Chirped-Pulse and Cavity-Based Fourier Transform Microwave

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Chirped-Pulse and Cavity Based Fourier Transform Microwave Spectroscopy of a Chiral Epoxy Ester: Methyl Glycidate Javix Thomas, Jensen Yiu, Johannes Rebling, Wolfgang Jaeger, and Yunjie Xu J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp402552t • Publication Date (Web): 25 Apr 2013 Downloaded from http://pubs.acs.org on April 26, 2013

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The Journal of Physical Chemistry

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Chirped-Pulse and Cavity Based Fourier Transform Microwave Spectroscopy of a Chiral Epoxy Ester: Methyl Glycidate Javix Thomas, Jensen Yiu, Johannes Rebling, Wolfgang Jäger, and Yunjie Xu* Department of Chemistry, University of Alberta. Edmonton, Alberta, Canada, T6G 2G2.

*Corresponding author: Dr. Yunjie Xu Email:[email protected]

ACS Paragon Plus Environment


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ABSTRACT: Rotational spectra of a chiral epoxy ester, methyl glycidate, were measured using a chirped-pulse and a cavity based Fourier transform microwave spectrometer. The two lowest energy conformers where the epoxy oxygen and the ester oxygen atoms are in the syn and anti relative orientation with respect to each other were identified experimentally. Spectra of four 13C isotopologues of the lowest energy conformer of methyl glycidate were also measured and assigned. All of the observed rotational transitions are split into doublets due to the presence of the ester methyl internal rotor. The rotational constants and the internal rotation barrier height for the ester methyl group were determined for both conformers of methyl glycidate and for the four 13

C isotopologues of the most stable conformer. A value for the interconversion barrier between

the two most stable conformers was estimated. Furthermore, comparison to strawberry aldehyde, a larger derivative of methyl glycidate, shows how the syn-anti conformational equilibrium shifts as a result of the additional bulky substituents at the epoxy ring and at the ester oxygen atom. KEYWORDS: Rotational spectroscopy, chirped-pulse, chiral epoxy ester, conformation, internal rotation


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INTRODUCTION Most important biological molecules are chiral and they perform their biological functions in an aqueous medium. A detailed description of the effects of solvation of chiral molecules in water is therefore critical for understanding and modelling outcomes of biological events.1 A significant amount of current research efforts focuses on providing such a detailed and accurate description using, for example, Terahertz spectroscopy,2 infrared3 and vibrational circular dichroism spectroscopy,4 and 2D-infrared spectroscopy.5 In recent years, high resolution rotational spectroscopic studies of a number of model chiral molecule—water clusters6 and pure water clusters7 have been reported. In these studies, multiple conformations of such solvated and pure water clusters were probed. High resolution spectroscopic assignments rely on internal consistency of tens and hundreds of rotational transitions observed and can in turn provide precise structural information and definitive conformational identification of the molecular targets. It is, however, often challenging to achieve these spectral assignments and it is helpful to study the chiral molecules themselves before moving on to their solvated clusters. Such studies also allow one to examine how the conformational landscapes of the chiral molecules alter upon solvation by water molecules. Rotational spectroscopy, together with high level ab initio calculations, has been used to investigate the conformational landscapes of chiral molecules such as amino acids8 and chiral esters9,10 in great detail. Methyl glycidate, a chiral epoxy ester, is the subject of the current study. This chiral epoxy ester is chosen because it is relatively small, making it amenable to high level ab initio calculations. Furthermore, it has multiple hydrogen-bonding sites and can exist in several conformations. These properties make it particularly interesting for the subsequent solvation



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studies planned. Esters in general are also of spectroscopic interest because of the internal rotation motion of the methyl tops. Rotational or ro-vibrational spectra of a few small to mid-size esters containing one or two methyl tops have been reported previously.9,11-17 For example, internal rotation splittings due to one or two methyl rotors were detected and analyzed in methyl salycylate,13 methyl lactate,9 methyl acetate,14 methyl difluoroacetate,15 and methyl pyruvate.17 In this paper we report a detailed high resolution rotational spectroscopic study of methyl glycidate and all its singly substituted 13C isotopologues using a broadband chirped-pulse Fourier transform microwave (FTMW) and a narrow band cavity FTMW spectrometer. Methyl glycidate is a versatile chiral building block used in syntheses of many important organic molecules.18 So far, there are no high resolution spectroscopic studies reported on this chiral molecule. A broad band rotational spectrum of a larger derivative of methyl glycidate, i.e. strawberry aldehyde, was reported recently.10 Five different isomers, some of which are based on the same rotameric frame as in methyl glycidate, were identified experimentally. It would thus be of interest to see if the conformational preference would alter due to the addition of the bulky substituents at the epoxy ring sites and at the ester oxygen atom. Although no internal rotation tunneling splittings were detected in the case of strawberry aldehyde, one may expect to see such splittings in methyl glycidate due to the presence of the ester methyl group.

EXPERIMENTAL DETAILS Preliminary scans for rotational spectra of methyl glycidate were carried out using a newly constructed chirped-pulse FTMW instrument.19 Briefly, a frequency chirped-pulse (200–1000 MHz, 4 µs) generated by an arbitrary waveform generator (Tektronix AWG 710B) is mixed with


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the output of a MW synthesizer to produce a 2 GHz chirp (with a 400 MHz gap) in the 8-18 GHz range. These pulses are amplified with a 20 W solid state MW amplifier (MW Power Inc., L0818-43) and then propagated into free space using a wide band, high gain, MW horn antenna (radiofrequency(rf)/MW instrumentation, ATH7G18). The chirped MW pulse interacts with a molecular ensemble generated by a pulsed expansion of a sample gas mixture through a pulsed nozzle (General Valve, Series 9). The subsequent molecular emission signal is collected by a second, identical horn antenna, passes through a power limiter and a protective MW switch, and is then amplified by a low noise MW signal amplifier. The signal is digitized at a rate of 40 Giga sample per second with a fast digital oscilloscope (Tektronix, TDS 6124C), transferred to a computer, averaged, and Fourier transformed to yield a frequency spectrum. Typically 10,000 to 200,000 time domain signals are averaged to achieve a sufficiently high signal-to-noise ratio. The frequency resolution of the broadband spectrometer is 50 kHz. Recently, we have improved the spectrometer performance by implementing the capability of executing up to 20 chirped pulse excitation – signal detection and digitization cycles during a single molecular pulse. It also proved advantageous to record background signals prior to each molecular pulse rather than in a separate set of experiments. The final high resolution frequency measurements were done with a Balle–Flygare type,20 cavity based pulsed molecular beam FTMW spectrometer, which has been described previously.21 The high resolution capability of this spectrometer allows one to unravel the tunnelling splittings due to the methyl top in methyl glycidate. The experimental uncertainty in the rotational transition frequencies is estimated to be about 2 kHz. The full line width at half height is about 10 kHz for well resolved lines. Sample mixtures consisting of 0.6% methyl glycidate at stagnation pressures of 4 to 8 bars were used for the broadband chirped-pulse MW



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experiments and 2.5 to 4 bar for the higher resolution experiments. Methyl glycidate (97% purity, Sigma Aldrich), and helium (99.9990 %) were used without further purification.

THEORETICAL CALCULATIONS High level ab initio calculations were carried out using the Gaussian 0922 suite of programs. Calculations were performed using second order Møller-Plesset perturbation theory (MP2)23 with the 6-311++G(2d,p) basis set to aid the experimental search for rotational transitions. In order to identify all the possible rotameric conformations of the methyl glycidate monomer, we rotated the carboxylic oxygen atom around the C3-C7 bond (see Figure 1) and the ester methyl group around the C7-O9 bond in steps of 18.6 and 20 degrees, respectively, from 0 to 360 degree. Four rotameric conformers were identified. Subsequent harmonic frequency calculations confirm that these are all true minima without imaginary vibrational frequencies. The structures, relative dissociation energies, molecular rotational constants, and the electric dipole moment components for each of the conformers are given in Table 1. We named the conformers CI, CII, CIII, and CIV in order of decreasing stability. The shape and atom numbering of the lowest energy conformer of methyl glycidate, CI, in its principal inertial axes system, are shown in Figure 1.


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Figure 1. Atom numbering and shape of the methyl glycidate conformer CI in its principal inertial axes system.

In CI and CII, the ester oxygen takes on an anti and syn relative orientation, respectively, with reference to the epoxy oxygen atom. Similarly, these oxygen atoms have syn and anti relative orientations in the CIII and CIV conformers, respectively. The main difference between the pair of CI and CII versus that of CIII and CIV is that the ester methyl group turns towards the epoxy ring in the latter. From Table 1, one can see that the conformers CIII and CIV are much less stable compared to the other two. The energy difference is mainly due to the strong steric hindrance experienced by the methyl group in the vicinity of the epoxide CH group in CIII and CIV. The percentage room temperature Boltzmann populations for these four conformers are also provided in Table 1. It is of no surprise that conformers CIII and CIV have no appreciable population at room temperature. The interconversion barrier height from CII to CI was estimated to be about 13.5 kJ/mol by scanning the dihedral angle O1C3C7O8 in steps of 18.6 degrees and re-optimizing the structure of the remaining portion of the molecule at the MP2/6-311++G(2d,p) level. The resulting energy scan for the interconversion path between CI and CII is given in Figure S1, Supporting Information. Previous studies suggest that the degree of conformational cooling in a jet expansion depends on a number of factors, such as source temperature, carrier gas, and interconversion barrier.24 Empirically, one would not expect significant conformational cooling from CII to CI in a helium jet expansion because of the relatively high conformational conversion barrier of 13.5 kJ/mol. Transitions of CI and CII are therefore expected to be detected in the helium jet, while those of CIII and CIV are likely to be too weak to be observed.



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Table 1. Calculated relative, ∆De, and zero-point vibrational corrected, ∆Do, dissociation energies, Gibbs free energies, ∆G, at 298K, and the corresponding percentage Boltzmann population factors, f, as well as rotational constants, electric dipole moment components, and structures of the four lowest energy conformers of methyl glycidate.a Const. ∆De(kJ/mol) ∆Do(kJ/mol) ∆G(kJ/mol) f (%) A (MHz) B (MHz) C (MHz) µa (Debye) µb (Debye) µc (Debye)

a

CI 0 0 0 57.4 7607 1640 1499 1.26 -2.28 2.09

CII 0.36 0.53 0.74 42.6 5430 1959 1612 0.03 -1.23 1.84

CIII 35.4 34.7 33.9 0.00 4103 2500 1684 -0.62 -3.18 2.16

CIV 38.5 37.9 37.5 0.00 4676 2124 1627 1.64 -5.75 0.96

Calculated at the MP2/6-311++G(2d,p) level of theory.

RESULTS AND DISSCUSSIONS A broadband rotational spectrum of methyl glycidate was initially recorded in the 8-13 GHz region. While all a-, b- and c-type transitions are expected for CI, the electric dipole moment component along the a-axis, µa, is predicted to be close to zero for CII. A detailed analysis of the spectrum resulted in the assignments of these two lowest energy conformers of methyl glycidate. A 0.7 GHz section of the broadband spectrum centered at 9.35 GHz is shown in


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Figure 2. A few of the initially assigned transitions of CI and CII are indicated in the spectrum. Each rotational level is expected to split into two, which are labelled as A and E. Indeed, all transitions measured show such doublets. In Figure 2, one can see very different magnitudes of the methyl internal rotor splitting for different types of transitions. The final frequency measurements were done by using the cavity based FTMW instrument. The measured frequencies were fitted with the internal rotation program XIAM by Hartwig and Dreizler,25 which is based on the combined axis method (CAM). This program works adequately for high barrier cases such as the present case. Watson's S-reduction in the Ir representation26 was used for the fit. The Hamiltonian used can be written as H=Hrot+Hcd+Hi. Here, Hrot is the rigid rotor Hamiltonian, Hcd refers the centrifugal distortion part, and Hi corresponds to the internal rotation Hamiltonian of the methyl top. We observed a total of 66 transitions, which include a-, b-, and c-types transitions for CI and 60 b- and c-type transitions for CII. The missing a-type transitions of CII and the relative intensities of the different types of transitions of CI and CII are consistent with the ab initio electric dipole predictions shown in Table I, thus further confirming the identities of the two conformers. The standard deviation for the fit is 3.4 and 3.5 kHz for CI and CII, respectively. The measured transition frequencies and quantum number assignments are provided in Tables S1 and S2, Supporting Information.



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Figure 2. A 0.7 GHz section of a broadband spectrum of methyl glycidate recorded using the chirped-pulse FTMW spectrometer. The intensity is truncated for the strong transitions. The strongest transition,

12

CI, 303-202, at 9403.708 MHz has a relative intensity of 1.7 x 1016. The

noise level is 2.0 x 1011. Lines marked with asterisks are not yet assigned. The line marked with # is due to residual methyl lactate from previous experiments in the sample system.


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Table 2. Experimental spectroscopic constantsa of the observed conformers and isotopologues of methyl glycidateb

Conformer I 12

13

C

A (GHz)

13

C2

Conformer II 13

C3

13

C7

12

C10

C

7.6440143(11)

7.5846793(16)

7.5982077(15)

7.6428224(16)

7.6425902(10)

5.4372486(13)

B (GHz)

1.63905466(26)

1.61369995(81)

1.63435025(49)

1.63852931(53)

1.60211277(26)

1.95999448(30)

C (GHz)

1.49875152(25)

1.47976431(67)

1.49308499(40)

1.49827222(57)

1.46776327(36)

1.61151250(31)

DJ (kHz)

0.1314(58)

0.118(15)

0.135(14)

0.133(18)

0.1364(80)

0.3180(73)

DJK (kHz)

1.429(38)

1.43(14)

1.45(14)

1.59(14)

1.31(11)

-0.615(24)

DK (kHz)

2.41(25)

2.41c

2.41c

2.41c

2.41c

4.03(24)

d1 (kHz)

-0.0041(19)

-0.034(27)

-0.0041

c

-0.0041

c

0.0158

c

c

-0.0041c 0.0158

c

-0.0691(31)

d2 (kHz)

0.0158(34)

-0.92(64)

0.0158

V3 (kJ/mol)

4.8672(95)

4.862(16)

4.877(18)

4.877(17)

4.883(13)

0.0179(20)

F0 (GHz)

160.08(31)

159.96(58)

160.35(68)

160.33(61)

160.40(46)

159.19(20)

Ρ

0.045104(26)

0.044753(47)

0.044851(57)

0.045011(57)

0.045056(42)

-0.033319(18)

4.8268(57)

β (rad)

0.07636(94)

0.0763(18)

0.0751(21)

0.766(18)

0.0738(13)

3.0554(13)

Nd

66

36

34

33

35

60

σ (kHz)

3.4

3.5

3.7

4.1

2.8

3.3

a

Errors in parenthesis are expressed in units of the least significant digit.

b

The initial scans for the normal isotopologue were done with the chirped-pulsed spectrometer.

The final experimental frequencies for all isotopologues were measured using the cavity-based spectrometer. c

Fixed at the corresponding values of the parent species.

d

Number of transitions included in each fit.

The signal-to-noise ratio achieved with the broadband measurements was not high enough to allow one to assign the singly substituted singly substituted

13

13

C isotopologues confidently. Searches for four

C isotopologues of CI were carried out using the cavity based FTMW



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spectrometer. The related 13C lines of CII were not detected despite substantial efforts because of the lower abundance and smaller dipole moment components in general. The measured frequencies of transitions of the

13

C isotopologues are given in Table S3-S6 of the Supporting

Information. The experimental spectroscopic constants, including the internal rotor constants, for CI and CII are summarized in Table 2, together with the results for the four 13C isotopologues of CI. With the 13C substituted isotopologues, the rs-coordinates, i.e. the a-, b-, and c-coordinates of the four carbon atoms, can be calculated using Kraitchman’s equations.27 These resulting values are summarized and compared to the corresponding ab initio, re, values in Table 3. With Kraitchman’s equations, the signs of the coordinates cannot be determined, although they can generally be sorted out based on the prior knowledge of typical chemical bond lengths. The rs coordinates are listed with +/- signs. Because of zero-point vibrations, the rs procedure may give unphysical results when an atom lies too close to a principal axis. For example, the C2 atom lies very close to the b-principal inertial axis. The square of its b-coordinate calculated from the Kraitchman’s equation gives an unphysical value of -0.00036 Å2. This b-coordinate is therefore set to 0 Å in Table 3. An alternative and commonly utilized approach was taken here to obtain a partially refined ro structure. In this procedure, some selected structural parameters are fitted to the rotational constants of all isotopologues while the remaining parameters are kept at their ab initio values. The partially refined ro structure is listed in Table S7, Supporting Information, and the corresponding ro coordinates for the four C atoms are included in Table 3 for comparison. With the partially refined ro structural parameters, the experimental rotational constants can be reproduced to within 1 MHz. Some structural parameters obtained from the rs approach differ from those obtained from ro and re. This is a result of some carbon atoms lying close to the


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Table 3. Comparison of the rs coordinates of the four substituted carbon atoms and structural parameters related to the ro and re coordinates and structural parameters of CI. Coord.a a (Å)

C2 C3 C7 C10 ±2.0849 ±0.9427 ±0.3144 ±2.6747 -2.085(1) -0.9446(7) 0.3473(3) 2.676(1) -2.0767 -0.9495 0.3435 2.6731 b (Å) rs 0d ±0.6319 ±0.0986 ±0.1063 ro -0.058(9) -0.635(3) 0.119(1) -0.112(4) re -0.0607 -0.6341 0.1185 -0.1031 ±0.7275 ±0.0571 ±0.0256 ±0.0383 c (Å) rs ro 0.7330(3) -0.013(2) -0.0136(4) 0.03322(5) re 0.7376 -0.0157 -0.0145 0.0330 Struct. param. rs ro re C3C2 (Å) 1.5230 1.4807 1.4721 C3C7 (Å) 1.4543 1.4961 1.4961 C7C10 (Å)e 2.3700 2.3409 2.3406 C2C3C7 (˚) 115.4 117.9 117.7 e C3C7C10 (˚) 144.9 144.0 144.3 a The substitution coordinates according to Kraitchman.27 b

rsa rob rec

The coordinates obtained from the partially refined ro structures presented in Table S7. Errors in

parenthesis are expressed in units of the least significant digit. All experimental rotational constants are reproduced within 1 MHz with the ro structural parameters. c

The equilibrium coordinates obtained from the MP2/6-311++G(2d,p) calculation.

d

This value is fixed at 0. See the text for details.

e

This is not a chemical bond or a valence angle.

principal axes, as outlined above. On the other hand, the structural parameters obtained from the ro approach are quite similar to the re values, suggesting that the molecule is relatively rigid. The experimentally determined abundances of CI and CII are approximately 60% and 40%, respectively. This is in good agreement with the calculated percentage Boltzmann factors of 57.4% for CI and 42.6% for CII at room temperature. This suggests that there is little



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conformational relaxation from CII to CI, consistent with the fairly high interconversion barrier of 13.5 kJ/mol predicted theoretically. The rotational temperature, on the other hand, is estimated to be about 1 K, based on relative intensities of the observed rotational transitions. It is also of great interest to compare the conformational preference of methyl glycidate with its larger derivative, strawberry aldehyde. The four conformations obtained for methyl glycidate can in principle also exist for strawberry aldehyde. CI corresponds to the anti (or “a” in Ref. 10) conformation of strawberry aldehyde where the ester oxygen atom is in the anti orientation with respect to the epoxy oxygen atom, whereas CII corresponds to the syn (or “s” in the notation used in Ref. 10) conformer. A rotation about the alpha-COester bond, i.e. the C7O9 bond in methyl glycidate, generates CIV from CI. Because of the steric hindrance resulting from the close proximity of methyl group and epoxy ring, CIV is significantly higher in energy than CI. This particular conformation where the ethyl group in strawberry aldehyde is close to the epoxy ring was not among the 24 most stable conformations considered in the previous study.10 In Figure 3, we compare the geometries of the most stable syn and anti conformers of the homoand heterochiral strawberry aldehyde with the two most stable conformers of methyl glycidate. Strawberry aldehyde has one additional stereogenic center where the two hydrogen atoms of the epoxy ring of methyl glycidate are substituted with a phenyl ring and a methyl group. The homo- and heterochiral diastereomers are designated as cis(c) and trans(t) in Ref. 10. The abbreviations used for strawberry aldehyde in Ref. 10 are also used in Figure 3 for consistency. In the homochiral strawberry aldehyde, the syn conformation is strongly favored over the anti conformation with percentage populations of 72% and 27% calculated at a source temperature of 393K. Experimentally, the percentages are 88% and 12%.10 In the heterochiral strawberry aldehyde, on the other hand, the anti conformation is preferred over the syn conformation with


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calculated percentages of 69% and 21% at 393 K, while no syn conformers were detected experimentally. For comparison, in the case of methyl glycidate, there is a minor preference for the anti conformation over the syn conformation, with a calculated ratio of 57% : 43% for anti versus syn at 298 K and a corresponding experimental ratio of about 60% : 40%.

Figure 3. Comparison of the most stable syn and anti conformations of homo- and heterochiral strawberry aldehyde with the syn and anti conformations of methyl glycidate. The abbreviations used for strawberry aldehyde are the same as in Ref. 10. The syn and anti conformations are



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marked with dashed squares, whereas the orientations of the ester methyl/ethyl groups are highlighted with ovals. See text for detailed discussions.

Clearly, there is a shift in the conformational equilibrium towards the syn conformation in going from methyl glycidate to the homochiral strawberry aldehyde. This observation can be rationalized in terms of the additional stabilization provided by the long range dispersion interactions between the phenyl ring and the ethyl group (see Figure 3). Such long rang attractive interaction is not available in methyl glycidate. Going from methyl glycidate to heterochiral strawberry aldehyde, the conformational equilibrium shifts further towards the anti conformation. This is likely because of the extra repulsion between the terminal ethyl group and the methyl group of the epoxy ring. These additional bulky groups are not present in methyl glycidate. Clearly, the substitution of the phenyl and ethyl groups plays an important role in choosing the preference of the syn or anti conformation, even though there are no additional intramolecular hydrogen-bonding interactions involving these two groups. The internal rotation barrier heights of the ester methyl group for CI and CII are 4.86 and 4.82 kJ/mol, respectively. This indicates that there is not much difference in the electronic and steric environment around the ester methyl group in both conformers. These barriers are also quite similar in magnitude to the related ester methyl group barriers of other methyl carboxylates, such as methyl lactate (4.76 kJ/mol),9 methyl pyruvate (4.883 kJ/mol).17 and methyl acetate (5.0500 kJ/mol).14

CONCLUSIONS


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High resolution rotational spectra of methyl glycidate, a chiral epoxy ester, were recorded and assigned. Two important conformations, i.e. syn and anti, were identified and the interconversion barrier between them estimated. The rotational constants and the internal rotational barrier heights for the ester methyl rotor were determined experimentally for both of the two major conformers, as well as for the four 13C isotopologues of the most stable conformer. The syn-anti conformational equilibrium of methyl glycidate is compared to that of strawberry aldehyde, a larger derivative of methyl glycidate with a number of bulky substitutions at the epoxy ring. This comparison highlights the effects of these bulky substituents on the syn-anti conformational equilibrium and emphasizes the importance of long-range dispersion interactions and steric hindrance introduced by these large substituents.

ASSOCIATED CONTENT Supporting information of transition frequencies and conversion barrier height provided by the authors at (http://pubs.acs.org/page/jacsat/submission/authors.html).

ACKNOWLEDGMENT This research was funded by the University of Alberta and the Natural Sciences and Engineering Research Council (NSERC) of Canada. JY thanks NSERC for an undergraduate summer research award. JR thanks the Alberta-Saxony International Internship Alliance for partial financial support of his summer studentship. We thank Dr. S. T. Shipman for providing us the Gaussian files for strawberry aldehyde. YX and WJ are holders of Canada Research Chairs (Tier I).



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REFERENCES:

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26 Watson, J. K. G. Aspects of Quartic and Sextic Centrifugal Effects on Rotational Energy Levels. In Vibrational Spectra and Structure, Durig, J. R. Ed.; Elsevier: Amsterdam, Netherland, 1977, Vol.6, pp 39. 27 Kraitchmann, J. Determination of Molecular Structure from Microwave Spectroscopic Data. Am. J. Phys. 1953, 21, 17-25.


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TOC Graphic:


Chirped-Pulse and Cavity-Based Fourier Transform Microwave

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