O. Faurskov Nielsen

Hydrogen bonding in liquid formamide. A low frequency Raman study

Low frequency Raman spectra from 20 to 400 cm−1 of liquid HCONH2, HCOND2, and HCO 15NH2 were studied. Experimental data were directly transformed to the R(v) representation, previously described. Assignments of low frequency bands are discussed based on a comparison between frequencies for the isotopic species and measurements of depolarization ratios. Chain structures are the dominant species. An out‐of‐plane intermolecular mode involving atoms in the hydrogen bond was assigned. The frequency for this mode decreased with increasing temperature.

Low-frequency Raman spectroscopy

Abstract Some intensity expressions for low-frequency Raman, far-IR and inelastic neutron scattering are discussed. Low-frequency near infrared (NIR)-FT-Raman spectra (∼80 – 400 cm −1 ) of some proteins and their aqueous solutions are presented, along with low-frequency visible Raman spectra (10 – 400 cm −1 ) of their model systems, some liquid amides. A low-frequency band is observed in the R( \ gn)-representation of these spectra at approximately 100 cm −1 . This band is assigned to a mode involving atoms in hydrogen bonds. Three more quantitative models are used to describe it. The first model describes these low-frequency bands in terms of longitudinal and transverse mode density of states curves of one-dimensional hydrogen-bonded chains, the second model is a damped harmonic oscillator model, and the third model is the Gaussian cage model. A band with a maximum around 290 cm −1 might be sensitive to the water structure in aqueous protein solutions.

Intermolecular Raman active vibrations of hydrogen bonded acetic acid dimers in the liquid state

Low frequency Raman spectra (20–250 cm−1) of liquid CH3COOH, CH3COOD, and CD3COOD at room temperature were studied. The experimental data were directly transformed to the R(ν) representation, previously described. The three Raman active intermolecular vibrations of the cyclic dimer (C2h symmetry) were assigned by use of depolarization ratios and a comparison of vibrational frequencies for the different isotopic molecules.

Low-frequency vibrational spectra of some hydrogen-bonded amides

Abstract Low-frequency infrared and Raman spectra at 10–400 cm −1 are presented for some simple amides. Both the Stokes and the anti-Stokes sides are given for N -methylformamide to ensure that no fluorescence bands are present in the spectrum. Spectra are presented for compounds in the liquid state and that for N -methylacetamide is compared with spectra of the crystalline compound at −15°C−20°C. The liquid state spectra are all given in the R( v )-representation. A complex is formed between lithium ions and formamide molecules and the vibrational spectra are discussed in terms of this complex. Hydrogen bonding in these simple amides is used as a model for more complicated hydrogen-bonded biological systems. Some synthetic polymers such as nylon, are also very useful in this context. The spectrum of nylon-6 is presented and a low-frequency band with a maximum at 103 cm −1 is assigned to a vibration involving H-bonded atoms.

Low-frequency Raman spectra of liquid formamides and aqueous solutions of formamide

Abstract Low-frequency Raman scattering of liquid formamide, aqueous solutions of formamide, liquid N-methylformamide and N,N-dimethylformamide are studied by construction of a quantity R ( V ). Previous experimental observations (far-infrared and Raman) in this frequency region are compared to those obtained from the R( v ) representation.

Temperature dependence of the dielectric function of C6H6(l) and C6H5CH3(l) measured with THz spectroscopy

We report on an experimental investigation of the temperature dependence of the intermolecular dynamics in liquid benzene and toluene. With the use of THz time domain spectroscopy we measured the complex dielectric function (0.2–3.3 THz) of the liquids, at temperatures between −6 °C and 75 °C. By analyzing the dielectric loss (as opposed to the absorption coefficient) we found three contributions to the dielectric function for toluene and two for benzene. In the dipolar liquid toluene we observed a contribution from rotational diffusion at lower frequencies in addition to the two high-frequency librations also observed in benzene. The temperature and density dependence were different for the two librational bands, probably due to the different effect of three-body interactions for the two processes. Furthermore, we present measurements of the low-frequency depolarized Raman spectra as a function of temperature for benzene and toluene. These have been compared with the dielectric loss at similar temperatur...

Comments on the R(ν̄) spectral representation of the low frequency Raman spectrum

A few misunderstandings concerning our use of the R (?) spectral representation are clarified. Two different spectral representations for the low frequency Raman spectrum are compared.

Raman spectroscopic studies of some biochemically relevant molecules

Abstract Near-infrared (NIR) Raman spectra of the protein aprotinin, in both powder form and aqueous solutions, are presented. The amide I and amide III bands give information about the secondary structure. The conformation around the sulphur bridges and the environment of tyrosine were also studied. Due to the low scattering efficiency, only aqueous solutions in the concentration range 2–20% (w/w) were used. Use of a windowless cell improved the quality of the spectra, as compared to spectra obtained with quartz cells. Fluorescence can be a serious problem in Raman studies of biologically relevant molecules. Some examples are shown, which illustrate that the use of NIR excitation can frequently eliminate this fluorescence. Heating effects give rise to serious problems with excitation at 1064 nm in the NIR-FT-Raman spectrum of some strongly coloured macromolecules, like haemoglobin. In order to avoid complications due to both heating and fluorescence, an excitation wavelength around 800 nm is suggested. A preliminary surface enhanced Raman (SER) spectrum of a peptide nucleic acid (PNA) in aqueous silver colloid solution is shown. Low-frequency Raman spectra of aprotinin in aqueous solution are presented. The low-frequency limit in the NIR-FT-Raman spectrum is ∼80 cm −1 . Several models are used to describe the bands assigned to hydrogen bonding in the systems. The low-frequency modes can be of importance for the formation and breaking of hydrogen bonds, and thus may be of importance for biological activity.

Fourier Transform Raman Spectroscopy of 15th Century Mummies from Qilakitsoq, Greenland

To investigate molecular changes in mummified skin, near-infrared Raman spectroscopy was applied to the skin obtained from four mummies found in Qilakitsoq in Greenland. The mummies date from AD 1475 (±50 years) and are the oldest preserved bodies in the Arctic region. The spectra of the skin obtained from the different mummies were very similar, but they were distinctly different from those of fresh and freeze-dried contemporary skin. Especially in the spectra of the ancient skin the amide I (1640–1680 cm-1) and amide III (1220–1300 cm-1) bands had very low intensity, indicating loss of protein and/or changes in the secondary protein structure. Similar spectral changes have previously been found in the 5200-year-old skin of the Iceman. This may suggest that most changes in molecular structure take place in a relatively short time after mummification.

Vibrational spectra of solutions of formamide in DMSO

Abstract The infrared and Raman spectra of HCONH 2 , HCOND 2 , and an equimolar mixture of the two in dimethylsulphoxide (DMSO) at concentrations ranging from the pure formamide to a very dilute solution have been measured. The carbonyl stretching band of formamide is asymmetric. At very low concentrations in DMSO the band becomes symmetric. The behaviour of the asymmetry upon dilution can be explained in terms of two “sites”. One corresponding to formamide molecules which have no hydrogen bonds to their carbonyl groups and another to a hydrogen-bonded species forming a chain of formamide molecules. The carbonyl stretching band of the equimolar mixture does not become symmetric upon dilution. This is because it consists of the contribution from four different molecules. Subtraction of the contributions from HCONH 2 and HCOND 2 produces the spectrum of cis- and trans -HCONHD.