Spectroscopic investigations of hydrogen bonding interactions in the gas phase. III. The identification of the hydrogen-bonded heterodimer (CH3)3CCN • • • HF and the determination of its geometry by microwave and infrared spectroscopy

The existence of the hydrogen-bonded heterodimer (CH3)3CCN • • • HF has been established from a study of its infrared and microwave spectra and from the latter the collinearity of the C—C = N • • • HF fragment has been demonstrated. Rotational constants for the ground state and the vibration-rotation constants αβ and ασ for states associated with the hydrogen bond modes νβ (bending) and νσ (stretching) are as follows for the species (CH3)3CCN• • • HF and (CH3)3CCN• • •DF: B0/MHz ασ/MNz αβ/MNz (CH3)3CCN• • •HF 876.25 ± 0.12 12.3 ± 0.2 — 4.3 ± 0.1 (CH3)3CCN• • •DF 862.10 ± 0.09 12.4 ± 0.5 — 4.1 ± 0.1 From the intensity distribution within the vβ satellite progression the value 55 + 5 cm-1 is determined for the vibrational separation vβ =1 ← 0 (vβ is assumed harmonic). If monomer geometries are assumed unchanged on dimer formation the rotational constants B0 lead to r0 (N• • •F) = 2.725 ± 0.003 Å and 2.725 ± 0.003 Å for the hydrogen- and deuterium-bonded species respectively. It is thereby established that the N • • • 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 • • • HF from 2.795 A when R = H, through 2.760 Å when R = CH3 to 2·725 Å when R = (CH3)3C. 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 ƒ8 (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 r0(N • • • F) to deuterium substitution is discussed in relation to the previous results on this topic and their interpretation.