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Conformational studies of monosubstituted three-membered rings by variable temperature FT-IR spectra of rare gas solutions
Journal of Molecular Structure 563±564 (2001) 141±145
www.elsevier.nl/locate/molstruc
Conformational studies of monosubstituted three-membered rings by variable temperature FT-IR spectra of rare gas solutions J.R. Durig a,*, Z. Yu a, S. Shen a, R. Warren a, V.N. Verma b, G.A. Guirgis a a
Department of Chemistry, University of Missouri Ð Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA b Department of Chemistry, University of Guyana, Georgetown, Guyana, South America Received 27 September 2000; accepted 23 October 2000
Abstract Variable temperature (255 to 21008C) studies of the infrared spectra (3500±400 cm 21) of cyclopropane carboxaldehyde, c-C3H5CHO, and ¯uoromethyl cyclopropane, c-C3H5CH2F, dissolved in liquid xenon have been recorded. Utilizing the conformer pair at 959 cm 21 (cis with the oxygen atom over the three-membered ring) and 925 cm 21 (trans) for the aldehyde, the enthalpy difference has been determined to be 105 ^ 18 cm 21 (1.26 ^ 0.22 kJ/mol) with the trans conformer the more stable rotamer. For ¯uoromethyl cyclopropane the conformer pair at 927 cm 21 (gauche) and 961 cm 21 (cis with ¯uorine atom over the ring) was utilized to obtain the enthalpy difference of 250 ^ 25 cm 21 (2.99 ^ 0.30 kJ/mol) with the gauche conformer the more stable form. Ab initio calculations have been carried out with several different basis sets with full electron correlation for c-C3H5CHO, c-C3H5CH2F, and c-C3H5CH2NH2, from which conformational stabilities have been determined and compared to the experimental results. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Conformational stability; Variable temperature FT-IR spectra; Xenon solutions; Cyclopropane carboxaldehyde; Fluoromethyl cyclopropane; Aminomethyl cyclopropane
1. Introduction Organic molecules containing three-membered rings have been of interest to chemists and physicists for several years because of the `ring-strain' as well as the nature of the bonding in the rings. The near 608 angles for the bonds in the ring compared to the near tetrahedral angles for four-coordinated carbon bonds lead to interesting possibilities for bonds in the ring molecules. Of particular interest have been molecules which have a methyl group or a monosubstituted methyl group attached to the three-membered ring, * Corresponding author. Tel.: 11-816-235-1136; fax: 11-816235-5502. E-mail address: [email protected] (J.R. Durig).
i.e. c-C3H5CH2X where X is equal to F, Cl, Br, CN, CuCH, CH3. All of these molecules have two conformers in the ¯uid states where the cis form has the X atom over the three-membered ring and the two equivalent gauche forms (Fig. 1) are obtained by the rotation around the C±CH2X bond by approximately 1208 from the cis form with a hydrogen atom over the three-membered ring. The conformational stabilities [1±4] of many of these molecules with the exception of the ¯uoride have been determined from the temperature-dependant infrared spectra of rare gas solutions (Table 1). The only one of these molecules which has the cis conformer as the more stable form is ethynylmethyl cyclopropane, c-C3H5CH2 ±CuCH. In search of the factors that in¯uence the determination of the conformer which will be the more stable
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00875-9
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J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
transform interferometer equipped with a Globar source, a Ge/KBr beamsplitter and a DTGS detector. The spectra were recorded at variable temperatures ranging from 255 to 21008C. All the spectra were collected with 100 scans at a resolution of 1.0 cm 21. The temperature-dependent spectral data for the conformer bands are listed in Table 2. Fig. 1. Newman projections for the stable conformations of monosubstituted methylcyclopropanes.
rotamer, we initiated the studies of the conformational stability of cyclopropane carboxaldehyde, cC3H5CHO, ¯uoromethyl cyclopropane, c-C3H5CH2F, and aminomethyl cyclopropane, c-C3H5CH2NH2. Variable temperature FT-IR spectral studies of xenon solutions have been carried out and the results compared to those obtained from ab initio calculations with several different basis sets with full electron correlation. 2. Experimental The samples of c-C3H5CHO and c-C3H5CH2NH2 were purchased from Aldrich Chemical Co., Milwaukee, WI. The sample of c-C3H5CH2F was prepared from the alcohol, c-C3H5CH2OH, by ¯uorination with diethylaminosulfur tri¯uoride (DAST) supplied by Aldrich Chemical Co. The samples were puri®ed by a low temperature, low pressure fractionation column and the purity was checked by mass spectrometry and NMR data. The mid-infrared spectra of the three samples dissolved in xenon as a function of temperature were recorded on a Bruker model IFS 66 Fourier Table 1 Enthalpy differences of the conformers of several monosubstituted methyl cyclopropanes, i.e. c-C3H5CH2X Substituent X
DH value (cm 21)
Stable conformer
(%) gauche at 258C
Reference
Cl Br CN CuCH CH3
274^21 383^29 54^4 147^14 385
gauche gauche gauche cis gauche
88 92 72 52 92
[1] [1] [2] [3] [4]
3. Results and discussion The c-C3H5CHO molecule is a mono-substituted methyl cyclopropane where two of the hydrogen atoms on the methyl group have been replaced by an oxygen atom. For this molecule the two stable conformers are the cis form where the oxygen atom is over the three-membered ring and the trans form where the oxygen atom is eclipsing the hydrogen atom on the three-membered ring (Fig. 1). The ab initio calculations with the largest basis set, MP2/63111G(d,p), indicates that the cis conformer of this aldehyde is more stable by about 84 cm 21 (Table 3) than the trans conformer in the vapor state. However, it should be noted that the vibrational spectrum is not consistent with this prediction since many of the infrared bands of the trans conformer have considerably higher intensity in the spectrum of the gas or xenon solution (Fig. 2) than the corresponding modes for the cis rotamer. For example, there are two pairs of bands at 959/925 and 669/ 504 cm 21 where, the ®rst listed band is due to the cis conformer and in each of these pairs the band Table 2 Temperature and intensity ratios from the conformational study of cyclopropane carboxaldehyde and ¯uoromethyl cyclopropane T (8C)
1000/T (K)
c-C3H5CHO I959(cis)/I925(trans)
c-C3H5CH2F I927(gauche)/I961(trans)
255 260 265 270 275 280 285 290 295 2100 DH (cm 21)
4.58 4.69 4.80 4.92 5.05 5.18 5.31 5.46 5.61 5.78
± 2.524 ± 2.500 ± 2.662 2.368 2.323 ± 2.138 105^18
2.872 2.920 3.051 3.174 3.312 3.445 3.668 3.861 4.055 4.352 250^25
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
143
Table 3 Predicted total energies (hartree) and energy differences (cm 21) for the two stable conformers of cyclopropane carboxaldehyde Method/basis set
Energy (trans)
Energy (cis)
Energy (cm 21)
RHF/6-31G(d,p) MP2(full)/6-31G(d,p) MP2 (full)/6-311G(d,p) MP2 (full)/6-311G(d,p) MP2 (full)/6-3111G(d,p) MP2 (full)/6-3111G(2d,2p) B3LYP/6-31G(d)
2229.797950 2230.545610 2230.705321 2230.562054 2230.714216 2230.773597 2231.217520
2229.798398 2230.547131 2230.706683 2230.562639 2230.714601 2230.773923 2231.218154
98 334 299 128 84 72 139
due to the cis conformer is about one-half of the intensity of the corresponding mode for the trans conformer. These relative intensity data provide a strong indication that the trans conformer is the more stable rotamer in the vapor state. Spectral data were obtained at six different temperatures ranging from 2100 to 2608C of the infrared spectrum from 3500 to 400 cm 21 and the recorded spectral data for the pair of bands at 959 (cis) and 925 cm 21 (trans) are listed in Table 2. The enthalpy difference between the cis and trans conformers was calculated by using the van't Hoff equation, 2ln K DH=RT 2 DS=R: A plot of 2ln K versus 1/ T, where K is the ratio of the intensity of a band due to the cis conformer to that of the anti conformer, has a slope, which is proportional to the enthalpy difference. The data for the conformer pair at 959/ 925 cm 21 yielded a value of 105 ^ 18 cm 21 (1.26 ^ 0.21 kJ/mol) with the listed uncertainty the
Fig. 2. Infrared spectrum (1000±480 cm 21) of cyclopropane carboxaldehyde in liquid xenon.
standard deviation and the trans rotamer the more stable form. This value should be close to the value in the vapor [5±9]. We also carried out conformational stability studies on ¯uoromethyl cyclopropane and aminomethyl cyclopropane. For the latter molecule, there is not only the conformational stability around the XC±C (ring) bond where there will be gauche (G) and cis (C) rotamers, but there will also be conformers associated with the relative orientation of the substitutent X which is this amino group with rotation around the C±N bond. This latter rotation results in a trans (t) conformation and two gauche (g1 and g2) conformations which will be equivalent for the cis conformation around the C±C (ring) bond, but two nonequivalent gauche forms for the gauche orientation around the C±C (ring) bond. Thus, there are ®ve possible conformers of aminomethyl cyclopropane where the ®rst designation (capital letter) will be rotation around the C±C (ring) bond and the second one (lower case letter) around the C±N bond, i.e. gauche± gauche(Gg1),.gauche±trans (Gt), gauche±gauche (Gg2), cis±gauche (Cg) and cis±trans (Ct). The relative stability of these ®ve conformers has been predicted from RHF/6-31G(d) and MP2/6-31G(d) ab initio calculations which are listed in Table 4. Both calculations indicate that the most stable conformation relative to the rotation around the C±C (ring) bond is the gauche form, but the results differ on whether the gauche or trans form relative to the rotation around the C±N bond will be more stable. The ab initio predicted Raman spectra of the three gauche conformers (C±C ring) is shown in Fig. 3 along with the combined spectra using the RHF/6-31G(d) predicted energy differences. Current studies are
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J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
Table 4 Total ab initio predicted energies (hartree) and energy differences (cm 21) for the ®ve conformers of aminomethyl cyclopropane Conformer
RHF/6-31G(d)
DE
MP2/6-31G(d)
DE
Gauche/gauche-1 (Gg1) Gauche/trans (Gt) Gauche/gauche-2 (Gg2) Cis/gauche (Cg) Cis/trans (Ct)
2 211.115529 2 211.115456 2 211.113665 2 211.113702 2 211.113127
0 16 409 401 518
2 211.821758 2 211.822197 2 211.819771 2 211.820671 2 211.820969
96 0 532 335 269
Fig. 3. Predicted Raman spectra of aminomethyl cyclopropane: (A) combined spectrum of the three lowest energy conformers; (B) gauche/ gauche-2 conformer; (C) gauche/trans conformer; and (D) gauche/gauche-1 conformer.
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
145
initial studies of the temperature-dependent infrared spectra of xenon solutions (Fig. 4) clearly indicates that the gauche form is the more stable rotamer (Table 2). This result is at variance in that the electronegativity of the substituent group determines the conformer stability [10] with a large electronegativity favoring the cis form. More complete studies of the ¯uoride will be reported in the future.
References
Fig. 4. Temperature-dependent infrared spectrum (915±975 cm 21) of ¯uoromethyl cyclopropane in liquid xenon.
being carried out on both the temperature-dependent Raman spectrum and infrared spectrum of xenon solutions. For the ¯uoromethyl cyclopropane molecule the
[1] J.R. Durig, S. Shen, X. Zhu, C.J. Wurrey, J. Mol. Struct. 485/486 (1999) 501. [2] C.J. Wurrey, S. Shen, X. Zhu, H. Zhen, J.R. Durig, J. Mol. Struct. 449 (1998) 203. [3] G.A. Guirgis, C.J. Wurrey, Z. Yu, X. Zhu, J.R. Durig, J. Phys. Chem. 103 (1999) 1509. [4] C.J. Wurrey, S. Shen, T.K. Gounev, J.R. Durig, J. Mol. Struct. 406 (1997) 207. [5] W.A. Herrebout, B.J. van der Veken, J. Phys. Chem. 100 (1996) 9671. [6] W.A. Herrebout, B.J. van der Veken, A. Wang, J.R. Durig, J. Phys. Chem. 99 (1995) 578. [7] M.O. Bulanin, J. Mol. Struct. 347 (1995) 73. [8] B.J. van der Veken, F.R. DeMunck, J. Chem. Phys. 97 (1992) 3060. [9] M.O. Bulanin, J. Mol. Struct. 19 (1973) 59. [10] W. Caminati, R. Danieli, M. Dakkouri, R. Bitschenauer, J. Phys. Chem. 99 (1995) 1867.
www.elsevier.nl/locate/molstruc
Conformational studies of monosubstituted three-membered rings by variable temperature FT-IR spectra of rare gas solutions J.R. Durig a,*, Z. Yu a, S. Shen a, R. Warren a, V.N. Verma b, G.A. Guirgis a a
Department of Chemistry, University of Missouri Ð Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA b Department of Chemistry, University of Guyana, Georgetown, Guyana, South America Received 27 September 2000; accepted 23 October 2000
Abstract Variable temperature (255 to 21008C) studies of the infrared spectra (3500±400 cm 21) of cyclopropane carboxaldehyde, c-C3H5CHO, and ¯uoromethyl cyclopropane, c-C3H5CH2F, dissolved in liquid xenon have been recorded. Utilizing the conformer pair at 959 cm 21 (cis with the oxygen atom over the three-membered ring) and 925 cm 21 (trans) for the aldehyde, the enthalpy difference has been determined to be 105 ^ 18 cm 21 (1.26 ^ 0.22 kJ/mol) with the trans conformer the more stable rotamer. For ¯uoromethyl cyclopropane the conformer pair at 927 cm 21 (gauche) and 961 cm 21 (cis with ¯uorine atom over the ring) was utilized to obtain the enthalpy difference of 250 ^ 25 cm 21 (2.99 ^ 0.30 kJ/mol) with the gauche conformer the more stable form. Ab initio calculations have been carried out with several different basis sets with full electron correlation for c-C3H5CHO, c-C3H5CH2F, and c-C3H5CH2NH2, from which conformational stabilities have been determined and compared to the experimental results. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Conformational stability; Variable temperature FT-IR spectra; Xenon solutions; Cyclopropane carboxaldehyde; Fluoromethyl cyclopropane; Aminomethyl cyclopropane
1. Introduction Organic molecules containing three-membered rings have been of interest to chemists and physicists for several years because of the `ring-strain' as well as the nature of the bonding in the rings. The near 608 angles for the bonds in the ring compared to the near tetrahedral angles for four-coordinated carbon bonds lead to interesting possibilities for bonds in the ring molecules. Of particular interest have been molecules which have a methyl group or a monosubstituted methyl group attached to the three-membered ring, * Corresponding author. Tel.: 11-816-235-1136; fax: 11-816235-5502. E-mail address: [email protected] (J.R. Durig).
i.e. c-C3H5CH2X where X is equal to F, Cl, Br, CN, CuCH, CH3. All of these molecules have two conformers in the ¯uid states where the cis form has the X atom over the three-membered ring and the two equivalent gauche forms (Fig. 1) are obtained by the rotation around the C±CH2X bond by approximately 1208 from the cis form with a hydrogen atom over the three-membered ring. The conformational stabilities [1±4] of many of these molecules with the exception of the ¯uoride have been determined from the temperature-dependant infrared spectra of rare gas solutions (Table 1). The only one of these molecules which has the cis conformer as the more stable form is ethynylmethyl cyclopropane, c-C3H5CH2 ±CuCH. In search of the factors that in¯uence the determination of the conformer which will be the more stable
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00875-9
142
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
transform interferometer equipped with a Globar source, a Ge/KBr beamsplitter and a DTGS detector. The spectra were recorded at variable temperatures ranging from 255 to 21008C. All the spectra were collected with 100 scans at a resolution of 1.0 cm 21. The temperature-dependent spectral data for the conformer bands are listed in Table 2. Fig. 1. Newman projections for the stable conformations of monosubstituted methylcyclopropanes.
rotamer, we initiated the studies of the conformational stability of cyclopropane carboxaldehyde, cC3H5CHO, ¯uoromethyl cyclopropane, c-C3H5CH2F, and aminomethyl cyclopropane, c-C3H5CH2NH2. Variable temperature FT-IR spectral studies of xenon solutions have been carried out and the results compared to those obtained from ab initio calculations with several different basis sets with full electron correlation. 2. Experimental The samples of c-C3H5CHO and c-C3H5CH2NH2 were purchased from Aldrich Chemical Co., Milwaukee, WI. The sample of c-C3H5CH2F was prepared from the alcohol, c-C3H5CH2OH, by ¯uorination with diethylaminosulfur tri¯uoride (DAST) supplied by Aldrich Chemical Co. The samples were puri®ed by a low temperature, low pressure fractionation column and the purity was checked by mass spectrometry and NMR data. The mid-infrared spectra of the three samples dissolved in xenon as a function of temperature were recorded on a Bruker model IFS 66 Fourier Table 1 Enthalpy differences of the conformers of several monosubstituted methyl cyclopropanes, i.e. c-C3H5CH2X Substituent X
DH value (cm 21)
Stable conformer
(%) gauche at 258C
Reference
Cl Br CN CuCH CH3
274^21 383^29 54^4 147^14 385
gauche gauche gauche cis gauche
88 92 72 52 92
[1] [1] [2] [3] [4]
3. Results and discussion The c-C3H5CHO molecule is a mono-substituted methyl cyclopropane where two of the hydrogen atoms on the methyl group have been replaced by an oxygen atom. For this molecule the two stable conformers are the cis form where the oxygen atom is over the three-membered ring and the trans form where the oxygen atom is eclipsing the hydrogen atom on the three-membered ring (Fig. 1). The ab initio calculations with the largest basis set, MP2/63111G(d,p), indicates that the cis conformer of this aldehyde is more stable by about 84 cm 21 (Table 3) than the trans conformer in the vapor state. However, it should be noted that the vibrational spectrum is not consistent with this prediction since many of the infrared bands of the trans conformer have considerably higher intensity in the spectrum of the gas or xenon solution (Fig. 2) than the corresponding modes for the cis rotamer. For example, there are two pairs of bands at 959/925 and 669/ 504 cm 21 where, the ®rst listed band is due to the cis conformer and in each of these pairs the band Table 2 Temperature and intensity ratios from the conformational study of cyclopropane carboxaldehyde and ¯uoromethyl cyclopropane T (8C)
1000/T (K)
c-C3H5CHO I959(cis)/I925(trans)
c-C3H5CH2F I927(gauche)/I961(trans)
255 260 265 270 275 280 285 290 295 2100 DH (cm 21)
4.58 4.69 4.80 4.92 5.05 5.18 5.31 5.46 5.61 5.78
± 2.524 ± 2.500 ± 2.662 2.368 2.323 ± 2.138 105^18
2.872 2.920 3.051 3.174 3.312 3.445 3.668 3.861 4.055 4.352 250^25
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
143
Table 3 Predicted total energies (hartree) and energy differences (cm 21) for the two stable conformers of cyclopropane carboxaldehyde Method/basis set
Energy (trans)
Energy (cis)
Energy (cm 21)
RHF/6-31G(d,p) MP2(full)/6-31G(d,p) MP2 (full)/6-311G(d,p) MP2 (full)/6-311G(d,p) MP2 (full)/6-3111G(d,p) MP2 (full)/6-3111G(2d,2p) B3LYP/6-31G(d)
2229.797950 2230.545610 2230.705321 2230.562054 2230.714216 2230.773597 2231.217520
2229.798398 2230.547131 2230.706683 2230.562639 2230.714601 2230.773923 2231.218154
98 334 299 128 84 72 139
due to the cis conformer is about one-half of the intensity of the corresponding mode for the trans conformer. These relative intensity data provide a strong indication that the trans conformer is the more stable rotamer in the vapor state. Spectral data were obtained at six different temperatures ranging from 2100 to 2608C of the infrared spectrum from 3500 to 400 cm 21 and the recorded spectral data for the pair of bands at 959 (cis) and 925 cm 21 (trans) are listed in Table 2. The enthalpy difference between the cis and trans conformers was calculated by using the van't Hoff equation, 2ln K DH=RT 2 DS=R: A plot of 2ln K versus 1/ T, where K is the ratio of the intensity of a band due to the cis conformer to that of the anti conformer, has a slope, which is proportional to the enthalpy difference. The data for the conformer pair at 959/ 925 cm 21 yielded a value of 105 ^ 18 cm 21 (1.26 ^ 0.21 kJ/mol) with the listed uncertainty the
Fig. 2. Infrared spectrum (1000±480 cm 21) of cyclopropane carboxaldehyde in liquid xenon.
standard deviation and the trans rotamer the more stable form. This value should be close to the value in the vapor [5±9]. We also carried out conformational stability studies on ¯uoromethyl cyclopropane and aminomethyl cyclopropane. For the latter molecule, there is not only the conformational stability around the XC±C (ring) bond where there will be gauche (G) and cis (C) rotamers, but there will also be conformers associated with the relative orientation of the substitutent X which is this amino group with rotation around the C±N bond. This latter rotation results in a trans (t) conformation and two gauche (g1 and g2) conformations which will be equivalent for the cis conformation around the C±C (ring) bond, but two nonequivalent gauche forms for the gauche orientation around the C±C (ring) bond. Thus, there are ®ve possible conformers of aminomethyl cyclopropane where the ®rst designation (capital letter) will be rotation around the C±C (ring) bond and the second one (lower case letter) around the C±N bond, i.e. gauche± gauche(Gg1),.gauche±trans (Gt), gauche±gauche (Gg2), cis±gauche (Cg) and cis±trans (Ct). The relative stability of these ®ve conformers has been predicted from RHF/6-31G(d) and MP2/6-31G(d) ab initio calculations which are listed in Table 4. Both calculations indicate that the most stable conformation relative to the rotation around the C±C (ring) bond is the gauche form, but the results differ on whether the gauche or trans form relative to the rotation around the C±N bond will be more stable. The ab initio predicted Raman spectra of the three gauche conformers (C±C ring) is shown in Fig. 3 along with the combined spectra using the RHF/6-31G(d) predicted energy differences. Current studies are
144
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
Table 4 Total ab initio predicted energies (hartree) and energy differences (cm 21) for the ®ve conformers of aminomethyl cyclopropane Conformer
RHF/6-31G(d)
DE
MP2/6-31G(d)
DE
Gauche/gauche-1 (Gg1) Gauche/trans (Gt) Gauche/gauche-2 (Gg2) Cis/gauche (Cg) Cis/trans (Ct)
2 211.115529 2 211.115456 2 211.113665 2 211.113702 2 211.113127
0 16 409 401 518
2 211.821758 2 211.822197 2 211.819771 2 211.820671 2 211.820969
96 0 532 335 269
Fig. 3. Predicted Raman spectra of aminomethyl cyclopropane: (A) combined spectrum of the three lowest energy conformers; (B) gauche/ gauche-2 conformer; (C) gauche/trans conformer; and (D) gauche/gauche-1 conformer.
J.R. Durig et al. / Journal of Molecular Structure 563±564 (2001) 141±145
145
initial studies of the temperature-dependent infrared spectra of xenon solutions (Fig. 4) clearly indicates that the gauche form is the more stable rotamer (Table 2). This result is at variance in that the electronegativity of the substituent group determines the conformer stability [10] with a large electronegativity favoring the cis form. More complete studies of the ¯uoride will be reported in the future.
References
Fig. 4. Temperature-dependent infrared spectrum (915±975 cm 21) of ¯uoromethyl cyclopropane in liquid xenon.
being carried out on both the temperature-dependent Raman spectrum and infrared spectrum of xenon solutions. For the ¯uoromethyl cyclopropane molecule the
[1] J.R. Durig, S. Shen, X. Zhu, C.J. Wurrey, J. Mol. Struct. 485/486 (1999) 501. [2] C.J. Wurrey, S. Shen, X. Zhu, H. Zhen, J.R. Durig, J. Mol. Struct. 449 (1998) 203. [3] G.A. Guirgis, C.J. Wurrey, Z. Yu, X. Zhu, J.R. Durig, J. Phys. Chem. 103 (1999) 1509. [4] C.J. Wurrey, S. Shen, T.K. Gounev, J.R. Durig, J. Mol. Struct. 406 (1997) 207. [5] W.A. Herrebout, B.J. van der Veken, J. Phys. Chem. 100 (1996) 9671. [6] W.A. Herrebout, B.J. van der Veken, A. Wang, J.R. Durig, J. Phys. Chem. 99 (1995) 578. [7] M.O. Bulanin, J. Mol. Struct. 347 (1995) 73. [8] B.J. van der Veken, F.R. DeMunck, J. Chem. Phys. 97 (1992) 3060. [9] M.O. Bulanin, J. Mol. Struct. 19 (1973) 59. [10] W. Caminati, R. Danieli, M. Dakkouri, R. Bitschenauer, J. Phys. Chem. 99 (1995) 1867.