Reinvestigation of the microwave spectrum of 2-methylmalonaldehyde (original) (raw)
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Journal of the American Chemical Society, 1981
The microwave rotational spectra of malonaldehyde and a number of its isotopic forms have been investigated. In the vapor phase the molecule is found to exist in a planar, intramolecularly hydrogen-bonded form with two equivalent, individually asymmetric equilibrium configurations between which tunneling occurs. The data indicate that the hydrogen bond may be described by a double-minimum potential function. The large amplitude tunneling motion complicates structure determination, but with use of symmetrically substituted isotopic species and asymmetrical species in which the tunneling is believed to be quenched, a modified r, structure has been obtained. Dipole moment measurements of several isotopic species are also reported. The effect of tunneling and of various degrees of quenching of tunneling on the dipole moment is demonstrated and interpreted. From relative intensity measurements the tunneling energy separation of the normal isotopic species was calculated to be 26 f 10 cm-I. Far-infrared observations showed an absorbtion band near 21 cm-I. Rotation-vibration interactions were observed which perturb the rotational spectra of a number of isotopic species. A brief report2 has appeared on a microwave spectroscopic study of the structure and nature of the hydrogen bond in the sixmembered ring compound malonaldehyde (3-hydroxy-2-propenal). It was shown that in the vapor phase the molecule occurs in the internally hydrogen bonded form (see Figure 1) and that it is planar or essentially planar. A 3:l intensity ratio was observed between states with different K-l + K+l parity, where K-l and K+l are the limiting prolate and oblate rotational quantum numbers. A second set of rotational transitions, almost as strong as those of the ground vibrational state but with opposite nuclear spin weight, was also measured. From the observation of alternating intensities and the existence of two spectra of nearly the same average intensities but opposite nuclear spin weights, it was inferred that the molecule exists in two equivalent forms shown in Figure lA, B, so that the proton (atom 6) is rapidly tunneling between the two equilibrium configurations shown. The symmetric configuration, C of Figure 1, represents a transition state of higher energy between the two energy minima. There are various ways to define an "effective" one-dimensional potential function for this large amplitude tunneling motion of the proton. This potential was found to have the double-minimum form shown in Figure 2. The variable x is approximately the displacement of the hydrogen atom 6 in the a direction shown in Figure 1. Microwave spectra of 2-methylmal~naldehyde~ and 2-nitro-malonaldehyde4 showed additional effects arising from hindered rotation about the C-X bond (where X is CH3 or NOz). With CH3, the tunneling of the hydrogen was coupled with this rotation. The main aim of this paper5 (hereafter called paper 2) is to obtain a reasonably complete and fairly accurate quantitative molecular structure which can be compared with various ab initio quantum chemical calculations.8-'0 Evidence will be presented (1) (a) Los Alamos National Laboratory, Los Alamos, NM 87545.
The Journal of Physical Chemistry A, 2009
The microwave spectrum of the sugar alcohol 1,3-propanediol (CH 2 OHCH 2 CH 2 OH) has been measured over the frequency range 6.7 to 25.4 GHz using both cavity and broadband microwave spectrometers. The tunneling splittings from two structurally chiral conformer (enantiomeric) pairs of 1,3-propanediol have been fully resolved and assigned. The tunneling frequency of the lowest-energy inverting pair is 5.4210(28) MHz and found to increase by more than 7-fold to 39.2265(24) MHz for the higher-energy form. From the observed selection rules, three possible inversion pathways along the two OH concerted torsional modes have been identified and theoretically investigated. Quantum chemical calculations (MP2/aug-cc-pVTZ level) have been performed on the eight lowest-energy forms and three transition-state structures. Two of these pathways cross through C S transition states associated with each of the enantiomeric pairs and a third common pathway of lowest energy has a transition state of C 1 symmetry. For only the C 1 pathway is good agreement found between predictions from a 1D WKB analysis and the observed tunneling frequencies and 7-fold ratio. The conformer interconversion barrier is calculated to be about 3-fold smaller than that for the inversion suggesting the wave functions of the four inversion levels are partially delocalized over the four surface minima. Accurate dipole moment components have also been obtained from Stark effect measurements for the lowest-energy form.
ChemInform Abstract: The Ground Torsional State of Acetaldehyde
ChemInform, 1991
New microwave measurements on the ground state of acetaldehyde have been carried out using a Fourier transform spectrometer in the region from 7 to 26 GHz (typical measurement uncertainty 4 kHz), and a conventional Stark spectrometer in the region from 45 to 116 GHz (typical measurement uncertainty 40 kHz). These new ground state measurements and remeasurements have permitted a much better fit to two theoretical models of a data set containing far-infrared combination differences from the literature, microwave transitions from the literature, and the new microwave transitions. Root-mean-square residuals obtained here for all these data (which come from a large number of sources) are only slightly larger (for either model) than the estimated measurement uncertainties. The first theoretical model is essentially a high-barrier effective Hamiltonian for one vibrational state only, based on Fourier expansions in terms of the form cos( 2*n/ 3)(pK -6). The second model is based on calculations using the internal-rotation potential function, and is in principle much more powerful than the first. The present successful fits using either model indicate that earlier fitting difficulties using the second model and a combined infrared and microwave data set were caused by problems in the microwave data set, rather than problems in the model. It is hoped that similar success can be achieved with the more powerful second model when data from higher excited torsional states are considered. o
A two-tunneling path internal-axis-method-like treatment of the microwave spectrum of divinyl ether
Journal of Molecular Spectroscopy, 1988
The microwave spectrum of the cis-trans conformer of divinyl ether, previously measured by Hirose and co-workers, has been fitted using a two-dimensional internal-axis-method (IAM) like treatment which accounts for the 27-MHz tunneling splitting displayed by this molecule. This two-dimensional IAM-like treatment begins by first determining the various feasible tunneling path(s) connecting the two frameworks of the molecule. For the tunneling process corresponding to an antigeared rotation of each vinyl unit about axes coinciding with the respective CO bonds, two limiting cases are considered: if the molecule goes through a planar configuration during the tunneling, only one tunneling path arises; if the intermediate configuration is not planar, two equivalent tunneling paths occur. The consequences for the J and K dependence of the splitting are examined, and using this formalism the microwave data are fitted with a root-mean-square deviation of 0.156 MHz. The parameters related to the J and K dependence of the splitting are also determined and their values are interpreted in favor of a two-tunneling path system. A comparison with the values obtained theoretically for those parameters is also carried out.
Journal of Molecular Spectroscopy, 2004
The jet-cooled Fourier-transform microwave spectrum of N-methylacetamide (CH 3 ANHAC(@O)ACH 3 ), a molecule containing two methyl tops with relatively low barriers to internal rotation, has been recorded and fit to nearly experimental uncertainty. Measurements were carried out between 10 and 26 GHz, with the nitrogen quadrupole splittings resolved for many transitions. The permutation-inversion group for this molecule is G 18 (not isomorphic to any point group), with irreducible representations A 1 , A 2 , E 1 , E 2 , E 3 , and E 4 . One of these symmetry species and the usual three asymmetric rotor quantum numbers J KaKc were assigned to each torsion-rotation level involved in the observed transitions. F values were assigned to hyperfine components, where F ¼ J þ I N . Transitions involving levels of A 1 and A 2 species could be fit to an asymmetric rotor Hamiltonian. The other transitions were first fit separately for each symmetry species using a Pickett-like effective rotational Hamiltonian. Constants from these fits show a number of additive properties which can be correlated with sums and differences of effects involving the two tops. A final global fit to 48 molecular parameters for 839 hyperfine components of 216 torsion-rotation transitions involving 152 torsion-rotation levels was carried out using a newly written two-top computer program, giving a root-mean-square deviation of observed-minus-calculated residuals of 4 kHz. This program was written in the principal axis system of the molecule and uses a free-rotor basis set for each top, a symmetric-top basis set for the rotational functions, and a single-step diagonalization procedure. Such an approach requires quite long computation times, but it is much less prone to subtle programming errors (a consideration felt to be important since checking the new program against precise fits of low-barrier two-top molecules in the literature was not possible). The two internal rotation angles in this molecule correspond to the Ramachandran angles w and / often defined to describe polypeptide folding. Barriers to internal rotation about these two angles were found to be 73 and 79 cm À1 , respectively. Top-top coupling in both the kinetic and potential energy part of the Hamiltonian is relatively small in this molecule.
Journal of Molecular Spectroscopy, 2003
Line assignments were carried out for about 600 Fourier-transform microwave transitions for dimethyl methylphosphonate involving levels of all six symmetry species in the G 18 molecular symmetry group appropriate for three large-amplitude motions and covering J and K values of 1 6 J 6 6 and 0 6 jKj 6 3. The assignments are based on combination-difference loops, variations in line shape for different symmetry species caused by the small internal-rotor splitting patterns of the third, high-barrier methyl top, and agreement with theoretically expected positions. A global fit of 609 lines to a phenomenological tunneling-rotational Hamiltonian with 54 constants was performed, yielding a standard deviation of 8.0 kHz, which is close to the experimental measurement uncertainty. The A/E tunneling splitting for the lowest barrier methyl group internal rotation motion was determined indirectly to be about 34 GHz. The much smaller tunneling splitting for the methoxy interchange motion was determined (with some assumptions) to be 3.3 MHz. ) of [2] by S À1 ðp=2; p=2; 0Þ, as in Eq. (10) of [2]. This procedure is equivalent to abandoning the use of a principal axis system for frameworks 1 through 9, so that the terms D ab , D ac , and D bc in the phenomenological Hamiltonian operator of Eq. (1) become more important. Group-theoretically it leads to definite symmetry species for the rotational angular momentum operators, so that J x , J y , and J z are of species A 2 , A 2 , and A 1 , respectively. Furthermore, since the approximate equality in Eq. (9b) of [2] becomes exact, the symmetric-top rotational basis functions jJ ; Ki are of species A 1 for even K and A 2 for odd K. The following transformation properties of components of angular momentum operators under the combined operation (z) of time-reversal and Hermitian conjugation are also useful in deriving selection rules for Hamiltonian matrix elements:
Microwave and infraared spectra of C2H4--HCCH: Barrier to two-fold internal rotation of C2H4
Journal of Molecular Spectroscopy
Mtcrowave spectra of CzHJ HCCH, C2H4.. DCCH, C2H,. .DCCD, DLC=CH2...HCCH. and tram-HDC=CHD...HCCH have been recorded usmg a pulsed-nozzle Fourter-transform mtcrowave spectrometer. An a-type, A,YO=O spectrum ts observed. wtth a number of transitions being spht mto doublets due to tunnehng arising from the hindered mternal rotatton of the ethylene untt about tts C=C bond. For the normal tsotoptc species we find .1=25981 MHz, B+C=3478.2560( 13) MHz. and B-C= 89 45 ( 18 ) MHz. The complex 1s shown to have a Czv structure m whtch the HCCH unit hydrogen bonds to the ethylene x cloud. wtth the HCCH axis normal to the plane of the ethylene. The hydrogen bond length 1s found to be 2.78 A. Centnfugaldtstortton analysts yields a weak-bond stretching force constant of 2.5 N/m (0.025 mdyn/A), corresponding to a stretching frequency of 56 cm-'. Stark effect measurements determine the electrtc dipole moment of the complex to be 8.852 (21 )X 10m3' C m (0 2654(6) D). The observed tunneling-Induced splittings yteld an internal rotation barrier of 240 cm-'. An infrared spectrum of the asymmetric acetylemc C-H stretch m the complex has also been measured usmg an optothermal color-center laser spectrometer
Microwave and infrared spectra of C2H4…HCCH: barrier to twofold internal rotation of C2H4
Chemical Physics, 1992
Mtcrowave spectra of CzHJ HCCH, C2H4.. DCCH, C2H,. .DCCD, DLC=CH2...HCCH. and tram-HDC=CHD...HCCH have been recorded usmg a pulsed-nozzle Fourter-transform mtcrowave spectrometer. An a-type, A,YO=O spectrum ts observed. wtth a number of transitions being spht mto doublets due to tunnehng arising from the hindered mternal rotatton of the ethylene untt about tts C=C bond. For the normal tsotoptc species we find .1=25981 MHz, B+C=3478.2560( 13) MHz. and B-C= 89 45 ( 18 ) MHz. The complex 1s shown to have a Czv structure m whtch the HCCH unit hydrogen bonds to the ethylene x cloud. wtth the HCCH axis normal to the plane of the ethylene. The hydrogen bond length 1s found to be 2.78 A. Centnfugaldtstortton analysts yields a weak-bond stretching force constant of 2.5 N/m (0.025 mdyn/A), corresponding to a stretching frequency of 56 cm-'. Stark effect measurements determine the electrtc dipole moment of the complex to be 8.852 (21 )X 10m3' C m (0 2654(6) D). The observed tunneling-Induced splittings yteld an internal rotation barrier of 240 cm-'. An infrared spectrum of the asymmetric acetylemc C-H stretch m the complex has also been measured usmg an optothermal color-center laser spectrometer
Chemphyschem : a European journal of chemical physics and physical chemistry, 2016
The molecular beam Fourier transform microwave spectra of 2-acetyl-5-methylfuran were recorded in the frequency range 2-26.5 GHz. Quantum chemical calculations calculated two conformers with a trans and a cis configuration of the acetyl group, both of which were assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two non-equivalent methyl groups. Using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, yielding well-determined rotational and internal rotation parameters, inter alia the V3 potentials. While the V3 barrier height of the ring methyl rotor does not change for both conformers, that of the acetyl methyl rotor differs by about 100 cm1. The predicted values from quantum chemistry are only in the correct order of magnitude.