Journal of Molecular Structure, 32 (1976) 191-202
@Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
MICROWAVE SPECTRUM, CONFORMATION, BARRIER TO INTERNAL
ROTATlON, CENTRIFUGAL DISTORTION, AND DIPOLE MOMENT OF
METHYLPROPARGYLETHER
K.-M. MARSTOKK and H. M(j)LLENDAL
Department of Chemistry, The University of Oslo, Blindern, Oslo 3 (Norway)
(Received 30 September 1975)
ABSTRACT
The microwave spectrum of methylpropargyl ether, CH3OCH2C=CH, has been
investigated in the 11.9-26.5 GHz region. Only the gauche rotamer with a dihedral
angle of 68 o :t 2 o from the syn position was assigned. Other forms are not present in
concentrations
exceeding 10 % of the total. The barrier to internal rotation of the methyl
group was determined to be 2512 :t 75 cal mol-I. The dipole moment components
are
/la = 0.290 :t 0.003 D, /lb = 0.505 :t 0.012 D, and /le = 1.016 :t 0.003 D. The total dipole
moment is 1.171 :t 0.013 D. Extensive centrifugal distortion analyses have been carried
out for the ground as well as for two vibrationally
excited states. For the ground state,
transitions up to J = 77 were assigned and a large centrifugal distortion exceeding 9 GHz
enabled the determination
of accurate quartic and significant sextic distortion coefficients.
INTRODUCTION
Molecules of the general form CH3OCH2X, where X can be any substituent,
are generally found to exist in either the anti or gauche conformation (Fig. 1)
or as a mixture of these two. These molecules thus offer an attractive
opportunity for investigating the influence of oxygen on conformational
behaviour. Several of them have therefore been studied in recent years by
spectroscopic and diffraction techniques. It has been found that rather
complex equilibria situations exist. For example, CH3OCH2CI [1-3] and
CH3OCH2Br [3] prefer the gauche conformation although the anti forms
cannot be ruled out. CH3OCH2CN also exists predominantly as the gauche
form, [4, 5] while the anti form has be en determined to be 1.3 :t 0.4 kcal
mol-1 less stable [4]. On the other hand, for CH3OCH2CH3 the anti form is
the more stable [6, 7] .
In the case of the title compound, very recent vibrational spectroscopic
studies made independently by Seth-Paul et al. [8] and Bjeprseth and Gustavsen
[9] conclude that the gauche rotamer predominates in solution, and in the
crystalline, liquid, and gaseous states. The present work was undertaken as a
confirmation and extention of these investigations. In particular, we wanted
to leam more about the structure, the barrier to internal rotation of the
192
"$"
x
Anti
HMH
HMH
"'VCH3 CH0V
X
x
Gauche
Fig.l. Anti and gauche rotamers of molecules of the CH3OCH2X type viewed alon g the
XCH20-0 bond.
methyl group, the dipole moment, and to search for additional rotamers.
further forms were found although their possible existence cannot be
completely eliminated by the present study.
No
EXPERIMENT AL
The sample of methylpropargyl ether used in this work was the same as
that used by BjØrseth and Gustavsen [9]. The microwave spectrum is
relatively weak and the measurements were therefore made at dry ice temperature. The spectral region 11.9-26.5 GHz was investigated utilizing a
conventional spectrometer described briefly in ref. 10. Resolution as good as
about 0.3 MHz was obtained in favourable cases with free-running klystrons
used as frequency sources.
RESULTS
Microwave spectrum and assignment of the ground vibrational state
Based on the findings of the IR and Raman works [8,9], the gauche
rota mer was presumed to be the more stable. The dihedral angle was
expected to be dose to 69 o from the syn form because this is the value
found in the dosely related molecule CH3OCH2CN [4] . The di pole moment
and its components along the princiPal axes were predicted by vectorial
addition of bon d moments taken from ref. 11. In addition it was assumed
that the -CH2C=CH part of the molecule has a dipole moment of about 0.8
D with the acetylene group as the negative end. In this way, the dipole
moment components were predicted to be roughly /.la= 0.4 D, /.lb = 0.2 D,
and /.le = 1.0 D, respectively, for the gauche conformation. Analogous
computations
lead to /.la = 1.3 D, /.lb = 1.2 D, and, of course, /.le= O D for the
hypothetical anti rotamer.
Despite these rather accurate predictions for the gauche form, the assignment was not easily derived. This was due mainly to the relatively dens e
spectrum, the comparatively weak low J transitions, and the rather large
193
centrifugal distortion perturbations of most lines. The initial assignment was
achieved for the K_l = 2
3 c-type R-branch series using a systematic search
procedure resembling the one introduced by van Eijck [12]. With A-C and K
thus obtained, further b- and c-type Q-branch transitions were predicted and
assigned readily. A rather weak line at 15080.58 MHz was then attributed to
the 00,1 11,0transition on the basis of its Stark effect and position in the
spectrum. Preliminary rotational constants were the n derived from these
assignments and used to prediet further low J R-branch transitions. While the
weak b- and the stronger c-type lines were found with ease, no a-type transitions were identified with certainty presurnably because of the 0.29 D dipole
moment component producing insufficient intensities.
Initially, the assigned transitions were fitted to Watson's [13] first-order
centrifugal distortion formula and it became evident that there is a rather
large contribution of centrifugal distortion to the spectrum. As higher J
transitions were subsequently assigned, this expression was found to be
insufficient and Watson's sec ond-order formula had to be utilized in order to
obtain a standard deviation of the fit (a) comparable to the experimental
uncertainty. Ultimately, a total of about 200 transitions were assigned for
the ground state in the 11.9-26.5 GHz spectral region. The c-type Q-branches
were followed up to the 7710,67"""*
7711,67transition, while the highest J-values
assigned for the P- and R-branches involved a 65
64 and a 68
69 transition,
respectively. At such high values of J the rotational energies are so high that
quite weak transitions were observed. Searches for even higher J lines were not
successful presurnably because intensities were too low to make definite
assignments. Table 1lists 47 selected transitions* of the 193 used to derive
the spectroscopic constants shown in Table 2. Computer programrnes MB07
[10] and ROTFIT [14] were used in the analysis and the latter program was
employed to derive the constants of Table 2.
Inspection of Table 1 reveals that centrifugal distortion can amount to
more than 9 GHz even for such a large molecule as methylpropargyl ether.
Contribution from the sextic coefficients are of importanee even for medium
J values and amount to about 90 MHz for the highest Q-branch transitions.
As shown in Table 2 this made it possible to determine significant values for
all seven sextie coefficients.
It should be mentioned that all transitions except several of the high J
c-type Q-branch transitions were not split within the resolution obtainable
with our instrument. The lo west J transition exhibiting res ol vable splitting
was 466 40 467 40found to be split by 0.29 MHz. Splittings in the 0.290.45 MHz range 'were subsequently resolved for about half of the K_l = 6"""*7
to K_l = 9"""*10 Q-branch transitions. The other half of these lines appeared
"""*
"""*
"""*
"""*
"""*
*The complete list of frequencies for the ground as well as for the vibrationally excited
states is available from the authors upon request or from the Microwave Data Center,
Molecular Spectroscopy Section, National Bureau of Standards, Washington D.C. 20234
(U.S.A.), where it has been deposited.
194
TABLE
1
Selected
transitions
Transition
for the ground
vibrational
state of CH3OCH2G=CH
Observed
frequencya
(MHz)
Obs.-calc.
frequency
(MHz)
23775.26
23059.70
21763.90
24165.59
0.03
--0.04
0.04
0.03
-1.78
0.83
1.82
-6.73
0.00
0.00
0.00
0.00
15080.58
21728.00
13851.41
26152.23
23500.48
14114.28
24269.45
24269.45
25695.4 7
14332.72
14332.72
17391.63
17391.63
15804.86
15804.86
18503.28
18503.28
25145.30
25145.30
20093.40
20093.40
16796.46
16796.46
17780.64
17780.64
25297.82
23496.69
19082.33
13288.71
23595.54
14820.33
20896.68
23387.10
12761.16
13794.19
14716.32
19385.82
0.10
0.01
-0.02
--0.08
0.02
--0.08
0.05
--0.02
-0.15
0.01
0.01
-0.06
0.03
0.02
0.02
-0.01
--0.01
-0.05
-0.05
0.07
0.07
0.02
0.02
-0.04
-0.04
-0.14
0.00
0.07
0.07
0.00
0.01
--0.01
--0.05
-0.11
0.04
0.04
--0.20
-0.09
-0.15
-0.47
-0.27
-42.65
-17.96
-64.98
-64.98
-180.43
-3.22
-3.22
-342.16
-342.15
54.79
54.79
105.36
105.36
-1348.18
-1348.18
287.34
287.34
-1804.11
-1804.11
-2072.45
-2072.45
-1.02
0.28
4.95
13.70
61.62
90.43
269.39
648.23
586.53
1096.61
1856.19
3261.26
0.00
0.00
0.00
0.00
0.02
0.10
0.44
0.44
0.37
1.01
1.01
1.21
1.21
3.57
3.57
8.15
8.15
12.89
12.89
18.14
18.14
19.82
19.82
25.76
25.76
0.00
0.00
0.00
0.00
--0.03
--0.05
--0.29
-1.15
-1.26
-3.40
-7.82
-17.13
Centrifugal
Total
(MHz)
dis tort ion
Sextic
(MHz)
b- type
4.,4 -> 50,5
5..4 -> 52.3
8.,7-> 82,6
11,.,0 -> 112.9
c- type
00.0 -> 1..0
10,. -> 2.,.
3..2 -> 40.4
6.,5 -> 70,7
124.8 -> 133,10
166.11 -> 157,9
208..2 -> 199,10
208.13 -> 199,,,
248,'6 -> 257..8
3011,'9 -> 29'2,17
30".20 -> 29'2,'8
34'2,22 -> 35",24
3412.23 -> 35",25
4115.26 -> 40'6,24
4115,27 -> 40.6.25
49'8,31 -> 48,9.29
49'8.32 -> 48'9.30
55'9.36 -> 56'8.38
55.9.37 -> 56.8.39
6022,38 -> 5923.36
6022.39 -> 5923.37
6523.42 -> 6622,44
6523.43 -> 6622.45
6824.44 -> 6923,46
6824.45 -> 6923.47
2... -> 22..
4..3 -> 42,3
7..6 -> 72.6
10..9 -> 102,9
152,13 -> 153.13
182,,6 -> 183,,6
243.2. -> 244.2.
314.27 -> 315,27
344.30 -> 345,30
415.36 -> 416.36
486.42 -> 487,42
547,47 -> 548.47
195
TAB LE 1 (continued)
Transition
60. 52
62;54""
69:60""
70:61""
60. 52
62:54
69,~ 60
70\0'61
76,~.66
76,:.66
77'0,67"" 77".67
Observed
frequencya
(MHz)
Obs.-calc.
frequency
(MHz)
25149.73
16659.04
17881.11
14389.50
19412.86
15713.48
0.08
0.08
0.08
0.06
0.06
-0.09
Centrifugal
Total
(MHz)
5426.37
4425.57
6458.76
5664.61
9220.59
8091.31
distortion
Sextic
(MHz)
-34.74
-30.46
-54.54
-49.44
-93.62
-84.65
a:t0.07 MHz.
to be somewhat broad and are suspected to be split by a very
unresolvable amount. The reason for this splitting is ascribed
of the methyl gro up through its threefold barrier. In deriving
Table 2 the lo w-frequency or so-called A-species components
were utilized.
First excited heavy-atom
small but
to tunnelling
the constants of
of the split lines
torsiona/'state
According to the vibrational spectroscopic work [9] the first excited state
of the heavy-atom torsional mode is located at about 105 cm-1 abo.ve the
ground vibrational state. In accord with this, a series of lines of roughly half
the intensities of their ground state counterparts were assigned relatively
easily to this mode. A total of 121 transitions were assigned. The maximum
J Q-branch line was the 567,49-+ 588.49transition while J = 54 -+ 55 R-branch
and J = 53 -+ 52 P-branch lines were subsequently assigned. As in the case of
the ground state these very high J transitions were quite weak. With the
exception of several c-type Q-branch transitions the lines were not split. The
low-frequency components of the named split lines were employed in the
least-squares procedure with results shown in Table 2.
Barrier to internal rotation of the methyl group
In order to treat the effect of internal rotation observed in the spectrum of
methylpropargyl et her a modification of our computer program described
in ref. 15 was made. Instead of diagonalizing the full matrix of Herschbach
[16] as was done in our previous work [15], the treatment was restricted
to include only the even order terms of Pz to the fourth order. The necessary
formulae employed were taken from Table 11-3 of ref. 17. This Hamiltonian
was considered to be sufficient in this case because of the comparatively high
barrier of methylpropargyl ether.
It is generally advisable [18] to determine the barrier from the ground
state transitions because possible coupling with other vibrational modes is
196
TABLE
2
Molecular constants for CH,OCH,O=CH in the ground and the first excited heavy-atom
torsional states
Vibrational state
Number of transitions
a (MHz)
Ground
First ex. C-O
193
0.0697
121
0.1196
11756.7861
:!:0.0070
3323.7926
:!:0.0020
2795.3321
:!:0.0016
2.8783 :!:0.0060
-20.712
:!:0.045
98.475:!: 0.020
0.90293 :!:0.00094
8.98 :!:0.11
0.0018 :!:0.0015
0.206 :!:0.028
-1.263
:!:0.063
4.15 :!:0.11
0.00381 :!:0.00038
-0.245
:!:0.035
4.43 :!:0.32
Ay (MHz)
By (MHz)
Cy (MHz)
ÅJ (kHz)
ÅJK (kHz)
ÅK (kHz)
/j J (kHz)
/j K (kHz)
HJ (Hz)
HJK (Hz)
HKJ (Hz)
HK (Hz)
hJ (Hz)
hJK (Hz)
hK (Hz)
torso
11830.487
:!: 0.014
3317.5594:!:
0.0042
2790.8917
:!: 0.0036
3.002 :!: 0.014
-22.11
:!: 0.15
110.433
:!: 0.086
0.9257
:!: 0.0036
10.96:!:
0.38
0.018
:!: 0.015
0.11 :!: 0.40
-0.80
:!: 0.76
5.27 :!: 0.94
0.0008
:!: 0.0035
0.32 :!: 0.24
0.1 :!: 4.7
Uncertainties
represent one standard deviation.
a is the standard deviation of the fit.
T ABLE 3
Split ground vibrational
.
of CH,OCH,C,=CH
Observed
Transition
46..40
47..41""
54, 4'
56;49""
61:53""
689' 59
state transitions
467.40
477.41
548 47
56;49
61;53
681~ 59
.
Calculateda
-
vA
(MHz)
vE-vA b
(MHz)
vE-v A
(MHz)
22410.86
18284.03
19385.82
12469.90
20586.22
22003.98
0.29
0.30
0.45
0.29
0.44
0.32
0.20
0.18
0.19
0.13
0.19
0.19
av, = 2512 cal mol-'. b:!:0.15MHz.
normally minimal in this state. As shown in Table 3, splittings of less than
0.5 MHz were the only on es observed for the ground state. These splittings
are small and inaccurately determined and therefore no precise value for the
barrier can be derived from them. Instead we used the first excited state of
the methyl group torsional mode where splittings are larger than in the ground
state as indicated in Table 4. As criteria for the assignments of these trans i-
197
TABLE 4
Microwave spectrum of the first excited state of the methyl group torsional mode and
calculated barriers of CH3OCH2C=CH
Transition
Observed
Barrier
(cal mor')
frequenciesb
vA
(MHz)
15121.77
00.0 --+1'.0
--+
114,. --+123,10
4.,3 --+ 42.3
6,.. --+ 62,s
7.,.--+72,.
8'.7 --+ 82.7
9,.. --+92..
101 9 --+102 9
162:,4 --+ 16;.,4
172.,5 --+173,15
182.,.
20796.12
192,17 --+193,17
233,20 --+234,20
243,21 --+244,21
253,22 --+ 254.22
263,23 --+264,23
324.28 --+32..28
334,29 --+ 335.29
34..30 --+ 345.30
385,33 --+38.,33
405.35 --+ 40.,35
415,3.
--+
aSplitting
0.7'4
-1.21
o.oa
o.oa
O.Oa
0.94
1.27
1.37
3.33
3.59
3.59
3.45
6.08
6.14
5.89
5.47
8.21
7.41
6.62
11.62
9.26
8.17
17833.76
14993.89
12350.90
24886.25
21174.96
17692.82
14513.39
19774.89
16181.00
13025.25
25856.44
17540.25
14123.85
183.,.
--+
o.oa
18451.03
16038.80
12775.60
23619.84
20876.55
19199.10
17363.31
15409.57
13395.22
9'.9
82,7 --+
o.oa
O.Oa
21769.13
2,.,
4'.3 --+ 50..
10.,
vA-vE
(MHz)
41..36
not resolved.
b:!:0.10 MHz.
tions, spectral positions, intensities, Stark
centrifugal
tunnelling
The
distortion formula,
were
and
direction
first order
the characteristic splittings due
cosines of the methyl
discussed
this axis was
group
around
the splittings exactly. These
to be 2512
but :t75 cal mol-1
seems
assumed
The
reasonable
to be 3.20
to
u N.
when
axis were
taken
from
of inertia of the methyl
V3 was
in Table
the n varied to
4. The
average
error limit is difficult to estimate,
taking into account
possible
of the barrier height calculation
that the calculated
using this barrier are of the same
symmetry
moment
values are shown
cal mol-l. The
errors. The parameters
in Table 5.
It is interesting to note
group
below.
match
systematic
summarized
effects, fit to Watson's
used.
the plausible structure
barrier is found
2553
2514
2531
2521
2509
2507
2512
2515
2509
2509
2510
2506
2508
2507
2498
2505
2499
are
splittings for the ground
order of magnitude
as the observed
state
ones
198
TABLE 5
Molecular
constants
Rotational
AA
AE
for the first excited
constants
= 11798.74:t
= 11798.87
:!:
state of the methyl
torsion
of CR, OCR, C=CR
(MHz)
0.07
BA
0.10
BE
= 3323.75:!:
= 3323.81
:!:
0.03
CA = 2794.83
0.05
CE
= 2794.81
:!:
0.03
:!:
0.04
Centrifugal distortion constants (kHz)
A-species
AJ = 4.82 :t 0.29
oK = 18.5 :!: 1.8
AJK
= -32.49
:!:
0.77
D.K
= 244.2
:!:
6.5
OJ
= 0.7185:!:
= -31.31
:!:
0.97
AK
= 233.1
:!:
8.9
OJ
= 0.735:t
.'
0.0077
E-species
D.J
= 4.46
oK = 18.3
:t 0.38
:t 2.2
AJK
0.010
Direction cosines
Aa
V,
= 0.2738
= 2512
:!:
Ab = 0.8674
75 ca! mol-'
Ac
= 0.4163
I~ = 3.20 uA'
a Assumed moment of inerti a of methyl top about
b Reduced barrier.
Uncertainties
represent one standard deviation.
Sb = 72.33
its symmetry
axis.
as shown in Table 3. Moreover, the torsional fundamental frequency is
calculated to be 195 cm-i with this barrier. The solution value of 178 cm-i
found by Bj«;6rsethand Gustavsen [9] seems to be in reasonable agreement
with the calculated frequency since low-lying frequencies generally increase
by 10-20 % on conversion from solution to the gas phase. The frequency at
232 cm-i assigned to this mode by Seth-Paul et al. [8] seems to be inc ompatible with our result.
As mentioned above, several Q-branch transitions of the first excited state
of the heavy-atom torsional state were split. It is shown in Table 6 that this
effect is several times larger than in the ground state. The reason for this
enhanced splitting is presumed to be coupling between the heavy-atom and
methyl group torsional modes. A similar but mue h larger coupling was
observed for CH3OCH2Cl [2]. As the coupling is much smaller than for
chloromethyl methyl ether, no attempts have been made to determine the
coupling parameters which can probably be determined only very inaccurately,
if at all. However, we have fitted an apparent barrier to the observed
splittings. As shown in Table 6 the splittings can be fitted reasonably well
assuming an average apparent barrier of 1851 cal mol-i some 26 % lower than
found from the first excited state of the torsional mode.
Finally, we would like to compare the barrier of methylpropargyl ether of
2512:t 75 cal mori to that of three other molecules. For dimethyl ether [19]
the barrier is slightly higher, viz. 2720 :t 150 cal mori. In anti ethyl methylether it is 2702 :t 7 cal mol-i [7], while a low value of 1.84 :t 0.05 kcal mor!
was found for gauche chloromethyl
methyl ether [2] .
199
T ABLE
6
Split c-type Q-branch transitions of the first excited
apparent barrier of CH3OCH2=CH
Transition
192,17
243,21
253,22
273,2'
31.,27
--+193,17
-+
-+
-+
-+
24.,21
25.,22
27.,24
31,,27
32.,.. -+ 32,,28
34.,30 -+ 34,,30
39, 34 -+ 396 34
40,:"
-+ 406:"
41,,36 -+ 416,36
466,40 --+ 467,40
476.1 --+477..
54;.7 -+ 54:.7
55':.. -+ 558:.1
56,,-'9 -+ 568,'9
heavy-atom
torsional
mode and
vaA
(MHz)
(MHz)
Apparent barrier
(ca! mol-')
12615.87
21651.64
18142.34
12062.24
24453.11
20416.19
13542.33
22320.00
18312.89
14804.14
24039.90
19732.76
21164.74
17184.04
13790.81
0.51
0.99
0.99
0.60
1.18
1.25
1.10
1.53
1.50
1.27
1.76
1.69
1.73
1.56
1.38
1861
1830
1819
1916
1888
1852
1826
1862
1835
1842
1867
1841
1860
1840
1820
Average
1851
VE-VAa
a:!:0.10 MHz.
Further rotamers
Dipole moment calculations made as described above predict that
additional hypothetical rotamers should have rather sizable dipole moments.
A search was therefore made for them. In particular, spectral regions where
the intense low J a-type R-branch transitions of the presurnably fairly polar
anti form were predicted were searched thoroughly, however with negative
results. Moreover, by the extensive assignments made for the gauche form,
all the strongest, most medium intensity, and many weak lines were
accounted for, We therefore feel that if additional forms were present, they
would have been detected if their individual concentrations exceeded about
10 % of the total. This conclusion is supported by absolute intens it y
measurements which were made as described in ref. 20. Although this method
is rather crude, agreements between calculated and observed intensities were
good assuming the presenee of only one, viz., the gauche rotamer,
Structure
The rotational constants of Table 2 fumish insufficient information for
a detailed structure determination. The -CH2-o-CH3 part of the molecule
was therefore assumed to be largely the same as in dimethyl ether [21]. The
geometry of the C-G=C-H
moiety was taken from propyne [22] and was
--
200
assumed to be linear. The C-C-O angle and the dihedral angle were then
varied systematically until the best agreement with the observed rotational
constants was found. The results were: LC-C-O = 112 o :t 2 o, and dihedral
angle 68 o :t 2 o from syn. These values as well as the plausible structure and
the calculated and observed rotational constants are found in Table 7.
Dipole moment
Stark coefficients of the 10,1 ,. 21,1, 31,2""" 32,2 and 61,s ,. 62,s transitions
were used to determine the di pole moment. A d.c. voltage was applied
between the Star k septum and the cell with the modulating square wave
voltage superimposed. The d.c. voltage was calibrated using the OCS 1 ,. 2
transition with fJ.ocs = 0.71521 D [23]. While the 10,1 ,. 21,1 and (h,s""" 62,s
transitions were found to have the usual sec ond order Star k effect, a marked
fourth order contribution was found for the Stark lobes of the 31,2""" 32,2
transition. The second order coefficients for this transition were determined
by plotting t:.v/E2 versus E2. The sec ond order coefficients and their standard
deviations are given in Table 8. A least-squares fit using a diagonal weight
matrix was performed. The weights were chosen as the inverse squares of
the experimental standard deviations of the coefficients appearing in Table 8.
From the fit, unrealistic low standard deviations were obtained. However,
the standard deviations presente d in this table have been derived by taking
possible systematic errors into account.
TABLE 7
Plausible structural
CH, OCHz C=CH
parametersa
Assumed
parameters
structural
c-o
C=C
C-C
C-Hacetylene
and observed
1.410A
1.205A
1.459A
1.055A
1.093A
C-Hmethylene
and predicted
rotational
constants
of
LCOC
111.8o
LCCC
LHCC
LCCH
LOCH
180.0
180.0
o
109.5
109.5
o
Fitted structural parameters
LCCO
112
Dihedral
angle 68
Rotational
AD
Bo
Co
aSee text.
o
o
constants
from syn
(MHz)
Observed
Calculated
Difference
11756.7861
3323.7926
2795.3321
11847.996
3384.674
2854.334
0.8 %
1.8 %
2.1 %
o
o
201
TABLE 8
Stark coefficients and dipole moment of CH,OCH,C=CH
Transition
Av/E'
(MHzV-' cm') X lO'
Observed
31" -+ 3,,2
IMI = 2
IMI= 3
6, " -+ 6",
IMI= 5
IMI= 6
va
ve
0.152
IMI= O
IMI= 1
10,1 -+ 21,1
1.162:!:
-1.86
-4.21
represent
vb
Vtot
one standard
0.152
1.163
0.005
:!:0.02
:!:0.02
-1.82
-4.23
0.171 :!:0.001
0.345 :!:0.001
= 0.290 :!: 0.003 D
= 1.016:!: 0.003 D
Uncertainties
:!: 0.001
= 0.505
= 1.171
:!:
Calculated
0.170
0.345
0.012 D
0.013 D
:!:
deviation.
DISCUSSION
It is not easy to explain the remarkable stability of the gauche form of
methylpropargyl ether in all investigated states. Perhaps a model which
assumes that localized dipoles exist within a molecule can give areasonable
explanation for this preference. Presumably , the dipole of the -CH2C=C-H
part lies roughly along the line C-C=C-H
with the negative end pointing in
this direction. Areasonable assumption would be that the di pole of the
-CH2-O-CH3
moiety lies in the plane of the heavy atoms bisecting the
C-o-C angle with its negative end towards
the oxygen
atom.
In the gauche
form these dipoles would be almost antiparallel which is very favourable and
leads to a stabilisation of this conformation. On the other hand, in the anti
rotamer the se dipoles are predicted to be roughly 55 o from being parallel
and they should thus repel each other.
It is interesting to note that in the related compound CH3CH2CH2C=CH
where the electronegative oxygen atom is replaced by a methylene group,
the gauche and anti forms are approximately equally stable [24, 25]. In this
molecule a destabilizing repulsion of dipoles is very unlikely in the anti form
and may perhaps be the reason why this rotamer coexists with the gauche
contrary to the findings for methylpropargyl ether.
ACKNOWLEDGEMENT
Cand. scient. Claus J. Nielsen is thanked for making the computer program
ROTFIT available to us and for helpful discussions. Mrs. Jorunn Gustavsen
and cand. real. Alf BjiPrseth are thanked for donating the sample and
discussions.
202
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1
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6
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13
14
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17
18
19
20
21
22
23
24
25
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