Charles H. DePuy

Bio: Charles H. DePuy is an academic researcher from University of Colorado Boulder. The author has contributed to research in topics: Nucleophile & Alkyl. The author has an hindex of 17, co-authored 42 publications receiving 1690 citations. Previous affiliations of Charles H. DePuy include University of Minnesota.

Bond Strengths of Ethylene and Acetylene

Abstract: Negative ion photoelectron spectroscopy and gas-phase proton transfer kinetics were employed to determine the CH bond dissociation energies of acetylene, ethylene, and vinyl radical: Do(HCC-H) = 131.3 f 0.7 kcal mol-', Do(CH2CH-H) = 109.7 f 0.8 kcal mol-', and Do(CH2C-H) = 81.0 f 3.5 kcal mol-'. The strengths of each of the other CH and CC bonds in acetylene and ethylene and their fragments were derived. The energy required to isomerize acetylene to vinylidene was also determined: HCWH - H2C=C: AHbo = 47.4 f 4.0 kcal mol-'. As part of this study, proton transfer kinetics in a flowing afterglow/selected-ion flow tube apparatus were used to refine the acidities of ethylene, acetylene, and vinyl. The gas-phase acidity of acetylene was tied to the precisely known values for hydrogen fluoride, AGsdd298(HF) = 365.6 f 0.2 kcal mol-', and water, AG-(H20) = 383.9 f 0.3 kcal mor', yielding AG-(HCC-H) = 369.8 f 0.6 kcal mol-'. The gas-phase acidity equilibria of acetylene with isopropyl alcohol and terr-butyl alcohol were also measured. Combined with relative acidities from the literature, these measurements yielded improved acidities for the alcohols, AGad,298( (CH3)2CHO-H) = 370.1 f 0.6 kcal mol-', AGaa298((CH3)3CO-H) = 369.3 f 0.6 kcal mol-', hGadd,298(C2HSO-H) = 372.0 f 0.6 kcal mol-', and AGacid.298(CH30-H) = 375.1 f 0.6 kcal mol-'. The gas-phase acidity of ethylene was measured relative to ammonia, AGacid.298(NH3) = 396.5 f 0.4 kcal mol-', giving AGadJ98(C2H1) = 401.0 f 0.5 kcal mol-'. The gas-phase acidity of vinyl radical was bracketed, 375.1 f 0.6 kcal mol-' I AGaaB8(CH2C-H) I 380.4 f 0.3 kcal mol-'. The electron affinities of ethynyl, vinyl, and vinylidene radicals were determined by photoelectron spectroscopy: EA(HCC) = 2.969 f 0.010 eV, EA(CH2CH) = 0.667 f 0.024 eV, and EA(CH2C) = 0.490 f 0.006 eV.

Gas-phase SN2 and E2 reactions of alkyl halides

Abstract: Rate coefficients have been measured for the gas-phase reactions of methyl, ethyl, n-propyl, isopropyl, tert-butyl, and neopentyl chlorides and bromides with the following set of nucleophiles, listed in order of decreasing basicity: HO − , CH 3 O − , F − , HO − (H 2 O), CF 3 CH 2 O − , H 2 NS − , C 2 F 5 CH 2 O − , HS − , and Cl − . For methyl chloride the reaction efficiency first falls significantly below unity with HO − (H 2 O) as the nucleophile and for methyl bromide with HS − as the nucleophile; in both cases the overall reaction exothermicity is about 30 kcal mol −1 . Earlier conclusions that these halides react slowly with stronger bases are shown to be in error. In the region where the rates are slow oxygen anions react with the alkyl chlorides and bromides by elimination while sulfur anions of the same basicity react by substitution. This difference is due to a slowing down of elimination with the sulfur bases; sulfur anions show no increased nucleophilicity as compared to oxy anions of the same basicity

The gas-phase acidities of the alkanes

Abstract: The gas-phase acidities of 15 simple alkanes have been determined in a flowing afterglow-selected ion flow tube (FA-SIFT) by a kinetic method in which alkyltrimethylsilanes were allowed to react with hydroxide ions to produce a mixture of trimethylsiloxide ions by loss of alkane and alkyldimethylsiloxide ions by loss of methane. The reaction is proposed to proceed by addition of hydroxide ion to the silane to form a pentacoordinate siliconate ion intermediate which decomposes through two transition states, one in which negative charge is placed on a methyl group and the other in which negative charge is placed on the alkyl group. The ratio of siloxide ions produced is proposed to be correlated to the relative basicity of the methyl and alkyl anions. The method is calibrated by making use of the known acidities of methane (/Delta/H/sub acid/) = 416.6 kcal/mol and benzene (/Delta/H/degree//sub acid/ = 400.7 kcal/mol). In general, methyl substitution is found to stabilize alkyl anions in the gas phase except that the ethyl anion is found to be more basic than the methyl anion. By combining the gas-phase acidities with the bond dissociation energies, the electron affinities (EA) of the corresponding alkyl radicals can be calculated. Many simplemore » alkyl radicals are found to have negative EA's. The results for the alkyl groups studied are given.« less

Experimental Studies of Allene, Methylacetylene, and the Propargyl Radical: Bond Dissociation Energies, Gas-Phase Acidities, and Ion-Molecule Chemistry

Abstract: Electron affinities and A&ld are combined in a thermochemical cycle to arrive at bond dissociation energies for allene, methylacetylene, and the propargyl radical: Do(CH2eC=CH-H) = 88.7 f 3 kcal mol-', Do(HCH2CECH) = 90.3 f 3 kcal mol-', Do(CH3CrC-H) = 130.2 f 3 kcal mol-', and Do(CHz=C=C-H) = 100 f 5 kcal mol-'. Electron affinity measurements were determined using negative ion photoelectron spectroscopy and yielded the following for the propargyl, I-propynyl, and propadienylidene radicals: EA(CH2=C=CH) = 0.918 f 0.008 eV, EA(CH3Cd) = 2.718 f 0.008 eV, and EA(CHz=C=C) = 1.794 f 0.008 eV. Gas-phase acidity measurements were made using proton transfer kinetics in a flowing afterglow/selected-ion flow tube and yielded the following for allene, methylacetylene, and the propargyl radical: AGacld(CH2=C=CH-H) = 372.8 f 3 kcal mol-', AGacld(H-CH2C'CH) = 374.7 f 3 kcal mol-', AGaCld(CH3C~C-H) = 373.4 f 2 kcal mol-', and AGacld(CH2=C=CH) = 364 f 5 kcal mol-'. AGac,d was converted to hHac,d by employing ASac,d: AHac,d(CH2%%H-H) = 381.1 f 3 kcal mol-', AZ-Zyld(H-CHzC*H) = 382.7 f 3 kcal mol-', AHacld(CH3CSH) = 381.1 f 3 kcal mol-', and &i!,,,d(CH2=C=CH) = 372 f 5 kcal mol-'. Evidence is provided for the isomerization of the allenyl anion (CH2%4H-) to the 1-propynyl anion ( C H F S ) in the proton transfer reactions of CH2=C=CHwith CH30H and CH3CH20H. This complexity limits the precision of experimental measurements. This study explores the intricacies of determining gas phase acidity values by proton transfer reactions for systems in which isomerization can occur.

Relative gas-phase acidities of the alkanes

Abstract: : In the gas phase alkyl trimethyl silanes are cleaved by hydroxide ions to siloxides with loss of methane or alkane. The alkane to methane loss ratio in this reaction is proposed as a measure of the relative acidity of RH and CH4. A linear correlation is established for loss of substituents of known acidity (CH4, H2, C2H4, C6H6). This relationship allows the acidity of several alkanes to be predicted, as well as the electron affinity of the corresponding alkyl radicals. Ethane and the secondary position in propane are predicted to be less acidic than methane, and the ethyl, isopropyl and t-butyl radicals are predicted to have negative electron affinities.

Chemistry with ADF

Abstract: We present the theoretical and technical foundations of the Amsterdam Density Functional (ADF) program with a survey of the characteristics of the code (numerical integration, density fitting for the Coulomb potential, and STO basis functions). Recent developments enhance the efficiency of ADF (e.g., parallelization, near order-N scaling, QM/MM) and its functionality (e.g., NMR chemical shifts, COSMO solvent effects, ZORA relativistic method, excitation energies, frequency-dependent (hyper)polarizabilities, atomic VDD charges). In the Applications section we discuss the physical model of the electronic structure and the chemical bond, i.e., the Kohn–Sham molecular orbital (MO) theory, and illustrate the power of the Kohn–Sham MO model in conjunction with the ADF-typical fragment approach to quantitatively understand and predict chemical phenomena. We review the “Activation-strain TS interaction” (ATS) model of chemical reactivity as a conceptual framework for understanding how activation barriers of various types of (competing) reaction mechanisms arise and how they may be controlled, for example, in organic chemistry or homogeneous catalysis. Finally, we include a brief discussion of exemplary applications in the field of biochemistry (structure and bonding of DNA) and of time-dependent density functional theory (TDDFT) to indicate how this development further reinforces the ADF tools for the analysis of chemical phenomena. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 931–967, 2001

Gaussian-2 theory for molecular energies of first- and second-row compounds

Abstract: The Gaussian‐2 theoretical procedure (G2 theory), based on a b i n i t i o molecular orbital theory, for calculation of molecular energies (atomization energies, ionization potentials,electron affinities, and proton affinities) of compounds containing first‐ (Li–F) and second‐row atoms (Na–Cl) is presented. This new theoretical procedure adds three features to G1 theory [J. Chem. Phys. 9 0, 5622 (1989)] including a correction for nonadditivity of diffuse‐s p and 2d f basis set extensions, a basis set extension containing a third d function on nonhydrogen and a second p function on hydrogen atoms, and a modification of the higher level correction. G2 theory is a significant improvement over G1 theory because it eliminates a number of deficiencies present in G1 theory. Of particular importance is the improvement in atomization energies of ionic molecules such as LiF and hydrides such as C2H6, NH3, N2H4, H2O2, and CH3SH. The average absolute deviation from experiment of atomization energies of 39 first‐row compounds is reduced from 1.42 to 0.92 kcal/mol. In addition, G2 theory gives improved performance for hypervalent species and electron affinities of second‐row species (the average deviation from experiment of electron affinities of second‐row species is reduced from 1.94 to 1.08 kcal/mol). Finally, G2 atomization energies for another 43 molecules, not previously studied with G1 theory, many of which have uncertain experimental data, are presented and differences with experiment are assessed.

Bond Dissociation Energies of Organic Molecules

Abstract: In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

Bond dissociation energies of organic molecules

Abstract: In this Account we have compiled a list of reliable bond energies that are based on a set of critically evaluated experiments. A brief description of the three most important experimental techniques for measuring bond energies is provided. We demonstrate how these experimental data can be applied to yield the heats of formation of organic radicals and the bond enthalpies of more than 100 representative organic molecules.

Atomic and molecular electron affinities: photoelectron experiments and theoretical computations.

Abstract: I. Introduction and Scope 231A. Definitions of Atomic Electron Affinities 233B. Definitions of Molecular Electron Affinities 233II. Experimental Photoelectron Electron Affinities 235A. Historical Background 235B. The Photoeffect 236C. Experimental Methods 237D. Time-of-Flight Negative Ion PhotoelectronSpectroscopy239E. Some Thermochemical Uses of ElectronAffinities241F. Layout of Table 10: ExperimentalPhotoelectron Electron Affinities242III. Theoretical Determination of Electron Affinities 242A. Historical Background 2421. Theoretical Predictions of Atomic ElectronAffinities2422. Theoretical Predictions of MolecularElectron Affinities243B. Present Status of Theoretical Electron AffinityPredictions243C. Basis Sets and Theoretical Electron Affinities 244D. Density Functional Theory (DFT) andElectron Affinities245E. Layout of Tables 8 and 9: Theoretical DFTElectron Affinities247F. Details of Density Functional MethodsEmployed in Tables 8 and 9247IV. Discussion and Observations 248A. Statistical Analysis of DFT Results ThroughComparisons to Experiment and OtherTheoretical Methods248B. Theoretical EAs for Species with UnknownExperimental EAs251C. On the Applicability of DFT to Anions and theFuture of DFT EA Predictions251D. Specific Theoretical Successes 251E. Interesting Problems 2521. C