Structural Isomerism in Transition Metal
Clusters
Dan Harding, Jay Bomphrey,Tiffany Walsh* and Stuart Mackenzie
Dept. of Chemistry,
* Dept. of Chemistry and Centre for Scientific Computing,
University of Warwick,
Coventry, CV4 7AL, UK
Experimental Evidence of Isomerism
Density Functional Theory Calculations
Transition metal cluster reactivity studies have found numerous clusters whose behaviour suggests the
presence of multiple forms of the cluster with different reactivity.1,2 These forms could be isomers or
electronic states. Reactivity studies can provide no direct structural information. Rh6 was chosen for
computational study due to the interesting reactions seen with NO (Fig.1)and N2O (Fig.2)and it's small size
making it computationally convenient
1.2
DFT calculations at the PBE/ SDD level of theory have been performed for a range of structures predicted by basin hopping
with a Sutton Chen model potential. Electronic states from singlet to septet and doublet to octet have been considered for
neutrals and cations respectively. Optimised structures are shown in Fig. 3 .
1
Relative Abundance
Relative
RelativeAbundance
Abundance
1
0.8
0.6
0.4
0.1
0.2
0
5
10
15
0
20
20
40
60
80
100
120
Octahedron
Time/ s
Time / s
Time/
s
Capped square
pyramid
Prism
Boat
Fig. 3 Optimised low energy structures from DFT PBE/ SDD calculations
A number of isomers and electronic states are found to be close in energy. For the
cations the octet octahedron is found to be the lowest energy structure.
Experimental measurements show Rh9 clusters to have large magnetic moments.3 T
The binding energies of the cationic clusters are shown in Fig. 4.
Vibrational frequencies have been calculated to confirm the structures are true
minima at this level of theory.
These frequencies can be used to produce a simulated IR spectrum for each
cluster. Fig. 5 shows the simulated spectra for the octet isomers which may differ
sufficiently to aid identification of cluster structures in IR spectroscopy
experiments.4
The simulated spectra of the different octahedron electronic states, Fig. 6, show
the 4Oh+ structure to have very much higher IR absorbance than the other
electronic states. This may due to the presence of a permanent dipole or the
structures lower symmetry.
●
●
●
Reaction Calculations
●
The interaction of Rh clusters with reactant gas molecules can be investigated using DFT calculations.
8
+
●A range of possible initial geometries of the [Rh N O]
cluster complex have been optimised at the PBE level
6 2
of theory with SDD effective core potentials for Rh atoms and aug-cc-pVDZ basis functions for N and O
atoms. Some structures are shown in Fig. 7 .
●Many optimised structures show the cleavage of the N-O bond seen in reactivity experiments where the
product is [RhnO] + .
●Adsorbate binding energies can be calculated
●Transition states to dissociation, Fig. 8, give the energy barriers to reaction.
●Low binding energy of N
suggests it would easily desorbed.
2
●
●
oh
prism
csp
boat
Relative Intensity
Fig. 2 Kinetic plot of the
decay of [Rh6+ ] on reaction
with N2O
Fig. 1 Kinetic plot of the decay of
[Rh6+ ] on reaction with NO fitted
to a sum of two exponentials
50
100
200
250
300
-1
Wavenumber/ cm
Fig. 5 Simulated IR spectra for 8Rh6+
isomers.
Fig. 6 Simulated IR spectra for different electronic states
of the Rh6+ octahedron cluster.
Fig. 4 Binding energies of Rh6+ clusters relative to 5
Rh atoms and 1 Rh+ ion at PBE/ SDD level of theory.
Relative Intensity
Et= -847.891635 au
Eb(N2) = 1.0eV
150
8
6
4
2
50
Et = -847.832916 au
Eb(N2O) = 0.8eV
100
150
200
250
300
Wavenumber/ cm-1
1
6
0
2
4
0
Et = -847.867071 au
Eb(N2) = 0.8eV
Et= -847.808073 au
Eb(N2O) = 1.9eV
Fig. 7 Input and DFT optimised structures for Rh6N2O+ clusters
Transition States and Isomerism
Transition states between isomers can be identified using DFT and the energy of the TS used to calculate the energy barriers to
rearrangement. Fig. 9
●Pulses of inert gas can be used in FT-ICR experiments to thermalise the clusters, causing an apparent change in the proportion of
reactive and unreactive forms of the clusters. Fig. 10
●Gas pulse experiments may make it possible to determine if the “bi-exponential” reactions seen are due to the presence of only
two isomers or a larger number.
●
+
Fig. 8 Transition state for dissociation of N2O on a Rh6 cluster. Arrows
show the vectors along the reaction co-ordinate. Reaction appears to
encourage structural rearrangement of the cluster. Et= -847.861163 au.
1
ts
●
●
●
●
Collisions of cluster complexes with inert gas atoms can cause dissociation of the complex. Fig.11
shows the experimental sequence. Fig. 12 shows a typical mass spectrum after CID.
Yield information about the nature of bonding on the cluster, molecular or dissociative adsorption
Adsorbate binding energies may be measured by finding the threshold excitation energy for
dissociation under single collision conditions.5
May allow comparison of experimental and caclulated binding energies and structures, Fig. 13.
oh
0.7 eV
Relative Intensity
Collision Induced Dissociation And Binding Energy
0.5 eV
0.1
+
+
Rh12 no pulse
Rh11 with pulse
Rh+12 with pulse
Rh+11 no pulse
0.01
(a)
(b)
prism
(c)
2500000
+
Fig. 9 Transition state and energy barriers to rearrangement of 5Rh6.
Rh8
0
2
4
6
8
10
0
Time/s
2
4
6
8
10
Time/s
Fig. 10 Experimental evidence changes in reactivity of
Rhn+ clusters after thermalisation
(f)
(e)
Intensity /arb. units
2000000
(d)
Conclusions
1500000
1000000
+
500000
(g)
Rh 9+
+
0
500
Rh 9CO +
[Rh8CO]
+
Rh6
(b)
(b)
Rh 8+
Calculations predict two low energy geometries and several low energy electronic states for Rh6+ and this is consistent with experimental results.
The effect of a large external magnetic field, as present in FT-ICR experiments, on the spin states of the clusters is unclear.
●Reaction with gas molecules may induce changes in the metal cluster geometry.
● Thermalisation of clusters may cause a change in the proportion of reactive and unreacive forms of clusters.
●
600
Rh7
700
800
900
1000
m/z
Future Work
●
●
Fig. 12 Mass spectrum showing the products of CID
of [Rh8CO] +
Fig. 11 Illustration of CID process in the FT-ICR cell.
a) Clusters injected into cell
b) Pulse of reactant gas
c) Pumping delay
d) Cluster of interest isolated
e) Pulse of inert gas
Fig. 13 [Rh6CO] + opitimised at
f) Clusters excited to known KE during gas pulse
g) CID products detected in mass spectrum
LDA/ SDD, aug-cc-pVDZ
level of theory. Eb(CO) = 2.9 eV
●
Calulate the O atom binding energies on the clusters.
Use inert gas pulse reactivity experiments to investigate the change in reactivity of clusters.
Simulate ZEKE/ MATI spectra of TM clusters for comparison with experiments at Warwick and elsewhere.6 This can provide information
about the accuracy of theoretical model and may aid assignment of the spectra.
References
1) For example Berg, Schindler, Kanthlehner,Niedner-Schatteburg and Bondybey , Chem. Phys. , 262(1) p.143, 2000
2) Ford, Anderson, Barrow, Woodruff, Drewello, Derrick and Mackenzie, Phys. Chem. Chem. Phys., 7, p. 975, 2005
3) Cox, Louderback, Apsel and Bloomfield, Phys. Rev. , 49(17), p.12295, 1994
4) Ratsch, Fielicke, Kirilyuk, Behler, von Helden, Meijer and Scheffler, J. Chem. Phys., 122, p. 124302, 2005
5) Vakhtin, Markin and Sugawara, Chem. Phys., 262, p. 93, 2000
6) Yang and Hackett, J. Electron Spectroscopy and Related Phenomena, 106, p. 153, 2000
I would like to acknowledge th e computer time provided by the Centre for Scientific Computing at Warwick.