Analysis of the rotational structure in the high-resolution infrared spectra of cis,cis- and trans,trans-1,4-difluorobutadiene-1-d1 and trans,trans-1,4-difluorobutadiene-1,4-d2

Journal of Molecular Spectroscopy 288 (2013) 18–27

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Analysis of the rotational structure in the high-resolution infrared spectra of cis,cis- and trans,trans-1,4-difluorobutadiene-1-d1 and trans,trans-1,4-difluorobutadiene-1,4-d2 Norman C. Craig a,⇑, Yihui Chen a, Yuhua Lu a, Christopher F. Neese a,1, Deacon J. Nemchick a,2, Thomas A. Blake b a b

Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074, USA Pacific Northwest National Laboratory, Richland, WA 99352, USA

a r t i c l e

i n f o

Article history: Received 14 February 2013 In revised form 13 March 2013 Available online 4 April 2013 Keywords: Cis,cis- and trans,trans-1,4difluorobutadiene-1-d1 Trans,trans-1,4-difluorobutadiene-1,4-d2 Syntheses High-resolution infrared spectroscopy Analysis of rotational structure Rotational constants

a b s t r a c t Samples of cis,cis- and trans,trans-1,4-difluorobutadiene-1-d1 and of trans,trans-1,4-difluorobutadiene1,4-d2 have been synthesized, and high-resolution (60.0018 cm1) infrared spectra of these substances have been recorded in the gas phase. Analysis of the rotational structure, mostly in C-type bands, has yielded ground state rotational constants. For the two 1-d1 species more than one band has been analyzed. For the 1,4-d2 species only one band was available for analysis. However, good agreement between the experimental centrifugal distortion constants and those predicted with a B3LYP/cc-pVTZ model give strong support to the analysis of the very dense spectrum. The ground state rotational constants are a contribution to finding semiexperimental equilibrium structures of the two nonpolar isomers of 1,4difluorobutadiene. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction Substitution by fluorine, the most electronegative element, is expected to have a significant influence on bond properties of carbon atom backbones. An investigation of the structure of the isomers of 1,4-difluorobutadiene (DFBD) was initiated to explore this effect on the three isomers of this substance. For comparison, an equilibrium structure of butadiene is known [1]. Another reason for interest in the isomers of DFBD is the unusual energy relationship among them [2,3]. Despite the fluorine atoms being most distant in the trans,trans (tt) isomer, it has the highest energy. The cis,cis (cc) isomer, which is most compact, has the lowest energy (6.5 kJ/mol lower than the tt isomer [3]). Rotational constants for a full set of isotopologues are required for determining a good structure from rotational spectroscopy. Such an investigation of the cis,trans (ct) isomer, which is polar, has been done by microwave (MW) spectroscopy, and a complete equilibrium structure has been proposed [4,5]. For the cc and tt

⇑ Corresponding author. Address: Oberlin College, 119 Woodland St., Oberlin, OH 44074, USA. Fax: +1 440 775 6682. E-mail address: [email protected] (N.C. Craig). 1 Present address: Ohio State University, USA. 2 Present address: Yale University, USA. 0022-2852/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2013.03.006

isomers of DFBD, which are nonpolar, IR spectroscopy is needed. The feasibility of the IR method at room temperature with practical high-resolution methods is limited to molecules of modest size and mass. The cc and tt isomers of DFBD are barely within range. A study of the two nonpolar isomers by IR spectroscopy demonstrated the success of the method [6] for these substances. An investigation of the two 2-d1 species of these isomers has also recently been reported [7]. The synthesis of the 1-d1 isotopologues of DFBD traces to the ‘‘assembly’’ strategy of Viehe and Franchimont [2]. In this method fluoroethylene and 1-fluoro-2-iodoethylene react photochemically to form 1,4-difluoro-4-iodobutene, from which hydrogen iodide is removed to make DFBD. This strategy succeeds for isotopic synthesis because the isotopic content of the fluoroethylene can be altered selectively. For the present work fluoroethylene-1-d1 was prepared by two different exchange methods. Direct exchange of fluoroethylene with sodium deuteroxide was limited to approximately two-thirds enrichment. An alternative pathway, which started with exchange of bromoethylene, gave high isotopic replacement. The sample of the 1,4-d2 isotopologue came from the partial exchange of the tt isomer [4].

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2. Experimental 2.1. Syntheses Monofluoroethylene-1-d1 (b.p. 72 °C) was prepared with approximately 2/3 deuterium content by repeated exchanges [3]. The exchange reaction was done in a 300-mL heavy-wall, cylindrical quartz vessel (46 cm O.D.), which had an attached seal-off and a breakseal made of borosilicate glass. 10 mL of fresh 3 M sodium deuteroxide was used for each exchange step. 13 mmol of fluoroethylene (Matheson) was condensed onto the frozen D2O solution, and the vessel was sealed with a torch. Each exchange step was carried out in a rocking oven at 120 °C for 5 days. Recovered fluoroethylene was dried by distillation through a column packed with phosphorus pentoxide. IR spectra were used to monitor progress of the exchange, principally in the C@C stretching region. By-product acetylene-d2 gave a distinct perpendicular band at 537 cm1. The exchange was repeated eight times. Acetylene-d2 was removed by gas chromatography on a 7 m column packed with dicyanoether on Chromosorb at 0 °C [8]. Acetylene-d2 eluted second. NMR spectra were used to assess the extent of exchange. The proton spectrum of HFC@CH2 consisted of three octets with dH (gem) = 5.54 ppm, dH(cisH) = 4.821 ppm, and dH(transH) = 4.484 ppm with JHF(gem) = 85.5 Hz, JHF(cis) = 20.6 Hz, JHF (trans) = 53.9 Hz, JHH(cis) = 4.7 Hz, JHH(trans) = 11.3 Hz, and JHH (gem) = 3.2 Hz. The 19F spectrum of HFC@CH2 was an octet at dF = 115.84 ppm. The proton spectrum of DFC@CH2 showed deuterium isotope shifts with dH(cisH) = 4.812 ppm and dH(transH) = 4.480 ppm. Each band was a quartet of triplets with JHD(trans) = 1.8 Hz and JHH(gem) = 3.3 Hz. The 19F spectrum of DFC@CH2 had a deuterium isotope shift and a quartet of triplets at dF = 116.441 ppm with JDF(gem) = 13.2 Hz, JHF(cis) = 20.5 Hz, and JHF (trans) = 53.8 Hz. In addition, weak spectra from trans-DFC@CHD, cis-DFC@CHD, trans-HFC@CHD, and cis-HFC@CHD were present. From integrals of peaks in the 19F spectrum, the sample was estimated to be 51% DFC@CH2 and 31% HFC@CH2 with the remainder divided among the four other species. A mixture of 1-fluoro-2-iodoethylene isomers was prepared and reacted by photolysis with 12 mmol of DFC@CH2 [7]. The halobutene product, which was obtained in low yield, was separated from unreacted DFC@CH2 by bulb-to-bulb distillation at dry ice temperature and from the other reactants by gas chromatography on a column packed with tricresylphosphate on Fluoropak at 85 °C [7]. The product mixture, dominated by the cis isomer, was confirmed by the familiar elution times in gas chromatography. Hydrogen iodide was eliminated from the halobutene with sodium hydroxide in Ascarite (Thomas) [7] to give a mixture of the three isomers of DFBD-1-d1. This mixture was separated by gas chromatography on a 5 m tricresylphosphate-on-Fluoropak column at room temperature [3]. Amounts of the least abundant tt isomer were increased by repeated iodine-catalyzed isomerization followed by gas chromatographic fractionation [3]. Species were confirmed by their IR spectra. Figs. S1a and S1b in the Supplementary material are the gas phase IR spectrum of ccDFBD-1-d1 with a ratio of d1/d0 of approximately 1.65. A sample of 0.45 mmol of the cc species was used for high-resolution IR spectroscopy. Because of troublesome overlap of bands of the d1 and d0 species in the IR spectrum of the tt isomer, a second synthesis was developed for this species. 30 mmol of bromoethylene (Matheson, b.p. 16 °C) was sealed in a heavy-wall borosilicate cylinder with 9 mL 1.5 M NaOD and exchanged at 65 °C five times for 3–4 days in each cycle. A small amount of acetylene-d2 was distilled off at toluene-slush temperature. 24 mmol of DBrC@CH2 were recovered and confirmed by the proton NMR spectrum. The pattern for the HD(cis) proton was a doublet of triplets at dH = 5.988 ppm

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(0.035 ppm upfield isotope shift) with JHD(cis) = 1.1 Hz and JHH(gem) = 1.96 Hz. For the HD(trans) proton the pattern was a doublet of triplets at dH = 5.852 ppm (0.011 ppm upfield isotope shift) with JHD(trans) = 2.3 Hz and JHH(gem) = 1.9 Hz. All of the DBrC@CH2 was reacted in two portions with 12 mmol each of bromine (Aldrich) in a 500 mL borosilicate flask, which had a standard taper joint attached to a stopcock adaptor and contained short pieces of Teflon tubing to aid mixing. The reaction was initiated in a darkened room with illumination from a flashlight and with shaking. The DBr2CCBrH2 (b.p. 189 °C) product was confirmed by its proton NMR spectrum. AgF2 (Aldrich) was used to transform DBr2CCBrH2 into DF2CCBrH2 at 70 °C [9]. The product (b.p. 57 °C) was confirmed by its NMR spectra. The proton signal of DF2CCBrH2 was a triplet of triplets at dH = 3.486 ppm (0.0098 ppm upfield isotope shift) with JHF = 14.1 Hz and JHD = 0.59 Hz. The 19F signal was a triplet of triplets at dF = 116.624 ppm (0.739 ppm upfield isotope shift) with JHF = 14.3 Hz and JDF = 8.3 Hz. Yield was 98%. DF2CCBrH2 was dehalogenated to DFC@CH2 [10] with a yield of 47%. All of the DFC@CH2 was used in the photochemical reaction with HFC@CIH [7]. The halobutene, which was obtained in a small yield, underwent HI elimination by distillation through Ascarite [7]. The yield was about 0.7 mmol of a mixture of isomers of DFBD-1-d1. After repeated iodine-catalyzed isomerizations and gas chromatography separations, a sample of 0.2 mmol of ttDFBD-1-d1 was obtained for use in high-resolution IR spectroscopy. Figs. S2a and S2b in the Supplementary material are the IR spectrum of pure ccDFBD-1-d1, which is an advance over the impure sample in Figs. S1a and S1b. Figs. S3a and S3b are the IR spectrum of pure ttDFBD-1-d1. Both samples had high isotopic purity. A mixture of predominately ttDFBD-1-d1 and -1,4-d2 species was prepared by several exchanges of ttDFBD [4]. Exchange occurs about five times as fast for a terminal CH bond as for an interior one. Thus, relatively little of the species with exchange of the interior CH bond has formed when most of the d0 species is gone. Figs. S4a and S4b are the IR spectrum of the result of an exchange process, comparable to the one used for the sample for the highresolution spectrum. 2.2. Spectroscopy NMR spectra were obtained on a Varian 400 MR instrument with samples dissolved in CDCl3 in standard 5 mm tubes. The proton spectra were referenced to residual CHCl3 in the solvent, and the 19F spectra were referenced to external CFCl3. Medium-resolution IR spectra were recorded at 0.1 cm1 resolution on a Nicolet 6700 Fourier transform (FT) instrument or on a Perkin–Elmer 1760 FT instrument with 0.5 cm1 resolution. The cell had 25 mm potassium bromide windows and a 10 cm pathlength. The instruments were purged with dry nitrogen. High-resolution IR spectra were obtained on an evacuated Bruker IFS 125HR FT instrument at PNNL at 22 °C. The source was a globar, the beamsplitter used potassium bromide, the detector was a liquid-nitrogen-cooled HgCdTe device, and the resolution was 0.0015 cm1. The White cell had a 16 m path length and a pressure of 0.15 Torr. For spectra of ccDFBD-1-d1 4096 scans were accumulated. CO2 was the calibration gas for the spectra of the Ctype bands [11]; N2O was the calibrant for the spectrum that included the A-type band [12]. For the spectra of ttDFBD-1-d1 3776 scans were accumulated. For the spectrum that included the band at 920 cm1 H2O was the calibrant [13]; for the bands at 684 and 619.5 cm1 CO2 was the calibrant [11]. The spectrum of the mixture of ttDFBD-1-d1 and -1,4-d2 was recorded on a Bruker IFS 120HR instrument in a White cell with 0.0018 cm1 resolution at Justus Liebig Universität in Giessen, Germany by Dr. Michael Lock. See Ref. [14] for the set-up of this

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instrument. Details of the path length in the White cell and the low pressures are no longer available. Scans were made with two intensities. 2.3. Computations Anharmonic frequencies, adjusted for Fermi resonances, were predicted with the B3LYP/cc-pVTZ model and Gaussian 03 (G03) software [15]. The ultrafine grid and tight convergence limits were used. The window for detecting Fermi resonances was 100 cm1. Cartesian coordinates were rotated into the principal axis system by an external program before computing centrifugal distortion constants. Loomis-Wood (LW) pattern recognition software was an essential aid to finding subband series in the dense spectra with marginal S/N [16]. A number of locally written Fortran programs facilitated the handling of large data sets. An adaptation of Dr. Arthur Maki’s ASYMBD7 Fortran program was used to fit ground state (GS) constants to ground state combination differences (GSCD) and to fit upper state (US) rotational constants to observed lines. When fitting US rotational constants to lines, the GS rotational constants were held fixed. The asymmetric top reduction in the Ir representation for the Watson-type Hamiltonian was employed even though the molecules studied are near prolate symmetric tops. 3. General considerations Fig. 1 displays schematic representations of the three molecules investigated in this study: ccDFBD-1-d1, ttDFBD-1-d1, and ttDFBD1,4-d2. The approximate orientations of the principal a and b rotation axes are also shown. The two 1-d1 species have Cs symmetry. As a consequence, they have 17 in-plane modes of the a0 symmetry species and 7 out-of-plane modes of the a00 symmetry species. In principle, all of the fundamental transitions are IR active. However, those that are close to the Raman-active modes of the parent mol-

ecule have negligible IR intensities. Band shapes for the a0 modes are hybrid A/B-type, and band shapes for the a00 modes are C-type. Because C-type bands are easiest to analyze and give good determinations of all three rotation constants, this work is focused on C-type bands. Table 1 lists the predicted anharmonic frequencies and harmonic intensities for the a’’ modes in comparison with observations of band centers. Where observed frequencies are given in Table 1 with decimal significance, the values come from a rotational analysis of the band. The ttDFBD-1,4-d2 molecule has the full C2h symmetry of the parent molecule. As a consequence, the out-of-plane modes include four IR-active modes of the au symmetry species, which have C-type band shapes. The other three out-of-plane modes belong to the bg symmetry species and are IR-inactive. Predicted anharmonic frequencies and harmonic intensities in comparison with observations are in Table 1. The rotational structure in bands in the high-resolution IR spectrum of ccDFBD-1-d1 proved to be analyzable despite the d0 species being present as approximately one-third of the sample. In addition, there were some unidentified impurity bands in the spectrum (Figs. S1a and S1b). The IR spectrum of the isotopically pure material (Figs. S2a and S2b) confirmed the interpretation of the spectrum of the impure material. Bands analyzed were at 868, 773, and 1293 cm1. The first two were C-type bands for the m19 and m20 out-of-plane modes, respectively, and the third was an Atype band of the m8 in-plane mode. Analysis was not attempted for the C-type band at 661 cm1, which was distorted by some underlying bands. Because of the successful analysis of bands in the spectrum of the impure sample, a high-resolution spectrum of the pure material was not recorded.

Table 1 Vibration frequencies and infrared intensities for out-of-plane modes of ccDFBD-1-d1, ttDFBD-1-d1, and ttDFBD-1,4-d2. Mode

Calculated a

Freq. (cm

Observed

1

)

b

I (km mol

1

)

Freq. (cm1)

Ic

d

Fig. 1. Schematic structures for the ccDFBD-1-d1, ttDFBD-1-d1, and ttDFBD-1,4-d2 species and approximate orientations of the a and b principal rotation axes.

ccDFBD-1-d1 a00 m18 924 m19 885 m20 781 m21 671 m22 561 m23 314 m24 82

2.1 7.4 32 9.3 0.01 21 2.3

907 867.6 773.0 661

w m s m

ttDFBD-1-d1e a00 m18 936 m19 890 m20 835 m21 700 m22 396 m23 220 m24 123

89 1.9 0.01 7.0 0.00 3.3 0.22

920.4 872

s vw

684.4

wm

ttDFBD-1,4-d2 aum10 912 m11 697 m12 206 m13 123 bgm14 888 m15 703 m16 396

72 13 3.2 0.20 0.0 0.0 0.0

893.00 681

s m

a Anharmonic frequencies adjusted for Fermi resonance, as computed with the B3LYP/cc-pVTZ model. b Intensities calculated at the harmonic level. c Qualitative intensities: s, strong; m, medium; w, weak. d m8(a0 ) is at 1293 cm1 and predicted at 1298 cm1. e m13(a0 ) is near 958 cm1 and predicted at 958 cm1. m14(a0 ) is near 619.5 cm1 and predicted at 624 cm1.

N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27

For the ttDFBD-1-d1 species the medium-resolution spectrum of the sample of two-thirds isotopic purity looked too congested to analyze. Thus, for this species the high-resolution spectrum was recorded for the isotopically near-pure sample (Figs. S3a and S3b). The rotational structure in the C-type bands at 920 and 684 cm1 for m18 and m21, respectively, was analyzed. An attempt was made to analyze the A-type band for m14 at 619.5 cm1. However, the dispersal of its QQK branches and the lack of discriminating changes in GSCDs as a function of Ka blocked analysis. Because the available spectrum for the ttDFBD-1,4-d2 species was a complex result of exchange reactions at more than one site, only one C-type band was available for analysis. This one band was at 893 cm1. The C-type band at 681 cm1 might have been analyzable, but it was not included in the available high-resolution spectrum. The overall spectrum in medium resolution is in Figs. S4a and S4b in the Supplementary material. The C-type band for the 1-d1 species is seen at 920 cm1. Our first attempt to analyze this band was made with the corresponding high-resolution spectrum of the mixture. Selection rules for rotational transitions for C-type bands are DJ = 0, ±1; DKa = ±1; DKc = 0. For A-type transitions, the rules are DJ = 0, ±1; DKa = 0; DKc = ±1.

4. Results 4.1. Analysis of the rotational structure in bands of ccDFBD-1-d1 The analysis of rotational structure began in the C-type band of medium intensity for m19(a00 ) at 868 cm1. This vibrational transition is principally out-of-plane flapping of the CH bond adjacent to the CD bond. The overall structure of this band is shown in absorbance in Fig. 2. Combs mark the progressions of subband Q branches from K 00a ¼ 4 to 25 in the R branch and 6 to 27 in the P branch. The P branch has sharper Q-branch features, which could be indexed despite the abundance of competing Q branches from hot bands, than does the R branch. In an expanded plot, the Q branches in the R branch are observable despite being more dispersed than those in the P branch. The center of the band has an obvious sequence of hot band structure in addition to the center for the fundamental at 868 cm1. The five central Q branches degrading to low frequency are undoubtedly caused by thermal excitation of the low frequency CAC torsion mode m24, which must

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have a frequency somewhat lower than the 78 cm1 value for the normal species [3]. A similar hot-band progression occurs in the Q branch of the equivalent C-type band in the normal species [6]. The intense hot band structure in the center of the band at 868 cm1 prevented extending the analysis of subbands below K 0a ¼ 5. This band is unperturbed inside K 0a ¼ 20, thereby making its analysis rather straightforward. PPK and RRK subband series for all of the Ka values shown in Fig. 2 were assigned. Series inside K 0a ¼ 10 were found with guidance from predictions made while fitting US rotational constants. The detail in Fig. 3 from the P branch is an example of some assignments for two subband series. This region includes the sharp Q branches for PQ7 and PQ8. Sequences are shown for PP6 and PP7. This figure reveals how dense the spectra are and how marginal the S/N is. Without the aid of the LW pattern-recognition computer displays, finding series in these spectra would have been impossible. Beyond the range shown in Fig. 3, asymmetry splitting for the P P6 subband was apparent at J00 = 29. Asymmetry splitting was also observed in RR4 and RR5. This splitting helped define the difference between B0 and C0. A total of 1920 lines were assigned in the first C-type band. From these lines 862 GSCDs were derived. They were used along with 458 GSCDs from the C-type band at 773 cm1 to determine the GS rotational constants. Table 2 gives these GS constants in the first column of data. The Hamiltonian contains all five quartic centrifugal distortion constants. However, the small dJ and dK values, which could not be fit adequately to the data, were taken from computations with the B3LYP/cc-pVTZ model. The small negative value for the inertial defect is consistent with 0.08848 amu Å2 found for the d0 species of ccDFBD [6] and with values for other planar molecules [17]. All the GSCDs and the details of their fit to GS rotational constants are in Table S1 in the Supplementary material. The observed centrifugal distortion constants are in good agreement with the values computed with the B3LYP/cc-pVTZ model. The calculated values (in cm1) and percent obs–calc differences in parentheses are DK = 0.6399  106 (2.0% diff), DJK = 0.7927  107 (1.5% diff), and DJ = 0.7017  108 (3.3% diff). US rotational constants were fitted to 1446 lines from K 0a ¼ 5 to 20 of the C-type band at 868 cm1. These rotational constants are in the second column of data in Table 2. The band center from the fit is 887.61 cm1, which is within 0.005 cm1 of the corresponding peak of the central Q branch. The lines used and the details of fitting them are in Table S2 in the Supplementary material.

Fig. 2. The C-type band for m19 at 868 cm1 of ccDFBD-1-d1.

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Fig. 3. A portion of the P branch in the C-type band for m19 of ccDFBD-1-d1 with parts of two subband series.

Table 2 Rotational constants for ccDFBD-1-d1.

1

A (cm ) B (cm1) C (cm1) dJ  108 (cm1) dK  107 (cm1) DK  106 (cm1) DJK  107 (cm1) DJ  108 (cm1)

j m0 (cm1) s.d. (cm1) Dc (amu Å2) No. trans. K 0a Jmax a b c d

Ground state

m19(a00 ) – C-type

m20(a00 ) – C-type

m8(a0 ) – A-type

0.4193536(8) 0.0527471(15) 0.0468697(10) 0.1042a 0.2085a 0.627(1) 0.781(5) 0.726(8) 0.96844

0.4184285(4) 0.0527265(4) 0.0468786(4) 0.1042a 0.2085a 0.618(1) 0.767(1) 0.7263(6) 0.96852 867.60672(3) 0.00032 0.4044 1446 5–20 78

0.418522(4) 0.052779(3) 0.046849(4) 0.1042a 0.2085a 0.44(1) 0.78(1) 0.760(4) 0.96809 773.0029(3) 0.000405 0.1484 514 10–15 70

0.419360(3) 0.0528975(6) 0.0467912(7) 0.1042a 0.2085a 0.6271b 0.7807b 1.186(3) 0.9672 1292.9251(2) 0.00047 1.3901 216 6–8 67

0.00039 0.12205 1320d 5–26 72

Calculated with the B3LYP/cc-pVTZ model. Ground state values. Inertial defect, D = Ic  Ia  Ib. 862 GSCDs from m19 and 458 from m20.

An addendum to Table S2 contains the other assigned lines that were involved in computing GSCDs but not in the fitting to US rotational constants. The vibrational transition for the second C-type band at 773 cm1 is carried by m20(a00 ), which is largely out-of-plane flapping for the two hydrogen atoms in the half of the molecule without deuterium substitution. This C-type band is shown in Fig. 4 along with an overlapping C-type band at 763 cm1 from the impurity d0 species [6]. The band from the 1-d1 species is at higher frequency, which was initially surprising. However, when all the au and bg out-of-plane modes of the d0 species are assembled into a single a00 set and put in descending order, the higher frequency for the 1-d1 transition is consistent with the Rayleigh rule [18]. In the bands of both the d0 and 1-d1 species, the Q branches of the subbands are more prominent in the R branches. Subband Q branches from the band of the normal species spill over the central Q branch of the 1-d1 species and are designated with the upper comb in Fig. 4. The central Q branch for the band of the 1-d1 species shows much hot band structure, as is seen in the C-type band of ccDFBD. In Fig. 4, subband Q-branch sequences for the 1-d1 species

are shown below with combs extending from K 00a ¼ 9 to 21 in the R branch and 11 to 23 in the P branch. Despite the d0 impurity band, a useful analysis of the rotational structure in the band for the 1-d1 species was possible. Subband series were assigned in the R and P branches of the band for the 1-d1 species at 773 cm1 for K 0a ¼ 10 to 22. For the most part, subbands were found first in the R branch. Predictions from GSCDs and RRK series helped identify PPK+2 series. A fit of US rotational constants was made with 514 lines from a truncated set of subband series for K 0a ¼ 10 to 15. The US rotational constants are in the third data column in Table 2. The positive value for the inertial defect in the US is suspicious for this planar molecule. Perturbations in the US compromise this fit, which shows some signs of systematics, despite the small standard deviation (s.d.) of 0.000405 cm1. Although this fit is provisional and goes no closer than K 0a ¼ 10 to the band center, it probably gives a reasonable estimate for m0 of 773.0 cm1. The strongest feature in the central Q branch is the highest frequency component, which is at this frequency. Table S3 in the Supplementary material supplies the details of the fit of US rotational constants. An addendum to this

N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27

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Fig. 4. The C-type band for m20 at 773 cm1 of ccDFBD-1-d1.

table provides the other assigned lines that contributed to the GSCDs. A total of 1170 lines were assigned in this band, and 458 GSCDs from this band contributed to the overall set from the two C-type bands analyzed. The GSCDs from the m20 band are included in Table S1. Success in assigning the second C-type band, which yielded GSCDs compatible with those of the first C-type band, was strong evidence of a correct analysis of both bands. The third band considered for the ccDFBD-1-d1 molecule was the A-type band at 1293 cm1, which is displayed in Fig. 5. The m8 vibrational transition of the a0 symmetry species for this band is principally the in-plane, in-phase bending of the two CH bonds in the half of the molecule without deuterium substitution. At the outset this band appeared to be a good candidate for analysis of rotational structure. The central Q branch was compact thereby localizing the centers of subbands and enabling the assignment of m values of the polynomials of each subband. Rotational structure was readily seen, including obvious series not far from the band center in both branches. An analysis of this band promised to com-

pensate for the failure to extend the analysis of the band at 773 cm1 below K 0a ¼ 10. In the LW display the lines for the obvious series in Fig. 5 were dominant, and alternative subband series were difficult to discern. After much searching aided with A-type GSCDs, which were computed in association with fitting GSCDs for the C-type bands at 868 and 773 cm1, some series that linked between the R and P branches were found. They were the QP6/QR6, QP7/QR7, QP8/QR8, and QP9/QR9 subbands. A weakness in these assignments is the sharing of lines between substantial parts of series, in accord with the dominance of the series seen not far from the band center in Fig. 5. A Doppler broadening of 0.0017 cm1 at this frequency [19] compromises the resolution. Measurements of some line widths in the dominant series gave values greater than 0.0025 cm1, presumably reflecting blending of lines as well as Doppler broadening. In addition, the A-type GSCDs change only in the third decimal place as Ka varies, thereby placing assignments of Ka values in doubt. Attempts to extend the assignments inward

Fig. 5. The A-type band for m8 at 1293 cm1of ccDFBD-1-d1.

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N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27

to Ka = 5 and outward to Ka = 10 failed. From the assignments 109 GSCDs were derived, but they were omitted from the final fit of GS rotational constants. The US rotational constants from a fit to 216 lines from the Ka = 6 to 8 series are in the fourth data column in Table 2. The assigned lines and the details of this fit are in Table S4 in the Supplementary material. The US fit points tentatively to 1292.9 cm1 as the band center, a value that coincides with a prominent Q branch. 4.2. Analysis of the rotational structure in bands of ttDFBD-1-d1 Fig. 6 displays the C-type band of ttDFBD-1-d1 at 920 cm1. The vibrational transition is m18 of the a00 symmetry species, a mode which is out-of-plane flapping for all three CH bonds. Several hot bands are signaled by the density of subband Q branches in the two wings of the band. The central Q branches for these hot bands are on the low frequency side of the central Q branch of the fundamental. The distinct, weak Q branch near 916 cm1 seems too separated from the principal Q branch to be a hot band. It can be explained as a ternary combination band gaining intensity from Fermi resonance with the fundamental. Possibilities are 2m22 + m24 = 912 cm1 or m16 + m17 + m22 = 922 cm1, where the anharmonic frequencies are approximations from the calculations. An A-type band of medium intensity for m13(a0 ) at 958 cm1 overlaps the R-branch of the C-type band near the edge of Fig. 6. This transition is principally in-plane bending of the CD bond. Fig. 6 has combs for the Q branches of the assigned subbands. A strong perturbation suppresses the intensity for the two subbands with K 0a ¼ 11, and the corresponding subband series were only tentatively identified. These series are not included in this report. Although possible Q branches are apparent, the subbands for K 0a ¼ 13 were not found despite several candidates, which must arise from hot bands. Subband assignments in the two wings ranged from K 00a ¼ 3 to 16 in the R branch and 5 to 18 in the P branch. The marginal S/N level for lines in the subband series was comparable to that displayed in Fig. 3. Observed asymmetry splitting for the RR3, RR4, RR5, PP5, and PP6 subbands helped define the difference between B0 and C0. From 1844 lines assigned in the C-type band at 920 cm1, 819 GSCDs contributed to fitting GS rotational constants. An additional 710 GSCDs from the analysis of the second C-type band at 684 cm1 were also used in fitting the GS rotational constants.

The GS rotational constants obtained from the fit of the combined GSCDs are in the first data column in Table 3. The dJ and dK centrifugal distortion constants computed with B3LYP/cc-pVTZ model were used in the fit. The other three centrifugal distortion constants, which were found in the fit, were in good agreement with the calculated values (in cm1) with obs–calc percent differences (in parentheses): DK = 0.3013  105 (-2.8% diff), DJK = -0.2595  107 (4.0% diff), and DJ = 0.1242  108 (3.0% diff). Table S5 in the Supplementary material gives all the GSCDs and the details of fitting GS rotational constants. An attempt to predict subbands with K 0a ¼ 3, while fitting subbands with K 0a ¼ 4 to 6 failed. Above K 0a ¼ 6 a perturbation has an observable impact. The limited data set of 451 lines for the US fit was no doubt insufficient to determine the exact spacing between subbands, which would affect the predictions for the RR2 and PP4 subbands. The approximate US rotational constants found in this fit are in the second data column in Table 3. The band center was estimated as 920.4 cm1. Table S6 in the Supplementary material contains the subbands used for the upper state fit, the details of the fit of US rotational constants, and an addendum with the additional assignments for the C-type band at 920 cm1, which contributed to GSCDs. The second band to be analyzed for the ttDFBD-1-d1 species was the weak C-type band at 684 cm1. The vibrational transition is m21, which is largely out-of-plane flapping of the CD bond. Fig. 7 shows the overall structure of this band with combs in the two wings for Q-branch features from K 00a ¼ 5 to 15 in the R branch and 7 to 17 in the P branch. This band is severely overlapped, especially in the P branch, by lines from the bending mode of CO2 and pure rotational transitions of H2O, as a consequence of air leaking into the White cell during the long accumulation of the spectrum. The lines from CO2 are marked with ‘‘c,’’ and the lines from H2O are marked with ‘‘w’’ in Fig. 7. Despite this CO2 and H2O contamination and the weakness of the spectrum, a useful analysis of the spectrum was possible. For the second C-type band, 1552 lines were assigned. From these lines 710 GSCDs were combined with 819 from the first Ctype band to determine the GS constants, which are in the first data column in Table 3. The comparability of the GSCDs derived from the two C-type bands is strong evidence in support of a correct analysis of rotational structure. The small inertial defect of 0.06772 amu Å2 for the GS of the ttDFBD-1-d1 species compares

Fig. 6. The C-type band for m18 at 920 cm1 of ttDFBD-1-d1.

N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27 Table 3 Rotational constants for ttDFBD-1-d1.

1

A (cm ) B (cm1) C (cm1) dJ  1010 (cm1) dK  108 (cm1) DK  105 (cm1) DJK  107 (cm1) DJ  108 (cm1)

j m0 (cm1) s.d. (cm1) Dc (amu Å2) No. trans. K 0a Jmax b

Ground state

m18(a00 ) – C-type

m21(a00 ) – C-type

0.9216655(12) 0.0386725(7) 0.0371207(4) 0.6382a 0.8254a 0.2932(3) 0.249(4) 0.128(3) 0.99649

0.918742(15) 0.038659(1) 0.037127(2) 0.1042a 0.2085a 1.79(3) 0.42(3) 0.137(1) 0.99652 920.3948(2) 0.00032 0.3575 451 4–6 95

0.920663(3) 0.038685(1) 0.037122(1) 0.6382a 0.8254a 0.380(2) 0.305(5) 0.137(5) 0.96814 684.2849(1) 0.00035 0.04639 878 6–10 94

0.00038 0.06772 1529d 4–17 96

Ground state values. a Calculated with the B3LYP/cc-pVTZ model. c Inertial defect, D = Ic  Ia  Ib. d 819 GSCDs from m18 and 710 from m21.

favorably with the value of 0.07395 for the GS of the ttDFBD species [6]. A fit of US rotational constants was made to 878 lines for the Ctype band at 684 cm1 with K 0a ¼ 6 to 10. A weak perturbation above K 0a ¼ 10 limited the data set in this way. The RR4 and PP6 series were predicted in association with fitting the US rotational constants. However, these predictions were unconvincing in the spectrum. The fit of US rotational constants predicted a band center at 684.3 cm1, 0.1 cm1 below the lowest Q-branch feature in the band center. Because the subband assignments do not go deeper than K 0a ¼ 5, the four decimal significance of the fitted band center is illusory. We take the band center as 684.4 cm1. The US rotational constants are in the third column of data in Table 3. Table S7 in the Supplementary material gives the details of fitting the US rotational constants, the series from the second C-type band used in the fitting, and an addendum of the series used to compute GSCDs but not used in the US fitting. We also attempted to analyze the rotational structure in the A-type band at approximately 619.5 cm1 in the spectrum of the ttDFBD-1-d1 species. The vibrational transition is m14(a0 ), which is largely in-plane bending of the two central CH bonds. The overall

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structure of this band is shown in Fig. 8. Although some subband series could be found in the two wings of the band, we were unable to carry the analysis any further. The absence of a strong central Q branch implies that the centers of the subbands are dispersed over the band center. No evidence of Q branches for individual subbands could be discerned in the spectrum, and predicted A-type GSCDs differed so little with Ka that GSCDs could not be used to assign Ka values or to help link up QPK and QRK parts of subbands. We abandoned further work on the A-type band. 4.3. Analysis of the rotational structure in a C-type band of ttDFBD1,4-d2 The rotational structure in a C-type band of ttDFBD-1,4-d2 at 893 cm1 was analyzed. The vibrational transition is m10(au), which is largely out-of-plane flapping of the two interior CH bonds. The spectrum analyzed contained a significant contribution from the d1 species, and smaller amounts of the d0 species and other d2 species. Fig. 9 gives the overall spectrum for the lower intensity scan in transmittance display for the C-type band of ttDFBD-1,4-d2. This spectrum includes a substantial overlap of the P branch of the 1-d1 species centered at 920 cm1. Combs designate the Q branches for the C-type band of the 1,4-d2 species for K 00a ¼ 3 to 16 in the R branch and 5 to 18 in the P branch. A comb also gives the subband Q branches in the P branch of the C-type band of the 1-d1 species centered at 920 cm1. In contrast to Fig. 6, the comb for the P branch of the band for the d1 species in Fig. 9 is cut off at K 00a ¼ 11, beyond which a perturbation begins to interfere. Subband assignments were made on a higher intensity spectrum than is shown in Fig. 9. Observations of asymmetry splitting for several subbands, RR4, RR5, and PP6 helped define the difference between B0 and C0. The assignments of RR3 and PP5 were not carried beyond the split because of doubt about the paths. Table 4 gives the rotational constants for ttDFBD-1,4-d2, as determined from the experimental data. From 1675 assigned lines 669 GSCDs were found and used in fitting GS rotational constants. The GS rotational constants are in the first column of data in Table 4. The dJ and dK centrifugal distortion constants were from the B3LYP/cc-pVTZ predictions. Although the s.d. of the fit is acceptable at 0.000434 cm1, it is significantly larger than in the corresponding work with the other two species. The GSCDs and details of the fit are in Table S9 in the Supplementary material.

Fig. 7. The C-type band for m21 at 684 cm1 of ttDFBD-1-d1.

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N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27

Fig. 8. The A-type band for m14 at 619.5 cm1 of ttDFBD-1-d1.

Fig. 9. The C-type band for m10 at 893 cm1 of ttDFBD-1,4-d2. Lower-intensity scan.

We consider reasons why the frequencies for the lines in the spectrum of the ttDFBD-1,4-d2 species are somewhat degraded. The spectrum for this species was recorded with a lower resolution of 0.0018 cm1 than the 0.0015 cm1 resolution available for the other two molecules. Thus, more lines were blends. Interference from the lines of the d1 species occurred in the R branch. In the P branch the subband series for the 1,4-d2 species were entangled, similar to vines twisted around a post. Blending was a consequence of this effect. The 669 GSCDs were a distinctly smaller number than the 1000+ that are routinely used when fitting GS rotational constants to IR data. In addition to having a spectrum of lower quality for the ttDFBD-1,4-d2 species, only one band was available for analysis. Thus, the confirmation that comes from assigning a second band is absent for this species. Fortunately, strong support for the experimental GS rotational constants is available. As was seen above for the other two molecules, the experimental centrifugal distortion constants agree with the values computed with the B3LYP/cc-pVTZ model within 5%.

(Values computed with the higher-level B3LYP/aug-cc-pVTZ model are negligibly different.) The calculated values (in cm1) and the percent difference for obs–calc (in parentheses) are DK = 0.22926  105 (2.4%), DJK = 0.20276  107 (0.68%), and DJ = 0.12154  108 (9.2%). In addition, the inertial defect is appropriately small and negative at 0.04863 amu Å2 for this planar molecule. In conclusion, we have confidence in the GS rotational constants determined for the ttDFBD-1,4-d2 species. A reviewer drew attention to the large differences between centrifugal distortion constants for the GS of ccDFBD-1-d1 (Table 2) and the GSs of ttDFBD-1-d1 (Table 3) and ttDFBD-1,4-d2 (Table 4). Of course, the PAS for the cc species is significantly rotated with respect to the tt species, as seen in Fig. 1. This sensitivity of centrifugal distortion constants to structure provides further support for using centrifugal distortion constants to evaluate the fit of rotational constants. US rotational constants for the C-type band of ttDFBD-1,4-d2 were fitted to 886 lines in the subbands with K 0a ¼ 4 to 10. At

N.C. Craig et al. / Journal of Molecular Spectroscopy 288 (2013) 18–27 Table 4 Rotational constants for ttDFBD-1,4-d2.

1

A (cm ) B (cm1) C (cm1) dJ  1010 (cm1) dK  108 (cm1) DK  105 (cm1) DJK  107 (cm1) DJ  108 (cm1)

j m0 (cm1) s.d. (cm1) Db (amu Å2) No. trans. K 0a Jmax a b

Ground state

m10(au) – C-type

0.820299(2) 0.038372(1) 0.0366609(8) 0.6835a 0.7675a 0.2239(5) 0.201(8) 0.134(6) 0.99563

0.818999(3) 0.0383459(5) 0.0366704(5) 0.6835a 0.7675a 0.217(2) 0.140(4) 0.1311(4) 0.99572 892.9827(2) 0.00035 0.4970 886 4–10 94

0.00044 0.04863 669 4–17 88

Calculated with the B3LYP/cc-pVTZ model. Inertial defect, D = Ic  Ia  Ib.

K 0a ¼ 11, a perturbation becomes apparent. In addition to these lines, 789 more lines were available for computing the 669 GSCDs. The US rotational constants fitted for the C-type band are in the second data column of Table 4. The m0 for this band was found to be 892.983 cm1 in this fit. The decimal significance for this band is artificial because the subband assignments are no closer to the band center than K 0a ¼ 4. This value falls 0.023 cm1 below the peak of the central Q branch, which is only an estimate of m0. Thus, a best value for m0 is 893.00 cm1. Table S9 in the Supplementary material gives lines used in fitting the US rotational constants and the details of the fitting. Assigned lines not used in the US fit are an addendum to this table. 5. Summary From the analysis of rotational structure in several bands of the high-resolution IR spectra, rotational constants have been determined for ccDFBD-1-d1, ttDFBD-1-d1, and ttDFBD-1,4-d2. Prior work produced the rotational constants for the normal species of the two isomers [6] and for the ccDFBD-2-d1 and ttDFBD-2-d1 species [7]. In the light of the MW findings for ctDFBD, where the experimental rotational constants are known for 13C substitution at each position, good ground state rotational constants for the 13 C species can be predicted from ab initio calculations supplemented with scale factors derived from the calculated and observed values for the normal species. Thus, it will be unnecessary to synthesize and investigate the 13C species of the tt and cc isomers as a prelude to finding semiexperimental equilibrium structures. However, the mixed estimation method [5] will be needed because the fluorine atoms cannot be substituted isotopically, and the assumption about the adequacy of computed rotational constants for the 13C species needs to be reinforced. Acknowledgments We are grateful to Dr. Michael Lock for recording the spectra of the mixture of partly deuterated ttDFBD at Justus Liebig Universi-

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tät in Giessen, Germany. At Oberlin College Ethan Glor, Erik Hernandez, Zoë McLaughlin, Petros Svoronos, and Herman van Besien contributed to the syntheses. The work at Oberlin College was supported by Dreyfus Senior Scholar Mentor grants and by the college. National Science Foundation Grant 0420717 provided for the purchase and technical support of the Beowulf computer cluster at Oberlin College. The high-resolution spectroscopy was done at the W.R. Wiley Environmental Molecular Science Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at the Pacific Northwest Laboratory (PNNL). PNNL is operated for the United States Department of Energy by Battelle under contract DE-AC05-75RLO-1830. Appendix A. Supplementary material Supplementary data for this article are available on Science Direct (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://library.osu.edu/ sites/msa/jmsa+hp.htm). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.jms.2013.03.006. References [1] N.C. Craig, P. Groner, D.C. McKean, J. Phys. Chem. A 110 (2006) 7461–7469. [2] H.G. Viehe, E. Franchimont, Chem. Ber. 97 (1964) 602–608. [3] N.C. Craig, C.F. Neese, T.N. Nguyen, C.M. Oertel, L. Pedraza, A.M. Chaka, J. Phys. Chem. A 103 (1999) 6726–6739. [4] N.C. Craig, C.M. Oertel, D.C. Oertel, M.J. Tubergen, R.J. Lavrich, A.M. Chaka, J. Phys. Chem. A 106 (2002) 4230–4235. [5] J.F. Demaison, N.C. Craig, J. Phys. Chem. A 115 (2011) 8049–8954. [6] N.C. Craig, M.C. Moore, C.F. Neese, D.C. Oertel, L. Pedraza, T. Masiello, J. Mol. Spectrosc. 254 (2009) 39–46. [7] N.C. Craig, C.C. Easterday, D.J. Nemchick, D.F.K. Williamson, R.L. Sams, J. Mol. Spectrosc. 272 (2012) 2–10. [8] R.D. Suenram, B.H. Pate, A. Lesarri, J.L. Neill, S. Shipman, R.A. Holmes, M.C. Leyden, N.C. Craig, J. Phys. Chem. A 113 (2009) 1864–1868. [9] N.C. Craig, J.I. Chuang, C.C. Nwofor, C.M. Oertel, J. Phys. Chem. A 104 (2000) 10092–10103. [10] N.C. Craig, E.A. Entemann, J. Am. Chem. Soc. 83 (1961) 3047–3050. [11] L.S. Rothman, C.P. Rinsland, A. Goldman, S.T. Massie, D.P. Edwards, J.M. Flaud, A. Perrin, C. Camy-Peyret, V. Dana, J.Y. Mandin, J. Schroeder, A. McCann, R.R. Gamache, R.B. Wattson, K. Yoshino, K.V. Chance, K.W. Jucks, L.R. Brown, V. Nemtchinov, P. Varanasi, JQSRT 60 (1998) 665–710. [12] A.G. Maki, J.S. Wells, Wavenumber Calibration Tables from Heterodyne Frequency Measurements, NIST Special Publication 821, US Department of Commerce, 1991. . [13] R.A. Toth, J. Opt. Soc. Am. B 8 (1991) 2236–2255. [14] N.C. Craig, J.L. Davis, K.A. Hanson, M.C. Moore, K.J. Weidenbaum, M. Lock, J. Mol. Struct. 695–696 (2004) 59–69. [15] M.J. Frisch, G.W. Trucks, H. B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K. N. Kudin, J.C. Burant, et al., Gaussian 03, revision C.02; Gaussian, Inc., Wallingford CT, 2004. [16] B.P. Winnewisser, J. Reinstädtler, K.M.T. Yamada, J. Mol. Spectrosc. 136 (1989) 12–16. [17] T. Oka, J. Mol. Struct. 352–353 (1995) 225–233. [18] D. Steele, The Theory of Vibrational Spectroscopy, W. Saunders, Philadelphia, 1971. p. 109. [19] J.I. Steinfeld, Molecules and Radiation, second ed., The MIT Press, Cambridge, MA, 1974. p. 35.