A study of the ozonolysis of isoprene in a cryogenic buffer gas cell by high resolution microwave spectroscopy

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, CH2=C(CH3)-CH=CH2, 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-CH=CH2 + CH2=OO] + [CH2=O + CH3COO-CH=CH2]. Alternatively, O3 could attack at the 3,4 position to produce methacrolein (MACR), formaldehyde, and two carbonyl oxides [CH2=C(CH3)-CHO + CH2=OO] + [CH2=O + CH2=C(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 CH2=OO, and formaldehyde (CH2=O) 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

THERMAL DECOMPOSITION OF METHYL ACETATE (CH3COOCH3) IN A FLASH-PYROLYSIS MICRO-REACTOR

The thermal decomposition of methyl acetate (chem{CH_3COOCH_3}) has been studied in a set of flash pyrolysis micro-reactors. Samples were diluted to (0.06 – 0.13%) in carrier gases (He, Ar) and subjected to temperatures of 300 - 1600 K at roughly 20 Torr. After residence times of approximately 25 – 150 $mu$seconds, the unimolecular pyrolysis products were detected by vacuum ultraviolet photoionization mass spectrometry at 10.487 eV (118.2 nm). Complementary product identification was provided by matrix isolation infrared spectroscopy. Decomposition began at 1000 K with the observation of (chem{CH_2}=C=O, chem{CH_3OH}), products of a four centered rearrangement with a $Delta_{textrm{rxn}}$H$_{298}$ = 39.1 $pm$ 0.2 kcal mol$^{-1}$. As the micro-reactor was heated to 1300 K, a mixture of (chem{CH_2}=C=O, chem{CH_3OH}, chem{CH_3}, chem{CH_2}=O, H, CO, chem{CO_2}) appeared. A new novel pathway is calculated in which both methyl groups leave behind chem{CO_2} simultaneously, $Delta_{textrm{rxn}}$H$_{298}$ = 74.5 $pm$ 0.4 kcal mol$^{-1}$. This pathway is in contrast to step-wise loss of methyl radical, which can go in two ways: $Delta_{textrm{rxn}}$H$_{298}$ (chem{CH_3COOCH_3} $rightarrow$ chem{CH_3} + chem{COOCH_3}) = 95.4 $pm$ 0.4 kcal mol$^{-1}$, $Delta_{textrm{rxn}}$H$_{298}$ (chem{CH_3COOCH_3} $rightarrow$ chem{CH_3COO} + chem{CH_3}) = 88.0 $pm$ 0.3 kcal mol$^{-1}$. _x000d

High Sensitivity Microwave Spectroscopy in a Cryogenic Buffer Gas Cell

We describe an instrument which can be used to analyze complex chemical mixtures at high resolution and high sensitivity. Molecules are collisionally cooled with helium gas at cryogenic temperatures (~ 4-7 K), and subsequently detected using chirped pulse microwave spectroscopy. Here we demonstrate three significant improvements to the apparatus relative to an earlier version: (1) extension of its operating range by more than a factor of two, from 12-18 GHz to 12-26 GHz, which allows a much wider range of species to be characterized; (2) improved detection sensitivity owing to use of cryogenically-cooled low-noise amplifiers and protection switches, and (3) a versatile method of sample input that enables analysis of solids, liquids, gases, and solutions, without the need for chemical separation (as demonstrated with a 12-16 GHz spectrum of lemon oil). This instrument can record broadband microwave spectra at comparable sensitivity to high Q cavity spectrometers which use pulsed supersonic jets, but up to 3000 times faster with a modest increase in sample consumption rate.Comment: 12 pages, 6 figure

The thermal decomposition of the benzyl radical in a heated micro-reactor. II. Pyrolysis of the tropyl radical.

Cycloheptatrienyl (tropyl) radical, C7H7, was cleanly produced in the gas-phase, entrained in He or Ne carrier gas, and subjected to a set of flash-pyrolysis micro-reactors. The pyrolysis products resulting from C7H7 were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by infrared absorption spectroscopy. Pyrolysis pressures in the micro-reactor were roughly 200 Torr and residence times were approximately 100 μs. Thermal cracking of tropyl radical begins at 1100 K and the products from pyrolysis of C7H7 are only acetylene and cyclopentadienyl radicals. Tropyl radicals do not isomerize to benzyl radicals at reactor temperatures up to 1600 K. Heating samples of either cycloheptatriene or norbornadiene never produced tropyl (C7H7) radicals but rather only benzyl (C6H5CH2). The thermal decomposition of benzyl radicals has been reconsidered without participation of tropyl radicals. There are at least three distinct pathways for pyrolysis of benzyl radical: the Benson fragmentation, the methyl-phenyl radical, and the bridgehead norbornadienyl radical. These three pathways account for the majority of the products detected following pyrolysis of all of the isotopomers: C6H5CH2, C6H5CD2, C6D5CH2, and C6H5 (13)CH2. Analysis of the temperature dependence for the pyrolysis of the isotopic species (C6H5CD2, C6D5CH2, and C6H5 (13)CH2) suggests the Benson fragmentation and the norbornadienyl pathways open at reactor temperatures of 1300 K while the methyl-phenyl radical channel becomes active at slightly higher temperatures (1500 K)

Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO-CH2OH), and Glyoxal in a Heated Microreactor.

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO-CH2OH and HCO-CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH2═O, CH2═C═O, and HCO-CHO. Thermal decomposition of HCO-CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)(CHO-CH2OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is ΔfH298(CHO-CH2OH) = -75.8 ± 1.3 kcal mol(-1). Fully ab initio, coupled cluster calculations predict ΔfH0(CHO-CH2OH) of -73.1 ± 0.5 kcal mol(-1) and ΔfH298(CHO-CH2OH) of -76.1 ± 0.5 kcal mol(-1). The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO-CH2OH. We find that the O-H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia

Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol.

The nascent steps in the pyrolysis of the lignin components salicylaldehyde ( o-HOC6H4CHO) and catechol ( o-HOC6H4OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID × 3 cm long); transit times through the reactors are about 100 μs. Temperatures in the microreactors can be as high as 1600 K, and pressures are typically a few hundred torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: o-HOC6H4CHO (+ M) → H2 + CO + C5H4═C═O (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HC≡C-C≡C-CH3, HC≡C-CH2-C≡CH, and HC≡C-CH═C═CH2. These alkynes decompose to a mixture of radicals (HC≡C-C≡C-CH2 and HC≡C-CH-C≡CH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: o-HOC6H4CHO + H → C6H5OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: o-HOC6H4OH (+ M) → H2O + C5H4═C═O. These findings have implications for the pyrolysis of lignin itself

Synchrotron-Based High Resolution Far-Infrared Spectroscopy of trans-Butadiene

International audienceThe high resolution far-infrared spectrum of trans-butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm−1 , ν10 and ν24 , have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν24 analysis was rather straightforward, ν10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν9 + ν12 state. Effective rotational constants have been derived for both the v10 = 1 and v24 = 1 states. Since only one weak, infrared active fundamental band (ν23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form

Isomerization and Fragmentation of Cyclohexanone in a Heated Micro-Reactor

The thermal decomposition of cyclohexanone (C₆H₁₀O) has been studied in a set of flash-pyrolysis microreactors. Decomposition of the ketone was observed when dilute samples of C₆H₁₀O were heated to 1200 K in a continuous flow microreactor. Pyrolysis products were detected and identified by tunable VUV photoionization mass spectroscopy and by photoionization appearance thresholds. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures were roughly 100 Torr, and contact times with the microreactors were roughly 100 μs. Thermal cracking of cyclohexanone appeared to result from a variety of competing pathways, all of which open roughly simultaneously. Isomerization of cyclohexanone to the enol, cyclohexen-1-ol (C₆H₉OH), is followed by retro-Diels–Alder cleavage to CH₂CH₂ and CH₂C­(OH)–CHCH₂. Further isomerization of CH₂C­(OH)–CHCH₂ to methyl vinyl ketone (CH₃CO–CHCH₂, MVK) was also observed. Photoionization spectra identified both enols, C₆H₉OH and CH₂C­(OH)–CHCH₂, and the ionization threshold of C₆H₉OH was measured to be 8.2 <i> ± </i> 0.1 eV. Coupled cluster electronic structure calculations were used to establish the energetics of MVK. The heats of formation of MVK and its enol were calculated to be Δ_f<i>H</i>₂₉₈(<i>cis</i>-CH₃CO–CHCH₂) = −26.1 ± 0.5 kcal mol^–1 and Δ_f<i>H</i>₂₉₈(<i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) = −13.7 ± 0.5 kcal mol^–1. The reaction enthalpy Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) is 53 ± 1 kcal mol^–1 and Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>cis</i>-CH₃CO–CHCH₂) is 41 ± 1 kcal mol^–1. At 1200 K, the products of cyclohexanone pyrolysis were found to be C₆H₉OH, CH₂C­(OH)–CHCH₂, MVK, CH₂CHCH₂, CO, CH₂CO, CH₃, CH₂CCH₂, CH₂CH–CHCH₂, CH₂CHCH₂CH₃, CH₂CH₂, and HCCH

Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

The nascent steps in the pyrolysis of the lignin components salicylaldehyde (<i>o</i>-HOC₆H₄CHO) and catechol (<i>o</i>-HOC₆H₄OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID × 3 cm long); transit times through the reactors are about 100 μs. Temperatures in the microreactors can be as high as 1600 K, and pressures are typically a few hundred torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: <i>o</i>-HOC₆H₄CHO (+ M) → H₂ + CO + C₅H₄CO (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HCC–­CC–CH₃, HCC–­CH₂–CCH, and HCC–­CHCCH₂. These alkynes decompose to a mixture of radicals (HCC–­CC–CH₂ and HCC–CH–­CCH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: <i>o</i>-HOC₆H₄CHO + H → C₆H₅OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: <i>o</i>-HOC₆H₄OH (+ M) → H₂O + C₅H₄CO. These findings have implications for the pyrolysis of lignin itself

Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO−CH₂OH), and Glyoxal in a Heated Microreactor

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO–CH₂OH and HCO–CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH₂O, CH₂CO, and HCO–CHO. Thermal decomposition of HCO–CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)­(CHO–CH₂OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is Δ_f<i>H</i>₂₉₈(CHO–CH₂OH) = −75.8 ± 1.3 kcal mol^–1. Fully ab initio, coupled cluster calculations predict Δ_f<i>H</i>₀(CHO–CH₂OH) of −73.1 ± 0.5 kcal mol^–1 and Δ_f<i>H</i>₂₉₈(CHO–CH₂OH) of −76.1 ± 0.5 kcal mol^–1. The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO–CH₂OH. We find that the O–H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia