The basicity of unsaturated hydrocarbons as probed by hydrogen-bond-acceptor ability: bifurcated N-H+ ...pi hydrogen bonding

Abstract

The competitive substitution of the anion (A(-)) in contact ion pairs of the type [Oct3NH+]B(C6F5)4 (-) by unsaturated hydrocarbons (L) in accordance with the equilibrium Oct3NH+...A(-) + nL right arrow over left arrow [Oct3NH+...Ln]A(-) has been studied in CCl4. On the basis of equilibrium constants, K, and shifts of nuNH to low frequency, it has been established that complexed Oct3NH...+Ln cations with n=1 and 2 are formed and have unidentate and bifurcated N--H+...pi hydrogen bonds, respectively. Bifurcated hydrogen bonds to unsaturated hydrocarbons have not been observed previously. The unsaturated hydrocarbons studied include benzene and methylbenzenes, fused-ring aromatics, alkenes, conjugated dienes, and alkynes. From the magnitude of the redshifts in the N--H stretching frequencies, Delta nuNH, a new scale for ranking the pi basicity of unsaturated hydrocarbons is proposed: fused-ring aromatics<or=benzene<toluene<xylene<mesitylene<durene<conjugated dienes approximately 1-alkynes<pentamethylbenzene<hexamethylbenzene<internal alkynes approximately cycloalkenes<1-methylcycloalkenes. This scale is relevant to the discussion of pi complexes for incipient protonation reactions and to understanding N--H+...pi hydrogen bonding in proteins and molecular crystals.

Scheme 1

Structures of H-bonded alkenes (A), alkynes (B) and benzene (C).

Figure 1

Evolution of the IR spectrum in the νNH region of 0.4 M [Oct₃NH⁺]{F₂₀⁻} in CCl₄ as benzene is added. Benzene concentrations increase from zero (a) to 100% (b).

Figure 2

Representative IR spectra showing the formation of [Oct₃NH⁺⋯nL]{F₂₀⁻} complexed ion pairs with their νNH frequenses for arenes L = benzene (a), toluene (b), mesitylene (c), pentamethyl-benzene (d), hexamethylbenzene (e). Spectra, with the exception of (a), are normalized to νNH of the uncomplexed contact ion pair at 3323 cm⁻¹.

Figure 3

Slopes of function (4) for benzene (1, n = 0.99), hexamethyl-benzene (2, n = 0.98) and 1,3,5,7-cyclooctatetraene (3, n = 0.50 and 0.98).

Figure 4

Plot of band width at half height (S_½) for νNH of [Oct₃NH⁺⋯C₆H₆]{F₂₀⁻] with increasing concentration of benzene (2.24-11.2 M).

Figure 5

The slope of function (4) with 1,3-cyclohexadiene concentration.

Figure 6

The slope of function (4) with 3-hexyne concentration.

Figure 7

The slope of function (3) with n = 1 for 3-hexyne.

Figure 8

The slope of the function (3) with n = 2 for 3-hexyne.

Figure 9

IR spectra of (a) CCl₄ solution of 0.04 M Oct₃NH⁺{F₂₀} with 10 vol% of 3-hexyne (solid) and (b) CCl₄ solution of 10 vol% 3-hexyne (dotted). The dashed spectrum shows the band at 1655 cm⁻¹ that was isolated by sequential subtraction of the spectrum of free hexyne and the band at 1642 cm⁻¹ of {F₂₀}⁻.

Figure 10

Dependence of m on 3-hexyne concentration.

Figure 11

IR spectrum of 0.0156 M (Oct)₃NH⁺{F₂₀⁻} in CCl₄ with 0.0055 M water in the frequency region of νOH (a) and νNH (b). Solid line: measured spectrum. Dotted: spectrum of 0.0055 M of water in CCl₄. Dashed: spectrum of [(Oct)₃NH⁺⋯OH₂]{F₂₀⁻} ion pair obtained by subtraction of dotted from solid.

Figure 12

Dependence of the width of the νNH band on the ΔνNH frequency for set of arenes C₆H_6-n(CH₃)_n with n = 0-6.

Figure 13

Dependence of ΔνNH shifts on the number of CH₃ groups in methylbenzenes.

Figure 14

Dependence of K_eq on the number of CH₃ groups in methylbenzenes.