Microwave Spectrum and Molecular Structure of the Chiral Tagging

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A: Molecular Structure, Quantum Chemistry, and General Theory

Microwave Spectrum and Molecular Structure of the Chiral Tagging Candidate, 3,3,3-Trifluoro-1,2-Epoxypropane and Its Complex with the Argon Atom Mark D. Marshall, Helen O. Leung, Kevin Wang, and Mbatang Desmond Acha J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02550 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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The Journal of Physical Chemistry

Microwave Spectrum and Molecular Structure of the Chiral Tagging Candidate, 3,3,3Trifluoro-1,2-Epoxypropane and Its Complex with the Argon Atom Mark D. Marshall,* Helen O. Leung,* Kevin Wang, Mbatang Desmond Acha Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, MA 01002-5000

Address for correspondence: Prof. Mark D. Marshall Department of Chemistry Amherst College P.O. Box 5000 Amherst, MA 01002-5000 Telephone: (413) 542-2006 Fax: (413) 542-2735 E-mail: [email protected] *

Corresponding authors. Fax: +1-413-542-2735; e-mail addresses: [email protected] (H.O. Leung), [email protected] (M.D. Marshall). The authors declare no competing financial interest.

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Abstract The rotational spectrum of the chiral tagging candidate molecule, 3,3,3-trifluoro-1,2epoxypropane (TFO) and of its heterodimer with the argon atom, is obtained using Fourier transform microwave spectroscopy from 5.6 to 18.1 GHz. With a strong, simple rotational spectrum, TFO shows promise for applications in chiral analysis through the conversion of enantiomers into spectroscopically distinct diastereomeric species through non-covalent attachment. The structure of the argon complex of TFO, determined from analysis of the microwave spectrum, is extremely similar to that previously found for ethylene oxide-argon, but quite different from that suggested for propylene oxide-argon.


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I.

Introduction The high resolving power of microwave spectroscopy has long been harnessed to

determine precise molecular structures. Recently, the use of three-way mixing to detect rotational coherence has advanced the technique as a means for fast and precise chiral analysis.1-7 Nevertheless, although two enantiomers will give free induction decay (FID) signals completely (i.e., 180°) out of phase with the other, the assignment of the observed phase of the FID in an experiment to a specific absolute configuration of a molecule is nontrivial. This difficulty can be circumvented by converting a pair of enantiomers into diastereomers through complexation with a chiral molecular tag of known configuration.8 Because diastereomers have different microwave spectra, they can be easily identified and their absolute configurations determined.9 Furthermore, the intensities of the microwave signals also can serve to measure enantiomeric excess, a quantity that is of much value to the scientific and pharmaceutical communities. Thus, chiral tagging shows promise as an efficient and reliable method for chiral analysis. A good choice for a chiral tag will depend on several characteristics. The tag should be a small molecule so that the rotational constants in the complex are not too small, and obviously, it must be chiral and readily available in enantiopure form. The tag needs to be easily incorporated into the free-jet expansion used to form the complex and introduce it into the spectrometer. It will be helpful for the tag to be functionalized in a manner to facilitate noncovalent interactions with the analyte, and it should have a simple rotational spectrum. This latter requirement suggests a molecule with no complications from internal rotation or hyperfine splittings (although one could imagine using hyperfine patterns to distinguish between diastereomers). Similarly, although naturally occurring 13C and/or 18O are useful in structure determination, keeping isotopic dilution to a minimum will aid in keeping the spectrum of the complex intense



and simple. Likewise, unless there is an unfortunate cancellation of moments, a large permanent electric dipole moment for the tag can contribute to a strong spectrum for the complex. Finally, the tag molecule must have a known rotational spectrum, and its molecular structure must be determined. As our first stage in utilizing this technique, we turn to 3,3,3-trifluoro-1,2epoxypropane [CH2CH(CF3)O, or TFO from the non-systematic name, 2-(trifluoromethyl)oxirane], a potentially useful chiral tag, and determine the structure of this molecule. Additionally, we employ argon as a structureless probe to explore the electron distribution of this species. We have an additional, fundamental motivation for studying 3,3,3-trifluoro-1,2epoxypropane. Ethylene oxide, or oxirane, (CH2CH2O), has two planes of symmetry, one of which, the σv plane, contains the oxygen atom and bisects the C–C bond. The structure of oxirane has been well determined.10 While this symmetry is broken when a hydrogen atom in the molecule is substituted with a methyl group to give propylene oxide [CH2CH(CH3)O],11 the average heavy atom structure of the three-membered ring is not affected to any significant extent. Nevertheless, the argon complexes of these two species are very different. For ethylene oxide, argon lies in the σv plane, and it exhibits a tunneling motion, sampling both sides of the oxirane heavy atom plane.12-13 The breaking of symmetry in propylene oxide, not unexpectedly, results in the loss of tunneling, but it also leads to a distinctly different binding configuration. Initial results suggested that argon binds to the three-membered ring on the same side as the CH3 substiutent,14 but more thorough calculations indicate that the experimental structure is consistent with one where argon does not interact directly with the ring; instead, it lies outside of the ring, to one side of a C−O bond, interacting with these two atoms and the C atom in the CH3 group.15 In light of the similarities of the structures of ethylene oxide and propylene oxide but the


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differences in their interactions with argon, we seek to examine the case when CF3, instead of CH3, is used as a substituent to explore the effect of the electronegative F atoms on the ring and on intermolecular interactions. II.

Ab Initio Calculations The structure of 3,3,3-

trifluoro-1,2-epoxypropane is optimized using ab initio calculation at the MP2/6311++G(2d,2p) level with GAUSSIAN 16.

16

The

predicted structure is shown in Figure 1, together with the

Figure 1. (a) The labelling scheme used to describe the structure of 3,3,3-trifluoro-1,2-epoxypropane (TFO). The labeling of the F atoms is illustrated in (b) using a Newman projection along the C2−C3 bond. Atom colors: C, dark gray; H, light gray; O, red; F, light blue.

labeling scheme. Specifically, the three F atoms are labeled according to their positions in the Newman projection along the C2−C3 bond (Figure 1b): the F atom approximately anti to the H atom of C2 is labeled as Fa, while those on the same sides as O and C1, respectively, are labeled as FO and FC. The resulting structural parameters are listed in Table 1, and the A, B, and C rotational constants are 4568, 2187, and 2068 MHz. (The atomic positions in the principal axis system are available as Supporting Information.) The calculated dipole moment is 2.633 D, with components of 1.494, 1.826, and 1.169 D, respectively, along the a, b, and c inertial axes. These components are large; thus, we expect to see strong a, b, and c type transitions for this molecule. Indeed, the predicted structure enables us to identify readily and assign rotational transitions in the chirped pulse



Table 1. Structural parameters for 3,3,3-trifluoro-1,2-epoxypropane obtained using ab initio calculation and a structure fit to the moments of inertia of five isotopologues of the molecule. Experimenta

Theory C1−O / Å C2−O / Å C1−C2 / Å C2−C3 / Å C3−FO / Å

1.4484 1.4252 1.4621 1.4990

1.4374(28) 1.4342(66) 1.4555(65) 1.4795(41)

1.3427

1.3503(48)

C3−FC / Å

1.3475

1.3551(68)

C3−Fa / Å

1.3379 1.0791 1.0786 1.0811

1.3454(25) 1.0791 1.0786 1.0811

C1−H1 / Å C1−H2 / Å C2−H3 / Å ∠C1C2O / °

60.201

59.65(17)

∠C2OC1 / °

61.161

60.91(41)

∠OC1C2 / °

58.638

59.44(41)

∠H1C1C2 / °

118.295

118.295

∠H2C1C2 / °

119.099

119.099

∠H3C2C1 / °

120.657

120.657

∠C3C2C1 / °

120.147

121.46(43)

∠FOC3C2 / °

111.072

111.98(48)

∠FCC3C2 / °

109.710

110.94(43)

∠FaC3C2 / °

112.500

112.61(40)

∠H1C1C2O / °

−103.199

−103.199

∠H2C1C2O / °

102.587

102.587

∠H3C2C1O / °

104.865

104.865

∠C3C2C1O / °

−103.581

−102.05(66)

∠FOC3C2C1 / °

145.108

143.59(69)

∠FCC3C2C1 / °

−96.198

−98.16(64)

∠FaC3C2C1 / °

24.157

21.96(96)

a

1σ standard deviations in the parameters are given in parentheses. The parameters without uncertainties are fixed to the ab initio values.


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spectrum due to several isotopologues of the molecule and to determine its average structure, as described in later sections. Since F has only one naturally occurring isotope, 19F, it is unlikely that we will be able to determine the positions of the three F atoms experimentally. To ascertain the barrier height for internal rotation of the CF3 group, we carry out a series of calculations where the dihedral angle formed by one of the F atoms and C3, C2, and C1 is scanned from 0o to 360o while all other structural parameters are optimized. The resulting one-dimensional potential

Figure 2. A relaxed scan of the dihedral angle formed by an F atom with C3, C2, and C1 in 3,3,3-trifluoro-1,2-epoxypropane, showing the barrier to internal rotation of the –CF3 group.

curve can be found in Figure 2. The mass of three fluorine atoms, and to a lesser extent a barrier height over 1100 cm−1, make any internal rotation unlikely. To explore the interaction potential between argon and TFO, we once again carry out ab initio calculations at the MP2/6-311++G(2d,2p) level with GAUSSIAN 16.16 TFO is fixed at its experimental average structure (Section IV.B.1). The origin of the coordinate system employed is the center of mass of the TFO molecule, and the principal a, b, c inertial axes of TFO correspond to the x, y, z axes used in the calculations (Figure 3). The position of argon is specified by its spherical polar coordinates relative to this axis system: R, its distance from the origin; θ, the polar angle formed between R and the z axis; and φ, the azimuthal angle between the x axis and the projection of R onto the x-y plane. The value of θ is scanned from 5o to 175o



and that of φ from 0o to 360o, both in 10o increments, while allowing R to optimize. The resulting potential energy contour plot is displayed in Figure 3. Six minima, labeled (a) – (f), in order of increasing energy are identified and

Figure 3. Left: the spherical polar coordinate system used to locate Ar with respect to 3,3,3-trifluoro-1,2-epoxypropane. The x, y, and z axes are the a, b, and c axes for the propane, with the origin at its center of mass. (The y axis is directed out of the plane of the figure.)

optimized. The structures

R is the distance between Ar and the origin, and θ and φ (not shown) corresponding to these minima are shown in Figure 4, with the interaction lengths between Ar and each heavy atom listed in Table 2. (The atomic

are, respectively, the polar and azimuthal angles formed by R and the coordinate system. Right: a contour plot of the potential energy as a function of the angles with R optimized. Six minima are located, and the geometry is optimized at each. The corresponding structures are shown in Figure 4. Atom colors: C, dark gray; H, light gray; O, red; F, light blue; Ar, purple.

positions for each isomer, in its principal coordinate system, are available as Supporting Information.) The rotational constants, dipole moment components, and relative energies of these structures are listed in Table 3. In addition, we correct for basis set superposition error (BSSE) for these isomers,17 and the relevant interaction lengths and molecular properties are also listed in Tables 2 and 3. The optimized structures with BSSE correction are similar to those


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Figure 4. The optimized structures (without BSSE correction) corresponding to the six minima found in the potential scan for Ar3,3,3-trifluoro-1,2-epoxypropane. The more important intermolecular interactions are indicated using dashed lines. Atom colors: C, dark gray; H, light gray; O, red; F, light blue; Ar, purple 9 ACS Paragon Plus Environment

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Table 2. Interaction lengths between Ar and heavy atoms in four isomers of Ar-3,3,3-trifluoro-1,2-epoxypropane obtained from ab initio calculations and from a structure fit to experimental moments of inertia of four isotopologues. No BSSE BSSE Correction Correction Structure (a)

No BSSE BSSE Correction Correction Structure (b)

No BSSE BSSE Correction Correction Structure (c)

Ar−C1

3.720

3.851

3.556

3.660

3.778

3.883

Ar−C2

3.661

3.807

3.877

4.001

4.392

4.553

Ar−C3

5.058

5.205

4.112

4.296

4.408

4.613

Ar−O

3.489

3.633

4.858

4.975

3.487

3.617

Ar−FO

5.400

5.546

5.380

5.556

4.950

5.169

Ar−FC

5.678

5.814

3.501

3.687

5.503

5.703

Ar−Fa

5.950

6.098

4.353

4.573

3.442

3.656

Structure (d)

Structure (e)

Structure (f)

Experiment

Ar−C1

4.622

4.748

5.277

5.419

6.205

6.436

3.7644(11)

Ar−C2

3.653

3.790

3.941

4.102

5.314

5.554

3.72693(40)

Ar−C3

4.160

4.330

3.892

4.096

3.834

4.075

5.12414(9)

Ar−O

3.418

3.564

5.053

5.202

6.064

6.296

3.4744(12)

Ar−FO

3.497

3.677

3.536

3.748

3.556

3.787

5.4445(16)

Ar−FC

5.101

5.246

3.567

3.765

3.566

3.795

5.7863(17)

Ar−Fa

5.042

5.232

5.210

5.417

3.557

3.774

5.99046(47)


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Table 3. Rotational constants, dipole moment components, and relative energies for six isomers of for Ar-3,3,3-trifluoro-1,2epoxypropane obtained from ab initio calculations at the MP2/6-311++G(2d,2p) level without and with BSSE correction.

A/MHz

No BSSE BSSE Correction Correction Structure (a) 3052 3042

No BSSE BSSE Correction Correction Structure (b) 2200 2208

No BSSE BSSE Correction Correction Structure (c) 2465 2502

B/MHz

614

585

861

802

790

732

C/MHz

582

556

703

665

700

657

µa / D

1.790

1.797

2.318

2.384

0.288

0.217

µb / D

0.572

0.560

1.206

1.144

2.253

2.244

µc / D

1.977

1.982

0.863

0.789

1.477

1.504

0.0

0.0

1.0

7.0

17.9

13.7

Energya/ cm−1

A/MHz

Structure (d) 2231 2231

Structure (e) 2321 2285

Structure (f) 4065 4034

B/MHz

857

801

798

746

642

595

C/MHz

708

671

693

651

641

594

µa / D

0.467

0.395

0.001

0.086

1.991

1.991

µb / D

1.907

1.889

1.797

1.816

0.926

0.789

µc / D

1.881

1.916

2.012

2.006

1.690

1.750

Energya/ cm−1

19.7

11.2

50.6

49.0

117.5

93.3

a

The energy of the most stable isomer is set to 0 for the structures computed respectively with and without BSSE correction. 11 ACS Paragon Plus Environment


without the correction. In general, the distances between Ar and the heavy atoms are longer by 0.10 – 0.24 Å when calculated with BSSE correction than those without the correction. Consequently, the BSSE corrected rotational constants are typically smaller than those without the correction. The differences, however, are not too large: up to 2% for the A constants and 8% for the B or C constant. We can use van der Waals radii of the heavy atoms (Ar: 1.88 Å, C: 1.70 Å, F: 1.47 Å, O: 1.52 Å)18 to assess the importance of the intermolecular distances in each isomer. Estimates of optimal interaction lengths are simply the sum of the van der Waals radii: 3.58 Å for Ar−C, 3.35 Å for Ar−F, and 3.40 Å for Ar−O. Here, for simplicity, we will consider bond lengths calculated without BSSE correction, and we have indicated in Figure 4 the heavy atom distances that are between optimal and approximately 10% longer. In the lowest energy structure, Structure (a), Ar forms strong interactions with the three atoms in the epoxide ring: the Ar−O (3.489 Å), Ar−C2 (3.661 Å), and Ar−C1 (3.720 Å) distances are only 0.09, 0.08, and 0.14 Å, respectively, longer than van der Waals contacts. This configuration is similar to that observed for Ar-ethylene oxide. Structure (b) is higher in energy only by 1.0 cm−1 (7.0 cm−1) without (with) BSSE correction. Once again, argon interacts with three heavy atoms in this structure. Two of them involve C1 and C2, as in Structure (a), but the third is with FC instead of O. The Ar−C1 distance (3.556 Å) is similar to the optimal length while the Ar−FC (3.501 Å) and Ar−C2 (3.877 Å) distances are 0.10 Å and 0.30 Å longer than the respective van der Waals contacts. Thus, C1 and FC interact strongly with Ar, but not so for C2 and Ar. This weaker Ar−C2 interaction might be one of the reasons that renders Structure (b) slightly higher in energy than Structure (a). It is interesting to note that Structures (c) and (d) both include interactions with the O atom, a C atom, and an F atom and are of similar energy (17.9 and 19.7 cm−1 without BSSE correction,


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respectively, with the energy ordering reversed when BSSE correction is taken into account). Finally, although Structures (e) and (f) each has four argon-heavy atom interactions, those involving Ar−C are 0.25 – 0.36 Å longer than optimal and are not expected to be important for stabilizing these structures. As a result, the major interactions for these two structures are between Ar and F atoms: two in Structure (e) and three in Structure (f). These isomers are significantly higher in energy (50.6 and 117.5 without BSSE correction, respectively). Because Structures (c) – (f) are much higher in energy, we do not expect to observe them under our experimental conditions. The rotational constants for the two lowest energy structures, Structures (a) and (b) are very different; thus, their spectra can be easily distinguished if both are observed. III.

Experiment The vapor pressure over a room-temperature liquid sample of 3,3,3-trifluoro-1,2-

epoxypropane (SynQuest Laboratories, Achala, FL) is used to prepare a 1% gas mixture in argon, which is used at a backing pressure of 1 – 2 atmospheres in this work. A spectrum, ranging from 5.6 GHz to 18.1 GHz, is collected using a broadband, chirped pulse Fourier transform microwave spectrometer.19-21 After expanding the gas mixture through two pulsed valves, each with a 0.8 mm diameter nozzle, the sample is polarized using a chirped microwave polarization pulse of 4 µs duration and 20 – 25 W of power. The resulting FID is digitized at 50 Gs s–1 for 10 µs beginning 0.5 µs after the end of the excitation pulse. Ten FIDs are collected during each 700 µs opening of the pulsed valves, which typically operate at 4 Hz, although this is reduced to 0.8 Hz for overnight operation. 618,000 to 900,000 FIDs are averaged for each segment, and as described previously,20 the average is Fourier transformed to give a frequency domain spectrum with a resolution element of 23.84 kHz and typical line widths (FWHM) of 225



kHz. We are able to identify and assign rotational transitions for five isotopologues of TFO (the most abundant species, three isotopologues singly substituted with 13C, and one with 18O) and for the most abundant isotopologue of the argon complex in this spectrum. The rotational constants for the argon complex are similar to those for Structure (a). We do not observe a spectrum consistent with Structure (b). The chirped pulse spectrum of the most abundant isotopologue of the argon complex allows us to predict readily those for the species singly substituted with 13C, and we subsequently collect their spectra in natural abundance in the 6.9 – 17.3 GHz region using a narrow band, Balle-Flygare spectrometer.20, 22 Using only one pulsed valve in this instrument, the backgroundcorrected time domain signals spectra are digitized for 1024 data points and zero-filled to a 2048-point record length before Fourier transformation to give a frequency domain signal with a resolution element of 4.8 kHz. IV.

Results

A.

Spectral Analysis

1.

3,3,3-Trifluoro-1,2-epoxypropane We have observed and assigned 213 rotational transitions in the chirped pulse spectrum

for the most abundant isotopologue of 3,3,3-trifluoro-1,2-epoxypropane, and 86 – 95 transitions each for those species singly substituted with 13C. Because of the low abundance of 18O, there are only 34 transitions observed for this isotopologue. All three types of transitions, a, b, and c, are present, sampling J from 0 to at least 10, and Ka from 0 to at least 3. Figure 5a shows the quality of the spectrum. This 1 GHz region contains mostly Ka = 3 – 2 b and c type Q branch transitions. The simulated b and c type transitions are shown in panels (Figure 5b & c) below the experimental spectrum.


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Figure 5. (a) A 1 GHz segment of the chirped pulse spectrum taken with 1% 3,3,3-trifluoro1,2-epoxypropane in Ar. The spectrum contains mostly Q branch Ka = 3 – 2 b and c type transitions for the most abundant 3,3,3-trifluoro-1,2-epoxypropane isotopologue and J = 10 – 9 a type transitions for the most abundant isotopologue of its Ar complex. A portion of the spectrum is expanded 10 fold and displayed in green; (b) blue line: b type stick spectrum for the 3,3,3-trifluoro-1,2-epoxypropane monomer, brown: a type stick spectrum for the Ar-3,3,3trifluoro-1,2-epoxypropane complex; (c) c type stick spectrum for the 3,3,3-trifluoro-1,2epoxypropane monomer displayed in red.



The spectrum of each isotopologue is analyzed using the Watson A-reduced Hamiltonian23 and Pickett’s nonlinear SPFIT program.24 The rotational constants are well determined. Four (for the 18O isotopologue) or five (for all other isotopologues) quartic centrifugal distortion constants are also found. The rms deviation of each fit is between 6 and 11 kHz, commensurate with the resolution of the chirped pulse spectrometer. The spectroscopic constants are listed in Table 4, and tables of observed and calculated transition frequencies with assignments for all isotopologues studied are in Supporting Information. 2.

Ar-3,3,3-trifluoro-1,2-epoxypropane For the Ar-3,3,3-trifluoro-1,2-epoxypropane complex, we have assigned 258 rotational

transitions in the chirped pulse spectrum. Once again, all three types of transitions (a, b, and c) are present, and large ranges of J (1 – 16) and Ka (0 – 6) are accessed. Figure 5a contains eight a type, J = 10 – 9 transitions due to the most abundant species. The rotational spectra for the 13C isotopologues, because of their small number density, are collected using the more sensitive Balle-Flygare spectrometer. For each of these species, we have assigned 50 – 61 rotational transitions, also over a large J range (1 – 12) but smaller Ka range (0 – 3). The spectrum of each of the four isotopologues is similarly fitted using the Watson Areduced Hamiltonian23 and Pickett’s nonlinear SPFIT program.24 The three rotational constants and five quartic centrifugal distortion constants are reported in Table 5. The rms errors are consistent with the resolutions of the spectrometers (8.2 kHz for the most abundant species studied using the chirped pulse instrument, and less than 1.5 kHz for the other isotopologues using the Balle-Flygare spectrometer). Tables of observed and calculated transition frequencies with assignments for all isotopologues studied are in Supporting Information.


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Table 4. Spectroscopic constants (in MHz, unless as otherwise noted) for five isotopologues of 3,3,3-trifluoro-1,2-epoxypropane.a CH2CH(CF3)O

13

CH2CH(CF3)O

CH213CH(CF3)O

CH2CH(13CF3)O

CH2CH(CF3)18O

A

4595.69375(43)

4575.75379(79)

4577.75383(99)

4596.08287(86)

4554.9283(13)

B

2177.88275(21)

2144.71422(50)

2168.34203(71)

2174.86702(52)

2120.91207(89)

C

2063.53764(21)

2029.81723(50)

2058.54030(70)

2060.83065(56)

2004.49405(82)

∆J / 10-3

0.2531(22)

0.235(10)

0.263(14)

0.2370(92)

0.283(30)

∆JK / 10-3

1.5833(19)

1.549(19)

1.493(20)

1.554(16)

1.586(49)

∆K / 10-3

−0.9442(88)

−0.926(42)

−0.908(45)

−0.916(43)

δJ / 10-3 δK / 10-3

0.02160(11)

0.0221(26)

0.0239(20)

0.0249(16)

−1.50(18) 0.0225(16) −0.8504b

−0.8504(57)

−0.79(10)

−0.57(12)

−0.559(86)

No. of rotational transitions

213

86

93

95

34

No. of a type

15

15

15

15

13

No. of b type

126

45

50

51

12

No. of c type

72

26

28

29

9

J range

0 – 16

0 – 10

0 – 10

0 – 10

0 – 10

Ka range

0–8

0–4

0–4

0–4

0–3

rms/kHz

8.43

8.44

10.93

9.89

6.38

a

1σ standard deviations in the parameters are given in parentheses.

b

Fixed at the value appropriate to the most abundant isotopologue. 17 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 18 of 37

Table 5. Spectroscopic constants (in MHz, unless as otherwise noted) for four isotopologues of the Ar-3,3,3-trifluoro-1,2epoxypropane complex.a Ar−CH2CH(CF3)O

Ar−13CH2CH(CF3)O

Ar−CH213CH(CF3)O

Ar−CH2CH(13CF3)O

A

3105.06577(36)

3060.66880(21)

3104.15793(27)

3104.09772(25)

B

600.48925(11)

600.21588(11)

600.49728(13)

598.50592(13)

C

571.12061(11)

569.76461(11)

571.15084(13)

569.28788(13)

0.66395(30)

0.65736(24)

0.66297(29)

0.65993(28)

∆J / 10-3 ∆JK / 10-3

−2.0995(20)

−2.1322(24)

−2.1026(33)

−2.1150(31)

∆K / 10-3

31.812(15)

31.773(22)

31.885(30)

31.867(29)

δJ / 10-3

0.07320(12)

0.07502(10)

0.07284(15)

0.07270(13)

δK / 10-3

0.896(28)

0.864(52)

0.860(58)

0.860(59)

No. of rotational transitions

258

61

50

56

No. of a type

104

33

24

30

No. of b type

41

0

0

0

No. of c type

113

28

26

26

J range

1 – 16

1 – 12

1 – 12

1 – 12

Ka range

0–6

0–3

0–3

0–3

rms/kHz

8.23

1.14

1.41

1.39

a

1σ standard deviations in the parameters are given in parentheses. 18 ACS Paragon Plus Environment

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As we show below, the values of the asymmetry parameter for the isotopologues of the Ar-TFO complex are very close to −1, suggesting a consideration of the Watson S-reduced Hamiltonian.23 The spectroscopic constants obtained in this reduction are available as Supporting Information. It is worth noting that for each isotopologue, the rotational constants obtained using either the Aand S-reduced Hamiltonian agree within 2 kHz, and they both give the same structure for the complex. We will proceed with reporting our analysis with the constants obtained using the Areduced Hamiltonian. B.

Structure Determination

1.

3,3,3-Trifluoro-1,2-epoxypropane TFO is a near prolate asymmetric top with an asymmetry parameter value between

−0.913 and −0.909 for the isotopologues. The availability of the spectra for the 13C and 18O containing isotopologues allow us to determine the positions of the three C atoms and the O atom, but the lack of isotopic data for the H and F atoms causes the experiment to be less sensitive to their locations. Consequently, many structural parameters involving these atoms are fixed to ab initio values. We are able to fit ten parameters to a total of fifteen moments of inertia for the five isotopologues of TFO using Kisiel’s STRFIT program.25 For the atoms in the threemembered ring, we fit O−C1, O−C2, and ∠C2−O−C1. The position of the C atom in the CF3 group (C3) can be specified relative to any one of the heavy atoms in the ring. We make the non-intuitive selection of O because the fit then shows no correlations among the parameters. Other choices lead to unacceptably large correlations. Specifically, we fit the distance between O and C3, the angle formed by C3, O, and C1, and the dihedral angle formed by C3, O, C1, and C2. Additionally, we are able to fit the three ∠F−C3−O angles and a collective set of three C–F bond lengths, while restricting the difference between each pair of C−F bond lengths to be the



Table 6. The coordinates of the atoms in 3,3,3-trifluoro-1,2-epoxypropane determined from a structure fit of five isotopologues and the substitution coordinates for four atoms from a Kraitchman analysis.a a/Å (i) From structural fit

b/Å

c/Å

C1 C2 C3 O FO

−1.8956(12) −0.7699(28) 0.5682(34) −1.77159(61)

−0.7054(25) −0.027(11) −0.00888(72) 0.7247(14)

0.0353(69) 0.6606(27) 0.0295(11) −0.0383(50)

1.2036(55)

1.1663(41)

0.2254(78)

FC

1.3694(70)

−0.9612(64)

0.5655(55)

Fa

0.5177(75) −2.6987(38) −1.7460(42) −0.7296(56)

−0.232(10) −1.0524(61) −1.2126(91) 0.100(19)

−1.2963(15) 0.667(11) −0.905(10) 1.7335(29)

−0.7046(21) −0.047(32) nonphysical 0.7241(21)

nonphysical 0.6581(23) nonphysical −0.031(48)

H1 H2 H3

(ii) Substitution coordinatesb C1 C2 C3 O a

−1.89602(79) −0.7705(20) 0.5729(26) −1.77180(85)

Costain errors26 in the parameters are given in parentheses.

b

Although only the absolute values of the substitution coordinates can be determined from the

Kraitchman analysis, the relative signs are assigned using physically reasonable atomic distances. The b coordinate of C2 and c coordinate of O are not well determined; their signs are set to be the same as those from the structure fit.


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same as that determined by theory. The results of the fit, after converting the parameters to chemically relevant ones using Kisiel’s EVAL program,27-28 are listed in Table 1 and the principal coordinates of the atoms are reported in Table 6. The rms deviation of the fit is 0.0038 u Å2. Because each 13C or 18O isotopologue contains a single isotopic substitution in the most abundant species, the position of the substituted atom in the principal axis system of the parent species can be determined using a Kraitchman analysis.29 Although we can only determine absolute values for Kraitchman’s coordinates, many of the relative signs can be assigned based on reasonable chemical distances. These coordinates are presented in Table 6. Because a Kraitchman analysis does not take into account the difference in zero-point vibrational motions upon isotopic substitution, when the value of a coordinate is close to zero, it is either not well determined or appears as a nonphysical value in the analysis. The well-determined Kraitchman coordinates agree with those from the structure fit to within experimental uncertainty, indicating that the vibrational motion involving the heavy C and O atoms are reasonably harmonic. 2

Ar-3,3,3-trifluoro-1,2-epoxypropane Ar-TFO is also a near prolate asymmetric top, and the values of the asymmetry

parameters are almost identical for all the isotopologues studied (between −0.977 and −0.976). The rotational constants of the most abundant isotopologue unambiguously demonstrate that the observed species has a configuration similar to Structure (a). Fixing the structure of TFO as determined above, we locate Ar by using Kisiel’s STRFIT program,25 to fit the distance between Ar and C3, the angle formed by Ar, C3, and C1, and the dihedral angle formed by Ar, C3, C1, and O to the twelve moments of inertia from the four isotopologues. The rms deviation of the fit is 0.055 u Å2. The chemically relevant heavy atom distances calculated by Kisiel’s EVAL27-28



program are listed in Table 2, and the atomic positions are in Table 7. The average structure, with the most important intermolecular distances labeled, is shown in Figure 6a. Ar interacts

Table 7. The coordinates of the atoms in Ar-3,3,3-trifluoro-1,2-epoxypropane determined from a structure fit of four isotopologues and the substitution coordinates for the three C atoms from a Kraitchman analysis.a a/Å (i) From structural fit

b/Å

c/Å

C1 C2 C3

0.17909(98) −0.24720(38) −1.67438(7)

−1.4494(15) −0.09809(67) 0.23245(8)

−0.5701(12) −0.2373(15) −0.03028(13)

O

0.3228(11)

−0.9466(19)

0.76878(24)

FO FC

−1.8445(15)

1.1480(24)

0.9476(22)

−2.2130(17)

0.7754(28)

−1.14889(66)

Fa H1

−2.41476(40) 1.1093(18)

−0.84612(79) −1.5759(29)

0.2839(37) −1.10222(14)

H2 H3 Ar

−0.5650(12) 0.34163(81) 3.44866(6)

−2.2231(17) 0.7562(15) 0.33835(9)

−0.6759(43) −0.5410(45) −0.03696(21)

−1.4260(11) −0.078(19) 0.2360(64)

−0.5791(26) −0.2043(73) nonphysical

(ii) Substitution coordinatesb C1 C2 C3 a

0.2533(59) nonphysical −1.67401(90)

Costain errors26 in the parameters are given in parentheses.

b

Although only the absolute values of the substitution coordinates can be determined from the

Kraitchman analysis, the relative signs are assigned using physically reasonable atomic distances. 22 ACS Paragon Plus Environment

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most strongly with the heavy atoms of the threemembered ring, and lies 3.473 Å above the ring on the side opposite to the CF3 group. When it is projected onto the plane of the ring, its position almost completely overlaps that of the O atom (Figure 6b). With the most abundant species as the parent, the Kraitchman’s coordinates of the three C atoms are calculated and listed in Table 7. While many of the coordinates compare well with those obtained from the structural fit, the a substitution coordinates for C1 and C2 show a

Figure 6. (a) The experimental structure of

deviation that is puzzling at first glance.

Ar-3,3,3-trifluoro-1,2-epoxypropane with

Specifically, the a substitution coordinate for C1,

(b) the O atom projected onto the three-

0.2533(59) Å is 0.074 Å (41%) longer than that

membered ring. (c) The experimental

from the structural fit and that for C2 is

structure of Ar-ethylene oxide (drawn here

nonphysical even though the value is not

using the parameters given in Ref. 13) with

particularly close to zero in the average structure.

(d) the O atom projected onto the three-

To explore a possible reason for these dramatic

membered ring. Atom colors: C, dark gray;

disagreements between the Kraitchman and

H, light gray; O, red; F, light blue; Ar,

average coordinates, we first calculate the position purple. of Ar using only the moments of inertia of the



Page 24 of 37

Table 8. The coordinates of the atoms in Ar-3,3,3-trifluoro-1,2-epoxypropane calculated from the moments of inertia of the most abundant isotopologue. a/Å 0.1789

b/Å −1.4496

c/Å −0.5701

C2 C3 O FO

−0.2472 −1.6744 0.3227

−0.0982 0.2325 −0.9468

−0.2373 −0.0303 0.7688

−1.8444

1.1480

0.9476

FC

−2.2129 −2.4149 1.1091 −0.5653 0.3417 3.4488

0.7756 −0.8460 −1.5762 −2.2232 0.7560 0.3384

−1.1489 0.2839 −1.1023 −0.6759 −0.5410 −0.0370

C1

Fa H1 H2 H3 Ar

Table 9. Spectroscopic constants (in MHz) of three singly substituted 13C isotopologue calculated from the structure of the most abundant Ar-3,3,3-trifluoro-1,2-epoxypropane complex tabulated in Table 8. Ar−13CH2CH(CF3)O

Ar−CH213CH(CF3)O

Ar−CH2CH(13CF3)O

A

3059.6028

3103.8127

3104.0254

B

600.2461

600.4057

598.5012

C

569.7417

571.0751

569.2881


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most abundant isotopologue. The resulting structure is then used to predict the rotational constants for the three singly substituted 13C species. The atomic coordinates derived from the most abundant isotopologue (Table 8) agree completely with average coordinates determined using all four isotopologues (Table 7). Similarly, to no surprise, the rotational constants of the 13

C substituted isotopologues calculated from the structure of the most abundant species (Table

9) agree very well with the experimental values (Table 5). The differences between corresponding constants fall within 0.000 – 1.066 MHz. Although these deviations are small, they are comparable or sometimes even greater than the difference between the values of the same rotational constant for a 13C species and for the most abundant isotopologue. Because Kraitchman coordinates depend on these differences, or more precisely, the differences between moments of inertia, it is not surprising that some substitution coordinates deviate significantly from their corresponding average coordinates. V.

Discussion We have determined the structure of 3,3,3-trifluoro-1,2-epoxypropane, a potentially

useful chiral tag, and have ascertained the manner in which it interacts with argon. The experimental rotational constants for both species agree well with those predicted by ab initio calculations. Specifically, for the TFO monomer, the ab initio values differ from the experimental ones by 0.2 – 0.6%. The differences are slightly greater for the complex, 1.7 – 2.2% without BSSE correction and 2.0 – 2.6% with BSSE correction. In fact, the average value of the B constant determined with and without BSSE calculation is only 1 MHz smaller than the experimental value, and when the same is done for the predicted values of the C constant, the average is 2 MHz smaller than the experimental value. The average of the two predicted A constant values, however, does not agree as well (it is 58 MHz smaller than the experimental



value). This is expected because the large angular motions of the subunits of a weakly bound complex contribute to significant differences between the predicted equilibrium value and the observed vibrationally averaged value for the A constant. Nevertheless, theory has guided our experimental work exceedingly well. It is useful to compare our findings with those of similar compounds, ethylene oxide and propylene oxide, and their Ar complexes. By comparing the structures of the three-membered rings in these species, we can discover how much their structures are affected by different substituents. The lengths of the C−C bonds in ethylene oxide10 and propylene oxide11 are both 1.470(3) Å, which is slightly longer than the 1.4555(65) Å bond length in 3,3,3-trifluoro-1,2epoxypropane, namely by 2σ of the uncertainty of the latter. While the lengths of both C−O bonds are the same in ethylene oxide [1.434(3) Å], a substituent breaks this symmetry. In the cases when the substituent is CH3 or CF3, the C1−O bond appears to be slightly longer than the C2−O bond, although when uncertainties are taken into account, these two bonds continue to have practically the same length. Specifically, the C1−O and C2−O bond lengths are 1.441(2) Å and 1.435(3) Å, respectively, for propylene oxide, and 1.4374(28) Å and 1.4342(66) Å for 3,3,3trifluoro-1,2-epoxypropane. Taking the lengths of all bonds in the three-membered ring into consideration, we can therefore conclude that the CH3 and CF3 substituents do not affect the heavy atom structure of the ring to any significant extent. The C2−C3 bond lengths, however, are different for the two substituents: 1.505(2) Å for propylene oxide, which is 0.026 Å longer than the bond length of 1.4795(41) Å in 3,3,3-trifluoro-1,2-epoxypropane. Ar interacts with the three-membered ring in TFO as it does with ethylene oxide,12-13 but because of the lack of symmetry in TFO, no tunneling motion is observed. In Ar-ethylene oxide, the length, R, between Ar and the center of mass of ethylene oxide (CM) is 3.6062(8) Å, with R


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forming an angle of 72.34(4)o with the line connecting O and CM.13 These parameters place Ar 3.436 Å above the three-membered ring, which is 0.037 Å closer to the ring than in Ar-TFO. Once again, Ar is almost directly above the O atom (Figure 6b and d). We can additionally calculate the distances between Ar and the heavy atoms in Ar-ethylene oxide: the Ar−O interaction length is 3.4485 Å and the two equivalent Ar−C lengths are 3.8292 Å. Because the Ar−O distance is just 1% longer than the van der Waals contact distance of 3.40 Å, this interaction is significant. On the other hand, the Ar−C lengths are 0.25 Å longer than the sum of the van der Waals radii of the atoms, making these interactions considerably weaker. Similar types of interactions are observed in Ar-TFO. The Ar−O distance is 3.4744(12) Å, 0.75% longer than that in the Ar-ethylene oxide complex, and the Ar−C distances are 3.7644(11) Å and 3.72693(40) Å for C1 and C2, only 1.7% and 2.7%, respectively shorter than the equivalent lengths in Ar-ethylene oxide. It appears than the CF3 substituent has only a small effect on the manner in which Ar binds to the three-membered ring. Blanco et al.14-15 determined a structure for Ar-propylene oxide that is different from Ar3,3,3-trifluoro-1,2-epoxypropane. In this structure, the Ar atom does not bind to the threemembered ring in the same manner as it does in ethylene oxide and 3,3,3-trifluoro-1,2epoxypropane. Initial conclusions regarding the structure of Ar-propylene oxide14 were refined following work on Kr-propylene oxide15 where Kr is found to interact with O, C2, and C3, and indeed, the conformation of the Ar complex appears to be similar to that of the Kr complex. We have examined two possible conformations of the Ar-propylene oxide complex here, one similar to the Kr-propylene complex and the other analogous to the structures of Ar-TFO and Arethylene oxide. To ensure a valid comparison, we repeat the similar calculations of reference 15, but using the same level of theory [MP2/6-311++G(2d,2p)] and version of Gaussian (G16) as



we use here for Ar-TFO. Without BSSE correction, the configuration of Ar-propylene oxide similar to that of the Ar-TFO complex is 18.7 cm−1 higher than the global minimum (20.0 cm−1

Figure 7. The predicted structures (without BSSE correction) of Ar-propylene oxide (a) where Ar interacts with the three-membered ring and (b) at the global minimum. (c) A structure consistent with the rotational constants reported in references 14 and 15. Atom colors: C, dark gray; H, light gray; O, red; F, light blue; Ar, purple. with BSSE correction). This structure is shown in Figure 7a. The global minimum structure, shown in Figure 7b, allows Ar to interact closely with O and C2, giving interaction lengths of 3.3552 Å and 3.5742 Å, respectively, that are very close to the van der Waals contact distances. The distance between Ar and C3 is 3.9058 Å, which is perhaps a much weaker interaction. The rotational constants for this global minimum structure (without BSSE correction) are 6694 MHz, 1424 MHz, and 1235 MHz, which agree satisfactorily with the experimental values of 6791.62(1) MHz, 1382.041(6) MHz, and 1200.374(7) MHz.14 As was apparently done earlier,15 we use this theoretical structure as a guide, and calculate three geometric parameters locating the argon atom relative to the ethylene oxide molecule from the three experimentally available rotational constants. These parameters are shown in Figure 7c. Additional spectra from other 28 ACS Paragon Plus Environment

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isotopologues would be helpful in providing redundancy in the data and to confirm this is indeed the correct structure for Ar-propylene oxide. Despite the lack of experimental confirmation, the theoretical evidence for the striking difference between the binding modes observed for Ar-propylene oxide and those for Arethylene oxide and Ar-TFO suggests that an examination of the electron density distributions in

Figure 8. The electrostatic potential, mapped onto a total electron density isosurface for (a) ethylene oxide, (b) propylene oxide, and (c) 3,3,3-trifluoro-1,2epoxypropane. The same value of electron density is used for the isosurface in all molecules and identical color scales are used. Blue color represents positive electrostatic potential and red, negative electrostatic potential. these species may provide important information. We thus map the electrostatic potential of each species onto its total electron density, calculated at the MP2/6-311G++(2d,2p) level (Figure 8). For each of the three molecules, the O atom is the most negative; thus, it is not surprising that Ar forms an interaction with this atom in all three complexes. It is noteworthy that the O atom is much less negative in TFO, a result of the presence of three electronegative F atoms. For 29 ACS Paragon Plus Environment


the symmetric ethylene oxide, Ar can form the strongest interaction with the molecule by locating in the σv plane even though the presence of the H atoms prevents close interactions with the C atoms. For TFO, additional binding configurations are available for argon. Balancing the steric bulk of the CF3 group with the electronegative F atoms, argon can occupy different configurations, as shown in Figure 4, and some of these configurations have similar energies. Experimentally, the binding configuration is similar to that of Ar-ethylene oxide, but with a diminished negative potential on the O atom a longer, and hence weaker, Ar-O interaction is found for Ar-TFO than for Ar-ethylene oxide. When the CF3 group is replaced by the CH3 group, giving propylene oxide, the steric bulk is eliminated, allowing Ar to approach C2 more closely, giving an entirely different binding mode. Nevertheless, the argon atom does continue to interact with O, as observed for the other two species, but is able to take advantage of additional close approaches as well. VI.

Conclusion The successful analysis of the microwave spectrum of 3,3,3-trifluoro-1,2-epoxypropane

shows that it possesses many of the required properties of a useful chiral tag. The spectrum is simple and strong and is well predicted by ab initio calculation. Many of the molecule’s structural parameters have been refined by the experimental results, and the positions of the carbon and oxygen atoms well determined via analysis of singly substituted isotopologues. The ability to participate in non-covalent bonding is demonstrated by the straightforward observation of the argon complex of TFO, although additional work, currently in progress, will be necessary to show that complexes with more complicated species are similarly easy to observe and analyze. With the exception of the absence of tunneling, the heterodimer of TFO with the argon atom is nearly completely analogous to that formed between argon and ethylene oxide. The


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argon atom in both interacts with all three atoms in the ring, almost directly above the oxygen atom. Argon makes a closer approach to the ring in the complex with TFO. This could be a result of an enhanced polarization due to the presence of the electronegative –CF3 group or a reduction in the effective van der Waals contact distance as electrons are withdrawn from the side of the ring opposite the –CF3, or some combination of these effects. Despite careful searching, no evidence was found for the presence of Structure (b) (Fig. 4) of Ar-TFO in our spectra. Under our experimental conditions in which these weakly bound complexes are generally believed to undergo repeated formation and dissociation in the argon expansion, we typically only see the lowest energy conformer of a species. The conformational temperature is expected to be on the order of only a few Kelvin, or kT < 3 cm–1. The energy difference between structures (a) and (b) is so small, 1.0 cm–1 and 7.0 cm–1 without and with BSSE correction, respectively, that we might have expected to observe both. The failure to observe structure (b) does suggest, albeit with negative evidence, that the BSSE-corrected energy difference is more realistic or that the conformational temperature is lower than our estimate. Perhaps because of the steric bulk due to the three fluorine atoms, no minimum corresponding to a structure analogous to that of propylene oxide-argon was found on the ArTFO potential energy surface. Although there are several higher energy structures with the argon atom off to the “side” of the ring as in the propylene oxide complex, none has the argon interacting with a carbon atom and the oxygen atom in the ring as well as the carbon atom of the trifluoromethyl group. Those minima on the surface for Ar-TFO with interactions to two ring atoms have the third interaction with one of the fluorine atoms. The previously reported rotational spectrum of propylene oxide-argon14-15 is consistent with ab initio predictions of its structure, but with the spectrum of only a single isotopologue for this species, there is no



redundancy in the data for locating the three-dimensional position of the argon atom relative to the propylene oxide from three available rotational constants. Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. CHE-1465014. KW acknowledges the support of an Amherst College Summer Undergraduate Research Fellowship. Supporting Information. Tables of observed and calculated transition frequencies for all isotopologues of 3,3,3-trifluoro1,2-epoxypropane and argon–3,3,3-trifluoro-1,2-epoxypropane that are reported in this study, the atomic coordinates for the structures shown in Figures 1 and 4, the spectroscopic constants in the S reduction for all isotopologues studied of argon–3,3,3-trifluoro-1,2-epoxypropane, and the complete citation for Gaussian 16.


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Shubert, V. A.; Schmitz, D.; Medcraft, C.; Krin, A.; Patterson, D.; Doyle, J. M.; Schnell, M. Rotational Spectroscopy and Three-Wave Mixing of 4-Carvomethanol: A Technical Guide to Measuring Chirality in the Microwave Region. J. Chem. Phys. 2015, 142, 214201.

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Shubert, V. A.; Schmitz, D.; Patterson, D.; Doyle, J. M.; Schnell, M. Identifying Enantiomers in Mixtures of Chiral Molecules with Broadband Microwave Spectroscopy. Angew. Chem. Int. Ed. 2014, 53, 1152-1155.

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Shubert, V. A.; Schmitz, D.; Pérez, C.; Medcraft, C.; Krin, A.; Domingos, S. R.; Patterson, D.; Schnell, M. Chiral Analysis Using Broadband Rotational Spectroscopy. J. Phys. Chem. Lett. 2015, 7, 341-350.

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Shubert, V. A.; Schmitz, D.; Schnell, M. Enantiomer-Sensitive Spectroscopy and Mixture Analysis of Chiral Molecules Containing Two Stereogenic Centers – Microwave ThreeWave Mixing of Menthone. J. Mol. Spectrosc. 2014, 300, 31.36.

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Hirota, E. Triple Resonance for a Three-Level System of a Chiral Molecule. Proc. Jpn. Acad. B 2012, 88, 120-128.

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TOC Graphic


Microwave Spectrum and Molecular Structure of the Chiral Tagging

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