Tobias N. Wassermann

The last globally stable extended alkane.

Mother of all folding: cold isolated linear alkanes C(n)H(2n+2) prefer an extended all-trans conformation before cohesive forces between the chain ends induce a folded hairpin structure for longer chains. It is shown by Raman spectroscopy at 100-150 K that the folded structure becomes more stable beyond n(C) = 17 or 18 carbon atoms. High-level quantum-chemical calculations yield n(C) = 17 ± 1 as the critical chain length.

Ethanol monomers and dimers revisited: a Raman study of conformational preferences and argon nanocoating effects.

The gauche-trans conformational distribution in ethanol can be determined from the OH stretching Raman spectrum of seeded supersonic jet expansions, which thus provides a sensitive conformational thermometer. Depending on the rare gas mixture, one, two or four ethanol dimer conformations are abundant. Their conformational assignment is facilitated by the observation of hydrogen bond acceptor modes, which have similar Raman cross sections but much inferior infrared intensities than donor modes. Ethanol monomers and dimers can be progressively Ar-coated, and the resulting spectra may be compared with those in a bulk argon matrix. The low frequency range of torsional transitions provides some evidence for conformation-changing transitions in Raman jet spectra.

Vibrational tuning of the Hydrogen transfer in malonaldehyde – a combined FTIR and Raman jet study†

Tunnelling splittings in vibrationally excited malonaldehyde are derived from complementary FTIR and Raman spectra utilising jet cooling and matrix isolation. Values from 0 to 69 cm−1 (i.e. from complete quenching of the proton tunnelling to an acceleration by a factor of 3) are firmly assigned, underscoring a strong coupling between hydrogen transfer and vibrational excitation. More tentative assignments involve even larger effects for the O⋅⋅⋅O breathing vibration. The results are discussed with respect to the corresponding normal modes and compared to previous theoretical predictions.

The stiffness of a fully stretched polyethylene chain: A Raman jet spectroscopy extrapolation

Linear alkanes with n=5-16 C-atoms are partially relaxed into their stretched all-trans conformation by supersonic jet expansion. Their longitudinal acoustic modes are identified by spontaneous Raman scattering and deperturbed from transverse bending mode components and Fermi resonance with combination states of the same symmetry. Comparison with quantum chemical predictions of the longitudinal modes in hydrocarbon chains with up to 54 C-atoms allows for a reliable extrapolation to the limiting product nnu(n)=2310+/-30 cm(-1) for large n, from which the elastic modulus of an ideal polyethylene chain in vacuum may be estimated at 309+/-8 GPa. Differences to solid state determinations of this quantity are discussed.

Weak Hydrogen Bonds Make a Difference: Dimers of Jet-Cooled Halogenated Ethanols

Abstract Hydrogen-bonded clusters of fluorinated and chlorinated ethanols exhibit rich isomerism in terms of monomer conformation, secondary contacts between the OH and CH groups and the halogen atoms, hydrogen bond topology, chirality recognition and acceptor lone electron pair choice. By expanding the six alcohols involving one to three fluorine or chlorine atoms at the methyl group in a supersonic slit jet expansion and by probing their monomer, dimer and trimer IR spectra between 800 and 4000 cm−1, this isomerism is unravelled in substantial detail. Argon relaxation experiments and complementary cluster Raman spectroscopy provide further information on the individual dimer conformations and on trimer assignments. Energy sequences, helicity- and topology-dependent OH red-shifts, differences between fluorine and chlorine, the influence of dispersion-like interactions and halogen number trends are uncovered and compared to systematic quantum-chemical calculations up to MP2/6–311+G* level. The experimental data provide rigorous reference values for an accurate and balanced quantum-mechanical description of weak hydrogen bond interactions to halogens in the presence of a strong hydrogen bond between oxygen atoms.

Brightening and locking a weak and floppy N-H chromophore: the case of pyrrolidine.

The N-H stretching signature of the puckering equilibrium between equatorial and axial pyrrolidine is analyzed via FTIR and Raman spectroscopy in supersonic jets as a function of aggregation. Vibrational temperatures along the expansion axis can be extracted from the Raman spectra and allow for a localization of the compression shock waves. While the equatorial conformation is more stable in the ground state monomer, this preference is probably switched in the excited state with one N-H stretching quantum. Furthermore, the dominant dimer involves an axial donor and the trimer and tetramer structures seem to prefer uniform axial conformations. The IR intensity is boosted by up to 3 orders of magnitude upon aggregation, whereas the Raman scattering intensity shows only moderate hydrogen bond effects. B3LYP and MP2 calculations provide a reasonable description of the N-H vibrational dynamics under the influence of self-aggregation. In mixed dimers with pyrrole, pyrrolidine assumes the role of a hydrogen bond acceptor.

Uv and IR Photochemistries of Malonaldehyde Trapped in Cryogenic Matrices

UV and IR photochemistries of malonaldehyde, the simplest molecule exhibiting an intramolecular proton exchange, have been studied in four cryogenic matrices at 4.3 K, N2, Ne, Ar, and Xe. Samples have been irradiated using a UV and IR OPO type tunable laser, and with a broad band UV mercury lamp. UV and IR spectra have been recorded and compared with theoretical calculations carried out at the SAC-CI/6-31++G(d,p) (UV transitions) and B3LYP/6-311++G(2d,2p) (IR spectra) levels of theory. After deposition, the intramolecularly H-bonded form is found exclusively, while several open forms are formed upon UV irradiation. These open forms show ability to interconvert upon UV irradiation too. Some of them are also able to isomerize upon selective IR irradiations. The whole set of results allowed us to identify seven isomers among the eight postulated. The photodynamics of the electronic relaxation of malonaldehyde have also been investigated. By following the decay or rise of suited specific vibrational bands in the IR spectra, and by comparing the results with an earlier study of the homologous acetylacetone, we deduced that the electronic relaxation of malonaldehyde proceeds through singlet states, most probably through a 3-fold conical intersection, as postulated from theoretical calculations. In contrast with acetylacetone, malonaldehyde does not show fragmentation after UV excitation.