ISSN:0021-2148    

DOI:10.1002/ijch.199300029

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

ISSN:0021-2148    

DOI:10.1002/ijch.199300030

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractAn interpretation of the first‐order wave function is presented in terms of correlation factors in the two‐body distribution function. An argument is given for usingN(N‐1)/2 instead ofN/2 as the number of pairs in approximate corrections to Single and Double Configuration Interaction (

ISSN:0021-2148    

DOI:10.1002/ijch.199300031

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe electronic structure of theX̃1A1andã3B2states ofo‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state ofo‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐planesp2‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns correspon

ISSN:0021-2148    

DOI:10.1002/ijch.199300032

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe applicability of modern methods of quantum computational chemistry to the problem of modelling photochemical mechanisms is discussed. It is demonstrated that one of the central mechanistic features is the surface‐crossing region which takes the form of a conical intersection. It is shown that this mechanistic feature can yield a new rationalization of common experimental observations such as the simultaneous production of different photoproducts, the occurrence of rigid stereochemistry, and the occurrence of very low fluorescence quantum yields. In addition, a simple VB model can often lead to accurate predictions about the nature of the surface‐crossing region and the nature of the recoupling pathways on the ground‐state su

ISSN:0021-2148    

DOI:10.1002/ijch.199300033

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe potential energy surface (PES) of [CH2NO]+ions with an NCO connectivity has been explored by GAUSSIAN‐1 type calculations. Six isomers having an NCO backbone are found and characterized as minima on the PES; three of them are separated by isomerization barriers reasonably large enough to allow an experimental verification. The favored protonation site of neutral isocyanic acid (HNCO) is the nitrogen atom (proton affinity, calculated = 171.4 kcal/mol; experimental = 173 ± 1 kcal/mol) leading to the most stable isomer H2NCO+(1). For the protonation of NCOH at the nitrogen atom we predict a proton affinity for the cyanic acid of 179.7 kcal/mol. The resulting ion HNCOH+(2) is 15.9 kcal/mol higher in energy than1.The third stable isomer5having the NCO connectivity can be formally viewed as a complex between CN and the water radical cation;5is 72.9 kcal/mol less stable than H2NCO+(1). In addition, two high‐energy isomers HN‐C(H)O+(3) and HO‐C(H)N+(4) have been found, but according to the shape of the PES, they are predicted not to be accessible experimentally. The calculated thermochemical data are in excelled agreement with experimentally available values. The calculations are extended to the unimolecular fragmentation channels, which should be observed in mass‐spectrometric experiments. The analysis of the results suggests that the hydrogen‐atom elimination process is favored over all other dissociation channels, followed by loss of

ISSN:0021-2148    

DOI:10.1002/ijch.199300034

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe electrocyclic reactions ofcis,cis,cis‐1,3,5‐cyclooctatriene have been studied using ab initio molecular orbital theory.cis,cis,cis‐1,3,5‐cyclooctatriene can undergo an electrocyclic ring opening in a conrotatory fashion to formcis,cis‐1,3,5,7‐octatetraene and a disrotatory electrocyclization to form bicyclo[4.2.0]octa‐2,4‐diene. The transition structures for these electrocyclic reactions have been located. Geometry optimizations employed restricted Hartree‐Fock calculations and the 3–21G and 6–31G* basis sets. Electron correlation energies were calculated using second‐order, and in some cases fourth‐order, Møller‐Plesset theory. Scaled RHF/6–31G* force constants were employed in the prediction of secondary deuterium isotope effects for the conrotatory ring opening. The ground state ofcis,cis,cis‐1,3,5‐cyclooctatriene exists in a twist‐boat conformation with staggering at the saturated linkage. The transition structure for the conrotatory electrocyclic ring opening to formcis,cis‐1,3,5,7‐octatetraene has a helical structure, which has implications for the stereoselectivities of ring closure of 1‐substituted‐cis,cis‐1,3,5,7‐octatetraenes. The disrotatory transition structure for the electrocyclization to form bicyclo[4.2.0]octa‐2,4‐diene is strongly distorted from Cssymmetry, in contrast to the transition structure for the disrotatory electrocyclization ofcis‐1,3,5‐hexatriene. This distor

ISSN:0021-2148    

DOI:10.1002/ijch.199300035

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe syn elimination of HX from CH3CH2X produces a C2H4‐HX complex, which is stabilized relative to ethylene and HX. The primary kinetic H/D isotope effects in the complex‐forming steps of such reactions have been examined ab initio for X = H, BH2, CH2, NH2, NH3+, OH, OH2+, F, Cl, and Br, using 3–21G, 3–21G(d), 6–31G(d), and MP2/6–31G(d)optimized structures and vibration frequencies. Four‐centered transition structures are found for all but X = H, BH2, and CH3; with BH2, no transition structure exists at MP2/6–31G(d); with H and CH3, the transition structures are three‐centered. In addition, although the syn mechanism has lower energy for X=H than the four‐centered Woodward‐Hoffman allowed anti mechanism, C‐H, C‐C, and H‐H bond breaking would be favored over either pathway. The semiclassical primary kinetic isotope effect increases systematically as the central atom of a neutral X is varied from left to right along a row, or down a column, of the periodic table. Concurrently, the X‐H‐C angle in the transition state increases. The increase in kH/kDas the X‐H‐C angle becomes more linear is the direction predicted by E.S. Lewis for hydride (deuteride) transfer, and by More O'Ferrall for hydron transfer. One‐dimensional corrections to these isotope effects predict significant quantum mechanical tunneling only in the cases of F and OH, for which the Bell and Eckart barriers are narrower than those obtained from intrinsic reaction coordinate (IRC) calculations. In contrast, for NH2, where tunneling seems unimportant, the Eckart barrier is wider than the IRC. A quantitative measure of barrier width is the imaginary frequency of the transition state. Tunneling occurs when this is large, when the barrier is large, and, most importantly, when th

ISSN:0021-2148    

DOI:10.1002/ijch.199300036

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractBy using CASSCF (for optimization of geometries) and MR‐SDGI‐CASSCF (for energies) methods we have studied and compared the mechanism of reaction MCH2++ H2, as well as the electronic and geometrical structure of the MCH2+complex, where M = Co, Rh, and Ir. It has been found that the mechanisms of reaction MCH2++ H2→ M++ CH4(1) for M = Co and Rh are similar and follow the path: MCH2++H2→ (H2)MCH2+→ [TS1, H2‐activation] → MCH4+→ M++ CH4. The key step is activation of the H‐H bond, which has a barrier about twice as high for M = Co as for M = Rh; reaction (1) occurs more easily for M = Rh than M = Co. M = Ir completely changes the mechanism of reaction (1), which now follows the path: IrCH2+(3A2) + H2→ (H2) IrCH2+(3A2) → [TS1, H2‐activation] → (H)2IrCH2+(1A') → [TS2, H‐migration] → HIrCH3+(3A) → [TS3, CH4‐elimination] → IrCH4+(3A2) → Ir+(5F, s1d7) + CH4. The reaction (1) is exothermic for M = Co and Rh, but endothermic for M = Ir. For M = Co and Rh, the reverse reaction M++ CH4can give only one product MCH4+and does not proceed further easily; for M = Co, at elevated temperature CoCH4+may give CoH+and CoCH3+. However, for M = Ir the reverse reaction can proceed further to give hydridomethyl HIrCH3+and bishydri

ISSN:0021-2148    

DOI:10.1002/ijch.199300037

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractEthynylvinylidene, an energetically low‐lying isomer of diacetylene, has been studied using ab initio molecular electronic structure theory. Geometries of diacetylene, ethynylvinylidene, and the transition state between them have been optimized using the double‐ζ plus polarization (DZP) and triple‐ζ plus double polarization function (TZ2P) basis sets with the self‐consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple substitutions [CCSD(T)] methods. At the TZ2P CCSD(T) level of theory only single point energies have been determined at the TZ2P CCSD geometries. Harmonic vibrational frequencies and infrared (IR) intensities are reported for the three stationary points at the DZP SCF, TZ2P SCF, and DZP CISD levels of theory. For ethynylvinylidene, rotational constants, dipole moments, and dipole moment components along the principal axes are also given. This isomer is found to be an extremely shallow minimum on the C4H2potential energy surface, and the classical barrier to rearrangement (to diacetylene) is vanishingly small at the highest level

ISSN:0021-2148    

DOI:10.1002/ijch.199300038

出版商:WILEY‐VCH Verlag    

年代:1993

数据来源: WILEY