Proton/Hydrogen‐Transfer Coordinate of 2,5‐Dihydroxybenzoic Acid Investigated in a Supersonic Beam: Combined IR/UV Spectroscopy in the S₀, S₁, and D₀ States

Abstract

As a model system for intramolecular proton/hydrogen‐transfer coordinates, the structure of 2,5‐dihydroxybenzoic acid is investigated for the ground, first electronically excited and also the ionic state. Combined IR/UV spectroscopy in molecular‐beam experiments is applied and the experimental results are interpreted by the application of DFT and CASPT2 methods. No proton or hydrogen transfer is observed, but evidence is given for a hydrogen dislocation of the intramolecular hydrogen bond in the S1 state and to lesser extent in the D0 state. To obtain direct information on the proton/hydrogen‐transfer coordinate, IR spectra are recorded both in the region of the OH and especially the CO stretching vibrations by also applying two new variants of combined IR/UV spectroscopy for the S1 and D0 states. The CO groups are directly involved in the hydrogen bond and, in contrast to the hydrogen‐bonded OH groups, the CO stretching frequencies can be observed in all electronic states.

Structure of Isolated Xanthone in the T1 State Obtained via Combined UV/IR Spectroscopy

“…These experimentally obtained vibrational transitions fit excellently to the calculated frequencies of xanthone in the T 1 state (DFT/B3LYP/TZVP, scaled by 0.99, [9,10,49] Figure 4 b). The transitions can be assigned to combined CC stretching (ring deformation vibrations) and CÀH bending modes.…”

“…It is interesting to note that this value is a further hint that the ionisation starts from the T 1 state. The origin of this state is at 25 808 cm À1 [29,30] and the ionisation potential was determined by photoelectron spectroscopy in two different publications to be at 69 686 cm À1 (8,64 eV) [32] or 68 718 cm À1 (8.52 eV), [35] leading to an energy of 43 [9,10,49] cf. Figure 3 b).…”

IR Spectroscopic Techniques to Study Isolated Biomolecules

“…The combination of IR and UV spectroscopies can also be applied to the study of vibrational structure of electronically excited states, although technical challenges have limited the broad application of these methods as compared to ground-state IR absorption studies [84,[101][102][103][104][105]. As compared to ground state IR-UV ion dip spectroscopy, a two-color R2PI scheme must be chosen, so that the IR photon can be timed after the first UV excitation pulse, but before the second UV ionization pulse (see Fig.…”

“…Although the opportunity to characterize the electronically excited state is very valuable, both the excited-state lifetime constraint and intersystem crossing effects have resulted in a limited, but promising, application of this method to biomolecular systems [84,103,106,107]. In addition to the UV-IR-UV scheme described above, various other multiple-resonance IR/UV schemes have been developed to study selectively the IR absorption of the electronically excited state.…”

MALDI ionization mechanisms investigated by comparison of isomers of dihydroxybenzoic acid

“…However, ESPT is known to be very dependent on a strongly solvating environment for ions, which is not the case in the solid matrix nor in the early MALDI plume . Also, evidence for ESPT has been sought in 2,5‐DHB, but was not found …”