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Microwave spectrum and structure of furfural
Journal of Molecular Spectroscopy 240 (2006) 93–101 www.elsevier.com/locate/jms
Microwave spectrum and structure of furfural R.A. Motiyenko b
a,*
, E.A. Alekseev a, S.F. Dyubko a, F.J. Lovas
b
a Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA
Received 16 June 2006; in revised form 22 August 2006 Available online 8 September 2006
Abstract The microwave spectrum of furfural was investigated in the frequency range 7 GHz–21 GHz and 49 GHz–330 GHz. The ground and first torsional state of trans-furfural and ground state of cis-furfural were assigned and analyzed. A total of 1720 rotational lines with J up to 100 and Ka up to 53 were assigned to the ground state of trans-furfural, 1406 rotational lines with J up to 100 and Ka up to 48 were assigned to the first torsional state of trans-furfural and 2103 rotational lines with J up to 90 and Ka up to 48 to the ground state of cisfurfural. Accurate sets of centrifugal distortion constants for both conformations have been determined for the first time. The spectra of all 13C and 18O singly substituted isotopic species were observed in natural abundance in the 7 GHz–21 GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman substitution type (rs) and pseudo-Kraitchman type (rpKr) are derived from the isotopic studies. 2006 Elsevier Inc. All rights reserved. Keywords: Furfural; Microwave spectrum; Rotational spectrum; Molecular structure;
1. Introduction In recent studies of the C5 sugar, ribose, both at National Institute of Standards and Technology (NIST) and at Institute of Radio Astronomy of NASU (IRA NASU), furfural was found to be a major dehydration product. In the past few years a growing interest is developed in searching for simple sugars and their decomposition products in interstellar hot molecular cores. This interest has been stimulated by detection of the simplest C2 sugar glycolaldehyde (CH2OHCHO) in the Sgr B2 (N-LMH) molecular cloud by Hollis et al. [1], as well as by identification by Cooper et al. [2] of the C3 ketone sugar 1,3-dihydroxy-2-propanone (dihydroxyacetone), sugar alcohols and sugar acids in the Murchison meteorite. Another possible candidate for interstellar identification is C3 aldehyde sugar glyceraldehyde [3]. Recently the tentative detection of dihydroxyacetone (DHA) in Sgr B3(N-LMH) has been reported from *
Corresponding author. Fax: +38 057 706 1415. E-mail address: [email protected] (R.A. Motiyenko).
0022-2852/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.09.003
13
C-isotopologs;
18
O-isotopologs
millimeter wave astronomical studies [4]. In a follow up study to confirm this detection of DHA, Apponi et al. [5] have found no plausible emission at 97% of the 63 transitions sought, so the ketone form of the C3 sugar has not been detected. Several attempts to detect the aldehyde form, glyceraldehyde, at low frequency have been reported and at all frequencies searched, no signal could be detected [6,7]. Radio astronomy searches for higher order sugars are hampered by the lack of spectroscopic information. Another interest in the microwave spectra of furfural results from the complexity of laboratory spectral investigations of ribose due to its high instability and decomposition rate during vaporization [8]. Thus, the assignment of rotational spectra of ribose would be impossible without prior identification and exclusion of rotational lines of decomposition products, including furfural. Previous spectroscopic investigations of the furfural molecule (C4H3OCHO) have dealt mainly with infrared and Raman spectra [9–11]. These investigations have enabled the determination of relative stabilities of the cisand trans-conformations of furfural in the gas phase (see
94
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a
trans- and cis-conformers were assigned and analyzed. However, in both publications the rotational analysis was limited to rigid rotor constants and therefore such data sets could not provide a reliable prediction basis in a wide frequency range. In this paper, we report the results of a new investigation of the microwave spectrum of furfural. We have assigned and analyzed the ground state of both conformations and also the first torsional state of trans-furfural, since the latter provides rather intense spectrum which is comparable to the spectrum of the ground state of cis-furfural and thus may represent an interest to radio astronomers. We have remeasured some of the previously measured lines and extended the frequency range up to 330 GHz. Centrifugal distortion constants up to eighth order were required to fit the data within experimental accuracy. As a result, a reliable basis for radio astronomy search for interstellar furfural has been provided. In addition, the spectra of all 13C and 18O singly substituted isotopic species were observed in natural abundance in the 7 GHz–21 GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman type (rs) and pseudo-Kraitchman type (rpKr) fits are derived from the isotopic studies.
b H9 H10 C3 C4 O7
a
C2 C6
C5 O1
H11
H8
b
b H9
H8
2. Experimental details
C3 H10
C6
C4
C2
a O7
C5
O1
H11
Fig. 1. (a) Trans-furfural structure and atom numbering. (b) Cis-furfural structure and atom numbering.
Fig. 1 for their geometry) and also measurements of the barrier to internal rotation. It has been determined that the trans-conformer is the most stable in the gas phase. The most recent experimental value for the energy difference between cis- and trans-conformations is 3.4260(28) kJ/mol [11] and the value of barrier to internal rotation is 38.9660(25) kJ/mol [11]. The pure rotational spectrum of furfural was studied previously only in two separate publications by Mo¨nnig, Dreizler and Rudolph [12,13]. In their first work [12], the rotational spectrum of the ground state of both conformers and their several deuterated species was investigated in the frequency range 7 GHz–26 GHz providing the partial determination of the molecular structure. In the second work [13], several torsional and vibrational states of
In order to provide better predictions for the high frequency range, we undertook new measurements a using Fourier transform microwave spectrometer (FTMW) at NIST. Thus, frequencies of most of rotational lines of furfural reported in [12] were remeasured with an accuracy of 2 kHz, which is an order of magnitude better in comparison with previous results. Spectral measurements were carried out with a Fabry– Perot cavity, Fourier-transform microwave spectrometer of the Balle–Flygare type [14] designed by Lovas and Suenram [15,16]. 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 furfural entrained in argon (or neon) 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 molecular beam was polarized by a short microwave pulse when the microwave frequency was near-resonant (Dm < 400 kHz) with a rotational transition. The free induction decay signal from the cavity was digitized in 0.5 ls increments for 2048 channels. Typically, 50–100 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
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
element of 2 kHz per 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 uncertainty with coverage factor k = 2 [18], i.e., a two standard deviation estimate). All measurements in the frequency range between 50 GHz and 330 GHz have been carried out using an automated, synthesizer-based, millimeter-wave spectrometer at the Institute of Radio Astronomy of the National Academy of Science of Ukraine at Kharkov. A brief description of the spectrometer is presented in Ref. [19]. Since the spectrometer has been extensively modified, compared to the original version described in Ref. [19], we will provide below a brief description of the spectrometer and its modification. A block diagram of the current spectrometer is shown in Fig. 2. This is a usual absorption frequency-modulated
spectrometer with lock-in detection. As was mentioned previously in Ref. [19] (and references therein) the synthesis of frequencies in the millimeter region is carried out by a twostep frequency multiplication of the reference synthesizer 390–420 MHz in two phase-lock-loop (PLL) stages. The first PLL stage utilizes a klystron in the range of 3.4– 5.2 GHz. In the second stage, the ISTOK backward-wave oscillator (BWO) is locked to a harmonic of the klystron. By changing the BWO and the corresponding waveguide components, the frequency region from 49 GHz to 149 GHz can be covered. In addition, the frequency range above 200 GHz may be covered using a frequency doubler. The klystron PLL contains a special electromechanical system which provides continuous mechanical adjustment of the klystron resonator. This allows us to obtain a wide range of continuous spectral records in a fully automated mode.
Frequency Doubler
Shottky Detector
BWO, 50 – 150 GHz
Absorbing Cell
Harmonic Mixer PLL, IF=25MHz
Klystron 3.4 – 5.2 GHz
Amplifier
Harmonic Mixer FMModulated Synthesizer 25MHz
PLL, IF=5MHz
5MHz
Sine Wave Synthesizer 7 - 120 kHz
Lock-in Detector
5MHz
Band-pass Amplifier 390–420 MHz
Rb - Standard 5MHz
Balanced Mixer Synthesizer 360MHz
Frequency Divider f/2
180MHz
95
DDS AD9851 30 – 60MHz
Reference Synthesizer Fig. 2. Block diagram of the Kharkov spectrometer.
ADC
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As an absorption cell, we used a quasioptic dielectric hollow waveguide. In order to significantly reduce the well-known problem of standing waves, we used special dielectric horns for matching with the radiation source and detector. In addition we have employed special polytetrafluoroethylene vacuum windows with a thickness of about 0.8 mm. As detectors we use Schottky diodes placed in home-made waveguide mounts. In order to minimize distortions of the line shape due to modulation, we employ a special frequency-modulated synthesizer which provides a very low distortion sinusoidal modulation. It is possible to use both first and second derivative spectra recording, but usually we obtain the first derivative spectra. The most important modification of the spectrometer is the application of the new reference synthesizer. Previously we employed a commercial direct analog synthesizer, which was modified in order to improve its spectral purity. The best spectral purity was available in the narrow frequency range only (390–400 MHz). That is why there were socalled ‘‘dead zones’’ in the operating frequency range of the spectrometer, i.e., a few rather narrow frequency ranges where frequency synthesis was impossible. In order to overcome this disadvantage it is sufficient to employ a reference synthesizer with slightly expanded operating range, for example, 390–405 MHz and more. We undertook a number of attempts to apply various other reference synthesizers (mainly PLL-synthesizers) as the reference source, but every time the spectral purity of the new reference synthesizer was much worse than the initially chosen synthesizer. Some years ago there appeared highly integrated chips of a direct digital synthesizer (DDS). Application of such a synthesizer gives a number of advantages in comparison with other methods of frequency synthesis [20]. Since narrow-band spectral purity of the DDS is very good, we applied this type of synthesizer as a reference source in our spectrometer, namely, a DDS AD9851 from Analog Devices, Inc [21]. The simplified block diagram of the reference synthesizer is shown in Fig. 2. In fact it is just a usual up-converter which allows us to convert the output signal of the AD9851 from a frequency range of 30–60 MHz to 390–420 MHz, keeping its initial spectral purity. This up-conversion reduces the total multiplication factor by an order of magnitude. However, as it is known from the literature [20], the wideband output spectrum of a DDS contains a lot of rather intense spurious components, that is why high factor multiplication of the DDS output signal usually produces rather spurious spectrum. In order to overcome this obstacle it is necessary to apply narrow-band adaptive filtering. As was mentioned above, on the first stage of frequency multiplication in our spectrometer, a klystron is employed. Due to rather low phase noise level of a klystron, it is convenient to use this phase-lock loop as a narrow-band filter (its closed loop bandwidth is about 2 kHz). This provides effective filtering of spurious components.
For measurements at Kharkov a sample of furfural was obtained commercially and purified at the Department of Organic Chemistry of V.N. Karazin Kharkov National University. The Type B, coverage factor k = 1 [18] (or one sigma) uncertainty of the measurements for a strong isolated line is estimated to be: (i) 5 kHz for trans-furfural and 10 kHz for cis-furfural and first torsional state of transfurfural in the frequency range between 49 GHz and 149 GHz; (ii) 15 kHz for all rotational transitions in the frequency range between 200 GHz and 249 GHz; (iii) 60 kHz for all rotational transitions in the frequency range 300–330 GHz. 3. Results and analysis Assignment of the observed spectra was rather straightforward and based on frequency predictions obtained from fitting a combination of data from [12] and [13] and frequencies remeasured at NIST. A sample spectrum recorded at Kharkov between 84 800 MHz and 85 600 MHz is presented in Fig. 3. With increasing values of the quantum number J, several fitting/measurement cycles for improving the rotational constants were needed. A total of 1720 lines have been assigned to the ground state of trans-furfural, 1406 lines to the first torsional state of trans-furfural and 2103 lines to the ground state of cisfurfural. For both conformers most of the assigned lines belong to a-type R-branch transitions due to relatively large values of the electric dipole moment component, la = 3.20(3) D for trans-furfural and la = 3.41(3) D for cis-furfural [12]. However for the cis-conformer the set of b-type transitions is much larger than for the trans-conformer. This fact arises from the great difference between values of lb: 1.93(2) D for cis-conformer and 0.40(4) D for transconformer [12]. It should be noted that about 100 b-type transitions for the ground state of trans-furfural have been assigned for the first time. For the first torsional state of trans-furfural, no b-type transitions have been observed. The Watson A-reduction Hamiltonian in the Ir representation [22] was used for rotational analysis. ASROT and ASFIT programs from the PROSPE website were used for predicting and fitting spectral data [23]. The sets of rotational constants which have been determined for the first time include quartic and sextic constants for both conformations and in addition two octic constants for the ground and first torsional states of trans-furfural. The results of the fits are presented in Table 1. The overall root-mean-square (rms) deviations obtained are 9.9 kHz (weighted rms deviation is 0.58) for the ground state of trans-furfural, 10.3 kHz (weighted rms deviation is 0.59) for the first torsional state of trans-furfural and 12.6 kHz (weighted rms deviation is 0.65) for the ground state of cis-furfural. The overall quality of the fits is illustrated in Tables 2–4, which give the rms deviations for transitions grouped according to their measurement uncertainties (weight in the fit was proportional to reciprocal of uncertainty squared).
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97
Fig. 3. A sample spectrum of furfural recorded between 84 800 MHz and 85 600 MHz. The strongest lines correspond to Ka series of aR0,1 J = 23 ‹ 22 transitions of trans-furfural.
Table 1 Rotational parameters of furfural Parameter
trans-furfural vt = 0 a
trans-furfural vt = 1
cis-furfural vt = 0
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) hJ (Hz) hJK (Hz) hK (Hz) LJ (mHz) LKKJ (mHz) lKJ (mHz)
8191.77386(19) 2045.929569(10) 1637.183876(11) 0.1361873(52) 0.706617(24) 1.7835(14) 0.0314688(24) 0.82192(18) 0.0000175(10) 0.000657(46) 0.00509(15) 0.00000868(32) 0.000403(30) 0.01335(98) 0.000000191(62) 0.0001018(82) —
8115.4200(12) 2045.583482(26) 1639.627551(22) 0.1376876(74) 0.544931(49) 1.055(10) 0.0315427(35) 0.42739(42) 0.0000263(10) 0.00459(12) 0.01653(41) 0.00001085(49) 0.000345(53) 0.0945(26) — 0.002808(19) 0.000587(46)
8143.738729(46) 2098.724250(11) 1668.872904(11) 0.1726593(47) 0.499948(24) 1.81402(15) 0.0403044(34) 0.80893(14) 0.00002719(75) 0.0003230(48) 0.002259(17) 0.00001091(51) 0.000139(29) — — — —
Numberb rms (MHz) weighted rmsc
1729 0.0099 0.583
1406 0.0103 0.589
2110 0.0125 0.649
a b c
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits. Number of rotational transitions treated for each conformer. Unitless root-mean-square deviation of the fit.
All singly substituted 13C and 18O isotopologs of both isomers were measured in natural abundance. For the 13 C species 21–25 transitions were measured and fit to a quartic Watson Hamiltonian [22], and the rotational constants are listed in Table 5. Due to the lower natural abundance of 18O, only 14–19 transitions were observable, and two of the centrifugal distortion parameters, DK and dK, were fixed in the fits at the values for the normal isotopic species. The fit results are shown in Table 6.
Tables with line assignments, measured frequencies their uncertainties and deviations are provided as a Supplementary material to this paper. 4. Structure determination In the earlier study of furfural isomers, Mo¨nnig et al. [12] reported results on two deuterated forms of the trans isomer and one deuterated form of the cis isomer. They
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Table 2 Root-mean-square deviations from the fit for trans-furfural vt = 0 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
Table 3 Root-mean-square deviations from the fit for trans-furfural vt = 1 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
Table 4 Root-mean-square deviations from the fit for cis-furfural vt = 0 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a
Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
were able to derive limited structural information, primarily on the aldehyde group from the isotopolog data. With our new 13C and 18O rotational analysis, coupled with the earlier analysis of several deuterated forms, we can derive a rather complete structure of the heavy atom frame. The most common type of structure analysis is the Kraitchman substitution, rs structure [24] in which the
atom coordinates are determined from differences in the planar second moments of inertia, e.g., Paa = 1/2(Ibb + Icc Iaa) where Iaa, Ibb and Icc are the principal axes moments of inertia, between the parent form and the isotopically substituted form. In Table 7, columns 2 and 3, the rs coordinates of the substituted atoms shown in the first column are listed. From these coordinates the bond distances and angles shown in Table 8, columns 2 and 5, can be derived. The uncertainties shown for the rs values were determined from the Costain rule [25] which is dxn,k = 0.0015/jxn,kj where xn,k is the kth coordinated of atom n. The statistical uncertainties are usually smaller than those derived from the Costain rule. A second type of structural analysis was carried out in which all the isotopologs were treated in a global fit using the STRFTQ program developed by Schwendeman [26] whereby the internal coordinates, bond distances and angles are fit to either moments of inertia (Ixx) or planar second moments of inertia (Pxx) to give an r0 structure, or to differences in these quantities between the parent species and each of the isotopologs to give a pseudo-Kraitchman (rpKr) structure which is similar to an rs structure (see also the discussion of these types of structural analyses by Rudolph [27]). In the current case we chose the differences in second moments (DP nxx ¼ P pxx P nxx where p is the parent moment and n is the substituted atom moment), or pseudoKraitchman analysis which most closely parallels the rs analysis and is often better than an rs structure when small coordinates occur as in furfural. These results are given in Tables 7 and 8 under the rpKr column headings. For the trans-furfural a fit of 15 parameters gave an overall stan˚ 2 and for the cis isomer it dard deviation of 0.0064 uA 2 ˚ in fitting 13 parameters. The uncertainties was 0.0048 uA shown for the rpKr analysis bond lengths and angles are a combination of the statistical uncertainties and the Costain rule uncertainties, which sometimes dominate. The largest uncertainties will occur with the atoms having the smallest coordinates, namely the C2, C3 and O7 atoms of the trans isomer and the O1, C2 and O7 atoms of the cis isomer. The coordinates for the rs and rpKr structures in Table 7 are quite similar except for the C2 atom, where a difference ˚ occurs for the trans form and 0.012 A ˚ for the cis of 0.020 A form. As a result the bond distances in Table 8 which involve atom C2 will also disagree by similar amounts. The rs values agree with the rpKr values within the combined uncertainties given for both structures. The largest differences are found for r(C2C6), \(O1C2C3) and \(C2O1C5) for both isomers. Montero et al. [28] report ab initio SCF-MO calculations of the structure of furfural with several basis sets. In columns 4 and 7 of Table 8 we list their results for the trans and cis forms, respectively, with the 6-31G** basis set. Also for comparison, the last column of Table 8 lists the rs values of furan derived by Bak et al. [29]. The largest changes in the furfural ring compared to the furan structure occur in the bonds and angles involving the C2 atom where the aldehyde group is attached. Unfortunately, due to several
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101 Table 5 Rotational parameters of Parameter A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) Numberb rms (kHz) Wtd. rmsc
99
13
C substituted furfural species
t-C2-furfural a
8191.2984(14) 2045.6074(6) 1636.9647(6) 0.1401(45) 0.692(30) 1.48(31) 0.0259(26) 0.98(26) 24 1.4 0.58
t-C3-furfural
t-C4-furfural
t-C5-furfural
t-C6-furfural
8025.2399(14) 2044.7201(5) 1629.6481(4) 0.1443(41) 0.710(21) 1.78(30) 0.0286(32) 0.91(18)
8097.8411(9) 2019.7916(3) 1616.6913(3) 0.1365(32) 0.638(17) 1.69(19) 0.0289(21) 0.84(15)
8159.5106(11) 2016.4563(4) 1616.9968(4) 0.1300(35) 0.720(18) 1.47(22) 0.0240(25) 0.76(16)
8163.1167(13) 2024.0110(5) 1621.9939(4) 0.1356(40) 0.641(20) 2.34(29) 0.0266(31) 0.99(18)
25 1.3 0.56
25 0.9 0.44
24 1.1 0.48
25 1.4 0.55
Parameter
c-C2-furfural
c-C3-furfural
c-C4-furfural
c-C5-furfural
c-C6-furfural
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz)
8132.6075(11)a 2098.4658(4) 1668.2472(4) 0.1741(33) 0.467(20) 1.73(23) 0.0374(26) 0.83(14)
7963.9761(14) 2092.7573(5) 1657.4365(4) 0.1681(43) 0.534(27) 1.96(32) 0.0341(35) 0.99(17)
8122.5291(15) 2063.7000(5) 1645.7865(5) 0.1684(46) 0.497(28) 1.84(32) 0.0330(34) 0.83(19)
8043.2439(12) 2076.2141(4) 1650.4150(4) 0.1761(36) 0.478(22) 1.87(27) 0.0369(28) 0.72(15)
8108.5514(15) 2075.5316(5) 1652.7208(5) 0.1694(43) 0.471(27) 2.05(32) 0.0324(34) 0.77(18)
Numberb rms (kHz) Wtd. rmsc a b c
23 1.1 0.45
23 1.3 0.60
21 1.4 0.59
23 1.2 0.50
23 1.4 0.59
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits shown. Number of rotational transitions fit for each isotopolog. Unitless root-mean-square deviation of the weighted fit.
Table 6 Rotational parameters of
18
O substituted furfural species
Parameter
t-O1-furfural
t-O7-furfural
c-O1-furfural
c-O7-furfural
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz)
7894.1527(13)a 2038.3484(3) 1620.1360(2) 0.1324(32) 0.758(29) 1.78b 0.0288(35) 0.82b
8165.7441(21) 1944.8085(5) 1570.8227(4) 0.1308(38) 0.608(36) 1.78b 0.0249(45) 0.82b
7903.7962(7) 2097.6148(2) 1657.8573(1) 0.1740(15) 0.642(42) 1.81b 0.0412(16) 0.81b
8114.7031(5) 1997.8153(1) 1603.3011(1) 0.1632(10) 0.415(26) 1.81b 0.0369(10) 0.81b
Numberc rms (MHz) Wtd. rmsd a b c d
17 1.2 0.58
19 1.8 0.90
14 0.6 0.29
14 0.4 0.19
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits shown. Value fixed in the fit. Number of rotational transitions fit for each isotopolog. Unitless root-mean-square deviation of the weighted fit.
small coordinates for atoms 1, 2 and 3 mentioned above these derived values also show some of the largest uncertainties. While it appears that the r(O1C2) and r(C2C3) are reduced by the addition of the aldehyde group, the large uncertainties do not allow for this conclusion, especially for the trans isomer. In general the ring structure of furan is preserved quite well in the furfural isomers and our structure results compare quite favorably with the ab initio results.
Recently Demaison and Rudolph [30] have shown that ‘‘an rs structure can be quite unreliable when the molecule is heavy and has low-frequency normal mode(s)’’. This might be the case for furfural which is rather heavy and has a low torsion frequency for the HCO group, namely, 127 cm1 for cis and 146 cm1 for trans [11]. Demaison, Rudolph and co-workers [30–34] applied the mass-dependent (rm) method of Watson et al. [35] to a variety of molecules (diatomic, linear, symmetric top
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Table 7 Principal axis Kraichman substitution rs and global rpKr coordinates of trans- and cis-furfural Parameter
t-furfural rs ˚) a (A
t-furfural rs ˚) b (A
t-furfural global rpKr ˚) a (A
t-furfural global rpKr ˚) b (A
O1 C2 C3 C4 C5 C6 O7 H8 H11
0.6798(22)a 0.1979(75) 0.3823(39) 1.7908(8) 1.9056(8) 1.6404(9) 2.5564(6) 1.8267(19) 2.7493(13)
1.0912(14) 0.060(25) 1.1359(13) 0.8562(18) 0.5005(30) 0.4704(32) 0.3273(46) 1.5628(10) 1.1673(13)
0.6795(22) 0.2175(65) 0.3782(39) 1.7906(8) 1.9052(8) 1.6406(9) 2.5565(6) 1.8246(19) 2.7490(13)
1.0915(14) 0.078(19) 1.1362(13) 0.8571(18) 0.4982(30) 0.4722(32) 0.3249(46) 1.5603(10) 1.1667(13)
Parameter
c-furfural rs ˚) a (A
c-furfural rs ˚) b (A
c-furfural global rpKr ˚) a (A
c-furfural global rpKr ˚) b (A
O1 C2 C3 C4 C5 C6 O7 H8
0.2533(59) 0.1728(87) 0.8283(18) 2.0278(7) 1.6180(9) 1.6449(9) 2.4875(6) 1.9273(9)
0.9799(15) 0.2925(51) 1.1900(13) 0.4086(37) 0.8901(17) 0.5247(29) 0.3476(43) 1.5985(9)
0.2528(57) 0.1855(74) 0.8277(18) 2.0275(7) 1.6180(9) 1.6450(9) 2.4875(6) 1.9257(9)
0.9799(15) 0.2964(50) 1.1907(13) 0.4071(37) 0.8904(17) 0.5274(29) 0.3443(43) 1.5964(9)
a
Numbers in parentheses are one standard deviation Costain uncertainties (Type B, coverage factor k = 1 [18]) and apply to the last digits shown.
Table 8 ˚ ) and angles for trans- and cis-furfural compared with theoretical values and rs furan values Kraichman substitution rs and global rpKr bond distances (A Parameter
t-furfural rs
t-furfural global rpKr
t-furfural Theory [28]
c-furfural rs
c-furfural global rpKr
c-furfural Theory [28]
Furan rs [29]
r(O1C2) r(C2C3) r(C3C4) r(C4C5) r(C5O1) r(C2C6) r(C6O7) r(C6H8) r(C5H11) \(O1C2C3) \(C2O1C5) \(C2C3C4) \(C3C4C5) \(O1C2C6) \(O1C5C4) \(C2C6O7) \(C2C6H8) \(O1C5H11)
1.354(15)a 1.329(17) 1.436(4) 1.362(3) 1.361(3) 1.500(8) 1.215(4) 1.108(3) 1.075(2) 113.7(5) 104.7(6) 104.7(5) 106.7(11) 114.5(13) 110.9(1) 123.1(8) 115.6(7) 116.0(3)
1.353(19) 1.353(23) 1.440(28)b 1.360(8) 1.362(4) 1.477(11) 1.214(7) 1.104(8) 1.076(4) 112.4(10) 105.7(8) 105.0(14)b 106.0(14)b 116.0(16) 111.0(2) 123.5(11) 115.1(10) 115.8(6)
1.35 1.33 1.43 1.35 1.35 1.46 1.19 1.09 1.07 110.3 — 105.9 105.6 117.2 111.2 123.3 114.7 116.4
1.342(6) 1.345(7) 1.432(3) 1.362(4) 1.368(6) 1.490(7) 1.213(4) 1.110(3) — 113.4(6) 104.8(4) 105.0(3) 105.6(1) 117.5(5) 111.3(1) 125.0(7) 113.7(3) —
1.349(9) 1.351(11) 1.433(23)b 1.361(6) 1.368(7) 1.478(11) 1.212(6) 1.105(5) 1.073(2)b 112.5(10) 105.2(6) 105.4(10)b 105.6(5)b 117.9(8) 111.3(2) 125.0(6) 113.7(5) 116.2(2)b
1.35 1.33 1.43 1.35 1.35 1.46 1.19 1.09 1.07 110.2 — 106.0 105.2 119.4 111.3 124.8 113.4 116.3
1.362(1) 1.361(1) 1.431(2) 1.361(1) 1.362(1) — — — 1.075(2) 110.68(7) 106.55(7) 106.05(7) 106.05(7) — 110.68(7) — — 115.92(14)
a Numbers in parentheses are one standard deviation Costain uncertainties (Type B, coverage factor k = 1 [18]) for the rs entries and combined statistical and Costain uncertainties for the global fit entries, applying to the last digits shown. The distances and angles for the C3, C4, H9 and H10 atoms were fixed at the furan rs values [29]. b Parameter not fit.
and asymmetric tops) with considerable success. In an effort to see if we can improve our structure results, we employed the STRFIT program developed by Kisiel ð2Þ [36] to carry out rð1Þ m and rm structure fits which require fitting three ro-vibrational constants ca, with isotopic ð1Þ dependence I a1=2 (a = a, b, or c) for rm or six constants ð2Þ (three ca and three da) for the rm structure as shown in Eq. (31) of Ref. [35]. In all fits we found that the ca
and da were not determinable with uncertainties larger than the values themselves. We also included the Laurie correction term, dH, for D atom substitution with similar poor results and unreasonable size for the dH correction term. A number of authors who have carried out rm structure analyses have noted high correlations in parameters and rather large uncertainties as well. Often a large set of isotopically substituted species (often multiple
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
substitutions) were used in the analysis compared to the rather small set we have for furfural. With the exception of the Laurie correction analysis, these rm results are listed in the Supplementary tables and compared to an r0 structure (fit of moments of inertia only). 5. Summary The rotational spectrum for cis- and trans-furfural has been studied from 7 GHz to 330 GHz. This new investigation of the microwave spectra of furfural provides a reliable basis for searching interstellar furfural. Most of the strongest lines which may be of interest to radio astronomers have been measured with accuracy 5–10 kHz. The accuracy of prediction of the strong submillimeter lines (above 330 GHz) is estimated to be better than 0.5 MHz and interpolation below 330 GHz would give rotational frequencies which are accurate to 30 kHz or better. The study of all of the singly substituted 13C and 18O isotopologs has allowed the determination of the heavy atom rs and rpKr structure for the first time, is in good agreement with the theoretical structure. The mass-dependent (rm) structure fits were examined but provide uncertainties larger than the parameters. Acknowledgments Authors thank Dr. V.M. Kotlyar for purification of furfural sample. Authors also like to thank V.I. Piddyachiy for his assistance in preparation of detectors. The authors also thank the anonymous referee who provided several helpful comments. Appendix A. Supplementary data 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). References [1] J.M. Hollis, F.J. Lovas, P.R. Jewell, Astrophys. J. (Letters) 540 (2000) L107–L110. [2] G. Cooper, N. Kimmich, W. Bellsle, J. Sarinanna, K. Brabham, L. Garrel, Nature 414 (2001) 879–883. [3] F.J. Lovas, R.D. Suenram, D.F. Plusquellic, H. Møllendal, J. Mol. Spectrosc. 222 (2003) 263–272. [4] S.L. Widicus Weaver, G.A. Blake, Astrophys. J. (Letters) 624 (2005) L33–L36. Astrophys. J. (Letters) 632 (2005) L163. [5] A.J. Appone, D.T. Halfen, L.M. Ziurys, J.M. Hollis, A.J. Remijan, F.J. Lovas, Astrophys. J. (Letters) 643 (2006) L29–L32. [6] J.M. Hollis, P.R. Jewell, F.J. Lovas, A. Remijan, H. Møllendal, Astrophys. J. (Letters) 610 (2004) L21–L24.
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[7] J.M. Hollis, A.J. Remijan, P.R. Jewell, F.J. Lovas, Astropyhs. J. 642 (2006) 933–939. [8] J. Larralde, M.P. Robertson, S.L. Miller, Proc. Natl. Acad. Sci. USA 92 (1995) 8158–8160. [9] F.A. Miller, W.G. Fateley, R.E. Witkowski, Spectrochim. Acta A 23A (1967) 891–908. [10] R.J. Abraham, T.M. Siverns, Tetrahedron 28 (1972) 3015–3024. [11] T.S. Little, J. Qiu, J.R. Durig, Spectrochim. Acta A 45A (1989) 789– 794. [12] F. Mo¨nnig, H. Dreizler, H.D. Rudolph, Z. Naturforsch. 20a (1965) 1323–1326. [13] F. Mo¨nnig, H. Dreizler, H.D. Rudolph, Z. Naturforsch. 21a (1966) 1633–1640. [14] T.J. Balle, W. Flygare, Rev. Sci. Instrum. 52 (1981) 33–45. [15] R.D. Suenram, F.J. Lovas, G.T. Fraser, J.Z. Gillies, C.W. Gillies, M. Onda, J. Mol. Spectrosc. 137 (1989) 127–137. [16] F.J. Lovas, R.D. Suenram, J. Chem. Phys. 87 (1987) 2010–2020. [17] J.-U. Grabow, private communication (2003), see
Microwave spectrum and structure of furfural R.A. Motiyenko b
a,*
, E.A. Alekseev a, S.F. Dyubko a, F.J. Lovas
b
a Institute of Radio Astronomy of NASU, Chervonopraporna 4, 61002 Kharkov, Ukraine Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD 20899-8441, USA
Received 16 June 2006; in revised form 22 August 2006 Available online 8 September 2006
Abstract The microwave spectrum of furfural was investigated in the frequency range 7 GHz–21 GHz and 49 GHz–330 GHz. The ground and first torsional state of trans-furfural and ground state of cis-furfural were assigned and analyzed. A total of 1720 rotational lines with J up to 100 and Ka up to 53 were assigned to the ground state of trans-furfural, 1406 rotational lines with J up to 100 and Ka up to 48 were assigned to the first torsional state of trans-furfural and 2103 rotational lines with J up to 90 and Ka up to 48 to the ground state of cisfurfural. Accurate sets of centrifugal distortion constants for both conformations have been determined for the first time. The spectra of all 13C and 18O singly substituted isotopic species were observed in natural abundance in the 7 GHz–21 GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman substitution type (rs) and pseudo-Kraitchman type (rpKr) are derived from the isotopic studies. 2006 Elsevier Inc. All rights reserved. Keywords: Furfural; Microwave spectrum; Rotational spectrum; Molecular structure;
1. Introduction In recent studies of the C5 sugar, ribose, both at National Institute of Standards and Technology (NIST) and at Institute of Radio Astronomy of NASU (IRA NASU), furfural was found to be a major dehydration product. In the past few years a growing interest is developed in searching for simple sugars and their decomposition products in interstellar hot molecular cores. This interest has been stimulated by detection of the simplest C2 sugar glycolaldehyde (CH2OHCHO) in the Sgr B2 (N-LMH) molecular cloud by Hollis et al. [1], as well as by identification by Cooper et al. [2] of the C3 ketone sugar 1,3-dihydroxy-2-propanone (dihydroxyacetone), sugar alcohols and sugar acids in the Murchison meteorite. Another possible candidate for interstellar identification is C3 aldehyde sugar glyceraldehyde [3]. Recently the tentative detection of dihydroxyacetone (DHA) in Sgr B3(N-LMH) has been reported from *
Corresponding author. Fax: +38 057 706 1415. E-mail address: [email protected] (R.A. Motiyenko).
0022-2852/$ - see front matter 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2006.09.003
13
C-isotopologs;
18
O-isotopologs
millimeter wave astronomical studies [4]. In a follow up study to confirm this detection of DHA, Apponi et al. [5] have found no plausible emission at 97% of the 63 transitions sought, so the ketone form of the C3 sugar has not been detected. Several attempts to detect the aldehyde form, glyceraldehyde, at low frequency have been reported and at all frequencies searched, no signal could be detected [6,7]. Radio astronomy searches for higher order sugars are hampered by the lack of spectroscopic information. Another interest in the microwave spectra of furfural results from the complexity of laboratory spectral investigations of ribose due to its high instability and decomposition rate during vaporization [8]. Thus, the assignment of rotational spectra of ribose would be impossible without prior identification and exclusion of rotational lines of decomposition products, including furfural. Previous spectroscopic investigations of the furfural molecule (C4H3OCHO) have dealt mainly with infrared and Raman spectra [9–11]. These investigations have enabled the determination of relative stabilities of the cisand trans-conformations of furfural in the gas phase (see
94
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
a
trans- and cis-conformers were assigned and analyzed. However, in both publications the rotational analysis was limited to rigid rotor constants and therefore such data sets could not provide a reliable prediction basis in a wide frequency range. In this paper, we report the results of a new investigation of the microwave spectrum of furfural. We have assigned and analyzed the ground state of both conformations and also the first torsional state of trans-furfural, since the latter provides rather intense spectrum which is comparable to the spectrum of the ground state of cis-furfural and thus may represent an interest to radio astronomers. We have remeasured some of the previously measured lines and extended the frequency range up to 330 GHz. Centrifugal distortion constants up to eighth order were required to fit the data within experimental accuracy. As a result, a reliable basis for radio astronomy search for interstellar furfural has been provided. In addition, the spectra of all 13C and 18O singly substituted isotopic species were observed in natural abundance in the 7 GHz–21 GHz range. Molecular structure co-ordinates, bond lengths and angles of the Kraitchman type (rs) and pseudo-Kraitchman type (rpKr) fits are derived from the isotopic studies.
b H9 H10 C3 C4 O7
a
C2 C6
C5 O1
H11
H8
b
b H9
H8
2. Experimental details
C3 H10
C6
C4
C2
a O7
C5
O1
H11
Fig. 1. (a) Trans-furfural structure and atom numbering. (b) Cis-furfural structure and atom numbering.
Fig. 1 for their geometry) and also measurements of the barrier to internal rotation. It has been determined that the trans-conformer is the most stable in the gas phase. The most recent experimental value for the energy difference between cis- and trans-conformations is 3.4260(28) kJ/mol [11] and the value of barrier to internal rotation is 38.9660(25) kJ/mol [11]. The pure rotational spectrum of furfural was studied previously only in two separate publications by Mo¨nnig, Dreizler and Rudolph [12,13]. In their first work [12], the rotational spectrum of the ground state of both conformers and their several deuterated species was investigated in the frequency range 7 GHz–26 GHz providing the partial determination of the molecular structure. In the second work [13], several torsional and vibrational states of
In order to provide better predictions for the high frequency range, we undertook new measurements a using Fourier transform microwave spectrometer (FTMW) at NIST. Thus, frequencies of most of rotational lines of furfural reported in [12] were remeasured with an accuracy of 2 kHz, which is an order of magnitude better in comparison with previous results. Spectral measurements were carried out with a Fabry– Perot cavity, Fourier-transform microwave spectrometer of the Balle–Flygare type [14] designed by Lovas and Suenram [15,16]. 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 furfural entrained in argon (or neon) 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 molecular beam was polarized by a short microwave pulse when the microwave frequency was near-resonant (Dm < 400 kHz) with a rotational transition. The free induction decay signal from the cavity was digitized in 0.5 ls increments for 2048 channels. Typically, 50–100 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
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
element of 2 kHz per 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 uncertainty with coverage factor k = 2 [18], i.e., a two standard deviation estimate). All measurements in the frequency range between 50 GHz and 330 GHz have been carried out using an automated, synthesizer-based, millimeter-wave spectrometer at the Institute of Radio Astronomy of the National Academy of Science of Ukraine at Kharkov. A brief description of the spectrometer is presented in Ref. [19]. Since the spectrometer has been extensively modified, compared to the original version described in Ref. [19], we will provide below a brief description of the spectrometer and its modification. A block diagram of the current spectrometer is shown in Fig. 2. This is a usual absorption frequency-modulated
spectrometer with lock-in detection. As was mentioned previously in Ref. [19] (and references therein) the synthesis of frequencies in the millimeter region is carried out by a twostep frequency multiplication of the reference synthesizer 390–420 MHz in two phase-lock-loop (PLL) stages. The first PLL stage utilizes a klystron in the range of 3.4– 5.2 GHz. In the second stage, the ISTOK backward-wave oscillator (BWO) is locked to a harmonic of the klystron. By changing the BWO and the corresponding waveguide components, the frequency region from 49 GHz to 149 GHz can be covered. In addition, the frequency range above 200 GHz may be covered using a frequency doubler. The klystron PLL contains a special electromechanical system which provides continuous mechanical adjustment of the klystron resonator. This allows us to obtain a wide range of continuous spectral records in a fully automated mode.
Frequency Doubler
Shottky Detector
BWO, 50 – 150 GHz
Absorbing Cell
Harmonic Mixer PLL, IF=25MHz
Klystron 3.4 – 5.2 GHz
Amplifier
Harmonic Mixer FMModulated Synthesizer 25MHz
PLL, IF=5MHz
5MHz
Sine Wave Synthesizer 7 - 120 kHz
Lock-in Detector
5MHz
Band-pass Amplifier 390–420 MHz
Rb - Standard 5MHz
Balanced Mixer Synthesizer 360MHz
Frequency Divider f/2
180MHz
95
DDS AD9851 30 – 60MHz
Reference Synthesizer Fig. 2. Block diagram of the Kharkov spectrometer.
ADC
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As an absorption cell, we used a quasioptic dielectric hollow waveguide. In order to significantly reduce the well-known problem of standing waves, we used special dielectric horns for matching with the radiation source and detector. In addition we have employed special polytetrafluoroethylene vacuum windows with a thickness of about 0.8 mm. As detectors we use Schottky diodes placed in home-made waveguide mounts. In order to minimize distortions of the line shape due to modulation, we employ a special frequency-modulated synthesizer which provides a very low distortion sinusoidal modulation. It is possible to use both first and second derivative spectra recording, but usually we obtain the first derivative spectra. The most important modification of the spectrometer is the application of the new reference synthesizer. Previously we employed a commercial direct analog synthesizer, which was modified in order to improve its spectral purity. The best spectral purity was available in the narrow frequency range only (390–400 MHz). That is why there were socalled ‘‘dead zones’’ in the operating frequency range of the spectrometer, i.e., a few rather narrow frequency ranges where frequency synthesis was impossible. In order to overcome this disadvantage it is sufficient to employ a reference synthesizer with slightly expanded operating range, for example, 390–405 MHz and more. We undertook a number of attempts to apply various other reference synthesizers (mainly PLL-synthesizers) as the reference source, but every time the spectral purity of the new reference synthesizer was much worse than the initially chosen synthesizer. Some years ago there appeared highly integrated chips of a direct digital synthesizer (DDS). Application of such a synthesizer gives a number of advantages in comparison with other methods of frequency synthesis [20]. Since narrow-band spectral purity of the DDS is very good, we applied this type of synthesizer as a reference source in our spectrometer, namely, a DDS AD9851 from Analog Devices, Inc [21]. The simplified block diagram of the reference synthesizer is shown in Fig. 2. In fact it is just a usual up-converter which allows us to convert the output signal of the AD9851 from a frequency range of 30–60 MHz to 390–420 MHz, keeping its initial spectral purity. This up-conversion reduces the total multiplication factor by an order of magnitude. However, as it is known from the literature [20], the wideband output spectrum of a DDS contains a lot of rather intense spurious components, that is why high factor multiplication of the DDS output signal usually produces rather spurious spectrum. In order to overcome this obstacle it is necessary to apply narrow-band adaptive filtering. As was mentioned above, on the first stage of frequency multiplication in our spectrometer, a klystron is employed. Due to rather low phase noise level of a klystron, it is convenient to use this phase-lock loop as a narrow-band filter (its closed loop bandwidth is about 2 kHz). This provides effective filtering of spurious components.
For measurements at Kharkov a sample of furfural was obtained commercially and purified at the Department of Organic Chemistry of V.N. Karazin Kharkov National University. The Type B, coverage factor k = 1 [18] (or one sigma) uncertainty of the measurements for a strong isolated line is estimated to be: (i) 5 kHz for trans-furfural and 10 kHz for cis-furfural and first torsional state of transfurfural in the frequency range between 49 GHz and 149 GHz; (ii) 15 kHz for all rotational transitions in the frequency range between 200 GHz and 249 GHz; (iii) 60 kHz for all rotational transitions in the frequency range 300–330 GHz. 3. Results and analysis Assignment of the observed spectra was rather straightforward and based on frequency predictions obtained from fitting a combination of data from [12] and [13] and frequencies remeasured at NIST. A sample spectrum recorded at Kharkov between 84 800 MHz and 85 600 MHz is presented in Fig. 3. With increasing values of the quantum number J, several fitting/measurement cycles for improving the rotational constants were needed. A total of 1720 lines have been assigned to the ground state of trans-furfural, 1406 lines to the first torsional state of trans-furfural and 2103 lines to the ground state of cisfurfural. For both conformers most of the assigned lines belong to a-type R-branch transitions due to relatively large values of the electric dipole moment component, la = 3.20(3) D for trans-furfural and la = 3.41(3) D for cis-furfural [12]. However for the cis-conformer the set of b-type transitions is much larger than for the trans-conformer. This fact arises from the great difference between values of lb: 1.93(2) D for cis-conformer and 0.40(4) D for transconformer [12]. It should be noted that about 100 b-type transitions for the ground state of trans-furfural have been assigned for the first time. For the first torsional state of trans-furfural, no b-type transitions have been observed. The Watson A-reduction Hamiltonian in the Ir representation [22] was used for rotational analysis. ASROT and ASFIT programs from the PROSPE website were used for predicting and fitting spectral data [23]. The sets of rotational constants which have been determined for the first time include quartic and sextic constants for both conformations and in addition two octic constants for the ground and first torsional states of trans-furfural. The results of the fits are presented in Table 1. The overall root-mean-square (rms) deviations obtained are 9.9 kHz (weighted rms deviation is 0.58) for the ground state of trans-furfural, 10.3 kHz (weighted rms deviation is 0.59) for the first torsional state of trans-furfural and 12.6 kHz (weighted rms deviation is 0.65) for the ground state of cis-furfural. The overall quality of the fits is illustrated in Tables 2–4, which give the rms deviations for transitions grouped according to their measurement uncertainties (weight in the fit was proportional to reciprocal of uncertainty squared).
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
97
Fig. 3. A sample spectrum of furfural recorded between 84 800 MHz and 85 600 MHz. The strongest lines correspond to Ka series of aR0,1 J = 23 ‹ 22 transitions of trans-furfural.
Table 1 Rotational parameters of furfural Parameter
trans-furfural vt = 0 a
trans-furfural vt = 1
cis-furfural vt = 0
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) hJ (Hz) hJK (Hz) hK (Hz) LJ (mHz) LKKJ (mHz) lKJ (mHz)
8191.77386(19) 2045.929569(10) 1637.183876(11) 0.1361873(52) 0.706617(24) 1.7835(14) 0.0314688(24) 0.82192(18) 0.0000175(10) 0.000657(46) 0.00509(15) 0.00000868(32) 0.000403(30) 0.01335(98) 0.000000191(62) 0.0001018(82) —
8115.4200(12) 2045.583482(26) 1639.627551(22) 0.1376876(74) 0.544931(49) 1.055(10) 0.0315427(35) 0.42739(42) 0.0000263(10) 0.00459(12) 0.01653(41) 0.00001085(49) 0.000345(53) 0.0945(26) — 0.002808(19) 0.000587(46)
8143.738729(46) 2098.724250(11) 1668.872904(11) 0.1726593(47) 0.499948(24) 1.81402(15) 0.0403044(34) 0.80893(14) 0.00002719(75) 0.0003230(48) 0.002259(17) 0.00001091(51) 0.000139(29) — — — —
Numberb rms (MHz) weighted rmsc
1729 0.0099 0.583
1406 0.0103 0.589
2110 0.0125 0.649
a b c
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits. Number of rotational transitions treated for each conformer. Unitless root-mean-square deviation of the fit.
All singly substituted 13C and 18O isotopologs of both isomers were measured in natural abundance. For the 13 C species 21–25 transitions were measured and fit to a quartic Watson Hamiltonian [22], and the rotational constants are listed in Table 5. Due to the lower natural abundance of 18O, only 14–19 transitions were observable, and two of the centrifugal distortion parameters, DK and dK, were fixed in the fits at the values for the normal isotopic species. The fit results are shown in Table 6.
Tables with line assignments, measured frequencies their uncertainties and deviations are provided as a Supplementary material to this paper. 4. Structure determination In the earlier study of furfural isomers, Mo¨nnig et al. [12] reported results on two deuterated forms of the trans isomer and one deuterated form of the cis isomer. They
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Table 2 Root-mean-square deviations from the fit for trans-furfural vt = 0 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
Table 3 Root-mean-square deviations from the fit for trans-furfural vt = 1 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
Table 4 Root-mean-square deviations from the fit for cis-furfural vt = 0 Sourcea
Rangeb (GHz)
Linesc
Uncertaintyd (MHz)
RMSe (MHz)
a
Sources of data: Kharkov and NIST—this study, Dreizler et al.— references [12] and [13]. b Frequency range for the group of measurements in a given row. c Number of rotational lines in each group. d One-sigma standard measurement uncertainty in MHz used in the fit (Type B estimates with coverage factor k = 1 [18]). e Root-mean-square deviation in MHz for each group.
were able to derive limited structural information, primarily on the aldehyde group from the isotopolog data. With our new 13C and 18O rotational analysis, coupled with the earlier analysis of several deuterated forms, we can derive a rather complete structure of the heavy atom frame. The most common type of structure analysis is the Kraitchman substitution, rs structure [24] in which the
atom coordinates are determined from differences in the planar second moments of inertia, e.g., Paa = 1/2(Ibb + Icc Iaa) where Iaa, Ibb and Icc are the principal axes moments of inertia, between the parent form and the isotopically substituted form. In Table 7, columns 2 and 3, the rs coordinates of the substituted atoms shown in the first column are listed. From these coordinates the bond distances and angles shown in Table 8, columns 2 and 5, can be derived. The uncertainties shown for the rs values were determined from the Costain rule [25] which is dxn,k = 0.0015/jxn,kj where xn,k is the kth coordinated of atom n. The statistical uncertainties are usually smaller than those derived from the Costain rule. A second type of structural analysis was carried out in which all the isotopologs were treated in a global fit using the STRFTQ program developed by Schwendeman [26] whereby the internal coordinates, bond distances and angles are fit to either moments of inertia (Ixx) or planar second moments of inertia (Pxx) to give an r0 structure, or to differences in these quantities between the parent species and each of the isotopologs to give a pseudo-Kraitchman (rpKr) structure which is similar to an rs structure (see also the discussion of these types of structural analyses by Rudolph [27]). In the current case we chose the differences in second moments (DP nxx ¼ P pxx P nxx where p is the parent moment and n is the substituted atom moment), or pseudoKraitchman analysis which most closely parallels the rs analysis and is often better than an rs structure when small coordinates occur as in furfural. These results are given in Tables 7 and 8 under the rpKr column headings. For the trans-furfural a fit of 15 parameters gave an overall stan˚ 2 and for the cis isomer it dard deviation of 0.0064 uA 2 ˚ in fitting 13 parameters. The uncertainties was 0.0048 uA shown for the rpKr analysis bond lengths and angles are a combination of the statistical uncertainties and the Costain rule uncertainties, which sometimes dominate. The largest uncertainties will occur with the atoms having the smallest coordinates, namely the C2, C3 and O7 atoms of the trans isomer and the O1, C2 and O7 atoms of the cis isomer. The coordinates for the rs and rpKr structures in Table 7 are quite similar except for the C2 atom, where a difference ˚ occurs for the trans form and 0.012 A ˚ for the cis of 0.020 A form. As a result the bond distances in Table 8 which involve atom C2 will also disagree by similar amounts. The rs values agree with the rpKr values within the combined uncertainties given for both structures. The largest differences are found for r(C2C6), \(O1C2C3) and \(C2O1C5) for both isomers. Montero et al. [28] report ab initio SCF-MO calculations of the structure of furfural with several basis sets. In columns 4 and 7 of Table 8 we list their results for the trans and cis forms, respectively, with the 6-31G** basis set. Also for comparison, the last column of Table 8 lists the rs values of furan derived by Bak et al. [29]. The largest changes in the furfural ring compared to the furan structure occur in the bonds and angles involving the C2 atom where the aldehyde group is attached. Unfortunately, due to several
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101 Table 5 Rotational parameters of Parameter A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz) Numberb rms (kHz) Wtd. rmsc
99
13
C substituted furfural species
t-C2-furfural a
8191.2984(14) 2045.6074(6) 1636.9647(6) 0.1401(45) 0.692(30) 1.48(31) 0.0259(26) 0.98(26) 24 1.4 0.58
t-C3-furfural
t-C4-furfural
t-C5-furfural
t-C6-furfural
8025.2399(14) 2044.7201(5) 1629.6481(4) 0.1443(41) 0.710(21) 1.78(30) 0.0286(32) 0.91(18)
8097.8411(9) 2019.7916(3) 1616.6913(3) 0.1365(32) 0.638(17) 1.69(19) 0.0289(21) 0.84(15)
8159.5106(11) 2016.4563(4) 1616.9968(4) 0.1300(35) 0.720(18) 1.47(22) 0.0240(25) 0.76(16)
8163.1167(13) 2024.0110(5) 1621.9939(4) 0.1356(40) 0.641(20) 2.34(29) 0.0266(31) 0.99(18)
25 1.3 0.56
25 0.9 0.44
24 1.1 0.48
25 1.4 0.55
Parameter
c-C2-furfural
c-C3-furfural
c-C4-furfural
c-C5-furfural
c-C6-furfural
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz)
8132.6075(11)a 2098.4658(4) 1668.2472(4) 0.1741(33) 0.467(20) 1.73(23) 0.0374(26) 0.83(14)
7963.9761(14) 2092.7573(5) 1657.4365(4) 0.1681(43) 0.534(27) 1.96(32) 0.0341(35) 0.99(17)
8122.5291(15) 2063.7000(5) 1645.7865(5) 0.1684(46) 0.497(28) 1.84(32) 0.0330(34) 0.83(19)
8043.2439(12) 2076.2141(4) 1650.4150(4) 0.1761(36) 0.478(22) 1.87(27) 0.0369(28) 0.72(15)
8108.5514(15) 2075.5316(5) 1652.7208(5) 0.1694(43) 0.471(27) 2.05(32) 0.0324(34) 0.77(18)
Numberb rms (kHz) Wtd. rmsc a b c
23 1.1 0.45
23 1.3 0.60
21 1.4 0.59
23 1.2 0.50
23 1.4 0.59
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits shown. Number of rotational transitions fit for each isotopolog. Unitless root-mean-square deviation of the weighted fit.
Table 6 Rotational parameters of
18
O substituted furfural species
Parameter
t-O1-furfural
t-O7-furfural
c-O1-furfural
c-O7-furfural
A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) dJ (kHz) dK (kHz)
7894.1527(13)a 2038.3484(3) 1620.1360(2) 0.1324(32) 0.758(29) 1.78b 0.0288(35) 0.82b
8165.7441(21) 1944.8085(5) 1570.8227(4) 0.1308(38) 0.608(36) 1.78b 0.0249(45) 0.82b
7903.7962(7) 2097.6148(2) 1657.8573(1) 0.1740(15) 0.642(42) 1.81b 0.0412(16) 0.81b
8114.7031(5) 1997.8153(1) 1603.3011(1) 0.1632(10) 0.415(26) 1.81b 0.0369(10) 0.81b
Numberc rms (MHz) Wtd. rmsd a b c d
17 1.2 0.58
19 1.8 0.90
14 0.6 0.29
14 0.4 0.19
Numbers in parentheses are one standard deviation uncertainties (Type A, coverage factor k = 1 [18]) and apply to the last digits shown. Value fixed in the fit. Number of rotational transitions fit for each isotopolog. Unitless root-mean-square deviation of the weighted fit.
small coordinates for atoms 1, 2 and 3 mentioned above these derived values also show some of the largest uncertainties. While it appears that the r(O1C2) and r(C2C3) are reduced by the addition of the aldehyde group, the large uncertainties do not allow for this conclusion, especially for the trans isomer. In general the ring structure of furan is preserved quite well in the furfural isomers and our structure results compare quite favorably with the ab initio results.
Recently Demaison and Rudolph [30] have shown that ‘‘an rs structure can be quite unreliable when the molecule is heavy and has low-frequency normal mode(s)’’. This might be the case for furfural which is rather heavy and has a low torsion frequency for the HCO group, namely, 127 cm1 for cis and 146 cm1 for trans [11]. Demaison, Rudolph and co-workers [30–34] applied the mass-dependent (rm) method of Watson et al. [35] to a variety of molecules (diatomic, linear, symmetric top
100
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
Table 7 Principal axis Kraichman substitution rs and global rpKr coordinates of trans- and cis-furfural Parameter
t-furfural rs ˚) a (A
t-furfural rs ˚) b (A
t-furfural global rpKr ˚) a (A
t-furfural global rpKr ˚) b (A
O1 C2 C3 C4 C5 C6 O7 H8 H11
0.6798(22)a 0.1979(75) 0.3823(39) 1.7908(8) 1.9056(8) 1.6404(9) 2.5564(6) 1.8267(19) 2.7493(13)
1.0912(14) 0.060(25) 1.1359(13) 0.8562(18) 0.5005(30) 0.4704(32) 0.3273(46) 1.5628(10) 1.1673(13)
0.6795(22) 0.2175(65) 0.3782(39) 1.7906(8) 1.9052(8) 1.6406(9) 2.5565(6) 1.8246(19) 2.7490(13)
1.0915(14) 0.078(19) 1.1362(13) 0.8571(18) 0.4982(30) 0.4722(32) 0.3249(46) 1.5603(10) 1.1667(13)
Parameter
c-furfural rs ˚) a (A
c-furfural rs ˚) b (A
c-furfural global rpKr ˚) a (A
c-furfural global rpKr ˚) b (A
O1 C2 C3 C4 C5 C6 O7 H8
0.2533(59) 0.1728(87) 0.8283(18) 2.0278(7) 1.6180(9) 1.6449(9) 2.4875(6) 1.9273(9)
0.9799(15) 0.2925(51) 1.1900(13) 0.4086(37) 0.8901(17) 0.5247(29) 0.3476(43) 1.5985(9)
0.2528(57) 0.1855(74) 0.8277(18) 2.0275(7) 1.6180(9) 1.6450(9) 2.4875(6) 1.9257(9)
0.9799(15) 0.2964(50) 1.1907(13) 0.4071(37) 0.8904(17) 0.5274(29) 0.3443(43) 1.5964(9)
a
Numbers in parentheses are one standard deviation Costain uncertainties (Type B, coverage factor k = 1 [18]) and apply to the last digits shown.
Table 8 ˚ ) and angles for trans- and cis-furfural compared with theoretical values and rs furan values Kraichman substitution rs and global rpKr bond distances (A Parameter
t-furfural rs
t-furfural global rpKr
t-furfural Theory [28]
c-furfural rs
c-furfural global rpKr
c-furfural Theory [28]
Furan rs [29]
r(O1C2) r(C2C3) r(C3C4) r(C4C5) r(C5O1) r(C2C6) r(C6O7) r(C6H8) r(C5H11) \(O1C2C3) \(C2O1C5) \(C2C3C4) \(C3C4C5) \(O1C2C6) \(O1C5C4) \(C2C6O7) \(C2C6H8) \(O1C5H11)
1.354(15)a 1.329(17) 1.436(4) 1.362(3) 1.361(3) 1.500(8) 1.215(4) 1.108(3) 1.075(2) 113.7(5) 104.7(6) 104.7(5) 106.7(11) 114.5(13) 110.9(1) 123.1(8) 115.6(7) 116.0(3)
1.353(19) 1.353(23) 1.440(28)b 1.360(8) 1.362(4) 1.477(11) 1.214(7) 1.104(8) 1.076(4) 112.4(10) 105.7(8) 105.0(14)b 106.0(14)b 116.0(16) 111.0(2) 123.5(11) 115.1(10) 115.8(6)
1.35 1.33 1.43 1.35 1.35 1.46 1.19 1.09 1.07 110.3 — 105.9 105.6 117.2 111.2 123.3 114.7 116.4
1.342(6) 1.345(7) 1.432(3) 1.362(4) 1.368(6) 1.490(7) 1.213(4) 1.110(3) — 113.4(6) 104.8(4) 105.0(3) 105.6(1) 117.5(5) 111.3(1) 125.0(7) 113.7(3) —
1.349(9) 1.351(11) 1.433(23)b 1.361(6) 1.368(7) 1.478(11) 1.212(6) 1.105(5) 1.073(2)b 112.5(10) 105.2(6) 105.4(10)b 105.6(5)b 117.9(8) 111.3(2) 125.0(6) 113.7(5) 116.2(2)b
1.35 1.33 1.43 1.35 1.35 1.46 1.19 1.09 1.07 110.2 — 106.0 105.2 119.4 111.3 124.8 113.4 116.3
1.362(1) 1.361(1) 1.431(2) 1.361(1) 1.362(1) — — — 1.075(2) 110.68(7) 106.55(7) 106.05(7) 106.05(7) — 110.68(7) — — 115.92(14)
a Numbers in parentheses are one standard deviation Costain uncertainties (Type B, coverage factor k = 1 [18]) for the rs entries and combined statistical and Costain uncertainties for the global fit entries, applying to the last digits shown. The distances and angles for the C3, C4, H9 and H10 atoms were fixed at the furan rs values [29]. b Parameter not fit.
and asymmetric tops) with considerable success. In an effort to see if we can improve our structure results, we employed the STRFIT program developed by Kisiel ð2Þ [36] to carry out rð1Þ m and rm structure fits which require fitting three ro-vibrational constants ca, with isotopic ð1Þ dependence I a1=2 (a = a, b, or c) for rm or six constants ð2Þ (three ca and three da) for the rm structure as shown in Eq. (31) of Ref. [35]. In all fits we found that the ca
and da were not determinable with uncertainties larger than the values themselves. We also included the Laurie correction term, dH, for D atom substitution with similar poor results and unreasonable size for the dH correction term. A number of authors who have carried out rm structure analyses have noted high correlations in parameters and rather large uncertainties as well. Often a large set of isotopically substituted species (often multiple
R.A. Motiyenko et al. / Journal of Molecular Spectroscopy 240 (2006) 93–101
substitutions) were used in the analysis compared to the rather small set we have for furfural. With the exception of the Laurie correction analysis, these rm results are listed in the Supplementary tables and compared to an r0 structure (fit of moments of inertia only). 5. Summary The rotational spectrum for cis- and trans-furfural has been studied from 7 GHz to 330 GHz. This new investigation of the microwave spectra of furfural provides a reliable basis for searching interstellar furfural. Most of the strongest lines which may be of interest to radio astronomers have been measured with accuracy 5–10 kHz. The accuracy of prediction of the strong submillimeter lines (above 330 GHz) is estimated to be better than 0.5 MHz and interpolation below 330 GHz would give rotational frequencies which are accurate to 30 kHz or better. The study of all of the singly substituted 13C and 18O isotopologs has allowed the determination of the heavy atom rs and rpKr structure for the first time, is in good agreement with the theoretical structure. The mass-dependent (rm) structure fits were examined but provide uncertainties larger than the parameters. Acknowledgments Authors thank Dr. V.M. Kotlyar for purification of furfural sample. Authors also like to thank V.I. Piddyachiy for his assistance in preparation of detectors. The authors also thank the anonymous referee who provided several helpful comments. Appendix A. Supplementary data 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). References [1] J.M. Hollis, F.J. Lovas, P.R. Jewell, Astrophys. J. (Letters) 540 (2000) L107–L110. [2] G. Cooper, N. Kimmich, W. Bellsle, J. Sarinanna, K. Brabham, L. Garrel, Nature 414 (2001) 879–883. [3] F.J. Lovas, R.D. Suenram, D.F. Plusquellic, H. Møllendal, J. Mol. Spectrosc. 222 (2003) 263–272. [4] S.L. Widicus Weaver, G.A. Blake, Astrophys. J. (Letters) 624 (2005) L33–L36. Astrophys. J. (Letters) 632 (2005) L163. [5] A.J. Appone, D.T. Halfen, L.M. Ziurys, J.M. Hollis, A.J. Remijan, F.J. Lovas, Astrophys. J. (Letters) 643 (2006) L29–L32. [6] J.M. Hollis, P.R. Jewell, F.J. Lovas, A. Remijan, H. Møllendal, Astrophys. J. (Letters) 610 (2004) L21–L24.
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