23 December I994
ELSEVIER
CHEMICAL
PHYSICS
LETTERS
Chemical Physics Letters 231 (1994) 164-170
The millimeter-wave spectrum of 25MgNC and 26MgNC:
bonding in magnesium isocyanides
M.A. Anderson,
L.M. Ziuxys ’
Department of Chemistry, Arizona State University, Tempe, AZ 85287-I604, USA
Received 11August 1994
Abstract
Rotational spectra of the 25Mg and 26Mg isotopomers of MgNC have been recorded in the frequency range 298-386 GHz using
millimeter/sub-millimeter
direct absorption spectroscopy. The species were created in their natural isotopic ratios in a dc discharge of cyanogen gas and magnesium vapor. Rotational and fine structure parameters were determined for these radicals, as
well as magnesium 25 hyperfine constants for *‘MgNC. For this isotopomer, b+Z - 298 ( 16)) indicating a large electron density
at the nucleus and an ionic type Mg+NC- structure, as expected for a linear configuration. RO bond lengths were also derived,
which are in good agreement with theoretical calculations.
1. Introduction
The bonding in metal cyanides and isocyanides is
of chemical interest because of the difference in
structure between the alkali and alkaline earth compounds of this type. These molecules are supposed to
have primarily ionic bonding resulting from the large
electron affinity of CN (3.82 eV) and the relatively
low ionization
potential of the alkali and alkaline
earth metals. The ionic interaction has both a long
range (Coulombic)
attractive force and a short range
repulsive (exchange) force between the M+ and CNmoieties [ 1,2], and the competition between these
two forces determines the overall structure. For the
alkali metal cyanide compounds, such as KCN and
NaCN, experiments have shown that these molecules
are basically ‘T-shaped’ [ 3-51. Such structures can
be explained as arising from a potential surface that
is relatively flat in the angular coordinate defining the
orientation of the metal atom with respect to the CN’ NSF Presidential Faculty Fellow.
group [ 6,7]. Thus, the M+ moiety essentially ‘orbits’
the CN- and there is no preferred structural formula,
resulting in a triangular shape and a floppy ‘polytopic’ bond. In this case it is thought that the short
range repulsive exchange forces dominate over the
Coulombic ones.
For the alkaline earth cyanides/isocyanides,
the
bonding is different. For these systems, the longer
range Coulombic forces are thought to be more important. Theoretical calculations have suggested that
these species are linear, with the most stable structure
being the isocyanide as opposed to the cyanide [ 8,9].
The presence of the unpaired electron evidently alters the highly symmetrical closed shell structure of
the alkali cyanides such that one orientation is preferred. The MNC geometry is lowest in energy because the polarity of the CN- anion is C6+-N6-,
which places the slightly negative N atom in the direction of the metal with the positive charge [ 71. The
linear cyanide compounds are calculated to be only
slightly higher in energy ( ~0.3 eV), which may result from a small amount of covalent character in the
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
ZYDI0009-2614(94)01259-8
M.A. Anderson, L.M. Ziurys /Chemical Physics Letters 231 (1994) 164-I 70
metal-carbon bond [ 81. Calculations also suggest that
barriers exist for the MNC/MCN conversion in the
Mg and Be compounds, but do not appear to be present for Ca and Ba.
Experimental measurements have verified a linear
structure for several of the alkaline earth isocyanide
radicals. Lower resolution LIF spectra showed evidence of linear geometries for CaNC and SrNC [ 1O121. Recent millimeter/sub-millimeter
measurements by Steimle et al. [ 131, and MODR/Fourier
transform data obtained by Scurlock et al. [ 141 of
the pure rotational spectrum of CaNC have confirmed this structure. The combined astrophysical and
mm/sub-mm
laboratory measurements
of the rotational spectrum of MgNC [ 15,16 ] have established
its linear geometry as well. Moreover, very recently
the pure rotational spectrum of the metastable isomer MgCN has been recorded [ 17 1. These data indicate that this molecule is also linear, and the difficulty in obtaining spectra of this compound relative
to the isocyanide is evidence that MgCN is indeed
the less stable isomer.
In order to better understand
the structure and
bonding of alkaline earth isocyanideslcyanides,
we
have measured the pure mm/sub-mm
rotational
spectrum of the magnesium
isotopomers 26MgNC
and 25MgNC in their 2C+ ground electronic states.
(Kawaguchi et al. [ 161 mentioned recording some
spectra of these species, but no data was presented.)
Seven rotational transitions of 26MgNC, and three for
“MgNC were recorded in the frequency range 298386 GHz. Fine structure was observed in both molecules, and the hyperfine splittings were also recorded
for 25MgNC, which arise from the 25Mg nucleus
(I= 5 /2). Rotational
and fine/hyperfine
structure
constants have been determined, as well as estimates
of bond lengths. Based on these measurements,
both
26MgNC and “MgNC have been identified in the interstellar
medium
towards
the late-type
star
IRC + 102 16 by Gutlin et al. [ 18 1. In this Letter, we
present the laboratory data and discuss implications
for bonding in magnesium isocyanides.
2. Experimental
The ASU direct absorption
mm/sub-mm
spectrometer was used for the measurements,
and is de-
165
scribed in detail by Ziurys et al. [ 191. Very briefly,
the system consists of a phase-locked Gunn oscillator/Schottky
diode multiplier source, a gas cell, and
an InSb detector. The cell is a double pass system
through which the radiation is propagated quasi-optically by lenses and feed horns. FM modulation of
the Gunn oscillators is employed to achieve phasesensitive detection, which is accomplished at 2f:
The MgNC isotopomers were created in a dc discharge using a mixture of magnesium vapor, cyanogen gas, and argon. The metal vapor was produced in
a Broida-type oven and entrained in = 30 mTorr Ar
carrier gas which was added to = 10 mTorr cyanogen. The discharge current used was ~200 mA. A
bright blue-green color was observed in the reaction
mixture when running the discharge. The isotopomers were observed with the magnesium isotopes in
their natural abundances ( 24Mg: 78.6%; 25Mg: 10.1%;
26Mg: 11.3%). Strong lines of 24MgNC were also recorded. Typical linewidths observed were x 800-900
kHz.
3. Results
Table 1 lists the seven rotational transitions measured for 26MgNC. As the table illustrates, each transition is split into doublets, separated by x 14 MHz,
because of the interaction of the unpaired electron
spin with the rotational angular momentum. As found
for the main isotope of 24MgNC, no hyperline interactions were observed in these data, which would arise
from the nitrogen spin of I= 1. In Table 2, the three
transitions
recorded for 25MgNC are presented. In
addition
to spin-rotation
interactions,
hypertine
structure is also resolved in these data. The hf splittings result from the interaction of the 25Mg nucleus
(I= 512) with the electron spin. As a consequence,
each rotational transition consists of twelve hyperline components,
some of which are blended in the
recorded spectra.
Fig. 1 shows the spectrum of the N= 3 1+ 32 rotational transition of 26MgNC. As the figure illustrates,
the spin-rotation
doublets are clearly resolved. In Fig.
2, the N= 32+ 33 rotational lines of 25MgNC are presented. This spectrum consists of twelve hf components, of which two sets of lines are completely
blended.
M.A. Anderson, L.M. Ziurys /Chemical
166
Table 1
Observed
transition
frequencies
Physics L-eiters 231(1994)
164-l 70
of 26MgNC: X ‘Z+ (0, 0, 0)
N-N’
J-J
F-rF’
~0,. (MHz)
25-26
49/2+51/2
4712-4912
49/2+51/2
51/2+53/2
298844.284
0.077
298858.961
0.084
310316.349
0.015
310331.025
0.021
51/2+53/2
26-27
51/2+53/2
53/2-+55/2
27-28
53/2+55/2
5512-5712
28-29
55/2+57/2
57/2+59/2
29-30
57/2+59/2
59/2+61/2
30+31
59/2+61/2
61/2+63/2
31-32
61/2-6312
6312-6512
49/2+51/2
51/2+53/2
5312-5512
I
49/2+51/2
51/2+53/2
53/2+55/2
I
51/2+53/2
5312-5512
55/2+57/2
I
51/2+53/2
53/2+55/2
5512-5712
I
5312-5512
55/2+51/2
5712-5912
I
5312-5512
55/2-+57/2
51/2+59/2
I
5512-5712
5?/2+59/2
59/2+6112
I
5512-5712
5712-5912
59/2+61/2
I
5712-5912
59/2+61/2
61/2+63/2
I
5712-5912
59/2+61/2
61/2-6312
I
59/2+61/2
61/2+63/2
6312-6512
I
59/2-+61/2
61/2+63/2
63/2+65/2
I
61/2+63/2
6312-6512
65/2+67/2
v&a- ycalc (MHz)
321785.901
-0.051
321800.610
-0.012
333252.907
-0.061
333267.511
-0.068
344717.206
-0.083
344731.878
- 0.082
356178.843
0.020
356193.461
-0.033
367637.561
0.085
367652.239
0.093
M.A. Anderson, L.M. Ziurys /Chemical Physics Letters 231(1994) 164-l 70
161
Table 2
Observed transition frequencies of “MgNC: X 2Z+ (0,0,O)
N-rN’
J-4
F-F’
v,br (MHz)
v&s- vc0k (MHz)
29-30
V/2+59/2
26-2-l
27+28
28-29
29-+30
30-?31
31-32
350879.420
350880.728
350882.155
350883.838
- 0.084
0.037
0.107
0.173
-0.127
0.381
59/2+61/2
31-32
61/2+63/2
6312-6512
32-33
63/2+65/2
65/2-+67/2
30-31
31-32 >
29-30
28+29
21-28
32-33 >
28-29
29+30
30+31
31-32
32-33
33-34 >
32-33
33-34 >
31+32
30-+31
29-30
34+35 >
29-30
30-31
31-32
32+33
33-34
34-35 1
33-34
34-35 I
32-33
31-32
30-31
35+36
350885.593 =
350888.087 ’
350889.895
350891.560
350894.032 ’
374208.371
374209.586
374211.031
374212.612
-0.136
-0.057
0.087
0.116
0.114
0.073
374214.593”
374217.211 ’
374219.314
374220.920
374223.273 a
385868.275
385869.558
385870.993
385872.589
385874.556 ’
385877.232 ’
385879.321
385880.310
385882.352
385883.379
-0.080
-0.580
-0.325
-0.274
0.833
-0.349
a
a
=
’
-0.205
-0.157
-0.082
-0.026
1.020
-0.112
-0.176
- 0.004
0.158
0.223
-0.255
-0.063
-0.248
0.075
-0.103
-0.632
0.131
0.050
’ Blended lines.
Molecular constants were determined for “MgNC
and 26MgNC from a non-linear least squares fit using
a *C Hamiltonian.
The results of this analysis are
given in Table 3, which lists the derived spectrascopic parameters. For 26MgNC, the rotational constants B,, and D, and the spin-rotation
parameter y
were determined. In the case of *‘MgNC, hypertine
structure for the *‘Mg nucleus had to be considered,
which was analyzed
Hamiltonian
[ 20 ] :
fihf=bf*#+
using the following
cIzS, + 41(;f*)
(3&f*).
hyperfine
(1)
In this expression, the first two terms define magnetic interactions
(b and c are Frosh and Foley notation [ 2 1 ] ), while the third term described the elec-
168
M.A. Anderson, L.&l. Ziurys /Chemical Physics Letters 231(1994) 164-l 70
J,6’,62
2
Table 3
Molecular constants for 25MgNC and 26MgNC: X ‘Z+ ’
J,h3_$$
2
2
2
=MgNC
Bo
Do
“MgNC
Bo
DO
YO
YO
b
C
eclQ
I
I
I
367,595
367,645
I
a In units of MHz; errors quoted are 3a statistical uncertainties
and apply to last digits quoted.
b Held fixed, see text.
I
367,695
Frequency (MHz)
Fig. 1. Spectrum of the N=31+32 rotational transition of
26MgNC (X ?)
observed in this work. The spin-rotation doublets are clearly resolved. This spectrum represents one single 4minute scan covering 100 MHz in range.
F={
:::::
5752.3798(39)
0.0038719(22)
14.671(61)
5855.312(35)
0.003998( 17)
14.89(49)
-303( 16)
14.72 b
- 19.5(2.9)
F = { :::::
not be independently
determined. Therefore, it was
fixed to the value found for MgF, which is 14.72 MHz
1221.
The derived constants reproduce the frequencies
measured for 26MgNC to v&s- Y,~=< 93 kHz. For
25MgNC, because of the blending of components, the
difference between measured and calculated values is
uobs - Vcalc -< 1 MHz. Thus, the derived spectroscopic
parameters appear to reproduce the observed transition frequencies fairly accurately. Errors in the measurements for 25MgNC arise primarily from the line
blending.
4. Discussion
385,864
385,879
Frequency (MHz)
385.894
Fig. 2. Spectrum of the N=32-+33 rotational transition of
25MgNC (X *Z+ ) observed in this work. This transition consists
of twelve hf components which arise from the “Mg nuclear spin
of I= 5/2, some of which are blended. These data represent an
average of eight S-minute scans covering 30 MHz in range.
tric quadrupole splitting whose magnitude is related
to the constant eqQ. Consequently,
for 25MgNC, in
addition to B,,, Do, and yo, the hf constants b and eqQ
were also derived. However, because the data concerns only high Ntransitions, the hf parameter c could
Measurement of the “Mg hf structure gives some
insight into the bonding of MgNC. Determination
of
the Fermi contact term & (related to Frosh and Foley
b constant via bF= b+ SC) enables an evaluation of
the amount of electron density at the nucleus, in this
case that of 25Mg. As shown in Table 4, bF=
- 298 ( 16) for MgNC. This value is relatively large,
and indicates a substantial electron density at the
25Mg nucleus. Thus, the unpaired electron must primarily reside on magnesium nucleus, resulting in an
Mg+NC- structure, as is predicted for a linear geometry. Also, the orbital must have mostly o character, because b, is much larger than c. (Although the c
constant has been fixed for the final data analysis, allowing it to vary in the fit always resulted in much
smaller values than for b,. )
Also shown in Table 4 are the bF and c hyperfine
169
MA. Anderson, L.M. Ziurys /Chemical Physics Letters 231 (1994) 164-l 70
Table 4
Hypefine constants of alkaline earth radicals a
Molecule
Nucleus
6,
c
eqQ
“MgNC b
25MgOH ’
=MgF ’
“M&l =
CaNC f
25Mg
-298( 16)
-304.4(4.6)
-304.10(26)
-319.1(2.9)
12.4815(28)
14.72 g
0.0 8
14.72(22)
0.0 B
2.0735(42)
- 19.5(2.9)
-41(17)
-20.02(50)
- 19.0( 1.5)
-2.6974(33)
N
p In MHz; errors quoted are 3~. b This work. ’ Ref. [ 27 1.
d Ref. [22]. “Ref. [26]. f Ref. [ 141. 8Value fixed.
constants for “Mg in several other magnesium radicals with 2Cf ground electronic states, including
MgOH and certain magnesium halides. Both theory
[ 23-251 and experiment [ 22,261 suggest that MgCl
and MgF are very ionic compounds with the dominant configuration being M+X-. MgOH is found to
be ionic as well, with an Mg+OH- linear structure
[ 271. Thus, comparing the hf constants of these compounds with those of MgNC should give a qualitative
evaluation of how ionic magnesium isocyanide is.
As Table 4 illustrates, &(MgOH) = -304.4 MHz,
&(MgF)=
-304.lOMHz,
and &(MgCl)=
-319.1
MHz. Thus, the “Mg Fermi contact terms in these
highly ionic species are all very close in value to that
of MgNC.
Another indication of ionic bonding is the value of
the electric quadrupole term eqQ, which is related to
the electric field gradient across the nucleus with the
spin. Again, for the ionic magnesium halide and monohydroxide radicals, this parameter has the value
eqQ z 20 to - 40 MHz. For MgNC, the quadrupole
parameter is - 19.5 MHz, i.e. within the range of the
other species. The similarity between the quadrupole
constants is additional evidence that the bonding in
MgNC is ionic.
Table 5
Bond lengths for MgNC (A)
Millimeter-wave
Theoretical a
ro(MgN)
r&NC)
r,(MgN)
r&NC)
1.925 b
1.924 d
1.169 b
1.172 d
1.945 =, 1.934 c
1.170 =, 1.179 c
a Ref. [ 91. b Determined from 24MgNC/2SMgNC.
’ Ref. [ 181. ’ Determined from 24MgNC/26MgNC.
MgNC contains a nitrogen atom which has a spin
of one. If the unpaired electron were distributed more
uniformly across the molecule, hyperfine structure
arising from the nitrogen nucleus would be observed.
The hf structure found for 25MgNC, however, can be
completely accounted for by the magnesium nucleus.
Moreover, no hf interactions
were apparent in the
spectra of 26MgNC or 24MgNC [ 161. For CaNC,
however, nitrogen hf splittings were resolved [ 141,
and the constants are given in Table 4. As the table
shows, these parameters
are quite small, with
bF=12.48
MHz, ~~2.07 MHz, and eqQ= -2.70
MHz. Therefore, for this radical, there is only a small
electron density at the N nucleus. The nitrogen hf parameters for CaNC were obtained by measuring the
lowest rotational transitions using very high resolution FTMW measurements
[ 141. Presumably,
the
nitrogen hf constants for magnesium isocyanide are
similar in magnitude to those of CaNC, and could be
obtained if similar experiments were performed on
the molecule.
Table 5 lists the r, bond lengths determined for
MgNC from both the 25Mg and 26Mg substitutions.
The Mg-N bond length is estimated to be z 1.925 8,
and the N-C length was found to be z 1.17 1 A, considering both magnesium isotopes. These values are
in reasonable agreement with the calculated equilibrium bond lengths of r,( Mg-N) z 1.945 and 1.934 8,
andr,(N-C)xl.l70and
1.179A [9,18].Thesmall
differences between the measured and theoretical
values are likely partly due to the neglect of zero-point
vibrations in the experimental determinations.
Acknowledgement
This work was supported by NSF grant No. AST92-53682 and NASA grant No. NAGW 2989. The
170
M.A. Anderson, L.M. Ziurys /Chemical Physics Letters 231(1994) 164-I 70
authors thank Dr. Michel GuClin for suggesting these
measurements
and for providing his astrophysical
data for comparison of frequencies, and P. Valiron
for his calculations prior to publication.
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