J. Phys. Chem. 1994,98,4845-4853
4845
Methylthiomethyl Radical. A Variable-Time Neutralization-Reionization and ab Initio Study David W. Kuhns, Thuy B. Tran, Scott A. Shaffer, and Frantisek TureZek' Department of Chemistry, BG-10, University of Washington, Seattle, Washington 98195 Received: December 17, 1993;In Final Form: March 8,1994'
Stable CH3SCH2' (l), relevant to the high-energy chemistry of dimethyl sulfide, is prepared as an isolated species in the gas phase by flash-vacuum pyrolysis of CH~SCHZCH~ONO a t 550 OC, collisionally activated dissociation of CH~SCHZCH~SCH~'+, and collisional neutralization of its CH3SCH2+ cation. Loss of methyl is the major dissociation of 1 on thermal or collisional excitation, requiring 104 k J mol-' at the thermochemical threshold. Variable-time neutralization-reionization mass spectrometry is introduced and used to distinguish the dissociations of neutral 1 from those of its cation following collisional reionization with oxygen. The latter deposits -270 k J mol-' in the CH3SCH2+ ion formed due to collisional activation and moderate FranckCondon effects, calculated as 36-40 kJ mol-'. A b initio calculations at the G2(MP2) level of theory give the adiabatic ionization energy of 1 as IE, = 6.84 eV and the enthalpy of formation of 1+ as Af?f,298 = 809 k J mol-'.
Introduction Sulfides and thiols are efficient scavengers of hydroxyl radicals produced by high-energy processes in the gas phase and solution.14 In the atmosphere, the reaction of OH' with dimethyl sulfide (DMS) triggers an oxidation cascade that eventually results in the formation of methanesulfonic and sulfuric acids that contribute to natural cloud acidity.' Reactions of OH' with cysteine and methionine in aqueous solution play an important role in tissue damage under conditions of oxidative stress induced chemically or by irradiation.2 While the reaction of OH' with DMS has been a paradigm for sulfide oxidations in both gas phase3 and aqueous ~ o l u t i o nits , ~ intrinsic mechanism has been a matter of argument . The reaction is thought to proceed via two competingchannels, the major one involving hydrogen atom abstraction from DMS to form the methylthiomethyl radical (1, eq 1) and the other leading to the formation of a (CH3)2S*-OH adduct (2, eq 2).3a
CH3SCH3+ 'OH
-
CH3SCH,' 1
+ H,O
(1)
Thermochemical data indicate that reaction 1 is 94 kJ mol-' e~othermic,s-~ and kinetic studies estimated its activation energy to be as low as 3 kJ mol-l.3a Recent ab initio9 and experimental studies10 have indicated that 2 does not exist as a bound structure in the gas phase, while a weak dipole-dipole complex of 14 kJ mol-' binding energy may be transiently formed in aqueous ~ o l u t i o n .In ~ contrast to 2, the methylthiomethyl radical 1 is predicted by ab initio calculations to be a bound structure9J1 analogous to those of a-alkoxymethy111J2and a-(alky1amino)methyl r a d i c a l ~ . ~ ~Shum J 3 and Benson studied the gas-phase kineticsof the DMS reaction with iodine a t 630-650 K and derived the standard heat of formation for 1 as AHf,298 = 149 f 4 kJ A recent photofragmentation time-of-flight study of DMS yielded an estimate for the AHf,o of 1 as 142 f 10 kJ mol-1.14 These data indicate that the 94 kJ mol-' exothermic reaction 1 will generate radical 1with an internal energy close to the threshold of dissociation to CHzS and CHI', estimated as AH, = 102 kJ mol-1.6J5 Very recently, Baker and Dyke have generated 1 as a short-lived intermediate by the reaction of DMS with fluorine
-
* To whom correspondence should be addressed. .Abstract published in Aduance ACS Abstracts, April 15, 1994. 0022-365419412098-4845$04.50/0
atoms and measured its adiabatic and vertical ionization energies by photoelectron spectroscopy.16 In regard to the key role 1 plays in DMS oxidation, its intrinsic properties and reactivity in an isolated state are of interest. In the present work we take three different approaches to prepare radical 1 as an isolated species in the gas phase. First, we utilize flash pyrolysis under molecular flow conditions to generate 1, examine its stability on the millisecond time scale, and analyze it by electron ionization mass ~pectrometry.'~ Second, collisional dissociation of a CH3SCH2CH2SCH3*+ionic precursor at 8 keV is used to prepare neutral 1 of 4-keV kinetic energy, separate it from ionic fragments, and study its unimolecular dissociations on the microsecond time scale by collision-induced dissociative ionization.18J9b Third, neutralization of the stable CH3SCH2+ cation by collisions with atoms, molecules, and pyrolytically generated radical 1 is employed to prepare 1 and investigate its unimolecular and collision-induced dissociations by neutralization-reionization mass spectrometry.19 A new technique allowing variable-time measurements is introduced here and employed to assess dissociations of neutral 1 as compared with those of the cation 1+ formed by collisional reionization. To complement and interpret the experimental results, the electronic structure, molecular geometry, ionization energy, and heat of formation of 1 are assessed by a b initio calculations carried out at the G2(MP2) level of theory.*O
Experimental Part
Mass Spectrometry. Electron ionization mass spectra were measured on a tandem quadrupole acceleration-deceleration mass spectrometer described in detail recently.lobJ The ionization conditions wereas follows: electron energy 70 or 12 eV (nominal), electron current 400 PA, ion source temperature 150 O C . Collisionally activated dissociation (CAD) spectra were obtained on a Kratos Profile HV-4 double-focusing mass spectrometer furnished with a grounded collision cell mounted in the first fieldfree region. Oxygen a t a pressure such as to achieve 70% transmittance of the ion beam was used for CAD, and the magnet (B) and electric (E) sectors were scanned in a linked mode while keeping the B/E ratio constant. The product-ion mass resolution was typically >200. Pyrolysis. Flash-vacuum pyrolyses were carried out in a special microoven adapted to fit a standard direct insertion probe. The pyrolytic probe consists of an alumina tube (McDanel Refractory, 25 cm long, 5.6-mm o.d., 2.7-mm i.d.) embedded in a 0.5-in. (12.7 mm)-0.d. polished stainless steel tube and sealed into a Conflat flange. A 25-mm-long hot zone at the end of the alumina 0 1994 American Chemical Society
4846
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
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Kuhns et al.
T depends on the neutral mass;21T = mf/mpeUa, - AT, where mf and mp are the neutral fragment and precursor masses, respectively, U,, is the acceleration voltage, and ATis the -20eV kinetic energy defect due to endothermic neutralization and r e i o n i ~ a t i o n . ~Ions ~ ~ .formed ~~ at potentials other than Udm are rejected. This is further utilized in neutral collisional activation, where helium is admitted in the conduit floated at +250 V to Reionization gas achieve 50% transmittance of the neutral beam and the neutral Neutralization gas products are reionized in the reionization cell. Ions formed by reionization within the conduit have kiloelectronvolttotal energies and are rejected.10a Variable-Time Measurements. For variable-time studies, reionization is carried out within the conduit to which oxygen is Voltage combinations admitted at a uniform pressure to achieve 70% transmittance of the ion beam at m / z 61 (Figure 1). Reionization thus can occur -0 kV - ions transmitted at any point in the conduit with probability proportional to the H neutral path length. The first variable-length part of the conduit a t +250 V stops and reflects the incoming precursor cations. Any ) variable-time NRMS ions formed by reionization in this part of the conduit have kiloelectronvolt total energies and are rejected, while reionization +250V I in the second, variable-length,part gives rise to detectable products I - standard NRMS at UdEsatisfying eq 1. The time of flight through the first conduit Figure 1. Segmented collision cell for variable-time reionization. segments charged to +250 V defines the minimum delay between neutralization and reionization, corresponding to the shortest tube is heated with 0.1-mm tantalum wire (Goodfellow) that is observable neutral lifetime. Likewise, the time of flight through wound in a bifilar groove and insulated by rolled mica sleeves, the negatively charged conduit segments, in which detectable a tantalum radiation reflector, and a quartz shield. The hot zone cations are formed by reionization, defines the maximum extends to within 8 mm of the ionization chamber. The mean observable ion lifetime. Extending the +250-V potential seresidence time within the hot zone is calculated as 1.6 ms at 823 quentially to the second, third, and fourth segment lengthens the K.17b922A flow restrictor consisting of a 0.5-mm4.d. alumina observation window for neutral dissociations, while shortening tube was inserted to separate the hot zone from the sample reservoir that for ion dissociations (Figure 1). From thevelocity of 1having and prevent back-diffusion of pyrolytic products. Hot zone m / z 61 and 8220-eV kinetic energy (v = 1.613 X lo5 m/s) one temperature was measured with a chromel-alumel thermocouple calculates the minimum neutral lifetimes in these measurements embedded in the alumina tube. Liquid samples were placed in as 0.36,1.05,1.75, and 2.44 ps, while the corresponding maximum a small bulb furnished with a Teflon needle valve (Chemglass) ion lifetimes are 3.78, 3.09, 2.39, and 1.70 ps, respectively. By and attached through a glass-metal seal and a 0.25-in. Swagelok comparison, in standard NRMS, in which reionization takes place adaptor to the flange. Sample vapor pressures in the ion source in the down-beam collision cell, neutral and ion dissociations of were maintained at (5-8) X 10-6 Torr. species of the above velocity are observed over 3.72 and 0.42 ps, Neutralization-Reionization. Standard and variable-time respectively.21 neutralization-reionization mass spectra were obtained with a new arrangement of segmented collision cells23shown in Figure Materials. Common chemicals were obtained from Aldrich 1 and referred to as the conduit. Cations produced in the ion and used as received. 1,2-Bis(methylthio)ethane (3) and 1,2source with -90-eV kinetic energy are accelerated to achieve bis(methy1-d3-thio)ethane (3a) were prepared by standard me8220-eV kinetic energy and subsequently neutralized in the first thylation with CHJ and CD31, respectively, of 1,a-ethanedithiol 20-mm-long collision cell floated at -8130 V. Xenon, cyclo(CH30Na, CH3OH, 1-h reflux under Ar). The products were propane, nitric oxide, dimethyl disulfide, and trimethylamine purified by distillation, bp 85 OC/12 Torr; ref 25 gives 80-82 (TMA) were admitted to the cell from a differentially pumped OC/ll Torr. 2-(Methy1thio)ethyl nitrite (4) was prepared by manifold at a pressure such as to achieve 70% transmittance of careful nitrosation of 2-methylthioethanol with aqueous NaN02 the ion beam of 1+. For collisional neutralization with 1, the and H2SO4 at -5 OC and purified by vacuum distillation to give pyrolytic probe was inserted through a vacuum lock into the a pale yellow liquid boiling at Zki, the integrated equations (1 6) and (1 7) predict that increasing the time interval for neutral dissociations ( t l ) , while decreasing that for ion dissociations ( t 2 - t , ) , will result in a decreasing [M+/XFi+]ratio; an opposite trend is expected for Ck, < Cki. Figure 7a,b shows the changes in the product ion relative intensities upon varying the dissociation lifetimes of 1 and reionized 1+. The ion precursor of 1 was produced from 3*+at 12 eV to minimize its internal energy. The relative abundanceof surviving 1+exhibits a 6-fold decrease upon decreasing the minimum neutral lifetime from 2.44 to 0.36 ps, while increasing that of reionized 1+from 1.70 to 3.78 ps. Hence, extending the observation time for ion dissociations while proportionately shortening that for neutral dissociations results in more extensive overall decomposition. It should be noted that changing the dissociation time scale between 0.4 and 3.7 ps has the greatest effect on the fractions of 1 and 1+dissociating with k, and ki in the 105-106-s-' range. By contrast, very fast ( k > lo7 s-l) neutral and ion dissociations are not affected by the changing time scales, and hence they cannot be distinguished. The variable-time spectra show that the fraction of 1dissociating with k lO5-lO6is small but increases in reionized l+. Although the energy content and distribution in 1 and 1+ are unknown, crude estimates using the simplified quasi-equilibrium theory formula49indicate that in order to dissociate with the above rate constants radicals 1 must have internal energies
