Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening

Fluorescence of tryptophan in aqueous solution

In this work, the absorption and emission spectra of Tryptophan (Trp) in aqueous solution were studied. Moreover, a hydrogen-bonded zwitterionic Trp(H2O)9 model was proposed and its ground-state and excited-state properties were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, respectively. All spectroscopic data in our experiments can be well explained by the hydrogen bond strengthening in the excited state of the model complex. The delocalization of electron density between indole moiety and neighboring H2O molecules in fluorescent state was proposed to be facilitated by the strengthened hydrogen-bond chain, and thus resulting in the large red-shift fluorescence of Trp in aqueous solution.

A TDDFT study on the excited-state intramolecular proton transfer (ESIPT): excited-state equilibrium induced by electron density swing

One important issue of current interest is the excited-state equilibrium for some ESITP dyes. However, so far, the information about the driving forces for excited-state equilibrium is very limited. In this work, the time-dependent density functional theory (TDDFT) method was employed to investigate the nature of the excited-state intramolecular proton transfer (ESIPT). The geometric structures, vibrational frequencies, frontier molecular orbitals (MOs) and the potential-energy curves for 1-hydroxy-11H-benzo[b]fluoren-11-one (HHBF) in the ground and the first singlet excited state were calculated. Analysis of the results shows that the intramolecular hydrogen bond of HHBF is strengthened from E to E*. Moreover, it is found that electron density swing between the proton acceptor and donor provides the driving forces for the forward and backward ESIPT, enabling the excited-state equilibrium to be established. Furthermore, we proposed that the photoexcitation and the interchange of position for electron-donating and electron-withdrawing groups are the main reasons for the electron density swing. The potential-energy curves suggest that the forward ESIPT and backward ESIPT may happen on the similar timescale, which is faster than the fluorescence decay of both E* and K* forms.

Effects of intermolecular hydrogen bonding on the excited-state proton transfer properties of the cinnamonitrile–methanol complex

The time-dependent density functional theory (TD-DFT) method was used to study the excited-state proton transfer (ESPT) properties of the hydrogen-bonded cinnamonitrile (3TPAN)–methanol (MeOH) complex (3TPAN–MeOH). The intermolecular hydrogen bonds N1···H11 in both the ground state S0 and the excited state S1 were demonstrated by the optimized geometric structures of the hydrogen-bonded 3TPAN–MeOH complex. While in the excited state S3, a new hydrogen bond H11···O1 was formed after the ESPT took place from the hydrogen-bonded MeOH molecule to the 3TPAN moiety. It was demonstrated that the electronic transitions of the S1 states for both the 3TPAN monomer (including the S3 state) and the hydrogen-bonded 3TPAN–MeOH complex should be of a localized-excited (LE) nature on the 3TPAN molecule, while the S3 state of the hydrogen-bonded 3TPAN–MeOH complex should be of charge transfer (CT) character from the hydrogen-bonded MeOH molecule (through O1···H11) to the 3TPAN moiety. The S3-state proton transfer and charge transfer due to the intermolecular hydrogen-bonding interaction should be the reasons for the remarkable redshift (0.91 eV) of the S3-state electronic energy for the hydrogen-bonded 3TPAN–MeOH complex compared with that of the 3TPAN monomer.

Protochlorophyllide in model systems--an approach to in vivo conditions

Absorption and fluorescence properties of protochlorophyllide (Pchlide) monomers and aggregates in various model systems are presented in this study. The absorption and fluorescence maxima, and fluorescence lifetimes of Pchlide monomers were not dependent on liposome composition. Fluorescence quenching experiments using KI and SASLs as fluorescence quenchers, revealed that Pchlide molecules entered a lipid bilayer and were localized close to the polar lipid headgroup area. The process of Pchlide aggregation was evident for high (i.e. at least 9 mol%) Pchlide content in liposomes prepared from galactolipids. To our knowledge, this is the first study of Pchlide aggregation in membrane-mimicking model systems. The aggregates showed absorption maxima at 480 and 650 nm. Fluorescence of the aggregates measured for excitation at 480 nm had a maximum at 656 nm and was characterized with two fluorescence lifetime components, i.e. 0.1 and 1-2 ns. Pchlide aggregates observed in the buffer had similar position of absorption and fluorescence bands to those observed in liposomes, although the overall fluorescence intensity was considerably lower. Some differences in the relative intensity of Soret absorption bands were observed. These results showed that the presence of liposomes decreased the efficiency of the process of Pchlide aggregation. Water bound at the interface region of AOT/isooctane/water reversed micelles induced disaggregation of the Pchlide aggregates indicating that Pchlide aggregates are buried into hydrophilic core of micelles. The results are discussed with respect to the role of lipids in Pchlide aggregation found in plant etioplasts and their significance for light-induced Pchlide photoreduction.

DRASTIC INFLUENCE OF BORON ATOM ON THE ACIDITY OF ALCOHOL IN BOTH GAS PHASE AND SOLUTION PHASE, A DFT STUDY

In this study, the drastic influence of the boron atom on the acidity of alcohol has been considered. The calculated ΔH acid (320.9–338.1 kcal/mol) and pKa range of boron containing alcohol (-0.1–9.4) indicate that the boronation of alcohol leads to considerable enhancement of its acidity. For instance, we have obtained the ΔH acid values 338.1, 335.2 kcal/mol and the pKa values 4.12, 2.81 for BH2CH2OH , BF2CH2OH alcohols, respectively, which are much smaller than that of CH3OH (with ΔH acid = 374.9 kcal/mol and pKa = 15). The increase in the acidity of boronated alcohol can be related to the stabilization of alkoxy ion due to overlap of unoccupied orbital of boron atom with the electron pairs of negative oxygen. All gas phase computations were performed at MP2/6-311++G(d,p)//(B3LYP/6-31+G(d)) level. The primary results indicate that the presence of boron atom in an alcohol might make it as acidic as nitric acid. The geometry optimization of studied structures was performed with DFT computation and optimized structures were used to carry out natural bond orbital (NBO) analysis. NBO analysis revealed that the increase in the acidity of boron-containing alcohols is due to the charge transfer from the negative oxygen (in deprotonated structure) to the empty orbital of - BH2 and - BF2 . Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of bonds formed in the deprotonated structure.

A theoretical study of hydration effects on structural stability and hydrogen-bonding dynamics of 2′-deoxyguanosine 5′-monophosphate with different negative charges

The effects of hydration on the ground-state structural stability and excited-state hydrogen-bonding dynamics of 2′-deoxyguanosine 5′-monophosphate (dGMP) carrying different negative charges were investigated with B3LYP/6–31+G(d,p) using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, respectively. Particularly, we not only considered the solvent effects by the polarizible continuum model (PCM), but also the first solvation shell was included explicitly. We demonstrated that the intramolecular hydrogen bond O2–H1···O3 will be weakened with the strengthening of the hydration. From the view of bond length, we can make a valid presumption that the site of negative charge will be the more preferable site of the hydration, and the preferable site may be changed because of the presence of other hydrogen bonds. Furthermore, we found that the first solvation shell had very little effect on the geometric structures except for the hydrogen bond P–O5···H5. By comparing the excitation energies, one important finding is that the changes in different electronic states are not obvious with the increase in n value when considering the PCM. Another finding is that the average interactions of hydrogen bonds may be strengthened with an increase of negative charge because of a decrease in excitation energies.

Time-dependent density functional theory (TD-DFT) study on the excited-state intramolecular proton transfer (ESIPT) in 2-hydroxybenzoyl compounds: significance of the intramolecular hydrogen bonding

The excited-state properties of intramolecular hydrogen bonding (IMHB) in methyl salicylate (MS) and its effects on the excited-state intramolecular proton transfer (ESIPT) have been investigated using theoretical methods. From the geometric optimization and IR spectra in the ground and excited states calculated by density functional theory (DFT) and time-dependent DFT (TD-DFT) methods respectively, the IMHB is demonstrated to be significantly strengthened upon excitation to excited state S(1). Thereby, the ESIPT is facilitated by the excited-state IMHB strengthening since ESIPT takes place through IMHB. In addition, the absorption and fluorescence peaks of the S(1) state are also calculated using the TD-DFT method. It is noted that the calculated spectra are in good agreement with the experimental results, which has confirmed the ESIPT mechanism of MS first proposed by Weller. Moreover, other four 2-hydroxybenzoyl compounds forming strong IMHB are investigated to understand the effect of substituent R on the ESIPT process. We find that the hydrogen bond strength can be controlled by the inductive field effect of the substituent. Thus it is inferred that the ESIPT reaction can be facilitated by the inductive effect of electron-donating substituent.

A DFT/TDDFT study on the excited-state hydrogen bonding dynamics of 6-aminocoumarin in water solution

Intermolecular hydrogen bonding, as a site-specific solute-solvent interaction, plays an extremely important role on the spectra of chemical and biological molecules. A detailed theoretical study on the hydrogen bonds is extremely necessary for understanding the unusual spectra blue-shift for some molecules. In the present paper, the hydrogen bonding dynamics of 6-aminocoumarin (6AC) in the water was investigated by the time-dependent density functional theory (TDDFT) and density functional theory (DFT) methods. According to our results, it is demonstrated that C6-N1⋯H9 bond (AHB) is the strongest hydrogen bonds among three ones formed between 6AC and water molecules in the ground state. Upon the photoexcitation, due the photo-induced charge transfer from N1 to C6, AHB is proved to be extremely weakened which is significantly greater than the strengthening of C9=O2⋯H11 bond (BHB) and N1-H6⋯O5 bond (CHB) in S1 states. This is expected to be theoretical explanation for the unusual blue-shift of a long-wavelength band in absorption spectra for 6AC in the water observed in the experiment.

Protochlorophyllide and protochlorophyll in model membranes - an influence of hydrophobic side chain moiety

In the present work, a comparative study of protochlorophyllide- and protochlorophyll-lipid interaction was performed on liposomes prepared from phospholipids and galactolipids, which had a pigment content varying from 0.1 to 4mol%. The incorporation of pigment molecules into the lipid bilayer and pigment-pigment interactions were investigated. Protochlorophyllide entered the lipid bilayer spontaneously and showed fluorescence spectra characteristic of its monomers. Similar spectra were observed for protochlorophyll where its concentration was low. However, the fluorescence maxima of protochlorophyll monomers were blue-shifted compared to those of protochlorophyllide by about 5nm. Protochlorophyll at high concentrations formed transient aggregates that showed an additional fluorescence band with a maximum at around 685nm, especially in liposomes prepared from phospholipids. For both compounds, the Stern-Volmer constant for KI quenching was much lower in liposomes than in solution, which confirmed the incorporation of these compounds into the lipid bilayer. Two populations of protochlorophyll that differed in their accessibility to quenching by KI were determined, and the proportions between them for different lipids are discussed. Protochlorophyllide showed such heterogeneity only in DPPC membranes. Quenching with 5- and 16-SASL revealed a localization of the porphyrin ring of both Pchl and Pchlide in the polar headgroup area of the lipid bilayer. The side chain of protochlorophyll forced these molecules to localize deeper in the bilayer in the case of DPPC in gel phase.

ELECTRON TRANSFER THROUGH NON-HYDROGEN AND HYDROGEN BONDED INTERMOLECULAR TUNNEL JUNCTIONS: A COMPUTATIONAL STUDY

The nature of electron transfer through intermolecular tunnel junctions has been studied. Intermolecular junctions feature non-hydrogen and hydrogen bonded interactions and are defined by the end groups of pairs of functionalized conjugated alkenes. We compared the distant dependent electron transfer from donor to acceptor group attached to the other sides of those functionalized conjugated alkenes. Study shows electron transfer coupling matrix element for these intermolecular junctions decays exponentially as a function of junction separation and quite substantial in energetically favourable hydrogen bonded intermolecular junctions.

TD-DFT study on the excited-state proton transfer in the fluoride sensing of a turn-off type fluorescent chemosensor based on anthracene derivatives

The sensing mechanism for fluoride chemosensor based on anthracene structure has been investigated by DFT and TDDFT methods. The results show that the similar geometries in S(0) and S(1) states in the absence of the fluoride anion induce the local excited (LE) state over the anthracene moiety, which is responsible for the strong fluorescence. The fluorescence quenching phenomenon for F-coordinated complex can be explained by the photoinduced electron transfer (PET) process from benzylic amide to anthracene moiety. Moreover, the strong electronegativity for fluoride anion in the F-complex constructs the intermolecular hydrogen bond of N-H…F in the ground state. In contrast, the upper proton in the urea group close to phenyl group prefers to bind fluoride anion in S(1) state of the F-complex, and it presents excited-state proton transfer (ESPT) to form another hydrogen bond like N…H-F, which has been confirmed by natural bond orbital (NBO) analysis and the potential energy curve of S(1) state for the function of N-H bond. In this novel and efficient fluoride fluorescent chemosensor based on anthracene, the anthracene moiety is regarded as not only the fluorescent source, but the template for introducing the binding sites for fluoride anion.

Response of an environment-sensitive intramolecular charge transfer probe towards solubilization of liposome membranes by a non-ionic detergent: association and dissociation kinetics

The present report describes an endeavor to follow the solubilization of DMPC and DMPG liposome membranes by a non-ionic detergent Triton X-100 on the lexicon of environment-sensitive intramolecular charge transfer (ICT) photophysics of an extrinsic molecular probe 5-(4-dimethylamino-phenyl)-penta-2, 4-dienoic acid methyl ester (DPDAME). The prospective applicability of the probe to function as a reporter for detergent-sequestered solubilization of liposome membranes is argued on the basis of comparison of the spectral properties of the probe in various environments. Fluorescence anisotropy study delineates the degree of motional restriction imposed on the probe in different microheterogeneous assemblies. The kinetics of association of the probe with the liposome membranes and the dissociation kinetics of TX-100-sequestered solubilization process of the liposomes have been monitored by the stopped-flow fluorescence technique and the results are rationalized in relevance to fluorescence anisotropy study.

Mechanistic reappraisal of early stage photochemistry in the light-driven enzyme protochlorophyllide oxidoreductase

The light-driven enzyme protochlorophyllide oxidoreductase (POR) catalyzes the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide). This reaction is a key step in the biosynthesis of chlorophyll. Ultrafast photochemical processes within the Pchlide molecule are required for catalysis and previous studies have suggested that a short-lived excited-state species, known as I675*, is the first catalytic intermediate in the reaction and is essential for capturing excitation energy to drive subsequent hydride and proton transfers. The chemical nature of the I675* excited state species and its role in catalysis are not known. Here, we report time-resolved pump-probe spectroscopy measurements to study the involvement of the I675* intermediate in POR photochemistry. We show that I675* is not unique to the POR-catalyzed photoreduction of Pchlide as it is also formed in the absence of the POR enzyme. The I675* species is only produced in samples that contain both Pchlide substrate and Chlide product and its formation is dependent on the pump excitation wavelength. The rate of formation and the quantum yield is maximized in 50∶50 mixtures of the two pigments (Pchlide and Chlide) and is caused by direct energy transfer between Pchlide and neighboring Chlide molecules, which is inhibited in the polar solvent methanol. Consequently, we have re-evaluated the mechanism for early stage photochemistry in the light-driven reduction of Pchlide and propose that I675* represents an excited state species formed in Pchlide-Chlide dimers, possibly an excimer. Contrary to previous reports, we conclude that this excited state species has no direct mechanistic relevance to the POR-catalyzed reduction of Pchlide.

DENSITY FUNCTIONAL STUDY ON THE PROTON ASSISTED ISOMERIZATION OF 1H-IMIDAZO(4,5-b)PYRIDINE DERIVATIVES

The proton assisted isomerization reactions of 1H-imidazo(4,5-b)pyridine (IMP) derivatives have been studied by using B3LYP/6-31G + (d,p) calculations, and the transition states of the reactions are analyzed with B3LYP/6-31+G(d,p) opt=qst3 route. It has been found that the prototropic transformation could be the feasible pathway of isomerization, since the energy gaps between the various protonated isomers are found closer compared to free molecules. The conversion of IMP-a1 to IMP-b1 may pass through several protrotopic isomerization, since the activation energy as well as the relative energy levels of these isomers are not small compared to other pathways. However, the results suggest that some of the reactions may take place simultaneously through protrotopic transformation. The relative variations of energy gaps in the excited states are smaller than the ground states. The protrotropic transformation in the excited states may be more feasible than the ground state.

FACILITATED PHOTOLYSIS OF 9-FLUORENOL IN ALCOHOLS BY EXCITED-STATE HYDROGEN BOND REORGANIZATION

Time-dependent density functional theory (TDDFT) and second-order coupled cluster method with resolution-of-the-identity approximation (RICC2) were used to investigate the photolysis dynamics of 9-fluorenol (FOH) in alcohols. In this work, a novel mechanism for the accelerated photolysis dynamics of FOH in alcohols is proposed for the first time. The two hydrogen bonds present different effects in the dissociation process of C9–O bond in MeOH⋯FOH⋯MeOH trimer: formation of hydrogen bond MeOH⋯FOH could weaken the C9–O bond, while, hydrogen bond FOH⋯MeOH fastens the bond. Moreover, the thermodynamic equilibrium can be accomplished in both ground and excited states between hydrogen-bonded complexes, since the hydrogen bond reorganization occurs in hundreds of femtosecond upon the excitation. The excited-state potential energy (PE) curves along C9–O bond have been optimized in S1 state. The cleavage of C9–O bond upon the photoexcitation would be facilitated effectively in MeOH⋯FOH dimer. This leads the thermodynamic equilibrium between hydrogen-bonded complexes leaning to the side of MeOH⋯FOH dimer to quench the fluorescence. Therefore, the photolysis of 9-fluorenol in alcohols can be facilitated effectively by MeOH⋯FOH hydrogen bond via excited-state hydrogen bond reorganization. Additionally, the excited-state hydrogen bond reorganization is also the rate-controlling step in photolysis of FOH in alcohols, since there is no barrier in the PE curve of MeOH⋯FOH dimer.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON THE ELECTRONIC EXCITED-STATE HYDROGEN BONDING DYNAMICS OF METHYL ACETATE IN AQUEOUS SOLUTION

The time-dependent density functional theory (TDDFT) method has been carried out to investigate the hydrogen-bonding dynamics of methyl acetate ( CH 3 CO 2 CH 3) in hydrogen-donating water solvent. The ground-state geometry optimizations, electronic transition energies and corresponding oscillation strengths of the low-lying electronically-excited states for the isolated CH 3 CO 2 CH 3 and H2O monomers, the hydrogen-bonded CH3CO2CH3-(H2O)1, 2 complexes have been calculated using DFT and TDDFT methods respectively. One intermolecular hydrogen bond C=O⋯H–O is formed between CH3CO2CH3 and one water molecule in CH3CO2CH3-H2O dimer. Meanwhile, in CH3CO2CH3-(H2O)2 trimer, two intermolecular hydrogen bonds C=O⋯H–O are formed between CH3CO2CH3 and two water molecules. By theoretically monitoring the excitation energy changes among the CH3CO2CH3 monomer, the CH3CO2CH3-H2O dimer, and the CH3CO2CH3-(H2O)2 trimer, we have demonstrated interestingly that in some electronically-excited states, the intermolecular hydrogen bonds are strengthened inducing electronic spectral redshifts, while in others weakened with electronic spectral blueshifts. The phenomenon that hydrogen bonds are strengthened in some electronic states while weakened in others can arouse further probe into CH3CO2CH3-(H2O)1, 2 complexes.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON DYNAMICS OF HYDROGEN BONDING IN EXCITED STATES OF TRANS-ACETANILIDE IN METHANOL SOLVENT

The hydrogen-bonding dynamics in both singlet and triplet excited states of the trans-acetanilide ( AA ) in methanol ( MeOH ) solvent was investigated using the time-dependent density functional theory (TDDFT) method. Geometric optimizations of the hydrogen-bonded AA–MeOH complexes considered here as well as the isolated AA and MeOH molecules were performed using density functional theory (DFT) method. At the same time, the TDDFT method was performed to calculate the electronic transition energies and corresponding oscillation strengths of all the compounds in the low-lying electronically excited states. In this study, only the intermolecular hydrogen bonds C=O⋯H–O and N–H⋯O–H can be formed. A theoretical forecast that changes of hydrogen bonds in the low-lying electronic excited states was proposed. We discussed not only ground-state geometric structures and electronic excitation energies but also frontier molecular orbitals and electron density transition. The intermolecular hydrogen bonds between AA and MeOH molecules play an important role in the geometric structures and electronic excitation energies. Zhao et al. have put forward the relationship between the electronic spectra and hydrogen bonding dynamics for the first time. According to Zhao's rule, a redshift of the relevant electronic spectra will appear if hydrogen bond is strengthened, while the hydrogen bond weakening can make an electronic spectra shift to blue.

Hydrogen bonding in the electronic excited state

Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated. Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtosecond time-resolved vibrational spectroscopy is used to directly monitor the ultrafast dynamical behavior of hydrogen bonds in the electronic excited state. It is important to note that the excited-state hydrogen-bonding dynamics are coupled to the electronic excitation. Fortunately, the combination of femtosecond time-resolved spectroscopy and accurate quantum chemistry calculations of excited states resolves this issue in laser experiments. Through a comparison of the hydrogen-bonded complex to the separated hydrogen donor or acceptor in ground and electronic excited states, the excited-state hydrogen-bonding structure and dynamics have been obtained. Moreover, we have also demonstrated the importance of hydrogen bonding in many photophysical processes and photochemical reactions. In this Account, we review our recent advances in electronic excited-state hydrogen-bonding dynamics and the significant role of electronic excited-state hydrogen bonding on internal conversion (IC), electronic spectral shifts (ESS), photoinduced electron transfer (PET), fluorescence quenching (FQ), intramolecular charge transfer (ICT), and metal-to-ligand charge transfer (MLCT). The combination of various spectroscopic experiments with theoretical calculations has led to tremendous progress in excited-state hydrogen-bonding research. We first demonstrated that the intermolecular hydrogen bond in the electronic excited state is greatly strengthened for coumarin chromophores and weakened for thiocarbonyl chromophores. We have also clarified that the intermolecular hydrogen-bond strengthening and weakening correspond to red-shifts and blue-shifts, respectively, in the electronic spectra. Moreover, radiationless deactivations (via IC, PET, ICT, MLCT, and so on) can be dramatically influenced through the regulation of electronic states by hydrogen-bonding interactions. Consequently, the fluorescence of chromophores in hydrogen-bonded surroundings is quenched or enhanced by hydrogen bonds. Our research expands our understanding of the nature of hydrogen bonding by delineating the interaction between hydrogen bonds and photons, thereby providing a basis for excited-state hydrogen bonding studies in photophysics, photochemistry, and photobiology.

ELECTRON TRANSFER MAKES D3h (78:5) CAGE EASY TO FORM M2@C78(M = La, Ce): A RELATIVISTIC DENSITY-FUNCTIONAL THEORY STUDY

Applying relativistic density functional theory to isomers of C 78 and M 2@ C 78( M = La , Ce ), we calculate and analyze the relative energies and HOMO–LUMO gaps of neutral and hexaanion ( -6 charged) C 78 isomers. Our results indicate that the [Formula: see text] (5) isomer is the most stable, and it illustrate that electron transfer plays an important role in controlling the stability of endohedral metallofullerenes. We also calculate the electronic structures of there neutral isomers, and based on their LUMO + 2 and LUMO + 3 gaps, we explain why it is easier to encage two metal atoms in D3h′ (78:5). To further elucidate this issue, we theoretically characterize M 2@ C 78( M = La , Ce ) and compare the relative energies and the HOMO–LUMO gap of the two isomers M 2@ C 78 (4) and the M 2@ C 78 (5) ( M = La , Ce ). The results indicate that M 2@ C 78 (5) is more stable than M 2@ C 78 (4). Furthermore, the good agreement between the experimental and computed 13C NMR chemical shift of the isomer M 2@ C 78 (5) provided strong evidence that M 2@ C 78 forms a D3h′ (78:5) cage.

Theoretical reconsideration on the hydrogen bonding and coordination interactions of chlorophyll a in aqueous solution

In the present work, explicit water molecule and solvent-field effects on the absorption spectrum of chlorophyll a have been studied using time-dependent density functional theory (TDDFT) method. Calculated results show that the one complex and two water coordinated complexes formed by concerted coordination and hydrogen-bonding interactions would be the most preferable conformations of chlorophyll a in aqueous surroundings. Moreover, four obvious absorption bands are assigned by comparing the theoretically simulated absorption spectra with the experimental ones. The theoretical study shows that the explicit water molecule interactions slightly influence the first absorption band. However, the water coordination and hydrogen-bonding interactions can significantly affect the second absorption band which has a strong red-shift. The solvent-field effect due to the polarity of water on absorptions in Q-bands is relatively smaller than that on absorptions in B-bands. As a consequence, our theoretical study on the absorption spectra in the 350–400 nm region presents that the absorption strength in this region was influenced by the explicit coordination and hydrogen bonding interactions from water molecules, significantly.