Ketenes triplet methylene formation

Energy-resolved rate constant measurements near the threshold for triplet methylene formation from ketene have been used to provide confirmation of the fundamental hypothesis of statistical transition state theory (that rates are controlled by the number of energetically accessible vibrational states at the transition state).6 The electronic structure and aromaticity of planar singlet n2-carbenes has been studied by 7t-electron coupling perturbation theory.7 The heats of formation of three ground-state triplet carbenes have been determined by collision-induced dissociation threshold analysis.8 The heats of formation of methylene, vinylcarbene (H2C=CHCH), and phenylcarbene were found to be 92.2 3.7, 93.3 3.4, and 102.8 33.5 kcal mol-1, respectively. 

A chemical test of a quite different nature is used for the identification of excited methylene, CH2. Methylene in the ground state is a triplet, but the photolysis of diazomethane or ketene (the standard methods for CH2 production) may lead to the formation of appreciable amounts of the excited singlet methylene. The fraction of singlet methylene has been determined by a study of the stereospecific addition of CH2 to double bonds eig.. the addition of methylene to tronj-butene-2 to yield dimethylcyclopropane ). It is assumed that spin will be conserved in the addition thus triplet methylene will form initially a triplet adduct (i.e. a biradical), and rotation may occur around the 2 3 C-C bond in the radical before ring closure. On the other hand, singlet methylene may form the cyclopropane directly, so... 

On the basis of eqn. (36), the yields of triplet methylene from ketene are found to be roughly 15% at 2800 A, 15% at 3130 A, 30% at 3340 A and 40% at 3660 A . Equation (36) neglects the formation of singlet-type products from triplet methylene via radical recombination (35). If the reduction in yield of rra/ti-2-pentene and 2-methyl-2-butene by added oxygen is also considered to be the result of triplet CH2 reactions, a recalculation of triplet methylene fractions yields 20% triplet at 2800 A, 55% at 3340 A and 60% at 3660 A for ketene photolysis . Assuming that 10% added oxygen suppresses all triplet products and measuring product yields relative to an internal standard, Eder and Carr " arrived at 29 3% triplet CH2 at 3130 A, and 87 2% at 3660 A. For the diazomethane-troni-2-butene system, the fraction of triplet methylene was found to be 12% at 3550-4000 A . 

The electronic spectroscopy of ketene is of particular interest because the upper state is photochemically active, leading to the formation of singlet and triplet methylene. There has been much controversy over the wavelength dependence of the relative yields of the two species of methylene, which to this day has not been satisfactorily resolved. For such a relatively simple molecule, the difficulties in unraveling its spectroscopy continue to be both surprising and challenging. 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

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