Vibrational analysis of crystalline diketopiperazine—II. Normal mode calculations

Based on our Raman and i.r. [1] data and assignments for DKP and five of its isotopic derivatives, we have refined an intramolecular force field in a non-redundant basis. This analysis shows that the cis peptide group may be best differentiated from the trans by the presence of an NH out-of-plane bend mode in the 800 cm⁻¹ region. Several atom—atom potentials have been evaluated for their ability to explain observed lattice frequencies and internal mode splittings. Observed splittings in CO stretch modes can be accounted for by dynamical charge transfer and transition dipole coupling interactions.     

Vibrational spectroscopic analysis of an amber necklace—a forensic historical study

The vibrational infrared spectroscopic analysis of an important historical necklace of 102 beads that are purported to be made of amber indicated strong signal characteristics of cellulose nitrate with dark green-coloured areas of a naphthylamine dye. Confocal Raman depth-profiling spectroscopy using a 785-nm laser excitation, a novel application first applied here for the analysis of inclusions in amber resin, confirmed that the beads were amber resin and that residues of cellulose nitrate, camphor plasticiser and a naphthylamine dyestuff were present in surface cracks and inclusions in the bead matrix. The bead stringing material was confirmed as cellulose, which was stained green in part with the dyestuff. Comparison of the Raman spectra of the amber beads with a resin database suggested that the amber was sourced from Northern England. The scientific evidence supports the stylistic opinion that the necklace is an important example that could date from the 19th Century and that efforts had been made to coat it with a synthetic dyed polymer; this provides a rather unusual example of the chemical masking of a genuine article—a procedure that renders the article of particular interest.     

Vibrational levels and anharmonicity in SF6—I. Vibrational band analysis

The vibrational spectrum of SF₆ has been recorded with a Fourier-transform i.r. spectrometer at a resolution of 0.05 cm⁻¹ and pressure—path length products of up to 2 × 10⁵ Torr-cm. Twenty-nine bands were observed. Rotational structure was resolved for 11 of these and polynomials were fitted to the observed frequencies to yield the scalar spectroscopic constants, including the band origins m and derived values of B′B₀ and the Coriolis constants ζ. For 12 other unresolved bands accurate estimates of the origins could be made from the frequency of a sharp Q-branch edge. Three more bands (ν₃, 2ν₁ + ν₃, and 3ν₃) were not resolvable at our resolution but have been previously analyzed from Doppler-limited or sub-Doppler spectra. In addition, about 10 assignable hot bands were observed whose frequency shifts relative to the principal transitions could be accurately measured; two of these were sufficiently resolved for full scalar analyses. These frequencies were combined with results of several high-resolution Raman studies by other authors to yield the most complete data set on SF₆ vibrational levels yet obtained. Isotopic frequency shifts have also been measured. The effective Coriolis constants for combination and overtone bands of octahedral molecules are discussed.     

Vibrational spectra and force constants of symmetric tops—XXIV. Vibrational and rotational analysis of parallel overtones and combination bands of CF3Br

The i.r. spectra of monoisotopic CF₃⁷⁹Br and of natural CF₃Br have been investigated in the gas phase with a resolution of ∼ 0.04 cm⁻¹. The rotational J structure of 2ν₁, 2ν₃, ν₁ - ν₃, ν₁ + ν₃, ν₁ + ν₂, ν₂ + 2ν₃ and 2ν₂ + ν₃ of CF₃⁷⁹Br has been resolved, Furthermore vibrational transitions of hot bands and of further parallel bands have been measured. Numerous excited state molecular parameters ν⁰_i, x_ij, α^A_i and α^B_i have been determined. Due to the accidental equivalence of ν₁/2ν₅ and ν₂ + ν₃, strong anharmonic resonance correlates all (n + 2)ν₃/ν₁ + (n - 1)ν₃/2ν₅(∥)+(n - 1)ν₃/ν₂ + nν₃ levels.     

Vibrational analysis of chlorinated para-benzoquinones—III. Infrared and Raman spectra of the five fully deuterated chloroparabenzoquinones

The five fully deuterated chloroparabenzoquinones: 2-chloro-d₃-, 2,3-dichloro-d₂-, 2,5-dichloro-d₂-, 2,6-dichloro-d₂- and 2,3,5-trichloro-d₁-para-benzoquinone, have been prepared, and their i.r. and Raman spectra are reported. Spectroscopic information comes from powder and solution spectra, as well as from vapour band envelopes in the i.r. and, in the case of the three dichloro-derivatives, from polarized single-crystal i.r. spectra.A normal coordinate analysis based on the valence force field previously used for the light chloroparabenzoquinones results is useful in evaluating the approximate spectral regions where the few fundamentals not directly attributable should occur. The extended experimental data collected lead to a definite vibrational assignment for all the five deuterated species which adds to that previously reported of the corresponding light molecules.     

Vibrational spectra of N-2′-methyl-propyliden-propan-2-amine: conformational analysis and influence of the nitrogen hybridization

Infrared and Raman spectra of the (CH₃)₂C^2′HC^1′HNCH(CH₃)₂ aldimine (NPP) and of two deuterated derivatives at C^1′ and C^2′ in the liquid, solid and solution phases have been recorded and assigned between 4000 and 130 cm⁻¹. NPP adopts the E configuration and two conformers at the C_sp2 and N sides are in equilibrium. Some vibrational modes are specifically assigned to the anticlinal (ac) or synperiplanar (sp) conformers at the C_sp2 side. The ac(C_sp2) form is dominant in the pure liquid whilst the sp(C_sp2) form is favoured in the solid and in chloroform. The vibrational dynamics of the isopropyl group on both sides of the CN bond are partially similar to that of the (CH₃)₂CHCHO aldehyde on the one hand and of the (CH₃)₂CHNH₂ amine on the other hand. When moving from the amine to the corresponding aldimine, changes about νNC and wCC₂ modes (at the N side) are related to electronic and geometrical effects as a consequence of the nitrogen hybridization change from sp₃ to sp₂.     

Vibrational spectrum and normal coordinate analysis of the P4S4−8 anion

The I.R. and Raman spectra of K₄P₄S₈ · 2H₂O and (NH₄)₄P₄S₈ · 2H₂O have been recorded and the vibrations of the anion have been assigned for D_4h symmetry. A normal coordinate analysis has been performed, and agreement between observed and calculated frequencies has been achieved by employing “standard” values for the PP and PS force constants, 2.05 and 2.8 [10² N m−1], respectively.     

Vibrational absorption intensities in chemical analysis—VIII. Dimethyl sulphide

The structure and electric dipole moment of dimethyl sulphide have been calculated with a variety of basis sets. It is shown that the force fields computed with the 3-21G, 3-21G and 6-311G** bases are very similar after scaling. The vibrational assignments are discussed. The force field is compared with that of trimethyl phosphine. There is an excellent accord for the CH stretching modes, but the ab initio calculations indicate that the X-C/X-C (X = S,P) interaction terms have opposite signs. This appears to be incompatible with the experimental data. Effective atomic charges and atomic polar tensors (APT) are computed with the above bases and also with bond centred sp functions added to the 3-21G* basis. It is shown that the APT are much more sensitive to the basis set than are the effective atomic charges. The effects of the lone pair electrons on the bond electrical properties are much less than with oxy-ethers, though the overall perturbation of the sulphur atom on the intensities is greater. A number of inadequacies of the SCF calculations are identified.     

Spectra and structure of organophosphorus compounds—XXVIII. Vibrational and conformational analysis of dimethylmethoxyphosphine and some isotopically substituted derivatives

The i.r. spectra (3600-50 cm⁻¹) of the gaseous and solid states, and the Raman spectra (3600-10 cm⁻¹) of the gaseous, liquid and solid states of (CH₃)₂POCH₃, (CH₃)₂POCD₃, (CD₃)₂POCH₃ and (CD₃)₂POCD₃ have been recorded. Thirty-five of the 36 normal modes have been assigned and the asymmetric torsion has been observed as a broad weak feature centered at 106 cm⁻¹ in the far i.r. spectrum of gaseous (CH₃)₂POCH₃. A comparison of the vibrational spectra obtained for the fluid phases with those obtained for the amorphous and annealed solids indicates the existence of a second conformer present in a small amount in the gas phase but becoming more abundant in the liquid phase and the only remaining rotamer in the annealed solid. Asymmetric top i.r. band contour simulation provides evidence that the dominating species in the gas phase is one in which the methoxy methyl group is oriented approximately 60° (gauche) away from the lone pair of electrons on the phosphorus atom. The rotamer which remains in the spectra of the annealed solid has been assigned to a structure in which the lone pair on the phosphorus atom is oriented trans to the methoxy methyl group. These results are compared to similar data obtained for methoxy phosphoryl compounds and discussed in terms of both canonical molecular orbital and VSEPR thoery.     

First observation of the SeO+ emission spectrum. Vibrational analysis of the A2Π-X2Π system

The low-resolution emission spectrum of the selenium monoxide cation, SeO⁺, excited in the reaction of discharged helium with SeOCl₂, has been recorded from 15000 to 25500 cm⁻¹. Thirty-five bandheads have been assigned to the A²Π-X²Π system of SeO⁺. Vibrational constants of the new A²Π and X²Π states have been estimated from least-squares fits of the measured head positions.     

Vibrational analysis of sodium α-, β- and γ-hydroxybutyrates. Inter- and intramolecular hydrogen bonds

The infrared and Raman spectra of sodium α-, β- and γ-hydroxybutyrates and their deuterated analogues are examined in the 4000-100 cm⁻¹ range and an assignment of the fundamental vibrations is given. Based on the localization of the asymmetric stretching vibrations ν_asOH and the out-of-plane vibration γOH, inter- and/or intramolecularly hydrogen-bonded forms are proposed: the low frequencies of ν_asOH (<3200 cm⁻¹) and high frequencies of γOH (≈800 cm⁻¹) argue in favour of the existence of intramolecular hydrogen bonding. Sodium α-hydroxybutyrate exhibits as a chelate ring with an intramolecular hydrogen bond between hydroxyl and carboxyl groups, whereas sodium, β-hydroxybutyrate has the two association forms with inter- and intramolecular hydrogen bonds. Sodium γ-hydroxybutyrate exists as a hydrogen-bonded polymer, with an intermolecular hydrogen bond between the hydroxyl groups and between the hydroxyl and carbonyl groups. At a crystallization temperature above 50°C, only the α- salt showed a structural change indicating the existence of intra- and intermolecular hydrogen bonds. This result is confirmed by differential scanning analysis.     

Vibrational dynamics of adsorbates – Quo vadis?

Vibrational energy is a prime reservoir for activating surface processes such as adsorption, desorption and reaction. On metal surfaces, vibrational energy flow occurs on a femto-to picosecond time scale and competing energy dissipation channels in this time range determine the outcome of chemical reactions at surfaces. Fundamental questions of relaxation time, mode selectivity, importance of intra- versus intermolecular coupling and coupling between electronic and vibrational states can now be tackled for relatively complex adsorbates and surfaces. This review looks at the state-of-the-art of surface vibrational dynamics across a wide range of vibrational spectroscopies and the challenges and exciting prospects that lie ahead to further not only our understanding but also the control of vibrational energy flow in model systems as well as real-world problems.     

Vibrational spectrum of α- and δ-TiCl3

The vibrational spectrum of α-TiCl₃ is assigned on the basis of a normal coordinate analysis. The i.r. bands shift to higher frequency from α- to δ-TiCl₃. This is discussed in terms of a dispersion relation for k_c in α-TiCl₃ and force constants, where the former is carried out for examining effect on disorder to the c axis in δ-TiCl₃. The shift is explained in terms of a change in force constants rather than disorder.     

Vibrational Raman spectra of AgIAgPO3 glasses

Raman spectra of fast ion conducting AgIAgPO₃ glasses have been investigated as a function of composition and temperature. Only an average anionic environment is indicated for Ag⁺ ions. The spectroscopic variations have been found to be consistent with the model of structural unpinning proposed earlier by the authors.     

Vibrational absorption intensities in chemical analysis—II. n-alkanes,n-alkones and n-alkals

The absolute absorption intensities of homologous series of n-alkanes, n-alkan-2-ones and n-alk-1-als, as well as of cyclohexane and cyclohexanone, have been measured in tetrachloromethane in 25 cm⁻¹ intervals. Procedures to deduce the separate contributions from various structural groupings are defined. The resulting intensity parameters are then used to predict the spectra of methyl- and ethyl-cyclohexanes, di-n-hexylketone and cycloheptanone. Agreement is within a few per cent of the strongest peak when allowance is made for small frequency shifts arising from such as the alkyl to cyclohexyl link. Significant perturbation of characteristic band intensities occur for CH₂ and CH₃ groupings β to the carbonyl. The available data leaves some uncertainty as to the distribution of intensity between groupings α and β to the perturbing group.     

Vibrational modes of the anthracene—tetrachlorophthalic anhydride charge-transfer complex

Raman and i.r. studies of the anthracene—tetrachlorophthalic anhydride charge-transfer complex have been carried out. Five lattice modes are identified in the low-frequency Raman spectra. The 21 cm⁻¹ lattice band is tentatively assigned to the librational vibration of the donor and acceptor molecules along their stacking axis. The intramolecular modes are assigned on the basis of the free-molecule symmetries D_2h and C_2ν for anthracene and tetrachlorophthalic anhydride, respectively. The intramolecular frequencies do not change significantly on complexation. This suggests that the molecular symmetry and the force constants between interacting molecules are not greatly affected by the formation of the complex.     

Vibrational spectra and intermolecular potential of the α-form crystal of glycine

Polarized Raman and far i.r. spectra of glycine and N-deuterated glycine crystallized in the α-form were recorded. The normal coordinate analysis of the optical active intra- and intermolecular vibrations of the α-form crystal of glycine was carried out. A modified Urey—Bradley type intramolecular potential was adopted. The observed factor group splittings were elucidated fairly well by using the intermolecular potential including the exchange repulsion—dispersion interaction between non-bonded atoms and the Coulomb interaction. The stretching potential for the hydrogen bonds was found to contribute appreciably to the lattice frequencies.