head of the laboratory:(1960-2000)
Danica Doskočilová a Bohdan Schneider (Institute of Macromolecular Chemistry, Prague)
In introductions to scientific papers, the state of the art is invariably described, together with clearly defined aims. To show how things sometimes actually work, let us tell the story of our development of the magic angle spinning method for applications in proton NMR. At Christmas time of 1962 we were lucky to install in the Spectroscopy Department of our Institute, engaged in studies of synthetic polymers, the first commercial NMR spectrometer in the then Czechoslovakia. JEOL JNM-3-60 it was named, and was equipped both for classic
1
H broad-line measurements, and for
1
H high-resolution work in liquids; without lock, of course, but with easy switching broad-line to high-resolution regimes, and a very good variable temperature outfit.
By that time the basic of CW 1 H high-resolution work in liquids was already conveniently summarized in the classic book by Pople, Schneider and Bernstein, and we quickly proceeded to apply it in structural studies of polymers, and especially of their model compounds. The weird observation that the PVC dimer model compound - meso -2,4-dichloropentane - a molecule with only a mirror plane of symmetry, showed equal vicinal coupling constant, J AX = J BX in its ABX 2 spectral pattern, led us to realize the existence of rapid conformational interconversions in polymer model compounds, and ultimately in polymer molecules themselves. For some years to follow, we concentrated on the study of the conformational structure of polymer model compounds by a combination of vibrational and 1 H high-resolution NMR spectroscopy, thus establishing a basis for both experimental and theoretical studies of polymer conformation and configuration.
Nevertheless, it quickly became clear that our dear JNM-3-60 was becoming obsolete for high-resolution work in liquids. Moreover, many synthetic polymers do not dissolve in NMR solvents, and even when they do, for most practical purposes they are applied as solids and therefore should be studied as such. Looking around for a way to utilize the remaining excellent properties of our JNM-3-60, the imagination of one of us (B.S.) was caught by the "magic" of rotational line narrowing (MAR), described by Andrew and Lowe in the late fifties for some inorganic solid. According to theoretically versed physicists, for 1 H NMR work, the odds were against us: in rigid samples the large dipolar broadening would require practically inaccessible spinning speeds, whereas in samples with naturally narrowed NMR signals (like polymers), further line narrowing would be prevented by the existing internal motional frequencies. Nevertheless, we would not be dissuaded from our MAR gadget, and actually, our first turbine, machined directly from methacrylate glass and spun about a central wire did give some kind of effect.
During a short visit with Professor Andrew in Nottingham in 1967, B.S. inspected his spinning mechanism with a lower conical bearing, and discussed with him all contemporary premises of the success (or failure in the case of 1 H) of MAR. Agreeing on the fact that a good theoretician should always find an explanation for a positive experiment, they parted as friends. Meanwhile in Prague, in collaboration with our electronics engineer Ruzicka and the head of the Institute workshop Babka, making use of the excellent skills of the workshop mechanics, we built 1 within seven months an apparatus suitable for proton MAR-NMR. The probe could be inclined continuously from 90 to 53° with respect to the magnetic field, and permitted direct transition from broad-line to high-resolution measurement, which was important in our preliminary experiments and attempts at theoretical interpretation. The nitrogen-gas-propelled turbines were suspended in two conical dynamic gas bearing and reached spinning speeds of up to 10 kHz when machined directly from a suitable solid polymer. But we also developed 2 hollow glass turbines for the measurement of liquids in suspensions or sorbed on powders which could be spun up to 5 kHz. To achieve at the sample site a field homogeneity necessary for high-resolution work, the probe housing and gas jet construction were built from nonmagnetic materials. In our very limited conditions of those times this was only possible thanks to the very profound knowledge and love of exotic materials by Ruzicka, so that almost anything could be found in his garage (no car!). At perfect magic angle setting, we could reach a resolution of 3 Hz. Within short time the apparatus was also modified for operation at variable temperature 3. To this day, spinning assemblies of very similar type are used in all modern solid-state high-resolution NMR spectrometers.
Already in our first communication on the subject 1, most prospective applications of 1 H NMR line narrowing by magic angle rotation were probed, and the criterion for a positive line narrowing effect was stated in terms of the good old theory of Gutowsky and Pake 4 : to obtain line narrowing by magic angle rotation at practically attainable spinning speeds, the static line width must be partially narrowed by internal motion that is anisotropic in space, with residual static dipolar interactions. The resulting residual line width in the MAR NMR spectrum is limited by the frequency of the internal motion that caused the partial narrowing of the static spectrum. The applications included studies of the phase structure in solid polymers, of motional frequencies of various proton groups in liquid-crystalline polymer solutions, and of the structure of swollen crosslinked polymer gels, the latter two thanks to the turbine construction permitting the measurement of more or less liquid samples 5. In later developments it was shown that by combining the information from MAR NMR spectra with static line-shape analysis, information could be obtained both on the dynamics of internal motions, and on the degree of their spatial anisotropy 6.
In this work, we were travelling a very lonely path. We did make mistakes. We did not patent our apparatus abroad, only in Czechoslovakia, and even that rather late (1971). In the late seventies, the advent of pulsed 13 C NMR was followed by an explosive interest in the magic angle spinning method (from then on called MAS), and its success quenched any interest in 1 H MAR. Having at that time no suitable 13 C NMR spectrometer, we did not participate in that boom, though some of our previous experience was almost tacitly absorbed in it.
By that time the basic of CW 1 H high-resolution work in liquids was already conveniently summarized in the classic book by Pople, Schneider and Bernstein, and we quickly proceeded to apply it in structural studies of polymers, and especially of their model compounds. The weird observation that the PVC dimer model compound - meso -2,4-dichloropentane - a molecule with only a mirror plane of symmetry, showed equal vicinal coupling constant, J AX = J BX in its ABX 2 spectral pattern, led us to realize the existence of rapid conformational interconversions in polymer model compounds, and ultimately in polymer molecules themselves. For some years to follow, we concentrated on the study of the conformational structure of polymer model compounds by a combination of vibrational and 1 H high-resolution NMR spectroscopy, thus establishing a basis for both experimental and theoretical studies of polymer conformation and configuration.
Nevertheless, it quickly became clear that our dear JNM-3-60 was becoming obsolete for high-resolution work in liquids. Moreover, many synthetic polymers do not dissolve in NMR solvents, and even when they do, for most practical purposes they are applied as solids and therefore should be studied as such. Looking around for a way to utilize the remaining excellent properties of our JNM-3-60, the imagination of one of us (B.S.) was caught by the "magic" of rotational line narrowing (MAR), described by Andrew and Lowe in the late fifties for some inorganic solid. According to theoretically versed physicists, for 1 H NMR work, the odds were against us: in rigid samples the large dipolar broadening would require practically inaccessible spinning speeds, whereas in samples with naturally narrowed NMR signals (like polymers), further line narrowing would be prevented by the existing internal motional frequencies. Nevertheless, we would not be dissuaded from our MAR gadget, and actually, our first turbine, machined directly from methacrylate glass and spun about a central wire did give some kind of effect.
During a short visit with Professor Andrew in Nottingham in 1967, B.S. inspected his spinning mechanism with a lower conical bearing, and discussed with him all contemporary premises of the success (or failure in the case of 1 H) of MAR. Agreeing on the fact that a good theoretician should always find an explanation for a positive experiment, they parted as friends. Meanwhile in Prague, in collaboration with our electronics engineer Ruzicka and the head of the Institute workshop Babka, making use of the excellent skills of the workshop mechanics, we built 1 within seven months an apparatus suitable for proton MAR-NMR. The probe could be inclined continuously from 90 to 53° with respect to the magnetic field, and permitted direct transition from broad-line to high-resolution measurement, which was important in our preliminary experiments and attempts at theoretical interpretation. The nitrogen-gas-propelled turbines were suspended in two conical dynamic gas bearing and reached spinning speeds of up to 10 kHz when machined directly from a suitable solid polymer. But we also developed 2 hollow glass turbines for the measurement of liquids in suspensions or sorbed on powders which could be spun up to 5 kHz. To achieve at the sample site a field homogeneity necessary for high-resolution work, the probe housing and gas jet construction were built from nonmagnetic materials. In our very limited conditions of those times this was only possible thanks to the very profound knowledge and love of exotic materials by Ruzicka, so that almost anything could be found in his garage (no car!). At perfect magic angle setting, we could reach a resolution of 3 Hz. Within short time the apparatus was also modified for operation at variable temperature 3. To this day, spinning assemblies of very similar type are used in all modern solid-state high-resolution NMR spectrometers.
Already in our first communication on the subject 1, most prospective applications of 1 H NMR line narrowing by magic angle rotation were probed, and the criterion for a positive line narrowing effect was stated in terms of the good old theory of Gutowsky and Pake 4 : to obtain line narrowing by magic angle rotation at practically attainable spinning speeds, the static line width must be partially narrowed by internal motion that is anisotropic in space, with residual static dipolar interactions. The resulting residual line width in the MAR NMR spectrum is limited by the frequency of the internal motion that caused the partial narrowing of the static spectrum. The applications included studies of the phase structure in solid polymers, of motional frequencies of various proton groups in liquid-crystalline polymer solutions, and of the structure of swollen crosslinked polymer gels, the latter two thanks to the turbine construction permitting the measurement of more or less liquid samples 5. In later developments it was shown that by combining the information from MAR NMR spectra with static line-shape analysis, information could be obtained both on the dynamics of internal motions, and on the degree of their spatial anisotropy 6.
In this work, we were travelling a very lonely path. We did make mistakes. We did not patent our apparatus abroad, only in Czechoslovakia, and even that rather late (1971). In the late seventies, the advent of pulsed 13 C NMR was followed by an explosive interest in the magic angle spinning method (from then on called MAS), and its success quenched any interest in 1 H MAR. Having at that time no suitable 13 C NMR spectrometer, we did not participate in that boom, though some of our previous experience was almost tacitly absorbed in it.
- D.Doskocilova and B.Schneider,
Chem.Phys.Lett, 1970,
6,381.
- B.Schneider, D.Doskocilova and J.Jakes,
Abstracts, XXth Congress AMPERE, Tallin, 1978,
- B.Schneider, D.Doskocilova, J.Babka and Z.Ruzicka,
J.Magn.Reson., 1980,
37,41.
- H.S.Gutowsky and G.E.Pake,
J.Chem.Phys., 1950,
18,162.
- D.Doskocilova and B.Schneider,
Adv.Colloid.Interface Sci., 1978,
9,63.
- D.Doskocilova and B.Schneider, Pure Appl.Chem., 1982, 54,575.
