Low frequency raman spectra of isotopic mixed benzophenone crystals

In this paper, we present an experimental study of vibrational lattice modes in isotopic mixed crystals of benzophenone-h₁₀ and -d₁₀. Our results are discussed using theoretical and experimental data concerning other molecular isotopic mixed crystals. The spectra show two regions; in the first (ν < 100 cm^?1) we did not observe an interaction between the various vibrational lattice bands; in the second region (ν > 100 cm^?1), the interaction appears clearly. The 111.5 cm^?1 and 73 cm^?1 torsional modes of benzophenone-h₁₀ have the same behaviour as the external modes throughout the whole concentration range (0–100% of benzophenone-d₁₀).     

Efficient implementation of the analytical second derivatives of hartree–fock and hybrid DFT energies within the framework of the conductor-like polarizable continuum model

Calculation of vibrational frequencies for solvated systems is essential to study reactions in complex environments. In this paper, we report the implementation of the analytical self-consistent field Hessian at the Hartree–Fock and density functional theory levels in the framework of the conductor-like polarizable continuum model (C-PCM) into the ORCA quantum chemistry suite. The calculated vibrational frequencies agree very well with those computed through numerical differentiation of the analytical gradients. The deviation between both sets of data is smaller than 3 cm⁻¹ for frequencies larger than 200 cm⁻¹ and smaller than 5 cm⁻¹ for the low-frequency regime (100 cm⁻¹ < ω < 200 cm⁻¹). The accuracy of the frequencies is not significantly affected by the size of the density functional theory (DFT) integration grid, with a deviation lower than 0.5 cm⁻¹ between data computed with the smallest and that with the largest DFT grid size. The calculation of the analytical Hessian is between 3 and 12 times faster than its numerical counterpart. The C-PCM terms only add an overhead of 10–30% relative to the gas phase calculations. Finally, for acetone, the (B3LYP) values for the frequency shifts obtained in going from the gas phase to liquid acetone are in agreement with experiment. © 2019 Wiley Periodicals, Inc.     

Temperature and pressure dependence of the Raman active modes of vibration of α-quartz

The Raman spectra of a single crystal of natural quartz have been very carefully investigated as a function of temperature (range 77–300 K), and as a function of hydrostatic pressure (atmospheric up to 10 kbar). The temperature results shows that cooling the quartz increases the vibrational frequencies with corresponding reductions in linewidth, and that the 206 cm⁻¹, vibrational mode (the soft mode) is very strongly temperature dependent. The pressure dependent Raman data has been used to calculate the mode Gruneisen parameters γ_i, for the three strongest vibrational modes; these are 3.63, 0.64 and 1.55 for the 206, 464 and 128 cm⁻¹ lines, respectively.     

Infrared spectra of pyridinium salts in solution—I. The region of middle frequencies

Infrared spectra of h₅- and d₅-pyridinium salts Py⁺H ⋯ A⁻ (A⁻= SbCl⁻₆, ClO⁻₄, BF⁻₄, J⁻, Br⁻, Cl⁻) and their N-deuterated analogues have been measured in organic aprotic solvents in the region of 1650-400 cm⁻¹ and assignments proposed for internal fundamental modes of the pyridinium ions. Notable differences of in-plane δ_NH and out-of plane γ_NH deformation mode frequencies in i.r. spectra of the salts in solution and in solid state have been established. In particular, the γ_NH mode frequencies in solution are 40–80 cm⁻¹ higher than those in solid state. In the series of the salts studied the γ_NH mode frequencies change from 903 to 1080 cm⁻¹ depending upon the H-bonding strength. A strong vibrational coupling γ_NH↔-ν₅ (γ_CH) has been shown to occur in h₅-pyridinium salts and a γ_ND↔v_10b (γ_CH) coupling in their N-deuterated analogues. Isotopic shifts and changes of the bandwidths of the v₁₄, v_19b and v_8b internal modes show that these modes are strongly mixed with the in-plane deformation mode δ_NH in h₅- and d₅-pyridinium salts.     

A reinvestigation of the vibrational spectra of cyclooctatetraene and cyclooctatetraene-d8

Band shapes in the gas phase infrared spectrum of cyclooctatetraene (COT) have been recorded in the 3500–250 cm⁻¹ range. With these and the existing vibrational data for COT and COT-d₈, revised assignments of the fundamental modes have been made for both compounds. A normal coordinate analysis based on D_2d symmetry has been carried out and used to support the vibrational assignments.     

Infrared absorption spectra of the spinels Fe2SiO4 and Co2SiO4

The i.r. absorption spectra of the spinel polymorphs of Fe₂SiO₄ and CO₂SiO₄ have been measured from 200 to 4000 cm⁻¹. Three strong absorption bands are observed at 835, 503 and 340 cm⁻¹ and 810, 503 and 357 cm⁻¹, respectively, and correspond to 3 of the 4 i.r. active T_1u modes. The observed frequencies lie close to those for Ni₂SiO₄ [5] and are consistent with the normal spinel structure. The cation size effect on the i.r. frequencies and the nature of the vibrational modes are discussed for II–IV spinels.     

Velocity modulation laser spectroscopy of vibrationally excited CF+ determination of the molecular potential function

The lowest six vibrational hot bands of CF⁺ have been measured in a helium/C₂F₆ discharge by velocity modulation laser spectroscopy. A total of 56 transitions has been fitted to Dunham expansion for v = 0–7, yielding the parameters: ω_e = 1792.6654(18) cm⁻¹B_e = 1.7204176(75) cm⁻¹, Y₂₀, = −13.22968(54) cm⁻¹, and D₀ = 62086(30) cm⁻¹. The rotational temperature of CF⁺ in the plasma is near 650 K and the vibrational temperature is approximately 5200 K.     

Raman spectrum of 1,1′-diethyl-2,2′-cyanine iodide near the π-π* electronic transition

The Raman spectrum of 1,1′-diethyl-2,2′-cyanine iodide in a methyl alcohol solution (4 × 10⁻⁵m) in which the lines of the argon ion laser at 457.9, 488.0, 496.5, 501.7 and 514.5 nm were employed as excitation sources has been investigated. The strongest lines in the Raman spectrum appear in the region from 1200–1700 cm⁻¹ and are strongly polarized (ϱ= 0.2). Two vibrational modes have been identified with reasonable certainty: one a doublet at 1361 and 1380 cm⁻¹ which involves the stretching of the conjugated chain in the ring of the quinolinic end groups, and the other at 1639 cm⁻¹ which characterizes the stretching of the conjugated chain which bridges the quinolinic end groups. The splitting of the line associated with the ring stretching mode probably arises from the resonance interaction between the two quinolinic end groups. The intensity and polarization of the lines in the spectrum indicate that the vibrational modes of the strongest lines are totally symmetric. The visible electronic spectrum shows maxima at 490.6 and 522.9nm; an analog resolution of the spectrum shows that the contour of the vibronic band can be accounted for by a vibronic progression with a spacing of approximately 1200 cm⁻¹. Both the observed variation of intensity and an approximate calculation based on A-term scattering of totally symmetric vibrational modes support the conclusion that a maximum enhancement of the intensity will be attained when the frequency of exciting radiation is equal to that of the pure electronic transition at 522.9 nm.     

Optimized structures and vibration frequencies of the ether–water complex: a DFT and FTIR study

The infrared spectrum of ether was studied using Fourier transform infrared spectroscopy in conjunction with the density functional theory (DFT). The optimized structures and vibrational frequencies of the ether·(H₂O) _n (n = 1–3) complexes were obtained at B3LYP/6-31G(d) theory levels. Compared to those of free-form ether, the C–O stretching vibrational frequencies of the ether–water complexes are found to shift to red by up to 39 cm^?1 with an increase in the C–O length of 0.016 Å. Meanwhile, the frequency of the O–H stretching modes of water in the complexes appears significantly redshifted to a varying degree. The DFT calculations suggest that these shifts are caused by the hydrogen bonding between ether and water.     

Molecular mechanics investigation of some acrylic polymers using SPASIBA force field

The First generation SPASIBA force field is used to study normal vibrational modes of PMMA, and then extended to other thermoplastic polymers, namely PMA, PMAA and PAA, in order to determine its parameters transferability. To this end, FTIR and FTR spectra of pure PMMA samples, prepared by the emulsion polymerization of MMA and initiated by sodium, are recorded in 400–3500 cm⁻¹ and 200–3500 cm⁻¹, respectively. A detailed vibrational analysis was performed on the obtained spectra and the observed frequencies are assigned to their respective vibrational modes, supported by potential energy distribution (PED) analysis. Our numerical results reveal an RMS value of 7.8 cm⁻¹ corresponding to IR wavenumbers and 8.7 cm⁻¹ relatively to Raman wavenumbers. Our vibrational calculations on PMA, PMAA and PAA polymers reveal that the parameters transferability criterion, established by Shimanouchi, is verified for the SPASIBA force field.     

Vibrational spectroscopy of lapachol, α- and β-lapachone: Theoretical and experimental elucidation of the Raman and infrared spectra

Raman and infrared spectroscopy were applied for the vibrational characterization of lapachol and its pyran derivatives, α-lapachone and β-lapachone. Experimental spectra of solid state samples were acquired between 4000 and 100 cm⁻¹ in Raman experiments, and between 4000 and 600 cm⁻¹ (mid-infrared) and 600–100 cm⁻¹ (far-infrared) with FTIR spectroscopy, respectively. Full structure optimization and theoretical vibrational wavenumbers were calculated at the B3LYP/6-31 + + G(d,p) level. Detailed assignments of vibrational modes in an experimental and theoretical spectra were based on potential energy distribution analyses, using Veda 4.1 software. Clear differentiation between the three compounds was verified in the region between 1725 and 1525 cm⁻¹, in which the ν(CO) and ν(CC) modes of the quinone moiety were assigned.     

Theoretical DFT study of phosphorescence from porphyrins

Geometrical structure of free-base porphin (H₂P) and Mg- and Zn-porphyrins together with their vibrational frequencies and vibronic intensities in phosphorescence are investigated by density functions theory (DFT) with the standard B3LYP functional. These molecules have a closed-shell singlet ground state (S₀) and low-lying triplet (T₁) excited states of ππ* type. The S₀–T₁ transition probability and radiative lifetime of phosphorescence (τ_p) of these molecules are calculated by time-dependent DFT utilizing quadratic response functions for account of spin–orbit coupling (SOC) and electric-dipole transition moments including displacements along active vibrational modes. The infrared and Raman spectra in the ground singlet and first excited triplet states are also studied for proper assignment of vibronic patterns. The long radiative lifetime of free-base porphin phosphorescence (τ_p ∼ 360 s at low temperature limit, 4.2 K) gets considerably shorter for the metalloporphyrins. An order of magnitude reduction of τ_p is predicted for Mg-porphyrin but no change of phosphorescence polarization is found. A forty times enhancement of the radiative phosphorescence rate constant is obtained for Zn-porphyrin in comparison with the H₂P molecule which is accompanied by a strong change of polarization and spin-sublevel radiative activity. A strong vibronic activity of free-base porphin phosphorescence is found for the b_2g mode at 430 cm⁻¹, while the 679 and 715 cm⁻¹ vibronic bands of b_3g symmetry are less active. These and other out-of-plane vibrations produce considerable changes in the radiative constants of different spin sublevels of the triplet state; they also promote the S₁ → T₁ intersystem crossing. Among the in-plane vibrations the a_g mode at 1614 cm⁻¹ is found very active; it produces a long progression in the phosphorescence spectrum. The time-dependent DFT calculations explain the effects of the transition metal atom on phosphorescence of porphyrins and reproduce differences in their phosphorescence and EPR spectra.     

The rotational spectrum of the hydrogen-bonded heterodimer H2O…HF in the frequency range 40–80 GHz

The μ_a R-branch rotational spectra of the ground and first excited states of the three lowest vibrational modes of the H₂O…HF heterodimer have been observed in the frequency range 40–80 GHz. Coriolis perturbations between the ground vibrational state (υ = 0) and the first excited state of the out-of-plane bending vibration (υ_β(0) = 1) show that for a given J the K₋₁ = 2 levels of υ_β(0) = 1 lie approximately 3 cm⁻¹ above the K₋₁ = 3 levels of υ = 0. The vibrational separation between these two states is estimated to be 70±3 cm⁻¹. This value is consistent with those determined by other methods and reinforces the conclusion that ν_β(0) is governed by a double-minimum potential energy function with the quantitative form previously published. A perturbation is also observed in the first excited state of the hydrogen-bond stretching vibration υ_σ = 1. This manifests itself as a large, negative centrifugal distortion constant D_JK = −8.5 MHz compared with 2 MHz in the other vibrational states.     

Pressure-tuning Raman and infrared spectra of N-thiocyanato[tris(o-tolyl)]tin(IV), (o-CH3C6H4)3SnNCS

The effect of high external pressures on the Raman and IR spectra of the title compound (I) has been examined at ambient temperature. A pressure-induced phase transition was observed at 13–16 kbar, which is most likely second-order, resulting from slight rotations of the phenyl rings and/or the CH₃ groups under the influence of pressure. No new peaks were observed in the spectra with increasing pressure indicating that no pressure-induced linkage isomerism or SnNCS⋯Sn bridging took place. The average pressure sensitivity (dν/dP) of the Raman-active vibrational modes is lower in the low-pressure region (0.23 cm⁻¹/kbar) than in the high-pressure one (0.47 cm⁻¹/kbar). In general, the IR-active modes are less sensitive to increasing pressure than are the Raman-active modes and the average dν/dP value for the IR-active modes in the low-pressure region is quite similar to that in the high-pressure region, i.e., about 0.23 cm⁻¹/kbar.     

DFT and Raman spectroscopy of porphyrin derivatives: Tetraphenylporphine (TPP)

Raman spectrum of the meso tetraphenylporphine (TPP) deposited onto smooth copper surface as thin film were recorded in the region 200–1700 cm⁻¹. To investigate the effect of meso-phenyl substitution rings on the vibrational spectrum of free base porphyrin, we calculated Raman and infrared (IR) spectra of the meso-tetraphenylporphine (TPP), meso tetramethylporphine (TMP), copper (II)porphine (CuPr) and free base porphine (FBP) at the B3LYP/6-311+G(d,p) level of the density functional theory (DFT). The observed Raman spectrum of the TPP is assigned based on the calculated its Raman spectrum in connection with the calculated spectra of the TMP, CuPr and FBP by taking into account of their corresponding vibrational motions of the Raman modes of frequencies. Results of the calculations clearly indicated that the meso tetraphenyl substitution rings are totally responsible for the observed Raman bands at ∼1593, 1234 and 1002 cm⁻¹. The calculated and observed Raman spectra also suggested that the observed Raman band with a medium intense at 962 cm⁻¹ might result from the surface plasmon effect. Furthermore, the observed Raman bands with medium intense at ∼334 and ∼201 cm⁻¹ are as results of the dimerization or aggregation of the TPP or would be that related to intramolecular interaction. We also calculated IR spectra of these molecules at same level of the theory. To investigate the solvent effect on the vibrational spectrum of porphine, the Raman and IR spectra of the TPP and FBP are calculated in solution phase where water used as solvent. The results of these calculation indicated that there is no any significant effect on the vibrational spectrum of the TPP.     

Vibrational (i.r. and Raman) spectra and conformational studies of 1-formyl-3-thiosemicarbazide

The i.r. and Raman spectra (30–4000 cm⁻¹) of 1-formyl-3-thiosemicarbazide (FTSC) and deuterated ftsc-d₄, have been studied. Most of the vibration modes reveal pairs of bands and show strong temperature dependence. A band group {ν(NNH₂)} at ∼ 1100 cm⁻¹ exhibits well resolved doublet (1095 and 1112 cm⁻¹) structure below 100 k. The intensity in the 11 12 cm⁻¹ band decreases regularly (band disappears at 150 K) with the rise in temperature. Two new bands at 955 and 1070 cm⁻¹ appear while measured above 400 K. The system eventually exists in several conformers in simultaneous equilibria. Moreover, a few bands {e.g. ν(CO), ν(CS) and ν(CH)} that show strong intensifies in i.r. exhibit weak (or zero) intensifies in the Raman and vice-versa. The features (characteristic of u and g vibration species) could be explained by a C_2h pseudo symmetry space group proposed for the system. Both the FTSC and FTSC-d₄ represent strong molecular associations. This favours the maximum abundance in the dimer stabilized conformers.     

Spectroscopy and decay dynamics of jet-cooled carbazole and N-ethylcarbazole and their homocyclic analogues

Fluorescence excitation spectra of supersonic jet-cooled carbazole (C) and N-ethylcarbazole (EC) are reported together with those of their homocyclic analogues fluorene (F) and 9-ethylfluorene (EF). Fluorescence spectra of C and EC have been measured following excitation at energies up to ≈ 1300 cm⁻¹ above the S₁ origin and reveal that the onset of intramolecular vibrational redistribution occurs at around 900 cm⁻¹ for both molecules, with redistribution being more extensive in the ethylated molecule. In C and EC, a number of modes are active in vibronically coupling the S₁ and S₂ states and Duschinsky mixing of these modes is apparent in the spectra. The fluorescence lifetimes of both C and EC show a slowly decreasing trend with increasing excitation energy in the range 0–1500 cm⁻¹ excess vibrational energy; vibrational redistribution does not appear to enhance the rate of non-radiative decay in either molecule. Comparison of lifetime values under supersonic jet conditions with solution phase results indicates that solvation produces a considerable increase in the rate of intersystem crossing in these molecules.     

Vibrational studies,barrier height and thermodynamic functions for biomolecules: 5-Trifluoromethyl uracil

Laser Raman (50–4000 cm⁻¹) and IR (200–4000 cm⁻¹) spectra of 5-trifluoromethyl uracil have been recorded and analysed. It has been possible to assign all the 39 (26a′+13a″) normal modes of vibration. Consistent assignments have been made for the internal modes of the CF₃ group, especially for the antisymmetric CF₃ stretching and bending modes. Using thus assigned vibrational frequencies and assumed structural parameters, thermodynamic functions, in the temperature range 100–1000 K, have been computed and the barrier to the internal rotation for the CF₃ top has been determined.     

Infrared spectroscopy and intramolecular vibrational relaxation of c-C6F12 excited above the dissociation threshold

The IR spectrum of c-C₆F₁₂ at a vibrational energy of twice the dissociation threshold was investigated. Absorption of cw CO₂ laser radiation was measured at various frequencies. Our experimental conditions were chosen such that during absorption measurements all vibrational degrees of freedom were in equilibrium, the molecular rotation being at room temperature. The Boltzmann vibrational distribution allowed computer simulations of the spectrum to be made to determine the homogeneous contribution. The homogeneous half-width of the spectrum is γ=13±0.5 cm⁻¹ and the homogeneous spectrum of c-C₆F₁₂ at E= 60000 cm⁻¹ is non-Lorentzian. We attribute this to the influence of higher-order anharmonicities on the relaxation from the excited mode (v₂₇) to other modes in the molecule.