Valentin S. Dimitrov

Ab initio study of the barrier to internal rotation in simple amides: 1. N,N-dimethylformamide and N,N-dimethylcarbamic halogenides

Abstract Free energies of activation for rotation about the amide C–N bond in X–C(O)N(CH 3 ) 2 (X=H, F, Cl and Br) were calculated at the MP2(fc)/6-31+G*//6-31G* and MP2(fc)/6-311++G**//6-311++G** levels and compared with NMR gas-phase data. The results of calculations indicate that the repulsion between X and methyl group in ground state and the repulsion between X or oxygen and nitrogen lone pair in transition states (TS) are largely responsible for the difference in the free energies of the studied amides. For X=H (DMF), the anti TS is more stable; for the cases X=Cl, Br, the syn TS is more stable, while for the case X=F the two transition states are energetically almost equivalent.

Dynamic NMR: comparison of the experimental barriers to internal rotation in N,N-dimethylformamide with those calculated by the ab initio SCF MO method

Abstract The GAUSSIAN 70 package of ab initio computer programs has been used to determine the total energies, force constants and dipole moments of the ground and transition states of DMF following complete geometry optimisation. The 4–31G split-valence basis set was employed. There is good agreement between the resulting geometry and that determined by electron diffraction. The barriers to internal rotation about the CN amide and NMe bonds so derived are compared with those determined experimentally by NMR methods.

A double-fitting procedure for two-site exchange analysis

Abstract The total lineshape (TLS) analysis of a two-site system undergoing chemical exchange is subject to difficulties associated with the temperature dependence of the lineshape parameters. A logarithmic approach is developed in order to deal with this problem and its use is demonstrated by application to the case of N,N -dimethyltrichloroacetamide. The extraction of the temperature dependence of T 2 at each site from the TLS results provides information on the molecular reorientation which could otherwise have been overlooked.

Dynamic NMR: Integral parameter sensitivities and their use in the total line shape analysis

Abstract In applying the total line shape analysis of dynamic NMR problems the choice of parameters to iterate upon is usually subjective. The inclusion of insensitive parameters in the iteration process lengthens the procedure and quite often leads to erroneous results. The Integral Parameter Sensitivity (IPS) described here is a measure of the sensitivity of the line shape to any particular parameter at any given lifetime τ and it may be used to identify automatically which parameters need to be iterated upon. The utility of this ITS approach is demonstrated for the case of N,N-dimethyltrichloroacetamide.

Nuclear magnetic resonance spectra of tropic acid and some derivatives

Abstract The NMR spectra of tropic acid, its methyl ester and acetyltropic acid methyl ester were measured at 100 and 220 MHz in various solvents. The spectra were analysed by means of an iterative computer procedure. The results indicate, for all the compounds studied, a predominance of the conformation where the phenyl and hydroxyl (or acetoxyl) groups are in anti.positions to each other. The solvent and concentration effects upon the vicinal coupling constants and hence upon the position of the conformational equilibria are rather weak.

Ab initio study of the barrier to internal rotation in simple amides. Part 2. N,N-dimethylacetamide and N,N-dimethyl-2-(mono-, di- and tri-) halogenacetamides

Abstract The free energies of activation for rotation about the amide C–N bond in R–C(O)N(CH 3 ) 2 (R=CH 3 , CH 2 F, CHF 2 , CF 3 and CCl 3 ) were calculated at the MP2(fc)/6-31+G ∗ //6-31G ∗ level and compared with the NMR gas-phase data. The results of calculations indicate that the repulsion between the substituent R and methyl group in ground state and the conformation of R are largely responsible for the difference in the free energies of the studied amides. The anti transition state is the preferred one in all the studied compounds.

Combined use of complete lineshape analysis of 1D spectra subjected to reference deconvolution and linear prediction, 2D-EXSY spectra and a double fitting method for the study of chemical exchange. Application to an eight-site exchange system

Obtaining unknown NMR parameters from experimental spectra of exchanging systems is, from a mathematical point of view, associated with the solution of an inverse problem. Inverse problems are usually solved by iterative procedures and calculations may not always converge and the solutions are rarely unique. To attain reliable rate constants we adopted the following experimental–computational scheme: acquirement of perfect NMR lineshapes for complete lineshape analysis (CLSA) by elimination of undesirable effects due to magnetic field inhomogeneity through reference deconvolution and linear prediction; the combined use of 2D exchange spectroscopy (2D‐EXSY) and 1D CLSA for the determination of the rotation rate constants; and application of the double fit method, which is based on the knowledge that the exchange parameters depend on thermally activated processes.

Proton magnetic resonance spectra and conformational properties of spiro[2.5]octan-6-ol

Abstract The free energy difference (-Δ G 0 ) for the axial-equatorial conformational equilibrium of spiro[2.5]octan-6-ol in CS 2 has been found with the aid of PMR spectra taken at low temperatures, to be 0.79 kcal mole −1 at −75° C. An Arrhenius activation energy of ring inversion of 14.4 ± 1.3 kcal mole −1 has been obtained by the variable-temperature NMR total line-shape method.

Ab initio SCF study of the barrier to internal rotation in simple amides. Part 3. Thioamides

Abstract The free energies of activation for rotation about the thiocarbonyl C–N bond in X–C(S)N(CH 3 ) 2 (X=H, F, Cl, CH 3 , CF 3 ) were calculated at the MP2(fc)/6-31+G*//6-31G* and MP2(fc)/6-311++G**//6-311++G** levels and compared with literature NMR gas-phase data. The results of calculations indicate that the nonbonded interactions in ground state (GS) are mainly responsible for the differences in the rotational barriers. For X=H, CH 3 and CF 3 , the anti transition state (TS) is more stable; for the case X=Cl, the syn TS is more stable, while for the X=F, the two TS are energetically almost equivalent.