sample NMR paper 1149

1H NMR Study of 4-Aminopyrimidine Coordinated to [SiW 11CoIIO39]6− Bull. Korean Chem. Soc. 1999, Vol. 20, No. 10 1149 1H NMR Study of 4-Aminopyrimidine Coordinated to the Paramagnetic Undecatung- stocobalto(II)silicate Anion: Rates of Internal Rotation of the Amine Group Byung Ahn Kim and Hyunsoo So* Department of Chemistry, Sogang University, Seoul 121-742, Korea Received July 21, 1999 1H NMR spectrum of a DMF-d 7 solution containing 4-aminopyrimidine and [SiW 11CoIIO39]6− (SiW 11Co) shows separate peaks from two linkage isomers, a and b, in which N(1) and N(3) of the pyrimidine ring are coordinated to SiW 11Co, respectively. The signal from the amine group in the isomer a exhibits temperature dependence that is characteristic of a two-site exchange problem. Rates of internal rotation of the amine groupwere determined by simulating the NMR spectra at 5-35 oC. The amine group of free 4-aminopyrimidine also shows temperature-dependent spectra at lower temperatures; rates of internal rotation at (-25)-25 oC were de- termined. The internal rotation of the amine group in the complex is much slower than that for free 4-aminopy-rimidine, indicating that π-character of the C-N bond increases on coordination to SiW 11Co. The amine group in the isomer b does not show such behavior. It is probable that hydrogen bonding between N-H and a bridging oxygen atom of SiW 11Co prevents it from rotating at low temperatures. Introduction Structure and internal motions of the amine groups in aro- matic compounds have been subjects of extensive studies,both experimental and theoretical. 1,2 The amine group hydrogens are out of the aromatic plane by an amount whichdepends on a balance between π-electron delocalization across the C-N bond and the tendency of the amine group toform sp 3 hybrid orbitals. Because of the pyramidalization the amine groups in aminobenzenes and aminopyrimidines canundergo inversion and internal rotation at relatively low tem-peratures. Calculated values of vibrational levels and barri-ers to amine group internal rotation and inversion for someaminobenzenes and aminopyrimidines were reported in tworecent papers. 1,2 Experimental values for the barriers in aniline were given, but no experimental data were presentedfor aminopyrimidines. While studying 1H NMR spectra of 4-aminopyrimidine coordinated to the paramagnetic polyoxometalate, [SiW 11- CoIIO39]6− (SiW11Co), in dimethylformamide-d 7 (DMF-d 7), we have observed that the amine group shows a tempera-ture-dependent NMR signal that is characteristic of a two-site exchange problem. Rates of internal rotation of theamine group were determined by simulating the NMR spec-tra. Subsequent study has shown that the NMR signal fromthe amine group in free 4-aminopyrimidine is also split intotwo peaks at low temperatures. Rates of internal rotation atlow temperatures were determined from this signal and com-pared with those from the complex. Experimental Section K 6[SiW11Co(H2O)O39]·nH2O was prepared according to the literature method3 and identified by its IR spectrum. 4- Aminopyrimidine was purchased from Aldrich. NMR sam-ples were prepared by dissolving the polyoxometalate and aligand in a 1:1 molar ratio in DMF-d 7. The concentration was 10 mM for each of the polyoxometalate and the ligand. 1H NMR spectra were obtained with a Varian Unity- INOVA (500 MHz) NMR spectrometer. NMR measure-ments were made at -55 oC to 40 oC. Tetramethylsilane (TMS) was used as an internal reference. Results and Discussion Free 4-Aminopyrimidine .T h e 1H NMR spectrum of 4- aminopyrimidine in DMF-d 7 at room temperature consists of the following resonances:1 2-H 8.32 (s), 6-H 8.02 (d), NH 2 6.80 (s), and 5-H 6.40 ppm (dd).4 The temperature depen- dence of the signal from the amine group is shown in Figure1. The signal splits into two peaks below 0 oC, and each peak exhibits spin-spin splitting ( J=2.1 Hz) due to the other Figure 1 . Temperature dependence of the 1H NMR signal from the amine group of free 4-aminopyrimidine in DMF-d 7. 1150 Bull. Korean Chem. Soc . 1999, Vol. 20, No. 10 Byung Ahn Kim and Hyunsoo So amine proton below -35 oC. It is noted that both chemical shifts and line shapes vary with temperature. The chemicalshifts of the amino group are shifted upfield by 0.7 ppm astemperature increases from -55 oC to 25 oC, whereas the chemical shifts of the ring protons are temperature-indepen-dent. Temperature-dependent line shapes were simulated using the modified Bloch equations. 5,6 The half line-width in the absence of exchange was taken as 3.0 Hz from the spectrumat -45 oC.7 Accurate transition probabilities could be deter- mined for the temperature range where the signal is split intotwo peaks. For the merged spectra at higher temperatures,separations of the two peaks in the absence of exchangemust be estimated. Since the separation between the twopeaks increases linearly with increasing temperature at -25 oC to -5 oC, the separations at higher temperatures estimated by extrapolation should be accurate. The resulting transitionprobabilities, P, between the two sites are listed in Table 1. According to the transition state theory the Arrhenius equation can be expressed as 8 When values of ln(Ph/kT) are plotted as a function of 1/T,they fall on a straight line at -25 oC to 0 oC (Figure 2). The activation parameters determined from this temperaturerange are ΔH* = 79.9 kJ mol −1 and ΔS* = 94.4 J K−1 mol−1. SiW11Co Complexes .T h e 1H NMR spectrum of a DMF- d7 solution containing 4-aminopyrimidine and SiW 11Co at room temperature is shown in Figure 3. The four resonancesat 8.28, 7.97, 6.80, and 6.41 ppm originate from the freeligand (see above). The remaining spectrum consists of twosets of resonances, indicating that two linkage isomers, a and b, are formed. All resonances were assigned by saturation transfer tech-nique. The stronger set of resonances is attributed to the iso- mer a. The resonances at 85.7, 31.6, and 14.2 ppm originate from 6-H, 2-H, and 5-H, respectively. The two resonances at-3.70 and -5.87 come from the amine group. The weaker res-onances at 46.2, 21.5, -12.5, and -22.3 ppm are assigned to2-H, 5-H, 6-H, and one of the amine protons of the isomer b, respectively. Another signal originating from the other amineP = kT h-------expΔS∗R⁄() exp-ΔH∗RT⁄()Table 1. Transition Probabilities (in Hz) for Internal Rotation of the Amine Group in 4-Aminopyrimidine Temp. (oC) Free SiW11Co Complex Lower line-width Upper line-width - 25 0007 - 20 0013 - 15 0035 - 10 0065 0 - 5 0133 00 0 0250 00 5 0440 0060 - 0 10 0- 0120 0060 0 15 1050 0200 0160 0 20 0- 0400 0350 0 25 1650 0770 0700 0 30 1360 1300 0 35 2360 2300 ΔH* (kJ mol−1)0079.9 085.1 0 95.8 ΔS* (JK−1 mol−1) 0094.4 095.8 130.5 Figure 2 . Least squares fits of ln(Ph/kT) as a function of 1/T for free 4-aminopyrimidine and its SiW 11Co complex. Two straight lines were obtained for two sets of transition probabilities for thelatter (see text). Symbols represent values determined by simulat-ing the spectra:
, free 4-aminopyrimidine; , the complex with the lower limit of the line-width; Δ, the complex with the upper limit of the line-width. Figure 3 .1H NMR spectrum of a DMF-d 7 solution containing 4- aminopyrimidine and [SiW 11Co(H2O)O39]6− in a 1 :1 molar ratio at 25 oC. 1H NMR Study of 4-Aminopyrimidine Coordinated to [SiW 11CoIIO39]6− Bull. Korean Chem. Soc. 1999, Vol. 20, No. 10 1151 proton is probably hidden under the residual peak of DMF or HDO.9 The amine group in the isomer a exhibits two separate res- onances at room temperature, which merge at 40 oC (Figure 4). Temperature-dependent line shapes of the signal from theamine group were simulated using the modified Bloch equa-tions. 5,6 The resulting transition probability between the two sites depends on the line-width in the absence of exchange,the upper and lower limits of which were estimated as fol-lows. The minimum line-width of the amine group observedat 5 oC was taken as the upper limit. This choice of line- width assumes that no internal rotation occurs below thistemperature. The line-widths at other temperatures wereestimated by assuming that the temperature-dependentincrement or decrement of the line-width is the same as thatof 5-H (not shown in Figure 4). The lower limit was deter-mined by decreasing the line-width gradually until the simu-lated spectrum deviates significantly from the measured spectrum at 5 oC. The line-widths at other temperatures were estimated as above. The lower limit was about 10 Hz smallerthan the upper limit. The resulting transition probabilities are listed in Table 1. It is noted that the transition probabilities for the lower limitof line-width are larger than the corresponding values for theupper limit by ca. 60 Hz. This agrees with the relation that the line-width increases by P/2 π for a slow exchange. Rates of internal rotation of the amine group in the complexdecrease remarkably in the complex: for example, the transi-tion probability at 5 oC is reduced from 440 to 60 Hz. This result suggests that the π-character of the C-N bond is increased on coordination of 4-aminopyrimidine to SiW 11Co. It seems that the π-dπ interaction between the pyrimidine ring and the cobalt atom strengthens the C-N π-bond. Activation parameters obtained by least squares fit of ln(Ph/kT) as a function of 1/T (Figure 2) are also listed inTable 1. It is noted that activation enthalpies for free 4-ami-nopyrimidine and the complex, especially the value for thelower limit of line-width, are very close, although their tran-sition probabilities are quite different. However, the physical meaning of the activation enthalpies for this system is notclear; see below. Barfield and Fagerness calculated inversion and internal rotation barriers (0.793 and 47.9 kJ mol −1, respectively) for the amine group in 4-aminopyrimidine.1 Since the inversion barrier is very small, our transition probabilities should berelated with the internal rotation. The question arises howthe activation enthalpy is related with the calculated energybarrier. A similar problem was encountered in interpretingthe experimental rates of intramolecular electron transfer ina mixed-valence Cu(I)-Cu(II) system. 10 It was shown that the activation energy (10.9 kJ mol−1) from the Arrhenius equation is much smaller than the energy barrier (32.7 kJmol −1) obtained using the PKS model. When there are sev- eral vibrational levels below the energy barrier, the experi-mental transition probability is an average for these levels.As the vibrational quantum number increases, the transitionprobability increases rapidly although the populationdecreases. Therefore, contribution of excited vibrational lev-els to the average transition probability may be important.Since the Arrhenius equation can be derived from a modelhaving a single energy level below the energy barrier, thebarrier for a system having several vibrational levels must bemuch higher than the activation energy to produce the exper-imental transition probabilities. The same argument may be applied to the internal rotation of the amine group. Since there are several vibrational levelsbelow the energy barrier, 1 the barrier should be much larger than the activation energy.11 The activation energies deter- mined here are already much larger than the calculatedenergy barrier (83 vs. 47.9 kJ mol −1). Therefore, the real energy barrier should be much higher than the calculatedvalue. Theoretical work is needed to determine the energybarrier that agrees with the experimental transition probabil-ities. Temperature dependence that is characteristic of a two-site exchange problem is not observed for the amine group in the isomer b. As was noted before, 12 2-methylpyridine does not bind to SiW 11Co. If such complex were formed, one of the methyl protons should be within 2.3 Å from a bridging oxy-gen atom on SiW 11Co. This distance is much shorter than 2.6 Å suggested by Pauling for O…H van der Waals contact dis-tance. 13 On the other hand, 2-aminopyridine, 2-aminopyrim- idine, and 4-aminopyrimidine { via N(3)} bind readily to SiW11Co. When these ligands coordinate to SiW 11Co, the amine group may form a hydrogen bond with a bridgingoxygen atom on SiW 11Co or it may be rotated to form a high-energy conformation in order to avoid the O…H con-tact. We have calculated the distances between the aminegroup and a bridging oxygen atom in the SiW 11Co complex, using the structural data for 2-aminopyridine1 and a Co…N distance of 2.201 Å.14 The shortest distance between the amine nitrogen atom and a bridging oxygen atom onSiW 11Co is 2.8 Å and one of the protons is displaced 9o from the N…O vector. The structure being favorable for hydrogenbonding, it is quite likely that hydrogen bonding plays an Figure 4 . Temperature dependence of the 1H NMR signal fro m the amine group of 4-aminopyrimidine coordinated to SiW 11Co in DMF-d 7. The signal at the high field is due to 6-H of the isomer b. 1152 Bull. Korean Chem. Soc . 1999, Vol. 20, No. 10 Byung Ahn Kim and Hyunsoo So important role in complex formation between SiW 11Co and these ligands. In summary, we have determined the transition probabili- ties for internal rotation of the amine group in both free 4-aminopyrimidine and its SiW 11Co complex. More work is needed to interpret the experimental transition probabilitiesin terms of a theoretical model. Acknowledgment . Financial support of the Korea Minis- try of Education (1998-015-D00155) is gratefully acknowl-edged. References 1. Barfield, M.; Fagerness, P. J. Am. Chem. Soc. 1997, 119, 8699. 2. McCarthy, W. J.; Lapinski, L.; Nowak, M. J.; Adamowicz, L. J. Chem. Phys . 1998, 24, 10116. 3. Simmons, V. E. Ph.D. Thesis ; Boston University: 1963. 4. s and d represent singlet and doublet, respectively. 5. Gutowsky, H. S.; McCall, D. W.; Slichter, C. P. J. Chem. Phys. 1953, 21, 279. 6. Gutowsky, H. S.; Saika, A. J. Chem. Phys . 1953, 21, 1688. 7. Since this line-width contains the spin-spin splitting due to the other amine proton, the line-width of a single reso- nance (1.7 Hz) may be used especially for the spectra athigh temperatures. But the transition probabilities for the high temperature spectra are not significantly affected by such small change in line-widths. 8. Eyring, H. J. Chem. Phys. 1935, 3, 107. 9. The other signal originating from the amine group is seen at -3.08 ppm for 2-aminopyrimidine coordinated to SiW 11Co in DMSO-d 6. This signal disappears, if a small amount of D2O is added to the solution. 10. So, H. Bull. Korean Chem. Soc . 1992, 13, 385. 11. The activation enthalpy and the activation energy are essentially the same for internal rotation of the amine group in solution. 12. Ko, M.; Rhyu, K. I.; So, H. Bull. Korean Chem. Soc . 1993, 14, 500 13. Pauling, L. The Nature of Chemical Bonds ; Cornell Uni- versity Press: Ithaca, New York, 1960. 14. Lumme, P. O. Polyhedron 1995, 14, 1553.