Sec-butyl Radicals Research Articles

First-principles computational studies of the torsional potential energy surface of the sec-butyl radical

First-principles calculations were carried out to investigate the torsional potential energy surface (PES) of the sec-butyl radical. All the wave function methods employed predict a cis-like stable conformation with a dihedral angle of about 47° in addition to the trans-like global minimum conformation and a gauche conformation. However, most of the popular density functional approaches predict only the latter two local minima and lack the cis conformation that was experimentally observed. On the other hand, some density functional methods that incorporate the exact exchange and asymptotically corrected correlation functionals can locate the cis conformation successfully. The basis-set effect was also measured using popular B3LYP and MP2 Hamiltonians: only moderate shape changes were found for PES profiles upon basis-set variations. The stationary structures and their Hessians were obtained at both MP2 and B3LYP levels, with or without incorporating the zero-point energies. Opposite to the relative stability within the Born–Oppenheimer approximation, the cis conformation is more stable than the gauche conformation upon the zero-point correction, consistent with the experiment observations.

Isotope Effects and the Temperature Dependences of the Hyperfine Coupling Constants of Muoniated sec-Butyl Radicals in Condensed Phases

Reported here is the first μSR study of the muon (A(μ)) and proton (A(p)) β-hyperfine coupling constants (Hfcc) of muoniated sec-butyl radicals, formed by muonium (Mu) addition to 1-butene and to cis- and trans-2-butene. The data are compared with in vacuo spin-unrestricted MP2 and hybrid DFT/B3YLP calculations reported in the previous paper (I), which played an important part in the interpretation of the data. The T-dependences of both the (reduced) muon, A(μ)′(T), and proton, A(p)(T), Hfcc are surprisingly well explained by a simple model, in which the calculated Hfcc from paper I at energy minima of 0 and near ±120° are thermally averaged, assuming an energy dependence given by a basic 2-fold torsional potential. Fitted torsional barriers to A(μ)′(T) from this model are similar (~3 kJ/mol) for all muoniated butyl radicals, suggesting that these are dominated by ZPE effects arising from the C−Mu bond, but for A(p)(T) exhibit wide variations depending on environment. For the cis- and trans-2-butyl radicals formed from 2-butene, A(μ)′(T) exhibits clear discontinuities at bulk butene melting points, evidence for molecular interactions enhancing these muon Hfcc in the environment of the solid state, similar to that found in earlier reports for muoniated tert-butyl. In contrast, for Mu−sec-butyl formed from 1-butene, there is no such discontinuity. The muon hfcc for the trans-2-butyl radical are seemingly very well predicted by B3LYP calculations in the solid phase, but for sec-butyl from 1-butene, showing the absence of further interactions, much better agreement is found with the MP2 calculations across the whole temperature range. Examples of large proton Hfcc near 0 K are also reported, due to eclipsed C−H bonds, in like manner to C−Mu, which then also exhibit clear discontinuities in A(p)(T) at bulk melting points. The data suggest that the good agreement found between theory and experiment from the B3LYP calculations for eclipsed bonds in the solid phase may be fortuitous. For the staggered protons of the sec-butyl radicals formed, no discontinuities are seen at all in A(p)(T), also demonstrating no further effects of molecular interactions on these particular proton Hfcc.

Electron transfer processes in photoinitiating systems composed of hemicyanine sec-butyltriphenylborate ion pairs

Borate hemicyanine salts, namely sec-butyltriphenyl styrylbenzoxazole borates are shown to be effective photoinitiators for the polymerization of vinyl monomers. Mechanism of the photoinitiation involves sec-butyl radicals formed from the heterolytic cleavage of carbon-boron bond that follows the electron transfer process. The capability of the salts to act as initiators for the polymerization of multifunctional monomer is documented.

Kinetic study of free-radical polymerization photoinitiated by cyanine-borate salts. II.

A series of cyanine butyltriphenylborate salts were prepared and tested as initiators of free-radical polymerization photoinitiated via a photoinduced electron-transfer process. For the majority of the tested series, the highest rate of photoinitiated free-radical polymerization was observed when sec-butyl radicals were formed. Essentially, there was no influence of the quantum yield of the free-radical formation on the rate of the free-radical polymerization initiated by the cyanine-borate salts. The experimental data revealed that the relationship between the rate of polymerization and the free energy change for the electron transfer displayed typical Marcus region kinetic behavior. The photoreduction of the cyanine butyltriphenylborate salts produced colorless products. The efficiency of the bleached-dye formation had no effect on the overall efficiency of photoinitiated polymerization. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2365–2374, 2000

Structure, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the sec-butyl radical: an ab initio study. [Erratum to document cited in CA113(9):77363e

Structure, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the sec-butyl radical: an ab initio study. [Erratum to document cited in CA113(9):77363e

ChemInform Abstract: Structure, Barriers for Internal Rotation, Vibrational Frequencies, and Thermodynamic Functions of the sec‐Butyl Radical: An ab initio Study.

ChemInformVolume 21, Issue 46 Physical Organic Chemistry ChemInform Abstract: Structure, Barriers for Internal Rotation, Vibrational Frequencies, and Thermodynamic Functions of the sec-Butyl Radical: An ab initio Study. Y. CHEN, Y. CHEN Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this authorA. RAUK, A. RAUK Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this authorE. TSCHUIKOW-ROUX, E. TSCHUIKOW-ROUX Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this author Y. CHEN, Y. CHEN Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this authorA. RAUK, A. RAUK Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this authorE. TSCHUIKOW-ROUX, E. TSCHUIKOW-ROUX Dep. Chem., Univ. Calgary, Calgary, Alberta T2N 1N4, Can.Search for more papers by this author First published: November 13, 1990 https://doi.org/10.1002/chin.199046057AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume21, Issue46November 13, 1990 RelatedInformation

Structure, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the sec-butyl radical: an ab initio study

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStructure, barriers for internal rotation, vibrational frequencies, and thermodynamic functions of the sec-butyl radical: an ab initio studyYonghua. Chen, Arvi. Rauk, and E. Tschuikow-RouxCite this: J. Phys. Chem. 1990, 94, 16, 6250–6254Publication Date (Print):August 1, 1990Publication History Published online1 May 2002Published inissue 1 August 1990https://doi.org/10.1021/j100379a019RIGHTS & PERMISSIONSArticle Views78Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (583 KB) Get e-Alerts

Isomerization of chemically activated secondary butyl radical

Photolytic decomposition of hydrogen sulfide followed by the addition of energized hydrogen atoms to cisbut-2-ene gives excited sec-butyl radicals. The radicals undergo a 1,3 hydrogen migration as is evidenced by the good agreement between the RRKM calculations and experiment. The contribution of a 1,2 hydrogen shift cannot be excluded. The threshold energies are 153.8 and 176.8 kJ mol−1 for 1,3 and 1,2 isomerization, respectively.

Theory of chemically activated unimolecular reactions. Weak collisions and steady states

The prediction of decomposition/stabilization ratios for chemically activated unimolecular reactions is usually based on the assumption of steady state and irreversible deactivation. The levels below the activation energy Eo are treated as a sink. In the present work we assume that the reaction can be described by a weak-collision master equation. An exact analysis shows that one can expect three timescales: an initial transient, an intermediate steady state, and an asymptotic steady state. The intermediate steady state is well defined only if the eigenvalues of the corresponding thermal rate matrix can be separated into two groups one of which contains eigenvalues of substantially smaller magnitude. An efficient and accurate method for the estimation of intermediate steady-state populations and branching ratios is described. It avoids the irreversible deactivation assumption which is shown to lead to errors particularly for weak collisions. The new analysis is illustrated in a series of calculations for a model of the sec-butyl radical system. The numerical solutions are readily obtained by use of a recently developed equilibrium finite-basis-set method.

Deductions from the master equation for chemical activation

The master equation for unimolecular decomposition and collisional deactivation of an energetic species A formed by chemical reaction was investigated. Particular attention was paid to the validity of the approximation of neglecting upward transitions, i.e., collisional transitions in which energy is added to A. Calculations of κa , the collision frequency ω times the ratio of the rate of decomposition of A to that of deactivation, were performed for the stepladder model for very large and very small ω and for the separable model for small ω. Neglect of upward transitions reduces κa. For the stepladder model the amount by which κa is reduced is greater for large ω than it is for small ω. It was also found that κa is more sensitive to changes in 〈Δε〉−, the average internal energy loss from A in deactivating collisions, when ω is small than it is when ω is large. Thus, when fitting computational results to experimental data, one derives a 〈Δε〉− which is too small if one neglects upward transitions, and the amount by which 〈Δε〉− is too small is considerably greater for large ω than it is for small ω. This result explains both qualitatively and semiquantitatively the discrepancies in 〈Δε〉−’s derived from stepladder model calculations of low pressure and high pressure κa’s for sec-butyl radicals in the presence of rare gas diluents.

A method to estimate intermolecular potential well depths for species in both ground and excited electronic states

The relationship lnσM=lnC+[(εA*A*) (εMM)]1/2/kT correlates the cross sections σM for a state change A*→B induced by a series of added M gases with the intermolecular potential well depths for A*...A* pairs and M...M pairs. This correlation is used with literature data concerning A*→B to deduce εA*A* for electronically excited atoms (Na, Ne, Ar, Xe) and electronically excited molecules (I2, SO2, CH3OH, glyoxal, propynal, benzene). The well depths are generally observed to exceed the ground state values by factors of 2–10. Large well depths are also observed for sec-butyl radicals and for the C5H9+ ion with high vibrational excitation. The correlation also provides an alternate means to measure ground state well depths εMM. In cases where secure comparisons are available, the well depths so derived usually lie within 20% of values found from transport measurements or virial coefficients. The correlation seems a useful alternative to empirical estimating procedures when data from conventional methods are not available.

Radical scavenging reactions of alpha-tocopherol. II. The reaction with some alkyl radicals.

alpha-Tocopherol was reacted with some alkyl radicals (ethyl, n-propyl, iso-propyl, n-butyl, and sec-butyl radical) to study its radical scavenging reactivity. The two types of products (alkyl ethers of alpha-tocopherol and cyclohexadienones) were obtained on treatment of each radial. These structures were determined by the spectral analysis. It was observed that alpha-tocopherol is very sensitive to the alkyl racidals and that the yields of the cyclohexadienones are decreased and that of the alkyl ethers are not much varied with an increase of carbon numbers of the alkyl radicals.

Effect of thermal activation on the reactions of chemically activated sec-butyl radicals

Effect of thermal activation on the reactions of chemically activated sec-butyl radicals

Studies on the Kolbe Electrolysis. XI. Racemization of Optically Active sec-Butyl Radicals in a Mixed Coupling Reaction.

Studies on the Kolbe Electrolysis. XI. Racemization of Optically Active sec-Butyl Radicals in a Mixed Coupling Reaction.

An Electron Spin Resonance Study of Hydrogen Atom Reactions with Butenes Trapped in Aqueous Sulfuric Acid Glasses

Hydrogen atom reactions with butenes trapped in sulfuric acid glasses yield sec-butyl radicals by addition and methallyl radicals by abstraction. The character of the e.s.r. spectra due to the CH3ĊHCH2CH3 radicals depends on whether cis-2-butene or trans-2-butene was the precursor. Computer simulated spectra indicate that the most probable conformation for the CH3ĊHCH2CH3 radicals derived from cis-2-butene and trans-2-butene are 'oblique' and 'trans', respectively. Qualitative agreement between the simulated and experimental spectra is achieved by adding together the spectra computed for CH3ĊHCH2CH3 and [Formula: see text] in the percentage ratio 40:60. In computing the spectra for CH3ĊHCH2CH3, it is assumed that the radicals are distributed over a range of conformations and can undergo torsional motion.

Unimolecular Decomposition and Intramolecular Energy Relaxation in the Suprahigh-Pressure Region

The unimolecular rate constant ka for decomposition of chemically activated sec-butyl radicals was measured at pressures 55–203 atm of hydrogen. The reaction enters its high-pressure region above 0.01 atm. This study extends the range of earlier work. Butyl radicals were produced by mercury photosensitization of hydrogen: cis-2-butene mixtures. The value of ka was obtained from the ratio of stabilization (S) to decomposition (D) according to the equation ka = βωD / S, where β represents the relative collisional efficiency of hydrogen. The average value of ka was found to be 1.78(± 0.2) × 107 sec−1. No systematic change occurred in the value of ka with increase of pressure; this is taken as support for the continued validity of the random lifetime assumption at average intervals between collision of 2 × 10−13 sec, at least in this case. The disporportionation-recombination ratio of sec-butyl radicals was measured as 0.57 in agreement with earlier values from this laboratory.

Secondary Isotope Effects in the Chemically Activated sec-Butyl-d and -d8 Systems and sec-Butyl-d1 and -d9 Systems

The decomposition of chemically activated sec-butyl-d8 and -d9 radicals, produced by H- and D-atom addition to cis-butene-2-d8, has been studied at 195° and 300°K over a range of pressures. These measurements, as well as internal comparison experiments with cis-butene-2-d0, reveal a large normal secondary intermolecular kinetic-isotope ratio which varies with the pressure and temperature, and whose minimum value is a factor of 4 and whose maximum value averages around 7. The absolute magnitude of the ratio, and its temperature, pressure, and energy dependence are in satisfactory agreement with the theory of these chemical-activation systems; some aspects of this dependence are discussed. Evidence against a significant mechanistic isotope effect on the energetics is presented. Calculated isotope effects for all of the possible sec-butyl radical systems, to be formed by H- and D-atom addition to butene-d0 and -d8 isomers, are given.

Collisional Transition Probabilities for Vibrational Deactivation of Chemically Activated sec-Butyl Radicals. Diatomic and Polyatomic Molecules

The study of collisional transitional probabilities for the de-excitation by inert gases of chemically activated sec-butyl radicals, excited to internal energies in excess of 40 kcal mole−1, has been extended to H2, D2, N2, CO2, CH4, CD3F, CH3Cl, and SF6. The diatomic gases display behavior similar to the rare gases, and on a preferred exponential model of collisional transition probabilities the average amount of energy transferred per collision is 〈ΔE〉expon≃1.3 kcal mole−1. On a stepladder model the corresponding amount is ΔE≃2.5 kcal mole−1. From higher-pressure data, the efficiency for CD3F, CH3Cl, and SF6 is deduced to be comparable with that for butene and, on a preferred stepladder model, ΔE>9 kcal. For CO2 and CH4 the behavior is intermediate. The possible importance of the role of internal rotation of butyl in facilitating energy transfer is noted; some uncertainty concerning the role of over-all rotations and vibrational modes of the deactivator in the relaxation process exists.

Collisional Transition Probabilities for Vibrational Deactivation of Chemically Activated sec-Butyl Radicals. The Rare Gases

Vibrationally excited sec-butyl radicals, having a narrow range of energies above 40 kcal mole−1, were produced in a heat bath of cis-butene-2-rare-gas mixtures by H addition to cis-butene-2. He, Ne, Ar, and Kr were used. Two competing reactions of the chemically activated radicals, decomposition by C–C rupture with critical energy of 33 kcal mole−1 and collisional stabilization, were studied as a function of pressure. Apparent rate constants for decomposition ka were obtained at three temperatures over a range of bath pressures by analysis of the decomposition and stabilization products. Strong and weak multistep collisional deactivation processes, corresponding to transfer of all or only part of the initial excess vibration energy (≥7 kcal mole−1) of the radicals, were considered. Multistep models of various step sizes (ΔE) give a family of calculated curves of ka as a function of p, each of which turns up from a quasi-constant value at higher pressures to higher values at low pressures. Comparison with the low-pressure experimental ``turnup'' yielded information on ΔE, independent of the assumed collision cross section. Comparison of relative rates in the higher-pressure region gave minimum estimates of ΔE or, alternatively, conventional ``efficiency'' factors, βmin; these conclusions are somewhat dependent on the assumed cross section. Values of ΔE from the lower-pressure turnup data bunch at 2.5–3.5 kcal mole−1 on an assumed stepladder model of transition probabilities. Such a model corresponds only formally to harmonic oscillator restrictions, and encompasses the characteristics of models in which small steps are less probable than large ones. Values of 〈ΔE〉exp for an exponential weighting of transition probabilities group at 1.2–1.8 kcal mole−1; this distribution describes the characteristics of models in which small steps are more probable than large ones and seems to be the preferred description of the rare-gas behavior here. He and Ne appear to transfer less energy than Kr and Ar. ΔE for cis-butene itself was confirmed to be ≥9 kcal, as derived in a previous study of deactivation of sec-butyl-d1 radicals by Harrington, Rabinovitch, and Hoare; and by contrast, the stepladder model is more appropriate here. From the higher-pressure data, values of βmin which varied from 0.16 for He at 100°C to 0.62 for Kr at −78°C were found. The efficiencies increase with increasing atomic weight of the rare gas and with decreasing bath temperature. Energy transfer between rare-gas atoms and butyl radicals is thus surprisingly efficient, with relatively large amounts of energy transferred per collision (〈ΔE〉>kT). The findings provide further detailed evidence concerning the inapplicability of the Landau-Teller theory and related models to polyatomic molecules at high vibrational energies (cf. Herzfeld and Litovitz). The possible role of rotational degrees of freedom is considered.

Unimolecular Decomposition in Nonequilibrium Systems. sec-Butyl and sec-Butyl-d1 Radicals Produced by Chemical Activation at Different Levels of Vibrational Excitation

Results of earlier studies on the decomposition of activated sec-butyl radicals formed in nonequilibrium systems by the addition of H or D atoms to trans-butene-2, to cis-butene-2, and to butene-1 are summarized together with new data. The work constitutes a program of study in which butyl radicals have been produced at six different levels of minimum internal excitation energy. Two choices of ambient temperature (300° and 195°K), together with pressure variation over the (ideal) limiting range of zero to infinity, give rise in these systems to a further increase in the number of values of the average energy of the reacting radicals; these energies correspond to a range from 7.2 to 14.2 kcal mole—1 for the average excess energy of the radicals with respect to decomposition. Twenty-one of such average energy states out of the possible twenty-four values, corresponding to two pressure extremes at each of two temperatures, have been obtained experimentally. These data permit a more consistent test of the theoretical models which were presented in the earlier work. A comparison and discussion of the observed rates is given. The agreement is satisfactory. An ``energy'' isotope effect which arises upon replacement of reactant H atom by D is well accounted for. It is concluded that the merits of the chemical activation technique for clarifying the nature of unimolecular reaction, at high levels of vibrational excitation of polyatomic species, are well borne out. The present evidence, together with other work, shows the usefulness of the Marcus—Rice rate expression in all systems where the assumptions may apply, when accurate calculations are made with quantum statistical models. The effect of variation of thermochemical and molecular parameters on the calculations is described. Not only are all vibrational degrees of freedom to be taken as active, as the best a priori assumption, but there is evidence that where they exist, internal rotations and possibly one of the over-all rotations should be so treated also.