Sum-frequency vibrational spectroscopy of chiral liquids off and close to electronic resonance and the antisymmetric Raman tensor

The strength of the chiral vibrational peaks in infrared-visible sum-frequency (SF) vibrational spectra from isotropic chiral liquids is proportional to the square of the corresponding antisymmetric Raman element. Under the Born-Oppenheimer adiabatic approximation with nonadiabatic corrections, the antisymmetric Raman tensor is much weaker than the symmetric counterpart, but becomes significantly stronger as the input frequency (or the sum-frequency in SF generation) approaches electronic resonance. We verify the theory with experimental results obtained from infrared-visible doubly resonant sum-frequency generation from an isotropic solution of chiral molecules.     

Theoretical investigation of doubly resonant IR-UV sum-frequency vibrational spectroscopy of binaphthol chiral solution

The doubly resonant IR-UV sum-frequency vibrational spectroscopy (SFVS) of 1,1'-bi-2-naphthol (BN) solution and its dispersion spectra are analyzed and computed using the ZINDO//AM1 calculation and the direct approach of Raman scattering tensor calculation, which is based on calculations of Franck-Condon factors and on differentiation of the electronic transition moments with respect to the vibrational normal modes. The calculated results indicate that, for the most intense vibrational bands observed in the SFVS experiment, the calculated frequencies, symmetry, order, intensities, and pattern of the enhanced vibrational modes agree with experiment qualitatively, and due to the Franck-Condon progression, there are the doublet peaks in the corresponding resonant sum-frequency dispersion spectra. The polarization resonance Raman spectra of BN for the vibrational modes appearing in SFVS are also computed and associated with the experiment SFVS of BN. This direct evaluation approach of Raman tensors may provide a way of assigning the doubly resonant IR-UV SFVS.     

A new ultrafast technique for measuring the terahertz dynamics of chiral molecules: the theory of optical heterodyne-detected Raman-induced Kerr optical activity

Optical heterodyne-detected Raman-induced Kerr optical activity (OHD-RIKOA) is a nonresonant ultrafast chiroptical technique for measuring the terahertz-frequency Raman spectrum of chirally active modes in liquids, solutions, and glasses of chiral molecules. OHD-RIKOA has the potential to provide much more information on the structure of molecules and the symmetries of librational and vibrational modes than the well-known nonchirally sensitive technique optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). The theory of OHD-RIKOA is presented and possible practical ways of performing the experiments are analyzed.     

FT-Raman spectroscopy of biological molecules

Raman spectroscopy of biological molecules is often very difficult if not impossible due to a large fluorescence background from absorbing species, either from the molecule itself or an impurity. Photobleaching is occasionally successful in photochemically removing fluorescent impurities, but the majority of samples are not responsive to such treatment. Resonance enhancement of an absorbing species allows acquisition of Raman spectra in spite of competing fluorescence. However, the resonance Raman spectrum is characteristic of the chromophore only and little structural information is obtained from the spectrum about other parts of the molecule which are not resonantly enhanced. The newly developed technique of FT-Raman spectroscopy proves to be a solution to both of these problems for biological materials. Excitation with infrared wavelengths prevents electronic absorptions which give rise to fluorescence. In addition, the obtained spectra are completely nonresonant, allowing detection of vibrational modes of all parts of the molecule including the chromophore. We will present nonresonant, fluorescence free spectra of a range of biologically significant molecules including phospholipids and porphyrins.     

Comparison of the Electronic and Vibrational Optical Activity of a Europium(III) Complex

The geometry and the electronic structure of chiral lanthanide(III) complexes are traditionally probed by electronic methods, such as circularly polarised luminescence (CPL) and electronic circular dichroism (ECD) spectroscopy. The vibrational phenomena are much weaker. In the present study, however, significant enhancements of vibrational circular dichroism (VCD) and Raman optical activity (ROA) spectral intensities were observed during the formation of a chiral bipyridine–Eu^III complex. The ten‐fold enhancement of the vibrational absorption and VCD intensities was explained by a charge‐transfer process and the dominant effect of the nitrate ion on the spectra. A much larger enhancement of the ROA and Raman intensities and a hundred‐fold increase of the circular intensity difference (CID) ratio were explained by the resonance of the λ=532 nm laser light with the ⁷F₀ → ⁵D₀ transitions. This phenomenon is combined with a chirality transfer, and mixing of the Raman and luminescence effects involving low‐energy ⁷F states of europium. The results thus indicate that the vibrational optical activity (VOA) may be a very sensitive tool for chirality detection and probing of the electronic structure of Eu^III and other coordination compounds.     

Combination of Resonance and Non-Resonance Chiral Raman Scattering in a Cobalt(III) Complex

Resonance Raman optical activity (RROA) spectra with high sensitivity reveal details on molecular structure, chirality, and excited electronic properties. Despite the difficulty of the measurements, the recorded data for the Co(III) complex with S,S-N,N-ethylenediaminedisuccinic acid are of exceptional quality and, coupled with the theory, spectacularly document the molecular behavior in resonance. This includes a huge enhancement of the chiral scattering, contribution of the antisymmetric polarizabilities to the signal, and the Herzberg-Teller effect significantly shaping the spectra. The chiral component is by about one order of magnitude bigger than for an analogous aluminum complex. The band assignment and intensity profile were confirmed by simulations based on density functional and vibronic theories. The resonance was attributed to the S₀→S₃ transition, with the strongest signal enhancement of Raman and ROA spectral bands below about 800 cm⁻¹. For higher wavenumbers, other excited electronic states contribute to the scattering in a less resonant way. RROA spectroscopy thus appears as a unique tool to study the structure and electronic states of absorbing molecules in analytical chemistry, biology, and material science.     

Two-dimensional Raman and infrared vibrational spectroscopy for a harmonic oscillator system nonlinearly coupled with a colored noise bath

Multidimensional vibrational response functions of a harmonic oscillator are reconsidered by assuming nonlinear system-bath couplings. In addition to a standard linear-linear (LL) system-bath interaction, we consider a square-linear (SL) interaction. The LL interaction causes the vibrational energy relaxation, while the SL interaction is mainly responsible for the vibrational phase relaxation. The dynamics of the relevant system are investigated by the numerical integration of the Gaussian-Markovian Fokker-Planck equation under the condition of strong couplings with a colored noise bath, where the conventional perturbative approach cannot be applied. The response functions for the fifth-order nonresonant Raman and the third-order infrared (or equivalently the second-order infrared and the seventh-order nonresonant Raman) spectra are calculated under the various combinations of the LL and the SL coupling strengths. Calculated two-dimensional response functions demonstrate that those spectroscopic techniques are very sensitive to the mechanism of the system-bath couplings and the correlation time of the bath fluctuation. We discuss the primary optical transition pathways involved to elucidate the corresponding spectroscopic features and to relate them to the microscopic sources of the vibrational nonlinearity induced by the system-bath interactions. Optical pathways for the fifth-order Raman spectroscopies from an "anisotropic" medium were newly found in this study, which were not predicted by the weak system-bath coupling theory or the standard Brownian harmonic oscillator model.     

Resonance Raman study of the A-band short-time photodissociation dynamics of 2-iodothiophene

Resonance Raman spectra were obtained for 2-iodothiophene in cyclohexane solution with excitation wavelengths in resonance with the A-band absorption spectrum. These resonance Raman spectra indicate that the Franck-Condon region photodissociation dynamics have multidimensional character with motion mainly along the nominal symmetric C=C stretch of the thienyl ring and accompanied by a moderate amount of motion along the nominal symmetric CSC stretch, the nominal antisymmetric CSC stretch, and the nominal C-I stretch vibrational modes. A preliminary resonance Raman intensity analysis was done for the A-band resonance Raman spectra of 2-iodothiophene. These results were compared to previous results for related iodobenzene and iodoalkane molecules that also contain a C-I chromophore and the similarities and differences in the short-time photodissociation dynamics were discussed.     

Excited-state metal-to-ligand charge transfer dynamics of a ruthenium(II) dye in solution and adsorbed on TiO2 nanoparticles from resonance Raman spectroscopy

The dynamics of metal-to-ligand charge transfer (MLCT) in a cis-bis(4,4'-dicarboxy-2,2'-bipyridine)-bis(isothiocyanato)ruthenium(II) dye (N3) are compared for the free dye in solution and the dye adsorbed on the surface of the TiO(2) nanoparticles from resonance Raman spectroscopy. The 544-nm MLCT absorption band of N3 adsorbed on TiO(2) is slightly blue-shifted from that of the free N3, indicating a weak electronic coupling between N3 and TiO(2). The resonance Raman spectra of N3 and the N3|TiO(2) complex obtained upon excitation within the lowest-lying MLCT singlet state of the dye are similar except for slight shifts in band positions. Resonance Raman cross sections have been obtained for the vibrational modes of both N3 and N3|TiO(2) with excitation frequencies spanning the 544-nm MLCT band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism over two electronic states yields mode-specific vibrational and solvent reorganization energies. Despite the weak electronic coupling between N3 and TiO(2) in N3|TiO(2), adsorption strongly affects the reorganization energies of N3 in the intramolecular MLCT state. Adsorption of N3 onto TiO(2) increases the absolute Raman cross section of each mode by a factor of ca. 1.6 and decreases the vibrational and solvent reorganization energies by factors of 2 and 6, respectively. The excited-state dynamics of N3 adsorbed on the surface of TiO(2) nanoparticles were observed to be independent of the number of N3 molecules adsorbed per TiO(2) nanoparticle. The effect of TiO(2) on the dynamics of the adsorbed N3 is primarily due to both mode-specific vibrational and electronic pure dephasing, with the dominant contribution from the latter process.     

Understanding excited-state structure in metal polypyridyl complexes using resonance Raman excitation profiles,time-resolved resonance Raman spectroscopy and density functional theory

This article discusses the use of Raman spectroscopy, in concert with density functional theory, as a strategy for understanding excited-state structure in metal polypyridyl complexes. The first sections of the article discuss how one can use resonance Raman spectra of the ground-state molecule to understand the resonant Franck-Condon excited state. The theories behind these analyses are based on the sum-over-states and time-dependent approaches; a brief introduction to each of these methods is given. The use of density functional theory and its use in the determination of normal modes of vibration and infrared and Raman band intensities are discussed, with reference to a number of recent papers. The application of these methods is illustrated through the analysis of a number of selected examples which exemplify the strategies used to extract data from probing the Franck-Condon region. These data include the displacements of the resonant excited state with respect to the electronic ground state, the reorganisation energies associated with photoexcitation, bond length changes with excitation and other electronic parameters. The use, and limitations, of these methods are discussed. The direct calculation of resonance Raman band intensities is introduced. The direct measurement of excited-state vibrational spectra through time-resolved methods is discussed in the latter section of the article; with particular regard to the use of transient resonance Raman and time-resolved resonance Raman techniques to probe structural changes in metal polypyridyl complexes.     

Using internal coordinates to describe photoinduced geometry changes in MLCT excited states

A resonance Raman intensity analysis of the metal-to-ligand charge-transfer (MLCT) transition for the rhenium compound Re(2-(2'-pyridyl)quinoxaline)(CO)(3)Cl (RePQX) is presented. Photoinduced geometry changes are calculated, and the results are presented using the vibrational normal modes and the redundant internal coordinates. A density functional theory calculation is used to determine the ground-state nonresonant Raman spectrum and a transformation matrix that transforms the redundant internal coordinates into the normal modes. The normal modes nu(37) (rhenium coordination sphere distortion) and nu(75) (ligand skeletal stretch) show the largest photoinduced geometry change (Delta = 1.0 and 0.7, respectively). A single carbonyl mode is enhanced in the resonance Raman spectra. Time-dependent density functional theory is used to calculate excited-state geometry changes, which are subsequently used to determine the signs of the photoinduced normal mode displacements. Transforming to internal coordinates reveals that all the CO bond lengths are displaced in the excited state. The Re-C and C-C ligand bond lengths are also displaced in the excited state. The results are discussed in terms of a simple one-electron picture for the electronic transition. Many bond angles and torsional coordinates are also displaced by the metal-to-ligand charge transfer, and most of these are associated with the rhenium coordination sphere. It is demonstrated that using internal coordinates presents a clear picture of the geometry changes associated with photoinduced electron transfer in metal polypyridyl systems.     

Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene

The utility of recording Raman spectroscopy under liquid nitrogen, a technique we call Raman under nitrogen (RUN), is demonstrated for ferrocene, uranocene, and thorocene. Using RUN, low-temperature (liquid nitrogen cooled) Raman spectra for these compounds exhibit higher resolution than previous studies, and new vibrational features are reported. The first Raman spectra of crystalline uranocene at 77 K are reported using excitation from argon (5145 A) and krypton (6764 A) ion lasers. The spectra obtained showed bands corresponding to vibrational transitions at 212, 236, 259, 379, 753, 897, 1500, and 3042 cm(-1), assigned to ring-metal-ring stretching, ring-metal tilting, out-of-plane CCC bending, in-plane CCC bending, ring-breathing, C-H bending, CC stretching and CH stretching, respectively. The assigned vibrational bands are compared to those of uranocene in THF, (COT)2-, and thorocene. All vibrational frequencies of the ligands, except the 259 cm(-1) out-of-plane CCC bending mode, were found to increase upon coordination. A broad, polarizable band centered about approximately 460 cm(-1) was also observed. The 460 cm(-1) band is greatly enhanced relative to the vibrational Raman transitions with excitations from the krypton ion laser, which is indicative of an electronic resonance Raman process as has been shown previously. The electronic resonance Raman band is observed to split into three distinct bands at 450, 461, and 474 cm(-1) with 6764 A excitation. Relativistic density functional theory is used to provide theoretical interpretations of the measured spectra.     

Intermolecular Vibrational Energy Transfers in Melts and Solutions

Resonant and nonresonant intermolecular vibrational energy transfers in Gdm-SCN/KSCN=1/1, GdmSCN/KS¹³CN=1/1 and GdmSCN/KS1¹³C¹⁵N=1/1 mixed crystals in melts and in aqueous solutions are studied with the two dimensional infrared spectroscopy. The energy transfers in the samples are slower with a larger energy donor/acceptor gap, independent of the Raman spectra. The energy gap dependences of the nonresonant energy transfers cannot be described by the phonon compensation mechanism. Instead, the experimental energy gap dependences can be quantitatively described by the dephasing mechanism. Temperature dependences of resonant and nonresonant energy transfer rates in the melts are also consistent with the prediction of the dephasing mechanism. The series of results suggest that the dephasing mechanism can be dominant not only in solutions, but also in melts (pure liquids without solvents), only if the molecular motions (translations and rotations) are much faster than the nonresonant energy transfer processes.     

Electronic Circular Dichroism-Circularly Polarized Raman (eCP-Raman): A New Form of Chiral Raman Spectroscopy

This Concept article summarizes recent work on the development of a new form of chiral Raman spectroscopy, e CP-Raman, which combines two spectroscopies: electronic circular dichroism (ECD) and circularly polarized Raman (CP-Raman). First, some puzzling observations while carrying out Raman optical activity (ROA) measurements of several transition metal complexes under resonance are described, as well as the search for the mechanisms responsible. Then an equation for quantifying the e CP-Raman contribution is presented, followed by several examples of how e CP-Raman influences the I_R−I_L spectra of achiral and chiral solvent molecules and of a number of chiral solutes under resonance. The conditions to extract resonance ROA, when the e CP-Raman contribution is minimized, are also discussed. Finally, we comment on the potential applications of e CP-Raman.     

New chiral porphyrin-brucine gelator characterized by methods of circular dichroism

Herein, we report the first use of chiral alkaloid brucine to synthesize novel porphyrin-brucine conjugate capable of acting as a gelator of methanol and acetonitrile at extremely low level of concentration. The synthesis, characterization and spectral properties of gelator based on a novel structural motif, quaternized alkaloid conjugates, are described. Different spectroscopic methods (¹H NMR spectroscopy, Raman and infrared spectroscopy, and spectroscopy of electronic and vibrational circular dichroism) were used for characterization of the prepared organogel. The aggregation of the gelator studied by UV-vis spectroscopy and electronic circular dichroism showed the formation of chiral J-aggregates in water and water-methanol (1:1) mixture. A new methodology for the determination of functional groups involved in gel formation based on vibrational circular dichroism is presented.     

Resonance Raman studies of xanthine oxidase: The reduced enzyme-product complex with violapterin

A study of the molecular, electronic, and vibrational characteristics of the molybdenum-containing enzyme complex xanthine oxidase with violapterin has been carried out using density functional theory calculations and resonance Raman spectroscopy. The electronic structure calculations were carried out on a model consisting of the enzyme molybdopterin cofactor [in the four-valent, reduced state; Mo(IV)O(SH)] covalently linked to violapterin (1H,3H,8H-pteridine-2,4,7-trione in the neutral form) via an oxygen bridge, Mo-O-C7. Full geometry optimizations were performed for all models using the SDD basis set and the three-parameter exchange functional of Becke combined with the Lee, Yang, and Parr correlational functional. Harmonic vibrational frequencies were determined for a variety of isotopes in an attempt to correlate experimentally observed shifts upon 18O-labeling of the Mo-OR bridge to bound product as well as shifts seen upon substitution of solvent-exchangeable protons in samples prepared in D2O. The theoretical vibrational frequencies compared favorably with experimentally observed vibrational modes in the resonance Raman spectra of the reduced xanthine oxidase-violapterin complex prepared in H2O and D2O and with 18O-labeled product. Correlating the isotopic shifts from the calculations with those from the resonance Raman experiments resulted in complete normal mode assignments for all modes observed in the 350-1750 cm(-1) range. The present work demonstrates that a model in which the violapterin is coordinated to the molybdenum of the active site in a simple end-on manner via the hydroxyl group introduced by an enzyme accurately predicts the observed resonance Raman spectrum of the complex. Given the numerous modes involving the bridging oxygen, a side-on binding mode can be eliminated.     

Field-resolved coherent Raman spectroscopy of high frequency vibrational resonances

Electric fields of coherent Raman signals are resolved with sensitivity for high-frequency vibrational resonances utilizing a four-pulse, trapezoidal beam geometry in a diffractive optic-based interferometer. Our experiments show that the heterodyne detected signal phase is stabilized for particular terms in the third-order response function by the cancellation of inter-pulse phases. The C-H stretching modes of cyclohexane and benzene are studied under two polarization conditions. The temporal profiles of signal fields for cyclohexane exhibit a low-frequency recurrence due to the interference between the signals associated with the symmetric and asymmetric C-H stretching modes. In contrast, the electronically nonresonant polarizability response of benzene gives rise to a significant broadband signal component in addition to that associated with its C-H vibrational resonance. Time-frequency shapes of the Raman signal fields are strongly dependent on the properties of the liquid and the polarizations of the laser pulses.     

Resonant Raman spectra and first molecular hyperpolarizabilities of strongly charge-transfer molecules

The effect of vibrational structure on the frequency dependence of the first molecular hyperpolarizability of two thiophene-based charge-transfer chromophores is investigated. A time domain formulation is used to express the polarizability. The new expression includes the solvent-induced inhomogeneous distribution of electronic transition frequencies as well as the effect of the motion of solvent molecules that modulates the vibrational and electronic transition frequencies of the nonlinear optical molecule on which the first molecular hyperpolarizability depends. Resonance Raman scattering and one-photon absorption spectra of the chromophores are measured. By simultaneously fitting the experimental one-photon absorption spectrum and Raman cross sections of vibrational lines derived from resonance Raman scattering to a theoretical model, important parameters needed for the calculation of the first molecular hyperpolarizability are obtained. The first molecular hyperpolarizability is calculated as a function of frequency covering both nonresonance and two-photon resonance regions. The calculated result is compared with the measured hyperpolarizability as a function of frequency of the excitation laser. The resonance Raman-based analysis is shown to account reasonably well for the dispersion of the hyperpolarizability of the two charge transfer chromophores.     

Quantum theory of (femtosecond) time-resolved stimulated Raman scattering

We present a complete perturbation theory of stimulated Raman scattering (SRS), which includes the new experimental technique of femtosecond stimulated Raman scattering (FSRS), where a picosecond Raman pump pulse and a femtosecond probe pulse simultaneously act on a stationary or nonstationary vibrational state. It is shown that eight terms in perturbation theory are required to account for SRS, with observation along the probe pulse direction, and they can be grouped into four nonlinear processes which are labeled as stimulated Raman scattering or inverse Raman scattering (IRS): SRS(I), SRS(II), IRS(I), and IRS(II). Previous FSRS theories have used only the SRS(I) process or only the "resonance Raman scattering" term in SRS(I). Each process can be represented by an overlap between a wave packet in the initial electronic state and a wave packet in the excited Raman electronic state. Calculations were performed with Gaussian Raman pump and probe pulses on displaced harmonic potentials to illustrate various features of FSRS, such as high time and frequency resolution; Raman gain for the Stokes line, Raman loss for the anti-Stokes line, and absence of the Rayleigh line in off-resonance FSRS from a stationary or decaying v=0 state; dispersive line shapes in resonance FSRS; and the possibility of observing vibrational wave packet motion with off-resonance FSRS.