- Email: [email protected]
Vibrational circular dichroism in hydrogen bond systems
Journal of Molecular Structure,
146 (1986)
Elsevier
Amsterdam
Science
Publishers
B.V.,
85-90 -
Printed
in The Netherlands
VIBRATIONAL CIRCULAR DICHROISM IN HYDROGEN BOND SYSTEMS Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol*y**
YOSHIHIKO
NAKAO,
YOSHIMASA
KYOGOKU
and HIROMU
SUGETA
Institute for Protein Research, Osaka University, Suita, Osaka 565 (Japan) (Received
29 January
1986)
ABSTRACT Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G-G’ conforr-------1 mation in the intramolecular hydrogen bond system H-O$#-Ogives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as AH = -7.8 kJ mol.’ and AS = -18 kJ K-’ mol-‘. INTRODUCTION
Since the first successful measurement of vibrational circular dichroism (VCD) by Holtzworth and co-workers [2, 31, the methodology in VCD has been extensively developed along both experimental and theoretical lines over the past decade and it has been demonstrated that this new spectroscopy will provide a new aspect of absolute configuration and conformational details in chiral molecules [ 4-91. In our laboratory VCD studies of hydrogen bonded systems are currently in progress [ 11. In the present study the correlation between the VCD of the OH stretching vibration and the conformation of the intramolecular hydrogen bond in 2,2dimethyl-1,3-dioxolane-4-methanol has been worked out. Also the conformational equilibrium between the free and intramolecularly hydrogen-bonded species has been studied by IR spectroscopy. Several studies associated with intramolecular hydrogen bonds in 1,3-dioxolane-4methanol and related molecules have been carried out by IR [l&--14] and NMR [15, 161 spectroscopy. *See ref. 1 for Part I. **Dedicated to the memory 0022-2860/86/$03.50
of Prof. 0 1986
T. Shimanouchi. Elsevier
Science
Publishers
B. V.
EXPERIMENTAL
(R)- and (S)-2,2-dimethyl-1,3-dioxolane-4-methanol was obtained from Tokyo Kasei Co. Spectroscopic grade carbon tetrachloride was dried over 3 a molecular sieves and used as the solvent. Infrared spectra were measured on a JASCO A3 IR spectrophotometer interfaced to a personal computer (NEC PC-9801). The VCD spectra were measured with ca. 18 cm-’ resolution on a JASCO J-200E IR spectropolarimeter interfaced to a personal computer (NEC PC-9081) as described previously [ 11. The IR and VCD spectra in the OH stretching frequency region were studied in 9 mM (1 M = 1 mol dme3) concentration in carbon tetrachloride. At this concentration intermolecular hydrogen bonding was negligible. A thermostated IR quartz cell of 5 mm path length was used. The absorbances were measured at different temperatures in the region lO--70°C. The digitized absorbance data were simulated by the method of least-squares assuming the Lorentzian curves, in order to resolve overlapped bands and obtain integrated absorption coefficients. The curves simulated with the Lorentzian band contours reproduced the observed spectra well. The equilibrium constants were calculated by the method proposed by Hartman et al. [ 171. RESULTS
AND
DISCUSSION
In Fig. 1 is shown in the OH stretching 9 mM Ccl4 solution. stretching vibrations groups, respectively. recognizable at 3500 +
5
0 60-
the temperature dependence of the absorption spectra region of 2,2-dimethyl-1,3-dioxolane-4-methanol in The bands at 3640 and 3600 cm-l are assigned to the for the free and intramolecularly hydrogen-bonded OH An intermolecularly hydrogen-bonded band was just cm-’ at low temperatures.
r
2,2-Dimethyl-1,3-dioxolane -4-methanol
3600 Wave
3400
nbmber/cm-1
Fig. 1. IR spectra in the OH stretching region in 9 mM Ccl, solution at various temperatures.
of 2,2-dimethyl-1,3-dioxolane-4-methanol
The equilibrium constant K in the conformational equilibrium between free and intramolecularly hydrogen-bonded species is given by K = C,/c, = (Ab/‘-%,)/(Af/Qf)
(1)
in which the subscripts f and b represent free and intramolecularly hydrogenbonded species, C is the concentration, A is the apparent (observed) integrated molar absorption coefficient, and (Yf and (Yb are integrated molar absorption coefficients for free and bonded bands respectively. Af = afCfl A, = ‘$,C,,l
(2)
Cf + c,
(3)
= C, = 1
where the total concentration C, = 1 M and the path length I= 1 cm are employed because the observed absorbances are normalized to the molar absorption coefficients. If the integrated molar absorption coefficients (IIfand (Ybare independent of temperature, the plot of A, versus Af will give a straight line Af/af + A,/&,
= 1
(4)
and one can obtain the integrated molar absorption coefficients (Yf and &b and calculate the equilibrium constants from eqn. (1) [ 171. From a linear plot of A, versus Af as shown in Fig. 2 the integrated molar absorption coefficients were calculated as of = 1660 and (Yb= 3330 cmv2 M-’ and (Yb/&f= 2.0. With these values the equilibrium constants were obtained as listed in Table 1. From the equilibrium constants obtained the population ratio of the bonded to free conformers was calculated as 72:28 at 30°C. The van? Hoff plot of In (K) versus l/T gave thermodynamic parameters, AH = -7.8 kJ mol-’ and AS = -18 kJ K-l mol-‘. The VCD spectrum in the OH stretching region of (R)-2,2-dimethyl-1,3dioxolane-4-methanol as shown in Fig. 3 exhibits only one positive band at 2.2-Dimethyi-1.3-dloxoione-4-methanol Ab af = 1660 cm-‘M-’ Q = 3330 cm-2M-1 Ub/“, = 2.0
7GOO-
2400-
400
500
600
A,
Fig. 2. Integrated intensity plot of A,, versus Af.
TABLE
1
Equilibrium Q-methanol
constants(K)
and thermodynamic
parameters
for 2,2-dimethyl-1,3-dioxolane-
K = C&/C,
Parameter
10 30
3.24 2.59
AH (kJ mol-‘)
40 70
2.15 1.83
Temperature
Value
(“C)
AS (kJ K-r
mol-‘)
-7.3 -18
(R)-2.2~Dimethyl-1.3-dioxolane -4-methanol
Wave number/cm-l Fig. 3. IR circular dichroism and IR absorption spectra in the OH stretching (R)-2,2-dimethyl-1,3-dioxolane-4-methanol in 9 mM Ccl, solution at 30°C.
region
of
3600 cm-’ corresponding to the intramolecularly hydrogen-bonded OH stretching. The apparent rotational strength is 0.37 X 1O-43 (esu cm)* and the anisotropy factor is g = 2.4 X lo-‘, comparable to those for VCD bands reported so far. The correction for the population of the intramolecularly hydrogen-bonded species gives the rotational strength R = 0.5 X 1O-43(esu cm)2. Neither VCD bands could be observed for the free OH nor for the inter-molecularly hydrogen-bonded band in a concentrated solution. It has been established that the 3-position oxygen atom in the dioxolane ring is a proton acceptor site in the intramolecular hydrogen bond in 1,3dioxolane-4-methanol derivatives [ lO--141 . Two conformations are possible in the intramolecular hydrogen bond of (R)-2,2-dimethyl-1,3-dioxolane-4methanol, one is the G+ conformation with the dihedral angle of + 60” about the O,-C,-C-OH bond and the other is G- with -6O”, as shown in Fig. 4. The G’ conformation would be a stable form while the G- conformer might
89
CH3
CH3
G'
G-
(R)-2.2-Dlmethyl-1.3-dioxolane-4-methanoL Ca&lsopropylideneglycerol) Fig. 4. Two possible conformations, (R)-2,2-dimethyl-1,3-dioxolane-4-methanol.
G’ and G-, for the intramolecular
hydrogen
bond in
be excluded because of steric hindrance between the hydroxyl group and ring atoms. Therefore we conclude that the OH stretching vibration gives a positive VCD band for the G-G’ conformation in the intramolecular hydrogen bond system I%--$?!&6-. Recently, Freedman et al. [18] proposed an interesting theory on VCD intensity enhancement by a vibrationally generated ring current mechanism for the intramolecular hydrogen bond ring systems. The prediction of VCD sign by their theory for the present hydrogen bond system is not decisive because of the unfavorable orientation of the OH bond, which is approximately coplanar with the expected ring current loop. However, the observation that only the intramolecularly hydrogen-bonded OH stretching band exhibits a distinct VCD but indistinct features for the free band could mean that the vibrational ring current mechanism might apply in this system. ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific the Ministry of Education, Science and Culture, Japan.
Research
from
REFERENCES 1 Y. Nakao, H. Sugeta and Y. Kyogoku, Chem. Lett., (1984) 623. 2 E. C. Hsu and G. Holtzwarth, J. Chem. Phys., 59 (1973) 4778. 3 G. Holzwarth, E. C. Hsu, H. S. Mosher, T. R. Faulkner and A. Moscowitz, J. Am. Chem. Sot., 96 (1974) 251. 4 P. J. Stephens and R. Clark, in S. F. Mason (Ed.), Optical Activity and Chiral Discrimination, Reidel, Dordrecht, 1979, p. 263. 5 S. F. Mason, in R. J. H. Clark and R. E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. 8, Heyden, London, 1980, p. 283. 6 L. A. Nafie, in J. R. Durig (Ed.), Vibrational Spectraand Structure, Elsevier, Amsterdam, Vol. 10, 1981, p. 153. 7 T. A. Keiderling, Appl. Spectrosc. Rev., 17 (1981) 189. 8 P. L. Polavarapu, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier, Amsterdam, Vol. 13, 1984, p. 103.
90
9 L. A. Nafie, in R. J. H. Clark and R. E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. 11, Heyden, Chichester, 1984, p. 49. 10 S. A. Baker, J. S. Brimacombe, A. B. Foster, D. H. Whifen and G. Zweifel, Tetrahedron, 7 (1959) 10. 11 J. S. Brimacombe, A. B. Foster and A. H. Haines, J. Chem. Sot., (1969) 2582. 12 G. Aksnes, P. Albriktsen and P. Juwik, Acta Chem. Stand., 19 (1965) 920. 13 G. Aksnes and P. Albriktsen, Acta Chem. Stand., 20 (1966) 1330. 14 J. Gelas and R. Rambaud, Bull. Sot. Chim. Fr., (1969) 1300. 15 R. R. Fraser, R. U. Lemieux and J. D. Stevens, J. Am. Chem. Sot., 83 (1961) 3901. 16 J. Gelas and R. Rambaud, C. R. Acad. Sci., 262 (1966) 128. 17 K. 0. Hartman, G. L. Carlson, R. E. Witkowski and W. G. Fateley, Spectrochim. Acta, Part A, 24 (1968) 157. 18 T. B. Freedman and G. A. Balukjian and L. A. Nafie, J. Am. Chem. Sot., 107 (1985) 6213.
146 (1986)
Elsevier
Amsterdam
Science
Publishers
B.V.,
85-90 -
Printed
in The Netherlands
VIBRATIONAL CIRCULAR DICHROISM IN HYDROGEN BOND SYSTEMS Part II. Vibrational circular dichroism of the OH stretching vibration in 2,2-dimethyl-1,3-dioxolane-4-methanol*y**
YOSHIHIKO
NAKAO,
YOSHIMASA
KYOGOKU
and HIROMU
SUGETA
Institute for Protein Research, Osaka University, Suita, Osaka 565 (Japan) (Received
29 January
1986)
ABSTRACT Vibrational circular dichroism (VCD) of the OH stretching band in 2,2-dimethyl-1,3dioxolane-4-methanol has been studied. The OH stretching vibration for the intramolecularly hydrogen-bonded species in the (R)-enantiomer gives rise to a positive VCD band but no VCD band for the hydrogen-bond free species. It is shown that the G-G’ conforr-------1 mation in the intramolecular hydrogen bond system H-O$#-Ogives a positive VCD band for the OH stretching vibration. The thermodynamic parameters in equilibrium between the free and intramolecularly hydrogen-bonded species have been determined as AH = -7.8 kJ mol.’ and AS = -18 kJ K-’ mol-‘. INTRODUCTION
Since the first successful measurement of vibrational circular dichroism (VCD) by Holtzworth and co-workers [2, 31, the methodology in VCD has been extensively developed along both experimental and theoretical lines over the past decade and it has been demonstrated that this new spectroscopy will provide a new aspect of absolute configuration and conformational details in chiral molecules [ 4-91. In our laboratory VCD studies of hydrogen bonded systems are currently in progress [ 11. In the present study the correlation between the VCD of the OH stretching vibration and the conformation of the intramolecular hydrogen bond in 2,2dimethyl-1,3-dioxolane-4-methanol has been worked out. Also the conformational equilibrium between the free and intramolecularly hydrogen-bonded species has been studied by IR spectroscopy. Several studies associated with intramolecular hydrogen bonds in 1,3-dioxolane-4methanol and related molecules have been carried out by IR [l&--14] and NMR [15, 161 spectroscopy. *See ref. 1 for Part I. **Dedicated to the memory 0022-2860/86/$03.50
of Prof. 0 1986
T. Shimanouchi. Elsevier
Science
Publishers
B. V.
EXPERIMENTAL
(R)- and (S)-2,2-dimethyl-1,3-dioxolane-4-methanol was obtained from Tokyo Kasei Co. Spectroscopic grade carbon tetrachloride was dried over 3 a molecular sieves and used as the solvent. Infrared spectra were measured on a JASCO A3 IR spectrophotometer interfaced to a personal computer (NEC PC-9801). The VCD spectra were measured with ca. 18 cm-’ resolution on a JASCO J-200E IR spectropolarimeter interfaced to a personal computer (NEC PC-9081) as described previously [ 11. The IR and VCD spectra in the OH stretching frequency region were studied in 9 mM (1 M = 1 mol dme3) concentration in carbon tetrachloride. At this concentration intermolecular hydrogen bonding was negligible. A thermostated IR quartz cell of 5 mm path length was used. The absorbances were measured at different temperatures in the region lO--70°C. The digitized absorbance data were simulated by the method of least-squares assuming the Lorentzian curves, in order to resolve overlapped bands and obtain integrated absorption coefficients. The curves simulated with the Lorentzian band contours reproduced the observed spectra well. The equilibrium constants were calculated by the method proposed by Hartman et al. [ 171. RESULTS
AND
DISCUSSION
In Fig. 1 is shown in the OH stretching 9 mM Ccl4 solution. stretching vibrations groups, respectively. recognizable at 3500 +
5
0 60-
the temperature dependence of the absorption spectra region of 2,2-dimethyl-1,3-dioxolane-4-methanol in The bands at 3640 and 3600 cm-l are assigned to the for the free and intramolecularly hydrogen-bonded OH An intermolecularly hydrogen-bonded band was just cm-’ at low temperatures.
r
2,2-Dimethyl-1,3-dioxolane -4-methanol
3600 Wave
3400
nbmber/cm-1
Fig. 1. IR spectra in the OH stretching region in 9 mM Ccl, solution at various temperatures.
of 2,2-dimethyl-1,3-dioxolane-4-methanol
The equilibrium constant K in the conformational equilibrium between free and intramolecularly hydrogen-bonded species is given by K = C,/c, = (Ab/‘-%,)/(Af/Qf)
(1)
in which the subscripts f and b represent free and intramolecularly hydrogenbonded species, C is the concentration, A is the apparent (observed) integrated molar absorption coefficient, and (Yf and (Yb are integrated molar absorption coefficients for free and bonded bands respectively. Af = afCfl A, = ‘$,C,,l
(2)
Cf + c,
(3)
= C, = 1
where the total concentration C, = 1 M and the path length I= 1 cm are employed because the observed absorbances are normalized to the molar absorption coefficients. If the integrated molar absorption coefficients (IIfand (Ybare independent of temperature, the plot of A, versus Af will give a straight line Af/af + A,/&,
= 1
(4)
and one can obtain the integrated molar absorption coefficients (Yf and &b and calculate the equilibrium constants from eqn. (1) [ 171. From a linear plot of A, versus Af as shown in Fig. 2 the integrated molar absorption coefficients were calculated as of = 1660 and (Yb= 3330 cmv2 M-’ and (Yb/&f= 2.0. With these values the equilibrium constants were obtained as listed in Table 1. From the equilibrium constants obtained the population ratio of the bonded to free conformers was calculated as 72:28 at 30°C. The van? Hoff plot of In (K) versus l/T gave thermodynamic parameters, AH = -7.8 kJ mol-’ and AS = -18 kJ K-l mol-‘. The VCD spectrum in the OH stretching region of (R)-2,2-dimethyl-1,3dioxolane-4-methanol as shown in Fig. 3 exhibits only one positive band at 2.2-Dimethyi-1.3-dloxoione-4-methanol Ab af = 1660 cm-‘M-’ Q = 3330 cm-2M-1 Ub/“, = 2.0
7GOO-
2400-
400
500
600
A,
Fig. 2. Integrated intensity plot of A,, versus Af.
TABLE
1
Equilibrium Q-methanol
constants(K)
and thermodynamic
parameters
for 2,2-dimethyl-1,3-dioxolane-
K = C&/C,
Parameter
10 30
3.24 2.59
AH (kJ mol-‘)
40 70
2.15 1.83
Temperature
Value
(“C)
AS (kJ K-r
mol-‘)
-7.3 -18
(R)-2.2~Dimethyl-1.3-dioxolane -4-methanol
Wave number/cm-l Fig. 3. IR circular dichroism and IR absorption spectra in the OH stretching (R)-2,2-dimethyl-1,3-dioxolane-4-methanol in 9 mM Ccl, solution at 30°C.
region
of
3600 cm-’ corresponding to the intramolecularly hydrogen-bonded OH stretching. The apparent rotational strength is 0.37 X 1O-43 (esu cm)* and the anisotropy factor is g = 2.4 X lo-‘, comparable to those for VCD bands reported so far. The correction for the population of the intramolecularly hydrogen-bonded species gives the rotational strength R = 0.5 X 1O-43(esu cm)2. Neither VCD bands could be observed for the free OH nor for the inter-molecularly hydrogen-bonded band in a concentrated solution. It has been established that the 3-position oxygen atom in the dioxolane ring is a proton acceptor site in the intramolecular hydrogen bond in 1,3dioxolane-4-methanol derivatives [ lO--141 . Two conformations are possible in the intramolecular hydrogen bond of (R)-2,2-dimethyl-1,3-dioxolane-4methanol, one is the G+ conformation with the dihedral angle of + 60” about the O,-C,-C-OH bond and the other is G- with -6O”, as shown in Fig. 4. The G’ conformation would be a stable form while the G- conformer might
89
CH3
CH3
G'
G-
(R)-2.2-Dlmethyl-1.3-dioxolane-4-methanoL Ca&lsopropylideneglycerol) Fig. 4. Two possible conformations, (R)-2,2-dimethyl-1,3-dioxolane-4-methanol.
G’ and G-, for the intramolecular
hydrogen
bond in
be excluded because of steric hindrance between the hydroxyl group and ring atoms. Therefore we conclude that the OH stretching vibration gives a positive VCD band for the G-G’ conformation in the intramolecular hydrogen bond system I%--$?!&6-. Recently, Freedman et al. [18] proposed an interesting theory on VCD intensity enhancement by a vibrationally generated ring current mechanism for the intramolecular hydrogen bond ring systems. The prediction of VCD sign by their theory for the present hydrogen bond system is not decisive because of the unfavorable orientation of the OH bond, which is approximately coplanar with the expected ring current loop. However, the observation that only the intramolecularly hydrogen-bonded OH stretching band exhibits a distinct VCD but indistinct features for the free band could mean that the vibrational ring current mechanism might apply in this system. ACKNOWLEDGEMENTS
This work was supported by a Grant-in-Aid for Scientific the Ministry of Education, Science and Culture, Japan.
Research
from
REFERENCES 1 Y. Nakao, H. Sugeta and Y. Kyogoku, Chem. Lett., (1984) 623. 2 E. C. Hsu and G. Holtzwarth, J. Chem. Phys., 59 (1973) 4778. 3 G. Holzwarth, E. C. Hsu, H. S. Mosher, T. R. Faulkner and A. Moscowitz, J. Am. Chem. Sot., 96 (1974) 251. 4 P. J. Stephens and R. Clark, in S. F. Mason (Ed.), Optical Activity and Chiral Discrimination, Reidel, Dordrecht, 1979, p. 263. 5 S. F. Mason, in R. J. H. Clark and R. E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. 8, Heyden, London, 1980, p. 283. 6 L. A. Nafie, in J. R. Durig (Ed.), Vibrational Spectraand Structure, Elsevier, Amsterdam, Vol. 10, 1981, p. 153. 7 T. A. Keiderling, Appl. Spectrosc. Rev., 17 (1981) 189. 8 P. L. Polavarapu, in J. R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier, Amsterdam, Vol. 13, 1984, p. 103.
90
9 L. A. Nafie, in R. J. H. Clark and R. E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, Vol. 11, Heyden, Chichester, 1984, p. 49. 10 S. A. Baker, J. S. Brimacombe, A. B. Foster, D. H. Whifen and G. Zweifel, Tetrahedron, 7 (1959) 10. 11 J. S. Brimacombe, A. B. Foster and A. H. Haines, J. Chem. Sot., (1969) 2582. 12 G. Aksnes, P. Albriktsen and P. Juwik, Acta Chem. Stand., 19 (1965) 920. 13 G. Aksnes and P. Albriktsen, Acta Chem. Stand., 20 (1966) 1330. 14 J. Gelas and R. Rambaud, Bull. Sot. Chim. Fr., (1969) 1300. 15 R. R. Fraser, R. U. Lemieux and J. D. Stevens, J. Am. Chem. Sot., 83 (1961) 3901. 16 J. Gelas and R. Rambaud, C. R. Acad. Sci., 262 (1966) 128. 17 K. 0. Hartman, G. L. Carlson, R. E. Witkowski and W. G. Fateley, Spectrochim. Acta, Part A, 24 (1968) 157. 18 T. B. Freedman and G. A. Balukjian and L. A. Nafie, J. Am. Chem. Sot., 107 (1985) 6213.