2,4-二硫代尿嘧啶的紫外吸收光谱和共振拉曼光谱

硫代嘧啶碱基是光动力疗法潜在的重要光敏剂,其最低单重激发态的光物理研究已有广泛报道。然而,其较高激发态的跃迁性质和反应动力学研究较为稀少。因此,本文采用共振拉曼光谱和密度泛函理论计算方法研究2,4-二硫代尿嘧啶的紫外光谱和几个较高单重激发态的短时结构动力学。首先,基于共振拉曼光谱强度与电子吸收带振子强度f的关系,将紫外光谱去卷积成四个吸收带,分别为358 nm(f=0.0336)中等强度吸收带(A带),338 nm(f=0.1491)、301 nm(f=0.1795)和278 nm(f=0.3532)强而宽的吸收带(B、C和D带)。这一结果既吻合密度泛函理论计算结果,又符合共振拉曼光谱强度模式对紫外光谱带的预期。据此,去卷积得到的四个吸收带被分别指认为S0→S2跃迁、S0→S6跃迁、S0→S7跃迁和S_0→S_8跃迁。同时,分别对B,C和D带共振拉曼光谱进行了详细的指认,获得了短时动力学信息。结果表明,S_8态短时动力学的显著特征是在Franck-Condon区域或附近发生了S8(ππ~*)/S(nπ~*)势能面交叉引发的、伴随超快结构扭转的非绝热过程。S7和S6态短时动力学的主要特征是反应坐标的多维性,它们分别沿C_5C_6/C_2S_8/C_4S_(10)/N_2C_3+C_4N_3H_9/N_1C_2N_3/C_2N_1C_6/C_6N_1H_7/C_5C_6H_(12)和C_5C_6/N_3C_2/C_4S_(10)/C_2S_8+C_6N_1H_7/C_5C_6H_(12)/C_5C_6N_1/C_5C_6H_(12)/C_2N_1C_6/N_1C_2N_3/C_4N_3H_9/N_1C_2N_3等内坐标演化。     

三维吸附紧密高分子单链的力学性质研究

采用PERM(pruned-enriched Rosenbluth method)算法,研究了吸附在界面附近的紧密高分子链力学行为.发现当界面的吸附能比较大时,紧密高分子链从紧贴于吸附界面到逐渐远离的过程中,其外形会经历4种典型的变化.同时紧密高分子链的尺寸大小如/N、xy/N、z/N,形状参数<δ*>,热力学性质如每个键的平均自由能A/N,平均相互作用能/N等,甚至所受外力的大小都会同时做出相应的变化,其出现变化的位置也一致.特别是随着紧密高分子链离开吸附界面的过程中,作用于高分子链上的外力明显出现几个力学平台,这与实验得到的结果完全一致.同时还研究了弱吸附能的情况,在这种情况下实验是很难进行的.     

Photochemical E/Z isomerization of aryl-substituted methanimines. AM1-CI potential energy curves along the CN double bond twisting and the N atom in-plane inversion

Potential energy curves of aryl-substituted methanimines along the C = N bond twisting and the in-plane inversion of the N atom were obtained by AM1-SDCI calculations, and the photochemical E/Z isomerization paths were elucidated. An aromatic group introduced at the C atom of the C = N group has little effect on the S₀ surface for the inversion and rotation paths, while it has a significant effect on the shape of the T₁ curve along the rotation path. It is suggested that phenylmethanimine and 2-anthrylmethanimine undergo photoisomerization by the rotation mechanism. The methoxy group introduced at the N atom raises the inversion barrier on the S₀ state, while it lowers the 90°-twistedT₁ energy. TheT₁ potential energy curves of N-methoxy-1-phenylethanimine and N-methoxy-1-(2-anthryl)ethanimine indicate that the former undergoes a two-way isomerization and the latter a one-way isomerization by the rotation mechanism, which is in accordance with experiment.     

Electroabsorption study of metal-to-ligand charge transfer in an organic complex of iridium (III)

The dipole moment and polarizability changes have been determined from electroabsorption (EA) spectroscopy of solid films of fac tris(2-(phenyl)pyridinato,N,C^2′)iridium (III) [Ir(ppy)₃]. The maximum changes in the dipole moment |Δμ|_S=(5.0±0.5) D/f (f is the local field correction factor: 1.3–1.7) accompany ground state to the lowest singlet, and |Δμ|_T=(1.7±0.5) D/f ground state to the lowest triplet metal-to-ligand charge transfer (MLCT) excited states formation, while the average polarizability change ų/f² follows from the fitting procedure throughout the visible absorption spectrum range. The experimental values of |Δμ| as well as energy positions of the MLCT states correlate with the literature results of time-dependent density functional theory.     

2-甲基呋喃分子激发态超快非绝热动力学

Excited-state dynamics of 2-methyl furan has been studied by femtosecond time-resolved photoelectron imaging. The molecule 2-methyl furan was simultaneously excited to the n=3 Rydberg series of S₁[¹A"(π3s)], ¹A'(π3p_x), ¹A"(π3p_y) and ¹A"(π3p_z) and the valence state of ¹A'(ππ^*) by two 400 nm photons and subsequently probed by two 800 nm photons. The average lifetime of the Rydberg series and the valence state was measured to be on the time scale of 50 fs by the time-dependent ion yield of the parent ion. Ultrafast internal conversions among these excited states were observed and extracted from the time-dependences of the photoelectron kinetic energy components of these excited states in the photoelectron kinetic energy spectra. Furthermore, it is identified that the ¹A'(ππ^*) state might play an important role in internal conversions among these excited states. The Rydberg-valence mixings, which result in numerous conical intersections, act as the driving force to accomplish such ultrafast internal conversions.     

Elastic behavior of adsorbed polymer chains

Elastic behaviors of single polymer chains adsorbed on the attractive surface are first investigated using Monte Carlo simulation method based on the bond fluctuation model. We investigate the chain size and shape of adsorbed chains, such as mean-square radius of gyration S2, mean-square bond length b2, shape factors sf(i) and delta*, and the orientation of chain segments P2, to illuminate how the shape of polymer chains changes during the process of tensile elongation. There are some special behaviors of the chain size and shape at the beginning of elongation, especially for strong attraction interaction. For example, mean fraction of adsorbed segments decreases abruptly in the region of small elongation ratio and then decreases slowly with increasing elongation ratio. In fact, the chain size and shape also changes abruptly for small elongation ratio with strong attraction interaction. Some thermodynamics properties are also investigated here. Average Helmholtz free energy increases fast for elongation ratio lambda<1.15, especially with strong attraction, and increases slowly for lambda>1.15. Similar behaviors are obtained for average energy per bond. Elastic force (f ) and energy contribution to force (f(U)) are also studied, and we find that elastic force decreases abruptly for lambda<1.15, and there is a minimum of elastic force for strong attraction interaction, then increases very slowly with increasing elongation ratio. However, there are different behaviors for weak attraction interaction. For energy contribution to force (f(U)), there is a maximum value for strong attraction interaction in the region of lambda<1.15. Some comparisons with the atomic force microscopy experiments are also made. These investigations may provide some insights into the elastic behaviors of adsorbed polymer chains.     

Metal complexes of biologically important ligands: Synthesis of amino acidato complexes of Pd containing a C,N-cyclometallated group as an ancillary ligand

New amino acidato complexes of Pd^II of stoichiometry [Pd(C---N)(Aa)] (C---N=C,N-cyclometallated ligand, Aa = N,O-amino acidato ligand) have been obtained by reaction of [Pd(C---N)(acac)] (C---N=N,N-dimethylbenzylamine-C²,N (dmba) (1) or N,N-dimethyl(S--phenylethyl)amine-C²,N (S-dmphea) (2)) with glycine, chiral amino acids (alanine, phenylalanine and valine), and amino acid derivatives (N-acetylglycine and N-acetyl-,β-dehydroalanine) in MeOH. The compounds are characterized by IR, ¹H and ¹³C NMR. The geometry of these complexes has been unambiguously determined by NOE difference experiments and NOESY measurements.     

Chiral Non-racemic Bis(vicinal 1,2-Diamines): 4,5-Diamino-N-(3,4-diaminobutyl)pentanamide Tetrahydrochloride, N,N′-Bis[3,4-bis (t-butoxycarbonylamino)butyl]urea and N,N′Bis

The reaction of (R) or (S)-N⁴,N⁵-bis(t-butoxycarbonyl)-4,5-diaminopentanoic acid (6) with (R) or (S)-N³,N⁴-bis(t-butoxycarbonyl)-3,4-diaminobutylisocyanate (8) catalyzed by 4-dimethylamino pyridine (DMAP), leads to the synthesis of (R,R), (S,S), (R,S) and (S,R) isomeric amides (11 a — d) The addition of adipic acid monomethyl ester to (R) or (S) isocyanate, followed by saponification, acidification and subsequent reaction with the second molecule of (R) or (S) isocyanate allows isolation of the (R,R), (S.S) and the meso isomers of N,N′-bis[3,4-bis(t-butoxycarbonylamino) butyl]hexanediamide (17) Removal of protecting groups with HCl/EtOH affords chiral non-racemic molecules having two free vicinal diamine units.     

Biocatalytic procedure for obtaining all four diastereoisomers of 1-(1-hydroxyethyl)-3-ethylferrocene: synthons for chiral 1,3-disubstituted ferrocenes

Lipase from Candida antarctica has been successfully used to realise the selective esterification of the four 1-(1-hydroxyethyl)-3-ethylferrocene isomers, possessing central/planar chirality. In this reaction, the lipase recognises only the two isomers possessing an (R)-configuration, independently from planar chirality. This allows us to split the mixture into two couples of diastereoisomers, namely the (S_p,R_c)-/(R_p,R_c)- and (R_p,S_c)-/(S_p,S_c)-isomers, conveniently separated in the single constituents by crystallisation from hexane.     

Synthesis of Pt compounds containing chiral (2S,4S) -pentane-2,4-diyl-bis(5H-dibenzo[b]phosphindole) as ligand and their use in asymmetric hydroformylation of styrene derivatives

Unlike bis(diphenyl)phosphine derivatives in general, (2S,4S)-pentane-2,4-diyl-bis(5H-dibenzo[b]phosphindole), S,S-BDBPP, gives a trans oligomeric compound [PtCl₂(S,S-BDBPP)]_n, 1, in reaction with dichloro-Pt precursors such as PtCl₂(PhCN)₂, PtCl₂(CH₃CN)₂ and PtCl₂(COD) at room temperature. Compound 1, which could be readily isolated, slowly rearranges in solutions at room temperature to the expected cis-monomer PtCl₂(S,S-BDBPP), 3. Heating or the presence of PtCl₂(COD) accelerates the transformation of compound 1 to 3. SnCl₂ adducts of both compounds, trans-[PtCl(SnCl₃)(S,S-BDBPP)]_n, 2, and cis-PtCl(SnCl₃)(S,S-BDBPP), 4, as well as the known cis-PtCl(SnCl₃)(S,S-BDPP), 5, (S,S-BDPP = (2S,4S)-2,4-bis(diphenylphosphino)pentane) have been tested as catalysts in the asymmetric hydroformylation of p-isobutylstyrene. The phenyl analog 5 provides up to 75% e.e. but moderate yields to chiral 2-(4-isobutylphenyl)-2-propanal. Compared to this, the regioselectivity to the branched aldehyde is remarkably increased; however, the enantioselectivity is drastically decreased by the use of both dibenzophosphole derivatives 2 and 4. The similarities in the selectivities provided by 2 and 4 indicate that the trans oligomer 2 transforms to the cis-monomer 4 during the catalytic process. X-ray crystal structure determination of compound 3 shows a half-chair conformation for the chelate ring with a symmetric arrangement of dibenzophosphole groups. Besides a preference for the latter achiral conformation, the planar structure of the dibenzophosphole groups can also be considered as reason for the moderate enantioselectivities provided by 4.     

EPR and ESEEM study of the plastoquinone anion radical QA in photosystem II treated at high pH

The effect of treatment at pH = 11 on the photosystem II was studied by EPR and electron spin echo envelope modulation (ESEEM). The magnetic interaction between the semiquinone Q_A^−. and the non-heme Fe²⁺ (S = 2) was absent. ESEEM showed that the Q_A^−. interacts magnetically with two ¹⁴N nuclei. The first interaction has a hyperfine coupling tensor (A_XX, A_YY, A_ZZ)=(2.0, 1.7, 2.3 MHz) and nuclear quadrupole interaction parameters e²qQ/h=3.24 MHz and η = 0.45 while those of the second are (A_XX, A_YY, A_ZZ)=(1.2, 1.5, 2.3 MHz), e²qQ/h = 1.56 MHz and η = 0.71. These are assigned to an amide nitrogen of the peptide backbone and the amino nitrogen of an imidazole respectively. By analogy to the bacterial reaction centre, these nitrogens are attributed to the Ala 261 and His 215 of the D2 protein. It was shown earlier that the imidazole coupling is absent in cyanide-treated PSII, its presence here is attributed to a difference in the position of the imidazole group itself.     

Conformational structures and thermodynamic properties of compact polymer chains confined between two parallel plane boundaries

In this paper, the authors investigated the adsorption phenomenon of compact chains confined between two parallel plane boundaries using a pruned‐enriched Rosenbluth method. The authors considered three cases with different adsorption energies of ε = 0, ?1, and ?3 (in units of k_BT) for the confined compact chains of different chain lengths N, respectively. Several parameters were employed to describe the size and shape of compact chain, and some special behaviors in the conformational structures were investigated for the first time. For example, the size and shape of confined compact chains undergo distinct changes in the adsorption cases of ε = ?1 and ?3, and pass through the maximum values at the characteristic distances D_c. The authors found that this characteristic distance D_c could be scaled as D_c～ (N + 1)^ν (ν = 0.56 ± 0.01) in the case of ε = ?3. In addition, the microstructures of chains were investigated, and several significant results were obtained by analyzing the segment density distribution and the mean fractions of segment in tails, trains, bridges, and loops structures. On the other hand, the thermodynamic properties were also investigated for the confined compact chains, such as average energy per bond, Helmholtz free energy per bond, and elastic force per bond. Results show that elastic forces f have different behaviors in three cases, indicating that it is not necessary to exert an external force on the boundaries in the nonadsorption case. At the same time, the average contact energy of compact chain obviously changes when the distance between the two parallel boundaries D increases, which is similar to those of the size and shape parameters. The authors also conclude that these thermodynamic properties of compact chains depend strongly on not only the adsorption energies but also the chain lengths and the confined condition. In addition, several results of the conformational and thermodynamic parameters, such as the segment density distribution and free energy, were compared with the results from the self‐consistent field theory. These investigations may help us to deepen the knowledge about the adsorption phenomenon of confined compact chains. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2888–2901, 2006     

Metal imidazolato polymers: synthesis, characterization and crystal structure of new silver(I) triphenylphosphine derivatives

When the polymeric complex [Ag(im)]_n (Him = imidazole) is reacted with PPh₃ (PPh₃ = triphenylphosphine), it yields the [Ag₂(μ₂-im)₂(PPh₃)₃]_n and [Ag(μ₂-im)(PPh₃)₂]_n species, shown to contain wavy chains of metal ions, singly bridged by N,N′-exo-bidentate imidazolate ligands. The former, crystallised as the CH₂Cl₂ solvate, contains two non-equivalent silver(I) ions, differing in the number of coordinated phosphines (one, in trigonal planar stereochemistry, or two, having tetrahedral geometry). The latter has a unique independent silver(I) ion in a tetrahedral environment, with two coordinated PPh₃ ligands. The reactivity of known silver(I) azolates with PPh₃, as well as the solution behaviour and (when available) the crystal structures of the corresponding derivatives are taken into consideration for a due comparison.     

Update of the anharmonic force field parameters of the ozone molecule

A new set of spectroscopic constants of the ¹⁶O₃ molecule (ω_i, x_ij, y_ijk, γ^DD, _i^X, β_ij^X,…), which determine vibrational dependence of band centres and rotational parameters, is derived from recent accurate analysis of high-resolution experimental ro-vibrational spectra through the theoretical approach based on second-order perturbation expansions in normal coordinates accounting for Darling–Dennison resonance interactions. These values are used to update empirical values of anharmonic coefficients (k_ijl, k_ijlm) of the potential function expansion in normal coordinates. Quadratic f_rr, f_r, f_rr′, f as well as cubic f_rst and quartic f_rstl force constants in internal (bond lengths, bond angle) coordinates are also derived. A detailed discussion is devoted to the accuracy of parameter determination for each of four steps of calculations. It is emphasised that the conventional method based on the inversion of formulae of the perturbation theory gives the largest uncertainties at the last step of calculations: the determination of the anharmonic force field in internal coordinates.     

Synthesis, crystal structures and magnetic properties of Cu(II) compounds bridged by the terephthalate ligand and hydrogen bonds

A 3D network [Cu(tmen)(tp)(H₂O)₂]_n (1) (tmen = N,N,N′,N′-tetramethylethylenediamine; tp = terephthalate) and a 2D sheet [Cu(pyrazole)₂(tp)]_n (2), featuring 1D chains interwoven by hydrogen bonds, have been prepared and characterized by means of X-ray analyses and magnetic measurements. For 1, coordinative zigzag chains contain Cu(II) centers capped by the chelate ligand tmen, in which the tetragonal structure is elongated due to Jahn–Teller distortion. Coordinated water molecules are hydrogen-bonded to two free carboxylate oxygens of tp bridges, leading to the observed 3D structure. The use of the non-chelating capping ligand pyrazole produced the covalent-bonded 1D linear compound 2 with hydrogen bonds. A severe octahedral distortion of the Cu(II) center arises from a small bite angle (52.3(1)°) of two carboxylate oxygen atoms of tp, which are in turn hydrogen-bonded to the N–H groups of pyrazole ligands coordinated to Cu(II) atoms in neighboring chains. Magnetic data were fitted with the high-temperature series expansion for the Heisenberg chain spin Hamiltonian H = −J∑_iS_i · S_i + 1 together with consideration of the molecular field approximation (zJ′). Both compounds interestingly exhibit ferromagnetic interactions with g = 2.17, J = 4.08 cm⁻¹, zJ′ = −0.28 cm⁻¹ for 1 and g = 2.09, J = 1.47 cm⁻¹, zJ′ = −0.04 cm⁻¹ for 2. By taking into account structural parameters of distances between Cu atoms, it is reasonably assigned that the ferromagnetic couplings (J > 0) in these systems originate from the hydrogen bonds. The spin density of the d_x²-y² orbital on a Cu(II) atom in a chain is propagated and induced over the d_z² orbital of another Cu(II) atom in an adjacent chain. This orbital orthogonality gives rise to such interactions. The negative zJ′ term suggests that the tp bridges communicate only tiny antiferromagnetic interactions.     

Heat capacities of NaNO3 and KNO3 from 350 to 800 K

The heat capacities of NaNO₃ and KNO₃ were determined from 350 to 800 K by differential scanning calorimetry. Solid-solid transitions and melting were observed at 550 and 583 K for NaNO₃ and 406 and 612 K for KNO₃, respectively. The entropies associated with the solid-solid transitions were measured to be (8.43± 0.25) J K⁻¹ mole⁻¹ for NaNO₃ and (13.8±0.4) J K⁻¹ mole⁻¹ for KNO₃. At 298.15 K the values of C⁰_P S⁰_P, {H⁰(T)-H⁰(0)}/T and -{G⁰(T)-H⁰(0)}/T, respectively, are 91.94, 116.3, 57.73, and 58.55 J K⁻¹ mole⁻¹ for NaNO₃ and 95.39, 133.0, 62.93, and 70.02 J K⁻¹ mole⁻¹ for KNO₃. Values for S⁰_T, {H⁰(T)-H⁰(0)}/T, and -{G⁰(T)-H⁰(0)}/T were calculated and tabulated from 15 to 800 K for NaNO₃ and KNO₃.     

Diaziridines thermal cleavage possibilities: Disrotatory or conrotatory?

Ab initio calculations are carried out at UB3LYP/6-311++G (3df, 2p) levels of theory, on electrocyclic thermal cleavage of four (S) derivatives of diaziridines, 1_X-R, to their corresponding (Z) and (E) azomethine imides (2_X-Z, 2_X-E, 3_X-Z and 3_X-E), where X=–H, –Me, t-Bu and Ph. Cleavage of 1_X-R Series to 2_X-Z (Path 1) emerged as the more favored, for producing the most stable products, 2_X-Z. In IRC calculations that were shown in Paths 1 and 2, C₆–N₁ bond was cleavage, before reaching reaction rate determinating step (transition state).     

Evaluation of carrier gases for use in high-field asymmetric waveform ion mobility spectrometry

Effects of carrier gas type (N₂, O₂, CO₂, N₂O, and SF₆) on changes in the ratio of high- to low-field ion mobility, K_h/K, of cesium, gramicidin S, tetrahexylammonium, heptadecanoic acid, and aspartic acid in fields of up to 67 Td are presented. The theory of the mobility of ions at high E/N in different gases is discussed. Plots of K_h/K as a function of the ionic energy parameter, E/N, for the five ions in each of the gases were derived from experimental data collected using a high-field asymmetric waveform ion mobility spectrometer. The change in the ratio of high- to low-field ion mobility of cesium in carrier gases of O₂ and N₂ showed excellent agreement with literature values. The behavior of cesium in O₂ and N₂ is used to illustrate that the ratio K_h/K as a function of effective temperature is invariant with gas type as long as the well depth of the interaction potential significantly exceeds thermal energy. From these results, it appears that the well depth of the interaction potential of the heavier ions studied here, including gramicidin S, tetrahexylammonium, and heptadecanoic acid, with bath gases such as N₂ and O₂, is shallow relative to thermal energy.     

Asymmetric synthesis catalyzed by chiral ferrocenylphosphine-transition metal complexes VII. New chiral ferrocenylphosphines with C2 symmetry

A new chiral ferrocenylphosphine ligand, 2,2′-bis[1-N,N-dimethylamino)ethyl]-1,1′-bis(diphenylphosphino)ferrocene (2), which has C₂ symmetry and a functional group on the side chain, was prepared by ortho-lithiation and phosphination of 1,1′-bis[1-N,N-dimethylamino)ethyl]ferrocene followed by optical resolution; recrystallization of the diammonium salt with tartaric acid. An X-ray diffraction study of PdCl₂[(+)-2] showed that the complex has square-planar geometry with two cis chlorine and two phosphorus atoms and ligand (+)-2 has an (S) configuration on the 1-dimethylaminoethyl side chain and (R) ferrocene planar chirality.     

Splay elasticity in an oligomeric liquid crystal

A magnetically-induced Freedericksz transition was used in conjunction with capacitance techniques to determine the splay elastic constant of the 4,4'-dialkoxyphenylbenzoate monomer '5OO5rsquo; [C₅H₁₁OC₆H₄COOC₆H₄OC₅H₁₁] and its dimer. As a function of reduced temperature in the nematic phase, it was found that K₁₁ of the dimer is only about 20 per cent larger than that of the monomer. Moreover, the values of K₁₁/S² versus reduced temperature, where S is the scalar order parameter, are equal to within experimental noise. These results suggest that for short oligomers Meyer's entropic model for splay elasticity in polymer liquid crystals is not applicable. A lowest order correction to the entropic model is suggested, which partially accounts for excluded volume and dispersive effects.