The ozonolysis of isoprene in a cryogenic buffer gas cell by high resolution microwave spectroscopy

Abstract

We have developed a method to quantify reaction product ratios using high resolution microwave spectroscopy in a cryogenic buffer gas cell. We demonstrate the power of this method with the study of the ozonolysis of isoprene, CH₂C(CH₃)–CHCH₂, the most abundant, non-methane hydrocarbon emitted into the atmosphere by vegetation. Isoprene is an asymmetric diene, and reacts with O₃ at the 1,2 position to produce methyl vinyl ketone (MVK), formaldehyde, and a pair of carbonyl oxides: [CH₃CO–CHCH₂ + CH₂OO] + [CH₂O + CH₃COO–CHCH₂]. Alternatively, O₃ could attack at the 3,4 position to produce methacrolein (MACR), formaldehyde, and two carbonyl oxides [CH₂C(CH₃)–CHO + CH₂OO] + [CH₂O + CH₂C(CH₃)–CHOO]. Purified O₃ and isoprene were mixed for approximately 10 seconds under dilute (1.5–4% in argon) continuous flow conditions in an alumina tube held at 298 K and 5 Torr. Products exiting the tube were rapidly slowed and cooled within the buffer gas cell by collisions with cryogenic (4–7 K) He. High resolution chirped pulse microwave detection between 12 and 26 GHz was used to achieve highly sensitive (ppb scale), isomer-specific product quantification. We observed a ratio of MACR to MVK of 2.1 ± 0.4 under 1 : 1 ozone to isoprene conditions and 2.1 ± 0.2 under 2 : 1 ozone to isoprene conditions, a finding which is consistent with previous experimental results. Additionally, we discuss relative quantities of formic acid (HCOOH), an isomer of CH₂OO, and formaldehyde (CH₂O) under varying experimental conditions, and characterize the spectroscopic parameters of the singly-substituted ¹³C trans-isoprene and ¹³C anti-periplanar-methacrolein species. This work has the potential to be extended towards a complete branching ratio analysis, as well towards the ability to isolate, identify, and quantify new reactive intermediates in the ozonolysis of alkenes.