Single photon ionization of methyl isocyanide and the subsequent unimolecular decomposition of its cation: Experiment and theory

Article abstract of DOI:10.1039/c9cp04310a

Methyl isocyanide, CH₃NC, is a key compound in astrochemistry and astrobiology. A combined theoretical and experimental investigation of the single photon ionization of gas phase methyl isocyanide and its fragmentation pathways is presented. Vacuum ultraviolet (VUV) synchrotron radiation based experiments are used to measure the threshold photoelectron photoion coincidence (TPEPICO) spectra between 10.6 and 15.5 eV. This allowed us to experimentally determine the adiabatic ionization energy (AIE) and fragment ion appearance energies (AE) of gas-phase methyl isocyanide. Its AIE has been measured with a precision never achieved before. It is found to be AIE_exp = 11.263 ± 0.005 eV. We observe a vibrational progression upon ionization corresponding to the population of vibrational levels of the ground state of the methyl isocyanide cation. In addition, four fragment ion appearance energies (AEs) were measured to be AE (m/z 40) = 12.80 ± 0.05 eV, AE (m/z 39) = 13.70 ± 0.05, AE (m/z 15) = 13.90 ± 0.05 eV, AE (m/z 14) 13.85 ± 0.05 eV, respectively. In order to interpret the experimental data, we performed state-of-the-art computations using the explicitly correlated coupled cluster approach. We also considered the zero-point vibrational energy (ZPVE), core-valence (CV) and scalar relativistic (SR) effects. The results of theoretical calculations of the AIE and AEs are in excellent agreement with the experimental findings allowing for assignment of the fragmentations to the loss of neutral H, H₂, CN and HCN upon ionization of CH₃NC. The computations show that in addition to the obvious bond breakings, some of the corresponding ionic fragments result from rearrangements-upon photon absorption-either before or after electron ejection.

Full text of DOI:10.1039/c9cp04310a

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Single photon ionization of methyl isocyanide and
the subsequent unimolecular decomposition of its
cation: experiment and theory†
c
Cite this: Phys.Chem.Chem.Phys.,
2019, 21, 26017
A. Bellili,a Z. Gouid,ab M. C. Gazeau,b Y. Benilan,b N. Fray,b J. C. Guillemin,
´
M. Hochlaf *a and M. Schwell
*
b
Methyl isocyanide, CH3NC, is a key compound in astrochemistry and astrobiology. A combined theoretical and
experimental investigation of the single photon ionization of gas phase methyl isocyanide and its
fragmentation pathways is presented. Vacuum ultraviolet (VUV) synchrotron radiation based experiments are
used to measure the threshold photoelectron photoion coincidence (TPEPICO) spectra between 10.6 and
15.5 eV. This allowed us to experimentally determine the adiabatic ionization energy (AIE) and fragment ion
appearance energies (AE) of gas-phase methyl isocyanide. Its AIE has been measured with a precision never
achieved before. It is found to be AIEexp = 11.263 Æ 0.005 eV. We observe a vibrational progression upon
ionization corresponding to the population of vibrational levels of the ground state of the methyl isocyanide
cation. In addition, four fragment ion appearance energies (AEs) were measured to be AE (m/z 40) = 12.80 Æ
0.05 eV, AE (m/z 39) = 13.70 Æ 0.05, AE (m/z 15) = 13.90 Æ 0.05 eV, AE (m/z 14) 13.85 Æ 0.05 eV,
respectively. In order to interpret the experimental data, we performed state-of-the-art computations using
the explicitly correlated coupled cluster approach. We also considered the zero-point vibrational energy
(ZPVE), core-valence (CV) and scalar relativistic (SR) effects. The results of theoretical calculations of the AIE
and AEs are in excellent agreement with the experimental findings allowing for assignment of the
fragmentations to the loss of neutral H, H2, CN and HCN upon ionization of CH3NC. The computations show
that in addition to the obvious bond breakings, some of the corresponding ionic fragments result from
rearrangements – upon photon absorption – either before or after electron ejection.
Received 2nd August 2019,
Accepted 26th September 2019
DOI: 10.1039/c9cp04310a
rsc.li/pccp
–NRC, called isocyanides (or isonitriles; R–NRC), have also
1. Introduction
been detected in objects of the ISM. They include HNC,3,8
Among the molecules observed in the interstellar medium (ISM), CH3–NC5,9–14 HCRC–NC,15 CH2CH–NC5 (tentatively) as well
organic and inorganic cyanides (or nitriles) are an important as inorganic isocyanides.1 CH3CH2–NC has also been searched
class of compounds. They represent approximately 20% of the for and upper limits of the column densities were derived
more than 200 molecules observed today in the ISM gas phase.1 recently.13 Basic photophysical data are often unknown for these
Cyanides (R–CRN) are easily detectable in a number of inter- compounds.
stellar environments because of their large abundances.2–5 Very
Methyl isocyanide, CH3–NRC, the object of this vacuum
recently, even fairly complex cyanides like hydroxyacetonitrile6 ultraviolet (VUV) spectroscopic study and denoted ‘‘MIC’’ in
and benzonitrile7 have been detected. Since the observation of the following, is a thermodynamically less stable isomer of
R–CRN molecules, a few isomers with the functional group CH3–CRN, the latter being one of the most common molecules
in the ISM. MIC was claimed to be detected for the first time in
1984 towards the cold cloud TMC19 but the first secure detection
towards the high-mass star-forming region Sgr B2 was reported
in 1988.10 The presence of MIC in the cold clouds of Sgr B2(N)
was confirmed seventeen years later by Remijan et al.11 MIC has
a
´
´
Universite Paris-Est, Laboratoire Modelisation et Simulation Multi Echelle,
´
MSME UMR 8208 CNRS, 5 bd Descartes, 77454 Marne-la-Vallee, France.
E-mail: hochlaf@univ-mlv.fr
b
c
`
´
Laboratoire Interuniversitaire des Systemes Atmospheriques, LISA,
´
´
´
UMR CNRS 7583, Universite Paris-Est-Creteil, Universite de Paris, Institut Pierre
also been detected in the Horsehead nebula photodissociation
´
region (PDR)12 and in Orion-KL. Very recently, Margules et al.
Simon Laplace, 94010 Creteil, France. E-mail: martin.schwell@lisa.u-pec.fr
5
`
´
Univ Rennes, Ecole Nationale Superieure de Chimie de Rennes, CNRS,
confirmed the detection of MIC in Sgr B2 and Orion-KL.13 These
authors analyzed in depth all available MIC observations using
isomeric abundance ratios r = [R–NRC]/[R–CRN] as a tool to
ISCR UMR6226, F-35000 Rennes, France
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cp04310a
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constrain chemical routes for formation and destruction in high-level ab initio calculations. The ionization and the appearance
order to explain the presence of R–NC (R = methyl, vinyl, ethyl) energies of cationic fragments have been computed using explicitly
in a large range of interstellar environments. For example, the correlated coupled cluster calculations, considering different
observation of MIC (with r = 0.01) in warm regions of Sgr B2 dissociative ionization pathways. We also took into account the
points to a possible grain chemistry pathway for its production zero-point vibrational energy, core-valence and scalar relativistic
because isomerization from CH3CN to CH3NC is very unfavorable effects. In order to interpret the observed vibronic structure,
under thermal conditions. In this context, dedicated laboratory harmonic and anharmonic frequencies of the ground state of the
experiments have shown that desorption of MIC from interstellar cation have been calculated.
grains is slightly facilitated compared to the isomer CH3CN
because MIC is less tightly bound to the studied grain
analogues.16,17 This would lead to a gas phase enrichment of
2. Methods
2.1. Experimental methods
CH3NC at a given grain temperature compared to the abundance
ratio in the condensed phase. On the other hand, Remijan et al.,11
The experiments were carried out at the DESIRS beamline of
have concluded, from their Sgr B2(N) observations, that large-
the French synchrotron facility SOLEIL in connection with its
scale, non-thermal processes such as shocks or enhanced UV
6.65 m normal incidence monochromator and the photoelectron
flux may account for the conversion of CH3CN to CH3NC. This
photoion coincidence (PEPICO) spectrometer DELICIOUS III.31
conclusion is confirmed by the observation of MIC in the
This spectrometer allows for full momentum detection of the
Horsehead nebula PDR, with a fairly large relative abundance
of r = 0.15.12 Laboratory experiments performed by Hudson and
ions and velocity map imaging (VMI) of the electrons at the same
Moore,18 on a number of different organic cyanides, have shown
time, but in exactly opposite directions. Ions and electrons are
detected in coincidence and their collection is perpendicular to
that these can undergo, with significant yields, isomerization to
(i) the molecular beam inlet and (ii) the VUV light propagation
isocyanides upon irradiation with 0.8 MeV protons and also
with VUV radiation. We also mention that the interconversion
direction (all 901 setup). More details on this set-up and the
experimental operating conditions can be found elsewhere.31 For
CH3CN 3 CH3NC has been studied by high-level theoretical
our measurements, we used the 200 g mmÀ1 grating of the
calculations very recently.19
monochromator with entrance/exit slit widths of typically 100 mm/
100 mm yielding a spectral resolution (photons) of 0.72 Å (about
6 meV at 10 eV). (T)PEPICO spectra are normalized by the photon
In this astrophysical context, the knowledge of basic photo-
physical data, including ionization and fragmentation energies,
is crucial to go further in the interpretation of interstellar
flux measured by a photodiode (AXUV, IRD) placed after the
abundances and to model the chemistry occurring in these
photoionization region. For better spectral purity of the beamline,
media. The study of the photoionization and dissociative
we use a gas filter that is filled with 0.25 mbar of Ar. This allows for
photoionization of MIC is particularly relevant since the edges
effective suppression of either higher energy stray light of the
of molecular clouds, for example, are subject to intense VUV
electron storage ring or higher order radiation from the undulator.
irradiation. In the past, our group has investigated the VUV
The presence of Ar absorption lines in the spectrum arising from
photophysics of several cyanides that are key compounds in
this filter furthermore allows for calibration of the energy scale to
astrochemistry and astrobiology. We have studied methyl cyanide
(CH3CN20,21), cyanoacetylenes (HC3N22 and HC5N23), acetyl cyanide
an absolute accuracy of about 1 meV.
(CH3C(O)CN24), and aminoacetonitrile (H2NCH2CN25). To date,
2.2. Preparation of methyl isocyanide
isonitriles were rarely studied by VUV spectroscopy, therefore
laboratory photophysical data are scarce. This is the case of MIC CH3NC has been synthesized following Schuster et al.32 with
since to the best of our knowledge, only HeI and HeII photoelectron some modification. N-Methylformamide (5.9 g, 100 mmol),
spectra26–28 and a VUV absorption spectrum in arbitrary intensity p-toluenesulfonyl chloride (38.1 g, 200 mmol), and trioctylamine
units in the energy range 7.3–11.8 eV29 have been reported in (70.7 g, 200 mmol) were introduced into a one-necked round-
literature.
bottomed flask equipped with a stirring bar. The flask was fitted
Here we want to step forward towards the understanding of on a vacuum line (0.1 mbar) equipped with two U-tubes with
methyl isocyanide photochemistry. The main aim of this work stopcocks. The first one was immersed in a liquid nitrogen cold
is to give insight into the stability and reactivity of MIC under bath. The apparatus was evacuated, the pressure was then
interstellar radiation fields. Since the early studies mentioned stabilized at 2 mbar and the mixture allowed to warm to 80 1C
above, the VUV spectroscopy of MIC has not been reinvesti- over about 1 h. MIC was thus evacuated from the reaction
gated. In this work, we use threshold photoelectron photoion mixture as it was formed and condensed in the trap. At the
coincidence (TPEPICO) spectroscopy30 in connection with VUV end of the reaction, the trap was allowed to warm to room
synchrotron radiation from the DESIRS beamline of the French temperature, and the pure compound was condensed in the
synchrotron SOLEIL. The adiabatic ionization energy (AIE) has second trap cooled to À100 1C. Yield: 3.3 g, 80%. MIC, a
been measured with high precision as well as fragment ion colorless, bad-smelling liquid, is kept under nitrogen in the
appearance energies up to a photon energy of 15.5 eV. Several freezer for storage.
fragment ions are observed in this energy domain. The assign-
For the measurements, a quartz glass recipient with a few mL of
ment and the interpretation of all experimental data require MIC is allowed to warm up to room temperature. The evacuated
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recipient is directly connected to the molecular beam inlet via an and the subsequent unimolecular decomposition processes.
adjustable needle valve. The saturation vapor pressure of MIC is Calculations were carried out in order to assign the observed
such that no additional heating is necessary for the measurements. ionization and fragmentation thresholds. To this end, a cali-
Helium is used as a carrier gas in the molecular beam inlet.
brated method of theory was applied. Further calculations were
performed with the aim to elucidate the structure of fragment
ions experimentally observed as well as the observed vibra-
tional progression.
2.3. Theoretical methods
The main aim of the present computations is to provide an
interpretation of the experimental spectra. For that purpose, we
map the potential energy surfaces (PESs) of MIC and of its
cation, MIC+. These PESs cover the equilibrium structures and
the lowest cationic fragmentation channel regions. All electro-
nic computations are carried out using GAUSSIAN0933 and
MOLPRO (version 2015) package34 in the C1 point group.
We start our computations by geometry optimizations (opt)
of MIC and MIC+ molecular structures, as well as the fragments
of MIC+ at the PBE0/aug-cc-pVDZ level35–37 of theory (using the
ultrafine integral grids). These computations are followed by
harmonic frequencies calculations to attest the nature of the
stationary points found (either a minimum or a transition
state). The anharmonic frequencies are obtained from the
derivatives (second, third, and fourth) of the potentials and
second-order perturbation theory treatment of nuclear motions
as implemented in GAUSSIAN 09. Note that this procedure did
not allow to account for possible anharmonic resonances or
large amplitude motion vibrational modes. The latter need full
variational treatments, which are out of the scope of this work.
Afterwards, we perform single point (SP) computations at the
(R)CCSD(T)-F12(b)/cc-pVTZ-F12 (+CV+SR+ZPVE) level to derive more
accurate energetics. Briefly, single-point calculations, based on the
optimized structures, are done using the (R)CCSD(T)-F12 (approxi-
mation b) explicitly correlated method38–40 in conjunction with the
cc-pVTZ-F12 explicitly correlated basis sets41 and the corresponding
auxiliary basis sets and density-fitting functions.42–45 Then, we
include the core-valence (CV, as the difference between CCSD(T)/
cc-pwCVTZ46,47 energies with and without considering 1s electrons)
and scalar-relativistic (SR, as the difference between CCSD(T)/
cc-pVTZ-DK48–50 and CCSD(T)/cc-pVTZ energies) corrections.
The zero-point vibrational energy correction (ZPVE) is obtained
at the PBE0/aug-cc-pVDZ level. Through benchmarks and com-
parisons to experiment, we showed earlier that the PBE0/aug-
cc-pVDZ(opt)//(R)CCSD(T)-F12/cc-pVTZ-F12 (+CV+SR+ZPVE) (SP)
composite scheme allows for prediction of ionization energies and
appearance energies (to within Æ0.005 eV) with reduced cost.51–57
We also computed the photoelectron spectrum of MIC following
the method implemented in GAUSSIAN 09, which consists on
a Franck Condon (FC) analysis followed by a simulation of
the vibrationally resolved electronic spectrum by means of the
Time-Independent Adiabatic Hessian Franck–Condon (TI-AH|FC)
model.58–61
3.1. Photoionization mass spectra
Fig. 1a shows the photoionization mass spectra of MIC collected
after single photon ionization at different photon energies in the
range from 12 to 15.5 eV (12, 13.5, 14, 14.5, 15 and 15.5 eV). All
the spectra have been normalized to the base peak, at m/z 41
corresponding to the parent ion CH3NC+. The mass region of the
parent ion is shown in Fig. 1a, between m/z 37 and m/z 43. We
can identify four peaks with different intensities: the most
intense peak, at m/z 41, corresponds to the parent ion CH3NC+.
At increasing photon energy two peaks at m/z 40 and m/z 39 with
lower intensity appear, corresponding respectively to the loss of
atomic hydrogen and molecular hydrogen (or potentially 2H).
They are assigned to fragments with elemental formulas C2H2N+
3. Results and discussion
We present a combined theoretical and experimental investiga-
tion on the single photon ionization of the methyl isocyanide
molecule in the gas-phase, in the 10.5 to 15.5 eV energy range
Fig. 1 Time-of-flight mass spectra of methyl isocyanide recorded
between 12 and 15.5 eV photon energies (0.5 eV step width). (a) Region
of the parent cation m/z 41. (b) Region below m/z 17.
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and C2HN+. The 13C isotopomer of the parent ion CH3NC+ is also
visible at m/z 42 with the expected natural isotopic abundance
ratio. In Fig. 1b, we show a mass spectrum recorded at 15.5 eV
from m/z 11 to m/z 17. In this zoom two fragments ions at m/z 15
and m/z 14 are seen but with very weak intensity. These mass
peaks are assigned to the fragments CH3+ and CH2+ respectively
formed by the loss of CN and HCN (or HNC) molecules.
3.2. Ionization and fragmentation thresholds
Fig. 2 presents the threshold photoelectron photoion coincidence
(TPEPICO) spectrum of m/z 41. One can see the vibronic progres-
sion of the ground electronic state of CH3NC+. This band structure
has been observed in the earlier HeI photoelectron spectra, but
with much less spectral resolution.26,27 The first band corresponds
to the MIC 000 - MIC+ photoionization transition. The other bands
are due to the population of the lowest vibrational levels of the
cation upon ionizing the neutral MIC. Their tentative assignment
shown also in Fig. 2 is discussed in Section 3.3.
The experimental adiabatic ionization energy (AIE) of MIC
can be measured from the TPEPICO spectrum being the center
of the first and most intense band (indicated by a vertical
dotted line in Fig. 2). This band is assigned to the 000 origin of
the progression and corresponds to the transition from the
neutral ground state to the ground state of the cation. We can
deduce AIEexp = 11.263 Æ 0.005 eV. This value has never been
measured which such a precision. In the earlier HeI photoelectron
spectra, values with a large spread of 11.24 eV,26 11.27 eV,27 and
11.32 eV,28 are reported by three different groups (no error bars are
given in the earlier studies). Also, our experimental finding is in
Fig. 3 PEPICO spectra of the CH3NC+ parent ion and 4 fragment ions
observed in the 10.5 to 15.5 eV energy range. These spectra are recorded
with a fixed energy VMI image taken at a photon energy of hn = 15.5 eV.
The energy scale is constructed as the difference of the kinetic energy
scale of the position sensitive electron detector and hn = 15.5 eV (for
details see ref. 31).
This is in line with recent precise determinations of AIEs of
medium-sized molecular systems using similar experimental
and theoretical approaches.52–57
Fig. 3 shows the PEPICO spectra of the five cations observed
in Fig. 1 (m/z 41, 40, 39, 15, 14), in the 10.6–15.5 eV energy
range. These PEPICO spectra are derived from a fixed photon
energy VMI image (at hn = 15.5 eV) taken at a repeller voltage
permitting the detection of all photoelectrons on the circular
detector, as a function of their kinetic energy KEel and in
coincidence with the cation of the respective m/z ratio. The
energy scale in Fig. 3 is constructed as the difference of the
kinetic energy scale of the position sensitive electron detector
and hn = 15.5 eV (for details see ref. 31). In this type of PEPICO
spectra the spectral resolution is low (the step-width is 18 meV)
but sufficient for AE determinations with 0.05 eV precision. The
m/z 41 PEPICO spectrum shows two bands peaking at 11.29 and
12.53 eV corresponding respectively to the ionization of nC and
pNC electrons according to Bevan et al..27 We note that, below
12.8 eV, the parent ion m/z 41 PEPICO spectrum represents a
relative photoionization efficiency curve being proportional to
the total photoionization cross section of MIC in this energy
region. Dissociative photoionization processes take place only
above 12.8 eV. Relative partial and total ionization cross sections
can be determined from these data. The experimental appearance
energies (AEs) are set to the energy position where the ion signal
exceeds the background level and starts rising (see Fig. F1a–d
provided in the ESI†).
excellent agreement with the theoretical AIE value being AIEcalc
=
11.265 eV. In this study AIEcalc has been calculated at the PBE0/
aug-cc-pVDZ(opt)//(R)CCSD(T)-F12/cc-pVTZ-F12 (+CV+SR+ZPVE)
(SP) level (for more details see Table S1 of the ESI†). It falls
into the precision interval of the experimental AIE given above.
Table 1 summarizes the measured AEs together with the
results from the quantum-chemical calculations (cf. Table S2 in
the ESI† for more details). The appearance energy of the intense
fragment ion m/z 40 has been measured to be AEexp (C2H2N+) =
12.80 Æ 0.05 eV. This mass is due to an ion formed by H loss
reaction corresponding to the CH3NC + hn - C2H2N+ + H + eÀ
reaction. The question as to which C2H2N+ isomer is formed is
Fig. 2 TPEPICO spectrum of the methyl isocyanide parent ion (m/z 41).
The experimental adiabatic ionization energy (AIE) is taken as the median
of the 000 band (indicated by a bar and a vertical dotted line). The assign-
ment of the vibrational progression is discussed in the text. Red sticks
corresponds to the computed photoelectron spectrum of MIC (Franck
Condon analysis followed by a simulation of the vibrationally resolved
electronic spectrum).
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Table 1 Calculated and measured appearance energies (AEs) of the different fragmentation pathways of the resulting cation of methyl isocyanide.
These AEs (in eV) are computed at the PBE0/aug-cc-pVDZ(opt)//(R)CCSD(T)-F12/cc-pVTZ-F12 (+CV+SR+ZPVE) (SP) level. For more details, see Table S2
of the ESI
40
39
m/z
15
14
H + H2CNC+
13.499
H + H2CCN+
12.881
H2 + HCCN+
13.940
H2 + HCNC+
13.708
CN + CH3
14.05
HCN + CH2
13.812
+
+
Fragmentation channel
Cal.a
Exp.
12.80 Æ 0.05a
13.21 Æ 0.04b
13.21c
13.70 Æ 0.05a
14.56 Æ 0.08b
14.46c
13.90 Æ 0.05a
13.85 Æ 0.05a
14.76c
a
b
c
This work. Electron impact ionization, ref. 62. Electron impact ionization, ref. 63.
discussed below. The second observed fragmentation is associated Furthermore, ion transmission efficiencies of quadrupole mass
with the reaction channel CH3NC + hn - C2HN+ + H2 + eÀ. analyzers used in the past are poor and thus the sensitivity of
Elimination of two separate hydrogen atoms can be discarded since these instruments is further lowered in comparison with modern
this channel would be much higher in energy given the H2 Time-of-Flight instruments. Both effects lead to too high AE
dissociation energy of 4.52 eV. We measure AEexp (C2HN+) = values which can be off by 1 eV easily.
13.70 Æ 0.05 eV. Also in this reaction, at least two isomers of the
In order to understand in more detail these fragmentation
fragment C2HN+ can be expected either HCCN+ or HCNC+. The AE processes and in particular the chemical structure of the
of the m/z 15 ion, visible upon zooming in the baseline in Fig. 1b, is cationic and neutral fragments formed by the dissociative
determined experimentally to be AEexp = (13.90 Æ 0.05) eV. This photoionization processes, we have undertaken quantum-
ion is produced a priori by loss of CN giving rise to the reaction chemical calculations of the different fragmentation pathways
CH3NC + hn - CH3+ + CN + eÀ. The AE of m/z 14, with similar, presented in Fig. 4. The results are summarized in Table 1. For
low intensity in the TOF-MS shown in Fig. 1b, is determined the m/z 40 (C2H2N+) formed by H loss reaction from the parent
experimentally to be AEexp = 13.85 Æ 0.05 eV. This fragmentation cation, two isomeric ions can potentially be formed (cf. Fig. 4
pathway corresponds to the loss of a species of the elemental and Table 1), namely H2CNC+ or H2CCN+. According to our
formula HCN giving rise to the dissociative reaction CH3NC + theoretical results, the dissociative photoionization pathway (a)
hn - eÀ + CH2+ + HCN (or HNC). Table 1 shows that our AEs are CH3NC + hn - H2CCN+ + H + eÀ is energetically lower, by
much lower than those measured earlier using electron impact 0.618 eV, compared to the channel (b) CH3NC + hn - H2CNC+ +
(EI) ionization by Harland and McIntosh62 and by Heerma and H + eÀ forming an isomeric C2H2N+ ion. Indeed, the computed
deRidder.63 We have discussed recently53 that AEs measured by appearance energy at the PBE0/aug-cc-pVDZ(opt)//(R)CCSD(T)-
EI ionization are generally too high, mainly because with EI F12/cc-pVTZ-F12 (+CV+SR+ZPVE) (SP) level of theory for the
fewer ions are formed in the ion source and by consequence the formation of H2CCN+ is AEcalc (H2CCN+ + H) = 12.881 eV.
instrument’s sensitivity is lower. Since reaction rates increase This value is in excellent agreement with the experimentally
smoothly in the threshold region, this yields too high AEs. measured energy AEexp = 12.80 Æ 0.05 eV. The precision interval of
Fig. 4 Dissociative photoionization pathways and their AE calculated in this study.
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Table 2 Main geometrical parameters (distances in Å and angles in
degrees) of neutral and cationic methyl isocyanide in their electronic
ground states. These data are computed at the PBE0/aug-cc-pVDZ level.
See Fig. 5 for the numbering of the atoms
the latter is only 30 meV below AEcalc. We can thus unambiguously
assign the mass peak at m/z 40 to H2CCN+, at least in the threshold
region. This finding means that there is a possibility that CH3NC
isomerizes to CH3CN before the loss of the H atom. This
isomerization may occur after formation of the CH3NC+ ion
or on the highly excited electronic states of neutral CH3NC
populated by photon absorption followed by electron ejection.
At higher energies above the threshold, notably at E 4 13.5 eV,
energetically both isomeric ions could be potentially formed
according to our measurements.
The experimental AE of m/z 39 (13.7 Æ 0.05 eV) is in excellent
agreement with the calculated AE value for the CH3NC + hn -
HCNC+ + H2 + eÀ fragmentation channel (cf. Table 1). The
calculated AEcalc = 13.708 eV is inside the precision interval of
the experimental AE for this cation. The formation of the
HCCN+ isomeric form is predicted at 13.940 eV i.e., about
25 meV higher than the measured AE. Thus, there would be
no NC isomerization before dissociation. Hence, this fragmenta-
tion exhibits a different behavior upon absorption of photons
compared to the H loss, at least at threshold. Nevertheless, the
HCCN+ + H2 channel may contribute at hn 4 13.94 eV.
The third observed fragmentation m/z 15 is associated with
the fragmentation pathway CH3NC - CH3+ + CN. We measured
for this fragment AEexp = 13.9 Æ 0.05 eV, which is close to the
calculated AEcalc = 14.050 eV. The difference between experi-
mental and theoretical values is only B0.1 eV.
Parameter
CH3NC
CH3NC+
C1–H6
C1–H5
C1–H4
C1–N2
1.096
1.096
1.096
1.415
1.174
180.0
109.2
109.2
109.2
1.097
1.097
1.097
1.431
1.144
180.0
111.4
111.4
111.4
N2–C3
C1–N2–C3
H4–C1–H6
H4–C1–H5
H5–C1–H6
of the neutral MIC and its cation are very close. Thus, upon
ionization, MIC undergoes only very minor structural modifica-
tions. For neutral MIC, our computed geometry is close to the one
65
`
calculated earlier by Margules et al. Inspecting the geometrical
parameters, we observe a small change in the bond length of the
single bond C1–N2, going from 1.415 to 1.431 Å in MIC+ and in the
N2–C3 triple bond, going from 1.174 to 1.144 Å. The HCH bond
angle of the methyl group changes from 109.21 to 111.41. These
small changes lead to the short vibrational progression in the
TPEPICO spectrum of the ground state of MIC+.
We present in Table 3 the anharmonic frequencies calcula-
tions, at the PBE0/aug-cc-pVDZ level, of the eight vibrational
modes of MIC+ in its electronic ground state. These values were
obtained using vibrational second-order perturbation theory.
The frequencies are arranged in a descending order respectively
under a1 and e symmetry. The e symmetry normal modes n+5 to
n+8 are doubly degenerated. In the ESI† we also give, in Table S5,
the anharmonic frequencies of neutral MIC and compare to
experimental data from the literature.66 This table shows a
reasonable agreement between our theoretical calculations and
the available experimental data.
With these data, we can proceed to the assignment of the
vibrationally resolved TPEPICO spectrum in the 11.1–11.8 eV
photon energy range (Fig. 2). We present in Fig. 2 and Table 4 a
tentative assignment of the observed progression. Guided by
the geometry changes between neutral MIC and MIC+ cation
depicted above, we can fully assign these vibrational features to
the pure or combination of the normal modes n+3 (CH3 sym-
metric deformation) and n+4 (C1N2 stretching) excited upon
photoionization. As noted above, the most intense transition
(at hn = 11.263 eV) in the TPEPICO spectrum is assigned to the
000 origin of the progression of the transition from the neutral
The experimental AE of m/z 14 (13.85 Æ 0.05 eV) is also in
excellent agreement with the calculated AE value for the frag-
mentation pathway corresponding to the loss of neutral HCN
(and not HNC) according to CH3NC+ - CH2+ + HCN. Here, we
calculated AEcalc = 13.812 eV being in excellent agreement with
the experiment. Elimination of isomeric HNC, an important
astrophysical molecule but less stable than HCN (by about
0.64 eV),64 can be discarded at least in the threshold region
since the corresponding fragmentation channel would be
higher in energy.
3.3. Tentative assignment of the vibrational structure of the
ground state TPEPICO spectrum
In order to analyze the observed vibronic structure in the
TPEPICO spectrum shown in Fig. 2 we studied the neutral and
cationic forms of MIC in their electronic ground states. Fig. 5
presents the calculated optimized structures of MIC and of MIC+
as computed at the PBE0/aug-cc-pVDZ level of theory. Both
species belong to the C3v symmetry point group. All geometric
parameters are summarized in Table 2. The molecular structures
Fig. 5 Equilibrium structure of the ground state of neutral methyl isocyanide and its cation. We also give the atom numbering used in the present study.
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Table 3 PBE0/aug-cc-pVDZ anharmonic frequencies (n, cmÀ1) of the
vibrational modes of CH3NC+ and the type of vibration. Those of the
neutral species are given in Table S5 of the ESI
can be assigned to 2n+3 + n4+ with a calculated frequency of
3696 cmÀ1
.
Fig. 2 presents the simulated spectrum as described in
Section 2.2. An overall agreement between the measured and
calculated spectra is noticeable in Fig. 2. However, one can see
that some of the computed bands did not match the experi-
mental ones e.g. the one assigned to 310 + 401 combination mode.
This is may be due to the non-consideration of anharmonic
resonances during the computations of the FC factors. Further
extensive treatments are needed as those performed recently
for adenine.57
No.
Sym.
n+
Type of vibration
n1
n2
n3
n4
n5
n6
n7
n8
a1
a1
a1
a1
e
e
e
e
3071
2426
1404
888
3188
1418
1102
324
CH3 s-stretching
NC stretching
CH3 s-deformation
CN stretching
CH3 d-stretching
CH3 d-deformation
CH3 rocking
CNC bending
ground state to the fundamental state of the cation. For the low
intensity band peaking at 11.369 eV (855 cmÀ1 above the origin)
we propose excitation of the C1N1 stretching mode n+4 with
4. Conclusion
one quantum. The computed value for this mode in the cation In this work, we have presented a combined experimental and
(888 cmÀ1) confirms this assignment. We can also propose the theoretical study of the photoionization and dissociative photo-
assignment of the band at 11.437 eV (1403 cmÀ1 above the ionization of methyl isocyanide in the gas phase, from its IE up
origin) to the population of the vibrational mode n+3 corres- to 15.5 eV. The experiments were done by means of VUV
ponding to the excitation of the deformation vibration of the synchrotron radiation from the DESIRS beamline of the Syn-
CH3 with one quantum. This assignment has also been chrotron Soleil in connection with the DELICIOUS 3 spectro-
proposed in ref. 26 and is supported by the observation of an meter. Photoionization mass spectra, TPEPICO and PEPICO
isotopic red shift, of nH/nD = 1.33, observed for CD3NC in their spectra were recorded. The experiments were interpreted with
study. The other bands can be assigned as the combination the help of ab initio quantum chemical calculations. Notably,
excitation of the n+3 with the n+4 modes. The intense band at we calculated the adiabatic ionization energy and appearance
11.544 eV (2266 cmÀ1 above the origin) can be assigned to a energies of fragment cations. The experimental adiabatic ioni-
combined excitation of the n+3 + n+4 modes, with respectively one zation energy is found at AIEexp = 11.263 Æ 0.005 eV, from the 000
quantum. This is in good agreement with our frequency band origin of the vibrational progression corresponding to the
calculations n+3 + n4+ calculated to be 2292 cmÀ1. We note, CH3NC(X) + hn - CH3NC+(X) + eÀ photoionization transition.
however, that it has been proposed earlier (based on experi- It is in excellent agreement with a PBE0/aug-cc-pVDZ (opt)//
ments only) that this band could be due to N2–C3 stretching (R)CCSD(T)-F12/cc-pVTZ-F12 (+CV+SR+ZPVE) (SP) level of the-
excitation.26 This assignment is supported by the absence of an ory determination (AEcalc = 11.265 eV). From the ionization
isotopic shift in the HeI photoelectron spectrum of CD3NC26 for energy up to 12.8 eV the CH3NC+ cation is stable. Above this
this band, and in line with the change of the N2–C3 bond energy, the CH3NC+ intensity drops to zero and four dissocia-
length found in our study. But it is in slight disagreement with tive photoionization reactions have been observed in the energy
our frequency calculation of the NC stretching vibration of range of our study, with the H loss reaction being highest in
n+2 = 2426 cmÀ1 which is off by 160 cmÀ1 from the experimental yield. The respective appearance energies (AEs) have been
band. We therefore favor the n+3 + n4+ assignment for the band at measured, for m/z 40, 39, 15 and 14 cations. In order to analyze
11.544 eV (cf. also the expected accuracy of our frequency further the corresponding reactions, AE calculations have been
calculations from Table S5, ESI†).
performed at the same level of theory as the AIE calculations. As
We finally propose that the band at 11.638 eV (3024.6 cmÀ1 discussed in this study, the excellent agreement between mea-
above the origin) can be assigned to n+3 + 2n4+ calculated to lie at sured and calculated appearance energies allowed for the
3180 cmÀ1. The band at 11.715 eV (3645.6 cmÀ1 above the origin) assignment of the particular fragmentation pathways by dis-
sociative photoionization of CH3NC to form respectively
H2CCN+ + H, HCNC+ + H2, CH3 + CN, and CH2 + HCN with
ascending photon energy. Furthermore, we calculated equili-
brium geometries of the CH3NC neutral molecule and the
monocation as well as the anharmonic frequencies of both
species. Thanks to these theoretical results, the vibrational
progression of the cation ground state TPEPICO spectrum has
been fully assigned. Finally, an important result of the present
study that deserves to be further stressed is the very good
performance of the PBE0/aug-cc-pVDZ (opt)//(R)CCSD(T)-F12/
cc-pVTZ-F12 (+CV+SR+ZPVE) (SP) composite scheme. This
approach, despite the low computational cost, is suitable for
providing precise predictions for ionization and fragmentation
+
+
Table 4 Tentative assignment for the experimentally observed vibrational
frequencies of the photoionization bands from the ground state TPEPICO
(Fig. 2)
Band origin [eV]
Exp. frequency [cmÀ1
]
Assignment
11.263
11.369
11.437
11.544
11.638
11.715
0.0a
855
1403
2266
3025
3646
Adiabatic IE, 000
n4+
n3+
n+3 + n+4
n+3 + 2n+4
2n+3 + n+4
a
Used as reference.
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PCCP
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thresholds close to experimental accuracy, in the case of
medium-sized molecules.
Conflicts of interest
´
10 J. Cernicharo, C. Kahane, M. Guelin and J. Gomez-Gonzalez,
There are no conflicts to declare.
Tentative detection of CH3NC towards Sgr B2, Astron. Astro-
phys., 1988, 189, L1–L2.
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Acknowledgements
This work was supported by the Programme National ‘‘Physique
et Chimie du Milieu Interstellaire’’ (PCMI) of CNRS/INSU with
INC/INP co-funded by CEA and CNES. We thank the COST
ACTION CM 1401 ‘‘Our Astro-chemical history’’ and COST
ACTION CM 1405 ‘‘Molecules in Motions’’ for support. We are
indebted to the general technical staff of Synchrotron SOLEIL for
running the facility under proposals 20141309 and 20170203.
J.-C. G. thanks the Centre National d’Etudes Spatiales (CNES) for
a financial support. We would like also to thank Jean-François
Gil for his technical help on the SAPHIRS molecular beam
`
13 L. Margules, B. Tercero, J. C. Guillemin, R. A. Motiyenko and
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´
chamber. We thank Lionel Poisson, Dusan Boˇzanic and Gustavo
Garcia for precious help during the measurement campaigns at
SOLEIL. We acknowledge the use of the computing center
MesoLUM of the LUMAT research federation (FR LUMAT 2764).
15 K. Kawaguchi, M. Ohishi and S.-I. Ishikawa, Detection of
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