3
+ %,2
31,2
+ 21,1
32,l 41,4 82,6 83,5
+2
2,0
+ 31,3 + 72,5 *
73,4
‘2,8
+ 82,7
92,7
+ 82,6
‘3,7 g3,6 10o,loc 10
1,10+
+ ‘3,6 + 83,5 90,9 g1,9
lo1 , 9 + g1,8 102,9 l”2,8 103,s
+ g2,8 + ‘2,7 + 93,7
A = 3000.510~0.046 B = 2046.652*0.007
Frequency
De”.”
Transition
Frequency
De”.a
35770.58
-0.13
7538.88
0.06
9546.42
0.08
llo,ll+lOo,lo
31793.66
-0.07
9237.02
0.12
lll,llflO1,10
31793.66
-0.04
33838.83
-0.05
11122.32
-0.01
11126.62
-0.16
12140.29 27888.32 30007.50 28238.83
0.04 -0.05 0.01 -0.06
30511.37
0.06
30205.02
0.17
33078.28
-0.13
28996.29
-0.09
103,7
+ ‘3,6
111,10*101,9 112,10+102,9 112,9
+102,8
113,9
+l”3,8
“3,8
+103,7
33836.95
0.02
35934.11
0.00
35886.78
0.00 0.05
38330.52
120,12f110,11
34591.11
-0.02
121,12+111,11
34591.11
-0.01
121,11+111,10
36634.05
-0.04
i22,11*112,10
36633.52
0.00
38705.31
0.04
=2
, lo*l~z,Y
28996.29
0.03
38688.72
-0.16
31045.99
0.05
13,,13+120,12
37388.44
-0.08
31039.58
0.03
131,13+121,12
37388.44
-0.07
33193.80
0.02
‘31,12+121,1r
39430.29
0.07
33066.85
0.17
132,12*122,11
39430.29
0.24
123,10’“3,9
and
k = -0.1911056b DJ =
(1.417+0.299)
x
10
-4
C = 1398.87510.007 =Dev. = Observed - calculated frequency. The from the rotational and centrifugal constants deviation of the fit is 0.093 MHz.
calculated frequencies are listed here. The standard
bDimensionless.
The 3,,3 +- 2x,2 transition was studied first. The levels of this transition each interact with a nearby level via pC, and the data were analyzed by the usual near-degenerate perturbation theory (12). The iA41 = 1 and 2 lobes were measured at fields up to 454 V/cm. However, the Stark displacements could not be measured at high enough fields to provide sufficient data to specify both pa and pC uniquely; the correlation coefficients for these quantities were very high. To provide additional data the 3,,Z c 22,1 transition was studied. This transition could be studied to fields of only 100 V/cm. With larger fields, interference precluded further measurements and only the 1A4 1 = 1 lobe could be studied. This
435
SPECTRUM OF 2-CYCLOHEPTENE-l-one
transition was found to be extremely sensitive to pa-the lower level interacting with a nearby level via pa. Several attempts were made with both transitions to study the M = 0 lobes, but nearby lines were modulated at the higher fields, thus preventing these studies. At extremely low voltages, the [M( = 2 lobe for the 3,,, + 2,,, transition was located but could not be followed. However, this did further confirm the M assignment. A least-squares analysis of the splitting data of both transitions was carried out to evaluate pa and J.L,.The Stark effect measurements and the resulting dipole moment components are given in Table III. The data were not sensitive to ,!&,.
TABLE III Stark Shifts and Dipole Moment of 2-Cycloheptene-l-one Field (Volts/cm)
Ohs.*
Dev.b
ww
miz)
31,3
Field mlts/cm)
Dev .
(MHz)
(MHz)
31,3
+ 21,2=
+ 21,2c
jM/
/MI = 2
= 1
43.9
0.49
96.4
0.64
0.08
50.7
0.71
0.04
120.5
0.90
0.03
64.4
1.19
0.11
153.0
1.51
0.10
77.6
1.50
86.4
2.00
96.4
2.41
120.5
3.77
130.7
4.18 3
-0.01
-0.07 0.05 -0.01 0.00 -0.25 d
2,2
-
[M/
22,1
= 1
36.3
1.84
48.3
3.44
0.10
61.9
5.69
0.26
73.2
7.66
0.14
84.1
9.82
-0.04
107.3
15.71
Stark -
174.9
1.97
0.13
195.8
2.39
0.09
216.1
2.67
239.3
3.69
0.26
292.1
5.40
0.32
349.3
7.20
402.0
9.59
454.3
11.78
pa = 3.626D
-0.13
-0.02 0.09 -0.26
0.023De
L
“b
-
uc
= 0.8820
-0.24
12.54
96.4
aObserved b Observed
-0.06
b
Obs.a
0.040De
0.08
shift.
Calculated
shift.
‘The 3 level interacts inten?&& with 2. 2, both d The 22 1 level interacts f No transition ‘In Deb&. ference restricted Stark
with the nearby level interactions via uc. with
the
nearby
found with measurements
3.
22 o via
ubdependence. to very low
’
3 and
21 2 Ieve
II,. In addition, voltages.
inter-
436
KITCHIN, ?&ALLOY, AND COOK TABLE IV Comparison of Calculated to Observed Values for the Structures of 2-Cycloheptene-l-ones B NW
C ww
Half-chair 3243
2077
1451
3.58
0.68
0.63
Chair
3007
2017
1366
3.25
0.28
1.74
Twist
2983
2085
1424
3.35
0.49
1.49
Observed
2997
2049
1400
3.63
."-
0.83
Avg. Dav. Half-ChairChair of A, B, C 108 25
UC (D)
Twist 25
%he dipolemouientcomponentswere calculatadasmmin~ the total dipole mmwit (3.7D)lies along the C=O bond,
The results compare favorably with a microwave study made on the six-membered analog, 2-cyclohexene-l-one (If), for which pa = 3.75 2 0.02 D, JL~ = 0.31 + 0.01 D, and the ,ob component was also undetermined. The “t&al” dipole moment, excluding ,u&,for each molecule is faund to be very nearly the same-3.76 D for 2-cyclohexene-l-one and 3.73 D for %cycloheptene-l-one. However, the “total” dipole moment of 2-cycloheptene-l-one measured from the Stark effect is considerably larger than the total dipole moment found by dielectric absorption measurements, where p = 3.45 + 0.05 D (13). In contrast, the dielectric absorption measurement for 2-cyciohexene-l-one is p = 352 ? 0.05 D (13). Another similar molecule, 2-~yc~opentene-~-one, has a dipole moment of J.45 I?: .05 D as determined by a dielectric absorption measurement (M). The reported microwave study of this compound did not include a dipole moment study (IO).
CHAIR
FIG. 4. Chair co~tio~
of 2-cycioheptene-l-one.
SPECTRUM OF 2-CYCLUHEPTENE-l-one
437
CHAIR
I%. 5. MoIecular parameters used for the chair form of 2-cycloheptene-l-one. = 2.2”, *(1,4,3,2) = 3.1”, 4(7,1,4,3) = 128.8”. and Q(I,7,5,6) = 119.5”. V. ~OL~~U~R
4(1,7,5,4)
STRUCTURE
There are a number of possible conjurations that one might envisage for 2cycloheptenone. Two of the more reasonable forms would be a chair and a halfchair configuration. The molecule can quite easily be arranged into a chair form as illustrated in Fig. 4. Furthermore, reasonable structural parameters can be assumed (see Fig. 5), and little angle strain or torsion strain would be present. Alternatively, the half-chair form used previously in the initial calculations of the rotational constants may be considered. Here again reasonable parameters as given in Fig. 2 can be selected for the bond distances and angles, and little angle strain is expected. A comparison of the rotational constants and dipole components of these structures with the observed quantities is given in Table IV. The dipole components were calculated assuming that the total dipole moment lies along the C=O bond. It is seen that the agreement, in the case of the rotational constants, is fair, and it is not possible to choose between the half-chair and the chair form solely on the basis of the rotational constants. A comparison
FIG. 6. projection of the haIf&air form of kycioheptene-f-one in the ac principaf axis plane. The closeness of the two inner hydrogen atoms on C, and Ci is easily seen. The eclipsing of the out-of-p&me metfiyiene groups is also apparent.
KITCHIN,MALLOY,ANDCOOK
FIG. 7. Twist configuration of 2-cycloheptene-l-one. found in the half-chair form are relieved in this form.
The nonbonded
interaction and the eclipsing
of dipole components would favor the half-chair form. However, it is found that the pu, component is quite sensitive to the exact chair form assumed. In any case, comparison of the dipole components is not particularly significant if delocalization is important. A typical C=O bond moment is about 2.9 D. The large dipole moment of 3.7 D would indicate substantial delocalization of the m-electrons of the C=C and C=O bonds. This would not be possible for the chair form. However, there are serious problems in the case of the halfchair form, as shown in Fig. 6. Considerable delocalization is possible, but there are significant torsional and Van der Waals strains. The hydrogens on C, and C, are clearly eclipsed, and there is severe cross-ring interaction between the inner hydrogens of C, and Cr. Calculations indicate that it would be difficult to distort this structure sufficiently to relieve these interactions and maintain reasonable bond angles and bond distances. One could relieve these interactions by appropriate rotation of the CH, groups, or by relaxing the planarity of five-
1.349
c
1400
talc. 22888~ 1424
FIG. 8. Molecular parameters for the twist configuration of 2-cycloheptene-l-one. This form is consistent with the microwave data and has no serious angle or torsional strain. 4(4,3,2,1) = X7”, X(7,6,5,4) = 42.W, and 4(8,1,2,3) = 157”.
439
SPECTRUM OF 2-CYCLOHE~ENE-l-one
ring carbon atoms and the oxygen atom. If one takes a model of the half-chair form and twists about an axis perpendicular to the C,--C, bond without keeping the oxygen atom in the plane, the twisted form shown in Fig. 7 is obtained. In this form, the torsional and Van der Waals strains have been removed, and this can be accomplished without moving the carbonyl system too far out of the plane, thereby maintaining substantial delocalization. Just how much the carbony1 group is kicked out of the plane becomes a delicate balance between minimization of torsional and Van der Waals strains while one is trying to maint~n as much delo~~ization as possible. We have recently made a molecular mechanics calculation for this molecule using a program kindly provided by Professor N. L. Allinger (15). Starting with our initial half-chair structure, the program converges to the type of twisted configuration shown in Fig. 7. The details of the structure are shown in Fig. 8. It is apparent that most of the molecular parameters are reasonable, and the observed and calculated rotational constants, given in Table IV, are in quite good agreement. This twisted configuration is therefore consistent with the microwave data and would seem to have no serious drawbacks with regard to angle strain, torsional strain, etc. ACKNOWLEDGMENT We wouId like to thank the Langley Research Center, Hampton, Virginia, for an eq~pment loan which has aided us greatly in our studies. One of us (T.B.M.) would like to acknowledge the Research Corporation for a Cottrell Grant. RECEIVED:
August 17, 1978 REFERENCES
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89, 7036-7061
(1%7).
2. D. F. BOCIAN, H. M. PICKETT,T. C. ROUNDS, AND H. L. STRAUSS,J. Amer. Chem. Sot. 97, 687-695 (1975). 3. N. L. ALLINGERAND J. T. SPRAGUE,J. Amer. Chem. Sot. 94, 5734-f747 (1972). 4. S. S. BUTCHER,f. Chem. Phys. 42, 1830-1832 (1965). 5. C. L. NORRIS, R. C. BENSON, P. BECK, AND W. H. FLYCARE, J. Amer. Chem. SM. 95, 2766-2112 (1973). 6. T. K. AVIRAH, T. B. MALLOY, JR., AND R. L. COOK, J. Chem. Phys., to be published. 7. R. A. CRESSWELL,1. Mol. Spectrosc. 56, 133- 137 (1975). 8. T. K. AVIRAH, R. L. COOK, AND T. B, MALLOY, JR., “Symposium on Molecular Structure
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95, 3893-3907
(1973).