Gas phase detection of the unstable halofulminate BrCNO by millimeter wave spectroscopy

Article abstract of DOI:10.1016/S0009-2614(98)00085-2

The millimeter wave spectrum of the unstable halofulminate BrCNO was observed in a low-pressure pyrolysis of Br₂CNOH. Ground-state rotational constants of ⁷⁹BrCNO and ⁸¹BrCNO were determined to be 1739.93007(10) and 1726.96186(11) MHz, respectively. Least-squares fits of the bromine quadrupole hyperfine splittings gave χ values of 656.8(88) and 551(10) MHz for the ground states of ⁷⁹BrCNO and ⁸¹BrCNO, respectively. The observed vibrational satellite pattern arising from the BrCN bending mode indicates that the molecule exhibits extremely quasilinear behavior, roughly analogous to that of OCCCO.

Full text of DOI:10.1016/S0009-2614(98)00085-2

27 March 1998
Ž
.
Chemical Physics Letters 285 1998 391–397
Gas phase detection of the unstable halofulminate BrCNO by
millimeter wave spectroscopy
1
2
C.W. Gillies , J.Z. Gillies , H. Lichau, B.P. Winnewisser, M. Winnewisser
Physikalisch-Chemisches Institut, Justus-Liebig-UniÕersitat, Heinrich-Buff-Ring 58, D-35392 Gießen, Germany
¨
Received 15 December 1997; in final form 16 January 1998
Abstract
The millimeter wave spectrum of the unstable halofulminate BrCNO was observed in a low-pressure pyrolysis of
81
Br2CNOH. Ground-state rotational constants of 79BrCNO and BrCNO were determined to be 1739.930 07 10 and
Ž
.
Ž
.
1726.961 86 11 MHz, respectively. Least-squares fits of the bromine quadrupole hyperfine splittings gave x values of
656.8 88 and 551 10 MHz for the ground states of 79BrCNO and 81BrCNO, respectively. The observed vibrational
satellite pattern arising from the BrCN bending mode indicates that the molecule exhibits extremely quasilinear behavior,
roughly analogous to that of OCCCO. q 1998 Elsevier Science B.V.
Ž
.
Ž .
1. Introduction
in the vibrational ground state to be rather flat with a
barrier to linearity of 11.49 19 cmy1 3 . They
derived an equilibrium HCN bending potential from
the excited vibrational state data which has a barrier
to linearity of 0.2 cmy1, with an estimated uncer-
tainty of 2 cmy1. It is not easy to determine the
equilibrium structure and HCN bending potential
from ab initio calculations. In a recent study using
Ž
.
w x
Fulminic acid, HCNO, is the simplest member of
the nitrile oxides, a versatile class of compounds
noted for their synthetic utility particularly in cy-
cloaddition reactions 1 . Gas phase investigations of
its rotational and vibrational spectra have provided a
basis for a fuller understanding of quasilinearity in
w x
w x
linear polyatomic molecules 2 . A pronounced an-
Ž .
the CCSD T method and Dunning’s cc-pVQZ basis
harmonicity in the two-dimensional HCN bending
potential causes a large amplitude HCN bending
motion and an irregularity of the vibrational energy
levels. Analysing the rotational and vibrational data
using a semirigid bender model, Bunker et al. deter-
mined the effective HCN bending potential of HCNO
set, HCNO was found to have a planar trans equilib-
rium structure with / HCN s 165.138 and
Ž
.
w x
/ CNO s176.588 4 . The calculated HCN poten-
Ž
.
tial function is quite anharmonic with a barrier to
linearity of the HCNO chain of 7.6 cmy1. Upon
extrapolation to an infinite basis set, a barrier of f3
cmy1 was obtained, which is in good agreement
with the experimental equilibrium potential function
described above.
1 Permanent address: Department of Chemistry, Rensselaer
Polytechnic Institute, Troy, NY 12180, USA.
2 Permanent address: Department of Chemistry, Siena College,
Loudonville, NY 12211, USA.
Gas phase spectroscopic studies of simple substi-
tuted fulminates include a microwave investigation
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII S0009-2614 98 00085-2

(
)
392
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
3
w x
w x
5 and a millimeter wave study 6 of acetonitrile
based millimeter wave synthesizers from AMC ,
Ž
.
oxide methylfulminate , CH3CNO, which provide
Schottky barrier diode detectors and a 2.5 m free-
space glass absorption cell 9 . The output frequency
w x
w x
evidence for large amplitude bending 2 . The halo-
fulminates ClCNO and BrCNO were first detected
by Maier and Teles in an argon matrix at 10 K by
infrared spectroscopy 7 . Recently, BrCNO has been
identified in the gas phase by photoelectron and
photoionization mass spectroscopy and by low-reso-
of the synthesizers was phase-locked to a frequency
modulated reference signal, thus providing source
modulation. Two synthesizers were used to cover the
spectral ranges of 52–79, 117–148 and 160–179
GHz. The operating parameters of the spectrometer
were set at 25 kHz steps, a modulation frequency of
100 kHz, a modulation deviation of 250 kHz, 2f
demodulation and a lock-in amplifier time constant
of 100 ms, resulting in an overall time of 190 ms
required per step. Spectrometer control, digital data
acquisition and data reduction were carried out with
computer programs from AMC and the Gießen labo-
ratory.
w x
w x
lution infrared spectroscopy 8 . Two intense infrared
bands observed at 2211 and 1321 cmy1 have been
assigned to the antisymmetric
n
and symmetric
Ž
.
1
n
CNO stretches, respectively. The overtone of
Ž
.
2
n2 is assigned to a weak band observed at 2632
cmy1. These gas phase measurements correlate well
w x
with the argon matrix infrared work 7 . None of the
remaining normal modes have been observed includ-
ing the BrCN bend n5, which is important to the
question of quasilinearity. Ab initio calculations of
the equilibrium BrCN bending potential result in a
zero to moderate barrier to linearity, depending on
the extent to which triple excitations are included in
BrCNO was synthesized in a flow system by gas
phase pyrolysis of the volatile solid Br2CNOH as
w
x
described in the literature 7,8 . The quartz pyrolysis
tube had an inner diameter of 11 mm and was heated
to 1100 K along a length of 20 cm. Owing to the
unstable nature of BrCNO, the furnace was located
directly at the inlet of the cell and the full pumping
capacity of the liquid nitrogen trapped vacuum sys-
tem had to be utilized. The consumption of Br2CNOH
was f0.6 grhour when a pressure of 0.01 mbar
was maintained in the absorption cell.
w x
the calculation 8 . This indicates that BrCNO might
exhibit quasilinear behaviour. Harmonic vibrational
wavenumbers and intensities calculated at the MP2
and QCISD levels of theory for 79 BrCNO are in fair
agreement with experiment. However, the BrCN bend
calculated to be 177 cmy1 at the MP2 level and 83
cmy1 at the QCISD level of theory is not likely to be
reliable for a molecule which is possibly quasilinear,
since anharmonicities are not considered in the cal-
3. Results
w x
culations 8 .
This Letter reports the first investigation of the
rotational spectrum of BrCNO. Spectral assignments
of the ground vibrational state lines are presented for
the bromine 79 and bromine 81 isotopomers, yield-
ing accurate ground state constants. The observed
vibrational satellite pattern of the low-lying BrCN
bending mode is briefly discussed to show that the
molecule exhibits quasilinear behavior. Detailed
analyses of the excited vibrational state data for
BrCNO and similar investigations on ClCNO and
NCCNO, are in progress.
The millimeter wave spectrum of BrCNO was
found to be dense due to the presence of a large
number of intense vibrational satellite lines arising
from an extremely low-energy BrCN bending mode
n5. The spectrum is further complicated by the over-
lap of the spectra of the 79 BrCNO and 81 BrCNO
isotopomers which have approximately the same nat-
ural abundance. Additional lines from the known
w
x
millimeter wave spectra of HCNO 10 and HNCO
w
x
11 were also identified in agreement with the pyrol-
w
x
ysis chemistry reported in previous work 7,8 . Iso-
topic shifts could be used effectively to identify the
two sets of isotopomer lines for approximately one
2. Experimental procedures
3 Analytik und Meßtechnik GmbH, Stollberger Straße 4a, D-
09119 Chemnitz, Germany
The rotational spectra of BrCNO were recorded at
room temperature using backward wave oscillator

(
)
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
393
Fig. 1. The assignment of the ground state and the most intense n5 satellites is demonstrated for the Js15§14 rotational transition of the
BrCNO isotopomers. The Õ5 and l5 quantum numbers are given in the form Õ5 , erf denoting the parity. 79 BrCNO lines are labelled above
l
5
the spectrum, 81 BrCNO lines below the spectrum.
hundred vibrational levels, defined by the vibrational
quantum number Õ5 and the vibrational angular mo-
mentum quantum number l5. A portion of the rota-
tional spectrum in the lower frequency range is
shown in Fig. 1. The assignments of the most intense
n5 satellite lines as well as the ground state line are
shown for the Js15§14 rotational transition of
79 BrCNO. A number of the stronger n5 satellite lines
of the 81 BrCNO isotopomer also appear in this spec-
trum. These assignments were obtained by combin-
ing information from line positions, line intensities,
l-type resonance splitting and the l-dependence of
The first order quadrupole coupling energy of a
linear molecule with one coupling nucleus is given
4
by
3l2
EQ sx
y1
J Jq1
Ž
.
3C Cq1 y4I Iq1 J Jq1
. Ž .
Ž
.
Ž
=
,
1
Ž .
8I 2 Iy1 2 Jy1 2 Jq3
. Ž . Ž
Ž
.
CsF Fq1 yI Iq1 yJ Jq1 ,
Ž
.
Ž
.
Ž
.
where x is the quadrupole coupling constant and I
3
w
x
is the nuclear spin quantum number 12,13 . Is 2
for the 79 Br nucleus and for the 81 Br nucleus. F is
the total angular momentum quantum number which
w
x
the bromine nuclear quadrupole splitting 12 . The
nuclear quadrupole splitting, which decreases with
increasing J, was of particular importance since in
the lower frequency range, it could be at least partly
resolved for all l5 states. In contrast, the l-type
resonance splitting, which increases with increasing
J, could only be resolved in the higher frequency
range for l5 F3.
4 The coupling of the 14 N nucleus is expected to be two orders
of magnitude smaller than that of the Br nuclei and could not be
observed in the present spectra.

(
)
394
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
Fig. 2. The nuclear quadrupole splitting of the Js15§14 transitions and of the Js36§35 transitions is shown for 79 BrCNO in several
Õ5l vibrational states to demonstrate the l5 dependence of the splitting. Note that the splitting of the Js36§35 transition in the 22
5
vibrational state arises from l-type resonance, while that of the 64 vibrational state is still quadrupole splitting.
results from the coupling of J to I. For 79 BrCNO
For the ground state and some n5 vibrational satel-
lites of 79 BrCNO, the nuclear quadrupole splitting of
the Js15§14 transition and the splitting of the
Js36§35 transition are shown in detail in Fig. 2.
and for 81 BrCNO, it can take the values Jq 2 , Jq 2 ,
3
1
1
3
Jy 2 and Jy 2 . The selection rules are D Jsq1,
D Is0 and DFs0,"1, but in the case of BrCNO,
only the four DFsq1 components have apprecia-
ble intensity in the investigated frequency range. The
frequency expression for a hyperfine transition there-
fore is
Ž .
Eq. 2 was used to determine x values from
least-squares fits of the nuclear hyperfine compo-
nents observed for the lowest Jq1§J rotational
transition. For the ground states of 79 BrCNO and
81
Ž
.
Ž .
BrCNO, the fits gave xs656.8 88 and 551 10
nsn0 q EQ Jq1,I,Fq1 yE J,I,F
Q Ž
.
2
Ž .
Ž
.
.
MHz, respectively. For each rotational transition, the
The hypothetical unsplit frequency n0 of each transi-
center frequencies n0 were calculated using these
tion would be observed without quadrupole coupling
Ž .
values of x and Eq. 2 . The center frequencies of
3
w
x
12 . For l5 s0, the FsJq 2 component of a
the ground state transitions of 79 BrCNO and
81 BrCNO are listed in Table 1. They were least-
squares fit to the usual frequency equation for a
1
transition coincides with the FsJq 2 component
1
3
and the FsJy 2 component with the FsJy 2
component, whereas all four components are ob-
served at different frequencies for l5 )0. However,
in some cases even at the lowest investigated fre-
quencies only two or three components could be
resolved. In the frequency range above 117 GHz, no
hyperfine structure at all could be resolved for l5 F3.
w
x
linear molecule 13 ,
3
n0 s2 B0 Jq1 y4D Jq1 3 qH0 Jq1
Ž
.
0 Ž
.
Ž
.
3
=
Jq2 yJ 3
.
3
Ž .
Ž
.

(
)
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
395
Table 1
Rotational transition frequencies of 79 BrCNO and 81 BrCNO in the vibrational ground state
Transition
79 BrCNO
81BrCNO
a
b
a
b
Ž
.
Ž
.
Ž
.
Ž
.
Jq1§J
n0 MHz
Dn kHz
n0 MHz
Dn kHz
15§14
16§15
17§16
18§17
19§18
20§19
21§20
22§21
34§33
35§34
36§35
37§36
38§37
39§38
40§39
41§40
42§41
47§46
48§47
49§48
50§49
51§50
a
52193.3163
55672.1923
59150.9396
62629.5630
66108.0481
69586.3434
73064.4866
76542.4722
118262.0171
121737.0524
125211.8073
128686.2703
132160.4454
135634.3139
139107.8628
142581.0993
146054.0049
163413.3111
166884.0967
170354.4784
173824.4898
177294.1000
y1.0
y6.3
y1.0
4.6
1.9
2.0
y4.5
1.6
4.6
y2.2
y3.1
y6.6
y9.4
3.2
5.0
5.9
8.5
y9.3
2.8
y7.2
1.2
5.1
55257.2877
58710.1143
62162.8180
65615.3853
69067.7719
72519.9969
75972.0740
117380.9249
120830.0940
124278.9816
127727.5870
131175.9065
134623.9153
138071.6206
141519.0058
144966.0813
162196.2881
165641.2405
169085.8231
172530.0254
175973.8520
y6.0
y9.8
y7.9
1.7
6.5
y5.8
1.9
y1.3
y1.0
y4.6
y5.0
1.6
y1.6
y1.2
y2.7
8.5
1.2
y1.1
y5.8
y8.3
4.3
The center frequencies, n0, were calculated for each transition using the x values obtained from a hyperfine fit of the lowest
Ž .
frequencytransition and Eq. 2 . See text.
b
Ž .
Dn is n0 minus the frequency calculated from Eq. 3 using the constants in Table 2.
Table 2 summarizes the spectral constants obtained
from these fits for the two isotopomers.
the rotational constants in excited states of the low-
est-lying bending mode of BrCNO with the corre-
w
x
sponding patterns of HCNO 10,14 and of OCCCO
w
x
15 shows that BrCNO is more similar to OCCCO
4. Discussion
than to HCNO. This is confirmed by a preliminary
analysis of the vibrational dependence of the l-type
doubling constant q5 which is plotted in Fig. 3. All
the currently available information strongly suggests
that BrCNO is much further from the linear limit
The observed rotational spectra of 79 BrCNO and
81 BrCNO are consistent with the type of rotational
spectrum one would expect for
a quasilinear
w x
molecule. A comparison of the pattern presented by
than HCNO and perhaps more so than OCCCO 2 .
Hence, BrCNO should have a value for the quasilin-
w
x
earity parameter g0 16 that is roughly equal to the
Table 2
Rotational and centrifugal distortion constants of 79 BrCNO and
81 BrCNO in the vibrational ground state
w
x
value y0.20 found for OCCCO 15 , which means
that BrCNO is the most quasilinear polar molecule
known.
79 BrCNO
81BrCNO
Ž .
1726.96186 11
The fundamental transition frequency v5 of the
low-lying BrCN bending mode can be roughly esti-
mated from the relation
a
Ž
.
Ž
.
B0 MHz
1739.93007 10
Ž
.
Ž
.
Ž .
335.10 74
D0 Hz
339.88 68
Ž
.
Ž
6.8
.
Ž
7.1
.
H0 mHz
758 14
747 15
b
Ž
.
s
kHz
B02
a
v5 sf
,
4
Ž .
Quantities in parentheses are one standard deviation.
q5
b s is the standard deviation of the least-squares fit.

(
)
396
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
˚
1.199 A, respectively, and adjusting the Br–C dis-
tance to get agreement with the 79 BrCNO and
81 BrCNO ground-state rotational constants. Using
the HCNO values for BrCNO seems reasonable since
˚
the C–N distances of 1.155 A in HCN is not very
˚
different from the value of 1.158 A determined for
w
x
BrCN 20 . The calculations give Br–C distances of
1.732 and 1.739 A for 79 BrCNO and 81BrCNO,
˚
respectively, which are indeed significantly shorter
˚
than the Br–C distance of 1.790 A found for BrCN.
According to recent results, the vibrational satel-
lite pattern of the low-lying ClCN bending mode in
ClCNO is remarkably similar to that of the BrCN
bending mode in BrCNO. This strongly indicates
that ClCNO, like BrCNO, is an extremely quasilin-
ear molecule, both definitely more quasilinear than
HCNO. The rotational constants of the ground states
of 35ClCNO and 37ClCNO are found to be
Fig. 3. The vibrational dependence of the l-type doubling constant
qt is shown for the lowest-lying bending mode nt of the linear
molecule OCCCS and of the quasilinear molecules HCNO, OC-
CCO and BrCNO. All values are normalized to the l-type dou-
bling constant in the first excited bending state. The open symbols
indicate interpolated values.
Ž
.
Ž .
2 572.781 23 26 and 2 511.445 82 37 MHz, respec-
tively. The pattern found for the low-lying CCN
bending mode of NCCNO, however, exhibits only
slight deviations from the linear limiting case, sug-
gesting that NCCNO is significantly less quasilinear
than HCNO. A detailed analysis of the data for
ClCNO and NCCNO will be published separately.
where B0 is the ground state rotational constant and
q5 is the l-type doubling constant for the first excited
w
x
state of the bending mode n5 13 . From the splitting
of the l-type doublets in the rotational spectrum, q5
of 79 BrCNO was found to be 5.52 MHz. The con-
stant f, which is f2 for a harmonic bending poten-
tial, has empirically been found to decrease with
increasing quasilinearity of the molecule. A value of
w
x
fs1.32, calculated for OCCCO 15 , was used as an
approximation of f for BrCNO. Using these con-
stants, v5 of 79 BrCNO is calculated to be 24.15
cmy1. Relative intensity measurements are in agree-
ment with this estimate which is only slightly larger
Acknowledgements
than the 18.18 cmy1 found for OCCCO 17 . How-
ever, this value is much lower than the harmonic
wavenumbers obtained from the ab initio calcula-
w
x
CWG and JZG thank Professor M. Winnewisser,
Dr. B.P. Winnewisser and all members of the spec-
troscopy group at the Physikalisch-Chemisches Insti-
w x
tions 8 .
tut der Justus-Liebig-Universitat for the wonderful
hospitality during their sabbatical leave. One of the
¨
Another indication of quasilinearity in BrCNO
Ž
.
comes from an estimate of the Br–C internuclear
authors CWG is grateful to Professor P.F. Bernath
for his collaborative work with BrCNO at Rensse-
laer. The inability to observe this reactive species
using pulsed beam microwave spectroscopy led us to
pursue the present study at Gießen. We also thank
Professor N.P.C. Westwood for his interest and the
information which he provided to us. We acknowl-
edge the Deutscher Akademischer Austauschdienst
distance. In HCNO, it was found that the rs H–C
˚
w
x
internuclear distance is 1.027 A 18 , which must be
˚
w
x
compared with 1.063 A in HCN 19 . It is abnor-
mally short due to the effect of the large amplitude
bending motion of the hydrogen atom. So far, the
79 Br13CNO or Br13CNO ground state rotational
81
constants needed to determine an rs Br–C internu-
clear distance are not available. However, it is possi-
ble to obtain an estimate by fixing the C–N and
N–O distances at the HCNO values of 1.169 and
Ž
.
for a study visit scholarship CWG and the Deutsche
Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie for financial assistance.

(
)
C.W. Gillies et al.rChemical Physics Letters 285 1998 391–397
397
w
w
x
10 M. Winnewisser, B.P. Winnewisser, Z. Naturforsch. 26a
References
Ž
.
1971 128.
x
11 W.H. Hocking, M.C.L. Gerry, G. Winnewisser, Can. J. Phys.
w x
1
Ž
.
P. Caramella, P. Grunanger, in: A. Padwa Ed. , 1, 3-Dipolar
Cycloadditions, vol. 1, Wiley, New York, 1984, p. 291.
Ž
.
53 1975 869.
w
w
x
x
Ž
.
12 A. Javan, Phys. Rev. 99 1955 1302.
13 W. Gordy, R.L. Cook, Microwave Molecular Spectra, vol. 18
w x
2
Ž
.
B.P. Winnewisser, in: K. Narahari Rao Ed. , Molecular
Spectroscopy: Modern Research, vol. 3, Academic Press,
New York, 1985, p. 321.
of Techniques of Chemistry, Wiley, New York, 1984.
w
w
w
w
w
x
14 M. Winnewisser, B.P. Winnewisser, J. Mol. Spectrosc. 41
w x
3
P.R. Bunker, B.M. Landsberg, B.P. Winnewisser, J. Mol.
Ž
.
1972 143.
Ž
.
Spectrosc. 74 1979 9; P.R. Bunker, J. Mol. Spectrosc. 80
Ž .
x
15 L. Fusina, I.M. Mills, G. Guelachvili, J. Mol. Spectrosc. 79
Ž
.
1980 422 E .
Ž
.
1980 101.
w x
4
J. Koput, B.P. Winnewisser, M. Winnewisser, Chem. Phys.
Lett. 255 1996 357.
x
Ž
.
16 K. Yamada, M. Winnewisser, Z. Naturforsch. 31a 1976
Ž
.
139.
w x
5
Ž
.
H.K. Bodenseh, K. Morgenstern, Z. Naturforsch. 25a 1970
150.
x
17 A.V. Burenin, E.N. Karyakin, A.F. Krupnov, S.M. Shapin, J.
Ž
.
Mol. Spectrosc. 78 1979 181.
w x
6
M. Winnewisser, E.F. Pearson, J. Galica, B.P. Winnewisser,
J. Mol. Spectrosc. 91 1982 255.
x
Ž
.
18 M. Winnewisser, H.K. Bodenseh, Z. Naturforsch. 22a 1967
Ž
.
1724.
w x
7
Ž
.
.
G. Maier, J.H. Teles, Angew. Chem. Int. Ed. Engl. 26 1987
155.
w
w
x
x
Ž
.
19 C.C. Costain, J. Chem. Phys. 29 1958 864.
20 C.H. Townes, A.N. Holden, F.R. Merritt, Phys. Rev. 74
w x
8
Ž
T. Pasinszki, N.P.C. Westwood, J. Phys. Chem. 99 1995
6401.
Ž
.
1948 1113.
w x
9
M. Winnewisser, H. Lichau, F. Wolf, J. Mol. Spectrosc., to
be submitted.