Cu+ reactivity trends in sp, sp2, and sp3 nitrogen, phosphorus, and arsenic containing bases

Density functional theory (DFT) has been used to analyze the reactivity trends of sp, sp(2), and sp(3) nitrogen bases with respect to their phosphorus and arsenic analogues, both when the reference acid is H+ or Cu+. Geometries were fully optimized at the B3LYP/6-311G(d, p) level and the final energies were obtained in B3LYP/6-311 + G(2df, 2p) single point calculations. For some model systems the G2 molecular orbital theory was used to assess the reliability of the DFT approach used. Significant differences in the reactivity trends of the nitrogen bases with respect to their phosphorus and arsenic analogues have been found. While for H2C=NH only the nitrogen attached species is stable, for H2C=PH and H2C=AsH both, the species in which the cation is bonded to the heteroatom and the carbon attached structures are local minima of the potential energy surface. Furthermore, for the arsenic derivative the latter is the most stable form. Similarly, while for HCN only the nitrogen attached species is stable for HCP and HCAs only the carbon attached structures are found. The basicities of XH3 and CH3XH2 were also studied for comparison. Methyl substituent effects for phosphorus and arsenic compounds are larger than for the corresponding nitrogen derivative. Proton affinities and Cu+ binding energies are Linearly correlated, although the slope of the correlation for phosphorus and arsenic derivatives is significantly different from that of nitrogen bases. The unsaturated systems are systematically less basic than the corresponding saturated counterparts. The observed differences in the reactivity trends reflect significant dissimilarities in the charge redistribution undergone by the neutral when it interacts with the attaching ion, which are also mirrored in the optimized geometries and in the shiftings of the harmonic stretching frequencies. (C) 2000 Elsevier Science B.V.