Experimental and theoretical evidence of basic site preference in polyfunctional superbasic amidinazine: N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine.

The gas-phase basicity (GB) of the flexible polyfunctional N(1),N(1)-dimethyl-N(2)-beta-(2-pyridylethyl)formamidine (1) containing two potential basic sites (the ring N-aza and the chain N-imino) is obtained from proton-transfer equilibrium constant measurements, using Fourier-transform ion-cyclotron resonance mass spectrometry. Comparison of the experimental GB obtained for 1 with those reported for model amidines and azines indicates that the chain N-imino in the amidine group is the favored site of protonation. Semiempirical (AM1) and ab initio calculations (HF, MP2, and DFT), performed for 1 and its protonated forms, confirm this interpretation. These results are in contrast to those found previously for N(1),N(1)-dimethyl-N(2)-azinylformamidines (containing the amidine function directly linked to the azinyl ring), in which the ring N-aza is the most basic site in the gas phase. The separation of the two potential basic sites in 1 by the ethylene chain interrupts the resonance conjugation between the two functions and changes their relative basicities and, thus, the preferable site of protonation. It also increases the chelation effect against the proton and the gas-phase basicity of 1 in such a magnitude that consequently 1 may be classified as a superbase (GB = 241.1 kcal mol(-)(1)). A transition state corresponding to the internal transfer of the proton (ITP) between the ring N-aza and the chain N-imino in 1 is investigated at the DFT(B3LYP)/6-31G level. The energy barrier calculated for the ITP between the two basic sites is small and vanishes when zero-point vibrational terms and thermal corrections are applied to obtain the enthalpy or Gibbs energy of activation for the proton transfer. Additional calculations at the DFT(MPW1K)/6-31G level confirm this behavior. This indicates that the quantum-chemical ITP in 1 has a single-well character. The proton is located on the N-imino site, and the H-bond is formed with the N-aza site.

Enhanced Li+ binding energies in alkylbenzene derivatives: the scorpion effect.

The gas-phase lithium cation basicities (LCBs; Gibbs free energy of binding) of ethyl-, n-butyl-, and n-heptylbenzene have been measured by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of the corresponding complexes and their relative stabilities were investigated through the use of B3LYP/6-311G(+)(3df,2p)//B3LYP/6-31G(d) density functional theory calculations. For n-butylbenzene and n-heptylbenzene, the most stable adducts correspond to pi complexes in which the alkyl chain coils toward the aromatic ring to favor its interaction with the metal cation. The extra stabilization provided by the flexible alkyl chain polarized by the charge on Li(+) is named the "scorpion effect". Conversely, these coiled conformations are among the least stable in the neutral system; they are not all stationary points on the potential-energy surface. The formation of complexes with a coiled alkyl chain leads to a significant enhancement of the Li(+) bonding energies (LBEs), which are approximately 20-30 kJ mol(-1) higher than those calculated for alkylbenzene pi complexes in which an uncoiled chain remains distant from the cation and thus minimizes the scorpion effect. This enhancement is less significant when LCBs are concerned, because the scorpion effect is entropically disfavored. There is very good agreement between the experimental Li(+) gas-phase basicities and the calculated values, provided that the statistical distribution of the conformers present in the gas phase is taken into account in this calculation.

Lithium-cation/pi complexes of aromatic systems. The effect of increasing the number of fused rings.

The gas-phase lithium cation basicities (LCBs) of naphthalene, azulene, anthracene, and phenanthrene were measured by means of Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The structures of the corresponding complexes and their relative stabilities were investigated at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d) level of theory. In the theoretical survey, pyrene, coronene, [3]phenylene, angular [3]phenylene, and circumcoronene were also included. The strength of the binding to a given aromatic cycle decreases as the number of cycles directly fused to it increases. Hence, the stability of the outer pi-complexes, in which Li(+) is attached to the peripheral rings, is systematically greater than that of the complexes in which the metal is attached to the inner rings. The energy gap between these local minima decreases as the number of fused rings in the system increases. This result seems to indicate that, as the size of the system increases, the rings tend to lose their peculiarities, in such a way that in the limit of a graphite sheet all rings would exhibit identical characteristics and reactivity. The good agreement between calculated LCBs and experimental values lends support to the enhanced stability of the outer complexes. The activation barriers connecting these local minima decrease as the number of fused cycles increases, but seems to tend toward a limit. [3]Phenylene and angular [3]phenylene exhibit enhanced LCBs reflecting nonnegligible Mills-Nixon effects that increase the electron-donor properties of these annelated benzenes.