Chemically induced dynamic nuclear polarization in the photolysis of tetrafluoro-1,4-benzoquinone with plane polarized light evidence of phototriplet mechanism

CIDNP in the photolysis of tetrafluoro-1,4-benzoquinone was studied by using plane polarized light for excitation. Experiments show that the magnitude of CIDNP depends upon the polarization of the excitation light and thus provides evidence for the phototriplet mechanism in the initial electron spin polarization which subsequently leads to nuclear polarization by cross relaxation.     

Time-resolved CIDNP in laser flash photolysis of di-tert-butyl ketone. Multiplet versus net effects

Based on the principles of Fourier spectroscopy of non-equilibrium systems a method for the separate determination of net and multiplet type CIDNP effects in chemical reactions is developed. Further, a theoretical model for the analysis of time evolution of high-field CIDNP following pulse initiated reactions is extended to cover multiplet effects. Experimental data on multiplet and net CIDNP effects created by laser photolysis of di-tert-butyl ketone are presented and verify the theoretical conclusions. The observed magnitudes of net and multiplet effects support the Pedersen-Freed high-field radical pair theory.     

Thermal decomposition of diisopropyl peroxydicarbonate. CIDNP and product studies

The thermal decomposition of diisopropyl peroxydicarbonate (IPP) in two solvents (chlorobenzene and tetrachloroethylene) was studied by [¹H]NMR. In both solvents the major decomposition products, acetone and isopropanol, showed interesting CIDNP effects during the first few minutes of the decomposition at 60 and 75 respectively. For decomposition in the presence of excess di-t-butyl-p-cresol, no acetone was formed while the other CIDNP effects were completely suppressed. From these results and further analysis of the by-products, it is concluded that the observed CIDNP effects are closely connected with induced decomposition of IPP, starting by abstraction of the secondary H-atom of the isopropyl group.     

Measurement of rate processes of free radicals in homogeneous and micellar solutions by cidnp

CIDNP is used to study rate processes of free radicals in both homogeneous and micellar solution. An estimate of the lifetime of the phenyl-acetyl radical at ambient temperature (τ__co?10^?7 sec) produced during photolysis of dibenzyl ketone is made based on quantitative CIDNP measurements and computer simulations. Observation of CIDNP in micellar solution is shown to be consistent with an isotropic medium which restricts diffusion on a short time scale, allowing for an increased tendency toward cage reaction. In the case of t-butyl/pivaloyl radical pairs, escape of the radical fragments from the micelle is shown to be competitive with decarbonylation of the pivaloyl radical Likewise, CIDNP is consistent with product yield results which show the enhanced tendency of triplet born benzyl radical pairs to undergo cage reaction when they are sequestered in a micelle.     

An exception to the CIDNP sign rules

It is demonstrated experimentally that the CIDNP sign rules can give erroneous results. Net effects observed in the ¹H NMR spectrum of 1-phenyl[2-¹³C]propan-2-one after photolysis are correctly predicted at 360 MHz but not at 100 MHz by the rules. The converse is true for the ¹H–¹³C multiplet effects. The radical pair theory can account for the CIDNP effects in all cases.     

Chemically-induced dynamic nuclear polarisation. Photoreactions of organomercury compounds

The photolysis of symmetric and asymmetric organomercury compounds has been studied by CIDNP. No indications of the existence of RHg· radicals have been found. Depending on the solvent, reactions occur from excited states with different spin multiplicities.     

CIDNP during photoreactions of formaldehyde in solution

CIDNP has been studied during photolysis of formaldehyde in solution. The primary photochemical process is the hydrogen abstraction by predominant triplet excited formaldehyde from ground state formaldehyde. In the presence of bromotrichloromethane a singlet reaction occurs.     

The effett of medium on cidnp kinetics in geminate recombination of biradicals. Experiment and calculation

Submicrosecond flash-CIDNP was used to study the dependences of the kinetics and amplitude of CIDNP, arising in biradicals produced in the photolysis of cyclic aliphatic ketones on the temperature and viscosity of the medium. It has been found that for biradicals the geminate CIDNP increases with the diffusion coefficient of the medium. The theoretical consideration of geminate recombinaiion kinetics is based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the spin density matrix. The dependences of nuclear polarization on the lifetime of the triplet state of initial ketone, on electronic paramagnetic relaxation times, on scavenging rate, and on exchange interaction have been analyzed and compared with erperimental data. The conditions of quantum beats manifestaiion in CIDNP kinetics have been considered theoretically.     

CIDNP-Effekte bei Photolyse und Thermolyse von substituierten Phenyldiazoniumsalzen

The existence of radical pairs during the photolysis and the thermolysis of substituted benzenediazonium tetrafluoroborates and hydroquinone in acetonitrile is shown by means of CIDNP. The reaction is assumed to occur via a one-electron-step to form first a radical pair consisting of phenyldiazene and p-benzosemiquinone. This radical pair is able to lose nitrogen either by photolysis or thermolysis and can lead either to a disproportionation or to various recombinations.     

Deuterium isotope effect on the magnetic field dependence of CIDNP derived from biradicals

Incorporation of deuterium into the enolizable positions of several cyclic and bicyclic ketones alters the magnetic field dependence of CIDNP resulting from photolysis of the ketones. These results cannot be explained in terms of D-induced changes in the conformational populations of the biradicals formed in the photolyses.     

13C CIDNP effects in the decomposition products of acetylbenzoyl peroxide

¹³C CIDNP effects in the decomposition of acetylbenzoyl peroxide in cyclohexanone are described. A comparison between our data, and those on the ¹H CIDNP effects and the ¹³C CIDNP observed in the decomposition of ABP in C₂Cl₄¹ leads to definite conclusions on the signs of the HFI constants of the benzoyl radical and to an identification of the separate products of the radical-induced reactions with the solvent.     

The effects of chemically induced polarisation in the out-of-cage products of methyl and other alkyl radicals during the decomposition of di-t-butylperoxalate

The out-of-cage CIDNP effects, and kinetics of the out-of-cage reactions between the methyl radicals and the other radicals produced from the donor admixtures have been studied during the thermal decomposition of di-t-butylperoxalate (DTBPO). The DTBPO decomposition proves to be a good source of methyl radicals. The CIDNP effects on the methyl protons and on the nuclei of the other alkyl radicals are determined both by the out-of-cage radical collisions and the kinetics of out-of-cage recombination.     

CIDNP studies of macromolecular systems. I. Photolysis of poly(methyl isopropenyl ketone) and its model compounds in solution

The photoreactions of poly(methyl isopropenyl ketone) (PMIK) and two low-molecular-weight model compounds have been investigated in solution with proton NMR spectroscopy. Chemically induced dynamic nuclear polarization (CIDNP) has been observed in all three systems, but the CIDNP intensities have been found to depend on the chain lengths of the reaction products: long-chain products fail to show CIDNP at room temperature, whereas simultaneously formed small fragments exhibit nuclear spin polarization. The CIDNP originates from the Norrish type I decomposition of the ketones, which for the polymer occurs from the triplet state in all solvents. In the model compounds this cleavage can occur from the triplet or singlet state. The main photoreaction of PMIK, namely, scission of the polymer backbone, does not give rise to CIDNP. The failure of long-chain products to display CIDNP is attributed in part to a relaxation phenomenon: the short nuclear spin relaxation times of the long-chain product molecules destroy the CIDNP before it can be detected. In high-temperature studies above 150°C, however, in which the relaxation times are longer, CIDNP has been detected in long-chain molecules as well.     

Reversal of low-field CIDNP due to deuterium substitution

A perturbation method is applied to the problem of low-field CIDNP. It is shown that in the intermediate magnetic field region the most important deviation from the high-field formulae is due to a perturbation of the S—T₀ mixing by interaction with the T_± levels of the radical pair. In the case of a symmetrical radical pair this contribution decreases to zero and higher order contributions become dominant. An experimental example is the reversal of low-field CIDNP of ethane formed during the thermal decomposition of acetyl peroxide compared with that of ethane-d₃ during the decomposition of acetylperoxide-d₃. A qualitative rule is given for net CIDNP effects in the intermediate field region.     

The effects of chemically induced polarisation in the cage recombination products of the methyl radical

A study of the CIDNP effects on methyl protons was carried out for the cage recombination products formed during the thermal decomposition reactions of peracetyls and peracetylcarbonates. The increase of both the cage recombination product yield and the enhancement of CIDNP on the CH₃ protons is shown to be connected with the RCO₂ radical lifetime, where R is an alkyl or alkoxy radical. A general theory of CIDNP is presented and the calculated data prove to be in satisfactory agreement with the experimental results.     

Diazoalkanes in the Reactions with Dialkyl (Thio)-Phosphites

Abstract

The photochemical, thermo-catalytical and thermal reactions of diazoalkanes with dialkyl (thio)phosphites were studied.

Thus, the photolysis of dimethyldiazomalonate and methyl-diazoacetate with dialkyl(thio)phosphites gives P-H insertion products only upon triplet-sensitized conditions (i), whereas diphenyldiazomethane gives P-H insertion products upon direct photolysis as well (ii). The investigation of 31 the reaction mechanism is based on CIDNP ³¹P method. The catalytical reactions (iii) are a convenient method of preparing (thio)phosphonates.     

13C CIDNP derived from a 1,4-biradical generated in a norrish type II photolysis

NorRIsh type II photolysis of 1-benzoyl-1-methylcyclopentane, labeled with ¹³C in the carbonyl position, produces emissive CIDNP in the cyclization product. The polarization is consistent with the formation of a 1,4-biradical with a singlet ground state. By comparing this result with those for other biradicals, inferences are drawn about possible factors governing the singlet—triplet splittings of biradicals.     

Polarisation nucleaire induite chimiquement: Decomposition thermique du peracide dodecanoique dans l'hexachloroacetone

Undecyl chloride formed by decomposition of perdodecanoic acid in hexachloroacetone showed CIDNP signals. Thermal decomposition of aliphatic peracids was studied by this way. The first step is the formation of a singlet radical pair of undecyl and hydroxyl radicals which do not recombine into ROH and give exclusively transfer on the solvent.     

The field dependence of CIDNP in gas-phase reactions of biradicals

The field dependence of CIDNP effects in gas-phase biradical reactions is for the first time detected during the photolysis of the homological series of cyclic aliphatic ketones from cyclodecanone to cyclopentadecanone. The influence of magnetic field on nuclear polarization is studied up to 0.47 T. The experimental data are interpreted on the basis of nuclear-spin-selective ST₋ transitions in intermediate biradicals. In contrast to liquid-phase reactions, mainly protons in the γ-position are polarized. The relatively weak α-CH₂ and β-CH₂ protons in the gas are explained by compensation of various conversion channels in biradicals. The strong spin—rotation interaction in gases excludes polarization transfer observed in solutions. The field dependence of CIDNP effects differs noticeably in gases and liquids. It is associated with the different dynamics of conformational motion in liquids and gases.     

Norrish Type I Cleavage of Aliphatic Carboxylic Acids and Esters in Solution. A CIDNP.-study

UV.-irradiation of pivalic and isobutyric acid and their methyl esters as well as of α-hydroxy isobutyric and malonic acid in solution lead to chemically induced dynamic nuclear polarization (CIDNP.) of parent compounds and of various reaction products. CIDNP.-effects and product distributions confirm α-cleavage of the C(α), CO bond to be the major mode of photodecomposition. Scavenger experiments indicate that decomposition of the molecules from a triplet excited state is roughly equally or more probable than from a singlet excited state. Quantum yields of educt disappearance are also given for the methyl esters of pivalic and isobutyric acid.