Akiko Y. Hirakawa

Molecular Geometry in an Excited Electronic State and a Preresonance Raman Effect

Observations of Raman spectra of various molecules at different exciting laser wavelengths lead to an empirical rule. If a Raman line becomes stronger when the exciting frequency is brought closer to the frequency of an electronic band, this means that the equilibrium conformation of the molecule is distorted along the normal coordinate for the Raman line in the transition from the ground to the excited electronic state.

Infrared and Raman spectra of adenine and its 15N and 13C substitution products

Abstract Infrared (down to 40 cm −1 and Raman spectra (down to 10 cm −1 have been examined of adenine, adenine-1,3-[ 15 N 2 , adenine-2-[ 13 C], adenine-8-[ 13 C], and their amino- d 2 -9- d derivative in crystalline powder. All of the 34 fundamental frequencies of the molecule expected in the 200–1800 cm −1 range have been observed with their isotope shifts. The “first guess” assignments of them have been nearly completed, so that we are now ready for a least squares adjustment of the force constants.

Internal rotation in ethylamine: A treatment as a two-top problem

The infrared absorption spectra of eight isotopic ethylamine molecules, i.e., CH3CH2NH2, CD3CH2NH2, CH3CD2NH2, CD3CD2NH2, CH3CH2ND2, CD3CJ2ND2, CH3CD2ND2, and CD3CD2ND2, have been examined in the vapor phase in the 300–100 cm−1 region. Several Q‐branch peaks were observed for each isotopic species and assigned to the torsional oscillations of the methyl and amino groups of the trans and gauche isomers. The energy levels were calculated on the basis of a coupled two‐top system. The analysis yielded a probable potential function for the internal rotation about the C–N bond of the form V (α) = (316.5/2)(1−cosα)−(11.3/2)(1−cos2α)+(713.7/2)(1−cos3α) −(25.0/2)(1−cos4α)+(25.0/2)(1−cos5α)−(3.7/2)(1−cos6α). The difference between the potential energy minima of the trans and gauche conformations is about 230 cm−1, the trans being the more stable form. It has been concluded that the axis of internal rotation of the amino group does not coincide with the N–C bond but is along a line about 4.5° from the N–C bond and i...

Amino Wagging and Inversion in Methylamines

Fine structures have been observed of the amino‐wagging bands at 780 cm—1 of CH314NH2, at 776 cm—1 of CH315NH2, at 625 cm—1 of CH314ND2, and at 689 cm—1 of CH314NHD. For each vibration—rotation line an appreciable inversion splitting is observed. There is a marked coupling found between the inversion and the internal rotation; and the amount of the inversion splitting of each absorption line depends greatly upon the internal rotation quantum number σ (=0, +1, or —1) and the over‐all rotation quantum number K of the state in question. A hypothetical inversion splitting when the coupling were absent is estimated to be 31.6 cm—1 for the first excited state of the amino‐wagging vibration of CH314NH2.

Vibrational analysis of ethylamines: Trans and gauche forms

Abstract Starting from force constant values calculated by an ab initio MO method ( 4-31G(N ∗ ) ), and by adjusting the diagonal elements, a practical force constant matrix (F) has been reached which could explain the observed infrared and Raman spectra (in the frequency range lower than 2000 cm−1) of the gauche form of the ethylamine CH3CH2NH2 molecule and five isotopic species CH313CH2NH2, CH3CH215NH2, CH3CD2NH2, CH3CH2ND2, and CD3CD2NH2. The F matrix for the trans form of ethylamine was constructed by transferring ab initio 4-31G(N ∗ ) values and by revising diagonal elements with conversion factors whose values are equal to the corresponding values of gauche form. A nearly complete set of assignments was achieved of the vibrational bands of ethylamines, observed so far in the spectral range 2000–100 cm−1. In matrix isolation spectroscopy, two bands assignable to the NH2 wagging vibrations of gauche and trans forms have been found at 775 and 782 cm−1, respectively, for CH3CH2NH2. They are at 768 and 774 cm−1, respectively, for CD3CD2NH2. From the intensity changes of these bands observed on changing the nozzle temperature in the matrix formation, the energy difference ΔE (gauche-trans) of these two conformers has been estimated to be 100 ± 10 cm−1.

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 in Methylamine‐A Determination by the Use of 15N Isotope Shifts

Infrared absorption spectrum of methylamine—15N has been examined in the gaseous state, and the amounts of 15N isotope shifts have been determined for 12 absorption bands of undeuterated methylamine. Force constants in the methylamine molecule were adjusted on the bases of the vibrational frequencies of CH3NH2, CH3ND2, CD3NH2, and CD3ND2 and of the 15N isotope shifts of CH3NH2 by the method of the least squares. The 15N isotope shifts have been found to be useful data for selecting the best set of force constants among several sets which are equally good as far as only the agreement of the observed and calculated frequencies are taken into account.

Amino wagging and inversion in methylamines: Part II. CH3ND2 and CD3NH2

Abstract Fine structure of the amino-wagging band at 625 cm −1 of CH 3 ND 2 has been reexamined, and that at 740 cm −1 of CD 3 NH 2 has been newly observed. Analysis of the spectra was made on the basis of the previously described theory for the coupling of the internal rotation and inversion of the methylamine molecule. A hypothetical inversion splitting when the coupling were absent is estimated to be 13 cm −1 for the first excited state of CH 3 ND 2 , and 18 cm −1 for the first excited state of CD 3 NH 2 . The barrier height is estimated to be 1688 cm −1 in the wagging-inversion double-minimum potential in CH 3 NH 2 .

Amino wagging and inversion in hydrazines: Antisymmetric wagging band of NH2NH2

Abstract The fine structure of the 937 cm−1 band of undeuterated hydrazine in the gaseous state has been observed with a resolving power of 0.3 cm−1. It has been found that every vibration-rotation line splits into four components, and that the amounts of the splittings depend only slightly upon the rotational quantum number K. This fine structure was explained by considering (1) that the band is caused by a vibration in which the wagging motions of the two amino groups take place antisymmetrically with respect to the C2 axis, (2) that along this coordinate an appreciable inversion takes place, (3) that such an inversion results in four equivalent equilibrium conformations of the molecule one after another, and (4) that the barrier height between every two adjacent equilibrium conformations is 2620 cm−1.

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.