Insights into the C–H⋯F–C hydrogen bond by Cambridge Structural Database analyses and computational studies

Abstract

Fluorine bound to a carbon atom (C–F group) behaves differently than its heavier analogues. Weak interactions involving C–F groups in crystal structures have been found to be of immense interest in recent literature. Herein, a series of dimers of ethylene and fluoroethylene have been studied by an ab initio method to calculate the stabilization energy offered by weak C–H⋯F interactions with benchmark accuracy by using a complete basis set (CBS) extrapolation technique. The model complexes have been studied in a systematic fashion to explore the structural and electronic parameters. The total interaction energies of all the complexes have been decomposed to obtain information about the nature of such interactions. Dispersion energy has been found to be the major component in the stabilization energy and C–H⋯F interactions have been found to be of closed shell type in the atoms in molecules (AIM) framework.

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Introduction

Hydrogen bonds and other non-covalent interactions are considered to play a very important role in supramolecular chemistry, chemical biology and material science.^1–5 The “hydrogen bond” was first defined by Pauling in 1939 as “under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them”.⁶ Earlier theoretical and experimental studies have indicated that strong (−14.1 kcal mol⁻¹ to −50.0 kcal mol⁻¹) and moderate (−4.1 kcal mol⁻¹ to −14.0 kcal mol⁻¹) hydrogen bonds are stabilized majorly by an electrostatic component, whereas the weak hydrogen bonds (≤−4.0 kcal mol⁻¹) have a more significant contribution from dispersion energy.^2–5,7 Hydrogen bonds (X–H⋯Y) are generally associated with the red shift of the stretching frequency of the X–H bond. On the contrary a new set of hydrogen bonded complexes (mainly C–H⋯O and C–H⋯N) has been found to exhibit a blue shift of the C–H stretching frequency.^8–10 Hobza et al., have termed this type of interaction as an “anti” or “improper” hydrogen bond.¹¹

Fluorine mediated hydrogen bond is of special interest in crystal engineering because of its interesting role in the solid state.¹² Fluorine stands separated from its congeners due to its small size and very low polarizability.¹³ It has been debated over decades that “organic fluorine”¹⁴ hardly accepts hydrogen bond and thus offers insignificant interactions towards the stability of crystal structures.^15–18 This was mostly based on the analyses of the crystal structures containing fluorine, reported in the database available in 1990s. As there were only a few structures containing fluorine in the database, the analyses done by the authors were based on insufficient data and hence the occurrences of hydrogen bonds involving “organic fluorine” were refuted. They also commented on the non-polarizability of fluorine bonded to a carbon atom. A recent review¹⁹ and a couple of other results^20,21 highlight the importance of organic fluorine in crystal engineering and in designing of functional materials. Fluorine mediated weak interactions of the types C–H⋯F–C, C–F⋯F–C and C–F⋯π have been shown to influence the packing of supramolecular assemblies in the absence of strong hydrogen bonds.²² The evidence for the formation of sigma hole on fluorine has also been shown recently.^22d It is also realised by extensive study that crystal packing is influenced by the weak non-covalent interactions involving organic fluorine even in the presence of typical hydrogen bonds like N–H⋯O=C and C–H⋯O=C.²³

Although various experimental evidences have been provided in the favour of fluorine mediated interactions, suitable computational study to reveal the vivid description of the nature and the properties of such type of hydrogen bonds is still lacking. Systematic fluorination on the model complexes of CH₄ using density functional theory (DFT) has revealed the effect of Lewis acidity of the proton donating bond and basicity of the acceptor.²⁴ Novoa et al., studied the C–H⋯F–C hydrogen bonds using computational methods in neutral and charged model complexes of real molecules, selected from Cambridge Structural Database (CSD).²⁵ They have shown that the stabilization energy increased manifold in the cases of interactions between the charged species compared to those between the neutral species. If the stabilization energy for these hydrogen bonds is decomposed in physically meaningful components, the nature of the interaction is well understood. In weak hydrogen bonds, the dispersion energy is more prominent while in strong hydrogen bridges the electrostatic term contributes the most.²⁶ Novoa et al.,²⁷ also classified a new class of hydrogen bond between a cation and an anion, where the stabilization energy extends beyond the covalent limit (−80 to −210 kcal mol⁻¹).

Fluorinated compounds generally exhibit better biological activity compared to their hydrogen analogues but are more resistant to biological degradation.²⁸ Presence of fluorine alters electronic arrangement and thus has a pronounced effect on intra and intermolecular interactions. Based on our experience with the exploration of aromatic C–F groups a number of different supramolecular synthons are proposed involving one or more C–H⋯F–C hydrogen bonds (Scheme 1). A search within the latest update (May 2014) of CSD has been done on the above proposed synthons within a distance of H and F within 2.2–2.8 Å and the ∠C–H⋯F–C within 90–180°. The R factor was restricted to ≤10%, organic structures only and all the structure from powder diffraction, structures with ions and disorders etc. have been rejected from the search criteria. The synthon S1 was found in the cases of 532 compounds, totalling to 614 interactions, S2 was present in 220 compounds resulting into 282 interactions, S3 was found in 138 molecules yielding 194 interactions and S4 was observed in 388 compounds totalling to 442 interactions. A search in the database for S5 was carried out such that the hits did not have any of the searches for S1–S4. This resulted into 1855 compounds which had 3342 interactions within them. The relevant scatter plots are reported in the ESI.† It is evident from the search that most of these interactions are common in the reported crystal structures in the database. The synthons of S1 being a centrosymmetric dimer, the database study showed the preference for centre of symmetry in the reported structures. This is evident from the two equal H⋯F distances and the two equal (∠C–H⋯F) angles. The scattergram of distance and angle shows higher population the distance range 2.5–2.7 Å and in the angle range 130–160°. The synthon S2 being non-centrosymmetric in nature, the two H⋯F distances and the two ∠C–H⋯Fs do not show any correlation. The population is more above 2.6 Å and in the angle range of 130–160°. The bifurcated C–H⋯F–C interactions, S3 and S4 show significant population over the larger distance range and non-linearity in the geometries (population is higher in the lower angle region). The last synthon S5 as well establishes the fact that non-linearity is preferred for ∠C–H⋯F (much less than 180°).

Scheme 1 Different fluorine mediated synthons observed in CSD.

It would be interesting to look into the C–H⋯F–C hydrogen bonds similar to the above mentioned synthons. Hence, ethylene, fluoroethylene and difluoroethylene (1,2-cis; 1,2-trans and 1,1-difluoroethylene) as model compounds to form several different dimers involving C–H⋯F–C hydrogen bonds similar to those shown in Scheme 1. We intend to compute the energetics and the intrinsic nature of the C–H⋯F–C hydrogen bonds in a series of molecular complexes between the selected set of molecules using complete basis set (CBS) extrapolation method.²⁹

Methodology

Based on the synthons indicated in Scheme 1, a series of different dimers formed by molecular pairs ethylene (e) & fluoroethylene (fe), ethylene & 1,1-difluoroethylene (1,1-dfe), ethylene & 1,2-difluoroethylene (1,2-dfe) (cis and trans), fluoroethylene & fluoroethylene, fluoroethylene & 1,2-difluoroethylene (cis and trans), fluoroethylene & 1,1-difluoroethylene, 1,2-difluoroethylene (cis and trans) & 1,2-difluoroethylene (cis and trans), 1,1-difluoroethylene & 1,2-difluoroethylene (cis and trans) were considered for all computational purposes described herein.

Geometry optimization for all the complexes was carried out at the Møller–Plesset second order perturbation theory (MP2) using the aug-cc-pVDZ and aug-cc-pVTZ using Gaussian 09 suite of programmes.³⁰ Gaussview 5.0 (ref. 31) was used for visualization of the geometries. Vibrational frequency analysis at the aug-cc-pVDZ basis set level ensured that the geometries obtained corresponded to true minima. Optimized geometries of both the basis sets exhibited comparable geometrical parameters (ESI†). Hence, geometry at aug-cc-pVTZ level is considered for further calculations. Single point energy of the optimized geometry obtained using aug-cc-pVTZ basis set was calculated at aug-cc-pVDZ level and these energy values were extrapolated to the infinite basis set (CBS) limit according to the method proposed by Varandas.²⁹ The Hartree–Fock energy was extrapolated according to the equation , where b = 9.0, X denotes the cardinal number of basis set and A is a constant. The correlation energy was extrapolated using the equation E^Cor_X = E^Cor_∞ + A₃Y, where Y = (X + α)⁻³[1 + τ₅₃(X + α)⁻²], α = −3/8 and τ₅₃ = A₅/A₃ and A₅ and A₃ are related according to A₅ = A₅(0) + cA₃ⁿ where A₅(0), c and n are taken from the article by Varandas.²⁹ This scheme has only two parameters E^Cor_∞ and A₃ for all (X − 1, X) pairs. The extrapolated correlation is given by the total energy was determined by adding the E^HF_∞ and E^Cor_∞. Stabilization energy was calculated as ΔE_S = E^Dimer_CBS − {E^Monomer1_CBS + E^Monomer2_CBS}. Contribution of the specific C–H⋯F hydrogen bond (E^C_H) to the total stabilization energy was calculated by replacing F by H³² to calculate single point energy values at aug-cc-pVDZ and aug-cc-pVTZ. Obtained energy values were then extrapolated to the CBS limit. It is calculated as E^C_H = E^{Optimized geometry}_CBS − E^{Replaced geometry}_CBS. Wavefunctions were generated from the Gaussian output files of the optimised geometries at aug-cc-pVTZ for topological analysis of the electron density according to “Atoms in Molecules” (AIM) Quantum Theory³³ using the AIM2000 programme.³⁴ Dispersion energy (E_D) of the optimised structure was calculated as E_D = {E^Dimer_MP2 − [E^Monomer1_MP2 + E^Monomer2_MP2]} − {E^Dimer_HF − [E^Monomer1_HF + E^Monomer2_HF]}, where all the energy values correspond to CBS limit. Total molecular energy was decomposed in different components by Morokuma³⁵ energy decomposition analysis using WinGamess³⁶ at HF/cc-pVTZ basis set.

Results and discussion

Fig. 1 lists all the optimized geometries arising from their parent starting dimers. The dimers are formed either by only one C–H⋯F–C interaction, or by a bifurcated C–H⋯F–C interaction or two C–H⋯F–C interactions. Geometrical parameters of all the C–H⋯F–C interactions are listed in the Table 1, including the ΔE_s values for each dimer. Two distinct dimers A and B have been obtained from the optimization of ethylene (donor) and fluoroethylene (acceptor). Three different optimized dimers namely, C, D and E have been obtained by replacing one of the hydrogen atoms of the acceptor molecule by F, i.e. dimers between ethylene (e) and difluoroethylenes (1,1dfe, 1,2 dfe, cis and trans). These dimers explain the effect of fluorination in the acceptor molecule. The third set of dimers F, G and H have been obtained by replacement of one of the H atoms by F in the donor molecule, i.e. the dimers are between two fe molecules. The fourth set of optimized dimers, I, J and K have been obtained between cis-1,2-dfe and fe. The optimized dimers L, M and N are the molecular complexes between fe and trans-1,2-dfe. The next series (O, P and Q) comprises of three complexes between 1,1-dfe and fe. The next series of optimized dimers (R, S, T, and V) are between two 1,2-dfe molecules (cis–cis, trans–trans, trans–trans, cis–trans, cis–trans respectively). The last set of optimized dimers (U, W, X, and Y) are between the combinations of molecules 1,1-dfe and 1,2-dfe (cis and trans). It is interesting to note that approximately 40% (dimers A, B, D, E, F, K, N, Q and Y) of the optimized dimers have bifurcated C–H⋯F–C hydrogen bonds with bifurcation at the acceptor F atom. These C–H⋯F–C interactions have been found to have ∠C–H⋯F close to 120° and H⋯F distance varying from 2.5–2.75 Å. All of these dimers also have a weak C–H⋯π interaction involving the ethylenic C=C bond. The dimers C, G, M, P, T, W and X contain two similar C–H⋯F interactions between the two interacting molecules. The dimers H, L, O, R, S, U and V are unique as the donor and acceptor atoms are connected to the same C atom.

Fig. 1 Geometry and structures of series of molecular complexes of ethylene.

Table 1 Stabilization energies, structural and topological parameters of different bimolecular complexes

Geometry		d(HF) (Å)	∠CHF (°)	Gradient path (Å)	Electron density (eÅ^−3)	Laplacian (eÅ^−5)	ΔEs^a (kcal mol^−1)	ΔEs^b (kcal mol^−1)
a Stabilization energy at aug-cc-pVTZ level. b Stabilization energies in CBS extrapolation.
A	1	2.75	120.00	2.82	0.031	0.445	−2.1	−5.6
	2	2.75	120.03	2.81	0.031	0.446
B	1	2.78	115.70	2.89	0.031	0.454	−2.1	−5.6
	2	2.78	115.69	2.90	0.031	0.454
C	1	2.68	118.78	2.76	0.036	0.520	−2.3	−5.8
	2	2.62	139.03	2.79	0.037	0.530
D	1	2.72	119.93	2.79	0.033	0.472	−2.4	−5.8
	2	2.72	119.95	2.85	0.033	0.472
E	1	2.77	119.40	2.84	0.029	0.425	−2.2	−5.6
	2	2.77	119.12	2.64	0.030	0.430
F	1	2.58	120.38	2.81	0.042	0.618	−2.5	−6.0
	2	2.72	118.55	2.44	0.033	0.479
G	1	2.42	158.46	2.44	0.053	0.754	−2.5	−6.1
	2	2.42	158.46	2.48	0.053	0.754
H	1	2.44	133.44	2.48	0.052	0.763	−2.6	−6.0
	2	2.44	133.38	2.55	0.053	0.772
I	1	2.51	134.74	2.65	0.047	0.671	−2.9	−6.4
J	1	2.57	118.18	2.66	0.045	0.689	−3.0	−6.4
K	1	2.58	118.27	2.66	0.043	0.624	−2.8	−6.3
	2	2.58	118.27	2.46	0.043	0.624
L	1	2.43	143.93	2.38	0.051	0.746	−2.7	−6.1
	2	2.35	151.67	2.46	0.060	0.886
M	1	2.43	168.47	2.35	0.049	0.704	−2.6	−6.1
	2	2.32	168.21	2.85	0.061	0.904
N	1	2.74	118.35	2.68	0.033	0.478	−2.5	−5.9
	2	2.61	116.90	2.41	0.041	0.594
O	1	2.39	156.32	2.52	0.055	0.805	−2.3	−5.8
	2	2.49	144.21	2.39	0.044	0.652
P	1	2.37	172.80	2.51	0.055	0.811	−2.2	−5.7
	2	2.48	169.14	2.73	0.043	0.624
Q	1	2.67	118.00	2.84	0.036	0.616	−2.2	−5.6
	2	2.75	117.65	2.50	0.031	0.532
R	1	2.45	125.53	2.50	0.052	0.791	−2.8	−6.3
	2	2.45	125.52	2.47	0.052	0.791
S	1	2.46	125.17	2.51	0.051	0.769	−1.9	−5.9
	2	2.46	125.17	2.39	0.0501	0.769
T	1	2.36	149.58	2.39	0.060	0.874	−2.5	−6.6
	2	2.36	149.51	2.47	0.060	0.873
U	1	2.41	153.71	2.48	0.052	0.763	−3.0	−5.8
	2	2.45	140.20	2.44	0.047	0.716
V	1	2.39	140.25	2.40	0.054	0.822	−2.5	−6.3
	2	2.37	149.45	2.44	0.057	0.837
W	1	2.45	167.24	2.47	0.045	0.677	−2.3	−5.4
	2	2.44	167.20	2.51	0.045	0.680
X	1	2.44	147.44	2.41	0.050	0.732	−2.8	−6.1
	2	2.39	154.98	2.71	0.056	0.814
Y	1	2.66	116.58	2.76	0.037	0.539	−2.3	−5.7
	2	2.66	116.56	2.43	0.037	0.539

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The complexes show stabilization energy in the range of 5–6 kcal mol⁻¹ (Table 1), and can be considered to be moderately strong.^25,37 Stabilization energy range does not exhibit any obvious trend among the synthons observed. It is interesting to note that the organic fluorine which was debated to be weak acceptor of hydrogen bond, actually exhibits moderately strong hydrogen bond in aliphatic compounds. All the optimised structures show the evidence of non-collinear ∠C–H⋯F–C angles.

Hydrogen bond contribution

It is important to find the contribution of the C–H⋯F–C interaction to the overall stabilization. When the interacting F is replaced by H atom, the concerned C–H⋯F interaction is lost. Hence the difference in the stabilization energy of the original dimer and the modified dimer will reflect the contribution of the particular C–H⋯F–C interaction which has been removed from the parent dimer (Table 2). It can be seen from this table that an addition of F in the acceptor molecule does not in general result in the increase in the value of E^C_H (geometries D and E are compared with A and B) but an additional F in the donor molecule results into the increase in the value of E^C_H (geometries F, K, N, and Q are compared with A and B and Y with N). Therefore it may be concluded that the addition of more F atom(s) in the donor molecule acts in a cooperative nature and increases the strength of the concerned C–H⋯F interactions. This may be explained by the electron withdrawing nature of the additional F in the donor molecule. The additional F attracts the electron density towards itself thereby increasing the partial positive character of the concerned H atom of the C–H bond, which in turn is responsible for the increased stability of the C–H⋯F–C interactions. Few of the geometries like A, B, D, E, F, K, N, Q, and Y have bifurcated hydrogen bonds and I, J have single hydrogen bond.

Table 2 Hydrogen bond contribution with different no. of F substitution in donor and acceptor molecules

Structures	Donor substitution	Acceptor substitution	E ^CH (kcal mol^−1)
A	0	0	−1.3
B	0	0	−1.3
D	0	1	−1.5
E	0	1	−1.9
F	1	0	−2.3
K	2	0	−2.1
N	1	1	−1.7
Q	1	1	−1.6
Y	2	1	−2.0

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The trend in E^C_H values for the structures having closed hydrogen bonded synthon has also been studied carefully. Structure C, G, H, L, M, O, P, R, S, T, U, V, W, and X have this kind of closed synthon, where it is not possible to identify any fragment as donor or acceptor. These structures show an interesting pattern when E^C_H values are plotted against the bond angle (Fig. 2) and bond distance (Fig. 3). It is important to note that most of these interactions are found in the angular range 140–160° and in the distance range from 2.4–2.55 Å and E^C_H ranging between −1.9 and −2.72 kcal mol⁻¹. Even the most stabilizing C–H⋯F–C interaction is at 120°. This observation is in concurrence with the CSD analysis on similar type of dimers. This indicates that the C–H⋯F hydrogen bonded dimers prefer nonlinear geometries as previously pointed out by Novoa et al.²⁵

Fig. 2 Angular dependence of hydrogen bond contribution for structures having closed hydrogen bonded synthons.

Fig. 3 Variation of hydrogen bond contribution with the H⋯F distances for structures having closed hydrogen bonded synthons.

Dispersion energy

Dispersion energy is the origin of weak intermolecular interactions. Hence, dispersion energies of all the complexes have been calculated to study the details of the interaction. It is noteworthy that the dispersion energy component ranges from 1.8–3.8 kcal mol⁻¹ (Table 3). The structures having dispersion energy >3 kcal mol⁻¹ are C, I and J, while those having dispersion energy <2 kcal mol⁻¹ are H, L, M, O, P, R, S, U, V and W. These molecular dimers have two C–H⋯F–C hydrogen bonds, like synthon type S1 and the interacting molecules lie almost in the same plane.

Table 3 Dispersion energy and different energy component by Morokuma energy decomposition in all the structures

Geometry	E D ^a	ES	EX	PL	CT	MIX	E T
a E D has been calculated as mentioned in the methodology section.
A	−2.8	−2.2	3.0	−0.2	−0.4	0.1	−2.5
B	−2.5	−2.3	2.8	−0.2	−0.4	0.1	−2.5
C	−3.2	−1.9	2.9	−0.2	−0.4	0.1	−2.7
D	−2.8	−2.5	3.2	−0.3	−0.5	0.2	−2.7
E	−2.9	−2.2	3.0	−0.2	−0.5	0.1	−2.7
F	−2.7	−2.6	2.7	−0.2	−0.4	0.1	−3.1
G	−2.8	−2.8	2.9	−0.3	−0.4	0.1	−3.3
H	−1.8	−3.7	2.2	−0.3	−0.4	0.0	−4.0
I	−3.8	−3.2	3.9	−0.3	−0.6	−0.1	−4.1
J	−3.8	−3.4	4.1	−0.4	−0.8	0.3	−4.0
K	−2.7	−3.5	2.8	−0.3	−0.5	0.0	−4.2
L	−1.9	−3.9	2.4	−0.4	−0.5	0.0	−4.3
M	−2.0	−3.6	2.4	−0.4	−0.5	0.0	−4.1
N	−2.8	−2.5	3.0	−0.3	−0.5	0.1	−3.0
O	−1.9	−3.1	2.1	−0.3	−0.4	0.0	−3.6
P	−2.0	−3.0	2.1	−0.3	−0.4	0.0	−3.6
Q	−2.8	−2.0	2.7	−0.2	−0.5	0.1	−2.7
R	−1.8	−4.3	2.1	−0.3	−0.4	0.0	−4.7
S	−1.8	−3.5	2.0	−0.3	−0.4	0.0	−4.0
T	−3.00	−3.3	3.2	−0.3	−0.6	0.0	−4.0
U	−1.9	−3.3	2.0	−0.3	−0.4	0.0	−3.9
V	−1.9	−4.0	2.3	−0.4	−0.5	0.0	−4.5
W	−1.9	−2.4	1.8	−0.2	−0.4	0.0	−3.1
X	−2.9	−2.5	2.7	−0.3	−0.5	0.0	−3.5
Y	−2.8	−2.2	2.7	−0.3	−0.5	0.0	−3.1

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Morokuma energy decomposition

Partitioning of interaction stabilization energy gives immense information about the nature of the interaction. The Morokuma energy decomposition analysis partitions the interaction energy in electrostatic (ES), polarization (PL), exchange–repulsion (EX) and charge transfer (CT) components, along with higher order terms (MIX). The ES term represents the total Coulombic interaction between the free monomer charges and the multipoles and may be either attractive or repulsive. The PL term depicts interaction between all permanent charges and induced multipoles. It describes the distortion of the electron cloud of monomer1 by monomer2 and vice versa and is always attractive in nature. The origin of EX is the short range repulsion originating from the electron orbital overlap of one monomer with the other satisfying Pauli's principle. CT is originated from the electron delocalization from the filled orbitals of monomer1 to the vacant orbitals of the other monomer. MIX term contributes to the other coupling terms. All the decomposed components of all the optimized dimers are listed in the Table 3. ES and EX contributions are of the same order but of opposite sign. Hence they cancel each other. The other components, like PL, CT and MIX are one order smaller in magnitude than E_D. Hence dispersion energy becomes the major component of stability. The molecules of S1 type mentioned in the previous comparison of having low dispersion energy are comprised of high ES values which exceeds the destabilizing component EX.

Atoms in molecule calculation

A great deal of information about the nature of bonding in a molecule or a complex can be obtained from the topological analysis of the electron density. Koch and Popelier suggested a set of criteria for the existence of hydrogen bond.³⁸ AIM2000 calculations on all the dimers show the presence of BCPs (point corresponding to ∇ρ = 0) between the hydrogen atom and the F atom. The concomitant bond path links the two atoms as well. Again ring critical points are observed for the geometries with the synthons, which form a closed ring between the two molecules. This has made the topology consistent. The calculated values of ρ and ∇²ρ and the optimized H⋯F distances are listed in the Table 1. ρ values lie in the range of 0.03–0.06 eÅ⁻³ and ∇²ρ lies within 0.42–0.90 eÅ⁻⁵. Electron density shows an inverse exponential relation with the H⋯F distance with R² = 0.96 (Fig. 4).³⁹ This correlation is expected as with the increase in bond length distance the orbital overlap reduces and hence electron density is reduced. Surprisingly ∇²ρ shows poor correlation with H⋯F distance, while it shows excellent exponential inverse relation with gradient path with R² = 0.91 (Fig. 5). Close inspection to the molecular graph reveals bending nature of bond paths which deviates significantly from the linear chemical bond paths (ESI†). Laplacian of the electron density (∇²ρ(r)) at the bond critical points give us information about the nature of bonding. The Laplacian values are all positive indicating a closed shell of interaction (Table 1).

Fig. 4 Correlation plot of electron density and H⋯F distance.

Fig. 5 Correlation plot of Laplacian and gradient.

Summary and conclusion

Twenty five fluorinated dimers have been computationally analysed to gain insight into the stabilizing effect of C–H⋯F–C interactions. Our results show that the increase in the number of fluorine atoms in the donor molecule increases the stabilization energy of the dimers. The C–H⋯F–C interaction majorly comprises of polarization and dispersion forces. Study of these interactions by AIM theory reveals the closed shell nature of the interaction. The electron density at the BCPs show excellent correlation with the H⋯F distance and the values of Laplacian as well correlate with the gradient path. We would further emphasise that the dimers of the type S1 prefer H⋯F distance in the range 2.4–2.6 Å and the ∠C–H⋯F is preferred to be between 130° and 160°. This trend was observed in the CSD analysis as well. Structures of this particular synthon have appreciable contribution from the electrostatic component and lower dispersion energy. Further the other synthons (S2–S5) are majorly stabilised by large dispersion energy. In our knowledge, this is the first attempt to compute the C–H⋯F–C interaction energy and to explore the nature of the interaction by several probes. Novoa et al., have found preference of linear geometries in methane–fluoromethane complexes in contrary to the database study. Our results indeed follow the trend observed in the database study to support the fact that non-linearity is not due to the crystal packing but is inherent in molecular geometry preference and stabilization of different synthons in different geometrical preferences. The study on similar aromatic compounds and other fluorinated dimers need to be conducted to emphasize this observation. The extension of this study using benzene, fluorobenzene and difluorobenzene as model systems to investigate the effect of conjugation and systematic substitution (fluorination, chlorination etc.) in donor and acceptor to C–H⋯F–C interaction is in progress. This extension work is necessary to correlate the observations from ab initio study with the database.

Acknowledgements

We thank Prof. A. J. C. Varandas for useful discussion and for providing the code for CBS calculation. We thank Dr P. Balanarayan and Dr Saurav Srivastava for their generous help. SM thanks DST INSPIRE for fellowship. Computing facility of IISER Mohali is gratefully acknowledged for providing computational facilities and IISER Mohali is acknowledged for funding and infrastructural supports.

References

1. G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biology and Chemistry, Springer-Verlag, Berlin, 1991.
2. G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford University Press, New York, 1997.
3. S. Scheiner, Hydrogen Bonding. A Theoretical Perspective, Oxford University Press, Oxford, 1997.
4. G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in Structural Chemistry and Biology, Oxford University Press, New York, 1999.
5. T. Stiner, Angew. Chem., Int. Ed., 2002, 41, 49.
6. L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, NY, 3rd edn, 1960, p. 449.
7. G. R. Desiraju, Acc. Chem. Res., 2002, 35, 565.
8. Y. Gu, T. Kar and S. Scheiner, J. Am. Chem. Soc., 1999, 121, 9411.
9. P. Hobza and Z. Havlas, Theor. Chem. Acc., 2002, 108, 325.
10. N. Karger, A. M. Amorim da Costa and J. A. Riberio-Claro, J. Phys. Chem. A, 1999, 103, 8672.
11. (a) P. Hobza and Z. Havlas, Chem. Rev., 2000, 100, 4253; (b) I. V. Alabugin, M. Manoharan, S. Peabody and F. Weinhold, J. Am. Chem. Soc., 2003, 125, 5973.
12. (a) A. R. Choudhury, U. K. Urs, K. Nagarajan and T. N. Guru Row, J. Mol. Struct., 2002, 605, 71; (b) A. R. Choudhury and T. N. Guru Row, Cryst. Growth Des., 2004, 4, 47; (c) A. R. Choudhury and T. N. Guru Row, CrystEngComm, 2006, 8, 265.
13. A. R. Choudhury, K. Nagarajan and T. N. Guru Row, Cryst. Eng., 2003, 6, 43.
14. J. D. Dunitz and R. Taylor, Chem.–Eur. J., 1997, 3, 89.
15. R. Taylor and O. Kennard, J. Am. Chem. Soc., 1982, 104, 5063.
16. G. R. Desiraju and R. Parthasarathy, J. Am. Chem. Soc., 1989, 111, 8725.
17. J. A. K. Howard, V. J. Hoy, D. O'Hagan and G. T. Smith, Tetrahedron, 1996, 52, 12613.
18. L. Shimoni and J. P. Glusker, Struct. Chem., 1994, 5, 383.
19. R. Berger, G. Resnati, P. Metrangolo, E. Weber and J. Hulliger, Chem. Soc. Rev., 2011, 40, 3496.
20. D. Chopra and T. N. Guru Row, CrystEngComm, 2011, 13, 2175 and references therein.
21. D. Chopra, Cryst. Growth Des., 2012, 12, 541.
22. (a) G. Kaur, P. Panini, D. Chopra and A. R. Choudhury, Cryst. Growth Des., 2012, 12, 5096; (b) V. R. Hathwar, D. Chopra, P. Panini and T. N. Guru Row, Cryst. Growth Des., 2014, 14, 5366; (c) V. R. Hathwar, R. G. Gonnade, P. Munshi, M. M. Bhadbhade and T. N. Guru Row, Cryst. Growth Des., 2011, 11, 1855; (d) V. R. Hathwar and T. N. Guru Row, Cryst. Growth Des., 2011, 11, 1338.
23. (a) D. Chopra and T. N. Guru Row, Cryst. Growth Des., 2005, 5, 1679; (b) P. Panini and D. Chopra, CrystEngComm, 2012, 14, 1972.
24. I. Hyla-Kryspin, G. Haufe and S. Grimme, Chem.–Eur. J., 2004, 10, 3411.
25. J. J. Novoa and E. D'Oria, CrystEngComm, 2008, 10, 423.
26. T. V. Mourik and F. B. Duijneveldt, J. Mol. Struct.: THEOCHEM, 1995, 341, 63.
27. J. J. Novoa and E. D'Oria, J. Phys. Chem. A, 2011, 115, 13114.
28. K. Reichenbacher, H. I. Suss and J. Hulliger, Chem. Soc. Rev., 2005, 34, 22.
29. A. J. C. Varandas, J. Chem. Phys., 2007, 126, 244105.
30. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman Jr J. A. Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03, Revision B.05, Gaussian, Inc., Pittsburgh, PA, 2003.
31. R. Dennington, T. Keith and J. Millam, GaussView, version 5, Semichem Inc., Shawnee Mission KS, 2009.
32. M. Karanam and A. R. Choudhury, Cryst. Growth Des., 2013, 13, 4803.
33. R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, New York, 1990.
34. F. Biegler-Konig, AIM2000, University of Applied Sciences, Bielefeld, Germany.
35. K. Kitaura and K. Morokuma, Int. J. Quantum Chem., 1976, 10, 325.
36. M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. Su, T. L. Windus, M. Dupuis and J. A. Montgomery, J. Comput. Chem., 1993, 14, 1347.
37. R. Parthasarathi, V. Subramanian and N. Sathyamurthy, J. Phys. Chem. A, 2006, 110, 3349.
38. U. Koch and P. L. A. Popelier, J. Phys. Chem., 1995, 99, 9747.
39. L. Senthilkumar, T. K. Ghanty and S. K. Ghosh, J. Phys. Chem. A, 2005, 109, 7575.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra03063c

‡ These authors have contributed equally.

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