Re-evaluation of the standard thermochemical properties of the Al 2 cluster on the basis of CBS thermochemistry of isogyric reactions and correlation dependencies

Abstract

Abstract In the present study, the value of ΔfHo298.15(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 ΔfHo298.15(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 ΔfHo298.15(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 ΔfHo298.15(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 ΔfHo298.15(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 ΔfHo298.15(3B2) and ΔfHo298.15(3Ga2). As a result, the lowest limit of the experimental values of ΔfHo298.15(3Al2) = 525 kJ/mol and ΔfHo298.15(3Ga2) = 430 kJ/mol, as well as the upper limit of the value of ΔfHo298.15(3B2) = 851.2 kJ/mol have been accepted as the best available estimate. The temperature dependence of ΔfHo298.15(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 So298.15(3Al2) = 236.6 ± 0.1 J/(mol K) and its temperature dependence, calculated in the present work, agree with those reported previously.