Dissociative Photoionization of X(CH3)3 (X ) N, P, As, Sb, Bi): Mechanism, Trends, and Accurate Energetics




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Dissociative Photoionization of X(CH3)3 (X ) N, P, As, Sb, Bi): Mechanism, Trends, and Accurate Energetics
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Threshold photoelectron photoion coincidence spectroscopy is used to study the dissociation of energy-selected
X(CH3)3
+ ions (X ) As, Sb, Bi) by methyl loss, the only process observed up to 2 eV above the ionization
energy. The ion time-of-flight distributions and the breakdown diagrams are analyzed in terms of the statistical
RRKM theory to obtain accurate ionic dissociation energies. These experiments complement previous studies
on analogous trimethyl compounds of the N group where X ) N and P. However, trimethylamine was observed
to lose only an H atom, whereas trimethylphosphine was shown to lose methyl radical, H atom, and, to a
lesser extent, methane in parallel dissociation reactions. Both kinetic and thermodynamic arguments are needed
to explain these trends. The methyl radical loss has two channels: either a H transfer to the central atom,
followed by CH3 loss, or a direct homolytic bond cleavage. However, the H transfer channel is blocked in
trimethylamine by an H loss channel with an earlier onset, and, thus, the methyl loss is not observed. Bond
energies are defined based on ab initio reaction energies and show that the main thermodynamic reason
behind the trends in the energetics is the significantly weakening CdX double bond in the ion in the N f As
direction. The first adiabatic ionization energies of Sb(CH3)3 and Bi(CH3)3 have also been measured by
ultraviolet photoelectron spectroscopy to be 8.02 ( 0.05 and 8.08 ( 0.05 eV, respectively.
Introduction
Although the chemical properties of homologous compounds
are often similar, small changes in the electronic structure can
result in large differences in chemical properties. We discovered
one such case during the investigation of the dissociation
patte
s of trimethylamine and trimethylphosphine ions.1,2 While
energy-selected trimethylamine ions lose exclusively an H atom,
forming H2CdN(CH3)2
+ ions (isoelectronic with isobutene) over
a large inte
al energy range, the trimethylphosphine ions
dissociate not only by H atom loss but also via methyl and
methane loss, the methyl loss being the lowest energy and
dominant channel. The methyl loss channel can involve a simple
X-C bond break to form X(CH3)2
+ or can take place via a
prior H atom transfer step to produce the pentavalent intermediate,
(CH3)2X(H)dCH2
+, followed by methyl loss to finally
produce the tetravalent (CH3)(H)XdCH2
+ ion. An overview of
these processes is presented in Figure 1. The difference in the
products for the trimethylamine and phosphine cannot be
explained by the thermodynamics of the product species, as the
calculated stability differences between the H atom loss and an
H transfer followed by CH3 loss (the preferred path for methyl
loss in these species) suggest a strong preference for the latter
in trimethylamine, the opposite of what is observed. Therefore,
explaining the trends in the dissociation patte
involves not
only knowing how bond energies change from the second to
the third row central atom but also kinetic arguments.
In order to shed more light on this trend, we have embarked
on a study of three higher mass homologues: trimethylarsane,
-stibane, and -bismane, which completes the series X(CH3)3, X
) N, P, As, Sb, Bi. As in the previous study, ions are produced
in the gas phase, and energy selected by threshold photoelectron
photoion coincidence (TPEPICO) using a monochromatic
photon beam in the vacuum UV range from 7 to 14 eV.3-10
The ion inte
al energy in a photoionization process is given
by Eion ) hν + Eth - IE - Eel, where the quantities are the
photon energy, the neutral molecule’s thermal energy, the
adiabatic ionization energy, and the electron kinetic energy,
respectively. By measuring the ions in coincidence with initially
* Author to whom correspondence should be addressed. E-mail: bsztaray@
pacific.edu.
† Eo¨tvo¨s University.
‡ Paul Scherrer Institut.
§ Current address: Department of Chemistry, Stanford University, Stanford,
CA 94305.
| University of North Carolina.
⊥ University of the Pacific.
Figure 1. Possible dissociation reaction schemes for X(CH3)3
+ (X )
N, P, As, Sb, Bi) at low inte
al energies.
J. Phys. Chem. A XXXX, xxx, 000 A
10.1021/jp900920r CCC: $40.75  XXXX American Chemical Society
Downloaded by AUSTRIA CONSORTIA on July 6, 2009
Published on June 11, 2009 on http://pubs.acs.org | doi: 10.1021/jp900920r
zero-energy electrons (threshold electrons), the ion is effectively
energy selected. The time-of-flight (TOF) mass spectra as a
function of the photon energy provide information about the
ion dissociation energies. The data are then analyzed with the
aid of ab initio calculations of the various reaction paths.
Experimental Section
Trimethylarsane was purchased from Sigma-Aldrich and used
without further purification. Trimethylstibane was synthesized
according to Fleming:11 antimony trichloride was purified by
vacuum sublimation; 25.4 g (0.111 mol) of the halide was
dissolved in 55 cm3 of diethyl ether and added to a 190 cm3
ether solution of methylmagnesium iodide (0.334 mol). The
reaction mixture was subjected to fractional distillations in order
to obtain pure trimethylstibane (16.5 g, 88% yield). Trimethylbismane
was prepared similarly via the method described by
Amberger:12 a solution made of 33.1 g (0.105 mol) of dry
bismuth(III) chloride and 250 cm3 of diethyl ether was added
dropwise to a methylmagnesium iodide solution (0.320 mol in
190 cm3 diethyl ether). The reaction product was purified with
fractional distillation to produce 18.1 g of trimethylbismane
(68% yield).
Photoelectron Spectroscopy. In order to overcome the
significant uncertainty conce
ing the adiabatic IEs, photoelectron
spectra were recorded for Sb(CH3)3 and Bi(CH3)3, using
He(I) radiation with the ATOMKI ESA-32 UV photoelectron
spectrometer, equipped with a Leybold-Heraeus UVS 10/35
high-intensity gas discharge photon source and a hemispherical
analyzer.13 The energy resolution was better than 30 meV, and
the sample was introduced through a gas inlet system at room
temperature. Nitrogen was used as an inte
al standard.
Threshold Photoelectron Photoion Coincidence (TPEPICO).
The TPEPICO apparatus at the University of North
Carolina is only briefly presented, as it has been described in
detail elsewhere.14,15 The sample is introduced in the ionization
chamber through an inlet needle. The sample gas was in thermal
equilibrium with the inlet system; therefore, room temperature
was assumed in the data analysis. Vacuum UV light from a
hydrogen discharge lamp dispersed by a 1 m normal incidence
monochromator is used to ionize the gas-phase thermal sample.
The wavelength resolution of 1 Å translates to 8 meV (0.8 kJ
mol-1) at the onset energies obtained in this study (approximately
10 eV). The photon energy was calibrated using
the intense Lyman-R emission line of the light source at 1215.67
Å. Electrons and ions are accelerated out of the ionization region
in a homogeneous 20 V cm-1 field. Threshold electrons are
velocity focused along the extraction axis onto a 1.5 mm
aperture located 12 cm from the ionization region and detected
by a Channeltron electron multiplier. Because energetic electrons
with initial velocity vectors directed parallel to the extraction
axis are also focused onto this aperture, the threshold electron
signal is contaminated with “hot” electrons. They are accounted
for by collecting a second set of electrons, whose trajectory ends
in a 2 mm × 5 mm rectangular aperture close to the central
aperture where a second Channeltron detects them. Electrons
collected by the off-axis rectangular aperture have been found
to be a good representation of the hot electrons at the central
collector,16 so that the off-axis signal can be used to correct for
hot electron contamination to yield a pure threshold electron
signal.
Ions are accelerated in the constant 5 cm long 20 V cm-1
field to -100 V, and then within 5 mm to -260 V as they
enter the main drift region of the linear time-of-flight analyzer.
The ions travel through a 26 cm long field-free drift region and
are detected by tandem Burle multichannel plates. The two
electron signals are fed into two Ortec 467 time-to-pulse height
converters and used as start signals for the ion TOF measurement,
and the ion TOF distributions are obtained for both the
central and the off-axis electron signals with Ortec TRUMP-
8K multichannel analyzers.
The purpose of the 5 cm long acceleration region is to
disperse slowly dissociation ions into asymmetric fragment ion
peaks, the shape of which provides information about the ion
dissociation rates in the 104-107 s-1 range. In order to extend
the rate measurements to 103 s-1, it is possible to decelerate
ions some 20 cm into the drift region.17 Without this deceleration
stage, daughter ions formed in the field-free region have the
same time-of-flight as their parents. But because their mass is
smaller, they can be decelerated more than the parent ions and
thus distinguished from them. This extends rate information to
nearly the full parent ion TOF.
The experimental results consist of the ion TOF distributions
at various photon energies. The fractional abundances of the
various parent and fragment ions were corrected for the hot
electron contribution and plotted as a function of the photon
energy to produce the breakdown diagram. In some cases, the
TOF distribution of the fragment ion exhibited an asymmetry,
which is characteristic of slow dissociations. The peak shapes
are modeled to obtain the ion dissociation rate constants as a
function of the ion inte
al energy.

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