Potential energy surface survey of Al₃H₇: borane analogue or not?

Abstract

Much recent interest has focused on the understanding of the borane analogues of alanes. Being potential hydrogen-storage species, smaller alanes are also precursors or key intermediates of larger alanes. However, here we show that even for the simple trialane Al₃H₇, our knowledge is far from sufficient. Early low-level calculations considered an Al₃-chain (classical) isomer and a less-stable Al₃-ring (non-classical) isomer, indicative of the non-borane analogue of Al₃H₇. In the present work, we locate 11 new Al₃H₇ isomers within 15 kcal mol⁻¹ and construct the first isomerization potential energy surface of Al₃H₇ at the G3B3///B3PW91/TZVP level. It is shown that the lowest-energy isomer of Al₃H₇ (Al-1) has a non-classical structure with an Al₃ ring, similar to B₃H₇. Therefore, according to the Al₃-ring skeleton, Al₃H₇ can be viewed as the borane analogue. Moreover, the second (Al₃-ring, non-classical, Al-2) and third (Al₃-chain, classical, Al-3) low-lying Al₃H₇ isomers lie only 2.04 and 4.31 kcal mol⁻¹, respectively, above the ground isomer Al-1, and are separated from each other by barriers of around 10.0 kcal mol⁻¹. Thus, in low-temperature matrix experiments, the three Al₃H₇ isomers (Al-1, Al-2 and Al-3) might all be observable. The difference between the trialane and triborane (X₃H₇, X = Al, B) is also disclosed, i.e. for X = Al, the X₃-ring isomer with one XH₂ is more stable than that with two or three XH₂, whereas the case is the opposite for X = B. The calculated spectroscopic properties at the CCSD/TZVP level should provide useful information for future laboratory identification of Al-1, Al-2 and Al-3, the possible formation strategies of which are also discussed.

Keywords:

• Al₃H₇
• potential energy surface
• isomerization

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos. 20103003, 20573046 and 20773054), the Doctor Foundation of the Ministry of Education (20070183028), the Excellent Young Teacher Foundation of the Ministry of Education of China, the Excellent Young People Foundation of Jilin Province (20050103), and the Program for New Century Excellent Talents in University (NCET). The reviewer's comments are greatly appreciated.