4.2.1 o-Nitrobenzaldehyde and the Equivalence Law
The formulation by Einstein of the “equivalence law” induced extensive work attempting to reach an experimental verification, as reported in Chap. 2, and the distinction between the primary process—for which the law holds, and secondary processes that led to final products in an overallquantum yield that might well differ from 1, as in practice was the case for most photochemical reactions. In this sense, it was perhaps more interesting that the measured value could be a simple number, e.g., 2 or½, rather than 1, because a round number led to the expectation that a simple rationalization could be arrived at, thus supporting the trust in the theoretical frame that was being built. The most interesting case in this respect is probably the rearrangement of o-nitrobenzaldehyde to o-nitrosobenzoic acid, a clean reaction that had been reported by Ciamician and Silber in 1901 [58,59]
and occurred in the same way in solution and in solid. The original workers had noticed its efficiency (in part, as we can appreciate now, due to the good absorption of solar light by this yellow compound).
It is thus not surprising that Weigert chose this process as a suitable model for the verification of the equivalence law. This was a mechanistically “very simple photochemical reaction,” in the sense that appeared to be an intramolecular rearrangement with a shift of an oxygen atom, involving no interaction with the medium that might introduce a departure from the simplest path. Thus, it closely corresponded to the idea of primary photoprocess as expressed above. The actual measurement of the quantum yield had required detailed studies, in order to establish how precise and dependable were the results and the suitability of the analytical method chosen (in particular, potentiometry for the aldehyde) [60–65], as well as the dependence on conditions. It appeared that wavelength and intensity of light caused little effect, as did the concentration of the solution (though some disagreement with the results by Kaplan occurred) [60,62–64]. Experiments under different conditions consistently gave a quantum yield close to 0.5 (exemplificative data obtained in various laboratories over several decades, which did not vary much from the accurate study by Bowen and by Leighton and Lucy [66], are shown in Table 4.1). Now the question was, which confidence had to be given to such a fractional value. As always in science, as in any manifestation of the human mind, similar data could be interpreted in opposite ways.
4.2 Born to Measure 79
Thus, Bowen et al. [67] studied the photorearrangement ofo-nitrobenzaldehyde in the crystal state and measured the quantum yield (the amount of acid formed was determined by extraction with water and measurement by conductivity, the light intensity by means of an air thermometer). The results obtained varied between 0.38 and 0.68, in fact again not far from 0.5, but one had to take into account the loss of energy by scattering owing to the irregular disposition of the crystals, difficult to assess quantitatively, but estimated to be <50 % by comparison with a white substance. From these data, the Authors concluded that “when allowance has been made for energy not absorbed and for experimental errors, it appears that for each quantum of absorbed radiation very nearly one molecule of product is formed.” On the other hand, Wegscheider [68] found again values close to½, but opined that it was not appropriate to round off the experimental value, since the actual measurements varied between 0.503 and 0.610, exceeding the experimental error (he also found that the quantum yield in the solid phase was close to unitary).
Furthermore, Ku¨chler and colleagues [73] carried out the photolysis in the gas phase and found that even in that case the quantum yield remained below 1. Thus, a mechanism should exist that made deactivation to some extent unavoidable. As can be easily understood, this was disturbing when considering in a simple-minded way the equivalence law.
Table 4.1 Measured quantum yields for the rearrangement ofo-nitrobenzaldehydea
Light, conditions
Measured quantum yield
Year, reference
“Violet light,” r T 0.5 1924, [67]
366 nm, 22–24C, ligroin 0.52 1934, [68]
366 nm, 22–24C, acetone 0.51 1934, [68]
350 nm, acetonitrile, r T 0.500.06 1980, [69]
313 nm, water, 20C 0.400.03 1994, [70]
366 nm, water, 201C 0.420.02 1994, [70]
390–440 nm, crystals r T 0.440.06a 1934, [68]
366 nm, crystals, 22–24C 0.50 1924, [67]
334 or 366 nm, poly(methyl methacrylate) film, r T 0.500.06 1968, [71]
31320 nm, water-ice,ca.25C 0.410.04 2010, [72]
aTo be corrected by taking into account the loss of light by scattering
80 4 Some Paradigmatic Topics
4.2.2 Mechanism: Early Studies
The next question was whether a quantum yield of more or less exactly 0.5 had to do with the physics or with the chemistry of the molecule. Physical mechanisms that were invoked were either a partial absorption or a partial deactivation. Weigert considered the first one a viable hypothesis, in the sense that absorption may depend on the reciprocal orientation of the light beam and molecules. Indeed, Padoa had shown that this was the case in crystals and using polarized light (see Table4.1and references therein, [74]). Alternatively, Φ¼0.5 may be the result of mere colli- sional deactivation that happened to occur at the same rate as reaction, a mechanism preferred by Ku¨chler and Patat [73]. More precisely, these Authors thought that excitation by light essentially involved the nitro group (see further below) and this reaction required previous intramolecular energy transfer from the nitro group, freely rotating between the two planar conformations, to the actually reacting formyl group. They calculated that transfer had to occur within ns, in order to compete with intermolecular deactivation by collision with inactive molecules (N2) and this may well be the source of the lower than unitary efficiency.
Other scientists, however, found that a straightforward rationalization had to be found for a primary photoreaction with quantum yield one half, not an accidental compensation (perhaps a version of Einstein’s “God does not play dice with the universe”). The number of possibilities that were considered is noteworthy. One was that the photoreactive species was noto-nitrobenzaldehyde, but an intermedi- ate formed from it that absorbed light and reacted. To this, Tananescu pointed out that no “active” hydrogen was present in the structure ofo-nitrobenzaldehyde [75]
and suggested that some intermediate was formed in equilibrium and absorbed light, as an example formula18.
Weigert [76] objected that, were this the case, the quantum yield should be quite dependent on the irradiation wavelength, because the two forms would absorb differently, whereas this was not the case foro-nitrobenzaldehyde. Such a depen- dence was observed for the less reactive 4-nitrophthalaldehyde (quantum yield around 0.1) and indeed the hypothesis of the intermediate was considered in that case [77].
A mechanism based directly on the molecule structure was proposed by Zimmer (Scheme4.10), who suggested that the reaction involved detachment of an oxygen atom from the nitro group followed by recombination with equal likelihood at the C and N atom [78]. The hypothesis of the liberation of a gas was ill tenable, however, in view of the uniformity of the reaction quantum yield under so many different conditions.
4.2 Born to Measure 81
In the meantime, theoretical chemistry was developing and this mechanism was considered. Actually, the rearrangement ofo-nitrobenzaldehyde was one of the first photochemical reactions for which a theoretical mechanistic investigation was carried out [79]. It was felt that radiation at the long-wavelength end of the spectrum was absorbed by the nitro group (from the comparison of the UV spectra of various substituted derivatives—the association of a UV band with a specific vibration seemed to be appropriate at the time). The next question was whether the groups were freely rotating. Steric hindrance was expected to have a role, but could not be calculated with the methods available at the time. However, the sum of the dipole, induction, and dispersion effect was calculated and a (ground) energy surface was described (see Fig. 4.4). It resulted that the most stable form was coplanar and rotation was greatly hindered for the CHO group (the ordinate in the figure), but confronted a smaller barrier for rotation of the nitro group (along the abscissa, 1.36 kcal/M). It may be expected that electronic excitation would lead to a state with some vibrational excitation. In particular, a vibration may “swing the oxygen nucleus near enough to the aldehyde hydrogen, so that an hydroxyl group with normal nuclear separation could form by a mere redistribution of the electron cloud. . .at the same time a chemically saturated nitroso group could form from the remainder of the nitro group, leaving the hydroxyl to react with the carbonyl.” Now, starting from the planar conformation it may be that excitation of the N–O bond pointing toward the formyl group led to reaction, while excitation of the other one led to decay with no reaction, explaining the experimentally observed½quantum Scheme 4.10 Some of the mechanisms put forward to explain the quantum yield 0.5 of the conversion of nitrobenzaldehyde intoo-nitrosobenzoic acid. Thus, it may be thought that of the two rotamers (19,190), only one absorbed light; or only one had a significant likelihood to react. Or that previous rearrangement to the actually photoreactive species was required (18). Or that an atom of oxygen was detached and recombined with the same efficiency at the nitrogen and at the carbon atom (see20). Or that the nitro group abstracted a hydrogen atom and that a hydroxyl group was split off and recombined
82 4 Some Paradigmatic Topics
yield [79]. A model was built, which included transfer and deactivation (electronic to vibrational energy, then to the medium) and predicted a reaction lifetime of ca. 1013s.
The quantum yield of this reaction thus was a hot topic for a couple of decades and was the ground of several disagreements. Thus, polemics were published between Weigert and respectively Kailan, about the quantitative determination of the photoproducts [60,62–64]; Jannsen about the role of an intermediate [61,65, 77]; and Wegschneider about whether the quantum yield was exactly 0.5 and this had a mechanistic significance that required a rationalization [76]. The mechanism was seen as quite important because, as pointed out by Kogel (see Sect.2.5) [80], this may distinguish at which point the actual chemical change occurred, at the same time as excitation or afterward from an excited state, as yet chemically unchanged.
4.2.3 Mechanism: Modern Studies
When advanced instruments were available after 1970, the mechanism issue was confronted again and some elements of the previously reported hypotheses were incorporated in the new proposals. Thus, the reaction is in fact intramolecular and goes through distinct intermediates. Transients were detected by flash photolysis and it was noticed that, while the quantum yield of photoreaction of the aldehyde was rather insensitive to the nature of the solvent, the transient signals in the same systems were extremely sensitive to the presence of water in the medium. Thus, the initial (photochemical) step was irreversible, but generated further “active” species, the evolution of which depended on conditions. Scaiano [69] proposed that the intermediate was the aci-nitroketene (21), an identification that was further supported by experiments in matrix with IR detection (see Scheme4.8) [81]. In the following years, spectral and time resolutions grew spectacularly, arriving at femtosecond resolution both with UV–Vis detection (see Fig.4.4) [82] and with the highly structure informative Raman detection [83,84].
On the other hand, the½quantum yield (or rather 4/10, see Table4.1and below) seems have no special significance, or maybe it does in the sense that quite often excited molecules funnel down to the potential energy surface of the ground state at a geometry corresponding, or close, to a maximum, and thus the likelihood that the system comes back to the starting configuration (reagent) or to a new one (products) is similar. The state-of-the-art science (see Schemes4.11and4.12) [85] considers that the singlet excited aldehyde decays within less than 1 ps via three channels, viz.
internal conversion, intramolecular hydrogen transfer to give the ketene, and (minor) intersystem crossing to the triplet state (that in turn forms a further amount of ketene). The end product, nitrosobenzoic acid, is formed from the ketene via a further cyclized intermediate or directly in the presence of water (and thus its lifetime strongly depends on the solvent, from 13 ps in a 1:1 ethanol/water mixture to 24 ns in acetonitrile) [82,83,85–87].
4.2 Born to Measure 83
Scheme 4.11 Photochemistry ofo-nitrobenzaldehyde
Fig. 4.4 Altitude, energy ofo-nitrobenzaldehyde; abscissa, angular rotation of nitro; ordinate, angular rotation of aldehyde. By permission from [79]
84 4 Some Paradigmatic Topics
The photochemical step is ultrafast (~0.5 ps), which explains the independence of conditions and in particular the fact that Kasha’s rule is obeyed (the quantum yield is independent of the irradiation wavelength), since IC from higher singlet states is likewise ultrafast, actually faster (<0.1 ps) than reaction. The ketene intermediate is formed irreversibly, otherwise the quantum yield would depend on the solvent and thus the branching occurs during the decay of the electronically excited state. The decay involves a displacement of the aldehyde hydrogen atom toward the nitro group that brings the ground and excited state surfaces in energetic vicinity opening a path toward internal conversion, leading in part to the starting material, in part to the ketene intermediate. The last species gives the nitrosoacid quantitatively. As a result, only a limited change in the quantum yield is observed when exciting at shorter wavelength (Fig.4.5a, dark blue line).
In the meantime, the computational contribution became a versatile and rela- tively easy to use instrument. An example is shown in Fig.4.5b, where the potential energy surfaces for the intramolecular hydrogen transfer reaction of o-nitroben- zaldehyde in ground and singlet excited state are reported [86]. Calculations by the more reliable MS-CASPT2//CASSCF approach have been likewise carried out [87]
as well as molecular dynamics simulations [85] (Fig.4.6).
4.2.4 o-Nitrobenzaldehyde as an Actinometer
Apart from the role in mechanistic photochemistry, the scarce dependence on conditions continues to make this compound a convenient actinometer. A recent study has confirmed this fact, supporting its choice as a practical, photochemically sensitive, and thermally robust standard [72]. The study showed that the molar absorptivities of the aldehyde were only weakly dependent upon temperature and that the quantum yield was virtually independent of temperature as well as of the wavelength. This compound was used as an actinometer in field experiments on photochemistry in ice, with excellent results. The value of 0.41 was recommended Scheme 4.12 Photochemistry ofo-nitrobenzaldehyde. Kinetics
4.2 Born to Measure 85
for both solution and water ice [70,72] and, although this may take away some of the magic of the 0.5 value, this was another of the many applications this clean reaction has found as an excellent actinometer, easy to use and well suited for different conditions, ranging from polymeric films [71], or KBr matrix [71], to TiO2 suspension [88].
Fig. 4.5 (a) Transient absorption data of o-nitrobenzaldehyde (48 mM) dissolved in THF.
Transient spectra at indicated delay times are plotted. The femtosecond excitation pulse had a wavelength of 388 nm and was in resonance with the lowest excited state. (b) Formation of the ketene intermediate. From [81] (b) Computed potential energy surface for the rearrangement ofo- nitrobenzaldehyde. The surface for the S0state was obtained from B3LYP/6-311+G(d,p) opti- mized structures and for the S1state were constructed by the TD (time dependent)-B3LYP/6-311 +G(d,p) method based on the S0optimized geometries. Energy is given in kcal mol1and bond lengths in angstroms. The solvation effects are included using the polarizable continuum model from [86]
86 4 Some Paradigmatic Topics