Solvent effects on resonant first hyperpolarizabilities and Raman and hyper-Raman spectra of DANS and a water-soluble analog

The two-photon-resonant first hyperpolarizabilities associated with hyper-Rayleigh and hyper-Raman scattering are reported for 4-dimethylamino-4-nitrostilbene in 1,4-dioxane, dichloromethane, acetonitrile, and methanol, and for an ionic analog, 4-N,N-bis(6-(N,N,N-trimethylammonium)-hexyl)amino-4-nitrostilbene dibromide in methanol and water. Resonance Raman and hyper-Raman excitation profiles are also measured and modeled. The resonance Raman and hyper-Raman spectra show very similar relative intensities which do not vary much as the excitation frequency is tuned across the lowest-energy strong linear absorption band, suggesting that a single resonant electronic state dominates the one- and two-photon absorptions in this region. The absorption, resonance Raman, and hyper-Raman profiles can be simulated reasonably well with a common set of parameters. The peak resonant (absolute value of beta)2, measured by hyper-Rayleigh scattering, varies by about 50% over the range of solvents examined and shows a weak correlation with the linear absorption maximum, with the redder-absorbing systems exhibiting larger peak hyperpolarizabilities. The experimental hyper-Rayleigh intensities are higher than those calculated, possibly reflecting contributions from nonresonant electronic states.     

Resonance hyper-Raman scattering from conjugated organic donor-acceptor "push-pull" chromophores with large first hyperpolarizabilities

Resonance hyper-Raman spectra have been obtained using 1064 nm excitation for several electron donor-acceptor-substituted, pi-conjugated "push-pull" molecules that have large second harmonic hyperpolarizabilities. The hyper-Raman spectra are nearly identical to the resonance Raman spectra measured with 532 nm excitation. This indicates that both the second harmonic hyperpolarizability and the linear absorption are dominated by the same, single electronic transition that is both one- and two-photon allowed. Comparison of resonance Raman and resonance hyper-Raman spectra is proposed as an experimental test of the common two-electronic-state model for the first hyperpolarizability.     

Solvent effects on the resonance Raman and hyper-Raman spectra and first hyperpolarizability of N,N-dipropyl-p-nitroaniline

Linear absorption spectra, resonance Raman spectra and excitation profiles, and two-photon-resonant hyper-Rayleigh and hyper-Raman scattering hyperpolarizability profiles are reported for the push-pull chromophore N,N-dipropyl-p-nitroaniline in seven solvents spanning a wide range of polarities. The absorption spectral maximum red shifts by about 2700 cm(-1), and the symmetric -NO2 stretch shifts to lower frequencies by about 11 cm(-1) from hexane to acetonitrile, indicative of significant solvent effects on both the ground and excited electronic states. The intensity patterns in the resonance Raman and hyper-Raman spectra are similar and show only a small solvent dependence except in acetonitrile, where both the Raman and hyper-Raman intensities are considerably reduced. Quantitative modeling of all four spectroscopic observables in all seven solvents reveals that the origin of this effect is an increased solvent-induced homogeneous broadening in acetonitrile. The linear absorption oscillator strength is nearly solvent-independent, and the peak resonant hyperpolarizability, beta(-2omega;omega,omega), varies by only about 15% across the wide range of solvents examined. These results suggest that the resonant two-photon absorption cross sections in this chromophore should exhibit only a weak solvent dependence.     

Contributions of the 8-methyl group to the vibrational normal modes of flavin mononucleotide and its 5-methyl semiquinone radical

Resonance Raman spectroscopy is a powerful tool to investigate flavins and flavoproteins, and a good understanding of the flavin vibrational normal modes is essential for the interpretation of the Raman spectra. Isotopic labeling is the most effective tool for the assignment of vibrational normal modes, but such studies have been limited to labeling of rings II and III of the flavin isoalloxazine ring. In this paper, we report the resonance and pre-resonance Raman spectra of flavin mononucleotide (FMN) and its N5-methyl neutral radical semiquinone (5-CH 3FMN(*)), of which the 8-methyl group of ring I has been deuterated. The experiments indicate that the Raman bands in the low-frequency region are the most sensitive to 8-methyl deuteration. Density functional theory (DFT) calculations have been performed on lumiflavin to predict the isotope shifts, which are used to assign the calculated normal modes to the Raman bands of FMN. A first assignment of the low-frequency Raman bands on the basis of isotope shifts is proposed. Partial deuteration of the 8-methyl group reveals that the changes in the Raman spectra do not always occur gradually. These observations are reproduced by the DFT calculations, which provide detailed insight into the underlying modifications of the normal modes that are responsible for the changes in the Raman spectra. Two types of isotopic shift patterns are observed: either the frequency of the normal mode but not its composition changes or the composition of the normal mode changes, which then appears at a new frequency. The DFT calculations also reveal that the effect of H/D-exchange in the 8-methyl group on the composition of the vibrational normal modes is affected by the position of the exchanged hydrogen, i.e., whether it is in or out of the isoalloxazine plane.     

Resonance hyper-Raman spectra of zinc phthalocyanine

Hyper-Raman spectra were obtained for zinc phthalocyanine in a dilute pyridine solution at excitation wavelengths that are two-photon resonant with the one-photon-allowed B band (360-380 nm) as well as with the two-photon absorption near 440 nm reported by Drobizhev et al. ( J. Chem. Phys. 2006, 124, 224701 ). In both regions, the hyper-Raman spectra were very different from the linear resonance Raman spectra at the corresponding excitation frequencies. While the resonance Raman spectra show only g symmetry modes, almost all of the hyper-Raman frequencies can be assigned as fundamentals of E u symmetry that also are observed in the infrared absorption spectrum or E u symmetry combination bands. These results contrast sharply with previous observations of highly noncentrosymmetric push-pull conjugated molecules and are consistent with a structure for phthalocyanine in solution that is centrosymmetric or nearly so. The hyper-Raman spectra show different intensity patterns in the two excitation regions, consistent with different Franck-Condon and/or vibronic coupling matrix elements for the different resonant states.     

Molecular structure and vibrational raman and infrared spectra of bromochlorofluoromethane and its silicon and germanium analogs: quantum-mechanical DFT and MP2 calculations

DFT(B3LYP) and MP2 calculations with the 6-311G(2d, 2p)-type basis set have been carried out for the prediction of molecular parameters (bond distances, bond angles, rotational constants, and dipole moments) and vibrational Raman and infrared spectra (harmonic wavenumbers, absolute intensities, Raman scattering activities, and depolarization ratios) of bromochlorofluoromethane (HCBrCIF) and its silicon and germanium analogs (HSiBrClF and HGeBrCIF). The predicted geometry and vibrational Raman and infrared spectra of HCBrClF agree well with the available experimental data for this molecule and their deuterated derivatives. This agreement allows one to believe that the predicted molecular parameters and vibrational spectra of HSiBrClF, HGeBrClF, and their deuterated derivatives will guide their future experimental studies.     

Resonance hyper-Raman excitation profiles and two-photon states of a donor-acceptor substituted polyene

Resonance Raman and resonance hyper-Raman spectra and excitation profiles have been measured for a "push-pull" donor-acceptor substituted conjugated polyene bearing a julolidine donor group and a nitrophenyl acceptor group, in acetone at excitation wavelengths from 485 to 356 nm (two-photon wavelengths for the nonlinear spectra). These wavelengths span the strong visible to near-UV linear absorption spectrum, which appears to involve at least three different electronic transitions. The relative intensities of different vibrational bands vary considerably across the excitation spectrum, with the hyper-Raman spectra showing greater variation than the linear Raman. A previously derived theory of resonance hyper-Raman intensities is modified to include contributions from purely vibrational levels of the ground electronic state as intermediate states in the two-photon absorption process. These contributions are found to have only a slight effect on the hyper-Rayleigh intensities and profiles, but they significantly influence some of the hyper-Raman profiles. The absorption spectrum and the Raman, hyper-Rayleigh, and hyper-Raman excitation profiles are quantitatively simulated under the assumption that three excited electronic states contribute to the one- and two-photon absorption in this region. The transition centered near 400 nm is largely localized on the nitrophenyl group, while the transitions near 475 and 355 nm are more delocalized.     

Vibrational spectroscopic studies and DFT calculations on tribromoacetate and tribromoacetic acid in aqueous solution

Aqueous solutions of sodium tribromoacetate (NaCBr3CO2) and its corresponding acid (CBr3COOH) have been studied using Raman and infrared spectroscopy. The spectra of the species in solution were assigned according to symmetry Cs. Characteristic bands of CBr3CO2-(aq) and the tribromoacetic acid, CBr3COOH(aq), are discussed. For the hydrated anion, the CO2 group, the symmetric CO2 stretching mode at 1332 cm(-1) and the asymmetric stretching mode at 1651 cm(-1) are characteristic while the CO mode at 1730 cm(-1) is characteristic for the spectra of the acid. The stretching mode, νC-C at 912cm(-1) for CBr3CO2-(aq) is 10 cm(-1) lower in the anion compared with that of the acid. These characteristic modes are compared to those in acetate, CH3CO2-(aq). Coupling of the modes are fairly extensive and therefore DFT calculations have been carried out in order to compare the measured spectra with the calculated ones. The geometrical parameters such as bond length and bond angles of the tribromoacetate, and tribromoacetic acid have been obtained and may be compared with the ones published for other acetates and their conjugated acids. CBr3COOH(aq) is a moderately strong acid and the pKa value derived from quantitative Raman measurements is equal to -0.23 at 23°C. The deuterated acid CBr3COOD in heavy water has been measured as well and the assignments were given.     

Resonance Raman observation of the allyl cation produced after ultraviolet photodissociation of cyclopropyl bromide in acetonitrile solution

We have obtained transient resonance Raman spectra of the [CH₂CHCH₂]⁺ (allyl cation) produced following C-band excitation of cyclopropyl bromide. The experimental resonance Raman spectrum display an overtone progression in the nominal [CCC]⁺ stretch mode and its combination bands with the CH/CH₂ rocking modes. Density functional theory computations were performed to estimate the vibrational frequencies for the allyl cation, the allyl radical, the cyclopropyl radical, the cyclopropyl bromide molecule and the gauche-allyl bromide molecule and compared to the experimental vibrational frequencies. This comparison indicates that the allyl cation can be formed as a product of cyclopropyl bromide photodissociation in acetonitrile solution.     

The vibrational spectra and conformations of ethylbenzene

Infrared spectra (4000-250 cm-1) of the liquid, amorphous, crystalline solids and solutions in liquid krypton and Raman spectra (2500-20 cm-1) of the liquid as well as the amorphous and crystalline solids of ethylbenzene and its deuterated analogue-ethylbenzene-d(10) have been recorded. The spectra indicate that in the liquid and amorphous solids a small amount of a second conformer is present, whereas only one conformer remains in the crystalline phases. Assignments of the observed band wave numbers are discussed by comparison with normal mode wave numbers and IR and RS intensities calculated from ab initio 6-31G force fields and optimised geometries for both conformers for two species. All of the normal modes of conformers have been assigned.     

Normal coordinate analysis of the indole ring

Normal coordinate calculations have been performed for indole and 11 deuterated indoles. The set of valence force constants obtained is able to reproduce i.r. and Raman frequencies with an average error of about 6 cm⁻¹. Extension of the force field to skatole has also been made and the calculated frequency shifts on isotopic substitutions are compared with the experimental ones of tryptophan. The general agreement between the calculation and experiment suggests that the vibrational modes obtained here are of practical use in the interpretation of Raman spectra of proteins containing tryptophan residues.     

Vibrational analysis of the imidazolate ring

IR and Raman spectra have been investigated for imidazolate and 4-methylimidazolate including five and three deuterated analogs, respectively. Assignment of the observed IR and Raman bands has been made on the basis of isotopic frequency shifts, Raman polarization properties, and normal coordinate calculations. The calculated normal frequencies are in good agreement with experimental ones: the average error below 1600 cm⁻¹ is 4.5 cm⁻¹ for 104 in-plane vibrations and 3.8 cm⁻¹ for 43 out-of-plane vibrations. The calculated vibrational modes are useful in analyzing the Raman bands of histidine residues in proteins.     

Vibrational spectroscopic analysis of lithium hydrazinium sulfate

The low temperature polarized Raman spectra of lithium hydrazinium sulfate, LiN₂H₅SO₄, have been measured in the range 5–3500 cm⁻¹. The symmetry-based assignments for the observed modes are given. The room temperature polarized i.r. reflectivity spectra in the range 200–4000 cm⁻¹ were also measured to verify the presence of extensive TO—LO splitting in the sulfate ion ν₃ and ν₄ internal optic modes. In the lattice mode region, the lithium ion external optic modes are identified, and the spectra of the deuterated analog allow the identification of several modes due primarily to the external motions of either the sulfate or hydrazinium ions. Other lattice modes are found to be a mixture of sulfate and hydrazinium ion external motions. Coupling of the sulfate ion ν₃ modes and the hydrazinium ion group bending modes prevents an unambiguous assignment of the bands in the 1050–1200 cm⁻¹ spectral region. The N—H stretching region spectra contain a larger number of modes than predicted by a factor group analysis. These extra modes are discussed in terms of resonance interactions of the N—H stretching fundamental modes with overtones and combination modes.     

Vibrational spectra and structures of urazole and 4-methylurazole: DFT calculations of the normal modes in aqueous solution and in the solid state,and the influence of hydrogen bonding

Solid-state IR and Raman as well as aqueous solution state Raman spectra are reported for urazole, 4-methylurazole and their deuterated derivatives. DFT calculations, at the B3-LYP/cc-pVTZ level, established that the structures and vibrational spectra of the molecules can be interpreted using models with hydrogen-bonded water molecules, in conjunction with the polarizable continuum solvation method. The vibrational spectra were computed at the optimised molecular geometry in each case, enabling normal coordinate analysis, which yielded satisfactory agreement with the experimental IR and Raman data. Computed potential energy distributions of the normal modes provided detailed vibrational assignments. Solid-state pseudopotential-plane-wave DFT calculations, using the PW91 functional were also carried out, reflecting the importance of intermolecular hydrogen bonding in the solid state.     

Polarization effects on the hyper-Raman spectra of carbon tetrachloride: a joint experimental-theoretical study

Hyper-Raman spectra of pure carbon tetrachloride in the liquid phase are recorded for different combinations of the polarizations of the incident and scattered lights and are compared to ab initio time-dependent Hartree-Fock simulations. Both the calculated intensities of the Raman and hyper-Raman spectra give indeed a quite satisfactory agreement with polarized experimental spectra.     

Spectra and structure of phosphorus—boron compounds—XXVI. Infrared and Raman spectra,conformational stability and normal coordinate analysis of ethyldichlorophosphine-borane

The Raman (3500-10 cm⁻¹) and infrared (3500-50 cm⁻¹) spectra of solid ethyldichlorophosphine-borane, CH₃CH₂P(BH₃)Cl₂ and its deuterated analog, CH₃CH₂P(BD₃)Cl₂ have been recorded. Additionally, the infrared spectra of the gases and the Raman spectra of the liquids have been recorded and qualitative depolarization ratios have been obtained. Based on the fact that several distinct Raman lines disappear on going from the liquid to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers, with the trans conformer being more stable in the liquid phase, and the only one present in the solid phase. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be nearly zero. Based on Raman depolarization data, group frequencies, isotopic shift factors and infrared band contours, a complete vibrational assignment has been proposed for the trans conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The BH₃ torsion has been observed at 188 cm⁻¹, while the BD₃ torsion was not observed. The methyl torsions in the spectra of the solids have been observed at 209 and 202 cm⁻¹ for the “light” and deuterated species, respectively. From the torsional data, barriers to internal rotation have been calculated. The asymmetric torsional mode has been observed for the trans conformer in the infrared spectra of the gas phase at 108 and 104 cm⁻¹ for the BH₃ and BD₃ species, respectively. These results are compared with similar quantities for some corresponding organophosphine—borane compounds.     

Semi-empirical and vibrational studies of flavone and some deuterated analogues

The infrared solid state, Raman solid state and tetrachloride solution spectra of flavone have been obtained. Assignments of most of the vibrational data have been performed by comparison between the spectra of flavone and three isotopic species, deuterated on the A, B and C rings, respectively. The vibrational frequencies for all the investigated compounds have been calculated from the conformational analysis of flavone using the semi-empirical AM1 method and compared with experimental values. The correlation is more or less satisfactory; however, for some vibrational modes, the calculated isotopic shifts agree better with experiment than do the frequencies themselves. Specific vibrational modes which retain a benzene ring mono-substituted and ortho-distributed character have been recognized in the spectra, according to literature data, isotopic frequency shifts and graphic representation of the atomic displacements.     

Fourier transform infrared and Raman spectra, vibrational assignment and density functional theory calculations of naphthazarin

FT Raman and FTIR spectra of Naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) and its deuterated analogue are recorded. Comparison between the spectra obtained by two techniques, a series of density functional theory (DFT) calculations and the spectral behavior upon deuteration were used for the assignment of the vibrational spectra of this compound. The calculated vibrational frequencies by the B3LYP, B3PW91, G96LYP, G96P86, and MPWLYP density functionals are generally consistent with the observed spectra. Infrared and Raman vibrational transitions predicted by B3LYP/6-311++G** are reported for the titled compound and its deuterated analogous and the assignments are discussed. All experimental and theoretical results support a relatively weak hydrogen bond in naphthazarin (NZ), compared with that in the enol form of normal beta-diketones. The observed nuOH/nuOD and gammaOH/gammaOD appear at about 3060/2220 and 790/560 cm(-1), respectively, which are consistent with the calculated hydrogen bond geometry and proton chemical shift results. Two bands at about 350 and 290 cm(-1) are assigned to the O...O stretching modes belong to A1 and B2 species, respectively.     

Tunable resonance hyper-Raman spectroscopy of second-order nonlinear optical chromophores

Two-photon-resonant hyper-Raman spectra are reported for three "push-pull" conjugated organic chromophores bearing -NO(2) acceptor groups, two dipolar and one octupolar. The excitation source is an unamplified picosecond mode-locked Ti:sapphire laser tunable from 720 to 950 nm. The linear resonance Raman spectra of the same molecules are measured using excitation from the laser second harmonic. Excitation on resonance with the lowest-lying band in the linear absorption spectrum yields nearly identical resonance Raman and resonance hyper-Raman spectra. However, excitation into a region that appears to contain more than one electronic transition gives rise to different intensity patterns in the linear and nonlinear spectra, indicating that different transitions contribute differently to the one-photon and two-photon oscillator strength. The promise of the hyper-Raman technique for examining electronic transitions that are both one- and two-photon allowed is discussed.