The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
Conformation of Ethyl Vinyl Ether
1483
Microwave Spectrum, Conformation, and Barrier to Internal Rotation of Ethyl Vinyl Ether Noel L. Owen*+ School of Physical and Molecular Sciences, University Coilege of North Wales, Bangor, Gwynedd, United Kingdom
and (2. Ole Stkensen H. C. Orsted Institute, Department of Chemical Physics, University of Copenhagen, DK-2 100 Copenhagen, Denmark (Received September 25, 1978)
The most stable molecular conformation of ethyl vinyl ether in the vapor phase is shown to be a “sickle” shaped structure with all the heavy atoms coplanar. This conformation dominates the microwave rotational spectrum of this compound and no definitive evidence was found for the presence of other rotameric forms. Combined use of rotational and vibrational data has enabled several of the low frequency modes to be assigned to normal vibrations and evidence is presented to show that two of the skeletal torsional modes are comprehensivelycoupled together. The barrier to internal rotation of the methyl group (12.85 f 0.1 kJ mol-’; 3.070 f 0.025 kcal mol-’) has been determined from an analysis of the doublet lines seen for the two coupled torsional excited states, and also from splittings of high J high K transitions of the ground vibrational state. The dipole moment of C m). ethyl vinyl ether was measured through the Stark effect to be 0.98 f 0.02 D (3.25 f 0.04 X
Introduction Ethyl vinyl ether is an example of a compound containing several axes of internal rotation and consequently there are many possible molecular conformations (Figure 1). Earlier infrared work on this compound produced evidence of rotational isomerism and the conclusion was made that the most stable rotamer had a plane of symmetry and a conformation similar to that shown in Figure la.’ This microwave spectroscopic study was carried out to confirm or to establish the correct conformation of the molecule and to measure the barrier hindering internal rotation of the methyl group. It was felt that, since this is a fairly flexible molecule with several axes of internal rotation, the methyl lbarrier might be affected by molecular interactions. Muchi of our interest in the spectra and structure of alkyl vinyl ethers originated in the laboratory of Professor E). B. Wilson, where one of us (N.L.0) carried out microwave measurements on the methyl analogue of this compound.2 Experimental Section Ethyl vinyl ether was obtained from BDH Co. Ltd., and the compound was redistilled before use (bp 35-36 “C). At Bangor, the rotational spectrum was measured using a conventional 100-kHz Stark modulated microwave spectrometer with ,a 14-ft aluminum X band cell and oscilloscope presentation of spectra. At Copenhagen a Hewletl Packard MRR 8460A spectrometer was used for the final frequency measurements and for obtaining low resolution survey spectra. Experiments were carried out with the samlple temperature approximately -40 O C. Ground State Spectrum Ethyl vinyl ether has a rich microwave spectrum, and a series of strong Q branches of the type J1,J-,-J0,Jand J2,~-2-JL,J-1 were easily assigned from their Stark effects. The assignment was completed by identifying the p B R branch line 212-101, again from its Stark effect, and this enabled many more lines to be predicted and observed. The observed pattern of lines for the vibrational ground state may be seen to fit well with that predicted for a molecular model with coplanar heavy atoms and with the ethyl group orientated as shown in Figure la. 0022-365~/79/2083-1483$01 ~. .OO/O-
The experimental and predicted rotational constants are compared in Table I and the agreement is such that there can be little doubt that the planar “sickle” shaped form (Figure l a ) represents a stable rotamer of this molecule. The rotational constants predicted for all the other likely conformations were significantly different from the experimental values. A careful search of the spectrum between 13 and 40 GHz enabled many more lines to be assigned, and it soon became apparent that the vast majority of the strong absorption lines are associated with this rotameric form of the molecule. Many characteristic satellite lines were observed near the ground state lines, and a summary of all the assigned transitions is given in Table 11, and it can be seen from the quoted standard deviation values that the absorption lines are fitted very
Vibrational Satellite Spectra The main satellites to the ground state absorptions showed precisely the same Stark effects as the parent lines and, consequently, were easily assigned. By extrapolating the frequency shifts between the two series of lines, two further satellite series were found and assigned to higher excited states of the lowest molecular vibration v(1). Three more series of satellite spectra were found which were considerably weaker than the v(1) series and positioned closer to the ground state lines. The behavioral pattern of these lines, as well as their relative intensities, showed convincingly that they represented three states of quite different normal vibrations. Careful measurements at low sample pressures enabled most of the lines from two of these series [v(2) and ~(311to be resolved into close doublets. That one set of satellite spectra (arising from torsion about the C-C bond) would be split into doublets is to be expected, but the appearance of two such series of lines with almost identical splittings was a t first sight rather puzzling. The analysis, and our interpretation, of these lines is outlined in subsequent sections of this paper. A typical satellite pattern is shown in Figure 2, together with other lines observed within a range of 1000 MHz near to the 909-818transition. Confirmation of the assignment of some of the vibrational satellites was effected by means of double resonance. 0 1979 American Chemical Society
1484
The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
N. L. Owen and G. 0. Strensen
TABLE I: Rotational Constants of Ethyl Vinyl Ethera state
AIMHz
B/MHz
C/MHz
AJ/kHz
AJK/kHz
AK/kHz
fij/kHz
fi,q/kHz
predictedb v(0)
16179.69 16307.085 7 16115.581 13 15946.862 16 15772.706 58 16313.757 5 16292.923 14 16318.040 3
2827.99 2816.9430 13 2814.995 2 2813.614 4 2812.619 9 2811.5780 10 2810.200 3 2812.6116 7
2482.76 2478.0885 12 2481.020 3 2484.17 9 4 2487.935 8 2476.1551 11 2477.211 3 2476.9370 9
0.4790 30 0.445 12 0.499 13 0.697 140 0.4757 43 0.428 11 0.4863 30
-2.208 52 -2.30 10 - 2.44 11 -6.22 34 -2.903 36 - 1.62 10 - 2.387 25
37.03 14 25.92 35 34.50 38 36.87 (fixed) 36.58 12 28.55 26 27.39 10
0.08218 38 0.0871 22 0.0787 22 0.0770 41 0.0824 6 0.0687 20 0.0851 5
0.515 76 0.4632 (fixed) 0.4632 (fixed) 0.4632 (fixed) 0.4407 (fixed) 0.4407 (fixed) 0.4632 (fixed)
V(1)
. ,
V(l)(*)C
v(~)(~)c
v(2)
a The uncertainties in the measurements are written below the constants. For a model with parameters as those shown in Figure 6. The superscripts ( 2 ) and ( 3 ) respectively imply the second and third excited states of the V(1)vibrational level.
TABLE 11: Summary of Absorption Lines Measured for Ethyl Vinyl Ether
state
k a lines
fit, lines
total no. of lines
24 R
16P, 30Q, 36RC 2 P, 27 Q, 1 2 R 2 P, 22 Q, 6 R 9 Q, 3 R 3 P, 24 Q, 7 R 4 P, 1 9 Q, 11 R 2 P, 1 6 Q, 4 R
106 43 31 19 35 35 23
1R ~ ( l ) ( ~ 7 R) ~ 1 R v(2) 1R v(4)
’
\c=C
\o
H
H.
H
H
H
H/
\o/C
\c--c/
/H
vapor
0.087 0.080 0.076 0.086 0.027 0.076 0.014
115 * 203dp 210+ -200 195 203 245 280 247 246p 263* 340 3 4 7 ( ~ 3) 3 8 ( ~ )3 3 6 p 3 4 8 i
\
a
CH3
b H
I-!
d
CH3
H‘ C
H
I-!
e
h
Figure 1. Some of the possible molecular conformations of ethyl vinyl ether. Structures a and c have a planar M O C C framework, whereas b and d have the methyl group out of the C=COC plane. Structure e has the ethyl group out of the plane of the vinyl group.
The 606-615 transition for a vibrational state (located around 17500 MHz) was pumped with a high power klystron, and a connecting transition (606-515)searched for and observed near 20 100 MHz. In a similar manner, the 404-413transition (at about 15400 MHz) was pumped and the resonance connected transition 515-404observed near 37 600 MHz. Appropriate “cutoffs” and isolators were employed to prevent the pumping power from affecting observation of the connecting transitions. The assignment of the vibrational states to specific normal vibrations of ethyl vinyl ether is discussed in the following section. Low Frequency Vibrations Of the 33 normal vibrations of ethyl vinyl ether only five are expected to fall below 500 cm-’, and these include three torsional vibrations (tl, tz,t3)and two skeletal bending
solid
interferomr Raman MW int (vapor) (liquid) (vapor)
MHz
EH
H I H p C
IR
a’/
a Standard deviation of fit. The superscripts ( 2 ) and ( 3 ) respectively imply the second and third excited states of the V(1)vibrational level. R-R branch, Q-Q branch, P-P branch. H
TABLE 111: Low Frequency Vibrations (cm-I) of Ethyl Vinyl Ethera
400 397
a
400
399
10 10 15 20
assignment t 0-C,H, tC-CH, tC-0 6 C-0
s
‘l
0-c\
w = weak; dp = depolarized; p = polarized.
1
~(1)
~(2)
v(3) ~(4) v(5)
C
modes (al, 1 3 ~ ) . The three torsions (which are illustrated in Figure 3) are associated with motion about the 0-C, C-C, and C-0 single bonds whereas the two bending modes largely involve deformation of the LC-0-C and LO-C-C angles. The torsional oscillation about the single C-C bond (tz) should be comparable in all respects to the corresponding mode in other monosubstituted ethanes and should therefore be found at approximately 250 cm-’. However, internal rotation about the C-0 bond adjacent to the double bond in ethyl vinyl ether will be affected by the lone pair electrons on the oxygen atom interacting with the x electron system of the double bond. This tends to strengthen the C-0 bond as was found to be the case for methyl vinyl ether where the vibration was found at about 220 cm-1.2 However, the reduced mass for this mode is larger for ethyl vinyl ether than for the methyl analogue and the vibration should consequently be found at a lower frequency. The final torsional mode (tl)associated with movement about the O-C2H5 bond is most likely to be the lowest molecular mode, since the reduced mass is fairly large and there is no possibility of any strengthening effects to raise the frequency. By analogy with other molecules, the two low frequency skeletal bending modes will both probably occur at frequencies higher than 300 cm-’. We have obtained information about the low frequency modes of ethyl vinyl ether from several different sources (Table 111). Four of the five low frequency vibrations were observed for vapor and solid samples by infrared spectroscopy, for the liquid by Raman spectroscopy, and for the vapor by interferometry. We also carried out careful intensity studies on the microwave satellite spectra and obtained approximate frequencies for the four lowest modes from such measurements. It can be seen (Table 111) that there
The Journal of Physical Chemistry, Vol. 83, No. 1 I,
Conformation of Ethyl Vinyl Ether
1979
1485
VI01
I
22500
23000
22000
MHz
Figure 2. Part of the microwave spectrum of ethyl vinyl ether showing the satellite pattern associated with the 908-81stransition. Lines marked with “a” represent those assigned to one of the observed seven vibrational states. The broken horizontal lines give an indication of the noise level for this particular pattern.
M
\
/”=“ HI
/H
,g 3;T3
/
H
Figure 3. Ethyl ‘vinyl ether showing the three low frequency torsional modes.
is good correlation for the frequencies of three of the modes between all the techniques; the lowest mode (below 200 cm-l) could only be observed indirectly from microwave satellite spectra, and the highest frequency vibration was not identifiable through its satellite spectra in the microwave region. The assignment of these vibrations to specific normal modes of the molecule is a matter of some interest. The strongest series of satellite spectra in the microwave region correspond to an energy level having a wavenumber value of 115 f 10 cm-I. The direct absorption a t this frequency was apparently too weak to be observed by any of the other spectroscopic techniques, but in view of the foregoing argument, there can be little doubt that it corresponds to the torsion albout the 0-C2H5 bond (tJ. The next two lowest vibrations ccirrespond to the states v(2) and v(3) seen in the rniicrowave spectrum and they represent the other two torsions (tzand t3).The fact that they lie within about 50 cm-l of ealch other, having the same symmetry
(A” for a molecule with a plane of symmetry), and the fact that both states give rise to doublets in their rotational spectra implies that the two torsions are highly coupled and that the two modes are thoroughly mixed. Both sets of doublet transitions have very similar but not identical splittings, and although both sets behave as perfectly normal internal rotation doublets it is not possible to derive a unique value for the barrier from either set. Assignment of the other two vibrations v(4) and v(5) to specific molecular motions is also complicated by the fact that they occur fairly close to each other (-60 cm-l apart), and are almost certainly of the same symmetry (A’ for a molecule with C, symmetry). Consequently they are probably coupled and it is not possible to definitely assign either one to the following bending modes: C
0
C
I \
I \
C o r 0
C
The pseudo-inertial defect has been calculated for those vibrational states observed in the microwave spectrum and it may be significant that the value for v(4) is “odd” in that it is surprisingly negative for an “in-plane’’ vibration (Table IV) . Barrier to Internal Rotation
Both sets of doublets v(2) and v(3) showed similar splittings and this initially posed an assignment problem since an acceptable value for the methyl barrier could be obtained from either set. It seemed apparent that both torsional states would have to be considered coupled together before a realistic value for V , could be derived from these splittings. If the unperturbed states are described in terms of the torsional or vibrational quantum numbers,
The Journal of Physical Chemistry, Vol. 83, No.
1486
1 I, 1979
N. L. Owen and G. 0. SCrensen
TABLE IV: Pseudo-Inertial Defect of Ethyl Vinyl Ether vibratnl state v(0) v(1) V(
assignment
1)(’)
t 0-C,H, t 0-C,H, t 0-C,H, t C-CH,
s
c-0
s
0-c\
\
-6.4592 -7.1926 -7.8715 - 8.5921 -6.6296 - 6.8444 -6.6199
-0.073 -0.807 -1.486 - 2.206 -0.244 - 0.458 -0.234
I,
ground state
v(l)(,)
- I, -
I, - I, Ib 8dlHx2a
M.
3 7 8 3 0 36927
3 7 8 2 9 36928
iEh
C C
Flgure 4. Internal rotation splitting pattern associated with the 378-369 K doublet.
The pseudo-inertial defect was calculated for la:= 3.193 amu A 2 . a
then the v(2) state, corresponding to torsion about the C-CH3 bond, may be depicted as lO,l,g),while the v(3) state, corresponding to torsion about the C-0 bond, is given by ll,O,u), where g denotes the symmetry (A or E) of the wave function in the C3 symmetry group of the internal rotor. If there were no interaction between these two states then the internal rotation splittings associated with the ll,O,u) state would be identical with those seen for the ground state of the molecules, while the 10,1,0) state should show much large splittings commensurate with the first excited state of the methyl torsion. However since the two levels are obviously interacting, we can combine the individual wave functions in order to give a better description for each state:
11,~)= alO,l,u) III,g) = blO,l,g)
+ bll,O,g) -
all,O,g)
+
where a2 b2 = 1. For a methyl group with a medium or high barrier the splittings observed on the rotational spectra are adequately described by the second-order terms of the perturbation series in the reduced angular momentum of the ~ y s t e m , ~ Wv,(2). Thus, for our new states
WIo(2)= a2Wlg(2)+ b2W0,,(2) WIIo(2)= b2W l o (2) + a2Woo(2)
(1)
The coefficient Wvo(2)is directly related to the difference in the rotational constants of the A and E torsional subspecies for a rigid molecular model, e.g.
AA = F p 2 ( W v ~ ( 2 )W,E‘~’)
(2)
where F = h2/(87r2rI,) is the reduced rotational constant, I , is the moment of inertia of the methyl group, and r = 1- ~ , ( A , I , / I Jwhere A, represents the directional cosine of the methyl group with respect to the principal axes, and pa is given by AaIJIa, and for the model employed for these calculations, A, = cos 16.6O and I , = 3.193 amu A2 (3.193 x amu (nm)2). The value of AA (and AB) for the two observed series of doublets is very similar:
I , is a principal moment of inertia. The parameter
V(2)
AA
-
AE =
-0.6568
BA
AA
-
AE =
-0.5928
BA - BE = -0.0018 MHz
-
BE = -0.0017 MHz
43) thus implying that the coefficients a and b (eq 1) have approximately the same value (l/d5).Thus, these two
Figure 5. Energy level scheme wRh transitions for the 37,-36, K doublet.
states are comprehensively coupled and the mixing is almost complete. From eq 1 we note that
+
w1,(2)
w
110(2)
=
w
10(2)
+
w
(2)
Oa
Thus the sum of the two AA’s gives a value for A(W1i2) of -0.74417 X from eq 2 and using Fp? = 1.6792 X lo3 MHz. This in turn may be related to the reduced barrier s through published tables of the coeff i c i e n t ~ .In ~ this instance the sum of the W@)’S for both ground and first excited states was plotted as a function of s. The value thus obtained is s = 81.62 and since s is directly proportional to V , ( V , = 9/4Fs)a barrier of 12.82 kJ mo1-l (3064 cal mol-I) is obtained. The coefficients of eq 1 are then found as a 2 = 0.5248 and b2 = 0.4752. While searching for medium and high J lines for the ground state rotational spectrum some absorptions were found to be multiplets. These are found to consist of high K,P branch doublets as, for example, 378,30-369,27and 378,29-369,28. These are lines where the asymmetry splitting is small and may even be comparable in magnitude to the internal rotation splittings (Figure 4).5 The result is that the E levels are perturbed by a linear term in the internal rotation contribution (the A levels are not affected), and “forbidden” kLclines appear. The energy level scheme describing these transitions is illustrated in Figure 5. The
+
The Journal of Physical Chemistry, Vol. 83, No. 11, 1979
Conformation of Ethyl Vinyl Ether
TABLE V: Fit of K-Doublet Splittings (MHz) in the Ground State of Ethyl Vinyl Ethera ___-
a
-
TABLE VII: Dipole Moment and Stark Coefficients of Ethyl Vinyl Ether ( A u / E 2 (MHz
rot at ional states
obsd splitting
obsd - calcd
308 31, 31, 3 2, 338 34, 359 36, 369 37 h 389 398 431, 441 1
1.115 3.845 1.085 5.805 1.160 13.140 1.000 1.550 1.060 2.140 1.070 4.340 0.980 1.690
0.015 0.026 -0.019 0.023 0.022 -0.006 -0.068 - 0.067 - 0.009 0.006 - 0.002 - 0.002 - 0.022 - 0.011
1487
VZ
-
cm2)x 1O6Ia transitn
M state
obsd
calcd
0 0 5 4
12.067 5.584 7.624 8.334
12.652 5.598 7.649 8.681
1,i-Onn
2,,-1ni 51 4-505 41 3-404 pa pb a
= 0.51 i 0.02 D = 0.84 i 0.02 D
kc =
0.0 (assumed) 0.98 f 0.03 D
@tot=
Stark coefficients. H
Standard deviation 0.031 MHz.
TABLE VI: 13arrier to Internal Rotation of CH, Group source
V,/kJ mol- ’
1. combinatio:n of both V(2) and V(3)i excited
12.82 state splittings, using molecular model as per Figure 6 2. high J, high K ground state splittings, with 12.87 model as per Figure 6 3. effective ground state constants with new angles 12.90 between top and A axis adjusted to 15.4 for consistancy between ground state and excited state V, values final value = 12.85 i 0.1 kJ mol-’ (3070 c 25 cal mol-’)
splitting of the E lines is a very sensitive function of the barrier height, and these high K high J lines may be analyzed using the perturbation coefficients of the internal rotation in essentially a similar way to that outlined for the excited states.6 Such an analysis carried out on several transitions with 30
