Conformational Equilibria in Diols: The Rotational Spectrum of Chiral

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Conformational Equilibria in Diols: The Rotational Spectrum of Chiral 1,3-Butandiol Biagio Velino,† Laura B. Favero,‡ Assimo Maris,§ and Walther Caminati*,§ †

Dipartimento di Chimica Fisica e Inorganica dell'Universit
a, Viale Risorgimento 4, I-40136 Bologna, Italy Istituto per lo Studio dei Materiali Nanostrutturati (ISMN, Sezione di Bologna), CNR, Via Gobetti 101, I-40129 Bologna, Italy § Dipartimento di Chimica “G.Ciamician” dell'Universit
a, Via Selmi 2, I-40126 Bologna, Italy ‡

bS Supporting Information ABSTRACT: The rotational spectra of five conformers of 1,3butandiol have been measured by pulsed jet Fourier transform microwave spectroscopy. All of them are stabilized by an internal hydrogen bond and all of them have a GG0 or a G0 G arrangement of the two hydroxyl oxygens, which means that both oxygen atoms are on the same side with respect to the C1C2C3 plane. Apart from the spectroscopic constants, the relative abundances in the supersonic expansion are provided.

’ INTRODUCTION Aliphatic diols are interesting chemical systems, in relation to (i) internal hydrogen bonding, (ii) transient chirality and potential energy surfaces of the internal motions, (iii) permanent chirality and molecular recognition, (iv) relation and similarity with important biological building block molecules (they can be classified as sugar alcohols), and (v) astrochemistry, which is the possibility for the smaller members to be observed in interstellar space. The investigation of their rotational spectra provides a wealth of information on these topics and on the intramolecular interactions which govern the shapes of their most stable conformers. Extensive qualitative and quantitative investigations of hydrogen bonding in diols have been conducted through the infrared spectroscopic study of the hydroxyl group stretching vibrations (see, for example, Tichy1). Two peaks have usually been observed for the OH stretching and assigned to the free and to the bonded hydroxyl hydrogens. The relative intensities of the two peaks and their frequency difference were the relevant data. The absorbance ratios of the OH-bonded and OH-free peaks and their frequency shifts have been used sometimes to obtain information on the percentage of hydroxyl hydrogens involved in hydrogen bonds.2 More recently, IR investigations of diols have been combined with low-temperature inert matrixes, providing plenty of information on the conformational equilibria and on the conformational cooling dynamics.5
7 Also calorimetric studies are available, performed to size the strengths of the internal hydrogen bonding.6 r 2011 American Chemical Society

Several diols have been characterized by rotational spectroscopy, which provides, besides precise structural data, unambiguous conformational assignments. The simplest one, ethylene glycol, has been the first diol to attract the interest of microwave spectroscopists, and it is the most extensively investigated member of the family.5
11 Its spectrum is indeed extremely complicated by the presence of two conformers, both of them tunneling through a concerted internal rotation of the two hydroxyl groups among two equivalent minima.10,11 The first report gave just a list of measured transition frequencies without a spectral assignment.7 Then the investigation of the doubly8 and singly9 O-deuterated species, with reduced or quenched tunneling effects, allowed the assignment of the spectrum of the most stable conformer. Finally, Christen, Coudert, and collaborators supplied an exhaustive conformational and internal dynamics description of this complicated molecular system.10,11 As to diols with a C3 aliphatic frame, we have two chemical isomers: 1,3-propandiol and 1,2-propandiol. The former one possesses, similarly to ethylene glycol, two equivalent hydroxyl groups. Its rotational spectrum revealed two conformers with small tunneling frequencies due to the concerted rotation of the two hydroxyl groups between two equivalent minima.12,13 As to 1,2Special Issue: David W. Pratt Festschrift Received: January 7, 2011 Revised: February 21, 2011 Published: March 18, 2011 9585

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Table 1. MP2/6-311þþG** Shapes, Rotational Constants, Zero Point Corrected Relative Energies (ΔE0, from Reference 4), Relative Abundances at 70 °C, and Electric Dipole Moment Components of the 16 Most Stable Conformers of 1,3-Butanediol

propanediol, three microwave (MW) investigations are available.14
16 The first one was performed 30 years ago with conventional MW spectroscopy and allowed for the assignment of two conformers and of several skeletal torsion vibrationally excited states.14 The advent of pulsed jet Fourier transform MW spectroscopy allowed assignment of a third conformer.15 After that the simplest sugar, glycolaldehyde, and the simplest diol, ethylene glycol, were detected toward the interstellar molecular cloud Sgr B2 (NLMH), Lovas and collaborators became interested in the rotational spectra of sugar alcohols and sugar acids and, in particular, in extending the conformational assignment of 1,2-propanediol. They succeeded in observing the rotational spectra of seven conformers.16 Related to the C3 diols is glycerol, an important biochemical block molecule. Two MW investigations are available for this molecule.17,18 An early millimeter (mmw) absorption free jet investigation allowed for the detection of two conformers,17 while Lovas and collaborators, still again for astrochemical finalities, assigned the spectra of five conformers with pulsed Fourier transform microwave (FTMW) spectroscopy.18 One of these conformers was characterized by tunneling splittings, due to the concerted motion of the three hydroxyls from all trans to the opposite. Very little MW work is available concerning diols with a C4 aliphatic frame. In this case, three chemical isomers are possible for the n-butane chain and two are possible for isobutane. The only available MW investigation concerns 1,3-butandiol, for which one conformer with a hydrogen-chelated six-membered

Figure 1. MP2 6-311þþG** relative abundances (red) and predicted relative FTMW intensities of the μa (dark blue), μb (blue), and μc (light blue) type transitions of the 16 most stable conformers of 1,3-butanediol.

ring in a distorted chair configuration has been observed.19 However, for the IR investigation in low-temperature inert matrixes of 1,3-butandiol mentioned above, behind the conformational cooling dynamics, spectral signatures of four conformers have been observed.4 For this reason, after the investigations with pulsed jet FTMW spectroscopy of 1,2propandiol, 1,3-propandiol, and glycerol allowed the rotational assignment to be extended to many more conformers than those observed with MW absorption techniques, we decided to extend our early investigation of 1,3-butandiol (from now on 13Bol) to the FTMW technique. The obtained results are reported below. 9586

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Table 2. Experimental Spectroscopic Constants of the Observed Conformers of 1,3-Butanediol (S-Reduction, Ir Representation) I

II

III

IV

VI

A/MHz

6604.9080(9)a

6541.8393(9)

6544.798(2)

6497.768(3)

5169.682(5)

B/MHz

2229.0431(4)

2264.0596(4)

2241.4905(5)

2239.8548(6)

2525.132(2)

C/MHz

1815.3657(4)

1812.8437(3)

1808.1532(4)

1809.8292(5)

2152.733(2)

DJb/kHz

0.37(1)

0.36(1)

0.33(1)

0.35(1)

1.71(5)

0.66(7)

0.37(4)


1.3(2)

DJK/kHz

0.29(3)

0.53(2)

d1/kHz

0.060(9)

0.089(6)

Nc

21

25

20

21

12

σd/kHz

2

2

4

4

4


0.11(5)

Error in parentheses in units of the last digit. b The remaining quartic centrifugal distortion parameters have been fixed to zero since undetermined from the fit. c Number of transitions in the fit. d Root-mean-square deviation of the fit. a

Table 3. Measured and Calculated Ratios of the Dipole Moment Components I

II

III

IV

VII

measd calcd measd calcd measd calcd measd calcd measd calcd μa/μb

6.9

μa/μc ≈3

Figure 2. Region of the spectrum of 1,3-butanediol showing the 40,4 r 30,3 transitions for the four species (I, II, III, IV) of the GG0 family.

’ EXPERIMENTAL SECTION We measured the rotational spectrum of the diol with a pulsed jet Fourier transform20 microwave spectrometer in a coaxial arrangement of the MW radiation and of the molecular beam,21 described elsewhere.22 Commercial samples of 13Bol (Aldrich) have been used without further purification. Helium at a pressure of ≈2 bar was allowed to flow over 13Bol heated at about 70 °C 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 point time domain signal, recorded with 100 ns sample intervals. Each rotational transition is split by the 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 frequency of the Doppler components. The estimated accuracy of frequency measurements is better than 3 kHz. Lines separated by more than 7 kHz are resolvable. ’ RESULTS AND DISCUSSION A. Theoretical Calculations. The complete conformational space of (R)-1,3-butanediol has been explored by Rosado et al.4 at the MP2/6-311þþG(d,p) level. Seventy three unique stable conformers were found, and all the relevant transition states for the conformational interconversion reaction paths were investigated. In the present work we recalculated, at the same level of theory, the structure of the most stable conformers in order to

5.1 2.4

1.1 ≈4

1.0 1.9

0.6 ≈4

0.4

≈3

1.7

≈5

1.7

≈10

8.8

>10 >10

3.6

obtain the rotational constants and the dipole moment components which are needed for the assignment and identification of the conformers present in the jet expansion. The obtained results are listed in Table 1. We labeled each conformer according to ref.,4 both with a Roman number and with an tag like gGG0 t, where the capital letter indicate the gauche (G, G0 , ≈ 60° or ≈
60°) or trans (T, ≈ 180°) arrangements of the two OC
CC dihedral angles, and the small letters indicate the same for the two HO
CC dihedral angles. The ab initio calculations were performed using the Gaussian 03 suite of programs.23 B. Assignment of the Spectra. We first searched for the transitions of the conformer previously observed (conf. I or g0 GG0 t) at room temperature in a static cell.19 We could easily observe the transitions of this conformer and optimized the working conditions, such as the expansion parameters and the temperature (best T ≈ 70 °C) on these lines. We could measure, beyond the μa-type spectrum observed in ref 19, also μb- and μctype transitions. At this point, the search for the spectra of other conformers was based on the data of Table 1, which can be graphically represented in Figure 1, where relative abundances at 70 °C and predicted FTMW intensities of the μa (dark blue), μb (blue), and μc (light blue) type transitions of the 16 most stable conformers of 1,3-butanediol are given as a histogram. The predicted “μg (g = a, b, c) intensities” are just the relative abundance multiplied by the value of the g dipole moment component. After the spectrum of species I, we could observe and assign the μa-R-type transitions of all members of the GG0 family, that is, species II, III, and IV (see Table 1). All these conformers have appreciable values for the μa-component of the electric dipole moment. For all of them we could also measure several μb- and μctype transitions. The measured transitions, given in the Supporting Information, have been fitted with Watson’s semirigid Hamiltonian24 in the S reduction and Ir representation. The obtained spectroscopic constants are listed in Table 2. 9587

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The Journal of Physical Chemistry A The rotational constants of the GG0 family are very close to each other, so that we could observe the same transitions for all four species in a small frequency range, as shown in Figure 2 for the 40,4 r 30,3 transition. Since the rotational constants are quite similar to each other, the conformational assignment has been done on the basis of the relative intensities of the μa-, μb-, and μctype transitions of each species, as described in the next section. Then we dedicated our attention to the rotational spectra of the remaining predicted relatively low energy conformers. Among them, species V
IX are predicted to be less than 6 kJ/mol higher in energy than species I and II. However, after an extensive and careful search for the rotational lines of these conformers, we could assign only one additional spectrum, whose transition frequencies and obtained spectroscopic constants are also reported as Supporting Information and given in Table 2, respectively. The rotational constants of this species are very unique and different from those of all other species and can be straightforwardly assigned to the first member (VI) of the G0 G family. The different nature of this conformer and of its large amplitude motions manifold is reflected in the considerably different values of its DJ and DJK centrifugal distortion parameters (see Table 2). It was not possible to observe any species of the G0 G0 family. Such a failure will be discussed in more detail in a subsequent section. C. Conformational Analysis. As said above, by comparing the experimental rotational constants to their theoretical values (Tables 2 and 1, respectively), one can easily see that the assignment of the last spectrum to species VI leaves no doubt. Vice versa, the rotational constants alone do not allow discriminating between the four species of the GG0 family. However, the assignment of conformer I to the first observed spectrum was already proven in ref 19 from the rotational spectra of the OD species. Then from relative intensity measurements of the μatype transitions (one example is shown in Figure 1), and relying somewhat on the ΔE0 values of Table 1, one can assume the conformational assignment of Table 2 to be correct. However, an additional confirmation of the assignment is given by the intensity measurement performed on μa, μb, and μc transitions of each species, reported in Table 3. In some cases, μb and μc transitions are very weak, and only rough values of the ratios have been obtained, From the relative intensities of the μa transitions, combined with the values of the μa dipole moment components of the various conformers, it has been possible to establish the relative abundance ratios in the jet: NII/NI ≈ 0.86, NIII/NI ≈ 0.19, NIV/NI ≈ NVI/NI ≈ 0.14. The estimated errors on these ratios are about 20%, so that we are not completely sure, for example, that conformer I is more stable than conformer II. As to the structures, the discrepancies between experimental and calculated rotational constants are within 1% for all conformers, so that we did not make any structural adjustment. D. Discussion. As for 1,2-propandiol,15,16 1,3-propandiol,13 and glycerol,18 the investigation of the rotational spectrum of 13Bol with pulsed jet FTMW spectroscopy allowed to extend the measurements of the rotational transitions to a considerable number of conformer species with respect to the room temperature measurements with absorption spectroscopy.12,14,17,19 The rotational spectra of five conformers of 13Bol have been indeed assigned. Among them, all the members of the GG0 family (I, II, III, IV), and one member of the G0 G family (VI). The observed species have a spatial configuration, with both hydroxyl groups on the same side of the C1C2C3 plane, which allows for the formation of a classical intramolecular O
H 3 3 3 O H-bond, with

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canonical values of the H 3 3 3 O H-bond lengths. In addition, such an intramolecular hydrogen bond takes part in a six-membered ring structure with a chair (generally the most stable in sixmembered “aliphatic” rings) configuration. Vice versa, in the species of the G0 G0 family (V, VII, VIII, X, XII, XIII, XV), the two hydroxyl groups are on opposite sides of the C1C2C3 plane, and the formation of a canonical O
H 3 3 3 O H-bond is not longer favored. As a consequence, many of these species, and among them the most stable member of the family (V), do not possess an internal O
H 3 3 3 O H-bond. This could be the reason why we did not observe any of these conformers. Moreover, the calculated values of the dipole moment components of species V are very small (see Table 1), and this could also be a valid reason why we did not observe its spectrum. One could also impute such a failure to conformational relaxations which can take place upon supersonic expansions in the case of low interconformational path barriers.25 Such a relaxation is invoked for 13Bol in the case of matrix deposition.4 However, since we did not observe conformational relaxation among the members of the GG0 family, which would require internal rotations of light (and thus more easily crossing a barrier) hydroxyl groups, we think its improbable for an interfamily relaxation to occur. In the lowtemperature inert matrix IR investigation of 13Bdol, spectral features attributed to species I and II (GG0 family) and to species V and XIV (G0 G0 family) have been observed. The observation of species V, although apparently in contrast with the MW results, can be rationalized by its small dipole moment, which can prevent its MW detection.

’ CONCLUSIONS In summary, we have assigned the rotational spectra of five conformers of 13Bol. As expected, intramolecular O
H 3 3 3 O H-bond was found to be the lead factor in determining the molecular configuration of the most stable species. Pulsed jet FTMW spectroscopy allowed for the extension of the conformational assignment with respect to static cell experiments. After that, a rather complete conformational investigation has been performed for the two propane diols. This is the first extended conformational MW study of a butane diol. As outlined in refs 13, 16, and18 the assignment of their spectra is fundamental for the astrochemical search of such organic molecules in the interstellar media. Diols are plausible candidates for this search, after the detection of rotational lines of ethylene glycol in an interstellar molecular cloud. ’ ASSOCIATED CONTENT

bS

Supporting Information. Complete ref 23 and table of transition frequencies. 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 Italian MIUR (PRIN08, project KJX4SN_001) and the University of Bologna (RFO) for financial support. 9588

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