Fluorescence-dip infrared spectroscopy of the tropolone-H2O complex

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Abstract
Fluorescence dip infrared spectroscopy (FDIRS) is used to probe the effect of a solvent water molecule on intramolecular H‐atom tunneling in tropolone. As with the bare molecule discussed in paper I, the FDIR spectrum of the tropolone‐H2O complex is recorded in the O–H and C–H stretch regions. Three OH stretch fundamentals are observed in the spectrum, and can be assigned nominally to a free OH stretch of the water molecule (3724 cm−1), a hydrogen bonded OH stretch of water (3506 cm−1), and the OH stretch of tropolone (∼3150 cm−1). The breadth and complexity of the bands is highly mode specific. The free OH stretch transition is sharp (1.8 cm−1 FWHM) and has weak combination bands built on it at +73 and +1600 cm−1. The former is assigned to a combination band with the in‐plane bending mode of the tropolone‐H2O hydrogen bond, while the latter is the free OH/intramolecular water bend combination band. The water hydrogen‐bonded OH fundamental is also a sharp transition which, after correction for the decreased infrared power at its frequency, is clearly the strongest transition in the spectrum. It is flanked by three close‐lying satellite bands 13, 23, and 34 cm−1 above it, and also supports a weak combination band at +69 cm−1 due to the in‐plane intermolecular bending mode. The tropolone OH absorption is in the same frequency region as in the bare molecule, but broadened to over 100 cm−1 in TrOH–H2O. Distinct substructure in the band is present, with spacings reminiscent of those in the water H‐bonded OH stretch region. Ab initio calculations on tropolone‐H2O are carried out at both the MP2 and Becke3LYP levels of theory. Two isomers with similar binding energies and vibrational frequencies are identified. In one isomer (isomer I), the water molecule serves as a hydrogen‐bonded bridge between the tropolone OH and keto groups. In the other (isomer II), the water molecule is exterior to the tropolone and hydrogen bonded to the keto oxygen. The experimental evidence does not conclusively distinguish between these two possibilities, though the exterior structure seems somewhat more in keeping with the data as a whole.