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    First observation of the elusive iodocarbene: ground state multiplicity and singlet–triplet gap of CHIw
    Chong Tao, Carlena Ebben, Hsiang-Ting Ko and Scott A. Reid*
    Received 24th July 2008, Accepted 7th August 2008 First published as an Advance Article on the web 3rd September 2008 DOI: 10.1039/b812734d We report the first observation of iodocarbene (CHI) using fluorescence excitation and emission spectroscopy, which indicate a singlet ground state and a lower bound to the singlet– triplet gap of 3.76 kcal molÀ1. Carbenes are among the most important reactive intermediates,1 and a central issue in carbene chemistry has been determining the energy splitting (or gap, DEST) between the low-lying singlet and triplet states associated with the divalent carbon. In this quest, the monohalocarbenes CHX (X = F, Cl, Br, I) have served as valuable benchmarks,2–5 and recent studies of CHCl and CHBr using single vibronic level (SVL) emission4,5 and stimulated emission pumping (SEP) spectroscopy6 have provided precise estimates of DEST which are largely in excellent agreement with high level ab initio calculations. Of the triatomic halocarbenes, only the iodocarbenes CXI (X = H, F, Cl, Br, I) have not been observed, and (due in part to the cost associated with computational investigation of these many-electron molecules) there has been considerable controversy in the literature concerning the ground state multiplicity of these species.2,3 The most recent theoretical studies of CHI have pointed to a singlet ground state, with DEST between 0.8 and 6.3 kcal molÀ1.3 This is in agreement with early radiochemical studies,7 but in disagreement with the photoelectron studies of CHIÀ by Lineberger and co-workers, which suggest that DEST lies between À2 and À10 kcal molÀ1 (i.e., a triplet ground state).2 From a broader perspective, the use of iodocarbenes in stereoselective organic synthesis8 has been demonstrated; however, the utility of these compounds has been limited by a lack of direct experimental observation and characterization. In this work, we report the first spectroscopic detection of an iodocarbene, CHI. We have recorded fluorescence excitation and SVL emission spectra of jet-cooled CHI produced in a pulsed discharge source. Our spectra conclusively demonstrate a singlet multiplicity for the ground state, and a deperturbation analysis of the vibronic structure observed in SVL emission provides a lower bound on the singlet–triplet gap, which we compare with ab initio and density functional theory (DFT) calculations. The experimental approach used in this work is similar to that employed in recent studies from our laboratory.5,6 The halocarbene CHI was produced using a pulsed electrical discharge through a precursor seeded in high purity He. Initially, we observed very weak CHI signals using CH2I2 as precursor; the signals were enhanced using a solution of CHI3 in CH2I2. In either case the mixture was formed by passing He through a stainless steel bubbler containing the precursor solution. The discharge was initiated by a 50–100 ms wide, +1000 V pulse generated from a high voltage (HV) pulser (Directed Energy with a Glassman HV power supply). The discharge products expanded into a vacuum chamber evacuated by a 600 diffusion pump, and were crossed at right angles by a laser beam generated from a Nd:YAG pumped dye laser (Continuum NY-61/Lambda-Physik Scanmate 2E or Spectra Physics INDI/Syrah Cobra-stretch). The resulting fluorescence was collected by a 2-inch diameter f/2.4 plano-convex lens and focused using a second 2-inch diameter f/3.0 lens either: (a) through a long-pass cutoff filter onto a photomultiplier tube detector for monitoring total fluorescence; or (b) onto the slit of a 0.3 m spectrograph equipped with a gated intensified charge coupled device detector for wavelength-resolved emission spectra. Background spectra were obtained with the laser blocked to identify emission lines from the discharge. Fig. 1 displays a comparison of the first CHI band we recorded, lying near 13 638 cmÀ1, with spectra previously ˜ ˜ obtained in our laboratory for bands in the A1A00 ’ X1A 0 systems of CHCl and CHBr. The striking similarity between these spectra is apparent. Each displays prominent pP1, pQ1, and pR1 bands of a c-type transition, along with a qQ0 subband which arises from axis tilting, due to the large change in equilibrium bond angle between the component electronic states. The appearance of this sub-band affords an estimate of the A rotational constant in the lower state, which we place at 15.3(2) cmÀ1, consistent with expectations for the lowest 1A 0 state.3 In contrast, calculations predict a much larger A constant (B28 cmÀ1) for the 3A00 state.3 As found also for CHCl and CHBr, sub-bands terminating in levels with Ka 0 4 0 were not observed, indicating that this level lies above the Renner–Teller intersection.5 To place this assignment on stronger footing, we fit the measured spectrum to determine rotational constants for the upper and lower states, using the PGopher program of Colin Western.9 A stick simulation based upon the fit is shown together with the experimental spectrum of the 13638 cmÀ1 band in Fig. 2, and the parameters determined from the fit are compared in Table 1 with theoretical expectations for the lowest 1A 0 state. Based upon the quality of the derived fit and the observed lower state rotational constants, we confirm a singlet multiplicity for both states in the transition. A list of observed and calculated line positions is provided in the ESI.w
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    Department of Chemistry, Marquette University, Milwaukee, WI 53201-188, US. E-mail: scott.reid@mu.edu; Fax: +1 (0)414 288 7066; Tel: +1 (0)414 288 7565 w Electronic supplementary information (ESI) available: List of observed and calculated line positions for the 13 638 cmÀ1 band. See DOI: 10.1039/b812734d
    
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    Table 1 Rovibrational parameters determined from a fit of the 13 638 cmÀ1 band and theoretical predictions Parameter Te A00  B00 DJ00  B0 DJ 0 N s
    a
    
    This work 13 638.72(2) 15.3(2) 0.3281(20) À2.1(15) Â 10À5 0.3401(18) À1.4(10) Â 10À5 21 0.04
    
    Theorya 15.02 0.3213
    
    Equilibrium values calculated at B3LYP/Sadlej-pVTZ level.
    
    Fig. 1 Fluorescence excitation spectra of the 203 band of CHCl, 204 band of CHBr, and 13 638 cmÀ1 band of CHI. Details and a tentative assignment for the CHI band are provided in the text.
    
    blue of the laser excitation line, indicating that the bands excited are cold bands (i.e., originating from the vibrationless level), and that the lower state accessed in these transitions is most likely the true ground state of this molecule. This strongly suggests a singlet multiplicity for the ground state. Further evidence is provided in an analysis of the vibrational structure in the emission spectra. Previous studies of CHCl and CHBr have shown that the bending levels are the most strongly perturbed by nearby triplet levels, due to more favorable vibrational overlap, while the pure C–X stretching levels are largely unperturbed.4–6 Indeed, in Fig. 3 we can easily identify a progression in the C–I stretching mode (3n) which extends up to n = 6. A fit of the observed levels to a standard anharmonic effective Hamiltonian model yields: o3 = 594.0(4) cmÀ1, x33 = À3.1(1) cmÀ1. The observed frequency is in excellent agreement with a theoretical value of 589 cmÀ1 calculated at the B3LYP/Sadlej-pVTZ level using the Gaussian 98 program suite.10–12 Note that our previous
    
    Fig. 2 Fluorescence excitation (upper) and stick simulation of the 13 638 cmÀ1 band. The simulation is the result of a rotational fit described in the text. The fit constants are given in Table 1.
    
    The data presented thus far do not provide conclusive evidence regarding the multiplicity of the CHI ground state, since low-lying electronic states may be thermally populated in the discharge. To determine this, we measured SVL emission spectra. Example spectra from bands at B15 045 and 15 200 cmÀ1 are shown in Fig. 3. In these spectra we find no transitions to the
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    Fig. 3 (a) SVL emission spectrum obtained following excitation of the 15 200 cmÀ1 band. Vibrational assignments are noted, and lines marked with a black dot indicate background discharge emission. (b) SVL emission spectrum following excitation of the 15 045 cmÀ1 band.
    
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    halocarbene studies have demonstrated that the B3LYP functional, in combination with basis sets of triple zeta quality, quantitatively reproduces harmonic vibrational frequencies.5,6 In contrast to the C-I stretching states, all states containing bending excitation are severely perturbed. We can identify the bending fundamental at 948 cmÀ1, significantly lower than predicted (1048 cmÀ1) at the B3LYP/Sadlej-pVTZ level. This level is being pushed down by interaction with the triplet origin, which we observe at 1407 cmÀ1. At higher energies, it is very difficult to make any assignments in the bending manifold. Based upon the estimated singlet–triplet splitting (detailed below) and harmonic vibrational frequencies, eight singlet levels and three triplet levels are predicted to lie in the region 0–2500 cmÀ1 above the singlet vibrationless level. Indeed, in this region we observe a total of 11 lines, suggesting that all possible states are observed. Although we can reasonably assign the triplet origin to the band at 1407 cmÀ1, this is obviously not its true (i.e., unperturbed) position, the determination of which requires a deperturbation analysis. Our results suggest that, in addition to the 21 level, the origin is also perturbing higher lying singlet vibrational states, indicating the need for a multilevel deperturbation analysis, which will be the focus of future work. However, we can set a rough lower limit on the unperturbed position of the triplet origin by assuming a two level interaction with the singlet 21 level. Using the formulas obtained from two level nondegenerate perturbation theory, we place a lower ˜ bound on the triplet origin (i.e., T00(a˜ À X)) of 1315 cmÀ1, or 3.760 kcal molÀ1. Our lower limit for the singlet–triplet gap lies firmly within the range of previous theoretical predictions.3 As part of our recent SEP studies of CHCl,6,13 which gave a very precise determination of the singlet–triplet gap, we found that several DFT methods including BLYP, BVWN5, and MPWLYP could reproduce the experimental singlet–triplet gap to within B100 cmÀ1. Calculations for CHI using these three methods with the Sadlej pVTZ basis set yield values for the zero-point corrected DEST between 1620 and 1730 cmÀ1, or 4.64–4.94 kcal molÀ1, consistent with our experimental lower bound. Finally, while a detailed study of the bands observed in the ˜ ˜ A1A00 ’ X1A 0 systems of CHI must necessarily be left to a future report, it is appropriate to comment here on a probable assignment. Drake, et al. reported calculations of the adiabatic ˜ ˜ A1A00 ’ X1A 0 transition energies for a number of halocarbenes,14 and their CASPT2(18,12) calculations generally show agreement with experiment to within 500 cmÀ1 or better. Their CASPT2(18,12) value for CHI is 10856 cmÀ1. Our measurements of this system show polyads with an intrapolyad spacing of roughly B160 cmÀ1, suggesting excited state frequencies of B790 cmÀ1 (bend, n2) and B630 cmÀ1 (C–I stretch, n3). The emission spectrum obtained following excitation of the 13 638 cmÀ1 band is similar to that shown in Fig. 3(a), where the lack of nodal structure in the intensities of the C–I stretching progression is consistent with assignment to a pure bending level. Extrapolating back based upon the measured frequencies, we tentatively assign the 13 638 cmÀ1 band to 23, which would imply an origin frequency near 0 B11 300 cmÀ1. An alternative assignment to 24 would give an 0
    
    origin frequency near B10 500 cmÀ1. Additional work will be ˜ ˜ needed to clarify the position of the A1A00 ’ X1A 0 origin. The authors acknowledge support of this work by the National Science Foundation (CHE-0717960).
    
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