In-cage Recombination Research Articles

Structural dynamics probed by X-ray pulses from synchrotrons and XFELs

This review focuses on how short X-ray pulses from synchrotrons and XFELs can be used to track light-induced structural changes in molecular complexes and proteins via the pump–probe method. The upgrade of the European Synchrotron Radiation Facility to a diffraction-limited storage ring, based on the seven-bend achromat lattice, and how it might boost future pump–probe experiments are described. We discuss some of the first X-ray experiments to achieve 100 ps time resolution, including the dissociation and in-cage recombination of diatomic molecules, as probed by wide-angle X-ray scattering, and the 3D filming of ligand transport in myoglobin, as probed by Laue diffraction. Finally, the use of femtosecond XFEL pulses to investigate primary chemical reactions, bond breakage and bond formation, isomerisation and electron transfer are discussed.

1,2-Addition of Diethylzinc to a Bis(Imidazolyl)ketone Ligand.

In this study, the selective 1,2‐addition of diethylzinc to the ketone functionality of BMdiPhIK [bis(1‐methyl‐4,5‐diphenylimidazolyl)ketone] is shown. The reaction product is isolated in a dimeric form with a planar Zn2(µ‐O)2‐motif keeping the two monomers together. This compound can serve as a model for reactive intermediates in the catalytic alkylation of ketones with diorganozinc reagents. Hydrolysis of this binuclear zinc compound leads to isolation of the C‐alkylated product in 89 % yield. A reaction pathway is proposed in which BMdiPhIK initially coordinates to diethylzinc as a bidentate bis(nitrogen) ligand. This is followed by the homolytic cleavage of the Zn–Et bond and in‐cage recombination of the Et‐radical and the Zn‐coordinated ligand‐centered radical, which is mainly localized on the carbonyl moiety of the ligand.

Fluorescence Quenching by Reversible Charge Separation Followed by Ions Recombination and Their Separation Suppressed by Coulomb Attraction

The Stern-Volmer constant is specified for the luminescence quenched by reversible ionization of excited molecules. The exergonic branch of the Rehm-Weller free energy dependence of this constant is known to be a plateau determined by irreversible ionization being under diffusion control. In the endergonic region the ionization is reversible and competes with the irreversible in-cage recombination of ions and their escape from the cage. At strong Coulomb attraction the latter phenomenon is shown to be negligible compared to the former that determines the shape and location of the descending branch of the Rehm-Weller curve. At weaker Coulomb attraction (at higher solvent polarity), this curve turns down at larger endergonicity. The experimental data obtained in solvents of different polarities are put in order and in full accordance with present theory.

On the effect of 2-propanol on phenol photonitration upon nitrate photolysis

It is shown that the enhancement by 2-propanol of the nitration of phenol upon nitrate photolysis is compatible with the inhibition by the alcohol of in-cage recombination between O − and NO 2. This effect would increase the availability of NO 2 that is involved into phenol nitration, despite the enhancement by the alcohol of the production of superoxide that is a scavenger of NO 2. A kinetic model is proposed to describe the experimental data and to get insight into the processes involved. Kinetic calculations suggest that in the absence of 2-propanol less than 25% of cage O − and NO 2 would evolve into bulk species, the remainder undergoing recombination to nitrate. If reaction between cage O − and phenol were not negligible, the percentage of bulk species formation would be even lower. The data also show that most of the recombination between OH/ O − and NO 2 would take place in the solvent cage instead of the solution bulk.

ChemInform Abstract: Comparison of the Photochemistry of Diarylchloronium, Diarylbromonium, and Diaryliodonium Salts.

The direct and triplet-sensitized photochemistry of diphenylchloronium, diphenylbromonium, bis(4-methylphenyl) chloronium, and bis(4-methylphenyl) bromonium salts was studied. Direct photolysis of the diarylhalonium salts gave haloarene 2-, 3-, and 4-halobiaryls, and acetanilide, predominantly from a heterolytic cleavage mechanism via the aryl cation-haloarene pair, via in-cage recombination and cage-escape reactions. However, triplet-sensitized photolysis gave haloarene, 2-, 3-, and 4-halobiaryl, and reduced arene mainly from homolytic cleavage to the arene radical-haloarene radical cation pair, which also gave in-cage recombination and cage-escape products. There is evidence for singlet-triplet interconversion of the excited states by spin-orbit coupling and interconversion of the singlet-triplet radical pairs by spin-spin coupling. The effect of halogen and arene substituent on these interconversions is discussed

Suppression of inhibition of substrate photodegradation by scavengers of hydroxyl radicals: the solvent-cage effect of bromide on nitrate photolysis

In this paper we show that bromide scavenges the ·OH radicals formed upon photolysis of nitrate, before they leave the solvent cage. Bromide can thus inhibit the in-cage recombination between ·OH and ·NO2. The consequence is an increased generation of ·NO2 and nitrite and of Br2 −· + ·OH, compared to ·OH alone in the absence of bromide. We show that this effect compensates for the lower reactivity of Br 2 −· compared to ·OH toward certain organic substrates, e.g. phenol and tryptophan. Our findings could lead to a deep revision of the present views of the role of bromide in saltwater photochemistry.

Solution and surface photochemistry of fenarimol: A comparative study

The photochemistry of the pesticide fenarimol was studied in methanol and on a cellulose surface. The ground state absorption and fluorescence spectra are similar in solution and at the solid/gas interface. Room temperature phosphorescence emission was observed in cellulose. Transient absorption of fenarimol and of its chromophoric units, chlorobenzene and pyrimidine, showed a common absorption band centered at 320 nm in methanol. An absorption band above 350 nm was observed only for fenarimol and was assigned to a recombination intermediate. On cellulose the transient absorption is broader and extends up to 600 nm. The main photodegradation pathway in methanol involves the homolytic cleavage of the bond to the pyrimidyl ring, followed by a fast in-cage recombination of the initially formed pyrimidine and ketyl radicals. On cellulose surfaces, dechlorination is also one of the important photodegradation pathways.

To the Core of Autocatalysis in Cyclohexane Autoxidation

Despite their industrial importance, the detailed reaction mechanism of autoxidation reactions is still insufficiently known. In this work, complementary experimental and theoretical techniques are employed to address the radical-chain initiation in the autoxidation of cyclohexane with a particular focus on the "lighting-off" of the oxidation by (added) cyclohexanone. We used a newly developed method to quantify the intrinsic rate of chain initiation as well as the rate enhancement by cyclohexanone and several other (oxygenated) molecules. On the basis of first principles, the hitherto assumed perhemiketale mechanism was found to be many orders of magnitude too slow to account for the observed initiation enhancement by the ketone. Instead, it is shown that the pronounced chain-initiation enhancement by the ketone is attributable to a newly proposed concerted reaction between cyclohexyl hydroperoxide and cyclohexanone, in which the (.)OH radical breaking away from the hydroperoxide abstracts an alphaH atom from the ketone, thereby energetically assisting in the cleavage of the RO--OH bond. This reaction is highly efficient in generating radicals as it quasi-excludes geminate in-cage recombination. As a result, the ketone oxidation product at a level of 1 mol % increases the initiation rate by one order of magnitude, and so acts as a highly efficient "autocatalyst" in autoxidation reactions. An analogous reaction with cyclohexanol, although estimated to be even faster, has only a marginal effect on the overall kinetics, owing to the fast subsequent formation of HO(2) (.) radicals that very rapidly terminate with other ROO(.) radicals. Finally, solid evidence is presented that, also in absence of oxygenates, ROOH initiation is actually a bimolecular reaction, involving concerted H abstraction from the alkane substrate by the nascent (.)OH.

The photo-Fries rearrangement of 9-trimethylsilyl substituted xanthenes

In this paper, the absorption and fluorescence spectra of xanthene, 9-trimethylsilyl xanthene ( 1) and 9-methyl-9-trimethylsilyl xanthene ( 2) are presented and discussed with the help of CNDO/S and ab initio calculations. Introduction of the trimethylsilyl group at the 9-position of the xanthene molecule considerably shifts the short-wavelength band (246 nm) to the red and reduces the fluorescence quantum yields ( Φ fl = 0.03 for xanthene, 0.008 for 1 and <10 −4 for 2 in MeCN) and the fluorescence lifetimes ( τ fl = 7.4 ns for xanthene, 220 ps for 1 and <100 ps for 2 in MeCN), while it does not affect seriously the long-wavelength band and the singlet excited state energies ( E S = 97.7 kcal/mol for xanthene, 94.5 kcal/mol for 1 and 97.4 kcal/mol for 2 in MeCN). Ab initio calculations predict a ‘roof-like’ structure for 1 with folding angles 30° for the S 0 and 20° for the S 1 state. Laser (248, 266 and 308 nm) and lamp photolysis (254 nm) of 1 and 2 (MeCN or cyclohexane) results in [1,3]-trimethylsilyl rearrangement into the ortho-position of the xanthene moiety in the sense of a photo-Fries type reaction. The corresponding photo-Fries intermediates (exocyclic cyclohexatrienes: trimethyl-(1 H-xanthen-1-yl)-silane, 1CHT and trimethyl-(9-methyl-1H-xanthen-1-yl)-silane, 2CHT) are formed within the 20 ns laser pulse and show absorption spectra peaking up at 410 and 403 nm, respectively. Additionally, small amounts of the corresponding 9-xanthyl radicals were detected as a result of the C Si bond rupture. Using ps-laser flash photolysis (266 nm laser, MeCN) we observed a broad absorption spectrum peaking up at 960 nm and decaying monoexponentially with a lifetime of 130 ps, close to the measured fluorescence lifetime. We assigned therefore this transient to the singlet excited state of 1 (S 1 → S n absorption). We assume the S 1 state as the origin of the photo-Fries rearrangement, giving via C Si bond dissociation a singlet geminal radical pair (9-xanthyl radical + SiMe 3). In the next step, the radical pair undergoes predominantly in-cage recombination to the persistent photo-Fries intermediates with high quantum yields ( Φ 1CHT = 0.70 and Φ 2CHT = 0.50), while to a lesser extent it escapes the solvent cage ( Φ = 0.30 for 1) and undergoes typical free radical reactions (e.g., scavenging with O 2). The estimation of the above quantum yields was possible only after determination of the absorption coefficients ( ɛ) of the photo-Fries intermediates [ ɛ 1CHT (410 nm) = 16,800 M −1 cm −1 and ɛ 2CHT (403 nm) = 15,900 M −1 cm −1] using three independent methods; this represents the first example in the literature.

Triplet- vs. singlet-state imposed photochemistry. The role of substituent effects on the photo-Fries and photodissociation reaction of triphenylmethyl silanes

The photochemistry of three structurally very similar triphenylmethylsilanes 1, 2, 3 [p-X-C(6)H(4)-CPh(2)-SiMe(3): X = PhCO, 1; H, ; Ph(OCH(2)CH(2)O)C, 3] is described by means of 248 and 308 nm nanosecond laser flash photolysis (ns-LFP), femtosecond LFP, EPR spectroscopy, emission spectroscopy (fluorescence, phosphorescence), ns-pulse radiolysis (ns-PR), photoproduct analysis studies in MeCN, and X-ray crystallographic analysis of the two key-compounds 1 and 2. The photochemical behavior of 1, 2 and 3 is discussed and compared with that of a fourth one, 4, bearing on the p-position an amino group (X = Me(2)N) and whose detailed photochemistry we reported earlier (J. Org. Chem., 2000, 65, 4274-4280). Silane 1 undergoes on irradiation with 248 and 308 nm laser light a fast photodissociation of the C-Si bond giving the p-(benzoyl)triphenylmethyl radical (1*) with a rate constant of k(diss)= 3 x 10(7) s(-1). The formation of 1* is a one-quantum process and takes place via the carbonyl triplet excited state with high quantum yield (Phi(rad)= 0.9); the intervention of the triplet state is clearly demonstrated through the phosphorescence spectrum and quenching experiments with ferrocene (k(q)= 9.3 x 10(9) M(-1) s(-1)), Et(3)N (1.1 x 10(9) M(-1) s(-1)), and styrene (3.1 x 10(9) M(-1) s(-1)) giving quenching rate constants very similar to those of benzophenone. For comparative reasons radical 1* was generated independently from p-(benzoyl)triphenylmethyl bromide via pulse radiolysis in THF and its absorption coefficient at lambda(max)= 340 nm was determined ([epsilon]= 27770 M(-1) cm(-1)). We found thus that the p-PhCO-derivative 1 behaves similar to the p-Me(2)N one (the latter giving the p-(dimethylamino)triphenylmethyl radical with Phi(rad)= 0.9), irrespective of their completely different ground state electronic properties. In contrast, compounds 2, 3 that bear only the aromatic chromophore give by laser or lamp irradiation both, (i) radical products [Ph(3)C* and p-Ph(OCH(2)CH(2)O)C-C(6)H(4)-C(*)Ph(2), respectively] after dissociation of the central C-Si bond (Phi(rad)= 0.16), and (ii) persistent photo-Fries rearrangement products (of the type of 5-methylidene-6-trimethylsilyl-1,3-cyclohexadiene) absorbing at 300-450 nm and arising from a 1,3-shift of the SiMe(3) group from the benzylic to the ortho-position of the aromatic ring (Phi approximately 0.85 for 2). Using fs-LFP on 2 we showed that the S(1) state recorded at 100 fs after the pulse decays on a time scale of 500 fs giving Ph(3)C* through C-Si bond dissociation. In a second step and within the next 10 ps trityl radicals either escape from the solvent cage (the quantum yield of Ph(3)C* formation Phi(rad)= 0.16 was measured with ns-LFP), or undergo in-cage recombination to photo-Fries products. Thus, singlet excited states (S(1)) of the aromatic organosilanes (2, 3) prefer photo-Fries rearrangement products, while triplet excited states (1, 4) favor free radicals. Both reactions proceed via a common primary photodissociation step (C-Si bond homolysis) and differentiate obviously in the multiplicity of the resulting geminate radical pairs; singlet radical pairs give preferably photo-Fries products following an in-cage recombination, while triplet radical pairs escape the solvent cage (MeCN). The results demonstrate the crucial role which is played by the chromophore which prescribes in a sense, (i) the multiplicity of the intervening excited state and consequently that of the resulting geminate radical pair, and (ii) the dominant reaction path to be followed: the benzophenone- and anilino-chromophore present in silanes 1 and 4, respectively, impose effective intersystem crossing transitions (k(isc)= 10(11) s(-1) and 6 x 10(8) s(-1), respectively) leading to triplet states and finally to free radical products, while the phenyl chromophore in 2 and 3, possessing ineffective isc (k(isc)= 6 x 10(6) s(-1)) leads to photo-Fries product formation via the energetic high lying S(1) state [approximately 443 kJ mol(-1)(106 kcal mol(-1))].

Photodissociation Dynamics of Iodoform in Solution

The photodissociation dynamics of iodoform (CHI3) in acetonitrile and cyclohexane were investigated by femtosecond pump−probe spectroscopy and nanosecond laser flash photolysis. We have measured transient absorption spectra in the spectral region of 370−730 nm and kinetics at selected wavelengths on a femtosecond−microsecond time scale. We assign the observed photoproduct with absorption maxima at 450 and 600 nm, formed upon excitation of iodoform at 350 nm in acetonitrile and cyclohexane, to the isomer of iodoform, iso-iodoform (CHI2−I). The iso-iodoform is most probably formed via an in-cage recombination of the produced photofragments, the I atom and the CHI2 radical, with an ∼7 ps time constant. The lifetime of the iso-iodoform is found to be ∼1.8 and 0.2 μs in cyclohexane and acetonitrile, respectively.

Ultrafast Study of the Photodissociation of Bromoiodomethane in Acetonitrile upon 266 nm Excitation

We studied the A-band photodissociation of bromoiodomethane, CH2BrI, in acetonitrile solution at room temperature by femtosecond pump-probe spectroscopy, Initiated by the 266 nm light, this reaction leads to the CH2Br-I isomer, most likely produced via in-cage recombination of the CH,Br and I photofragments. The isomer is formed vibrationally excited with a time constant of similar to7.5 ps and the hottest isomer molecules are observed as early as I ps after the UV excitation, Vibrational relaxation of the isomer molecules takes place on two distinct time scales of a few and similar to40 ps. The ground-state lifetime of the isomer is about 2.5 ns.

Solvent Cage Effects in Organocobalt Corrinoid Chemistry: Thermal Homolysis of α- and β-(Cyanomethyl)cobinamides1

The equilibrium constant for the thermal isomerization of the diastereomeric alpha- and beta-(cyanomethyl)cobinamides (NCCH(2)Cbi(+)'s) has been measured over the temperature range 70-95 degrees C. Although the beta diastereomer is the thermodynamically more stable isomer, it is favored by the entropy change, but disfavored by the enthalpy change. In the presence of >/=5 x 10(-)(3) M concentration of the radical trap 4-hydroxy-2,2,6,6,-tetramethylpiperidinyloxy (4-HTEMPO), thermolysis of either isomer leads to cob(II)inamide and the trapped NCCH(2)(*) radical (NCCH(2)-4-HTEMPO) in high yield and no isomerization can be detected. The kinetics of the 4-HTEMPO-trapped thermal homolysis of alpha- and beta-NCCH(2)Cbi(+) have been studied in anaerobic glycerol/water mixtures of varying viscosity. The observed first-order rate constants for thermolysis show the expected inverse dependence on viscosity indicating that the process is at least partially diffusion controlled. From these data, the primary rate constant, k(1), for carbon-cobalt bond homolysis and the ratio of the rate constants for in-cage recombination and diffusional separation (k(c)/k(d)) can be extracted. The enthalpies of activation for Co-C bond homolysis are identical (29.0 +/- 0.3 kcal mol(-)(1)) while the entropy of activation is 2-fold higher for the alpha diastereomer. In water, the fractional cage efficiencies, F(c), are quite small (0.12 +/- 0.01, alpha; 0.049 +/- 0.008, beta) and invariant for each complex in the temperature range 75-95 degrees C. Assuming that the rate constant for diffusional separation of the caged radical pairs is the same for both isomers, the ratio of the in-cage recombination rate constants, k(c)(alpha)/k(c)(beta), can be calculated to be 2.6 +/- 0.6. This surprising kinetic preference for the alpha diastereomer results from enthalpic stabilization of the recombination transition state for the alpha diastereomer, since the beta diastereomer is entropically favored.

Dissociative electron attachment of O2: a solid-state effect on potential curve crossing

Previously published data on electron stimulated desorption (ESD) from condensed O2 or O2 in solid matrices are reanalyzed. In the gas phase, the O2−(2Σg+) resonant state at 8.5 eV is known to dissociate predominantly into the second lowest limit. In this paper we point out that this dissociation requires a non-adiabatic curve crossing. In the condensed phase or in solid matrices, this resonant state is found to dissociate adiabatically into the lowest limit as well as non-adiabatically into the second limit. Furthermore, we find that the branching ratio strongly depends on the kinetic energy of the dissociating atoms. The singlet excited oxygen atom, O∗(1D), formed upon dissociation into the second limit, is found to participate in an anion complex formation via in-cage recombination.

Comparison of the photochemistry of diarylchloronium, diarylbromonium, and diaryliodonium salts

The direct and triplet-sensitized photochemistry of diphenylchloronium, diphenylbromonium, bis(4-methylphenyl) chloronium, and bis(4-methylphenyl) bromonium salts was studied. Direct photolysis of the diarylhalonium salts gave haloarene 2-, 3-, and 4-halobiaryls, and acetanilide, predominantly from a heterolytic cleavage mechanism via the aryl cation-haloarene pair, via in-cage recombination and cage-escape reactions. However, triplet-sensitized photolysis gave haloarene, 2-, 3-, and 4-halobiaryl, and reduced arene mainly from homolytic cleavage to the arene radical-haloarene radical cation pair, which also gave in-cage recombination and cage-escape products. There is evidence for singlet-triplet interconversion of the excited states by spin-orbit coupling and interconversion of the singlet-triplet radical pairs by spin-spin coupling. The effect of halogen and arene substituent on these interconversions is discussed

The Anion Radicals of Pheophytin a and its Derivatives Studied by Means of CIDNP Technique

Abstract We applied the Chemically Induced Dynamic Nuclear Polarization (CIDNP) technique to the photochemical reaction of pheophytin a (P1) and 4-methoxyphenol (1) in benzene-d6. The observed CIDNP effects were explained in terms of reversible electron-transfer reactions from 1 to 3(P1)* and from (P1)\ewdot to (1)\underset.+. The polarized signals of P1 were induced from the in-cage recombination of the triplet radical ion pair.

The reaction of triphenylmethyl bromide with potassium O-ethyl dithiocarbonate (potassium xanthate) in benzene and cumene. A note of caution on the application of the radical trap dicyclohexylphosphine as a probe for electron-transfer-initiated reactions of triphenylmethyl halides

The title reaction was studied under conditions where homolysis of the triphenylmethyl-to-sulphur bond of the dithiocarbonate 1d, its main product does not take place, or is negligible. Triphenylmethane 1c, triphenylmethanol 1g and benzophenone were obtained as further products under argon in cumene and in the presence of oxygen in benzene, respectively. This, together with ESR studies, indicates that part of the dithiocarbonate 1d results from single-electron transfer from O-ethyl dithiocarbonate anion to triphenylmethyl bromide 1a and out-of-cage recombination of triphenylmethyl and xanthyl radicals 6a. The main pathways leading to dithiocarbonate 1d appear, under the conditions studied, to be either a polar pathway consisting of two successive SN2′-type steps [mechanism 2, Scheme 3] and/or an SET-type reaction [mechanism 3, Scheme 3] with rapid in-cage recombination of triphenylmethyl and xanthyl radicals 6a. The hydrogen-atom donor dicyclohexylphosphine (DCPH) may not be used for trapping of the triphenylmethyl radicals formed in the title reaction because DCPH acts as a single-electron donor towards bromide 1a, i.e. it itself generates triphenylmethyl radicals from the bromide.

Transient intermediates in the photolysis of iodonium cations

Pulsed picosecond laser photolysis of the diphenyliodonium cation indicates that excitation of the allowed transition is followed by rapid (ps) deactivation to a homolytically dissociative state, presumably T1. Steady-state photolysis yields iodobenzene and biphenyl from the primary radical products. Iodobiphenyl is also observed. We infer in-cage recombination of these radical products, with intervention of a carbocationic intermediate. Laser flash photolysis of diphenyliodonium iodide in its charge transfer band yields an initial transient with a lifetime of ca. 200 ps. A strongly absorbing secondary transient is observed on the nanosecond time scale. Iodobenzene is the exclusive reaction product. We infer that it is not formed directly from the primary photoproduct, but through in-cage recombination of phenyl cation with iodide anion, as in benzenediazonium iodide.

The photochemistry of sulphoxides. A CIDNP study of carbon–sulphur bond cleavage paths

A comparative photo-CIDNP study of triplet benzophenone sensitized C–S photocleavage of sulphoxides reveals significant structural dependence of the cage recombination and escape processes. In the photoreactive ortho-substituted phenyl methyl sulphoxides the triplet spin-correlated methyl–arylsulphinyl radical pair, 3RP1, [graphic omitted], is formed by triplet benzophenone sensitization or in some molecules, by direct excitation and intersystem crossing. Polarized methane and ethane are formed by the escape path of 3RP1, while the methyl- and aryl-H polarized starting molecule is obtained by the recombination path. The methyl-H polarized thioanisole product is obtained by deoxygenation of ArSO˙ escaped from 3RP1 and subsequent reaction of ArS˙ with a polarized methyl radical. In the absence of an active ortho-substituent, phenyl methyl sulphoxides give by direct excitation singlet state methyl–sulphinyl bond cleavage and form polarized methane as a minor escape product. The benzophenone triplet sensitized path takes place as a very inefficient process. The photocleavage reactivity of the methyl vinyl sulphoxides (3) and (4) is similar to that of the ortho-substituted methyl phenyl sulphoxides (1). In the methyl β-substituted ethyl sulphoxide (5a) photocleavage takes place only at the ethyl–sulphinyl bond. Polarized (5a) is obtained by the recombination path, while the escape path leads by the rare SO extrusion process to a 1H methyl polarized n-propyl product (5b). Fluorenyl cinnamyl sulphoxide (6) undergoes two parallel bond cleavage processes, at the fluorenyl–sulphinyl and at the cinnamyl–sulphinyl bonds. The in-cage recombination in both radical pairs gives rise to (6*) polarized on both fluorenyl and cinnamyl moieties. The present work indicates that the recently described complex photorearrangement of the closely related cyclic sulphoxides (7a–d) is not a free radical reaction but rather a concerted electrocyclic process. The bond cleavage processes of (6) were not observed in these systems. 4-Methylpentadienyl phenyl sulphoxide (9a) and its sigmatropic rearrangement isomer (9b) undergo photocleavage at the pentadienyl–sulphinyl bond. In the two isomers the alternating sign polarization pattern due to the recombination path was evidenced at 90 MHz and more clearly so in the recently obtained 270 MHz photo-CIDNP spectra. Good correlation can be established between the photocleavage reactivity and ground state bond dissociation energies for C–S bonds of sulphoxides, while the escape path reactivity can be correlated with the C–H bond dissociation energies of the hydrocarbon product.