Rotational Study of Dimethyl Ether–Chlorotrifluoroethylene: Lone Pair

Article pubs.acs.org/JPCA

Rotational Study of Dimethyl Ether−Chlorotrifluoroethylene: Lone Pair···π Interaction Links the Two Subunits Lorenzo Spada,† Qian Gou,†,§ Yannick Geboes,‡ Wouter A. Herrebout,‡ Sonia Melandri,† and Walther Caminati*,† †

Dipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy Department of Chemistry, University of Antwerp, Groenenborgerlaan 171, 2020 Antwerp, Belgium

‡

S Supporting Information *

ABSTRACT: The rotational spectra of two isotopologues of chlorotrifluoroethylene− dimethyl ether show that the two constituent molecules are held together by a lone pair···π interaction. The ether oxygen is linked to the (CF2) carbon atom, with a CO distance of 2.908 Å.

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INTRODUCTION Rotational spectroscopy is a powerful technique in characterizing noncovalent interactions through the investigation of small molecular complexes isolated in the gas phase. We will describe in this paper the intermolecular interactions that have been encountered in the gas-phase studies of adducts of dimethyl ether (DME) with various partner molecules and then discuss a new kind of interaction found in its complex with chlorotrifluoroethylene. First we should mention that DME itself has a wide range of industrial applications as starting material for conversion process as a methylating agent, propellant, refrigerant, and fuel for diesel engines.1 Coming back to the nonbonding interactions involved in the molecular complexes of DME, we can divide them in several categories. (i) van der Waals interactions characterize the adducts with rare gases, whose interaction energies are in the range 1.0−3.6 kJ/mol in going from DME−Ne2 to DME−Ar,3 DME−Kr,4 and DME−Xe,5,6 according to a linear dependence with the square of the rare gas atom polarizability. (ii) Weak hydrogen bonds of the type C−H···O or C−H···F (2 to 3 kJ/mol for each of them) link the subunits in the dimer of DME or in its adducts with hydrogen-containing freons.7−9 (iii) Conventional hydrogen bonds have been found to constitute the main linkage with the partner molecule in complexes of DME with hydrogen halides10,11 or alcohols.12−14 In these complexes the dissociation energies have been estimated to be in the range 15−25 kJ/mol. (iv) Finally, it has been found that DME is linked to perhalogenated freons through a O···X (X = Cl, Br, and I) halogen bond,15,16 with interaction energies on the order of 10 kJ/mol. Recently, it has been reported that in the gas phase chlorotrifluoroethylene (C2F3Cl) is linked to simple molecules like H2O17 or NH318 through an interaction between its π © XXXX American Chemical Society

electronic system and one oxygen or nitrogen lone pair (lp). This is related to the so-called π-hole effect.19 DME is heavier than H2O, but besides the same molecular symmetry (C2v), it has an electronic environment of the oxygen atom (two lone pairs) similar to that of H2O. C2v is assumed to be the symmetry group of DME on the time scale of MW because we do not observe any splitting due to the internal rotations of the two methyl groups. From the previously mentioned list of molecular complexes of DME with organic molecules, it appears that DME behaves like water, forming the same kind of interaction: weak hydrogen bonds with hydrogenated freons and halogen bonds with perhalogenated saturated freons. Will DME still behave like water, forming an lp···π link in complexes with unsaturated freons not containing hydrogen atoms, for example, C2F3Cl? Such an lp···π interaction has also been observed in the 1:1 complex of DME with C2F3Cl in cryogenic solutions.20 To characterize this linkage, we thought to investigate the rotational spectrum of C2F3Cl−DME. The obtained results are discussed in the following sections.

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EXPERIMENTAL SECTION

A 1:1 mixture of C2F3Cl and DME (commercial samples) in helium at a stagnation pressure of 6 bar was expanded through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry−Pérot cavity at 5 × 10−7 bar. The rotational spectrum in the 6.5−18 GHz frequency region was recorded using a COBRA-type21 pulsed jet Fourier-transform microwave Special Issue: Piergiorgio Casavecchia and Antonio Lagana Festschrift Received: December 23, 2015 Revised: January 22, 2016

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DOI: 10.1021/acs.jpca.5b12571 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A spectrometer,22 whose details are given elsewhere.6 Each rotational transition is split by the Doppler effect, enhanced by the coaxial arrangement of the supersonic jet and the resonator. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. The line position is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz. Lines separated by >7 kHz are resolvable.

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Figure 1. Ab initio electrostatic potential of C2F3Cl plotted on the molecular surface defined by the 0.001 electrons Bohr−3 contour of the electron density. The potential ranges from −2 × 10−2 (red) to 2 × 10−2 V (blue).

THEORETICAL CALCULATION Before recording the rotational spectra, we performed some theoretical calculations to evaluate the stable minima in the conformational potential energy surface. More exactly, we used the ab initio MP2/aug-cc-pVDZ level with Gaussian09.23 Basis set superposition errors (BSSEs) were accounted for using the counterpoise method proposed by Boys and Bernardi.24 These ab initio calculations yielded four stable isomers for the C2F3Cl− DME complex, the shape of which are reported in Table 1.

(−2 to 2) × 10−2 electrons Bohr−3. It clearly shows the existence of two separate areas with positive electrostatic potential, one of which is located on the external part of the Cl atom, while the second one is located perpendicular to the molecular plane, with a maximum above the CF2-carbon atom, the so-called “π-hole”.19

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ROTATIONAL SPECTRA We started the acquisition of the spectrum focusing our research on the plausible strongest transitions, the μb-type lines of the predicted most stable isomer (lp···π1 of Table 1). Two transitions, each consisting of four quadrupole component lines, were localized around 8218 and 8100 MHz (see Figure 2). They were immediately identified, according to the quadrupole pattern due to the nuclei 35Cl and 37Cl, as the 505-414 transitions of both isotopologues of this isomer, respectively. Several more μb-type R-branch transitions were then assigned, and, later on, some much weaker μa and μc transitions have been measured for both species. The agreement between the experimental rotational and quadrupole coupling constants and the ab initio values of isomer lp···π1 lead to an unambiguous assignment. All transition frequencies (given as Supporting Information) have been fitted using Pickett’s SPFIT program25 with Watson’s S-reduction and Ir-representation.26 The results are reported in Table 2. Searches of the other conformers were unsuccessful. We will discuss this point in a following section.

Table 1. MP2/aug-cc-pVDZ Calculated Energies and Spectroscopic Parameters of the Four Stable Conformers of C2F3Cl−DME

A/MHz B/MHz C/MHz μa/D μb/D μc/D χaa/MHz χbb−χcc/MHz χab/MHz χac/MHz χbc/MHz Δ-0.1Ea/cm−1 ΔE0b/cm−1 ΔEBSSEc/cm−1 ED/kJ·mol−1

lp···π 1

lp···π 2

lp···π 3

O···Cl

1397 900 823 −0.3 −1.6 0.2 19.1 −71.0 34.1 11.0 −25.1 0 0 0 12.1

1644 818 699 1.1 −1.2 1.0 12.6 −64.5 −39.1 13.1 24.0 222 218 50.3 11.5

1681 778 731 −1.1 −0.4 0.3 −6.6 38.4 21.4 45.3 −23.8 180 168 54.6 11.6

2892 455 424 −1.7 0.7 0.0 −59.6 −8.8 −29.5 −0.2 −0.1 1000 966 261 9.4

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STRUCTURAL INFORMATION The theoretical rotational constants of lp···π1 are quite close to the experimental values (maximum discrepancy ∼37 MHz for rotational constant A); however, it has been possible, with reference to Figure 3, to adjust three structural parameters, ∠O7C1C2, ∠O7C1C2Cl3, and ∠DuO7C1 (Du is a dummy atom along the bisector of the COC angle of DME) to effective r0 values, which are compared in Table 3 to the ab initio (re) ones. With this effective structure, the discrepancies between the experimental and calculated values have been reduced to a maximum of ∼0.5 MHz. All of the other structural data were fixed to their ab initio values (see the Supporting Information) In Table 4 we report the three most interesting structural parameters concerning the lone pair···π interaction that takes place in the adducts of C2F3Cl with DME, H2O, and NH3. One can note that these parameters are quite similar for the three complexes; however, the length of O···C1 linkage in C2F3Cl− DME is shorter by ∼0.04 Å than that in C2F3Cl−H2O. This could suggest that weak C−H···F hydrogen bonds may play a role in a further stabilization of C2F3Cl−DME; however, the effective C−H···F distances (H12−F5 = 3.182, H13−F6 = 3.002,

Absolute energy: −989.179788Eh. bAbsolute energy zero pointcorrected: −989.078983Eh . cAbsolute energy BSSE-corrected: −989.174121Eh.

a

Vibrational frequency calculations (harmonic approximation) proved these four isomers to be the real minima and gave the zero-point-corrected energies (E0). The two subunits are linked to each other by a lp···π interaction in the three most stable isomers and by a halogen bond contact (Cl···O) in the higher energy one. We also calculated the dissociation energies, which are reported in Table 1, together with the rotational and quadrupole coupling constants, the relative energies, and the components of the electric dipole moment. The ab initio calculated electrostatic potential of C2F3Cl is plotted on the molecular surface in Figure 1. The isodensity surface shown is obtained using a value of 0.001 e Bohr−3. The electrostatic potential mapped on this surface ranges between B

DOI: 10.1021/acs.jpca.5b12571 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 2. F′ ← F″ component lines of the 505 ← 414 transitions of C2F337Cl−DME (left) and C2F335Cl−DME (right). Each line appears as a doublet due to the Doppler effect.

Table 3. r0 and re Values of the Fitted Angles of C2F3Cl− DMEa

Table 2. Experimental Spectroscopic Parameters of C2F335Cl−DME and C2F337Cl−DME C2F335Cl−DME A/MHz B/MHz C/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/kHz χaa/MHz χbb−χcc/MHz χab/MHz χac/MHz χbc/MHz σ/kHzc Nd

a

1434.4456(2) 903.5197(1) 823.4024 (1) 0.9123(9) 7.104(5) −4.642(6) −0.1032(6) 0.0129(4) 18.483(5) −81.276(6) 41.5(7) 11.0b −24.5(5) 2.0 339

C2F337Cl−DME

∠O7C1C2/deg

∠O7C1C2Cl3/deg

∠DuO7C1/°

100.5 103.5(1)a

88.1 87.2(1)

115.3 107.7(1)

re r0

1421.3262(2) 893.0386(1) 810.43402(9) 0.9397(7) 6.334(4) −3.577(4) −0.1195(4) 0.0123(2) 13.029(6) −62.996(7) 35(1) 8.8b −18.6(7) 2.3 330

a a

Uncertainties (in parentheses) are expressed in units of the last digit. Full re geometry is given in the Supporting Information.

Table 4. Structural Parameters Concerning the Lone Pair···π Interaction in C2F3Cl−DME, C2F3Cl−H2O, and C2F3Cl− NH3a C2F3Cl−DME C2F3Cl−H2O C2F3Cl−NH3 a

a

Error in parentheses in units of the last digit. bUndetermined in the fit; fixed to the ab initio value. cRMS error of the fit. dNumber of fitted lines.

X···C1/Å

∠XC1C2/deg

∠XC1C2Cl/deg

ref

2.908 2.947 2.987

103.5 100.6 100.9

87.2 88.5 88.3

this work 17 18

X = O or N.

contribution to the stability of this adduct is plausibly less important than the lone pair···π interaction. The Cartesian coordinates of the chlorine atom in the principal axes system of the parent species have been calculated by the substitution method (rs).27 The obtained values are listed in Table 5, where they are compared with the values given by the r0 and re structures of the adduct. Table 5. rs, r0, and re Coordinates of the Chlorine Atom in the Principal Inertial Axes System of C2F335Cl−DME rs r0 re

a/Å

b/Å

c/Å

±1.8102(8)a −1.8075 −1.7676

±1.305(1) −1.3234 −1.3480

0.0b −0.1323 −0.2175

a Error in parentheses in units of the last digit. bImaginary value, fixed to zero.

Their good agreement further supports the conformational assignment.

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OTHER (MISSING) ISOMERS One could be surprised that conformers lp···π2 and lp···π3 (frequency calculations show that they are real minima), just slightly less stable than lp···π1, have not been detected. This can be explained by analyzing the ab initio potential energy pathway shown in Figure 4. The MP2/aug-cc-pVDZ potential energy is

Figure 3. Sketch, atom numbering, and principal axes system of the observed isomer of C2F3Cl−DME.

H15−F4 = 2.989, and H16−F4 = 3.046 Å, respectively) are larger than those typical of weak hydrogen bonds so that their C

DOI: 10.1021/acs.jpca.5b12571 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 4. Potential energy of C2F3Cl-DME as a function of the C9O7−C1C2 dihedral angle.

Author Contributions

represented as a function of the C9O7−C1C2 dihedral angle (see Figure 2). The structural relaxations of the remaining parameters have been taken into account. One can see that although lp···π2 and lp···π3 are minima, they can easily reach lp···π1 through low-energy barriers. It is well known that conformational relaxation easily takes place upon supersonic expansion28 when the interconformational barriers are below 2kT (where T is the temperature prior to the expansion), and this is largely the case also for the O···Cl → lp···π1 conformational relaxation.

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. Y.G. thanks the FWO-Vlaanderen for a doctoral fellowship (11O9514N). W.H. acknowledges financial support through FWO-Vlaanderen and the Special Research Fund BOF (UA). The Hercules foundation and the VSC are thanked for generously providing the required CPU resources. Q.G. thanks the China Scholarships Council (CSC) for financial support.

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CONCLUSIONS This investigation represents further evidence that complexes of molecules containing oxygen or nitrogen lone pairs form lp···π linkages with perhalogenated alkenes, like C2F3Cl. This effect is related to the presence of a π-hole (that is low electronic density region) in the π-system of this kind of compounds; such a π-hole attracts the high electronic dense lone pairs of oxygen and nitrogen. Alternative nonbonding interactions such as weak hydrogen bonds of the type C−H···X or halogen bonds (O···X, X = F or Cl) are shown to be quite weaker than the lp···π linkage. This molecular system is one of the few investigated by rotational spectroscopy describing this kind of linkage. It is similar to the Bürgi−Dunitz n−π* interaction that has been found to stabilize one or two conformers in amino acids.29,30

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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.5b12571. Complete ref 23, two tables of transition frequencies, and MP2/aug-cc-pVDZ geometry of the observed conformer. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: +39-2099480. E-mail: [email protected]. Present Address §

Q.G.: School of Chemistry and Chemical Engineering, Chongqing University, Chongqing, P. R. China, 400030. D

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on February 4, 2016 with production errors in Table 1. The corrected version was reposted on February 5, 2016.

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DOI: 10.1021/acs.jpca.5b12571 J. Phys. Chem. A XXXX, XXX, XXX−XXX