- Email: [email protected]
High resolution synchrotron FTIR spectroscopy of the far infrared ν10 and ν11 bands of R152a (CH3CHF2)
Chemical Physics Letters 465 (2008) 203–206
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
High resolution synchrotron FTIR spectroscopy of the far infrared m10 and m11 bands of R152a (CH3CHF2) Tarekegn Chimdi a, Evan G. Robertson a, Ljiljana Puskar a,b, Christopher D. Thompson a, Mark J. Tobin b, Don McNaughton a,* a b
Centre for Biospectroscopy and School of Chemistry, Monash University, Clayton, 3800, Victoria, Australia Australian Synchrotron, 800 Blackburn Rd, Clayton, 3168, Victoria, Australia
a r t i c l e
i n f o
Article history: Received 8 August 2008 In final form 6 October 2008 Available online 11 October 2008
a b s t r a c t High resolution (0.0019 cm1) Fourier transform infrared spectra of R152a (CH3CHF2) at room temperature were measured using far infrared radiation from the IR beamline at the Australian Synchrotron. The a-type fundamentals m10 = 569.12 cm1 and m11 = 468.34 cm1 were assigned and analysed. Hot-band transitions centred at 471.18 cm1 and 469.27 cm1 were identified as 1110 1811 and 1110 1711 , respectively. Transitions were assigned with the aid of Loomis Wood plots, confirmed by combination differences and fitted with Pickett’s SPFIT/SPCAT to determine molecular constants. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the original recognition by Yarwood et al. [1] some 20 years ago that synchrotron light provides a bright source of infrared radiation, synchrotron infrared radiation (SIR) has been increasingly used to advantage in a vast range of fields such as surface science, geology, cell biology, materials science and conservation science [2–6]. Design features and areas of expertise for operating IR synchrotrons around the world have also been discussed [7–11] and many synchrotrons around the world now have at least one, and usually more, IR beamlines. There are clear advantages for synchrotron radiation over conventional thermal radiation sources such as the Globar in the mid-infrared (MIR) and mercury discharge lamp in the far-infrared (FIR) regions. These advantages are: two to three orders of magnitude greater brightness across the whole infrared region; a greater radiation flux in the far-IR; the possibility of polarized light; pulsed radiation down to a sub ns timescale. The increased brightness has led to great advantages in synchrotron FTIR micro-spectroscopy, the pulsed nature of the light has led to time resolved infrared spectroscopy and the selection of polarization allows for polarized micro-spectroscopic studies and ellipsometry. The greater flux in the FIR and the point source nature of the radiation source has led to advantages for high resolution spectroscopy, which has always been extremely challenging in this spectral region. Synchrotron source high resolution molecular spectroscopy studies have been carried out at MAX-lab in Sweden [11–15] and very recently
* Corresponding author. Fax: +61 3 9905 4597. E-mail address: [email protected] (D. McNaughton). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.10.011
at the Canadian light source [16,17]. This work includes the m5 band of HClO4 [14], the m11 band of allene [13], the torsional band of CH3SiD3 [15] and bands of acrolein [16,17] in the 150–600 cm1 region. The newly commissioned Australian Synchrotron has an IR beamline, outlined by Creagh et al. [18], utilizing edge radiation for far infrared spectroscopy. This letter presents the first results arising from high resolution synchrotron far-infrared FTIR spectroscopy conducted at the Australian synchrotron. We report rovibrational analyses for the m10 and m11 bands of the hydrofluorocarbon R152a (CH3CHF2) from spectra recorded on the high resolution beamline in its commissioning phase. R152a was chosen because of its importance as an atmospheric infrared absorber and the observed 5-fold increase in its atmospheric concentration since its introduction as a CFC replacement over a decade ago [19]. 2. Experimental R152a (CH3CHF2), (99% commercial grade, BOC gases Australia) was maintained at a pressure of (115 ± 9) Pa in a glass multi-pass White cell set to a path length of 4 m. Room temperature absorption spectra in the 350–750 cm1 region were recorded at the Australian synchrotron using the infrared synchrotron edge radiation continuum source, a Bruker IFS125HR interferometer equipped with a multilayer Mylar beam splitter and a liquid helium-cooled external Cu:Ge bolometer. During the commissioning period in which experiments were conducted, full ring current injections (200 mA) were interspersed with half current injections (100 mA). Interferometer ADC counts (and S/N) decreased proportionally with the current. High resolution measurements were therefore recorded in the high current periods, with beam current decreasing from 192 mA upon injection to 172 mA over a period of
204
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
several hours. 26 coadded scans were recorded with a four point apodization function at a resolution of 0.0019 cm1. The calibration of the wavenumber scale was achieved using 32 rotational lines of H2O and CO2 spectra (present as residual background gas in the evacuated instrument) and compared with the values listed in HITRAN spectroscopic database [20]. The corresponding rotational lines were located with an average uncertainty of 0.0003 cm1. 3. Results and discussion Despite the early commissioning stage of the beamline with non optimum conditions and the synchrotron beam apertured
Fig. 1. High resolution room temperature FTIR spectra of CH3CHF2 in the region of (a) the m10 band and (b) the m11 band.
down to attain the desired resolution, the synchrotron provided a flux (and S/N) advantage of ca. 2.5 over the internal globar source in the wavenumber range of the experiment. High resolution spectra of the R152a bands m11 (468.34 cm1) and m10 (569.12 cm1) due to the CF2 bend and wag respectively are shown in Fig. 1a and b. Both bands are a-type bands with a prominent central Q branch. Some of the sub structure visible in the spectra is associated with hot bands emanating from v17 = 1 (390 cm1) and v18 = 1, 2 (221, 442 cm1) levels, which are quite heavily populated at room temperature. For the rovibrational analysis of these bands, the absorbance spectra were post-zerofilled by a factor of 32 and peaks picked in OPUS 5.5. In total over 18 000 peak positions and intensities were selected for input into the interactive assignment tool MacLoomis [21]. MacLoomis generates Loomis Wood plots which help to identify and select series of regularly spaced transitions that occur even in asymmetric top molecules [22,23]. In this case, transitions that share specific values of the quantum number Ka, but with differing values of J and Kc are separated by ca. 2C (0.345 cm1). Ground state rotational constants for R152a are known from earlier microwave [24] and mid-IR high resolution FTIR studies [25,26]. A total of 1701 lines with Jmax = 67, Ka max = 14 and 4594 lines with Jmax = 82, Ka max = 22 were assigned to a-type rovibrational transitions of m10 and m11, respectively, with selection rules of DJ = ±1, DKa = 0 and DKc = ±1. Assignments were confirmed by comparison of ground state combination differences (GSCD) with those calculated from the microwave rotational constants of Villamanan et al. [24]. The fitting of assigned transitions was performed with Pickett’s SPFIT program [27], using Watson’s A reduced Hamiltonian in the Ir representation [28], whilst predictions and simulations were carried out using Pickett’s SPCAT program. IR lines were given an experimental uncertainty of 0.0004 cm1 in the least squares fits. Ground state constants and some higher order centrifugal distortion constants for the excited state were fixed to the values of Villamanan et al. [24]. Preliminary fits of the IR data yielded small r.m.s. residuals of 3–4 104 cm1, suggesting that both of the bands m11 and m10 have regular rotational structure with no apparent avoided crossings and no obvious major perturbations. The fitted constants for m10 and m11 correlated well with the constants of the vibrational satellites designated ‘vb’ and ‘vc’ by Villamanan et al., allowing unique vibrational quantum number assignments for these transitions. As a consequence, it was possi-
Table 1 Molecular constants (cm1) of the m10 and m11 bands of R152a fitted jointly with microwave transitions obtained from satellite bands [23] Parameters
Ground state [23]
vb = 1 [23]
m0 A B C DJ 106 DJK 106 DK 106 dJ 106 dK 106 UJ 1012 UJK 1012 UKJ 1012 uJ 1012 uK 1012 No. of transitions Jmax Ka max Standard deviation R.m.s. error a b c
0.316618424 0.298964583 0.172467785 0.158504 0.0603364 0.0971422 0.05764504 0.1550315 0.2537 0.3231 0.5200 0.1204 1.8506
0.315970705(86) 0.299027286(84) 0.172440996(81) 0.158504b 0.072166(34) 0.084112(48) 0.05764504b 0.153389(20) 0.2537b 0.3231b 0.5200b 0.1204 b 1.8506b 37 32 25
v10 = 1 569.118459(29)a 0.315970744(58) 0.299027337(55) 0.172441241(43) 0.156827(42) 0.06737(17) 0.08704(33) 0.056772(20) 0.155134(107) 0.2537b 0.3231b 0.5200b 0.1204 b 1.8506b 1701/37c 67/32c 14/25c 0.798 3.1 104/0.042c
vb = 1 and vc = 1, respectively.
vc = 1 [23] 0.3165157220(83) 0.2989925267(79) 0.1727200165(75) 0.158504b 0.053824(37) 0.108632(53) 0.05764504b 0.148733(21) 0.2537b 0.3966(40) 0.5200b 0.1204b 1.8506 b 42 35 27
Figures in brackets are one standard deviation from the least squares fit. Constrained to the corresponding ground state values [23]. The two numbers refer to infrared and microwave transitions respectively, with r.m.s. errors quoted in cm1 and MHz accordingly.
v11 = 1 468.341641(21) 0.316515938(40) 0.298992913(38) 0.17272025(36) 0.159795(17) 0.04747(14) 0.11205(26) 0.0568028(50) 0.151477(75) 0.2966(17) 0.3231b 0.5200b 0.1204b 1.8506b 4594/41c 82/32c 22/25c 0.824 3.1 104/0.084c
205
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
Fig. 2. Expanded spectrum of the m10 bands of R152a (a), compared to an a-type band simulation (b) generated with spectroscopic parameters from Table 1 at T rot = 298 K, and Gaussian lineshape with FWHM 0.0025 cm1.
Fig. 3. Expanded spectrum of the m11 band of R152a (a), compared to a-type band simulations based on either the m11 fundamental only (c), or on the m11 ð1110 Þ fundamental combined with the v11 = 1,v17 = 1 v17 = 1 ð1110 1711 Þ and v11 = 1,v18 = 1 v18 = 1 ð1110 1811 Þ hot band transitions (b). Labels indicate some individual lines associated with 1110 (solid line, 41,4 51,5 & 41,4 51,5), the 1110 1811 hotband (dashed line, 103,7 113,8 & 104,7 114,8) and the 1110 1711 hotband (dotted line, 70,7 80,8 & 71,7 81,8).
ble to perform a combined fit of the infrared transitions and the vibrational satellite data, appropriately weighted (0.04 MHz). The resulting rovibrational constants are summarized in Table 1. It can be seen that inclusion of the infrared data results in a more complete set of constants than those from microwave vibrational satellites alone. The band centre is accurately determined, along with all the quartic centrifugal distortion constants. Most of the sextic constants were fixed to ground state values, except UJ in m11. It was necessary to freely vary this constant in order to treat IR transitions with high rotational quantum number (Jmax = 82). The r.m.s. residual for IR transitions of both m10 and m11 is excellent - just 0.00031 cm1. In Fig. 2, a short section of the m10 spectrum is compared with a simulation based on the constants of Table 1, where predicted lines are convolved with a Gaussian line shape of appropriate linewidth. A similar comparison is made for m11 in Fig. 3a and c. The simulations do not match perfectly as hot band transitions generate extra structure in the experimental spectra. To remove the fundamental band transitions from the experimental spectrum and simplify the hot band analysis in the m11 region the ‘spectral analysis by subtraction of simulated intensities’ (SASSI) [23,29] approach was used. In this approach a simulated spectrum is subtracted from the original data and the resultant difference spectrum peak picked for new MacLoomis analysis. Rovibrational transitions for the two most intense hotbands were then assigned in an analogous manner to the fundamental transitions. Villamanan et al. [24] in their microwave work fitted vibrational satellite data and labeled the associated vibrational states as vt, va, vb, vc rather than commit to an absolute assignment to particular modes. By comparing our lower state combination differences for hot-band transitions against predictions using the constants of Villamanan et. al. [24] their transitions for vt = 1 and va = 1 are assigned as v18 = 1 and v17 = 1 respectively. A total of 1404 lines with Jmax = 67, Ka max = 8 and 686 lines with Jmax = 56, Ka max = 5 were assigned to a-type rovibrational transitions for hot bands emanating from v18 = 1 and v17 = 1 respectively. Results of the fits for hot bands associated with m11 are summarized in the Table 2. The lower state constants in these fits were fixed to the values from the microwave study, whilst most of the centrifugal distortion constants for the upper states had to be held at the lower state values also. In order to assess the quality and accuracy of the data the calculated constants from all the assigned levels associated with the m11
Table 2 Molecular constants (cm1) for the hot bands associated with the m11 band of R152a. Parameters
va (v17) = 1 [23]
m0 A B C DJ 106 DJK 106 DK 106 dJ 106 dK 106 UJ 1012 UJK 1012 UKJ 1012 uJ 1012 uK 1012 No. of transitions Jmax Ka max Standard deviation R.m.s. error a b c
0.317226179 0.299212864 0.171645946 0.156408204 0.065511988 0.096556799 0.059407765 0.160958018 0.253742207 0.323090183 0.520026424 0.120449995 1.850646960 85 40 31
v11 = 1, v17 = 1 469.265135(43)a 0.317087(76) 0.299229(66) 0.1718456(99) 0.157590(175) 0.065511988b 0.096556799b 0.059455(97) 0.160958018b 0.253742207b 0.323090183b 0.520026424b 0.120449995b 1.850646960b 686c 56c 5c 1.053 4.2 104
Figures in brackets are one standard deviation from the least squares fit. Constrained to corresponding lower state value from microwave vibrational satellite data [23]. Corresponding only to infrared transitions.
vt (v18) = 1 [23] 0.316629309 0.298015160 0.172247082 0.155350806 0.063760777 0.091653406 0.056203215 0.148501067 0.253742207 1.09542449 0.520026424 0.086393101 1.40897474 206 40 32
v11 = 1, v18 = 1 471.177649(37) 0.3166425(163) 0.2978785(139) 0.172514893(55) 0.1054(46) 0.9107(149) 0.091653406 0.056203215b 0.148501067b 0.253742207b 1.09542449b 0.520026424b 0.086393101b 1.40897474b 1404c 67c 8c 1.026 4.1 104
206
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
Acknowledgements We acknowledge the financial support of the Australian Research Council, the Australian Synchrotron for the beam time, Mr. Finlay Shanks for technical assistance and Dr. Dominique Appadoo for assistance in the initial setting up of the beamline. References
Fig. 4. Experimental spectrum of the m11 band of R152a (a), together with a simulation including the hot bands (b) based on spectroscopic results from Tables 1 and 2 at Trot = 298 K, and Gaussian lineshape with FWHM 0.0020 cm1. Hot-band components were scaled according to the calculated Boltzmann populations for vibrational states originating at 221 and 390 cm1.
band were used to simulate the band with all the transitions weighted by the appropriate 298 K Boltzmann factors. The simulated spectrum, together with the experimental spectrum, is shown in Fig. 4 where the gross structure including two additional Q-branches compares well. A portion of the simulation consisting of fundamental and hot bands transition lines, shown in Fig. 3b, reveals the usefulness of the constants in predicting many of the additional lines in the spectrum. 4. Conclusion Despite the IR beamline being in a commissioning phase at the time of these experiments, and the synchrotron itself being at an early stage of routine operation, the synchrotron beam provided flux sufficiently higher than that from a globar source to enable us to record spectra of two R152a bands in the far infrared 350– 750 cm1 region with good signal-to-noise at high spectral resolution. Both the m10 and m11 fundamental bands and the most intense hot bands associated with m11 have been successfully assigned and fitted to produce sets of well determined molecular constants. We expect a further improvement in synchrotron flux and stability after full commissioning of the beamline.
[1] J. Yarwood, T. Shuttleworth, J.B. Hasted, T. Nanba, Nature 312 (5996) (1985) 742. [2] N.S. Marinkovic, S. Gupta, C. Zhan, M.R. Chance, Nucl. Instrum. Meth. Phys. Res. B 241 (2005) 242. [3] P. Roy et al., Nucl. Instr. Meth. Phys. Res. A (2001) 467. [4] L.M. Miller, R.J. Smith, Vib. Spectrosc. 38 (2005) 237. [5] G.L. Carr, Vib. Spectrosc. 19 (1999) 53. [6] D. Creagh, J. McKinlay, P. Dumas, Vib. Spectrosc. 41 (2006) 213. [7] A. Nucara, S. Lupi, P. Calvani, Infr. Phys. Technol. 45 (2004) 375. [8] R.W. Larsen, F. Hegelund, B. Nelander, Mol. Phys. 102 (2004) 1743. [9] J. Cutler, E. Hallin, M. de Jong, W. Thomlinson, T. Ellis, Nucl. Instr. Meth. Phys. Res. A 582 (2007) 11. [10] S.K. Deb et al., Infr. Phys. Technol. 49 (2006) 82. [11] B. Nelander, Vib. Spectrosc. 9 (1995) 29. [12] B. Nelander, V. Sablinskas, M. Dulick, V. Braun, P.F. Bernath, Mol. Phys. 93 (1998) 137. [13] S. Nissen, F. Hegelund, M.S. Johnson, B. Nelander, J. Mol. Spectrosc. 216 (2002) 197. [14] M.S. Johnson, F. Hegelund, B. Nelander, J. Mol. Spectrosc. 190 (1998) 269. [15] J. Schroderus, V.-M. Horneman, M.S. Johnson, N. Moazzen-Ahmadi, I. Ozier, J. Mol. Spectrosc. 215 (2002) 134. [16] A.R.W. McKellar, D.W. Tokaryk, D.R.T. Appadoo, J. Mol. Spectrosc. 244 (2007) 146. [17] A.R.W. McKellar, D.W. Tokaryk, Li-Hong Xu, D.R.T. Appadoo, T. May, J. Mol. Spectrosc. 242 (2007) 31. [18] D. Creagh, M. Tobin, A. Broadbent, J. McKinlay, AIP Conference Proceedings, 879 (Synchrotron Radiation Instrumentation, Part 1), 2007, p. 615. [19] B.R. Greally et al., J. Geophys. Res. 112 (2007) D06308. [20] L.S. Rothman et al., J. Quant. Spec. Rad. Trans. 96 (2005) 139. [21] D. McNaughton, D. McGilvery, F.S. Shanks, J. Mol. Spectrosc. 149 (1991) 458. [22] D. McNaughton, C. Evans, E.G. Robertson, J. Chem. Soc., Faraday Trans. 91 (1995) 1723. [23] E.G. Robertson, D. McNaughton, J. Mol. Spectrosc. 238 (2006) 56. [24] R.M. Villamanan, W.D. Chen, G. Wlodarczak, J. Demaison, A.G. Lessarri, J.C. Lopez, J.L. Alonso, J. Mol. Spectrosc. 171 (1995) 223. [25] D. McNaughton, C. Evans, J. Phys. Chem. 100 (1996) 8660. [26] D.R.T. Appadoo, E.G. Robertson, D. McNaughton, J. Mol. Spectrosc. 217 (2003) 96. [27] H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371. [28] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, Amsterdam, 1977, p. 1. [29] C.D. Thompson, E.G. Robertson, D. McNaughton, PhysChemChemPhys 5 (2003) 1996.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
High resolution synchrotron FTIR spectroscopy of the far infrared m10 and m11 bands of R152a (CH3CHF2) Tarekegn Chimdi a, Evan G. Robertson a, Ljiljana Puskar a,b, Christopher D. Thompson a, Mark J. Tobin b, Don McNaughton a,* a b
Centre for Biospectroscopy and School of Chemistry, Monash University, Clayton, 3800, Victoria, Australia Australian Synchrotron, 800 Blackburn Rd, Clayton, 3168, Victoria, Australia
a r t i c l e
i n f o
Article history: Received 8 August 2008 In final form 6 October 2008 Available online 11 October 2008
a b s t r a c t High resolution (0.0019 cm1) Fourier transform infrared spectra of R152a (CH3CHF2) at room temperature were measured using far infrared radiation from the IR beamline at the Australian Synchrotron. The a-type fundamentals m10 = 569.12 cm1 and m11 = 468.34 cm1 were assigned and analysed. Hot-band transitions centred at 471.18 cm1 and 469.27 cm1 were identified as 1110 1811 and 1110 1711 , respectively. Transitions were assigned with the aid of Loomis Wood plots, confirmed by combination differences and fitted with Pickett’s SPFIT/SPCAT to determine molecular constants. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Since the original recognition by Yarwood et al. [1] some 20 years ago that synchrotron light provides a bright source of infrared radiation, synchrotron infrared radiation (SIR) has been increasingly used to advantage in a vast range of fields such as surface science, geology, cell biology, materials science and conservation science [2–6]. Design features and areas of expertise for operating IR synchrotrons around the world have also been discussed [7–11] and many synchrotrons around the world now have at least one, and usually more, IR beamlines. There are clear advantages for synchrotron radiation over conventional thermal radiation sources such as the Globar in the mid-infrared (MIR) and mercury discharge lamp in the far-infrared (FIR) regions. These advantages are: two to three orders of magnitude greater brightness across the whole infrared region; a greater radiation flux in the far-IR; the possibility of polarized light; pulsed radiation down to a sub ns timescale. The increased brightness has led to great advantages in synchrotron FTIR micro-spectroscopy, the pulsed nature of the light has led to time resolved infrared spectroscopy and the selection of polarization allows for polarized micro-spectroscopic studies and ellipsometry. The greater flux in the FIR and the point source nature of the radiation source has led to advantages for high resolution spectroscopy, which has always been extremely challenging in this spectral region. Synchrotron source high resolution molecular spectroscopy studies have been carried out at MAX-lab in Sweden [11–15] and very recently
* Corresponding author. Fax: +61 3 9905 4597. E-mail address: [email protected] (D. McNaughton). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.10.011
at the Canadian light source [16,17]. This work includes the m5 band of HClO4 [14], the m11 band of allene [13], the torsional band of CH3SiD3 [15] and bands of acrolein [16,17] in the 150–600 cm1 region. The newly commissioned Australian Synchrotron has an IR beamline, outlined by Creagh et al. [18], utilizing edge radiation for far infrared spectroscopy. This letter presents the first results arising from high resolution synchrotron far-infrared FTIR spectroscopy conducted at the Australian synchrotron. We report rovibrational analyses for the m10 and m11 bands of the hydrofluorocarbon R152a (CH3CHF2) from spectra recorded on the high resolution beamline in its commissioning phase. R152a was chosen because of its importance as an atmospheric infrared absorber and the observed 5-fold increase in its atmospheric concentration since its introduction as a CFC replacement over a decade ago [19]. 2. Experimental R152a (CH3CHF2), (99% commercial grade, BOC gases Australia) was maintained at a pressure of (115 ± 9) Pa in a glass multi-pass White cell set to a path length of 4 m. Room temperature absorption spectra in the 350–750 cm1 region were recorded at the Australian synchrotron using the infrared synchrotron edge radiation continuum source, a Bruker IFS125HR interferometer equipped with a multilayer Mylar beam splitter and a liquid helium-cooled external Cu:Ge bolometer. During the commissioning period in which experiments were conducted, full ring current injections (200 mA) were interspersed with half current injections (100 mA). Interferometer ADC counts (and S/N) decreased proportionally with the current. High resolution measurements were therefore recorded in the high current periods, with beam current decreasing from 192 mA upon injection to 172 mA over a period of
204
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
several hours. 26 coadded scans were recorded with a four point apodization function at a resolution of 0.0019 cm1. The calibration of the wavenumber scale was achieved using 32 rotational lines of H2O and CO2 spectra (present as residual background gas in the evacuated instrument) and compared with the values listed in HITRAN spectroscopic database [20]. The corresponding rotational lines were located with an average uncertainty of 0.0003 cm1. 3. Results and discussion Despite the early commissioning stage of the beamline with non optimum conditions and the synchrotron beam apertured
Fig. 1. High resolution room temperature FTIR spectra of CH3CHF2 in the region of (a) the m10 band and (b) the m11 band.
down to attain the desired resolution, the synchrotron provided a flux (and S/N) advantage of ca. 2.5 over the internal globar source in the wavenumber range of the experiment. High resolution spectra of the R152a bands m11 (468.34 cm1) and m10 (569.12 cm1) due to the CF2 bend and wag respectively are shown in Fig. 1a and b. Both bands are a-type bands with a prominent central Q branch. Some of the sub structure visible in the spectra is associated with hot bands emanating from v17 = 1 (390 cm1) and v18 = 1, 2 (221, 442 cm1) levels, which are quite heavily populated at room temperature. For the rovibrational analysis of these bands, the absorbance spectra were post-zerofilled by a factor of 32 and peaks picked in OPUS 5.5. In total over 18 000 peak positions and intensities were selected for input into the interactive assignment tool MacLoomis [21]. MacLoomis generates Loomis Wood plots which help to identify and select series of regularly spaced transitions that occur even in asymmetric top molecules [22,23]. In this case, transitions that share specific values of the quantum number Ka, but with differing values of J and Kc are separated by ca. 2C (0.345 cm1). Ground state rotational constants for R152a are known from earlier microwave [24] and mid-IR high resolution FTIR studies [25,26]. A total of 1701 lines with Jmax = 67, Ka max = 14 and 4594 lines with Jmax = 82, Ka max = 22 were assigned to a-type rovibrational transitions of m10 and m11, respectively, with selection rules of DJ = ±1, DKa = 0 and DKc = ±1. Assignments were confirmed by comparison of ground state combination differences (GSCD) with those calculated from the microwave rotational constants of Villamanan et al. [24]. The fitting of assigned transitions was performed with Pickett’s SPFIT program [27], using Watson’s A reduced Hamiltonian in the Ir representation [28], whilst predictions and simulations were carried out using Pickett’s SPCAT program. IR lines were given an experimental uncertainty of 0.0004 cm1 in the least squares fits. Ground state constants and some higher order centrifugal distortion constants for the excited state were fixed to the values of Villamanan et al. [24]. Preliminary fits of the IR data yielded small r.m.s. residuals of 3–4 104 cm1, suggesting that both of the bands m11 and m10 have regular rotational structure with no apparent avoided crossings and no obvious major perturbations. The fitted constants for m10 and m11 correlated well with the constants of the vibrational satellites designated ‘vb’ and ‘vc’ by Villamanan et al., allowing unique vibrational quantum number assignments for these transitions. As a consequence, it was possi-
Table 1 Molecular constants (cm1) of the m10 and m11 bands of R152a fitted jointly with microwave transitions obtained from satellite bands [23] Parameters
Ground state [23]
vb = 1 [23]
m0 A B C DJ 106 DJK 106 DK 106 dJ 106 dK 106 UJ 1012 UJK 1012 UKJ 1012 uJ 1012 uK 1012 No. of transitions Jmax Ka max Standard deviation R.m.s. error a b c
0.316618424 0.298964583 0.172467785 0.158504 0.0603364 0.0971422 0.05764504 0.1550315 0.2537 0.3231 0.5200 0.1204 1.8506
0.315970705(86) 0.299027286(84) 0.172440996(81) 0.158504b 0.072166(34) 0.084112(48) 0.05764504b 0.153389(20) 0.2537b 0.3231b 0.5200b 0.1204 b 1.8506b 37 32 25
v10 = 1 569.118459(29)a 0.315970744(58) 0.299027337(55) 0.172441241(43) 0.156827(42) 0.06737(17) 0.08704(33) 0.056772(20) 0.155134(107) 0.2537b 0.3231b 0.5200b 0.1204 b 1.8506b 1701/37c 67/32c 14/25c 0.798 3.1 104/0.042c
vb = 1 and vc = 1, respectively.
vc = 1 [23] 0.3165157220(83) 0.2989925267(79) 0.1727200165(75) 0.158504b 0.053824(37) 0.108632(53) 0.05764504b 0.148733(21) 0.2537b 0.3966(40) 0.5200b 0.1204b 1.8506 b 42 35 27
Figures in brackets are one standard deviation from the least squares fit. Constrained to the corresponding ground state values [23]. The two numbers refer to infrared and microwave transitions respectively, with r.m.s. errors quoted in cm1 and MHz accordingly.
v11 = 1 468.341641(21) 0.316515938(40) 0.298992913(38) 0.17272025(36) 0.159795(17) 0.04747(14) 0.11205(26) 0.0568028(50) 0.151477(75) 0.2966(17) 0.3231b 0.5200b 0.1204b 1.8506b 4594/41c 82/32c 22/25c 0.824 3.1 104/0.084c
205
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
Fig. 2. Expanded spectrum of the m10 bands of R152a (a), compared to an a-type band simulation (b) generated with spectroscopic parameters from Table 1 at T rot = 298 K, and Gaussian lineshape with FWHM 0.0025 cm1.
Fig. 3. Expanded spectrum of the m11 band of R152a (a), compared to a-type band simulations based on either the m11 fundamental only (c), or on the m11 ð1110 Þ fundamental combined with the v11 = 1,v17 = 1 v17 = 1 ð1110 1711 Þ and v11 = 1,v18 = 1 v18 = 1 ð1110 1811 Þ hot band transitions (b). Labels indicate some individual lines associated with 1110 (solid line, 41,4 51,5 & 41,4 51,5), the 1110 1811 hotband (dashed line, 103,7 113,8 & 104,7 114,8) and the 1110 1711 hotband (dotted line, 70,7 80,8 & 71,7 81,8).
ble to perform a combined fit of the infrared transitions and the vibrational satellite data, appropriately weighted (0.04 MHz). The resulting rovibrational constants are summarized in Table 1. It can be seen that inclusion of the infrared data results in a more complete set of constants than those from microwave vibrational satellites alone. The band centre is accurately determined, along with all the quartic centrifugal distortion constants. Most of the sextic constants were fixed to ground state values, except UJ in m11. It was necessary to freely vary this constant in order to treat IR transitions with high rotational quantum number (Jmax = 82). The r.m.s. residual for IR transitions of both m10 and m11 is excellent - just 0.00031 cm1. In Fig. 2, a short section of the m10 spectrum is compared with a simulation based on the constants of Table 1, where predicted lines are convolved with a Gaussian line shape of appropriate linewidth. A similar comparison is made for m11 in Fig. 3a and c. The simulations do not match perfectly as hot band transitions generate extra structure in the experimental spectra. To remove the fundamental band transitions from the experimental spectrum and simplify the hot band analysis in the m11 region the ‘spectral analysis by subtraction of simulated intensities’ (SASSI) [23,29] approach was used. In this approach a simulated spectrum is subtracted from the original data and the resultant difference spectrum peak picked for new MacLoomis analysis. Rovibrational transitions for the two most intense hotbands were then assigned in an analogous manner to the fundamental transitions. Villamanan et al. [24] in their microwave work fitted vibrational satellite data and labeled the associated vibrational states as vt, va, vb, vc rather than commit to an absolute assignment to particular modes. By comparing our lower state combination differences for hot-band transitions against predictions using the constants of Villamanan et. al. [24] their transitions for vt = 1 and va = 1 are assigned as v18 = 1 and v17 = 1 respectively. A total of 1404 lines with Jmax = 67, Ka max = 8 and 686 lines with Jmax = 56, Ka max = 5 were assigned to a-type rovibrational transitions for hot bands emanating from v18 = 1 and v17 = 1 respectively. Results of the fits for hot bands associated with m11 are summarized in the Table 2. The lower state constants in these fits were fixed to the values from the microwave study, whilst most of the centrifugal distortion constants for the upper states had to be held at the lower state values also. In order to assess the quality and accuracy of the data the calculated constants from all the assigned levels associated with the m11
Table 2 Molecular constants (cm1) for the hot bands associated with the m11 band of R152a. Parameters
va (v17) = 1 [23]
m0 A B C DJ 106 DJK 106 DK 106 dJ 106 dK 106 UJ 1012 UJK 1012 UKJ 1012 uJ 1012 uK 1012 No. of transitions Jmax Ka max Standard deviation R.m.s. error a b c
0.317226179 0.299212864 0.171645946 0.156408204 0.065511988 0.096556799 0.059407765 0.160958018 0.253742207 0.323090183 0.520026424 0.120449995 1.850646960 85 40 31
v11 = 1, v17 = 1 469.265135(43)a 0.317087(76) 0.299229(66) 0.1718456(99) 0.157590(175) 0.065511988b 0.096556799b 0.059455(97) 0.160958018b 0.253742207b 0.323090183b 0.520026424b 0.120449995b 1.850646960b 686c 56c 5c 1.053 4.2 104
Figures in brackets are one standard deviation from the least squares fit. Constrained to corresponding lower state value from microwave vibrational satellite data [23]. Corresponding only to infrared transitions.
vt (v18) = 1 [23] 0.316629309 0.298015160 0.172247082 0.155350806 0.063760777 0.091653406 0.056203215 0.148501067 0.253742207 1.09542449 0.520026424 0.086393101 1.40897474 206 40 32
v11 = 1, v18 = 1 471.177649(37) 0.3166425(163) 0.2978785(139) 0.172514893(55) 0.1054(46) 0.9107(149) 0.091653406 0.056203215b 0.148501067b 0.253742207b 1.09542449b 0.520026424b 0.086393101b 1.40897474b 1404c 67c 8c 1.026 4.1 104
206
T. Chimdi et al. / Chemical Physics Letters 465 (2008) 203–206
Acknowledgements We acknowledge the financial support of the Australian Research Council, the Australian Synchrotron for the beam time, Mr. Finlay Shanks for technical assistance and Dr. Dominique Appadoo for assistance in the initial setting up of the beamline. References
Fig. 4. Experimental spectrum of the m11 band of R152a (a), together with a simulation including the hot bands (b) based on spectroscopic results from Tables 1 and 2 at Trot = 298 K, and Gaussian lineshape with FWHM 0.0020 cm1. Hot-band components were scaled according to the calculated Boltzmann populations for vibrational states originating at 221 and 390 cm1.
band were used to simulate the band with all the transitions weighted by the appropriate 298 K Boltzmann factors. The simulated spectrum, together with the experimental spectrum, is shown in Fig. 4 where the gross structure including two additional Q-branches compares well. A portion of the simulation consisting of fundamental and hot bands transition lines, shown in Fig. 3b, reveals the usefulness of the constants in predicting many of the additional lines in the spectrum. 4. Conclusion Despite the IR beamline being in a commissioning phase at the time of these experiments, and the synchrotron itself being at an early stage of routine operation, the synchrotron beam provided flux sufficiently higher than that from a globar source to enable us to record spectra of two R152a bands in the far infrared 350– 750 cm1 region with good signal-to-noise at high spectral resolution. Both the m10 and m11 fundamental bands and the most intense hot bands associated with m11 have been successfully assigned and fitted to produce sets of well determined molecular constants. We expect a further improvement in synchrotron flux and stability after full commissioning of the beamline.
[1] J. Yarwood, T. Shuttleworth, J.B. Hasted, T. Nanba, Nature 312 (5996) (1985) 742. [2] N.S. Marinkovic, S. Gupta, C. Zhan, M.R. Chance, Nucl. Instrum. Meth. Phys. Res. B 241 (2005) 242. [3] P. Roy et al., Nucl. Instr. Meth. Phys. Res. A (2001) 467. [4] L.M. Miller, R.J. Smith, Vib. Spectrosc. 38 (2005) 237. [5] G.L. Carr, Vib. Spectrosc. 19 (1999) 53. [6] D. Creagh, J. McKinlay, P. Dumas, Vib. Spectrosc. 41 (2006) 213. [7] A. Nucara, S. Lupi, P. Calvani, Infr. Phys. Technol. 45 (2004) 375. [8] R.W. Larsen, F. Hegelund, B. Nelander, Mol. Phys. 102 (2004) 1743. [9] J. Cutler, E. Hallin, M. de Jong, W. Thomlinson, T. Ellis, Nucl. Instr. Meth. Phys. Res. A 582 (2007) 11. [10] S.K. Deb et al., Infr. Phys. Technol. 49 (2006) 82. [11] B. Nelander, Vib. Spectrosc. 9 (1995) 29. [12] B. Nelander, V. Sablinskas, M. Dulick, V. Braun, P.F. Bernath, Mol. Phys. 93 (1998) 137. [13] S. Nissen, F. Hegelund, M.S. Johnson, B. Nelander, J. Mol. Spectrosc. 216 (2002) 197. [14] M.S. Johnson, F. Hegelund, B. Nelander, J. Mol. Spectrosc. 190 (1998) 269. [15] J. Schroderus, V.-M. Horneman, M.S. Johnson, N. Moazzen-Ahmadi, I. Ozier, J. Mol. Spectrosc. 215 (2002) 134. [16] A.R.W. McKellar, D.W. Tokaryk, D.R.T. Appadoo, J. Mol. Spectrosc. 244 (2007) 146. [17] A.R.W. McKellar, D.W. Tokaryk, Li-Hong Xu, D.R.T. Appadoo, T. May, J. Mol. Spectrosc. 242 (2007) 31. [18] D. Creagh, M. Tobin, A. Broadbent, J. McKinlay, AIP Conference Proceedings, 879 (Synchrotron Radiation Instrumentation, Part 1), 2007, p. 615. [19] B.R. Greally et al., J. Geophys. Res. 112 (2007) D06308. [20] L.S. Rothman et al., J. Quant. Spec. Rad. Trans. 96 (2005) 139. [21] D. McNaughton, D. McGilvery, F.S. Shanks, J. Mol. Spectrosc. 149 (1991) 458. [22] D. McNaughton, C. Evans, E.G. Robertson, J. Chem. Soc., Faraday Trans. 91 (1995) 1723. [23] E.G. Robertson, D. McNaughton, J. Mol. Spectrosc. 238 (2006) 56. [24] R.M. Villamanan, W.D. Chen, G. Wlodarczak, J. Demaison, A.G. Lessarri, J.C. Lopez, J.L. Alonso, J. Mol. Spectrosc. 171 (1995) 223. [25] D. McNaughton, C. Evans, J. Phys. Chem. 100 (1996) 8660. [26] D.R.T. Appadoo, E.G. Robertson, D. McNaughton, J. Mol. Spectrosc. 217 (2003) 96. [27] H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371. [28] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, Amsterdam, 1977, p. 1. [29] C.D. Thompson, E.G. Robertson, D. McNaughton, PhysChemChemPhys 5 (2003) 1996.