The rotational spectrum of CF3ClAr

Chemical Physics Letters 653 (2016) 1–4

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

The rotational spectrum of CF3ClAAr Luca Evangelisti ⇑, Qian Gou, Gang Feng, Walther Caminati 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 17 February 2016 In final form 19 April 2016 Available online 20 April 2016 Keywords: van der Waals interactions Rotational spectroscopy Pulsed jets Freons

a b s t r a c t The microwave spectrum of the van der Waals complex CF3ClAAr has been investigated by pulsed jet Fourier transform microwave spectroscopy. The observed spectra of the 35Cl and 37Cl isotopologues are typical of asymmetric tops, with rotational constants A, B, C = 3373.118(4), 988.2529(4), 879.5788(3) and 3286.66(4), 985.50(3), 871.359(8) MHz, respectively. The Ar atom is almost ‘‘L-shaped” with respect to the ClAC bond, at a r0 distance of 3.824(2) Å from the center of mass (CM) of CF3Cl and with the angle ClACMAAr = 81(2)°. The dissociation energy has been estimated to be 2.3 kJ mol
1. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Several adduct of rare gases (RG) with organic molecules have been characterized by rotational spectroscopy combined with supersonic expansions: they are stable species in the plume formed in the jet. Generally, complexes with 5–6 membered ring aromatic molecules are relatively rigid, and splittings caused by tunneling motions of RG are not observed within the time scale of microwave spectroscopy [1]. Conversely, the rotational spectra of molecular adducts of RGs with small asymmetric molecules are generally characterized by inversion splittings due to the low barriers between equivalent minima in the two-dimensional (2D) potential energy surface of angular motions. These splittings are useful to quantify the inversion barriers. Indeed, tunneling splittings have been observed in several complexes of rare gases with freons. In the family difluoromethane (CH2F2)ARG, the inversion splittings range from 39.316(1), to 79.19 (4) and to 193.740(1) MHz in going from CH2F2AXe [2], to CH2F2AKr [3] and CH2F2AAr [4], respectively. In going from difluoromethane to chlorofluoromethane (CH2ClF), that is substituting a fluorine with a chlorine atom in the RG partner molecule, one can observe a considerable decrease of the inversion splittings, to 0.1360(2), 0.6298(6) and 2.9219(5) MHz for the most abundant isotopologues of CH2ClFAXe [5], CH2ClFAKr [6] and CH2ClFAAr [7], respectively. Substituting a second fluorine atom with a chlorine atom, that is in going from CH2F2 to CH2ClF to CH2Cl2, the splittings of the corresponding complexes with rare gases become very small, DE01 = 6.8900(5) MHz for CH2Cl2ANe [8], but not observable in the pulsed jet Fourier ⇑ Corresponding author. E-mail address: [email protected] (L. Evangelisti). http://dx.doi.org/10.1016/j.cplett.2016.04.067 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

transform microwave (FTMW) spectrum of CH2Cl2AAr [9]. Freons like CH2F2, CH2ClF and CH2Cl2 are asymmetric tops, and their adducts with RGs are also asymmetric tops. Making complexes with symmetric top freons, what would be the nature of the adduct? In the case of trifluoromethaneAAr, the spectrum of an asymmetric top has been observed, with the Ar atom almost T-shaped with respect to the CAH bond, and with the CF3AH symmetric top undergoing a hindered internal rotation [10]. What will it happen when an Ar atom is making a complex with ClCF3, a perhalogenated freon that with ammonia [11], CO [12] and even with water [13] forms halogen bonded effective symmetric top adducts? The answer is given below, following the investigation of the rotational spectrum of ClCF3AAr.

2. Experimental The rotational spectrum of the complex was observed in a FTMW spectrometer [14] with a COBRA (Coaxial Oriented Beam and Resonator Axes) configuration [15] in the frequency range 6– 18 GHz which has been described previously [12]. Briefly, the experimental detection is made up by three stages. The first step is the creation of the molecular pulse by opening the injection valve allowing the adiabatic expansion of our gas sample in the cavity. The macroscopic polarization of the molecular beam is the next stage. It is generated by applying a microwave pulse into the Fabry–Perot resonator where the sample was previously introduced. Finally, the following spontaneous molecular emission is digitised in the time-domain and Fourier transformed in order to obtain the frequencies of the rotational transitions of the system under study. Additionally, the coaxial arrangement in the cavity

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L. Evangelisti et al. / Chemical Physics Letters 653 (2016) 1–4

causes the splitting of the molecular signals into two different components due to the Doppler effect. Transitions separated by more than 7 kHz are resolvable with an estimated accuracy above 0.5 kHz. A mixture of 2% CF3Cl in Ar at pressures of ca. 0.3 MPa was expanded through a pulse valve, where the supersonic jet was created, to the cavity, reaching a pressure of about 5 mPa. 3. Preliminary calculations of the conformational energies Before search for the rotational spectrum, we ran some model calculations, in order to have a reliable starting conformation. Serving this purpose, we first utilized the simple Distributed Polarizability Model (DPM) [16,17], followed by ab initio calculations in the vicinity of the DPM minima. (a) DPM calculations were performed using the computer program RGDMIN [18]. Within the model underlying this program, the structures are determined by minimizing the multicenter interaction energy arising from dispersive attraction counterbalanced by hard-sphere repulsion. The geometry of CF3Cl was fixed to the experimental r0 structure [19] while the distance (RCM) between its center of mass (CM) and the rare gas was free to relax for energy minimization in the full range h = 0–180°, / =
90 to 270° at steps of D/ = Dh = 10°. RCM, h and / are the spherical coordinates shown in Fig. 1. The obtained two dimensional (2D) potential energy surface is shown in Fig. 2. The most stable conformer is an asymmetric top (A), constituted of three equivalent minima, according to the threefold potential energy surface of the internal rotation of the CF3 group. The Ar atom is far away 3.748 Å from the CM point, with an angle ClACMAAr = 81.5°. Another minimum corresponds to a symmetric top (S1), with the Ar atom in the incave of the ACF3 group (on the opposite side of the Cl atom), far away 4.111 Å from CM. From these geometries we calculated the two corresponding sets of rotational constants, reported at the bottom of Table 1. (b) Ab initio optimization at the MP2/6-311++G(d,p) level of theory was performed with the Gaussian09 suite of programs [20] on the two DPM minima. These calculations were also useful to evaluate, besides the rotational constants, the set of quadrupole coupling constants (vhg, h, g = a, b, c) and the components of the electric dipole moment. The ab initio calculations suggested the S1 conformation to be the global minimum, slightly lower in energy than A. In addition, they found also the symmetric species with Ar attached to the Cl atom to be a stable form (S2). The frequency calculation

Fig. 1. Sketch of CF3ClAAr with the polar coordinates defining the position of Ar with respect to CF3Cl. The origin of the axes identifies the CM of the CF3Cl.

Fig. 2. Distributed polarizability model calculations: two- and three-dimensional representation of potential energy surface as a function of the spherical coordinates h and /. The blue areas correspond to energy minima. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

proved these three to be real minima, providing the zeropoint corrected energies. In Table 1, we list the spectroscopic parameters useful for the assignment of the spectrum, that is the rotational constants, the 35Cl quadrupole coupling constants, the values of the la and lc components of the electric dipole moment, and the centrifugal distortion constants.

4. Results The DPM model suggests the asymmetric top species (A) to be the most stable species, in contrast with the ab initio calculations, which indicated one of the two symmetric top (S1) to be the absolute minimum. After scanning more than a 2B spectral region, we did not observe any feature associable to a symmetric top spectrum. Vice versa, following the predictions of the spectrum with the rotational constants of the A species, and searching for the more intense lb-type transitions, we could assign the set of transitions (J + 1)1,(J+1) J0,J, with J in the range 2–7. The hyperfine structure due to the quadrupolar 35Cl (I = 3/2) nucleus was in agreement with the calculated quadrupole coupling constants. The quadrupole component lines of the 414 303 transition are shown in Fig. 3. Other types of lb-transitions, and a few, weak la-transitions have been measured later. After completing the analysis of the parent species, it has been possible to assign also the spectrum of the 37Cl isotopologue. The experimental transition frequencies (available in the Supplementary Material) have been fitted using Pickett’s SPFIT program [21] within semirigid Watson´s Hamiltonian [22] in the symmetric reduction and Ir representation. An additional correction takes into account the quadrupole coupling effect [23] due to the non-spherical charge distribution in the Cl nucleus. All obtained spectroscopic parameters are reported in Table 2. The experimental rotational constants are in good agreement with the theoretical values of conformer A, showing that the conformer identified in the jet corresponds to the most stable species predicted by DPM but not with MP2. No other conformers were detected in the spectrum, despite the small energy gap between

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L. Evangelisti et al. / Chemical Physics Letters 653 (2016) 1–4 Table 1 Theoretical spectroscopic parameters and relative energies of the three most stable conformations of CF3ClAAr.

a b c d e

A

S1

S2

Ab initio: MP2/6-311++G(d,p) A/B/C/MHz vaa/vbb–vcc/vab/MHz la/lb/Da DJ/DJK/DK/kHz d1/d2/Hz DE/DE0b/cm
1 Dee/kJ mol
1

3330/998/887 36.9/
110.4/14.8
0.01/0.61 2.54/31.11/
33.13
0.31/
0.07 9/7 3.2

5716/810/810
73.6/0.0/0.0
0.63/0.00 1.06/13.82/
13.54 – 0c/0d 3.3

5713/560/560
73.5/0.0/0.0
0.70/0.00 0.78/12.76/
11.51 – 94/83 2.2

Distributed polarizability model A/B/C/MHz DE/cm
1 De/kJ mol
1

3366/1020/907 0.0 5.4

5716/790/790 59 4.7

– – –

lc = 0 by symmetry. Zero point relative energies. Absolute energy =
1323.637724 Eh. Zero point absolute energy =
1323.622232 Eh. Dissociation energy.

Table 2 Experimental spectroscopic parameters of the two isotopologues of the detected conformer of CF3ClAAr. CF335ClAAr A/MHz B/MHz C/MHz vaa/MHz vbb–vcc/MHz vab/MHz DJ/kHz DJK/kHz DK/kHz d1/kHz d2/Hz Nd re/kHz

CF337ClAAr a

3373.118(4) 988.2529(4) 879.5788(3) 36.57(1)
114.40(1) 15.4(2) 2.529(3) 25.39(7)
25.2(7)
0.301(3)
0.053(3) 75 2.0

3286.66(4) 985.50(3) 871.359(8) 28.99(2)
90.30(2) 12.1b 2.48(1) [25.39]c [
25.2] [
0.301] [
0.053] 24 6.5

a

Error in parentheses in units of the last digit. Fixed at the value extrapolated from the corresponding value of the parent species. c Values in brackets fixed at the corresponding value of the parent species. d Number of lines in the fit. e Root-mean-square deviation of the fit. b

Fig. 3. The quadrupole component lines of the 414 species.

303 transition of the CF335ClAAr

A and S1. Probably a conformational relaxation takes place upon supersonic expansion, as usual when low barriers to the conformational transformation are met [24].

Table 3 Experimental (rs and r0) and theoretical (re and rDPM) coordinates of the Cl atom in principal axes system of the parent species (c is zero by symmetry).

rs – exptl. r0 – exptl. re – ab initio re – DPM

5. Structural information Some structural information has been obtained from the six available rotational constants. First the Kraitchman [25] coordinates of the Cl atom were calculated in the principal axes system of the parent species. The obtained values are reported in Table 3, where they are compared to the ab initio, DPM and partial r0 (see below) structure data. The ab initio values correspond to the bottom of the vibrational potential energy surface and are indicated by the notation re (equilibrium structure). A partial r0 structure, suitable to reproduce the rotational constants in the vibrational ground state, has been also determined by adjusting, with respect to the ab initio geometry (given in

a

a

b

±0.854(3)a 0.802 1.057 0.795

±1.422(2) 1.419 1.415 1.421

Error in parentheses in units of the last digit.

Supplementary material), the ArACM distance and the ArACMACl angle. These parameters are labeled as RCM and h in Fig. 1, and the ab initio geometry is given in Supplementary material. The maximum discrepancy between the calculated and experimental values was, after the fitting, 2 MHz. The values RCM = 3.824(2) Å and h = 81 (2)° have been obtained, with increases of 27 mÅ and of 0.6° with respect to the ab initio values.

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6. Dissociation energy

Appendix A. Supplementary data

When the intermolecular stretching motion leading to dissociation is almost parallel to the a-axis of the complex, it is plausible to derive the corresponding force constant within the pseudo diatomic approximation, through the equation [26]:

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.04. 067.

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

References

ð1Þ

l is the pseudo diatomic reduced mass, RCM is the distance between the centers of the mass of the two subunits, and DJ is the centrifugal distortion constant. The value ks = 1.9 N/m was obtained. Then, assuming a Lennard–Jones type potential, the zero point dissociation energy of the complex can be derived applying the approximate expression [27]:

ED ¼ 1=72 ks R2CM

ð2Þ

Hence, the dissociation energy of the complex was found to be 2.3 kJ mol
1, relatively in good agreement with the ab initio value. This value is also in agreement with the dissociation energies found for complexes of Ar with other freons [4,7,9]. 7. Conclusions The observed conformer of CF3ClAAr is an asymmetric top, with the line ArACM almost perpendicular to the CACl bond. Relative energies of conformers are predicted quite well even with DPM model which is a simple calculation respect to MP2/6-311++G(d, p). Also the orientation of Ar with respect to CF3Cl – for the asymmetric top – is well described by DPM. In principle, the internal rotation of the symmetric top CF3Cl around its symmetry axis could produce A–E splittings. Such a kind of splitting has been observed, indeed, in the adduct CF3HACH3F, for which the V3 barriers to internal rotation of both the CH3 and CF3 groups have been experimentally determined [28]. In other adducts, such as benzeneACHF3, the features of a free rotation have been observed [29]. The rotational spectrum of the ‘‘twin” complex CF3HAAr displays the features of both the internal rotation of CF3H and of the Ar inversion [10]. In the case of CF3ClAAr no splittings of any type appeared in the spectrum. This indicates that the barrier to internal rotation is larger than 70 cm
1 [30]; both ab initio and DPM calculations suggest that this is the case. Now the question is: why when substituting an H atom with a Cl atom the V3 barrier to internal rotation of the CF3 group increases so much? The analysis in Ref. [10] suggests that in CF3HAAr the up M down inversion of CF3H is coupled with the internal rotation of the CF3 group. Replacing the H atom with a Cl atom increases considerably the reduced mass of such an up M down inversion, and probably also the barriers to inversion and to internal rotation. The modified tunneling pathway, combined with higher reduced masses and tunneling barriers, results in tunneling splittings too small to be observed with our resolving power. Acknowledgements We thank Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. L.E. was supported by Marie Curie fellowship PIOF-GA-2012-328405.

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