The impact of solvation on the structure and electric field strength in LiGlyGly complexes.

To scrutinise the impact of electric fields on the structure and vibrations of biomolecules in the presence of water, we study the sequential solvation of lithium diglycine up to three water molecules with cryogenic infrared action spectroscopy. Conformer-specific IR-IR spectroscopy and HO/DO isotopic substitution experiments provide most of the information required to decipher the structure of the observed conformers. Additional confirmation is provided by scaled harmonic vibrational frequency calculations using MP2 and DFT.

The impact of the electric field of metal ions on the vibrations and internal hydrogen bond strength in alkali metal ion di- and triglycine complexes.

Using infrared predissociation spectroscopy of cryogenic ions, we revisit the vibrational spectra of alkali metal ion (Li, Na, K) di- and triglycine complexes. We assign their most stable conformation, which involves metal ion coordination to all C=O groups and an internal NH⋯NH hydrogen bond in the peptide backbone. An analysis of the spectral shifts of the OH and C=O stretching vibrations across the different metal ions and peptide chain lengths shows that these are largely caused by the electric field of the metal ion, which varies in strength as a function of the square of the distance.

Slow monomer vibrations in formic acid dimer: Stepping up the ladder with FTIR and Raman jet spectroscopy.

In an effort to extend the cold gas phase spectroscopic database of the cyclic formic acid dimer (FAD), we present and analyze the jet-cooled vibrational infrared and Raman spectrum of (HCOOH) in the monomer fingerprint region between 600 and 1500 cm. The present study bridges the gap between the intermolecular dimerization-induced and the carbonyl stretching fundamentals that have already been reexamined using jet-cooled or high-resolution spectroscopy. This completes the characterization of the jet-cooled vibrational (HCOOH) spectrum below the complex OH (CH) stretching fundamentals, and we report resonance-induced FAD combination/overtone transitions that will serve as a valuable reference for a theoretical modeling of its vibrational dynamics.

CC-stretched formic acid: isomerisation, dimerisation, and carboxylic acid complexation.

The cis-trans-isomerism of the propiolic acid monomer (HC[triple bond, length as m-dash]C-COOH) is examined with linear Raman jet spectroscopy, yielding the first environment-free vibrational band centres of a higher-energy cis-rotamer beyond formic acid (HCOOH) in addition to all fundamentals and a large number of hot and combination/overtone bands of the trans-conformer. Two near-isoenergetic trans-fundamentals of different symmetry (CC[double bond, length as m-dash]O bend and OH torsion) prove to be a sensitive benchmarking target, as their energetic order is susceptible to the choice of electronic structure method, basis set size, and inclusion of vibrational anharmonicity. For the infrared- and Raman-active C[double bond, length as m-dash]O stretching fundamentals of the cyclic (C) trans-propiolic acid dimer, resonance couplings are found that in part extend to the C-symmetric heterodimer of trans-propiolic and trans-formic acid.

Increasing the weights in the molecular work-out of - and -formic acid: extension of the vibrational database deuteration.

The higher-energy cis- as well as the global minimum trans-rotamers of the four H/D isotopologues of the formic acid monomer have been examined with Raman jet spectroscopy extending the vibrational gas phase reference database by eleven new cis-band positions for HCOOD, DCOOH, and DCOOD. With these new additions, all O-H/D, C-H/D, and C[double bond, length as m-dash]O stretching as well as the O-D in-plane bending vibrations of these higher-energy rotamers are known in addition to the previously determined C-O stretch and OH torsion of cis-HCOOH. Further, a comparison of the vibrational spectra of all four H/D isotopologues of the globally stable trans-rotamer of formic acid is shown to be very helpful in revealing similarities and differences in these systems, particularly with regard to Fermi resonances.

Shifting formic acid dimers into perspective: vibrational scrutiny in helium nanodroplets.

A metastable dimer of formic acid has been prepared inside superfluid helium nanodroplets and examined using IR spectroscopy and quantum chemical calculations. This dimer has one strong O-HO[double bond, length as m-dash]C hydrogen bond and one weak C[double bond, length as m-dash]OH-C bond, which is the same bonding motif that exists between adjacent molecules in catemer chains found in the crystalline phase. The strongly bound OH stretching vibration of the metastable dimer shows clear evidence of significant coupling to other vibrational modes, but it is far less extensive than that seen for the doubly hydrogen bonded global energy minimum dimer structure, which dominates in the gas phase but is not observed in helium droplets.

Stretching of -formic acid: warm-up and cool-down as molecular work-out.

A new technique to rotationally simplify and Raman-probe conformationally and vibrationally excited small molecules is applied to the - isomerism of formic acid. It quintuples the previously available gas phase vibrational data base on this excited form of a strongly anharmonic planar molecule despite its limited spectral resolution. The newly determined -formic acid fundamentals allow for a balanced vibrational benchmark on both rotamers of formic acid.

Vibrational exciton coupling in homo and hetero dimers of carboxylic acids studied by linear infrared and Raman jet spectroscopy.

The jet-cooled band positions of the C=O stretching vibrations in the three hetero dimers composed of formic, acetic, and pivalic acid have been determined. Resonance patterns in the symmetric stretching modes have been corrected for by assuming a single bright state. An analysis of their Davydov or vibrational exciton splitting shows that the hetero dimer values can be averaged from the respective homo dimer splittings (ranging from 56 cm for the acetic to 75 cm for the formic acid dimer) with an error of ≤7%.

Formic acid aggregation in 2D supersonic expansions probed by FTIR imaging.

C=O stretching vibrations of formic acid trimers are assigned on the basis of FTIR and Raman jet spectroscopy and further validated by an FTIR imaging study based on their aggregation behavior in supersonic expansions. The effect of shock waves on cluster formation and decomposition is probed by shifting them into the field of view of the focal plane array detector. A double slit nozzle is presented that merges two supersonic jets for a more localized study of such shock waves.