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Composition of Tautomeric Mixtures Studied by Nitrogen Chemical Shifts and Ab Initio Molecular Orbital Calculations

Wiench J. W., Koprowski M., Stefaniak L., Webb G. A.

Polish Journal of Chemistry

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2002

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Vol. 76 / nr 4

525-536

EN

Some nitrogen and carbon chemical shifts and ab initio Gauge Invariant Atomic Orbitals - Coupled-Perturbed Hartree-Fock (GIAO-CPHF) calculations are reported for benzotriazole 1, 5-nitrobenzimidazole 2, 5-nitrobenzotriazole 3, 4-nitrobenzotriazole 4, and N-methyl derivatives of compounds 1-3. A good correlation is found between the calculated and observed 13C and 15N chemical shifts. Two methods were employed for calculation of equilibrium constants. The prototropic equilibria exhibited by the system studied are found to be controlled by enthalpy rather than entropy.

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Electron correlation effects on geometries and 19F shieldings of fluorobenzenes

Webb G. A., Karadakov P. B., England J. A.

Bulletin of the Polish Academy of Sciences. Chemistry

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2000

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Vol. 48, No. 1

101-112

EN

In order to include the effects of electron correlation in ab initio molecu- lar orbital calculations it is necessary to go beyond the single determinant Hartree-Fock (HF) level of theory. In the present investigation the influences of both dynamic and nondynamic correlation effects on the optimised geometries and F nuclear shielding calculations of the twelve fluorobenzenes are reported. The non-dynamic electron correlation effects are represented by complete-active space self-consistent field (CASSCF) calculations. Second- and fourth-order Moller-Plesset (MP2 and MP4) calculations are used to describe the dynamic electron correlation effects. Some density-functional (DFT) results are also reported which do not distinguish between dynamic and non-dynamic electron correlation. Following the correlated geometry optimisations F nuclear shielding calculations were performed using the gauge-included atomic orbitals (GIAO) procedure, these were undertaken with wavefunctions which include various levels of electron correlation including HF, CASSCF and MP2. For the calculations of the optimised geometries, and some of the nuclear shieldings the 6-31G** basis set is used whereas the locally-dense [6-31G** on C and H and 6-311++G(2d,2p) on F] set is used for some of the shielding calculations. A comparison of the results of HF shielding calculations using other basis sets is included. Comparison of the calculated geometry and shielding results with relevant, reported, experimental data is made

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N NMR study of three proton sponges and some of their salts

Klimkiewicz J., Stefaniak L., Grech E., Brzeziński B., Webb G. A.

Bulletin of the Polish Academy of Sciences. Chemistry

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1998

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Vol. 46, No. 1

75-78

EN

15N NMR data are reported for the three proton sponges DMAMN, DMAMB and DEAMB together with some of their salts with HNO3 and HC104. Low temperature 15N measurements on the monoperchlorate salts reveal the presence of a (N-H-N)+ structure from the values observed for 1J(15N-1H). The diperchlorate salts show monoprotonation of each nitrogen atom.