Development of millimeter- and submillimeterwave spectroscopy and its application to isotopically-substituted nonpolar molecules, deuterated cubane and deuterated cyclobutanes

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-d₂ are presented. The transitions of J = 14 ← 13 up to 26 ← 25 are observed for cubane-d, yielding the rotational and centrifugal distortion constants B₀ = 3220.720 08 (85) and D_JO = 0.000 180 99 (93) MHz; values in parentheses represent three standard deviations. Trans cyclobutane-1,2-d₂ 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-d₂ 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.