Construction of Carbon-13 Nuclear Magnetic Resonance Database System with Intensities

Transcription

1 ANALYTICAL SCIENCES OCTOBER 1988, VOL Construction of Carbon-13 Nuclear Magnetic Resonance Database System with Intensities Osamu YAMAMOTO, Kikuko HAYAMIZU and Masaru YANAGISAWA National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305 O. Y. Present address Kanda University of International Studies, Wakaba, Chiba 260 A 13C-NMR database system is described for the spectra of pure organic compounds obtained under the complete decoupling of the 1H-NMR. In this database the peak positions, the relative intensities, and the line widths for broad lines are included, together with the spectral assignments given in the structural formula. Since the relative intensities depend on the measuring conditions, the sample and measuring conditions are also included in the database. The spectral pattern with a Lorentzian line shape is generated for user's access. The data structure and the problems in constructing the 13C-NMR database are discussed, especially for the spectral assignments, the splittings due to the coupling with the magnetic nuclei other than 1H, the relative intensities, and other important points concerning the reliability of the database. Keywords Spectral database, 13C-nuclear magnetic resonance, file structure At the present time, 13C-NMR is widely used by chemists for the structure determination or the functional analysis of organic compounds, because the information on the carbon skeletons and the substruc tures of the compounds is directly derived from the 13C-NMR. Up to now enormous amounts of data have been accumulated in the literature; some of these, together with some newly measured data, have been compiled into the databases as the spectral databases have been developed. 1-5 The 13C-NMR databases have been increasingly utilized by chemists, along with the spectral databases for IR, MS, or the other spectra. 13C-NMR spectra are usually obtained under the condition of complete 1H decoupling, so that generally one peak has no fine structure and corresponds to magnetically non-equivalent carbon(s) in a molecule. The peak positions, which are generally the 13C chemical shift values, are the most important spectral data. The simplicity of the spectrum greatly favors the interpretation of the spectrum. Thus many people tend to be interested only in information on the 13C chemical shift values. The 13C-NMR databases2,4 and data books6-8 so far compiled have concentrated on accumulating only the chemical shift values. This is unavoidable when the compilation of the data is performed mainly from the literature, in which the intensities of 13C-NMR peaks can rarely be found. However, if the intensities are not included in the database, it will be difficult to reproduce the actual spectral patterns. Although the chemical shifts are the most important information obtained from the 13C NMR spectroscopy, the intensities and the line widths should be included in the databases, since they give information that cannot be obtained only from the chemical shift values. In principle, it is possible to make assignments for individual 13C-NMR peaks which are very informative for general users. The changes in the spectrum can be easily interpreted from the changes of the chemical structure, if the correct assignments are given. Fur thermore, it is possible to correlate the chemical shift values to the substructure from the assignments, which are essential in order that the database system will develope into an intelligent system. Along the line, we have constructed the 13C-NMR database as a subsystem of the integrated spectral database named SDBS.9 In our 13C-NMR database construction, most of the spectra are measured, the assignments are added, and then the data are stored in our laboratory. In this paper the file structure and various problems in constructing the 13C-NMR data base are presented Results and Discussion in some detail. The data files of the 13C-NMR database consist of two files, a MASTER FILE and an ACCESS FILE. The MASTER FILE is similar in structure to MASTER FILE I of the 1H-NMR spectral database reported previously10, except that it contains all of the information on a 13C-NMR spectrum. That is, MASTER FILE comprises sets of the position, the intensity, and the line width of a peak and its

2 462 ANALYTICAL SCIENCES OCTOBER 1988, VOL. 4 assignment, and the other complementary data which characterize the spectrum of a compound other than the spectral pattern. ACCESS FILE is a file through which users can access the data in the MASTER FILE. Data reduction from 13C-NMR spectral pattern Most of the 13C-NMR spectra obtained under the complete 1H decoupling condition consist of sharp lines. Generally, the number of the lines of one spectrum is equal to the number of magnetically non equivalent carbon atoms in the compound, if the magnetic nuclei other than 1H such as 19F and 31P are not included. The line shape can be assumed as a Lorentzian in the extremely narrowing condition, where all the line widths may be assumed to be approximately the same as long as the 1H decoupling conditions are properly set, except for some lines which will be described later. In general, the spectral pattern can be reduced to a set of lines, each of which consists of a pair: the position and the intensity. In the database, therefore, 13C-NMR spectra can be stored in the form of the lines. This leads to a greater saving in the memory size as compared with 1H-NMR spectral database.10 When displaying the spectrum, the pattern can be generated as a Lorentzian line shape with a suitable line width. Some exceptional line broadenings, however, may arise in some circumstances including the following: (1) The carbon atoms perturbed by a quadrupole nucleus nearby. An example is a signal for some aromatic carbon substituted by a nitro group. (2) The carbon atoms undergoing chemical ex change. (3) The carbon atoms contained in a molecule of high molecular weight, in which spectrum the extremely narrowing condition does not hold. The line width information should be included in the database for the carbons under the circumstances described above. The spectral pattern is always generated using a Lorentzian line shape. When the carbon in question undergoes the chemical exchange, only the line shapes at the fast or slow exchange limits can be treated in the database. Since the line shapes are not Lorentzians for the intermediate chemical exchange processes, they cannot be registered in the database. Actually, in these cases, we observe the spectrum at the higher temperature, and we may often extract the peak positions, the intensities and the line widths. In our 13C-NMR database, when the line width of a signal seems to exceed about 5 Hz, its information is extracted from the spectrum and stored in the database. By inspecting the observed spectrum, the signals which seem to have the line width larger than about 5 Hz are picked up. When the line width between about 5 10 Hz, the signal is assigned to be "slightly broad". For - the signal whose line width is larger than about 10 Hz, an effort is made to obtain the exact value by expanding the signal and measuring the full line width at the half height. But the S/N ratio of the broad lines is generally not very good, so that only approximate values are often extracted. Theoretically the line broadening is a frequency-dependent phenomenon and actually in many cases the line broadenings are observed at the lower resonance frequencies. Relative intensities and measuring conditions The intensities of 13C-NMR are not proportional to the number of the corresponding carbon atom(s), since both the nuclear Overhauser effect (NOE) and the spin lattice relaxation time (T1) differ from carbon to carbon within a molecule. Because of the different values of T1, the relative intensities of 13C-NMR spectrum depend very much on the measuring conditions. Under various measuring conditions which are used in many different laboratories, the intensities also vary. Ac cording to the results that were investigated by the NMR Data Committee of the Chemical Society of Japan, the relative intensity of the quaternary carbon in ethylbenzene observed by various laboratories was shown to deviate about 45% for the FT 13C-NMR. 12 This fact presents us with a serious question: whether the intensity can be a datum suitable for the database. Although the intensity of the 13C-NMR signal is strongly perturbed, as described above, it contains useful information about the molecule. As is true for the 1H-NMR, the 13C-NMR intensities properly reflect the type and the number of carbons giving the signal. NOE strongly affects the intensity, but the variation of the intensity by NOE is itself the important information on the geometrical relationship between the atoms or groups in a molecule. The same is true for the effect of the T1 on the signal intensity. Finally the assignment of the signal is greatly aided by the information of the intensity. One of the solutions for the difficulties about the 13C-NMR signal intensity will be that the same or nearly the same measuring conditions are used through out the collection of data in the database. It is possible to estimate the deviation of the intensities arising from the different measuring conditions. Therefore, if the measuring conditions are approximately the same throughout the database, it is relatively easy for the users to compare the spectra measured by their own measuring conditions with those in the database. Along the line we decided to use several sets of the standard measuring conditions, which are suitable for sample conditions such as the solvent, the molecular weight, and other factors. The measuring conditions are described for each entry in the database. The most important factors affecting the observed signal intensity are the flip angle and the repetition time of the r.f. pulse, in connection to the spectral width and the data length. The flip angle and the repetition time of the pulse are selected to be and 4-7 s, respectively, after our long experience with the 13C NMR spectra and from the consideration of the efficiency in constructing the database. The line broadening constant for the improvement of the S/ N

3 ANALYTICAL SCIENCES OCTOBER 1988, VOL ratio of the signals is typically between 0.5 and 1 Hz, except for very broad lines. When a spectrum contains many sharp and a few broad lines, two line broadening constants are used to obtain the precise positions and the intensities for the sharp lines, and accurate line widths for the broad lines. Another factor which must be taken into account is the reproducibility of the relative intensities. If the data points for the FID signal are not enough, the signal intensities deviate from one measurement to the other especially for sharp signals. Good digital resolution is important for the accuracy of both the peak positions and the relative intensities. The digital resolution is set to ppm, depending on the spectrometers. The spectra are observed at the resonance frequencies of , 22.50, and/ or MHz. The accuracy of the peak positions is which means that the peak positions of the spectra for the same sample measured by the different spectrometers agree with each other within the value mentioned. 13C-NMR spectra are not so sensitive to the solvent and the concentration as in 1 H-NMR. It is preferable, however, that the solvent and the concentration are similar throughout the database. In our 13C-NMR database, CDCl3 is generally used as a solvent D2O, DMSO-d6 and dioxane/ CDCl3 mixture (50/50 in volume) are used as other solvent systems. The concentration of the sample is adjusted to about 25% or less. Tetramethylsilane (TMS) is the internal reference of the chemical shifts for the organic solvents. In the D2O solutions, dioxane or, less frequently, t-butanol (its methyl signal) are used as the internal references. The peak positions referred to the second reference substances are converted to the values referred to TMS by adding 67.4 and 30.8 ppm for dioxane and t-butanol, respectively. These values were determined experimentally by using the mixed solvents of D2O and ethanol including TMS, in which a small amount of dioxane or t-butanol is added. The concentration of ethanol is decreased gradually and the 13C shifts of the second reference substance referred to TMS are plotted versus to the concentration of ethanol and extrapolated to the infinite dilution. MASTER FILE MASTER FILE includes the records given below, for which similar items have similar meanings as in the 1 H-NMR database. 10, 11 Record 1: Spectral key to each spectrum. Record 2: Structural formula with assignments. Record 3: Quality level of the spectral data. In the 13C-NMR database, this is determined by the number of signals which cannot be assigned. Record 4: Sample conditions. Record 5: F, AMP, IAS, ML, IC, ID, WID for each signal. F is the peak position in ppm, which is given to two digits below the decimal point AMP is the relative intensity in an arbitrary unit, which is normalized to 1000 for the most intense peak in ACCESS FILE. IAS is the assignment of the signal. ML of the spin multiplicity of the signal, i.e., the number of protons attached to the carbon atom. IC is the coupling information. If the signal is one of a multiplet split by the coupling with hetero-nuclei such as 19F and 31P, the nuclei are specified in IC. ID is the mark given to a set of signals whose assignments may be interchangeable. WID is the line width of the signal, which is not given for the "normal" signal with a narrow line width. Record 6: Origin of the spectrum. Record 7: Comment, if any. The important com ment is the description on the spectral assignments are made. The data are input in EDITOR form10, and then compressed and registered to MASTER FILE; an ACCESS FILE for the users is made by a program, as in the 1 H-NMR database. 10, 11 Output of the spectrum For output of the spectra in SDBS, the users can select several output devices, i.e., a character terminal, a line printer, a graphic display, an x-y plotter and a laser printer. Examples of output to the last three devices have already been given In our 13C-NMR database, the spectrum is plotted with a Lorentzian line shape, where the "normal" signal is assumed to have the line width of 1 Hz. The signal assigned to have a "slightly broad" line width is regarded to have a width of 5 Hz, and the signal whose line width is explicitly given is plotted with that value. Since the spectrum is regarded as a set of lines in the 13C-NMR database, the output to a character terminal or a line printer is useful, where the information not given in the graphic form can be provided. A bar graph spectrum is also helpful in the 13C-NMR. Figures 1 and 2 are the examples of the outputs of the bar graph and the line printer modes, respectively. Problems in constructing 13 C-NMR database One of the main bottlenecks in constructing the 13C NMR database is, of course, to make assignments of signals. In a sense, the assignment of signals is more difficult in 13C-NMR than in 1H-NMR spectra, since the coupling information is lost due to the low natural abundance of 13C nucleus (13C-13C coupling) and to the complete decoupling of the 1H resonance (13C-1H coupling). At the beginning of our building up the 13C-NMR database, we consulted the assignments in the usual textbooks13, 14, databooks6-8 and the literature, especi ally in review papers15, but a long time was not necessary to find out that the assignments are not always consistent with each other. Although the published papers are important sources to make assignments, we have made efforts to confirm the assignments experimentally. The most useful method is to apply pulse sequences to distinguish the carbon types. The DEPT or INEPT pulse sequences are famous, but we use the GASPE16 pulse sequence for the

4 464 ANALYTICAL SCIENCES OCTOBER 1988, VOL. 4 Fig. 1 An example of the output in the bar graph mode from the 13C-NMR database. database. In the GASPE method, all the peaks including the quaternary carbons can be observed by one measurement and the adjustment of the phase of the spectrum is easy to do. Sometimes we observe the undecoupled spectrum to observe the splittings due Fig. 2 An example of the output in the line printer mode (the character mode) for the same compound in Fig. 1. to the long-range 13C-1H spin coupling. For the assignment of some signals of a compound including fluorine or phosphorus atoms, we observe the spectra at different resonance frequencies, since the coupling constant between 19F or 31P and 13C spins is frequency independent while the 13C chemical shift is frequency dependent. The two-dimensional 13C-1H shift correla tion spectroscopy is also very helpful to make assign ments, but the measuring time is long, so that we observe the spectrum for one representative in a class of the similar compounds. In addition to the experimental procedures, we estimate the 13C chemical shift values for the spectrum which is waiting to be compiled, using the data in our database. The principles of the estimation are based on the substituent constants, but in our case we use the substituent constants for each compound derived by selecting the several key compounds in the homologues and estimating the substituent constants from the most similar compounds. A simpler method is provided in the access system named "NMRS"9, the details of which will be reported elsewhere.17 In this system the output data are the values of the chemical shifts estimated from the chemical structure. Sometimes we use this

5 ANALYTICAL SCIENCES OCTOBER 1988, VOL intelligent system for the assignment of newly observed spectra. Since the connections between the substruc tures and the chemical shifts are provided in the NMRS system, the systematic checks can be made by NMRS to avoid careless mistakes. The splitting of the 13C signal due to abundant magnetic nuclei other than 1H presents some problems. In particular, 19F and 31P give rise to definite splittings of the signals. One of the advantages of 13C-NMR spectra compared with the 1H-NMR spectra is the frequency-independence of the spectra if the scaling is made by ppm units. In other words, generally the frequency-dependence of the 13C-NMR spectra can be removed by expressing in ppm units. On the other hand, the splittings of the signals due to the spin-spin coupling are independent of the applied magnetic field. Thus, when the signal split by the spin-spin coupling is scaled in ppm units, the signal, in turn, will show an apparent "frequency-dependence". In order to remove the difficulty, the signals split by the spin-spin coupling are distinguished from the usual signals. It is desirable to give the true chemical shift values and the spin-spin coupling constants explicitly for the split peaks. This, however, clearly leads to a complex structure of the database system. We did not adopt this way, but all of the signals are simply expressed in ppm units in the database. Careful attention should be paid when the database system is used for the identification of the compounds by the spectral search method. The spectra which are supposed to include the signal splitting due to the other spins should be compared with each other by considering the observing resonance frequency. The satellite signals due to less abundant nuclei with spin 1 / 2, such as 29Si, 117Sn, and 119Sn, also present a problem. Actually we have no criterion for which satellite signals should be taken into the database, since they depend more or less on the S/ N ratio obtainable with a particular spectrometer. In our 13C-NMR database, when the satellite signals can be observed, the two pairs of the peak position and the intensity are added as the spectral data, together with the item IC in Record 5 in the MASTER FILE. The output of the spectral pattern includes the satellite peaks. As described in a previous paper9, SDBS is the database for pure organic compounds whose structural formula can be explicitly given. However, there are Fig. 3 An example of the output to a laser printer for the keto-enol tautomers.

6 466 ANALYTICAL SCIENCES OCTOBER 1988, VOL. 4 some problems inherent to NMR spectra. The tran sactions of the problems including keto-enol tautomers, and the anti-syn, the rotational, the optical or some other isomers are made in the 13C-NMR database as is the 1H-NMR database system.10 An example of the output for the keto-enol tautomers is shown in Fig. 3. Since the population of the tautomers or the isomers depends on the temperature, the concentration and other factors, the sample conditions are especially important for these systems. All the NMR information concerning the isomers can be obtained from the data in our database. At the end of June 1988, the number of the 13C-NMR data registered in our SDBS -NMR is about The authors express their hearty thanks to Ms. A. Yabe, Ms. M, Tochigi and Ms. H. Hatanaka for their efforts in spectral measurement, signal assignment and data input in constructing this 13C-NMR database. References 1. W. Bremser, Angew. Chem. Int Ed. Engl. 27, 247 (1988). 2. "Databank INKADATA", Fachinformationszentrum fur Energie, Physik, Mathematik GmbH, Eggenstein Leopoldshafen. 3. J. R. Rumble, Jr. and D. R. Lide, Jr., J. Chem. Inf. Comput. Sci., 25, 231 (1985). 4. "C-13 NMR SEARCH SYSTEM in Chemical Information System", Chemical Information System, Inc., Baltimore. 5. "Sadlter Spectra, 13 CNMR Search Libraries", Sadtler Research Laboratories, St. Philadelphia. 6. E. Breitmaier, G. Haas and W. Voelter, "Atlas of Carbon-13 NMR Data", Vol. 1-3, Heyden, London (1979). 7. B. E. Mann and B. F. Taylor, "13CNMR Data for Organometallic Compounds", Academic Press, London (1981). 8. F. Toda and T. Oshima, "Handbook of 13CNMR Spectra", Sankyo-Shuppan, Tokyo (1981). 9. O. Yamamoto, K. Someno, N. Wasada, J. Hiraishi, K. Hayamizu, K. Tanabe, T. Tamura and M. Yanagisawa, Anal. Sci., 4, 233 (1988). 10. O. Yamamoto, K. Hayamizu and M. Yanagisawa, Anal. Sci., 4, 347 (1988). 11. O. Yamamoto, K. Hayamizu and M. Yanagisawa, Anal. Sci. 4, 455 (1988). 12. O. Yamamoto and K. Hayamizu, Kagaku to Kogyo, 31, 152 (1978), in Japanese. 13. J. B. Stothers, "Carbon-13 NMR Spectroscopy", Academic Press, New York (1972). 14. E. Breitmaier and W. Voelter, "13C-NMR Spectroscopy", 2nd ed., Verlag Chemie, Weinheim (1978). 15. For example, J. W. Blunt and J. B. Stothers, Org. Magn. Reson., 9, 439 (1977), F. D. Gunstone, M. R. Pollard, C. M. Scrimgeour and H. S. Vedanayagam, Chem. Phys. Lipids, 18, 115 (1977), D. F. Ewing, Org. Magn. Reson., 12, 499 (1979) and P. E. Hansen, Org. Magn. Reson., 12, 109 (1979). 16. D. J. Cookson and B. E. Smith, Org. Magn. Reson., (1981). 17. O. Yamamoto, K. Hayamizu and M. Yanagisawa, to be published. (Received July 8, 1988) (Accepted August 1, 1988)