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.     

Solvent effects on the proton magnetic resonance spectra of thio- and dithio-oxamides

The chemical shifts of the N-methyl signals for a number of thio-oxamide and oxamide derivatives are unambiguously determined in the solvents chloroform, carbon tetrachloride, benzene, chlorobenzene, o-dichlorobenzene and nitrobenzene. The ASIS effect is discussed. The temperature dependence of the absolute shift and of the relative shift differences of both methyl groups of the N,N-dimethyl derivatives are studied.     

Solvent effects on the proton magnetic resonance spectra of p-substituted phenyltin chlorides

A PMR study of solvent effects on some p-substituted phenyltin chlorides (substituents = CH₃, (CH₃)₃C, CH₃O) is reported. Co-ordination of solvent molecules to the tin atom leads to an unusual low field shift of the o-ring protons. The results for p-tolytin trichloride have been compared with those for p-tolylsilyl trichloride and discussed. In o-, m- and p-tolyltin trichlorides long range spin-spin couplings between the ring methyl protons and the tin atom have been observed. The order of magnitude is o- > p- > m-methyl protons.     

Solvent effects on Raman optical activity spectra calculated using the polarizable continuum model

The integral equation formulation of the polarizable continuum model (IEFPCM) has been extended to the calculation of solvent effects on vibrational Raman optical activity spectra. Gauge-origin independence of the differential scattering intensities of right and left circularly polarized light is ensured through the use of London atomic orbitals. Density functional theory (DFT) calculations have been carried out for bromochlorofluoromethane, methyloxirane, and epichlorhydrin. The results indicate that solvent effects on the ROA differential scattering intensities can be substantial, and vary in sign and magnitude for different vibrational modes. It is demonstrated that both direct and indirect effects are important in determining the total solvent effects on the ROA differential scattering intensities. Local field effects are shown to be in general small, whereas electronic nonequilibrium solvation has a profound effect on the calculated solvent effects compared to an equilibrium solvation model. For molecules with several conformations, the changes in the relative stability of the different conformers also lead to noticeable changes in the ROA spectra.     

Experimental and theoretical investigation of the Raman and hyper-Raman spectra of acetonitrile and its derivatives

The Raman and hyper-Raman spectra of acetonitrile and its deuterated analog have been investigated by combining experimental analysis and theoretical interpretation. It has been observed that the Raman spectra can easily be reproduced at both the Hartree-Fock and Moller-Plesset second-order levels of approximation and that for these fundamental transitions, inclusion of anharmonicity effects is not essential. On the other hand, the hyper-Raman spectra are more difficult to simulate and interpret. In particular, electron correlation has to be included in order to describe properly the intensity of the CN stretching mode. Then, a pseudo-C(infinity v) symmetry was assumed to better fit the experimental observations. This accounts for the fact that the a1- and e-symmetry modes correspond to time-decoupled vibrations. The e-symmetry modes, associated with nuclear motions perpendicular to the molecular axis are indeed subject to relaxation processes and, except the CCN bending mode, not visible in the hyper-Raman spectra of acetonitrile or of its deuterated analog. This assumption is supported by the gradual decrease of the phenomenon when going from acetonitrile to trichloroacetonitrile, where the presence of the heavier chlorine atoms in the latter reduces the relaxation processes.     

Raman,resonance Raman and infrared spectra of potassium uranyl croconate

The Raman, resonance Raman and IR spectra of potassium uranyl croconate, UO₂(H₂O)K₂(C₅O₅)₂ were obtained and interpreted. Several croconate modes are split indicating a substantial decrease in the oxocarbon symmetry, as is to be expected from a recent crystallographic investigation, revealing the coordination of the oxocarbon to be two non-equivalent UO²⁺₂ moieties in a monodentate fashion. In terms of vibrational frequency shifts it can be concluded that the UO²⁺₂ moiety behaves as an isolated oscillator.The resonance Raman results suggest that the strong band centered around 450 nm in the UV—vis spectrum should be assigned to a charge transfer transition from the oxocarbon to the uranyl ion. In fact, as resonance is approached, both uranyl and croconate modes are enhanced. It can also be inferred that the chromophore is rather delocalized into the oxocarbon ring, rather than localized in the carbonyl groups as previously observed for other croconate complexes.     

Substituent effects on the resonance Raman spectra of bis(dithiobenzil)nickel

The influence of substituents on the resonance Raman spectra of bis(p-substituted dithiobenzil)nickels has been examined. The assigned sulfur—nickel stretching vibrations in the complexes appeared in the range 390–410 cm⁻¹ with a shift to higher frequency being observed for the electron-donating substituent. It was found that Raman intensities at vibrations of the benzene ring for ligands excited with a 457.9 nm laser line are about 1.5–3.0 times larger than with a 514.5 nm laser line. The assignments of electronic transitions in the visible region of the nickel complexes were made on the basis of observed resonance Raman intensity patterns.     

Solvent effects in nuclear magnetic resonance spectra of some pyrazines,pyrimidines and their N-oxides

The benzene-induced solvent effects upon the proton chemical shifts of various pyrazines, pyrimidines and their N-oxides are described. Larger chemical shift effects, implying closer benzeneheterocycle association, are noted in the N-oxides as compared to the non-oxidized heterocycles. The solvent-induced chemical shift changes can be used to establish the site of N-oxidation in those instances where different isomers can be formed.     

Solvent effects on the NMR spectra of heterocyclics

The NMR spectra of pyridine, pyrazine and imidazole are examined in acetone-d₆. The differential shifts of the ring protons are accounted for in terms of an interaction between the lone pair on the ring nitrogen atom and the carbonyl group of acetone.     

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.     

Pre-processing of ultraviolet resonance Raman spectra

The application of UV excitation sources coupled with resonance Raman have the potential to offer information unavailable with the current inventory of commonly used structural techniques including X-ray, NMR and IR analysis. However, for ultraviolet resonance Raman (UVRR) spectroscopy to become a mainstream method for the determination of protein secondary structure content and monitoring protein dynamics, the application of multivariate data analysis methodologies must be made routine. Typically, the application of higher order data analysis methods requires robust pre-processing methods in order to standardize the data arrays. The application of such methods can be problematic in UVRR datasets due to spectral shifts arising from day-to-day fluctuations in the instrument response. Additionally, the non-linear increases in spectral resolution in wavenumbers (increasing spectral data points for the same spectral region) that results from increasing excitation wavelengths can make the alignment of multi-excitation datasets problematic. Last, a uniform and standardized methodology for the subtraction of the water band has also been a systematic issue for multivariate data analysis as the water band overlaps the amide I mode. Here we present a two-pronged preprocessing approach using correlation optimized warping (COW) to alleviate spectra-to-spectra and day-to-day alignment errors coupled with a method whereby the relative intensity of the water band is determined through a least-squares determination of the signal intensity between 1750 and 1900 cm(-1) to make complex multi-excitation datasets more homogeneous and usable with multivariate analysis methods.     

On mode mixing effects in absorption-emission spectra and resonance Raman excitation profiles

The Dushinsky effect is studied in absorption and fluorescence spectra and in resonance Raman excitation profiles of totally-symmetric fundamentals, overtones and combination bands. It is demonstrated that even for strong mode mixing the absorption or emission spectrum of a strongly allowed electronic transition can be analyzed in terms of displaced harmonic oscillators, but in that case the displacement parameters for the two spectra will be quite different. If no emission spectrum can be obtained, Raman excitation profiles of combination bands provide a sensitive probe of mode mixing.     

Environmental and excitonic effects on the first hyperpolarizability of polar molecules and related dimers

We investigate the second-order nonlinear optical properties of a push-pull chromophore in different external and supramolecular environments, through a combined experimental and theoretical approach. In particular, we compare the first hyperpolarizability (beta) of a model dipolar and polarizable chromophore with that of a charged analogue and of a molecular dimer based on the chromophore itself. We find that the beta value of the model chromophore in solutions of low-polarity solvents is strongly affected by association effects, already at concentrations of 10 (-3) M. The presence of a positive charge in close proximity to the chromophore is found to lead to a 100% increase of the beta response of the model push-pull chromophore. This effect is of major importance for biological applications, in particular when chromophores are used as markers in charged anisotropic environments. Finally, excitonic effects, beyond the Frenkel exciton approximation, are discussed for the dimer and found to be more important the higher the order of nonlinearity is.     

Solvent effects on the absorption spectra of merocyanine dyes

Solvent effects on the lowest-energy electronic transitions of two merocyanines derived from quinoline have been studied using nine different solvent parameters reported in the literature. Relative merits and deficiencies of different parameters have been assessed. From the correlation results, attempts have been made to propose a solute-solvent interaction mechanism. For an assorted set of 19 solvents, as well as for a set of nine hydroxylic solvents, excellent results are obtained with correlation equation involving E_T(30) and Hilderbrand's solubility parameter _H.     

Resonance Raman and resonance hyper-Raman intensities: structure and dynamics of molecular excited states in solution

Resonance Raman scattering is discussed as a vibronic spectroscopy that can provide detailed information about the structure and dynamics of excited electronic states of molecules. The emphasis is on molecules in liquid solution. The theory of resonance Raman intensities and experimental and interpretive methods are discussed both in a historical context and in their present and future implementations. The related but much less developed technique of two-photon-resonant hyper-Raman scattering is also discussed in a similar context.     

Electronic and vibrational resonance Raman spectra of octamethyluranocene

The Raman spectrum of bis(tetramethylcyclo-octatetraene)uranium (U(TMCOT)₂), excited in resonance with its visible charge-transfer transitions shows an anomalously polarized electronic band at 473 cm^?1, twice as broad as the analogous band of uranocene, at 466 cm^?1. The broadening is attributed to crystal-field splitting associated with the lowered symmetry introduced by the methyl group, and/or a distribution of rotamer populations. Totally symmetric vibrational modes are observed at 879, 750, 580, 513 and 95 cm^?1; possible assignments are discussed.     

Pure surface plasmon resonance enhancement of the first hyperpolarizability of gold core-silver shell nanoparticles

We report the optical second harmonic (SH) response from gold core-silver shell nanoparticles supported at a liquid-liquid interface in the spectral region where the second harmonic generation (SHG) frequency is resonant with the surface plasmon (SP) resonance excitation of the nanoparticles. We compare these results with that obtained by classical linear optical absorption spectroscopy and show that the nonlinear optical response is dominated by the SP resonance enhancement with negligible contributions from the interband transitions. As a result, the SH spectrum exhibits two clear SP resonance bands attributed to the two SP resonances of the composite nanostructure formed by the gold core-silver shell nanoparticles. Absolute values of the hyperpolarizabilities are measured by hyper Rayleigh scattering (HRS) and compared that of pure gold nanoparticles. The hyperpolarizability measured at a harmonic energy of 3.0 eV, enhanced through excitation of the high energy SP resonance of the nanoparticle, increases with the silver content whereas the hyperpolarizability measured at a harmonic energy of 2.4 eV, enhanced through the excitation of the low energy SP resonance of the nanoparticle, decreases because of the shift of this resonance away from the harmonic frequency. The hyperpolarizability determined by HRS and the square root of the SHG intensities, scaling with the nanoparticle hyperpolarizability, have similar trends with respect to the silver content indicative of closely related adsorption properties yielding similar surface concentrations at the liquid-liquid interface.     

Solvent effects on the infrared spectra of thionyl and seleninyl chlorides

Infrared spectra of thionyl and seleninyl chlorides are reported and discussed. Complete assignment for both molecules is given on the basis of C_s symmetry. Solvent effects on XO bond (X = S, Se) have been studied in comparison with CO bond for carbonyl compounds and by means of the Kirkwood, Bauer and Magat relationship. Acceptor properties of oxychlorides for pyridinic adducts are shown.     

Theoretical studies on absorption, emission, and resonance Raman spectra of Coumarin 343 isomers

The vibrationally resolved spectral method and quantum chemical calculations are employed to reveal the structural and spectral properties of Coumarin 343 (C343), an ideal candidate for organic dye photosensitizers, in vacuum and solution. The results manifest that the ground-state energies are dominantly determined by different placements of hydrogen atom in carboxylic group of C343 conformations. Compared to those in vacuum, the electronic absorption spectra in methanol solvent show a hyperchromic property together with the redshift and blueshift for the neutral C343 isomers and their deprotonated anions, respectively. From the absorption, emission, and resonance Raman spectra, it is found that the maximal absorption and emission come from low-frequency modes whereas the high-frequency modes have high Raman activities. The detailed spectra are further analyzed for the identification of the conformers and understanding the potential charge transfer mechanism in their photovoltaic applications.