## Abstract

The existence of the hydrogen-bonded heterodimer (CH$_3$)$_3$CCN $\cdots$ HF has been established from a study of its infrared and microwave spectra and from the latter the collinearity of the C-C????N $\cdots$ HF fragment has been demonstrated. Rotational constants for the ground state and the vibration-rotation constants $\alpha_\beta$ and $\alpha_\sigma$ for states associated with the hydrogen bond modes $\nu_\beta$ (bending) and $\nu_\sigma$ (stretching) are as follows for the species (CH$_3$)$_3$CCN $\cdots$ HF and (CH$_3$)$_3$CCN$\cdots$DF: From the intensity distribution within the $\nu_\beta$ satellite progression the value 55$\pm$5 cm$^{-1}$ is determined for the vibrational separation $\nu_\beta$ = 1 $\leftarrow$ 0 ($\nu_\beta$ is assumed harmonic). If monomer geometries are assumed unchanged on dimer formation the rotational constants B$_0$ lead to r$_0$(N$\cdots$F) = 2.725$\pm$0.003 A and 2.725 $\pm$ 0.003 A for the hydrogen- and deuterium-bonded species respectively. It is thereby established that the N$\cdots$F distance is on the one hand unaffected when D replaces H while on the other hand this distance progressively decreases in the series RCN$\cdots$HF from 2.795 A when R = H, through 2.760 A when R = CH$_3$ to 2.725 A when R = (CH$_3$)$_3$C. The latter conclusion provides an indication of increase of the strength of the hydrogen bond along the series. This is in agreement with the concomitant decrease of the force constants f$_s$ (778, 744 and 736 N m$^{-1}$ respectively) associated with the high frequency hydrogen motion along the symmetry axis as derived from the modified H-F stretching band in the infrared spectra of these dimers, and can be readily understood in terms of the inductive effect. The invariance of r$_0$(N$\cdots$F) to deuterium substitution is discussed in relation to the previous results on this topic and their interpretation.