Rotational spectrum of the tetrafluoromethane-ethylene oxide

Journal of Molecular Spectroscopy xxx (2017) xxx–xxx

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Rotational spectrum of the tetrafluoromethane-ethylene oxide Qian Gou a, Gang Feng a,⇑, Luca Evangelisti b, Walther Caminati b a b

School of Chemistry and Chemical Engineering, Chongqing University, Daxuecheng South Rd. 55, 401331 Chongqing, China Dipartimento di Chimica ‘‘G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 16 January 2017 In revised form 13 February 2017 Accepted 1 March 2017 Available online xxxx Keywords: Molecular clusters Halogen bonding Freons Supersonic expansion Rotational spectroscopy

a b s t r a c t The rotational spectrum of one conformer of the CF4–ethylene oxide complex has been measured by using a pulsed jet Fourier-transform microwave spectrometer. The observed conformer is stabilized by a CF3


O halogen bond, with a distance rC


O of 3.341 Å. No experimental evidence of the internal rotation of CF4 with respect to ethylene oxide has been observed, but it is expected to be almost free (V3  14 cm1 from ab initio calculations). Ó 2017 Elsevier Inc. All rights reserved.

1. Introduction Freons have attracted considerable attention due to their potential of depleting the ozone layer and the greenhouse effect that contribute to the radioactive forcing of climate [1,2]. Freons contain more than one fluorine or chlorine atoms, allowing forming weakly bonded molecular complexes with atmospheric molecule such as water or other pollutants. The atmospheric fate of this kind of molecular complexes is suggested to be very different from those of isolated molecules [3,4]. Complexes of freons also provide useful models for studying weak hydrogen bonding or halogen bonding. The investigations of such weak interaction bonded molecular complexes have been mainly performed by IR spectroscopy in rare gas solutions and rotational spectroscopy combined with supersonic expansions. The gas-phase investigations can generate details on molecular structure and specifically of weak intermolecular interactions, free from solvent or crystal effects. Interesting results have been obtained from rotational spectroscopic studies of some freon complexes. OAH


Cl [5,6] and OAH


F [7,8] hydrogen bonds were found to be the main forces that stabilized the complex of freon containing hydrogen atoms. Per-halogenated freon prefer to form halogen bonds with the nucleophilic part of other molecular entities [9–12]. While in special cases when per-halogenated freon have p orbitals, a lone pair– p linkage is favoured [13]. Freon molecules also tend to weakly ⇑ Corresponding author. E-mail address: [email protected] (G. Feng).

bond to each other forming homo- [14–16] or hetero-oligomers [17,18]. The intermolecular interaction strength between freon and other molecules have been estimated to be a few kJ/mol. A tiny structural change of the subunit can dramatically affect the way of intermolecular interaction, reflecting the delicate balance of these weak intermolecular forces and their cooperativity. For example, Freon-32 (chlorofluoromethane) forms intermolecular complexes with water via OAH


Cl linkage [5]. Freon-151 (1-Chloro-1fluoroethane) has similar structure as Freon-32, but a OAH


F linkage is preferred in its complex with water [7]. Tetrafluoromethane (CF4, Freon-31) is a refrigerant which does not deplete ozone layer but has an extremely efficient greenhouse potential with a very long atmospheric lifetime of 50,000 years. Microwave spectroscopic investigation proved that CF4 forms stable complex with water through CF3


O halogen bonding with the O atom of water situating in the center of CF3 cavity [19]. The internal dynamics of these complexes gives rise to dramatic effect in the rotational spectra. The complex of CF4–H2O, theoretically an asymmetric top, displays a symmetric top spectrum due to the almost free internal rotation of water moiety. The rotational studies of CF4–H217O complex further evaluated the orientation of the water molecule in the complex [20]. In its complex with pyridine, with the two subunits linked through a CF3


N halogen bond [21], CF4 rotates freely leading to a rotational constant A much larger than that expected for a rigid complex. In order to get more details on the nature of weak noncovalent bonding and on the internal dynamics of the complexes of CF4, we investigated the complex CF4–Ethylene Oxide (EO) by high resolution microwave spectroscopy and ab initio calculations. The results are presented below.

http://dx.doi.org/10.1016/j.jms.2017.03.002 0022-2852/Ó 2017 Elsevier Inc. All rights reserved.

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Q. Gou et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx Table 1 MP2/6-311++(d,p) level of spectroscopic parameters of the two conformers of CF4–EO.

2. Experimental The molecular complex was generated in a supersonic expansion. A gas mixture of 1% CF4 and 1% EO in Helium with 0.2 MPa backing pressure was allowed to expand through a pulse valve to produce the CF4–EO complex. It was necessary to use a stainless steel bottle, in order to avoid the chemical decomposition of EO. The rotational spectrum was recorded by a COBRA-type [22] pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer [23] in the range 6.5–18.6 GHz. The details of the FTMW spectrometer has been described elsewhere [24]. Each rotational transition is split by the Doppler effect due to the configuration of spectrometer. The frequency is calculated as the arithmetic mean of the frequencies of the Doppler components. The accuracy of frequency measurements is better than 3 kHz and lines separated by more than 7 kHz are resolvable. 3. Theoretical calculation Geometry optimizations of the plausible conformers of CF4–EO complex were carried out at the MP2/6-311++G(d,p) level of theory. The shapes of the three complex structures are presented in Fig. 1. Conformer I is the global minimum, displaying a CF3


O halogen bond and two CAH


F weak hydrogen bonds from the two methylene groups to the same fluorine center. Conformer I-TS, 32 cm1 higher in energy, has a similar structure as conformer I, with two F atoms interacting with two H atoms of EO. Harmonic vibrational frequencies calculations confirm that conformer I is the real minimum, while conformer I-TS is the transition state for the internal rotation of the CF4 of conformer I. The conformer (II) lying in higher energy is characterized by CAH


F interactions. Counterpoise corrections [25] were also calculated to remove the basis set superposition error (BSSE). The theoretical rotational constants, dipole moment components and the dissociation energies are reported in Table 1. All calculations were carried out using the Gaussion09 suite of program package [26]. 4. Rotational spectrum Ab initio calculations suggested that conformer I is the most stable species, with a high value of the la dipole moment component. Therefore, the spectral searching was firstly targeted on the la-R-type bands of this conformer. Several groups of transitions, each of them falling in narrow frequency regions and separated by (B + C) of conformer I, were observed and identified as the Ka = 0, 1 transitions of the (J + 1) J with J from 3 to 8. Each transition appears as a doublet due to the above mentioned instrumental Doppler effect. The frequencies of all transitions, reported in

I

Δ E/ Δ E0 /ΔEBSSE

0/0/0

A/MHz B/MHz C/MHz la/D lb/D lc/D D0a/kJ/mol a

I

II

4217 1048 1031 1.4 0.0 1.4 9.6

4510 855 833 2.4 0.0 0.5 5.3

The zero point corrected dissociation energies.

Table 2, were fitted with Pickett’s SPFIT program [27], according to the following Hamiltonian:

H ¼ HR þ HCD

ð1Þ

HR represents the rigid rotational part of the Hamiltonian and HCD represents the centrifugal distortion contributions, analyzed according to the Watson’s S-reduction and Ir-representation [28]. With this set of la–transitions, rotational constants B and C can be well determined, but not the rotational constant A. This is most likely due to the effect of almost free internal rotational of CF3 top around one of the CAF bonds, likely the non-interacting C8AF9 bond (see Fig. 2), which results in effective ground state (m = 0 and r = 0) rotational constants very different from those of a rigid top. Such discrepancies can be accounted according to

A00 ¼ Ar þ W 00 F q2a ð2Þ

B00 ¼ Br þ W 00 F q2b ð2Þ

C 00 ¼ C r þ

ð2Þ W 00 F

q

ð2Þ

2 c

where Ar, Br and Cr are the ‘‘rigid” rotational constants in the limit of an infinite barrier. The W(2) 00 is the Hersbach’s barrier-dependent perturbation sums relative to the sublevels of the A-symmetry (r = 0) species of the torsional ground state (v = 0) [29] with qg = kgIa/Ig depending on the moment of inertia of CF4 along the axis of internal rotation (Ia), the inertial moments of the complex along the principal axis (Ig) and the direction cosine kg, giving the orientation of the internal rotation axis in the principal inertial axis system of the complex. F = ⁄/[2(1  RgkgIa/Ig) Ia] is the reduced constant of the motion. The barrier to internal rotation around C8AF9 is calculated, at MP2/6-311++G(d,p) level of theory, to be 32 cm1, while those for C8AF10 and C8AF11 are much larger than that around C8AF9. Fig. 2 shows the calculated potential-energy curve, which describes the internal rotation of CF3 group around C8AF9 bond. This barrier was reduced to 14 cm1 from the single point energies calculations to the minimum and the transition state at MP2/6-311 ++G(3df,3pd) including the BSSE corrections. This indicates that

II

425/361/187

I-TS

32/ -/-

Fig. 1. The structures of energy minima of CF4–EO complex and transition state for the internal motion of –CF3.

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Q. Gou et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx Table 2 Measured transition frequencies (m, MHz) and observed – calculated values (Dm, kHz) of CF4–EO. J0 (K0 1, K0 1) – J00 (K00 1, K00 1)

v/MHz

Dv/kHz

4(0,4)-3(0,3) 4(1,4)-3(1,3) 4(1,3)-3(1,2) 5(0,5)-4(0,4) 5(1,5)-4(1,4) 5(1,4)-4(1,3) 6(0,6)-5(0,5) 6(1,6)-5(1,5) 6(1,5)-5(1,4) 7(0,7)-6(0,6) 7(1,7)-6(1,6) 7(1,6)-6(1,5) 8(0,8)-7(0,7) 8(1,8)-7(1,7) 8(1,7)-7(1,6) 9(0,9)-8(0,8) 9(1,9)-8(1,8) 9(1,8)-8(1,7)

7951.0996 7932.7410 7969.6935 9938.3670 9915.5712 9961.7837 11925.2948 11898.1509 11953.6496 13911.8168 13880.4232 13945.2495 15897.8489 15862.3245 15936.5361 17883.3351 17843.7891 17927.4664

5 4 3 2 0 1 2 2 0 2 2 2 6 1 2 5 2 2

internal rotation around C8AF9 bond is the most likely motion affecting the rotational constants of ground torsional state. As indicated in Fig. 1, the b-axis is perpendicular to the internal rotation axis. Then kb and thus also qb are zero, and the rotational constant 2 B is not affected by the internal rotation. The values of W(2) 00 F qg for rotational constants A and C are calculated to be 1907.66 and 7.16 MHz respectively on the basis of the ab initio prediction, indicating that A is strongly affected by the free internal rotation. Therefore rotational constant A was approximately estimated to be 6124 MHz based on ab initio value and was fixed in the fit. The obtained spectroscopic constants were reported in Table 3. In spite of a careful search, K1 > 1 la-transitions and lc-transitions were not observed. The failure in detecting lc-transitions is clearly related to the large uncertainty of the effective value of the A rotational constant, which strongly depend on the barrier to the CF4 internal rotation. The impossibility to locate la-transitions with K1 > 1 is plausibly related that for near free barriers the energy of the torsional states is given, in a first approximation, by Em = Fm2, where F is very small for a heavy top as CF3. As a consequence, the K1 > 1 rotational levels of the ground state (m = 0) interact with the K1 = 0, 1 rotational levels of the upper torsional states. Such an effect is clearly described in Fig. 3 of Ref. [30].

Table 3 Experimental spectroscopic parameters of the CF4–EO. A/MHz B/MHz C/MHz DJ/kHz DJK/kHz d1/kHz HJ/Hz HJK/kHz h1/Hz Nc rd/kHz a b c d

This value fixed in the fit. See text. Errors in parentheses in units of the last digit. Number of fitted frequencies. Standard deviation of the fit.

5. Structural information The re structure of the observed conformer is reported in Table 4, according to the atomic numbering given in Fig. 2. In order to reproduce the experimental B and C rotational constants, we modified the bonding distance r and the angle a of Fig. 1, while keeping the geometry of CF4 and EO fixed to the re structure in the complex. We noted that for CF4–H2O complex, based on its re structure, it was necessary to increase about 0.2 Å of the rC


O halogen bonding length to reproduce the experimental rotational constant B. Approximately, we applied this structural variation for the CF4–EO complex and estimated the r0 value of rC


O to be 3.341 Å. With rC


O value being fixed to 3.341 Å, the bending angle of the two subunits a is calculated to be 108 (1)°. As reported in Table 5, the CF3


O halogen bonding length is very closed to that of CF3


N halogen bond in CF4–Pyridine [21]. 6. Dissociation energy By inspection of Fig. 1, the stretching motion leading to dissociation occurs almost along the a-axis. Assuming that the complex is a ‘pseudo- diatomic molecule’ made from two rigid parts, the stretching force constant (ks) can be estimated with Eq. (3): [31]

ks ¼ 16p4 ðl RCM Þ2 ½4B4 þ 4C 4  ðB  CÞ2 ðB þ CÞ2 =ðhDJ Þ;

30 25

V3 /cm-1

ð3Þ

where l is the reduced mass, RCM (3.670 Å) is the distance between the centers of mass of the two subunits, DJ, B and C are

35

20 15 10 5 0 -20 0

6124.66a 998.590(1)b 989.358(1) 2.24(1) 11.6(4) 0.032(7) 0.45(7) 0.468(4) 0.59(5) 18 3

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320

τ ( F10C8F9C2)/ ° Fig. 2. The potential energy surface for the internal motion of –CF3.

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Q. Gou et al. / Journal of Molecular Spectroscopy xxx (2017) xxx–xxx

Table 4 Geometries of the observed CF4–EO complex from MP2/6-311++G(d,p) level of calculations. Bond length/Å

Valence angle/°

C2O1 C3O1 H4C2 H5C2 H6C2 H7C2 C8O1 F9C8 F10C8 F11C8 F12C8

1.435 1.435 1.086 1.086 1.086 1.086 3.141 1.329 1.327 1.318 1.318

Dihedral angle/°

C3O1C2 H4C2O1 H5C2O1 H6C2O1 H7C2O1 C8O1C2 F9C8C2 F10C8F9 F11C8F9 F12C8F9

61.5 114.8 114.8 114.8 114.7 111.8 158.8 108.7 109.2 109.2

Table 5 Structural parameters of CF3


O(N) halogen bonding and their dissociation energies.

r/Å

a/° D/kJ/mol

CF4–H2O

CF4–pyridine

CF4–EO

3.371 125(2) 5.0

3.372(1) 180.0 10.6

3.341 108(1) 3.0

the experimental spectroscopic constants from Table 3. A ks value of 2.62 Nm1 was obtained which corresponds to a stretching frequency of 39 cm1. The dissociation energy of the complex could be estimated by assuming of a Lennard-Jones type potential relates the dissociation energy D to ks:[32]

D ¼ 1=72 ks R2CM

ð4Þ

The value D = 3.0 kJ/mol has been obtained. In Table 5, the dissociation energies of several CF4 complexes estimated with the same approximation were compared. The dissociation energy of CF4–EO is close to that of CF4–H2O. 7. Conclusions The conformation and structure of CF4–EO complex were investigated by rotational spectroscopy. The most stable configuration adopts a CF3


O halogen bond structure with a bonding distance rC


O of 3.341 Å and a  108°. The barrier to internal rotational is calculated to be 14 cm1. The dissociation energy of the observed complex was estimated to be 3.0 kJ/mol, a value considerably smaller than those of other complexes with a CF3


N or O


Cl halogen bond linkages. Acknowledgements This work was supported by the Foundation of 100 Young Chongqing University (Project 0220001104441), the Italian MIUR (PRIN08, project KJX4SN_001) and the University of Bologna (RFO). We acknowledge the CINECA award under the ISCRA initiative, for the availability of high performance computing resources and support. References [1] D.A. Good, J.S. Francisco, Chem. Rev. 103 (2003) 4999. [2] G.J. Velders, D.W. Fahey, J.S. Daniel, M. McFarland, S.O. Andersen, Proc. Natl. Acad. Sci. USA 106 (2009) 10949.

H4C2O1C3 H5C2O1C3 H6C2O1C2 H7C2O1C2 C8O1C2C3 F9C8C2O1 F10C8F9C2 F11C8F9F10 F12C8F9F10

110.7 110.4 110.7 110.4 103.8 177.8 31.4 119.7 119.7

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