Article pubs.acs.org/JPCA
VUV Photofragmentation of CH2I2: The [CH2I−I]•+ Iso-diiodomethane Intermediate in the I‑Loss Channel from [CH2I2]•+ Antonella Cartoni,*,† Anna Rita Casavola,*,‡ Paola Bolognesi,‡ Stefano Borocci,§ and Lorenzo Avaldi‡ †
Dipartimento di Chimica, Sapienza Università di Roma, P. le Aldo Moro 5, Rome 00185, Italy CNR-ISM, Area della Ricerca di Roma 1, Monterotondo Scalo (RM) 00015, Italy § Dipartimento per l’ Innovazione nei sistemi Biologici, Agroalimentari e Forestali (DIBAF), Università della Tuscia, L.go dell’Università, s.n.c., 01100 Viterbo, Italy ‡
ABSTRACT: Diiodomethane is an important halocarbon responsible for several atmospheric processes like ozone depletion and aerosol particle formation. Despite this, the thermochemical data and a detailed analysis of the pathways for the decomposition of this halomethane and its molecular ion [CH2I2]•+ are scarce. In this paper an investigation of the photodissociation dynamics of the CH2I2 molecule focused on the I-loss channel by the photoelectron−photoion coincidence (PEPICO) technique and computational methods is reported. The experimental results show that upon VUV irradiation the dissociation of the lower electronic ionic states of diiodomethane leads only to the CH2I+ ion and the I atom. The theoretical calculations point out that isomerization of [CH2I2]•+ into iso-diiodomethane [CH2I−I]•+ may play an important role in the emission of iodine atom as compared to direct C−I bond breaking.
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INTRODUCTION
iodine, the iodine atom, or both. In the literature many computational studies can be found on the CH2I2 molecule,14−17 but to the best of our knowledge a detailed theoretical study of the fragmentation of [CH2I2]•+ has not been reported.18,19
Iodine plays an important role in different fields of chemistry. It is a fundamental element for humans, a useful catalyst in organic synthesis and its radioelements are widely used for the preparation of radiopharmaceuticals.1−3 Moreover, iodine participates in several atmospheric reactions and physical processes.4−7 Among the organohalide compounds, diiodomethane is one of the main precursors of iodine in the environment and it can influence the oxidative capacity of the atmosphere catalytically destroying ozone molecules and reacting with OH radicals. Recently, Reid et al.8,9 reported that the isomerization of several important halons (i.e., CF2Cl2) into iso-halons, which feature a halogen−halogen bond, in the gas phase is a pathway to molecular products comparable to the simple C-halo bond fission. Moreover, many studies demonstrated that iso-halons, i.e., iso-diiodomethane, are reactive photointermediates in the condensed phase mainly used in cyclopropanation of alkenes.10−12 In a previous work we have addressed the physical chemistry of halomethanes and their implication on the atmospheric chemistry.13 The results on the VUV photodissociation of the CH2I2 molecule showed that (i) several fragmentation pathways occur when the molecular ion [CH2I2]•+ is generated by a VUV photon and (ii) the I-loss channel leading to CH2I+ ions is one of the favored channels. In this work we continue our study of the physical chemistry of diiodomethane via a photoelectron−photoion coincidence (PEPICO) experiment and a theoretical investigation on the radical cation [CH2I2]•+ produced by VUV irradiation. The aim is to understand if the isomerization of diiodomethane into isodiiodomethane occurs in the radical cation [CH2I2]•+ and if isodiiodomethane [CH2I−I]•+ can be a source of molecular © 2015 American Chemical Society
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EXPERIMENTAL METHODS The experiments have been performed at the gas phase photoemission beamline of the Elettra synchrotron radiation facility, Trieste (Italy).20 The light source is an undulator of period 12.5 cm, 4.5 m long. The 100% linearly polarized radiation from the undulator is deflected to the variable anglespherical grating monochromator by a prefocusing mirror. The monochromator consists of entrance/exit slits and two optical elements: a plane mirror and a spherical grating. Five interchangeable gratings cover the energy region 13−1000 eV, with a typical resolving power of 10 000. Two refocusing mirrors after the exit slit provide a circular focus (radius about 300 μm) at the interaction region in the experimental chamber. In the present experiments, a fixed photon energy of 40 eV, with an energy resolution of about 20 meV was used for all measurements. The experimental chamber is maintained at a background pressure of less than 1 × 10−7 mbar. The target molecule CH2I2, which is in the form of a liquid sample at room temperature, is kept in a test tube outside the vacuum chamber Received: November 5, 2014 Revised: March 30, 2015 Published: April 13, 2015 3704
DOI: 10.1021/jp511067d J. Phys. Chem. A 2015, 119, 3704−3709
The Journal of Physical Chemistry A
Article
Table 1. Electronic Energy (Ee), Zero Point Energy (ZPE), and Thermal Correction (TC) of All the Stationary Points Investigated (Hartree)a [CH2I2]•+ (1) [CH2I−I]•+ (2) [CH2I--I]•+ (3) I CH2I+ CH2•+ TS1 TS2 CH2I2 I2 a
Ee-CCSD(T) full
Ee-CCSD(T) frozen
ZPE (MP2)
E0 = Ee(full) + ZPE
TC (MP2)
−628.7308038 −628.6909807 −628.6873233 −294.8934684 −333.7793545 −38.6996627 −628.6680154 −628.6805700 −629.0649287 −589.8585012
−628.5178119 −628.4779697 −628.4761582 −294.7998600 −333.6641934 −38.6809923 −628.4545554 −628.4691416 −628.8517613 −589.6661461
0.027529 0.023351 0.025111
−628.7032748 −628.6676297 −628.6622123 −294.8934684 −333.7551355 −38.6822764 −628.6441634 −628.6556700 −629.0375727 −589.8579852
0.003999 0.005384 0.004987 0.001416 0.002995 0.002868 0.004116 0.004204 0.004055 0.002881
0.024219 0.016899 0.023852 0.024900 0.027356 0.000516
The numbering 1−3 refers to the structures indicated in Figure 3.
and admitted to the interaction region via a leak valve and a gas line that can be baked up to 60−70 °C on both the vacuum and air sides. In the present experiments, the sample was maintained at room temperature, producing a typical residual gas pressure of 3 × 10−6 mbar in the experimental chamber and enough gas density in the interaction region to perform the measurements. The sample was purchased from Alpha Aesar with a purity of 99% and used without further purification. The temperature of the gas line and the needle was kept at 50 °C only to guarantee a proper cleaning of the gas line, before and after the experiments. The effusive beam of the target molecules is admitted in the interaction region through a 0.7 mm stainless steel needle, placed between the repeller and extractor electrodes of the time of flight (TOF), about 2 mm below the photon beam. The end station is equipped with a commercial 150 mm mean radius hemispherical electron energy analyzer (VG 220i),21 with six channel electron multiplier detectors that allow for multidetection. The VG analyzer is mounted behind the repeller electrode of the TOF, the electrons passing through a 98% transmission gold mesh. A homemade Wiley− McLaren22 time of flight mass spectrometer is mounted opposite to the VG electron analyzer. The TOF spectrometer, working in conjunction with the “virtually” continuous ionization source provided by the multibunch operation mode of the synchrotron radiation, is operated in pulsed extraction mode. The repeller and extractor electrodes are polarized with antisymmetric voltages (manufacturer Directed Energy Inc., model PVM4210) driven by an external trigger and providing a typical extraction field of 700 V/cm. The electron and ion mass analyzers can be operated independently, for photoelectron (PES) and mass spectroscopy measurements, respectively. In this operation mode (i) the hemispherical analyzer is normally operated with a pass energy of 5 eV, corresponding to a kinetic energy resolution of about 150 meV and (ii) the TOF spectrometer extraction field is triggered using a 1 kHz random pulse generator (Stanford ResearchDG535) to extract the ions. The two analysers can be operated simultaneously for coincidence experiments. In this mode a residual penetration field from the drift tube of the TOF produces a kinetic energy shift of the photoelectron spectrum (easily taken into account by the calibration procedure) and a degradation of the energy resolution of the electron analyzer. Therefore, in the coincidence mode the electron energy analyzer has been operated at pass energy of 20 eV, with a gain in efficiency, but no further loss of resolution. The final energy resolution is estimated to be around 0.5 eV.
The photoelectron spectrum was energy calibrated using the well-known position of the water photoelectron peak. Ionization of the valence shell produces a photoelectron and a correlated ion in each event. To perform electron−ion coincidence experiments, the following electronic chain has been used. The amplified and discriminated signals from the channeltrons of the electron analyzer provide the start signals for six independent channels of a time-to-digital converter (TDC, model AM-GPX, ACAM Messelectronic) as well as the trigger for the extraction of the ions from the interaction region, after being combined in a suitable “OR” logic unit. The signal of the extracted ions provides the stop signal to all the TDC channels. Random coincidences are estimated by randomly pulsing at 100 Hz, i.e., a frequency comparable to the photoelectron count rate, and counted in a separate channel of the TDC. The measured mass spectra are normalized to the number of starts provided by photoelectrons, and the random coincidences to the number of random starts. The normalized spectra are then subtracted to produce the photoelectronphotoion coincidence (PEPICO) spectra. A schematic of the present arrangement can be found in Figure 1 of ref 21.
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THEORETICAL METHODS All calculations were performed using Gaussian 09.23 The geometries were optimized at the MP2 level using 6-311++G** basis set for C and H atoms. The iodine atom was treated by the small-core (28 electrons) scalar-relativistic effective potential (ECP-28) in conjunction with the aug-cc-pVTZ-PP basis set.24,25 Accurate total energies were obtained by singlepoint coupled cluster calculations, CCSD(T)26 using the same basis sets and pesudopotential for the MP2 calculations. Both MP2 and CCSD(T) were employed within the frozen-core approximation by using the 4s4p4d frozen-core orbitals for the I atom. CCSD(T) has been also performed in full mode (Table 1). The energy profile and the appearance energy AE298 for the CH2I+ fragment ion are obtained by the CCSD(T,full) calculations. All critical points were characterized as energy minima or transition structures (TS) by calculating the corresponding MP2 harmonic frequencies, also used to evaluate the zero-point energy correction. The TS were unambiguously related to their interconnected energy minima by intrinsic reaction coordinates (IRC) calculations.27,28 Small spin contamination was revealed, at the MP2 level of theory, in radicals and radical cations as indicated by the ⟨S2⟩ operator close to the theoretical value for the pure doublet spin state (0.75). The CCSD T1 diagnostic is within the recommended 3705
DOI: 10.1021/jp511067d J. Phys. Chem. A 2015, 119, 3704−3709
The Journal of Physical Chemistry A
Article
Table 2. Atomic Charges (e) Computed by Natural Bond Orbital (NBO) Analysis C II III H H
[CH2I2]•+ (1)
TS1
[CH2II-III]•+ (2)
TS2
[CH2II---III]•+ (3)
CH2II+
−0.728 0.614 0.614 0.250 0.250
−0.515 0.638 0.423 0.227 0.227
−0.411 0.779 0.194 0.219 0.219
−0.303 0.717 0.131 0.228 0.227
−0.447 0.842 0.147 0.229 0.229
−0.319 0.853 0.233 0.233
Figure 1. Mass spectrum (a) and low resolution photoelectron spectrum PES (b) of CH2I2 recorded in the PEPICO conditions (Experimental Methods) at hν = 40 eV. The lower energy states, which are the ones involved in the present study, are reported as vertical bars in Figure 1b and can be assigned as 3b2, 2b1, 1a2, and 4a1 orbitals from the lowest to the higher binding energy one, respectively.32
threshold of 0.02.29 This suggests that these species are correctly described methods. The atomic charges e (Table natural bond orbital (NBO) analysis function.30
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spectra acquired in the BE region between 9 and 11 eV, are shown in Figure 2.
the wave functions of by a single-reference 2) were computed by using the MP2 wave
RESULTS AND DISCUSSION
In a first experiment, at the photon energy of hν = 40 eV the CH2I2 mass spectrum was acquired using the TOF analyzer (Figure 1a). Although the m/z resolution does not allow the complete separation of the multiplet in the region of m/z = 140, the peaks relative to different fragment ions, formed from [CH2I2]•+ (m/z = 268), are clearly discernible (see also inset in Figure 1a) and can be assigned as I+ (m/z = 127), CI+ (m/z = 139), CHI+ (m/z = 140), CH2I+ (m/z = 141) and I2+ (m/z = 254). The PES of diiodomethane recorded using the hemispherical analyzer is shown in Figure 1b. In these experiments the distribution of the kinetic energies, KE, of the electrons emitted from the photoionization of the CH2I2 molecules maps out the binding energy, BE, of the different molecular states, where BE = hν − KE (Figure 1b). Although the energy resolution is not sufficient to resolve all of the overlapping electronic states, the first feature in Figure 1b includes the ground and first excited states of the ion.31−34 Indeed, the ionization potential for the diiodomethane molecule, which corresponds to the removal of an electron from the outermost orbital 3b2, is reported to be 9.46 eV,35 whereas the other non-bonding electrons (lone pair) have all binding energies
