Photoelectron Spectrum and Energetics of the meta-Xylylene Diradical

Article abstract of DOI:10.1021/jacs.7b06714

The meta-xylylene diradical m-C₈H₈ is a prototypical organic triplet that represents a building block for organic molecule-based magnets and also serves as a model compound for test and refinement of quantum chemical calculations. Flash vacuum pyrolysis of 1,3-bis-iodomethyl-benzene (m-C₈H₈I₂) produces m-C₈H₈ in gas phase; we used photoelectron spectroscopy to probe the first two electronic states of the radical cation, and resolve the vibrational fine structure of the ground state band. The determined adiabatic ionization energy of m-C₈H₈ is (7.27 ± 0.01) eV. Heat of formation of the diradical was established measuring C-I bond dissociation thresholds in the precursor cation and utilizing a thermochemical cycle to yield ΔH_f,298K = (325 ± 8) kJ mol^-1, ca. 10 kJ mol^-1 below the previous value.

Full text of DOI:10.1021/jacs.7b06714

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Communication
On the Photoelectron Spectrum and Energetics of the meta-Xylylene Diradical
Mathias Steglich, Victoria B. F. Custodis, Adam J Trevitt, Gabriel daSilva, Andras Bodi, and Patrick Hemberger
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06714 • Publication Date (Web): 30 Sep 2017
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On the Photoelectron Spectrum and Energetics of the meta-Xylylene
Diradical
Mathias Steglich,† Victoria B. F. Custodis,‡ Adam J. Trevitt,¶ Gabriel daSilva,§ Andras Bodi,†
and Patrick Hemberger∗,†
†Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland
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‡Department of Chemistry and Applied Biosciences, Institute for Chemical and Bioengineering, ETH
Zurich, HCI D 130, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland
¶School of Chemistry, University of Wollongong, New South Wales 2522, Australia
§Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010,
Australia
Received September 27, 2017; E-mail: patrick.hemberger@psi.ch
tion was obtained by pulsed-ion cyclotron double-resonance
spectroscopy.18 Later, it was more precisely reported at
∆Hf,298 K(m-C8H8) = (338 10) kJ mol by combining the
electron affinity of the diradical, (0.919 0.008) eV,7 and
acidity measurements of the 3-methylbenzyl radical as well
as by collision-induced dissociation threshold energy mea-
surements of 3-(chloromethyl)benzyl.19
Abstract: The meta-xylylene diradical m-C8H8 is a prototyp-
ical organic triplet that represents a promising building block
for organic molecule-based magnets and also serves as a model
compound for the test and refinement of quantum chemical
calculations. Flash vacuum pyrolysis of 1,3-bis-iodomethyl-
benzene (m-C8H8I2) produces m-C8H8 in the gas phase and
we used photoelectron spectroscopy to probe the first two
electronic states of the radical cation, and resolve the vibra-
tional fine structure of the ground state band. The experimen-
tally determined adiabatic ionization energy of meta-xylylene
is (7.27 0.01) eV. The heat of formation of the diradical was
established by measuring the C–I bond dissociation thresholds
in the precursor cation and utilizing a thermochemical cycle to
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−1
Unlike the closed-shell isomers para- and ortho-xylylene,
m-C8H8 cannot be synthesized via unimolecular decompo-
sition from the corresponding meta-xylyl radical C8H9.20
The m-xylyl radical isomerizes to ortho- or para-xylyl prior
to hydrogen elimination both in model flames and microre-
actors.21,22 Flash vacuum pyrolysis of 1,3-bis-iodomethyl-
benzene (m-C8H8I2) was found to be an efficient synthesis
route instead.17 Due to the lower dissociation energy of the
C–I bond compared to the C–H bond, the loss of the second
iodine atom is favored to yield the diradical. Here, we used
this method to produce m-C8H8 cleanly in an SiC reactor
heated to (900 100) K and probe it immediately thereafter
with vacuum ultraviolet (VUV) synchrotron light.
Photoelectron spectra of diradical species are scarce, but
much-needed to understand their electronic and magnetic
properties.23–25 In addition to insights into the electronic
and vibrational structure, isobaric species may be identified
isomer-selectively based on their characteristic photoelec-
tron spectral signature. This makes mass spectrometry in
conjunction with photoelectron spectroscopy a highly selec-
tive and sensitive analytical technique.26 Before using photo-
electron photoion coincidence (PEPICO)27 to address com-
plex reactive systems,28 it is often worthwhile to synthesize
and characterize potential reactive intermediates purely.29
In this study, photoion mass-selected threshold photoelec-
tron (ms-TPE) spectroscopy and photoelectron (PE) imag-
ing was applied to obtain the adiabatic ionization energy
(AIE) and characterize the ground and first excited state of
the m-xylylene cation.
−1
yield ∆Hf,298 K = (325 8) kJ mol−1, ca. 10 kJ mol below
the previous value.
Their unique chemistry and intriguing electronic struc-
ture have thrust diradicals in the limelight, as evidenced by
a comprehensive review by Abe from 2013.1 Aromatic di-
radicals, such as meta-xylylene (m-C8H8), have received in-
tense attention by theoreticians.2–5 They are considered as
building blocks for future organic molecule-based magnetic
materials, which possess unique properties and could replace
conventional metallic magnets in a number of applications.6
For example, m-C8H8 can act as a ferromagnetic coupling
unit in a polymeric network3 due to its large triplet–singlet
gap of 0.42 eV.7,8 As a consequence, small perturbations of
the m-C8H8 structural motif will not change the spin na-
ture of the ground state. For comparison, the isoelectronic
meta-benzoquinone has also been studied9–11 and a triplet-
singlet gap of 0.39 eV has been obtained.12 A tetraphenyl-
substituted, stable derivative of m-C8H8 was first observed
by Schlenk and Brauns in 1915.13 The confirmation of the
triplet character of Schlenk’s hydrocarbon by electron spin
resonance (ESR) spectroscopy dates back to 1970.14 The
parent m-C8H8 itself is more difficult to investigate, because
it has to be trapped applying matrix isolation or studied in
the isolated phase using molecular beam techniques in vac-
uum conditions. In a 77 K n-pentane matrix, UV-vis absorp-
tion and fluorescence spectra of m-C8H8 have been reported,
agreeing with a triplet ground state,15 which was eventually
confirmed by ESR spectroscopy.16 Vibrations of the triplet
ground state and first singlet state have been characterized
PEPICO is a powerful technique to measure energetics
data, such as enthalpies of formation or proton affinities
−1
with sub- kJ mol
accuracy in favorable cases.30 Its ap-
plication to open-shell systems, however, has been limited
by the often low signal levels and plagued by large uncer-
−1 31,32
.
tainties in excess of 15 kJ mol
This is because, first,
doublet radicals form closed-shell cations upon ionization,
which rarely dissociate into two open-shell fragments at the
thermochemical threshold. Second, thermochemical cycles,
which bypass this complication by employing the ionization
energy of the radical, rely on a well resolved origin transition
by argon matrix17 and anion photoelectron spectroscopy.7
lower limit of 318 kJ mol for the m-xylylene heat of forma-
A
−1
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(182 kJ mol−1). Interestingly, the first C–I bond breaking
To summarize, we have investigated the photoioniza-
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energy of the precursor cation is only 9.18 eV − 8.50 eV =
0.68 eV (66 kJ mol−1). The significant lower bond energy is
explained by charge stabilization on the benzene ring. The
tion of the m-xylylene diradical using imaging PE and
ms-TPE spectroscopy. The adiabatic ionization energy is
(7.27 0.01) eV. A symmetric in-plane bending vibration of
the methylene groups was found to have comparable vibra-
tional energy in the neutral and cation ground states. The
multiplicity change upon charge transfer in a magnetic net-
work consisting of m-C8H8 units is expected to alter the
electronic and magnetic properties of the material, but not
the structural ones, as is indicated by the high Franck–
Condon factor of the 000 transition in the PE spectrum. The
first excited ion state can be populated only 0.3 eV above
the origin transition, producing electrons with a weak an-
gular anisotropy. Utilizing a cation cycle, we could obtain
the heat of formation of the diradical and found agreement
with computational results using isodesmic and isogyric re-
actions. Reliable thermochemical data as well as information
on the electronic states of organic diradicals are still scarce in
the literature, but are essential, e.g., to evaluate theoretical
methods or to describe magnetic properties. Furthermore,
adiabatic ionization energies are required in general when-
ever spectroscopic techniques rely on ionization detection
schemes, such as resonance-enhanced multiphoton ionization
spectroscopy.
+
positive charge in m-C8H8I2 is about evenly distributed
between the benzene ring and the iodine atoms, while in
m-C8H8I+ it is almost exclusively located on the ring (see
calculated electrostatic potentials in the supporting infor-
mation), i.e., the number of π electrons basically increases
from 5.x to 6, leading to a stabilization due to aromatic-
ity in the closed-shell cation and thus to a lower C–I bond
energy in m-C8H8I2+. Removal of the second iodine then
creates a radical cation, which should be energetically less
favorable than the first iodine removal. Indeed, the second
removal costs 11.05 eV − 9.18 eV = 1.87 eV (180 kJ mol−1),
as an additional resonance stabilization cannot be achieved.
The enthalpy of formation of the precursor 1,3-bis-
iodomethyl benzene has not yet been determined experi-
mentally. Here, we obtained it by calculating the reaction
enthalpy of the isodesmic reaction
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(2)
and using known heats of formation of benzyliodide40 and
benzene.41 Utilizing different theoretical approaches, the
Acknowledgement This project was funded by the
Swiss Federal Office of Energy (SFOE, Contract Number
SI/501269-01) and by the Laboratory for Thermal Processes
and Combustion (LTV) located at PSI. The authors thank
Patrick Ascher for technical support.
mean reaction enthalpy for equation 2 is ∆HR,298 K = (0.3
−1
1) kJ mol
(see supporting information for more details).
After correcting to 0 K,42,43 the enthalpy of formation of
the precursor is then derived as ∆Hf,0K (m-C8H8I2) =
198.6 kJ mol−1. The corresponding value of the iodine radi-
−1 44
.
cal is ∆Hf,0 K(I) = 107.2 kJ mol
Equation 1 can now be evaluated, resulting in ∆Hf,0 K(m-
C8H8) = (349 8) kJ mol−1. The room temperature value is
(325 8) kJ mol−1, somewhat lower than what has been ob-
tained by Hammad et al.,19 but still above the lower limit
of Pollack et al. (see Table 1).18 To compare with the ex-
perimental value, we also obtained the heat of formation of
m-xylylene by computations applying three different reac-
tions:
Supporting Information Avail-
able
The following files are available free of charge.
SuppInfo.pdf. Contains experimental information and de-
tails on appearance energy modelling and formation en-
thalpy calculations.
SuppInfo.zip. Contains output files of quantum chemical
calculations, including energies and coordinates of calculated
molecular structures.
(3)
(4)
(5)
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