Hydrogen Bonding in the Indole−Water Complex:  A High Resolution UV Study of the Hydrogen Donor Conformer

Abstract

The nature of the intermolecular bonding in the indole-water complex is investigated by highly resolved UV-REMPI spectra with rotational fine structure. First, we determine the coordinates of the amino hydrogen atom by isotope substituting it with deuterium for the S 0 electronic ground and S 1 excited state of indole. In a second step, we calculate the rotational fine structure resulting from possible cluster structures and compare it with the experimental results. A fit routine adapts the spectroscopic constants within the condition of a planar complex which is a reasonable assumption for the observed rotational a,b-type band structure. The best agreement between simulation and experiment is found when the water molecule oxygen is located 2.93(5) Å away from the amino hydrogen of indole. This clearly demonstrates that in the indole-water complex a hydrogen bond is formed between the hydrogen donor indole and the oxygen atom of the water molecule. Upon electronic excitation to S 1 this bond length is nearly constant.

Mass-analyzed threshold ionization of deuterium substituted indazole and benzimidazole and site-specific H/D exchange reaction

“…Oppositely, it yields a blue shift. Table 3 lists the measured E 1 's and IEs of indole, indazole, benzimidazole, and aniline and their mono-and di-deuterium substituted isotopomers [4,[16][17][18][19][20]. These values were measured by using the R2PI and MATI spectroscopic techniques and have an uncertainty of only a few wavenumbers.…”

“…Comparing these data with those of indole [18][19][20], indazole [14,16,21], and benzimidazole [15,17,22] can lead us to a better understanding about the site-specific H/D exchange reaction in these aza-aromatic bicyclic system.…”

Threshold photoionization in time‐of‐flight mass spectrometry

“…þ measured via the vibrationless intermediate state S 1 , 0 0 of the cluster at 35099 cm À1 (Hager et al, 1985;Tubergen & Levy, 1991;Braun, Grebner, & Neusser, 1998;Helm et al, 1998;Short & Callis, 1998) is shown in the middle trace of Figure 11. We identify intramolecular modes in the (indole Á H 2 O) þ spectrum by a comparison with the indole þ MATI spectrum.…”

FTIR and fluorescence studies on the ground and excited state hydrogen-bonding interactions between 1-methylindole and water in water–triethylamine mixtures

“…Typically, the hydrogenbonding ability of the indole ring has been ascribed to the donor properties of its pyrrolic NH group. However, in recent years, experimental and theoretical studies have stressed the hydrogen-bonding acceptor properties of the indole p-cloud [2][3][4][5][6][7][8][9]. These hydrogen-bonding acceptor properties, apart from playing an important role in the stabilisation processes of indoles in their natural environments, are also fundamental for understanding the spectral and excited state dynamical properties of indole derivatives.…”

“…Indeed, the indole-water system has occupied a prominent role in the studies of the hydrogen-bonding p-acceptor properties of the indole ring [3][4][5][6][7][8][9][16][17][18][19][20][21][22]. However, despite theoretical calculations have suggested the existence of ptype complexes for indole-water system, no direct experimental evidences exist for these p-bonded structures.…”