- Email: [email protected]
Microwave spectroscopy and curious molecular dynamics of ethyl trifluoroacetate
Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Microwave spectroscopy and curious molecular dynamics of ethyl trifluoroacetate Robert K. Bohn a,⇑, John A. Montgomery Jr b, H. Harvey Michels b, Christian Acharte a a b
Dept. of Chemistry, Univ. of Connecticut, Storrs, CT 06269-3060, United States Dept. of Physics, Univ. of Connecticut, Storrs, CT 06269-3046, United States
a r t i c l e
i n f o
Article history: Received 13 January 2017 In revised form 10 March 2017 Accepted 15 March 2017 Available online xxxx
a b s t r a c t The first ethyl ester whose structure was determined by microwave spectroscopy is ethyl formate. It exists in two conformations. In the 1970s, that study was used as a model to determine the structures of other ethyl esters, ethyl cyanoformate, chloroformate, and trifluoroacetate. They display the same conformations as ethyl formate. But under the experimental conditions used, Stark modulation with a maximum electric field, static low pressure gas, rapid sweeping, and long detector time constants, each of those esters displays bands of an additional third species. A careful, high resolution study of ethyl cyanoformate only observed two conformers. A model has been proposed that the third species derives from a dense array of torsionally excited states with broadened transitions due to short lifetimes. The present study of ethyl trifluoroacetate in a pulsed jet Fourier Transform spectrometer is intended to clarify the earlier results. Two conformers are observed including all their monosubstituted 13C and 18O isotopologs. In a pulsed jet Fourier Transform spectrometer using argon as the carrier gas, only one conformer is observed. Switching to helium as the carrier gas, another, higher energy conformer is also observed. Ó 2017 Published by Elsevier Inc.
1. Introduction A microwave study of ethyl formate by Riveros and Wilson [1] characterized the two conformers shown in Fig. 1. Similar conformations have been characterized in other ethyl esters [2], the more stable conformer having s1 (O@CAOAC) in a syn planar configuration (s1 = 0°) with s2 (CAOACAC) = 180° (anti) and s2 90° (gauche) in the less stable conformer. In a low resolution study [2] with experiments carried out on a static gas at room and dry ice temperatures in a Stark modulated spectrometer, we expected to observe bands of two conformers (syn-anti and syn-gauche), as had been seen in ethyl formate, and were surprised to observe three a-type, R-branch band series suggesting three conformers, all nearly prolate tops. In order to understand this puzzle of observing spectra of three forms rather than two as in ethyl formate, ethyl cyanoformate was selected and characterized in a high resolution study [3] with the same Stark modulated spectrometer, Hewlett-Packard’s HP8460A, used earlier [2]. Spectra of the syn-anti and syn-gauche
⇑ Corresponding author.
forms were identified and characterized but no spectral trace of the third species was observed. Nancy True of UCDavis recognized that the third form can be understood and characterized by short lifetimes of torsionally excited states [4]. The short lifetimes broaden the rotational transitions rendering them invisible or merely a broad background when scanned slowly under conventional high resolution conditions in a Stark modulated spectrometer. But when the spectra are scanned rapidly with a large electric field for the Stark modulation, the broadened and overlapping transitions appear and are the source of the unexpected third set of spectral bands. The puzzling results with ethyl cyanoformate [3] encouraged us to study another example, ethyl trifluoroacetate, using a pulsed jet Fourier Transform spectrometer [5,6] which cools the sample to a rotational temperature near 2 K. The pulsed jet FT spectrum was easily assigned to the syn-anti conformer but showed nothing else. The syn-gauche and exchange averaged species seen earlier did not appear. It seemed as if nature was enjoying our conundrum that three a-type, R-branch band series are observable in the static gas of ethyl esters at room (300 K) and dry ice (220 K) temperatures, more species than expected, but only one conformation in the pulsed jet FT spectrometer, fewer than expected. These apparently contradictory observations deserve explanation.
http://dx.doi.org/10.1016/j.jms.2017.03.009 0022-2852/Ó 2017 Published by Elsevier Inc.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
2
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
H
H
O
C
C
H
H 3C
O
CH3
H
C
C
O
formers is not changed very much in the expansion. The assignment of a-type R-branch transitions of the syn-gauche form was straightforward and lines with a- and b-type selection rules were observed. A search for c-type lines for the C1 species failed. The assigned spectrum and derived parameters are given in Tables 1 and 2.
O
H
H
Ethyl Formate (syn-gauche)
Ethyl Formate (syn-anti) H O
C C
3. Results
H H3 C
CH3 O
O
H
C C
F3C
H
O
F3C
Ethyl Trifluoroacetate (syn-anti)
Ethyl Trifluoroacetate (syn-gauche)
Fig. 1. Conformers of ethyl formate and ethyl trifluoroacetate.
2. Experimental A sample of ethyl trifluoroacetate was purchased from SigmaAldrich and used directly. A 7 L tank was evacuated, 1 g (0.007 mol) of sample distilled into it, and then filled to 4 atm with Ar as the carrier gas to form a 0.6% mixture. Pulses of this mixture at 1 atm were fed at 5 Hz into the pulsed jet Fourier transform microwave spectrometer of the Southern New England Microwave Consortium [6]. Five microwave pulses were recorded for each gas pulse. Rotational transitions were observed between 7.6 and 16.2 GHz but the complete spectral range was not scanned. The rotational temperature of the expanded gas is estimated to be 2 K. Measured rotational frequencies are estimated to be accurate to <2 kHz. The Illinois spectroscopy group studied vibrational relaxation in a pulsed jet expansion and noted that a higher energy conformer will relax to a more stable form in the gas expansion if the barrier between them is less than 1 kcal/mol [7]. A second sample of ethyl trifluoroacetate was prepared using He rather than Ar as the carrier gas and spectra of both the syn-anti form with Cs symmetry and the syn-gauche conformer, C1 symmetry, were observed. Apparently the CAOACAC torsional barrier between the syn-anti and syn-gauche conformers is such that collisions with Ar in the pulsed jet expansion relax the syn-gauche species present in the room temperature gas to the lower energy syn-anti but collisions with He are less effective and the room temperature distribution of con-
The early low resolution microwave study [2] reported B + C values of 1842, 1904, and 1947 MHz for the species present in ethyl trifluoroacetate. Since the molecule is a near prolate top in both conformers with large dipole moment components along the a principal axis, intense a-type R-branch transitions cluster about the frequencies predicted by (J + 1) (B + C). Assigning and fitting the 5 4 transitions, a host of other lines were predicted and measured between 8.9–9.5 GHz and readily assigned to the symmetric Cs conformer, the 1842 MHz species. Assigning those transitions allowed accurate predictions of many others and a representative set of a- and b-type transitions were measured, assigned, and fit to experimental accuracy using the rotational constants and four centrifugal distortion constants. The values of B + C from the present pulsed jet FT spectra are 1839.246 and 1949.072 MHz compared to 1842 and 1947 MHz from the low resolution study. The values of Ray’s asymmetry parameters are 0.9001 and 0.9473 for the Cs syn-anti and the C1 syn-gauche species, respectively, close enough to the prolate symmetric top value of 1.0 to produce overlapping a-type, R-branch bands from any of the forms. The intensities of the transitions of the parent compound are sufficient so that transitions of all of the singly substituted 13C and 18O isotopologs could be observed and measured. Atom labels are shown in Fig. 2. Refining the rotational constants and four centrifugal distortion constants produced fits to the frequencies within estimated experimental uncertainties. Between 9 and 20 lines were measured for each isotopolog and assigned to each of the six singly substituted 13C and 18O isotopologs of both conformers. The rotational constants were used to determine the moments of inertia and second moments. The second moments listed in Table 1 establish that the species labeled Cs has a plane of symmetry because the Pcc values of the parent and its six isotopologs are all identical to within experimental error, 47.982 to 47.996 lÅ2. This will only happen if each substituted atom has a zero value for its c principal axis coordinate. That insight is not easily revealed
Table 1 Rotational constants, moments of inertia, second moments, and centrifugal distortion constants for the CS conformer of ethyl trifluoroacetate and all its monosubstituted 13C and 18 O isotopologs. Parameter A/MHz B/MHz C/MHz Ia/lÅ2 Ib/lÅ2 Ic/lÅ2 Paa/lÅ2 Pbb/lÅ2 Pcc/lÅ2 DJ/kHz DJK/kHz d1/kHz d2/kHz kappa No. Lines StdDev/kHz a b
CS (Parent)
13
13
13
3094.174(9) 975.51164(7) 863.28018(7) 163.3324(5) 518.06558(4) 585.41713(4) 470.0752(3) 115.3420(3) 47.9904(3) [0.0776]b [0.965] [0.0071] [0.01813]
3098.236(29) 964.5641(2) 855.0079(2) 163.1183(15) 523.94551(13) 591.08109(14) 475.9541(10) 115.1269(10) 47.9914(10) [0.0776] [0.965] [0.0071] [0.01813]
16 1.1
18 3.2
C8
a
3100.9681(5) 975.47072(12) 863.77508(13) 16 2.97459(3) 518.08731(6) 585.08171(8) 470.0972(6) 114.9845(6) 47.9901(6) 0.0776(10) 0.965(5) 0.0071(4) 0.01813(10) 0.9001 39 2.3
C6
13
18
3091.301(28) 953.6762(2) 845.9190(2) 163.4842(15) 529.92728(11) 597.43191(14) 481.9384(8) 115.4944(8) 47.9903(8) [0.0776] [0.965] [0.0071] [0.01813]
3100.506(13) 972.6756(1) 861.5331(1) 162.9989(7) 519.57611(6) 586.60430(6) 471.5908(4) 115.0135(4) 47.9853(4) [0.0776] [0.965] [0.0071] [0.01813]
2983.538(18) 975.40718(12) 854.33512(12) 169.3892(10) 518.12106(7) 591.54656(8) 470.1392(5) 121.4073(5) 47.9818(5) [0.0776] [0.965] [0.0071] [0.01813]
3098.716(35) 970.9178(3) 860.0460(2) 163.0930(18) 520.51700(17) 587.61855(13) 472.5213(9) 115.0973(9) 47.9957(9) [0.0776] [0.965] [0.0071] [0.01813]
20 3.2
17 1.4
12 1.6
9 2.4
C7
C9
O5
18
O4
Numbers within parentheses are single standard errors in units of the last printed digit. Centrifugal distortion constants determined for the parent species and displayed in square brackets were held constant for all the minor isotopologs.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
3
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
Table 2 Rotational constants, moments of inertia, second moments, and centrifugal distortion constants for the C1 conformer of ethyl trifluoroacetate and all its monosubstituted 13C and 18 O isotopologs. Parameter A/MHz B/MHz C/MHz Ia/lÅ2 Ib/lÅ2 Ic/lÅ2 Paa/lÅ2 Pbb/lÅ2 Pcc/lÅ2 DJ/kHz DJK/kHz Dk/kHz d2/kHz kappa No. Lines StdDev/kHz a b
13
C1 (Parent) a
3146.5687(13) 1003.52076(12) 945.55069(11) 160.61274(7) 503.60593(6) 534.48113(6) 438.73716(6) 95.74397(6) 64.86877(6) 0.1003(8) 0.4297(17) 0.85(23) 0.0053(6) 0.9473 39 2.
C8
13
C6
13
13
C7
C9
18
18
O5
O4
3142.672(19) 1003.42401(7) 945.22542(7) 160.8119(10) 503.65451(4) 534.66506(4) 438.7538(11) 95.9112(11) 64.900711) [0.1003]b [0.430] [0.85] [0.0053]
3138.319(35) 991.35959(12) 935.15192(13) 161.0349(18) 509.78375(6) 540.42449(6) 444.5866(9) 95.8378(9) 65.1971(9) [0.1003] [0.430] [0.85] [0.0053]
3136.081(27) 983.40015(5) 928.55339(6) 161.1499(14) 513.90983(3) 544.26489(4) 448.5124(7) 95.7525(7) 65.3974(7) [0.1003] [0.430] [0.85] [0.0053]
3146.623(46) 1000.60918(16) 942.95904(18) 160.6099(24) 505.07133(8) 535.95011(10) 440.2058(12) 95.7444(12) 64.8656(12) [0.1003] [0.430] [0.85] [0.0053]
3051.660(31) 1002.40882(12) 936.02199(8) 165.6079(17) 504.16456(6) 539.92215(5) 439.2394(8) 100.6828(8) 64.9252(8) [0.1003] [0.430] [0.85] [0.0053]
3126.520(44) 998.97756(8) 940.64227(8) 161.4266(23) 505.89625(4) 537.27014(4) 440.7619(11) 96.5083(11) 65.1343(11) [0.0776] [0.965] [0.0071] [0.01813]
20 1.1
18 1.8
16 0.7
18 2.4
10 1
12 1
Numbers within parentheses are single standard errors in units of the last printed digit. Centrifugal distortion constants determined for the parent species and displayed in square brackets were held constant for all the minor isotopologs.
Fig. 2. Ethyl trifluoroacetate in the Cs (syn-anti) conformation. Atom labels are shown.
Table 3 Selected bond lengths, bond angles, and dihedral angles for ethyl trifluoroacetate PBE0/aug-cc-pVTZ level of theory (Raw) for the Cs and C1 conformers of ethyl trifluoroacetate. The ‘‘scaled” columns are from the PBE0 second moment values multiplied by the appropriate scale factor. Syn-anti (Cs) PBE0
Syn-gauche (C1) PBE0
Raw
Scaled
Raw
Scaled
Bond Length/Å
C8@O5 C8AO4 O4AC6 C6AC7 C8AC9 C9AF1
1.1946 1.3186 1.4431 1.5031 1.5465 1.3219
1.1952 1.3201 1.4452 1.5047 1.5486 1.3234
1.1948 1.3189 1.4442 1.5075 1.5469 1.3220
1.1948 1.3185 1.4428 1.5130 1.5455 1.3215
Bond Angle/°
O5@C8AC9 O5@C8AO4 C8AO4AC6 O4AC6AC7
123.04 127.34 115.80 107.74
123.01 127.31 115.86 107.80
122.85 127.73 116.55 111.10
122.85 127.78 116.51 110.89
Dihedral Angle/°
O5@C8AO4AC6 C8AO4AC6AC7
0.0 180.0
0.0 180.0
0.78 85.62
0.79 85.58
Cs (syn-anti) scale factors: Sfa = 1.0016353, Sfb = 1.0005015, Sfc = 0.9951195. C1 (syn-gauche) scale factors: Sfa = 0.9987970, Sfb = 1.0000390, Sfc = 1.0051790.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
4
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
by the rotational constants or moments of inertia. The spectral constants for the asymmetric C1 form are all different from each other and reveal no simple feature except that it has no plane of symmetry. Using PBE0/aug-cc-pVTZ density functionals [8], structures for both conformers were calculated. Using 2nd moments [9] those structures were scaled to give models which exactly reproduce the observed rotational constants. Table 3 lists selected structural parameters predicted by the density functional calculation and scaled to fit the observed rotational constants. Since the rotational constants have been determined to up to 7 significant figures, scaled parameters need to be similarly accurate. But the specific coordinate values will differ for different starting models. Table 3 lists bond lengths and angles to 5 significant digits, a compromise between the requirements of the scaling method structure and the ambiguity caused by using a different starting model. It appears that reported bond lengths are accurate to thousandths of an Å and angles to hundredths of a degree. There is enough spectral data to determine an rs structure using Kraitchman’s method [10] but we believe the scaling method is more accurate. Kraitchman’s method uses differences between second moments of the parent and isotopolog to determine coordinates of the substituted atom in the inertial axis system of the parent species. But differences between seven significant digit parameters have fewer significant digits compromising the accuracy of the rs structure. 4. Conclusions Ethyl trifluoroacetate displays rotational spectra from two stable conformations consistent with published studies of other ethyl esters. Bands of a third species, interpreted as a collection
of vibrationally excited states with short lifetimes [4], require experimental conditions no longer routinely available. The potential barrier between the syn-anti and syn-gauche conformers allows relaxation of the higher energy conformer (syn-gauche) to the lower (syn-anti) when Ar is the carrier gas in the pulsed jet expansion but that relaxation does not occur with He. Acknowledgments Nancy True of UCDavis and Stewart Novick and Pete Pringle of Wesleyan University provided thoughtful questions and discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2017.03.009. References [1] J.M. Riveros, E.B. Wilson Jr., J. Chem. Phys. 46 (1967) 4605. [2] N.S. True, R.K. Bohn, Low resolution microwave spectroscopy III. Three conformers in ethyl trifluoroacetate, ethyl chloroformate and ethyl cyanoformate, J. Am. Chem. Soc. 98 (1976) 1188–1194. [3] R.D. Suenram, N.S. True, R.K. Bohn, J. Mol. Spec. 69 (1978) 435–440. [4] N.S. True, Conformer lifetimes of ethyl cyanoformate from exchange averaged rotational spectra, J. Phys. Chem. A 113 (2009) 6936–6946. [5] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. [6] A.R. Hight-Walker, Q. Lou, R.K. Bohn, S.E. Novick, J. Mol. Struct. 346 (1995) 187. [7] R.S. Ruoff, T.D. Klots, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [8] M. Frisch et al., Gaussian 98, Rev. A.6, Gaussian, Inc., Pittsburgh, PA. [9] R.K. Bohn, J.A. Montgomery Jr., H.H. Michels, J.A. Fournier, Second moments and rotational spectroscopy, J. Molec. Spectrosc. 325 (2016) 42–49. [10] J. Kraitchman, Am. J. Phys. 21 (1953) 17.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Microwave spectroscopy and curious molecular dynamics of ethyl trifluoroacetate Robert K. Bohn a,⇑, John A. Montgomery Jr b, H. Harvey Michels b, Christian Acharte a a b
Dept. of Chemistry, Univ. of Connecticut, Storrs, CT 06269-3060, United States Dept. of Physics, Univ. of Connecticut, Storrs, CT 06269-3046, United States
a r t i c l e
i n f o
Article history: Received 13 January 2017 In revised form 10 March 2017 Accepted 15 March 2017 Available online xxxx
a b s t r a c t The first ethyl ester whose structure was determined by microwave spectroscopy is ethyl formate. It exists in two conformations. In the 1970s, that study was used as a model to determine the structures of other ethyl esters, ethyl cyanoformate, chloroformate, and trifluoroacetate. They display the same conformations as ethyl formate. But under the experimental conditions used, Stark modulation with a maximum electric field, static low pressure gas, rapid sweeping, and long detector time constants, each of those esters displays bands of an additional third species. A careful, high resolution study of ethyl cyanoformate only observed two conformers. A model has been proposed that the third species derives from a dense array of torsionally excited states with broadened transitions due to short lifetimes. The present study of ethyl trifluoroacetate in a pulsed jet Fourier Transform spectrometer is intended to clarify the earlier results. Two conformers are observed including all their monosubstituted 13C and 18O isotopologs. In a pulsed jet Fourier Transform spectrometer using argon as the carrier gas, only one conformer is observed. Switching to helium as the carrier gas, another, higher energy conformer is also observed. Ó 2017 Published by Elsevier Inc.
1. Introduction A microwave study of ethyl formate by Riveros and Wilson [1] characterized the two conformers shown in Fig. 1. Similar conformations have been characterized in other ethyl esters [2], the more stable conformer having s1 (O@CAOAC) in a syn planar configuration (s1 = 0°) with s2 (CAOACAC) = 180° (anti) and s2 90° (gauche) in the less stable conformer. In a low resolution study [2] with experiments carried out on a static gas at room and dry ice temperatures in a Stark modulated spectrometer, we expected to observe bands of two conformers (syn-anti and syn-gauche), as had been seen in ethyl formate, and were surprised to observe three a-type, R-branch band series suggesting three conformers, all nearly prolate tops. In order to understand this puzzle of observing spectra of three forms rather than two as in ethyl formate, ethyl cyanoformate was selected and characterized in a high resolution study [3] with the same Stark modulated spectrometer, Hewlett-Packard’s HP8460A, used earlier [2]. Spectra of the syn-anti and syn-gauche
⇑ Corresponding author.
forms were identified and characterized but no spectral trace of the third species was observed. Nancy True of UCDavis recognized that the third form can be understood and characterized by short lifetimes of torsionally excited states [4]. The short lifetimes broaden the rotational transitions rendering them invisible or merely a broad background when scanned slowly under conventional high resolution conditions in a Stark modulated spectrometer. But when the spectra are scanned rapidly with a large electric field for the Stark modulation, the broadened and overlapping transitions appear and are the source of the unexpected third set of spectral bands. The puzzling results with ethyl cyanoformate [3] encouraged us to study another example, ethyl trifluoroacetate, using a pulsed jet Fourier Transform spectrometer [5,6] which cools the sample to a rotational temperature near 2 K. The pulsed jet FT spectrum was easily assigned to the syn-anti conformer but showed nothing else. The syn-gauche and exchange averaged species seen earlier did not appear. It seemed as if nature was enjoying our conundrum that three a-type, R-branch band series are observable in the static gas of ethyl esters at room (300 K) and dry ice (220 K) temperatures, more species than expected, but only one conformation in the pulsed jet FT spectrometer, fewer than expected. These apparently contradictory observations deserve explanation.
http://dx.doi.org/10.1016/j.jms.2017.03.009 0022-2852/Ó 2017 Published by Elsevier Inc.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
2
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
H
H
O
C
C
H
H 3C
O
CH3
H
C
C
O
formers is not changed very much in the expansion. The assignment of a-type R-branch transitions of the syn-gauche form was straightforward and lines with a- and b-type selection rules were observed. A search for c-type lines for the C1 species failed. The assigned spectrum and derived parameters are given in Tables 1 and 2.
O
H
H
Ethyl Formate (syn-gauche)
Ethyl Formate (syn-anti) H O
C C
3. Results
H H3 C
CH3 O
O
H
C C
F3C
H
O
F3C
Ethyl Trifluoroacetate (syn-anti)
Ethyl Trifluoroacetate (syn-gauche)
Fig. 1. Conformers of ethyl formate and ethyl trifluoroacetate.
2. Experimental A sample of ethyl trifluoroacetate was purchased from SigmaAldrich and used directly. A 7 L tank was evacuated, 1 g (0.007 mol) of sample distilled into it, and then filled to 4 atm with Ar as the carrier gas to form a 0.6% mixture. Pulses of this mixture at 1 atm were fed at 5 Hz into the pulsed jet Fourier transform microwave spectrometer of the Southern New England Microwave Consortium [6]. Five microwave pulses were recorded for each gas pulse. Rotational transitions were observed between 7.6 and 16.2 GHz but the complete spectral range was not scanned. The rotational temperature of the expanded gas is estimated to be 2 K. Measured rotational frequencies are estimated to be accurate to <2 kHz. The Illinois spectroscopy group studied vibrational relaxation in a pulsed jet expansion and noted that a higher energy conformer will relax to a more stable form in the gas expansion if the barrier between them is less than 1 kcal/mol [7]. A second sample of ethyl trifluoroacetate was prepared using He rather than Ar as the carrier gas and spectra of both the syn-anti form with Cs symmetry and the syn-gauche conformer, C1 symmetry, were observed. Apparently the CAOACAC torsional barrier between the syn-anti and syn-gauche conformers is such that collisions with Ar in the pulsed jet expansion relax the syn-gauche species present in the room temperature gas to the lower energy syn-anti but collisions with He are less effective and the room temperature distribution of con-
The early low resolution microwave study [2] reported B + C values of 1842, 1904, and 1947 MHz for the species present in ethyl trifluoroacetate. Since the molecule is a near prolate top in both conformers with large dipole moment components along the a principal axis, intense a-type R-branch transitions cluster about the frequencies predicted by (J + 1) (B + C). Assigning and fitting the 5 4 transitions, a host of other lines were predicted and measured between 8.9–9.5 GHz and readily assigned to the symmetric Cs conformer, the 1842 MHz species. Assigning those transitions allowed accurate predictions of many others and a representative set of a- and b-type transitions were measured, assigned, and fit to experimental accuracy using the rotational constants and four centrifugal distortion constants. The values of B + C from the present pulsed jet FT spectra are 1839.246 and 1949.072 MHz compared to 1842 and 1947 MHz from the low resolution study. The values of Ray’s asymmetry parameters are 0.9001 and 0.9473 for the Cs syn-anti and the C1 syn-gauche species, respectively, close enough to the prolate symmetric top value of 1.0 to produce overlapping a-type, R-branch bands from any of the forms. The intensities of the transitions of the parent compound are sufficient so that transitions of all of the singly substituted 13C and 18O isotopologs could be observed and measured. Atom labels are shown in Fig. 2. Refining the rotational constants and four centrifugal distortion constants produced fits to the frequencies within estimated experimental uncertainties. Between 9 and 20 lines were measured for each isotopolog and assigned to each of the six singly substituted 13C and 18O isotopologs of both conformers. The rotational constants were used to determine the moments of inertia and second moments. The second moments listed in Table 1 establish that the species labeled Cs has a plane of symmetry because the Pcc values of the parent and its six isotopologs are all identical to within experimental error, 47.982 to 47.996 lÅ2. This will only happen if each substituted atom has a zero value for its c principal axis coordinate. That insight is not easily revealed
Table 1 Rotational constants, moments of inertia, second moments, and centrifugal distortion constants for the CS conformer of ethyl trifluoroacetate and all its monosubstituted 13C and 18 O isotopologs. Parameter A/MHz B/MHz C/MHz Ia/lÅ2 Ib/lÅ2 Ic/lÅ2 Paa/lÅ2 Pbb/lÅ2 Pcc/lÅ2 DJ/kHz DJK/kHz d1/kHz d2/kHz kappa No. Lines StdDev/kHz a b
CS (Parent)
13
13
13
3094.174(9) 975.51164(7) 863.28018(7) 163.3324(5) 518.06558(4) 585.41713(4) 470.0752(3) 115.3420(3) 47.9904(3) [0.0776]b [0.965] [0.0071] [0.01813]
3098.236(29) 964.5641(2) 855.0079(2) 163.1183(15) 523.94551(13) 591.08109(14) 475.9541(10) 115.1269(10) 47.9914(10) [0.0776] [0.965] [0.0071] [0.01813]
16 1.1
18 3.2
C8
a
3100.9681(5) 975.47072(12) 863.77508(13) 16 2.97459(3) 518.08731(6) 585.08171(8) 470.0972(6) 114.9845(6) 47.9901(6) 0.0776(10) 0.965(5) 0.0071(4) 0.01813(10) 0.9001 39 2.3
C6
13
18
3091.301(28) 953.6762(2) 845.9190(2) 163.4842(15) 529.92728(11) 597.43191(14) 481.9384(8) 115.4944(8) 47.9903(8) [0.0776] [0.965] [0.0071] [0.01813]
3100.506(13) 972.6756(1) 861.5331(1) 162.9989(7) 519.57611(6) 586.60430(6) 471.5908(4) 115.0135(4) 47.9853(4) [0.0776] [0.965] [0.0071] [0.01813]
2983.538(18) 975.40718(12) 854.33512(12) 169.3892(10) 518.12106(7) 591.54656(8) 470.1392(5) 121.4073(5) 47.9818(5) [0.0776] [0.965] [0.0071] [0.01813]
3098.716(35) 970.9178(3) 860.0460(2) 163.0930(18) 520.51700(17) 587.61855(13) 472.5213(9) 115.0973(9) 47.9957(9) [0.0776] [0.965] [0.0071] [0.01813]
20 3.2
17 1.4
12 1.6
9 2.4
C7
C9
O5
18
O4
Numbers within parentheses are single standard errors in units of the last printed digit. Centrifugal distortion constants determined for the parent species and displayed in square brackets were held constant for all the minor isotopologs.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
3
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
Table 2 Rotational constants, moments of inertia, second moments, and centrifugal distortion constants for the C1 conformer of ethyl trifluoroacetate and all its monosubstituted 13C and 18 O isotopologs. Parameter A/MHz B/MHz C/MHz Ia/lÅ2 Ib/lÅ2 Ic/lÅ2 Paa/lÅ2 Pbb/lÅ2 Pcc/lÅ2 DJ/kHz DJK/kHz Dk/kHz d2/kHz kappa No. Lines StdDev/kHz a b
13
C1 (Parent) a
3146.5687(13) 1003.52076(12) 945.55069(11) 160.61274(7) 503.60593(6) 534.48113(6) 438.73716(6) 95.74397(6) 64.86877(6) 0.1003(8) 0.4297(17) 0.85(23) 0.0053(6) 0.9473 39 2.
C8
13
C6
13
13
C7
C9
18
18
O5
O4
3142.672(19) 1003.42401(7) 945.22542(7) 160.8119(10) 503.65451(4) 534.66506(4) 438.7538(11) 95.9112(11) 64.900711) [0.1003]b [0.430] [0.85] [0.0053]
3138.319(35) 991.35959(12) 935.15192(13) 161.0349(18) 509.78375(6) 540.42449(6) 444.5866(9) 95.8378(9) 65.1971(9) [0.1003] [0.430] [0.85] [0.0053]
3136.081(27) 983.40015(5) 928.55339(6) 161.1499(14) 513.90983(3) 544.26489(4) 448.5124(7) 95.7525(7) 65.3974(7) [0.1003] [0.430] [0.85] [0.0053]
3146.623(46) 1000.60918(16) 942.95904(18) 160.6099(24) 505.07133(8) 535.95011(10) 440.2058(12) 95.7444(12) 64.8656(12) [0.1003] [0.430] [0.85] [0.0053]
3051.660(31) 1002.40882(12) 936.02199(8) 165.6079(17) 504.16456(6) 539.92215(5) 439.2394(8) 100.6828(8) 64.9252(8) [0.1003] [0.430] [0.85] [0.0053]
3126.520(44) 998.97756(8) 940.64227(8) 161.4266(23) 505.89625(4) 537.27014(4) 440.7619(11) 96.5083(11) 65.1343(11) [0.0776] [0.965] [0.0071] [0.01813]
20 1.1
18 1.8
16 0.7
18 2.4
10 1
12 1
Numbers within parentheses are single standard errors in units of the last printed digit. Centrifugal distortion constants determined for the parent species and displayed in square brackets were held constant for all the minor isotopologs.
Fig. 2. Ethyl trifluoroacetate in the Cs (syn-anti) conformation. Atom labels are shown.
Table 3 Selected bond lengths, bond angles, and dihedral angles for ethyl trifluoroacetate PBE0/aug-cc-pVTZ level of theory (Raw) for the Cs and C1 conformers of ethyl trifluoroacetate. The ‘‘scaled” columns are from the PBE0 second moment values multiplied by the appropriate scale factor. Syn-anti (Cs) PBE0
Syn-gauche (C1) PBE0
Raw
Scaled
Raw
Scaled
Bond Length/Å
C8@O5 C8AO4 O4AC6 C6AC7 C8AC9 C9AF1
1.1946 1.3186 1.4431 1.5031 1.5465 1.3219
1.1952 1.3201 1.4452 1.5047 1.5486 1.3234
1.1948 1.3189 1.4442 1.5075 1.5469 1.3220
1.1948 1.3185 1.4428 1.5130 1.5455 1.3215
Bond Angle/°
O5@C8AC9 O5@C8AO4 C8AO4AC6 O4AC6AC7
123.04 127.34 115.80 107.74
123.01 127.31 115.86 107.80
122.85 127.73 116.55 111.10
122.85 127.78 116.51 110.89
Dihedral Angle/°
O5@C8AO4AC6 C8AO4AC6AC7
0.0 180.0
0.0 180.0
0.78 85.62
0.79 85.58
Cs (syn-anti) scale factors: Sfa = 1.0016353, Sfb = 1.0005015, Sfc = 0.9951195. C1 (syn-gauche) scale factors: Sfa = 0.9987970, Sfb = 1.0000390, Sfc = 1.0051790.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009
4
R.K. Bohn et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx
by the rotational constants or moments of inertia. The spectral constants for the asymmetric C1 form are all different from each other and reveal no simple feature except that it has no plane of symmetry. Using PBE0/aug-cc-pVTZ density functionals [8], structures for both conformers were calculated. Using 2nd moments [9] those structures were scaled to give models which exactly reproduce the observed rotational constants. Table 3 lists selected structural parameters predicted by the density functional calculation and scaled to fit the observed rotational constants. Since the rotational constants have been determined to up to 7 significant figures, scaled parameters need to be similarly accurate. But the specific coordinate values will differ for different starting models. Table 3 lists bond lengths and angles to 5 significant digits, a compromise between the requirements of the scaling method structure and the ambiguity caused by using a different starting model. It appears that reported bond lengths are accurate to thousandths of an Å and angles to hundredths of a degree. There is enough spectral data to determine an rs structure using Kraitchman’s method [10] but we believe the scaling method is more accurate. Kraitchman’s method uses differences between second moments of the parent and isotopolog to determine coordinates of the substituted atom in the inertial axis system of the parent species. But differences between seven significant digit parameters have fewer significant digits compromising the accuracy of the rs structure. 4. Conclusions Ethyl trifluoroacetate displays rotational spectra from two stable conformations consistent with published studies of other ethyl esters. Bands of a third species, interpreted as a collection
of vibrationally excited states with short lifetimes [4], require experimental conditions no longer routinely available. The potential barrier between the syn-anti and syn-gauche conformers allows relaxation of the higher energy conformer (syn-gauche) to the lower (syn-anti) when Ar is the carrier gas in the pulsed jet expansion but that relaxation does not occur with He. Acknowledgments Nancy True of UCDavis and Stewart Novick and Pete Pringle of Wesleyan University provided thoughtful questions and discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jms.2017.03.009. References [1] J.M. Riveros, E.B. Wilson Jr., J. Chem. Phys. 46 (1967) 4605. [2] N.S. True, R.K. Bohn, Low resolution microwave spectroscopy III. Three conformers in ethyl trifluoroacetate, ethyl chloroformate and ethyl cyanoformate, J. Am. Chem. Soc. 98 (1976) 1188–1194. [3] R.D. Suenram, N.S. True, R.K. Bohn, J. Mol. Spec. 69 (1978) 435–440. [4] N.S. True, Conformer lifetimes of ethyl cyanoformate from exchange averaged rotational spectra, J. Phys. Chem. A 113 (2009) 6936–6946. [5] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. [6] A.R. Hight-Walker, Q. Lou, R.K. Bohn, S.E. Novick, J. Mol. Struct. 346 (1995) 187. [7] R.S. Ruoff, T.D. Klots, T. Emilsson, H.S. Gutowsky, J. Chem. Phys. 93 (1990) 3142. [8] M. Frisch et al., Gaussian 98, Rev. A.6, Gaussian, Inc., Pittsburgh, PA. [9] R.K. Bohn, J.A. Montgomery Jr., H.H. Michels, J.A. Fournier, Second moments and rotational spectroscopy, J. Molec. Spectrosc. 325 (2016) 42–49. [10] J. Kraitchman, Am. J. Phys. 21 (1953) 17.
Please cite this article in press as: R.K. Bohn et al., J. Mol. Spectrosc. (2017), http://dx.doi.org/10.1016/j.jms.2017.03.009