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A peptide molecule: N -ethylformamide HCONHCH 2 CH 3 (NEFA) was investigated by Fourier transform microwave spectroscopy in order to determine molecular structure, potential barrier to methyl internal rotation, and nuclear quadrupole coupling constant of the nitrogen atom. All the three ( a , b and c ) types of transitions were observed; they were split into hyperfine structure components due to nitrogen nuclear quadrupole coupling. The rotational constants of NEFA were determined to be A =9904.8373(6), B =3521.0995(2) and C =2984.9808(2) MHz, with three standard deviations in parentheses. The inertial defect Δ = I cc − I aa − I bb was calculated from the rotational constants to be −25.24492(2) uA 2 , which indicates the ethyl group to be bent out of the peptide linkage plane. A comparison of the observed rotational constants with those calculated by an ab initio molecular orbital method also led us to conclude that the most stable form of NEFA is trans - sc , a conformer with a nonplanar heavy atom skeleton. No evidence has so far been obtained for the existence of other conformers, as was the case for a related molecule: N -ethylacetamide. We have also observed spectra of five singly substituted isotopomers, three 13 C and one for each of 15 N and 18 O, from which we derived a partial r s structure, in fair agreement with an ab initio result.

The rotational spectrum of the SiFCl molecule in the ground vibronic state was observed for the first time in the frequency region from 340 to 356 GHz. The analysis of the observed spectrum, which consisted of both a-type and b-type transitions, yielded rotational constants and centrifugal distortion constants of the 35Cl and 37Cl isotopic species in the ground state. The harmonic force field of SiFCl was derived from the centrifugal distortion constants and the inertial defects determined for both isotopic species and was used to calculate the vibrational frequencies of the molecule. The average structure in the ground vibrational state was determined to be r z ( SiF ) = 1.5960 A , r z ( SiCl ) = 2.0714 A , ϑz(FSiCl) = 100.85° and σr z ( SiCl )[ 35 Cl  37 Cl difference ] = 0.000065 A by using the observed rotational constants and the harmonic force field derived in the present study.

The gas-phase vibration-rotation spectrum of FDF− was observed in the 930 and 1460 cm−1 regions with a tunable diode laser spectrometer. The observed bands in the 930 cm−1 region were assigned to the v3 antisymmetric stretching and the v2 bending modes. The 1460 cm−1 region was assigned to the v1 + v3 combination band. The previous assignment of the 1397 cm−1 band was corrected to the 2v1 + v3 − v1 band. All observed transitions were analyzed simultaneously by including a Coriolis interaction between the v2 and v3 states to determine molecular constants in the ground as well as v2, v3, v1 + v3, 2v1 + v3, and v1 states. The fundamental frequencies were determined to be v2 = 928.7303(17) cm−1 and v3 = 934.1933(7) cm−1, with three standard deviations in parentheses. From the rotational constants obtained, the equilibrium F ··· F internuclear distance was calculated to be 2.277 52(10) A, which agreed with the value (2.277 71(9) A) of FHF− reported previously.

The contributions of Professor Morino's research work to the understanding of molecular structure are reviewed chronologically: early studies in Tokyo (1930–1943), studies at Nagoya University (1943–1948), at the University of Tokyo (1948–1969), and at Sagami Chemical Research Center (1969–present).

The cw dye laser excitation spectrum of the A 2 A 1 (000) ← X 2 B 1 (000) vibronic transition of the PH2 radical was observed between 18 158 and 18 443 cm−1 at Doppler-limited resolution. The PH2 radical was produced by the reaction of discharged hydrogen with powdered red phosphorus. About 1000 lines were observed for Ka up to 7 and N up to 9. Using the ground-state parameters reported by Endo et al. [Y. Endo, S. Saito and E. Hirota, J. Mol. Spectrosc., 97 (1983) 204–212], upper-state term values were calculated from the observed transition frequencies and were subjected to a least-squares analysis to derive rotational constants, centrifugal distortion constants, and spin-rotation interaction constants in the A state. The perturbations observed for the upper state are briefly discussed.

The change in the centrifugal distortion constants of the allyl radical with the vibrational excitation to the ν 11 state was interpreted as being caused by the interaction of the ν 11 state with the overtone state of the ν 7 mode. This interpretation requires revision of the ν 7 frequency, in conformity with the recent results reported by Getty and co-workers (J. Am. Chem. Soc., 113 (1991) 801; Chem. Phys., 168 (1992) 357) using resonance Raman spectroscopy. The normal coordinate analysis was performed on the allyl radical based upon the vibrational assignment revised by Fjogstad and Ystenes (Spectrochim. Acta, Part A, 46 (1990) 47) and Getty and co-workers in order to calculate the inertial defect and centrifugal distortion constants for comparison with the observed values.

Studies of molecular structure are discussed by paying special attention to the dynamical behavior of molecules and/or molecular systems. Two classic examples of large-amplitude motions are discussed: internal rotation in ethane and ring puckering in cyclobutane. Microwave studies on deuterated species are described. The discussion is then directed to a reaction intermediate, the vinyl radical, which exhibits a unique tunnelling motion. Finally, an application of time-resolved or kinetic spectroscopy to silane discharge plasma is presented, where the infrared spectra of SiH, SiH 2 and SiH 3 are monitored.

A spectroscopic method has been developed to observe rotational spectra of molecules in the millimeter- and submillimeter-wave regions. Advantages and disadvantages of the method are discussed and compared to those of Stark-modulation and Fourier-transform spectroscopy in the centimeter-wave region. As examples of the method's applicability, the results for cubane-d and cis and trans cyclobutane-1,2-d 2 are presented. The transitions of J = 14 ← 13 up to 26 ← 25 are observed for cubane-d, yielding the rotational and centrifugal distortion constants B 0 = 3220.720 08 (85) and D JO = 0.000 180 99 (93) MHz; values in parentheses represent three standard deviations. Trans cyclobutane-1,2-d 2 is found to exist in an equatorial-equatorial and an axial-axial conformation, with only the b-dipole component being nonvanishing, as expected from previous results on the structure of cyclobutane. The observed spectra lead to the rotational constants A = 9 794.3831 (58), B = 9 758.3272(56), and C = 5 782.6540(73) and A = 9 738.242 (15), B = 9 693.452(14), and C = 6 048.564 (14) MHz, for the two forms, respectively; values in parentheses again represent three standard deviations. Cis cyclobutane-1,2-d 2 is expected to exist in two equivalent forms, axial-equatorial and equatorial-axial. The observed spectrum exhibits the effect of puckering. Preliminary values of the rotational constants are A = 9 938.757 (47), B = 9 577.008(47), and C = 5 913.212(41) MHz.

A critical review is presented on the studies of the internal rotation potential function which is indispensable in understanding rotational isomerism from a dynamical point of view. Molecules so far investigated are classified into a few groups to clarify what factors govern the potential function. As an application of these results, the molecular structures of some bicyclic compounds are discussed, where the internal rotation is shown to play an important role. The recent microwave studies on two molecules of fundamental importance in the internal rotation problem, namely ethane and dimethylacetylene, are briefly mentioned.

Observed vibration—rotation constants for HCO/DCO and HO2/DO2 are combined with vibrational frequencies and ground-state rotational constants in order to derive harmonic as well as third-order anharmonic potential constants, and also equilibrium structure parameters for the formyl and hydroperoxyl radicals. The “diagonal” terms of the third-order anharmonic constants thus obtained for the stretching modes are discussed in terms of the diatomic-molecule model.