Multiple Hydrogen Bond Tethers for Grazing Formic Acid in Its

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Multiple Hydrogen Bond Tethers for the Grazing Formic Acid in Its Complexes with Phenylacetylene Ginny Karir, Gaurav Kumar, Bishnu Prasad Kar, and Kodumudi S Viswanathan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b11428 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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The Journal of Physical Chemistry

Multiple Hydrogen Bond Tethers for the Grazing Formic Acid in its Complexes with Phenylacetylene

Ginny Karir*, Gaurav Kumar1, Bishnu Prasad Kar2 and K. S. Viswanathan* Department of Chemical Sciences, Indian Institute of Science Education and Research, Sector 81, Mohali 140306, Punjab, India

Corresponding Authors: Ginny Karir ([email protected]) K. S. Viswanathan ([email protected])

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ABSTRACT

Complexes of Phenylacetylene (PhAc) and Formic Acid (FA) present an interesting picture, where the two submolecules are tethered, sometimes multiply, by hydrogen bonds. The multiple tentacles adopted by PhAc-FA complexes stems from the fact that both submolecules can, in the same complex, serve as proton acceptors and/or proton donors. The acetylenic and phenyl π systems of PhAc, can serve as proton acceptors, while the ≡C-H or – C-H of the phenyl ring can act as a proton donors. Likewise, FA also is amphiprotic. Hence, more than 10 hydrogen bonded structures, involving O-H...π, C-H...π and C-H...O contacts, were indicated by our computations; some with multiple tentacles. Interestingly, in spite of the multiple contacts in the complexes, the barrier between some of the structures is small, and hence FA grazes around PhAc, even while being tethered to it, with hydrogen bonds. We used matrix isolation infrared spectroscopy, to experimentally study the PhAc-FA complexes, using which, we located global and a few local minima, involving primarily an O-H…π interaction. Experiments were corroborated by ab initio computations which were performed using MP2 and M06-2X methods, with 6-311++G (d,p) and aug-cc-pVDZ basis sets. Single point energy calculations were also done at MP2/CBS and CCSD(T)/CBS levels. The nature, strength and origin of these non-covalent interactions were studied using AIM, NBO and LMO-EDA analysis.


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1. INTRODUCTION Formic acid (FA)-the simplest organic carboxylic acid, is an interesting model system to study non-covalent interactions. Even though it is a small molecule, it presents multiple bonding sites for weak interactions and therefore lends sufficient complexity to make its study challenging. For instance, the OH group of FA can play the dual role of a proton donor as well as a proton acceptor, through the H or O, respectively. The C=O group provides a proton acceptor site on the O, while the C-H group can serve as a proton donor. In addition, FA exists in two isomeric forms, cis and trans, which adds to the complexity. 1 The study of FA has been of interest since it has been identified in interstellar medium2-4 and its aqueous reactions on aerosols and cloud droplets affect halogen activation and ozone concentrations.5,6 Hence understanding its bonding characteristics is imperative in understanding many of its functions. Consequently, a number of reports exist in the literature, where this molecule has been studied from the point of view of its weak non-covalent interactions.7-13 Techniques such as supersonic jet spectroscopy and matrix isolation have been employed to study non-covalent interactions.14-18 Using such techniques, FA has been studied for its interactions with precursors such as H2O, C2H2, C6H6, to name a few.19-21 Lisa et. al. performed matrix isolation experiments in Ar, to study the interactions of FA with C2H2, which is a nonaromatic π system.20 FA was shown to act as a bidentate hydrogen bonding ligand, forming two contacts. They observed three isomeric structures, having either an OH…π or a CH…π contact as a primary interaction, together with a C-H…O as a secondary interaction. Amongst all the complexes, a structure having an OH…π interaction was the global minimum, with an interaction energy of -4.9 kcal/mol. This complex showed an experimental red shift of 137 cm-1 in the OH stretch of FA, relative to that observed in uncomplexed FA. Recently Banerjee et. al. studied hydrogen bonded complexes of C6H6-FA



which involves an aromatic π electron system.21 They observed a T-shaped complex, having an OH…π interaction with the C6H6 ring along with a C-H…O interaction, involving C=O of FA and C-H of C6H6. The complex was evidenced by a red shift of 120 cm-1 in the O-H stretch of FA. Both these studies gave an insight into the dominant O-H…π interaction of FA. With an aim to deduce the competitive preference of FA towards a molecule having multifunctional π system, we studied hydrogen bonded complexes of FA with PhAc. A number of structures are possible for the PhAc-FA complexes, since both molecules have multiple bonding sites. While the multitude of structures is a theorists‟ delight, it can be an experimentalists‟ nightmare. It was also found interesting to compare this system with the FA complexes of C2H2 and C6H6, the two constituent moieties in PhAc. It was also interesting to explore whether PhAc-FA system shows floppy behaviour in its non-covalently bonded complexes. We had indicated the possibility of fluxional behaviour in our earlier study on the PhAc-HCl complex.22 The floppiness in weakly bonded systems such as in the C6H6 dimer and C6H6-H2O has been studied in the literature, as these systems served as benchmarks for studying the dynamics.23-25 2. EXPERIMENTAL DETAILS Matrix isolation experiments were performed at ~12 K employing a He compressor cooled cryostat, CH-202 W/HC 4E1 Model Sumitomo Heavy Industries Ltd. The cryostat was housed in a vacuum chamber where the base pressure was typically 1 x 10-6 torr. The details of the experiments are described elsewhere.26-29 In particular, the present set of experiments was performed using N2 and Ar (Grade –I, Sigma Gases and Services, 99.999%) as matrix gases. Phenylacetylene (Sigma Aldrich, 98%), Phenylacetylene-D (Sigma Aldrich, 99% Atom-D), Formic-Acid (Merck,100% GR) were used after subjecting them to several freeze-pump-thaw cycles, before preparing the matrix gas/sample mixture.


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Sample to matrix gas ratios were varied from 0.5:1000 to 3:1000 using manometric procedures, for each of the two precursors. Only low concentrations of FA and PhAc were used in all the experiments, to minimize self-aggregation or formation of complexes beyond 1:1 stoichiometry. Typical deposition rates of ~3 mmol/hr of the matrix gas were employed using a flow-control needle valve (Model: EVN 116, Pfeiffer Vacuum). The spectra of the matrix isolated species, were recorded in the region 4000-400 cm-1 using a Bruker Tensor 27 FTIR spectrometer, operating at a resolution of 0.5 cm -1. The matrix was annealed at 27 K for N2 or 32 K for Ar, typically for 1 hr, and the spectrum again recorded after recooling the matrix to 12 K. Annealing was performed to encourage diffusion of the precursors and allow for complex formation which was identified by the observation of new product bands. Experiments were performed in both Ar and N2 matrices, both of which showed evidence for the formation of the PhAc-FA complexes. However, since the spectra were well resolved in the Ar matrix, we show the spectra obtained in this matrix. 3. COMPUTATIONAL DETAILS The computational study was carried out using Gaussian-09 suite of programs.30 All computations were performed using M06-2X and MP2 methods, employing 6-311++G(d,p) and aug-cc-pVDZ basis sets. At the outset, geometry optimization of the monomers was done. It must be noted that the tFA is more stable than cis-FA (cFA) by ~4 kcal/mol. Consequently, the population of the cis isomer would be insignificant in the matrix to be discerned. Hence our discussion will be focused on the complexes involving tFA. Using the optimized monomer geometries, the structure of the PhAc-tFA complexes were then optimized (Fig. 1-3). Frequency calculations were performed at the same level of theory as that used for geometry optimization. Vibrational frequency computations were done to ensure that the computed structures corresponded to minima on the potential surface and also to assign the vibrational features observed in the experiments. For the M06-2X calculations, the



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2.43Å

T4 (O-H...πPh) (-4.45)

2.23Å 2.29Å

2.77Å

T3 (O-H...πAc) (-5.44) 2.64Å 2.31Å

2.19Å

2.86Å

T2 (O-H...πAc) (-5.53)

2.54Å 2.23Å

2.34Å

T1 (O-H...πAc) (-5.63)

Figure 1. Optimized geometries of PhAc-tFA complexes of Group 1 manifesting an O-H...π interaction. Geometry optimizations were done at MP2/aug-cc-pVDZ level of theory. Interaction energies computed at CCSD (T)/CBS limit (kcal/mol) are given in parenthesis (See text for details).


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2.64Å

2.47Å

T8 (C-H...πAc) (-2.51)

2.62Å

T7 (C-H...πPh) (-3.19) 2.65Å

2.36Å T6 (C-H...πAc) (-3.25)

2.85Å

T5 (C-H...πPh) (-3.36)

Figure 2.Optimized geometries of PhAc-tFA complexes of Group 2 manifesting a C-H...π interaction. Geometry optimizations were done at MP2/aug-cc-pVDZ level of theory. Interaction energies computed at CCSD(T)/CBS limit (kcal/mol) are given in parenthesis (See text for details).



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2.26Å T10 (C-HAc...OH) (-1.57)

2.16Å T9 (C-HAc...O=C) (-3.07)

Figure 3. Optimized geometries of PhAc-tFA complexes of Group 3 manifesting a C-H...O interaction. Geometry optimizations were done at MP2/aug-cc-pVDZ level of theory. Interaction energies computed at CCSD(T)/CBS limit (kcal/mol) are given in parenthesis. (See text for details).


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optimization and harmonic frequency calculations were done using opt=tight and int=ultrafine keywords.31 The computed frequencies of the complexes, PhAc-tFA were scaled by using appropriate scaling factors to enable a comparison with experimentally observed features. The scaling factors were calculated by comparing the computed frequencies of monomeric PhAc and tFA with the observed experimental frequencies, in a given matrix, to bring the two in agreement. For a better comparison, we have used three different scaling factors in this work; one applicable for the region 3800-3000 cm-1 derived by using OH stretch of tFA for comparison, another for the region 3000-2500 cm-1 obtained by comparing the computed and experimental wavenumber of the ≡CD stretch of PhAcD . A third scaling factor was used for spectral region ranging from 2500-1000 cm-1, and it was based on C-O stretch of tFA. These scaling factors were then used to scale the computed features of all the complexes. The scaling factors have been mentioned in the table listing the experimental and computed wavenumbers. Vibrational spectra were simulated using the scaled wavenumbers and the intensity obtained from the vibrational analysis implemented through Gaussian. The interaction energies of the complexes were calculated by subtracting the sum of energies of monomers, from the energy of the complexes, obtained at the same level of theory. These interaction energies were corrected separately for basis set superposition error (BSSE), using counterpoise method proposed by Boys and Bernardi32 and zero point energies (ZPE).

Single

point

calculations

at

MP2/aug-cc-pVTZ,

MP2/aug-cc-pVQZ,

and

CCSD(T)/aug-cc-pVDZ were performed using optimized geometries at MP2/aug-cc-pVDZ level. Interaction energies at MP2/CBS limit were calculated using two point extrapolation method by Helgaker et al.33, which employs the MP2/aug-cc-pVXZ level where X represents triple and quadruple zeta basis set , given by the equation, EMP

E (p Q )

E (p T )

C S



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To evaluate interaction energies at a higher basis set, we calculated interaction energies at CCSD(T)/CBS limit as well using the following equation ECCSD T

C S

EMP

C S

ECCSD T

aug cc p D

EMP

aug cc p D

An examination of electron density topology was performed using the atoms-inmolecules (AIM) analysis implemented through AIM2000,34 to identify and characterize the nature of hydrogen bonding interactions in the complexes. Estimation of interaction energies corresponding to the hydrogen bond in question, were also performed using topological parameters, such as electron density, ρ rc) and Laplacian,

ρ rc , as proposed by Espinosa et

al.35 Natural Bond Orbital (NBO) analysis36 and Localized Molecular Orbital Energy Decompoistion Analysis (LMO-EDA)37, implemented using Gaussian-09 and GAMESS,38 respectively, were also performed to understand the nature of bonding in these non-covalent interactions. 4. RESULTS 4.1 EXPERIMENTAL The OH stretch of tFA appears as a doublet at 3550.7cm-1 and 3548.6 cm-1 (Fig. 4). The doublet is due to site splitting in the Ar matrix. The ≡C-H stretch of PhAc appears as a multiplet in Ar matrix due to Fermi resonance and site splitting.39 Codeposition of PhAc and tFA, in an Ar matrix, at 12 K, followed by annealing at 32 K for 1 hr, resulted in the appearance of two strong product features in the O-H stretching region of tFA, at 3375.1 and 3368.1 cm-1. In addition, two weak features at 3439.7 and 3394.9 cm-1 were also observed. In the C-O stretching region of tFA, a strong product feature was observed at 1131.3 cm-1, in addition to weak features at 1123.4, 1120.2 cm-1, as shown in Fig. 5 (Block A). The C-O stretch in uncomplexed FA occurs at 1103.6 cm-1. In the C=O stretching region of FA, product features were observed at 1751.8 and 1758.2 cm-1 , while the corresponding feature in uncomplexed FA occurs at 1767.8 cm-1 (Fig. 5, Block B). In the ≡CH stretching of PhAc, weak product features were observed at 3306.6, 10 ACS Paragon Plus Environment

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Figure 4. Spectra of PhAc-tFA complexes in an Ar matrix in the OH stretching region of FA (3575-3350 cm-1) (a) PhAc:Ar (3:1000) (b) FA:Ar (0.6:1000) (c) PhAc:FA:Ar(3:0.6:1000) (as deposited) (d) PhAc:FA:Ar (3:0.6:1000) (e) PhAc:FA:Ar(3:0.9:1000). All spectra, except that shown in trace (c), were recorded after annealing the matrix at 3 K. Spectrum in trace „c‟ was recorded after depositing the matrix before annealing .



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Figure 5. Spectra of PhAc-tFA complexes in an Ar matrix in (A) C-O stretching region of FA (1150-1075 cm-1) (B) C=O stretching region of FA (1700-1800 cm-1). Features marked with an asterisk are due to FA dimers. (a) PhAc:Ar (3:1000) (b) FA:Ar (0.6:1000) (c) PhAc:FA:Ar (3:0.6:1000) (as deposited) (d) PhAc:FA:Ar (3:0.6:1000) (e)PhAc:FA:Ar (3:0.9:1000). All spectra, except that shown in trace (c), were recorded after annealing the matrix at K. Spectrum in trace „c‟ was recorded after depositing the matrix before annealing .


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Figure 6. Spectra of PhAc-tFA complexes in an Ar matrix in the ≡CH stretching region of PhAc 60-3280 cm-1) (a) FA:Ar (0.6:1000) (b) PhAc:Ar (3:1000) (c) PhAc:FA:Ar (3:0.6:1000) (as deposited) (d) PhAc:FA:Ar (3:0.6:1000)(e) PhAc:FA:Ar (3:0.9:1000). All spectra, except that shown in trace (c), were recorded after annealing the matrix at 32 K. Spectrum in trace „c‟ was recorded after depositing the matrix before annealing .



3304.7 and 3290.8 cm-1 (Fig, 6). All the features mentioned above were assigned to the PhAc-tFA complexes, as these features were observed only when both precursors were present. On increasing the concentration of either monomer, the intensity of the product bands increased, confirming them to be features due to the PhAc-tFA complexes. As these features were seen even at very low concentrations of both monomers, they were considered to be due to a 1:1 complex of PhAc-tFA. Experiments with isotopic PhAcD (acetylenic hydrogen substituted by deuterium) were also performed to observe isotopic shifts. The feature observed at 2586.1 cm-1 in ≡C-D stretching of PhAcD corresponds to PhAcD-tFA adduct (Fig.7). It should be noted that cis-FA (cFA) was not observed in the matrix due to the large energy difference (~4 kcal/mol) between cis and trans FA isomers. Hence only complexes of PhAc-tFA were observed in the matrix. It is interesting to note that some of the complexes of PhAc-cFA had the largest interaction energies (Fig. S1 in Supporting Information), but the higher energy of the cis isomer of FA rendered these complexes inaccessible to experiments. Fig.8 shows the correlation between the energy asymptote of the monomers and the energy of the complexes, which clearly indicates that the complexes involving the cFA were indeed placed well above the energies of the complexes involving the tFA. It may be worthwhile mentioning that while the cFA has insignificant population in the matrix, owing to its higher energy relative to tFA, experiments have been performed where by irradiating the tFA with a narrow band IR radiation, the cis isomer was populated.40 The cFA isomer was shown to have a lifetime of ~10 min at the cryogenic temperatures of the matrix; the lifetime being limited by a hydrogen tunnelling mechanism. Similarly, when the tFA-H2O complex was irradiated, the cFA-H2O was produced, which involved the conversion of the tFA to the cis isomer, followed by an extensive rearrangement of the matrix, to allow for the relocation of water. Furthermore, when the cFA was complexed with


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water, its lifetime was found to be at least 4-5 orders of magnitude higher than that of the cFA monomer, with the tunnelling mechanism being efficiently aborted, in the water complex.41 A similar process may be envisaged in the PhAc-FA complex, where the complex of the cFA not observed in our experiments may be produced by irradiation. However, the efficiency of the process may be expected to be somewhat less than that observed for FAH2O, since relocation of the PhAc to follow the trans-cis isomerization may require a more extensive rearrangement of the matrix than with the water complex. These are interesting experiments, which may be performed in future. 4.2 COMPUTATIONAL Several hydrogen bonded structures were indicated as minima by our calculations. The structures calculated at MP2/aug-cc-pVDZ level of theory are shown in Fig. 1-3, with Table 1 listing the important geometrical parameters. Table 2 summarizes the interaction energies of the PhAc-tFA complexes at various levels of theory. On the basis of primary interaction in the complex, we can classify the various structures, into three groups. Group 1 (O-H…π consists of structures having an O-H…π contact as the primary interaction, involving the O-H of tFA as the proton donor (Fig. 1). In some cases, as will be explained below, the O-H…π interaction, involved the π system of the acetylenic group as the proton acceptor, which is denoted as an O-H…πAc interaction. Similarly, O-H…πPh denotes an O-H…π interaction which involved the phenyl π system as the proton acceptor. In addition to this primary interaction, some structures also show a secondary n-σ* interaction. Structures T1, T2 and T3 belong to the O-H…πAc category, while T4 shows an O-H…πPh interaction. Group 2 (C-H…π

consists of structures, dominated by a C-H…π contact between

the C-H moiety of tFA and acetylenic phenyl π cloud of PhAc, as observed in complexes T5, T6, T7 and T8 (Fig. 2). In complex T5, C-H of tFA points towards C4 of phenyl ring, whereas complex T7 involves C-H pointing towards the C1 of PhAc. Structure T6 shows two 15 ACS Paragon Plus Environment


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Figure 7. Spectra of PhAcD-tFA complexes in an Ar matrix in the ≡CD stretching region of PhAcD (2625-2575 cm-1) (a) FA:Ar (0.6:1000) (b) PhAcD:Ar (3:1000) (c) PhAcD:FA:Ar(1:0.6:1000) (as deposited). (d) PhAcD:FA:Ar (1:0.6:1000) (e) PhAcD:FA:Ar (3:0.6:1000). All spectra, except that shown in trace (c), were recorded after annealing the matrix at K. Spectrum in trace „c‟ was recorded after depositing the matrix (before annealing).


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Figure 8. Correlation diagram showing uncorrected energies of the PhAc-FA complexes involving

both cis and trans FA, computed at MP2/aug-cc-pVDZ level. The asymptote corresponds to the sum of the energies of the monomers. C1, C2 and C3 correspond to PhAc-cFA complexes, while T1 to T10 correspond to PhAc-tFA complexes.



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1 2 3 4 5 6 Table 1. Important Geometrical Parameters, Bond Lengths (Å), Bond Angles (˚), Dihedral Angles (˚), for 7 Phac-tFA Complexes Computed at the MP2/aug-cc-pVDZ Level. Labelling of Atoms is shown in Fig. 1, 2 8 and 3. 9 10 parameter T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 11 C13-H14 1.07 1.08 1.08 1.07 1.07 1.07 1.07 12 13 O16-H15 0.98 0.98 0.98 0.98 0.97 14 O18-H7 2.54 15 C12-H15 2.23 2.31 2.29 16 C12-H15-O16 155.06 159.0 163.70 17 7 C6-H7-O18 118.29 18 O16-H15-C12-C1 21.60 0.03 0.01 19 C -C -H -O -75.97 1 6 7 18 20 O18-H14 2.86 2.77 21 H7-O16 2.64 -22 107.7 107.35 C13-H14-O18 23 1 24 141.1 C6-H7-O16 81.03 25 7 H15-O16-H7-C6 0.00 --26 C17-O18-H14-C13 0.00 -0.00 27 C -O -H -C 0.03 17 16 15 13 28 H15-C3 2.43 29 C 172.19 3-H15-O16 30 C3-H15-O16-C17 -16.66 31 H -C 2.85 19 4 32 C17-H19-C4 140.56 33 O18-C17-H19-C4 -79.35 34 35 H19-C1 2.62 36 H19-C1-C17 152.77 37 H11-C2-C1-H19 -110.11 38 H19-C13 2.65 2.64 39 O18-H11 2.36 40 H 0.00 19-C17-O18-H11 41 C -H -O 176.89 2 11 18 42 169.4 C13-H19-C17 173.78 43 4 -180.00 44 O16-C17-O18-H11 C17-H19 1.10 45 46 2.47 16-H11 176.0 CO 2-H11-O16 47 9 H11-O16-C17-H19 0.00 48 2.16 O18-H14 49 0.00 C -O -H -C 17 18 14 13 50 146.76 C13-H14-O18 51 2.26 H14-O16 52 175.94 53 C13-H14-O16 54 C13-H14-O16-C17 -19.43 55 56 57 58 59 60


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contacts simultaneously - a C-H…πAc contact with acetylenic π cloud of PhAc and an n-σ* interaction involving C=O of tFA and C-H of phenyl ring. Structure T8 resembles T6 except that the secondary n-σ* interaction involves the OH of tFA as proton acceptor. It may be relevant to note that such H…π interactions play an important role in many supramolecular structures.42,43 We also obtained complexes which belonged to Group 3, i.e. T9 and T10, having nσ* interaction as the only contact between tFA and PhAc. In both the complexes, PhAc acts as proton donor through ≡C-H group. In T9 the proton acceptor is the oxygen of C=O group of tFA, while in complex T10, O-H group of tFA is the proton acceptor. Of all these structures, (restricted to tFA) the complexes belonging to group 1 have the largest BSSE corrected interaction energies, with T1 as a global minimum followed by T2, T3 and T4 as local minima. It must however be said that all the three structures (T1, T2 &T3) are nearly isoenergetic, with energy differences between them being as small as ~0.2 kcal/mol. All the three structures vary in the orientation of tFA with respect to the plane of PhAc. In T1, tFA is at an angle of ~7

with the plane of PhAc, manifesting two interactions

an OH…πAc, with a secondary interaction n-σ* interaction, between carbonyl of tFA and a hydrogen of the phenyl ring . Complex T2 has a three point contact, as shown in the Fig.1, where PhAc and tFA are coplanar. In this case, OH…πAc is the primary interaction, accompanied by two n-σ* secondary interactions. Complex T3 is similar to T2, the difference being in the orientation of the FA relative to PhAc. The perpendicular orientation allows only two hydrogen bonded contacts - an OH…πAc and a ≡C-H…O꞊C (n-σ* . It must be mentioned that when uncorrected or only ZPE corrected interaction energies are considered at the MP2/aug-cc-pVDZ level, T4 which is an O-H…πPh complex, shows the largest interaction energy followed by structures T1, T5, T2, T3 and T7. The difference between the strongest and weakest bound complexes is just ~1.2 kcal/mol. When 20 ACS Paragon Plus Environment

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BSSE correction is applied, the ordering changes, as follows; T1, T3, T2, T4, T5 and T7, given in order of decreasing stability, with the difference between the strongest and weakest bound complex being ~1.9 kcal/mol.

The same ordering was also obtained when the

interaction energies were computed by extrapolation to the complete basis set limit. We therefore consider this ordering as being reliable, which is used for comparison with experiments. The CCSD(T)/CBS limit values closely follow the energy ordering obtained at MP2/CBS limits, excepting for T6 and T7, which are interchanged. With the difference between the various structures, being as small as they are and dependent on the computational methodology, experimental evidences, as presented in this work, are necessary to corroborate the computational findings. 5. DISCUSSIONS 5.1 PhAc-tFA Complexes 5.1a OH stretch of tFA In an Ar matrix, the OH stretch of tFA occurs as a doublet at 3550.7 cm-1 and 3548.6 cm-1 (Fig.4). The codeposition of PhAc and FA at 12 K and annealing at 32 K, produces strong product features at 3375.1 and 3368.1 cm-1, which are red shifted from the OH stretch in uncomplexed tFA by 175.6 cm-1 and 182.6 cm-1 respectively. These are in agreement with the computed red shifts for complex T3 and T2 respectively, thereby lending credence to the assignment of these features to the O-H…πAc stretch in the PhAc-tFA complexes (Table 3). Of course, that these may be two site split features of T3/T2 complexes cannot be ruled out. In addition, there is a weak product feature at 3394.9 cm-1, which amounts to a red shift of 155.8 cm-1, corroborating with the calculated red shift of 148.5 cm-1 for complex T1. A feature at 3439.7 cm-1 was also observed, whose shift of 110 cm-1, is in agreement with the computed shift for T4, which is an O-H…πPh complex. We conclude that in the Ar matrix, complexes T2 and T3 were unambiguously identified, together with, probably, complexes T1 21 ACS Paragon Plus Environment


and T4. It may be seen from Table 2 that the three complexes, T1, T2 and T3 are nearly isoenergetic global minimum structures, with T4 being less stable by ~1.2 kcal/mol at CCSD(T)/CBS level. Experimental results are consistent with the energy ordering predicted by MP2/CBS, CCSD(T)/CBS levels and BSSE corrected MP2 and M06-2X methods, using aug-cc-pVDZ basis sets. 5.1 b C-O stretch of tFA The C-O stretch of uncomplexed tFA in Ar occurs at 1103.6 cm-1 (Fig. 5, Block A), as assigned by George et al.19 while this mode has been assigned to a CO-COH deformation by Redington et al.44 A strong product feature of the PhAc-tFA complex, corresponding to this mode, was observed at 1131.3, with weaker features at 1123.4 and 1120.2 cm-1 (Fig. 5, Block A) . Assignment of the feature at 1131.3 cm-1 to the PhAc-tFA complex, requires to be made with caution, as a feature at this wavenumber, has already been reported in the literature to belong to an acyclic dimer of tFA.45 However, in our experiments, the feature at 1131.3 cm-1 shows a strong dependence on both PhAc and tFA concentration, indicating it to be due to a PhAc-tFA complex. We also confirmed it to be a product feature of PhAc-tFA, by performing experiments at very low concentrations of FA, when this feature was barely visible. At the same concentration of tFA, when PhAc was added, this feature grew in strength, indicating it to be due to PhAc-tFA complex. This feature is blue shifted by 27.7 cm-1 from the monomer feature and once again can be assigned to any of the three PhAc-tFA complexes, T1, T2 or T3, confirming that O-H…πAc complexes are indeed prepared in the matrix. No experimental feature was observed that could be assigned to the T4 complex. 5.1c C=O stretch of tFA The C=O stretch of uncomplexed tFA in Ar is observed at 1767.8 cm-1 (Fig. 5, Block B). In our experiments, new features at 1751.8 cm-1 and 1758.2 cm-1 were observed, only


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in the presence of both tFA and PhAc. These features are red shifted from the monomer feature by ~16 and 10 cm-1 respectively, which agree well with the computed shifts of all the three O-H…πAc complexes, T1, T2 and T3, more so with T2 and T3 (Table 3). Hence, while the product features in the C=O region indicate the formation of the PhAc-tFA complex, it does not allow for an unambiguous assignment to any particular complex, but confirms the presence of complexes T2 and T3. No evidence was observed for the T4, which was barely observable in the O-H stretching region. It must be mentioned that FA is known to associate to form homodimers and complexes with water.45 It is already reported in the literature that tFA on annealing yields features at 1728.5 cm-1 and 1745.5 cm-1 which correspond to cyclic and acyclic dimer of tFA respectively;20,45 an observation which we also made in our experiments. However, at the concentrations employed in our experiments, the intensity of the dimer features were very small. Further, no evidence of PhAc dimer was observed in our experiments. It is therefore unlikely that ternary complexes of FA dimer with PhAc or PhAc dimer with FA would be formed in our experiments. 5.1d ≡C-H stretch of PhAc In Ar matrix, ≡C-H stretch of PhAc occurs as a Fermi resonance diad at 3340.2 and 3323.9 cm-1 (Fig. 6). These peaks also suffer from site effects and as a result the ≡C-H stretch of PhAc occurs as bunch of sharp peaks spread between 3343.4 and 3308.3 cm-1. The product features appear as weak peaks at 3306.6, 3304.8 and 3290.8 cm-1. It is rather difficult to compute the red shifts as the feature for the monomer occurs as a bunch of multiplets. Computations show this feature to be red shifted by ~18 cm-1 for complexes T2 and T3. It should be noted that due to spectral congestion in this region the assignment of these feature


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is at best tentative (Table 3). The shifts in the T4 complex was too small to be observed. A feature observed at 3290.8 cm-1 which was red shifted by ~60 cm-1 can be tentatively assigned to an n-σ* structure (T9) involving carbonyl oxygen of tFA (Fig. 6, Table 3). 5.2 PhAcD-tFA Complexes 5.2a ≡C-D stretch of PhAcD Spectra were also recorded with PhAcD, where the acetylenic hydrogen was replaced with deuterium. It was observed that although PhAcD was free of any Fermi resonance, it still suffered from site effects. The ≡C-D stretch of PhAcD, occurs as a doublet at 2609.3 and 2607.9 cm-1 (Fig.7). The product feature occurs at 2586.1 cm-1 which amounts to a red shift of 21.8 cm-1. The computed red shifts for complex T3 and T2, are 12.4 and 14.3 cm-1 (Table 3) which is in general agreement with the experimental value for these complexes. The experimental observations therefore clearly point to the presence of complexes T2 and T3 in the matrix. As indicated by the features in the O-H region (Fig.4) it is likely that complex T1 may be present in minor amounts. The shift in the feature for the T4 complex was computed to be negligible and hence not observed. 6. AIM ANALYSIS Hydrogen bonding interactions between PhAc and tFA were examined by the charge density topology, performed using the atoms-in-molecules (AIM) theory of Bader.46 The wavefunctions were generated at the MP2/aug-cc-pVDZ level of theory and subsequently (3,1) bond critical points were located (Fig.9). Values of topological parameters, such as electron density ρ rc), eigenvalues of the Hessian, λ1, λ2, λ3, the Laplacian ( 2ρ rc)) of electron density (defined as the sum of the eigenvalues), the ellipticity, [ λ1 λ2)- ] and ǀλ1 λ3ǀ, for all the complexes, computed at the bond critical points, are shown in Table 4. All the complexes are characterized by a small positive value for

2

ρ rc), falling in the range of 0.024-0.139 a.u.,

which indicates all the interactions to be of a closed shell type, as seen in hydrogen bonds.47



Complexes T1 to T3, which are hydrogen bonded interactions involving an O-H…πAc as the primary contact are nearly isoenergetic, with similar values of ρ rc) and ( 2ρc). All these complexes also manifest multiple interactions. It can be seen that the bond critical points of all these interactions are characterized by ellipticities which fall in the range of 0.3 to 0.5, which indicates near cylindrical symmetry. On the other hand, complex T4, which involves a O-H…πPh interaction as its primary interaction, has an ellipticity of 1.9, indicating concentration of electron density in a plane.

This complex too manifests a secondary

interaction. The hydrogen bond molecular graphs of all the isomeric forms of the complexes are shown in Fig. 9. Using the topological parameters, we also computed the interaction energy for each contact ( EHB), through the use of local kinetic energy density, G(rCP) and local potential energy density V(rCP), as proposed by Espinosa et al.35 Total EHB given in the table is the sum of the energies of all the hydrogen bonding contacts in the complex. Though the values of interaction energies obtained through a topological analysis do not exactly match with the values presented in Table 2, they still indicate complexes T1, T2 and T3 to be the most stable. Clearly the most stable complexes in the PhAc-tFA system manifest bi or tridentate contacts in the hydrogen bonded complexes. 7. NBO ANALYSIS We used natural bond orbital analysis (NBO analysis) implemented through Gaussian 09, to quantify interactions between donor and acceptor orbitals of PhAc-tFA complexes (Table 5). The acetylenic π system is the donor orbital in complex T , T and T , whereas in case of complex T4 the phenyl π system acts as a donor orbital. In all the complexes mentioned above, σ* O-H orbital of tFA acts as acceptor of electron density. Electron transfer to the σ* OH orbital of tFA corroborate the observed and computed red shifts in the O-H


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T1

T4

T7

T3

T2

T5

T8

T6

T9

T10 Figure 9. AIM analysis on the optimized geometries of the PhAc-tFA complexes, obtained at MP2/aug-cc-pVDZ level of theory. Bond critical points corresponding to the intermolecular interactions have been indicated. 27 ACS Paragon Plus Environment


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Table 4. Summary of AIM Calculations (MP2/aug-cc-pVDZ level) for the PhAc-tFA Complexes. Hydrogen Bond Energies (∆EHB) are in kcal/mol . All other Quantities are in a.u. See Text for Definition of each of the Quantities.

complex

ρ rc)

T1 bcp1

0.0171

bcp2

2

λ1

λ2

λ3

λ‫׀‬1 λ3‫׀‬

λ 1/ λ2)-1

0.0482

-0.0169

-0.0111

0.0762

0.2218

0.5278

-3.30

0.0088

0.0316

-0.0078

-0.0066

0.0460

0.1700

0.1899

-1.51

bcp1

0.0187

0.0512

-0.0187

-0.0136

0.0835

0.2244

0.3821

-3.71

bcp2

0.0058

0.0239

-0.0051

-0.0037

0.0327

0.1553

0.3684

-0.96

bcp3

0.0058

0.0214

-0.0044

-0.0015

0.0274

0.1621

1.9601

-0.90

T3 bcp1

0.0179

0.0494

-0.0175

-0.0121

0.0791

0.2218

0.4450

-3.49

bcp2

0.0068

0.0251

-0.0054

-0.0026

0.03313

0.1632

1.0949

-1.10

T4 bcp1

0.0118

0.0334

-0.0094

-0.0032

0.04601

0.2043

1.9357

-1.97

bcp2

0.0072

0.0213

-0.0041

-0.0033

0.02868

0.1434

0.2550

-1.03

T5

0.0089

0.0315

-0.0034

-0.0022

0.03701

0.0915

0.5592

-1.51

-1.51

T6 bcp1

0.0085

0.0228

-0.0062

-0.0051

0.0346

0.1913

0.2899

-1.23

-3.24

bcp2

0.0119

0.0342

-0.0123

-0.0120

0.0584

0.2103

0.0259

-2.01

T7 bcp1

0.0097

0.0295

-0.0073

-0.0015

0.03823

0.1900

3.8009

-1.56

bcp2

0.0074

0.0276

-0.0022

-0.0005

0.03031

0.0719

2.9626

-1.23

T8 bcp1

0.0088

0.0237

-0.00695

-0.00526

0.035869

0.1938

0.3216

-1.30

bcp2

0.0084

0.0284

-0.00838

-0.0079

0.044686

0.1874

0.0601

-1.37

T9

0.0142

0.0422

-0.0149

-0.0148

0.0718

0.2069

0.0079

-2.61

-2.61

T10

0.0125

0.0410

-0.0138

-0.0130

0.0677

0.2032

0.0620

-2.28

-2.28

T2

ρ rc)


EHB total EHB -4.81

-5.57

-4.59

-3.00

-2.79

-2.67

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stretch discussed earlier. For example, it can be seen that complex T2 has the largest red shift amongst all the three acetylenic complexes and it can be related to the largest electron occupancy in the acceptor σ* OH, followed by complex T . Complex T1 which manifests a smaller shift in the O-H frequency can also be seen to have smaller occupancy in the σ* OH relative to complexes T2 and T3 (Table 5). In structures T5 and T7, the second order perturbation energy for π-π* interaction, between phenyl π cloud of PhAc and carbonyl group of tFA, was larger than the π-σ* interaction between phenyl π cloud of PhAc and the C-H of tFA. These structures were hence largely stabilized by π stacking interaction along with π-σ* interaction. 8. LMO-EDA The interaction energy of each of the PhAc-tFA complexes was partitioned into contributions from various interactions, such as electrostatic, exchange-repulsion, dispersion and polarization. LMO-EDA analysis was performed at MP2/aug-cc-pVDZ level of theory using GAMESS. Complex T1, T2 and T3 having O-H…πAc as major interaction receive the largest contribution from electrostatic interactions, whereas complexes T4, T5 and T7 have nearly equal contributions from dispersion and electrostatic components (Table 6). 9. COMPARISON OF THE HYDROGEN BONDED COMPLEXES OF PhAc-FA, C2H2-FA AND C6H6-FA. It would be instructive to compare the complexes of PhAc-tFA with those of C2H2tFA and C6H6-tFA, C2H2 and C6H6 being constituent moieties of PhAc. The most strongly bound complex of C2H2-tFA was very similar to the PhAc-tFA complex (T2 and T3), with OH…πAc being the primary interaction. In addition, there was a secondary interaction between the carbonyl oxygen and the acetylenic hydrogen in both PhAc-tFA and C2H2-tFA. In the case of C2H2-tFA, the cylindrical symmetry of C2H2, results in an infinite-fold degeneracy for this complex, with regard to rotation around the C≡C bond. In the case of PhAc-tFA the 29 ACS Paragon Plus Environment


presence of the phenyl system removes the cylindrical symmetry of the acetylenic system, giving rise to two distinct complexes, T2 and T3. T3 in PhAc-tFA is almost identical to the global minimum in C2H2-tFA, with the tFA lying in a plane perpendicular to the phenyl ring and employing two interactions in the complex. In T2 on the other hand, tFA lies in the plane of the phenyl ring and enjoys yet another interaction, where the phenyl C-H interacts with the hydroxyl oxygen, much like the interaction observed in the PhAc-H2O complex.39 T2 therefore manifests a triple interaction and highlights the important role of secondary interactions in stabilizing complexes. However, the energy difference between T2 and T3 is small. Furthermore, the barrier for conversion of T2 to T3 is ~0.1 kcal/mol, while that for the reverse conversion is even smaller with a value of 0.01 kcal/mol, which potentially leads to a free rotation of FA around the acetylenic system, as in the case of PhAc-HCl.22 Floppiness of hydrogen bonded complexes has been observed in many systems before, such as in the C6H6H2O and borazine-H2O.24,25,48 The presence of the phenyl ring, gives rise to another structure T1, not seen in C2H2-tFA. This structure can convert to the T3 structure, by a rotation of the FA, wherein the O-H is kept tethered to the πAc cloud and the CHO group is rotated to yield T3. This transformation however has a barrier of about 1.4 kcal/mol for going from T1 to T3 but the reverse conversion is much less (~0.5 kcal/mol). The general picture that emerges is that while the FA is tethered to PhAc in a variety of isomers, there seems to be a certain amount of free motion of FA around the PhAc submolecule. The complex T4 which is O-H…πPh complex is very similar to the C6H6-tFA complex observed by Banerjee et al.21 It is also interesting to compare the red shifts in OH stretch of the FA in the above mentioned complexes, which is seen to follow the order : C6H6:tFA (-120 cm-1) < C2H2:tFA (-137 cm-1) < PhAc:tFA (-155 cm-1). Clearly the PhAc-tFA complex shows the largest red shift, as evidenced by the large occupancy of the σ* orbital of O-H (Table 5).


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Table 5. NBO Analysis of PhAc-tFA Complexes at MP2/aug-cc-pVDZ Level of Theory. The Atom Numbering Indicated in the Table is as shown in Fig.1, 2 and 3. E(j) is the Second Order Perturbation Energy (kcal/mol), E(j)-E(i) is the Donor-Acceptor Energy Difference and F(i,j) is the Off Diagonal NBO Fock Matrix Element.

complex

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

orbital involved donor C12-C13 π C12-C13 π C12-C13 π C3-C4 π C1-C2 π C3-C4 π C12-C13 π C4-C5 π C1-C6 π C12-C13 π O18(n) O16(n)

acceptor H15-O16 σ* H15-O16 σ* H15-O16 σ* H15-O16 σ* C17-O18 σ* C17-H19 σ* C17-H19 σ* C17-O18 σ* C17-H19 σ* C17-H19 σ* H14-C12 σ* C13-H14 σ*

E(kcal/ mol)

E(j)-E(i) a.u.

F(i,j) a.u.

electron occupancy in σ* orbital

6.94 9.49 8.33 3.10 1.56 0.90 2.75 1.27 1.22 3.01 2.98 3.60

1.07 1.07 1.07 1.00 0.53 0.97 1.05 0.54 0.98 1.06 1.22 1.66

0.078 0.090 0.085 0.054 0.027 0.029 0.048 0.025 0.034 0.051 0.055 0.069

0.02261 0.02612 0.02348 0.01695 0.12148 0.05072 0.04961 0.1183 0.04932 0.04967 0.01079 0.01047

Table 6. LMO-EDA Analysis for the Various PhAc-tFA Complexes at the MP2/aug-ccpVDZ. All Energies are in kcal/mol.

complex

EES

EER

EPol

EDisp

ETotal

EES/ETotal

EDisp/ETotal

EPol/ETotal

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10

-7.92 -8.87 -8.61 -4.96 -4.84 -5.03 -4.50 -3.84 -4.71 -2.61

10.43 10.67 9.84 8.74 9.02 5.73 8.03 4.92 4.37 2.70

-3.69 -3.82 -3.76 -2.68 -1.57 -1.48 -1.66 -1.09 -1.14 -0.61

-3.87 -2.75 -2.32 -5.31 -5.93 -1.89 -4.91 -2.04 -1.21 -0.96

-5.05 -4.78 -4.86 -4.22 -3.31 -2.68 -3.04 -2.04 -2.68 -1.48

1.57 1.86 1.77 1.18 1.46 1.88 1.48 1.88 1.76 1.76

0.77 0.58 0.48 1.26 1.79 0.71 1.62 1.00 0.45 0.65

0.73 0.80 0.77 0.64 0.47 0.55 0.55 0.53 0.43 0.41



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The T2 complex in PhAc-tFA is reminiscent of the PhAc-H2O complex, where the H2O is oriented in a very similar fashion with a dual contact involving O-H…πAc and CH…O.39 However, while PhAc-tFA also forms the complex T3, where FA is in a plane perpendicular to the phenyl ring, PhAc-H2O does not manifest a complex with a similar geometry. In fact, in PhAc-H2O system such a geometry was a saddle point with two imaginary frequencies. The reason could be that while FA uses its carbonyl oxygen to form a secondary contact with the acetylenic hydrogen, water cannot.

All these observations

indicate the important role made by secondary interactions in the stabilization of complexes with multiple contacts. 10. CONCLUSIONS Hydrogen bonded interactions in PhAc-FA heterodimer were explored using matrix isolation FTIR spectroscopy and ab initio calculations. PhAc and FA, both being multifunctional molecules, gave rise to a large number of hydrogen bonded geometries. Our computations reveal that 10 isomeric complexes were possible on the PhAc-tFA potential surface. Amongst PhAc-tFA complexes – the structures constituting the global minima were composed of a primary O-H…πAc interaction together with a secondary C-H…O interaction (n-σ* . In our experiments, we observed two of the three nearly isoenergetic structures (complexes T2 and T3), in which OH of tFA interacts with acetylenic π cloud of PhAc. The secondary C-H…O interactions involved either the C=O or the OH group of tFA on the one hand and –C-H of phenyl or the ≡CH of the acetylenic group of PhAc on the other. Two other structures T1 and T4 were also probably observed. In all these complexes, AIM calculations show that these structures are multiply bonded. The LMO-EDA calculations indicate these complexes to be dominated by electrostatic contributions. As in the case of our earlier work on PhAc-HCl, where HCl was indicated to have a free rotation around the acetylenic π system, here too it was found that the energy varied little as the tFA was rotated around the 32 ACS Paragon Plus Environment

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cylindrical π acetylenic system. This phenomenon may have implications in the experimental observation of structures T2 and T3 showing large intensities compared with T1, even though all the three structures are isoenergetic. It is likely that structures T2 or T3 are produced by the approach of FA along any plane, while T1 may require the approach of FA along specific directions. In other words, the trapping of T2 and T3 is kinetically directed and features corresponding to these structures have a greater intensity than T1. T3 may in fact be produced through a conversion from T1. Local minima having O-H…πPh interaction was also observed with a red shift of ~110 cm-1 in the O-H mode, a shift which is significantly lower than that observed in the case of the O-H…πAc structures. Structure T9 having an n-σ* interaction was also seen in the matrix. Earlier work had reported FA to show bidentate hydrogen bonds with acetylene and benzene. In the case of PhAc-tFA complex, the system is seen to expand its tentacles to construct more than two hydrogen bonds in a given structure, as was observed in complex T2. Clearly FA manifests structures with multiple contacts and shows itself to be a good case study for such multiply bonded architectures. Interestingly, in spite of the multiple tentacles, FA appears to have a free motion where it can possibly visit multiple isomeric structures, through near-barrierless conversions.



Supporting Information Optimized structures of PhAc-cFA complexes with interaction energies given as Uncorrected/ZPE corrected/BSSE Corrected energies in kcal/mol.

AUTHOR INFORMATION Corresponding Author *Ginny Karir: Phone: +91-83605 67173 E-mail: [email protected]

FAX: +91-172-2240266.

*K. S. Viswanathan: Phone: +91-95308 02088. Email: [email protected]

FAX: +91-172-2240266.

Present Address: 1

Department of Chemistry, University of Southern California, Los Angeles, CA-90089-0482, USA 2

Government Science College, Chatrapur, Ganjam, Odisha, 761020 India

ACKNOWLEDGEMENTS Ginny Karir, Gaurav Kumar and Bishnu Prasad Kar gratefully acknowledge the fellowships from MHRD. The authors are also grateful to IISER Mohali, India for the facilities and MHRD for funding. We also thank Dr Balanarayan and Dr Sugumar for useful discussions.


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References (1) Marushkevich, K.; Khriachtchev, L.; Lundell, J.; Domanskaya, A.; Räsänen, M. Matrix Isolation and Ab Initio Study of Trans-Trans and Trans-Cis Dimers of Formic Acid. J. Phys. Chem. A. 2010, 114, 3495–3502. (2) Crovisier, J.; Bockele´e-Morvan, D. Remote Observations of the Composition of Cometary Volatiles. Space Sci. Rev. 1999, 90, 19-32. (3) Ehrenfreund, P. E.; Charnley, S. B. Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to Early Earth. Annu. Rev. Astron. Astrophys. 2000, 38, 427-483. (4) Andrade, D. P. P.; Boechat-Roberty, H. M.; da Silveira, E. F.; Pilling, S.; Iza, P.; Martinez, R.; Farenzena, L. S.; Homem, M. G. P.; Rocco, M. L. M. Astrophysical Icy Surface Simulation under Energetic Particles and Radiation Field in Formic Acid. J. Phys. Chem. C. 2008, 112, 11954-11961. (5) Jacob, D. J. Heterogenous Chemistry and Tropospheric Chemistry. Atmos. Environ. 2000, 34,

− 59.

(6) Lelieveld, J.; Crutzen, P. J. The Role of Clouds in Tropospheric Photochemistry. J. Atmos. Chem. 1991, 12,

9− 67.

(7) Miyazawa, T.; Pitzer, K. S. Internal Rotation and Infrared Spectra of Formic Acid Monomer and Normal Coordinate Treatment of Out-of-Plane Vibrations of Monomer, Dimer and Polymer. J. Chem. Phys. 1959, 30, 1076-86. (8) Lopes, S.; Domanskaya, A. V.; Fausto, R.; Räsänen, M. Khriachtchev, L. Formic-Acid and Acetic Acids in a Nitrogen Matrix: Enhanced Stability of the Higher Energy Conformer. J. Chem. Phys. 2010, 133, 144507-14. (9) Tsuge, M.; Marushkevich, K.; Räsänen, M.; Khriachtchev, L. Infrared Characterization of the HCOOH...CO2 Complexes in Solid Argon: Stabilization of the Higher-Energy Conformer of Formic Acid. J. Phys. Chem.A. 2012, 116, 5 05−5

.

(10) Lundell, J.; Räsänen, M.; Latajka, Z. Complexes between Formic Acid and Carbon Monoxide: An Ab Initio Investigation. J. Phys. Chem. 1993, 97,

5 − 57.

(11) Ito, F.; Nakanaga, T. A Jet Cooled Infrared Spectrum of the Formic Acid Dimer by Cavity Ring-Down Spectroscopy. Chem. Phys. Lett. 2000, 318, 571-577. (12) Ito, F. Modelling and Spectral Simulation of Matrix Isolated Molecules by Density Functional Calculations : A Case Study on Formic Acid Dimer. J. Chem. Phys. 2010, 133, 214502-8. 35 ACS Paragon Plus Environment


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(13) Mackeprang, K.; Xu, Z-H.; Maroun, Z.; Meuwly, M.; Kjaergaard, H. G. Spectroscopy and Dynamics of Double Proton Transfer in Formic Acid Dimer. Phys. Chem. Chem. Phys. 2016, 18, 24654-24662. (14) Sundararajan, K.; Sankaran, K.; Viswanathan, K. S. H-π Complexes of AcetyleneEthylene : A Matrix Isolation and Computational Study. J. Phys. Chem. A. 2002, 106, 15041510. (15) Khriachtchev, L. Matrix-Isolation Studies of Noncovalent Interactions: More Sophisticated Approaches. J. Phys. Chem. A. 2015, 119, 2735-2746. (16) Karir, G.; Viswanathan, K. S. The Elusive ≡CH...O Complex in the Hydrogen onded Systems of Phenylacetylene : A Matrix Isolation Infrared and Ab Initio Study. J. Chem. Sci. 2016, 128, 1557-1569. (17) Singh, P. C.; Bandyopadhyay, B.; Patwari, G. N. Structure of the Phenylacetylene-Water Complex as Revealed by Infrared-Ultraviolet Double Resonance Spectroscopy. J. Phys. Chem. A. 2008, 112, 3360-3363. (18) Goswami, M.; Arunan, E. Microwave Spectroscopic and Theoretical Studies on the Phenylacetylene...H2O Complex: C-H...O and O-H...π Hydrogen

onds as Equal Partners.

Phys. Chem. Chem. Phys. 2011, 13, 14153-14162. (19) George, L.; Sander, W. Matrix Isolation Infrared and Ab Initio Study of the Hydrogen Bonding between Formic Acid and Water. Spectrochim. Acta A. 2004, 60, 3225-3232. (20) George, L.; Sanchez-Garcia, E.; Sander , W. Matrix Isolation Infrared and Ab Initio Study of Formic Acid-Acetylene Interaction: Example of H...π and C-H...O Interaction. J. Phys. Chem. A. 2003, 107, 6850-6858. (21) Banerjee, P.; Bhattacharya, I.; Charaborty, T. Matrix Isolation Infrared Spectroscopy of an O-H...π Hydrogen-Bonded Complex between Formic Acid and Benzene. J. Phys. Chem. A. 2016, 120, 3731-3739. (22) Karir, G.; Viswanathan, K. S. H-π Landscape of the Phenylacetylene-HCl System : Does This Provide Gateway to The Markovnikov Addition ? J. Phys. Chem. A. 2017, 121, 57975808. (23) Schnell, M.; Erlekam, U.; Bunker, P. R.; von Helden, G.; Grabow, J-U.; Meijer, G.; van der Avoird, A. Unravelling the Internal Dynamics of the Benzene Dimer: A Combined Theoretical and Microwave Spectroscopy Study. Phys. Chem. Chem. Phys. 2013, 15, 1020710223.


Page 37 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(24) Augspurger, J. D.; Dykstra, C. E.; Zwier, T. S. Hydrogen-Bond Swapping in the Benzene-Water Complex: A Model Study of the Interaction Potential. J. Phys. Chem. A. 1992, 96, 7252-7257. (25) Engdahl, A.; Nelander, B. A Matrix Isolation Study of Benzene-Water Interaction. J. Phys. Chem. 1985, 89, 2860-2864. (26) Venkatesan, V.; Sundararajan, K.; Viswanathan, K. S. Conformations of 1,1Dimethoxyethane : A Matrix Isolation and Ab Initio Studies. J. Phys. Chem. A. 2002, 106, 7707-7713. (27) Kar, B. P.; Ramanathan, N.; Sundararajan, K.; Viswanathan, K. S. Conformations of Dimethyl Carbonate and its Complexes with Water: A Matrix Isolation Infrared and Ab Initio Study. J. Mol. Struct. 2012, 1024, 84-93. (28) Sundararajan, K.; Viswanathan, K. S. A Matrix Isolation and Ab Initio Study of the C2H2: MeOH Complex. J. Mol. Struct. 2006, 798, 109-116. (29) Saini, J.; Viswanathan, K. S. Does a Hydrogen Bonded Complex with Dual Contacts Show Synergism ? A Matrix Isolation Infrared and Ab Initio Study of Propargyl AcoholWater Complex. J. Mol. Struct. 2016, 1118, 147-156. (30) Frisch, M. J. et al., Gaussian 09, Gaussian, Inc., Wallingford CT, 2010. (31) Vincent, M. A.; Hillier, I. H. The Structure and Interaction Energies of Weak Complexes of CHClF2 and CHF3 with HCCH: A Test of Density Functional Theory Methods. Phys. Chem. Chem. Phys. 2011, 13, 4388-4392. (32) Boys, S. F.; Bernardi, F. The Calculation of Small Molecular Interactions by the Differences of Separate Total Energies. Some Procedures with Reduced Errors. Mol. Phys. 1970, 19, 553-566. (33) Helgaker, T.; Klopper, W.; Koch, H.; Noga, J. Basis-Set Convergence of Correlated Calculations of Water. J. Chem. Phys., 1997, 106, 9639-46. (34) Blieger-König, F.; Bayles, D.; Schönbohn, J. AIM2000 (Version 1.0): Chemical Adviser: Bader R. F. W. (35) Espinosa, E.; Molins, E.; Lecomte, C. Hydrogen Bond Strengths Revealed by Topological Analyses of Experimentally Observed Electron Densities Chem. Phys. Lett., 1998, 285, 170-173. (36) Glendening, E. D.; Carpenter, J. E.; Weinhold, F. NBO (Version 3.1). (37) Su, P.; Li, H. Energy Decomposition Analysis of Covalent Bonds and Intermolecular Interactions. J. Chem. Phys. 2009, 131, 014102-15. 37 ACS Paragon Plus Environment


(38) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.;Su, S.; et al. General Atomic and Molecular Electronic Structure System. J. Comput. Chem. 1993, 14, 1347−1363. (39) Karir, G.; Viswanathan, K. S. Phenylacetylene-Water Complex : Is it n...σ or H...π in the Matrix ? J. Mol. Struct. 2016, 1107, 145-156. (40) Pettersson, M. Maçȏas, E. M. S. Khriachtchev, L.; Lundell, J.; Fausto, R.; Räsänen, M. Cis-Trans Conversion of Formic Acid by Dissipative Tunneling in Solid Rare Gases: Influence of Environment on the Tunneling Rate. J. Chem. Phys. 2002, 117, 9095-8. (41) Marushkevich, K.; Khriachtchev, L.; Räsänen, M. Hydrogen Bonding between Formic Acid and Water: Complete Stabilization of the Intrinsically Unstable Conformer. J. Phys. Chem. A. 2007, 111, 2040-2042. (42) Nishio, M.; Hirota, M.; Umezawa, Y. The CH π Interaction, Evidence, Nature and Consequences. Wiley-VCH, New York. 1998. (43) Nishio, M. CH π Hydrogen onds in Crystals. CrystEngComm. 2004, 6, 130-158. (44) Redington, R. L. Vibrational Spectra and Normal Coordinate Analysis of Isotopically Labeled Formic Acid Monomers. J. Mol. Struct. 1977, 65, 171-189. (45) Gatenberg, M.; Halupka, M.; Sander, W. Dimerization of Formic Acid - An Example of a “NonCovalent” Reaction Mechanism. Chem. Eur. J. 2000, 6, 1865-1869. (46) Bader, R. F. W.Atoms in Molecules, A Quantum Theory, Clarendon Press, Oxford, 1994. (47) Koch, U.; Popelier, P. L. A. Characterization of C-H-O Hydrogen Bonds on the Basis of the Charge Density. J. Phys. Chem. 1995, 99, 9747–9754. (48) Mishra, P.; Verma, K.; Bawari, D.; Viswanathan, K. S. Does Borazine-Water Behave like Benzene-Water? A Matrix Isolation Infrared and Ab Initio Study. J. Chem. Phys. 2016, 144, 234307-9.


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TOC Graphic Multiple Tethers

Phenylacetylene-Formic Acid Complexes


Multiple Hydrogen Bond Tethers for Grazing Formic Acid in Its

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