Vibrational circular dichroism in hydrogen bond systems

75

Journal of Molecular Structure, 242 (1991) 75-86 Elsevier Science Publishers B.V., Amsterdam

VIBRATIONAL CIRCULAR DICHROISM IN HYDROGEN BOND SYSTEMS Part III. Vibrational circular dichroism of the OH stretching vibrations of 1,2-diols and /3-methoxyalcohols*

KIMIKO YAMAMOTO, SUGETA***

YOSHIHIKO

NAKAO”,

YOSHIMASA

KYOGOKU

and HIROMU

Institute for Protein Research, Osaka University, Suita, Osaka 565 (Japan) (Received 21 May 1990)

ABSTRACT Vibrational circular dichroism (VCD ) of the OH stretching vibrations in intramolecular hydrogen bond systems, i.e. 1,2-diols andfi-methoxyalcohols, has been studied. The VCD feature of the OH stretching mode is discussed in relation to the molecular conformation, with the aid of the results of ‘H NMR spectroscopy. The positive VCD observed for the OH stretching vibration of the donor hydroxyl group in intramolecular hydrogen bond systems is attributable to the Gconformation about the O-C-C-O bond. The rotational strengths calculated on the basis of the dynamic polarization model account for the observed VCD feature.

INTRODUCTION

Recent progress in the measurement of vibrational optical activities, vibrational circular dichroism (VCD) and Raman optical activities (ROA) has demonstrated the great utility of these spectroscopies for studying the structure of chiral molecules in solution [l-4]. Many semiclassical and empirical models have been proposed for interpretation of the origin of vibrational optical activities [ 5-151, and the theoretical calculation of the rotational strength has recently been reported [ 16-181. It is very important to establish, from an experimental point of view, the correlation between VCD Cotton effects and molecular structures for clarifying the validity and limitations of the proposed models and theoretical calculations, and for developing a new model for interpretation of vibrational optical activities as well as for the practical application of VCD to stereochemical studies. *Dedicated to Professor Masamichi Tsuboi on the occasion of his 65th birthday. **Present address: Central Research Laboratories, Kuraray Co., Ltd., Kurashiki, Japan. ***To whom correspondence should be addressed.

0022-2860/91/$03.50

0 1991-

Elsevier Science Publishers

B.V.

Okayama 710,

76

The OH stretching mode is one of the vibrational chromophores suitable for such studies, because it is a well characterized and isolated band, with characteristic spectroscopic features arising from hydrogen bonding and solvent effects, etc. [ 19-22 1. In the present study VCD spectra for the OH stretching vibrations of intramolecular hydrogen bond systems were obtained. The VCD feature of the OH stretching band is discussed in relation to the molecular conformation determined by means of combined IR and NMR spectroscopic analysis. The dynamic polarization model [7-lo] was applied to the systems and accounted for the experimental VCD features well.

MATERIALS

AND METHODS

The optically active 1,2-diols, 1,2-propanediol, 2,3-butanediol and 1,2-cyclohexanediol, were obtained from commercial sources and used without further purification. (S) -l-Methoxy-2-propanol was obtained by resolution of a racemic mixture according to the procedure reported in the literature [ 231 and purified by gas chromatography. (S ) - and (RS ) -2-methoxy-1-propanol were prepared by reduction of ethyl cu-methoxypropionate derived from ethyl lactate by the procedure described below [ 241. Ethyl lactate (26.6 g) was added dropwise to a mixture of methyl iodide (45.3 g) and a suspension of NaH (11.0 g) in dry THF (120 ml), and then the mixture was stirred at 45-50°C for 30 min. After the reaction, water was added slowly to remove the residual NaH, and then the product was extracted with ether and dried. After removal of the ether, the crude product was distilled to give ethyl a-methoxypropionate: b.p. 81-83”C/80 mmHg; yield 8.3 g (27.9% ). Ethyl (S) -cu-methoxypropionate (10 g) was reduced with lithium aluminum hydride to give (S) -2-methoxy-1-propanol [ 241: b.p. 71-74”C/80 mmHg; yield 4.2 g; [cx]~=l9.6” (c=O.47, CHCl,); e.e .=93.0%. Finally, the sample was purified by gas chromatography. Infrared absorption spectra, in dilution carbon tetrachloride solutions, were measured on a JASCO A-3 infrared spectrophotometer. VCD spectra were obtained on a JASCO J-200E infrared spectropolarimeter [21] at 8-15 cm-l resolution. The CD base lines were obtained from the average of VCD spectra for both enantiomers Rand S or from the VCD spectra for RS racemic samples. All VCD spectra given are for R enantiomers. Proton NMR spectra, in Ccl, solutions, were measured using Bruker WM360 and JEOL GX500 spectrometers. All the spectroscopic measurements were made in dilute solutions, in which no intermolecular association was observed. For 2,3-butanediol, the coupling constants, J( HOCH) and J( HCCH ), were obtained by computer analysis of the NMR spectra for the AA’XX’ spin system of HxO-CH, (CH3)-CHA ( CH,)-OHx with decoupling of CH, protons.

77 RESULTS AND DISCUSSION

VCD and absorption spectra The VCD and IR absorption spectra of R enantiomers of diols and P-methoxyalcohols in CC& solutions are shown in Fig. 1. Absorption bands were resolved by computer fitting to obtain integrated absorption intensities. The dipole and rotational strengths were calculated from the integrated intensities. The results are summarized in Table 1. Two OH stretching bands, at about 3600 and 3630 cm-‘, were observed in the IR absorption spectra of Ccl, solutions of diols. The former band is assignable to the OH stretching vibration of the donor hydroxyl group involved in the intramolecular hydrogen bond, and the latter band to that of the accep-

m

0 L: a

2 0 -2

Wavenumberlcm-' 2 0 0 t: a -2

m

*Ot

t

Fig. 1. VCD (top) and absorption (bottom) spectra in Ccl, solutions of 1,2-dials and/$methoxyalcohols; (a) 0.005 M (lR,2R)-trans-1,2-cyclohexanediol; (b) 0.0077 M (R)-1,2-propanediol; (c) 0.0099 M (2R,3R)-2,3-butanediol; (d) 0.010 M (R)-1-methoxy-2-propanol; (e) 0.010 M (R)2-methoxy-l-propanol.

CCI,

ccl,

ccl,

ccl,

ccl,

Ccl,

(lR,2R)-1,2-Cyclohexanediol

(R)-1,2-Propanediol

(2R,3R)-2,3-Butanediol

(R)-1-Methoxy-2-propanol

(R)-2-Methoxy-1-propanol

(R)-2,2-Dimethyl-1,3-dioxolane-4-methanolb

61.3 85.3 56.2 55.2 55.3 57.7 16.0 55.8 18.2 54.8 22.9 51.9

3632 3600 3640 3600 3636 3594 3626 3598 3638 3598 3640 3602 0.45 0.88 0.58 0.70 0.49 1.09 0.01 0.62 0.05 0.74 0.10 0.62 3605

3600

3620 3590 3620 3590 3620 3590

Freq. (cm-‘)

(esu cm)’

e (cm’ mol-‘)

Freq. (cm-‘)

Dx103*

IRCD

Absorption

2.0

1.8

-1.7 2.1 -1.0 1.3 - 1.0 2.4

Acx103 (cm’ mol-‘)

0.37

0.31

-0.36 0.36 -0.13 0.17 -0.24 0.42

(esu cm)’

Rx 1O43

2.4

2.0

-3.2 1.6 -0.91 0.96 -1.9 1.5

gx105 Free Bonded Free Bonded Free Bonded Free Bonded Free Bonded Free Bonded

Assignment”

“‘Bonded)) refers to the donor OH stretching vibration of an intramolecular hydrogen bond and “Free” to the OH stretching vibration of a free and/or acceptor hydroxyl group in an intramolecular hydrogen bond. bFrom ref. 21.

Solvent

Molecule

Observed frequencies, peak intensities (E and AE), and dipole (D) and rotational (R) strengths, and assignments of the OH stretching vibrations of 1,2-diols and&methoxyalcohols

TABLE I

3.0 3.8 3.8

7.4

4.7

6.9

3.1

8.2

7.8

54

56

64

51

60

18

18

10

11

T

G-

J.4

JB

Population

J(HC-CH)

From NMR

28

26

26

49”

29

G+

HO-CH,HO-CHCH,-

7.0

4.3

5.2

J.4

2.9

3.9 3.4

6.1

8.2

6.1

JB

J(HO-CH)

40

22

8

31 18 13

g-

49

61

40

g+

92b

82b 87b

Population

11

16

28

t

28d

24

4

43

36,44”

28

(%o)

Non-hydrogen bond form

From IR

“The sum of populations of T and G + conformers. bThe sum of populations of t and g + conformers. ‘Calculated from the standard intensities for primary and secondary alcohols, respectively. dFrom ref. 21.

(lR,2R)-trans-1,2Cyclohexanediol (R)-1,2Propanediol (2R,3R)-2,3Butanediol (R)-l-Methoxy2-propanol (R)-2-Methoxy1-propanol (R)-2,2-Dimethyl1,3-dioxolane-4-methanol

Molecule

Observed vicinal coupling constants (J in Hz) and conformer populations (% )

TABLE 2

80

tor in the intramolecular hydrogen bond and/or the hydroxyl group in the nonintramolecularly hydrogen-bonded structure [ 25-271 (hereafter we refer to the OH stretching vibrations of the hydroxyl group of the acceptor involved or not involved in hydrogen bonding as the “free OH band”). The corresponding VCD bands are observed as a positive band for the stretching vibration of the donor hydroxyl group and a negative band for the free and/or acceptor hydroxyl group. For /3-methoxyalcohols, the free OH stretching band is observed as a weak shoulder and the corresponding VCD band is hardly observable. The main band in the absorption spectra assignable to the donor OH stretching vibration in the intramolecular hydrogen bond exhibits a positive VCD band for (R) -lmethoxy-2-propanol, but no clear VCD band for (R)-2-methoxy-1-propanol. On comprehensive analysis of the IR absorption intensity for the OH stretching band of a hundred saturated alcohols in Ccl, solutions by Singelenberg et al. [ 281, it was found that the integrated intensity, A, of the free OH stretching band remains constant, depending on the local structure of primary, secondary and tertiary alcohols; A = 16.8,13.8 and 11.2 km mol-‘, respectively. By using their results we can determine the amounts of the hydrogen-bondfree and acceptor hydroxyl groups from the absorption intensity of the free OH stretching band and then estimate the fraction of non-intramolecularly hydrogen-bonded species. The estimated populations of non-hydrogen-bonded species are listed in the last column of Table 2. The obtained fractions prove that more than half the molecules take on intramolecular hydrogen bond structures in all the compounds studied. For the formation of an intramolecular hydrogen bond the conformation about the H-O-C-C-O bond should be either g +G - or g -G +, standard staggered conformations, truns ( T= 180’ ) and gauche (G + = + 60 o and G - = - 60’ ) , being assumed (Fig. 2 ) . The hydroxyl group in all the trans conformers ( T) about the O-C-C-O bond, and the conformers of tG + , g +G + , tG - and g -G - for the H-O-C-C-O bond is free from hydrogen bond or can be an acceptor for intramolecular hydrogen bonding. (lR,2R) -Cyclohexanediol must have the G - conformation to form an intramolecular hydrogen bond owing to the ring constraint whose fraction is estimated to be 72% from IR intensity. The OH stretching vibration of the donor hydroxyl group in intramolecular hydrogen bonding exhibits a positive VCD band for all the (R)-enantiomers except for (R) -2-methoxy-1-propanol. A negative VCD band is observed for the free and acceptor OH stretching bands in all the diols. Conformer populations The coupling constants of proton NMR were analyzed to estimate the molecular conformation (Fig. 2). The vicinal coupling constants, J( HCCH) and

81

9+

b

t

g-

C,H’

C,H”

G-

G’

Fig. 2. (a) Conformations about the C-C and C-O bonds in 1,2-diols: HB for the primary hydroxyl and CH, for the (R)-secondary hydroxyl groups. (b) Conformation of (R)-2,2-dimethyl-1,3dioxolane-4-methanol.

J( HOCH ) , provide information on the conformer populations, P ( 7’) , P (G + ) and P (G - ) , about the O-C-C-O bond, and P( t ) , P (g + ) and P (g - ) about the H-O-C-C bond, respectively. The observed coupling constants, JA and JB, in the spin systems, -CHAHB-CHRand HO-CHAHB-, give the conformer populations as

P(G-)[=P(g-)l=(J,-J,)l(J,-J,)

P(n[=Pk+)l

=(JrJgMJt-Jg)

P(G+)[=P(t)l

=(J,+J,-J,-J,MJt-J,)

For the -CHR-CHR’tions of populations

(1)

and HO-CHRsystems, only the following combinacan be calculated from the observed coupling constant, J

P(G-)[=P(g-)l=(J-J,)l(J,-J,) P(G+)+P(T)[=P(t)+P(g+)l=(Jt-J)/(Jt-J,) The standard

coupling

constants,

(2)

J, and Jg, for tram and gauche were taken

82

from the literature, i.e. Jt = 11.7 and Jg= 2.0 Hz for H-C-C-H [ 291, and Jt= 12.1 and J,=2.05 Hz for H-C-O-H [30,31]. Through analysis of coupling constants using eqn. (1)) the conformer population, P( G+ ), about the O-C-C-O bond can be uniquely determined, but the conformers, G - and T, are not distinguishable unless the individual assignments of J to HA and Hn protons in the methylene group are made. However, since the T conformation about the O-C-C-O bond cannot form an intramolecular hydrogen bond, the population of the T conformer estimated from NMR must be less than the population of non-hydrogen-bonded conformers obtained from the IR intensity. Also the population of intramolecular hydrogen bond structure is determined to be more than 50% from the IR intensity. Then we could make the assignments for the HA and Hn protons and determine the populations for conformers by combined NMR and IR analysis, as shown in Table 2. In all the compounds studied more than half the molecules take on intramolecularly hydrogen bonded structures, the most predominant structure being the G- conformation about the O-C-C-O bond for (R)-enantiomers. Then we can conclude that the g +G - conformation about the H-O-C-C-O bonds is the most predominant. In the previous study, the G + conformation (Fig. 2b) was assumed to be the most stable in (R)-2,2-dimethyl-1,3-dioxolane-4-methanol [22], but NMR analysis in the present study proved that the G - conformation is the most stable one for the (R)-enantiomer. Therefore the conclusion in the previous study [ 221 should be revised such that the positive VCD feature observed for the OH stretching band is associated with the G- conformation about the OC-C-O bond. Calculation of rotational strength The dynamic polarization model was proposed as a model of electronic optical activities for electric allowed transitions by Weigang [ 7,8] and applied to VCD bands by Barnett et al. [9,10]. In this model, the optical activity of a symmetric chromophore is based on the chiral arrangement of polarizable substituent groups with anisotropic polarizability tensors with respect to the chromophore. The electric field due to the transition electric dipole moment of a vibrational chromophore induces an electric dipole on each substituent group. If the polarizability tensor of the substituent is anisotropic, the induced dipole moment might have a chiral arrangement with respect to the chromophore electric dipole vector, producing a nonvanishing rotational strength. Assuming the cylindrical symmetry of the polarizability of the substituent, the rotational strength derived in the literature [ 7,8] may be rewritten as R= (n/2) I~l”vPlr”{


(u.r)/r2}

(3)

83

where r is the position vector of the substituent from the chromophore, /r the transition electric dipole moment vector of the chromophore vibrational transition at wavenumber Y, u the unit vector along the symmetry axis of the polarizability tensor in the substituent, and j?= cy//- CX~the anisotropy of the polarizability. Denoting the angle between line r connecting the chromophore to substituent centers and transition dipole ,Min the chromophore as &, the angle between r and cylindrical symmetry axis u in the substituent as &, and the dihedral angle between p and u across line r as r (Fig. 3), the anisotropy factor, g, is expressed as g=4RID

(4)

= 2nv@/r 2sin$, sin@, sinz( 2cos@, cos& + sin@, sin@,cosr) D= IpI2

(5)

where D is the dipole strength. In the present calculation, the bond polarizability model was employed, and the centers of the chromophore and substituents were set at the centers of their bonds. The bond polarizabilities used in the calculation, i.e. a,,=0.99 and ay, =0.27x 1O24cm3 for the C-C bond, cu,,=O.89 and cxI =0.46 for the C-O bond, and LY//= (YI - 0.64 for the C-H bond were taken from the review of Le Fevre [32]. The anisotropy of polarizability of the O-H bond is I ~0.04 [33]. The bond lengths, r(C-C) =1.536, r(C-0) = 1.427, /?=a,,-+! r (C-H) = 1.085 and r (O-H) = 0.956 A, and tetrahedral angles were assumed. The calculated anisotropy factors are presented in Table 3. The results obtained for the conformations which are possible to form an intramolecular hydrogen bond are given in the table, but the results for those unable to form a hydrogen bond owing to steric hindrance have been omitted. Equation (4) indicates that the rotational strength is proportional to the anisotropy of polarizability of a substituent and the inverse square of the distance of the substituent from the chromophore. Then the rotational strength for the OH stretching vibration is primarily determined by the chiral arrangement of the bond polarizabilities directly bonded to the C, atom relative to the O-H bond across the C&-O bond. The bonds separated by two or more bonds from the OH bond have secondary effects. Since the anisotropy of the C-H

Fig. 3. The orientation of a chromophore and a substituent.

84 TABLE 3 Calculated anisotropy factor (g) for the OH stretching mode in the donor and acceptor hydroxyl groups in the intramolecular hydrogen bonded structure of 1,2-diols and /I-methoxyalcohols Molecule

Conformation”

gx105 Donor

Acceptor

(lR,ZR)-trans-1,2Cyclohexanediol

g+G-t g+G-g-

2.47 2.49

-2.32 0.08

(2R,3R)-2,3Butanediol

g+G-t g+G-gg-G+t g-G+g+

2.06 2.08 0.10 0.08

-2.08 0.25 -2.08 2.19

(R)-1,2Propanediolb

g+G-t g+G-gg-G+t g-G+g+ tG-g+ g-G-g+ tG+gg+G+g-

2.23 2.25 0.02 0.00 2.06 2.01 -2.15 -2.17

0.19 -2.19 0.19 2.19 - 2.08 0.25 -2.46 2.19

(R)-1-Methoxy-2propanol

g+G-t g+G-gg-G+t

2.03 2.39 0.22

(R)-2-Methoxy-lpropanol

g+G-t g+G-gg-G+t g-G+g+

1.86 2.28 - 1.95 -2.31

“Conformations are about the successive bonds in H-O-C-C-O-H or H-0-C-C-0-CH, bConformations are about the successive bonds in H-0-CH (CH,) -CH,-O-H.

bond polarizability can be ignored, the conformation about the H-0-C,-C bond system determines the vibrational optical activity. Equation (4) indicates that the T orientation about the H-0-C,-C bond makes no contribution to the rotational strength and the G+ orientation makes a positive contribution to the rotational strength, while the G- orientation makes a negative contribution. In a primary hydroxyl group, H-0-CH,-C, the OH stretching band for the t conformation is expected to give a vanishing VCD, a positive VCD for the g + conformation and a negative VCD for the g- conformation. In the (R)-secondary hydroxyl group, H-0-CH (CH, )-CO, the g - conformation has two CC bonds (one is the C-CO bond of the G - orientation and the other the CCH, bond of the G+ orientation against the O-H bond) which cancel each other out, resulting in a vanishing VCD; the g + conformation has the C-CO bond of G+ and the C-CH, bond of T, resulting in a positive VCD; and the t

85

conformation has the C-CO bond of T and the C-CH, bond of G-, resulting in a negative VCD. The most abundant conformation, g +G -, about the H-O-C-C-O bond found on combined NMR and IR analysis is expected to give a positive VCD for the donor OH stretching vibration on calculation based on the dynamic polarization model. This is in good agreement with the positive VCD band observed for the donor OH stretching vibration in intramolecular hydrogen bonds except for that in (R)-2-methoxy-1-propanol. The rotational strengths for the acceptor OH stretching vibrations in the G- conformers about the O-C-C-O bond with the intramolecular hydrogen bond structure were calculated to be negative or nearly zero, as shown in Table 3. The resulting VCD band for the acceptor OH stretching vibration might be expected to be negative, in good agreement with the observed VCD in the diols. The order of magnitude of calculated anisotropy factors is also comparable with the observation. The fact that no VCD was observed for the donor OH stretching vibration in (R)-2methoxy-1-propanol may be due to the compensation of the contributions from G + and G - conformers. CONCLUSIONS

VCD of the OH stretching of 1,2-diols and P-methoxyalcohols was studied in relation to the molecular conformation of intramolecular hydrogen bonds. It was revealed on combined analysis of IR intensities and NMR coupling constants that more than half the molecules assume an intramolecularly hydrogen bonded structure, the most stable structure being the g +G- conformation about the H-O-C-C-O bond for the (R) -enantiomers examined. The positive VCD band observed for the OH stretching vibration of the donor hydroxyl group in the intramolecular hydrogen bond is due to the g +G - conformation about the H-O-C-C-O bond. This conclusion was supported by the rotational strengths calculated on the basis of the dynamic polarization model. The negative VCD band observed for the free and/or acceptor OH stretching vibration is in good agreement with the result obtained for the intramolecularly hydrogen-bonded structure with the G- conformation about the O-C-C-O bond. ACKNOWLEDGEMENTS

This work was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. The authors wish to express their gratitude to Professor T. Harada, Ryukoku University, for helpful advice on the preparation of 2-methoxy-1-propanol and 1-methoxy-2propanol.

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