Theoretical study on the reaction mechanism of BH2+ and ethylene in gas-phase

4 May 2001

Chemical Physics Letters 339 (2001) 140±146

www.elsevier.nl/locate/cplett

Theoretical study on the reaction mechanism of BH2‡ and ethylene in gas-phase Zheng-wang Qu a,*, Hui Zhu c, Ze-sheng Li b, Qi-yuan Zhang a a

State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People's Republic of China b State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People's Republic of China c Key Lab for Supramolecular Structure and Spectroscopy of Ministry of Education, Jilin University, Changchun 130023, People's Republic of China Received 3 January 2001; in ®nal form 28 February 2001

Abstract The potential energy surface (PES) for the reaction of the borohydride cation BH2‡ and ethylene (C2 H4 ) in the gasphase has been investigated at the B3LYP/6-311G(d, p) and single-point CCSD(T)/6-311G(2df, p) levels. Based on the calculated PES, the stabilities of various BC2 H6‡ isomers are determined and the mechanism of the formation of the association product BC2 H6‡ and the dissociation product BC2 H4‡ ‡ H2 in this reaction are also discussed. This study provides the ®rst theoretical results on the reaction mechanism of the electron-de®cient borohydride cations with alkenes. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction The electron-de®cient boron-containing species have attracted many interests in recent years mainly due to their novel chemical behavior and potential applications [1]. A myriad of borohydride cations Bn Hm‡ have been found ®rstly by Dunbar in an ICR instrument [2] and recently by DePuy et al. [3] in a FA-SIFT instrument. Though the hydroboration reactions of neutral boranes (such as BH3 ) with alkenes are well known and widely used in organic syntheses, very little is known about the reaction mechanism of these *

Corresponding author. Fax: +86-10-6258-8930. E-mail address: [email protected] (Z.-w. Qu).

borohydride cations with alkenes. To our best knowledge, only the gas-phase reaction BH2‡ ‡ C2 H4 has been studied by DePuy et al. [4] using an FA-SIFT instrument in 1998, where the adduct ‰B; C2 ; H6 Š‡ and the dissociation product ‡ ‰B; C2 ; H4 Š ‡ H2 are produced with the branching ratios of 0.15 and 0.85, respectively. To our best knowledge, no theoretical study has been performed on the reaction mechanism of borohydride cations with alkenes. Recently, we have performed some B3LYP and CCSD(T) studies on several electron-de®cient reaction systems such as B2 H3 ‡ CO2 [5], B2 H3 ‡ CS2 [6] and BH2‡ ‡ C2 H2 [7], and good agreement with available experimental data has been reached. Since BH2‡ is an extremely interesting electrophilic

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 1 ) 0 0 3 1 5 - 3

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species and C2 H4 is the simplest alkene, further theoretical study on the reaction BH2‡ ‡ C2 H4 may be helpful for understanding the chemical behavior of electron-de®cient borohydride cations towards alkenes.

2. Computational methods All structures are full-optimized at the B3LYP/ 6-311G(d, p) level [8] of theory using the GA U S S I A N 98 program package [9]. For stationary points, the harmonic frequency calculations are performed to verify whether they are minima with all positive frequencies or transition states with only one imaginary frequency. The intrinsic reaction coordinate (IRC) [10] calculations are further carried out at the B3LYP/6-311G(d, p) level to con®rm that the transition states connect the right minima. In order to give more reliable energetics, the ®nal relative energies are evaluated at the single-point CCSD(T)/ 6-311G(2df, p) level [11] with inclusion of the B3LYP zero-point vibrational energies, by taking the most low-lying isomer c as zero for reference. Unless otherwise speci®ed, the B3LYP-optimized geometries and CCSD(T) relative energies are used in the following discussions.

3. Results and discussion The full-optimized geometries of the reactant (R) BH2‡ ‡ C2 H4 , dissociation product (P) ‡ BHCHCH2‡ ‡ H2 and ®ve ‰B; C2 ; H6 Š isomers are presented in Fig. 1 and of ®ve optimized transition states in Fig. 2. Note that TSa/c (Cs ) is referred to as the Cs -symmetrized transition state connecting the isomers a and c, and so on. The B3LYP and QCISD energies of various structures are listed in Table 1. By means of CCSD(T) relative energies, the schematic potential energy surface (PES) of the reaction BH2‡ ‡ C2 H4 is shown in Fig. 3. For simplicity, the calculated harmonic frequencies are omitted here. Wherever necessary, the comparison between our calculations and available theoretical and experimental results will be made.

141 ‡

3.1. Relevant structures on the ‰B; C2 ; H6 Š PES To verify the reliability, a comparison of our B3LYP-geometries with some available data is made. As shown in Fig. 1, our calculated B±H  of the ion reactant BH ‡ bond length (1.1745 A) 2 (D1h ) is very close to the MP2(full)/6-311G(d, p) [4], MP2/6-311++G(3df, 2pd) and CCSD(T)/cc repVTZ [12] values (1.173, 1.169 and 1.174 A, spectively). Moreover, our calculated geometries  of the neutral reactant C2 H4 (D2h ) (C±C: 1.3269 A;  C±H: 1.0850 A; C±C±H: 121.76°) are also in good accordance with the experimental [13] values  C±H: 1.086 A;  C±C±H: 121.0°). (C±C: 1.340 A; Thus very reliable geometries may be provided by the B3LYP calculations. As pointed out in our recent calculations [7], ‡ among the four possible ‰B; C2 ; H4 Š isomers ‡ ‡ BH2
C2 H2 …C2v †, BH2 CCH2 …Cs †, BHCHCH2‡ …Cs † and H2
HCCH ‡ …Cs †, only the most lowlying isomer BHCHCH2‡ is kinetically stable enough (66 kJ/mol) to be observed in SIFT experiments. Thus only (P) BHCHCH2‡ ‡ H2 is considered in this study as the dissociation product of the reaction BH2‡ ‡ C2 H4 . ‡ Five ‰B; C2 ; H6 Š isomers, i.e., a …C2v † ‡ BH2
C2 H4 , b …Cs † BH2 CHCH3‡ , c …Cs † BHCH2 CH3‡ , d …Cs † cis-H2
BHCHCH2‡ and d0 …Cs † trans-H2
BHCHCH2‡ are newly found in our calculations as shown in Fig. 1. The long  suggest isomer a is a B. . .C distances (1.8062 A) complex between BH2‡ and C2 H4 . All four isomers b, c, d and d0 possess a B±C±C skeleton. However, the C±C single-bond of isomer b (1.4463  is dramatically shorter than that of isomer c A)  mainly due to the strong hyperconju(1.5785 A), gation e€ects induced by the respective highly charged carbon atom of b and boron atom of c. Isomers d and d0 are typical complexes between H2 and BHCHCH2‡ through three-center-two-elec distances. tron bond with long B. . .H (1.49 A) Their relative energies (in kJ/mol) may be given in parentheses as follows: c …0:0† < a …‡30:6† < b …‡60:4† < d0 …‡92:9† < d …‡97:9†, with isomer c to be the global minimum. Six transition states, i.e., TSa/b, TSa/c, TSa/d, TSa=d0 , TSb/c, and TSc/P are obtained as shown in Fig. 2. IRC calculations suggest that TSa/d,

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Fig. 1. Geometries of various structures calculated at the B3LYP/6-311G(d, p) level. Bond lengths are in angstroms and bond angles in degrees.

TSa=d0 , and TSc/P are related to the H2 -elimination processes from isomers a and c while TSa/b, TSa/c, and TSb/c to the H-shift processes. Notice that isomer a may directly dissociate into (R) with a barrier of 229.8 kJ/mol or isomerize to isomers b, c, d and d0 with the barriers of 62.4, 25.6, 144.0 and

168.2 kJ/mol via TSa/b, TSa/c, TSa/d and TSa=d0 , respectively. Thus a low barrier of 25.6 kJ/mol stabilizes isomer a. The kinetic stabilities of other isomers may also be determined in the same way and given as follows: b …3:2† < d0 …12:1† < d …17:1† < a …25:6† < c …56:2†.

Z.-w. Qu et al. / Chemical Physics Letters 339 (2001) 140±146

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Fig. 2. Geometries of the transition states calculated at the B3LYP/6-311G(d, p) level. Bond lengths are in angstroms and bond angles in degrees.

Since the calculated ‰B; C2 ; H6 Š‡ isomers, transition states and dissociation product (P) in this study are all lower in energy than the initial reactant (R), if the energy released in the association process of BH2‡ and C2 H4 is not rapidly removed, all of these relevant structures can be reached in principle.

3.2. Reaction paths for the formation of (P) BHCHCH2 ‡ ‡ H2 Four energetically feasible pathways for the formation of the observed major dissociation product (P) BHCHCH2‡ ‡ H2 [4] are obtained from Fig. 3 as follows:

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Table 1 Zero-point vibrational energies (ZPVE, in hartree), total energies (TE, in hartree) and relative energies (RE, in kJ/mol) of various structures calculated at the B3LYP/6-311G(d, p) and single-point CCSD(T)/6-311G(2df, p) levels B3LYP

Species

(R) BH2‡ ‡ H2 C ˆ CH2 a b c d d0 TSa/b TSa/c TSa/d TSa=d0 TSb/c TSc/d (P) HBCHCH2 ‡ ‡ H2

CCSD(T)

ZPVE (hartree)

TE (hartree)

RE (kJ/mol)

TE (hartree)

RE (kJ/mol)

0.070137 0.073447 0.070137 0.074288 0.070010 0.070597 0.070388 0.072546 0.069260 0.068625 0.070271 0.068962 0.062735

)104.201462 )104.348462 )104.342461 )104.362115 )104.319982 )104.322909 )104.322102 )104.339814 )104.292894 )104.284947 )104.339096 )104.263538 )104.309608

+274.6 +33.7 +40.7 0.0 +99.4 +93.3 +97.5 +54.0 +168.6 +187.7 +49.9 +244.9 +107.6

)103.983714 )104.076473 )104.061803 )104.088985 )104.047405 )104.049908 )104.049648 )104.065825 )104.017452 )104.007618 )104.060719 )103.989533 )104.035520

+260.4 +30.6 +60.4 0.0 +97.9 +92.9 +93.0 +56.2 +174.6 +198.8 +63.6 +247.1 +110.0

PathP1 : …R† ! a ! TSa=b ! b ! TSb=c ! c ! TSc=P ! …P† PathP2 : …R† ! a ! TSa=c ! c ! TSc=P ! …P† PathP3 : …R† ! a ! TSa=d ! d ! …P† PathP4 : …R† ! a ! TSa=d0 ! d0 ! …P†

In all these pathways, the common intermediate a is directly formed from the association of the reactant (R) BH2‡ ‡ C2 H4 . Notice that from this common intermediate, the overall barriers involved in Path P1, Path P2, Path P3 and Path P4 for H2 -elimination are 216.5, 216.5, 144.0 and 168.2 kJ/mol, respectively. It seems that Path P3 is the most feasible pathway leading to the product (P). Since the barrier for …a ! c† conversions is 118.4 lower than that for …a ! d† conversion and

Fig. 3. Schematic PES for the reaction BH2 ‡ ‡ C2 H4 calculated at the CCSD(T)/6-311G(2df, p)//B3LYP/6-311G(d, p) level including the B3LYP zero-point vibrational energies. The relative energies are in kJ/mol.

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the barrier for …c ! a† conversion is 190.9 kJ/mol lower than that for …c ! P† conversion, some inter-conversion between isomers a and c may also occur before H2 -elimination as follows: Path P5 : …R† ! a ! TSa=c ! c ! TSa=c ! a ! TSa=d ! d ! …P† Path P6 : …R† ! a ! TSa=c ! c ! TSa=c ! a ! TSa=d0 ! d0 ! …P† Both pathways may contribute to H-scrambling and to the depletion of isomer c in the reaction BH2‡ ‡ C2 H4 . The total reaction of …R† BH2‡ ‡ C2 H4 ! …P† BHCHCH2‡ ‡ H2 is largely exothermic by 150.4 kJ/mol. 3.3. Reaction paths for the formation of adduct ‡ ‰B; C2 ; H6 Š Since isomers a, b, d and d0 are kinetically rather unstable (<26 kJ/mol), we only consider the most low-lying isomer c with the highest kinetic stability ‡ (>56 kJ/mol) as the observed adduct ‰B; C2 ; H6 Š in FA-SIFT experiment [4]. The feasible pathways leading to isomer c are obtained from Fig. 3 as follows: Path c1 : …R† ! a ! TSa=b ! b ! TSb=c ! c Path c2 : …R† ! a ! TSa=c ! c From the common intermediate a, a single 1,3-Hshift step is needed in Path c2 while two ordinal 1,2-H-shift steps are required in Path c1 to form adduct c. Notice that the …a ! c† conversion barrier is 36.8 and 7.4 kJ/mol lower than those for the …a ! b† and …b ! c† conversions, respectively, it seems Path c2 is energetically more preferable than Path c1. The total reaction of (R) BH2‡ ‡ C2 H4 ! c BHCH2 CH3‡ is also highly exothermic by 260.4 kJ/mol. 3.4. Mechanism of the reaction BH2 ‡ ‡ C2 H4 We now discuss the overall mechanism of the reaction BH2‡ ‡ C2 H4 . Considering the B3LYPcalculated frontier orbitals of the reactant BH2‡

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(HOMO: Ru , )0.7485 hartree; LUMO: Pu , )0.4098 hartree) and C2 H4 (HOMO: B3u , )0.2778 hartree; LUMO: B2g , +0.0037 hartree), the interaction between the LUMO of BH2‡ (mainly on the vacant p-orbital of boron atom) and the HOMO of C2 H4 (mainly on two carbon atoms with equal coecient) may play the crucial role in the reaction BH2‡ ‡ C2 H4 , leading to the vertical attack of BH2‡ towards the bonding p-orbital of C2 H4 . Since the initial ……R† ! a† association step is highly exothermic by 229.8 kJ/ mol, the released energy may overcome the barriers for intermediate a to take further isomerization to the most stable isomer c and dissociation into product (P). The feasible reaction pathways leading to both the dissociation product (P) and the most likely adduct c have been discussed in Sections 3.2 and 3.3, respectively. Note that the formation of the adduct c in the reaction BH2‡ ‡ C2 H4 is very similar to the hydroboration reaction of neutral boranes with alkenes. Since the low-lying isomers a, b and c are at least 200 kJ/mol lower in energy than the initial reactant (R) BH2‡ ‡ C2 H4 , some energy dissipation ways are required to stabilize these ‡ electronically or vibrationally excited ‰B; C2 ; H6 Š adducts which otherwise would take further dissociation into (P) BHCHCH2‡ ‡ H2 via the energetically feasible pathways Paths (P1, P2, P5, P6) as discussed in Section 3.2. Under the conditions of FA-SIFT experiments [4], collision with the helium bu€er gas (200±500 mTorr) is the most likely way. However, this collisional stabilization process still cannot compete with the dissociation process, mainly due to the high exothermicity of the reaction. As observed in the FA-SIFT experiment [4], (P) BHCHCH2‡ ‡ H2 is the major product with a branching ratio of 0.85 though the most likely adduct c BHCH2 CH3‡ is 110.0 kJ/mol lower in energy. It should be pointed that the competition between collisional stabilization and dissociation are very common in gas-phase ion±molecule reactions. For example, HCCCH2‡ may react with C2 H2 to produce the adduct C5 H5‡ under high-pressure [14] but only c-C3 H3‡ ‡ C2 H2 under rather lowpressure [15].

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4. Conclusions

References

We have studied in detail the mechanism of the gas-phase reaction BH2‡ ‡ C2 H4 at the B3LYP/ 6-311G(d, p) and single-point CCSD(T)/ 6-311G(2df, p) levels. The stabilities of various ‡ ‰B; C2 ; H6 Š isomers are determined. The reaction pathways leading to the most stable adduct BHCH2 CH3‡ and the dissociation product BC2 H4‡ ‡ H2 are also discussed. The calculated mechanism for the reaction BH2‡ ‡ C2 H4 are in good agreement with available experimental data and may be helpful for understanding the chemical behavior of electron-de®cient borohydride cations towards alkenes.

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Acknowledgements This project was support by the National Natural Science Foundation of China (NNSFC numbers: 29892168, 20073014, 29733100, 39890390) and the State Key Basic Research and Development Plan (G1998010100).