The rotational spectrum of dichlorocarbene, C³⁵Cl₂, observed by molecular beam-Fourier transform microwave spectroscopy

Abstract

The rotational spectrum of the most abundant isotopomer of dichlorocarbene, C³⁵Cl₂, in its X̃¹A₁ ground electronic state has been recorded using a molecular beam—Fourier transform microwave spectrometer. The CCl₂ was generated by flash pyrolysis in a pulsed free jet expansion using CBr₂Cl₂ or CHCl₂COCl as precursors. 43 hyperfine structure lines of 6 rotational transitions were recorded in the frequency range 8–40 GHz. The analysis of the observed spectrum yielded the rotational constants, the complete ³⁵Cl nuclear quadrupole coupling tensor, and the nuclear spin-rotation interaction constants. In addition, a refined r₀ structure of CCl₂ was derived.

---

1. Introduction

Carbenes are highly reactive intermediates whose physical properties and chemical reactions have been extensively studied for quite some time.¹ Depending on the nature of the substituents, the carbenes may have either singlet or triplet electronic ground states. In many cases, these states are very close in energy. Singlet and triplet carbenes exhibit very different chemical properties.^1–4

Dichlorocarbene, CCl₂, is a prototypical halogen-containing carbene. Its electronic structure is particularly important for understanding the unique chemical behavior of carbenes. Over the years, CCl₂ has therefore been the subject of numerous spectroscopic^5–22 and theoretical^23–36 investigations. Most of the theoretical studies focused on the energy gap between the X̃¹A₁ and the ã³B₁ electronic states. A consensus seemed to have emerged that the ã³B₁ electronic state should be at about T₀(ã) = (7000 ± 1000) cm⁻¹ above the X¹A₁ ground state. A more sensitive and higher resolution investigation of the photoelectron spectrum of the CCl₂⁻ anion has recently been published, however, by the Lineberger group,²² in which a value for the singlet–triplet splitting of T₀(ã) = (1000 ± 1000) cm⁻¹ was derived, much lower than the theoretical estimates. Barden and Schaefer³⁶ have argued, on the other hand, that this low experimental value should be outside the error limits of the theoretical calculations. In this situation, further experimental work on the spectroscopy of CCl₂ clearly seems desirable.

The present paper is part of our continuing studies of the rotational spectra of halogen-containing carbenes. Previously, we investigated the rotational spectrum of CF₂ (X̃¹A₁).³⁷ In the observed CF₂ spectra, we detected unusually large nuclear spin–rotation interaction parameters for the two identical ¹⁹F nuclei. This also motivated us to analyze the rotational spectrum of the corresponding CCl₂ carbene.

In this work, high resolution molecular beam-Fourier transform microwave (MB-FTMW) spectroscopy has been used to observe a series of low lying rotational transitions of C ³⁵Cl₂ (X̃¹A₁) in its ground vibrational state in the frequency range from 8 to 40 GHz. This investigation complements an earlier study by Fujitake and Hirota, who reported a weak pure rotational spectrum of CCl₂ in the millimeter and submillimeter wave region.²¹ Because of the high resolution of the MB-FTMW spectrometer, even very small perturbations could be detected. All of the observed lines were split into several components due to the nuclear quadrupole hyperfine interaction of the two identical ³⁵Cl nuclei. Furthermore, additional perturbations were observed caused by the ³⁵Cl nuclear spin–rotation interaction and by the influence of the off-diagonal element in the nuclear quadrupole coupling tensor. The results of the measurements were used to determine precise rotational and ³⁵Cl nuclear spin–rotation interaction constants as well as the complete nuclear quadrupole coupling tensor. Diagonalization of this tensor revealed a small deviation of the principal axes of the χ tensor from the C–Cl internuclear axis. In addition, an r₀ structure of CCl₂ in the singlet ground state was derived by using the effective moments of inertia obtained from the rotational constants.

2. Experimental

The rotational spectra were recorded with a pulsed MB-FTMW spectrometer described in some detail previously.³⁸ Some modifications have been made in order to increase its sensitivity and, in addition, the accessible frequency range has been extended from the previous 7–26 GHz up to 40 GHz. These changes are described in the following.

2.1 Microwave circuits

The electronic set-up of the spectrometer in its extended version for the frequency range from 26.5 to 40 GHz is shown in Fig. 1. The detection cavity is formed by a flat and a concave aluminium mirror (1 and 2) with diameters of 45 cm at a distance of ≈70 cm. Since in the region from 26.5 to 40 GHz some MW components are not available or very expensive, the polarizing frequency is produced by frequency doubling. The primary MW signal at frequency ν is generated with a synthesizer (3). A single pole double throw (SPDT) PIN diode switch (4) guides the signal alternately into the polarization or detection sections. In the former, the MW pulses are prepared for excitation of the molecules; in the latter, a superheterodyne detection of the response signal of the molecular ensemble is carried out. For polarization, the MW power can be adjusted with the variable attenuators (5) and (6). The single side-band modulator (7) generates a MW signal at a frequency ν + 80 MHz. The SPDT switch (8) modulates the 80 MHz signal synchronously to the MW pulse creation. The power amplifier (9) compensates for the conversion loss of (7). The polarization frequency is then generated by doubling this primary MW frequency. The frequency doubler (10) provides sufficient MW power for the excitation of weakly polarized molecules. The power level can be monitored [italic v]ia the directional coupler (11) and the power meter (12). The MW pulses are reshaped by the SPDT PIN diode switch (13). The MW signal is then coupled into the spectrometer cavity [italic v]ia an L-shaped antenna.

Fig. 1 Microwave circuit of the MB-FTMW spectrometer for the frequency range from 26.5–40 GHz. (1) Flat aluminium mirror; (2) concave aluminium error; (3) MW synthesizer; (4) SPDT switch; (5 and 6) variable attenuators; (7) single side-band modulator; (8) SPDT switch; (9) power amplifier; (10) MW frequency doubler; (11) directional coupler; (12) power meter; (13) SPDT switch; (14) MW low-noise amplifier; (15) SPDT switch; (16) detector diode; (17) second harmonic mixer; (18) RF amplifier; (19) RF attenuator; (20) RF amplifier; (21) band pass filter; (22) RF mixer; (23) RF synthesizer; (24) low pass filter; (25) digitizer. See the text for a detailed explanation.

During the detection period, the molecular response is received by a second antenna and passed to the low-noise MW amplifier (14). The MW antennas are mounted near the center of the flat mirror. To avoid any degradation of the molecular signal, the amplifier is directly mounted to the back-side of the mirror, with no protection switch installed. The recovery time of the unprotected MW amplifier after the microwave pulse is fast enough to enable the detection of the free induction decay (FID) of the molecular sample without additional delay. The SPDT switch (15) passes the excitation power to the diode (16) to protect the subsequent detection electronics from saturation. Following a short delay after the MW pulse, the signal path is set by (15) to the second harmonic mixer (17) which down-converts the molecular MW signal to an intermediate frequency band around 160 MHz. This signal is amplified (18–20) and band-pass filtered (21). An RF mixer (22) uses a 157.5 MHz frequency signal (23) to convert the molecular signal down to a frequency around 2.5 MHz. This signal is low-pass filtered (24), digitized (25) and sent to a PC microcomputer for data averaging, storage and analysis. For frequency tuning of the resonator, the switches (4), (8) and (13) are set for cw power, while switch (15) passes the transmitted MW signal to the detector diode (16). The diode output is A/D converted to provide the required tuning feedback.

With slight modifications, the spectrometer can also be operated in the 7 to 26.5 GHz frequency range. The frequency doubler (10) has to be removed and the components (11–15) have to be replaced by similar ones specified for the lower frequencies. Furthermore, instead of using the second harmonic mixer, an image rejection mixer is used. The RF components (18–24) are the same for both set-ups. For the single sideband generation (7), an RF signal at 160 MHz (instead of 80 MHz) is used.

2.2 Generation of CCl₂ and measurement of the FTMW spectra

The spectrometer offers two options for generating free radicals.³⁷ Using the first source, transient species can be produced by laser photolysis in the expanding molecular beam. In the second source, radicals are produced by high-temperature flash pyrolysis in a SiC extension tube attached to the face plate of the pulsed molecular beam valve (General Valves Series 9).³⁹ The nozzle is located in the center of the spherical mirror so that the expanding gas forms a free jet along the cavity axis.

In the present study, CCl₂ was produced by the flash pyrolysis method using dibromodichloromethane (CBr₂Cl₂) or dichloroacetylchloride (CHCl₂COCl) as precursors. The latter was found to give somewhat stronger signals. Using CHCl₂COCl, the pyrolysis initially leads to dichloroketene (Cl₂C[double bond, length half m-dash]C[double bond, length half m-dash]O) by elimination of HCl. At higher temperatures, the Cl₂C[double bond, length half m-dash]C[double bond, length half m-dash]O loses CO to yield the CCl₂ carbene. The gas flow was maintained by bubbling Ar at a backing pressure of 5 bar through liquid CHCl₂COCl or CBr₂Cl₂ contained in a stainless-steel reservoir at room temperature. The radicals in the jet were excited with microwave pulses of 0.2 μs duration at an optimal power of ≈5 mW. After the decay of the polarization pulse in the cavity (7–10 μs), the molecular emission signal was amplified and down-converted to the frequency band around 2.5 MHz, as described above. This signal was sampled at a rate of 50 (or 100) MHz using a transient digitizer. Free induction decay signals were averaged over 1024 cycles for the strongest rotational transitions and over 8192 cycles for the weakest ones, transferred to the PC microcomputer, and Fourier transformed to obtain power or amplitude spectra. Linewidths of ca. 4 kHz (HWHM) were obtained. The precision of the measurements for strong, well resolved lines is ca. 2 kHz. The precise transition frequencies were determined from fits to the time-domain signals.⁴⁰

3. Results

3.1 Observed spectra

The measurements of the CCl₂ FTMW spectra were initiated using the spectroscopic constants of Fujitake and Hirota²¹ to predict approximate transition frequencies in the frequency range of our spectrometer. After adjusting all pulse lengths and delay times, signals which were due to CCl₂ were readily detected. Fig. 2 shows as an example the power spectrum of the J_Ka,Kc = 3_1,3 − 4_0,4 transition with different hyperfine structure components. All observed frequencies are listed in Table 1. A total of 43 hyperfine structure components were resolved and assigned to 6 different rotational transitions.

Fig. 2 Different hyperfine components of the J_Ka,Kc = 3_1,3–4_0,4 transition of C³⁵Cl₂. Shown are the Doppler doublets. The components are denoted by F′–F. Recording conditions: molecular pulse width: 990 μs; molecular pulse–MW pulse delay: 500 μs; polarization frequencies: 17368.36, 17371.28, 17374.20 MHz, respectively; MW pulse width: 0.2 μs; MW power: 5 mW; MW FID delay: 9 μs; sample interval: 10 ns, 8 k = 8192 data points; number of cycles averaged: 3164, 4214, 8192, respectively.

Table 1 Observed and fitted rotational transitions J_{Ka^′,Kc^′}–J_Ka,Kc of C³⁵Cl₂. F and I denote the hyperfine quantum numbers. Δν gives the difference of the observed minus calculated frequencies

J′	Ka ^′	Kc ^′	J	Ka	Kc	F′	I′	F	I	ν obs/MHz	Δν/kHz
1	1	1	2	0	2	1	0	2	2	32277.8185	−0.5
3	2	4	2	32266.3445	−1.4
2	2	3	2	32266.2685	−0.2
2	1	2	0	32254.6628	−0.8
2	1	2	3	0	3	4	3	4	3	24885.8464	4.3
3	3	4	3	24885.4179	1.0
5	3	5	3	24884.7243	−1.5
1	1	2	1	24883.0832	−0.4
2	3	3	3	24882.9971	−1.9
4	3	5	3	24882.7135	−0.3
3	1	4	1	24879.6759	1.5
5	3	6	3	24877.1207	−0.8
2	1	3	1	24875.7305	3.6
3	1	3	4	0	4	4	2	4	2	17377.1627	−0.4
3	2	4	2	17374.2478	0.9
5	2	6	2	17371.3947	0.5
4	2	5	2	17371.3348	−2.2
3	0	4	0	17368.4787	0.2
4	1	4	5	0	5	5	3	6	3	9746.2240	−1.2
4	3	5	3	9745.7052	−0.1
3	1	4	1	9745.3181	0.4
6	3	7	3	9744.3190	0.3
3	3	4	3	9743.5268	−4.0
5	1	6	1	9743.4949	2.2
2	3	3	3	9742.6413	0.0
7	3	8	3	9741.6703	1.0
4	1	5	1	9741.2656	−0.6
8	0	8	7	1	7	8	2	7	0	13809.2160	0.8
6	2	5	2	13807.5659	−0.4
7	2	6	2	13807.5659	1.5
10	2	9	2	13807.5474	−0.9
9	2	8	2	13807.5354	0.8
8	0	7	2	13805.8884	−0.9
9	0	9	8	1	8	6	3	5	3	21862.6046	0.1
9	1	8	1	21862.4057	−1.5
12	3	11	3	21862.2254	0.2
7	3	6	3	21861.2014	−1.7
8	3	7	3	21860.8600	0.4
10	1	9	1	21860.8432	−1.9
11	3	10	3	21860.5039	3.8
8	1	7	1	21859.4576	1.8
9	3	8	3	21859.3088	−2.2
10	3	9	3	21859.1228	1.5

---

The observed characteristic quadrupole hyperfine patterns are caused by the coupling of the nuclear spin angular momenta I₁ and I₂ of the Cl nuclei with the rotational angular momentum J. For molecules with two identical nuclei, like C³⁵Cl₂, it is most convenient to analyze the spectra using the symmetric coupling scheme

which means that the nuclear spins I₁ and I₂ of the two ³⁵Cl atoms couple to form a total nuclear spin I which then couples with the rotational angular momentum J to form the resultant total angular momentum F. Since CCl₂ has a plane of symmetry which contains the a and b inertial axes, the quadrupole coupling tensors of the chlorine nuclei have only one nonzero off-diagonal element, χ_ab. Moreover, since the χ tensors are traceless, only two of the three diagonal elements of each tensor are independent. In the present work, χ_bb and χ_cc were treated as the two adjustable diagonal elements and χ_aa was fixed accordingly. The first-order quadrupole splittings are completely determined by the diagonal elements χ_gg. Measurements of higher-order effects are necessary to determine the χ_ab element.

For the symmetric molecule C³⁵Cl₂, the hyperfine patterns consist of a symmetric triplet of strong lines (with the central line being much more intense than the others) or a more complex pattern. The former occurs for transitions between states in which K_a and K_c are both odd (oo) or both even (ee), whereas the more complex pattern occurs when K_a and K_c are of different parity (eo; oe). In a totally symmetric vibronic state, only antisymmetric nuclear spin states (I = 0, 2) exist for the symmetric rotational levels A(ee) and B_b(oo), while only symmetric nuclear spin states (I = 1, 3) exist for the antisymmetric rotational levels B_a(eo), and B_c(oe).

3.2 Theory

The effective Hamiltonian employed for analyzing the spectra is:

The asymmetric rotor Hamiltonian H_rot contains the rotational constants A, B and C of the molecule:

The components of the total angular momentum are denoted by P_g, referring to the principal inertia axes g = a, b, c. The centrifugal distortion term H_cd in the Hamiltonian (1) contains the fourth-order terms of the standard Hamiltonian of Watson in the A reduced form:⁴¹

[,]₊ denotes an anticommutator.

The quadrupole Hamiltonian H_nqhfs(³⁵Cl) represents the interaction between the nuclear quadrupole moments and the electric field gradients at two identical ³⁵Cl nuclei. It can be expressed as the scalar product of two spherical tensor operators of rank 2 as follows:

V _i ⁽²⁾ denotes the electric field gradient tensors and Q_i⁽²⁾ the nuclear quadrupole moment tensors of the ith nuclei. Due to the symmetry of dichlorocarbene, only three of the five components of V⁽²⁾ are different from zero. Instead of these spherical components, we used the Cartesian components of the “negative” field gradient tensor, defined in the principal axes inertial system of the molecule, q_gg′ = (∂²V/∂g∂g′) (g = a, b, c). The interpretation of the quadrupole hyperfine structure is then made in terms of the quadrupole coupling tensors χ⁽¹⁾ and χ⁽²⁾, χ_gg′ = eQq_gg′ , where e is the elementary charge and Q the quadrupole moment of the ³⁵Cl nuclei. In the case of C³⁵Cl₂, for the two equivalent nuclei, χ_gg⁽¹⁾ = χ_gg⁽²⁾ and χ_ab⁽¹⁾ = − χ_ab⁽²⁾.

In addition, the end-over-end rotation of the molecule generates a weak magnetic field with which the ³⁵Cl nuclear magnetic moments interact. Therefore, an extra term due to the nuclear spin–rotation interaction was included. The Hamiltonian H_nsr(³⁵Cl) can be written in the form

where C_i is the nuclear spin–rotation coupling tensor of the ith nuclei. Due to the small g-factor for the ³⁵Cl nucleus, this magnetic perturbation is very small and only the diagonal elements of the C tensor were taken into consideration.

3.3 Analysis—Determination of hyperfine constants

Using the Hamiltonian as defined by eqn. (1), all observed rotational transitions were assigned and fitted to the spectroscopic constants using the program SPFIT.⁴² The present MB-FTMW spectra were taken into account in the least squares analysis with an uncertainty of 2 kHz. In addition, the rotational transitions reported by Fujitake and Hirota²¹ in the millimeter and submillimeter wave region were taken into consideration with uncertainties of 500 kHz. A compilation of the millimeter and submillimeter frequencies (Table S1) can be found as Electronic Supplementary Information.† The total data base consists of 169 transition frequencies.

The molecular constants from the final fit are given in Table 2. We fitted simultaneously the rotational constants, the fourth-order centrifugal distortion constants, the χ tensor components (including the off-diagonal element χ_ab), and the nuclear spin–rotation interaction parameters. As seen from the standard errors, all constants are well determined. Diagonalization of the complete χ tensor gave the principal ³⁵Cl quadrupole coupling constants which are also included in Table 2. The small residual deviations between observed and calculated transition frequencies for the molecular beam spectra (Table 1) indicates that all relevant effects have been accounted for. Effects of nuclear spin–nuclear spin coupling and centrifugal distortion contributions to the quadrupole coupling constants were found to be negligible.

Table 2 Spectroscopic constants for C³⁵Cl₂ from the fit to the observed transition frequencies. Uncertainties in parentheses correspond to the 3σ standard deviations

C^35Cl2
Ref. 21	This work
Rotational constants:
A/MHz	50229.524(65)	50229.503(3)
B/MHz	3695.3440(31)	3695.3459(5)
C/MHz	3438.3633(32)	3438.364(1)
Centrifugal distortion constants:
ΔJ/kHz	1.5048(17)	1.505(1)
ΔJK/kHz	−33.324(50)	−33.2(1)
ΔK/MHz	1.5633(56)	1.557(1)
δJ/kHz	0.15602(18)	0.1558(3)
δK/kHz	8.8517(516)	9.3(3)
ΦJ/Hz	0.00057(30)	—
ΦJK/Hz	−0.0027(99)	—
ΦKJ/Hz	−6.8(24)	—
ΦK/kHz	0.25(17)	—
ϕJ/Hz	0.000198(37)	—
Quadrupole coupling constants:
χbb/MHz	17.40(85)	17.319(1)
χcc/MHz	5.21(85)	5.540(2)
∣χab∣/MHz	46.2(5)
χzz/MHz	−63.37	−53.1(4)
χxx/MHz	58.16	47.6(4)
χyy/MHz	5.21	5.540(2)
Nuclear spin–rotation interaction parameters:
Caa/kHz	58.2(9)
Cbb/kHz	2.8(2)
Ccc/kHz	1.4(3)
Fit properties:
number of lines	169
σ/kHz	116.3

---

3.4 Structure determination

For CCl₂ as a planar C_2v molecule, the geometry is determined by two parameters, the C–Cl bond lengths and the Cl–C–Cl angle. Since the molecule is vibrating, and only ground vibrational state rotational constants were obtained, any structure obtained is an effective structure. The ground state or r₀ structure of CCl₂ was determined using the program MWSTR.⁴³ The C–Cl bond length and the Cl–C–Cl angle were determined to be r₀(C–Cl) = 1.714(1) Å and Θ₀(Cl–C–Cl) = 109.35(4)°, where the values in parentheses denote the 3σ standard deviations.

4. Discussion

The derived rotational constants are in good agreement with the published results of Fujitake and Hirota.²¹ However, due to the higher resolution and sensitivity of the present study, the rotational constants are now much better defined. Compared to ref. 21, the better defined rotational constants lead to refined structural parameters. In the most recent theoretical investigation, Barden and Schaefer³⁶ predicted the equilibrium geometry at different levels of theory. Their calculated equilibrium values of r_e(C–Cl) = 1.720 Å and Θ_e(Cl–C–Cl) = 109.1° are in close agreement with our experimental values for the ground vibrational state.

Fujitake and Hirota²¹ used only first-order perturbation theory to determine the diagonal elements of the nuclear quadrupole coupling tensor. They could not detect higher-order contributions from the off-diagonal element. In this work, we found it necessary to include the nuclear spin–rotation interaction and higher-order nuclear quadrupole interaction effects in the Hamiltonian to describe the rotational spectrum to within the experimental error. Only with these additional terms did the standard error of the fit become comparable with the estimated errors of the measurements.

Knowledge of χ_ab allowed us to diagonalize the χ tensor for C³⁵Cl₂ and to determine the principal values and the angle θ_za between the z principal axis of the coupling tensor and the a principal axis of the inertia tensor. The coupling constants χ_gg (g = x, y, z) in the principal axes system of the coupling tensor are given in Table 2. The z axis of the χ tensor makes an angle of 56.76(3)° with the b inertial axis that bisects the Cl–C–Cl angle. According to the r₀ structure, the C–Cl internuclear axis makes an angle of 54.68° with the b inertial axis. This indicates a small deviation of the z axis of the χ tensor from the C–Cl internuclear axis of 2.16° in a direction away from the bisector of the Cl–C–Cl angle. Although this deviation is derived from r₀ structural data, it is considered to be significant since the equilibrium bond angles are only slightly different (<0.5°) as seen from data for other triatomic molecules.⁴⁴ In general, it is common to assume coincidence between the principal axes of the χ tensor and the internuclear axes for terminally bonded atoms. On the other hand, the conclusion that there is a deviation of the principal axes of the χ tensor from the C–Cl internuclear axis in CCl₂ and that that deviation is in the direction away from the bisector of the Cl–C–Cl angle is supported by work on related molecules. By analyzing the microwave spectrum of phosgene (COCl₂), Ferguson and Flygare⁴⁵ determined a deviation of the z axis of the χ tensor from the C–Cl axis of 2.0(4)° in a direction away from the bisector of the Cl–C–Cl angle. Merke and Dreizler⁴⁶ showed that the z axis of the coupling tensor in SCl₂ is tilted by 1.3° with respect to the S–Cl bond direction. Again, the angle between both z tensor axes is larger than the bond angle.

Considering the magnetic interaction between the nuclear magnetic moment of the Cl nuclei and the molecular rotation, due to the weakness of this interaction, only a few molecules are known so far, where this effect has to be taken into consideration (see, for example, ref. 47–57). Normally the nuclear spin–rotation coupling constants are very small. Noticeable in the present results is the relatively large C_aa constant. For comparison, the C_aa constant in CCl₂ is significantly (about one order of magnitude) larger than in SOCl₂.⁵² Due to the lack of nuclear spin–rotation interaction parameters in similar molecules, we reanalyzed the SCl₂ spectra of Merke and Dreizler⁴⁶ and found nuclear spin–rotation coupling constants similar to those derived for the SOCl₂.⁵² However, considering our results of CF₂³⁷ in comparison to those for CCl₂, this is the second time that large nuclear spin–rotation interaction constants were determined for an unstable carbene. This may be further evidence that low lying excited electronic states of CCl₂ and CF₂ play an important role for this magnetic hyperfine interaction.

5. Conclusion

High resolution molecular beam-Fourier transform microwave (MB-FTMW) spectroscopy has been used to observe rotational transitions of C³⁵Cl₂ in its ground vibrational state. The measured lines were split into several components due to the nuclear quadrupole hyperfine interaction of the two identical ³⁵Cl nuclei. Because of the high resolution of the MB-FTMW spectrometer, even small perturbations were detected so that the off-diagonal quadrupole coupling constant could be determined. The results were used to derive the complete nuclear quadrupole coupling tensor, which upon diagonalization revealed a slight deviation of the z axis of the χ tensor from the C–Cl internuclear axis. In the process, a set of refined and more precise rotational constants, and the ³⁵Cl nuclear spin–rotation constants have been determined. As in CF₂, the large coupling constants may hint at an influence of low lying electronic states in these unstable carbenes. In addition, an accurate r₀ structure of CCl₂ for the singlet ground state was determined. This success is a result of the high resolution and precision obtainable with the MB-FTMW technique.

Acknowledgements

The authors thank Dr. H. S. P. Müller (Universität Köln) for helpful information regarding the SPFIT program. Financial support of this work by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References

1. P. P. Gaspar and G. S. Hammond, in Carbenes, ed. R. A. Moss and M. Jones, Jr., Wiley, New York, 1975, vol. 2, p. 207..
2. T. Böhland, S. Dóbé, F. Temps and H. Gg. Wagner, Ber. Bunsen-Ges. Phys. Chem., 1985, 89, 1110.
3. T. Böhland, F. Temps and H. Gg. Wagner, Ber. Bunsen-Ges. Phys. Chem., 1986, 90, 468.
4. M. Koch, F. Temps, R. Wagener and H. Gg. Wagner, Z. Naturforsch. A, 1989, 44, 195.
5. D. E. Milligan and M. E. Jacox, J. Chem. Phys., 1967, 47, 703.
6. L. Andrews, J. Chem. Phys., 1968, 48, 979.
7. M. E. Jacox and D. E. Milligan, J. Chem. Phys., 1970, 53, 2688.
8. J. S. Shirk, J. Chem. Phys., 1971, 55, 3608.
9. A. K. Maltsev, O. M. Nefedov, R. H. Hauge, J. L. Margrave and D. Seyferth, J. Phys. Chem., 1971, 75, 3984.
10. L. Andrews, J. M. Grzybowski and R. O. Allen, J. Phys. Chem., 1975, 79, 904.
11. D. E. Tevault and L. Andrews, J. Mol. Spectrosc., 1975, 54, 110.
12. V. E. Bondybey, J. Mol. Spectrosc., 1977, 64, 180.
13. R. E. Huie, N. J. T. Long and B. A. Thrush, Chem. Phys. Lett., 1977, 51, 197.
14. J. J. Tiee, F. B. Wampler and W. W. Rice, Chem. Phys. Lett., 1979, 65, 425.
15. D. A. Predmore, A. M. Murray and M. D. Harmony, Chem. Phys. Lett., 1984, 110, 173.
16. K. K. Murray, D. G. Leopold, T. M. Miller and W. C. Lineberger, J. Chem. Phys., 1988, 89, 5442.
17. D. J. Clouthier and J. Karolczak, J. Phys. Chem., 1989, 93, 7542.
18. D. J. Clouthier and J. Karolczak, J. Chem. Phys., 1991, 94, 1.
19. J. Choe, S. R. Tanner and M. D. Harmony, J. Mol. Spectrosc., 1989, 138, 319.
20. J. Choe, S. R. Tanner and M. D. Harmony, J. Mol. Spectrosc., 1990, 140, 193.
21. M. Fujitake and E. Hirota, J. Chem. Phys., 1989, 91, 3426.
22. R. L. Schwartz, G. E. Davico, T. M. Ramond and W. C. Lineberger, J. Phys. Chem. A, 1999, 103, 8213.
23. C. W. Bauschlicher, Jr., H. F. Schaefer III and P. S. Bagus, J. Am. Chem. Soc., 1977, 99, 7106.
24. T. Ha, H.-U. Gremlich and R. E. Bühler, Chem. Phys. Lett., 1979, 65, 16.
25. M. T. Nguyen, M. C. Kerins, A. F. Hegarty and N. J. Fitzpatrick, Chem. Phys. Lett., 1985, 117, 295.
26. E. A. Carter and W. A. Goddard III, J. Phys. Chem., 1986, 90, 998.
27. E. A. Carter and W. A. Goddard III, J. Phys. Chem., 1987, 91, 4651.
28. E. A. Carter and W. A. Goddard III, J. Chem. Phys., 1988, 88, 1752.
29. G. L. Gutsev and T. Ziegler, J. Phys. Chem., 1991, 95, 7220.
30. N. Russo, E. Sicilia and M. Toscano, J. Chem. Phys., 1992, 97, 5031.
31. Z.-L. Cai, X.-G. Zhang and X.-Y. Wang, Chem. Phys. Lett., 1993, 210, 481.
32. A. Gobbi and G. Frenking, J. Chem. Soc., Chem. Commun., 1993, 14, 1162.
33. V. M. Garcia, O. Castell, M. Reguero and R. Caballol, Mol. Phys., 1996, 87, 1395.
34. M. Schwartz and P. Marshall, J. Phys. Chem. A, 1999, 103, 7900.
35. K. Sendt and G. B. Bacskay, J. Chem. Phys., 2000, 112, 2227.
36. C. J. Barden and H. F. Schaefer III, J. Chem. Phys., 2000, 112, 6515.
37. N. Hansen, H. Mäder and F. Temps, Chem. Phys. Lett., 2000, 327, 97.
38. N. Hansen, U. Andresen, H. Dreizler, J.-U. Grabow, H. Mäder and F. Temps, Chem. Phys. Lett., 1998, 289, 311.
39. D. W. Kohn, H. Clauberg and P. Chen, Re[italic v]. Sci. Instrum., 1992, 63, 4003.
40. J. Haekel and H. Mäder, Z. Naturforsch. A, 1988, 43, 203.
41. J. K. G. Watson, in Vibrational Spectra and Structure, ed. J. Durig, Elsevier, Amsterdam, 1977, vol. 6, p. 1..
42. H. M. Pickett, J. Mol. Spectrosc., 1991, 148, 371.
43. V. Typke, MWSTR program for the determination of r₀ structures using experimentally derived rotational constants..
44. W. Gordy and R. L. Cook, Microwa[italic v]e Molecular Spectra, Wiley, New York, 1984, ch. XIII, p. 678..
45. A. C. Ferguson and W. H. Flygare, J. Phys. Chem., 1979, 83, 3161.
46. I. Merke and H. Dreizler, Z. Naturforsch. A, 1992, 47, 1141.
47. A. Dubrulle, D. Boucher, J. Burie and J. Demaison, Chem. Phys. Lett., 1977, 45, 559.
48. H. E. Gillis Singbeil, W. D. Anderson, R. Wellington Davis, M. C. L. Gerry, E. A. Cohen, H. M. Pickett, F. J. Lovas and R. D. Suenram, J. Mol. Spectrosc., 1984, 103, 466.
49. G. Wlodarczak, N. Heineking and H. Dreizler, J. Mol. Spectrosc., 1991, 147, 252.
50. N. Heineking, W. Jäger and M. C. L. Gerry, J. Mol. Spectrosc., 1993, 158, 69.
51. H. S. P. Müller and M. C. L. Gerry, J. Chem. Soc., Faraday Trans., 1994, 90, 2601.
52. H. S. P. Müller and M. C. L. Gerry, J. Chem. Soc., Faraday Trans., 1994, 90, 3473.
53. B. Gatehouse, H. S. P. Müller, N. Heineking and M. C. L. Gerry, J. Chem. Soc., Faraday Trans., 1995, 91, 3347.
54. H. S. P. Müller and M. C. L. Gerry, J. Mol. Spectrosc., 1996, 175, 120.
55. J. Preusser and M. C. L. Gerry, J. Chem. Phys., 1997, 106, 10037.
56. H. S. P. Müller, G. O. Sørensen, M. Birk and R. R. Friedl, J. Mol. Spectrosc., 1997, 186, 177.
57. B. Gatehouse, H. S. P. Müller and M. C. L. Gerry, J. Mol. Spectrosc., 1998, 190, 157.

---

Footnote

† Electronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b0/b007492f

---