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The microwave spectrum and molecular structure of 3,3-difluoro-1,2-epoxypropane and its complex with the argon atom
Journal of Molecular Spectroscopy 350 (2018) 18–26
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The microwave spectrum and molecular structure of 3,3-difluoro-1,2epoxypropane and its complex with the argon atom Mark D. Marshall, Helen O. Leung Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, MA 01002-5000, United States
a r t i c l e
i n f o
Article history: Received 3 April 2018 In revised form 4 May 2018 Accepted 8 May 2018 Available online 16 May 2018
a b s t r a c t As a small, chiral molecule with a strong and simple microwave rotational spectrum, 3,3-difluoro-1,2epoxypropane is a promising candidate as a chiral tag for conversion of enantiomeric analytes into spectroscopically distinct diastereomers via formation of a non-covalently bound complex. The Fourier transform microwave spectra of this species and its complex with the argon atom are obtained from 5.1 to 21.0 GHz, and their structures determined. Not surprisingly, these structures for both the monomer and the argon complex show strong similarities with those previously determined for 3,3,3-trifluoro-1,2epoxypropane and 3,3,3-trifluoro-1,2-epoxypropane–argon, but are different from those of the analogous propylene oxide species. This suggests that fluoromethyl and methyl substitution affect the bonding in the epoxy moiety and intermolecular interactions in distinctly different ways. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Recently, chiral tagging has been proposed as a fast, efficient method for chiral analysis [1]. In this method, a molecule of known absolute configuration (the tag) is bound via non-covalent interactions to a chiral analyte. This converts the two enantiomers of the analyte into spectroscopically distinct diastereomers that can be unambiguously characterized via microwave spectroscopy. The method can provide a determination of absolute stereochemistry for an enantiopure analyte and a measurement of entiomeric excess for a sample of mixed chirality. Furthermore, chiral tagging avoids the ‘‘zero-time” problem associated with three-wave mixing [2–8] where the determination of absolute stereochemistry requires detailed knowledge of all the phase shifts in the spectrometer. Obviously, chiral tagging depends on the availability of suitable tags. These molecules must be readily available in enantiopure form. The tag should be a small molecule so that the rotational constants of the non-covalent complex are not too small, and the tag itself and its complexes with the carrier gases typically used for free-jet expansion need to have well characterized rotational spectra. Recently, we have begun exploring a series of tag candidates based upon substituted ethylene oxides. Our first example was 3,3,3-trifluoro-1,2-epoxypropane [also known as 2(trifluoromethyl)-oxirane, or TFO] [9]. Because it will be useful to have several alternatives tags available as the technique is further
E-mail addresses: [email protected] (M.D. Marshall), hleung@amherst. edu (H.O. Leung) https://doi.org/10.1016/j.jms.2018.05.003 0022-2852/Ó 2018 Elsevier Inc. All rights reserved.
developed, we present here the microwave spectrum of 3,3difluoro-1,2-epoxypropane [2-(difluoromethyl)-oxirane or DFO] and its complex with the argon atom. 2. Ab initio calculations To guide our experimental work, we use ab initio calculations at the MP2/6-311++G(2d,2p) level with GAUSSIAN 16 [10] to provide theoretical structures for both the 3,3-difluoro-1,2-epoxypropane monomer and its complex with argon. The equilibrium structure of the monomer, together with the labeling scheme, is shown in Fig. 1 and bond lengths and angles are listed in Table 1. (The coordinates of the atoms in the principal inertial axis system are given as Supplementary material.) The H atoms, H3 and H4, respectively bonded to C2 and C3, are almost anti to each other when viewed in a Newman projection along C2AC3 (Fig. 1b). Using this configuration as a reference, we label the F atom on the same side as the O atom as FO and that on the same side as C1 as FC. This configuration makes possible two intramolecular HAF bonds: 2.705 Å for H3AFC and 2.643 Å for H3AFO. Additionally, H4 is 2.615 Å from O, well positioned to interact with it. The stability conferred by these interactions can be observed by examining the energy of the molecule when the H4-C3-C2-C1 dihedral angle is varied from 0° to 360° in steps of 10° while the structure of the rest of the molecule is allowed to relax (Fig. 2). There are three minima. The global minimum is at 31°, with H4 almost anti to H3 as described in Fig. 1. The two local minima are at 137° and 264°, corresponding to H4 occupying approximately the positions of FO and FC in Fig. 1, respectively. As mentioned earlier, the global minimum permits three
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
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Fig. 1. (a) The labeling scheme used to describe the structure of 3,3-difluoro-1,2-epoxypropane. The labeling of the F atoms is illustrated in (b) using a Newman projection along the C2AC3 bond. Atom colors: C, dark gray; H, light gray; O, red; F, light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Structural parameters for 3,3-difluoro-1,2-epoxypropane using ab initio calculation and a structure fit to the moments of inertia of 5 isotopologues of the molecule.
C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å C3AFO/Å C3AFC/Å C1AH1/Å C1AH2/Å C2AH3/Å C3AH4/Å \C1C2O/° \C2OC1/° \OC1C2/° \H1C1C2/° \H2C1C2/° \H3C2C1/° \H4C3C2/° \C3C2C1/° \FOC3C2/° \FCC3C2/° \H1C1C2O/° \H2C1C2O/° \H3C2C1O/° \H4C3C2C1/° \C3C2C1O/° \FOC3C2C1/° \FCC3C2C1/°
Theory
Experimenta
1.4460 1.4358 1.4626 1.4919 1.3644 1.3683 1.0791 1.0806 1.0815 1.0862 59.846 60.998 59.156 118.693 119.320 120.346 112.669 118.972 110.009 109.384 103.410 103.006 104.318 30.713 102.441 151.778 90.779
1.41990(40) 1.4270(23) 1.4753(25) 1.4893(24) 1.3644 1.3683 1.0791 1.0806 1.0815 1.0862 58.55(10) 62.425(83) 59.023(70) 118.693 119.320 120.346 112.669 119.007(56) 110.009 109.384 103.410 103.006 104.318 30.713 102.441 151.778 90.779
a 1r standard deviations in the parameters are given in parentheses. The parameters without uncertainties are fixed to the ab initio values
interactions, but there are only two interactions for the local minima, FAH2 and FAH3, while H4 does not interact with an electronegative atom in either case. The global minimum structure is significantly lower in energy than the other two structures (by 171 and 479 cm 1, respectively). Furthermore, the barrier to interconversion between the global minimum structure and either higher energy structure is more than 800 cm 1. As a result, the CHF2 group is not expected to exhibit internal rotation. The three rotational constants of DFO are calculated to be 6910, 2705, and 2105 MHz, and the dipole moment is 2.400 D, with components of 1.508, 1.850, and 0.248 D along the a, b, and c inertial axes, respectively. Accordingly, we are able to observe both a and b type transitions in both the chirped pulse and the Balle-Flygare spectrometers for the most abundant isotopologue and those sin-
Fig. 2. A relaxed scan of the dihedral angle formed by H4 with C3, C2, and C1 in 3,3difluoro-1,2-epoxypropane from 0° to 360° in 10° steps. The minima are optimized and displayed in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
gly substituted with 13C, but the much weaker c type transitions of these species and the spectrum of the 18O isotopologue show up much more clearly in the more sensitive Balle-Flygare instrument. After establishing the average structure of DFO, as described in Section 4.2.1, we use ab initio calculations to determine its interaction energy with argon. The coordinate system we use is given in Fig. 3. The origin is the center of mass of the epoxypropane, with the x, y, and z-axes coincident with the a, b, and c inertial axes of the average structure of the molecule. The position of Ar is described using spherical polar coordinates, where R is the distance between Ar and the origin of the axis system, h is the polar angle formed by R and the z axis, the / is the azimuthal angle between the projection of R onto the x-y plane and the x axis. The potential energy of the system is then calculated by varying the values of h from 5° to 175° and the values of / from 0° to 360°, both in 10° steps, while allowing R to optimize. The resulting energy contour (Fig. 3) is remarkably similar to that obtained for the interaction between Ar and 3,3,3,-trifluoro-1,2-epoxypropane [also known as 2-(trifluoromethyl)-oxirane or TFO] [9] except that
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Fig. 3. Left: the spherical polar coordinate system used to locate Ar with respect to 3,3-difluoro-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. R is the distance between Ar and the origin, and h and / (not shown) are, respectively, the polar and azimuthal angles formed by R and the coordinate system. Right: the potential energy contour as a function of the angles with R optimized. Eight minima are located and optimized, and their corresponding structures are shown in Fig. 4. Atom colors: C, dark gray; H, light gray; O, red; F, light blue; Ar, purple. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
in the present case, eight minima, instead of six, can be identified. These are labeled (a) – (h) in order of increasing energy. The two minima (e) and (f) observed here have no equivalent in the ArTFO interaction potential; they are in shallow wells and are separated by low barriers (10–15 cm 1) from the nearby deeper wells labeled (b) and (c), respectively.
The structures at the eight minima are subsequently optimized; they are shown graphically in Fig. 4, and the interaction lengths between Ar and each heavy atom are listed in Table 2. (The atomic positions for each isomer, in its principal coordinate system, are available as Supplementary material.) The rotational constants, dipole moment components, and relative energies of these
Fig. 4. The optimized structures and relative energies (without BSSE correction) corresponding to the eight minima found in the potential scan for Ar-3,3-difluoro-1,2epoxypropane. 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. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 2 Interaction lengths between Ar and heavy atoms in eight isomers of Ar-3,3-difluoro-1,2-epoxypropane obtained from ab initio calculations and from the experimental moments of inertia of the most abundant isotopologue. No BSSE Correction
BSSE Correction
No BSSE Correction
3.851 3.825 5.254 3.628 5.583 5.887
4.642 3.676 4.181 3.398 3.513 5.263
Structure (a) ArAC1 ArAC2 ArAC3 ArAO ArAFO ArAFC
No BSSE Correction
4.760 3.798 4.348 3.543 3.688 5.398
3.565 3.936 4.246 4.867 5.458 3.531
Structure (b)
3.721 3.676 5.104 3.483 5.433 5.747 Structure (e)
ArAC1 ArAC2 ArAC3 ArAO ArAFO ArAFC
BSSE Correction
4.745 4.390 3.950 3.707 3.725 5.199
4.098 4.462 3.815 4.920 4.971 3.531
No BSSE Correction
3.681 4.061 4.404 4.988 5.606 3.687
3.818 4.316 4.118 3.549 4.781 5.096
Structure (c)
Structure (f)
4.596 4.212 3.771 3.553 3.543 5.027
BSSE Correction
Structure (d)
Structure (g) 4.240 4.629 3.985 5.064 5.135 3.703
5.253 3.959 3.936 5.182 3.586 3.573
structures are listed in Table 3. In addition, we correct for basis set superposition error (BSSE) [11] for these isomers, and the relevant interaction lengths and molecular properties are also listed in Tables 2 and 3. As we have previously observed with TFO [9], the Ar-heavy atom distances in a BSSE corrected structure are typically longer than those in a structure without the correction, this time by 0.12–0.21 Å. The rotational constants for a BSSE corrected structure are thus typically smaller than those obtained without the correction, up to 4% for the A constants and 5–8% for the B and C constants. As with Ar-TFO [9], we 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) [12] to examine the importance of the intermolecular distances in each Ar-DFO isomer. Estimates of optimal interaction lengths are simply the sum of the van der Waals radii: 3.58 Å for ArAC, 3.35 Å for ArAF, and 3.40 Å for ArAO. We consider here bond lengths calculated without BSSE correction, and indicate in Fig. 4 the heavy atom distances that are between optimal to approximately 10% longer. The lowest energy structure, Structure (a), has a similar configuration as the lowest energy theoretical structure for Ar-TFO [9], with Ar interacting with the three atoms in the epoxy ring and with distances differing only by 0.001–0.015 Å from those in the TFO counterpart. All
BSSE Correction
3.942 4.472 4.287 3.687 4.953 5.256 Experimenta
Structure (h) 5.391 4.116 4.134 5.333 3.796 3.758
5.785 5.082 3.616 5.681 3.568 3.557
5.937 5.263 3.805 5.834 3.764 3.755
3.7577 3.7270 5.1551 3.4950 5.4768 5.8167
other structures are much higher in energy; thus, we expect to observe only this isomer under our experimental conditions.
3. Experiment The vapor pressure of DFO (SynQuest Laboratories) at room temperature is introduced into a sample cylinder and is diluted with seven atmospheres of argon for our work. We are unaware of any reported data for the vapor pressure of this chemical, and it is too low to measure when we prepare our sample. The gas mixture is likely to be about 0.1% or less of the epoxypropane. Nonetheless, it is adequate for our spectroscopic study. Using a backing pressure of 1–2 atm, we collect a broadband spectrum from 5.6 to 18.1 GHz using a chirped pulse Fourier transform microwave spectrometer. 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 ls duration and 20–25 W of power. The resulting free induction decay (FID) is digitized at 50 Gs s–1 for 10 ls beginning 0.5 ls after the end of the excitation pulse. Ten FIDs are collected during each 700 ls opening of the pulsed valves, which operate
Table 3 Rotational constants, dipole moment components, and relative energies for eight isomers of Ar-3,3-difluoro-1,2-epoxypropane obtained from ab initio calculations at the MP2/6311++G(2d,2p) level without and with BSSE correction. No BSSE Correction
BSSE Correction
Structure (a) A/MHz B/MHz C/MHz ma/D mb/D mc/D Energya/cm
1
3518 708 678 0.850 1.908 0.997 0.0
a
1
2424 1033 797 1.048 1.705 1.205 37.6
BSSE Correction
No BSSE Correction
2627 946 737 1.284 1.941 0.418 9.8
2722 940 721 1.971 1.129 0.512 23.0
2380 887 712 1.342 1.671 1.001 36.3
2822 882 806 1.207 0.908 1.738 61.8
Structure (b) 3499 672 645 0.847 1.890 1.041 0.0
Structure (e) A/MHz B/MHz C/MHz ma/D mb/D mc/D Energya/cm
No BSSE Correction
2650 1011 773 1.326 1.940 0.336 16.4
2390 952 753 1.339 1.685 0.984 53.7
No BSSE Correction
2721 883 688 1.971 1.120 0.556 14.1
2509 883 774 0.824 1.449 1.618 24.0
2734 830 756 1.146 0.997 1.737 52.8
3870 722 716 1.135 1.077 1.688 78.4
Structure (c)
Structure (f) 2417 955 750 1.018 1.706 1.231 32.6
BSSE Correction
BSSE Correction
Structure (d)
Structure (g)
2513 827 731 0.825 1.438 1.630 17.1
Structure (h)
The energy of the most stable isomer is set to 0 for the structures computed respectively with and without BSSE correction.
3730 683 673 1.103 1.079 1.715 64.4
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M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
at 4 Hz. To conserve sample, we use a much smaller number of averages (72,000–78,000) than we typically accumulate [13–15]. The averaged FID 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. This allows us to assign line centers with an estimated measurement uncertainty of 10–15 kHz. We are able to identify readily and assign rotational transitions for four isotopologues of the epoxypropane (the most abundant species and three isotopologues singly substituted with 13C) and for the most abundant isotopologue of the argon complex in this spectrum. The dynamic range and quality of the chirped pulse signals can be seen from a sample spectrum spanning 800 MHz (Fig. 5). The signals are typically strong, containing transitions from all singly substituted 13C and 18O isotopologues of the epoxypropane and the argon complex of the most abundant one. (The weak signals due to DFO singly substituted with 18O, can be seen and are labeled in Fig. 5, but were not initially assigned until identified later after further spectroscopic work described below.)
We turn to the more sensitive, higher resolution narrowband Balle-Flygare spectrometer [14,16], operating in the 5–21 GHz range, to measure more precisely transitions observed in the chirped pulse instrument and also weaker ones, including those due to DFO singly substituted with 18O, that are not immediately apparent in the chirped pulse spectrum. The backgroundcorrected time domain signals from the Balle-Flygare instrument are digitized for 1024 data points and zero-filled to a 2048-point record length before Fourier transformation to give frequency domain signals with a resolution element of 4.8 kHz. In our experience, line centers in this instrument are determined with a measurement uncertainty estimated to be 2 kHz. The spectra of singlysubstituted 13C isotopologues of Ar-DFO would have been valuable in determining the structure of this species, but despite extensive searching, none were observed. Given the low signal strength of the parent, the available signal to noise ratio is not sufficient to see transitions for the 13C isotopologues in natural abundance. 4. Results 4.1. Spectral analysis
Fig. 5. A 800 MHz segment of the chirped pulse spectrum taken with 3,3-difluoro1,2-epoxypropane in Ar. The spectrum with its intensity scaled to display all lines is shown in the lower display. It is magnified 50 times in the upper display to show more clearly the weaker signals. The lines marked in red, blue, green, purple, and orange, respectively, are due to the most abundant species of 3,3-difluoro-1,2epoxypropane, its isotopologues substituted with 13C in the C-1, C-2, C-3 positions, and that substituted with 18O. Those marked in brown are due to Ar-3,3-difluoro1,2-epoxypropane. All marked lines have also been observed with the Balle-Flygare instrument, but two of them, marked with upside down triangles, have broad linewidths with distorted shapes; they are not included in the spectral fits.
4.1.1. 3,3-Difluoro-1,2-epoxypropane We have observed and assigned 94 rotational transitions in the Balle-Flygare spectrum for the most abundant isotopologue of DFO, and 40 or more transitions for each of the isotopologues singly substituted with 13C and 18O. All three types (a, b, and c) of transitions are observed, sampling J from 0 to 10, and Ka from 0 to 4 for the most abundant species, and narrower ranges for the less abundant species. The spectrum of each isotopologue is analyzed using the Watson A-reduced Hamiltonian [17] and Pickett’s nonlinear SPFIT program [18]. We determine 3 rotational constants and 4–5 quartic centrifugal distortion constants with an rms deviation of less than 2 kHz, commensurate with the resolution of the narrowband 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 Supplementary material. 4.1.2. Ar-3,3-difluoro-1,2-epoxypropane We have assigned 269 rotational transitions in the Balle-Flygare spectrum for the most abundant isotopologue of Ar-DFO. Once again, all three types of transitions (a, b, and c) are present in large ranges of J (1–16) and Ka (0–4). As described in Section 4.2.2, the complex is very nearly prolate. Thus, the transitions are analyzed
Table 4 Spectroscopic constants (in MHz, unless as otherwise noted) for five isotopologues of 3,3-difluoro-1,2-epoxypropane.a,b
A B C DJ/10 3 DJK/10 3 DK/10 3 dJ/10 3 dK/10 3 No. of rotational transitions No. of a type No. of b type No. of c type J range Ka range rms/kHz a b
CH2CH(CHF2)O
13
CH213CH(CHF2)O
CH2CH(13CHF2)O
CH2CH(CHF2)18O
6967.48141(19) 2693.447203(61) 2103.668834(57) 0.35909(74) 2.9209(32) 0.0822(93) 0.08393(21) 1.8157(51) 94 21 46 27 0–10 0–4 1.60
6920.50953(21) 2647.65783(11) 2071.62091(12) 0.3566(20) 2.7898(91) [0.0822] 0.08466(96) 1.750(26) 43 15 26 2 0–6 0–3 1.11
6945.99119(30) 2684.84564(16) 2100.32966(17) 0.3632(30) 2.867(13) [0.0822] 0.0837(14) 1.735(37) 43 15 26 2 0–6 0–3 1.63
6959.38537(28) 2686.34920(15) 2100.15878(16) 0.3677(27) 2.958(12) [0.0822] 0.0846(13) 1.871(34) 43 15 26 2 0–6 0–3 1.50
6872.87626(23) 2614.78590(12) 2047.48850(15) 0.3535(24) 2.649(12) [0.0822] 0.0773(16) 1.748(34) 40 15 23 2 0–6 0–3 1.18
CH2CH(CHF2)O
1r standard deviations in the parameters are given in parentheses. Quantities in square brackets are fixed to the value appropriate to the most abundant isotopologue.
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M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26 Table 5 Spectroscopic constants (in MHz, unless as otherwise noted) for the most abundant isotopologue of the Ar-3,3-difluoro-1,2-epoxypropane complex.a A-reduction A B C DJ/10 3 DJK/10 3 DK/10 3 dJ/10 3 dK/10 3 UJ/10 6 UJK/10 6 UKJ/10 6 UK/10 6 No. of rotational No. of a type No. of b type No. of c type J range Ka range rms/kHz a
S-reduction 3565.00650(13) 695.214576(33) 665.584346(33) 1.17477(22) 8.7524(17) 98.734(17) 0.136210(31) 2.4117(94) 0.01306(54) 0.3964(87) 5.424(64) 17.74(69) transitions
A B C DJ/10 3 DJK/10 3 DK/10 3 d1/10 3 d2/10 3 HJ/10 6 HJK/10 6 HKJ/10 6 HK/10 6 269 87 117 65 1–16 0–4 1.30
Table 6 The coordinates of the atoms in 3,3-difluoro-1,2-epoxypropane determined from a structural fit and the substitution coordinates for four atoms from a Kraitchman analysis.a a/Å
3565.00653(13) 695.209740(27) 665.589157(26) 1.16858(22) 8.7152(17) 98.704(17) 0.136210(31) 0.003086(12) 0.01320(54) 0.3954(87) 5.419(64) 17.73(69)
1r standard deviations in the parameters are given in parentheses.
using both the Watson A-reduced and S-reduced Hamiltonians [17] and Pickett’s nonlinear SPFIT program [18]. For each fit, we obtain 3 rotational constants, and 5 quartic and 4 sextic centrifugal distortion constants (Table 5) with an rms deviation of less than 2 kHz. A table containing observed and calculated transition frequencies (using the A-reduced Hamiltonian) with assignments is in Supplementary material. (A similar table using the S-reduced Hamiltonian would be identical to the 0.0001 MHz precision used except for seven lines for which the difference is 0.0001 MHz, much smaller than the resolution of the spectrometer.) 4.2. Structure determination 4.2.1. 3,3-Difluoro-1,2-epoxypropane 3,3-Difluoro-1,2-epoxypropane is a near prolate asymmetric top with an asymmetry parameter value between 0.765 and 0.757 for the various isotopologues. The experimental A, B, and C rotational constants of the most abundant species agree excellently with the theoretical values, differing by 58 MHz (0.83%), 12 MHz (0.43%), and 2 MHz (0.07%), respectively. We fit 5 structural parameters of the molecule using 15 moments of inertia of the 5 isotopologues with Kisiel’s STRFIT program [19]. These parameters involve the C and O atoms: the C1AO, C2AO, C2AC3 bond lengths and the C2AOAC1 and C3AC2AC1 angles; all other parameters are fixed to their ab initio values. The rms deviation of the fit is 0.0078 u Å2. The resulting structural parameters, calculated by Kisiel’s EVAL program [20,21] are listed in Table 1, and the positions of all atoms in the principal axis system are in Table 6. Alternatively, the positions of the C and O atoms can be determined with a Kraitchman analysis [22]: by treating each less abundant isotopologue as a single isotopic substitution in the most abundant species, the absolute values of the coordinates of the substituted atom in the principal axis system of the parent molecule can be calculated. Using chemically reasonable bond lengths, the signs of the coordinates can usually be assigned (Table 6). The substitution coordinates thus derived compare well with those from the structural fit, either agreeing to within experimental uncertainties or differing by no more than 0.007 Å from each other. The unphysical Kraitchman b coordinate of C3 indicates that this atom lies very close to the a-c plane. Indeed, according to the structural fit, its b coordinate is 0.01272(12) Å.
b/Å
c/Å
(i) From structural fit C1 1.8054(14) C2 0.6244(28) C3 0.64698(58) O 1.68749(84) FO 1.37773(66) FC 1.3980(10) H1 2.5598(11) H2 1.7250(26) H3 0.5148(39) H4 0.4824(18)
0.70007(27) 0.0308(11) 0.01272(12) 0.71486(27) 1.09202(53) 1.10675(55) 1.08714(82) 1.17205(12) 0.0543(11) 0.00918(25)
0.10061(35) 0.47718(48) 0.29831(45) 0.11453(19) 0.02887(53) 0.03535(38) 0.5669(11) 1.06938(27) 1.54969(40) 1.37192(64)
(ii) Substitution coordinatesb C1 1.80120(83) C2 0.6177(24) C3 0.6397(23) O 1.68483(89)
0.7064(21) 0.051(29) Nonphysical 0.7209(21)
0.104(14) 0.4732(32) 0.3003(50) 0.117(13)
a
Costain errors [27] in the parameters are given in parentheses. 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. b
4.2.2. Ar-3,3-difluoro-1,2-epoxypropane Ar-3,3-difluoro-1,2-epoxypropane is a near prolate asymmetric top with an asymmetry parameter of 0.980. The experimental rotational constants (either from the A or S-reduction) agree very well with the theoretical values for Structure (a). Specifically, the experimental A constant is greater than those predicted theoretically: by 47 MHz (1.3%) without BSSE correction and 66 MHz (1.9%) with BSSE correction. While the experimental B and C values are smaller than the theoretical ones without BSSE correction by 13 (1.8%) and 12 (1.8%) MHz, respectively, they are greater than those with BSSE correction by 23 (3.5%) and 21 (3.2%) MHz, respectively. Thus, the rotational constants strongly support a structure where Ar interacts with the atoms in the epoxy ring and on the side opposite from the CHF2 group. We can still determine the position of Ar using the moments of inertia of the most abundant species, albeit without any estimate of the uncertainties of its coordinates because we do not observe the spectra of any minor isotopologues. The values of the moments of inertia derived using the Watson A and S-reduced Hamiltonians are slightly different, but the resulting principal atomic coordinates differ by no more than 0.0003 Å (and by no more than 0.0001 Å for the heavy atoms). In fact, the distances between Ar and each of the atoms in the epoxy ring are identical when they are calculated from the moments of inertia determined by either Hamiltonian, and they are shown in Fig. 6. Argon lies more or less above the O atom, and is 3.493 Å above the epoxy plane. The principal coordinates of all atoms in the complex derived from the moments of inertia using the A-reduced Hamiltonian are listed in Table 7. 5. Discussion Well guided by theory, we have determined the structures of 3,3-difluoro-1,2-epoxypropane and its complex with argon. We will first examine the effects of substituents on the epoxy ring. In a previous study, we have compared the ring size for compounds with no substituent (ethylene oxide) [23], a methyl substituent (propylene oxide) [24], and a trifluoromethyl substituent (3,3,3-tri fluoro-1,2-epoxypropane) [9]. Here, we extend the list with a difluoromethyl substituent, and we have listed the CAC and CAO bond lengths and the C1C2O bond angle of all four species in Table 8. While the lengths of all bonds in the epoxy ring in propylene oxide are practically the same within experimental uncertainties as
24
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
Fig. 6. The experimental structures of (a) Ar-3,3-difluoro-1,2-epoxypropane (this work), (b) Ar-3,3,3-trifluoro-1,2-epoxypropane [9], and (c) Ar-ethylene oxide [25]. Atom colors: C, dark gray; H, light gray; O, red; F, light blue, Ar: purple. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 7 The coordinates of the atoms in Ar-3,3-difluoro-1,2-epoxypropane determined from the experimental moments of inertia of the most abundant isotopologue.
C1 C2 C3 O FO FC H1 H2 H3 H4 Ar
a/Å
b/Å
c/Å
0.0506 0.5553 2.0161 0.0872 2.2284 2.6146 0.8708 0.7486 0.0163 2.4965 3.1151
1.6228 0.2366 0.0006 0.8003 1.2116 0.0380 1.8363 2.4406 0.5436 0.7643 0.3982
0.0753 0.0688 0.0995 1.0739 0.6886 1.1304 0.5950 0.0325 0.5888 0.7042 0.1951
those in ethylene oxide, it is not the case for the other two species. The length of the CAC bond in DFO is 1.4753(25) Å, which is the same, to experimental uncertainty, as that [1.470(3) Å] in ethylene oxide [23]. The CAO bonds in DFO are not equivalent, with C1AO shorter than C2AO by 0.007 Å, which is 3 times the greater uncertainty of the two lengths. These are, respectively, 0.014 Å and 0.007 Å shorter than those bonds in ethylene oxide. The addition of a ACHF2 substituent therefore has a slight effect on the epoxy ring. In the presence of a ACF3 substituent, the CAO bonds are
not affected: they are the same, to experimental uncertainty, as those in ethylene oxide. The CAC bond, however, becomes shorter by 0.015 Å. At first, these results seem puzzling because the ACHF2 and ACF3 substituents appear to have an opposite effect on the ring. However, it is important to realize that these experimental bond lengths are affected by differences in vibrational averaging in addition to changes in electron distribution. To eliminate the effect of vibrational averaging, we compare the ab initio equilibrium structures of these four species, all determined at the MP2/6-311++G(2d,2p) level. The lower portion of Table 8 lists the resulting equilibrium values for the same CAC and CAO bond lengths in these molecules. Now it is possible to see sensible trends. At the point of attachment (C2), the ACH3 results in a very slight, 0.0004 Å, lengthening of the ring CAC bond (C2AC1) with a somewhat greater 0.0031 Å effect on the C2AO bond length. The C1AO bond length increases by 0.0051 Å in response to these changes and an accompanying slight increase (0.15°) in the C1C2O angle. Now both ACHF2 and ACF3 are seen to have the same effect, with the latter being greater. For both these fluorinated substituents, the ring bonds involving C2 shorten. The C2AC1 bond is 0.0023 Å (0.0028 Å) shorter in DFO (TFO) than in ethylene oxide, and the C2AO bonds 0.0064 Å (0.0170 Å) shorter in DFO (TFO). Although these bonds ae getting shorter, the C1C2O angle opens up by 0.37° in DFO compared to ethylene oxide and that for TFO
Table 8 The experimental (average) and theoretical (equilibrium) CAC and CAO bond lengths and the C1C2O bond angle in the epoxy ring for ethylene oxide [23], propylene oxide [24], 3,3-difluoro-1,2-epoxypropane, and 3,3,3-trifluoro-1,2-epoxypropane [9]. C1, C2, and O form the epoxy ring, and C3 is in the substituent is connected to C2. Ethylene oxide
Propylene oxide
3,3-Difluoro-1,2-epoxypropane
3,3,3-Trifluoro-1,2-epoxypropane
Average structure C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å \C1C2O/°
1.434(3) 1.434(3) 1.470(3) – 59.17
1.441(2) 1.435(3) 1.470(3) 1.505(2) 59.46
1.41990(40) 1.4270(23) 1.4753(25) 1.4893(24) 58.55
1.4374(28) 1.4342(66) 1.4555(65) 1.4795(41) 59.65
Equilibrium structure C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å \C1C2O/°
1.4422 1.4422 1.4649 – 59.48
1.4473 1.4453 1.4653 1.4987 59.63
1.4460 1.4358 1.4626 1.4919 59.85
1.4484 1.4252 1.4621 1.4990 60.20
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
by 0.72°, resulting in longer C1AO bonds in each case. We see then that the electron donating methyl group lengthens the ring bonds at the point of attachment, but electron withdrawing fluoromethyl groups shorten those bonds to a degree that seems to depend on the number of fluorine atoms. Both types of substitution lead to an increase in the ring bond angle at that point. The trend in the C2AC3 bond length, between the ring carbon and the carbon of the substituent, shows similar contrary behavior. The values for the vibrationally averaged structures show a monotonic decrease from ACH3 to ACHF2 to ACF3 substitution, but those for the equilibrium structures are longest for TFO and shortest for DFO. This could be a result of an electrostatically induced shortening of the bond upon fluorine substitution competing with an unfavorable steric interaction in TFO that places the third fluorine atom close to C1 and O. This can be seen by comparing of Fig. 1 here with Fig. 1 of Ref. [9]. Ethylene oxide [25,26], 3,3-difluoro-1,2-epoxypropane, and 3,3,3-trifluoro-1,2-epoxypropane [9] all offer the same site for argon to bind (Fig. 6): on one side of the epoxy ring. While the ArAC bonds in Ar-ethylene oxide are longer by 0.065–0.102 Å than those in ArDFO and Ar-TFO, the ArAO bond in Ar-ethylene oxide is the shortest of the three, by 0.026–0.047 Å. The corresponding ArAC distances in both Ar-DFO and Ar-TFO are quite similar, with the ArAC1 bond longer (by 0.031–0.037 Å) than the ArAC2 bond. The ArAO bond in Ar-DFO is 0.021 Å longer than that in Ar-TFO. The Ar atom is located 3.436 Å, 3.493 Å, and 3.473 Å above the epoxy ring of ethylene oxide, Ar-DFO, and Ar-TFO, respectively, with projections onto the ring only 0.29 Å, 0.13 Å, and 0.10 Å away from the O atom; thus, Ar locates approximately above the O atom in all three cases. This atom is the most negative portion of the molecules, as evidenced from the mapped electrostatic potential surfaces of ethylene oxide and TFO given in Ref. [9]. (Although not presented here, the surface for DFO is similar to that for TFO.) Argon, in this case, serves well as a probe for electron density. Of course, steric considerations might well prevent it from favorable interactions with both O and F atoms in the case of the substituted ethylene oxide. 6. Conclusion 3,3-difluoro-1,2-epoxypropane has a strong, simple microwave rotational spectrum that is well predicted by ab initio calculations. Although work needs to be done to show that it readily forms noncovalently bound heterodimers with potential analyte species, it meets the first test necessary for use as a chiral tag. Many of the molecule’s structural parameters have been refined using the experimental results, and the positions of the carbon and oxygen atoms well determined via analysis of singly substituted isotopologues. As could be expected, the structure (and spectrum) is very similar to those of the analogous molecule 3,3,3-trifluoro-1,2epoxypropane [9] as is the nature of the complex with the argon atom. Although partially masked by the effects of vibrational averaging, ab initio structures for the series of molecules ethylene oxide, propylene oxide, 3,3-difluoro-1,2-epoxypropane, and 3,3,3trifluoro-1,2-epoxypropane can be rationalized in terms of the relative electron donating versus electron withdrawing nature of the substituent. Competing financial interest The authors declare no competing financial interest. Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. CHE-1465014.
25
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jms.2018.05.003.
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Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, G.A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.V. Marenich, J. Bloino, B.G. Janesko, R. Gomperts, B. Mennucci, H.P. Hratchian, J.V. Ortiz, A.F. Izmaylov, J.L. Sonnenberg, F. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V.G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M.J. Bearpark, J.J. Heyd, E.N. Brothers, K.N. Kudin, V.N. Staroverov, T.A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A.P. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, J.M. Millam, M. Klene, C. Adamo, R. Cammi, J.W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J.B. Foresman, D.J. Fox, Gaussian 16, Wallingford, CT, 2016. [11] S.F. Boys, F. Bernardi, The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors, Mol. Phys. 19 (1970) 553–566. [12] A. Bondi, van der Waals volumes and radii, J. Phys. Chem. 68 (1964) 441–451. [13] M.D. Marshall, H.O. Leung, B.Q. Scheetz, J.E. Thaler, J.S. Muenter, A chirped pulse Fourier transform microwave study of the refrigerant alternative 2,3,3,3tetrafluoropropene, J. Mol. Spectrosc. 266 (2011) 37–42. [14] M.D. Marshall, H.O. Leung, C.E. Calvert, Molecular structure of the argon-(Z)-1chloro-2-fluoroethylene complex from chirped-pulse and narrow-band Fourier transform microwave spectroscopy, J. Mol. Spectrosc. 280 (2012) 97– 103. [15] H.O. Leung, M.D. Marshall, J.P. Messinger, G.S. Knowlton, K.M. Sundheim, J.C. Cheung-Lau, The microwave spectra and molecular structures of 2-chloro-1,1difluoroethylene and its complex with the argon atom, J. Mol. Spectrosc. 305 (2014) 25–33. [16] H.O. Leung, D. Gangwani, J.U. Grabow, Nuclear quadrupole hyperfine structure in the microwave spectrum of Ar-N2O, J. Mol. Spectrosc. 184 (1997) 106–112. [17] J.K.G. Watson, Aspects of quartic and sextic centrifugal effects on rotational energy levels, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier Scientific Publishing, Amsterdam, 1977, pp. 1–89. [18] H.M. Pickett, The fitting and prediction of vibration-rotation spectra with spin interactions, J. Mol. Spectrosc. 148 (1991) 371–377. [19] Z. Kisiel, Least-squares mass-dependence molecular structures for selected weakly bound intermolecular clusters, J. Mol. Spectrosc. 218 (2003) 58–67. [20] Z. Kisiel, PROSPE – Programs for ROtational SPEctroscopy, 2018. (accessed February 23, 2018). [21] Z. Kisiel, Assignment and analysis of complex rotational spectra, in: J. Demaison, K. Sarka, E.A. Cohen (Eds.), Spectroscopy from Space, Kluwer Academic Publishers, Dordrecht, 2001. [22] J. Kraitchman, Determination of molecular structure from microwave spectroscopic data, Am. J. Phys. 21 (1953) 17–24. [23] C. Hirose, The microwave spectra and r0, rs, and rm structures of ethylene oxide, Bull. Chem. Soc. Jpn. 47 (1974) 1311–1318.
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[24] M. Imachi, R.L. Kuczkowski, The microwave spectrum and structure of propylene oxide, J. Mol. Struct. 96 (1982) 55–60. [25] P. Moreschini, W. Caminati, P.G. Favero, A.C. Legon, Pathways for inversion in the oxirane-argon complex, J. Mol. Struct. 599 (2001) 81–87.
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The microwave spectrum and molecular structure of 3,3-difluoro-1,2epoxypropane and its complex with the argon atom Mark D. Marshall, Helen O. Leung Department of Chemistry, Amherst College, P.O. Box 5000, Amherst, MA 01002-5000, United States
a r t i c l e
i n f o
Article history: Received 3 April 2018 In revised form 4 May 2018 Accepted 8 May 2018 Available online 16 May 2018
a b s t r a c t As a small, chiral molecule with a strong and simple microwave rotational spectrum, 3,3-difluoro-1,2epoxypropane is a promising candidate as a chiral tag for conversion of enantiomeric analytes into spectroscopically distinct diastereomers via formation of a non-covalently bound complex. The Fourier transform microwave spectra of this species and its complex with the argon atom are obtained from 5.1 to 21.0 GHz, and their structures determined. Not surprisingly, these structures for both the monomer and the argon complex show strong similarities with those previously determined for 3,3,3-trifluoro-1,2epoxypropane and 3,3,3-trifluoro-1,2-epoxypropane–argon, but are different from those of the analogous propylene oxide species. This suggests that fluoromethyl and methyl substitution affect the bonding in the epoxy moiety and intermolecular interactions in distinctly different ways. Ó 2018 Elsevier Inc. All rights reserved.
1. Introduction Recently, chiral tagging has been proposed as a fast, efficient method for chiral analysis [1]. In this method, a molecule of known absolute configuration (the tag) is bound via non-covalent interactions to a chiral analyte. This converts the two enantiomers of the analyte into spectroscopically distinct diastereomers that can be unambiguously characterized via microwave spectroscopy. The method can provide a determination of absolute stereochemistry for an enantiopure analyte and a measurement of entiomeric excess for a sample of mixed chirality. Furthermore, chiral tagging avoids the ‘‘zero-time” problem associated with three-wave mixing [2–8] where the determination of absolute stereochemistry requires detailed knowledge of all the phase shifts in the spectrometer. Obviously, chiral tagging depends on the availability of suitable tags. These molecules must be readily available in enantiopure form. The tag should be a small molecule so that the rotational constants of the non-covalent complex are not too small, and the tag itself and its complexes with the carrier gases typically used for free-jet expansion need to have well characterized rotational spectra. Recently, we have begun exploring a series of tag candidates based upon substituted ethylene oxides. Our first example was 3,3,3-trifluoro-1,2-epoxypropane [also known as 2(trifluoromethyl)-oxirane, or TFO] [9]. Because it will be useful to have several alternatives tags available as the technique is further
E-mail addresses: [email protected] (M.D. Marshall), hleung@amherst. edu (H.O. Leung) https://doi.org/10.1016/j.jms.2018.05.003 0022-2852/Ó 2018 Elsevier Inc. All rights reserved.
developed, we present here the microwave spectrum of 3,3difluoro-1,2-epoxypropane [2-(difluoromethyl)-oxirane or DFO] and its complex with the argon atom. 2. Ab initio calculations To guide our experimental work, we use ab initio calculations at the MP2/6-311++G(2d,2p) level with GAUSSIAN 16 [10] to provide theoretical structures for both the 3,3-difluoro-1,2-epoxypropane monomer and its complex with argon. The equilibrium structure of the monomer, together with the labeling scheme, is shown in Fig. 1 and bond lengths and angles are listed in Table 1. (The coordinates of the atoms in the principal inertial axis system are given as Supplementary material.) The H atoms, H3 and H4, respectively bonded to C2 and C3, are almost anti to each other when viewed in a Newman projection along C2AC3 (Fig. 1b). Using this configuration as a reference, we label the F atom on the same side as the O atom as FO and that on the same side as C1 as FC. This configuration makes possible two intramolecular HAF bonds: 2.705 Å for H3AFC and 2.643 Å for H3AFO. Additionally, H4 is 2.615 Å from O, well positioned to interact with it. The stability conferred by these interactions can be observed by examining the energy of the molecule when the H4-C3-C2-C1 dihedral angle is varied from 0° to 360° in steps of 10° while the structure of the rest of the molecule is allowed to relax (Fig. 2). There are three minima. The global minimum is at 31°, with H4 almost anti to H3 as described in Fig. 1. The two local minima are at 137° and 264°, corresponding to H4 occupying approximately the positions of FO and FC in Fig. 1, respectively. As mentioned earlier, the global minimum permits three
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
19
Fig. 1. (a) The labeling scheme used to describe the structure of 3,3-difluoro-1,2-epoxypropane. The labeling of the F atoms is illustrated in (b) using a Newman projection along the C2AC3 bond. Atom colors: C, dark gray; H, light gray; O, red; F, light blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Structural parameters for 3,3-difluoro-1,2-epoxypropane using ab initio calculation and a structure fit to the moments of inertia of 5 isotopologues of the molecule.
C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å C3AFO/Å C3AFC/Å C1AH1/Å C1AH2/Å C2AH3/Å C3AH4/Å \C1C2O/° \C2OC1/° \OC1C2/° \H1C1C2/° \H2C1C2/° \H3C2C1/° \H4C3C2/° \C3C2C1/° \FOC3C2/° \FCC3C2/° \H1C1C2O/° \H2C1C2O/° \H3C2C1O/° \H4C3C2C1/° \C3C2C1O/° \FOC3C2C1/° \FCC3C2C1/°
Theory
Experimenta
1.4460 1.4358 1.4626 1.4919 1.3644 1.3683 1.0791 1.0806 1.0815 1.0862 59.846 60.998 59.156 118.693 119.320 120.346 112.669 118.972 110.009 109.384 103.410 103.006 104.318 30.713 102.441 151.778 90.779
1.41990(40) 1.4270(23) 1.4753(25) 1.4893(24) 1.3644 1.3683 1.0791 1.0806 1.0815 1.0862 58.55(10) 62.425(83) 59.023(70) 118.693 119.320 120.346 112.669 119.007(56) 110.009 109.384 103.410 103.006 104.318 30.713 102.441 151.778 90.779
a 1r standard deviations in the parameters are given in parentheses. The parameters without uncertainties are fixed to the ab initio values
interactions, but there are only two interactions for the local minima, FAH2 and FAH3, while H4 does not interact with an electronegative atom in either case. The global minimum structure is significantly lower in energy than the other two structures (by 171 and 479 cm 1, respectively). Furthermore, the barrier to interconversion between the global minimum structure and either higher energy structure is more than 800 cm 1. As a result, the CHF2 group is not expected to exhibit internal rotation. The three rotational constants of DFO are calculated to be 6910, 2705, and 2105 MHz, and the dipole moment is 2.400 D, with components of 1.508, 1.850, and 0.248 D along the a, b, and c inertial axes, respectively. Accordingly, we are able to observe both a and b type transitions in both the chirped pulse and the Balle-Flygare spectrometers for the most abundant isotopologue and those sin-
Fig. 2. A relaxed scan of the dihedral angle formed by H4 with C3, C2, and C1 in 3,3difluoro-1,2-epoxypropane from 0° to 360° in 10° steps. The minima are optimized and displayed in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
gly substituted with 13C, but the much weaker c type transitions of these species and the spectrum of the 18O isotopologue show up much more clearly in the more sensitive Balle-Flygare instrument. After establishing the average structure of DFO, as described in Section 4.2.1, we use ab initio calculations to determine its interaction energy with argon. The coordinate system we use is given in Fig. 3. The origin is the center of mass of the epoxypropane, with the x, y, and z-axes coincident with the a, b, and c inertial axes of the average structure of the molecule. The position of Ar is described using spherical polar coordinates, where R is the distance between Ar and the origin of the axis system, h is the polar angle formed by R and the z axis, the / is the azimuthal angle between the projection of R onto the x-y plane and the x axis. The potential energy of the system is then calculated by varying the values of h from 5° to 175° and the values of / from 0° to 360°, both in 10° steps, while allowing R to optimize. The resulting energy contour (Fig. 3) is remarkably similar to that obtained for the interaction between Ar and 3,3,3,-trifluoro-1,2-epoxypropane [also known as 2-(trifluoromethyl)-oxirane or TFO] [9] except that
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Fig. 3. Left: the spherical polar coordinate system used to locate Ar with respect to 3,3-difluoro-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. R is the distance between Ar and the origin, and h and / (not shown) are, respectively, the polar and azimuthal angles formed by R and the coordinate system. Right: the potential energy contour as a function of the angles with R optimized. Eight minima are located and optimized, and their corresponding structures are shown in Fig. 4. Atom colors: C, dark gray; H, light gray; O, red; F, light blue; Ar, purple. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
in the present case, eight minima, instead of six, can be identified. These are labeled (a) – (h) in order of increasing energy. The two minima (e) and (f) observed here have no equivalent in the ArTFO interaction potential; they are in shallow wells and are separated by low barriers (10–15 cm 1) from the nearby deeper wells labeled (b) and (c), respectively.
The structures at the eight minima are subsequently optimized; they are shown graphically in Fig. 4, and the interaction lengths between Ar and each heavy atom are listed in Table 2. (The atomic positions for each isomer, in its principal coordinate system, are available as Supplementary material.) The rotational constants, dipole moment components, and relative energies of these
Fig. 4. The optimized structures and relative energies (without BSSE correction) corresponding to the eight minima found in the potential scan for Ar-3,3-difluoro-1,2epoxypropane. 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. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 2 Interaction lengths between Ar and heavy atoms in eight isomers of Ar-3,3-difluoro-1,2-epoxypropane obtained from ab initio calculations and from the experimental moments of inertia of the most abundant isotopologue. No BSSE Correction
BSSE Correction
No BSSE Correction
3.851 3.825 5.254 3.628 5.583 5.887
4.642 3.676 4.181 3.398 3.513 5.263
Structure (a) ArAC1 ArAC2 ArAC3 ArAO ArAFO ArAFC
No BSSE Correction
4.760 3.798 4.348 3.543 3.688 5.398
3.565 3.936 4.246 4.867 5.458 3.531
Structure (b)
3.721 3.676 5.104 3.483 5.433 5.747 Structure (e)
ArAC1 ArAC2 ArAC3 ArAO ArAFO ArAFC
BSSE Correction
4.745 4.390 3.950 3.707 3.725 5.199
4.098 4.462 3.815 4.920 4.971 3.531
No BSSE Correction
3.681 4.061 4.404 4.988 5.606 3.687
3.818 4.316 4.118 3.549 4.781 5.096
Structure (c)
Structure (f)
4.596 4.212 3.771 3.553 3.543 5.027
BSSE Correction
Structure (d)
Structure (g) 4.240 4.629 3.985 5.064 5.135 3.703
5.253 3.959 3.936 5.182 3.586 3.573
structures are listed in Table 3. In addition, we correct for basis set superposition error (BSSE) [11] for these isomers, and the relevant interaction lengths and molecular properties are also listed in Tables 2 and 3. As we have previously observed with TFO [9], the Ar-heavy atom distances in a BSSE corrected structure are typically longer than those in a structure without the correction, this time by 0.12–0.21 Å. The rotational constants for a BSSE corrected structure are thus typically smaller than those obtained without the correction, up to 4% for the A constants and 5–8% for the B and C constants. As with Ar-TFO [9], we 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) [12] to examine the importance of the intermolecular distances in each Ar-DFO isomer. Estimates of optimal interaction lengths are simply the sum of the van der Waals radii: 3.58 Å for ArAC, 3.35 Å for ArAF, and 3.40 Å for ArAO. We consider here bond lengths calculated without BSSE correction, and indicate in Fig. 4 the heavy atom distances that are between optimal to approximately 10% longer. The lowest energy structure, Structure (a), has a similar configuration as the lowest energy theoretical structure for Ar-TFO [9], with Ar interacting with the three atoms in the epoxy ring and with distances differing only by 0.001–0.015 Å from those in the TFO counterpart. All
BSSE Correction
3.942 4.472 4.287 3.687 4.953 5.256 Experimenta
Structure (h) 5.391 4.116 4.134 5.333 3.796 3.758
5.785 5.082 3.616 5.681 3.568 3.557
5.937 5.263 3.805 5.834 3.764 3.755
3.7577 3.7270 5.1551 3.4950 5.4768 5.8167
other structures are much higher in energy; thus, we expect to observe only this isomer under our experimental conditions.
3. Experiment The vapor pressure of DFO (SynQuest Laboratories) at room temperature is introduced into a sample cylinder and is diluted with seven atmospheres of argon for our work. We are unaware of any reported data for the vapor pressure of this chemical, and it is too low to measure when we prepare our sample. The gas mixture is likely to be about 0.1% or less of the epoxypropane. Nonetheless, it is adequate for our spectroscopic study. Using a backing pressure of 1–2 atm, we collect a broadband spectrum from 5.6 to 18.1 GHz using a chirped pulse Fourier transform microwave spectrometer. 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 ls duration and 20–25 W of power. The resulting free induction decay (FID) is digitized at 50 Gs s–1 for 10 ls beginning 0.5 ls after the end of the excitation pulse. Ten FIDs are collected during each 700 ls opening of the pulsed valves, which operate
Table 3 Rotational constants, dipole moment components, and relative energies for eight isomers of Ar-3,3-difluoro-1,2-epoxypropane obtained from ab initio calculations at the MP2/6311++G(2d,2p) level without and with BSSE correction. No BSSE Correction
BSSE Correction
Structure (a) A/MHz B/MHz C/MHz ma/D mb/D mc/D Energya/cm
1
3518 708 678 0.850 1.908 0.997 0.0
a
1
2424 1033 797 1.048 1.705 1.205 37.6
BSSE Correction
No BSSE Correction
2627 946 737 1.284 1.941 0.418 9.8
2722 940 721 1.971 1.129 0.512 23.0
2380 887 712 1.342 1.671 1.001 36.3
2822 882 806 1.207 0.908 1.738 61.8
Structure (b) 3499 672 645 0.847 1.890 1.041 0.0
Structure (e) A/MHz B/MHz C/MHz ma/D mb/D mc/D Energya/cm
No BSSE Correction
2650 1011 773 1.326 1.940 0.336 16.4
2390 952 753 1.339 1.685 0.984 53.7
No BSSE Correction
2721 883 688 1.971 1.120 0.556 14.1
2509 883 774 0.824 1.449 1.618 24.0
2734 830 756 1.146 0.997 1.737 52.8
3870 722 716 1.135 1.077 1.688 78.4
Structure (c)
Structure (f) 2417 955 750 1.018 1.706 1.231 32.6
BSSE Correction
BSSE Correction
Structure (d)
Structure (g)
2513 827 731 0.825 1.438 1.630 17.1
Structure (h)
The energy of the most stable isomer is set to 0 for the structures computed respectively with and without BSSE correction.
3730 683 673 1.103 1.079 1.715 64.4
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M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
at 4 Hz. To conserve sample, we use a much smaller number of averages (72,000–78,000) than we typically accumulate [13–15]. The averaged FID 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. This allows us to assign line centers with an estimated measurement uncertainty of 10–15 kHz. We are able to identify readily and assign rotational transitions for four isotopologues of the epoxypropane (the most abundant species and three isotopologues singly substituted with 13C) and for the most abundant isotopologue of the argon complex in this spectrum. The dynamic range and quality of the chirped pulse signals can be seen from a sample spectrum spanning 800 MHz (Fig. 5). The signals are typically strong, containing transitions from all singly substituted 13C and 18O isotopologues of the epoxypropane and the argon complex of the most abundant one. (The weak signals due to DFO singly substituted with 18O, can be seen and are labeled in Fig. 5, but were not initially assigned until identified later after further spectroscopic work described below.)
We turn to the more sensitive, higher resolution narrowband Balle-Flygare spectrometer [14,16], operating in the 5–21 GHz range, to measure more precisely transitions observed in the chirped pulse instrument and also weaker ones, including those due to DFO singly substituted with 18O, that are not immediately apparent in the chirped pulse spectrum. The backgroundcorrected time domain signals from the Balle-Flygare instrument are digitized for 1024 data points and zero-filled to a 2048-point record length before Fourier transformation to give frequency domain signals with a resolution element of 4.8 kHz. In our experience, line centers in this instrument are determined with a measurement uncertainty estimated to be 2 kHz. The spectra of singlysubstituted 13C isotopologues of Ar-DFO would have been valuable in determining the structure of this species, but despite extensive searching, none were observed. Given the low signal strength of the parent, the available signal to noise ratio is not sufficient to see transitions for the 13C isotopologues in natural abundance. 4. Results 4.1. Spectral analysis
Fig. 5. A 800 MHz segment of the chirped pulse spectrum taken with 3,3-difluoro1,2-epoxypropane in Ar. The spectrum with its intensity scaled to display all lines is shown in the lower display. It is magnified 50 times in the upper display to show more clearly the weaker signals. The lines marked in red, blue, green, purple, and orange, respectively, are due to the most abundant species of 3,3-difluoro-1,2epoxypropane, its isotopologues substituted with 13C in the C-1, C-2, C-3 positions, and that substituted with 18O. Those marked in brown are due to Ar-3,3-difluoro1,2-epoxypropane. All marked lines have also been observed with the Balle-Flygare instrument, but two of them, marked with upside down triangles, have broad linewidths with distorted shapes; they are not included in the spectral fits.
4.1.1. 3,3-Difluoro-1,2-epoxypropane We have observed and assigned 94 rotational transitions in the Balle-Flygare spectrum for the most abundant isotopologue of DFO, and 40 or more transitions for each of the isotopologues singly substituted with 13C and 18O. All three types (a, b, and c) of transitions are observed, sampling J from 0 to 10, and Ka from 0 to 4 for the most abundant species, and narrower ranges for the less abundant species. The spectrum of each isotopologue is analyzed using the Watson A-reduced Hamiltonian [17] and Pickett’s nonlinear SPFIT program [18]. We determine 3 rotational constants and 4–5 quartic centrifugal distortion constants with an rms deviation of less than 2 kHz, commensurate with the resolution of the narrowband 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 Supplementary material. 4.1.2. Ar-3,3-difluoro-1,2-epoxypropane We have assigned 269 rotational transitions in the Balle-Flygare spectrum for the most abundant isotopologue of Ar-DFO. Once again, all three types of transitions (a, b, and c) are present in large ranges of J (1–16) and Ka (0–4). As described in Section 4.2.2, the complex is very nearly prolate. Thus, the transitions are analyzed
Table 4 Spectroscopic constants (in MHz, unless as otherwise noted) for five isotopologues of 3,3-difluoro-1,2-epoxypropane.a,b
A B C DJ/10 3 DJK/10 3 DK/10 3 dJ/10 3 dK/10 3 No. of rotational transitions No. of a type No. of b type No. of c type J range Ka range rms/kHz a b
CH2CH(CHF2)O
13
CH213CH(CHF2)O
CH2CH(13CHF2)O
CH2CH(CHF2)18O
6967.48141(19) 2693.447203(61) 2103.668834(57) 0.35909(74) 2.9209(32) 0.0822(93) 0.08393(21) 1.8157(51) 94 21 46 27 0–10 0–4 1.60
6920.50953(21) 2647.65783(11) 2071.62091(12) 0.3566(20) 2.7898(91) [0.0822] 0.08466(96) 1.750(26) 43 15 26 2 0–6 0–3 1.11
6945.99119(30) 2684.84564(16) 2100.32966(17) 0.3632(30) 2.867(13) [0.0822] 0.0837(14) 1.735(37) 43 15 26 2 0–6 0–3 1.63
6959.38537(28) 2686.34920(15) 2100.15878(16) 0.3677(27) 2.958(12) [0.0822] 0.0846(13) 1.871(34) 43 15 26 2 0–6 0–3 1.50
6872.87626(23) 2614.78590(12) 2047.48850(15) 0.3535(24) 2.649(12) [0.0822] 0.0773(16) 1.748(34) 40 15 23 2 0–6 0–3 1.18
CH2CH(CHF2)O
1r standard deviations in the parameters are given in parentheses. Quantities in square brackets are fixed to the value appropriate to the most abundant isotopologue.
23
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26 Table 5 Spectroscopic constants (in MHz, unless as otherwise noted) for the most abundant isotopologue of the Ar-3,3-difluoro-1,2-epoxypropane complex.a A-reduction A B C DJ/10 3 DJK/10 3 DK/10 3 dJ/10 3 dK/10 3 UJ/10 6 UJK/10 6 UKJ/10 6 UK/10 6 No. of rotational No. of a type No. of b type No. of c type J range Ka range rms/kHz a
S-reduction 3565.00650(13) 695.214576(33) 665.584346(33) 1.17477(22) 8.7524(17) 98.734(17) 0.136210(31) 2.4117(94) 0.01306(54) 0.3964(87) 5.424(64) 17.74(69) transitions
A B C DJ/10 3 DJK/10 3 DK/10 3 d1/10 3 d2/10 3 HJ/10 6 HJK/10 6 HKJ/10 6 HK/10 6 269 87 117 65 1–16 0–4 1.30
Table 6 The coordinates of the atoms in 3,3-difluoro-1,2-epoxypropane determined from a structural fit and the substitution coordinates for four atoms from a Kraitchman analysis.a a/Å
3565.00653(13) 695.209740(27) 665.589157(26) 1.16858(22) 8.7152(17) 98.704(17) 0.136210(31) 0.003086(12) 0.01320(54) 0.3954(87) 5.419(64) 17.73(69)
1r standard deviations in the parameters are given in parentheses.
using both the Watson A-reduced and S-reduced Hamiltonians [17] and Pickett’s nonlinear SPFIT program [18]. For each fit, we obtain 3 rotational constants, and 5 quartic and 4 sextic centrifugal distortion constants (Table 5) with an rms deviation of less than 2 kHz. A table containing observed and calculated transition frequencies (using the A-reduced Hamiltonian) with assignments is in Supplementary material. (A similar table using the S-reduced Hamiltonian would be identical to the 0.0001 MHz precision used except for seven lines for which the difference is 0.0001 MHz, much smaller than the resolution of the spectrometer.) 4.2. Structure determination 4.2.1. 3,3-Difluoro-1,2-epoxypropane 3,3-Difluoro-1,2-epoxypropane is a near prolate asymmetric top with an asymmetry parameter value between 0.765 and 0.757 for the various isotopologues. The experimental A, B, and C rotational constants of the most abundant species agree excellently with the theoretical values, differing by 58 MHz (0.83%), 12 MHz (0.43%), and 2 MHz (0.07%), respectively. We fit 5 structural parameters of the molecule using 15 moments of inertia of the 5 isotopologues with Kisiel’s STRFIT program [19]. These parameters involve the C and O atoms: the C1AO, C2AO, C2AC3 bond lengths and the C2AOAC1 and C3AC2AC1 angles; all other parameters are fixed to their ab initio values. The rms deviation of the fit is 0.0078 u Å2. The resulting structural parameters, calculated by Kisiel’s EVAL program [20,21] are listed in Table 1, and the positions of all atoms in the principal axis system are in Table 6. Alternatively, the positions of the C and O atoms can be determined with a Kraitchman analysis [22]: by treating each less abundant isotopologue as a single isotopic substitution in the most abundant species, the absolute values of the coordinates of the substituted atom in the principal axis system of the parent molecule can be calculated. Using chemically reasonable bond lengths, the signs of the coordinates can usually be assigned (Table 6). The substitution coordinates thus derived compare well with those from the structural fit, either agreeing to within experimental uncertainties or differing by no more than 0.007 Å from each other. The unphysical Kraitchman b coordinate of C3 indicates that this atom lies very close to the a-c plane. Indeed, according to the structural fit, its b coordinate is 0.01272(12) Å.
b/Å
c/Å
(i) From structural fit C1 1.8054(14) C2 0.6244(28) C3 0.64698(58) O 1.68749(84) FO 1.37773(66) FC 1.3980(10) H1 2.5598(11) H2 1.7250(26) H3 0.5148(39) H4 0.4824(18)
0.70007(27) 0.0308(11) 0.01272(12) 0.71486(27) 1.09202(53) 1.10675(55) 1.08714(82) 1.17205(12) 0.0543(11) 0.00918(25)
0.10061(35) 0.47718(48) 0.29831(45) 0.11453(19) 0.02887(53) 0.03535(38) 0.5669(11) 1.06938(27) 1.54969(40) 1.37192(64)
(ii) Substitution coordinatesb C1 1.80120(83) C2 0.6177(24) C3 0.6397(23) O 1.68483(89)
0.7064(21) 0.051(29) Nonphysical 0.7209(21)
0.104(14) 0.4732(32) 0.3003(50) 0.117(13)
a
Costain errors [27] in the parameters are given in parentheses. 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. b
4.2.2. Ar-3,3-difluoro-1,2-epoxypropane Ar-3,3-difluoro-1,2-epoxypropane is a near prolate asymmetric top with an asymmetry parameter of 0.980. The experimental rotational constants (either from the A or S-reduction) agree very well with the theoretical values for Structure (a). Specifically, the experimental A constant is greater than those predicted theoretically: by 47 MHz (1.3%) without BSSE correction and 66 MHz (1.9%) with BSSE correction. While the experimental B and C values are smaller than the theoretical ones without BSSE correction by 13 (1.8%) and 12 (1.8%) MHz, respectively, they are greater than those with BSSE correction by 23 (3.5%) and 21 (3.2%) MHz, respectively. Thus, the rotational constants strongly support a structure where Ar interacts with the atoms in the epoxy ring and on the side opposite from the CHF2 group. We can still determine the position of Ar using the moments of inertia of the most abundant species, albeit without any estimate of the uncertainties of its coordinates because we do not observe the spectra of any minor isotopologues. The values of the moments of inertia derived using the Watson A and S-reduced Hamiltonians are slightly different, but the resulting principal atomic coordinates differ by no more than 0.0003 Å (and by no more than 0.0001 Å for the heavy atoms). In fact, the distances between Ar and each of the atoms in the epoxy ring are identical when they are calculated from the moments of inertia determined by either Hamiltonian, and they are shown in Fig. 6. Argon lies more or less above the O atom, and is 3.493 Å above the epoxy plane. The principal coordinates of all atoms in the complex derived from the moments of inertia using the A-reduced Hamiltonian are listed in Table 7. 5. Discussion Well guided by theory, we have determined the structures of 3,3-difluoro-1,2-epoxypropane and its complex with argon. We will first examine the effects of substituents on the epoxy ring. In a previous study, we have compared the ring size for compounds with no substituent (ethylene oxide) [23], a methyl substituent (propylene oxide) [24], and a trifluoromethyl substituent (3,3,3-tri fluoro-1,2-epoxypropane) [9]. Here, we extend the list with a difluoromethyl substituent, and we have listed the CAC and CAO bond lengths and the C1C2O bond angle of all four species in Table 8. While the lengths of all bonds in the epoxy ring in propylene oxide are practically the same within experimental uncertainties as
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M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
Fig. 6. The experimental structures of (a) Ar-3,3-difluoro-1,2-epoxypropane (this work), (b) Ar-3,3,3-trifluoro-1,2-epoxypropane [9], and (c) Ar-ethylene oxide [25]. Atom colors: C, dark gray; H, light gray; O, red; F, light blue, Ar: purple. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 7 The coordinates of the atoms in Ar-3,3-difluoro-1,2-epoxypropane determined from the experimental moments of inertia of the most abundant isotopologue.
C1 C2 C3 O FO FC H1 H2 H3 H4 Ar
a/Å
b/Å
c/Å
0.0506 0.5553 2.0161 0.0872 2.2284 2.6146 0.8708 0.7486 0.0163 2.4965 3.1151
1.6228 0.2366 0.0006 0.8003 1.2116 0.0380 1.8363 2.4406 0.5436 0.7643 0.3982
0.0753 0.0688 0.0995 1.0739 0.6886 1.1304 0.5950 0.0325 0.5888 0.7042 0.1951
those in ethylene oxide, it is not the case for the other two species. The length of the CAC bond in DFO is 1.4753(25) Å, which is the same, to experimental uncertainty, as that [1.470(3) Å] in ethylene oxide [23]. The CAO bonds in DFO are not equivalent, with C1AO shorter than C2AO by 0.007 Å, which is 3 times the greater uncertainty of the two lengths. These are, respectively, 0.014 Å and 0.007 Å shorter than those bonds in ethylene oxide. The addition of a ACHF2 substituent therefore has a slight effect on the epoxy ring. In the presence of a ACF3 substituent, the CAO bonds are
not affected: they are the same, to experimental uncertainty, as those in ethylene oxide. The CAC bond, however, becomes shorter by 0.015 Å. At first, these results seem puzzling because the ACHF2 and ACF3 substituents appear to have an opposite effect on the ring. However, it is important to realize that these experimental bond lengths are affected by differences in vibrational averaging in addition to changes in electron distribution. To eliminate the effect of vibrational averaging, we compare the ab initio equilibrium structures of these four species, all determined at the MP2/6-311++G(2d,2p) level. The lower portion of Table 8 lists the resulting equilibrium values for the same CAC and CAO bond lengths in these molecules. Now it is possible to see sensible trends. At the point of attachment (C2), the ACH3 results in a very slight, 0.0004 Å, lengthening of the ring CAC bond (C2AC1) with a somewhat greater 0.0031 Å effect on the C2AO bond length. The C1AO bond length increases by 0.0051 Å in response to these changes and an accompanying slight increase (0.15°) in the C1C2O angle. Now both ACHF2 and ACF3 are seen to have the same effect, with the latter being greater. For both these fluorinated substituents, the ring bonds involving C2 shorten. The C2AC1 bond is 0.0023 Å (0.0028 Å) shorter in DFO (TFO) than in ethylene oxide, and the C2AO bonds 0.0064 Å (0.0170 Å) shorter in DFO (TFO). Although these bonds ae getting shorter, the C1C2O angle opens up by 0.37° in DFO compared to ethylene oxide and that for TFO
Table 8 The experimental (average) and theoretical (equilibrium) CAC and CAO bond lengths and the C1C2O bond angle in the epoxy ring for ethylene oxide [23], propylene oxide [24], 3,3-difluoro-1,2-epoxypropane, and 3,3,3-trifluoro-1,2-epoxypropane [9]. C1, C2, and O form the epoxy ring, and C3 is in the substituent is connected to C2. Ethylene oxide
Propylene oxide
3,3-Difluoro-1,2-epoxypropane
3,3,3-Trifluoro-1,2-epoxypropane
Average structure C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å \C1C2O/°
1.434(3) 1.434(3) 1.470(3) – 59.17
1.441(2) 1.435(3) 1.470(3) 1.505(2) 59.46
1.41990(40) 1.4270(23) 1.4753(25) 1.4893(24) 58.55
1.4374(28) 1.4342(66) 1.4555(65) 1.4795(41) 59.65
Equilibrium structure C1AO/Å C2AO/Å C1AC2/Å C2AC3/Å \C1C2O/°
1.4422 1.4422 1.4649 – 59.48
1.4473 1.4453 1.4653 1.4987 59.63
1.4460 1.4358 1.4626 1.4919 59.85
1.4484 1.4252 1.4621 1.4990 60.20
M.D. Marshall, H.O. Leung / Journal of Molecular Spectroscopy 350 (2018) 18–26
by 0.72°, resulting in longer C1AO bonds in each case. We see then that the electron donating methyl group lengthens the ring bonds at the point of attachment, but electron withdrawing fluoromethyl groups shorten those bonds to a degree that seems to depend on the number of fluorine atoms. Both types of substitution lead to an increase in the ring bond angle at that point. The trend in the C2AC3 bond length, between the ring carbon and the carbon of the substituent, shows similar contrary behavior. The values for the vibrationally averaged structures show a monotonic decrease from ACH3 to ACHF2 to ACF3 substitution, but those for the equilibrium structures are longest for TFO and shortest for DFO. This could be a result of an electrostatically induced shortening of the bond upon fluorine substitution competing with an unfavorable steric interaction in TFO that places the third fluorine atom close to C1 and O. This can be seen by comparing of Fig. 1 here with Fig. 1 of Ref. [9]. Ethylene oxide [25,26], 3,3-difluoro-1,2-epoxypropane, and 3,3,3-trifluoro-1,2-epoxypropane [9] all offer the same site for argon to bind (Fig. 6): on one side of the epoxy ring. While the ArAC bonds in Ar-ethylene oxide are longer by 0.065–0.102 Å than those in ArDFO and Ar-TFO, the ArAO bond in Ar-ethylene oxide is the shortest of the three, by 0.026–0.047 Å. The corresponding ArAC distances in both Ar-DFO and Ar-TFO are quite similar, with the ArAC1 bond longer (by 0.031–0.037 Å) than the ArAC2 bond. The ArAO bond in Ar-DFO is 0.021 Å longer than that in Ar-TFO. The Ar atom is located 3.436 Å, 3.493 Å, and 3.473 Å above the epoxy ring of ethylene oxide, Ar-DFO, and Ar-TFO, respectively, with projections onto the ring only 0.29 Å, 0.13 Å, and 0.10 Å away from the O atom; thus, Ar locates approximately above the O atom in all three cases. This atom is the most negative portion of the molecules, as evidenced from the mapped electrostatic potential surfaces of ethylene oxide and TFO given in Ref. [9]. (Although not presented here, the surface for DFO is similar to that for TFO.) Argon, in this case, serves well as a probe for electron density. Of course, steric considerations might well prevent it from favorable interactions with both O and F atoms in the case of the substituted ethylene oxide. 6. Conclusion 3,3-difluoro-1,2-epoxypropane has a strong, simple microwave rotational spectrum that is well predicted by ab initio calculations. Although work needs to be done to show that it readily forms noncovalently bound heterodimers with potential analyte species, it meets the first test necessary for use as a chiral tag. Many of the molecule’s structural parameters have been refined using the experimental results, and the positions of the carbon and oxygen atoms well determined via analysis of singly substituted isotopologues. As could be expected, the structure (and spectrum) is very similar to those of the analogous molecule 3,3,3-trifluoro-1,2epoxypropane [9] as is the nature of the complex with the argon atom. Although partially masked by the effects of vibrational averaging, ab initio structures for the series of molecules ethylene oxide, propylene oxide, 3,3-difluoro-1,2-epoxypropane, and 3,3,3trifluoro-1,2-epoxypropane can be rationalized in terms of the relative electron donating versus electron withdrawing nature of the substituent. Competing financial interest The authors declare no competing financial interest. Acknowledgements This material is based on work supported by the National Science Foundation under Grant No. CHE-1465014.
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Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jms.2018.05.003.
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