4.4 Basic Features of Atom–Diatom Reactions
139
It is worth pointing out here that a simple qualitative interpretation of the effect of
vibration and translation is that while excess energy allocated to translation helps surmounting the energy barrier located in the entrance channel, excess energy allocated
to vibration helps surmounting (due to the “sideway” nature of the corresponding
degree of freedom) the energy barrier located in the exit channel. More illustrative
examples of such mechanisms will be given later together with the evidence that an
excess of vibration or translation energy may end up preventing reaction. In some
cases, not discussed here, however, the cooperative interplay of energy between the
two degrees of freedom may help reaction (e.g., an attack from the side of repulsive
interaction may slow down the collision and allow vibration to become effective
once the attacked molecule has slightly rotated so as to offer its attractive side to the
collision partner).
4.4.2 Quantum Effects
The smoothness of the quantum reactive probability plots of the N + N 2 system
should not be taken as a general feature of chemical reactions. There are cases, in fact,
in which the quantum reactive probability plots are much quite structured and it would
be interesting to trace back the origin of such structures. It is interesting, in fact, to find
out the nature of the underlying dynamical effect. This is indeed the case of the H + H 2
reaction in which the “lightness” of the collision partners exalts the quantum nature
of the collision process. This makes the hydrogen atom–hydrogen molecule reaction
the most popular benchmark for the calculation of quantum effects in atom–diatom
collisions [69]. For illustrative purposes, the quantum reactive vibrational state-tostate probabilities of the collinear H + H 2 (v) → H + H 2 (v
) case computed using
the time-independent APH method are plotted in Figs. 4.12 and 4.13, respectively,
as solid lines.
In Figs. 4.12 and 4.13 also, the IVR-SC values are plotted providing a qualitative
comparison of the accuracy of the semiclassical treatment for both reactive and
nonreactive processes. Systematic applications of the IVR-SC method have been
made to the N + N 2 collisions [70].
In order to help the rationalization of the dynamical effects in elementary reactions
let us consider the reaction Li + FH. This system bears the feature of being made
of three different atoms (H that is by definition the lightest stable atom and Li and F
which are respectively 7 and 19 times heavier than H and can, therefore, be considered
both heavy). Such system has reaction channels largely differing from those of H +
H 2 . As shown by the minimum energy path plotted in Fig. 4.14, the channel Li +
FH → LiF + H that connects the reactants to the LiF product is slightly endoergic
(about 0.15 eV). The MEP, in fact, while Li approaches FH forms a well (associated
with a slightly bent LiFH triatom) before rising to a double barrier (associated with
a tightly bent triatom) sandwiching a small well of about 0.05 eV. The other channel
connecting the reactants to the LiH product is instead highly endoergic and is not
considered here.
139
It is worth pointing out here that a simple qualitative interpretation of the effect of
vibration and translation is that while excess energy allocated to translation helps surmounting the energy barrier located in the entrance channel, excess energy allocated
to vibration helps surmounting (due to the “sideway” nature of the corresponding
degree of freedom) the energy barrier located in the exit channel. More illustrative
examples of such mechanisms will be given later together with the evidence that an
excess of vibration or translation energy may end up preventing reaction. In some
cases, not discussed here, however, the cooperative interplay of energy between the
two degrees of freedom may help reaction (e.g., an attack from the side of repulsive
interaction may slow down the collision and allow vibration to become effective
once the attacked molecule has slightly rotated so as to offer its attractive side to the
collision partner).
4.4.2 Quantum Effects
The smoothness of the quantum reactive probability plots of the N + N 2 system
should not be taken as a general feature of chemical reactions. There are cases, in fact,
in which the quantum reactive probability plots are much quite structured and it would
be interesting to trace back the origin of such structures. It is interesting, in fact, to find
out the nature of the underlying dynamical effect. This is indeed the case of the H + H 2
reaction in which the “lightness” of the collision partners exalts the quantum nature
of the collision process. This makes the hydrogen atom–hydrogen molecule reaction
the most popular benchmark for the calculation of quantum effects in atom–diatom
collisions [69]. For illustrative purposes, the quantum reactive vibrational state-tostate probabilities of the collinear H + H 2 (v) → H + H 2 (v
) case computed using
the time-independent APH method are plotted in Figs. 4.12 and 4.13, respectively,
as solid lines.
In Figs. 4.12 and 4.13 also, the IVR-SC values are plotted providing a qualitative
comparison of the accuracy of the semiclassical treatment for both reactive and
nonreactive processes. Systematic applications of the IVR-SC method have been
made to the N + N 2 collisions [70].
In order to help the rationalization of the dynamical effects in elementary reactions
let us consider the reaction Li + FH. This system bears the feature of being made
of three different atoms (H that is by definition the lightest stable atom and Li and F
which are respectively 7 and 19 times heavier than H and can, therefore, be considered
both heavy). Such system has reaction channels largely differing from those of H +
H 2 . As shown by the minimum energy path plotted in Fig. 4.14, the channel Li +
FH → LiF + H that connects the reactants to the LiF product is slightly endoergic
(about 0.15 eV). The MEP, in fact, while Li approaches FH forms a well (associated
with a slightly bent LiFH triatom) before rising to a double barrier (associated with
a tightly bent triatom) sandwiching a small well of about 0.05 eV. The other channel
connecting the reactants to the LiH product is instead highly endoergic and is not
considered here.