J. Phys. Chem. 1987, 91, 5487-5489
5487
Orientatlon Effects and Reaction Mechanisms for the H 4- IC1 Reaction J . M . Alvariiio Departamento de Quimica Fisica, Facultad de Quimica, Uniuersidad de Salamanca. Salamanca. Spain
and A. Lagana’* Dipartimento di Chimica, Uniuersita’ di Perugia, Perugia. Italy (Received: December 23, 1986)
The competition between insertion and abstraction mechanisms for reactions of a hydrogen atom and an interhalogen molecule has been investigated for two extreme orientations of the target molecule.
Introduction Measurements of reactive scattering properties for oriented molecules’ has stimulated a great deal of theoretical work. One of the aims of these theoretical studies has been the investigation of reaction paths associated with collisions starting at a given value of the angle of attack. Such an investigation is usually based on the analysis of exact and model classical trajectory results2 A recent classical trajectory study of the H H2 system3 has shown that reaction can occur either by exchange of the hydrogen atom with the nearest molecular end or (especially at higher collision energy) through its insertion into the hydrogen molecule. A tight correlation between the angle of attack of the incoming atom and the reactive outcome was found. According to these results, one can define a reaction cone inside which the large majority of trajectories react with the atom located on its apex. For this system the contribution to reaction of reorientation and migration paths was found to be negligible. This is likely to be due to the strong collinear bias of the H H2 reaction and to the high barrier to insertion of its potential energy surface. Recently we have undertaken a dynamical study of M + H Y ( M = alkali, Y = halogen) family of reaction^.^ For the lightest of these systems (Li + H F ) we found that, when the H atom is bound to a significantly heavier partner and the transition state has a bent geometry, reaction mechanisms can be driven by reorientation effect^.^ In order to investigate the role played by migration mechanisms in reactions of a hydrogen atom when little or no barrier to insertion exists, we have performed a detailed classical trajectory study of the H IC1 system. A LEPS potential energy surface for this system is available from the literaturee6 This surface shows a small barrier (1.59 kcal/mol) along the collinear minimum energy path when the attack occurs from the C1 side. The barrier gradually vanishes when the initial angle of attack decreases becoming fully attractive on the I side. Reactions leading to H I and HCl products are both exoergic. Previously, trajectory calculations for hydrogen isotopes reacting with halogenic mol-
+
+
+
(1) Brooks, P. Science 1976, 193, 11. Stolte, S. Ber. Bunsenges. Phys. Chem. 1982,86,413. Stolte, S.; Chakravorty, K. K.; Bernstein, R. B.; Parker, D. H. Chem. Phys. 1982, 71, 353. Parker, D. H.; Chakravorty, K. K.; Bernstein, R. B. J. Phys. Chem. 1981,85,466. Chem. Phys. Lett. 1982,86, 113. Van den Ende, D.; Stoke, S.; Cross, J. B.; Kwei, G. H.; Valentini, J. J. J. Chem. Phys. 1982, 77,2206, Van den Ende, D.; Stolte, S. Chem Phys. Lett. 1980, 25, 378. Chem. Phys. 1984, 89, 121. Jalink, H.; Parker, D. H ; Meiwes-Broer, K. H.; Stolte, S.J. Phys. Chem. 1986, 90, 552. (2) Levine, R. D.; Bernstein, R. B. Chem. Phys. Lett. 1984, 105, 467. Bernstein, R.B. J. Chem. Phys. 1985, 82, 3656. Kuppermann, A.; Levine, R. D. J . Chem. Phys. 1985, 83, 1671. Schechter, I.; Kosloff, R ; Levine, R. D. Chem. Phys. Lett. 1985, 121, 297. Blais, N. C.; Bernstein, R. B.; Levine, R. D. J. Phys. Chem. 1985,89, 10. Loesch, H. J. Chem Phys. 1986, 104, 203. (3) Schechter, I.; Kosloff, R.; Levine, R. D. J. Phys. Chem. 1979,90, 1006. (4) Alvarifio, J. M.; Hernandez, M. L.; Garcia, E.; Lagana’, A. J. Chem. Phys. 1986,84, 3059. Garcia, E.; Lagana’, A,; Palmieri, P Chem. Phys. Lett. 1986, 127, 73. ( 5 ) Alvarifio,J. M.; Basterrechea, F. J.; Lagana’, A. Mol. Phys. 1986, 59, 559. Alvarifio, J. M.; Basterrechea, F. J.; Hernandez, M. L.; Lagana’, A. J Mol. Struct. 1985, 120, 187. (6) McDonald, J. D. Faraday Discuss., Chem SOC.1973, 55, 372.
0022-3654/87/2091-5487$01.50/0
ecules have been performed by McDonaldS6 Results were rationalized by modeling the target molecule as an ovoid and assigning different reactive properties to sections of its surface. Classical trajectory calculations for the H IC1 system with reactants thermalized at 300 K have been performed on this surface by Polanyi and co-workers’ with the aim of rationalizing infrared chemiluminescence results. In their study, evidence for a migration of the H atom from I to C1 during the collision was already given though limited to events starting on the C1 side. In this paper we discuss the possibility that H reacts with the molecular partner opposite to the side of the initial attack even under severe steric requirements.
+
Calculations and Results Calculations were carried out by running batches of 2500 trajectories for the two values of the angle of attack 29 (this is the angle formed by the vector pointing from H toward the center of mass of IC1 and the internuclear axis of the diatom) corresponding to the two opposite collinear geometries HICl (29 = OD) and HClI (29 = l8OD). The value of the impact parameter was selected randomly in the interval 0-6 A. At 29 = OD 3.5% and 5.8% of the total events led respectively to HC1 and H I products suggesting that a migration of the attacking hydrogen atom from the I to the C1 end of the diatom is, indeed, possible not only (as in the case reported by Polanyi) when the hydrogen is first oriented toward C1 but also when the collision starts from the opposite side. Graphical evidence for this is given in the lower panel of Figure 1 where plots of both HI (dashed-dotted line) and HCI (dashed line) internuclear distances C1 migratory mechanism are reported as a for a typical I function of the elapsed time (IDT plots). In the figure, the H I distance is, at first, shorter than that of HCl. However, after a few oscillations of the H I diatom around its equilibrium position, the HC1 distance becomes shorter than that of HI. From this moment on, the H I internuclear distance starts increasing while that of HC1 keeps oscillating around its equilibrium value. A more detailed analysis of the migration mechanism can be performed by restricting the calculations of two dimensions. In this case, in fact, the plane of the collision is frozen to its initial orientation and trajectories can be represented by an X and Y graph (daisy plot). Points of the different trajectories corresponding to the same amount of elapsed time are labeled with identical numbers. A two-dimensional (2D) trajectory integrated starting from the same initial conditions of the previously reported three-dimensional (3D) case is illustrated in Figure 2. In the upper panel of the figure the usual internuclear distance vs. elapsed time plot is reported in order to evidence differences due to the reduced dimensionality of the calculation. The 2D IDT plot shows the same migratory structure of the 3D calculation implying that also the coplanar reactive collision is governed by a migratory mechanism: i.e. the HCl internuclear distance becomes shorter than that of HI after that H has been first bound to I for a significant time interval.
-
(7) Polanyi, J. C.; Schreiber, J. L.; Skrlac, W. J. Faraday Discuss., Chem. SOC.1979, 67, 66.
0 1987 American Chemical Society
AlvariHo and Lagana'
5488 The Journal of Physical Chemistry, Vol. 91, No. 21, 1987
0 0
I
8.
.2
t / ns
.4
.5
"0
1
t / ns
I
4r
oa \
0
-4
-8
L
X / K Figure 2. Two-dimensional trajectory calculations of a migratory attack from the I side. HC1 (dashed line) and HI (dashed-dotted line) diatomic internuclear distances reported as a function of the elapsed time (upper panel). Daisy plot of the same trajectory (lower panel). Positions of H, I, and C1 atoms at some identical values of the elapsed time are marked by circles, triangles, and squares, respectively.
Figure 1. HCl (dashed line) and H I (dashed-dotted line) diatomic internuclear distances reported as a function of the elapsed time for a direct attack from the I side (top panel), for a direct attack from the CI side (intermediate panel), and for a migratory attack from the I side (bottom panel) from a 3D calculation.
The daisy plot of this trajectory (lower part of the figure) shows that t h e migratory behavior is due to a repeated reflection of the H atom from the barrier at the entrance of the H + C1I reaction channel. The process was successful in finding a reactive path to the HC1 product only by entering the insertion window opening when the IC1 bond is slightly stretched. As already mentioned, however, the majority of reactive collisions starting from the I side follow a direct path to form H I (a graphical representation of this type of events is given in the top panel of Figure 1) by abstracting the nearest atom. This is true also for attacks on the C1 side (19 = 1 8 0 O ) . The largest part of the reactive trajectories (4.9% of the total) follows a direct abstraction path to form hydrogen chloride (see the central panel of Figure 1). However, our results show that migration can occur also for reactive processes starting at the C1 side. We have found, in fact,
8
,
I
o a
I
\ I
4
0
0
.2
.4 t / ns
Figure 3. As in the lower panel of Figure 1 for a migratory attack from the I side.
J. Phys. Chem. 1987,91, 5489-5495 that 0.5% of total trajectories starting from the C1 side lead to the H I product. This fraction, although not large, still amounts to a rough 10% of the total reactive events. The Occurrence of this mechanism, not evidenced by Polanyi's calculations, is in agreement with the Walsh rule.* A IDT plot for this reactive mechanism is given in Figure 3. It shows how the HCl distance (shorter than that of the HI diatom in the initial part of the trajectory) becomes large after a few oscillations while the H atom binds to I. (8) McDonald, J. D.; Le Breton, P. R.; Lee, Y. T.; Herschbach, D. R. J . Chem. Phys. 1972, 56, 169.
5489
Conclusions A graphical analysis of the reaction path for the hydrogen atom colliding with the IC1 molecule has shown that this system evolves through both a direct and a migratory path even for opposite geometries of attack. This fact implies that it is possible to obtain both hydrogen halide products irrespective of the angle of attack of the incoming atom (although the relative product fraction depends on it and on the electronegativity of the halogen atom). An obvious consequence of these results is the need for defining model reaction functions which have built in an angular-dependent branching weight. Registry No. IC1, 7790-99-0; H atomic, 12385-13-6.
Effects of Diatomic Reagent Alignment on the A
+ BC Reaction
M. D. Pattengill, Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506
R. N. Zare,* Department of Chemistry, Stanford University, Stanford, California 94305
and R. L. Jaffe NASA Ames Research Center, Moffett Field, California 94035 (Received: February 25, 1987)
-
Effects of diatomic reagent alignment on the prototype reaction A + BC AB + C have been investigated by running classical trajectories on a modified version of the extended LEPS potential which permits the minimum energy path to be varied from collinear (Bo = 180') to broadside attack (Bo = 90'). Preliminary model calculations are reported for the mass combination H H'L HH' + L where H and H' are heavy atoms and L is a light one. Two sets of empirical potential functions are chosen, one set for endothermic reactions @e., late barriers to reaction), the other set for exothermic reactions (Le,, early barriers), For each set, the minimum energy path is adjusted to be either Bo = 90', 135', or 180' and the BC reagent diatomic is in either the ground vibrational state (u = 0) or the first excited state (u = 1). For corresponding u = 0 and v = 1 cases, the vibrational energy difference is ascribed to relative A,BC translational energy. For all cases, the initial BC rotational energies are chosen at random from a 300 K Boltzmann distribution. Thus, BC(u=O) and BC(u=l) collisions are chosen to have nearly equal total collision energies. Since the energetic characteristics of the minimum energy paths for each set of surfaces, corresponding to different values of Bo, are quite similar, the reported calculations permit direct investigation of the effects of varying minimum energy reaction path geometry on observed reaction attributes. Separate calculations have been performed for alignments in which the BC rotational angular momentum vector is either asymptotically parallel (or, equivalently, antiparallel) or asymptotically perpendicular to the direction of the initial relative A,BC velocity vector. For reaction on endothermic surfaces, a distinct effect [for both BC(u=O) and BC(v=l)] on the extent of product vibrational excitation is observed. Although the present studies are clearly model ones, the observed effect is in qualitative agreement with preliminary results obtained in alignment experiments on Sr + HF(v=l). A similar but weaker effect is observed for BC(u=O) on the exothermic surfaces but is absent for BC(u=l). Beyond product energy disposals, the results obtained also demonstrate a marked effect, both for endothermic and exothermic potentials, of diatomic reagent alignment on reactive cross sections. Analysis of the observed trends suggests a possible underlying steric basis of the Polanyi rules which, for elementary bimolecular gas reactions, relate mode-specific excitation to extent of reactivity. In addition, the results obtained are suggestive of generalizations relating to the applicability of the Polanyi rules to reactions which favor noncollinear collisions.
+
-
Introduction Steric requirements on chemical reactivity have been inferred, for the most part, from indirect evidence. Examples are the need to include a steric factor in the hard-sphere model for calculating or the observation that one type of bimolecular rate isomer predominates over another in the products of a reaction m i x t ~ r e . Molecular ~ beam studies under single collision conditions (1) Bunker, D.L. Theory of Elementary Gas Reaction Rates; Pergamon: Oxford, 1966. (2) Smith, I. W.M. Kinetics and Dynamics of Elementary Gas Reactions; Butterworths: London, 1980. ( 3 ) See: Steric Effects in Organic Chemistry; Newman, M. S., Ed.; Wiley: New York, 1956.
0022-365418712091-5489$01.50/0
are beginning to provide the first direct measurements on the role of reagent approach geometry in controlling the outcome of a chemical reaction, a field which might be called reaction stere ~ d y n a m i c s . ~Here, reagents can be oriented by using external electric and magnetic fields or aligned by the absorption of plane-polarized light.5 In spite of a few widely celebrated successes, experimental results are still sparse and the understanding of steric requirements remains presently at a primitive stage. [In accord with ref 4, by orientation of a molecule, we refer to the distribution of rotational angular momenta J which depends on Zare, R. N. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 422. (5) J. Phys. Chem., this issue and references cited therein. (4)
0 1987 American Chemical Society
