COLLIMUNICA’I:ION~ T O THE EDITOR the electron affinity of the acceptor. Such data are of course not available for excited electronic states, but it has been shown2‘ Lhat for ground state fluorobenzenee, the electron affinity increases by approximately 0.4 eV per fluorincb substituent,. Assuming the same to hold true for excited states of these compounds, increase in fluorine substitution should lead to enhanced quenching. The effect would be expected to be even more pronounced for substitution of CFa groups into the benzene rirg for which the estimated increase in electron affinity per sJbstituent group is approximately 1 Figure 4 shows the Stern-Volmer plots for the quenching of the excited singlet states of a series of mono- and disubstituted benzcnes by but-2-ene and buta-1,3-dienej and qLenching parameters are given in Table 111. I t is cviclmt that monofluoro substitution in benzene, toluene, and (trifluoromethy1)benzene does apparently eiihnnce the efficiency of quenching of the aromatic moleciile by the olefins, and it is also evident that CF3 substitution has a similar but much more dramatic effect, iericling support to the proposed mechanism. Reaction 8. .Is far as can be ascertained electronic energy transfer from T31u benzene to all olefins studied here (with the thxonption of 1,l-difluoroethylene) is an exothermic process, arid it thus seems likely that this mechanism is piredoininantly responsible for the observed quenching of triplet benzene in these experiments. In no c a e w a the ~ phosphorescence of biacetyl excited directly at 404.7 nm quenched by the addition of the olefins, arid thus in the case of benzene-biacetyl
3673 mixtures quenching is a directly attributable to quenching of an excited state of benzene. Perfluorobut-2-ene and czs- and trans-dichloroethylene are seen to be efficient yuenchers of triplet benzene, indicating low-lying triplet olefin energy levels The fact that cis- and trans-dichloroethylene quench with the same rate, whereas slightly different rates are observed for singlet quenching, is indicative that different mechanisms pertain for singlet and triplet quenching. Vinyl fluoride and allene are less efficient triplet quenchers and 1,l-difluoroethylene is very inefficient, indicating perhaps that in this cas? electronic energy transfer would be an endothermic process. The results for allene support the contention that in the benzene-senstitized photolysis of this type of triplet energy transfer rather than singlet sensitization is the mechanism, although it has recently been reported that in solution allene may undergo photoaddition to the excited singlet state of benzene by a nonconcerted process.24 Acknowledgments. The authors gratefully acknoa 1edge partial financial support from NATO European Office of Scientific Affairs and payment of fees by the British Council during the tenure of a, researeh studentship by Miss Das Gupta. Helpful discussions uith Dr. M. Godfrey are gratefully aclinowledged. (21) W. T. Naff and C. P. Cooper, J. Chem. Phys., 49, 2787 (1968). (22) M. Godfrey, private communication. (23) H. R. Ward and E. Karafiath, J . Amer. Chem. SOC.,91, 7475 (1969). (24) D. Bryce-Smith, B. E. Foulger, and A. Gilbert, J . Chem. Sac., Chem. Commun., 664 (1972).
COM nls u I C A T I O N S T O T H E E D I T O R Ultraviolet Absorption Spectra of Palyphosphnte Solutions
Sir: Although .he IN absorption spectra of pyrophosphate and ortliophosphate have been given’ 40 detail is givcn of polyphospha-te absorption bands. The shoulder on the a-a* peak reported here for trimethyl phosphate and pyrophosphate has not previously been demonstrated. The effect of solvents on the n-a* absorption bands in the uv i.s to give the well-known “blue shift.’’ The effect i 6 well documented for organic molecule^,^^^ nitrates, and nitrites4 but no n-a* transition is given in the literature for ortho and polyphosphate anions. 41ihx~ghthe peak in the region of
45,000 cm-’ reported here is a suspect’edn-a* trransition no definite blue shift with change of solvent has been established. The trimethyl phosphate was British Drug House technical reagent grade soluble in three solvents, distilled water, spectroscopically pure ethan-ol, and nhexane. Sodium met,aphosphate was prepared by heating AR quality NaH2P04in a clean platinum dish in (1) R. P. Buck, S. Singadya, and L. B. Rogus, Anal. Chem., 26, 1240
(1954). (2) J. N. Murrell, “The Theory of the Electronic Spectra of Organic Molecules,” Methuen, London, 1963. (3) J. W. Sidman, Chem. Rea., 58, 689 (1958). (4) S. J. Strickler and M , Kasha, “Molecular Orbitals in. Chemistry, Physics, and Biology,” F.0 . Lowdin and B. Pullman, Ed., Academic Press, Xew York, iX.V., 1964.
The Journal of Physical Chemistrg/,Val. 76, No. 94, 1972
3674
COMMUNICATIONS TO THE EDITOR
a muffle furnace a t 950" for 8 hr. Five sodium phosphales with varying composition were prepared by heating the metaphosphate with calculated weights of AR Na2C03to give glasses with NazO:Pz05 ratios of 1.1:l; 1 . 2 : l ; 1.3:l; 1*4:1;1.5:l. Thechainlengths of these glasjes were determined by pH titration5 and the chain lengths axe recorded in Table I, together with the n-T 'uv absorption peak. Potassium metaphosphate and related phosphates were prepared in a similar manner by using AIL EIH2P04 and AR K&O3 as above.
2.0
t .O
0
45
50
40
35
Table I : 'Variation of Absorption Peak Wavenumber, wavelength, and Molar Absorptivity with Composition and Chain Length for a Series of Sodium Phosphates Molar
Chain
ratio
length,
Naz0:PeOs
6
57.1 17.70 9.79 7.23 5.20
1.1 I 1:l 1.2:1 1.3:1 1.4:1 1.5:l
2.01
Wavenumbers, cm-1
Wavelength, nm
49,380 49,660 49,800 49,960 50,900 51,680
202.5 201.4 201.0 200.2 196.5 193.5
Molar
absorptivity
1.400 X 1.278 X 1.204 X 3.742 X 1.516 X 6.304 X
lo8 lo3 lo3 IO2
lo2 lo1
A11 spectra were recorded on an Optica CF, and SP800 spectrop hocometers at room temperature in matched 1-cm quartz absorption cells. The effect of solvents on the suspected n-a* peak near 45,000 cm-l is shown in Table II and Figure 1 for trimethyl phosphate. The intensity of the band near 45,500 cm-I is much lesci than that near 50,000 em-' and the 45,500 cm-l barid may be due to an n-n* transition. The peak is fo'md in all spectra we have measured. The variation 111 the T-T* peak near 50,000 cm-I with decrea3ing chain length for 5 X polyphospha,te solution is shown in Table 1 and Figure 2. A similar trend in tkie absorption spectrum of solutions of potassium phosphates is shown in Table 111. As the number of phosphorus atoms in the chain decreases, so the T-T* peak decreases in wavelength. The decrease of phosphate chain length of potassium phosphates with increasing metal oxide content is a well-established effect be1ng shovl7n by Westman and coworkersBaby paper chromatographic studies of K2O P205 solutions and by VliiLiam, et aZ.,6bfrom both infrared studies of
0
45
50 W
A
V
J x~
103 ~
4c
35
~
Figure 1. Spectrum of (a) trimethyl phosphate in ethanol, (b) pure trimethyl phosphate, and (e) trimethyl phosphate in n-hexane.
solids and glasses and by surface tension studiesec of molten potassium phosphates. Similar trends in the position of a n- T * transition have been shown by Murel (see ref 2, chapter 5 ) for conjugated phenyl and carboxyl groups, for polyene acids, and for a-w dimethyl polyenes. For conjugated organic molecules both free electron theory and simple MO theory predict that the numerical value of the wavelength of the first absorption (5) (a) J. R. Van Wazer, "Phosphorus and its Compounds," Vol. 1, Interscience, New York, N. Y . , 1958, p 419 ff.; (b) ibid., p 717 ff (6) (a) A. E. R. Westman and P. A . Gartaganis, J . Amer. Ceram Soc., 40, 293 (1957); (b) D. J. Williams, B. T. Bradbury, and W. R Maddocks, J . SOC.Glass Tech., 43, 337 (1959); (c) B. T. Bradbury and W. R. Maddocks, ibid., 43, 325 (1959).
1 1 _ _ 1 _ _ -
Table I1 : Veriation of Absorption Peak Wavenumber, Wavelength, and Molar Absorbtivity with Solvent for Trimethyl Phosphate Solvent
Distilled €LO Spectroscopically pure ethanol %-Hexane Nonsolvated (CH3)3Pi04 Th,e Journal
6.f
Wavenumbers, om-1
Wavelength, nm
51,000 50,500 50,000 50,500
196.1 198.0 200 198.0
Phy,oical Chemistry, Vol. 76, N o . 94, 1979
Molar
Wavenumbers,
absorptivity
om-1
2.56 X 1 0 - I 3.04 X 10-' 1.25 X 10-1
45 ,500 45,500 45,500 45,500
Wavelength, nm
219.8 219.8 219.8 219.8
Molar
absorptivity
7 . 5 x 10-2 8.0 X
7.5
x
10-2
COMMUNICATICING TOITHE EDITOR
3675
peak is proportional to the number of carbon atoms in the chain of the molecule. With increasing chain length of the molecule the first absorption peak tends to a limited value and theoretically this limit can be calcu-
OB G U S S (lip) Ill
1.211
1.3:1 14:1 1.5~1
1ao W'4rJimGwl
225
Giixl'if 1 2
J
4
H
330
nM
Figure 2. Uv absorption spectra of polyphosphttte solutions.
Table I11 : Variation of Absorption Peak, Wavenumber, and Wa,velength for a Series of Potassium Phosphates Mole ratio oNf
potassium phosphate
Wavenumhers,
Ka0 : P z O ~
1:l 1.1:l x.2:1 1.3:1
P.4:I
-
P.5:1
----
cm-1
51,300 51,679 51 ,800 51,813 52 ,100 52,219
Infrared Band of HDO Publication costs assisted by UniversitB Lava1
Sir: I n a recent communication, Bonnerl reports measurements of the near-infrared spectrum of dilute (5%) solutions of HzO in DzO in which he has observed
MOIM NLSIO
1.1:1
On the Alleged Resolution of an
Wavelength, nm
195 193.5 193 193 192 191.5
lated by allowing for an alteration in bond length along the conjugated chain of the molecule. It is likely that similar theoretical interpretations can be used to explain the shift in n-r* absorption peak with chain length in polyphosphates Dietailed calculations are being performed in these laboratories.'
two adjacent bands with opposite temperature behavior. He interprets them as resolved components of Lhe same combination band due to hydrogen-bonded and nonbonded HDO molecules. He claims this to be the first instance of a single band resolved into such Components. However, his results and interpretation are a t variance with those of a recent study12which he does not quote. The two bands in question, at 2030 nrn (4935 c") and 1905 nm (5250 cm-'), have already been described by Luck and Ditter (their Figure 10) who found their intensity to be rather insensitive to temperature over the range from 0 to 80". In contrast, Bonner observes a large temperature effect over a narrower range. Thus, his weak band a t 5250 cm-' increases nearly tenfold from 25 to 70" while that a t 4935 em-' decreases by roughly 40%. Since Bonner's conclusions are based mainly on this temperature effect, the discrcpancy with the previous authors raises serious doubt about his interpretation. Following Bnyly, et u L . , ~ he assigns the main band a t 4935 em-' to the combination of O-H stretching (at 3410 cm-l) and bending modes (at 1460 cm-I) of the hybrid molecule HOD. Now, this assignment had first been suggested on the assumption that the assignment of the corresponding (?) vapor band (at 5089 cm-l)' was correct. However, assignment to the overtone of the O-D stretching (at 5363.6 cm-l in the vapor) gives a more realistic frequency shift on condensation, and likewise, a more reasonable anharmonicity (2 X 2500 cm-l) for the liquid bands. As for the weaker band a t 5250 cm-l, Bonner simply mentions that the corresponding vg) = 5332 cm-l in the vapor state5 combination ( v:! shows the right frequency shift upon condensation. That conclusion is unfounded because the vapor band belongs in fact t o the NzQ molecule, not t o HDOe6
+
(1) 0. D. Bonner, J.Phys. Chem., 76, 1228 (1972). (2) W. A. P. Luck and W. Ditter, Z. Naturforsch., 24b, 482 (1969). (3) 3. G. Bayly, V. B. Kartha, and W. H. Stevens, Infrared Phys., 3,311 (1963). (4) W. 8. Benedict, N. Gaylar, and K. Plyler, J. Chem. .Phys., 24, 1139 (1956).
(5) G. Herzberg, "Infrared and Raman Spectra of Polyatomic Molecules," Van Nostrand, Princeton, N. J., 1959. (7) L. 3. Btanley, personnltl communication.
CHEMISTRY DEPARTMENT UNIVERSITY O F BRADFORD BRADFORD
7,
ENGIAND
(6) The confusion here arises from the erroneous nomenclature adopted by infrared spectroscopists for the fundamental modes of the
M. BENNOSON hybrid HDO molecule: namely, YL for the O-D stretching, Y C for the D. J. WILLIAMS* bending, and va for the O-H stretching. Since the fundamentals of HDO are all of the same species, they should normally be numbered in the order of their decreasing frequency, but this also would be confusing in relation to the fundamentals of H20 and D:O. It is preferable to identify them simply as VO-H, YO-D, and ken. The Journal of Physical Chemistry, Vol. 76, N o . 84,2978
