Rotational spectrum of 2,5-difluorobenzyl alcohol

Journal of Molecular Structure 1023 (2012) 15–17

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Rotational spectrum of 2,5-difluorobenzyl alcohol Luca Evangelisti, Gang Feng, Qian Gou, 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: Available online 9 January 2012 Keywords: Rotational spectroscopy Large amplitude motions Supersonic expansions Molecular structure

a b s t r a c t The rotational spectra of 2,5-difluorobenzyl alcohol and of its OD isotopologue have been assigned and measured in a supersonic expansion by Fourier transform microwave spectroscopy. The hydroxyl oxygen is gauche (with a dihedral angle of 65°) with respect to the ring, but tilted towards the F atom in ortho (position 2) rather than towards the F atom in meta (position 5). The hydroxyl hydrogen points towards the 2-F atom, in such a way to form a OH  F hydrogen bridge. No tunnelling effects due to the two equivalent configurations with the OH group above and below the aromatic ring have been observed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The rotational spectra of several a-substituted derivatives of toluene (C6H5–CH2X) have been reported, and a variety of configurations and internal dynamics have been described. Ethyl benzene (X = CH3) [1] and benzyl fluoride (X = F) [2] have perpendicular shapes of the substituent with respect to the ring plane, but in the latter case tunnelling splittings due to the low barrier to the internal rotation of the –CH2X have been measured. For benzyl amine two different conformers have been identified [3]. In one of them the aminic group is perpendicular to the aromatic plane with the aminic hydrogens pointing towards the ring p cloud, while in the other one the aminic group is rotated ca. 40° from the aromatic plane with the nitrogen lone pair pointing towards the hydrogen atom in the ortho position. This latter conformer exhibits a doubling due to the interconversion of the roles of the two amino hydrogens [3]. Benzyl alcohol (BA) displays only a skew configuration but its rotational spectrum resisted for a long time to the assignment because of severe Coriolis interactions between the four equivalent tunnelling states originated by the equivalent gauche conformations [4]. Actually, only the torsion of the –CH2OH group when it remains on the same side of the ring produces a measurable tunnelling, according to what shown in Fig. 1. The substitution of the ring hydrogen with fluorine atoms revealed some interesting effects. p-F-BA has the same symmetry of BA, with the same tunnelling motion [5]. However, the tunnelling splitting is reduced by 1/3 with respect to that of BA, and correspondingly the barrier to the tunnelling motion is about 10% larger. In o-F-BA, the replacement of the ring hydrogen in position two with a F atom makes the two gauche minima not longer equivalent. Only the zusammen conformation of the OH group with re⇑ Corresponding author. Tel.: +39 051 2099480; fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). 0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2012.01.004

spect to the F atom has been observed [6], can be because of the OH  F interaction. Again, in m-F-BA, only the zusammen conformation of the OH group with respect to the F atom has been observed, but in this case it is the motion between the equivalent forms with the OH group above and below the ring which generates a tunnelling splitting [7]. This is likely due to the smaller dihedral angle between the OH group and the ring plane with respect to BA, o-F-BA and p-F-BA, which makes such a tunnelling pathways shorter. In 2,5-difluorobenzyl alcohol (25FBA) a conformational equilibrium is expected, with the hydroxyl group pointing either to the side of the ortho (conformer O) or to the side of the meta F atom (conformer M). We decided to investigate its rotational spectrum because it appears interesting: (i) to determine which one of the two conformers is the most stable one; (ii) to check if tunnelling splittings are observable. 2. Experimental part The microwave spectra of the 25FBA has been recorded in the frequency range 6–18 GHz using a COBRA version [8] of Balle– Flygare type [9] molecular beam Fourier transform microwave spectrometer already described elsewhere [10]. Helium at ca. 0.1 MPa flowed over 25FBA (supplied by Sigma–Aldrich and used without further purification) at 368 K and expanded through a solenoid valve (General Valve series 9) into the Fabry–Perot type resonator chamber, reaching an estimated ‘‘rotational temperature’’ of about 1 K. The OD species was obtained by direct exchange with D2O. 3. Theoretical calculations Before the initial search of the rotational transitions, we performed quantum chemical calculations at the MP2/6-311++G(d, p) level, in order to obtain information on the relative energies of

16

L. Evangelisti et al. / Journal of Molecular Structure 1023 (2012) 15–17 Table 1 MP2/6-311++G(d, p) spectroscopic parameters and relative energies of 25FBA.

Tunneling

No Tunneling

A (MHz) B (MHz) C (MHz) la (D) lb (D) lc (D) E (cm1)

I

II

a b

III

O

M

2351.2 951.1 705.1 0.48 1.37 1.01 0a

1752.0 1120.7 609.4 0.08 1.31 0.84 157/20b

Absolute energy: 544.012106 Eh. B3LYP value; the absolute energy of the O species is 545.41140 Eh.

IV

Fig. 1. Tunnelling motions in benzyl alcohol.

different conformations and their geometries. The Gaussian 09 software package was used [11], and the nature of all stationary points was verified by subsequent harmonic frequency calculation. We found two energy minima, corresponding to the two above mentioned O and M forms, shown in Fig. 2 (where the atom labelling are reported). The calculated rotational constants, dipole moment components and relative energies are reported in Table 1. The ab initio geometries of the two conformers are given as Supplementary Material. Apparently, the relatively small energy difference between the two conformers could allow the observation of the two species. However, it is well known that when the barrier to interconversion between two conformers is smaller than 2 kT (ca. 500 cm1 in our case), conformational relaxation to the most stable minimum (conformer O in this case) can take place upon supersonic expansions [12]. For this reason we calculated the B3LYP potential energy pathway corresponding to internal rotation about the C7–C1 bond (represented by the O8C7–C1C2 dihedral angle) which interconverts conformers O and M obtaining the results plotted in Fig. 3. The calculations were carried out in steps of about 5° over the full range of the O8C7–C1C2 dihedral angle of the organic main frame. While the dihedral angle was kept fixed at every step, all other geometric parameters were re-optimized for each point along the path. The rel-

Fig. 3. B3LYP potential energy surface of the O8C7–C1C2 internal rotation.

Table 2 Experimental frequencies (m, MHz) of the measured transitions of the OH and OD species of 25FBA. Several transitions of the OD species are split into D-quadrupole component lines. J0 (K 0a ; K 0c )

a

Fig. 2. Draws of the two most stable conformers of 25FBA (O on the left, M on the right, respectively).

J00 (K 00a ; K 00c )

OH

OD

m

F0

5(1, 5)–4(0, 4)

8227.3379

6(0, 6)–5(1, 5)

8830.1279

6(1, 6)–5(1, 5) 6(0, 6)–5(0, 5)

9068.8774 9230.2637

6(1, 6)––5(0, 5)

9469.0087

4(1, 3)–3(0, 3) 7(0, 7)–6(1, 6) 6(1, 5)–5(1, 4) 7(1, 7)–6(1, 6) 7(0, 7)–6(0, 6) 7(1, 7)–6(0, 6) 4(2, 2)–3(1, 2) 5(2, 4)–4(1, 3)

9487.9691 10384.5482 10440.1473 10519.7319 10623.2960 10758.4778 10885.8450 11623.3897

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

11884.8182 11900.3856 11946.3415 12004.1695 12093.6700 12563.9749 12691.3062 15642.5898

5(3, 2)–4(2, 3)

16139.7785

F00

m

4–3 6–5 5–4 6–5 and 7–6 5–4

8152.9684 8152.9743 8153.0089 8683.7672 8683.7796

5–4 and 7–6 6–5 5–4 and 7–6 6––5 5–4 Alla

9112.3426 9112.3501 9373.2415 9373.2622 9343.6029 10227.2438

Alla

10638.8818

4–3 and 6–5 5–4 Alla

11572.3089 11572.3734 11715.5156

Alla

11950.0572

4–3 and 6–5 5–4

15593.2124 15593.2426

All D quadrupole component lines are overlapped to each other.

atively low barrier suggest that conformer M could relax to conformer O within our experiments.

L. Evangelisti et al. / Journal of Molecular Structure 1023 (2012) 15–17 Table 3 Experimental spectroscopic constants of the observed isotopologues of 25FBA (Sreduction, Ir representation).

A (MHz) B (MHz) C (MHz) DJ (Hz) DK (kHz) d1 (Hz) 1.5vaa (MHz) 0.25 (vbb–vcc) (MHz) r (kHz)d Ne a b c d e

OH

OD

2345.4206(9)a 958.2698(4) 709.0729(3) 39(4) 2.10(8) 11(2)

2345.3995(6) 940.1550(4) 699.8986(1) [39]b [2.10] [11] 0.11(6) 0.102c 3 30

2 22

Error in parentheses in units of the last digit. Values in bracket fixed to the value of the parent species. Keep fixed to the ab initio value. Root-mean-square deviation of the fit. Number of lines in the fit.

a (Å) b (Å) c (Å)

±3.1255(1) ±0.6i ±0.647(2)

coordinates match, indeed, the structural model values only for conformation O, as shown in Table 4. In addition, the relative intensities are in agreement with the calculated value of the dipole moment components of species O (la/lb = 0.35; lc/lb = 0.74, see Table 1). The maximum discrepancy between the experimental and ab initio values of the rotational constants is less than 0.8% for the B constant, so that we did not pursue a structural improvement by fitting some structural parameters to the experimental values of the rotational constants. As in 2FBA [7], the imaginary experimental |b| value is probably related to the low barrier separating the two isomers (see Fig. 2), with the ground state of O (positive b value) having some character of M (negative b value). 6. Conclusions

Table 4 Experimental substitution (rs) and MP2/6-311++G(d, p) (re) coordinates for the hydroxyl hydrogen atom of 25FBA in the principal axes system of the normal species. rs

17

re O

M

3.0002 0.290 0.735

1.9796 2.084 1.197

4. Rotational spectra Preliminary trial calculations of the spectrum were based on the rotational constants of Table 1. It was possible to assign only one spectrum, that of the O species. According to the theoretical values of the calculated dipole moment components the first search has been targeted to the lb-R band. The J = 5 4 was observed first and then the assignment was extended to higher, up to J = 8. Then, it was possible to measure some perpendicular la and lc-type transitions, for a total of 22 measurements. The S-reduction of Watson’s quartic Hamiltonian in the Ir representation [13] was fitted against the experimental frequencies of Table 2, giving the spectroscopic constants reported in Table 3. To obtain supplemental structural information, we investigated the rotational spectrum of the -OD mono-deuterated species, whose rotational transitions are also reported in Table 2. In addition, several transitions displayed small splittings due to the quadrupole coupling of the D atom (I = 1) with the overall rotation. Its spectroscopic constants, also given in Table 3, were obtained using the same fit procedure as for the normal species. 5. Conformation and structure By comparing the experimental rotational constants with those of the conformational predictions given in Table 1 for the two species, one can see that they agree almost perfectly with the calculated values of species O. Such an agreement is reinforced from its substitution coordinates [14] of the hydroxyl hydrogen, obtained through the H ? D isotopic substitution. The experimental

The rotational spectra of the most stable conformer (O) of 25FBA and of deuterated -OD species have been experimentally observed and reported. The conformational preferences are foremost driven by a O–H  p interaction, but the hydroxyl hydrogen also ‘‘likes’’ to be in the proximity of the F atom. The failure to observe the M conformer is likely due to the conformational relaxation process to the O species upon supersonic expansion. Moreover, the replacement of the hydrogen atoms in positions 2 and 5 of benzyl alcohol with fluorine atoms makes the molecule asymmetric enough to quench any tunnelling detectable with our instrumental resolving power. Acknowledgements We thank the Italian MIUR (PRIN08, Project KJX4SN_001) and the University of Bologna (RFO) for financial support. G.F. and Q.G. also thank the China Scholarship Council (CSC) for financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2012.01.004. References [1] W. Caminati, D. Damiani, G. Corbelli, B. Velino, C.W. Bock, Mol. Phys. 74 (1991) 885. [2] R.K. Bohn, S.A. Sorenson, N.S. True, T. Brupbacher, M.C.L. Gerry, W. Jäger, J. Mol. Spectrosc. 184 (1997) 167. [3] S. Melandri, A. Maris, P.G. Favero, W. Caminati, ChemPhysChem 2 (2001) 172. [4] K.A. Utzat, R.K. Bohn, J.A. Montgomery Jr., H.H. Michels, W. Caminati, J. Phys. Chem. A 114 (2010) 6913. [5] R.G. Bird, A.E. Nikolaev, D.W. Pratt, J. Phys. Chem. A 115 (2011) 11369. [6] L. Evangelisti, L.B. Favero, W. Caminati, J. Mol. Struct. 978 (2010) 279. [7] S. Tang, Z. Xia, A. Maris, W. Caminati, Chem. Phys. Lett. 498 (2010) 52. [8] J.-U. Grabow, W. Stahl, Z. Naturforsch. A 45 (1990) 1043. [9] T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. [10] W. Caminati, A. Millemaggi, J.L. Alonso, A. Lesarri, J.C. Lopez, S. Mata, Chem. Phys. Lett. 392 (2004) 1. [11] M.J. Frisch et al., Gaussian09, revision A.1, Gaussian Inc., Wallingford, CT, 2009. [12] R.S. Ruoff, T.D. Klots, T. Emilson, H.S. Gutowski, J. Chem. Phys. 93 (1990) 3142. [13] J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, vol. 6, Elsevier, New York/Amsterdam, 1977, pp. 1–89. [14] J. Kraitchman, Am. J. Phys. 21 (1953) 17.