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Internal rotation in propargyl alcohol from microwave spectrum
JOURNAL
OF MOLECULAR
Internal
SPECTROSCOPY
26,
335-350
(1968)
Rotation in Propargyl Alcohol Microwave Spectrum
from
EIZI HIROTA Department
of Chemistry,
Faculty of Science, The University of Tokyo, Hongo, Tokyo, Japan
The microwave spectrum of propargyl alcohol has been observed for the normal species as well as a deuterated species HC=CCHZOD to study the internal rotation of the hydroxyl group in the molecule. The a-type R-branches indicated that the stable form was gauche, while no lines due to the trams isomer were observed. In addition to the pure rotation spectrum simultaneous transitions of t.he overall and internal rotation were detected. The separation between the lowest two levels of the internal rotation was determined to be 21.49 cm-’ for the normal species and 7.14 cm-’ for the deuterated species. An analysis leads to the result that the potential function has a minimum at 121” f 3” from the trans position, a low hump of 90 f 10 cm-l at, the cis and a much higher barrier of (6 f 2) X lo2 cm-1 at the trans. INTRODUCTION
Little has been known about the internal rotation of the hydroxyl group in molecules, since the mass of the hydrogen atom is too small to investigate its internal motion by the methods currently available. Microwave spectroscopy is sensitive enough to detect a small perturbation of the OH motion on the overall rotation. Although the OH group is an asymmetric internal rotor, an approximate treatment can be carried out on the OH internal rotation, if the framework is heavy compared with the OH group. Propargyl alcohol is a molecule suitable for the study of the OH internal rotation, since it has only two CCC bending modes as the low-frequency vibration. The frequency of the in-plane bending is about 210 cm-‘, whereas that of the out-of-plane vibration will be 300-350 cm-‘. Other vibrational states would be probably higher than 400 cm-’ above the ground state (1). Propargyl alcohol may be considered as a derivative of methyl alcohol; it is derived from methyl alcohol by replacing one of the methyl hydrogen atoms by an acetylenic group. It is thus expected that the molecule exists in two rotameric forms, truns and gauche (see Fig. l), and the barrier height separating these isomers is about 1 kcal/mole (2). This is, however, not the case, as will be shown later. 335
336
HIROTA
H
aH
Y
Y
Y
$ C
F C H
@
H
H
H
GAUCHE FIG. 1. Schematic
diagram
EXPERIMENTAL
TRANS of propargyl
alcohol
METHOD
A sample of propargyl alcohol, obtained from Wako Pure Chemicals Industries, Ltd., was used without further purification. The deuterated species, HC=CCHzOD, was synthesized by mixing the normal species with deuterium oxide. The mixing was directly made in the absorption cell. The vapor pressure of propargyl alcohol was about 60 mmHg at room temperature, whereas that of DzO was about 100 mmHg. Four volumes of the DzO vapor were thus mixed with nine of the propargyl alcohol vapor to obtain the maximum concentration of the deuterated species. The HC=CCHzOD species will hereafter be referred to as the D-species, whereas the normal species as the H-species. Microwave spectrometer used was of conventional Hughes-Wilson type, with both sine-wave and square-wave Stark modulators of 120 kc/set (3). The spectrum was observed at room temperature and displayed on a cathode ray oscilloscope or on a recorder. ROTATIONAL SPECTRUM, U-TYPE R-BRANCHES
By assuming a set of reasonable parameters, the rotational constants and the dipole moment (components along the principal axes) were computed as functions of the internal rotation angle +, where 4 was taken to be zero at the tram position. Since the molecule is a near-prolate symmetric rotor (at any 4) with sizable or, component both at the truns (+0.50 0) and the gauche ( - 1.17 O), search was made on the a-type R-branches. The observed lines were listed in Table I, where the strongest set was designated by G, the second strongest by V( 1)) and the third one by V( 2). There were found many weaker satellites. The spectral pattern of the 2-l transitions of the H-species is illustrated in Fig. 2. The data on the D-species were also included in Table 1.l 1The a-type R-branches have also been observed by J. Sheridan and K. Bolton for the normal and three deuterated species. The author wishes to thank Profe&sor Sheridan for sending their results prior to publication.
INTERNAL
ROTATION
IN PROPARGYL TABLE
337
ALCOHOL
I
OHSERVED FREQUENCIES OF THE ~-TYPE R-BRANCHES OF PROP.~RGYL ALCOHOL (Mc/sec)
G
10,&h 213-11, 2”2-la, kl,o 313-212 3or20z 3~221 32,-2x 31r2,, 414-313 401-303 423-322 4x331 4,1-330i 4PT321 41n-312
HC=CCHzOD
HC=CCHzOH
Transition
V(1)
V(2)
G
V(1) 8728.50 16984.44 17450.35 17930.41 25472.11 26158.62 26189.39 26214.81 26891.25 33955.32 34847.06 34913.07
8932.48 17397.05 17858.96 18333.13 26091.82 26773.63 26798.86 26821.71 27495.72 34782.16 35670.59 35726.55
8912.18 17368.10 17818.56 18280.65 26048.52 26713.77 26737 70 26759.54 27417.22 34724 90 35592.24 35645.46
8942.60 17411.67 17879.13 18359.45 26113.48 26803.43 26829.20 26852.66 27534.95 34810.72 35709.67 35766.87
8743.30 17004.73 17479.75 17968.81 25502.34 26202.14 26230.36 26256.60 26948.51 33995.12 34903.49 34967.52
35745.01
35663.59
35785.79
34985.50
35784.77 36653.70
35700.70 36549.35
35826.64 36705.55
35036.11 35923.08
a Overlapped
V(2) 8769.52 17040.24 17531.79 18039.09 25555.27 26279 00 26310.05 26338.10 27053.62 (34064.83)” 35003.87 35073.71 35093.88 35147.87 36062.66
34979.12 35847.05
by other line
G
2,,4--IKI
V(l) V(2) I
18II
kMc/sec
I
Id.4 2 ozcL
I 17.3
I 17.4
17.5
FIG. 2. Spectral pattern of the 2-1 transit’ions of propargyl
17.6 alcohol,
HC=dKXLOH
338
HIROTA
The a-type R-branches were well accounted for by the rigid-rotor formula with allowance for small centrifugal distortion effects. This was the case not only for the G spectrum, but also for V( 1) and V(2). The rotational constants determined are listed in Table II. The observed values of B and C agree with those calculated for the gauche form. Furthermore, the observed value of I, + Ir, - I, for the H-species, 4.3 amu A’, is definitely larger than 3.162 amu A’, a value expected for the “planar” forms, such as trans or cis. No a-type R-branch lines were observed for the trans form. It is thus concluded that the molecule exists mainly in the gauche form. The rotational constants of the D-species indicate that the deuteration has taken place only for the hydroxyl hydrogen, whereas neither the methylene nor acetylenic hydrogens were replaced by deuterium atoms. Although the intensity of the V( 1) spectrum amounted to 60-70 % of that of G, no spectrum due to the 2 X V( 1) state was found at the frequency twice as far as that of the V( 1) spectrum from the G line. A relative-intensity measurement (4) on the 20~-101 transition of the H-species gave 0.67 for the ratio of V( 1)/G at room temperature. This figure should be corrected for several factors to derive the energy difference between the V( 1) and G states. The most important correction factor is the dipole moment, which was estimated from the Stark effect of the M = 1 components of the 211-11~and 212-111transitions. It was concluded that V( 1) was located at 28 f 10 cm-’ above the ground state. The V(2) state is assigned reasonably to the first excited state of the in-plane bending. SERIES
OF LINES
WITH
FIRST-ORDER
STARK EFFECT
a. P- and R-branches. In order to determine the A constant accurately, the b- or c-type &-branches were searched carefully. This attempt was proved not successful, instead series of strong lines were observed. These lines were characterized by their first-order Stark effect. As an example, a line at 33549.48 MC/ TABLE
II
ROTATIONAL CONSTANTS OF PROPARGYL ALCOHOL (Mc/sec) Rotational constant
G
VU)
V(2)
HChCCH20H
B (A -
(WC+ Q/2)
4700.29 4232.25 26.8 X lo3
4684.23 4227.96 26.9 X lo3
4708 *34 4234.45 26.2 X lo3
4612 71 4130.67 24.8 X lo3
4600.85 4127.86 23.7 X lo3
4634.63 4135.20 23.8 X lo3
HCkCCHzOD
B (A -
c @ + (J/2)
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
339
set, is shown in Fig. 3. The line was recorded by using a sine-wave modulation and by applying adc bias of about 1000 V/cm. The Stark effect is nearly of the first order with a small contribution from the second order, thus producing an asymmetry in the pattern. The IMI = J components (the outermost ones) are clearly weaker than the IlMl = J - 1 components, hence the transition is a P- or R-branch line. The number of components, which is equal to 2J* + 1, is 21, therefore the lower J* value of the transition is 10. As shown in Fig. 4, the lines of this sort fall on nearly straight lines; here the abscissa denotes the lower J* value and the ordinate the frequency of the transition. The open circle means that all Stark components are resolved and identified. The separation between the successive members of one series is nearly equal to B + C obtained from the u-type R-branches. The transition frequencies are listed in Table III.
FIG. 3. Stark effect of the (-)J = 10, K = 9 +- (+).I= 11, K component in the high-frequency side is overlapped by other line.
= 10 line. The M = 5
340
HIROTA
/
; K-IO-11 40
= / K’b-IO
30 20 IO
- J*
0 IO 20 30 40 50 *
FIG. 4. The five series, I-V, of P- and R-branches with the first-order Stark effect (for the H-species). The abscissa is the lower value of J of the transition and the ordinate is the transition frequency. The upper part is R-branch and the lower part P-branch. (0), a transition for which all Stark components are resolved, and (0) other observed transitions; (0) transitions are not observed; and (X), termination of the Series I and II.
It is important to point out that the J* = 9 member of the Series I and the J* = 8 member of the Series II are missing, as indicated by cross marks in Fig. 4. This fact is compatible with the assignment that the K value should be smaller than, or equal to J*. The asymmetry splitting for all members of the Series I-IV is negligible, whereas for the Series V the splitting was 2.77 Mc/sec for J = 27-25 and 41.87 Mc/sec for J = 34-35 transition, respectively. These doublet separations are exactly of the order of magnitude expected from the a-type R-branches. These series of lines are thus expected to be accounted for by the symmetric-rotor formula. The results described above can be explained on the basis of the following assumptions: (i) There are two internal-rotation states: one is symmetric and the other antisymmetric with respect to the internal-rotation angle 4. The states, G and V(l), correspond to the symmetric and antisymmetric state, respectively. (ii) Each of the internal-rotation states is accompanied by overall-rotational states that are expressed by a symmetric-rotor formula. These assumptions are certainly drastic and will be refined in a subsequent paper.
INTERNAL
ROTATION
IN PROPARGYL
TABLE ORSERVED
FREQUENCIES
341
ALCOHOL
III
OF P- AND R-BRANCH SIMULTANEOUS TRANSITIONS FOR HCkCCH20H (Mc/sec)
J’, K’ + J”, K”
~_ ~-.
-
Series I
Series IV
(+) 11, II-(--) (+) 12, II-(--) (+) 13, ll-(-) Series II (-) (-) (--)
Y! 9-(+) 10, 9-(+) 11, 9-(+)
(-) (+) (+)
12, 9-(-t-) 16. lo-(-) 17, IO-(-)
19 008.09 28 120.81 37 247.53
10, 10 11, 10 12, 10 13, 10 15, 9 16, 9
42 G79.99 33 549.48 24 401.12
9 9 9
15 12 21 30 40 49
234.85 367.81 602.64 853.84 121.46 404.91
16, 8-(+) 17, 9 17, 8-(-t) 18, 9 18, 8-(f) 19, 9 22, 9%(-) 21, 8 23, 9%(--) 22, 8 24, 9-(-) 23, 8
31 22 12 15 24 33
308.69 037.52 749.16 216.86 571.15 940.88
(f) 18, lo-(-) (+) 19, l@(--_) (t-) 20, lo-(-) Series III (-) (-) (-) f+) (+) (+)
10, 10 11, 10 12, 10
17, 18, 19,
(-) (-) (-) (--) (+)
21, 7-(+) 22,7-(+) 23, 7-c+) 24, 7-c+) 27, 8-(-)
(f) 28,8-w (f) 29, %(-) (+) 30, 8-(-) Series V
38 173.14 28 814.59 19 441.18 10 053.55 87G2.22 18 189.04 27 627.56 37 077.20
22, 8 23,8 24, 8 25, 8 26, 7 27,7 28, 7 29, 7
(-)
27, 6-c+)
28, 7
(--)
28, e-(+1
29, 7
(--)
29, 6-(+)
30, 7
(+)
33, 7-c-j
32, 6
(+)
34, 7-c+)
33, 6
(+)
35, T-(-)
34, 6
35 35 26 26 16 16 11 11 20 20 30 30
806.83 804.06 395.58 391.30 977.46 971.09 325.24 304.78 766.23 736.80 210.22 168.35
The five series of Table III are assigned as (-I- )J + 1, K -I- 1 tf ( - )J, K, where (+ j and ( - ) mean the symmetric and antisymmetric states and the K value is 10 for the Series I, 9 for II, 8 for III, 7 for IV, and 6 for V. An alternative choice of the assignment, (+)J + 1, K +-+ (->J, K + 1, is rejected by the consideration of the magnitude of the Stark effect. As is stated above, the K-splitting observed for the Series V gives further support for the present assignment. The frequency of the transition ( ->.J, K + (+>J + 1, K -I- 1 is then given bJ l&F = Do +
%(J
+
1) +
WJ
+
1)” +
Q(J
+
1)3,
(1)
where g,
= A -
(2K
1)8
+
P1 = -2B 9 _ 2 = -2b r,.3 = 40,.
(2K2 + 2K + 1)~ + (4K3 + 6K2 + 4K + l)DK,
+ (2K2 + 2K + l)DJK, + (2K + l)DJK,
(2a) (2b) (2c) (2d)
342
HIROTA
In deriving Eq. (1) the centrifugal distortion is assumed to be expressed by
-D.,J’(J
+ 1)’ - D.,KJ(J + 1)K2 - D,cK4,
(3)
for both states. The internal-rotation splitting A is taken to be independent of the rotational states, and g=LGB,
(4)
and B = (B + C)/2,
(5)
are average values for the (Jr ) and ( - ) states, that is 8* = B f
a,
(6)
B* = B f
b.
(7)
The constants ZJ, in Eq. (1) were determined for the five series and are listed in Table IV. For the series I b3 was assumed to be the same as that for the Series II ( +0.041865 Mc/sec). The K-type doublets of the SeriesV were averaged in the determination of LD,. The expansion Eq. (1) converges slowly, particularly for high-J lines. Therefore, it is difficult to give much significance to the centrifugal-distortion constants derived from 50,,. For the D-species two series were observed and assigned: the Series I included R-branches, (+)J + 1, K = 4 + (-)J,K = 3 with J = 5, 6, and 7 and the Series II consisted of P-branches, ( - ) J, K = 2 t ( + ) J + 1, K = 3 with J = 5, 6, 7, and 8, and R-branches, (+) J + 1, K = 3 +- ( - ) J, K = 2 with J = 10, 11, 12, and 13. All three lines of the Series I and the lower J = 6 and higher J = 11 member of the K-type doublets of the Series II were definitely assigned by observing well-resolved Stark components. The two Series of transitions were all doublets because of the K-type doubling. Since the deuteration on the hydroxyl group reduced the internal-rotation splitting to a large extent, the simultaneous transitions with lower-6 values were observed in the microwave region. The magnitudes of the K-type splittings gave further support for the assignment. The frequencies of the transitions are listed in Table V. TABLE
COEFFICIENTS %YLFOR THE P-
I II III IV V
80 132 185 238 291
235.61 938.16 859.65 998.76 608.39
IV
AND R-BRANCHES
- 8933.61 -8926.111 - 8901.087 -8854.885 -8709.715
FOR HCzCCH20H
-8.51 -10.3895 -12.3165 -14.7609 -20.7183
(Me/see)
(+0.041865) +0.041865 +0.066543 +o .105753 +0 .196438
INTERNAL
ROTATION
IN PROPARGYL
TABLE
343
ALCOHOL
V
OBSERVEDFREQUENCIESOF P- AND R-BRANCH SIMULTANEOUSTRANSITIONSAND COEFFICIENTSD, FOR HC=CCHz OD (Mc/sec)
J’, K’ t
J”,
K”
J', K'
Vobs
c
J”, K”
Vobs
Series II
Series I (+I
6,4-(-1
5,3
(f)
7, 4-c-1
6, 3
(+I
8,4-!--j
7,3
Series II (-1 5, 2-_(f) c-)
6, 2-c+)
6, 3 7, 3
15 15 24 24 32 32
414.34 413.33 197.30 193.05 983.22 972.67
34 34 26 25
764.39 540.16 082.10 635.31
17 471.46 16 675.19 8957.28 -
(-1
7, 2-c+)
8, 3
c-1
8, 2-c+)
9, 3
(+I
11, 3-c-1
10, 2
10 661.52 -
(+I
12, 3-c-1
11, 2
(+I
13,3-(--J
12, 2
(+I
14, 3-c-1
13, 2
15 19 23 29 31
756.38 970.22 654.24 400.10 366.95
-
Series I Series II
37 244.17 87 585.69
- 8772.045 - 8846.805
-0.715 4.095
The average frequencies of the K-type doublets were subjected to an analysis based on Eq. (1). For both Series I and Series II Z& was assumed to be zero. The derived an’s are given in Table V. b. Q-branches. It is predicted from the analysis of the P- and R-branch Series that at least two series of the Q-branches of the H-species fall in the frequency region accessible with the present spectrometer. The frequency of the transition (-)J,Kc(+)J,K+lisgivenby &
= Ro +
&J(J
+
1) +
RzP(J
+
1Y +
&P(J
+
l)“,
(8)
where &I = %O)
(9)
91 = 302.
(10)
and
In fact two series were observed; the first one, the Q-l series, was (-)J, K = 11 t (+)J, K = 12 and the second one, the Q-II series, (+)J, K = 13 c (-)J, K = 12. Th e members of J = 1240 were observed for the Q-I series and those of J = 13-24 for the Q-II series. Observed frequencies are listed in Table VI. Figure 5 shows a Q-branch line at 25 383.03 Mc/sec, recorded by
HIROTA Table VI Observed Frequencies of Q-branch Simultaneous Transitions (in Mc/sec) IllI=11131P3=I====PI_p==I=Dp===III================= ===========PIPI==.===P== J value
HECCHeOH
HC%CH,OH
(-)3,K=l?1~+)J,K=12 (+)J,X=l;&,X=12
HCfCCHzOD (+)J,K=5-(-)J,K=4
5
13106.28
6
13392.01
7 8 9 10 11
13717.03 14077.91 14471.35 14894.16 15344.26 15340.55 i15816.55 15810.19 i16309.30 16298.27
12
27121.28
13
27145.70
25383.03
14
27176.39
26107.62
i16819.75 16800.97
15
27214.08
26866.29
{i;;i:%i .
16
27259.85
27657.15
i 17833.50 17886.69
17
27314.56
28478.30
I ::gi
18
27379.56
29328.06
19
27455.59
30204.93
*;z . t18871.50 1g005.65
i19378.74 19583.95 20 27543.94 31107.38 I19869.39 20176.co 21 27645.71 32034.2c {20335.01 20784.49 22 27761.94 32984.14 i20766.34 21412.83 23 27893.88 33956.24 i21152.56 22066.49 24 28042.63 34949.61 {21430.83 22752.70 25 28209.33 I21737.04 23481.4 -------~_"'_~"~~~~"'~_~"--_'--_~______~-----_ __ _____________---_____________________~~~~~~~-~~---~--~-----------------
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
345
Table VI (continued) ____________________-_____-__-__-__-___------__---___---__-~_--~~~ ___________________________-_-_____-_--__-~_--_~-_-__---_----~-~__ J value -
HCSCCH,OD
iiCXCH,OH (-)J,K=ll-(+)J,K=12
--
(+)J,&5-(-)J,K=4 21905.41 24264.2
26
28395.13
I
27
28600.98
'21968.32 I 25116.6
28
28828.03
21907.25 I26058.6
29
29077.27
30
29349.55
31
29645.78
32
29966.86
33
30313.45
34
30686.
35
31085.48
36
31512.20
37
31966.32
38
32448.41
39
32958.69
40
33497.31
lo
using sine-wave modulation and a DC bias of about 500 V/cm; here nine components, M = f13, f12, ..a 1t5, are clearly seen on both high- and lowfrequencies sides of the zero-field frequency vo . The J value is unambiguously determined to be 13, as illustrated in Fig. 6. Both series terminate at the expected J-values (J = 12 and J = 13), leading to the assignment of the K values. Fitting to Eq. (8) gave coefficients shown in Table VII. The convergence was very slow for the Q-II series. The values of &, and & in parentheses are estimated from the P- and R-branch series mentioned earlier. The agreement is satisfactory in view of the model chosen. Only one Q-branch series was assigned for the D-species. The frequencies are given in Table VI. The J values were determined by observing the Stark effects for the J = 9 and 11 lines. The series terminates at Jmin = 5, hence it is assigned to ( +)J, K = 5 t ( -)J, K = 4, of which J = 5-28 were observed. This assignment is also consistent with the magnitudes of the K-type doubling separations. The series again converges slowly, and it is difhcult to determine the coefficients R,, of Eq. (8). The results are given in TabIe VII. I he values of $I!,,and RI are compared with those estimated from the P- and R-series and given in parentheses.
HIROTA
346
-v FIQ. 5. Stark effect of the (+)J the zero-field frequency.
= 13, K
= 13 +- (-)J
= 13, K
= 12line. Here ~0means
MC/S
25410-
380-
360-
FIG. 6. Stark shifts of M components of the (+)J line are plotted against M values.
= 13, K = 13 +- (-)J
= 13, K = 12
INTERNAL ROTATION IN PROPARGYL ALCOHOL
347
TABLE VII COEFFICIENTS J?, FOR THE Q-BRANCHES(Mc/sec)
HCkCCHtOH (-)J, K = ll(+)J, K = 12 HGCCHzOH (+)J, K = 13(-)J, K = 12 HCkCCH 201) (+)J, K = 5(-)J, K = 4
-0.0535 (-6.68)8
+0.693640
-0.344
x l(r”
-26 273.77 (-24 507p
-30.229 (-4.9op
+0.01279
-5.314
x lo-6
-12 372.23 (- 12 975)”
-25.224 (--5.53p
+0.02253
-14.524
X l(r6
27 057.75 (27 753)”
* Estimated from the data on the P- and R-branches. POTENTIAL FUNCTION FOR THE INTERNAL ROTATION
For the normal species five L% values are available; when the first four values of L&, (those for the Series I-IV) are substituted into Eq. (2a), the following parameters are determined : A = 644 319.69
Me/see
8 = 27 416.33
Mc/sec
a=-
51.60 Mc/sec
DK =
0.055 Mc/sec
The value of DOfor the Series V, calculated by using these parameters, is 292 354.7 MC/see, which is to be compared with the observed value, 291 608.39 Mc/sec. A similar discrepancy has already been mentioned on the value of &( = a,,) for the Q-I and Q-II series. The value of B = A - (B + C)/2 is consistent with that obtained from the a-type R-branches, 26.8 X lo3 Mc/sec. For the D-species only two Is)0values are available. By assuming DK = 0.03 Mc/sec and a = -30 Mc/sec,2 A and B are calculated to be 213 955.79 Mc/sec and 25 352.41 Mc/sec, respectively. The value of Z!lis in fairly good agreement with that obtained from the a-type R-branches, 24.8 X lo3 Me/see, whereas a good estimate of it, 25 490 f 150 Mc/sec, is obtained from the K-type splittings observed for the P- and R-branch Series II and the Q series. The internal-rotation splitting A for the H-species is about 644.3 Gc/sec, or 21.49 cm-‘, in agreement with the result of relative intensity measurement. This is unexpectedly large, because the kinetic energy of the internal rotation 2These values were chosen by referring to the results on the H-species. If both DR and a are equal to zero, A and % are 213 439.49 Mc/sec (or 7.12 cm-‘) and 25 170.76 Me/see, respectively.
348 is of the order of 635 p2 Gc/sec, where p2 is and the between the two levels 635 Gc/sec the free takes place. information is from the on the If the rotation is free, the of A this species be about of 644.3 or 322.2 However, observed of A, Gc/sec or cm-‘, is smaller than above value the free-rotation. simplified Hamiltonian assumed for internal rotation: = Fp2 where
potential function V(4)
=
V(4)
consists of
(1 -
+
:
-
~0~24)
cosine terms: z
(l-
(12)
The group was as a top; a F value 21.198 cm-’ used for H-species and of, 10.856 for the The,matrix elements X were by choosing free-rotation waveas basis. determinant of X 20 were diagonalised using a 5020E electronic at the of Tokyo. only two data, the between the two levels, available, equilibrium angle was adjusted 120”, the position. As in Table three potential are very to 4min. is important note that becomes negative 4min decreases 117“. On other hand, +min of a second occurs at truns posiand is at 108 above the minimum. The shows that level is at the position. This not consistent the experimental that no R-branches of trans form are observed. Since a negative value of Va is not reasonable, and the observed rotational TABLE
VIII
‘POTENTIALCONSTANTS FOR THE INTERNAL ROTATION OF PROP~RGYL AWOHOL 123”
117 V1 (cm-l) Vz (cm-l) V? (cm?) V,i, (cm-‘) V kB”% (cm-*) g&is (cm-’ degree)
-795.15 -425.64 -42.16 78.93 916.24 2636
8 The height of the trams minimum. gauche.
There
-608.50 -304.25 8.14 84.24 684.60 2614 is a barrier
-415.94 -173.95 66.06 93.31 443.18 2716 of 224 cm-l
-147.53 8.61 146.42 106.79 (107.90)” 3022
between
lrans and
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
349
V(9). cni’ 800
600-
400-
0
1 -120°
I 180”
t+ min
I l2C.f
60°
I
O0
FIG. 7. Potential function for the internal rotation of propargyl is assumed to be 120”. Energy levels for the H-species are indicated.
alcohol.
Here
+,,,i
constants of the two species indicate strongly +rninof near 120°,3 it is concluded that the most plausible set of potential constants is &in = 121” f
3”,
Vcis= 90 f 10 cm-l, V trans = (6 & 2)
X lo2 cm-‘.
A potential function with +min = 120” is shown in Fig. 7, where energy levels of the H-species are also indicated. DISCUSSION
For propargyl alcohol the framework is heavy compared with the top, the OH group. Therefore, the internal rotation is approximately separated from the 3 The rs coordinates of the hydroxyl hydrogen atom were evaluated by using the rotational constants of the H- and D-species. It was found that the a, and b, coordinates were in good agreement with those estimated for &,,;,, of 120°, but +min of about 150” was favored by the cJ coordinate. This discrepancy is probably due to the fact that the internal rotation of such a large amplitude as in the present case makes obscure the geometrical meaning of the rotational constants, and the effect is the largest for c, . since c is the “out-ofplane” coordinate.
350
overall rotation, as assumed in the present paper. The coupling of the two motions will be treated in a subsequent paper.4 The a- and b-components of the dipole moment are symmetric with respect to the internal rotation angle, whereas the c-component is antisymmetric. The b-component happens to be small in the region of 4 of 120-180” (gauche to czk), whereas the c-component is large around 120”. Therefore, the transitions between the symmetric and the antisymmetric states were observed simultaneously with transitions between the rotational levels for which the c-type selection rule was applied, AK = fl. It is obvious that no change of the internal-rotation levels is accompanied in the a-type R-branch transitions (5). The potential function shown in Fig. 7 is essentially a double-minimum potential, and the internal rotation splitting is due to the tunneling of the two gauche forms through the cis barrier. It is, therefore, reasonable that detailed information is obtained only for the cis barrier, but not for the trans barrier. In fact, the area of the cis barrier Xois is reasonably constant, as shown in Table VIII. The low cis barrier might be explained by the interaction between the OH group and 7~electrons of the acetylenic group. ACKNOWLEDQMENTS The author would like to express his sincere thanks to Professor Yonezo Morino for encouragement throughout the course of this research. A part of the expenses for the present work was defrayed from the Scientific Research Grant of the Ministry of Education to which the author’s thanks are also due. RECEIVED: November 25, 1967 REFERENCES 1. Estimated from the data on propargyl halides, see for example G. ZERBI AND M. GusSONI, J. Chem. Phys. 41, 456 (1964); J. C. EVANS AND R. A. NYQUIST,Spectrochim. Acta 19, 1153 (1963). 2. E. V. IVASHANDD. M. DENNISON,J. Chem. Phys. 21,13&i (1953). S. C. MATSUMURA, Ph.D. thesis, University of Tokyo, 1964; S. S~ITO, Ph.D. thesis, University of Tokyo, 1966. 4. Y. MORINO ANDE. HIROTA,J. Chem. Sot. Japan 86,535 (1964). 6. E. HIROTA,J. Chem. Phys. 43.2071 (1965). 4 E. Hirota, to be published.
OF MOLECULAR
Internal
SPECTROSCOPY
26,
335-350
(1968)
Rotation in Propargyl Alcohol Microwave Spectrum
from
EIZI HIROTA Department
of Chemistry,
Faculty of Science, The University of Tokyo, Hongo, Tokyo, Japan
The microwave spectrum of propargyl alcohol has been observed for the normal species as well as a deuterated species HC=CCHZOD to study the internal rotation of the hydroxyl group in the molecule. The a-type R-branches indicated that the stable form was gauche, while no lines due to the trams isomer were observed. In addition to the pure rotation spectrum simultaneous transitions of t.he overall and internal rotation were detected. The separation between the lowest two levels of the internal rotation was determined to be 21.49 cm-’ for the normal species and 7.14 cm-’ for the deuterated species. An analysis leads to the result that the potential function has a minimum at 121” f 3” from the trans position, a low hump of 90 f 10 cm-l at, the cis and a much higher barrier of (6 f 2) X lo2 cm-1 at the trans. INTRODUCTION
Little has been known about the internal rotation of the hydroxyl group in molecules, since the mass of the hydrogen atom is too small to investigate its internal motion by the methods currently available. Microwave spectroscopy is sensitive enough to detect a small perturbation of the OH motion on the overall rotation. Although the OH group is an asymmetric internal rotor, an approximate treatment can be carried out on the OH internal rotation, if the framework is heavy compared with the OH group. Propargyl alcohol is a molecule suitable for the study of the OH internal rotation, since it has only two CCC bending modes as the low-frequency vibration. The frequency of the in-plane bending is about 210 cm-‘, whereas that of the out-of-plane vibration will be 300-350 cm-‘. Other vibrational states would be probably higher than 400 cm-’ above the ground state (1). Propargyl alcohol may be considered as a derivative of methyl alcohol; it is derived from methyl alcohol by replacing one of the methyl hydrogen atoms by an acetylenic group. It is thus expected that the molecule exists in two rotameric forms, truns and gauche (see Fig. l), and the barrier height separating these isomers is about 1 kcal/mole (2). This is, however, not the case, as will be shown later. 335
336
HIROTA
H
aH
Y
Y
Y
$ C
F C H
@
H
H
H
GAUCHE FIG. 1. Schematic
diagram
EXPERIMENTAL
TRANS of propargyl
alcohol
METHOD
A sample of propargyl alcohol, obtained from Wako Pure Chemicals Industries, Ltd., was used without further purification. The deuterated species, HC=CCHzOD, was synthesized by mixing the normal species with deuterium oxide. The mixing was directly made in the absorption cell. The vapor pressure of propargyl alcohol was about 60 mmHg at room temperature, whereas that of DzO was about 100 mmHg. Four volumes of the DzO vapor were thus mixed with nine of the propargyl alcohol vapor to obtain the maximum concentration of the deuterated species. The HC=CCHzOD species will hereafter be referred to as the D-species, whereas the normal species as the H-species. Microwave spectrometer used was of conventional Hughes-Wilson type, with both sine-wave and square-wave Stark modulators of 120 kc/set (3). The spectrum was observed at room temperature and displayed on a cathode ray oscilloscope or on a recorder. ROTATIONAL SPECTRUM, U-TYPE R-BRANCHES
By assuming a set of reasonable parameters, the rotational constants and the dipole moment (components along the principal axes) were computed as functions of the internal rotation angle +, where 4 was taken to be zero at the tram position. Since the molecule is a near-prolate symmetric rotor (at any 4) with sizable or, component both at the truns (+0.50 0) and the gauche ( - 1.17 O), search was made on the a-type R-branches. The observed lines were listed in Table I, where the strongest set was designated by G, the second strongest by V( 1)) and the third one by V( 2). There were found many weaker satellites. The spectral pattern of the 2-l transitions of the H-species is illustrated in Fig. 2. The data on the D-species were also included in Table 1.l 1The a-type R-branches have also been observed by J. Sheridan and K. Bolton for the normal and three deuterated species. The author wishes to thank Profe&sor Sheridan for sending their results prior to publication.
INTERNAL
ROTATION
IN PROPARGYL TABLE
337
ALCOHOL
I
OHSERVED FREQUENCIES OF THE ~-TYPE R-BRANCHES OF PROP.~RGYL ALCOHOL (Mc/sec)
G
10,&h 213-11, 2”2-la, kl,o 313-212 3or20z 3~221 32,-2x 31r2,, 414-313 401-303 423-322 4x331 4,1-330i 4PT321 41n-312
HC=CCHzOD
HC=CCHzOH
Transition
V(1)
V(2)
G
V(1) 8728.50 16984.44 17450.35 17930.41 25472.11 26158.62 26189.39 26214.81 26891.25 33955.32 34847.06 34913.07
8932.48 17397.05 17858.96 18333.13 26091.82 26773.63 26798.86 26821.71 27495.72 34782.16 35670.59 35726.55
8912.18 17368.10 17818.56 18280.65 26048.52 26713.77 26737 70 26759.54 27417.22 34724 90 35592.24 35645.46
8942.60 17411.67 17879.13 18359.45 26113.48 26803.43 26829.20 26852.66 27534.95 34810.72 35709.67 35766.87
8743.30 17004.73 17479.75 17968.81 25502.34 26202.14 26230.36 26256.60 26948.51 33995.12 34903.49 34967.52
35745.01
35663.59
35785.79
34985.50
35784.77 36653.70
35700.70 36549.35
35826.64 36705.55
35036.11 35923.08
a Overlapped
V(2) 8769.52 17040.24 17531.79 18039.09 25555.27 26279 00 26310.05 26338.10 27053.62 (34064.83)” 35003.87 35073.71 35093.88 35147.87 36062.66
34979.12 35847.05
by other line
G
2,,4--IKI
V(l) V(2) I
18II
kMc/sec
I
Id.4 2 ozcL
I 17.3
I 17.4
17.5
FIG. 2. Spectral pattern of the 2-1 transit’ions of propargyl
17.6 alcohol,
HC=dKXLOH
338
HIROTA
The a-type R-branches were well accounted for by the rigid-rotor formula with allowance for small centrifugal distortion effects. This was the case not only for the G spectrum, but also for V( 1) and V(2). The rotational constants determined are listed in Table II. The observed values of B and C agree with those calculated for the gauche form. Furthermore, the observed value of I, + Ir, - I, for the H-species, 4.3 amu A’, is definitely larger than 3.162 amu A’, a value expected for the “planar” forms, such as trans or cis. No a-type R-branch lines were observed for the trans form. It is thus concluded that the molecule exists mainly in the gauche form. The rotational constants of the D-species indicate that the deuteration has taken place only for the hydroxyl hydrogen, whereas neither the methylene nor acetylenic hydrogens were replaced by deuterium atoms. Although the intensity of the V( 1) spectrum amounted to 60-70 % of that of G, no spectrum due to the 2 X V( 1) state was found at the frequency twice as far as that of the V( 1) spectrum from the G line. A relative-intensity measurement (4) on the 20~-101 transition of the H-species gave 0.67 for the ratio of V( 1)/G at room temperature. This figure should be corrected for several factors to derive the energy difference between the V( 1) and G states. The most important correction factor is the dipole moment, which was estimated from the Stark effect of the M = 1 components of the 211-11~and 212-111transitions. It was concluded that V( 1) was located at 28 f 10 cm-’ above the ground state. The V(2) state is assigned reasonably to the first excited state of the in-plane bending. SERIES
OF LINES
WITH
FIRST-ORDER
STARK EFFECT
a. P- and R-branches. In order to determine the A constant accurately, the b- or c-type &-branches were searched carefully. This attempt was proved not successful, instead series of strong lines were observed. These lines were characterized by their first-order Stark effect. As an example, a line at 33549.48 MC/ TABLE
II
ROTATIONAL CONSTANTS OF PROPARGYL ALCOHOL (Mc/sec) Rotational constant
G
VU)
V(2)
HChCCH20H
B (A -
(WC+ Q/2)
4700.29 4232.25 26.8 X lo3
4684.23 4227.96 26.9 X lo3
4708 *34 4234.45 26.2 X lo3
4612 71 4130.67 24.8 X lo3
4600.85 4127.86 23.7 X lo3
4634.63 4135.20 23.8 X lo3
HCkCCHzOD
B (A -
c @ + (J/2)
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
339
set, is shown in Fig. 3. The line was recorded by using a sine-wave modulation and by applying adc bias of about 1000 V/cm. The Stark effect is nearly of the first order with a small contribution from the second order, thus producing an asymmetry in the pattern. The IMI = J components (the outermost ones) are clearly weaker than the IlMl = J - 1 components, hence the transition is a P- or R-branch line. The number of components, which is equal to 2J* + 1, is 21, therefore the lower J* value of the transition is 10. As shown in Fig. 4, the lines of this sort fall on nearly straight lines; here the abscissa denotes the lower J* value and the ordinate the frequency of the transition. The open circle means that all Stark components are resolved and identified. The separation between the successive members of one series is nearly equal to B + C obtained from the u-type R-branches. The transition frequencies are listed in Table III.
FIG. 3. Stark effect of the (-)J = 10, K = 9 +- (+).I= 11, K component in the high-frequency side is overlapped by other line.
= 10 line. The M = 5
340
HIROTA
/
; K-IO-11 40
= / K’b-IO
30 20 IO
- J*
0 IO 20 30 40 50 *
FIG. 4. The five series, I-V, of P- and R-branches with the first-order Stark effect (for the H-species). The abscissa is the lower value of J of the transition and the ordinate is the transition frequency. The upper part is R-branch and the lower part P-branch. (0), a transition for which all Stark components are resolved, and (0) other observed transitions; (0) transitions are not observed; and (X), termination of the Series I and II.
It is important to point out that the J* = 9 member of the Series I and the J* = 8 member of the Series II are missing, as indicated by cross marks in Fig. 4. This fact is compatible with the assignment that the K value should be smaller than, or equal to J*. The asymmetry splitting for all members of the Series I-IV is negligible, whereas for the Series V the splitting was 2.77 Mc/sec for J = 27-25 and 41.87 Mc/sec for J = 34-35 transition, respectively. These doublet separations are exactly of the order of magnitude expected from the a-type R-branches. These series of lines are thus expected to be accounted for by the symmetric-rotor formula. The results described above can be explained on the basis of the following assumptions: (i) There are two internal-rotation states: one is symmetric and the other antisymmetric with respect to the internal-rotation angle 4. The states, G and V(l), correspond to the symmetric and antisymmetric state, respectively. (ii) Each of the internal-rotation states is accompanied by overall-rotational states that are expressed by a symmetric-rotor formula. These assumptions are certainly drastic and will be refined in a subsequent paper.
INTERNAL
ROTATION
IN PROPARGYL
TABLE ORSERVED
FREQUENCIES
341
ALCOHOL
III
OF P- AND R-BRANCH SIMULTANEOUS TRANSITIONS FOR HCkCCH20H (Mc/sec)
J’, K’ + J”, K”
~_ ~-.
-
Series I
Series IV
(+) 11, II-(--) (+) 12, II-(--) (+) 13, ll-(-) Series II (-) (-) (--)
Y! 9-(+) 10, 9-(+) 11, 9-(+)
(-) (+) (+)
12, 9-(-t-) 16. lo-(-) 17, IO-(-)
19 008.09 28 120.81 37 247.53
10, 10 11, 10 12, 10 13, 10 15, 9 16, 9
42 G79.99 33 549.48 24 401.12
9 9 9
15 12 21 30 40 49
234.85 367.81 602.64 853.84 121.46 404.91
16, 8-(+) 17, 9 17, 8-(-t) 18, 9 18, 8-(f) 19, 9 22, 9%(-) 21, 8 23, 9%(--) 22, 8 24, 9-(-) 23, 8
31 22 12 15 24 33
308.69 037.52 749.16 216.86 571.15 940.88
(f) 18, lo-(-) (+) 19, l@(--_) (t-) 20, lo-(-) Series III (-) (-) (-) f+) (+) (+)
10, 10 11, 10 12, 10
17, 18, 19,
(-) (-) (-) (--) (+)
21, 7-(+) 22,7-(+) 23, 7-c+) 24, 7-c+) 27, 8-(-)
(f) 28,8-w (f) 29, %(-) (+) 30, 8-(-) Series V
38 173.14 28 814.59 19 441.18 10 053.55 87G2.22 18 189.04 27 627.56 37 077.20
22, 8 23,8 24, 8 25, 8 26, 7 27,7 28, 7 29, 7
(-)
27, 6-c+)
28, 7
(--)
28, e-(+1
29, 7
(--)
29, 6-(+)
30, 7
(+)
33, 7-c-j
32, 6
(+)
34, 7-c+)
33, 6
(+)
35, T-(-)
34, 6
35 35 26 26 16 16 11 11 20 20 30 30
806.83 804.06 395.58 391.30 977.46 971.09 325.24 304.78 766.23 736.80 210.22 168.35
The five series of Table III are assigned as (-I- )J + 1, K -I- 1 tf ( - )J, K, where (+ j and ( - ) mean the symmetric and antisymmetric states and the K value is 10 for the Series I, 9 for II, 8 for III, 7 for IV, and 6 for V. An alternative choice of the assignment, (+)J + 1, K +-+ (->J, K + 1, is rejected by the consideration of the magnitude of the Stark effect. As is stated above, the K-splitting observed for the Series V gives further support for the present assignment. The frequency of the transition ( ->.J, K + (+>J + 1, K -I- 1 is then given bJ l&F = Do +
%(J
+
1) +
WJ
+
1)” +
Q(J
+
1)3,
(1)
where g,
= A -
(2K
1)8
+
P1 = -2B 9 _ 2 = -2b r,.3 = 40,.
(2K2 + 2K + 1)~ + (4K3 + 6K2 + 4K + l)DK,
+ (2K2 + 2K + l)DJK, + (2K + l)DJK,
(2a) (2b) (2c) (2d)
342
HIROTA
In deriving Eq. (1) the centrifugal distortion is assumed to be expressed by
-D.,J’(J
+ 1)’ - D.,KJ(J + 1)K2 - D,cK4,
(3)
for both states. The internal-rotation splitting A is taken to be independent of the rotational states, and g=LGB,
(4)
and B = (B + C)/2,
(5)
are average values for the (Jr ) and ( - ) states, that is 8* = B f
a,
(6)
B* = B f
b.
(7)
The constants ZJ, in Eq. (1) were determined for the five series and are listed in Table IV. For the series I b3 was assumed to be the same as that for the Series II ( +0.041865 Mc/sec). The K-type doublets of the SeriesV were averaged in the determination of LD,. The expansion Eq. (1) converges slowly, particularly for high-J lines. Therefore, it is difficult to give much significance to the centrifugal-distortion constants derived from 50,,. For the D-species two series were observed and assigned: the Series I included R-branches, (+)J + 1, K = 4 + (-)J,K = 3 with J = 5, 6, and 7 and the Series II consisted of P-branches, ( - ) J, K = 2 t ( + ) J + 1, K = 3 with J = 5, 6, 7, and 8, and R-branches, (+) J + 1, K = 3 +- ( - ) J, K = 2 with J = 10, 11, 12, and 13. All three lines of the Series I and the lower J = 6 and higher J = 11 member of the K-type doublets of the Series II were definitely assigned by observing well-resolved Stark components. The two Series of transitions were all doublets because of the K-type doubling. Since the deuteration on the hydroxyl group reduced the internal-rotation splitting to a large extent, the simultaneous transitions with lower-6 values were observed in the microwave region. The magnitudes of the K-type splittings gave further support for the assignment. The frequencies of the transitions are listed in Table V. TABLE
COEFFICIENTS %YLFOR THE P-
I II III IV V
80 132 185 238 291
235.61 938.16 859.65 998.76 608.39
IV
AND R-BRANCHES
- 8933.61 -8926.111 - 8901.087 -8854.885 -8709.715
FOR HCzCCH20H
-8.51 -10.3895 -12.3165 -14.7609 -20.7183
(Me/see)
(+0.041865) +0.041865 +0.066543 +o .105753 +0 .196438
INTERNAL
ROTATION
IN PROPARGYL
TABLE
343
ALCOHOL
V
OBSERVEDFREQUENCIESOF P- AND R-BRANCH SIMULTANEOUSTRANSITIONSAND COEFFICIENTSD, FOR HC=CCHz OD (Mc/sec)
J’, K’ t
J”,
K”
J', K'
Vobs
c
J”, K”
Vobs
Series II
Series I (+I
6,4-(-1
5,3
(f)
7, 4-c-1
6, 3
(+I
8,4-!--j
7,3
Series II (-1 5, 2-_(f) c-)
6, 2-c+)
6, 3 7, 3
15 15 24 24 32 32
414.34 413.33 197.30 193.05 983.22 972.67
34 34 26 25
764.39 540.16 082.10 635.31
17 471.46 16 675.19 8957.28 -
(-1
7, 2-c+)
8, 3
c-1
8, 2-c+)
9, 3
(+I
11, 3-c-1
10, 2
10 661.52 -
(+I
12, 3-c-1
11, 2
(+I
13,3-(--J
12, 2
(+I
14, 3-c-1
13, 2
15 19 23 29 31
756.38 970.22 654.24 400.10 366.95
-
Series I Series II
37 244.17 87 585.69
- 8772.045 - 8846.805
-0.715 4.095
The average frequencies of the K-type doublets were subjected to an analysis based on Eq. (1). For both Series I and Series II Z& was assumed to be zero. The derived an’s are given in Table V. b. Q-branches. It is predicted from the analysis of the P- and R-branch Series that at least two series of the Q-branches of the H-species fall in the frequency region accessible with the present spectrometer. The frequency of the transition (-)J,Kc(+)J,K+lisgivenby &
= Ro +
&J(J
+
1) +
RzP(J
+
1Y +
&P(J
+
l)“,
(8)
where &I = %O)
(9)
91 = 302.
(10)
and
In fact two series were observed; the first one, the Q-l series, was (-)J, K = 11 t (+)J, K = 12 and the second one, the Q-II series, (+)J, K = 13 c (-)J, K = 12. Th e members of J = 1240 were observed for the Q-I series and those of J = 13-24 for the Q-II series. Observed frequencies are listed in Table VI. Figure 5 shows a Q-branch line at 25 383.03 Mc/sec, recorded by
HIROTA Table VI Observed Frequencies of Q-branch Simultaneous Transitions (in Mc/sec) IllI=11131P3=I====PI_p==I=Dp===III================= ===========PIPI==.===P== J value
HECCHeOH
HC%CH,OH
(-)3,K=l?1~+)J,K=12 (+)J,X=l;&,X=12
HCfCCHzOD (+)J,K=5-(-)J,K=4
5
13106.28
6
13392.01
7 8 9 10 11
13717.03 14077.91 14471.35 14894.16 15344.26 15340.55 i15816.55 15810.19 i16309.30 16298.27
12
27121.28
13
27145.70
25383.03
14
27176.39
26107.62
i16819.75 16800.97
15
27214.08
26866.29
{i;;i:%i .
16
27259.85
27657.15
i 17833.50 17886.69
17
27314.56
28478.30
I ::gi
18
27379.56
29328.06
19
27455.59
30204.93
*;z . t18871.50 1g005.65
i19378.74 19583.95 20 27543.94 31107.38 I19869.39 20176.co 21 27645.71 32034.2c {20335.01 20784.49 22 27761.94 32984.14 i20766.34 21412.83 23 27893.88 33956.24 i21152.56 22066.49 24 28042.63 34949.61 {21430.83 22752.70 25 28209.33 I21737.04 23481.4 -------~_"'_~"~~~~"'~_~"--_'--_~______~-----_ __ _____________---_____________________~~~~~~~-~~---~--~-----------------
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
345
Table VI (continued) ____________________-_____-__-__-__-___------__---___---__-~_--~~~ ___________________________-_-_____-_--__-~_--_~-_-__---_----~-~__ J value -
HCSCCH,OD
iiCXCH,OH (-)J,K=ll-(+)J,K=12
--
(+)J,&5-(-)J,K=4 21905.41 24264.2
26
28395.13
I
27
28600.98
'21968.32 I 25116.6
28
28828.03
21907.25 I26058.6
29
29077.27
30
29349.55
31
29645.78
32
29966.86
33
30313.45
34
30686.
35
31085.48
36
31512.20
37
31966.32
38
32448.41
39
32958.69
40
33497.31
lo
using sine-wave modulation and a DC bias of about 500 V/cm; here nine components, M = f13, f12, ..a 1t5, are clearly seen on both high- and lowfrequencies sides of the zero-field frequency vo . The J value is unambiguously determined to be 13, as illustrated in Fig. 6. Both series terminate at the expected J-values (J = 12 and J = 13), leading to the assignment of the K values. Fitting to Eq. (8) gave coefficients shown in Table VII. The convergence was very slow for the Q-II series. The values of &, and & in parentheses are estimated from the P- and R-branch series mentioned earlier. The agreement is satisfactory in view of the model chosen. Only one Q-branch series was assigned for the D-species. The frequencies are given in Table VI. The J values were determined by observing the Stark effects for the J = 9 and 11 lines. The series terminates at Jmin = 5, hence it is assigned to ( +)J, K = 5 t ( -)J, K = 4, of which J = 5-28 were observed. This assignment is also consistent with the magnitudes of the K-type doubling separations. The series again converges slowly, and it is difhcult to determine the coefficients R,, of Eq. (8). The results are given in TabIe VII. I he values of $I!,,and RI are compared with those estimated from the P- and R-series and given in parentheses.
HIROTA
346
-v FIQ. 5. Stark effect of the (+)J the zero-field frequency.
= 13, K
= 13 +- (-)J
= 13, K
= 12line. Here ~0means
MC/S
25410-
380-
360-
FIG. 6. Stark shifts of M components of the (+)J line are plotted against M values.
= 13, K = 13 +- (-)J
= 13, K = 12
INTERNAL ROTATION IN PROPARGYL ALCOHOL
347
TABLE VII COEFFICIENTS J?, FOR THE Q-BRANCHES(Mc/sec)
HCkCCHtOH (-)J, K = ll(+)J, K = 12 HGCCHzOH (+)J, K = 13(-)J, K = 12 HCkCCH 201) (+)J, K = 5(-)J, K = 4
-0.0535 (-6.68)8
+0.693640
-0.344
x l(r”
-26 273.77 (-24 507p
-30.229 (-4.9op
+0.01279
-5.314
x lo-6
-12 372.23 (- 12 975)”
-25.224 (--5.53p
+0.02253
-14.524
X l(r6
27 057.75 (27 753)”
* Estimated from the data on the P- and R-branches. POTENTIAL FUNCTION FOR THE INTERNAL ROTATION
For the normal species five L% values are available; when the first four values of L&, (those for the Series I-IV) are substituted into Eq. (2a), the following parameters are determined : A = 644 319.69
Me/see
8 = 27 416.33
Mc/sec
a=-
51.60 Mc/sec
DK =
0.055 Mc/sec
The value of DOfor the Series V, calculated by using these parameters, is 292 354.7 MC/see, which is to be compared with the observed value, 291 608.39 Mc/sec. A similar discrepancy has already been mentioned on the value of &( = a,,) for the Q-I and Q-II series. The value of B = A - (B + C)/2 is consistent with that obtained from the a-type R-branches, 26.8 X lo3 Mc/sec. For the D-species only two Is)0values are available. By assuming DK = 0.03 Mc/sec and a = -30 Mc/sec,2 A and B are calculated to be 213 955.79 Mc/sec and 25 352.41 Mc/sec, respectively. The value of Z!lis in fairly good agreement with that obtained from the a-type R-branches, 24.8 X lo3 Me/see, whereas a good estimate of it, 25 490 f 150 Mc/sec, is obtained from the K-type splittings observed for the P- and R-branch Series II and the Q series. The internal-rotation splitting A for the H-species is about 644.3 Gc/sec, or 21.49 cm-‘, in agreement with the result of relative intensity measurement. This is unexpectedly large, because the kinetic energy of the internal rotation 2These values were chosen by referring to the results on the H-species. If both DR and a are equal to zero, A and % are 213 439.49 Mc/sec (or 7.12 cm-‘) and 25 170.76 Me/see, respectively.
348 is of the order of 635 p2 Gc/sec, where p2 is and the between the two levels 635 Gc/sec the free takes place. information is from the on the If the rotation is free, the of A this species be about of 644.3 or 322.2 However, observed of A, Gc/sec or cm-‘, is smaller than above value the free-rotation. simplified Hamiltonian assumed for internal rotation: = Fp2 where
potential function V(4)
=
V(4)
consists of
(1 -
+
:
-
~0~24)
cosine terms: z
(l-
(12)
The group was as a top; a F value 21.198 cm-’ used for H-species and of, 10.856 for the The,matrix elements X were by choosing free-rotation waveas basis. determinant of X 20 were diagonalised using a 5020E electronic at the of Tokyo. only two data, the between the two levels, available, equilibrium angle was adjusted 120”, the position. As in Table three potential are very to 4min. is important note that becomes negative 4min decreases 117“. On other hand, +min of a second occurs at truns posiand is at 108 above the minimum. The shows that level is at the position. This not consistent the experimental that no R-branches of trans form are observed. Since a negative value of Va is not reasonable, and the observed rotational TABLE
VIII
‘POTENTIALCONSTANTS FOR THE INTERNAL ROTATION OF PROP~RGYL AWOHOL 123”
117 V1 (cm-l) Vz (cm-l) V? (cm?) V,i, (cm-‘) V kB”% (cm-*) g&is (cm-’ degree)
-795.15 -425.64 -42.16 78.93 916.24 2636
8 The height of the trams minimum. gauche.
There
-608.50 -304.25 8.14 84.24 684.60 2614 is a barrier
-415.94 -173.95 66.06 93.31 443.18 2716 of 224 cm-l
-147.53 8.61 146.42 106.79 (107.90)” 3022
between
lrans and
INTERNAL
ROTATION
IN PROPARGYL
ALCOHOL
349
V(9). cni’ 800
600-
400-
0
1 -120°
I 180”
t+ min
I l2C.f
60°
I
O0
FIG. 7. Potential function for the internal rotation of propargyl is assumed to be 120”. Energy levels for the H-species are indicated.
alcohol.
Here
+,,,i
constants of the two species indicate strongly +rninof near 120°,3 it is concluded that the most plausible set of potential constants is &in = 121” f
3”,
Vcis= 90 f 10 cm-l, V trans = (6 & 2)
X lo2 cm-‘.
A potential function with +min = 120” is shown in Fig. 7, where energy levels of the H-species are also indicated. DISCUSSION
For propargyl alcohol the framework is heavy compared with the top, the OH group. Therefore, the internal rotation is approximately separated from the 3 The rs coordinates of the hydroxyl hydrogen atom were evaluated by using the rotational constants of the H- and D-species. It was found that the a, and b, coordinates were in good agreement with those estimated for &,,;,, of 120°, but +min of about 150” was favored by the cJ coordinate. This discrepancy is probably due to the fact that the internal rotation of such a large amplitude as in the present case makes obscure the geometrical meaning of the rotational constants, and the effect is the largest for c, . since c is the “out-ofplane” coordinate.
350
overall rotation, as assumed in the present paper. The coupling of the two motions will be treated in a subsequent paper.4 The a- and b-components of the dipole moment are symmetric with respect to the internal rotation angle, whereas the c-component is antisymmetric. The b-component happens to be small in the region of 4 of 120-180” (gauche to czk), whereas the c-component is large around 120”. Therefore, the transitions between the symmetric and the antisymmetric states were observed simultaneously with transitions between the rotational levels for which the c-type selection rule was applied, AK = fl. It is obvious that no change of the internal-rotation levels is accompanied in the a-type R-branch transitions (5). The potential function shown in Fig. 7 is essentially a double-minimum potential, and the internal rotation splitting is due to the tunneling of the two gauche forms through the cis barrier. It is, therefore, reasonable that detailed information is obtained only for the cis barrier, but not for the trans barrier. In fact, the area of the cis barrier Xois is reasonably constant, as shown in Table VIII. The low cis barrier might be explained by the interaction between the OH group and 7~electrons of the acetylenic group. ACKNOWLEDQMENTS The author would like to express his sincere thanks to Professor Yonezo Morino for encouragement throughout the course of this research. A part of the expenses for the present work was defrayed from the Scientific Research Grant of the Ministry of Education to which the author’s thanks are also due. RECEIVED: November 25, 1967 REFERENCES 1. Estimated from the data on propargyl halides, see for example G. ZERBI AND M. GusSONI, J. Chem. Phys. 41, 456 (1964); J. C. EVANS AND R. A. NYQUIST,Spectrochim. Acta 19, 1153 (1963). 2. E. V. IVASHANDD. M. DENNISON,J. Chem. Phys. 21,13&i (1953). S. C. MATSUMURA, Ph.D. thesis, University of Tokyo, 1964; S. S~ITO, Ph.D. thesis, University of Tokyo, 1966. 4. Y. MORINO ANDE. HIROTA,J. Chem. Sot. Japan 86,535 (1964). 6. E. HIROTA,J. Chem. Phys. 43.2071 (1965). 4 E. Hirota, to be published.