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Optical absorption spectrum of γ-irradiated crystalline glycine
Research Notes
This explanation would also account for the absence of any significant frequency shifts of ammonia in carbon tetrachloride with the addition of chloroform, as no change in hybridization would be required for the formation of the hydrogen bond. It is of interest that the “free” N-H stretching frequencies of aniline in chloroform are the same as in carbon tetrachloride (within experimental error). The only major solvent effect of chloroform compared with carbon tetrachloride on the N-H stretching bands of aniline would therefore appear to be the hydrogen bond between the proton of CHCl, and the nitrogen atom of aniline. Acknowledgementa-The author wishes to thank I.C.I.A.N.Z. and the Royal Commissioners for the Exhibition of 1851 for the award of scholarships. The infra-red spectrometer was purchased with the aid of a grant from the Rockefeller
Foundation. A. G. MORITZ*
Department of Organic Chemistry University of Adelaide, South Australia * Present address: University Chemical Laboratory, Lensfield Road, Cambridge.
Optical absorption spectrum of y-irradiated crystalline &wine (Received
7 Janumy 1961)
RECENTLY DODD [l] has reported the ultra-violet absorption spectrum of solid glycine irradiated with X-rays at room temperature up to a total dose of 7 Wads. He found that, during the week following irradiation, the spectrum gradually changed, presumably owing to slow chemical reactions within the crystal, but that the final spectrum did not change over long intervals. This final spectrum had two main peaks at 2550 A and 3200 8; the former was ascribed by DODD to a dicarboxylic acid whilst the latter was unassigned. The electronspin resonance (ESR) spectrum of y-irradiated glycine has been described by GHOSH and WEJIFFEN [Z, 31, the irradiation again being at room temperature. These authors found the spectrum to be complex and to become more so on heating, but nevertheless they were able to show that at least two free radicals were present initially. The concentration of the first,
tentatively
identified as fiH,,
diminished
within a few days of irradiation
while the second,
which could be identified as NH,+.CH.CO,-, was indefinitely stable at room temperature. HORSFIELD [4] has shown that glycine crystals irradiated at liquid-nitrogen temperature, and then allowed to warm up to room temperature, contain but one free radical, namely NH,+.(?H.CO,-. Consequently it was decided to examine the ultra-violet and visible absorption spectra of crystals so treated with a view to a comparison with the spectra reported by DODD. Glycine crystals were grown from aqueous solution and were found to be elongated along the c-axis.* Crystals of good optical quality were sealed in Pyrex tubes under vacuum and * Dr. J. J. WINULE has pointed out to D. H. W. that the statement by ALBIZECHTand COREY151,that glycine elongates along the a-axis, is unreliable and that the crystals used by GHOSH and WHIFFEN [3] were in fact elongated in the c-direction. This has been confirmed in this laboratory in collaboration with D. E. HENN. D. M. DODD, S~ectrochim. Actn 16, 413 (1960). [21 D. K. GHOSHand D. H. WHIFFEN, J. Chem. Sot. 1869 (1960). 133 D. K. GHOSH and D. H. WHIFFEN, Mol. Phys. 2, 285 (1959). 141A. HORSBIELD. Personal communication.
[l]
L5] G. ALBRECHT and R. C. COREY, J. Am. Chew Not. 61, 1087 (1939).
36’i
Research Notes the setubes immersed in liquid nitrogen in a glass dewar vessel. This vessel was then irradiated with a dose of 5 Mrads of y-rays at the Spent Fuel Irradiation Centre, Harwell. After irradiation the crystals were a dark brown colour when cold, but on warming to room temperature they rapidly turned pale yellow. ESR and optical spectra were taken immediately, the latter with a Unicam S.P. 500 spectrophotometer adapted with the aid of an extra quartz lens to study small crystals. The ESR spectrum showed that the only free radical species present was indeed NH,+.(?H.CO,-. The crystal had absorption maxima at 2600 A and 3300 A, the former being twice as intense as the latter. This spectrum is the same as DODD’S final spectrum. A saturated solution of these crystals in water had an absorption spectrum identical with that of a saturated solution of glycine and therefore the species responsible for the optical absorption are not stable to water. It seems most likely that the optical absorption is due to the same free radical as the ESR spectrum. To confirm this, heating experiments were carried out. In these an irradiated and a non-irradiated crystal (to serve as a blank in the spectrophotometer) were sealed in a Pyrex tube under vacuum and this tube placed in the thermometer compartment of a reflux distillation apparatus. Temperature control was then achieved by refluxing a liquid having the appropriate boiling point. In this way, the crystals were heated for two-hour periods at lOO”C, 12O”C, 140°C and 160°C. At the three lower temperatures no change occurred in either spectrum, but, at the highest temperature, between 4 and 4 of the radicals disappeared, as shown by the ESR spectrum, and a corresponding decrease was noted in the intensity of each of the optical absorption maxima. A further heating for two more hours at 160°C caused a further reduction in both spectra without any new features appearing in either. It seems established therefore that the peaks observed by DODD, additional to the absorption of glycine itself, in his “rested” crystals are part of the absorption spectrum of the free radical. NH$.&H.CO,-. AclenowZedgement-This work forms part of the research programme of’ the Basic Physics Division of the National Physical Laboratory and is published by permission of the Director. Basic Physics Division National Physical Laboratory Teddington, Middlesex
G. W. CHANTRY D. H. WHIFFEN
368
This explanation would also account for the absence of any significant frequency shifts of ammonia in carbon tetrachloride with the addition of chloroform, as no change in hybridization would be required for the formation of the hydrogen bond. It is of interest that the “free” N-H stretching frequencies of aniline in chloroform are the same as in carbon tetrachloride (within experimental error). The only major solvent effect of chloroform compared with carbon tetrachloride on the N-H stretching bands of aniline would therefore appear to be the hydrogen bond between the proton of CHCl, and the nitrogen atom of aniline. Acknowledgementa-The author wishes to thank I.C.I.A.N.Z. and the Royal Commissioners for the Exhibition of 1851 for the award of scholarships. The infra-red spectrometer was purchased with the aid of a grant from the Rockefeller
Foundation. A. G. MORITZ*
Department of Organic Chemistry University of Adelaide, South Australia * Present address: University Chemical Laboratory, Lensfield Road, Cambridge.
Optical absorption spectrum of y-irradiated crystalline &wine (Received
7 Janumy 1961)
RECENTLY DODD [l] has reported the ultra-violet absorption spectrum of solid glycine irradiated with X-rays at room temperature up to a total dose of 7 Wads. He found that, during the week following irradiation, the spectrum gradually changed, presumably owing to slow chemical reactions within the crystal, but that the final spectrum did not change over long intervals. This final spectrum had two main peaks at 2550 A and 3200 8; the former was ascribed by DODD to a dicarboxylic acid whilst the latter was unassigned. The electronspin resonance (ESR) spectrum of y-irradiated glycine has been described by GHOSH and WEJIFFEN [Z, 31, the irradiation again being at room temperature. These authors found the spectrum to be complex and to become more so on heating, but nevertheless they were able to show that at least two free radicals were present initially. The concentration of the first,
tentatively
identified as fiH,,
diminished
within a few days of irradiation
while the second,
which could be identified as NH,+.CH.CO,-, was indefinitely stable at room temperature. HORSFIELD [4] has shown that glycine crystals irradiated at liquid-nitrogen temperature, and then allowed to warm up to room temperature, contain but one free radical, namely NH,+.(?H.CO,-. Consequently it was decided to examine the ultra-violet and visible absorption spectra of crystals so treated with a view to a comparison with the spectra reported by DODD. Glycine crystals were grown from aqueous solution and were found to be elongated along the c-axis.* Crystals of good optical quality were sealed in Pyrex tubes under vacuum and * Dr. J. J. WINULE has pointed out to D. H. W. that the statement by ALBIZECHTand COREY151,that glycine elongates along the a-axis, is unreliable and that the crystals used by GHOSH and WHIFFEN [3] were in fact elongated in the c-direction. This has been confirmed in this laboratory in collaboration with D. E. HENN. D. M. DODD, S~ectrochim. Actn 16, 413 (1960). [21 D. K. GHOSHand D. H. WHIFFEN, J. Chem. Sot. 1869 (1960). 133 D. K. GHOSH and D. H. WHIFFEN, Mol. Phys. 2, 285 (1959). 141A. HORSBIELD. Personal communication.
[l]
L5] G. ALBRECHT and R. C. COREY, J. Am. Chew Not. 61, 1087 (1939).
36’i
Research Notes the setubes immersed in liquid nitrogen in a glass dewar vessel. This vessel was then irradiated with a dose of 5 Mrads of y-rays at the Spent Fuel Irradiation Centre, Harwell. After irradiation the crystals were a dark brown colour when cold, but on warming to room temperature they rapidly turned pale yellow. ESR and optical spectra were taken immediately, the latter with a Unicam S.P. 500 spectrophotometer adapted with the aid of an extra quartz lens to study small crystals. The ESR spectrum showed that the only free radical species present was indeed NH,+.(?H.CO,-. The crystal had absorption maxima at 2600 A and 3300 A, the former being twice as intense as the latter. This spectrum is the same as DODD’S final spectrum. A saturated solution of these crystals in water had an absorption spectrum identical with that of a saturated solution of glycine and therefore the species responsible for the optical absorption are not stable to water. It seems most likely that the optical absorption is due to the same free radical as the ESR spectrum. To confirm this, heating experiments were carried out. In these an irradiated and a non-irradiated crystal (to serve as a blank in the spectrophotometer) were sealed in a Pyrex tube under vacuum and this tube placed in the thermometer compartment of a reflux distillation apparatus. Temperature control was then achieved by refluxing a liquid having the appropriate boiling point. In this way, the crystals were heated for two-hour periods at lOO”C, 12O”C, 140°C and 160°C. At the three lower temperatures no change occurred in either spectrum, but, at the highest temperature, between 4 and 4 of the radicals disappeared, as shown by the ESR spectrum, and a corresponding decrease was noted in the intensity of each of the optical absorption maxima. A further heating for two more hours at 160°C caused a further reduction in both spectra without any new features appearing in either. It seems established therefore that the peaks observed by DODD, additional to the absorption of glycine itself, in his “rested” crystals are part of the absorption spectrum of the free radical. NH$.&H.CO,-. AclenowZedgement-This work forms part of the research programme of’ the Basic Physics Division of the National Physical Laboratory and is published by permission of the Director. Basic Physics Division National Physical Laboratory Teddington, Middlesex
G. W. CHANTRY D. H. WHIFFEN
368