Microwave spectrum of salicyl aldehyde: Structure of the hydrogen bond (original) (raw)
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Journal of Molecular Spectroscopy, 2017
The pure rotational spectrum of salicyl aldehyde, C 7 H 6 O 2 , a classic molecule with an OH• • •O intramolecular hydrogen bond, was investigated in detail with several different spectrometers. Supersonic expansion measurements at 8-18 GHz were used to determine rotational constants for 26 different isotopologues, either in natural abundance or prepared synthetically, and including single substitution of each of the 15 atoms in the molecule. Further measurements of the room temperature rotational spectrum at 8-230 GHz allowed assignment of the five lowest excited vibrational states. Their rotational constants were used to calibrate anharmonic force field calculations, which were then used to determine the semi-experimental equilibrium r SE e geometry. This is compared with several other structural determinations and with quantum chemistry calculations. In addition, Stark measurements in supersonic expansion were used to determine the electric dipole moment of salicyl aldehyde.
Reinvestigation of the microwave spectrum of 2-methylmalonaldehyde
Journal of Molecular Spectroscopy, 2008
The molecule 2-methylmalonaldehyde (2-MMA) exists in the gas phase as a six-membered hydrogen-bonded ring [HOACH@C(CH 3 )ACH@O] and exhibits two large-amplitude motions, an intramolecular hydrogen transfer and a methyl torsion. The former motion is interesting because transfer of the hydrogen atom from the hydroxyl to the carbonyl group induces a tautomerization in the ring, which then triggers a 60°internal rotation of the methyl group attached to the ring. We present a new experimental study of the microwave spectra of the 2-MMA-d0 [HOACH@C(CH 3 )ACH@O] and 2-MMA-d1 [DOACH@C(CH 3 )ACH@O] isotopologs of the molecule. The new measurements were carried out by Fourier-transform microwave (FTMW) spectroscopy in the 8-24 GHz frequency range and by conventional absorption spectroscopy in the 49-149 GHz range. 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 2578 2-MMA-d0 transitions and 2552 2-MMA-d1 transitions to measurement uncertainty, obtaining a root-mean-square deviation of 0.015 MHz for both isotopologs. This represents a significant improvement in fitting accuracy over past attempts. Some problems associated with calculating barrier heights from the observed tunneling splittings and assumed tunneling paths are also considered.
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.
Hydrogen bonding in salicylaldehyde: A molecular orbital study
Journal of Molecular Structure: THEOCHEM, 1985
Calculations were done at the 6-31G level on hydrogen-bonded salicylaldehyde and the rotamer in which the O-H group is rotated 180" about the C-O bond axis, assuming that the structures are planar but otherwise with full geometry optimization. The parameters for the hydrogen bridge, the six-membered hydrogen-bonded ring, and the changes in converting the rotamer into the hydrogen-bonded structure are compared with values obtained for the aliphatic analog, fl-hydroxyacrolein, calculated using the 4-31G basis set. The concomitant changes in bond length in the benzene ring are similar in magnitude, showing that the perturbation of the O-H group spreads throughout the entire molecule. Mulliken population analysis indicates that the changes in total atomic charge and overlap populations are due primarily to n-electron and u-electron effects, respectively. The hydrogen bond energy is evaluated using this rotamer as the reference state, the rotamer in which the H-C=0 group is rotated 180" about the C-C bond axis, and also benzene, benzaldehyde and phenol as a composite reference state. The procedures by which intramolecular hydrogen bond energies are derived, in part or entirely, from experimental measurements are compared and contrasted with the methodology involved in molecular orbital calculations. 0166-1280/85/$03.50 0 1985 Elsevier Science Publishers B.V.
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
The hydrogen bond: a molecular beam microwave spectroscopist’s view with a universal appeal
Physical Chemistry Chemical Physics, 2009
In this manuscript, we propose a criterion for a weakly bound complex formed in a supersonic beam to be characterized as a 'hydrogen bonded complex'. For a 'hydrogen bonded complex', the zero point energy along any large amplitude vibrational coordinate that destroys the orientational preference for the hydrogen bond should be significantly below the barrier along that coordinate so that there is at least one bound level. These are vibrational modes that do not lead to the breakdown of the complex as a whole. If the zero point level is higher than the barrier, the 'hydrogen bond' would not be able to stabilize the orientation which favors it and it is no longer sensible to characterize a complex as hydrogen bonded. Four complexes, Ar 2-H 2 O, Ar 2-H 2 S, C 2 H 4-H 2 O and C 2 H 4-H 2 S, were chosen for investigations. Zero point energies and barriers for large amplitude motions were calculated at a reasonable level of calculation, MP2(full)/aug-cc-pVTZ, for all these complexes. Atoms in molecules (AIM) theoretical analyses of these complexes were carried out as well. All these complexes would be considered hydrogen bonded according to the AIM theoretical criteria suggested by Koch and Popelier for C-HÁ Á ÁO hydrogen bonds (U. Koch and P. L. A. Popelier, J. Phys. Chem., 1995, 99, 9747), which has been widely and, at times, incorrectly used for all types of contacts involving H. It is shown that, according to the criterion proposed here, the Ar 2-H 2 O/H 2 S complexes are not hydrogen bonded even at zero kelvin and C 2 H 4-H 2 O/H 2 S complexes are. This analysis can naturally be extended to all temperatures. It can explain the recent experimental observations on crystal structures of H 2 S at various conditions and the crossed beam scattering studies on rare gases with H 2 O and H 2 S.
Microwave spectrum of 1,2-propanediol
Journal of Molecular Spectroscopy, 2009
The microwave spectrum of the sugar alcohol 1,2-propanediol (CH 3 CHOHCH 2 OH) has been measured over the frequency range 6.5-25.0 GHz with several pulsed-beam Fourier-transform microwave spectrometers. Seven conformers of 1,2-propanediol have been assigned and ab initio electronic structure calculations of the 10 lowest energy forms have been calculated. Stark effect measurements were carried out on several of the lowest energy conformers to provide accurate determinations of the dipole moment components and assist in conformer assignment.