A Study of the Monohydrate and Dihydrate Complexes of

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A Study of the Monohydrate and Dihydrate Complexes of Perfluoropropionic Acid Using Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectroscopy Garry S Grubbs II, Daniel A Obenchain, Derek S Frank, Stewart E. Novick, Stephen Anthony Cooke, Agapito Serrato, and Wei Lin J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b08347 • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 7, 2015

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The Journal of Physical Chemistry

A Study of the Monohydrate and Dihydrate Complexes of Perfluoropropionic Acid using Chirped-Pulse Fourier Transform Microwave (CP-FTMW) Spectroscopy G. S. Grubbs II,∗,† Daniel A. Obenchain,‡ Derek S. Frank,‡ Stewart E. Novick,‡ S. A. Cooke,¶,‡ Agapito Serrato III,§ and Wei Lin§ Department of Chemistry, Missouri University of Science and Technology, 400 W. 11th St., Rolla, MO, 65401, USA, Department of Chemistry, Wesleyan University, 52 Lawn Ave., Middletown, CT, 06459, USA, School of Natural and Social Sciences, State University of New York-Purchase College, 735 Anderson Hill Road, Purchase, NY, 10577, USA, and Department of Chemistry, University of Texas Rio Grande Valley, Brownsville, TX 78520 E-mail: [email protected] Phone: 573-341-6281. Fax: 573-341-6033

∗

To whom correspondence should be addressed Missouri S&T ‡ Wesleyan University ¶ SUNY-Purchase § University of Texas Rio Grande Valley †

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Abstract This work reports the first known spectroscopic observation of the monohydrate and dihydrate complexes of perfluoropropionic acid (PFPA). The spectra have been observed using a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer in the 7750 to 14250 MHz region. The structures of the species have been confirmed with the aid of ab initio quantum chemical calculations. Rotational constants A, B, and C have been determined and reported for both species along with centrifugal distortion constants ∆J , ∆JK , ∆K , δJ , δK for H2 O-PFPA and ∆J , ∆JK , and δJ for (H2 O)2 -PFPA. Effects due to large amplitude motions were not observable in these experiments. Structures of the complexes have been determined using a combination of experimental second moment values and ab initio results. The complexation of the -OH of one or two water molecules has been found to occur in the plane of the carboxylic acid group forming a 6- or 8-member ring.

Keywords van der Waals Complexes, Water-Acid Interactions, Rotational Spectroscopy, Hydrogen Bonding

Introduction Perfluorinated carboxylic acids are an integral part of many chemical processes due to their common use as surfactants. This is because they possess a highly hydrophobic end (the fluorinated alkane) and a highly hydrophilic end (the carboxylic acid end). In recent years, however, there has been an increasing health concern with these chemicals as they have been found in groundwater, lake water, wildlife, and even human blood serum. 1 Furthermore, it has been suggested that these chemicals may prove to be carcinogenic to humans. 1 Spectroscopic studies of these molecules with water will aid in understanding how these molecules


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can be traced and transported in our environment. 2 In this work we report the microwave rotational spectroscopic observation of the gas phase mono- and dihydrate complexes of perfluoropropionic acid (labelled H2 O-PFPA and (H2 O)2 -PFPA, respectively). This is the first known spectroscopic observation of either of these complexes. Similar spectroscopic experiments recently have focused on the fluorinated carboxylic acid monomers perfluoropropionic acid (PFPA), 3 perfluoropentanoic acid, 4 and 3,3,3-trifluoropropionic acid. 5 Rotational spectroscopic investigations pertaining to both monomers and hydrated species of trifluoroacetic acid(TFA), 6 perfluorobutyric acid (PFBA), 2 and the unfluorinated propanoic acid 7 have also been reported. In the studies pertaining to hydration of the perfluorinated carboxylic acids, it has been found that the hydrogen bonding -OH of the water molecules add planar to the carboxylic acid group forming a ring structure. 6 In this paper we attempt to find similarities between all three hydrated studies by investigating the structure and binding energies of each and make comparisons within the family. In the TFA-water species, tunneling splitting was observed arising from the free hydrogen atom moving through the ab-plane, the plane of the interaction. We address in this report the lack of tunneling splitting in the spectra and our reasoning for this absence.

Experimental All experiments were performed on the chirped-pulse Fourier transform microwave spectrometer housed at Wesleyan University. This instrument has been described in detail elsewhere. 8,9 For these experiments, 97% perfluoropropionic acid purchased from SigmaAldrichTM was used without further purification. The original setup used by the authors consisted of a 1:1 ratio of deionized water and perfluoropropionic acid, however, observation of spectra was achieved only while sampling/running another species of interest, Allyl Phenyl Ether. All liquids were placed inside of a U-shaped tube approximately 40 cm upstream from



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R Series 9 solenoid valve. Argon was bubbled through the mixture a 0.8 mm diameter Parker

at a pressure of 1.5 atm and used as a carrier gas for the experiments. Microwave excitation pulse lengths of 8 µs were used for each approximately 2 GHz chirp region. Each region was collected for 30,000 averaging cycles with 1 microwave pulse per gas pulse and transitions were observed from 7750 to 14250 MHz with an average FWHM of 90 kHz.

Quantum Chemical Calculations Quantum chemical calculations were employed to aid in experimental spectral assignment and geometric structure as detailed in the Results and Discussion section. The calculations utilized in this work were geometry optimizations performed at the MP2/6-311G++(2df,2pd) level holding the core electrons frozen. No counterpoise corrections were used. All calculac program suite. 10 The calculated equilibrium tions were performed using the Gaussian09 structures are given in Table 1 and shown in Figures 1 and 2 while the rotational constants are presented in Table 2 with the experimentally determined values for comparison.

2.15 Å

1.69 Å

1.69 Å

2.15 Å

Figure 1: The ab initio structure of H2 O-PFPA in the ab-, ac-, and bc-planes. The intermolecular O—H distances are labelled.


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1.57 Å

1.74 Å 1.92 Å

1.57 Å 1.74 Å 1.92 Å

Figure 2: The ab initio structure of (H2 O)2 -PFPA in the ab-, ac-, and bc-planes. The intermolecular O—H distances are labelled.

Table 1: Quantum Chemical Calculations for (H2 O)n -PFPA. a

Level A / MHz B / MHz C / MHz Paa / u˚ A2 Pbb / u˚ A2 Pcc / u˚ A2 ∆ / u˚ A2 κ / Unitless |µa | / D |µb | / D |µc | / D a

H2 O-PFPA MP2/6-311G++(2df,2pd) 1734 681.4 650.3 613.7 163.5 128.0 -256.0 -0.943 3.7 0.62 0.09

(H2 O)2 -PFPA MP2/6-311G++(2d,2pd) 1418 467.3 450.0 924.1 199.0 157.4 -314.8 -0.964 3.2 0.30 0.51

All were run with core electrons frozen.



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Results and discussion Table 1 presents the calculated dipole moment magnitudes along each principal axis. The a dipole moment for each complex is 3.7 D and 3.2 D for H2 O-PFPA and (H2 O)2 -PFPA, respectively. In accordance with this strong calculated dipole moment, the observed experimental spectra contained predominantly a-type transitions. a- and b-type, R-branch transitions were observed for the H2 O-PFPA complex with over three-quarters of the transitions being a-type. All observed transitions of (H2 O)2 -PFPA were a-type, R-branch, as b-type transitions were too weak to be observed in the trimer. A sample spectrum of each species can be found in Figures 3 and 4. H2 O-PFPA and (H2 O)2 -PFPA spectra were fit utilizing a Watson-A reduced Hamiltonian 11 with Pickett’s SPFIT and SPCAT software. 12 Kisiel’s AABS 13 package was used as a front-end to help with manual assignment of the broadband spectra. Links to SPFIT/SPCAT and AABS can be found at the PROSPE website. 14 67 transitions were measured and assigned for H2 O-PFPA while 48 were measured and assigned for (H2 O)2 -PFPA. Line centers were given an attributed uncertainty of 10 kHz. Rotational constants A, B, and C were determined for both species and centrifugal distortion terms ∆J , ∆JK , ∆K , δJ , δK were determined for H2 O-PFPA. Centrifugal distortion terms ∆J , ∆JK , and δJ were determined for (H2 O)2 -PFPA. ∆K and δK were attempted to be fit as was needed for the dimer species, but these parameters were not determinable. As mentioned before, a listing of all the experimentally determined parameters can be found in Table 2. All materials associated with transition assignments and fits for the species can be found in the Supporting Information. Assignments were carried out by utilizing the ab initio calculations as a starting point. To aid in this, perfluoropropionic acid (PFPA) monomer transitions were removed from the spectra using an in-house spectra subtraction program and the known fit/assignments of PFPA from our previous study. 3 After this, H2 O-PFPA was the first to be assigned because its spectrum was, in general, much more intense than the dihydrated species. A comparison of Figures 3 and 4 has been presented in Figure 5 on the same intensity scale to show this. 6 ACS Paragon Plus Environment

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Table 2: Experimental and Calculated Parameters for H2 O-PFPA and (H2 O)2 PFPA. Parameter A / MHz B / MHz C / MHz ∆J / kHz ∆JK / kHz ∆K / kHz δJ / kHz δK / kHz Nc RMSd / kHz

H2 O-PFPA Experimental Calculateda 1738.31182(77)b 1734 676.51447(39) 681.4 640.12175(36) 650.3 0.0922(14) — 0.2850(72) — 0.201(47) — 0.00923(67) — 1.16(13) — 67 — 4.5 —

(H2 O)2 -PFPA Experimental Calculateda 1414.176(82) 1418 459.55736(90) 467.3 438.45625(87) 450.0 0.05480(91) — 0.2628(34) — — — -0.0115(16) — — — 48 — 7.7 —

a

MP2/6-311G++(2df,2pd) for H2 O-PFPA and MP2/6-311G++(2d,p) for (H2 O)2 -PFPA both with core electrons frozen. b Numbers in parentheses give standard errors (1σ, 67% confidence level) in units of the least significant figure. c Number of observed transitions used inr the fit. P h i 2 d Root mean square deviation of the fit, (observed frequencies − calculated frequencies) /N

J=7-6 K aKc = 2,6 - 2,5 K aKc = 2,5 - 2,4 K aKc = 4,3 - 4,2 K aKc = 3,5 - 3,4 K aKc = 5,2 - 5,1

K aKc = 3,4 - 3,3

Not a H2O-PFPA or (H2O)2-PFPA transition

K aKc = 6,1 - 6,0

Frequency (MHz) Figure 3: A section of the J = 7 − 6 transitions of H2 O-PFPA located in the region of 9200-9260 MHz.



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J = 10 - 9 K aKc = 2,9 - 2,8

KaKc = 5,5 - 5,4 Not a H2O-PFPA or (H2O)2-PFPA transition

KaKc = 3,8 - 3,7

KaKc = 6,4 - 6,3 KaKc = 4,6 - 4,5 KaKc = 4,7 - 4,6

KaKc = 7,3 - 7,2

KaKc = 3,7 - 3,6

Frequency (MHz) Figure 4: A section of the J = 10 − 9 transitions of (H2 O)2 -PFPA located in the region of 8970-8990 MHz. For H2 O-perfluorobutyric acid (H2 O-PFBA), the monohydrate interaction was so strong that there was a noticeable weakening effect on the monomer transitions, 2 this effect was also observed in these experiments. After completely assigning the monohydrated species, the subtraction program was run again on the spectrum making it easier to assign the dihydrated species. During assignment, it was noticed that some transitions in the spectra did not belong to either the mono- or dihydrated species. These were determined to belong to a possible transition of the trihydrate species or a transition belonging to Allyl Phenyl Ether.

Signal-to-Noise and Intensity Signal-to-noise and intensity were used to aid in the assignment of the spectra. Generally, tranisitions of dimer species were of larger intensity than that of the trimer species. This, again, is shown in Figure 5. As stated before, it was also noticed that the monomer PFPA transitions weakened upon performing these experiments. To monitor this, monomer PFPA signal-to-noise experimental values were compared to those observed in this work. The very


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Frequency (MHz)

Frequency (MHz) Figure 5: A comparison of the monohydrate-PFPA (top) and dihydrate-PFPA (bottom) spectra on the same intensity scale. intense b-type transitions 32,1 - 21,2 and 41,4 - 30,3 at 9224.134 and 9642.915 MHz, respectively, assigned in reference 3 were chosen. The 32,1 - 21,2 S:N decreased from 246:1 to 95:1 while the 41,4 - 30,3 decreased from 333:1 to 103:1, representing ≈

2 3

loss in S:N.

One explanation for this large change in the intensity of the PFPA monomer specta may be due to the strong interaction energy of the dimer and trimer complexes. Using the same type, theory, and level of quantum chemical calculations for H2 O and PFPA as those used for the complexes, equilibrium dissociation energies of 49.4 kJ/mol and 101.8 kJ/mol were determined for the dimer and trimer, respectively. This is in agreement with those found for H2 O-trifluoroacetic acid and (H2 O)2 -trifluoroacetic acid which were calculated to be 41.9 and 86.1 kJ/mol, respectively. 6 These values are substantially larger than those typically found in van der Waals interactions and more on the order of the dissociation energy of that found for the H2 -MX interactions studied previously by the authors 15–17 where it was determined that those interactions are more like a chemical bond. 9 ACS Paragon Plus Environment


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As mentioned before, overall signal intensity also decreased as frequency increased. This can be seen by the transition listings given for each complex in the Supporting Information. There is a substantial dropoff in the number of observed transitions in the spectra of both complexes above 11 GHz. For (H2 O)2 -PFPA, there are no observed transitions above 12 GHz. Because of this intensity dropoff, transitions like 818 - 707 and 90,9 - 81,8 of the H2 OPFPA species that would complete transition loops are missing from the assignment in the Supporting Information.

A Comment on Tunneling As mentioned in the Introduction, the spectra of the H2 O-triflouroacetic acid (H2 O-TFA) interaction exhibited tunneling splitting arising from the free hydrogen atom in the water molecule moving from an orientation above the ab-plane, through the ab-plane, to a position below the ab-plane while maintaining the interaction with the trifluoroacetic acid. 6 However, there is an absence of splitting in the spectra observed for H2 O-PFPA in this work which agrees with that reported for the H2 O-PFBA interaction. 2 However, there are differences from this work and H2 O-PFBA that may address the absence. In reference 2 it was stated that the tunneling in H2 O-perfluorobutyric acid was probably not a factor because there was no tunneling observed and a semirigid Hamiltonian fit the data well. While the experiments with H2 O-PFPA in this work did not observe any additional splittings and was fit to a semirigid Hamiltonian, H2 O-PFPA did have a larger rms value in the fit than that of H2 OPFBA. The reason for this is the CP-FTMW spectrometer data were used for final transition assignment in these experiments while the higher-resolution cavity FTMW spectrometer was used for final transition assignments in H2 O-PFBA. 2 The rms values of the fits are 4.5 kHz and 1.9 kHz for H2 O-PFPA and H2 O-PFBA, respectively, with the available data set. The splittings, however, were on the order of 3.2 kHz/(J+1) for H2 O-TFA. 6 Given that the largest J+1 transition quantum number is 11 for the H2 O-PFPA transition data set, if the splittings for the H2 O-PFPA transitions are of a similar magnitude as that for the H2 O-TFA 10 ACS Paragon Plus Environment

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interaction transitions, then this would be ≈35 kHz. Given the resolution of the experiment (90 kHz FWHM), the authors cannot and do not rule out the possibility of similar tunneling motion existing in these interactions.

Structure The major driving force behind these investigations was to understand structurally how the water molecules interacted with the PFPA molecule as well as how this structure compared to that of the family of studied perfluorinated carboxylic acids. Since only the main isotopolouge spectra were observed in natural abundance, experimental second moments 18 arguments in accordance with Bohn et al 19 were used in conjunction with the ab initio structures to better understand the (H2 O)n -interactions. In the cases of (H2 O)n -TFA 6 and (H2 O)n -propanoic acid ((H2 O)n -PPA) 7 it was specified that the water molecules add in a ring with one hydrogen from each water sticking out of the plane of the ring. In the case of (H2 O)n -PPA, it is straightforward to use Pcc values to show the water adding into the ab plane. According to Bohn et al, 19 propane has a Pcc value of 4.69 u˚ A2 . In propane, essentially all contribution to Pcc is due to the hydrogens, so we can attribute ∼0.4-0.6 u˚ A2 for each hydrogen. This means if water binds to a molecule with an OH fragment in the ab-plane, it will be noted by an increase of ∼0.4-0.6 u˚ A2 due to the outof-plane hydrogen contribution to Pcc . In the hydration of PPA, Pcc goes from 3.1813 u˚ A2 in PPA to 3.5485 u˚ A2 in H2 O-PPA and 4.01566 u˚ A2 in (H2 O)2 -PPA. 7 Similarly, in (H2 O)n TFA, 6 Pcc goes from 44.7350 u˚ A2 in TFA to 45.0321 u˚ A2 in H2 O-TFA to 45.5175 u˚ A2 in (H2 O)2 -TFA providing evidence toward the formation of a new ab-planar ring structure with each successive addition of a water molecule. We will now address how the addition of water molecules change the PFPA monomer structure. For this we will refer to Table 3 which gives a comparison of structural parameters of this study to that of the monomer. Figures 1, 2, and 6, 3 showing the ab initio structures of H2 O-PFPA, H2 O-PFPA, and PFPA, respectively, will be used as visual comparisons as 11 ACS Paragon Plus Environment


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they have been already shown to be very near their respective experimental structures. Examination of Table 3 shows quantitatively that there is little change in the Pcc value ˚2 vs. 124.12941(32) u˚ going from PFPA to H2 O-PFPA (122.39674(12) uA A2 ). In order to interpret this result and compare the monomer structure to that of the hydrate complexes, therefore, we must break the PFPA molecule up into its corresponding functional groups and look at each functional group’s contributions to the Pcc value. According to Bohn et al, 19 perfluorinated methyl groups with their carbon approximately in the ab-plane each ˚2 to Pcc . Since PFPA only contains 2 perfluorinated methyl groups (with contribute 45 uA their carbon in the ab-plane) and 1 carboxylic acid group, the remainder of ∼32-33 u˚ A2 represents the contribution of the carboxylic acid functional group in PFPA to Pcc . When the first water is added, Pcc only has the contribution of the two hydrogens of the water molecule being added to it because the oxygen is very close to the ab plane (-0.05 ˚ A for ˚2 to Pcc giving the c-coordinate in the ab initio calculations). This adds approximately 1 uA ≈ 124 u˚ A2 , very close to the measured value and reinforcing the accuracy of the ab initio structure. To confirm this structure, the authors are currently attempting experiments on the D2 O-PFPA species. Table 3: Structural Comparisons for (H2 O)n -PFPA vs. PFPA. Parameter A / MHz B / MHz C / MHz Paa / u˚ A2 Pbb / u˚ A2 Pcc / u˚ A2 ∆ / u˚ A2 κ / Unitless

H2 O-PFPA 1738.31182(77)b 676.51447(39) 640.12175(36) 622.90414(32) 166.60037(32) 124.12941(32) -248.25882(63) -0.9337

(H2 O)2 -PFPA 1414.176(82) 459.55736(90) 438.45625(87) 947.487(11) 205.145(11) 152.221(11) -304.442(21) -0.9567

PFPAa 1893.5299(4) 1175.7031(4) 1118.2017(4) 307.45584(12)c 144.50109(12)c 122.39674(12)c -244.79347(23)c -0.8517c

a

Values in column from Ref. 3 unless otherwise stated. Numbers in parentheses give standard errors (1σ, 67% confidence level) in units of the least significant figure. c Value derived from Ref. 3.

b

To interpret the second moment structural parameters for (H2 O)2 -PFPA, more caution


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Figure 6: The ab initio structure of PFPA in the ab- and bc-planes.



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must be used as the addition of the second water molecule has caused the carbonyl group to be almost completely aligned with the b-axis as opposed to being closer to the c-axis as in the monohydrate and PFPA monomer species. The interaction of the water molecules with the carboxylic acid group of PFPA now lies in more with the ab plane rather than the ac plane. Pbb (not Pcc ), therefore, of the PFPA monomer is used as a starting point for the computation of the Pcc value in the trimer complex. This was measured to be 144.50109(12) ˚2 . 3 According to the ab initio structure, the oxygen on one of the water molecules has uA a very small c-coordinate position so it is ignored. The carboxylic acid group, because it is now very close to the ab-plane, can also be considered to have negligible contribution to Pcc . The second water molecule’s oxygen atom has a c-coordinate of ≈0.5 ˚ A and each hydrogen ˚2 which is close contributes 0.5 u˚ A2 to Pcc . This gives a Pcc value of 144.5+4+2=150.5 uA ˚2 given all the assumptions made. This also shows to the measured value of 152.221(11) uA that the water molecules in the experimental structure are very close to the ab-plane and join together with the carboxylic acid group of the PFPA molecule in a ring-like fashion with one hydrogen from each water molecule positioned out of the plane of the ring as shown by the ab initio structure. This is in agreement with that shown for the (H2 O)2 -TFA 6 species. Finally, structural comparisons across the family of studied perfluorinated carboxylic acids were made. Table 4 presents these comparisons. As mentioned above, TFA hydration into the form of a ring can be rationalized by one hydrogen atom on each water molecule sticking out of the ab-plane. To understand the trends, we will use a similar approach as was used for the hydration of the PFPA monomer above. Going from H2 O-TFA to H2 O-PFPA, there is a change in Pcc from 45.0321 u˚ A2 to 124.12941 u˚ A2 . The value of 45.0321 u˚ A2 can be understood by the fact that the molecular species is ab-planar, the perfluorinated methyl group contributes ≈45 u˚ A2 , and there is a small addition due to the out of plane hydrogen. There is no contribution from the carboxylic acid group -COOH because it lies in the ab plane. The second water molecule adds in the same ab plane as the first and, again, only adds the 1 hydrogen out of plane contributing ≈0.5 u˚ A2 to Pcc . The experimental Pcc


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confirms this with a measurement of 45.5175 u˚ A2 . However, when the complex is formed with PFPA, the carboxylic acid group is tilted out of the ab plane and is now more in the ˚2 above. The extra perfluorinated ac-plane. This contribution was calculated as ∼32-33 uA methyl contributes another ∼45 u˚ A2 to Pcc . The sum of these is 123.03 u˚ A2 very close to the measured value and showing that a similar structure is maintained between H2 O-PFPA and H2 O-TFA. When going from H2 O-PFPA to H2 O-PFBA, the molecular interaction becomes more ab-planar again giving Pcc of the perfluorinated methyls accounting for 135 u˚ A2 and the remainder of the 168.48539 u˚ A2 from the water and carboxylic acid being slightly out of the ab-plane, but, again, forming a planar ring-type structure. For the dihydrate species, it is more useful to move to a comparison of Pbb . The reasoning for this is that the water molecules have been shown to add in the ab plane for (H2 O)2 -TFA and, if (H2 O)2 -PFPA is a similar ring structure mostly in the ab plane as shown by the ab initio structure, then Pbb will provide the most straightforward derivation from one species to another. This is because the contribution by the interaction of the water molecules with the carboxylic acid group in both species should be similar as the carbon atom has a small ˚2 b-coordinate in both ab initio structures. 6 Starting with the Pbb value of 153.94402(79) uA for (H2 O)2 -TFA, all that needs to be considered, then, is the effect of the substitution of one of the fluorine atoms by a second perfluorinated methyl group. However, one of the fluorine atoms from this perfluorinated methyl group lies on the a-axis and can be ignored. The remaining carbon and fluorine atoms have calculated b-coordinates of 0.13 ˚ A for C, 1.41 ˚ A for F1, and -0.63 ˚ A for F2. This gives a contribution of 0.20 u˚ A2 for C, 37.77 u˚ A2 for F1, and 7.54 u˚ A2 for F2. Summing the values, we arrive at 199.45 u˚ A2 for (H2 O)2 -PFPA compared with the experimental value of 205.145(11) u˚ A2 . Most of this error can be attributed to our assumption that the interaction of the water molecules with the carboxylic acid group occurred in the ab plane, but there is a slight error due to the small tilt of the carboxylic group with respect to the b-axis (see Figure 2).



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Table 4: Structural Comparisons Amongst the Family of Previously Studied Perfluorinated Carboxylic Acid Hydrates.

Parameter A / MHz B / MHz C / MHz Paa / u˚ A2 Pbb / u˚ A2 Pcc / u˚ A2 ∆ / u˚ A2 κ / Unitless

6,a

H2 O-TFA 3835.107(76)c 1082.58147(81) 993.78411(77) 421.7953(13)d 86.7447(13)d 45.0321(13) -90.0646(27)d -0.9375d

Monohydrates H2 O-PFPAb 1738.31182(77) 676.51447(39) 640.12175(36) 622.90414(32) 166.60037(32) 124.12941(32) -248.25882(63) -0.9337

2,a

H2 O-PFBA 1310.1124(1) 406.23021(6) 390.90242(5) 1075.58501(12)d 217.26701(12)d 168.48539(12)d -336.97079(25)d -0.9667d

Dihydrates (H2 O)2 -TFA 6,a (H2 O)2 -PFPAb 2533.7133(189) 1414.176(82) 718.45639(36) 459.55736(90) 622.50323(32) 438.45625(87) 657.9056(79)d 947.487(11) d 153.94402(79) 205.145(11) 45.5175(79) 152.221(11) -91.0355(16)d -304.442(21) -0.8996d -0.9567

a

Value in column reported directly from reference unless otherwise stated. This Work. c Numbers in parentheses give standard errors (1σ, 67% confidence level) in units of the least significant figure. d Value derived from reference values. b

Conclusions The rotational spectra have been observed and reported for the mono- and dihydrate complexes of PFPA for the first time using CP-FTMW spectroscopy. The transitions were predominately a-type wtih some b-type transitions reported for the monohydrated species. No tunneling was observed in the spectra for either species. Experimental structural parameters have been determined and reported and are compared to those of ab initio quantum chemical calculations as well as the PFPA monomer and the family of perfluorinated carboxylic acids. Structural arguments based on second moments suggest that the hydrogen bonding -OH of the water molecule(s) complex in a planar, ring-type fashion to the carboxylic acid functional group similar to those found in the trifluoroacetic acid-water complexes. 6

Acknowledgement The authors thank Wallace “Pete” Pringle, Herb Pickett, and Robert Bohn for many useful and insightful discussions. GSGII gratefully acknowledges financial support through the University of Missouri Research Board. W. L. gratefully acknowledges financial support 16 ACS Paragon Plus Environment

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from the Welch Foundation. SEN acknowledges financial support from the National Science Foundation (CHE-1011214). The authors also acknowledge NSF Grant # CNS-0619508, which supports the cluster at Wesleyan.

Supporting Information Available All assignments, fits, and input files for SPFIT can be found in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org/.

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of Dihydrogen Complexation to Transition Metals, a Gas Phase Case Study: H2 -CuF. Inorg. Chem. 2013, 52, 816–822. (16) Grubbs, G. S., II; Obenchain, D. A.; Pickett, H. M.; Novick, S. E. H2 -AgCl: A Spectroscopic Study of a Dihydrogen Complex. J. Chem. Phys. 2014, 141, 114306–1–114306– 10. (17) Grubbs, G. S., II; Obenchain, D. A.; Pickett, H. M.; Novick, S. E. Erratum:“H2 -AgCl: A Spectroscopic Study of a Dihydrogen Complex” [J. Chem. Phys. 141, 114306 (2014)]. J. Chem. Phys. 2015, 143, 029901–1–029901–2. (18) Gordy, W.; Cook, R. L. Microwave Molecular Spectra; Techniques of Chemistry Vol. XVIII ; Wiley: New York, 1984. (19) Bohn, R. K.; Montgomery, J. A., Jr.; Michels, H. H.; Byrd, J. Second (Planar) Moments and Their Applications in Spectroscopy. 68th Ohio State University International Symposium on Molecular Spectroscopy 2013, WH03, Columbus, OH.



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Graphical TOC Entry

Frequency (MHz)

Frequency (MHz)


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A Study of the Monohydrate and Dihydrate Complexes of

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