5518
J. Phys. Chem. 1991,95, 5518-5523
Photoreactlon of (p-Nltrophenyl)glyoxyiic Acid to p-Nltrosobenzoic AcM In Aqueous Solutlon Helmut Giirner,* Leslie J. CurreU, and Hans Jochen Kuhn Max-Planck-instirut fiir Strahlenchemie. 0-4330 Mtilheim an der Ruhr, Germany (Received: December 28, 1990; In Final Form: February 26, 1991)
(pNitropheny1)glyoxylicacid (NO2-PA) was studied by time-resolved and steady-state photochemical methods in water and mixtures with acetonitrile at room temperature. Irradiation of NO2-PA leads to pnitrosobenzoic acid (NO-BA) and C02 in substantial yield (@ = 0.28 in aqueous solution at pH 2-12, Xi, = 313, 280, or 254 nm). An intermediate (A= 350 nm, T I 2 M), observed by nanosecond laser flash photolysis (h, = 308 or 248 nm), is assigned to a quinoid aci-nitro species. It is suggested to be formed by decarboxylation of the excited NOz-PA anion in aqueous solution and to rearrange to NO-BA in a reaction with water. In the acidic pH range it decays by protonation (rate constant 4 X 1O'O M-'s-l). The proposed mechanism is based on transient conductivity measurements (range 20 ns-10 M) in aqueous solution as a function of pH and in mixtures with acetonitrile. The photoreaction of the anion of NO2-PA (pK, = 1.1) to the NO-BA anion (pK, = 3.3) is suggested to involve protonation of the observed aci-nitro species, subsequent rearrangement, and deprotonation of the acid form of NO-BA. Thus, the overall photoreaction at pH 4-9 is accompanied by neutralization but not by a photoinduced change of ion mobility on an extended time scale. At pH 9-12, however, a slow conductivity decrease (in the 0.01-10 s range) was observed, which is indicative for the reaction of C02 with OH-.
Introduction The photochemistry of phenylglyoxylic acid (PA, benzoylformic acid) in solution has been studied by Leermakers and Vesleyl and several other In the presence of water PA exhibits decarboxylation with formation of benzaldehyde in a straightforward photoreaction.H The quantum yield is large (i) when PA in aqueous solution is present as acid (at pH < 1) or (ii) in mixtures (e.g., 1:3) with acetonitrile. Under the latter conditions PA is mainly present as anion and photoreaction 1 is accessible C6H5 300 nm. No emission could be detected for NO2-PA in argon-saturated acetonitrile at room temperature (and in aqueous solution at pH 1 or 12). This is in contrast with the case of PA, where phosphorescence was observed in fluid nonaqueous solution! However, in several rigid media at -196 OC a structured emission spectrum was recorded; the two maxima are at 494 and 534,484 and 526, and 482 and 522 nm in ethanol, butyronitrile, and mixtures of (23) Mark, G.; Schuchmann, M.N.;Schuchmann, H. P.;von Sonntag, C.
Aquu (London) 1990,39,309.
(24) G h e r , H. J. Photochcm. Photobiol. 1990, BS, 359. (25) Oehme, 0.;Filchcr, 0.;Schellenbcrger, A. Chcm. Bcr. 1967, 100, 425.
350 0
300
2
1
I a b s I M O I PhOtOnS810~'~ ~
A(nm)
Figure 2. (a) Absorption spectra of N0,:PA (7 X M) in aqueous solution at pH 3.8 prior to (-) and after irradiation (broken lines: 10, 20, 30, and 100 s; 4, = 254 nm). (b) Conversion of NOz-PA (0)to NO-BA (A) (using HPLC analysis) as a function of the number of absorbed photons; Xi, = 280 nm.
-.
TABLE I: Quantum Yields of the N02-Pa NO-BA Pbotoructioa in Aqueous Solutiod pH @wb @NO-" @co1
0.0 0.5 1 .o 2.0 3.7
0.03 0.06 0.12 0.23 0.29
0.10 0.1 1 0.11 0.20 0.25
0.07 0.08 0.09 0.17 0.22
d
0.25 0.26* 0.29 0.28 0.25
0.21
0.20
0.29 0.25 0.18
0.23 0.24 0.15
3.8 5.0 10 12
#auv,@NorPA, @NO-BA,and @-,
0.08 0.17 0.17 0.18 0.30 O.3lc 0.27
reproducibility fO.O1, were o b
tained by using UV spectra, HPLC detection of NO,-PA and NOBA, and GC detection of COz, respectively. [NO,-PA] = 0.25 mM, argon-saturated, xi, = 280 nm unless indicated otherwise. b&, = 254 nm. cA,m = 313 nm; [NOz-PA] = 10 mM. DzO at natural pD.
*In air-saturated solution. ethylene glycol and water (2:1), respectively, with h0between 250 and 330 nm. This emission is present in low yield ( 10 ps in deoxygenated solution) is comparable to that of other ketyl radicals under our conditions.* Pulsed-her-InducedConductivity Cbaages. These changes were detected in the 20 ns-10 ccs time domain in water, acetonitrile, and their mixtures. The amplitude of the transient conductivity for N02-PA (0.1-0.2 mM) in neat acetonitrile shows a small increase (+AK) at the completion of the laser pulse and a decay to its initial value within 2 ps. On addition of 5% water AK increases within the pulse width and then becomes negative, whereas for 25% water the initial increase of AKdisappears and the remaining negative amplitude is even larger (insets of Figure 5). In addition, the reciprocal half-life increases with increasing [H20] (Figure 5 ) . In neat water the amplitude and kinetics of the conductivity signal are strongly dependent on H. At pH 3, the lowest value possible for these measurementsJn the signal becomes negative on completion of the pulse (at low intensities), the decay follows fmt-order kinetics, and the rate constant (k(h)dr d indicates decay and i inmase) is within the time resolution of = 3 X lo76'. When increasing pH to about 4.7, the signal initially becomes slightly positive (this only at high intensities) and then strongly negative; k(Ar)d equals 7 X 106 s-I at pH 3.8 and 1.0 X 106 s-' at pH 4.7 (insets of Figure 4). The effect of proton concentration on the
9
10
PH
11
12
Figure 8. Semilogarithmic plot of the rate constant of the slow conductivity decrease for NOz-PA (0.1 mM) in aqueous solution as a function of the pH. Insets: conductivity signals at pH 10.6 and 11.6.
rate constant is (within experimental error) the same as on k*(T3%) (Figure 4). It follows that the rate constant, obtained from log k(hK)d VS log ([H+])-', is ah0 4 x 10" M-' S-'. At pH 5-9 when relatively high pulse intensities (13are applied, there is an instantaneous increase in AK and a second increase within 0.5-0.8 ps (insets of Figure 6), before AKdecreases within a further few microseconds to the value prior to the pulse (AK = 0). However, variation of 1, showed that only the slow component is monophotonic, whereas the instantaneous one has a biphotonic character, Le., the amplitudes of A ~ ( 2 0ns) and A K ( ps) ~ are essentially proportional to IL2 and I,, respectively. This is illustrated in Figure 6 for aqueous solution at pH 7. The origin of the conductivity just after the high-intensity pulse is not known. Because of these mixed processes, the relative amplitudes were taken from their initial dependences on 1,. At pH 4.7-5 the conductivity signal initially increases (again from biphotonic excitation),then decreases within 100 ns, inmases again to a second maximum at 300 ns, and subsequently decays with k(&)d = 1 x 106 s-5 however, at reduced 1, only the increase (from AK = 0 to the maximum value) and the decrease of AK remain (insets of Figure 7). At pH 9.2-1 1 the increase in AK, associated with the monophotonic step, is somewhat faster than at neutral pH. The complete pH dependence of the signal amplitude at two characteristic times, using a sufficiently low intensity, to avoid the biphotonic side reaction, is shown in Figure 7. While AKis zero at completion of the laser pulse throughout, it has reached its maximum value within 1 ps in neutral solution. With increasing pH AK( 1 ps) decreases slightly; with decreasing pH it decreases strongly, passes through the zero value at pH = 4.6, and a p proaches the above-mentioned negative values.
5522 The Journal of Physical Chemistry, Vol. 95. No. 14. 1 9'91
On an extended time scale (up to 0.1 s) in the pH range 5-8 the transient conductivity is zero. At pH 3, however, the signal becomes negative during the pulse and -AK remains constant from e100 ns to 0.1 s, indicating removal of a proton from the bulk of the solution and no further reaction. On the other hand, at pH 8-12, after the fast increase and the subsequent decrease within a few microseconds, AKbecomes negative on the extended time scale. Two examples are shown at pH 10.6 and 11.6 (insets of Figure 8). The decay is pseudo-first-order, and the rate constant ~ ( A Kof) this slow conductivity decrease increases from 10.3 s-l at pH 9.0 to 90 s-I at pH 12.0 (Figure 8). This conductivity behavior strongly suggests a reaction of C 0 2 with OH- yielding bicarbonate (q3), as has already been determined with parent C 0 2 + OH-
-
HC03-
(3) PA? The bimolecular rate constant, obtained from the linear plot of log ~ ( A Kvs) log [OH-], is k3 = 9 X lo3 M-' s-l; a literature value is 6.3 X IO3 M-' s-Iez6
Discussion Ground-State Equilibria in Aqueous Solution. In aqueous solution NO2-PA exists in the acid form at pH values smaller than pK, = 1.1 (Figure 1) and as the anion in the pH range 1.2-12 (eq 4). In this respect NO2-PA behaves in a similar manner to O Z N - C ~ H ~ - C O C O+ ~ H02N-CsH4-COC02- + H+ (4) other para-substituted PA's (e.g., OCH3, CH3, F, CI, Br, CN).879 For the parent PA we have previously shown that the equilibrium lies on the acid side in neat acetonitrile and is shifted to the anion when water is addede8 This is also suggested for NOz-PA. In order to correctly interpret the conductivity results, the pK, value of the acid-base equilibrium (5) of the product NO-BA must be known. From the (small) changes in the absorption spectra of NO-BA in aqueous solution we obtained pK, = 3.3, in agreement with the literature value of 3.27.27 03.N--C6Hd--C02H
*N*&4*02-
+ H+
(5) Photoreactions of the Anion of NO2-PA in Aqueous Solution. Clear evidence for decarboxylation of NO2-PA is given by the results described above on the basis of UV, HPLC, and GC analyses. The Q, values from the four methods (UV spectra and determination of the concentrations of NO2-PA, NO-BA, and COz), compiled in Tables I and 11, are substantial in the presence of water, in particular neat water over a wide pH range. The photochemical formation of NO-BA and C 0 2 is favored when NOz-PA is present as anion. This follows from the pH dependence of Q, in neat aqueous solution; here Q, decreases with increasing H+ concentration as expected from the acid-base equilibrium and assuming a photoreaction purely from the excited anion (Table I). Furthermore, Q, increases when water is added to acetonitrile (Table 11). Whether the photoreaction of NO2-PA originates from an excited singlet or triplet state of the anion cannot be distinguished by our means. The Occurrence of weak phosphorescence at -196 OC does not exclude efficient intersystem crossing but is likewise not in favor of the intermediacy of the triplet in reaction 2. It follows from laser flash photolysis (no T-T absorption) that, if the triplet anion were involved, its lifetime should be shorter than a few nanoseconds, assuming a molar extinction coefficient comparable with that of PA.8 Upon laser excitation of NO2-PA in aqueous solution, the main intermediate formed within the pulse duration (120 ns) is T393 (Figure 3a). Assignment of T3%to the excited singlet state is ruled out by the fact that its lifetime (Table 11) is too long. The triplet state is also an unlikely candidate since T350does not react with oxygen. However, the absorption spectrum of T3%is compatible with the quinoid ketene structure (abbreviated as -02N=
