Article pubs.acs.org/JPCA
Decomposition of Diazomeldrum’s Acid: A Threshold Photoelectron Spectroscopy Study Melanie Lang,† Fabian Holzmeier,† Ingo Fischer,*,† and Patrick Hemberger*,‡ †
Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany Molecular Dynamics Group, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland
‡
S Supporting Information *
ABSTRACT: Derivatives of meldrum’s acid are known precursors for a number of reactive intermediates. Therefore, we investigate diazomeldrum’s acid (DMA) and its pyrolysis products by photoionization using vacuum ultraviolet (VUV) synchrotron radiation. The threshold photoelectron spectrum of DMA yields an ionization energy (IE) of 9.68 eV. Several channels for dissociative photoionization are observed. The first one is associated with loss of CH3, leading to a daughter ion with m/z = 155. Its appearance energy AE0K was determined to be 10.65 eV by fitting the experimental data using statistical theory. A second parallel channel leads to m/z = 69, corresponding to N2CHCO, with an AE0K of 10.72 eV. Several other channels open up at higher energy, among them the formation of acetone cation, a channel expected to be the result of a Wolff-rearrangement (WR) in the cation. When diazomeldrum’s acid is heated in a pyrolysis reactor, three thermal decomposition pathways are observed. The major one is well-known and yields acetone, N2 and CO as consequence of the WR. However, two further channels were identified: The formation of 2-diazoethenone, NNCCO, together with acetone and CO2 as the second channel and E-formylketene (OCCHCHCO), propyne, N2 and O2 as a third one. 2-Diazoethenone and E-formylketene were identified based on their threshold photoelectron spectra and accurate ionization energies could be determined. Ionization energies for several isomers of both molecules were also computed. One of the key findings of this study is that acetone is observed upon decomposition of DMA in the neutral as well as in the ion and both point to a Wolff rearrangement to occur. However, the ion is subject to other decomposition channels favored at lower internal energies.
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INTRODUCTION Meldrum’s acid (OC1OC(OC(O)C1)(C)C) and its derivates are known to be suitable precursors for the generation of reactive intermediates, such as carbenes, ketenes, and propadienones.1−4 Here we employ diazomeldrum’s acid (DMA), a α-diazo carbonyl compound depicted in Scheme 1, and study its photoionization and decomposition in a pyrolysis reactor by VUV synchrotron radiation. The chemistry of DMA
is of particular interest due to its ability to undergo a Wolff rearrangement (WR) after thermal or photochemical excitation yielding a ketene intermediate that further decomposes to acetone.5 This induced rearrangement after nitrogen loss is utilized, i.e., in deep UV positive photoresists. 6 The spectroscopic investigations of DMA comprise infrared spectroscopy,7 several photolysis,7−14 and thermolysis8−10,12,13 experiments in solution, pyrolysis under vacuum conditions,15 laser ablation reactions on polymer matrices containing DMA as a dopant,16−19 theoretical investigations8,9,20 and the dynamics of the WR in the liquid21−23 as well as recently in the gas phase.24 Solution-phase studies showed that DMA loses nitrogen and rearranges to a ketene intermediate on a femtosecond time scale, but evidence for a carbene intermediate has also been found.21,23 A recent femtosecond gas phase photoionization study investigated the photoinduced nitrogen loss and reported the formation of a ketene within a few femtoseconds.24 The aim of the present study was first to investigate the photoionization and dissociative photoionization of DMA, to
Scheme 1. Chemical Structure of Diazomeldrum’s Acid (DMA)a
Received: September 15, 2014 Revised: November 4, 2014 Published: November 4, 2014
a The molecule has Cs symmetry and assumes an envelope conformation.
© 2014 American Chemical Society
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COMPUTATIONAL DETAILS Quantum chemical computations were performed with the Gaussian 09 program.46 Harmonic frequencies and rotational constants of all relevant molecules and fragments were computed at the B3LYP/6-311(2d,d,p) or the ωB97xD/6311g(d,p) level of theory. Ionization (IE) and appearance energies (AE) were both corrected for the zero-point vibrational energy. Franck−Condon simulations were performed with the program FCFit version 2.8.20.47 Transition states were located by relaxed potential energy surface scans and subsequent geometry optimization. The complete basis set composite method CBS-QB3 was used to obtain more reliable energies.48,49 For ionization energies computed by this method Montgomery et al.48 determined a mean absolute deviation of 1 kcal·mol−1 (±0.04 eV) based on a test set of more than 30 atoms and small molecules. No such information is yet available for the computed appearance energies. A 2D potential energy surface of the DMA cation was computed on the B3LYP/6311g(d,p) level of theory. Here the C1−O4 as well as the C1− N6 coordinate were fixed in steps during the scan, while the other degrees of freedom were fully optimized. Statistical rate theory, namely the rigid activated complex Rice−Ramsperger− Kassel−Marcus (RAC-RRKM) method, was applied to model breakdown curves and the ion time-of-flight distributions simultaneously to yield the appearance energies.50
derive ionization and appearance energies and to gain insight into the electronic structure and the fragmentation of the cation. By analyzing the dissociative photoionization with the imaging photoelectron photoion coincidence (iPEPICO) technique25,26 it is possible to unravel this mechanism.27 The second goal was to study the decomposition of the neutral diazomeldrum’s acid by pyrolysis and to identify the thermal reaction products. By thermal reaction in a flash pyrolysis reactor it is possible to generate reactive intermediates from a precursor molecule.28−31 This has previously been used to study several hydrocarbon radicals by threshold photoelectron spectroscopy (TPES).30,32−40 It has been shown that even mixtures of reactive molecules produced in the pyrolysis or flames can be analyzed by using photoelectron−photoion coincidence techniques.38,41,42
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Article
EXPERIMENTAL METHODS
Experiments were conducted at the imaging photoelectron photoion coincidence (iPEPICO) endstation of the X04DB beamline located at the Swiss Light Source (SLS) storage ring. Experimental details are described in the literature25,26,43 and are thus only briefly summarized in this report. A bending magnet provides the vacuum ultraviolet radiation (VUV) that is collimated onto a normal incidence monochromator with a 600 grooves/mm plane grating, offering a maximum resolution E/ ΔE of 104. A rare-gas filter was operated either with a mixture of Ne/Ar/Kr for photon energies below 12.5 eV or pure Ne above 12.5 eV to cut off higher order radiation. The 11s’ Rydberg state of Argon at 15.764 eV was used to perform the energy calibration. Diazomeldrum’s acid (5-diazo-2,2-dimethyl1,3-dioxane-4,6-dione) was purchased from TCI Europe (CAS 7270-63-5) and used without further purification. To investigate the dissociative photoionization the precursor was effusively expanded into the source chamber. Pyrolysis experiments were carried out by heating the precursor to 120 °C. The sample was seeded in Argon at a pressure of 0.6 bar passing the precursor through an electrically heated SiC tube connected to a 100 or 150 μm orifice before expansion. Details on the pyrolysis source can be found elsewhere.44 The temperature on the outside of the SiC tube was measured by a Type C thermocouple giving an accuracy of ±100 K. All species were ionized by VUV synchrotron radiation. Photoions were detected in a Wiley−McLaren time-of-flight mass spectrometer (Jordan C-726) in coincidence with photoelectrons. The latter were velocity map imaged onto a position sensitive detector (Roentdek DLD40). For the extraction of all charged particles in the flight tube a field of 120 V cm−1 was applied. Threshold photoelectrons were selected by taking only the central part of the photoelectron image into account that corresponds to electrons with a kinetic energy of less than 5 meV. The contribution of hot electrons was accounted for as described by Sztáray and Baer.45 The threshold photoelectron signal was not corrected for the photon flux, because the flux is considered to be almost constant over the investigated energy range.43 The data for the dissociative photoionization of the precursor molecule was recorded with a step size of 0.1 eV and an acquisition time of 6 min per data point. In the pyrolysis experiments the data was recorded in 5 meV steps and averaged for 3 min per data point. The pyrolysis power was chosen to yield a temperature of around 450 K in the pyrolysis region.
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RESULTS AND DISCUSSION a. Photoionization of Diazomeldrum’s Acid. The threshold photoelectron spectrum (TPES) of DMA, recorded in the energy range from 9.5 to 10.4 eV is shown in Figure 1.
Figure 1. TPE spectrum of diazomeldrum’s acid (DMA) integrated over all masses (bold black line) and Franck−Condon simulation (red line).
The ionization energy (IE) is obtained from the first peak in the spectrum at (9.68 ± 0.02) eV, which is in good agreement with the computed value of 9.70 eV (CBS-QB3). Several vibrational bands appear in the spectrum that are well represented by the Franck−Condon simulation shown as a red line in Figure 1. The 0−0 transition is accompanied by a low-wavenumber progression of the ν48+ with a computed energy of +7 meV, representing the bending motion of the C NN unit. It cannot be resolved in the spectrum. The bands at +80 meV, +170 meV, +240 meV, +280 meV and +320 meV are caused by various vibrational modes of the cation, as a result of the considerable change of several bond length and angles upon ionization. The Cs symmetry of the neutral molecule is retained 11236
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the daughter fragments in coincidence with threshold electrons as a function of the photon energy. The parent signal starts to decrease at around 10.2 eV, while the first daughter fragment, m/z = 155 appears, which corresponds to a methyl loss. At slightly higher energies a second parallel dissociation channel (m/z = 69) emerges. Mass 69 corresponds to the composition N2HC2O, generated after H atom migration of the parent cation (vide infra). A third parallel dissociation channel at m/z = 58+ (acetone cation) opens up at around 11 eV and might be explained by a WR initiated reaction. Around 12 eV the m/z= 155 signal has vanished, because a sequential reaction channel to m/z = 43 opens up, corresponding most likely to the acetyl cation. The signal at m/z = 69 also starts to decrease at around 11 eV, but does not completely disappear. Below 12 eV m/z = 43 originates either from a direct fragmentation of DMA or from a secondary decomposition of the initially formed fragment ion m/z = 155. However, above 12 eV the acetyl signal at m/z = 43 increases further and is due to the sequential decomposition of the acetone fragment ion.51 Due to the emergence of overlapping parallel and sequential dissociation channels the breakdown diagram was only modeled up to 11 eV to determine the appearance energies AE0K of fragment ions m/z = 155 and m/z = 69 (Figure 3a,b and the mechanism of dissociative ionization. In the first step a methyl loss takes place, forming the m/z = 155 daughter ion. Since the parent cation possesses Cs symmetry and assumes an envelope conformation with C5 being located above the ring plane, the two methyl substituents are not energetically equivalent. When performing redundant coordinate scans on the B3LYP/6-311g(d,p) level of theory for the loss of the C8 methyl group and the C12 methyl group (see Scheme 1 for atom labeling), we find isoenergetic transition states with a reverse barrier of only 11 kJ mol−1 and the abstraction of either the C8- or the C12-methyl group leads to an identical product geometry of m/z = 155+. In contrast, the more accurate CBSQB3 computations predict a barrierless decomposition pathway. Therefore, we did not consider the reverse barrier in the analysis of the methyl loss channel, since its impact is supposed to be negligibly small. The second fragmentation channel leads to the m/z = 69+, which cannot be associated with a simple bond breaking leading to N2C2OH or C3O2H. For both channels an H atom shift has to occur in the parent cation prior to the dissociation. To find the correct fragment cation all possible structures of mass 69
upon ionization. The most pronounced geometry changes according to ωB97xD/6-311g(d,p) computations comprise the elongation of the C1−C2 bond by about 4 pm and the decrease of the C2−O4 bond by about 4 pm (see Scheme 1 for the numbering of atoms). The CN double bond has a length of 132 pm in the neutral and increases by 3 pm upon ionization, thus developing partial single bond character in the cation. Furthermore, the C5−O4 bond increases by about 2 pm. In addition, the angle (C1−C2−O3) decreases about 5° and (O4− C2-O3) widens about 5°. A table containing all computed geometry parameters of the neutral and the cationic molecule is given in the Supporting Information (SI). The threshold photoelectron (TPE) signal increases further beyond 10.2 eV, but the structure disappears. Most likely the signal is due to an electronically excited state in the DMA cation. The dip at 10.03 eV is caused by a resonant absorption in krypton that was used in the gas filter. b. Dissociative Photionization (DPI) of Diazomeldrum’s Acid. A breakdown diagram of DMA, depicted in Figure 2, shows the fractional abundance of the parent ion and
Figure 2. Breakdown diagram of diazomeldrum’s acid. The parent cation m/z = 170+ (black dots) fragments into several parallel channels and a sequential one. The most important parallel channels are (I) loss of a methyl group (m/z = 155+, red dots), (II) the appearance of fragment m/z = 69+ (blue dots), referring to N2CHCO+, and (III) the formation of the m/z = 58 acetone ion (green dots). The formation of m/z = 43+ from direct fragmentation of the parent cation contributes to less than 5% to the breakdown diagram below 11 eV, but at higher energies m/z = 155 and 58 yield m/z = 43 daughter ions in a sequential process.
Figure 3. (a) Experimental time-of-flight (TOF) distributions (squares) of the parent cation and the fragment cations with the fitted TOF distributions (red lines) for photon energies from 10.3 to 11.0 eV. (b) Experimental (dots) and fitted (lines) breakdown curves of the first two dissociation channels after photoionization of DMA. The derived AE0K are given in the figure. 11237
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Scheme 2. Mechanism of the Second Dissociation Channel in the Diazomeldrum’s Acid Cationa
a
Energies are given with respect to the neutral parent in eV and were computed on CBS-QB3 level of theory.
frequency (
