ARTICLE pubs.acs.org/JPCA
Chirped Pulse Fourier Transform Microwave Study of 2,2,2-Trifluoroethyl Formate Luca Evangelisti,† Adam Grabowiecki, and Jennifer van Wijngaarden* Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
bS Supporting Information ABSTRACT: The rotational spectra of 2,2,2-trifluoroethyl formate and its three 13C isotopologues have been measured with a molecular-beam-based, chirped-pulsed Fourier transform microwave spectrometer in combination with a conventional Balle Flygare-type instrument up to 18 GHz. Although ab initio calculations predict the presence of two low-energy conformers (analogous to the trans and gauche forms of ethyl formate), the trans isomer was the only stable conformer observed. The rs geometry of the molecular main carbon frame was precisely derived based on a coplanar heavy-atom backbone of this conformer. ESPs of the two lowest energy conformers were calculated to obtain information about the role of through-space effects on their structures and relative stability.
’ INTRODUCTION For years, microwave spectroscopy has been employed to study problems of conformational and tautomeric equilibrium.1 In general, the substitution of the methyl groups by heavier rotating tops (for example, CF3 instead of CH3) has a drastic effect on the pattern of the rotational spectrum allowing the influence of the inertial mass and potential energy to be evaluated. This is the case, for example, for acetylacetone hexafluoroacetylacetone2,3 or acetone 1,1,1-trifluoroacetone,4,5 where internal rotation and keto enol tautomerism questions have been clearly resolved. As a logical extension of these studies to understand further the influence of the fluorine atom substitution, we have decided to investigate 2,2,2-trifluoroethyl formate for comparison with ethyl formate, which we will label TFEF and EF, respectively. EF is a good candidate for interstellar detection and so has been previously studied in both the microwave and millimeter/ submillimeter-wave ranges of the electromagnetic spectrum.6 9 Whereas several conformers are possible, only two have been observed in experimental rotational studies. The global minimum of the potential energy surface corresponds to the trans conformer with a planar heavy-atom backbone. The other lowenergy minimum corresponds to the gauche conformer that is produced mainly by an out-of-plane rotation of the ethyl group around the O C bond. Despite the fact that there are two equivalent gauche forms corresponding to rotation in two different directions, no splitting has been observed in high-resolution spectroscopic studies due to the high barrier for interconversion between these forms. The most recent geometries reported for EF are based on a joint analysis of electron diffraction, rotational, and vibrational spectroscopic studies.10 Previous work has also r 2011 American Chemical Society
shown that EF consists of 61% trans and 39% gauche conformers at room temperature. Although TFEF is stable at ambient temperatures, a survey of the literature has revealed that this compound has not been extensively used in chemical studies, even though it is a versatile and selective reagent for the formylation of alcohols, amines, and N-hydroxylamines. Moreover in contrast with other formylating reagents, TFEF is simple to prepare from formic acid and 2,2, 2-trifluoroethanol using a straightforward literature procedure.11 Despite its potential uses in the laboratory, no spectroscopic or photochemical investigations are available for TFEF to our knowledge. We report here the results of our recent rotational spectroscopic investigation of 2,2,2-trifluoroethyl formate. The observation of transitions arising from the 13C isotopolgoues has allowed for heavy atom substitution analysis to be carried out to derive key geometric parameters related to the organic mainframe of the lowest energy trans conformer of TFEF. Moreover, electrostatic potentials (ESPs) have been used to develop a conceptual model and provide a simple means of understanding the role of intermolecular interactions on the geometries and stabilities of the lowest energy conformers of EF and TFEF.12
’ EXPERIMENTAL METHODS The rotational spectrum of TFEF was first recorded using the recently built chirped pulse FTMW spectrometer at the University of Manitoba and described elsewhere.13 In brief, the Received: May 20, 2011 Revised: June 13, 2011 Published: July 06, 2011 8488
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Table 1. MP2/6-311++G(d,p) Spectroscopic Parameters and Relative Energies of TFEF
a
Absolute energy =
564.994802Eh. b Absolute energy =
564.926311Eh (ZPE-corrected). c Atom numbering shown in Figure 4.
Figure 1. Molecular electrostatic potential surface plotted on the van der Waals surface of the EF trans (left) and gauche (right) conformers. Select values (kilojoules per mole) are shown.
spectrometer has three main components: (a) the chirped-pulse microwave source, (b) the molecular beam spectrometer, and (c) the broadband receiver (6 GHz bandwidth). This instrument operates from 8 to 18 GHz and follows the previous design of Pate and coworkers14 with later modifications by Cooke and coworkers.15 A commercial sample of TFEF (purchased from Sigma Aldrich) was used without further purification. Vapor from the TFEF liquid at room temperature was combined with the carrier gas (Ne) to make a 0.5% gas mixture. The spectra of all 13C species were measured in natural abundance. Typical linewidths in the cp-FTMW spectrum were ∼150 kHz when 40 μs FIDs were recorded. The transition frequencies were verified (and more transitions were sought) using a Balle Flygare-type Fourier transform microwave (FTMW) spectrometer that has already been described.16 Because the supersonic jet is arranged coaxially to the cavity axis, each rotational transition is split by the Doppler effect. The estimated accuracy of frequency measurements is better than 2 kHz, and lines separated by >7 kHz are typically resolvable.
’ THEORETICAL CALCULATIONS Before searching for the rotational spectra, the properties of different conformations of TFEF were predicted in silico. The computations provided their geometries, the rotational constants, the dipole moment components along the principal axes, and the relative energies. All ab initio quantum chemical calculations were performed at MP2/6-311++G(d,p) level using the Gaussian09 software package.17 The nature of all stationary points was verified by subsequent harmonic frequency calculation.
Figure 2. Molecular electrostatic potential surface plotted in the van der Waals surface of TFEF conformers I and II. Select values (kilojoules per mole) are shown.
The results for conformers I, II, and III are shown in Table 1. There is a small difference in the relative energies of conformers I and II, which may allow the observation of both species, as seen experimentally for EF (trans and gauche conformers, respectively). Conformer III lies considerably higher in energy. To understand better the underlying interactions that govern these low-energy conformers, a molecular ESP analysis was performed using the cubegen utility in Gaussian09. In the gas phase, inter- and intramolecular interactions may be separated into four components: repulsion between the electron densities at close distances of approach, induction effects between the components, dispersion interactions, and electrostatic interactions. In general, the electrostatic component plays a dominant role overall.18 In general, molecular ESP maps can be used to (i) identify electron-rich and electron-deficient sites of various compounds, (ii) analyze the influence of conformation on these sites, and (iii) shed light on the inter- and intramolecular hydrogen bonding interactions occurring in the system.19 In displaying the ESP surfaces, we have adopted the convention that the maximum (positive) potential is blue and the minimum (negative) potential is red.20 The ESP on the van der Waals surface of EF is shown in Figure 1. In the trans conformer, the maximum is located near the hydrogen atom of the methyl group, and the minimum is over a lone pair of electrons on the carbonyl oxygen atom. The gauche conformer appears to be partially stabilized by the electrostatic interaction between these two terminal groups but remains 72 cm 1 higher in energy than the trans form at this level of theory. By comparison, Figure 2 reports the ESPs of the two most stable conformers of TFEF. In conformer II (similar to the gauche form of EF), the minimum (91 kJ/mol) is located on the 8489
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Figure 3. Energetic profile obtained at MP2/6-311++G(d,p) level for the rotation around the C2O3 C4C5 dihedral angle of TFEF to interchange conformers I and II. The structure and selected stationary points are represented.
carbonyl oxygen atom on the same side as the CF3 terminal group. This value is lower than the minimum value (84 kJ/mol) on the ESP of conformer I (similar to the trans form of EF), which is also located near this oxygen atom. The increase in the nucleophilic character of conformer II of TFEF relative to conformer I can be attributed to the repulsion between the lone pairs of electrons on the proximal oxygen and fluorine atoms in the former. This repulsion in TFEF prevents a favorable electrostatic interaction between the terminal groups as in the gauche conformer of EF. This is supported by the zero-point-corrected ab initio calculations that show that conformer II is 125 cm 1 higher in energy than conformer I in TFEF versus 72 cm 1 for the comparable forms of EF. The increased stability of the gauche form in EF relative to TFEF is further evidenced by the calculated O 3 3 3 H C distance in EF (2.709 Å), which is 0.556 Å shorter than the O 3 3 3 F C distance in TFEF and by comparison the increased CO CC dihedral angle for EF (98.7°) compared with that of TFEF (77.7°). These results suggest that the hydrogen bond donor/ acceptor ability of the O 3 3 3 CH groups of EF appears to be appreciably greater than that of the O 3 3 3 CF group in TFEF. This is a consequence of the greater electron-withdrawing effect of the three fluorine atoms in TFEF compared with the hydrogen atoms in EF. The potential energy pathway corresponding to internal rotation about the O3 C4 bond (corresponding to the C2O3 C4C5 dihedral angle) that interconverts conformers I and II was calculated via ab initio methods and is plotted in Figure 3. The ab initio scan was carried out in steps of 2° over the full range of the C2O3 C4C5 dihedral angle of the organic mainframe. Whereas the dihedral angle was kept fixed at every step, all other geometric parameters were reoptimized for each point along the path. The low barrier to interconversion (54 cm 1) suggests that in the absence of additional favorable intramolecular interactions, conformational relaxation to the most stable minimum (conformer I in this case) will be facile during the supersonic jet expansion in our experiments.
’ ROTATIONAL SPECTRA The expected spectra of the TFEF conformers were calculated based on the rotational constants in Table 1. The most obvious transitions in the cp-FTMW spectrum were soon assigned to conformer I. The most intense transitions are from the μa-R-type spectrum with J ranging from 4 to 6, and the weaker μb-R-type
Figure 4. Sketch of TFEF in its principal axis system with the atom numbering.
transitions were subsequently identified. The relative intensities are in agreement with the calculated value of the dipole moment components (μa/μb = 3.48). The assigned transitions are reported in the Supporting Information, and see Figure 4 for atom numbering. After assignment of the main conformer, a number of weaker lines remained unassigned in the broadband spectrum. These transitions were soon attributed to the less abundant 13C containing species. In the end, the spectra of all three 13C isotopologues were assigned, and all transition frequencies are also reported in the Supporting Information. After the initial broadband survey spectra were collected using the cp-FTMW instrument, all transition frequencies were carefully measured, and new transitions of TFEF were sought using the higher resolution Balle Flygare FTMW instrument. The S-reduction of Watson’s quartic Hamiltonian in the Ir representation21 was used to fit the experimental frequencies of TFEF listed in the Supporting Information using SPFIT Pickett’s program.22 The resulting spectroscopic constants of all four isotopologues are reported in Table 2.
’ CONFORMATION AND STRUCTURE By comparing the experimental rotational constants with those of the conformational predictions given in Table 1, one can see that they are in close agreement with the calculated values of conformer I. As expected, μc-type transitions were not observed for TFEF because the calculated dipole moment along this axis was predicted to be zero for conformer I. This further supports an organic backbone that is planar such that the symmetry plane contains the a and b axes of the principal axis system. This is analogous to the observed spectra attributed to the trans conformer of EF. 8490
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Table 2. Experimental Spectroscopic Constants of the Four Observed Isotopologues of TFEF (S-Reduction, Ir Representation) species
A (MHz)a
B (MHz)
C (MHz)
Pcc (uÅ2)
Nb
Normal
4536.6624(4)c
1123.4181(1)
1077.7016(1)
46.158
97
13
4529.938(2)
1108.6098(2)
1063.6977(2)
46.158
27
13
4525.266(2)
1123.2091(2)
1076.8821(2)
46.161
26
13
4536.886(2)
1120.4426(2)
1074.9355(3)
46.153
27
C2 C4 C5
a
Five quartic centrifugal distortion constants were determined in the fit of the normal isotopologue: DJ = 89.0(9) Hz, DJK = 2.860(3) kHz, DK = 1.67(4) kHz, d1 = 4.4(6) Hz, and d2 = 7.3(3) Hz. These values for DJ and DJK were fixed in the subsequent analysis of the 13C species. The standard deviation of the global fit is 1.7 kHz. b Number of transitions of each isotopologue in the fit. c Errors in parentheses are expressed in the units of the last digit.
Table 3. Structural Parameters of TFEF: the Experimental Substitution Coordinatesa rs
re conformers I
13
C2
13
C4
13
C5
IIb
IIIb 2.098
as/Å
(2.456(1)
2.460
2.250
bs/Å
(0.411(4)
0.414
0.278
0.115
as/Å
(0.290(5)
0.331
0.171
0.034
bs/Å as/Å
(0.531(2) (1.095(1)
0.520 1.110
0.308 0.997
0.866 1.098
bs/Å
(0.07i (2)c
0.066
0.053
0.026
Calculated by fixing to zero the |c| value of atoms. b |c| values of atoms are not reported. c Imaginary value. Set to zero when deriving subsequent geometric parameters. a
Table 4. Structure of Conformer I (trans) of TFEF Based on the MP2/6-311++G(d,p) Geometry bond distance (Å)
angles (deg)
’ CONCLUSIONS We report here information on the structure and relative energies of the low-energy conformers of TFEF. Only the most stable form, conformer I (trans) from Table 1, was experimentally observed in our molecular-beam-based spectrometers. On the basis of the observed rotational spectrum and ab initio theoretical calculations, we have shown that (i) the energetic minima of the molecule corresponds to a planar mainframe and (ii) the unfavorable electrostatic repulsion between the oxygen and fluorine atoms in TFEF destabilizes the gauche conformer more than in the analogous conformer of EF. As a result, the failure to observe conformer II (despite the estimation that the population of conformer II should be ca. 50% of that of conformer I) may be due to a conformational relaxation process to conformer I during the supersonic expansion. It is established, for example, that different conformers separated by low interconversion barriers can relax into the global minimum if the barriers to relaxation are smaller than 2kT.25 In our case, such a relaxation is likely because the barrier (Figure 3) is estimated to be only 54 cm 1. ’ ASSOCIATED CONTENT
bS
Supporting Information. Assigned transition frequencies for 2,2,2-trifluoroethylformate and its three 13C isotopologues. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
C2O1
1.204
O3C2O1
125.0
O3C2
1.351
C403C2
113.6
C403 C4C5a
1.425 1.510
C5C4O3 C2C4C5a
106.7 138.9
F6C5
1.345
F6C5C4
108.9
F7C5
1.339
F7C5C4
111.9
F8C5
1.338
F8C5C4
111.9
H9C4
1.091
H9C4C5
109.0
H10C4
1.091
H10C4C5
109.0
H11C2
1.095
H11C2O3
108.8
F7(F8)C5C4O3 H9(H10)C4C5F6
C2C4C5 = 135.4(6)o of the carbon mainframe of TFEF. These experimentally derived parameters match the calculated equilibrium values for conformer I, as shown in Table 4, where the C4 C5 distance and C2C4C5 angle are 1.510 Å and 138.9°, respectively. The maximum discrepancy between the experimental and ab initio values of the rotational constants in TFEF is ∼0.7% (for the A constant) and
