2.1. Experimental Methods
As discussed above, the existence of an intramolecular H-bond was accepted in this review on the basis of gas-phase experiments. In a number of the studies the spectra were recorded for jet-cooled systems. References to these are provided when the gas-phase and in-solution molecular structures are compared in Section 3. The jet-cooling technique [23] allows reaching local temperatures as low as about 5 K, with the advantage that the molecules assume their vibrational ground states. Under these conditions, the vibrational spectrum becomes simpler and different conformers can be more easily identified and characterized. Microwave spectra were recorded at room temperature or somewhat below [24,25,26,27]. Gas-phase electron diffraction structure determinations, sometimes at two or three different temperatures, were performed in the range of 297–733 K [28,29,30,31].
For in-solution IR spectra, the temperature was generally room temperature or not far from it in the experiments surveyed here. In these cases, the boiling point of the solvent imposes a limit for the upper temperature. It is known that signals can split in NMR studies by lowering the temperature. The lowest operational temperature is constrained, however, by the freezing point of the NMR solvent [32].
In summary, the gas-phase structural parameters were obtained from experiments conducted in a very large temperature range of 5–733 K. The in-solution investigations were mainly performed near room temperature. Thus, comparisons of the structural data between gas-phase and in-solution experiments, as well as to theoretical calculations referring to 0 K, need caution.