Article pubs.acs.org/JPCA
Rotational Spectrum and Internal Dynamics of Methylpyruvate Biagio Velino,† Laura B. Favero,‡ Paolo Ottaviani,§ Assimo Maris,§ and Walther Caminati*,§ †
Dipartimento di Chimica Fisica e Inorganica dell’Università, Viale Risorgimento 4, I-40136 Bologna, Italy Consiglio Nazionale delle Ricerche − Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN) via P. Gobetti 101, I-40129 Bologna, Italy § Dipartimento di Chimica “G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy ‡
S Supporting Information *
ABSTRACT: The rotational spectra of five isotopologues (normal and all monosubstituted 13C species) of methylpyruvate have been measured with the pulsed jet Fourier transform microwave technique. Rotational transitions are split into quintets due to the internal rotations of the two methyl groups. The corresponding barriers to internal rotation have been determined to be V3(H3C−O) = 4.883(8) kJ mol−1 and V3(H3C−C) = 4.657(8) kJ mol−1, respectively. Information on the skeletal heavy atom structure has been obtained from the 15 available rotational constants.
■
INTRODUCTION Conformational equilibria and molecular structures of carboxylic esters (RCOOR′) have been investigated extensively. Already in 1973 Jones and Owen wrote a review on the available investigations of this class of compounds.1 The molecular systems mentioned in that review comprise simple formates and acetates, with R′ as saturated or unsaturated alkyl groups; in some cases, a halogen atom replaces a aliphatic hydrogen in R or in R′. The cited investigations included theoretical calculations and a variety of experimental methods, such as dielectric measurements, infrared and Raman spectroscopy, ultrasonic relaxation nuclear magnetic resonance spectroscopy, electron diffraction, electrical double refraction, and microwave spectroscopy. It was established that a planar heavy-atom skeleton arrangement is preferred and that an equilibrium between a cis and a trans structure occurs (see Figure 1), with the cis form generally more stable, either in vapor or in the solution or in the solid states. Measurements of the rotational spectra of several esters have been reported more recently.2−9 The lighter members of the family are of astrophysical interest. The simplest ester, methyl
formate, has been detected, indeed, in several Galactic center molecular clouds,10 and even both 13C species have been detected in Orion.11 These astrochemical observations raised the interest in the rotational spectra of small ester molecules. Esters are interesting also from a pure spectroscopic point of view, because the presence of one or more methyl groups produces complicated tunnelling effects on this kind of molecule. When a methyl group is attached to a carbonyl group, the V3 barrier is generally low, and large internal rotation splittings are expected. However, for a relatively simple but important ester, methyl pyruvate (MEPY, see Figure 2 for the canonic form), no rotational study has been reported. For this reason, we decided to investigate the rotational spectrum of MEPY.
Figure 2. Shape and atom numbering of the canonical conformer of MEPY. Received: October 11, 2012 Revised: January 3, 2013 Published: January 4, 2013
Figure 1. Cis and trans forms of carboxylic esters (RCOOR′). © 2013 American Chemical Society
590
dx.doi.org/10.1021/jp310074z | J. Phys. Chem. A 2013, 117, 590−593
The Journal of Physical Chemistry A
■
Article
EXPERIMENTAL DETAILS The rotational spectrum of MEPY was first observed as an impurity in a commercial sample of methyl (S)-(−)-lactate (98%, Sigma-Aldrich).12 Later, the assignment was confirmed when repeating the experiments with a technical 90% Sigma-Aldrich sample. MEPY, diluted in helium, was expanded through a solenoid valve (General Valve series 9) into the Fabry−Perot resonator chamber. The backing pressure was kept at 3.5 bar to reach a concentration of about 0.3% of MEPY in the gas mixture prior to the expansion. The microwave spectra of normal and all 13 C species in natural abundance have been recorded in the frequency range 6−18 GHz using a COBRA version13 of a BalleFlygare type14 molecular beam Fourier transform microwave spectrometer already described elsewhere.15 The estimated accuracy of the measured frequency was about 2 kHz, and the resolution of the hyperfine components was 7 kHz.
internal rotations of the two methyl groups, as shown in Figure 3 for the 51,4←41,3 transition.
■
THEORETICAL CALCULATION Full geometry optimizations were carried out at the MP2/6-311+ +G** level using the Gaussian 03 suite of programs.16 Two stationary points, corresponding to the molecular shapes shown at the bottom of Table 1, were found. In the same table, the
Figure 3. Each rotational transition of MEPY is split into five component lines, due to the internal rotation of the two methyl groups.
The roto-torsional energy levels relate to the MS group G18, and their torsional wave functions are symmetry classified as A, E1, E2, E4, and E3.17−19 Despite such a 5-fold structure, the spectrum is intense enough for the measure of all 13C species in natural abundance. A global fit of all component lines was performed with the computer program XIAM, based on the combined axis method (CAM).20 It results in a “rigid” limit set of rotational constants, common to A and E sublevels, and in the first-order centrifugal distortion constants (S reduction Ir representation of Watson’s quartic Hamiltonian21). The parameters concerning the internal rotation of the two (H3C−C and H3C−O) methyl groups are also determined. They include the V3 barrier, the Iα moment of inertia of the methyl group, as well as the angles ∠(g i) (g = a, b, c), which are the angles that the methyl group internal rotation axis forms with the principal axes. In the case of planar molecules, such as MEPY, it is enough to provide ∠(a i), becauase ∠(b i) is the complement to 90° of ∠(a i) and ∠(c i) is 90°. The parameter Dpi2J is one of the so-called internal rotation−overall rotation interaction distortion constants.20 All obtained parameters are reported in Table 2 for the parent species. For the 13C species, the numbers of measured lines were considerably lower than those of the parent species, so that
Table 1. MP2/6-311++G** Structures, Energies, and Spectroscopic Constants of the Two Most Stable Conformers of MEPY
a
Table 2. Spectroscopic Parameters of the Parent Species of MEPY
Absolute energy = −380.816988Eh.
internal rotation parameters of the two tops rotational and centrifugal distortion constants
spectroscopic parameters relevant to the assignment of the rotational spectrum are also listed. In the more stable form (ΔE = 28.4 kJ mol−1), all heavy atoms lie in the ab inertial plane, and the molecule has a Cs symmetry.
A/MHz B/MHz C/MHz DJ/kHz DJK/kHz d1/kHz
■
ROTATIONAL SPECTRA As mentioned above, the rotational spectrum of MEPY was first observed as an impurity in a commercial sample of methyl lactate, while searching for high energy conformers.12 The data reported here are based, however, on the measurement performed on a sample of MEPY. The spectrum has a peculiar aspect, because each transition is split into five component lines, due to the
5328.6576(3)a 1939.3670(2) 1450.1115(2) 0.124(3) 0.38(2) −0.025(3)
H3C−O (1) V3/kJ L mol−1 Dpi2J/kHz Iα/uÅ2 ∠(a i)/degb
H3C−C (2)
4.883(8) 4.657(8) 48(4) 29(5) 3.208(4) 3.221(5) 14.9(2) 41.3(1) statistical parameters α/kHz = 2.5c Nd = 105
Errors in parentheses are expressed in units of the last digit. b∠(b i) is the complement to 90° of ∠(a i) and ∠(c i) is 90°, due to the Cs symmetry of MEPY. cStandard deviation of the fit. dNumber of fitted transitions. a
591
dx.doi.org/10.1021/jp310074z | J. Phys. Chem. A 2013, 117, 590−593
The Journal of Physical Chemistry A
Article
Table 3. Spectroscopic Parameters of the Four 13C Isotopologues of MEPYa 13
13
C(1)
A/MHz B/MHz C/MHz DJ/kHz ∠(a i)(1)c/deg ∠(a i)(2)/deg α/kHzd Ne
13
C(2)
b
5295.972(2) 1902.7956(3) 1427.2238(3) 0.098(6) 15.4(2) 40.7(1) 2.5 35
5325.088(1) 1929.6070(2) 1444.3875(2) 0.109(4) 15.0(2) 41.5(1) 2.0 35
13
C(4)
5323.963(2) 1938.9174(3) 1449.5194(2) 0.124(5) 15.1(2) 41.3(1) 3.0 45
C(7)
5326.437(2) 1892.2614(4) 1423.4521(4) 0.112(7) 15.6(3) 41.1(1) 3.0 35
The parameters DJK, d1, V3(1), V3(2), Dpi2J(1), Dpi2J(2), Iα(1), and Iα(2) have been fixed to the values of the parent species. bErrors in parentheses are expressed in units of the last digit. c∠(b i) is the complement to 90° of ∠(a i) and ∠(c i) is 90°, due to the Cs symmetry of MEPY. dStandard deviation of the fit. eNumber of fitted transition.
a
several spectroscopic parameters have been fixed to the corresponding values for the normal species (see the heading of Table 3). The obtained results, as well the list of the fixed parameters, are given in Table 3.
Table 5. MP2/6-311++G** Structural Parameters of MEPY bond distances (Å) C2C1 O3C2 C4C2 O5C4 O6C4 C7O6 H8C1 H9C1 H10C1 H11C7 H12C7 H13C7
■
STRUCTURAL ANALYSIS Structural information on MEPY can be obtained from the 15 rotational constants of the five isotopologues. Actually, only 10 of these data supply an independent information because, due to the plane of symmetry, for all of the species, the combination of the moments of inertia supplies a fixed value of the Pcc second moment (Pcc ≈ 3.46 uÅ2). First, the rs-coordinates22 of the four carbon atoms have been calculated and are reported in Table 4. These rs-coordinates are
b/Å
a
rs r0 re rs r0 re
C1
C2
C4
C7
±2.2450(7)a −2.2902 −2.2355 ±0.780(2) −0.744 −0.791
±1.154(1) −1.162 −1.159 ±0.255(6) 0.254 0.262
±0.247(6) 0.275 0.280 ±0.291(5) −0.308 −0.297
±2.5592(6) 2.5486 2.5513 ±0.204(7) 0.193 0.206
valence angles (deg) O3C2C1 C4C2C1 O5C4C2 O6C4C2 C7O6C4 H8C1C2 H9C1C2 H10C1C2 H11C7O6 H12C7O6 H13C7O6
125.4a 117.2 122.6 111.9 114.2 109.3 109.8 109.8 105.2 110.2 110.2
dihedral angles (deg)
C4C2−C1O3 O5C4−C2O3 O6C4−C2C1 C7O6−C4C2 H8 C1−C2C4 H9C1−C2H8 H10C1−C2H8 H11C7−O6C4 H12C7−O6H11 H13C7−O6H11
180 180 180 180 180 121.5 −121.5 180 119.6 −119.6
a The three parameters in bold have been fitted to reproduce the 15 experimental rotational constants within 1 MHz. The corresponding ab initio values are 124.7°, 114.4°, and 114.1°, respectively.
Table 4. rs Coordinates of the C Atoms as Compared to the r0 and re (MP2/6-311++G**) Values a/Å
1.5058 1.2150 1.5438 1.2155 1.3325 1.4388 1.0898 1.0936 1.0936 1.0876 1.0912 1.0912
Table 6. re, rs, and r0 Geometries of the Four Carbon Atoms Frame C2C1/Å C4C2/Å C7C4/Åa C4C2C1/deg C7C4C2/dega
Error in parentheses are in units of the last digit. a
the a, b, and c coordinates of the atom of interest in the principal axis system of the parent isotopologue (generally the most abundant isotopic species), and are obtained, according to Kraitchman,22 from the changes of the rotational constants upon a single atom isotopic substitution. These coordinates are compared to the ab initio values (indicated as re, because this is the nature of the ab initio data) and to the values obtained with the partially refined r0 structure discussed presented in Table 5. The c-coordinates have been fixed to zero, due to the planarity of the heavy atoms frame. In Table 5, we propose a partial r0 geometry with all structural parameters from the MP2/6-311++G** calculations, except the parameters given in bold. This set of structural parameters can reproduce the ab initio values within 1 MHz. From the structure of Table 5, one can see that, despite the conceptually different meanings of re (ab initio in this case) and r0 (empirical) geometries, the experimental results fit quite well the ab initio data. The re (ab initio), rs, and r0 (from the geometry of Table 5) geometries of the four carbon atoms frame, provided from our data, are shown in Table 6.
re
rs
r0
1.5058 1.5438 2.327 114.4 146.3
1.504 1.503 2.365 115.2 146.7
1.5058 1.5438 2.328 117.2 146.2
This is not a chemical bond or a valence angle.
The rs values of the C4C2 and C7C4 distances are smaller and larger, respectively, with respect to the re and r0 ones. This could be due to the fact that the small values of the a and b coordinates of atom C4 (see Table 4) are more indeterminate than what it appears from the estimated errors.
■
DISCUSSION AND CONCLUSIONS The V3 barriers to internal rotation have been determined for both methyl groups. The barrier of the O-methyl group is compared in Table 7 to the corresponding values in several methyl carboxylates. We note an increase of the V3 barrier, with respect to methyl formate, when the substituent is a hydrocarbon (aliphatic or aromatic) chain. As to the V3 barrier of the methyl group in the CH3−CO− surrounding of MEPY, its value (4.657(8) kJ mol−1) is much higher than the corresponding values in several acetates. The CH3−CO V3 values are, indeed, 1.2171(4), 1.1909(13), and 1.855(1) kJ mol−1 in methyl acetate,3 ethyl acetate,23 and vinyl 592
dx.doi.org/10.1021/jp310074z | J. Phys. Chem. A 2013, 117, 590−593
The Journal of Physical Chemistry A
■ ■
Table 7. V3 Internal Rotation Barriers of the O-Methyl Group in Several Methyl Carboxylates (X−CO−O−Me) X
V3/kJ mol−1
ref
H CH3 CClF2 F HCC CH2CH CH3−CHOH NC CH3−CH2 o-HO-C6H4 CH3−CO
4.53809(2) 5.0500(7) 4.43(2) 4.51(13) 5.30(13) 5.10(13) 4.80(5) 4.86(1) 5.1253(2) 5.38(2) 4.883(8)
2 3 4 5 5 5 6 7 8 9 this work
ACKNOWLEDGMENTS We thank the Italian MIUR (PRIN08, Project KJX4SN_001) and the University of Bologna (RFO) for financial support.
Table 8. V3 Internal Rotation Barriers of the C-Methyl Group in Several Methyl α-Diketones (X−CO−CO−Me) V3/kJ mol−1
ref
H CH3 OH CH3−O−
3.218(4) 3.81(2)a 4.0228(6) 4.657(8)
25 26 27 this work
a
Actually, this value is obtained from the rotational spectrum of the complex of trans-diacetyl with water.
We conclude this article by outlining that now the rotational spectrum has been assigned also for this ester, MEPY. The investigation was prompted by the observation of an unknown impurity rotational spectrum in commercial methyl lactate. Now the spectral location of the rotational lines of MEPY is available for its interstellar search. In addition, we gave precise information on the potential energy functions of the two large amplitude motions underlying the rotation spectrum of MEPY, the internal rotations of the two methyl groups. In addition, we showed how the V3 barrier to internal rotation of a methyl group in a CH3− CO−O− chain increases considerably when this chain is modified to CH3−CO−CO− chain, that is, replacing the oxygen atom of the chain with a CO group.
■
ASSOCIATED CONTENT
S Supporting Information *
Complete ref 16 and tables of transition frequencies. This material is available free of charge via the Internet at http://pubs. acs.org.
■
REFERENCES
(1) Jones, I. G. I. L.; Owen, N. L. J. Mol. Struct. 1973, 18, 1−32. (2) Karakawa, Y.; Oka, K.; Odashima, H.; Takagi, K.; Tsunekawa, S. J. Mol. Spectrosc. 2001, 210, 196−212. (3) Tudorie, M.; Kleiner, I.; Hougen, J. T.; Melandri, S.; Sutikdja, L. W.; Stahl, W. J. Mol. Spectrosc. 2011, 269, 211−225. (4) Long, B. E.; Powoski, R. A.; Grubbs, G. S., II; Bailey, W. C.; Cooke, S. A. J. Mol. Spectrosc. 2011, 266, 21−26. (5) Williams, G.; Owen, L.; Sheridan, J. Trans. Faraday Soc. 1971, 67, 922−949. (6) Ottaviani, P.; Velino, B.; Caminati, W. Chem. Phys. Lett. 2006, 428, 236−240. Borho, N.; Xu, Y. Phys. Chem. Chem. Phys. 2007, 9, 1324− 1328. (7) Durig, J. R.; Groner, P.; Lin, J.; van der Veken, B. J. J. Chem. Phys. 1992, 96, 8062−8071. (8) Nguyen, H. V. L.; Stahl, W.; Kleiner, I. Mol. Phys. 2012, 110, 2035− 2042. (9) Melandri, S.; Giuliano, B. M.; Maris, A.; Favero, L. B.; Ottaviani, P.; Velino, B.; Caminati, W. J. Phys. Chem. A 2007, 111, 9076−9079. (10) Requena-Torres, M. A.; Martín-Pintado, J.; Rodríguez-Franco, A.; Martín, S.; Rodríguez-Fernández, N. J.; de Vicente, P. Astron. Astrophys. 2006, 455, 971−985 and refs therein. (11) Carvajal, M.; Margulès, L.; Tercero, B.; Demyk, K.; Kleiner, I.; Guillemin, J. C.; Lattanzi, V.; Walters, A.; Demaison, J.; Wlodarczak, G.; et al. Astron. Astrophys. 2009, 500, 1109−1118. (12) Caminati, W., unpublished work. (13) Grabow, J.-U.; Stahl, W. Z. Naturforsch., A: Phys. Sci. 1990, 45, 1043−1044. Grabow, J.-U. Ph.D. thesis, Christian-Albrechts-Universität zu Kiel, 1992. Grabow, J.-U.; Stahl, W.; Dreizler, H. Rev. Sci. Instrum. 1990, 67, 4072−4084. (14) Balle, T. J.; Flygare, W. H. Rev. Sci. Instrum. 1981, 52, 33−45. (15) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; Lopez, J. C.; Mata, S. Chem. Phys. Lett. 2004, 392, 1−6. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.01; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) Longuet-Higgins, H. C. Mol. Phys. 1963, 6, 445−460. (18) Bunker, P. R.; Jensen, P. Molecular Symmetry and Spectroscopy, 2nd ed.; NRC Research Press: Ottawa, 1998. (19) Schnell, M.; Grabow, J.-U.; Hartwig, H.; Heineking, N.; Meyer, M.; Stahl, W.; Caminati, W. J. Mol. Spectrosc. 2005, 229, 1−8. (20) Hartwig, H.; Dreizler, H. Z. Naturforsch. 1996, 51a, 923−932. (21) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1977; Vol. 6, pp 1−89. (22) Kraitchmann, J. Am. J. Phys. 1953, 21, 17−25. (23) Jelisavac, D.; Cortés Gómez, D. C.; Nguyen, H. V. L.; Sutikdja, L. W.; Stahl, W.; Kleiner, I. J. Mol. Spectrosc. 2009, 257, 111−115. (24) Velino, B.; Maris, A.; Melandri, S.; Caminati, W. J. Mol. Spectrosc. 2009, 256, 228−231. (25) Dyllick-Brenzinger, C. E.; Bauder, A. Chem. Phys. 1978, 30, 147− 153. (26) Favero, L. B.; Caminati, W. J. Phys. Chem. A 2009, 113, 14308− 14311. (27) Kisiel, Z.; Pszczółkowski, L.; Białkowska-Jaworska, E.; Charnley, S. B. J. Mol. Spectrosc. 2007, 241, 220−229.
acetate,24 respectively. However, while in simple acetates the CH3−CO group is linked to a oxygen atom giving a CH3−CO− O− chain, in MEPY the CH3−CO group is linked to a second carbonyl group, originating a CH3−CO−CO− chain. For molecules containing such a chain, the V3 barriers are very similar to that of MEPY, as shown in Table 8. It seems that, within the trans arrangement of the −CO−CO− group, the interaction of the methyl group favors the energy minimum.
X
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 593
dx.doi.org/10.1021/jp310074z | J. Phys. Chem. A 2013, 117, 590−593
