The microwave spectrum of lactic acid

JOURNAL

OF MOLECULAR

SPECTROSCOPY

101,

133- I38 (1983)

The Microwave Spectrum of Lactic Acid B.P. VAN ELJCK Department of StructuralChemistry, University of Utrecht, Padualaan 8. 3584 CH Utrecht. The Netherlands The microwave spectrum of lactic acid has been identified with aid of double resonance techniques. Satellite spectra have been tentatively assigned to the two C-C torsional modes. From deuterium substitution into the two hydroxyl groups the conformation was determined: the molecule shows a hydrogen bond from the a-hydroxyl group to the carbonyl oxygen atom. The c coordinate of the hydrogen atom involved has the appalling imaginary value 0.3 1i, thus once more casting doubt on the reliability of the substitution method for accurate structure determination.

INTRODUCTION Lactic acid is a biologically important substance, and the history of its study by the early chemists is closely linked to the development of the concept of optical activity (I). To our knowledge no X-ray study of this key compound has been published yet, probably due to the difficulty of obtaining well-defined crystals. Therefore we thought it worthwhile to study the microwave spectrum, although, of course, no information about the absolute configuration of a substance can be obtained by this method. The related molecule a-fluoropropionic acid has recently been studied in our laboratory (2). Here two conformations were found, with the F atom cis and tram to the carbonyl oxygen atom. The latter conformation was stabilized by an intramolecular hydrogen bond from the hydroxyl group. In lactic acid the corresponding cis conformation can have a similar advantage from a different type of hydrogen bonding, so the conformation shown in Fig. 1 is expected. This structure is quite analogous to the one predicted (3) and observed (4) for glycolic acid, and is indeed found in this study. ANALYSIS OF MICROWAVE SPECTRA

A sample of L(+)-lactic acid (Fluka A. G.) containing about 10% water was used without further purification. The vapor pressure was just sufficient to obtain satisfactory microwave spectra by means of a flow system at room temperature. The Stark spectrum was recorded from 27 to 39 GHz, but turned out to be too complex for straightforward identification procedures. Therefore modulation by radiofrequency-microwave double resonance was tried, and for ~-MHZ pump frequency the spectrum was found to be sufficiently simple to assign Q-type lines with J = 11 through 14, accompanied by quite a few vibrational satellites. Microwave-microwave double resonance was then employed to find many other transitions, including R-type lines, and finally strong 133

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1983 by Academic Press, Inc.

of reproduction m any

form rruwed.

134

B. P. VAN H

EIJCK

b

FIG. I. Projection of the L(+)-lactic acid molecule in the (a, b) plane.

lines with J up to 36 were measured with aid of Stark modulation. Both a- and btype transitions were observed; from relative intensities a dipole moment ratio &pb of about 314 was deduced. No c-type lines were found. The observed frequencies are given in Table I. Two excited states (denoted by v = 1 and v = 2) belonging to the lowest vibrational mode were easily observed. This is no doubt the torsion of the carboxyl group about the C-C bond, from relative intensities a torsional frequency of about 60 cm-’ was estimated. A weaker satellite spectrum (denoted by v’ = 1) was also identified. In fluoropropionic acid (2) the structure of the satellite spectrum is quite analogous, and from splittings in the u’ = 2 state this vibrational mode was assigned as the methyl torsion with a barrier VJ = 1130 f 25 cm-‘. So we take over this assignment to lactic acid, although here the lines corresponding to v’ = 2 were too weak to be observed. From the relative intensities of the 2)’= 1 state we estimate a torsional frequency of about 220 cm-’ and a barrier height V3 in the region 1000-l 300 cm-‘. To obtain more information about the structure lactic acid was dissolved in DzO and the vapor of the mixture was passed through the microwave absorption cell. After the excess D,O was removed and the pressure was stabilized, the spectrum was easily assigned using the same methods as for the parent molecule. Just to make sure that the correct ground state was identified, lines due to the torsional states u = 1 and D = 2 were observed but not analyzed in detail. From the rotational constants (see below) it was obvious that the spectrum corresponded to the Dz-substituted compound CHFHODCOOD. Therefore the procedure was repeated with an equimolecular mixture of HZ0 and D20, and in this way the spectra of the two monosubstituted compounds were obtained. Of course, a decrease of intensity by a factor of 4 was inevitable and the assignment was somewhat tedious. All rotational constants and centrifugal distortion constants (representation 17 are collected in Table II. The rotational frequencies of the excited states and deuterated compounds are available from the author upon request. For R-type transitions the highest rotational quantum number was J = 8, for Q-type transitions about J = 36. For the parent molecule 41 of the 45 strongest lines in the Stark spectrum (selected before the identification was done!) are now assigned. MOLECULAR

STRUCTURE

Table III gives the changes in the planar moments of inertia which occur substitution of the carboxyl hydrogen atom (H,), the a-hydroxyl hydrogen atom

upon (Hh)

135

MICROWAVE SPECTRUM OF LACTIC ACID TABLE I Ground State Transitions (MHz) of Lactic Acid Transition

3

0

3

14

0

5

I5 0

5

I5 0

6

I6 0

6

I6 15 2

5

I5 2 0

5

obs

\)

C.%lC

-”

4

I

38763.1

+o.ll

I6

6

4

0

38845.1

+o.oa

16

8

6

0

6

28729.0

+0.06

17

4

6

0

6

28650.8

-0.07

17

8

6

16

20755.2

-0.07

I8

5

I3

6

16

28616.9

-0.10

I8

7

7

0

7

33202.5

-0.02

I8

7

0

7

33176.3

+o.ll

7

I7

33210.8

7

I7

7 7

4

"

Transition

obs

obs

”

CalC

--v

7

IO

32275.8

+0.01

9

7

34485.3

+0.11

5

12

38102.0

-0.09

9

8

32101.1

-0.12

I8

6

12

35026.2

-0.11

I2

I8

8

II

34991.0

-0.12

8

IO

I8

9

9

29071.6

-0.15

I9

6

I3

I9

7

I2

30445.9

+0.03

-0.03

I9

8

I2

I9

9

II

36538.7

-0.09

33184.6

+0.10

19

9

10

19

IO

9

36514.5

-0.02

I6

36936.0

-0.03

20

6

14

20

7

I3

35942.2

-0.02

I6

36226.2

-0.13

20

8

I3

20

9

I2

37776.6

-0.01

20

IO

IO

33443.8

-0.02

13

30809.7

-0.03 -0.12

20

9

II

16

816 I3

17

917

II

7

2

6

37225.3

co.04

7

2

6

36515.5

-0.06

21

7

I4

21

8

-0.11

21

9

I2

21

IO

II

30044.5

7 IO

I5 I2

22 22

8 II

I4 II

36513.2

+0.08

37938.8

-0.05

7

8

0

8

37688.2

8

0

8

37679.7

+0.10

7

8

18

37690.7

-0.07

22 22

I7 0

”

37682.2

co.15

23

8

15

23

9

14

31071.6

-0.12

IO

7

410

8

3

34077.7

+0.01

23

IO

I3

23

II

I2

34320.1

+0.08

IO

7

310

8

2

34054.8

-0.02

24

8

I6

24

9

I5

36787.7

+0.02

II

I

IO

II

2

9

31527.3

+0.13

24

9

I5

24

10

I4

27635.4

-0.12

2

IO

II

3

9

31633.3

+0.06

24

IO

I4

24

II

I3

30909.7

+o.ll

6

6

ll

7

5

28685.4

-0.07

25

9

I6

25

IO

I5

31366.2

-0.05

7

11

8 8

4 3

33700.7

+o.m

25

10

15

25

11

I4

28855.3

-0.19

33606.0

+0.02

25

II

I4

25

I2

I3

38822.4

+0.02

9

3

38752.3

-0.00

26

IO

16

26

II

I5

29059.0

-0.18

I7

8

I8

I I

7

5 41

II

8

411

II

8

3

ll

9

2

38747.6

+0.18

26

II

15

26

I2

I4

34936.5

+0.06

I?

3

IO

12

4

9

30730.8

+o.ll

27

IO

17

27

II

I6

31835.5

-0.09

12

7

612

8

5

33248.3

-0.03

512

8

4

32927.0

+0.02

16 I8

27 28

I2 II

I5 I7

-0.12

7

II IO

31946.8

12

27 28

36813.9

-0.22

12

8

412

9

3

38382.8

-0.01

28

II

I7

28

12

16

30967.2

-0.02

I3

2

3

IO

33962.5

+0.10

29

II

18

29

I2

I7

32615.5

+0.02

I3

7

613

8

5

31854.9

-0.11

29

12

I7

29

I3

I6

35555.3

-0.05

13

8

613

9

5

+0.08

30

II

I9

30

I2

I8

36819.8

-0.12

I3

8

513

30

I2

I8

30

I3

17

33423.5

-0.03

3

II

I4

4 IO

-0.10

I4

9 4

37963.5 37882.6 32118.7

-0.00

31

I2

I9

31

13

I8

33827.1

-0.03

14

4

II

14

5

IO

33175.8

+0.17

32

12

20

32

I3

I9

37015.8

+0.03

32

14

18

36457.1

+o.ll

I9

II

I4

5

II

IO

I3

I4

6

9

29605.8

14

8

7

9

6

37436.3

I4

8

614

9

5

37171.2

IS

8

715

9

6

36110.7

16

4

I2

I6

5

II

16 -. -.

5

12

I6

6

II

14

+0.04

32

13

19

+0.04

33

I3

20

33

I4

35567.3

+0.02

+0.19

34

I3

21

34

I4

20

37546.7

+0.32

-0.18

35

I4

21

35

I5

20

37904.0

-0.02

33748.8

+O.l6

36

14

22

36

I5

21

38542.6

to.18

35526.8

-0.01

obs

and both atoms simultaneously (H, + Hs). The values of A PCare all negative, leading to imaginary c-coordinates. In Table IV the a and b coordinates are compared with values calculated from a model which was constructed by taking over geometry parameters from glycolic acid (4) and propionic acid (5). The correspondence is fairly rough, but sufficient to establish the conformation shown in Fig. 1 beyond doubt. Improvement of the model might easily be accomplished, but the choice of geometry parameters to be adapted is so large that the resulting structure would be quite arbitrary. The decrease of PCupon deuterium substitution, which must be due to vibrational effects since it can never follow from the structure, is extraordinarily large for substitution of Hi,. An obvious explanation could be a misassignment of the spectrum of the deuterated compound, where no vibrational satellites were observed due to

136

B. P. VAN EJJCK TABLE II Rotational Constants (MHz) and Centrifugal Distortion Parameters &Hz) for Lactic Acid

Parent

molentle

Crounll

state

v=

1

v’=

v=2

I

A

5140.652 + 0.009

5140.751 * 0.012

5140.835 i 0.019

5130.406 ? 0.021

B

3287.746 f 0.008

3274.878 + 0.010

3262.992 ?;0.019

3291.274 f 0.019

C

2246.704 ?:0.008

2252.563 f 0.010

2257.818 f 0.019

2243.775 f 0.019

AJ A

JK

AK 6J 6K NR

NQ S

1.44 f 0.08

1.42 + 0.09

1.53 2 0.16

1.51 f 0.17

-2.18 f 0.03

-2.35 + 0.03

-2.36 ? 0.05

-1.96 + 0.05

2.10 ? 0.05

2.37 i 0.06

2.28 ? 0.10

1.83 i 0.11

0.384 f 0.003

0.392 * 0.004

0.406 t 0.004

0.370 ? 0.006

4.77 f 0.03

4.71 k 0.03

4.68 f 0.04

4.53 f 0.04

18

I6

IO

IO

67

46

41

29

0.11

0.10

0.11

0.12

Deuterated molecules, ground state

CH3CHODCOOH

CH3CHOHCOOD

CHjCHODCOOD

A

4995.452 + 0.012

5120.382 f 0.008

4979.081 + 0.012

B

3273.633 + 0.010

3166.516 * 0.007

3150.908 _+0.010

C

2210.335 k 0.010

2185.539 + 0.007

2150.443 ? 0.010

AJ AJK AK &.J 6K NR

NQ S

1.52 ? 0.10

1.28 + 0.07

1.36 ? 0.09

-2.33 f 0.05

-1.98 + 0.03

-2.23 f 0.03

2.05 ? 0.08

2.01 i 0.07

2.32 f 0.07

0.412 + 0.005

0.357 f 0.003

0.372 ? 0.004

4.30 ? 0.04

4.53 f 0.03

4.18 f 0.03

12

I2

I6

47

47

48

0.10

0.07

0.11

Representation Ir was used. NR and NQ denote the numbers of R- and Q-type transitions; s is the root mean square deviation in MHz.

the weakness of the lines discussed above. However, should the observed spectrum of CH3CHODCOOH inadvertently have belonged to u = 1 instead of u = 0, we could relate it to the corresponding excited state in the parent molecule. We then calculate AP, = -0.684 uA*, which is only worse. Another agreement to rule out trivial errors follows from a calculation, from the substitution coordinates in Table IV, of the inertial moments of the disubstituted molecule CH,CHODCOOD. If we incorporate the effect of vibrations by formally using the imaginary c coordinates, the calculated AP values agree within 0.004 uA* with the observed ones if the sign combinations of the substitution coordinates are taken as predicted from the molecular model (see Tables III and IV).

137

MICROWAVE SPECTRUM OF LACTIC ACID TABLE III Results of Deuterium Substitution in Lactic Acid AP

“c+“h

Values of

for

H c

a,-‘ah

Hb

in

0.0003

0.3997

?

0.0006

-0.0106

?

0.0012

0.7532

t

0.0003

2.9480

?

0.0007

-0.0905

r

0.0014

6.7773

?

0.0003

3.2919

*

0.0007

-0.1017

t

0.0014

substitution various

AP

+

+ +

AP denotes

the

c

?

b,‘bh

+

AP

b

5.8955

double

and

AP

a

sign

calculated

from

a

coordinates

AP

APb

6.677

3.255

6.523

3.459

6.613

3.409

6.779

3.294

increase

substitution

combinations.

in

a planar

moment

of

c

-0.105

inertia

upon

substitution

&I

So once again we encounter vibrational effects that are many times larger than expected (6), and which are only apparent because the corresponding coordinate happens to be so small as to produce a net negative isotopic shift (here -0.09 uA*) in a planar moment of inertia. A similar situation has been encountered in dichloTABLE IV Comparison of Observed and Model Values in Lactic Acid Observed

Model

98.3097

*

0.0002

95.96

‘b

153.7150

?

0.0004

153.68

Ic

224.9411

2 0.0009

225.20

H c

“h

a

2.425

2.429

b

0.667

0.676

c

0.106

i

0.028

a

-0.847

-0.886

b

-I .734

-I

c

0.309

The

model

acid

(4)

(5)

(2,

and

was by

a

constructed

by

tetrahedral

methyl

a dihedral

angle

i

replacing

O-C-C-C

a hydrogen

group of

60°.

.670 0.137

with

a

C-C-C

atom

in

angle

glycolic of

112.7’

138

B. P. VAN EIJCK

roacetic acid (7). It is disheartening to think of other structure determinations where a comparable effect might occur, but remain unnoticed due to the larger values of the substitution coordinates. In some cases entirely unreasonable structures could be produced, as exemplified by our recent study of difluoroacetic acid (8). For lactic acid it is clear that a traditional structure determination by the substitution method would be doomed to failure. RECEIVED:

April 5, 1983 REFERENCES

1. L. F. FIESERAND M. RESER, “Organic Chemistry”, 3rd edition, pp. 255-263, Reinhold, New York, 1956. 2. E. VAN ZOERENAND B. P. VAN EIJCK, to be published. 3. T.-K. HA, C. E. BLOM, AND Hs. H. GONTHARD,J. Mol. Strut. 85,285-292 (1981). 4. C. E. BLOM AND E. BAUDER,Chem. Phys. L&t. 82,492-495 (1981); J. Amer. Chem. Sot. 104,29932996 (1982). 5. 0. L. STIEIVATER,J. Chem. Phys. 62, 244-256 (1974). 6. B. P. VAN EIJCK, J. Mol. Spectrosc. 91, 348-362 (1982). 7. E. VAN ZOEFCEN AND B. P. VAN EUCK, J. Mol. Struct. 97, 315-322 (1983). 8. B. P. VAN EUCK, A. J. MAAGDENBERG,G. JANSSEN,AND T. J. VAN G~ETHEM-WIERSMA,J. Mol.

Spectrosc. 98, 282-303 (1983).