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1.
(2+2)- and (4+2)-cycloadducts are formed by thermal reaction of xanthenethione with allenes. Activation parameters for ths “concerted two step” (π2s+π2s+π2s) reaction are presented.  相似文献   

2.
[2+2] Cycloaddition reactions of P2 with alkenes were predicted to have concerted paths, that is, pseudoexcitation, distorted 2πs+2πs, and 2πs+2πa processes without any interventions of intermediates. The pseudoexcitation and/or distorted 2πs+2πs paths with retention of configuration of alkenes are kinetically preferred to the 2πs+2πa path with inversion of configuration. The reactions were predicted from the appreciable difference in the calculated enthalpies of activation to be stereospecific.  相似文献   

3.
Irradiation (λ>400 nm) of solid pleiadiene ( 1 ) yields a single, head-to-head [π4s+π4s]-photodimer ( 2 ) the structure of which was determined by X-ray analysis. The formation of 2 is entirely suppressed at 77 K, since properly oriented pairs of molecules arise only from thermal disorder in crystals of 1 . Upon pyrolysis (80°), the strained photodimer 2 rearranges to the [π2s + π4s] dimer 3 by a ‘forbidden’ suprafacial [1,3]-C-atom migration. Both 2 and 3 are reconverted to 1 by UV. irradiation in solution, but the latter, ‘forbidden’ photoreaction is suppressed at 77 K . Discrepancies of the experimental observations with the predictive schemes of Kaupp or Michl are discussed.  相似文献   

4.
Near UV. irradiation of N-methyl isoindole ( 1 ) in deaerated solution has yielded two constitutionally isomeric [π4s+π4s] dimers 3 and 4 (Scheme 2). No transient or stable photoisomers of 1 were detected. The photodimers were reconverted to 1 both by pyrolysis and photolysis. The photocleavage of dimer 3 proceeds (predominantly) by anonadiabatic pathway yielding 1 in its electronic ground state. Prolonged pyrolysis of 1 afforded 11H-indeno [1,2-c]-isoquinoline ( 5 ) as a major product.  相似文献   

5.
The chemoselectivities in the (π4s + π2s) cycloaddition reactions of tetraphenylcyclopentadienone (TPCD) with ethyl- and 1,1-dimethylallene have been determined and are compared with those observed for reaction with maleic anhydride.  相似文献   

6.
All electron ab initio calculations have been applied to elucidate the electronic states and the nature of the chemical bonds in the molecules NiC, NiSi, and NiGe. The calculations have revealed that the ground states of all three molecules are1Σ+, but due to the open 3d shell of the Ni atom the molecules have many low-lying electronic states. The NiC molecule is strongly polar, and the low-lying electronic states have been identified as those arising when the angular momenta of the3Fg Ni+ ion are coupled to the angular momenta of the4SuC? anion. The chemical bond in the NiC molecule has triple bond character due to the valence bond couplings between the Ni 4s and 3 electrons and theC 2p electrons. The chemical bonds in the molecules NiSi and NiGe are very much alike; they are double bonds composed of oneσ and oneπ bond. Theσ bond is due to the doubly occupied delocalized molecular orbital composed of the Ni 4s orbital and the Si 3 or the Ge 4 orbital. Theπ bond originates from the valence bond coupling between the localized hole in the Ni 3 orbital and the valence electron of Si or Ge.  相似文献   

7.
Photoelectron spectroscopy is used to demonstrate the mechanistic consequences of the level ordering in a given molecule on its reactivity, using the recently synthesized hypostrophene, which contains two CC double bonds in a rigid, cisoid conformation, as an example. The inability of this molecule to close photochemically to the saturated analog is traced to the presence of an exceptionally high-lying σ level which is ideally oriented for an effective through-bond coupling of the two π orbitals. Contrary to the norbornadiene case, this through-bond coupling overrides the direct through-space interaction, placing the in-phase combination of the two π orbitals above the out-of-phase combination, and thus converts the π2s+π2s photocycloaddition from a symmetry-allowed to a symmetry-forbidden reaction.  相似文献   

8.
The inner-shell excitation and decay of the CO molecule have been studied in electron impact experiments. The dipole-forbidden transition (1sσc)−1(2pπ) 3Π has been characterized by angular resolved electron energy loss spectroscopy and its decay via the measurement of resonant Auger spectra. The contribution of the (1sσc)−1(2pπ) 3Π state to the CO resonant Auger spectrum in the region of the “spectator transitions” has been isolated and the population of CO+ quartet final states has been observed.  相似文献   

9.
A new synthesis of 3-oxatricyclo[7.2.2.01,7]tridecenones via intramolecular π4s2s cycloaddition in cyclohexa-2,4-dienones is described. Their photochemical reactions leading to oxepane-diquinane and oxepane-sterpurane hybrids are depicted. The crystal structure of a key intermediate is also presented.  相似文献   

10.
The cation···π interactions of alkali metal cations (Li+, Na+, and K+) with five-membered heteroaromatic rings [furan(C4H4O), thiophene(C4H4S), pyrrole(C4H5N)] were examined by high level ab initio calculations, to investigate the different roles of C4H4O, C4H4S, and C4H5N as the electron donor, the influential factors that affect these interactions, the nature of this kind of cation···π interaction, and to determine topological and energetical properties to characterize these interactions. The sulfur atom in C4H4S plays a certain role in the cation···π interactions except the C–C π bond, which is different from C4H4O and C4H5N. The size of cation and the character of heteroaromatic ring are two influential factors that affect the cation···π interactions. The studied cation···π interactions can be classified as “closed-shell” and noncovalent interactions. The electron density and its Laplacian at the bond critical points and ring critical points generated upon complexation are useful measurements for the strength of cation···π interactions.  相似文献   

11.
C2(a 3πu) disappearance rate constants of 1.44, 0.96, 0.0296, 0.0130 and < 10?6(x10?10cm3s?1) are reported for reactions with C2H4, C2H2, O2, C2H6, and CH4, respectively at 298 K. C2(a 3πu) fragments are generated by multiphoton ArF excimer laser photodissociation at C2H2, and monitored by dye laser induced fluorescence. Arguments are presented which favor chemical reactions over the C2(a 3πu) → (X 1σ+g) quenching channel. C2 + C2H2 represents the one possible exception to the reactive channel.  相似文献   

12.
Under Ammonia chemical Ionization conditions the source decompositions of [M + NH4]+ ions formed from epimeric tertiary steroid alchols 14 OHβ, 17OHα or 17 OHβ substituted at position 17 have been studied. They give rise to formation of [M + NH4? H2O]+ dentoed as [MHsH]+, [MsH? H2O]+, [MsH? NH3]+ and [MsH? NH3? H2O]+ ions. Stereochemical effects are observed in the ratios [MsH? H2O]+/[MsH? NH3]+. These effects are significant among metastable ions. In particular, only the [MsH]+ ions produced from trans-diol isomers lose a water molecule. The favoured loss of water can be accounted for by an SN2 mechanism in which the insertion of NH3 gives [MsH]+ with Walden inversion occurring during the ion-molecule reaction between [M + NH4]+ + NH3. The SN1 and SNi pathways have been rejected.  相似文献   

13.
High-resolution absorption spectra of the following diphenylmethylenes (DPMs) dispersed in benzophenone crystals at liquid-helium temperatures are presented: DPM-h10, DPM-d10, 4-chloro-DPM, and 4-bromo-DPM. The substituent effects concerning the electronic structure, transition energy and intensity are discussed. From polarization measurements, the electronic configurations of the ground and the first excited triplet states of these DPMs are assigned as (pπ)1(pσ)1 and (pσ)1(π*)1, respectively. Further studies reveal a second excited triplet state, designated as (pπ)1(π*)1, which lies less than 1000 cm-1 above the first excited triplet state of DPM. Diffuse broad bands appear as common features in all the spectra. Such diffuseness is discussed in terms of electron-phonon coupling of the low-lying excited states.  相似文献   

14.
E. Dunkelblum 《Tetrahedron》1976,32(8):975-978
The stereochemistry of the cycloaddition of dichloroketene (1) and chlorosulfonyl isocyanate (2) to 4-t-butylmethylenecyclohexane (4) has been investigated as a model for the 2πs + 2πa cycloaddition to the methylenecyclohexane system. The reactions are kinetically controlled and proceed mainly by axial attack to yield the thermodynamically less stable isomers 7 and 10, as major products, respectively.  相似文献   

15.
Calculations using the MRD CI method are reported for the ground and low lying excited states of C3. Transitions from the 3σu, 4σg and 1πu MO's into 1πg are considered, as well as the 1πu → 3s Rydberg species and the corresponding ionization, and good agreement with experimental data is obtained where comparison is possible. Potential curves calculated for the ground and (1πu → 1πg) 1Σ+u excited state are discussed.  相似文献   

16.
The vertical valence ionization potentials of trans-N2F2 and cis-N2F2 have been computed by a many-body Green function method. For trans-N2F2 the agreement with experiment is very satisfactory in general and the calculations permit an analysis and assignment of the experimental photoelectron spectrum. The ionization potentials of cis-N2F2 are predictions. The ordering of the ionization potential is for trans-N2F2 5ag(n+), 2au(π), 4bu(n?), 4ag, 1bg(π), 1au(π), 3bu, 3ag, 2bu and for cis-N2F2 4b2 (n?), 2b1 (π) + 5a1(n+), 3b2, 1a2 (π), 1b1(π), 4a1, 3a1, 2b2, n+ and n? denote lone pairs on the N atoms except for the 4bu(n?) orbital which has the largest contribution from the F atoms.  相似文献   

17.
Valence-bond calculations are reported for the isoelectronic series of molecules and ions: N2, CO, BF, NO+ and CN?. The most important structures are N?N, C?O, Bπ? F, N+?O and C?N. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ-orbital or gains a π-orbital are favored. π-bonds tend to be favored over σ-bonds. The bond in NO+ resembles that in CO, whereas that in CN? resembles the bonding in N2.  相似文献   

18.
The complexes [Pt(tBu3tpy){C?C(C6H4C?C)n?1R}]+ (n=1: R=alkyl and aryl (Ar); n=1–3: R=phenyl (Ph) or Ph‐N(CH3)2‐4; n=1 and 2, R=Ph‐NH2‐4; tBu3tpy=4,4’,4’’‐tri‐tert‐butyl‐2,2’:6’,2’’‐terpyridine) and [Pt(Cl3tpy)(C?CR)]+ (R=tert‐butyl (tBu), Ph, 9,9’‐dibutylfluorene, 9,9’‐dibutyl‐7‐dimethyl‐amine‐fluorene; Cl3tpy=4,4’,4’’‐trichloro‐2,2’:6’,2’’‐terpyridine) were prepared. The effects of substituent(s) on the terpyridine (tpy) and acetylide ligands and chain length of arylacetylide ligands on the absorption and emission spectra were examined. Resonance Raman (RR) spectra of [Pt(tBu3tpy)(C?CR)]+ (R=n‐butyl, Ph, and C6H4‐OCH3‐4) obtained in acetonitrile at 298 K reveal that the structural distortion of the C?C bond in the electronic excited state obtained by 502.9 nm excitation is substantially larger than that obtained by 416 nm excitation. Density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations on [Pt(H3tpy)(C?CR)]+ (R= n‐propyl (nPr), 2‐pyridyl (Py)), [Pt(H3tpy){C?C(C6H4C?C)n?1Ph}]+ (n=1–3), and [Pt(H3tpy){C?C(C6H4C?C)n?1C6H4‐N(CH3)2‐4}]+/+H+ (n=1–3; H3tpy=nonsubstituted terpyridine) at two different conformations were performed, namely, with the phenyl rings of the arylacetylide ligands coplanar (“cop”) with and perpendicular (“per”) to the H3tpy ligand. Combining the experimental data and calculated results, the two lowest energy absorption peak maxima, λ1 and λ2, of [Pt(Y3tpy)(C?CR)]+ (Y=tBu or Cl, R=aryl) are attributed to 1[π(C?CR)→π*(Y3tpy)] in the “cop” conformation and mixed 1[dπ(Pt)→π*(Y3tpy)]/1[π(C?CR)→π*(Y3tpy)] transitions in the “per” conformation. The lowest energy absorption peak λ1 for [Pt(tBu3tpy){C?C(C6H4C?C)n?1C6H4‐H‐4}]+ (n=1–3) shows a redshift with increasing chain length. However, for [Pt(tBu3tpy){C?C(C6H4C?C)n?1C6H4‐N(CH3)2‐4}]+ (n=1–3), λ1 shows a blueshift with increasing chain length n, but shows a redshift after the addition of acid. The emissions of [Pt(Y3tpy)(C?CR)]+ (Y=tBu or Cl) at 524–642 nm measured in dichloromethane at 298 K are assigned to the 3[π(C?CAr)→π*(Y3tpy)] excited states and mixed 3[dπ(Pt)→π*(Y3tpy)]/3[π(C?C)→π*(Y3tpy)] excited states for R=aryl and alkyl groups, respectively. [Pt(tBu3tpy){C?C(C6H4C?C)n?1C6H4‐N(CH3)2‐4}]+ (n=1 and 2) are nonemissive, and this is attributed to the small energy gap between the singlet ground state (S0) and the lowest triplet excited state (T1).  相似文献   

19.
The ammonia chemical ionization (CI/[NH4+]) mass spectra of a series of diastereomeric methyl and benzyl ethers derived from 3-hydroxy steroids (unsaturated in position 5 and saturated) have been studied. The adduct ions [M+NH4]+ and [MH]+ and the substitution product ions [M+NH4? ROH]+ (thereafter called [MsH]+) are characterized by an inversion in their relative stabilites in relation to their initial configuration. [M+NH4]α+ and [MH]α+ formed from the α-Δ5-steroid isomers are stabilized by the presence of a hydrogen bond which is not possible for the β-isomers. This stereochemical effect has also been observed in the mass analysed ion kinetic energy (MIKE) spectra of [M+NH4]+ and [MH]+. The MIKE spectra of [MsH]+ indicate that those issued from the β-isomers are more stable than the one originating from the α-isomers. This behavior is also observed in the first field free region (HV scan spectra) for [MH]+, [MsH]+ and [M+NH4]+ which are precursors of the ethylenic carbocations (base peak in the conventional CI/[NH4]+ spectra). Mechanisms, such as SN1 and SNi, have been ruled out for the formation of [MsH]+, but instead the data support an SN2 mechanism during the ion-molecule reaction between [M+NH4]+ and NH3.  相似文献   

20.
The kinetic and the exchange energy functionals are expressed in the form T[ρ] = CTF∫ drρ5/3(r)ft(s) and K[ρ] = CD∫ drρ4/3(r)fK(s), where CTF = (3/10)(3π2)2/3 and CD = −(3/4)(3/π)4/3 are the Thomas-Fermi and the Dirac coefficients, respectively, and s = |∇ρ(r)|/Csρ4/3(r), with Cs = 2(3π2)1/3. These expressions are used to perform a comparison of fT(s) and fK(s) in terms of their generalized gradient expansion approximations. It is shown that fκ(s) and is congruent to fT(s) in the range characteristic of the interior regions of atoms and many solids and that the second-order gradient expansion of the kinetic energy provides a rather reasonable approximation to the generalized gradient expansion approximation of both the kinetic and the exchange energy functionals. © 1996 John Wiley & Sons, Inc.  相似文献   

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