Enthalpies of formation and bond energies in lithium, beryllium, and boron derivatives. 2. Dative, single, and triple bonds

For a set of small XYH(n) singlet-state molecules (where X stands for Li, Be, or B and Y is one of the second-row atoms), we have calculated the enthalpies of formation at the MP4(sdtq)/6-311++G(3df,2p) level by using MP2(full)/6-31G(d,p) fully optimized structures. At this level of theory, the heats of formation are expected to be in the range of the so-called ''chemical accuracy'' (+/- 1 kcal/mol). Some alkyl derivatives of the previous XYH(n) compounds have been studied at the RHF/6-31G(d) level (with fully optimized geometries). The theoretical enthalpies of formation reproduce the available experimental results quite satisfactorily. In addition to compounds with usual dative single bonds, we also describe molecules with covalent (nondative) single B-N and B-O bonds. We use all the available (experimental and theoretical) data to calculate new bond energies for lithium, beryllium, and boron derivatives.