ARTICLE pubs.acs.org/JPCA
High-Resolution Fourier Transform Infrared Absorption Spectroscopy of the ν6 Band of c-C3H2 Pradeep R. Varadwaj,†,‡ Ryuji Fujimori,† and Kentarou Kawaguchi*,† † ‡
Department of Chemistry, Faculty of Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama, 700-8530, Japan Centre for Research in Molecular Modeling, Department of Chemistry & Biochemistry, Concordia University, 7141 Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada
bS Supporting Information ABSTRACT: The gas-phase high-resolution absorption spectrum of the ν6 band of cyclopropenylidene (c-C3H2) has been observed using a Fourier transform infrared spectrometer for the first time. The molecule has been produced by microwave discharge in an allene (3.3 Pa) and Ar (4.0 Pa) mixture inside a side arm glass tube. The observed spectrum shows a pattern of c-type ro-vibrational transitions in which the Q-branch lines strongly and distinctly stand out in the spectrum. A combined least-squares analysis of the observed 216 ro-vibrational transitions together with 28 millimeter-wave rotational transitions from the previous study has resulted in an accurate determination of the molecular constants in the ν6 state. The band center is found to be at 776.11622(13) cm1 with one standard deviation in parentheses, which is 2.3% lower than the matrix isolation value. The intensity ratio I3(ν3)/I6(ν6) obtained from the observed ν3 and ν6 bands, 1.90(9), is somewhat lower than the ratio estimated from ab initio (2.42.6) and DFT (2.8) calculations.
’ INTRODUCTION The chemistry of cyclopropenylidene, c-C3H2, is of significant experimental and theoretical interest.110 It is available in two different spin states, singlet and triplet.810 The cyclic form of the singlet molecule (C2v symmetry) is found to be the lowest-energy conformer,810 and because there is a neutral dicoordinate carbene center with a lone pair and an accessible vacant orbital, it has profound application in organic synthetic chemistry.11 c-C3H2 is the smallest hydrocarbon ring widely distributed in interstellar cold quiescent clouds, in star-forming regions, and in circumstellar envelopes.17 Numerous ab intio studies at the HF-SCF, MP2, and CCSD levels have been carried out for the singlet c-C3H2 molecule to determine its equilibrium structure, dipole moment, vibration rotational interaction constants, vibrational frequencies, and intensities using a variety of basis sets.810,1217 The C2v symmetry of c-C3H2 leads to nine fundamental vibrational degrees of freedom (four modes of A1 symmetry, three modes of B2 symmetry, one mode of A2 and the other of B1 symmetry) in the 1A1 electronic ground state. Several experimental studies have been performed to determine the ground state rotational and centrifugal distortion constants from the microwave spectrum,1820 the dipole moment from Stark measurements,21 and the vibrational frequencies of the linear and cyclic forms from low-temperature Ar matrix isolation studies.22,23 Reisenauer et al.22 observed a few r 2011 American Chemical Society
infrared active bands at 1279, 1063, 888, and 789 cm1 in lowtemperature Ar matrix. They assigned their spectra to the ν3, ν8, ν4, and ν6 bands of the molecule, respectively, and their observation was in line with the ab initio results.12 Bogey et al.24,25 analyzed the structure of the molecule using the rotational constants of the parent and isotope (13C and D) substituted species. Hirahara et al.26 have observed and analyzed the high-resolution FTIR spectrum of the ν3 (symmetric CC stretch and inplane symmetric CH bend) band in the 12501305 cm1 region with its band origin at 1277.3711 cm1. Mollaaghababa et al.20 have reported rotational constants of c-C3H2 in several of its excited vibrational states ν2, ν3, ν5, and ν6 from a millimeter-wave study. However, the fundamental frequencies have not been determined yet experimentally in gas-phase except for the ν3 mode. In this study, we have observed the high-resolution FTIR spectrum of the lowest ν6 bending mode of c-C3H2. The observed spectrum was analyzed by using Watson’s A-reduced Hamiltonian to determine the molecular constants in the ν6 state. The intensity ratio, I3(ν3)/I6(ν6), of the two bands ν3 and ν6 was evaluated using the observed gas-phase spectra and was Received: May 2, 2011 Revised: June 24, 2011 Published: June 24, 2011 8458
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compared with the results of quantum chemistry calculations performed at several levels of theory (ab initio (MP2, QCISD, and CCSD) and density functional theory (B3LYP, X3LYP, PBEPBE, and PW91PW91)).
’ EXPERIMENTAL DETAILS The high-resolution FTIR absorption spectrum of c-C3H2 was recorded with a Fourier transform spectrometer Bruker IFS 120 HR at Okayama University. The schematic diagram of the spectrometer is reported elsewhere.26 The infrared beam generated by a water-cooled infrared source (Globar) reaches a HgCdTe detector after passing though a KBr beam splitter, a KBr window, a multipass cell, and a band-pass filter (below 800 cm1). The length of the multipass cell, made of Pyrex, was of 1.5 m with an inner diameter of 14.2 cm. The concave mirrors were placed at the two ends of the cell (a T-shape mirror fixed in the glass cell at one end close to the detector while the other two circular mirrors were fixed at the opposite extreme). In the present experiment, the number of reflections (White-type multireflections) was increased to 18 (before it reaches the detector) compared with our previous setup (eight reflections),26 and was determined by counting the numbers with the help of a visible tungsten source together with a CaF2 beam splitter. This arrangement in turn has resulted in an increase of the effective path length to 54 m (previously 24 m), and thus a relatively weak signal is expected to be measurable. Attached to the multipass cell was a side arm Pyrex glass tube of 40 cm in length and of 1 cm for the inner diameter where c-C3H2 was produced by microwave discharge in a continuous flow of a low-pressure mixture of allene (C3H4) 3.3 Pa (25 mTorr) and argon (Ar) 4.0 Pa (30 mTorr). The buffer gas Ar with 99.999% purity was used throughout the experiment. A microwave discharge power of 60 W (2450 MHz) was sufficient for our measurement purpose. After an hour of operation, discharge products containing carbon were deposited on the inner surface of the discharge tube, and replacements of new Pyrex glass tubes were necessary for efficient production and measurement of c-C3H2. The reaction products were continuously pumped out with a mechanical booster pump followed by a rotary pump, and the effective pumping speed was about 10 L/s at 5.3 Pa (40 mTorr). The spectrum was observed with a resolution of 0.008 cm1 in the 700820 cm1 region, and the measured wavenumbers were calibrated using C2H227 which was produced inside the discharge tube as a byproduct. A lower resolution was employed in the measurement, compared with the previous ν3 study with a 0.004 cm1 resolution,26 because of the fact that there was relatively low source power in this region and that the intensity of the ν6 band is weaker than that of the ν3 band. ’ RESULTS AND DISCUSSION 1. Observed Spectrum and Analysis of the ν6 Band. A large number of absorption lines were recorded in the 700820 cm1 region, composed of the spectral lines predominantly originated from the parent species and from the products of the microwave discharge in the C3H4 and Ar mixture. Some species among discharge products were found to have long lifetime to survive in a sealed condition after short time discharge in the allene and Ar mixture. So, we recorded a spectrum containing the parent species and the long-lived discharged products in the sealed
Figure 1. A part of the observed c-type spectrum of the v6 band of c-C3H2 in the band origin region, showing strong Q-branch ro-vibrational transitions. In these transitions, J1,J1J2,J1 (J0,JJ1,J) is overlapping with J2,J1J1,J1 (J1,JJ0,J), because of small K-splitting.
condition. The transmittance spectra were obtained by taking ratios of the background spectrum (without samples) with those generated separately in a continuous flow discharge and in a sealed condition. After converting the transmittance spectra to absorbance spectra, the spectrum of short-lived species was obtained by subtracting it from the sealed spectrum, where an appropriate multiplication factor was used to cancel out the spectra of stable species. A part of the subtracted spectrum is shown in Figure 1. Since the ν6 band is of out-of-plane in-phase CH deformation having B1 vibrational point group symmetry,13 the band must have a c-type ro-vibrational spectral pattern. The molecular constants of the ν6 state, reported by Mollaaghababa et al.,20 were used for the prediction and assignment of the ro-vibrational structure of the molecule. The observed band was found to be around 776 cm1 and is somewhat closer to the value of 789 cm1 reported by the matrix isolation study. The difference value of 13 cm1 between them could be attributed to a shift caused by matrix isolation. From Figure 1, we could readily see that a dominant feature of the spectrum is a series of closely spaced strong Q-branch transitions with Ka = 1 r 0, and 0 r 1, Kc = J r J starting from J = 16 to J = 27, and fit these transition frequencies simultaneously with the reported pure rotational transition frequencies.20 In this fitting procedure a maximum weight of 3 106 was used for a rotational transition with a measurement error Δν of 0.01 MHz, relative to the weight of one for those of the ro-vibrational transitions. Weights for other transitions were also estimated from a 1/(Δν)2 relation. When we used only the millimeter wave spectral lines in the v6 = 1 state for fitting, the molecular constants reported by Mollaaghababa et al.20 were reproduced exactly, and we noticed that the errors listed in Table II of ref 20 for quadratic centrifugal distortion constants are misprinted by a factor of 1000. The Q-branch transitions from J = 115 were not assigned because of an unresolved strong overlapping of all these 15 transitions centered at 776.5 cm1. The line was broad, intense, and clearly stands out in the observed spectrum compared to the other transitions. It is worthwhile to note that the striking peak can be detectable even in a low resolution spectrometer and may 8459
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Table 1. Molecular Constants of c-C3H2 in the ν6 Statea present
millimeterb
groundc
A
34157.202(36)
34157.182(61)
35092.5083(32)
B
31944.446(49)
31944.546(75)
32212.9468(32)
C
16778.0170(58)
16777.999(10)
16749.0286(32)
ΔJ 103
211.07(14)
210.4(26)
41.689(65)
ΔJK 103
409.3(29)
412.8(47)
44.017(55)
ΔK 103
523.4(82)
499(14)
61.871(38)
δJ 103
101.30(72)
101.0(13)
16.4338(86)
δK 103 HJ 106
43.0(20) 0.511(94)
41.3(36) [0.00]d
58.610(20) 0.00
HJK 106
[6.37]d
[6.37]d
6.37(84)
HKJ 10
119.6(90)
[14.7]d
14.7(12)
HK 106
551(28)
[10.08]d
10.08(56)
hJ 106
[ 0.477]d
[0.477]d
0.477(75)
hJK 106
[0.88]d
[0.88]d
0.88(37)
hK 106
[1.22]d
[1.22]d
1.22(31)
ν0
776.11622(13)
constant
6
MHz units except for ν0 with a cm1 unit. Numbers in parentheses denote 1 standard deviation and apply to last significant digits. b Reference 20. c Reference 25. d Fixed to the ground state value. a
also be usable for a monitoring purpose. The intensities of other Q-branch transitions of Kc = J r J were decreased in higher wavenumbers with increasing the J values (J < 27), (Figure 1). Successively, Q-branch transitions with higher Ka values were assigned and included in the fit to refine the ro-vibrational parameters. Ortho and para components of the K-doublets were not resolved in the spectrum because of very small K-type splitting. In the analysis of the millimeter-wave spectrum, Mollaaghababa et al.20 have used transitions of Ka = 0, 1, and 2 with the maximum J value of 11 to determine the rotational and quadratic centrifugal distortion constants, where sextic distortion constants were fixed to the ground state values. The reported molecular constants were found to be accurate to explain the presently observed transitions with Ka e 4, J e 11. However, inclusion of transitions up to J = 27 in the fitting procedure required a higher order centrifugal distortion constant HJ as the fitting parameter, where the other sextic centrifugal distortion constants were fixed to the ground state values. The determined value HJ = 0.225(55) 106 cm1 is found to be the smallest compared with the other sextic constants, while its value was fixed to zero in an analysis of the ground state spectrum of the molecule.25 The standard deviation of fitting was found to be 0.0011 cm1. Finally, when we included transitions up to Ka = 7, the sextic centrifugal distortion constants HJK and HK had to be adjusted. After the assignments of the Q-branch transitions, some R- and P-branch transitions were then fitted by confirming the ground state combination differences. A total of 244 lines including 216 ro-vibrational transitions of this study and 28 pure rotational lines from ref 20 were analyzed by using Watson’s A-reduced Hamiltonian to determine the molecular constants of the ν6 state, as listed in Table 1. The ground state constants were fixed to those reported by Bogey et al.25 The standard deviation of the fit was found to be 0.0016 cm 1 . The larger standard deviation compared with the results (0.0003 cm1) of the ν3 band26 may be due to the Coriolis interaction caused by other states indicated below and a lower resolution (0.008 cm1) employed
in the measurement. The assigned transitions and their calculated minus observed values are provided in Table S1 of the Supporting Information. Anomalous quadratic centrifugal distortion constants in the ν6 state have been reported by Mollaaghababa et al.20 They have discussed the effect of a-type Coriolis interaction on the centrifugal distortion constant τaaaa in the ν6 state, and estimated the change in τaaaa (Δτaaaa = 1.53 MHz) from the ground state by using the Coriolis coupling constant obtained by ab initio calculations and energy difference between ν4 and ν6 obtained from the change in the A rotational constant. The value was largely different from the observed change of Δτaaaa = 3.4 MHz. The reason is not clear. In the present study, we also estimated the change in τaaaa by using newly determined quadratic constants. However, results similar to those of Mollaaghababa et al.20 were obtained. Although a satisfactory agreement is found between the rotational and quadratic centrifugal distortion constants obtained from this work and those reported in the millimeter-wave study, the sextic centrifugal distortion constants HJK, and HK determined in the present study are more than orders of magnitude larger than the ground state parameters. These values might be affected by the Coriolis interaction similar to those of the quadratic centrifugal distortion constants. The origin of the band is determined to be 776.11622(13) cm1, which is 2.3% lower than the matrix isolation value of 789 cm1.23 2. Theoretical Calculations and Intensity Ratio of ν3 and ν6. There is a wide variation in the quantitative estimation of normal mode vibrational frequencies and intensities of the molecule determined by various ab initio procedures. Lee et al.13 have carried out ab initio calculations at the SCF level together with the DZ and DZ+P basis sets to determine the normal mode vibrational frequencies of c-C3H2 and their band intensities. According to their harmonic calculations, the strongest mode (ν3) with band intensity 65.92 km mol1 appears around 1419 cm1 (unscaled) while the bands appearing at 1191 (B2, ν8), 983 (A1, ν4), and 854 cm1 (B1, ν6) have intensities of 21.1, 20.3, and 26.2 km mol1, respectively (at the SCF/DZ+P level). At the HF/6-311++G(d,p) level of theory, Talbi and Pauzat15 found that the fundamental mode at 1271.5 cm1 has the strongest band intensity of 61 km/mol among other intense bands while the bands appearing at 1061.6 cm1(ν8), 886.9 cm1 (ν4), and 786.6 cm1 (ν6) have intensities 23, 18, and 27 km/mol, respectively. Later, Gauss and Stanton16 have carried out computations at a higher level of theory (CCSD(T)/ cc-pVTZ) and they obtained normal-mode frequencies at 1316.1 (ν3), 1089.4 (ν8), 912.9 (ν4), and 798.1 cm1 (ν6) with band intensities 48.1, 9.0, 19.3, and 20.1 km mol1, respectively. From all these studies, it is clearly understood that the intensity trends predicted by the various ab initio procedures are somewhat contradictory for the bands of ν8 and ν4. To gain insight about the intensity of the observed band compared with the ν3 band previously assigned at 1277.3711 cm1, and because of an apparent inconsistency in the intensity trends predicted by various ab initio procedures for the ν8, ν4, and ν6 bands at 1061.5, 886.4, and 776.1 cm1, we have recalculated the vibrational properties by geometry optimizing the structure of c-C3H2 with various density functionals (B3LYP, X3LYP, PBEPBE, and PW91PW91) and ab initio (MP2, QCISD, and CCSD) procedures in conjunction with a moderate basis set augcc-pVTZ using GAUSSIAN 03.28 The results are listed in Table 2 which includes the calculated harmonic vibrational frequencies 8460
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Table 2. Calculated Harmonic Frequencies (cm1) and Infrared Intensities (in parentheses in units of km/mol) of c-C3H2 DFT
a
ab initio
experiment
X3LYP
B3LYP
PBEPBE
PW91PW91
MP2(fc)
QCISD(fc)
CCSD(fc)
CCSD(T)a
matrixb
gasc
1277.4
ω1(A1)
3260(0.3)
3255(0.2)
3176(0.1)
3180(0.1)
3311(1.2)
3302(1.0)
3305(1.0)
3309(0.6)
ω2(A1)
1647(0.3)
1644(0.3)
1603(0.4)
1606(0.4)
1615(0.7)
1647(0.8)
1651(0.8)
1633(0.4)
ω3(A1)
1316(47.5)
1314(47.5)
1280(47.1)
1281(47.0)
1319(39.8)
1320(50.5)
1323(50.8)
1316(48.1)
1278.8
ω4(A1)
897(22.5)
896(22.7)
857(23.7)
861(23.5)
905(23.6)
916(18.0)
917(18.1)
913(19.3)
886.4
ω5(A2)
1006(0.0)
1004(0.0)
967(0.0)
970(0.0)
993(0.0)
1003(0.0)
1005(0.0)
997(0.0)
ω6(B1)
806 (17.1)
804(17.0)
774(17.0)
778(16.8)
798(16.4)
798(16.4)
799(19.5)
798(20.1)
ω7(B2)
3225(0.3)
3219(0.2)
3142(0.1)
3146(0.1)
3272(3.0)
3262(1.5)
3264(1.6)
3262(0.7)
ω8(B2) ω9(B2)
1088(13.1) 911(2.9)
1085(13.0) 908(2.9)
1057(9.9) 864(4.9)
1056(10.1) 869(4.5)
1090(6.3) 908(4.8)
1103(11.3) 922(3.7)
1106(11.3) 923(3.8)
1089(9.0) 913(4.3)
I3/I6d
2.78
2.79
2.77
2.80
2.43
2.59
2.60
2.40
787.4
776.1
1061.5 1.90(9)
Reference 16. b Reference 23. c Reference 26 and present. d Band intensity ratio of the ν3 and ν6 bands.
and intensities together with the experimentally observed band origin frequencies and intensity ratio. The geometry of the stable isomer of c-C3H2 has been optimized by utilizing C2v point group symmetry and restricted spin formalism (singlet). Normal mode frequency calculations were performed analytically from the second derivatives of the DFT-X3LYP, B3LYP, PW91PW91, PBEPBE, and MP2 potential energy surfaces with respect to the atom-fixed nuclear coordinates to check whether each of the minimized structures corresponded to genuine stationary points. The Hessian calculations were performed with numerical differentiations at the CCSD and QCISD levels. The structure resulted is a genuine minimum energy configuration (IMAG=0). Since the experimentally observed frequencies include anharmonic effects, they should, in general, be compared with the calculated anharmonic frequencies. The anharmonic corrections were performed at the vibrationally averaged geometries. The ν3 and ν6 anharmonic frequencies were calculated to be 1284 and 794 cm1 (X3LYP), 1281 and 792 cm1 (B3LYP), 1248 and 758 cm1 (PBEPBE), 1249 and 764 cm1 (PW91PW91), and 1286 and 796 cm1 (MP2). Comparing these wavenumbers with the experimental values, 1277 and 776 cm1, we conclude that in X3LYP, B3LYP, and MP2, the ν3 frequency is well reproduced. On the other hand, the ν6 frequency is in poor agreement with the observed one. This indicates that the correlated methods seem to have problems in handling the anharmonic effects to a high degree of accuracy while the harmonic frequencies (Table 2) obtained from the PBEPBE and PW91PW91 level calculations are in good agreement with the observed gas-phase values within 4 cm1. Although the agreement may be accidental, the result can be used for the assignment of other nonreported bands in the infrared region. The ν4 band is expected to have similar intensity as the ν6 band (see Table 2). The band origin is expected to be in line with the results of the PBEPBE and PW91PW91 calculations. We have attempted to assign the band by assuming the band origin of 859 cm1, but in the allene discharge the region was predominantly occupied by ro-vibrational transitions of the parent species, and some other byproduct, and therefore, we came up with an ambiguous situation. We have exposed different precursors such as benzene, normal hexane, acetylene and cyclopropane to microwave discharges; however, none of these compounds could produce detectable amounts of c-C3H2.
Table 3. Observed Intensities of VibrationRotation Transitions in the ν3 and ν6 Bands of c-C3H2 ν3 band a
ν6 band b
c
transition
S
obs. int
transitiona
Sb
obs. intc
ratiod
151,14142,13
13.4
0.280
141,13142,13
23.3
0.260
1.87
241,24230,23
23.5
0.302
231,23230,23
45.0
0.300
1.93
223,20212,19
19.3
0.213
213,19212,19
33.6
0.197
1.88
222,21211,20 212,19203,18
20.4 18.3
0.261 0.248
212,20211,20 202,18203,18
37.2 31.6
0.264 0.220
1.80 1.95
203,18192,17
17.3
0.229
193,17192,17
29.6
0.211
1.86
201,19192,18
18.4
0.267
191,18192,18
33.2
0.238
2.04
192,17183,16
16.4
0.255
182,16183,16
27.7
0.213
2.02
191,18182,17
17.4
0.326
181,17182,17
31.2
0.329
ave
1.78 1.90(9)
a
Ortho transitions are listed, where para transition, for example 152,14141,13 is overlapped with the ortho line. b Line strength. c Observed integrated intensity with an unit of absorbance 3 cm1 103. d Intensity ratio of the ν3 and ν6 bands.
The infrared intensity listed in Table 2 does not include the effect of Coriolis interaction between vibrational modes, so we estimated the effect on the observed intensity. The ν6 state is subjected to the a-type Coriolis interaction with ν4(A1: 861 cm1) and ν3(A1: 1277 cm1) and b-type Coriolis interaction with ν9(B2: 869 cm1). The change in rotational constant A is expressed as follows29 4ðςa4, 6 Þ2 A2 4ðςa3, 6 Þ2 A2 ΔA ¼ E4 E6 E3 E6 where ζa4,6, ζa3,6 are Coliolis coupling constants, and E3, E4, and E6 are energies of the ν3, ν4, and ν6 states, and effects from other two A1 states are neglected. By using the energy difference of E4 E6 = 83 cm1 (Table 2) and calculated values of ζa4,6 = 0.6805 and ζa3,6 = 0.5411 with the X3LYP level anharmonic analysis, we obtained ΔA = 916.7 + 96 = 1012 MHz, which is compared with the observed value of 935 MHz. When we consider uncertainties (at least 4 cm1 for ν6) in the energy difference and the Coriolis coupling constant, the observed change in A will agree with the calculated value in the error limit. In the previous study, Mollaaghababa et al.20 used ζa4,6 = 0.65 to estimate the energy 8461
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The Journal of Physical Chemistry A difference E4 E6 = 76 cm1, where the effect from the ν3 state was neglected. Since the Coriolis interaction mixes the ν6 state with the ν4 state, the ν6 state wave function will be expressed as follows ψ ¼ ð1 a2 Þ1=2 jν6 ¼ 1, Ka i ajν4 ¼ 1, Ka i where a = |2ζa4,6AKa|/(E4 E6). In the case of the observed maximum Ka = 7, the a value is estimated to be 0.133, which reduces the c-type transition intensity of ν6 by 2%. The b-type Coriolis interaction with the ν9 state leads to a decrease of the ν6 intensity. However, the effect is smaller than the a-type interaction because of larger energy difference and smaller Coriolis coupling constant ζb9,6 = 0.493. This means that the effect of the Coriolis interaction on the observed intensity is small compared with the intensity measurement error (57%). Table 3 lists the observed gas-phase integrated intensities for ro-vibrational transitions of the ν3 and ν6 bands, where we have used observed transitions with the same lower rotational quantum states common to both the vibrational bands, which makes direct comparison possible without incorporating the Boltzmann factor. In the case of the ν3 band, the ratios of the P and R branch transition intensities were in good agreement (within intensity measurement error) with line strength (S) ratios, if both transitions have a common lower state. We considered only R-branch intensities as representative of the ν3 band. The transitions listed in Table 3 are ortho lines, and corresponding para lines are not listed explicitly because they are degenerate, and the intensities of para lines were assumed to be 1/3 of the ortho lines in the calculation. By considering line strength (S) for each transition, we obtained a band intensity ratio I(ν3)/I(ν6) = 1.90(9) with 1 standard deviation in parentheses. From Table 2, it is evident that DFT methods gave a larger I(ν3)/I(ν6) ratio than the experimental value, and the high level computations at the QCISD and CCSD levels are relatively accurate in predicting the infrared band intensity ratio. Although some previously reported calculations predicted that the ν8 (1063 cm1) band is stronger than the ν4 (888 cm1), our calculations predicted stronger intensity of the ν4 band than ν8, which is in agreement with Gauss and Stanton’s result.16 These calculations indeed guide for an understanding of intensity profiles of various bands required for an experimental search of the high-resolution infrared spectrum of the compound. In 2009, Bernstein and Lynch30 proposed that small carbonaceous compounds c-C2H4O (ethylene oxide) and c-C3H2 may be the sources of the unidentified infrared bands (UIRs) which were observed as emission in some astronomical objects, including planetary nebula such as NGC 7027. The c-C3H2 assumption may be supported by the fact that the pure rotational transitions of c-C3H2 have been detected in numerous astronomical objects. So they compared gas-phase c-C3H2 vibrational frequencies obtained from ab initio calculations by Talbi and Pauzat15 with UIRs wavelengths, 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 μm. Although Talbi and Pauzat’s predicted value for ν6, 786.7 (12.7 mm), agrees with UIRs 12.7 μm in wavelength, the present gas-phase value 776.1 cm1 (12.9 μm) is significantly different from the 12.7 μm peak. This indicates that c-C3H2 is not a major species responsible for the UIR 12.7 μm band, although there may be a small contribution on the shoulder of the band.
ARTICLE
4. SUMMARY The high-resolution spectrum of the ν6 band of c-C3H2 has been observed in the gas phase for the first time. Least-squares analyses of the ro-vibrational transitions together with the millimeter-wave spectral lines led us to determine accurate molecular constants in the ν6 state. The estimated intensity ratio, 1.90(9), of the ν3 and ν6 bands is found to be lower than the ratio obtained from theoretical calculations with a moderate basis set aug-cc-pVTZ and with those reported previously. The GGAtype PBEPBE and PW91PW91 functionals have adequately reproduced the experimental frequencies of ν3 and ν6 within 4 cm1 at the harmonic level. ’ ASSOCIATED CONTENT
bS
Supporting Information. Tabulation of the assigned transitions and their calculated minus observed values. This information is available free of charge via the Internet at http:// pubs.acs.org
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT P.R.V. acknowledges gratefully the Japan Society for the Promotion of Science for an award of a Postdoctoral Fellowship during the course of this work and for funding and rewarding support and thanks the Osaka University Computer Center for supercomputing facilities. We thank Tsuneo Hirano for valuable comments on ab initio calculations. The present study was partly supported by a Grant-in-Aid (Grant Nos. 2008349 and 21104003) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. ’ REFERENCES (1) Winnewisser, G.; Mezger, P. G.; Breuer, H. D. Top. Curr. Chem. 1974, 44, 1–81. (2) Mattews, H. E.; Irvine, W. M. Astrophys. J. Lett. 1985, 298, L61–L65. (3) Herbst, E.; Leung, L. M. Astrophys. J., Suppl. Ser. 1989, 69, 271–300. (4) Vrtilek, J. M.; Gottlieb, C. A.; Thaddeus, P. Astrophys. J. 1987, 314, 716–725. (5) Madden, S. C.; Irvine, W. M.; Mathews, H. E.; Friberg, P.; Swade, D. A. Astron. J. 1989, 97, 1403–1422. (6) Oike, T.; Kawaguchi, K.; Takano, S.; Nakai, N. Publ. Astron. Soc. Jpn. 2004, 56, 431–438. (7) Morisawa, Y.; Fushitani, M.; Kato, Y.; Hoshina, H.; Simizu, Z.; Watanabe, S.; Miyamoto, Y.; Kasai, Y.; Kawaguchi, K.; Momose, T. Astrophys. J. 2006, 642, 954–965. (8) Seburg, R. A.; Patterson, E. V.; Stanton, J. F.; McMahon, R. J. J. Am. Chem. Soc. 1997, 119, 5847–5856. (9) Mohajeri, A.; Jenabi, M. J. J. Mol. Struct: THEOCHEM. 2007, 820, 65–73. (10) Ochsenfeld, C.; Kaiser, R. I.; Lee, Y. T.; Suits, A. G.; HeadGordon, M. J. Chem. Phys. 1987, 106, 4141–4151. (11) Carbenes; Jones, M., Moss, R. A., Eds.; Wiley: New York, 1973; Vols. I and II. (12) Hehre, W. J.; Pople, J. A.; Lathan, W. A.; Radom, L.; Wasserman, E.; Wasserman, Z. R. J. Am. Chem. Soc. 1976, 98, 4378–4383. (13) Lee, T. J.; Bunge, A.; Schaefer, H. F., III J. Am. Chem. Soc. 1985, 107, 137–142. (14) DeFrees, D. J.; McLean, A. D. Astrophys. J. 1986, 308, L31–35. 8462
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