PAPER
    
    www.rsc.org/pccp | Physical Chemistry Chemical Physics
    
    Probing spin–orbit mixing and the singlet–triplet gap in dichloromethylene via Ka-sorted emission spectra
    Calvin Mukarakate, Yulia Mishchenko, Danielle Brusse, Chong Tao and Scott A. Reid*
    Received 24th July 2006, Accepted 9th August 2006 First published as an Advance Article on the web 21st August 2006 DOI: 10.1039/b610582c The magnitude of the singlet–triplet gap in dichloromethylene (CCl2) has been a point of controversy in the recent literature. In this study, we report single vibronic level emission spectra ˜ ˜ of the A1B1 - X1A1 system of the carbene C35Cl2, which probes the vibrational structure of the ˜ X1A1 state up to E10 000 cmÀ1 above the vibrationless level. By the careful selection of bands where complete isotope and Ka 0 selectivity in excitation was possible, we measured Ka 0 -sorted emission spectra in order to test the previously established hypothesis [M.-L. Liu, C.-L. Lee, A. Bezant, G. Tarczay, R. J. Clark, T. A. Miller and B.-C. Chang, Phys. Chem. Chem. Phys., 2003, 5, 1352] that unassigned lines lying above E5000 cmÀ1 belong to levels of the a˜3B1 state. The  Ka 0 -sorting method discriminates between singlet and triplet levels via the (A00 À B00 ) rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond ˜ angle. In the region between 3500 and 9000 cmÀ1 above the vibrationless level of the X1A1 state,  we find only a very modest increase in (A00 À B00 ), and B86% of the lines observed between 5000 ˜ and 9000 cmÀ1 can be assigned to X1A1 levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total. A nearly complete set of Dunham ˜ parameters was determined for the C35Cl2 isotopomer, and the X1A1 state term energies up to 4000 cmÀ1 are in excellent agreement with recent variational calculations of Tarczay, et al. [G. ´ ´´ Tarczay, T. A. Miller, G, Czako and A. G. Csaszar, Phys. Chem. Chem. Phys., 2005, 7, 2881]. Finally, the implication of our results for the singlet–triplet gap in dichloromethylene is discussed.
    
    Introduction
    Carbenes are important reactive intermediates in a vast array of chemical processes.1–6 The divalent carbon gives rise to singlet and triplet configurations of similar energy but very different reactivity, and the magnitude of the singlet–triplet gap (DEST) is thus an important quantity in predicting the reactivity of carbenes in environments where both states can be populated. Over the past decade, significant experimental progress has been made in determining DEST for a number of simple carbenes,7–21 and the agreement between experiment and theory is, in some cases, impressive. For example, in CHCl ˜ the experimental value for T00(a À X) of 2172(2) cmÀ1 is ˜ À1 within 2 cm of a theoretical estimate at the CCSD(T)/augcc-pVQZ level that included relativistic, core correlation and diagonal Born–Oppenheimer corrections.17,19,21 Dichloromethylene (CCl2) is one of the best studied of all the simple carbenes,7,9,16,19,22–66 yet the magnitude of its singlet–triplet gap is arguably the most controversial.56 The majority of theoretical calculations over the past two decades predict a singlet ground state with DEST B 20 kcal molÀ1 (E7000 cmÀ1),19,24,28,29,32,33,36,38,40,43,45,48,50,53–57,63,64 and a recent calculation at the CCSD(T)/aug-cc-pVQZ level including relativistic, core correlation and diagonal Born–OppenDepartment of Chemistry, Marquette University, Milwaukee, WI 53201-1881. E-mail: scott.reid@mu.edu
    
    heimer corrections located the gap at 7050 cmÀ1.19 This lies in stark contrast to the photoelectron studies of CCl2À by Lineberger and co-workers,7,9 which place the gap at 3(Æ3) kcal molÀ1 (E1050 cmÀ1).9 Recently, it has been suggested that the photoelectron spectrum may contain contributions from a quartet state of the anion, which might explain this discrepancy.63,64 On another experimental front, Chang, Miller and co-workers16 and Kable and co-workers61 have obtained emission ˜ spectra that probed the vibrational structure of the X1A1 state. The former study focused on the high energy region (i.e., above B5000 cmÀ1), finding many levels that could not be ˜ assigned to the X1A1 state, although only levels containing symmetric stretch and/or bending excitations were considered. These authors suggested that the unassigned levels might belong to the a˜3B1 state, and this hypothesis was carried forward in a recent theoretical study,19 where a band in the emission spectra at 7051 cmÀ1 was tentatively assigned to the triplet origin based upon the calculated a˜ 3B1 state parameters. In this study, we set out to test for the presence of a˜ 3B1 levels in the high energy region of the CCl2 emission spectrum, and thus report new Single Vibronic Level (SVL) emission spectra ˜ ˜ of the A1B1 - X1A1 system of C35Cl2 that probe the vibra˜ tional structure of the X1A1 state up to E10 000 cmÀ1 above the vibrationless level. By the careful selection of bands affording full isotope and Ka 0 selectivity in excitation, in turn made possible by a complete rotational simulation of all bands
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    ˜ ˜ in the CCl2 A1B1 ’ X1A1 system, we measured Ka 0 -sorted 61 emission spectra. As recently shown in a confirmation of the triplet origin in CHCl,21 these spectra discriminate between  singlet and triplet levels via the (A00 À B00 ) rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle. In the region between B3500 ˜ and 9000 cmÀ1 above the vibrationless level of the X1A1 state,  we find only a very modest increase in (A00 À B00 ), and B86% of the lines observed between 5000 and 9000 cmÀ1 can be ˜ assigned to X1A1 levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total. A nearly complete set of Dunham parameters was determined for the C35Cl2 isotopomer, and these are compared ˜ with theoretical predictions, as are the X1A1 state term energies up to 4000 cmÀ1. Finally, the implication of our results for the controversial singlet–triplet gap in CCl2 is discussed.
    
    using a 600 lines mmÀ1 grating blazed at 500 nm and photon counting was employed, with typical accumulation over 10 000 laser shots. The integration gate (generally 8 ms) was set to fully encompass the fluorescence decay of the emitting level under our experimental conditions, and the spectrograph was operated in a ‘‘step and glue’’ mode, where the grating was stepped sequentially and spectra recorded at each grating step in order to cover the entire spectral region of interest. Spectra were calibrated in each range by first fitting the Ne I emission lines to a Gaussian lineshape function, using Origin 7.5 software. The observed positions were then compared against the known values,67 and the deviations fitted to a second order polynomial to obtain a calibration curve that was applied to the corresponding emission spectrum. Bands in the emission spectra were also fit to a Gaussian lineshape function.
    
    Results and discussion Experimental
    The apparatus, pulsed discharge nozzle, and data acquisition procedures have been described in detail in recent studies.20,21 The carbene CCl2 was produced using a pulsed electrical discharge through a B1–2% mixture of CCl4 (Aldrich) seeded in high purity He. The precursor was kept in a stainless steel bubbler, through which pure He gas was passed at a pressure of 3 bar. The discharge was initiated by a +1 kV pulse of B90 ms duration, through a current-limiting 10 kO ballast resistor. The timing of laser, nozzle, and discharge firing was controlled by a digital delay generator (Stanford Research Systems DG535), which generated a variable width gate pulse for the high voltage pulser (Directed Energy GRX-1.5K-E). The ´ laser system consisted of an etalon-narrowed dye laser (Lambda-Physik Scanmate 2E) pumped by the third harmonic of a Nd:YAG laser (Continuum NY-61). The laser beam was not focused, and typical pulse energies were B1–2 mJ in a B3 mm diameter beam. A mutually orthogonal geometry of laser, molecular beam, and detector was used, where the laser beam crossed the molecular beam at B1 cm downstream. Fluorescence was collected and collimated by a f/2.4 plano-convex lens, and focused into the spectrograph using a f-matching f/3.0 plano-convex lens. Insertion of an aluminum mirror into the beam path at 451 allowed collection of the total fluorescence, which was filtered via an appropriate long-pass cutoff filter (Corion or Edmund Scientific) prior to striking a photomultiplier tube detector (Oriel) held at typically À700 V. In acquiring emission spectra, the fluorescence signal was first optimized on the band of interest, and the mirror subsequently removed to allow fluorescence to enter the spectrograph. A second removable mirror assembly was used to direct the output of an Fe:Ne hollow cathode lamp into the spectrograph for wavelength calibration; these spectra were typically obtained immediately after the emission spectra. Background spectra were obtained with the laser blocked to check for emission lines from species in the discharge. The spectrograph used in this work was an Acton SR303i equipped with an ISTAR-intensified CCD camera. Calibration spectra were acquired with a slit width of 10 mm and 500 shot accumulation; photon counting was not used. The emission spectra were typically acquired with a slit width of 50 mm
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    As noted by Kable and co-workers,61 the fluorescence excita˜ ˜ tion spectrum of the A1B1 ’ X1A1 system of CCl2 can be separated into three distinct regions. Region 1, lying below 20 300 cmÀ1 and studied extensively by Clouthier and coworkers,34,37 displays regular vibrational structure, and a Dunham expansion fit of the band origins for the C35Cl2 and C35Cl37Cl isotopomers reproduces the experimental term energies to within a standard deviation, o1 cmÀ1. In region 2, lying between 20 300 cmÀ1 and B21 500 cmÀ1, the rotational structure of the bands is largely unperturbed; however, vibrational mixing is extensive due to near resonances among the states 1n2m having the same polyad number p = 2n + m. In region 3, above 21 500 cmÀ1, the rotational structure of all bands changes markedly, such that above 22 500 cmÀ1 only subbands terminating in Ka 0 = 0 appear strongly in the spectra, indicating that the Renner–Teller (RT) intersection has been exceeded. In collaboration with Kable and co-workers, we simulated the rotational structure of all bands in the excitation spectrum, and recorded fluorescence lifetimes that show a pronounced lengthening for levels with Ka 0 4 0 at the RT intersection.68 Our simulations show that the isotope splittings increase almost linearly with energy, so that bands of the various isotopomers are well separated in regions 2 and 3. For this study, we chose bands in the 20 600, 21 200 and 21 500 cmÀ1 polyads, where complete isotope and Ka 0 selectivity was possible. Fig. 1 shows experimental and simulated spectra for the 21 200 cmÀ1 polyad, where clean excitation of levels with Ka 0 = 0, 2, and 3 was possible for the C35Cl2 isotopomer in the higher energy band. This is a critical point, as we are attempting to measure weak lines in the high energy region of the emission spectrum. In all future discussion, it is assumed that our spectra refer only to the C35Cl2 isotopomer. Fig. 2 shows sample SVL emission spectra obtained by pumping the Ka 0 = 0 level of the 20 643 and 21 253 cmÀ1 bands. These spectra show an excellent signal-to-noise ratio, and reveal structure extending up to B10 000 cmÀ1 above the ˜ X1A1 state origin. Fig. 3 shows expanded views of the 6300–7700 cmÀ1 region for spectra obtained by pumping different Ka 0 = 0, 2, and 3 in the 21 253 cmÀ1 band. The splitting of all peaks in the spectrum with increasing Ka 0 is clearly discernable, and the splitting patterns are illustrated for
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    Fig. 1 Fluorescence excitation spectrum (upper) and total simulation ˜ ˜ of the 21 200 cmÀ1 polyad in the A1B1 - X1A1 system of CCl2. The individual simulations of bands for the C35Cl2 and C35Cl37Cl isotopomers are shown.
    
    some isolated transitions. From the DKa = Æ1 selection rule, levels in the Ka 0 = 0 spectrum are not split, while those in the   Ka 0 = 2 and 3 spectra are split by 8(A00 À B00 ) and 12(A00 À B00 ), respectively. Thus, for non-overlapping transitions (of which there are fewer and fewer with increasing energy), these spectra
    
    Fig. 3 Expanded single vibronic level emission spectra of C35Cl2 from the Ka 0 = 0, 2, and 3 levels of the 21 253 cmÀ1 band. With increasing Ka 0 , an obvious splitting of all lines in the spectra is observed; the splitting patterns are illustrated for three transitions.
    
     allow a determination of (A00 À B00 ). In Fig. 4, we show a plot  of the derived (A00 À B00 ) constants for lines in the 3500–9000 cmÀ1 region; the lower dashed line shows the measured value ˜ for the vibrationless level of the X1A1 state58 and the upper line the predicted value for the vibrationless level of the a˜ 3B1 state.55 With increasing energy, a very modest increase in
    
    Fig. 2 Single vibronic level emission spectra of C35Cl2 from the Ka 0 = 0 level of the 20 643 and 21 253 cmÀ1 bands.
    
     Fig. 4 Measured (A–B) rotational constants for C35Cl2 levels in the 3000–9000 cmÀ1 region. The lower dashed line shows the experimental ˜ value for the vibrationless level of the X1A1 state, the upper line the calculated value for the vibrationless level of the a˜3B1 state.
    
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     (A00 À B00 ) is observed, arising from the approach to linearity, the barrier to which lies some 23 000 cmÀ1 above the vibra˜ tionless level of the X1A1 state.55,61 However, with the possible exception of one band at B7444 cmÀ1, no obvious increase in  (A00 À B00 ) due to mixing with triplet levels can be seen over this range. The results in Fig. 4 would suggest that the vast majority of intense lines observed in the high energy region belong to ˜ levels of the X1A1 state. It remains, then, to assign these levels. We fit the observed term energies using a non-linear least squares routine to a standard anharmonic potential function (Dunham expansion) of the form:69 Gðu1 ; u2 ; u3 Þ ¼    3 3 X X 1 1 1 uj þ xij u i þ oi þ ui þ 2 2 2 i¼1 j!i;i¼1 ð1Þ where oi is the harmonic frequency of mode i, xii is a diagonal anharmonicity constant, and xij is an off-diagonal or crossanharmonicity constant. The x33 constant could not be determined and was thus fixed at the ab initio value.19 Assignments and fit deviations for the Dunham expansion fit of the C35Cl2 term energies are given in Table 1, while Table 2 compares the vibrational parameters derived from our Dunham fit with theoretical predictions.19 The standard deviation of the fit to 142 levels, extending up to B9200 cmÀ1 in energy, is 2.9 cmÀ1. Comparing our term energies to those of the variational calculations of Tarczay, et al.,19 the standard deviation for 48 levels lying below 4000 cmÀ1 is 3.6 cmÀ1. The corresponding deviations for the data sets of Chang, Miller, and coworkers19 and Kable and co-workers61 are 3.4 cmÀ1 (comparing 41 levels) and 7.2 cmÀ1 (comparing 47 levels), respectively. The calculated term values extend to much higher energy; however, a detailed wavefunction analysis has not yet been completed, and assignments are thus questionable above E4000 cmÀ1.19 In the region between 5000 cmÀ1 and 9000 cmÀ1, a total of 91 lines are observed that we are confident belong to unrelaxed C35Cl2 SVL emission. Of these, 78 (or 86%) can be assigned to singlet lines in the (n,m,0) and (n,m,2) progressions (n,m represent integers Z 0) within three standard deviations of the Dunham fit. Of the remaining lines, those for which (A00 À  B00 ) was determined do not show a larger value than other lines in this region, and thus are probably also singlet levels. The lack of obvious triplet lines in the spectrum is perhaps not surprising based on the intensity of triplet levels in CHCl, which are around 100 times smaller than adjacent singlet levels.17,21 Concerning the previous (tentative) hypothesis that unassigned structure in the emission spectra of CCl2 at energies ˜ B5000 cmÀ1 above the X1A1 state origin arises from the a˜ 3B1 state,16,19 our study shows that this hypothesis is incorrect. Of course, there are many weak unresolved lines in our  spectra above 7000 cmÀ1 for which the (A00 À B00 ) constants could not be determined; it is likely that some of these are triplet levels. Due to the high density of levels, higher resolution spectra, obtained for example using Stimulated Emission Pumping spectroscopy, will be needed to identify the presence of triplet lines in this region.
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    ˜ Table 1 Vibrational term energies (cmÀ1) for the X1A1 state of C35Cl2 derived from SVL emission spectra. Assignments and deviations from a Dunham expansion fit are given Level (0,1,0) (0,2,0) (1,0,0) (0,3,0) (1,1,0) (0,4,0) (1,2,0) (2,0,0) (0,0,2) (1,3,0) (2,1,0) (0,6,0) (1,4,0) (2,2,0) (0,2,2) (3,0,0) (1,0,2) (0,7,0) (1,5,0) (2,3,0) (3,1,0) (1,1,2) (0,8,0) (1,6,0) (2,4,0) (3,2,0) (4,0,0) (2,0,2) (0,9,0) (1,7,0) (2,5,0) (3,3,0) (4,1,0) (2,1,2) (0,10,0) (1,8,0) (3,4,0) (4,2,0) (5,0,0) (0,11,0) (1,9,0) (2,7,0) (3,5,0) (1,5,2) (4,3,0) (5,1,0) (3,1,2) (1,10,0) (2,8,0) (3,6,0) (4,4,0) (5,2,0) (2,4,2) (6,0,0) (0,13,0) (1,11,0) (2,9,0) (0,9,2) (3,7,0) (4,5,0) (2,5,2) (5,3,0) (6,1,0) (0,14,0) (1,12,0) (2,10,0) (0,10,2) (3,8,0) (4,6,0) (2,6,2) (6,2,0) Term energya 335(2) 671(3) 730(3) 1006(2) 1061(2) 1342(1) 1396(1) 1448(3) 1513(1) 1729(4) 1781(2) 2006(1) 2059(2) 2110(3) 2164(2) 2164(2) 2232(1) 2337(2) 2392(1) 2442(2) 2494(1) 2558(1) 2670(1) 2723(1) 2774(2) 2824(1) 2875(2) 2940(1) 3002(2) 3054(1) 3103(1) 3150(3) 3201(4) 3265(3) 3329(3) 3383(3) 3482(3) 3529(3) 3581(2) 3662(1) 3712(2) 3761(2) 3805(2) 3851(1) 3851(1) 3903(2) 3960(1) 4042(1) 4092(1) 4136(2) 4179(2) 4227(2) 4227(2) 4277(2) 4325(2) 4369(1) 4418(2) 4435(2) 4464(3) 4505(5) 4552(2) 4552(2) 4597(2) 4647(2) 4696(3) 4747(3) 4760(2) 4790(2) 4825(3) 4868(2) 4912(6) Observed À Calculated 0 1 1 2 À1 4 1 À3 2 2 À1 1 0 À3 0 À2 4 À1 2 0 À1 4 0 1 1 1 0 À2 0 2 1 À2 0 1 À4 1 2 1 4 À2 0 1 À3 3 À4 1 À8 1 5 2 À1 1 À2 4 1 À1 3 0 4 À1 2 1 1 À7 À3 4 3 3 À6 À2 À6
    
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    Table 1 (continued ) Level (7,0,0) (1,13,0) (0,11,2) (3,9,0) (4,7,0) (5,5,0) (6,3,0) (7,1,0) (2,12,0) (0,12,2) (3,10,0) (1,10,2) (2,8,2) (5,6,0) (6,4,0) (7,2,0) (8,0,0) (5,2,2) (2,13,0) (6,0,2) (3,11,0) (4,9,0) (2,9,2) (3,7,2) (7,3,0) (8,1,0) (2,14,0) (3,12,0) (5,8,0) (3,8,2) (8,2,0) (9,0,0) (2,15,0) (2,11,2) (5,9,0) (6,7,0) (7,5,0) (4,7,2) (9,1,0) (0,16,2) (7,1,2) (3,14,0) (4,12,0) (5,10,0) (4,8,2) (7,6,0) (5,6,2) (9,2,0) (3,15,0) (8,0,2) (4,13,0) (6,9,0) (6,10,0) (4,10,2) (5,8,2) (9,4,0) (10,2,0) (6,11,0) (8,7,0) (10,3,0) (6,12,0) (5,10,2) (8,8,0) (9,6,0) (6,8,2) (6,13,0) (10,6,0) (8,6,2) (9,4,2) (11,0,2) (7,13,0) (10,7,0)
    a
    
    Term energya 4967(3) 5020(2) 5077(1) 5115(2) 5158(3) 5196(4) 5239(1) 5285(2) 5401(1) 5401(1) 5436(1) 5460(3) 5505(1) 5524(4) 5560(4) 5604(3) 5649(1) 5671(1) 5725(1) 5725(1) 5764(4) 5800(2) 5829(2) 5875(4) 5921(4) 5962(2) 6049(4) 6087(3) 6166(6) 6199(1) 6279(2) 6322(2) 6370(2) 6461(1) 6487(3) 6523(3) 6555(3) 6569(3) 6636(1) 6686(1) 6713(2) 6741(2) 6771(2) 6802(1) 6879(3) 6879(3) 6931(3) 6956(2) 7058(4) 7076(2) 7091(3) 7164(1) 7480(3) 7512(3) 7564(1) 7589(1) 7617(3) 7809(2) 7870(6) 7939(1) 8123(1) 8183(3) 8183(3) 8214(3) 8233(2) 8437(3) 8873(2) 8936(3) 8973(2) 9057(2) 9100(2) 9184(5)
    
    Observed À Calculated 5 À6 À2 2 2 À2 À1 2 5 0 À2 5 À5 3 À3 0 5 À1 3 À1 1 À3 0 À7 À3 À1 1 0 1 0 À3 1 À2 À5 0 À2 À8 2 À1 2 À4 7 0 À6 À3 À2 0 2 1 À2 À3 À1 À4 0 6 3 À3 7 2 6 2 À2 À1 À2 4 À1 0 3 À2 À1 À1 À1
    
    Table 2 Comparison of calculated C35Cl2 vibrational parameters with those determined from the Dunham expansion fits Parameter o1 o2 o3 x11 x12 x11 x22 x23 x33
    a
    
    Dunham fit (this work) 738.48(28) 338.88(19) 776.13(69) À3.270(27) À2.031(19) À4.95(10) À0.218(13) À4.362(68) À5.15a
    
    Ab initio (ref. 19) 733.37 336.92 772.17 À3.21 À1.34 À4.8 À0.19 À4.64 À5.15
    
    Fixed at the ab initio value.
    
    Recently, Chang and co-workers reported SVL emission spectra of CBr2, in which congested and unassigned vibrational structure observed at energies B3650 cmÀ1 above the ˜ X1A1 state origin was suggested to arise from the a3B1 ˜ state.70,71 This is surprising, as recent high level theoretical calculations suggest a singlet–triplet gap of B17 kcal molÀ1, or B5900 cmÀ1,54,55,57 again in serious disagreement with the photoelectron studies of Lineberger and co-workers, which place the gap at 2(Æ3) kcal molÀ1 (E700 Æ 1050 cmÀ1).9 Given that spin–orbit coupling is expected to be significantly larger in CBr2 than CCl2, and that Ka and isotope selectivity is also possible in excitation,72 we believe that CBr2 will be a perfect system for application of the Ka sorting method, and studies of this carbene are underway in our laboratory. Finally, while these results (disappointingly) do not provide a value for the singlet–triplet gap in CCl2, they do provide insight into the discrepancy between theory and previous photoelectron studies.7,9 If the singlet–triplet gap were only 3(Æ3) kcal molÀ1 (E1050 Æ 1050 cmÀ1), in the low energy region we would expect to observe, based upon previous studies of CHCl,16,21 extra lines and obvious shifts in level positions indicative of spin–orbit mixing. However, all lines below 3000 cmÀ1 can be fitted to a Dunham expansion with a standard deviation of around 1 cmÀ1 (less than our experimental uncertainty), suggesting that the gap derived from the photoelectron studies is indeed too small. This, in fact, has already been shown in the previous SVL emission studies.16,61
    
    Conclusions
    We have recorded new single vibronic level emission spectra of ˜ ˜ the A1B1 - X1A1 system of C35Cl2 that probe the vibrational ˜ structure of the X1A1 state up to 10 000 cmÀ1 above the vibrationless level. By the careful selection of bands affording complete isotope and Ka 0 selectivity in excitation, we measured Ka 0 -sorted emission spectra in order to test the previously established hypothesis16,19 that unassigned lines lying above B5000 cmÀ1 belong to levels of the a˜ 3B1 state. The Ka 0 -sorting method discriminates between singlet and triplet levels via the  (A00 À B00 ) rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle.21 In the region between B3500 and 9000 cmÀ1, we find only a  very modest increase in (A00 À B00 ), and B86% of the lines ˜ observed between 5000 and 9000 cmÀ1 can be assigned to X1A1
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    levels within 3 standard deviations of our Dunham expansion fit, which included a total of more than 140 levels. A nearly complete set of Dunham parameters was determined for the ˜ C35Cl2 isotopomer, and the X1A1 state term energies up to 4000 cmÀ1 are in excellent agreement with recent variational calculations of Tarczay, et al.19 Although this study has not provided a value for the singlet–triplet gap, our results do suggest that the value derived from the photoelectron studies of Lineberger and coworkers is too small. Higher resolution spectroscopic studies of the high energy region will be needed to gain further insight into the singlet–triplet gap in CCl2.
    
    Acknowledgements
    The National Science Foundation (grant CHE-0353596) is gratefully acknowledged for support of this research. The authors thank Scott Kable and Terry Miller for useful discussions.
    
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