J . Phys. Chem. 1987,91, 6683-6686
6583
lution by laser flash photolysis. In an ideal experiment, one would radical pairs that can be detected by EPR spectroscopy. 'DBC use a wavelength where only 1 is absorbing and exclude the does not abstract D atoms from perdeuteriated matrices but inpossibility of secondary photolysis of 'DBC. Unfortunately, it stead reacts to give a complex mixture of products. Photolysis has become evident from flash photolysis studies that this condition of 'DBC or 3Fl at 77 K gives 'DBC* or 'Fl* which will abstract cannot be met for 'DPC and 'DBC; the overlap in the absorption D atoms from deuteriated matrices. The formal C D insertion spectra of diazo precursor and triplet carbene is too s e ~ e r e . ~ , ~ ~adducts , ~ ~ formed in low yields arise either from a singlet-state It is reasonable to expect 'DBC to absorb light of 365 nm because insertion reaction pathway or the secondary photochemical process A, of this species is between 345 and 350 This was mentioned above. confirmed by measuring the radical pair EPR signal intensities as a function of light intensity (365, 405, and 436 nm). Experimental Section In the case of DAF, it is possible to cleanly convert DAF to DAF, DPDM, and 1 were prepared by known method^.^ 'Fl without secondary carbene photolysis by the use of longSamples for EPR were sealed in 4-mm quartz tubes after three wavelength UV light (A > 360 nm). Short wavelength (A > 295 freeze-pumpthaw cycles. They were photolyzed (1000-W Hgnm) photolysis of matrix-isolated DAF and 'FI produces a RP-like Xe lamp with a CuS04 and an Oriel 350-nm long pass filter) at spectrum. This is the primary basis for attributing the spectra 77 K in the cavity of a Varian E-12 EPR spectrometer; thus, derived from Fl to radical pairs. The assignment is strengthened samples were exposed to light of 365,405, and 436 mm. An Oriel by the fact that R P spectra derived from DAF vary as the matrix 295-nm long pass filter was used for short-wavelength photolysis; is changed and the R P signal intensity depends on the light inthis transmitted the 303- and 313-nm mercury lines. Sample tensity squared. The RP's derived from F1 are less stable than concentrations were varied between 0.1 and 0.001 M. Product those formed from DBC. This prevented our correlating 'Fl decay studies were conducted by photolysis of 0.01 M solutions of DPDM with 'RP formation. We suspect that the fluorenyl radical, being and 1 in toluene or toluene-d8 containing naphthalene as an incompletely planar, diffuses more rapidly than the nonplanar DBC ternal standard sealed in Pyrex tubes after three freeze-pump derived radical. The decay of F1 derived RP's is accompanied thaw cycles. Samples were photolyzed by using Rayonet RPR by the formation of an isolated free-radical absorption in the center 3500 lamps (principal emission at 365 nm) and analyzed by GLC of the R P spectrum. This suggests that diffusive separation of on a Hewlett-Packard 5830 A chromatograph with a 6-ft by l/s-in. the F1-derived RP's (that can be detected by EPR) is responsible SE-30 column. Chromatogram peaks were identified by coinfor their disappearance. jection with authentic samples and by GCMS. Conclusion Registry No. 'DBC,15306-40-8;'Fl, 2762-16-5;D1,7782-39-0;2Photolysis of 1 and DAF in organic matrices at 77 K produces MTHF,96-47-9;toluene, 108-88-3;toluene-d8,2037-26-5;2-propanol, 'DBC and 3Fl which react by H atom transfer to give triplet 67-63-0; 2-propanol-d8,22739-76-0; methyicyclohexane, 108-87-2; methylcyclohexane-d14,10120-28-2;methanol, 67-56-1;methanol-d4,81 1(24) Horn, K. A.; Allison, B. D. Chem. Phys. Lett. 1985, 216, 114. 98-3; ether, 60-29-7; ether-dlo,2679-89-2.
Structures, Energies, and Vlbrational Frequencies of Intermediates and Transition States in the Reaction of NH2 and NO John A. Harrison, Robert G. A. R. Maclagan,* and Andrew R. Whyte Department of Chemistry, University of Canterbury, Christchurch, New Zealand (Received: December 9, 1986; In Final Form: September 1, 1987)
The optimized geometries for reactants, intermediates, transition states, and possible products for the reaction of NH, and NO, obtained at the HF/6-31G* level of theory, are given. HF, MP2, and MP4SDQ energies are given. Harmonic vibrational frequencies are reported for all species. The study confirms that the pathway leading to the production of N2 and H 2 0 via intramolecular rearrangement of N2H20species can occur with a small or no activation barrier. The channel to N2H OH will be at most a secondary pathway.
+
Introduction The reaction between N H 2 and NO is currently of considerable theoretical as well as experimental interest, both as a key step in the thermal DeNO, process and as an example of a bimolecular reaction which is fast in spite of having a complex mechanism.' Early experiments2*' suggested that N 2 and H 2 0 were the only products. On the basis of these results, and the observed negative temperature dependence of the reaction, Miller and co-workers4 suggested that the mechanism involves a sequence of isomerisations
starting from an initially formed H2N-NO (N-nitrosamide) complex (I), with the possibility of multiple product channels arising from the decomposition of various stable (relative to N H 2 NO) N 2 H 2 0isomers. Recently significant yields of OH have been reported by several groups5-' but further studies' suggest a branching ratio of about 0.13 which is lower than those reported in the earlier works. A systematic study of the NH2 NO potential surface was subsequently carried out by Casewit and Goddard8 using the
(1) Gilbert, R. G.; Whyte, A. R.; Phillips, L. F. Int. J. Chem. Kine?. 1986, 18, 721-37. (2) Fenimore, C. P.; Jones, G . W. J . Phys. Chem. 1961, 65, 298-303. (3) Gehring, M.; Hoyermann, K.; Schacke, H.; Wolfrum, J. Fourreenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1972; pp 99-105. (4) Miller, J. A.; Branch, M. C.; Kee, R. J. Combust. Flame 1981, 43, 81-98.
( 5 ) Silver, J. A.; Kolb, C. E. J . Phys. Chem. 1982,86, 3240-3246; 1987, 91, 3713-3714. (6) Stief, L. J.; Brobst, W. D.; Nava, D. F.; Borkowski, R. P.; Michael, J. V. J . Chem. Soc., Faraday Trans. 2 1982, 78, 1391-1401. (7) Andresen, P.; Jacobs, A,; Kleinermanns, C.; Wolfrum, J. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1982; pp 11-22. (8) Casewit, C. J.; Gcddard, W. A. J . A m . Chem. SOC.1982, 104, 3280-3287.
0022-3654/87/2091-6683$01.50/0
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0 1987 American Chemical Society
6684 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
GVB-CI method with a DZP basis set, and geometries optimized with a 4-31G basis set at the R H F level of theory. They calculated structural and thermochemical data for nine isomers of formula N 2 H 2 0 ,including structure I for which a planar structure was found. The structure of I was also studied by Ha and mworkersg in an S C F study using the 3-21G and 6-31G* basis sets. However, these calculations did not take account of the importance of polarization functions for an accurate description of N-nitrosamide structure. We have recently reported an ab initio study of the geometry and vibrational frequencies of this molecule using basis sets ranging from STO-3G to 6-31 1G**,I0 which showed that inclusion of polarization functions results in a nonplanar structure for H2N-NO (I), the planar structure predicted with the 4-31G basis set being a local maximum on the potential energy surface (one imaginary frequency). The structures of N-nitrosamide (I) and the four isomers of hydroxydiimide (11-V), and the reaction path for the conversion of I I1 111, were studied by Thomson and co-workers' in an S C F study using STO-4G and 4-31G basis sets. Melius and Binkley12 have calculated the heats of formation and the free energies for an extensive set of reactants, products, intermediates, and associated activated complexes occurring along possible pathways for the reaction of N H 2 and N O using their BAC-MP4 method. However, they did not provide any details of calculated geometries or vibrational frequencies, data which are necessary if analysis of the reaction mechanism is to proceed beyond simple thermodynamic feasibility considerations. The first attempt to apply a statistical kinetic theory to the study of the reaction of N H 2 and N O was made by Abou-Rachid and co-workersI3 who used the results of SCF-CI calculations in an RRKM-type treatment of a simplified version of the mechanism proposed by Miller and c o - ~ o r k e r s .This ~ study, however, was based upon the planar 4-3 1G structure for N-nitrosamide, and several assumptions of questionable validity were made in the RRKM calculations. More recently the pathway leading to the experimentally observed major products NH2 N O N2 + H 2 0 (1) has been studied by Gilbert and co-workers1 using the master equation formulation of unimolecular rate theory to treat correctly collisional effects and the presence of significant anharmonicity in the activated complex leading from I toward product formation. This treatment, though, is still approximate, as subsequent isomerization processes were not explicitly considered; Le., a single "effective" barrier to rearrangement and subsequent product formation was assumed. A recent study by Miller and co-workersI4 on the multipleproducts, combining statistical channel reaction 0 H C N (Le,, RRKM) theory with the results of ab initio calculations, shows that a detailed analysis of such a system is feasible, although the N H 2 N O system is considerably more complex and the analysis is further complicated by the effects of anharmonicity. Toward this end, and following on from our previous study of I, we have investigated the potential energy surface for reaction 1, calculating energies, optimized geometries, and vibrational frequencies for four intermediates (11-V)and six activated complexes (VI-XI) occurring along the pathway leading to N2 H20. We compare this pathway to that giving N 2 OH H as products subsequent t o formation of N2H + OH: NH2 N O N2H + OH (2)
--
H1
\
0
/
H1 \
N1-N2
-
-
+
+
+
+
+
+
+
-N,+H+OH
(3)
~~
(9) Ha, T.-K.; Nguyen, M T.; Ruelle, P. J . Mol. Strucr. 1984, 109, 339-350.
(10) Harrison, J. A.; Maclagan, R. G. A. R.; Whyte, A. R. Chem. Phys. Lett. 1986, 130, 98-102. (1 1) Thomson, C.; Provan, D.; Clark, S.Inr. J. Quantum Chem., Quantum Biol. Symp. 1977, 4, 205-215. (12) Melius, C.; Binkley, J. S. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 575-583. (13) Abou-Rachid, H.; Pouchan, C.; Chaillet, M. Chem. Phys. 1984,90, 243-255. (14) Miller, J. A,; Parrish, C.; Brown, N. J. J . Phys. Chem. 1986, 90, 3339-3 345.
P
I
H2
H2
II
111
\
/o
Ht
V
IV
hll 0
'
+
~
Harrison et al.
0 / N1-N2
XI
Figure 1. Structures of N 2 H 2 0intermediates and transition states.
loot
n
NLH
+ OH
.....
-200
t
N
-500
i
-
Figure 2. Relative energies, calculated at the MP4DQ/6-31G8//HF/ 6-31G8 level of theory, for reaction pathways for N H 2 + NO products. Vertical energy scale is in kJ mol-'.
Details of Calculations The geometries were optimized at the HF/6-31G* level of theory (RHF for closed shell, U H F for open shell) and harmonic vibrational frequencies and MP2 to MP4SDQ energies were calculated at these optimized geometries. All calculations were done using the GAUSSIAN 82 p r ~ g r a m . ' ~The 6-31G* basis set is of double-zeta quality and includes d polarization functions on non-hydrogen elements. This basis set has been shown by DeFrws and McLeanI6 to yield vibrational frequencies with a mean accuracy within 49 cm-' for first-row molecules in calculations at the HF level of theory when the results are scaled by an empirical factor of 0.89. Results and Discussion Previous s t ~ d i e s ~ * "have - ' ~ shown that at room temperature the most energetically feasible pathways are (l), (2), and (3), while at high temperatures other pathways could occur. Reaction 1 has been postulated to occur by rearrangement of a N 2 H 2 0 intermediate and final 1,2 elimination of H 2 0 , while N2H (reaction 2) could be formed from decomposition of one of several of the same intermediates involved in the reaction scheme of (1). Our reaction scheme for (1) is qualitatively equivalent to that used by Melius and Binkley,Iz differing in that the rotation of OH about (!5),Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; Whiteside, R. A.; Schlegel, H. B.; Fluder, E. M.; Pople, J. A. GAUSSIAN 82; Carnegie-Mellon University, Pittsburgh, PA, 1983. (16) DeFrees, D. J.; McLean, A. D. J . Chem. Phys. 1985, 82, 333-341.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6685
Reaction of N H 2 and N O TABLE I: 00timized Geometric Parameters NH2,’
structure
NH2N0,b
NO
N1-N21 A Ni-Hi, A N I - H ~ ,A N2-0,A 0-HI, 8, LN2NIH1,deg LN2N1H2,deg LN,N,O, deg LN,OH, deg
1.013 1.013 1.127
HNNOH
I
I1
111
IV
V
N29 H,O‘
N2H9 OH
1.316 1.000 0.994 1.184
1.206
1.203
1.202
1.199
1.078
1.179
1.007 1.334 0.958
1.008 1.351 0.949
1.020 1.347 0.957
1.017 1.366 0.949
107.0 112.8 107.5
106.0 110.4 104.6
113.4 118.6 110.8
111.0 112.8 105.4
117.3 115.4 114.5
1.029 0.947
0.958 113.0
Transition States structure
X
VId 1.240 1.002 1.638
VIP 1.201 1.011
VI11 1.179 0.982
IXf 1.199 1.017
1.172 0.981
XIS 1.128 (1.140)
1.260 (1.327) 118.6 50.8
1.383 0.951 106.0
1.380 0.956 179.9
1.391 0.953 111.9
1.406 0.950 176.9
1.724 1.837 0.953 (98.9)
103.3 (78.5)
110.6 106.3
115.2 107.4
114.8 107.8
111.7 103.9
40.8 90.0 139.8
‘LHINH, = 104.3’. bwON2NIH1 = -13.4; w O N ~ N I H=~195.2. ‘LHIOH2 8.75. f w N I N 2 0 H I = 65.0. ELHOH = 158.7’. 0-H2 = 1.485 A.
TABLE 11: HF, MP2, and MP4SDQ Total and Relative Energies‘ structure E(HF) I
-184.805 586 -184.826480
-185.256 365 -185.337 296
E(MP4SDQ) -185.274 534 -185.341 926
AE(HF) 0.0 -54.9
I1 111 IV V
trans-cis trans-trans cis-cis cis-trans transition states
-184.825 151 -184.822464 -184.810008 -184.822 198
-185.334729 -185.331 126 -185.322680 -185.333 654
-185.341 571 -185.338 654 -185.329480 -1 85.340 984
I I1 I1 111 I1 IV IV v I11 v V products
-184.747 581 -184.809 332 -184.744409 -184.804977 -184.746 270 -184.750 187
-185.284015 -185.315 552 -185.262915 -185.315405 -185.264672 -185.289 940
-185.283406 -185.324 125 -185.267 914 -185.323 416 -1 85.270 022 -185.290 197
-184.807 677
-185.236 141
-185.262669
-184.786 185 -184.825 503 -184.954696
-185.226 567 -185.276760 -185.453037
-185.248 575 -185.287050 -185.457 825
+
VI VI1 VI11 IX
X XI
----
N2H
+ OH
transition state
+
N2H O H N2 O H H N2 + H2O
+
+
-+
N2
+ H + OH
N2 1.119 1.386
4
+H
113.3
105.5’. dN1-H1 = 1.285 A; LN2NlH1 = 80.8’. ‘wNIN20HI =
NH2 N O NH2NO HNNOH
~
N2H
E(MP2)
AE(MP2) AE(MP4SDQ)
0.0 -212.5
0.0 -176.9
-5 1.4 -44.3 -11.6 -43.6
-205.7 -196.3 -174.1 -202.9
-176.0 -168.3 -144.3 -174.5
+152.3 -9.8 +160.6 +1.6 +155.7 +145.5
-72.6 -155.4 -17.2 -155.0 -21.8 -88.2
-23.3 -130.2 +17.4 -128.3 +11.8 -37.2
+5.5
+53.1
+31.2
+50.9 -52.3 -391.5
+78.2 -53.5 -516.4
+68.2 -32.9 -48 1.2
“Total energies ( E ) in hartrees and relative energies (AE) in kJ mol-’. the N O bond, after the initial 1,3 H migration, is formally included. The optimized HF/6-31G* geometric parameters for the N 2 H 2 0 isomers and transition states as well as those for the reactants and possible products are given in Table I. The structures of the intermediates and transition states are given in Figure 1. Structure IX is similar to VI1 with H2 cis to 0. Structure X is similar to VI11 with H I trans to N1. The HF/6-31G*//HF/6-31G*, MP2/6-3lG*//HF/6-3lG*, and MP4SDQ/6-3lG*//HF/6-3 lG* energies are given in Table 11. The relative MP4SDQ/6-3lG*//HF/6-31G* energies are plotted in Figure 2. The MP2 energies include core contributions, but the MP4SDQ energies do not. Intermediate I has been discussed in our previous paper.1° In going from N-nitrosamide (I) to the hydroxydiimide isomers (11-V), the N-N bond shortens as it goes from a single to a double bond, and the N - 0 bond lengthens in going from a double to a single bond. The geometric parameters for the transition state VI are intermediate between those of I and 11, except that the N N O bond angle is about 10’ less than in both I and 11. In the transition states the N-0 bond lengths are slightly longer and the N N O bond angles slightly larger than in the intermediates.
In the final transition state H2 is still relatively strongly attached to N1. At high temperatures this transition state could lead to the formation of N2H O H rather than N 2 H 2 0 . The geometric parameters for isomers 11-V are in good agreement with published HF/4-3 1G optimized s t r u c t ~ r e s 8 -The ~ ~ only noticeable differences between the 4-31G and 6-3 lG* geometries for I-V are a shortening (about 0.05 A) in the N - 0 bond length, and a decrease (about 3’) in the N N H and N O H bond angles, both of which may be attributed to the inclusion of d polarization functions on the nitrogen and oxygen atoms in the 6-31G* basis set. The STO-4G bond lengths of Thompson are longer for the transition state VI, and the N-0 bond length is 0.06 A longer in the 4-31G structure than in the 6-31G* structure, the rest of the parameters being in reasonable agreement. Table I1 shows that at the HF/6-31G* level the activation energy barriers to rearrangement are large. Inclusion of correlation substantially lowers the energies of the transition states and intermediates relative to the reactants. While at the MP2/631G*//HF/6-31G* level our results imply that reaction 1 can proceed without an activation barrier, in accord with the BACMP4 study of Melius and Binkley,I2 the MP4SDQ/6-3 1G*//
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6686 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Harrison et a].
TABLE HI: HF/6-31G* Vibrational Frequencies (em-') NO (CJ, NH2 (C2")
A,, 1712
E+,2221 A,, 3605
BZ, 3705
NHzNO (I) 370 690 744 1251 1392 1781 1882 3772 3955
TS VI
TS VI1
A', 2444i A", 705 A', 1058 A", 1282 A', 1343 A', 1507 A', 1693 A', 2316 A'. 381 1
5391' 746 1066 1074 1462 1592 1939 3663 4058
HNNOH (11) A", 668 A', 738 A", 1080 A', 1136 A', 1547 A', 1618 A', 1925 A', 3710 A', 3953
HNNOH (111) HNNOH (IV) A", 498 A", 475 A', 735 A', 761 A", 1096 A", 1089 A', 1107 A', 1100 A', 1510 A', 1455 A', 1618 A', 1635 A', 1966 A', 1937 A', 3692 A', 3505 A', 4108 A', 3946
TS VI11 A', 1561i A", 544 A", 6 13 A', 727 A', 971 A', 1508 A', 2001 A', 3976 A'. 4103
- -
+
--
-+
-
+
+
+
(17) Curtiss, L. A.; Drapcho, D. L.; Pople, J. A. Chem. Phys. Lett. 1984,
103, 437-442.
TS X
TS IX 461i 725 1030 1103 1441 1585 1943 3559 4035
HF/6-3 1G* calculations actually give a small activation barrier. It is interesting to note that, although structures I-V have their relative energies lowered by between 122 and 133 kJ mol-' when the H F and MP4SDQ energies are compared, the effect on the transition states VI-XI is far more erratic, with relative energies lowered by between 120 and 183 kJ mol-'. The MP2 lowerings are even larger. The H F level of theory is known to give a poor description of molecules incorporating bonds between electronegative elements.I6 The difference between the HF results and those including correlation is not surprising. The activation barriers are also lowered by the inclusion of the thermal (vibrational and rotational) energies. The addition of the thermal energies gives reasonable agreement with the barrier heights given by Melius and Binkley: the barrier for I VI I1 is 141.0 kJ mol-l at 0 K, compared with 118.8 kJ mol-'; the barrier I11 X V is 169.1 kJ mol-' compared with 161.1 kJ mol-'; and the barrier V XI N 2 H 2 0 is 114.8 kJ mol-' compared with 92.0 kJ mol-'. Agreement is actually better with the MP2 energies (127.2, 164.0, and 92.3 kJ mol-', respectively). Relative to the reactants, the enthalpies of the intermediates calculated by us are about 52-56 kJ mol-' higher than those reported by Melius and Binkley. The other major systematic error in these calculations is the quality of the 6-31G* basis set. The existence of the comparison by DeFrees and McLeanI6 between experimental harmonic vibrational frequencies and frequencies computed at the HF/6-31G* level of theory was a major factor in the choice of the 6-31G* basis set for this study. Going to the 6-31G** basis set changes AE (MP2) for (I) from -212.5 to -211.5 kJ mol-'. Including thermal energies reduces the change to 0.5 kJ mol-l level of theory, at 0 K. At the MP4SDQ/6-31G**//HF/6-31G** structure I1 is actually 3.45 kJ mol-' lower than structure I. As was found by Melius and Binkley, the major barrier to rearrangement for pathway I is the in-plane migration of the H atom (VIII). The barrier to rotation about the N-N double bond is higher than the barrier for in-plane migration. Our MP4SDQ calculations for reactions 2 and 3 are in reasonable agreement with the calculations of Melius and Binkley and Curtiss and co-workers.17 The extensive study of the N2H N2 H system by Curtiss and co-workers has shown that the MP2 level of theory overestimates the dissociation energy of N2H and underestimates the height of the transition state relative to N2H + OH. As noted by Curtiss and co-workers, there is a large amount of spin contamination ((Sz) = 0.879) in the U H F wave function. N 2 H + OH lie above the reactants in energy and N, + O H + H lie just below the energy of the reactants. With the bigger 6-31 1G** basis set, the relative energy of N 2 H OH at
HNNOH (V) A", 475 A', 711 A', 1060 A", 1147 A', 1477 A', 1607 A', 1975 A', 3576 A', 4101
A', 15541 A", 500 A", 631 A', 717 A', 898 A', 1482 A', 2036 A', 4088 A'. 4120
Nz(Dd, HzO ( C b )
N2H (CS), OH (C")
A,, 1827 Z,+, 2758 A,, 4070
A', 1264
B,, 4189 TS XI
A', 1615i A", 91
A', 1661 A', 3216 Z+, 3997 TS N2 H A', 19191
+
A', 616 A', 859 A", 1131 A', 1184 A', 2122 A', 2326 A'. 4048
A', 896 A', 2136
MP4SDQ level of theory is lowered to 30.0 kJ mol-', that of N2H(TS) OH is raised to 71.6 kJ mol-' and that for N 2 O H H is raised to -16.2 kJ mol-'. When vibrational energies are added to the transition state X for reaction 1, it is still about 12 kJ mol-' below the energy of the barrier for the reaction of N2H going to N2 and H and the height of the activation barrier for the formation of N2H + OH from the hydroxydiimides will be at least as large. Curtiss and co-workers have argued that it is likely that once formed any N2H would spontaneously dissociate. From our results, therefore, we must conclude that the channel corresponding to reactions 2 and 3 occurs with an activation barrier higher than that for reaction 1 and thus will not be as important as channel 1. The barrier heights for pathway 1 are, from the MP2 calculations, in agreement with a reaction that exhibits a negative temperature dependence and, due to the absence of any activation energy requirements the reaction, could be expected to be fast, as o b ~ e r v e d . ' ~ JThe ~ MP4SDQ calculations do give a barrier without corrections such as those used by Melius and Binkley. The unscaled HF/6-3 1G* harmonic vibrational frequencies are presented in Table 111. The vibrational frequencies for structure I have been discussed in our previous paper.1° Once scaled, these frequencies should be reliable enough for use on RRKM and other unimolecular theories, especially those of higher frequency, as these are less likely to be affected by anharmonicity than the low-frequency vibrations. Our frequencies are significantly higher, especially at low frequencies, than those given by Abou-Rachid and co-workers. l 3
+
+
+
Conclusions This study suggests that (i) the pathway leading to the production of N 2 and H 2 0via intramolecular rearrangement of the stable (with respect to NH2 NO) N 2 H 2 0species can occur with a small or no activation barrier; (ii) the channel to N2H will be at most a secondary pathway, with the intermediate N2H molecule dissociating very rapidly to N2 H, and is thus not an important intermediate at low temperatures for reaction schemes involving products; (iii) the harmonic frequencies, once NH2 N O scaled, and the energy data presented here will be adequate for the calculation of the various rates of intramolecular rearrangement and of the overall rate constant.
+
+
-
+
Acknowledgment. We acknowledge the Department of Civil Engineering, University of Canterbury, for the generous provision of computer time. Registry No. NH2, 13770-40-6; NO, 10102-43-9. (18) Whyte, A. R.; Phillips, L. F. Chem. Phys. Lett. 1983, 102, 451-454. (19) Whyte, A. R.; Phillips, L. F. J . Phys. Chem. 1984, 88, 5670-5673.
