Laboratory microwave spectroscopy of aluminium cyanide

Article abstract of DOI:10.1016/S0009-2614(99)00013-5

The pure rotational spectrum of aluminium cyanide (AlCN) between 10 and 21 GHz has been investigated by Fourier transform microwave spectroscopy. Molecular samples were produced by reacting ablated Al metal with cyanogen present in an Ar supersonic jet. R

Full text of DOI:10.1016/S0009-2614(99)00013-5

1
9 February 1999
Chemical Physics Letters 301 Ž1999. 200–204
Laboratory microwave spectroscopy of aluminium cyanide
Kaley A. Walker , Michael C.L. Gerry )
Department of Chemistry, The UniÕersity of British Columbia, 2036 Main Mall, VancouÕer, BC V6T 1Z1, Canada
Received 14 December 1998
1
Abstract
The pure rotational spectrum of aluminium cyanide ŽAlCN. between 10 and 21 GHz has been investigated by Fourier
transform microwave spectroscopy. Molecular samples were produced by reacting ablated Al metal with cyanogen present in
2
7
14
an Ar supersonic jet. Rotational and centrifugal distortion constants and Al and N nuclear quadrupole coupling constants
2
7
12 14
and nuclear spin–rotation constants have been determined for the main isotopomer, Al C N. The spectrum of AlCN was
found to be much weaker than that of AlNC thus confirming that AlCN is the less stable of the two isomers. q 1999 Elsevier
Science B.V. All rights reserved.
1
. Introduction
IRCq10216, as have both MgNC w4,5x and its
metastable linear cyanide isomer MgCN w6x. The two
linear isomers of aluminium monocyanide have also
been suggested as possible candidates for detection
in this object w8x. As was found for magnesium
monocyanide, the linear isocyanide species is again
the lowest energy isomer w9x: calculations on the
formation of metal monocyanide species in this
molecular cloud suggest that most of the aluminium
monocyanide should be in the form AlNC not AlCN
w10x. Therefore sensitive searches need to be made
for both linear isomers to verify this prediction.
Until recently, only theoretical calculations of
ground-state structural parameters were available for
AlCN and AlNC w8,11,12x. In the past year, several
Over the past decade, transitions of several
metal-containing species, including aluminium,
sodium, and potassium monohalides w1,2x and sodium
and magnesium monocyanides w3–6x, have been
identified in the circumstellar envelope of IRCq
1
0216. The metal monocyanides are particularly in-
teresting because they have three possible structural
isomers: they can take the linear cyanide, the linear
isocyanide or the non-linear T-shaped configurations.
Experimentally the most stable isomer of sodium
monocyanide is T-shaped w7x but that of magnesium
monocyanide is the linear isocyanide species w5x. The
T-shaped NaCN isomer w3x has been detected in
experimental studies of gas-phase AlNC have been
published. Millimeter wave transitions, in both the
ground vibrational state and excited states of the
)
Corresponding author. Fax: q1 604 822 2847; e-mail:
mgerry@chem.ubc.ca
1
bending mode Žn ., have been measured by Robin-
2
Present address: Molecular Spectroscopy Group, Steacie Insti-
son et al. w9x. The hyperfine structure in the pure
tute for Molecular Sciences, 100 Sussex Drive, Ottawa, Ont.,
Canada K1A 0R6.
rotational spectrum was investigated by Walker and
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00013-5

K.A. Walker, M.C.L. GerryrChemical Physics Letters 301 (1999) 200–204
201
Gerry w13x using Fourier transform microwave
ŽFTMW. spectroscopy. Fukushima observed the vi-
racy is estimated to be better than "1 kHz. The
microwave synthesizer is referenced to a Loran fre-
quency standard, which is accurate to one part in
bronic structure of an electronic band system at
y1
12
2
8754 cm
using laser-induced fluorescence ŽLIF.
10 .
1
X
1
q
w14x. He assigned this system as the A– S transi-
The pulsed nozzle laser ablation source has been
described elsewhere w20x. An Al rod ŽGoodfellow,
99.999%. was ablated, using a frequency-doubled
Nd:YAG laser Ž532 nm, ;5–10 mJrpulse., in the
tion of AlNC based on the vibrational analysis of the
ground electronic state and the results of his own ab
initio calculations. There has also been a matrix
isolation infrared study of the monocyanides of all
the group 13 metals, including AlNC and AlCN w15x.
presence of a less than 0.03% cyanogenrAr mixture
Žstagnation pressure, 5–7 bar.. Both AlCN and AlNC
Very recently, Gerasimov et al. obtained rotationally
resolved fluorescence excitation spectra of an elec-
were produced in the ablation source. The AlCN
signals were particularly sensitive to the amount of
y1
tronic band system at 36389 cm
which they as-
ŽCN. in the gas mixtures: lower concentrations of
2
1
1
q
˜
˜
w x
signed as the A P–X S transition of AlNC 16 .
They also reassigned the band reported by Fukushima
to the P– S transition of AlCN. This spectrum is
the only reported observation of gas phase AlCN so
far.
cyanogen produced stronger signals.
1
1
q
3
. Observed spectra and analyses
This Letter is the first report of the microwave
spectrum of AlCN. The two lowest frequency rota-
tional transitions, Js1–0 and Js2–1, have been
measured using a cavity Fourier transform mi-
crowave ŽFTMW. spectrometer. The nuclear hyper-
fine structure due to Al and N nuclei has been
observed and nuclear quadrupole and nuclear spin–
rotation constants have been calculated. The molecu-
lar constants determined from the FTMW spectrum
of AlCN will aid in the search for this molecule in
circumstellar regions.
The initial search parameters were determined
using the ab initio calculations by Ma et al. w8x, since
the experimental results of Gerasimov et al w16x.
were not available when our study was undertaken.
For AlNC, it was found that the calculations made
at the TZ2PqfCISD level of theory produced
rotational constants closest to the experimental
values w13x. Accordingly, the predicted value at the
2
7
14
same level of theory was used for AlCN. Nuclear
2
7
quadrupole coupling constants eQq Ž Al. from AlNC
0
1
4
w13x and eQq Ž N. from HCN w21x were used to
0
2
. Experimental
The transitions of AlCN were measured using a
pulsed jet cavity FTMW spectrometer w17x which has
been described in detail elsewhere w18x. The spec-
trometer cavity is formed by two spherical alu-
minium mirrors 28 cm in diameter and placed ;30
cm apart. A pulsed nozzle laser ablation source is
mounted near the centre of one of the mirrors so that
the molecular jet travels parallel to the axis of mi-
crowave propagation. This configuration enhances
the sensitivity of the spectrometer w19x. It also gives
rise to Doppler splitting in the observed spectra so
each line appears as a doublet. Transition frequencies
were determined by averaging the frequencies of the
measured Doppler components. Measurement accu-
1
4
Fig. 1. Composite spectrum showing the N hyperfine splitting in
the Js1–0 F s7r2–5r2 transition of AlCN. The results of
1
two different microwave experiments were used to produce this
composite. Each spectrum was obtained with 2000 averaging
cycles and 4K data points.

2
02
K.A. Walker, M.C.L. GerryrChemical Physics Letters 301 (1999) 200–204
Table 1
Measured frequencies of Js1–0 and Js2–1 transitions of AlCN
Transition
Frequency
Obs.–calc.
X
X
1
X
XX
XX
XX
J
F
F
J
F1
F
ŽMHz.
ŽkHz.
1
1
1
1
1
1
1
1
1
2
2
2
2
5r2
3r2
0
0
0
0
0
0
0
0
0
1
1
1
1
5r2
5r2, 3r2
5r2, 7r2
5r2, 7r2, 3r2
5r2, 7r2
5r2, 7r2, 3r2
7r2
10043.9734
10044.4797
10045.6314
10052.0147
10052.7302
10052.9498
10055.8463
10056.2743
10056.7580
20102.1140
20102.4836
20103.1588
20103.3738
0.1
y0.5
0.5
5r2
7r2
5r2
5r2
5r2
5r2
7r2
7r2
5r2
0.1
7r2
5r2
5r2
y0.1
y0.0
0.3
y0.1
y0.2
0.2
y0.3
0.0
0.1
7r2
9r2
5r2
3r2
3r2
5r2
5r2, 3r2
3r2
3r2
1r2
5r2
3r2
5r2
5r2
5r2, 7r2, 3r2
7r2
9r2
9r2
7r2
9r2
11r2
7r2, 3r2
9r2
7r2
9r2
9r2, 1r2
7r2
7r2, 3r2
5r2
5r2
7r2
estimate the magnitude of the hyperfine splitting.
Transitions for Js1–0 were found within ;5 MHz
of the prediction.
confirm that AlCN is less stable than AlNC, the
energy difference between these isomers could not
be quantified because the intensities of FTMW spec-
tra are also functions of excitation and cavity param-
eters as well as molecular populations. Since the
signals were so weak for the main isotopomer, species
containing the minor isotopes of N and C were not
sought.
Thirteen hyperfine components belonging to the
two lowest rotational transitions, Js1–0 and Js
2
7
12 14
2
–1, have been observed for Al C N Ž98.53%
natural abundance.. The spectra obtained were sig-
nificantly weaker than those of AlNC w13x. Several
hundred averaging cycles were required to observe
the strongest Js1–0 transitions of AlCN whereas
only a few cycles were necessary to observe similar
transitions of AlNC. These strongest lines, compo-
2
7
Nuclear hyperfine structure due to both Al ŽIs
1
4
5r2. and N ŽIs1. was observed. Because the
2
7
nuclear quadrupole moment of Al is an order of
1
4
magnitude larger than that of N, the observed lines
could be easily assigned in terms of the coupling
scheme JqI sF ; F qI sF. The measured
nents of the Js1–0 F s7r2–5r2 transition, are
1
shown in Fig. 1. Although the relative intensities
Al
1
1
N
Table 2
Molecular constants calculated for AlCNa
Parameter
FTMW
Theoreticalb
LIFc
ŽMHz.
ŽMHz.
ŽMHz.
d
e
B0
D0
eQq Ž Al.
eQq0Ž N.
5025.41235Ž25.
0.002751Ž42.
y37.2225Ž29.
y5.2321Ž29.
5025 , 4982
5019Ž138.
2
7
0
1
4
2
7
C Ž Al.
0.00438Ž16.
0.00147Ž36.
I
1
4
CI Ž N.
a
One standard deviation in parentheses, in units of least significant digit.
b
c
d
e
B values taken from Ma et al. w8x. B was calculated since it was determined that B and B differ by less than 5 MHz.
e
e
e
0
XX
Ground-state rotational constant, B , taken from Gerasimov et al. w16x.
Result calculated at the TZ2PqfCISD level.
Result calculated at the TZ2Pqf CCSDŽT. level.

K.A. Walker, M.C.L. GerryrChemical Physics Letters 301 (1999) 200–204
203
frequencies and their assignments are given in Table
. The spectral data were analysed using Pickett’s
ble bond character’ w15x. This bonding picture can
1
be examined qualitatively using the Al nuclear
quadrupole coupling constants of AlCN and AlNC
and Eq. Ž1.. For these species, the most significant
global least-squares fitting program SPFIT w22x. A fit
was made to the rotational and centrifugal distortion
constants, B and D , and the nuclear quadrupole
2
7
contribution to n , and thus to eQqŽ Al., is sp-hy-
0
0
z
and nuclear spin–rotation constants, eQq and C ,
bridisation of the bonding orbitals on Al. Without
this, there would be little electron density in the 3pz
orbital and the Al nuclear quadrupole coupling con-
stant would be negligible w13x. Because the ionic
characters of the Al–CN and Al–NC bonds are
similar, the sp-hybridisation should not change sig-
nificantly between the linear cyanide and the linear
0
I
for both the Al and N nuclei. The lines observed for
the overlapped hyperfine components of Js1–0
transition were fit as blended lines using predicted
intensities as weighting factors. The constants ob-
tained from the fit are listed in Table 2.
Also listed in Table 2 are the rotational constants
obtained in earlier theoretical and experimental stud-
ies. As was also found for AlNC, the rotational
constant calculated at the TZ2PqfCISD level of
theory was the closest to that obtained from experi-
ment. The theoretical value deemed to be the most
reliable, that calculated at the TZ2PqfCCSDŽT.
level, was more than 40 MHz lower in frequency
than the experimentally determined constant. The
rotational constant from the LIF study, which has a
very large uncertainty, is within 6 MHz of that
determined by FTMW. This confirms that AlCN is
isocyanide configurations and, therefore, n should
z
be the same for both isomers. The values of n and
x
ny depend on the degree of double bond character in
the Al–C or Al–N bond. If these are taken to be
single bonds for both species then n sn s0 and
x
y
2
7
the eQqŽ Al. values would be the same. However, if
the Al–ŽCN. bond in one of the species had some
double-bond character then Žn qn .r2 would be
x
y
2
7
non-zero and this would lower eQqŽ Al.. Experi-
mentally, the Al nuclear quadrupole coupling con-
stant of AlNC Žy35.627 MHz w13x. was found to be
;1.6 MHz lower than that of AlCN Žy37.223
MHz.. These results are consistent with the Al–N
the molecular carrier of the electronic band system at
y1
2
8754 cm
.
bond in AlNC having more ‘double-bond character’
than the Al–C bond in AlCN. Although the Al–C
bond order in AlCN cannot be determined unam-
biguously, it can be assumed to be ;1 because the
Al quadrupole coupling constant of AlCN is very
close to that of AlF Žy37.49 MHz w25x..
Since AlCN is the first linear metal cyanide
molecule for which the nuclear hyperfine parameters
have been determined, comparisons with similar
molecules cannot be made. However, the bonding in
the C[N group in AlCN should be similar to that
found in HCN. To verify this, Eq. Ž1. can be used to
4
. Discussion
2
7
14
The Al and N nuclear quadrupole coupling
constants can be used to examine the bonding in
AlCN. These constants can be interpreted in terms of
valence p-electron densities using the Townes–Dai-
ley model w23x. This relates the measured nuclear
quadrupole coupling constant, eQqŽmol., to the nu-
clear quadrupole coupling constant of one atomic np
electron
nx qny
estimate the N nuclear quadrupole coupling con-
stants of these two linear cyanide compounds. The N
atom has an sp-hybrid orbital involved in the C–N s
bond and the counterhybridised orbital has a lone
pair of electrons. Assuming that the bonds in the
C[N group are completely covalent then each sp
hybrid orbital has 50% p character. One of the sp
hybrid orbitals has one-half of a bond pair and the
other has the lone pair, so n s3r2. The p and p
eQq
Ž
mol
.
s n y
eQqn10
Ž
atom
.
,
1
Ž .
z
ž
/
2
where n , n , and n are the number of electrons in
the np , np and np orbitals, respectively, and z
x
y
z
x
y
z
axis is along the molecular axis. Values of eQqn10
are tabulated in Ref. w24x.
It has been suggested that, for aluminium mono-
cyanide and other group-13 metal monocyanide
species, the M–C bonds in the MCN species are
z
x
y
‘
single bonds’ while the M–N bonds in the cor-
orbitals have one-half of a bond pair each so
1
4
responding MNC molecules have a ‘trace of dou-
n sn s1. Since eQq Ž N. is y10 MHz w24x,
x
y
210

2
04
K.A. Walker, M.C.L. GerryrChemical Physics Letters 301 (1999) 200–204
1
4
eQqŽ N. for these terminal nitrogen containing
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Acknowledgements
This work has been supported by the Natural
Sciences and Engineering Research Council of
Canada ŽNSERC. and by the Petroleum Research
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