Two-dimensional tunneling Hamiltonian treatment of the microwave spectrum of 2-methylmalonaldehyde

Abstract

The molecule 2-methylmalonaldehyde (2-MMA) exists in the gas phase as a six-membered hydrogen-bonded ring [HO-CH=C(CH(3))-CH=O] and exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is interesting because the transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the ring, i.e., HO-CH=C(CH(3))-CH=O-->O=CH-C(CH(3))=CH-OH, which then triggers a 60 degrees internal rotation of the methyl group attached to the ring. The microwave spectra of 2-MMA-d0, 2-MMA-d1, and 2-MMA-d3 were studied previously by Sanders [J. Mol. Spectrosc. 86, 27 (1981)], who used a rotating-axis-system program for two-level inversion problems to fit rotational transitions involving the nondegenerate A(+) and A(-) sublevels to several times their measurement uncertainty. A global fit could not be carried out at that time because no appropriate theory was available. In particular, observed-minus-calculated residuals for the E(+) and E(-) sublevels were sometimes as large as several megahertz. In the present work, we use a tunneling-rotational Hamiltonian based on a G(12) (m) group-theoretical formalism to carry out global fits of Sanders' 2-MMA-d0 and 2-MMA-d1 [DO-CH=C(CH(3))-CH=O] spectra nearly to measurement uncertainty, obtaining root-mean-square deviations of 0.12 and 0.10 MHz, respectively. The formalism used here was originally derived to treat the methylamine spectrum, but the interaction between hydrogen transfer and CH(3) torsion in 2-MMA is similar, from the viewpoint of molecular symmetry, to the interaction between CNH(2) inversion and CH(3) torsion in methylamine. These similarities are discussed in some detail.

Reinvestigation of the microwave spectrum of 2-methylmalonaldehyde

“…In addition, the Fourier-transform microwave (FTMW) measurements present a 20-fold improvement in measurement precision, together with a 5-fold improvement given by the millimeter wave measurements. We have also shown that the formalism employed in [2] is capable of fitting these new higher precision measurements to nearly experimental accuracy. This allowed us: (i) to confirm the validity of discarding a few lines from Sanders' tables, (ii) to verify the stability of the molecular constants when more data were added, (iii) to confirm a surprising anomaly in the internal rotation splitting for the normal and singly deuterated species, and (iv) to confirm the validity of the two-dimensional tunneling model for hydrogen transfer and internal rotation to the 10 kHz accuracy level.…”

“…Sanders [1] presented the first microwave measurements on this system, discussed the basic theoretical concepts necessary to understand the rotational spectrum of a molecule with these two large-amplitude motions, and derived a number of molecular parameters from his spectrum. Much later, two of the present authors showed [2] that Sanders' data could be fit with much greater precision using a high-barrier tunneling formalism originally developed for methylamine. The reader is referred to these two papers for further introductory material and background concerning this problem.…”

“…We use here the Hamiltonian described in detail in [2]. The physical model assumes that the molecule is confined for many vibrations to one of six equilibrium frameworks, but that it occasionally tunnels from one of these six minima to another.…”

“…Even though the fit in [2] was quite satisfactory, questions arising from the exclusion of a few of Sanders' measurements from the fit, from the relatively small number of measurements available from Sanders' tables [1], and from the measurement accuracy available at that time, still remained. In the present paper, we have increased the number of measured lines by almost a factor of 30, and increased their frequency range from 14 6 m 6 40 GHz to 8 6 m 6 149 GHz.…”

Understanding High‐Resolution Spectra of Nonrigid Molecules Using Group Theory

“…From this, the complete nuclear permutation-inversion (CNPI) group has been developed, which has its origins in the work of LonguetHiggins [10] and Hougen. [11] In the recent past, PI group theory has been successfully applied to a broad variety of complex nonrigid molecules, such as the water dimer (H 2 O) 2 (see also Section 3), [12,13] the methanol dimer (CH 3 OH) 2 , [14] 2-methylmalonaldehyde, [15,16] the dimer CH 3 FÀCHF 3 , [4] acetaldehyde CH 3 COH, [17,18] and the benzene dimer (C 6 H 6 ) 2 , [19] to mention just some to give an impression of its broad applicability. In a recent article, Hougen summarized strategies for advanced applications of PI groups to the microwave spectra of molecules with large-amplitude motions.…”