9352
J. Phys. Chem. 1991,95, 9352-9359
SCHEME 11 344BPY* ..*H-OH + amine
I
+
3(44BPYo-***H-OH,amin@+)
MBPYH' + OH-,^,
+
-
amineo* rdv
44BPYo-,dv
+ amineo+rdv
k-
44BPYHO
is the main reduction product ObSeWed in this case indicates that the rate constant for proton transfer is higher than the dissociation rate constant. In conclusion, the reaction mechanism proposed in Scheme I is quite similar to that established for the reduction of triplet ketones by amines in P l a r solvents.3640 From femtosecondpicosecond photolysis studies, Mataga et have recently characterized the triplet ion pair formed on photoreduction of benzophenone by dimethylaniline in aceonitrile. The ion-pair (lifetime 140 f 13 PS) was found to decay by proton transfer to form a ketyl radical (kl = (5.4 0.5) x IO9 s-') competing with ionic dissociation (k2 = (1.4 f 0.2) x io9 s-'). Assuming kinetic constants of comparable order of magnitude in the case of 44BPY is consistent with the fact that the ion pair has not been detected
*
Cohen, S. G.; Cohen, J. 1. J . Phys. Chem. 1968, 72,3782. Cohen, S. G.; Chao, H. M. J . Am. Chem. Soc. 1968, 90,165. Davidson, R. F.; Lambeth. P. F. Chem. Commun. 1968, 51 1 . Dcvadoss, C.; Fessenden, R. W. J . Phys. Chem. 1990, 94, 4540. (40)M i y a k , H.; Morita, K.;Kamada, K.;Mataga, N. Bull. Chem. Sac. Jpn. 1990,63,3385.
(36) (37) (38) (39)
in the present study limited to a time resolution of 20 ns. This is also consistent with our observation that the triplet-state decay and the 44BPY'- and 44BPYH' formation have similar kinetics. Finally, the results obtained in the presence of small amounts of water ( 10-2-10-' M) in aceonitrile indicate that, in the case of reduction by DABCO, protonation by water follows rapidly the transfer of electron. In the case of reduction by TEA, the fact that in acetonitrile/D20 solutions the 44BPYD' species is observed at short times instead of 44BPYH' indicates that intra-ion-pair proton transfer from the amine cation is quenched by proton transfer from water and suggeststhat hydrogen bonding in the excited state favors this transfer, according to Scheme 11, These results do not elucidate the origin of the formation of 4 4 ~ p y from ~ - reduction of 4 4 ~ p by y DABCO. They confirm that, if trace amounts of water are present in aceonitrile, rapid protonation of the anion radical via hydrogen bonding must take place within the ion pair, according to Scheme 11. However, direct protonation by DABCO*+(Scheme 1) may also be considered.39 I,, any case, the observation in ~i~~~~2 that the 44BpYH' R~~~~ spectrum does not grow as the 44BPY'- spectrum decays, but is maximum a t the shortest time delay (40 ns), confirms that the formation of radical 44BPYH' does not arise from protonation of the free 44BPY'- species in the solution but takes place within the initial ion pair.
Acknowledgment. We thank N. Leygue for technical assistance in the synthesis and solvent purification. Registry NO. 44BPY, 553-26-4; DABCO, 280-57-9; TEA, 121-44-8; 44BPY'-, 34475-1 1-1; 44BPYH', 58814-81-6; 44BPYH2,64429-05-6; 44BPYH;+, 35862-62-5; 44BPYH?+, 46040-54-4.
Improved Source of NF(b'Z+) Molecules: Spectroscopic Constants and Evidence for € 4Energy-Transfer Reactions with CH4 and H20 K. Du and D.W.Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received: May 13, 1991; In Final Form: July 5, 1991)
A low-power dc discharge in a dilute flow of NF2 in Ar carrier gas provides a source of NF(b) molecules in the concentration ~ , is 20-50 times larger than the concentration from the reaction of Ar('POz) with range (2-5) X 10" molecules ~ m - which NF2 for the same conditions in the flow reactor. The discharge source gives high vibration levels, v' 5 11, and the NF(b'Z++X3Z-) spectrum had strong bands for the Au = f 2 sequences. The extended spectrum from high-v'levels provides improved vibrational constants, a", = 1 1 36.7 f 3.3 and ~ " ~ x '=: 8.9 f 0.9 cm-l; w', = 1191 .I f 3.7 and w ' p : = 8.2 f 1 .O cm-I. The dependence of the NF(b-.X) transition dipole upon the f centroid was assigned as -5.4075 + 9.0695F - 3.21839 for 1.0I ? I 1.8 A. The direct observation of NF(alA) formation from the quenching of NF(blZ+) by CHI and H 2 0 proved that E-Y energy transfer is the major channel for NF(b) deactivation. The E-V quenching data are consistent with the accepted radiative lifetime of NF(a).
Introduction The reactions of NF(alA) and molecules have been the subject of several recent studies,'-* since the NF(a,b) states could ( I ) Du, K.; Setser, D. W. J . Phys. Chem. 1990,94,2425; 199L95.4729. (2) (a) Cha, H.; Setser, D. W. J . Phys. Chem. 1987, 91,3758. (b) Cha, H.; Setser. D. W. J . Phys. Chem. 1989.93, 235. (c) Bao, X. Y.; Setser, D. W. J . Phys. Chem. 1989, 93, 8162. (3) (a) Pritt, A. T., Jr.; Benard, D. J. J . Chem. Phys. 1986,85,7159. (b) Young, R. A.; Blauer, J.; Lin, C. L. J . Chem. Phys. 1987, 87, 4634. (4) (a) Clpe, M. A. A.; White, 1. F. Chem. Phys. Lelr. 1970, 26,465. (b) Cheah, C. T.; Clyne, M. A. A. J . Phorochem. 1981, IS, 12. ( 5 ) Pritt, A . T., Jr.; Patel, D.; Cwmbe, R. W. Int. J . Chem. Kine!. 1984, 16, 977. (6) Habdas, J.; Setser, D. W . J . Phys. Chem. 1989, 93, 229. (7) Benard, D. J.; Winker. B. K.; M e r , T. A,; Cohn, R. H. J . Phys. Chem. 1989, 93, 4790.
have application as energy storage systems for chemical lasers. The quenching rate constants and mechanisms for NF(a) and N W ) have been systematically determined for a variety of reagents in this laboratory by using flow reactor methods.Is2 In our-laboratory the NF(a) molecules.are generated by the reaction of HN3 with excess F atoms,' and concentrations as high as 1 X l O I 3 molecules cm-3 can be obtained. The NF(b) molecules were generated by the reaction of Ar(3Po,2)with NF2; the [NF(b)] was by the [Ar,3Po,2] limited to less than 1 X 1Olo molecules provided by the dc discharge. There is a need for a clean NF(b) source that gives higher concentrations in order to study self~
~~
~~
(8) (a) Heidner, R. F.; Helvajian, H.; Holloway, J. S.; Koffend, J. B. J . Phys. Chem. 1989, 93, 7813. (b) Herbelin, J. Chem. Phys. Left. 1987,133, 331. (c) Holloway, J. S.;Koffend, J. B. J . Phys. Chem. 1991, 95, 1645.
0022-365419 112095-9352%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9353
Improved Source of NF(b’Z+) Molecules destruction reactions and to identify product states from quenching reactions. We found that passing NF2 with Ar through the dc discharge enhanced the NF(b) concentration, relative to the [NF(b)] generated by reacting NF2 with the Ar(3Po,2)from the same discharge by a factor of 20-50. On the basis of strong correlation between the quenching rate constants and the magnitude of the vibrational frequencies of the reagent,2 E-V transfer with formation of NF(a) has been postulated to be the major product channel for NF(b) deactivation., However, there is no direct proof of this mechanism from observation of the vibrationally excited product molecules (except for HF)6 or for observation of NF(a). The higher [NF(b)] from the discharge source in the present work enabled us to observe NF(a) formation and show that the quenching of NF(b) by CH4 (and C2H6)and H,O does proceed via E-V transfer. This discharge source gave vibrationally excited NF(b,u’ll I ) , and measurement of the new emission bands gives improved vibrational constants for NF(X) and NF(b) and the dependence of the transition dipole upon i centroid. The currently accepted spectroscopic constants of the NF(X) and -(b) states were reported in 1966 by Douglas and Jones9 from high-resolution measurements on bands from u ’ l 2 . Some qualitative observations also are made about vibrational relaxation of NF(b,u’
