How Inert, Perturbing, or Interacting Are Cryogenic Matrices? A Combined Spectroscopic (Infrared, Electronic, and X-ray Absorption) and DFT Investigation of Matrix-Isolated Iron, Cobalt, Nickel, and Zinc Dibromides

Article abstract of DOI:10.1021/acs.jpca.7b09734

The interactions of FeBr₂, CoBr₂, NiBr₂, and ZnBr₂ with Ne, Ar, Kr, Xe, CH₄, and N₂ matrices have been investigated using IR, electronic absorption, and X-ray absorption spectroscopies as well as DFT calculations. ZnBr₂ is linear in all of the matrices. NiBr₂ is linear in all but N₂ matrices, where it is severely bent. For FeBr₂ and CoBr₂ there is a more gradual change, with evidence of nonlinearity in Xe and CH₄ matrices as well as N₂. In the N₂ matrices, the presence of ν_NN modes blue-shifted from the "free" N₂ values indicates the presence of physisorbed species, and the magnitude of the blue shift correlates with the shift in the ν₃ mode of the metal dibromide. In the case of NiCl₂ and NiBr₂, chemisorbed species are formed after photolysis, but only if deposition takes place below 10 K. There was no evidence for chemisorbed species for NiF₂ and FeBr₂, and in the case of CoBr₂ the evidence was not strong.

Full text of DOI:10.1021/acs.jpca.7b09734

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Article
How Inert, Perturbing or Interacting Are Cryogenic Matrices? A Combined
Spectroscopic (Infrared, Electronic and X-ray Absorption) and DFT
Investigation of Matrix Isolated Iron, Cobalt, Nickel and Zinc Dibromides
Owen M. Wilkin, Neil Harris, John F. Rooms, Emma L. Dixon, Adam J. Bridgeman, and Nigel Andrew Young
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b09734 • Publication Date (Web): 28 Dec 2017
Downloaded from http://pubs.acs.org on December 30, 2017
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How Inert, Perturbing or Interacting are Cryogenic Matrices? A
Combined Spectroscopic (Infrared, Electronic and Xꢀray
Absorption) and DFT Investigation of Matrix Isolated Iron,
Cobalt, Nickel and Zinc Dibromides.
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(a)
(a)
(a)
(a)
Owen M. Wilkin, Neil Harris, John F. Rooms, Emma L. Dixon, Adam J.
(
b)
(a)*
Bridgeman, and Nigel A. Young
a
Department of Chemistry, The University of Hull, Kingston upon Hull, HU6 7RX, UK. Fax:
44 1482 466410; Tel: 44 1482 465442; E-mail: n.a.young@hull.ac.uk
b
School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
email: n.a.young@hull.ac.uk
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Abstract
2 2 2 2 4 2
The interaction of FeBr , CoBr , NiBr and ZnBr with Ne, Ar, Kr, Xe, CH and N matrices
has been investigated using IR, electronic absorption and Xꢀray absorption spectroscopies, as
well as DFT calculations. ZnBr is linear in all the matrices. NiBr is linear in all but N
matrices where it is severely bent. For FeBr and CoBr there is a more gradual change, with
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evidence of nonꢀlinearity in Xe and CH matrices as well as N . In the N matrices the
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2
2
presence of νNN modes blue shifted from the “free” N values indicates the presence of
physisorbed species, and the magnitude of the blueꢀshift correlates with the shift in the ν
3
mode of the metal dibromide. In the case of NiCl and NiBr chemisorbed species are formed
2
2
after photolysis, but only if deposition takes place below 10 K. There was no evidence for
chemisorbed species for NiF
strong.
2 2 2
and FeBr and in the case of CoBr the evidence was not
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Introduction
Whilst the notion that the spectroscopic data obtained from species trapped in (noble) gas
matrices are always directly comparable to their vapor phase counterparts has been
acknowledged as naïve, the need to identify and understand how matrices can affect the
geometric and electronic structures of the trapped species remains. The matrix isolation
literature is replete with asides and comments about site effects, matrix splittings etc., usually
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invoked to explain some spectral artefact. For example PdCO is found in two sites in Ar as is
2
3ꢀ6
CO
2
, and SiH
4
.
In the case of PdCO more recent calculations indicate the possible
7
8, 9
formation of ArꢀPdCO. However, in the related NiCO case the computational work
1
0, 11
indicated the possible presence of ArꢀNiCO, but the experimentalists disputed this.
The
ability of "inert" matrices to affect the structure of the trapped species has been demonstrated
by Beattie et al. for both actinide tetrahalides and hexafluorometallates. ThCl and UCl are
4
4
1
2, 13
tetrahedral in neon, but have a distorted C2v geometry in argon.
The alkali metal
14
hexafluorouranates are C3v in solid argon, but C2v in nitrogen matrices. The alkali metal
1
5
hexafluoroniobates have a tridentate coordination in neon and argon matrices, but a
bidentate coordination mode in nitrogen and carbon monoxide matrices. Using CsClO as a
probe molecule they showed that the order of hostꢀguest interaction with the matrix was Ne
4
1
5
<Ar < O
2
≈ F
2
<Kr <Xe <N
2
<CO. NiCl
2
is bent in nitrogen matrices but linear in argon
1
6, 17
17
4ꢀ6
matrices,
GeH
as is CoCl
2
.
SiH
4
adopts a D2d geometry in argon but is C3v in N
2
,
and
4, 5
4
also C3v in N
2
.
The absorption and excitation spectra of Mn atoms have been used to
18
identify the different sites in which the Mn is trapped in Ar, Kr and Xe matrices. Fausto et
al. have demonstrated how the choice of argon or xenon matrices can be used to control the
1
9
photochemistry of 4ꢀmethoxybenzaldehyde. Kofranek has carried out calculations on 1,3ꢀ
butadiene embedded in an Ar254 matrix, which confirmed that the argon matrix stabilizes the
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sꢀcis structure over the normally more stable gauche conformation.
It has normally been assumed that these matrix effects are due to different interactions
between the trapped guest and the host, rather than specific bond formation. However, there
2
1ꢀ24
are a number of reports of noble gas compound formation in matrices. Räsänen et al.
have beautifully demonstrated the formation of HArF when HF is photolyzed in argon. Their
24
work also includes a wide range of other Kr and Xe compounds. Perutz et al. showed that
photolysis of matrix isolated metal hexacarbonyls yielded metal pentacarbonyls with IR and
UVꢀvis spectra that were dependent on the matrix material, indicating the vacant site was
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occupied by a matrix atom with a weak metalꢀmatrix bond, implying the formation of
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M(CO) Ng complexes.
This work is often overlooked as one of the first reports of metalꢀ
5
noble gas bonding interactions. Later gas phase measurements on M(CO)
5
Xe indicated MꢀXe
−
1
−1
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binding energies of ca. 38 kJ mol for Cr, and ca. 33ꢀ34 kJ mol for Mo and W. The work
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on noble gas complexes in solutions and superꢀcritical fluids has been reviewed.
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Andrews has shown that Be atoms can interact with argon matrices to form ArBeO and
3
1
more recently Wang has extended this to include NgBeS (Ng = Ne, Ar, Kr, Xe) complexes.
Andrews et al. have also carried out a detailed study of the interaction of noble gases with
32ꢀ39
40, 41
+ 42
43
CUO,
addition of the noble gases and detailed calculations there is evidence for the formation of
CUO(Ng) and UO (Ng) complexes with UꢀAr distances of ca. 3.2 Å and weak binding
energies of ca. 17 kJ mol per UꢀAr bond. In contrast UO binds to five argon atoms with
UO
2
,
2 2 2
and UO , and H UO . On the basis of IR spectra showing stepwise
4
2
4
−1
+
2
about twice the binding energy as for CUO(Ng)
4
2 4
and UO (Ng) . There is also evidence for an
4
4
(
Ar)
n 2 2
(Th(C H )) complex. More recently Andrews and Riedel have observed ArꢀAuF and
45, 46
NeꢀAuF compounds, but the interaction was much weaker with CuF and AgF.
A series of experiments by Zhou et al. using a combination of IR experimental data and DFT
calculations has indicated the formation of transition metal oxideꢀnoble gas complexes. The
+
IR spectrum of ScO had 5 new absorptions when Kr was doped into the argon matrix, and
+
47
this was taken to imply the presence of [ScO(Ng)
5
] where Ng = Ne, Ar, Kr. This was
confirmed by DFT calculations, which indicated ScꢀNg bond lengths of 2.839 and 3.297 Å
47, 48
for Ar, 2.979 and 3.433 Å for Kr and 3.197 and 3.663 Å for Xe.
For the neutral scandium
oxide/dioxygen complexes the experimental evidence for argon complexes was less
49
+
compelling. In analogous experiments with YO 6 new bands were observed in mixed
+
Ar/Kr matrices, but only 5 for Ar/Xe matrices, implying the presence of [YO(Kr)
6
] and
+
48
5
[YO(Xe) ] . For neutral monoxides it was shown that ArMO species were formed for Cr to
5
0
Ni, but not for ScO, TiO and VO. The calculated NgꢀM distances were: ArCrO, 3.300 Å;
KrCrO, 3.164 Å; XeCrO, 3.257; ArMnO, 2.973 Å; KrMnO, 2.987 Å; XeMnO, 3.089 Å;
ArFeO, 2.691 Å; KrFeO, 2.779 Å; XeFeO, 2.910 Å; ArCoO, 2.561 Å; KrCoO, 2.623 Å;
50
XeCoO, 2.784 Å; ArNiO, 2.421 Å; KrNiO, 2.527 Å; XeNiO, 2.695 Å. VO
2
coordinated
two argon or xenon atoms to form VO (Ng) with DFT calculated VꢀAr distances of 2.694 Å
2
2
5
1
and VꢀXe distances of 2.939 Å. VO only coordinated one argon or xenon resulting in
4
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VO
4
(Ng) with calculated VꢀAr distances of 2.730 and VꢀXe distances of 2.951 Å. Whilst
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niobium formed NbO
2
(Ng)
2
complexes with calculated NbꢀAr distances of 2.925 Å, there
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was no experimental evidence for the analogous tantalum species. However, for both
4
niobium and tantalum MO (Ng) complexes were observed with calculated NbꢀAr distances
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of 2.788 Å, NbꢀXe distances of 3.089 Å, TaꢀAr 2.723 Å and TaꢀXe 3.006 Å. As well as
forming complexes it has also been shown that noble gas atoms can participate in reactions,
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for example, xenon induced disproportionation of the sideꢀon bonded disuperoxo (η ꢀ
1
2
O
2
)
2
2 2 2
CrO complex into (η ꢀOO)(η ꢀO )CrO (Xe) which can be regarded as containing a
5
3
weakly bound dioxygen unit as well as a sideꢀon bonded peroxo ligand. Calculations
5
3
indicated a CrꢀXe bond length of 2.892 Å. Work on 4d oxides indicated the formation of
2
2
2
54
2
2
Pd(η ꢀO
2
)(Ng)
2
, Pd
2
(η ꢀO
2
)(Ng)
2
and Pd
2
(η ꢀO
2
)
2
(Ng)
2
,
2 2
and Rh(η ꢀO )(Ng) , Rh(η ꢀ
2
1
55
O
2
)
2
(Ng)
2
and Rh(η ꢀO
)
2 2
(η ꢀOO)(Ng). A detailed bonding analysis of these data indicated
that the noble gas and transition metal oxides interact by both ionꢀinduced dipole as well as
more formal chemical bonding interactions, where the noble gas atoms act as electron donors
56
into localised metal based orbitals. It was suggested that if the transition metal species
trapped in the noble gas host has electrophilic character, combined with low lying empty or
partially filled orbitals, then it will be able to coordinate with noble gas atoms, resulting in the
formation of noble gasꢀtransition metal complexes. An important observation was that whilst
the coordination of the noble gas atom results in vibrational frequency shifts, there was no
5
6
direct correlation between matrix shifts and the strength of the noble gas coordination.
In the proceeding work, the evidence for interaction between the noble gas and the matrix
isolated species came from a combination of experimental (usually IR) data and calculations
as there are very few techniques which are able to directly yield bond distances from noble
gas matrices. Xꢀray absorption spectroscopy is one of the few techniques that can yield direct
structural information about both the matrix isolated species, and its interaction with the
matrix host. The application of Xꢀray absorption spectroscopy to matrix isolated species has
57
recently been reviewed.
Gerry has carried out extensive microwave studies of the gas phase complexes between
58
Group 11 halides and noble gases (see reference and references therein), and the complexes
59ꢀ62
with xenon as a ligand are also important examples.
Therefore, there is considerable evidence that the noble gases whilst providing an excellent
medium for trapping reactive and unstable species, may actually be exerting a benign and
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often poorly characterized effect on the trapped species. Whilst N is often a very good
matrix material, it is well known that it can also have a considerable impact on the trapped
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3ꢀ65
species. For example; it can lift the degeneracy of degenerate modes,
some 5d transitionꢀ
metal pentachlorides adopt a squareꢀpyramidal geometry in N
2
but a variety of conformers in
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Ar.
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N matrices are also capable of exerting considerable influence on the structures of
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coordinatively unsaturated metal halides
and the formation of more formal
69ꢀ72
complexes.
Therefore, it may be possible to identify two distinct aspects of interaction between the
matrix and the trapped species, one that gives rise to compounds/complexes with significant
bonding interaction and a second one that results in the perturbation of the guest by the
matrix. This could be compared to chemisorption and physisorption for surface species,
especially for dinitrogen.
Experimental
The anhydrous metal salts were prepared by heating the relevant metal foil (Goodfellow,
2
99.9+%) in a flowing atmosphere of HBr, except for ZnBr where a commercial (Fluka)
anhydrous sample was used. The samples were sublimed under vacuo and stored in sealed
silica ampoules prior to use. The metal salts were loaded into silica holders in a glove box
and placed within a tantalum wound silica furnace mounted within a water cooled vacuum
jacket mounted on the stainless steel or aluminium vacuum chamber. The furnace was heated
resistively and the temperature monitored with a type K thermocouple. The matrix gases Ar
(Energas 99.999%, Distillers MG 99.999%), N
2
(Energas 99.999%, Distillers MG 99.999%),
1
5
2
N (Euroisotp 99%), Ne (Distillers MG 99.99%), Kr (Distillers MG 99.99%), Xe (Distillers
MG 99.99%), CH
4
2
(Distillers MG 99.995%), O (Distillers MG 99.998 %) were used as
supplied and admitted via a needle valve at the rate of ca. 3 – 5 mmole/hr. Kr matrices were
deposited at 30 K and Xe matrices deposited at 50 K, both were then cooled to ca. 10 K for
spectroscopy. Photolysis was carried out with a LOTꢀOriel 200 W HgꢀXe lamp equipped
with a liquid light guide and appropriate filters.
The IR spectra were recorded using a Bruker IFS66 FTIR spectrometer equipped with KBr
(MIR) and 6 ꢀm Mylar (FIR) beamsplitters, and DTGS detectors. The vacuum shroud used
−7
CsI windows and had a base pressure of ca. 2x10 mbar before the APD DE204SL Displex
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closed cycle helium cryostat, (base temperature of ca. 9 K) was turned on. Temperature
measurement and control was achieved via Si diodes and a Scientific Instruments 9650
controller. Depending on the experimental requirements the deposition substrate was either a
CsI polished window for transmission experiments (vacuum chamber within spectrometer
sample compartment) or a polished copper plate for reflectance experiments (vacuum
chamber mounted externally).
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Synchrotron radiation far IR spectra (SRꢀFIR) were collected using a Nicolet vacuum FIR
73
bench on station 13.3 of the Daresbury Laboratory SRS operating in gapped beam mode at
GeV with circulating currents of ca. 180 mA. The deposition substrate was a polished
2
copper block held at ca. 13 K with the incident and reflected beams at 135º to allow for inꢀ
situ monitoring of the sample during both deposition and photolysis. The vacuum chamber
was mounted between the optical bench and He cooled Ge bolometer and the whole optical
path was evacuated.
The UVꢀVisꢀNIR spectra were recorded using a Varian Cary 5E spectrometer, with a similar
2
vacuum and pumping regime to the lab based FTIR experiments, except that CaF was used
for the deposition substrate and the external windows on the vacuum chamber which was
mounted in the spectrometer sample compartment.
The Xꢀray absorption experiments utilized station 9.2 of the Daresbury Laboratory SRS,
operating in either multiꢀbunch or gapped beam mode at 2 GeV with circulating currents of
68, 74
ca. 180 mA. A combined FTIR/XAFS facility
was used to collect simultaneous in-situ IR
and XAFS data from the same sample, which allowed for the monitoring of the deposition as
well as any subsequent annealing or photolysis. The metal and bromine Kꢀedge XAFS
spectra were collected in fluorescence mode (Tl/NaI scintillator) using a Si(220) double
crystal monochromator detuned by ca. 50 % to remove harmonic contamination. 8ꢀ12 spectra
were collected at each edge and then averaged. The 3d metal Kꢀedges were calibrated using
the first derivative in the spectra of 5 or 10 ꢀm metal foils (Fe (7112.0 eV), Co (7709.0 eV),
Ni (8333.0 eV), Zn (9659.0 eV) and the Br Kꢀedge was calibrated using the first derivative of
75
the L
2
edge of a 5 ꢀm Au foil (13734.0 eV). Background subtraction was carried out using
76
PAXAS by fitting the preꢀedge region to a quadratic polynomial, subtracting this from the
data and approximating the atomic component of the postꢀedge region with a high (typically
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sixth) order polynomial. This approximation was optimised in order to minimise the lowꢀr
features in the Fourier transforms by an iterative process, although it should be noted that
77, 78
atomic XAFS features may be expected in this part of the FT.
Fitting of the experimental
7
9ꢀ81
data was carried out with EXCURV98
making use of multiple scattering curved wave
theory, a vonꢀBarth ground state and a HedinꢀLundqvist exchange potential. For some
samples the metal Kꢀedge and the Br Kꢀedge were refined simultaneously. Although
experiments utilizing Ne matrices are attractive, these were not conducted as complete
isolation of the metal dibromide could not be guaranteed using the cryostat with a base
temperature of 9 K under optimum conditions and data acquisition times of up to 24 h. The
production of aggregates in these experiments would make the analysis of the data at best
difficult and at worst meaningless. Experiments involving Ar are also intrinsically difficult
for the 3d transition metals due to the short penetration depth of the matrix due to the high Xꢀ
ray absorption crossꢀsection of argon. For example the attenuation length (1/e) for Ar at 8000
0
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82
eV is ca. 50 ꢀm. The length of time for deposition is severely limited as the increased
thickness of the matrix soon cancels out any additional fluorescence due to the increase in
deposited metal dihalide. For this reason, Kr and Xe experiments were also not possible as
these matrices have even higher Xꢀray absorption crossꢀsections, with attenuation lengths of
50 and 10 ꢀm, respectively at 8 keV. In many ways CH
4
is an ideal matrix for XAFS work as
its Xꢀray absorption crossꢀsection is even lower than Ne. Therefore, in this work XAFS
studies were conducted using CH
matrices for FeBr and CoBr .
4 2 2 2 2
and N matrices for FeBr , CoBr and NiBr , and Ar
2
2
Force field calculations using the SVFF approach were carried out using the Wilson GF
83
84ꢀ86
method within SOTONVIB. DFT calculations were performed using the ADF program,
at the BP86 level with tripleꢀζ (TZ2P) Slaterꢀtype orbital basis sets and relativistic effects at
87
the ZORA level. TDꢀDFT calculations of the XANES spectra were carried out with ORCA
88ꢀ94
2.9.1 using the BP86 def2ꢀTZVP(ꢀf) def2ꢀTZVP/J ZORA TightSCF methodology.
Setting the scene
2
The transition metal dihalides (MX ) are an ideal probe of matrix interactions as they allow
for variation in both hardness of the metal centre as the halogen is changed, as well as dꢀ
electron count as one goes across the series. Whilst many matrix studies of the difluorides
and dichlorides have been carried out, there are no complete sets of data available for the
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2 2
related dibromides and diiodides. Whilst there is data available for CaBr and CaI in Ar and
95
96, 97
Kr, and ZnBr and ZnI in Ar and Kr,
there is much less data on the dibromides and
2
2
diodes of the transition metals with the only reports appearing to be CrBr in Ar and N
2
2
98
68, 99
68
matrices, and NiBr
2
in Ar,
2
and N matrices.
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Infrared spectroscopy
2
MX linear molecules have four vibrational degrees of freedom, whereas bent molecules have
three vibrational degrees of freedom. For the linear D∞h molecules, the vibrational modes
+
g
+
u
transform as Σ
(ν
1
symmetric stretching mode), Π
u
(ν
2
bending mode) and Σ
(ν
3
asymmetric stretching mode). Of these only ν and ν are infrared active, but in general the ν
2 3
2
−1
mode is too low in energy (<100 cm ) to be observed on most spectrometers. The vibrational
modes of the bent C2v molecules span 2A
mode) and B (ν asymmetric stretching mode) all of which are IR active. As in the linear
case the low frequency bending mode is expected to be beyond the range of most commercial
spectrometers. Although the observation of the ν mode in the IR spectrum is an obvious
1 1 2
(ν symmetric stretching mode and ν bending
2
3
1
criterion for unambiguous evidence of a bent geometry in such molecules, its observation has
been historically difficult, and assignment of both ν and ν has produced controversy even
3
1
100
with such extensively studied systems as the alkaline earth dihalides. The intensity of the
mode is critically dependent on the bond angle, and is only expected to be observed at
fairly acute angles. Our DFT calculations indicate that the relative intensities of the ν and ν
modes for 155º and for 130º bond angles in metal dibromides (and dichlorides) are 50:1 and
0:1, respectively. Therefore, unless the molecule is severely bent it will be almost
impossible to identify the ν mode with any certainty. Therefore, the absence of ν is not
unambiguous evidence for linearity in MX compounds. Whilst the chlorine isotope patterns
for the 3d transition metal chlorides can be employed relatively routinely to obtain the
ν
1
3
1
1
1
1
2
1
6, 17
estimates of the bond angles,
the isotopic splitting arising from bromine is much smaller
and is often an experimental challenge to observe. Therefore, unless it is possible to identify
unambiguously the ν mode, the linearity or otherwise cannot be determined from IR data
1
alone unless either metal or bromine isotopic data are available. Although we have access to
1
FTꢀRaman matrix facilities, which in principle allow for the identification of ν directly, this
is currently limited to chromophore:matrix ratios of about 1:200 (due to the incident laser
power) which are too high for these type of experiments. As a result of these limitations other
spectroscopic techniques were employed in addition to IR.
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Electronic absorption spectroscopy
UVꢀvisꢀNIR data on matrix isolated transition metal dihalides in the literature is limited to the
chlorides, although some vapor phase data is available for the dibromides. For linear MX
2
species the intensity of any dꢀd bands is expected to be very low due to the orbital and
Laporte selection rules, but in the case of the nonꢀlinear molecules the Laporte selection rule
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01ꢀ103
will be relaxed. Therefore, as noted previously
the electronic absorption spectra will be
dominated by charge transfer bands, with weak dꢀd bands only observed in some cases. As a
result of this, the discussion of the spectra will be mainly based on assigning bands as either
charge transfer or dꢀd bands and using the spectra as a 'fingerprint' indicative of a linear and
nonꢀlinear structure, rather than a detailed theoretical treatment.
Xꢀray absorption spectroscopy
In the past we have demonstrated that Xꢀray absorption fine structure spectroscopy (XAFS) is
6
7, 68, 74, 104ꢀ109
a powerful tool for determining interatomic distances of matrix isolated species,
57
and this has recently been reviewed. Xꢀray absorption spectroscopy is able to provide a
detailed picture of both the local electronic and geometric structure of the matrix isolated
species. The electronic structure, which can also yield valuable information about the
coordination geometry and oxidation state, is obtained from the Xꢀray absorption near edge
structure (XANES) part of the spectrum located around the absorption edge. In contrast, bond
length data is obtained from the oscillations in the absorption crossꢀsection after the
absorption edge which is known as extended Xꢀray absorption fine structure (EXAFS).
XANES
Whilst metal Kꢀedge XANES spectra are well known to be diagnostic of metal oxidation
1
10
111
112
state, coordination number and geometry for iron, cobalt, and nickel complexes, there
are very few examples in the literature of XANES data for triatomic first row transition metal
species. The best examples of these are for copper where Kau et al. demonstrated that the
intensity of the characteristic 1s → 4p edge feature in Cu(I) complexes was dependent on the
113
copper coordination number. This has been widely used as a diagnostic probe of electronic
1
14
and geometric structure of copper metalloproteins. Of more relevance to this work is that
−
115
− 116
of Fulton et al. who have investigated [CuCl
2
] ,
and [CuBr
2
] ,
formed under
117
hydrothermal conditions. Brugger has also carried out calculations on Cu(I) . In both cases
1
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−
115
a very intense edge feature was observed at 8984 eV for [CuCl
2
]
and 8982 eV for
−
116
113
[
CuBr
transitions. With the use of TDꢀDFT calculations
edge and edge features in XANES spectra. Fig. 1 shows the TDꢀDFT calculated XANES
spectra for NiBr at 180° and 140° showing both the individual transitions and also the
2
]
and based on the work of Kau et al.
these were assigned to 1s → 4p
8
8ꢀ94
it is now possible to simulate the preꢀ
2
1
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0
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9
0
molecular orbitals involved. The TDꢀDFT calculated spectra do not look like conventional
experimental XANES spectra as they only model the electric dipole, electric quadrupole and
magnetic dipole transitions which will be superimposed on the 1s → continuum (edge)
transition in the experimental spectra. In addition, there is an energy offset from the
experimental values and whilst this can be adjusted for empirically, we have not done so. At
1
80° the spectrum is dominated by the dipole allowed 1s → 4p transitions, in particular the
intense transition involving MOs with substantial Ni 4px,y character at 8299.3 eV, and those
with Ni 4p character at 8302.6 eV. The transitions to the MOs with substantial 3d
z
xz,yz
th
character at 8292.8 eV are weak (one 400 of the main peak) and those to MOs involving the
th
4s at 8297.3 eV are very very weak (one 2300 ) as they are dipole forbidden but gain
intensity from quadrupole and magnetic dipole transitions. On bending of the NiBr molecule
2
x y
the intensity of the transition from the 1s → 4p p orbitals is reduced as the two MOs are no
longer degenerate, and as the bond angle becomes more acute they resolve into two separate
peaks. The most noticeable increase in intensity is for the 1s → 4s transition at 8296.5 eV,
which is dipole forbidden in linear geometry but on bending it becomes dipole allowed, with
a very small (≈0.3%) quadrupole contribution. Of the two transitions at 8293 eV involving
the 3d orbitals within 0.3 eV of each other, one is much more intense by about an order of
magnitude than the other, but both are very much weaker than the 1s → 4s and 1s → 4p
transitions. The lower energy transition is essentially electric dipole forbidden, and gains its
intensity from magnetic dipole (36%) and electric quadrupole (64%) transitions, and this is an
order of magnitude less intense than the second 1s → 3d transition which has now become
dipole allowed (93%) with small magnetic dipole (2.5%) and electric quadrupole (4%)
contributions.
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Figure 1. TDꢀDFT calculated Ni Kꢀedge XANES spectra for NiBr
angles.
2
with 180° and 140° bond
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FeBr2
CoBr2
NiBr2
o
o
o
o
1
80
180
180
1
1
1
1
1
1
1
1
1
1
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2
2
2
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0
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o
o
1
70
170
170
o
60
o
o
1
160
160
o
o
o
o
o
150
150
150
o
o
1
1
1
40
140
140
o
o
30
130
130
o
o
o
20
120
120
o
o
o
110
110
110
o
o
o
100
100
100
o
o
o
9
0
90
90
7055
7060
7065
7070
7075
7660
7665
7670
7675
7680
8290
8295
8300
8305
Photon energy / eV
Photon energy / eV
Photon energy / eV
Figure 2. TDꢀDFT calculated metal Kꢀedge XANES spectra for FeBr
different bond angles. The same bond length was used for all bond angles of each metal,
which was set to the ADF calculated ground state (FeBr , 2.254 Å; CoBr , 2.210 Å, NiBr
.180 Å).
2 2 2
, CoBr and NiBr at
2
2
2
,
2
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Fig. 2 shows how the TDꢀDFT calculated XANES spectra vary with bond angle for FeBr
2
,
CoBr and NiBr (the individual transitions and MOs are very similar to those for NiBr
2
2
2
shown in Fig. 1). Therefore, the metal Kꢀedge XANES spectra are expected to be very
sensitive to changes in geometry. In particular, the presence of an intense edge feature in the
experimental XANES spectra can be used diagnostically as an indicator of a linear geometry,
and any reduction in its intensity as due to nonꢀlinearity. In addition, there is also expected to
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be an increase in the 1s
→ 3d based transitions as well the 1s → 4s transitions, however the
latter may get masked by other changes in the edge structure.
EXAFS
In addition to the bond length information obtainable using EXAFS data it is also possible to
extract bond angle information. Whilst this has been demonstrated previously from
68, 105, 118, 119
triangulation using the Br⋅⋅⋅Br and BrꢀM distances from the Br Kꢀedge data,
multiple scattering paths through the central metal atom also yield valuable information from
both the metal and bromine Kꢀedges. The importance of these pathways as a diagnostic tool
in determining the geometry at the metal centre has been demonstrated in tetrahedral
NiBr (PPh transꢀNiBr (PEt and cisꢀNiBr (dppe) (dppe 1,2ꢀ
bis(diphenylphosphino)ethane) complexes. The Fourier transforms of the Ni Kꢀedge
data of transꢀNiBr (PEt and cisꢀNiBr (dppe) contained features at approximately twice the
ꢀ
2
3
)
2
,
2
3
)
2
2
=
120, 121
2
3
)
2
2
first shell distance which were absent in the Fourier transform of the Ni Kꢀedge XAFS data of
tetrahedralꢀNiBr (PPh ) . These features are due to multiple scattering involving pathways
2
3 2
through the central absorbing atom (as shown in Fig. 3 for CoBr
as a diagnostic test of linearity for MBr units.
2
), and can therefore be used
2
Figure 3. Representative multiple scattering pathways in CoBr at the Br Kꢀedge
2
and the Co Kꢀedge. (Absorbing atoms are shown with *.)
To illustrate the sensitivity of these multiple scattering pathways with respect to the bond
angles in the transition metal dihalides studied in this work, theoretical Fourier transforms of
1
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Co and Br Kꢀedge data for CoBr
2
over a range of bond angles (180° ꢀ 90°), are shown in Fig.
4. The model for the calculation of the spectra used a constant CoꢀBr bond length of 2.25 Å
2
and constant DebyeꢀWaller factors for the CoꢀBr and Br⋅⋅⋅Br shells of 0.010 and 0.015 Å ,
respectively. The angle of the BrꢀCoꢀBr unit was then defined and the theoretical spectrum
calculated. The multiple scattering parameters used were also consistent for both Kꢀedges and
1
1
1
1
1
1
1
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1
2
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2
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0
2
for every angle calculated. The Co Kꢀedge theoretical Fourier transform for CoBr calculated
without multiple scattering had just the peak corresponding to the CoꢀBr distance at 2.25 Å
for all bond angles.
(b)
(a)
(c)
o
180
o
175
o
1
1
1
1
1
1
1
1
1
1
1
90
70
o
65
o
60o
55
o
50
o
45o
40
30
20o
10
o
o
o
00
o
0
1
2
3
4
5
6 0
1
2
3
4
5
6 0
1
2
3
4
5
6
r /Å
r /Å
r /Å
Figure 4. Calculated FTs of EXAFS data for CoBr2 at different bond angles. (a) Co Kꢀedge
with multiple scattering, (b) Br Kꢀedge without multiple scattering, (c) Br Kꢀedge with
multiple scattering.
However, when the multiple scattering contribution was included a second peak at
approximately twice the CoꢀBr bond distance is observed for nearꢀlinear geometries (Fig.
4(a)). As the bond angle is reduced this feature diminishes very rapidly, particularly for bond
angles less than 160 and completely disappears in the spectra calculated for bond angles of
°
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less than 155
°
, where the FTs are essentially identical to those calculated for no multiple
scattering. It can therefore be concluded that when analysing good quality metal Kꢀedge
XAFS data for the metal dibromides isolated in inert matrices, the presence of a multiple
scattering feature at approximately twice the MꢀBr distance is indicative of a bond angle of
160° or greater. The effect of multiple scattering in the Br Kꢀedge data is more complex as
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there are no additional peaks solely due to multiple scattering. In this case the multiple
scattering events affect the intensity of the Br…Br shell as the intervening metal atom in a
linear arrangement acts as a lens, strongly forward scattering the photoelectron, thus
increasing the intensity of the backscattering Br shell. This is a much more well known
manifestation of multiple scattering than that involving the central absorbing element, and is
−
common in complexes containing CO, CN and imidazole ligands. Fourier transforms were
calculated without multiple scattering theory, shown in Fig. 4(b), as well as with multiple
scattering, shown in Fig. 4(c). Fig. 4(b) shows a simple and expected increase in the intensity
2
of the Br…Br peak as the distance is shortened upon a more acute bond angle due to the 1/r
effect. Fig. 4(c) shows an altogether different effect due to multiple scattering. The data for
1
80
the bond angle is decreased from 180
is particularly evident between 165 and 155
°
shows an enhanced intensity of the Br…Br feature due to the multiple scattering. But as
the Br…Br peak shows a reduction in intensity, which
, to such an extent that the Br…Br shell would
°
°
°
not be observed above the noise level in the experiment. At smaller angles the shape and size
of the Br…Br peak corresponds closely to that of spectra calculated without multiple
scattering theory. Therefore, the effect of multiple scattering at the Br Kꢀedge is to increase
the intensity of the Br…Br peak at near linear geometries, reduce it around 160
have little effect below 150 . Therefore, these calculations suggest that the absence of a
Br…Br shell in the Br Kꢀedge spectra is indicative of a bond angle between 155 and 165
and that peaks with intensities above what was expected when calculated without multiple
scattering are indicative of a bond angle between 170 and 180
°, and then to
°
°,
°.
It is to be noted that the sensitivity of the XAFS multiple scattering effects is at a maximum,
where the trigonometric functions used for bond angle calculations with IR isotopic data, are
at their least sensitive, i.e. for nearly linear systems. The validity of the model was tested with
67
the other metals employed in this study. As it has been shown previously that the MꢀHal
bond length increases upon deviation from linearity, the calculations were carried out with
1
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slightly different bond lengths, and no appreciable difference in the trends shown in Fig. 4
were detected.
DFT calculations
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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8
9
0
In addition to the experimental results, DFT calculations on the ground and low lying excited
electronic states of the dibromides and diiodides have been carried out to support the
interpretation of the experimental data.
This paper describes the results of the isolation of the 3dꢀtransition metal dibromides of Fe,
Co, Ni and Zn in noble gas matrices (Ne, Ar, Kr, Xe) as well as CH
being employed in the NiBr studies. Whilst the difluorides and dichlorides of the 3d
transition metals have been widely studied, the available data in the matrix isolation literature
4 2 2
and N , with O also
2
99
96, 97, 122,
for the dibromides is much more scarce and is limited to NiBr
2
in Ar, ZnBr
2
in Kr,
1
23
98
and CrBr
2
in Ar. We have published two preliminary pieces of work on NiBr
2
in N
2
68, 69
matrices.
The paucity of spectroscopic data for the transition metal dibromides has also
124
been highlighted. Therefore, the combination of the published data on the difluorides and
dichlorides together with the data presented herein will make it possible to compare the effect
of changing the dꢀelectron count as well as the hardness of the metal on going from fluoride
1
24, 125
through the chloride to the bromides. Previous electron diffraction work
on the
molecular geometry of gasꢀphase metal halides has shown that all the dihalides studied in this
126
127
work are linear in the gas phase. Mass spectrometry indicates that for FeBr2 and CoBr2
the predominant vapor species is the monomer, and that the dimer species only become
127
significant close to the melting points. No dimeric species were observed for NiBr
2
.
In
128
2
contrast, the vapor above ZnBr can contain a considerable proportion of dimeric species.
Results
The results and discussion are divided into sections concerning each of the metals, and the
metals are presented in the reverse order of their d electron configuration. However, before
considering the experimental data, the DFT results on the ground and low lying excited states
will be discussed.
DFT calculations on ground and low lying excited states of MBr and MI .
2
2
1
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DFT calculations have been employed to calculate the geometries and spectroscopic
properties of the ground and lowest lying excited electronic states for the dibromides and
diiodides. Wang and Schwarz have carried out DFT calculations for the difluorides and
129
dichlorides of the 3d transition elements. Although there have been many subsequent
papers on chlorides and fluorides there has not been a systematic study of all of the
dibromides and diiodides, although individual reports detailing calculations on the
0
1
2
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4
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0
130
131, 132
133
132
134, 135
dibromides and diiodides of manganese, iron,
cobalt, nickel and zinc
have
been published, as well as a comparison of different DFT methods for calculating heats of
1
.
36
formation and ionization potentials of third row transition elements
Therefore, we have
carried out a comprehensive and consistent set of calculations on the ground and lowest lying
excited states, for all of the 3d transition metal dibromides and diiodides (as well as Ca and
Zn). The optimized geometries, vibrational frequencies and IR intensities are given in Tables
129
1
and 2, which are in good agreement with those of Wang and Schwarz for the analogous
dichlorides and difluorides. The number of low lying excited states, both linear and bent,
should be noted.
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Zinc dibromide
1
37
Vapor phase electron diffraction studies on zinc dichloride, dibromide and diiodide have
shown them all to be linear with a ZnꢀBr bond length ( ) for ZnBr of 2.204(5) Å. XAFS
in argon and nitrogen matrices have obtained bond
r
g
2
67
studies on matrix isolated ZnCl
distances that are in agreement with those obtained by vapor phase electron diffraction ( g
Loewenschuss and coꢀworkers have conducted vibrational spectroscopic studies on all the
2
1
37
r
).
1
1
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1
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0
9
6, 97
zinc dihalides isolated in krypton matrices,
with particular interest shown in the
97, 123
formation of mixed dihalides.
Infrared Spectroscopy
14
15
The IR spectra of ZnBr isolated in neon, argon, krypton, xenon, methane, N and N2
2
2
matrices are shown in Fig. 5, and the observed frequencies of the
ν
3
asymmetric ZnBr
2
stretching mode are given in Tables 3. These data are in very good agreement with the Kr
96, 97
matrix data reported by Loewenschuss et al.
What is immediately clear is that all the
vibrational frequencies are very close to each other, indicating only a very small effect of the
matrix on the vibrational energy levels of the trapped molecules.
1
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(a)
(
b)
414
408
402
0
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(
c)
(d)
(e)
(f)
(g)
(h)
4
30
420
410
400
390
380
370
360
350
−1
Wavenumber / cm
2
Figure 5. IR spectra of ZnBr isolated in (a) Ne, (b) Ne at high resolution, (c) Ar, (d) Kr, (e)
14
15
4 2 2
Xe, (f) CH , (g) N and (h) N matrices.
−1
At 2 cm resolution the Zn isotope pattern is clearly resolved with absorption intensities in
6
4
66
68
line with the isotopic natural abundance ( Zn, 48.6%; Zn, 27.9%; Zn, 18.8%). SVFF
calculations (Table 3) using these data clearly indicate the linearity of ZnBr in all of the
2
matrices studied. ZnCl
2
vapor has been shown to contain a considerable concentration of
128
−1
dimer molecules,
and Loewenschuss et al. have assigned a band at 326 cm in the
96, 97
spectrum of ZnBr isolated in krypton as belonging to the ZnBr dimer.
None of the
2
2
spectra recorded in this study showed any significant absorption bands in this region. The
−1
broad features at ca. 380 cm in the spectrum of ZnBr
2
isolated in neon in Fig. 5(a) are most
likely to be associated with the problems of using Ne as a matrix with an 8ꢀ9 K cryostat.
−
1
However, the 0.25 cm resolution spectrum of ZnBr
afforded both zinc and bromine isotopic fine structure (Table 3) both indicating a linear
geometry and that the ZnBr was well isolated. This spectrum clearly shows the presence of
2
isolated in neon, shown in Fig. 5(b)
2
1
5
2 2
multiple trapping sites in Ne which are often observed. When ZnBr was isolated in a N
14
−1
matrix, (Fig. 5(h)) the shift of the
ν
3
modes from a N
2
matrix, (Fig. 5(g)) was ca. 0.1 cm .
2
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*
*
(
a)
(b)
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0
ZnBr2
ZnBr2
*
*
*
NiBr2
NiBr2
*
NiCl2
NiCl2
NiF2
*
*
NiF2
*
*
CoBr2
CoBr2
FeBr2
*
*
FeBr2
2340
2330
2320
2270
2260
2250
2240
−1
−1
Wavenumber / cm
Wavenumber / cm
1
4
15
Figure 6. IR spectra of MX
2
2 2
species trapped in (a) solid N and (b) solid N matrices at ca.
14
1
0 K. (Each spectrum has an absorbance of ca. 0.01, * indicates impurity induced N and
2
1
5
N modes)
2
14
15
Photolysis studies of ZnBr
2 2 2
in N and N matrices produced no change in the absorption
bands in the FIR or MIR. In addition to the usual H
2
O and CO
2
bands in the MIR, additional
1
5
−1
bands were observed in the N matrices at 2252.2 and 2250.0 cm (Fig. 6, Table 4). The
2
1
5
latter of these (marked with *) has been previously assigned as an impurity induced
N
2
138
14
−1 138
mode. In a
N
2
2
matrix the impurity induced N mode was observed at 2327.8 cm ,
−1
together with a new band at 2329.9 cm .
The bands to high wavenumber of the impurity induced ones are discussed in more detail in
later sections, but their assignment was only possible once they had been clearly identified in
1
5
the N
2
matrices, because of the problems of gas and matrix phase CO
2
masking this region
14
for
N . DFT calculations indicate that the reaction enthalpy for the formation of either a
2
−
1
monoꢀ or bisꢀ dinitrogen ZnBr
2
complexes is about −1 kJ mol .
2
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The isolation of a metal dihalide in a matrix can be viewed as being analogous to solvation in
2
many ways. The small consistent matrix shift observed for ZnBr is analogous to a solvent
shift and indicated that there was no significant change in the geometry of ZnBr
the matrices studied.
2
between all
0
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0
XAFS Spectroscopy
2
ZnBr was isolated in a nitrogen matrix and both the Zn and Br Kꢀedge XAFS spectra were
obtained. However, the data for this system were of such poor quality, particularly the Zn Kꢀ
edge spectrum due to several large unnormalized glitches of instrumental origin, that the data
was essentially unanalyzable, except for an estimate of the ZnꢀBr bond length (2.19(2) Å)
which is in good agreement with the electron diffraction data (2.204(5) Å) previously
137
obtained by Hargittai and coꢀworkers. Therefore, no other matrices were studied using
XAFS due to the instrumental origin of the glitches, and hence no firm conclusions can be
drawn from the XAFS data.
Conclusion
2
From the IR results, the consistent matrix shift observed for ZnBr in all the matrix hosts
studied, combined with the bond angle calculations, suggest that ZnBr has minimal
2
interaction with the matrix and preserves its linear geometry in all cases. The order of
interaction for ZnBr
order proposed by Beattie and Millington. In conclusion, there is no experimental evidence
that ZnBr is anything but linear in Ne, Ar, Kr, Xe, CH and N matrices.
2 4 2
is Ne < Ar < Kr <CH < Xe < N , which is in very good agreement with
1
5
2
4
2
Nickel dibromide
2
Nickel dihalides, and in particular NiCl with its easily resolved nickel and chlorine isotope
patterns, have received the most attention in the matrix isolation literature of all the dihalides
101, 139
investigated in this work. The IR spectra of NiF , isolated in neon and argon matrices
2
1
40, 141
and also in argon matrices doped with N
2
and O
2
,
have been studied. A bond angle of
1
52
°
°
was calculated for NiF isolated in Ne or Ar matrices by Margrave, et al. compared to
2
139
157
for ZnF2,
but as discussed below, these are at the margins of what could be
considered as linear.
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Whilst NiF
doped Ar matrices,
induced mode, even in a pure N
2
.N
2
and NiF
2
.O
2
complexes were proposed by DeKock and Van Leirsburg using
1
40, 141
N
2
no
ν
NN modes were observed for NiF
2
.N
2
other than the impurity
2
matrix. NiCl
2
has been the subject of numerous studies in an
9
9, 101, 140ꢀ142
102, 103,
143
argon matrix with IR,
electronic absorption
and fluorescence spectra all
indicating a linear or near linear structure. NiCl
very elegant molecular beam experiments with vibrational and rotational resolution on the
60 and 360 nm bands which showed that it was linear in both the ground and exited states,
2
has also been the subject of a number of
1
1
1
1
1
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0
1
2
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0
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2
3
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5
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9
0
1
2
3
4
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9
0
1
2
3
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5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
3
−
144ꢀ150
with a Σ ground state.
g
1
51
Green and coꢀworkers proposed that NiCl
2
isolated in an argon matrix has a nonꢀlinear
17, 148
structure with a calculated bond angle of 161
°. However, it has been shown
that a
−1
difference between the calculated isotope shifts of only ca. 0.1 cm results in a change of
67
bond angle from 180
indicated a linear structure whereas in a N
and O complexes for NiCl analogous to those of NiF
and Van Leirsburg. Clearly, the nature of the geometry of nickel dihalides isolated in
Ne, Ar, and N matrices has been a contentious issue. The literature reports on matrix NiBr
°
to 160
°
. EXAFS data for NiCl
matrix, NiCl
have also been postulated by DeKock
2
isolated in a methane matrix have
67
2
68
2
2
and NiBr
are strongly bent. N
2
2
2
2
140, 141
2
2
99
68, 69
concern its isolation in an argon matrix, and our more recent preliminary work.
152
Electron diffraction data indicates linearity in the vapor phase.
Infrared Spectroscopy
2 2 4
The IR spectra of NiBr isolated in Ar, Kr, Xe, O , and CH matrices are shown in Fig. 7, and
the observed frequencies of the ν3 asymmetric NiꢀBr stretching vibration, are given in Table
9
9
5
and there is reasonable agreement with the previous argon data. The spectra obtained at 2
−1
58
60
cm resolution, shown in Fig. 7, produced clear bands due to the Ni and Ni isotopes
6
2
(natural abundance ꢀ 68.3% and 26.1%, respectively) with the Ni isotope (natural
abundance 3.6 %) also observed after prolonged deposition. It is clear from the relative
positions of the
3 2
ν modes in the spectra that a simple matrix shift is observed for NiBr ,
58
60
analogous to the ZnBr case. Bond angle determination from the Ni and Ni isotope pattern
2
58 79
58 81
and also from the Ni Br
2 2 2
and Ni Br absorption bands resolved in Ar (Fig. 7(a)), O (Fig.
7(b)), Kr (Fig. 7(c)) and Xe (Fig. 7(e)) matrices all indicated a linear structure (Table 5). In
140, 141
line with the previous report on NiCl isolated in a Ar/O matrix,
no OꢀO stretches were
2
2
seen for NiBr isolated in an O matrix, nor after broad band photolysis. This was perhaps not
2 2
2
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surprising as the interaction between the guest molecule and the matrix seems to be weak as
the gas phase molecular geometry was preserved. Indeed, the broad absorption band observed
−
1
−1
for NiCl
2 2
in mixed Ar/O matrices at 480.2 cm (compared to 520.7 cm for a pure Ar
1
40, 141
matrix) and assigned
as the asymmetric stretch of NiCl
2
O
2
may be simply due to the
perturbed asymmetric stretch of NiCl
environment.
2
in a predominantly O
2
environment, or mixed Ar/O
2
0
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(a)
422 420 418 416 414 412
(b)
416 414 412 410 408
(c)
414 412 410 408 406
(d)
(e)
408 406 404 402 400
440
430
420
410
400
390
(f)
(g)
340
330
320
310
300
−
1
Wavenumber / cm
14
Figure 7 IR Spectra of NiBr
2
isolated in (a) Ar, (b) O
2 4 2
, (c) Kr, (d) CH , (e) Xe, (f) N , (g)
1
5
N
2
matrices. The inserts show higher resolution (0.25 cmꢀ1) spectra.
−1
−1
The νNiꢀBr modes for NiBr
2
in nitrogen matrices are shifted by ca. 90 cm (419.1 cm (Ar) to
−1
331.8 cm (N
very bent geometry for NiBr
by Beattie et al. also found that NiCl
2
)) (Fig. 7(f) and (g)). As identified in our preliminary IR data this indicates a
68
2
in a nitrogen matrix with a bond angle of ca.125°. IR studies
isolated in a N matrix was also strongly bent with a
2
2
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6, 17
bond angle of ca. 130
°
.
These observations are in contrast with what is seen for the other
matrices described above, where bond angle calculations indicated linear geometries. When
15
14
−1
−1
N
2
was used instead of
N
2
(Fig. 7(g)) the
ν
3
mode shifted by 0.3 cm to 331.5 cm ,
guest and N host as compared to the
. The separation of the features due to the Ni isotopes was
matrices than previously seen for the other matrices and is a direct
indicating an enhanced interaction between the NiBr
2
2
−1
0
.1 cm shift observed for ZnBr
2
1
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
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2
3
4
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8
9
0
less defined in the N
2
consequence of the change in bond angle as the central element isotope splitting reduces on
bending, whereas the terminal atom isotope splitting increases. The Br isotopic splitting
pattern was not able to be resolved in nitrogen matrices. The nature of the very weak bands
−1
centred around 315 cm is uncertain, but they are probably not due to dimers as these would
99
be expected to be at lower energy and their shape and intensity are also not consistent with
dimer modes. These bands were more likely to be some sort of matrix site effect which is
dependent on the experimental conditions. It is conceivable that they are the
1
ν modes, but
our calculations show that for bond angles of ca. 130° the separation is expected to be
3 1
ν ꢀ ν
−
1
in excess of a 100 cm .
The low energy bending mode of NiBr in argon matrices has been has been reported
2
−
1 99
previously to be 69 cm . This is below the range of most commercial spectrometers so in
order to determine how the bending mode was affected by varying the matrix gas synchrotron
radiation far infrared (SRꢀFIR) experiments were carried out. The SRꢀFIR spectra of NiBr2
trapped in Ar, N
deposits as the DFT calculations indicated that the intensity of the
about 1ꢀ2% of that of the mode. Unfortunately, it is clear from the SRꢀFIR argon matrix
2
and CH
4
matrices are shown in Fig. 8. These experiments utilised heavy
ν
2
bending mode will be
ν
3
−1
spectrum (Fig. 8(a)) that the low energy part of the spectrum (< 100 cm ) where the bending
−1
modes are expected, is dominated by the Ar phonon bands (32.4 and 70.8 cm ), nearly freely
−1
−1 153, 154
rotating water molecules (< 40 cm ) and argon hydrates between 40 and 100 cm .
Whilst the water bands could be reduced by prolonged pumping and baking of the vacuum
system, this was not feasible in the limited amount of SRꢀFIR time available, and water bands
are essentially impossible to remove completely. In nitrogen matrices (Fig. 8(b)) the spectrum
−1
is also badly affected by the N lattice modes at 49 and 71 cm and the water libration bands
2
−
1 153, 154
−1
at 147 and 225 cm .
The 225 cm bands are usually obscured when CsI optics are
−
1
used down to 200 cm . Therefore, it is not possible to identify the ν mode with any certainty
2
in Ar or N matrices. However, in the spectrum of NiBr isolated in CH (Fig. 8(c)) there is a
2
2
4
2
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1
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1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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3
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5
5
5
5
5
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6
−
1
band at 55 cm which is in reasonable agreement with the earlier Ar matrix value of 69
−1 99
cm .
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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8
9
0
1
2
3
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8
9
0
1
2
3
4
5
6
7
8
9
0
(a)
(b)
(c)
500 450 400 350 300 250 200 150 100 50
−1
Wavenumber / cm
2 2 4
Figure 8. SRꢀFIR spectra of NiBr isolated in (a) Ar, (b) N and (c) CH matrices.
If the molecule is severely bent in nitrogen, the ν mode would be expected to gain some
1
intensity, but there is no convincing evidence for it in any of the IR spectra. The DFT
calculations indicate that the intensity of the ν1 mode compared to the ν3 mode is ca. 10% at
130° and ca. 2% at 155°. One explanation for the apparent absence of
1
ν is that it is masked
−1
2 2
by the broad libration modes of H O in N around 225 cm or it could possibly be the weak
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features at 315 cm . The DFT calculations indicate that the separation of
ν
1
and
on bending, and the separation is expected to be around 100 cm for bond angles around
30°. Therefore, the most likely explanation is that the mode is masked by the broad
3
ν reduces
−1
1
ν
1
−
1
modes around 250ꢀ200 cm in nitrogen matrices.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
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8
9
0
1
2
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1
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0
1
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8
9
0
Experiments were also carried out on NiF
2
in nitrogen matrices to provide a comparison with
5
8
60
14
the NiCl and NiBr data. The Ni isotope pattern on ν for NiF and NiF in N at 705.8
2
2
3 2 2 2
−1
−1
15
cm and 701.3 cm gave a bond angle of 152
°
, and for N
2
matrices the bands at 705.3 and
−1
−1
7
00.7 cm yielded 162° (but these values only reflect a difference of 0.2 and 0.1 cm from
linearity, respectively). Previous work had calculated a bond angle of 152
°
for the
ν
3
modes
5
8
−1 60
−1
58
of NiF
2
in solid Ne ( NiF
2
, 800.7 cm ; NiF
2
, 795.5
5
cm ) and solid Ar ( NiF
2
, 780.0
−1 60
−1 139, 141
cm ; NiF
2
, 775.0
4
cm ).
These values indicate a 10% reduction in wavenumber and
an 18% drop in force constant (assuming linearity) between NiF in Ar and NiF in N . The
2
2
2
corresponding shifts in wavenumber for NiCl
2
and NiBr
2
between Ar and N
2
matrices are
1
7% and 21%, respectively, with a drop in force constant of 32% for NiCl and 37% for
2
NiBr . This is a clear indication that the level of interaction between the nickel dihalide and
2
2
the dinitrogen matrix is in the order Br > Cl > F. NiI would be of obvious interest as it would
possibly coꢀordinate to dinitrogen the most readily. Performing successful experiments with
bulk NiI was a challenge as it decomposed on heating to liberate I , resulting in very poor
2
2
throughput of the infrared radiation, and no meaningful data was obtained.
Having identified that there is some form of interaction between the N
2
matrix and the
trapped NiBr , and that the NiBr unit is bent, the νNN region was studied in some detail. A
2
2
69
preliminary report of this has been published previously, but the main findings are
summarized below to put the rest of the data in context.
15
−1
The νNN region of NiBr in a N matrix contained two bands at 2263.8 and 2259.4 cm
2
2
15
after deposition (Fig. 6(b) and 9(a), Table 4) in addition to the perturbed νNN mode of N at
2
−1 138
2
250.0 cm . In contrast, in the ZnBr
2
data (Fig. 6(a)) the only additional bands were a
NN modes in the NiBr spectra to
shoulder on the impurity induced modes. The position of
ν
2
high frequency of the “free” N modes is unusual, and in comparison with the isoelectronic
2
carbonyl species maybe indicative of “nonꢀclassical” bonding with very limited πꢀback
155
bonding. When this sample was photolyzed with broad band HgꢀXe radiation, a radical
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change was observed (Fig. 9(b)) compared to the spectrum recorded after initial deposition
−1
(
Fig. 9(a)), with the appearance of two new bands at 2205.7 and 2186.4 cm (these have low
intensity, about 1 ꢀ 2% of the absorbance of the NiBr
2
3
ν mode) together with a slight
−
1
reduction in the intensity of the 2259.4 cm band shown in the difference spectrum (Fig.
14
9
(c)). When NiBr
2
was isolated in an N
2
matrix there were features at 2336.9 and 2342.8
0
1
2
3
4
5
6
7
8
9
0
1
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9
0
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7
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0
1
2
3
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5
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
−1
−1
cm as well as the perturbed νNN mode at 2327.8 cm on deposition (Fig. 6(a)), but due to
2
the ubiquitous presence of matrix isolated CO features these were only identifiable once the
15
corresponding
2
N features had been identified. After photolysis (Fig. 9(d)), new bands
−1
14
15
appeared at 2281.4 and 2261.4 cm . When a N
2 2
/ N matrix was used (Fig. 9(e)), bands
−1
after photolysis were observed at 2273.3 and 2195.3 cm , in addition to those present in the
14
15
pure N
2 2 2
or N matrices, giving rise to a pair of triplets indicating the presence of two N
14
15
units. Attempts to use N N matrices to identify the N bonding mode (endꢀon vs. sideꢀon)
2
1
56, 157
were unsuccessful due to the very weak nature of the features.
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(
a)
(b)
c)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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5
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5
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0
1
2
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8
9
0
1
2
3
4
5
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7
8
9
0
1
2
3
4
5
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9
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2
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0
1
2
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4
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6
7
8
9
0
(
(d)
(e)
(f)
(g)
2
280
2260
2240
2220
2200
2180
2160
−1
Wavenumber / cm
15
Figure. 9 IR spectra of NiBr
photolysis, (c) difference spectrum (photolysis – deposition). (d) difference IR spectrum of
2 2
isolated in N (a) after deposition, (b) after broad band
1
4
NiBr
isolated in N
2
isolated in N
2
after broad band photolysis. (e) difference IR spectrum of NiBr
(1:1) after broad band photolysis. (f) difference IR spectrum of NiCl
after broad band photolysis. (g) difference IR spectrum of NiCl isolated in
2
after broad band photolysis.
2
14
15
2
: N
2
2
1
4
isolated in N
2
2
1
5
N
1
4,15
Failure to observe
N
2
species with weak
ν
NN modes has been previously noted by
138
Andrews and coꢀworkers in their study of beryllium dinitrogen complexes. No changes
were observed in the NiBr
2
ν
3
region after photolysis. In all cases the intensity of the
ν
NN
bands observed after photolysis was markedly reduced after annealing to 15 K and
disappeared completely after warming the matrix to ca. 20 K, but could be regenerated (with
less intensity) after further photolysis at ca. 8 K, but only for 3 or 4 times. The intensity of the
photoactive bands was greater when UV rather than visible irradiation was used and was also
slightly enhanced if photolysis was carried out during deposition. The new bands after
2
9
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