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Far infra-red absorption of dimethyl mercury
Sp~t~hi~ic& Acts,1963,vol. IQ,pr, 1903to 1960.Pergamon Pnm Ltd. Printedin Northernl&and
RESEARCH NOTES
Far in&a-red
absorption
of &methyl
mercury
(Received 29 May 1963) THE H&--Hg-CIIs molecule doubtless has a linear G-Hg-C skeleton and the investigation of infra-red perpendicular C-H stretching bands [l] indicates that the methyl groups are freely rotating at ordinary temperature. BAUMAN [ 21 has concluded that the effective point group for selection rules in such cases is Dsd. Thus the molecule effectively possesses a centre of symmetry, and consequently all frequencies permitted in the Raman effect should be forbidden in infrs-red absorption and vice versa. In general the paucity of observed coincidences between the two types of spectra (see GUTOWSKY [3]) is in accordance with expectation; but special interest attaches to the appearance of a low frequenoy at about 160 cm-l in the Ram&n spectrum of liquid dimethyl mercury. This is most naturally assignable to the skeletal bending fundamental (class e, for point group Ds& which however should be forbidden in the Raman effect and permitted only in the infrared. The Raman line in question was not observable by GUrowsKY [33 owing to its being fortuitously coincident with a grating ghost. It was however reported by FEHBR et oZ. [4] & A9 = 156 cm-r (weak) and its reality has been confirmed by the present writer both photographically and also with the Gary Node1 81 recording Raman spectrome~r (Av = 160 cm-l, weak). GUTOWSKY[3] suggested that the line might be due not to the skeletal bending fundamental but to some permitted difference. He put forward rs(un,) - Zrs(ar,) = 1184 - Z(Sl5) = 154 cm-l as a possibility. This would be Ramsn active (as a polarized line) and forbidden in the infrared. The observed Raman line, however, shows no sign of being polarized; nor does the corresponding sum frequency 11s+ 2~s = 2214 cm-l appear in the Raman spectrum. GUTOWSKY writes, “Verification of the assignments could be made’by observing the region in the infrared with the aid of residual rays.” This Note reports the result obtained in this way. The far infm-red spectrometer used has been described by &%ILLER et al. [5] The sample of dimethyl mercury v&pour was contained in a 10-m path-length cell at IS pressure of 28 mm Hg. Using a 20 lines@n grating, a well-pronounced absorption band was observed in the KC1 Reststrahlung region. The band extended from about 14.5to 170 cm-r and appeared somewhat unsymmetrical in shape, being steeper on the low-frequency side. The “oentre”, which is rather difficult to locate precisely, is at about 153 f 3 om-l. There seems to be no doubt that this is the skeletal bending fundamental. The practical coincidence of its frequency with that of the Raman line at 160 cm-l indicates that the latter must be due to a breakdown of the D,, selection rules. This is reminiscent of the breakdown causing the appearance of the bending fundamental of carbon disulphide in the Raman spectrum [I] D. [2] R. [3]H. [4] F. [5] F.
R. J. BOYD, H. W. TEOMPSONand R. L. WILLIAMS, D&wme.s.Faraday 5’00. 9, 154 (1960). P. BAXJMAN,J. Cftern. P&s. 24, 13 (1956). S. GUTOWSKY,J.G%~TPL P&p. 17, 128 (1949). FEH~R, W. KOI;B and L. LEVERENZ, 2;.~~t~rfor~c~. 2A, 454 (1947). A. MILLER, G. L. CAIZLSON and W. B. WKITE, Spectrochim. dcta 15,709(1959). 1963
1964
Research notes
of the liquid, which has been shown [S] to be due to molecular ~~ractions. same cause is operative in liquid dimethyl mercury.
It is likely that the
Acknowledgeme&+--Thiswork was carriedout at the Mellon Institute, Pittsburgh, U.S.A. during the tenure of a Research Fellowship, for which the writer expresses his thanks. In partioular he is grateful to Dr. FOILA. MILLERfor putting at his disposalthe far infrared and Gary Raman spectrometers. L. A. Woonw~E~ Inorganic Chemistry Laboratory South Parka Road Oxford [6] J. C. EVANSand H. J. BERNSTEIN,Can. J. Chem. 54, 1127 (1956).
The &P--H and SI?--H coupliug constants and the direct measurement of JEH in diethyltin compounds (Received 28 February 1963) DIETHYXTINcompounds show complicated AaBs spectra, the complexity arising from the similarity of the chemical shifts of the methyl and methylene resonance. In diethyltin dichloride and dimethoxydiethyltin Et,Sn(OMe), the interpretation of the spectra is assisted by observations on the resonancebands arising from the tin-proton coupling. The presence of two sets of tin-proton spin coupling bands, arising from the Sn117and Snln?isotopes, both having spin +. and in relative abundance 7.67 and 8-68 per cent respectively, has been described for the me~ylt~ compounds [I, 21. In the ~ethyltin compounds the Sn-H couplingsgive rise to two AaBs patterns on either side of the main resonance, each component being doubled due to the presence of the two tin isotopes (see Fig. 1). The occurrenceof this Sn-H splitting permits the determination of the origin of the methyl and methylene resonance. In the dimethoxy compound the methylene resonanceoccurs at 2.3 c/s, to high field of the methyl resonance, and consequentlythe two resonancesoccur together to give rise to a broad line, although under very high resolution some fine structure is observed. A similar situation has been reported in tetraethyl lead in whioh SCH, - 6CHs m 0 [3]. In the dichloride the methylene resonance occurs at 14.6 c/s to low field of the methyl resonance. The couplingof the tin to the &protons ismuch greaterthan that to the Z-protons, themethylene proton couplingconstantsto Sn*17and Snlls of dimethoxydiethyltinare of the same order as the Sn117-Me and Snlls-Me coupling constants of dimethoxy~ethylt~ [a], namely 71.3 and 74.4 c/s respectively. The spectra were recordedat 40 Mcfs on a Perkin-Flmer spectrome~r, using 1.5 M solutions of the tin compounds in carbon ~tra~hloride. AckmwZedgement+--Wo thank the Department of Scientific and Industrial Research for a grant to purchase the NMR spectrometer. [l] J. R. HOLMXSand H. D. I(AESZ,J. Am. Chem. Sot. 85, 3903 (1961). [2] G. P. VAN DEE K&LEN, Nature 193,1069 (1962). [33 E. B. BAKER,J. Ghem. Phya. 26, 960 (1957). [4] W. GERRARD,E. F. MOONEYand R. G. REES. Unpublished work (1963). The Northern Polytechnic Holloway Road London N.7
W. GERRAED J. B. LEAXE E. F. MOONEY R. G. REES
RESEARCH NOTES
Far in&a-red
absorption
of &methyl
mercury
(Received 29 May 1963) THE H&--Hg-CIIs molecule doubtless has a linear G-Hg-C skeleton and the investigation of infra-red perpendicular C-H stretching bands [l] indicates that the methyl groups are freely rotating at ordinary temperature. BAUMAN [ 21 has concluded that the effective point group for selection rules in such cases is Dsd. Thus the molecule effectively possesses a centre of symmetry, and consequently all frequencies permitted in the Raman effect should be forbidden in infrs-red absorption and vice versa. In general the paucity of observed coincidences between the two types of spectra (see GUTOWSKY [3]) is in accordance with expectation; but special interest attaches to the appearance of a low frequenoy at about 160 cm-l in the Ram&n spectrum of liquid dimethyl mercury. This is most naturally assignable to the skeletal bending fundamental (class e, for point group Ds& which however should be forbidden in the Raman effect and permitted only in the infrared. The Raman line in question was not observable by GUrowsKY [33 owing to its being fortuitously coincident with a grating ghost. It was however reported by FEHBR et oZ. [4] & A9 = 156 cm-r (weak) and its reality has been confirmed by the present writer both photographically and also with the Gary Node1 81 recording Raman spectrome~r (Av = 160 cm-l, weak). GUTOWSKY[3] suggested that the line might be due not to the skeletal bending fundamental but to some permitted difference. He put forward rs(un,) - Zrs(ar,) = 1184 - Z(Sl5) = 154 cm-l as a possibility. This would be Ramsn active (as a polarized line) and forbidden in the infrared. The observed Raman line, however, shows no sign of being polarized; nor does the corresponding sum frequency 11s+ 2~s = 2214 cm-l appear in the Raman spectrum. GUTOWSKY writes, “Verification of the assignments could be made’by observing the region in the infrared with the aid of residual rays.” This Note reports the result obtained in this way. The far infm-red spectrometer used has been described by &%ILLER et al. [5] The sample of dimethyl mercury v&pour was contained in a 10-m path-length cell at IS pressure of 28 mm Hg. Using a 20 lines@n grating, a well-pronounced absorption band was observed in the KC1 Reststrahlung region. The band extended from about 14.5to 170 cm-r and appeared somewhat unsymmetrical in shape, being steeper on the low-frequency side. The “oentre”, which is rather difficult to locate precisely, is at about 153 f 3 om-l. There seems to be no doubt that this is the skeletal bending fundamental. The practical coincidence of its frequency with that of the Raman line at 160 cm-l indicates that the latter must be due to a breakdown of the D,, selection rules. This is reminiscent of the breakdown causing the appearance of the bending fundamental of carbon disulphide in the Raman spectrum [I] D. [2] R. [3]H. [4] F. [5] F.
R. J. BOYD, H. W. TEOMPSONand R. L. WILLIAMS, D&wme.s.Faraday 5’00. 9, 154 (1960). P. BAXJMAN,J. Cftern. P&s. 24, 13 (1956). S. GUTOWSKY,J.G%~TPL P&p. 17, 128 (1949). FEH~R, W. KOI;B and L. LEVERENZ, 2;.~~t~rfor~c~. 2A, 454 (1947). A. MILLER, G. L. CAIZLSON and W. B. WKITE, Spectrochim. dcta 15,709(1959). 1963
1964
Research notes
of the liquid, which has been shown [S] to be due to molecular ~~ractions. same cause is operative in liquid dimethyl mercury.
It is likely that the
Acknowledgeme&+--Thiswork was carriedout at the Mellon Institute, Pittsburgh, U.S.A. during the tenure of a Research Fellowship, for which the writer expresses his thanks. In partioular he is grateful to Dr. FOILA. MILLERfor putting at his disposalthe far infrared and Gary Raman spectrometers. L. A. Woonw~E~ Inorganic Chemistry Laboratory South Parka Road Oxford [6] J. C. EVANSand H. J. BERNSTEIN,Can. J. Chem. 54, 1127 (1956).
The &P--H and SI?--H coupliug constants and the direct measurement of JEH in diethyltin compounds (Received 28 February 1963) DIETHYXTINcompounds show complicated AaBs spectra, the complexity arising from the similarity of the chemical shifts of the methyl and methylene resonance. In diethyltin dichloride and dimethoxydiethyltin Et,Sn(OMe), the interpretation of the spectra is assisted by observations on the resonancebands arising from the tin-proton coupling. The presence of two sets of tin-proton spin coupling bands, arising from the Sn117and Snln?isotopes, both having spin +. and in relative abundance 7.67 and 8-68 per cent respectively, has been described for the me~ylt~ compounds [I, 21. In the ~ethyltin compounds the Sn-H couplingsgive rise to two AaBs patterns on either side of the main resonance, each component being doubled due to the presence of the two tin isotopes (see Fig. 1). The occurrenceof this Sn-H splitting permits the determination of the origin of the methyl and methylene resonance. In the dimethoxy compound the methylene resonanceoccurs at 2.3 c/s, to high field of the methyl resonance, and consequentlythe two resonancesoccur together to give rise to a broad line, although under very high resolution some fine structure is observed. A similar situation has been reported in tetraethyl lead in whioh SCH, - 6CHs m 0 [3]. In the dichloride the methylene resonance occurs at 14.6 c/s to low field of the methyl resonance. The couplingof the tin to the &protons ismuch greaterthan that to the Z-protons, themethylene proton couplingconstantsto Sn*17and Snlls of dimethoxydiethyltinare of the same order as the Sn117-Me and Snlls-Me coupling constants of dimethoxy~ethylt~ [a], namely 71.3 and 74.4 c/s respectively. The spectra were recordedat 40 Mcfs on a Perkin-Flmer spectrome~r, using 1.5 M solutions of the tin compounds in carbon ~tra~hloride. AckmwZedgement+--Wo thank the Department of Scientific and Industrial Research for a grant to purchase the NMR spectrometer. [l] J. R. HOLMXSand H. D. I(AESZ,J. Am. Chem. Sot. 85, 3903 (1961). [2] G. P. VAN DEE K&LEN, Nature 193,1069 (1962). [33 E. B. BAKER,J. Ghem. Phya. 26, 960 (1957). [4] W. GERRARD,E. F. MOONEYand R. G. REES. Unpublished work (1963). The Northern Polytechnic Holloway Road London N.7
W. GERRAED J. B. LEAXE E. F. MOONEY R. G. REES