A Rotational Study of Formic Acid–Pyridine - ACS Publications

Article pubs.acs.org/JPCA

Interactions between Carboxylic Acids and Heteroaromatics: A Rotational Study of Formic Acid−Pyridine Lorenzo Spada,† Qian Gou,†,‡ Barbara M. Giuliano,† and Walther Caminati*,† †

Dipartimento di Chimica, “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy College of Chemistry and Chemical Engineering, Chongqing University, Daxuechengnan Road 55, Chongqing, 401331, China

‡

S Supporting Information *

ABSTRACT: The rotational spectra of four isotopologues of the 1:1 complex between formic acid and pyridine show that the two units are linked together through a “classical” (OH···N) and a weak (CH···O) hydrogen bond. The molecular system appears quite rigid and no effects of the internal motions have been observed in the spectrum. The dissociation energy obtained from the centrifugal distortion by applying an approximate model, 39.8 kJ/mol, is quite similar to the ab initio value, 41.7 kJ/mol. Its relatively high value suggests a small charge transfer to take place.

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INTRODUCTION Several molecular complexes involving carboxylic acids have been investigated by rotational spectroscopy,1−24 to understand the nature of their noncovalent interactions and to have information on their internal dynamics and on their conformational equilibria. Most attention has been paid to their dimers1−14 and adducts with water.15−21 Plenty of data have been obtained on the dimers, concerning proton tunneling, the Ubbelohde effect, and conformational equilibria. The adducts of formic acid have been investigated mainly: apart from its hydrated adducts and dimers with carboxylic acids, the complexes with partner molecules containing various functional groups, like anhydrides,22 aldehydes,23 and amides24 have been characterized, indeed, by rotational spectroscopy. In this paper we characterize the intermolecular interactions that link the prototype carboxylic acid, formic acid (HCOOH), to the prototype aromatic azine, pyridine (PYR).

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frequencies of the two Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz, resolution is better than 7 kHz.

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RESULTS AND DISCUSSIONS

a. Theoretical Calculations. The results of B3LYP/6-311+ +G(d,p) theoretical calculations on HCOOH-PYR have been already reported.28 The authors found five stable species; two of them, shown in Figure 1, have binding energies much higher than those of the remaining ones. Both conformers are characterized by a conventional hydrogen bond O−H···N (which in ref 28 is depicted as nN → σ*O−H electronic transfer) and by a weak C−H···O (for the planar conformation P) or C−H···π (for the orthogonal conformation O) hydrogen bond. The reported binding energy (De) of species P is 46.4 kJ/mol, much higher than that of

EXPERIMENTAL SECTION

Commercial samples of HCOOH, DCOOH, PYR, and (15N)PYR were obtained from Aldrich and used without further purification. HCOOD was prepared by mixing HCOOH with D2O in excess. The rotational spectra in the 6−18.5 GHz frequency region were measured with a COBRA-type25 pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer,26 described elsewhere.27 Helium at a total pressure of 0.6 MPa was streamed over PYR and then over HCOOH, with both samples at 273 K. The obtained mixture was expanded through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry−Pérot-type cavity. Due to the coaxial arrangement of the molecular beam expansion and the resonator axis, each rotational transition displays an enhanced Doppler splitting. The rest frequency was calculated as the arithmetic mean of the © XXXX American Chemical Society

Figure 1. Two most stable conformers of HCOOH-PYR (from ref 28). Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: January 13, 2016 Revised: February 12, 2016

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DOI: 10.1021/acs.jpca.6b00387 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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following the procedure described above. For these latter isotopologues a smaller number of transitions have been measured, so that quartic centrifugal distortion constants and χab quadrupole coupling constants (for DCOOH-PYR and HCOOD-PYR) have been fixed to the corresponding values of the parent species. No splittings due to the quadrupolar effect of the D nucleus have been observed. The obtained results are listed in Table 2. We report in Table 2 also the inertial defects, Δc, of the four isotopologues, which indicate a planar configuration of HCOOH-PYR. This is another proof, in addition to the comparable rotational and quadrupole coupling constants between the theoretical and experimental values, for the conformational assignment (P). The values of Δc are slightly smaller than zero, but the negative value can be due to an outof-plane butterfly-like motion of the two planar molecules with respect to each other. One can see, indeed, that the value of Δc, decreases more for the isotopologues with the substituted atom far away from the center of mass. c. Structural Information. From the rotational constants of the four isotopologues, it is possible to calculate the substitution coordinates, rs, of the three substituted atoms in the principal axes of the parent species.33 The atom numbering is given in Figure 3. The obtained data are summarized in Table 3 where the values of a partial r0 structure (Table 4) and of the ab initio (re) structure are also given for comparison. c-coordinates have been set to zero by symmetry. The most remarkable discrepancy is on the a coordinate of the acid hydrogen involved in hydrogen bonds, as usual for this kind of proton. The largest discrepancy between experimental and MP2/6311++G(d,p) rotational constants is up to ca. 20 MHz for A. Although r0 and re values have conceptually different meanings, it is quite common, for molecular adducts, to minimize the differences between experimental (r0) and ab initio (re) values of the rotational constants by adjusting the structural parameters which connect the two constituting molecules. In our case, given the planarity of the complex, three structural parameters are needed to specify the distance and orientation of one moiety with respect to the second one, for example, the N1−O12 distance (rN1O12 of Figure 3) and the N1−O12−C13 (α of Figure 3) and C3−N1−O12 “valence angles”. We have 12 rotational constants, which, due to the planarity of the complex, are reduced to 8 independent data. In principle, we could fit the three parameters, but their values are strongly correlated, so that we have chosen to fit only rN1O12 and α. The effective structure of the cluster (partial r0) has been derived by a linear square fit for the determination−from the experimental rotational constantsof these two parameters (Table 4) while keeping the remaining ones fixed to their ab initio values. The rotational constants of isotopologues HCOOH-PYR, DCOOH-PYR, and HCOOH-(15N)PYR were reproduced within a maximum discrepancy of 0.4 MHz. However, the differences of those ones relative to the deuterated species O− D···N, with the D atom inserted in the hydrogen bond, are about 10 times larger. Actually, to reproduce the rotational constants also for the HCOOD-PYR isotopologue, an increase of about 7 mÅ of the rN1O12 distance is required. This behavior is surprising, because generally an increase of the distance between the two heavy atoms involved in the hydrogen bond is observed upon deuteration (Ubbelohde effect34) for double minima potential describing the proton transfer, as in dimers of carboxylic acids.9−15 Vice versa, in the cases of complexes with

species O, 26.9 kJ/mol. It is thus plausible that the rotational spectrum can be observed only for species P. In ref 28, no data useful for the rotational investigation (apart from the binding energy) have been reported. For this reason, we ran MP2/6311++G(d,p)29 calculations to estimate the spectroscopic parameters needed for a preliminary calculation of the rotational spectrum. The results, including the zero-point dissociation energy (D0) corrected for the basis set superposition error,30 are summarized in Table 1 for species P. The Table 1. MP2/6-311++G(d,p) Spectroscopic Parameters and Dissociation Energy (D0) of the Most Stable Conformer of HCOOH-PYR A, B, C/MHz χaa, χbb − χcc, χab/MHz μa, μb/D D0/kJ mol−1

4013, 805, 671 −3.73, −2.12, −1.75 −4.16, 0.51 41.7

corresponding MP2/6-311++G(d,p) data for three additional high energy conformers, as well as the geometry of conformer P, are given in the Supporting Information. b. Rotational Spectrum. The μa component of the electric dipole moment is much larger than μb (Table 1). For this reason, our survey started with the search of the J = 6 ← 5 μa-Rband. The 606 ← 505 rotational transition was observed first, which appeared split into three 14N quadrupole component lines (Figure 2), separated according to the quadrupole coupling constants given in Table 1.

Figure 2. ΔF = +1 14N quadrupole component lines of the 606 ← 505 transition of the parent species of HCOOH-PYR.

Then, several other μa-type R-branch ΔF = +1 and some ΔF = 0 transitions were found and, later on, some weaker μb lines were measured. All transition frequencies (listed in the Supporting Information), have been fitted with Pickett’s SPFIT program,31 within the Ir-representation of Waltson’s S reduction.32 Later on, the rotational spectra of three more isotopologues, DCOOH-PYR, HCOOD-PYR, and HCOOH(15N)PYR) have been assigned, measured, and fitted by B

DOI: 10.1021/acs.jpca.6b00387 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 2. Experimental Spectroscopic Parameters of Four Isotopologues of HCOOH-PYR HCOOH-PYR A/MHz B/MHz C/MHz χaa/MHz χbb − χcc/MHz χab/MHz DJ/kHz DJK/kHz d1/kHz d2/kHz Δc/u Å2 σd/kHz Ne

4032.0599(7) 811.1656(3) 675.8181(2) −3.574(8) −2.11(1) −2.6(8) 0.096(1) 0.30(2) −0.018(1) −0.0023(8) −0.565 3.4 99

DCOOH-PYR

HCOOD-PYR

HCOOH-(15N)PYR

4029.371(4) 789.0255(2) 660.3307(1) −3.7(1) −2.13(4) −1.75b [0.096]c [0.30] [−0.018] [−0.0023] −0.592 3.1 43

4007.921(3) 806.9132(3) 672.1974(1) −3.5(1) −2.09(4) −1.74b [0.096] [0.30] [−0.018] [−0.0023] −0.575 3.4 39

4026.130(1) 811.1282(2) 675.6278(1)

a

[0.096] [0.30] [−0.018] [−0.0023] −0.568 1.3 22

a Error in parentheses in units of the last digit. bAb initio data. cData in brackets fixed to the corresponding values of the parent species. dRMS error of the fit. eNumber of fitted lines.

two heavy atoms involved in the hydrogen bond−called the reverse Ubbelohde effectis generally observed upon deuteration.35 However, in the case of HCOOH-PYR, plausibly a proton transfer takes place, and we are no longer dealing with a “normal” single hydrogen bond. This aspect is pointed out in an NMR investigation of adducts of carboxylic acids with (15N)PYR.36 The authors of this paper outline that the heteronuclear scalar 1H−15N coupling constants between the hydrogen bond proton and the 15N nucleus of pyridine show that the proton is remarkably shifted from the acid to pyridine. d. Information on the Dissociation Energy. The intermolecular stretching motion which leads to the dissociation is almost parallel to the a-axis of the complex. In this case, a pseudodiatomic approximation can be used to roughly estimate the force constant of the stretching mode leading to the dissociation through the equation:37

Figure 3. Sketch of the observed conformer of HCOOH-PYR, atom numbering, main structural parameters, and principal axes system.

Table 3. rs, r0, and re Coordinates of the Substituted Atoms in the Principal Axes System of the Parent Species N1 H15 H16

|a|b/Å |b|/Å |a|/Å |b|/Å |a|/Å |b|/Å

rs

re

r0a

0.170(9)c 0.432(3) 1.812(1) 0.872(2) 4.184(1) 0.294(5)

0.254 0.425 1.416 0.856 4.163 0.357

0.253 0.424 1.367 0.807 4.165 0.345

ks = 16π 4(μD R CM)2 [4B4 + 4C 4 − (B − C)2 × (B + C)2 ]/(hDJ )

where μD is the pseudodiatomic reduced mass, RCM (=4.26 Å) is the distance between the centers of the mass of the two subunits, and B, C, and DJ are the spectroscopic parameters of Table 2. We obtained ks = 26.2 N m−1, corresponding to a harmonic stretch fundamental ν = 123 cm−1. Assuming a Lennard-Jones type potential the zero point dissociation energy of the complex can be estimated to be 39.8 kJ mol−1 by applying the approximate expression:38

a

Calculated from the partial r0 structure in Table 4. bCoordinate c set to zero by symmetry. cError in parentheses are in units of the last digits.

Table 4. Partial r0 Geometry (Compared to the Corresponding re Values) and Derived Hydrogen Bond Parameters N1−O12/Å 2.662(1) α/deg 111.4(1) Hydrogen Bond (Derived) Parameters N1−H15/Å 1.661 N1−H15−O12/deg 175.3 O14−H7/Å 2.460 C3−H7−O14/deg 132.0

E B = ksR CM 2/72

b

(2)

Such dissociation energy is quite higher than that of complexes with a single conventional hydrogen bond. It is then plausible, as mentioned above, that a considerable proton transfer of the carboxylic hydrogen to the PYR nitrogen takes place. This proton transfer makes it difficult to comment on the Ubbelohde effect. Probably the HCOOH unit acts as a resonance assisted protic wire (both a hydrogen-bond donor and acceptor), an effect described previously in 7-hydroxyquinoline−ammonia.39 The changes of the 14N quadrupole coupling constants in going from pyridine to its complexes have been used to evaluate the charge transfer upon complexation, for example, in the case of PYR-SO3,40 or directly the related proton transfer in the case of PYR-HCl.41 In these adducts the comparison of the

rea

r0

(1)

2.733 109.0 1.730 179.1 2.438 131.8

a MP2/6-311++G(d,p) values. bError in parentheses in units of the last digit.

the two subunits held together by single O−H···O or O−H···N single hydrogen bonds a shortening of the distance between the C

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quadrupole coupling constants of the complex with those of bare pyridine was easier because the inertial principal axes have the same orientation of the quadrupolar tensor. In our case only the direction of the c-axes of PYR and HCOOH-PYR coincides, so that an easy comparison is possible between χcc(PYR) and χcc(HCOOH-PYR). These values are 3.474(3)42 and 2.84(1) (from the data of Table 2) MHz, respectively. With χcc = eQ(∂2V/∂c2)c=0, one can deduce that the formation of the complex decreases the field gradient along c at the N nucleus. This would correspond to a depletion of the π electrons at N, according to the above-mentioned resonance assisted proton transfer, which would imply a further π-delocalization toward the seven-membered ring protic wire. Another interesting effect is the considerably higher stability (about 20 kJ/mol) of species P with respect to species O. Both adducts include one classical O−H···N hydrogen bond and one weaker (C−H···O or C−H···π) hydrogen bond, and at a first sight should not differ so much in energy. However, in species O the HCOOH unit looses its most stable configuration (with the hydroxyl hydrogen close to the carbonyl oxygen), which is 16.3(4) kJ/mol lower in energy than the form with the hydroxyl hydrogen trans to the carbonyl oxygen.43

AUTHOR INFORMATION

Corresponding Author

*Walther Caminati. Email: [email protected]. Tel: + 39-0512099480. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support.

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REFERENCES

(1) Costain, C. C.; Srivastava, G. P. Study of Hydrogen Bonding. The Microwave Rotational Spectrum of CF3COOH-HCOOH. J. Chem. Phys. 1961, 35, 1903−1904. (2) Bellott, E. M., Jr.; Wilson, E. B. Hydrogen Bonded Bimolecular Complexes of Carboxylic Acids in the Vapor Phase: Observation and Characterization by Low Resolution Microwave Spectroscopy. Tetrahedron 1975, 31, 2896−2898. (3) Martinache, L.; Kresa, W.; Wegener, M.; Vonmont, U.; Bauder, A. Microwave Spectra and Partial Substitution Structure of Carboxylic Acid Bimolecules. Chem. Phys. 1990, 148, 129−140. (4) Antolinez, S.; Dreizler, H.; Storm, V.; Sutter, D. H.; Alonso, J. L. The Microwave Spectrum of the Bimolecule Trifluoroacetic Acid··· Cyclopropanecarboxylic Acid. Z. Naturforsch., A: Phys. Sci. 1997, 52a, 803−806. (5) Daly, A. M.; Douglass, K. O.; Sarkozy, L. C.; Neill, J. L.; Muckle, M. T.; Zaleski, D. P.; Pate, B. H.; Kukolich, S. G. Microwave Measurements of Proton Tunneling and Structural Parameters for the Propiolic Acid-Formic Acid Dimer. J. Chem. Phys. 2011, 135, 154304. (6) Tayler, M. C. D.; Ouyang, B.; Howard, B. J. Unraveling the Spectroscopy of Coupled Intramolecular Tunneling Modes: A Study of Double Proton Transfer in the Formic-Acetic Acid Complex. J. Chem. Phys. 2011, 134, 054316. (7) Feng, G.; Favero, L. B.; Maris, A.; Vigorito, A.; Caminati, W.; Meyer, R. Proton Transfer in Homodimer of Carboxylic Acids: The Rotational Spectrum of the Dimer of Acrylic Acid. J. Am. Chem. Soc. 2012, 134, 19281−19286. (8) Evangelisti, L.; Ecija, P.; Cocinero, E. J.; Castaño, F.; Lesarri, A.; Caminati, W.; Meyer, R. Proton Tunneling in Heterodimers of Carboxylic Acids: A Rotational Study of the Benzoic Acid-Formic Acid Bi-Molecule. J. Phys. Chem. Lett. 2012, 3, 3770−3775. (9) Feng, G.; Gou, Q.; Evangelisti, L.; Xia, Z.; Caminati, W. Conformational Equilibria in Carboxylic Acid Bimolecules: a Rotational Study of Acrylic Acid-Formic Acid. Phys. Chem. Chem. Phys. 2013, 15, 2917−2922. (10) Gou, Q.; Feng, G.; Evangelisti, L.; Caminati, W. Conformational Equilibria in Bimolecules of Carboxylic Acids: A Rotational Study of Fluoroacetic Acid−Acrylic Acid. J. Phys. Chem. Lett. 2013, 4, 2838− 2842. (11) Gou, Q.; Feng, G.; Evangelisti, L.; Caminati, W. A Rotational Study of cis and trans Acrylic Acid-Trifluoroacetic Acid. J. Phys. Chem. A 2013, 117, 13500−13503. (12) Gou, Q.; Feng, G.; Evangelisti, L.; Caminati, W. Conformers of Dimers of Carboxylic Acids in the Gas Phase: A Rotational Study of Difluoroacetic Acid-Formic Acid. Chem. Phys. Lett. 2014, 591, 301− 305. (13) Feng, G.; Gou, Q.; Evangelisti, L.; Caminati, W. Frontiers in Rotational Spectroscopy: Shapes and Tunneling Dynamics of the Four Conformers of the Acrylic AcidDifluoroacetic Acid Adduct. Angew. Chem., Int. Ed. 2014, 53, 530−534.

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CONCLUSIONS The Fourier transform microwave spectra of four isotopologues of the 1:1 complex between HCOOH and PYR show that it is planar, with its two subunits held together through a conventional O−H···N hydrogen bond and a weak C−H···O hydrogen bond. The complex is characterized by dissociation energy smaller than those characterizing dimers of carboxylic acids, similar to that of formic acid−formamide (42.4 kJ·mol−1, obtained by applying eqs 1 and 2 to the data from ref 24) but considerably higher than those of complexes of formic acid with other functionalized molecules such as carbon anhydride (8.2 kJ·mol−1)22 or formaldehyde (19.2 kJ·mol−1).23 It is plausible that proton transfer takes place in considerable degree from the HCOOH to the nitrogen atom. One could consider the HCOOH unit as a resonance assisted protic wire (both a hydrogen-bond donor and acceptor), which favors the proton transfer process. It is interesting to observe a multivariate behavior in the adducts of carboxylic acids with organic compounds: from the double proton transfer of symmetric carboxylic acid dimers, to the conformational equilibria in dimers of asymmetric carboxylic acid dimers, to the apparent rigidity of adducts of HCOOH with CO2, HCOOH, and HCONH2, to the asymmetric proton transfer in HCOOH-PYR. In all these cases, the features embedded in the rotational spectra disclosed the way to the harvest of chemical properties difficult to achieve with other techniques.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.6b00387. (1) Completion of ref 29; (2) table with MP2/6-311+ +G(d,p) spectroscopic parameters of four different conformation of HCOOH-PYR; (3) table with optimized geometry of the most stable species of HCOOHPYR; (4) tables of transition frequencies. (PDF) D

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