3-二甲氨基-2-甲基丙烯醛S2激发态结构动力学

采用共振拉曼光谱和完全活性自洽场理论计算研究了3-二甲氨基-2-甲基丙烯醛（DMAMP）光激发到S2（ππ^＊）态后的光物理性能,在B3LYP／6—311＋＋G（d,p）水平计算确定了DMAMP与其三种异构体之间的基态异构化能垒,指认了振动光谱．采用涵盖紫外强吸收带的激光波长,获得了DMAMP在环己烷、乙腈和甲醇溶剂中的A-带共振拉曼光谱,含时密度泛函方法计算确定了该光谱中基频的相对强度,发现振动-电子耦合发生在S2（）态的Franck—Condon区域．CASSCF计算方法确定低单重和三重激发态、势能面锥形交叉点和系间窜跃点的激发能．共振拉曼光谱强度模式分析和CASSCF计算获得了DMAMP的A一带短时结构动力学和其后的衰变动力学表明,C1=06和C2=C3之间的瞬时去共轭效应发生在S2（ππ^＊）态的Franck—Condon区域,激发态电荷重分布机制表明,C3和二甲氨基之间以及C1和C2之间的共轭增强效应发生在波包离开Franck-Condon区域后．C1=06和C2=C3之间的去共轭效应使得-C3=N（CH3）2沿着C2-C3键旋转更加容易,C1-C2之间以及C3和N（CH3）的共轭增强效应使得绕C1-C2和C3-N5旋转变得比较困难．这些表明DMAMP初始结构动力学沿着CI-1（S2／S0）交叉点展开,而沿CI-2（S2／S0）和[CI-3（S2／S0）交叉点展开的几率可以忽略,提出了DMAMP分子受光激发从S2,FC（ππ^＊）经由各锥形交叉点和各系间窜跃点回到S0或T1,min的两个衰变通道。     

黄曲霉素B2分子吸附在银团簇的表面增强拉曼散射机理

采用密度泛函理论（DFT）B3LYP方法,6-311G（d,p）（C,H,O）/LANL2DZ（Ag）基组,计算了黄曲霉素B₂（AFB₂）分子吸附在Ag₂团簇的表面增强拉曼散射（SERS）光谱和预共振拉曼光谱,并与实验结果比较. 结果显示：AFB₂分子在基态Ag₂团簇表面吸附时,增强因子最大达到10²,对应吡喃（pyrane）环C=O伸缩振动,主要是由AFB₂分子周围化学环境改变而引起的基态静极化率改变导致的化学增强. 不同激发波长下的AFB₂分子预共振拉曼光谱的增强强度不同：电荷转移态激发波长为1144 和544 nm时拉曼信号增强了10²倍,而选择电荷转移预共振波长432和410 nm作为入射光时,其拉曼信号增强了10⁴倍,增强机理为银团簇和黄曲霉素分子之间的电荷转移共振增强. 因此通过改变入射光波长,选择电荷转移共振激发波长,更有利于强致癌物AFB₂分子的痕量检测.     

黄曲霉素B_2分子吸附在银团簇的表面增强拉曼散射机理

采用密度泛函理论(DFT)B3LYP方法,6-311G(d,p)(C,H,O)/LANL2DZ(Ag)基组,计算了黄曲霉素B2(AFB2)分子吸附在Ag2团簇的表面增强拉曼散射(SERS)光谱和预共振拉曼光谱,并与实验结果比较.结果显示:AFB2分子在基态Ag2团簇表面吸附时,增强因子最大达到102,对应吡喃(pyrane)环C=O伸缩振动,主要是由AFB2分子周围化学环境改变而引起的基态静极化率改变导致的化学增强.不同激发波长下的AFB2分子预共振拉曼光谱的增强强度不同:电荷转移态激发波长为1144和544 nm时拉曼信号增强了102倍,而选择电荷转移预共振波长432和410 nm作为入射光时,其拉曼信号增强了104倍,增强机理为银团簇和黄曲霉素分子之间的电荷转移共振增强.因此通过改变入射光波长,选择电荷转移共振激发波长,更有利于强致癌物AFB2分子的痕量检测.     

N-甲基吡咯-2-甲醛激发态结构动力学及其溶剂效应的共振拉曼光谱和密度泛函理论研究

获取了覆盖N-甲基吡咯-2-甲醛(NMPCA)A-带和B-带电子吸收共7个激发波长的共振拉曼光谱,并结合含时密度泛函理论(TD-DFT)方法研究了的A-带和B-带电子激发和Franck-Condon区域结构动力学.TD-B3LYP/6-311++G(d,p)计算表明:A-带和B-带电子吸收的跃迁主体为π→π*.共振拉曼光谱可以指认为,11-13振动模式(A-带激发)或者7-11振动模式(B-带激发)的基频、倍频和组合频,其中C=O伸缩振动(ν7)、环的变形振动+N1-C6伸缩振动(ν17)、环的变形振动(ν21)和C6-N1-C2/C2-C3-C4不对称伸缩振动(ν14)占据了绝大部分.这表明NMPCA的Sπ激发态结构动力学主要沿C=O伸缩振动、环的变形振动和环上N1-C6伸缩振动等反应坐标展开.在同一溶剂的共振拉曼光谱中随激发波长由长变短,ν7与ν14的强度比呈现出由强变弱再变强的现象,这种变化规律被认为与Franck-Condon区域Sn/Sπ态混合或势能面交叉有关.溶剂对Sn/Sπ态混合或势能面交叉具有调控作用.     

极性分子键角与键偶极矩的关系

研究了基态极性分子的键角和键偶极矩之间的关系。我们采用原子偶极矩校正的Hirshfeld （ADCH）电荷来计算键偶极矩,利用电子的局域函数和键临界点处的局域函数值来分析键的电子结构。通过对IVA族（IVA = C,Si,Ge）、VA族（VA = N,P,As ）、VIA族（VIA = O,S,Se）和VIIA族（VIIA = F,Cl,Br）元素形成的系列共价型基态分子,以及环状基态分子的键角和键偶极矩数据进行分析,发现在键的电子结构类似的情况下,由于键偶极矩的排斥作用,这些分子的键角随键偶极矩的增加而增大。这一发现有助于加深我们对分子几何结构的认识。     

γ-巴豆酰内酯激发态动力学的共振拉曼光谱和完全活性空间自洽场(CASSCF)计算研究

采用共振拉曼光谱和完全活性空间自洽场(CASSCF)方法研究了γ-巴豆酰内酯的光吸收S2态的结构动力学和衰变机制.采用含时密度泛函理论方法结合光谱实验确认了紫外光谱和振动光谱.获得了涵盖A-带吸收的4个激发波长下的共振拉曼光谱.用CASSCF计算得到了S1,min,S2,min,T1,min,T2,min和T3,min及其相关势能面交叉点的结构与能量.研究了A-带共振拉曼光谱强度模式与S2,min和CI(S2/S1)交叉点结构的关系.借助El-Sayed规则分析了各系间窜跃路径的效率,提出了γ-巴豆酰内酯从S2,FC弛豫到基态S0的2条主要路径:内转换路径和系间窜跃路径.     

I2-环己烯复合物的共振拉曼光谱和密度泛函理论计算研究

实验得到I2-环己烯电荷转移复合物的电子吸收光谱和共振拉曼光谱.用密度泛函方法计算了复合物的基态结构、振动频率和电子跃迁能.计算和吸收光谱实验结果表明,I2-烯烃复合物在约300nm处的强吸收带为pz(I17)→π*(C=C)跃迁,即由靠近C=C双键端的碘原子(I17)上的一个pz电子向C=C双键反键轨道跃迁引起的吸收.在约300nm共振拉曼光谱的强度模式表现为I—I伸缩振动模和C=C伸缩振动模的基频、泛频及其组合频,表明在该激发态上I2-环己烯复合物经历了显著的I—I和C=C的价键变化.     

间甲基苯甲醚分子旋转异构体的质量分辨阈值电离光谱

间甲基苯甲醚分子有顺式和反式两个转动异构体.利用单光共振双光子电离技术和质量分辨阈值电离技术,研究了间甲基苯甲醚分子顺反异构体的基态到第一电子激发态(S1←S0)的跃迁和阈值电离.得到顺式、反式间甲基苯甲醚分子S1态的激发能(E1)分别为(36049±2)和(36117±2)cm-1,绝热电离能(Ip)分别为(64859±5)和(65110±5)cm-1.结合从头算法和密度泛函理论的量子化学计算,解释了顺式、反式间甲基苯甲醚分子E1和Ip存在差异的原因,并且对S1态和离子基态D0态出现的谱峰进行了标识.间甲基苯甲醚分子顺反异构体在S1态和D0态的活性振动主要是甲基转动、面内环的运动和与取代基相关的弯曲振动.间甲基苯甲醚分子的S1态振动光谱、D0态离子光谱以及理论计算均表明这两个转动异构体在D0态的几何构型与S1态的中性几何构型相比有较大改变,取代基与取代基、取代基与苯环间的相互作用强度高低次序为:S0S1D0.     

四磺基苯基卟啉的表面增强拉曼效应

以三电极电池系统在不同电位下的在位测量测得四磺基苯基卟啉钠水溶液（浓度为５×１０＾－５ｍｏｌ／Ｌ）的表面增强拉曼谱。结果表明，在接近第一还原态电位处对吸附分子活化后，可得到较大倍数的增强信号（１０＾７数量级）。对照简正坐标计量结果，发现与Ｎ原子有关的振动模式有较大的增强，在此基础上提出了增强机理。     

卟啉化合物的共振能

具有芳香性的卟啉环的共振能是卟啉类化合物的重要结构参数,据此可以讨论卟啉的光谱和化学反应性能等问题。本文根据分享键能(Contributing Bond Energy)的概念,对复杂分子卟啉的键能进行了计算。采用分享键能和键解离能(Bond Dissociation Energy)的差值,计算了某些卟啉化合物的共振能。计算方法简便,计算结果与以前文献报道的以及实验值符合。     

Femtosecond degenerate four-wave mixing of carbon disulfide: high-accuracy rotational constants

Femtosecond degenerate four-wave mixing (fs-DFWM) rotational coherence spectroscopy (RCS) has been used to determine the rotational and centrifugal distortion constants of the 00 (0)0 ground and 01 (1)0 vibrationally excited states of gas-phase CS(2). RCS transients were recorded over the 0-3300 ps optical delay range, allowing the observation of 87 recurrences. The fits yield rotational constants B(00 (0)0)=3.271 549 2(18) GHz for (12)C(32)S(2) and B(00 (0)0)=3.175 06(21) GHz for the (12)C(32)S(34)S isotopomer. The rotational constants of the degenerate 01 (1)0 bending level of (12)C(32)S(2) are B(01 (1)0)=3.276 72(40) and 3.279 03(40) GHz for the e and f substrates, respectively. These fs-DFWM rotational constants are ten times more accurate than those obtained by CO(2) laser/microwave heterodyne measurements and are comparable to those obtained by high-resolution Fourier transform infrared spectroscopy. Ab initio calculations were performed at two levels, second-order Moller-Plesset theory and coupled-cluster singles, doubles, and iterative triples [CCSD(T)]. The equilibrium and vibrationally averaged C=S distances were calculated using large Dunning basis sets. An extrapolation procedure combining the ab initio rotational constants with the experiment yields an equilibrium C=S bond length of 155.448 pm to an accuracy of +/-20 fm. The theoretical C=S bond length obtained by a complete basis set extrapolation at the CCSD(T) level is r(e)(C=S)=155.579 pm, or 0.13 pm longer than that in the experiment.     

Resonance Raman spectroscopy and density functional theory calculation study of photodecay dynamics of tetra(4-carboxyphenyl) porphyrin

The 397.9 nm, 416.0 nm and 435.7 nm resonance Raman spectra were acquired for meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP) in tetrahydrofuran solution, and the Raman effect of relaxation dynamics was analyzed according to Herzberg-Teller (vibronic coupling) contributions. Density functional calculations were done to help the elucidation of the Soret (B(x) and B(y)-band) electronic transitions and the corresponding photo relaxation dynamics of TCPP. The spectra indicate that the Franck-Condon region photo relaxation dynamics upon S(0) → S(4) electronic transition are predominantly along the totally symmetric C(m)-ph stretch and Porphin ring breath stretch, and simultaneously along the asymmetric ν(C(m)-Phenyl) + δ(N-H) and ν(C(α)-C(m)-C(α))(as) + def (pyr) vibrational relaxation processes. The excited state structural dynamics of TCPP determined from the resonance Raman spectra show that the internal conversion between the B(y) and B(x) electronic states occurs in tens of femtoseconds, and the electronic relaxation dynamics were firstly interpreted taking into account the time-dependent wave packet theory and Herzberg-Teller (vibronic coupling) contributions.     

Theoretical study of 1,6-anhydrosugar formation from phenyl D-glucosides under basic condition: reasons for higher reactivity of β-anomer

Degradation of anomeric phenyl d-glucosides to levoglucosan under basic condition is theoretically studied. MP4(SDQ)//DFT(B3LYP)-computational results indicate that the degradation of phenyl α-glucoside (R(α)) occurs via the S(N)icB mechanism. In this mechanism, the oxyanion at the C6, which is formed through deprotonation of the OH group, directly attacks the anomeric carbon. On the other hand, the degradation of phenyl β-glucoside (R(β)) occurs via the S(N)icB(2) mechanism. In this mechanism, the oxyanion at the C2 attacks the anomeric carbon in a nucleophilic manner to afford 1,2-anhydride intermediate and then the oxyanion at the C6 attacks the anomeric carbon to afford levoglucosan. The activation barrier is much lower in the reaction of R(β) (ΔG(0++) = 25.6 kcal/mol and E(a) = 26.5 kcal/mol) than in the reaction of R(α) (ΔG(0++) = 38.1 kcal/mol and E(a) = 37.2 kcal/mol), which is consistent with the experimental observation that β-glucoside is generally much more reactive than the corresponding α-glucoside. The lower activation barrier of the reaction of R(β) arises from the stereoelectronic effect, which is induced by the charge transfer from the ring oxygen to the anomeric carbon, and the staggered conformation around the C1-C2 bond. When the stereoelectronic effect is absent, the degradation needs larger activation energy; for instance, the degradation of phenyl 5a-carba-β-d-glucoside (R(Cβ)) occurs with large ΔG(0++) and E(a) values like those of α-glucosides, because the methylene group of R(Cβ) does not contribute to the stereoelectronic effect. Also, the conformation around the C1-C2 bond is staggered in the transition state of the R(β) reaction but eclipsed in that of the R(α) reaction, which also leads to the larger reactivity of R(β).     

Conformational analysis of the sixteen C(4)-C(6) and C(4)-C(8) linked dimers of (+)-catechin and (−)-epicatechin

A conformational analysis has been performed for sixteen dimers of (+)-catechin and/or (?)-epicatechin using molecular mechanics (MM2). Monomer units are linked by 4α-6, 4α-8, 4β-6, and 4β-8 bonds. THe four possible combinations of (+)-catechin and/or(?)-epicatechin are used for each bonding pattern. The objectives are characterization of (1) the two rotational isomers at the bond between the two monomer units and (2) the conformations of the heterocyclic rings. There is a twofold rotation about the bond between monomer units. Differ4ences in the energies at the two minima range from a few tenths of a kcal/mol to several kcal/mol, depending on the dimer Heterocyclic rings occupy a range of conformations that can be described as half chairs with varying degrees of distoration toward C(2) or C(3) sofas. The more frequent distortion is toward the C(2) sofa. Interconversion between most of the heterocyclic ring conformations can be obtained by coordinated motion of C(2) and C(3), over a range of about 40 pm, with respect to the mean plane of the fused aromatic ring system.     

Equilibrium Structure and Fundamental Vibrational Wavenumbers in Difluorosilanone: An ab Initio Study

High‐level ab initio calculations with large basis sets have been performed for difluorosilanone, F₂SiO. Based on these calculations, an empirically corrected theoretical equilibrium structure is derived: r_e(SiO) = 149.8(1) pm, r_e(SiF) = 155.5(1) pm, α_e(FSiF) = 104.7(3)°. Furthermore, these calculations confirm the experimental assignments of the observed infrared bands to the fundamentals in F₂SiO, except for ν₃. The previous assignment of ν₃ appears to be incorrect and should be reinvestigated.     

Application of Dicyanohexasulfane for the Synthesis of cyclo-Nonasulfur. Crystal and Molecular Structures of S(6)(CN)(2) and of alpha-S(9)(1)

A novel synthesis for dichlorotetrasulfane is reported. Careful chlorination of cyclo-hexasulfur yields S(4)Cl(2) (besides S(2)Cl(2)), which is used to prepare S(6)(CN)(2) by reaction with Hg(SCN)(2). An X-ray diffraction analysis of S(6)(CN)(2) shows nonhelical chainlike molecules with the following molecular parameters: SS bond lengths 203.4-207.4 pm, SSS valence angles 104.95-105.96 degrees, SS torsion angles 81.2-94.5 degrees (motif: + + - - +). The chain-terminating SCN groups exhibit a parallel orientation within the molecules and are antiparallel in neighboring molecules. S(6)(CN)(2) reacts with titanocene pentasulfide to give S(9) and titanocene diisothiocyanate. alpha-S(9) was obtained as single crystals, the structure of which was determined by X-ray diffraction. The two independent molecules occupy sites of C(1) symmetry, but the molecular symmetry is approximately C(2), in agreement with predictions by density functional and ab-initio MO calculations. Molecular parameters: bond lengths 203.2-206.9 pm, valence angles 103.7-109.7 degrees, torsion angles 59.7-115.6 degrees (motif: + + - - + + - + -). The average SS bond lengths in S(6)(CN)(2) and alpha-S(9) agree with the single-bond value of 205 pm as observed in H(2)S(2) and in alpha-S(8).     

Conformation-specific spectroscopy and populations of diastereomers of a model monolignol derivative: chiral effects in a triol chain

Single-conformation spectroscopy of two diastereomers of 1-(4-hydroxy-3-methoxyphenyl)propane-1,2,3-triol (HMPPT) has been carried out under isolated, jet-cooled conditions. HMPPT is a close analog of coniferyl alcohol, one of the three monomers that make up lignin, the aromatic biopolymer that gives structural integrity to plants. In HMPPT, the double bond of coniferyl alcohol has been oxidized to produce an alkyl triol chain with chiral centers at C(α) and C(β), thereby incorporating key aspects of the β-O-4 linkage between monomer subunits that occurs commonly in lignin. Both (R,S)- and (R,R)-HMPPT diastereomers have been synthesized in pure form for study. Resonant two-photon ionization (R2PI), UV hole-burning (UVHB)/IR-UV hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been carried out, providing single-conformation UV spectra in the S(0)-S(1) region (35200-35800 cm(-1)) and IR spectra in the hydride stretch region. Five conformers of (R,S)- and four conformers of (R,R)-HMPPT are observed and characterized, leading to assignments for all nine conformers. Spectroscopic signatures for α-β-γ, γ-β-α, and α-γ-β-π chains and two cyclic forms [(αβγ) and (αγβ)] of the glycerol side chain are determined. Infrared ion-gain (IRIG) spectroscopy is used to determine fractional abundances for the (R,S) diastereomer and constrain the populations present in (R,R). The two diastereomers have very different conformational preferences. More than 95% of the population of (R,R) configures the glycerol side chain in a γ-β-α triol chain, while in (R,S)-HMPPT, 51% of the population is in α-β-γ chains that point in the opposite direction, with an additional 21% of the population in H-bonded cycles. The experimental results are compared with calculations to provide a consistent explanation of the diastereomer-specific effects observed.     

Arylthiazylamides: syntheses, structures, and bonding properties

Air-sensitive, thermally unstable tris(dimethylamino)sulfonium (TAS) salts (3) of the title anions [ArNSN]- have been prepared from corresponding sulfurdiimides Ar-N=S=N-SiMe3 (2) by Si-N bond cleavage with [(Me2N)3S]-[Me3SiF2]- (TASF). They are characterized by low-temperature X-ray crystallography as Z isomers. Because of the very short terminal S-N distance (144.2 (3h)-147.9 (3i)pm) and the relatively long internal S-N distance (158.3 (3i)-160.3 (3c) pm) the [ArNSN]- ions should be regarded as thiazylamides 1b, rare species containing a S triple bond N triple bond. A bonding model is developed and the experimental results are compared with those of restricted Hartree-Fock (RHF), density functional theory (DFT), and M?ller-Plesset second-order (MP2) calculations.     

Reactivity of titanium dimer and molecular nitrogen in rare gas matrices. Vibrational and electronic spectra and structure of Ti2N2

The reactivity of diatomic titanium with molecular nitrogen has been investigated in rare gas matrices. The formation of Ti2N2 from the condensation of effusive beams of Ti and N2 in neon and argon matrices is observed after sample deposition. Our results also show that the in situ formation results from the spontaneous reaction at 9 K of ground state Ti2 with N2. Several low-lying excited states of Ti2N2 are also observed between 0.78 and 1.1 eV above the ground state, leading to a complex sequence of interacting vibronic transitions, merging into a broad continuum above 1.25 eV. Observations of Ti2(14)N2, Ti2(15)N2 and Ti2(14)N(15)N isotopic data enable the determination of all fundamental vibrations in the ground electronic state. Semi-empirical harmonic potential calculations lead to estimates of 3.22 N cm(-1) for the Ti-N bond force constant and 90 +/- 5 degrees for the bond angles. Comparisons with TiN diatomic data suggest a near square-planar structure with 175 +/- 3 pm TiN bond distance. Quantum chemical calculations at various levels indicate a 1A(g) ground state with a Ti-N distance close to 180 pm and 89 degrees for the NTiN bond angle, and give fundamental frequencies in excellent agreement with the experimentally observed values. Further MRCI calculations on all low-lying states enable an interpretation of the complex electronic spectrum in the NIR region.     

The molecular structure and conformation of acetamides in the vapour phase: Part III. Thioacetamide (ethanethioamide)

Thioacetamide has been studied by electron diffraction in the gas phase, utilizing a new nozzle construction and using a broad electron beam. The molecule has C_s symmetry, and one C-H bond eclipses the CS bond. The most important structural parameters are: r_g(C-N) = 135.6(3) pm, r_g(C—C) = 151.2(4) pm, r_g(CS) = 164.7(3) pm,∠_αCCS = 122.9(3)° and ∠_αCCN = 114.8(4)°. Parenthesized values are one standard deviation where correlation among data and uncertainty in the electron wavelength have been included. The methyl barrier, V₃, is found from the electron diffraction data to be 4.56 kJ mol^?1. This corresponds to a torsional frequency of 131 cm^?1.