J. Phys. Chem. 1995,99, 5883-5888
5883
Ab Initio Molecular Orbital Study of the Chemical Reactions of Diborane with Ammonia Shogo Sakai Department of Information Systems Engineering, Faculty of Engineering, Osaka Sangyo University, Daito, 574 Japan Received: February 16, 1994; In Final Form: January 31, I995@
The potential energy surface for the reaction of diborane B2H6 with ammonia has been calculated by MP4/ 6-3 1 I+G(d,p)//MP2/6-3 lG(d,p) method with zero-point energy corrections. Two steps for the formation of aminoborane, HzB=NH2 were studied. The first step is the formation of borane-ammonia adduct H,B: NH3 and complex H3BHBH2NH3, and the second is three type (1,l-, 1,2-, and 1,3-) subsequent hydrogen eliminations. The adduct H3B:NH3 formation occurs via the dissociation (BH3 BH3) of diborane or via the complex H3BHBHzNH3 formation. Two transition states for the complex H3BHBH2NH3 formation from diborane and ammonia were found, which are an ionic and a covalent (keeping of one B-H-B bridge bond) types. The transition state of the covalent type is about 5 kcal/mol lower in energy than that of the ionic one. For 1,l-hydrogen elimination, two eliminations from B atom and from N atom of H3B:NH3 are examined. The energy barriers for both eliminations are extremely high (107- 103 kcal/mol from the isolated diborane and ammonia). For 1,2-hydrogen elimination, four reaction paths were calculated. These reaction paths have also high energy barriers (33-46 kcal/mol), since essentially they are forbidden reactions by the Woodward-Hoffmann rules. The reaction pathway of 1,3-hydrogen elimination includes two concerted reactions (hydrogen elimination and hydrogen shift), which have a low energy barrier of 16.7 kcal/mol at the MP4/6-31 l+G(d,p) ZPE level. Two elimination pathways (1,Zhydrogen elimination from the adduct and 1,3-hydrogen elimination from the complex) lead to aminoborane in one step, except for 1,l-hydrogen elimination with extremely high barrier.
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Introduction
H3B:NH3
Diborane, B2H6, is the simplest stable member of borane hydrides and has been of interest to chemists's2 for a long time. B2H6 is electron deficient, and it reacts readily with a range of Lewis bases, both in solution and in gas phase. Recently Carpenter and Ault studied, by an infrared matrix isolation, the reactions of B2H6 with NH3,394m e t h a n ~ land , ~ SH2,6which lead to the formation of aminoborane, HzB=NH2, methoxyborane, HzB=OCH3, and mercaptoborane, H2B=SH, respectively. It was concluded from their results that these reactions produce directly aminoborane, methoxyborane,and mercaptoborane, but not by a one-step mechanism; the first process is the formation of a borane-Lewis bases adduct, and the second is the hydrogen elimination from the adduct. The first step is slow and the second is rapid. This conclusion comes from the fact that they did not observe the evidence of the adduct as H3B:NH3 for the reaction of B2H6 with NH3. To clarify the proposed mechanism, Carpenter and Ault? studied the pyrolysis of the adduct H3B: NH3, and observed the evidence of H2B=NH2. They also studied the reaction of diborane with methylamine' (CH3NH2, (CH3)2NH, and (CH3)3N) by an infrared matrix isolation and observed evidence of the formation of H2B=NHCH3, H2B=N(CH3)2, and H3B:N(CH3), respectively. From these results, they concluded a mechanism for the reaction of &H6 with NH3 in which the adduct H3B:NH3 is formed in initial slow step and is followed by rapid elimination of H2 to form H2B=NH2. Experimentally estimated activation barrier for the reaction of diborane with ammonia is only 5.6 kcallmol: B,H6
+ NH, - H3B:NH3+ BH3
@Abstractpublished in Advance ACS Absrrucrs, March 15, 1995.
(1)
-
H2B=NH2
+ H2
(2)
In this mechanism estimated experimentally, the second step is puzzling. The second step, 1,Zhydrogen elimination, is a 2s 2s type reaction and is forbidden thermally by the Woodward-Hoffmann rules. One can assume a very high barrier for this reaction step. On the other hand, an ab initio MO study on the reaction mechanisms of with NH3 by McKee* has provided a minimum energy reaction path, which initiates with the formation of NH3(BH3)2, followed by 1,Zhydrogen elimination to give B ~ H ~ : N H z +and H ~ eventually leads to H2B=NH2 BH3 Hz with very high barriers (42 and 48 kcdmol). Recently, Morokuma and co-workersgcalculated minimum energy reaction paths for the formation of HzB=SH through 1,Zhydrogen elimination from H3B:SH2 and SH2(BH3)2 complex by ab initio MO methods. They provided the reaction paths for HzB=SH formation which are similar to those calculated by McKee* for the reaction of B2& and NH3. The calculated activation barriers for the formation of H2B=SH were 41 and 45 kcallmol. The reaction barriers calculated by these groups are very high, because their paths are 1,2-hydrogen eliminations. Quite recently, for the reaction of diborane with ammonia, a new transition statelo of I-i2B=NH2 formation with a low energy barrier (17 kcallmol) was reported in my previous letter by ab initio MO methods. The potential energy surface of possible reaction paths for H2B=NHz formation at the same theoretical level should be useful to understand the common mechanisms of B2H6 with Lewis bases. To explain the overall reaction for diborane and ammonia systematically, I Calculated the potential energy surface of the initial step reactions and three type (1,l-, 1,2-, and 1,3-) hydrogen elimination reactions by ab initio methods.
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0022-365419512099-5883$09.00/0 0 1995 American Chemical Society
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Sakai
5884 J. Phys. Chem., Vol. 99, No. 16, 1995
-2
n)-$
Hn7NH , 108.7
m,c, 0.761
$(HBNH)=I15.6
1.501
i
,,''
1.139
,,,aH,b
,
H
H H
1.605
, 2.258
4
9
T,,,
,
>
-N
H
H
b
m
H
2.284
0.921
H E'l
H
7-H 1.186
H
:
,*'
'H
.-a= 93.9
Is5
1.5
H H
>-H
H
TS8, cs
Figure 1. MP2/6-3 IG(d,p) optimized geometric structures (in angstroms and degrees) of various species.
Computational Approach
Results and Discussion
The basis sets used were the split-valence plus polarization 6-3lG(d,p)" and the split-valence plus polarization and diffuse functions 6-3 1l+G(d,p) sets.12 All molecular structures, including those for transition states, were obtained at the MP21631G(d,p) 1 e ~ e l . ITo ~ verify the minima and transition states, it was established that the matrixes of energy second derivative^'^ have zero and one negative eigenvalue, respectively. The reaction energies were determined by using fourth-order MdlerPlesset perturbation theory corrections with the 6-31l+G(d,p) basis set. The intrinsic reaction coordinate (IRC)I5 was followed from the transition state toward bath reactants and products. All calculations were made by GAUSSIAN92 program.16
The calculated stationary point geometries for the reaction of with ammonia are illustrated in Figure 1. The total energies with the MP2l6-3 lG(d,p) optimized geometries are listed in Table 1. The potential energy profile along the minimal energy path for the reaction of BzH6 with NH3 is displayed in Figure 2. A. Initial Step Reaction of B& NH3. The dissociation (dimerization) energy of BH3 from B2H6 is 34.4 kcal/mol at the MP4/6-31 l+G(d,p)//MP2/6-3 lG(d,p) ZPE level. For borane-ammonia adduct H3B:NH3, the complexation energy between BH3 and NH3 is 25.6 kcaYmo1. Accordingly diborane is about 9 kcaYmol more stable in energy than the adduct, and the formation of the adduct H3B:NH3 is an endothermic reaction. On the other hand, the first step of the reaction of B2H6 NH3
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Reactions of Diborane with Ammonia
J. Phys. Chem., Vol. 99, No. 16, 1995 5885
TABLE 1: Total Energies (hartree) and Zero-Point Energies (kcahol) for Various Species MP2/ MP4/ sym 6-3IG(d,p) 6-311+G(d,p) ZPE" HZ Dmh -1.15766 -1.16769 6.6 BH3 D3h -26.486 16 -26.518 28 17.2 NH3 C3" -56.383 22 -56.434 26 22.3 BzH6 (1) Dzh -53.038 51 -53.102 20 41.2 H3B:NH3 (2) C3" -82.932 29 -83.002 75 45.4 H2B=NH2 (3) Cz, -81.774 37 -81.834 57 31.1 HBNH3 (4) CI -81.588 30 -81.675 57 29.8 H3BHBHzNHs (5) CI -109.440 95 -109.552 89 66.8 HsBHBHzNH3 (5') C, -109.430 38 -109.542 30 66.1 p-HzBHBH2NHz (6) Cz, -108.309 90 -108.407 20 54.6 HzBHzBWz (7) C, -108.28407 -108.385 77 52.7 HzBNH2BH3 (8) C, -108.274 11 -108.373 59 52.3 HzBHzBHzNH3 (TS1) C, -109.419 13 -109.53092 65.4 HzBHzBHm3 (TS2) C1 -109.410 88 -109.449 86 65.7 ( H d B W 3 (TS3) C1 -82.746 57 -82.843 51 39.6 HB(HINH2 ( T W CI -81.571 31 -81.655 61 28.1 HzB(H)NH(Hz) (TS5) C1 -82.745 71 -82.834 11 37.5 HzB(H2)NHz(TS6) C, -82.855 81 -82.936 67 41.4 H3BHBH(Hz)NHz (TS7) CI -109.358 93 -109.472 58 62.4 HzB(H2)NHzBH3 (TSS) C, -109.361 19 -109.472 67 62.9 HzB(H2)NHzBHs(TS9) C, -109.371 59 -109.482 95 62.9 H~B(H)BH~NHZ(H~) (TS10) C1 -109.393 77 -109.508 45 62.6 Zero-point correction in kcal/mol.
energy difference between 5 and 5' is 6.6 kcdmol at the MP4/ 6-31 l+G(d,p) calculation. The force constant matrix of 5 has no negative eigenvalue. The product 5 is a complex between H3B:NH3 and BH3 compounds, which has a large complexation energy of 15.8 kcaVmo1 at the MP4/6-31 l+G(d,p) ZPE level. This complexation energy is about half of the dimerization energy (34.4 kcaymol) of BH3 for diborane included two B-H-B bridged bonds. That is to say, the complexation energy in 5 corresponds to one B-H-B bridged bond energy. The mechanism for the formation of the complex 5 through TS1 is similar to that for the reaction of the Menschutkin" SNZ type. The lone-pair orbital of nitrogen pushes the one bridged H atom of the reverse side for B atom, and the other side B-H-B bridged bond is kept through the reaction pathway. The transition state TS2 is an ionic B&-:B+H*NH3 type and is characterized by two asymmetric B-H bridged bonds. The energy barrier at TS2 is about 5 kcdmol higher than that at TS1 at the MP4/6-31 l+G(d,p)//MP2/6-31G(d,p) ZPE level. This higher energy transition state of TS2 type was also found by McKee at MP2/6-31G(d) calculation level. However, he did not find the lower energy transition state of TS1 type. The energy barrier at TS2 is 10.3 kcdmol from the isolated B& and NH3 and corresponds to the value (12.8 kcdmol) calculated by McKee. TS2 leads directly to the complex 5 along the IRC pathway. In comparison between TS1 and TS2, the reaction through TS1 keeps one B-H-B bridged bond. On the other hand, TS2 has no B-H-B bridged bonds and is an ionic state (the natural charge of B atom not attacked by NH3 -0.272e, and the dipole moment 5.36 D for TS1 and the natural charge -0.457e, and 6.67 D for TS2 at HF/6-31G(d,p) level). It is considered that the covalent (SNZtype) TS1 is lower in energy than the ionic TS2 because of the B-H-B bridged bond energy. From the B-N and the B-B bond distances in TS1 and TS2, TS2 is a later transition state than TS1. These results are consistent with the Hammond postulation,18 since the product 5' of TS1 is higher in energy than the product 5 of TS2. In the initial step reaction of BzH6 NH3, we could not find a one-step reaction pathway to form the adduct H3B:NH3. It is considered that the adduct produces through the dissociation of BH3 from diborane or from the complex 5 : B2H6 NH3 2BH3 NH3 H3B:NH3 BH3 (barrier 34.4 kcdmol); B2H6 NH3 5 H3B:NH3 BH3 (barrier 8.8 kcdmol). The latter is the favorable reaction pathway for the formation of the adduct H3B :NH3. In comparison with Gibbs free energies at 298.15 K for these species the energy barriers at TS1 and TS2 increase with increasing temperature. The relative energy of the dissociation state (2BH3 NH3) decreases with an increase in the temperature. The energy barrier height at TS1 is the same as that at the dissociation state at the temperature value of 174 "C. Under the experimental condition (160-300 "C) by Carpenter and Ault, the formation of the complex 5 or the adduct may occurs through both TS1 and the dissociation state (2BH3 NH3). The relative energy of the complex 5 increases with increasing temperature, and that of the adduct 2 BH3 change little for the variation of temperature. The complex 5 equalizes with the adduct 2 BH3 in energy at the temperature value of 237 "C. Carpenter and Ault did not observed the evidence of the complex 5 under their experimental condition (160-300 "C). The experimental detection of the complex 5 may be possible at low temperatures. The dissociation state (2BH3 NH3) is the same to the adduct 2 BH3 in energy at the temperature value of 485 "C. B. Second Step: Hydrogen Elimination. B.Z. 1,l-Hydrogen EZimination. In this section, 1,l-hydrogen elimination pathways from boron atom and from nitrogen atom in H3B:NH3 were
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Figure 2. Potential energy profile along the minimal energy path for the reaction of Bz& with N H 3 at MP4/6-311+G(d,p) ZPE level.
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- --
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-
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Figure 3. Gibbs free energy profile along the minimal energy path at
298.15 K. is the formation of H3BHBHzNH3 complex 5 through the transition states TS1 or TS2. The transition state TS1 has a C, symmetry and is characterized by one asymmetric and one symmetric B-H-B bridged bonds. TS1 leads to a complex 5' along the IRC pathway. The force constant matrix of 5' has one negative eigenvalue (179 cm-'). This vector has a" symmetry and corresponds to the rotation of BH3 for the B-B bond axis. A little geometric distortion of 5' from C, symmetry leads to a C1 symmetry structure of 5 without a barrier. The
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Sakai
5886 J. Phys. Chem., Vol. 99, No. 16, 1995 studied. The transition state TS3 of 1,l-hydrogen elimination from boron in H3B:NH3 has the energy barrier of 94.1 kcaV mol above H3B:NH3 at the MP4/6-31 l+G(d,p) ZPE level, and leads to 4 and Hz along the IRC path. TS3 is only 0.3 kcal/mol higher in energy than the products (4 H2) at the MP2/6-31G(d,p) calculation, while at the MP4/6-31 l+G(d,p) level, TS3 is 0.2 kcdmol lower than the products. At the MP4/ 6-3 11+G(d,p) ZPE level, the products is 4 kcaYmol lower in energy than TS3. The energy barrier (4 kcdmol) at TS3 from the products causes from the difference of the zero-point energies for TS3 and the products (4 H2). This reaction is endothermic reaction (91.1 kcaYmo1). Hence, it is consistent with the facts that TS3 is a late transition state from the H-H (0.761 A) and B-N (1.750 A) bond lengths. In the subsequent formation of H2B=NH2 from 4, the energy barrier at the transition state TS4 for 1,2-hydrogen migration is 10.8 k c d mol above 4. The overall barrier height of the reactions is about 111 kcal/mol from the reactant (diborane and NH3). In 1,l-hydrogen elimination from nitrogen in H3B:NH3, the transition state TS5 leads directly to H2B=NH2 H2 for the product side along the IRC path. The energy barrier at the transition state TS5 is also very high (97.9 kcdmol above 2). This reaction pathway includes two elementary mechanisms: 1,l-hydrogen elimination and 1,Zhydrogen shift. From the geometry variation along the IRC path, the reaction occurs via two steps. After one N-H bond breaking occurs at first, the other N-H bond breaking and 1,Zhydrogen migration occur at the same time. As a result, the reaction leads to H2B=NHz and Hz. For 1,l-hydrogen elimination, both reaction pathways have very high barriers because of the loss of one bond energy (the breaking of two X-H bonds (X = N or B atom) and the formation of one H-H bond). Accordingly, aminoborane formation does not occur through 1,l-hydrogen elimination. B.ZZ.1,2-Hydrogen Elimination. Four transition states for 1,2-hydrogen elimination were studied in this section. The transition state of 12-hydrogen elimination from the adduct H3B: NH3 is TS6, and the geometrical parameters are close to those of the previous calculations.*JOThe transition state TS6 is characterized by the hydrogen atom bridged the nitrogen and boron atoms symmetrically. From the structure of TS6, the N-H bond breaking and the H-H bond formation occur at the same time, and subsequently the H2 molecule leaves the B atom. The energy barrier height at TS6 is 37.5 kcaYmol from H3B: NH3 at the MP4/6-311+G(d,p) ZPE level, and this high barrier corresponds to the calculated value (37.9 kcaYmo1) by McKee. From the experimental result^,^,^ it was assumed that this reaction step is rapid. However the calculated barrier does not correspond to the experimental assumption. At the temperature of 298.15 K, the Gibbs free energy (shown in Table 2) for TS6 is 37.1 kcal/mol above H3B:NH3. The energy barrier is still very high. The difference of entropy energies of TS6 and H3B:NH3 is only 1.3 cal/(mol "C), so that this barrier does not depend on the temperature in the range of the experimental condition. This is the same to the results for the reaction of B2H6 SH2 in the previous paper.g The total energy of the final products (aminoborane H2 BH3) is very close to that of the reactants (B2H6 NH3). The transition state, TS7, of 1,Zhydrogen elimination from the complex 5 leads directly to aminodiborane 7 and H2 along the IRC path. The transition state TS7 is characterized by the hydrogen atom bridged the nitrogen and boron atoms asymmetry. The active H-H (formation of H2) bond length in TS7 is very close to that in the transition state TS6 for H2 elimination from H3B:NH3. The B-B bond length in TS7 is shorter by
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0.128 8, than that in the complex 5 and is longer by 0.373 A than that in aminodiborane 7. The energy barrier at TS7 is also high (39.0 kcaYmol from the isolated B2& and NH3) and is about 7 kcaYmol lower than that at TS6. This energy difference corresponds to the complexation energy between TS6 and BH3. Accordingly this reaction step includes two process; the first is 1,Zhydrogen elimination, and the second process is the subsequent rearrangement to aminodiborane. Aminodiborane 7 has two B-H-B bridged bonds which conjugate strongly with the lone pair orbital (pn) !o -NH2 group. The B-N bond distance of 7 is only 0.023 A longer than that of aminoborane 3. The rotational barrier (nconjugation energy) of -NH2 group for the B-N axis is 12.0 kcdmol at the MP4/ 6-31 l+G(d,p) calculation, which is about half energy of that (31.7 kcdmol) of aminoborane. The products (7 H2) is 14.8 kcaYmo1 lower in energy than the separated B2H6 and NH3. For the other transition states for 1,2-hydrogen elimination, we found TSS and TS9. The transition state TSS is an early transition state from the H-H bond length: H-H distances are 0.963 8, for TS6,0.955 8, for TS7, and 1.118 8, for TSS. TS8 leads to the adduct 2 BH3 for the reactants side and to 8 H2 for the products along the IRC path. The force constant matrix of 8 has one negative eigenvalue (255 cm-'), and the vector corresponds to the rotation of BH3 part for the N-B axis. After the rotation of BH3 part, the reoptimized geometry reached p-aminodiborane 6. McKee found a transition state similar to that of TS8, which was related to the complex 5 and p-aminodiborane. However, TSS does not lead to the complex 5, but leads to 2 and BH3 along the IRC path. The formation of the complex 5 from 2 and BH3 has no barriers. Consequently, after the formation of 2 and BH3 in this reaction step, it may produce the complex 5 easily. From a careful analysis of the IRC pathway, it is found that the mechanism through TS8 is not the similar to that through TS6. The hydrogen atoms eliminated at TSS are provided from the H-N bond of the adduct 2 and the H-B bond of the attacked BH3:
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I m
.u
The energy of TS8 is 39.5 kcal/mol higher than that of the separated B2H.5 and NH3 at the MP4/6-3 11+G(d,p) ZPE level. The formation energy of p-aminodiborane is -26.4 kcaYmol from B2H6 and NH3, and p-aminodiborane will be produced easily from the reaction of aminoborane and BH3 molecules. On the other hand, TS9 leads also to H3B:NHs BH3 for the reactant side and to p-aminodiborane H2 through the compound 8 for the product side along the IRC path. This reaction mechanism is similar to that of TSS. Namely, the hydrogen atoms eliminated are also provided from the H-N bond of 2 and the H-B bond of the attacked BH3:
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I
H
The difference of the mechanisms via TSS and TS9 is the approaching direction of BH3. That is to say, for TS8, BH3 attacks the front side of the N-B bond, and for TS9, BH3 attacks the back side of NH3. The reaction via TS9 includes two-
J. Phys. Chem., Vol. 99, No. 16, 1995 5887
Reactions of Diborane with Ammonia process mechanism. One is hydrogen elimination, and the other is exchange reaction of BH3. The energy barrier height for exchange reaction, H3B':NH3 BH3 B'H3 H3N:BH3, is 14.1 kcdmol above the separated H3B:NH3 and BH3 at the MP4/6-311+G(d,p) ZPE level. The transition state of the exchange reaction is about 10 kcal/mol lower in energy than TS9. The B-N bond length at the transition state of the exchange reaction is 2.055 A and is significantly longer than that in TS9. From the H-H bond length, TS9 is an early transition state, as is TS8. Two elementary mechanisms (the exchange reaction and the hydrogen elimination) decrease the energy barriers. Because the effect of symmetry forbidden for 1,Zhydrogen elimination decreases with the weaker B -N bond by the exchange process, and the inversion energy of NH3 part for the exchange reaction decreases with the weaker N-H bond by the elimination. The energy barrier height at TS9 is 33.0 kcdmol from the separated B2H6 and NH3 at the MP4/63ll+G(d,p) ZPE level. The energy barrier at TS9 is lower by 6.5 kcdmol than that at TS8. As above shown, 1,2-hydrogen eliminations from the boraneammonia adduct and the complex 5 have large energy barriers. These reactions are essentially symmetry forbidden by the Woodward-Hoffmann rules. In above four 1,2-hydrogen eliminations, only TS6 leads to HzB=NH2 in one step. B.III. 1,3-HydrogenElimination. The transition state structure, TS10, of 1,3-hydrogen elimination from the complex 5 is a cyclic type. The transition state is characterized by the B-H-B bridged bond. The B-N and the H-H bond lengths of TSlO are the shortest in the corresponding bond lengths of the other hydrogen elimination transition states (TS5-TS9). The dissociation of the B-H (bridged BH3) and the N-H bonds occurs in concert: B+- - -H- and N-- - -H+. The interacting BH3 in 5 becomes a B€&- anion type at TS10. Consequently, the reaction is characterized by an ionic cyclic mechanism. The ionic cyclic mechanism is the movements of three electron pairs. The electron pair in the breaking H-B (bonded to N) bond moves to the new B-H bond region, and the electron pair in the breaking H-B (interacting to H3B:NH3) bond moves to the new H-H bond region. The electrons in the N-H bond move likewise to the B-N x bond region:
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TABLE 2: Relative Energies and Gibbs Free Energies at 298.15 K (kcaYmo1) for Various Stationary Points on the Potential E n e m Surface MP4t 6-311+G(d,p) +ZPE -TAS
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99.9 100.1 112.6 105.8 41.5 0.3 31.5 0.0 3.5 8.1 - 10.3 40.1 -10.7 40.0 -24.1 33.6 17.6 10.0
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0.0 0.0 0.0 94.1 -1.9 92.2 80.9 91.1 -10.2 101.9 -8.9 93.0 97.4 97.9 -0.5 37.5 -0.4 37.1 -7.4 -8.5 -15.9 15.5 25.6 -10.1 0.0 0.0 0.0 14.8 5.4 9.4 10.3 9.9 20.2 -7.0 9.4 2.4 9.7 48.7 39.0 1.6 -13.2 -14.8 39.5 7.9 47.4 3.1 -23.3 -26.4 33.0 8.9 41.9 16.7 10.2 26.9 -6.9 1.5 -8.4
from the complex 5 is only 6.5 kcdmol. The equilibrium between the complex 5 and 2 BH3 depends on the entropy term (temperature). Accordingly, evidence of the complex 5 will be observed under experimental conditions including a low temperature. In comparison with Gibbs free energies at 298.15 K the energy barrier of TSlO increases with increasing temperature. The difference in Gibbs free energies for TS6 at 0 and 298.15 K is only 0.3 kcdmol. The energy barrier at TSlO is the same as that of TS6 BH3 at 568 "C. This temperature (568 "C) is very high in comparison with the experimental condition (160 "C-300 "C) by Ault and co-workers. Consequently, the reaction path for the formation of aminoborane occurs through TSlO under the experimental condition. In summary, the present calculation shows that the reaction of diborane with ammonia occurs through the six-ring transition state (1,3-hydrogen elimination) instead of 1,l- and 1,Zhydrogen elimination from borane-ammonia adduct.
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Conclusions BH2-H
(4
The energy barrier height at TSlO is 16.7 kcdmol from the isolated B2H6 and NH3 at the MP4/6-31l+G(d,p) ZPE level. In comparison with Gibbs free energies, the energy barrier height at TSlO is the same as that at TS6 at 568 "C. Experimentally, Ault and co-workers4 observed evidence of aminoborane formation from H3B:NH3 and proposed the mechanism of direct 1,2-hydrogen elimination process. However, the dissociation state (2BH3 -tNH3) is about 12 kcal/mol lower in energy than TS6. At 298.15 K, the Gibbs free energy of the dissociation state is 15.6 kcaymol above that of 2 BH3. In the reaction of aminoborane formation from H3B:NH3, the energy barrier height on the reaction pathway through the complex 5 and TSlO is 25.6 kcal/mol; 2H3B:NH3 BH3 N H 3 H3B:NH3 NH3 5 TSlO NH3 3 H2 H3B:NH3. The direct H2 elimination reaction through TS6 has an energy barrier of 37.5 kcdmol. Therefore the former reaction path is more favorable than the latter one. Ault and co-workers did not observe evidence of the complex 5 under the experimental condition of high temperature. At 298.15 K, the dissociation energy of BH3
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The reaction mechanisms of B2H6 with NH3 were studied by ab initio MO methods. In the initial step, the complex 5 is formed through the transition state TS1 or TS2. The energy barrier at TS1 is about 5 kcdmol lower than that at TS2. The formation of borane-ammonia adduct H3B:NH3 is not a onestep process from diborane with ammonia. The lowest energy path for the formation of H3B:NH3 is the dissociation of BH3 from the complex 5. The energy barriers of 1,l-hydrogen eliminations from the adduct 2 are extremely high (103-107 kcal/mol), so that the formation of aminoborane does not occur through 1,l-hydrogen elimination. The energy barrier of 1,2hydrogen elimination from the adduct 2 is also high (37.5 k c d mol from the adduct H3B:NH3). This energy is larger than that (25.6 kcal/mol) of the dissociation (BH3 NH3) of H3B:NH3. The structure of the active site in the transition state TS7 of 1,2-hydrogen elimination from the complex 5 is similar to that (TS6) from the adduct 2. The total energy of TS7 is about 7 kcal/mol lower than that of TS6 BH3. The transition state TS7 leads to aminodiborane 7 along the IRC path. For other 1,Zhydrogen elimination, the transition states (TS8 and TS9) from the reactants (the adduct 2 BH3) lead to p-aminodiborane but do not lead to aminoborane directly. The mechanisms of HZ elimination at TS8 and TS9 are completely different from
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Sakai
5888 J. Phys. Chem., Vol. 99, No. 16, 1995 those at TS6 and TS7. The energy barrier heights at TS8 and TS9 are 30.7 and 24.2 kcal/mol above the adduct 2 -I- BH3, respectively. TS9 is about 1.4 kcal/mol lower in energy than the dissociation (2BH3 NH3) species. Therefore, the formation of p-aminodiborane may also occur from the adduct 2 and BH3. The transition state of 1,3-hydrogen elimination from the complex 5 is the ionic cyclic structure and has an energy barrier of 16.7 kcal/mol from B2H6 NH3. This transition state has the lowest energy barrier in the transition states (TS4-TS10) treated here. It is concluded that the aminoborane formation occurs through this transition state TS10. As above shown, the direct reaction paths for the formation of aminoborane in the elimination step are 1,2-hydrogen elimination from the adduct H3B:NH3 and 1,3-hydrogen elimination from the complex 5 except for 1,l-hydrogen eliminations with extremely high barrier. The Gibbs free energy at the transition state TSlO of 1,3-hydrogen elimination increases with increasing temperature. However, the energy barrier at TSlO is lower than that of the transition state TS6 at the condition under about 570 “C. Therefore the formation of aminoborane occurs through TSlO at the experimental condition by Ault and co-workers.
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Acknowledgment. The present research is supported in part by a Grant-in-Aid for Scientific Research on Priority Area “Theory of Chemical Reactions” from the Ministry of Education, Science and Culture. This work is also supported in part by a Grant-in-Aid for Special Research from the Sangyo Institute of Osaka Sangyo University. The computer time was made available by the Computer Center of the Institute for Molecular Science and by the Information Systems Engineering Depart-
ment of Osaka Sangyo University with its CONVEX C240 minisupercomputer.
References and Notes (1) Muetterties, E. Boron Hydride Chemistry; Academic Press: New York, 1975. (2) Lane, C. F. Chem. Rev. 1976, 76, 773. (3) Carpenter, J. D.; Ault, B. S. J. Phys. Chem. 1991, 95, 3502. (4) Carpenter, J. D. Ault, B. S. Chem. Phys. Lett. 1992, 197, 171. (5) Carpenter, J. D. Ault, B. S. J. Phys. Chem. 1992, 96, 4288. (6) Carpenter, J. D. Ault, B. S. J. Phys. Chem. 1992, 96, 7913. (7) Carpenter, J. D. Auk, B. S. J. Phys. Chem. 1992, 95, 3507. (8) McKee, M. L. J. Phys. Chem. 1992, 96, 5380. (9) Mebel, A. M. Musaev, D. G.; Morokuma, K. J. Phys. Chem. 1993, 97, 7543. (10) Sakai, S. Chem. Phys. Lett. 1994, 217, 288. (11) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (12) McLean, A. D.; Chandler, G.S. J. Chem. Phys. 1980, 72, 5639. Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3261. (13) Pople, J. A.; Seeger, R.; Krishnan, R. Int. J. Quantum Chem. 1979, S11, 149. (14) Pople, J. A,; Binkley, J. S.; Seeger, R. Int. J. Quantum Chem. 1975, 9, 229. Pople, J. A.; Krishnan, R.; Schegel, H. B.; Binkley, J. S. Int. J. Quantum Chem. 1979, S13, 225. (15) Fukui, K. J. Phys. Chem. 1970, 74, 4161. Ishida, K.; Morokuma, K.; Komomicki, A. J. Chem. Phys. 1977, 66, 2153. (16) Frisch, K. J.; Trucks, G.W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Comperts, E.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, L. R.; Fox, D. J.; DeFrees, D. J.; Baker, I.; Stewart, J. J. P.; Pople, J.A. GAUSSIAh‘92;Gaussian, Inc.: Pittsburgh, PA, 1992. (17) Menschutkin, N. Z. Phys.Chem. 1980, 6, 41. (18) Hammond, G. S. J. Am. Chem. SOC. 1955, 77, 334.
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