π-Stacked Dimers of Fluorophenylacetylenes: Role of Dipole Moment

Article pubs.acs.org/JPCA

π‑Stacked Dimers of Fluorophenylacetylenes: Role of Dipole Moment Sohidul Islam Mondal,§,† Saumik Sen,§,† Anirban Hazra,*,‡ and G. Naresh Patwari*,† †

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Department of Chemistry, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pune 411008, India

‡

S Supporting Information *

ABSTRACT: The homodimers of singly fluorine-substituted phenylacetylenes were investigated using electronic and vibrational spectroscopic methods in combination with density functional theory calculations. The IR spectra in the acetylenic C−H stretching region show a marginal red shift for the dimers relative to the monomers. Further, the marginal red shifts indicate that the acetylenic group in all the dimers is minimally perturbed relative to the corresponding monomer. The observed spectra were assigned to a set of πstacked structures within an energy range of 1.5 kJ mol−1, which differ in the relative orientation of the two monomers on the basis of M06-2X/aug-cc-pVTZ level calculation. The observed red shift in the acetylenic C−H stretching vibration of the dimers suggests that the antiparallel structures contribute predominantly based on a simple coupled dipole model. Energy decomposition analysis using symmetry-adapted perturbation theory indicates that dispersion plays a pivotal role in π−π stacking with appreciable contribution of electrostatics. The stabilization energies of fluorophenylacetylene dimers follow the same ordering as their dipole moments, which suggests that dipole moment enhances the ability to form π-stacked structures. on π−π stacking using a variety of theoretical methods.40−46 Incidentally, gas-phase experiments are very valuable, as they can be directly compared with the quantum chemical calculations, and useful information about the role of electronic effects can be extracted to understand the nature of π−π stacking. Interaction between two aromatic molecules in the gas phase has been subject of intense investigations, and benzene dimer is a prototypical example. In the case of benzene dimer only the tilted T-shaped structure has been reported experimentally,47−50 even though high-level ab initio calculations indicate that parallel displaced π-stacked dimer is almost isoenergetic.51−54 Apart from benzene dimer there have been several reports on aromatic dimers in the gas phase, which include, phenol,55 2-pyridone,56 benzoic acid,57 anthranilic acid,58 7azaindole,59 1-indanol,60 pyrazine,61 1,2-difluorobenzene,62 anisole,63 phenylacetylene,64 propargylbenzene,65 and several others. These aromatic dimers have large variation in their intermolecular structures, which include O−H···O, N−H···N, O−H···π, C−H···π, and π···π interactions. Among these dimers, the π−π stacked gas-phase dimers are relatively sparse.61−65 To this end, investigations on dimers of singly fluorine-substituted phenylacetylenes, specifically, 2-fluorophenylacetylene (2FPHA), 3-fluorophenylacetylene (3FPHA), and 4-fluorophenylacetylene (4FPHA), were performed using a double-barrel approach of gas-phase experiments combined with density

1. INTRODUCTION The molecular assemblies that are held together by π−π stacking play a significant role in chemistry,1−5 biology,6−9 and material science.10,11 The three-dimensional structure and stability of DNA,12−15 RNA,16,17 proteins,18−20 crystal packing of molecules,21,22 drug binding,23,24 and host−guest complexes25 containing aromatic rings are determined by π−π stacking interactions. Several specific examples have been reported in the literature, wherein π−π stacking determines structure and reactivity, such as amyloid fibril formation,26 aggregation of metalloporphyrins,27 catalytic asymmetric hydroformylation,28 and many others. It has also been reported that π-stacked self-assemblies of aromatic units function as electron conducting units.29,30 Aromatic π−π stacking also controls surface adsorption of dyes to carbon nanotubes and graphene, which are used in dye-sensitized solar cells.31−35 Given the wide range of utility of π−π stacking it is essential to understand the nature of π−π stacking at the molecular level. One of the earliest models of aromatic π−π stacking was primarily a polar electrostatic model (Hunter−Sanders model), wherein the quadrupole−quadrupole interaction leads to formation of π−π stacking.36,37 In the multipole expansion of the potential the quadrupole−quadrupole term has distance dependence of r−7. Evidently, the dispersion term, which has r−6 distance dependence, would dominate over the quadrupole−quadrupole interaction term.38 The transformation of helical B-DNA to ladderlike structure in the absence of dispersion term in the potential clearly establishes the role of dispersion on π−π stacking.39 A large number of reports in the literature address the role of dispersion vis-à-vis electrostatics © XXXX American Chemical Society

Received: January 8, 2017 Revised: April 13, 2017 Published: April 13, 2017 A

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geometry optimization. The analysis of the interaction energies of various dimer structure was performed using symmetryadapted perturbation theory (SAPT).76,77 The simplest of SAPT approach, that is, SAPT0, was performed using cc-pVTZ basis set along with the cc-pVTZ-JKFIT basis for the Hartree− Fock and cc-pVTZ-RI basis for the SAPT procedure.78 The density-fitting approach was used to reduce the computational expense. The main advantage of SAPT calculations is that it allows for the separation of interaction energy (ESAPT0) into physically well-defined components, such as those arising from the electrostatic (Eelec), induction (Eind), dispersion (Edisp), and exchange (Eexch) as given in eq 1

functional theory (DFT) calculations. The motivation to investigate dimers of fluorphenylacetylenes (FPHAs) arises from the fact that the unsubstituted phenylacetylene (PHA) forms a π-stacked dimer;64 therefore, the present work is to investigate the effect of fluorine substitution on π−π stacking ability. Investigating the effect of fluorine substitution on π−π stacking ability of PHA is particularly interesting due to following: (a) Organic fluorine (C−F bond) being a very poor hydrogen bond acceptor,66−69 it is therefore expected not to form strongly hydrogen-bonded dimers. (b) Substitution of fluorine increases the dipole moment, which is position dependent; therefore, it is expected to increase the electrostatic contribution. (c) Substitution of fluorine also reduces the π electron density of the phenyl ring;70 therefore, it is expected to lower the dispersion component.

(10) (20) (20) (20) (20) (10) ESAPT0 = Eelec + Eexch + E ind + Eexch − ind + Edisp + Eexch − disp

(1)

The perturbative method SAPT0 treats the monomers at the Hartree−Fock level and then separates the overall intermolecular interaction energy into the different components using second-order perturbation theory. It has been found that the SAPT0 calculations with reasonable basis sets provide good estimates of stabilization energies.78 In the present analysis the exchange-induction and exchange-dispersion terms were added to the parent induction and dispersion terms. Further, the charge-transfer component is given by the eqs 2 and 3.79

2. METHODS A. Experimental Section. The details of the experimental setup have been described elsewhere.71 A supersonic jet expansion of the reagents (2FPHA, 3FPHA, and 4FPHA; Aldrich) doped in helium was performed using 0.5 mm diameter pulsed nozzle (Series 9, Iota One; General Valve Corporation) operating at 10 Hz. The electronic excitation spectra of the monomers 2FPHA, 3FPHA, and 4FPHA were recorded using laser-induced fluorescence (LIF) method by monitoring the total fluorescence with a photomultiplier tube (9780SB + 1252−5F; Electron Tubes Limited) and a filter (BG3 + WG-305) combination. On the other hand the electronic excitation spectra of the dimers were recorded following one-color resonant two-photon ionization (1C-R2PI) method and monitoring the parent mass ion signal at 240 Da, using a two-stage Wiley−McLaren time-of-flight mass spectrometer (TOFMS) fitted with a channel electron multiplier (CEM-KBL-25RS; Sjuts Optotechnik) and a preamplifier (SR445A; Stanford Research Systems). The LIF and the TOFMS signals were digitized by a digital storage oscilloscope (TDS-1012; Tektronix) that was interfaced to a personal computer using a data acquisition program written in LabView. IR−UV double resonance spectroscopic method was used to record the infrared spectra in the acetylenic C−H stretching region,72−74 and the spectrum is called fluorescence-dip infrared (FDIR)/ion-dip infrared (IDIR) spectrum depending on the detection method. For achieving the electronic excitation, the tunable UV laser used was a frequency-doubled output of a dye laser (Narrow Scan GR; Radiant Dyes) operating with the Rhodamine-19 dye, pumped with second harmonic of a Nd:YAG laser (Brilliant-B; Quantel). The tunable IR light was generated by LiNbO3 OPO (Custom IR OPO; Euroscan Instruments), as an idler component, pumped with an injection-seeded Nd:YAG laser (Brilliant-B; Quantel). The typical bandwidth of both UV and IR lasers is ∼1 cm−1, and the absolute frequency calibration is within ±2 cm−1. B. Computational. A detailed conformational search was performed by generating several initial structures by taking snapshots from MM2-MD trajectory followed by geometry optimization using M06-2X/aug-cc-pVDZ level with ultrafine integration grids.76 The vibrational frequency calculations were performed at M06-2X/aug-cc-pVDZ level of theory. The stabilization energy was determined as the difference between the dimer energy and the sum of monomer energies. The stabilization energies were corrected for the vibrational zeropoint energy (ZPE) and the basis-set superposition error (BSSE) using counterpoise method and was made after

(20) (20) (20) ECT = E ind (dimer) − E ind (monomer)

(2)

(20) (20) (20) Eexch − CT = Eexch − ind (dimer) − Eexch − ind(monomer)

(3)

Geometry optimization and frequency calculations were performed using the Gaussian 09 suite of programs80 with its graphical interface GaussView 5.81 SAPT0 calculations were performed using PSI4 ab initio package.82,83 The structures and the vibrations were visualized by ChemCraft.84

3. RESULTS AND DISCUSSION The electronic spectra of the monomers and the dimers are presented in Figure 1, specifically, 2FPHA (Figure 1A), 2FPHA-dimer (Figure 1B), 3FPHA (Figure 1C), 3FPHAdimer (Figure 1D), 4FPHA (Figure 1E), and 4FPHA-dimer (Figure 1F). The electronic spectra for the monomers were recorded using LIF method, since the adiabatic ionization energies of all the three monomers is higher than energy available in 1C-R2PI experiment.85,86 The LIF spectra of the three monomers (2FPHA, 3FPHA, and 4FPHA) show narrow bands corresponding to the band origin of S1←S0 transition. The electronic excitation spectra of the dimers were recorded using 1C-R2PI method by monitoring the mass signal at 240 Da. The 1C-R2PI spectra of the three dimers remain largely in the same wavelength region; however, they are considerably broadened, similar to spectrum observed for the PHA dimer.64 The broadening of the electronic transition can possibly be attributed to variety of factors such as (a) distributed Franck− Condon factors, which can be important for the π-stacked structures due to presence of low-frequency vibrations, (b) multiple isomers, and (c) excitonic coupling.87 To determine the structure of the three homodimers, the IR spectra in the acetylenic C−H stretching region were recorded using IR−UV double-resonance spectroscopic method, using either fluorescence detection (for monomers) or ion detection (for dimers), and the results are presented in Figure 2. The FDIR spectrum of the 2FPHA (Figure 2A) shows a strong band at 3334 cm−1 accompanied by weaker bands at 3316 and B

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broader bands centered around 3332 and 3346 cm−1. The FDIR spectrum of 3FPHA monomer (Figure 2C) shows a strong band at 3336 cm−1 corresponding to the acetylenic C− H stretching vibration. The IDIR spectrum of the 3FPHA dimer (Figure 2D) shows a single broad band centered around 3333 cm−1. The FDIR spectrum of the 4FPHA (Figure 2E) shows multiple bands, wherein the two strong bands at 3329 and 3347 cm−1 are due to Fermi resonance, while the weaker band was assigned to be originating from higher-order coupling terms.89 The FDIR spectrum of the 4FPHA dimer (Figure 2F) shows two slightly broader bands at 3322 and 3341 cm−1, which clearly indicates the presence of Fermi resonance coupling, similar to monomer, albeit marginally shifted. This spectrum is also similar to the IDIR spectrum of the PHA dimer.64 Several IDIR spectra were recorded for each of the three dimers setting the probe laser at various positions in the excitation spectrum. Specifically in the case of 4FPHA dimer, the IR spectra were recorded by monitoring the signal from the narrow bands in the red and also the broad bands. These spectra are depicted in Figures S1−S3 (see Supporting Information). The appearance of the IR spectra for each dimer was found to be independent of probe position. Comparison of the FDIR spectra of the three monomers with the IDIR spectra of the corresponding dimers suggest that in all the three cases the acetylenic C−H stretching vibrations are marginally red-shifted relative to the corresponding monomers. These observations suggest that the monomer units in the dimer are marginally perturbed relative to corresponding monomer. To interpret the IR spectra, the structures of 2FPHA, 3FPHA, and 4FPHA dimers were optimized using M06-2X level of theory using aug-cc-pVDZ basis set. These calculations converged to 19 structures each for the 2FPHA and 3FPHA dimers and 14 structures for the 4FPHA dimer. The structures are shown in Figures S4−S6 (see Supporting Information), and the ZPE- and BSSE-corrected stabilization energies are given in Table S1 (see Supporting Information) along with their dipole moments. The dimers of 2FPHA, 3FPHA, and 4FPHA can be generally classified into three sets of structures. The first set consists of structures that are π-stacked, the second set consists of structures that have C−H···π interaction, while the third set consists of structures with C−H···F hydrogen bond. In all the three cases the π-stacked structures are most stable, while the C−H···F hydrogen-bonded structures are least stable. Structures of the PHA dimer were also calculated at the same level of theory for the sake of comparison. Figure 3 shows the six most stable π-stacked structures of all the three sets of dimers along with the dimers of unsubstituted phenylacetylene, which differ in the relative orientations of the two monomers in the dimer. Table 1 lists the ZPE- and BSSE-corrected stabilization energies calculated at M06-2X/aug-cc-pVDZ level of theory. Further, structures of the six most stable π-stacked structures optimized at M06-2X/aug-cc-pVTZ level of theory and the corresponding ZPE- and BSSE-corrected stabilization energies are listed in Table S2 (see Supporting Information). The stabilization energies calculated at M06-2X/aug-cc-pVTZ level are lower by ∼2−3 kJ mol−1 for the corresponding values calculated at M062X/aug-cc-pVDZ level. However, the energy ordering remains almost the same, with some exceptions for the 3FPHA dimer. In the case of 2FPHA dimer (see Figure 3), the most stable structure 2D1 is almost an antiparallel π-stacked structure with two 2FPHA monomers at a small angle relative to each other, resulting in a dipole moment of 0.28 D. The second most stable

Figure 1. Electronic excitation spectra of (A) 2FPHA, (B) 2FPHAdimer, (C) 3FPHA, (D) 3FPHA-dimer, (E) 4FPHA, and (F) 4FPHAdimer. (A, C, E) Recorded by LIF method. (B, D, F) Recorded by 1CR2PI method by monitoring ion signal at 240 Da.

Figure 2. IR spectra in the acetylenic C−H stretching region of (A) 2FPHA, (B) 2FPHA-dimer, (C) 3FPHA, (D) 3FPHA-dimer, (E) 4FPHA, and (F) 4FPHA-dimer. (A, C, E) Recorded by FDIR method. (B, D, F) Recorded by IDIR method.

3338 cm−1. The strong band at 3334 cm−1 was assigned to the C−H stretching frequency of the acetylenic moiety.88 The IDIR spectrum of the 2FPHA dimer (Figure 2B) shows two C

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Figure 3. Six lowest-energy structures of 2FPHA, 3FPHA, 4FPHA, and PHA dimers optimized at M06-2X/aug-cc-pVTZ level of theory.

Table 1. ZPE- and BSSE-Corrected Stabilization Energies (kJ mol−1) at M06-2X/aug-cc-pVDZ and SAPT0 Interaction Energy Components (kJ mol−1) for Various π-Stacked Dimers structure

ΔE

Eelec

Eind

ECT

Edisp

Eexch

ESAPT0

2D1 2D2 2D3 2D4 2D5 2D6 (1) 3D1 3D2 3D3 3D4 3D5 3D6 4D1 4D2 4D3 4D4 4D5 4D6 PD1 PD2 PD3 PD4 PD5 PD6

−24.1 −24.0 −23.4 −23.4 −23.0 −22.4 −23.6 −23.5 −23.1 −23.0 −22.4 −21.8 −22.9 −22.4 −22.3 −20.1 −19.2 −18.7 −20.4 −20.1 −19.2 −17.5 −17.3 −14.4

−27.0 −27.6 −27.0 −23.9 −24.8 −25.5 −25.2 −23.5 −25.0 −25.0 −23.3 −24.5 −22.3 −24.7 −22.6 −23.2 −19.5 −16.1 −22.5 −18.7 −18.9 −18.4 −17.3 −10.2

−5.6 −6.0 −5.0 −4.8 −5.0 −5.4 −5.4 −5.0 −4.9 −5.2 −4.8 −4.9 −4.9 −5.0 −4.6 −7.0 −4.4 −3.5 −5.3 −5.1 −4.7 −4.4 −4.8 −4.0

−0.5 −0.6 −0.5 −0.5 −0.5 −0.5 −0.6 −0.6 −0.5 −0.6 −0.5 −0.5 −0.6 −0.5 −0.5 −0.5 −0.5 −0.4 −0.5 −0.5 −0.4 −0.3 −0.4 −0.4

−64.3 −65.4 −62.5 −62.9 −63.4 −62.6 −64.8 −63.6 −61.5 −61.8 −60.5 −62.3 −60.8 −63.6 −58.3 −50.8 −55.9 −53.7 −58.1 −61.1 −58.4 −50.8 −54.4 −51.8

61.1 63.4 59.4 57.8 59.3 60.6 61. 7 58.7 58.0 58.5 55.4 60.1 55.1 60.8 52.7 52.7 51.2 48.1 54.4 55.3 52.6 46.5 50.0 45.1

−35. 8 −35.6 −35.1 −33.8 −33.9 −32.9 −33.8 −33.4 −33.3 −33.5 −33.2 −31.6 −33.0 −32.5 −32.9 −28.3 −28.6 −25.2 −31.4 −29.6 −29.3 −27.2 −26.5 −20.9

structure 2D2 is also π-stacked in which the two acetylenic groups are antiparallel to each other, while the fluorine atoms are on the same side leading to dipole moment of 2.11 D. On the one hand, the most stable structure of 3FPHA dimer 3D1 is a perfectly antiparallel π-stacked structure with 0.00 D dipole moment. On the other hand for the 4FPHA dimer the

antiparallel π-stacked structure 4D2 is marginally higher in energy (0.5 kJ mol−1) than the global minimum in which the two FPHA moieties are at 60° relative to each other. The IR spectra of 2FPHA and 4FPHA monomers shows the presence of Fermi resonance coupling, which has been discussed in some detail in our earlier work.88 The IR spectra D

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The Journal of Physical Chemistry A of the 2FPHA and 4FPHA dimers also show the presence of Fermi resonance coupling; however, the band positions and the relative intensities are marginally changed relative to the corresponding monomer. The IR spectrum of the 3FPHA monomer shows a single prominent band, while the corresponding dimer also shows a single broad band. The distinct feature of the IR spectra of the dimers is the marginal red shift (2−6 cm−1) in the acetylenic C−H stretching vibrations relative to the corresponding monomer. The appearance of the IR spectra of the dimers is indicative of the fact that the acetylenic C−H group of the monomer is weakly perturbed in the dimer relative to the monomer. The IR spectra are clearly indicative of absence of C−H···π interaction involving the acetylenic C−H group. This is based on the fact that in the aromatic molecules of the acetylene complexes, the acetylenic C−H stretching vibrations always show a large red shift of ∼50 cm−1.90 Therefore, the presence of C−H···π interaction would result in lowering of C−H stretching frequency of at least one of the monomers in the dimer by ∼30−40 cm−1, which is not observed in the present case. The observed spectra suggest the possibility of dimer with either C− H···π interaction involving aromatic C−H groups or structures with C−H···F hydrogen bonds, or those which are π-stacked. While based on the appearance of the spectra, none of these structures can be ruled out; the energetics readily favor the πstacked structures by ∼10 kJ mol−1 in comparison to the other structures. Therefore, the observed dimers of 2FPHA, 3FPHA, and 4FPHA can be logically assigned to the π-stacked structures. The electronic spectra of the dimers, presented in Figure 1, show considerable broadening relative to the monomers. It has been pointed out earlier, this broadening can be attributed to variety of factors such as (a) distributed Franck−Condon factors, (b) multiple isomers, and (c) excitonic coupling.87 However, the broadening of the IR spectra can only be attributed to the presence of multiple isomers that have closely lying acetylenic C−H stretching vibrations. Our group has recently shown that, under similar experimental conditions, for the ammonia complexes of fluorophenylacetylenes, structures within 0.5 kJ mol−1 of global minimum (calculated at CCSD(T)/CBS level) were populated.91 For the M06-2X method the mean error for the π-stacked systems is ∼1.5 kJ mol−1.75 Figure 4 shows the plot of stabilization energies of six lowest-energy minima for the dimers of PHA, 2FPHA, 3FPHA, and 4FPHA. Considering an energy cutoff of 1.5 kJ mol−1 from the global minimum for the present level of calculation, the observed spectrum can be assigned to the presence of five isomers each for the 2FPHA and 4FPHA dimers and three isomers for the 3FPHA dimer for the structures optimized at M06-2X/aug-cc-pVTZ level as can be inferred from Figure 4. In an effort to understand the origin of broadening of electronic transitions of the dimers time-dependent DFT calculations using CAM-B3LYP/6-311+G(d,p) level were performed for the lowest-energy structures. The corresponding transition energies and oscillator strengths are listed in Table S3 (see Supporting Information). Here the lowest-energy transition for the monomer and two lowest-energy transitions for the dimers were considered. It can be seen that the spread in the transition energies for various geometries of 2FPHA, 3FPHA, and 4FPHA dimers is ∼650, 750, and 560 cm−1, respectively. This spread in transition energies is comparable to the boarding of ∼800 cm−1 seen in the R2PI spectra of the dimers (see Figure 1). The broadening of the electronic and IR spectra thus

Figure 4. Plot of M06-2X/aug-cc-pVTZ energies for the six lowestenergy minima of the dimers of PHA (○), 4FPHA (□), 3FPHA (△), and 2FPHA (◇). The error bars of ±1.5 kJ mol−1 represents the relative accuracy of the M06-2X method.

Figure 5. 2FPHA dimer is modeled using two antiparallel dipoles, where each dipole consists of two opposite charges connected by a harmonic spring. A distortion in the equilibrium length of each of the isolated dipoles takes place when they are brought together in the geometry shown.

can be attributed to the presence of the multiple isomers and the excitonic splitting. As noted earlier, all the structures are essentially π-stacked but differ in relative orientations of the two monomer units. One of the interesting observations that can be made from the IR spectra is the marginal red shift of the acetylenic C−H stretching vibrations in the dimers relative to the monomers. The red shifts in the IR spectra can be rationalized by modeling the FPHA dimer by two antiparallel shifted dipoles, where each dipole consists of two equal and opposite point charges connected by a harmonic spring. Figure 5 shows the configuration of this system. The equilibrium length of each isolated dipole is considered to be x0, and the distorted length when the two dipoles interact is x. Supposing that the distance between the two parallel lines passing through the dipoles is d, and the shift between the dipoles is s, the energy of each dipoles due to the restoring force of spring and electrostatic interaction between the charges is E

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1 k(x − x0)2 − 2

q2 (s 2 + d 2 )

+

q2

included in the plot. For smaller s values this energy is lower than the energy of the non-interacting system as shown in Figure S7 (see Supporting Information). In this simple model the energy of the dipole increases at s = 4, due to repulsion between the two negative ends of the dipoles (see Figure 5). Further, a comparison between non-interacting system and antiparallel and parallel arrangements of dipoles is depicted in Figure S8 (see Supporting Information). On the basis of this simple model, the observed red shift in the acetylenic C−H stretching vibration of the dimers can be assigned to formation of antiparallel dimers. It must be pointed out that a parallel arrangement leads to shortening of the dipole (minimum at x = 4.985) and increase in the force constant (k = 1.007), which should result in a blue shift in the stretching frequency. Decomposition of the interaction energy for the dimers of 2FPHA, 3FPHA, and 4FPHA along with PHA was performed using SAPT0 method, and the results are listed in Tables S4− S6 (see Supporting Information). The SAPT0 energy components for the six lowest-energy minima are listed in Table 1. For the π-stacked structures, dispersion dominates among all the stabilizing components, and the dispersionless interaction energy [Eelec + Eind + Eexch] is destabilizing, which clearly indicates that π-stacking is driven by dispersion. Figure 7 shows the plot of electrostatic and dispersion components of the SATP0 interaction energy and the total stabilization energy as a function of the dipole moment. A general trend of increase with the dipole moment is exhibited by all the three data sets. However, only the electrostatic components are linearly correlated with the dipole moment. Comparison of the various energy components for the most stable dimers suggests that the magnitude of electrostatics, dispersion, and exchange components increase upon substitution of fluorine and are positiondependent. On the relative scale, electrostatic component of 2FPHA dimer is ∼20% larger than in the case of PHA dimer in the order 2FPHA > 3FPHA > 4FPHA ≈ PHA. However, the corresponding increase in the dispersion component is ∼11% in the same order. Therefore, it can be inferred that substitution of fluorine does lead to increase in the electrostatic component

2 ((s + x)2 + d 2)

q2 2 ((s − x)2 + d 2)

(4)

where k is the force constant of each spring, and q is the magnitude of the point charges in the dipole. Taking the values of x0, d, and s to be 5, 3.5, and 4 units, respectively, which are approximations to the 2D1 geometry (see Figure 5), and taking k = 1 and q = 1 units, the energy of the dipole system as a function of x is shown in Figure 6. It is evident from Figure 6

Figure 6. Potential energy plots of a non-interacting dipole of length 5 units and an interacting dipole in parallel and antiparallel orientations. There is an elongation and contraction of the dipole due to interaction in the antiparallel and parallel orientations, respectively. The dotted lines indicate the minimum-energy positions.

that the antiparallel arrangement leads to lengthening of the dipole (minimum at x = 5.015) and a consequent lowering of the effective force constant (k = 0.993), which results in a red shift in the stretching frequency. The energy of the interacting dipole is greater than the non-interacting one, which is because only harmonic potential energy and electrostatic energy are

Figure 7. Plot of energy electrostatic (□; scale on the left) and dispersion (△; scale on the right) components of SAPT0 interaction energies and the ZPE- and BSSE-corrected stabilization energies (scale on the left) for the most stable dimers of PHA, 4FPHA, 3FPHA, and 2FPHA against the dipole moment of the monomer. The dashed lines are trend lines, while the solid line is a linear fit (R2 = 0.965) to the electrostatic components. F

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ACKNOWLEDGMENTS This material is based upon work supported by Board of Research in Nuclear Sciences (Grant No. 2012/34/14) and Department of Science and Technology (Grant No. SB/S1/ PC-29/2012). Authors wish to thank Dr. D. Gosh and Prof. K. Szalewicz for valuable discussions. S.I.M. thanks UGC, and S.S. thanks IIT Bombay for research fellowship. High-performance computing facility of IIT Bombay is gratefully acknowledged.

to a larger extent than dispersion (on a relative scale), which can be attributed to the increased dipole moment. This also explains the fact that the stabilization energies of the dimers follow order 2FPHA > 3FPHA > 4FPHA > PHA, which is same as the dipole moment ordering. It can therefore be concluded that dipole moment enhances the ability of substituted benzenes to π-stack.

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4. CONCLUSIONS Electronic (1C-R2PI) and vibrational (IR−UV double resonance) spectroscopic methods were applied to the dimers of three fluorophenylacetylenes (2FPHA, 3FPHA, and 4FPHA) to determine the nature of interaction. Comparison of IR spectra of the dimers with the corresponding monomers reveals marginal perturbation of the acetylenic C−H group, which rules out the possibility of formation of structures involving C− H···π interaction. On the basis of minimal perturbation of the acetylenic C−H groups it was inferred that all the homodimers form π-stacked structures, which was amply supported by the calculations. In all the cases the observed dimers were assigned to the set of π-stacked structures in the energy band of 1.5 kJ mol−1, which also explains the inhomogeneous broadening of the R2PI spectra due to presence of multiple isomers. Further, the observed red shift in the acetylenic C−H stretching vibration of the dimers suggests that the antiparallel structures contribute predominantly. Partitioning of the interaction energy using SAPT0 scheme reveals that π−π stacking in the dimers is dispersion-dominated with reasonable contribution from electrostatics. The stability trends indicate that the dipole moment enhances the ability to form π-stacked structures.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b00209. IR spectra of monomers and dimers recorded at various probe wavelengths, structures of optimized dimers, potential energy surfaces of model interacting dipoles, mathematica code for evaluating potentials of model interacting dipoles, stabilization energies of various optimized dimers, calculated electronic excitation energies and the oscillator strengths, SAPT0 energy decomposition of various dimers, coordinates for the optimized geometries of the all the dimers (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (A.H.) *E-mail: [email protected]. (G.N.P.) ORCID

G. Naresh Patwari: 0000-0003-0811-7249 Author Contributions §

Equal contribution.

Author Contributions

The problem was formulated by G.N.P.; S.I.M. performed all the experiments, and S.S. performed all the calculations. The dipole model was given by A.H. The results were interpreted jointly by S.I.M., S.S., A.H., and G.N.P. Notes

The authors declare no competing financial interest. G

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