Hydrogen Bond Networks Formed by Several Dozens to Hundreds of Molecules in the Gas Phase

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol–(Acetylene)n(∼16 ≤n≤ ∼30) and (Acetylene)n+(10 ≤n≤ 70) Clusters

“…As has been demonstrated in the previous IR studies of the small-sized (acetylene) n + and related cluster cations, extensive isomerization can occur in the ionization of acetylene clusters. − Therefore, the ion core of the observed (acetylene) n + would not be an acetylene cation (C 2 H 2 + ). Though the identification of the ion core structure is practically difficult at the present stage (we have tried isomerization reaction path search of the n = 3 and 4 cations by the automatic reaction path search program (the GRRM program), and we have found more than 40 stable isomer structures for each size), it is reasonably expected that the contribution of the ion core decreases with increasing cluster size, and the spectrum will finally converge to that of neutral (acetylene) n , as has been found in the protonated water and methanol clusters. − Figure actually shows that the features of the acetylene CH band of (acetylene) n + seem very similar to those of phenol–(acetylene) n in n ≥ 20; the band peak is at around ∼3260 cm –1 and the characteristic red shade of the band profile is seen. The red shade of the CH band is more extended in (acetylene) n + in comparison with the same size of phenol–(acetylene) n .…”

“…It has been demonstrated for protonated water and methanol clusters that their IR spectra become almost identical to those of the corresponding neutral clusters in n ≥ 30–50. − This means that the influence of the excess charge to hydrogen bonds is practically limited to surrounding 30–50 molecules. Here, we assume that this trend can be applied also to the present system; that is, the spectrum of n ≥ 50 of (acetylene) n + can be regarded as a good approximation of the spectrum of neutral (acetylene) n at the same size.…”

“…In addition, it has been demonstrated that significant isomerization can occur in ionization of acetylene, and the ion core of (acetylene) n + is no longer an acetylene cation (C 2 H 2 + ). − However, with the increase of cluster size, the contribution of the ion core becomes relatively smaller. In the protonated water and methanol clusters, it has been reported that their IR spectra almost converges to those of the corresponding neutral clusters in the size range 30–50. − Therefore, also for (acetylene) n + , we expect that we can estimate the influence of the ion core by the comparison with the spectra of neutral phenol–(acetylene) n , and their spectra can be practically regarded as those of neutral (acetylene) n in a large size region. In the present study, we also observe IR spectra of size-selected (acetylene) n + (10 ≤ n ≤ 70) in the acetylene CH region and explore the size dependence of the CH stretch band.…”