Microwave spectrum and rotational isomerism of ethoxy ethyne

61 Jownal of Molecular &I Elsevier Scientific

Strucfrrre, 20 (1974)

Pubhshing

Company,

MICROWAVE SPECTRUM ETHOXY ETHYNE*

AND

6 1-74 Amsterdam

- Prmted

ROTATIONAL

III The Netherlands

ISOMERISM

OF

ALF BJQRSETH** Department of Chemistry Texas 78712 (U.S.A.)

(Received

and Center for

Stractrrral Studies,

The

Unioersity

of

Texas,

Amtin,

7 May 1973)

ABSTRACT

The microwave spectrum of ethoxy ethyne, CHaCH20C=CH, has been investigated in the region 12.4-32.0 GHz_ The spectrum demonstrates the coexistence of two rotational isomers, an anti form and a gauche form. The rotational constants for the anti form are A = 29500 k 200 MHz, B = 2502.178 f 0.007 MHz and 2380.034~0.008 MHz and for the gauche form A = 11799.84&0.02 MHz, c= B = 3422.405f0.007 MHz and C = 2887.691 f0.007 MHz. From these rotational constants a plausible structure has beenderived which gives the gauche dihedral angle as 108+3” (from the ant1 position). The dipole moment components for the D and pL, = 0 D (assumed) anti form are p, = 1.62+_0.03 D, p,, = 1.01+0.03 with pr = 1.9lkO.05 D, and for the gauche form p, = 1.72f0.04 D, jib = 0.64f 0.07 D and p, = 0.85+0.02 D with p, = 2.02f0.07 D. Several vibrational satellite lines were also assigned, and relative intensity measurements yielded the torsional frequencies 100 cm-l for the anti and 78 cm-’ for the gauche form. The energy difference between the two rotamers was determined to be 138 +90 cal mole-l, the anti form the more stable. A Fourier expansion of the potential for the V, = 0.04, G-Cclhyl torsion gave the first four coefficients to be: V, = -0.41, Y3 = 1.42 and V, = 0.77 kcal mole-‘.

INTRODUCTION

For a molecule with at least one mode of torsional motion, the possibility of rotational isomerism exists. Conformational equrlibria in the gas phase can be studied by several different techniques, as recently reviewed by Bastiansen, Seip

* This research has been supported by a grant from the Robert A. Welch Foundation. Permanent address: Department of Chemistry, University of Oslo, Oslo 3, Norway.

l*

62

Tb

-2 Fig.

4

I. The ethoxy ethync moIecuIe

in the anti form.

and Boggs’. Microwave spectroscopy is well suited for studying rotational isomerism, since this method can often provide information about the structure of and the energy difference between the coexisting rotamers. In favorable cases, a complete potential function can also be found. The present work is part of a microwave spectroscopic investigation of rotational isomerism about single bonds between carbon and hetero atoms to study conformational consequences of different substituents. To the best of our knowledge, no structural or conformational information is available on ethoxy ethyne. However, the microwave spectra of methoxy ethyne are reported’p 3, and can supposedly give information concerning the structure of the acetylenic part of the molecule. Rotational isomerism about the 0-Cethy, bond has been studied in ethyl formate by microwave spectroscopy and in ethy1 methyl ether by Raman, infrared and far infrared spectroscopy59 6. The anti form of ethyl methyl ether has also been studied by microwave spectroscopy7. Figure 1 shows the ethoxy ethyne molecule in the anti form.

EXPERIMENTAL

A sample of ethoxy ethyne was kindly donated by Professor .J. F. Arens. The synthesis, and chemical and physical properties have been reviewed recentlya. The sample was pumped under vacuum at Iiquid nitrogen temperature to remove volatile gases, vacuum transferred to the sample tube, and used without further purification. The sample decomposed slowly at room temperature and had to be stored at dry ice temperature. Broad-banded gas phase microwave spectra in the frequency region 26.5 to 40 GHz were recorded at room temperature on a Hewlett-Packard 8460A spectrometer at the University of Gothenburg. More detailed studies were done in the frequency region 12.4-32 GHz using a conventional 25 kHz Stark modulation

63 spectrometer described earlier ‘_ Most measurements tion cell cooled with dry ice.

GROUND-STATE

Spectrum

were made with the absorb-

SPECTRA

of the anti form

The broad-banded recorded spectra of ethoxy ethyne revealed distinctive clumps of lines spaced every B+ C = 4900 MHz, characteristic of a-type Rbranch transitions in very nearly prolate rotor molecules. The value of B+ C is in very good agreement with the value 4.8 GHz predicted for the anti form by combining structural parameters of methoxy ethyne3 and I-pentyne”. Measurements of the frequencies of the individual a-type R-branch transitions gave accurate values of B and C as well as a rough estimate of the A rotational constant. The b-type transitions proved to be very difficult to assign on account of the exceedingly rich spectrum and the uncertamty In the A rotatronal constant. By allowing some degree of centrifugal distortion, several sets of lutes could be fitted to the Q-branch serves J,-,, , + JIS,_r (J = 8 to 11) and a final assignment will have to await confirmation by double resonance experiments. The measured frequencies for the assigned transitions of the anti form are given in Table 1. TABLE

1

OBSERVED

AND

(in MHz)

FOR

Transirion

THE

DIFFERENCE

THE

ANTI

FORM

BETWEEN OF

Obsemed’

14463.30 14829.70 19 772.43 19283.84 19524.84 24402.75 24714.65 24103.80 28923.52 29278.74 29305.81

29 656.32 34597.40

p f0.10

E~HOXY

OBSERVED

AND

CALCULATED

GROUND-STATE

FREQUENCIES

ETHYNE

Obserced-calcrrlated

0.14 0.11 0.13 0.09 0.13 -0.03 0.05

-0.10 - 0.03 -0.05 -0.08 -0.04 -0.10

MHz.

Spectrum

of the gauche form

The possibility of rotational isomers rotated around the O-Cctibyl bond. Rotational

appears when the ethyl group is constants for the gauche form were

64

calculated by using the structural parameters already known from the anti form, and attention was focused on a dihedral angle around 115”, as in 1-pentyne” (the anti form has a dihedral angle of O”). Search in the IS-19 GHz region of the spectrum revealed several R-branch transitions with characteristic and well-resolved Stark effect, whrch were assigned to 2 + 3 a-type transitions. The measured frequencies of these transitions were used to predict the other a-type transitions, which in turn gave accurate values for the B and C rotational constants and a good estimate of the A rotational constant. The J,,,_, + J1,,_2 b-type Q-branch series (J = 6-l 1) was found by a Q-plot’ ’ and yrelded a more accurate value of the A rotational constant_ This was used to predict other b- and c-type transitrons, and measurements of several transitions confirmed the assrgnment. The assigned frequencies for the gauche form are tabulated m Table 2. Ground-state

rotational constants

The frequencies given in Tables 1 and 2 for the anti and gauche form respectrvely were used to obtain the final values of the rotational constants for each rotamer. The frequencies for the anti form deviate little from a rigid rotor spectrum and a fit of only the three rotational constants was found to be satisfactory. The frequencies for the gauche form deviate more from a rigid rotor spectrum, and were fitted to Watson’sr2 eight-parameter energy expression for a centrifugal distorted rotor: W = W,-d,J’(J+

1)2 -d,KJ(J+l)-dK-dw,WoJ(J+l)-d,,K

(1)

In eqn. (I) IV, is the energy of the rigid rotor and a function of the rotational constants A, B and C only, and d,, CIIJK, dK, dw, and dWK are the centrifugal distortion constants. The fitting procedure was the method of least squares1 3. The results are given inTable 3 and the calculated frequency differences are listed inTables 1 and 2.

STRUCTURE

Since no isotopically substituted forms of ethoxy ethyne were assigned, an accurate calculation of its molecular structure is not possible. However, the molecular parameters of methoxy ethyne’ can be assumed with some confidence since the replacement of a methyl group with an ethyl group is not expected to affect the framework appreciably. In addition, the methyl group was considered to be tetrahedral with C-H distance 1.094 A and the O-C-C,,,,, angle was taken to be 111.0”. The experimental moments of inertia of the anti rotamer can be reproduced satisfactorily if the COC angle is decreased to 111.6” (from 113.3”) and the C-Cmclhyl

65 TABLE 2 OBSERVED

AND

THE

(in MHz)

FOR

THE

Transrtion

DIFFERENCE GAUCHE

BETWEEN FORM

OF

Obserl;eda

OBSERVED

ETHOXY

AND

CALCULATED

GROUND

E~HYNE

Obserwd

Centrifugal

-caIcrrlated

distortion correction

lo.1 -

20.2

12595.35

0.06

-

II.0 -+

2 1.L

13 154.85

0.05

-

2 I.2 -

3 1.3

18113.07

20.2

-+

3 0.3

18831.23

0.11

22.1

+

32.2

I

8 930.65

0.01

22.0 +

32.1

19029.23

0.14

2 1.1 +

3 1.2

19715.98

30.3 -

40.4

24994.87

31.2

-

41.3

3 2.2

+

42-3

-0.02

-0.05

O.lZ 0.10 0 02

-

0.37 0.36 0.29

-

0.46

0.01

-

0.80

26256.85

0.01

-

25221.16

0.00

1.17 0.07

31.3 -

4 1.4

24 123.23

-0.02

-

022

4 1.d *

515

30111.87

-0.02

-

0.65

40.4 -+

50.5

31068.71

42.3 -

52..+

3 1494.84

5 0.5 + 60.6 -

5 1.4 61.5

13318.52

0.01

-

1.51

15516.21

0.01

-

335

61.5 +

62.4

22201.54

0.04

7 1.6 +

72.6

I8 852.76

004

70.7 +

7 1.6

18254.76

0.03

71.6 +

72.5

21750.05

0.10

8 0-E +

81.7

21546.62

0.04

8 1.7 -

82.6

21637.45

0.00

2.66

8 1.7 + 90,s -

82,7 9 I.8

17004.62 25 364.94

0.00 0.04

11.72 - 18.27

9 I.8 9 I.0 +

9 2.7

21956.98

-0.03

0.72

92.8

15 043.70

-0.05

16 35

0 05 -0.01

-

141

-

055

2.32 7.76 -

6.53 2.98

-11.43

22785.13

-0.07

-

11 l.lO-fll2.9 12 1.113122.10

24 184.15 26201.64

-0.08 -0.14

-11.26 -23.14

142,1~-+143,1z 15 2.1~-fl53.13

26053.99 23 075.33

0.01 -0.04

162,143

20076.61

-0.04

101.9

-

102.a

163~4

17,.,,+17,.,,

17 136 05

0.00

18 z.,~+~%Is

14333.21

0.14

183.16+ 183.15

21773.71

0.06

m fO.10

MHz.

3.62

79.39 96.41 113.02 127.86

139.39 -209.03

STAllI

FREQUENCIES

66 TABLE

3

GROUND-STATE

ROTATIONAL

CONSTANTS

AND

CENTRIFUGAL

DISTORTION

CONSTANTS

OF

ETHOXY

ETHYNE=

A (MHz) B (MHz) C (MHz) :, &K 4c dw, &K

Anti

Gauche

29 500 &200 2502.178 &0.007 2380 034&0.008

I 1799.84&0.02 3422.405f0.007 2887.69 I =tO.O07

-0.990984

-0.880003 -29.3 f0.3

(kHz)

--473.8&g 6 -796.1 +23.9 10.58 kO.05

W-fz) H-W (x

IO61

110.7+3.1

(Xl@)

a Uncertainties

are standard

deviations.

is 1.527 A. No attempt was made to refine the A rotational constant further since the observed value is encumbered with rather large uncertainty. The same parameters were then transferred to the gauche structure, and the dihedral angle was varied to get the predicted and observed rotational constants in agreement. It was, however, found necessary to open both the COC and the OCC angle by l.O”, and to increase the 0-CcthY, bond length by 0.01 A to make the gauche structure agree better than 0.1 %. This observation fits with the slight structural differences between rotameric forms found in other molecules studied4m lo, i4. r5_ Due to steric repulsion, which may exist between the methyl group and the acetylenic group, it is reasonable to expect a slight change in the gauche structure. A torsional angle for the gauche form of 108 +3” (from the anti form) results from the fitting. In Table 4 plausible structural parameters are listed, as well as the predicted and observed moments of inertia.

DIPOLE

MOMENT

as a function of the applied electric The displacement of the Stark lobesi field was used to determine the dipole moment of both rotamers. A number of measurements was made on several Stark components from at least three different lines for each rotameric form, and the data were used in a least squares fitting of the dipole moment components. For the anti form the PC-component, perpendicular to the plane of symmetry, was assumed to be zero. The 1 + 2 transition of OCS was used to calibrate the electric field in the cell, using po, = 0.71521 D17. The observed and calculated values of AvIE’ as well as the dipole moment components of the two rotamers are given in Table 5. The total dipole moment for

67 TABLE

4

PLAUSIBLE

STRUmURAL

PARAMETERS

AND

CALCULATED

1.210 A 1.060 1.527 1.313 1.100 1.095 I.415 anti 1.425 gauche

CGC S-H c-c EC-0

C-H mclhyl C-Hmc,hr,e.e o-C,,w o-CclhyI

AND

111.6” 112.6” 111.0” I 12.0” 109.5” 109.5’

LCOC LCOC iocc LOCC LCCH LOCH

Dihedral angle: 0” anti; 108*3” of inerria (amrr A’):

Monwnrs

OBSERVED

MOMENTS

OF

INERTIA

ant1 gauche anti

gauche

gauche

Obserced

Calculated

17.1 202.036 212.405

16.857 202.029 212.412

42.842 147.712 175.064

42.836 147.889 174.606

Anti I, Ib 1, Gauche 1. Ib I.

TABLE STARK

5

EFFECi-

AND

DIPOLE

MOMENTS

OF

ANTI

An:i form Transition

2 11 +

312

4 13 *

M

0

414

2 0

La

2 0

1

1 2

Dipole =

ISOMERS

OF

ETHOXY

h/E2

(Hz

V-2

cm2)

Obseroed

Calculated

-0 969 -44.4 - 173.7

-44.2 -173.9

10.9 16.5 32.5 -0 489 -1.51 -4.48

-0.945

11.5 16.6 31.7 -0.442 -1.31 -4-07

Transition

101+

202

3 03 *

404

312 *

413

M

0 1 0 1 2 0 1

moments

D j‘b = D pc = 0.0 (assumed) ,+ = 1.91&:0.05 D

p.

GAUCHE

1.62f0.03 1.01&0.03

ETHYNE

Gauche form

1 3 13 +

AND

p. = pb = pc = pLT=

1.72f0.04 0.64f0.07 0.85&0.02 2.02fO.07

D D D D

h/E2

(Hz

V-2

cm’)

Obsemed

Calculated

-12.6 24.0 1.74 -0.60 2 76 4.30 2.91

- 12.4 24.3 - 1.60 -0.53 2 68 4.36 2.96

68 the anti form, I .91 f0.05 D, is not significantly different from that for the gauche form, 2.02 k 0.07 D. These dipole moments may be compared to the value 1.94 D found by dielectric-constant measurements made in benzene solution’s. For comparison, the dipole moment of methoxy ethyne’ is p, = 1.41 D, IQ, = 1.32 D and /I,,~, = 1.93 +0.02 D. Th e relatively large difference between the dipole moments of cthoxy ethyne and diethyl ether (1.25 D”) has been attributed to an appreciable contribution of the resonance structure C,H,-&C&H derived from dipole momentrs

VIBRATIONAL

and NMR

data”.

SATELLITE SPECTRA

Anri form During the assignment of the a-type transitions, three sets of weaker lines with similar Stark effect were observed. The lines appeared consistently on the high frequency side of the ground state line. Their decreasing intensity as they became farther displaced from the ground state line was taken as an indication that these transitions belong to successive excited states of the same vibration. The lines were all sharp and unsplit. These vibrational satellites were assrgned to the torsion about the 0-Gel,.,,,, bond. The measured frequencies were fitted to a rigid rotor energy expression. As for the ground state, the fit was satisfactory. The observed frequencies and the difference between observed and calculated frequencies are given in Table 6 and the rotational constants in Table 7.

Gauche form For the gauche form, two vrbrational satellites were assigned on the low frequency side of the ground state line. The presence of these lines was not so readily apparent as for the anti form due to the larger displacement of the vibrational satellites from the ground state line. The measured transitions were fitted to a rigid rotor energy expression_ As for the ground state, the deviation from rigid rotor was significant. The mean square deviations were 0.3 MHz and 0.4 MHz for the first and second torsional excited states, respectively. The measured frequencies and the difference between observed and calculated frequencies are reported in Table 6, the rotational constants in Table 7.

RELATIVE IN-IENSLTY MEASUftEhlENTS

The measured played on of several

relative intensities of the vibrational satellites described above were both at dry ice and room temperature and the transitions were drsthe oscilloscope or recorder. Measurements were performed at intervals days with different settings on the spectrometer. Despite somewhat low

69 TABLE

6

OBSERVED TIONAL

AND

EXCITED

THE DIFFERENCE STATES

BETWEEN

OF ETHOXY

OBSERVED

ETHYNt?

(in

Gauche form 101 -+ 202 I 10 3211 2 12+3,, 202 - 303 221 - 322 220 + 3,, 2 11 -+ 31, 3 13 -+ 4,5 303 -+ 40, 322 - 4~~ 3 12 +4,x 41* + 51, 4OJ + 505 4 23 + s2,

CALCULATED

TRANSITIONS

1- = 2

T=l

Transitions

AND

IN THE VIBRA-

MHZ)

7=3

Obserued

Obs. talc.

Observed

Obs.talc.

Obserced

14506.75 14 867.20 19341 87 19 822.55 24 176.47 24485.41 24777.41 29010.74 29732.10

0.08 -0.07 0.09 -0.03 -0.04 -0.04 -0.09 -0.04 0.15

14553.93 14914.02 19404.91 19 884.95 24255.3 I

-0.01 -a.05 0.10 -0.03 -0.01 -

14606.62 19475.05 19933.89 24343.16 -

0.01 -oo.05 -0.05 -

0.02 -o_os

24916.84 -

-

24 855.52 29 105.30 29825 62

0.05

12521 14 13063.00 18021 43 I 8 724.43 18816.68 18907 63 19581.59 14003.35 24861.00 25070.66 26079.89 29 965.89 30913.55 3 I 308.95

-005 0.06 -0.04 - 0.02 0.61 007 0.08 -0.46 0 05 0.45 -0.46 0.07 0.05 -0.11

0.07 -

29 899.36

0.01 0.03

M=l

T=2

T=l

Obs. talc.

12955.29

-0.89 0.60

17914.13

-

-

-

18 103.30 I8 822.93

0.10 0.05

19709.71

0.22

-

19421.97 23 862.07 24702.07 25095.43 25 869.36 29791.46 30729.27

0.92 0.33 0.16 -0.52 0.08 -0.36 -0 36

-

24983.83

-0.10

25455.30 26248.80

-0_09 -0.09

-

-

-

’ T means internal torsion and M means methyl torsion. TABLE

7

ROTATIONAL

Rorarner

CON5rAMS

(in MHz)

T=l

FOR

ETHOXY

ETHYNE

IN

T=2

Anti A B c

(31 OOOf300) 2507.968 f0.008 2387.767&0.008

(31 lOOf900) 25 15.728 f0.007 2395.685 +0.007

Gauche A B c

11958.3 h7.8 3396.06&0.05 2875.97+0&I

12255.9&16.0 3364.72&0.09 2862.03 LO.08

z T means Internal torsion and M means methyl torsion.

VIBRATIONAL

EXCITED

T=3

(33000~2000) 2520.503f0.012 2405.776+0.007 -

STATES=

M=l -

1 I 807.5 k4.8 3421.54&0.03 2885.80~0.04

70 intensity, the most consistent

set of data was obtained

at room temperature and

with recorder tracing. The results for the gauche isomer are less satisfactory than for the anti due to the larger displacement from the ground state line to the satellite lines. The results for both rotamers are given in Table 8. The enerw difference between the lowest states of the two rotamers was determined by measuring the relative intensity of adjacent lines. Four pairs of transitions were used and the measurements, presented in Table 9 yielded Einli =

AE = E;.ushc-

TABLE

INTENSLN

MEASUREMENTS

Rorurmv

G+T=I

Anti Gauche

loo&

(Cm-

(2)

‘)

G+M=l

T=I+T=2 7

-

103p10 105f20

78&15

a T means

ENERGY

cal mole-‘.

8

RELATIVE

TABLE

138+90

270f30

internaltorsion and M means methyl torsion.

9 DIFFERENCE

Tratzsirionr

BEI-WEEN

GAUCHE

ETHOXY

ETTHYNE

compared

Anri

Garrche

3 12 -413

211 *31r

Average AE”

=

POTENTIAL

ANTI AND

138k90

FUNCTION

160*50

cal mole-‘.

FOR INTERNAL

ROTATION

OF THE ErHYL

GROUP

The rotational isomers of ethoxy ethyne can be considered as the conformations of the molecule at the minima of the potential function for the internal rotation of the ethyl group about the O-Cclhy, bond. A general form for the potential function governing this rotation is given by V(0) = c $V”(l -cos ”

n0)

(3)

71 where 0 is the torsional angle, taken as 0” in the anti form. The evaluation of the coefficients V, that characterize the potential function is based on an idealized model necessary to simplify the problem. The molecule is regarded as consisting of two rigid parts (the ethyl and the ethynyl groups) and only the ethyl torsron is allowed. Hence, this treatment neglects the interactions of all the other vibrations with the internal rotation. Furthermore, since only a small number of Fourier coefficients can be determined, it is assumed that eqn. (3) converges rapidly, so that only a few terms are needed to characterize the potentral. The coefficients in eqn. (3) are determined by the procedure given by HiTotal by relating some of the experimental qualities reported in earlierparts of this paper to the potential function. By expanding the potential in a Taylor series at both minima, the torsional frequencies can be related to the curvature of the potential. Thus (a’ F/l&92)B,00 = C _1(1Z2Y”)= 47E2V2p,~i(Gr,l)anr,_ (a’ V/ad2)e=

l,,Bo 1

2 +(n2Vn)

”

(4)

cos (II - 108”) = ~~~~~~~~~~~~~~~~~~~~~ (5)

where G,’ represents the diagonal element of the ethyl torsron of the G- ’ matrix2’. Two more relationships can easily be derived by making use of the difference in energy between the ground states of the rotamers and the condition that the potential has a minimum at v = lOga, V(108”) (8 v/av),,

= c +V,[l -cos

@I - lOSo)] = (E”--~~v)B”uchc-(Eo-~h~),,li

,,,l=

c n V, sm (n - 108”) = 0 n

(6) (7)

The values of the (GLr) elements needed in eqns. (4) and (5) were calculated according to the method of Polo” and yielded (G,l),,u = 14.65 amuA2 radB2 and (GLr%auche = 13.70 amu A2 rad2. The equations to obtain four of the coefficients in the potential function expansion are: 1 $zv, c+V,(l c +z2v, c

sin [n(lO8&3”)] -cos

= 0

[n(lO8+3’)])

= 12.44f2.61

+I’ V, cos [N( 108 +3”)]

(8) = 0.170+_0.090

kcal mole-r

kcal mole-l = 7.08 + 1.93 kcal mole-’

(9) (IO) (11)

and the deduced values are: VI = -0.41+0.10

v, =

0.04~040

v, =

1.42f0.38

v4

0.77+0.30

=

kcal mole-’

(12)

72 The uncertainties given in eqns. (10) and (I 1) reflect an uncertainty in the vibrational frequencies of 10 cm-‘. It is seen from eqn. (12) that the most important term is V, _ The V4 term is of significant magnitude, however, indicating a slower convergence of the Fourier expansion than assumed, and this fact introduces addrtional uncertainty in the potential function. Figure 2 shows a plot of the four-term potential.

DISCUSSION

The four-term Fourier expansion of the potential function for the torsion gives a barrier height between the anti and gauche wells of 2.0f0.5 kcal mole-‘, while the syn barrier (0 = 180’) is l.Of 1.5 kcal mole-’ from the anti minimum energy. Tt is important at this point to note that only classical, and no quantum mechanical, quantities have been used to evaluate the potential function, and that the barrier heights are determined by extrapolating the potential function determined from data obtamed in the anti and gauche minima. The dotted curve in Frg. 2, calculated by taking the maximum contribution from the given uncertainties in the Fourier coefficients, illustrates the uncertainty in the potential function. WilsonZ3 has proposed that the V3 term in the potential is approximately transferable for molecules with rotation about the same bond. It is interesting to note

kcal

mole-’

t-t I I I I 2.0

-

\

’ 1

I

/

,t

I

1.0

Fig. 2. The derived potential function for rotation about the 0-Cclhyl bond. The observed torsional levels are indicated. The dotted curve is the uncertainty, see text.

73 that the I/, term determined for ethoxy ethyne (1.42kO.38 kcal mole-‘) is in good agreement with the V, term found in methoxy ethyne3 (1.440+_0.030 kcal mole-l). The dihedral angle of 108” and the apparent structural changes going from anti to gauche conformation indicate that steric repulsions exist in the gauche form. The distance between the methyl carbon and the nearest acetylene carbon is 2.96 A, considerably less than the sum of the van der Waals radii (3.4 A). Therefore, some other factor must account for the increased stabilrty of the gauche form over what might be expected on the basis of steric considerations alone. For n-propyl derivatrves, CH3CH,CH2X, it is well known that increased electronegativity of X (X = CH,, C=CH, C=N) gives an increased stability to the gauche rotamer’“~24~2s. It has been suggested that the greater stability of the gauche form of I-pentyne compared to butane may be due to attractive dipoIedipole interactions’ O. It is believed that this argument is also valid for ethoxy ethyne. For CH,CH20CHa the anti form is found to be 1.35 kcal mole-’ more stable than the gauche form5. For ethoxy ethyne, where the energy difference IS only on the order of 0.1 kcal mole- ‘, it appears to be an even higher stabilization of the gauche form. This may be reasonable since the bond moment of 0-CCH is larger than the bond moment of C-CCH. A qualitative calculation of the classical dipole-dipole interaction ener_gy of two dipoles ji, and 2, was performed using the expressionz6 E

3(&

. R)(k lR12

- RI I)

where I i?I is the distance between the two dipoles. Using /icHJ = 0.1 D and po_,-c, = 2.0 D the interaction energy is on the order of 200 cal mole-‘. This result indicates that drpole-dipole interactions are of significance for the energy difference between the anti and gauche form of ethoxy ethyne.

ACKNOWLEDGEMENTS

The author wishes to express his appreciation to Professor J. E. Boggs for his hospitality and for numerous helpful discussions. Dr. C. 0. Britt is thanked for several discussions of instrumental problems. Professor J. F. Arens is gratefully acknowledged for providing the sample of ethoxy ethyne. Fil. lit. Hasse Karlson and Professor Carl Lagerkrantz are thanked for making available the commercial spectrometer used in parts of this work. Finally, United States Educational Foundation in Norway is thanked for a Fullbright Travel Grant.

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