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Microwave spectra of mono-13C substituted acetone, (CH3)2CO
Journal of Molecular Spectroscopy 236 (2006) 173–177 www.elsevier.com/locate/jms
Microwave spectra of mono-13C substituted acetone, (CH3)2CO F.J. Lovas a, Peter Groner a
b,*
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA b Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA Received 18 November 2005; in revised form 23 January 2006 Available online 3 March 2006
Abstract The microwave spectra of the two mono-13C isotopic forms of acetone are reported for the first time. Measurements were carried out from 11 to 25 GHz with a pulsed-beam Fourier-transform microwave spectrometer. Because overall rotation interacts with the internal rotations of the methyl groups, the spectra were analyzed with an effective rotational Hamiltonian for molecules with two periodic largeamplitude internal motions. In acetone-2-13C, the methyl groups are equivalent; in acetone-1-13C, they are not. The molecular structure has been re-examined by including rotational constant data on other isotopic forms reported previously. An equilibrium structure for acetone has been derived from the observed rotational constants and vibration–rotation constants calculated from ab initio force fields. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Acetone isotopic species; Fourier transform microwave spectroscopy; Internal rotation; Rotational spectrum; Structure; Radio astronomy
1. Introduction Acetone is among the earliest species studied by microwave spectroscopy with four lines observed, but not assigned, by Weatherly and Williams [1]. The first microwave structural study of acetone was reported by Swalen and Costain [2], who examined the normal species and the d6 isotopic form and reported a partial structure. Later Nelson and Pierce [3] studied two additional isotopic forms, (13CH3)2CO and (CH3)2C18O, and combined the results with those of the earlier studies to reanalyze the structure. Subsequently, there have been more extensive studies of normal acetone [4] and deuterated species [5,6], but there have been no reports on single 13C substituted acetone. In 1987, Combes et al. [7] reported the tentative detection of interstellar acetone in the source Sgr B2, and Snyder et al. [8] recently confirmed this result with the detection of 13 emission features assigned to 20 acetone transitions in Sgr B2(N). The North position in Sgr B2 is the richest in
*
Corresponding author. Fax: +1 816 235 2290. E-mail address: [email protected] (P. Groner).
0022-2852/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.01.009
molecular species and the warmest since it shows a number of vibrationally excited states of a number of large molecules, e.g., methanol, vinyl cyanide, and cyanoacetylene. This is most clearly illustrated in the recent survey from 218 to 263 GHz by Nummelin et al. [9], who examined three molecular emission positions in the Sgr B2 molecular cloud, with the North source having more than 1500 identified lines and 337 unidentified lines. These unidentified lines may be due to uncharacterized isotopic species, low lying vibrational states of the established interstellar species, or new interstellar molecules. In a recent summary of all interstellar lines reported through 2002, Lovas [10] reports more than 1700 unidentified lines for all interstellar molecular sources. Acetone has also been detected toward the star forming region Orion-KL by lines assigned to both the vibrational ground state and the lowest vibrational excited state [11]. We have undertaken the current study to aid astronomers in identifying lines that may be attributed to 13C acetone isotopomers and to re-examine the structure of acetone. A new radio astronomy instrument, Green Bank Telescope, has recently begun operation with coverage up to 48 GHz and planned coverage to 90 GHz. Two millimeter-wave array instruments are under construction,
174
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
Combined Array for Research in Millimeter-wave Astronomy (CARMA) [12] in California which has nine 6.1 m antennas combined with six 10.4 m antennas and Atacama Large Millimeter Array (ALMA) [13] in Chile, which is planned to have sixty-four 12 m antennas. Each of these new instruments have (or will have) substantially improved sensitivity over present instruments and will quite likely lead to the detection of the 13C forms of acetone in hot-core interstellar clouds. 2. Experimental Spectral measurements were carried out with a Fabry– Perot cavity, Fourier-transform microwave (FTMW) spectrometer designed by Lovas and Suenram [14,15] of the Balle–Flygare type [16]. A new PC based system for timing and control of the mirrors, pulsed nozzle, microwave synthesizer, and signal processing has been incorporated and uses the FTMW++ software system designed by Grabow [17]. A pulsed solenoid valve was used to produce a supersonic molecular beam from a mixture of about one volume percent acetone (with 13C in natural abundance) entrained in argon carrier gas at a total pressure of 100 kPa (1 atm) behind a 1 mm nozzle orifice and injected along the axis of the Fabry–Perot cavity and parallel to the microwave field. Molecular beam pulses with about 400 ls duration were employed with repetition rates up to 10 Hz. The acetone beam was polarized by a short microwave pulse when the microwave frequency was near-resonant (Dm < 400 kHz) with a rotational transition of acetone. The free induction decay signal from the cavity was digitized in 0.5 ls increments for 2048 channels. Typically, 100 pulses were signal averaged, after a background microwave pulse was subtracted from each signal pulse, to yield signal-to-noise ratios of 10 or more. The averaged data were Fourier transformed to obtain the amplitude spectrum in the frequency domain with a resolution element of 2 kHz/point. Molecular transitions observed as Doppler doublets had line widths of 5 kHz, and the frequency measurement uncertainties were estimated to be 2 kHz in most cases (type B with coverage factor k = 2), with the resolution element for the digitization time described above. Some transitions exhibited partially resolved spin–spin hyperfine structure, which caused larger uncertainties in the reported line centers. 3. Spectrum and fit Because acetone-2-13C has the same symmetry as normal acetone, its rotational transitions are split by the interactions with the internal rotors into four components [4]. The components are labeled by combinations of the symmetry numbers r1r2 with the usual symmetry label notation [18] in parentheses: 00 (AA), 01 (EE), 11 (AE), and 12 (EA). Acetone-1-13C on the other hand has a reduced symmetry identical to the symmetry of ethyl methyl ether, CH3CH2OCH3, [19]. Its rotational transitions are split into
five components labeled 00, 01, 10, 11, and 12. When nonequivalent internal rotors become equivalent, the components 01 and 10 become degenerate. Spectroscopic parameters of an effective rotational Hamiltonian for two periodic internal motions [20] were obtained by a global weighted non-linear least-squares fit to the measured frequencies. For acetone-2-13C, the Hamiltonian was identical to the one used for the ground state of normal acetone [4]. For acetone-1-13C, the Hamiltonian of ethyl methyl ether [19] was used. Because of the limited number of observed transitions, some parameters held constant were transferred from the ground state of normal acetone [4]. Measured frequencies of assigned transitions of both isotopomers are listed in Table 1 together with estimated experimental uncertainties and the residuals of the least-squares fits. The spectroscopic parameters are listed in Table 2. The dimensionless standard deviations of 1.54 and 0.96 for acetone-1-13C and acetone-2-13C, respectively, indicate that the fit is almost as good as the experimental uncertainty. Initially, fits were performed using all spectroscopic parameters of normal acetone [4], even though most were kept constant. However, even with several frequencies deleted from the fits, the dimensionless standard deviations were in the order of 2.5 or higher. Better standard deviations were achieved when only the sextic centrifugal distortion constants and the tunneling energy parameters e11, e11, and e20 = e02 were transferred from normal acetone and used as constant parameters. However, the tunneling contributions, rotational constants, and quartic centrifugal distortion constants were fitted. Apart from the rotational constants, the agreement of the fitted parameters with those of normal acetone is quite good. The resulting spectroscopic constants were used to predict transitions of 13C-substituted acetone between 5 and 115 GHz with Ka 6 10. They are characterized in terms of quantum number assignments, symmetry numbers, predicted frequencies with estimated uncertainties (type A, k = 1 [21]), spin statistical weights, and line strengths and upper state energies (Appendix A). 4. Structure The rotational constants of normal acetone [4], acetone1-13C and -2-13C (this work), and refitted rotational constants for acetone-18O and acetone-1,3-13C2 [3], acetone-d6 [5,6], and partially deuterated acetone [6] were used to determine an approximate re-structure according to Groner and Warren [22]. For that purpose, approximate equilibrium rotational constants were obtained with vibration–rotation constants derived from quadratic and cubic force fields obtained from MP2(full)/6-31G(d) ab initio calculations with Gaussian 98 [23]. All quadratic force constants were scaled by the factor 0.95. Because the rotational constants for acetone-18O [3], -1,3-(13C)2 [3], -d6 [5,6], -d1 [6], and -d5 [6] were either reported without standard errors or determined for representations other than I r,
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
175
Table 1 Measured and assigned transition frequencies mobs (MHz), experimental uncertainties Unc. (kHz), and residuals Dm (kHz) of the least-squares fit Transitiona
211–202
111–000
221–212
422–413
330–321
331–322
321–312
312–303
322 313
202–111
212–101
a b c d
r1r2b
13
CH313COCH3
CH3COCH3
mobs
Unc.c
Dmd
mobs
Unc.c
Dmd
00 01 10 11 12
10962.347 10950.743 10947.299 10929.990 10941.532
4 4 4 4 4
1.8 0.3 0.8 0.3 1.7
11287.650 11273.974
4 4
0.8 1.9
11254.047 11266.776
4 4
0.5 0.4
00 01 10 11 12
14914.887 14890.834 14895.092 14884.214 14857.719
2 2 2 2 2
0.8 0.5 0.6 1.0 1.6
15095.127 15072.915
2 2
1.7 0.5
15063.771 15037.437
2 2
0.4 2.5
00 01 10 11 12
15875.777 15789.908 15805.368 15786.136 15658.123
2 2 2 2 2
3.1 1.5 2.7 2.6 2.4
15823.901 15747.196
2 2
0.1 0.7
15732.531 15612.459
2 2
2.7 2.1
00 01 10 11 12
17902.639 17895.725 17883.988 17867.847 17886.737
4 2 4 2 2
1.0 0.3 0.3 3.0 2.1
18672.758 18659.484
4 4
2.3 1.4
18636.195 18656.831
4 4
2.9 3.9
00 01 10 11 12
11916.235 11989.607 11989.913 11791.291 12218.598
4 2 2 2 2
3.2 0.7 2.7 0.4 1.0
11188.690 11246.752
4 4
1.4 2.5
11063.596 11456.595
4 4
0.5 2.0
00 01 10 11 12
18911.388 18688.012 18729.158 18780.194 18346.549
4 2 4 2 2
4.5 5.6 5.5 0.8 3.1
18573.297 18385.101
4 4
5.1 1.9
18441.509 18039.936
4 4
5.2 3.2
00 01 10 11 12
10580.566 10568.517 10569.391 10547.722 10566.912
4 2 4 2 4
0.8 6.8 16.2 1.0 9.3
10762.832 10751.797
4 4
0.5 3.1
10731.524 10750.057
4 4
3.7 0.3
00 01 10 11 12
19834.081 19811.438 19803.903 19775.333 19787.358
4 2 4 2 4
5.8 3.0 4.4 1.1 1.1
20457.835 20429.424
4 4
0.2 1.0
20395.078 20407.178
4 4
1.8 2.3
00 01 10 11 12
22210.622 22152.842 22158.259 22112.168 22088.548
4 4 4 4 2
3.7 2.5 6.3 0.4 3.6
22467.269 22411.533
4 4
3.0 4.4
22366.407 22344.901
4 4
2.1 6.4
00 01 10 11 12
22165.540 22176.423 22170.152 22167.671 22194.548
2 2 2 2 2
0.1 0.0 4.3 2.1 0.8
22795.444 22802.510
2 2
0.6 3.6
22796.473 22822.898
2 2
2.8 0.1
00 01 10 11 12
24537.601 24519.057 24522.815 24509.900 24498.438
2 2 2 2 2
2.3 2.7 3.7 1.7 1.4
24915.363 24898.406
2 2
2.7 0.5
24887.159 24875.493
2 2
1.5 0.7
Asymmetric rotor notation. Torsional substate. Uncertainties are type B estimates with a coverage factor, k = 2 [21]. Dm = mobs mcalc.
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Table 2 Spectroscopic parameters of acetone-1-13C and -2-13Ca Parameter
CH313COCH3
13
q1/q2 b1/1-b2 (degree) A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) UJ (Hz)b UJK (Hz)b UKJ (Hz)b UK (Hz)b /J (Hz)b /JK (Hz)b /K (Hz)b e10/e01 (MHz) e11 (MHz)b e11 (MHz)b e20 = e02 (MHz)b [A (B + C)/2]10 (kHz) [(B + C)/2]10 (kHz) [(B C)/4]10 (kHz) nc sd
0.062074(27) 25.8224(33) 10164.00791(76) 8516.08462(99) 4910.23681(74) 4.957(98) 3.08(12) 9.829(94) 2.042(16) 0.617(61) 0.0506046 0.336741 0 0.423395 0.0253760 0.0273291 0.221468 763.36(33) 0.0799732 1.049511 0.766643 60.6(20) 18.73(33) 2.17(16) 44 0.96
0.060591(37) 29.5461(36) 10083.0347(11) 8277.5070(13) 4811.4692(10) 4.62(14) 2.60(14) 9.34(11) 1.901(21) 0.253(77) 0.0506046 0.336741 0 0.423395 0.0253760 0.0273291 0.221468 756.85(49) 0.0799732 1.049511 0.766643 57.8(26) 18.25(37) 2.21(18) 55 1.54
a b c d
13
CH3COCH3 (1st rotor)
CH3COCH3 (2nd rotor) 0.062047(34) 21.3035(49)
763.15(41)
0.766643 57.8(26) 18.25(37) 2.21(18)
Standard uncertainties in parentheses are in units of the last significant digit (uncertainty type A, k = 1 [21]). Transferred from ground state of 12CH312CO12CH3 [4]. Number of transitions used in the least-squares fit. Dimensionless standard deviation.
the frequencies reported for these isotopomers were used to refit the spectroscopic parameters. For acetone-18O [3], -1,3-13C2 [3], and -d6 [5,6], the effective rotational Hamiltonian was the same as the one used for normal acetone and acetone-2-13C. For the symmetric forms of acetone-d1 and -d5, the effective Hamiltonian used for acetone-1-13C was modified by setting all parameters
pertaining to the second periodic internal motion (q2, b2, e01, and r2) to zero. Because of the small number of measured transitions for the isotopomers acetone-18O and -1,3-13C2 [3], many parameters transferred from normal acetone had to be kept constant. The reported frequencies, assignments and residuals as well as the derived spectroscopic parameters for these isotopomers are listed in Appendix A.
Table 3 Structural parameters of acetonea r0/rsb r(CO) r(CC) r(CHi) r(CHo) a(CCC) a(CCO) a(CCHi)h a(CCHo)h 0 a(HoCH o)h a(HiCHo)h x a b c d e f g h
1.222(3) 1.507(3) 1.085(7) 1.085(7) 117.20(33) 121.40
r0c
1.10(5) 1.091(7)
108.77(50) 108.77(50) 120.49
red 1.2117(21) 1.5080(11) 1.0830(50) 1.0912(3) 116.48(13) 121.76(6) 110.03(30) 109.89(4) 107.04(4) 109.97(17) 119.95(2)
MP2(full)/6-31G(d)e
MP2/6-31G(d,p)f
MP2FC/cc-pVTZg
1.2265 1.5118 1.0899 1.0945 116.48 121.76 109.48 110.32 107.21 109.74
1.2257 1.5112 1.0847 1.0892 116.38 121.81 109.59 110.19 107.24 109.79
1.2174 1.5093 1.0850 1.0899 116.147 121.926 109.937 109.998 107.065 109.900
˚ , angles a and x in °, and x = angle between internal rotation axes from effective rotational Hamiltonian fit. Distances r in A Ref. [3]. Ref. [6]. Standard uncertainties in parentheses are in units of the last significant digit (uncertainty type A, k = 1 [21]). This work and Ref. [24]. Refs. [25,26]. Ref. [27]. Hi and Ho refer to the in-plane and out-of-plane H atoms, respectively.
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
Also listed in Appendix A are BeB0 differences as obtained from the calculated vibration rotation constants and the resulting equilibrium rotational constants Be. The pseudo-re structure was obtained by fitting the independent structural parameters directly to the equilibrium rotational constants in a non-linear least-squares fit. The squared inverses of the standard uncertainties (type B, k = 1 [21]) of the ground state rotational constants were used as weights of the equilibrium rotational constants. The results for the structural parameters are shown in Table 3 in comparison with the optimized structures from theoretical MP2 calculations [24–27]. The difference between the angle between the internal rotation axes, x, derived from the data in Table 2, and the angle a(CCC), indicates that the methyl groups are tilted away from each other towards the carbonyl group by 1.75°. The re CO dis˚ is 0.0151 A ˚ shorter than the distance tance of 1.2114 A from the MP2(full)/6-31G(d) optimization. The difference ˚ is consistent with the differences of 0.0155–0.019 A obtained by the same methods for five other compounds with CO double bonds [22]. Therefore, the re-structure listed in Table 3 is expected to be a good approximation to the true equilibrium structure. Initially, we used the rotational constants for the other isotopomers as reported in the original publications [3,5,6] and assumed standard uncertainties to derive the re-structure. Even though some rotational constants changed by as much as a few MHz, the structure was almost identical to the one reported in Table 3. The differences in the parameters were much smaller than the quoted uncertainties. However, the uncertainties of some parameters were 50–70% smaller than those reported in Table 3, most likely because the assumed uncertainties for the rotational constants were much too optimistic for the -18O and -1,3-13C2 species.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
5. Summary We report the microwave spectra of 13C1- and 13C2-acetone for the first time. Predictions for their spectra up to 115 GHz are provided as Supplementary material (see Appendix A) to aid in astronomical identification. A re structure has been derived by combining the new data with literature data on other isotopomers of acetone.
[24] [25]
Appendix A. Supplementary data [26]
Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://msa.lib.ohio-state.edu/jmsa_hp.htm).
177
[27]
T.L. Weatherly, D. Williams, J. Chem. Phys. 20 (1952) 755. J.D. Swalen, C.C. Costain, J. Chem. Phys. 31 (1959) 1562–1574. R. Nelson, L. Pierce, J. Mol. Spectrosc. 18 (1965) 344–352. P. Groner, S. Albert, E. Herbst, F.C. De Lucia, F.J. Lovas, B.J. Drouin, J.C. Pearson, Astrophys. J. Suppl. Ser. 142 (2002) 145–151. R. Peter, H. Dreizler, Z. Naturforsch. A 20 (1965) 301–312. J.M. Vacherand, B.P. Van Eijck, J. Burie, J. Demaison, J. Mol. Spectrosc. 120 (1986) 118–126. F. Combes, M. Gerin, A. Wootten, G. Wlodarczak, F. Clausset, P.J. Encrenaz, Astron. Astrophys. 180 (1987) L13–L16. L.E. Snyder, F.J. Lovas, D.M. Mehringer, N.Y. Miao, Y.-J. Kuan, J.M. Hollis, P.R. Jewell, Astrophys. J. 578 (2002) 245–255. ˚ . Hjalmarson, P. Friberg, W.M. Irvine, A. Nummelin, P. Bergman, A T.J. Millar, M. Ohishi, S. Saito, Astrophys. J. Suppl. Ser 117 (1998) 427–529. F.J. Lovas, J. Phys. Chem. Ref. Data 33 (2004) 177–355. D.N. Friedel, L.E. Snyder, A.J. Remijan, B.E. Turner, Astrophys. J. Lett. 632 (2005) L95–L98. See
Microwave spectra of mono-13C substituted acetone, (CH3)2CO F.J. Lovas a, Peter Groner a
b,*
Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA b Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA Received 18 November 2005; in revised form 23 January 2006 Available online 3 March 2006
Abstract The microwave spectra of the two mono-13C isotopic forms of acetone are reported for the first time. Measurements were carried out from 11 to 25 GHz with a pulsed-beam Fourier-transform microwave spectrometer. Because overall rotation interacts with the internal rotations of the methyl groups, the spectra were analyzed with an effective rotational Hamiltonian for molecules with two periodic largeamplitude internal motions. In acetone-2-13C, the methyl groups are equivalent; in acetone-1-13C, they are not. The molecular structure has been re-examined by including rotational constant data on other isotopic forms reported previously. An equilibrium structure for acetone has been derived from the observed rotational constants and vibration–rotation constants calculated from ab initio force fields. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Acetone isotopic species; Fourier transform microwave spectroscopy; Internal rotation; Rotational spectrum; Structure; Radio astronomy
1. Introduction Acetone is among the earliest species studied by microwave spectroscopy with four lines observed, but not assigned, by Weatherly and Williams [1]. The first microwave structural study of acetone was reported by Swalen and Costain [2], who examined the normal species and the d6 isotopic form and reported a partial structure. Later Nelson and Pierce [3] studied two additional isotopic forms, (13CH3)2CO and (CH3)2C18O, and combined the results with those of the earlier studies to reanalyze the structure. Subsequently, there have been more extensive studies of normal acetone [4] and deuterated species [5,6], but there have been no reports on single 13C substituted acetone. In 1987, Combes et al. [7] reported the tentative detection of interstellar acetone in the source Sgr B2, and Snyder et al. [8] recently confirmed this result with the detection of 13 emission features assigned to 20 acetone transitions in Sgr B2(N). The North position in Sgr B2 is the richest in
*
Corresponding author. Fax: +1 816 235 2290. E-mail address: [email protected] (P. Groner).
0022-2852/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.01.009
molecular species and the warmest since it shows a number of vibrationally excited states of a number of large molecules, e.g., methanol, vinyl cyanide, and cyanoacetylene. This is most clearly illustrated in the recent survey from 218 to 263 GHz by Nummelin et al. [9], who examined three molecular emission positions in the Sgr B2 molecular cloud, with the North source having more than 1500 identified lines and 337 unidentified lines. These unidentified lines may be due to uncharacterized isotopic species, low lying vibrational states of the established interstellar species, or new interstellar molecules. In a recent summary of all interstellar lines reported through 2002, Lovas [10] reports more than 1700 unidentified lines for all interstellar molecular sources. Acetone has also been detected toward the star forming region Orion-KL by lines assigned to both the vibrational ground state and the lowest vibrational excited state [11]. We have undertaken the current study to aid astronomers in identifying lines that may be attributed to 13C acetone isotopomers and to re-examine the structure of acetone. A new radio astronomy instrument, Green Bank Telescope, has recently begun operation with coverage up to 48 GHz and planned coverage to 90 GHz. Two millimeter-wave array instruments are under construction,
174
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
Combined Array for Research in Millimeter-wave Astronomy (CARMA) [12] in California which has nine 6.1 m antennas combined with six 10.4 m antennas and Atacama Large Millimeter Array (ALMA) [13] in Chile, which is planned to have sixty-four 12 m antennas. Each of these new instruments have (or will have) substantially improved sensitivity over present instruments and will quite likely lead to the detection of the 13C forms of acetone in hot-core interstellar clouds. 2. Experimental Spectral measurements were carried out with a Fabry– Perot cavity, Fourier-transform microwave (FTMW) spectrometer designed by Lovas and Suenram [14,15] of the Balle–Flygare type [16]. A new PC based system for timing and control of the mirrors, pulsed nozzle, microwave synthesizer, and signal processing has been incorporated and uses the FTMW++ software system designed by Grabow [17]. A pulsed solenoid valve was used to produce a supersonic molecular beam from a mixture of about one volume percent acetone (with 13C in natural abundance) entrained in argon carrier gas at a total pressure of 100 kPa (1 atm) behind a 1 mm nozzle orifice and injected along the axis of the Fabry–Perot cavity and parallel to the microwave field. Molecular beam pulses with about 400 ls duration were employed with repetition rates up to 10 Hz. The acetone beam was polarized by a short microwave pulse when the microwave frequency was near-resonant (Dm < 400 kHz) with a rotational transition of acetone. The free induction decay signal from the cavity was digitized in 0.5 ls increments for 2048 channels. Typically, 100 pulses were signal averaged, after a background microwave pulse was subtracted from each signal pulse, to yield signal-to-noise ratios of 10 or more. The averaged data were Fourier transformed to obtain the amplitude spectrum in the frequency domain with a resolution element of 2 kHz/point. Molecular transitions observed as Doppler doublets had line widths of 5 kHz, and the frequency measurement uncertainties were estimated to be 2 kHz in most cases (type B with coverage factor k = 2), with the resolution element for the digitization time described above. Some transitions exhibited partially resolved spin–spin hyperfine structure, which caused larger uncertainties in the reported line centers. 3. Spectrum and fit Because acetone-2-13C has the same symmetry as normal acetone, its rotational transitions are split by the interactions with the internal rotors into four components [4]. The components are labeled by combinations of the symmetry numbers r1r2 with the usual symmetry label notation [18] in parentheses: 00 (AA), 01 (EE), 11 (AE), and 12 (EA). Acetone-1-13C on the other hand has a reduced symmetry identical to the symmetry of ethyl methyl ether, CH3CH2OCH3, [19]. Its rotational transitions are split into
five components labeled 00, 01, 10, 11, and 12. When nonequivalent internal rotors become equivalent, the components 01 and 10 become degenerate. Spectroscopic parameters of an effective rotational Hamiltonian for two periodic internal motions [20] were obtained by a global weighted non-linear least-squares fit to the measured frequencies. For acetone-2-13C, the Hamiltonian was identical to the one used for the ground state of normal acetone [4]. For acetone-1-13C, the Hamiltonian of ethyl methyl ether [19] was used. Because of the limited number of observed transitions, some parameters held constant were transferred from the ground state of normal acetone [4]. Measured frequencies of assigned transitions of both isotopomers are listed in Table 1 together with estimated experimental uncertainties and the residuals of the least-squares fits. The spectroscopic parameters are listed in Table 2. The dimensionless standard deviations of 1.54 and 0.96 for acetone-1-13C and acetone-2-13C, respectively, indicate that the fit is almost as good as the experimental uncertainty. Initially, fits were performed using all spectroscopic parameters of normal acetone [4], even though most were kept constant. However, even with several frequencies deleted from the fits, the dimensionless standard deviations were in the order of 2.5 or higher. Better standard deviations were achieved when only the sextic centrifugal distortion constants and the tunneling energy parameters e11, e11, and e20 = e02 were transferred from normal acetone and used as constant parameters. However, the tunneling contributions, rotational constants, and quartic centrifugal distortion constants were fitted. Apart from the rotational constants, the agreement of the fitted parameters with those of normal acetone is quite good. The resulting spectroscopic constants were used to predict transitions of 13C-substituted acetone between 5 and 115 GHz with Ka 6 10. They are characterized in terms of quantum number assignments, symmetry numbers, predicted frequencies with estimated uncertainties (type A, k = 1 [21]), spin statistical weights, and line strengths and upper state energies (Appendix A). 4. Structure The rotational constants of normal acetone [4], acetone1-13C and -2-13C (this work), and refitted rotational constants for acetone-18O and acetone-1,3-13C2 [3], acetone-d6 [5,6], and partially deuterated acetone [6] were used to determine an approximate re-structure according to Groner and Warren [22]. For that purpose, approximate equilibrium rotational constants were obtained with vibration–rotation constants derived from quadratic and cubic force fields obtained from MP2(full)/6-31G(d) ab initio calculations with Gaussian 98 [23]. All quadratic force constants were scaled by the factor 0.95. Because the rotational constants for acetone-18O [3], -1,3-(13C)2 [3], -d6 [5,6], -d1 [6], and -d5 [6] were either reported without standard errors or determined for representations other than I r,
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
175
Table 1 Measured and assigned transition frequencies mobs (MHz), experimental uncertainties Unc. (kHz), and residuals Dm (kHz) of the least-squares fit Transitiona
211–202
111–000
221–212
422–413
330–321
331–322
321–312
312–303
322 313
202–111
212–101
a b c d
r1r2b
13
CH313COCH3
CH3COCH3
mobs
Unc.c
Dmd
mobs
Unc.c
Dmd
00 01 10 11 12
10962.347 10950.743 10947.299 10929.990 10941.532
4 4 4 4 4
1.8 0.3 0.8 0.3 1.7
11287.650 11273.974
4 4
0.8 1.9
11254.047 11266.776
4 4
0.5 0.4
00 01 10 11 12
14914.887 14890.834 14895.092 14884.214 14857.719
2 2 2 2 2
0.8 0.5 0.6 1.0 1.6
15095.127 15072.915
2 2
1.7 0.5
15063.771 15037.437
2 2
0.4 2.5
00 01 10 11 12
15875.777 15789.908 15805.368 15786.136 15658.123
2 2 2 2 2
3.1 1.5 2.7 2.6 2.4
15823.901 15747.196
2 2
0.1 0.7
15732.531 15612.459
2 2
2.7 2.1
00 01 10 11 12
17902.639 17895.725 17883.988 17867.847 17886.737
4 2 4 2 2
1.0 0.3 0.3 3.0 2.1
18672.758 18659.484
4 4
2.3 1.4
18636.195 18656.831
4 4
2.9 3.9
00 01 10 11 12
11916.235 11989.607 11989.913 11791.291 12218.598
4 2 2 2 2
3.2 0.7 2.7 0.4 1.0
11188.690 11246.752
4 4
1.4 2.5
11063.596 11456.595
4 4
0.5 2.0
00 01 10 11 12
18911.388 18688.012 18729.158 18780.194 18346.549
4 2 4 2 2
4.5 5.6 5.5 0.8 3.1
18573.297 18385.101
4 4
5.1 1.9
18441.509 18039.936
4 4
5.2 3.2
00 01 10 11 12
10580.566 10568.517 10569.391 10547.722 10566.912
4 2 4 2 4
0.8 6.8 16.2 1.0 9.3
10762.832 10751.797
4 4
0.5 3.1
10731.524 10750.057
4 4
3.7 0.3
00 01 10 11 12
19834.081 19811.438 19803.903 19775.333 19787.358
4 2 4 2 4
5.8 3.0 4.4 1.1 1.1
20457.835 20429.424
4 4
0.2 1.0
20395.078 20407.178
4 4
1.8 2.3
00 01 10 11 12
22210.622 22152.842 22158.259 22112.168 22088.548
4 4 4 4 2
3.7 2.5 6.3 0.4 3.6
22467.269 22411.533
4 4
3.0 4.4
22366.407 22344.901
4 4
2.1 6.4
00 01 10 11 12
22165.540 22176.423 22170.152 22167.671 22194.548
2 2 2 2 2
0.1 0.0 4.3 2.1 0.8
22795.444 22802.510
2 2
0.6 3.6
22796.473 22822.898
2 2
2.8 0.1
00 01 10 11 12
24537.601 24519.057 24522.815 24509.900 24498.438
2 2 2 2 2
2.3 2.7 3.7 1.7 1.4
24915.363 24898.406
2 2
2.7 0.5
24887.159 24875.493
2 2
1.5 0.7
Asymmetric rotor notation. Torsional substate. Uncertainties are type B estimates with a coverage factor, k = 2 [21]. Dm = mobs mcalc.
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Table 2 Spectroscopic parameters of acetone-1-13C and -2-13Ca Parameter
CH313COCH3
13
q1/q2 b1/1-b2 (degree) A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) UJ (Hz)b UJK (Hz)b UKJ (Hz)b UK (Hz)b /J (Hz)b /JK (Hz)b /K (Hz)b e10/e01 (MHz) e11 (MHz)b e11 (MHz)b e20 = e02 (MHz)b [A (B + C)/2]10 (kHz) [(B + C)/2]10 (kHz) [(B C)/4]10 (kHz) nc sd
0.062074(27) 25.8224(33) 10164.00791(76) 8516.08462(99) 4910.23681(74) 4.957(98) 3.08(12) 9.829(94) 2.042(16) 0.617(61) 0.0506046 0.336741 0 0.423395 0.0253760 0.0273291 0.221468 763.36(33) 0.0799732 1.049511 0.766643 60.6(20) 18.73(33) 2.17(16) 44 0.96
0.060591(37) 29.5461(36) 10083.0347(11) 8277.5070(13) 4811.4692(10) 4.62(14) 2.60(14) 9.34(11) 1.901(21) 0.253(77) 0.0506046 0.336741 0 0.423395 0.0253760 0.0273291 0.221468 756.85(49) 0.0799732 1.049511 0.766643 57.8(26) 18.25(37) 2.21(18) 55 1.54
a b c d
13
CH3COCH3 (1st rotor)
CH3COCH3 (2nd rotor) 0.062047(34) 21.3035(49)
763.15(41)
0.766643 57.8(26) 18.25(37) 2.21(18)
Standard uncertainties in parentheses are in units of the last significant digit (uncertainty type A, k = 1 [21]). Transferred from ground state of 12CH312CO12CH3 [4]. Number of transitions used in the least-squares fit. Dimensionless standard deviation.
the frequencies reported for these isotopomers were used to refit the spectroscopic parameters. For acetone-18O [3], -1,3-13C2 [3], and -d6 [5,6], the effective rotational Hamiltonian was the same as the one used for normal acetone and acetone-2-13C. For the symmetric forms of acetone-d1 and -d5, the effective Hamiltonian used for acetone-1-13C was modified by setting all parameters
pertaining to the second periodic internal motion (q2, b2, e01, and r2) to zero. Because of the small number of measured transitions for the isotopomers acetone-18O and -1,3-13C2 [3], many parameters transferred from normal acetone had to be kept constant. The reported frequencies, assignments and residuals as well as the derived spectroscopic parameters for these isotopomers are listed in Appendix A.
Table 3 Structural parameters of acetonea r0/rsb r(CO) r(CC) r(CHi) r(CHo) a(CCC) a(CCO) a(CCHi)h a(CCHo)h 0 a(HoCH o)h a(HiCHo)h x a b c d e f g h
1.222(3) 1.507(3) 1.085(7) 1.085(7) 117.20(33) 121.40
r0c
1.10(5) 1.091(7)
108.77(50) 108.77(50) 120.49
red 1.2117(21) 1.5080(11) 1.0830(50) 1.0912(3) 116.48(13) 121.76(6) 110.03(30) 109.89(4) 107.04(4) 109.97(17) 119.95(2)
MP2(full)/6-31G(d)e
MP2/6-31G(d,p)f
MP2FC/cc-pVTZg
1.2265 1.5118 1.0899 1.0945 116.48 121.76 109.48 110.32 107.21 109.74
1.2257 1.5112 1.0847 1.0892 116.38 121.81 109.59 110.19 107.24 109.79
1.2174 1.5093 1.0850 1.0899 116.147 121.926 109.937 109.998 107.065 109.900
˚ , angles a and x in °, and x = angle between internal rotation axes from effective rotational Hamiltonian fit. Distances r in A Ref. [3]. Ref. [6]. Standard uncertainties in parentheses are in units of the last significant digit (uncertainty type A, k = 1 [21]). This work and Ref. [24]. Refs. [25,26]. Ref. [27]. Hi and Ho refer to the in-plane and out-of-plane H atoms, respectively.
F.J. Lovas, P. Groner / Journal of Molecular Spectroscopy 236 (2006) 173–177
Also listed in Appendix A are BeB0 differences as obtained from the calculated vibration rotation constants and the resulting equilibrium rotational constants Be. The pseudo-re structure was obtained by fitting the independent structural parameters directly to the equilibrium rotational constants in a non-linear least-squares fit. The squared inverses of the standard uncertainties (type B, k = 1 [21]) of the ground state rotational constants were used as weights of the equilibrium rotational constants. The results for the structural parameters are shown in Table 3 in comparison with the optimized structures from theoretical MP2 calculations [24–27]. The difference between the angle between the internal rotation axes, x, derived from the data in Table 2, and the angle a(CCC), indicates that the methyl groups are tilted away from each other towards the carbonyl group by 1.75°. The re CO dis˚ is 0.0151 A ˚ shorter than the distance tance of 1.2114 A from the MP2(full)/6-31G(d) optimization. The difference ˚ is consistent with the differences of 0.0155–0.019 A obtained by the same methods for five other compounds with CO double bonds [22]. Therefore, the re-structure listed in Table 3 is expected to be a good approximation to the true equilibrium structure. Initially, we used the rotational constants for the other isotopomers as reported in the original publications [3,5,6] and assumed standard uncertainties to derive the re-structure. Even though some rotational constants changed by as much as a few MHz, the structure was almost identical to the one reported in Table 3. The differences in the parameters were much smaller than the quoted uncertainties. However, the uncertainties of some parameters were 50–70% smaller than those reported in Table 3, most likely because the assumed uncertainties for the rotational constants were much too optimistic for the -18O and -1,3-13C2 species.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
5. Summary We report the microwave spectra of 13C1- and 13C2-acetone for the first time. Predictions for their spectra up to 115 GHz are provided as Supplementary material (see Appendix A) to aid in astronomical identification. A re structure has been derived by combining the new data with literature data on other isotopomers of acetone.
[24] [25]
Appendix A. Supplementary data [26]
Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://msa.lib.ohio-state.edu/jmsa_hp.htm).
177
[27]
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