THE JOURNAL OF CHEMICAL PHYSICS 124, 224314 2006
    
    Fluorescence excitation and single vibronic level emission spectroscopy ˜ ˜ of the A 1A ] X 1A system of CHCl
    Chong Tao, Calvin Mukarakate, and Scott A. Reida
    Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881
    
    Received 17 March 2006; accepted 21 April 2006; published online 12 June 2006 We report new fluorescence excitation and single vibronic level emission spectra of the ˜ 1A ↔ ˜ 1A system of CHCl. A total of 21 cold bands involving the pure bending levels 2n with X A 0 n = 1 – 7 and combination bands 2n31 n = 4 – 7 , 2n32 n = 4 – 6 , 112n n = 5 – 7 , 112n31 n = 4 – 6 , and 00 00 00 000 112n32 n = 4 were observed in the 450– 750 nm region; around half of these are reported and/or 000 rotationally analyzed here for the first time. Spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotational analysis typically yielded band origins and rotational A constants for both isotopomers CH35Cl, CH37Cl . The derived ˜ 1A vibrational intervals are combined with results of Chang and Sears J. Chem. Phys. 102, 6347 1995 to determine the A excited state barrier to linearity Vb = 1920 50 cm−1 . The ˜ 1A state C–H stretching frequency is determined here for the first time, in excellent agreement with ab initio predictions. Following our observation of new bands in this system, we obtained the single vibronic level SVL emission spectra which probe the vibrational structure of the ˜ 1A state up to 9000 cm−1 above the X vibrationless level. The total number of ˜ 1A levels observed is around three times than that X previously reported, and we observe five new ˜ 3A state levels, including all three fundamentals. a The results of a Dunham expansion fit of the ground state vibrational term energies, and comparisons with the previous experimental and recent high level ab initio studies, are reported. Our aX data confirm the previous assignment of the ˜ 3A origin, and our value for T00 ˜ − ˜ a = 2172 2 cm−1 is in excellent agreement with theory. By exploiting SVL spectra from excited state levels with Ka = 1, we determine the effective rotational constant A – ¯ of the triplet origin, also in B good agreement with theory. Our results shed new light on the vibrational structure of the ˜ 1A , X ˜ 1A , and ˜ 3A states of CHCl, and, more generally, spin-orbit coupling in the a A monohalocarbenes. © 2006 American Institute of Physics. DOI: 10.1063/1.2204916
    INTRODUCTION
    
    There is much current interest in the spectroscopy, photochemistry, and photophysics of simple carbenes, species which play an important role in diverse areas of chemistry.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 is thus an important quantity in predicting the reactivity of carbenes in environments where both states can be populated. The prototypical carbene, methylene CH2 , has been extensively studied over many years,7–46 however, as the smallest carbenes with singlet ground states, the monohalocarbenes CHX X = F , Cl, Br, I have received increasing attention.47–123 These are prototypical systems for examining spectroscopy and dynamics on three coupled potential energy surfaces, involving a fascinating interplay between the Renner-Teller RT and spin-orbit interactions. The seminal work of Merer and Travis showed that the two lowest singlet states ˜ 1A , ˜ 1A are the RT pair derived from a 1 state in the X A
    a
    
    Author to whom correspondence should be addressed. Electronic mail: scott.reid@mu.edu
    
    linear configuration.47,48 Subsequent studies have shown that the ˜ state barrier to linearity drops from 6900 cm−1 above A the vibrationless level for CHF88,92,108,119 to 2000 cm−1 for CHCl Ref. 80 and 1640 cm−1 for CHBr,100,123 and thus CHF has proved the best system of the three for systematic study of the onset of RT interactions as the barrier is approached, as demonstrated in a number of recent articles.88,92,105,109–112,115,117,119 The spectroscopy of CHCl and CHBr has also been extensively investigated, most notably by Sears and co-workers.80–82,84,87,90,91,96,100,102–104,120–122 With much smaller excited state barriers to linearity, the dominant tranX sitions observed in their ˜ 1A ← ˜ 1A systems are Ka = 1 A → Ka = 0. In a recent paper, we reported a comprehensive X survey of the ˜ 1A ← ˜ 1A system of CHBr using fluoresA cence excitation and single vibronic level SVL emission spectroscopy.123 That work involved the analysis of 30 cold bands, many of which were reported or rotationally analyzed for the first time. We reported the first measurement of the ˜ 1A state C–H stretching frequency, following the observaA tion of bands in the C–H stretching progressions 112n and 00 112n3m, which were earlier observed for CHF.109,115,119 Ex000
    © 2006 American Institute of Physics
    
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    124, 224314-1
    
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    224314-2
    
    Tao, Mukarakate, and Reid
    
    J. Chem. Phys. 124, 224314 2006
    
    ploiting these as pump transitions in SVL emission spectroscopy, we examined the vibrational level structure up to X 9000 cm−1 above the vibronic origin of ˜ 1A , thereby reassigning the C–H stretching frequency in the ˜ 1A state X and reporting many previously unobserved levels in both the ˜ 1A and ˜ 3A states. a X In this report, we turn our attention back to CHCl. Following the original work of Merer and Travis,47 Hirota and co-workers reported a high resolution study of the 25 band 0 and probed perturbations in this system using Zeeman measurements.58,62 Over the past decade, Sears and coworkers reported a beautiful series of high resolution measurements of cold and hot bands of CHCl and CDCl near the electronic origin 12 280 cm−1 .80,81,86,102,122 In the past two years, reports from our group108 and Lin et al.114 yielded new information on the higher energy region of ˜ 1A A ˜ 1A spectrum. Recently, some of our assignments were ←X called into question by new calculations of the transition dipole surface and the Franck-Condon factors.122 To clarify these assignments and search for bands in the C–H stretching progression, we reinvestigated the visible spectroscopy of CHCl, following experimental improvements which led to a tenfold increase in signal over our previous report. Here, then, we report new LIF spectra of jet-cooled CHCl in the 450– 750 nm region. Our measurements yielded rotationally resolved spectra for 21 cold bands, around half of which are either observed or rotationally analyzed here for the first time. We were typically able to analyze the bands for both 35 Cl, 37Cl isotopomers, and report on the vibrational state dependence of the isotope shift in the excited state. We also report the first observation of bands containing C–H stretching excitation, from which the ˜ state C–H stretching freA quency was determined, in excellent agreement with recent ab initio calculations.120 Following our observation of new bands in this system, we obtained a series of SVL emission spectra which probe a the vibrational structure of the ˜ 1A and ˜ 3A states up to X X 9000 cm−1 above the vibrationless level of the ˜ 1A state. As in our previous study of CHBr,123 we observe many prea viously unassigned levels in both the ˜ 1A and ˜ 3A maniX folds. We assign five new levels of the ˜ 3A state, including a all three fundamentals, and our data confirm the previous assignment of the ˜ 3A origin,114 with a value for T00 ˜ a a − ˜ in excellent agreement with recent ab initio X calculations.118,120 Moreover, by exploiting SVL emission spectra from excited state levels with Ka = 1, we determine the effective rotational constant A − ¯ of the triplet origin, B also in excellent agreement with theory. A detailed comparison of vibrational term energies for both states is made with the predictions of two recent ab initio studies,118,120 and a Dunham expansion fit to the observed term energies was used to determine a complete set of ˜ 1A state vibrational X parameters for both isotopomers.
    EXPERIMENTAL SECTION
    
    The apparatus, pulsed discharge nozzle, and data acquisition procedures have been described in detail in earlier
    
    studies.105,108–112,117,119,123 The carbene CHCl was produced using a pulsed electrical discharge through 1 % – 2% mixture of a suitable precursor seeded in high purity He. Several precursors were tried in this experiment, including CH2Cl2 Aldrich, 99.5% stated purity , CH3Cl Aldrich, 99.5% , and CHClBr2 Aldrich, 98% ; all were used without further purification, and all gave reasonable CHCl signals. We typically used the dichloromethane precursor, however, the discharge of CH2Cl2 also produced some CCl2, which overlapped the CHCl spectra in certain spectral regions; in those regions we used CHClBr2. In each case the precursor was kept in a temperature controlled stainless steel bubbler, through which pure He gas was passed at a pressure of 2 – 4 bars. The discharge pulse width fully encompassed the gas pulse from the nozzle, and the discharge was initiated by a +1 kV pulse of 700 s duration, through a current limiting 10 k ballast resistor. Typically, the discharge nozzle could be used continuously for one to two weeks before cleaning was required. The timing of laser, nozzle, and discharge firing was controlled by a digital delay generator Stanford Research Systems DG535 , which also generated a variable width gate pulse for the high voltage pulser Directed Energy GRX1.5K-E . The laser system consisted of an etalon narrowed dye laser Lambda-Physik Scanmate 2E pumped by the second or third harmonic of a Nd:YAG yttrium aluminum garnet laser Continuum Powerlite 7010 or NY-61 . The laser beam was not focused, and typical pulse energies were 1 – 2 mJ in a 3 mm diam. beam. A portion of the dye laser fundamental was directed into a Fe–Ne lamp for absolute wavelength calibration using the optogalvanic effect; the optogalvanic and fluorescence signals were recorded simultaneously. These measurements utilized a mutually orthogonal geometry of laser, molecular beam, and detector, where the laser beam crossed the molecular beam at a distance of 10 mm downstream. Fluorescence was collected and collimated by a f / 2.4 planoconvex lens, and focused into the spectrograph using a f-matching f / 3.0 planoconvex lens. Insertion of an aluminum mirror into the beam path at 45° 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 −600 V. In acquiring fluorescence excitation spectra, the photomultiplier tube PMT signal was terminated into 15 k , and digitized by a fast oscilloscope HP 54521A , and 20 laser shots were typically averaged at each step in wavelength 0.002 nm . In acquiring emission spectra, the fluorescence signal was first optimized and the wavelength then set on the band of interest. The mirror was 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 before or 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
    
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    224314-3
    
    Spectroscopy of CHCl TABLE I. Fit parameters in cm−1 for CHCl ˜ 1A ← ˜ 1A A X Band 21 0 22 0 23 0 24 0 25 0 2 43 1 00 26 0 2 53 1 00 2 43 2 00 27 0 2 63 1 00 2 53 2 00 2 73 1 00 2 63 2 00 1 12 43 1 000 1 12 5 00 1 12 6 00 1 12 53 1 000 1 12 43 2 000 1 12 7 00 1 12 63 1 000
    a
    
    J. Chem. Phys. 124, 224314 2006 bands. B +C /2 0.6004 0.6091 ¯ 0.6097 ¯ 0.6096 ¯ 0.6065 0.5932 0.6015 0.5942 0.6039 0.5896 0.5983 0.5880 0.5994 0.5880 0.6033 0.5822 0.5934 0.5809 0.5903 0.5750 0.5870 0.6063 0.5790 ¯ 0.5978 0.5992 0.5901 0.5957 0.6045 0.5932 0.6032 ¯ 0.6023 ¯ 0.5998 0.5882 0.5971 0.5887 0.5976 36 10 14 4 9 17 14 5 5 10 5 7 7 11 10 7 6 8 7 13 16 61 9 17 24 64 6 5 19 19 11 18 48 16 16 29 Nb 9 16 ¯ 14 ¯ 15 ¯ 17 13 18 16 18 15 21 18 19 16 16 16 21 18 19 13 14 5 17 ¯ 13 10 7 12 16 9 12 ¯ 15 ¯ 12 4 15 10 13
    c
    
    Isolope Cl Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl 37 Cl 35 Cl
    35 37
    
    Ta 13 156.32 14 13 157.00 5 14 011.6 3 14 012.67 7 14 859.8 2 14 861.32 5 15 715.2 2 15 717.90 5 16 584.79 6 16 589.57 8 16 638.63 3 16 646.53 3 17 454.89 3 17 464.97 3 17 514.33 3 17 525.55 5 17 557.94 5 17 570.77 4 18 313.88 3 18 329.97 4 18 379.74 4 18 398.01 4 18 425.06 4 18 448.66 5 19 233.38 30 19 255.46 14 ¯ 19 303.03 16 19 486.94 16 19 493.67 20 19 500.21 14 19 502.97 14 20 354.99 10 20 360.18 11 ¯ 20 387.87 15 ¯ 20 420.07 16 21 205.82 20 21 216.03 12 21 266.09 4 21 276.90 8
    
    0.07 0.04 ¯ 0.05 ¯ 0.03 ¯ 0.04 0.05 0.08 0.02 0.03 0.03 0.03 0.03 0.05 0.04 0.04 0.03 0.04 0.04 0.03 0.03 0.04 0.09 0.03 ¯ 0.04 0.03 0.06 0.02 0.03 0.02 0.04 ¯ 0.04 ¯ 0.04 0.04 0.05 0.02 0.06
    
    Three standard errors given in parenthesis. Number of transitions included in the fit. c Standard deviation of the fit.
    b
    
    SR303i equipped with an iSTAR intensified charge-coupled device CCD camera. Calibration spectra were acquired with a slit width of 10 m and 500 shot accumulation; photon counting was not used. The emission spectra were typically acquired with slit widths of 100– 200 m, and photon counting was used, with typical accumulation over 7500– 10 000 laser shots. Lower and higher resolution spectra were obtained using, respectively, a 600 lines/ mm grating blazed at 500 nm and an 1800 lines/ mm holographic grating. The integration gate typically 2 s 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 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 line shape function, using the ORIGIN 7.5 software. The observed positions were then compared against the known values,124 and the deviations fit to a second order polynomial to obtain a calibration curve which was applied to the corresponding emission spectrum. Bands in the emission spectra were also fit to a Gaussian line shape function. In analyzing the fluorescence excitation spectra, transition frequencies were fit to a standard asymmetric top Hamiltonian using a least squares routine in the ASYROTWIN program package of Judge and Clouthier,125 incorporating the
    
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    224314-4
    
    Tao, Mukarakate, and Reid
    
    J. Chem. Phys. 124, 224314 2006
    
    ground state rotational constants determined in the high resolution study of Kakimoto et al.62
    RESULTS AND DISCUSSION
    
    Fluorescence excitation spectra. We obtained and rotationally analyzed the fluorescence excitation spectra of 21 cold bands involving the pure bending levels 2n with n 0 = 1 – 7 and combination bands 2n31 n = 4 – 7 , 2n32 n 00 00 = 4 – 6 , 112n n = 5 – 7 , 112n31 n = 4 – 6 , and 112n32 n = 4 00 000 000 in the ˜ 1A ← ˜ 1A system. The fit parameters for these A X bands are listed in Table I. As illustrated in Fig. 1, which displays an experimental and simulated spectrum of the 27 0 band, the signal levels were enhanced by around one order of magnitude compared to our previous report. For each band we typically determined band origins and effective rotational constants ¯ = 1 / 2 B + C for both chlorine isotopomers B 35 Cl, 37Cl . As found in previous studies,47,108,114 only subbands terminating in Ka = 0 appear strongly in the spectra for levels above the barrier. For levels near and below the barrier, we observed the perturbed subbands with Ka 0, as first reported by Merer and Travis.47 Due to the limited range of J in our jet-cooled spectra Trot 20 K , the small 10−6 centrifugal distortion constant DJ was not well determined and was set to zero in the fits. Where they overlap, our results are in reasonable agreement with previous rotationally resolved studies.62,81,108,114,122 A complete table of observed line positions is available in the EPAPS archives.126 The relative band intensities in this system are in good agreement with the calculations of Wang et al.,122 who reported the transition dipole moment surface and the FranckX Condon factors for the ˜ 1A ← ˜ 1A system of CHCl at the A MRCI/cc-pVTZ level. As suggested in that work, our previous assignments Ref. 108 for weak bands in the regions 18 300 and 19 300 cm−1 were in error; with the increase in signal in the present study we were able to rotationally analyze bands for both isotopomers, and consideration of the isotope shifts, level spacings, and SVL emission spectra led to the reassignments given in Table I. Specifically, bands previously assigned as 27 and 2631 are reassigned as 2631 and 0 00 00 2532 the 27 band was found at slightly lower energy, as 00 0 shown in Fig. 1 , while the previously assigned 28 band is 0 reassigned as 2731 Table I . 00 Following the procedure used in earlier studies of CHF/ CDF and CHBr,109,119,123 we searched at higher energy for bands in the C–H stretching progression 112n. Indeed, new 00 bands at 19 500, 20 360, and 21 220 cm−1 were observed which had a 35Cl– 37Cl isotope shift comparable to levels in the pure bending progression, indicating that the C–Cl stretch was not excited. These were assigned to members of the 112n progression with n = 5 – 7, and the assign00 ments were confirmed by SVL emission spectroscopy, as described below. A linear extrapolation to n = 0 of the difference in term energies of the 112n and 2n bands with n = 5 – 7 re0 00 turns a value for 1 of 2980 cm−1, in excellent agreement with that 2986 cm−1 predicted at the MRCI/cc-pVTZ 120 level. Figure 2 displays a Dixon plot127 of the vibrational intervals for the 2n progression which combines our data with
    
    FIG. 1. Fluorescence excitation spectrum upper and simulation of the 27 0 band in the ˜ 1A ← ˜ 1A system of CHCl. The simulation is based upon the A X rotational constants given in Table I.
    
    the term energy of the 00 band derived by Chang and Sears.80 0 In this plot we used the average term energy of the two Cl isotopomers for each band, and a minimum indicating the position of the barrier to linearity is clearly observed. Following the procedure used in our previous studies,109,119,123 the data were fit to a third order polynomial using a nonlinear least squares routine, and the minimum position located by
    
    FIG. 2. Dixon plot of the vibrational intervals for the 2n progression, combining our data with the term energy of the 00 band derived by Chang et al. 0 Ref. 80 . The fit to a third order polynomial is shown as the solid line in this figure, while the open symbols are the calculated results from Yu et al. Ref. 120 .
    
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    224314-5
    
    Spectroscopy of CHCl
    
    J. Chem. Phys. 124, 224314 2006
    
    FIG. 3. Excited state 35Cl– 37Cl isotope splitting for all observed 2n, 2n31, 2n32, and 112n bands plotted as a function of n.
    
    differentiation. The fit results and those from recent calculations of Yu et al.120 are shown in Fig. 2. The derived barrier height relative to the vibrationless level of the ˜ 1A state is A 1920 50 cm−1. Note that the intervals appear to decrease again at higher energies—this may arise from anharmonic interactions with C–Cl stretch containing combination states.
    
    FIG. 5. Single vibronic level emission spectra of CH 35Cl from the 2532 level upper panel and 1125 level. The x axis labels the shift in frequency from the excitation line.
    
    FIG. 4. Single vibronic level emission spectra from the 27 level for CH35Cl upper panel and CH37Cl. The x axis labels the shift in frequency from the excitation line.
    
    Figure 3 displays a plot of the excited state 35Cl– 37Cl isotope splitting for the 2n, 2n3m m = 1 , 2 , and 112n bands which combines our results with data for the 00 band from 0 Chang and Sears.80 Unlike our previous findings for CHBr, where the isotope splitting is well reproduced by a simple model which assumes that the C–Br stretch is not strongly coupled to other modes,123 the isotope splitting in CHCl exhibits a strong dependence on bending quantum number n , particularly for n 4. This indicates a significant degree of stretch-bend coupling in the ˜ state. A SVL emission spectra. We obtained SVL emission specA tra for CH 35Cl from the ˜ state levels: 25, 2431, 26, 2531, 2432, 27, 2631, 2532, 112431, 1125, 1126, 112531, 1127, and 112631; and spectra for CH 37Cl from the levels: 25, 26, 2531, 2432, 27, 2631, 112431, and 1125. Sample spectra are shown in Figs. 4 and 5. The various spectra allow us to observe many ˜ state levels, and eliminate spurious peaks due to the disX charge background or collision-induced relaxation in the ˜ A 123 state. Here, unlike our CHBr study, the discharge background was generally negligible. The ˜ state term energies X derived for CH 35Cl are compared in Table II with previous work from Lin et al.;114 where they overlap the agreement is very good, although we observe around three times the total number of levels. The corresponding CH 37Cl term energies are available in the EPAPS archives,126 and for simplicity we use the notation 1 , 2 , 3 to label the vibrational states. All term energies were derived from SVL emission spectra which show predominantly Ka = 0 → Ka = 1 transitions, and there is thus a small bias in the term energies for excited bending levels due to the increase in A rotational constant
    
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    224314-6
    
    Tao, Mukarakate, and Reid
    
    J. Chem. Phys. 124, 224314 2006
    
    TABLE II. Vibrational term energies in cm−1 for the ˜ 1A state of CH 35Cl derived from SVL emission spectra. Assignments and deviations from the X calculations of Refs. 118 and 120 and the Dunham expansion fit this work are given. Term energy This worka 813 1 1198 1 1614 1 2001 1 2384 1 2401 5 2793 4 2793 4 3185 4 3185 4 3540 1 3577 3 3606 3 3970 1 3977 3 4331 3 4350 1 4408 1 4687 1 4729 2 4781 4 5110 2 5110 2 5132 4 5200 2 5447 2 5469 3 5498 1 5577 2 5822 3 5873 4 5934 1 6240 2 6258 4 6288 4 6364 3 6587 1 6630 2 6726 4 6934 1 7002 2 7055 5 7073 4 7354 3 7405 2 7698 3 7767 3 7843 3 8033 3 8186 2 o.-c. so c −2 −4 −2 −2 −2 4 2 −15 1 −1 8 −4 −8 −34 −13 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 11 Assignments and Deviations Ref. 118 o.-c. Ref. 114 810 1195 1613 1999 2383 2402 2791 2791 3181 3181 3538 3575 3604 ¯ 3976 4329 ¯ ¯ 4684 ¯ ¯ ¯ ¯ ¯ ¯ ¯ 5466 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ Fit standard deviation
    a
    
    Ref. 120 0,0,1 0,1,0 0,0,2 0,1,1 0,2,0 0,0,3 0,1,2 1,0,0 0,0,4 0,2,1 0,3,0 0,1,3 1,0,1 0,2,2 1,1,0 0,3,1 ¯ ¯ 0,4,0 ¯ ¯ ¯ ¯ ¯ ¯ ¯ 0,4,1 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯
    
    o.-c.
    
    b
    
    Dunham 0,0,1 0,1,0 0,0,2 0,1,1 0,2,0 0,0,3 0,1,2 1,0,0 0,0,4 0,2,1 0,3,0 0,1,3 1,0,1 0,2,2 1,1,0 0,3,1 0,1,4 1,0,2 0,4,0 0,2,3 1,1,1 0,1,5 0,3,2 1,2,0 1,0,3 2,0,0 0,4,1 0,2,4 1,1,2 0,5,0 0,3,3 1,2,1 0,4,2 2,0,1 1,3,0 1,1,3 0,5,1 0,3,4 1,2,2 0,6,0 0,4,3 2,0,2 1,3,1 0,5,2 1,4,0 0,6,1 2,2,0 1,3,2 0,7,0 1,4,1
    
    o.-c. 0 0 0 −2 4 −3 −3 −1 1 9 −4 −2 0 9 2 −1 0 −1 −4 −7 2 0 1 −6 1 −1 −2 2 4 2 −3 0 1 −3 4 9 −5 0 7 1 4 −7 1 1 −8 1 11 −6 5 −7 5
    
    VPT2 −5 0 −8 −3 −5 −8 −4 −5 −14 −14 0 −9 −14 −16 −5 0 −12 −17 2 −3 −8 −3 −8 12 −21 −19 −3 −10 −12 −9 −7 4 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 10
    
    Var4 −3 2 −4 1 −2 −1 −8 2 −8 −5 9 −3 −7 −10 2 5 −4 −9 14 5 1 −2 2 14 −10 0 12 0 −2 19 7 8 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 7
    
    Var4+ −3 2 −4 1 −3 −1 −8 2 −9 −5 6 −3 −7 −11 2 2 −4 −9 8 4 0 −2 0 12 −10 −1 6 −1 −3 8 4 6 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 6
    
    −2 −3 −2 −1 −7 4 3 −15 1 −13 8 −4 −8 −22 −4 ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ 9
    
    One standard errors given in parenthesis right justified . Observed-calculated. c Calculations including spin-orbit interaction.
    b
    
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    224314-7
    
    Spectroscopy of CHCl
    
    J. Chem. Phys. 124, 224314 2006
    
    FIG. 6. Stick diagram of observed and calculated term energies for CH35Cl. The left-hand side shows the predicted ˜ 1A term energies derived from the X Dunham expansion parameters given in Table III. The right-hand side displays the predicted ˜ 3A term energies derived from the calculations of Ref. a 118. The observed term energies are shown in the middle.
    
    with bending excitation, which is not explicitly accounted for in our analysis. We fit the observed ˜ state term energies using a nonX linear least squares routine to a standard anharmonic potential function Dunham expansion of the form,128
    3 3 i i=1
    
    FIG. 7. Expanded view of Fig. 6, showing the regions encompassing the ˜ 3A origin lower panel , and ˜ 3A 0 , 0 , 1 and 0 , 1 , 0 levels upper a a panel .
    
    G
    
    1, 2, 3
    
    =
    
    + 1/2
    
    i
    
    +
    j i,i=1
    
    i
    
    + 1/2
    
    j
    
    + 1/2 xij , 1
    
    where i is the harmonic frequency of mode i, xii is a diagonal anharmonicity constant, and xij is an off-diagonal or cross-anharmonicity constant. Assignments and fit deviations for the Dunham expansion fit of the CH 35Cl term energies are compared in Table II with the results of two recent ab initio studies. The calculations of Yu, et al.120 at the MRCI/
    
    aug-cc-pVTZ level were reported with and without incorporation of spin-orbit coupling, while Tarczay et al. reported a more extensive set of term energies calculated from a complete quartic force field computed at the CCSD T /aug-ccpVQZ level using second-order vibrational perturbation theory VPT2 and variational methods VAR4, VAR4+ , but without consideration of spin-orbit effects.118 The standard deviation for the Dunham expansion fit to 50 levels is 5 cm−1, which may be compared to our experimental uncertainty of 2 cm−1 and the corresponding fit deviation for CHBr of 16 cm−1. The calculations of Yu et al.120 give a standard deviation of 9 cm−1 for the set of 15 term energies calculated without consideration of spin-orbit coupling.
    
    TABLE III. Comparison of calculated CH 35Cl/ CH 37Cl vibrational parameters with those determined from the Dunham expansion fits. Dunham fit CH 35Cl 2943 7 1229 2 827 2 −70 2 −18 1 01 −8.6 2 −8.2 3 −5.5 4 Ab initio Ref. 118 2924 1223 821 −68 −8.5 1.1 −8.5 −7.7 −5.1 DFTa this work 2912.6 1228.8 791.6 ¯ ¯ ¯ ¯ ¯ ¯ Dunham fit CH 37Cl 2924 7 1228 2 820 2 −65 2 −16 1 31 −9.0 3 −7.9 4 −5.6 3 DFTa this work 2917.6 1227.5 786.9 ¯ ¯ ¯ ¯ ¯ ¯
    
    Parameter
    1 2 3
    
    x11 x12 x13 x22 x23 x33
    a
    
    Calculated at the B3LYP/aug-cc-pVTZ level.
    
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    224314-8
    
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    J. Chem. Phys. 124, 224314 2006
    
    TABLE IV. Comparison of experiment and theory for the vibrational frequencies of CH 35Cl in ˜ 1A and ˜ 3A . X a Method CH 35Cl ˜ 1A X SCF TCSCF QCISD/ 6-311G d , p CASSCF/cc-pVTZ CASPT2/cc-pVTZ CCSD T /cc-PVTZ CCSD T /aug-cc-pCVQZ MRCI/aug-ccpVTZ B3LYP/aug-cc-VTZ Experiment SVLE Experiment SVLE Experiment LIF Experiment IR Experiment PE a CH 35Cl ˜ 3A SCF QCISD/ 6-311G d , p CASSCF/cc-pVTZ CASPT2/cc-pVTZ CCSD T /cc-pVTZ CCSD T /aug-cc-pCVQZ MRCI/aug-cc-pVTZ B3LYP/aug-cc-pVTZ Experiment SVLE Experiment PE
    a b 1 2 3
    
    RC–H
    
    RC–Cl
    
    Ref.
    
    3160 3145 2800 3129 2892 2919 2924 2778a 2913 2942 7 2792 1 a ¯ ¯ ¯ 3362 3080 3346 3190 3229 3203 ¯ 3176 3083 5 ¯
    
    1309 1309 1183 1272 1212 1229 1223 1195a 1229 1229 2 1203 1 ¯ 1201a ¯ 1089 968 1052 966 985 970c 948 951 972 4 a ¯
    
    812 797 784 798 811 831 821 810a 791 830 2 818 1 ¯ 815a 810 25
    
    1.090 1.092 1.114 1.093 1.107 1.111 1.109 1.119 1.108 ¯ ¯ 1.119 ¯ ¯ 1.070 1.085 1.071 1.078 1.083 1.082 1.080 1.082 ¯ ¯
    
    1.710 1.725 1.711 1.719 1.702 1.707 1.695 1.696 1.704 ¯ ¯ 1.696 ¯ ¯ 1.699 1.678 1.692 1.661 1.671 1.659 1.665 1.664 ¯ ¯
    
    103.1 102.1 102.0 103.1 101.9 101.8 102.3 102.1 102.2 ¯ ¯ 101.4 ¯ ¯ 124.4 125.7 125.4 126.9 126.3 126.5 128.4 126.7 ¯ ¯
    
    66 66 93 95 95 95 118 120 PSb PS 114 62 50 70 and 75
    
    a
    
    869 855 857 891 905 893c 912 875 886 4 a 850 60
    
    66 93 95 95 95 118 120 PS PS 70 and 75
    
    Anharmonic frequency. Present study. c Ordering reversed from that given in Ref. 118 see Ref. 130 .
    
    Their calculations incorporating spin-orbit coupling well reproduce the shifts in specific regions, as described below, yet the overall standard deviation is slightly larger 11 cm−1 . The calculations of Tarczay et al.118 are slightly better in terms of overall deviation; the standard deviations for a set of 33 levels of the 35Cl isotopomer are, respectively, 10, 7, and 6 cm−1 for the VPT2, VAR4, and VAR4+ methods. We can anticipate that the incorporation of spin-orbit coupling into these calculations would lead to further improvement.
    
    Figures 6 and 7 display energy level diagrams reflecting the data reported in Table II for the CH 35Cl isotopomer. Each diagram compares the observed term energies to the calcua lated ˜ and ˜ state term energies without incorporation of X spin-orbit effects. The latter reflects the data of Tarczay et al.,118 the former were calculated from our Dunham expansion parameters Table III . The lower panel of Fig. 7 shows an expanded view of the region of the ˜ state origin, where a
    
    TABLE V. Vibrational term energies in cm−1 for the ˜ 3A state of CHCl derived from SVL emission spectra. a Assignments and deviations from the calculations of Refs. 118 and 120 are shown. Term energy CH 35Cl Term energy CH 37Cl
    
    Assignments and Deviations CH 35Cl Ref. 120 o.-c.b 12 −14 45 −54 ¯ ¯ Ref. 118 o.-c. VPT2 2 7 −31 5 15 −13 Var4 2 5 −31 0 14 3
    c
    
    This worka 2172 3058 3144 3926 5255 5799
    a b
    
    Ref. 114 2167 ¯ ¯ ¯ ¯ ¯
    
    This worka 2170 3049 3137 3912 5254 5792 3 4 4 4 3 4
    
    Ref. 114 2167 ¯ ¯ ¯ ¯ ¯
    
    Assignment ˜ a ˜ a ˜ a ˜ a ˜ a ˜ a 0,0,0 0,0,1 0,1,0 0,0,2 1,0,0 0,3,1
    
    Var4+ 2 5 −31 0 14 3
    
    2 1 1 4 3 1
    
    One standard deviation in parenthesis right justified . Observed minus calculated. c Labels for 2 and 3 reversed from that given in Ref. 118 See Ref. 130 .
    
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    224314-9
    
    Spectroscopy of CHCl
    
    J. Chem. Phys. 124, 224314 2006
    
    FIG. 8. Expanded view of the CH35Cl single vibronic level emission speca trum from 112431, showing the ˜ 3A 0 , 0 , 0 , 0,0,1 , and 0,1,0 levels. The x axis labels the shift in frequency from the excitation line.
    
    spin-orbit interaction shifts the ˜ 0 , 2 , 0 up by 4 cm−1 and X the 0,1,1 level down by 2 cm−1 according to the Dunham expansion fit, while the 0,0,3 level is essentially unperturbed. The shifts calculated by Yu et al.120 for ˜ 0 , 2 , 0 , X 0,1,1 , and 0,0,3 are, respectively, +5, −1, and 0 cm−1, in good agreement with our results. The upper panel of Fig. 7 displays the region of the ˜ 0 , 0 , 1 and 0,1,0 levels, which a are reported here for the first time see below . The ˜ 0 , 2 , 1 X −1 and 0,1,2 levels are shifted by +9 and −3 cm , respectively, while the 1,0,0 and 0,0,4 are essentially unperturbed. The observed shifts are again in good agreement with the predictions of Yu et al. +8 and −1 cm−1, respectively, for ˜ 0 , 2 , 1 and 0,1,2 .120 X The ˜ state Dunham fit parameters for both CH 35Cl and X CH 37Cl are compared in Table III with the results of Tarczay et al.118 and our own density functional theory DFT calculations at the B3LYP/aug-cc-pVTZ level, previously shown to reproduce experimental harmonic vibrational frequencies in the ˜ state of CHF/CDF and CHBr.117,123 The calculations X were carried out using the GAUSSIAN 98 electronic structure package129 on a personal computer. The calculated frequencies and geometrical parameters are in good agreement with previous ab initio values Table IV , and with experiment Tables III and IV . Note that the CCSD T method, in combination with the cc-pVXZ and aug-cc-pVXZ basis sets,95,118 also well reproduces the experimental ˜ state vibrational freX quencies. Turning to the ˜ 3A state, we have identified five new a levels Table V and Fig. 8 , including all three fundamentals. These are compared with ab initio values in Tables IV and V; again the CCSD T method in combination with cc-pVXZ and aug-cc-pVXZ basis sets well reproduces the experimental vibrational frequencies.95,118 Note that the labels for 2
    
    FIG. 9. Expanded view of the CH35Cl single vibronic level emission speca trum from the Ka = 1 level of 24, showing the ˜ 3A 0 , 0 , 0 level. The strongest peaks are assigned as transitions to Ka = 0 and 2 and the observed spacing gives a value for A − ¯ of 26 2 cm−1, in excellent agreement with B ab initio predictions. The spectrum was averaged over 300 000 laser shots, and the x axis labels the shift in frequency from the excitation line.
    
    and 3 given in Tables 10 and 12 of Ref. 118 should be reversed; i.e., the C–Cl stretch is lowest in frequency.130 Our own calculations at the B3LYP/aug-cc-pVTZ level are in good agreement with experiment and previous ab initio calculations Table IV .121 Finally, we note that the identification of the triplet origin in both CHCl and CHBr has, to date, relied on comparisons with the predictions of high level ab initio calculations, as the rotational structure of the triplet bands has not been obtained. This is because for CHBr and, at higher energy, CHCl, only subbands with Ka = 0 appear strongly in the spectra, and SVL emission spectra show predominantly Ka = 0 → Ka = 1 transitions, as described above. However, at energies close to and below the barrier to linearity, perturbed subbands with Ka 0 do appear in the spectra of CHCl.47,80 We obtained SVL emission spectra from several of the Ka = 1 subbands in order to probe ground state levels with Ka = 0 and 2 and thus estimate A − ¯ for the triplet origin. The B triplet spectrum from these levels was very weak and required significant averaging; an example spectrum averaged over 300 000 laser shots is shown in Fig. 9. The ˜ 1A state X peaks not shown exhibit a splitting consistent with the B known ˜ 1A state A − ¯ constant.62 The two strongest tranX sitions in Fig. 9 are assigned as transitions to Ka = 0 and 2, and the splitting gives a value for A − ¯ of 26 2 cm−1, in B excellent agreement with the ab initio prediction of 25 cm−1.95 We also appear to observe a weak parallel Ka = 0 component.
    CONCLUSIONS
    
    We have obtained new fluorescence excitation spectra of CHCl in the 450– 750 nm region; a total of 21 cold bands
    
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    224314-10
    
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    10
    
    J. Chem. Phys. 124, 224314 2006 J. W. C. Johns, D. A. Ramsay, and S. C. Ross, Can. J. Phys. 54, 1804 1976 . 11 R. K. Lengel and R. N. Zare, J. Am. Chem. Soc. 100, 7495 1978 . 12 D. Feldmann, K. Meier, H. Zacharias, and K. H. Welge, Chem. Phys. Lett. 59, 171 1978 . 13 D. Feldmann, K. Meier, R. Schmiedl, and K. H. Welge, Chem. Phys. Lett. 60, 30 1978 . 14 M. N. R. Ashfold, M. A. Fullstone, G. Hancock, and G. Duxbury, Mol. Phys. 45, 887 1982 . 15 T. J. Sears, P. R. Bunker, A. R. W. McKellar, K. M. Evenson, D. A. Jennings, and J. M. Brown, J. Chem. Phys. 77, 5348 1982 . 16 A. R. W. McKellar, P. R. Bunker, T. J. Sears, K. M. Evenson, R. J. Saykally, and S. R. Langhoff, J. Chem. Phys. 79, 5251 1983 . 17 H. Petek, D. J. Nesbitt, P. R. Ogilby, and C. B. Moore, J. Phys. Chem. 87, 5367 1983 . 18 D. Feller and E. R. Davidson, J. Chem. Phys. 80, 1006 1984 . 19 H. Petek, D. J. Nesbitt, D. C. Darwin, and C. B. Moore, J. Chem. Phys. 86, 1172 1987 . 20 H. Petek, D. J. Nesbitt, C. B. Moore, F. W. Birss, and D. A. Ramsay, J. Chem. Phys. 86, 1189 1987 . 21 G. Duxbury and C. Jungen, Mol. Phys. 63, 981 1988 . 22 P. Jensen and P. R. Bunker, J. Chem. Phys. 89, 1327 1988 . 23 H. Petek, D. J. Nesbitt, D. C. Darwin, P. R. Ogilby, C. B. Moore, and D. A. Ramsay, J. Chem. Phys. 91, 6566 1989 . 24 W. Xie, C. Harkin, H.-L. Dai, W. H. Green, Jr., Q. K. Zheng, and A. J. Mahoney, J. Mol. Spectrosc. 138, 596 1989 . 25 W. H. Green, Jr., I.-C. Chen, H. Bitto, D. R. Guyer, and C. B. Moore, J. Mol. Spectrosc. 138, 614 1989 . 26 L. B. Knight Jr., M. Winiski, P. Miller, C. A. Arrington, and D. Feller, J. Chem. Phys. 91, 4468 1989 . 27 W. Xie, C. Harkin, and H.-L. Dai, J. Chem. Phys. 93, 4615 1990 . 28 A. Alijah and G. Duxbury, Mol. Phys. 70, 981 1990 . 29 W. H. Green, Jr., N. C. Handy, P. J. Knowles, and S. Carter, J. Chem. Phys. 94, 118 1991 . 30 G. V. Hartland, D. Qin, and H.-L. Dai, J. Chem. Phys. 98, 2469 1993 . 31 I. Garcia-Moreno, E. R. Lovejoy, C. B. Moore, and G. Duxbury, J. Chem. Phys. 98, 873 1993 . 32 I. Garcia-Moreno and C. B. Moore, J. Chem. Phys. 99, 6429 1993 . 33 D. Qin, G. V. Hartland, and H.-L. Dai, J. Mol. Spectrosc. 168, 333 1994 . 34 H. Nakatsuji, M. Ehara, and T. Momose, J. Chem. Phys. 100, 5821 1994 . 35 B.-C. Chang, M. Wu, G. E. Hall, and T. J. Sears, J. Chem. Phys. 101, 9236 1994 . 36 P. Jensen, M. Brumm, W. P. Kraemer, and P. R. Bunker, J. Mol. Spectrosc. 171, 31 1995 . 37 G. V. Hartland, D. Qin, and H.-L. Dai, J. Chem. Phys. 102, 6641 1995 . 38 S. A. Perera, L. M. Salemi, and R. J. Bartlett, J. Chem. Phys. 106, 4061 1997 . 39 C. Fockenberg, A. J. Marr, T. J. Sears, and B.-C. Chang, J. Mol. Spectrosc. 187, 119 1998 . 40 A. J. Marr, T. J. Sears, and B.-C. Chang, J. Chem. Phys. 109, 3431 1998 . 41 G. Duxbury, B. D. McDonald, M. Van Gogh, A. Alijah, C. Jungen, and H. Palivan, J. Chem. Phys. 108, 2336 1998 . 42 G. Duxbury, A. Alijah, B. D. McDonald, and C. Jungen, J. Chem. Phys. 108, 2351 1998 . 43 J.-P. Gu, G. Hirsch, R. J. Buenker, M. Brumm, G. Osmann, P. R. Bunker, and P. Jensen, J. Mol. Struct. 247, 517 2000 . 44 K. Kobayashi, L. Pride, and T. J. Sears, J. Phys. Chem. A 104, 10119 2000 . 45 A. Kalemos, T. H. Dunning, Jr., A. Mavridis, and J. F. Harrison, Can. J. Chem. 82, 684 2004 . 46 E. Ionescu and S. A. Reid, J. Mol. Struct.: THEOCHEM 725, 45 2005 . 47 A. J. Merer and D. N. Travis, Can. J. Phys. 44, 525 1966 . 48 A. J. Merer and D. N. Travis, Can. J. Phys. 44, 1541 1966 . 49 R. Hoffmann, G. D. Zeiss, and G. W. Van Dine, J. Am. Chem. Soc. 90, 1485 1968 . 50 M. E. Jacox and D. E. Mulligan, J. Chem. Phys. 47, 1626 1967 ; 50, 3252 1969 . 51 J. F. Harrison, J. Am. Chem. Soc. 93, 4112 1971 . 52 V. Staemmler, Theor. Chim. Acta 35, 309 1974 . 53 C. W. Bauschlicher, Jr., H. F. Schaefer III, and P. S. Bagus, J. Am. Chem. Soc. 99, 7106 1977 .
    
    involving the pure bending levels 2n with n = 1 – 7 and com0 bination bands 2n31 n = 4 – 7 , 2n32 n = 4 – 6 , 112n n 00 00 00 = 5 – 7 , 112n31 n = 4 – 6 , and 112n32 n = 4 in the ˜ 1A A 000 000 ˜ 1A system were observed. The spectra were measured ←X under jet-cooled conditions using a pulsed discharge source, and analysis typically yielded band origins and rotational constants for both chlorine isotopomers CH 35Cl, CH 37Cl . The derived ˜ state vibrational intervals were combined with A the term energy of the origin band reported by Chang and Sears Ref. 80 to derive the excited state barrier to linearity; the barrier height with respect to the ˜ state vibrationless A −1 ˜ 1A state C–H stretching frelevel is 1920 50 cm . The A quency was determined for the first time, and the experimental value of 2980 cm−1 is in excellent agreement with theory.120 Single vibronic level SVL emission spectra were obtained from several ˜ 1A state levels using a 0.3 m specA trograph equipped with a gated intensified CCD detector. These spectra reveal rich new information on the vibrational structure of the ˜ 1A state up to 9000 cm−1 above the X vibrationless level, as we observe around three times the number of levels previously reported. The results of a fit of the ground state vibrational term energies to the Dunham expansion, and comparisons with previous experimental and recent high level ab initio studies, are reported. As first reported by Chang and co-workers,97,101,114 the spectra show significant perturbations due to spin-orbit interaction with the low-lying ˜ 3A state. Our results confirm the position of the a ˜ 3A state origin, and our value for T00 ˜ − ˜ is in excellent aX a agreement with recent ab initio calculations. We report the first measurement of the rotational constant A − ¯ of the B ˜ 3A origin, also in excellent agreement with theory. Finally, a we observed five new levels of the ˜ 3A state, including all a three fundamentals, and the derived vibrational term energies are also in good agreement with ab initio predictions.
    ACKNOWLEDGMENT
    
    The National Science Foundation Grant No. CHE0353596 is gratefully acknowledged for support of this research.
    1
    
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    224314-11
    54 55
    
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    98
    
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