Chemical Physics Letters 514 (2011) 244–246

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Chemical Physics Letters

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A rotational study of the molecular complex tert-butanol� � �1,4-dioxane

Luca Evangelisti, Walther Caminati ⇑Dipartimento di Chimica 0 0G. Ciamician0 0 dell’Università, Via Selmi 2, I-40126 Bologna, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 July 2011In final form 26 August 2011Available online 30 August 2011

0009-2614/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.cplett.2011.08.081

⇑ Corresponding author. Fax: +39 051 209 9456.E-mail address: walther.caminati@unibo.it (W. Cam

We report the results of a pulsed jet Fourier transform microwave investigation of the molecular complextert-butanol� � �1,4-dioxane and of its isotopologue tert-butanol(OD)� � �1,4-dioxane. The rotational spectrahave been observed only for one conformer in which tert-butanol acts as a proton donor to one of theaxial lone pairs of one ring oxygen. The D quadrupole coupling constants have been determined forthe OD isotopologue, and a 0.006 Å shrinking of the O� � �O distance upon deuteration of the hydroxylgroup (Ubbelohde effect) has been observed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Relatively large hydrogen bonded molecular complexes havebeen investigated by FTMW spectroscopy combined with super-sonic expansions. Some of these complexes were dimers of chiralalcohols and so related to the topic of molecular recognition. Thiswas the case of butan-2-ol dimer, for which one heterochiral dimerwas assigned [1]. The rotational study of the dimers of glycidol leadto the observation of dimers arising from the combination of differ-ent conformers [2], so giving insight to the molecular recognitionof chiral conformers. Subsequently, the rotational spectra of in-duced chiral dimers have been observed: three conformers forthe ethanol dimer [3], five conformers for the isopropanol dimer[4] and one conformer for the tert-butyl alcohol dimer [5].

In addition, two FTMW investigations are available, concerningadducts formed by the combination of alcohols and ethers, that isisopropanol–dimethylether [6] and ethyl alcohol–dimethyl ether[7]. In both cases, the alcohol was, obviously, the proton donorand in isolation it was constituted by two nearly isoenergetic con-formers. Only one conformer has been observed for isopropanol–dimethylether [6], while two conformers were detected for ethylalcohol–dimethyl ether [7], with ethyl alcohol preserving bothgauche and trans configurations in the complex.

In all complexes mentioned above, the OD deuterated species ofthe proton donor hydroxyl group had the B and C rotational con-stants larger than those of the corresponding OH species. This ef-fect allowed for a quantitative description of the Ubbelohdeeffect [8], that is a shortening of ca. 5–7 mÅ of the O� � �O distanceupon H ? D substitution has been given. Such an effect is not ob-served in complexes where one of the partner molecules is light,e.g. H2O, because the combination of the small Ubbelohde O� � �Oshortening with the small mass of the light partner of the alcohol

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inati).

does not produce an inversion of the size of the B and C rotationalconstants upon H ? D substitution. This is the case, for example, oftert-butyl alcohol–water [9]. Viceversa, the Ubbelohde effect hasbeen observed in the case of heavy bi-molecules, such as carboxylicacid dimers [10] and 2-pyridone dimer [11]. However, in the caseof carboxylic acid dimers, an increase of the O� � �O distance upondeuteration was observed.

Among the alcohol–ether type dimers, no a case where theether subunit can lead to a conformational equilibrium has beenconsidered. However, if we consider a complex like tert-butyl alco-hol� � �1,4-dioxane (TBA-DXN), a conformational equilibrium cantake place, because the alcoholic hydroxyl proton can be donatedboth to the axial or equatorial lone pair of the cyclic ether. Sucha kind of conformational equilibrium has been observed previouslyfor complexes between cyclic ethers and halogenidric acids [12–14], and generally the axial species has been found to be the abso-lute minimum.

Tert-butyl alcohol itself has a very complicated spectrum [15],but when is linked in a molecular complex, produces rotationalspectra amenable to be interpreted. This is the case of its dimer[5], of its complex with NH3 [16], and, in a lesser extent, of its com-plex with water [9]. The rotational spectra in systems larger thanTBA become simpler because of the loss of symmetry. So, the rota-tional spectrum of TBA-DXN should be relatively simple and couldallow for the detection of the axial and equatorial conformers. Wereport here the obtained results.

2. Experimental details

The microwave spectrum of TBA-DXN has been recorded in thefrequency range 6–18 GHz using a COBRA version [17] of a Balle–Flygare type [18] molecular beam Fourier transform microwavespectrometer already described elsewhere [19], recently updatedwith the FTMW++ set of programs [20].

Table 2Spectroscopic parameters of TBA-DXN (Ir representation, S reduction). N is thenumber of transitions in the fit, and r is the standard deviation.

OH OD

A/MHz 1891.7197(2) 1893.0438(5)B/MHz 492.72784(8) 493.5044(3)C/MHz 483.3829(1) 484.0215(4)DJ/kHz 0.1317(6) 0.131(2)DJK/kHz 0.031(3) 0.022(7)

L. Evangelisti, W. Caminati / Chemical Physics Letters 514 (2011) 244–246 245

Helium at ca 0.3 MPa was flowed over a 50% sample of TBA (orTBA-OD) and DXN (supplied by Sigma–Aldrich and used withoutfurther purification) at room temperature, and expanded througha solenoid valve (General Valve series 9) into the Fabry–Perot typeresonator chamber.

The estimated accuracy of the measured frequency was about2 kHz and the resolution of the hyperfine components was about7 kHz.

DK/kHz 1.328(6) 1.30(1)d1/kHz 0.0048(2) 0.0103(7)d2/kHz 0.00104(8) 0.0008(2)r/kHz 2.2 4.0N 62 44

3. Theoretical calculations

MP2/6-311++G⁄⁄ preliminary ab initio calculations were run toobtain information about the geometry and the relative bindingenergies of the complex. For this purpose, the GAUSSIAN 03 packageof programs [21] was used. Two stable structures, both with a O–H� � �O hydrogen bond, labeled Axial and Equatorial according towhich of the axial or equatorial lone pair of the oxygen atom isbond to the hydroxyl proton, have been found. Their energiesand calculated spectroscopic constants and shapes are shown inTable 1.

Figure 1. Ubbelohde effect and deuterium hyperfine structure (right) of the42,2 31,2 transition of TBA-DXN.

4. Rotational spectra

We searched first the spectrum of the axial conformer, theoret-ically predicted to be the most stable one. This spectrum was ex-pected to be prevalently a lc-type one, so that the first search forthe rotational transitions was focused on the lc R type transitions.We could easily identify the family of transitions (J + 1)1,J J0,J (e.g.61,5 50,5), with J in the range from 5 to 11. After that, many morelc-type transitions with higher Ka values, as well as several la-typetransitions have been assigned and measured.

All the measured frequencies (given as Supplementary mate-rial) were fitted using the SPFIT program [22] within Watson’s Sreduction and Ir representation [23]. The derived spectroscopicparameters are reported in Table 2. The experimental rotationalconstants fit very well the theoretical ones of the axial speciesand are so markedly different with respect to those of the equato-rial conformer, that there are no doubts about the conformationalassignment. We searched then for both la- and lc-spectra of theequatorial complex, but unsuccessfully.

After a refinement of the intermolecular parameters, we pre-dicted the spectra of the OD deuterated species. The family of tran-sitions (J + 1)1,J�1 J0,J, first observed for the normal species, wereexpected at lower frequencies with respect to those of the normal

Table 1MP2/6-311++G⁄⁄ calculated spectroscopic parameters and relative energies of the twostable forms of TBA-DXN.

Axial Equatorial

A/MHz 1953.5 2346.0B/MHz 493.1 435.9C/MHz 480.5 411.0la/D 0.5 1.6lb/D 0.0 0.0lc/D 1.7 �1.6DE/cm�1 0a 232

a Absolute energy = �539.960327 Eh.

species, due to the decrease of the (B + C) rotational parameter. TheH ? D isotopic substitution should, indeed, originate a 0.26 MHzdecrease of the (B + C) value, because the mass of the complexincreases.

Viceversa, those transitions were observed at higher frequen-cies than those of the normal species. This effect is related to achange of the distance between the two heavy atoms (generallyoxygen atoms) involved in the hydrogen bond, upon H ? D isoto-pic substitution: then, instead of a 0.26 MHz decrease of the (B + C)value we observed a 1.41 MHz increase of (B + C). This unexpectedchange is accounted when we decrease the O� � �O distance by�0.006 Å. In addition, several transitions displayed small splittingsdue to the quadrupole coupling of the D atom (I = 1) with the over-all rotation. Figure 1 shows these effects for the 42,2 31,2 transi-tion. The quadrupole effects were in perfect agreement with the abinitio values of the quadrupole coupling constants (0.098, �0.161,and 0.062 MHz for the vaa, vbb, and vcc, respectively), so that wedid not need a refinement of these parameters in the fit. All hyper-fine corrected central frequencies were fitted with the same proce-dure used for the normal species, obtaining the spectroscopicconstants reported in the right column of Table 2. Transitions fre-quencies are available as Supplementary material. The rotationalconstants B and C of the deuterated species are larger than thoseof the OH species, according to the Ubbelohde effect, discussedbelow.

Figure 2. Shape of the observed conformer and adjusted parameter in the geometryof axial TBA-DXN.

246 L. Evangelisti, W. Caminati / Chemical Physics Letters 514 (2011) 244–246

5. Structure and Ubbelodhe effect

As said above, the observed spectrum belongs to the axial spe-cies. The discrepancies between the experimental and theoreticalvalues of the rotational constants of the normal (OH) species canbe considerably reduced (smaller than 1 MHz) when adjustingthe a and b angles of Figure 2 from the ab initio values (164.0�and 126.6�) to 161.9� and 128.1�, respectively. The full ab initiogeometries are available as Supplementary material.

The substitution coordinates of the hydroxyl hydrogen, whichin principle could be obtained with the Kraitchmann method[24] upon H ? D substitution, do not have any meaning in thiscase, because the O� � �O distance is shrink by DrO� � �O = �0.006 Åupon deuteration. Such a shortening has been evaluated accordingto the standard equation for r0 fittings:

DðBþ CÞobs � DðBþ CÞcalc ¼ ½@ðBþ CÞ=@rO���O�pi¼costDrO���O ð1Þ

where the suffix pi indicates that all the other structural parametersare kept constant.

Such a reduction, generally labeled Ubbelohde effect, was called‘Reverse Ubbelohde effect’ in the original Letter [8]. Its contribu-tions to the moments of inertia overcome those of the OH ? ODisotopic modification. The origin of the Ubbelohde effect is relatedto the different zero point energies between the O–H/O–D andO� � �H/O� � �D stretchings.

6. Dissociation energy

The intermolecular stretching motion, directed along the hydro-gen bond, is almost parallel to the a axis of the complex. In thiscase we can use the pseudo diatomic approximation which consid-ers the two molecular subunits of the complex as two rigid parts.Using this model the hydrogen bond stretching force constantcan be derived from the B and C rotational constants and the DJ

centrifugal distortion constant through the equation [25]:

ks ¼ ð4pÞ4ðlRcmÞ2½4B4 þ 4C4 � ðB� CÞ2ðBþ CÞ2�=hDJ ð2Þ

where l is the reduced mass of the complex and Rcm is the distancebetween the centers of mass of the two subunits.

From the partial structural fitting a Rcm value of 4.50 Å is de-rived, and the ks value is 7.4 N/m. From this value the correspond-ing harmonic stretching frequency ms = 1/2p (ks/l)1/2 is determinedto be 56 cm�1.

Assuming a Lennard–Jones type potential the zero point disso-ciation energy of the complex can be derived applying the approx-

imate expression [26] ED = 1/72 ks Rcm2 which supplies ED = 12.5 kJ/

mol. This value is about one third of the ab initio value (36.7 kJ/mol). Chemical intuition suggests the ‘real’ value to be intermedi-ate between the two indications.

7. Conclusions

The rotational spectrum observed for the TBA-DXN describesone complex with the two units held together by a O–H� � �O hydro-gen bond and with the axial lone pair of one of the DXN oxygenatoms acting as proton acceptor. We missed the assignment ofthe equatorial conformer, which is predicted to be higher in en-ergy. It should have been observable, but probably is still higherin energy. One could also invoke a conformational relaxation uponsupersonic expansion [27]. However, such a kind of conformationrelaxation was not observed, when using He as carrier gas, whenlight proton donor groups were involved [12–14], so that we be-lieve this process to have little effects in our case. To explain thehigher stability of the axial form one could invoke two additionalinteractions of the oxygen of the hydroxyl group with two CHhydrogens of the ring which are not possible in the equatorial con-former (see Figures at the bottom of Table 1).

Finally, we quantitatively determined the decrease (�0.006 Å)of the O� � �O distance upon H ? D substitution, known as Ubbe-lohde effect.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.cplett.2011.08.081.

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