Microwave Structure for the Propiolic Acid–Formic Acid Complex - The

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Microwave Structure for the Propiolic Acid−Formic Acid Complex Stephen G. Kukolich,* Erik G. Mitchell,† Spencer J. Carey, Ming Sun, and Bryan A. Sargus Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States ABSTRACT: New microwave spectra were measured to obtain rotational constants and centrifugal distortion constants for the DCCCOOH···HOOCH and HCCCOOD···DOOCH isotopologues. Rotational transitions were measured in the frequency range of 4.9−15.4 GHz, providing accurate rotational constants, which, combined with previous rotational constants, allowed an improved structural fit for the propiolic acid−formic acid complex. The new structural fit yields reasonably accurate orientations for both the propiolic and formic acid monomers in the complex and more accurate structural parameters describing the hydrogen bonding. The structure is planar, with a positive inertial defect of Δ = 1.33 amu Å2. The experimental structure exhibits a greater asymmetry for the two hydrogen bond lengths than was obtained from the ab initio mp2 calculations. The best-fit hydrogen bond lengths have an r(O1−H1···O4) of 1.64 Å and an r(O3−H2···O2) of 1.87 Å. The average of the two hydrogen bond lengths is rav(exp) = 1.76 Å, in good agreement with rav(theory) = 1.72 Å. The center of mass separation of the monomers is RCM = 3.864 Å. Other structural parameters from the least-squares fit using the experimental rotational constants are compared with theoretical values. The spectra were obtained using two different pulsed beam Fourier transform microwave spectrometers.

1. INTRODUCTION There has been considerable recent interest in doubly hydrogen bonded complexes because they can provide simple models for the hydrogen bonding in the A-T DNA base pairs. Doubly hydrogen bonded carboxylic acids are not simply static structures but exhibit interesting tunneling dynamics. The simpler complexes propiolic acid−formic acid1−3 and acetic acid−formic acid4 have been shown to exhibit resolvable concerted proton tunneling splittings in microwave spectra. Concerted proton tunneling was studied experimentally for the formic acid dimer by Havenith et al.5−7 Using highresolution IR spectroscopy on formic acid−formic acid dimers, they obtained νT = 85(8) MHz for (DCOOH)2 and 473(12) MHz for (HCOOH)2, in the two lowest vibrational states. Such a large difference for these similar dimers, with D substitution only on the carbon site, is quite surprising. Recently, the formic acid−acetic acid complex has been shown to exhibit simultaneous proton transfer4 with a barrier to exchange of 8000 cm−1, but the spectrum is complicated by additional splittings due to the CH3 internal rotation. The propiolic acid−formic acid dimer (Prop−FA), is a simpler system that exhibits a concerted proton tunneling splitting of 291.43 MHz.3 The Prop−FA complex is a nearprolate top with a dipole moment of 0.82 D along the a axis.3 The basic structure of the complex is shown in Figure 1. A preliminary report ascribed splittings observed upon rotational transitions to the concerted proton tunneling,1,2 and a much more compete study of this proton tunneling was published more recently.3 Microwave spectra for Prop−FA were measured in the 1−21 GHz range using four different Fourier transform spectrometers. The a-dipole transitions exhibited splittings of a few MHz into pairs of lines, and the b-type dipole transitions were split by approximately 580 MHz. The b-dipole © 2013 American Chemical Society

Figure 1. Basic structure of the Prop−FA complex, showing the three variable parameters used in the least-squares fit to determine the structure of the complex. XCM and YCM are the translations of the formic acid center of mass (COM) relative to the propiolic acid COM and allow for rotation and translation of propiolic acid relative to the COM of the complex.

rovibrational transitions involve a change in both the rotational state and the tunneling level; therefore, the splitting of these transitions provides direct information on the concerted proton tunneling frequency. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for the normal isotopologue. The small splittings of 1−1.5 MHz for the a-dipole transitions are due to the differences in rotational constants for the upper and lower tunneling states. No similar splittings were observed for a-dipole transitions when deuterium was substituted for either or both of the hydrogen Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 11, 2012 Revised: January 28, 2013 Published: January 29, 2013 9525

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the product was determined using 1H NMR (Bruker AVIII-400 MHz, CDCl3), which revealed a deuterium content of 79%. Propiolic acid-OD samples were prepared both by using simple hydrogen exchange with methanol (OD) (99.5%) (Sigma-Aldrich) (CAS: 1455-13-6) and by making sodium propiolate using NaOH in methanol, removing the methanol under reduced pressure and adding D2SO4 (99% d, 96% assay) (Cambridge Isotopes) to the dry salt at −78 C, following the literature procedure.12 All glassware was heated in an oven to remove residual water from its surface. About 4.0 g of sodium hydroxide was dissolved in 150 mL of methanol, followed by the addition of 7g of propiolic acid. The large quantity of methanol is needed to prevent catalysis of the Fischer esterification of propiolic acid to methyl propiolate. After stirring for several minutes, the methanol was removed under a reduced atmosphere, leaving a white solid. Then, 10−15 mL of D2SO4 was slowly added to the salt, which was left under reduced pressure and cooled to −78 C. Propiolic acid-OD was distilled from the reaction vessel onto a 77 K trap. Formic acid (OD) was purchased from Sigma-Aldrich and used without further purification. For the CD-propiolic acid−formic acid heterodimer (DCCCOOH···HOOCH), eight a-dipole transitions were measured in the 4−12 GHz range using the transverse-beam configuration. Transitions for the OD-propiolic−OD-formic dimer (HCCCOOD···DOOCH) were measured in the 6−16 GHz range using a newly constructed coaxial beam spectrometer (spectrometer #2, resolution = 10 kHz). The 37 a-dipole transitons and 8 b-dipole transitions are listed in Table 1.

bonding protons. In recent experiments, additional splittings of the b-dipole transitions for HCCCOOD···DOOCH were measured, and these will be reported separately. In the previous work3 on Prop−FA, measured rotational constants for seven isotopologues were used to obtain structural parameters. The data for that structural fit included only two isotopologues for propiolic acid but four isotopologues for formic acid. The only substitution for propiolic acid was D for the acid proton, which lies close to the b axis and COM of the complex, reducing the usefulness of this substitution for determining the propiolic acid orientation. This is very likely the reason that the structural fit results were fairly independent of the rotation of propiolic acid about an axis perpendicular to the plane of the complex and passing through its center of mass (COM). In the present work, the DCCCOOH isotopologue was synthesized, and new spectra were obtained for the DCCCOOH monomer and for the DCCCOOH−HOOCH doubly hydrogen bonded dimer. The substitution with D at H3 (which is far from the COM of the complex) provides important data for a much better determination of the orientation for the propiolic acid moiety. The three variable parameters (described below) for the new structural fit allowed for rotation of both the propiolic acid and formic acid about axes perpencdicular to the plane of the complex and changes in the separation fot the COMs of the monomers. The three variable parameters used to fit the structure of the complex to the data are shown in Figure 1.

2. EXPERIMENTAL SECTION Spectra for the heterodimers were measured using pulsed beam Fourier transform microwave spectrometers. In addition to the previous transverse beam spectrometer,8,9 a new spectrometer was constructed in our lab with the pulsed beam orientation coaxial with the cavity axis, and this provides 10 kHz resolution. For the D−C propiolic−formic acid dimer, the vapor, seeded in neon, was introduced into the spectrometer resonator cavity transverse to the microwave radiation at a 2 Hz pulse rate using a pulsed valve (General valve series 9) (spectrometer #1). The pressure inside of the spectrometer chamber was maintained at 10−6−10−7 Torr prior to the valve opening, and the backing pressure of neon was kept at about 0.8 atm during the measurements. Following the molecular pulse, a π/2 microwave excitation pulse (1 μs duration) was injected into the resonator to coherently excite the molecules, using a Herley SPDT microwave switch. The molecular FID signal was transmitted via the same SPDT switch, passed to a Miteq 6−18 GHz lownoise amplifier, and sent to the RF circuits for further signal processing. The details of the homodyne mixing and detection system and spectrometer have been given previously.9 Propiolic acid-CD (DCCCOOH) was prepared in 32−54% yield by the decarboxylation of acetylenedicarboxylic acid monopotassium salt (95%) (TCI America) (CAS: 928-04-1) in D2O (99.9%) (Cambridge Isotope Laboratories) (CAS: 778920-0) following the literature procedure cited.10,11 About 3.0 g of acetylene dicarboxylic acid monopotassium salt (95%) (Sigma-Aldrich) (CAS: 142-45-0) was dissolved with 10 mL of D2O and gradually heated to 100 °C over 5 h. It was then refluxed for 1 h. The D2O was then removed via reduced pressure, resulting in an orange tinted salt. Then, 2 mL of sulfuric acid (95%) was added, followed by 20 mL of deionized water. It was then extracted using approximately 20 mL of diethyl ether and dried with magnesium sulfate, and then, the diethyl ether was removed via reduced pressure. The purity of

3. DATA ANALYSIS AND NEW ROTATIONAL CONSTANTS FOR TWO ISOTOPOLOGUES OF THE PROP−FA The Prop−FA is a near-prolate asymmetric top that exhibits splittings of the rotational transitions due to the proton tunneling effects. The measured transitions for DC-propiolic acid−formic acid (DCCCOOH···HOOCH) are for the lower tunneling state (+) only, as listed in Table 2. No b-dipole transitions were searched for or measured for this dimer due to lower isotopic purity of the DCCCOOH and reduced sensitivity due to the possible deuterium quadrupole structure on the lines. These transitions were fit to determine the B and C rotational constants with other parameters fixed to values obtained for the normal isotopologue,3 and the results of this fit are shown in Table 3. Many more transitions were obtained for the OD-propiolic− OD-formic dimer (HCCCOOD···DOOCH) (see Table 1.) and molecular parameters obtained by fitting these transitions are listed in Table 4. A more detailed analysis of these results, including the tunneling splitting, will be presented elsewhere.13 4. LEAST SQUARES FIT TO DETERMINE STRUCTURAL PARAMETERS FOR THE PROP−FA The new rotational constants for DCCCOOH···HOOCH and HCCCOOD···DOOCH along with those from the earlier work3 were used in a least-squares fit to determine structural parameters for the complex. For the structural fit, the complex was assumed to be planar. This is a good approximation because the experimental inertial defect for this complex is positive with Δ = 1.33 amu Å2. 9526

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Table 1. Observed Transitions for HCCCOOD···DOOCH, Measured Transitions (νobs) (MHz), and Deviations (Obs− Calc) of the Best-Fit Calculated Frequencies (νo−c) (MHz)

a

J′KaKc

v′

J″KaKc

v″

νobs

νo−c

414 414 404 404 413 413 212 212 515 515 505 505 514 514 616 616 313 606 606 615 615 414 414 717 717 707 707 716 716 515 515 818 818 808 808 616 616 817 817 919 919 909 909 717 717

1 0 1 0 1 0 0 1 1 0 1 0 1 0 1 0 1 1 0 1 0 0 1 1 0 1 0 1 0 0 1 1 0 1 0 0 1 1 0 1 0 1 0 0 1

313 313 303 303 312 312 101 101 414 414 404 404 413 413 515 515 202 505 505 514 514 303 303 616 616 606 606 615 615 404 404 717 717 707 707 505 505 716 716 818 818 808 808 606 606

1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0

6614.528 6614.573 6846.485 6846.498 7116.361 7116.384 8208.978a 8216.002a 8263.886 8263.928 8540.441 8540.470 8890.789 8890.807 9910.538 9910.578 9746.201a 10222.968 10223.008 10661.945 10661.965 11210.619a 11217.547a 11554.082 11554.139 11892.331 11892.380 12429.020 12429.043 12628.068a 12634.939a 13194.208 13194.277 13547.328 13547.382 13998.200a,b 14005.000a,b 14191.110 14191.136 14830.682 14830.757 15187.473 15187.542 15329.450a,b 15336.074a,b

−0.011 0.000 0.000 −0.011 −0.002 0.008 −0.003 −0.002 −0.004 −0.004 −0.004 −0.006 0.001 0.002 0.001 −0.009 0.005 −0.001 0.000 0.000 −0.001 0.000 −0.008 −0.002 −0.004 0.004 0.006 0.001 0.000 0.002 0.003 −0.006 −0.005 0.011 0.009 −0.009 0.004 0.001 −0.002 −0.006 −0.007 0.016 0.019 0.067 0.002

Table 2. DC-propiolic acid−formic acid (DCCCOOH···HOOCH) Measured Transitions and Deviations for the Fit to Determine Rotational Constants (see Table 5)a

a

J′KaKc

J″KaKc

νobs

νo−c

303 404 515 505 606 615 707

202 303 414 404 505 514 606

4991.294 6646.169 8036.543 8293.489 9931.552 10326.320 11558.805

−0.008 −0.002 0.000 0.000 0.014 0.000 −0.008

These transitions are for the lower (+) tunneling state only.

Table 3. Spectroscopic Constants for DCCCOOH···HOOCH Obtained by Fitting the Measured Lower Tunneling State Transitions Listed in Table 1 of CDProp−FA in MHza parameter

value (MHz)

A B C DJ DJKb σ (for fit)

5993.6(6) 890.536(1) 775.7803(8) 0.00005(1) 0.0007 0.007

a Errors are 1σ in the last quoted decimal places. bValues from the main isotopologue (ref 3) were fixed during fitting.

Table 4. Spectroscopic Constants for HCCCOOD···DOOCH (MHz), Obtained by Fitting the Individual Tunneling States (+,−) Separately Using Measured Lines in Table 2 to Obtain Rotational Constants in the PAS A B C DJb DJKb DKb

Pro-OD−FA-OD(+)

Pro-OD−FA-OD(−)

5825.40(7) 921.5295(3) 796.0360(2) 0.000024(14) 0.0015(6) 1.1(4)

5825.52(7) 921.5291(3) 796.0306(2) 0.000024(14) 0.0015(6) 1.1(4)

a

Errors are 1σ in the last quoted decimal places. bValues from the lowest tunneling state are fixed during fitting.

translation of the propiolic acid moiety. The third parameter ϕFA allows for the rotation of formic acid about its COM (which is near C4). The internal structural parameters for both monomer acids were fixed during the fitting process. Fits were attempted with the COH angles opened up slightly to 108°, as was done with other similar complexes,14 but the best results were obtained using the measured 106° COH angles for both monomers in the complex. Structural parameters for the propiolic acid monomer were obtained from the work of D. G. Lister et al.,15 and structural parameters for formic acid were from the work of Gerry et al.16 Small adjustments to the propiolic acid structure were made to improve the fit to the monomer rotational constants. The details of monomer structural parameters can be obtained from Table 6 by selecting the appropriate atoms. The Cartesian coordinates that are obtained using the best-fit structural parameters for all atoms in the complex are listed in Table 6.

Indicates b-dipole transitions. bMeasured previously (ref 3).

The measured rotational constants (using only the 0+ state values) for the seven isotopologues are listed in Table 5 and were used in a least-squares fit to determine three structural parameters, XCM, YCM, and ϕFA, shown in Figure 1. In order to parametrize the structure in the x−y plane, propiolic acid was taken as the reference molecule with the x axis passing through the three carbon atoms C1, C2, and C3. The formic acid was then displaced along the x and y axes by xcc,ycc, such that xcc and ycc are the displacements of the formic acid COM from the propiolic acid COM. With respect to the COM of the complex, the variables xcc and ycc allow for 9527

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values for the rC1−C4 separation, given in the last column of Table 7.

Table 5. Measured Rotational Constants for Seven Isotopologues of the Prop−FA Complex (MHz)a isotopologue B C B C B C B C B C B C B C

normal HCCCOOH···DOOCH HCCCOOD···HOOCH HCCCOOH···HOO13CH HCCCOOD···DOOCH HCCCOOH···HOOCD DCCCOOH···HOOCH

meas

calc−meas

927.7824 803.7199 923.4015 799.1703 925.6587 800.7534 915.2551 794.3132 922.1900 796.0357 903.1300 786.0260 890.5360 775.7803

0.86 0.01 0.71 0.67 −1.13 −0.59 0.63 −0.18 −1.09 −0.05 −0.21 0.27 0.20 −0.07

Table 7. COM Separations of the Monomers for the Various Isotopologuesa complex

ICC (amu Å2)

RCM (Å)

r(C1−C4) (Å)

HCCCOOH−HOOCH (normal) HCCCOOH−HOO13CH HCCCOOH−DOOCH HCCCOOD−HOOCH HCCCOOH−HOOCD HCCCOOD−DOOCH DCCCOOH−HOOCH

628.585 636.247 631.130 632.380 642.955 634.872 651.543

3.8720 3.8815 3.8487 3.8562 3.9026 3.8326 3.9188

3.822 3.822 3.818 3.830 3.817 3.822 3.819

a

Prop−FA COM separation (RCM) from ICC = ICC(Pro) + ICC(FA) + μR2CM in Å.

a

The calc−meas (MHz) is the deviation of the measured value from the “best-fit” calculated value from the least-squares structure fit. σ for the fit is 0.7 MHz.

The listed rC1−C4 values are in quite good agreement and do not exhibit a significant shortening of the hydrogen bonds with deuterium substitution. The RCM value listed for the normal isotopologue of 3.8720 Å agrees well with value 3.8645 from the fit using all seven isotopologues.

Table 6. Cartesian Coordinates of Atoms of the Prop−FA Complex in the Principal Inertial Axis System As Obtained from the Structure Fita

a

atom

a

b

H1 H2 H3 H4 O1 O2 O3 O4 C1 C2 C3 C4

−0.2280 −1.5717 4.7349 −3.7732 0.7440 0.2563 −2.5326 −1.8220 1.0625 2.4953 3.6940 −2.7104

−1.3592 1.2979 0.5024 −0.4520 −1.3149 0.8863 1.1505 −0.9921 0.0079 0.1916 0.3577 −0.1800

6. COMPARISON OF EXPERIMENTAL STRUCTURE PARAMETERS WITH THEORETICAL VALUES Results of MP2 calculations for the structure of the Prop−FA complex were reported earlier.1 These calculations were done using the Gaussian 0317 suite with MP2/6-311++G**(Gaussian) basis set, with the counterpoise correction included on the University of Arizona ICE high-performance computing cluster. The calculated structural parameters are shown in Figure 2a and Table 8. The Cartesian coordinates obtained from the least-squares structure fit (above) to the experimental data are given in Table 6. The final structural parameters for the Prop−FA complex are shown Figure 2b and Tables 6 and 8. A comparison of the

Values are in Å, and the atom numbering is shown in Figures 1 and 2.

The experimental B and C rotational constants and the deviations of the best-fit calculated values from the experimental values are listed in Table 5. Values of the adjustable parameters obtained in the structural fit are XCM = 3.832(15) Å, YCM = 0.502(97) Å, and ϕFA = 8.0(11)°. The angle ϕFA represents a rotation of the formic acid moiety from the starting configuration with near equal hydrogen bond lengths. The x axis makes an angle of 6° with respect to the a axis of the complex; therefore, the rotation of formic acid relative to the a axis is only a few degrees. The above measured XCM and YCM yield the separation of the monomer COMs of RCM = 3.864 Å.

5. COM SEPARATIONS OF THE MONOMERS The measured C rotational constants can be readily analyzed using the parallel axis theorem to determine the COM separations for the various isotopologues. This also provides some additional confidence in the assignment of the isotopologues. The formula used is ICC = ICC(Pro) + ICC(FA) + μR2CM. These values are listed in Table 7 and can be used with the COM isotopic shifts for the monomers to obtain comparison

Figure 2. Comparison of the Gaussian MP2-calculated ab initio structure (a) of the Prop−FA complex with the structure obtained from the best−fit to the measured rotational constants (b). The horizontal axis is the a principal axis, and the vertical axis is the b principal axis. 9528

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C2v(M) symmetry of the complex, but the tunneling transitions are readily observed. The other, monomer, bond lengths in Table 6 exhibit excellent agreement between calculated and experimental values. The listed interbond angles also agree quite well.

Table 8. Comparison of the Experimental Interatom Distances (Å) and Interbond Angles with Corresponding Values Obtained from the Gaussian MP2 Calculationsa parameter Bond Length (Å) H1−O4 H2−O2 O1−O4 O2−O3 H3−C3 C2−C3 C1−C2 C1−O2 C1−O1 O1−H1 H2−O3 O3−C4 C4−O4 C4−H4 Angles ∠O2−C1−O1 ∠C1−O1−H1 ∠H2−O3−C4 ∠O3−C4−O4 ∠O3−C4−H4 a

experimental value 1.64 1.87 2.59 2.80 1.05 1.21 1.45 1.20 1.35 0.97 0.97 1.34 1.20 1.10 124° 106° 106° 125° 112°

calculated value

7. SUMMARY Measurements of the rotational constants for two new isotopologues for the propiolic acid−formic acid complex have allowed a much improved determination of the molecular structure of the complex. The experimental structure for this complex is planar, with more asymmetric hydrogen bond lengths than were obtained from ab initio calculations.

1.71 1.73 2.70 2.72 1.07 1.22 1.45 1.23 1.32 0.99 O.99 1.32 1.22 1.10

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AUTHOR INFORMATION

Present Address †

Patrick Air Force Base, 32925, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the National Science Foundation under Grants CHE-0721505, CHE0809053, and CHE-10557796 at the University of Arizona. We thank Adam Daly for helpful discussions on this work and for previous work on this complex.

125° 109° 109° 126° 111°

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The atom numbering is shown in Figures 1 and 2.

MP2-calculated structure and the experimental best-fit structure is shown in Figure 2. The two hydrogen bond lengths for the experimental structure are significantly more asymmetric than those for the calculate structure. Experimental hydrogen bond lengths are 1.64 Å for (O1−H1···O4) and 1.87 Å for (O3− H2···O2). The corresponding values for the calculated structure are 1.71 and 1.73 Å. We note that the average of the two experimental hydrogen bond lengths is rav(exp) = 1.76 Å, which is in better agreement with rav(theory) = 1.72 Å. The uncertainty in the COM separation (0.015 Å) is smaller than the relative uncertainty of ϕFA of 1.1°. This experimental uncertainty in ϕFA however would only be propagated to 0.02 Å uncertainties in the H-bond lengths, not enough to account for the differences between experimental and ab initio calculated results. Possible sources of the discrepancies are (a) the experimental results represent a near r0 structure whereas ab initio calculations predict the re structure and the amplitude of H1 and H2 vibrational motion is very large, (b) the assumption that the monomer structures do not vary on complex formation may not be sufficiently accurate, and (c) the accuracy of the ab initio calculations for hydrogen-bonded complexes may not be as good as expected even though the H-bonds are fairly strong for this complex. Most of the other calculated structural parameters are in quite good agreement with the experimental values. The structural parameters for the monomers for the experimental and calculated values are in much better agreement, generally within 0.02 Å. Both the experimental and calculated structural parameters are given in Table 6. Following the trend of the large difference in the experimental H1···O4 and H2···O2 hydrogen bond lengths, the O1−O4 and O3−O2 distances are also quite different for the experimental structure and quite close in the calculated structure. These large differences cause significant deviations from the nominal

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