Article pubs.acs.org/JPCA
Matrix Isolation Infrared Spectroscopy of an O−H···π HydrogenBonded Complex between Formic Acid and Benzene Pujarini Banerjee,* Indrani Bhattacharya, and Tapas Chakraborty* Department of Physical Chemistry, Indian Association for the Cultivation of Science, Kolkata 700032, India S Supporting Information *
ABSTRACT: Mid-infrared spectra of an O−H···π hydrogen-bonded 1:1 complex between formic acid and benzene were measured by isolating the complex in an argon matrix at a temperature of 8 K. The O−H stretching fundamental of formic acid (νO−H) undergoes a red shift of 120 cm−1, which is the largest among the known π-hydrogen bonded complexes of an O−H donor with respect to benzene as acceptor. Electronic structure theory methods were used extensively to suggest a suitable geometry of the complex that is consistent with a recent study performed at CCSD(T)/CBS level by Zhao et al. (J. Chem. Theory Comput. 2009, 5, 2726−2733), as well as with the measured IR spectral shifts of the present study. It has been determined that density functional theory (DFT) D functionals as well as parametrized DFT functionals like M06-2X, in conjunction with modestly sized basis sets like 6-31G (d, p), are sufficient for correct predictions of the spectral shifts observed in our measurement and also for reproducing the value of the binding energy reported by Zhao et al. We also verified that these low-cost methods are sufficient in predicting the νO−H spectral shifts of an analogous O− H···π hydrogen-bonded complex between phenol and benzene. However, some inconsistencies with respect to shifts of νO−H arise when diffuse functions are included in the basis sets, and the origin of this anomaly is shown to lie in the predicted geometry of the complex. Natural bond orbital (NBO) and atoms-in-molecule (AIM) analyses were performed to correlate the spectral behavior of the complex with its geometric parameters.
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the prototypical water−Bz binary complex.2,4−8,21 A wide range of spectroscopic methods were employed to investigate various attributes of this species prepared either in a cold supersonic jet expansion5−8 or in inert gas matrixes.2 The binding energy of the complex predicted by calculation at CCSD(T) level of theory is ∼3.2 kcal/mol,21 which is somewhat smaller compared to that of the water dimer (5.02 kcal/mol), for calculations performed at the same level.28 The complex has been shown to prefer a T-shaped structure, where the O−H bond of water points toward the benzene π-electronic orbitals in a shallow intermolecular potential. In consequence, the O− H stretching fundamentals of water are split into multiple components, as revealed in an ion-dip infrared spectrum, and these were assigned as signatures of exchange between the isoenergetic structures.5 A larger analogue, the binary complex between phenol and Bz, also attracted considerable attention for spectroscopic studies. An obvious point of interest is to see how the OH···π interaction and spectral changes depend on the acidity of the O−H donors. The binding energy of the phenol− Bz binary complex is ∼5.5 kcal/mol at CCSD(T)/CBS level,17 and the spectral shift of the donor (ΔνO−H) is significantly larger (−78 cm−1) compared to that for the water−Bz complex (18−26 cm−1 for the ν1 mode and 18−29 cm−1 for the ν3 mode).2,9,23 A correlation study of the macroscopic acidity
INTRODUCTION Hydrogen bond, depicted conventionally as X−H···Y−Z, is an attractive noncovalent interaction between two molecules or between two segments of the same molecule mediated by a covalently bound electropositive hydrogen. In commonly encountered hydrogen bonds, X and Y are the atoms of electronegative elements, and the bond energies vary in the range of 5−15 kcal/mol.1 This energy range of intermolecular binding, under ambient conditions in liquids, allows exchange of the interacting partners with little hindrance, and this subtle trait of hydrogen bonding is considered to be a key factor for functioning of biological macromolecules in aqueous media of living cells. Several nonconventional variants of hydrogen bonds have been identified, where X or Y, or even both, are different from the conventional electronegative elements. A much-studied variant is the one where Y represents the filled πmolecular orbitals of alkene, alkyne, or arene moieties.1−24 Energetically, although the hydrogen bonds of this class are weaker, with binding energies less than 5 kcal/mol,1 they have been suggested to be significant for structures and stabilities of numerous organic molecular crystals, biological macromolecules, and industrially important polymeric materials.1,3,10,20,25−27 The present article is concerned with the structure and energetic and spectroscopic issues of a model binary complex between formic acid (FA) and benzene (Bz) stabilized by OH···π hydrogen bonding. To investigate the structural attributes of the OH···π hydrogen-bonded linkages, much attention has been paid to © XXXX American Chemical Society
Received: April 5, 2016 Revised: May 10, 2016
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DOI: 10.1021/acs.jpca.6b03447 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A parameter (pKa) of the phenolic O−H donor with ΔνO−H induced by Bz π-electrons was investigated recently using a series of different fluorophenol−Bz 1:1 complexes.23 Measurements revealed that ΔνO−H values display an almost linear correlation with the aqueous-phase pKa values of the phenol donors. Among the family of the model organic O−H donors, acidity as well as hydrogen-bonding ability of carboxylic acids are known to be the highest.29,30 The interactions of carboxylic acids with aromatic residues are important in determining the properties of different pigment and polymeric materials like resins.20 Therefore, it is of general interest to know in what terms the O−H···π interaction of carboxylic acid−Bz complexes could differ from those of water−Bz and phenol− Bz complexes. However, reports of spectroscopic as well as electronic structure studies on carboxylic acid−Bz complexes are rather scarce in literature. Recently, energetic issues of FA− Bz complex were addressed by Zhao et al. for the purpose of benchmarking its binding energy.20 Among the probable isomeric geometries, a T-shaped π-hydrogen-bonded structure was found to have the lowest energy, which for the complete basis set (CBS) limit at CCSD(T) level is 4.79 kcal/mol, and this binding energy is quite comparable with that of the water dimer (5.02 kcal/mol),28 but somewhat larger compared to that of the water−Bz complex (3.16 kcal/mol).21 In addition to the energetic issue, the other major concern is the structure of this complex. Unlike water and phenols, the carboxylic acid functional group (−COOH) has a carbonyl (CO) segment, which is a strong hydrogen bond acceptor. Thus, new interaction channels could open between lone pair, π, and π* molecular orbitals (MOs) of this segment with different π and π* MOs of Bz moiety. Second, a weak C−H···O hydrogen-bonding interaction between CO of FA and C−H bonds of Bz could also occur. We show in the subsequent discussion that these additional orbital overlaps between the CO group and Bz as well as weak C−H···O interactions are responsible for disagreements between measured and calculated νO−H frequencies when higher basis sets are used. Therefore, accurate information on some key experimental parameters like vibrational frequencies of the modes involving atoms and groups at the binding sites of the complex are important to corroborate the predictions of theory. To this objective, we report here an infrared spectroscopic characterization of the complex for the first time by trapping it in an argon matrix at a temperature below 10 K. The measured spectral shifts of the O−H as well as CO and C−O subgroups of the carboxyl functional were used as probes to understand the nature of interaction between the two molecules and also to validate the structure predicted by electronic structure theories.
partial pressures of FA and Bz vapors in the gas-mixing cell were monitored using a high-pressure capacitive diaphragm gauge (model CMR 361, Pfeiffer Vacuum). Care was taken to avoid dimerization of FA molecules in the initially deposited matrix, and this was done by maintaining a very low partial pressure of FA in the matrix, that is, a mixing ratio of ∼1:2000, and by carrying out a slow and uniform deposition (at
