Shuqiong Lin's research while affiliated with National Research Council Canada and other places

Differences in O-H bond dissociation enthalpies (ΔBDEs) between the hydroxylamine of (15)N-labeled TEMPONE and 10 N,N-di-tert-alkyl hydroxylamines were determined by EPR. These ΔBDEs, together with the g and a(N) values of the derived nitroxide radicals, are discussed in relation to various geometric, intramolecular dipole/dipole, and steric effects and in relation to the results from DFT calculations. We find that dipole/dipole interactions are the dominant factors in dictating a(N) values and O-H BDEs in all of these structurally similar nitroxides and hydroxylamines, respectively. The importance of including the Boltzmann distribution of conformations for each nitroxide in the a(N) calculations is emphasized.

(Figure Presented) Old wisdom teaches a new lesson : The rate constant for the isomerization of thermally generated triphenylmethoxyl to Ph 2(PhO)C has been determined both experimentally and by DFT calculations (see scheme), and the results vindicated Wieland's 1911 observations and conclusions-in contrast to photochemically based findings.

Thermolysis of 4-aminophenyl benzyl sulfide at 523 K in the hydrogen donor solvent (HDS), 9,10-dihydroanthracene (AnH2), gave 4-aminothiophenol and toluene as the predominant products of the homolytic S-C bond cleavage. Under these conditions, a portion of the 4-aminothiophenol was desulfurized to aniline with first-order kinetics and with a rate constant estimated by kinetic modeling to be 7.0x10(-6) s-1. Starting with 4-NH2C6H4SH at 523 K, it was found that sulfur loss was more efficient in the non-HDSs, anthracene and hexadecane, than in AnH2. Under similar (competitive) reaction conditions, YC6H4SHs with Y=H, 4-CN, and 3-CF3 were completely inert; with Y=4-CH3O, there was some very minor desulfurization, whereas with Y=4-N(CH3)2 and 4-N(CH3)(H), the sulfur extrusions were as fast as that for Y=4-NH2. We tentatively suggest that this apparently novel reaction is a chain process initiated by the bimolecular formation of diatomic sulfur, S2, followed by a reversible addition of ground state, triplet 3S2 to the thiol sulfur atom, 4-NH2C6H4S upward arrow(SS upward arrow)H, and insertion into the S-H bond, 4-NH2C6H4SSSH. In a cascade of reactions, aniline and S8 are formed with the chains being terminated by reaction of 4-NH2C6H4S upward arrow(SS upward arrow)H with 4-NH2C6H4SH. Such a reaction mechanism is consistent with the first-order kinetics. That this reaction is primarily observed with 4-YC6H4SH having Y=N(CH3)2, N(CH3)(H), and NH2 is attributed to the fact that these compounds can exist as zwitterions.

There are conflicting reports on the origin of the effect of Y substituents on the S-H bond dissociation enthalpies (BDEs) in 4-Y-substituted thiophenols, 4-YC(6)H(4)S-H. The differences in S-H BDEs, [4-YC(6)H(4)S-H] - [C(6)H(5)S-H], are known as the total (de)stabilization enthalpies, TSEs, where TSE = RSE - MSE, i.e., the radical (de)stabilization enthalpy minus the molecule (de)stabilization enthalpy. The effects of 4-Y substituents on the S-H BDEs in thiophenols and on the S-C BDEs in phenyl thioethers are expected to be almost identical. Some S-C TSEs were therefore derived from the rates of homolyses of a few 4-Y-substituted phenyl benzyl sulfides, 4-YC(6)H(4)S-CH(2)C(6)H(5), in the hydrogen donor solvent 9,10-dihydroanthracene. These TSEs were found to be -3.6 +/- 0.5 (Y = NH(2)), -1.8 +/- 0.5 (CH(3)O), 0 (H), and 0.7 +/- 0.5 (CN) kcal mol(-1). The MSEs of 4-YC(6)H(4)SCH(2)C(6)H(5) have also been derived from the results of combustion calorimetry, Calvet-drop calorimetry, and computational chemistry (B3LYP/6-311+G(d,p)). The MSEs of these thioethers were -0.6 +/- 1.1 (NH(2)), -0.4 +/- 1.1 (CH(3)O), 0 (H), -0.3 +/- 1.3 (CN), and -0.8 +/- 1.5 (COCH(3)) kcal mol(-1). Although all the enthalpic data are rather small, it is concluded that the TSEs in 4-YC(6)H(4)SH are largely governed by the RSEs, a somewhat surprising conclusion in view of the experimental fact that the unpaired electron in C(6)H(5)S(*) is mainly localized on the S. The TSEs, RSEs, and MSEs have also been computed for a much larger series of 4-YC(6)H(4)SH and 4-YC(6)H(4)SCH(3) compounds by using a B3P86 methology and have further confirmed that the S-H/S-CH(3) TSEs are dominated by the RSEs. Good linear correlations were obtained for TSE = rho(+)sigma(p)(+)(Y), with rho(+) (kcal mol(-1)) = 3.5 (S-H) and 3.9 (S-CH(3)). It is also concluded that the SH substituent is a rather strong electron donor with a sigma(p)(+)(SH) of -0.60, and that the literature value of -0.03 is in error. In addition, the SH rotational barriers in 4-YC(6)H(4)SH have been computed and it has been found that for strong electron donating (ED) Ys, such as NH(2), the lowest energy conformer has the S-H bond oriented perpendicular to the aromatic ring plane. In this orientation the SH becomes an electron withdrawing (EW) group. Thus, although the OH group in phenols is always in-plane and ED irrespective of the nature of the 4-Y substituent, in thiophenols the SH switches from being an ED group with EW and weak ED 4-Ys, to being an EW group for strong ED 4-Ys.

The antioxidant properties of Hantzsch 1,4-dihydropyridine esters and two dibenzo-1,4-dihydropyridines, 9,10-dihydroacridine (DHAC) and N-methyl-9,10-dihydroacridine (N-Me-DHAC), have been explored by determining whether they retard the autoxidation of styrene or cumene at 30 degrees C. Despite a claim to the contrary [(2003) Chem. Res. Toxicol. 16, 208-215], the Hantsch esters were found to be virtually inactive as chain-breaking antioxidants (CBAs), their reactivity toward peroxyl radicals being some 5 orders of magnitude lower than that of the excellent CBA, 2,2,5,7,8-pentamethyl-6-hydroxy-chroman (PMHC). DHAC was found to be about a factor of 10 less reactive than PMHC. From kinetic measurements using DHAC, N-deuterio-DHAC, and N-Me-DHAC, it is concluded that it is the N--H hydrogen in DHAC that is abstracted by peroxyl radicals, despite the fact that in DHAC the calculated C-H bond dissociation enthalpy (BDE) is about 11 kcal/mol lower than the N-H BDE. The rates of hydrogen atom abstraction by the 2,2-diphenyl-1-picrylhydrazyl radical (dpph*) have also been determined for the same series of compounds. The trends in the peroxyl and dpph* rate constants are similar.

Six O-phenyl ketoxime ethers, RR'C=NOPh 1-6, with RR' = diaryl, dialkyl, and arylalkyl, together with N-phenoxybenzimidic acid phenyl ether, PhO(Ph)C=NOPh, 7, have been shown to thermolyze at moderate temperatures with "clean" N-O bond homolyses to yield iminyl and phenoxyl radicals, RR'C=N(*) and PhO(*). Since 1-6 can be synthesized at room temperature, these compounds provide a new and potentially useful source of iminyls for syntheses. The iminyl from 7 undergoes a competition between beta-scission, to PhCN and PhO(*), and cyclization to an oxazole. Rate constants, 10(6) k/s(-1), at 90 degrees C for 1-6 range from 4.2 (RR' = 9-fluorenyl) to 180 (RR' = 9-bicyclononanyl), and that for 7 is 0.61. The estimated activation enthalpies for N-O bond scission are in satisfactory agreement with the results of DFT calculations of N-O bond dissociation enthalpies, BDEs, and represent the first thermochemical data for any reaction yielding iminyl radicals. The small range in k (N-O homolyses) is consistent with the known sigma structure of these radicals, and the variations in k and N-O BDEs with changes in RR' are rationalized in terms of iminyl radical stabilization by hyperconjugation: RR'C=N(*) <--> R(*)R'C[triple bond]N. Calculated N-H BDEs in the corresponding RR'C=NH are also presented.

Thermolyses of seven dialkyl, two alkyl-aryl and two diaryl O-benzyl ketoxime ethers, R(1)R(2)C[double bond, length as m-dash]NOCH(2)Ph, have been examined in three hydrogen donor solvents: tetralin, 9,10-dihydrophenanthrene, and 9,10-dihydroanthracene. All the oxime ethers gave the products expected from homolytic scission of both the O-C bond (viz., R(1)R(2)C[double bond, length as m-dash]NOH and PhCH(3)) and N-O bond (viz., R(1)R(2)C[double bond, length as m-dash]NH and PhCH(2)OH). The yields of these products depended on which solvent was used and the rates of decomposition of the O-benzyl oxime ethers were greater in 9,10-dihydrophenanthrene and 9,10-dihydroanthracene than in tetralin. These results indicated that a reverse radical disproportionation reaction in which a hydrogen atom was transferred from the solvent to the oxime ether, followed by [small beta]-scission of the resultant aminoalkyl radical, must be important in the latter two solvents. Benzaldehyde was found to be an additional product from thermolyses conducted in tetralin. This, and other evidence, indicated that another induced decomposition mode involving abstraction of a benzylic hydrogen atom, followed by [small beta]-scission of the resulting benzyl radical, became important for some substrates. Participation by minor amounts of enamine tautomers of the oxime ethers was shown to be negligible by comparison of thermolysis data for the O-benzyloxime of bicyclo[3.3.1]nonan-9-one, which cannot give an enamine tautomer, with that of the O-benzyloxime of cyclohexanone.

Abstract: Density functional theory indicates that the minimum energy structure of the diphenylaminyl radical, Ph2N, has a "staggered" conformation in which the two phenyl rings are twisted relative to each other by an angle, , of 40. In this conformation, the aromatic rings are oriented so as to maximize interaction with the unpaired electron while minimizing repulsion between the 2- and 2'-hydrogen atoms. This calculated ground state structure of Ph2N differs from that, which has been accepted for the past 15 years, which had the two rings orthogonal ( = 90) with one ring conjugating with the nitrogen's lone pair and the other conjugating with the unpaired electron. This structure was based on unexpected differences between the UV-vis absorption spectra of Ph2N and the diphenylmethyl radical. However, our calculations indicate that this orthogonal structure lies 3.5 kcal/mol above the global minimum. Further support for the staggered conformation of Ph2N is provided by the similarities between absorption transition wavelengths determined theoretically and the experimental absorption bands of Ph2N and other diarylaminyl radicals generated by laser flash photolysis. The long wavelength transition of Ph2N, resulting in a structure that can be represented as (Ph2)+N-, is red-shifted as compared to the related transition from Ph2CH to (Ph2)+CH- due to the electronegativity of the N atom. The absorption bands for PhCH2, PhNH, and PhO in the 300-450 nm region are similar in position, which has been taken to indicate that for isoelectronic species the electronic transition energies should be little affected by heteroatom substitution. Our calculations show, however, that these sets of absorption bands arise from different transitions. Therefore, the experimentally similar 300-450 nm absorption bands for these three radicals are fortuitous and do not reflect some common, unifying traits, a fact that further serves to emphasize the importance of theory in the assignment of bands due to electronic transitions.

The thermal decomposition of bis(4-carboxybenzyl)hyponitrite (SOTS-1) in aerated water under physiological conditions has previously been shown to give the superoxide radical anion in a yield of 40 mol % (Ingold, K. U.; et al. J. Am. Chem. Soc. 1997, 119, 12364). The absolute kinetics of the elementary reactions involved in the cascade of events leading from the first-formed water-soluble benzyloxyl radical to superoxide have been determined by laser flash photolysis. On the basis of these kinetics it is concluded that SOTS-1 will be suitable for studies of superoxide-induced oxidative stress in most biological systems. A water-assisted 1,2-H shift converting benzyloxyl into the benzyl ketyl radical is an important step in the above reaction cascade. The kinetics of the 1,2-H shift assisted by H2O, D2O, and a number of nucleophilic alcohols have been measured for the first time. These data have led to a proposed new mechanism involving the initial formation of a ketyl radical anion and an oxonium cation which generally collapse to give the neutral ketyl radical as the first observable product on the time scale of our experiments (ca. 80 ns).