LETTER pubs.acs.org/JPCL
Photoionization of CH3N3 Produces 3B2 N3: A Theoretical and Experimental Study of the Ion-Pair Channel Alfredo Quinto-Hernandez,*,† Yin Yu Lee,‡ Tzu-Ping Huang,‡ Wan-Chun Pan,‡ Ricardo A. Mata,§ and Alec M. Wodtke†,§,|| †
Department of Chemistry and Biochemistry, University of California, Santa Barbara, California, 93106-9510, United States National Synchrotron Radiation Research Center, 101-Hsin Ann Road, 30077, Hsinchu, Taiwan, Republic of China § Institut f€ur Physikalische Chemie, Universit€at G€ottingen, Tammannstrasse 6, D-37077, G€ottingen, Germany Max-Planck-Institut f€ur Biophysikalische Chemie, Karl Friedrich-Bonhoeffer-Institut, Am Fassberg 11, 37077, G€ottingen, Germany
)
‡
bS Supporting Information ABSTRACT: We observe two ion formation thresholds when monochromatized synchrotron radiation is tuned through the onset of the methyl azide ion-pair channel (CH3N3 + hν f CH3+ + N3). We assign these to production of the two lowest electronic states of the azide anion: the singlet linear (1Σg+) and the triplet bent (3B2) forms of N3. This finding is supported by extensive quantum chemical calculations on the N3 singlet and triplet potential energy surfaces. Quantum chemical calculations are also used to rule out alternative ionization channels that exhibit the same m/z ratios: NH+ + isomers of CH2N2. A value of 292.9 kJ/mol for the D0(CH3N3) is suggested. SECTION: Dynamics, Clusters, Excited States
R
ing formation in poly-nitrogen (poly-N) is nearly unknown, a fact that is related to the paucity of poly-N molecules that have so far been discovered.1 Recently, photochemical production of the simplest ring-form poly-N molecule, cyclic-N3, has been reported,2 based on a variety of indirect measurements35 and supported by theoretical calculations.68 High resolution spectra of cyclic-N3’s rovibrational states have not yet been reported. Such spectra are interesting as they would provide direct proof of this molecule’s structure as well as reveal the influence of the geometric phase effect on its rovibrational dynamics. Theoretical predictions including the treatment of the geometric phase effect are now available.9,10 These calculations suggest that infrared spectroscopy may be challenging as the transition frequencies are inconvenient and the transition strengths are not especially strong. Unfortunately, there has also been no suggestion of potential rovibronic spectroscopic approaches using resonance enhanced multiphoton ionization, laserinduced fluorescence, or direct absorption spectroscopy. Photoelectron spectroscopy (PES) of the bent (C2v) (3B2) or cyclic (D3h) (3A20 ) forms of N3 might be the most promising and practical approach to obtaining the rovibrational energy spectrum of cyclic-N3. Furthermore, the structure of the 3B2 N3 is close to the transition state structure for cyclization of neutral N3. Thus, PES of this state of the azide ion might provide interesting dynamical information about the nature of poly-N cyclization in the vicinity of this transition state. r 2011 American Chemical Society
The azide radicals large electron affinity ion (EA = 2.68 ( 0.01 eV11,12) means that preparation of excited electronic states of the azide ion may be possible; yet, experimental evidence for the bent 3B2 or cyclic 3A20 states of the azide anion are indirect. Specifically, IR spectra of 15N isotopically enriched cryogenic nitrogen matrices that had been bombarded with 5 kV Ne atoms revealed anomalous intensity patterns, implying the presence of a cyclic N3.13 Quantum chemical calculations were used to interpret these isotopic anomalies as due to the presence of cyclic 3A20 , although this isomer of the azide anion is significantly higher in energy than the bent 3 B2 state. In this article, we report experiments on molecular beamcooled methyl azide (CH3N3), where the photoion yield is obtained as a function of vacuum ultraviolet (VUV) photon energy in the vicinity of the lowest energy ion-pair formation threshold. CH3 N3 þ hν f CH3 þ þ N3
ðR1Þ
High signal-to-noise photoion yield spectra are obtained for both m/z = +15 (CH3+) and 42 (N3). Both photoion yield spectra exhibit two thresholds below the threshold for dissociative Received: July 7, 2011 Accepted: August 16, 2011 Published: August 16, 2011 2311
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LETTER
Table 1. Theoretical Results for the Energetics of CH2N2 Isomers Cyanamide, Carbodiimide, and Diazomethane, the Enthalpy of Formation of the Neutral Species, As Well As the Vertical Electron Affinity (VEA)b,a species
a
radical ΔHf (kJ/mol)
VEA (eV)
est. anion ΔHf (kJ/mol)
cyanamide
143.1
0.17
159.4
carbodiimide
154.4
0.43
196.7
diazomethane
277
0.59
333.9
N3 N3 N3
ΔHf (kJ/mol)
— (NNN) ()
RNN (Å)
(D∞h)
197.5
180.0
1.191
(C2v) (D3h)
444.3 507.1
127.2 60.0
1.265 1.411
Also included are the heats of formation for the lowest energy-lying N3 isomers and the value for the largest angle. b VEA is defined as the energy difference between the anion radical and the neutral molecule, keeping with the neutral geometry. See the Supporting Information for details of calculations.
CH3+
Figure 1. (a) ion yield from CH3N3 as a function of VUV photon energy. (b) Logarithmic scale representation of the data. The upward pointing arrow is an estimate of the dissociative ionization threshold (reaction R2) based on the value of D0(CH3N3) calculated from quantum chemical methods in this work. This bond energy is combined with the experimentally known value for the ionization potential of CH3. When this quantity is adjusted by the experimentally known electron affinity of N3, one obtains the downward pointing arrow, which is an estimate of the ion-pair threshold (reaction R1). The 2.56 eV scale-bar represents the energy difference between the singlet linear (1Σg+) and the triplet bent (3B2) forms of N3, which has also been calculated in this work.
ionization, which is also observed. Quantum chemical calculations of the singlettriplet splitting (1Σg+ T 3B2) in N3 compare favorably with the energy spacing between the experimentally observed thresholds. VUV photoionization of methyl azide thus appears to be a viable route to laboratory production of the bent triplet azide anion. Figure 1 shows the methyl azide photoionization efficiency (PIE) spectrum for m/z = +15 between photon energies of 9 and 14.5 eV. Quantum chemical calculations on CH3N3, N3 and CH3 radicals lead to a computed dissociation energy, D0(CH3N3) = 292.9 kJ/mol. Using an experimental value for the ionization energy of the methyl radical, we obtain an energy threshold for dissociative ionization of methyl azide (reaction R2). CH3 N3 þ hν f CH3 þ þ N3 þ e
ðR2Þ
AEðCH3 þ þ N3 Þ ¼ D0 ðCH3 N3 Þ þ IEðCH3 Þ14 ¼ 12:87 eV This energy is shown as an upward pointing solid arrow in Figure 1a. While the m/z = +15 ion signal rises rapidly above the expected threshold of 12.87 eV consistent with this assignment, due to the high sensitivity of the instrument, the ion signal at lower photon energies is readily apparent. Figure 1b shows the same data magnified by 10 and on a base-10 logarithmic scale to emphasize the large dynamic range of these experiments. Our determination of the true background ion signal level is shown by the thick horizontal line. Here the
m/z = +15 ion signal appearing at photon energies below 12.87 eV is more easily seen. The data clearly show that m/z = +15 ions are produced at photon energies as low as ∼10 eV. The electron affinity of N3 is known from experiment (2.68 eV11,12). We use this value and the theoretical dissociation energy to estimate the position of the ion-pair threshold energy. AEðCH3 þ þ N3 Þ ¼ D0 ðCH3 N3 Þ þ IEðCH3 Þ14 EAðN3 Þ11, 12 ¼ 10:19 eV
This is shown as a downward pointing solid arrow in Figure 1b. This combined experimental and theoretical analysis is strong evidence that the m/z = +15 ions observed between 10 and 13 eV result predominantly from the ion-pair channel (R1). The mass spectrometric signal at m/z = +15 can in principle also arise from NH+. Thus, in the course of this assignment we considered ionization processes that involve NH+ with any of the anion isomers of the [C, H2, N2] family: NNCH2 (diazomethane), NCNH2 (cyanamide), CNNH2 (isocyanamide), HNCNH (carbodiimide), HCNNH (nitrilimine), cyc-CH2NN (diazirine), and cyc-CHNNH (isodiazirine). We label these processes generically as one class of ion pair channels as follows: CH3 N3 þ hν f NHþ þ CH2 N2
ðR10 Þ
The large difference between ΔHf(CH3+) = 1095.4 kJ/mol and ΔHf(NH+) = 1652.7 kJ/mol means that for an (R10 ) ionpair channel to appear at a photon energy as low as 10 eV (as seen in this work), the corresponding (CH2N2) anion would have to possess a heat of formation much lower than that of N3. Specifically, it would need to exhibit a ΔHf(CH2N2) = 338.9 kJ/mol. Likewise, for the threshold to appear at 12.5 eV—the upper range of interest—a value of ΔHf(CH2N2) = 96.2 kJ/mol is implied. Additional quantum chemical calculations were carried out to evaluate the stability of many such species. None were found that are energetically reasonable candidates for an alternative ion pair channel R10 . We have included some of the results of these calculations in Table 1. Three of the most stable neutral isomers with the CH2N2 molecular formula are diazomethane, carbodiimide, and cyanamide. We discuss these three species in detail to illustrate how these and other anion candidates were ruled out. 2312
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Figure 2. (a) Comparing the CH3+ and N3 ion yields in the ion pair region. In both sets of data two thresholds are seen below the 12.87 eV, the dissociative ionization threshold of methyl azide. In panel b we subtract the CH3+ signal from the N3 signal. The remaining signal is assigned to the dissociative electron attachment of CH3N3 (see text).
The heat of formation for diazomethane calculated in this work (277 kJ/mol) agrees well with a previous theoretical estimate from Dixon et al. (279 kJ/mol).15 The cyanamide isomer is, as previously reported by Kawauchi et al., slightly lower in energy (143.1 kJ/mol) than its tautomer carbodiimide (154.4 kJ/ mol).16 Extensive ROHF-UCCSD(T)/aug-cc-pVTZ calculations have been performed in order to find stable anions for these species. However, we always found the electron affinity to be negative at this level of theory. That is, an electron is not bound to these molecules. Even if one were to assume that a slightly positive electron affinity occurred in these molecules and the theory was not sufficient to capture this structural feature— an eventuality we consider highly unlikely—the heats of formations of the postulated anions would still be far too high to be consistent with the low energy ion pair thresholds seen in this work (see Table 1). Therefore, we exclude the possibility of finding CH2N2 within the energy range of the experiment. Instead of reporting on the structures found, we have added the energy difference between the neutral species and the anion, computed in the neutral geometry. A comprehensive report on the electronic affinity of these species is beyond the scope of this work. Returning to Figure 1, we see that Figure 1b clearly shows two thresholds below that of dissociative ionization: one near 10 eV and another near 12.5 eV. The threshold near 10 eV is assigned to the formation of ground-state linear azide anion (N3 + CH3+). Full valence CASSCF/cc-pVTZ calculations were carried out on the singlet and triplet potential energy surfaces of N3 in order to identify possible candidates for the higher energy threshold. The structure of the lowest lying species was later refined at the ROHF-UCCSD(T)/aug-cc-pVTZ level of theory. The higher energy threshold is consistent with the formation of a bent triplet
LETTER
isomer of the azide anion with C2v symmetry, 3B2. All other minima on the PES were significantly higher in energy, including the D3h triplet cyclic isomer. The structures for the investigated minima as well as the heat of formation for each of the isomers are given in Table 1. The calculations indicate that the 3B2 bent anion is 2.56 eV above the linear ground state. A scale bar of this length is placed in Figure 1b to indicate the assignment of the two thresholds. The same set of calculations are found to reproduce the electron affinity of the azide radical EA(N3) = 2.69 eV, which validates our computational approach. We also observed photoion yield spectra for m/z = 42 (N3) (see Figure 2a.). Here the efficiency to produce N3 is compared to that of producing CH3+. The two signals have been carefully normalized to one another so that the relative intensities are meaningful. As for production of CH3+, N3 production exhibits two thresholds below the dissociative ionization threshold at 12.87 eV. The positions of the two thresholds in the m/z = 42 data are consistent with the two thresholds seen for CH3+ production that have been assigned above to the production of linear and bent (C2v) N3. However, the m/z = 42 and +15 ion yield curves are not identical, as there is a second contribution to the m/z = 42 signal; namely, dissociative attachment of electrons to methyl azide forming N3. In Figure 2b, we show the best effort to subtract the ion-pair channel contribution from the azide signal, which shows the remaining dissociative electron attachment contribution to the azide signal (shown as a solid black line). We compare this to the photoionization yield spectrum of methyl azide (dashed gray line) also obtained in this work. The electron attachment spectrum mirrors the dissociative attachment spectrum, shifted by about 1 eV to higher energy. Thus, when photoelectrons with about 1 eV of kinetic energy are produced from CH3N3 photoionization, the dissociative attachment is most effective. This is similar to previous work,17 where azide anion from dissociative attachment to phenylazide was observed to be optimal with 1 eV electrons. We also point out that negative ion formation from photoelectron attachment has been previously reported in experiments using intense synchrotron sources. For example, our observations of “extra azide ion signal” are similar to studies of negative ion mass spectrometry where synchrotron radiation was used to illuminate beams of SF6.18 In that work, SF6 was observed. That work furthermore showed that SF6 was formed by electron attachment due to low energy photoelectrons produced in the photoionization of SF6. Furthermore, the SF6 yield mirrored the positive ion formation spectrum,18 similar to what is seen here. Consequently, we assign the second contribution to the azide ion signal to a two step process, reaction R3: CH3 N3 þ hν f CH3 N3 þ þ e
ðR3Þ
followed by reaction R4 CH3 N3 þ e f CH3 þ N3
ðR4Þ
The high sensitivity afforded by the synchrotron-based tunable VUV photoionization end station at the NSRRC (National Synchrotron Radiation Research Center, Taiwan) allows high signal-to-noise photoion yield spectra from molecular beamcooled CH3N3 well below the first dissociative ionization threshold. Examining both positive and negative ions, we obtain a 2313
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The Journal of Physical Chemistry Letters remarkably clear look at the ion-pair channel in methyl azide photoionization. The large electron affinity of azide allows us to observe the formation of an excited electronic state of the anion. The observations of this work point to the formation of both the singlet linear (1Σg+) and the triplet bent (3B2) forms of N3 when methyl azide is ionized at photon energies between 10 and 13 eV. This suggests a convenient approach to the production of 3 B2 bent N3, from which a variety of interesting PES experiments might be carried out. In particular, the calculated structure of the 3B2 N3 is close to the transition state structure for cyclization of neutral linear N3. Thus, PES of this state of the azide ion might provide interesting dynamical information about the nature of poly-N cyclization in the vicinity of this transition state.
’ EXPERIMENTAL DETAILS Methyl azide was synthesized using degassed solutions (
