J . Phys. Chem. 1987, 91, 3758-3767
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extensive complex vibrational redistribution prior to dissociation leading to the population of a wide distribution of fragment 2AP* vibrational levels. 2. Hydrogen-Bonded Complexes: H 2 0 , MeOH, EtOH. a. Bond Energies. Changes of fluorescence lifetimes and one-color photoionization have been used to place upper limits on the hydrogen-bond energies of these complexes. Results show excited-state complex bond energies of approximately 1300 cm-I for 2AP-H20 and 1500 cm-’ for 2AP-MeOH and -EtOH. From electronic spectral shifts and ionization potectial stabilization energies, hydrogen-bond energies for So and X+ have been determined. Ionic complexes have bond energies of about 3 times those of the excited-state neutral species. b. Geometries. Calculations show that excited-state geometries of these complexes are probably planar and in a cyclic double bond arrangement. Ground-state geometries are likely more complicated due to the nonplanar nature of the 2AP molecule about the amino nitrogen atom. The nonplanar-to-planar geometry change
upon electronic excitation is suggested to be the source of the high degree of excitation of intermolecular complex vibrations in our spectra. 3. 2-Aminopyridine Dimer. We have reported the electronic spectrum of the dimer of 2AP. The dimer shows a very large spectral shift of almost -1 700 cm-I. The geometry of the dimer has been suggested to be a double hydrogen-bonded arrangement involving an amino hydrogen and the pyridine lone electron pair on each monomer subunit. The large excited-state stabilization energy of the dimer may arise from the achievement of planarity in the excited state compared with a nonplanar-twisted geometry in the ground electronic state.
Acknowledgment. We acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada. Registry No. 2AP, 504-29-0; H20,7732-18-5; MeOH, 67-56-1; EtOH, 64-17-5; Ar, 7440-37-1; CH,, 74-82-8.
NF(b‘Z+) Quenchhg Rate Constants by Halogens and Interhalogens and the Excitation Rate Constant for IF(B) Formation Hyungki Cha and D. W. Setser* Chemistry Department, Kansas State University, Manhattan, Kansas 66506 (Received: December 29, 1986; In Final Form: March 13, 1987)
The flowing afterglow technique with AI carrier gas (300 K) was used to measure the total quenching rate constants of NF(b’Z+) by halogen and interhalogen molecules The NF(b’Z+) radicals were prepared by the dissociative excitation-transfer reaction between Ar(3Po,2)metastable atoms and NF,. The quenching rate constants, which are in the (0.35-1 5 ) X IO-’’cm3molecule-’ s-’ range, increase with molecular weight of the halogens. In the reaction zone the NF(b) vibrational distribution was u,,’-uJI = 1.00:0.24:0.14:0.09; however, there was no obvious dependence of the quenching rate constants on 0’. The excitation rate constant for IF(B3110+)formation from NF(b), with a u4-u3’ distribution of 1.00:0.14:0.08:0.04,was measured as (1.4 0.2) X lo-’* cm3 molecule-’ s-I, which corresponds to a branching fraction of about 0.02. The only other reaction that gave emission was I2 with a similarly small 12(B3110,+)formation branching fraction. Qualitative measurements of the quenching rate constants for I atoms, CN radicals, ICN, and BrCN were obtained.
Introduction Efficient excitation-transfer reactions between a metastable species and a suitable acceptor molecule are of current interest because of possible applications to new laser systems. The success of the oxygen-iodine laser’ has encouraged study of NF, which is isoelectronic with O2but with higher energy metastable states. The NF(a’A) (1 1435 cm-’) and NF(b’8’) (18905 cm-I) states, with lifetimes of 5.6 s and 20 ms, respectiveIy,*q3can be studied by flowing afterglow techniques. Halogens and interhalogens are possible acceptors for NF(b) because the energy of NF(b) is sufficiently high that several halogen excited states can be produced. The I F system is particularly interesting because the IF(B3n0+)state is one of the more stable excited states4 and is in energy resonance with NF(b,u’=O).
+
-
NF(b’B+,u’=O) IF(X’Z+,u’’=O) NF(X3X-,u”=O) IF(B3110+,v’=O); AHoo = 48 cm-I ( l a )
+
-
other products
(1b)
The radiative and quenching properties of the IF(B) state have (1) (a) McDermott, W. E.; Pchelkin, N. R.; Benard, D. J.; Bousek, R. R. Appl. Phys. Lett. 1978, 32, 469. (b) Benard, D. J.; McDermott, W. E.; Pchelkin, N . R.; Bousek, R. R. Appl. Phys. Lett. 1979, 34,40. (c) Bachar, J.; Rosenwaks, S. Appl. Phys. Lett. 1982, 41, 16. (2) Malins, R. J.; Setser. D. W. J . Phys. Chem. 1981, 85, 1342. (3) Lin, D.; Setser, D. W. J . Phys. Chem. 1985, 89, 1561.
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been studied,eg and the feasibility of an electronic transition laser operating on the B-X transition has been demonstrated.’OJ1 Energy transfer from NF(b) to IF(X) has been reported,’* but quantitative studies are needed to ascertain the efficiency. This laboratory has developed a flowing afterglow source of This NF(b,v’l3) that is based upon the NF2 Ar(3P0,2)rea~tion.~ technique has been shown to be suitable for measurement of quenching rate constants, and in this work the method was adapted for a comprehensive study of NF(b) quenching by halogens. The total quenching rate constants at room temperature were measured
+
(4) Clyne, M. A. A.; McDermid, I. S. J . Chem. SOC.,Faraday Trans. 2 1976, 72, 2242. (5) Clyne, M. A. A.; Coxon, J . A.; Townsend, L. W. J . Chem. SOC., Faraday tram. 2 1912, 68, 2134. (6) Clyne, M. A. A.; McDermid, 1. S . J . Chem. SOC.,Faraday Trans. 2 1916, 72, 2252. (7) Clyne, M. A. A.; McDermid, I. S. J . Chem. Soc., Faraday Trans. 2 1977, 73, 1094. (8) Clyne, M. A. A,; McDermid, I. S. J . Chem. Sor., Faraday Trans. 2 1978, 74, 1644. (9) (a) Davis, S. J.; Hanko, L.; Shea, R. F. J . Chem. Phys. 1983, 78, 172. (b) Wolf, P. J. Ph.D. Dissertation, Air Force Weapons Lab, 1984. (10) Wolf, P. J.; Glover, J. H.; Hanko, L.;Shea, R. F.; Davis, S. J . J . Chem. Phys. 1985,82, 2321. (1 1) Davis, S. J.; Hanko, L.; Wolf, P. J . J . Chem. Phys. 1985, 82, 4831. (12) (a) Pritt, A. T. Jr.; Patel, D.; Benard, D. J. Chem. Phys. Lerr. 1983, 97,471. (b) Pritt, A. T.; Benard, D. J. In Gas-Phase Chemiluminescence and Chemiioniration; Fontijn, A., Ed.; North-Holland: Amsterdam, 1985; Chapter 15. (c) Pritt, A. T. Jr.; Benard, D. J. J. Chem. Phys. 1986, 85, 7159. (13) Trickl, T.;Wanner, J. J . Chem. Phys. 1983, 78, 6091.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 14, 1987 3759
NF(blZ+) Quenching Rate Constants from the decay of the NF(b) concentration in a flow reactor with Ar carrier gas in the presence of known reagent concentration. The NF(b) vibrational distribution varied from vo:vl:v2:v3= 1.00:0.24:0.14:0.09 to 1.00:0.12:0.03:0.01 over the length of the reactor used to observe quenching. Electronic quenching by halogens was found to be more rapid than vibrational relaxation, and the NF(b) vibrational distribution did not change with added halogen. There was no obvious dependence of the quenching rate constants on the vibrational level of NF(b). We report the total quenching rate constants for all diatomic halogens, for all diatomic interhalogens except BrC1, and for IF5 and BrF5. In the course of these studies, estimates for the quenching rate constants for 1 atoms, C N radicals, BrCN, and ICN were obtained. The unstable interhalogen molecules were generated by the rapid reaction of excess fluorine atom with halogen-containing precursor molecules in the reactor: C1F ( F Cl2),I4BrF ( F Br2I4and F CCI3BrIS),and I F ( F + 12,14F CF31,16and F ICl14). The quenching rate constants for the other product from the precursor reactions were examined for the I F case in order to be able to assign the quenching rate constant to just IF. It is for this reason that several precursor systems yielding I F were examined. The branching fraction for reaction l a and the vibrational distribution of IF(B) also were measured. The excitation-transfer rate constant for IF(B) formation was deduced from the IF(B-X) emission intensity, the NF(b-X) emission intensity, and the concentration of IF. The branching fraction for IF(B) formation was small (
