Conformations of indan and 2-indanol: A combined study by UV laser spectroscopy and quantum chemistry calculation

Three conformational isomers of 2-indanol are identified by use of resonance enhanced two-photon ionization (R2PI) and single vibronic level dispersed fluorescence spectroscopy in a supersonic jet expansion. By combining the experimental results with the predictions of the ab initio quantum chemistry calculations at the MP2/6-311++G(d,p) level of theory, the major species is identified as a conformational isomer in which the hydroxyl hydrogen is involved in an intramolecular hydrogen bonding with the π-electrons of the aromatic ring. The theoretical estimate of the hydrogen bond energy is ∼6.5 kJ/mol. A comparative investigation with indan reveals that this weak hydrogen bonding in the former significantly affects the puckering potential of the five-member side ring. The dispersed fluorescence data indicate for a much higher ring-puckering barrier in the ground state than what has been suggested recently by measuring rotational spectra of the unsubstituted indan.