Microwave Measurements of the Spectra and Molecular Structure for

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Microwave Measurements of the Spectra and Molecular Structure for the Monoenolic Tautomer of 1,2-cyclohexanedione Aaron M Pejlovas, Michael Barfield, and Stephen George Kukolich J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp504880z • Publication Date (Web): 09 Jul 2014 Downloaded from http://pubs.acs.org on July 14, 2014

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The Journal of Physical Chemistry

Microwave Measurements of the Spectra and Molecular Structure for the Monoenolic Tautomer of 1,2-Cyclohexanedione Aaron M. Pejlovas, Michael Barfield and Stephen G. Kukolich* Department of Chemistry and Biochemistry University of Arizona, Tucson, AZ 85721 *Corresponding author. Fax: +1 520 621 8407 E-mail address: [email protected] (S.G. Kukolich) Keywords: microwave spectrum, rotational constants, molecular structure, keto-enol tautomers

Abstract: The microwave spectrum for the monoenolic tautomer of 1,2-cyclohexanedione was measured in the 4-14 GHz regime using a pulsed-beam Fourier transform (PBFT), Flygare-Balle type microwave spectrometer. The molecular structure and moments of inertia were initially calculated using Gaussian 09 using MP2 and 6-311++G** basis sets and these calculations were used to predict the rotational constants and microwave spectra. Rotational transition frequencies were measured and used to determine rotational constants (A, B and C) and centrifugal distortion constants (DJ and DK). The rotational constants for the parent isotopologue, one singly substituted deuterium, and six singly substituted 13C isotopologues were used in a least squares fit to determine gas phase structural parameters for this molecule. All hydrogen atoms were held fixed to the calculated positions, as well as the carbon atoms at positions 1 and 10 and the oxygen atoms at positions 6 and 7. The rotational constants for the parent isotopologue are A=3161.6006(12), B=2101.5426(3), and C=1320.7976(4) MHz. The distortion constants obtained from the fit are DJ=0.0436 and DK=0.436 KHz. Structural parameters from the MP2 calculations are in fair agreement with the measured parameters. ~1~

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1. Introduction: The important organic compound 1,2-cyclohexanedione (CDO) is widely used in organic syntheses and has many industrial applications. It can be prepared by a number of different reactions. One method of CDO preparation is by the ring-opening of epoxides, which are versatile organic intermediates, easily prepared from olefins or carbonyl compounds using Bi catalysts1,2. It can also undergo many organic reactions, including a Michael addition of CDO to β-nitrosytrenes3. It is also added to a number of different consumer products to alter the scent and flavor, such as foodstuffs, perfumes, and tobacco products lacking in certain flavors4,5. CDO has gained attention in biological chemistry, as it was found to selectively and reversibly modify arginine residues in proteins under specific conditions at the guanidino group6, allowing for primary protein sequences to be determined7. CDO is a solid at room temperature and melts at about 35 °C into a yellowish liquid. The vapor pressure was sufficiently high at 35 °C that strong single pulse signals of the parent isotopologue could be obtained by pulsed-beam Fourier transform (PBFT) microwave spectroscopy. CDO can exist in either the diketo or monoenol forms8, but in the gas phase it is mostly monoenolic. Searches for rotational transitions for the diketo form were not successful. The gas phase structure of the monoenolic form of CDO has been measured using electron diffraction9. Since there are limitations to electron diffraction data, it is beneficial to measure microwave data also for this compound. Infrared spectroscopy and quantum chemistry calculations for CDO were reported by Chakraborty et al10.

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___________________________________ [FIGURE CAPTION] Figure 1. The best fit structure of 1,2-cyclohexanedione (enol tautomer) showing the best fit A) bond lengths (Å) and B) the interbond angles (degrees) for the cyclohexane ring. The C3-C5-C2 bond angle passes over the C5-H16 bond. All C-H bond lengths are 1.09(1) Å. ___________________________________

2. Microwave Measurements: The microwave spectrum was measured for the parent isotopologue, all singly substituted 13C isotopologues, and a deuterium substitution at H9 in the 4-14 GHz region, using a PBFT-type spectrometer11,12. There were 72 transitions measured in total, 16 for the parent isotopologue, 14 for the deuterium substitution at H9, and 7 for each single substituted 13C isotopologue. The numbering scheme used in the analysis is given in Figure 1. The sample was purchased from Sigma Aldrich (97%) and was used without further purification. It was loaded into a glass sample cell and attached to a pulsed valve (General Valve series 9). The sample cell and valve were heated to ~35 to 40 °C to obtain sufficient vapor pressure of the sample in the neon gas stream. The pressure inside the spectrometer was maintained at 10-6 to 10-7 torr prior to the pulsed injection of the sample vapor in the Ne carrier gas. The backing pressure was maintained at ~1 atm during microwave measurements. The signal strength was sufficiently strong that the

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singly substituted 13C isotopologue transitions could be measured under the same conditions in 100 beam pulse cycles using a sample with a natural abundance of 13C. A deuterated sample of 1,2-cyclohexanedione was prepared by mixing equimolar amounts of 1,2-cyclohexanedione (Sigma Aldrich, 97%) and CH3OD (Cambridge Isotope Laboratories, Inc., 99%) overnight. The CH3OH was removed under vacuum and the deuterated 1,2-cyclohexanedione crystallized and transferred to a small vial. Similar instrumental conditions were used to obtain the deuterated transitions. All measured transitions are shown in Tables 1-3.

3. Calculations: Ab initio calculations were performed to obtain initial values of rotational constants for 1,2-cyclohexanedione using the Gaussian 09 suite13 with MP2 and 6311++G** basis sets. The calculations predicted the lowest energy structure to be the enol tautomer. This was determined to be true due to the intense signals observed for the rotational transitions corresponding with the enol structure, as well as the strong signals observed with each of the singly substituted 13C isotopologues. Predicted transitions were not observable for the diketo structure. Comparisons of the structural parameters with those determined from the least squares structure fit are given in Tables 6 and 7.

4. Rotational Constants: The experimental rotational and centrifugal distortion constants for the parent isotopologue were determined using a least squares fitting program, FITSPEC14, and are given in Table 4. Similar analyses were carried out for all singly substituted ~4~


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isotopologues, but the centrifugal distortion constants were held fixed to the values obtained from the parent isotopologue for each fit. The deviations (νo-c) between the best fit calculated (c) and observed (o) frequencies are listed in Tables 1-3 for each of the respective isotopologues. The J and K values for reported transitions ranged from 0 to 4 and this range is believed to be sufficient to obtain good values for the distortion constants. Higher J-values could be obtained but signal strength would be lower.

5. Molecular Structure: The rotational constants obtained for each isotopologue from fitting the measured rotational transitions were used in a nonlinear least squares fit program to determine the best fit coordinates of the atoms in 1,2-cyclohexanedione relative to the parent isotopologue’s center of mass. During the least squares fit, all hydrogen atom coordinates were held fixed relative to the atom it was bonded to, as was determined from the Gaussian 09 calculation. The coordinates of atoms C1, C6, and O7 were also held constant at calculated values. The relative coordinates of carbon atoms at positions 3 and 5 were fixed and the coordinates of both were varied using the same variable parameters. The relative coordinates of as C4, O8, and H9 were also held fixed at calculated values and coordinates of these were varied using the a different set of variable parameters. With these constraints, only 9 variable parameters remained and were precisely determined using the least squares structure fit. The variable parameters were the x, y, and z Cartesian coordinates (along the a, b and c axes) for C2, C3 and C5, and C4.

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The experimental A, B, and C rotational constants and the deviations of the “best fit” calculated values from the experimental values are listed in Table 4. Values of the atomic coordinates obtained in the structural fit are listed in Table 5. Coordinates for the substituted atoms were also obtained from performing Kraitchman analyses using the Kiesel KRA program15 and these values are also shown in Table 5. Agreement between the structural fit values and Kraitchman values is very good, with the exception of several of the c-coordinates.

6. Discussion: The rotational spectrum of 1,2-cyclohexandione has been measured using PBFT microwave spectroscopy and all of the measured lines were assigned. The experimental rotational transitions of all the isotopologues are shown in Tables 1-3, as well as the best fit structural parameters given in Tables 6 and 7. The best fit structure obtained from microwave data (Figure 1) obtained from the microwave data is in good agreement with the structure obtained from the electron diffraction data9. The intercarbon bond lengths also agreed well with values calculated from Gaussian 09. An exception is the C1-C4 bond length which was 0.025 Å larger than the calculated value. The dihedral angles for the best fit structure are

Microwave Measurements of the Spectra and Molecular Structure for

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