On the fast addition reactions of BH₃

Abstract

Borane is a rather unique reactive species in that it has a low lying empty boron 2p atomic orbital [ l ] and, thus, serves as a good model for an electrophilic species. One advantage of BHB as a reagent is that the addition reactions are examined in the gas phase, thereby eliminating solvent effects. A second advantage is that the amount of energy deposited in the initial adducts is much less than in the case of oxygen atoms, for example. Thus the initial adduct can usually be directly observed. Finally, as BH3 adds rapidly to reagents with differing electronic structures to form observable adducts, the rate constants of these reactions constitute an unusual probe of the relationship of reaction rates to electronic structures.We have recently measured the rate constants for the rapid addition of BH3 to various substrates. The experimental absolute rate constants for eight of these reactions are gathered in Table I. In all these cases the simple adduct was observed, and all the data are consistent with the formation of a donor-acceptor adduct in the first instance. In two cases the activation energy was measured and was found to be small.It is known that the rates of the fast reactions of many reactive species correlate to some extent with the ionization potential of the substrate [9]. However, as illustrated in Figure l(a), this is not the case for BH3. On the other hand [ Figure 1 (b)] there does seem to be a good correlation of rate with the nuclear charge on the atom to which BH3 adds, i.e., the rate for P = 0 < N < C < B. This observation can be qualitatively explained as follows.The reactions considered are all very rapid in an absolute sense and therefore an early transition state is expected. Thus it will be the initial interactions that will control the barrier height and reaction rate. Although repulsive interactions .are important, it appears that the attractive interactions are the controlling factors [ 10-121. The attractive interaction between borane and the donor species can be approximately represented by the mixing of the B (2p) orbital on BHa and the highest filled orbital on the substrate [lo]. The attractive interaction at constant temperature may be written as Eat,, = -aS2/(E + I ) 633

Experimental and Modeling Studies of B Atom Number Density Distributions in Hot Filament Activated B2H6/H2 and B2H6/CH4/H2 Gas Mixtures

“…BH 3 is not expected to react with CH 4 itself. Diode laser absorption studies of the BH 3 + C 2 H 4 reaction returned BH 3 removal rate constants >10 -11 cm 3 s -1 , at room temperature and at N 2 buffer gas pressures as low as 6 Torr, while previous flow-tube studies suggested that the adduct, ethylborane, was the major reaction product . Theory generally supports this view, though the reliability of the early picture of the reaction proceeding via a two-step mechanism involving initial π-complex formation and subsequent rearrangement via a four center transition state has been called into question by higher level calculations which suggest a steady decline in energy with decreasing BH 3 −C 2 H 4 separation .…”

Spectroscopic and Modeling Investigations of the Gas Phase Chemistry and Composition in Microwave Plasma Activated B2H6/CH4/Ar/H2 Mixtures

“…Best current estimates for the reaction enthalpies Δ r H (6) and Δ r H (7) when x = 3 are, respectively, −134 kJ mol −1 − and −180 kJ mol −1 . Diode laser absorption studies of reaction returned BH 3 removal rate constants >6 × 10 12 cm 3 mol −1 s −1 at room temperature and N 2 buffer gas pressures as low as 6 Torr, and flow-tube studies suggest that the adduct, ethylborane, is a major reaction product . BH 3 addition to C 2 H 2 (reaction ) has received little experimental attention but is a recognized route to forming vinylborane and is expected to show many similarities with reaction .…”

“…41 Diode laser absorption studies of reaction 6 returned BH 3 removal rate constants >6 × 10 12 cm 3 mol -1 s -1 at room temperature and N 2 buffer gas pressures as low as 6 Torr, 42 and flow-tube studies suggest that the adduct, ethylborane, is a major reaction product. 43 BH 3 addition to C 2 H 2 (reaction 7) has received little experimental attention but is a recognized route to forming vinylborane and is expected to show many similarities with reaction 6. We assume that H-shifting reactions within the CH 3 CH 2 BH x-1 and CH 2 CHBH x-1 families favor the more stable speciessthat is, ethylborane and vinylborane (x ) 3).…”