Raman Quantum Research Articles

Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of l-ascorbic acid and its anion and cation

IR and spectra of the l-ascorbic acid ( l-AA) also known as vitamin C have been recorded in the region 4000–50 cm −1. In order to make vibrational assignments of the observed IR and Raman bands computations were carried out by employing the RHF and DFT methods to calculate the molecular geometries and harmonic vibrational frequencies along with other related parameters for the neutral l-AA and its singly charged anionic ( l-AA −) and cationic ( l-AA +) species. Significant changes have been found for different characteristics of a number of vibrational modes. The four ν(O–H) modes of the l-AA molecule are found in the order ν(O 9–H 10) > ν(O 19–H 20) > ν(O 7–H 8) > ν(O 14–H 15) which could be due to complexity of hydrogen bonding in the lactone ring and the side chain. The C O stretching wavenumber ( ν 46) decreases by 151 cm −1 in going from the neutral to the anionic species whereas it increases by 151 cm −1 in going from the anionic to the cationic species. The anionic radicals have less kinetic stabilities and high chemical reactivity as compared to the neutral molecule. It is found that the cationic radical of l-AA is kinetically least stable and chemically most reactive as compared to its neutral and anionic species.

Carotenoids and β-Cyclodextrin Inclusion Complexes: Raman Spectroscopy and Theoretical Investigation

In the present study, the inclusion processes of β-carotene, astaxanthin, lycopene, and norbixin (NOR) into the β-cyclodextrin (β-CD) cavity were investigated by means of Raman spectroscopy and quantum mechanics calculations. The Raman ν(1) band assigned to C═C stretching was sensitive to the host-guest interaction and in general undergoes a blue shift (3-13 cm(-1)) after inclusion takes place, which is the consequence of the localization of single and double bonds. This is supported by the molecular modeling prediction, which inclusion complexes show the ν(1) band blue shifted by 1-8 cm(-1). The calculated complexation energies was small for most of derivatives and was found to be -11.1 kcal mol(-1) for inclusion of AST and +0.27 kcal mol(-1) for NOR. The stability order was qualitatively correlated to topological parameters accounting for the opening angle of the chain. This means that after inclusion the guest molecules assume a slightly more extended conformation, which enhances the host-guest contact, improving the interaction energy. The results discussed here clearly demonstrate the matrix effect on the carotenes' spectroscopic profile and should contribute to fully characterize the raw samples.

Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of nicotinic acid and its N-oxide

FTIR and Raman spectra of nicotinic acid and its N-oxide have been recorded and analyzed. The stabilities, optimized molecular geometries, APT charges and vibrational characteristics for the two possible conformers of nicotinic acid and its N-oxide have been computed using DFT method. The E (trans) conformers of both the molecules are found to be more stable and less polar than their respective Z (cis) conformers. Due to addition of an O atom at the N 1 site in nicotinic acid the magnitudes of atomic charges on all the H atomic sites of the nicotinic acid N-oxide molecule are found to increase. Most of the vibrational frequencies have nearly the same magnitude for the two conformers of both the molecules. However, significant changes are noticed in their IR intensities, Raman activities and depolarization ratios of the Raman bands. The calculated frequencies have been correlated with the experimental frequencies.

π-conjugation and charge polarization in fluorene-dibenzothiophene-S,S-dioxide co-oligomers by Raman spectroscopy and quantum chemistry

The Raman spectra of a series of fluorine (F)∕dibenzothiophene-S,S-dioxide co-oligomers (S) of different length and alternation sequences in their backbones (FSF, FFSFF, FSFSF, and FASAF; A is the -C≡C- bridge) have been recorded and simulated theoretically. It is shown that Raman spectroscopy is useful to probe π conjugation and ground state electron polarization in these molecules, phenomena directly related with the existence of intramolecular charge-transfer processes owing to the combination of electron donor (fluorene) and acceptor (dibenzothiophene-S,S-dioxide) groups. Their geometric, electronic, and vibrational properties have been studied by density functional theory B3LYP∕6-311G(2d,p) quantum chemical calculations and compared with those for fluorene homo-oligomers. Comparative studies in solution∕solid∕melted phase led to the conclusion that the Raman wavenumbers are also sensitive to intermolecular interactions.

Theoretical analysis of quantum well fountain and Raman laser schemes for far-infraredand THz generation

A theoretical analysis of fountain and Raman laser schemes based onsemiconductor heterostructures with quantum wells and suited to generationin the far-infrared and THz range is conducted. It is shown that for ametal–metal waveguide and a heterostructure design optimal for lasing at100 µm, a threshold mid-infrared pump intensity (inside the active region) for generation at thiswavelength is . The maximum output power (in the pulsed regime, from one strip perone facet) is estimated to be 0.4 W and achieved at the pump intensity4.8 MW cm − 2 when the generation wavelength is shifted to85 µm. Stability analysis is carried out and domains of steady one-frequency generation aswell as non-stationary lasing are found. The fountain and Raman quantum welllasers of the far-infrared and THz range could become an alternative for quantumcascade lasers whose advances in this frequency region encounter serious problems.

Static and Dynamic Response Analysis of Raman Injection Quantum Cascade Laser Using Circuit Level Modeling

In this paper a new approach for modeling of Raman quantum cascade lasers (QCLs) is presented which integrates electrical, optical and Raman effects. The equivalent circuit model of laser by employing simplified two-level rate equations of pump and Raman regions is developed. The total equivalent circuit is composed of pump laser circuit coupled to Raman laser circuit which models the behaviors of each component and whole device. By using the presented model the static and dynamic performances of laser for different parameters are investigated. The results of simulation show that the main characteristics of laser such as threshold current, quantum efficiency, output power and modulation response are affected by photon lifetime and variation of population inversion conditions. Static results of model agree favorably with experimental data from devices reported in the literatures.

Vibrational study of fluorobenzene and its solvation with methanol via polarized Raman measurements and quantum chemical calculations

Raman spectra of neat fluorobenzene (C 6H 5F, FB) and its binary mixtures with methanol (CH 3OH, M) at varying mole fractions of FB from 0.1 to 0.9 were recorded in order to understand the influence of intermolecular interaction on spectral features corresponding to some selected vibrational bands of FB in the region 1200–450 cm −1. Only few vibrational bands of fluorobenzene show a significant change in their peak position in going from neat liquid to the complexes. The 803, 829 and 994 cm −1 bands show blue shift upon complexation which indicates significant amount of charge transfer between the reference molecule and the solvent. However, the linewidths do not show any appreciable change. Density functional theory (DFT) calculations were performed employing B3LYP method and high level basis set 6-311++G(d,p) to obtain the ground state geometry of neat FB and its hydrogen bonded complexes with methanol in gas phase. In order to account for the solvent effect and also to realize a condition quite close to the experiment, polarizable continuum model (PCM) calculations considering bulk solvation as well as explicit (specific plus bulk) solvation approaches were also performed. A detailed vibrational assignment of the various normal modes has been performed on the basis of potential energy distribution (PED) calculations. Depolarization ratios for the different vibrational bands were calculated and the values match nicely with the depolarization ratio determined from the experimental data.

Quantum chemical and experimental studies on the structure and vibrational spectra of efavirenz

Efavirenz, (S)-6-chloro-4(cyclopropylethynyl)-1, 4-dihydro-4-(trifluoromethyl)-2H-3,1-benzoxazin-2-one, is a anti HIV agent belonging to the class of the non-nucleoside inhibitors of the HIV-1 virus reverse transcriptase. Structural and spectral characteristics of efavirenz have been studied by methods of Raman spectroscopy and quantum chemistry. The optimized geometries, electronic charge distribution, dipole moments and electrostatic potential surface were calculated by Hartree–Fock and Density functional B3LYP method with the 6-311++G(d,p) basis sets. Electrostatic potential surfaces have been mapped over the electron density isosurfaces to obtain information about the size, shape, charge density distribution and chemical reactivity of the molecules.

Infrared and Raman spectroscopic and quantum chemical investigations of zinc halide complexes of 3-aminoquinoline

In this study detailed structure and vibrational spectra of free and coordinated 3-AQ (3-AQ = 3-aminoquinoline; C 9H 8N 2) have been reported. Vibrational wavenumbers of free 3-AQ, its Zn halide complexes {Zn(3-AQ) 2X 2; where X = Cl or Br} and 3-AQ interacting with water molecules have been calculated using a harmonic approach at B3LYP/6-31G(d,p) and B3LYP/6-31++G(d,p) theory levels. The wavenumbers of free 3-AQ have also been calculated using a harmonic approach at B3LYP/6-31++G(d,p) theory levels. The fundamental vibrational modes were characterized by their total energy distribution (TED%). The Zn(3-AQ) 2X 2 complexes have been prepared for the first time and their vibrational wavenumbers were reported, in comparison with the calculated results. Coordination through the ring nitrogen effects on the vibrational wavenumbers of 3-AQ were investigated and the coordination sensitive modes of 3-AQ were determined. It was found that, the wavenumber-shifts due to complex formation depend on the halogen group and decreases in the order Cl > Br for the Zn(3-AQ) 2X 2 complexes.

Molecular and electronic structures of germylene and stannylene complexes (CO)5MECl2·nTHF (M = Cr, W; E = Ge, Sn; n = 1, 2) as studied by IR and Raman spectroscopy, X-ray analysis, and quantum chemistry

The Raman and IR spectra of the complexes (CO)5CrSnCl2·THF (1), (CO)5WSnCl2·THF (2), (CO)5CrGeCl2·THF (3), (CO)5WGeCl2·THF (4), (CO)5CrSnCl2·2THF (5), and (CO)5WSnCl2·2THF (6) were measured and interpreted using quantum chemical calculations. Complexes 3 and 5 were characterized by X-ray analysis. The stretching vibrations of the CO groups in the spectra of solutions of complexes 1–6 obey the selection rules for C4ν local symmetry. For the complexes containing 0 (type A), 1 (type B), and 2 (type C) THF molecules, a comparison was made of the calculated and experimental M\( \underline \cdots \)EII bond lengths and energies, as well as the ν(CO) vibrational frequencies. The contribution of the π-component to the M\( \underline \cdots \)EII bond decreases in the order A→B→C and leads to enhancement of the donor ability of the carbene-like ligand and to a slight elongation and weakening of this bond. An attempt to grow crystals of complex 6 in air unexpectedly resulted in a polynuclear complex [(CO)5WSn(Cl)(μ-OH)2SnCl2(μ-OH)]2·6THF, which was characterized by X-ray analysis and Raman spectroscopy.

A spectroscopic and computational study of Al(III) complexes in sodium cryolite melts: Ionic composition in a wide range of cryolite ratios

The structure of sodium cryolite melts was studied using Raman spectroscopy and quantum chemical calculations performed at the density functional theory level. The existence of bridged forms in the melts was argued first from the analysis of experimental Raman spectra. In the quantum chemical modelling emphasis was put on the construction of potential energy surfaces describing the formation/dissociation of certain complex species. Effects of the ionic environment were found to play a crucial role in the energetics of model processes. The structure of the simplest possible polymeric forms involving two Al centres linked through F atoms (“dimers”) was thoroughly investigated. The calculated equilibrium constants and model Raman spectra yield additional evidence in favour of the dimers. This agrees with a self-consistent analysis of a series of Raman spectra for a wide range of the melt composition.

Conformational landscape of small organosilicon compounds from the combined use of gas electron diffraction, IR and Raman spectroscopies and quantum chemical calculations: diethyldichlorosilane

AbstractA conformational study of diethyldichlorosilane and the elucidation of the gas‐phase molecular structures of its four conformers have been performed using the combined approach of gas‐phase electron diffraction and computational techniques. Moreover, the Raman spectrum of the liquid and the IR spectra of the gas and liquid phases have been recorded and thoroughly analyzed on the basis of the scaled quantum‐mechanical force field methodology. The results of the vibrational assignment have given spectroscopic evidence of the presence of the different conformers in the samples. Copyright © 2009 John Wiley & Sons, Ltd.

Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

2,4-Bis(trimethylsilyl) substituted n-pentasilane Me3SiSiMe(SiMe3)SiMe2SiMe(SiMe3)SiMe3 (1) has been prepared from Me2SiCl2 and (SiMe3)2SiMeK, and its solid state molecular structure has been determined by single crystal X-ray diffraction. By reaction of 1 with KOtBu and subsequent treatment of the resulting silyl anion with (MeO)2SO2, the novel hexasilane Me3SiSiMe(SiMe3)SiMe2SiMe2SiMe3 (2) has been synthesized. By again using the reaction sequence KOtBu/(MeO)2SO2, known n-pentasilane Me3SiSiMe2SiMe2SiMe2SiMe3 (3) has been prepared in a novel and very convenient way from 2. All silanes 1, 2, and 3 as well as the intermediate silyl anions obtained from 2 and 3 have been characterized by 29Si and 1H NMR spectroscopy, 1 and 2 by elemental analyses also. Raman spectra in the temperature range 210–370 K of the pure liquids 1, 2 and 3 as well as of solutions in toluene (1 and 2) or cyclopentane (3) reveal that all of the silanes exist as mixtures of conformers. The symmetric SiSi stretch observed in the range 341 to 363 cm-1 and possessing by far the largest Raman intensity of all SiSi stretching vibrations splits into five bands for 3 and two bands for 1 and 3. Quantum chemical DFT B3LYP/6–311G(d) calculations located 15 conformational minima for 2 with relative energies up to 19.9 kJmol-1, and five minima for 1 with relative energies up to 15.5 kJmol-1. Calculated SiSiSiSi torsion angles vary between 26.2° (smallest value found) and 179.3°, demonstrating the flexibility of backbones composed of silicon.

Raman Spectroscopy Study and First-Principles Calculations of the Interaction between Nucleic Acid Bases and Carbon Nanotubes

In this work, we have used Raman spectroscopy and quantum chemical methods (MP2 and DFT) to study the interactions between nucleic acid bases (NABs) and single-walled carbon nanotubes (SWCNT). We found that the appearance of the interaction between the nanotubes and the NABs is accompanied by a spectral shift of the high-frequency component of the SWCNT G band in the Raman spectrum to a lower frequency region. The value of this shift varies from 0.7 to 1.3 cm(-1) for the metallic nanotubes and from 2.1 to 3.2 cm(-1) for the semiconducting nanotubes. Calculations of the interaction energies between the NABs and a fragment of the zigzag(10,0) carbon nanotube performed at the MP2/6-31++G(d,p)[NABs atoms]|6-31G(d)[nanotube atoms] level of theory while accounting for the basis set superposition error during geometry optimization allowed us to order the NABs according to the increasing interaction energy value. The order is: guanine (-67.1 kJ mol(-1)) > adenine (-59.0 kJ mol(-1)) > cytosine (-50.3 kJ mol(-1)) approximately = thymine (-50.2 kJ mol(-1)) > uracil (-44.2 kJ mol(-1)). The MP2 equilibrium structures and the interaction energies were used as reference points in the evaluation of the ability of various functionals in the DFT method to predict those structures and energies. We showed that the M05, MPWB1K, and MPW1B95 density functionals are capable of correctly predicting the SWCNT-NAB geometries but not the interaction energies, while the M05-2X functional is capable of correctly predicting both the geometries and the interaction energies.

The intramolecular charge transfer in a donor–π–acceptor dianion probed by resonance Raman spectroscopy and quantum chemical calculations

AbstractThe dideprotonation of 4‐(4‐nitrophenylazo)resorcinol generates an anionic species with substantial electronic π delocalization. As compared to the parent neutral species, the anionic first excited electronic transition, characterized as an intramolecular charge transfer (ICT) from the CO− groups to the NO2 moiety, shows a drastic red shift of ca. 200 nm in the λmax in the UV‐vis spectrum, leading to one of the lowest ICT energies observed (λmax = 630 nm in dimethyl sulfoxide (DMSO)) in this class of push‐pull molecular systems. Concomitantly, a threefold increase in the molar absorptivity (εmax) in comparison to the neutral species is observed. The resonance Raman enhancement profiles reveal that in the neutral species the chromophore involves several modes, as ν(CN), ν(NN), ν(CC) and νs(NO2), whereas in the dianion, there is a selective enhancement of the NO2 vibrational modes. The quantum chemical calculations of the electronic transitions and vibrational wavenumbers led to a consistent analysis of the enhancement patterns observed in the resonance Raman spectra. Copyright © 2009 John Wiley & Sons, Ltd.

Synthesis, Spectroscopic Characterization, and Conformational Properties of Trichloromethanesulfenyl Acetate, CCl3SOC(O)CH3

Trichloromethanesulfenyl acetate, CCl 3SOC(O)CH 3, belongs to the family of sulfenic esters. This molecule has been characterized by vibrational spectroscopy. The conformational and geometrical properties of this species have been determined by IR and Raman spectroscopy, X-ray diffraction, and quantum chemical calculations. Geometry optimizations of the most stable forms were performed with ab initio (HF, MP2) and density functional theory (B3LYP) methods. According to our data, this compound results in a gauche-syn conformer with C 1 symmetry (gauche orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the most stable geometry, and trans-syn conformer with C s symmetry (trans orientation around the S-O bond and syn orientation of the CO double bond with respect to the S-O single bond) for the second stable conformer (1.1 and 0.53 kcal/mol higher in energy than the most stable C 1 form according to the matrix FTIR spectroscopy and MP2/6-31G* level of the theory, respectively). The crystalline solid (monoclinic, P2 1/ n, a = 8.0152(17) A, b = 5.7922(13) A, c = 17.429(4) A, alpha = gamma = 90 degrees , beta = 100.341(3) degrees ) consists exclusively of the main form. The geometrical parameters (X-ray diffraction) are d C-Cl = 1.767(19) A, d C-S = 1.797(2) A, d S-O = 1.663(14) A, d CO = 1.189(2) A, d O-C = 1.389(3) A, d C-C = 1.483(3) A, angles Cl-C-Cl = 110.3(11) degrees , Cl-C-S = 111.8(12) degrees , C-S-O = 97.4(8) degrees , S-O-C = 116.7(11) degrees , O-CO = 122.8(19) degrees , OC-C = 127.1(2) degrees , and the main torsion angles are delta(CSOC) = 105.9(15) degrees and delta(SOC(O)) = 7.6(3) degrees . The geometrical data calculated with B3LYP/6-31G++(3df,3pd), B3LYP/6-311G++(3df,3pd), B3LYP/aug-cc-pVTZ, and MP2/6-31G* are in good agreement with diffraction data.

Homogeneous Decomposition Mechanisms of Diethylzinc by Raman Spectroscopy and Quantum Chemical Calculations

The gas-phase decomposition pathways of diethylzinc (DEZn), a common precursor for deposition of Zn-VI compounds, were investigated in detail. The homogeneous thermal decomposition of DEZn in N2 carrier was followed in an impinging-jet, up-flow reactor by Raman scattering. Density Functional Theory calculations were performed to describe the bond dissociation behavior using the model chemistry B3LYP/6-311G(d) to estimate optimal geometries and Raman active vibrational frequencies of DEZn, as well as anticipated intermediates and products. Comparison of the measured DEZn decomposition profile to that predicted by a 2-D hydrodynamic simulation revealed that simple bond dissociation between zinc and carbon atoms is the dominant homogeneous thermal decomposition pathway. The calculations suggest several reactions involving intermediates and Raman scattering experiments confirming the formation of the dimer (ZnC2H5)2. In a different set of experiments, photolysis of DEZn gave evidence for decomposition by beta-hydride elimination. The results suggest that beta-hydride elimination is a minor pathway for the gas-phase homogeneous pyrolysis of diethylzinc. A reasonable transition state during beta-hydride elimination was identified, and the calculated energies and thermodynamic properties support the likelihood of these reaction steps.

FT-IR and Raman spectroscopic and quantum chemical studies of zinc halide complexes with 2,2′-biquinoline

The molecular structure, vibrational frequencies and the corresponding vibrational assignment of Zn(biq)X 2 (X = Cl and Br; biq = 2,2′-biquinoline) have been studied by employing the hybrid density functional theory (B3LYP) method and the complete basis set (DFT) method together with the 6-31G(d,p) basis set for X = Cl and Br. The FT-IR (400–4000 cm −1) and Raman (100–3200 cm −1) spectra of compounds were recorded and compared with that of the calculated spectra, which allowed authors to assign most of the observed bands. It was demonstrated that cis conformer is suitable for the Zn(biq)X 2 compounds. The fundamental vibrational modes were characterized by their total energy distribution. The coordination effects on vibrational wavenumbers of biq were discussed in detail.

The electronic delocalization in para‐substituted β‐nitrostyrenes probed by resonance Raman spectroscopy and quantum‐chemical calculations

AbstractThe electronic (UV‐vis) and resonance Raman (RR) spectra of a series of para‐substituted trans‐β‐nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO2, COOH, Cl, OCH3, OH, N(CH3)2, and O−) on the extent of the charge transfer to the electron‐withdrawing NO2 group directly linked to the ethylenic (C = C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the νs(NO2) and ν (C = C)sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH3)2 that the symmetric stretching of the NO2, νs(NO2), is the most substantially enhanced mode, whereas for X = O−, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd.

Vibrational spectra and theoretical calculations of [(trifluoromethyl)sulfonyl] phosphorimidic trichloride (CF 3SO 2N [dbnd]PCl 3)

Vibrational, structural and configurational properties of [(trifluoromethyl)sulfonyl]phosphorimidic trichloride (CF 3SO 2N PCl 3) have been studied by FT-IR and Raman spectroscopy (liquid phase) and quantum chemical calculations using the 6-311++G(d) basis set with MP2, HF and B3LYP approximations. CF 3SO 2N PCl 3 exists as a single conformer belonging to the C 1 symmetry group. The assignment of the vibrational fundamental modes was rationalized in terms of the comparison with reported data for similar molecules and quantum chemical calculations at different levels of approximation. Stabilization energy values for the main orbital interactions derived from a natural bond orbital analysis (NBO) are presented.