Resonance Raman spectra of uracil based on Kramers-Kronig relations using time-dependent density functional calculations and multireference perturbation theory

The use of time-dependent density functional calculations for the optimization of excited-state structures and the subsequent calculation of resonance Raman intensities within the transform-theory framework is compared to calculations of Hartree-Fock/configuration interaction singles-type (CIS). The transform theory of resonance Raman scattering is based on Kramers-Kronig relations between polarizability tensor components and the optical absorption. Stationary points for the two lowest excited singlet states of uracil are optimized and characterized by means of numerical differentiation of analytical excited-state gradients. It is shown that the effect of electron correlation leads to substantial modifications of the relative intensities. Calculations of vibrational frequencies for ground and excited states are carried out, which show that the neglect of Duschinsky mixing and the assumption of equal wave numbers for ground and excited state are not in all cases good approximations. We also compare the transform-theory resonance Raman intensities with those obtained within a simple approximation from excited-state gradients at the ground-state equilibrium position, and find that they are in qualitative agreement in the case of CIS, but show some important differences in calculations based on density functional theory. Since the results from CIS calculations are in better agreement with experiment, we also present approximate resonance Raman spectra obtained using excited-state gradients from multireference perturbation theory calculations, which confirm the CIS gradients.     

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.     

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.     

The resonance Raman effect of uranyl formate in dimethyl sulfoxide

The resonance Raman scattering spectra of uranyl formate (UO(2)(HCOO)(2)) in dimethyl sulfoxide ((CH(3))(2)SO, DMSO) have been measured under laser excitation of the uranyl ion in resonance with the 1Sigma(g)(+)-->(1)Phi(g) Laport forbidden f-f electronic transitions (ranging from 510 to 450 nm) by using ten output lines with wavelength ranging from 528.7 to 454.5 nm of the argon-ion laser at room temperature. The observed resonance excitation profile resembles the vibronic structure of the electronic absorption spectrum (ABS) but does not completely superimpose on it. Such a discrepancy is quantitatively explained by the interference effect, which occurs noticeably in the UO(2)L(2) (L=NO(3), CH(3)COO, Cl or HCOO)-DMSO system. Transform theory that makes use of the electronic ABS of the resonant electronic state has been applied to predict the Raman excitation profile (REP) of the uranyl totally symmetric stretching vibrational mode. Comparing the experimental REP with the transform theory prediction, it is found that the resonance Raman intensities of this stretching mode depend mainly on the vibronic interaction (non-Condon effect) in excited electronic states. Reliable value of the nuclear displacement on going the 1Sigma(g)(+)-->(1)Phi(g) electronic transition and the amount of charge transferred from the ligand to uranium of uranyl ion both in the ground and excited states are obtained. Elongation of the U-O equilibrium bond length due to the electronic transition is related to the magnitude of the change in the excitation profile, and has linear relation to the change in the amount of charge transferred from the ligand to uranium of uranyl ion in UO(2)L(2) type uranyl compounds in DMSO.     

Excited-State Raman Spectroscopy of Inorganic Compounds

Abstract— Raman spectra of inorganic complexes in excited electronic states are discussed. A brief overview of the field of transient Raman spectroscopy and experimental considerations are presented. Two examples from the author's laboratory are used to illustrate the type of information that can be obtained. The first example, an excited-state Raman spectroscopic study of K₃[Mn(CN)₅NO], is chosen because it illustrates the connections between excited-state molecular structure and vibrational properties. The pump pulse causes a change from a linear sp-hybridized NO containing a triple bond to a bent sp²-hybridized NO containing a double bond. Both the NO stretch and normal modes involving other ligands are measured and interpreted. The second example is chosen to illustrate the vibrational consequences of photoinduced electron transfer. The Raman spectra of W(CO)₄(diimine) complexes (diimine = 2,2'-bipyridine, 4,4'-dimethyl-2,2'-bipyridine, and isopropyl-pyridine-2-carbaldehyde imine) in the lowest tungsten to diimine charge transfer excited state are discussed. The excited-state peaks are assigned to ligand ring deformation modes and to carbonyl stretching modes. The totally symmetric cis -carbonyl stretching mode in the charge transfer excited state is about 50 cm' higher in energy than that of the molecule in the ground electronic state. The increase is interpreted in terms of loss of metal-car-bonyl back-bonding in the charge transfer excited state. Finally, a summary of the field's strengths and difficulties and a brief discussion of the future perspectives are presented.     

Simultaneous resonance Raman optical activity involving two electronic states

In the present work, the first observation of strong resonance Raman optical activity (RROA) involving more that one resonant electronic state is reported. The chiral transition metal complex bis-(trifluoroacetylcamphorato) copper(II), abbreviated Cu(tfc)(2), exhibits both resonance Raman (RR) and RROA spectra with laser excitation at 532 nm. Vibrational assignments for this complex were carried out by comparing the non-RR spectra of Cu(tfc)(2) excited at 1024 nm to density functional theory (DFT) calculations. The theory of the single-electronic-state (SES) RROA is extended to the next simplest level of theory involving two resonant electronic states (TES) without interstate vibronic coupling as an aide to the interpretation of the observed TES-RROA spectra. Based on measured UV-vis electronic absorbance spectra and corresponding TD-DFT calculations, the most likely two states associated with the RROA spectra are identified.     

Theory and method for calculating resonance Raman scattering from resonance polarizability derivatives

We present a method to calculate both normal Raman-scattering (NRS) and resonance Raman-scattering (RRS) spectra from the geometrical derivatives of the frequency-dependent polarizability. In the RRS case, the polarizability derivatives are calculated from resonance polarizabilities by including a finite lifetime of the electronic excited states using time-dependent density-functional theory. The method is a short-time approximation to the Kramers, Heisenberg, and Dirac formalism. It is similar to the simple excited-state gradient approximation method if only one electronic excited state is important, however, it is not restricted to only one electronic excited state. Since the method can be applied to both NRS and RRS, it can be used to obtain complete Raman excitation profiles. To test the method we present the results for the S2 state of uracil and the S4, S3, and S2 states of pyrene. As expected, the results are almost identical to the results obtained from the excited-state gradient approximation method. Comparing with the experimental results, we find in general quite good agreement which enables an assignment of the experimental bands to bands in the calculated spectrum. For uracil the inclusion of explicit waters in the calculations was found to be necessary to match the solution spectra. The calculated resonance enhancements are on the order of 10(4)-10(6), which is in agreement with experimental findings. For pyrene the method is also able to distinguish between the three different electronic states for which experimental data are available. The neglect of anharmonicity and solvent effects in the calculations leads to some discrepancy between theory and experiment.     

Excited-state structural dynamics of cytosine from resonance Raman spectroscopy

Cytosine, a nucleobase found in both DNA and RNA, is known to form photoproducts upon UV irradiation, damaging the nucleic acids and leading to cancer and other diseases. To determine the molecular mechanism by which these photoproducts occur, we have measured the resonance Raman spectra of cytosine at wavelengths throughout its 267 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism yields both the excited-state structural changes and electronic parameters. From this analysis, we have been able to determine that, at most, 31% of the reorganization energy upon excitation is directed along photochemically relevant modes.     

Interpretation of unusual absorption bandwidths and resonance Raman intensities in excited state mixed valence

Excited state mixed valence (ESMV) occurs in molecules in which the ground state has a symmetrical charge distribution but the excited state possesses two or more interchangeably equivalent sites that have different formal oxidation states. Although mixed valence excited states are relatively common in both organic and inorganic molecules, their properties have only recently been explored, primarily because their spectroscopic features are usually overlapped or obscured by other transitions in the molecule. The mixed valence excited state absorption bands of 2,3-di-p-anisyl-2,3-diazabicyclo[2.2.2]octane radical cation are well-separated from others in the absorption spectrum and are particularly well-suited for detailed analysis using the ESMV model. Excited state coupling splits the absorption band into two components. The lower energy component is broader and more intense than the higher energy component. The absorption bandwidths are caused by progressions in totally symmetric modes, and the difference in bandwidths is caused by the coordinate dependence of the excited state coupling. The Raman intensities obtained in resonance with the high and low energy components differ significantly from those expected based on the oscillator strengths of the bands. This unexpected observation is a result of the excited state coupling and is explained by both the averaging of the transition dipole moment orientation over all angles for the two types of spectroscopies and the coordinate-dependent coupling. The absorption spectrum is fit using a coupled two-state model in which both symmetric and asymmetric coordinates are included. The physical meaning of the observed resonance Raman intensity trends is discussed along with the origin of the coordinate-dependent coupling. The well-separated mixed valence excited state spectroscopic components enable detailed electronic and resonance Raman data to be obtained from which the model can be more fully developed and tested.     

Isotope effects on the picosecond time-resolved emission spectroscopy of tris(2,2'-bipyridine)ruthenium (II)

The excited-state properties of the transition metal complexes tris(2,2'-bipyridine) ruthenium(II) and tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) are examined using picosecond time-resolved luminescence spectroscopy. For both complexes, direct observation of a short-lived high-energy emission with a lifetime of less than 4 ps is reported. Upon deuteriation of the complexes the lifetime of the high-energy emission shows a marked increase with a biexponential decay (20 and approximately 300 ps components). Examination by time-resolved excited-state resonance Raman shows that for the perprotio complexes features attributable to the 3MLCT excited state are formed within 4 ps, while for the perdeuterio a rise time of approximately 20 ps is observed in the 3MLCT features. The results indicate that the emission in both cases may be 1MLCT in origin and are discussed with respect to heterogeneous electron transfer.     

Theoretical investigation of resonance Raman scattering of dye molecules absorbed on semiconductor surfaces

A method in time domain is proposed to investigate resonance Raman spectra of absorbed molecules on semiconductor surfaces. The charge transfer at the molecule-surface interface is incorporated with the use of an Anderson-Newns type Hamiltonian, where the surface continuum state is dealt with an expansion of Legendre polynomials for fast numerical convergence. From a model test, it is found that the intensities of Raman modes in the sole molecule generally decrease as the molecule-surface interaction is switched on, except that the energy gaps between the molecular excited state and the bottom of the band are at special values. New Raman peaks which are not observed in the sole molecule, however, appear and are greatly enhanced. The enhancement depends on the electronic coupling and the energy gap. It is also highly sensitive to the mode-specific reorganization energy in the charge transfer state, and a thousand times enhancement can be obtained at a certain reorganization energy. The corresponding electron dynamics is revealed by the population decay from the absorbed molecule.     

Surface-enhanced Raman scattering and surface-enhanced resonance Raman scattering excitation profiles of Ag-2,2'-bipyridine surface complexes and of [Ru(bpy)3]2+ on Ag colloidal surfaces: manifestations of the charge-transfer resonance contributions to the overall surface enhancement of Raman scattering

Excitation profiles of SERS (surface-enhanced Raman scattering) and/or SERRS (surface-enhanced resonance Raman scattering) spectral bands of two forms of a Ag-bpy (bpy = 2,2'-bipyridine) surface complex and of [Ru(bpy)3]2+ on Ag nanoparticle (hydrosol) surfaces were determined from the spectra excited in the 458-600 nm region and are reported together with the FT-SERS spectra of the Ag-bpy surface complex and FT Raman spectra of [Ru(bpy)3] Cl2. Seven of the observed 11 fundamentals as well as their first overtones and combination bands are selectively enhanced in SERS of the Ag-bpy surface complex formed in the Ag colloid/HCl/bpy system. The profiles of these bands show a common maximum at approximately 540 nm. The selectively enhanced bands of the Ag-bpy surface complex have nearly the same wavenumbers as those enhanced in the SERRS and resonance Raman spectra of [Ru(bpy)3]2+ upon excitation close to the 453 nm maximum of its MLCT absorption band. Moreover, the intensity patterns of the bpy vibrations of the two species match both in resonance (541 nm excitation for Ag-bpy, 458 nm for [Ru(bpy)3]2+) and in off-resonance (458 and 1064 nm for Ag-bpy, 1064 nm for [Ru(bpy)3]2+). The distinct band shapes of the excitation profiles of the selectively enhanced vibrational modes of the Ag-bpy surface complex, as well as the observation of overtones and combination bands in the SERS spectra upon excitation into this "band", are interpreted in terms of a charge-transfer resonance contribution to the overall SERS enhancement. In view of the near-coincidence of the vibrational modes coupled to the resonant electronic transition of Ag-bpy with those coupled to the MLCT transition of [Ru(bpy)3]2+, the resonant electronic transition is tentatively assigned to a Ag metal to bpy (pi*) CT transition.     

Ab initio calculations of the ultraviolet resonance Raman spectra of uracil

An equation been derived to calculate, ab initio, the frequencies and intensities of a resonant Raman spectrum from the transform theory of resonance Raman scattering. This equation has been used to calculate the intensities of the ultraviolet resonance Raman spectra from the first π-π* excited state of uracil and 1,3-dideuterouracil. The protocol for this calculation is as follows: (1) The force constant matrix elements in Cartesian coordinate space, the vibrational frequencies, and the minimum energy ground and excited state geometries of the molecule are calculated ab initio using the molecular orbital program Gaussian 92, (2) the force constants in Cartesian coordinates are transformed into force constants in the space of a set of 3N – 6 nonredundant symmetrized internal coordinates, (3) the G matrix is constructed from the energy minimized ground state Cartesian coordinates and the GFL = LΛ eigenvalue equation is solved in internal coordinate space, (4) the elements of the L and L^?1 matrices are calculated, (5) the changes in all of the internal coordinates in going from the ground to the excited state are calculated, and (6) these results are used in combination with the transform theory of resonance Raman scattering to calculate the relative intensities of each of the 3N – 6 vibrations as a function of the exciting laser frequency. There are no adjustable parameters in this calculation, which reproduces the experimental frequencies and intensities with remarkable fidelity. This indicates that the Dushinsky rotation of the modes in the excited state of these molecules is not important and that the simplest form of the transform theory is adequate. © 1995 John Wiley & Sons, Inc.     

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.     

Ultrafast dynamics of myoglobin probed by time-resolved resonance Raman spectroscopy

Recent experimental work carried out in this laboratory on the ultrafast dynamics of myoglobin (Mb) is summarized with a stress on structural and vibrational energy relaxation. Studies on the structural relaxation of Mb following CO photolysis revealed that the structural change of heme itself, caused by CO photodissociation, is completed within the instrumental response time of the time-resolved resonance Raman apparatus used (approximately 2 ps). In contrast, changes in the intensity and frequency of the iron-histidine (Fe-His) stretching mode upon dissociation of the trans ligand were found to occur in the picosecond regime. The Fe-His band is absent for the CO-bound form, and its appearance upon photodissociation was not instantaneous, in contrast with that observed in the vibrational modes of heme, suggesting appreciable time evolution of the Fe displacement from the heme plane. The band position of the Fe-His stretching mode changed with a time constant of about 100 ps, indicating that tertiary structural changes of the protein occurred in a 100-ps range. Temporal changes of the anti-Stokes Raman intensity of the v4 and v7 bands demonstrated immediate generation of vibrationally excited heme upon the photodissociation and decay of the excited populations, whose time constants were 1.1 +/- 0.6 and 1.9 +/- 0.6 ps, respectively. In addition, the development of the time-resolved resonance Raman apparatus and prospects in this research field are described.     

Relaxation dynamics of the hydrated electron: femtosecond time-resolved resonance Raman and luminescence study

Femtosecond time-resolved resonance Raman measurements were carried out to examine the relaxation process of the hydrated electron in water. The rise of the intra- and intermolecular vibrational Raman bands of the solvating water molecules was successfully time-resolved with a time resolution as high as 250 fs. The temporal intensity change of Raman bands, as well as that of luminescence background, was compared with the time evolution of the transient absorption signal. It was found that (1) the Raman and luminescence signals exhibited the same temporal behavior, (2) the rise time of the Raman bands is faster than the appearance of the equilibrated hydrated electron, indicating that the precursor state also gives rise to resonance Raman signals, and (3) the rise of the transient Raman band is slower than that of the transient absorption at the probe wavelength of 800 nm. Because it has been shown that the Raman intensity enhancement arises from the resonance with the s --> p transition, fact 2 implies that the precursor state is the nonequilibrated s-state electron. The delayed rise of the Raman signal compared to the absorption was explained in terms of the temporal change of the resonance condition. In very early time when the absorption is largely red-shifted, the probe at 800 nm is resonant with the high energy part of the absorption that provides little resonance Raman enhancement. This explanation was consistent with the probe wavelength dependence of the temporal behavior of the Raman signal: the Raman bands measured with the higher energy probe (600 nm) rose even more slowly. The resonance Raman signal in the anti-Stokes side was also examined, but no anti-Stokes band was observable. It suggests that the temperature increase of the solvation structure around the nonequilibrated hydrated electron is less than 100 K.     

Polarization dispersion in resonance Raman scattering

A simple model for resonance Raman scattering by asymmetric and undisplaced symmetric vibrational modes is proposed. The model permits a detailed calculation of the polarization dispersion and the excitation spectrum in the region of the 0-0 and the 0-1 band of the resonating excited electronic state. It is directly amenable to experimental verification and shows good qualitative agreement with the experimental results available to date.     

Initial excited-state structural dynamics of 2'-deoxyguanosine determined via UV resonance Raman spectroscopy

The resonance Raman spectra of 2'-deoxyguanosine, a DNA nucleoside, were measured in aqueous solution at wavelengths throughout its 260 nm absorption band. Self-consistent analysis of the resulting resonance Raman excitation profiles and absorption spectrum using a time-dependent wave packet formalism with two electronic states yielded the initial excited-state structural dynamics in both states. The vibrational modes containing the N(7)═C(8) stretching and C(8)-H bending internal coordinates were found to exhibit significant initial structural dynamics upon photoexcitation to either state and are coincident with the photochemical reaction coordinate involving the formation of the 2'-deoxyguanosine cation radical.