K.-M. Marstokk

Microwave spectrum and dipole moment of glycolaldehyde

Abstract The microwave spectrum of glycolaldehyde, CH 2 OH-CHO, has been measured and the rotational and centrifugal distortion constants of the ground and three vibrational excited states have been obtained. Only one isomer, which has the carbonyl and the hydroxyl groups cis to one another, was identified. The dipole moment was determined to be 2.73 ± 0.04 D from Stark-effect measurements.

Microwave spectrum, conformational equilibrium, intramolecular hydrogen bonding, inversion tunnelling, dipole moments and centrifugal distortion of ethylenediamine

Abstract The microwave spectrum of ethylenediamine, CH 2 NH 2 CH 2 NH 2 , has been investigated in the 12.4–39.5 GHz spectral region. The two N-C-C-N gauche conformations denoted I and II and shown in Fig. 1, were assigned. The existence of large fractions of further conformations is ruled out. Both rotamers I and II possess an intramolecular hydrogen bond. I is favoured by 0.3 ± 0.2 kcal mol −1 relative to II. The N-C-C-N angles are 63 ± 2° in both conformers. The average CCN angles are 109 ± 1° in I and 111.5 ± 1° in II. The spectra of both rotamers display splittings caused by double minimum potentials. In conformation I the a - and c -dipole moment components were of the “inverting” type, while μ b , is “non-inverting”. The separation between the (+)- and the (-)-energy levels of the double minimum potential of I is 86.356 ± 0.021 MHz. In conformer II the a -axis component of the dipole moment “inverts”, while μ b is “non-inverting”. No c -type lines were observed for this conformation. The energy difference between the (+)- and the (−)-states of the double minimum potential of conformation II is 1.332 ± 0.018 MHz. The first excited state of the C-C torsional motion was assigned for this conformation and the energy difference between the (+)- and (−)-states determined as 1.564 ± 0.066 MHz. The dipole moments were μ b = 1.059 ± 0.007 D, μ b = 0.787 ± 0.032 D, μ c = = 1.179 ± 0.023 D and μ tot = 1.770 ± 0.033 D for conformation I; μ a = 1.952 ± 0.002 D, μ b = 0.867 ± 0.006 D, μ c = 0.538 ± 0.006 D and μ tot = 2.203 ± 0.006 D for II, respectively. All quartic and two sextic centrifugal distortion constants were determined for I, while the quartic distortion coefficients were found for II.

Rotational spectrum of butyronitrile: Dipole moment, centrifugal distortion constants and energy difference between conformers

Abstract The molecular rotational spectrum of butyronitrile has been investigated in the vibrational ground state up to 300 GHz. High J transitions have been measured for the two isomers and fitted to a centrifugally distorted Hamiltonian including some sextic coefficients. The results of the analysis are sufficient for the prediction of all strong transitions throughout the millimeter-wave range. The molecular dipole moment components were calculated from measured Stark effect shifts as μ a = 3.597(59) D and μ b = 0.984(15) D for the anti form and μ a = 3.272(37) D and μ b = 2.139(30) D with μ c preset at zero debye for the gauche form. It has been found from intensity measurements that the anti form is slightly more stable than the gauche form with an energy difference of 1.1(3) kJ mol −1 .

ON THE MICROWAVE SPECTRUM OF ETHYLENE GLYCOL

As a part of our interest in intramolecular hydrogen-bonding problems the microwave spectrum of ethylene glycol was investigated. Previously, electron diffraction investigations [1, 2] have shown that the preferred conformation of the free molecule has the two C-O bonds gauche to one another, and the dihedral angle between the two OCC planes is about 65°. Moreover, the infrared spectrum [3] of gaseous ethylene glycol presents strong evidence for the presenee of intramolecular hydrogen bonding stabilizing the gauche rotamer. In an attempt to assign the microwave spectrum the following procedure was used. A plausible structure of the gauche form was first assumed and th~ rotational constants calculated to be about A = 16.4 GHz, B = 5.1 GHz, and C = 4.4 GHz, respectively. A rigid rotor spectrum was then predicted with these rotational constants and a very thorough search was made for low J a-, b-, and c-type transitions in the 8-38 GHz spectral region. However, no lines with app~opriate Stark effects were found in the predicted frequency ranges, and we therefore feel that it is very unlikely that all three dipole moment components follow rigid rotor selection rulcs. lnstead of the expected rigid rotor spectral features, a very unusual spectrum was revealed. Of about 600 transitions* of medium and strong intensities occurring in the examined spectral range, more than 200 of the strongest lines fall in the 16-19 GHz range with the majority centered around 17.1 GHz. Study of the unresolved Stark effects of these as well as of the great majority of the other intense lines strongly indicated that they are high J transitions. Only the 12 lines ofTable 1 were found to possess resolved Stark lobes with relative intensity features characteristic for R-branch transitions [4]. Attempts to fit these lines to a rigid rotor spectrum of the gauche rotamer proved impossible.

Microwave spectrum, conformation, barrier to internal rotation and dipole moment of pyruvic acid

Abstract Microwave spectra of CH3COCOOH and CH3COCOOD are reported. The preferred conformation of the molecule is demonstrated to possess a planar HCCOCOOH skeleton with two out-of-plane hydrogens. The two carbonyl groups are trans to each other and a weak five-membered hydrogen bond is formed between the carboxyl group hydrogen atom and the carbonyl group oxygen atom. The methyl group conformation is discussed. A computer programme based on “the principal axis method” is described in some detail and the results of a least squares analysis of the observed spectra are outlined. The barrier to internal rotation was determined as V3 = 965±40 cal mol−1 for both isotopic species. Stark effect measurements yielded μa = 2.27±0.02 D, μb = 0.35±0.02 D and μtot = 2.30±0.03 D for the dipole moment and its components along the principal axes.

Microwave spectrum, conformation, dipole moment and centrifugal distortion of glyoxylic acid

Abstract Microwave spectra of CHO-COOH and CHO-COOD are reported. The molecule has a planar equilibrium conformation with the two carbonyl groups trans to each other. A weak five-member intramolecular hydrogen bond is formed between the hydroxyl proton of the carboxyl group and the oxygen atom of the carbonyl group thus stabilizing the trans planar form. Other conformations having a statistical weight of 1 ( cis and trans ) are at least 1.3 kcal mol −1 less stable, and rotamers with a statistical weight of 2 ( e.g. , gauche and skew ) have at least 1.7 kcal mol −1 higher energy. Four vibrationally excited states of CHO-COOH have been analyzed and relative intensity measurements yielded 167 ± 12 cm −1 for the C-C torsional mode and 288 ± 26 cm −1 for the lowest in-plane bending mode. The dipole moment was determined to be μ a = 1.85 ± 0.03 D, μ b = 0.20 ± 0.10 D, and μ tot = 1.86 ± 0.04 D. A seven-parameter centrifugal distortion analysis has been carried out for the ground vibrational state of CHO-COOD and for the ground and three vibrationally excited states of CHO-COOH.

MICROWAVE SPECTRUM, CONFORMATION, AB INITIO CALCULATIONS, BARRIER TO INTERNAL ROTATION AND DIPOLE MOMENT OF PROPIONAMIDE

Abstract The microwave spectrum of propionamide has been investigated in the 21.4–39 GHz spectral range. One conformer has been assigned. This rotamer has a C s equilibrium conformation and the methyl group is syn to the carbonyl group. The dipole moments are μ a = 2.121(20), μ b = 11.66(10) and μ tot. = 11.85(10) × 10 −30 C m. The nuclear quadrupole coupling constants of the 14 N nucleus are χ aa = 2.2(8) and χ bb = 2.3(5)MHz. The barrier to internal rotation of the methyl group is 9.1(5) kJ mol −1 . Six vibrationally excited states of the torsion around the C2–C6 bond were assigned and this fundamental frequency was found to be 45(7)cm −1 . These vibrationally excited states were used to approximate the potential function for torsion near its bottom as V = 7.0 (〈 z 4 〉 + 2.0〈 z 2 〉) cm −1 . This function implies that there is no (or a very small) potential hump at the heavy-atom planar conformation. The first vibrationally excited state of the methyl-group torsion was also assigned. Ab initio computations have been made at the 6–31G ∗ , 6&3ndash;11G ∗∗ , 6–311+ G ∗ , MP 2 6–311 ++ G ∗∗ and MP 3 6–311 ++ G ∗∗ levels of theory. The ab initio calculations imply that there is no second stable form of propionamide. These computations also predict a non-planar heavy-atom conformation at all levels of theory. The Hartree-Fock calculations predict a very small barrier to planarity which is not in complete disagreement with the microwave data. However, the computations involving electron correlation are in obvious disagreement with the experimental findings because a rather high barrier to planarity of 1.6 kJ mol −1 is predicted.

Microwave spectrum, structure and dipole moment of cyclopentadienylberyllium hydride

Abstract Microwave spectra of C5 H5 BeH, C5 H5 BeD, 13CC4 H5 BeH, and 13CC4 H5 BeD are reported. The molecule is a C5v symmetrical top. The BeH bond length was found to be 1.32 A with an error limit of 0.01 A and the CC bond length was determined as 1.423 A with one standard deviation of 0.001 A. The distance from the beryllium atom to the centre of the cyclopentadienyl ring, h, and the CH bond length were assumed to be 1.49 A and 1.09 A, respectively. The dipole moment was determined through the Stark effect to be 2.08 D with one standard deviation of 0.01 D. Four different vibrationally excited normal modes were identified and their frequencies determined by relative intensity measurements.

Microwave spectrum of ethyl thiocyanate

Abstract The microwave spectrum of ethyl thiocyanate, CH3CH2SCN, was investigated in the frequency region 9.8–36.3 GHz; rotational and centrifugal distortion constants in the ground vibrational state, and rotational constants for two excited vibrational states were obtained. Only one rotational isomer, having the CH3 group and CN group gauche to one another, was identified. The dihedral angle was found to be 122° (from anti position). The total dipole moment was found from Stark-effect measurements to be 4.01 D±0.12 D with the components μa = 3.80±0.10 D, μb = 1.13±0.05 D and μc = 0.61±0.04 D.

Microwave spectra of isotopic glyoxylic acids, structure and intramolecular hydrogen bond

Abstract Microwave spectra of CH 18 OCOOH, CHOC 18 OOH, CHOCO 18 OH, 13 CHOCOOH and CHO 13 COOH are reported and have been used in combination with data on CHOCOOH and CHOCOOD to determine the molecular structure as r(C=O) ald. = 1.174 ± 0.006 A, r (C=O) acid = 1.203 ±0.006 A, r (C—O) = 1.313 ± 0.010 A, r (C—C) = 1.535 ± 0.005 A, r (O—H) = 0.948 ± 0.004 A, r (C—H) = 1.104 ±0.010 A, ald. = 123.7 ± 0.4