J. Phys. Chem. 1983, 87, 1113-1120
point that the periodic variation in Q for a specific adsorbate is smak the Q vs. Ndplots should be nearly flat. Also, the absolute magnitudes of Q explicably are small (-5-40 kcal/mol) compared to the magnitude of bond energies for the light elements in the upper right of the periodic table. (ii) All closed-shell diatomic molecules will tend to chemisorb on flat metal surfaces in an upright fashion-at least all such diatomic molecules in which the adsorbate atoms are more electronegative than the transition-metal atom. The transition state for dissociative (bond breaking) chemisorption of such diatomic molecules may tend to be closer to reactants than to products. Surface topography may be a very important factor for such bond scission processes. (iii) Adsorbate antibonding molecular orbitals contribute significantly to the surface chemisorptionbond. The metal surface tends to be a donor. This pervasive electronic feature explicably leads to some degree of “bond activation” (i.e., bond order reduction in an adsorbate bond)-increasing the potential for bond scission in the adsorbate species. (76) In addition to the data in Table 111, note that QCO increases from Fe toward Ni (3d metals) and from Ru to Pd (4d metals): Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1982, 117, 639.
1113
We further note that, for initial simplicity, our model has been developed for the unique world of clean surfaces and, in some cases, for atomically flat clean metal surfaces. We are now extending the model to probe the effect of surface impurity atoms, of variations in surface topography such as steps and kinks, and of direct adsorbateadsorbate interactions. We anticipate no new conceptual problems in these extensions, but the technical difficulties may be formidable. Acknowledgment. This collaborative research has been generously supported by the Eastman Kodak Company, the National Science Foundation, and by the Director, Office of Basic Energy Sciences, Chemical Sciences Division of the US.Department of Energy under Contract No. DE-AC03-76SF00098. We especially acknowledge Drs. Bryant Rossiter and Evelio Perez-Albuerne for encouragement and support and Drs. Roald Hoffmann, Gerhardt Ertl, W. Henry Weinberg, John Horsley, John L. Gland, Thor N. Rhodin, Robert L. Burwell, Jr., Kenneth S. Pitzer, and Galen B. Fischer for information concerning unpublished work and for helpful discussions. Members of ELMS research group also contributed to this theoretical development: specifically, Thomas M. Gentle, Allen L. Johnson, Kirk L. Shanahan, Min-Chi Tsai, and Ronald M. Wexler are gratefully acknowledged for their contributions.
Interconversion Studies and Characterization of Asymmetric and Symmetric Dinitrogen Trioxide in Nitric Oxide Matrices by Raman and Infrared Spectroscopy E.
M. Now,+L.-H. Chen, and J.
Laane’
Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Recelvec: August 31, 1982; In Final Form: November 8, 1982)
The fmt laser Raman spectra of both asymmetric dinitrogen trioxide (0,NNO) and symmetric dinitrogen trioxide (ONONO) have been collected by stabilizing these species in nitric oxide matrices at 12 K. Laser irradiation at wavelengths from 5682 to 7526 8, converts as-Nz03to s-Nz03but wavelengths of 5145 or 4880 8, convert the symmetric isomer back to the asymmetric one. The Raman data along with the infrared spectra of several isotopic species (with 15Nand/or l8O) for both molecules made it possible to resolve previous ambiguities in the vibrational assignments of these isomers and to carry out meaningful force constant calculations for them. The N-N stretching frequency at 266 cm-l for as-Nz03and its stretching force constant off” = 0.57 mdyn/8, both are characteristic of a very weak bond, although these values are higher than previously proposed. The N-0 single bond stretching force constant for s-NzO3 was determined to be 3.61 mdyn/A.
Introduction The chemistry of the nitrogen oxides is highly unusual in that a large number of molecular or ionic species may be formed from just these two elements. Approximately 25 different species of the general formula N,O, have been characterized by vibrational spectroscopy.l The abundance and variety of such molecules arise from the fact that nitrogen can effectively form both u and ir bonds to other nitrogen or oxygen atoms. Eleven of the nitrogen oxide species (NO, NO+, ONO, ONO+, ONO-, ONOz-, NNO, ONN02-, ONNOZ2-,02NN02,and OZNONOz)are stable under normal conditions whereas the others are typically stabilized in inert gas matrices or by utilizing special ‘Present address: Department of Chemistry, Zagazig University, Zagazig, Egypt. 0022-385418312087-1113$01.50/0
techniques. For example, O=N-O=N is formed at low temperatures from NO in the presence of Lewis acids.2 Other less stable species may represent isomeric forms of more common nitrogen oxides, e.g., ONONOZ. as-Dinitrogen trioxide is a blue liquid or solid (mp 162 K) which is formed when NO and NO2 are combined at low temperatures. The highest concentration of Nz03 vapor can be obtained3 near 220 K, the temperature at which N204solidifies, since two primary equilibria are applicable: NO
+ NO2
F=
Nz03
(1)
(1)J. Laane and J. R. Ohlsen, Prog. Inorg. Chem., 27, 465 (1980). (2)J. R. Ohlsen and J. Laane, J. Am. Chem. SOC.,100,6948 (1978). (3)C. H.Bibart and G . E. Ewing,J . Chem. Phys., 61, 1293 (1974).
0 1983 American Chemical Society
1114
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
2N02 * N204
Nour et al.
(2)
The structure of m-NZO3vapor has been determined from its microwave spectrum;4 the molecule very closely resembles free NO and NO2 molecules held together by a weak (1.86 A) N-N bond. The infrared spectra of the ONN02molecule have been studied over a period of more than 20 years"" and the assignments of most vibrational bands are secure.l However, only one incomplete prelaser Raman study, that of N203in dichloromethane solution? has been reported. Moreover, the assignment of the N-N stretching and NO rocking modes has remained ambiguous since these may correspond to infrared bands near 260 and 160 cm-', respectively, or vice versa. The question is an important one since the N-N stretching frequency lends insight into the nature of this particular bond. The existence of a symmetric dinitrogen trioxide isomer (ONONO) was first proposed by Fateley, Bent, and Crawford,12who examined NO and Ozcocondensed in an argon matrix at 4 K. Hisatsune, Devlin, and Wada' cocondensed NO and NO2 at 150 K and also detected bands assigned to ONONO, in addition to as-N203bands. The most convincing evidence for the existence of s-N203, however, came from the infrared work of Varetti and Primentel (VP),lowho prepared seven different isotopically substituted forms of this species in nitrogen matrices. These workers found that as-Nz03could be converted into s-N203by irradiation near its absorption maximum of 720 nm, and back again by irradiation in the 370-480-nm region:
(3) \
0
The infrared spectra of s-N203were in support of the structure as O=N-0-N=O with external N=O double bonds and internal single bonds. Since no laser Raman spectra of either as-Nz03or sN203have been reported, such a study of both molecules was undertaken. In addition, further information on the infrared spectra and interconversion of these molecules was of interest. The studies were carried out in nitric oxide matrices at 12 K in order to inhibit the decomposition of the N2O3 species. The data obtained made it possible to assign the vibrational frequencies with confidence and to obtain meaningful force constants for both molecules.
Experimental Section Materials and Preparations. All preparations and purifications were carried out in a standard high-vacuum system using procedures previously described.2 Nitric oxide (14N0,99.0% from Matheson Gas Products; 16N0, ~
~~
1JL I I
1900
1700
,
,
1500
,
! 1300
,
! 1100
,
,
900
/
I fl)
,
l
Ua
,
, , 300
I
,
,
XK)
Cm-' Figure 1. Raman spectrum of NO -t O2 (1O:l)at 12 K. Laser excitation, 25 mW (unfocused) at 5145 A; slit width, 3.5 cm-'.
99.0% from Stohler Isotopic Chemicals) and oxygen (99.670, Airco) were used to prepare the various N203 species. The ratio of NO to O2 used ranged from 5 to 25. (Stoichiometrically a 4:l ratio of NO/02 produces N203.) In certain cases argon, nitrogen, and carbon dioxide were also used as matrix gases. The prepared gas mixtures were deposited over a 0.5-2-h period onto either a highly polished brass wedge (for Raman measurements) or a CsI window (for infrared studies) maintained at 12 K by an Air Products CSW-202 closedcycle helium refrigerator. Photolysis. The 568.2-, 647.1-, 676.4-, and 752.6-nm krypton ion laser lines were used to photolyze as-N203to other species while the 488.0-nm argon ion laser line was used to reconvert the material to as-Nz03. For Raman measurements photolysis times of 10 min using unfocused radiation at low intensities (5-75 mW) was sufficient to effect the conversion. Since the infrared sampling requires larger sample area, a lens was used to expand the laser beam to -1-in. diameter for these photolyses in order to cover the entire CsI window. In such cases 2-12-h photolysis times were utilized. Spectroscopic Measurements. Raman spectra were recorded with a Cary Model 82 spectrophotometer equipped with Coherent Radiation Model 52 krypton ion and Model 53 argon ion lasers. Laser excitation lines at 457.9,488.0, and 514.5 nm from the argon laser were used to record the spectra of as-Nz03while lines at 568.2,647.1, 676.4, and 752.6 nm from the krypton laser were used to produce and record the spectra of s-N203and associated species. The laser excitation was unfocused and the power kept at low levels (5-75 mW) to prevent decomposition of the various species. Infrared spectra in the 4000-450-cm-' region were recorded with a Digilab Model FTS-2OC high-resolution purged spectrometer. A germanium-coated KBr beam splitter was used for this region.
~~~~~
(4)A. H. Brittain, A. P. Cox, and R. L. Kuczkowski, Trans.Faraday Soc., 65,1963 (1969). (5)L. D'Or and P. Tarte, Bull. SOC.R. Sci. Liege, 22, 276 (1953). (6)I. C. Hisataune and J. P. Devli, Spectrochim. Acta, 16,401(1960). (7)I. C. Hisatsune, J. P. Devlin, and Y. Wada, J.Chem. Phys., 33,714 (1960). (8) J. P. Devlin and I. C. Hisatsune, Spectrochim. Acta, 17,218(1961). (9)W. A. Yeranos and M. J. Onich, Mol. Phys., 13, 263 (1967). (10)E.L.Varetti and G. C. Pimentel, J.Chem. Phys., 55,3813(1971). (11)G. M.Bradley, W. Sidall, H. L. Straw, and E. L. Varetti, J.Phys. Chem., 79,1949 (1975). (12)W. G. Fateley, H. A. Bent, and B. Crawford, J. Chem. Phys., 31, 204 (1959). (13)G. R. Smith and W. A. Guillory, J.Mol. Spectrosc., 68,223(1977). (14)E.L. Varetti, J. Mol. Struct., 53,275 (1979). (15)S. E.Novick, B. J. Howard, and W. Klemperer, J.Chem. Phys., 57,5619 (1972).
Results as-Dinitrogen Trioxide (ONNO,). The Raman spectra of as-N203 at 12 K were obtained from cocondensed NO/02 mixtures with ratios ranging from 5 to 15. A t higher ratios the N203 decomposed in the laser beam even at low laser intensities. A t ratios of 5 or less, bands related to Nz04from the dimerization of NOz (after 2N0 + O2 2NOZ)were observed. The use of other matrix materials including Ar, Nz, or COz also led to decomposition of the material in the laser beam. Apparently the presence of excess NO is necessary to rereact with any NO2produced from decomposition of the Nz03(the reverse of eq 1). This is suggested by the fact that the N203band intensities
-
The Journal of Physical Chemktry, Vol. 87, No. 7, 1983 1115
as- and s-N,03 in Nitric Oxide Matrices
TABLE I: Raman and Infrared Frequencies (cm-l) for as-N,O, in an NO Matrixa Raman 1 4 ~ ~ 0 ,
infrared
1 5 ~ ~ 0 ,
1826m 1557 w
1858 m 1590 w 1 2 8 8 ms 8 3 1 mw 784 s 627 vs 565 m 537 mw 405 w 370 w 266 vvs 205 s 70 sh
assignment
1 4 ~ ~ 0 ,
1861s 1593s
w l , NO stretch w,, NO, antisymmetric
stretch 1 2 7 3 m s 1 2 9 8 s w g , NO, symmetric stretch 823 mw ( w * + v,) 787 m w 4 , NO, deformation 773 s 615vs 6 2 5 w u 8 , NO, wag (out-of-plane) 553 m (va - ~ 9 ) 525 mw 2V6 w S r NO, rock (in-plane) 395 w 261 vvs 201 s 69 sh
(., - V 6 ) w 6 , N-N stretch
laloo '
u 7 , NO
wag (in-plane) torsion
lioo
'
1 L
'
,100
I
'
1100
/
900
,
/
700
,
I
500
'
shoulder.
c ni' Figure 2. Infrared spectrum of NO O2(1O:l) at 12 K and 1-cm-' resolution.
decrease upon exposure to the laser beam but are restored if the laser beam is blocked for a brief period of time. Furthermore, no previous laser Raman spectra of Nz03 have been published, indicating the instability of the material in other environments when exposed to laser radiation. Figure 1shows the Raman spectrum of a nitric oxide + oxygen mixture with a NO/Oz ratio of 10 taken by using 488.0-nm excitation. Under these conditions only bands due to as-N203and ONNO (from nitric oxide dimerization at low temperatures) can be observed. The spectra recorded with 457.9-, 488.0-, and 514.5-nm laser lines show no change with excitation wavelength. The corresponding infrared spectrum of the same mixture is shown in Figure 2, and Table I lists the vibrational assignments for both l4NZO3and 15Nz03.As the molecule has C, symmetry, it has nine modes of vibration distributed according to 7A' + 2A" symmetry species. All vibrations are both infrared and Raman active. The most significant features in the Raman spectra are the observations of the two most intense bands at 627 and 266 cm-l. Previously it was not clear whether v& the NO, out-of-plane wag, was a t 337 cm-', as suggested from gas-phase infrared spectra? or near 620 cm-'. The Raman band at 627 cm-l, however, gives unambiguous evidence that it is due to a fundamental vibration. Similarly, the N-N stretching vibration is expected to give a very strong Raman band and is thus almost certainly associated with the 266-cm-' band. The NO in-plane wag (vl), previously assigned" to 260 cm-', must then be associated with the weaker, but still strong, band at 205 cm-l. An infrared band near 160 cm-' for liquid Nz03has been reported" and this may also be due to v7, shifted due to the molecular
environment. The assignments of vg, v7, and as well as vg torsion are further supported by the observation of several combination and overtone bands involving these modes. Table I1 compares the vibrational frequencies from the present work to those reported previously. Where appropriate, previous assignments have been revised in order to achieve frequency correspondence. As is evident, the Raman spectra reported in this work represent the first example where all of the fundamental vibrational frequencies have been observed in one spectrum. Aside from the reassignment of vg, vl, and us, the other vibrational descriptions are in excellent correspondencewith previous work. Table I also lists the Raman frequencies for the 15N203 isotopic species. As will be described later, the observed shifts are entirely consistent with the force constant calculation carried out for as-dinitrogen trioxide. Moreover, the 15Nshifts for the nitroso (NO) and nitro (NO,) portions of the molecule are very similar to those in free NO and free NOz. Thus, the nitroso stretch at 1858 cm-' shifts 32 cm-' upon isotopic substitution whereas the corresponding shift for free NO is 33 cm-' (from 1876 cm-I). Similarly, the nitro bands of N203at 1590,1288, and 784 cm-l show isotopic shifts of 33, 15, and 11 cm-l, respectively, while free NO2 has corresponding frequencies of 1617,1320,and 750 cm-' and frequency shifts of 35, 14, and 10 cm-l, respectively. Because of the presence of excess NO in the matrix study, several bands due to the nitric oxide dimer were observed in the Raman and infrared spectra. Also very weak features, which may be due to unreacted oxygen in the matrix, were observed. These bands are listed in Table
a
w 9,
+
s = strong, m = medium; w = weak; v = very; sh =
TABLE 11: Vibrational Frequencies (cm-' ) for as-N,O, infrared gas A'
w1 v 2 v3 u4
v5 6'
A"
v7
wa y9
NO stretch antisym NO, stretch sym NO, stretch NO, deformation NO, rock (in-plane) N-N stretch NO, wag (in-plane) NO, wag (out-of-plane) torsion
reference CH,Cl, solution.
1832 1652 1305 773 414 241b
liquid
solid 1863 1589 1297 783 407
N2
Raman 0 2
matrix matrix NO matrix 1840 1630 1303 776 420
1632 1305
76 11
solnu
1841 1611 1291 7 68
1849
1858
1600 1291 772
1590 1288 784 405 266 205 627 70 this work
253
Derived from combination bands.
6
625 10
13
thiswork
NO matrix
solna
260 160 627
63b 3
1861 1593 1298 787
624
614
6
6
1116
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Nour et al.
TABLE 111: Raman and Infrared Bands (cm-') of Other Species in the NO Matrix Raman [1858 m]"
infrared
assignment
[1861 m]" 1754 s 17 35&
N,O,, v 1 (sym N=O) N,O,, v g (antisym N=O) N , 0 4 , v g (antisym NO, str) 0, 0, N,O,, v 1 (sym NO, str) N2°4, ' 1 1 (sym str) N2°49 '2 def) NZO,, 2v, NZO,, v 2 N,O,, v 3 (ONNO sym bend) N,O, (lattice)
1602 w 1570 w 137gb 1250b 810b 490 w [ 266 vvs]" 188 w 105 w
I
752.8 nm
" These overlap with N,O, bands. Weak bands observed only when NO/O, ratios are less than 5 .
Cm-'
Figure 4. Infrared spectrwnof NO i0 (10 1) at 12 K after photolysis with laser excitation of 6764 or 7526 Resolution: 1 cm-'.
1.
I
le00
I
1700
,
,
1500
,
1300
L
~
1100
-
_
900
_
700
_
_
500
-
-
300
-
~
-
-
100
Cm-' Figure 3. Raman spectrum of photolyzed NO 4- O2(1O:l) at 12 K. Laser excitation (unfocused): 5682 (30 mW), 6471 (20 mW), 6764 (13 mw), and 7526 (18 mw) A. Slit width: 4.5 (5682 A), 3.0 (6471 A), 6.0 (6764 A), 5.5 (7526 A) cm-I.
111. The v1 and v2 bands of ONNO overlap with the v1 and v6 bands of N203in the Raman spectrum. However, based on the relatively weak intensity of the other N202bands, these features are predominantly due to as-N203. The infrared band at 1861 cm-' similarly represents an overlap of N203and N202bands. In pure N202this feature is much weaker than the v5 antisymmetric stretch a t 1754 cm-'. Thus, most of this band intensity probably comes from N203although monomeric NO also absorbs in this region. The spectra in Figures 1 and 2 show no features due to N204. Such bands were only observed when the ratio of nitric oxide to oxygen was less than 5 to 1. s-Dinitrogen Trioxide (ONONO) and Other Species. Approximately 50 photolysis experiments were carried out in order to convert as-N203to s-N203in an NO matrix a t 12 K. Laser wavelengths of 568.2, 647.1, 676.4, and 752.6 nm were used to produce and record the Raman spectra of the new species. Figure 3 shows the Raman spectra
Figure 5. Structures of nitrogen oxides: (a) as-N,O,; (b) s-N203;(c) N304(s-N203 -t NO); (d) N,OB (2as-N20, or N20, N202);(e) N308 (3N02).
+
recorded with each of these wavelengths. The scan obtained with 752.6-nm excitation is terminated near 1400 cm-' due to the low-frequency limit of the monochromator. Figure 4 shows the infrared spectra of material photolyzed with 676.4- and 752.6-nm radiation. All of the Raman data are compiled in Table IV for both 14N and 15N species. Table V compares our infrared results to those of Varetti and Pimentel.'O The results first of all demonstrate that as-N203is readily converted to one or more other species when these wavelengths (568-752 nm) are used. Since the new material is readily converted back to as-N203upon photolysis with 488.0-nm radiation, it is highly likely that the photolysis products are isomers of as-N20B,a combination of these isomers (dimer or polymers), or combinations of isomers and NO molecules which exist in excess in the matrix. Some possible structures for photolysis products are shown in Figure 5 along with as-N203(a). From their studies Varetti and Pimentel'O have suggested that s-N203
as-
and s-N,03 in Nitric Oxide Matrices
No. 7,
The Journal of Physical Chemistry, Vol. 87,
1983
1117
TABLE IV: Raman Spectra (cm-') for s-N,O, and Other Photolysis Products from as-N,O, in NO Matrix" hkser = 647.1 nm
h
I5N
I4N
I4N
1863 s
1831 w
1862 ms
1830 w
1740 m
1710 m
1737 m 1675 w
1706 mw 1644 w
1290 mw
1275 w
1287 mw
1271 mw
9 7 3 mw
962 w
969 mw
950 w
1862 mw 1835 mw 1737 m 1682 m 1470 w 1450 w 1290 w 1240 w 974 mw
387 vs
625 w 492 mw 395 vs
612 w 482 w 386 ms
624 w 493 mw 395 vs
hkser =
568.2 nm
14N
625 w 494 mw 395 vs 332 m 300 sh
295 sh
268 vs
267 s
308 sh 275 sh 269 vs
187 m 1 4 1 sh
212 w 185 w 140 m
210 w 186 m 140 m
I5N
289 sh
w = 676.4 n m
184 m 137 sh
14N
assignment
1707 w
u s , as-N,O,
950 vw
610 vw 482 vw 387 m
305 m 275 m 267 m 235 vw 210 w 185 m 140 m
268 s
= 752.6 nm,
h~
15N
965 950 810 710
m w mw mw
492 mw 396 s 345 mw
'2,
s-N203
VZ?
NZO,
v s , s-Nz03
u 8 , US-N,O,
2v,, NZO, s-N,O,
v3,
s-N203
'4,
300 sh
305 mw
265 m
270 m
v 2 , N,O,; v 6 , as-N,O,
183 w 137 w
211 w 188 s 138 sh
V33
2 ~ , s-N,O,; , u,, as-N,O, NZO, u s , s-N,O,
s = strong, m = medium, w = weak, v = very, sh = shoulder,
TABLE V: Infrared Spectra (cm-l) of s-N,O, this worka 1687 vs 1295 m w 975 ms 865 vw 705 vw
hkser =
ref 1 0 1690 s 1661 vw
TABLE VI: Vibrational Assignment (cm-') for s-N,O,
assignment u 6 , antisym
pre- alterferred nate Varetti14
N=O str A,
u,, as-N,O,
969 877 704 387 366
w
vw
u , , sym N-0 str u , , antisym N-0 str u s , antisym O=N-0
m s
u , , NON bend v 4 , sym O=N-0
vw
u, u2
v,
bend
A, B,
bend
676.6 nm.
(b) is produced upon photolysis. As will be shown, our results are in agreement with the production of the ONONO along with the other nitrogen oxide species. As shown in Table V, Varetti and Pimentel'O observed seven infrared bands which they assigned to the s-N203 molecule. The infrared spectra in our study were only recorded above 400 cm-' and thus we did not observe the two infrared bands at 387 and 366 cm-'. In addition, the very weak 1661-cm-' band found in the VP spectra was not detected in our work. An infrared band was consistently observed at 1295 cm-' in our work. However, we have tentatively ascribed this to the v3 band of as-NzO3 although no other bands for this species were evident in the spectra. The Raman spectra shown in Table IV provide strong support for the assignment of s-N203bands. Bands near 1680,970, 395, and possibly at 710 cm-' confirm infrared assignments. In addition, Raman bands at 1737,210, and 140 cm-' help complete the vibrational assignment which is shown in Table VI. The main disagreements with the previous assignment^'^ are that the medium-intensity Raman band at 1737 cm-' (rather than a very weak infrared band) is ascribed to v1 and the Raman bands at 210 and 140 cm-' are assigned to 2v9 and v5, respectively. A previous analysis by Laane and Ohlsen had some of the symmetric and antisymmetric assignments reversed' but the additional Raman data and force constant calculations support the conclusions represented in Table VI. Nonetheless, we feel somewhat uncertain about the assignment of weak bands observed in the infrared at 870 and 704 cm-'. Since a shoulder at 275 cm-' was observed in most
u4 us
v6 u,
v8
B, a
up
s y m N = O str symN-0 str NON bend" sym O=N-0 benda torsion antisymN=O str antisym N-0 str antisym O=N-0 bend torsion
1740 973 395 366 140 1687 877 704 105
1740 973 395 275 140 1687 704 366 105
1661 969 387 366 1690 877 704
Coupled motions.
TABLE VII: Valence Force Constants" for as-N,O, Primary Constants fNO(No) fNO(No 2 ) fNN f, (deformation
thiswork
DHb
BSSVC
15.039 9.574 0.573 1.680 1.127 0.584 0.396 0.020
15.12 7.93 0.66 0.17 1.15 0.40 0.35 0.003
14.74 10.09 0.233 1.547 0.432 0.653
Interact ion Constantsd fN0, N N ( NO fN0,NO(N02) fNO,NN(Noz) fNO,@(NO) fNO,a(NOz)
0.072 1.595 0.137 0.038 0.541
fN0,p fNN,a:
f",@ f&@
-0.114 0.112 0.187 0.094
Stretching constants in mdyn/A; bending constants in mdyn A/rad2. Urey-Bradley constants from ref 8. Urey-Bradley constants from ref 11. This work.
Raman spectra, the alternate assignment shown in Table VI is also possible. This version revises the frequencies for v3, v7, and but retains the others for which we have considerable confidence. In summary, Raman bands at 1740,1682,973,395,210, and 105 cm-' and infrared bands at 1687,975, 387, and 366 cm-' quite certainly correspond to seven of the nine fundamentals. The other two fun-
1118
Nour et al.
The Journal of Fhysical Chemistry, Vol. 87, No. 7, 7983
TABLE VIII: Observed and Calculated Frequencies and Potential Energy Distribution for ( I S - ~ ~ N , ~ ~ O , potential energy distributiona frequency, cm-' A'
obsd 1858 1590 1288 784 405 266 205 627 70
VI
V2 V 3 u4 " 5
V6 1' 7
A"
1' 8 V9
a
calcd 1859 1590 1288 784 405 266 203 629 70
fN0-
fN0.
(NO) 99 1 0 0 1 0 0 0 0
(NO,) 0 115 85 4 4 12 0 0 0
fNN
fa
0 0 2 5 12 59 37 0 0
0 0 12 79 1 6 6 0 0
fo
1 3 0 0 29 46 23 0 0
f@
0 1 0
0 63 25 24 0 0
fY
fT
0 0 0 0 0 0 0 98 2
0 0 0 0 0 0 0 2 98
Potential energy distribution totals 100 including contributions from interaction constants, which are not shown.
TABLE IX: Observed" and Calculated Frequency Shifts (cm-l) for Isotopic Species of as-N,O, 16015NI4Nl60 16014NISN160 160lSN15N160 18O14N14N16O 16014N14N180160 16014N14N180 2
obsd
calcd
2
2
obsd
calcd
obsd
calcd
2
obsd
calcd
2
obsd
0 47 47 32 34 0 0 34 0 36 36 33 36 0 0 12 0 24 16 0 16 16 0 15 U3 0 0 13 10 10 11 10 "4 1 0 6 10 7 V , 2 6 1 5 5 1 4 1 '6 6 4 1 U.. 0 0 0 12 17 A" u, 1 16 1 0 1 1 vg 1 Data for 160'5N15N160, from this work. Other observed frequencies are from ref 10.
A'
v1 b2
32 0 0
TABLE X: Valence Force Constants for s-N20,* calcn I fR fr fa
fo
f,
fRr fRa frr fi-0
fro foo f a0 fTT
fRR fRr fR0 fRo' fi.0'
calcn 11
Varetti14
Primary Constants 12.11 12.47 3.61 2.89 1.87 1.29 1.61 0.80 0.05 0.05
12.10 4.14 1.74 1.51
Interaction Constants 0.19 0.28 -0.39 - 0.20 0.36 0.79 0.22 0.21 0.16 0.10 0.34 -0.16 0.02 -0.33 -0.01 --0.01
0.26 0.28 0.95 0.16 0.26 --0.29 -0.42 -0.01 0.16 0.12 0.01 0.19
a R : N = O str; r: N-0 str; a : NON bend; 0: 0 - N - - 0 bend, 7 : torsion, prime designations indicate the more distant interactions. Stretching constants in mdyn/A; bending constants in mdyn A /rad2.
damentals may be related to weak features near 870, 710, and/or 275 cm-' or possibly even to 1290 cm-'. Further analysis of Table IV and the spectra of the photolyzed N203makes it clear that other nitrogen-oxygen species in addition to s-N203,as-N20S,and the nitric oxide dimer are present. These species may resemble structures c, d, or e in Figure 5 but no attempt has been made to confirm their existence. Structures c and d represent possible combinations of s-N203with NO or with itself while e has previously been proposed to explain the presence of N306 in mass-spectral studies.15 Proposed structure d is interesting in that it can be thought of as a dimer of s-N203or as-N203,or as a combination of N204 and N20z.
calcd
obsd
0 12 24 14 3 1 5 4
0 31 45 26
calcd 0
29 44 29 6 7 5 7 1
0
TABLE XI: Observed and Calculated Frequencies and Potential Energy Distribution for s - ' ~ N , ' ~ O , frequency, cmobsd 1740 97 3 395 366 140 1687 877 704 105
calcd f R fr Calculation I 1743 90 1 974 0 68 394 8 23 3 0 363 143 0 0 1687 9 6 3 1 106 878 703 3 3 0 0 105
1740 97 3 395 215 140 1687 7 04 366 105
Calculation 1743 93 973 1 396 6 1 273 143 0 1689 9 9 0 706 363 0 105 0
fe
fa
f,
2 40 56 25 0 2 6 84 0
4 22 30 57 0 0 0 0 0
0 0 0 0 129 0 0 0 81
1 20 21 28 0 1 1 173 0
3 16 22 66 0 0 0 0 0
0 0 0 0 129 0 0 0 82
I1 1 54 24 1 0 2 137 0 0
Force Constant Calculations as-N203. Previous force constant c a l c u l a t i ~ n on s~~~~~~ OzNNO have been based on vibrational assignments different from those in this work since our Raman data were not available earlier. In addition, the previous calculations have used Urey-Bradley force fields which emphasize nonbonded interactions. In the present work we have chosen a general quadratic force field utilizing no nonbonded interactions. The latter is generally the preferred force field although certain approximations or assumptions are often necessary. The internal coordinates which were selected for the calculation are the obvious ones: four bond stretches, a (the O N 0 angle deformation), p (the NO2 in-plane wag), b, (the NO2 in-plane wag), y (the NO out-of-plane wag),
The Journal of Physical Chemistty, Vol. 87, No. 7, 7983 1119
a s - and s-N,03 in Nitric Oxide Matrices
TABLE XII: Observed and Calculated Frequency Shifts (cm-') for Isotopic Species of s - N 2 0 3 ON180N0 0 1 5 ~ 0 1 5 ~ 0 0 1 5 ~0 0 ~ obsd
calcd
30 15 I 6 3 28
35 17 4 3 9 32 15
11
9
calcd Calculation I
obsd
I 4 4 16
4
20
13 8 2 1 2 21 8 4 0
obsd
calcd
19
24 0 7 0 1 20 15 2
'80NON0 obsd
0 12 1
calcd 13
5 7 8
25
1 8 8 1 26 1 7 2
Calculation I1 30 15
I 3 28 11 6
34 17 5 2 9 31
9 9
I 4 16 4
4
20
and 7 (the torsion). The eight primary valence force constants and nine interaction constants were adjusted to fit the data in Table I along with the previously available data on frequency shiftdo for the various isotopic species. Table VI1 lists the force constants determined and compares the primary constants to those previously determined from Urey-Bradley type calculations.8J1 Table VI11 presents the observed and calculated frequencies for 14N21603and the potential energy distribution for each frequency. Table IX lists the observed and calculated frequency shifts for the six isotopic species. The average frequency deviation between observed and calculated values was less than 1.0 cm-' for all seven isotopic species. The force constants in Table VI1 differ from the previous analyses both because of the potential function used (quadratic constants vs. Urey-Bradley) and because of the new frequency assignments. It is of interest to compare the values of the primary valence force constants to those in other molecules. as-N203is generally regarded as NO2 and NO molecules loosely held together by a weak bond. Free NOz has fN0, fa, fNoS0, and fNop values' of 11.04,1.58, 2.14, and 0.57, respectively. These are 9.57, 1.68, 1.60, and 0.54 for as-N203,reflecting the similarity in the bonding of the NO2 group. Notably, f N 0 is about 14% lower, demonstrating the weakening of the N=O bonds upon formation of the N-N linkage. The N=O nitroso force constant, which is 16.0 mdyn/A for free NO, is lowered to 15.04 mdyn/A in as-N203. The N-N stretching force constant was previously1' calculated to be 0.23 mdyn/A but, on the basis of the revised assignment, is found to be 0.57 mydn/A in this work. It is nevertheless a very weak bond, much less than a single bond in character. s-N203. Varetti14 has previously carried out a force constant calculation for s-N203based on his assignments shown in Table VI. While he has reported only the symmetrized force constants, we have calculated his unsymmetrized constants from these and list them in Table X for comparison with our values. Two calculations were carried out in the present work, for both the preferred (calcn I) and alternate (calcn 11) assignments given in Table VI. The force constants for calcn I are thus to be considered the better set. On the whole, these agree qualitatively with those of Varetti. However, he has not carried out calculations for the torsional motions, and he has included five additional constants. Several of these such as fRR, fR,+ and f w represent long-range interactions
13 8 2 1 2 20 5 3 0
19
1 12
1 24 0 5 0 0 22 5 2
5 7 25 8
13 1 7 4 3 28 1 4 2
and should certainly be close to zero. Our value calculated for the N-O stretch (3.61 mdyn/A) is somewhat lower than Varetti's and is characteristic of a single N-0 bond, somewhat weaker than normal but similar in magnitude to values determined for H-0-N=0l6 and Cl-0N=O1' which range from 2.2 to 4.36 mdyn/A. Table XI lists the observed and calculated frequencies for the natural isotopic species of s-N203and also the potential energy distributions (PEDS). As is evident from the PED calculation, the NON bending and symmetric O=N-0 bending motions are highly coupled and the approximate description of v3 and v4 given in Table VI could easily be reversed. To a somewhat lesser extent the symmetric N-O stretch is also mixed in with these motions. Table XI1 presents the observed and calculated frequency shifts for the four isotopically substituted species and these are quite satisfactory. The average frequency deviation for the five isotopic species was about 2 cm-', which is an improvement over Varetti's calculation even though fewer adjustable force constants have been used in the present work.
Discussion The laser Raman spectra recorded for as-N203and sN203represent the first recorded for these molecules. The Raman spectra obtained for as-N203demonstrate that v6, the N-N stretching mode, occurs at 266 cm-' and that vg, the NO2 out-of-plane wagging, occurs at 627 cm-'. Knowledge of these assignments, which previously were uncertain, made it possible to calculate a meaningful set of force constants for this molecule. The values obtained are in good agreement with the view that the bonding of as-N203(0,NNO) may be described as slightly perturbed individual NO2and NO molecules held together by a weak N-N bond. The force constant of 0.57 mdyn/A for the N-N stretch is somewhat higher than that previously reported" but still represents a bond order considerablyless than unity. The relatively small decreases in the nitro and nitroso force constants with respect to free NO2 and NO also reflect only a minor redistribution of bonding electron density upon formation of as-N203.In addition, the relatively low force constant values for the out-of-plane wag (16)G.E.McGraw,D. L. Bemitt, and I. C. Hisataune, J. Chem. Phys., 45, 1392 (1966). (17) B. Janowski, H. D. Knauth, and H. Martin, Ber. Bunsenges. Phys. Chem., 81, 1262 (1977).
J. PhyS. Chem. 1983, 8 7 , 1120-1125
1120
v,)
and torsion (f,) are consistent with the view that nonbonded 0-0 interactions are not significant. The overall success of the quadratic force field used here, as opposed to the Urey-Bradley field which emphasizes nonbonded interactions, further supports this picture. The Raman spectra and force constant calculations on s-NZ03,which can be stabilized after formation by photolysis in the NO matrix, have also provided an improved perspective on the spectra and bonding of this molecule. Seven of the nine fundamental frequencies of s-N203have been assigned with considerable certainty, and those have permitted the calculation of force constants for two different vibrational models. The force constants (from calcn I) show the molecule to have a nitrogen-oxygen double bond (fR = 12.1 mdyn/A) stronger than in free NO2 CfR = 11.0 mdyn/A) or HO-N=O (fR = 11.7 mdyn/A) but weaker than in XO-N=O (fR = 13.0 mdyn/A for X = F or C1) or X-N=O molecules ( f R = 15.2-15.9 mdyn/A). The nitrogen-oxygen single bond constant o f f , = 3.6 mdyn/A may be compared' to the lower values found for X-O-NOZ molecules (f, = 2.3-3.2 mdyn/A for X = C1, F, H) or for Nz05(f, = 1.6 mdyn/A for OZN-O-NO2). However, the corresponding bond may be considerably stronger as in FO-NO (f, = 6.3 mdyn/A). Although as-Nz03 rather than s-N203is the species formed under normal conditions, it has the weakest bond
(the N-N linkage) in either molecule. Its higher thermodynamic stability thus must result from its three nitrogen-oxygen multiple bonds (see Figure 5a) vs. only two for s - N ~ O ~Lattice . effects may also contribute to its solidstate stability. In the vapor phase as-Nz03is also more stable than s-NZ03,based on the fact that infrared spectra of N2O3 vapors show only the presence of the asymmetric i ~ o m e r .In ~ the nitric oxide matrix as-N203is also preferrentially produced. However, photolysis with red radiation apparently results in rupture of the N-N bond and free NOz and NO are produced. Under continuing photolysis the OZN-NO cannot reform. Instead, one of the oxygen atoms in the NOz attaches to the nitrogen of NO producing O=N-0-N=O. The process is reversed when radiation in the green or blue region is used. Photolysis in this region ruptures a nitrogen-oxygen single bond in s-Nz03and the NOz and NO products rearrange to form as-Nz03. As should be apparent, the nitric oxide matrix plays an important role in these interconversions by providing excess NO molecules which may react with the NOz decomposition products. This is true even though much of the NO is dimerized as ONNO.
Acknowledgment. This research was supported by the National Science Foundation. Registry No. N,Os, 10544-73-7; NO, 10102-43-9.
Thermodynamics of Sodium Chloride Solutions in Steam Kenneth S. Pitzer Department of Chemistry and Lawrence Berkeley Laboratory, University of California. Berkeley, California 94720 (Received: September 7, 1982; I n Final Form: November 3, 1982)
Gas-phase data from mass spectrometry are used to calculate the Gibbs energy of hydration of Na+ and C1ions in steam. A similar hydration model for the ion pair NaCl is fitted to the experimental measurements of the solubility of NaCl in steam. The ionization constant calculated from these sources fits the directly measured values at 1073 K and densities above 0.3 g ~ m - At ~ . lower temperatures reasonable curves interpolate between the calculated values for low density and the direct measurements at higher density. Other thermodynamic properties are calculated for Na+, C1-, and NaCl in steam. The partial molal heat capacity of the ions is very large in the range 700-1000 K; this arises from the enthalpy of dissociation of water from the hydrated ions. The Born equation is compared with these results. A practical application to steam turbine technology is also considered.
Recently the writer' showed that the mass spectrometric measurements on the hydration equilibria for H30f and OH- could be used to calculate the self-ionization in steam in the region above the critical temperature and at pressures up to maxima increasing from 100 or 200 bar near T,to lo00 bar near lo00 K. Other thermodynamic properties of ions in steam were also calculated. In this paper similar methods are applied to the system NaC1-H20 in the same region of temperature and pressure. Kebarle and have measured the hydration equilibria of gaseous Na+ and C1-. For this system there is the additional problem of the hydration of the NaCl ion (1)K. S.Pitzer, J. Phys. Chem., 86, 4704 (1982). (2)I. Dzldic and P. Kebarle, J. Phys. Chem., 74, 1466 (1970). (3)M. Anshadi, R. Yamdagni, and P. Kebarle, J. Phys. Chem., 74, 1475 (1970). 0022-3654/83/2007-1120$01 S O / O
pair in steam. Reasonable estimates are made for these hydration equilibria for NaCl with guidance from experimental values for the solubility of solid NaCl in steam. Unfortunately there are large differences among the various experimental solubility but it is found that many data are reasonable on this theoretical basis while the others are not. Further experiments are very desirable (4)M.A. Styrikovich, I. Kh. Khaibullin, and D. G. Tskhvirashvili, Akad. Nauk. SSSR, 100, 1123 (1955). (5)M. A. Styrikovich and I. Kh. Khaibullin, Dokl. Akad. Nauk. SSSR, 109, 962 (1956);Engl. transl. 109, 507 (1956). (6)0. L.Martynova, Zh.Fir. Khim., 38, 1065 (1964);Russ. J. Phys. Chem., 38, 587 (1964). (7)M. A. Styrikovich, 0. I. Martynova, and E. I. Mingulina, Dokl. Akad. Nauk. SSSR, 171, 911 (1966);Engl. trans., 171, 783 (1966). (8)S. Sourirajan and G. C. Kennedy, A m . J. Sci., 260, 115 (1962). (9) J. F. Galobardes, D. R. Van Hare, and L. B. Rogers, J. Chem. Eng. Data, 26,363 (1981).
0 1983 American Chemical Society
