The microwave spectrum of germyl isocyanate, a quasi-symmetric top

Authors:

• Stephen Cradock,
• James R. Durig,
• A.B. Mohamad,
• Joann F. Sullivan,
• Jacek Koput

Abstract

The microwave spectrum of germyl isocyanate in the K, P, and R bands (12.4-40.0 GHz) has been investigated using natural isotopic samples and samples containing specific germanium isotopes, mass numbers 72 and 74, and samples containing GeD₃ groups. The very complex spectrum is analyzed as that of a quasi-symmetric top, in which the skeletal bending motion has an energy maximum at the linear configuration; the height of the maximum is some 339 cm^-1. Many vibrational states of the two-dimensional bend lie close in energy, resulting in strong interactions between rotational levels of different vibrational states. The program used for the analysis has been used previously to account for similar spectra of methyl and silyl isocyanates and methyl isothiocyanate; the behavior of the germyl compound is more like that of the methyl isocyanate than that of the silyl compound, where the height of the maximum is much less. Although we did not include centrifugal distortion operators in the Hamiltonian the program successfully predicts k = 0 line positions in J + 1 ← J bunches with J up to 9, using only lines in the J = 3 bunch to define the parameters of the model, which include the height of the hump, the bend angle of the skeleton at the energy minimum (142.18(5)°), the force constant for the bending motion at the minimum (0.1094(7) mdyn Å), and the GeN bondlength at equilibrium (1.8257(5) Å). The threefold barrier to internal rotation of the GeH₃ group at the equilibrium position is about 3 cm^-1, much lower than the value found for methyl isocyanate. The fit to the observations, about 100 lines in the 4 ← 3 bunch, is improved by allowing for a variation in the GeN bondlength as the bend angle changes. The quality of the least-squares fit corresponds to an estimated standard deviation for an observation of about 2 MHz, but not all observed lines can be accounted for precisely, showing that additional interactions are involved that are not yet included in the model. © 1988.