Article pubs.acs.org/JPCA
CH2D+, the Search for the Holy Grail Evelyne Roueff,*,† Maryvonne Gerin,‡ Dariusz C. Lis,§ Alwyn Wootten,∥ Nuria Marcelino,∥ Jose Cernicharo,⊥ and Belen Tercero⊥ †
Observatoire de Paris, LUTh and UMR 8102 due CNRS, Place J. Janssen, 92190 Meudon, France Département de Physique de l’ENS, ENS, LERMA, and UMR 8112 du CNRS, 24 rue Lohmond, Paris 75230 cedex 05, France § MC301-17, Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, California 91125, United States ∥ NRAO, North American ALMA Science Center, 520 Edgemont Road, Charlottesville, Virginia 22903, United States ⊥ CAB, INTA-CSIC, 28850 Torrejon de Ardoz, Madrid, Spain ‡
ABSTRACT: CH2D+, the singly deuterated counterpart of CH3+, offers an alternative way to mediate formation of deuterated species at temperatures of several tens of Kelvin, as compared to the release of deuterated species from grains. We report a longstanding observational search for this molecular ion, whose rotational spectroscopy is not yet completely secure. We summarize the main spectroscopic properties of this molecule and discuss the chemical network leading to the formation of CH2D+, with explicit account of the ortho/para forms of H2, H3+, and CH3+. Astrochemical models support the presence of this molecular ion in moderately warm environments at a marginal level.
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reaction where a temporary molecular complex CH5+ is formed and stabilized through infrared emission. Alternatively, CH3+ can exchange a deuteron with HD, producing CH2D+, and offering an efficient pathway to deuteration, as will be discussed later. It may also react with other abundant neutral molecules, suggesting a natural gas-phase path to molecular complexity.11
INTRODUCTION CH3+ is recognized as a key polyatomic molecular ion in astrophysical plasmas. It has been found in the innermost coma of comet Halley1−3 and is thought to be present in both diffuse and dense molecular clouds.4 Despite its importance, there is a lack of high-resolution spectroscopic data, primarily because of its polymerization in discharges.5 Laboratory infrared spectroscopy studies have first been conducted in Oka’s group.5 More recently, threshold photoionization studies of the methyl radical and its deuterated isotopologues in the 9.5−10.5 eV photon energy range have allowed us to investigate the vibrational spectroscopy of their corresponding cations by using the easily tunable and powerful sources of radiation, provided by the third generation of synchrotron sources.6 Rotational spectroscopy of CH3+, however, cannot be achieved as its fully symmetric D3h ground state planar structure does not allow for a permanent dipole moment. Deuterium substitution of a hydrogen atom in CH3+ breaks the symmetry and allows the presence of a small, but significant 0.3 D dipole moment, which gives potentially observable rotational transitions of CH2D+. Regrettably, the molecule is light and the rotational constants are large, producing a widely spaced level structure. Two independent recent laboratory studies7,8 have considerably improved our knowledge of the CH2D+ rotational spectrum, which is now an entry in the CDMS database (Cologne Database for Molecular Spectroscopy)9,10 (http://www.astro.uni-koeln.de/cdms/). CH3+ does not react with molecular nor atomic hydrogen, as the corresponding reactions are highly endothermic. Nevertheless, molecular hydrogen can radiatively associate in a slow © XXXX American Chemical Society
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SPECTROSCOPY OF THE CH3−nDn+ FAMILY Vibrational and Rotational Constants. Values of the ν3 fundamental frequency of CH3+ (ref 5), ν1 fundamental frequency of CH2D+ and CHD2+ (ref 12), and the ν4 frequency of CH2D+ (ref 12) have been derived from the infrared vibrational spectrum of these ions performed in the group of Oka. As a four-atom containing species, six vibrational modes are involved. For the fully substituted species CH3+ and CD3+, the ν3 stretching and ν4 bending modes are doubly degenerate. Threshold photoelectron spectroscopy studies with highly tunable radiation sources provided by the new third generation synchrotron facility SOLEIL allow us to record the photoionization spectra over a wide range of photoionization energies. All the vibrational frequencies of the methyl radical and its deuterium-substituded forms, as well as those of the corresponding ions have been reported by Cunha de Miranda Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: January 4, 2013 Revised: April 29, 2013
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dx.doi.org/10.1021/jp400119a | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
et al.6 Additional theoretical quantum calculations are also documented by these authors.6 We display the spectroscopic constants of the different deuterated isotopologues in Table 1. Theoretical harmonic Table 1. Spectroscopic Constants of CH3+ and Its Deuterated Isotopologues CH3+ electronic state point group ν01 (cm−1) ν1 (cm−1) symmetry ν02 (cm−1) ν2 (cm−1) symmetry ν03 (cm−1) ν3 (cm−1) symmetry ν04 (cm−1) ν4 (cm−1) symmetry ν5 (cm−1) symmetry ν6 (cm−1) symmetry A (cm−1) B (cm−1) B (cm−1)
CH2D+
CHD2+
CD3+
X̃ 1A′1
X̃ 1A′1
X̃ 1A′1
X̃ 1A′1
D3h 303713 29406 a′1 symmetric stretch 141813 1359 ± 714 a2″ OPLAa 324713 31085 e′ degenerate stretch 142913 137014 e′ degenerate bend
C2v
C2v
D3h
300512 a1
305612 a1
20976 a′1
22406 A1
21686 A1
10856 a″2
13896 a1
1036(2) a1
23456 e′ degenerate stretch
12996 b1
11886 b1
10306 e′ degenerate bend
310612 b1 11716 b2 9.36868657 5.77130187 3.52523327
235612 b2 128112 b2 7.2525112 4.6904612 2.81547012
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9.3622 4.715515
Figure 1. Diagram showing the fundamental vibrations of methyl cation isotopologues. The experimental vibrational frequencies are listed when available. Theoretical values are given in parentheses. Dotted lines correspond to degenerate modes of vibration. OPLA stands for “out of plane large amplitude” mode. Adapted from Brum et al.16
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4.731 2.36415
OPLA stands for “out of plane large amplitude” mode; ν0i stands for theoretical harmonic wavenumber. Experimental values of the fundamental wavenumbers νi, when available, are displayed. A, B, and C refer to the rotational constants. a
where ν0i is the harmonic frequency and the anharmonic constants χij are simple functions of the second, third, and semidiagonal fourth energy derivatives with respect to normal modes. One can then derive the expressions of the fundamental vibrational frequencies νi and ZPE:
frequencies ν0i of CH3+ have also been computed by Keceli et al.13 and are also reported. Finally, Table 1 also gives the rotational spectroscopic constants of the vibrational ground state of the different isotopologues. Figure 1, adapted from Brum et al.,16 summarizes the respective numerical values of the fundamental vibrational frequencies on a wavenumber scale for all deuterated isotopologues of the methyl cation. Zero Point Energies (ZPEs). Determining zero point energies (ZPEs) is important for predicting the relative stability of different isotopologues as they can be further used to determine the exothermicity of isotopic exchange reactions. Whereas ZPEs values are well-known and documented for diatomics, as summarized in Irikura,17 their estimates, computations, and experimental corrections are much less obvious for polyatomics (Csonka et al.).18 Using a fourth-order expansion of the potential energy, the second-order perturbative expression of the vibrational energy of asymmetric tops can be expressed as19 0⎛ ⎜
1⎞ En = χ0 + ∑ giνi ni + ⎟ + ⎝ 2⎠ i ⎛ 1 ⎞⎟ ⎜n + ⎝ j 2⎠
∑ i
1 2
νi = νi0 + 2χi +
∑ χij (2)
j≠i
and ZPE = χ0 +
1 2
⎛
∑ ⎜⎜νi0 + i
⎝
1 χ + 2 ii
∑ j
