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Gas phase infrared spectra of halogen nitrates revisited
Journal ofMolecular Structure, 74 (1981) 155-158 Elsevier Scientific Publishing Company, Amsterdam
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Printed in The Netherknds
Short communication
GAS PHASE INFRARED REVISITED
B. J. VAN DER VEKEN,
SPECTRA
R. L. ODEURS,
OF HALOGEN
J. SHAMIR*
NITRATES
and M. A. HERMAN
Laboratorium voor Anorganische Scheikunde, Rijksuniversitair Centrum Antwerpen, Groenenborgedizan 171. B 2020 Antwerpen (Belgium) (Received
27 November
1980)
Over a long period of time, the structure of halogen nitrates XONOz has been the subject of controversy. The gas phase infrared investigations either fail to produce evidence for the postulated planarity [l] or simply accept planarity as a starting point [ 21. Subsequent Raman studies have focused on depolarization ratios to forward either perpendicularity [3, 41 or planarity [ 5]_ More recently, a microwave study on chloronitrate led to inertial constants for the 35Cl and “Cl isotopic species [6] _ Although insufficient data were available for a complete structure determination, the inertial defect value (1,-I,-r,) suggests a planar conformation. It is the intention of this note to show that the previously published gas phase infrared spectra [ 1,2] allow an unambiguous assignment of the conformation of chloronitrate. Based on the structural model for chloronitrate used in ref. [2] , for a planar conformation the geometry parameters were adjusted to reproduce the inertial constants published by Suenram et al. [6]. The same bond distances and angles were also used to set up a perpendicular model. Projections of both are shown in Fig. 1. The characteristics of the gas phase infrared bands for these models can be obtained either by locating the nearest model in the catalogue of Ueda and Shimanouchi [ 71, or by using the PR separation relations of Seth-Paul [S] . Results for the latter are gathered in Table 1. The symmetric NO, stretching for both conformations is an A’ vibration and thus its 6ii’/6Q vector has to lie in the plane of symmetry, which is the (a, b) plane for the planar, and the (a, c) plane for the perpendicular conformation. The antisymmetric NOz stretching however is an A’ vibration for the planar, but an A” vibration for the perpendicular conformation_ This implies that for the planar conformation Sz’/SQ is in the (a, b) plane, while for the perpendicular conformation it has to be orthogonal to the (a, c) plane. In the latter case the vector is parallel to the b axis and thus the antisymmetric NOz stretching for this conformer has to show a pure B type character, while for the planar conformation it is an A, B hybrid. *Permanent address: Department of Inorganic and Analytical University
of Jerusalem,
Jerusalem,
0022-2860/81/0000--0000/$02.50
Chemistry, The Hebrew
Israel.
0 1981 Elsevier Scientific Publishing Comnanv
156
Fig. 1. Projections of chloronitrate models in their principal axes. (A) Planar model; (B) perpendicular model. TABLE
2
Calculated separations for chtoronitrate
@RA dRB
@Rc
AQQ
Planar
Perpendicular
14.0 11.6 20.9 10.8
14.4 11.8 21.6 10.4
It should be emphasized that for the perpendicular conformer the prediction is a consequence of the symmetry of the model and not of the umerical values for ‘the geometry parameters actually used. In the region of antisymmetric NO, stretches, ClONO, shows a profile with three maxima, those at 1731 and 1737 and 1743 cm-‘, as listed by Arvia et al. [ 11, and recoginised as a P, Q, R structure centered at 1737 cm-’ by Miller et al. [ 21 (unfortunately these authors do not list P 6nd A branch maxima). From the spectrum published by Arvia et al. [I], it is clear that the outer maxima (1731,1743) are more intense than the sharp central Q branch. The observed PR separation in this structure is in good agreement with the value predicted for an A type transition, as is clear from Table 1. All this shows that the antisymmetric NO2 stretching has a pronounced A type character (the possibility of an unresolved Q branch belonging to a
157
B type transition must be rejected because of the intensities of P and R branches and because of the high QQ’separation predicted from Ueda and Shimanouchi [ 71 and from the Seth-Paul relations; for a QQ separation to be much smaller than the PR separation in a B type band the molecule has to approach spherical top geometry, which actually it is far from). Consequently, with great certainty, from the gas phase infrared spectra the perpendicular conformation can be rejected for chloronitrate, leaving the planar one, in agreement with predictions from microwave. For fluoronitrate, for which isostructural possibilities arise, the gas phase spectra [I, 23 show a beautiful B type antisymmetric NO* stretching. It is then tempting to reverse the argument and to assign a perpendicular conformation to fluoronitrate. This however is dangerous: for a planar geometry the appearance of a B type v,,(NO,) cannot be ruled out on symmetry grounds. For this conformation the observed profile depends on the exact form of the vibration, which due to the symmetry of the molecule, may be coupled to other fundamentals and so the orientation of Sz/SQ and consequently the outlook of the profile is difficult to predict. The situation can be compared with methyl nitrate for which the planar geometry has been established beyond doubt [ 9, IO]. The orientation of the planar conformer of this molecule in its principal axis is very analogous to the one for our planar fluoronitrate model, so again for a pure antisymmetric stretch a highly hybrid A, B profile is expected. Nevertheless, the profile of this band has been classified as a B type transition due to the lack of a Q branch [ 111. It thus is not possible on symmetry grounds to draw conclusions on the conformation of fluoronitrate, both models being equally acceptable as it stands. We are now attempting to resolve this ambiguity by applying gas phase IR profile simulation to this compound. In conclusion, the observation of an A type y,,(NO,) proves the planarity of chloronitrate, whereas the appearance of another type of band in fluoronitrate does not prove anything either way. ACKNOWLEDGEMENTS
We gratefully acknowledge the fruitful discussions we had on this subject with W. A. Seth-Paul. One of us (J. S.) wishes to express his thanks to the “Nationaal Fonds voor Wetenschappelijk Onderzoek” for providing the financial support for his appointment as visiting professor. REFERENCES 1 A. J. Arvia, L. F. R. Cafferata and H. J. Schumacher, Chem. Ber., 96 (1963) 1187. 2 R. H. Miller, D. L. Bernitt and I. C. Hisatsune, Spectrochim. Acta, Part A, 23 (1967) 223. 3 J. Shamir, D. Yellin and H. H. Claassen, Isr. J. Chem., 12 (1974) 1015. 4 K. 0. Christe, C. J. Schack and R. D. Wilson, Inorg. C&em., 13 (1974) 2811. 5 D. W. Amos and G. W. Flewett, Spectrochim. Acta, Part A, 31 (1975) 213.
158
6 R_ D. Suenram and D. R. Johnson, J. Mol. Spectrosc., 65 (1977) 239. 7 T. Ueda and T. Shimanouchi, J. Mol. Spectrosc., 28 (1965) 350. 8 W. A. Seth-Paul, J. Mol. Struct., 3 (1969) 403; W. A. Seth-Paul and H. De Meyer; Spectrochim. Acta, Part A, 25 (1969) 1671; W. A. Seth-Paul, J. Mol. Struct., 50 (1978) 29. 9 W. B. Dixon and E. B. Wilson Jr., 3. Chem. Phys., 35 (1961) 191. 10 A. P. Cox and S. Waring, Trans. Faraday Sot., 67 (1971) 3441. 11 J. C. D. Brand and T. M. Cawthon, J. Am. Chem. Sot., 77 (1955) 319.
-
Printed in The Netherknds
Short communication
GAS PHASE INFRARED REVISITED
B. J. VAN DER VEKEN,
SPECTRA
R. L. ODEURS,
OF HALOGEN
J. SHAMIR*
NITRATES
and M. A. HERMAN
Laboratorium voor Anorganische Scheikunde, Rijksuniversitair Centrum Antwerpen, Groenenborgedizan 171. B 2020 Antwerpen (Belgium) (Received
27 November
1980)
Over a long period of time, the structure of halogen nitrates XONOz has been the subject of controversy. The gas phase infrared investigations either fail to produce evidence for the postulated planarity [l] or simply accept planarity as a starting point [ 21. Subsequent Raman studies have focused on depolarization ratios to forward either perpendicularity [3, 41 or planarity [ 5]_ More recently, a microwave study on chloronitrate led to inertial constants for the 35Cl and “Cl isotopic species [6] _ Although insufficient data were available for a complete structure determination, the inertial defect value (1,-I,-r,) suggests a planar conformation. It is the intention of this note to show that the previously published gas phase infrared spectra [ 1,2] allow an unambiguous assignment of the conformation of chloronitrate. Based on the structural model for chloronitrate used in ref. [2] , for a planar conformation the geometry parameters were adjusted to reproduce the inertial constants published by Suenram et al. [6]. The same bond distances and angles were also used to set up a perpendicular model. Projections of both are shown in Fig. 1. The characteristics of the gas phase infrared bands for these models can be obtained either by locating the nearest model in the catalogue of Ueda and Shimanouchi [ 71, or by using the PR separation relations of Seth-Paul [S] . Results for the latter are gathered in Table 1. The symmetric NO, stretching for both conformations is an A’ vibration and thus its 6ii’/6Q vector has to lie in the plane of symmetry, which is the (a, b) plane for the planar, and the (a, c) plane for the perpendicular conformation. The antisymmetric NOz stretching however is an A’ vibration for the planar, but an A” vibration for the perpendicular conformation_ This implies that for the planar conformation Sz’/SQ is in the (a, b) plane, while for the perpendicular conformation it has to be orthogonal to the (a, c) plane. In the latter case the vector is parallel to the b axis and thus the antisymmetric NOz stretching for this conformer has to show a pure B type character, while for the planar conformation it is an A, B hybrid. *Permanent address: Department of Inorganic and Analytical University
of Jerusalem,
Jerusalem,
0022-2860/81/0000--0000/$02.50
Chemistry, The Hebrew
Israel.
0 1981 Elsevier Scientific Publishing Comnanv
156
Fig. 1. Projections of chloronitrate models in their principal axes. (A) Planar model; (B) perpendicular model. TABLE
2
Calculated separations for chtoronitrate
@RA dRB
@Rc
AQQ
Planar
Perpendicular
14.0 11.6 20.9 10.8
14.4 11.8 21.6 10.4
It should be emphasized that for the perpendicular conformer the prediction is a consequence of the symmetry of the model and not of the umerical values for ‘the geometry parameters actually used. In the region of antisymmetric NO, stretches, ClONO, shows a profile with three maxima, those at 1731 and 1737 and 1743 cm-‘, as listed by Arvia et al. [ 11, and recoginised as a P, Q, R structure centered at 1737 cm-’ by Miller et al. [ 21 (unfortunately these authors do not list P 6nd A branch maxima). From the spectrum published by Arvia et al. [I], it is clear that the outer maxima (1731,1743) are more intense than the sharp central Q branch. The observed PR separation in this structure is in good agreement with the value predicted for an A type transition, as is clear from Table 1. All this shows that the antisymmetric NO2 stretching has a pronounced A type character (the possibility of an unresolved Q branch belonging to a
157
B type transition must be rejected because of the intensities of P and R branches and because of the high QQ’separation predicted from Ueda and Shimanouchi [ 71 and from the Seth-Paul relations; for a QQ separation to be much smaller than the PR separation in a B type band the molecule has to approach spherical top geometry, which actually it is far from). Consequently, with great certainty, from the gas phase infrared spectra the perpendicular conformation can be rejected for chloronitrate, leaving the planar one, in agreement with predictions from microwave. For fluoronitrate, for which isostructural possibilities arise, the gas phase spectra [I, 23 show a beautiful B type antisymmetric NO* stretching. It is then tempting to reverse the argument and to assign a perpendicular conformation to fluoronitrate. This however is dangerous: for a planar geometry the appearance of a B type v,,(NO,) cannot be ruled out on symmetry grounds. For this conformation the observed profile depends on the exact form of the vibration, which due to the symmetry of the molecule, may be coupled to other fundamentals and so the orientation of Sz/SQ and consequently the outlook of the profile is difficult to predict. The situation can be compared with methyl nitrate for which the planar geometry has been established beyond doubt [ 9, IO]. The orientation of the planar conformer of this molecule in its principal axis is very analogous to the one for our planar fluoronitrate model, so again for a pure antisymmetric stretch a highly hybrid A, B profile is expected. Nevertheless, the profile of this band has been classified as a B type transition due to the lack of a Q branch [ 111. It thus is not possible on symmetry grounds to draw conclusions on the conformation of fluoronitrate, both models being equally acceptable as it stands. We are now attempting to resolve this ambiguity by applying gas phase IR profile simulation to this compound. In conclusion, the observation of an A type y,,(NO,) proves the planarity of chloronitrate, whereas the appearance of another type of band in fluoronitrate does not prove anything either way. ACKNOWLEDGEMENTS
We gratefully acknowledge the fruitful discussions we had on this subject with W. A. Seth-Paul. One of us (J. S.) wishes to express his thanks to the “Nationaal Fonds voor Wetenschappelijk Onderzoek” for providing the financial support for his appointment as visiting professor. REFERENCES 1 A. J. Arvia, L. F. R. Cafferata and H. J. Schumacher, Chem. Ber., 96 (1963) 1187. 2 R. H. Miller, D. L. Bernitt and I. C. Hisatsune, Spectrochim. Acta, Part A, 23 (1967) 223. 3 J. Shamir, D. Yellin and H. H. Claassen, Isr. J. Chem., 12 (1974) 1015. 4 K. 0. Christe, C. J. Schack and R. D. Wilson, Inorg. C&em., 13 (1974) 2811. 5 D. W. Amos and G. W. Flewett, Spectrochim. Acta, Part A, 31 (1975) 213.
158
6 R_ D. Suenram and D. R. Johnson, J. Mol. Spectrosc., 65 (1977) 239. 7 T. Ueda and T. Shimanouchi, J. Mol. Spectrosc., 28 (1965) 350. 8 W. A. Seth-Paul, J. Mol. Struct., 3 (1969) 403; W. A. Seth-Paul and H. De Meyer; Spectrochim. Acta, Part A, 25 (1969) 1671; W. A. Seth-Paul, J. Mol. Struct., 50 (1978) 29. 9 W. B. Dixon and E. B. Wilson Jr., 3. Chem. Phys., 35 (1961) 191. 10 A. P. Cox and S. Waring, Trans. Faraday Sot., 67 (1971) 3441. 11 J. C. D. Brand and T. M. Cawthon, J. Am. Chem. Sot., 77 (1955) 319.