Diaminocarbenes;: Calculation of barriers to rotation about Ccarbene-N bonds, barriers to dimerization, proton affinities, and 13C NMR shifts

Barriers to rotation about the C-carbene-N bends in diaminocarbene (H2N)(2)C 1a, bis(dimethylamins)carbene ((CH3)(2)N)(2)C 1b, the related formamidinium ions (H2N)(2)CH+ 2a and ((CH3)(2)N)(2)CH+ 2b, and the Li+ complexes (H2N)(2)CLi+ 3a and ((CH3)(2)N)(2)CLi+ 3b have been calculated using density-functional theory in order to study the extent of pi-bond stabilization of the carbene center. Experimental barriers from DNMR are reported for 1b and 2b and compared with those for bis(diisopropylamino)carbene 1c and the N,N,N',N'-tetraisopropyl-formamidinium ion 2c; rotational barriers computed for 1b and 2b including thermal corrections compare well with experiment. The dimerization of 1a and 1b have been studied with (full) geometry optimization up to the levels QCISD(T)/cc-pVDZ//MP2/cc-pVDZ and B3LYP/cc-pVDZ//B3LYP/cc-pVDZ, respectively. The minimum-energy path for the dimerization of la has been computed using the BPW91/cc-pVDZ method. It is shown that the transition state geometries for the dimerizations of 1a and 1b have C-2 and C-1 symmetry, respectively, the latter being strongly polarized. The possible involvement of catalysis by protons and lithium ions in the dimerization processes is discussed. Calculations of the proton affinities of 1a, 1b, and some related species are reported. C-13 NMR shielding constant calculations on a series of diaminocarbenes have been performed using the gauge-including atomic orbitals (GIAO) method. The variation in the extremely downfield-shifted C-13 NMR signal of the carbene carbon in 1a, 1b, and related species is reproduced reasonably well by CIAO calculations, the latter being 2-8 ppm more upfield than the experimentally observed signals. It is shown that the paramagnetic contributions to the shielding tensor at the carbene nucleus play an important role in the chemical shift changes upon substitution in the RXC(NR2) species.