Some Geometric and Electronic Structural Effects of Perfluorinating

Jul 12, 2010 - ... UniVersity of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017 ... The 35Cl, 37Cl, and each of the monoisotopic 13...
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J. Phys. Chem. A 2010, 114, 8009–8015

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Some Geometric and Electronic Structural Effects of Perfluorinating Propionyl Chloride G. S. Grubbs II, R. A. Powoski, D. Jojola, and S. A. Cooke* Department of Chemistry, UniVersity of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017 ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: June 16, 2010

Propionyl chloride and perfluoropropionyl chloride have been characterized using chirped pulse Fourier transform microwave spectroscopy between the frequency range of 8 and 14 GHz. Molecules were studied in separate experiments in supersonic expansions of argon. The 35Cl, 37Cl, and each of the monoisotopic 13C substituted isotopologues of propionyl chloride were observed. The 35Cl and 37Cl isotopologues of perfluoropropionyl chloride were observed. Analyses of the resulting microwave spectra have yielded spectroscopic constants for the target molecules. The analyses indicate that, under the conditions of these experiments, both molecules are only detectable as cis conformers in which ∠CCCO ) 0°. Comparisons are made between the electronic and geometric structure of propionyl chloride and perfluoropropionyl chloride, and also other acyl chlorides. The data produced are relevant in regards to quantifying the known destabilizing effect of perfluoroalky chains on carbonyl groups. Distinct differences in electronegativity between the CH3CH2CO- and CF3CF2CO- groups are discussed. Methyl group internal rotation is observed for propionyl chloride and has been analyzed to produce a V3 barrier height. Introduction Perfluorination is the act of completely replacing all hydrogen atoms in a molecular system by fluorine atoms. Curiosity surrounds the numerous effects1 of perfluorination on (i) hydrocarbons2-4 and (ii) functionalized hydrocarbons.5-8 The structural effects of perfluorination include (i) differing lowest energy conformations and (ii) reduced conformational flexibility of perfluorinated compounds compared to the hydrogenated analogues. This is pertinent in regards to rationalizing the socalled “perfluoroalkyl effect” which is believed to be steric in origin and concerns the kinetic and thermodynamic stabilizing effect that fluorination imparts to a carbon skeleton.9 Also of structural interest is the required carbon chain length for the onset of helicity, a structural feature apparent in the polymer polytetrafluoroethene (Teflon),10 yet absent in hydrogenated carbon chains. The electron-withdrawing effects of fluorine and, by extension, perfluoroalkyl groups gives rise to the known perfluorination-induced, destabilizing effect on the carbonyl group.11,12 The electron withdrawal by fluorines close to the carbonyl group has the effect of opposing the pull of electrons by the carbonyl oxygen which normally causes substantial positive charge on the carbonyl carbon. As an example of this effect, note that many perfluoroenols resist ketonization in the presence of acids whereas nonfluorinated enols will readily undergo ketonization.13 The methods for studying this destabilization effect typically involve measuring the equilibrium constant between the ketone and the enol using 19F NMR. No experimental methods for quantifying the destabilizing effect have been described. We suggest that a useful metric for quantifying this perfluorodestabilizing effect includes measuring the difference in the electric field gradient at the chlorine center for an acyl chloride compared to a perfluoroacyl chloride. This goal has, in part, been the motivating force for this work. Insights into the structure of propionyl chloride, C2H5COCl, have been obtained from two prior low-resolution microwave * To whom correspondence should be addressed. E-mail: [email protected].

spectroscopic investigations, one by Karlsson,14 and a second by Mata and Alonso.15 In both cases the cis conformer, ∠CCCO ) 0°, was determined to be the lowest energy conformer and, indeed, the only one observed. The rotational spectra were recorded in the vibrational ground state and two vibrationally excited states. The ionic character of the C-Cl bond was found to be high (36%), with a π-bonding character, πc, of 0.104, and a C-Cl bond length of 1.789 Å. The terminal methyl group was determined to possess a V3 barrier to internal rotation of 867(14) cm-1. However, the barrier was determined to be too high to observe A-E internal rotation splittings in the ground vibrational state at the resolution available. Perfluoropropionyl chloride has not previously been structurally investigated. This work details the first structural characterization of perfluoroproprionyl chloride, together with a new, higher resolution study of propionyl chloride. We contrast the results. Experimental Methods All experiments were made on a chirped pulse Fourier transform microwave spectrometer. This instrument, inspired by the work of Brown et al.,16 has been described previously.17 However, several modifications have been made which are detailed here and shown in Figure 1. The signal generation method is unchanged17 in which linear frequency sweeps, spanning 2 GHz regions between 8 and 18 GHz in 5 µs, are created by mixing the output of an arbritrary waveform generator (3) with the fixed frequency output of a microwave synthesizer (4). This pulsed signal is amplified (6) and broadcast onto the supersonically expanding molecular sample issued from a solenoid valve (8). The receiving circuit has been adjusted to allow direct digitization. Resultant molecular free induction decays are collected and passed directly to a protected (9) low noise amplifier (10). The experiment can be repeated at 2-4 Hz and the molecular signals averaged and fast Fourier transformed on a digital oscilloscope (12). Our oscilloscope has a hardware limited 10000 permissible averaging cycles. To overcome this limitation a National Instruments LabView code

10.1021/jp103966e  2010 American Chemical Society Published on Web 07/12/2010

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Figure 1. The Univeristy of North Texas chirped pulse, Fourier transform microwave spectrometer: 1, Rb atomic clock, SRS FS725, and distribution amplifier, Wenzel Associates; 2, 3.96 GHz PLDRO Nexyn Corp. NXPLOS-0396-2381; 3, Tektronix AWG 710B; 4, MW synthesizer HP 8341A; 5, low pass filter, Minicircuits; 6, mixer, Miteq DB0418LW1; 7, power amplifier, Microwave Power, L0818-37; 8, horn antenna, Amplifier Research AT 4004; 9, series 9 solenoid valve, Parker-Hannefin; 10, SPST pin diode switch ATM S1517D; 11, LNA Miteq AMF-6F-06001800-15-10P; 12, dc block; 13, O-scope Tektronix TDS 6124C; 14, Tektronix AWG 2041; 15, BNC 505-8C; 16, Iota One valve driver, Parker Hannefin; 17, PLCRO 0.64 GHz Nexyn Corp. NXPLOS-0064-2381. Dotted lines show TTL pulse timing communication lines.

TABLE 1: M06-2X/6-311G** Calculated Cartesian Coordinates in Units of Angstroms in the Principal Inertial Axis System for Propionyl Chloride and Perfluoropropionyl Chloride CH3CH2COCl

CF3CF2COCl

atom

a

b

c

a

b

c

C C C O Cl H/F H/F H/F H/F H/F

-2.486 -1.054 -0.057 -0.269 1.652 -0.836 -0.836 -2.677 -2.677 -3.186

-0.300 -0.816 0.310 1.466 -0.296 -1.437 -1.438 0.315 0.313 -1.135

0.000 0.000 0.000 0.000 0.000 -0.874 0.872 -0.880 0.882 0.000

-1.450 -0.033 1.069 0.868 2.676 0.102 0.102 -1.631 -1.631 -2.337

0.141 -0.457 0.624 1.781 -0.091 -1.224 -1.223 0.882 0.883 -0.845

0.000 0.000 0.000 0.000 0.000 -1.090 1.091 -1.084 1.083 0.000

has been written to control the instrument through GPIB interfacing. In this way the instrument can collect an unlimited amount of 10000 averaged scans and save them to an external hard drive. The software can also automatically step up the frequency, i.e., run multiple 10000 acquisitions at 9-11 GHz, then step up and collect multiple 10000 acquisitions at 11-13 GHz and so on. In this way, the instrument may run uninterrupted for several days. The data may be removed from the hard drive of the oscilloscope and be processed (deep averaged) and “pieced together” in preparation for analysis. When 20 µs sections, at 25 ps/point, of the free induction decay are fast Fourier transformed, line widths with this instrument are approximately 80 kHz full width at half-maximum height (fwhm). Propionyl chloride (Sigma-Aldrich) is a volatile liquid, bp ) 77-79 °C. The liquid was sampled by pulsing argon gas through a 1/4 in. tube filled with approximately 5 mL of liquid about 40 cm up stream of the solenoid valve. Perfluoropropionyl chloride (Synquest Laboratories) is a gas, bp ) 7-9 °C. Sampling occurred by dissolving the gas in argon at about 3% by volume in a gas reservoir held at approximately 4-5 atm. Quantum Chemical Calculations Quantum chemical calculations were performed using the GAMESS software package,18 Version 12 January 2009 (R1).

TABLE 2: M06-2X/6-311G** Calculated Equilibrium Rotational Constants and Dipole Moments for Propionyl Chloride and Perfluoropropionyl Chloride parameter

CH3CH2COCl

CF3CF2COCl

A/MHz B/MHz C/MHz µa/D µb/D µc/D

8956.98 2368.72 1917.39 -2.29 -2.10 0.00

2035.79 847.30 759.74 0.29 -0.54 0.00

The objective of these calculations was to determine useful structures and dipole moments for propionyl chloride and perfluoropropionyl chloride. Information of this type is helpful in regards to experimental spectral assignments. The calculations employed a density functional method, M06-2X19,20 with a 6-311G** basis set.21,22 These basis sets were obtained from the EMSL Basis Set Library23,24 and were used without further modification. The resulting calculated Cartesian coordinates for propionyl chloride and perfluoropropionyl chloride are presented in Table 1. Also, calculated rotational constants and dipole moments are given in Table 2. The calculated structures for both molecules are shown in Figure 2. Further calculations were performed in connection with the methyl- and trifluoromethyl-group internal rotations in the target

Perfluorinating Propionyl Chloride

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Figure 3. A portion of spectra collected for propionyl chloride. The lower figure shows a broad scan (30000 averaging scans). The upper figure shows a blow up of a small region. Several transitions have been labeled, all transitions shown may be identified by consulting the Supporting Information. Figure 2. The M06-2X/6-311G** calculated structures of propionyl chloride and perfluoropropionyl chloride.

molecules. A relaxed potential energy scan was used with a B3LYP density functional25-28 and small 6-31+G basis set. Experimental Results and Analyses The spectra for both molecules were treated in a similar way. Both molecules are asymmetric tops, κ ≈ -0.87, and although the principal components of the electric dipole moment have different magnitudes, they are, in both cases, aligned only along the a and b axes. For both species harmonic sequences of aR0, 1type transitions, for the parent isotopologue (35Cl), were identified first. A portion of the observed propionyl chloride spectrum is shown in Figure 3. These initial assignments produced molecular parameters of sufficient quality to then allow successful assignment of the aQ-, b P-, bQ-, and bR-type transitions. The observed frequencies, together with their quantum number assignments are presented in the Supporting Information. Assignments were performed using the AABS package,29 which is freely available from the PROSPE Web site.30 The AABS package uses the spectral fitting and prediction programs of Pickett, namely, SPFIT and SPCAT.31,32 The Hamiltonian, H, was constructed in the coupled basis I + J ) F and had the form

H ) HR + HQ

(1)

The operator HR was that appropriate to a semirigid rotor, for which the Watson A reduction in the Ir representation was used.33 HQ is the usual operator describing the chlorine nuclear quadrupole coupling of the nuclear spin and framework angular momentum vectors I and J, respectively. The forms of HR and HQ are well-known.34 Following successful assignment of the parent isotopologues the 37Cl isotopologues for both molecules and all three 13C isotopologues for propionyl chloride (but not perfluoropropionyl chloride) were quickly identified. The signalto-noise ratio was not quite sufficient to observe transitions for

the 13C isotopologues for perfluoropropionyl chloride in a timely manner. This is unsurprising given the considerably smaller dipole moment components for perfluoropropionyl chloride compared to propionyl chloride; see Table 2. The constants determined in this way for propionyl chloride and perfluoropropionyl chloride are presented in Table 3 and Table 4, respectively. We note that structureless lines recorded with this instrument are typically about 80 kHz wide (fwhm). However, it was noted that several b-type transitions were broadened possessing line widths of ≈100 kHz (fwhm) or greater. Notably, four rotational transitions were clearly observed as distinct doublets. These transitions were the Jk-1k+1 ) 432 r 523, 532 r 625, 743 r 836, and the 927 r 836. These four lines provide only a limited data set; however, an analysis was undertaken using the extended internal axis method with the XIAM software.35 The recorded split transition frequencies and their tentative quantum number assignments are given in Table 5. Hyperfine structure from the Jk-1k+1 ) 432 r 523 transition is shown in Figure 4. Using centrifugal distortion and hyperfine constants held fixed at those values given in Table 3, and holding δi,a, the angle between the methyl rotor and the a-axis fixed at a calculated value of 19.85°, we have determined the V3 (barrier height) and methyl axis rotational constant, F, in propionyl chloride. The values of the determined parameters are given in Table 6. The value of F seems very reasonable at 160(7) GHz. The error on the V3 value is likely considerably larger than stated given that other rotational constants were held fixed. Notably the constants obtained predict the internal rotation splitting on the observed a-type and many of the b-type transitions to be below the resolution of our instrument in its current configuration, that is below ≈80 kHz. Discussion Molecular Geometric Structures. By use of the experimental rotational constants obtained in this work, a partial Kraitchman substitution structure36 for both molecules has been undertaken. The results of this investigation are given in Table 7. Also, experimental second moments along with inertial

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TABLE 3: Ground State Spectroscopic Parameters for Propionyl Chloride parameter A0/MHz B0/MHz C0/MHz ∆J/kHz ∆JK/kHz ∆K/kHz δJ/kHz δk/kHz χaa/MHz χbb/MHz χcc/MHz | χab |/MHz Nc σrms d σ/kHz e

CH3CH2CO35Cl a

8854.7845(14) 2377.94361(37) 1918.68343(24) 0.3016(98) 1.335(66) 13.83(25) 0.0591(16) 0.788(67) -49.0673(37) 26.4986(55) 22.5687(40) 29.94(57) 125 0.227 5.67

CH3CH2CO37Cl

CH3CH213CO35Cl

CH313CH2CO35Cl

8828.0790(21) 2320.2750(12) 1879.7511(13) 0.237(18) 1.09(12) 12.06(14) 0.0488(42) 0.28(55) -38.9625(51) 21.1725(80) 17.7900(61) 23.75(40) 92 0.270 6.74

8840.9115(74) 2378.1111(29) 1918.1392(20) 0.23(12) 1.335b 14.7(27) 0.0591b 0.788b -49.133(29) 26.448(47) 22.685(37) 33.2(28) 22 0.467 11.67

8755.7361(49) 2365.7836(17) 1906.1228(13) 0.320(73) 1.335b 13.8(20) 0.0591b 0.788b -48.747(35) 26.165(39) 22.582(34) 29.6(25) 24 0.345 8.62

13

CH3CH2CO35Cl

8841.1905(51) 2311.1544(16) 1874.3562(13) 0.282(73) 1.25(18) 13.7(18) 0.0591b 0.788b -48.814(14) 26.215(24) 22.599(19) 30.8(18) 31 0.319 7.98

a Numbers in parentheses give standard errors (1σ, 67% confidence level) in units of the least significant figure. b Fixed at the parental isotopologue value. c Number of observed transitions used in the fit. d The root-mean-square deviation of the fit is unitless and equal to ([((obs - calc)/error)2]/N lines)1/2. e This quantity is equal to ([(obs - calc)2]/N lines)1/2.

TABLE 4: Ground State Spectroscopic Parameters for Perfluoropropionyl Chloride parameter A0/MHz B0/MHz C0/MHz ∆J/Hz ∆JK/Hz ∆K/Hz δJ/Hz δk/Hz χaa/MHz χbb/MHz χcc/MHz |χab|/MHz Nb σrmsc σ/kHzd

35

37

Cl

2025.71840(19) 842.99816(13) 755.97374(14) 28.9(11) 290.7(30) -227.2(57) 4.07(23) -340(23) -49.7148(61) 25.4688(90) 24.2460(66) 37.77(21) 402 0.296 7.40

a

Cl

2025.58453(37) 823.58204(35) 740.30668(35) 26.6(21) 292(10) -224(14) 2.7(10) -381(98) -39.256(11) 20.147(16) 19.109(12) 29.84(13) 232 0.348 8.71

TABLE 5: Observed Transitions Frequencies for Those Propionyl Chloride Transitions Observed as Doublets Due to Methyl Group Internal Rotation J′K-1K+1 r J′′K-1K+1

2F′-2F′′

symmetry

frequency/ MHz

(obsd - calcd)/ kHza

532-625

7-9 13-15 11-13 7-9 13-15 11-15 19-17 17-15 21-19 15-13 19-17 17-15 21-19 15-13 5-7 11-13 7-9 9-11 5-7 11-13 7-9 9-11 11-13 17-19 13-15 15-17 11-13 17-19 13-15 15-17

A A A E E E A A A A E E E E A A A A E E E E A A A A E E E E

8077.5732 8079.2326 8086.6362 8077.5011 8079.1608 8086.5598 9885.0316 9885.4381 9888.5241 9888.9286 9885.1600 9885.5606 9888.6554 9889.0544 11367.4579 11371.1130 11379.3949 11383.0529 11367.2202 11370.8833 11379.1590 11382.8281 12248.4677 12249.4821 12254.4064 12255.4202 12248.5879 12249.5948 12254.5091 12255.5243

-5.3 -35.6 -33.1 15.7 -14.4 -16.4 -5.4 2.6 3.0 -0.9 3.9 5.9 2.6 -6.8 -10.3 -14.9 -14.9 -17.1 -1.7 1.7 -4.4 4.5 29.8 26.1 28.0 23.7 13.4 2.1 -6.1 -9.0

927-836

432-523

a Numbers in parentheses give standard errors (1σ, 67% confidence level) in units of the least significant figure. b Number of observed transitions used in the fit. c Root mean square deviation of the fit, ([((obs - calc)/error)2]/N lines)1/2. d This quantity is equal to ([(obs - calc)2]/N lines)1/2.

defects and Ray’s asymmetry parameters34 have been evaluated for each isotopologue of each molecule. The results are displayed in Table 8. The zero magnitudes of the c principal axes Cartesian coordinates for the CCC(dO)Cl backbone in the calculated structures of propionyl chloride and perfluoropropionyl chloride, Table 1, present evidence that both compounds possess a cis, ∠CCCO ) 0°, configuration for the lowest energy conformation. The good agreement between the calculated rotational constants, Table 2, and observed rotational constants, Table 6, present compelling evidence that it is the cis conformer that is observed in this work. To further the argument we note that the calculated rotational constants for the trans conformer of propionyl chloride are A ) 5036.76 MHz, B ) 3745.70 MHz, and C ) 2209.14 MHz, in very poor agreement with the observed rotational constants. If further evidence were required, the substitution structural properties of propionyl chloride and perfluoropropionyl chloride, Table 7, and the relative invariance of the second moment, Pcc, to substitution of Cl (perfluoropropionyl chloride) and Cl, C(1), C(2), and C(3) (propionyl chloride), Table 8, confirm the cis conformation for both molecules. The C-Cl bond length in both propionyl chloride and perfluoropropionyl chloride is found to be shorter than that

743-836

a The observed transition frequencies minus those transition frequencies calculated using those parameters in Table 6.

previously assumed for propionyl chloride, where r(C-Cl) was assessed to be 1.789 Å.14 We also note that the distribution of nuclear masses in perfluoropropionyl chloride differs from that in propionyl chloride in such a way as to slightly rotate clockwise the common backbone CCC(dO)Cl in the ab plane in the perfluoropropionyl chloride compared to propionyl chloride. From the calculated structures for perfluoropropionyl chloride, the angle between the C-Cl bond and the a axis is 24° and the angle of the CdO bond with the b axis is 30°. The analogous angles in propionyl chloride are 19.5° and 31°, respectively. It is this subtle difference that contributes to µb being larger in magnitude than µa for perfluoroprionyl chloride, whereas the reverse is true for propionyl chloride; see Table 2.

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Figure 4. The 432 r 523 transition. The upper figure shows the observed spectrum the lower figure shows the spectrum produced using the constants in Table 3. The transition is clearly doubled.

TABLE 6: Internal Rotation Parameters Related to the Internal Methyl Rotor in Propionyl Chloridea parameter

value

A/MHz B/MHz C/MHz F/GHz V3/cm-1 δi,a/deg Nc σ/kHzd σrmse

8854.7062(30) 2377.9342(38) 1918.6425(43) 160(7) 853(35) 19.85b 30 15.7 0.628

a The centrifugal distortion parameters and Cl nuclear quadrupole coupling tensor values were held fixed to those values given in Table 3. b This parameter, the angle between the symmetry axis of the methyl rotor and the a axis, was held fixed at this value, obtained from the calculated structure. c Number of observed transitions in the fit. d This quantity is equal to ([(obsd - calcd)2]/N lines)1/2. e Root mean square deviation of the fit, ([((obsd - calcd)/ error)2]/N lines)1/2.

This work is consistent with previous studies2,8 that have demonstrated that perfluoro-induced helicity of alkyl chains requires a chain length of at least four or five carbons. Chlorine Nuclear Quadrupole Coupling Tensors and the Destabilizing Effect of Perfluorination on the Carbonyl Group. Components of the chlorine nuclear quadrupole coupling tensor, χ, for both molecules in the inertial axes have been

rotated into their principal axes and are shown in Table 9. Also in Table 9 are the chlorine nuclear quadrupole coupling tensors for several other acyl halides, also rotated into their principal axes. We observe that in all cases, the angles between the z and a axes are very close to the angle formed between the C-Cl bond and the a axis. This means that fair comparison may be made between the different chlorine χzz values. If we first consider the series HCOCl, CH3COCl, CH3 CH2COCl, and syn-anti CH3CH2CH2COCl, we find that the magnitude of the chlorine χzz value is essentially invariant to the alkyl chain length. An acyl chloride “standard” χzz(Cl) value of ≈-59 MHz seems apparent. Although not as strikingly similar, the χxx(Cl) and χyy(Cl) values are also reasonably invariant to chain length, varying by only 10% or less across the series of nonfluorinated species shown. Recalling that the nuclear quadrupole coupling tensor is directly proportional to the electric field gradient at the nucleus in question, it seems likely that the reason for this “standard” acyl chloride χzz(Cl) value is probably that the local electronic environment at the Cl center is most affected by the neighboring carbonyl group and also somewhat, but not completely (see below), hidden from the alkyl chain on the other side of the carbonyl group. The two members of the perfluoroacyl chlorides suggest a different “standard” perfluoroacyl chloride χzz (Cl) of ≈-65 MHz. This small, yet significant, increase in χzz(Cl) values for perfluoroacyl chlorides compared to acyl chlorides may be rationalized in the following way. The electric field gradient, ∂2V/∂z2, at the nucleus in an isolated chloride ion, Cl-, will be zero owing to the spherically symmetric electron distribution about the Cl nuclei due to the full valence shell. This zero value of ∂2V/∂z2 translates to a zero value for the Cl nuclear quadrupole coupling component χzz in Cl-. In this way, deviations from zero may be interpreted in terms of the decreasing ionicity or, alternatively, increasing covalency, of the C-Cl linkage. The Cl center in the perfluoroacyl chlorides appears to be in a slightly more covalent environment than in the acyl chlorides. In the specific case of the propionyl chlorides this suggests that the difference in electronegativity of Cl and that of the CF3CF2C(dO)- group is smaller than the difference between Cl and the CH3CH2C(dO)- group. In short CF3 CF2C(dO)is more electronegative than CH3CH2C(dO)-. This argument is supported by the considerably smaller calculated dipole moments for perfluoroprionyl chloride compared to propionyl chloride; see Table 2. These findings are fully consistent with the perfluoro-destabilizing effect of carbonyl groups and provide a new way of looking at the phenomenon.

TABLE 7: Kraitchman Substitution Coordinatesa and Experimentally Derived Geometric Parameters for Propionyl Chloride and Perfluoropropionyl Chloride CH3CH2COCl Cl C(1) C(2) C(3) r(C(1)-Cl)/Å r(C(1)-C(2))/Å r(C(2)-C(3))/Å ∠(Cl-C(1)-C(2))/deg ∠(C(1)-C(2)-C(3))/deg D(Cl-C(1)-C(2)-C(3))/deg

CF3CF2COCl

a

b

c

a

b

c

1.6430(9) 0b -1.046(2) -2.4868(6)

-0.302(5) 0.300(5) -0.809(2) -0.303(5) 1.750(3) 1.525(5) 1.527(3) 113.2(3) 114.0(3) 180.0

0b 0b 0.03(4) 0.0(1)

2.6747(6)

-0.09(2)

0.02(6)

1.777(3)c

a In units of angstroms. Signs have been added on the basis of the calculated structures. C(1) is the carbon closest to the acyl chloride group. An imaginary value was obtained which is indicative that the value is very close to zero. c This value best reproduces the determined rotational constants for both isotopologues given in Table 4. b

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TABLE 8: Second Moments and Related Quantities for Propionyl Chloride and Perfluoropropionyl Chloride parameter 35

CH3CH2CO Cl CH3CH2CO37Cl CH3CH213CO35Cl CH313CH2 CO35 Cl 13 CH3CH2CO35Cl CF3CF2CO35Cl CF3CF2CO37Cl

Paa/u Å2 a

Pbb/u Å2

Pcc/u Å2

∆/u Å2 b

κc

209.4262 214.7087 209.4113 210.5175 215.5678 509.2673 523.3996

53.9726 54.1455 54.0622 54.6171 54.0602 159.2467 159.2621

3.1015 3.1013 3.1014 3.1027 3.1017 90.2347 90.2357

-6.2030 -6.2025 -6.2029 -6.2053 -6.2033 -180.4693 -180.4714

-0.8676 -0.8732 -0.8671 -0.8658 -0.8746 -0.8629 -0.8704

a The second moment, Paa ) ∑imiai2, and similarly for Pbb and Pcc. b ∆ ) Ic - Ia - Ib is the inertial defect. c κ is the asymmetry parameter and is equal to (2B - A - C)/(A - C).

TABLE 9: The Chlorine Nuclear Quadrupole Coupling Tensor in the Principal Axes System for Various Acyl Chlorides parameter

HCOCla,b

CH3COCla,c

C2H5COCl

C2F5COCl

syn-antiC3H7COCld

syn-antiC3F7COCle

χzz/MHz χxx/MHz χyy/MHz ηz h θzai/deg θ(Cl-C), a/degj

-59.8(7)g 39.0(2) 20.8(2) -0.303(2) 17.5 17.5k

-59.486 37.542 21.944 -0.2622 5.16 5.16k

-59.49(35) 36.92(35) 22.569(4) -0.2413(61) 19.20(27) 19.5k

-65.41(15) 41.17(15) 24.246(7) -0.2587(24) 22.569(81) 24.0k

-59.22(21) 36.73(21) 22.485(6) -0.2406(36) 32.082(60) 32.8k

-65.427(18) 41.129(19) 24.298(13) -0.2572(4) 33.185(8) 34.68k

(CH3)3CCOClf -60.05(22) 37.56(22) 22.4864(50) -0.2510(40) 31.64(7)

a Obtained from a first-order perturbation analysis. No off-diagonal χ components determined. No uncertainties given. b Reference 37. Reference 38. d Reference 39. e Reference 40. f Reference 41. g Standard errors (1σ, 67% confidence level) in units of the least significant figure. h The asymmetry of the χ tensor in the principal axes system, ηx ) (χxx - χyy)/χzz. i The angle between the z and a axis. j The angle between the Cl-C bond axis and the (a, b) axes. k Taken from the calculated structure. c

TABLE 10: Methyl Group Internal Rotation Barrier Heights in Related Molecules molecule

V3/cal mol

ethanoyl chloride cis-propanal cis-propionic acid cis-propionyl fluoride cis-propionyl chloride cis-propionyl chloridea trans-1-chloropropane

1269(10) 2280(100) 2340(30) 2400(60) 2440(100) 2480(40) 2760(20)

a

-1

ref 38 42 43 44 this work. 14 45

Determined from the molecule in an excited vibrational state.

To extend this argument further, the experimental data contained in Table 9 demonstrate that although the electronegativity of Cn F2n+1C(dO)- is greater than that for CnH2n+1C(dO)-, it seems to also be the case that the electronegativities of these two groups is quite invariant to the chain length n. Further experiments on longer chains will explore this.

Propionyl Chloride Methyl Group Internal Rotation. The observed splittings in four of the rotational transitions for propionyl chloride, see Table 5 and Figure 4, have been analyzed, as discussed above, to produce a 3-fold barrier to internal rotation of V3 ) 853(35) cm-1 or 2440(100) cal mol-1. The value appears reasonable and is the first determination of the methyl torsional barrier height for propionyl chloride data in the ground vibrational state. As mentioned in the Introduction, splittings in spectral transitions for propionyl chloride in excited vibrational states have been analyzed previously by Karlson14 and Mata and Alonso15 and a barrier to methyl group internal rotation of 867(14) cm-1, or 2480(40) cal mol-1, was established. The magnitude of the determined barrier to methyl internal rotation is compared to several related molecules in Table 10. The B3LYP/6-31+G calculated barriers to terminal methyl group internal rotation in propionyl chloride and perfluoropro-

Figure 5. The B3LYP/6-31+G calculated barrier to methyl and trifluoromethyl group internal rotation in propionyl chloride (blue line) and perfluoropropionyl chloride (green line).

Perfluorinating Propionyl Chloride pionyl chloride are shown in Figure 5. The calculated barrier for propionyl chloride of 817 cm-1 is in near agreement with the experimental value from this work of 853(35) cm-1. A considerably higher barrier of 1120 cm-1, or 3200 cal mol-1, is found for the trifluoromethyl group internal rotation in perfluoropropionyl chloride. This is as expected and accounts for the lack of observed A-E internal rotation splittings in the perfluoropropionyl chloride microwave spectrum. Conclusion The microwave spectrum of perfluoropropionyl chloride has been obtained and analyzed for the first time. The microwave spectrum of propionyl chloride has been revisited at higher resolution and increased sensitivity than before. Analyses have shown that both molecules have identical cis confirmations under our experimental conditions, these being the lowest energy confirmations. The Cl nuclear quadrupole coupling tensor has revealed a more covalent C-Cl linkage in perfluoropropionyl chloride than propionyl chloride. Somewhat “standard” χzz(Cl) values are becoming apparent in both acyl chlorides, χzz(Cl) ≈ -59 MHz, and perfluoroacyl chlorides, χzz(Cl) ≈ -65 MHz. We propose that measurements of these quantities are useful in understanding and quantifying the perfluorination-induced destabilization of the carbonyl group. The b-type transitions are stronger than the a-type transitions in perfluoropropionyl chloride; the reverse is true for propionyl chloride. However, overall, the propionyl chloride spectra were stronger than the perfluoropropionyl chloride. The barrier height to methyl group internal rotation has been obtained from the ground vibrational state of propionyl chloride, 853(35) cm-1. As expected the analogous barrier height is determined to be considerably higher, at 1120 cm-1, for perfluoropropionyl chloride which results in internal rotation splitting to be too small to determine in our experiments. Longer chain members of the acyl chlorides and perfluoroacyl chlorides will be studied in the near future. Acknowledgment. The authors thank the National Science Foundation for award NSF-0820833. We also thank Professor S. A. Peebles at Eastern Illinois University for providing a sample input for the XIAM software. Supporting Information Available: All measured transition frequencies and quantum number assignments for both molecules and all isotopologues. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lemal, D. M. J. Org. Chem. 2004, 69, 1–11. (2) Fournier, J. A.; Bohn, R. K.; Montgomery, J. A., Jr.; Onda, M. J. Phys. Chem. A 2010, 114, 1118–1122. (3) Fournier, J. A.; Bohn, R. K. Talk RH10. 65th International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010. (4) Munrow, M. R.; Subramanian, R.; Minei, A. J.; Antic, D.; MacLeod, M. K.; Michl, J.; Crespo, R.; Piqueras, M. C.; Izuha, M.; Ito, T.; Tatamitani, Y.; Yamanoh, K.; Ogata, T.; Novick, S. E. J. Mol. Spectrosc. 2007, 242, 129–138.

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