Comparable strength of OH-O versus OH-π hydrogen bonds in hydrogen-bonded 2,3-benzofuran clusters with water and methanol.

Two different types of hydrogen bond, which are classified into a familiar OH-O and a relatively weak OH-π one, have been compared in the 1:1 hydrogen-bonded 2,3-benzofuran clusters with water and methanol molecules. By applying fluorescence-detected infrared spectroscopy and dispersed fluorescence spectroscopy, two isomers having different types of hydrogen bonds are distinguished. From the calculated stabilization energy as well as the frequency shift of the OH stretching vibration in each cluster, these two isomers are almost equally stable, although that of OH-π type is usually thought to be relatively weak. It is suggested that the origin of the weak OH-O hydrogen bond is derived from the lower availability for a hydrogen bond acceptor on the oxygen atom of a heteroaromatic ring, which is attributed to the larger furan aromaticity.

Solvation effect on the NH stretching vibrations of solvated aminopyrazine, 2-aminopyridine, and 3-aminopyridine clusters.

The vibrational spectra of the hydrated and methanol-solvated aminopyrazine, 2-aminopyridine and 3-aminopyridine in supersonic jets have been measured in terms of IR-UV double-resonance spectroscopy. Comparing the IR spectrum of aminopyrazine with those of 2-aminopyridine and 3-aminopyridine clusters, we determine the solvation structure of aminopyrazine to be a similar cyclic structure as hydrated 2-aminopyridine clusters [Wu, et al., Phys. Chem. Chem. Phys. 2004, 6, 515]. In the case of monohydrated aminopyrazine cluster, one of the normal modes composed of the hydrogen-bonded OH and NH stretching local modes gives the anomalously weak IR intensity, which is ascribed to the cancellation of the dipole moment change between the OH and NH stretching local modes. The solvated 3-aminopyridine clusters forms the hydrogen-bond between the pyridyl nitrogen atom and the OH group, but the amino group is indirectly affected to induce slight blue shift of the NH(2) stretches. This phenomenon is explained by inductive effect where the electron withdrawing from the amino group upon the solvation results in a "quinoid-like" structure of the amino group.