Infrared spectroscopy of gas phase alpha hydroxy carboxylic acid homo and hetero dimers

New gas phase infrared spectroscopy is reported for an aromatic alpha hydroxy carboxylic acid homo dimer of 9-hydroxy-9-fluorene carboxylic acid (9HFCA)2, and the hetero dimer of 9HFCA with glycolic acid. In terms of the 9-hydroxy stretching frequency, the 16 cm−1 blue-shift in the homo dimer and the 17 cm−1 blue-shift in the hetero dimer, relative to that in 9HFCA monomer, are attributed to collective effects with anti-cooperativity stronger than cooperativity. Furthermore, for the hetero dimer, the two alpha hydroxy groups’ stretching frequencies are clearly resolved, and differ by 30 cm−1. This difference represents a modest, quantitative enhancement of the intramolecular H-bond by the fluorene moiety in 9HFCA monomer, as opposed to that in glycolic acid. Accurate vibrational frequencies of the alpha OH, 3568 cm−1 in the bare glycolic acid, and 3584 cm−1 in the glycolic acid homo dimer are determined for the first time by comparison to 9HFCA monomer, homo and hetero dimers. The quantitative studies by infrared spectroscopy reveal subtle interactions among intra- and intermolecular H-bonds in the alpha hydroxyl acid dimers, which are also uniquely extended to probe each monomer's subtle intramolecular interactions.