1770
J. Phys. Chem. 1981, 85, 1770-1771
COMMENTS Comment on “A Laser Raman Spectroscopic Study of Aqueous Orthophosphate Salts”’
Sir: Unfortunately our more recent work2s3on saturated solutions of NaH2P04had not come to the attention of Preston and Adams. Their observations of a marked change in contour (shift of maximum, increase of width a t half-height) of the broad Raman scattering region between 1000 and 1100 cm-l in going from concentrations of 4 to 6 mol L-l should be understood as a change between main constituents in an equilibrium mixture. The statementsl that “the 1 0 3 0 - ~ mband - ~ is most likely the PO2 vibration of H2PO4- which has been perturbed, possibly by hydrogen bonding”, and that “two previously reported bands have been shown to be perturbations ...” are only obscuring the experimental results. The occurrence of at least two different vibrations for saturated solutions in the region under discussion was established from the different amounts of anisotropic ~cattering.~ Only for saturated solution IR results are available a t this time.3 When we calculate a dimerization constant Qd by the “simple mode1”l from integrated IR intensities of the bands found at 1050 and 1082 cm-l we obtain Qd = 1.5 L mol-l, quite a t variance with the values cited.’ This invalidates the assignment proposed’ for the bands of H2P042-. It is quite normal that not all the regions of a spectrum show sure signs from an additional species. The effects occur just where we consider from IR e ~ i d e n c eoverlap,~ ping v,(PO2)-S(P0H), which band should be preferably involved by the formation of additional species through hydrogen bonding. (1) C. M. Preston and W. A. Adams, J. Phys. Chem., 83, 816 (1979). (2) E. Steger and K. Herzog, 2nd Symposium on Inorganic Phosphorus Compounds, Prague, Sept 1974, Abstracts. (3) E. Steger, K. Herzog, and J. Klosowski, 2.Anorg. Allg. Chem., 432, 42 (1977); cf. Chem. Abstr., 87, 93054 (1977). Chemistry Department Technical University Dresden, German Democratic Republic
E. Steger” K. Herzog
Received: June 24, 1980
I, using argon-flushed solutions in methylcyclohexane at room temperature in 10 X 10 mm cross-section quartz cells. Two concentrations were used to ensure almost equal absorbances (about 35%) at 360 nm (high concentration) and at 330 and 310 nm (low concentration). The areas beneath the emission spectra measured at these three wavelengths of excitation on a Perkin-Elmer-Hitachi MPF-44, 3-nm slits, were evaluated and corrected for small differences of absorbance. The relative intensity of the exciting light at the three wavelengths, Ix, was measured by means of the emission from a cell containing a solution of rhodamine B, as used for measuring corrected excitation and emission spectra. The relative quantum yields QFd calculated from the observations were as follows: 360 nm, 1.00; 330,0.95; 310,0.85; all rt0.05. In a second set of measurements on the dilute solution, emission spectra were taken a t a number of excitation wavelengths, the transmission values at the same wavelengths were measured digitally, and the light intensities I x recorded as above. The resultant QFrd’s were as follows: 360 nm, 1.00; 350,0.87; 340,0.91; 330,0.96; 320,0.87; 310,0.83; all *0.1. The values reported by Shekk et al. were 366 nm, 0.16(!); 334, 0.85; 313, 0.60. The discrepancy is much beyond the experimental uncertainty. We raised this matter with our above colleagues and together reached the conclusion that their very low value QFrel = 0.16 at 366 nm is probably due to a nonfluorescent impurity which absorbs in the range 360-370 nm much more strongly than compound I (366 nm is at the longwavelength edge of the absorption spectrum of I). The QFre%reported by us2 for compound I1 were A,, = 380, QFre’ = 1.18; 370, 1.13; 360, 0.97; 340, 1.02; 330, 1.00; 320, 0.90; 310,0.81. Most of these values fall within &lo% of those reported by Shekk et al.,l which were as follows: (relative to QF at 334) 405, 0.72; 366, 0.61; 334, 1.0; 313, 0.78. We conclude that the only trans-1,2-diarylethylenesin which a pronounced variation of the fluorescence quantum yield with A,, has been shown to exist2 are l-phenyl-2(2-naphthyl)- ahd 1,2-di-(2-naphthyl)ethylene,most probably because two or three almost isoenergetic conformers exist in equilibrium, and differ in their QF and lifetime TF values. This is not the case for compounds I and 11. Department of Structural Chemistry The Weizmann Institute of Science Rehovot, Israel
Varlatlon of Fluorescence Quantum Yield of Naphthylethylenes with the Frequency of the Exciting Light
Sir: In a recent paper1 Shekk and co-workers report a very large variation of the fluorescence quantum yield with the wavelength of excitation for l-phenyl-2-(l-naphthyl)ethylene, I, and a less pronounced one with 1,2-di-(1naphthyl)ethylene, 11. These results differ markedly from our own published2 and unpublished ones, and we therefore remeasured the relative emission quantum yields of (1) Kovalenko, N. P.; Alfimov, M. V.; Abdukadirov, A.; Shekk, Yu. B. Zzu. Akad. Nauk SSSR., Ser. Khim. 1979, 6, 1247.
(2) Haas, E.; Fischer, G.; Fischer, E . J . Phys. Chem. 1978, 82, 1638.
Ernst Fischer
Recelved: February 25, 198 1
Measurement of Partial Molar Volumes at Infinite Dllutlon in a Supercritlcal-Fluld Solvent near Its Crltical Point
Sir: Interest in solvent extraction with supercritical or near-critical fluids has been rapidly increasing, and has resulted in renewed research activity in phase equilibrium behavior for dilute solutions a t conditions near the critical point of the dense-fluid solvent. Recently, van Wasen and Schneiderl presented an experimental method for meas-
0022-3654/81/2085-1770$01.25/00 1981 American Chemical Society
