Branching ratios between C−C and C−H bond activation were measured for reactions of ground-state Y (a2D, s2d) atoms with two C3H6 isomers (cyclopropane and propene) in crossed molecular beams. For both isomers, C−C bond activation led to formation of YCH2 + C2H4, whereas C−H activation led to YC3H4 + H2 and YH2 + C3H4. The angular and velocity distributions for all three product channels and for nonreactive collisions were measured at several collision energies (E coll). For Y + cyclopropane, the branching ratio for YCH2 + C2H4 increased relative to YC3H4 + H2 with increasing E coll, this C−C activation channel becoming dominant at E coll ≥ 19 kcal/mol. For the propene reaction, φYCH 2 /φYC 3 H 4 also increased with E coll, reaching 0.75:1.00 at E c oll = 28.8 kcal/mol. For both C3H6 reactants, formation of YH2 + C3H4 was observed as a minor channel at the highest collision energies. Experimental results and Rice−Ramsperger−Kassel−Marcus (RRKM) modeling indicate that for propene reactions all three channels involve initial formation of π-association complexes that undergo insertion into one of the sp3-hybridized β-C−H bonds in the methyl substituent. Decay of the yttrium allyl hydride intermediate by β-H migration leads to the two C−H activation products, YC3H4 + H2 and YH2 + C3H4. We propose that the YCH2 + C2H4 channel involves reverse β-H migration forming the same strongly bound metallacyclobutane intermediate formed in the Y + cyclopropane reaction.
The competition between C-C and C-H insertion in model transition-metal reactions with cyclopropane and propene (C3H6) was studied as a function of total energy. Insertion of neutral transition metal atoms M (= Y, Zr, Nb, and Mo*) into the C-C bonds of cyclopropane led to formation of MCH2 + C2H4, whereas C-H insertion produced MC3H4 + H2. The measured product branching ratios verify the relative potential energy barrier heights for C-C and C-H insertion predicted by ab initio calculations.
The reactions of ground state yttrium atoms (Y) with formaldehyde (H2CO) have been studied in crossed molecular beams as a function of collision energy (Ecoll). The potential energy barrier for C–H insertion is found to lie below 12 kcal/mol. It is proposed that the reaction is initiated by C–H insertion, producing HYCHO followed by H atom migration forming H2YCO. Although Y–CO bond fission leading to YH2+CO is dominant, a secondary minor channel also leads to the production of YCO+H2. Formation of YCHO+H is not observed at 16 kcal/mol, but is clearly seen at 31 kcal/mol, indicating that D0(Y–CHO) lies between 58 and 73 kcal/mol.
The interactions of Mo(a 7S3) and Mo*(a 5S2) with methane, CH4, and ethane, C2H6, were studied under single collision conditions using the crossed molecular beams technique. Ground state Mo(a 7S3) atoms were found to be unreactive at all collision energies studied up to 〈Ecoll〉=35.4 kcal/mol. Nonreactive scattering of Mo(a 7S3) with methane and ethane was studied and compared to collisions with Ne and Ar. A forward peaking center-of-mass angular distribution, T(Θ), was necessary to simulate the elastic collisions with inert gases as well as inelastic collisions with the alkanes. At a collision energy of 14.4 kcal/mol with CH4 and 21.0 kcal/mol with C2H6, inelastic collisions were found to transfer ∼10% and ∼19% of the initial kinetic energy into alkane internal energy, respectively. For collisions of Mo*(a 5S2)+CH4, the dehydrogenation product, MoCH2, was observed at all collision energies studied down to 2.1 kcal/mol. The reaction Mo*(a 5S2)+C2H6→MoC2H4+H2 was observed down to 〈Ecoll〉=4.5 kcal/mol. For a given total energy (electronic+translational), it was found that electronic energy is highly effective in promoting this reaction whereas translational energy is ineffective.
The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.