Spectra Of Molecules In Solution Research Articles

Modeling solvation effects on absorption and fluorescence spectra of indole in aqueous solution.

Modeling the optical spectra of molecules in solution presents a challenge, so it is important to understand which of the solvation effects (i.e., electrostatics, mutual polarization, and hydrogen bonding interactions between solute and solvent molecules) are crucial in reproducing the various features of the absorption and fluorescence spectra and to identify a sufficient theoretical model that accurately captures these effects with minimal computational cost. In this study, we use various implicit and explicit solvation models, such as molecular dynamics coupled with non-polarizable and polarizable force fields, as well as Car-Parrinello molecular dynamics, to model the absorption and fluorescence spectra of indole in aqueous solution. The excited states are computed using the equation of motion coupled cluster with single and double excitations combined with the effective fragment potential to represent water molecules, which we found to be a computationally efficient approach for modeling large solute-solvent clusters at a high level of quantum theory. We find that modeling mutual polarization, compared to other solvation effects, is a dominating factor for accurately reproducing the position of the peaks and spectral line shape of the absorption spectrum of indole in solution. We present an in-depth analysis of the influence that different solvation models have on the electronic excited states responsible for the features of the absorption spectra. Modeling fluorescence is more challenging since it is hard to reproduce even the correct emitting state, and force field parameters need to be re-evaluated.

A study of the solvent effect on the low temperature spectra of benzoanthracene molecules

been taken at room temperature down to liquid nitrogen temperature (77K). Polar and nonpolar solvents have been used to study the solvent effect on the absorption and fluorescence spectra of solute molecules. Some of the spectroscopic parameters have been determined as functions of solvent polarity and temperature. The results indicate that the band width FWHM increases with increasing the solvent polarity and temperature, while the peak emission cross section decreases with increasing of solvent polarity and decreases with increasing the temperatures. Clear vibrational structure spectra of benzoanthracene molecules have been observed in Nonane and Hexane solvents at 77K.

Interferometric Measurement of Transient Absorption and Refraction Spectra with Dual Frequency Comb.

We demonstrate that a dual frequency comb-transient absorption (DFC-TA) technique can be combined with a time-domain interferometric detection to measure both the transient absorption and refraction spectra of molecules in solution. To do this, the pump-probe signal field of DFC-TA is allowed to interfere with a time-delayed local oscillator field in a time domain. We show that this DFC interferometric pump-probe spectroscopy (DFC-IPS) technique has a unique ability to extract the phase and amplitude information on the pump-probe signal using just a single-scan data, while conventional techniques require an independent signal measured without the pump field for the normalization of the pump-probe spectrum. As a proof-of-principle experiment, we here show that the DFC-IPS enables us to simultaneously measure the frequency-resolved (from 650 to 950 nm) transient absorption and refraction signals with an exceptionally broad dynamic range from femtosecond to nanosecond without using a mechanical translational stage for pump-probe time-scanning. We anticipate that our DFC-IPS technique with femtosecond time-resolution capability will be useful to investigate photoinduced chemical and biological reactions covering broad dynamic ranges.

Modeling absorption spectra of molecules in solution

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute–solvent interactions are key to understanding solution‐phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute–solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute–solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.

Electronically Excited States in Solution via a Smooth Dielectric Model Combined with Equation-of-Motion Coupled Cluster Theory.

We present a method for computing excitation energies for molecules in solvent, based on the combination of a minimal parameter implicit solvent model and the equation-of-motion coupled-cluster singles and doubles method (EOM-CCSD). In this method, the solvent medium is represented by a smoothly varying dielectric function, constructed directly from the quantum mechanical electronic density using only two tunable parameters. The solvent-solute electrostatic interactions are computed by numerical solution of the nonhomogeneous Poisson equation and incorporated at the Hartree-Fock stage of the EOM-CCSD calculation by modification of the electrostatic potential. We demonstrate the method by computing excited state transition energies and solvent shifts for several small molecules in water. Results are presented for solvated H2O, formaldehyde, acetone, and trans-acrolein, which have low-lying n → π* transitions and associated blue shifts in aqueous solution. Comparisons are made with experimental data and other theoretical approaches, including popular implicit solvation models and QM/MM methods. We find that our approach provides surprisingly good agreement with both experiment and the other models, despite its comparative simplicity. This approach only requires modification of the Fock operator and total energy expressions at the Hartree-Fock level-solvation effects enter into the EOM-CCSD calculation only through the Hartree-Fock orbitals. Our model provides a theoretically and computationally simple route for accurate simulations of excited state spectra of molecules in solution, paving the way for studies of larger and more complex molecules.

Anharmonic Infrared Spectroscopy through the Fourier Transform of Time Correlation Function Formalism in ONETEP.

Density functional theory molecular dynamics (DFT-MD) provides an efficient framework for accurately computing several types of spectra. The major benefit of DFT-MD approaches lies in the ability to naturally take into account the effects of temperature and anharmonicity, without having to introduce any ad hoc or a posteriori corrections. Consequently, computational spectroscopy based on DFT-MD approaches plays a pivotal role in the understanding and assignment of experimental peaks and bands at finite temperature, particularly in the case of floppy molecules. Linear-scaling DFT methods can be used to study large and complex systems, such as peptides, DNA strands, amorphous solids, and molecules in solution. Here, we present the implementation of DFT-MD IR spectroscopy in the ONETEP linear-scaling code. In addition, two methods for partitioning the dipole moment within the ONETEP framework are presented. Dipole moment partitioning allows us to compute spectra of molecules in solution, which fully include the effects of the solvent, while at the same time removing the solvent contribution from the spectra.

An efficient computational scheme for electronic excitation spectra of molecules in solution using the symmetry-adapted cluster-configuration interaction method: the accuracy of excitation energies and intuitive charge-transfer indices.

Solvent effects on electronic excitation spectra are considerable in many situations; therefore, we propose an efficient and reliable computational scheme that is based on the symmetry-adapted cluster-configuration interaction (SAC-CI) method and the polarizable continuum model (PCM) for describing electronic excitations in solution. The new scheme combines the recently proposed first-order PCM SAC-CI method with the PTE (perturbation theory at the energy level) PCM SAC scheme. This is essentially equivalent to the usual SAC and SAC-CI computations with using the PCM Hartree-Fock orbital and integrals, except for the additional correction terms that represent solute-solvent interactions. The test calculations demonstrate that the present method is a very good approximation of the more costly iterative PCM SAC-CI method for excitation energies of closed-shell molecules in their equilibrium geometry. This method provides very accurate values of electric dipole moments but is insufficient for describing the charge-transfer (CT) indices in polar solvent. The present method accurately reproduces the absorption spectra and their solvatochromism of push-pull type 2,2'-bithiophene molecules. Significant solvent and substituent effects on these molecules are intuitively visualized using the CT indices. The present method is the simplest and theoretically consistent extension of SAC-CI method for including PCM environment, and therefore, it is useful for theoretical and computational spectroscopy.

Electronic excitation spectra of molecules in solution calculated using the symmetry-adapted cluster-configuration interaction method in the polarizable continuum model with perturbative approach

A perturbative approximation of the state specific polarizable continuum model (PCM) symmetry-adapted cluster-configuration interaction (SAC-CI) method is proposed for efficient calculations of the electronic excitations and absorption spectra of molecules in solutions. This first-order PCM SAC-CI method considers the solvent effects on the energies of excited states up to the first-order with using the zeroth-order wavefunctions. This method can avoid the costly iterative procedure of the self-consistent reaction field calculations. The first-order PCM SAC-CI calculations well reproduce the results obtained by the iterative method for various types of excitations of molecules in polar and nonpolar solvents. The first-order contribution is significant for the excitation energies. The results obtained by the zeroth-order PCM SAC-CI, which considers the fixed ground-state reaction field for the excited-state calculations, are deviated from the results by the iterative method about 0.1 eV, and the zeroth-order PCM SAC-CI cannot predict even the direction of solvent shifts in n-hexane for many cases. The first-order PCM SAC-CI is applied to studying the solvatochromisms of (2,2(')-bipyridine)tetracarbonyltungsten [W(CO)4(bpy), bpy = 2,2(')-bipyridine] and bis(pentacarbonyltungsten)pyrazine [(OC)5W(pyz)W(CO)5, pyz = pyrazine]. The SAC-CI calculations reveal the detailed character of the excited states and the mechanisms of solvent shifts. The energies of metal to ligand charge transfer states are significantly sensitive to solvents. The first-order PCM SAC-CI well reproduces the observed absorption spectra of the tungsten carbonyl complexes in several solvents.

Absorption and Emission Spectra of Solvated Molecules with the EOM-CCSD-PCM Method.

The accurate calculation of transition energies and properties of isolated molecules is not enough for realistic simulations of their absorption and emission spectra in solution. In fact, the solvent influences the solute geometry, electronic structure, and response to external fields, and a proper description of the solvent effect is fundamental. However, the computational cost of including explicit solvent molecules around the solute becomes rather onerous when an accurate method such as the equation of motion coupled cluster singles and doubles (EOM-CCSD) is employed. The polarizable continuum model of solvation (PCM) may provide an efficient alternative to explicit models, since the sampling of solvent configurations is implicit and the solute-solvent mutual polarization is naturally accounted for. In this contribution, the absorption and emission spectra of molecules in solution are modeled through the EOM-CCSD-PCM method. The equilibrium solvation regime is employed for the geometry optimization of the solute molecule in the ground and excited states, while the nonequilibrium solvation regime is employed for vertical transitions. The theory, implementation, and prototypical applications of the method are presented. The numerical tests involve solvents that are particularly challenging for PCM: low-polar and protic polar solvents. Nonetheless, the experimental trends are well reproduced, and the overall agreement with the measured data is remarkable.

Femtosecond transient absorption with chirped pump and supercontinuum probe: Perturbative calculation of transient spectra with general lineshape functions, and simplifications

Femtosecond transient UV/vis absorption spectra of molecules in solution show vibronic structure initially while pump and probe pulses overlap, superimposed on bands which broaden and shift on several time scales. Third-order perturbation theory is used simulate and understand such spectra under experimental conditions, i.e., with chirped pump pulses and 40–80 fs pump/probe correlation time. The electronic system is coupled to a solvent bath whose fluctuations enter through a lineshape function g( t). This approach has two advantages. (i) In the general case when g( t) is modelled realistically it provides a distinction of coherent and sequential contributions to early transient spectra. The time-zero stimulated emission band can be extracted, giving access to the relaxation function for the emission frequency. (ii) For limiting forms of g( t) corresponding to the Bloch approximation and to inhomogeneous broadening the threefold time integrals underlying the theory reduce to a single integral. In such cases the lineshape formulation of transient absorption spectra can be used for data fitting.

Frequency dispersed transient absorption spectra of dissolved perylene: A case study using the density matrix version of the MCTDH method

A theoretical scheme is presented to calculate non-linear optical spectra of molecules in solution. Starting with electronic structure calculations of the ground and excited state, a subset of vibrational coordinates exhibiting the largest Huang–Rhys factors is assigned. It is used to set up a model Hamiltonian for density matrix multi configurational time-dependent Hartree (MCTDH) calculations. The expression derived for the dissipative part of the equation of motion goes beyond the earlier used Lindblad-form. In order to calculate the non-linear response the electric field strength is introduced into the density matrix equations used to directly determine the polarization. The whole scheme is applied to perylene as a reference case.

Calculation of the electronic spectra of molecules in solution and on surfaces

An approach is presented to calculate the electronic spectra of molecules in solution or adsorbed on surfaces. Through limiting electronic excitations to include only those between orbitals localized on the solute or adsorbant, large computational savings are achieved with minimal additional error. This approach has been implemented within time-dependent density functional theory and single excitation configuration interaction. Calculations of the electronic spectrum of formamide in water and carbon monoxide on the Ni(1 1 1) surface illustrate the method.

Resonance Raman four-wave mixing in molecular systems

Non-linear vibronic spectra of molecules in solution were considered. Probe profiles (PP) of CARS with three laser frequencies, treated as Raman four-wave mixing (Raman FWM), were calculated and compared with excitation profiles (EP) of CARS and of spontaneous Raman scattering. It is demonstrated that the PP of Raman FWM is a characteristic that can be used instead of the EP of spontaneous Raman scattering. The dependence of the lineshape for Raman FWM was modelled. The advantages of Raman FWM are mentioned. In contrast to CARS with two lasers, it is easy to penetrate the high-wavenumber side of the vibronic spectra and to avoid a significant absorption at the exciting wavenumbers.

Inherent sample heating and temperature calibration in excited state absorption (ESA) measurements between room temperature and 77 kelvin

Laser pulses of high intensities are usually required for recording excited state absorption (ESA) spectra of molecules in solution. Following the optical excitation non-radiative relaxation processes in the chromophores can lead to a remarkable and unintentional heating of the overall excited sample volume, especially at low temperatures. This temperature rise and its spatial distribution is investigated experimentally and compared to model calculations. The temperature of the excited sample volume was monitored by the temperature sensitive fluorescence of 4-methylumbelliferone (4-MU; synonymous with 7-hydroxy-4-methylcoumarine). A maximal temperature rise of several degrees was observed experimentally. Although the model was based on simple assumptions the agreement between theoretical results and experimental data proved to be satisfying.

Optical properties of a streptocyanine dye single crystal with tubelike aggregation: bis(dimethylamino)-heptamethinium tetrafluoroborate BDH+BF−4

Normal incidence reflectance spectra from different faces of an optically biaxial dye single crystal, bis(dimethylamino)-heptamethinium tetrafluoroborate, have been measured for two orthogonal polarizations of light. The spectra are very anisotropic with a wide band of quasi-metallic reflectance for one polarization of light. Strong coupling of light to electronic states leads to splitting of transverse and longitudinal excitons as large as 1.5 eV, resulting in strong directional dispersion of the spectra. The data were analyzed by Kramers-Kronig transformation of spectra from appropriate faces and checked by comparison of solutions of the wave equation and the experimental spectra. Electronic transition energies of two Davydov components were found at 2.07 and 2.18 eV shifted by 0.32 and 0.24 eV with respect to absorption spectra of molecules in solution. The results are compared with those obtained from the chloride of the same dye molecule.

Applications of multiple-pulse sequences to the simplification of liquid-crystal NMR spectra

Abstract It is shown that by changing their pulse timing, several multiple-pulse sequences normally used to obtain high-resolution NMR spectra of abundant spins in solids can afford spectra in which the effective dipolar couplings appear scaled by a factor ranging between 1 and −1 2 . This was used to design a family of par tial de coupling sequences (PARDES) which may be useful for recording simplified spectra of solute molecules oriented in liquid-crystal solvents. Upon exploring the experimental performance of these pulse sequences, it was found that their instrumental requirements are considerably less demanding than those necessary to perform dipolar averaging in the solid state. Moreover, it was observed that when proper experimental conditions are chosen, these PARDES perform a selective averaging of the dipolar couplings within the solute molecules without affecting the broad coupling patterns that characterize the nematic solvent, preserving thereby one of the main properties of liquid-crystal 1 H NMR experiments.

Consideration of the structure of the solution in investigations of the influence of hydrogen bonds with the solvent on shifts of the electronic spectra of solute molecules

Consideration of the structure of the solution in investigations of the influence of hydrogen bonds with the solvent on shifts of the electronic spectra of solute molecules

Application of non-photochemical hole burning to the two-photon spectroscopy of impurity molecules in low temperature glasses

One-photon non-photochemical hole burning (NPHB) is shown to be a viable method for producing narrow-line two-photon spectra of molecules in solution at low temperature. The TPE spectrum of phenanthrene at 2 K in a glassy medium is presented and is shown to manifest one-photon NPHB effects.

On the relation between absorption and emission spectra of molecules in solution

The universal relation between absorption and fluorescence spectra derived by Stepanov in 1957 is reexamined to assess the relative merits of two of the inferences usually drawn from its use in the analysis of data: elevated excited state temperatures (“warm fluorescence”) and inhomogeneous broadening of the molecular spectra. Of these, only the latter appears to give a satisfactory explanation of the anomalous temperatures derived using the Stepanov method in condensed media, and in particular, a purely “warm fluorescence” interpretation implies that a distinct curvature should appear in plots which experimentally appear to be straight lines.

Influence of Paramagnetic Species on the Internuclear Overhauser Effect

It is shown that internuclear Overhauser enhancements can be substantially reduced if a paramagnetic species is added to the sample. Such reduction or elimination of the Overhauser enhancement can be accomplished without significant broadening of a nuclear resonance, if the nuclei observed are embedded within a molecule and those decoupled are in the periphery. Thereby the intensity information and resolution can be retained in proton decoupled 13C spectra of molecules in solution. Also, the method should be applicable to decoupled spectra of species such as B, N, Si, and P. In cases where Overhauser enhancements are not reduced to zero, the actual magnitude of the enhancement can be calculated from a plot of signal intensity versus concentration of paramagnetic species. Characteristic spin-lattice relaxation times can also be inferred from such a plot. The theory is extended to multiple spin systems and to cases of complex formation, and it is shown that preferential reduction of Overhauser enhancements by a paramagnetic complexing species offers an extremely sensitive means for determining ligand binding sites and for inferring paramagnetic-diamagnetic spin separations. Observation of transient nuclear Overhauser effects in both the presence and absence of added paramagnetic material can also be used to determine the absolute magnitude of Overhauser enhancements and nuclear relaxation times.