Wavelength effects in the photolysis of ketones: stereoisomerization and magnetic isotope carbon-13/carbon-12 separation; a probe for adiabatic versus diabatic trajectories during bond dissociation

Abstract

The I3C enrichment in the photolysis of three ketones, dibenzyl ketone (DBK), (S)-(+)-a-methyldeoxybenzoin (MDB), and d,l-2,4-diphenylpentan-3-one (DPP), along with the photostereoisomerization of the latter two From the simple paradigm of eq 1, the value P,, the probability of combination of a primary geminate pair, is a potentially useful and intuitive mechanistic parametcfl~~ which clearly represents the competition between the knit process and all other competing processes.When the primary geminate pair is created in a singlet state (IRP), the value of Pr is often close to unity and is relatively lacking in mechanistic information. On the other hand, when the primary geminate pair is produced in a triplet state ORP), the intersystem crossing step, )RP -'RP, requires a magnetic interaction and may ku" rate determining and, before the

MIE versus CIE: Comparative Analysis of Magnetic and Classical Isotope Effects

“…Step et al 35 have studied the wavelength effects on the stereoisomerization and 13C/12C isotope separation under the photolysis of three ketones, methyldeoxybenzoin (MDB), dibenzyl ketone (DBK), and 2,4-dimethylpentanone (DPP) in SDS micelles. By monitoring the circular dichroism for MDB and DPP and by measuring isotope distribution for all ketones, a and RP recombination probabilities P have been evaluated.…”

“…Only the central part of the DBK molecule, -CH2COCH2-, should be taken into consideration, since in the precursor RP (PhCH2* 'COC^Ph), HFI is significant and, therefore, strongly influences the RP spin dynamics only for CH2 and COCH2 groups in the radical partners, while all other HFI couplings can be ignored. For each isotopic modification i of DBK listed above (i = 1 -6) eq 28 acquires the form = ln(l -F^/lnd -Ft) (35) where Fi and Ft are the conversions of the first (isotopically nonlabeled) and ith isotopic forms, respectively. The concentration of any isotopic form can be obtained from the relation C; = Ufi (36) where C is the total concentration of DBK molecules and coefficients Ui are determined statistically from the isotope distribution:…”

“…Figure12. Coefficients a (a) and recombination probabilities P (b) as a function of the light energy in the photolysis of methyldeoxybenzoin (1), dibenzyl ketone (2), and diphenylpentanone(3) 35. …”

Mass-Independent Isotope Effects

“…confirmed it in triplet sensitized photolysis of benzoyl peroxide. Nicholas Turro in 1978 was the first who used micelles as the microreactors for the photolysis reactions; he has shown that the reactions in micelles result in enormously large isotope separation. − In 1980 Pines and co-workers showed that in viscous solutions the magnetic isotope effect markedly increases (see, for details, ref ). Later this nuclear spin and, therefore, nuclear magnetic moment dependent isotope effect was discovered for oxygen, − silicon, , sulfur, , germanium, , tin, − mercury, , magnesium, , calcium, zinc, and uranium .…”

Magnetic Isotope Effect: Nuclear Spin Control of Chemical Reactions

“…Later, MIE was found in the photolysis of DBK on porous silica and zeolites and in the photolysis of phenyl adamantyl ketone and many others, including cyclic ketones . Temperature and wavelength effects on the MIE in the photolysis of ketones have also been studied (section 5).…”