DECISIVE ELECTRON CORRELATION-EFFECTS ON COMPUTED B-11 AND C-13 NMR CHEMICAL-SHIFTS - APPLICATION OF THE GIAO-MP2 METHOD TO BORANES AND CARBABORANES

The performance of B-11 NMR chemical shift computations at a correlated (GIAO-MP2/TZP') level of ab initio theory is assessed for the first time. For molecules where SCF based methods (e.g. IGLO or GIAO-SCF) perform well, correlation effects on delta are negligible and IGLO, GIAO-SCF, and GIAO-MP2 results are equally satisfactory. But for problem cases e.g. 1,5-C2B3H5 (1) and B4R4 (3c, R = t-Bu), the large (10-35 ppm) errors obtained with uncorrelated wave functions are corrected at GIAO-MP2: excellent accord is achieved with the experimental values. While correlation effects on delta B-11 are negligible for the third problem molecule, 1,2-C2B3H7 (2), the discrepancies proved to be due to inaccuracies in the early experimental delta B-11 values. Remeasurements show the published IGLO predictions to have been correct. However, the GIAO-MP2 computed delta C-13 Values for 2, and for 1 as well, are superior to the IGLO results. For the elusive B4F4 (3e), delta B-11 = 44.7 is predicted. Calibration is provided by B2F4, B2Cl4, and B4Cl4 (3d), as well as by B4H10 (4), B5H9 (5), the two C2B4H6 isomers (6 and 7), and C2B5H7 (8) whose accurate structures were established earlier by means of the combined ab initio/IGLO/NMR method: the correlated MP2/TZP' chemical shifts result in comparably good agreement with the experimental values. A number of the C-13 chemical shifts in these carbaboranes improve at the MP2/TZP' level as well.