Rotational spectra of gauche perfluoro-n-butane, C4F10; perfluoro-iso-butane, (CF3)3CF; and tris(trifluoromethyl)methane, (CF3)3CH

Journal of Molecular Spectroscopy 242 (2007) 129–138 www.elsevier.com/locate/jms

Rotational spectra of gauche perfluoro-n-butane, C4F10; perfluoro-iso-butane, (CF3)3CF; and tris(trifluoromethyl)methane, (CF3)3CH Michaeleen R. Munrow a,1, Ranga Subramanian a,2, Andrea J. Minei a, Dean Antic b, Matthew K. MacLeod b, Josef Michl b, Raul Crespo c, Mari Carmen Piqueras c, Mitsuaki Izuha d, Tomohiro Ito d, Yoshio Tatamitani d,3, Kenji Yamanou d, Teruhiko Ogata d, Stewart E. Novick a,* a

Department of Chemistry, Wesleyan University, Middletown, CT 06459, USA Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA c Departament de Quimica Fisica, Universitat de Valencia, E-46100 Burjassot, Spain d Department of Chemistry, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan

b

Received 5 February 2007; in revised form 22 February 2007 Available online 3 March 2007

Abstract The microwave spectra of the gauche conformer of perfluoro-n-butane, n-C4F10, of perfluoro-iso-butane, (CF3)3CF, and of tris(trifluoromethyl)methane, (CF3)3CH, have been observed and assigned. The rotational and centrifugal distortion constants for gauche n-C4F10 are: A = 1058.11750(7) MHz, B = 617.6832(1) MHz, C = 552.18794(1) MHz, DJ = 0.0257(5) kHz, dJ = 0.0052(3) kHz. A C–C–C–C dihedral angle, x, of 55 has been determined. These values agree well with those obtained from a coupled cluster (CCSD/cc-PVTZ) calculation. The rotational and centrifugal distortion constants for iso-C4F10 and iso-C4HF9 are: Bo = 816.4519(4) MHz, DJ = 0.023(2) kHz, and Bo = 903.6985(25) MHz, DJ = 0.043(4) kHz, respectively. The dipole moment of iso-C4F10 and iso-C4HF9 have been measured and found to be 0.0338(8) and 1.69(9) D, respectively.  2007 Elsevier Inc. All rights reserved. Keywords: C4F10; Perfluorobutane; (CF3)3CF; Perfluoro-iso-butane; (CF3)3CH; Tris(trifluoromethyl)methane; Conformations; Microwave spectroscopy; Fourier transform microwave spectroscopy; Dipole moments

1. Introduction In order to establish the flexible nature of organic molecules, many investigations have been carried out which aid our understanding of the conformational changes that

*

Corresponding author. Fax. +1 860 685 2211. E-mail address: [email protected] (S.E. Novick). 1 Present address: Norwalk High School, Norwalk, CT 06851, USA. 2 Present address: Department of Chemistry, Tulane University, New Orleans, LA 70118, USA. 3 Present address: Department of Applied Chemistry, Kanagawa Institute of Technology, Kanagawa 243-0203, Japan. 0022-2852/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2007.02.021

occur in linear chain molecules, specifically with alkanes of chain length N P 4. It is well known that these molecules are not freely rotating about C–C bonds, but rather that energy barriers to internal rotation cause there to be a preference for the molecule to exist at certain dihedral angles, x. For the well investigated molecule, n-butane [1–6], where both experimental and theoretical data are abundant, two low energy minima occur at configurations with dihedral angles equal to x  ±65 (racemic gauche) and x = 180 (achiral anti) [7]. The anti conformation, for which the dipole moment is zero, is the lowest energy conformation of n-butane. If the hydrogen atoms in nbutane are replaced by larger atoms, the anti conformer

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is found to deviate away from x = 180 towards a racemic transoid configuration at x = ±165 so as to alleviate unfavorable steric interactions that occur between substituents at carbons in positions 1 and 3. Perfluoro-n-butane, C4F10, is a linear chain molecule with a carbon backbone and fluorine saturation. Like its four-carbon alkane analogue, n-butane, n-C4F10 shows rotational isomerism about the central C–C bond. A general semiquantitative conformational analysis of linear chains based on the ratio of substituent size and backbone bond length suggests that this molecule should have three pairs of minima, transoid (x @ ±165), ortho (x @ ±90), and gauche (x @ ±55) [8]. Several theoretical papers have been published on the isomeric structures of n-C4F10 which disagree on the number of low energy conformational isomers that exist. Dixon and co-workers have performed several early calculations (SCF level with the 6-31G*(C) basis set) on the conformational isomers of n-C4F10. They showed that there is a twist in the carbon backbone of n-C4F10 about the central C–C bond which corresponds to a dihedral angle of 15 away from 180 [9]. In a later publication (optimization at the HF level with a DZ + Dc basis set), Dixon calculated the full torsional potential energy surface for rotation about the central C–C bond [10]. It was determined from these calculations that the low energy conformers occur at dihedral angles of 165 (twist anti) and 63 (gauche). Transition states (TS) were also calculated and found to be at dihedral angles of 180 (anti-TS), 119 (120 TS), and 0 (syn TS). More recently, Albinsson and Michl performed ab initio calculations at the MP2/6-31G* level of theory [11]. They found three low energy configurations with dihedral angles of 165 (anti), 95 (ortho), and 54 (gauche), as shown in Fig. 1. They proposed that the gauche and previously unobserved ortho conformers result from the ‘‘ordinary’’ gauche minimum at 60 splitting into two minima at 54 and 95 in order to alleviate unfavorable interactions between substituents at carbons in the 1 and 4 positions. To confirm these findings, Albinsson and Michl obtained a nitrogen matrix-isolation IR spectrum for the anti, ortho, and gauche conformers of n-C4F10. They assigned the spec-

tra by comparing the observed spectra with those expected from HF/6-31G* calculations and reported finding only minute amounts of the ortho conformer in the mixture trapped in the nitrogen matrix. Since much of the work on n-C4F10 has been done by theoretical calculation and the only experimental data are the matrix-isolation IR, which does not provide direct information about the backbone dihedral angle, a high resolution experimental search for the various conformations was called for. In this paper, we report the assignment and structural analysis of the gauche conformer of perfluoro-nbutane and the perfluoro-iso-butane molecule measured using a pulsed jet Fourier transform microwave (FTMW) spectrometer. Extensive searches for the ortho conformer of the n-C4F10 molecule have been performed but were unsuccessful. Additionally, the transoid conformer has little or no dipole moment and is expected to be unobservable in our spectrometer. iso-C4F10 was first observed as an impurity in our n-C4F10 sample tank and the spectrum of an oblate symmetric top was later assigned to the isoC4F10 molecule. In the present work, the microwave spectra of two 13C isotopomers in natural abundance have been obtained for iso-C4F10. In addition, Stark data on isoC4F10 were observed and the dipole moment for the molecule along the c axis has been measured. Both hydrofluorocarbons (HFC) and perfluorocarbons (PFC) have drawn attention in relation to environmental problems. Unlike the chlorofluorocarbons (CFC), they do not destroy ozone, which, in Earth’s stratosphere, provides protection for living creatures against harmful ultraviolet radiation. Large amounts of HFCs have been used as CFC-Halon replacement compounds [12]. However, HFCs and PFCs have high global warming potentials (GWP) and behave as greenhouse gases. Thus, the chemical industry has been forced to find other alternatives for HFCs that are friendlier to the environment. Unfortunately, the GWP of PFCs is even higher than that of HFCs: they are super-greenhouse gases. NASA recently sponsored a conference called ‘‘The Physics and Biology of Making Mars Habitable’’ [13]. It is believed that Mars can be made more habitable by raising the temperature at the Martian surface by pumping the atmosphere full of super-greenhouse gases: the best candidates are PFCs. Due to their long lifetimes, a small amount injected into the Martian atmosphere would account for considerable warming. 2. Experiment and calculations

Fig. 1. Three possible conformations of perfluoro-n-butane, guache, ortho, and anti, shown here with generic dihedral angles, x, of 60, 90, and 180, respectively. The CCSD/cc-PVTZ calculated values for these angles are 54.3, 96.1, and 165.6, with relative energies of 0.7, 1.9, and 0.0 kcal/mol.

Three different spectrometers were used in this study, pulsed-jet FTMW spectrometers at Wesleyan, [14] and at Shizuoka, [15] and a Stark modulation microwave spectrometer at Shizuoka [16]. In the FTMW spectrometers, a pulsed jet of gas with a low rotational temperature (4 K) is produced by a standard pulsed supersonic expansion and passes through a high-Q Fabry-Perot microwave cavity tunable between 5 and 26.5 GHz. A microwave pulse, coupled into the cavity by a small L-shaped antenna,

M.R. Munrow et al. / Journal of Molecular Spectroscopy 242 (2007) 129–138

is timed to coincide with the arrival of the gas pulse with a temporal width of about 1 ls and tuned to the cavity frequency. If a molecular absorption line lies within the 500 kHz bandwidth of the microwave pulse and FabryPerot cavity combination, a macroscopic polarization is induced in the molecules. The free induction decay (FID) of this polarization is collected, averaged over multiple pulses, and the Fourier transform of the FID is the frequency spectrum of the transition. The sample of (CF3)3CH was purchased from PCR Research Chemicals, Inc. The deuteriated material, (CF3)3CD, was prepared by the mixing (CF3)3CH with (CD3)2CO and D2O. All measurements on these compounds were made at dry ice temperature. The microwave spectrum of (CF3)3CH was measured in the frequency region of 8–50 GHz with a conventional 100 kHz Stark modulation spectrometer [16], controlled by a desktop computer, FM/V 5150DPT. All electronic devices were connected to the computer by HP-IB interfaces programmed by HP VEE software. Microwave radiation from the HP 83650L synthesizer was amplified and introduced to the 3 m 100 kHz Stark modulation absorption cell. The resulting signals were phase-sensitive detected with a lock-in amplifier, A/D converted, stored on the hard drive, and displayed on the computer’s monitor. In the FTMW experiments, the samples consisted of 1% mixture of n-C4F10 or iso-C4F10 (Flura Corporation) in a neon carrier gas. This was done to minimize any rare gas-C4F10 complexation. The gas mixture is expanded through a 0.5 mm diameter General Valve nozzle which expands through a small hole in one of the mirrors of the Fabry-Perot cavity, coaxial to the cavity axis. In this configuration, linewidths (FWHM) are typically 5–10 kHz, allowing for the determination of transition frequencies to better than 1 kHz. An input power amplifier and scans of 2500 averaged shots [17] were also needed due to the small predicted dipole moment and weak transitions of the n-C4F10 molecule. The normal 12C and both 13C isoC4F10 monomer transitions could easily been seen after a few hundred shots. Lines were tested in a 1% mixture of the C4F10 samples in an argon carrier. Lines present in both the neon and argon carriers are attributed to the C4F10 monomers, as opposed to an argon or neon van der Waals complex with the perfluorobutane. To aid in the assignment of the gauche n-C4F10 conformer, we only consider transitions that are found in both the Ar-n-C4F10 and the Ne-n-C4F10 samples. The spectrometer is automated to scan unattended over a broad frequency region which allows for the expeditious coverage of 5–26.5 GHz. Stark analysis in the Wesleyan FTMW spectrometer is done with a 0.5 mm diameter nozzle perpendicular to the cavity axis and with Stark plates outside the cavity separated by 24.4 cm with typical voltages of up to 6000 V. Geometrical parameters and electronic properties for the transoid, ortho, and gauche conformers of n-C4F10 were computed at the coupled-cluster method with singles

131

and doubles (CCSD) [18] employing Dunning’s correlation consistent triple-f basis set (cc-pVTZ) [19] using the Gaussian-03 program package [20]. The geometry optimizations were performed within the C2 symmetry point group constraints, with z as the twofold symmetry axis. 3. Results and analysis 3.1. Transition frequencies and assignments The transition frequencies and assignments for the gauche n-C4F10 conformer are listed in Table 1. Transitions for the gauche conformer are b-type. Using an asymmetric rotor fitting program [21] ,the transitions for the gauche nC4F10 in Table 1 are fitted to a Hamiltonian in an Ir representation, Watson A reduction, using three rotational constants and two quartic centrifugal distortion constants. This results in a 44 line fit with a standard deviation of less than 2 kHz. Spectroscopic constants for the gauche conformer are given in Table 2. The transition frequencies and assignments for iso-C4F10 are listed in Table 3a. The iso-C4F10 spectrum exhibits a typical (A + B)/2 progression of c-type transitions, typical for an oblate symmetric top. By utilizing Pickett’s SPFIT program [22], the seven transitions of iso-C4F10 were fitted to a standard deviation of 1 kHz using one rotational constant, Bo, which is (A + B)/2, and one centrifugal distortion constant, DJ. The 13C isotopomers were fitted using a Hamiltonian in an Ir representation, Watson S reduction, for the central carbon and an Ir representation, Watson A reduction, for the adjacent equivalent carbons. Spectroscopic constants for the all 12C iso-C4F10 are given in Table 4a. Table 3b and c lists the transition frequencies and assignments for the 13C iso-C4F10 isotopomer with the substitution at the central carbon and the 13C iso-C4F10 isotopomer with the single substitution at the adjacent carbon. Table 4b and c gives the spectroscopic constants for these two isotopomers. The 13C isotopomer of the carbon adjacent to the central carbon is a nearly symmetric top with many overlapping transitions that occur within a 5 MHz window. With overlapping transitions in a Fourier transform of a free induction decay signal, it has been shown that the FT power spectrum can lead to erroneous frequency measurements with errors amounting to as much as 20% of the apparent multiplet splitting [23]. One of the causes for the discrepancies between the apparent and actual transition frequencies is the phase difference of the FIDs of the multiplet. Haekel and Mader [24] have shown that accurate frequencies of closely spaced overlapping multiplets can be obtained by direct fitting of the time-domain signal. Muller and Gerry have used this technique to obtain accurate frequencies for overlapping transitions in the FT microwave study of BrF and IF [25]. We have also fitted the timedomain (FID) signal directly, using computer code written and supplied by Grabow [26]. For the transitions we have measured, the time-domain fit can differ from the direct FT

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Table 1 Transition frequencies and assignments for gauche n-C4F10 J0

Ka 0

Kc 0

J00

Ka00

Kc00

Observed frequency (MHz)

Observed  calculated (MHz)

3 3 5 4 6 4 6 6 5 4 4 7 7 6 4 4 7 5 5 8 7 8 8 9 6 6 9 9 9 7 6 6 10 10 11 11 12 12 13 13 14 14 15 15

3 3 1 2 1 2 0 1 2 3 3 1 0 2 4 4 1 3 3 1 2 0 1 2 3 3 1 0 1 3 4 4 0 1 0 1 0 1 0 1 0 1 0 1

1 0 5 3 5 2 6 6 4 2 1 6 7 5 1 0 7 3 2 7 6 8 8 7 4 3 8 9 9 5 3 2 10 10 11 11 12 12 13 13 14 14 15 15

2 2 4 3 5 3 5 5 4 3 3 6 6 5 3 3 6 4 4 7 6 7 7 8 5 5 8 8 8 6 5 5 9 9 10 10 11 11 12 12 13 13 14 14

2 2 0 1 2 1 1 0 1 2 2 2 1 1 3 3 0 2 2 2 1 1 0 3 2 2 2 1 0 2 3 3 1 0 1 0 1 0 1 0 1 0 1 0

0 1 4 2 4 3 5 5 3 1 2 5 6 4 0 1 6 2 3 6 5 7 7 6 3 4 7 8 8 4 2 3 9 9 10 10 11 11 12 12 13 13 14 14

5872.780 5879.996 5886.750 5901.733 6187.105 6391.433 6679.653 6934.916 6940.516 7028.341 7064.814 7500.847 7833.633 7950.840 7992.557 7993.022 7994.441 8158.741 8268.034 8800.402 8939.164 8970.419 9067.139 9086.782 9252.034 9502.600 10073.403 10094.825 10150.994 10301.355 10327.074 10340.255 11211.069 11242.832 12322.242 12339.839 13430.396 13439.989 14536.814 14541.971 15642.253 15644.996 16747.155 16748.601

0.001 0.001 0.002 0.001 0.001 0.000 0.001 0.001 0.003 0.002 0.002 0.002 0.001 0.002 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.001 0.000 0.003 0.002 0.002 0.000 0.001 0.001 0.003 0.001 0.001 0.000 0.001 0.001 0.001 0.000 0.000 0.001 0.001 0.001 0.001 0.001 0.000

power spectrum fit by as much as 10 kHz for lines the centers of which are apparently measured to 0.25 kHz. The Stark modulation microwave spectrum of (CF3)3CH for J = 20–19 is shown on Fig. 2. The transition frequencies for (CF3)3CH and (CF3)3CD in the ground

state and the excited states are listed in Tables 5 and 6, respectively. Utilizing Pickett’s SPFIT program [22], transition frequencies were fitted to Bo and DJ. The spectroscopic constants for (CF3)3CH and (CF3)3CD are listed in Table 7. 4. Structural analysis

Table 2 Spectroscopic constants for gauche n-C4F10

4.1. Gauche perfluoro-n-butane

A = 1058.11750 (7) MHz B = 617.6832 (1) MHz C = 552.18794 (1) MHz DJ = 0.0257 (5) kHz dJ = 0.0052 (3) kHz

Using the spectroscopic constants from the experimental transitions, the structure of the gauche conformer was fitted using Schwendeman’s STRFTQ program [27] and also Kisiel’s program of the same name [28] both of which fit the structure of the molecule from the experimentally deter-

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133

Table 3 Transition frequencies and assignments for perfluoro-iso-butane J0

J00

Observed frequency (MHz)

Observed  calculated (MHz)

3 4 5 6 7 8 9

6531.608 8164.508 9797.403 11430.294 13063.184 14696.067 16328.946

0.000 0.000 0.001 0.001 0.000 0.000 0.000

C iso-C4F10 (central carbon) 3 4 5 6 7 8 9

6530.428 8163.030 9795.631 11428.227 13060.820 14693.408 16325.991

0.000 0.001 0.000 0.000 0.000 0.000 0.000

(a) 4 5 6 7 8 9 10

12

(b) 4 5 6 7 8 9 10

13

C iso-C4F10

J0 (c) 5 5 5 5 5 6 6 6 7 7 7 7 7 7 7 8 8 8 8 8 8 8 9 9 9 10 10 10 10 10 11 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12

Ka 0

Kc 0

J00

Ka00

Kc00

Observed frequency (MHz)

Observed  calculated (MHz)

4 4 4 4 4 5 5 5 6 6 6 6 6 6 6 7 7 7 7 7 7 7 8 8 8 9 9 9 9 9 10 10 10 10 10 10 10 10 11 11 11 11 11 11 11 11

3 2 1 3 4 4 4 5 5 2 3 2 1 5 6 6 4 3 2 3 6 7 7 7 8 8 0 1 8 9 9 4 5 4 3 2 3 10 4 5 4 3 2 3 0 1

1 3 3 2 1 1 2 1 1 4 4 5 5 2 1 1 4 4 5 5 2 1 1 2 1 1 9 9 2 1 1 6 6 7 7 8 8 1 7 7 8 8 9 9 11 11

8143.173 8150.143 8150.143 8150.297 8157.535 9771.880 9780.399 9789.107 11400.625 11410.182 11410.182 11410.224 11410.224 11410.526 11420.716 13029.420 13040.160 13040.160 13040.222 13040.222 13040.681 13052.363 14658.271 14670.867 14684.054 16287.188 16300.308 16300.308 16301.089 16315.791 17916.182 17930.210 17930.210 17930.259 17930.259 17930.292 17930.292 17947.579 19560.230 19560.230 19560.270 19560.270 19560.302 19560.302 19560.331 19560.331

0.001 0.002 0.001 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.003 0.003 0.000 0.000 0.000 0.003 0.003 0.002 0.002 0.000 0.000 0.000 0.001 0.000 0.000 0.001 0.001 0.002 0.000 0.001 0.002 0.002 0.001 0.001 0.004 0.004 0.000 0.003 0.003 0.002 0.002 0.004 0.004 0.001 0.001

13

C iso-C4F10 (adjacent carbon) 4 1 3 3 2 3 4 2 5 1 5 1 5 2 6 1 6 1 3 4 4 4 3 5 2 5 6 2 7 1 7 1 5 4 4 4 3 5 4 5 7 2 8 1 8 1 8 2 9 1 9 1 1 9 2 9 9 2 10 1 10 1 5 6 6 6 5 7 4 7 3 8 4 8 11 1 5 7 6 7 5 8 4 8 3 9 4 9 1 11 2 11

18095.79 19905.22 21714.88 23524.30 25333.71 27143.20 28952.60 30761.94 32571.33 34380.80 36190.01 37999.40 0.01 0.03 0.01 0.02 0.00 0.04 0.03 0.04 0.07 0.09 0.06 0.13 0.01 0.04 39688.50

18087.20 19895.89 21704.51 23513.20 25321.81 27130.48 28939.01 30747.68 32556.13 34364.66 36173.22 37981.93 39790.30 0.11 0.02 0.12 0.10 0.10 0.08 0.06 0.09 0.04 0.03 19844.89 21649.02 23453.19 25257.18 27061.17 28864.96 30668.93 32472.84 34276.90 36080.71

0.16 0.04 0.06 0.01 0.02 0.12 0.08 0.07 0.01 0.16 0.12 0.11 0.05 18046.08 19850.50 21654.91 23459.50 25264.00 27068.32 28872.81 30677.39 32481.70 34285.93 36090.57 37894.88 39699.01 0.05 0.07 0.09 0.13 0.05 0.01 0.08 0.00 0.03 0.00 0.07 0.17 0.05 18052.80 19858.00 21663.38 23468.33 25273.69 27078.80 28883.99 30689.00 32494.10 34299.10 36104.19 37908.92 39714.10 0.14 0.07 0.09 0.04 0.08 0.02 0.11 0.11 0.01 0.10 0.10 0.15 0.02 18061.38 19867.3 21673.49 23479.40 25285.41 27091.50 28897.42 30703.42 32509.50 34315.58 36121.31 37927.48 39733.20 a

Frequencies in MHz.

0.00 0.04 0.05 0.01 0.00 0.10 0.04 0.11 0.04 0.06 0.02 0.04 0.01

Observed O–C Observed Observed O–C Observed

O–C

v=2 v=1 GS J ‹ J00

Table 5 Transition frequencies of (CF3)3CHa

mined rotational constants. Bond lengths and angles may be held constant or allowed to vary so that the resulting structure yields rotational constants that closely reproduce the experimentally determined values. Fig. 3 shows the four geometric parameters in n-C4F10 that were used: CF3CF2CF2–CF3 bond length, r1; CF3CF2–CF2CF3 bond length, r2; C–C–C angle, h; and the C–C–C–C dihedral angle, x. For this structural treatment, we have assumed that the r1 length between C1 and C2 equals the r1 length between C3 and C4. We also assumed that the angle h between C1–C2–C3 equals the angle h between C4–C3–C2. Since we experimentally determined three rotational constants, we can fit at most three structural parameters. We performed several calculations varying some of the geometric parameters while holding others constant at the calculated CCSD/cc-pVTZ ab initio values. Table 8 lists the results of our fits giving the geometric parameters and resulting error in the rotational constants (observed  calculated). The numbers in brackets are the parameters that were held constant to the ab initio values. It can be seen from Table 8 that there is an overall consistency between the fits. None of the structural parameters are unreasonable for bond lengths and angles of similar molecules. The average value of the dihedral angle for all of these fits is approximately 55. This value is also obtained if the dihedral angle is the sole geometric param-

v=3

Fig. 2. Stark modulation microwave spectrum of (CF3)3CH.

Observed

O–C

v=4

O–C

v0 = 1

O–C

(c) 13C (adjacent carbon) iso-C4F10 A = 816.4610(8) MHz B = 813.5882 (8) MHz DJ = 0.021(2) kHz

18073.8 19881.19 21688.41 23495.80 25303.10 27110.48 28917.61 30724.80 32532.20 34339.31 36146.60 37953.80 39760.90

(b) 13C (central carbon) iso-C4F10 Bo = 816.3042(4) MHz DJ = 0.023(2) kHz

10 ‹ 9 11 ‹ 10 12 ‹ 11 13 ‹ 12 14 ‹ 13 15 ‹ 14 16 ‹ 15 17 ‹ 16 18 ‹ 17 19 ‹ 18 20 ‹ 19 21 ‹ 20 22 ‹ 21

v0 = 2

(a) 12C iso-C4F10 Bo = 816.4519(4) MHz DJ = 0.023(2) kHz

Observed

O–C

Table 4 Spectroscopic constants for perfluoro-iso-butane

0.02 0.06 0.09 0.02 0.02 0.01 0.00 0.06 0.05 0.06 0.04 0.05

M.R. Munrow et al. / Journal of Molecular Spectroscopy 242 (2007) 129–138

Observed

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M.R. Munrow et al. / Journal of Molecular Spectroscopy 242 (2007) 129–138

135

Table 6 Transition frequencies of (CF3)3CDa J 0 ‹ J00

GS Observed

11 ‹ 10 12 ‹ 11 13 ‹ 12 14 ‹ 13 15 ‹ 14 16 ‹ 15 17 ‹ 16 18 ‹ 17 19 ‹ 18 20 ‹ 19 21 ‹ 20 22 ‹ 21 a

v0 = 1

v=1

21587.58 23386.40 25185.13 26984.10 28782.92 30581.90 32380.79 34179.50 35978.30 37777.28 39575.81

O–C 0.12 0.03 0.12 0.04 0.09 0.03 0.08 0.03 0.03 0.15 0.09

Observed

O–C

Observed

O–C

19775.00 21572.79 23370.52 25168.03 26965.59 28763.48 30561.19 32358.80 34156.30 35953.90

0.05 0.03 0.08 0.08 0.18 0.04 0.11 0.10 0.01 0.00

39549.01

0.04

19802.82 21603.20 23403.37 25203.80 27003.81 28803.91 30604.10 32404.30 34204.48 36004.59 37804.79 39604.72

0.12 0.02 0.02 0.19 0.01 0.07 0.05 0.01 0.02 0.00 0.09 0.07

Frequency in MHz.

Table 7 Spectroscopic constants of (CF3)3CH and (CF3)3CD (CF3)3CH GS v = 1a v=2 v=3 v=4 v 0 = 1b v0 = 2

(CF3)3CD

Bo/MHz

DJ/kHz

Bo/MHz

DJ/kHz

903.6985(25) 903.0707(25) 902.6557(25) 902.3075(25) 902.0555(27) 904.3680(25) 904.8003(27)

0.0428(36) 0.0443(36) 0.0660(36) 0.0579(36) 0.0485(35) 0.0447(36) 0.0612(44)

899.4880(29) 898.8741(27)

0.0369(42) 0.0330(42)

900.1421(26)

0.0343(37)

a Vibrational states labeled by v are presumably the low lying CF3 torsional states, m  110 cm1, in which the three CF3 units twist in the same direction. b Vibrational states labeled by v 0 are presumably the low lying CF3 torsional states, m  180 cm1, in which two of the three CF3 units twist in opposite directions and the third is stationary.

Fig. 3. Geometric parameters in n-C4F10.

eter allowed to vary in the fit. If we compare the dihedral angle we determined and our experimentally determined rotational constants to Albinsson and Michl’s calculated values, agreement between the two only exists for the case of the gauche conformer. Table 9 displays the optimized geometrical parameters of the transoid, ortho, and gauche conformers of n-C4F10 along with their energies, dipole moments, and rotational constants calculated at the CCSD/cc-pVTZ level. The coordinates of all the atoms of the three conformers are available as Supplementary material [29]. Except for the dihedral angle (165.56, 96.06, and 54.30 for transoid, ortho, and gauche, respectively) the geometrical parameters are very similar for all three conformers. The calculated values are consistent with those obtained previously at the MP2/631G* level [11], and presumably represent the most reliable theoretical values available to date. The predicted relative energies are transoid (0.0 kcal/mol) < gauche (0.74 kcal/ mol) < ortho (1.91 kcal/mol). These values are in between those obtained at the MP2/6-31G* level[11] (0.68 and 1.63 kcal/mol for gauche and ortho, respectively) and those calculated from single-point MP2/6-311G* calculations at the MP2/6-31G* geometries, which yielded 0.85 and 2.12 kcal/mol, respectively [11]. All those energies are small enough to expect measurable quantities of all conformers to be present at equilibrium at room temperature.

Table 8 Results of fitting geometric parameters in n-C4F10a,b ˚) ˚) r1 (A r2 (A h ()

x ()

DA (MHz)

DB (MHz)

DC (MHz)

[1.5455] 1.5654 1.5877 1.5392 [1.5455] 1.5604 1.5605 1.5588 [1.5455] [1.5455]

54.806 56.486 58.601 [54.301] 52.2 54.7 [54.301] [54.301] [54.301] 52.7

0 0 0 0 10.014 1.93 1.592 1.786 0.285 4.326

0 0 0 0 0.88 0.4 0.694 0.584 0.215 0.643

0 0 0 0 0.045 0.417 0.53 0.496 0.18 0.56

a b

1.6075 [1.5535] 1.4969 1.6254 [1.5535] [1.5535] [1.5535] 1.5575 1.6046 1.5818

116.494 117.17 [117.703] 116.241 118.7 [117.703] 117.77 [117.703] 116.68 [117.703]

Values given in square brackets are those calculated for the gauche configuration at the CCSD/cc-pVTZ level of theory. The values of the C–F bond lengths and FCF angles are held to those given in the ab initio calculation.

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Table 9 Computed properties of transoid, ortho, and gauche conformers of nC4F10 at the CCSD/cc-pVTZ level Total energy (a.u.) Relative energy (kcal/mol) ˚) r1 (A ˚) r2 (A h () x () Dipole moment (D)

Transoid

Ortho

Gauche

1149.41046 0.0 1.5489 1.5491 114.20 165.56 0.0460

1149.40742 1.91 1.5492 1.5618 115.55 96.06 0.1104

1149.40928 0.74 1.5455 1.5535 117.70 54.30 0.2799

1139.4 562.8 541.4

1065.5 622.3 556.3

Rotational constants (MHz) A 1273.3 B 520.0 C 501.6

Fig. 4. Labeling convention of iso-C4F10.

4.2. Perfluoro-iso-butane The iso-C4F10 monomer is an oblate symmetric top. Previous work has been chiefly restricted to theoretical calculations. Cooper et al. [30] performed SCF 3-21G+dF calculations on primary, secondary, tertiary, and quaternary fluorocarbons with iso-C4F10 being included in their study. They determined the C 0 –C bond length as well as the bond lengths between C 0 –F 0 and C–F, where these labels are explained in Fig. 4. The transitions were least-squares fitted to two spectroscopic constants, Bo and DJ, for the all 12C and for the two 13 C isotopomers. Once again Schwendeman’s STRFTQ program [27] was utilized to fit the rotational constants to the F–C and C–C 0 bond lengths as well as the F–C–C 0 bond angle. We used the (A + B)/2 constant for each of the all 12 C and central 13C isotopomers and the A and B rotational constants for the adjacent 13C isotopomer in the leastsquares fit to the molecular structure. Errors in the rotational constants (observed  calculated) are within 15 kHz and the resulting structural parameters are: F–C bond length, ˚ ; C–C 0 bond length, 1.534(5) A ˚ ; F–C–C 0 , 1.455(27) A 105.9(4). These values agree with the calculations by Cooper et al., [30] and by Petersson [31]. 4.3. Tris(trifluoromethyl)methane, (CF3)3CH and (CF3)CD The structure of tris(trifluoromethyl)methane was not well determined from the two experimental rotational

constants. The structure was calculated by at the MP2/6˚ and 311++G(d,p) level to be r(C3–C4) = 1.534 A \C2C3C4 = 111.8, where C3 is the central and C2 and C4 are the off-axis carbon atoms. The values calculated for ˚, (CF3)3CH can be compared with r(C3–C4) = 1.549 A \C2C3C4 = 106.47 for (CF3)3C–C„CH [32]. 5. Dipole moment measurements 5.1. Dipole moment measurements of iso-C4F10 The dipole moment of the iso-C4F10 monomer has been measured using the first order Stark data from the J = 7–6 and 6–5 rotational transitions. The applied electric field was calibrated using measurements of the Stark effect for the 1–0 transition of OCS and its dipole moment of 0.7124 D [33]. Components for DM = 0 and DM = ±1 were observed for the 7–6 transition while only DM = 0 components were observed for the 6–5 transition. The Stark components for both transitions have been simulated in order to aid in the assignment of the experimentally observed M levels and this results in a dipole moment of 0.0338 (8) D for the iso-C4F10 molecule. This agrees with the 0.0336 D dipole moment predicted by an ab initio calculation at a RHF/3-21G level of theory [31]. 5.2. Dipole moment measurements of (CF3)3CH The dipole moment of (CF3)3CH was determined from the first-order Stark effect of the J = 17–16 to J = 20–19 transitions. Since the Stark lobes of such high transitions were not resolved, the dipole moment was measured from the Stark shift of the peak of the unresolved components [34]. The peaks were calculated by summing the Lorentzian line shape function for each component. The dipole moment was determined to be 1.69(9) D with the error signifying one standard deviation. 6. Discussion and summary The pure rotation spectrum of the gauche n-C4F10 conformer has been detected for the first time. No internal rotation splittings have been observed. The signal intensities are weak because of the small, 0.3 D, dipole moment of the molecule, and because of the small population of the gauche conformation of n-C4F10 which has an energy 260 cm1 above that of the transoid conformation. The fact that we observe the gauche form at all implies that the relative populations of the transoid and gauche conformations in the jet are determined by something approaching a room temperature distribution of these two structures. This implies that the energy barrier separating the gauche and transoid forms is greater than 350 cm1 [35]. The ground state spectrum of n-C4F10 could be easily assigned to a centrifugal-distorted rigid rotor of the gauche conformer. This was originally done by comparing the

M.R. Munrow et al. / Journal of Molecular Spectroscopy 242 (2007) 129–138

experimental rotational constants and our estimate of the CCCC dihedral angle to those calculated by Albinsson and Michl [11]. This gave reasonable qualitative agreement within 9 MHz for the rotational constants and a few degrees for the dihedral angle. The coupled cluster CCSD/cc-pVTZ calculation presented here gives a better agreement between experiment and theory. As Table 8 shows, we calculate the dihedral angle of gauche n-C4F10 by alternately holding constant the bond lengths and bond angles at the coupled cluster values and then least-squares fitting the remaining structural parameters of n-C4F10 to the measured rotational constants. This gives a dihedral angle that varies between 52.2 and 58.8. We take the average of these fitted values, 54.9 ± 2.4, to be our best measure of the dihedral angle. This is to be compared to the coupled cluster ab initio result of 54.30for the C–C– C–C dihedral angle for the gauche conformer of n-C4F10. The coupled cluster values for the rotational constants, A, B, and C, of gauche n-C4F10 are larger than the experimental values by 7.6, 4.7, and 4.1 MHz (0.72%, 0.76%, and 0.75% errors), respectively. Extensive searches have been undertaken to observe the ‘‘ortho’’ conformer which were unsuccessful. We were able to observe only the gauche n-C4F10 conformer in our spectrometer. Several reasons for this are possible: (1) The actual dipole moment of the ortho conformer may be an order of magnitude smaller than the 0.1 D predicted dipole moment and thus the very weak transitions were hidden in the noise and missed. (2) Since the temperature of the molecular beam is only a few degrees Kelvin and the rotational barrier between the ortho and gauche conformers is expected to be very small, the ortho n-C4F10 molecules may be ‘‘cooling’’ down to the lower energy gauche conformer as a result of the supersonic expansion [35]. The very small amounts of the ortho conformer observed in the matrix experiments suggest that this may be the case. (3) Although the ortho conformer is a low energy minimum in a nitrogen matrix, it may, in fact, be a transition state in the gas phase. At the MP2/6-31G* level of theory Albinsson and Michl calculate the x = 95, ortho, conformation to be 0.95 kcal/mol above the x = 54, gauche, conformation [11]. At the same level of theory, Dixon calculated the x = 119 transition state conformation to be a similar 1.11 kcal/mol above his x = 63, gauche, conformation [10]. However, the present coupled cluster calculations support the existence of the ortho conformer of perfluoro-n-butane as a distinct gasphase species, albeit undoubtedly protected by only minimal potential energy barriers. Also the observed value of the dihedral angle in the gauche conformation, significantly smaller than 60, is compatible with the notion that 1,4 steric interactions in perfluoro-n-butane have split the single gauche minimum of n-butane into two, one at a smaller and one at a larger dihedral angle. In addition to this investigation on the conformations of perfluoro-n-butane, we report the measured structure of perfluoro-iso-butane, and the dipole moments of perfluoro-iso-butane and tris(trifluoromethyl)methane.

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