Infrared spectra of (NO)2+, (NO)2-, and (NO)3- trapped in solid neon

Article abstract of DOI:10.1063/1.478967

New studies of the infrared spectra of the products which result on codeposition at approximately 5 K of a Ne:NO sample with Ne atoms that have been excited in a microwave discharge have led to new and revised assignments for several ionic species. The appearance of the ν₁ absorption of ONNO⁺ for several new species with asymmetric isotopic substitution, but for no symmetrically substituted species, confirms the trans ground-state configuration for ONNO⁺. The behavior of a neon-matrix product absorption at 1227.5 cm^-1 parallels that of an argon-matrix absorption at 1221.0 cm^-1 which has recently been assigned to trans-ONNO^-. The identity of the carrier of a product absorption at 1424.1 cm^-1, contributed by a vibration of two symmetrically equivalent NO groups, has not been definitively established. This absorption exhibits complex photodestruction behavior. Three absorptions are assigned to cis,cis-(NO)₃^-, which has C_2v symmetry, with the aid of density functional calculations of the isotopic substitution pattern for the vibrational fundamentals of this species. Similar calculations of the isotopic substitution patterns for other structures result in poor agreement with the experiments. Photodestruction of cis,cis-(NO)₃^- trapped in solid neon yields the N₂O...NO₂^- complex.

Full text of DOI:10.1063/1.478967

Infrared spectra of (NO)2+, (NO)2−, and (NO)3− trapped in solid neon
Catherine L. Lugez, Warren E. Thompson, Marilyn E. Jacox, Anders Snis, and Itai Panas
Citation: J. Chem. Phys. 110, 10345 (1999); doi: 10.1063/1.478967
View online: http://dx.doi.org/10.1063/1.478967
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 21
1 JUNE 1999
؉
2
؊
2
؊
3
Infrared spectra of „NO… , „NO… , and „NO… trapped in solid neon
a)
a)
b)
Catherine L. Lugez, Warren E. Thompson, and Marilyn E. Jacox
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899-8441
c)
Anders Snis and Itai Panas
Department of Environmental Inorganic Chemistry, Chalmers University of Technology,
S-412 96 G o¨ teborg, Sweden
͑
Received 19 February 1999; accepted 10 March 1999͒
New studies of the infrared spectra of the products which result on codeposition at approximately
K of a Ne:NO sample with Ne atoms that have been excited in a microwave discharge have led
5
to new and revised assignments for several ionic species. The appearance of the ␯ absorption
1
ϩ
of ONNO for several new species with asymmetric isotopic substitution, but for no symmetrically
substituted species, confirms the trans ground-state configuration for ONNO . The behavior of a
neon-matrix product absorption at 1227.5 cm parallels that of an argon-matrix absorption at
221.0 cm which has recently been assigned to trans-ONNO . The identity of the carrier of a
ϩ
Ϫ1
Ϫ1
Ϫ
1
Ϫ1
product absorption at 1424.1 cm , contributed by a vibration of two symmetrically equivalent NO
groups, has not been definitively established. This absorption exhibits complex photodestruction
Ϫ
3
behavior. Three absorptions are assigned to cis,cis-(NO) , which has C symmetry, with the aid
2v
of density functional calculations of the isotopic substitution pattern for the vibrational
fundamentals of this species. Similar calculations of the isotopic substitution patterns for other
Ϫ
structures result in poor agreement with the experiments. Photodestruction of cis,cis-(NO)3 trapped
Ϫ
2
in solid neon yields the N O¯NO complex. © 1999 American Institute of Physics.
2
͓
S0021-9606͑99͒01021-1͔
I. INTRODUCTION
studied in a neon matrix at the time of the earlier paper, data
Ϫ1
were not considered for absorptions below 1300 cm . The
As part of a series of studies of the infrared spectra of
small molecular ions produced by the photoionization and
Penning ionization of various precursors and trapped in solid
neon, we have proposed assignments for several ionic spe-
cies derived from NO and (NO) . An extremely prominent
Ϫ
infrared spectrum of NO2 has since been the subject of de-
5
tailed study in this laboratory. Moreover, recent ab initio
6,7
calculations
indicate that the prominent antisymmetric
1
Ϫ
stretching absorptions of both cis- and trans-ONNO should
lie between 1200 and 1300 cm . Experiments conducted by
Andrews and co-workers yielded the infrared spectroscopic
identification of both cis- and trans-ONNO , formed upon
2
Ϫ1
Ϫ1
absorption which appeared at 1619.2 cm was assigned to
7
Ϫ
trans-ONNO . Detailed isotopic substitution experiments
Ϫ
were consistent with the attribution of this absorption to the
capture of electrons produced on laser ablation of a variety of
same product as that reported ͑but not identified͒ in an earlier
metal targets by (NO) and trapped in an argon matrix.
2
2
study of the codeposition of an Ar:NO sample with an
In order to obtain more definitive information regarding
the carriers of the product absorptions reported by Jacox and
Ar:H sample that had been passed through a microwave
2
discharge. The isotopic substitution pattern also corre-
sponded to that assigned to trans-ONNO produced in
1
Thompson, we have performed many new neon-matrix ex-
Ϫ
periments using the same discharge sampling configuration
Ar:N O experiments which used a chemical ionization dis-
2
and additional isotopic substitutions. The analysis of the re-
3
charge source optimized for the collection of anions. In the
neon-matrix study, a combination band of this product ap-
peared at 3683.9 cm . Subsequent to these studies, vibra-
tional structure which corresponded closely to the two neon-
Ϫ1
sulting data includes the spectral region below 1300 cm
.
New discoveries regarding the behavior of the product ab-
sorptions on exposure of the deposit to near infrared and
visible radiation aid in the analysis of the spectra. The results
of these studies are presented in this article.
Ϫ1
4
matrix bands was observed in the first photoelectron band of
cis-(NO) . This observation required that the absorptions at
2
Ϫ1
1619.2 and 3683.9 cm be contributed by the ONNO cat-
II. EXPERIMENTAL DETAILS
ion, rather than by the anion.
Several other circumstances also suggest the importance
The normal and isotopically enriched nitric oxide1
samples were the same as those used in the previous study,
1
of reviewing and extending the earlier study. Because of
Ϫ
2
14 18
spectral interference from NO , which had not yet been
except that N O was no longer available and that a heavily
enriched 15N O sample was obtained from Isotec, Inc.
18
8
These samples were not purified, except for freezing at 77 K
and pumping the resulting solid to remove small amounts of
relatively volatile materials. The neon-matrix gas ͑Spectra
a͒
Guest Researcher.
Electronic mail: marilyn.jacox@nist.gov
b͒
c͒
Technology Administration, U.S. Department of Commerce.
0021-9606/99/110(21)/10345/14/$15.00
10345
© 1999 American Institute of Physics
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10346
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
8
Gases, research grade͒ also was used without further purifi-
other product absorptions, the most prominent of which lie
near 1228, 1424, 1619, and 3684 cm , is discussed below.
Ϫ1
cation. Ne:NO samples of mole ratios ranging from 50 to
4
00 were prepared using standard manometric procedures.
Over the concentration range explored, the pattern of
absorption intensities varied relatively little. The spectra of
deposits resulting in experiments on samples with a Ne:NO
mole ratio of 50 or 100 included very prominent absorptions
of cis-(NO) , an enhanced contribution of N O , modest
Most of the experiments, including all of those involving
isotopic mixtures, were conducted on samples with an over-
all Ne:NO mole ratio of 200. These mixtures included
1
4
16 15 16
14 16 15 18
N O: N Oϭ1:1, 1:2, 2:1, and 3:1,
N O: N O
2
2
3
ϭ1:1, 1:2, and 3:1, and 15N O: N Oϭ1:1. The isotopic
distribution randomizes in the sample bulb. In some experi-
ments, randomization was avoided by codepositing
Ne: N O and Ne: N O mixtures from separate manifolds.
The sample mixtures were codeposited at approximately
16
15 18
absorptions of cis- and trans-HONO, and slight enhancement
Ϫ1
of weak product absorptions near 1370, 1002, and 694 cm
.
Little change was noted in the prominence of the product
1
4
16
15 18
Ϫ1
absorptions near 1424, 1619, and 3684 cm . When the de-
posit was slightly warmed, the absorptions of cis-(NO) and
2
5
K with a similar amount of pure neon that had been excited
the product absorptions near 694, 1002, 1370, 1424, and
Ϫ1
by a microwave discharge before streaming from the pinhole
in the end of the quartz discharge tube. Details of this depo-
sition procedure and of the discharge configuration have
1619 cm
1227.5 cm decreased slightly.
grew somewhat. The product absorption at
Ϫ1
All of the absorptions attributed to ionic species other
9,10
Ϫ
1
Ϫ1
been described previously.
A Helitran APD Cryogenics,
than NO2 in the earlier study, as well as the 1227.5 cm
8
Inc.͒ continuous-transfer liquid helium cell was used.
The absorption spectra of the resulting sample deposits
were obtained using a Bomem DA3.002 Fourier transform
product absorption, experienced photodestruction when the
sample was exposed for a few minutes to mercury-arc radia-
tion passed by the 420 nm cutoff filter. The positions and
approximate relative intensities of these photosensitive ab-
sorptions are summarized in the first column of Table I.
More detailed filtered photolysis studies permit the sepa-
ration of the absorptions of Table I according to their photo-
destruction behavior. As is shown in Fig. 1, the product ab-
8
interferometer with transfer optics which have been de-
scribed previously.11 Observations were conducted with a
resolution and frequency accuracy ͑determined by the Fou-
rier transform data acquisition process͒ of 0.2 cm between
Ϫ1
Ϫ1
400 and 5000 cm using a globar source, a KBr beamsplit-
Ϫ1
ter, and a HgCdTe detector cooled to 77 K. Under these
conditions, the positions observed for prominent, nonblended
atmospheric water vapor lines between 1385 and 1900 cm
sorption at 1424.1 cm behaved uniquely. It was readily
photobleached by the background infrared radiation. How-
ever, after a brief period of mercury-arc irradiation through
the 420 nm cutoff filter it reappeared, only to disappear irre-
versibly after prolonged irradiation through that filter. As is
shown in Fig. 2, tungsten-lamp radiation passed by the 850
and 695 nm cutoff filters also photobleached the 1424.1
Ϫ1
Ϫ1
Ϫ1
and between 3620 and 3900 cm agreed within 0.01 cm
with the high-resolution values reported by Toth.12 Data
were accumulated for each spectrum over a period of at least
15 min. The resulting spectrum was ratioed against a similar
one taken without a deposit on the cryogenic mirror.
In order to obtain information on photoinduced changes
in the matrix sample, after the spectrum of the initial deposit
had been recorded, the sample was exposed to various wave-
length ranges of near infrared, visible, and ultraviolet radia-
Ϫ1
cm absorption, with no significant change in the other ab-
sorptions. After a 1 min subsequent mercury-arc irradiation
Ϫ1
through the 420 nm cutoff filter, the 1424.1 cm absorption
grew to almost twice its intensity in the initial sample de-
posit, but as the period of irradiation was increased it once
8
tion. For this a tungsten lamp was used with a Schott
again disappeared. In this photolysis study, the absorption at
RG850, RG780, or RG695 filter or with a filter of Corning8
glass type 2403, 2424, or 3480 to provide a short wavelength
cutoff of approximately 850, 780, 695, 630, 580, or 560 nm,
respectively. In addition, a medium-pressure mercury arc
was used with a filter of Corning glass type 3384, 3389,
Ϫ1
1
369.9 cm also behaved uniquely among the subset of the
product absorptions included in Figs. 1 and 2. It was almost
completely destroyed after only 1 min of mercury-arc irra-
diation through the 420 nm cutoff filter. Weak to very weak
Ϫ1
absorptions at 1001.9 and 693.8 cm , not included in Figs.
7380, or without a filter to provide a short wavelength cutoff
1
and 2, showed the same photodestruction behavior as did
of approximately 490, 420, 345, or 240 nm or with a filter of
Corning glass type 9863 to provide a pass band between 420
and 240 nm.
Ϫ1
the 1369.9 cm absorption. Later experiments determined
that the threshold for the photodestruction of these three ab-
sorptions lies between 560 and 580 nm. A sharp, very weak
Ϫ1
absorption at 2243.9 cm grew in intensity after 1 min of
mercury-arc irradiation through the 420 nm cutoff filter. Its
intensity experienced little additional change upon prolonged
irradiation. This absorption persisted on the irradiation
through the 345 nm cutoff filter, but was destroyed on irra-
diation through the 240–420 nm band pass filter or upon
unfiltered irradiation of the deposit. Weak absorptions at
III. OBSERVATIONS
1
As in the earlier study, absorptions of N O , ONONO,
2
3
Ϫ
and NO2 were present, in addition to prominent absorptions
of NO and cis-(NO)2 and relatively weak absorptions of
N O, CO , CO, and NO produced because of atmospheric
Ϫ1
1718.4 and 1737.3 cm also grew after brief mercury-arc
2
2
2
Ϫ1
impurities in the discharge. In some experiments, absorptions
of small amounts of cis- and trans-HONO were also recog-
nized. These are not discussed further here. Absorptions
irradiation through the 420 nm cutoff filter. The 1227.5 cm
absorption decreased more slowly than the 1369.9 cm
Ϫ1
peak as mercury-arc irradiation through the 420 nm cutoff
3
,13,14
Ϫ
Ϫ1
assigned
to NNO2 did not appear. The behavior of
filter proceeded, and the 1619.2 cm absorption disappeared
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10347
TABLE I. Product absorptions ͑Ϯ0.2 cmϪ1͒ between 600 and 3700 cmϪ1 which appear when a normal or an isotopically enriched Ne:NOϭ200 or 400 sample
is codeposited at ϳ5 K with a beam of excited neon atoms and are destroyed on prolonged mercury-arc irradiation through a 420 nm cutoff filter. ͑vw—very
weak; w—weak; m—medium; s—strong; vs—very strong; sh—shoulder͒.
1
4
16
15 18
14 16
15 18
N Oϩ N O
͑nonrandom͒
N Oϩ N O
͑random͒
14
16
14 16
15 16
15 16
15 18
15 18
N O
N Oϩ N O
N Oϩ N O
N O
Isotopic sequence
-
669.7w
669.8w
͑15–18͒͑15–18͒͑15–18͒
͑15–16͒͑15–18͒͑15–18͒,
671.2
͑
15–18͒͑15–16͒͑15–18͒
͑15–16͒͑15–18͒͑15–16͒,
15–16͒͑15–16͒͑15–18͒
672.8
͑
674.5vvw
674.6w
͑15–16͒͑15–16͒͑15–16͒
675.7vw
6
6
80.1
81.6
6
82.7
83.3
6
83.0
84.6
683.0?
͑14–16͒͑15–16͒͑15–16͒
͑14–16͒͑14–16͒͑15–16͒
6
6
6
6
6
89.7
91.0
92.3
693.0
6
93.8w
693.8vvw
693.8
693.7
͑14–16͒͑14–16͒͑14–16͒
͑15–18͒͑15–18͒͑15–18͒
͑15–18͒͑15–16͒͑15–18͒
͑15–16͒͑15–18͒͑15–18͒
͑15–16͒͑15–16͒͑15–18͒
͑15–16͒͑15–18͒͑15–16͒
͑15–16͒͑15–16͒͑15–16͒
͑14–16͒͑15–16͒͑15–16͒
͑14–16͒͑15–16͒͑14–16͒
͑15–16͒͑14–16͒͑15–16͒
͑14–16͒͑14–16͒͑15–16͒
͑14–16͒͑14–16͒͑14–16͒
18–15–15–18
9
9
9
9
9
66.2vw
68.3
68.9
71.0
966.2w
71.5
973.6
976.7
979.8
995.4
998.5
973.6
1
001.9w
1001.9
1001.9
1173.5
1173.5
1173.5
1188.5
1173.5wm
1
1
1
183.4
188.5
195.4sh
18–14–15–18
16–15–15–18
18–14–14–18
1
195.8
1195.8
201.2
1206.2
210.3
16–14–15–18
16–15–14–18
16–15–15–16
16–14–14–18
1
1206.2
1206.2
1310.0
1325.0
1
1
1
211.6
215.7
1215.8
1227.5
16–14–15–16
16–14–14–16
͑15–18͒͑15–16͒͑15–18͒,
1
227.5wm
1227.5
1227.5
1310.2
1310.1wm
͑
15–18͒͑15–18͒͑15–18͒
1313.0
1313.0
1
1
1
319.0
321.7
325.0
͑15–16͒͑15–16͒͑15–18͒,
15–16͒͑15–18͒͑15–18͒
͑
1328.5
1
1
329.9
333.2
1
1
1
339.3
342.1
344.8
1346.2sh
1346.6
1347.1
1346.6
͑15–16͒͑15–16͒͑15–16͒,
15–16͒͑15–18͒͑15–16͒
͑
1
1
1
1
349.6
355.1
358.0
367.5
1349.7
1355.1
1358.1
1367.5
1370.0
͑15–16͒͑14–16͒͑15–16͒
͑14–16͒͑15–16͒͑15–16͒
͑14–16͒͑14–16͒͑15–16͒
͑14–16͒͑15–16͒͑14–16͒
͑14–16͒͑14–16͒͑14–16͒
1367.5
1369.9
1
369.9wm
1369.9
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10348
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
TABLE I. ͑Continued.͒
1
4
16
15 18
14 16
15 18
N Oϩ N O
͑nonrandom͒
N Oϩ N O
͑random͒
14
16
14 16
15 16
15 16
15 18
15 18
N O
N Oϩ N O
N Oϩ N O
N O
Isotopic sequence
1
1
362.4
374.5
1362.5
1374.9
1362.5
1362.4m
18–15–15–18
18–14–15–18
16–15–15–18
18–14–14–18
16–14–15–18
16–15–14–18
16–15–15–16
16–14–14–18
16–14–15–16
16–14–14–16
18–15–15–18
17–15–15–18
18–14–15–18
16–15–15–18
18–14–14–18
16–14–15–18
16–15–14–18
1
1
380.2
388.0
1380.2
1399.4
1391.1
1391.1
393.7
1399.4
405.3
1
1
1
399.4
411.4
1
1411.5
1424.1
1548.6
1
424.1m
1424.1
1424.1
1
548.6
1548.6
1568.7
1548.5vs
558.3m
1
1
1
1
562.4
568.7
577.4
1
580.5
1580.5
584.2
1
1586.2wm
1
591.2
604.7
1591.3
597.2
1604.7
1619.2
1591.3
16–15–15–16
16–14–14–18
16–14–15–16
16–14–14–16
16–14–14–16
16–14–14–16
16–14–14–16
16–14–14–16
18–14–15–18
16–15–15–18
16–15–14–18
16–14–15–18
16–14–14–18
16–14–15–16
1
1
1
1
1
1
1
619.2vs
718.4w
737.3w
760.6s
1619.2
1619.2
764.9m
2
2
024.5
033.9
2033.9
2
054.5sh
2054.9
071.3
2
054.9
523.7
2
2079.2
2079.2
3
439.0w
3
3523.8
3523.9
3573.2
3523.7
18–15–15–18
18–14–15–18
16–15–15–18
18–14–14–18
16–15–14–18
16–14–15–18
16–15–15–16
16–14–14–18
16–14–15–16
16–14–14–16
3
3
557.5
573.3
a
͓
3589.5͔
604.9
3
3607.1
3607.1
3620.2
3
620.1
652.5
3620.1
3638.2
3
3652.4
3
683.8wm
3683.8
3683.8
aFrom Ref. 1.
significantly less readily than the 1227.5 cmϪ1 peak during
prolonged mercury-arc irradiation through the 420 nm cutoff
filter. Despite the large disparity between the intensities of
cutoff filter in several of the isotopic substitution experi-
ments are included in Table I.
Ϫ1
Details of the spectra between 1165 and 1250 cm of
Ϫ1
the absorptions at 1619.2 and 3683.8 cm , their photode-
several isotopically enriched samples are shown in Fig. 3. In
Ϫ
2
struction behavior was sufficiently similar to support the pre-
vious assignment of these two absorptions to the same prod-
uct. Other experiments showed that quite efficient
photodestruction of both of these absorptions occurs when
the deposit is exposed to tungsten-lamp radiation passed by
the 630 nm cutoff filter.
Isotopic substitution studies yielded additional informa-
tion regarding these product absorptions. The positions and
approximate relative intensities of the product absorptions
destroyed by mercury-arc irradiation through the 420 nm
order to remove contributions from such species as NO and
heavy-isotopic N O, which add considerably to the complex-
2
ity of the absorption pattern in this spectral region, difference
plots showing the changes resulting from mercury-arc irra-
diation through the 420 nm cutoff filter are given. Upward-
pointing peaks grew during irradiation; downward-pointing
peaks diminished. The photosensitive product contributes
Ϫ1
sharp absorptions between 1173.5 and 1227.5 cm . In trace
͑a͒ of Fig. 3, for a nonrandom 14N O: N OХ1:1 sample,
16
15 18
three peaks appear, the central one most intense, consis-
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10349
FIG. 1. Photodestruction behavior of major peaks in the infrared spectrum
of 9.32 mmol Ne:NOϭ200 codeposited at ϳ5 K over a period of 195 min
with 9.05 mmol discharged Ne. ͑The filter cutoff wavelength and duration of
tungsten-lamp or, for 420 nm, mercury-arc irradiation are designated in the
plot. ‘‘IR’’ indicates sequential data collections taken with a Fourier-
transform infrared spectrometer.͒
FIG. 3. Isotopic substitution behavior of product absorptions between 1165
and 1250 cm which change in intensity upon mercury-arc irradiation,
Ϫ1
␭Ͼ420 nm. ͑In each plot, the spectrum after 15 or 20 min subsequent
mercury-arc irradiation, ␭Ͼ420 nm, was subtracted in order to remove in-
terfering absorptions of nonphotosensitive species. Peaks pointing in the
‘‘ϩ’’ direction grew upon irradiation through the 420 nm cutoff filter;
peaks pointing in the ‘‘Ϫ’’ direction decreased.͒ ͑a͒ 8.19 mmol
tent with the participation of two symmetrically equivalent
NO units in the vibration. In trace ͑b͒, for a randomized
1
4
16 15 18
N O: N Oϭ1:1 sample, a total of 10 peaks is present,
14
16
15 18
Ne: N Oϭ400 ϩ 7.49 mmol Ne: N Oϭ400 codeposited at ϳ5 K from
also consistent with the participation of two NO units in the
vibration. ͓The tenth peak is a shoulder on the low-frequency
side of the sharp peak at 1195.8 cmϪ1 in trace ͑b͒.͔ Traces ͑c͒
and ͑d͒ show triads with approximately 1:2:1 intensity ratios,
again consistent with the participation of two symmetrically
equivalent NO units in the vibration. Also shown in Fig. 3
are relatively broad, upward-pointing peaks, which corre-
spond to species which grow on 420 nm cutoff irradiation of
separate manifolds over a period of 397 min with 14.7 mmol discharged Ne
14 16 15 18
͑
nonrandomized sample͒. ͑b͒ 16.2 mmol Ne: N O: N Oϭ400:1:1 code-
posited at ϳ5 K over a period of 358 min with 15.2 mmol discharged Ne
14
16 15 16
͑
randomized sample͒. ͑c͒ 9.63 mmol Ne: N O: N Oϭ400:1:1 codepos-
ited at ϳ5 K over a period of 208 min with 9.67 mmol discharged Ne. ͑d͒
15
16 15 18
1
2.0 mmol Ne: N O: N Oϭ400:1:1 codeposited at ϳ5 K over a period
of 265 min with 11.0 mmol discharged Ne.
the deposit. The most prominent of these peaks are contrib-
Ϫ
uted by absorptions of isotopic species of NO , which were
2
previously studied in this laboratory.5
The absorption spectra recorded between 1305 and
Ϫ1
1372.5 cm
for various isotopically enriched Ne:NO
samples are shown in Fig. 4. The sharp absorption near 1323
Ϫ1
cm , present in all of the traces, is not photosensitive. Trace
1
5
16 15 18
͑
e͒ for a Ne: N O: N Oϭ400:1:1 sample, shows three
Ϫ1
absorptions between 1346.6 and 1310.2 cm with an ap-
proximate 1:2:1 intensity ratio. This behavior is appropriate
for the participation of two symmetrically equivalent oxygen
FIG. 4. Isotopic substitution behavior of product absorptions between 1305
Ϫ1
and 1372.5 cm . ͑a͒ 12.3 mmol Ne:NOϭ400 codeposited at ϳ5 K over a
period of 292 min with 13.2 mmol discharged Ne. ͑b͒ 8.19 mmol
14
16
15 18
Ne: N Oϭ400 ϩ 7.49 mmol Ne: N Oϭ400 codeposited at ϳ5 K from
separate manifolds over a period of 397 min with 14.7 mmol discharged Ne
14
16 15 18
͑
nonrandomized sample͒. ͑c͒ 16.2 mmol Ne: N O: N Oϭ400:1:1 code-
FIG. 2. Photodestruction behavior of major peaks in the infrared spectrum
of 12.3 mmol Ne:NOϭ400 codeposited at ϳ5 K over a period of 292 min
with 13.2 mmol discharged Ne. ͑The filter cutoff wavelength and duration of
tungsten-lamp or, for 420 nm, mercury-arc irradiation are designated in the
plot.͒
posited at ϳ5 K over a period of 358 min with 15.2 mmol discharged Ne
14
16 15 16
͑randomized sample͒. ͑d͒ 9.63 mmol Ne: N O: N Oϭ400:1:1 codepos-
ited at ϳ5 K over a period of 208 min with 9.67 mmol discharged Ne. ͑e͒
15
16 15 18
12.0 mmol Ne: N O: N Oϭ400:1:1 codeposited at ϳ5 K over a period
of 265 min with 11.0 mmol discharged Ne.
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10350
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
FIG. 5. Dependence of the pattern of product absorptions between 1330 and
Ϫ1
15
1
372.5 cm
on the extent of enrichment in N. ͑a͒ 8.56 mmol
14
16 15 16
Ne: N O: N Oϭ800:3:1 codeposited at ϳ5 K over a period of 183 min
1
4
16 15 16
FIG. 6. Dependence of the pattern of product absorptions between 668 and
with 8.86 mmol discharged Ne. ͑b͒ 9.63 mmol Ne: N O: N O
ϭ400:1:1 codeposited at ϳ5 K over a period of 208 min with 9.67 mmol
discharged Ne. ͑c͒ 8.56 mmol Ne: N O: N Oϭ600:1:2 codeposited at
ϳ5 K over a period of 180 min with 8.55 mmol discharged Ne.
Ϫ1
15
7
00 cm on the extent of enrichment in N ͓traces ͑a͒, ͑b͒, and ͑c͔͒ and on
15
18
1
4
16 15 16
the enrichment of
NO with
O ͓trace ͑d͔͒. ͑a͒ 8.56 mmol
14
16 15 16
Ne: N O: N Oϭ600:2:1 codeposited at ϳ5 K over a period of 194 min
1
4
16 15 16
with 8.79 mmol discharged Ne. ͑b͒ 9.63 mmol Ne: N O: N O
ϭ400:1:1 codeposited at ϳ5 K over a period of 208 min with 9.67 mmol
discharged Ne. ͑c͒ 8.56 mmol Ne: N O: N Oϭ600:1:2 codeposited at
ϳ5 K over a period of 180 min with 8.55 mmol discharged Ne. ͑d͒ 12.0
14
16 15 16
atoms in the vibration. In contrast, trace ͑d͒, for a
1
4
16 15 16
15 16 15 18
Ne: N O: N Oϭ400:1:1 sample, shows six absorptions
mmol Ne: N O: N Oϭ400:1:1 codeposited at ϳ5 K over a period of
Ϫ1
265 min with 11.0 mmol discharged Ne.
between 1369.9 and 1346.6 cm . These can be subdivided
into three pairs of absorptions, with the members of each pair
Ϫ1
separated by approximately 3 cm . The relative intensities
of comparable members of each pair are approximately
closely spaced peaks with an approximately 1:3:3:1 intensity
ratio. In Fig. 7, for the 965 to 1005 cm spectral region,
Ϫ1
1
:2:1.
The nature of this splitting was probed by further experi-
ments on samples with the same overall Ne:NO mole ratios
traces ͑a͒–͑c͒, taken together, give evidence of two triads on
nitrogen-isotopic substitution, with a large separation be-
tween these triads and separations of approximately 3.1
1
4
15
but different NO: NO ratios. Representative patterns of
Ϫ1
Ϫ1
cm between adjacent members of each triad. This total of
absorptions between 1330 and 1372.5 cm are shown in
Fig. 5. In trace ͑a͒, for a sample with an excess of 14NO, the
higher frequency member of each pair is more prominent
than the lower frequency member. Trace ͑b͒ of Fig. 5 repeats
the spectrum shown in trace ͑d͒ of Fig. 4 for a sample with
1
4
15
NO: NOϭ1:1. Trace ͑c͒ of Fig. 5 shows the absorption
15
pattern for a sample with an excess of NO. The lower
frequency member of each pair is more prominent than the
higher frequency member. ͑The absorption at 1369.9 cm
Ϫ1
was too weak to be distinguished from the background
noise.͒
In a search for other absorptions which share the photo-
Ϫ1
destruction behavior of the 1369.9 cm
absorption and
whose isotopic substitution behavior might clarify the nature
of the carrier of the species responsible for this absorption,
Ϫ1
weak absorptions were found at 693.8 and 1001.9 cm . The
isotopic substitution behavior of these two absorptions is
shown in Figs. 6 and 7, respectively. At the scale used for
Ϫ1
Fig. 6, for the 668–700 cm spectral region, a modest ab-
Ϫ1
sorption centered near 665 cm , contributed by ␯ of CO2
2
impurity in the deposit, spans the entire ordinate range. In
traces ͑a͒–͑c͒, a total of four peaks can be distinguished from
the background noise. There is a large nitrogen-isotopic shift
for the 693.8 cmϪ1 absorption. Trace ͑d͒ shows a modest
oxygen-isotopic shift associated with a pattern of four rather
FIG. 7. Dependence of the pattern of product absorptions between 965 and
Ϫ1
15
1
005 cm on the extent of enrichment in N ͓traces ͑a͒, ͑b͒, and ͑c͔͒ and
15
18
on the enrichment of NO with O ͓trace ͑d͔͒. The sampling conditions are
the same as in Fig. 6.
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10351
FIG. 10. Isotopic substitution behavior of product absorptions between 2020
and 2090 cm which change in intensity upon mercury-arc irradiation,
FIG. 8. Isotopic substitution behavior of product absorptions between 1350
and 1430 cm . The sampling conditions are the same as in Fig. 3.
Ϫ1
Ϫ1
␭Ͼ420 nm. ͑In each plot, the spectrum after 15 or 20 min subsequent
mercury-arc irradiation, ␭Ͼ420 nm, was subtracted in order to remove in-
terfering absorptions of nonphotosensitive species.͒ The sampling condi-
tions are the same as in Fig. 3.
six absorptions requires the presence of three or more N
atoms in the carrier of the absorption. Trace ͑d͒ shows that
six quite closely spaced absorptions result from oxygen-
isotopic substitution. This behavior requires the presence of
at least three oxygen atoms in the carrier of the absorption.
The isotopic substitution behavior of the absorptions be-
Figure 9 shows the product absorption pattern for the
Ϫ1
1
540 to 1625 cm spectral region. In order to remove con-
tributions from absorptions of NO and H O, invariant on
2
2
Ϫ1
mercury-arc irradiation through the 420 nm cutoff filter, dif-
ference plots are used. The isotopic substitution pattern is
similar to that for the absorptions of Fig. 8.
tween 1350 and 1430 cm is illustrated in Fig. 8. In traces
͑a͒–͑c͒, there is some overlap with the absorption pairs
shown in Fig. 5. Traces ͑a͒, ͑c͒, and ͑d͒ of Fig. 8 show triads
with approximately 1:2:1 relative intensity for the product
absorptions which have photodestruction behavior similar to
Ϫ1
Absorptions in the 2020–2090 cm spectral region, in
Ϫ1
which the ␯ absorption of the carrier of the 1619.2 cm
1
ϩ
Ϫ1
absorption ͑now identified as trans-ONNO ͒ lies, are shown
that of the 1424.1 cm peak of the unenriched samples. In
trace ͑b͒, for a randomized 14N O: N O sample, 10 peaks
can be identified. This behavior is appropriate for a vibration
contributed by two symmetrically equivalent NO groups.
16
15 18
in Fig. 10. The totally symmetric ␯ vibration is infrared-
1
active only for the unsymmetrically substituted isotopomers.
The greater the asymmetry, the more intense its absorption
should be. Here, the most highly asymmetric species is
1
6
14 15 18
ϩ
trans- O N N O . The exceptionally prominent absorp-
Ϫ1
tion at 2054.9 cm in trace ͑a͒ can be assigned to that iso-
topomer. There are no other absorptions in this spectral re-
gion in trace ͑a͒. The same absorption is the most prominent
one in trace ͑b͒ for a randomized sample. Two other absorp-
Ϫ1
tions, at 2033.9 and 2071.3 cm , are readily seen in that
Ϫ1
trace. In addition, there is a shoulder at 2054.5 cm , and
weaker product absorptions appear at 2079.2 and 2024.5
Ϫ1
cm
.
Ϫ1
Product absorptions between 3500 and 3700 cm are
shown in Fig. 11. Once again, difference plots are shown in
order to remove photostable absorptions of CO2 and
ϩ
(
H O) . The ␯ ϩ␯ combination band of trans-ONNO
2 n 1 5
1,4
has been assigned in this spectral region. A total of nine
sharp product absorptions can be identified in these four
plots. In addition, an absorption contributed by the
1
8
14 14 18
Ϫ1
O N N O species was identified at 3589.5 cm in the
1
earlier work.
Difference plots for the 2140–2255 cmϪ1 spectral region
FIG. 9. Isotopic substitution behavior of product absorptions between 1540
and 1625 cm which change in intensity upon mercury-arc irradiation,
are shown in Fig. 12. Prominent absorptions of N O and its
2
Ϫ1
isotopomers, which are already well developed in the initial
deposit, lie in this spectral region. A differential absorption
pattern persists for several of these absorptions, suggesting
␭
Ͼ420 nm. ͑In each plot, the spectrum after 15 or 20 min subsequent
mercury-arc irradiation, ␭Ͼ420 nm, was subtracted in order to remove in-
terfering absorptions of nonphotosensitive species.͒ The sampling condi-
tions are the same as in Fig. 3.
that the N O band contour changes somewhat during the
2
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10352
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
TABLE II. Product absorptions ͑Ne matrix, Ϯ0.2 cmϪ1; this work͒ between
Ϫ1
2
150 and 2250 cm which grow upon 420 nm irradiation of the deposit and
are destroyed by radiation in the 240–345 nm spectral region compared with
gas-phase band centers ͑Ref. 15͒ of ␯3 of isotopically substituted N O.
2
Isotopic
NNO
sequence
͑gas͒
Obs.
Obs.Ϫgas
1
4–14–16
4–14–18
5–14–16
5–14–18
4–15–16
4–15–18
5–15–16
5–15–18
2223.76
2216.71
2201.60
2194.04
2177.66
2171.04
2154.73
2147.7a
2243.9
20.1
1
1
1
1
1
1
1
2221.6
2214.1
2197.4
2190.9
2174.2
2167.2
20.0
20.1
19.7
19.9
19.5
19.5
aEstimated from separation between 14–14–16 and 14–14–18.
the 420 nm cutoff filter. Usually the isotopic substitution
counterparts of this peak are relatively weak. Broken lines
have been added to Fig. 12 to aid in their recognition. The
spectral region in which these peaks appear and their posi-
tions for the heavy-isotopic species suggest that the 2243.9
FIG. 11. Isotopic substitution behavior of product absorptions between 3500
and 3700 cm which change in intensity upon mercury-arc irradiation,
Ϫ1
␭
Ͼ420 nm. ͑In each plot, the spectrum after 15 or 20 min subsequent
mercury-arc irradiation, ␭Ͼ420 nm, was subtracted in order to remove in-
terfering absorptions of nonphotosensitive species.͒ The sampling condi-
tions are the same as in Fig. 3.
Ϫ1
cm absorption is contributed by the NNO moiety of a com-
plex. This hypothesis is tested in Table II. The positions of
the ␯ band centers for the gas-phase NNO species are taken
irradiation. In the spectrum of trace ͑a͒, for an unsubstituted
3
15
from the high-resolution study by Toth. The band center
Ne:NO sample, there is a sharp, negative-going peak at
for 15N N O, not given by Toth, was estimated by assum-
15
18
Ϫ1
2
243.9 cm . The sign of this absorption indicates that it
Ϫ1
ing that it is shifted downward by 7.0 cm from the band
grows upon mercury-arc irradiation of the deposit through
15
15 16
center for N N O; the corresponding separation between
1
4
14 16
14 14 18
Ϫ1
N N O and N N O is 7.05 cm . In the experiment on
14
16
15 18
a nonrandom Ne: N OϩNe: N O sample, shown in trace
b͒ of Fig. 12, N O isotopomers and their complexes derived
from N O by the addition of an end N atom ͑i.e.,
͑
2
14
16
14
14 16
15 14 16
15 18
N N O and N N O͒, and those derived from N O
i.e., 14N N O and N N O͒ were detected, but the four
15
18
15 15 18
͑
15
16
14 18
isotopomers derived from N O and N O did not appear.
Because of very strong absorptions of heavy-isotopic
species of cis-(NO) , isotopic data were inaccessible for the
2
Ϫ1
counterparts of the absorptions at 1718.4 and 1737.3 cm or
for the 1760.6–1764.9 cm
assigned to trans-ONNO.
Ϫ1
absorption, previously
1
IV. DISCUSSION
؉
A. Infrared spectrum of trans-ONNO
As already noted, the excellent correspondence of the
vibrational spacings measured by Bondybey and co-workers4
in the first photoelectron band of cis-(NO) to the absorp-
2
tions previously assigned1 to trans-ONNO requires the
,3
Ϫ
ϩ
reassignment of these absorptions to ONNO . In a study of
the ions produced from an Ar:CCl sample and trapped using
4
FIG. 12. Isotopic substitution behavior of product absorptions between 2140
and 2250 cm which change in intensity upon mercury-arc irradiation,
the same chemical ionization source operated in the negative
Ϫ1
16
ion mode, Hacaloglu and Andrews saw not only a number
␭
Ͼ420 nm. ͑In each plot, the spectrum after 15 or 20 min subsequent
of already familiar bands but also a weak absorption of
mercury-arc irradiation, ␭Ͼ420 nm, was subtracted in order to remove in-
terfering absorptions of nonphotosensitive species. Peaks pointing in the
ϩ
2
CCl . They noted that, when the chemical ionization source
‘‘ϩ’’ direction grew upon irradiation through the 420 nm cutoff filter; peaks
is operated in the negative ion mode, positive ions can result
from electron impact on the cryogenic surface. ͑In contrast,
when the source is operated in the positive ion mode, only
cations and neutrals should appear.͒
pointing in the ‘‘Ϫ’’ direction decreased.͒ ͑a͒–͑e͒ The sampling conditions
are the same as those of the corresponding traces of Fig. 4. ͑f͒ 9.05 mmol
15
18
Ne: N Oϭ200 codeposited at ϳ5 K over a period of 211 min with 8.46
mmol discharged Ne.
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10353
TABLE III. Comparison of the position of the (␯ ϩ␯ ) combination band
TABLE IV. Results of least-squares force constant fita of the positions
1
5
ϩ
Ϫ1
of isotopically substituted trans-ONNO with the sum of the ␯1 and ␯5
͑cm ͒ of the NO-stretching fundamentals of isotopically substituted
Ϫ1
ϩ
fundamental frequencies ͑cm ͒.
trans-ONNO .
Isotopic
sequence
Symmetric
Antisymmetric
obs
obs
ϩ␯ o5 bs
1
⌬a
Isotopic
sequence
(␯ ϩ␯ )
␯
1
5
Obs.b
Calc.
Obs.b
Calc.
1
1
1
1
1
1
1
1
1
1
6–14–14–16
6–14–15–16
6–15–15–16
6–14–14–18
8–14–14–18
6–14–15–18
6–15–14–18
6–15–15–18
8–14–15–18
8–15–15–18
3683.8
3652.4
3620.1
3638.2
3589.5
3607.1
3604.9
3573.2
3557.5
3523.7
1
1
1
1
1
1
1
1
1
1
6–14–14–16
6–14–15–16
6–15–15–16
6–14–14–18
8–14–14–18
6–14–15–18
6–15–14–18
6–15–15–18
8–14–15–18
8–15–15–18
2098.0
2079.3
2059.4
2072.6
2044.7
2055.2
2052.2
2033.5
2025.5
2005.0
1619.2
1604.7
1591.2
1597.2
1577.4
1580.5
1584.2
1568.7
1562.4
1548.6
1620.2
1605.1
1591.2
1597.6
1577.5
1580.3
1584.3
1568.2
1562.0
1547.7
3683.9
3668.5
31.5
30.3
2079.2
2071.3
3635.4
3638.7
3602.6
3586.9
28.3
33.8
29.4
29.4
2054.9
2054.5
2033.9
2024.5
a␯ ϩ␯5 Ϫ͑␯ ϩ␯ ͒obs. ␯1 is infrared-inactive for the centrosymmetric iso-
obs
obs
1
1
5
aAssuming valence angles of 105°. FNN fixed at 1.0ϫ10 N mϪ1;
2
ϩ
topomers of trans-ONNO .
2
Ϫ1
FNO,NOϭ3.74ϫ10 N mϪ1;
2
FNOϭ15.28ϫ10 N m
ϫ10 N m.
;
FNNOϭ0.56
Ϫ18
b
Ne matrix, Ϯ0.2 cmϪ1; this work.
Preliminary ab initio calculations by the Bondybey
4
group suggest that the ground state of the dimer cation is
likely to have the trans configuration. The more extensive
isotopic data of the present study ͑cf. Table I and Fig. 10͒
NN-stretching fundamental was calculated to lie near 600
include the identification of four new asymmetrically substi-
Ϫ1
2
Ϫ1
cm . For FNNϭ1.0ϫ10 N m , the average frequency er-
ϩ
tuted species of ONNO , for which ␯ of the trans configu-
1
Ϫ1
ror changed very slowly, and still rounded off to 0.6 cm as
ration becomes weakly infrared-active. The failure to detect
absorptions attributable to ␯1 of any of the symmetrically
substituted species would be consistent with the trans but not
the cis configuration, since for the latter ␯1 of all of the
isotopic species would be infrared-active. The positions of
the absorptions assigned to the ␯ ϩ␯ combination band of
the valence angles increased to 150° and increased only to
Ϫ1
0
.7 cm for the linear molecule. The calculations were also
quite insensitive to the value of FNN
.
When
2
Ϫ1
FNNϭ4.0ϫ10 N m and the valence angles were 105°, the
Ϫ1
average frequency error was also 0.6 cm , all values of the
1
5
ϩ
antisymmetric NO-stretching fundamental were identical to
those in Table IV, and no value of the symmetric NO-
stretching fundamental differed from the corresponding
10 isotopic species of trans-ONNO are compared in Table
III with the sum of the positions of the observed ␯ and ␯5
1
absorptions. The sum of the positions of the fundamental
absorptions exceeds the value for the position of the combi-
Ϫ1
value in Table IV by more than 0.5 cm . The infrared-
Ϫ1
inactive NN-stretching fundamental was calculated to appear
nation band by an average of 30.5 cm . Forney and
Ϫ1
5
at about 1000 cm for the unsubstituted cation. There was a
co-workers have discussed the results of a similar compari-
slow increase in the average frequency error out to angles of
son for gas-phase O , which shows a disparity of 34.44
3
Ϫ1
Ϫ1
150°, for which it amounted to 1.4 cm . The average fre-
cm . Deviations of similar magnitude and sign have been
Ϫ1
quency error for the linear molecule was 1.8 cm . An at-
observed for a number of other heavy-atom chain molecules.
A least-squares force constant adjustment was performed
tempt to refine the calculations by adjusting the NN-
stretching force constant changed the fit very little. Survey
calculations for the cis structures with valence angles of 105°
and 150° gave average frequency errors similar in magnitude
to those for the corresponding trans structures. It is con-
cluded that the normal coordinate calculations support the
ϩ
for the two NO-stretching fundamentals of trans-ONNO
using the program developed by Schachtschneider.17 These
force constant adjustment calculations are insensitive to bond
lengths. A four-constant valence force potential was fitted to
the experimental values for the in-plane vibrational funda-
ϩ
ϩ
identification of a highly bent ONNO cation. The detection
mentals of 10 isotopic species of trans-ONNO . Calcula-
of ␯1 only for the asymmetrically substituted isotopomers
tions were performed at increments of 15° assuming valence
angles ranging from 105° to 150°, as well as for the linear
molecule. Two series of calculations were run. For the first
strongly supports the trans configuration for this product.
18
Very recently, Xie, Schaefer, and co-workers have
ϩ
series, the NN-stretching force constant, F , was fixed at
conducted the first detailed study of ONNO using ab initio
NN
2
Ϫ1
1
.0ϫ10 N m , and for the second series, it was set at the
larger value of 4.0ϫ10 N m . In addition to the neon-
and density functional theory methods. The occurrence of
2
Ϫ1
19
inverse symmetry breaking required the use of more elabo-
matrix frequencies, the gas-phase value for the symmetric
rate calculational procedures. Their results indicate that, con-
in-plane bending vibration of the unsubstituted molecule
sistent with the experimental results, the ground-state struc-
Ϫ1
2
͑323 cm ͒ was included in the fit. The closest fit to the
ture ( A ) has the trans configuration, with valence angles
g
observed vibrational frequencies, with an average frequency
near 112°. They have also obtained vibrational fundamentals
for trans-ONNO which are in reasonable agreement with
the observed values. The cis structure was calculated to lie
approximately 4 kJ/mol above the trans ground state.
Ϫ1
ϩ
error of 0.6 cm , was obtained for valence angles of 105°
2
Ϫ1
and for FNNϭ1.0ϫ10 N m . Detailed results of this calcu-
lation are summarized in Table IV. The infrared-inactive
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10354
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
؊
TABLE V. Results of least-squares force constant fita of the positions
B. Infrared spectrum of trans-ONNO
Ϫ1
͑
cm ͒ of the antisymmetric NO-stretching fundamental of isotopically sub-
Ϫ
Concurrent with the present study, the results of another
ab initio and argon-matrix study of the infrared spectrum of
trans-ONNO became available. In that study, this species
stituted trans-ONNO .
7
Ϫ
Isotopic
sequence
Obs.b
Calc.
was detected among the products of the reaction between NO
and approximately 20 different species of laser-ablated metal
atoms trapped in solid argon. Electrons produced in the ab-
lation process are captured by dimeric NO in the system.
Sufficient energy was imparted to the system for the appear-
1
1
1
1
6–14–14–16
6–14–15–16
6–15–15–16
6–14–14–18
1227.5
1215.8
1206.2
1210.3
1195.4
1195.8
1201.2
1188.5
1183.4
1173.5
1227.0
1215.8
1205.0
1211.1
1194.7
1200.0
1199.7
1188.9
1183.3
1172.1
18–14–14–18
16–14–15–18
1
1
1
ϩ
ance of a modest yield of trans-ONNO , as well. The metal-
6–15–14–18
6–15–15–18
8–14–15–18
independent peaks were also seen in experiments on Ar:NO
samples subjected to a dc glow discharge maintained near
the cryogenic surface during deposition. Assignments were
proposed with the aid of isotopic substitution and detailed ab
18–15–15–18
aStructure calculated by Ref. 6 assumed. Values of 1300 and 800 cmϪ1
assumed for the other two stretching fundamentals of the 16–14–14–16
species. F ϭ4.29ϫ10 N m ; FNNϭ6.58ϫ10 N m ; FNO,NOϭϪ2.33
ϫ10 N m ; FNNOϭ0.82ϫ10 N m.
Ϫ
Ϫ
initio calculations for trans-ONNO , cis-ONNO , and
Ϫ
Ϫ
2
Ϫ1
2
Ϫ1
NNO . These calculations indicated that trans-ONNO
2
NO
Ϫ1
2
Ϫ18
should be the lowest energy dimer anion structure. Its ␯5
b
Ϫ1
Ϫ1
Ne matrix, Ϯ0.2 cm ; this work.
fundamental absorption was identified at 1221.0 cm
.
Ϫ
cis-ONNO was calculated to lie approximately 12.5 kJ/mol
͑
(
3 kcal/mol͒ higher in energy than the trans isomer. Its ␯1
؊
1
Ϫ1
C. Assignment of the 1424.1 cm absorption
a ) absorption was identified at 1300.3 cm , its ␯ (b )
1
5
2
Ϫ1
2
The identity of the carrier of the 1424.1 cmϪ1 absorption
remains uncertain. The isotopic substitution pattern of this
absorption is consistent with its assignment to a coupled NO-
stretching vibration of two symmetrically equivalent NO
groups. The only unassigned uncharged or ionic NO-dimeric
species which meets this criterion would be cis or linear
ONNO . This absorption did not appear in the recent argon-
matrix study, in which molecular anions were stabilized but
the cations were predominantly those formed from metal at-
oms and small metal clusters. The photobleaching of the
absorption at 1222.7 cm , and its ␯6 (b ) absorption at
8
absorptions, ␯ was calculated and observed to be the most
prominent. A very weak absorption at 2492.0 cm was as-
signed to the ␯ ϩ␯ combination band. The NNO isomer
was calculated to lie approximately 25 kJ/mol ͑6 kcal/mol͒
higher in energy than trans-ONNO . The calculations of the
positions of its stretching vibrational fundamentals yielded
values which were in good agreement with those previously
Ϫ1
84.4 cm for the molecule isolated in solid argon. Of these
5
Ϫ1
Ϫ
2
1
5
Ϫ
ϩ
7
3
,13,14
reported.
Ϫ1
The isotopic frequency pattern of the 1227.5 cmϪ1 ab-
1
424.1 cm absorption in the near infrared and visible spec-
tral regions and its reappearance and intensification after a
brief period of irradiation through the 420 nm cutoff filter
could be explained by photoisomerization or by the deacti-
vation and reactivation of a metastable species. The 1619.2
sorption observed in the present neon-matrix study tracks
Ϫ1
that of the 1221.0 cm
argon-matrix absorption very
closely. In contrast, however, absorptions which can be as-
Ϫ
signed to the higher energy N O2 isomers were not detected
2
Ϫ1
cm absorption is so prominent that a change in its intensity
in this study. A least-squares force constant adjustment was
also conducted for the in-plane vibrational fundamentals
resulting from a small amount of photoisomerization or
metastable excitation might not be detected. Preliminary
density functional calculations by Bondybey and
Ϫ
of trans-ONNO . The structure calculated by Snis and
6
Panas was assumed. A four-constant valence force potential
4
ϩ
co-workers for the cis- and trans-ONNO structures sug-
Ϫ1
was used in the fit. Values of 1300 and 800 cm , close
Ϫ1
gested that the cis isomer might lie as little as 160 cm
7
to the values obtained in ab initio calculations, were as-
above the trans isomer. For such a small energy difference, it
sumed for the infrared-inactive symmetric NO-stretching
would not be surprising for a detectable amount of the cis
(
␯ ) and NN-stretching (␯ ) fundamentals, respectively, of
ϩ
4
1
2
isomer to be stabilized. Both trans- and rectangular O , also
1
6
14 14 16
Ϫ
trans- O N N O . The results of the force constant ad-
justment calculations are summarized in Table V. Further
force constant adjustment calculations which included values
for the two in-plane bending fundamentals obtained in the ab
initio calculations did not yield a satisfactory fit. Experience
with related molecules suggests that a stretch-bending force
constant would need to be added in order to fit these two
lower frequency fundamentals. Since no experimental data
for them were available, attempts to achieve a fit for a five-
constant valence force potential were not made. Considering
these approximations, the average frequency error for the
almost isoenergetic, were stabilized in experiments analo-
20,21
gous to these on the Ne:O system.
Preliminary complete
2
active space self-consistent field ͑CASSCF͒ calculations by
the Bondybey group suggested a very flat potential, with
ϩ
almost free internal rotation, for ONNO . The symmetric
ϩ
NO-stretching fundamental of cis-ONNO would be infra-
Ϫ1
red active, but quite possibly it lies in the 1700–1850 cm
spectral region, where it would be obscured by very strong
absorption of cis-(NO) , as well as by rather prominent,
2
complicated absorption patterns of isotopically substituted
N O .
2
3
Ϫ1
calculation summarized in Table V, 0.9 cm , appears rea-
sonable.
Force constant adjustment calculations were also at-
Ϫ1
tempted for the 1424.1 cm absorption, assumed to be con-
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10355
Ϫ
tributed by ␯ , the antisymmetric NO-stretching fundamen-
tal, of cis-ONNO . All four force constants were adjusted to
5
spectra of (NO)3 isomers and related compounds is to be the
subject of a separate publication,2 as will be a study of the
application of calculations to predicting the properties of
ϩ
9
provide the best fit to the 10 frequencies of the various iso-
Ϫ1
Ϫ
30
topic counterparts of the 1424.1 cm absorption. A set of
transition metal complexes with (NO) .
These new density functional calculations indicate that
the most stable configuration of (NO)3 is an all-cis planar
structure in which the three nitrogen atoms are linked. The
two end NO units are symmetrically equivalent, resulting in
C2v symmetry for the ground-state anion. While this article
was being written, the results of MP2/6-31ϩG* calcula-
tions of the structures and energetics for two chain, three
3
force constants which can reproduce the observed isotopic
Ϫ1
Ϫ
pattern with an average frequency error of 0.6 cm was
found for assumed valence angles of 120°, 135°, 150°, and
Ϫ1
1
80°. The average frequency error rose to 0.7 cm in the
calculations for an angle of 105°.
While the factors discussed here are consistent with the
Ϫ1
assignment of the 1424.1 cm
absorption to ␯5 of
ϩ
cis-ONNO , this assignment cannot be regarded as defini-
Ϫ
N-bonded ͑branched͒, and one ring structures of (NO)3
tive.
23
were reported. Single-point energies were also obtained for
those structures at the MP4 level. In agreement with the
present density functional calculations, the all-cis N-bonded
structure was found to be the most stable. The all-trans chain
structure, at approximately 0.3 eV, was next higher in en-
ergy. The remaining structures fell within 1 eV of the lowest
energy structure, except that the ring structure was 5.55 eV
higher in energy.
؊
3
D. Infrared spectrum of „NO…
There have been relatively few experimental studies of
the NO cluster anions. Carman,22 who reviewed the earlier
work, studied low-energy electron attachment to NO clusters
using rubidium atoms in high Rydberg states to produce
electrons with energy between 9 and 80 meV. (NO)n clus-
ters with n odd were found to be more stable than those with
n even, a phenomenon attributed to spin pairing of the va-
Ϫ
The calculated positions and intensities of the vibrational
fundamentals of the all-cis structure obtained in the present
study are in reasonable agreement with the observations. The
Ϫ
Ϫ1
lence electrons. The mass spectral signal for (NO)3 was
ratio of 1369.9 to 1448.0 cm , the calculated position of the
very prominent. Above nϭ20. Carman found that only the
clusters with n odd appeared. He suggested that the electron
most strongly absorbing fundamental, is 0.9461. This value
is close to the expected scaling factor for vibrational funda-
mentals in density functional calculations. The second most
Ϫ
distribution in the larger clusters corresponds to (NO)3 sol-
Ϫ
Ϫ1
vated by (NO) units. The photoelectron spectra of (NO)n
intense fundamental is calculated to lie at 679.7 cm , close
2
Ϫ1
clusters were very recently reported by Tsukuda and
to the observed absorption at 693.8 cm . Next in calculated
2
3
Ϫ1
co-workers. The vertical detachment energy increased by
intensity is the fundamental at 1400.8 cm . If a scaling
Ϫ
Ϫ
3
0
.8 eV when going from (NO)2 to (NO) . The increment
factor of 0.9461 is also used for this fundamental, the ob-
Ϫ1
associated with the addition of more NO units to the cluster
anion decreased to approximately 0.1 eV. These workers
served absorption is predicted to lie near 1325 cm . In one
experiment, there was a very weak absorption with appropri-
Ϫ1
concluded that the larger NO cluster anions, with a total of n
ate photodestruction behavior at 1327.3 cm . Of similar in-
Ϫ
3
Ϫ1
NO units, have N O ...͑NO)
structures.
tensity is a peak calculated to lie at 1036.1 cm , in reason-
3
nϪ3
As already noted, the complexity of the isotopic substi-
tution patterns of the infrared absorptions near 1370, 1002,
and 694 cm , which exhibit the same photodestruction be-
able correspondence with the observed absorption at 1001.9
Ϫ1
cm . The remaining two mid-infrared fundamentals, calcu-
Ϫ1
Ϫ1
lated to lie near 1542 and 958 cm , have low intensities,
havior, requires the presence of at least three NO units in the
species which contributes them. Under the sampling condi-
tions typical of these experiments, the probability of stabiliz-
ing trimers and larger aggregates is generally small. How-
and are likely to be weak and undetectable. The positions of
the mid-infrared fundamentals for the 11 isotopomers which
14
16
15 16
15 16
15 18
can result from N Oϩ N O and N Oϩ N O mix-
tures, together with those for the 14N O trimer, are summa-
rized in Table VI. Scaled frequencies were derived using the
ratio of the observed frequency to the calculated frequency
for the unenriched molecule to determine the scaling factor.
The scaled frequencies are subtracted from the correspond-
ing observed frequencies to give a measure of the extent of
agreement of the calculated isotopic shifts with the observed
spectra. The agreement is excellent, providing strong support
18
ϩ
ever, two infrared absorptions of the O ...O complex were
2
4
2
4
identified in recent experiments on Ar:O ϭ100 samples
2
codeposited with discharged neon under conditions similar to
those used for the present experiments. This result suggests
Ϫ
that a detectable concentration of (NO)3 might be stabilized
in the Ne:NO system.
In order to obtain further information regarding the
Ϫ
3
Ϫ1
structure of (NO) , computational modeling of the isotopic
for the assignment of the 1369.9, 1001.9, and 693.8 cm
substitution pattern for this species was conducted. Earlier
absorptions to three vibrational fundamentals of this C2v
comparative studies6 of the NO oligomer anions by explic-
,25
structure of (NO) .
Ϫ
3
itly correlated wave function based methods and by B3LYP
Similar density functional calculations were also per-
formed for isotopic species of two different structures that lie
somewhat higher in energy. The first structure, which is all
trans, has an approximately planar ONNO group and an ap-
proximately planar NONO group, with a dihedral angle of
70.4° between the two planes. The magnitude of this dihedral
angle suggests approximate orthogonality between the vibra-
density functional2
6,27
methods gave good agreement be-
tween the two approaches. The B3LYP density functional
and the 6-311ϩG(d) basis set, provided in the GAUSSIAN’94
program package,28 were used exclusively for the calcula-
tions reported here. A combined density functional and ab
initio study addressing structures, stabilities, and electronic
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10356
J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
TABLE VI. Calculated ͓B3LYP/6Ϫ311ϩG(d)͔ and observed ͑Ne matrix, Ϯ0.2 cmϪ1; this work͒ mid-infrared
Ϫ1
Ϫ
vibrational fundamentals ͑cm ͒ of isotopomers of cis,cis-(NO)3
.
Isotopic sequence
␯1
␯2
␯3
␯4
␯5
␯6
͑14–16͒͑14–16͒͑14–16͒
͑14–16͒͑15–16͒͑14–16͒
͑14–16͒͑14–16͒͑15–16͒
͑14–16͒͑15–16͒͑15–16͒
͑15–16͒͑14–16͒͑15–16͒
͑15–16͒͑15–16͒͑15–16͒
Calc.
Scaled
Obs.
1542.4
1448.0
͑1369.9͒
1369.9
¯
1400.8
1325.3
1327.3?
2.0
1369.2
1295.4
1036.1
͑1001.9͒
1001.9
¯
958.0
679.7
͑693.8͒
693.8
¯
a
b
⌬
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
1542.3
1529.2
1529.1
1512.2
1512.2
1446.2
1368.2
1367.5
Ϫ0.7
1011.4
978.0
979.8
1.8
957.6
949.4
948.9
940.4
939.9
677.5
691.5
1434.7
1357.4
1358.0
0.6
1432.6
1355.4
1355.1
Ϫ0.3
1424.9
1348.1
1349.6
1.5
1422.6
1345.9
1346.6
0.7
1399.3
1323.9
1033.3
999.2
998.5
Ϫ0.7
1008.7
975.4
976.7
1.3
1030.4
996.4
995.4
Ϫ1.0
1005.8
972.6
973.6
1.0
670.0
683.9
684.6
0.7
667.9
681.7
683.0
1.3
660.8
674.5
674.6
0.1
658.7
672.3
674.6
2.3
1367.7
1294.0
1398.0
1322.6
Calc.
Scaled
Obs.
⌬
1366.4
1292.8
͑
͑
14–18͒͑14–18͒͑14–18͒
15–16͒͑15–18͒͑15–16͒
Calc.
Scaled
Calc.
Scaled
Obs.
⌬
1506.9
1511.4
1409.3
1333.3
1422.6
1345.9
1346.6
0.7
1379.9
1305.5
1345.4
1272.9
1028.2
994.3
1003.4
970.3
971.5
1.2
933.4
921.9
676.2
690.2
657.8
671.4
672.8
1.4
͑
͑
͑
͑
15–16͒͑15–16͒͑15–18͒
15–16͒͑15–18͒͑15–18͒
15–18͒͑15–16͒͑15–18͒
15–18͒͑15–18͒͑15–18͒
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
Calc.
Scaled
Obs.
⌬
1497.9
1497.1
1476.2
1475.4
1399.2
1323.8
1325.0
1.2
1399.2
1323.8
1325.0
1.2
1383.0
1308.4
1310.0
1.6
1383.0
1308.4
1310.0
1.6
1366.2
1292.6
1003.2
970.1
971.0
0.9
1000.7
967.7
968.9
1.2
1000.3
967.3
968.3
1.0
997.8
964.9
966.2
1.3
936.5
918.6
933.5
915.6
657.6
671.2
672.8
1.6
656.6
670.2
671.2
1.0
656.4
670.0
671.2
1.2
655.3
668.9
669.8
0.9
1345.0
1272.5
1365.9
1292.3
1344.7
1272.2
a
Scaling factors for ␯2 and ␯ , 0.9461; for ␯ , 0.9670; for ␯ , 1.0207.
3
4
6
b
⌬ϭObs.—scaled.
tions of the ONNO unit and that of the third NO unit. How-
ever, the free NO bond length of the ONNO unit is more
than 0.2 Å shorter than that of the complexed NO. This
elongated central NO would be consistent with the ready
decomposition of the complex into energetically more stable
intensity of 29.9 km/mol would have been weak and unde-
tectable. Despite this good agreement, the calculated isotopic
substitution pattern was inconsistent with the observed one.
Four well-resolved, equally intense absorptions were calcu-
lated to be associated with the antisymmetric stretching fun-
Ϫ
Ϫ1
N OϩNO . The calculated frequencies and intensities for
damental ͑1369.9 cm ͒ for the 1:1 isotopic mixtures. There
2
2
three of the mid-infrared vibrational fundamentals of this
are large disparities between the calculated and observed iso-
topic frequency shifts for all three of the observed fundamen-
tals. The second structure has a planar trans-ONNO group
Ϫ1
Ϫ1
structure, 1373.0 cm ͑250 km/mol͒, 987.9 cm ͑219 km/
Ϫ1
Ϫ
mol͒, and 693.5 cm ͑163 km/mol͒, are in close agreement
with observed absorptions. Two other calculated absorptions,
linked by a very long bond ͑2.734 Å to the N atom of an
out-of-plane NO group. The most intense fundamental of this
species is the stretching mode of the unique NO moiety,
Ϫ1
Ϫ1
at 1614.3 cm ͑378 km/mol͒ and 1215.8 cm ͑346 km/
mol͒, could have been obscured by other product absorp-
Ϫ1
Ϫ1
tions. A sixth absorption at 814.3 cm with a calculated
calculated to lie at 1697 cm . With appropriate scaling, this
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J. Chem. Phys., Vol. 110, No. 21, 1 June 1999
Lugez et al.
10357
Ϫ1
absorption might lie near the very strong 1619.2 cm
mately 5 K with a beam of discharged neon atoms confirms
ϩ
ϩ
trans-ONNO absorption. The next highest frequency fun-
damental, half as intense, lies at 1483 cm . The ratio of
the trans ground-state configuration for ONNO . The isoto-
Ϫ1
Ϫ1
pic substitution pattern for the 1227.5 cm product absorp-
1
424.1 to this value is 0.960, an appropriate magnitude for a
tion is parallel to that recently reported for an argon-matrix
Ϫ1
scaling factor. However, the calculated isotopic substitution
behavior for the 1:1 mixtures remains appropriate for sym-
metrically inequivalent NO units in the trans-ONNO
group. These results emphasize the importance of securing a
good match not only for the ordinary molecule but also for
several isotopically substituted species when ab initio or
density functional calculations are used for identification and
structural determination.
absorption at 1221.0 cm
trans-ONNO , produced by the codeposition of laser-
which has been assigned to
Ϫ
Ϫ
ablated metal atoms with an Ar:NO mixture. Although ab-
Ϫ
Ϫ
2
sorptions of cis-ONNO and of NNO were also detected in
the laser ablation experiments, they were not observed in the
Ϫ1
present neon-matrix study. An absorption at 1424.1 cm
which appears in the neon-matrix experiments but not in the
laser ablation studies is contributed by a vibration of two
symmetrically equivalent NO groups. The carrier of this ab-
sorption is readily destroyed by exposure of the deposit to
near infrared or visible radiation but can be regenerated by
brief exposure of the deposit to mercury-arc radiation passed
by a 420 nm cutoff filter. This behavior suggests a photoi-
somerization process. There are insufficient data for defini-
؊1
E. Carrier of the 2243.9 cm absorption
Evidence has already been presented for the formation of
a complex one member of which is N O upon photodestruc-
tion of the carrier of the 1369.9 cm absorption, now iden-
tified as cis,cis-(NO) . The most probable other member of
this complex is NO ; the decomposition of cis,cis-(NO)
2
Ϫ1
Ϫ
3
tive identification of the carrier of this absorption. Three in-
Ϫ
Ϫ
Ϫ
3
2
3
frared absorptions are assigned to cis,cis-(NO) . Density
Ϫ
into N OϩNO is exothermic. Photodestruction of the
2
2
functional calculations of the isotopic substitution pattern for
the vibrational fundamentals of this species confirm that the
lowest energy isomer, a nitrogen-bonded C2v structure, is
Ϫ1
2243.9 cm absorption occurs at photon energies greater
than 3.6 eV ͑␭Ͻ345 nm͒, consistent with the conditions un-
der which an electron photodetaches from NO2 trapped in a
neon matrix. The initial deposit shows a relatively broad
absorption near 2243.9 cm . There are also absorptions of
Ϫ
Ϫ
appropriate. Photodestruction of cis,cis-(NO)3 leads to the
5
Ϫ
2
stabilization of the N O••NO complex trapped in solid
Ϫ1
2
neon.
Ϫ
isolated and perturbed NO2 in the initial sample deposit.
These absorptions are so prominent that it was not possible
ACKNOWLEDGMENTS
to study changes in their intensities associated with the pho-
Ϫ
todestruction of cis,cis-(NO) . This suggests that an appre-
Two of the authors ͑A. S. and I. P.͒ wish to acknowledge
support of their work by the Swedish Natural Science Re-
search Council.
3
Ϫ
ciable fraction of the (NO)3 fragments before being depos-
ited on the cryogenic surface, possibly as a result of
formation in a low-lying excited electronic state.
Because NO and (NO) are, by preparation, the major
molecular species in the deposit, neutral and charged com-
1
2
M. E. Jacox and W. E. Thompson, J. Chem. Phys. 93, 7609 ͑1990͒.
M. E. Jacox and D. E. Milligan, J. Mol. Spectrosc. 48, 536 ͑1973͒.
J. Hacaloglu, S. Suzer, and L. Andrews, J. Phys. Chem. 94, 1759 ͑1990͒.
A. Strobel, N. Knoblauch, J. Agreiter, A. M. Smith, G. Niedner-
Schatteburg, and V. E. Bondybey, J. Phys. Chem. 99, 872 ͑1995͒.
D. Forney, W. E. Thompson, and M. E. Jacox, J. Chem. Phys. 99, 7393
͑1993͒.
A. Snis and I. Panas, Chem. Phys. 221, 1 ͑1997͒.
L. Andrews, M. Zhou, S. P. Willson, G. P. Kushto, A. Snis, and I. Panas,
J. Chem. Phys. 109, 177 ͑1998͒.
Certain commercial instruments and materials are identified in this article
in order to specify adequately the experimental procedure. In no case does
such identification imply recommendation or endorsement by the National
Institute of Standards and Technology, nor does it imply that the instru-
ments or materials identified are necessarily the best available for the
purpose.
2
3
plexes of N O with one or both of these species may also
2
4
contribute to the observed spectrum. Calculations by Hiraoka
3
1
and co-workers suggest that there should be two isomers of
5
Ϫ
N O••NO , the more stable of which possesses a w-shaped,
2
6
nitrogen-bonded structure which is closely related to that of
7
Ϫ
3
cis,cis-(NO) . Recent coincident photoelectron and photo-
fragment translational spectroscopy studies on a fast ion
8
3
2
beam found evidence of both of these isomers. In the
Ϫ1
present experiments, the proximity of the 2243.9 cm ab-
sorption to that of ␯ of isolated N O would favor the rela-
3
2
Ϫ
tively weakly bonded N O••NO structure as an alternate
2
Ϫ
9
assignment. However, photodetachment of NO would be
M. E. Jacox and W. E. Thompson, J. Chem. Phys. 91, 1410 ͑1989͒.
D. Forney, W. E. Thompson, and M. E. Jacox, J. Chem. Phys. 97, 1664
Ϫ
2
10
expected at a much lower photon energy than that of NO .
͑
1992͒.
Uncharged complexes involving NO and (NO)2 would be
expected to be photolytically stable under conditions in
11
12
13
M. E. Jacox and W. B. Olson, J. Chem. Phys. 86, 3134 ͑1987͒.
R. A. Toth, J. Opt. Soc. Am. B 8, 2236 ͑1991͒; 10, 2006 ͑1993͒.
D. E. Milligan and M. E. Jacox, J. Chem. Phys. 55, 3404 ͑1971͒.
M. E. Jacox, J. Chem. Phys. 93, 7622 ͑1990͒.
Ϫ1
which photodestruction of the carrier of the 2243.9 cm
1
1
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Ϫ1
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V. CONCLUSIONS
18
The appearance of the ␯ absorption of four additional
1
1
2
9
0
ϩ
asymmetrically substituted trans-ONNO species, but not
for any of the symmetrically substituted isotopomers, when
isotopic mixtures of Ne:NO are codeposited at approxi-
21
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