A. MARTIN DE P. NICHOLAS and DONALD R. ARNOLD. Can. J. Chem. 60, 2165Chem. 60, (1982. Three methods for estimating the pKa of a radical cation using thermochemical cycles are discussed. These methods enable the calculations of the pK, of radical cations which have not been determined experimentally. Radical cation acids are classified according to the nature of their respective conjugate bases. The application of these methods is illustrated for a o-acid (the benzene radical cation) and n-acid (the toluene radical cation). The potential of these methods to estimate relative and absolute solvation energies is also discussed. The use of thermochemical cycles in estimating standard oxidation potentials of organic compounds is outlined.Calculations show that the toluene radical cation is an extremely strong acid and the benzene radical cation a moderately strong acid in acetonitrile solution. Difficulties in directly determining the pK, value of aromatic hydrocarbon radical cations are also discussed.A. MARTIN DE P. NICHOLAS et DONALD R. ARNOLD. Can. J. Chem. 60, 2165Chem. 60, (1982. On discute de trois methodes d'evaluation du pKa d'un cation radicalaire en utilisant les cycles thermochimiques. Ces methodes permettent de calculer le pKa des cations radicalaires que l'on ne peut pas diterminer experimentalement. Les cations radicalaires acides sont classes selon la nature de leurs bases conjuguCes respectives. On illustre I'application de ces methodes dans le cas d'un acide o (le cation radicalaire du benzene) et dans le cas d'un acide n (le cation radicalaire du toluene). On discute egalement du potentiel de ces methodes en ce qui a trait B I'Cvaluation des Cnergies de solvatation relatives et absolues. On souligne I'utilisation des cycles thermochimiques dans I'Cvaluation des potentiels standard d'oxydation des composCs organiques.Les calculs indiquent que le cation radicalaire du toluene est un acide extrkmement fort et que le cation radicalaire du benzene est un acide moyennement fort en solution dans I'acetonitrile. On discute egalement des difficultCs rencontrees lors de la determination directe de la valeur du pKa des cations radicalaires des hydrocarbures aromatiques.[Traduit par le journal]

A thermochemical cycle is used to estimate the gas phase acidity of several hydrocarbon radical cations: hydrocarbon radical cation (pKa), methane (≥ 86.5 ± 1), ethane(101 ± 1, 102 + 1), ethylene (≥ 125 ± 2), acetylene (118 ± 3, 122 ± 1), propene(124 ± 1, 122 ± 1), cyclopropane (≥ 134, ≥ 131), benzene (147 ± 1), and toluene (139 ± 1, 140). Similarly, proton affinities of the conjugate base, the hydrocarbon radical, are estimated.An estimate of the proton affinity is obtained using abinitio MO calculations. A Hartree–Fock proton affinity, PAHF(R•)g is defined as the difference in the computed energies of a radical cation and its conjugate base at the Hartree–Fock level. Calculations at the SCF level are carried out using both the minimal (STO-3G) and extended basis sets, without (3-21G, 4-31G, and 6-31G) and with (4-31G* and 6-31G*) polarization functions on carbon.The agreement between the thermochemical and abinitio estimates of the proton affinity is satisfactory.

The relationship between radical stability and bond dissociation enthalpy (BDH) is reexamined. It is shown that relative stabilization energies of radicals are not equal to relative BDH values. Net stabilization energies of radicals, SE"[R', RX] are defined relative to the R components of closed shell species RX (R(RX)). These components are chosen such that they contain the same (or, approximately the same) net charge as that of the radical (R'). The following results, relative to R = C'H,, were obtained: Re, SEn[R', RX](kJ mol-I) for X = R (i.e., the dimer RR), CH,, and H; CH,', 23, 32, 37; n-C,H7', -2, -2, -3; i-C,H7', -9, -14, -19; t-C4H9', -25, -32, -38. These results show that the methyl radical is more destabilized and the n-propyl-, i-propyl-, and tert-butyl radicals are more stabilized than is predicted from the corresponding relative BDH (R-X) values. The intrinsic C-H bond strengths of chosen alkanes are considered. Relative to the C-H bond in ethane, the bond in methane is found to be weaker by 8.12 kJ mol-' and the primary and secondary bonds in propane and the tertiary bond in methyl propane are stronger by 2.56, 7.98, and 17.12 kJ mol-' respectively.A. MARTIN DE P. NICHOLAS et DONALD R. ARNOLD. Can. J. Chem. 62, 1850(1984.On Ctudie de nouveau la relation existant entre la stabilit6 du radical et I'enthalpie de dissociation de la liaison (HDL). On montre que les Cnergies relatives de stabilisation des radicaux ne sont pas Cgales aux valeurs relatives de HDL. On a dCfini les Cnergies nettes de stabilisation des radicaux, ESOIR', RX], par rapport aux composants R des espkces des couches complktes RX (R(RX)). On a choisi ces composants de telle sorte qu'ils contiennent la mCme (ou, approximativement la m&me) charge nette que celle du radical (R'). On a obtenu les rCsultats suivants, relatifs R = C2Hs: R', ESa[R', RX](kJ mol-') pour X = R (soit le dimkre RR),

The temperature dependence of nitrogen-14 nuclear magnetic resonance (nmr) spin-lattice relaxation times (TI) for dilute aqueous solutions of the linear triatomic anions, SeCN-, SCN-, OCN-, and N3-, is investigated in the temperature range 3-80°C. These nmr relaxation times (TI) yield rotational correlation times, ' i 2~, for the reorientation of the C , symmetry axis in these anions. The observed nonlinear dependence of log T , ( I 4~) on temperature suggests that the rotations of the anions are coupled to the transport properties of water. For the thiocyanate anion the nmr correlation times are compared with those obtained previously from Raman spectroscopic data and Rayleigh scattering results. 170 T2 and ' ' N T, measurements for the NCO-anion are used along with ab initio calculations to estimate the "0 nuclear quadrupolar coupling constant in this ion.A. MARTIN DF P. NICHOLAS et RODERICK E. WASYLISHEN. Can. J . Chem. 64, 1839 (1986) OpCrant B des tempkratures allant de 3 B 80°C, on a ktudik la variation des temps de relaxation spin/rCseau (TI), en rmn du I4N, pour des solutions aqueuses dilukes des anions linkaires triatomiques, SeCN-, SCN-, OCN-et N3-. En utilisant ces temps de relaxation (TI), on a pu obtenir les temps de corrklation rotationnelle (rZR) pour la rkorientation de l'axe de symCtrie C, de ces anions. On a observk que la relation entre la tempkrature et le log TI ( 1 4~) n'est pas linkaire et ce rksultat suggere que les rotations des anions sont couplCes avec les propriktes de transport de l'eau. Dans le cas de l'anion thiocyanate, on a cornpark les temps de corrklation rmn avec ceux obtenus antkrieurement pour les donnCes de spectromktrie Raman et avec les rksultats de diffraction Rayleigh. Dans le but d'kvaluer la constante de couplage quadrupolaire nuclkaire du NCO-, on utilise des calculs ah initio de concert avec des mesures de T2(170) et de T~ ( '~N ) effectukes sur cet ion.[Traduit par la revue]

Nitrogen-14 and oxygen-17 NMR spin-lattice relaxation measurements on nitrous oxide yield a *'O nuclpar quadrupolar coupling constant of 12.4 If: 1.2 MHz. This result compares well with the value of 12.7 MHz obtained by scaling the value calculated using ab inifio molecular orbital theory at the 6-316* level.KEY WORDS Nitrous oxide, NMR TI, quadrupolar coupling constant, calculation of electric field gradients. INTRODUCTIONOxygen-17 nuclear quadrupolar coupling constants (QCCs) vary from less than 1MHz in the MOO:-anion' to a value in excess of 16MHz in Vaska's iridium compound truns-IrC1(CO)(PPh3)2.2 Although 170 nuclear quadrupolar coupling constants have been determined for a variety of diatomics and triatomics, a value for nitrous oxide (N,O) is still ~navailable.~ In this study I4N and 170 NMR spin-lattice relaxation (T,) measurements on N20 dissolved in n-hexane were used to obtain an experimental estimate of the 170 QCC in this relatively simple molecule. This result is compared with the value obtained using ub initio molecular orbital (MO) calculations at the 6-31G* level.4 ~~ EXPERIMENTALA sample of N20 (Matheson, 98%) was prepared by condensing the gas, at 77 K, into a thick-walled 10 mm NMR tube containing n-hexane (which had been previously degassed using three freeze-pump-thaw cycles). All manipulations were carried out on a vacuum gas-rack. To insure that the sample would not explode in the NMR probe the sample was first warmed in a shielded water-bath (30-35°C) for 15min. The sample tube did not burst under these conditions.All NMR measurements were performed on a Nicolet 360 NB spectrometer (B, = 8.48 T) using a broad-band variable-temperature 10 mm probe. The temperature was maintained with an accuracy of f 2 K by a nitrogen gas flow. Spin-lattice relaxation times were measured using the inversion recovery pulse sequence (IRCA) which uses compensating pulses.' The x/2 pulse widths were 50-6Ops. Eight scans at t Author to whom correspondence should be addressed. each delay in the IRCA experiment were carried out in the 14N relaxation experiments, while about 12 000 were required in the natural abundance I7O measurement. The resulting signal-to-noise ratio in the z = ~0 measurements was greater than 200: 1 for 14N and approximately 30: 1 for I7O. About 10 h of spectrometer time were required for the 170 q measurement. All & values were calculated from peak heights obtained at twelve or more different z values, using a non-linear three-parameter least-squares fitting procedure available on the Nicolet software.6 RESULTS AND DISCUSSIONBoth 14N and 170 are quadrupolar nuclei which are expected to relax entirely by the quadrupolar me~hanism.~ Under conditions of extreme narrowing the rate of relaxation of a quadrupolar nucleus of spin 1 is given by the equation7(1) where e2qQ/h is the nuclear quadrupolar coupling constant (QCC), q is an asymmetry parameter which must by symmetry be zero in a linear molecule and z2 is the rotational correlation time. Accurate values of the 14N QCC are available from gas...