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.