Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with isobutane, isopentane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2,3,4-trimethylpentane, n-heptane, n-octane, cyclopentane, cyclohexane, and cycloheptane. The results have been used to calibrate a structure-reactivity rate constant estimation method for k(298 K) which, when combined with previously determined relationships between k(298 K) and the Arrhenius parameters, is capable of determining the temperature dependence accurately. The estimation method reproduces most of the observed rate data within experimental accuracy but appears to fail for 2,3-dimethylbutane, which has an anomalously high rate constant. Curvature in the Arrhenius plots at low temperatures is not present for compounds with a single type of C-H bond and, for compounds with different C-H bonds, is shown to be consistent with effects due to different group sites on the molecule.

Relative rate experiments were used to measure the rate constant and temperature dependence of the reaction of OH radicals with 2-fluoropropane (HFC-281ea), using ethane, propane, and ethyl chloride as reference standards. Measurements were made using both infrared spectroscopy and gas chromatography (GC). Results from the two measurement techniques were in good agreement, with the GC data being more precise. The rate at 298 K for HFC-281ea, based on the GC experiments, was found to be 5.7 × 10 -13 cm 3 /(molecule s), with Arrhenius A factor ) 3.06 × 10 -12 cm 3 /(molecule s) and E/R ) 503 K. A previously described estimation technique, along with an improved method to estimate the Arrhenius parameters, is used to predict a rate constant and temperature dependence for 2-fluoropropane, which is in good agreement with the experimental value. A prediction is also made for 1-fluoropropane, for which there are no data.

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Figure 4.-The coordination polyhedron about the Pr(II1) ion showing the monocapped square-antiprismatic configuration.is bent down toward plane B as can be seen in Figure 3. Since this complex contains constraints of chelating as well as three different kinds of atoms in the coordination sphere, it does not lend itself to detailed analysis of the coordination polyhedron. Day and Hoard2; have noted that the quasi-Cd axis [N(1)-Pr] corresponding to the Cdv symmetry of the idealized monocapped square antiprism must generate four axes [Pr-N(l), Pr-N(2), Pr-N(2)', and Pr-Cl(1)] normal to which are observed one-five-three layering of the ligated atoms. Examination of the model does show such layering.A reasonable pattern of three-dimensional hydrogen bonding (Table 111) can be proposed as O(1) + Cl(1)"' and O(5) O ( 2 ) + Cl(2) and O(4) O ( 3 ) + Cl(2) and C1(l)vr O(4) + C1(2)11 and C1(2)Ir1 O(5) + C1(2)Iv and C1(2)v This scheme thus links each complex ion to all six of its neighboring ions via hydrogen bonding. This threedimensional hydrogen bonding is manifested in the excellent crystals corresponding to the octahydrated complex. The crystal and molecular structure of tris( glycinato)chrornium( 111) monohydrate, Cr( C S H~N O~)~ HzO, has been determined by single-crystal X-ray analysis. The cell constants are a = 6.256 ( l ) , b = 14.649 (l), c = 12.267 (1) A, and fl = 100.39 (1)'.The space group is P&/c and with 2 = 4 the calculated density is 1.755 g/cin3 compared to the observed 1.76 (1) g/c1n3. Scintillation counter diffractonietry was used to measure the intensities of 2631 independent reflections significantly above background. The phase problem was solved by the application of direct methods and the structural parameters refined by a block-diagonal least-squares procedure to a final R of 0.0266. All hydrogen atoms in the structure were located and their positional parameters were refined. Anisotropic thermal parameters were used for all atoms except hydrogen. The chromium ion is octahedrally coordinated by three glycinato ligands so that the three nitrogen atoms are mutually cis.

An extensive study of ultraviolet charge-transfer spectra, visible circular dichroism (CD) and absorption spectra, titrations, and formation constants is exploited to ascertain the mode of copper(II) binding to a variety of potentially tridentate ligands. In the series of , -L-diaminocarboxylate anions, lysine and ornithine bind to copper(II) as bidentate substituted glycines. 2,4-Diaminobutyrate complexes in this mode and also as a tridentate ligand with either one nitrogen and one oxygen or two nitrogen donors in the chelate plane. 2,4-Diaminopropionate binds as a tridentate ligand with an apical carboxylate group. The corresponding zero net charged «.tv-diaminocarboxylic acids all bind as substituted glycines with unbound -ammonium groups. Similarly, the neutral histidine molecule binds to copper(II) through the glycine locus with an unbound protonated imidazolium group. Though dependent as in some of the above cases on the other ligands bound, the first histidinate anion serves mainly as a tridentate ligand with primary bidentate nitrogen donors in the chelate plane and a weaker apical coordination of the carboxylate group. The second histidinate anion binds as a bidentate ligand. The CD of copper(II) chelates of histidine containing dipeptides is nearly an additive function of independent contributions from amino and carboxyl terminal amino acid residues. Carnosine exhibits the largest CD magnitudes among the dipeptide complexes. Titration and CD evidence are presented for the occurrence in solution of the unique dimer structure found for carnosine in the solid.Potentially tridentate ligands such as the amino acid histidine may serve as bidentate chelates by three different combinations of pairs of donor atoms and as tridentate chelates. Thus uncertainties arise in determining bonding modes. The uncertainties are compounded when the metal ion is copper(II). Coordina-

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Relative rate experiments were used to measure ratios of chemical kinetics rate constants as a function of temperature for the reactions of OH with eight fluoroethers, including CF3OCF2CHF2, CF3OCF2CHFCF3, CHF2CF2OCHF2, CF3CHFCF2OCH2CF3, (CF3)2CHOCHF2, CF2HCF2OCH2CF3, CHF2CF2OCHFCF3, and CF3CH2OCH2CF3. The temperature ranges were about 270-400 K. Each compound was measured against at least two references. Results are compared with previous data where available. An approach using model compounds for the approximate estimation of rate constants for the fluoroethers is discussed. Observed temperature dependences for fluoroethers from the present work and some literature work are shown to be accurately predictable, based on a previously determined correlation of k298K with the pre-exponential factor, A, in the Arrhenius equation k = Ae(-E/RT).

The rates of reaction, Arrhenius preexponential factors, and activation energies of CH 2 FCH 2 F (HFC-152) and CH 3 CHF 2 (HFC-152a) for the abstraction reaction with hydroxyl radical were measured by a relative rate method. Temperature ranges were 287-409 and 286-403 K, respectively. Reference compounds for both reactants were ethane (k ) 1.00 × 10 -11 e -1094/T cm 3 molec. -1 s -1 ) and cyclopropane (k ) 7.28 × 10 -12 e -1356/T cm 3 molec. -1 s -1 ). Also, the JPL 97-4 recommendation for HFC-152a (k ) 2.4 × 10 -12 e -1260/T cm 3 molec. -1 s -1 ) was used as a reference for HFC-152 vs 152a experiments. For HFC-152, the combined results yielded an Arrhenius expression of k ) 3.55 × 10 -12 e -1084/T cm 3 molec -1 s -1 . The result for HFC-152a was k ) 1.86 × 10 -12 e -1183/T cm 3 molec -1 s -1 . For both reactants, uncertainties in k(298 K) and E/R are believed to be less than 10% and 100 K, respectively. The experimental results are compared with predicted values based on previously observed group effects as well as the dependence of preexponential factors on the magnitude of the rate constant. The results are also compared to previously published values for these same compounds obtained by absolute methods.