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13C spin-lattice relaxation in benzophenone and its isotopomers
Joumd of Molecular Structure, 293 (1993) 163-166 Elsevier Science Publishers B.V., Amsterdam
163
13C Spin-Lattice Relaxation’ in Benzophenone and Its lsotopomers Predrag VujaniC and Zlatko MeiC Ruder BogkoviC Institute, BijeniEka 54 P. 0. Box 1016, Zagreb, CROATIA
Abstract In order to analyze deuterium effects on 13C spin-lattice relaxation time (T,) in benzophenone (BPN), fiie BPN isotopomers were prepared and their 13C NMR spectra recorded. In BPN all the phenyl carbon atoms, except the quarternary one, are predominantly relaxed by the dipole-dipole mechanism due to C-H dipole-dipole relaxation. Other contributions to the total spin-lattice relaxation times of individual carbon atoms are discussed as well. Deuteration affects also the magnitude of spin-lattice relaxation times. An increase in Td by cca. 550% at the u/the (C,) and para (C4) carbon atoms in perdeuterated BPN has been observed.
1. INTRODUCTION 13C spin-lattice relaxation time measurements have shown that these relaxation studies can give valuable information about molecular structure and dynamics in liquids’, and are also useful for spectral assignment in complex NMR spectra2. Spin-lattice relaxation as a process of energy exchange between individual nuclear spins and their surroundings, called “the lattice”, requires efficient mechanism for its energy transfer. For a 13C nucleus four possible relaxation mechanisms occur: 1 =-=TDD T&r& 1
1 1
I +-+-+-& TCsA 1
1 TSR 1
deuterium, other .mechanism may occur.3 Deuteration may drastically affect spin-lattice relaxation time due to a smaller magnetic moment of deuterium and is useful for assignment of quarternary and nonprotonated carbons.4 An increase of 300% in T,DD has been observed upon deuteration5 In this paper we would like to present how the deuteration affects relaxation of benzophenone and its isotopomers, both in duration and kind of relaxing mechanism. In order to do it we have studied the following molecules:
(1) 1
where R,ObS.is the observed relaxation rate, whereas Tat terms refer to dipole-dipole (DD), chemical shift anisotropy (GA), spin-rotation (SR) and scalar coupling (SC) contributions, respectively, to the total relaxation time, T,ObS.. Many investigations have shown a predominant dipole-dipole relaxation for carbon atoms in large molecules and for protonated carbon atoms in small molecules. However, for unsaturated carbons having not directly attached protons, and for those bearing a OOZZ-2860/93/$06.00 1993 Elsevier Science Publishers B.V.
0
m
‘,I
I
D
5
D
5
164
2. EXPERIMENTAL 13C NMR Spectra. 13C NMR spectra were recorded on a Varian Gemini 300 and Varian Unity 400 spectrometers, operating at 75 and 100 MHz, respectively. 13C Spin-Lattice Relaxation. 13C spin-lattice relaxation experiments were conducted at 75 and 100 MHz under condition of complete ‘H decoupling at 20°C. The spin-lattice relaxation times were measured by the inversion recovery (IRFT) and fast inversion recovery (FIRFT) pulse methods7p8, and were analyzed by the three parameter fii procedureg. The pulse duration for a 180” flip angle was 30.6 w. For the IRFT method the delay (D) in pulse sequence (180”-z-90”-D)n was set to 5T,, and for FIRFT to 1.5T,. z values were varied from experiment to experimentab. The rms error in the calculated T, values was better then f 3Ok for IRFT and f 8% for FIRFT method, and the reproducibility was f 5%. NOE Measurements. Nuclear Overhauser enhancement (NOE) factors were determined at 75 MHz comparing individual integrated
peak intensities in ‘H decoupled spectra with and without the Overhauser enhancements. A delay of approximately 10 T, was used before 90” pulses. Samples. All of the BPN isotopomera except BPN-d,, which was a commercial product (Alfa Products, 99% D), were synthesized in our laboratory6, and were purified properly by crystallization and column chromatography. 0.20.5 mmol deutero-chloroform solutions in 5 mm NMR tubes were used without degassing.
Fourier transformation of the FlDs arising from the 180“~~-90~ pulse sequence (varying the t values) and exponential magnetization curve8 of BPN-DIO are shown in Figure 1. 13C spin-lattice relaxation times (T,) for BPN and its isotopomers are given in Table 1. Molecular tumbling which strongly depend8 on the shape of a molecule affects spin-lattice relaxation timeld. Therefore, in all observed BPN isotopomers a differences in T, between the par- and both the mete and orthe carbon atoms indicate phenyl group internal rotation.
Table 1 1% spin-lattice relaxation times T, (s) of BPN and its isotopomers T, C-atom
CO
Cl
Cl,
C2
Cz
C3
C3
C4
C4
31 .oo 31.62 39.46 41.99
25.87 28.12 35.59 38.37
25.87 25.80 39.81 38.37
3.45 10.88 23.15 22.62
3.45 3.10 3.60 22.62
3.41 3.01 24.69 21.93
3.41 3.03 3.56 21.93
2.57 2.32 15.19 16.80
2.57 2.32 2.65 16.80
molecule BPN BPN9-D BPN-D5 BPN-DIO
Estimated error: T, (a) f l-8%.
16.5
LO.0
20.0 .
7.5
I 1.86
I
0
-'I V
' OA7
I '11
' 0.12
I
-20.0
I 'W
I 1 'fl -40.0 160
Iso
1LO
120
100
OPPM
Figure 1. Determination of 13C spin-lattice relaxation times of BPN-DI 0 by the IRFT method. Nuclear Overhauser effects measured for all carbons in BPN (Table 2) indicate the predominant dipole-dipole relaxation of all protonated carbons inthe phenyl ring due to an efficient C-H dipole-dipole relaxation. NOE enhancement factors (NOE*) were calculated according to the formula NOE*.
= IINOE - I
(2)
where I is the intensity of the signal. The T,DD contribution to the total relaxation rate of all carbon atoms could then be extracted following the expression
TDD = 1
TobS* *1.98 1
(3)
NOEobS*
Dissotved oxygen does not significantly affect the relaxation of protonated carbon nuclei (Table 2). However, for unprotonated nuclei, such as quarternary and carbonyl carbons, degassing is essential because the relaxation become important.tip10 by oxygen can Therefore, the quantitative studies of other possible contributing mechanisms require degassed samples.
Table 2 Noe factors (NOEObS)and dipole-dipole relaxation times TIDD (s) for BPN atom
co cl,*
C, C, CU undegassed, CDC13
NOEObG 0.61 0.71 1.96 1.90 1.83
T,Ok
T,DD
31 .oo 25.87 3.45 3.41 2.57
102.30 72.14 3.48 3.55 2.78
166
It is known that the CSA mechanism depends on the square of the magnetic fiildll. Spiess et al.3d have shown that CSA gives significant contributions to the relaxation rate of ring carbons of the deuterated toluene at higher fields and tow temperatures. Our preliminary measurements at 100 MHz indicate an increasing of a CSA contribution to the total relaxation time, but accurate data are not yet available.
isotopic substitution of hydrogen with deuterium drastically increases spin-lattice relaxation times of the observed carbon atoms. An increase by cca. 300% has been found in the literatures. Our investigation shows an increase by cca. 550% in T, of o&~(C,) and para(CS carbons in perdeuterated BPN (Table 3). More detailed study is necessary to resolve conformation of BPN in liquid and possible changes upon deuteration.
Table 3 13C spin-lattice relaxation times T, (s) of BPN and BPN-II10 and the difference A(Oh)in T,. C-atom molecule BPN BPN-DlO
Tl T, A
co
C11'
21 .oo 41.99 35.45
25.87 38.37 48.32
Acknowledgements. This work was supported by the Ministry of Science of the Republic of Croatia through the Project l-07-139. We thank Dr. L. Radics for Unity 400 spectra.
REFERENCES 1.
2.
3.
a) G.C.Levy, R.L.Lichter and G.L.Nelson, “Carbon-l 3 Nuclear Magnetic Resonance Spectroscopy”, Wiley Chapter 8, Interscience, York New 1980. b) G.C.Levy, J.D.Cargioly and F.A.L.Anet, J.Am.Chem.Soc. 95 (1973) 1527. J.R.Lyerla,Jr., DMGrant and c) R.K.Harrii, J.Phys.Chem. 75 (1971) 585. d) P.Dais, Magn.Reson.Chem. 25 (1987) 141. G.C.Levy, “Topics in Carbon-13 NMR Spectroscopy”, Wiley Interscience, New York, Vol. 2. Chapter 6, 1976. M.J.Shapiro A.D.Kahle, and a) Org.Magn.Resonance, 12 (1979) 4. A.Olivson E. Lippmaa, and b) Chem.Phys.Let., 11 (1971) 241. c) G.C.Levy, D.M.White and F.A.L.Anet, J.Magn.Reson., 6 (1972) 453.
C22
3.45 22.62 555.65
4
c,
CM
3.41 21.93 543.11
2.51 16.80 553.70
H.W.Spiess, D.Schweitzer and U.Haeberten, J.Magn.Reson., 9 (1973) 444. 4. G.C.Levy and U.Edlund, J.Am.Chem.Soc., 50 (1975) 31. 5. R.E.Echols and G.C.Levy, J.Org.Chem., 39 (1974) 1321. 6. To be published elsewhere. 7. D.Canet, G.C.Levy and I.R.Peat, J.Magn.Reson., 18 (1975) 199. 8. a) H.Hanssum, J.Magn.Reson., 45 (1981) 461. b) J.M.Bernassau and F.Hyafil, J.Magn. Reson. 40 (1980) 245. 9. a) J. Kowalevski, G.C.Levy, L.F.Jonson and L.Palmer, J.Magn.Reson., 26 (1977) 533. b) M.Sass and D. Ziessov, J.Magn.Reson., 25 (1977) 263. c) T.P. Pitner and J.F.Whidby, Anal.Chem., 51 (1979) 2203. 10. J.Jaeckte U.Heeberlen and D.Schweitzer, J.Magn.Reson., 4 (1971) 198. 11. A.Abragam, “The Principles of Nuclear Magnetism”, Oxford University Press, Oxford 1961.
163
13C Spin-Lattice Relaxation’ in Benzophenone and Its lsotopomers Predrag VujaniC and Zlatko MeiC Ruder BogkoviC Institute, BijeniEka 54 P. 0. Box 1016, Zagreb, CROATIA
Abstract In order to analyze deuterium effects on 13C spin-lattice relaxation time (T,) in benzophenone (BPN), fiie BPN isotopomers were prepared and their 13C NMR spectra recorded. In BPN all the phenyl carbon atoms, except the quarternary one, are predominantly relaxed by the dipole-dipole mechanism due to C-H dipole-dipole relaxation. Other contributions to the total spin-lattice relaxation times of individual carbon atoms are discussed as well. Deuteration affects also the magnitude of spin-lattice relaxation times. An increase in Td by cca. 550% at the u/the (C,) and para (C4) carbon atoms in perdeuterated BPN has been observed.
1. INTRODUCTION 13C spin-lattice relaxation time measurements have shown that these relaxation studies can give valuable information about molecular structure and dynamics in liquids’, and are also useful for spectral assignment in complex NMR spectra2. Spin-lattice relaxation as a process of energy exchange between individual nuclear spins and their surroundings, called “the lattice”, requires efficient mechanism for its energy transfer. For a 13C nucleus four possible relaxation mechanisms occur: 1 =-=TDD T&r& 1
1 1
I +-+-+-& TCsA 1
1 TSR 1
deuterium, other .mechanism may occur.3 Deuteration may drastically affect spin-lattice relaxation time due to a smaller magnetic moment of deuterium and is useful for assignment of quarternary and nonprotonated carbons.4 An increase of 300% in T,DD has been observed upon deuteration5 In this paper we would like to present how the deuteration affects relaxation of benzophenone and its isotopomers, both in duration and kind of relaxing mechanism. In order to do it we have studied the following molecules:
(1) 1
where R,ObS.is the observed relaxation rate, whereas Tat terms refer to dipole-dipole (DD), chemical shift anisotropy (GA), spin-rotation (SR) and scalar coupling (SC) contributions, respectively, to the total relaxation time, T,ObS.. Many investigations have shown a predominant dipole-dipole relaxation for carbon atoms in large molecules and for protonated carbon atoms in small molecules. However, for unsaturated carbons having not directly attached protons, and for those bearing a OOZZ-2860/93/$06.00 1993 Elsevier Science Publishers B.V.
0
m
‘,I
I
D
5
D
5
164
2. EXPERIMENTAL 13C NMR Spectra. 13C NMR spectra were recorded on a Varian Gemini 300 and Varian Unity 400 spectrometers, operating at 75 and 100 MHz, respectively. 13C Spin-Lattice Relaxation. 13C spin-lattice relaxation experiments were conducted at 75 and 100 MHz under condition of complete ‘H decoupling at 20°C. The spin-lattice relaxation times were measured by the inversion recovery (IRFT) and fast inversion recovery (FIRFT) pulse methods7p8, and were analyzed by the three parameter fii procedureg. The pulse duration for a 180” flip angle was 30.6 w. For the IRFT method the delay (D) in pulse sequence (180”-z-90”-D)n was set to 5T,, and for FIRFT to 1.5T,. z values were varied from experiment to experimentab. The rms error in the calculated T, values was better then f 3Ok for IRFT and f 8% for FIRFT method, and the reproducibility was f 5%. NOE Measurements. Nuclear Overhauser enhancement (NOE) factors were determined at 75 MHz comparing individual integrated
peak intensities in ‘H decoupled spectra with and without the Overhauser enhancements. A delay of approximately 10 T, was used before 90” pulses. Samples. All of the BPN isotopomera except BPN-d,, which was a commercial product (Alfa Products, 99% D), were synthesized in our laboratory6, and were purified properly by crystallization and column chromatography. 0.20.5 mmol deutero-chloroform solutions in 5 mm NMR tubes were used without degassing.
Fourier transformation of the FlDs arising from the 180“~~-90~ pulse sequence (varying the t values) and exponential magnetization curve8 of BPN-DIO are shown in Figure 1. 13C spin-lattice relaxation times (T,) for BPN and its isotopomers are given in Table 1. Molecular tumbling which strongly depend8 on the shape of a molecule affects spin-lattice relaxation timeld. Therefore, in all observed BPN isotopomers a differences in T, between the par- and both the mete and orthe carbon atoms indicate phenyl group internal rotation.
Table 1 1% spin-lattice relaxation times T, (s) of BPN and its isotopomers T, C-atom
CO
Cl
Cl,
C2
Cz
C3
C3
C4
C4
31 .oo 31.62 39.46 41.99
25.87 28.12 35.59 38.37
25.87 25.80 39.81 38.37
3.45 10.88 23.15 22.62
3.45 3.10 3.60 22.62
3.41 3.01 24.69 21.93
3.41 3.03 3.56 21.93
2.57 2.32 15.19 16.80
2.57 2.32 2.65 16.80
molecule BPN BPN9-D BPN-D5 BPN-DIO
Estimated error: T, (a) f l-8%.
16.5
LO.0
20.0 .
7.5
I 1.86
I
0
-'I V
' OA7
I '11
' 0.12
I
-20.0
I 'W
I 1 'fl -40.0 160
Iso
1LO
120
100
OPPM
Figure 1. Determination of 13C spin-lattice relaxation times of BPN-DI 0 by the IRFT method. Nuclear Overhauser effects measured for all carbons in BPN (Table 2) indicate the predominant dipole-dipole relaxation of all protonated carbons inthe phenyl ring due to an efficient C-H dipole-dipole relaxation. NOE enhancement factors (NOE*) were calculated according to the formula NOE*.
= IINOE - I
(2)
where I is the intensity of the signal. The T,DD contribution to the total relaxation rate of all carbon atoms could then be extracted following the expression
TDD = 1
TobS* *1.98 1
(3)
NOEobS*
Dissotved oxygen does not significantly affect the relaxation of protonated carbon nuclei (Table 2). However, for unprotonated nuclei, such as quarternary and carbonyl carbons, degassing is essential because the relaxation become important.tip10 by oxygen can Therefore, the quantitative studies of other possible contributing mechanisms require degassed samples.
Table 2 Noe factors (NOEObS)and dipole-dipole relaxation times TIDD (s) for BPN atom
co cl,*
C, C, CU undegassed, CDC13
NOEObG 0.61 0.71 1.96 1.90 1.83
T,Ok
T,DD
31 .oo 25.87 3.45 3.41 2.57
102.30 72.14 3.48 3.55 2.78
166
It is known that the CSA mechanism depends on the square of the magnetic fiildll. Spiess et al.3d have shown that CSA gives significant contributions to the relaxation rate of ring carbons of the deuterated toluene at higher fields and tow temperatures. Our preliminary measurements at 100 MHz indicate an increasing of a CSA contribution to the total relaxation time, but accurate data are not yet available.
isotopic substitution of hydrogen with deuterium drastically increases spin-lattice relaxation times of the observed carbon atoms. An increase by cca. 300% has been found in the literatures. Our investigation shows an increase by cca. 550% in T, of o&~(C,) and para(CS carbons in perdeuterated BPN (Table 3). More detailed study is necessary to resolve conformation of BPN in liquid and possible changes upon deuteration.
Table 3 13C spin-lattice relaxation times T, (s) of BPN and BPN-II10 and the difference A(Oh)in T,. C-atom molecule BPN BPN-DlO
Tl T, A
co
C11'
21 .oo 41.99 35.45
25.87 38.37 48.32
Acknowledgements. This work was supported by the Ministry of Science of the Republic of Croatia through the Project l-07-139. We thank Dr. L. Radics for Unity 400 spectra.
REFERENCES 1.
2.
3.
a) G.C.Levy, R.L.Lichter and G.L.Nelson, “Carbon-l 3 Nuclear Magnetic Resonance Spectroscopy”, Wiley Chapter 8, Interscience, York New 1980. b) G.C.Levy, J.D.Cargioly and F.A.L.Anet, J.Am.Chem.Soc. 95 (1973) 1527. J.R.Lyerla,Jr., DMGrant and c) R.K.Harrii, J.Phys.Chem. 75 (1971) 585. d) P.Dais, Magn.Reson.Chem. 25 (1987) 141. G.C.Levy, “Topics in Carbon-13 NMR Spectroscopy”, Wiley Interscience, New York, Vol. 2. Chapter 6, 1976. M.J.Shapiro A.D.Kahle, and a) Org.Magn.Resonance, 12 (1979) 4. A.Olivson E. Lippmaa, and b) Chem.Phys.Let., 11 (1971) 241. c) G.C.Levy, D.M.White and F.A.L.Anet, J.Magn.Reson., 6 (1972) 453.
C22
3.45 22.62 555.65
4
c,
CM
3.41 21.93 543.11
2.51 16.80 553.70
H.W.Spiess, D.Schweitzer and U.Haeberten, J.Magn.Reson., 9 (1973) 444. 4. G.C.Levy and U.Edlund, J.Am.Chem.Soc., 50 (1975) 31. 5. R.E.Echols and G.C.Levy, J.Org.Chem., 39 (1974) 1321. 6. To be published elsewhere. 7. D.Canet, G.C.Levy and I.R.Peat, J.Magn.Reson., 18 (1975) 199. 8. a) H.Hanssum, J.Magn.Reson., 45 (1981) 461. b) J.M.Bernassau and F.Hyafil, J.Magn. Reson. 40 (1980) 245. 9. a) J. Kowalevski, G.C.Levy, L.F.Jonson and L.Palmer, J.Magn.Reson., 26 (1977) 533. b) M.Sass and D. Ziessov, J.Magn.Reson., 25 (1977) 263. c) T.P. Pitner and J.F.Whidby, Anal.Chem., 51 (1979) 2203. 10. J.Jaeckte U.Heeberlen and D.Schweitzer, J.Magn.Reson., 4 (1971) 198. 11. A.Abragam, “The Principles of Nuclear Magnetism”, Oxford University Press, Oxford 1961.