Hydrogen bonded complexes and the HF vibrational energy distributions from the reaction of F atoms with NH2 and NH3

321

Chemical Physics 114 (1987) 321-329 North-Holland, Amsterdam

HYDROGEN BONDED COMPLEXES AND THE HF VIBRATIONAL ENERGY DISTRIBUTIONS FROM THE REACTION OF F ATOMS WITH NH, AND NH, J.D. GODDARD Guelph - Waierloo Centre for Graduate GuelJvh, Ontario, Canada NI G 2 Wl

D.J. DONALDSON

Work in Chemistry,

Department

of Chemistry

and Biochemistty,

University

of Guelph,

* and J.J. SLOAN

Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR6

Received 8 December 1986

The gas phase reactions of fluorine atoms with amino radicals and ammonia molecules: F(*P)+NH,(‘B,) -f HF(‘Z+)+ NH(3X-) and F(2P)+NH3(‘A,) + HF(‘X+)+NH,(*B,) produce hydrogen fluoride with very different primary vibrational energy distributions as determined by low-pressure chemiluminescence studies. The reaction with NH, yields HF with an inverted primary vibrational energy distribution, P( v’ = 1: 2 : 3 : 4) = 0.23 : 0.68 : 0.08 : 0.01. The HF from the reaction with ammonia is cold (non-inverted), P( v’ = 1: 2) = 0.60 : 0.40. Recent experimental work on these reactions is critically assessed and some discrepancies between low-pressure chemiluminescence results and fast-flowing afterglow studies are resolved. The results of high-level ab initio calculations (up to 6-311G ** CISD) on reactants, products, and the hydrogen bonded complexes FH...NH and FH...NH, in the exit channels are reported. The most reliable of the computations predict that FH. ..NH, is significantly more bound than FH . . .NH (8.1 versus 4.1 kcal mol-’ in comparison with products at the 6-311G * * MP2 level). Also, the calculated vibrational frequencies for the two hydrogen bonded complexes indicate that the FH stretch and NH, asymmetric stretch are much closer in frequency in FH.. .NH, than are the FH and NH stretches in FH.. . NH. The strong interaction and the close match of vibrational frequencies in the FH.. . NH, case both will lead to fast internal vibrational relaxation (IVR) of the reaction exoergicity from the F-H-N bonds, where it is released, to the NH, fragment in the F/NH, reaction. Thus, the HF produced in this reaction is expected to have less vibrational excitation than that created in the F/NH, reaction, for which these IVR mechanisms are not as important, and simple direct abstraction dynamics are expected.

1. Introduction The measurement of the primary HF vibrational energy distribution created by the reaction of F atoms with NH, molecules has proven to be among the most difficult of its kind to carry out. The vibrational distributions of this reaction, and its deuterated analogue: F(‘P) + NH3(‘A,) AH,+

--, HF( U’< 2, lx+) + NH,,

- 31 kcal mol-‘,

(1)

F(2P)+ND,(1A,)+DF(~‘$3,12+)+NDz, (2) ’ Present address: Department

of Chemistry, University of Colorado, Boulder, CO 80309, USA.

have been discussed in seven publications [l-7] and have been the subject of dozens of measurements by the two experimental techniques capable of this work - low-pressure infrared chemiluminescence and flowing afterglow. Because the experimental results were often contradictory, and because of other problems associated with measurements on this system, a series of high-level quantum chemical calculations have been undertaken to explore relevant parts of the reactive potential energy surface in order to provide additional information with which to interpret the experimental results. Before these calculations are reported, however, the various experimental results will be listed in order to define fully the problem which the computations addressed. I

0301-0104/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

J.D. Goddard et al. / Reactions of F atoms with NH2 and NH,

322

The signal-to-noise ratio obtained in all experiments on the F/NH, and F/ND, systems is quite small by comparison with that of other comparable reactions such as F/CH,, although the rate constant of the ammonia reaction is about 50% larger than that of the latter reaction [7]. (There is some uncertainty in the value of the ammonia rate constant, but it lies between lo-i2 [8] and 10-‘” [4,7] cm3 molecule-’ SK’, with the latter value being preferred.) Furthermore, the measured activation energy of the ammonia reaction is small - less than 1 kcal mol-’ [9] - a value which has been confirmed by quantum chemical computations [6], which indicate that the barrier may be even less. During measurements on these reactions, it is observed that ammonium fluoride is rapidly deposited on all surfaces exposed to the reagents, suggesting that an NH,-HF complex is formed either in the gas phase or on the surfaces, leading to the formation of NH,F. Furthermore, the vibrational deactivation of HF by NH, is unusually efficient [5], which is also consistent with this suggestion. The formation of such a complex indicates, in turn, that the FH-NH, interaction is strongly attractive, possibly due to hydrogen bonding. The vibrational distributions obtained in these experimental measurements vary widely. The distributions and the experimental techniques used to obtain them are listed in table 1. Considering the F/NH, reaction first, it is clear that the various results may be crudely separated into two classes: Table 1 Previous experimental Reagent

measurements Reported

of the F + NH,

population

a vibrationally cold distribution, P( u’ = 1 : 2) = 0.6 : 0.4 [1,2,5-71, and a vibrationally inverted one which peaks in HF( u’ = 2) and has population in HF( u’ = 3) [4], although the HF(u’ = 3) level is not thermodynamically accessible to the products. (The threshold energy for creation of HF(u’ = 3, J’= 0) is 33 kcal mol-’ whereas the total energy available to the products of the F/NH, reaction, including reagent energy, is 31.3 + 0.5 kcal mol-’ [9,10].) The inverted vibrational distribution always occurred in conjunction with the creation of HF( u’ = 3). Although originally reported in the flowing afterglow experiment, the HF(u’ = 3) level (and HF( U’= 4) as well) can also be created in the low-pressure experiment, ,by operating under conditions of high reagent flow (especially, high F atom flow) which are favourable to the secondary reaction: F+NH,+HF(u’<4)+NH, AH,0 = -43 kcal mol-‘.

(3) This led us to propose [6] that this reaction was responsible for both the vibrational inversion and the creation of HF( u’ > 3). We then extracted a vibrational distribution for reaction (3) from a series of measurements of the mixed results of reactions (1) and (3) using a detailed numerical model of our experiment. The distribution which we obtained from this procedure was inverted, as expected, and populated all levels up to HF( u’ = 4) [6]. It is now agreed [7] that the vibrational distri-

and ND, vibrational

distributions

distribution

Experimental technique

Ref.

v’ = 4

_

0.47

__

FA LP LP

Ill PI

0.56 0.40

0.12 0.45

_

FA FA

0.60 0.36 0.60

0.40 0.27 0.40

( < 0.37) _

_ _ _

NH, NH,

0.23 0.57

0.68 0.43

0.08

0.01

ND,

0.265

0.40

0.335

LP LP LP LP FA FA

u’=l

v’ = 2

ND,

0.56 0.59 0.20

0.44 0.41 0.33

NH, ND,

0.33 0.15

NH, ND, NH,

NH, NH,

V’ = 3 _

_

131 [41 I41 [51 151

I61 I61 [71 [71

J.D. Goddard et al. / Reactions of F atoms with NH2 and NH,

bution created by the F/NH, reaction is P( u’ = 1 : 2) = 0.59 : 0.41, with estimated errors of If: 0.02 in each u level. The F/ND, reaction has had the same complicated history as its protonated counterpart, with the added problem that the DF Einstein transition probabilities are four times smaller than those of HF, requiring four times higher reagent flows for comparable signal-to-noise ratio in the observations. In the ND, measurement, as in the NH, case, the secondary reaction: F+ND,+DF(&5)+ND

(4)

becomes important under conditions of high flow. Thus in our early measurements on the deuterated system [3], we obtained an inverted distribution, P( u’ = 1 : 2 : 3 : 4) = 0.20 : 0.33 : 0.47 : < 0.06 resembling that obtained in a later flowing afterglow measurement: P(0’ = 1 : 2 : 3 : 4) = 0.15 : 0.40 : 0.45 : “trace” [4]. When we became aware of the interference by the fast secondary reaction in the NH, case, we attempted to reduce the reagent flows, and thereby eliminate this interference in the ND, measurement as well [5]. Although the observed vibrational distribution behaved the same way as that from the NH, reaction when the reagent flows were decreased, we were not able to demonstrate to our own satisfaction that our lowest-pressure distributions were uncontaminated by the secondary reaction. Therefore, we did not report an initial distribution for reaction (4) based on the low pressure measurements. The most recent flowing afterglow results, however, suggest this distribution is P( u’ = 1 : 2 : 3) = 0.265 : 0.40 : 0.335, which, like earlier measurements based on both techniques [3,4], is inverted. Ab initio quantum chemical computations on the reactants, hydrogen abstraction transition state, and products have been of value in rationalizing the strong vibrational excitation of HF in the F + NH, reaction [6]. Similar calculations on reactants and products are reported in this work for F + NH,. In addition, particular attention is focused on the nature (structures, binding energies, vibrational frequencies) of the hydrogen bonded complexes FH . . . NH, and FH . . . NH in the exit channels of the primary and secondary reactions respectively. Differences in the strength

323

of the interactions in these complexes are highly suggestive of roles for these dimers in explaining the very different HF vibrational energy distributions from F + NH, and F + NH,.

2. Computational

methods

All calculations were performed with the GAUSSIAN 82 program system [ll]. The small split valence 4-31G basis set [12] was used for preliminary self-consistent-field (SCF) level optimizations of geometries. The 6-311G * * [13] basis, a triply split-valence set with d functions on N and F as well as p functions on H, was employed in SCF, second-order Moller-Plesset perturbation theory (MP2), and configuration interaction with single and double substitutions (CISD) computations. Analytic energy gradients [14,15] were exploited in the geometry optimizations. Reactant (F, NH,, NH,) and product (HF, NH,, NH) structures were determined as separated molecules at four levels of theory: 4-31G SCF, 6-311G** SCF, 6-311G * * MP2, and 6-311** CISD. The complexes, FH.. . NH and FH.. . NH,, were optimized with the 4-31G and 6-311G* * SCF and 6-311G** MP2 methods. Size extensivity errors in CISD calculations on the hydrogen bonded complexes would make such computations problematic (e.g. ref. [16]) and so these were not carried out. The SCF and MP2 approaches are strictly size extensive. 4-31G and 6-311G** SCF harmonic vibrational frequencies were computed using analytic second-derivative methods [14]. The frequencies were 6-311G* * MP2 vibrational calculated as numerical first derivatives of analytic MP2 gradients [14].

3. Results and discussion The total energies in atomic units for the reactants (F(‘P), NH,(‘A,), NH,(‘B,)), hydrogen bonded exit channel complexes (FH . . _NH (‘Z-), FH . . . NH,(‘B,)), and products (HF (IX+), NH(32-)) are collected in table 2. These energies are at geometries consistently optimized at each of four levels: 4-31G SCF, 6-311G **

J. D. Goddard ei al. / Reactions of F atoms with NH, and NH,

324

11’

H-

1.010 1.009

0.942 0.909 0.922 0.896 0.912

0.909 ( 0.91694

0.929

F

1.789 1.883

\ )

H

/

IAl

(

106.1” 106.2” 106.T

0.991

,, )

SCF, 6-311G* * MP2, and (for the monomers only) 6-311G * * CISD. The structures calculated for the molecules at each of the four levels are shown in fig. 1 along with the experimental values (in parentheses). Fig. 2 illustrates the geometries of the two exit channel hydrogen bonded dimers. The geometries from the correlated level methods, 6-311G * * MP2 or CISD, are of greatest interest.

Table 2 Total energies (in au) for reactants, and CISD calculations. Geometries

1.025 1.019 1.030

2.024 1.945

Fig. 1. Optimized geometries for reactants and products at: 4-31G SCF, 6-311G * * SCF, 6-311G * * MP2 and 6-311G * * CISD. (Experimental values in parentheses.)

F(2P) NH,(%) NH,(‘B,) FH...NH(32-) FH...NH(2B,) HF(‘Z+) NH(3..-)

H--_-1Yq33--__H_H

F-

N 1.015 1.012 1.025 1.026 ( 1.024)

‘H

0.929 0.902 0.919

1.033 1.023 1.034 1.040 ( 1.0362 )

H

106.3” 1os.z

1.774

)

2B1 108.3” 104.0” 102.1” 102.1” ( 103.4”

H______1f;td10.6a

F-

H -N

,,

Fig. 2. Optimized channel complexes

geometries for the hydrogen at: 4-31G SCF, 6-311G** 311G * * MP2.

bonded exit SCF and 6-

The binding energies of these complexes relative to products are given in table 3 below. Table 4 presents calculated harmonic vibrational frequencies for HF, NH, NH,, and NH, along with the experimental frequencies and, where available, the harmonic experimental values. The computed vibrational frequencies for the two hydrogen bonded pairs are given in table 5. The frequencies determined with a large polarized basis set (6311G * *) and a method explicitly including electron correlation effects (MP2) are noteworthy. 3.1. Geometries The 4-31G SCF geometries for completeness. Comparable are available in an archive

hydrogen bonded exit channel complexes, and products. were optimized at each level of computation

Core orbitals

in fig. 1 are shown 3-21G SCF results [17] while 4-31G

were correlated

4-31G SCF

6-311G * * SCF

MP2

CISD

- 99.26548 - 56.10669 - 54.41473 - 154.78507 - 155.38431 - 99.88729 - 54.88495

- 99.39687 - 56.21040 - 55.57893 - 155.03209 - 155.64128 - 100.04690 - 54.97606

- 99.51262 - 56.42716 - 55.75123 - 155.40257 - 156.05494 - 100.28608 - 55.10645

- 99.57886 - 56.43255 - 55.76334 not calculated not calculated - 100.28148 - 55.12222

in the MP2

J. D. Goddard et al. / Reactions of F atoms with NH,

parameters for some of these species may also be found in the older literature (for example, ref. [12]). The 6-311G * * SCF bond lengths are too short by 0.01-0.02 A, relative to experiment as expected. Correlation, 6-311G* * MP2 or CISD, lengthens these bonds bringing them into better agreement with experiment. The 6-311G * * MP2 values for HF and NH, are slightly different from those previously reported [13] because the core was frozen in the earlier study, while in the present work all orbitals were correlated. For these simple XH molecules, the 6-311G* * MP2 and CISD results are in very good agreement. However, the 6-311G* * MP2 method is much less computationally demanding than CISD. The HNH angles in NH, and NH, differ by only 0.1” between 6-311G** MP2 and CISD. The bond lengths to be compared are (6-311G** MP2/ CISD) : HF 0.912/0.909, NH 1.034/1.040, NH, 1.025/1.026 and NH, 1.013/1.011 A. At the correlated levels, agreement with experiment is excellent; the largest difference is only 0.008 A in the case of HF (6-311G* * CISD versus expt.). The ammonia geometry at the 6-3116 * * MP2 level of r(NH) = 1.013 A and L(HNH) = 106.1” is in reasonable agreement with the results of an extremely large basis set MP2 calculation [lS]: r(NH) = 1.007 A, L(HNH) = 107.5 =‘. The geometries of the two hydrogen bonded complexes given in fig. 2 may be referenced back to the monomers of fig. 1 at each of the three levels (4-31G SCF, 6-311G* * SCF, 6-311G* * MP2). From the point of view of the present study, it is significant that in several ways the FH . . . NH, structure indicates a stronger interaction between the fragments than does the FH _. . NH one. The FH . . . N distances show large variations with basis. The 4-31G small basis set SCF result is too short due to basis set superposition effects [16,19]. Increasing the basis set size to 6-311G * * leads to longer (by = 0.1 A) FH.. . N distances at the SCF level. The inclusion of electron correlation (MP2) shortens these distances by 0.09-0.08 A leaving (fortuitously) the 4-31G SCF distances and the 6-311G * * MP2 lengths nearly equal. Concentrating on the 6-311G* * MP2 geometries, the FH.. .N distance in FH.. .NH, is 1.789 A, which is 0.156 A shorter than the com-

325

and NH,

parable value in FH . . . NH. Upon complexation, the FH distance in FH . . . NH, increases by 0.017 A relative to monomer, while in FH.. . NH this increase is 0.007 A or 2.4 times less. These geometrical parameters clearly indicate a stronger interaction between FH and NH, as compared to FH and NH. 3.2. Binding energies Table 3 contains the binding energies in kcal mol-’ relative to products for the two exit channel hydrogen bonded complexes, FH . . . NH, and FH . . . NH. The zero-point vibrational energy corrections (AZPE) were computed using the calculated harmonic vibrational frequencies to be discussed shortly. At all levels of calculation, FH.. . NH, is 1.7-2.1 times more strongly bound than FH . . . NH. The most reliable 6-311G* * MP2 + AZPE values are 8.1 kcal mol-’ for FH . . . NH, and only 4.1 kcal mol-’ for FH . . . NH. Although the absolute members for the binding energies may vary slightly with more sophisticated computations, there is no doubt that the basic conclusion that FH and NH, form a much stronger hydrogen bond than FH . . . NH will continue to hold. It is particularly instructive to compare these binding energies with the overall exothermicities [20] of the atom-molecule and atom-radical reac-

Table 3 Binding energies (in kcal/mol and relative to products) for the exit channel hydrogen bonded complexes. The zero-point vibrational corrections ( AZPE) employed the calculated harmonic vibrational frequencies AE

AE + AZPE

FH...NH, 4-31G SCF 6-311G * * SCF 6-311G * * MP2

14.0 9.1 11.1

11.6 6.1 8.1

FH...NH 4-31G SCF 6-311G * * SCF 6-311G * * MP2

8.1 5.7 6.3

5.5 3.5 4.1

326

J.D. Goddard et al. / Reactions of F atoms with NH? and NH,

tions, reaction (1) and reaction (2) experimental exothermicities [20] used in the following comparisons. CISD + SCC exothermicity for calculated earlier [6] to be -40.9

respectively.The are quoted and The 6-311G * * F + NH, was kcal mol-’ or

= 95% of experiment. The FH.. . NH, binding of 8.1 kcal mol-’ represents = 25% of the overall reaction exoergicity. However, the FH . . . NH well is only 4.1 kcal mol-’ which is only = 10% of the reaction exothermicity. The FH . _. NH, complex, therefore, is a much more significant feature (deeper well) on the F + NH, surface than the FH.. . NH dimer, on the F + NH, potential energy surface. Fig. 3 provides a schematic picture of these relative energies for F + NH, and F + NH,. 3.3. Vibrational frequencies

FH + NH,

F-H-NH2

FH + SH F-H-NH

Fig. 3. A schematic illustration of the energetics of the F+NH, and F+ NH, reactions including reactants, intermediate hydrogen bonded comulexes. I , and oroducts. 1

The calculated harmonic vibrational frequencies for HF, NH, NH, and NH, at the 4-31G SCF, 6-311G* * SCF, and 6-311G * * MP2 levels are given in table 4. It is very well known [26] that harmonic vibrational frequencies calculated at the split-valence basis set SCF level are = 10% too large relative to experiment. The difference reflects both the SCF method and the fact that the experimental frequencies are normally the anharmonic (observed) values. The harmonic (w,) experimental frequencies for HF and NH, are also included in table 4. No scaling of the calculated frequencies was made as the major interest in this paper is in frequency shifts when the monomers form hydrogen bonded complexes. It is far less common to determine vibrational frequencies at the MP2 level than at the SCF level. Existing studies [27] would imply that harmonic vibrational frequencies calculated at the 6-311G * * MP2 level should be 5-7% greater than experimental (anharmonic) values. Of course, some of that difference is due to anharmonicity. For HF and NH, where the harmonic experimental frequencies are best known, the comparison of 6311G* * MP2 results with experiment is most experisatisfactory (6-311G * * MP2/harmonic mental result): HF 4256/4138 102.9%; NH, 3532/3506 100.7%, 1125/1022 llO.l%, 3676/3577 102.8%, and 1678/1691 99.2%. The calculated harmonic vibrational frequencies for the hydrogen bonded exit channel complexes are presented in table 5. The modes of the complex which approximately correspond to the FH stretch and NH, asymmetric stretch in FH.. .NH, and to the FH and NH stretches in

J.D. Goddard et al. / Reactions of F atoms with NH,

Table 4 Calculated harmonic and experimental vibrational frequencies for the reactants and products. The “harmonic experimental frequencies” for HF and NH, are given in brackets 4-31G SCF

6-311G * *

Experiment

SCF

MP2

HF NH

vte vt e

4118 3339

4513 3501

4256 3394

3962[4138] a1 3282 a)

NH,

vt a, v2 aI

3525 1659 3643

3591 1644 3680

3456 1546 3554

3219 b*c) 1497 3286

3759 621 3956 1821

3686 1148 3808 1802

3532 1125 3676 1678

3337 d’[3506] e, 950[1022] 3444[3577] 1627[1691]

“3

NH,

b,

Y, at r2 al ~3

e

r4 e

a) Ref. [21]. b, Ref. [22]. ‘) Ref. [23]. d, Ref. [24]. ‘) Ref. [25].

FH . . . NH have been labelled. All the vibrational frequencies are real indicating that FH.. . NH, is a minimum-energy geometry under C,, symmetry and FH.. .NH has a linear (C,,) minimum. Focusing on the 6-311G * * MP2 vibrational frequencies, the FH stretch has shifted down by 336 cm-’ from 4256 cm-’ (monomer) .to 3890 cm-’ in the FH.. .NH, complex. In contrast, in the

Table 5 Calculated harmonic vibrational frequencies for the hydrogen bonded exit channel complexes

FH...NH, vt a, FH str. v2 al r3 aI r4 aI rs b, NH, asym. str. v6 b, “7

b

vs b, “9

b

FH...NH r1 (I FH str. ~a e NH str.

4-31G SCF

6-311G* * SCF

6-311G * * MP2

3101 3593 1665 272 3717 898 221 876 365

4205 3636 1646 222 3728 783 199 764 311

3890 3500 1548 247 3607 832 193 786 284

3989 3448

4379 3563

4120 3460 523 185 187

v3

=

v4

=

579 234

527 205

v5

IJ

214

175

and NH,

321

FH.. . NH dimer the FH stretch is at 4120 cm-’ which is only 136 cm -’ less than the value for free FH. This greater shift for FH.. . NH, again illustrates the stronger interaction in this complex relative to FH.. .NH. Also reflective of the stronger binding is the shift of the NH, asymmetric stretch up by 311 cm-’ from 3286 cm-’ in the monomer to 3607 cm-’ in FH.. .NH, while in FH.. . NH the NH stretch increases by 174 cm-’ upon hydrogen bonding. Similarly, that mode which is best identified as the stretching motion of the hydrogen involved in the hydrogen bonding is at 247 cm-’ (vq(al)) in FH.. .NH, and only 187 cm-‘(v,(a))inFH...NH. The calculated characteristics of the complexes presented above, together with the experimental results listed in the introduction, strongly suggest a self-consistent interpretation of the F/HN, interaction which explains all the experimentally observed behaviour of the reaction. The data which are important to this interpretation are: the FH frequency in FH.. . NH, is only 283 cm-’ larger than the NH, asymmetric stretch, whereas in FH.. . NH, the FH vibration is 650 cm-’ larger than the NH frequency - a mismatch in vibrational frequency which is 2.3 times greater than in the FH.. . NH, case. The binding in the FH . . . NH, complex, 8.1 kcal mall’, is twice that in FH.. . NH complex and the FH.. . N distance in the former is 0.156 A shorter than in the latter. These data indicate that the exoergicity, which is released initially at the F-H-N reaction site, will be more likely to be shared with (flow into) the remaining NH, fragment in reaction (1) than the NH fragment in reaction (3) because in the former case, the frequency match between HF and NH; vibrations is closer, the binding (which would prolong the collision) is stronger, and the bond length at the location of the minimum is shorter. Also, of course, the state density is the NH, radical is higher than that of the NH radical, which becomes important if the interaction is substantially protracted by the hydrogen bonding in the exit channel. In the context of the close match of vibrational frequencies, we note that since the HF frequency starts at zero (for large reactant separation) and eventually becomes larger than either of the NH, or the NH frequencies, it must

328

JB.

Goddard et al. / Reactions of F atoms with NH, and NH,

coincide with these at some point during each of the reactions. The calculations show that this coincidence occurs slightly before the H-bond minimum in each case, and the shapes of the surfaces in this region will, of course, strongly influence the probab~ty for the intr~ol~ular energy flow which we have postulated. We have concentrated on exit channel interactions in this study, to the exclusion of entrance channel effects, because of the unusual depth of the exit channel minima. Furthermore, the expected effects of these minima (enhanced probability for secondary encounters and intramolecular energy flow) will, ipso facto, reduce the influence of entrance channel effects. The entrance channels of the two systems are not the same, however, and this may have an influence on the interaction. In particular, the F/NH, approach coordinate may be more attractive than that of the F/NH, system. If so, this should cause the former to have higher product vibrational excitation than the latter (neglecting exit channel effects). ‘This could contribute to the observed results, therefore, but we do not believe it is adequate to explain the dramatic difference which we observe experimentally. It is most probably that the greater attractiveness in the entrance channel of the F/NH, surface, combined with the shallower FH.. .NH ~~~, both combine to produce the greater vibrational excitation observed in this system.

4. Conclusion As indicated in the introduction, the experimental determination of the energy distributions in the ~bration~ly excited products created by the F/ND, reactions is very difficult. It is presently impossible to determine the (u’ = 0) populations from these reactions experimentally. Although laser-gain measurements using HF chemical lasers might be attempted, the very rapid vibrational relaxation of HF by NH, would make such measurements extremely difficult. In the absence of such direct information, high-level ab initio quantum chemical computations have been used to provide the required data. The results of these calculations suggest that the F + NH, reaction produces HF which has low vibrational exci-

tation and that the NH, from this reaction should therefore contain a greater fraction of the available energy than, for example, the CH, from the F + CH, reaction. It is also possible in principle to measure the energy content of the NH, fragment by laser-educed fluorescence, but for the reasons mentioned above, this would also be a difficult measurement. Furthermore, since it would be expected that rapid IVR processes would cause most of the NH, excitation to go to the bending mode, it might be difficult to detect it for this reason as well. In light of the very difficult nature of further experiments, the techniques of high-level ab initio quantum chemistry were employed to provide further information. The geometries of the complexes and the shifts in bond lengths within the monomers upon hydrogen bonding, the calculated well depths, and the shifts in FH and NH stretching frequencies all indicate clearly a much stronger interaction between FH and NH, as opposed to FH and NH. This stronger interaction is highly suggestive of greater transfer of vibrational energy from FH to NH, in comparison to FH with NH. Such an energy flow would leave the FH from the F + NH, reaction colder (non-inverted) in comparison with that from F + NH,. Acknowledgement The authors are grateful to Professor D.W. Setser for the provision of a preprint of ref. 1’71 and for several helpful discussions. Financial support of a part of this research by the Natural Sciences and Engineering Research Council of Canada through operating grants to JDG and JJS is acknowledged. References [l] W.H. Duewer and D.W. Setser, J. Chem. Phys. 58 (1973) 2310. [2] D.J. Douglas and J.J. Sloan, Chem. Phys. 46 (1980) 307. [3] J.J. Sloan, D.G. Watson and J. Williamson, Chem. Phys. Letters 74 (1980) 481. [4] AS. Manocha, D.W. Setser and M. Wickramaaratchi, Chem. Phys. 76 (1983) 129. IS] D.J. Donaldson, J. Parsons, J_J. Sloan and A. Stolov, Chem. Phys. 85 (1984) 47.

J.D. Goddard et al. / Reactions of F atoms with NH, and NH, [6] D.J. Donaldson,

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