Effect of noncovalent interactions on conformers of the n-butylbenzene monomer studied by mass analyzed threshold ionization spectroscopy and basis-set convergent ab initio computations

Computational Study on the Unimolecular Decomposition of JP-8 Jet Fuel Surrogates III: Butylbenzene Isomers ( n-, s-, and t-C14H10)

Ab initio G3(CCSD,MP2)//B3LYP/6-311G(d,p) calculations of potential energy surfaces have been carried out to unravel the mechanism of the initial stages of pyrolysis of three C₁₀H₁₄ isomers: n-, s-, and t-butylbenzenes. The computed energy and molecular parameters have been utilized in RRKM-master equation calculations to predict temperature- and pressure-dependent rate constants and product branching ratios for the primary unimolecular decomposition of these molecules and for the secondary decomposition of their radical fragments. The results showed that the primary dissociation of n-butylbenzene produces mostly benzyl (C₇H₇) + propyl (C₃H₇) and 1-phenyl-2-ethyl (C₆H₅C₂H₄) + ethyl (C₂H₅), with their relative yields strongly dependent on temperature and pressure, together with a minor amount of 1-phenyl-prop-3-yl (C₉H₁₁) + methyl (CH₃). Secondary decomposition reactions that are anticipated to occur on a nanosecond scale under typical combustion conditions split propyl (C₃H₇) into ethylene (C₂H₄) + methyl (CH₃), ethyl (C₂H₅) into ethylene (C₂H₄) + hydrogen (H), 1-phenyl-2-ethyl (C₆H₅C₂H₄) into mostly styrene (C₈H₈) + hydrogen (H) and to a lesser extent phenyl (C₆H₅) + ethylene (C₂H₄), and 1-phenyl-prop-3-yl (C₉H₁₁) into predominantly benzyl (C₇H₇) + ethylene (C₂H₄). The primary decomposition of s-butylbenzene is predicted to produce 1-phenyl-1-ethyl (C₆H₅CHCH₃) + ethyl (C₂H₅) and a minor amount of 1-phenyl-prop-1-yl (C₉H₁₁) + methyl (CH₃), and then 1-phenyl-1-ethyl (C₆H₅CHCH₃) and 1-phenyl-prop-1-yl (C₉H₁₁) rapidly dissociate to styrene (C₈H₈) + hydrogen (H) and styrene (C₈H₈) + methyl (CH₃), respectively. t-Butylbenzene decomposes nearly exclusively to 2-phenyl-prop-2-yl (C₉H₁₁) + methyl (CH₃), and further, 2-phenyl-prop-2-yl (C₉H₁₁) rapidly eliminates a hydrogen atom to form 2-phenylpropene (C₉H₁₀). If hydrogen atoms or other reactive radicals are available to make a direct hydrogen-atom abstraction from butylbenzenes possible, the C₁₀H₁₃ radicals (1-phenyl-but-1-yl, 2-phenyl-but-2-yl, and t-phenyl-isobutyl) can be formed as the primary products from n-, s-, and t-butylbenzene, respectively. The secondary decomposition of 1-phenyl-but-1-yl leads to styrene (C₈H₈) + ethyl (C₂H₅), whereas 2-phenyl-but-2-yl and t-phenyl-isobutyl dissociate to 2-phenylpropene (C₉H₁₀) + methyl (CH₃). Thus, the three butylbenzene isomers produce distinct but overlapping nascent pyrolysis fragments, which likely affect the successive oxidation mechanism and combustion kinetics of these JP-8 fuel components. Temperature- and pressure-dependent rate constants generated for the initial stages of pyrolysis of butylbenzenes are recommended for kinetic modeling.