Computational thermodynamic studies of alkali and alkaline earth compounds, olefin metathesis catalysts, and borane -- azoles for chemical hydrogen storage
Geometry parameters, frequencies, heats of formation and bond dissociation energies are predicted for the alkali (Li, Na and K) hydrides, chlorides, fluorides, hydroxides, and oxides and alkaline earth (Be, Mg and Ca) fluorides, chlorides, oxides and hydroxides at the coupled cluster theory [CCSD(T)] level extrapolated to the complete basis set (CBS) limit. The calculations including core-valence correlation corrections with the aug-cc-pwCVnZ basis sets (n = D, T, Q and 5) are mostly in excellent agreement with the available experimental measurements. Additional corrections (scalar relativistic effects, vibrational zero-point energies, and atomic spin-orbit effects) were necessary to accurately calculate the total atomization energies and heats of formation. The results resolve a number of issues in the literature. CCSD(T)/CBS level calculations with additional corrections are used to predict the heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) and Bronsted acidities and fluoride affinities for the model Schrock-type metal complexes M(NH)(CRR')(OH)_2 (M = Cr, Mo, W; CRR' = CH_2, CHF, CF_2) and MO_2(OH)_2 transition metal complexes. The metallacyclobutane intermediates formed by addition of C_2H_4 to M(NH)(CH_2)(OH)_2 and MO_2(OH)_2 are investigated at the same level of calculation. The electronegative groups bonded to the carbene carbon lead to less stable Schrock-type complexes as compared to the complexes with a CH_2 substituent. The Schrock compounds with M = Cr are less stable than with M = W or Mo. The heats of formation and bond dissociation energies (BDEs) for the pyrrole, pyrazole, imidazole, triazole and tetrazole borane adducts were predicted using an isodesmic approach based on G3MP2 calculations. As potential hydrogen storage substrates, dehydrogenation energies for the elimination of one H_2 molecule were predicted as well as thermodynamic properties relative to their acid-base behavior. The H_3B-N bonds to an sp^2 nitrogen are much stronger than those to an sp^3 nitrogen for the 5-membered rings. The B-N BDEs for the azolylborate adducts are much larger than for the neutral azole borane adducts. The azole adducts with more number of nitrogens in the ring and with more BH_3 molecules to the azole nitrogens are more acidic.