Small diatomic and triatomic 3d transition metal species are excellent model systems for understanding metal-ligand interactions important to larger complexes. Because of the unpaired 3d electrons, these radicals often occur in states with high angular momentum (electron spin, orbital, or nuclear spin). Three questions are particularly relevant to studying monosubstituted 3d metal compounds. What are the fundamental geometric, bonding, and electronic properties? How accurately does currentquantum mechanical theory describe the interactions in high spin states? Assuming these molecules may be present in the interstellar medium, what are the precise transition frequencies that can be used for radioastronomy?To answer these questions, pure rotational spectroscopy has been applied to eleven simple molecules containing 3d transition metals. The small radicals were synthesized in the gas phase and examined in situ. Both direct absorption, submillimeter spectroscopy and Fourier transform microwave spectroscopy were used to cover thefrequency range 8-660 GHz. New synthetic techniques, including oven-insulating methods, use of a longitudinal AC discharge, and emphasis on organometallic precursors, were developed to improve reaction yields. Spectra were recorded for four categories of 3d metal compounds: vanadium molecules, cobalt radicals, zinc species, and several monocyanides. Frequently, the data exhibited signs of perturbations either from low-lyingexcited electronic states, a common feature with 3d electrons, or from avoided crossings of hyperfine levels. The data were analyzed using effective Hamiltonians, and spectroscopic constants have been determined for rotational, fine structure, and hyperfine interactions. The measurements haveprovided transition frequencies as references for astronomical studies; these values are accurate to within 50 kHz for direct measurements and usually within 100 kHz for frequencies calculated from determined molecular constants. Rotational constants have been used to establish precise molecular geometries. Fine structure and hyperfine data provided insight into 3d metal bonding properties (molecular orbital composition and electron distribution) and structure of electronic state manifolds. In some cases, it was necessary to develop new terms for the Hamiltonian expressions to accurately describe the interactions observed in the spectra. These terms include deperturbation parameters and the first complete description of lambda-doubling for Phi states.

It is a great challenge in chemistry to clarify every detail of reaction processes. In older days chemists mixed starting materials in a flask and took the resul tants out of it after a while, leaving all the intermediate steps uncleared as a sort of black box. One had to be content with only changing temperature and pressure to accelerate or decelerate chemical reactions, and there was almost no hope of initiating new reactions. However, a number of new techniques and new methods have been introduced and have provided us with a clue to the examination of the black box of chemical reaction. Flash photolysis, which was invented in the 1950s, is such an example; this method has been combined with high-resolution electronic spectroscopy with photographic recording of the spectra to provide a large amount of precise and detailed data on transient molecules which occur as intermediates during the course of chemical reac tions. In 1960 a fundamentally new light source was devised, i. e. , the laser. When the present author and coworkers started high-resolution spectroscopic stud ies of transient molecules at a new research institute, the Institute for Molecu lar Science in Okazaki in 1975, the time was right to exploit this new light source and its microwave precursor in order to shed light on the black box.

It is amazing how much information can be gleaned from a magnetic resonance spectrum by one who knows. That series of lines on chart paper may conceal anything from energies of activation and spin densities, to conformations and differentiation of isomers. In order to be able to deduce such things about the structure and properties of molecules in a sample, it is necessary to be familiar with the underlying principles, and to arrive at that state of understanding is not easy. This book was conceived and written in an attempt to clarify what is necessary theoretical equipment for anyone wishing to extract the maximum information from a magnetic resonance spectrum. It is also written for those who will find a fascination and great satisfaction in the way this subject, which involves so many sides of modern physics, holds together. It seems to the author, from experience, that the difficulty of getting to grips with the theory of magnetism and magnetic resonance is two sided. On the one hand one has forgotten, or never really known, the principles of electromagnetism on which it is based, and on the other, detailed analysis of the spectra requires a certain facility with the operator techniques of quantum mechanics. In both cases the principle difficulty appears to be unfamiliarity, so the chief aim in this book will be to introduce the enquirer to the relevent language in a reasonably connected fashion.