Raman spectra of glassy aqueous solutions

The Raman spectra of glassy LiCl and LiBr aqueous solutions are obtained in the OH (OD) stretching vibrational region. Spectral features are discussed in conjunction with those of amorphous ice and liquid water.     

Effect of pressure and solvent on Raman spectra of all-trans-beta-carotene

The ground state Raman spectra of all-trans-beta-carotene in n-hexane and CS2 solutions are measured by simultaneously changing the solvent environment and molecular structure under high hydrostatic pressure. The diverse pressure dependencies of several representative Raman bands are explained using a competitive mechanism involving bond length changes and vibronic coupling. It is therefore concluded that (a) the in-phase C=C stretching mode plays an essential role in the conversion of energy from S1 to S0 states in carotenoids, (b) internal conversion and intramolecular vibrational redistribution can be accelerated by high pressure, and (c) the environmental effect, but not the structural distortion or pi-electron delocalization, is responsible for the spectral properties of a given carotenoid species. These findings revealed the potential of high pressure in exploring the nature of the biological functions of carotenoids.     

The temperature-dependence of the far-infrared spectra of l-alanine

Far-i.r. spectra in the region 20–600 cm⁻¹ are reported for solid l-alanine as a function of temperature between 6 and 300 K. Understanding of the temperature-sensitive as well as the temperature-insensitive bands is aided by the normal coordinate analysis of l-alanine. Bands containing contributions from hydrogen bonds, skeletal deformation and NH³₊ torsion are found to be highly sensitive to temperature changes. Although the hydrogen bonds are stronger at lower temperatures, no evidence of amide formation is detected even at 6 K. Thus the zwitterion form seems to be more stable than the amide form in the temperature range studied here.     

Improved DFT calculation of Raman spectra of silicates

Very accurate vibrational spectra of silicates are obtained from DFT calculations if the appropriate Hamiltonian is used. Theoretical considerations suggest that the Hartree-Fock component of ACM1 hybrid functionals should be 1/6 instead of 1/4 for this class of compounds. When applied to the PBE functional this removes the scaling error of the calculated vibrational frequencies. Calculations using this PBE(n = 6) functional in combination with optimized Gaussian basis sets result in very small remaining deviations between observed and calculated Raman shifts, with standard uncertainties of ≈3.5 cm⁻¹, maximum deviations of ≈10 cm⁻¹, and no significant systematic trends. This has been confirmed for a wide range of silicate structures, for which high-quality Raman spectra have been published: forsterite α-Mg₂SiO₄ (nesosilicate), γ-Y₂Si₂O₇ (sorosilicate), K₂Ca₃Si₃O₁₀ (oligosilicate), K₂Ca₄Si₈O₂₁ (phyllosilicate), and α-quartz SiO₂ (tectosilictae).     

Raman spectra of amino acids and their aqueous solutions

Amino acids are the basic "building blocks" that combine to form proteins and play an important physiological role in all life-forms. Amino acids can be used as models for the examination of the importance of intermolecular bonding in life processes. Raman spectra serve to obtain information regarding molecular conformation, giving valuable insights into the topology of more complex molecules (peptides and proteins). In this paper, amino acids and their aqueous solution have been studied by Raman spectroscopy. Comparisons of certain values for these frequencies in amino acids and their aqueous solutions are given. Spectra of solids when compared to those of the solute in solution are invariably much more complex and almost always sharper. We present a collection of Raman spectra of 18 kinds of amino acids (L-alanine, L-arginine, L-aspartic acid, cystine, L-glutamic acid, L-glycine, L-histidine, L-isoluecine, L-leucine, L-lysine, L-phenylalanine, L-methionone, L-proline, L-serine, L-threonine, L-tryptophan, L-tyrosine, L-valine) and their aqueous solutions that can serve as references for the interpretation of Raman spectra of proteins and biological materials.     

The Raman spectra of serine and 3,3-dideutero-serine in aqueous solution

The Raman spectra of serine [α-amino-β-hydroxypropionic acid; HOCH₂CH(NH₃)⁺COO⁻] and 3,3-dideutero-serine [HOCD₂CH(NH₃)⁺COO⁻] in aqueous solution were studied in the range 4000–300 cm⁻¹. The data obtained for the deuterated compound are novel and provide compelling evidence that previously reported assignments for the undeuterated amino acid should be revised.     

Polarized Raman spectra of aqueous solutions and band component analysis

Polarized Raman spectra of water in ν (OH) and δ (OH₂) regions has been recorded for various aqueous solutions of electrolytes of types AB and AB₂ (A=Li, Na, K, Ca; B=F, Cl, Br, I, NO₃) over a wide concentration range. The isotropic and anisotropic components of the intensity of ν (OH) band has been analysed as a function of concentration and as function of the nature of the ions by means of the Fourier transform analysis.     

Adding explicit solvent molecules to continuum solvent calculations for the calculation of aqueous acid dissociation constants

Aqueous acid dissociation free energies for a diverse set of 57 monoprotic acids have been calculated using a combination of experimental and calculated gas and liquid-phase free energies. For ionic species, aqueous solvation free energies were calculated using the recently developed SM6 continuum solvation model. This model combines a dielectric continuum with atomic surface tensions to account for bulk solvent effects. For some of the acids studied, a combined approach that involves attaching a single explicit water molecule to the conjugate base (anion), and then surrounding the resulting anion-water cluster by a dielectric continuum, significantly improves the agreement between the calculated pK(a) value and experiment. This suggests that for some anions, particularly those concentrating charge on a single exposed heteroatom, augmenting implicit solvent calculations with a single explicit water molecule is required, and adequate, to account for strong short-range hydrogen bonding interactions between the anion and the solvent. We also demonstrate the effect of adding several explicit waters by calculating the pK(a) of bicarbonate (HCO(3)(-)) using as the conjugate base carbonate (CO(3)(2-)) bound by up to three explicit water molecules.     

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

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.     

Full ab initio calculation of second-order Raman spectra of semiconductors

We present the first full ab initio calculation of second-order Raman spectra in semiconductors based on density functional perturbation theory. The method is applied to the Γ spectra of the elemental semiconductors diamond, silicon, and germanium and reproduces the experimental data excellently. Using first-principles phonons, we also calculated the corresponding overtone densities of states and Γ Raman spectra employing phenomenological polarizability coefficients. Finally, we analyze the results of the different approaches pointing out the relevance of a full first-principles derivation. © 1995 John Wiley & Sons, Inc.     

Time-domain calculations of the infrared and polarized Raman spectra of tetraalanine in aqueous solution

The IR and polarized (isotropic and anisotropic) Raman spectra are calculated for the amide I band of tetraalanine ((Ala)4) in aqueous solution by using a time-domain computational method, which includes the effects of the diagonal frequency modulations (of individual peptide groups), the off-diagonal (interpeptide) vibrational couplings, and structural dynamics. It is shown that the calculated band profiles, especially the existence of a large negative noncoincidence effect (i.e., large frequency separations between the IR, isotropic Raman, and anisotropic Raman bands, with the isotropic Raman being higher in frequency), are in reasonable agreement with the experimental results. This negative noncoincidence effect derives from two conditions: the positive coupling between the amide I vibrations of peptide groups and the angle larger than 90 degrees between the transition dipoles of the coupled vibrations. This result means that the dynamically changing structures mainly in the polyproline II and beta-type conformations containing some repeated interconversions obtained from the molecular dynamics calculation are consistent with the existence of a large negative noncoincidence effect, as far as the structures satisfy the above two conditions. It is also shown that the electric fields from solvent water molecules induce larger frequency shifts than those of intrachain interactions, with rapid underdamped oscillatory modulations ( approximately 43 fs) due to the librational motions of water molecules that give rise to motional narrowing effect on the spectra. The reason for the difference from the behavior seen for the O-H stretching mode of liquid water is discussed. The time-domain analysis of the mode identity shows that the system proceeds halfway to complete mode mixing with a similar time scale ( approximately 60 fs), suggesting the importance of the nonadiabatic effect, which is included in a natural way in the present computational method.     

Raman spectra of aqueous lithium sulfate and potassium thiocyanate in strong electric fields

The effects of high-voltage pulsed discharge (HVPD) activation on the Raman spectra of saturated aqueous solutions of lithium sulfate (Li₂SO₄/H₂O) in the range of the ν₁(A) totally symmetric vibrations of the SO ₄ ^2? anion and on the spectra of potassium thiocyanate KSCN/H₂O in the region of the ν₁(C≡N) vibrations of the SCN^? anion have been studied. The temperature dependences of the width and frequency of the corresponding spectral lines have been investigated.     

QM/MM calculation of solvent effects on absorption spectra of guanine

Electronic spectra of guanine in the gas phase and in water were studied by quantum mechanical/molecular mechanical (QM/MM) methods. Geometries for the excited‐state calculations were extracted from ground‐state molecular dynamics (MD) simulations using the self‐consistent‐charge density functional tight binding (SCC‐DFTB) method for the QM region and the TIP3P force field for the water environment. Theoretical absorption spectra were generated from excitation energies and oscillator strengths calculated for 50 to 500 MD snapshots of guanine in the gas phase (QM) and in solution (QM/MM). The excited‐state calculations used time‐dependent density functional theory (TDDFT) and the DFT‐based multireference configuration interaction (DFT/MRCI) method of Grimme and Waletzke, in combination with two basis sets. Our investigation covered keto‐N7H and keto‐N9H guanine, with particular focus on solvent effects in the low‐energy spectrum of the keto‐N9H tautomer. When compared with the vertical excitation energies of gas‐phase guanine at the optimized DFT (B3LYP/TZVP) geometry, the maxima in the computed solution spectra are shifted by several tenths of an eV. Three effects contribute: the use of SCC‐DFTB‐based rather than B3LYP‐based geometries in the MD snapshots (red shift of ca. 0.1 eV), explicit inclusion of nuclear motion through the MD snapshots (red shift of ca. 0.1 eV), and intrinsic solvent effects (differences in the absorption maxima in the computed gas‐phase and solution spectra, typically ca. 0.1–0.3 eV). A detailed analysis of the results indicates that the intrinsic solvent effects arise both from solvent‐induced structural changes and from electrostatic solute–solvent interactions, the latter being dominant. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Raman spectra of isotopic bisulfite and disulfite ions in alkali salts and aqueous solution

The Raman spectra of solid lithium, sodium, potassium, cesium, rubidium, and ammonium disulfite and of rubidium and cesium bisulfite were recorded and compared with the spectrum of an aqueous solution saturated with 1.2m K₂S₂O₅. Deuterium, ³⁴S₈, and ¹⁸O₂ were used to label cesium bisulfite, CsHSO₃, cesium disulfite, Cs₂S₂O₅, and potassium disulfite. All six fundamentals of the bisulfite ion were observed and assigned. Eleven of the fifteen expected fundamentals of the disulfite ion were observed and correlated to older experimental data, as well as recent calculations. Isotope shifts confirm assignments in the SO stretching region. In the low frequency region, assignment is less conclusive and two frequencies have been tentatively reassigned.     

An analytical derivative procedure for the calculation of vibrational Raman optical activity spectra

We present an analytical time-dependent Hartree-Fock algorithm for the calculation of the derivatives of the electric dipole-magnetic dipole polarizability with respect to atomic Cartesian coordinates. Combined with analogous procedures to determine the derivatives of the electric dipole-electric dipole and electric dipole-electric quadrupole polarizabilities, it enables a fully analytical evaluation of the three frequency-dependent vibrational Raman optical activity (VROA) invariants within the harmonic approximation. The procedure employs traditional non-London atomic orbitals, and the gauge-origin dependence of the VROA intensities has, therefore, been assessed for the commonly used aug-cc-pVDZ and rDPS:3-21G basis sets.     

Calculation of Raman spectra and structure elucidation of guanine in the polycrystalline state and in aqueous solution

The frequencies and relative intensities of lines in the Raman spectra of the condensed states of guanine and its deutero substituted analog have been calculated. The effect of hydrogen bonding on the Raman spectrum has been studied. Guanine exists in different tautomeric forms (G-N₉H and G-N₇H) at different pH in aqueous solution (for acidic and alkaline media, respectively).     

Semi-empirical AM1 calculation of the solvent effect on the fluorescence spectra of some dihydroquinolinones

The COnductor-like Screening MOdel of solvent-solute interactions of Klamt and Schüürmann, COSMO, at the semiempirical AM1 level of MO calculations, augmented by limited singles and doubles configuration interaction, proves useful for the study of solvent induced shifts of fluorescence spectra. Optimization of geometry of ground S0 and excited S1 states for each solvent separately provides estimates of the changes of solvation energy accompanying the electron transition process and helps the understanding of the related solvent-solute reorganization and fluorescence mechanisms.