5870
J. Phys. Chem. 1987, 91, 5870-5872
Resonance Enhanced Multlphoton Ionlzatlon Spectroscopy of CHCI, and CDCI, George R. Longt and Jeffrey W. Hudgens* Chemical Kinetics Division, Center for Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: March 16, 1987; In Final Form: June 29, 1987)
Resonance enhanced multiphoton ionization spectra between 355 and 375 nm of CHC12and CDClz radicals are reported. The origin of the observed spectra was at 370.1 nm for CHC12and 370.4 nm for CDC12. The REMPI mechanism is determined to be a 2 + 1 photon absorption, with the resonant intermediate state tentatively assigned as a 3d Rydberg state. Vibrational structure originated from activity in the C-CI symmetric stretching and the out-of-planebending modes. The C-C1 symmetric stretch frequencies are 845 cm-I in CHC12 and 814 cm-’ in CDC12.
Introduction Resonance enhanced multiphoton ionization (REMPI) spectroscopy has proven to be an effective tool for the ultrasensitive detection of reactive intermediate species.’ As such, it may be used for the elucidation of reaction mechanisms and to provide new structural information on reactive intermediates. The technique shows great potential for eventually equaling laser excited fluorescence (LIF) in popularity as a laser diagnostic technique. But to achieve this level of popularity, a larger spectroscopic data base for the detection of reactive intermediates is needed. As a step toward this goal, this paper reports the first observation of the dichloromethyl radical (CHCl,) by REMPI spectroscopy. CHC12 has been proposed as an intermediate in a number of reactions including the pyrolysis of pentachloroethane2 and the photolysis of c h l ~ r o f o r m . ~ The dichloromethyl radical was first detected by infrared spectroscopy in an argon matrix by Carver and Andrews4 and by Rogers et aLs Carver and Andrews assigned the absorptions observed at 1226 cm-I to the ~s”(b2)H-C-CI deformation and at 902 cm-’ to the ug))(b2) C-Cl antisymmetric stretch. A normal mode analysis showed that the C-CI stretching force constant was significantly larger than that observed for chloromethanes. Carver and Andrews proposed that ( p - p ) ~bonding between the carbon and chlorine atom stabilized the CHC12 radical and accounted for the enhanced the C-C1 bond strength.6 Electron spin resonance (ESR)’ studies of dichloromethyl radical indicate that the radical is nearly planar. The nearly planar structure is also supported by the photoelectron spectroscopy of gas-phase CHC128and by molecular orbital calculation^.^ The dichloromethyl cation has been extensively studied in the gas phase by photoelectron spectroscopy9 and in Ar matrix by infrared absorption spectroscopy.2s’0*1’ Using photoelectron spectroscopy, Andrews et aL8 measured gas-phase v21/(al) C-CI stretching frequencies in CHCl,’ of 860 (30) cm-’ and in CDC12+ of 790 (30) cm-I. They also measured the adiabatic ionization potential to be 8.32 eV. A planar cation structure is expected. Little is known about the electronic states of the dichloromethyl radical. Emmi et al.” have recently reported an UV absorption spectrum of CHC12 in solution between 220 and 330 nm. No analysis of this electronic state was given. In this study we present the first analysis of an excited electronic state of the dichloromethyl radical. Experimental S e ~ t i o n ’ ~ The apparatus used in this study has been described e1~ewhere.l~ The apparatus consisted of a flow reactor which produced the free radical species, an excimer pumped dye laser or a Nd:YAG laser pumped dye laser which ionized the radicals, a time-of-flight mass spectrometer, and a computer/data-acquisition system. Free radicals produced in the flow reactor effused into the ion source of the mass spectrometer where they were ionized by a focused laser beam (band width = 0.2 cm-’ fwhm; fl = 250 mm). Pressure ‘Postdoctoral Associate. *Author to whom correspondence should be addressed.
0022-365418712091-5870$01.50/0
within the ion source was about 5 X Torr. The laser-generated ions were mass selected and detected by the mass spectrometer. The ion signal was recorded as a function of wavelength to produce the REMPI spectrum. Although spectra were observed only between 355 and 375 nm, we also examined the wavelength intervals: 345-550, 325-340, and 302-315 nm. The spectra reported here were observed by using an excimer laser pumped dye laser with DMQ (Exciton Chemical) laser dye. Between 355 and 375 nm the dye laser energy varied between 8 and 15 mJ/pulse. The intensities of the REMPI spectra are uncompensated for laser energy. In conformity with custom, wavelengths are reported in reference to air medium. Reagents used in this study were CH2C1, (Mallinckrcdt; reagent grade), CDzC12(99% D; MSD Isotopes), F2 (Spectra Gases, Inc.), and helium (99.99%; Matheson). Results and Analysis The reaction of atomic fluorine with dichloromethane is known to produce the dichloromethyl radical as its principal p r o d ~ c t . ~ ~ , ’ ~ CHzClz + F H F + CHCl2 (1) When the effluent of this reaction was irradiated, laser-generated ion signals appeared at m l z 83, 85, and 87. Since the natural abundance of 35Cland 37Clare 75.5% and 24.576, respectively,” these results indicate that the structure of the spectral carrier includes two chlorine atoms. The intensities of mlz 83, 85, and 87 depended upon the partial pressures of fluorine and dichloromethane and required the presence of a microwave discharge. This evidence indicates that the ion signal is carried by a product of the reaction of atomic fluorine and dichloromethane. When fluorine was reacted with CD2C12,the ion signal appeared +
(1) Ashfold, M. N. R. Mol. Phys. 1986, 58, 1. (2) Benson, S. W.; Weissman, M. Int. J . Chem. Kinet. 1982, 14, 1287. (3) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1971, 54, 3935. (4) Carver, T. G.; Andrews, L. J . Chem. Phys. 1969, 50,4235. ( 5 ) Rogers, E. E.; Abramowitz, S.; Jacox, M. E.; Milligan, D.E. J. Chem. Phys. 1970, 52, 2198. (6) Andrews, L.; Smith, D. W. J . Chem. Phys. 1970, 53, 2956. (7) Mishra, S. P.;Neilson, G. W.; Symons, M. C. R. J . Chem. SOC., Faraday Trans. 2 1973, 69. 1425. (8) Andrews, L.; Dyke, J. M.; Jonathan, N.; Keddar, N.; Morris, A. J . Chem. Phys. 1983, 79, 4650. (9) M o h o , L. M.; Poblet, J. M.; Canadell, E. J . Chem. SOC.,Perkin Trans. 2 1982, 1217. (10) Jacox, M. E. Chem. Phys. 1976, 12, 51. (11) Kelsall, B. J.; Andrews, L. J . Mol. Spectrosc. 1983, 97, 362. (12) Emmi, S. S.; Beggiato, G.; Casalbore-Miceli, G.; Fuochi, P. G. J . Radioanal. Nucl. Chem. Lett. 1985, 93, 189. (13) Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental procedure. In no case does an identification imply recommendation or endorsement by the National Bureau of Standards, nor does such an identification imply that the material or equipment identified is necessarily the best available for that purpose. (14) Long, G. R.; Johnson, R. D.; Hudgens, J. W. J . Phys. Chem. 1986,
-.(4901 I 5 ) LeBras, G.; Butkovskaya, N. I.; Morozov, I. I.; Talrose, V. L . Chem.
-90.
Phys. 1980, 50, 6 3 . (16) Nazar, M. A.; Polanyi, J. C. Chem. Phys. 1981, 55, 299. (17) Handbook of Chemistry and Physics; Weast, R.C., Ed.; Chemical Rubber Co.: Cleveland, Ohio, 1972; p B-254.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987 5871
REMPI Spectroscopy of CHC12 and CDC12 TWO PHOTON ENERGY ( l / c m )
56000 1
"
"
I
"
"
TABLE I: Observed REMPI Bands of CHClz and C D Q and Their Assignments
54000
55000 l
'
laser wavelength, nm
2b
CHC12
1
i Q
Z
u H ffl
z
two-photon energy, cm-' CHCli
freq interval, cm-l
spectral assignt
370.1 366.0 364.4 360.4
54 024 54 629 54 869 55 479
0 605 845 1455
08 41 2; 2l4;
358.7
55 742
1718
2;
370.4 366.8 364.9 361.4
53 980 54510 54 794 55 325
0 530 814 1345
08
359.2
55 664
1684
2;
0 H
1
,
,
,
,
/
355
,
,
1
1
1
1
1
1
1
1
# I
1
370 LASER WAVELENGTH (nml Figure 1. REMPI spectrum of the CHC12radical between 355 and 375 360
365
TWO PHOTON ENERGY I l / c m ) 55000 54000
56000 /
l
355
l
/
"
l
"
l
I
l
"
I
360
,
"
l
I
I
'
I
I
365
I
I
J
I
I
I
370
LASER WAVELENGTH (nml F i p e 2. REMPI spectrum of the CDClz radical between 355 and 375
nm. at m J z 84, 86, and 88, which showed that the spectral carrier contains one hydrogen. Thus, all evidence is consistent with our assignment of the spectral carrier to the dichloromethyl radical. The magnitude of the REMPI signal from dichloromethyl radical was somewhat lower than typically observed from other radicals (CH20H, CH2F, C H 3 0 ) under similar conditions in our apparatus. The lower signals may have been due either to a low two-photon transition cross section or to a low density of radical species in the mass spectrometer because of competition between radical production and consumption. Reaction 1 is known to proceed at a relatively slow rate, approximately 15% of the rate of the hydrogen abstraction reaction of methane and atomic fluorine.18 The depletion of CHCl, by reaction with fluorine atoms is 200 times faster than the production reaction.ls Thus, secondary reaction of the radical may reduce the concentration. Other radical sinks such as recombination reactions may also be unusually rapid and further reduce the radical concentration. No REMPI spectrum was observed at masses other than those of the molecular ion. The absence of daughter ions shows that dichloromethyl radicals do not undergo fragmentation subsequent to laser ionization between 355 and 375 nm. The REMPI spectra of CHCl, and CDCl, between 355 and 375 nm are shown in Figures 1 and 2. Since spectral differences in REMPI spectra were not resolved among the assorted chlorine isotopic analogues, the spectra reported here display the sum of the molecular ion masses as a function of laser wavelength; e.g., in CHCl,, the signal is the sum of m / z 83-87. Each REMPI spectrum consists of five distinct bands (Table I), and each pattem of five bands originates from two active radical vibrations. In CHC1, the bands at 370.1, 364.4, and 358.7 nm (18) Clyne, M. A. A.; McKenney, D.J.; Walker, R. F. Can. J . Chem. 1973, 51, 3596.
CDC12 4; 2; 2h4;
form a progression with a one-photon frequency interval of -420 cm-l. In CDC12 the bands at 370.4, 364.9, and 359.2 nm form a similar progression with a one-photon frequency interval of -410 cm-l. In CHC1, a second one-photon interval of 302 cm-l produces the band at 366.0 nm. This one-photon interval also combines with the vibrational mode responsible for the first progression to produce the band at 360.4 nm. In CDCl, a one-photon interval of 265 cm-I produces the band at 366.8 nm and it also combines with the first vibrational mode to produce the band at 361.4 nm (Table I). In order to derive the frequencies of molecular vibration, these one-photon intervals must be multiplied by the number of photons simultaneously absorbed to produce the REMPI spectrum. The red-most member of each REMPI progression is assigned to the electronic origin. In CHCl, the origin lies at 370.1 nm, and in CDCl, the origin lies at 370.4 nm. Since the first adiabatic ionization potential of CHC12 is 8.32 eV,8 a minimum of three photons are required to ionize CHC1, between laser wavelengths of 355 and 375 nm. When we assume that the spectral structure arises from simultaneous two-photon absorption which prepares a radical Rydberg state, we can account for the REMPI spectra. In this model the ion signal is generated when the excited Rydberg radical absorbs one more laser photon; Le., the REMPI signal arises from a 2 1 ionization mechanism. Quite often the resonant electronic states probed by REMPI spectroscopy are molecular Rydberg states.' The familiar Rydberg formula is given by v(cm-') = I P - 109737/(n - 6),
+
where IP is the adiabatic ionization potential, 6 is the quantum defect, and n = 3,4, 5, ... is the principal quantum number. Using this formula one may determine the type of Rydberg orbital being observed (s, p, or d) by calculating the quantum defect for the observed band. Assuming that the band origin lies at the sum of two laser photons, two reasonable solutions are obtained: 6 = 0.1 for n = 3 and 6 = 1.1 for n = 4. These values correspond to either a 3d Rydberg state or a 4s ,Al Rydberg state. If these bands originate from 4s ,A, Rydberg states, then the equation predicts that the 3s ,A1 Rydberg state origin should appear at 545 nm through a 2 2 ionization mechanism. Our data extend to 550 nm, and no other REMPI band system was observed. Thus, if no large interaction red-shifts the 3s 2Al Rydberg bands more than 2000 cm-I, the proposed assignment of the 355-375-nm REMPI spectrum to 3d Rydberg transitions is secure. Confirmation that the REMPI spectrum originates from two-photon excitation of a Rydberg state is found in the vibrational analysis. Rydberg states of molecules are approximately described by a cation core with an additional electron bound in a diffuse, weakly interacting Rydberg orbital. The vibrational spacings observed in a Rydberg state should be similar to those observed in the cation. In CHC1, the frequency interval of the threemembered vibrational progression, assuming a two-photon transition, is 845 (10) cm-'. This interval is identical with the frequency of the v2/l(al) = 860 (30)-cm-' C-Cl stretch of CHCl,'.'
+
5812
Long and Hudgens
The Journal of Physical Chemistry, Vol. 91, No. 23, 1987
In the REMPI spectrum of CDCl, the two-photon vibrational progression is 814 (10) cm-I, which is also nearly identical with the u2/l(al) = 790 (30)-cm-' C-Cl symmetric stretch observed in CDC12+.8 Using this evidence, we assign the observed transition to a two-photon resonant REMPI process, probably through a 3d Rydberg state. The three-membered vibrational progression observed in each REMPI spectrum is assigned to transitions from the vibrationless ground state to the ui(al) = 0, 1, 2 levels in the Rydberg state (Table I). Since the spectral structure originates from simultaneous two-photon absorption, the second vibrational interval is 605 cm-' in CHC1, and 530 cm-' in CDC12. These vibrational intervals do not match any of the frequencies of the four known vibrational modes of CHCI2+or CDCl,'. The candidate assignments for this second vibration are the u4(bl) out-of-plane bending mode and the u3'(al) CC12 scissors mode. The frequencies of these modes in the dichloromethyl species are unknown. To reach a vibrational assignment, we calculated the predicted isotopic frequency ratios between the vibrational modes of CDC12+ To do this, we performed a normal coordinate and CHCl,'. calculation, using the Wilson FG matrix methodI9 and the Schachtschneider program, G M A T , ~which calculated the G matrix. In the absence of extensive cation molecular orbital calculations, we adopted an approximate cation geometry that is identical with the radical structure computed by Molino et ale9except that the cation is assumed to be planar. Calculated isotope shifts vary somewhat with changes in the assumed structure, but these changes are small enough to assure that use of a more accurate structure cannot alter the conclusions presented here. The v{(al) CC12 scissors mode is predicted to show an isotope frequency shift ratio of [CDCI2+/CHCl2+]= 0.99. The predicted isotope shift ratio of the u4'(bl) out-of-plane bending mode is [CDCI2+/ CHC12+] = 0.81. The observed isotope shift ratio is [CDC12/ CHClz] = 0.88 (0.03). These values support an assignment to the u4(bl) out-of-plane bending mode and preclude an assignment to the u{(al) CCI2 scissors made. Assignment of the second frequency interval to the +'(al) CC12 scissors mode is also discarded because the observed frequency interval of -605 cm-' is much greater than expected for the +'(al) CClz scissors mode. Typical u{(al) CC12scissors mode frequencies lie near 280 cm-1.21,22 We assign the second active vibration to the u4(bl) out-of-plane bending mode. REMPI spectra of CHC12 are expected to show bands associated with the u4(bl) out-of-plane bending mode because of changes in the shape of the out-of-plane bending potential between the nearly planar ground-state geometry into the planar Rydberg state geometry. Since the u4(bl) out-of-plane bending made (in C2, symmetry and in the Born-Oppenheimer approxi(19) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill: New York, 1955. (20) Schachtscheider, J. H . "Vibrational Analysis of Polyatomic Molecules"; Technical Report Nos. 231-64 and 57-65, 1964; Shell Development Co., Emerysville. (21) Simanouchi, T.; Suzuki, I. J . Mol. Specfrosc. 1962, 8, 222. (22) Overend, J.; Evans, J. C. Trans. Faraday Soc. 1959, 55, 1817.
mation) is governed by the selection rule, Av4 = 0, f 2 , f 4 , ..., the REMPI bands in CHClz at 366.8 nm and in CDClz at 366.00 nm are either 41 hot bands or 4; overtone bands. Evidence favors the 4; hot-band assignment. Upon deuteriation the -366-nm bands exhibit a 10% increase of intensity relative to the origin. A relative intensity increase is expected of hot bands because the thermal uq/l = 1 population in CDC12 is larger than the thermal u.," = 1 population in CHCI,. The observation of 41, hot bands in the present, ambient temperature experiments implies that the uql'(bl) modes lie at low frequencies. If the proper band assignment is the 41 transition, then the out-of-plane bending frequency in CHC12 is 605 cm-' greater in the Rydberg state than in the ground state. This frequency increase also conforms to expectation. In the ground state the radical electron is involved in a slightly antibonding interaction with the chlorine atoms. The removal of the radical electron increases the carbon-chlorine bond strength.23a The stronger bonds are shown by increases in vibrational frequencies; e.g., the antisymmetric C-C1 stretch increases from 902 cm-I in CHC1; to 1044 cm-' in CHC12+.3*10 Previous studies have established that the out-of-plane bending modes of other methyl radical systems are very sensitive to bonding changes associated with the loss of the radical electron. The out-of-plane bending frequency is observed to increase between the ground state and Rydberg state radicals, e.g., CH2F (260 cm-I to 1259 ~ m - ' ) , ,CH3 ~ ~ (606.5 cm-' to 1334 cm-1),24925 and C H 2 0 H (569 cm-' to 950 cm-1).26,27Thus, our assignment of the -366nm bands to the 41 hot bands appears reasonable. Adopting this band assignment, the bands at 360.4 nm in CHC1, and at 359.2 nm in CDC12 are assigned as the 2A41 transitions. Conclusion
In this work we have reported the first REMPI spectrum of any electronic state of the dichloromethyl radical. The REMPI spectrum was shown to originate from a 2 + 1 REMPI mechanism which most probably probed a 3d Rydberg state of the radical. The spectra exhibited two active vibrational modes which were assigned to the Rydberg state u2/(al) = 845 (10)-cm-' C-Cl stretching mode and the u4(bl) out-of-plane bending mode. The out-of-plane bending frequency is much greater in the Rydberg state than it is in the ground state. This increase in frequency is indicative of increased bonding interactions between the carbon and chlorines. Accurate ground-state bending mode frequencies are needed to permit computation of the out-of-plane bending frequency in the Rydberg state. Registry No. CHC12, 3474-12-2; CDCl2, 26443-88-9 (23) (a) Kafafi, S. A.; Hudgens, J. W., in press. (b) Hudgens, J. W.; Dulcey, C. S.; Long, G. R.; Bogan, D. J. J . Chem. Phys., in press. (24) Yamada, C.; Hirota, E.; Kawaguchi, K. J . Chem. Phys. 1981, 75, 5256. (25) Hudgens, J. W.; DiGiuseppe, T.; Lin, M. C. J . Chem. Phys. 1983, 79, 571. (26) Jacox, M. Chem. Phys. 1981, 59, 213. (27) Dulcey, C. S.; Hudgens, J. W. J . Chem. Phys. 1986, 84, 5262.
