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Article

Ubbelohde Effect within Weak C-H•••# HydrogenBonds: the Rotational Spectrum of Benzene-DCF3

Qian Gou, Gang Feng, Luca Evangelisti, Donatella Loru,Jose Luis Alonso, Juan Carlos Lopez, and Walther Caminati

J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp407408f • Publication Date (Web): 21 Aug 2013

Downloaded from http://pubs.acs.org on August 27, 2013

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Ubbelohde Effect within Weak C−H···π Hydrogen Bonds: the

Rotational Spectrum of Benzene−DCF3

Qian Gou,a Gang Feng,

a Luca Evangelisti,

a Donatella Loru,

a José L. Alonso,

b Juan C. López,

b

Walther Caminatia,*

aDipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy.

bGrupo de Espectroscopía Molecular (GEM), Edificio Quifima, Laboratorios de Espectroscopia y

Bioespectroscopia, Parque Científico Uva, Universidad de Valladolid, E-45011 Valladolid, Spain

KEYWORDS: Ubbelohde effect, Weak hydrogen bond, Microwave spectroscopy, Supersonic

expansions, Freon’s adducts

ABSTRACT: The Fourier transform microwave investigation of C6H6-DCF3 outlines a shortening

of the distance between the two constituent molecules of about 0.0044(2) Å upon H→D substitution

of the hydrogen atom involved in the C−H···π hydrogen bond. This proofs that the Ubbelohde effect

takes place also within weak hydrogen bonding. The measure of the spectra of several 13C

isotopologues in natural abundance has been useful to obtain structural information.

Corresponding Authors: *Walther Caminati, Email: walther.caminati@unibo.it

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Introduction

It is well known that the H→D isotopic substitution of hydrogen atoms involved in relatively

strong hydrogen bonds (e.g. O-H···O) produces an increase (Ubbelohde effect1) or a decrease

(inverse Ubbelohde effect2) of the distance between the heavy atoms participating in the hydrogen

bond (HB). However, generally both effects are called “Ubbelohde effect”. The effect is related to

the fact that the ν0 fundamental frequency of a R-H stretching (R is a generic heavy group attached

to a hydrogen atom) is reduced by a factor ≈ 1.4 ≈ (mH/mD)1/2 in the R-D deuterated form. In the gas

phase, when the proton transfer connects two equivalent forms (double minimum potential, such as

in malonaldehyde or in the dimers of carboxylic acids) we have the Ubbelohde effect; when the

proton transfer leads to the dissociation (single minimum potential, like in dimers of alcohols), we

have the reverse effect. The Ubbelohde effect is mentioned also in a recent paper on the quantum

nature of the hydrogen bond,3 but there no distinction is given between the two cases mentioned

above.

Rotational spectroscopy is especially suitable to quantitatively describe these fine structural

effects. Already in 1961 Constain and Srivastava, from the low resolution microwave spectrum of

formic acid – trifluoroacetic acid,4 observed an increase of the O···O distances upon H→D isotopic

substitution of the hydrogen atoms involved in the HB. Precise quantitative values of this effect

have been supplied this year with a pulsed jet Fourier transform microwave (PJFTMW)

investigation of the conformers of the bimolecule formic acid-acrylic acid.5

As to the reverse Ubbelohde effect, several MW investigations are available. In 1976 Penn and

Olsen showed, with the investigation of many isotopologues of 2-aminoethanol, that the O···N

distance between the two heavy atoms involved in the O-H···N intramolecular hydrogen bond

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decreases 0.0057 Å upon H→D substitution of the hydroxyl group.6 Analogous shrinkages have

been evaluated from the rotational spectra of molecular complexes with the two subunits held

together by an O-H···O hydrogen bond, such as the dimer of tert-butyl alcohol,7 the dimer of

isopropanol,8 and the adducts of some alcohols with some ethers.9-12

No data are available on the effects of the H→D isotopic substitution of the hydrogen atom

involved in a weak hydrogen bond (WHB). A C−H···π interaction links the CHF3 and C6H6 units in

the benzene (Bz)-trifluoromethane complex, as proved by its rotational spectrum.13 To verify the

appearance of the Ubbelohde effect within WHB linkages, we decided to investigate the rotational

spectrum of its Bz-DCF3 isotopologue, and of the 13C species of both Bz-HCF3 and Bz-DCF3 forms.

The obtained data are reported below, and we will see that they prove the existence of the

Ubbelohde effect also in the case of WHB.

Experimental details

The rotational spectrum of Bz-DCF3 was measured with a COBRA-type14 PJFTMW

spectrometer,15 described elsewhere16 and updated with the FTMW++ set of programs. A mixture of

2% CDF3 (CDN Isotopes) in Helium was allowed to flow over benzene (cooled to 273 K) and

expanded through a solenoid valve (General valve, series 9, nozzle diameter 0.5 mm) into the

Fabry-Pérot cavity. Each rotational transition is split by the Doppler effect, enhanced by the

molecular beam expansion in the coaxial arrangement of the supersonic jet and resonator axes. The

rest frequency is calculated as the arithmetic mean of the frequencies of the Doppler components.

The estimated accuracy of frequency measurements is better than 3 kHz and lines separated by

more than 7 kHz are resolvable.

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Rotational spectrum

Before searching for the rotational spectrum of Bz-DCF3, we applied empirical corrections to the

rotational constants calculated with the geometry reported in the previous study on the most

abundant species.13 Five evenly spaced bands with the features of a symmetric top were observed.

Figure 1. The J = 7←6 band of Bz−DCF3 (upper trace). The high-resolution details (lower trace)

show the complex patterns due to free internal rotation. Each rotational transition, split by the

Doppler effect (┌┐), is labeled with the quantum numbers K and m.

In Figure 1 we report the J = 7←6 band, which, similarly to the case of parent species, shows the

complex pattern that arises from the free internal rotation of DCF3 with respect to C6H6.

Contrary to what expected from simple mass exchange, each rotational transition resulted to lie at

a frequency higher than that of the corresponding transition of the parent species (see Figure 2), so

immediately showing the presence of the Ubbelohde effect.

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Figure 2. The J = 7←6 band of Bz−DCF3 (black) lies at higher frequency than the corresponding

band of Bz−HCF3 (red), contrarily to what expected for a heavier isotopologues within a rigid rotor

approximation.

The measured transition frequencies could easily be fitted with Equation (1), which is valid for a

symmetric top with a free internal rotation.13, 17-20

ν = 2(J+1)[B-DJKK2-DJmm

2+HmJm4-DJKmKm]-4(J+1)3[DJ-HJKK

2-HJmm2-HJKmKm] (1)

where DJ, DJK, DJm and DJKm are the quartic centrifugal distortion constants. Higher order

centrifugal distortion parameters, such as HmJ, HJK, HJm and HJKm were required to fit the data, in

agreement with the high flexibility of the adduct. The parameters obtained from the least-squares fit

of the 97 measured lines are provided in the second column of Table 1. In the first column, we

report, for comparison, the molecular parameters of the parent species.

Table 1. Spectroscopic parameters of all measured isotopologues of benzene-trifluoromethane

Bz-HCF3a Bz-DCF3 Bz(13C)-HCF3

a Bz(13C)-DCF3 Bz-D13CF3 Bz-H13CF3

A/MHz - - 2837(6) 2700(76) - -

B/MHz 744.84012(6) 745.8548(9)b 741.33150(6) 742.3282(6) 742.9132(2) 741.8858(2) C/MHz 739.19544(6) 740.1868(6) DJ/kHz 0.4700(5) 0.475(3) 0.4640(4) 0.458(6) [0.475]f [0.4700]f

DJK/kHz 42.392(7) 41.90(1) 41.792(8) 41.27(6) 40.7(3) 41.73(3)

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aFrom Ref. 12. bStandard errors in parentheses are given in units of the last digit. cThe sign given for

these constants is arbitrary. The sign given here corresponding to the choice of the sign of m given

in Table 1S of the supporting material. dRoot-mean-square deviation of the fit. eNumber of fitted

transitions. fFixed to the values of parent species.

After the assignment of the most abundant species, it was possible to assign some weak

transitions (m = 0 state) belonging to the 13C isotopologues in natural abundance (ca. 1%) with

further signal accumulation. The spectrum of the Bz(13C)-DCF3 species in natural abundance is

expected to be intense about 6% of that of Bz-DCF3, because the benzene ring contains six

equivalent carbon atoms. In addition, its spectral features are completely different with respect to

those of the parent species, because the asymmetric isotopic substitution breaks down the C3v

symmetry of complex, leading to a slightly near prolate asymmetric top. The measured transitions

of this 13C species have been fitted with Watson’s semirigid Hamiltonian (S-reduction;

Ir-representation).21 The obtained parameters are reported in the fourth column of Table 1, and there

compared to the values of the Bz(13C)-HCF3 species.13 One should note that the A rotational

constant is much larger than the value expected for a rigid molecule. It corresponds – in a first

approximation – to the rotation of the frame (Bz) only, in agreement with the fact that for m = 0 the

top does not rotate. The value of (B+C) of the DCF3 species is quite larger than that of the HCF3

species, thus confirming the Ubbelohde effect.

DJm/kHz 166.82(2) 165.70(4) - - - - DJKm/kHz -164.70(2) -162.89(3)c - - - - HJK/Hz 1.78(6) 2.3(1) 1.49(6) - - - HKJ/Hz -1.0(2) - -

HJm/Hz 7.3(1) 8.3(3) - - - - HmJ/Hz -37.2(8) -39(3) - - - - HJKm/Hz 7.5(1) -6.6(3)c - - - - LJK

3m

3/Hz 2.0(4) - - - - - σd/kHz 1.9 3.4 0.9 3.5 4.1 2.2

Ne 125 97 34 17 6 7

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Later on, a few very weak transitions of the Bz-D13CF3 isotopologue (only K =0, 1) have been

identified. After the assignment of this weak spectrum, we searched for the spectrum of the

Bz-H13CF3 isotopologue, not observed in the previous investigation,13 and fulfilled its assignment.

Due to the very few measured transitions, we needed to fix the centrifugal distortion constant DJ to

the values of the respective parent species. The parameters obtained for these two last isotopologues,

with a reduced form of Eq. (1), are shown in the two last columns of Table 1. All measured

transitions are available in the Supporting Information.

Structural information and quantitative estimate of the Ubbelohde effect

A scheme of the complex is given in Figure 3.

Figure 3. Scheme of Bz−DCF3.

As a consequence of the Ubbelohde effect, the Kraitchman equations22 are not suitable to evaluate

the position of the HCHF3 hydrogen, because the H→D mass change is overwhelmed by the

shortening of the C···CMBz (CM = center of mass) distance: one would obtain meaningless large

imaginary values. Vice versa, the rs located position of the CCHF3 carbon can be reliably obtained in

the principle axis systems of Bz-HCF3 and Bz-DCF3, separately. The a-coordinate of the CCHF3

carbon atom is easily obtained from the equation for the substitution of an atom along the symmetry

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axes of a symmetric top. The a and b-coordinates of a CC6H6 carbon atom can be calculated by

Kraitchman’s equations for a symmetric/asymmetric top combination. We used for this purpose

KRA Kisiel’s program.23 The obtained results, which uncertainties include the Costain’s errors, are

shown in Table 2 and there compared to the values calculated with a rigid rotor model.

Table 2. Substitution coordinates of the carbon atoms in the complex

Exp. Calc.

a) In the principal axis system of Bz-HCF3

CHCF3 a/Å ±1.6466(8) 1.648

CC6H6 a/Å ±1.7915(7) -1.803

b/Å ±1.4104(11) 1.370

b) In the principle axis system of Bz-DCF3

CHCF3 a/Å ±1.6407(8) 1.645

CBz a/Å ±1.7936(7) -1.807

b/Å ±1.4103(11) 1.370

Since the a-coordinate of CMBz is the same as its six equivalent carbon atoms, the distance

between the carbon atom of trifluoromethane (CHCF3 or CDCF3) and CMBz can be easily estimated

from the substitution coordinates, leading to a shrinkage of 0.0038(30) Å of the C···CMBz distance

upon the H→D substitution. Unfortunately, the errors on the substitution coordinates are too high to

calculate a net Ubbelohde effect. However, the r0 shortening, calculated - according to Eq. (2) - in

order to reproduce the reduction of the B+C value which take place upon H→D substitution, is very

precise, as one can see from Table 3.

∆(B+C)obs-∆(B+C)calc = [∂(B+C)/∂rCM(Bz)···C(CHF3)]pi=cost ∆r CM(Bz)···C(CHF3) (2)

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Table 3. Shortening of the r(C…CMBz) distance upon H→D substitution. CM=center of mass.

rs r0

r(CHCF3…CMBz)/Å 3.4381(15) 3.4683(1)

r(CDCF3…CMBz)/Å 3.4343(15) 3.4639(1)

∆r/Å 0.0038(30) 0.0044(2)

Conclusions

With the present study on the isotopologues of the benzene-trifluoromethane adduct, we shown

that the Ubbelohde effect takes place also within weak hydrogen bond, such as the C−H···π one. In

addition, we supply a precise value of the shortening of the C···CMBz distance upon H→D

substitution. The determined shrinkage, 0.0044(2) Å is slightly smaller than that observed in dimers

of alcohols, or in adducts of alcohols with ethers (∆r = 5-7 mÅ).

Acknowledgment. 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.

Supporting Information Available: Tables of transition frequencies. This information is available

free of charge via the Internet at http://pubs.acs.org.

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.

References

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1) Ubbelohde, A. R.; Gallagher, K. J. Acid-base Effects in Hydrogen Bonds in Crystals. Acta

Crystallogr. 1955, 8, 71-83.

2) Limbach, H.-H.; Pietrzaka, M.; Benedicta, H.; Tolstoya, P. M.; Golubevb, N. S.; Denisov, G.

S. Empirical Corrections for Anharmonic Zero-Point Vibrations of Hydrogen and Deuterium

in Geometric Hydrogen Bond Correlations. J. Mol. Struct. 2004, 706, 115-119.

3) Li, X.-Z; Walker, B.; Michaelides, A. Quantum Nature of the Hydrogen Bond. PNAS 2011,

108, 6369-6373.

4) Costain, C. C.; Srivastava, G. P. Study of Hydrogen Bonding. The Microwave Rotational

Spectrum of CF3COOH-HCOOH. J. Chem. Phys. 1961, 35, 1903-1904.

5) Feng, G.; Gou, Q.; Evangelisti, L.; Xia, Z.; Caminati, W. Conformational Equilibria in

Carboxylic Acid Bimolecules: A Rotational Study of Acrylic Acid-Formic Acid. Phys.

Chem. Chem. Phys. 2013, 15, 2917–2922.

6) Penn, R. E.; Olsen, R. J. H/D Structural Isotope Effect in Hydrogen-Bonded

2-Aminoethanol. J. Mol. Spectrosc. 1976, 62, 423-428.

7) Tang, S.; Majerz, I.; Caminati, W. Sizing the Ubbelohde Effect: The Rotational Spectrum of

tert-Butylalcohol Dimer. Phys. Chem. Chem. Phys. 2011, 13, 9137-9139.

8) Snow, M. S.; Howard, B. J.; Evangelisti, L.; Caminati, W. From Transient to Induced

Permanent Chirality in 2-Propanol upon Dimerization: A Rotational Study. J. Phys. Chem. A

2011, 115, 47-51.

9) Evangelisti, L.; Feng, G.; Rizzato, R.; Caminati, W. Conformational Equilibria in Adducts of

Alcohols with Ethers: The Rotational Spectrum of Ethylalcohol-Dimethylether.

ChemPhysChem 2011, 12, 1916–1920.

10) Evangelisti, L.; Pesci, F.; Caminati, W. Adducts of Alcohols with Ethers: The Rotational

Spectrum of Isopropanol−Dimethyl Ether. J. Phys. Chem. A 2011, 115, 9510–9513.

11) Evangelisti, L.; Caminati, W. A Rotational Study of the Molecular Complex

tert-Butanol···1,4-Doxane. Chem. Phys. Lett. 2011, 514, 244–246.

Page 10 of 12

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12) Evangelisti, L.; Caminati, W. The Shape of the Molecular Adduct

tert-Butylalcohol-Dimethylether: A Rotational Study. J.Mol. Spectrosc. 2011, 270, 120–122.

13) López, J. C.; Caminati, W.; Alonso, J. L. The C-H···π Hydrogen Bond in the

Benzene-Trifluoromethane Adduct: A Rotational Study. Angew. Chem. Int. Ed. 2006, 45,

290-293.

14) Grabow, J.-U.; Stahl, W.; Dreizler, H. A Multioctave Coaxially Oriented Beam-Resonator

Arrangment Fourier-Transform Microwave Spectrometer.Rev. Sci. Instrum. 1996, 67,

4072-4084.

15) Balle, T. J.; Flygare, W. H. Fabry-Perot Cavity Pulsed Fourier Transform Microwave

Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33-45.

16) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; Lopez, J. C.; Mata, S. Molecular

Beam Fourier Transform Microwave Spectrum of the Dimethylether-Xenon Complex:

Tunnelling Splitting and 131Xe Quadrupole Coupling Constants. Chem. Phys. Lett. 2004,

392, 1-6.

17) Kirchhoff, W.; Lide, D. R., Jr. Microwave Spectrum and Barrier to Internal Rotation in

Methylsilylacetylene. J. Chem. Phys. 1965, 43, 2203-2212.

18) Fraser, G. T.; Lovas, F. J.; Suenram, R. D.; Nelson, D. D., Jr.; Klemperer, W. Rotational

Spectrum and Structure of CF3H–NH3. J. Chem. Phys. 1986, 84, 5983-5988.

19) Forest, S. E.; Kuczkowski, R. L. The Microwave Spectrum of Cyclopropane···Ammonia. A

Novel Structure for Cyclopropane Complexes. Chem. Phys. Lett. 1994, 218, 349-352.

20) Blanco, S.; Sanz, M. E.; Lesarri, A.; López, J. C.; Alonso, J. L. Free Internal Rotation in

CH3–C≡C–CF3. Chem. Phys. Lett. 2004, 397, 379-381.

21) Watson, J. K. G. in Vibrational Spectra and Structure; Vol. 6, (Ed.: Durig, J. R.), Elsevier,

New York/Amsterdam, 1977, pp. 1-89.

22) Kraitchman, J. Determination of Molecular Structure from Microwave Spectroscopic Data.

Am. J. Phys. 1953, 21, 17-25.

23) PROSPE, http://www.ifpan.edu.pl/~kisiel/prospe.htm.

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Table of Contents artwork

Graphical abstract:

Abstract for web publication: A shrinkage of 0.0044(2) Å occurs between the two units of

C6H6-HCF3 when moving to C6H6-DCF3, showing that the Ubbelohde effect takes place also within

weak hydrogen bonds.

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