Infrared spectrum of the hyponitrite dianion, N2O22-, isolated and insulated from stabilizing metal cations in solid argon

Article abstract of DOI:10.1021/ja0029331

Ultraviolet irradiation of a rigid 7 K argon matrix containing alkali or alkaline earth metal atoms and (NO)₂ isolated from each other by one or two layers of argon forms N₂O₂^2- dianions insulated from two M⁺ cations by argon atoms, and visible photolysis reverses this electron-transfer process likely involving the N₂O₂^- anion intermediate. The isolated N₂O₂^2- dianion is identified from isotopic substitution and isotopic mixtures, which show that the new 1028.5 cm^-1 metal independent absorption involves two equivalent NO subunits. DFT calculations predict a strong 1078.1 cm^-1 fundamental for the Li(NO)₂Li molecule and isotopic frequency ratios in excellent agreement with the observed values, which provides a model for the matrix dianion system. The spectrum of solid Na₂N₂-O₂, exhibits a 1030 cm^-1 infrared band, which strongly supports the present N₂O₂^2- dianion assignment. The electrostatic stabilization of N₂O₂^2-, which is probably unstable in the gas phase, is made possible by metal cations separated by one or two insulating layers of argon in the rigid 7 K matrix.

Full text of DOI:10.1021/ja0029331

J. Am. Chem. Soc. 2001, 123, 1997-2002
1997
2-
Infrared Spectrum of the Hyponitrite Dianion, N2O2 , Isolated and
Insulated from Stabilizing Metal Cations in Solid Argon
Lester Andrews* and Binyong Liang
Contribution from the UniVersity of Virginia, Department of Chemistry,
CharlottesVille, Virginia 22904-4319
ReceiVed August 7, 2000
Abstract: Ultraviolet irradiation of a rigid 7 K argon matrix containing alkali or alkaline earth metal atoms
and (NO)2 isolated from each other by one or two layers of argon forms N2O22- dianions insulated from two
M+ cations by argon atoms, and visible photolysis reverses this electron-transfer process likely involving the
-
2-
N2O2 anion intermediate. The isolated N2O2 dianion is identified from isotopic substitution and isotopic
mixtures, which show that the new 1028.5 cm-1 metal independent absorption involves two equivalent NO
subunits. DFT calculations predict a strong 1078.1 cm-1 fundamental for the Li(NO)2Li molecule and isotopic
frequency ratios in excellent agreement with the observed values, which provides a model for the matrix
dianion system. The spectrum of solid Na2N2O2 exhibits a 1030 cm-1 infrared band, which strongly supports
2-
the present N2O2 dianion assignment. The electrostatic stabilization of N2O22-, which is probably unstable
in the gas phase, is made possible by metal cations separated by one or two insulating layers of argon in the
rigid 7 K matrix.
Introduction
isolated from perturbations of the adjacent cations that are
required to stabilize it?
Dianions are ubiquitous in crystalline solids, and common
examples in mineralogy include oxide (O2-), carbonate (CO32-),
and sulfate (SO42-).1 In the solid phase the counterions necessary
to stabilize dianions are in adjacent sites in the crystal lattice.
However, there is no firm evidence for atomic and diatomic
dianions in the gas phase, but larger molecular dianions have
been observed.2,3 Although atoms and most small molecules
have an affinity for electrons and capture one electron in an
exothermic process, a second electron is repulsive and the
acquisition of a second electron is an endothermic process.2,4
Even CO32- and SO42- are not large enough to be stable in the
gas phase because of the strong Coulomb repulsion between
the two excess electrons.2,5,6 However, larger dianions are often
stable and examples recently observed by photodetachment
photoelectron spectroscopy in the gas phase include dicarboxyl-
ate, PtX42-, and the heavy transition metal MX62- dianions (X
) Cl, Br).3,7-10 Nevertheless, stable dianions are often short-
The matrix isolation technique has been employed to study
molecular anions both as isolated anions surrounded by at least
one layer of matrix atoms and as ion pairs where cation and
anion are electrostatically bound together. The latter includes
M+O2- alkali superoxide, M+O3- alkali ozonide, and M+NO2
-
ion-pair molecules12-14 prepared in atom-molecule reactions,
-
-
and the former includes isolated O3 and NO2 made by the
capture of electrons from a photoionization process.15,16 Anion
spectra in the M+O2-, M+O3-, and M+NO2 molecules show
-
a clear dependence on the geminate metal cation; however, the
isolated anion spectra are independent of the electron source.
-
-
In fact both M+NO2 and isolated NO2 have been formed in
2-
the same sample.14 Several small dianions, including O2
,
NO2-, carbonate, and sulfite, have been investigated by matrix
isolation spectroscopy.12,17-19 The Li+(O22-) Li+ and Li+(NO2-)-
Li+ ion-trio species have been identified in the matrix systems
from isotopic shifts.12,17 Subsequent DFT calculations in this
2-
lived: the recently observed MF4 (M ) Be, Mg) dianions
laboratory have confirmed these bonding descriptions.20
A
have lifetimes in the 0.1 ms range.11 How then can we prolong
the lifetime and investigate the properties of a small dianion
carbonate moiety has been prepared by the reaction of thallium
suboxide (Tl2O) and carbon dioxide (CO2) in the condensing
matrix gas as the molecule (Tl+)(CO32-)(Tl+), which is trapped
in the solid matrix. The carbonate dianion spectrum reveals
perturbations by the adjacent thallous cations.18
(1) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley: New York, 1999.
(2) Scheller, M. K.; Compton, R. N.; Cederbaum, L. S. Science 1995,
270, 1160.
(3) Wang, L. S.; Wang, X. B. J. Phys. Chem. A 2000, 104, 1978 and
references therein.
(4) Hotop, H.; Lineberger, W. C. J. Phys. Chem. Ref. Data 1985, 14,
731.
(5) Boldyrev, A. I.; Simons, J. J. Phys. Chem. 1994, 98, 2298.
(6) Boldyrev, A. I.; Gutowski, M.; Simons, J. Acc. Chem. Res. 1996,
29, 497.
(7) Wang, X. B.; Wang, L. S. J. Chem. Phys. 1999, 111, 4497.
(8) Ding, C. F.; Wang, X. B.; Wang, L. S. J. Phys. Chem. A 1998, 102,
8633.
(12) Andrews, L. J. Am. Chem. Soc. 1968, 90, 7368. Andrews, L. J.
Chem. Phys. 1969, 50, 4288.
(13) Spiker, R. C., Jr.; Andrews, L. J. Chem. Phys. 1973, 59, 1851.
(14) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971, 55, 3404.
(15) Milligan, D. E.; Jacox, M. E.; Guillory, W. A. J. Chem. Phys. 1970,
52, 3864.
(16) Andrews, L.; Ault, B. S.; Grzybowski, J. M.; Allen, R. O. J. Chem.
Phys. 1975, 62, 2461.
(17) Tevault, D. E.; Andrews, L. J. Phys. Chem. 1973, 77, 1640 (Li +
NO).
(9) Wang, X. B.; Wang, L. S. J. Am. Chem. Soc. 2000, 122, 2339.
(10) Skurski, P.; Simons, J.; Wang, X. B.; Wang, L. S. J. Am. Chem.
Soc. 2000, 122, 4499.
(18) David, S. J.; Ault, B. S. J. Phys. Chem. 1982, 86, 4618.
(19) David, S. J.; Ault, B. S. Inorg. Chem. 1984, 23, 1211.
(20) Andrews, L.; Zhou, M. F.; Wang, X. J. Phys. Chem. A 2000, 104,
8475 (Tl + NO).
(11) Middleton, R.; Klein, J. Phys. ReV. A 1999, 60, 3515.
10.1021/ja0029331 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/10/2001

1998 J. Am. Chem. Soc., Vol. 123, No. 9, 2001
Andrews and Liang
Alkali and other metal hyponitrites are known in the solid
phase, and evidence has been presented for both cis- and trans-
hyponitrite anion structures; however, these compounds are
unstable.21-24 A hyponitrite species is suggested to form on solid
MgO based on infrared spectra following the adsorption of
NO.25 In laser-ablation experiments with Mg, Ca, and NO, a
sharp, weak 1028.5 cm-1 absorption appeared on ultraviolet
photolysis.26 The isotopic shifts and splittings for this absorption
and its agreement with the spectrum of sodium hyponitrite21-23
suggested a hyponitrite species.
To prepare a small isolated dianion, a rigid matrix host cage
and the electrostatic stabilization of counterions separated by
one or two matrix gas layers from the dianion guest are needed.
This requires the isolation of separate metal atoms and electron
receptor guests followed by appropriate radiation to transfer
valence electrons. Here we describe such experiments to prepare
2-
the isolated hyponitrite dianion N2O2 for the first time.
Figure 1. Infrared spectra in the 1420-1000 cm-1 region for laser-
ablated lithium co-deposited with 0.4% NO in argon at 7 K: (a)
spectrum of sample deposited for 70 min, 256 scans, (b) spectrum after
λ > 240 nm photolysis for 15 min, 192 scans, (c) spectrum after 240-
380 nm photolysis for 15 min, 128 scans, (d) spectrum after annealing
to 25 K, 128 scans, (e) spectrum after annealing to 35 K, 128 scans,
(f) spectrum after 240-380 nm photolysis, 256 scans, and (g) spectrum
after 40 K annealing, 256 scans.
Experimental and Theoretical Methods
The laser-ablation matrix isolation apparatus and experiment have
been described in previous reports.27-29 Metal targets (Li, Na, Mg, Ca,
Ba) positioned 2 cm from the cold window were ablated by focused
1064 nm radiation from a pulsed YAG laser. Nitric oxide (Matheson)
samples were prepared after fractional distillation from a coldfinger;
15NO (MSD Isotopes, 99% 15N) was treated similarly; 15N18O (Isotec,
99.9% 15N, 98.5% 18O) was used as received. A Nicolet 550 FTIR
instrument operating at 0.5 cm-1 resolution (frequency accuracy (0.1
cm-1) with a liquid nitrogen cooled HgCdTe detector was used to record
spectra. The instrument was purged continuously with a Balston 75-
20 compressed air-dryer. Matrix samples were co-deposited on a 7 K
CsI window, irradiated by filtered light from a medium-pressure
mercury lamp (Philips, 175 W) with the globe removed or a tungsten
lamp (Wiko, 90 W), and more spectra were collected.
Density functional theory (DFT) calculations were performed on
potential product molecules by using the Gaussian 94 program system.30
Most calculations employed the hybrid B3LYP functional but com-
parisons were done with the BPW91 functional as well.31,32 The
6-311+G* basis set was used for N, O, and metal atoms.33,34 Geometries
were fully optimized and the vibrational frequencies computed by using
analytical second derivatives.
Alkali Metals. Figure 1 shows the 1420-1000 cm-1 region
of the spectrum from a sample prepared by co-depositing laser-
ablated lithium and 0.4% NO in argon at 7 K. The strong 1351.0
cm-1 band has been assigned to Li(NO) in three previous
investigations by using thermal lithium atoms.14,17,35 The sharp
1589.3, 1243.7, and 1221.0 cm-1 and weak 1300.3, 1221.7, and
1205.0 cm-1 absorptions observed in a large number of laser-
-
ablation experiments with NO are due to isolated (NO)2+, NO2
,
trans-(NO)2-, cis-(NO)2-, and NNO2-, respectively.14,36 The
-
relative absorbance of trans-(NO)2 is 4-6 times greater than
that for (NO)2+ with Li and Na whereas with transition metals
+
the (NO)2 absorption is substantially stronger (10 times for
Ni).36,37 Not shown are strong 795.3 and 650.7 cm-1 bands due
to Li+(NO2-)Li+ and Li(NO), respectively,17 and a weak NO2
band at 1610.8 cm-1. Irradiation with the full light of a medium-
pressure mercury arc lamp (λ > 240 nm) produced a sharp weak
1028.5 cm-1 absorption (Figure 1b) and decreased the 1221.0
cm-1 absorption of trans-(NO)2-. It was noticed that the 1028.5
cm-1 peak intensity decreased during the recording of the scans,
which suggested that visible light from the infrared source might
destroy the species responsible for the new 1028.5 cm-1
absorption. Further irradiation employed a 240-380 nm trans-
mitting black filter and the 1028.5 cm-1 absorption was stronger
(Figure 1c). Annealing to 25 K markedly decreased the 1028.5
cm-1 absorption and increased the 1351.0 cm-1 band, and
annealing to 35 K virtually destroyed the 1028.5 cm-1 band
and produced new 1404.1 and 1392.6 cm-1 features. Another
240-380 nm irradiation (Figure 1f) brought back some of the
1028.5 cm-1 absorption, but final annealing to 40 K destroyed
it (Figure 1g).
Results and Discussion
The new absorptions observed here will be identified by
isotopic substitution and comparison with DFT calculated
isotopic frequencies.
(21) Kuhn, L.; Lippincott, E. R. J. Am. Chem. Soc. 1956, 78, 1820.
(22) Miller, D. J.; Polydropoulos, C. N.; Watson, D. J. Chem. Soc. 1960,
687.
(23) McGraw, G. E.; Bernitt, D. L.; Hisatsune, I. C. Spectrochim. Acta
1967, 23A, 25.
(24) Laane, J.; Ohlsen, J. R. Prog. Inorg. Chem. 1980, 27, 465.
(25) Cerruti, L.; Modone, E.; Guglielminotti, E.; Borello, E. J. Chem.
Soc., Faraday Trans. 1 1974, 70, 729.
(26) Kushto, G. P.; Ding, F.; Liang, B.; Wang, X.; Citra, A.; Andrews,
L. Chem. Phys. 2000, 257, 223.
(27) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
(28) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177.
(29) Zhou, M. F.; Andrews, L. J. Am. Chem. Soc. 1998. 120, 13230.
(30) Gaussian 94, ReVision B.1; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople J. A.,
Gaussian, Inc.: Pittsburgh, PA, 1995.
(31) Lee, C.; Yang, E.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(32) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. Becke, A. D. J. Chem.
Phys. 1993, 98, 5648.
(33) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639.
(34) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650.
Figure 2 illustrates the 1410-1000 cm-1 region of the
spectrum of a matrix containing laser-ablated sodium and 0.4%
NO. The strong 1358.3 cm-1 band is due to Na(NO).14,38 The
same NO2 and (NO)2 bands are observed.14,36 Full-arc
-
-
(35) Andrews, W. L. S.; Pimentel, G. C. J. Chem. Phys. 1966, 44, 2361.
(36) Andrews, L.; Zhou, M. F.; Willson, S. P.; Kushto, G. P.; Snis, A.;
Panas, I. J. Chem. Phys. 1998, 109, 177.
(37) Zhou, M. F.; Andrews, L. J. Phys. Chem. A 2000, 104, 3915 (Fe,
Co, Ni + NO).
(38) Tevault, D. E.; Andrews, L. J. Phys. Chem. 1973, 77, 1646 (Na +
NO).

2-
N2O2 Insulated from Stabilizing Metal Cations in Solid Ar
J. Am. Chem. Soc., Vol. 123, No. 9, 2001 1999
Table 1. Infrared Absorptions (cm-1) Observed for Reactions of
Laser-Ablated Li and Na with NO in Excess Argon
14N16O
15N16O
15N18O
identification
+
1589.3
1404.1
1392.6
1390.6
1358.3
1351.0
1300.3
1243.7
1222.7
1221.0
1028.5
795.3
1561.9
1381.4
1520.3
1344.1
(NO)2
(Li(Ar)x(NO)2(Ar)xLi)
(Li(Ar)x(NO)2(Ar)xLi) site
(Na(Ar)x(NO)2(Ar)xNa)
Na(NO)
Li(NO)
c-(NO)2
1366.6
1327.7
1278.9
1218.3
1198.6
1199.9
1010.4
790.0
1291.6
1242.7
1191.9
1172.0
1167.4
983.7
-
-
NO2
-
c-(NO)2
-
t-(NO)2
2-
N2O2
785.0
674.1
Li(NO)Li
? broad
690.0
690.0
650.7
649.8
643.9
Li(NO)
Figure 2. Infrared spectra in the 1410-1000 cm-1 region for laser-
ablated sodium co-deposited with 0.4% NO in argon at 7 K: (a)
spectrum of sample deposited for 70 min, 256 scans, (b) spectrum after
λ > 240 nm photolysis for 15 min, 64 scans, (c) spectrum after 240-
380 nm photolysis for 15 min, 64 scans, (d) spectrum after annealing
to 35 K, 64 scans, (e) spectrum after 240-380 nm photolysis, 64 scans,
and (f) spectrum after 40 K annealing, 256 scans.
q
Table 2. Photochemical Behavior of Three Important (NO)2
Absorptions (cm-1) in the Ba + NO Experimenta
action
1776.2b
1221.0c
1028.5d
deposition
30
26
25
24
25
24
25
24
150
3.5
5.5
6.0
6.1
7.3
6.6
7.3
6.8
3.4
0.0
4.6
2.0
4.6
3.7
4.0
0.0
3.8
1.2
240 < λ < 380 nm
λ > 380 nm
240 < λ < 380 nm
λ > 290 nm
240 < λ < 380 nm
W lamp, λ > 630 nm
240 < λ < 380 nm
anneal to 35 K
a Absorbances listed in mau (au × 1000). b q ) 0. c q ) -1. d q )
-2.
is shown in Figure 3c; the 1010.4, 996.5, and 983.7 cm-1 triplet
indicates the involvement of two equivalent oxygen atoms.
Finally, the pure 15N18O isotopic sample gave a single band at
983.7 cm-1
.
The barium experiment explored different photolysis wave-
lengths. First, 240-380 nm photolysis for 20 min produced the
new sharp 1028.5 cm-1 band (0.0046 absorbance units, au),
decreased the 1776.2 cm-1 (NO)2 band from 0.030 to 0.026
-
au, and increased the 1221.0 cm-1 (NO)2 band from 0.0035
Figure 3. Infrared spectra in the 1040-960 cm-1 region for laser-
ablated metals co-deposited with isotopic nitric oxide in argon at 7 K
after ultraviolet photolysis for 15 min: (a) Na + 0.4% 14N16O, (b) Na
+ 0.2% 14N16O + 0.2% 15N16O, (c) Mg + 0.2% 15N16O + 0.2% 15N18O,
and (d) Mg + 0.3% 15N18O.
to 0.0055 au. The next λ > 380 nm photolysis decreased the
1028.5 cm-1 band to 0.0020 au and increased the 1221.0 cm-1
band to 0.0060 au. A subsequent 240-380 nm photolysis
restored the 1028.5 cm-1 band and decreased the 1776.2 cm-1
band to 0.024 au. A low-intensity tungsten lamp (λ > 630 nm,
red + near-IR) photolysis also destroyed the 1028.5 cm-1 band
and increased the 1776.2 cm-1 band to 0.025 au and the 1221.0
cm-1 band to 0.0073 au. A final 240-380 nm irradiation again
reversed the process giving 0.0038 au for the 1028.5 cm-1 band
and 0.024 au for the 1776.2 cm-1 absorption. A final annealing
to 35 K reduced the 1028.5 cm-1 band to 0.0012 au and
increased the 1776.2 cm-1 band to 0.115 au. The band
absorbances are collected in Table 2.
irradiation produced a weak 1028.5 cm-1 band and reduced the
-
trans-(NO)2 absorption, but 240-380 nm irradiation gave a
much stronger 1028.5 cm-1 absorption (Figure 2b,c) and reduced
the 1776.2 cm-1 (NO)2 band absorbance by 2%. Annealing to
35 K almost destroyed the 1028.5 cm-1 band and produced a
new 1390.6 cm-1 absorption (Figure 2d) and increased a weak
band at 1225.8 cm-1 probably due14 to Na+NO2-. Another 240-
380 nm photolysis again reproduced most of the 1028.5 cm-1
band, but final 40 K annealing destroyed it (Figure 2 e,f). The
sodium spectra with 14N16O and 14N16O + 15N16O are shown
in Figure 3 a,b; the sharp triplet mixed isotopic absorption at
1028.5, 1019.2, and 1010.4 cm-1 clearly demonstrates the
involvement of two equivalent nitrogen atoms in this vibrational
mode.
Group 13 Metals. A new 1028.5 cm-1 band (0.002 au) was
produced on deposition of a Tl + NO sample; this band
disappeared on annealing to 25 K and was almost regenerated
on λ > 240 nm photolysis.20 A weak 1028.5 cm-1 band
appeared on UV photolysis in Al + NO experiments.39
Calculations. DFT calculations with the B3LYP functional
3
were performed on A′′ Li(NO) and Na(NO), and the results
Alkaline Earth Metals. Experiments were done with Mg,
Ca, and Ba and isotopic modifications of NO. The 1028.5 cm-1
band was observed after ultraviolet photolysis (λ > 240 nm) in
all of these experiments and it disappeared on annealing to 35
K; the band was regenerated in part on subsequent UV
photolysis. The mixed 15N16O + 15N18O experiment with Mg
are given in Table 3. The calculated frequencies for the N-O
mode, 1391.4 and 1378.6 cm-1, differ by -12.8 cm-1 compared
to the +7.3 cm-1 difference between the observed 1351.0 and
(39) Andrews, L.; Zhou, M. F.; Bare, W. D. J. Phys. Chem. A 1998,
102, 5019.

2000 J. Am. Chem. Soc., Vol. 123, No. 9, 2001
Andrews and Liang
Table 3. Calculated Geometries and Isotopic Frequencies for M(NO), M(NO)M, M(NO)2, and M(NO)2M (M ) Li, Na, and Mg)a
isotopic freq, cm-1 (intensities, km/mol)b
species
Li(NO)
elec. state
3A′′
point group
geometry (Å, deg)
14N16O
15N16O
15N18O
Cs
Li-N: 1.888, Li-O: 1.813,
N-O: 1.271, NLiO: 40.1
a′
a′
a′
1391.4(97)
679.8(139)
352.9(25)
913.1(53)
827.3(276)
674.1(31)
641.7(9)
409.0(154)
173.5(171)
1326.1(67)
1270.8(253)
935.1(63)
833.3(0)
620.6(127)
512.3(19)
461.3(18)
251.5(0)
230.5(56)
1369.2(0)
1238.1(0)
1078.1(509)
733.7(0)
607.9(254)
583.2(0)
520.6(66)
1378.6(232)
385.2(44)
249.4(2)
1313.7(0)
1206.2(0)
1083.6(584)
713.4(0)
534.6(14)
346.1(0)
1367.0
677.8
348.8
899.1
822.0
673.1
634.6
406.5
172.8
1303.6
1243.0
916.7
805.4
620.4
511.6
460.2
244.9
229.3
1323.2
1205.0
1058.9
731.9
607.5
581.9
511.8
1354.3
380.6
246.6
1269.6
1174.3
1064.0
710.4
526.8
345.3
358.1
1540.3
244.1
215.8
1339.0
1177.2
1108.6
749.6
556.0
401.4
347.6
1329.0
673.9
342.5
877.3
816.2
669.8
620.9
403.4
170.8
1266.0
1217.9
894.2
805.0
610.2
504.2
440.6
241.8
223.6
1322.7
1190.8
1031.4
698.7
595.0
573.7
504.5
1316.7
378.3
238.9
1269.1
1159.6
1035.5
674.6
510.0
337.0
354.1
1496.2
242.0
211.4
1337.5
1162.8
1078.5
712.4
541.1
392.0
342.3
Li(NO)Li
2B1
C2V
N-O: 1.439,
Li-O: 1.746,
Li-N: 1.811
a1
b2
a1
b2
a1
b1
a1
b2
b2
a1
a1
b2
a1
a2
b1
ag
ag
bu
ag
bu
ag
bu
a′
c
Li(NO)2
2B1
C2V
N-N: 1.386,
N-O: 1.266,
Li-O: 1.834,
O-Li-O: 83.9°,
Li-O-N: 113.1°,
O-N-N: 114.9°
Li(NO)2Li d
1Ag
C2h
N-N′: 1.273, N-O: 1.342,
Li-O: 1.816, Li-N: 2.261,
Li-N′: 1.974, ONN′: 113.9,
NLiO: 36.4, N′NLi: 60.5e
Na(NO)
3A′′
Cs
Na-N: 2.233, Na-O: 2.201,
N-O: 1.269, NNaO: 33.3
a′
a′
Na(NO)2Na d
1Ag
C2h
N-N′: 1.277, N-O: 1.332,
Na-O: 2.179, Na-N: 2.629,
Na-N′: 2.283, ONN′: 116.1,
NNaO: 30.4, N′NNa: 60.3e
ag
ag
bu
ag
bu
ag
bu
a′
361.1(96)
1567.0(1708)
250.2(16)
217.0(16)
1384.9(0)
1209.3(0)
1128.7(194)
753.2(0)
Mg(NO)
2A′′
Cs
Mg-O: 2.173, N-O: 1.196,
MgNO: 128.8
a′
a′
Mg(NO)2Mg d
1Ag
C2h
N-N′: 1.268, N-O: 1.316,
Mg-N: 2.577, Mg-N′: 2.303,
Mg-O: 2.052, ONN′: 115.3,
NMgO: 30.4, N′NMg: 63.2e
ag
ag
bu
ag
bu
ag
bu
566.1(9)
401.9(0)
349.8(77)
a DFT calculation, B3LYP functional and 6-311+G* basis set. b Intensities only listed for the 14N16O sample. Highest seven frequencies listed
2
2
for Li(NO)2Li, and their corresponding bands for M(NO)2M (M ) Na and Mg). c The A1 state, C2V cis structure is 16.7 kcal/mol higher; the A′
state, Cs trans structure is 18.2 kcal/mol higher. d Dissociation energies for the reactions: M(NO)2M f 2M(NO) are 68.8, 68.0, and 52.4 kcal/mol
for M ) Li, Na, and Mg, respectively. e N′ refers to the nitrogen atom in the other M(NO) unit.
1358.3 cm-1 bands. Both Li-cis-(NO)2 and Li-trans-(NO)2
were calculated and the cis isomer is lower in energy by 18.2
kcal/mol presumably because of more intimate Li interaction
with more of the (NO)2 subunit in this configuration. The Li-
cis-(NO)2 species gave a strong b2 frequency at 1270.8 cm-1
.
A C2V structure for Li(NO)Li and C2h structure for Li(NO)2Li
were also converged, and the parameters are also given in Table
2. The BPW91 functional gave 1-11 cm-1 lower bu frequencies
and 0.02-0.04 Å longer bonds for the M(NO)2M species. The
3
Li(NO)2Li molecule is 68.8 kcal/mol lower than two A′′ Li-
(NO) subunits. Figure 4 illustrates the structures of the lithium
species calculated here, and Table 4 contrasts the Mulliken30
and natural bond orbital40 atomic charges and shows that these
molecules exhibit a high degree of ionic character based on the
more reliable NBO analysis.41
Figure 4. Structures of Lix(NO)y species calculated at the B3LYP/6-
311+G level (bond lengths in Å).
(40) Reed, A. J.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899.
(41) Frenking, G.; Frohlich, N. Chem. ReV. 2000, 100, 717.

2-
N2O2 Insulated from Stabilizing Metal Cations in Solid Ar
J. Am. Chem. Soc., Vol. 123, No. 9, 2001 2001
Table 4. Atomic Charges (e) in the Mx(NO)y Species (M ) Li,
Na) Calculated at the B3LYP/6-311+G* Level
but this can only be tentative without isotopic data. The
analogous Ag(NO)2Ag molecule has been observed as a major
product at 1117 cm-1 in solid argon and predicted at 1146 cm-1
by DFT calculations.43
Mullikena
natural bond orbitalb
molecule
M
N
O
M
N
O
Identification of the 1028.5 cm-1 Absorber. The 1028.5
cm-1 absorption becomes a sharp triplet at 1028.5, 1019.2, and
1010.4 cm-1 with 14N16O + 15N16O and a sharp triplet at 1010.4,
996.5, and 983.7 cm-1 with 15N16O + 15N18O (Figure 3). The
14N16O/15N16O ratio (1.0179) and 15N16O/15N18O ratio (1.0271)
are nearly the same as the ratios for diatomic NO (1.0179 and
1.0277). This clearly demonstrates the involvement of two
equivalent NO oscillators in the 1028.5 cm-1 vibrational mode.
Li(NO), 3A′′
Li(NO)Li, 2B1
Li(NO)2, 2B1
Li(NO)2Li, 1Ag
Na(NO), 3A′′
+0.44 -0.24 -0.20 +0.91 -0.31 -0.60
+0.31 -0.27 -0.35 +0.91 -0.88 -0.94
+0.50 -0.12 -0.13 +0.94 +0.09 -0.56
+0.33 -0.12 -0.21 +0.89 -0.77 -0.12
+0.67 -0.41 -0.26 +0.92 -0.33 -0.59
Na(NO)2Na, 1Ag +0.67 -0.24 -0.43 +0.85 -0.10 -0.75
a Reference 30. b Reference 40.
Recent quantum chemical calculations of cis- and trans-
The cis-(NO)2 dimer absorbs at 1776.2 cm-1 and the cis- and
trans-(NO)2- anions at 1222.7 and 1221.0 cm-1 (antisymmetric
N-O modes). The 1028.5 cm-1 band requires still weaker N-O
bonds and is in almost exact agreement with the bu fundamental
(1030 cm-1) for trans-(NO)22- in solid Na2N2O2.21-23 Our DFT
frequency calculations predict 1078.1 and 1083.6 cm-1 for the
strongest modes of the Li(NO)2Li and Na(NO)2Na molecules
and the 14N16O/15N16O and 15N16O/15N18O isotopic frequency
ratios (1.0181, 1.0184 and 1.0267, 1.0275) almost exactly match
the observed ratios. However, the 1028.5 cm-1 absorption is
clearly independent ((0.1 cm-1) of the metal employed. Hence,
2-
N2O2 dianions have suggested that the trans form is slightly
more stable;42 however, such calculations with limited atomic
orbital basis sets may not provide a reasonable description of
2-
the isolated N2O2 dianion.2,5 We attempted to calculate the
Li(Ar)(NO)2(Ar)Li molecule, but the calculation failed to con-
verge. We performed several approximate calculations with
fixed dimensions and the most promising involved a fixed Li-O
distance at 5.58 Å (sum of the Li-O distance and one argon
diameter), fixed N-O-Li angle at 90°, and constrained Ar to
lie on the line between O and Li. We found three low (<42
cm-1) imaginary frequencies and a strong bu mode at 1316 cm-1
(1318 cm-1 for the Na counterpart), which is more appropriate
for an N2O2- species. This structure is 161 kcal/mol higher than
2 Ar atoms and Li(NO)2Li and shows less charge transfer. Next
we calculated Li-(NO)2-Li with fixed Li-O distances (4.0
Å) and Li-O-N angles (100 and 120°), and obtained two low
imaginary frequencies and bu modes of 1332 ( 1 cm-1; these
structures with more remote Li atoms are 160 ( 1 kcal/mol
higher than the optimized Li(NO)2Li species. Finally, we
reduced the Li basis to 6G (1s function) with 6-31G* on N, O
and fixed the Li-O distance (5.0 Å) and N-O-Li angle (90°);
this calculation gave five low-frequency imaginary Li motions,
a 1010 cm-1 bu frequency, and Mulliken charges of +1 on Li
(of course), -0.18 on N, and -0.82 on O.
2-
the metal cations are not mechanically coupled with the N2O2
2-
vibration, and the N2O2 dianion observed here must be
insulated by the argon matrix. How, then, does the isolated
N2O22- dianion not undergo autoionization to N2O2- and e- or
spontaneous decomposition to a pair of NO- anions? There must
be electrostatic stabilization by M+ cations separated by one or
2-
two layers of argon atoms from the N2O2 dianion observed
here.
Our attempts to model the observed (Li+)(Ar)x(N2O22-)(Ar)x-
(Li+) species have been only partially successful. Although the
Li(NO)2Li and Na(NO)2Na calculations predict strong bu
infrared absorptions at 1078.1 and 1083.6 cm-1, respectively,
which scale appropriately44 (by 0.95) to the observed 1028.5
cm-1 frequency, there is too much calculated metal dependence.
Accordingly, the metal was moved further away with a
concomitant increase in energy and bu frequency. The same was
found when Ar atoms were placed between Li, (NO)2, and Li.
However, when the lithium basis is restricted to 1s functions,
appropriate charge transfer over a fixed remote distance does
occur and an appropriate bu dianion frequency is found. We
were not able to calculate the ionic species reached by UV
photolysis, namely (Li+)(Ar)x(N2O22-)(Ar)x(Li+), which is
physically stable and separated by a barrier from the more stable
Li(NO)2Li molecule, but we have offered several approximate
models to support the argon matrix spectrum of N2O22-. Our
Assignment of the 1404.1, 1392.6, 1390.6, and 795.3 cm-1
Absorptions. The sharp 1404.1 cm-1 absorption appears on
annealing to 35 and 40 K at the expense of Li(NO) under
conditions where NO decreases and (NO)2 increases. The 1404.1
cm-1 band forms a 1/2/1 triplet at 1404.1, 1391.3, and 1381.4
cm-1 in 14NO + 15NO samples and at 1381.4, 1360.4, and
1344.1 cm-1 in 15N16O + 15N18O experiments. These observa-
tions confirm the participation of two equivalent NO subunits
in this vibrational mode. The 14NO/15NO and 15N16O/15N18O
ratios 1.0164 and 1.0278 are appropriate for an N-O vibration.
The C2V Li(NO)2 molecule is calculated to absorb strongly at
1270.8 cm-1 just above the isolated (NO)2- species,36 so more
metal atoms are involved. Accordingly, the 1404.1 cm-1 band
and 1392.6 cm-1 site are probably due to Li(Ar)x(NO)2(Ar)xLi,
and the analogous Na species absorbs at 1390.6 cm-1. The
spectra in Figure 1e,f,g and Figure 2d,e,f show that annealing
to 35 K produces this species, UV photolysis decreases these
absorptions in favor of the 1028.5 cm-1 band, and annealing to
40 K produces more of this M(Ar)x(NO)2(Ar)xM species.
The 885.6, 795.3, and 415.5 cm-1 bands assigned17 to
(Li+)(NO2-)(Li+) are well modeled by similar calculations,
which predicted infrared absorptions at 878.1, 800.6, and 399.0
cm-1 at the BPW91 level20 and at 931.1, 827.3, and 409.0 cm-1
at the B3LYP level for the a1 (N-O)2- and b2 and a1 Li-(NO)
stretching modes.
system is similar to the stabilization of the sulfate dianion by
2- 45
an alkali cation as M+(SO4
)
although we have two metal
2-
cations and two or more argon insulators around N2O2
.
Reactions in the Matrix. During condensation, diffusion of
reagents allows spontaneous reactions and the following prod-
ucts discussed above are formed: (NO)2, Li(NO), and Li(NO)-
Li.
NO + NO f cis-(NO)2
Li + NO f Li(NO)
(1)
(2)
(43) Citra, A.; Andrews, L. J. Phys. Chem. A, 2001, in press (Ag +
NO).
(44) Bytheway, I.; Wong, M. W. Chem. Phys. Lett. 1998, 282, 219.
(45) Wang, X. B.; Ding, C. F.; Nicholas, J. B.; Dixon, D. A.; Wang, L.
S. J. Phys. Chem. A 1999, 103, 3423.
The weak 1055 cm-1 band that appears on annealing (Figure
1g) is the only possible assignment for the Li(NO)2Li molecule,
(42) Snis, A.; Panas, I. Chem. Phys. 1997, 221, 1.

2002 J. Am. Chem. Soc., Vol. 123, No. 9, 2001
Li(NO) + Li f Li(NO)Li
Andrews and Liang
Scheme 1
(3)
Li(NO) + NO + Li + XAr f Li(Ar)x(NO)2(Ar)xLi (4)
However, under these very cold conditions for condensing argon,
some Li atoms are trapped in the 7 K argon matrix separated
by one or two layers of argon atoms from (NO)2 as suggested
by reaction 4 and Scheme 1. The known size of the (NO)2 dimer
allows a comfortable fit into a two-argon vacancy.46,47 If
photoelectron transfer occurs with one argon atom insulating,
7.5 Å separation (two argon diameters, i.e., 3.76 + 3.76 Å)
between Li+ and N2O22- centers provides electrostatic stabiliza-
tion of approximately 44 kcal/mol per Li+ or 88 kcal/mol total,
which is an overestimate of the separation and an underestimate
of the stabilization. Simply squeezing one argon atom between
each Li and (NO)2 in Li(NO)2Li predicts a 5.6 Å separation
and a stabilization of approximately 59 kcal/mol per Li+ or 118
2-
kcal/mol total. Hence, the N2O2 dianion formed here by
photoionization of metal atoms is isolated and insulated by the
argon matrix (one or two layers) from the metal cations that
stabilize the dianion. Irradiation with 240 nm photons is capable
of direct ionization of Li with a 1700 cm-1 red shift by the
matrix48 and direct ionization of Na with no matrix shift. In
contrast photolysis with red and near-infrared radiation destroys
2-
the N2O2 species most likely by a reverse electron-transfer
2-
process. The slight decrease in (NO)2 on formation of N2O2
and an increase on photodestruction are consistent with the
reversible photochemical reaction 5. The slight increase in the
-
isolated N2O2 absorption with UV photolysis that forms
new 1028.5 cm-1 metal independent absorption involves two
equivalent NO subunits. Our DFT calculations model only the
M(NO)2M species which predict strong 1078.1 and 1083.6 cm-1
bands for Li and Na, respectively. The spectrum of solid
Na2N2O2 exhibits a 1030 cm-1 infrared band,13-15 which
strongly supports the present assignment.
2-
2-
N2O2 and visible photolysis that destroys N2O2 (Table 2)
-
suggests that reaction 5 may proceed through isolated N2O2
anion36 in two electron-transfer steps. The isolated N2O2
2-
dianion is extremely sensitive to photodetachment by red light.
Li(Ar)x(NO)2(Ar)xLi {240-380 nm
}
λ > 630 nm
This work presents the first infrared spectroscopic measure-
ment on a dianion sufficiently isolated from cations that no
cation perturbation is observed in the dianion spectrum. The
electrostatic stabilization of N2O22-, which is probably unstable
in the gas phase, is made possible by metal cations separated
by one or two insulating layers of argon in the rigid 7 K matrix.
(Li+)(Ar)x(N2O22-)(Ar)x(Li+) (5)
Conclusions
Ultraviolet irradiation of a rigid 7 K argon matrix containing
alkali or alkaline earth metal atoms and (NO)2 isolated from
each other by one or two layers of argon forms N2O22- insulated
from two M+ by argon, and red visible photolysis reverses the
Acknowledgment. We gratefully acknowledge the National
Science Foundation and the Petroleum Research Fund for
support of this research, C. O. Trindle for suggesting and
performing the lithium 6G basis set calculation, and C. W.
Bauschlicher, Jr., for helpful correspondence.
-
electron-transfer process probably involving the N2O2 anion
2-
intermediate. The isolated N2O2 dianion is identified from
isotopic substitution and isotopic mixtures, which show that the
(46) East, A. L. L. J. Chem. Phys. 1998, 109, 2185 and references therein.
(47) Pollack, G. L. ReV. Mod. Phys. 1964, 36, 748.
(48) Gedanken, A.; Raz, B.; Jortner, J. J. Chem. Phys. 1973, 58, 1178.
JA0029331