An infrared spectroscopic and theoretical study on (CH₃)₃N–H⁺–(H₂O)_n, n = 1–22: highly polarized hydrogen bond networks of hydrated clusters

Abstract

Infrared spectra of protonated trimethylamine (TMA)-water clusters, (CH3)3N-H(+)-(H2O)n (n = 1-22) were measured in the OH stretching vibrational region by size-selective photodissociation spectroscopy. Density functional theory calculations of stable structures were performed, and temperature dependence of the isomer populations and infrared spectra was also simulated by the harmonic superposition approximation approach to analyze hydrogen bond network structures in the clusters. It was shown that the excess proton (H(+)) in this system localizes on the TMA moiety regardless of cluster size. In the small-sized clusters, many isomers coexist and their hydrogen bond networks are highly polarized to induce the large charge-dipole interaction to stabilize the excess proton. Magic number behavior is not observed at around the magic number size (n = 21) of protonated water clusters and its implication on the hydrogen bond network structures is discussed.

Hydrogen-bonded ring closing and opening of protonated methanol clusters H+(CH3OH)n (n = 4–8) with the inert gas tagging

“…7 For the bare cluster, the temperature range between 150 K and 200 K is a reasonable estimate in many previous studies on protonated water and methanol clusters produced in a jet. 1,2,7,8 2. Gross features of the observed IR spectra and comparison with the simulations ), and the lower traces are the expanded plots of the high frequency region (3200-3840 cm À1 ).…”

“…40 In the present study, to consider the contribution of all structural isomers, we used Q-HSA to sum up their thermal contributions. [6][7][8] IR spectrum intensity can be expressed as I total (o,T) = S a I a (o)P a (T), where I a (o) and P a (T) are the spectral intensity and statistical population weights of the a-th isomer, respectively. In all DFT-calculated IR spectra, harmonic frequencies are rescaled by a factor of 0.973.…”

“…[1][2][3][4][5][6][7][8][9] Though the minimum energy structure tends to the focus in spectroscopic studies of clusters, the most preferred structure actually depends on the vibrational temperature of the cluster because higher energy structures can be dominant by the temperature-dependent entropy factor in the free energy. [1][2][3][4][5][6][7][8][9] Though the minimum energy structure tends to the focus in spectroscopic studies of clusters, the most preferred structure actually depends on the vibrational temperature of the cluster because higher energy structures can be dominant by the temperature-dependent entropy factor in the free energy.…”

Stepwise Internal Energy Change of Protonated Methanol Clusters By Using the Inert Gas Tagging

“…With elevation of temperature of the cluster, however, higher energy isomers can be populated because of entropy. Therefore, to fully understand the hydrogen bond structure of the cluster, not only the most stable structure but also the temperature dependence of preferred structures is very important. − …”

“…<100 K), and produced protonated clusters are typically warm (ca. 200 K). ,,, Since inert gas tagging provides a cut off for the internal energy according to the binding energy with the tag, it would be possible to stepwise change the internal energy distribution of the protonated cluster by systematic changes of the tag species from strongly bound one to weakly bound one. The internal energy distribution of such a tagged cluster is not necessarily under thermal equilibrium.…”