Draft · 2020. 6. 23. · a sterically unfavourable C-O-C bond angle of about 60o),oxiranes are...

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A combined solid-state 17O NMR, crystallographic, and computational study of oxiranes

Journal: Canadian Journal of Chemistry

Manuscript ID cjc-2020-0114.R1

Manuscript Type: Article

Date Submitted by the Author: 13-Apr-2020

Complete List of Authors: Rinald, Andrew; Queen's UniversityTerskikh, Victor; University of Ottawa, ChemistrySchatte, Gabriele; Queen's UniversityWu, Gang; Queen's University,

Is the invited manuscript for consideration in a Special

Issue?:Not applicable (regular submission)

Keyword: Oxirane, solid-state 17O NMR, quadrupole coupling tensor, chemical shift tensor

https://mc06.manuscriptcentral.com/cjc-pubs

Canadian Journal of Chemistry

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A combined solid-state 17O NMR, crystallographic, and

computational study of oxiranes

Andrew Rinald,1 Victor Terskikh,1,2 Gabriele Schatte,1 and Gang Wu1*

1Department of Chemistry, Queen’s University, 90 Bader Lane,

Kingston, Ontario, Canada K7L 3N6; 2Department of Chemistry, University of Ottawa,

Ottawa, Ontario, Canada K1A 0R6

*Corresponding author: [email protected]

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Abstract

We report synthesis and solid-state 17O NMR characterization of three 17O-labeled

oxiranes: (2S*, 3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane, (2S*, 3R*)-2,3-bis(4-

nitrophenyl)-[17O]oxirane, and 2,2,3-triphenyl-[17O]oxirane. In addition, we have

determined the crystal structure of (2S*, 3R*)-2,3-bis(4-nitrophenyl)oxirane by X-ray

crystallography. When the experimentally determined 17O NMR tensors for oxiranes

(where the C-O-C bond angle is about 60) are compared with those for dimethyl ether

(where the C-O-C bond angle is 113) and other R-O-R functional groups, we found that

the highly constrained geometry of oxiranes results in distinct tensor orientations in the

molecular frame of reference. The experimental results are complemented by quantum

chemical computations. This study represents the first time that 17O chemical shift and

quadrupole coupling tensors are simultaneously determined for oxirane compounds.

Keywords: oxirane, solid-state 17O NMR, quadrupole coupling tensor, chemical shift tensor

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1. Introduction

Oxiranes (also known as 1,2-epoxides) are a class of organic compounds

containing a three membered heterocyclic ring of two carbon atoms and one oxygen

atom. Oxiranes are important building blocks used in organic syntheses. An example of

their utility is in the use of ring opening mechanisms to make regioselective

ethers/alcohols.1 Due to the strain present in the three membered ring system (containing

a sterically unfavourable C-O-C bond angle of about 60o), oxiranes are very reactive

electrophiles that can serve as useful reaction intermediates. Ring opening of epoxides is

the basis for the formation of epoxy glues and glycols. Oxiranes are also biologically

important molecules. They are present in numerous biological hormones, notably in

many juvenoids and insect sex pheromones.2-3 An example of oxirane incorporation into

insect sex pheromones is (+/-)-disparlure. Gypsy moth pheromone-binding proteins

enantioselectively recognize (+)-disparlure, whereas (-)-disparlure cancels the attraction

of (+)-disparlure. Clearly the conformation of the oxirane ring is directly linked to

disparlure function, but the specific source of the enantioselectivity is unknown.3

The C-O-C bond angle in oxiranes is approximately 60o, which is highly unusual,

because it is considerably smaller than the angle between orbitals with 100% p-character

(90o). Coulson and Moffit4 in 1949 described the bonding in three-membered rings as

“bent”, as depicted in Scheme 1, referring to chemical bonds that contain hybrid orbitals

whose maxima do not lie in the directions of the bonds. They used this explanation to

justify the strain in non-linear molecules, and how it is possible to have bond angles

smaller than 90o. They defined “strain energy” to be the difference between the observed

heat of formation and that of a strainless reference compound. More recent calculations

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suggested the primary source of strain energy in oxiranes to be due to the distortion of

electron populations within the three-membered ring as compared with open chain

structures.5 Also related to the synthetic utility of oxiranes is that their bonding properties

are much closer to what are observed for olefins than for typical aliphatic systems. This

was first observed in early electron diffraction studies that found large bond shortening

effects in three membered oxygen-containing rings.6-7 The carbon hybridization was also

found to be closer to sp2 than sp3. A later UV-Vis spectroscopic study also found

uncharacteristically high π-character for the oxygen atoms valence electrons in oxiranes.8

Due to the inherent topological features of three-membered rings, there is a high level of

electron density in the plane of the ring, which leads to a level of surface delocalization,

strengthening the bonds within the ring. Increasing the electronegativity of the X group

in the three-membered ring (for C2H4X), increases the electron donation to the X atom

and decreases the back-donation to the carbon atoms, thus the oxygen atom in oxiranes is

more nucleophilic than one would expect, which leads to its utility as a nucleophilic

attack and coordination site.9

~ 60o

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Scheme 1. Cartoon representation depicting the formation of bent bonds in oxiranes.

More recent studies have analyzed the effect that substituting electron withdrawing

groups on oxiranes have on the structural parameters of the central oxirane ring.

Computational and experimentally derived results suggest that increasing the electron

withdrawing character of oxirane residues (thus removing electron density from the

oxirane ring) lengthens the C-C bond lengths (and makes the bond less covalent), widens

the C-O-C angles, and in general weakens the C-O bond strength (the effect is additive).

It was also found that by adding asymmetric substituents to the oxirane ring leads to

asymmetric local electron densities within the oxirane ring that mirror the orientation of

the electron withdrawing ability of the substituents.10

While 1H and 13C NMR studies of oxiranes have been widely reported,11-21 very

limited information about 17O NMR parameters for oxiranes is available in the literature.

In an early microwave spectroscopic study of ethylene oxide (the smallest oxirane

molecule), Creswell and Schwendeman reported the following 17O quadrupole coupling

(QC) tensor: zz = 12.6, yy = –7.4, and xx = –5.2 MHz (or CQ = 12.6 MHz and Q =

0.17).22 There were also several early 17O NMR studies of oxiranes in solution.23-25

Owing to the unusual oxygen bonding in oxiranes, several computational studies were

aimed at calculating accurate CQ(17O) values.26-29 In addition, the 17O QC tensor in

oxiranes was analyzed with the aid of the Townes-Dailey model.30-31 Oxiranes were also

used to test magnetically corrected basis sets in magnetic shielding calculations.32 In the

present work, we set out to achieve two goals. First, we investigated synthetic methods

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for 17O-labeling of oxiranes. In this regard, we report herein synthesis of three

representative oxiranes: (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane (Compound 1),

(2S*,3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane (Compound 2), and racemic 2,2,3-

triphenyl-[17O]oxirane (Compound 3); see Scheme 2. The second objective of this study

was to experimentally determine 17O chemical shift (CS) and quadrupole coupling (QC)

tensors in these oxiranes. To the best of our knowledge,33-35 no solid-state 17O NMR

studies have ever been reported on oxirane compounds.

1

2

3

Scheme 2. Molecular structures of (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane

(Compound 1), (2S*,3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane (Compound 2), and [17O]-

2,2,3-triphenyl-[17O]oxirane (Compound 3). Note that Compound 3 prepared in this study

is a racemic mixture.

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2. Experimental section

Synthesis

All common chemicals and solvents were purchased from Sigma-Aldrich (Oakville,

ON). Oxygen-17 enriched water (40% 17O, 40% 18O, 20% 16O) and oxygen-18 enriched

water (97% 18O, 3% 16O) were purchased from CortecNet (Voisons-Le-Bretonneux,

France).

Synthesis of [17O]-(2S*,3S*)/(2S*,3R*)-2,3-bis(4-nitrophenyl)oxirane. p-Nitro-

[17O]benzaldehyde was first prepared following a literature method.36 In particular, p-

nitrobenzaldehyde (478 mg, 3.14 mmol) was dissolved in a minimum of dichloromethane

and 120 µL 40% H217O (6.25 mmol). The solution was stirred vigorously for three days

at room temperature. Upon removal of the solvent, the resultant white powders were

further dried in vacuo. (480 mg, 100% yield). 17O NMR (67 MHz, CH2Cl2), δ = 585 ppm.

(2S*,3S*)/(2S*,3R*)-2,3-Bis(4-nitrophenyl)-[17O]oxirane were prepared by using a

ZnBr2-mediated reaction of p-nitro-[17O]benzaldehyde with triethyl phosphite as reported

by Raju et al.37 Triethylphosphite (1.05 g, 6.37 mmol) was added to a mixture of p-Nitro-

[17O]benzaldehyde (502 mg, 3.30 mmol) and zinc bromide (87 mg, 0.33 mmol) under

nitrogen atmosphere. After the solution was stirred for three hours, it was poured over 50

g ice and left for two hours. The organics were extracted with 2 50 mL ethyl acetate,

then washed with 2 25 mL 20% brine. The solvent was then removed in vacuo.

Isolation of the (2S*,3S*) isomer (or cis-isomer) was achieved by treating the resultant

solid with 10 mL cold methanol, leaving behind the cis-isomer as a white precipitate that

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was collected by filtration. The filtrate was then dried in vacuo and the (2S*, 3R*)

isomer (or trans-isomer) was isolated as a white powder using silica column

chromatography (mobile phase 10% ethyl acetate: 90% hexanes). The trans compound

was recrystallized from hexanes. (2S*,3S*)-2,3-bis(4-nitrophenyl)-[17O]oxirane: (120

mg, 27% yield). 1H NMR (300 MHz, CDCl3): δ = 8.24 (d, 3JHH = 8.66 Hz, 4H), 7.54 (d,

3JHH = 8.66 Hz, 4H), 3.98 (s, 2H) ppm. 13C NMR (75.4 MHz, CDCl3): δ = 148.38,

143.22, 126.38, 124.07, 62.06 ppm. 17O NMR (67 MHz, CDCl3) δ = 35 ppm. (2S*,

3R*)-2,3-bis(4-nitrophenyl)-[17O]oxirane: (70 mg, 16% yield). 1H NMR (300 MHz,

CDCl3): δ = 8.11 (d, 3JHH = 8.66 Hz, 4H), 7.40 (d, 3JHH = 8.66 Hz, 4H), 4.55 (s, 2H) ppm.

13C NMR (75.4 MHz, CDCl3) δ = 147.5, 140.7, 127.5, 123.4, 59.0 ppm. 17O NMR (67

MHz, CDCl3) δ = 9 ppm. The level of 17O isotopic enrichment was determined to be

about 9% for both compounds by mass spectrometry performed on a Micromass GCT

(GC-EI TOF Mass Spectrometer) with + polarity. Details are provided in the Supporting

Information.

Synthesis of [Hydroxy(mesyloxy)iodo]benzene. (Diacetoxy)iodobenzene (1.950 g,

6.0931 mmol) was suspended in 11.25 mL acetonitrile, to which were added

methanesulfonic acid (1.20 g, 12.5 mmol) and water (225 mg, 12.5 mmol) in 2.5 mL

acetonitrile. The mixture was stirred overnight. The off white powder was then filtered,

washed with acetone and ether, and dried in vacuo. (1.591 g, 82% yield) 1H NMR (300

MHz, DMSO-d6): δ = 9.75 (s, 1H), 8.22 (d, 3JHH = 8.20 Hz, 2H), 7.72 (t, 3JHH = 7.50 Hz,

1H), 7.612 (dd, 3JHH = 8.20, 7.50 Hz, 2H), 2.29 (s, 3H) ppm. 13C NMR (75.4 MHz,

DMSO-d6): δ = 137.57, 131.15, 128.18, 39.96 ppm.

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Synthesis of [18O]-Iodosylbenzene. Because 18O-labeled water is considerably

cheaper than 17O-labeled water, we first tested the synthesis with 18O-labeled water.

[Hydroxyl(mesyloxy)iodo]benzene (350 mg, 1.0937 mmol) was dissolved in H218O (500

mg, 25 mmol) contained in a centrifuge tube. Sodium hydroxide (50 mg, 1.25 mmol) was

added and the tube was shaken and centrifuged. The liquid layer was removed and the

powder was washed repeatedly with ether and then water (206 mg, 84% yield). FTIR

(powder): 3049 s, 1566 m, 1434 s, 734 s, 689 s, 565 w, 485 m, 424 m cm-1. The

observation of IR bands at 565 and 424 cm-1 confirmed the > 90% 18O isotopic

enrichment in iodosylbenzene.

Synthesis of [17O]-Iodosylbenzene. [Hydroxyl(mesyloxy)iodo]benzene (350 mg,

1.0937 mmol) was dissolved in H217O (500 mg, 26.3 mmol) contained in a centrifuge

tube. Sodium hydroxide (50 mg, 1.25 mmol) was added and the tube was shaken and

centrifuged. The liquid layer was removed and the powder was washed repeatedly with

ether and then water. (153 mg, 63% yield). FTIR (powder): 3049 s, 1566 m, 1434 s, 733

s, 689 s, 587 w, 487 m, 436 m cm-1.

Synthesis of 2,2,3-Triphenyl-[17O]oxirane: Triphenylethylene (200 mg, 0.780

mmol) was combined with Jacobsen’s catalyst (50 mg, 0.078 mmol).38 [17O]-

iodosylbenzene (172 mg, 0.778 mmol) was added over two minutes and the solution was

refluxed for 7 days. 17O-iodosylbenzene (83 mg, 0.38 mmol) was then added and the

mixture was stirred for three days further. The solvent was removed in vacuo and the

crude product was isolated using a silica column (mobile phase 10% ethyl acetate: 90%

hexanes). The remaining impurities were dissolved using a minimum of hexanes. (104.8

mg, 49% yield). 1H NMR (300 MHz, CDCl3), δ = 7.37 (m, 6H), 7.24 (m, 3H), 7.18 (m,

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3H), 7.08 (m, 3H), 4.37 (s, 1H) ppm. 13C NMR (75.4 MHz, CDCl3), δ = 141.03, 135.83,

135.45, 127.87, 127.82, 127.75, 127.68, 127.60, 127.53, 126.79, 126.37, 68.70, 68.09

ppm. 17O NMR (67 MHz, CDCl3) δ = 47 ppm.

X-ray crystallography

Among the three oxirane compounds investigated in this study, only Compound 1 has

its crystal structure reported in the literature.37 We were able to obtain a single crystal of

Compound 2 and determine its crystal structure. Attempts to obtain a single crystal of

Compound 3 were unsuccessful. A colorless, rod-like crystal of Compound 2 having the

approximate dimensions of 0.125 0.118 0.075 mm, coated with oil (Paratone 8277,

Exxon), was collected onto the aperture of a mounted MicromountTM (diameter of the

aperture: 100 microns; MiTeGen - Microtechnologies for Structural Genomics; USA) and

quickly transferred to the cold nitrogen gas stream of the Oxford Cryostream 800

operating at 93.16 °C. The mounted MicromountTM had previously been inserted into a

reusable magnetic goniometer base (B3S-R, MiTeGen - Microtechnologies for Structural

Genomics; USA). All measurements were made on a Bruker AXS D8 Venture Duo

diffractometer using Mo K radiation ( = 0.71073 Å) generated by a high brilliance

Incoatec Is microfocus tube equipped with a HELIOS multilayer mirror optics (power:

50 kV 1 mA). Data were recorded with a Bruker AXS PHOTON II Charge-Integrating

Pixel Array Detector (CPAD) (frame size: 768 × 1024). The structure was solved using

direct methods and refined by full-matrix least-squares method on F2 with SHELXL-

2014 using ShelXle as the graphical user interface.39 Hydrogen atoms of the CH groups

were included at geometrically idealized positions (C-H bond distances: 0.95 Å) and

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were not refined. The isotropic thermal parameters of these hydrogen atoms were fixed

at 1.2 times (CH-groups) that of the preceding carbon atom. The hydrogen atoms

attached to the carbon atoms labeled as C(7) and C(8) were located in the difference

Fourier map. The coordinates and isotropic parameters for these hydrogen atoms were

refined. Data collection and refinement conditions are listed in Table 1. Other detailed

crystallographic data are given in the Supporting Information.

Solid-state 17O NMR

Solid-state 17O NMR spectra were recorded at 14.1 and 21.1 T, operating at the 17O

Larmor frequencies of 81.38 and 122.02 MHz, respectively. Experiments performed at

14.1 T utilized a 4 mm Bruker HX probe and samples were packed into 4 mm o.d.

zirconia rotors. On this probe, the B1 field at the 17O Larmor frequency was about 50

kHz. All experiments at 21.1 T were performed at the National Ultrahigh-Field NMR

Facility for Solids (Ottawa, Ontario, Canada) using a home-built solenoid 5 mm HX

static probe for static samples and a 3.2 mm MAS Bruker HX probe for magic angle

spinning (MAS). On both probes, the B1 field at the 17O Larmor frequency was

approximately 42 kHz. For the static experiments, a 5-mm Teflon tube was used as

sample holder. In the MAS experiments, the sample spinning frequency was 22 kHz. In

the 17O MAS experiment for Compound 3, the sample was cooled to 5 oC using a Bruker

BCU05 cooling unit while its MAS spectrum was recorded. NMR spectra were analysed

using Bruker Topspin 2.0. Solid-state 17O NMR spectra were fitted using DMFit.40

Quantum chemical computations

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All property calculations were performed on molecules that were either optimized

using the same method/basis set combination as was used for the specific property

calculation, or were performed on original crystal structures. All quantum chemical

calculations were carried out using Gaussian 09.41 The licensing for the software

packages is provided by the Center for Advanced Computing (CAC) (Queen’s

University, Kingston, Ontario, Canada). Calculations are submitted directly to the

Frontenac Cluster. Typically, 12 processors were used for each calculation. All

optimization and NMR calculations were performed for gas phase molecules. Structural

optimizations and NMR calculations were performed using B3LYP/6-311++G(3df,3pd)

and the GIAO method. Structural optimizations and NMR calculations for ethylene oxide

and dimethyl ether were performed using B3LYP/6-311G(d,p) and the GIAO method.

3. Results and discussion

Crystal structure of Compound 2

In this section, we describe the new crystal structure of Compound 2. Figure 1 shows

the molecular structure of Compound 2. The geometrical parameters around the central

three-membered ring are: rC7-O1 = 1.435, rC8-O1 = 1.435, rC7-C8 = 1.480 Å, C7-O1-C8 =

62.08. These are very similar to those found in the crystal structure of the cis isomer:37

rC7-O1 = 1.432, rC8-O1 = 1.441, rC7-C8 = 1.471 Å, C7-O1-C8 = 61.60. It is also

interesting to note that the two crystal structures are also similar to the geometry of the

parent compound, ethylene oxide (Compound 4), which was determined by microwave

spectroscopy in the gas phase: rCO = 1.434, rCC = 1.470 Å, ∠COC = 61.67o.42

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NMR spectral analysis

Figure 2 shows the solid-state 17O NMR spectra of Compounds 1, 2, and 3 recorded at

two magnetic fields, 14.1 and 21.1 T, under both MAS and static conditions. In general,

the major features in the static 17O NMR spectra for these oxirane compounds are typical

of those arising from the second-order quadrupole interactions.33 This observation

immediately suggests that the 17O chemical shift anisotropy is rather small for these

compounds. At 21.1 T, as the second-order quadrupole broadening is reduced, we were

able to obtain high-quality 17O MAS NMR spectra for these compounds. As seen from

Figure 2, the experimental 17O MAS NMR spectra display characteristic line shapes

suggesting the presence of relatively large CQ but rather small ηQ values. We should

point out that the spectral quality for Compound 3 is rather low because of the low 17O

enrichment level of the product. Nonetheless, all solid-state 17O NMR spectra obtained

for Compounds 1-3 can be properly simulated and the final results for the 17O QC and CS

tensors are listed in Table 2. Some geometric parameters are also given in Table 2.

Among the three oxirane compounds studied (Compounds 1-3), it appears that changing

the regiochemistry and the nature of the substituents on the oxirane ring does not have a

large effect on its structural features. We also found that Compounds 1-3 display very

similar 17O QC and CS tensors. For example, the CQ(17O) values in these oxirane

compounds are about 12-13 MHz and the asymmetry parameters are around 0.2. In

addition, the 17O CS tensors have rather small span ( = 11 – 33 ≈ 200-250 ppm). Table

2 also lists computational results for the 17O QC and CS tensors for Compounds 1-3. In

general, the agreement between the experimental and computed 17O NMR results is

reasonably good.

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Now we can compare the observed 17O QC and CS tensor data in 1-3 with those

previously reported for the R-O-R (R, R = H or C) type of oxygen atoms. The CQ(17O)

values in oxiranes (≈ 13 MHz) are found to be considerably larger than those in water (H-

O-H, CQ ≈ 7-10 MHz),43-49 hydroxyl groups (C-O-H, CQ ≈ 9-10 MHz),50-55 and ethers (C-

O-C, CQ ≈ 10-12 MHz),56-57 but still smaller than those found in the H-O-X type (X = N

and O) of bonding (for example, the hydroxylammonium cation, HONH3+, has

CQ(17O) = 14.7 MHz;58 hydrogen peroxide, HOOH, has CQ(17O) = 16.3 MHz).59

Another distinct feature of the 17O QC tensors in 1-3 is that the ηQ value is rather small,

ηQ ≈ 0.2. In comparison, all the compounds mentioned above have ηQ > 0.6. We will

further discuss this difference in a later section. In the literature, there are fewer studies

where 17O CS tensors are reported for the R-O-R (R, R = H or C) type of oxygen atoms.

For the water molecules in crystalline hydrates, the spans of the 17O CS tensor, , are

typically less than 80 ppm.47, 49 Similarly, the value of (17O) in the hydronium ion,

H3O+, is also small, 87 ppm.60 For the C-O-H groups in phenol and hemiketal

compounds, (17O) is typically on the order of 70-90 ppm,52, 54 which is still smaller than

those found in Compounds 1-3. It is interesting to note that the 17O NMR tensor

parameters in 1-3 are somewhat similar to those reported for the ether type O atom in [5-

17O]-D-glucose.61 In general, compounds with high CQ but small values would benefit

the most by going to ultrahigh magnetic fields such as 35.2 T.62

One of the advantages of the solid-state 17O NMR method employed in this study is

that one can obtain information about tensor orientation in the molecular frame of

reference. Here we will further examine the 17O QC and CS tensor orientations in

oxiranes. In particular, we are interested in comparing the tensor orientations in the

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highly strained geometry of oxiranes (with a C-O-C angle of 60) with those in a model

ether, dimethyl ether (5), where the C-O-C angle is about 113. The computed 17O NMR

tensors for dimethyl ether are also listed in Table 1. As seen from Figure 3, for oxiranes,

the least shielded component, 11, of the 17O CS tensor lies in the molecular plane

bisecting the C-O-C angle and the most shielded component, 33, is perpendicular to the

oxirane ring. In contrast, for dimethyl ether, 11 lies in the plane but along the tangent

direction of the C-O-C angle and 22 is perpendicular to the C-O-C plane. Because the

17O chemical shift anisotropies are rather small in both oxiranes and dimethyl ether, it is

not possible to identify predominant molecular orbital contributions to individual tensor

components. For this reason, we will not further discuss them. As seen from Figure 3,

the 17O QC tensors of oxirane and dimethyl ether share a common feature in that both

tensors have their largest components, zz, aligning perpendicular to the molecular plane.

Computations also confirm that the sign of CQ(17O) is positive in both cases; see Table 2.

However, one key difference between the 17O QC tensors in these two systems is the

interchange of orientations of xx and yy. As seen in Figure 3, in oxiranes, yy bisects

the C-O-C angle whereas in dimethyl ether it is xx. We note that the 17O QC tensor

orientation in dimethyl ether is the same as that observed for the water molecule.43 The

difference in the 17O QC tensor orientations shown in Figure 3 between dimethyl ether

and ethylene oxide is also related to the fact that oxirane compounds often exhibit ηQ ≈ 0

but dimethyl ether has ηQ ≈ 1. The distinct 17O QC tensor orientation in oxiranes (in

particular, ethylene oxide) was also noted in earlier studies by Gready.63-64 In the

following section, we will provide an explanation as to the origin of the different 17O QC

tensors in oxirane and dimethyl ether.

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On the basis of the Townes-Dailey model,65 it is well-established that, for oxygen

atoms in the R-O-R type of bonding geometry, ηQ of the 17O QC tensor depends on the

bond angle, , in the following fashion:66-68

[1] 𝜂𝑄 =3(1 + 𝑐𝑜𝑠𝜃)1 ― 3𝑐𝑜𝑠𝜃 for 109.47° ≤ θ ≤ 180°

[2] 𝜂𝑄 = ―3𝑐𝑜𝑠𝜃 for 90° ≤ θ ≤ 109.47°

Using the above equations, one predicts that ηQ is 0.84 for dimethyl ether ( = 113),

which is in good agreement with the ab initio computational results shown in Table 2.

For ethylene oxide, although the above equation is not strictly valid (as the three

membered ring has < 90), the observed small ηQ value does resemble the extreme case

of 90 (ηQ = 0). Of course, for the “bent” bonds, the hybrid atomic orbitals are no longer

aligned with the bond directions. To further understand the 17O QC tensors observed for

dimethyl ether and ethylene oxide, it is important to analyze the electron lone-pairs

around the oxygen atom under study. The results from a Natural Bonding Orbital (NBO)

analysis are shown in Figure 4. The oxygen atom in each compound has two electron

lone-pairs. The common feature between the two compounds is that they both have one

electron lone-pair, LP(2), with an essentially full occupancy (1.92 electrons) in the pure

pz atomic orbital pointing in the direction perpendicular to the molecular plane. Because

this direction clearly has an excess of valence p-orbital populations, it corresponds to the

direction along which the largest 17O QC tensor component lies, as we explained in a

recent study using the concept of valence p-orbital population anisotropy (VPPA).31 The

key difference between the two compounds, however, lies in the configuration of LP(1).

As seen from Figure 4, while dimethyl ether has its LP(1) in an sp1.39 hybrid orbital (42%

2s and 58% 2p) pointing along the y-axis, the LP(1) of ethylene oxide has much less p

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characters (an sp0.39 hybrid orbital has 72% 2s and 28% 2p). Because of the lack of p-

character in LP(1) of ethylene oxide, the valence p-orbital populations in the px and py

orbitals are primarily from those making the C-O bonds, which are also mainly pure p

atomic orbitals (the hybrid orbital on the oxygen atom to make the C-O bond has 23% 2s

and 87% 2p). As a result, the valence p-orbital populations in the px and py orbitals are

essentially the same, which leads an approximately axially symmetric 17O QC tensor in

ethylene oxide (i.e., ηQ 0). For dimethyl ether, on the other hand, because its LP(1) has

a considerable amount of p-character, the valence p-orbital population in py is

significantly greater than that in px, resulting in a high ηQ value; see data in Table 2.

4. Conclusions

In this study, we have reported synthesis of three 17O-labeled oxirane compounds.

Using solid-state 17O NMR, we were able to measure the 17O QC and CS tensors in these

compounds. During the course of this study, we have also determined the crystal

structure for (2S*,3R*)-2,3-bis(4-nitrophenyl)oxirane (Compound 2). The 17O QC tensor

in an oxirane molecule typically displays CQ = 12-13 MHz and ηQ = 0.1-0.2, whereas the

17O CS tensor has a relatively small anisotropy ( = 200-250 ppm). We found that the

oxirane structural parameters and the 17O NMR tensors are similar in all the three oxirane

compounds examined in this study, despite the differences in regiochemistry and

substituents on the oxirane ring. In general, oxirane compounds show similar 17O NMR

parameters as those of the R-O-R (R, R = H and C) type of functional groups. However,

when compared with dimethyl ether, oxiranes display distinct 17O QC and CS tensor

orientations as a result from the constrained C-O-C bond angle. In addition, dimethyl

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ether and oxirane also represent two extreme cases of the 17O QC tensor in terms of their

asymmetry parameters (dimethyl ether has ηQ 1, but oxiranes have ηQ 0). In the field

of solid-state 17O NMR for organic and biological molecules, there are continuing efforts

to synthesize new 17O-labeled functional groups and to characterize 17O NMR tensors.

These studies will ultimately lead to understanding of the relationship between 17O NMR

tensors and molecular structure/chemical bonding.

Acknowledgement

This work was supported by the Natural Sciences and Engineering Research Council

(NSERC) of Canada. A.R. thanks the Government of Ontario for an Ontario Graduate

Scholarship. Access to the 900 MHz NMR spectrometer was provided by the National

Ultrahigh Field NMR Facility for Solids (Ottawa, Canada), a national research facility

funded by a consortium of Canadian universities, National Research Council Canada and

Bruker BioSpin and managed by the University of Ottawa (http://nmr900.ca).

Supporting Information Available. MS data analysis for Compounds 1 and 2. Detailed

crystallographic data for Compound 2. A crystallographic information file (CIF) for

Compound 2 has been deposited to the Cambridge Crystallographic Data Centre (CCDC-

1997472).

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Tables and Figure Captions

Table 1. Crystallographic data for (2S*, 3R*)-2,3-bis(4-nitrophenyl)oxirane (Compound 2).

Empirical formula C14H10N2O5Formula weight 286.24Crystal color, habit colorless, rod-likeCrystal dimensions (mm) 0.125 0.118 0.071Crystal system triclinicSpace group P1Unit cell parameters

a (Å) 7.2465(3)7.9199(4)b (Å)

c (Å) 11.4449(5)α (°) 77.367(2)β (°) 80.038(2)γ (°) 81.173(2)V (Å3) 630.73(5)Z 2

F(000) 380Density (ρcalcd) 1.507 Mg/m3

Absorption coefficient (μ) 0.117 mm-1

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Table 2. Experimental and computed (G09) 17O NMR parameters and selected structural parameters for Compounds 1, 2, and 3, ethylene oxide (4), and dimethyl ether (5). The uncertainties in the experimental 17O CS tensor components were estimated to be 10 ppm by visual inspection of the agreement between experimental and simulated spectra.

Compd δiso (ppm)

δ11 (ppm)

δ22 (ppm)

δ33 (ppm)

CQ (MHz)

ηQ ∠COC (o)

rCO (Å)

1 Exp. 30 2 160 –15 –55 12.1 0.2 0.18 0.10 61.60a 1.436aCal. 30 193 –33 –70 13.4 0.22 61.59b 1.432b

2 Exp. 30 2 180 –33 –70 13.4 0.2 0.22 0.10 62.09b 1.435bCal. 31 171 4 –81 13.3 0.22 62.97b 1.428b

3 Exp. 43 2 173 –9 –35 12.8 0.2 0.20 0.10 Cal. 42 183 –13 –44 12.9 0.17 63.14b 1.424b

4 Exp. 12.6c 0.17c 61.67d 1.434dCal. –11 176 –52 –157 14.1 0.28 59.46b 1.490b

5 Cal. –16 7 –5 –49 12.3 0.90 113.0b 1.450baFrom ref. 37. bThis work. cFrom ref. 22. dFrom ref. 42.

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Figure 1. Crystal structure of Compound 2. Thermal ellipsoids are shown at the 30% probability level.

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* *

δ(17O)/ppm-10000

δ(17O)/ppm

-1000-50005001000

(a)

0500 -500 -10001000

δ(17O)/ppm

* *

1000 500 -500

(b) (c)

MAS at 21.1 T

Static at 21.1 T

Static at 14.1 T

*

Figure 2. Experimental (blue trace) and simulated (red trace) 17O solid-state NMR spectra of (a) 1, (b) 2, and (c) 3. The peak marked by * was due to the ZrO2 rotor used in MAS experiments. The sample spinning frequency was 22 kHz. All spectra were recorded with a Hahn-echo pulse sequence with central-transition selective pulses and a recycle delay of 10 s. Other data acquisition parameters are given below. In (a), 8000 transients for the MAS at 21.1 T; 13000 transients for the static experiment at 21.1 T; 41432 transients in the static experiment at 14.1 T. In (b), 24000 transients for the MAS at 21.1 T; 14000 transients for the static experiment at 21.1 T; 17262 transients in the static experiment at 14.1 T. In (c), 33000 transients for the MAS at 21.1 T; 16000 transients for the static experiment at 21.1 T; 21960 transients in the static experiment at 14.1 T.

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δ11

δ22 χxx

χyy(a) (b)

(c) (d)

113° 113°

δ11

δ33

χyy

χxx

60° 60°

Figure 3. Depiction of the 17O CS (a and c) and QC (b and d) tensor orientations in the molecular frames of reference of oxirane (a and b) and dimethyl ether (c and d). Tensor components that are perpendicular to the paper plane are not shown for clarity.

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(a) (b)

O

y

x

LP(1): spy0.39

LP(2): pz

O

y

x

LP(1): spy1.39

LP(2): pz

113° 60°

Figure 4. NBO results on electron lone-pairs in (a) dimethyl ether and (b) ethylene oxide.

TOC graphics

17O NMRO

C C

References

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Draft1

2

3

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* *

δ(17O)/ppm -1000 0

δ(17O)/ppm -1000 -500 0 500 1000

(a)

0 500 -500 -1000 1000 δ(17O)/ppm

* *

1000 500 -500

(b) (c)

MAS at 21.1 T

Static at 21.1 T

Static at 14.1 T

*

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δ11

δ22 χxx

χyy (a) (b)

(c) (d)

113° 113°

δ11

δ33

χyy

χxx

60° 60°

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O

y

x

LP(1): spy0.39 LP(2): pz

O

y

x

LP(1): spy1.39 LP(2): pz

113° 60°

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Draft · 2020. 6. 23. · a sterically unfavourable C-O-C bond angle of about 60o),oxiranes are...

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