John M. Dyke

Direct Measurements of Conformer-Dependent Reactivity of the Criegee Intermediate CH3CHOO.

More Criegee Sightings The reaction of ozone with unsaturated hydrocarbons produces short-lived molecules termed Criegee intermediates. The simplest such molecule, H2CO2, was recently detected and monitored in the laboratory. Su et al. (p. 174; see the Perspective by Vereecken) have obtained its vibrational spectrum, which could ultimately enable direct measurements of its reactivity in the atmosphere. Taatjes et al. (p. 177; see the Perspective by Vereecken) report on the laboratory preparation and reactivity of the next heavier Criegee intermediate, which bears a methyl group in place of one of the hydrogen atoms. The reaction kinetics of an intermediate implicated in atmospheric ozone chemistry has been measured in the laboratory. [Also see Perspective by Vereecken] Although carbonyl oxides, “Criegee intermediates,” have long been implicated in tropospheric oxidation, there have been few direct measurements of their kinetics, and only for the simplest compound in the class, CH2OO. Here, we report production and reaction kinetics of the next larger Criegee intermediate, CH3CHOO. Moreover, we independently probed the two distinct CH3CHOO conformers, syn- and anti-, both of which react readily with SO2 and with NO2. We demonstrate that anti-CH3CHOO is substantially more reactive toward water and SO2 than is syn-CH3CHOO. Reaction with water may dominate tropospheric removal of Criegee intermediates and determine their atmospheric concentration. An upper limit is obtained for the reaction of syn-CH3CHOO with water, and the rate constant for reaction of anti-CH3CHOO with water is measured as 1.0 × 10−14 ± 0.4 × 10−14 centimeter3 second−1.

Direct measurement of Criegee intermediate (CH2OO) reactions with acetone, acetaldehyde, and hexafluoroacetone

Criegee biradicals, i.e., carbonyl oxides, are critical intermediates in ozonolysis and have been implicated in autoignition chemistry and other hydrocarbon oxidation systems, but until recently the direct measurement of their gas-phase kinetics has not been feasible. Indirect determinations of Criegee intermediate kinetics often rely on the introduction of a scavenger molecule into an ozonolysis system and analysis of the effects of the scavenger on yields of products associated with Criegee intermediate reactions. Carbonyl species, in particular hexafluoroacetone (CF(3)COCF(3)), have often been used as scavengers. In this work, the reactions of the simplest Criegee intermediate, CH(2)OO (formaldehyde oxide), with three carbonyl species have been measured by laser photolysis/tunable synchrotron photoionization mass spectrometry. Diiodomethane photolysis produces CH(2)I radicals, which react with O(2) to yield CH(2)OO + I. The formaldehyde oxide is reacted with a large excess of a carbonyl reactant and both the disappearance of CH(2)OO and the formation of reaction products are monitored. The rate coefficient for CH(2)OO + hexafluoroacetone is k(1) = (3.0 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1), supporting the use of hexafluoroacetone as a Criegee-intermediate scavenger. The reactions with acetaldehyde, k(2) = (9.5 ± 0.7) × 10(-13) cm(3) molecule(-1) s(-1), and with acetone, k(3) = (2.3 ± 0.3) × 10(-13) cm(3) molecule(-1) s(-1), are substantially slower. Secondary ozonides and products of ozonide isomerization are observed from the reactions of CH(2)OO with acetone and hexafluoroacetone. Their photoionization spectra are interpreted with the aid of quantum-chemical and Franck-Condon-factor calculations. No secondary ozonide was observable in the reaction of CH(2)OO with acetaldehyde, but acetic acid was identified as a product under the conditions used (4 Torr and 293 K).

A new method of calculation of Franck–Condon factors which includes allowance for anharmonicity and the Duschinsky effect: Simulation of the He I photoelectron spectrum of ClO2

A new method of Franck-Condon (FC) factor calculation for nonlinear polyatomics, which includes anharmonicity and Duschinsky rotation, is reported. Watsons Hamiltonian is employed in this method with multidimensional ab initio potential energy functions. The anharmonic vibrational wave functions are expressed as linear combinations of the products of harmonic oscillator functions. The Duschinsky effect, which arises from the rotation of the normal modes of the two electronic states involved in the electronic transition, is formulated in Cartesian coordinates, as was done previously in an earlier harmonic FC model. This new anharmonic FC method was applied to the simulation of the bands in the He I photoelectron (PE) spectrum of ClO2. For the first band, the harmonic FC model was shown to be inadequate but the anharmonic FC simulation gave a much-improved agreement with the observed spectrum. The experimentally derived geometry of the (X) over tilde (1)A(1) state of ClO2+ was obtained, for the first time, via the iterative FC analysis procedure {R(Cl-O)=1.414 +/- 0.002 Angstrom, angle O-Cl-O=121.8 +/- 0.1 degrees}. The heavily overlapped second PE band of ClO2, corresponding to ionization to five cationic states, was simulated using the anharmonic FC code. The main vibrational features observed in the experimental spectrum were adequately accounted for in the simulated spectrum. The spectral simulation reported here supports one of the two sets of published assignments for this band, which was based on multireference configuration interaction (MRCI) calculations. In addition, with the aid of the simulated envelopes, a set of adiabatic (and vertical) ionization energies to all five cationic states involved in this PE band, more reliable than previously reported, has been derived. This led also to a reanalysis of the photoabsorption spectrum of ClO2.

Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 7.—The methyl radical

The HeI vacuum ultraviolet photoelectron spectrum of the methyl radical produced by pyrolysis of azomethane has been investigated. Three ionization potentials, corresponding to the formation of the 1A′1, 3E′ and 1E′ ionic states, have been observed with vertical ionization potentials of 9.84, 14.76 and 16.10 eV respectively. The observed band positions have been interpreted with the aid of both ab initio and semiempirical molecular orbital calculations.The band associated with the first ionization potential is the only one for which vibrational structure could be resolved. This has been investigated in both CH3 and CD3 using Hel and NeI radiation. From the Franck–Condon envelope of this band and the observed shifts on deuteration, it is concluded that the methyl radical is essentially planar in its ground electronic state. The factors which control the values of the out-of-plane deformation frequencies in the ground state of CH3 and CH+3 are discussed in detail. Jahn–Teller splitting has been detected in the band associated with the second ionization potential of CH3.

Vacuum ultraviolet photoelectron spectroscopy of transient species: Part 11. The NH2(X 2B1) radical

The HeI photoelectron spectrum of the NH2(X 2 B 1) radical produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of NH2(X 2 B 1) to the X 3 B 1, 1 A 1 and 1 B 1 states of NH2 +. Measurement of the first two adiabatic ionization potentials of NH2 gave a value for the separation of the ground vibrational levels in NH2 +(X 3 B 1) and NH2 +(1 A 1) of 0·99 ±0·02 eV. Ionization energies of NH2 have been estimated using ab initio calculations which include the effects of electron correlation. These calculations indicate that two other bands arising from ionization of NH2, corresponding to formation of the 3 A 2 and 1 A 2 stages, should be observed in the HeI region but no clear evidence of these was obtained experimentally because of overlapping band problems. Spectra of the deuteriated radical were obtained and are consistent with these conclusions. Calculated and observed separations of the two lowest states of NH2 + are compared with those for the analogous ...

Vacuum ultraviolet photoelectron spectroscopy of transient species. Part 4.—Difluoromethylene and ozone

He I and He II spectra are reported for the isoelectronic molecules CF2 and O3. New values are reported for the first and higher ionization potentials. The spectra are interpreted with the aid of semi-empirical molecular orbital calculations.

A photoelectron spectroscopic study of the ground state of CF+ via the ionization process CF+(X 1Σ+)←CF(X 2Π)

The CF (X 2Π) radical has been investigated with vacuum ultraviolet photoelectron spectroscopy and ab initio molecular orbital calculations. Only one band was observed and it was assigned to the ionization CF+(X 1Σ+)←CF(X 2Π). For this band, the adiabatic and vertical ionization potentials were measured as (9.11±0.01) and (9.55±0.01) eV, respectively, and analysis of the vibrational structure in this band allowed we and re for CF+(X 1Σ+) to be determined as (1840±30) cm−1 and (1.148±0.005) A. Comparison is made with other first row diatomic fluorides that have also been studied with PES.

Vacuum ultraviolet photoelectron spectroscopy of transient species

The He(I) photoelectron spectrum of the FO(X 2Π i ) radical, produced from the rapid reaction has been recorded. Three bands were observed corresponding to ionization of FO(X 2Π) to the X 3Σ-, 1Δ and 1Σ+ states of FO-. Analysis of the vibrational structure in each band gave estimates of , r e and D e in the corresponding ionic state.

A Study of the Products of the Gas-Phase Reactions M + N2O and M + O3, Where M = Na or K with Ultraviolet Photoelectron Spectroscopy,

Products of the gas‐phase reactions M+N2O and M+O3, where M=Na or K, have been investigated with UV photoelectron spectroscopy and bands have been assigned with the assistance of results from ab initio molecular orbital calculations. For the M+N2O reactions, the observed products were MO+N2. Measurement of the photoelectron bands associated with the metal monoxide MO allowed determination of the first adiabatic ionization energies of NaO and KO. The values obtained were AIE[NaO(X 2Π)]=(7.1±0.1) eV and AIE[KO(X 2Π)]=(6.9±0.1) eV. A similar study of the Li+N2O reaction gave AIE[LiO(X 2Π)]=(7.6±0.2) eV. The reactions M+O3, with M=Na or K, were observed to give MO+O2 as the major reaction products. However, for each reaction, a band was observed which was assigned to the first ionization energy of the secondary reaction product MO2. From the spectra obtained, the first adiabatic ionization energies of NaO2 and KO2 were measured as AIE[NaO2(X 2A2)]=(6.2±0.2) eV and AIE[KO2(X 2A2)]=(5.7±0.1) eV. For both the M+...

Vacuum ultraviolet photoelectron spectroscopy of transient species. XVII. The SiH3(X 2A1) radical

Abstract The first band in the vacuum ultraviolet photoelectron spectrum of the silyl radical, corresponding to the process SiH 3 (X 1 A′ 1 ) ← SiH 3 (X 2 A 1 ), has been observed with HeI radiation. Extensive vibrational fine structure associated with the SiH 3 + deformation vibration was observed in this band and analysis of the structure gave a value of ω = 820 = 40 cm -1 for the out-of-plane deformation mode in the ion. The vertical and adiabatic ionization energies were measured as 8.74 = 0.01 eV and 8.14 ± 0.01 eV respectively and use of the latter value together with the established heat of formation of the silyl radical allows an improved heat of formation of SiH 3 - . Δ H 298 0 (SiH 3 - ) to be derived as 980 ± 7 kJ mol −1 .