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J. Phys. Chem. 1991, 95, 3600-3606
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log
(
lifetime / days
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Figure 7. Lifetime of PAN as a function of altitude up to 12 km: (solid line) reaction with OH radicals; (dashed line) photolysis; (dotted line)
thermal decomposition.
Atmospheric Implications. PAN is a major component of heavily polluted air, reaching mixing ratios of 50 ppbv and higher. Typical mixing ratios are 2 ppbv for rural areas36and 30 pptv for the clean marine atmosphere? For the free troposphere, mixing ratios between 10 and loo0 p p t ~ ~have ~ J 8been measured at heights of 1-5 km above the ground, and a nearly constant mixing ratio of ca. 20 pptv was calculated for the background air from the ground up to the ~tratosphere?~Furthermore, NO, can be camed (36) Temple, P. J.; Taylor, 0. C. Atmos. Emiron. 1983, 17, 1583. (37) Singh, H.B.;Salas, L. J. Atmos. Emiron. 1983, 17, 1507. (38) Meyrahn, H.; Hahn, J.; Helas, G.;Wameck, P.; Penkett, S. A. Proc. 3rd Eur. Conf Physico-Chem. Behaviour Atmos. Pollutants, Varese 1984,
38.
(39) Aikin, A. C.; Herman, J. R.; Maeir, E. J. R.; McQillan, C. J. Planet. Space Sei. 1983, 31, 1075.
to unpolluted areas by long-range transport." The only reaction producing PAN is the recombination of acetylperoxy radicals with NO2, reaction 1. The thermal decomposition of PAN, reaction -1, is considered to be the only important gas-phase loss process of PAN close to the ground. For this reason, reactions 1 and -1 are important input parameters for atmospheric model calculations. While the k-l values a t atmospheric pressure of the present work agree with those of previous work, the recombination rate constant kl is higher by almost a factor of 2 as compared with the most recent literature values,'* thus leading to a corresponding increase of PAN formation a t the expense of the competing reaction of acetylperoxy radicals with NO, reaction 2. As the pressure dependences of kl and klhave also been determined, the thermal lifetime of PAN can be calculated with confidence for all conditions of temperature and pressure relevant to the troposphere and stratosphere. They are shown in Figure 7 together with the lifetimes corresponding to the other loss processes, e.g., the reactions with OH radicals" and photoly~is.~~ Decomposition is thus the dominant loss prooess for PAN a t low altitude while it becomes negligible near the tropopause.
Acknowledgment. We thank M.-T. Rayez for performing the MNDO/UHF calculations. Work in Bordeaux was supported in part by the Action Thtmatique Programmte "Phase Atmosphtrique des Cycles BiogBochimiques" and by the Environment program of the CEC. The work in Wuppertal was supported within the LACTOZ program by a CEC grant. (40) Nielsen, T.; Samuelsson, U.; Grennfelt, P.; Thomsen, E.
1981, 293, 553.
(41) Senum, G . I.; Lee, Y.N.; 1269.
L. Nature
Gaffney, J. S.J. Phys. Chem. 1984,88,
Unlmdecular Dissociation Kinetics and Collision-Induced Dlssociation of [Mn,( CO) Determination of I ndividuai Bond Dissociation Energies Wen YuJ Xiangqiu Liang,t and Royal B. Fceas* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: February 6, 1990; In Final Form: November 9, 1990) Unimolecular and collision-induced dissociations of the dimanganese decacarbonyl molecular ion [Mn2(CO)lo]+ have been studied by using tandem mass spectrometry. The molecular ion [Mn2(CO)lo]+was formed at different ion source temperatures from the neutral parent carbonyl by using electron ionization with 12-,30-,SO-,and 70-eV ionizing electrons. Gas-phase unimolecular dissociations of mass-selected, long-lived metastable [Mn2(CO)lo]+species were detected from 10 to 1000 ps after entering a radiofrequency-only quadrupole ion guide. The unimolecular dissociation rate constants of [Mn2(CO)lo]+ produced under different ionization/excitation conditions are reported. Low-energy (0-10 eV) collision-induceddissociation (CID) threshold energies of [Mn2(CO)lo]+were also measured. Statistical rate (RRKM) theory was used to relate the unimolecular dissociation rate constants to the internal energy of the metastable ions. The fragment ions appear to arise by sequential stepwise losses of CO. The CID threshold energies of fragment ions from [Mn2(CO),o]+corrected for ion internal energy are reported, yielding individual bond dissociation energies. The results suggest the presence of isomers containing bridging carbonyls.
Introduction The gas-phase chemistry of dimanganese-containing ions has been a subject of considerable intere~t.l-~Moreover, the strength of the Mn-Mn bond and Mn-CO bonds are the subject of some question. Reports of the Mn-Mn bond energy in M I I ~ ( C O ) ~ ~ range from 0.9S8 to 1.8 f 0.4 eV? The average Mn-CO bond dissociation energy in Mn2(CO)lowas reported to be 1.03 eV.IO On the other hand, laser/ion-beam photodissociation and colliPresent address: The Genetics Institute, Andover, MA 01810-5901. *Present address: Triangle Laboratories, Inc.. Houston, TX 77401. *To whom correspondence should be addressed.
sion-induced dissociation studies have determined bond dissociation energies for ions generated from M~I~(CO),,.~,~ The appearance (1) Larsen, B. S.;Freas. R. B.; Ridge, D. P.Kinetics of the Reactions of n-Donor Bases with Mn2+.Absolute Mn+ Affinities. J . Phys. Chem. 1984,
88,6014-6018.
( 2 ) Meckstroth, W. K.; Frcas, R. B.; Reents, Jr., W. D.; Ridge, D. P. Relationship between Structure and Reactivity for Metal Clusters Formed in Ion-Molecule Reactions in Decacarbonyldimanganese and Pentacarbonyl(pentacarbony1dimanganio)rhenium. Inorg. Chem. 1985, 24, 3139-3 146. (3) Lichtin, D. A.; Bernstein, R. B.; Vaida, V. Multiphoton Ionization Time-of-Flight Mass Spectrometry of Transition-Metal Complexes: Mn2(CO),o and Re,(CO),,. J . Am. Chem. Soc. 1982, IO#, 1830-1834.
0022-3654/91/2095-3600$02.50/00 1991 American Chemical Society
Kinetics of [Mn2(CO)lo]+
The Journal of Physical Chemistry, Vol. 95, No. 9, 1991 3601
B
namics of [Mn(CO),]+ ( x = 1-6) was examined." In these experiments, potential surfaces for ligand loss were probed by measuring the kinetic energy release during metastable decomposition. Calculations using RRKM theory to determine metastable ion internal energy followed by phase space calculations yielded individual D[(CO),IMn+-CO] bond dissociation energk. The ion internal energy was estimated from the constraints of observation of metastable decay for the ion lifetime (5-20 ps) in the second field-free region of a mass spectrometer. Thus, statistical rate theoriesi8describing unimolecular dissociations can be used to calculate the internal energy of metastable ions from unimolecular dissociation rate constants." In this paper, we present the use of RRKM theory to correlate experimentally determined unimolecular dissociation rate constants with the internal energy distribution of metastable ions. The apparent threshold energies obtained from collision-induced dissociation (CID) experiments are corrected for ion internal energy to yield corrected CID threshold energies and bond dissociation energies.
E
Figure 1. Schematic diagram of the instrument used in these studies.
potential for the formation of the molecular ion [Mn2(CO)lo]+ is 8.5 f 0.1 eV.11J2 Reports of the Mn-Mn bond dissociation energy in [Mn2(CO)lo]+range from 0.8212 to 0.96 eV.* The average bond dissociation energy of the first five Mn-CO bonds in (Mn2(CO)lo]+is reported to be 0.7 eV.I2 Photodissociation experiments have provided upper limits for the Mn-Mn bond dissociation energy (>1.9 eV) and for the average Mn-CO bond energy (
