Pyrolysis of 3-Methoxypyridine. Detection and Characterization of the

Article pubs.acs.org/JPCA

Pyrolysis of 3‑Methoxypyridine. Detection and Characterization of the Pyrrolyl Radical by Threshold Photoelectron Spectroscopy Fabian Holzmeier,† Isabella Wagner,† Ingo Fischer,*,† Andras Bodi,‡ and Patrick Hemberger*,‡ †

Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland D-97074, Germany Molecular Dynamics Group, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland

‡

S Supporting Information *

ABSTRACT: Pyrolysis of 3-methoxypyridine in a heated pyrolysis reactor was found to be an efficient way to generate the pyrrolyl radical, c-C4H4N, in the gas phase. The threshold photoelectron (TPE) spectrum of this radical was recorded using vacuum ultraviolet synchrotron radiation. The spectrum revealed a singlet ground state at 9.11 ± 0.02 eV (X̃ + 1A) and an excited triplet state (ã+ 3A) at 9.43 ± 0.05 eV. Vibrational structure was observed for both cationic states and could be assigned to ring deformation modes. Furthermore, (E)- and (Z)-1cyanoallyl radicals were found to contribute to the TPE spectrum below 8.9 eV. In addition, we have identified two parallel decomposition channels of the pyrrolyl radical, yielding either hydrogen cyanide and propargyl radical or acetylene and cyanomethyl radical. The reaction energy profiles have also been calculated for these reactions. In addition, the dissociative photoionization of the precursor 3-methoxypyridine is reported.

■

INTRODUCTION Pyrrolic (50−80%), pyridinic (20−40%), and quaternary (0− 20%) nitrogen are the typical structural motives of nitrogen found in fossil fuels.1 The pyrrolyl radical c-C4H4N (insert in Figure 1) can therefore be seen as a model for nitrogencontaining fuels and is assumed to be an important combustion intermediate. Fuel nitrogen leads to the formation of nitrogen oxides NOx, which are harmful to the environment and can poison catalysts.1,2 It is hence important to understand the chemistry occurring during combustion. The oxidation of pyridine can lead to the pyridyl radical (C5H4N), which reacts

with oxygen atoms to form pyridoxy (C5H4NO). The latter subsequently decomposes yielding pyrrolyl, according to reaction 1.3 C5H4N + O(3P) → C5H4NO → C4 H4N + CO

Furthermore, hydrogen abstraction from pyrrole (reaction 2) can also be a source of the c-C4H4N in flames.4 C4 H5N + X → C4 H4N + X−H

(2)

In this work, we generate the pyrrolyl radical in a pyrolysis reactor and investigate the photoionization of the radical utilizing VUV (vacuum ultraviolet) synchrotron radiation. Valence photoionization is a powerful tool to selectively probe even transient species in flames online,5−8 which permits us to draw conclusions on combustion mechanisms. Consequently, photoelectron photoion coincidence (PEPICO) techniques at synchrotron radiation facilities are emerging to analyze reactive environments containing elusive radicals and carbenes. Accurate ionization energies (IEs) and knowledge of the vibrational structure of characteristic transitions, which act as fingerprints, are therefore necessary for an unambiguous identification. Several nitrogen containing intermediates relevant in combustion have been studied previously by our group.9−13 In earlier photoionization mass spectrometry (PIMS) experiments on the pyrolysis of pyrrole, Fei Qi and co-workers observed traces of a compound with m/z = 66, for which they determined an ionization energy of 9.17 eV and Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift

Figure 1. Total ion yield mass spectra of 3-methoxypyridine as measured at 10 eV at room temperature (upper panel) and with active pyrolysis (lower panel). The most intense peak in the mass spectrum at 550 °C corresponds to m/z = 66, the pyrrolyl radical. © XXXX American Chemical Society

(1)

Received: November 3, 2015 Revised: December 21, 2015

A

DOI: 10.1021/acs.jpca.5b10743 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A

from Sigma−Aldrich and used without further purification. The precursor was seeded in 1.9 bar of argon and was expanded through a 100 μm orifice and pyrolyzed in an electrically heated silicon carbide (SiC) tube. Details on the pyrolysis source, which was operated at 550 °C as measured by a Type C thermocouple on the outside of the pyrolysis tube, can be found elsewhere.32 In a second experiment, 3-methoxypyridine was effusively expanded at room temperature into the ionization vacuum to investigate the dissociative photoionization (DPI) of the precursor. Cations were detected in a Wiley−McLaren time-of-flight mass spectrometer after extraction in an electric field of 120 V cm−1. Electrons were accelerated in the opposite direction by the same extraction field and velocity map imaged utilizing a delay line anode (Roentdek DLD40). Cations and electrons from the same ionization event were correlated employing a multiple start/multiple stop data acquisition scheme.33 Threshold photoelectrons were selected with a resolution of 5 meV and the contribution of hot electrons was subtracted as described in the literature.29,34,35 Mass-selected threshold photoelectron (ms-TPE) spectra of the radical were recorded with a 5 meV photon energy step size and an acquisition time of 180 s per data point for the pyrolysis experiment. ms-TPE spectra of the precursor and its DPI fragment ions were recorded in 20 meV steps and were averaged for 60 s. Longer integration times of 10 min were employed for additional mass spectra between 12.0 and 13.5 eV in 0.1 eV steps. The Gaussian 09 suite of programs was utilized for all quantum chemical computations.36 CBS-QB3 was employed for the computation of ionization and appearance energies.37,38 This composite method optimizes the molecular geometries and computes harmonic frequencies and rotational constants on the B3LYP/6-311G(2d,d,p) level of theory. Franck− Condon simulations of the TPE spectra at 0 K vibrational temperature were performed with the program FCfit version 2.8.20.39 The breakdown diagram of 3-methoxypyridine was modeled employing statistical rate theory.40 Additional details on the fitting procedure can be found in the Supporting Information of this paper.

assigned it to cyanoallyl, an open-chain isomer of the pyrrolyl radical.14 The ionization energy of the pyrrolyl radical, however, has not yet been reported, as the radical is difficult to generate. Computations showed that pyrrolyl has a C2v-symmetric, nonaromatic electronic ground state with five π-electrons (X 2 A2).15 The nitrogen lone pair is thus located in a σ-orbital. Cleavage of the N−H bond leading to the pyrrolyl radical and atomic hydrogen cannot proceed on the 1A1 pyrrole ground state, as this would generate pyrrolyl in the 2A1 excited state. Studies have shown that excitation of pyrrole with 243 nm laser light leads to dissociation to pyrrolyl in the electronic ground state and atomic hydrogen.16−18 The photodissociation must hence happen in an electronically excited state of pyrrole. Alternatively, isomerization of pyrrole to pyrrolenine has to precede the H atom loss,15,19 making the generation of isolated pyrrolyl radicals challenging. Pyrrolyl was first generated and detected indirectly by radical−radical reaction products in the flash photolysis of 1-phenylpyrrole.20 The 0 K heat of formation of the pyrrolyl radical was derived from the measured bond dissociation energy in pyrrole and the enthalpies of formation of pyrrole and hydrogen. Values of 259 ± 816 and 301.9 ± 0.5 kJ mol−1 21 were reported; the latter one compares well with ab initio computations (296 ± 2 kJ mol−1).15 The electronic ground state of pyrrolyl was investigated by photodetachment photoelectron spectroscopy of the pyrrolide anion.22 Motzke et al. carried out a Franck−Condon analysis of the vibrational structure of the pyrrolyl ground state, which shows excellent agreement with the photodetachment spectrum.23,24 To find an efficient pathway for the generation of pyrrolyl radicals beyond the rather elaborate photolysis of pyrrole, we were guided by the work on the decomposition of phenoxy radicals yielding cyclopentadienyl radical c-C5H5 and CO. This radical is isoelectronic with pyrrolyl and was observed in the pyrolysis of anisole (methoxybenzene),25−27 where methyl and carbon monoxide are subsequently cleaved off in a two-step decomposition reaction. The nitrogen analogue, methoxypyridine, was hence considered as a potential pyrolysis precursor for the generation of pyrrolyl. As the isomer with the lowest boiling point and presumably the highest vapor pressure, 2-methoxypyridine, is reported to isomerize to N-methylpyridone under pyrolytic conditions,28 3-methoxypyridine was used in this study as a precursor. Mass-selected threshold photoelectron spectroscopy (ms-TPES) was employed to verify the isomeric form of the composition C4H4N, check the efficiency of the radical generation, determine the ionization energy of the pyrrolyl radical, and to characterize the vibrational structure in the cation. In addition, the dissociative photoionization of the precursor, 3-methoxypyridine, was investigated.

■

RESULTS AND DISCUSSION a. Mass Spectra. Figure 1 depicts mass spectra of 3methoxypyridine at a photon energy of 10.0 eV at room temperature without pyrolysis and for a pyrolysis temperature of 550 °C. While the precursor signal at m/z = 109 is the only peak in the spectrum at room temperature, the pyrolysis mass spectrum shows rich chemistry, although a strong precursor signal is still observed. The observed signals agree with the expected decomposition pathway of 3-methoxypyridine in analogy to the pyrolysis of anisole.25,26 In a first step, the weak methyl−oxygen bond is cleaved, yielding methyl (m/z = 15) and 3-pyridoxy (m/z = 94). The latter radical eliminates carbon monoxide (m/z = 28 is observed in the mass spectrum only at high photon energies because of its IE = 14 eV), which leads to the pyrrolyl radical (m/z = 66). The reaction mechanism from 3-pyridoxy to the pyrrolyl radical is summarized in Scheme 1. In agreement with this mechanism, the m/z = 15 signal in the mass spectrum could be assigned to methyl by measuring the ms-TPE spectrum,41,42 whereas the m/z = 94 signal was too low for an unambiguous assignment, because it rapidly decomposes further to m/z = 66. On the basis of the observed ionization onsets of m/z = 67 and 81, these peaks in the mass spectrum

■

EXPERIMENTAL AND THEORETICAL METHODS Experiments were performed at the imaging photoion photoelectron (iPEPICO) endstation of the X04DB beamline at the Swiss Light Source (SLS) storage ring. Details on the beamline layout and on the iPEPICO spectrometer can be found in the literature and are only briefly described here.29−31 Vacuum ultraviolet (VUV) synchrotron radiation is provided by a bending magnet and collimated onto a 150 lines mm−1 grating monochromator, yielding a photon energy resolution of about E/ΔE = 103. Higher order radiation is suppressed by a rare gas filter operated with a mixture of Ne/Ar/Kr. The 11s′-13s′ argon autoionization resonances in first and second order are used for energy calibration. 3-Methoxypyridine was purchased B


Article


Although the structure in which pyrrolyl is substituted in the 2position is 12 kJ mol−1 more stable than the isomer substituted in the 3-position, the transition state (TS2a: +201 kJ mol−1 relative to 3-pyridoxy) leading to the latter is lower in energy (TS2b: +209 kJ mol−1). Both intermediates, however, lead to the same products: pyrrolyl and carbon monoxide. The CO loss reaction from 3-pyridoxy is computed to be endothermic by 63 kJ mol−1. The activation barrier for the formation of pyrrolyl and carbon monoxide from 3-methoxypyridine is in the same range as for the formation of cyclopentadienyl from anisole46 and should be surmounted at the chosen pyrolysis temperature. Because the first barrier, i.e., the methyl loss, computed at 273 kJ mol−1, is already higher than the following barriers corresponding to rearrangements and CO loss, it is likely that rethermalization in the reactor occurs by collisions of 3pyridoxy with thermalized carrier gas molecules. 3-Pyridoxy radicals then rearrange promptly in the sweet spot of the reactor47 and subsequently lose CO. We estimate the collisional rate in the microtubular reactor to be in the range of 109 s−1, leading to instantaneous rethermalization on the pyrolysis time scale, estimated to be on the order of 100 μs.48 At an energy of 273 kJ mol−1 relative to the pyridoxy radical, RRKM calculations yield a rate of 8 × 105 s−1 for the reaction to pyrrolyl and CO. This rate makes it impossible to detect measurable amounts of 3-pyridoxy.49,50 Mass spectra at higher photon energies show additional peaks at m/z = 26, 27, 40. These were assigned to acetylene, HCN, and the cyanomethyl radical. Together with propargyl observed already at 10 eV (Figure 1), these fragments can be explained by thermal decomposition of the pyrrolyl radical. Cyclopentadienyl was observed to decompose to acetylene and propargyl.25,26,50 The substitution of a C−H moiety by N leads to two possible decomposition pathways in the isoelectronic pyrrolyl radical: HCN + propargyl and C2H2 + cyanomethyl. The respective mechanisms were computed on CBS-QB3 level of theory and are depicted in Scheme 2. Both pathways are initiated by a [1,5] sigmatropic H shift leading to σ-radicals. Ring opening is followed by CC bond cleavage and leads to the product pairs HCN + propargyl and C2H2 + cyanomethyl. This results in almost equally stable products for both routes (HCN + propargyl, +183 kJ mol−1; C2H2 + cyanomethyl, +191 kJ mol−1), but the HCN + propargyl channel comprises higher energy barriers and less

Scheme 1. CBS-QB3 Computed Activation Barriers (kJ mol−1) for the Reaction Pathways from 3-Pyridoxy to Pyrrolyl and Carbon Monoxidea

a

As they are relatively low (compared to the O−CH3 bond energy of 273 kJ mol−1 in the precursor, see text), and in the absence of alternative, lower lying dissociation pathways, the formation of pyrrolyl from 3-methoxypyridine is likely to occur almost quantitatively due to rethermalization once the methyl group has been cleaved.

are identified as pyrrole and N-methylpyrrole, which are presumably formed in the pyrolysis tube from bimolecular chemistry. Both have been studied in detail previously.43,44 We assume that the m/z = 95 peak originates from a hydrogen abstraction reaction of the radical species m/z = 94 with precursor molecules, but the signal/noise ratio was again too low for a spectral assignment. Propargyl radicals, m/z = 39, were also identified,45 which seem to be a decomposition product of m/z = 66, as the signal decreases at higher pyrolysis temperatures, while the propargyl signal increases (vide infra). 550 °C was found to be the pyrolysis temperature at which the m/z = 66 signal is maximal. As the formation of pyrrolyl from 3-methoxypyridine must proceed via a complex pyrolysis mechanism, the energetics of the second decomposition step, i.e., the CO loss of 3-pyridoxy, was computed on CBS-QB3 level of theory as sketched in Scheme 1. Similar to the decomposition of the phenoxy radical, the first intermediate in the decomposition of 3-pyridoxy is a bicyclic structure, favored over the open chain isomer presumably because of the delocalization of the unpaired electron.46 However, in contrast to anisole, two possibilities exist for the following opening of the three-membered ring, as the presence of the nitrogen in the ring system leads to symmetry breaking.

Scheme 2. Decomposition Mechanisms of the Pyrrolyl Radical Computed by CBS-QB3 (All Values in kJ mol−1)

C


Article


800 cm−1 apart. A second sharp peak can be determined at 9.48 eV, which is followed by another peak at 9.53 eV and a broad band around 9.6 eV. At lower photon energies, further minor features can be identified at 8.84 and 8.87 eV. CBS-QB3 computations predict an ionization energy of 9.17 eV to the singlet ground state and 9.43 eV to the first excited, triplet cationic state. These values are in good agreement with the observed prominent features in the spectrum. For the isoelectronic cyclopentadienyl, by contrast, the triplet state is the electronic ground state of the cation and the cyclopentadienyl cation is thus a biradical. Due to the D5h symmetry of the cyclopentadienyl radical in the X̃ 2E1″ neutral ground state, the HOMO is degenerate. This degeneracy is lifted by replacing a CH by N in pyrrolyl and the b2 orbital is stabilized with respect to the a2 orbital.22 Ejecting an electron from the HOMO leads therefore inevitably to a singlet ground state in pyrrolyl, whereas different electronic configurations are possible in cyclopentadienyl.52,53 The first excited singlet state in cyclopentadienyl is subject to pseudo-Jahn−Teller distortion, to avoid the unfavorable antiaromatic electronic structure of C5H5+. However, the stabilization by distortion is lower than the electron repulsion in the singlet state, which is why the triplet cationic state is lower in energy. For pyrrolyl, computations predict a C2v symmetry for the neutral ground state, whereas both singlet and triplet cations have distorted nonplanar geometries, which again circumvents antiaromaticity. The corresponding vibrational progressions are therefore expected to be observed in the photoelectron spectrum. Franck−Condon simulations for the transitions from the neutral ground state to the singlet and triplet cation, also shown in Figure 2, assign the observed vibrational progressions to ring deformation modes in the cationic states, and match the experimental spectrum between 9.1 and 9.7 eV well. Four components of a vibrational progression with a spacing of 800 cm−1 (100 meV) are observed in the spectrum and can be assigned to the C1−C2 (next to the N atom) stretching mode in the cationic singlet state, for which a harmonic frequency of 808 cm−1 was computed at the B3LYP/6-311G(2d,d,p) level of theory. Several other stretching modes of the ring backbone are predicted to be active, mostly in combination bands, and explain the relatively high background level. The Stark shift due to the 120 V cm−1 extraction field has to be taken into account for an accurate assignment of the ionization energy. Previous studies using the same experimental setup have shown that dc fields of this size can lead to a red shift of the IE of up to 8 meV.54,55 Considering also the photon energy resolution, an error bar of 20 meV is assumed for the ionization energy of the pyrrolyl radical, which is therefore determined to be IE = 9.11 ± 0.02 eV. The 400 cm−1 (50 meV) progression observed at higher photon energies is assigned to an out-of-plane ring deformation mode of the nitrogen atom on the triplet potential energy surface (cf. a computed value of 393 cm−1). Setting the origin of this progression to 9.43 eV yields a good agreement of experiment and simulation. The ionization energy to the triplet cation is therefore assigned to be 9.43 ± 0.05 eV. A relatively high error bar is assumed, which equals the energy of one quantum of the excited deformation mode. The small singlet/ triplet gap indicates the biradicaloid character of the pyrrolyl cation. The Franck−Condon simulation for pyrrolyl does not explain the observed structure between 8.8 and 9.0 eV, where two peaks at 8.84 and 8.87 eV are observed. Hot and sequence bands are expected at energies below the 0−0 transition due to

stabilized intermediates. Especially the open chain intermediate is energetically disfavored because of the proximity of the nitrogen atom to the radical center. In comparison to the decomposition of C5H5 to acetylene and propargyl, the activation barrier for the pyrrolyl decomposition is lower by about 120 kJ mol−1 (C2H2 + cyanomethyl) and 50 kJ mol−1 (HCN + propargyl).27,50,51 The largest difference in activation energy compared to the C5H5 radical is observed for the cyanomethyl and acetylene formation, which mirrors the fact that the formation of a CN bond is more exothermic than the formation of a CCH bond. In addition, the bond energy of a CC bond is higher compared to the CN single bond, leading to smaller barriers and higher product stabilization in the latter case. So far, HCN has been assumed to be the main nitrogen-containing decomposition product of pyrrolyl.4 The identification of cyanomethyl in this experiment and the mechanistic studies show, though, that there is a second at least equally important decomposition pathway of pyrrolyl. Note that the activation barriers for the decomposition of pyrrolyl are lower than the barrier for the methyl cleavage from the precursor; i.e., the lifetime of the pyrrolyl radical should be very short. Possible reasons why pyrrolyl is nevertheless observed are the tight transition states and relatively stable intermediates leading to the decomposition products, which leads to longer lifetimes at the same internal energy in comparison to the precursor. In addition, pyrrolyl has fewer rotational and vibrational degrees of freedom than 3-methoxypyridine and therefore the internal energy is lower at the same temperature. This also serves to illustrate how the generation of a radical upon pyrolysis relies essentially on the choice of precursor and its thermal decomposition energetics. b. Threshold Photoelectron Spectrum. The photoion mass-selected threshold photoelectron spectrum of the m/z = 66 peak was recorded to confirm the assignment of the pyrolysis product channel as yielding the pyrrolyl radical. As seen in Figure 2, a sharp peak at 9.11 eV is observed in the TPE spectrum. Although the signal/noise ratio is rather low, further broad bands are visible at 9.21, 9.31, and 9.43 eV, i.e., roughly

Figure 2. Mass-selected TPE spectrum of m/z = 66 and Franck− Condon simulation for the pyrrolyl radical (blue sticks, singlet cation (X̃ + 1A); green sticks, triplet cation (ã+ 3A); red line, stick spectrum convoluted with a Gaussian of 20 meV fwhm) show a good agreement. The ionization energy of pyrrolyl is determined to be 9.11 ± 0.02 eV. The bands at 8.84 and 8.87 eV marked by arrows are assigned to (E)and (Z)-1-cyanoallyl. D


Article

The Journal of Physical Chemistry A int int E ion = Eneutral + hν − E kin(e−) − IE

inefficient cooling of the radicals produced in the pyrolysis. However, the red shift of these bands relative to the origin is too large for this explanation. Hence, the contribution of other isomers has to be considered. Sixteen further structures of the composition C4H4N were found to have both stable neutral and cationic states. Computed ionization energies for all of these isomers are given in the Supporting Information. Three isomers were found to have almost the same absolute energy as the pyrrolyl radical, within the accuracy of the employed computational level: (E)-1-cyanoallyl, (Z)-1-cyanoallyl, and 2-cyanoallyl. Although a CN or CH2 shift has to occur for the formation of 2-cyanoallyl, only an H atom migration is necessary for the formation of the other two isomers (see Scheme 3 for the

(3)

Figure 3 shows the experimental breakdown curve, i.e., a plot of the fractional abundance of parent and daughter TPE signal

Scheme 3. Chemical Structures of (E)-1-Cyanoallyl and (Z)1-Cyanoallyl

chemical structures). The Franck−Condon simulation of 2cyanoallyl yields no convincing match with the experimental spectrum, although the computed ionization energy IE(2cyanoallyl) = 8.94 eV lies in the expected region. The ionization energies of (E)- and (Z)-1-cyanoallyl are also computed to lie in that energy region (IE(E-isomer) = 8.92 eV, IE(Z-isomer) = 8.96 eV). The respective Franck−Condon simulations predict no distinct vibrational structure as the geometries of the two stereoisomers do not change significantly upon ionization, which is in agreement with the observation of two individual peaks separated by about 30 meV. Two close-lying stereoisomers have also been observed in the TPE spectrum of 1methylallyl.56 It is thus imaginable that the product of the ring opening at 127.4 kJ mol−1 (Scheme 2) undergoes a [1,3]sigmatropic hydrogen shift to both 1-cyanoallyl isomers. A contribution of (E)- and (Z)-1-cyanoallyl to the ms-TPE signal of m/z = 66 is therefore assumed and the ionization energies of (E)-1-cyanoallyl and (Z)-1-cyanoallyl (see Scheme 3 for the chemical structures) are tentatively assigned to 8.84 ± 0.02 and 8.87 ± 0.02 eV. These values for the cyanoallyl stereoisomers deviate significantly from the ionization energy of 9.17 ± 0.05 eV reported previously from PIMS experiments. In comparison to TPES, PIMS lacks the ability to resolve isomer specific features. On the basis of our data, the IE of the pyrrolyl radical of 9.11 eV matches the signal in the m/z = 66 channel in the PIMS experiments much better than cyanoallyl. c. Dissociative Photoionization of 3-Methoxypyridine. The photoionization of the precursor was studied to determine appearance energies (AE) of fragment ions, which can be used to derive thermochemical data such as heats of formation.57−60 The ionization energy of 3-methoxypyridine was determined to be IE = 8.63 ± 0.02 eV from the TPE spectrum (Supporting Information). Interestingly, the TPES is noticeably red-shifted compared to the conventional PES study reported previously.61 The precursor dissociatively ionizes above 11.5 eV, yielding four fragment ions. Dissociative photoionization can be observed when the internal energy of the ion is above the bond dissociation energy or the energy of the highest transition state involved in the dissociation mechanism. The ions’ internal energy Eint ion can be derived from the internal energy of the neutral Eint neutral and the difference of photon energy hν and ionization energy IE, when only threshold electrons (Ekin(e−) = 0) are considered:

Figure 3. Breakdown diagram and fitted appearance energies of four parallel channels in the dissociative photoionization of 3-methoxypyridine (symbols, experimental data; lines, fit). The total TPE signal is depicted in the background (thin black line).

against the photon energy. The parent cation (m/z = 109) starts to decrease at a photon energy of about 11.7 eV. Three fragment ions appear simultaneously in the breakdown diagram: m/z = 108, corresponding to hydrogen atom loss, m/z = 94, which is formed after cleavage of methyl, and m/z = 79, which has to coincide with a neutral fragment of 30 amu. Around 12.5 eV, these three mass channels slowly decrease while the signal at m/z = 66 is rising, which would correspond to C4H4N+ and the neutral fragments CH3 and CO. As the slope of the m/z = 66 abundance curve in the breakdown diagram is rather steep and none of the other three decreases with the same but negative slope, but in fact they decrease similarly, it is assumed that the emerging pyrrolyl cation fragment is another parallel channel and not a sequential one from the m/z = 94 daughter ion. The AE of a daughter ion does not only depend on the internal energy of the dissociating ion but also on the thermal energy distribution of the neutral molecules (room temperature sample) and the dissociation rate. The latter one leads to a kinetic shift of the observed dissociation onsets in the breakdown curve. Dissociation rates in the range 104−107 s−1 manifest themselves in broad and asymmetric time-of-flight distributions, because the parent ion stays intact in the ionization volume, and decomposes while it is accelerated in an electric field. 40,62 The time-of-flight distributions (Supporting Information) become more symmetric and less broad with increasing internal energy of the ion. In this experiment, the TOF distributions for all four channels show a distinct asymmetry (Supporting Information), confirming the presence of a kinetic shift. The appearance energies of the observed daughter ions can be determined by modeling the unimolecular dissociation rates employing statistical rate theory. Transition-state theory is a suitable tool to compute the rate constants k(E) utilizing the following equation. k(E) = E

σN ‡(E − AE0K ) hρ(E)

(4) DOI: 10.1021/acs.jpca.5b10743 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A Scheme 4. Parallel Dissociative Photoionization Pathways of 3-Methoxypyridine Computed by CBS-QB3

Here, N‡ is the number of states in the transition state, ρ(E) the density of states of the parent ion, σ a degeneracy factor for equivalent reaction pathways, and h Planck’s constant. Numbers and densities of state can be easily computed from vibrational frequencies and rotational constants of the relevant structures. Therefore, the dissociation pathways have to be modeled employing quantum chemistry. On the basis of the shape and abundances of the experimental breakdown curves (vide supra), we assume that there are four parallel dissociation channels for the DPI of 3-methoxypyridine leading to the observed daughter ions m/z = 108, 94, 79, and 66. As depicted in Scheme 4, the hydrogen loss was found to occur on the methyl group, and an appearance energy of 11.43 eV was computed for the m/z = 108 fragment ion. For this dissociation channel as well as for the m/z = 94 methyl loss channel, relaxed coordinate scans of the corresponding bonds predict a dissociation without reverse barrier. The appearance energy for the pyridoxy cation (m/z = 94) was computed to be 11.73 eV. A hydrogen shift has to occur for the formation of the fragment ion m/z = 79, pyridine cation. As the associated transition state is higher in energy than the products, pyridine cation and neutral formaldehyde, the appearance energy corresponds to the transition-state energy, computed at 11.47 eV. Thus, no bond energy can be determined on the basis of the breakdown diagram modeling, because the reverse barrier is not known experimentally. A neutral fragment of the composition C2H3O (43 amu), i.e., COCH3 or CO + CH3, is most likely cleaved off in the dissociation channel leading to m/ z = 66, the pyrrolyl cation. Hence, a rearrangement of the dissociating ion to a five-membered ring system has to take place first as depicted in Scheme 4. Similar to the formation of neutral pyrrolyl in the pyrolysis, a stable five-membered ring intermediate forms, which then dissociates to the pyrrolyl cation, carbon monoxide and methyl. The last step is not associated with a reverse barrier and the product energy is higher than the transition state leading to the ring rearrangement. The computed appearance energy for m/z = 66 is 12.65 eV. The breakdown diagram and TOF distributions were then fitted employing statistical rate theory (details on the procedure can be found in the Supporting Information). The fit of the breakdown diagram shown in Figure 3 enables us to determine appearance energies for the four parallel dissociation channels. The error bars for the appearance energies were obtained by varying one barrier and fitting all other parameters and

determining the highest and lowest barrier to yield an acceptable fit. Because the pathways to the fragment ions m/z = 108 and 94 do not possess reverse barriers, bond dissociation energies in the 3-methoxypyridine cation can be directly determined from the AEs. The obtained values are BDE(Cmethyl−H) = 269 ± 24 kJ mol−1 (2.79 ± 0.25 eV) and BDE(O−Cmethyl) = 298 ± 5 kJ mol−1 (3.09 ± 0.05 eV). In addition, dissociative photoionization to the pyrrolyl cation is neither associated with a reverse barrier. Hence, it is possible to determine the enthalpy of formation of the pyrrolyl radical ΔfH0K(C4H4N) via eq 3 utilizing the ionization and appearance energy determined in this study: Δf H0K(C4H4N) = Δf H0K(C6H 7NO) + AE0K (C4 H4N+) − Δf H0K(CH3) − Δf H0K(CO) − IE(C4 H4N)

(5)

Tabulated values for the heat of formation of methyl and carbon monoxide at 0 K were employed (149.03 ± 0.10 and −113.81 ± 0.17 kJ mol−1).63 For the precursor, though, only an estimated value at 298 K of −10.8 ± 5.1 kJ mol−1 is reported in the literature,64 which was converted to 0 K using the heat capacity as described by Chase.63 Following eq 5, the heat of formation for the pyrrolyl radical at 0 K was determined to be ΔfH0K(C4H4N) = 301 ± 14 kJ mol−1. A comparison of all appearance energies obtained by the fit of the breakdown diagram with computations is shown in Table 1. Although fit and computations agree well for the m/z = 94 and 79 daughter ions, the computations slightly overestimate the AE of the m/z = 108 and 66 fragment ions. The determination of the experimental fractional abundance for the m/z = 108 channel is complicated by the overlap with the parent ion peak (m/z = 109, see Supporting Information for Table 1. Comparison of the Appearance Energies Obtained by a Fit of the Experimental Data with Computed Ones on the CBS-QB3 Level of Theory m/z 108 94 79 66 F

AEfit /eV

AEcalc /eV

± ± ± ±

11.43 11.73 11.47 12.65

11.23 11.68 11.48 12.35

0.25 0.05 0.02 0.03


Article

The Journal of Physical Chemistry A details), which might explain the imperfect fit. The heat of formation derived from the appearance energy value for the m/ z = 66 daughter ion agrees excellently with the one determined previously by Ashfold and co-workers (301.9 ± 0.5 kJ mol−1),21 which supports the modeled dissociation mechanism. However, the suggested products, the pyrrolyl cation, methyl, and CO, are energetically disfavored compared to the other three parallel dissociation channels. One would therefore not expect the m/z = 66 channel to outcompete these other channels so quickly. The dissociative photoionization of 3-methoxypyridine is similar to that of anisole, in which fragment ions corresponding to methyl, formaldehyde, and sequential methyl + CO loss have been observed.65−68 The main argument why the m/z = 66 channel in the 3-methoxypyridine DPI is assumed to be a further parallel channel is, as stated above, that the slope of the m/z = 66 breakdown curve does not match the negative slope of m/z = 94, as would be expected for a sequential reaction. The TPES, plotted in the background in Figure 3, shows a band onset at about the same photon energy where the m/z = 66 channel appears. Thus, it is possible that this fragment arises as an apparent parallel channel because the electronically excited state opens up a new, nonstatistical CH3 + CO loss channel at this energy.69 The m/z = 94 daughter ion breakdown curve would then be amplified by the nonstatistical pathway and subsequently decrease because of a prompt sequential dissociation, leading to m/z = 66 and yielding the observed breakdown diagram. Without a detailed analysis of the kinetic energy release, we cannot completely exclude this “amplified sequential dissociation” mechanism, which would result in a blue shift in the model appearance energy. However, although bringing the AE more in line with the computed threshold (Table 1), the agreement of the derived heat of formation with the ΔfH0K(C4H4N) literature value would worsen in this alternative model.

potential energy surface was constructed. In addition, the ionization and dissociative ionization of the precursor 3methoxypyridine was also studied and an ionization energy of 8.63 ± 0.02 eV was measured. Four parallel decomposition channels were modeled by applying statistical theory to get the appearance energies of the fragment cations with the pyrrolyl ion among them at a tentative AE of 12.35 ± 0.03 eV. Although the underlying mechanism is not definitive, which could affect the derived AE, the heat of formation of the pyrrolyl radical, derived on the basis of the appearance energy via a thermochemical cycle to a value of ΔfH0K(C4H4N) = 301 ± 14 kJ mol−1, is in very good agreement with previous values.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b10743. Computed ionization energies of C4H4N isomers, TPE spectrum of 3-methoxypyridine, details on the dissociative photoionization of 3-methoxypyridine (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*I. Fischer. E-mail: ingo.fi[email protected]. *P. Hemberger. E-Mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The experiments were conducted at the VUV beamline of the Swiss Light Source storage ring, Paul Scherrer Institute. We thank Bálint Sztáray for his suggestions regarding the dissociative photoionization mechanism of the precursor. This work was supported by the Deutsche Forschungsgemeinschaft, contract FI575/7-3 and the Swiss Federal Office for Energy (BFE Contract Number 101969/152433 & SI/501269-01). Discussions with colleagues within the research training school GRK 2112 (Molecular Biradicals) are acknowledged. Travel subsidies were provided by the European Commission program “CALIPSO Transnational Access”. All computations were performed at the Linux-cluster of the Leibniz-Rechenzentrum der Bayerischen Akademie der Wissenschaften (LRZ).

■

CONCLUSIONS The pyrrolyl radical C4H4N was successfully generated in its singlet electronic ground state by pyrolysis of 3-methoxypyridine and characterized by threshold photoelectron spectroscopy. The precursor decomposes to 3-pyridoxy and methyl upon pyrolysis, of which the former further fragments to carbon monoxide and the pyrrolyl radical. The threshold photoelectron spectrum of the radical was recorded and an ionization energy of 9.11 ± 0.02 eV was determined for the singlet cation. The excitation energy to the triplet state was determined to be 9.43 ± 0.05 eV. The small singlet/triplet gap of only 0.31 eV confirms a biradicaloid character of the pyrrolyl cation. The TPE spectrum features two progressions of 800 and 400 cm−1, which can be assigned to ring deformation modes in the singlet and triplet cation, respectively. The pyrolysis of 3-methoxypyridine at 550 °C does not exclusively lead to the cyclic C4H4N isomer. (E)- and (Z)-1-cyanoallyl were additionally identified as carriers of the low-energy part of the spectrum. The corresponding ionization energies were determined to be 8.84 ± 0.02 eV for the (E)-isomer and 8.87 ± 0.02 eV for (Z)-1cyanoallyl, respectively. We identified two thermal decomposition pathways of the pyrrolyl radical, namely to (a) propargyl + hydrogen cyanide and to (b) cyanomethyl + acetylene. The latter has not been considered in the literature so far, which might be due to the fact that cyanomethyl (m/z = 40) is difficult to be detected in the close proximity of allene and propyne. 4,14 Both decomposition pathways were computed and the C4H4N

■

REFERENCES

(1) Glarborg, P.; Jensen, A. D.; Johnsson, J. E. Fuel Nitrogen Conversion in Solid Fuel Fired Systems. Prog. Energy Combust. Sci. 2003, 29, 89−113. (2) Perry, R. A.; Siebers, D. L. Rapid Reduction of Nitrogen Oxides in Exhaust Gas Streams. Nature 1986, 324, 657−658. (3) Ikeda, E.; Nicholls, P.; Mackie, J. C. A Kinetic Study of the Oxidation of Pyridine. Proc. Combust. Inst. 2000, 28, 1709−1716. (4) Tian, Z.; Li, Y.; Zhang, T.; Zhu, A.; Cui, Z.; Qi, F. An Experimental Study of Low-Pressure Premixed Pyrrole/Oxygen/ Argon Flames with Tunable Synchrotron Photoionization. Combust. Flame 2007, 151, 347−365. (5) Taatjes, C. A.; Osborn, D. L.; Cool, T. A.; Nakajima, K. Synchrotron Photoionization Measurements of Combustion Intermediates: The Photoionization Efficiency of HONO. Chem. Phys. Lett. 2004, 394, 19−24. (6) Taatjes, C. A.; Hansen, N.; Osborn, D. L.; Kohse-Höinghaus, K.; Cool, T. A.; Westmoreland, P. R. ″Imaging″ Combustion Chemistry Via Multiplexed Synchrotron-Photoionization Mass Spectrometry. Phys. Chem. Chem. Phys. 2008, 10, 20−34.

G


Article

The Journal of Physical Chemistry A (7) Oßwald, P.; Hemberger, P.; Bierkandt, T.; Akyildiz, E.; Köhler, M.; Bodi, A.; Gerber, T.; Kasper, T. In Situ Flame Chemistry Tracing by Imaging Photoelectron Photoion Coincidence Spectroscopy. Rev. Sci. Instrum. 2014, 85, 025101. (8) Felsmann, D.; Moshammer, K.; Krüger, J.; Lackner, A.; Brockhinke, A.; Kasper, T.; Bierkandt, T.; Akyildiz, E.; Hansen, N.; Lucassen, A.; et al. Electron Ionization, Photoionization and Photoelectron/Photoion Coincidence Spectroscopy in Mass-Spectrometric Investigations of a Low-Pressure Ethylene/Oxygen Flame. Proc. Combust. Inst. 2015, 35, 779−786. (9) Holzmeier, F.; Lang, M.; Hader, K.; Hemberger, P.; Fischer, I. H2CN+ and H2CNH+: New Insight into the Structure and Dynamics from Mass-Selected Threshold Photoelectron Spectra. J. Chem. Phys. 2013, 138, 214310. (10) Holzmeier, F.; Lang, M.; Fischer, I.; Tang, X.; Cunha de Miranda, B.; Romanzin, C.; Alcaraz, C.; Hemberger, P. Threshold Photoelectron Spectroscopy of Unstable N-Containing Compounds: Resolution of ΔK Subbands in HNCO+ and Vibrational Resolution in NCO+. J. Chem. Phys. 2015, 142, 184306. (11) Holzmeier, F.; Lang, M.; Fischer, I.; Hemberger, P.; Garcia, G. A.; Tang, X.; Loison, J. C. Assignment of High-Lying Bending Mode Levels in the Threshold Photoelectron Spectrum of NH2: A Comparison between Pyrolysis and Fluorine-Atom Abstraction Radical Sources. Phys. Chem. Chem. Phys. 2015, 17, 19507−19514. (12) Holzmeier, F.; Lang, M.; Fischer, I.; Hemberger, P. The Threshold Photoelectron Spectrum of Cyanovinylacetylene Leads to an Upward Revision of the Ionization Energy. Chem. Phys. Lett. 2015, 638, 201−204. (13) Schüßler, T.; Deyerl, H.-J.; Dümmler, S.; Fischer, I.; Alcaraz, C.; Elhanine, M. The VUV Photochemistry of the Allyl Radical Investigated Using Synchrotron Radiation. J. Chem. Phys. 2003, 118, 9077−9080. (14) Hong, X.; Zhang, L.; Zhang, T.; Qi, F. An Experimental and Theoretical Study of Pyrrole Pyrolysis with Tunable Synchrotron VUV Photoionization and Molecular-Beam Mass Spectrometry. J. Phys. Chem. A 2009, 113, 5397−5405. (15) Bacskay, G. B.; Martoprawiro, M.; Mackie, J. C. An Ab Initio Quantum Chemical Study of the Electronic Structure and Stability of the Pyrrolyl Radical: Comparison with the Isoelectronic Cyclopentadienyl Radical. Chem. Phys. Lett. 1998, 290, 391−398. (16) Blank, D. A.; North, S. W.; Lee, Y. T. The Ultraviolet Photodissociation Dynamics of Pyrrole. Chem. Phys. 1994, 187, 35− 47. (17) Wei, J.; Kuczmann, A.; Riedel, J.; Renth, F.; Temps, F. Photofragment Velocity Map Imaging of H Atom Elimination in the First Excited State of Pyrrole. Phys. Chem. Chem. Phys. 2003, 5, 315− 320. (18) Wei, J.; Riedel, J.; Kuczmann, A.; Renth, F.; Temps, F. Photodissociation Dynamics of Pyrrole: Evidence for Mode Specific Dynamics from Conical Intersections. Faraday Discuss. 2004, 127, 267−282. (19) Mackie, J. C.; Colket, M. B.; Nelson, P. F.; Esler, M. Shock Tube Pyrolysis of Pyrrole and Kinetic Modeling. Int. J. Chem. Kinet. 1991, 23, 733−760. (20) Santos, O.; Testa, A. C.; O’Sullivan, M. Flash Photolysis Study of 1-Phenylpyrrole. J. Photochem. 1983, 23, 257−263. (21) Cronin, B.; Nix, M. G. D.; Qadiri, R. H.; Ashfold, M. N. R. High Resolution Photofragment Translational Spectroscopy Studies of the near Ultraviolet Photolysis of Pyrrole. Phys. Chem. Chem. Phys. 2004, 6, 5031−5041. (22) Gianola, A. J.; Ichino, T.; Hoenigman, R. L.; Kato, S.; Bierbaum, V. M.; Lineberger, W. C. Thermochemistry and Electronic Structure of the Pyrrolyl Radical. J. Phys. Chem. A 2004, 108, 10326−10335. (23) Motzke, A.; Lan, Z.; Woywod, C.; Domcke, W. Simulation of the Photodetachment Spectrum of the Pyrrolide Anion. Chem. Phys. 2006, 329, 50−64. (24) Zhu, X.; Yarkony, D. R. The Photoelectron Spectrum of Pyrrolide: Nonadiabatic Effects Due to Conical Intersections. J. Phys. Chem. C 2010, 114, 5312−5320.

(25) Friderichsen, A. V.; Shin, E.-J.; Evans, R. J.; Nimlos, M. R.; Dayton, D. C.; Ellison, G. B. The Pyrolysis of Anisole (C6H5OCH3) Using a Hyperthermal Nozzle. Fuel 2001, 80, 1747−1755. (26) Scheer, A. M.; Mukarakate, C.; Robichaud, D. J.; Ellison, G. B.; Nimlos, M. R. Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth. J. Phys. Chem. A 2010, 114, 9043−9056. (27) Mackie, J. C.; Doolan, K. R.; Nelson, P. F. Kinetics of the Thermal Decomposition of Methoxybenzene (Anisole). J. Phys. Chem. 1989, 93, 664−670. (28) Lister, T.; Prager, R. H.; Tsaconas, M.; Wilkinson, K. L. On the Thermally Induced Rearrangement of 2-Alkoxypyridines to NAlkylpyridones. Aust. J. Chem. 2003, 56, 913−916. (29) Bodi, A.; Johnson, M.; Gerber, T.; Gengeliczki, Z.; Sztáray, B.; Baer, T. Imaging Photoelectron Photoion Coincidence Spectroscopy with Velocity Focusing Electron Optics. Rev. Sci. Instrum. 2009, 80, 034101. (30) Bodi, A.; Hemberger, P.; Gerber, T.; Sztáray, B. A New Double Imaging Velocity Focusing Coincidence Experiment: i2PEPICO. Rev. Sci. Instrum. 2012, 83, 083105. (31) Johnson, M.; Bodi, A.; Schulz, L.; Gerber, T. Vacuum Ultraviolet Beamline at the Swiss Light Source for Chemical Dynamics Studies. Nucl. Instrum. Methods Phys. Res., Sect. A 2009, 610, 597−603. (32) Kohn, D. W.; Clauberg, H.; Chen, P. Flash Pyrolysis Nozzle for Generation of Radicals in a Supersonic Jet Expansion. Rev. Sci. Instrum. 1992, 63, 4003−4005. (33) Bodi, A.; Sztaray, B.; Baer, T.; Johnson, M.; Gerber, T. Data Acquisition Schemes for Continuous Two-Particle Time-of-Flight Coincidence Experiments. Rev. Sci. Instrum. 2007, 78, 084102. (34) Sztáray, B.; Baer, T. Supression of Hot Electrons in Threshold Photoelectron Photoion Coincidence Spectroscopy Using Velocity Focusing Optics. Rev. Sci. Instrum. 2003, 74, 3763−3768. (35) Fogleman, E. A.; Koizumi, H.; Kercher, J. P.; Sztáray, B.; Baer, T. Heats of Formation of the Acetyl Radical and Ion Obtained by Threshold Photoelectron Photoion Coincidence. J. Phys. Chem. A 2004, 108, 5288−5294. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009. (37) Montgomery, J. J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A Complete Basis Set Model Chemistry. VI. Use of Density Functional Geometries and Frequencies. J. Chem. Phys. 1999, 110, 2822−2827. (38) Montgomery, J. J. A.; Frisch, M. J.; Ochterski, J. W.; Petersson, G. A. A Complete Basis Set Model Chemistry. VII. Use of the Minimum Population Localization Method. J. Chem. Phys. 2000, 112, 6532−6542. (39) Spangenberg, D.; Imhof, P.; Kleinermanns, K. The S1 State Geometry of Phenol Determined by Simultaneous Franck-Condon and Rotational Constants Fits. Phys. Chem. Chem. Phys. 2003, 5, 2505−2514. (40) Sztáray, B.; Bodi, A.; Baer, T. Modeling Unimolecular Reactions in Photoelectron Photoion Coincidence Experiments. J. Mass Spectrom. 2010, 45, 1233−1245. (41) Schulenburg, A. M.; Alcaraz, C.; Grassi, G.; Merkt, F. Rovibrational Photoionization Dynamics of Methyl and Its Isotopomers Studied by High-Resolution Photoionization and Photoelectron Spectroscopy. J. Chem. Phys. 2006, 125, 104310. (42) Cunha de Miranda, B. K.; Alcaraz, C.; Elhanine, M.; Noller, B.; Hemberger, P.; Fischer, I.; Garcia, G. A.; Soldi-Lose, H.; Gans, B.; Vieira Mendes, L. A.; et al. Threshold Photoelectron Spectroscopy of the Methyl Radical Isotopomers, CH3, CH2D, CHD2 and CD3: Synergy between VUV Synchrotron Radiation Experiments and Explicitly Correlated Coupled Cluster Calculations. J. Phys. Chem. A 2010, 114, 4818−4830. (43) Derrick, P. J.; Åsbrink, L.; Edqvist, O.; Jonsson, B.-Ö .; Lindholm, E. Rydberg Series in Small Molecules: XII. Photoelectron H


Article

The Journal of Physical Chemistry A Spectroscopy and Electronic Structure of Pyrrole. Int. J. Mass Spectrom. Ion Phys. 1971, 6, 191−202. (44) Baker, A. D.; Betteridge, D.; Kemp, N. R.; Kirby, R. E. Application of Photoelectron Spectrometry to Pesticide Analysis. Photoelectron Spectra of Five-Membered Heterocycles and Related Molecules. Anal. Chem. 1970, 42, 1064−1073. (45) Hemberger, P.; Lang, M.; Noller, B.; Fischer, I.; Alcaraz, C.; Cunha de Miranda, B. K.; Garcia, G. A.; Soldi-Lose, H. Photoionization of Propargyl and Bromopropargyl Radicals: A Threshold Photoelectron Spectroscopic Study. J. Phys. Chem. A 2011, 115, 2225− 2230. (46) Olivella, S.; Sole, A.; Garcia-Raso, A. Ab Initio Calculations of the Potential Surface for the Thermal Decomposition of the Phenoxyl Radical. J. Phys. Chem. 1995, 99, 10549−10556. (47) Guan, Q.; Urness, K. N.; Ormond, T. K.; David, D. E.; Barney Ellison, G.; Daily, J. W. The Properties of a Micro-Reactor for the Study of the Unimolecular Decomposition of Large Molecules. Int. Rev. Phys. Chem. 2014, 33, 447−487. (48) Bouwman, J.; Bodi, A.; Oomens, J.; Hemberger, P. On the Formation of Cyclopentadiene in the C3H5· + C2H2 Reaction. Phys. Chem. Chem. Phys. 2015, 17, 20508−20514. (49) Liang, S.; Hemberger, P.; Neisius, N. M.; Bodi, A.; Grützmacher, H.; Levalois-Grützmacher, J.; Gaan, S. Elucidating the Thermal Decomposition of Dimethyl Methylphosphonate by Vacuum Ultraviolet (VUV) Photoionization: Pathways to the PO Radical, a Key Species in Flame-Retardant Mechanisms. Chem. - Eur. J. 2015, 21, 1073−1080. (50) Hemberger, P.; da Silva, G.; Trevitt, A. J.; Gerber, T.; Bodi, A. Are the Three Hydroxyphenyl Radical Isomers Created Equal? - the Role of the Phenoxy Radical. Phys. Chem. Chem. Phys. 2015, 17, 30076−30083. (51) Moskaleva, L. V.; Lin, M. C. Unimolecular Isomerization/ Decomposition of Cyclopentadienyl and Related Bimolecular Reverse Process: Ab Initio MO/Statistical Theory Study. J. Comput. Chem. 2000, 21, 415−425. (52) Wörner, H. J.; Merkt, F. Photoelectron Spectroscopic Study of the First Singlet and Triplet States of the Cyclopentadienyl Cation. Angew. Chem., Int. Ed. 2006, 45, 293−296. (53) Wörner, H. J.; Merkt, F. Diradicals, Antiaromaticity, and the Pseudo-Jahn-Teller Effect: Electronic and Rovibronic Structures of the Cyclopentadienyl Cation. J. Chem. Phys. 2007, 127, 034303. (54) Bodi, A.; Shuman, N. S.; Baer, T. On the Ionization and Dissociative Photoionization of Iodomethane: A Definitive Experimental Enthalpy of Formation of CH3I. Phys. Chem. Chem. Phys. 2009, 11, 11013−11021. (55) Chupka, W. A. Factors Affecting Lifetimes and Resolution of Rydberg States Observed in Zero-Electron-Kinetic-Energy Spectroscopy. J. Chem. Phys. 1993, 98, 4520−4530. (56) Lang, M.; Holzmeier, F.; Hemberger, P.; Fischer, I. Threshold Photoelectron Spectra of Combustion Relevant C4H5 and C4H7 Isomers. J. Phys. Chem. A 2015, 119, 3995−4000. (57) Baer, T. Ion Dissociation Dynamics and Thermochemistry by Photoelectron Photoion Coincidence (PEPICO) Spectroscopy. Int. J. Mass Spectrom. 2000, 200, 443−457. (58) Lago, A. F.; Baer, T. A Photoelectron Photoion Coincidence Study of the Vinyl Bromide and Tribromoethane Ion Dissociation Dynamics: Heats of Formation of C2H3+, C2H3Br, C2H3Br+, C2H3Br2+, and C2H3Br3+. J. Phys. Chem. A 2006, 110, 3036−3041. (59) Shuman, N. S.; Bodi, A.; Baer, T. Heats of Formation of t-Butyl Peroxy Radical and t-Butyl Diazyl Ion: RRKM vs SSACM Rate Theories in Systems with Kinetic and Competitive Shifts. J. Phys. Chem. A 2010, 114, 232−240. (60) Borkar, S.; Sztáray, B. Self-Consistent Heats of Formation for the Ethyl Cation, Ethyl Bromide, and Ethyl Iodide from Threshold Photoelectron Photoion Coincidence Spectroscopy. J. Phys. Chem. A 2010, 114, 6117−6123. (61) Dunne, S. J.; Summers, L. A.; von Nagy-Felsobuki, E. I. Photoelectron Spectra of the Isomers of Methoxypyridine. Org. Mass Spectrom. 1993, 28, 316−320.

(62) Stevens, W.; Sztáray, B.; Shuman, N.; Baer, T.; Troe, J. Specific Rate Constants k(E) of the Dissociation of the Halobenzene Ions: Analysis by Statistical Unimolecular Rate Theories. J. Phys. Chem. A 2009, 113, 573−582. (63) Chase, M. W. NIST-Janaf Thermochemical Tables, 4th ed. J. Phys. Chem. Ref. Data, Monograph 9 1998. (64) Amaral, L. M. P. F.; Ribeiro da Silva, M. A. V. Thermochemistry of Some Methoxypyridines. J. Chem. Thermodyn. 2012, 48, 65−69. (65) McLafferty, F. W.; Bente, P. F.; Kornfeld, R.; Tsai, S.-C.; Howe, I. Metastable Ion Characteristics. XXII. Collisional Activation Spectra of Organic Ions. J. Am. Chem. Soc. 1973, 95, 2120−2129. (66) Fisher, I. P.; Palmer, T. F.; Lossing, F. P. The Vertical Ionization Potentials of Phenyl and Phenoxy Radicals. J. Am. Chem. Soc. 1964, 86, 2741−2742. (67) Harrison, A. G.; Haynes, P.; McLean, S.; Meyer, F. The Mass Spectra of Methyl-Substituted Cyclopentadienes. J. Am. Chem. Soc. 1965, 87, 5099−5105. (68) Das, P. R.; Gilman, J. P.; Meisels, G. G. Unimolecular Decomposition of Energy-Selected Anisole Ions. Breakdown Graph and Metastable Decay Rates. Int. J. Mass Spectrom. Ion Processes 1986, 68, 155−165. (69) Borkar, S.; Sztaray, B.; Bodi, A. Dissociative Photoionization Mechanism of Methanol Isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: Energetics, Statistical and Non-Statistical Kinetics and Isotope Effects. Phys. Chem. Chem. Phys. 2011, 13, 13009−13020.

I


Pyrolysis of 3-Methoxypyridine. Detection and Characterization of the

Recommend Documents