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The development of anion sensors for selective detection of a specific anion is a crucial research topic. We previously reported a selective photo-induced colorimetric reaction of 1-methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium (MNEI) having a cationic receptor in the presence of molecules having multiple carboxy groups, such as succinate, citrate, and polyacrylate. However, the mechanism underlying this reaction was not clarified. Here, we investigate the photo-induced colorimetric reaction of N-[2-(trimethylammonium)ethyl]-1,8-naphthalimide (TENI), which has a different cationic receptor from MNEI and undergoes the photo-induced colorimetric reaction, and its analogues to clarify the reaction mechanism. The TENI analogues having substituents on the naphthalene ring provide important evidence, suggesting that the colorimetric chemical species were radical anions generated via photo-induced electron transfer from carboxylate to the naphthalimide derivative. The generation of the naphthalimide-based radical anion is verified by 1H NMR and cyclic voltammetry analyses, and photo-reduction of methylene blue is mediated by TENI. In addition, the role of the cationic receptor for the photo-induced colorimetric reaction is investigated with TENI analogues having different hydrophilic groups instead of the trimethylammonium group. Interestingly, the photo-induced colorimetric reaction is observed in a nonionic analogue having a polyethylene glycol group, indicating that the colorimetric reaction does not require a cationic receptor. On the other hand, we reveal that the trimethylammonium group stabilizes the radical anion species. These generation and stabilization phenomena of naphthalimide-based radical anion species will contribute to the development of sophisticated detection systems specific for carboxylate.

1-Methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium (MNEI) salts show various photochemical phenomena depending on the guest anion. We used machine learning techniques to extract decisive factors that dominate the fate of MNEI salts after UV irradiation and prepared a decision tree diagram to deduce their photochemical phenomena before experiments.

1-Methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium can be a discriminator for anionic polysaccharides via monomer-excimer switching and photo-induced colorimetric reaction. In the system, the exci...

We have evaluated the strength of intramolecular hydrogen bond in a protein based on molecular dynamics and quantum chemical calculation. To estimate the intramolecular hydrogen bond strength in okadaic acid (OA), we analyzed the influence of solvent and protonation states on the hydrogen bond and the entire structure. We performed molecular dynamics calculation and analyzed the strength of the hydrogen bond by measuring bond length and bond angle. The stable structure differs depending on the kind of solvent used and the protonation state of OA. Using the mean interaction energy from the quantum chemical calculation, hydrogen bond length and angle were investigated against bond energy. Although dielectric constant slightly depends on bond energy, the estimation of the intramolecular hydrogen bond strength in OA is possible even in a protein environment. The Coulomb interaction between OA and surrounding arginine produced a more negatively charged O1 in OA. The hydrogen bond energy in the deprotonated state is larger than that in the protonated state.

We investigated the charge-transfer reactions of solar cells including a quaterthiophene copolymer with naphtho-bis-thiadiazole (PNTz4T) and naphtho-bis-oxadiazole (PNOz4T) using constrained density functional theory (CDFT). According to our calculations, the high electron-transfer rate results in a highly efficient solar cell, and the stable charge-transfer state results in low energy loss. Our computations imply that the following three factors are crucial to improve the performance of semiconducting polymers: (i) large structural changes following charge-transfer, (ii) narrow band gap, and (iii) spatially delocalized lowest unoccupied molecular orbital (LUMO) of the ground state.

We have investigated the dependencies of various computational methods in the calculation of acid dissociation constants (pKa values) of certain chemical groups found in protonatable amino acids based on our previous scheme [Matsui; Phys. Chem. Chem. Phys. 2012, 14, 4181−4187]. By changing the quantum chemical (QC) method (Hartree–Fock (HF) and perturbation theory, and composite methods, or exchange–correlation functionals in density functional theory (DFT)), basis sets, solvation models, and the cavities used in the solvent models, we have exhaustively tested about 2,200 combinations to find the best combination for pKa estimation among them. Of the tested parameters, the choice of the basis set and cavity is the most crucial to reproduce experimental values compared to other factors. Concerning the basis set, the inclusion of diffuse functions is quite important for carboxyl, thiol, and phenol groups judging from the mean absolute errors (MAEs) measured from the experimental values. Of the cavity models...

Development of host materials is crucial for organic electroluminescent devices. In order to generalize the way to molecular design the host materials, we here present a guideline to design effective host materials for blue phosphorescent devices based on 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) derivatives, comparing some theoretical parameters of seven CBP derivatives with experimental ones in terms of thermal stability, T1 energy level, and band gap. Although our computation at the density functional theory level could not reproduce the absolute values and phenomena that were experimentally obtained, theoretical relative correlations between different molecules agreed with experimental ones. Indeed, both experimental and theoretical approaches predicted that the same molecule (1,1-bis(4-(9-carbazolyl)-phenyl)cyclohexane) was viable as blue host material among the target molecules in terms of thermal stability, the T1 state with the LE character within carbazole moieties and T1 energy 3.00 eV higher t...

We have designed a novel 1,8-naphthalimide derivative with an open space for an anion. Computational calculation has predicted that the space could trap various anion species and photo-induced charge transfer depending on the anion's electrophilic properties. Indeed, the fluorescence behaviour of the 1,8-naphthalimide derivative complexes with each anion is consistent with the computational prediction.

We estimated the rate constant of a model consisting of an oligo(para-phenylenevinylene) (OPV) dimer in order to investigate the charge recombination of poly(para-phenylenevinylene) (PPV) using con...

1-Methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium (MNEI) has potential as a versatile sensor that can measure the electronegativity of anions based on the fluorescence intensity upon irradiation. To clarify the factors that determine the fluorescence intensity, constrained density functional theory (CDFT) was applied to explore the electron transfer (ET) states of MNEI halide species (MNEI–X; X = F, Cl, Br, I). According to the CDFT potential energy surface, intra-molecular ET (SM1) states on MNEI are responsible for the intensity of absorption and fluorescence spectra. However, inter-molecular ET (SET) states between MNEI and X are certainly responsible for fluorescence quenching. Hence, the energetic difference between the SM1 state and the SET state (ΔEM1_ET) is a crucial factor that determines the fluorescence intensity in the spectra of MNEI–X complexes. ΔEM1_ET decreases as the electronegativity of X decreases (i.e., F > Cl > Br > I). This explains the fluorescence intensity of MNEI–X.

We have investigated the dependencies of various computational methods in the calculation of acid dissociation constants (pK

A series of arylacetylenyl-substituted stilbenes 1–3 were synthesized as pure trans - and cis -isomers and their photochemistry was explored. The fluorescence spectra of trans- 1–3 are highly red-shifted and much more intense ( Φ f = 0.66–0.81) compared to that of the parent trans -stilbene ( Φ f = 0.04). The fluorescence lifetime analysis and the low-temperature excitation spectra indicate that both planar and perpendicular conformations at the arylacetylene moiety should exist in the ground state because of free rotation around the single bonds, whereas in the excited singlet state, an immediate structural change to the stable planar conformation takes place that results in efficient fluorescence. Most remarkably, the fluorescence quantum yields are 0.44 and 0.35 for cis - 2 and 3 , respectively, which are unexpectedly high and more than 3000 times more efficient than that of the parent cis -stilbene ( Φ f = 10 −5 ). All isomers exhibited mutual cis – trans photoisomerization. The trans / cis ratios at the photostationary state ([ trans ]/[ cis ]) pss are 93/7, 96/4 and 100/0 for 1 , 2 and 3 , respectively, which indicates that 3 undergoes one-way cis -to- trans photoisomerization. The transient absorption spectra suggest that both 2 and 3 undergo intersystem crossing to the triplet state where the planar triplet state ( 3 trans * and 3 cis *) and the perpendicular triplet state ( 3 p*) are in equilibrium, but the population of the planar triplet state is predominant. Thus, the photoisomerization should proceed partially from the excited triplet state.

Electron transfer (ET) states of 1-methyl-3-(N-(1,8-naphthalimidyl)ethyl)imidazolium iodide are responsible for its photophysics. Investigation of an ET state based on constrained density functional theory (CDFT) revealed that nonradiative decay from the ET excited state is mediated by the interaction of the iodine atom with the 1,8-naphthalimide or the imidazolium group.

The charge transfer property of the 9,10-diphenylanthracene (DPA) single-crystal system was investigated by density functional calculations. The hole mobility of DPA was predicted according to a hopping mechanism and compared with that of two standard organic single-crystal systems, namely, naphthalene and anthracene. The reorganization energy was calculated by the adiabatic potential energy surface method. The electronic coupling matrix elements were calculated by two methods, namely, the energy splitting in dimer (ESD) method and charge transfer integral (CTI) method. Using the coupling matrix calculated by the CTI method, we predicted a hole mobility of 2.15 cm2/(Vs) for DPA, whereas the CTI method gives the values of 0.35 and 1.39 cm2/(Vs) for naphthalene and anthracene, respectively. It is shown that the electronic coupling calculated by the CTI method gives the qualitatively satisfactory result for the hole mobilities of the three single-crystal systems.

In this work, we explore the validity of the application of the constrained density functional theory (CDFT) to the study of intramolecular charge transfer reactions. Accordingly, we carry out single-point calculation using CDFT and MRMP2 at each point in the intramolecular charge-transfer reaction coordinate of 1,3-dinitrobenzene anion radical, calculated using the complete active space self-consistent filed theory (CASSCF). We show that the CDFT//CASSCF and MRMP2//CASSCF energy profiles are qualitatively and quantitatively similar. Therefore, we conclude that CDFT is a good tool for studying the intramolecular charge-transfer reactions.

Poly-paraphenylenevinylene (PPV), a material used in organic light-emitting diodes (OLEDs), for which improving the efficiency is an important issue. In general, the molecular orientations of organic compounds in the crystal form are an essential factor determining electron and hole transfer, which are closely related to the efficiency of OLEDs. We have investigated the effects of the rotation of each molecule and the intermolecular distance in the dimer system of PPV, which consists of donor and acceptor molecules, on its charge-recombination process by performing constrained density functional calculations. Starting from the structure of the crystal, it was clarified that the rotation of the donor decreases the charge-recombination factor, to nearly zero, while that of the acceptor increases it to about 10(6) s(-1). We found that this is caused by the repulsive interaction between the donor and acceptor molecules and the formation of a transport pathway resulting from the acceptor rotation.

Fragment molecular orbital (FMO) method gives a powerful tool to investigate electronic structures for biological substances, and ABINIT-MP program has been developed to implement ab initio FMO calculations effectively. We introduced DFT code into ABINIT-MP and applied fragment-DFT (F-DFT) calculations to model polypeptides. The total accuracy of numerical integrations employed in those calculations was examined by the total numbers of electrons in the molecules. It is shown that the numerical integral of the total density function under the fragment approximation works as an indicator for the numerically total accuracy on the F-DFT implementation.

The Liu–Parr–Nagy (LPN) analysis was applied to the spin density at a nucleus ρs(0), for the first row atoms from carbon to fluorine and the diatomic hydrides, CH, NH, and OH. The LPN ρs(0) values for these radicals, were obtained with ab initio UHF and DFT calculations. In the DFT calculations, several fundamental exchange- and correlation-functionals were used with their combined or non-combined forms. Furthermore, various basis sets were also used. It can be shown that the LPN analysis presents the clear information to understand the differences of the ρs(0) values among the employed functional forms, and among basis sets.

The Liu–Parr–Nagy identity was applied to the DFT calculations of the spin densities for the NH radical with several exchange-correlation functional sets. The LPN spin density analyses give the important information to understand the differences of spin density values among the exchange-correlation functionals.

A combination of Monte Carlo (MC) simulation and ab initio spin-orbit coupling SCF (SOC-SCF) method was used to evaluate parity-violating energy, E pv , of l -alanine zwitterion ( l -ALAZ) in aqueous solution. In the MC simulation, the effective pair potential functions, which include the effect of the polarization of l -ALAZ caused by salvation were determined and employed. The 100 solution structures were picked up at every 1 M steps of the MC simulation of the l -ALAZ aqueous solution, and the E pv energy was calculated for the l -ALAZ-(H 2 O) n ( n =1–10) clusters in each solution structure by the SOC-SCF/6-31G method. The mean deviation of E pv for the l -ALAZ-(H 2 O) 10 clusters was large, 2×10 −20 hartree, and this comes from the fact the coordination of H 2 O at the CO 2 site brings about large effect on E pv . Although, the small change in the orientation of H 2 O to the CO 2 group may change the sign of E pv , the expectation value of E pv of l -ALAZ is positive for thermally stable conformations in aqueous solution at 298 K. The present MC/MO calculations have shown that the E pv value of l -ALAZ is positive when the solvent effect is introduced by explicit H 2 O molecules. This is same as that observed in previous calculations with the continuum model of solvation. The present calculations provide support for the destabilization of l -ALAZ compared to d -ALAZ at thermally accessible conformations in aqueous solution.

Singlet-oxygen [O2((1)Δg)] generation by valence-excited thiophene (TPH) has been investigated using multireference Moller-Plesset second-order perturbation (MRMP2) theory of geometries optimized at the complete active space self-consistent field (CASSCF) theory level. Our results indicate that triplet TPH(1(3)B2) is produced via photoinduced singlet TPH(2(1)A1) because 2(1)A1 TPH shows a large spin-orbit coupling constant with the first triplet excited state (1(3)B2). The relaxed TPH in the 1(3)B2 state can form an exciplex with O2((3)Σg(-)) because this exciplex is energetically more stable than the relaxed TPH. The formation of the TPH(1(3)B2) exciplex with O2((3)Σg(-)) whose total spin multiplicity is triplet (T1 state) increases the likelihood of transition from the T1 state to the singlet ground or first excited singlet state. After the transition, O2((1)Δg) is emitted easily although the favorable product is that from a 2 + 4 cycloaddition reaction.

Hyperfine coupling constants (hfcc's) of H2O+, CH3, NH2, allyl, NO2, HCO, CH2CH, H2CN and H2CO+ radicals were calculated with Density functional theory using Becke97-type exchange correlation functionals, B97, B97-1 and HCTH. These functionals gave the accuracy comparable with the conventional B3LYP functional for hfcc's of these small π and σ radicals. It was found that the calculated hfcc's of the heavy atoms depend largely on the parameters in the HCTH functional which were determined by referring the molecular properties selected in the parameter-fitting process.

The spin–orbit coupling SCF (SOC-SCF) wavefunction was obtained on the basis of Breit–Pauli Hamiltonian and generalized spin-orbitals and was used to calculate the parity-violating energy, E pv , of chiral molecules. For the twisted HOOH molecule, the magnitude of SOC-SCF E pv is comparable with those of RPA, MC-LR, and DHF methods, and 2 or 3 times as large as that calculated by sum-over-state perturbation theory (SOS-PT). Moreover, the SOC-SCF method gave a reliable variation in the E pv value for the conformational change of l -alanine zwitterion ( l -ALAZ), indicating that the SOC was introduced appropriately into the SOC-SCF wavefunction. Calculations for several chiral molecules have suggested that the [5s2p/3s] basis set was acceptable for the SCF–SCF E pv calculation of large molecules. The contribution of one- and two-electron spin–orbit coupling terms to E pv was analyzed by dividing the E pv values in parts. It was confirmed that only one-center terms of the spin–orbit coupling integrals are sufficient in the SOC-SCF E pv calculation, although both one- and two-electron terms should be included. The SOC-SCF method with the [5s2p/3s] basis set was applied to l -ALAZ in aqueous solution, and the E pv map was created as a function of two torsion angles, ϕ for the NH 3 group and θ for the CO 2 group. In the ( ϕ , θ ) map of E pv , the positive E pv area is wider than the negative one, and the averaged E pv value was +2.1×10 −20 hartree, indicating that l -ALAZ is more unstable than d -ALAZ in aqueous solution due to the parity-violating weak neutral current interaction.

Conformation of hydrated d -glyceraldehyde ( d -glyceraldehyde hydrate, d -GAH) in aqueous solution was analyzed by ab initio generalized Born (GB) method including the solvent effect by the continuum model using the generalized Born formula. The energy surface defined by six torsion angles of d -GAH was represented by the HF-GB/STO-3G energies for the conformations generated by changing each torsion angle in steps of 30 or 60° and the 164 local minima were found in this energy surface. The 10 low-energy structures of d -GAH in aqueous solution were obtained with the 6-31G ∗∗ basis set. In these structures, intramolecular hydrogen bondings were recognized. The parity-violating energy, E pv , was calculated by using the spin-orbit-coupling (SOC)-SCF/5s2p wavefunction. The calculated E pv values of the 164 conformations indicated that the E pv value of d -GAH depends largely on the conformation; half of the conformations have positive E pv and the other half have negative E pv . The lowest energy conformation determined by the 6-31G ∗∗ calculation has a negative E pv value of −1.63×10 −20 hartree and the energy-averaged E pv value is −1.12×10 −20 hartree, suggesting that hydrated d -glyceraldehyde is more stable than hydrated l -glyceraldehyde in aqueous solution due to the parity-violating weak neutral current interaction.

The potential energy curve for the reaction of the ethaniminium ion (CH 3 CH NH 2 + ) and the cyanide anion (CN − ) to give 2-amino-propanenitrile (APN) in aqueous solution which is the chirality-producing step of Strecker synthesis of l -alanine was obtained by ab initio GB 6-31G ∗∗ calculation including the solvent effect. The transition state was located at 2.13 A of the C α –C( N) distance. The barrier height for this reaction was 16.6 kcal/mol and the reaction is expected to occur easily in aqueous solution. The parity-violating interaction energy ( E pv ) was calculated by using the spin–orbit-coupling SCF wavefunction and the [5s2p1d/3s1p] basis set. The E pv value of the association state of the two ions is zero, while the reaction product l -APN has the positive value of +0.17×10 −20 hartree. E pv was positive for all reaction stages including the transition state, suggesting that the formation of l -APN is not more favorable than d -APN in the Strecker synthesis of alanine due to the parity-violating weak neutral current interaction.

Parity-violating interaction energy ( E pv ) of d -glyceraldehyde was calculated by using the spin–orbit coupling SCF wavefunction and the 5s2p basis set. The E pv value was negative for most of the low-energy conformations of d -glyceraldehyde, suggesting that d -glyceraldehyde is more stable than l -glyceraldehyde due to the parity-violating weak neutral current interaction.

Ab initio generalized Born (GB) method including solvent effects based on the GB formula was applied to two push–pull diazenes, methylcyanodiazene (MCD) and dimethylaminonitrodiazene (DAND), and the solvent effect on the cis – trans isomerization reaction was examined. In both MCD and DAND, differential solvation was recognized clearly; the transition state (TS) of the rotation mechanism was stabilized most by solvent. However, in the case of MCD, the TS for the inversion of the pull group was expected to be lower in energy than the rotation TS in aqueous solution. In order to analyze the solvent effect on the electronic structure and energy of the cis and trans forms and of TS, the solvation energy was divided into the polarization energy of the solute molecule and the solvation energy of the polarized solute molecule. The magnitude of these energy components was in the order, rotation TS>inversion TS> cis ∼ trans . The group charge population of the donor and acceptor groups was used to account the charge separation at each TS, which was strongly related to the stabilization by solvent.

Ab initio GB calculation that includes solvent effects by the generalized Born formula (GB) was applied to methyl yellow (4-dimethylaminophenylazobenzene, MY) and methyl orange (4-[4′-dimethylaminophenylazo]-benzenesulfonate, MO) and their protonated species, and the acid-catalyzed cis–trans isomerization of MY and MO was analyzed. The HF/3-21G(∗)/GB calculations determined the structures and relative energies of MY and MO, their ammonium and azonium ions, and the transitions state for the cis–trans isomerization of the azonium ions in aqueous solution. The calculations indicated that (i) the energy difference between the cis and trans forms of the azonium ion is much smaller than those of the ammonium ion and the unprotonated species, (ii) the cis-azonium ion is much more stable than the cis-ammonium ion, and (iii) the cis–trans isomerization occurs easily in the azonium ion which is produced by the protonation of the azo compound or by the acid-catalyzed proton transfer from the cis-ammonium ion.

A combination of Monte Carlo (MC) simulation and semi-empirical AM1 molecular orbital (MO) singly-excited configuration interaction (SCI) calculation was applied to dimethyl nitroxide (DMNO) in aqueous solution and the solvent effect on the hyperfine coupling constant (HFCC) of nitrogen in DMNO was analyzed. During the MC simulation, 100 solution structures were picked up. For each solution structure, the DMNO–(H 2 O) n ( n =1–40) clusters were cut out and the AM1-SCI calculation was applied to the cluster as a supermolecule. The HFCC of nitrogen was obtained by averaging the 100 solution structures. The H 2 O molecules included in the supermolecule were determined by two different types of selection and the contribution of H 2 O molecules to the HFCC was well elucidated in relation to the hydration shell structure.

Ab initio RHF and ROHF methods including solvent effects based on the generalized Born formula (GB) were applied to a typical push–pull azobenzene, 4-(dimethylamino)-4′-nitroazobenzene (DMANAB), and the solvent effect on the inversion and rotation mechanisms of thermal cis – trans isomerization of DMANAB was examined. The pull-group inversion path was more favorable than the push-group one in the gas phase and in solution, and the calculated activation energies were compared well with experimental values. Analysis of two-dimensional potential energy map and electron configurations of the transition states indicated that the transition state of the rotation path, if it exists, resembles closely to that of the inversion path, and it is unlikely that both transition states are found separately in the potential energy surface of the closed-shell electronic state. Although large stabilization by solvent was expected for the rotation transition state, searching for such a transition state on the closed-shell electronic state gave the inversion transition state even in polar solvent. The transition state for the rotation path in the diradical electronic state was calculated to be comparable in terms of energy to the transition state for the pull-group inversion path in the closed-shell electronic state.

Geometry optimizations at the UHF/6-31G* and UMP2/6-31G* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS + · 1 and TS + · 2 , were obtained which have C1 and C2 symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS + · 1 is the true transition state connecting N+· and Q+·, while TS + · 2 is a second order saddle point.

Ab initio UHF MIDI-4 calculations have been carried out for the dimethyl nitric oxide (DMNO) radical in the H 2 O and CHCl 3 solutions. The configurations of the solvent molecules were selected from the snapshots in the Monte Carlo (MC) simulation for the solution, and the electronic structure and the hyperfine coupling constant ( hfcc ) of DMNO were obtained by averaging the results of 300 configurations. The point charge approximation in which all solvent molecules were represented by point charges indicated that the electrostatic interaction polarizes the N–O bond and increases the spin density on the N atom. When some of the solvent molecules near DMNO were treated as a supermolecule together with DMNO and the remaining solvent molecules were approximated by point charges, the electron transfer occurs between DMNO and H 2 O molecules in two directions, from DMNO to H 2 O around the N–O group and from H 2 O to DMNO around the CH 3 groups. These electron transfers polarize the N–O bond in the same direction as the electrostatic interaction and increase the hfcc of the N atom. The contribution of the electron transfer accounts for 40% of the solvent effect on hfcc of DMNO.

Geometrical optimizations for the radical cation of 1,3,5-cycloheptatriene (CHT+) were performed at different levels of theory, UHF, UMP2, CASSCF and DFT/B3LYP, with the 6–31G(d) basis set. In all cases, planar structures with the C2ν symmetry were obtained. Vibrational analyses showed that the C2ν planar structure is at a local minimum and not very rigid for the ring inversion. Using the optimized structure, the hyperfine coupling constants for hydrogen atoms in CHT+ were calculated and were compared with the experimental values. The UHF and UMP2 calculations failed to predict the hyperfine coupling constants (hfcc) correctly, while DFT hfcc agreed well with experiments.

Hemi-π-bond properties of the sulfursulfur bond in organodisulfide radical cations of 1,2-dithietane ( 1 ), 1,2-dithiolane ( 2 ), and 1,2-dithiete ( 3 ) were examined by ab initio MO calculations. The UHF/MIDI-4(d) calculations predicted that 1 + and 2 + have a two-center three-electron π-bonding character between the sulfur atoms, while the π-anti-bonding character is dominant in the SS bond of 3 + .

The NMR 15N–6Li coupling constants 1JNLi in LiNH2 and LiN(CH3)2 monomers and their oligomers and mixed aggregates with LiCl were calculated using ab initio self-consistent perturbation theory and the relation between the molecular structure and 1JNLi values was examined. The calculated 1JNLi values in the monomers are influenced by solvation and agree well with experimetal values when model solvent molecules are coordinated at the lithium atom. The calculated 1JNLi values in cyclic oligomers also agree well with experiment and support the structures derived from NMR experiments in solution. For the mixed trimer, (LiNH2)2LiCl, a ladder form was obtained as the most stable structure when the lithium atoms are solvated by two H2O or two NH3 molecules.

Dissociation energies, equilibrium structures, equilibrium dipole moments, harmonic vibrational frequencies, infrared (IR) intensities, and zero‐point vibrational energies (ZPVEs) are presented for the hydrogen fluoride monomer and dimer at the coupled cluster with single and double excitations plus perturbative connected triple excitations [CCSD(T)] level of theory with a series of large basis sets. Discussion focuses on the results from the largest basis set, triple zeta plus double polarization and one set of higher angular momentum functions [TZ2P(f,d)]. The structural parameters and dissociation energy agree well with the recent theoretical values reported in a high level ab initio study by Peterson and Dunning [J. Chem. Phys. 102, 2032 (1995)] and with experimental results. Here, the harmonic vibrational frequencies and IR intensities for the dimer are also predicted. The predicted vibrational frequency shifts relative to the monomer for the HF and DF stretching coordinates in the dimer are close to the experimental estimates.

Ab initio and AM1 multi-reference SCI calculations have been applied to the stilbene cation radical and the potential energy curves for the lower doublet states were obtained as a function of the twist angle about the central C–C bond to elucidate the photochemical cis–trans isomerization process. The shape of the calculated potential energy curves has suggested that the cis→trans one-way isomerization proceeds very efficiently from an upper excited state of the cis-stilbene cation radical.

Ab initio calculations were carried out for the 2-, 3-, and 4-pyridyl cations, anions, and radicals in order to examine their molecular structures and relative energies among three isomers. The 4-31G//3-21G calculations have predicted that 2-pyridyl cation, 4-pyridyl anion, and 2-pyridyl radical are most stable among the isomers.

The ESR spectra of the radical anions of 2,9- and 5,6-dimethyl-1,10-phenanthrolines have shown that these radical anions are in the 2B1 state, as is the unsubstituted 1,10-phenanthroline radical anion. MO calculations have indicated that the two lower electronic states of these radical anions, 2B1 and 2A2, lie close to each other, and that the relative stability is largely affected by an interaction with the counter cation.

The reaction space map (RESMAP) representation is proposed for the comprehensive description of the chlorination and dechlorination reactions of polychlorobenzenes. By using the B3LYP/6-311G** ener...

Functional regulation of myoglobin (Mb) is thought to be achieved through the heme environment furnished by nearby amino acid residues, and subtle tuning of the intrinsic heme Fe reactivity. We have performed substitution of strongly electron-withdrawing perfluoromethyl (CF(3)) group(s) as heme side chain(s) of Mb to obtain large alterations of the heme electronic structure in order to elucidate the relationship between the O(2) affinity of Mb and the electronic properties of heme peripheral side chains. We have utilized the equilibrium constant (pK(a)) of the "acid-alkaline transition" in metmyoglobin in order to quantitatively assess the effects of the CF(3) substitutions for the electron density of heme Fe atom (rho(Fe)) of the protein. The pK(a) value of the protein was found to decrease by approximately 1 pH unit upon the introduction of one CF(3) group, and the decrease in the pK(a) value with decreasing the rho(Fe) value was confirmed by density functional theory calculations on some model compounds. The O(2) affinity of Mb was found to correlate well with the pK(a) value in such a manner that the P(50) value, which is the partial pressure of O(2) required to achieve 50% oxygenation, increases by a factor of 2.7 with a decrease of 1 pK(a) unit. Kinetic studies on the proteins revealed that the decrease in O(2) affinity upon the introduction of an electron-withdrawing CF(3) group is due to an increase in the O(2) dissociation rate. Since the introduction of a CF(3) group substitution is thought to prevent further Fe(2+)-O(2) bond polarization and hence formation of a putative Fe(3+)-O(2)(-)-like species of the oxy form of the protein [Maxwell, J. C.; Volpe, J. A.; Barlow, C. H.; Caughey, W. S. Biochem. Biophys. Res. Commun. 1974, 58, 166-171], the O(2) dissociation is expected to be enhanced by the substitution of electron-withdrawing groups as heme side chains. We also found that, in sharp contrast to the case of the O(2) binding to the protein, the CO association and dissociation rates are essentially independent of the rho(Fe) value. As a result, the introduction of electron-withdrawing group(s) enhances the preferential binding of CO to the protein over that of O(2). These findings not only resolve the long-standing issue of the mechanism underlying the subtle tuning of the intrinsic heme Fe reactivity, but also provide new insights into the structure-function relationship of the protein.

Ab initio single excited configuration interaction calculations of hyperfine coupling constants with Chipman's basis sets suggest that the oxirane radical cation has the ring-opened oxallyl structure, rather than a ring-closed structure.

The Σ and Π states of radical cations of formaldehyde (1), acetaldehyde (2), s-trans-acrolein (3), s-cis-acrolein (4) and cyclopentadienone (5) were examined by ab initio calculations with the MIDI-4 basis set. It was found that the Σ state is more stable than the Π state for the cation radicals 1–4, and the breakdown of Koopmans' theorem for the ionization of acrolein was confirmed.