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In this paper the excited state proton transfer (ESPT) of isolated 3-hydroxychromone (3-HC), the prototype of the flavonols, is investigated for the first time by combined IR/UV spectroscopy in molecular beam experiments. The IR/UV investigations are performed both for the electronically excited and electronic ground state indicating a spectral overlap of transitions of the 3-HC monomer and clusters with water in the electronic ground state, whereas in the excited state only the IR frequencies of the proton-transferred monomer structure are observed. Due to the loss of isomer and species selectivity with respect to the UV excitations IR/IR techniques are applied in order to figure out the assignment of the vibrational transitions in the S0 state. In this context the quadruple resonance IR/UV/IR/UV technique (originally developed to distinguish different isomers in the electronically excited state) could be applied to identify the OH stretching vibration of the monomer in the electronic ground state. In agreement with calculations the OH stretching frequency differs significantly from the corresponding values of substituted hydroxychromones.

Mixtures of acetaldehyde and water are reactive multicomponent systems because poly­(oxymethylmethylene) glycols are formed. A study on the kinetics of the formation of these oligomers was carried out using a new microreactor NMR probe head that combines online flow 1H NMR spectroscopy with microreaction technology. The study covers temperatures between 278 and 298 K and pH values between 3.5 and 10.3. From the peak areas in the 1H NMR spectra, quantitative results for the conversion of acetaldehyde were obtained. On the basis of the new data, a reaction kinetic model was developed and numbers for the kinetic constants of poly­(oxymethylmethylene) glycol formation were determined together with a correlation that describes their dependence on the temperature and pH value.

A liquid thermostated microreactor Nuclear Magnetic Resonance (NMR) probe head is presented that facilitates the investigation of kinetics of fast reactions. With this setup, reaction kinetics can be studied in a broad temperature (–20 – 100 °C) and pressure range (0 – 60 bar). The setup and NMR probe head are discussed from a reaction engineering standpoint based on experiments and simulations. The microreactor probe head widens significantly the range of applications of online NMR spectroscopy for reaction and process monitoring.