Ab Initio Study of the 1,3-butadienyl Radical Isomers

Abstract

Ab initio calculations at the B3LYP, QCISD, and MCSCF levels of theory and using the 6-311G(d,p) basis set were carried out on the ground state of the 1,3-butadiene derived radicals. The 1,3-butadien-1-yl and 1,3-butadien-2-yl radicals are obtained from 1,3-butadiene by abstraction of a hydrogen atom from a primary and secondary carbon, respectively. The 1,2-butadien-4-yl radical was also studied to examine the possibility of the relocalization of the unpaired electron from 1,3-butadien-2-yl. 1,2-Butadien-4-yl was consistently found to be the most stable isomer. The MCSCF relative energies are 29 kJ mol-1 for 1,3-butadien-2-yl and 35 kJ mol-1 for the most stable of the 1,3-butadien-1-yl configurational isomers. The 1,3-butadien-2-yl structure is found to be a local minimum in MCSCF calculations, but with an isomerization barrier of less than 1 kJ mol-1, and deforms to the 1,2-butadien-4-yl isomer at all other levels of theory used. The energy of the most stable 1,3-butadien-1-yl isomer relative to 1,3-butadien-2-yl ranges from 6 to 18 kJ mol-1 across all levels of theory used, substantially lower than previous predictions by ab initio and semiempirical means. Optimized geometries, relative energies, permanent dipole moments, Fermi contact terms and harmonic vibrational frequencies are reported.

Thermochemical properties from G3MP2B3 calculations, Set‐2: Free radicals with special consideration of CH2CHC•CH2, cyclo−•C5H5, •CH2OOH, HO•CO, and HC(O)O•

“…The free radical R1 (1,3-butadien-1-yl) is higher in energy by 23.1-55.0 kJ mol −1 with respect to R2. Free radical R2p (1,3-butadien-2-yl) is unstable (see above) as it spontaneously relaxes into R2 without a barrier except on the level of MCSCF theory in agreement with the present work [35]. The MCSCF theory predicts the energy of R2p and R1 to be 28.6 and 34.6 kJ mol −1 above R2, the latter of which represents a significant shortfall compared to the results of Melius [34] and the present work.…”

“…An exception is the extremely shallow R2p structure supporting just one vibrational state found by Parker and Cooksy [35] using the MCSCF(9,9) level of theory, which affords a stationary structure only for trans- [35] are not directly comparable to the present work as they only report absolute and relative electronic energies. They found R2 (1,2-butadien-4-yl), the most stable isomer derived from terminal H-abstraction from 1,3-butadiene, to be consistently the most stable isomer across all levels of theory examined.…”

“…Both butadienyl free radicals are intermediates in the pyrolysis of hydrocarbons such as 1,3-butadiene and potential precursors for the formation of benzene in flames [32]. Previous work on both butadienyl free radicals includes semiempirical [33] and ab initio [34,35] calculations whose energetics significantly differs but whose relative ordering of states is identical to the one found in this work. The 1,3-butadien-2-yl radical (R2p), a secondary radical with the double bonds conjugated, has another possible canonical structure, namely the 1,2-butadien-4-yl radical with cumulated double bonds akin to an allenic structure and the unpaired elecScheme 1 G3MP2B3 calculated resonance energies E r for allyl resonance stabilization.…”

Theoretical Study of the Straight-Chain C4H7 Radical Isomers and Their Dissociation and Isomerization Transition States

“…Isolated theoretical and experimental values for the energies of some C 4 H 7 radicals and transition states appear in the literature, 4,[13][14][15][16] and extensive computational studies have been performed on other four-carbon systems, [17][18][19] but thus far, there has been no complete theoretical study of the straight-chain C 4 H 7 radicals as has been done for the C 3 H 5 system. 3,11,12 It is such a study that we provide here.…”

Photocycloaddition of Cyclohex‐2‐enones to Penta‐1,2,4‐triene

“…In addition, calculations predict the buta-1,2-dien-4-yl radical (24) to be more stable than the isomeric buta-1,3-dien-2-yl radical (25) by ca. 9 -10 kcal/mol [8]. This implies that in 20 the spin density on C(4) of the biradical will be much higher than on C(6), again excluding any 1,6-cyclization.…”