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Vibrational Raman optical activity in backscattering
Volume 158, number 5
VIBRATIONAL
CHEMICAL
RAMAN OPTICAL
PHYSICS
ACTIVITY
LETTERS
I6 June I989
IN BACKSCATTERING
L. HECHT I, L.D. BARRON ChemutryDepartment, The University, Glasgow GI2 8QQ, UK and W. HUG Instifut de Chimie Physique,
Universitt4 de Fribourg, CH-I 700 Fribourg, Swtzerland
Received 7 April I989
We have succeeded in making measurements of vibrational Raman optical activity in the backscattering geometry. In accordance with theoretical predictions, a given signal-to-noise ratio is achieved approximately eight times faster than in rhe poiarized 90” scattering configuration with artefacts greatly reduced. Backscattered ROA spectra of both enantiomers of tram-pinane, and of alanine in water, are shown as first examples.
1. Introduction Although vibrational optical activity in typical small chiral molecules in the disordered phase was first observed using the Raman optical activity (ROA) technique [ 1,2 1, the relatively easier technique of vibrational circular dichroism (VCD) has attracted more attention. Nonetheless, a considerable amount of ROA work has been carried out, although it has tended to be concerned more with fundamentals than with the solving of specific stereochemical problems [ 3,4]. This Letter reports a breakthrough in ROA instrumentation based on the use of a backscattering geometry, instead of the standard 90” scattering arrangement, which should render the ROA method more widely applicable.
enhanced considerably in 180” scattering as compared with 90” and 0” scattering. The experimental quantity used here is the dimensionless circular intensity difference (CID) [ 5 ]
(1) where 1,” and Zk are the Raman-scattered intensities with linear a-polarization in right and left circularly polarized incident light. (An alternative cxperimental quantity q has been defined in terms of scattering cross sections that is related to A through q = - 24 for an enantiomerically pure sample [ 61.) In terms of molecular property tensor invariants, the CIDs for forward (0’ ), backward ( 180 ’ ), and polarized x and depolarized z right-angle (90” ) scattering are [ 7 ] 4(0")=8[45~G'+B(G')Z-p(A)21 2c[45aY2i-7P(a)']
2. The virtues of backscattered
ROA
It is evident from basic theory that the signal-tonoise ratio (SNR) of many ROA bands should be
d(180")=48[B(G')2+:13(A)'1 2c[45a2+7P(a)'] ’ d (90")=2[450C'+78(G')'+B(A)'1 .‘i c[45&+7/qa)']
’ Permanent
address: lnstitut fir Physikalische und Theoretische Chemie, UniversitZt Essen CiHS, FBB, D-4300 Essen 1, Federal Republic of Germany.
0 009-2614/89/$03.500 Elsevier Science Publishers (North-Holland Physics Publishing Division )
(?a>
’
A;(90”) =
B.V.
12[p(G’)*-&!t)‘] 6c/?(cr)’
’
(2bl ’
(2c)
(2d) 341
Volume 158. number
CHEMICAL
5
PHYSICS LETTERS
where o! and G’ are the isotropic invariants of the polarizability tensor and the electric dipole-magnetic dipole optical activity tensor, and p(c-u)‘, p( G’ )’ and P(A)’ are the anisotropic invariants of the polarizability, electric dipole-magnetic dipole and electric dipole-electric quadrupole optical activity tensor products. (Common factors in the numerators and denominators of (2) have not been cancelled so that the relative sum and difference intensities can be directly compared.) Only drpolurizeu’ 90” spectra were measured in the first few years of ROA studies because these are the least susceptible to artefacts, but polarized 90’ spectra are now measured routinely [ 6,8]. Within the bond polarizability theory of ROA intensities for the case of a molecule composed entirely of idealized axially symmetric bonds, the relations P(G’)2=/3(A)2 and cuG’=O are found [7,9], in which case the CIDs (2) reduce to
64/3( G’ )’ 2c[45&+7/3(~)‘]
.4,(90”)=
16fi( G’ )* c[45a2+7P(a)‘]
d,(90”)=
$$.
’
’
(3b) (3c)
(3d)
Hence the ROA intensity in 180” scattering is four times that for polarized 90” scattering with the associated conventional Raman intensity increased twofold, representing a 2$-fold SNR enhancement for the ROA measurements within the same recording time so that a given SNR is achieved eight times faster under the same conditions of laser power and collection efficiency. In contrast, the ROA intensity in the forward direction is zero. This result is expected to be a good approximation for pure saturated hydrocarbons, for which the predicted ratio of 2 : 1 for the polarized : depolarized 90” scattered ROA intensity has been confirmed experimentally [ 10 1. Large deviations from this ratio have been found for molecules containing oxygen or sulphur heteroatoms [ 1 l-l 31 so these arc not expected to exhibit such a large SNR enhancement in backscattered ROA measurements. 342
Another advantage of backscattered ROA is that the artefacts which plague ROA measurements in 90” scattering [ 14- 16 ] are greatly reduced [ 173. Finally, it can be seen from (2b) that, as in depolarized 90” scattering, there is no contribution to the backscattered ROA from aG’, the product ofthe isotropic parts of the polarizability and optical activity tensors (this is independenl of any particular model theory). This simplifies the analysis of the spectra since the isotropic contribution is the hardest to deal with. Thus backscattered ROA spectra are particularly favourable for comparison with the ab initio ROA calculations that are now being performed [ 18,191. Even greater SNR enhancement might be achieved using the recently suggested dual circular polariza-
(3a)
d(0”)=0,
4’180”)=
16 June IY89
Fig. 1.The backscattered Raman (a) and ROA spectra of the two enantiomers (b) and (c) of trans-pinane as a neat liquid. Experimental conditions: laser wavelength 488.0 nm, laser power 600 mW, spectral slit width 6 UK’, recording time 30 min.
Volume 158, number
5
CHEMICAL
tion technique in backscattering [ 201. However, this might be difficult to implement.
3. Experimental The Glasgow multichannel ROA instrument, described previously [ 81, was used for the measurements. The optical system employed for backscattering ROA was similar to that described some time ago [21] for use with an instrument that was destroyed by fire (together with the preliminary data) shortly after backscattering ROA mcasurcmcnts were first attempted. Figs. 1 and 2 show examples of backscattered ROA spectra for two quite different types of sample: trans-
PHYSICS LETTERS
16 June 1989
pinane as a neat liquid, and alanine in aqueous solution. Spectra of both enantiomers are presented to demonstrate the high degree of reproducibility and the almost complete absence of artefacts (the intense Raman band at 66 1 cm- ’ in trans-pinane is strongly polarized and shows large artefacts in 90” ROA spectra). As expected, the backscattered ROA spectra of trans-pinane are virtually identical with the 90” ROA spectra published previously [ lo] ; however, the SNR of the present spectra is much better, and since an acquisition time of 15 min gives roughly the same SNR as the earlier polarized 90” spectrum acquired in 2 h (recorded with similar conditions of laser power, slit width, etc. ) our results accord with the predicted eightfold increase in speed. Although weak, the backscattered ROA spectra of alanine show a number of reliable features. Despite considerable effort, we have never been able to observe significant ROA in 90” scattering from aqueous solutions of amino acids, so the backscattering method represents an important advance that should now render a wide range of biologically relevant molecules in aqueous media accessible to ROA studies.
Acknowledgement We thank the Science and Engineering Research Council and the Wolfson Foundation for Research Grants, and the Deutsche Forschungsgemeinschaft for a Postdoctoral Scholarship (III 02-He 1588/l-l ) for LH.
References [ I] L.D. Barron, M.P. Bogaard and A.D. Buckingham, Chem. Sot. 95 ( 1973) 603.
Fig. 2. The backscattered Raman (a) and ROA spectraofthe two enantiomers (b) and (c) ofalanine as a near-saturated solution in water. Experimental conditions: laser wavelength 488.0 nm, laser power 1W, spectral slit width 8 cm-‘, recording time 2 h.
J. Am.
[2] W. Hug, S. Kint, G.F. Bailey and J.R. Scherer, J. Am. Chem. Sot. 97 (1975) 5589. [3] L.D. Barron, in: Vibrational spectra and structure, Vol. 178, eds. H.D. Bist, J.R. Durig and 3.17. Sullivan (Elsevier, Amsterdam, 1989) p, 343. [4] L.A. NaIie and C.G. Zimba, in, Biological applications of Raman spectroscopy, Vol. 1, ed. T.G. Spiro (Wiley, New York, 1987) p. 307. [5] L.D. Rarron and A.D. Buckingham, Mol. Phys. 20 (1971) 1111. [6] W. Hug and H. Surbeck, Chem. Phys. Letters 60 ( 1979) 186.
343
Volume 158. number
5
CHEMICAL
[ 71 L.D. Barron, Molecular light scattering and optical activity (Cambridge Univ. Press, Cambridge, 1982). [8] L.D. Barron, D.J. Cutler and J.F. Torrance, J. Raman Spectry. 18 (1987) 281. [ 91 L.D. Barron and A.D. Buckmgham, Ann. Kev. Phys. Chem. 26 (1975) 381. [ lo] L.D. BarI-on, J.R. Escribano and J.F. Torrance, Mol. Phys. 57 (1986) 653. [ I I] L.D. Barron and P.L. Polavarapu, Mol. Phys. 65 (1988) 659. [ 121 L.D. Barron, L. Hecht and SM. Blyth, Speclrochim. Acla 45A (1989) 375. [ 13 ] L.D. Barron, L. Hecht and P.L. Polavarapu, Chem. Phys. Letters 154 (1989) 251.
344
PHYSICS LETTERS
I6 June 1989
[14] W. Hug,Appl. Spectry. 35 (1981) 115. [ 151L.D. Barron and J. Vrbancich, J. RamanSpectry. IS (1984) 47. [ 161 L. Hecht, 8. Jordanov and B. Schrader, Appl. Spectry. 41 (1987) 295. [ 17l.I.K. Escnbano, Chem. Phys. Letters 121 (1985) 191. [ 18 ] P.K. Bose, L.D. Barron and P.L. Polavarapu, Chcm. Phys. Letters 155 (1989) 423. [ 191 P.K. Bose, P.L. Polavarapu, L.D. Barron and L. Hecht. to be published. [20] L.A. Nafie and T.B. Freedman, Chem. Phys. Letters 154 (1989) 260. [ 2 1 ] W. Hug, in: Kaman spectroscopy, eds. J. Lascombe and P.V. Huong (Wiley-Heyden, Chichester, 1982) p. 3.
VIBRATIONAL
CHEMICAL
RAMAN OPTICAL
PHYSICS
ACTIVITY
LETTERS
I6 June I989
IN BACKSCATTERING
L. HECHT I, L.D. BARRON ChemutryDepartment, The University, Glasgow GI2 8QQ, UK and W. HUG Instifut de Chimie Physique,
Universitt4 de Fribourg, CH-I 700 Fribourg, Swtzerland
Received 7 April I989
We have succeeded in making measurements of vibrational Raman optical activity in the backscattering geometry. In accordance with theoretical predictions, a given signal-to-noise ratio is achieved approximately eight times faster than in rhe poiarized 90” scattering configuration with artefacts greatly reduced. Backscattered ROA spectra of both enantiomers of tram-pinane, and of alanine in water, are shown as first examples.
1. Introduction Although vibrational optical activity in typical small chiral molecules in the disordered phase was first observed using the Raman optical activity (ROA) technique [ 1,2 1, the relatively easier technique of vibrational circular dichroism (VCD) has attracted more attention. Nonetheless, a considerable amount of ROA work has been carried out, although it has tended to be concerned more with fundamentals than with the solving of specific stereochemical problems [ 3,4]. This Letter reports a breakthrough in ROA instrumentation based on the use of a backscattering geometry, instead of the standard 90” scattering arrangement, which should render the ROA method more widely applicable.
enhanced considerably in 180” scattering as compared with 90” and 0” scattering. The experimental quantity used here is the dimensionless circular intensity difference (CID) [ 5 ]
(1) where 1,” and Zk are the Raman-scattered intensities with linear a-polarization in right and left circularly polarized incident light. (An alternative cxperimental quantity q has been defined in terms of scattering cross sections that is related to A through q = - 24 for an enantiomerically pure sample [ 61.) In terms of molecular property tensor invariants, the CIDs for forward (0’ ), backward ( 180 ’ ), and polarized x and depolarized z right-angle (90” ) scattering are [ 7 ] 4(0")=8[45~G'+B(G')Z-p(A)21 2c[45aY2i-7P(a)']
2. The virtues of backscattered
ROA
It is evident from basic theory that the signal-tonoise ratio (SNR) of many ROA bands should be
d(180")=48[B(G')2+:13(A)'1 2c[45a2+7P(a)'] ’ d (90")=2[450C'+78(G')'+B(A)'1 .‘i c[45&+7/qa)']
’ Permanent
address: lnstitut fir Physikalische und Theoretische Chemie, UniversitZt Essen CiHS, FBB, D-4300 Essen 1, Federal Republic of Germany.
0 009-2614/89/$03.500 Elsevier Science Publishers (North-Holland Physics Publishing Division )
(?a>
’
A;(90”) =
B.V.
12[p(G’)*-&!t)‘] 6c/?(cr)’
’
(2bl ’
(2c)
(2d) 341
Volume 158. number
CHEMICAL
5
PHYSICS LETTERS
where o! and G’ are the isotropic invariants of the polarizability tensor and the electric dipole-magnetic dipole optical activity tensor, and p(c-u)‘, p( G’ )’ and P(A)’ are the anisotropic invariants of the polarizability, electric dipole-magnetic dipole and electric dipole-electric quadrupole optical activity tensor products. (Common factors in the numerators and denominators of (2) have not been cancelled so that the relative sum and difference intensities can be directly compared.) Only drpolurizeu’ 90” spectra were measured in the first few years of ROA studies because these are the least susceptible to artefacts, but polarized 90’ spectra are now measured routinely [ 6,8]. Within the bond polarizability theory of ROA intensities for the case of a molecule composed entirely of idealized axially symmetric bonds, the relations P(G’)2=/3(A)2 and cuG’=O are found [7,9], in which case the CIDs (2) reduce to
64/3( G’ )’ 2c[45&+7/3(~)‘]
.4,(90”)=
16fi( G’ )* c[45a2+7P(a)‘]
d,(90”)=
$$.
’
’
(3b) (3c)
(3d)
Hence the ROA intensity in 180” scattering is four times that for polarized 90” scattering with the associated conventional Raman intensity increased twofold, representing a 2$-fold SNR enhancement for the ROA measurements within the same recording time so that a given SNR is achieved eight times faster under the same conditions of laser power and collection efficiency. In contrast, the ROA intensity in the forward direction is zero. This result is expected to be a good approximation for pure saturated hydrocarbons, for which the predicted ratio of 2 : 1 for the polarized : depolarized 90” scattered ROA intensity has been confirmed experimentally [ 10 1. Large deviations from this ratio have been found for molecules containing oxygen or sulphur heteroatoms [ 1 l-l 31 so these arc not expected to exhibit such a large SNR enhancement in backscattered ROA measurements. 342
Another advantage of backscattered ROA is that the artefacts which plague ROA measurements in 90” scattering [ 14- 16 ] are greatly reduced [ 173. Finally, it can be seen from (2b) that, as in depolarized 90” scattering, there is no contribution to the backscattered ROA from aG’, the product ofthe isotropic parts of the polarizability and optical activity tensors (this is independenl of any particular model theory). This simplifies the analysis of the spectra since the isotropic contribution is the hardest to deal with. Thus backscattered ROA spectra are particularly favourable for comparison with the ab initio ROA calculations that are now being performed [ 18,191. Even greater SNR enhancement might be achieved using the recently suggested dual circular polariza-
(3a)
d(0”)=0,
4’180”)=
16 June IY89
Fig. 1.The backscattered Raman (a) and ROA spectra of the two enantiomers (b) and (c) of trans-pinane as a neat liquid. Experimental conditions: laser wavelength 488.0 nm, laser power 600 mW, spectral slit width 6 UK’, recording time 30 min.
Volume 158, number
5
CHEMICAL
tion technique in backscattering [ 201. However, this might be difficult to implement.
3. Experimental The Glasgow multichannel ROA instrument, described previously [ 81, was used for the measurements. The optical system employed for backscattering ROA was similar to that described some time ago [21] for use with an instrument that was destroyed by fire (together with the preliminary data) shortly after backscattering ROA mcasurcmcnts were first attempted. Figs. 1 and 2 show examples of backscattered ROA spectra for two quite different types of sample: trans-
PHYSICS LETTERS
16 June 1989
pinane as a neat liquid, and alanine in aqueous solution. Spectra of both enantiomers are presented to demonstrate the high degree of reproducibility and the almost complete absence of artefacts (the intense Raman band at 66 1 cm- ’ in trans-pinane is strongly polarized and shows large artefacts in 90” ROA spectra). As expected, the backscattered ROA spectra of trans-pinane are virtually identical with the 90” ROA spectra published previously [ lo] ; however, the SNR of the present spectra is much better, and since an acquisition time of 15 min gives roughly the same SNR as the earlier polarized 90” spectrum acquired in 2 h (recorded with similar conditions of laser power, slit width, etc. ) our results accord with the predicted eightfold increase in speed. Although weak, the backscattered ROA spectra of alanine show a number of reliable features. Despite considerable effort, we have never been able to observe significant ROA in 90” scattering from aqueous solutions of amino acids, so the backscattering method represents an important advance that should now render a wide range of biologically relevant molecules in aqueous media accessible to ROA studies.
Acknowledgement We thank the Science and Engineering Research Council and the Wolfson Foundation for Research Grants, and the Deutsche Forschungsgemeinschaft for a Postdoctoral Scholarship (III 02-He 1588/l-l ) for LH.
References [ I] L.D. Barron, M.P. Bogaard and A.D. Buckingham, Chem. Sot. 95 ( 1973) 603.
Fig. 2. The backscattered Raman (a) and ROA spectraofthe two enantiomers (b) and (c) ofalanine as a near-saturated solution in water. Experimental conditions: laser wavelength 488.0 nm, laser power 1W, spectral slit width 8 cm-‘, recording time 2 h.
J. Am.
[2] W. Hug, S. Kint, G.F. Bailey and J.R. Scherer, J. Am. Chem. Sot. 97 (1975) 5589. [3] L.D. Barron, in: Vibrational spectra and structure, Vol. 178, eds. H.D. Bist, J.R. Durig and 3.17. Sullivan (Elsevier, Amsterdam, 1989) p, 343. [4] L.A. NaIie and C.G. Zimba, in, Biological applications of Raman spectroscopy, Vol. 1, ed. T.G. Spiro (Wiley, New York, 1987) p. 307. [5] L.D. Rarron and A.D. Buckingham, Mol. Phys. 20 (1971) 1111. [6] W. Hug and H. Surbeck, Chem. Phys. Letters 60 ( 1979) 186.
343
Volume 158. number
5
CHEMICAL
[ 71 L.D. Barron, Molecular light scattering and optical activity (Cambridge Univ. Press, Cambridge, 1982). [8] L.D. Barron, D.J. Cutler and J.F. Torrance, J. Raman Spectry. 18 (1987) 281. [ 91 L.D. Barron and A.D. Buckmgham, Ann. Kev. Phys. Chem. 26 (1975) 381. [ lo] L.D. BarI-on, J.R. Escribano and J.F. Torrance, Mol. Phys. 57 (1986) 653. [ I I] L.D. Barron and P.L. Polavarapu, Mol. Phys. 65 (1988) 659. [ 121 L.D. Barron, L. Hecht and SM. Blyth, Speclrochim. Acla 45A (1989) 375. [ 13 ] L.D. Barron, L. Hecht and P.L. Polavarapu, Chem. Phys. Letters 154 (1989) 251.
344
PHYSICS LETTERS
I6 June 1989
[14] W. Hug,Appl. Spectry. 35 (1981) 115. [ 151L.D. Barron and J. Vrbancich, J. RamanSpectry. IS (1984) 47. [ 161 L. Hecht, 8. Jordanov and B. Schrader, Appl. Spectry. 41 (1987) 295. [ 17l.I.K. Escnbano, Chem. Phys. Letters 121 (1985) 191. [ 18 ] P.K. Bose, L.D. Barron and P.L. Polavarapu, Chcm. Phys. Letters 155 (1989) 423. [ 191 P.K. Bose, P.L. Polavarapu, L.D. Barron and L. Hecht. to be published. [20] L.A. Nafie and T.B. Freedman, Chem. Phys. Letters 154 (1989) 260. [ 2 1 ] W. Hug, in: Kaman spectroscopy, eds. J. Lascombe and P.V. Huong (Wiley-Heyden, Chichester, 1982) p. 3.