Stepwise hydration of the cyanide anion: A temperature-controlled photoelectron spectroscopy and ab initio computational study of $CN−(H2O)n$⁠, $n=2–5$

We report the study of microsolvated $CN−(H2O)n$ $(n=1–5)$ clusters in the gas phase using a combination of experimental and computational approaches. The hydrated cyanide clusters were produced by electrospray and their structural and energetic properties were probed using temperature-controlled photoelectron spectroscopy (PES) and ab initio electronic structure calculations. Comparison between the low temperature $(LT,T=12K)$ and the room-temperature (RT) spectra shows a 0.25 eV spectral blueshift in the binding energy of the $n=1$ cluster and a significant spectral sharpening and blueshift for $n=2$ and 3. The experimental results are complemented with ab initio electronic structure calculations at the MP2 and CCSD(T) levels of theory that identified several isomers on the ground state potential energy function arising from the ability of $CN−$ to form hydrogen bonds with water via both the C and N ends. In all cases the N end seems to be the preferred hydration site for the water network. The excellent agreement between the low temperature measured PES spectra and the basis set- and correlation-corrected [at the CCSD(T) level of theory] calculated vertical detachment energies, viz., 3.85 versus 3.84 eV $(n=0)$⁠, 4.54 versus 4.54 eV $(n=1)$⁠, 5.20 versus 5.32 eV $(n=2)$⁠, 5.58 versus 5.50 eV $(n=3)$⁠, and 5.89 versus 5.87 eV $(n=4)$⁠, allow us to establish the hydration motif of cyanide. Its microsolvation pattern was found to be similar to that of the halide anions (⁠$Cl−$⁠, $Br−$⁠, and $I−$⁠) as well as other diatomic anions having cylindrical symmetry such as $NO−$⁠, resulting to structures in which the ion resides on the surface of a water cluster. The exception is $CN−(H2O)2$⁠, for which one water molecule is bound to either side of the anion resulting in a quasilinear structure. For the $n=3$ cluster the anion was found to freely “tumble” on the surface of a water trimer, since the inclusion of zero-point energy even at $T=0K$ stabilizes the configuration of $C3$ symmetry with respect to the one having the anion tilted toward the water cluster. For $n=4$ this motion is more restricted since the corresponding barrier at RT is 1.2 kcal/mol. It is also possible that at RT other isomers (lying within $∼0.6kcal/mol$ above the global minima) are also populated, resulting in the further broadening of the PES spectra.