J. Phys. Chem. 1981, 85. 4051-4056
TABLE I: Extended Huckel Coulomb Integrals ( H i i ) and Slater Exponents Hii, eV exponent
c 2s c 2P c1 3s
c1 3p F 2s F 2~
- 21.4 -11.4 - 30.0 -13.0 -40.0 -18.0
1.625 1.625 2.0 2.0 2.425 2.425
7 shows the ionization potentials and electron affinities of the hal0gens.l' Assuming Koopmans theorem this places the a* and u* orbitals. Since the HOMO-LUMO gap decreases as we go from C12 to 12,we conclude that the barrier to insertion should decrease as we go from C12.to 12. This is in accord with the results of multiphoton dissociation experiments for CF2C125band CFzBr2.5eWork is currently under way for the CFz12system.12 Information about reaction dynamics can be inferred from the potential energy surface. Since a low-energy path involves an asymmetrical intermediate, we can imagine that a significant fraction of any energy released as the fragments separate will appear as internal excitation of the products. A crude estimate of the extent of this expected excitation can be derived from the properties of the potential energy surface. Taking our transition state as the initial geometry in 13, and recalling the previous arguments about the geometry of the initial attack, we see that the gradient of the potential (toward the lower energy of separated fragments) will be steepest for a displacement along the C-X coor(11) Electron affinities: W. A. Chupka, J. Berkowitz, and D. Gutman, J. Chem. Phys., 55,2725 (1971). Ionization potentiah: (a) R. P. Iczkowski and J. L. Margrave, J. Chem. Phys., 30,403 (1959); (b) K. Watanabe, T. Nakayama, and J. Mottl, J. Quantum Spectrosc. Radiat. Transfer, 2, 369 (1962); (c) I. P.Fisher, J. B. Homer, and F. P.Lossing, J. Am. Chem. SOC., 87,957 (1965). (12) S. Cain, R. J. S. Morrison, and E. R. Grant, to be published.
4051
dinate. The asymmetry of this force with respect to the centers of mass of the incipient fragments will result in the deposition of much of the energy associated with this coordinate into Xz and CF2 rotation. For an insertion barrier of a few kcal/mol, we can thus expect fragment rotational distributions rather hotter than room temperature. The final vibrational energy of Xz product will depend on coupling of exit channel relaxation with the X-X relative motion. Since formation of the X-X bond occurs fairly early in the dissociation (scission is late in the insertion), the X2 coordinate will be substantially relaxed in the critical configuration and we expect vibrational excitation to be less than rotational excitation. From the above, we predict that the higher the barrier to insertion, the higher the rotational excitation of dissociated products relative to vibrational and translational excitation. Hence we expect the ratio of rotational excitation to vibrational and translation excitation to decrease as X changes from C1 to Br to I. These predictions are testable by molecular beam experiments. Such experiments are now under development. For a thorough comparison of experimental and theoretically predicted dynamics, the qualitative features of the surface discussed in this paper provide the basis for a more complete surface on which trajectory calculations can be conducted. Such calculations are currently under way. Acknowledgment. We are grateful to the National Science Foundation for its support of this work through Research Grant CHE 7828048. Appendix In the extended Huckel calculation^^^ we used the parameters given in Table I. (13) R. Hoffmann and W. N. Lipscomb, J. Chem. Phys., 36, 2179 (1962).
Studies in the Vlbratlonal Spectroscopy of Sodium Trioxodinitrate' Francls T. Bonner," Mohammad Javald Akhtar, Tseng-Ven Klng, LI-Heng Chen, and Takanobu Ishida" Depafimenf of Chem/stfy, State Unlverdfy of New York at Stony Brook, Stony Brook, New York 11794 (Received: Ju/y 14, 1981; I n Final Form: September 11, 198 1)
Infrared and Raman spectra are reported for sodium trioxodinitrate,Na2Nz03,including nitrogen isotope shifts and solution Raman spectra exhibiting for the isotopic forms Naz015NN0z,NazON15N02and Naz015N15N0z, the effects of monoprotonation on the N2O3'- anion. A normal coordinate analysis has been carried out by the Wilson GF-matrix method, and the complex motions correspondingto the nine fundamentals are characterized in detail. A hitherto unreported out-of-plane fundamental at 210 cm-l was predicted in the course of the calculation and subsequently observed.
Sodium trioxodinitrate ("Angeli's saltn2),Na2N203,has been the subject of a number of structural studies, among them a brief account of its infrared and Raman spectra by Feltham.3 The crystallographic study of Hope and Sequeira4provided detailed information concerning sym-
metry, bond angles, and distances, and showed unambiguously that the predominant structural form of the dianion N203'- is
(1) Research supported by the National Science Foundation, Grant No. 78-24176, and Division of Basic Energy Sciences, U.S. Department of Energy, Contract No. DEAC02-80ER10612. (2) Aigeli, A. Gazz. Chim. Ital. 1896,26, 7. (3) Feltham, R. D. Inorg. Chem. 1964, 3, 900.
The dianion is in aqueous solution, but in the pH range in which the species HN203-predominates (pK1 =
0022-3654/81/2085-405 1$01.25/0
N ,=N+
-0
/o'0-
(4) Hope, H.; Sequeira, M. R. Inorg. Chem. 1973, 12, 286.
0 1981 American Chemical Society
4052
Bonner et ai.
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981
TABLE I: Infrared and Raman Frequencies of Na,N,03 (cm-') IR (ref 3 ) 1400 s 1280 s
Is 1
l1100 I2O s 980 s 970 s 747 w 630 m 610 w 430 w 367 m
IR (this study) 1397 s 1276 s
1126t
Raman, cryst. sample (this s t u d y )
Raman, soln (ref 3 )
Raman, soh (this s t u d y )
depolarization ratio
1389 s 1260 w 1084 m
1380 s 1240 w 1110 m
1380 s 1240 w 1095 m
0.29 + 0.04 0.28 i. 0.03 0.11 f 0.01
971 s
975 s
970 s
750 w
745 w
753 w
610 m
605 m 425 w
605 w 427 w 360 w
1102 s
9853
978 s 752 w 635 m (615 w?) 450 w 381 m 210 s
2.51, pK2 = 9.705)decomposition occurs via the reversible proce~s~-~ "203-
= HNO
+ NO2-
I
I
1
I
0.11
I
I
I
* 0.01
I
I
I
(1)
followed (in the absence of competing redox reactions) by HNO + HNO = N2O H2O (2)
+
Despite the fact that interactions with HN02 lead to the production of NO at low pH, there is evidence that the free acid H2N2O3 is more stable toward N-N bond cleavage than HN203-.' A recent 15N NMR studyloindicates that the site of protonation in this species is the nitrogen atom bound to a single oxygen. In view of the several rather unusual features of trioxodinitrate identified above, it is a matter of some interest to characterize its vibrational modes in detail. In the one existing publication on this subject, Feltham3 identified nine infrared frequencies as fundamentals and proposed assignments by analogy to known compounds. While he arrived at a correct structural conclusion, the fact that the N-N bond is very short (1.264 A)4 was not known at the time, and the assignments given in ref 3 are generally inappropriate. Unlike the case of N2O3l1-l4with its long N-N distance, the N2032-structure is tight and cannot be approximated in terms of quasi-independent nitro and nitroso groups. In this paper we present the results of infrared and Raman spectroscopic studies of N20$-, including nitrogen isotope shifts. We report a previously unobserved fundamental at 210 cm-l. The combination of frequency and isotope shift measuremenb has provided an adequate experimental basis for a normal coordinate analysis, which in turn has enabled us to characterize the generally complex motions corresponding to the nine fundamentals of N2032-in detail.
Experimental Section Na2N203at natural isotopic abundance, Na2015NN02, Na20N15N02,and Na2015N16N02were synthesized as ( 5 ) Sturrock, P. E.; Ray, J. D.; Hunt, H. R., Jr. Inorg. Chem. 1963,2, 649. (6)Bonner, F. T.;Ravid, B. Inorg. Chem. 1975, 14, 558.
(7) Hughes, M. N.; Wimbledon, P. E. J. Chem. SOC.,Dalton Trans.
1976, 703.
(8)Hughes, M.N.;Wimbledon, P. E. J. Chem. Soc., Dalton Trans.
1977,1650.
(9)Akhtar, M.J.; Lutz, C. A.; Bonner, F. T. Inorg. Chem. 1979, 18, 2369. (10)Bonner, F.T.;Degani, H.; Akhtar,M. J. J. Am. Chem. Soc. 1981, 103, 3739. (11)Hisataune, I. C.;Devlin, J. P.; Wada, Y. J. Chem. Phys. 1960,33,
.__.
71 A
(12)Varetti, E. L.; Pimentel, G. C. J. Chem. Phys. 1971, 55, 3813. (13)Bibart, C. H.; Ewing, G. E. J. Chem. Phys. 1974, 61, 1293. Siddall, W.; Straws, H. L.; Varetti, E. L. J. Phys. (14)Bradley, G.M.; Chem. 1975, 79,1949.
1400
1200 WAVE
1000 NUMBER
800
600
400
(CM-')
Flgure 1. KBr pellet infrared spectrum of Na2N203at natural abun-
dance.
described e l s e ~ h e r e . ~ *l5NH2OH.HC1 ~J~ and Hl5NO3at 99% enrichment (Prochem) were used in the isotopic syntheses. Infrared spectra were obtained by the KBr pellet method, on a Perkin-Elmer 567 spectrophotometer, using polystyrene for calibration at 1601.4 cm-l. Reproducibility was generally well within 1cm-'; a conservative estimate of absolute error in our reported frequencies is f 2 cm-l. Examination was extended to 200 cm-l with finely divided, microcrystalline Na2N203(natural abundance only) between two polyethylene plates, using a Perkin-Elmer 180 instrument. Raman spectroscopy was carried out on a Spex Ramalog 4 system employing an argon laser (exciting line 19 436 cm-'). Spectra of crystalline materials were obtained with the samples sealed in 2-mm capillaries. Solution spectra were obtained with about 0.4 mL of concentrated (1-2 M) deaerated solution contained in a capped segment of 10-mm NMR tube. Depolarization ratios (obtainable only for the modes of greatest intensity) were determined by measurement of band areas with parallel and perpendicular polarization of the laser source. The effect of protonation on the Raman spectrum of N203'- was explored by obtaining Raman spectra on 0.08 M Na2N203solutions at several pH values in the interval 12.8-8.4, using boric acid at various concentration levels to buffer the system. The solution spectra were generally noisy in the pH region in which the gasproducing thermal decomposition reaction takes place.
Results and Discussion (1)Infrared Spectrum. An infrared spectrum is shown for Na2N203at natural abundance in Figure 1, and observed Raman and infrared frequencies are listed in Table I, along with the values given in ref 3 for comparison. As seen, agreement between our (KBr pellet) spectra and the
Vlbrational Spectroscopy of Sodium Trioxodinitrate
400
300
200
100
WAVE NUMBER (CM-')
Figure 2. Band vg in Na2N,03 (natural abundance) observed with crystalline compound between polyethylene plates.
mull spectra of Feltham is generally satisfactory. The frequencies designated u3 and v4 show doublet character in all of our infrared spectra and have been treated as such for the purposes of our normal coordinate analysis. The feature that causes v2 to look like a doublet in Figure 1, on the other hand, appears as a shoulder or change of slope in most other spectra, and the main feature is taken to be the fundamental in this case. In order to identify a total of nine fundamentals, Feltham3 was understandably moved to include the weak feature that is seen at 615 cm-' in Figure 1. The fact that we have found a new frequency a t 210 cm-l (see below) removes that necessity. The 615 cm-' feature virtually disappears, or is reduced to a change of slope, in our infrared spectra of 15N labeled compounds. We have therefore decided to treat the main feature (635 cm-l) as the fundamental in this case. In the earlier stages of our normal coordinate analysis the possibility was raised that u4 might be an out-of-plane mode; the depolarization ratio shows this to be out of the question, however. Depolarization ratio measurements could not be made a t the lower frequencies with enough precision to identify out-of-plane modes unambiguously. However, a GF-matrix calculation suggested the possibility that the two out-of-plane fundamentals might be at 635 cm-' and a t 210 cm-'. The latter had not been seen prior to this prediction, but a band was observed, nearly exactly where predicted, when it was experimentally sought (Figure 2). While 635 cm-' seems high for an out-of-plane mode, the situation is interestingly analogous to that in the Nz03molecule, in which the out-of-plane modes have been identified by Bradley et al.,14 as occurring at 624 and 76 cm-'. (2)Raman Spectrum and Effects of Protonation. From the C, symmetry of Nz032-it is expected that all modes will be both Raman and infrared active. As seen in Table I this expectation has been realized in all cases except the new mode at 210 cm-' (IR),which we have not attempted to observe in the Raman. A number of differences in frequency between the IR and Raman spectra for corresponding modes may be noted, the latter generally somewhat lower; the frequencies u1 and v2 are also lower
The Journal of Physical Chemistry, Vol. 85,No. 26, 1981 4053
I400
1300
1200
1100
WAVE NUMBER
1000
900
(CM-')
Flgure 3. Raman spectra of NaN ,O ,, 0.080 M solution in borate buffer at several pH values in the monoprotonation interval (pK, = 9.7).
in Raman spectra acquired for dissolved Na2N20,than for microcrystalline samples. It is interesting to note that the doublet character observed at v3 and u4 in the IR is completely absent in Raman,and particularly so that only one band corresponding to u8 is seen in the Raman. The effect on solution Raman spectra of lowering pH through the interval 12.8-8.4, corresponding to formation of the monoprotonated anion HN20< (pK2 = 9.7) is seen in Figure 3. The figure shows only the region that includes v I - v ~ . While the spectra on which the Table I values are based were obtained using concentrated Na2N203solutions (1-2 M) a t high basicity, these spectra were obtained for relatively dilute solutions (0.08 M) because of the difficulty of realizing adequate buffering in the presence of the strong base N2032-. For the weak bands u5 and below it is not possible to discern a pH effect under these conditions. (The band u5 is not seen at all because of a competing band in borate at 744 cm-', despite the fact that the intensity of the latter decreases and a new one a t 876 cm-' appears during traversal of the pH interval.) It is also not possible to say anything with great certainty about the effects of pH on v2 and u3, although the intensity of u3 apparently drops to a very low level, and v2 appears to become sharper and possibly to undergo some blue shift. (The band a t 1124 cm-l in these spectra is a feature of the buffer.) The effects on u1 and u4 are dramatic and clear: the intensity of v1 is drastically reduced, and the intensity of u4 remains essentially constant while the frequency itself undergoes a blue shift to the extent of about 20 cm-'. (The loss of intensity at u1 due to HN203-decomposition during acquisition time was determined and found t~ be very small.) Isotope Shifts. Infrared frequencies for trioxodinitrate at natural abundance, and for 15Nlabeled compounds in
4054
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981
Bonner et ai.
TABLE 11: Experimental and Calculated Frequencies of Isotopic Molecules of N,03' frequencies,b cm-'
isotope shifts: cm-l
14-14'
15-14
14-15
1397 (1397.0) 1276 (1276.0)
1387 (13 86.9) 1273 (1273.2)
1370 (1365.9) 1249 (1246.4)
15-15 1356 (1355.1) 1243 (1243.1)
1 985 97 8 1 (978.1)
(1092.8)
1101 (1111.0)
1082 I1O2t (1091.7)
749 (742.0) 632 (632.3) 450 (448.8) 378 (373.3)
987 978 (975.6) 751 (744.5) 623 (619.3) 448 (449.9) 382 (372.9)
(961.5) 749 (741.8) 61 8 (616.5) 448 (447.7) 37 8 (372.4)
7.2
(207.3)
(210.0)
(206.9)
0.4
VU
1102 1126 (1112.5)
7 52 (744.7) 635 (635.0) 450 (451.1) 381 (373.8) 210 (210.4)
1102t ioao 2: I (963.5)
-
1125t t
-
E1
-
15-14
14-15
15-15
0
10 (10.1) 3 (2.8)
27 (31.1) 27 (29.6)
41 (41.9) 33 (32.9)
1.5
24 22 (19.7)
1 (1.5)
24 20 (20.8)
(2.5) 1 (0.2) 12 (15.7) 2 (1.2) 1 (0.9)
3 (2.9) 17 (18.5) 2 (3.4) 3 1.4
(0.4)
3.5
AV d
0
I
I
l3 14 (14.6) 3 (2.7 1 3 (2.7) 0 (2.3) 3 (0.5)
3.4 7.3
0
1.1
-
-
(3.1)
3
-
I l 4t 14 (16.6)
-
The numbering of v 8 and v g is in accordance with our conclusion that these are out-of-plane modes. Numbers in parentheses are calculated using the F-matrix of Table V. Those without the parentheses are experimental values. ' The two numbers designate the isotopic mass numbers of the two nitrogens, the first number being that of atom no. 2 (cf. Figure 3). A v = liobd - v c k d , natural abundance. e Isotope shift = (14-14) - (isotopic molecule). a Frequencies are arranged in descending order.
TABLE 111: Definition of Valence Coordinates no.
symbol
description NO bond stretching; 1-2a NN bond stretching; 2-3 NO bond stretching; 3-4 NO bond stretching; 3-5 ONN angle bending; 1-2-3,
1 2 3 4 5
P
6
PIR,
7
P,R,
NNO angle bending; 2-3-4, weighted by R , NNO angle bending; 2-3-5,
8
P,R,
O N 0 angle bending; 4-3-5,
R r1 r2
aR0
weighted by R , = 1.264 A
weighted by R , R(1-2)
weighted by R ,
9b
0 RO
1 Ob
TRO
NNO, out-of-planewagging; 2-3-4-5, weighted by Ro
ONNO torsion; 1-2-3-5, weighted by R ,
a The numbers following a semicolon in each coordinate definition identify the atoms involved, the latter being defined in Figure 4c. See footnote 15.
the two possible singly labeled and one doubly labeled configuration, are shown in Table 11. Isotope shifts were also determined for most of the Raman-detectable frequencies; since these were in generally good agreement with the infrared frequencies shown in Table 11,they have not been reported. The values given both for frequencies and for isotope shifts were then employed as the experimental information base to be matched in carrying out our normal coordinate analysis. (3) Normal Coordinate Analysis. The normal coordinate analysis for N2032-was performed by means of Wilson's GF-matrix method based on the valence coordinate system defined in Table III.16 A modified version (16)The order in which the atom numbers appear in the definitions of the out-of-plane wag and torsion is important, because it specifies the phases of these displacement coordinates relative to the others. The orders given here are in accordance with the input format for the Schachtschneider-Snyder G-matrix program.
= 1.3472,4(1-2-3) =112.9'
R ( 2 - 3 ) = 1.2642, &2-3-4 1 = 1 1 8.4' R ( 3 - 4 ) = 1.3102, q(2-3-51 =122.5' R(3-5)
= 1.3228,
4(4-3-51 =119.1'
Flgure 4. Equilibrium molecular geometry.
of the Schachtschneider-Snyder program16 was used to solve the vibrational secular problems. The molecular geometry used to calculate the G-matrices is shown in Figure 4.4 All oxygen atoms in all isotopic molecules are assumed to be lag. The similarity of the masses of the atoms in the system make N2032-vibrationally highly coupled, and it is almost impossible to characterize its normal vibrational modes by a single word or two. For the same reason any procedure of best-fitting an F-matrix for the set of experimental frequencies by an automatic iterative algorithm would lead to such hazardous situations as those of local minima, slow convergence, and divergence. Therefore, we employed a more deliberate method in which a table of first partial derivatives of every normal frequency with respect to every F-matrix element is constructed and examined to decide on a next move in improving the overall agreements. Typically, dwk/afij was calculated by solving the secular (16) Schachtschneider, J. H.; Snyder, R. G. Spectrochirn. Acta 1963, 19, 117.
Vibrational Spectroscopy of Sodium Trioxodinitrate
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981 4055
TABLE IV : Various Possible Combinations for Out-of-PlaneFrequencies frequencies, cm" no. 1
2
3
4
14-14a
15-14
14-15
15-15
isotope shifts,b cm-' 15-14
14-15
15-15
635 381
(;: :: :) 635 450
:64( E::) 752 450
(:!E) 752 381
5
(E:::) 752 7
a
See footnote c of Table 11.
See footnote e of Table 11.
problem twice, i.e., once by using a basis F-matrix and then by using a modified F-matrix in which the value of f i j is reduced to 99% of its original value. The reduced f i j was used a t all equivalent positions. The difference in two corresponding values of wk is divided by O.Olfij to approximate r30k/dfij. The table of derivatives was first formed based on a diagonal F-matrix as a zeroth-order approximation and then the off-diagonal elements were added step-by-step. Several alternative diagonal forms of F-matrix were investigated. A new table of first derivatives based on a new basis F-matrix was constructed whenever an existing table was considered obsolete. The in-plane and out-of-plane vibrations were treated separately. Since initial progress in best-fitting the 7 x 7 F-matrix for in-plane motions was severely hindered by our lack of certain identification of the two out-of-plane frequencies, 2 X 2 matrix calculations were carried out in an effort to identify them, as summarized in Table IV. In these calculations we have given more weight to a good agreement in isotope shifts than in absolute magnitudes of frequencies. In the cases of poor agreement between experimental and calculated values of isotopes shifts in Table IV, better agreement simply could not be achieved without letting agreement in the magnitudes of frequencies go completely out of hand. Thus, cases 1through 4, representing possible combinations of observed frequencies v5 through v8, are all found to represent unlikely out-ofplane combinations. We then explored the possibility that only one of the fundamental frequencies observed experimentally up to that time actually represented an out-ofplane mode. To do this, we have best-fitted three force constants in the 2 X 2 F-matrix for out-of-plane motions to the three experimental isotope shift data for each of the frequencies v5 through v8, under the additional constraint that agreement in magnitudes of frequencies be a t least
reasonable. The entries 5 through 8 of Table IV represent these attempts; in each case a second frequency emerges from the best-fit calculations as a possible, previously unobserved, out-of-plane motion. The agreement between calculated and observed isotope shifts for cases 5, 7, and 8 (v5, vg, and v,) is poor and cannot be further improved without significant sacrifice of agreements in absolute magnitude. The isotope shift agreements for case 6, however, are satisfactory, in view of the possible experimental uncertainty of f 2 cm-'. This is the only one of the choices shown in Table IV for which the calculation gave reasonable agreement with experiment. In a sense this agreement is amazingly good, when one considers the fact that the present 2 X 2 secular problem is overdetermined, since there are more conditions (>3) that have to be satisfied than the number of adjustable parameters (=3). This result convinced us that there must be a fundamental still undiscovered. We therefore examined a natural abundance NazNz03sample in the far-IR region and discovered a strong absorption band at 210 cm-l, which is almost exactly the frequency predicted (210.4 cm-') for case 6. This new fundamental has been included as vg in Table 11, along with its calculated isotopic frequencies. An Fmatrix for the in-plane motions was then best-fitted to the frequencies vl through ul by the method of first derivative table as described earlier. The results have been entered in Table 11. Table I1 thus represents the best agreement between the experimental values, i.e., those without parentheses, and the calculated values, i.e., those in the parentheses. The fiial F-matrix used to obtain the normal frequencies of Table I1 has been summarized in Table V. In Figure 5 we show the forms of the nine normal vibradrawn to the correct relative tional modes of 14Nz1s032scales of the eigenvectors.
4056
The Journal of Physical Chemistry, Vol. 85, No. 26, 1981
TABLE V : Best F-Matrixu diagonals in-plane
fpol = 0.5438
fr,p, = -0.1582
fR = 4.40
fRrl = 1.28
fr,@, = -0.076
frl = 5.00
fRr, = 1.82
fa@, =
0.14 fplp, = 0.061 fRp, = 0.823 fplp3 = 0.060 fRp3 = 0.2769 fOp,p, = -0.0626 frlr2 = 1.05 frlpl = 0.082 fe7 = -0.0626
fp
= 5.28
off-diagonals
f,., = 5.60 fa = 1.565 fp, = 0.6885 fP, = 1.0014 fb3 = 2.0029 out-of-plane fe = 0.3180 f7 = 0.1240
fRol = -0.085
a All elements in units of mdyn A - 1 . Note that all bending coordinates are weighted by R(NN) = 1.264 A , so that every coordinate has a dimension of length.
Figure 5. Normal modes of 14N2’60~-.
I t is seen in Figure 5 that the vibrational motions identified in this study are extremely intricate and complex. It would therefore be dangerous and misleading to characterize any of the normal modes by one or two words,
Bonner et al.
and we will refrain from any attempt to impose descriptive names upon these motions. The reason for this complexity lies in the facts that all five atoms are nearly alike in mass, that the two parts of the anion are held together by a short N=N bond, and that charge centers are present at oxygen atoms. The effects of the latter two circumstances can be seen in a comparison with the vibrational motions of the neutral N203molecule, in which the N-N distance is 1.864 A. In this case terms descriptive of “nitro” and “nitroso” motions are appropriately a~p1ied.l~ In the case of N2032-, u1 (1397 cm-l) was designated as “asym nitro stretch” in ref 3, yet its principal feature (Figure 5) could only be described as a staggering N-N stretch. The N-N stretch in N203is found at the extraordinarily low frequency 160 cm-’, and a clearly identifiable asymmetric NO2 stretch is observed at 1630 cm-l (u7 and u2 in ref 14). Our fundamental v2 (1276 cm-l) was designated “sym nitro stretch” in ref 3, and while N203exhibits a mode at 1303 cm-l that can appropriately be called that (us, ref 14), there is virtually no discernible character of this sort in the v2 vibration that emerges from our analysis (Figure 5). We should note, however, that the out-of-plane modes V8 and ug are similar to the motions described for N203. Our V8 (635 cm-l) is even closely similar in magnitude to the corresponding vibration in N203(624 cm-l), described as although in N2032- some motion “nitro out-of-plane in the nitroso group is also indicated. The effect of protonation on the Raman intensity of u1 (Figure 3) is interpretable in terms of a change in polarity at the N=N bond, since substantial motion in both N atoms is involved and protonation is known to occcur at N(l).10The intensity at u3 appears similarly affected and could be interpreted in the same manner since the nitroso N-0 bond is involved in a major way. There is some suggestion of blue shift at u2, and v4 definitely undergoes a blue shift without loss of intensity. In the case of u4 a strong NNO bending component is involved, and while the blue shift may reflect a tightening of bonding at the nitro end, the maintenance of intensity is difficult to explain in view of the substantial motion at N(l).
Acknowledgment. We wish to thank Ms. Tu-Ying Chang for useful preliminary work on the Raman spectroscopy. We are also grateful to Drs. Ralph E. Weston and David M. Hanson for helpful discussions and assistance.
