共查询到20条相似文献,搜索用时 624 毫秒
1.
Factors that influence the intramolecular [2+2] photochemical ring closure reaction of multiply bridgedsyn-tricyclo[4.2.0.02.5]octa-3,7-diene (2) leading to propella[34]prismane (1) are analyzed. The unfavorable, large increase in strain (89 kcal mol–1) calculated for this reaction is overcome by the favorable ordering of frontier molecular orbitals, which is caused primarily by bridging of the juxtaposed double bonds with trimethylene chains in2. This conclusion is based on detailed analysis of the frontier molecular orbitals (FMO) of cycloocta-1,5-diene (6), cyclodeca-1,6-diene (7), and tricyclo-[4.2.0.02.5]octa-3, 7-diene (3), bridged with alkylene groups of various lengths, in terms of Paddon-Row's theory of through-bond interaction by the use of AM1 semiempirical MO method. In addition to the -orbital interaction through an even number of C-C bonds, the double-bond-double-bond distance is recognized to be an important factor for the ordering of FMOs. It is suggested that reduction of this distance in6 by only a few tenths of an Ångstrom will change the FMO ordering and allow the intramolecular [2+2] cycloaddition to proceed. Potential energy calculations by MM2 indicate that the trimethylene bridges in1 and2 are so flexible that they appear flat on the NMR time scale. 相似文献
2.
Energetics and the charge distributions in azacubanes (C8NH8–) have been obtained using the ab initio Hartree–Fock, second-order Mø
øller–Plesset perturbation theory and hybrid density functional methods. For diazacubane to hexaazacubane the lowest-energy conformers have nitrogen atoms occupying the face opposite corners of a cube. The topography of the molecular electrostatic potential and the electron density of azacubane conformers have been investigated. The electrostatic potential studies have shown that successive substitution of nitrogen instead of CH groups of cubane engenders smaller and more localized electron-rich regions around the nitrogens of a cube. Further the bond ellipticity and the electron density at the bond critical point of the X–N bonds (X=C or N) in a cubanoid increase from azacubane to octaazacubane. The heats of formation of azacubanes calculated by the isodesmic reaction approach using different levels of theory correlate well with the electron density at the bond critical point of X–N (X=C or N) bonds in a cubanoid. 相似文献
3.
J. Eric D. Davies Vivienne A. Tabner 《Journal of inclusion phenomena and macrocyclic chemistry》1991,11(4):389-396
A pre-resonance Raman study of the yellow -quinol/SO2 clathrate has been carried out using 609.8, 586.8, 514.5, 488.0 and 457.9 run excitation. Pre-resonance enhancement is observed for the guest vl (Al) band at 1147 cm–1 and the host band at 1257 cm–1. These observations are consistent with a charge transfer interaction arising from the LUMO of S02 (S 3pz) and the HOMO of quinol, which consists mainly of the ring electrons. 相似文献
4.
The influence of the nature of the heteroatom on the Ni+ gas-phase binding energies of HCC–XH3 (X is C, Si, or Ge) compounds has been investigated through the use of high-level density functional theory methods. The structures of the corresponding Ni+ complexes were optimized at the B3LYP/6-311G(d,p) level of theory. Final energies were obtained in single-point B3LYP/6-311+G(2df,2p) calculations. Nonconventional complexes, in which the metal cation interacts simultaneously with the CC system and with one of the X–H bonds of the substituent XH3 group, play a significant role in the binding of Ni+ to HCC–XH3 (X is Si or Ge) derivatives. Conversely, such nonconventional complexes are not local minima of the propyne–Ni+ potential-energy surface. This establishes a clear distinction between unsaturated carbon derivatives and the Si- and Ge-containing analogues as far as bonding to transition-metal monocations is concerned. Actually, the attachment of Ni+ to HCC–XH3 (X is Si or Ge) compounds in the gas phase yields a mixture of conventional and nonconventional complexes. These agostic-type interactions can be viewed as a dative bond from the X–H bonding orbital toward the empty s orbital of the metal, and a back-donation from the valence electron pairs of the metal into the X–H antibonding orbital of the neutral species.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail 相似文献
5.
The composition of the volatile compoents of the oleoresin ofAbies alba Mill, has been studied. In this oleoresin 19 monoterpene compounds have been identified, the main ones of which are - and -pinenes and limonene. Among the 24 sesquitepene compounds identified in the oleoresin, 4 new terpenoids have been found: 10S,11S-himachala-2,3-diene, 10S,11S-himachala-3(12),4-diene, humula-4,9-dien-8-ol, and 4S,5S,10S-selin-6-en-4-ol. For the first time, , , and -himachalenes, 9-epicaryophyllene, intermedeol, and longiborneol have been found in the oleoresins of conifers of the genus Albies. A distinguishing feature of firs of the Albae series is the low content of oxygen-containing monoterpenoids and of sesquiterpenoids of the bisabolane series.Novosibirsk Institute of Organic Chemistry, Siberian Branch, USSR Academy of Sciences. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 193–197, March–April, 1988. 相似文献
6.
J. Laxmikanth Rao 《Central European Journal of Chemistry》2007,5(3):793-812
Density Functional Theory (DFT) calculations and Frontier Molecular Orbital (FMO) analysis have been carried out at B3LYP/6-31G(d,p)
level of theory on some Donor-Bridge-Acceptor (D-B-A) molecules for their electrical rectification behavior. The donor-acceptor-heterocyclics
(D/A-heterocyclics) (namely thiophene, furan and pyrrole rings) are attached as donor and acceptors to the two ends of cumulenic
bridge. FMO analysis indicates that the molecules having even number of double bonds in the bridge, possess a complete localization
of the MOs i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized
on the donor and the acceptor side of the molecules respectively, and LUMO+1 is localized on the donor side, where as in case
of odd number of double bonds in the bridge, both the HOMO and LUMOs are delocalized all over the molecule. The Potential
Drop (PD) in the former case decreases as the number of double bonds increases in the bridge and due to the presence of the
mutually orthogonal and noninteracting π-clouds, they can act as molecular rectifiers. For the molecules with the odd number
of double bonds due to the low-lying LUMO delocalized all over the molecule, may find application as molecular wires in molecular
electronics circuits.
相似文献
7.
Yongfan Zhang Liming Wu Junqian Li Zunxin Huang Lixin Zhou 《Journal of Cluster Science》1999,10(3):459-473
Using the ab initio method and natural bond analysis, the reactivity and chemical bond of dinuclear molybdenum sulfur clusters with Mo2O
n
S4–n
(n=1–3) core were studied. The results show that the Mo--Mo bonds in these clusters are significantly affected by the delocalization effects of the multicenter d--p and Mo--X
1 bonds, besides the direct interactions between Mo atoms. The substitution reactions of the cluster core are also discussed, and it is indicated that sulfur atoms have a preference for the bridging sites and the oxygen atoms tend to attach to the terminal sites. 相似文献
8.
Interfacial behavior of cysteine (Cys) between mild steel and sulfuric acid solution as a corrosion inhibitor has been studied with electrochemical AC (alternating current) and DC (direct current) techniques at (25.0±0.1) °C. The AC impedance results were evaluated using equivalent circuits in which a constant phase element (CPE) has been replaced with double layer capacitance (Cdl) to represent the frequency distribution of experimental data. Changes in impedance parameters (charge transfer resistance and double layer capacitance) indicated that cysteine molecules acted by accumulating at the metal/solution interface. The fractional coverage of the metal surface () was determined using AC impedance results and it was found that the adsorption of cysteine on the mild steel surface followed a Langmuir isotherm model with a standard free energy of adsorption (ΔG0ads) of −35.1 kJ·mol−1.To clarify the type of interaction between mild steel surface and cysteine molecules with a molecular orbital approach, electronic properties, such as, the highest occupied molecular orbital (HOMO) energy, the lowest unoccupied molecular orbital (LUMO) energy, and the frontier molecular orbital coefficients have been calculated. Energy gaps for the interaction of mild steel surface and cysteine molecules (ELUMO Fe−EHOMO Cys and ELUMO Cys−EHOMO Fe) were used to determine whether cysteine molecules acted as electron donors or electron acceptors when they interacted with the mild steel surface. The local reactivity was evaluated through the condensed Fukui indices. Theoretical calculations were carried out using the density functional theory (DFT) at B3LYP level with the 6-311++G(d,p) basis set for all atoms by Gaussian 03W program. 相似文献
9.
Volker Staemmler 《Theoretical chemistry accounts》1982,62(1):69-80
Ab initio calculations for the 2 ground states of SF and SCl have been performed on Hartree-Fock level and with inclusion of valence shell correlation effects by means of the CI and CEPA approaches. The calculated properties are: Equilibrium distances, vibration frequencies, and dipole moment curves in the vicinity of the respective equilibrium geometries. Our best estimates for the 0 1 infrared absorption frequencies o for SF and SCl are 786 cm–1 and 520 cm–1, respectively, both with an uncertainty of about 10 cm–1. This confirms a recent experimental value obtained by Willner for SF (791 cm–1), but indicates that for SCl both experimental values reported previously in the literature (617 cm–1 and 574 cm–1) are wrong. The S—F and S—Cl bonds in SF and SCl are very similar to the ones in SF2 and SCl2, being essentially single p-bonds in either case. In the analogous oxygen-halogen molecules the situation is different, the O—F and O—Cl bonds in the diatomic radicals OF and OCl have partial double bond character and are much stronger than those in OF2 and OCl2 or in HOF and HOCl. 相似文献
10.
Theoretical analysis of competing directions of electrocyclization of triazatetraene H2C=CH-N=N-CH=N-CH=CH2, triazatrienes H2C=CH-N=N-CH=NH and HN=N-CH=N-CH=CH2, and 1,4-diphenyl-1,2,4-triazabuta-1,3-diene Ph-N=N-C(R)=N-Ph was carried out. Specific features of cyclization of these compounds were explained by differences in the energies of -orbitals of C=C, C=N, and N=N bonds and in bond orders of atoms forming a new bond in the reaction. The activation barrier and the structure of the transition state of the electrocyclization of 1,2,4-triazahexa-1,3,5-triene were studied by AMI, MNDO, and MINDO/3 methods. The electrocyclization of triazatriene is an asymmetric disrotatory process. The rotation angle of the terminal CH2 group around the C=C bonds is twice as large as that of the NH group around the N=N bond. The 1,3-prototropic shift in dihydro-1,2,4-triazines is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1459–1465, August, 1995.The work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-5069). 相似文献
11.
Heidar-Ali Tajmir-Riahi 《Monatshefte für Chemie / Chemical Monthly》1987,118(2):245-254
The reaction betweenL-arabinose and hydrated uranyl salts has been investigated in aqueous solution and the solid complexes of the type UO2(L-arabinose)X
2 · 2 H2O, whereX=Cl–, Br–, and NO
3
–
, have been isolated and characterized. Due to the marked similarities with those of the structurally known Ca(L-arabinose)X
2 · 4 H2O and Mg(L-arabinose)X
2 · 4 H2O (X=Cl– or Br–) compounds, the UO
2
2+
ion binds obviously to twoL-arabinose moieties, through O1, O5 of the first and O3, O4 of the second molecule resulting into a six-coordinated geometry around the uranium ion with no direct U-X (X=Cl–, Br– or NO
3
–
) interaction. The intermolecular hydrogen bonding network of the freeL-arabinose is rearranged upon uranium interaction. The -anomer configuration is predominant in the freeL-arabinose, whereas the -anomer conformation is preferred in the uranium complexes.
Darstellung, spektroskopische und Strukturanalyse von Uran-Arabinose Komplexen
Zusammenfassung Es wurde die Reaktion zwischenL-Arabinose und hydratisierten Uranylsalzen in wäßriger Lösung untersucht und kristalline Komplexe des Typs UO2(L-Arabinose)X 2 · 2 H2O mitX=Cl–, Br– und NO 3 – isoliert und charakterisiert. Wie aus markanten Ähnlichkeiten der Komplexe mit den bekannten Verbindungen Ca(L-Arabinose)X 2 · 4 H2O und Mg(L-Arabinose)X 2 · 4 H2O (X=Cl– oder Br–) abzuleiten ist, bindet das UO 2 2+ -Ion mit zweiL-Arabinose Einheiten, wobei sich durch die O1,O5-Koordination des ersten und die O3,O4-Koordination des zweiten Moleküls eine sechs-koordinierte Geometrie um das Uranylion [ohne direkte U-X (X=Cl–, Br– oder NO 3 – ) Wechselwirkung] ausbildet. Die intermolekularen Wasserstoffbrücken zeigen nach der Wechselwirkung mit dem Uranylion eine Umgruppierung. In der freienL-Arabinose ist das -Anomere vorherrschend, in den Urankomplexen hingegen das -Anomere.相似文献
12.
The properties of alkylbenzenes were estimated, using the contributions of the σ bonds, the conjugated π bond and the steric effect in alkylbenzene molecule. And a novel bond orbital-connection matrix, conjugated π bond orbital-connection matrix in aromatic molecules, was proposed. The eigenvalues of the conjugated π bond orbital-connection matrix can well express the contribution of the conjugated π bond to the properties of aromatic molecules. Using this eigenvalue together with the parameters proposed in our early works, the bond orbital-connection matrix method was extended successfully to the QSPR studies of alkylbenzenes and a general model was obtained to evaluate the thermochemical properties of alkylbenzenes, that is,
p(alkylbenzene)=aNC–C+b∑X1CC+cNC–H+d∑X1CH+eSZ/E+k∑X1π,