Adducts of Alcohols with Ethers: The Rotational Spectrum of

ARTICLE pubs.acs.org/JPCA

Adducts of Alcohols with Ethers: The Rotational Spectrum of Isopropanol-Dimethyl Ether Luca Evangelisti, Federico Pesci, and Walther Caminati* Dipartimento di Chimica “G. Ciamician” dell'Universit
a, Via Selmi 2, I-40126 Bologna, Italy

bS Supporting Information ABSTRACT: The rotational spectra of three isotopologues of the isopropanol-dimethyl ether molecular complex have been measured with pulsed jet Fourier transform microwave spectroscopy. In the complex, isopropanol acts as a proton donor and takes a gauche conformation. The H f D isotopic substitution of the hydroxylic hydrogen participating in the O-H 3 3 3 O hydrogen bond produces an increase of the B and C rotational constants, according to the shrinkage of the O 3 3 3 O distance of about 7 mÅ, underlying and sizing the associated Ubbelohde effect.

’ INTRODUCTION Various types of molecular complexes have been studied by gas-phase high-resolution spectroscopy, which provides a wealth of information about their shapes and on the intermolecular interactions. Among hydrogen bonded complexes, in many cases the investigated systems were constituted of a relatively large organic molecule interacting with a small solvent molecule, such as H2O,1 NH3,2 or HX3 (X is a halogen atom or a pseudo-halogen group). More recently, some larger molecular complexes, made of two relatively large moieties, have been investigated by Fourier transform microwave (FTMW) spectroscopy combined with supersonic expansions. Some of these complexes were related to the topic of molecular recognition, in the sense that were made by chiral molecules4,5 or that a complex with induced chirality was obtained.6,7 The first high-resolution spectroscopic study of a chiral molecular complex, that is, butan-2-ol dimer, was reported by Howard and co-workers, where one heterochiral dimer was assigned.4 The rotational study of the dimers of glycidol led to the observation of dimers arising from the combination of different conformers,5 giving insight to the molecular recognition of chiral conformers. Subsequently, the rotational spectra of three conformers of induced chiral dimers of ethanol6 and of five conformers of induced chiral dimers of isopropanol7 have been reported. Very recently, the rotational spectrum of the dimer of tert-butyl alcohol has been presented, mainly to outline the Ubbelohde r 2011 American Chemical Society

effect,8 which is a shortening of ca. 7 mÅ of the O 3 3 3 O distance upon H f D substitution.9 However, no FTMW investigations are available, to date and to our knowledge, concerning adducts formed by the combination of relatively large molecules of alcohols and ethers. In order to obtain information on this class of molecular complexes, we investigated with this technique the adduct isopropanoldimethyl ether (ISO-DME). Isolated ISO is characterized by three conformational minima, two of which (gauche) are equivalent as illustrated in Figure 1, and correspond to H-C-O-H torsional angles of ca. 60° and -60°, respectively. The third minimum (trans conformation) corresponds to a H-C-O-H torsional angle of 180°, and it is outside of the range of Figure 1. The trans and gauche forms are almost isoenergetic (ET - EG ≈ 170 cm-1).10 The two gauche conformations form an enantiomeric pair. In the monomer, these are rapidly interconverted by quantum-mechanical tunneling which occurs at a frequency of around 47 GHz,10 constituting then a transient chirality system. However, in the complex, the

Special Issue: David W. Pratt Festschrift Received: December 23, 2010 Revised: February 14, 2011 Published: March 09, 2011 9510

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Table 1. MP2/6-311þþG** Structures, Energies, and Spectroscopic Constants of the Three Most Stable Conformers of ISO-DME

Figure 1. The two gauche equivalent conformations of isopropanol, with a qualitative indication of the two tunneling states.

reduced mass of the tunneling motion would be much larger and it could quench its tunneling effects. Then assuming that ISO will act as a proton donor in forming the O-H 3 3 3 O linkage, the related interesting problematics are (i) will ISO be in its trans or gauche configuration in the complex, (ii) in the case where ISO adopts a gauche configuration, what will it happen to its transient chirality, and (iii) will the Ubbelohde effect produce sizable features in the rotational spectra?

’ EXPERIMENTAL DETAILS We measured the rotational spectrum of the adduct with a pulsed jet Fourier transform11 microwave spectrometer in a coaxial arrangement of the MW radiation and of the molecular beam,12 described elsewhere.13 Commercial samples of ISO, ISO-OD, ISO-CD, and DME (Aldrich) have been used without further purification. A mixture of 1% of DME in He at a pressure of ≈2 bar was allowed to flow over ISO at room temperature and expanded through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the Fabry-Perot cavity. The frequencies were determined after Fourier transformation of the 8k data points time domain signal, recorded with 100 ns sample intervals. Each rotational transition is split by Doppler effect due to 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. Lines separated by more than 7 kHz are resolvable. ’ THEORETICAL CALCULATION Before collecting the rotational spectra, we ran some ab initio calculations, using the Gaussian03 package of programs,14 in order to estimate the plausible conformations of the complex and the values of its electric dipole moment components. The O-H 3 3 3 O hydrogen bond is the leading factor in determining the geometry of ISO-DME. The hydrogen bridge length estimated at MP2/ 6-311þþG** level is 2.831 Å, the OH and O 3 3 3 H are, for the most stable conformer, 0.970 and 1.861 Å, respectively. The results and the shapes of the three most stable conformers (hydrogen bonded) of the complex are shown in Table 1. The two conformations with a gauche arrangement of the ISO moiety differ in the orientation of the DME with respect to ISO. The nature of all stationary points was verified by subsequent harmonic frequency calculations, and all of them are real energy minima. The MP2/6-311þþG** geometries of the three conformers of ISO-DME are given as Supporting Information.

a

Absolute energy = -348.483644Eh.

Table 2. Spectroscopic Parameters (S reduction, Ir representation) for the (CH3)2CHOH 3 3 3 DME, (CH3)2CDOH 3 3 3 DME, and (CH3)2CHOD 3 3 3 3 DME Isotopologues of ISO-DME (CH3)2

(CH3)2

(CH3)2

CHOH-DME

CDOH-DME

CHOD-DME

A/MHz

4501.3866(8)a

4420.1(3)

4486.811(3)

B/MHz

941.9091(1)

934.3940(4)

943.7656(4)

C/MHz

863.3850(1)

858.5454(3)

864.8288(4)

DJ/kHzb

0.5252(6)

0.510(2)

0.512(1)

DJK/kHz

11.54(2)

9.95(3)

11.39(4)

d1/kHz

0.0394(5)

0.044(2)

0.039(2)

d2/kHz σ/kHz

0.0588(4) 2

0.055(1) 0.8

0.058(1) 2

Nc

34

13

20

Paa/μÅ2

504.811

507.586

503.612

Pbb/μÅ2

80.535

81.060

80.757

Pcc/μÅ2

31.737

33.277

31.880

a

Error in parentheses in units of the last digit. b DK is not determined in the fit and it has been fixed to 0. c Number of lines in the fit.

’ ROTATIONAL SPECTRA We analyzed first the normal species. According to the theoretical values of the rotational constants and of the calculated dipole moment components and the relative intensities of the predicted rotational transitions in the spectrometer frequency range, the first search has been targeted to the low J: 5 r 4 μa-R band, and then to higher J μa-R lines up to J = 9. Then it was possible to measure some perpendicular μb and μc type transitions, for a total of 34 transitions. The spectrum was fitted with a Watson’s type Hamiltonian (S reduction, Ir representation),15 obtaining the spectroscopic constants reported in Table 2. The rotational constants are compatible only with the ISOgauche species, so that the conformational problem is reduced to the discrimination between conformations I and II of Table 1. However, the neat higher intensities of μa with respect to the μb and μc type transitions support the assignment of the spectrum to 9511

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Figure 3. Sketch and atom numbering adopted for ISO-DME.

Figure 2. The μa-R type 505 r 404 transitions are shown for the (CH3)2CHOH 3 3 3 DME and (CH3)2CHOD 3 3 3 3 DME isotopologues. One can note the Ubbhelode effect shifts the frequencies of the heavier isotopologue, (CH3)2CHOD 3 3 3 3 DME, to values higher than those of the lighter one, (CH3)2CHOH 3 3 3 DME.

species I (see the values of the dipole moment components in Table 1) After a refinement of the intermolecular parameters, we predicted the spectra of the deuterated species, (CH3)2CDOH 3 3 3 DME and (CH3)2CHOD 3 3 3 3 DME. All μa-R type transitions were expected at lower frequencies with respect to those of the normal species, due to the decrease of the (B þ C) rotational parameter. We observed bands similar to those of the normal species but, for the OD 3 3 3 DME species, at higher frequencies than those of the normal species. This frequency inversion is shown in Figure 2. However, apart from this effect, all three spectra could be fit with the same procedure used for the normal species, and the obtained spectroscopic parameters are reported in the right side columns of Table 2. The line frequencies of all isotopologues are available as Supporting Information. One can see from the values of the rotational constants given in Table 2 that, while the H f D substitution of the hydroxyl hydrogen not involved in the H bond induces—as expected—a decrease of the B and C rotational constants, such a H f D replacement of the hydroxyl hydrogen participating in the hydrogen bond leads to an increase of these rotational constants. This effect is better outlined by the values of the Paa (=Σimiai2) planar moments of inertia, which represent the mass distribution along the a axis. Paa decreases, with respect to normal species, when deuterating the hydroxyl hydrogen involved in the H bond. This shows inequivocally a shrinkage of the complex along the a axis. To reproduce the value Paa expected in the case of a rigid system for a H f D substitution, a shortening of ca. 7 mÅ of the O 3 3 3 O distance is required.

’ STRUCTURAL ANALYSIS Usual methods for the determination of the coordinates of the indidual atoms upon isotopic substitution do not work, of course, for the H f D substitution of the hydrogen involved in the H bond (atom 5 of Figure 3), because the effect due to the H f D mass change is overhelmed by the shortening of the O 3 3 3 O distance. The meaningless Kraitchmann coordinates16 for this H

Table 3. The Experimental rs Coordinates of the H6 Hydrogen Atom Compared to the re (ab initio) Values MP2/6-311þþG** exptl

a

a/Å

(1.665(1)a

b/Å c/Å

(0.716(2) (1.253(1)

conf I

conf II

1.585

-1.414

0.690 -1.276

-0.721 -1.198

Error in parentheses in units of the last digit.

atom are indeed |a| = 1.096i, |b| = 0.470, and |c| = 0.370 Å, respectively, with a well far away from the ab initio value (a = 0.073, b = -0.442, c = 0.346 Å, respectively). Instead, the coordinates of the hydrogen (6 of Figure 3) attached to the intermediate carbon atom (numbered as 1 in Figure 3), which are given in Table 3, are in good agreement with the ab initio values, and possess the requisite to be useful for structural determinations. The experimental values are, on the average, closer to those of conformer I, supporting the conformational assignment to this species. In Table 4 we report the ab initio geometry of conformer I, where the atom numbering refers to Figure 3. The parameters O13H5 and C14O13H5, concerning the relative distance and orientation of the two moieties, have been fitted from the ab initio values (1.861 Å and 113.4°) to 1.917 Å and 115.9°, respectively. In this way, the discrepancies between observed and calculated values have reduced to a maximum of 2 MHz. The increase in the H bond length is expected, since the ab initio and the experimental values have different meanings and correspond to re and r0 structural parameters, respectively.

’ DISSOCIATION ENERGY Limited to conformer I, the observed one, we calculated the dissociation energy (EB) of the complex. At the same time, assuming that the stretching motion between the centers of mass of the two constituent molecules takes place along the inertial a axis of the complex, one can evaluate the dissociation energy with the approximate pseudo-diatomic model from the experimental value of the spectroscopic constants. The stretching force constant (ks) can be estimated with this approximation as17 ks ¼ 16π4 ðμRÞ2 ½4B4 þ 4C4 - ðB - CÞ2 ðB þ CÞ2 =ðhDJ Þ ð3Þ 9512

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Table 4. Structural Parameters of the Observed Conformer of ISO-DME from the MP2/6-311þþG** Calculations, Except the Parameters Given in Boldface bond lengths/Å

valence angles/deg

C2C1

1.525

C3C1

1.519

C3C1C2

112.0

dihedral angles/deg

It has been shown, indeed, that for barriers smaller than 2kT (ca. 420 cm-1 in our case), such a relaxation takes place in cases of a single degree of freedom for conformational relaxation.19

’ ASSOCIATED CONTENT

bS

-121.6

O4C1

1.424

O4C1C3

106.8

O4C1C3C2

H5O4

0.970

H5O4C1

107.0

H5O4C1C2

54.7

H6C1 H7C3

1.102 1.094

H6C1O4 H7C3C1

109.6 110.7

H6C1O4H5 H7C3C1C2

-65.3 -60.5

H8C3

1.093

H8C3C1

110.3

H8C3C1C2

179.0

H9C3

1.094

H9C3C1

109.7

H9C3C1C2

59.6

H10C2

1.095

H10C2C1

110.9

H10C2C1C3

60.9

H11C2

1.095

H11C2C1

109.6

H11C2C1C3

-59.3

H12C2

1.095

H12C2C1

110.5

H12C2C1C3

-179.1

O13H5

1.917a

O13H5O4

178.5

O13H5O4C1

-107.0

C14O13 C15O13

1.418 1.418

C14O13H5 C15O13C14

115.9a 111.5

C14O13H5O4 C15O13C14H5

-5.1 134.0

H16C14

1.091

H16C14O13

107.2

H16C14O13C15

179.2

H17C14

1.098

H17C14O13

110.9

H17C14O13C15

59.8

H18C14

1.098

H18C14O13

110.7

H18C14O13C15

-61.5

H19C15

1.090

H19C15O13

107.2

H19C15O13C14

-179.2

H20C15

1.098

H20C15O13

110.9

H20C15O13C14

-59.8

H21C15

1.098

H21C15O13

110.7

H21C15O13C14

61.5

a

The parameters in boldface have been adjusted to reproduce the experimental values of the rotational constants. Their ab initio values are O13H5 = 1.861 and C14O13H5 = 113.4, respectively.

where μ is the pseudo-diatomic reduced mass, DJ is the centrifugal distortion constant, and R is the distance between the centers of mass of the monomers. The value ks = 7.5 N m1- was obtained. The assumption of a Lennard-Jones type potential relates the dissociation energy to ks18 EB ¼ 1=72ks R 2

ð4Þ

from which a value EB = 10.4 kJ/mol was calculated. This value of EB appears too low for a O-H 3 3 3 O H bond. The ab initio values EB,MP2 (31.6 kJ/mol) and EB,MP2,ZPE (26.4 kJ/mol) are more in line with the bonding energies of this kind of H bond. Probably couplings of the stretching motion leading to the dissociation with the bending or internal rotation motions of one molecule with respect to the second one can make the pseudo-diatomic approximation quite crude.

’ CONCLUSIONS The rotational spectrum observed for the ISO-DME describes a complex with the two units held together by a OH 3 3 3 O Hbond and with a gauche configuration of ISO. Also in the five conformers observed for the dimer of ISO, the proton donor moiety always adopts a gauche conformation.7 However, as pointed out for the ISO dimer, the transient chirality of gauche isopropanol is induced to a permanent chirality in ISO-DME. We quantitatively determined the decrease of the measure of the O 3 3 3 O distance upon H f D substitution known as the Ubbelohde effect. It might appear surprising that only one conformer has been observed, but it is not unusual for different conformers separated by low interconversion barriers to relax into the global minimum.

Supporting Information. Completion of ref 14 and tables of transition frequencies and MP2/6-311þþG** geometries of the three found minima. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the Ministero dell’Istruzione, dell’Universit
a e della Ricerca (MIUR, PRIN 2008) and the University of Bologna (RFO) for financial support. ’ REFERENCES (1) See, for example: Evangelisti, L.; Caminati, W. Phys. Chem. Chem. Phys. 2010, 12, 14433and references therein. (2) See, for example: Giuliano, B. M.; Melandri, S.; Maris, A.; Favero, L. B.; Caminati, W. Angew. Chem., Int. Ed. 2009, 48, 1102and references therein. (3) See, for example: Antolínez, S.; L opez, J. C.; Alonso, J. L. Angew. Chem., Int. Ed. 1999, 38, 1772and references therein. (4) King, A. K.; Howard, B. J. Chem. Phys. Lett. 2001, 348, 343. (5) Maris, A.; Giuliano, B. M.; Bonazzi, D.; Caminati, W. J. Am. Chem. Soc. 2008, 130, 13860. (6) Hearn, J. P. I.; Cobley, R. V.; Howard, B. J. J. Chem. Phys. 2005, 123, No. 134324. (7) Snow, M. S.; Howard, B. J.; Evangelisti, L.; Caminati, W. J. Phys. Chem. A 2011, 115, 47. (8) Ubbelohde, A. R.; Gallagher, K. J. Acta Crystallogr. 1955, 8, 71–83. (9) Tang, S.; Majerz, I.; Caminati, W., submitted. (10) Kondo, S.; Hirota, E. J. Mol. Spectrosc. 1970, 34, 97. Hirota, E. J. Phys. Chem. 1979, 11, 83. Ulenikov, O. N.; Malikova, A. B.; Qagar, Ch. O.; Musaev, S. A.; Adilov, A. A.; Mehtiev, M. I. J. Mol. Spectrosc. 1991, 145, 262. Hirota, E.; Kawashima, Y. J. Mol. Spectrosc. 2001, 207, 243. (11) Balle, T. J.; Flygare, W. H. Rev. Sci. Instrum. 1981, 52, 33. (12) Grabow, J.-U.; Stahl, W. Z. Naturforsch., A: Phys. Sci. 1990, 45, 1043.Grabow, J.-U. Ph.D. thesis, Christian-Albrechts-Universit€at zu Kiel, 1992; Grabow, J.-U.; Stahl, W.; Dreizler, H. Rev. Sci. Instrum. 1990, 67, 4072.Grabow, J.-U. Habilitationsschrift; Universit€at Hannover: Hannover, 2004; http://www.pci.uni-hannover.de/∼lgpca/spectroscopy/ftmw. (13) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; Lopez, J. C.; Mata, S. Chem. Phys. Lett. 2004, 392, 1. (14) Frisch, M. J.; et al. Gaussian 03, Revision B.01; Gaussian, Inc., Pittsburgh, PA, 2003. (15) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R.; Elsevier: New York, 1977; Vol. 6, p 1. (16) Kraitchmann, J. Am. J. Phys. 1953, 21, 17. (17) Millen, D. J. Can. J. Chem. 1985, 63, 1477. (18) Novick, S. E.; Harris, S. J.; Janda, K. C.; Klemperer, W. Can. J. Phys. 1975, 53, 2007. (19) Ruoff, R. S.; Klots, T. D.; Emilson, T.; Gutowski, H. S. J. Chem. Phys. 1990, 93, 3142.

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