Interaction between Freons and Amines: The C–H··· N Weak

Article pubs.acs.org/JPCA

Interaction between Freons and Amines: The C−H···N Weak Hydrogen Bond in Quinuclidine−Trifluoromethane Qian Gou, Gang Feng, Luca Evangelisti, and Walther Caminati* Dipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy S Supporting Information *

ABSTRACT: The 1:1 complex between quinuclidine and trifluoromethane has been investigated using pulsed jet Fourier transform microwave spectroscopy. The two constituting molecules are held together through a weak C−H···N hydrogen bond, with a H···N length of 2.070(1) Å. The C3 symmetric axes of the two moieties are collinear, making the overall molecular system a symmetric top. The HCF3 subunit is freely rotating with respect to the amine moiety. Transitions for the ground (m = 0) and for the first excited (|m| = 1) torsional states, with K up to 2, have been measured, all of them showing the 14N nuclear quadrupole hyperfine structure. The dissociation energy of the complex has been estimated to be 10.2 kJ mol−1.

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INTRODUCTION The C−H group has been found to act as a proton donor toward electronegative atoms, forming weak hydrogen bonds (WHB), such as C−H···O, C−H···F, C−H···S, C−H···N, and C−H···π. Despite the fact that such interaction is generally quite weak, a few kJ mol−1, a vast literature based on X-ray investigations has shown that it has the same directional properties of “classical” hydrogen bond (HB).1 Another technique that supplied plenty of information on WHBs is IR spectroscopy in rare gas solutions of molecular adducts.2 In association with this technique, not exactly appropriate nomenclature, such as “anti hydrogen bond” or “improper blue shifted hydrogen bond” have been used, mainly in theoretical related papers.3,4 We believe, however, that the investigations of the microwave (MW) spectra of several molecular adducts generated in supersonic jets have provided the most precise information on the energies, structures and dynamics of such kinds of interaction, obtained in an environment free from the intermolecular interaction, which takes place in condensed phases.5 Trifluoromethane (HCF3) is a prototype weak Lewis acid, commonly involved in theoretical and experimental investigations of WHBs. Its aliphatic hydrogen atom has the proton donor ability enhanced by the electron withdrawing of the three fluorines attached to the carbon atom. With MW spectroscopic technique, it has been found that three C−H···F−C WHBs formed isolated cages in the dimer HCF3−CH3F,6 while two C−H···F−C and one C−H···O WHBs constitute small cages in HCF3−cyclobutanone,7 HCF3−oxirane,8 and HCF3−dioxane.9 The complex HCF3−OCS10 exhibits a single C−H···O WHB interaction, whereas the S atom embraces the two F atoms, reminiscent of the halogen bond interaction. The complex HCF3−thiirane (C2H4S)11 is stabilized by two C−H···F−C and © 2014 American Chemical Society

one C−H···S WHBs. In several cases, the rotational spectra display A−E splittings due to the internal rotation of HCF3 around an axis close to its C−H bond, supplying information on the strengths of the interactions of the F atoms of CHF3 with nearby H atoms of the partner molecule.6,8,11 In the case of HCF3−benzene12 with a C−H···π WHB, it has been found that the HCF3 moiety undergoes an almost fully free rotation. A few data are available on the C−H···N WHB, with N inserted in an aromatic ring, mainly in conjunction with the rotational spectra of pyridine with the family CHnF4−n. When n = 0 (tetrafluoromethane), a σ-type complex with a CF3···N trifurcated halogen bond is formed.13 For n = 1−3 (trifluoro, difluoro, and fluoromethane), two kinds of WHBs, C−H···N and C−H···F, are observed in a σ-type complex.14−16 Between methane (n = 4) and pyridine, a π-type complex is formed, with CH4 acting as a pseudo rare gas.17 As to a C−H···N WHB with N being an amino nitrogen atom, only the rotational spectrum of HCF3−NH3 has been reported.18 In order to extend the rotational analysis of this type of complex, we decided to investigate the rotational spectrum of the complex formed between quinuclidine (1-azabicyclo[2.2.2]octane, QUI) and HCF3. QUI is a cage amine, a rather heavy C3 symmetric top, with a rather rigid structure.19 In addition, since the Ubbelohde effect (a shrinkage of the hydrogen bond length upon H → D isotopic substitution)20 has been recently observed in the heavy complex HCF3− benzene,21 one could expect to observe the same effect in QUI−HCF3. Received: December 4, 2013 Revised: January 10, 2014 Published: January 13, 2014 737

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EXPERIMENTAL DETAILS Molecular complexes were generated in a supersonic expansion, under conditions optimized for the formation of the 1:1 adduct. The Fourier transform microwave spectrometer22 (COBRAtype23), which covers the range 6.5−18 GHz, has been described previously.24 A mixture of 2% HCF3 (or DCF3) in helium at a stagnation pressure of ∼0.25 MPa was passed over QUI (commercial sample bought from Aldrich, and used without further purification) and expanded through a solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry−Pérot cavity. The spectral line positions were determined after Fourier transformation of the time-domain signal with 8k data points, recorded with 100 ns sample intervals. Each rotational transition appears as doublets due to Doppler effect. 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 more than 7 kHz are resolvable.

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THEORETICAL CALCULATIONS Ab initio computations at the MP2/6-311++G(d,p) level25 of the complex QUI−HCF3 converged to a global minimum with a C3v symmetric top shape, depicted in Figure 1, with a C−H···

Figure 2. J = 10 ← 9 band of QUI−HCF3 is shown in the upper trace. The high-resolution details of the m = 0 state are given in the lower trace, displaying the 14N quadrupole hyperfine structure. Each component line, split by the Doppler effect (⌈⌉), is labeled with the quantum numbers K and F′ ←F″.

complex is specified by the quantum number m, which gives the projection of the angular momentum of the HCF3 subunit on the symmetric axis of the complex. Being the selection rules for the observed transitions ΔJ = +1, ΔK = 0, and Δm = 0, the rotational transitions are uniquely labeled by Jlower (indicated with J in the equation below), |K|, and |m|. The observed frequencies were first fitted assuming a symmetric top behavior, according to the following expression:12,27−30 ν0 = 2(J + 1)[B − DJK K 2 + HKJK 4 − DJmm2 − DJKmKm]

Figure 1. MP2/6-311++G(d,p) symmetric top shape of QUI−HCF3.

− 4(J + 1)3 [DJ − HJmm2]

N WHB. The theoretical rotational constants A = 1755.6 and B = C = 389.8 MHz as well as the quadrupole coupling constant χaa = −5.01 MHz were obtained. The theoretical value of the distance RH···N is 2.174 Å. We also calculated the V3 barrier to the reciprocal internal rotation of the two symmetric top units, which is predicted as 4 cm−1, with a set of ab initio calculations that included basis-set superposition error (BSSE) counterpoise corrections.26 We found a second stationary point, whose configuration is obtained from that of Figure 1 by flipping the CHF3 group up/down by 180°, forming a CF3···N trifurcated halogen bond. It is, however, much higher in energy than the first form, by 1297 cm−1 or by 1754 cm−1 when including the BSSE corrections. For this reason, we did not consider its observation achievable within the present study.

(1)

where ν0 is the unperturbed rotational frequency that would be observed if there was no quadrupole coupling effect. The quartic centrifugal distortion constants DJ, DJK, DJm, and DJKm as well as the higher order constants HKJ and HJm were found necessary to fit the measured transition frequencies. While correcting the frequencies of the quadrupole component lines to obtain the ν0 values, the χaa quadrupole coupling constant was determined according to the expressions below (eqs2−4):31 ⎡ 3K 2 ⎤ ν = ν0 − χaa ⎢ − 1⎥[Y (J + 1, I , F ′) ⎣ J(J + 1) ⎦

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− Y (J , I , F ″)]

ROTATIONAL SPECTRUM After several frequency scans, in the ranges corresponding to the B values predicted from the theoretical calculations, seven evenly spaced bands with the typical features of a symmetric top were observed. In Figure 2 we report the J = 10 ← 9 transition, which shows the complex pattern arising from the practically free internal rotation of HCF3 with respect to QUI as well as the quadrupolar effect of the 14N nucleus. Considering the QUI moiety as the framework, the internal rotation state of the

(2)

where F′ = F″, F″ ± 1, and Y (J , I , F ″ ) =

3 C(C 4

+ 1) − I(I + 1)J(J + 1)

2(2J − 1)(2J + 3)I(2I − 1)

(3)

with C = F ″(F ″ + 1) − J(J + 1) − I(I + 1)

(4)

With this set of equations, we could fit simultaneously the rotational transitions belonging to the m = 0 and |m| = 1 states. 738

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The 8 parameters determined from the least-squares fit of 99 lines are provided in Table 1. The 14N-quadrupole coupling

degrees of freedom of one subunit disappear. We have already seen the effects on the spectra of one of these motions, the internal rotation of the HCF3 subunit. Also, the stretching between the centers of mass of the two constituent molecules can be, in a first approximation, separated from the remaining vibrational modes. For symmetric-top complexes, such as QUI−HCF3, the stretching-force constant (ks) can be estimated by approximating the complex to a molecule made of two rigid parts. This can be done by using the so-called pseudo diatomic approximation, expressed by eq 533

Table 1. Simultaneous m = 0 and |m| = 1 States Fitting of the Two Isotopologues for QUI−HCF3 B (MHz) DJ (kHz) DJK (kHz) DJm (kHz) DJKm (kHz) HKJ (kHz) HJm (Hz) χaa (MHz) σ (kHz)b Nc a b

QUI−HCF3

QUI−DCF3

403.3514(9)a 0.111(1) 18.96(8) 323.9(2) −35.39(8) 0.82(2) −30.6(4) −4.4(2) 4.6 99

402.653(7) 0.085(9) 18.6(1) 291.8(4) −34.20(1) 0.92(2) −38(2) −4.7(3) 3.7 42

ks = 128π 4(μR CM)2 B4 /hDJ

(5)

where μ, RCM and DJ are the reduced mass, the distance between the centers of mass (RCM = 4.8036 Å), and the firstorder centrifugal distortion constant, respectively. Contributions to the DJ parameter from the free internal rotations of the HCF3 moiety were calculated to be zero as a consequence of the collinearity between the principal axis a of the complex and the internal rotation axis. The value k s = 5.3 N m −1 corresponding to a harmonic stretching frequency of 46 cm−1 has been obtained. By assuming a Lennard-Jones potential function, according to the approximation shown as eq 6,34

Standard errors in parentheses are given in units of the last digit. Root-mean-square deviation of the fit. cNumber of fitted lines.

constant of the complex χaa = −4.4(2) MHz is smaller than that of QUI monomer in the gas phase, χaa = −5.191(4) MHz.19 This may be attributed to torsional−vibrational averaging effects or to a charge transfer upon complexation. After a first structural adjustment, we calculated the rotational spectrum of the QUI−DCF3 isotopologue. The measured transition frequencies of the deuterated species were very close to those predicted by the model, according to a negligible Ubbelohde effect. All the measured transition frequencies are available in the Supporting Information.

E D = 1/72ksR2CM

(6)

the dissociation energy (ED) was evaluated to be 10.2 kJ mol−1. This value is almost a half of the MP2/6-311++G(d,p) BSSE corrected dissociation energy of 20.0 kJ mol−1 calculated in this work. However, it is in reasonable agreement with the fact that, this C−H···N interaction is stronger than the C−H···π linkage observed in benzene−HCF3 (ED = 8.6 kJ mol−1)12 and fluorobenzene−HCF3 (ED = 8.4 kJ mol−1).35

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STRUCTURE In order to have an effective structure satisfactorily reproducing the experimental rotational constant B, the distance between N atom of QUI and H of HCF3 (partial r0 geometry), which is involved in the WHB, has been modified with respect to the ab initio value (2.174) to be 2.070(1) Å (see Figure 3). From the

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CONCLUSIONS The pure rotational spectra of QUI−HCF3 adduct has been recorded and assigned with Fourier transform MW technique. The spectra show the complex pattern due to the free internal rotation of HCF3 and quadrupole coupling effect of 14N nucleus. A C−H···N WHB, with a length of 2.070(1) Å, which is collinear with the C3 symmetric axes of the two subunits, characterize the complex as symmetric-top. The dissociation energy (10.2 kJ mol−1) has been estimated within the pseudodiatomic approximation, which is stronger than the C−H···π linkage. In the case of the C−H···π linkage observed in benzene− HCF3, the H → D isotopic substitution of the hydrogen atom involved in the WHB leads to a marked Ubbelohde effect.21 Vice versa, no observable change in the distance C···N is evidenced by the rotational spectrum of QUI−HCF3 upon H → D isotopic substitution. Then, no Ubbelohde effect is associable to the C−H···N WHB in this complex. Differently with respect to the C−H···π interaction, it seems that the Ubbelohde effect vanishes within the C−H···N linkage. However, it appears difficult to extract a general law since these are the only two investigated molecular complexes, with the two subunits held together through a WHB, in which the Ubbelohde effect could be observable.

Figure 3. Shape of QUI−HCF3, with the effective hydrogen bond length, which reproduces the experimental rotational constants.

rotational constants of the parent and of the deuterated species, we obtained the rs substitution a coordinate of the freon hydrogen as ±1.474(8) Å with Kraitchman’s equation.32 The value is, within the Kraitchman error, the same as that calculated with the r0 structure (−1.478 Å). The ab initio geometry of the complex is also given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

DISSOCIATION ENERGY Six low-energy vibrational modes result from the formation of the adduct, as the three translational and the three rotational

Complete ref 25, table with the MP2/6-311++G(d,p) geometry of the complex, and tables of transition frequencies. This 739

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AUTHOR INFORMATION

Corresponding Author

*(W.C.) E-mail: [email protected]. Author Contributions

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. G.F. and Q.G. also thank the China Scholarships Council (CSC) for financial support.

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REFERENCES

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