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We present R2PI, IR–UV and UV–UV double resonance measurements of the guanine–cytosine (G–C) dimer formed in a supersonic jet. We show that there is only one isomer of G–C in the investigated wavelength range from 33200 to 34100 cm−1. We assigned the observed G–C isomer to a specific structure, based on comparisons of the IR spectra of the G and C monomers with the G–C dimer in the range of the OH and NH stretching vibrations and ab initio-calculated vibrational frequencies and dimer stabilities. The cluster exhibits an HNH⋯O/NH⋯N/CO⋯HNH bonding similar to the Watson–Crick G–C base pair bonding but with C as the enol tautomer. We did not observe any keto–keto or enol–enol G–C dimers in the investigated wavelength region.

In this paper we present detailed R2PI spectra with IR–UV and UV–UV double resonance measurements of the guanine dimer (GG) and its methylated derivatives. We show that there are two isomers of GG in the investigated wavelength range from 32565 to 33600 cm−1. We were able to assign the two isomers to specific structures based on comparison of the intermolecular vibronic patterns of the dimers with and without methylation, on analysis of the IR spectra in the range of the OH and NH stretching vibrations and on comparison with ab initio calculated dimer stabilities and vibrational frequencies. In both structures both guanine moieties are in the keto tautomeric form, even though the enol tautomers are also present in the beam. One isomer exhibits nonsymmetric hydrogen bonding with HNH⋯N, NH⋯N and CO⋯HNH interactions (K9K7-2). The other isomer has a symmetrical hydrogen bond arrangement with CO⋯NH/NH⋯OC bonding (K9K7-1). The most stable guanine dimer forms CO⋯NH/NH⋯OC hydrogen bonds and has C2h symmetry (K9K9-1). Due to its strong exciton splitting the allowed S0–S2 transition is outside the investigated spectral range. We did not observe any keto–enol or enol–enol dimers in the investigated wavelength region. The calculations predict these dimers to be considerably less stable.

The vibronic spectra of the S0 and S1 states of p-chlorophenol have been analyzed using laser induced fluorescence spectroscopy. The ground state frequencies were determined by dispersed fluorescence and assigned by comparison with frequencies from ab initio calculations on the MP2 and B3LYP (6-311 G(d,p) basis set) level. The S1 state was investigated theoretically using CASSCF calculations (6-31 G(d,p) basis set).

We present R2PI and IR–UV double resonance spectra of the nucleobase adenine seeded in a supersonic jet. We show that there is only one tautomer of adenine which absorbs in the wavelength range 36 050 to 36 700 cm−1. The IR spectra, measured in the range 3200 to 3700 cm−1, show bands at 3452, 3508 and 3569 cm−1, which we assign to the symmetric NH2 , N–H and antisymmetric NH2 stretching vibrations of a single tautomer of adenine. We compare the experimental IR–UV double resonance spectra with ab initio based normal mode calculations. The observed tautomer is most probably the 9H amino-form of adenine.

Using a variety of methods phenol water clusters phenol(H2O)n, n⩽7, are investigated with a focus on phenol(H2O)5,6. A comprehensive search for low-energy isomers is conducted on a polarizable intermolecular potential energy surface. Zero-point energy contributions are calculated rigorously with the rigid-body quantum Monte Carlo method. The OH stretch vibrational spectra of the isomers are calculated using a local-mode model and compared with experimental isomer-selective IR–UV spectral hole burning (SHB) spectra. The topology of the clusters phenol(H2O)5,6 is shown in deviate from the corresponding pure water clusters.

The structures of benzotriazole monomer and benzotriazole(H2O)1,2 clusters have been examined by comparison of R2PI spectra and IR–UV double resonance spectra with the results of ab initio based normal mode calculations. The structures of the binary benzotriazole–water cluster and the n = 2 cluster are shown to be cyclic. Based on these cyclic structures the intermolecular vibronic transitions have been assigned by comparison with ab initio normal mode analysis. The cyclic benzotriazole(H2O)1,2 clusters can be viewed as a precursor to a transition state for H-atom transfer from the 1- to the 2-position in benzotriazole. The activation energy for this tautomerism has been calculated for the monomer and the n = 1 and 2 cluster as the difference between the stabilization energy of the more stable tautomer and the respective transition state. It has been found to decrease rapidly, with increasing cluster size.

The structures of neutral catechol, catechol(H2O)1, and catechol(H2O)3 are investigated by IR/R2PI spectroscopy and ab initio calculations at the HF and MP2 level of theory. Furthermore, the structure of the ionic catechol(H2O)1+ cluster is derived from resonant IR-photodissociation spectroscopy as well as ab initio calculations. Trans-linear arrangements are obtained for the neutral (S0) and ionic (D0) ground state of the catechol(H2O)1 cluster. In the case of the neutral catechol(H2O)3 cluster (S0 state) a cyclic and a π-bonded structure are discussed which are of similar stability. From the IR spectrum of catechol(H2O)3 it can be derived that the cyclic structure is the most probable arrangement. The calculated OH stretching frequencies of this structure are in good agreement with the experimentally observed values.

Differential infrared absorption spectroscopy was used to study the formation of isocyanates and further photo-products in the oligonucleotides dG10, dC10 and dT10 and in their mononucleosides by ultraviolet light at 266 nm. We find that α-cleavage takes place in oligonucleotides and mononucleosides both in films and in solution. The very intense and spectrally isolated isocyanate (N=C=O) asymmetric stretch vibration at 2277 cm(-1) is used as a spectroscopic marker for detection of the photo-product. The band disappears upon reaction with small amounts of water vapour as expected for isocyanates. Quantum yields for isocyanate formation by nucleobase ring cleavage in the α-position to the carbonyl group are ∼5 × 10(-5) in the mononucleosides and up to 5 × 10(-4) in the oligonucleotides. In the mixed oligonucleotides dG10/dC10 and dA10/dT10 the quantum yield of α-cleavage drops by a factor of 10 compared to the single oligonucleotides. Implications for DNA repair and photo-induced DNA-protein cross-linking via isocyanate reaction with NH2 groups of amino acids are discussed.

Copolyimide membranes are established materials for the separation of gaseous and liquid mixtures. Cross-linking of the polymer strands improves the physical and chemical stability. The photo-cross-linking of a 6FDA-ODA/6FDA-DABA 4 : 1 copolyimide membrane containing maleimide side groups as linker was investigated by FTIR spectroscopy. IR absorption spectra of the copolyimide backbone, 3-hydroxypropyldimethyl maleimide and the copolyimide functionalized with 3-hydroxypropyldimethyl maleimide were measured before and after different irradiation times and compared to each other. For band assignment a normal mode analysis was performed. The backbone of the polymer and the maleimide linker can be well distinguished due to their different spectral band positions. Only the films containing a maleimide moiety perform a photoreaction, the polymer backbone does not interfere. Based on the difference spectra and the results of the DFT calculations it was shown that the trans- and the cis-cycloadduct as well as the previously suggested 2-2'-adduct without a cyclobutane ring are formed upon UV irradiation. Evidence for an oxetane-like photoproduct was not found. Different time constants for the increase of the product bands were observed. The cycloadduct accumulates with a shorter time constant (τ = 2 to 5 min) than the 2-2'-adduct (τ = 75 min). The yield of the photo-cross-linking reaction was determined by spectral deconvolution and kinetic fitting of several marker bands. For the copolyimide synthesized in this work, a maximum value of 6% was reached. The stiffness of the copolyimide backbone inhibits further photo-cross-linking.

In this paper we show that the stacked structures of 9-methyl adenine⋯adenine are comparably stable as the most stable hydrogen bonded structures and that the calculated IR pattern of the stacked structures is in a very good agreement with the experimental spectrum. The pair 7-methyl adenine⋯adenine on the other hand shows a nearly planar hydrogen bond arrangement and no stacked structure was observed.