Theoretical Investigations On The Comparison Of Alanes With Boranes

Detailed theoretical studies on the electronic properties of the alanes are performed by using the quantum chemical methods. Through the direct comparison with the boranes, we have a further understanding toward the borane-analogy of the alanes. The following are the main results:1. We report the first comparative study directly between AlnHn2- and BnHn2- (5≤n≤12), AlnHn+2 and BnHn+2 (4≤n≤12) covering diverse structural forms. It was shown that AlnHn2-each have a closo ground structure as BnHn2-, nicely consistent with the Wade-Mingos rule. However, AlnHn+2 adopt the closo-nido ground structures following the even-odd alternation in the number of Al atoms, showing distinct violation from Wade-Mingos rule for the odd-numbered Al-atoms. Interestingly, the corresponding BnHn+2 also have similar closo (even)-nido (odd) alternation. Therefore, our direct comparison showed that AlnHn2-(5≤n≤12) and AlnHn+2 (4≤n≤12) can be viewed as the borane analogues, though not all AlnHn+2 have the ability of being explained by Wade-Mingos rule. So, the analogy between alanes and boranes in form of XnHn+2 should not be simply judged by the Wade-Mingos rule, due to the significant influence of the additional two hydrogen atoms on the closo-structure.2. Contrasting the boranes BnHn+4 with rich chemistry, the alanes AlnHn+4 remain largely unknown in laboratory except the simplest Al2H6. Though recent experimental and theoretical studies have proved AlnHn+2 to be the borane analogues, whether or not the borane-analogy can exist for the more complicated AlnHn+4 is still unclear. In this paper, we find that at the B3PW91/TZVP level, AlnHn+4 each has a nido single-cluster ground structure as BnHn+4 for n<12. For n>12, the fusion cluster becomes energetically more competitive than the single cluster also as BnHn+4. Thus, concerning the ground structures, the alanes AlnHn+4 (n=5-19) could be considered as the borane analogues. Remarkably, Al8H12 has a novel closo(4)-closo(4) cluster fused by two Td-like subunits Al4H6 lying only 0.49 kcal/mol above the single cluster. The Born-Oppenheimer molecular dynamic simulation shows that the closo(4)-closo(4) fusion cluster intrinsically has high kinetic isomers within 15 kcal/mol 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, respectively, above the ground isomer Al-1, and are separated from each other by the barriers of around 10.0 kcal/mol. Thus, in low-temperature matrix experiments, the three Al3H7 isomers (Al-1, Al-2, Al-3) might be all 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 just 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, whose possible formation strategies are also discussed.5. We report the first detailed potential energy surface surveys of small dialane Al2H4 at CCSD(T)/aug-cc-pvTZ//CCSD/aug-cc-pvTZ level, and the dissociation potential energy surface which include the isomerization between each isomers, both H2-extrusion and H2-exchange processes at B3PW91/TZVP level. In the present work, we locate eight low-lying isomers, and confirm that the C3v HAIH3Al form (Al2H4-1) is certainly the global minimum. The second low-lying Al2H4 isomer (Al2H4-2) lies only 2.65 kcal/mol above the ground isomer Al2H4-1 and has the corresponding smallest barrier as 0.31 kcal/mol from Al2H4-2 to Al2H4-1, whereas it can not be easily converted to D2d (Al2H4-2) form with a high barrier of 41.15 kcal/mol at CCSD(T)/aug-cc-pvTZ//CCSD/aug-cc-pvTZ (single-point) level. The Al2H4-5 is a new isomer which has never been reported previously. Moreover, five dissociation pathways are considered:(1) HAIH3Al (Al2H4-1-C3v)→AlH+AlH3; (2) HAlH3Al (Al2H4-1-C3v)→2AlH2; (3) H2AlH2Al (Al2H4-2-C2v)→2AlH2; (4) H2AlAlH2 (Al2H4-3-D2d)→2AlH2; (5) HAlAlH3 (Al2H4-6-C3v)→AlH+AH3. Through these dissociation curves we scaned, we find that although they are barrierlessness, in fact, besides the dissociation products of Al2H4-3-D2d and Al2H4-6-C3v have only one imaginary frequency of stretching vibration between Al-Al bonding, the other isomers all have more than one negative frequency towards various directions. Thus, we propose that there have not much chance of other attack pattern happening. Some feasible stability, which can be ascribed to the rigidity of the Td-Al4H6 subunit. Since Td-Al4H6 has been experimentally characterized in gas phase very recently, we strongly recommend that the unprecedented non-Wade-Mingos alane Al8H12 can be effectively formed via the direct dimerization between two Td-Al4H6 with the reaction energy (-39.65 kcal/mol) very similar to that of the known dialane (2AlH3→Al2H6,-35.27 kcal/mol).3. Recently, growing attention has been laid to the aluminum hydrides due to their interesting borane-analogy and their potential as hydrogen-storage materials. However, the tetraalane Al4H8 has not received any previous consideration, though two analogous tetralanes Al4H6 and Al4H7- (isoelectronic) have been very recently characterized in gas phase. In this article, through the detailed structural and energetic study of X4H8 (X=Al, B) at the B3PW91/TZVP, CCSD/TZVP and G3B3 (single-point) levels, we find that the ground structure of Al4H8 contrasts sharply to that of B4H8. While B4H8 has a nonplanar bicyclic structure, Al4H8 comprises a perfectly planar and quasi-square (AlH)4-skeleton with a pair of H-bridges positioned at each of the opposite Al-Al bonds. Thus, Al4H8 is definitively not the borane analogy. Electronically, Al4H8 possesses quite similar orbitals to the isoelectronic C4H4, well suggestive of itsπanti-aromaticity. However, the long and equalized four peripheral Al-Al bonds (reasonably interpreted by the chemical bonding model), slightly negative NICS values and the anomalous hydrgoentation pattern all indicate noπanti-aromatic character in effect, it is a non-aromatic molecule. Notably, though isoelectronic to the recently photoelectron spectroscopy characterized Td-Al4H7-, the target Al4H8 has a less stable Td-structure. The more costly CCSD(T) single-point energy calculations in conjugation with aug-cc-pvTZ and aug-cc-pvQZ basis sets confirm the higher stability of the planar Al4H8 than the tetrahedral one. In addition, Al4H8 has comparative or superior adiabatic electron affinity, adiabatic ionization potential and HOMO-LUMO gap to the very recently characterized Td-Al4H6. Thus, the hitherto unknown tetra-alane Al4H8 should possess appreciable chemical stability and deserves future laboratory study. Possible formation strategies of Al4H8 are also proposed.4. Much of the recent interest has focused on the understanding of the borane-analogues of alanes. Being potential as 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 enough. Early low-level calculations considered an Al3-chain (classical) isomer and a less stable Al3-ring (non-classical) one, indicative of the non-borane-analogue of Al3H7. In the present work, we locate eleven new Al3H7 fragmentation processes of C3v HAlH3Al are also considered for an in-depth understanding of the stability. We suggest that the C3v and D2d form can coexist, and the former might be observed through microwave spectra for its relatively large dipole moment. The calculated spectroscopic properties of Al2H4-1, Al2H4-2 and Al2H4-3 at CCSD(T)/aug-cc-pvTZ level would provide some useful information for future laboratory identification.