Ion Chemistry of OV(OCH3)3 in the Gas Phase: Molecular Cations and Anions and Their Primary Fragmentations

Article abstract of DOI:10.1021/ic030264x

Trimethyl vanadate(V), OV(OCH₃)₃ (1), is examined by various mass spectrometric means. Photoionization experiments yield an ionization energy of IE(OV(OCH₃)₃) = 9.54 ± 0.05 eV for the neutral molecule. The primary fragmentation of the molecular cation 1⁺, i.e., loss of neutral formaldehyde, can occur via two independent routes of hydrogen migrations to afford the formal V^IV compounds HOV(OCH₃)₂⁺ and OV(OCH ₃)(CH₃OH)⁺, respectively. These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies, direct V-O bond cleavage of 1⁺ allows for expulsion of a methoxy radical concomitant with the generation of the cationic fragment OV(OCH₃)₂⁺, a formal V^V compound. Trimethyl vanadate can also form a molecular anion, 1^-, whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V^V compound OV(OCH₃) ₂O^-. Various mass spectrometric experiments and extensive theoretical studies provide detailed insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular cations and anions of 1.

Full text of DOI:10.1021/ic030264x

Inorg. Chem. 2004, 43, 1976−1985
Ion Chemistry of OV(OCH ) in the Gas Phase: Molecular Cations and
3
3
Anions and Their Primary Fragmentations
,
†
†
†
†
†
Detlef Schr o1 der,* Jessica Loos, Marianne Engeser, Helmut Schwarz, Hans-Christian Jankowiak,
†
‡
‡
§
§
Robert Berger, Roland Thissen, Odile Dutuit, Jens D o1 bler, and Joachim Sauer
Institut f u¨ r Chemie der Technischen UniVersit a¨ t Berlin, Strasse des 17 Juni 135,
D-10623 Berlin, Germany, Laboratoire de Chimie Physique, Bat. 350,
UniVersit e´ Paris-Sud, 91405 Orsay, France, and Institut f u¨ r Chemie der Humboldt UniVersit a¨ t
Berlin, Unter den Linden 6, D-10099 Berlin, Germany
Received August 28, 2003
Trimethyl vanadate(V), OV(OCH
experiments yield an ionization energy of IE(OV(OCH )
3
)
3
(1), is examined by various mass spectrometric means. Photoionization
) ) 9.54 ± 0.05 eV for the neutral molecule. The primary
fragmentation of the molecular cation 1 , i.e., loss of neutral formaldehyde, can occur via two independent routes
3 3
+
IV
+
+
3 2 3 3
of hydrogen migrations to afford the formal V compounds HOV(OCH ) and OV(OCH )(CH OH) , respectively.
These two pathways are associated with low-lying activation barriers of almost identical height. At elevated energies,
+
direct V−O bond cleavage of 1 allows for expulsion of a methoxy radical concomitant with the generation of the
+
V
-
cationic fragment OV(OCH
3
)
2
, a formal V compound. Trimethyl vanadate can also form a molecular anion, 1 ,
V
whose most abundant dissociation channel involves loss of a methyl radical, thereby leading to the formal V
-
compound OV(OCH
3
)
2
O . Various mass spectrometric experiments and extensive theoretical studies provide detailed
insight into the ion structures and the relative energetics of the primary dissociation reactions of the molecular
cations and anions of 1.
1. Introduction
Scheme 1
Transition-metal alkoxides are important in various areas
of chemistry as they exhibit interesting properties as bulk
materials. Our particular interest in metal alkoxides arises
from their potential role in metal-mediated oxidation reac-
tions. Activation of an alkane R-H by a reactive metal-
oxo species [M]dO, present on a catalytically active surface,
can occur via formal σ-bond metathesis for which two
options exist (Scheme 1): either hydrogen adds to oxygen
and the alkyl residue is bonded to the metal (route a), or
C-O bond formation yields a metal alkoxide with an
additional [M]-H bond (route b).
studies of Yoshizawa and co-workers3 predict a clear
preference for route a. Nevertheless, also the intermediacy
of metal alkoxides (route b) is not too energy demanding
+
for the methane/FeO system and therefore worth consider-
3
ing. Formation of metal alkoxides has also been proposed
+
In the case of the activation of methane by the bare FeO
cation, a prototype reaction for the bond activation of an
alkane by a transition-metal oxide in the gas phase,1,2 ab initio
4,5
in the reactions of other metal-oxo complexes with alkanes
and is of crucial importance in the dihydroxylation of olefins
6
by high-valent metal oxides such as OsO
4
. Furthermore,
intermediate metal alkoxides may hinder the release of
alkanols (ROH), often the desired products in partial oxida-
*
Author to whom correspondence should be addressed.
Institut f u¨ r Chemie der Technischen Universit a¨ t Berlin.
Laboratoire de Chimie Physique.
†
‡
§
Institut f u¨ r Chemie der Humboldt Universit a¨ t Berlin.
(3) Yoshizawa, K.; Shiota, Y.; Yamabe, T. Chem.sEur. J. 1997, 3, 1160.
(4) Deng, L.; Ziegler, T. Organometallics 1996, 15, 3011.
(5) Fiedler, A.; Kretzschmar, I.; Schr o¨ der, D.; Schwarz, H. J. Am. Chem.
Soc. 1996, 118, 9941.
(
1) Schr o¨ der, D.; Schwarz, H. Angew. Chem. 1995, 107, 2126; Angew.
Chem., Int. Ed. Engl. 1995, 34, 1973.
2) Schr o¨ der, D.; Shaik, S.; Schwarz, H. In Metal-Oxo and Metal-Peroxo
Species in Catalytic Oxidations; Meunier, B., Ed.; Structure and
Bonding, Vol. 97; Springer: Berlin, 2000; p 91.
(
(6) Smith, M. B.; March, J. March’s AdVanced Organic Chemistry;
Wiley: New York, 2001; p 1048 ff.
1976 Inorganic Chemistry, Vol. 43, No. 6, 2004
10.1021/ic030264x CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/18/2004

3 3
Ion Chemistry of OV(OCH ) in the Gas Phase
tion of RH, from the active metal centers present on the
surface of a catalyst. This aspect is quite important in
selective partial oxidation because the primary oxygenation
product ROH is usually more easily oxidized than the alkane
nominal value. At photon energies <11.8 eV, a LiF window was
used to effectively eliminate higher-order light emerging from the
grating of the monochromator. The experiments without a LiF filter
were inherently interfered with higher-order photons and are
reported without corrections. Neutral 1 is sufficiently volatile for
introduction via a regular gas inlet. The neutral species were ionized
by monochromatized photons, and the electrons and cations formed
were extracted in opposite directions by a field of 1 V/cm. In the
present experiments, electrons of low kinetic energies are detected
without detailed velocity analysis, and the cations are transferred
into a QOQ system (Q stands for quadrupole and O for octopole),
Q2 of which was scanned for mass analysis. Cations were detected
by a multichannel plate operating in the counting mode. Unfortu-
nately, compound 1 leads to a serious contamination of the ion
source of CERISES. Moreover, the electron detector mounted
spatially close to the ionization volume showed a continuous
decrease in performance when exposed to 1. Given the strict time
RH, e.g., D
0
(H-CH
2
OH) ) 4.10 eV versus D
0
(H-CH
3
) )
7
4.48 eV. In turn, this reaction is important when oxidation
to carbonyl compounds is desired, e.g., the conversion of
methanol into formaldehyde performed on copper contacts.
Our present investigation is part of collaborative experi-
8
mental and theoretical efforts in the Berlin area on the
mechanisms of oxidation reactions catalyzed by transition-
metal oxides. A particular focus lies on vanadium oxide
catalysts which have widespread applications in redox
9
reactions. For example, V-O-CH
3
surface species are
intermediates in the selective oxidation of methanol to
10
formaldehyde. Within this context, the gas-phase chemistry
of ionized trimethyl vanadate(V), OV(OCH (1), shall
1
schedule at a synchrotron facility, also traces of 3-methyl[4-D ]-
3 3
)
valeramide examined the days before in another project were still
present. For these reasons, the PI experiments were limited to the
provide a benchmark for various experimental and theoretical
studies on vanadium oxides.
+
formation of the molecular ion 1 and its primary photofragments.
Some experiments were also performed in the coincidence mode
2. Methods
17
(TPEPICO ) threshold-photoelectron photoion coincidence), but
Exploratory experiments were performed with a modified VG
due to the decreasing performance of the electron detector in the
presence of 1, only a few data points could be collected, which are
insufficient for the construction of a breakdown graph. Appearance
energies of the photofragments were determined by extrapolating
the linear parts of the respective single-ion monitoring curves to
the baseline. The error estimates given refer to these extrapolations;
neither thermal contributions nor kinetic shifts were considered.
ZAB/HF/AMD 604 four-sector mass spectrometer of BEBE con-
figuration (B stands for magnetic and E for electric sector) which
has been described elsewhere.11 In brief, cations were generated
by electron ionization (EI) of 1 introduced via a solid probe. After
acceleration to a kinetic energy of 8 keV, the ions of interest were
mass-selected and subjected to metastable ion (MI) and collisional
activation (CA) studies. MI spectra of B(1)/E(1) mass-selected ions
were recorded by detection of the charged fragments formed
unimolecularly in the field-free region between E(1) and B(2) by
scanning the latter sector. CA spectra were obtained in the same
manner, except that helium (80% transmittance, T) was used as a
Compound 1 was prepared by reaction of OVCl with sodium
3
methoxide in dry diethyl ether and purified by recrystallization from
pentane. Likewise, fully deuterated OV(OCD ) , 1 , was prepared
3
3
D
from OVCl and CD OLi and purified by sublimation (1 mbar, 80
3
3
°C). The synthesis and the transport of the samples require the use
of inert gases, while the compounds are stable enough to tolerate
short contact with the atmosphere during introduction into the mass
spectrometer. For the photoionization studies, however, the sample
was handled in a glovebox.
-
stationary collision gas. In addition, the molecular anion 1 was
2
generated using N O as a reagent gas and characterized by CA as
well as charge reversal (CR),12 neutralization-reionization (NR),
and the related NIDD method (NIDD ) neutral and ion decomposi-
tion difference);15 in CR and NR, oxygen (80% T) was used as a
collision gas.
13,14
All calculations employed the density functional theory (DFT)
18
modules of the Turbomole 5.5 suite, specifically Becke’s three-
Photoionization (PI) studies were performed in the CERISES
apparatus16 installed on the beam line SA63 of the synchrotron
radiation source SuperACO at LURE (Orsay, France). This beam
line is a normal incidence monochromator which operates in the
range of 7-30 eV photon energy. Slits were opened to 1 mm,
corresponding to a photon-energy resolution of about 500 (i.e., 20
meV at 10 eV). The accuracy of the photon energy was verified
by measuring the ionization energy of argon within 2 meV of its
parameter hybrid functional B3-LYP19,20 and Becke’s exchange
functional with Perdew’s gradient-corrected correlation functional
(B-P86)21 as implemented in Turbomole. The B-P86 functional was
used in conjunction with the resolution of the identity (RI)
technique,22,23 which approximates the computationally demanding
four-center integrals by more simple three-center integrals using
an auxiliary basis without a considerable degradation of the results.
All structures were optimized in all-electron Kohn-Sham calcula-
tions employing Ahlrichs’ valence triple-ú basis sets with polariza-
(
7) Berkowitz, J.; Ellison, G. B.; Gutman, D. J. Phys. Chem. 1994, 98,
744.
24
tion functions on all atoms (TZVP). The two different functionals
2
(
8) Sonderforschungsbereich 546. Structure, dynamics and reactivity of
transition-metal oxide aggregates. http://www.chemie.hu-berlin.de/
sfb546/index.html.
(17) Baer, T. In Gas-Phase Ion Chemistry; Bowers, M. T., Ed.; Academic
Press: New York, 1979; Vol. 1, p 153.
(
9) Appl. Catal. A 1997, 157 (the entire volume deals with this topic).
(18) Ahlrichs, R.; B a¨ r, M.; H a¨ ser, M.; Horn, H.; K o¨ lmel, C. Chem. Phys.
Lett. 1989, 162, 165.
(19) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(20) Stephens, P. J.; Derlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys.
Chem. 1994, 98, 11623.
(
(
10) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25.
11) Schalley, C. A.; Schr o¨ der, D.; Schwarz, H. Int. J. Mass Spectrom.
Ion Processes 1996, 153, 173.
12) Bursey, M. M. Mass Spectrom. ReV. 1990, 9, 555.
13) Goldberg, N.; Schwarz, H. Acc. Chem. Res. 1994, 27, 347.
14) Ture cˇ ek, F. Top. Curr. Chem. 2003, 225, 77.
15) Schalley, C. A.; Hornung, G.; Schr o¨ der, D.; Schwarz, H. Chem. Soc.
ReV. 1998, 27, 91.
16) Dutuit; O.; Alcaraz, C.; Gerlich, D.; Guyon, P. M.; Hepburn, J. W.;
(
(
(
(
(21) (a) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (b) Becke, A. D. Phys.
ReV. A 1988, 38, 3098.
(22) Eichkorn, K.; Treutler, O.; O¨ hm, H.; H a¨ ser, M.; Ahlrichs, R. Chem.
Phys. Lett. 1995, 240, 283.
(
(23) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem.
Acc. 1997, 97, 119.
Metayer-Zeitoun, C.; Ozenne, J. B.; Weng, T. Chem. Phys. 1996, 209,
177.
(24) Sch a¨ fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
Inorganic Chemistry, Vol. 43, No. 6, 2004 1977

Schr o1 der et al.
used (B3-LYP and B-P86) agree well as far as relative energies
are concerned. For the sake of brevity, only B3-LYP energies are
reported here because this hybrid functional shows a somewhat
better performance in the prediction of thermochemical data.25
3. Experimental Results
EI mass spectra of several trialkyl vanadates(V) have
already been reported by Adler et al. in 1976. Unlike the
larger homologues, the trimethyl compound OV(OCH
3
2
3
)
3
, 1,
Harmonic vibrational frequencies were calculated from analytical
+
gives rise to an abundant molecular ion, 1 (m/z ) 160),
with an intensity of ca. 20% relative to the base peak at m/z
) 130. The latter is part of a “quartet” of fragment ions
between m/z ) 127 and m/z ) 130 with intensities of ca.
70:20:20:100 upon EI, a spectral feature to which we return
further below. In addition, loss of atomic hydrogen, [1 -
second derivatives of the total electronic energy. The optimized
structures are visualized with the program Molden.26 The transition
+
structures for hydrogen migrations within the cation 1 (see below)
were located with Gaussian 98,27 using B3-LYP in conjunction with
the QST (quadratic synchronous transit) approach, followed by
energy calculations as well as gradient and second-derivative
calculations with Turbomole. Recalculation of the energy is
necessary due to different implementation of some functionals in
the two program packages. However, the gradients calculated with
Turbomole were small so that the effect on the structures was
considered negligible. Using the harmonic frequencies, thermody-
namic properties at 0 and 298 K were computed with the program
Viewmol 2.3.
+
H] (m/z ) 159), was observed by Adler et al. along with
some consecutive fragmentations. Our EI measurements are
fully consistent with these earlier data. Investigation of the
perdeuterated compound OV(OCD
quartet of fragments (shifted to m/z ) 131, 133, 135, and
37) to be assigned to formal losses of CX O (X ) H, D; n
5-2), respectively, from the molecular ion. In the
3 3
) (1) further allows the
1
)
n
On the basis of the B3-LYP results, the Franck-Condon factors,
which determine the vibrational fine structure of the photoionization
process, have been obtained along the lines of a procedure described
previously.28 Briefly, the equilibrium structures of the electronic
ground states of the neutral species and of the molecular cation
together with the corresponding unscaled harmonic force fields were
used to obtain the multidimensional Franck-Condon integrals with
the aid of the recurrence formulas derived by Doktorov, Malkin,
and Man’ko,29 which take mode mixing effects and geometrical
changes into account.30 Although the computational scheme
described in ref 28 allows for separable one-dimensional anhar-
monic treatments, all internal degrees of freedom of the molecular
systems studied here were considered as harmonic modes. Each of
following sections, the dissociation behavior of the molecular
+
ion 1 is examined by several mass spectrometric means. In
-
addition, the fragmentations of the molecular anion 1 and
of the potassium adduct [1‚K] are discussed briefly.
+
+
3
.1. Fragmentation Behavior of 1 . The MI spectrum
+
of mass-selected 1 recorded in the sector-field mass
spectrometer is dominated by expulsion of formaldehyde
(∆m ) -30) along with small amounts of H and CH O
2 4
losses (Table 1). The formal elimination of CH O (∆m )
-32) may be assigned to either consecutive expulsions of
4
2 2
CH O and H or the generation of an intact methanol
molecule. Upon CA, loss of atomic hydrogen to afford [1 -
1
the equivalent C -symmetric structures of the cation was treated
+
H] appears as a new channel, and the fragments observed
as an isolated minimum, and couplings between the vibrational
levels of these structures were neglected. The Franck-Condon
profiles have been calculated for 0 K; no major changes of the
Franck-Condon envelope for ionization of 1 and 1 were found
D
for a vibrational Boltzmann distribution at 298 K. The correspond-
ing quantum number combinations of the numerous vibrational
overlap integrals have been generated with an algorithm described
elsewhere.31 From the resulting Franck-Condon envelope, the
between ∆m ) -30 and ∆m ) -33 resemble the quartet
observed in the EI mass spectrum. The notable signal for
∆
m ) -31 evolving upon CA can clearly be assigned to
+
the loss of an intact methoxy group from 1 . Absence of
this channel in the MI spectrum implies that direct V-O
bond cleavage is a high-energy process which is inaccessible
+
for metastable 1 .
+
relative energy-dependent photoionization cross-sections have been
obtained assuming a stepwise increase of the photoionization cross-
section for each vibronic transition.
Similar to the molecular ion 1 , the MI and CA spectra
+
of [1 - H] are characterized by losses of formaldehyde
(∆m ) -30). Upon CA, also expulsions of CH O and CH O
3
4
are observed. In contrast, the MI spectra of the ions [1 -
+
+
(
(
(
25) Koch, W.; Holthausen, M. C. A Chemist’s Guide to Density Functional
Theory; Wiley-VCH: Weinheim, Germany, 2000; p 137.
26) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000,
2 3
CH O] and [1 - CH O] both show pronounced dehydro-
genations (∆m ) -2) in that considerable percentages of
the mass-selected ions decompose in the field-free region
14, 123.
27) Gaussian 98, Revision A.7: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski,
V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, Dapprich,
S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas,
O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.;
Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.;
Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul,
A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M.
A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.;
Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian,
Inc., Pittsburgh, PA, 1998.
(
see footnotes b and c of Table 1); usually, the fraction of
33
decomposing ions is far below 1% in MI spectra. Obvi-
ously, loss of molecular hydrogen is particularly facile for
these two ions formed upon dissociative EI of 1. Therefore,
not surprisingly, also the corresponding CA spectra are
dominated by dehydrogenation. The consecutive dehydro-
genation reactions provide a rationale for the quartet observed
between m/z ) 127 and m/z ) 130. Loss of formaldehyde
+
from 1 (m/z ) 160) gives rise to m/z ) 130, which then
(
28) Berger, R.; Fischer, C.; Klessinger, M. J. Phys. Chem. A 1998, 102,
(32) Adler, B.; Lachowicz, A.; Thiele, K.-H. Z. Anorg. Allg. Chem. 1976,
427, 241.
7157.
(
(
(
29) Doktorov, E.; Malkin, I.; Man’ko, V. J. Mol. Spectrosc. 1977, 56, 1.
30) Dushinsky, F. Acta Physicochim. URSS 1937, 7, 551.
31) Berger, R.; Klessinger, M. J. Comput. Chem. 1997, 18, 4887.
(33) For another example of a rapid, unimolecular dehydrogenation of
transition-metal complexes, see: Schr o¨ der, D.; Schwarz, H. J. Am.
Chem. Soc. 1990, 112, 5947.
1978 Inorganic Chemistry, Vol. 43, No. 6, 2004

3 3
Ion Chemistry of OV(OCH ) in the Gas Phase
a
+
Table 1. Mass Differences (∆m) Observed in the MI and CA Mass Spectra of B(1)/E(1) Mass-Selected 1 and Its Major Fragment Ions
-1
-2
-15
-18
-28
-29
-30
-31
-32
-33
1+
MI
CA
MI
CA
2
1
100
100
100
100
1
10
2
4
100
100
65
100
3
2
7
15
15
15
4
6
[
[
[
[
[
1 - H]+
2
1
3
3
1
20
1
1 - CH2O]+
1 - CH3O]+
1 - “CH4O”]+
1 - “CH5O”]+
MI
b
100
100
100
100
2
4
100
10
CA
5
1
5
1
5
c
MI
CA
MI
CA
MI
CA
CA
CA
CA
4
5
1
35
12
20
2
15
2
1
15
30
1
20
25
3
3
2
10
75
+
+
+
d
d
d
[1 - “C2H5O2”]
[1 - “C2H6O2”]
[1 - “C2H7O2”]
100
40
2
15
100
100
a
b
Given relative to the base peak (100). For the sake of brevity, only the major signals are listed and successive fragmentations are omitted. Unusually
c
intense metastable-ion decay; the ratio between the mass-selected parent ion and the fragment due to loss of H2 is about 18:1. Unusually intense metastable-
ion decay; the ratio between the mass-selected parent ion and the fragment due to loss of H2 is about 22:1. d The corresponding MI spectra are very weak
and most likely contaminated by CA due to residual gas present as a background in the mass spectrometer.
Scheme 2
+
)+ for n
loses H
2
to yield m/z ) 128. Likewise, loss of methoxy from
assignments of these [VCH
n
O
2
] ions as OV(OCH
2
+
+
+
1
leads to m/z ) 129 and then m/z ) 127 via dehydrogen-
) 2, OV(OCH ) and/or HOV(OCH ) for n ) 3, and HOV-
3
2
3
4
+
+
ation. The resulting product ions [1 - CH
CH
4
O] and [1 -
(OCH ) for n ) 4.
3
+
5
O] show more diverse fragmentation patterns, although
With this information, a tentative network of the frag-
mentation behavior of 1 can be deduced (Scheme 2).
Because of the obvious remaining ambiguities in terms of
structural assignments, we deliberately distinguish between
the experimental observables and the assigned ion structures.
For example, the ionic fragment [1 - CH
loss of formaldehyde may correspond to either 3 or isomeric
a . At low energies, the molecular ion 1 may undergo a
hydrogen rearrangement (1 f 2 ) followed by loss of
formaldehyde to afford 3 . In this redox reaction, the
formally hypervalent molecular ion 1 is reduced to V in
compounds 2 and 3 . Alternatively, 1 can undergo
hydrogen migration from one methoxy unit to the oxygen
atom of another one, leading to the isomeric species 2a ,
from which loss of formaldehyde affords the methanol
complex 3a . The subsequent dehydrogenation step either
can proceed from 3 with retention of the VOH unit
losses of formaldehyde (∆m ) -30) prevail once more.
Notable is the difference between the MI and CA spectra of
+
+
[1 - CH
5
O] , indicating that the losses of H
2
(∆m ) -2)
and CO (∆m ) -28) involve low-energy pathways associ-
ated with more complex rearrangements which cannot
compete with formaldehyde elimination upon CA. The next
generation of daughter ions, [1 - C
and [1 - C
+
2
O] formed upon
+
+
+
2
H
5
O
2
] , [1 - C
2
H
6
O
2
] ,
+
+
3
+
+
H O
2 7 2
] correspond to [VCH
n
O
2
] species (n )
+
+
+
2-4) and can be ascribed to losses of CO from [1 - CH
5
O]
+
+
and expulsions of CH
2
O from [1 - CH
4
O] and [1 -
+
IV
+
CH
5
O] . Dehydrogenation predominates in the CA spectrum
+
+
+
+
of [1 - C
H O
2 5 2
] , whereas losses of formaldehyde prevail
+
upon CA of the two other ions. For [1 - C
pronounced VO fragment (∆m ) -31) is observed,
suggesting that the elimination of CH O is either kinetically
3
or thermodynamically favorable for this ion. These frag-
mentation patterns are consistent with tentative structural
2
H
6
O
2
] , also a
+
+
+
+
+
(
formation of 4 ) or may be associated with a reconstruction
+
+
(
34) Consecutive losses of CH2O and H2 have already been reported in
the mass spectra of alkoxysilazanes as early as 1967; see: Silbiger J.;
Lifshitz, C.; Fuchs, J.; Mandelbaum, A. J. Am. Chem. Soc. 1967, 89,
of the vanadium-oxo unit in 4a . For 3a , dehydrogenation
+
+
is assumed to yield 4a as well. In addition, 4a may be
4308.
formed directly upon loss of methanol rather than CH O +
2
Inorganic Chemistry, Vol. 43, No. 6, 2004 1979

Schr o1 der et al.
+
2
H from 2a (not included in Scheme 2). Assignment of
+
+
structure 4a to the [1 - CH
4
O] fragment is supported by
the losses of CH O and CH O observed upon CA (Table 1).
2
3
However, the expulsion of water observed in the MI spectrum
+
of [1 - CH
4
O] may indicate contributions of isomeric
+
structures (e.g., 4 ). Subsequent loss of formaldehyde from
+
+
+
+
4
5
or 4a leads to [VCH
a appear conceivable (see above). Note that 5 corresponds
to a formal V compound, whereas conservation of the V
valence is implied in structure 5a . Upon collisional activa-
tion, eliminations of formaldehyde from 5 and of methoxy
from 5a lead to VOH and VO , respectively, as the quasi-
terminal fragments; the ultimate terminus is the atomic V
O
3 2
] for which structures 5 and
+
+
II
IV
+
+
+
+
+
+
3
5
cation.
The reactions of the corresponding ions resulting from
initial losses of open-shell fragments are quite similar.
+
+
+
Formation of [1 - H] is assigned to R-cleavage (1 f 6 ),
and the subsequently observed elimination of formaldehyde
+
+
leads to 7 , which is also accessible directly from 1 by
homolytic V-OCH bond cleavage at elevated energies. As
open-shell fragments are lost from the molecular ion,
3
+
+
V
structures 6 and 7 correspond to formal V compounds.
Figure 1. PI mass spectrum of OV(OCD ) at photon energies of (a) 17
3
3
eV and (b) 11 eV. Some of the background signals (labeled with an asterisk)
are due to residual 3-methyl[4-D1]valeramide which was present in the
machine from previous experiments.
The subsequent elimination of H
with a reduction to V . The assignment of structure 8 is
supported by the almost complete absence of methoxy loss
2
is assumed to be associated
III
+
+
(∆m ) -31) in the CA spectrum of [1 - CH
5
O] . However,
because of some interferences due to the remainder of a
sample from other experiments (see the Methods). Not
surprisingly, considerable changes occur at lower photon
+
the decarbonylation observed for [1 - CH
some contribution of isomeric structures; in this respect, a
5
O] points to
+
formal 1,1-elimination of H
2
from 7 to eventually afford
energies. The PI mass spectrum obtained with 11 eV photons
+
36
+
HOV(OCH
3
)(CO) appears reasonable. Loss of formalde-
(Figure 1b) is dominated by the molecular ion 1
D
as well
+
+
hyde from 8 is suggested to afford the formaldehyde
as [1
D
- CD
2
O] , whereas the other fragments are either
+
complex 9 , from which elimination of CH
2
O leads to the
largely suppressed or absent. Particularly, the absence of [1
D
+
+
+
VO cation.
- D] and [1
D
3
- CD O] ions confirms the qualitative
Even though these structural suggestions are consistent
with the data given in Table 1, several isomeric ion structures
and reaction mechanisms are conceivable. Therefore, let us
consider Scheme 2 as a first assumption which shall be tested
in light of the results of further experimental and theoretical
studies, where we restrict ourselves to the molecular ion 1+
and the primary fragmentations reported in this study.
deduction made above that the losses of open-shell fragments
from 1 correspond to high-energy processes.
To gain quantitative information about the fragmentation
routes, appearance energies (AEs) were determined by
+
variation of the photon energy in single-ion monitoring mode.
+
The parent ion 1
D
(m/z ) 169) shows a threshold of AE-
+
(1
D
) ) 9.56 ( 0.04 eV (Figure 2a). The rapid rise of the
ion yield above threshold is consistent with formation of the
3
D
.2. Photoionization of 1 . While the above discussion
+
+
molecular ion 1
D
D
and suggests that AE(1 ) is close to the
of the sector-field experiments is largely qualitative, PI
studies can provide reliable absolute energetic information
about ionized 1. In view of the limited mass resolution of
CERISES and the decreased sensitivity at elevated mass
resolution, all PI experiments were performed with the fully
deuterated compound OV(OCD ) , 1 .
3 3 D
The PI mass spectrum obtained at a photon energy of 17
eV (Figure 1a) resembles the EI mass spectrum of 1: the
molecular ion 1
comparable abundance, a weak signal for [1
adiabatic ionization energy IE
a
(1 ); a more detailed analysis
D
of this aspect is presented further below. In comparison, the
curves of the fragment ions rise more smoothly (Figure 2b-
3
7
f) in accordance with their origin in dissociative photo-
ionization of 1
from the molecular ion, AE([1
D
. The lowest onset is found for loss of D
2
+
D
- D
2
] ) ) 10.0 ( 0.2 eV
(not shown); however, this fragment never exceeds 1% of
+
+
the total photoions and is therefore not considered any
D
and the fragment [1
D
- D] are of
+
+
further, except stating that the direct dehydrogenation of 1D
D
- D
2
]
is
appears to be subject to serious kinetic restrictions associated
with a rather complex transition structure. This conclusion
is further supported by the low abundance of dehydrogena-
observed, and the characteristic quartet appears at m/z ) 131,
33, 135, and 137 for the deuterated compound. In the PI
studies, ions at lower masses are not considered any further
1
+
tion in the MI and CA spectra of 1 in the sector-field
(
35) Schr o¨ der, D.; Engeser, M.; Schwarz, H.; Harvey, J. N. Chem. Phys.
Chem. 2002, 3, 584.
(37) From the PI threshold, the maximum yield of 1+ is achieved in less
than 0.5 eV, whereas the ions formed upon dissociative PI arise more
smoothly; see Figure 2.
(36) See also: Pr u¨ sse, T.; Fiedler, A.; Schwarz, H. J. Am. Chem. Soc. 1991,
113, 8335.
1980 Inorganic Chemistry, Vol. 43, No. 6, 2004

3 3
Ion Chemistry of OV(OCH ) in the Gas Phase
Figure 3. Photoelectron spectrum of OV(OCD3)3 in the energy window
ranging from 9 to 17 eV. The photoelectron signal observed below AE(1D)
)
9.56 ( 0.04 eV is due to residual 3-methyl[4-D1]valeramide from previous
experiments as well as a deterioration of the electron detector in the presence
of the vanadium compound.
cally and prevail in the PI spectrum at 17 eV (Figure 1a).
While pronounced competition between fragmentations to
closed-shell neutrals via rearrangements and direct bond
cleavages to afford open-shell neutrals is well-known in mass
spectrometry, it is somewhat surprising to observe such an
effect more than 3 eV above the first fragmentation
thresholds. Furthermore, if one assumes an efficiently
Figure 2. Single-ion monitoring of selected photoions formed from neutral
D as a function of photon energy (eV) in the order of their appearance
3
competing V-OCD bond cleavage due to kinetic prefer-
1
+
+
+
+
ences of direct bond rupture, the difference of AE([1 -
D
energies: (a) 1D , (b) [1D - CD2O] , (c) [1D - CD4O] , (d) [1D - D] ,
+
+
+
+
(
e) [1D - CD3O] , and (f) [1D - CD5O] . Note that the two latter spectra
CD
3
O] ) ) 13.4 ( 0.2 eV and AE(1 ) ) 9.56 ( 0.04 eV
D
were recorded without a LiF window as a cutoff filter of higher-order
photons. The vertical scales refer to the relative ion currents with the LiF
window (a-d) and without (e, f). The corresponding appearance energies
are determined by extrapolation of the linear parts of the signal onsets to
the baseline (see the Methods).
would imply in the absence of reaction barriers a dissociation
+
energy of D(OV(OCD
3
)
2
-OCD ) ) 3.8 ( 0.2 eV, which
3
appears unrealistically large for the molecular cation of a
0
neutral 3d compound, because ionization to the cation state
must involve a bonding orbital. Inspection of the photoelec-
+
experiments. With AE([1
(
D
- CD
2
O] ) ) 10.1 ( 0.1 eV
trons recorded in parallel provides a hint for an alternative
Figure 2b), loss of formaldehyde occurs only about 0.5 eV
above the ionization threshold of neutral 1
and subsequent loss of formal-
dehyde are quite facile. Within experimental uncertainty, the
O] ) ) 10.1 ( 0.1 eV is
found for the fragment assigned to the combined losses of
CD O and D (Figure 2c), even though this photofragment
rises more smoothly with photon energy in comparison to
+
explanation. Thus, the appearance of [1
D
- CD
3
O] at 13.4
D
, suggesting that
eV coincides with the onset of a broad feature in the
+
deuterium migration in 1
D
photoelectron spectrum of 1 with a maximum at about 15
D
38
eV (Figure 3). This finding tentatively suggests that the
loss of methoxy radical may be nascent from the ionization
+
same threshold of AE([1
D
- CD
4
of the neutral compound into particular ion states which differ
2
2
+
from the ground state of 1
D
3
in that direct V-OCD bond
cleavage is preferred (see below).
+
[1
D
- CD
2
O] (Figure 2b). The finding that these two
-
3
.3. Anionic 1 and a Cationized Mimic of Neutral 1.
channels have more or less identical thresholds is somewhat
surprising and reiterates the option that CD OD rather than
CD O + D might be formed at threshold, an issue to which
we will return in more detail in the Discussion.
The energetically least demanding expulsion of a neutral
In the sector-field mass spectrometer, we also briefly
3
-
investigated the behavior of the molecular anion 1 , which
2
2
can be formed upon chemical ionization (CI) of neutral 1
-
2
with N O as reagent gas; the actual ionizing agent is O
formed upon dissociative electron attachment to N
CI conditions. Collisional activation of 1 affords losses of
2
O under
open-shell fragment upon photoionization of 1
D
is loss of a
-
+
D atom with AE([1
D
- D] ) ) 11.0 ( 0.2 eV (Figure 2d).
CH
3
(∆m ) -15, 100%) and CH O (∆m ) -31, 15%) as
3
Elimination of a methoxy radical and the subsequent dehy-
drogenation do not occur below 11.8 eV, which is the cutoff
of the LiF filter used to eliminate higher-order light emerging
the major fragments; two minor routes (∆m ) -45 and -47,
both 3%) are assigned to consecutive dissociations. The
-
IV
molecular anion 1 corresponds to a formal V compound
from the monochromator. Without the LiF filter, AE([1 -
D
V
such that the system returns to V in the anionic fragment
+
+
CD
3
O] ) ) 13.4 ( 0.2 eV and AE([1
D
- CD
5
O] ) ) 14.6
3
O and CD O
-
OV(OCH
The NR
3
)
2
1
O upon the loss of the open-shell fragment CH
and CR mass spectra of the anion 1 to
cationic species show rich fragmentation patterns with VO
3
.
(
+
0.1 eV are found for the expulsions of CD
, respectively (Figure 2e,f). Both values are well above
3
3,14
12
-
D
2
+
+
the appearance energies of 1
D
and the ionic fragments
formed upon losses of neutral closed-shell molecules. Hence,
significant kinetic shifts of the threshold energies are to be
expected due to competition. Nevertheless, both fragments
rapidly gain in abundance once they are accessible energeti-
(
38) Note that the photoelectron spectra (PES) obtained with CERISES
differ from conventional He(I) PES. In the latter, the electron energy
is measured for a constant wavelength, whereas in our experiments
the photon energy is varied while only electrons with kinetic energies
lower than ca. 50 meV are detected.
Inorganic Chemistry, Vol. 43, No. 6, 2004 1981

Schr o1 der et al.
Table 2. Relative Energies (eV) and Selected Bond Lengths (Å) and Angles (deg) of Neutral 1 and of Some Ionic Species Calculated with
B3-LYP/TZVP
1-
1+
TS 1 /2+
+
2+
3+
+
+
2a+
3a+
4+
4a+
7+
1
TS 1 /2a
symmetry
Cs
C3v
Cs
0
C1
C1
-1.47
1.78
2.00
1.72
C1a
-0.5
1.72
C1
C1
C1
0.17
1.57
Cs
C1
Cs
2.16
1.56
b
0.43c
1.77
d
Erel
-10.35
1.63
1.87
1.89
1.89
1.39
1.39
1.10
-9.22
1.59
1.77
1.77
1.77
1.41
1.41
1.09
0.83
1.64
1.91
1.72
0.82
1.56
1.90
1.71
1.85
1.33
1.45
1.28
1.50
115
109
-1
-1.4
1.58
2.03
1.75
2.05
1.23
1.47
0.37
1.57
2.01
1.73
1
e
r(V- O)
1.57
1.88
1.75
1.75
1.39
1.42
1.09
2
f
r(V- O
3
r(V- O)
1.70
1.72
2.01
1.99
1.23
1.71
1.44
4
g
r(V- O)
1
2
r( C- O)
1.32
1.43
1.32
1.45
90
1.22
1.42
1.23
1.43
2
3
r( C- O)
1.43
0.97
121
1.43
0.97
h
r(C-H)
1
r( O-H)
0.97
97
109
0.97
105
122
3
1
2
R( OV O)
117
110
13
108
108
0
108
111
5
1
3
R( OV O)
114
114
17
-10
129
20
-2
110
20
-2
111
-6
6
1
2
θ( OV CH)
1
2′
θ( OV CH)
-13
0
-5
28
33
a
b
Quasi-C2V symmetry, since the C2V-symmetrical structure is only very slightly higher in energy than the C1-symmetrical minimum. Energies (eV)
+
+
+
+
+
+
c
+
relative to the cation 1 at 0 K. The following neutral fragments are added: CH2O for 3 and 3a , CH3OH for 4 and 4a , CH3O for 7 . 1.25 eV for 4
d
+
e
1
f
2
+
CH2O + H2. 1.19 eV for 4a + CH2O + H2. The notation O stands for the oxygen atom of the vanadyl group. The notation O stands for the oxygen
g
atom of the methoxy group which isomerizes in the cation state. Bond lengths to the oxygen atom which becomes part of the methanol unit in the rearrangement
+
+ h
1
f 2a . Only the bond length of the hydrogen atom which migrates in the cation state is reported. Variations in the other C-H bond lengths are less
than 0.02 Å and thus not displayed.
as the base peak. For the sake of brevity, we therefore refrain
from displaying the spectra as figures and list the major
fragments instead (m/z ratios and abundances relative to VO ,
(1.77 Å) to the methoxy groups (Table 2). The methoxy units
are hardly perturbed compared to either free methoxy radical
or methanol. Even though the crystal structure of 1 is known,
direct comparison is not sensible because solid 1 forms a
bridged dimer with additional coordinations. However, the
two vanadyl bond lengths of 1.51 and 1.57 Å and the V-O
distances to the nonbridging methoxy ligands (1.74 Å)
determined in the X-ray analysis lend some confidence to
our computational results.
+
m/z ) 67, in NR/CR): 159 (6/2), 130 (1/2), 129 (8/5), 128
43
(8/8), 127 (20/12), 113 (6/12), 99 (14/18), 98 (37/50), 97
(36/52), 84 (18/21), 83 (22/18), 68 (41/40), 67 (100/100),
5
1 (28/18), 31 (3/1), 30 (9/4), 29 (15/6). While neither of
+
the two spectra bears a survivor ion (1 ), the observed [1 -
H] fragment (m/z ) 159) suggests that some species can
+
remain almost intact. Nevertheless, quite extensive fragmen-
tations prevail in both spectra which are assigned to occur
at the cationic stage after the high-energy collision events
4.1. Molecular Ions. The highest occupied molecular
orbital of neutral 1 consists of nonbonding p-orbitals located
at the oxygen atoms of all three methoxy groups, whereas
the occupied orbitals of the vanadyl group are much lower
in energy. Therefore, ionization to the molecular cation
involves the methoxy manifold and is associated with a
3
(see below).
The difference of the NR and CR spectra is relatively
small, which can be seen as an indication that the transient
-
neutral formed upon electron detachment from 1 does not
considerable lengthening of one V-OCH bond from 1.77
1
5
+
undergo pronounced unimolecular decompositions. This
conclusion is consistent with the thermal stability of neutral
Å in neutral 1 to 1.88 Å in 1 . As a compensation, the two
other V-OCH bonds and the V-O bond of the vanadyl
3
3
9
trialkyl vanadates(V) in the condensed phase; in the
group are slightly shortened by 0.02 Å. These changes in
particular case of neutral 1, thermolysis was reported to
geometry are reflected in the difference of the adiabatic and
the vertical ionization energies of 1. With B3-LYP/TZVP,
40,41
afford loss of methanol.
Similar behavior is found for
+
mass-selected [1‚K] , a cationized mimic of neutral 1 which
can be generated by electrospray ionization of a methanolic
solution of 1 containing a trace of KBr.42 Thus, the only
the corresponding values are predicted as IE (1) ) 9.25 eV
a
and IE (1) ) 9.35 eV; thus, ∆IE (1) ) 0.10 eV. To evaluate
v
v/a
the basis set dependence, further calculations with Dunning-
type correlation-consistent basis sets were performed includ-
ing geometry reoptimization of neutral 1. This approach leads
v
+
fragments observed upon low-energy CA of [1‚K] at various
collision energies (5-20 eV) correspond to a weak expulsion
of methanol (∆m ) -32) and the prevailing loss of the entire
to a slight increase of IE (1) from 9.350 eV (TZVP) to 9.381
+
44
45
ligand 1 to afford bare K as the ionic product.
eV (cc-pVTZ) and 9.406 eV (cc-pVQZ), suggesting an
extrapolated value of IE (1) ) 9.425 eV for the complete
v
4
. DFT Results
46
basis-set limit. Further, inclusion of zero-point energy
(ZPE) lowers the adiabatic IE associated with the 0 f 0
transition of 1 by 0.113 eV (B3-LYP/TZVP) because the
vibrational modes are somewhat weakened in the cation
The calculated structure of the neutral compound 1 belongs
to the point group C3V with one short V-O bond (1.59 Å)
of the vanadyl unit and three equivalent, longer V-O bonds
(
(
(
39) Lachowicz, A.; Thiele, K.-H. Z. Anorg. Allg. Chem. 1977, 434, 271.
40) Adler, B. Z. Anorg. Allg. Chem. 1976, 427, 247.
41) See also: Spandl, J.; Br u¨ gdam, I.; Hartl, H. Z. Anorg. Allg. Chem.
(43) Caughlan, C. N.; Smith, H. M.; Watenpaugh, K. Inorg. Chem. 1966,
12, 2131.
(44) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
(45) Pykavy, M. Private communication, October 2002.
(46) Halkier, A.; Helgaker, T.; Jørgensen, P.; Klopper, W.; Koch, H.; Olsen,
J.; Wilson, A. K. Chem. Phys. Lett. 1998, 286, 243
2000, 626, 2125.
(42) Further details of the electrospray experiments will be described in a
forthcoming paper.
1982 Inorganic Chemistry, Vol. 43, No. 6, 2004

3 3
Ion Chemistry of OV(OCH ) in the Gas Phase
experimental measure for the barrier height associated with
+
+
+
TS 1 /2 . The experimental data AE(1 ) ) 9.56 ( 0.04
D
eV and AE(m/z ) 137) ) 10.1 ( 0.1 eV suggest a barrier
of 0.54 ( 0.11 eV, which is somewhat lower than the
computed value (0.83 eV at 0 K and 0.80 eV at 298 K).
Note, however, that the experimental values are determined
+
for 1
D
generated from the neutral sample at room temper-
ature, such that ion dissociation may profit from the thermal
energy content of the neutral compound (0.35 eV at 298 K).
Therefore, some underestimation of the barrier height derived
from the experimental appearance energy is expected.
Nevertheless, the difference between experiment and
theory with respect to the barrier height also suggests a
Figure 4. B3-LYP/TZVP energetics of calculated pathways from 1+ to
+
+
3
. The neutral fragments are omitted for the sake of clarity.
consideration of alternative rearrangements of 1 . In fact,
DFT predicts that a hydrogen migration from one of the
(
D
0.100 eV for 1 ). With ∆IEv/a(1) ) 0.094 eV (B3-LYP/
methoxy groups to the oxygen atom of another CH
3
O group
cc-pVQZ) and a ZPE correction of 0.113 eV (B3-LYP/
TZVP), our best theoretical estimate of the ionization energy
of neutral 1 at 0 K is accordingly IE
+
+
via TS 1 /2a is associated with a barrier (0.82 eV) virtually
+
+
+
isoenergetic to TS 1 /2 . The rearranged product 2a can
a
(1) ) 9.22 eV.
3
be described as a bisligated complex of a OV(OCH ) core
+
Electron attachment to afford the molecular anion results
in the population of low-lying d-orbitals on vanadium
concomitant with formation of a VIV species. Compared to
with one methanol and one formaldehyde ligand as reflected
+
by the elongation of two of the four V-O bonds in 2a
+
(Table 2). Energetically, 2a (-1.40 eV) is slightly less stable
+
+
+
neutral 1, this also leads to a reduced symmetry of the anion
than 2 (-1.47 eV). While both isomers 2 and 2a can
-
1
, but the differences between the methoxy groups are much
account for the loss of formaldehyde, the latter can also expel
+
+
smaller than in the cationic case 1 . Instead, all V-O bond
distances in the anion are lengthened to a similar extent.
Because these bond elongations are significant (up to 0.12
a methanol unit to directly afford the [1 - CH O] fragment
4
discussed above. Thus, theory sheds some doubt on the
exclusive occurrence of the stepwise losses of CH O + H2
2
Å), the vertical electron affinity EA
considerably lower than the adiabatic value EA
v
(1) ) 0.31 eV is
(1) ) 1.13
in the generation of this particular fragment because the
+
+
+
+
+
a
routes 1 f 2 f 3 + CH O f 4 + CH O + H and 1
2
2
2
+
+
+
eV (1.21 eV with inclusion of ZPE). For the same reasons,
f 2a f 3a + CH O f 4a + CH O + H2 are
2
2
-
the vertical electron detachment energy of the anion, DE
2.02 eV, is significantly larger than EA
v
(1 )
significantly more endothermic (1.25 and 1.19 eV, respec-
+
+
+
)
a
+
(1) ) 1.13 eV.
Finally, theory predicts that the cation 1 formed upon
vertical removal of two electrons from 1 has an excess
tively) than elimination of methanol, i.e., 1 f 2a f 4a
+ CH OH (0.37 eV). The difference of these channels is
3
-
related to the heat of hydrogenation of formaldehyde, where
internal energy of 1.14 eV.
the computed value of 0.82 compares reasonably well, though
not perfectly, with 0.95 eV derived from the literature.7
In the mass spectrometric experiments, also direct V-O
bond cleavage concomitant with loss of a methoxy radical
4
.2. Primary Fragmentations. Loss of formaldehyde
+
from the cationic species 1 proceeds via an initial hydrogen
migration involving the methoxy group with the elongated
+
+
+
V-O bond. The corresponding transition structure (TS) 1 /
is observed, which presumably leads to OV(OCH
3 2
) , 7
+
+
2
is computed to be 0.83 eV above 1 as schematically
(Scheme 2). At 0 K, theory predicts an energy demand of
+
+
shown in Figure 4; also see Table 2 for further details. The
optimized structure of the resulting ion is fully consistent
2.16 eV for the direct V-O bond cleavage 1 f 7 + CH
3
O.
+
The computed structure of 7 is hardly perturbed compared
to 1 with slightly shortened V-O and slightly elongated
+
+
with structure 2 assumed in Scheme 2: two hardly perturbed
methoxy units with rVO ) 1.72 Å and rCO ) 1.42 Å are
attached to the vanadium center, which bears the newly
formed hydroxy ligand with rVO ) 1.78 Å and rOH ) 0.97
Å and an additional formaldehyde ligand with rVO ) 2.00
C-O bonds. Accordingly, loss of a methoxy radical should
occur once the threshold energy is available. Quite remark-
+
ably, however, the computed bond energy D(OV(OCD
OCD
3.8 ( 0.2 eV derived from AE([1
0.2 eV and AE(1 ) ) 9.56 ( 0.04 eV in the photoionization
experiments with the deuterated compound. This “delayed
onset” of the CD O loss has already been pointed out in the
)
3 2
-
3
) ) 2.16 eV is significantly lower than the value of
+
+
Å and rCO ) 1.22 Å. Energetically, 2 is 1.47 eV more stable
D
- CD O] ) ) 13.4 (
3
+
than 1 such that the unimolecular rearrangement via
D
hydrogen migration is in fact quite exothermic. Loss of the
+
formaldehyde ligand from 2 barely perturbs the geometry
3
of the remaining skeleton and requires 0.97 eV. Accordingly,
context of the photoionization experiments, and we return
to it further below.
+
+
not only 2 but also the primary fragmentation channel 3
+
+
CH
2
O is lower in energy than 1 . These considerations
5
. Discussion
lead to the important conclusion that the appearance energy
of the CH O loss determined in the photoionization experi-
2
The combination of different mass spectrometric methods
ments is subject to kinetic rather than thermodynamic control.
As a consequence, the threshold can be regarded as a direct
with theoretical studies provides quite detailed insight into
the fragmentation behavior of ionized 1 and the associated
Inorganic Chemistry, Vol. 43, No. 6, 2004 1983

Schr o1 der et al.
above). Nevertheless, the observed difference is unexpectedly
large. Moreover, once the photofragmentation threshold is
reached, the elimination of methoxy rapidly gains in abun-
dance, thereby contradicting the assumption of a pronounced
kinetic shift of this channel due to mere competition. As
mentioned above, photoionization to excited cation states
may provide an alternative explanation for the experimentally
observed behavior. Thus, the AE of the CD
with the onset of an additional feature in the photoelectron
spectrum of 1 . This observation suggests that at about 4
eV above the photoionization threshold some ion states can
be formed for which direct V-OCD bond rupture is
3
O loss coincides
D
Figure 5. Low energy part of the calculated Franck-Condon envelope
for the ionization of neutral OV(OCD3)3 from its vibronic ground state to
the monocation in its lowest electronic doublet state. The abscissa shows
the photon energy (eV) in excess of the 0 f 0 transition energy. Each of
the vertical lines in the spectrum represents the sum of the Franck-Condon
factors belonging to transitions within a given energy interval (constant
width of 1.24 meV) with the most intense peak being normalized to 100.
The inset displays the resulting sum profile.
3
preferred. This aspect can be evaluated more accurately by
means of time-dependent DFT calculations of the electroni-
cally excited states of the cation. In addition to the ground-
state configuration, these calculations reveal eight excited
+
cation states within 3.2 eV relative to 1 , while a much larger
energetics. Yet, several problems remain, which are ad-
dressed here as far as the molecular ions and the primary
fragments are concerned.
number of states emerge above 3.7 eV (e.g., 24 excited states
lying between 4 and 5 eV relative to 1 ). Accordingly, we
+
propose that, at photon energies below 13.4 eV, photoion-
+
5.1. Photoionization. Above, we indirectly implied that
ization leads to 1 and its primary photofragments involving
photoionization of neutral 1 can proceed more or less
losses of neutral closed-shell molecules. Even though theory
suggests that the loss of methoxy could already occur at
photon energies above 11.7 eV, competition and energy
carried away by the leaving photoelectron suppress this
channel. Instead, the manifold of excited cation states arising
+
adiabatically; hence, IE
However, the theoretical study reveals a change in symmetry
between the neutral and cationic species 1 and 1 . To address
a
(1) ≈ AE(1
D
) ) 9.56 ( 0.04 eV.
+
this aspect more quantitatively, the Franck-Condon envelope
+
+
for the transition 1
D
f 1
D
has been calculated. Note,
ca. 4 eV above 1 may allow for a quasi-resonant adsorption
however, that the Franck-Condon profile has been computed
of photons and thereby the formation of highly excited
cations which rapidly dissociate. For these ions with high
internal energy content, direct bond rupture leading to
methoxy loss is likely to be entropically preferred compared
to the more complex hydrogen rearrangements involved in
the formation of the neutral closed-shell fragments discussed
above. While a solid foundation of this argument would
require a dynamical treatment of the excited cation states,
on the basis of the experimental and theoretical information
available we consider this picture as a quite reasonable
explanation for the “delayed” but then steep onset for the
within the harmonic approximation and has therefore to be
+
viewed with some caution. In particular, for 1
D
the
interconversion between various symmetry-equivalent struc-
tures is feasible at lower internal energies, which is not taken
into account in the harmonic approach. Inspection of Figure
5
shows that the 0 f 0 transition has a nonnegligible
intensity, but other transitions are much more favorable.
The computed Franck-Condon profile can be used to
refine the evaluation of IE
To this end, the onset of the photoion yield of 1
a
(1) from the experimental data.
+
D
(Figure
2
a) is overlaid with a summation of the calculated Franck-
loss of CD
3
O (and CD
3 2
O + D ) upon photoionization of
+
Condon envelope for the transition 1
D
f 1
D
. With this
D
1 .
approach, IE
converts to a final value of IE
unlabeled compound after inclusion of the calculated dif-
ferences in ZPE. This experimental value is in reasonable,
though not perfect, agreement with the theoretical estimate
a
(1
D
) ) 9.55 ( 0.05 eV is derived, which
(1) ) 9.54 ( 0.05 eV for the
5.3. CH
2
O + H versus CH
2
3
OH Loss. One specific
a
divergence between the various experimental and theoretical
results has been addressed several times so far, while not
being treated explicitly. Thus, the B3-LYP calculations
+
predict that the formation of 3 + CH
2
O is exothermic and
+
of IE
a
(1) ) 9.22 eV derived above.
that of 3a + CH
2
O only slightly endothermic relative to
+
+
5
.2. Losses of Odd-Electron versus Even-Electron
1 , whereas the subsequent dehydrogenations to either 4
+
Neutrals. The photoionization experiments reveal a surpris-
or 4a are considerably endothermic (between 1.25 and 1.19
ingly large appearance energy for the elimination of the open-
eV, respectively, for CH
2
O + H
2
as the neutral products).
+
+
shell fragment CD
3
O from the molecular ion 1 . Formally,
In contrast, the photoionization thresholds of [1
D
2
- CD O]
+
this process would correspond to a homolytic V-O bond
cleavage for which the difference between the AE of about
and [1
D
- CD
4
O] coincide within experimental error.
+
+
As outlined above, the formations of either 3 or 3a are
1
3.4 eV and IE
a
(1) ) 9.54 ( 0.05 eV imply an energy
subject to kinetic control. The barrier heights computed for
+
+
+
demand of almost 4 eV. Instead, the DFT calculations
suggest D(OV(OCH
the threshold for the formation of 7 might be attributed to
a kinetic shift4 arising from the competing unimolecular
the dissociations of energized 1 via 2 to 3 + CH O and
2
+
+
+
)
3 2
-OCH
3
) ) 2.16 eV. Some delay in
via 2a to 3a + CH O therefore indicate that the fragments
2
+
+
+
3 and 3a may contain considerable excess internal energy.
7
+
rearrangement in the course of CH
2
O loss from 1 (see
(47) Lifshitz, C. Eur. J. Mass Spectrom. 2002, 8, 85 and references therein.
1984 Inorganic Chemistry, Vol. 43, No. 6, 2004

3 3
Ion Chemistry of OV(OCH ) in the Gas Phase
Combination of the photoionization threshold AE([1
D
-
about 1.14 eV excess internal energy; in addition, a consider-
able amount of energy may be deposited in the kiloelec-
tronvolt collisions. Because the photoionization experiments
+
CD
2
O] ) ) 10.1 ( 0.1 eV and IE
a
+
(1) ) 9.54 ( 0.05 eV
+
+
with the calculated energetics of 1 , 3 , and 3a suggests
excess internal energies of about 1.1 eV for 3 and 0.4 eV
for 3a . Nevertheless, even these amounts of internal energy
+
+
reveal that the threshold for loss of formaldehyde from 1
+
is only on the order of 0.5 eV, the excessive fragmentation
behavior can therefore be attributed to the facile dissociation
cannot explain the identical appearance energies of the
+
+
-
fragments [1
D
- CD
2
O] and [1
D
- CD
4
O] . An alternative
of the cationic species, whereas the molecular anion 1 and
explanation of the experimental findings is that at threshold
neutral 1 appear to be quite stable.
+
energies not only loss of formaldehyde from 1 can occur
+
+
+
+
+
+
6. Conclusions
(
either 1 f 2 f 3 or 1 f 2a f 3a ), but hydrogen
migration can also lead to the formation of a methanol unit
Combined experimental and theoretical approaches elu-
cidate the energetics and mechanisms of the primary
+
which is then expelled as an intact neutral molecule (1 f
+
+
2
a
f 4a ). In fact, the difference of ∆
f 3
H(CH OH) and
fragmentation of the molecular cation of trimethyl vanadate-
∑
∆
f
H(CH O + H ) of 0.95 eV fills the gap between the
2
2
+
(
V), 1 , to a considerable extent. A particular hallmark is
calculated energy demand for the combined losses of CH
2
O
the quite important experimental information about the
+
H
2
and the experimentally observed threshold for dis-
+
barrier height for loss of CH
2
O from the cation 1 , which
+
sociative photoionization to [1
D
4
- CD O] . Accordingly, we
provides a direct observable for a unimolecular rearrange-
ment in a prototype V/O system. The present results may
therefore be used to evaluate the performance of various
theoretical approaches for the description of barrier heights
in vanadium oxide catalysis. In this respect, our DFT
calculations appear to somewhat overestimate the reaction
propose that loss of methanol is preferred close to the
fragmentation threshold, whereas the sequential route of first
losing formaldehyde and then dihydrogen can only compete
at elevated energies.
5.4. Reaction Mechanism. Quite interestingly, the theo-
retical studies indicate that the initially occurring hydrogen
barrier for the rearrangement of the methoxy substituents in
+
migration of ionized 1 can proceed via two independent
+
1
. Surprisingly, however, the theoretical results also indicate
routes, which are more or less isoenergetic. In both cases,
hydrogen migration commences from the methoxy group in
that at the very threshold two kinds of more or less
+
isoenergetic rearrangements of 1 can compete with each
+
1
which bears an elongated V-O bond after ionization and
other: (i) hydrogen transfer from one methoxy group to the
+
+
then proceeds either to the vanadyl unit (1 f 2 ) or to the
+
+
vanadyl unit via TS 1 /2 and (ii) hydrogen transfer from
+
+
oxygen atom of another methoxy group (1 f 2a ). The
former reaction involves oxidation of one methoxy unit to
formaldehyde in analogy to oxidation reactions mediated by
high-valent transition-metal oxides.1 The latter proceeds as
a disproportionation of two methoxy units in analogy to the
reactions of metal ions with dimethyl peroxide. Apparently,
both mechanisms can effectively compete in the particular
case of 1 , indicating the ambivalent behavior of this
one methoxy group to the O atom of another one via TS
+
+
1
/2a . On the basis of the present experimental data, we
cannot decide which of the two TSs is more relevant, but
,2
most likely both of them play a role in the dissociation of
+
1
.
48
As far as the consecutive fragmentations are concerned,
the sector-field experiments only provide limited insight
because the number of conceivable structural isomers is too
large, while the fragmentations observed may be determined
by thermochemical preferences or kinetic criteria including
possible restrictions imposed by spin multiplicity. Accord-
ingly, a more concise analysis of these secondary fragments
requires both complementary experimental studies and ad-
ditional theoretical work which will be addressed in a
forthcoming paper.
+
compound. On one hand, the vanadium center bears a
tendency for oxidation reactions, while conservation of the
vanadyl unit is favorable on the other hand. Even though
the experimental data do not allow a distinction between both
mechanisms, the apparent loss of methanol directly from the
molecular ion 1+ may serve as an indication for the
+
+
participation of TS 1 /2a .
.5. Vertical Electron Transfer. Finally, let us briefly
5
return to the extensive fragmentation observed in the NR
Acknowledgment. Continuous financial support by the
Sonderforschungsbereich 546, the Deutsche Forschungsge-
meinschaft, the Fonds der Chemischen Industrie, the Volk-
swagen-Stiftung, and the Gesellschaft von Freunden der
Technischen Universit a¨ t Berlin is gratefully acknowledged.
Further, we thank the Konrad-Zuse Institut for the allocation
of computer time and the staff of LURE for operating the
Super-ACO ring and the adjacent facilities.
-
and CR spectra of the molecular anion 1 . After having
+
understood the energetics for the dissociation of 1 , we
attribute the fragmentations upon NR and CR to the energy
deposition upon vertical electron transfer occurring in the
collisions. Thus, theory predicts that vertical detachment of
-
two electrons from 1 results in a cationic species having
(48) Schalley, C. A.; Wesendrup, R.; Schr o¨ der, D.; Weiske, T.; Schwarz,
H. J. Am. Chem. Soc. 1995, 117, 7711.
IC030264X
Inorganic Chemistry, Vol. 43, No. 6, 2004 1985