RRKM Analysis Research Articles

Proton Affinities and Photoelectron Spectra of Phenylalanine and N-Methyl- and N,N-Dimethylphenylalanine. Correlation of Lone Pair Ionization Energies with Proton Affinities and Implications for N-Methylation as a Method to Effect Site Specific Protonation of Peptides

A Fourier transform ion cyclotron resonance (FT-ICR) technique for measuring gas-phase proton affinities is presented which utilizes collisional dissociation of proton-bound clusters by off-resonance translational excitation. A simplified RRKM analysis relates unimolecular dissociation rates to proton affinities. This technique is used to measure values for the proton affinities of phenylalanine and N-methyl- and N,N-dimethylphenylalanine of 220.3, 223.6, and 224.5 kcal/mol, respectively (relative to the proton affinity of NH_3 = 204.0 kcal/mol). The proton affinity measured for phenylalanine is in excellent agreement with reported literature values. The photoelectron spectra of these three molecules are also presented and analyzed. Assignments of bands to specific ionization processes are aided by comparison with model compounds such as methyl-substituted amines and 2-phenylethylamines. These data are employed to examine the correlation of adiabatic nitrogen lone pair ionization energies with gas-phase proton affinities for phenylalanine, N-methylphenylalanine, and N,N-dimethylphenylalanine in comparison to correlations for other amino acids and selected aliphatic amines. Although amine nitrogen methylation increases the potential for localizing charge at the amine terminus of protonated peptides by increasing the gas-phase proton affinity, the present study establishes that the increase is not sufficient to compete with protonation of some of the more basic side chains in peptides.

Unimolecular decay of energy-selected dimethylformamide cations: a combined molecular orbital and RRKM analysis

The unimolecular dissociation chemistry of N,N-dimethylformamide cations, (CH[sub 3])[sub 2]NCHO[sup +], has been investigated by photoelectron-photoion coincidence in the energy range 11.11-11.49 eV. The measured rates for CHO and CH[sub 3] loss were modeled with RRKM statistical theory. The vibrational frequencies for both the molecular ion and two transition states were obtained from ab initio MO calculations so that the only adjustable parameters in the RRKM analysis were the activation energies. This analysis shows that these apparently simple dissociation reactions proceed via substantial reverse activation energies and tight transition states. Ab initio MO calculations (3-21G* basis set) of the transition states suggest that these reactions proceed via an intramolecular (not associated with the expelled radical) hydrogen atom transfer to produce the CH[sub 3]NHCH[sub 2][sup +] and CH[sub 3]NHCO[sup +] product ions, rather than (CH[sub 3])[sub 2]N[sup +] and CH[sub 3]NCHO[sup +] expected from simple loss of CHO and CH[sub 3]. The measured kinetic energy release was found to be only slightly greater that that predicted by statistical theory. 27 refs., 6 figs., 2 tabs.

A theoretical study of nitrososilane and six isomers of SiH 3NO

Geometries have been obtained for seven SiH 3NO molecules using HF/6-31G* and MP2/6-31G* theory: nitrososilane, isonitrososilane, silaformaldoxime, silaformaldonitrone, siloxaziridine, iminosilanol and silaformamide. Vibrational frequencies and some internal rotations have been characterized, and relative energies were obtained with MP4/6-31G* and MP4/6-311G** theory at the minimum energy geometries. A theoretical SiH 3NO bond strength of 124 kJ mol −1 in nitrososilane was obtained, in accord with an RRKM analysis of SiH 3 + NO recombination kinetics which yielded 117 ± 11 kJ mol −1.

Investigation into the pressure dependence between 1 and 10 Torr of the reactions of NO2 with CH3 and CH3O

The kinetics and pressure dependence of the reactions of NO2 with CH3 and CH3O have been investigated in the gas phase at 298 K, at pressures from 1 to 10 Torr. A low-pressure discharge-flow laser-induced fluorescence (LIF) technique was used. In a consecutive process, CH3 reacted with NO2 to form CH3O, CH3+ NO2→ CH3O + NO (1), which further reacted with NO2 to form products, CH3O + NO2→ products (2). Reaction (1) displayed no discernible pressure dependence over the pressure range 1–7 Torr, and k1 was calculated to be (2.3 ± 0.3)× 10–11 cm3 molecule–1 s–1. Reaction (2) displayed a strong pressure dependence and an RRKM analysis yielded the following limiting low- and high-pressure rate constants in He, k0= 5.9 × 10–29 cm6 molecule–2 s–1 and k∞= 2.1 × 10–11 cm3 molecule–1 s–1. It is unrealistic to quote errors for this type of analysis. Parametrisation in the standard Troe form with Fc= 0.6 yielded k0=(5.3 ± 0.2)× 10–29 cm6 molecule–2 s–1 and k∞=(1.4 ± 0.1)× 10–11 cm3 molecule–1 s–1. Atmospheric implications and possible reaction mechanisms are discussed.

Determination of the Arrhenius parameters for disilane .dblarw. silane + silicon dihydride and .DELTA.H.degree.f (SiH2) by RRKM analysis for forward and reverse reaction rate data

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDetermination of the Arrhenius parameters for disilane .dblarw. silane + silicon dihydride and .DELTA.H.degree.f (SiH2) by RRKM analysis for forward and reverse reaction rate dataHarry K. Moffat, Klavs F. Jensen, and Robert W. CarrCite this: J. Phys. Chem. 1992, 96, 19, 7683–7695Publication Date (Print):September 1, 1992Publication History Published online1 May 2002Published inissue 1 September 1992https://doi.org/10.1021/j100198a038RIGHTS & PERMISSIONSArticle Views163Altmetric-Citations28LEARN 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 (2 MB) Get e-Alerts

The pyrolysis of organic hydroperoxides (ROOH)

The pyrolysis of peracetic, perpropionic acids (RCO3−H) and t-butyl hydroperoxide (ROOH) has been studied in the gas phase. Under the conditions studied both peracetic and perpropionic acids exhibit pressure dependent unimolecular rate constants. This is particularly marked for peracetic acid. An RRKM analysis, using the Gilbert method, provided the best fit with the data with an A factor of 1015.6 s−1 and ΔEdown−200 cm−1. In fact the A factors for all three decompositions are all −1015.6 s−1 identical to that found by dialkyl peroxides (ROOR). The O−O bond strengths for the three types of peroxides are RO−OH, 180±2 kJ mol, RCO2−OH, 170±2 kJ mol−1 and RO−OR, 160±2 kJ mol−1 i.e. there is a subtle difference of −10 kJ mol−1 between each group. This difference in bond dissociation energies explain why acyl hydroperoxides decompose faster than the alkyl hydroperoxides in the “cool” flame region. Under high pressure conditions the ratio of the rate constants varies from 10 to 5 from 500–700 K in favour of the acyl peroxides—a subtle difference! Pressure dependent decomposition modifies this conclusion.

High temperature pyrolyses of acetylene and diacetylene behind reflected shock waves

A time-of-flight mass spectrometer coupled to a shock tube was used to study the thermal decompositions of C2H2 and C4H2 over the temperature range 1880–2620 K at total reflected shock pressures of ∼ 0.4 atm. Comparison of the experimental results obtained from 3.2% C2H2 mixtures, one of which contained synthesized, acetone-free acetylene, and the other comprised of purified tank acetylene containing 30 ppm acetone, reveals no discernible differences in the reaction profiles. The results are modeled satisfactorily with a new 40 step reaction mechanism which incorporates a recently proposed value for ΔfH0(C2H) of 130.4 kcal/mol. The concentration profiles obtained for C2H2, C4H2 and C6H2 are not affected by inclusion of molecular reactions in the mechanism over the temperature range covered herein. The rate employed for the dominant chain initiation at moderate temperatures, 2 C2H2→ C4H3+H, is that calculated from an RRKM analysis of new measurements on the dissociation of vinylacetylene. Strongly non-Arrhenius rate constants are employed for the crucial reaction, C2H+H2→C2H2+H, which exceed previous estimates by more than a factor of three at 2000 K. Additional support for the mechanism is provided by the kinetic data obtained from mixtures containing C4H2 as an initial reagent. It is concluded that a fragment chain reaction mechanism is clearly dominant in the high temperature region.

Deuterium isotope effect on the photoisomerization rates of stilbene: an RRKM analysis based on computed vibrational frequencies

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTDeuterium isotope effect on the photoisomerization rates of stilbene: an RRKM analysis based on computed vibrational frequenciesFabrizia Negri and Giorgio OrlandiCite this: J. Phys. Chem. 1991, 95, 2, 748–757Publication Date (Print):January 1, 1991Publication History Published online1 May 2002Published inissue 1 January 1991https://doi.org/10.1021/j100155a048RIGHTS & PERMISSIONSArticle Views105Altmetric-Citations26LEARN 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 (1 MB) Get e-Alerts

A Mathematical Model of the Fluid Mechanics and Gas‐Phase Chemistry in a Rotating Disk Chemical Vapor Deposition Reactor

We describe a mathematical model of the coupled fluid mechanics and gas‐phase chemical kinetics in a rotating disk chemical vapor deposition reactor. The analysis is for the flow between an infinite radius, heated nonporous rotating disk and a parallel infinite radius porous surface through which reactive fluid is injected normal to the disk. The analysis extends the usual von Karman transformation to allow specification of the normal velocity at the porous disk, and reduces to a stagnation point flow in the limit of zero rotating rate. The deposition of silicon from silane is used as an example system. A new reaction mechanism and set of rate constants are given for the thermal decomposition of silane. We present an RRKM analysis of several of the unimolecular reactions in the mechanism. Calculated velocity and temperature profiles, chemical species density profiles, and deposition rates as functions of susceptor temperature, spin rate, and inlet flow velocity are presented.

Theoretical analysis of the isomerizations and fragmentations of the C 3H 6O 2+· radical cation

A MINDO/3 theoretical study has been performed on the potential surface of the C 3H 6O 2 +· radical cation. CH 2OH ·, CH 3O ·, CHO · and CH 2O losses have been studied. Complex rearrangements occur before fragmentations. Kinetic aspects based on an RRKM analysis are discussed.

The least endothermic fragmentation pathways of the diazine cations

The least endothermic fragmentation pathways of the diazine cations have been investigated by variable residence time photoelectron-photoion coincidence spectroscopy. A detailed RRKM analysis of the corresponding results provides the rate energy functions for the loss of N 2 from the 1,2-diazine cation and for the loss of HCN from the 1,3- and 1,4-diazine cations. The outcome of the analysis further implies that the processes in question involve a rate-determining cleavage of the aromatic rings with a critical energy of ∼ 2.7 eV and a correspondingly loose transition state. Quantitative accord between computed and measured rates can only be achieved when it is assumed that the reactions occur on the ground state manifold of the respective parent ions, suggesting rapid internal conversion of any initial excess electronic energy.

Unimolecular dissociations of excited C3H6O+: A photoelectron—photoion coincidence study of cyclopropanol and allyl alcohol

Abstract The photoelectron spectrum of cyclopropanol has been determined and the first few bands have been assigned to the respective ionization processes. The dissociative photoionization of cyclopropanol and allyl alcohol has been studied by means of HeIα photoelectron—photoion coincidence spectroscopy. The breakdown diagrams of the two corresponding radical cations are practically indistinguishable within the entire energy range investigated (= 5 eV). This implies that extensive isomerizations to a single or to a mixture of common precursor structure(s) precede the dissociative processes. The currently available information on the structure of this reactant and, in particular, the role of other C 3 H 6 O + isomers is discussed. The kinetic and thermodynamic aspects of the present results are outlined. Although three generations of daughter ions are involved, the relative importance of the different fragmentation pathways could be determined. The RRKM analysis of the coincidence data suggests that the formation of the ethylene cation by loss of a CO molecule from the parent ion hardly competes with the other four primary fragmentation reactions in the sense of the statistical theory of unimolecular reactions.

Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations

AbstractA most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He‐Iα photoelectron‐photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3‐butadiene cation preceding the dissociative processes. The rate‐energy functions of four competitive primary fragmentation reactions, leading to C3H, C4H, C4H and C2H are established. There is compelling evidence that the production of C2H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.

A high pressure rate constant for CH 3 + H and an analysis of the kinetics of the CH 3 + H → CH 4 reaction

The rate of reaction of CH 3 H has been measured at 300 K over the pressure range 50–1000 Torr with Ar and SF 6 as diluent gases. Flash photolysis of azomethane/ethylene mixtures was employed, coupled with gas chromatographic analysis and numerical simulation of the product yields. The rate data were compared with a RRKM analysis, using a minimum density of state criterion for locating the activated complex. The limiting high pressure rate constant so obtained is 1.5 ± 0.7) × 10 −10 cm 3 molecule −1s −1. The RRKM model was found to give high temperature, high pressure rate constant for the dissociation of CH 4 which are significantly higher than those found experimentally.

The flash photolysis of azomethane. Minor products from the photolysis of methyl radicals and a rate constant for CH3 + NO

AbstractThe flash photolysis of azomethane in a quartz reaction vessel produces mainly ethane (>75%) plus smaller quantities of methane, ethylene, and acetylene. The minor products are interpreted quantitatively in terms of methyl radical photolysis at 216 nm to give CH2 and H. This interpretation is substantiated by the dependence of the minor products on flash intensity. The reduction of the ethane yield on adding NO is employed to obtain a rate constant for CH3 + NO as a function of total pressure, based on a value for methyl radical recombination of 4.2 × 10−11 cm3/molec · sec. An RRKM analysis is used to extrapolate the data to give a limiting high‐pressure rate constant for CH3 + NO of (1.2 ± 0.1) × 10−11 cm3/molec · sec at 298°K.