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1.
Both ab initio 6-31G, 3-21G and STO-3G basis sets and semiempirical PM3 and AM1 molecular orbital calculations are carried out on the C 24N 4 molecule of the Td symmetry group. Results on the fully optimized structure which constrained Td symmetry, molecular orbitals and vibrational frequency were obtained by both ab initio and semiempirical methods. The binding energy and various thermodynamic properties were also calculated via the PM3 and AM1 semiempirical methods. All the evidence of this work proves that the C 24N 4 molecule is stable and that its four six-membered rings with a remarkable delocalized C…C bond are similar to the related rings in the C 60 buckminsterfullerene structure. 相似文献
2.
Several symmetrical dimers comprised of salicylaldimine-moieties connected through ester linkages to the termini of odd-parity alkanediols have been synthesized and investigated for their thermal behaviour. In order to understand the structure-property relations, the lengths of the central alkylene spacers (C 3 and C 5) as well as those of the terminal alkoxy chains (C 6 to C 22) have been varied. The dimers with a C 3-alkylene spacer are non-liquid crystalline, while some of the compounds having a C 5-alkylene spacer exhibit liquid crystalline properties. The dimer, with a C 5-alkylene spacer and C 6-alkoxy tails, shows an intercalated smectic C (SmC c) phase, whereas the C 8, C 10, C 11, and C 12 homologues are non-mesomorphic. The higher homologues of this series with C 16, C 18, C 20 and C 22 alkoxy tails show a mesophase that has the signatures of a two-dimensional banana (B 1) phase. This mesophase is enantiotropic in the C 16 and C 18 homologues while it is monotropic in the other homologues. In these dimers, the spacer length has a remarkable influence on the thermal behaviour. 相似文献
3.
Hydrogen and fluorine addition reactions with C 28( Td) have been investigated by the density function theory method at B3LYP/6-31G level. The interaction potential between C 28( Td) and atom X (X=H and F) shows that there are three possible stable isomers of C 28( Td)X (X=H and F) and the average binding energy calculations suggest that C 28( Td)H 4 is the most stable hydrogen adduct among C 28( Td)H n ( n=1–28). Furthermore, by comparisons of the energy between C 28( Td)H and C 28( Cs)H we found that the former are more stable than the later, and the structural and energy analysis further indicate that C 28( Cs)H is only with a small distortion of C 28( Td)H symmetry. In addition, the transition states, as well as reaction pathways of X transfer reactions between different key points on C 28( Td) representative patch are given to explore the possible reaction mechanism. 相似文献
4.
Among unsymmetric oligomesogens, chiral dimers formed by connecting a cholesteryl ester fragment with various aromatic mesogenic cores through a polymethylene spacer have been attracting much attention due to their remarkable thermal behaviour. In particular, dimers containing a diphenylacetylene segment having an alkoxy chain have shown interesting mesomorphic behaviour. In view of this a new series of unsymmetric dimers consisting of a diphenylacetylene moiety having an alkyl chain and a cholesteryl ester unit joined through a paraffinic spacer have been synthesized and their liquid crystalline properties characterized. The lengths of the central methylene spacer (C 3, C 4, C 5 and C 7) as well as that of the alkyl chain ( n-butyl, n-pentyl, n-hexyl and n-heptyl) have been varied to establish structure-property relationships. These investigations have revealed that all the dimers exhibit smectic A, twist grain boundary and chiral neamtic (N*) phases with the exception of one of the dimers for which only the N* phase was observed. Some differences in the mesomorphic properties of the unsymmetric dimers containing odd or even parity methylene spacers have been observed. The majority of dimers having an even (C 4) parity paraffinic spacer show a blue phase while the dimers with odd (C 3, C 5 and C 7) parity spacers exhibit the chiral smectic (SmC*) phase. In some cases, the SmC* phase exists well below (-60°C) and above room temperature. 相似文献
5.
Asymmetric triphenylene imidazolium salts with different spacer lengths were successfully synthesized through quarternization of ω-bromo-substituted triphenylenes with 1-methyl imidazole. The asymmetry in ω-bromo-substituted triphenylenes tended to destroy liquid crystallinity in the sample. However, highly ordered columnar mesophases with a lamellar microphase segregation were induced by ionic interactions among the imidazolium salts, and the lamellar morphology was visualized by transmission electron microscopy. On the basis of an X-ray diffraction study on shear oriented samples, a novel rectangular columnar phase with a plane group of pm was observed for a triphenylene imidazolium salt with a spacer length of C 11, while an oblique columnar phase was determined for a triphenylene imidazolium salt with a C 8 spacer. Due to the asymmetric molecular shape and ionic interactions in the triphenylene imidazolium salts, the columnar liquid crystalline phase was extended to below room temperature ( c. -20°C) for samples with spacer lengths of C 8 and C 11. 相似文献
6.
The thermolysis of C 60H 2 to yield C 60 and H 2 was studied by hybrid density functional theory (B3LYP/6-311G**//B3LYP/3-21G). The concerted loss of dihydrogen requires an activation energy of 92 kcalmol −1 at T=452 K. An alternative radical mechanism, which is first order in the C 60H 2 concentration, has an activation energy at 452 K of only 61 kcalmol −1. Monitoring of the C 60H 2 decomposition in 1,2-dichloro-[D 4]-benzene solution by NMR spectroscopy indicates a pseudo first-order reaction with an activation energy of 61.38±2.35 kcalmol −1. 相似文献
7.
In the last thirty years, Gemini surfactants with various structures have been designed, synthesized, and demonstrated to show superior physicochemical properties. However, the utilization of non-degradable surfactants, including these Gemini surfactants, poses a threat to the environment; hence, degradable Gemini surfactants are desirable. Herein, biodegradable cationic Gemini surfactants with amide or ester groups in the hydrophobic chains or the spacer were synthesized. A monomeric surfactant containing an amide group and a Gemini surfactant with amide groups both in the hydrophobic chains and the spacer were synthesized for comparison. The effects of amide group location on the aggregation behavior of Gemini surfactants were studied systematically. The differences between the Gemini surfactants with amide groups and Gemini surfactants with ester groups were evaluated by comparing their aggregation behavior and hydrogen bonding formation. The Gemini surfactants with amide groups (C 12A-C n-AC 12) in the chains showed much larger exothermic Δ Hmic and more negative Δ Gmic values than those of the corresponding monomeric surfactant C 12A; besides, their critical micelle concentration (cmc) was more than one order of magnitude lower than that of C 12A. The amide groups located in the hydrophobic alkyl chains promoted hydrogen bonding formation and self-assembly of the Gemini surfactants C 12A-C n-AC 12. Moreover, 1H NMR spectra revealed that the co-effect of a short spacer and hydrogen bonding leads to slow exchange of the C 12A-C 2-AC 12 molecules between the monomer and the aggregate. For the Gemini surfactant series C 12-AC nA-C 12, the amide groups notably increased the spacer length, and largest cmc value and smallest exothermic Δ Hmic value were observed for C 12-AC 2A-C 12 instead of C 12-AC 6A-C 12. In C 12-AC 12A-C 12, the spacer was long and sufficiently flexible to adopt a "U"-shaped conformation above the cmc, and it acted as the hydrophobic part of the surfactant, as confirmed by 1H NMR spectra. Among the Gemini surfactant with amide groups in both the spacer and the hydrophobic alkyl chains, C 12A-AC 6A-AC 12 had a smaller cmc and I1/ I3 ratio as well as more exothermic Δ Hmic values than those of C 12A-C 6-AC 12 and C 12-AC 6A-C 12. 1H NMR spectra indicated that an ester-alcohol structural equilibrium exists during aggregation for the Gemini surfactants with ester groups. In addition, the Gemini surfactants with ester groups formed water-mediated hydrogen bonds in the aggregates. This water-mediated hydrogen bonding between ester groups was weaker than the direct hydrogen bonding between amide groups. Therefore, the Gemini surfactants with ester groups, C 12E-C 6-EC 12 and C 12-EC 6E-C 12, exhibited lower surface activity, a larger micelle ionization degree, higher micropolarity, and smaller exothermic Δ Hmic and less negative Δ Gmic values than their counterparts with amide groups, C 12A-C 6-AC 12 and C 12-AC 6A-C 12. 相似文献
8.
The conformation of N-glycoproteins and N-glycopeptides has been the subject of many spectroscopic studies over the past decades. However, except for some preliminary data, no detailed study on the vibrational spectroscopy of glycosylated peptides has been published until recently. This paper reports FTIR spectroscopic properties in DMSO and TFE of the N-glycosylated cyclic peptides cyclo[Gly-Pro-Xxx(GlcNAc)-Gly-δ-Ava] 3a and 3b in comparison with data on the non-glycosylated parent peptides cyclo(Gly-Pro-Xxx-Gly-δ-Ava) 2a and 2b [a, Xxx = Asn; b, Xxx = Gln; δ-Ava = NH-(CH2)4-CO] and N-acetyl 2-acetamido-2-deoxy-β-
-gluco pyranosylamine (GlcNAc-NHAc, 4). The assignment of amide I band frequencies to conformation is based on ROESY experiments and determination of the temperature coefficients in DMSO-d6 solution. (For the synthesis and NMR characterization of 2a and 3a see Ref. [19].) Cyclic peptides are expected to adopt folded (β- and/or γ-turn) conformations which may be fixed by intramolecular H-bonding(s). A comparison of the temperature coefficients of the NH protons and amide I band frequencies and intensities suggests that in DMSO there is no significant difference in the backbone conformation and H-bond system of the N-glycosylated models and their parent cyclic peptides. The common feature of the backbone conformation of models 2 and 3 is the predominance of a 1 ← 4 (C10) H-bonded type II β-turn encompassing Pro-Xxx or Pro-Xxx(GlcNAc), respectively. The ROESY connectivities in the Asn(GlcNAc) model (3a) have not been found to reflect intramolecular H-bondings between the peptide and the sugar. The unique feature of the FTIR spectra in DMSO of the cyclic models is the lack or weakness of low-frequency (< 1640 cm−1) amide I component bands. In TFE the amide I region of the FTIR spectra shows an increased number of components below 1650 cm−1 reflecting a mixture of open and H-bonded β- and γ-turn conformers. Because of its destabilizing effect upon γ-turns and other weakly H-bonded structures, DMSO decreases the number of backbone conformers. DMSO also destroys side-chain-backbone H-bondings of type C7, C6 or C8. Possible ‘glyco’ C7 H-bondings in GlcNAc-NHAc (4) or in glycopeptides 3a and 3b cannot resist the effect of DMSO either. The FTIR data in TFE of models 2–4 suggest that the acceptor amide group of strong C7 H-bondings in peptides and glycopeptides absorbs at 1630 ± 5 cm−1 and that of bifurcated H-bondings between 1600–1620 cm−1. 相似文献
9.
A series of symmetrical dimers consisting of salicylaldimine moieties connected by flexible alkylene central spacer via ether linkages has been synthesized. In order to validate the empirical rule suggested by Date et al. to account for the smectic behaviour of such dimers, the chain length of the terminal alkoxy chain has been kept constant (C 8) while the number of methylene units in the central spacer was varied from C 3 to C 11. Another aim of the present investigation was to understand structure-property relationship in these dimers in which the salicylaldimine mesogenic segment has been used for the first time in dimers. The mesomorphic behaviour of these dimers was evaluated using optical microscopy and differential scanning calorimetry and the structure of some of the mesophases has been further investigated with the help of X-ray diffraction. Our studies reveal that the dimers consisting of 3 to 8 methylene units in the flexible spacer show only smectic (smectic C and smectic A) phases. For the dimers containing 4, 6 and 8 methylene units in the central spacer, a unique filament growth pattern has been observed in the smectic A phase while cooling from the isotropic phase. The dimers containing of C 9 to C 11 methylene groups exhibit the nematic phase in addition to smectic modifications. This observation indicates that when the terminal chains are shorter than the spacer, the tendency to form smectic phases is not fully extinguished but is perhaps reduced. 相似文献
10.
The details of weak C–H π interactions that control several inter and intramolecular structures have been studied experimentally and theoretically for the 1:1 C 2H 2–CHCl 3 adduct. The adduct was generated by depositing acetylene and chloroform in an argon matrix and a 1:1 complex of these species was identified using infrared spectroscopy. Formation of the adduct was evidenced by shifts in the vibrational frequencies compared to C 2H 2 and CHCl 3 species. The molecular structure, vibrational frequencies and stabilization energies of the complex were predicted at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels. Both the computational and experimental data indicate that the C 2H 2–CHCl 3 complex has a weak hydrogen bond involving a C–H π interaction, where the C 2H 2 acts as a proton acceptor and the CHCl 3 as the proton donor. In addition, there also appears to be a secondary interaction between one of the chlorine atoms of CHCl 3 and a hydrogen in C 2H 2. The combination of the C–H π interaction and the secondary ClH interaction determines the structure and the energetics of the C 2H 2–CHCl 3 complex. In addition to the vibrational assignments for the C 2H 2–CHCl 3 complex we have also observed and assigned features owing to the proton accepting C 2H 2 submolecule in the acetylene dimer. 相似文献
11.
The molecular structure of 3-methylthiophene
has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: rg(S---C2) = 1.719(2) Å, rg(C2=C3) = 1.370(3) Å, rg(C3---C6) = 1.497(6) Å, rg(C2---H) = 1.101(5) Å, CSC = 91.6(2)°, SC 2C 3 = 113.3(5)°, SC 5C 4 = 111.3(3)°, C 2C 3C 6 = 123.2(11)° and C 3C 6H = 112(2)°. The values of r(S---C 2) - r(S=C 5) and r(C 2=C 3)- r(C 4 =C 5) were fixed at the 3–21G* value of 0.002 Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit. 相似文献
12.
A structural study of odd-numbered n-alkane (C n) binary mixtures (C 21 : C 23) was carried out on powder samples using a Guinier-de Wolff camera with increasing concentration of n-C 23 at 293 K. Despite the reports in the literature, these molecular alloys do not form an orthorhombic continuous homogeneous solid solution to C21 from C23 at “low temperature”. Instead, as already observed in two even-numbered Cn systems, X-ray diffraction results show the existence of seven solid solutions as the molar concentration of C23 increases: four terminal solid solutions, denoted β0(C21)β0(C23), isostructural with the “low temperature” phase of pure C21 and C23 (Pbcm), β′0(C21) and β′0(C23), identical to the phase β′0 which appears in pure C23 above the δ transition, and three orthorhombic intermediate solid solutions, designated β″1, β′1 and β″2. On the basis of powder X-ray photographs, the phases β″1 and β″2 (C21 : C23) are indistinguishable, and they are isostructural with the intermediate solid solution β″ of the even-numbered Cn binary systems (C22 : C24) and (C24 : C26). The phase β′1(C21 : C23) is also isostructural with the two indistinguishable intermediate solid solutions β′1 and β′2 of the molecular alloys (C22 : C24) and (24 : C26). From this study and our other laboratory results, the sequences of appearance of the solid solutions and the structural identities between these phases are established at “low temperature” for all the binary molecular alloys of consecutive Cn (odd-odd, even-even or odd-even: 19 < n < 27) when increasing the solute concentration. 相似文献
13.
采用密度泛函理论(DFT)的B3LYP方法,在6-31G*和6-311+G(3 df)水平上对C nB(n=1~6)团簇及其阴离子和阳离子的几何构型和电子结构进行了优化和振动频率计算.得到了C nB(n=1~6)团簇的电离能,绝热电子亲合势以及C nB δ(δ=0,±1)团簇的能隙.结果表明C nB -(n=1~6)团簇的基态构型均为线形,这与等电子的C n簇合物的结构是一致的; C nB(n=1~6)团簇的基态构型中,除C 2B为不对称的三角形,C 6B为具有C 2v对称性的环状结构外,其余均为线形结构.阳离子团簇中n=2、3、6的基态结构具有C 2v对称性外,其它几个均为线形结构.从几何参数和振动频率上发现,采用密度泛函B3LYP方法在6-311+G(3 df)和6-31G*两种基组上计算得到的键长参数和振动频率非常接近,说明B3LYP方法在计算C nB簇合物结构参数上对于基组的选择是不太敏感的.通过对C nB(n=1~6)的光电子能谱性质的研究发现,C 4B容易获得一个电子形成阴离子团簇,但失去一个电子是很困难的,这与实验上观测到的结果非常吻合. 相似文献
14.
The molecular structure of 3-methylthiophene
has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: rg(S---C2) = 1.719(2) Å, rg(C2=C3) = 1.370(3) Å, rg(C3---C6) = 1.497(6) Å, rg(C2---H) = 1.101(5) Å, CSC = 91.6(2)°, SC2C3 = 113.3(5)°, SC5C4 = 111.3(3)°, C2C3C6 = 123.2(11)° and C3C6H = 112(2)°. The values of r(S---C2) − r(S---C5) and r(C2=C3) − r(C4=C5) were fixed at the 3–21G* value of 0.002Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit. 相似文献
15.
C 60S + was synthesized through the gas-phase ion-molecule reaction of C 60 with the plasmas of carbon disulfide under self-chemical-ionization (self-CI) conditions in the ion source of a mass spectrometer. Semi-empirical PM3-UHF and density functional B3LYP levels of theory with 6-31G(d) basis set calculations were performed on all the possible structures and electronic properties of the product. The results showed that the most stable structure among the possible isomers was the 6/6 closed derivative. The reaction energies of C 60+S +→C 60S + and C 60+S→C 60S were also calculated to suggest the possibility of C 60S synthesis in condensed phase. 相似文献
16.
Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C 4) or n-pentyl (C 5) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C 4 central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C 5 central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures. 相似文献
17.
A Co-based two-dimensional (2D) microporous metal-organic frameworks (UPC-32) with narrow distance between layers and layers (3.8 Å) exhibits high selectivity of C 3H 6/CH 4 (31.46) and C 3H 8/CH 4 (28.04) at 298 K and 1 bar. It is the first 2D Co-MOF that showed selective separation of C3 hydrocarbon from CH 4. 相似文献
18.
Potential energy surfaces of the reaction of SiH 2 and C 2H 2 (and C 2D 2) have been calculated by means of ab initio molecular orbital theory at the QCISD/6-311G++(2df, 2p)//MP2/6-31G(d, p) level with corrections for the triple excitations to the QCISD energies. The barrier heights for the two reaction channels of the adduct, thus calculated, were further utilized for the dynamical calculation of the rate constants in the framework of quantum statistical Rice-Ramsperger-Kassel theory. Contributions of the rate constants of the various pathways to the total rate constant ( KT) for the disappearance of the reactants are critically examined and compared with experiment. The pressure dependence of KT(C 2H 2) is primarily due to the formation of silirene. KT(C 2D 2) is consistently higher than KT(C 2H 2). The standard heat of formation of silirene is predicted to be 72.1 ± 3 kcal/mol. Rearrangement of silirene to vinylsilylene requires an activation energy smaller than that to silylacetylene. 相似文献
19.
Self-assembled supramolecular networks are promising spacer layer for electronic decoupling from the metal substrate.However,the mechanism behind of how the intrinsic electronic structure of spacer layers affects the adsorbate is still unclear.Here a hydrogen bonded network composed of n-type semiconducting molecules 3,4,9,10-perylene-tetracarboxylic-dianhydride(PTCDA)is prepared under ultra-high vacuum to serve as a spacer layer for functional organics C60 on Au(111).The geometric and electronic information of C60 was investigated by scanning tunneling microscopy and scanning tunneling spectroscopy(STM/STS)at 5 K.Effective decoupling from the metal surface yields an energy gap of 3.67 eV for C602nd,merely considering the HOMO-LUMO peak separation.The broadening of resonance peaks in STS measurements however indicates unneglected interlayer interactions in this hetero-organic system.Moreover,we scrutinize the nucleation sites of C60 on PTCDA layer and attribute this to the decreased diffusion capability on a less dense molecular arrangement possessing inhomogeneous spatial distribution of unoccupied molecular orbitals. 相似文献
20.
用表面张力法、电导法和稳态荧光法研究了手性Gemini表面活性剂[C 12-m-C 12] Na 2(m=2,4,6)和[C 12-T-C 12] Na 2的表面性能及临界胶束聚集数,并计算胶束形成的热力学参数,用圆二色谱法考察了[C 12-2-C 12] Na 2在不同浓度下的立体构型. 结果表明,手性Gemini表面活性剂的临界胶束浓度(cmc)和临界表面张力γ cmc随着连接基链长增加或刚性增强而增大;ΔG m0和ΔH m0为负值,|ΔH m0|比|-TΔS m0|小很多,说明胶束化过程为熵驱动的自发放热过程;随着连接基链长增加或刚性增强,ΔG m0和ΔH m0逐渐增大,ΔS m0和临界胶束聚集数逐渐减小,表明其胶束化能力随之降低;当浓度大于cmc时,手性Gemini表面活性剂可形成手性超分子聚集体. 相似文献
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