Molecules In Liquid Solutions Research Articles

Quantum Simulation of Molecules in Solution.

Quantum chemical calculations on quantum computers have been focused mostly on simulating molecules in the gas phase. Molecules in liquid solution are, however, most relevant for chemistry. Continuum solvation models represent a good compromise between computational affordability and accuracy in describing solvation effects within a quantum chemical description of solute molecules. In this work, we extend the variational quantum eigensolver to simulate solvated systems using the polarizable continuum model. To account for the state dependent solute-solvent interaction we generalize the variational quantum eigensolver algorithm to treat non-linear molecular Hamiltonians. We show that including solvation effects does not impact the algorithmic efficiency. Numerical results of noiseless simulations for molecular systems with up to 12 spin-orbitals (qubits) are presented. Furthermore, calculations performed on a simulated noisy quantum hardware (IBM Q, Mumbai) yield computed solvation free energies in fair agreement with the classical calculations.

Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha-Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochromic reactions occurring with and without bond breaking or bond formation, namely ring-opening reactions and cis-trans isomerizations, respectively.

Translational, rotational and vibrational relaxation dynamics of a solute molecule in a non-interacting solvent.

Spectroscopically observing the translational and rotational motion of solute molecules in liquid solutions is typically impeded by their interactions with the solvent, which conceal spectral detail through linewidth broadening. Here we show that unique insights into solute dynamics can be made with perfluorinated solvents, which interact weakly with solutes and provide a simplified liquid environment that helps to bridge the gap in our understanding of gas- and liquid-phase dynamics. Specifically, we show that in such solvents, the translational and rotational cooling of an energetic CN radical can be observed directly using ultrafast transient absorption spectroscopy. We observe that translational-energy dissipation within these liquids can be modelled through a series of classic collisions, whereas classically simulated rotational-energy dissipation is shown to be distinctly faster than experimentally measured. We also observe the onset of rotational hindering from nearby solvent molecules, which arises as the average rotational energy of the solute falls below the effective barrier to rotation induced by the solvent.

Radical Cationic Pathway for the Decay of Ionized Glyme Molecules in Liquid Solution.

Chemical stability of primary radical cations (RCs) generated in irradiated matter determines substantially the radiation resistance of organic materials. Transformations of the RCs of the glyme molecules, R(-O-CH2-CH2-)nO-R (R = CH3, n = 1-4) has been studied on the nanosecond time scale by measuring the magnetic field effects in the recombination fluorescence from irradiated liquid solutions of the glymes. In all cases, the RCs observed were different from that expected for the primary ones and revealed very similar hyperfine couplings independent of the poly(ethylene oxide) chain length and of the substitution of terminal methyl groups by C2H5 or CH2CH2Cl, as has been shown with diglyme as an example. Quantum chemical analysis of possible chemical transformations for the monoglyme RC as a model system allowed us to discover the reaction pathway yielding the methyl vinyl ether RC. The pathway involves intramolecular proton transfer followed by C-O bond cleavage. Only one (-O-CH2-CH2-O-) fragment is involved in this transformation, which is nearly barrierless due to the catalytic effect of adjacent glyme molecules. The rapid formation of the methyl vinyl ether RC in the irradiated monoglyme was confirmed by the numerical simulation of the experimental curves of the time-resolved magnetic field effect. These findings suggest that the R'-O-CH═CH2(•+) formation is a typical decay pathway for the primary RCs in irradiated liquid glymes.

High-field liquid state NMR hyperpolarization: a combined DNP/NMRD approach.

Here we show how fast dynamics between radicals and solvent molecules in liquid solutions can be detected by comparison of coupling factors determined by nuclear magnetic relaxation dispersion (NMRD) measurements and dynamic nuclear polarization (DNP) enhancement measurements at high magnetic field (9.2 T). This is important for a theoretical understanding of the Overhauser DNP mechanism at high magnetic fields and thus for optimization of the DNP agent/target system for high resolution liquid state NMR applications. Mixtures of the solution of TEMPOL radicals in water, toluene, acetone and DMSO have been investigated. The results are compared to the classical hard-sphere model and molecular dynamic simulations. Our results clearly indicate that fast sub-ps dynamics, which are not related to classical rotational or translational motion of the molecules, significantly contribute to the Overhauser DNP mechanism at high magnetic fields.

Mass Transport across the Porous Oxide Shells of Core–Shell and Yolk–Shell Nanostructures in Liquid Phase

The mass transport of a number of molecules dissolved in a liquid solvent through the shells of Pt@SiO2 core–shell and Pt@Void@TiO2 yolk–shell nanostructures was characterized in situ by infrared absorption spectroscopy. Our samples, which were used here to represent the core–shell and yolk–shell nanostructures that have become so popular in recent times, were determined to exhibit a dual distribution of pore sizes, with a majority of micropores with diameters of less than 1 nm and a second group of mesopores with windows approximately 4 nm in diameter. The uptake of carbon monoxide from CCl4 or ethanol solutions onto the surface of the metal of these nanostructures was characterized first. It was determined that adsorption is possible with both samples, and that saturation on the metal surface precedes saturation on the oxide shell. This latter observation suggests that the access of the CO molecules (and all other molecules studied here) to the inside of the yolk–shell structures may be controlled by the mass transport of the liquid through the porous network of the shells, not by the diffusion of the dissolved gas alone. Additional studies were carried out with a family of cinchona alkaloid derivatives and with a porphyrin, covering a range of molecular sizes of approximately 5–20 Å in diameter. Adsorption of all those molecules on the Pt surfaces of both nanoarchitectures was deemed possible by a combination of tests, including the detection of changes in peak frequencies or peak intensities and measurements of the reversibility of the adsorption. Our results indicate that although the sol–gel synthetic method used to prepare the shells usually produces solids with microporous structures, a secondary mesoporous network that also develops in these nanostructures can provide a direct path to the metal nanocores, affording the mass transport of large molecules in liquid solutions in and out of the inside volumes. We also show that the attoliter-size volumes encased by typical shells in yolk–shell nanostructures are sufficient to afford adsorption displacement processes.

Evidence of concerted inversion for the photon-induced molecular switching of azobenzene using rotation-free azobenzene derivatives

The excited state relaxation of two types of azobenzene derivative excited at 350 nm in the short-wavelength (S2(π, π*)) band was studied by transient absorption spectroscopy. We compared the relaxation dynamics and the time-resolved anisotropic relaxation properties of these two molecules in liquid solution and in bulk film. The average relaxation life-time for Azo-2 in bulk film (13.2 ps) is much slower than that of Azo-2 (4.06 ps) and Azo-1(1.31 ps) in liquid solution. The time-resolved polarization anisotropy spectroscopy measurements of these samples also showed significant differences. The value of anisotropy for Azo-2 remained almost constant at about 0.2 in bulk film but decayed slowly from 0.3 in liquid solution. The value for Azo-1 in liquid solution decayed very fast from 0.4 to 0.2 within 6 ps. Our results gave new evidence for the existence of a converted inversion channel. We also concluded that the concerted inversion process and the conventional rotation process govern the relaxation for the photon-induced switches of azobenzene and its derivatives under short-wavelength band excitation.

Relaxation processes with participation of excited S 1 and T 1 states of spatially distorted meso-phenyl-substituted octamethylporphyrins

We have performed comparative analysis of the spectral properties and photophysical parameters of spatially distorted octamethylporphyrins (OMPs) upon varying the number (from one to four) and position of bulky meso-phenyl substituents in liquid solutions at 293 K and in solid media at 77 K. It has been substantiated that, for the series of studied compounds, which are characterized by a high degree of nonplanar distortions, considerable changes in the spectral-kinetic properties of singlet and triplet states at 293 K in liquid solutions are determined by the dynamic relaxation of the tetrapyrrole macrocycle that is caused by steric interactions of meso-phenyls with neighboring bulky groups β-CH3. The decay of T1 states of mono- and di-meso-phenyl-substituted (trans) OMP molecules in liquid solutions has a biexponential character, which is indicative of conformational rearrangements of the macrocycle in the T1 state. We are the first to measure the phosphorescence of spatially distorted metal-free OMP molecules in solid solutions at 77 K. It has been sub-stantiated that the decrease in the phosphorescence lifetime and quantum yield observed in the series of studied compounds with increasing number of meso-phenyl substituents at 77 K is related to a gradual increase in the probability of the nonradiative intersystem crossing, caused by the lowering of the energy of the T1 level. We have revealed that spatially distorted tetrapyrrole macrocycles in rigid media at 77 K in the ground state are represented by two conformations. It has been found that the oxygen quenching rate constants of excited S1 and T1 electronic states of planar and spatially distorted porphyrin molecules are close to each other and depend weakly on the character distortion of the macrocycle. We have shown that the singlet oxygen generation quantum yield (γδ) of studied compounds depends on the number of meso-phenyl substituents in the OMP molecule and is determined by the character of the conformational dynamics of the tetrapyrrole macrocycle at 293 K.

Effects of the structure of entrapped substituted porphyrins on the textural characteristics of silica networks

The outstanding photoluminescent properties of porphyrin molecules in liquid solution were extended to solid media when these species were chemically trapped inside translucent, monolithic SiO 2 pore networks. The entrapment was made in situ through the sol–gel technique by adding to the relevant macrocycle solution a functionalized silicon alkoxide, water, HCl, and (if required) a chemical agent that inhibited the aggregation or degradation of the porphyrin compound. The success in attaining solid materials that encapsulate and immobilize individual porphyrin molecules was not only based on the identity but also on the selection of the appropriate molar concentrations of the above reactants. It was found that the cavity sizes in which porphyrin molecules were encapsulated depended on the structure of the macrocyclic species as well as on the type of alkoxide used to structuralize the silica network. In principle, the sizes of the cavities containing the porphyrin molecules could be modulated in four possible ways: (i) by the physical insertion of the macrocycle species in the pore network; (ii) by linking porphyrin molecules endowed with specific end groups of given lengths to the pore walls; (iii) by using functionalized silicon alkoxides that create linking chains extending from the pore walls toward the macrocycle species (possibility not treated here); (iv) by affording the porphyrin and silicon alkoxide molecules with suitable functional groups that established a long covalent linking between the macrocycle and the pore walls. In these ways, the average cavity width usually varied from 2.0 to 3. 6 nm but this interval could even be extended from 3.6 to 9.4 nm. The whole picture of porphyrin encapsulation in porous media involved: (i) the creation and modulation of assorted cavity sizes, (ii) the modification of the polarity of the pore surface by the attachment of functionalizing molecules, and (iii) the permanence of the luminescent properties of porphyrin molecules when these are covalently encapsulated inside silica substrates.

Dynamic NMR spectroscopy of three 2,2′-dialkyl/aryl-1-picrylhydrazines in liquid solution

We have studied and compared the proton nuclear magnetic resonance spectra of a series of three 2,2'-di(alkyl/aryl)-1-(2,4,6-trinitrophenyl)hydrazines at 500 MHz in liquid toluene-d(8), with a view to learning details of the internal conformation changes taking place, as a function of temperature. Herein "alkyl" denotes 'methyl', and "aryl" denotes 'phenyl'. The synthesis and characterization of 2-phenyl,2'-methyl-1-picrylhydrazine is reported. One goal of the present study was to afford a comparison with the oxidation products, i.e., the three corresponding hydrazyl free radicals - which show striking differences. All the hydrazine molecules in liquid solution occur as interconverting enantiomorphic pairs (atropisomers). Advanced techniques for obtaining the relevant activation energies and pseudo-thermodynamic parameters (Arrhenius and Eyring) have been utilized, and these parameters are listed and discussed. The three hydrazines turn out be very similar with respect to their internal dynamics.

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.

Aggregation and spectral properties of Mg(II) tetracarboxyphthalocyanine in aqueous solutions and solid xerogels

Based on spectroscopic studies of magnesium(II) tetracarboxyphthalocyanine molecules in liquid solutions, we have determined the role of the carboxyl groups and the aqueous medium during formation of associates. We have shown that the Q band of the associated form is split into two components of frequency separated by ∼250 cm−1. The effect of the nature of the gel matrices on the spectral luminescence properties of magnesium(II) tetracarboxyphthalocyanine has been studied by incorporating the pigment into a series of xerogels differing in the chemical structure of the inorganic framework and the nanopore surface area. We discuss the reasons for the appearance of associated forms of the pigment in different xerogels.

Dynamic NMR spectroscopy of 2,2′-dimethyl-1-picrylhydrazine in various solvents

We have studied the 1H NMR spectra of 2,2′-dimethyl-1-(2,4,6-trinitrophenyl)hydrazine at 300 and 500 MHz in seven liquid solvents, with a view to learning details of the internal conformational changes taking place as a function of temperature and of the solvent. These molecules in liquid solution occur as interconverting enantiomorphic pairs (atropisomers). Advanced techniques for obtaining the correct activation energies and pseudo-thermodynamic parameters have been utilized, and these parameters are listed and discussed. Our results point to a transformation between the pair of atropisomers that is not quite as complicated as one might have encountered in that the solvent does not affect ΔG‡ in any major fashion. Molecular orbital calculations clarified some of the chemical shifts observed for both 1H and 13C. One goal of this study was to enable a detailed comparison with similar results available for 2,2′-diphenyl-1-(2,4,6-trinitrophenyl)hydrazine.Key words: dynamic NMR, dimethylpicrylhydrazine, hindered rotation, atropisomers, activation parameters.

Vibrational Energy Relaxation of Polyatomic Molecules in Liquid Solution via the Linearized Semiclassical Method

Vibrational energy relaxation (VER) of polyatomic, as opposed to diatomic, molecules can occur via different, often solvent assisted, intramolecular and/or intermolecular pathways. In this paper, we apply the linearized semiclassical (LSC) method for calculating VER rates in the prototypical case of a rigid, symmetrical and linear triatomic molecule (A-B-A) in a monatomic liquid. Starting at the first excited state of either the symmetric or asymmetric stretches, VER can occur either directly to the ground state or indirectly via intramolecular vibrational relaxation (IVR). The VER rate constants for the various pathways are calculated within the framework of the Landau-Teller formalism, where they are expressed in terms of two-time quantum-mechanical correlation functions. The latter are calculated by the LHA-LSC method, which puts them in a "Wignerized" form, and employs a local harmonic approximation (LHA) in order to compute the necessary multidimensional Wigner integrals. Results are reported for the LHL/Ar model of Deng and Stratt [J. Chem. Phys. 2002, 117, 1735], as well as for CO(2) in liquid argon and in liquid neon. The LHA-LSC method is shown to give rise to significantly faster VER and IVR rates in comparison to the classical treatment, particularly at lower temperatures. We also find that the type and extent of the quantum rate enhancement is strongly dependent on the particular VER pathway. Finally, we find that the classical and semiclassical treatments can give rise to opposite trends when it comes to the dependence of the VER rates on the solvent.

Recalling the appropriate representation of electronic spectra

This paper presents a critical discussion on the representation of UV–vis–NIR light absorption and fluorescence emission spectra of polyatomic molecules in liquid solution. A general overview how to handle the experimental spectra in order to extract physical data from them is presented together with some simulations to show the errors that can arise from unfortunate manipulation.

Development of SH-SAW sensors for underwater measurement

We developed SH (shear horizontal) surface acoustic wave (SAW) sensors to detect protein molecules in liquid solutions applying a particular antibody thin film on the delay line of transverse SAW devices. The antibody investigated was human-immuno-globulin G (HigG) to hold the antigens (anti-HigG) in the protein solution to be measured. The sensor showed stable response to the mass loading effects of the anti-HigG molecules with the sensitivity up to 10.8 ng/ml/Hz.

Study of interaction of aliphatic alcohols with primary radical cations of n-alkanes using MARY spectroscopy

MARY spectroscopy (the radical ion pair level crossing technique) was employed to probe the reaction of proton transfer from primary radical cations of n-alkanes to alcohol molecules in liquid solution. Alcohols were demonstrated to react with the primary radical cation of the solvent, leaving the counterion of the radical ion pair unaffected. The broadening of the zero field MARY line, tentatively attributed to the proton transfer reaction, was found to be independent of the proton affinity of the species in the studied systems, estimated from gas-phase data. The rate constant of the reaction is close to the diffusion controlled limit within the experimental accuracy for all the studied alcohol/solvent combinations.

Contribution of Diffusion to the Quenching by Oxygen of the Lowest Electronically Excited Singlet and Triplet States of Aromatic Molecules in Liquid Solution under High Pressure

The quenching by oxygen of the lowest electronically excited singlet (S1) and triplet (T1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S1 state, Δ , at 0.1 MPa fell in the range from 14 to 16 cm3/mol, which is significantly smaller than Δ (25 cm3/mol) determined from the pressure dependence of the solvent viscosity, η, whereas that for the T1 state, Δ , at 0.1 MPa changed from +6.1 (anthracene) to −15.2 cm3/mol (triphenylene) and correlated approximately linearly with the triplet energy, ET. However, Δ at 350 MPa was positive and almost independent of ET (3−6 cm3/mol). The pressure dependence of was interpreted in the framework reported previously by us, and that of was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T1 state molecules is involved.

Magnification in excess of 100-times of the microscopic photothermal lensing signal from solute molecules by two-color excitation with continuous-wave lasers.

A microscopic photothermal lensing measurement under two-color continuous-wave laser excitation was performed to investigate a signal enhancement owing to the transient absorption by photoexcited solute molecules in liquid solutions. An intensity-modulated 409 nm laser beam and an un-modulated 532 nm laser beam were used for excitation, and a 670 nm probe beam was used for detecting the modulation amplitude of thermal lensing signals generated with a microscopic objective lens focusing laser beams into a capillary flow cell of 0.1 mm optical path length. The amplitude of the modulated signal increased as the power of the un-modulated laser beam increased, and a 143-times magnification was observed for an iso-propanol solution of naphthacene having 4.6 x 10(-4) absorbance at 409 nm and a negligible absorbance at 532 nm. A four-level model explaining the signal enhancement is proposed, and an important role of the transient absorption by photoexcited molecules is discussed.

Time-dependent density functional theory for molecules in liquid solutions

A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n→π* transition of diazabenzenes in different solvents. The computed transition energies are in fairly good agreement with experimental values.