Michael Henchman- Brandeis University
Michael Henchman
- Brandeis University
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Publications (86)
Throughout its 40-year history, gas-phase ion chemistry has been explored with a battery of different techniques. From the
very beginning, however, a technique has been needed to bridge the energy / temperature gap between beam experiments at the
lowest energies (~0.3 eV) and swarm experiments at the highest temperatures (~900 K). Today, this need...
The chemistry of hot hydrogen atoms has been studied using tritium of high kinetic energy as produced by nuclear recoil. The possibilities and limitations of this technique are discussed using a collision theory for reactions of atoms having a very high initial energy. Using this theory and certain experimental data, it is concluded that hot hydrog...
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
We have measured the rate constants for the endothermic proton-transfer reactions of PO3- with HI, PO2- with HCl, and NO3- with HCl as a function of average center-of-mass kinetic energy in a drift tube. Activation energies were derived from the data. Using the known endothermicity of the last reaction as a calibration, the endothermicities of the...
The article, Evidence That the Vinland Map Is Medieval (Anal. Chem. 2003, 75, 6745-6747), is constructed with conjecture and with false logic. It imagines that the ink of the Vinland Map is an iron-gall ink where conclusive evidence shows the ink to be not an iron-gall ink but a carbon-based ink. It asserts that the ink was homogeneous where conclu...
For at least 30,000 years, artists have created sculpture out of stone. In some cases, geochemical techniques can be used to characterize the work of art. Traditional methods for investigating authenticity range from the subjective approach of the connoisseur to the objective approach of the scientist. A variety of procedures can be used to establi...
We describe the design and construction of a teaching mass spectrometer from components that are available commercially. The instrument is transportable, robust, and inexpensive. It yields a mass spectrum 3 minutes after being switched on and is designed to be used by undergraduates and maintained by faculty without special instrumental skills.Seve...
The main features of the stability diagram for a quadrupole mass filter can be understood qualitatively using two concepts: (i) moderate RF voltages stabilize a trajectory; (ii) large RF voltages destabilize a trajectory. The trajectories of the ions traveling in the quadrupole are readily displayed pictorially for any set of operating conditions....
Fifteen asymmetric resonant charge transfer reactions, involving the five title atomic ions A+ and the three title molecules B have been studied in the relative (center-of-mass) energy range 1-10 eV. Two different tandem mass spectrometers were used to measure limiting cross-sections at the highest energies. Time-of-flight and retarding-potential-a...
A tandem mass spectrometer has been used to measure cross sections and product distributions for reactions of the selectively solvated anions OH-(H2O)(n) with the neutral molecules CH3Cl (n = 0, 1, or 2) and CH3Br (n = 0 or 1) over the relative energy range 0.1-5 eV. The reactions observed include nucleophilic displacement, proton transfer, and col...
The reactions of the cyclic molecules C6H6 (benzene), c-C3H6 (cyclopropane) and c-C6H12 (cyclohexane) with ArH+ (ArD+), H3+, N2H+, CH5+, HCO+, OCSH+, C2H3+, CS2H+ and H3O+ have been studied at 300 K using a SIFT apparatus. All the reactions except those of C2H3+ proceed via proton transfer and all are fast except the H3O+ and CS2H+ reactions with c...
As rate constants for reactions occurring on repulsive potential-energy surfaces show an Arrhenius-type temperature dependence, what correspondingly may be said for reactions occurring on attractive surfaces? Experiment reveals no general answer to that question. Here a limited answer is sought for a restricted family of exoergic reactions (ΔE°0 <...
Using art as a "conductor" to lead the nonscientist into the heart of science, to show what science does and how it does it; includes content and a list of experiments. Keywords (Audience): First-Year Undergraduate / General
We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35&percnt...
While the business of this institute is to study the dynamical properties of the electron, a fundamental constraint is imposed on these dynamical properties by the thermodynamic properties of the electron. It may be surprising to learn that in 1993 the thermodynamic properties of the electron are not well established and remain a matter of some con...
In a tandem mass spectrometer, a beam of OH− ions was reacted with methyl chloride and methyl bromide at collision energies in the range 0.2 < E, < 5 eV. For both of the methyl halides, excitation functions, σ(ET), were measured for the two competing channels, endoergic proton transfer and exoergic nucleophilic displacement: The threshold energies...
Using a selected ion flow tube at 368 K, we have measured rate constants for ion-molecule reactions between I-, PO3-, HSO4-, FSO3-, CF3SO3-, NO3-.HNO3, FSO3-.FSO3H, and H2SO4, FSO3H, CF3SO3H. Channels observed include proton transfer and clustering. These results allow free energies (enthalpies) of deprotonation (kJ/mol) to be estimated (within +/-...
Using a selected ion flow tube at 368 K, we have measured rate constants for ion-molecule reactions between I-, PO-, HSO4-, FSO3-, CF3SO3-, NO3-·HNO3, FSO3-·FSO3H, and H2SO4, FSO3H, CF3SO3H. Channels observed include proton transfer and clustering. These results allow free energies (enthalpies) of deprotonation (kJ/mol) to be estimated (within ±10...
A kinetic model, with only one kinetic variable κ, is developed to describe the isotope-exchange reaction CD⁺5 + CH4 → products (and its mirror-isotope counterpart). Each reaction yields four products and, at a particular temperature, a single value of κ is able to specify all eight rate constants. According to the model, the reactants form the int...
Rate constants for the reactions of the ions CH3OH2+ and CH3OH2+.CH3OH with methanol have been measured by using a variable-temperature selected ion flow tube (SIFT) instrument at 300 and 450 K. The measurements were conducted as a function of helium buffer gas pressure in the range 0.26-0.59 Torr. For the reaction of CH3OH2+ with methanol, two cha...
The authors have measured the rate constants for the endothermic proton-transfer reactions of PO{sub 3}{sup {minus}} with HI, PO{sub 2}{sup {minus}} with HCl, and NO{sub 3}{sup {minus}} with HCl as a function of average center-of-mass kinetic energy in a drift tube. Activation energies were derived from the data. Using the known endothermicity of t...
The authors have measured the rate constants for the endothermic proton-transfer reactions of POââ» with HI, POââ» with HCl, and NOââ» with HCl as a function of average center-of-mass kinetic energy in a drift tube. Activation energies were derived from the data. Using the known endothermicity of the last reaction as a calibration, the endotherm...
This report presents results of a study to analyze available data on electron affinities, and thermochemical properties of species of interest in the chemical modification of plasmas. Using these data, together with related information on chemical and physical properties, candidate species are identified for tests on modification of free electron d...
A crossed-beam study of the reaction CH+4(CH4,CH3)CH+5 was carried out in the collision energy range 0.6–2.3 eV. Three distinct patterns were observed, which may be interpreted in terms of three competing mechanisms for CH+5 formation: proton stripping, H-atom pickup, and intermediate complex decomposition. The existence of a C2H+8 intermediate, st...
A crossed-beam study of the reaction CH{sup +}â(CHâ,CHâ)CH{sup +}â was carried out in the collision energy range 0.6--2.3 eV. Three distinct patterns were observed, which may be interpreted in terms of three competing mechanisms for CH{sup +}â formation: proton stripping, H-atom pickup, and intermediate complex decomposition. The existence of a CâH...
The collision mechanism of the reaction CH4(+)+ Methane = CH5(+)+ CH3 has been investigated as a function of temperature by measuring rate constants and product distributions for the reactant pairs CD4(+) + CD4 and CD4(+) + CH4 at 80 and 300 K. At 80K, both reactions give the same product distribution, which is close to the statistical distribution...
A simple model is used to predict the fractionation of isotopes as a function of temperature via ion–molecule reactions. It is developed for reactions proceeding via a single intermediate, where the isotopic mixing is completed within the lifetime of the intermediate such that the reaction is under thermodynamic control. The fractionation efficienc...
In a tandem mass spectrometer we have measured the excitation functions (reaction cross section as a function of collision energy) for the following solvated-ion reactant pairs: OH-.(H2O) + H2; OD-.(D2O) + D2; and OH-.(H2O) + D2—in the collision energy range 0–2 eV. Product channels include H3O--type production, collision-induced dissociation of re...
The formation of deuterated interstellar molecules is considered. This contribution (which, in the summarizing remarks of the conference, was admonished for being itself admonitory) has a simple purpose - to apply gas-phase ion chemistry to distinguish deuterium exchange reactions that can occur from those that cannot. Thus the exoergic reaction HC...
Reactions of the hydroxide ion OH–(H2O)0,1,2,3, solvated with 0–3 water molecules, have been studied in the gas phase using flow-tube and ion-beam techniques. The reactions studied included proton transfer, nucleophilic displacement and isotope exchange. The experiments measure, as a function of the reactant solvation number, the dependence of rate...
In a tandem mass spectrometer we have measured the excitation functions (reaction cross section as a function of collision energy) for the following solvated-ion reactant pairs: OH** minus . (H//2O) plus H//2; OD** minus . (D//2O) plus D//2; and OH** minus . (H//2O) plus D//2 - in the collision energy range 0-2 eV. Product channels include H//3O**...
A selected ion flow tube was used to measure the rate constants and product distributions for the reactions of OH− (H2O)n with CH3CN over the temperature range 240–363 K for the case n = 1 and at 298 K for n = 0 and 2. Proton transfer was the only primary reaction channel observed; this process was found to be fast (efficiency ≅ 70%) for n = 0 and...
CF3SO2)2O (I) has been reacted with a series of negative ions in the gas phase at 353 K. CF3SO3- is rapidly formed by reaction of (I) with O2-, NO2-, NO3-, PO3-, and HSO4-, whereas (I) reacts slowly with NO3-(HNO3) and not at all with WF6-.
Triflic anhydride, (CF3SO2)2O, has been reacted with a series of negative ions in the gas phase at 353 K. This species yields CF3SO3- rapidly with O2-, NO2-, NO3-, PO3-, and HSO4-. It reacts slowly with NO3-(HNO3) and not at all with WF6-. These results show that the electron affinity of CF3SO3- exceeds 4.9 ± 0.3 eV and is probably less than 6.0 eV...
Many exothermic ion-molecule reactions occur at the collision limit. Can endothermic ion-molecule reactions still be fast, if “driven” by a compensating entropy increase ? This question continues to excite controversy. To some it is anathema. For others it is as self-evident as the sweating of a feverish brow or the charring of a cake —“entropy-dri...
The influence of hydration on the rate constants and the product distributions of the title proton transfer reaction has been investigated in a selected ion flow tube.
Using the flowing afterglow/Langmuir probe (FALP) technique, we have determined (at variously 300 and 570 K) the dissociative attachment coefficients β for the reactions of electrons with the common acids HNO3 (producing NO−2) and H2SO4 (HSO−4), the superacids FSO3H (FSO−3), CF3SO3H (CF3SO−3), ClSO3H (ClSO−3,Cl−), the acid anhydride (CF3SO2)2O (CF3...
Proton-transfer reactions which are exothermic in the gas phase are exceedingly efficient - most reacting at essentially every collision. Several studies have shown how this extreme reactivity is influenced by solvation in the gas phase at 300 K. Here we report how hydration influences the rate constants and the product distributions of the reactio...
Nucleophilic displacement reactions which are exothermic do not react on every collision in the gas phase. They exhibit a negative temperature dependence, rate constants decreasing with increasing temperature, and reaction at 300 K is quenched dramatically by the addition of only one-three solvate molecules. In each respect nucleophilic displacemen...
Unter thermischen Bedingungen (300 K) zeigt das System H0′ + CH3Cl in der Gasphase nucleophile Substitution.
WF6 reagiert schnell mit F′, Cl′, Br", I′, CN′, N03′ , N02′ , SF; und SP6" . Als Reaktionswege werden Assoziation, Ladungsübertragung und F′-Übertragung beobachtet.
The rate coefficients and branching ratios have been measured for the reactions of WFâ with Fâ», Clâ», Brâ», Iâ», CNâ», NOââ», NOââ», SFââ» and SFââ». WFâ was found to react rapidly with these ions. Three channels were observed: association, charge transfer, and fluoride ion transfer. The reactivity of WFââ» and WFââ» with several neutra...
Mesures experimentales des proprietes de PO 3 − et comparaison avec celles de NO 3 − • PO 3 − est une espece de faible potentiel chimique. Il ne subit pas de reactions chimiques avec divers reactifs (H 2 , HCl, HBr, NH 3 , HNO 3 , CH 3 CN)
A procedure is outlined by which enthalpy changes ΔHoo are estimated for several positive-ion-molecule reactions in which a hydrogen-deuterium exchange occurs and for which the experimentally determined rate coefficients and product-ion distributions are reported in an accompanying paper (N.G. Adams, D. Smith and M.J. Henchman, Int. J. Mass. Spectr...
Results are presented for the rate coefficients and production distributions of the near-thermoneutral reactions: H3O+ + D2O and D3O+ + H2O at 295 K; NH4+ + ND3 and ND +4 + NH3 at 204, 295 and 475 K; and CH5+ + CD4 and CD5+ + CH4 at 80, 204, 295 and 475 K. The data obtained are interpreted in terms of the lifetimes τd with respect to unimolecular d...
The reactions of H+ with D2 and of D+ with H2 have been studied in a helium‐buffered drift experiment in the regime from near thermal up to 0.3 eV relative kinetic energies KEcm of the reactants. The rate coefficient of the first, endoergic reaction, increase with KEcm, up to 8.5×10−10 cm3 sec−1 at 0.3 eV, and extrapolates smoothly at near thermal...
The energy dependences of the exothermic proton transfer from ArH(+) to
various neutrals was investigated in the energy regime from thermal to
approx. 1 eV. All rate coefficients obtained are fast (within a factor
of two of the Langevin or ADO limiting values) and show only slight
energy dependences. From the equilibrium ArH(+) / H3(+) in Ar-H2
mix...
Forward and reverse rate coefficients have been measured in the temperature range 200–300 K for the two reactions H++D2&rlarr2;HD+D+ and D++H2&rlarr2;HD+H+. Equilibrium constants derived therefrom agree with theoretical van’t Hoff plots calculated from statistical mechanics and confirm the temperature calibration of the SIFT apparatus used. It is s...
The rate coefficients and product ion distributions for the binary reactions of H3O+⋅(H2O)0,1,2 and D3O+⋅(D2O)0,1,2 ions with D2O and H2O, respectively, and with NH3 have been studied at 300 K using a selected ion flow tube (SIFT) apparatus. The ions were created in a flowing afterglow ion source and after mass filtering were injected at low energy...
A theoretical investigation is given of a new technique for producing monoenergetic ion beams in a longitudinal tandem mass spectrometer. An ion swarm, produced by a pulsed electron beam, is accelerated impulsively and then subjected to a further accelerating field as it begin to leave the ionization region. Results are presented for the magnitude...
More is known about the rates of ion-molecule reactions than about those of any other family of chemical reactions. This chapter will not, however, consist of a joyful celebration of that pleasant reality, but rather, the reverse—it will take a somewhat severe look at what we need to know, at what we sometimes think we know; and at what in fact we...
In a tandem mass spectrometer, a beam of OHâ» ions was reacted with methyl chloride and methyl bromide at collision energies in the range 0.2 < ET < 5 eV. For both of the methyl halides, excitation functions, sigma (ET), were measured for the two competing channels, endoergic proton transfer and exoergic nucleophilic displacement: OHâ» + CH3X yie...
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A new pulsing technique is described for the study of ion-neutral collision processes at energies of ≈ 1 eV. Short, high voltage pulses applied to the repeller plate of a mass-spectrometer ion source can produce reactant ion beams of constant velocity. Velocity analysis of both reactant and product ions is achieved through pulsed de-focusing of the...
Lead nitrate crystals have been irradiated with electrons at room temperature and their electron spin resonance spectra examined at 77°K to reveal three paramagnetic species. One species shows no hyperfine structure and has an isotropic g tensor with a g value at the free spin value: little can be said of its nature. The others are located, by thei...
The stereochemistry of the displacement of hydrogen by hot hydrogen ; atoms has been studied at an sp³ carbon atom in a gaseous molecule. The ; system used was the reaction of recoil tritium, from the nuclear reaction He/sub ; 3/(n,p)Hâ, with optically active 2-butanol. There is 91 plus or minus 6% ; retention of configuration for displacement of...
Questions
Question (1)
There are experimental values and calculated values. These values have changed with time, as techniques have become more sophisticated. Twenty years ago, theory managed to agree with experiment but always a year after the experimental values were published. Ideally theory should reign supreme because experiment necessarily may be shrouded in uncertain error. How are we doing today?
