Molecular Structure and Internal Rotation of CF3 Group

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Molecular Structure and Internal Rotation of CF3 Group of Methyl Trifluoroacetate: Gas Electron Diffraction, Microwave Spectroscopy and Quantum Chemical Calculation Studies Nobuhiko Kuze, Atsushi Ishikawa, Maho Kono, Takayuki Kobayashi, Noriyuki Fuchisawa, Takemasa Tsuji, and Hiroshi Takeuchi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp508447b • Publication Date (Web): 01 Dec 2014 Downloaded from http://pubs.acs.org on December 3, 2014

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The Journal of Physical Chemistry

Molecular Structure and Internal Rotation of CF3 Group of Methyl Trifluoroacetate: Gas Electron Diffraction, Microwave Spectroscopy and Quantum Chemical Calculation Studies †

†

†

†

Nobuhiko Kuze*, , Atsushi Ishikawa , Maho Kono , Takayuki Kobayashi , Noriyuki ‡

‡

‡

Fuchisawa , Takemasa Tsuji and Hiroshi Takeuchi †

Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia

University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan ‡

Graduate School of Science, Hokkaido University, Kita 10, Nishi 8, Kita-ku, Sapporo, Hokkaido

060-0810, Japan Received

ABSTRACT: The molecular structure of methyl trifluoroacetate (CF3COOCH3) has been determined by gas electron diffraction (GED), microwave spectroscopy (MW) and quantum chemical calculations (QC).

QC study provides the optimized geometries and force constants of the molecule.

They

were used to estimate the structural model for GED study and to calculate the vibrational corrections for GED and MW data.

In addition, potential energy curves for the internal rotations

of CF3 and CH3 groups have been calculated for anti (dihedral angle of α(CCOC) is 180˚) and syn (α(CCOC) = 0˚) conformers of methyl trifluoroacetate. existence of the anti conformer.

Both the GED and MW data revealed the

Molecular constants determined by MW are: A0 = 3613.4(3)

MHz, B0 = 1521.146(8) MHz, C0 = 1332.264(9), ∆J = 0.09(2) kHz and ∆JK = 0.23(6) kHz.

The

GED data were well reproduced by the analysis where a large-amplitude motion of the CF3 group was taken into account.

The barrier of internal rotation of the CF3 group was determined to be V3

= 2.3(4) kJ mol-1, where V3 is the potential coefficient of the assumed potential function, V(φ) = (V3/2)(1 - cos3φ) and φ is a rotational angle for the CF3 group.

The values of geometrical

parameters (re structure) of the anti conformer of CF3COOCH3 are: r((O=)C-O) = 1.326(6) Å, r(O-CH3) = 1.421(4) Å,

r(C-Hin-plane) = 1.083(14) Å, r(C-Hout-of-plane) = 1.087(14) Å, r(C=O) =

1.190(7) Å, r(C-C) = 1.533(4) Å, r(C-Fin-plane) = 1.319(4) Å, r(C-Fout-of-plane) = 1.326(6) Å, ∠COC

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= 116.3(5)°, ∠OCHin-plane = 105.2°(fixed), ∠OCHout-of-plane = 110.0°(fixed), ∠O=C-CH3 = 123.7°(fixed), ∠O-C-CH3 = 111.2(5)°, ∠CCF = 110.1(3)° and OCCF (out-of-plane dihedral angles) =

±121.5(1)°.

Numbers in parentheses are three times standard deviations of the data

fit.

Keywords Large amplitude motion Potential energy curves Substituent effect Ab initio method


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1. INTRODUCTION We have determined the molecular structures of acetates (CH3COOR); ethyl acetate1 (R = CH2CH3), isopropyl acetate2 (R = CH(CH3)2) and tert-butyl acetate3 (R = C(CH3)3) by gas electron diffraction (GED). CH3COOR.3

We found some remarkable substituent effects on molecular skeletons of

The r(C(=O)-O) bond distances of tert-butyl acetate (1.334(6) Å) and isopropyl

acetate (1.334(6) Å) are smaller than the values of ethyl acetate (1.345(3) Å) and methyl acetate4 (R = CH3, 1.360(6) Å). groups.

This result is attributable to the electron-releasing inductive effect of alkyl

On the other hand, the steric effect of the alkyl group is seen in the bond angles in

CH3COOR such as ∠COC and ∠O=C-C; the COC angle increases in the order of ethyl acetate (115.7(5)°), isopropyl acetate (119.0(11)°), and tert-butyl acetate (122.3(9)°) and the O=CC angle decreases gradually from 125.6° in methyl acetate to 120.4(24)° in tert-butyl acetate.

Then, we

were interested in the molecular structures of R’COOCH3 to elucidate the substituent effect of R’. Methyl trifluoroacetate (CF3COOCH3, abbreviated as MTFA) is used as an intermediate molecule of the chemical reactions in pharmaceutical and agricultural fields because of the important substituents of trifluoromethyl5,6 and ester groups.

Since the trifluoromethyl group has

significant electronegativity, it can affect the molecular structure.

Stable conformers of MTFA are

considered to take the Cs-symmetric structures of anti (Z-conformer) in which the CF3 group has the anti orientation with respect to the CH3 group and syn (E-conformer) in which the CF3 group has the syn orientation with respect to the CH3 group (see Fig. 1).

MTFA has been investigated

previously by means of infrared and Raman spectroscopies,7–9 and microwave spectroscopy9 (MW). Recently Defonsi Lestard et al.10 reported the molecular structure determined by gas electron diffraction (GED) with the aid of the quantum chemical calculations.

They observed the infrared

spectra in the gas and liquid phases, and the Raman spectrum in the liquid phase.

According to the

quantum mechanical calculations in the study, the anti conformer is the most stable and the energy difference between the anti and syn conformers is about 30 kJ mol-1; the preference for the anti conformation was clarified by the torsional potential about the C(O)–O bond of MTFA at the MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels.10

The anti conformer was also adopted

in the interpretation for site-selective dissociation in the total-ion-yield spectrum and TOF



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mass-spectra of MTFA.11

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The explanation for the anti conformational preference for the ester

compounds is based on the hyperconjugative model but recent quantum chemical studies suggest that the contribution of electron delocalization is less important than other electronic and steric effects.10, 12 In the present work, we have observed GED intensities and rotational spectrum of MTFA, and performed two kinds of data analyses which were not considered in the previous GED study.10 One is based on combined use of GED and MW data. information about the molecular structure of MTFA.

It can be expected to supply precise

The other is based on a dynamic model

which takes into account a large-amplitude internal rotation in the molecule with the GED data. The large-amplitude vibrational motion of the CF3 group must be present in MTFA because of the low potential barrier hindering internal rotation about the C-C bond, 1.93(77) kJ mol-1, estimated by the previous IR study.9

Accordingly the potential barrier of the CF3 internal rotation and precise

molecular structure of MTFA can be derived by the dynamic model. After the determination of the structural parameters, we discuss the substituent effect of the CF3 group on some geometrical parameters of MTFA by comparing the molecular structures for acetate compounds.

Furthermore,

since the quantum chemical investigation on the internal rotations for MTFA has never been reported, we analyze the theoretical potential curves for the CF3 and CH3 groups in this study.

2. EXPERIMENTAL SECTION 2.1 Gas Electron Diffraction.

Commercial samples of MTFA of Sigma-Aldrich (99% purity)

and Tokyo Chemical Industry Co., Ltd. (>98% purity) were used in the GED and MW experiment, respectively. The electron diffraction intensities were observed on 8-inch square Kodak projector slide plates using the GED apparatus in Hokkaido University with an r3-sector.13 the electron beam was held at approximately 37 kV. µA.

The accelerating voltage of

The beam current of the experiment was 1.7

Gas sample of MTFA was injected through the nozzle in the vacuum chamber at 294 K.

The distance between the nozzle and the plate was 244.7 mm.

Exposure times needed for

recording the diffraction pattern for MTFA were 40 s, and background pressures in the vacuum chamber during exposure were 4.0 x 10-3 Pa.

Calibration of the wavelength of incident electrons 4

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(0.0590 Å) was carried out by analyzing the diffraction patterns of carbon disulfide (ra(C-S) = 1.5570 Å)14 at room temperature.

Uncertainty in the scale factor (3σ) was estimated to be

0.027% from the data analysis of diffraction patterns for CS2. Photographic plates were developed for 4.5 min in Kodak Dektol developer diluted 1:1.

The

optical densities were measured and averaged at intervals of 36˚ along the ark concentric with the center of the pattern.

Five average densities, obtained at intervals of 100 µm along the diameter,

were further averaged and converted to a total intensity. Thus total intensities were obtained at intervals of 0.5 mm.

Then the total intensities were divided by theoretical backgrounds to obtain

leveled intensities.

The leveled intensities from four photographic plates of MTFA were

averaged.

The leveled total intensities, It(s), and the backgrounds, Ib(s), are presented in the

supporting information (Table S1): s is defined by s = (4π/λ)sin(θ/2) where λ is the wavelength of the incident electron and θ is a scattering angle.

Elastic and inelastic atomic scattering factors

were taken from refs. 15 and 16, respectively. The experimental molecular scattering intensities, sM(s), are shown in Fig. 2 along with the calculated ones in the final data analysis where M(s) = (It(s) - Ib(s))/Ib(s).

The s-range of sM(s) is

4.46-33.75 Å-1 and data intervals are about 0.20 Å-1. 2.2 Rotational Spectrum. The microwave spectrometer employed was a conventional 100 kHz square-wave Stark modulation type with a phase sensitive detector. The spectrum was observed with the X-band wave guide cell at room temperature in the frequency range of 28 to 40 GHz by varying the Stark voltage. The sample pressure was from 3 to 10 Pa, using a flowing system.

In this work, 37 new absorption lines have been observed in the vibrational ground state

for the normal species of MTFA.

These transition frequencies and 31 frequencies from 16 to 28

GHz reported in the literature9 have been used to determine molecular constants of MTFA (see Table S2).

3. QUANTUM CHEMICAL CALCULATIONS 3.1 Conformation.

The potential energy curve of the internal rotation about the C(=O)–O

bond was calculated at the MP2(fc)/cc-pVTZ17,18 level of theory with the Gaussian09 (Revision B.01) program22.

In the calculation, the geometrical parameters except for the dihedral angle



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α(C3O2C1C8) were relaxed.

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Figure S1 in the supporting information shows the energy curve as

the function of the dihedral angle α.

Two conformers, anti (α = 180˚) and syn (α = 0˚), were

identified as energy minima and no other energy-minimum conformer was found.

The potential

maximum between anti and syn conformers was found at around α = 80˚ and the energy difference between the anti conformer and the maximum-energy conformer was about 54 kJ mol-1. The geometry optimizations for the anti and syn conformers were carried out at the MP2(fc)/6-311++G(d,p)17,19,20, B3LYP/6-311++G(d,p)19-21, and MP2(fc)/cc-pVTZ levels of theory. The optimized geometries are summarized in Table S3.

The results for all the levels of theory

show that the anti conformer corresponds to the global minimum.

The energy difference, ∆E,

between the anti and syn conformers was calculated to be 37.1, 33.6 and 34.1 kJ mol-1 for the MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), and MP2(fc)/cc-pVTZ levels of theory, respectively.

Since the abundances of the conformers determined by GED were related to the

Gibbs free energy difference, we convert ∆E into ∆G using the thermal correction.

The

theoretical ∆G value was estimated to be 39.8, 36.3 and 36.8 kJ mol-1 for the MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), and MP2(fc)/cc-pVTZ levels of theory, respectively.

These data indicate that the abundances of the anti and syn conformers are 100%

and 0%, respectively. 3.2 Internal rotation for the anti and syn conformers.

The potential energies

associated with the internal rotations of the CF3 and CH3 groups were calculated for the anti and syn conformers as a function of the dihedral angle φ (O7C1C8F11) or τ(C1O2C3H4) at the MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), and MP2(fc)/cc-pVTZ levels.

In these

calculations, the torsion angle φ or τ was frozen whereas other independent geometrical parameters were allowed to relax.

The torsion angle was scanned at an interval of 15˚.

Table S4 lists the

optimized geometrical parameters of the conformers with different values of φ. The potential energy curves for the internal rotations of the CF3 and CH3 groups are shown in Fig. 3.

It reveals that the energy minimum corresponds to conformation with the torsional

coordinates of φ = 0˚ and τ = 180˚, i.e., the in-plane C-F bond eclipses the C=O bond and the in-plane C-H and C-O bonds are in the anti position.

Regardless of the theoretical levels, both

the potential energy curves for the internal rotations of the CF3 and CH3 groups have the threefold 6 ACS Paragon Plus Environment

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periodic barrier.

Hence these curves are deconvolved using the following two potential

functions: ܸଷ ܸ଺ ሺ1 − cos 3߶ሻ + ሺ1 − cos 6߶ሻ 2 2 ܸଷ ′ ܸ଺ ′ ሺ1 − cos 3߬ሻ + ሺ1 − cos 6߬ሻ ܸ′ሺ߬ሻ = 2 2 ܸሺ߶ሻ =

Table 1 lists the set of potential coefficients (V3, V6) and (V’3, V’6) calculated for the anti and syn conformers

of

MTFA

at

the

MP2(fc)/6-311++G(d,p),

B3LYP/6-311++G(d,p),

and

MP2(fc)/cc-pVTZ levels of theory; the V3 and V’3 terms mainly contribute to the potential functions for the internal rotations of the CF3 and CH3 groups, respectively, compared with the V6 and V’6 terms. The V3 parameter for the CF3 rotation of each of the conformers takes similar values at the three levels of theory.

The V3 value of the syn conformer is about 6 times larger than the corresponding

one of the anti conformer; it suggests that steric interactions between the CF3 and CH3 groups strongly affect the potential barrier V3 of the syn conformer. On the other hand, the V3’ value of the CH3 rotation of the anti conformer is similar to that of the syn conformer at each of the levels of theory.

As seen in Figure 1, the shortest distance between

the hydrogen atoms and lone pair electrons is 2.2 Å for the anti conformer.

For the syn

conformer, the shortest distance between the hydrogen and fluorine atoms is 2.3 Å.

These short

contacts might have contributions of similar magnitude to the V3’ values of the anti and syn conformers. 3.2 Vibrational study.

Quadratic and cubic force constants of MTFA, and vibrational

frequencies are calculated at the MP2/6-31G(d,p) level of theory.

The harmonic force constants

in the Cartesian-coordinate system were converted into those in the local-symmetry coordinates. The internal coordinates and local-symmetry coordinates are defined in Tables S5 and S6, respectively.

The frequencies of the 27 normal modes of vibration (18A’ + 9 A”) were calculated

using scaled force constants adjusted so as to fit the observed wavenumbers (Table S7).10

The

linear scaling formula fij(scaled) = (cicj)1/2fij(unscaled) was used, where ci is a scale factor.

The

values of the scale factors are listed in Table S6.

The resultant force constants of MTFA were

used to calculate the vibrational corrections for atom pairs i and j, i.e., the root-mean-square vibrational mean amplitudes,23,24 lij = 1/2, and shrinkage corrections23,25 Kij as follows: 7 ACS Paragon Plus Environment


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Kij = rα0 – ra = -(3/2)a(T - 0) - (0 + 0)/2rij + T/rij - δrT where ra is an effective internuclear distance in the expression of the molecular contribution to electron scattering intensities23,24 and rα0 (rz) is a distance between average nuclear positions in the ground vibrational state,25 0 and 0 are perpendicular mean amplitudes at T = 0 K, T and 0 are parallel mean amplitude at T = 294 and 0 K, δrT is a centrifugal stretching term, and a is a Morse anharmonicity constant (a = 2 Å-1 for bonded pairs whereas a is zero for non-bonded pairs).

Evaluation of vibrational corrections was carried out using our program,

nvma-all2011, and the results were used in the small-amplitude model that the CF3 internal rotation is treated as a normal vibrational mode (see sections 4.2 and 4.3). Vibrational amplitudes and shrinkage corrections were also calculated using SHRINK-07 program written by Sipachev.26-28

Three types of the shrinkage corrections could be calculated

with the program; (1) first approximation (harmonic corrections), Kij = rα – ra = -(T + T)/2rij + T/rij, where rα is a distance between average nuclear positions in the thermal equilibrium at temperature T23,24; (2) second approximation with curvilinear distance correction using cosine function for the transformation between internal and Cartesian coordinates, Kij,h1 = rh1 – ra, where rh1 is an interatomic distance corrected for curvilinear distance corrections, and this type of correction includes centrifugal distortions due to vibrations28 and (3) anharmonic corrections27, re – ra, from quadratic and cubic force fields, where re is an equilibrium internuclear distance.

The corrections for the centrifugal distortion effects due to the overall rotation were

also included.29 in Table 2.

These three types of corrections as well as vibrational mean amplitudes are listed

They were used in a dynamic model in GED data analysis (see section 4.4).

In the

model, pseudo-conformers are used to describe the large-amplitude motion of the CF3 internal rotation and vibrational corrections for each pseudo-conformer were calculated neglecting the contributions of the CF3 rotation.

Our aim is to examine the effect of the anharmonic vibrational

corrections on the results.

4. DATA ANALYSIS 4.1 MW study.

The observed characteristic spectral lines were assigned to K-1 = 0, 1, and 2

using their Stark effect, and the spectral lines of higher K-1 values of K-structure were resolved


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separately.

In our observation, we could not find the line splitting due to the internal rotation of

the CF3 group.

Since the line splitting by the internal rotation of CH3 group could not be

similarly assigned, barrier height of the CH3 group is probably high. A total of 68 frequencies from 16 to 40 GHz were fitted to Watson’s A-reduced Hamiltonian (Ir representation).30,31

Table 3 gives the rotational constants in the ground vibrational state, B0, and

newly determined centrifugal distortion constants, ∆J and ∆JK, for the normal species of MTFA (see B0(MW) column).

The values of the rotational constants (MHz), A0 = 3613.4(3), B0 =

1521.146(8) and B0 = 1332.264(9), were almost the same as those in the previous MW study.9 The figures in parentheses are the standard deviations of the spectral fit. The rotational constants, B0, were converted to those in the zero-point average structure, Bz, by the harmonic vibrational corrections.25

The values of the vibrational correction, Bz - B0, are

13.451, 1.307 and -0.117 MHz for A, B and C, respectively, using the geometry and scaled quadratic force constants obtained at the MP2/6-31G(d,p) level of theory.

The uncertainty of Bz

was estimated as σobs. = (σ12 + σ22)1/2; where σ1 was the standard deviation of the observed rotational constant and σ2, the uncertainty accompanying the force constants, was taken to be 10% of the vibrational correction.

Converted Bz values with the estimated errors are Az = 3626.9(14),

Bz = 1522.45(13) and Cz = 1332.15(1) MHz. Theoretical rotational constants of the anti conformer of MTFA in the equilibrium state, Be, were also shown in Table 3 (see Be(QC) column).

They were calculated at the MP2/6-31G(d,p),

MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), and MP2(fc)/cc-pVTZ levels of theory.

In

order to compare the theoretical and experimental values, the rotational constants in the present study were converted to Be by using harmonic and anharmonic vibrational corrections.28 Vibrational corrections were obtained from the SHRINK-07 calculation with the optimized geometry and scaled quadratic and cubic force constants at the MP2/6-31G(d,p) level of theory. The estimated values of Be(MW) are Ae = 3651.5, Be = 1531.96 and Ce = 1341.54 MHz.

The

corresponding Be(QC) values at the MP2/cc-pVTZ level (Ae = 3614.489, Be = 1531.743 and Ce = 1338.382 MHz) are in satisfactory agreement with the experimental Be(MW) values and the differences between the values of Be(QC) and Be(MW) are 1.0, 0.01 and 0.2% for A, B and C constants,

respectively.

Similarly, the

Be(QC) values at the MP2/6-31G(d,p) and 9



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MP2(fc)/6-311++G(d,p) levels are in agreement with the observed ones; the differences between the experimental and theoretical values for the A, B and C constants are 2.8, 0.5 and 0.8% for the MP2/6-31G(d,p) level of theory and 1.7, 0.5 and 0.7% for the MP2(fc)/6-311++G(d,p) level of theory, respectively.

However the B3LYP/6-311++G(d,p) calculation underestimates all the

rotational constants by about 2% deviation (see Table 3).

This means that the inclusion of the

electron correlation in the calculation is important to estimate accurate rotational constants of this compound. Furthermore, theoretical centrifugal distortion constants, ∆J, ∆JK and ∆K, were estimated from the geometry and scaled quadratic force constants at the MP2/6-31G(d,p) level of theory (see Be(QC) [1] column in Table 3).

The theoretical value of the ∆J constant, 0.087 kHz, is in

excellent agreement with the experimental one, 0.09(2) kHz. constant is not consistent with the experimental one.

However theoretical value of ∆JK This is probably because of the

overestimation of centrifugal distortion constants such as τaaaa, which correspond to the a-axis and are included in the ∆JK constant.31 4.2 GED study.

Least-squares calculations on molecular scattering intensities were carried

out to determine the geometrical parameters, vibrational mean amplitudes and index of resolution. The index of resolution k is defined by the equation of sM(s)calc = ksM(s)obs. There are 27 structural parameters in MTFA as follows: ten bond lengths, C=O, C-C, C1-O2, C3-O2 and three C-H and three C-F; ten bond angles, COC, OCO, O=CC, O-CC, three CCF and three OCH angles; seven dihedral angles, O=C-O-C, three COCH and three OCCF angles.

The

Cs symmetry is assumed for the anti and syn conformers and thus it reduces the number of the independent structural parameters.

We adopted the definition of the bond distance parameters in

used the previous GED study,10 that is, the C–F, C–O, C=O and C–C distances were combined to give an overall average value and a series of differences (p1–p6).

These are defined as follows:

p1 = [r(C1=O7) + r(C1–O2) + r(C8–F11) + 2 r(C8–F9) + r(C3–O2) + r(C1–C8)]/7 p2 = r(C1–C8) – [r(C1=O7) + r(C1–O2) + r(C8–F11) + 2 r(C8–F9) + r(C3–O2)]/6 p3 = r(C3–O2) − [ r(C1=O7) + r(C1–O2 + r(C8–F11) + 2 r(C8–F9)]/5 p4 = r(C8–F9) − [r(C1=O7) + r(C1–O2) + r(C8–F11)]/3 p5 = r(C8–F11) – [r(C1=O7) + r(C1–O2)]/2 10 ACS Paragon Plus Environment

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p6 = r(C1–O2) − r(C1=O7) Several parameters are required for the methyl group.

A single parameter for r(C–H) is

defined (p7) by the averaged value of the C-H bonds and the difference between in-plane C-H distance and out-of-plane C-H distance is fixed at the theoretical value by the ab initio calculations. For two different OCH angles, the following two parameters are used to define them: the average of the OCH angles (p8) and the difference between the in-plane and out-of-plane OCH angles (p9). The dihedral angle for the out-of-plane hydrogen atoms (p10) is also an independent parameter. Similarly to the CH3 group, the CF3 group is described using the average of the CCF angles (p13), the difference between the two CCF angles (p14), and the OCCF dihedral angle for the out-of-plane fluorine atoms (p15).

The molecular model also includes the C1O2C3 (p11), O2C1C8 (p12), and

O7C1C8 (p16) bond angles. symmetry of the molecule.

The O7C1O2 angle is a dependent parameter because of the Cs Consequently 16 independent geometrical parameters are defined as

shown in Table 4. A few geometrical parameters such as C1O2C3 and OCH angles took unreasonable values during the least-squares refinement of the GED data because of the correlations among the structural parameters. Therefore geometrical restraints were applied to the bond lengths and angles that could not be refined (Table 4).

The difference between the C1O2C3 (p11) and

O2C1C8 (p12) angles was fixed at the value obtained by the quantum chemical calculation and the value of the O2C1C8 angle was refined. p10) were fixed at the theoretical values. the theoretical one.

The parameter values for the OCH angles (p8, p9 and Similarly, the difference between p5 and p14 was fixed at

The geometrical restraints were based on the results at the

MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), or MP2(fc)/cc-pVTZ level of theory.

At the

final stage of the GED data analysis, six distance parameters (p1-p4, p6, p7) and three angle parameters (p12, p13 and p15) were selected as adjustable parameters. In addition, vibrational mean amplitudes of the atom pairs lij were refined by dividing them into five groups corresponding to the interatomic distances of MTFA; (1) ra < 1.6 Å; (2) ra = 1.6-2.5 Å; (3) ra = 2.5-3.0 Å; (4) ra = 3.0-3.8 Å; and (5) ra > 3.8 Å (see Table 2). values between the atom pairs Kij are fixed at the calculated values.

Shrinkage correction

The anharmonicity constants

of the bonding atom pairs were estimated by the conventional method,32 while those of the 11 ACS Paragon Plus Environment


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non-bonded atom pairs were assumed to be zero.

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As a result, nine geometrical parameters, five

groups of vibrational amplitudes and the index of resolution were refined. The previous GED work10 showed that MTFA exists as a single anti conformer.

In our

preliminary data analysis based on the small-amplitude vibrational model (SAM), we examined a model where the anti and syn conformers coexist.

The population of the syn conformer was

determined to be 0.2(43)% in the two-conformer model where the number in parentheses was a standard deviation.

Since our experimental data confirmed that the single anti conformer model

was reasonable, the model was used in the further steps of the GED data analysis. 4.3 Combined analysis of GED and MW data.

Experimental rotational constants Bα0 can

be obtained from the rα0 structure determined by GED: this structure is defined by average nuclear positions in the ground vibrational state (see Table 3).

To derive the rα0 structure, shrinkage

corrections, Kij = rα0 – ra, were used (see section 3.2).

The GED+MW columns in Table 2 list

the values of the vibrational corrections used in this type of data analysis. Since the definitions of Bα0 and Bz are the same, we can compare these constants.

Although the

values of Bα0(GED) do not agreed with those of Bz(MW) within the experimental errors (Table 4), we performed the combined analysis of GED and MW data in the small-amplitude vibrational model.

The relative weights of the Bz constants were adjusted so as to make the errors for

Bz(GED) equal to the original errors for the Bz(MW).25

They were estimated to be 1.0×103, 1.0

×104, and 1.0×105 for the rotational constants, A, B and C, respectively, whereas data weights of sM(s) were 1.0 in the range of s = 6-18 Å-1, and less than 1.0 for other data ranges. Independent parameters fitted for the GED + MW data are listed in Table 5.

The R-factor in

the GED data fit was large, 0.070, indicating a poorly refined structure against the GED data (R = {ΣiWi(∆sM(s)i)2/ΣiWi(sM (s)iobs.)2}1/2, where ∆sM(s)i = sM(s)i obs. - sM(s)icalc. and Wi is a diagonal element of the weight matrix).

Moreover, the difference between the C-F distance, ∆r(C-F) =

rg(C-Fout-of--plane) - rg(C-Fin-plane) = 0.035 Å, is considerably larger than that in the previous GED study (0.012 Å) or quantum chemical calculations (0.011-0.013 Å).

The above results indicate

that the combined analysis of the GED and MW data was not valid under the assumption of the small-amplitude vibrational model. 4.4 Dynamic model for GED data analysis.

Another type of data analysis is a dynamic


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model, a large-amplitude vibrational model (abbreviated as LAM in the Tables 2 and 5). the internal rotation of the CF3 group as a large-amplitude vibration.33

It treats

The effect of this

large-amplitude mode on the interatomic distances was described by a set of pseudo-conformers. Since the experimental rotational constants of MTFA were not determined in excited vibrational states for the CF3 torsion and it was difficult to properly estimate the rotational constants corrected for the large-amplitude vibration properly, only the GED data were used in a dynamic model. The potential function for the internal rotation of the CF3 group was assumed to be V(φ) = (V3/2) (1 – cos3φ) The pseudo-conformers take the dihedral angles φ ranging from 0˚ to 60˚ at an interval of 15˚. The coefficient V3 was added as the adjustable parameter.

The population of each

pseudo-conformer was derived from the V3 value assuming the Boltzmann distribution at 294 K. The bond distances and angles were dependent on φ and the dependence of these parameters on φ (geometrical restraints) was taken from the optimized structures (Table S4).

Vibrational

amplitudes and shrinkage corrections for each pseudo-conformer were calculated from the force constants by neglecting the contributions of the CF3 torsion.

On the other hand, the CH3 internal

rotation was treated as a small-amplitude motion in this analysis because of the small scattering power of the hydrogen atom. Three types of the vibrational corrections26-28 described in subsection 3.2 were used in the dynamic model (see Table 2).

The complete set of the vibrational mean amplitudes and

shrinkage corrections for all the pseudo-conformers are listed in Table S8.

The derived

geometrical parameters are: (1) rg distances and ∠α angles using harmonic vibrational corrections (abbreviated to 1st), where rg is an average internuclear distance at temperature T23,24; (2) rh1 structure26,27 using harmonic vibrational corrections with curvilinear distance correction terms (abbreviated to 2nd); (3) re structure using anharmonic corrections26-28 (abbreviated to Anh), respectively.

Table 2 shows that the values of the vibrational mean amplitudes for first, second

and Anh approximations are close to those calculated by nvma-all2011 program except for the mean amplitudes for a few atom pairs including the F atoms.

This is because of the

large-amplitude vibrational treatment for the CF3 internal rotation. Furthermore, we examined the three types of geometrical restraints in the dynamic model, based 13 ACS Paragon Plus Environment


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on the MP2(fc)/6-311++G(d,p), B3LYP/6-311++G(d,p), or MP2(fc)/cc-pVTZ calculations.

The

fitted structural parameters are listed in Table S9. Figure 4 shows the final radial distribution curve.

Table 2 lists the values of the vibrational

mean amplitudes obtained with the best model. Table 5 lists the geometrical parameter values for the best model (see GED(LAM)) as well as those in the previous GED study10 and quantum chemical calculations.

Table S10 lists the values of the geometrical parameters for every

pseudo-conformer in the best model.

The least-squares correlation matrix of the analysis is given

in Table S11.

5. DISCUSSION 5.1 Structural restraints in the GED data analysis.

The

data

analyses

under

the

B3LYP/6-311++G(d,p) restraints gave the R-factors of around 0.030 using the 1st, 2nd and Anh vibrational corrections.

As shown in Table S9, the ∆r(C-F) are smaller than zero in the 1st, 2nd

and Anh analyses (-0.010, -0.007 and 0.000 Å) whereas the three theoretical calculations and the data analyses aided with the results at the different theoretical levels result in positive differences of 0.001 to 0.011 Å.

Furthermore the COC angle obtained with the B3LYP/6-311++G(d,p)

restraints has a substantially large value, over 117˚, compared with the present GED results with the different restraints and the predictions of MP2 calculations (about 114˚).

Accordingly the

structural model with the B3LYP/6-311++G(d,p) restraints is unacceptable. In the analyses with the MP2(fc)/6-311++G(d,p) and MP2(fc)/cc-pVTZ restraints, the values of geometrical parameters obtained by the analyses with the 1st and 2nd harmonic vibrational corrections were almost the same (see Tables 5 and S9).

Since the main difference between these

analyses is the calculated values of the shrinkage corrections, the geometrical parameters of MTFA are little affected by this difference.

Next we examined the effect of the anharmonic vibrational

corrections on the geometrical parameters by comparing the result derived with the Anh corrections with that with the 2nd corrections.

For the MP2(fc)/6-311++G(d,p) restraints (see

Table 5), the re bond distances obtained by the Anh-correction analysis are 0.003 - 0.020 Å shorter than the rh1 bond distances by the 2nd-correction analysis and the re values of the bond angles are close to the rh1 parameter values.

Similar trends are found for the results with the


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MP2(fc)/cc-pVTZ restraints.

The shortening found for the bond lengths is due to the

anharmonicity of the potentials of the bonded atom pairs.

The R-factor found for the data

analysis with the Anh corrections was smaller than the corresponding factors of the 1st- and 2nd-correction analyses (see Table 5).

The R-factor with the MP2(fc)/6-311++G(d,p) restraints

(R = 0.032) is smaller than that with the MP2(fc)/cc-pVTZ restraints (R = 0.034, see Table S9). With the R-factors of the above models, we selected the structural model obtained with the MP2(fc)/6-311++G(d,p) restraints and the anharmonic vibrational corrections as the best model. 5.2 Comparison with the previous GED data.

We compare the previous GED data10

(see GED(SAM)) with the present result in the re structure (see GED(LAM) Anh).

Although the

physical definition of the structure obtained in ref 10 (rh1 structure, see Section 3.2) is different from that in the present study (re structure), the bond lengths and angles of the present results are likely to be similar to the corresponding ones of the previous results except for ∠C1O2C3, r(C-F), and r(C2-O3).

These differences would be due to the difference between the structural models

adopted in ref 10 and the present study (the small-amplitude vibrational and dynamic models for the CF3 internal rotation). 5.3 Substituent Effects on Geometrical Parameters.

Table 6 list selected geometrical

parameters of RCOOR’ obtained by GED and ab initio calculation: MTFA (R = CF3, R’ = CH3, experimental re structure), anti-2,2,2-trifluoroethyl trifluoroacetate34 (R = CF3, R’ = CH2CF3, ra3,1 structure), methyl acetate4 (R = R’ = CH3, rg distances and ∠α0 angles), ethyl acetate1 (R = CH3, R’ = CH2CH3, rg distances and ∠α angles), isopropyl acetate2 (R = CH3, R’ = CH(CH3)2, rg distances and ∠α angles) and tert-butyl acetate3 (R = CH3, R’ = C(CH3)3, rg distances and ∠α angles).

The

ra3,1 structure determination includes corrections for cubic anharmonic effects and the amplitudes of vibration calculated in the harmonic approximation.35 to the re structure. lij2/2re.

Accordingly the ra3,1 structure is similar

Average internuclear distance, rg, can be related to the ra distance as rg = ra +

For the definitions of ra, rα and rα0 structures, see Section 3.2.

We consider the electron-withdrawing inductive effect of the CF3 group with a following resonance scheme.



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Page 16 of 39

When the structure of CH3COOR is compared with that of CF3COOR, the r(C(=O)-O) distance would decrease by the substitution of the CF3 group for the CH3 group since the inductive effect of the CF3 group enhances the double-bond character of the (O=)C-O bond.

On the other hand, the

CF3 group could lengthen the r(C=O) and r(O-R) bonds. The definitions of the bond distances of MTFA and methyl acetate are different from each other (re and rg distances, respectively).

However, the structural difference of rg - re for the r(C(=O)-O)

value is easily calculated to be 0.009 Å using the vibrational corrections in Table 2. Consequently the rg(C(=O)-O) value of MTFA is determined to be 1.335(6) Å. the value of methyl acetate (1.360(6) Å). scheme for CF3COOR molecule.

It is smaller than

This result is consistent with the above resonance

The MP2/6-311++G(d,p) calculations show the same structural

tendency. The rg(C(=O)-O) value for anti-CF3COOCH2CF3 takes an intermediate value between MTFA and methyl acetate.

Defonsi Lestard et al.10 pointed out that the shortening of the r(C(=O)-O)

value in MTFA relative to anti-CF3COOCH2CF3 can be explained by the larger hyperconjugative interaction between the lone pairs on the O2 atom and the σ* orbital of the C=O bond in MTFA than in anti-CF3COOCH2CF3.

The electron-withdrawing inductive effect of the CF3 group and

the hyperconjugative interaction must result in the difference between the C(=O)-O bond lengths of MTFA and CH3COOCH3. The rg(C=O) value of MTFA is determined to be 1.195(7) Å using the rg - re correction (0.005 Å).

This value is smaller than the corresponding values of methyl acetate (1.209(6) Å).

The

MP2/6-311++G(d,p) results show that the re(C=O) value of MTFA (1.206 Å) is slightly smaller than that of methyl acetate (1.212 Å). structural trend for r(C=O).

Thus the resonance scheme is not applicable to the

To explain the above discrepancy, we consider the electrostatic

repulsion in RCOOR’ molecules.

Table 7 lists the Mulliken atomic charges calculated for MTFA,

anti-2,2,2-trifluoroethyl trifluoroacetate and methyl acetate at the MP2/6-311++G(d, p) level of theory.

A significant difference is found for the C8 carbon atom; the charge on the carbon atom

in the CF3 group of MTFA, 0.483 e, is positive whereas the charge on the C8 atom of CH3COOCH3, -0.601 e, is negative.

On the other hand, the net charges for the C1 (0.257 e) and

O7 (-0.300 e) atoms in the C=O group of MTFA are slightly different from those of methyl acetate 16 ACS Paragon Plus Environment

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(0.372 e and -0.200 e).

The charge for the O2 atom (-0.216 e) in the (O=)C-O bond is close to

that of methyl acetate (-0.196e).

Consequently the electron density on the C1-C8 bond is reduced

by the substitution of the CF3 group for the CH3 group.

This indicates that the electrostatic

repulsion between the C1-C8 and C1=O7 bonds and that between the C1-C8 and (O=)C1-O2 bonds in CF3COOCH3 are smaller than the corresponding repulsions in CH3COOCH3.

The

decrease of the repulsions in MTFA may shorten the C=O and (O=)C-O bonds. Using the rg - re value of 0.012 Å, the rg(C-O) value of MTFA is determined to be 1.433(4) Å. The corresponding value of CH3COOCH3 is 1.442(7) Å.

Considering the experimental errors and

uncertainties for shrinkage corrections, no significant difference would be found between the rg(C-O) values of MTFA and methyl acetate.

The MP2/6-311++G(d,p) results suggest that the

r(C-O) value of MTFA is slightly longer than that of methyl acetate. The electrostatic repulsion between the C-CF3 and C=O bonds and that between the C-CF3 and (O=)C-O bonds may affect the O7=C1-C8 and O2-C1-C8 angles, respectively.

The value

of ∠O=C-C of MTFA, 123.7˚, is smaller than that of methyl acetate, 125.6˚; the value of ∠O-C-C of MTFA, 111.2(5)˚ is equal to that of methyl acetate 111.4(9)˚ within the experimental uncertainties.

The MP2/6-311++G(d, p) calculations confirm these structural trends.

The result

that the O=C-C angle of MTFA is narrower than that of methyl acetate can be explained in terms of the electrostatic repulsion between C-CF3 and C=O bonds. The experimental value of ∠O=C-C (126.1˚) in CF3COOCH2CF3 is larger than that of MTFA by 2.4˚.

The MP2/6-311++G(d, p) results do not reproduce this trend.

The values of ∠O=C-C

suggest that the effect of another CF3 group on the structures is underestimated in the quantum chemical calculation. As shown in Table 5, our value of ∠COC in MTFA, 116.3(5)°, is significantly larger than the previous GED value of 112.6(4)˚.

The MP2/6-311++G(d,p) and MP2/cc-pVTZ calculations give

the ∠COC values of 114.2˚ and 113.8˚, respectively, and they are smaller than our GED value. Similar trend is shown for the COC angle of CH3COOCH329; the GED value of ∠COC (rα0 structure) is 116.4(9)˚ and the calculated value for CH3COOCH3 is 114.3˚ at the MP2/6-311++G(d,p) level (see Table 6).

Furthermore the experimental ∠COC values for

CH3COOR (R = CH2CH3, CH(CH3)2 and C(CH3)3) are larger than the MP2/6-311++G(d,p) values. 17 ACS Paragon Plus Environment


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Page 18 of 39

According to the above trends, the ∠COC value of the previous GED study would be unreasonable. The experimental COC value of MTFA is 1.9° larger than that of CF3COOCH2CF3, 114.4(7)°. The theoretical calculations support this tendency (114.2° and 113.7° for MTFA and CF3COOCH2CF3, respectively).

This may originate from the electrostatic effect of the fluorine

atoms in the CH2CF3 group. 5.4 Potential barrier of the CF3 group.

The barrier of the internal rotation of the CF3

group, V3, was determined to be 2.3(4) kJ mol-1 in the best model with the MP2(fc)/6-311++G(d,p) restraints.

In the previous IR study5, the CF3 torsional frequency in the vapor sample of MTFA is

25 cm-1 and the potential barrier of the hindered rotation about the C-C bond was estimated to be 1.93(77) kJ mol-1, a value calculated from V3 = 2.25Fs in Mathieu’s equation, where the F parameter was related to the reduced moments of inertia, and s is a reduced barrier.

The present

study provides the more precise value for the barrier of the internal rotation of the CF3 group. Quantum chemical calculations indicate that the value of V3 is 2.1 to 2.4 kJ mol-1 (see Table 1). The V3 value from the GED data is in agreement with the theoretical data.

6. CONCLUSION The investigation on the molecular structure of methyl trifluoroacetate (CF3COOCH3) was carried out by gas electron diffraction, microwave spectroscopy and quantum chemical calculations.

The potential energy curve for the internal rotation of the CF3 group was calculated

for two possible conformers, anti (α(CCOC) = 180˚) and syn (α= 0˚).

Optimized geometrical

structures and force constants were used to construct the structural model for the GED study and to calculate the vibrational corrections for the GED and MW data.

Both the GED and MW

experimental data show that this molecule exists as the anti conformation.

Molecular constants

in the ground vibrational state were determined and used in the combined analysis of GED and MW data.

However the data fitting of the GED + MW data gave a high R-factor.

examined a dynamic model for the internal rotation of the CF3 group.

Therefore we

The best structure obtained

in the present study is based on the structural model with anharmonic vibrational corrections and


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the MP2/6-311++G(d,p) geometrical restraints.

The barrier of the internal rotation of the CF3

group was determined to be 2.3(4) kJ mol-1. We discussed the structural parameters of the related acetates in terms of the inductive effect and electrostatic repulsions by the CF3 group.

The values

of the structural parameters in the present study are close to the corresponding ones of the previous GED results except for the C1O2C3 angle.

This angle would be underestimated in the previous

study.

ASSOCIATED CONTENT * Supporting Information The total intensities (IT) and the backgrounds (IB) of methyl trifluoroacetate (Table S1), observed rotational transitions frequencies with residuals (MHz) of methyl trifluoroacetate in the ground vibrational state for normal species (Table S2), structural parameters, rotational constants, dipole moments, energies and populations for the anti and syn conformers of methyl trifluoroacetate obtained by the ab initio and DFT calculations (Table S3), structural parameters for the anti orientation of methyl trifluoroacetate as a function of φ(O7C1C8F11) obtained by the ab initio and DFT calculations (Table S4), internal coordinates of methyl trifluoroacetate (Table S5), local-symmetry coordinates of methyl trifluoroacetate (Table S6), calculated vibrational wavenumbers (cm-1) for the anti conformer of methyl trifluoroacetate (Table S7), vibrational mean amplitudes and shrinkage corrections (in Å) for the anti orientation of methyl trifluoroacetate (Table S8), structural parameters for methyl trifluoroacetate (Table S9), structural parameters for pseudo-conformers of anti orientation of methyl trifluoroacetate (Table S10), correlation matrix of methyl trifluoroacetate (Table S11) and torsional potential curve about the C(=O)–O bond of methyl trifluoroacetate calculated at the MP2/cc-pVTZ level of theory (Figure S1). is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Nobuhiko Kuze, E-mail: [email protected]

phone, +81332383458

Notes 19 ACS Paragon Plus Environment

This material


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This work supported by JSPS KAKENHI Grant Number 25410026.

ACKNOWLEDGMENT The authors thank Prof. Shinkoh Nanbu and members of Laboratory for Theory-Aided Molecular Design (Department of Materials and Life Sciences, Sophia University) for the use of their computing server of the quantum chemical calculations.

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Incoherent Scattering and Intensity Factors for the First Thirty-Six Elements J. Chim. Phys. Phys.-Chim. Biol., 1967, 64, 540-554. (17) Møller, C.; Plesset, M. S. Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev., 1934, 46, 618-622. (18) Kendall, R. A.; Dunning Jr., T. H.; Harrison, R. J. Electron Affinities of the First‐Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys., 1992, 96, 6796-6806. (19) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular Orbital Hydrogenation Energies. Theor. Chim Acta, 1973, 28, 213-222. (20) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Selfconsistent Molecular Orbital Methods. XX. A Basis Set For Correlated Wave Functions. J. Chem. Phys., 1980, 72, 650-654. (21) Becke, A.D. Density Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys., 1993, 98, 5648-5652. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Revision B.01, Gaussian, Inc., Wallingford CT, 2010. (23) Hargittai, I., in Hargittai, I., Hargittai, M., Eds., Stereochemical Applications of Gas-Phase Electron Diffraction Part A-The Electron Diffraction Technique, VCH Publishers, Inc., New 21 ACS Paragon Plus Environment


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York, 1988, Chapter 1. (24) Kuchitsu, K. in Domenicano, A.; Hargittai, I., Eds., Accurate Molecular Structures: Their Determination and Importance, Oxford University Press, Oxford, 1992, Chapter 2. (25) Kuchitsu, K.; Nakata, M.; Yamamoto, S. in Hargittai, I., Hargittai, M., Eds., Stereochemical Applications of Gas-Phase Electron Diffraction Part A-The Electron Diffraction Technique, VCH Publishers, Inc., New York, 1988, Chapter 7. (26) Sipachev, V.A. Calculation of Shrinkage Corrections in Harmonic Approximation. J. Mol. Struct. (THEOCHEM), 1985, 121, 143-151. (27) Sipachev, V.A. Anharmonic Corrections to Structural Experiment Data. Struct. Chem., 2000, 11, 167-172. (28) Sipachev, V.A. Local Centrifugal Distortions Caused by Internal Motions of Molecules. J. Mol. Struct., 2001, 567–568, 67-72. (29) Iwasaki, M.; Hedberg, K. Centrifugal Distortion of Bond Distances and Bond Angles. J. Chem. Phys., 1962, 36, 2961-2963. (30) Watson, J. K. G. in Durig, J. R. Ed., Vibrational Spectra and Structure; Vol. 6, Elsevier, Amsterdam, 1977, pp 33-35. (31) Gordy, W.; Cook, R. L., Microwave Molecular Spectra, Wiley Interscience, New York, 3rd ed. 1984, pp 324-333. (32) Kuchitsu K.; Bartell, L.S. Effects of Anharmonicity of Molecular Vibrations on the Diffraction of Electrons. II. Interpretation of Experimental Structural Parameters. J. Chem. Phys., 1961, 35, 1945-1949. (33) Lowrey, A. H. in Hargittai, I., Hargittai, M., Eds., Stereochemical Applications of Gas-Phase Electron Diffraction Part A-The Electron Diffraction Technique, VCH Publishers, Inc., New York, 1988, Chapter 12. (34) Defonsi Lestard, M. E.; Tuttolomondo, M. E.; Varetti, E. L.; Wann, D. A.; Robertson, H. E.; Rankin, D. W. H.; Ben Altabef, A. Experimental and Theoretical Studies of the Vibrations and Structure of 2,2,2-trifluoroethyl Trifluoroacetate, CF3CO2CH2CF3 J. Mol. Struct., 2009, 917, 183-192. (35) McCaffrey, P. D; Mawhorter, R. J.; Turner, A. R.; Brain, P. T.; Rankin, D. W. H. Accurate Equilibrium Structures Obtained from Gas-Phase Electron Diffraction Data: Sodium Chloride. J. Phys. Chem. A, 2007, 111, 6103-6114.


Page 22 of 39

Page 23 of 39

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-1

Table 1: Potential coe fficients for CF3 and CH 3 internal rotations of methyl trifluoloacetate (in kJ mol ) anti conformer CF3 MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) V3 V6

2.15 -0.17

2.09 -0.05

MP2/cc-pVTZ 2.39 -0.11

CH3

MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) V3' V6'

5.38 0.09

MP2/cc-pVTZ

3.02 0.09

4.48 0.12

CH3 MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) 5.39 3.90 -1.13 -0.37

MP2/cc-pVTZ 4.28 -0.99

syn conformer

V3 V6

CF3 MP2/6-311++G(d,p) B3LYP/6-311++G(d,p) 13.35 12.25 -2.04 -0.57

MP2/cc-pVTZ 14.00 -1.92

V3' V6'



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

Table 2: Vibrational mean amplitude s (l ), inte ratomic distance s (r ) and shrinkage corrections (in Å) for the anti conformer of methyl trifluoloacetate GED GED GED+MW GED+MW a

b

SAM Atom pair C 3C 3C 3C 1C 8C 8C 8C 1O 2C 1H 5H 4H 4O 2O 2O 2F 9F 9F 10 O 2C 1C 1C 1C 1O 2O 7O 7C 1C 1H 5H 6C 3O 2O 2C 1O 7O 7O 2C 3H 4H 5H 6C 3C 3H 6H 5H 4H 4H 4C 3H 5H 6H 5H 6H 4-

H H H O F F F O C C H H H H H H F F F O F C F F C C F H H O O O F F H F F F C O C C F F F F C F F F F F F F F

c

ra 4 5 6 7 11 9 10 2 3 8 6 6 5 4 6 5 10 11 11 7 11 3 9 10 8 8 11 5 6 7 7 7 9 10 4 9 10 11 8 7 8 8 9 10 10 9 8 9 10 11 10 9 11 11 11

d

1.099 1.102 1.102 1.194 1.325 1.326 1.326 1.334 1.430 1.542 1.797 1.806 1.806 2.016 2.077 2.077 2.122 2.169 2.169 2.248 2.352 2.352 2.352 2.361 2.368 2.412 2.662 2.663 2.664 2.691 2.693 2.704 2.750 2.750 3.207 3.294 3.295 3.505 3.705 3.718 4.072 4.072 4.089 4.089 4.326 4.326 4.351 4.551 4.552 4.701 4.725 4.725 4.920 4.921 5.477

l calc. 1st 0.077 0.078 0.078 0.037 0.044 0.045 0.045 0.043 0.050 0.051 0.123 0.123 0.123 0.104 0.103 0.103 0.057 0.056 0.056 0.051 0.064 0.065 0.072 0.072 0.064 0.061 0.099 0.191 0.191 0.306 0.306 0.099 0.235 0.235 0.100 0.222 0.222 0.061 0.068 0.117 0.169 0.169 0.164 0.164 0.229 0.229 0.119 0.290 0.290 0.078 0.198 0.198 0.228 0.228 0.111

a

LAM l calc. 1st, 2nd 0.075 0.076 0.076 0.037 0.044 0.045 0.045 0.044 0.049 0.050 0.122 0.121 0.121 0.102 0.101 0.101 0.058 0.058 0.058 0.051 0.064 0.064 0.068 0.068 0.064 0.061 0.101 0.194 0.194 0.313 0.314 0.098 0.098 0.098 0.099 0.079 0.079 0.062 0.068 0.115 0.167 0.167 0.122 0.122 0.221 0.221 0.118 0.184 0.184 0.078 0.153 0.153 0.214 0.215 0.110

b

SAM l calc. Anh 0.076 0.077 0.077 0.038 0.045 0.045 0.045 0.045 0.051 0.051 0.123 0.123 0.123 0.105 0.103 0.103 0.060 0.058 0.058 0.053 0.067 0.068 0.071 0.071 0.068 0.064 0.112 0.207 0.208 0.341 0.342 0.109 0.106 0.106 0.102 0.082 0.082 0.065 0.072 0.124 0.174 0.174 0.129 0.129 0.226 0.225 0.122 0.197 0.196 0.083 0.161 0.161 0.229 0.229 0.113

l obs.

d

0.074 0.075 0.075 0.036 0.042 0.043 0.043 0.043 0.049 0.049 0.116 0.117 0.117 0.098 0.097 0.097 0.054 0.052 0.052 0.047 0.061 0.064 0.064 0.062 0.062 0.058 0.095 0.191 0.191 0.324 0.325 0.093 0.089 0.089 0.095 0.076 0.076 0.058 0.065 0.117 0.169 0.169 0.123 0.123 0.220 0.220 0.117 0.191 0.191 0.078 0.156 0.155 0.223 0.224 0.107

n (4)

(6)

(22)

(19)

(50)

e

0 rα

1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5

- ra 1st -0.021 -0.022 -0.022 -0.006 -0.003 -0.003 -0.003 -0.004 -0.005 -0.001 -0.031 -0.031 -0.031 -0.014 -0.017 -0.017 -0.003 -0.005 -0.005 -0.008 -0.001 -0.002 -0.001 -0.001 -0.002 -0.003 -0.003 -0.001 -0.001 0.022 0.022 -0.002 0.015 0.015 -0.011 0.011 0.011 0.000 -0.002 -0.011 -0.004 -0.004 0.003 0.003 0.001 0.001 -0.007 0.007 0.007 0.000 -0.001 -0.001 0.003 0.003 -0.004

rα - ra 1st -0.064 -0.071 -0.071 -0.003 -0.002 -0.003 -0.003 -0.002 -0.013 -0.001 -0.121 -0.111 -0.111 -0.043 -0.049 -0.049 -0.003 -0.002 -0.002 -0.002 -0.001 -0.006 -0.001 -0.001 -0.001 0.000 0.002 -0.023 -0.023 0.011 0.012 -0.001 0.001 0.001 -0.024 0.000 0.000 -0.001 -0.002 -0.016 -0.016 -0.016 0.002 0.002 -0.008 -0.008 -0.015 -0.008 -0.008 0.000 -0.014 -0.014 -0.006 -0.006 -0.011

a

Obtained by the normal coordinate analysis using nvma-all2011 program and used for the the small-amplitude vibrational model.

b

Obtained by the normal coordinate analysis using SHRINK program and used for the large-amplitude vibrational model.

These data were used for the anti conformer with φ = 0 degree. For the every pseudo-conformer, see Table S9. c

See Fig. 1 for the atom numbering.

d

Obtained by the final data fit. The figures in the parentheses are three times standard deviation of the fit.

e

The group of vibrational amplitudes. The vibrational amplitudes belonging to the same group have the same experimental error.


LAM r h1 - r a

re - ra

2nd 0.001 0.002 0.002 0.001 0.001 0.000 0.000 0.000 0.000 0.000 0.008 0.008 0.008 0.007 0.006 0.006 0.002 0.002 0.002 0.002 0.002 0.004 0.002 0.002 0.002 0.005 0.010 0.007 0.007 0.018 0.019 0.003 0.004 0.004 0.035 0.008 0.008 0.005 0.011 0.049 0.014 0.014 0.017 0.017 0.013 0.013 0.036 0.035 0.035 0.014 0.036 0.036 0.013 0.014 0.048

Anh -0.016 -0.015 -0.015 -0.003 -0.006 -0.006 -0.006 -0.008 -0.010 -0.009 -0.021 -0.017 -0.017 -0.018 -0.015 -0.015 -0.008 -0.007 -0.007 -0.013 -0.011 -0.028 -0.011 -0.011 -0.007 -0.006 -0.004 -0.034 -0.035 -0.035 -0.037 -0.047 -0.004 -0.004 -0.008 -0.007 -0.008 -0.012 -0.017 -0.015 -0.030 -0.030 -0.004 -0.004 -0.022 -0.022 0.004 0.015 0.014 -0.030 -0.004 -0.004 -0.045 -0.046 0.001

Page 25 of 39

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a Table 3: Molecular constants for the anti conformer of methyl trifluoroacetate ∆J /kHz ∆JK /kHz ∆K /kHz A /MHz B /MHz C /MHz Previous work (Ref 9) B 0(MW) 3615.1 (4) 1521.14 (1) 1332.23 (1) This work B 0(MW) 3613.4 (3) 1521.146 (8) 1332.264 (9) 0.09 (2) 0.23 (6) b

B z(MW)

3626.9

(14)

1522.45

(13) 1332.15

(1)

0 c B α (GED) d B e(MW) e B e(QC) [1] e B e(QC) [2] e B e(QC) [3] e B e(QC) [4]

3622

(8)

1541.5

(36) 1345.6

(22)

3651.5

(14)

1531.96

(13) 1341.54

(1)

3553.743

1525.164

1330.678

3592.116

1524.240

1331.660

3575.817

1504.902

1316.867

3614.489

1531.743

1338.382

0.087

1.452

a

The figures in parentheses represent the standard deviations. MW: microwave spectroscopy. GED: gas electron diffraction. QC:quantum chemical calculation

b c d e

Harmonic corrections calculated from the MP2/6-31G(d,p) quadratic force field were used. 0

Caculated from the GED geometry. Definitions of rotational constants B α and B z are the same. Harmonic and anarmonic corrections were calculated from the MP2/cc-pVTZ force field. [1] MP2/6-31G(d,p); [2] MP2/6-311++G(d,p); [3] B3LYP/6-311++G(d,p); [4] MP2/cc-pVTZ


-1.057


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

Table 4: Structural restraints for anti orientation of methyl trifluoroacetate a b

Geometrical parameter Bond lengths/Å C1O2 O2C3 C3H4 C3H5 C3H6 C1O7 C1C8 C8F9 C8F10 C8F11 Bond angles/˚ C1O2C3 O2C3H4 O2C3H5 O2C3H6 O2C1O7 O7C1C8 O2C1C8 C1C8F9 C1C8F10 C1C8F11 Dihedral angles/˚ C1O2C3H4 C1O2C3H5 C1O2C3H6 C3O2C1O7 C3O2C1C8 O7C1C8F9 O7C1C8F10 O7C1C8F11 Population% a

φ = 0°

Structural restraint φ = 15° φ = 30°

φ = 45°

φ = 60°

r1 = r2 = r3 = r4 = r5 = r6 = r7 = r8 = r9 = r 10 =

p 1 - p 2/ 7 - p 3/6 - p 4×2/5 - p 5/3 + p 6/2 p 1 - p 2/7 - p 3×5/6 p 7 - 0.0024 p 7 + 0.0012 p 7 + 0.0012 p 1 - p 2/ 7 - p 3/6 - p 4×2/5 - p 5/3 - p 6/2 p 1 + p 2×6/7 p 1 - p 2/7 - p 3/6 + p 4×3/5 p 1 - p 2/7 - p 3/6 + p 4×3/5 p 1 - p 2/7 - p 3/6 - p 4×2/5

∆r 1 = ∆r 2 = ∆r 3 = ∆r 4 = ∆r 5 = ∆r 6 = ∆r 7 = ∆r 8 = ∆r 9 = ∆r 10 =

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

-0.0001 0.0005 -0.0001 -0.0001 0.0000 0.0002 -0.0003 -0.0028 0.0022 0.0007

-0.0006 0.0008 -0.0001 -0.0002 0.0000 0.0003 -0.0004 -0.0049 0.0025 0.0026

-0.0010 0.0013 -0.0001 -0.0002 -0.0001 0.0007 -0.0003 -0.0072 0.0015 0.0061

-0.0011 0.0015 -0.0001 -0.0002 -0.0002 0.0008 0.0000 -0.0079 -0.0017 0.0102

θ1= θ2= θ3= θ4= θ5= θ6= θ7= θ8= θ9= θ 10 =

p 11 = p 12 + 4.47 p 8 - p 9×2/3 p 8 + p 9/3 p 8 + p 9/3 360.0 - p 12 - p 13 p 16 p 12 p 13 + p 14/3 p 13 + p 14/3 p 13 - p 14×2/3

∆θ 1 = ∆θ 2 = ∆θ 3 = ∆θ 4 = ∆θ 5 = ∆θ 6 = ∆θ 7 = ∆θ 8 = ∆θ 9 = ∆θ 10 =

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

-0.05 -0.01 0.00 -0.04 0.01 -0.34 0.30 1.12 -1.03 -0.08

-0.24 -0.06 -0.01 -0.03 -0.05 -1.17 1.14 2.08 -1.61 -0.36

-0.34 -0.07 -0.04 -0.05 -0.01 -2.11 2.08 2.78 -1.70 -0.88

-0.37 -0.08 -0.05 -0.05 0.00 -2.48 2.48 3.05 -1.33 -1.46

ϕ1= ϕ2= ϕ3= ϕ4= ϕ5= ϕ6= ϕ7= ϕ8=

180.00 p 10 = -60.32 -p 10 = 60.33 0.00 180.00 p 15 -p 15 0.00

∆ϕ 1 = ∆ϕ 2 = ∆ϕ 3 = ∆ϕ 4 = ∆ϕ 5 = ∆ϕ 6 = ∆ϕ 7 = ∆ϕ 8 =

0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 19.0

-0.57 -0.56 -0.58 0.77 2.88 15.69 15.74 15.00 33.0

-0.65 -0.67 -0.68 0.80 3.94 31.00 31.31 30.00 23.7

-0.37 -0.37 -0.42 0.34 2.62 46.06 46.81 45.00 17.0

0.04 0.06 0.02 -0.03 -0.20 -58.92 62.06 60.00 7.3

See Figure 1 for the atom numbering.

b

Structural differences from the geometry of the anti conformer with φ = 0°.are listed. They are based on the MP2/6-311++G(d,p) calculation (see Table S4).


Page 27 of 39

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a

Table 5: Structural paramete rs for me thyl trifluoroace tate b GED (SAM) GED+MW (SAM) This work (r g, ∠z) Bond lengths/Å C1- O2 O2- C3 C3- H4 C3- H5 C3- H6 C1- O7 C1- C8 C8-F9 C 8 - F 10 C 8 - F 11 Bond angles/˚ C1- O2- C3 O2- C3- H4

1.308 1.435 1.116 1.120 1.120 1.202 1.539 1.348 1.348 1.313

( 11 ) ( 7)

115.0 105.4

( 4)

( 27 ) ( 11 ) ( 8) ( 9) ( 7)

f

c

Ref. 6 (r h1, ∠h1) 1.328 1.444 1.089 1.089 1.089 1.208 1.544 1.341 1.341 1.329 112.6 106.6

( 3) ( 3)

( 6) ( 4)

( 3)

1.330 1.434 1.107 1.111 1.111 1.196 1.545 1.331 1.331 1.324

( 4) ( 8)

116.2 105.2

( 6)

116.2 105.2

( 6)

( 4) ( 2) ( 2) ( 2)

O2- C3- H5

110.0

f

O2- C3- H6

110.0

f

O2- C1- O7

125.7

O7- C1- C8 O2- C1- C8 C1- C8- F9 C 1 - C 8 - F 10 C 1 - C 8 - F 11 Dihedral angles/˚ C1- O2- C3- H4

123.8 110.5 111.1 111.1 111.1 180.0

f

180.0

f

C1- O2- C3- H5

-60.3

f

-58.6

( 9)

C1- O2- C3- H6

60.3

f

58.6

O7- C1- O2- C3

0.0

f

0.0

C8- C1- O2- C3 O7- C1- C8- F9

-180.0 118.4

f

-180.0 121.2

O 7 - C 1 - C 8 - F10

-118.4

-121.2

O 7 - C 1 - C 8 - F11

0.0

( 14 ) e f

( 14 ) ( 4)

( 2) ( 2) e f

-1

V 3 /kJ mol R a b c d e f

0.070

GED (LAM) 2nd (r h1, ∠h1)

1st (r g, ∠α)

( 16 ) ( 7) ( 5) ( 6) ( 4)

f

1.329 1.432 1.103 1.107 1.107 1.196 1.543 1.330 1.330 1.323

( 6) ( 4) ( 15 ) ( 7) ( 5) ( 6) ( 4)

f

d

Anh (r e, ∠e) 1.326 1.421 1.083 1.087 1.087 1.190 1.533 1.320 1.320 1.319 116.3 105.2

113.75 105.38

110.0

109.95

109.96

110.03

110.0

f

109.95

109.96

110.03

127.19

127.30

126.97

123.68 109.13 110.50 110.50 110.57

123.19 109.51 110.73 110.73 110.52

123.75 109.28 110.44 110.44 110.41

( 8)

110.0

110.0

111.2

( 8)

110.0

f

110.0

f

126.5

f

125.2

( 6) e

125.2

( 6) e

125.2

122.1 110.3 111.0 111.0 111.0

( ( ( ( (

123.7 111.1 110.2 110.2 110.2

f

123.7 111.1 110.2 110.2 110.2

f

123.7 111.2 110.1 110.1 110.1

0.0

( 3)

( 3)

( 4) ( 5)

1.3291 1.4393 1.0823 1.0859 1.0859 1.2042 1.5386 1.3358 1.3358 1.3245

116.15 105.16

111.2

( 6)

( 7) ( 4) ( 6)

1.3272 1.4471 1.0864 1.0900 1.0900 1.1976 1.5529 1.3459 1.3459 1.3330

114.22 105.15

f

( 6)

( 14 )

1.3302 1.4420 1.0871 1.0907 1.0907 1.2057 1.5420 1.3402 1.3402 1.3287

f

f

4) 4) 2) 2) 3)

( 6) ( 4)

QC [1] (r e, ∠e) [2] (r e, ∠e) [3] (r e, ∠e)

f

( 5) e f

( 5) ( 3)

-180.0

f

-180.0

f

180.0

f

180.00

180.00

180.00

-60.4

f

-60.4

f

-60.4

f

-60.44

-60.50

-60.32

60.4

f

60.4

f

60.4

f

60.44

60.50

60.32

f

0.0

f

0.0

f

0.0

f

0.00

0.00

0.00

f

f

180.0 121.5

f

( 1)

-180.0 118.5

f

( 3)

-180.0 118.4

180.00 120.15

180.00 120.07

180.00 120.17

( 3) e

-118.4

( 1) e

-118.5

-120.15

-120.07

-120.17

0.00

0.00

0.00

2.15

2.09

2.39

( 9) e

f

0.0 3.8 0.036

f

0.0

( 5)

3.6 0.035

( 1) ( 1) e f

( 6)

See Fig.1 for the atom numbering. The figures in parentheses represent 3 times the standard deviation. Combined analysis of GED data and rotaional constants. SAM:small-amplitude vibrational model. Large-amplitude vibrational model using MP2/6-311++G(d,p) restraints. Structural parameters for the φ = 0˚ conformer are listed. Level of theory: [1] MP2/6-311++G(d,p); [2] B3LYP/6-311++G(d,p) and [3] MP2/cc-pVTZ. Dependent parameter. Estimated error is shown. Parameters fixed at the theoretical values.


-121.5 0.0 2.3 0.032

( 1) ( 1) e f

( 4)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

Table 6: Selected geometrical parameters for methyl trifluoroacetate and its related compounds CF3COOCH3 CF3COOCH2CF3 CH3COOCH3 a

Parameter Bond lengths/Å r (C-F)in-plane r (C-F)out-of-plane r (C-C(C=O)) r (C=O) r (C(=O)-O) r (C-O) Bond angles/˚ ∠C-C-F ∠C-O-C ∠O=C-O

b

GED r e , ∠e 1.319 1.320 1.533 1.190 1.326 1.421

( ( ( ( ( (

110.1 ( 116.3 ( 125.2 (

QC r e , ∠e 4 6 4 7 6 4

1.329 1.340 1.542 1.206 1.330 1.442

1.322 1.333 1.527 1.212 1.336 1.423

) ) ) ) ) )

1.328 1.338 1.542 1.203 1.338 1.429

3) 5) 5)

110.5 114.2 127.2

111.5 ( 5 ) 114.4 ( 7 ) 123.2 ( 10 )

123.7

126.1

h

123.7

∠O-C-CX3 (X = F, H)

111.2 ( 5 ) 109.1 CH3COOCH2CH3 e

( ( ( (

1 1 3 3 5 5

i

f

QC r e , ∠e 2 2 3 3

( ( ( ( ( (

1.496 1.209 1.360 1.442

( ( ( (

7 6 7 7

116.4 ( 9 ) 123.0 ( 9 )

114.3 123.3

123.6

125.6

i

126.1

111.4 ( 9 ) 110.6 CH3COOC(CH3)3 g

b

GED r g , ∠α

QC r e , ∠e

) ) ) )

1.507 1.213 1.354 1.443

1.512 ( 1.203 ( 1.334 ( 1.438 (

) ) ) )

1.507 1.213 1.354 1.452

1.512 ( 1.198 ( 1.334 ( 1.456 (

) ) ) )

1.510 1.213 1.352 1.466

115.7 ( 5 ) 124.0 ( 3 ) 124.1 ( 10 ) 111.9 i

114.8 123.5 126.0 110.6

119.0 ( 11 ) 124.1 ( 8 ) 121.3 ( 23 ) 114.6 i

116.2 124.0 125.6 110.4

122.3 ( 9 ) 126.1 ( 5 ) 120.4 ( 24 ) 113.5 i

120.4 125.2 125.2 109.7

a

This work. Obtained by the final data fit. The figures in parentheses are 3σ. These data were average values for the pseudo-conformer from φ = 0˚ to 60˚.

b

This work. The MP2/6-311++G(d,p) calculation.

c

Ref 30. The figures in parentheses are 1σ.

d


e


f


g


h

Fixed parameter.

i

b

QC r e , ∠e

0 ∠α

110.3 113.7 126.5

QC r e , ∠e 2 5 6 6

rg ,

d

1.506 1.212 1.354 1.436

b

GED r g , ∠α

GED

) ) ) )

110.7 ( 6 ) 109.9 CH3COOCH(CH3)2

b

GED r g , ∠α 1.508 1.203 1.345 1.448

b

QC r e , ∠e

) ) ) ) ) )

∠O=C-C

Parameter Bond lengths/Å r (C-C(C=O)) r (C=O) r (C(=O)-O) r (C-O) Bond angles/˚ ∠C-O-C ∠O=C-O ∠O=C-C ∠O-C-CH3

c

GED r a3,1 , ∠a3,1

Dependent parameter.


2 5 6 8

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Table 7: Mulliken atomic charges the for a

methyl trifluoroace tate and me thyl acetate CF3COOCH3 CH3COOCH3 C1 0.257 0.372 O2 -0.216 -0.196 C3 -0.226 -0.200 H4, C4 0.165 0.143 H5 0.182 0.165 H6 0.182 0.165 O7 -0.300 -0.376 C8 0.483 -0.601 F9, H9 -0.176 0.167 F10, H10 -0.175 0.179 F11, H11 -0.175 0.179 a

Calculated at the MP2/6-311++G(d,p) level of theory. See Fig.1 for atom numberings.



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FIGURES AND FIGURE CAPTIONS

Figure 1. Molecular structure, including numbering scheme, of the anti (upper) and the syn (lower) conformers of methyl trifluoroacetate, CF3COOCH3. The dihedral angles α, φ and τ mean C3O2C1C8, C1C8F11 and C1O2C3H4, respectively.


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Figure 2.

Experimental (dots) and theoretical (solid line) molecular scattering intensities for

methyl trifluoroacetate.

The theoretical curve was calculated from the best fitting parameters.



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Figure 3.

Potential energy curves for the internal rotation of the CF3 or CH3 group of methyl

trifluoroacetate as a function of dihedral angles φ(O7C1C8F11) or τ (C1O2C3H4) at different levels of theory.


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Figure 4.

Experimental Radial distribution curve and difference curve for methyl trifluoroacetate.

Distance distributions are shown by vertical bars.



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The TOC Graphic

Original graphics in a one- or two-column format are available from Fig.1.pdf, Fig.2.pdf, Fig.3.pdf, Fig.4.pdf and TOCgraphic.pdf files.


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a 6

C3

O7

b

5



C1

O2

4





F11 c

C8 F10

F9



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Gas Elecrton Diffraction

Microwave Spectroscopy Bz

sM(s)

B0

GED + MW Drij lij , Kij

Dynamic model for CF3 rotation

Bz - B0

Quantum Chemical Calculation Optimized Geometry Force constant


Molecular Structure and Internal Rotation of CF3 Group

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