24 J. Phys. Chem. A, Vol. 107, No. 1, 2003
Tweeten et al.
TABLE 4: Energy (cm-1) of Several Possible Product
product fluorescence spectra from both molecules showed a
mixture of I2 in the D, D′, and E states. The D′ peak in the
spectra arising from CHI3 was found to be weaker than that
from CI4, while the E state emission band was enhanced.
The ab initio results show that the dissociation proceeds
through a transition state to an ion-pair isomer. The isomer can
then dissociate into an atomic or molecular product channel with
the molecular channel having the largest barrier. The calculated
IRC pathway and the structure of the molecular transition state
agree well with the asynchronous concerted dissociation mech-
anism proposed by Dantus and co-workers.
a
Channels for the Photodissociation of CHI3 and CI4
photoproducts
energy
excess energyb
HCI (singlet) + I2 D(1441, 1Σu
)
82 519
92 064
97 710
73 095
80 767
86 413
21 108
11 563
5917
30 532
22 860
17 214
+
HCI (triplet) + I2 D′(1432, 3Π2g)
HCI (triplet) + I2 E(1432, 3ΠO g)
+
CI2 (singlet) + I2 D(1441, 1Σu
)
+
c
CI2 (triplet) + I2 D′(1432, 3Π2g)
CI2 (triplet) + I2 E(1432, 3ΠOg
)
a All reactant and product energies are from this work except the
singlet-triplet gaps for HCI and CI2, which are taken from ref 21.
Excited-state I2 energies were obtained by adding the vertical energy
values of Mulliken20 to the calculated ground-state values from this
work. Energies are calculated relative to the CHI3 (or CI4) minimum.
Acknowledgment. We thank Dr. Jean Standard for help with
the ab initio calculations and for a critical review of this paper
prior to publication. Acknowledgment is made to the Illinois
State University Research Grant program for support of portions
of this work.
b Starting with 103 627 cm-1 c Assuming singlet ground state.
.
in energy. The lower energy pathway is the nonsymmetric attack
of the Lewis acidic p orbital on the central carbon of the carbene
by a lone pair of the halogen. Their calculations predict an initial
structure similar to the molec TS structure shown in Figures 5
and 6. This is not surprising because in the reverse (dissociation)
mechanism one would expect the final structure to be similar
to the initial structure of the addition reaction.
References and Notes
(1) Molina, M. J.; Moline, L. T.; Kolb, C. E. Annu. ReV. Phys. Chem.
1996, 47, 327.
(2) See, for example: Bergmann, K.; Carter, R. T.; Hall, G. E., Huber,
J. R. J. Chem. Phys. 1998, 109, 474 and references therein.
(3) Marvet, U.; Zhang, Q.; Brown, E. J.; Dantus, M. J. Chem. Phys.
1998, 109, 4415.
The NPA also confirms the work of Cain et al.23 As the
dissociation proceeds from the ion-pair isomer, both C2 and I5
lose charge. At the molec TS, C2 has lost 0.220 electrons and
I5 has lost 0.239. Some of this loss is redistributed to I1 and I2
and the rest to I4. However, the key point is that I4 is still
positively charged, while C2 and I5 are still negatively charged.
This is not only in keeping with the work of Cain et al., but it
shows that at the molec TS, which is a late transition state, there
is still significant charge separation (0.429) between I4 and I5.
This strongly suggests that I2 is formed in an ion-pair state.
From the ion-pair isomer to the molec TS, there is an overall
shift of 0.459 electrons, which means that this transition state
would be expected to be high in energy. For CHI3, the molec
TS is 15 923 cm-1 above the ion-pair isomer and 28 137 cm-1
above initial reactant. For CI4, it is 15 624 cm-1 above the ion-
pair isomer and 28 133 cm-1 above the initial reactant. However,
on the basis of the experimental excitation energy, in both cases,
this leaves more than enough excess energy to form highly
excited photoproducts. Table 4 shows the remaining energy for
each product after 2 × 193 nm excitation. There is 5917-30 532
cm-1 of excess energy available to the photoproducts indicating
that all photofragments should have significant translational,
rotational (HCI only), and vibrational energy. It should also be
noted that for CHI3 forming the triplet HCI and I2 in the E state
requires 97 710 cm-1 of energy, which is 1556 cm-1 more
energy than that available in Dantus’ experiment. Because CH2I2
would be expected to have a larger barrier to molecular product
formation than CHI3, it is not surprising that they did not observe
fluorescence from E state I2.
(4) Zhang, Q.; Marvet, U.; Dantus, M. J. Chem. Phys. 1998, 109, 4428.
(5) Marvet, U.; Brown, E. J.; Dantus, M. Phys. Chem. Chem. Phys.
2000, 2, 885.
(6) Farmanara, P.; Stert, V.; Ritze, H.-H.; Radloff, W. J. Chem. Phys.
2000, 111, 1705.
(7) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(8) Hay P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270.
(9) Hay P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284.
(10) Hay P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(11) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; pp 1-28.
(12) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J.
E.; Weinhold, F. NBO 4.0; Theoretical Chemistry Institute, University of
Wisconsin: Madison, WI, 1996.
(13) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1983, 78, 4066.
(14) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J. Chem. Phys. 1985,
83, 735.
(15) Kawasaki, M.; Lee, S. J.; Bersohn, R. J. Chem. Phys. 1975, 63,
809.
(16) Okabe, H.; Kawasaki, M.; Tanake, Y. J. Chem. Phys. 1980, 73,
6162.
(17) Dyne, P. J.; Style, D. W. G. J. Chem. Soc. 1952, 2122.
(18) Style, D. W. G.; Ward, J. C. J. Chem. Soc. 1952, 2125.
(19) Hemmati, H.; Collins, G. J. Chem. Phys. Lett. 1980, 75 (3), 488.
(20) Mulliken, R. S. J. Chem. Phys. 1971, 55, 288.
(21) Hajgato, B.; Nguyen, H. M. T.; Veszpre´mi, T.; Nguyen, M. T. Phys.
Chem. Chem. Phys. 2000, 2, 5041.
V. Conclusions
(22) Maier, G.; Reisenauer, H. P. Angew. Chem., Int. Ed. Engl. 1986,
25, 819.
(23) Cain, S. R.; Hoffmann, R.; Grant, E. R. J. Phys. Chem. 1981, 85,
4046.
The two-photon photodissociations of CI4 and CHI3 have been
examined using both dispersed fluorescence and ab initio
methods. With 103 627 cm-1 of available energy, the photo-