Ching Yeh Lin

Ab Initio Evaluation of the Thermodynamic and Electrochemical Properties of Alkyl Halides and Radicals and Their Mechanistic Implications for Atom Transfer Radical Polymerization

High-level ab initio molecular orbital calculations are used to study the thermodynamics and electrochemistry relevant to the mechanism of atom transfer radical polymerization (ATRP). Homolytic bond dissociation energies (BDEs) and standard reduction potentials (SRPs) are reported for a series of alkyl halides (R-X; R = CH 2CN, CH(CH 3)CN, C(CH 3) 2CN, CH 2COOC 2H 5, CH(CH 3)COOCH 3, C(CH 3) 2COOCH 3, C(CH 3) 2COOC 2H 5, CH 2Ph, CH(CH 3)Ph, CH(CH 3)Cl, CH(CH 3)OCOCH 3, CH(Ph)COOCH 3, SO 2Ph, Ph; X = Cl, Br, I) both in the gas phase and in two common organic solvents, acetonitrile and dimethylformamide. The SRPs of the corresponding alkyl radicals, R (*), are also examined. The computational results are in a very good agreement with the experimental data. For all alkyl halides examined, it is found that, in the solution phase, one-electron reduction results in the fragmentation of the R-X bond to the corresponding alkyl radical and halide anion; hence it may be concluded that a hypothetical outer-sphere electron transfer (OSET) in ATRP should occur via concerted dissociative electron transfer rather than a two-step process with radical anion intermediates. Both the homolytic and heterolytic reactions are favored by electron-withdrawing substituents and/or those that stabilize the product alkyl radical, which explains why monomers such as acrylonitrile and styrene require less active ATRP catalysts than vinyl chloride and vinyl acetate. The rate constant of the hypothetical OSET reaction between bromoacetonitrile and Cu (I)/TPMA complex was estimated using Marcus theory for the electron-transfer processes. The estimated rate constant k OSET = approximately 10 (-11) M (-1) s (-1) is significantly smaller than the experimentally measured activation rate constant ( k ISET = approximately 82 M (-1) s (-1) at 25 degrees C in acetonitrile) for the concerted atom transfer mechanism (inner-sphere electron transfer, ISET), implying that the ISET mechanism is preferred. For monomers bearing electron-withdrawing groups, the one-electron reduction of the propagating alkyl radical to the carbanion is thermodynamically and kinetically favored over the one-electron reduction of the corresponding alkyl halide unless the monomer bears strong radical-stabilizing groups. Thus, for monomers such as acrylates, catalysts favoring ISET over OSET are required in order to avoid chain-breaking side reactions.

Mechanism of Carbon-Halogen Bond Reductive Cleavage in Activated Alkyl Halide Initiators Relevant to Living Radical Polymerization: Theoretical and Experimental Study

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(•-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(•-), which is shown here to be nonexistent.

EDF2: A density functional for predicting molecular vibrational frequencies

The majority of calculations of molecular vibrational spectra are based on the harmonic approximation but are compared (usually after empirical scaling) with experimental anharmonic frequencies. Any agreement that is observed in such cases must be attributable to fortuitous cancellation of errors and it would certainly be preferable to develop a more rigorous computational approach. In this paper, we introduce a new density functional model (EDF2) that is explicitly designed to yield accurate harmonic frequencies, and we present numerical results for a wide variety of molecules whose experimental harmonic frequencies are known. The EDF2 model is found to be significantly more accurate than other DFT models and competitive with the computationally expensive CCSD(T) method.

Benchmark Calculations of Absolute Reduction Potential of Ferricinium/Ferrocene Couple in Nonaqueous Solutions.

High-level ab initio molecular orbital theory is used to obtain benchmark values for the ferricenium/ferrocene (Fc(+)/Fc) couple, the IUPAC recommended reference electrode for nonaqueous solution. The gas-phase ionization energy of ferrocene is calculated using the high-level composite method, G3(MP2)-RAD, and two higher-level variants of this method. These latter methods incorporate corrections for core correlation and, in the case of the highest level considered, use (RO)CCSD(T)/6-311+G(d,p) in place of (RO)CCSD(T)/6-31G(d) as the base level of theory. All methods provide good agreement with one another and the corresponding experimental values. Solvation energies have been calculated using PCM, CPCM, SMD, and COSMO-RS. Using G3(MP2)-RAD-Full-TZ gas-phase energies and COSMO-RS solvation energies, the absolute redox potentials of the Fc(+)/Fc couple have been calculated as 4.988, 4.927, and 5.043 V in acetonitrile, 1,2-dichloroethane, and dimethylsulfoxide solutions, respectively.

Estimation of Standard Reduction Potentials of Halogen Atoms and Alkyl Halides

Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated E(o)(X•/X-) values of 3.66, 2.59, 2.04, and 1.37 V vs SHE for F•, Cl•, Br•, and I•, respectively. Additionally, we have computed the SRPs of Cl•, Br•, and I• in acetonitrile (CH3CN) and dimethylformamide (DMF) by correcting the values obtained in water for the free energies of transfer of X• and X- from water to the nonaqueous solvent S and the intersolvent potential between water and S. From the values of E(o)(X •/X-) in CH(3)CN and DMF, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other important processes such as pollution abatement have been calculated in these two solvents. This has been done with the aid of a thermochemical cycle involving the gas-phase homolytic dissociation of the C-X bond, solvation of RX, R•, and X•, and reduction of X• to X- in solution.

A comparison of methods for measuring relative radical stabilities of carbon-centred radicals

This article discusses and compares various methods for defining and measuring radical stability, including the familiar radical stabilization energy (RSE), along with some lesser-known alternatives based on corrected carbon-carbon bond energies, and more direct measures of the extent of radical delocalisation. As part of this work, a large set of R-H, R-CH(3), R-Cl and R-R BDEs (R = CH(2)X, CH(CH(3))X, C(CH(3))(2)X and X = H, BH(2), CH(3), NH(2), OH, F, SiH(3), PH(2), SH, Cl, Br, N(CH(3))(2), NHCH(3), NHCHO, NHCOCH(3), NO(2), OCF(3), OCH(2)CH(3), OCH(3), OCHO, OCOCH(3), Si(CH(3))(3), P(CH(3))(2), SC(CH(3))(2)CN, SCH(2)COOCH(3), SCH(2)COOCH(3), SCH(2)Ph, SCH(3), SO(2)CH(3), S(O)CH(3), Ph, C(6)H(4)-pCN, C(6)H(4)-pNO(2), C(6)H(4)-pOCH(3), C(6)H(4)-pOH, CF(2)CF(3), CF(2)H, CF(3), CCl(2)H, CCl(3), CH(2)Cl, CH(2)F, CH(2)OH, CH(2)Ph, cyclo-CH(CH(2))(2), CH(2)CH[double bond, length as m-dash]CH(2), CH(2)CH(3), CH(CH(3))(2), C(CH(3))(3), C[triple bond, length as m-dash]CH, CH[double bond, length as m-dash]CH(2), CH[double bond, length as m-dash]CHCH(3), CHO, CN, COCH(3), CON(CH(2)CH(3))(2), CONH(2), CONHCH(3), COOC(CH(3))(3), COOCH(2)CH(3), COOCH(3), COOH, COPh), and associated radical stability values are calculated using the high-level ab initio molecular orbital theory method G3(MP2)-RAD. These are used to compare the alternative radical stability schemes and illustrate principal structure-reactivity trends.

Side Reactions of Nitroxide-Mediated Polymerization: N-O versus O-C Cleavage of Alkoxyamines

Free energies for the homolysis of the NO-C and N-OC bonds were compared for a large number of alkoxyamines at 298 and 393 K, both in the gas phase and in toluene solution. On this basis, the scope of the N-OC homolysis side reaction in nitroxide-mediated polymerization was determined. It was found that the free energies of NO-C and N-OC homolysis are not correlated, with NO-C homolysis being more dependent upon the properties of the alkyl fragment and N-OC homolysis being more dependent upon the structure of the aminyl fragment. Acyclic alkoxyamines and those bearing the indoline functionality have lower free energies of N-OC homolysis than other cyclic alkoxyamines, with the five-membered pyrrolidine and isoindoline derivatives showing lower free energies than the six-membered piperidine derivatives. For most nitroxides, N-OC homolysis is normally favored above NO-C homolysis only when a heteroatom that is α to the NOC carbon center stabilizes the NO-C bond and/or the released alkyl radical is not sufficiently stabilized. As part of this work, accurate methods for the calculation of free energies for the homolysis of alkoxyamines were determined. Accurate thermodynamic parameters to within 4.5 kJ mol(-1) of experimental values were found using an ONIOM approximation to G3(MP2)-RAD combined with PCM solvation energies at the B3-LYP/6-31G(d) level.

The role of exchange in systematic DFT errors for some organic reactions

Serious (up to 87 kJ mol(-1)) systematic DFT errors in a series of isodesmic reactions are found to be due to the DFT exchange component, and can be largely corrected by substitution of the DFT exchange energy with the Fock exchange energy.

Infrared characterization of the guanine radical cation: finger printing DNA damage.

Oxidation of DNA represents a major pathway of genetic mutation. We have applied infrared spectroscopy in 77 K glass with supporting density functional theory (DFT) calculations (EDF1/6-31+G*) to provide an IR signature of the guanine radical cation G(+*), formed as a result of 193 nm photoionization of DNA. Deprotonation of this species to produce the neutral radical G(-H)(*) does not occur in 77 K glass. DFT calculations indicate that the formation of G(+*) within the double helix does not significantly perturb the geometry of the G/C pair, even though there is a significant movement of the N(1) proton away from G toward C. However, this is in stark contrast to drastic changes that are expected if full deprotonation of G/C occurs, producing the G(-H)(*)/C pair. These results are discussed in light of solution-phase time-resolved IR spectroscopic studies and demonstrate the power of IR to follow dynamics of DNA damage in natural environments.

Fast and catalyst-free hetero-Diels-Alder chemistry for on demand cyclable bonding/debonding materials†

A new dithioester possessing a cyano Z-group (cyano-dithioester (CDTE)) has been synthesized via a 2-step, one-pot reaction. The cyano-substituted dithioester has been found to undergo fast reversible hetero-Diels–Alder (HDA) reactions at ambient temperature, without the need for a catalyst, as demonstrated by ESI-MS and UV-Vis experiments. To apply the bonding/debonding on demand system to materials science, a cyano-dithioester di-linker was synthesized and employed as a di-functional dienophile in a HDA-based polymerization reaction with a bis-cyclopentadiene polymer. The reversible bonding of the polymer systems were demonstrated by on-line UV-Vis spectroscopy, on-line NMR spectroscopy, and on-line high temperature DLS, as well as via GPC in situ trapping experiments and high-level ab initio molecular orbital calculations.