Quantcast

Re-evaluation of the standard thermochemical properties of the Al2 cluster on the basis of CBS thermochemistry of isogyric reactions and correlation dependencies

Research paper by Gregory A. Poskrebyshev

Indexed on: 18 Apr '18Published on: 15 Apr '18Published in: Computational and Theoretical Chemistry



Abstract

Publication date: 15 April 2018 Source:Computational and Theoretical Chemistry, Volume 1130 Author(s): Gregory A. Poskrebyshev In the present study, the value of Δf H o 298.15(3Al2(3Πu))atom = 518.4, 521, 521.4, 521.3 and 517.7 kJ/mol have been calculated using, respectively, CBS-QB3, ROCBS-QB3, W1U, W1RO and CBS-Q approaches. The slightly lower values of Δf H o 298.15(3Al2)isog = 495 ± 15, 513 ± 7, 503 ± 17, 513 ± 9 and 507 ± 8 kJ/mol have been determined, respectively, by the same quantum mechanical approaches, but on the basis of thermochemistry of the isogyric reactions. The higher values of Δf H o 298.15(3Al2)CORR = 522.8 (CBS-QB3), 521 (ROCBS-QB3), 520.4 (W1U), 521.3 (W1RO) and 519.7 (CBS-Q) kJ/mol have been determined on the basis of the correlation dependence between tabulated and calculated values of the bond dissociation energy (BDE) of compounds contained double and triple bonds. The close value of Δf H o 298.15(3Al2) = 525 kJ/mol have also been determined within ROCBS-QB3 approach, on the basis of the perfectly linear correlation dependence between tabulated and calculated values of the BDE of dimers, formed by elements of the third row of the periodic table of elements. In contrast, the significantly higher value of Δf H o 298.15(3Al2)CORR = 549.9 kJ/mol have been determined on the basis of the correlation dependence between tabulated and theoretical values of BDE calculated for the lowest triplet states of B2, Al2 and Ga2. Such high difference, observed in the last case, has been attributed to the high uncertainty in the tabulated/experimental values of Δf H o 298.15(3B2) and Δf H o 298.15(3Ga2). As a result, the lowest limit of the experimental values of Δf H o 298.15(3Al2) = 525 kJ/mol and Δf H o 298.15(3Ga2) = 430 kJ/mol, as well as the upper limit of the value of Δf H o 298.15(3B2) = 851.2 kJ/mol have been accepted as the best available estimate. The temperature dependence of Δf H o 298.15(3Al2), has been calculated using the ROCBS-QB3 approach. The value of energy gap E(S-T) = 31.8, 29.6 and 29.1 kJ/mol between the lowest triplet (3Πu) and singlet (1Σg+) states of Al2 have been determined using, respectively, CBS-Q, ROCBS-QB3 and W1RO calculations. The slightly different values of E(S-T) = 32.1 ± 6, 28.9 ± 7 and 25.6 ± 4 kJ/mol have also been determined using correlation dependencies. The last of these values has been considered as the most accurate. The values of S o 298.15(3Al2) = 236.6 ± 0.1 J/(mol K) and its temperature dependence, calculated in the present work, agree with those reported previously. Graphical abstract