Rearrangements of a Water Molecule in Both Directions between Two Hydrogen-Bonding Sites of 5-Hydroxyindole Cation: Experimental Determination of the Energy Threshold for the Rearrangement

Abstract

Rearrangements of a water molecule in both directions between two hydrogen-bonding (H-bonding) sites of the 5-hydroxyindole (5HI) cation was investigated in the gas phase. IR-dip spectra of jet-cooled 5HI-(H2O)1 revealed that two structural isomers, 5HI(OH)-(H2O)1 and 5HI(NH)-(H2O)1, in which a water molecule is bound to either the OH group or the NH group of 5HI, were formed in the S0 state. The IR photodissociation spectrum of [5HI-(H2O)1](+) generated by two-color resonant two-photon ionization (2C-R2PI) via the S1-S0 origin of 5HI(NH)-(H2O)1 clearly showed that [5HI(OH)-(H2O)1](+) and [5HI(NH)-(H2O)1](+) coexist in the D0 state. The appearance of [5HI(OH)-(H2O)1](+) after R2PI via the S1-S0 origin of 5HI(NH)-(H2O)1 is explained by isomerization of [5HI(NH)-(H2O)1](+) to [5HI(OH)-(H2O)1](+), which corresponds to the rearrangement of the water. In addition, isomerization in the opposite direction was also observed when [5HI-(H2O)1](+) was generated via the S1-S0 origin of 5HI(OH)-(H2O)1. The upper limit of the energy threshold for the rearrangement of the water in [5HI(NH)-(H2O)1](+) was experimentally determined to be 2127 ± 30 cm(-1) from the adiabatic ionization energy of 5HI(NH)-(H2O)1. Above the energy threshold, the water molecule in [5HI-(H2O)1](+) may fluctuate between the two preferential H-bonding sites of 5HI(+).

Microhydration of protonated 5-hydroxyindole revealed by infrared spectroscopy

“…5 The distribution of the excess positive charge is very different in the individual protomers directly affecting the acidity of the OH and NH groups. In the neutral S 0 ground state, a5HI-W clusters are more stable than s5HI-W. 56 In contrast, ionization into the D 0 state and protonation reverse the relative stability of the rotamers. 22,23 The mechanism of the growth of the initial solvation shell (n o 4) is very similar for cationic and protonated hydrates.…”

“…While only two isomers, namely, syn and anti rotamers, exist in the S 0 and D 0 states of 5HI (+) , we observe (at least) six In the neutral ground state (S 0 ), NH-and OH-bound hydrates of both s-and a5HI rotamers could be identified by isomer-selective IR spectroscopy. 56 While neutral a5HI-W clusters are more stable by DE 0 4 1.1 kJ mol À1 , protonation reverses this trend, such that protonated s5HIH + -W are more stable by DE 0 4 1.5 kJ mol À1 . In the S 0 state, OHÁ Á ÁW bonds are stronger than NHÁ Á ÁW bonds.…”

Elevation of the Energy Threshold for Isomerization of 5-Hydroxyindole-(tert-butyl alcohol)1 Cluster Cations

“…The lower limit of E b for the isomerization of [5HI­(NH)-( t -BuOH) 1 ] + to [5HI­(OH)-( t -BuOH) 1 ] + ( E b NH ) is determined to be 3362 ± 30 cm –1 by fitting the baseline and the increase in the ion signal to two linear lines. The value of E b NH for [5HI-( t -BuOH) 1 ] + is approximately 1200 cm –1 higher than that for [5HI-(H 2 O) 1 ] + (2127 ± 30 cm –1 ), indicating that the rearrangement of the water molecule starts at a lower internal energy than the t -BuOH molecule . The disappearance of the backward isomerization of [5HI­(OH)-( t -BuOH) 1 ] + to [5HI­(NH)-( t -BuOH) 1 ] + is consistent with the elevation of E b .…”

“…Figure a presents a stable calculated structure in which the t -BuOH molecule is bound to syn -5HI, whereas it is bound to anti -5HI in Figure , parts b and c. The spectral features in parts a and b of Figure are very similar to one another, whereas the spectral pattern in Figure c is totally different from those in Figure , parts a and b. The vibrational transitions at 3525 cm –1 in Figure , parts a and b, are almost the same as those of the NH stretching vibrations (ν NH ) of syn - and anti -5HI . In addition, the H-bonded OH stretching vibrations (H-bonded ν OH ) are observed at 3426 and 3415 cm –1 in Figure , parts a and b, respectively.…”

Real-time observation of photoionization-induced water migration dynamics in 4-methylformanilide–water by picosecond time-resolved infrared spectroscopy and ab initio molecular dynamics simulations

“…Such an ionization-induced hydration site switching has been reported for several monohydrated amides (tFA 24,49, 50 , tAA 20,21,51 , N-(2-phenylethyl)acetamide) 52 and also for monohydrated 4-aminobenzonitrile, 22, 53 5hydroxyindol, 25,54,55 tryptamine, 56 aminophenol, 57,58 and phenylglycine. 59 The isomerization dynamics is investigated in section 3-3 by ps-TRIR spectroscopy.…”