No abstract

rue de la Houssiniere, 44072 Nantes, France The solvatochromic hydrogen-bonding shifts of p-nitrophenol and of p-nitroaniline have been measured by the thermosolvatochromic comparison method for an extended sample of oxygen, nitrogen, carbon, halogen, and sulphur bases. Their significance as a hydrogen-bonding parameter has been tested by their correlation with formation constants, n.m.r. shifts, vibrational shifts, and enthalpies for hydrogenbonding formation. Family-dependent correlations are generally found between the above properties. Thecorrelation of a hydrogen-bonding property for an OH donor versusthesame property for an NH donor is family (polar oxygen bases, ethers, pyridines, and tertiary aliphatic amines) dependent. The only significant family-independent correlation is for the solvatochromic shift of p-nitrophenol versus the enthalpy of hydrogen-bond formation of p-fluorophenol. It is shown that the b scale is mainly a scale of NH hydrogen-bond acceptor basicity. The averaging process used to define is criticized and it is recommended that correlation analysis of basicity is undertaken with clearly defined models.

Alkyl substitution on the amino nitrogen, vinylogy (but not benzology), iminology, and alkyl substitution on the functional carbon of the amidine skeleton in cyanamide iminologues increase the hydrogen-bonding basicity of cyanamide and produce, on the pKHB scale, super-basic nitriles more basic than tertiary amines. On the gas-phase basicity scale iminology also increases the basicity of cyanamide, but sp-nitrogen bases remain less basic than sp2-or sp3-nitrogen bases.The most basic nitriles presently known are cyanamides. ' *' This is generally explained by a 'push-pull' mechanism, schematically described by the resonant structures R,N-CkN -R2N+=C=N-. Interposition of a CH=N imino transmitter group between the 'pushing' NMe, and the 'pulling' C=N groups gives an 'iminologue' of the parent compound Me,N-CrN, in the same way as interposition of a phenylene or of a vinyl group gives, respectively, a cyanamide benzologue or vinylogue. We have recently shown that iminology greatly enhances the hydrogen-bond basicity of cyanamides. We report here other structural effects able to increase strongly the hydrogen-bond basicity of cyanamides and show that these effects are particular to the hydrogen-bond basicity scale.

temperature that would reasonably populate the lowest host vibrations rather than the lowest guest contributions, the simulations presented in this study do not conclusively explain the falloff in C/R as the temperature approaches 300 K. To study this effect more quantitatively would require a series of simulations beginning with the one presented here and proceeding to ones near room temperature. The boundary conditions for such simulations should certainly include neighboring unit cells.In all the calculations, we have made an assumption that the observed motions of the trapped molecules do not depend on any correlation between the motions of molecules in neighboring cells. This assumption could have led to some disagreement between the calculated and observed properties but probably not to the extent demonstrated by the disagreement between the observed

A thermodynamic hydrogen-bond basicity scale, ~KHs, and a spectroscopic hydrogen-bond basicity scale, Av(0H).were measured which permitted the construction of the solute hydrogen-bond basicity scale, @p, for 71 nitriles embracing a wide range of structures from trichloroacetonitrile to cyanamides. Field, resonance, and polarizability contributions of the X substituents to the hydrogen-bond basicity of XCN compounds were established. Steric effects do not contribute to the hydrogen-bond basicity of nitriles.