Yoko Sugawara

Ab initio MO calculation of force constants and dipole derivatives for formamide

Abstract Ab initio SCF MO calculations have been carried out for the equilibrium geometry, vibrational frequencies, force constants, dipole moment and its derivatives of formamide. The energy gradient method was employed and the 4-31G basis set was used. For in-plane vibrations: (1) Calculated normal frequencies were 10–20% greater than the observed fundamental frequencies. (2) Isotope shifts (− d 0 , − d 1 , − d 2 , and − d 3 species) were well reproduced. (3) The calculated dipole moment derivatives showed a good correspondence with the infrared intensity pattern. (4) The NH 2 rocking—OCN bending cross term, which should be zero in the Urey—Bradley force field, came out to be as large as −0.18 mdyne A. For out-of-plane vibrations, especially for the NH 2 wagging, it was found to be essential to include polarization functions for N, C and O atoms.

Force constants of trans and cis N-methylformamide from Ab initio SCF MO calculations

Abstract Ab initio SCF MO studies on the equilibrium geometries, force constants, vibrational frequencies, and dipole moment derivatives of trans and cis N-methylformamide have been carried out using the 4-31G and 4-31G* basis sets and an energy gradient method. The trans form was calculated to be 1.4 kcal/mol more stable than cis, the one in agreement with the experimental result. Discussions were made on the trans-cis differences in the geometry, force constants, and normal vibrations.

Ab initio SCF MO study on the force field of amides

Abstract Ab initio SCF MO calculations of the force fields, normal vibrations, and dipole moment derivatives of acetamide and N -methylacetamide were carried out using the 4–31 G basis set for in-plane models and the 4–31 G and 4–31 G ∗ basis sets for out-of-plane modes. The absolute values of the calculated normal frequencies were 10–15% higher than the observed fundamental frequencies, which is the common trend of such a calculation with the 4–31 G basis set under the assumption of harmonic vibration. However, vibrational modes, isotope shifts of vibrational frequencies, and infrared intensities were reproduced quite well. Comparing the force constants of a series of amides, i.e., formamide [Y. Sugawara, Y. Hamada, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 50 , 105 (1980)], N -methylformamide [Y. Sugawara, A. Y. Hirakawa, M. Tsuboi, S. Kato, and K. Morokuma, Chem. Phys. 62 , 339 (1981)], acetamide, and N -methylacetamide, systematic differences were found in the CONH moieties. On the other hand, the force constants of the CCH 3 and NCH 3 moieties were shown to be transferable among these amides.

Ab initio study on cyanamide and isocyanamide

Abstract Spectroscopic parameters, namely force constants and dipole-moment derivatives of cyanamide and isocyanamide at their equilibrium geometry, are studied by the ab initio MO method. The configuration of the former of the two structural isomers has amide character, and the latter amine character. The assignments of skeletal-deformation vibrations are reinvestigated experimentally as well. This study supports Ogilvies attribution of what he observed to the vibrational spectrum of isocyanamide and explains the anomalous infrared intensity in the spectrum. A special theoretical treatment of amino wagging motion is presented, which includes the inversion effect, and which enables the prediction of the inversion splitting of isocyanamide.

In-plane force constants of the peptide group: Least-squares adjustment starting from ab initio values of N-methylacetamide

Abstract Least-squares adjustment of the force constants of N-methylacetamide belonging to the A′ symmetry block was carried out starting from the force constant values obtained by an ab initio SCF MO calculation with the 4–31G basis set. Fourteen diagonal force constants were adjusted by the use of 122 vibrational frequencies of 9 isotope derivatives in the liquid state observed in the region 2000-100 cm−1. Here, all the off-diagonal elements were fixed at the initial values calculated with the 4–31G basis set. The greatest difference between the observed and calculated frequencies was 30 cm−1, and the differences for 94 modes were less than 10 cm−1. Most of the force constatns of bond stretching modes were reduced by 5–10%, and those of angle deformation modes by 20–30% in the course of the least-squares adjustment. The CO stretching force constant was lowered by 30% in the adjustment, and this is understandable because the adjustment involves the effect of bringing the peptide group from a free to a hydrogen-bonded state.

Determination of N-acetylneuraminic acid by gas chromatography-mass spectrometry with a stable isotope as internal standard☆

N-Acetylneuraminic acid was determined by gas chromatography-mass spectrometry using selected ion-monitoring technique with N-[2H3]acetylneuraminic acid as an internal standard. M-COOTMS fragments at m/z 624 of trimethylsilyl derivatives of N-acetylneuraminic acid and at m/z 627 of that of the internal standard were used as monitoring ions. The standard curve obtained was linear in the range of over 10(3), and the lower limit for quantitation was estimated to be a few hundred picograms. This method was used to measure total N-acetylneuraminic acid in the plasma of healthy humans and patients with lung cancer. The total N-acetylneuraminic acid level in the plasma was two to three times higher in the patients than in controls. A few hundred nanoliters of plasma was sufficient for the analysis. The mass fragmentogram of plasma gave a good signal/noise ratio, and measurements were very specific, accurate, and reproducible.

Occurrence of the cis-peptide in gaseous N-methylformamide as revealed by rotational structure in its infrared spectrum

Abstract In the 0.12 cm−1 resolution infrared absorption spectrum of gaseous N-methylformamide, a progression of peaks with a spacing of about 2.6 cm−1 has been found in the 610-540 cm−1 region. This is assignable to a conformer with the CO and NH bonds cis to each other.

Resonance Raman spectra of n-methylthioacetamide

Abstract Rigorous resonance, as well as preresonance Raman spectra of N-methylthioacetamide and its N-deuterated compound are reported. The observed selectivity in the resonance intensity enhancement is discussed in relation to the potential energy distributions obtained by normal coordinate treatment. The structural change concerning the CN and CS bond lengths is suggested to accompany the π* ← π electronic excitation.