COMMUNICATIONS TO THE EDITOR
2415
nonpolar analog and the solvent, respectively, and VP and CS are the limiting partial molar volume of P and the molar volume of pure S. Similarly, an expression for AG0p may be developed by considering L3'
the isothermal transfer of the NPA molecule from the ideal gaseous state to the pure liquid state a t the equilibrium vapor pressure and then to the ideal dilute solute state in solvent S a t unit molarity. This derivation leads to the result AGOP = Vp(6s -
~NPA)'
U+S
+ RT In ( ~ O K P A V S / R T(2))
where ONP PA is the vapor pressure of pure liquid NPA a t the temperature T a 4 Figure 1 shows results of applying eq 1 and 2 to predict AGOp from A E O p for various polar donor, acU+S
S
'
U
ceptor, and complex molecules transferred into nonpolar solvents a t 25". For simplicity, pp for each donor and acceptor has been equated to the molar volume of the pure liquid, and VP for each complex is taken to be the sum of the molar volumes of pure donor and acceptor; moderate changes in VP produce only slight changes in calculated thermodynamic properties. Values of ONP PA, in Torr, were obtained from the empirical relation log ONP PA (at 25")
=
- 1.038
x
io-4(62NPA
+
-ii,)''15 4.889
which provides an excellent fit of vapor pressure data for the nonpolar liquids for which 6 values are a ~ a i l a b l e . ~ Calculated and experimental values of AGOp in V"S
general agree to within a few tenths of a kilocalorie; the calculated results are in somewhat better agreement with an empirical linear free energy-energy correlation proposed previously'" (represented by the dashed line in Figure 1) than are the experimental values. Attempts are being made to provide a satisfactory theoretical basis for the NPA model6 and to apply it conjointly with several theories of nonpolar liquids and solutions. The model is also being used to predict limiting activity coefficients of polar solutes a t infinite dilution in nonpolar solvents, referred to the pure polar liquid standard state; this prediction requires merely that experimental thermodynamic properties of the polar liquid be combined with results of calculations based on eq 1 and 2, using known values of AEOp. V+S
Acknowledgment. The authors are indebted to Professor Roger Frech for numerous stimulating discussions. This \vork was supported by Kational Science Foundation Grant No. GP-23278. (4) Strictly speaking, P O S P A should be replaced by the fugacity of pure liquid NPA, but the effect of this correction on calculated free energies is usually negligible. (6) R. Frech, unpublished work.
COMMUNICATIONS TO THE EDITOR
Modification of Nitrobenzene Photochemistry
by Molecular Complexation Publication costs borne completely bu The Journal of Physical Chemistry
Sir: One of the intriguing possibilities for photochemists today is the modification of photochemistry
via inter- or intramolecular perturbations, i e . , protonation, complexation, heavy atom effects, etc. I n previous investigations from this laboratory we demonstrated that nitrobenzene in its lowest triplet n,p* state undergoes hydrogen abstraction from isopropyl alcohol and stannane, resulting in the formation of p h e n y l h y d r ~ x y l a m i n e . ~I ~n ~ the presence of acidic solutions, photoreduction of nitrobenzene and nitronaphthalene is observed, the latter showing no reactivity in alcoholic solutions.3,4
I n this report we present the results of a novel photochemical process involving nitrobenzene, after formation of the 1 : l donor acceptor complex with BC13, i e . , C6H5NOzs+ .BCl3", in cyclohexane solutions. The complex was prepared by bubbling BC13 through solutions having known concentrations of nitrobenzene in c y ~ l o h e x a n e . ~Photolysis ~~ of these solutions does not lead to photoreduction of nitrobenzene, but rather photooxidation to nitrosobenzene is observed. Irradiation of 366 nm of a vacuum degassed cyclohexane solution of 2.5 X M nitrobenzene complexed (1) R. Hurley and A. C. Testa, J . A m e r . Chem. SOC.,8 8 , 4330 (1966). (2) W.Trotter and A. C. Testa, ibid., 90, 7044 (1968). (3) R. Hurley and A. C. Testa, i b i d . , 89, 6917 (1967). (4) W.Trotter and A. C. Testa, J . P h y s . Chem., 74, 845 (1970). (5) H. C. Brown and R. R. Holmes, J . Amer. Chem. SOC.,7 8 , 2174
(1956). (6) E. F. htooney, M . A . Qaseem, and P. H. Winson, J . Chem. SOC. B , 224 (1968).
The Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
2416 with BC13 (0.58 M ) undergoes a photochemical change resulting in formation of a white precipitate and a reddish brown coloration of the solution. The white precipitate was identified by its ir spectrum to be H3B03, while disappearance of nitrobenzene is accompanied by the formation of nitrosobenzene. Photolysis after 130 min resulted in 11% disappearance of nitrobenzene, and the quantum yield was determined based on polarographic analto be 3.9 f 0.3 X ysis of the remaining nitrobenzene. The identification of nitrosobenzene was made by comparison of its uv spectrum and gas chromatographic behavior with that of an authentic sample. The photochemical process most likely involves oxygen atom transfer from nitrobenzene to BC13, concomitant with formation of a chlorine atom, which initiates a free radical attack on the solvent, cyclohexane, the source of hydrogen atoms for formation of H3B03, to form chlorocyclohexane. The presence of chlorocyclohexane was confirmed by gas chromatography using a 20% Carbowax 2011 column. Unphotolyzed solutions could be kept clear for a considerable length of time without any trace of hydrolysis of BC13. Thus, it is reasonable to consider that electronic excitation results in weakening of the N-0 bond and strengthening of the 0-B bond of the complex. Nmr studies have shown that the effect of coordination is confined to the nitro group and that one of the two initially equivalent oxygen atoms is the donor.6 There are no significant new peaks or shifts evident in the absorption spectrum upon complexation, although the 1 : l complex is isolable as a yelImr solid. Nitrobenzene in cyclohexane when photolyzed is unreactive, with a hydrogen abstraction quantum yield of consequently, it is clear that the excited complex is the source of modified photochemistry. The disappearance quantum yield for nitrobenzene increases with BC13 concentration, as is expected for an equilibrium process involving formation and dissociation of the complex. An alternate interpretation for the observations is that uncomplexed nitrobenzene excited to its lowest singlet or triplet state attacks BC13; however, we consider this process very unlikely since upon n, T* excitation electron migration within the nitrobenzene molecule results in the nitro group being electron deficient, whereas in the excited state of the complex it is an electron donor. Although
The Journal of Physical Chemistry, Vol. 76, N o . 16, 1971
COMMUNICATIONS TO THE EDITOR we have previously shown that the triplet state of nitrobenzene is responsible for hydrogen abstraction, it is difficult at present to identify which electronically excited state of the donor-acceptor complex is responsible for this new photochemical reaction. On the basis of experimental evidence, the following scheme is presented to describe the photochemical process of the electronically excited donor-acceptor complex
PhNO
3(HO)BC12
-
H,B03
+
+ ‘O-BC12 +
2BC13
C1. (1)
(4)
Reaction 2 is a known free radical reaction,’ while reaction 4 is a known rapid disproportionation reaction in which unsymmetrical boron compounds generally decompose to afford symmetrical product^.^^^ Since BCL is a strong Lewis acid capable of forming donor-acceptor complexes with many aromatic molecules, it becomes interesting to consider the photochemistry of aromatic molecules isolated and when complexed. Further photochemical studies using BC13 as an acceptor are currently in progress.
Acknowledgment. Support of this research by a Frederick Gardner Cottrell Grant from the Research Corporation and a Fellowship for Graduate Education and Fundamental Research from the Petroleum Research Fund, American Chemical Society, is gratefully ackno k+ledged. (7) W. A. Pryor, “Free Radicals,” NlcGraw-Hill, New York, N. Y., 1966, pp 185-188. (8) M. F. Lappert, Chem. Rev., 5 6 , 959 (1956). (9) W. Gerrard and M. F. Lappert, ibid., 58, 1087 (1958).
DEPARTMENT OF CHEMISTRY ST. JOHN’S UNIVERSITY JAYAICA, NEW YORK 11432 RECEIVED DECEMBER 17, 1970
W. TROTTER A. C. TESTA*
