Chemical Physics Letters 514 (2011) 202–206
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Fourier-transform microwave spectroscopy of FeCN (X4Di): Confirmation
of the quartet electronic ground state
L.N. Zack, J. Min, B.J. Harris 1, M.A. Flory 2, L.M. Ziurys ⇑
Department of Chemistry, Department of Astronomy and Steward Observatory, 933 North Cherry Avenue, University of Arizona, Tucson, AZ 85721, USA
a r t i c l e
i n f o
Article history:
Received 17 July 2011
In final form 16 August 2011
Available online 22 August 2011
a b s t r a c t
Spectra of the FeCN radical have been measured using Fourier-transform microwave (FTMW) techniques.
This species was created in a supersonic jet by laser-ablation of iron, coupled with a dc discharge, in the
presence (CN)2. The lowest rotational transition of FeCN near 36 GHz was recorded. The observation of
this transition, the J = 9/2 ? 7/2 line in the X = 7/2 spin–orbit component, conclusively establishes that
FeCN has a 4Di ground electronic state, rather a 6D state, as predicted by theory. The FTMW spectrum
of NiCN (X2Di) was also measured; small nitrogen hyperfine splitting was observed in both molecules.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Metal monocyanide/isocyanide species have varied structural
and electronic properties. The metal atom can bind to the CN moiety in one of three known stable geometric configurations: as a linear cyanide (MCN), a linear isocyanide (MNC), or in a T-shaped
structure where the metal ion ‘orbits’ the CN triple bond [1,2].
While metal-CN bonds are thought to be primarily ionic, the structure of a given metal monocyanide species can provide insight into
the degree of covalent character. For example, highly ionic species,
such as KCN and NaCN, are T-shaped [1,2], while molecules exhibiting the MNC form, MgNC and AlNC, for example, are thought to
have more covalent character [3,4]. The most covalent species appear to be the MCN isomers, where metal p backbonding could be
influencing the geometry [5]. Furthermore, many metal monocyanides have been found to have more than one stable structure, and
they often lie close in energy [4].
The iron cyanide/isocyanide system has been a case study in
this regard. In 2001, FeNC was identified for the first time in the
gas phase using laser-induced fluorescence spectroscopy [6]. Several vibrational bands of the X0 = 7/2 ? X00 = 9/2 system of this free
radical were recorded, including some for the FeN13C isotopologue.
Analysis of the rotational structure suggested a 6Di ground state
term for the molecule, in analogy to FeF [7] and FeCl [8,9]. This
experimental study concluded that the linear isocyanide FeNC
was the lowest energy isomer, as supported by subsequent theoretical calculations by DeYonker et al., Hirano et al., and Rayon
et al. [4,10,11]. In 2007, a new theoretical work by Hirano et al.
[12] suggested that, to the contrary, FeCN was the more stable species, also with a 6Di ground term. More recently, the pure rotational spectra of both FeCN and FeNC were recorded by Flory and
Ziurys [13], using millimeter-wave direct absorption techniques.
This study clearly demonstrated that FeCN is the lower energy isomer, and it confirmed the rotational constants of FeNC found by Lie
and Dagdigian [6]. Furthermore, the millimeter-wave spectra indicated a 4Di ground state for FeCN, not the 6Di term predicted by
theory [11,12]. The failure to observe FeCN in the previous LIF
experiment was likely a result of predissociation of the excited
state in the X0 ? X00 transition.
In order to conclusively establish the ground electronic term for
FeCN, the Fourier-transform microwave (FTMW) spectrum of this
molecule has been measured. The J = 9/2 ? 7/2 rotational transition in the lowest spin–orbit component, X = 7/2, was recorded
at 36 GHz, which exhibited distinct nitrogen hyperfine structure.
If the ground state of FeCN were 6Di, this transition would not exist, as in this case X = 9/2, and J P X. A combined analysis of the
microwave and previous millimeter/submillimeter data was carried out for iron cyanide, and the h hyperfine constant was established. The equivalent transition (J = 7/2 ? 5/2, X = 5/2) was also
measured in NiCN (X2Di) for comparison. These experimental results are presented here, along with a discussion of the hyperfine
constants and an interpretation of the quartet ground state in
FeCN.
2. Experimental
⇑ Corresponding author. Fax: +1 520 621 5554.
E-mail address: [email protected] (L.M. Ziurys).
Present address: Department of Chemistry, University of Virginia, Charlottesville,
VA 22904, USA.
2
Present address: CNA, Alexandria, VA 22311, USA.
1
0009-2614/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2011.08.040
Spectra of FeCN were measured using the Ziurys’ group Balle–
Flygare-type Fourier-transform microwave (FTMW) instrument,
described in detail elsewhere [14]. Briefly, the instrument is comprised of a vacuum chamber with two spherical aluminum mirrors
L.N. Zack et al. / Chemical Physics Letters 514 (2011) 202–206
forming a Fabry–Perot cavity. Two antennas, one embedded into
each mirror, launch the microwave radiation into the cavity and
detect resulting molecular emission. Molecules are pulsed into
the chamber at a 10 Hz rate using a supersonic nozzle at a 40° angle relative to the optical axis. The spectrometer is also equipped
with a laser ablation source based on the second harmonic
(532 nm) of a Nd:YAG laser (Continuum Surelite I-10), and
employing a rotating/translating metal rod. Radiation from the laser is introduced into the chamber perpendicular to the supersonic
jet [15]. Signals are recorded in the time domain and a fast Fourier
transform is applied to produce frequency-domain spectra with
2 kHz resolution. Each transition appears as a Doppler doublet
with a full width at half maximum of approximately 5 kHz, and
the transition frequencies reported are the average of the two
Doppler components.
FeCN was synthesized in a jet expansion of iron vapor and 0.25%
(CN)2 in argon, using the DALAS technique, or Discharge Assisted
Laser Ablation Spectroscopy; see Ref. [15]. The cyanogen/argon
mixture was expanded from the pulsed valve into the ablation region, where metal vapor was produced, and then through a DC discharge nozzle and into the cavity. For each single pulse, the valve
was opened for a duration of 750 ls at a stagnation pressure of
250 kPa (absolute); at the same time, the DC discharge (1000 V)
was switched on for 1390 ls. The laser (flash-lamp voltage
1.29 kV; 240 mJ/pulse) was fired 940 ls after opening the valve,
slightly sooner than used for other molecules such as CuCCH
(990 ls). Elimination of the DC discharge, removal of cyanogen,
and/or exclusion of the laser resulted in the loss of the FeCN signal.
Approximately 2000–5000 pulses for each transition were necessary to achieve an adequate signal-to-noise ratio.
NiCN was created in a nearly identical fashion, except a nickel
rod was used in place of iron. The DC discharge duration was
slightly longer at 1400 ls, and only 1000 pulses were needed to
produce reasonable signals.
3. Results and analysis
The ground state of FeCN was identified as 4Di in the experimental work of Flory and Ziurys [13] on the basis of the observed
fine structure spin–orbit components. Only four such components
could be found in their millimeter-wave spectra, hence the quartet
assignment. In similar direct absorption experiments done on FeF
and FeCl in the Ziurys group, all six fine structure lines were readily
apparent in the data [7,8]. However, it is possible that Flory and
Ziurys missed the two highest energy spin ladders, simply on the
basis of signal-to-noise ratios. Therefore, what was assigned to
FeCN as the X = 7/2 ladder in a 4Di state could instead arise from
the X = 9/2 component in a 6D state. These sub-levels are the lowest in energy for 4Di and 6Di (inverted) states, respectively.
Flory and Ziurys derived accurate rotational parameters for
FeCN in four spin–orbit components, independent of the exact X
assignments. Because these authors scanned continuously across
several rotational transitions, they identified the lowest-energy
spin–orbit component as it has the highest intensity in the direct-absorption experiments. The only spin components that could
have been ‘missed’ would be those lying much higher in energy, as
they are the weakest features. Because J P X, the lowest rotational
transition for a 4Di electronic state corresponds to J = 9/2 ? 7/2 in
the X = 7/2 ladder; for a 6Di term, the lowest is the J = 11/2 ? 9/2
line in the X = 9/2 ladder, and the J = 9/2 ? 7/2 transition does not
occur. Therefore, an unambiguous test of the identity of the ground
state in FeCN is to search for the J = 9/2 ? 7/2 transition. If the
ground state is 4Di, this transition will be present, but will not be
observed in a 6Di state.
Prior to the search for FeCN, chemical conditions and experimental timing were optimized on the J = 7/2 ? 5/2 transition of
203
NiCN in its X2Di state. The frequency of this line was calculated
using the millimeter-wave constants of Sheridan and Ziurys [5].
The three strong nitrogen hyperfine components of this transition
were readily found, and the signals maximized. The total frequency
separation of the three hyperfine lines was on the order of 2 MHz,
providing an estimate for such splittings in FeCN.
The frequency of the J = 9/2 ? 7/2 transition for FeCN was first
calculated from the rotational constants B, D, and H of the lowestenergy spin component observed in the millimeter-wave data [13].
Because this radical contains a nitrogen nucleus, which has a spin
of I = 1, magnetic and quadrupole hyperfine splittings were expected to be present in the spectrum, as observed in NiCN. Consequently, a range of 5 MHz (36 098–36 103 MHz), centered at the
predicted FeCN (X4Di) frequency, was surveyed in order to locate
the hyperfine lines. Three lines were observed in this search that
exhibited the expected splitting and relative intensities of the
strongest nitrogen hyperfine components, which follow the selection rule DJ = DF = +1, where DF = F0 F00 . Chemical tests were then
performed that demonstrated that the three lines were due to iron
and cyanogen. The weaker hyperfine transitions (DF = 0, 1) were
not found in the search, but they have significantly lower intensities than the observed features and would not be detected given
the noise level.
Figure 1 shows a qualitative energy level diagram of FeCN
(X4Di), highlighting the hyperfine lines observed in the J = 9/
2 ? 7/2 rotational transition of the X = 7/2 spin–orbit ladder. In
a case (a)bJ coupling scheme, total angular momentum F is defined
as F = I + J, and the hyperfine levels are indicated by this quantum
number. The other three ladders are shown, and the approximate
spin–orbit splitting.
The FTMW spectrum of the J = 9/2 ? 7/2 rotational transition of
FeCN measured here is displayed in Figure 2. The three nitrogen
hyperfine components with their appropriate F quantum numbers
are shown, exhibiting approximate theoretical relative intensities.
There are two frequency breaks in the spectrum so all the data can
be plotted on the same scale. Each hyperfine feature appears as a
Doppler doublet, indicated by the brackets. The lines are slightly
broader than the usual spectra obtained with the FTMW spectrom-
Figure 1. A qualitative energy level diagram of FeCN (X4Di), showing the rotational
ladders of the four spin–orbit levels X = 7/2, 5/2, 3/2, and 1/2 in this molecule. The
spin–orbit splitting is also indicated. The inset displays the hyperfine structure in
the two lowest rotational levels, J = 7/2 and 9/2, labeled by the quantum number F.
The hyperfine splittings are due to the nitrogen nuclear spin (I = 1). The arrows
indicate the microwave transitions of FeCN observed in this work.
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L.N. Zack et al. / Chemical Physics Letters 514 (2011) 202–206
Figure 2. FTMW spectrum of the J = 9/2 ? 7/5 transition of FeCN in the X = 7/2 spin–orbit ladder near 36 GHz. The transition consists of three strong hyperfine components,
labeled by quantum number F, arising from the nitrogen nuclear spin (I = 1). The brackets indicate the Doppler doublets. There are two frequency breaks in the spectrum so
that all three components could be displayed on the same frequency scale. The spectrum is a compilation of three 600 kHz scans, each of which is an average of 20,000 nozzle
pulses.
eter, indicative of a very short lifetime for FeCN, consistent with
the free induction decay.
Figure 3 shows the J = 7/2 ? 5/2 rotational transition of NiCN in
its lowest spin ladder, X = 5/2, measured with the FTMW system.
The hyperfine components are labeled by F, and the Doppler splitting is indicated by brackets. Again, there are two frequency breaks
in the displayed spectrum. In this case, the F = 7/2 ? 5/2 component appears somewhat stronger than expected, and has some
residual splitting, likely due to the Zeeman effect resulting from
the Earth’s magnetic field. The variation in intensities reflects differences in molecule production, arising from altering the experimental conditions (e.g. laser power, (CN)2 concentration,
discharge voltage, etc.) in order to optimize the signals.
The measured frequencies of the three hyperfine lines of FeCN
in the J = 9/2 ? 7/2 transition are listed in Table 1, and those for
NiCN (J = 7/2 ? 5/2) in Table 2. As the tables show, the overall
splitting of the hyperfine interactions is not large in either molecule – on the order of 2 MHz. This result is not unanticipated.
Table 1
Observed microwave transitions of FeCN (X4D7/2).a
a
J0 ? J00
F0 ? F00
mobs
mobs mcalc
9/2 ? 7/2
11/2 ? 9/2
9/2 ? 7/2
7/2 ? 5/2
36 099.746
36 101.106
36 101.618
0.135
0.091
0.119
In MHz; residuals from global fit to FTMW and mm-wave data.
The unpaired electrons in FeCN are thought to occupy p and d orbitals primarily of Fe d character. Similarly, the one unpaired electron in NiCN is in a dd non-bonding metal orbital [5]. Thus, the
electrons responsible for the hyperfine interactions are not located
near the nucleus with the spin, i.e. nitrogen.
A combined fit of the millimeter [13] and FTMW data of FeCN in
the X = 7/2 ladder was carried out, weighted according to the
Figure 3. FTMW spectrum of the J = 7/2 ? 5/2 transition of NiCN in the X = 5/2 spin–orbit ladder near 30 GHz showing the three nitrogen hyperfine components, labeled by
quantum number F. The brackets indicate Doppler doublets. There are two frequency breaks in the spectrum such that all three components could be displayed on the same
frequency scale. The F = 7/2 ? 5/2 line is somewhat stronger in intensity relative to the other components, a result of varying molecule production, and shows evidence of
slight Zeeman splitting. The spectrum is a compilation of three 600 kHz scans, each of which is an average of 1000 pulses.
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L.N. Zack et al. / Chemical Physics Letters 514 (2011) 202–206
Table 2
Observed microwave transitions of NiCN (X2D5/2).a
J0 ? J00
7/2 ? 5/2
a
F0 ? F00
9/2 ? 7/2
7/2 ? 5/2
5/2 ? 3/2
mobs
30 309.065
30 310.383
30 311.228
Table 3
Spectroscopic constants for FeCN (X4D7/2) and NiCN (X2D5/2).a
Parameter FeCN
mobs mcalc
0.016
0.048
0.030
B
D
H
L
h
rms of fit
In MHz; residuals from global fit to FTMW and mm-wave data.
experimental accuracies of each technique, estimated to be 100
and 5 kHz, respectively. Three microwave and 21 mm lines were
included in the analysis. An effective Hamiltonian composed of
terms for molecular frame rotation and magnetic hyperfine interactions was used to model the data:
Heff ¼ Hrot þ Hmhf
ð1Þ
Initially, a nitrogen quadrupole coupling term was included in
the Hamiltonian, but the quadrupole constant, eQq (N), could not
be determined to within a 3r error, and fixing the value did not improve the fit. Hence, only magnetic hyperfine coupling was considered. For the one spin ladder being analyzed, the parameter
involved is h. Higher order centrifugal distortion corrections to
rotation (H and L) were necessary to obtain a satisfactory fit –
not surprising, considering the range of rotational levels involved
(J00 = 7/2 through J00 = 123/2). H and L were also necessary in the
millimeter-wave fit of the X = 7/2 ladder data. A similar procedure
was used to analyze the NiCN data, using the millimeter data of
Sheridan and Ziurys [5].
The resulting spectroscopic constants for FeCN and NiCN are
summarized in Table 3. As the table shows, addition of the one
microwave transition slightly changed the rotational constants
for both molecules, and h is reasonably well established in either
case. Additional lower frequency data are needed to refine the fits.
Unfortunately, the next transition of FeCN in the X = 7/2 ladder is
out of range of the FTMW system, as is that of NiCN in the X = 5/2
component.
4. Discussion
This work clearly establishes the ground electronic state of
FeCN as 4Di, confirming the results of Flory and Ziurys [13]. It is
interesting to note that FeCN has the same electronic ground state
as the hydride, FeH [16], rather than the X6Di term of the halides
FeF or FeCl [7–9]. In general, the 3d hydride, halide, and cyanide
species have the same electronic ground state (e.g. ZnH, ZnF, ZnCN:
2 +
R ; CrH, CrF, CrCN: 6R+) [14,17–21], but not always. In the case of
nickel, the ground states of both NiH and NiCN are 2Di [5,22], while
those of the halides NiF and NiCl are 2Pi [23,24].
The competition between quartet and sextet terms for both
FeCN and FeNC is discussed in detail by DeYonker et al. [11]. As
these authors mention, calculations of the relative energies of
low-lying states of these two species are ‘treacherous’, because
the 6D state is dominated by a single electron configuration ([core]1d34p211r112r1), while the 4D term has massive multi-reference character. At almost all levels of theory, DeYonker et al.
predict the 6D state to lie lower in energy than the 4D state, except
when considering relativistic effects and core correlation. In this
case the quartet term drops below the sextet in energy. However,
DeYonker et al. also concluded that FeNC is more stable than FeCN,
which is not found experimentally. Hirano et al. [12] correctly predict FeCN to be the lower energy isomer, but did not consider the
4
D state in their computations. As noted by Hirano et al., FeCN becomes the favored species as higher levels of theory are considered,
a trend that may apply to the quartet state as well.
a
b
c
d
NiCN
MWb + MMWc
MMWc
MWb + MMWd
MMWd
4011.2066 (62)
0.0007035 (64)
2.16 (23) 108
8.9 (2.7) 1013
2.55 (19)
0.214
4011.2302 (16)
0.0007242 (11)
1.458 (23) 108
0.467 (73) 1020
4330.0092 (24)
0.0014450 (21)
5.81 (83) 109
4.9 (1.0) 1013
2.417 (42)
0.039
4330.0569 (16)
0.00147034 (27)
0.026
0.033
In MHz; errors are 3r to last quoted decimal place.
This work.
From Ref. [13].
From Ref. [5].
The 6D state of FeCN and FeNC has the electron configuration
[core] 8r29r210r23p41d34p211r112r1 [12]. According to Hirano
et al., FeCN has greater stability than FeNC because the 8r, 9r, and
10r orbitals have non-negligible Fe 4s and 4pr character and
participate in forming an actual bond between the iron and carbon
nuclei. In FeNC, the iron contribution to these orbitals is much
lower, and therefore the isocyanide is the more ionic species. The
electron configuration for the 4D state of FeCN is [core]
8r29r210r23p41d34p211r2 [11]. Here the 12r orbital in unoccupied, and 11r sub-shell is filled. According to theoretical calculations, the electron density of the doubly occupied 11r (4D)
orbital is localized between the Fe and C atoms, shortening the
FeAC bond. In contrast, in the 12r orbital, the density is principally
located on the side of the iron nucleus opposite to the carbon atom.
The quartet state therefore has additional covalent bonding character relative to the sextet, which may result in a greater stability.
As noted by Flory and Ziurys [13], the experimentally determined
bond lengths in FeCN match those predicted for the quartet state,
not the sextet.
The increase in covalent bonding in FeCN would result in a molecule more similar to FeH, as opposed to ionic FeCl or FeF. In the 6D
state of FeCN, Hirano et al. [12] calculate a +0.682 Mulliken population for the iron nucleus at the MR-SDCI/[Wachters + f (Fe), augcc-pVTZ (C,N)] level of theory, but +0.946 for FeNC. The population
in the 4D term may be even less electropositive. Backbonding of
the iron 3dp electrons into the open p antibonding orbital on the
carbon atom may also occur in the quartet state, although the calculations of Hirano et al. suggest that this effect does not occur in
the sextet state. Clearly additional high-level calculations are
needed for this complex system.
Insight into the bonding in FeCN can be obtained from examination of the nitrogen hyperfine structure. The hyperfine h parameter
is defined as [25]:
h ¼ aK þ ðb þ cÞR
ð2Þ
In this equation, a and c are the nuclear spin–orbit and dipolar constants, respectively, and b = bF c/3, where bF is the Fermi contact
term [26]. The a and c parameters take the form [26]:
1 X
1
li 3
K i
ri
ð3Þ
3
1 X 3 cos2 h 1
g s lB lN g N
3
2
n i
ri
ð4Þ
a ¼ 2lB lN g N
c¼
Here lB, lN, and gs are the Bohr magneton, nuclear magnetic
moment, and the electron g-factor, respectively, and gN = lI/I. The
a, b, and c parameters cannot be determined individually unless
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L.N. Zack et al. / Chemical Physics Letters 514 (2011) 202–206
Table 4
Nitrogen hyperfine parameters for metal cyanide species.a
Parameter
bF (MHz)
c (MHz)
h (MHz)
<1/r3> (m3)
a
b
FeCN
NiCN
5. Conclusion
ZnCNb
2.114 (16)
6.352 (33)
2.55 (19)
2.23 1029
2.417 (42)
2.46 1029
9.26 1029
3r uncertainties.
From Ref. [14].
hyperfine structure is measured in multiple spin–orbit components. Nonetheless, certain conclusions about these parameters
can be inferred.
Both a and c in FeCN arise from the unpaired electrons in the d
and p orbitals, as suggested by the proposed electron configuration. The spin dipolar constant, c, contains the angular factor
<3cos2h 1>. The 1d34p2 open-shell configuration in FeCN has
one unpaired 3dd electron, for which <3cos2h1> = 4/7. The
two 3dp electrons each contribute the quantity <3cos2h1> = 2/7.
Consequently, the angular factors effectively cancel each other,
such that c is approximately zero. The Fermi contact term, bF, is a
measure of s character in the bond; however, none of the unpaired
electrons reside in sr orbitals. As a consequence, bF would be expected to be zero or small and negative due to spin polarization effects. Thus, in FeCN, h 2a.
Nitrogen hyperfine structure has been measured for only two
other cyanide species, NiCN and ZnCN [14], and the corresponding
constants are provided in Table 4. For ZnCN (X2R+), the values for
bF and c were determined to be small (66 MHz: see Table 4); they
arise from the one unpaired electron residing in a sp-hybridized r
orbital [12]. In NiCN (X2Di), h has been measured in this work (see
Table 3). The hyperfine interactions in this case result from a single
dd electron [5], and a and c will contribute to h such that
h 2a + (1/3)c.
The expectation value of <1/r3>, where r is the distance of the
unpaired electrons from the nitrogen nucleus, can be determined
for all three molecules using the definitions of a and c and the
experimental hyperfine constants. The resulting <1/r3> values are
listed in Table 4. As seen in the table, <1/r3> is about the same
for NiCN and FeCN (2–2.5 1029 m3), but is significantly larger
for ZnCN (9.3 1029 m3). These results are consistent with the
individual electron configurations. In ZnCN, the unpaired electron
is in a metal r orbital with no angular momentum. For FeCN and
NiCN, the electrons are either in d or p orbitals on the metal atom.
Because they have angular momentum, their distributions are
more diffuse than the electron in ZnCN, hence the smaller <1/r3>
values.
Metal-cyanide complexes are important in a wide variety of
chemical disciplines. Yet, the basic properties of the simplest transition-metal cyanide/isocyanides systems are not easily discerned,
and surprising contradictions can be found. The experimental work
done here for FeCN has conclusively established its electronic
ground state as 4Di, in contrast to theoretical predictions. This
ground state suggests that FeCN more closely resembles FeH, as
opposed to the iron halides FeF and FeCl, and indicates of a significant degree of covalent bonding. This study of FeCN should provide a benchmark system for subsequent calculations of the
properties of transition metal cyanide species.
Acknowledgements
This work was supported by CHE-10-57924 and AST 09-06534.
We are indebted to the late J.M. Brown for the use of his Hamiltonian fitting program.
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