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Weak Hydrogen Bond Network: A Rotational Study of 1,1,1,2-Tetrafluoroethane Dimer Xiaolong Li, Yang Zheng, Junhua Chen, Jens-Uwe Grabow, Qian Gou, Zhining Xia, and Gang Feng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07007 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 23, 2017

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The Journal of Physical Chemistry

Weak Hydrogen Bond Network: A Rotational Study of 1,1,1,2-Tetrafluoroethane Dimer

Xiaolong Li,† Yang Zheng,† Junhua Chen,† Jens-Uwe Grabow,‡ Qian Gou,† Zhining Xia,† Gang Feng *, † †

Department of Chemistry, School of Chemistry and Chemical Engineering, Chongqing

University, Daxuecheng South Rd. 55, 401331, Chongqing, China. ‡

Institut für Physikalische Chemie und Elektrochemie, Universtät Hannover, Callinstr. 3A,

D-30167 Hannover, Germany.

AUTHOR INFORMATION Corresponding Author * Gang Feng, Email: [email protected]

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ABSTRACT The 1,1,1,2-tetrafluoroethane dimer has been investigated by pulsed jet Fourier transform microwave spectroscopy. One conformer, stabilized through a network of four C-H···F-C interactions, has been observed although several almost isoenergetic configurations were suggested by ab initio. The measurements, extended to four 13C species in natural abundance, allow to determine the carbon skeleton structures and to evaluate the weak hydrogen bond parameters. Information on the dissociation energy is also provided.

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I. INTRODUCTION Hydrogen bond (HB) interactions are fundamental important in chemistry and biology,1-3 which have attracted a significant amount of research efforts for nearly a hundred years. O-H···A and N-H···A (A=O, N, S) linkages have interactions energies of 15-25 kJ·mol-1 characterizing the “classical” HBs. C-H···A, C-H···X and C-X···H (X=F, Cl) linkages have interaction energies within a few kJ·mol-1, approaching to those of van der Waals forces, are generally classified as weak hydrogen bonds (WHBs). Experimental techniques including X-ray diffraction,4 NMR,5 laser spectroscopy,6,7 IR spectroscopy in rare gas solutions8,9 and rotational spectroscopy10 have been employed to study the structural and energetic nature of HB interactions. Among them, rotational spectroscopy, especially the development of pulsed jets Fourier transform microwave (pulse-FTMW) spectroscopy11,12 and recently the chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy13, is one of the most precise methods for characterizing such non-covalently bound molecular complexes. These techniques are also enable the investigations of relatively large molecular clusters formed by very weak forces, providing plenty of information which would shield a light on the link of microsystems and the molecular bulk. For example, OCS@(He)8 represents molecular cluster held by van der Waals forces, which links isolated OCS molecule and the helium nanodroplet environment.14 OCS itself also forms dimer15,16, trimer17-19 and tetramers19 through weak interactions. Water clusters describe molecular systems with the subunits held together by a network of O-H···O HBs,20-22 which is also the main forces

that

stabilize

phenol

trimer.23

The

2-fluoroethanol

3


trimer24

and


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2,2,2-trifluoroethanol trimer25 are stabilized through the union of O-H···O, C-H···F-C and C-H···O-H HBs. Difluoromethane (CH2F2) dimer,26,27 trimer28, and tetramer29,30 represent molecular clusters held together by pure WHBs. As the number and the molecular weight of the subunit increase, the challenges in detection and assignment of molecular clusters increase continuously in two terms: 1) a large number of possible configurations separated by low interconversion barrier may co-exist on the potential energy surface arising the difficulty in conformational assignment; and 2) a dense of weakly populated rotational transitions are expected in a narrow frequency range arising the difficulty in experimental measurement and rotational transitions assignment. For these reasons, rotational studies on those WHBs bounded complexes were mainly focused on methyl halides. No investigations, to our knowledge, concerning the dimer of alkyl halides with carbon chain greater than two are available. 1,1,1,2-Tetrafluoroethane (CF3CH2F, TFE) is an asymmetric rotor almost twice heavier than CH2F2 in molecular weight. The molecular structure of TFE can be taken as one of the fluorine atom of CH2F2 substituted by a -CF3 group. The increasing numbers of F atoms and its electronegativity may significantly affect the molecular property of the monomer and therefore alter its complexation behavior. In order to understand the complexation processes of TFE and to gain insight into the WHBs, here, we investigated TFE dimer by pulsed-Jet FTMW spectroscopy and ab initio calculations.

II. Experimental Section

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The rotational spectra were measured on a coaxially oriented beam-resonator arrangement (COBRA) type12,31 pulsed supersonic-jet Fourier-transform microwave (FTMW) spectrometer at Chongqing University, covering 2-20 GHz frequency region and operating with the FTMW++ set of programs. The details on the instrumental setup will be described elsewhere (unpublished results). All samples were obtained commercially and were used without further purification. Molecular clusters were generated in a supersonic expansion. In general, a gas mixture of ~2% TFE in Helium at a total pressure of 6 bar was allowed to expand through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry-Pérot-type cavity. Each rotational transition displays a Doppler splitting that originates from the supersonic jet expanding coaxially along the resonator axes. The rest frequency was calculated as the arithmetic mean of the frequencies of the two Doppler components. The estimated accuracy of the frequency measurements is better than 3 kHz, resolution is better than 6 kHz.

III. Results and Discussion Theoretical calculations TFE has a pair of weak proton donors and four weak proton acceptors, potentially forming C-H···F-C WHBs network. In addition, C-F···F-C interactions may also exist, which have been found playing important roles in stabilizing the 2,2,2-trifluoroethanol trimer.25 In order to spot the relative stability and structures of the most possible conformers of TFE dimer, geometry optimizations at MP2/6-311++G(d,p) level of theory have been performed by using Gaussian09 program package.32 The starting geometries were obtained by substituting two F atoms of CH2F2 dimer26 with -CF3 5



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group, and further supplemented by adjusting the position of one subunit with respect to another to form C-H···F-C or C-F···F-C interactions. Harmonic frequency calculations at the same level were performed to prove the obtained geometries to be really minima, and to provide zero-point energy and centrifugal distortion constants. Counterpoise corrections33 were also calculated in order to estimate the basis-set superposition error (BSSE). Eight stable conformers were located within 3 kJ·mol-1. Their shapes, relative energies (E0,BSSE and E0), rotational constants, electric dipole moment components and planner moments of inertia are present in Table 1. The full geometries of these isomers are available in Supporting Information. The first two most stable conformers are almost energetic degenerated while taking zero-point and BSSE corrections into account. Conformer I is formed through three C-H···F-C WHBs with bonding distances ranging from 2.46 to 2.72 Å. Conformer II is stabilized by four C-H···F-C WHBs with bonding distances ranging from 2.46 to 2.96 Å. Conformers III, IV, VI and VII are formed purely through C-H···F-C WHBs while Conformers V and VIII are formed through C-H···F-C and C-F···F-C interactions. All these species appear promising to be observed, because of their relatively low energies and high values of the dipole moment components.

Rotational spectra Frequency regions where µa-R bands of conformers I and II were expected, were recorded first. Several groups of lines belonging to (J+1)←J band, Ka = 0,1 transitions were found and assigned firstly. Then the measurements of µa-R type transitions have been extended to the J from 6 to 22, with Ka up to 9. After a first refinement of the

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spectroscopic constants, we were able to measure several weak µb-type and µc-type transitions. Figure 1 shows a part of the spectrum of the µa- J-12←11 band. The measured lines have been fitted to determine spectroscopic parameters by using Pickett’s SPFIT program,34 according to the Hamiltonian: H = HR + HCD

(1)

where HR represents the rigid rotational part of the Hamiltonian, HCD represents the centrifugal distortion contributions and is analyzed with the S reduction in the Ir representation.35 The obtained spectroscopic parameters were listed in Table 2. The determined rotational constants are quite close to the theoretical values of conformers I, II, and III (Table 1). Concerning the conformational assignment, the comparison of the experimental planner moments of inertia (Pgg) with the theoretical ones can excludes the possibility of conformer III. The measurements of the intensities of several nearby transitions of the µa, µb and µc types show that the µa transitions are the most intense lines while µb and µc transitions are much weaker, with µc lines slightly stronger than those of µb (Table 4s). These intensities are in agreement with the theoretical predicted dipole moment components values of the conformer II. Moreover, as shown in Table 2, the theoretical centrifugal distortion constants of conformer II match better with those of experimental values. All these evidences lead the conformational assignment to conformer II.

Besides the assignment of the most abundant species, we could also measure the spectra of four

13

C isotopic species in natural abundance. The observed lines were

analyzed with the same procedure as the normal species. Because of much smaller

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number of lines have been measured, the centrifugal distortion constants have been fixed at the values of the parent species. The resulted spectroscopic parameters of the 13

C isotopic species are reported in Table 3 with the atomic numbering given in Figure

2. In spite of a careful search, no transitions belonging to other conformers were detected. All the measured lines of five isotopologues are listed in Supporting Information.

Structure and weak hydrogen bond interactions The rs substitution coordinates36 of four substituted carbon atoms are calculated and compared with the theoretical values of conformers I and II in Table 4. A partial rs geometries of the substituted carbon atoms were therefore calculated and compared with the re structures of conformers I and II (Table 5). One can see that the re values of angle C9C1C2 and dihedral angle C10C9C1C2 of conformer II match the experimental values better with respect to Conformer I, which further confirms the conformational assignment. Based on the five sets of rotational constants, a partial r0 structure (Table 5) was calculated by refining the C9C1distance and angles of C9C1C2 and C10C9C1 with the structures of the monomers fixed at the ab initio geometry. The derived structures of the WHB linkages were reported in Table 6 and are compared to the re structures from MP2/6-311++G(d,p) level of calculation. Ab initio calculations reveal that the formation of dimer shortens the C-H bonds about ~0.001 Å and elongates those of C-F bonds about 0.005 Å (Table 3S). The C-H 8


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symmetric and antisymmetric stretching frequencies are blue shifted about 12 and 16 cm-1 respectively (Table S3), indicating that the C-H···F-C interactions are anti-hydrogen bond in nature. However, as pointed out by Alabugin et al.,37 these improper HBs are resulting from the dedicated balance of hyperconjunction and rehybridization, with no fundamental difference to those of red-shifting HBs. In Table 7, several C-H···X (X=F, Cl) bonded dimers are listed26, 38-41. For TFE dimer, the observed conformer is stabilized by a net of four C-H···F-C WHBs, with two of them tightly contacted (2.570 and 2.607 Å, respectively) and other two loosely contacted (3.000 and 3.005 Å). In comparison, three C-H···X (X=F, Cl) WHBs stabilize other dimers, with the C-H···F-C linkage length ranging from 2.43-3.05 Å and C-H···Cl-C linkage length of ~3.15 Å.

Dissociation energy The formation of the dimer reduces the translational and rotational motions of monomers into six low-energy vibrations. In a first approximation, one of these motions can be treated as the stretch between the two centers of mass of the subunits. As indicated in Figure 2, this stretching motion takes place along principal a-axis of the dimer, therefore a pseudodiatomic molecule model can be used to evaluate the stretching force constant (kS)42: ks = 16 π4 (µ RCM)2 [4B4+4C4-(B-C)2(B+C)2]/(hDJ),

(2)

where µ is the pseudo-diatomic reduced mass and RCM (= 5.013 Å) is the distance between the centers of mass of the two subunits, B, C and DJ the spectroscopic constants reported in Table 1. The value ks = 3.88 N·m-1 has been obtained, corresponding to a harmonic stretching 9



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frequency of 36 cm-1. The stretching force constant is used to estimate the dissociation energy (EB) by assuming of a Lennard-Jones type potential43: EB = 1/72 ks RCM2

(3)

The value EB ≈ 8.2 kJ·mol-1 has been obtained. The estimated value is quite similar to those complexes formed via C-H···X (X=F, Cl) WHBs, as shown in Table 7. All these dissociation energy values are fall into the range of 5-9 kJ·mol-1, smaller than typical EB values underlying classical (O-H···O, O-H···N, O-H···S, and N-H···O) HBs.

V. CONCLUSIONS We investigated the TFE dimer with pulsed-jet FTMW spectroscopy. Although ab initio predicts two energetically degenerated conformers to be the global minimum, only one conformer has been detected, in which the two monomers are united through a network of four C-H···F-C interactions. The rotational assignment of the normal species and four

13

C isotopic species allow evaluating the rs and r0 structures of the

observed conformer. The dissociation energy of the dimer has been estimated to be 8.2 kJ·mol-1, close to those of C-H···X (X=F, Cl) bound dimers. This kind of weakly bound molecular system can be hardly investigated by other means of technique: because there are several possible configurations with a tiny structural differences and a very close stable energy. They have very similar fingerprint vibrational frequency hardly can be resolved by IR. Quantum computations also have much difficulty in

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confirming which conformer is the real global minimum, without the assist of rotational assignment. Searching for other conformers have been carefully performed but failed, despite the ab initio calculations suggesting several additional configurations lie only slightly higher in energies than that of the observed one. This is possibly due to the fact that these conformers are separated by barriers smaller than 2kT (ca. 420 cm-1 in our case) which could efficiently relax into the global minimum.44 Indeed, a potential energy surface connecting the conformational relaxation of conformer I to conformer II calculated at MP2/6-311++G(d,p) level confirms a very flat relaxation barrier (~3 cm-1), indicating the likely existence of structural relaxation in the jet (Figure S1). In addition, the repeating of formation and dissociation of the dimer could also occur in the expansion which further leads to a strong preference for the most stable conformer.45

ASSOCIATED CONTENT Supporting Information MP2/6-311++G(d,p) calculated principal axes coordinates of TFE dimer; Transitions of all the observed isotopic species; MP2/6-311++G(d,p) calculated C-H and C-F bond lengths and the difference between TFE monomer and conformer II; The evidence of anti-hydrogen bond; Intensities measurements of µa, µb and µc types of transitions and Figure of potential energy surface connecting conformational relaxation of conformer I to II. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

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38. Caminati, W.; López, J. C.; Alonso, J. L.; Grabow, J.-U. Weak CH⋅⋅⋅F Bridges and Internal Dynamics in the CH3 F⋅CHF3 Molecular Complex. Angew. Chem. Int. Edit. 2005, 44, 3840-3844. 39. Spada, L.; Gou, Q.; Tang, S. Y.; Caminati, W. Weak Hydrogen Bonds in Adducts Between Freons: The Rotational Study of CH2F2-CH2ClF. New J. Chem. 2015, 39, 2296-2299. 40. Gou, Q.; Spada, L.; Vallejo-Lopez, M.; Kisiel, Z.; Caminati, W. Interactions Between Freons: A Rotational Study of CH2F2-CH2Cl2. Chem.-Asian J. 2014, 9, 1032-1038. 41. Christenholz, C. L.; Obenchain, D. A.; Peebles, S. A.; Peebles, R. A. Reduced Bandwidth Chirped-Pulse Microwave Spectroscopy for Analysis of Weakly Bound Dimers: Rotational Spectrum and Structural Analysis of CH2ClF⋯FHCCH2. J. Mol. Spectrosc. 2012, 280, 61-67. 42. Millen, D. Determination of Stretching Force Constants of Weakly Bound Dimers from Centrifugal Distortion Constants. Can. J. Chem. 1985, 63, 1477-1479. 43. Novick, S. E.; Harris, S. J.; Janda, K. C.; Klemperer, W. Structure and Bonding of KrClF: Intermolecular Force Fields in Van Der Waals Molecules. Can. J. Phys. 1975, 53, 2007-2015. 44. Ruoff, R. S.; Klots, T. D.; Emilsson, T.; Gutowsky, H. S. Relaxation of Conformers and Isomers in Seeded Supersonic Jets of Inert Gases. J. Chem. Phys. 1990, 93, 3142-3150. 45. Thomas, J.; Sunahori, F. X.; Borho, N.; Xu, Y. Chirality Recognition in the Glycidol⋅⋅⋅Propylene Oxide Complex: A Rotational Spectroscopic Study. Chem. – Eur. J. 2011, 17, 4582-4587.

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Figure Captions:

FIG. 1. Recorded spectrum of the 12←11 band of the observed conformer. Each line displays the Doppler doubling.

FIG. 2. Principal axes and atom numbering of the observed conformer. 17 and 18 represents the centers of mass of the two subunits respectively.

15



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TABLE 1. Shapes, relative stability and spectroscopic parameters of the low lying energy conformers of TFE dimer calculated at MP2/6-311++G(d, p) level of theory. I

II

III

IV

E0,BSSE (E0)/cm-1

0 (15)

2 (0)

91 (107)

99 (28)

A, B, C/MHz

1825,325,317

1875,328,321

1829, 324, 308

1552, 400, 390

|µa|, |µb|, |µc|/D

1.7, 1.4, 0.4

1.8, 0.3, 0.5

0.4, -0.4, 0.6

2.3, 1.1, 0.4

Paa, Pbb, Pcc/uÅ 2

1438.70, 160.60, 116.32

1422.82, 151.57, 117.97

1462.17, 178.67, 97.64

1116.83, 179.01, 146.62

V

VI

VII

VIII

E0,BSSE (E0)/cm-1

104 (19)

108 (136)

145 (43)

202 (58)

A, B, C/MHz

1585, 398, 375

1786, 326, 312

1508, 412, 401

1453, 464, 406

|µa|, |µb|, |µc|/D

1.2, 2.3, 0.1

-1.2, 2.3 -0.1

2.3, 0, 2.4

-0.8, 0.7, 0.1

Paa, Pbb, Pcc/uÅ 2

1149.31, 198.37, 120.48

1443.54, 176.26, 106.70

1075.91, 184.39, 150.74

993.07, 251.71, 96.11

16


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TABLE 2. Experimental values of the spectroscopic parameters of the observed conformer of TFE dimer and the theoretical values of conformers I and II.

[a]

Exp.

I

II

A/MHz

1863.1459(4) [a]

1825

1875 328

B/MHz

318.84696(5)

325

C/MHz

311.66078(5)

317

321

DJ/kHz

0.08634(5)

0.52

0.08

DJK/kHz

0.2422(8)

-4.81

0.04

DK/kHz

1.60(1)

22.16

1.78

d1/Hz

-3.14(4)

-36.29

-1.98

d2/Hz

0.19(4)

-0.33

0.31

Paa [b] /uÅ2

1467.67

1436.17

1422.82

Pbb/uÅ2

153.90

158.08

151.57

Pcc/uÅ2

117.35

118.84

117.97

σ [c] /kHz

2.2 1.7, 1.4, 0.4

1.8, 0.3, 0.5

N [d]

240

|µa|,|µ b|,|µc|/D

s, w, w e

Errors in parenthesis are expressed in units of the last digit; [b]planner moments of inertia, Paa= (-Ia + Ib + Ic)/2 =Σimiai2, Pbb = (Ia - Ib + Ic)/2 =Σimibi2,

Pcc= (Ia + Ib - Ic)/2 =Σimici2; [c]Number of transitions in the fit. [d]Standard deviation of the fit. [e]Experimentally, µ a lines is stronger than those of µb and µc, with µ c lines slightly stronger than those of µb.

17



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Page 18 of 25

TABLE 3. Experimental spectroscopic parameters of the

13

C isotopologues of the observed

conformer of TFE dimer.[a] C1

C2

C9

C10

A/MHz

1862.5604(4)[b]

1863.0397(4)

1853.6091(4)

1862.9046(3)

B/MHz

318.5302(1)

317.2605(1)

317.8096(2)

317.3573(1)

C/MHz

311.3598(1)

310.1482(1)

310.6572(1)

310.2303(1)

Paa/uÅ2

1469.20

1475.58

1472.18

1475.11

Pbb/uÅ2

153.94

153.90

154.63

153.93

2

/

[a] [c]

Pcc uÅ

117.40

117.37

118.03

117.35

σ[c]/kHz

1.8

2.2

2.1

2.6

N[d]

19

17

16

22

Centrifugal distortion constants have been fixed at the values of the parent species;

[b]

Errors in parenthesis are expressed in units of the last digit;

Standard deviation of the fit; [d]Number of transitions in the fit.

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TABLE 4. rs coordinates of the isotopically substituted carbon atoms of the observed conformer. a/Å

b/Å

c/Å

Exp.

C1

C2

C9

C10

±1.238(1)[a]

±2.8145(5)

±2.1238(7)

±2.7299(6)

I

1.338

2.800

-2.189

-2.689

II

1.308

2.773

-2.044

-2.694

Exp.

±0.196(8)

0.032i[b]

±0.849(2)

±0.191(8)

I

-0.287

0.098

-0.986

0.260

II

-0.349

-0.023

0.765

-0.171

Exp.

±0.217(7)

±0.129(12)

±0.826(2)

0.150i[b]

I

-0.188

-0.131

0.669

-0.030

II

-0.328

-0.137

0.915

-0.080

[a]

Errors in parenthesis are expressed in units of the last digit.

[b]

imaginary number. They are set to zero for structural evaluation.

19



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Page 20 of 25

TABLE 5. rs and r0 structural parameters of the observed conformer and those of theoretical (re, MP2/6-311++G(d, p)) structures of conformers I and II. r0[b]

rs C1C2/Å

[a]

1.592(2)

re (I)

re (II)

-

1.513

1.513

C9C10/Å

1.459(6)

-

1.513

1.514

C9C1/Å

3.671(3)

3.833(7)

3.695

3.740 143.3

C9C1C2/°

148.9(4)

144.3(3)

164.1

C10C9C1/°

91.0(2)

88.6(2)

93.1

88.6

C10C9C1C2/°

-179(1)

-

105.2

-176.8

[a]

Errors in parenthesis are expressed in units of the last digit.

[b]

Partial r0 structure were calculated from the five sets of experimental rotational constants, with the geometry of monomers fixed to the ab initio one.

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TABLE 6. Derived parameters of the WHBs linkages (r0) and the theoretical (re, MP2/6-311++G(d, p)) structural parameters. Distance/Å

r0

re

Valence Angles/°

r0

re

F5-H12

2.570(7) [a]

2.458

∠F5H12C9

119(1)

119

H4-F11

2.607(7)

2.521

∠F11H4C1

125(1)

114

H4-F16

3.005(7)

2.959

∠F16H3C1

91(1)

91

H3-F16

3.000(7)

2.928

∠F16H4C1

91(1)

89

[a]

Errors are estimated from r0 structure errors reported in Table 5

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Page 22 of 25

TABLE 7. Dissociation energies (EB) for some dimers formed through H···X (X=F, Cl) WHBs. dimers

EB /kJ·mol-1

WHBs

r0(H···Hal)/Å

Ref.

CF3CFH 2-CF3CFH2

8.2

4 C-H···F

2.570, 2.607, 3.000, 3.005

This work

CF2H 2-CF2H2

6.6

3 C-H···F

2.628, 2.759, 2.759

26

CF3H-CH3F

5.3

3 C-H···F

2.427, 3.052, 3.052

38

2 C-H···F

2.633, 2.861

1 C-H···Cl

3.169

CF2H 2-CH 2FCl

5.3

CF2H2-CH2Cl2

7.6

CH 2FCl-HFC=CH2

8.7

1 C-H···F

2.489

2 C-H···Cl

3.147, 3.147

2 C-H···F

2.764, 2.764

1 C-H···Cl

3.011

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FIG. 1. Recorded spectrum of the 12←11 band of the observed conformer. Each line displays the Doppler doubling. 63x50mm (600 x 600 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FIG. 2. Principal axes and atom numbering of the observed conformer. 17 and 18 represents the centers of mass of the two subunits respectively. 39x19mm (300 x 300 DPI)


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