Microwave spectrum, dipole moment, and structure of anti-vinyl alcohol

JOURNAL

OF MOLECULAR

SPECTROSCOPY

114,23-30 (1985)

Microwave Spectrum, Dipole Moment, and Structure of anti-Vinyl Alcohol M. RODLER Chemistry Department, Monash University. Clayton, Victoria 3168, Australia

Therotational spectra of the anti conformer of vinyl alcohol (ethenol, H,C=CHOH) and its OD modification have been studied by microwave spectroscopy. The compounds have been generated by very-low-pressure pyrolyses of the appropriate isotopic species of 3-thietanol. In both cases the 25 measured pco-and &-type transitions allowed the rotational constants and all five quartic centrifugal distortion constants to be determined. Stark effect measurements have yielded the electic dipole moment: cc,,= 0.547(2), & = I .702( l), and p = 1.788(I) D. By relative intensity measurements it has been found that the vibrational ground state of the anti conformer lies 4.5 f 0.6 kJ mol-’ above the syn conformer. In addition, ab initio calculations at the 6-3 IG** level have been performed to obtain the structure, relative energy, and dipole moment of both rotamers. 0 1985 Academic press. Inc. I. INTRODUCTION

Vinyl alcohol, a tautomer of acetaldehyde, has been subject of numerous spectroscopic investigations in recent years. In 1973, Blank and Fischer (1) observed its CIDNP-enhanced NMR spectrum in the liquid phase, while 3 years later Saito (2) reported the first gas phase-identification of syn-vinyl alcohol by means of microwave spectroscopy. More recently its infrared spectrum has been observed using the matrix isolation technique (3). In 1984 a complete substitution structure (4) and a general harmonic force field (5) of syn-vinyl alcohol were reported. Although vinyl alcohol belongs to a class of compounds which in principle exhibit rotational isomerism, the only rotamer observed so far has the syn conformation. Formic acid, which is isoelectronic to vinyl alcohol, has been found to exist in two strictly planar conformations. The truns rotamer (corresponding to the syn rotamer in vinyl alcohol) is about 16.3 kJ mol-’ (1365 cm-‘) lower in energy than the cis rotamer (corresponding to the anti rotamer) (6). Vinyl mercaptan, the sulfur analog of vinyl alcohol, is also known to exist in two conformations, but the less stable one (anli) has been found to have a nonplanar equilibrium structure (7, 8). The minimum of the potential function corresponds to approximately 26” of the torsional displacement from the planar position. The vibrational ground state lies above the barrier and the conformation is therefore referred to as anti. The isoelectronic monothioformic acid, HC( =O)SH, exists in two strictly planar conformations (9) and thus is, in this respect, more analogous to its oxygen counterpart formic acid. In this light it seemed of interest to search for anti-vinyl alcohol and to clarify its conformation. A molecule also isoelectronic to it is vinyl amine, it does not exhibit rotational isomerism, but the question of its planarity is still unanswered (10). 23

0022-2852/85 $3.00 Copyright 0

1985 by Academic Press. Inc.

All rights of reproduction in any form reserved.

M. RODLER

24

In 1978 Bouma and Radom (II) published optimized geometries of syn- and antivinyl alcohol using the STO-3G and 4-3 1G basis sets. No evidence for a nonplanar rotamer was found. The syn structure was corrected for systematic deficiencies by the use of experimental rotational constants of the parent and OD isotopic species. While the predicted bond lengths are in fair agreement with the substitution bond lengths (4), the C2CIH2 angle was estimated to be about 6” smaller than the experimental value. Due to the small a coordinates of C, and HZ, an accurate determination of this angle from the experimental data proved to be difficult. Nevertheless the discrepancy was much larger than expected. To clarify this matter a more complete basis set (including polarization functions) was used in this work to obtain optimized geometries for both conformers. 2. EXPERIMENTAL

DETAILS

Vinyl alcohol was generated by very-low-pressure pyrolysis of 3-thietanol (H&SCH&HOH) (12). Saturated four-membered rings are known to decompose smoothly via cleavage of opposite bonds (13). Pyrolyzing 3-thietanol therefore yields thioformaldehyde and viny1 alcohol. The latter isomerizes readily to acetaldehyde. Vinyl alcohol-OD was produced by pyrolyzing 3-thietanol-OD, which was synthesized from 3-thietanol by deuterium exchange with methanol-OD. An isotopic enrichment of about 85% proved to be sufficient. Vapor of the precursor was flowed at pressures of 0. I- 1.O Pa through a quartz tube of 3 cm inner diameter which was heated over a length of 30 cm with an electric oven. In order to obtain strong signals of the transient the oven was positioned as close to the Stark cell as possible and a liquid nitrogencooled trap was located immediately after the cell to allow for fast pumping speeds. At oven temperatures of 950-1050°C the lines of viny1 alcohol reached maximum strength. The l-m-long gold-plated wave guide cell of G-band size was kept at room temperature. The microwave spectrometer used was a conventional 30-kHz Stark modulated instrument. Phase-stabilized OKI klystrons were used as microwave sources. Accurate transition frequencies were obtained by a least-squares fit of a Lorentzian function to the average from repetitive scans in both directions. The frequencies should be accurate to within 20 kHz. Relative intensity measurements were made in order to determine an approximate energy difference between the vibrational ground state of syn- and anti-viny1 alcohol. The 2,-,*- loi and 2i2 - 1, 1 transitions were used because (i) the corresponding lines of both conformer lie fairly close together, (ii) the Stark components could be sufficiently moved away from the zero field line, and (iii) none of the lines was affected by transitions nearby. Care was taken not to saturate the lines. Each of them was recorded on chart paper. The linewidth and peak height were then directly read off. 3. ANALYSIS

AND ASSIGNMENT

OF THE

ROTATIONAL

SPECTRA

The differences between the ab initio structures of anti- and syn-vinyl alcohol (see Section 4) were added to the substitution structure of syn-vinyl alcohol. This model (see Fig. 1) for anti-vinyl alcohol was used to calculate the rotational constants. Broadband spectra recorded in the predicted region of the pa-type 2 - 1 transitions revealed several possible lines. Closing the cell on both sides and studying the time dependence

MICROWAVE

SPECTRUM

OF anti-VINYL

ALCOHOL

25

b

FIGURE 1.

of the line intensities allowed distinctions to be made between lines from stable and transient species. Then the Stark effect of the remaining lines was studied. For each transition only one line showed the expected Stark behavior. Measuring higher J Mutype transitions confirmed the assignment. Dealing with a near prolate top (K = -CL!%), the A rotational constant is poorly determined by pa-type transitions only. Therefore another broadband search was undertaken to find a cl&ype line. Having found possible candidates, the assignment was corroborated by measuring additional lines. Ultimately 25 transitions up to J = 22 have been measured for both isotopomers. They are listed in Table I. With these data the rotational constants and all five quartic centrifugal distortion constants could be determined. They are defined according to Watson’s asymmetric reduction in a prolate I’representation (14). In Table II the observed and calculated molecular constants for anti-H*C=CHOH and anti-H,C=CHOD are listed. The Stark effect of the 2,,, - loi and 40, - 3 13 transitions have been studied quantitatively to determine the electric dipole moment components. Three A4components were measured and they all showed a purely quadratic Stark effect within the experimental accuracy. The molecule was assumed to be planar as discussed later. Thus only pn and & were determined in a weighted least-squares fit from the measured Stark shifts. The calculated Stark coefficients were obtained from second-order perturbation theory (15). The Stark cell was calibrated with OCS assuming a value of 0.7 152 1 D for its dipole moment (16). The results are included in Table II along with the ab initio values. 4. AB INITIO

MOLECULAR

ORBITAL

CALCULATIONS

The geometries of syn- and anti-vinyl alcohol were optimized at the Hartree-Fock level using the 6-3 lG** (17) basis set which includes d and p polarization functions on the heavy atoms and hydrogen atoms, respectively. Planarity of the molecular structure was assumed. The minima of the potential surface were located by the use of analytical gradient methods. The GAUSSIAN 80 system of programs (18) was used

26

M. RODLER TABLE I Measured Rotational Transition Frequencies (in MHz) of anti-H*C=CHOH ant* -

-

anti -

CHOH

ohs.-talc.

ob* .

Transition J

Hz0

(K_ K+)

J

(K_

K+)

and anti-H*C=CHOD H*C

-

c800

ohs .

T~~ll*iC*O” J

(K

K+)

J

ohs.-c&c.

w_ K+)

1

(1

0)

-

1

(0

1)

53903.778

0.009

1

(1

O)-

1

(0

1)

53314.862

-0.044

2

(0

2) -

1

(0

1)

38806.499

0.017

2

(1

2)-

1

(0

1)

35143.882

-0.006

2

(1

2) -

1

(1

1)

37345.760

0.007

2

(0

2)-

1

(0

1)

36455.192

0.006

2

(1

1) -

1

(1

0)

40330.518

0.010

2

(1

I)-

1

(1

0)

37817.768

0.004

2

(1

1) -

2

(0

2)

55427.776

-0.018

2

(1

l)-

2

(0

2)

54677.500

0.015

3

(1

3) -

2

(1

2)

55998.854

-0.003

3

(0

3)-

2

(0

2)

54618.851

0.004

4

(0

4) -

3

(I

3)

28542.496

3

(1

2)-

2

(1

1)

56710.182

5

(2

4) -

6

(1

5)

27681.795

-0.047

3

(2

I)-

2

(2

0)

54823.451

6

(2

4) -

7

(1

7)

47214.269

-0.018

3

(2

2)-

2

(2

I)

54721.723

0.016

7

(1

6) -

7

(1

7)

41692.055

3

(1

2)-

3

(0

3)

56768.815

0.013

7

(2

5) -

8

(I

8)

35816.184

4

(2

3)-

5

(1

4)

56866.541

-0.009

9

(1

8) -

8

(2

7)

47098.755

5

(2

4)-

6

(1

5)

34775.221

0.000

10

(3

8)

-

11 (2

9)

36710.322

-0.006

6

(2

4)-

7

(1

7)

51233.014

-0.006

11

(3

8) -

12 (2

11)

41241.238

-0.049

7

(1

6)-

7

(1

7)

37364.727

0.022

13

(2

11) -

13 (2

12)

37041.440

7

(2

5)-

8

(1

8)

40013.118

14

(2

12) -

13 (3

11)

40553.149

-0.060

8

(2

6)-

9

(1

9)

30212.077

-0.006 -0.021

0.011

0.035 -0.033 0.019

0.053

0.017 -0.019

0.000

14

(2

12) -

14 (2

13)

47565.010

-0.011

9

(1

8)-

8

(2

7)

34881.175

15

(4

12) -

16 (3

13)

48672.601

0.059

10

(3

8) -

11

(2

9)

49840.764

0.019

16

(4

12) -

17 0

15)

40294.083

0.044

11

(3

8)

-

12

(2

11)

51171.227

-0.016

17

(2

15) -

18 (1

18)

35053.168

0.010

12

(3

9) -

13

(2

12)

35673.002

0.004

18

(2

17) -

17 (3

14)

36427.155

-0.015

13

(2

11) -

13

(2

12)

30698.658

0.001

18

(2

16) -

19 (1

19)

47302.725

-0.005

14

(2

12) -

14

(2

13)

39605.830

19

(2

18) -

18 0

15)

43691.783

0.001

16

(4

12) -

17

(3

15)

56560.126

20

(2

19) -

19 (3

16)

48807.430

0.014

18

(2

16) -

19

(1

19)

36331.816

0.000

21

(5

17) - 22 (4

18)

36713.016

19

(2

18) -

18

(3

15)

35579.673

0.000

-0.045

0.009 -0.002

for the calculations. The resulting geometries are listed in Table III. It is known (19) that bond lengths tend to be too short when computed at the Hartree-Fock level even using large basis sets. This is caused by the neglect of electron correlation. Bond angles usually agree to within 1S” with experimental data if polarization functions are included in the basis set. In order to reliably predict the rotational constants for antiviny1 alcohol these deficiencies were accounted for by adding the difference between the calculated and experimentally derived molecular parameters of the syn rotamer (4) to the calculated parameters of the anti rotamer. The resulting structure is shown in Fig. 1. The estimated rotational constants for the parent and OD isotopic species are listed in Table II, along with the predicted quartic centrifugal distortion constants. They were obtained by using the corrected ab initio geometry and transferring the known force field of syn-viny1 alcohol (5) to the anti rotamer. Because both molecules are planar the distortion constants only depend on the force constants of the totally symmetric A’ block. The inertia defect A(l, - I, - I& on the other hand, depends on the complete force field. The force constant governing the OH torsional motion was considered to be the most likely to change by going from the syn to the anti conformer.

27

MICROWAVE SPECTRUM OF anti-VINYL ALCOHOL TABLE II Observed and Calculated Rotational Constants (MHz), Centrifugal Distortion Constants (kHz), and Dipole Moment Components of anti-Vinyl Alcohol anti-H2c =

anti-H2C

CmH

Observed

Calculated(a)

Observed

= CHOD

Calculated(a)

62388.

A

62868.102(18)

63537.

61767.536(7)

B

10455.807(3)

10445.

9788.748(l)

97114.

C

8963.258(3)

8970.

R451.668(1)

845R.

*J

1124.(Z)

AK

1114.

1.495(Z)

65

A corrected

(b)

Inertia

995.(l)

991.

-0.01416(l) 0.75D

1.702(1)D

1.96D

initio

1.17 24.5

26.43(7)

29 .O

0.547(2)D

ab

Defect

-47.1

1.134(l)

0.00992(3)

(a)

6.12

-45.6(4)

1.49

31.1(l)

*K A(b)

.4

-5i

-5l3.3(5)

A.JK

5.90(2)

7.39

7.30(3)

stn~cture

(I,-I~-I,)

was 0 uA2

in

used

(see

(Conversion

text) factor

505379.05

ut

MHZ)

TABLE III Molecular Structures@) and Energies of anti- and syn-Vinyl Alcohol from Ab and Substitution Structure of the syn Conformer

anti-H2C=CHOH

Parameter(b)

Initio

6-31G** Calculations

Calculated

Experimental

Syn-H2C=CHOH

Syn-H2C-CHOH

c=c

1.315

1.318

1.326

C-O

1.352

1.346

1.372

O-H

0.941

0.944

0.960

C-H2

1.078

1.074

1.097

C-H3

1.073

1.073

1.079

C-H4

1.075

1.077

1.086


122.7

126.9

126.2


110.9

110.6

108.3


121.6

122.2

129.1


119.9

120.0

119.5

121.3

122.3

121.7

-152.8977

-152.9010


8.66

ml-‘)

ca) Atomicnumbering (b)

Bond

lengths

in

scheme 11, bond

is

given

angles

0.0

in in

Figure

degrees.

1.

(5)

28

M. RODLER

Of all the out-of-plane force constants it also has the largest influence on the inertia defect. Its value was thus calculated for both conformers using the corrected ab initio structures (which corresponds to the substitution structure in the case of the syn rotamer). Again the 6-3lG** basis set was used. The constant drops from 0.081 mdyn A rade2 (experimental value, 0.0727) in syn-vinyl alcohol to 0.022 mdyn A radw2 in anti-vinyl alcohol. While the trend is believed to be reliable the absolute value for the anti rotamer has to be taken with caution. It has been found (20) that very small diagonal force constants can change considerably once electron correlation is taken into account. 5. DISCUSSION

The close match between the calculated and observed constants listed in Table II suggest that the substance found is indeed anti-vinyl alcohol, but a decision can also be made on the grounds of the experimental data alone. The assignment of the observed spectra to a vibrationally excited state of the syn conformer can be excluded. First, the Stark behavior is very different, which is caused by the different dipole moment components (pa = 0.616, &, = 0.807 D versus pL,= 0.547, ,.q, = 1.702 D). Second, the knowledge of the rotational constants of both the parent and OD isotopomer allows the substitution coordinates of the hydrogen nucleus of the hydroxyl group to be determined. Using Rudolph’s formula (21) they amount to laHlI = 1.823 A, lb~,I = 0.380 A. The coordinates of the corrected ab initio structure are 1.819 and 0.398 A, respectively, whereas in syn-vinyl alcohol they are 1.049 and 1.090 A, respectively. The observed small inertia defect rules out the possibility of a nonplanar conformer with a high or intermediate barrier to planarity. The values for the isotopes of antivinyl alcohol are slightly smaller than the syn values: A(syn-H2C=CHOH) = 0.0463 uA2, A(anti-H2C=CHOH) = 0.0099 uA2, A(syn-H2C=CHOD) = 0.0415 uA2, and A(anti-H2C=CHOD) = -0.0142 I.&~. For the isoelectronic formic acid (6) and its sulfur analog, monothioformic acid (9), slightly smaller inertia defects of the cis rotamer than the tram rotamer (corresponding to the anti and syn conformations, respectively) are found as well. All of them are regarded as planar and have positive inertia defects. One might argue that the small but negative inertia defect in the case of the OD species is in favor of a small barrier, but this can also result from a fairly low lying harmonic torsional motion. Force field calculations show that the lowest vibration in vinyl alcohol is an almost pure OH bending motion. And, in fact, the ab initio calculations indicate that the OH torsion for the anti rotamer is much lower in energy than the syn rotamer. This leads to a smaller (or even negative) inertia defect provided all other vibrations are unchanged. If the diagonal force constant of the OH torsion is reduced from 0.0727 mdyn A radP2 (which is the experimental value for the syn conformer) to 0.039 mdyn A radm2, the vibrational contributions to the inertia defect for anti-H2C=CHOH and -0D become 0.011 and -0.013 uA2, which are in good agreement with the observed values. The fact that the change from the OH to the OD species is also well reproduced indicates that the observed data can be fully accounted for by a harmonic potential of the OH torsion without recourse to a double minimum potential. Nevertheless this does not rule out the possibility of a very small barrier, well below the vibrational ground state. In the case of anti-vinyl mercaptan (a), where

MICROWAVE SPECTRUM OF anti-VINYL ALCOHOL

29

a barrier height of about 19 cm-l at the planar geometry has been found, the vibrational ground state lies about 16 cm-’ above the top of the barrier. The inertia defects for the SH and SD isotopic species are much more negative than for their vinyl alcohol counterparts (-0.198 and -0.330 uA*, respectively). The relative intensity measurements of syn- and anti-vinyl alcohol allow the zero point energy difference to be determined. The following relation has been employed to analyze the data (22):

. ' Y - N".f",.f~.(u")2.AuS.(~a)2

r” _ NS.f”,.f~.(ys)2.Aya.(~S)2

the superscripts s and a denoting syn and anti, respectively, and y = the absorption coefficient at the absorption maximum, N = the total number of syn- or anti-vinyl alcohol molecules exposed to the microwave radiation field, fv = the fraction of molecules in the vibrational ground state, fr= the fraction of molecules in the lower rotational state, v = the rotational transition frequency, Au = the line width of the rotational transition, and P = the relevant component of the permanent electric dipole moment. An energy difference between the vibrational ground states of syn- and anti-vinyl alcohol of 4.5 4 0.6 kJ mol-’ ( 1.08 + 0.15 kcal mol-’ or 380 + 50 cm-‘) was calculated using the Boltzmann equation. Three factors are believed to contribute substantially to the estimated uncertainty: the measured linewidth and peak height ratios and the vibrational partition function of the anti rotamer. The latter is mainly due to the unknown frequency of the OH torsion. A comparison of the calculated and experimentally determined structures for the S_VZ rotamer listed in Table III reveals the large discrepancy of the CCH2 angle. The experimental value is 5.6” and 6.9” larger than the calculated ones using the 4-31G (II) and 6.3 lG** basis sets, respectively. The inclusion of polarization functions on both the heavy and hydrogen atoms apparently made the difference even larger. Differences of up to 5” have been encountered in the past with the 6-3 lG** basis set (e.g., N2H4 and BH2) (19). Whether the exclusion of electron correlation and the use of a possibly insufficient basis set or the problems encountered in reliably determining this angle from experimental data is responsible for the discrepancy still remains an open question. For anti-vinyl alcohol the agreement between the observed and calculated rotational constants using the corrected ab initio structure is fairly good. This is specially the case for B and C, which largely depend on the molecular geometry. The differences in the A rotational constant are explained by its sensitivity to the inertia defect, which is governed mainly by the force field of the molecule. It is therefore assumed that the corrected ab initio geometry shown in Fig. 1 (with the possible exception of the CCH2 angle) represents a reasonable model for the structure of anti-vinyl alcohol. ACKNOWLEDGMENTS The author is grateful to Professor R. D. Brown for the opportunity to carry out this work in his laboratory and for valuable discussions. He is also in debt to Mr. E. H. N. Rice for his help concerning the molecular

30

M. RODLER

orbital calculations. The assistance of a Monash University Vice-Chancellor’s Postdoctoral Fellowship is gratefully acknowledged. RECEIVED:

April

23, 1985 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Il. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

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