7388
J. Phys. Chem. 1995, 99, 7388-7394
Photoinduced Radical Cleavage of Iodobenzophenonesf Peter J. Wagner* and Carol I. Waite Chemistry Department, Michigan State University, East Lansing, Michigan 48824 Received: August 3, 1994; In Final Form: October 14, 1994@
The triplet state cleavage of m-iodo- (mIBP) and p-iodobenzophenone (pIBP) to benzoylphenyl and iodine radicals has been studied by both steady state and laser flash techniques. Phosphorescence spectra in methanol/ ethanol glasses at 77 K indicate triplet energies of 68.4 and 67.8 kcal/mol for mIBP and pIBP, respectively. In most organic solvents products are benzophenone and iodinated solvent; these are formed by hydrogen abstraction from solvent by the initially formed benzoylphenyl radicals. Quantum yields for product formation decrease with increasing solvent viscosity, ranging from 0.45 in acetonitrile to 0.28 in cyclopentane and 0.05 in cyclooctane; they appear to reflect significant in-cage recoupling of the initial radical pairs. Quenching of the reaction by naphthalene indicates 25 "C triplet lifetimes of 25 and 0.2 ns for mIBP and pIBP, respectively, which were confirmed by flash kinetics. Laser excitation produced transient signals assigned to the iodine atom-toluene complex (in toluene) and, for mIBP, the triplet ketone. Decay of the latter revealed activation parameters AfF = 4.0 f 0.2 k c d m o l and log A = 10.6 f 0.2 for mIBP in both methanol and toluene. The strong phosphorescence of both ketones at 77 K indicates that AfF for pIBP must be 2 3 k c d m o l , such that log A is -12, much higher than for mIBP. The different rates and A values reflect different n* spin densities at the meta and para positions in the lowest n,n* triplets, which react either by mixing with dissociative o,n* states or by thermally converting into a dissociative o,o*or m,o* state. The dependence on x* spin density and the low A values strongly suggest an inefficient state interconversion at a surface crossing between n,x* and o,o*states, because of their different symmetries. It is suggested that the low A values, which might indicate negative AS* values, are better understood as low transmission coefficients.
The photoinduced radical cleavage of carbon-iodine bonds in aryl iodides is w e l l - k n o ~ nbut ~ ~has ~ received surprisingly little mechanistic study. A basic mechanistic question about this process that awaits definitive solution is how triplet excitation can activate a C-X bond perpendicular to the x system. Bersohn first studied the photodissociation of iodoaromatics by molecular beam studies4 Iodobenzene itself (IB) appears to react in two stages: an instantaneous direct dissociation such as observed for many alkyl iodides and a slower -1 ps cleavage from a lower excited state presumed to be either a x ~ , o *or D,D* triplet. El-Sayed recently has found similar behavior for IB but describes the longer-lived state as x,n*, with its decay rate being determined by conversion to a x~,o* state: in agreement with recent calculation^.^ There have been very few careful studies of substituted iodobenzenes. Baum and Pitts reported that p-iodobenzophenone (pIBP) undergoes efficient photoinduced C-I bond cleavage.6 No excited state rate constants were measured; but the fact that pIBP reacted even in 1,3-pentadiene solvent indicated a very rapid cleavage process.6 The fact that the lowest triplet of most benzophenones is n,x* in character raises the obvious question as to how well such triplets couple with dissociative states compared with the x,x* triplets in IB and other compounds. We have begun a thorough study of various halophenyl ketones to explore energetic and electronic effects on radical cleavage rates as well as the behavior of arylhalogen radical pairs. In this paper we examine in detail the behavior of m- and p-iodobenzophenones. The most striking findings are a large metdpara rate difference and large solvent viscosity effects on the efficiency of trapping the acylphenyl radicals. This paper is submitted in recognition of Mostafa El-Sayed's unique contributions to Physical Chemistry and Photochemistry. Abstract published in Advance ACS Abstracts, May 1, 1995. @
We have also studied radical cleavage of several bromophenyl ketones, which show comparable meta and para reactivity, apparently from x,x* triplets. We have communicated the contrast between the VBr cleavages7
Experimental Section Chemicals. All solvents were commercial materials that were purified by standard procedures and distilled before use. Octanethiol (Aldrich) was vacuum distilled before use, and its purity was checked by GC. Naphthalene was recrystallized and then sublimed. Several alkyl benzoates were prepared and distilled for use as internal standards in chromatographic analyses. m-Iodobenzophenone. m-Iodobenzoic acid (3.25 g) was reacted with 5 mL of thionyl chloride in 15 mL of chloroform. The isolated acid chloride was then added slowly to a mixture of 25 mL of benzene and 2 g of anhydrous aluminum chloride in an ice bath. The mixture was heated at 50 "C for 4 h; normal workup produced a 70% yield of mIBP as white crystals, mp 39-40 "C; 'H-NMR (CDC13) 6 7.22 (t. J = 7.8 Hz, Hb') 7.49 (tt, J = 7.5, 1.5 Hz, 2 Hb), 7.61 (tt, J = 7.8, 1.5 Hz, Hc), 7.73 (dt, J = 7.8, 1.5 Hz, H,,) 7.77 (dt, J = 7.2, 1.5 Hz, 2 Ha), 7.91 ( d d d , J = 8.1, 1.8, l.OHz, Ha,) 8.13 (t, J = 1.5 Hz, &); 13CNMR (CDCl3) 6 93.99, 128.45, 129.08, 129.93, 130.01, 132.79, 136.96, 138.61, 139.58, 141.13, 195.0; IR (CC4 or CS2) 1665 (C=O), 1600, 1270, 1260, 945, 930, 715, 640 cm-1;8 MS mlz 308 (44),231 (19), 203, 181, 105(100), 77. Anal. Calcd for C&@I: C, 50.68; H, 2.94%. Found: C, 50.90%; H, 2.88.
0022-365419512099-7388$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 19, 1995 7389
Photoinduced Radical Cleavage of Iodobenzophenones
b
b'
p-Iodobenzophenone. p-Aminobenzophenone(2.75 g) in 20 mL of glacial acetic acid was cooled to 5 "C, and 1.26 g of sodium nitrite in 8.5 mL of sulfuric acid was added dropwise.
After 3 h the mixture was warmed to ambient temperature and 3.35 g of potassium iodide and 0.2 g of copper dust in 14 mL of water were added. The mixture was heated at 60 "C until all nitrogen evolution had ceased (1 h). The resulting suspension was filtered and the residue recrystallized three times from methanol. Final purification by chromatography on silica gel with ethyl acetatehexane eluent provided a 55% yield of pIBP as ivory crystals, mp 95-8 "C; 'H-NMR (CDC13) 6 7.88 (d, J = 8.7 Hz, 2 Hv), 7.80 (dt, J = 7.2, 1.5 Hz, 2 Ha), 7.63 (tt, J = 7.4, 1.5 Hz, HJ, 7.56 (d, J = 8.7 Hz, 2 Ha,), 7.52 (t, J = 7.2 Hz, 2 Hb); 13C-NMR(CDCl3) 6 100.04, 128.40, 129.93, 131.44, 132.65, 136.95, 137.19, 137.62, 195.83;IR (CCb and CS2) 1665, 1585,1280,1270,1010,650 cm-';* MS d e 308 (71), 231 (471, 203, 181, 105 (loo), 77. Anal. CalcdforC13H901: C, 50.68%; H, 2.94. Found: C, 50.96%; H, 2.74%.
a
9
300
350
400
h, nm
Figure 1. UV absorption spectrum of 0.005 M pIBP.
a'
h, nm
Figure 2. 77 K phosphorescence of
M mIBP (- - -) and pIBP
(-) in MeOWEtOH.
b
b'
I
Procedures. All samples were prepared by weighing reactants, quenchers, and standards in volumetric flasks. Aliquots (2.8 mL) of each sample were syringed into 13 x 100 mm Pyrex or Kimax test tubes which were then degassed by several freeze-pump-thaw cycles. After the tubes were sealed, the samples were irradiated in parallel on a merry-go round apparatus? in the center of which stood a water-cooled mediumpressure mercury arc. Its radiation was filtered either with a 1 cm path of 0.002 M potassium chromate in 1% aqueous potassium carbonate to isolate the 313 nm mercury line or with Corning #7-83 glass filters to isolate the 365 nm line. Products were identified by GC-MS analysis of photolysate solutions. The spectra of individual fractions were compared to those in the MS library, iodocyclopentane being the only exception. Its MS showed peaks at mlz 196, 127,69, and 41. GC and HPLC retention times were verified with independently obtained samples of products. GC analysis was performed on SE30 columns; HPLC analysis was performed on an Ultrasphere 5 pm 4.6 x 250 mm Si column held at 35 "C and eluted with a 98.5:1.5 hexane/ethyl acetate mixture, with a UV detector monitoring 270 nm. Phosphorescence spectra were obtained on a Perkin-Elmer MPF44A spectrophotometer on samples M in ketone, with 313 nm excitation. The emission intensities of both ketones were half-to-full scale at the lowest amplifier gain. Flash kinetics measurements were made on J. C. Scaiano's apparatus.'O Samples -0.005 M in ketone (OD = 0.5) in spectroscopic grade methanol or reagent grade toluene were placed in 7 x 7 mm Supracil cells fitted with serum caps. They were then deaerated with nitrogen before being excited with either a Molectron U-24 nitrogen laser (337 nm, 8 ns pulse, -8 mJ) or a Lumonics excimer laser (308 nm, 4 ns, -80 mJ).
TABLE 1: Electronic Spectra of Iodobenzophenones phos Ao.o? nm ketone A,," nm emax: M-* cm-' abs Ao.0, nm (kcdmol) BP mIBP pIBP a
346 347 348
60 106 173
-375 -375
413c (69.2) 419 (68.3) 422 (67.8)
In cyclopentane. In 1:1 methanollethanol at 77 K. Reference 11
Triplet decay was monitored at 600 nm, where there was little residual absorption.
Results Spectroscopy. Figure 1 displays the room-temperature nearUV spectrum of pIBP, which is very similar to those of mIBP and benzophenone (BP) itself. Figure 2 displays the phosphorescence spectra of mIBP and pIBP at 77 K, they are nearly identical in intensity to that of BP but differ somewhat in structure, although the 1670 cm-l vibrational mode identifies their n,z* origin. Table 1 lists the pertinent 0-0 and ,Im bands. Iodo substitution produces small decreases in the triplet excitation energy of benzophenone, with the lowest singlet and triplet transitions clearly being n,n* in character. The para isomer has a triplet energy 1 kcaUmol lower than that of p-chlorobenzophenone.ll The absorption intensity is enhanced, especially by para substitution. This effect is similar to that produced by a-halo substitution and suggests weak mixing of the n,z* absorption with transitions involving the C-I (7 bond, as well as some direct nI,a* absorption, as is characteristic of IB.3312 Steady State Studies. Irradiation (365 nm) of either iodo ketone in deaerated cyclopentane produced benzophenone in 95% yield. Iodocyclopentane was the major additional product, with no pinacols or HI and only traces of 1 2 being detected. Benzophenone remained the sole ketone-derived product in
1390 J. Phys. Chem., Vol. 99, No. 19, 1995
Wagner and Waite
TABLE 2: Quantum Yields for Product Formation from IodobenzophenoneP solvent CH3CN CH3CN + RSH cyclopentane MeOWEtOH CCL cyclooctane
viscosity, CP
mIBP
pIBP
0.34 0.34
0.07 0.12 0.28
0.24 0.45 0.27 0.30 0.07
0.44
0.37
0.60 0.96 2.16
0.05 0.05
':
@Om
0.08
"Product is benzophenone except in CCL, where it is m- or p-chlorobenzophenone.
