Anal. Chem. 1981, 53,2299-2304
2299
Spin-Lattice Relaxation Parameters in the Quantitative Determination of Condensed Aromatic Compounds by Carbon- 13 Nuclear Magnetic Resonance Spectrometry Terry ID. Alger,' Mark Solum, David M. Grant," Geoffrey D. Silcox, and Ronald J. Pugmlre2 Departnient of Chemistry and Department of Mining and Fuels Engineering, University of Utah, Salt Lake City, Utah 84 112
Carbon-13 spin-lattice relaxation times ( T , ) , and nuclear Overhauser enhancement (NOE) factors have been measured at 25.1 and 75.3 MHr and 32 O C for tetralln, hexahydropyrene, octahydrophenanthrene, and I-phenyloctane. For carbow13 nuclel wlth attached protons, the relaxatlon Is dornlnated by dipolar effects and T , and NOE values are relatlvely unaffected by fleld varlatlons. I n contrastyNOE and T1 values for nonprotonated carbon nuclei are profoundly affected by changes In field strengths. Chemical shift anisotropies of 200-250 ppm are adequate to account for the observed field dependency. Solutlons of the precedlng compounds and naphthalene, phenanthrene, pyrene, and xanthone were studied with 0.1 M chromium acetylacetonate added. By use of 0.1 M Cr3+ and gated-decoupling techniques, the Integrated and peak lntensltles of all carbon-13 siignals are proportional, on a quantltatlve basis, to the number of 13C nuclei In the compounds studied. These techniques permlt recycling times of under 2.5 s.
hydrophenanthrene, and 1-phenyloctane. Standard techniques are used to obtain T1and NOE measurements at 25.1 and 75.3 MHz and to separate the relaxation times into dipolar, chemical shift, and other components ( 2 ) . Evidence is again reported of sizable contributions from chemical shift anisotropy in these molecules for nonprotonated carbon-13 nuclei. Attention is also focused on the reliability of using 13C spectra as a means for quantitative measurements (1-3,12-16). The spin-lattice relaxation time, Tl, is measured at 75.3 MHz for the model compounds in a 0.1 M chromium acetylacetonate (Cr(AcAc)3)solution (1). This technique reduces the NOE factors of all 13Cnuclei uniformly toward unity and permits a shorter recycle time by shortening the 2'1 values. However, as in previous studies ( I ) this work verifies that gated-decoupling techniques are also necessary for optimizing 13C quantitative determinations. Finally, a test sample of tetralin, phenanthrene, and symmetrical hexahydropyrene was prepared by accurately weighing the components and was then compared with the NMR results to demonstrate the quantitative accuracy of the technique.
It has been demonstrated (1)that proton-decoupled carbon-13 NMR spectrometry can be used as a quantitative analytical tool for chemists only if proper recognition is given to the two basic problems which affect carbon-13 signal intensities, viz., (a) the great differences in relaxation times for different carbons observed in the same molecule, aind (b) the extensive variations in the nuclear Overhauser enhancement (NOE) parameters for different carbons in the same compound (2-6). Although workers have demonstrated that these problems can be removed in many cases by the addition of paramagnetic agents, by the use of gated-decoupling techniques, or by a combination of the two methods ( I - I I ) , the application to hydrocarbons in general and to aromatic hydrocarbons in particular requires a better knowledge of spin relaxation parameters and NOES for representative model compounds (1). A previous study by this laboratory examined such basic information at 25.1 and 75.3 MHz for naphthalene, phenanthrene, pyrene, acenaphthene, and xantlhone and delineated the relaxation mechanisms operating in such systemis ( 2 ) . The data demonstrated that all protonated carbon-13 nuclei in these molecules are relaxed predominately by the dipolar relaxation mechanisms at both field strengths. Of more fundamental interest, evidence was presented that the nonprotonated carbons are also relaxed by a chemical shift anisotropy mechanism which dominates the spin-lattice relaxation rate for the nonprotonated 13C nuclei at higher magnetic fields ( 2 ) . The previous study is now extended to other condensed aromatic compounds which also contain saturated ring systems or chains, i.e., tetralin, symmetrical hexahydropyrene, octa-
EXPERIMENTAL SECTION The Varian XL-100-15and Varian SC-300 NMR spectrometers used in this study have been previously described (2). Standard pulse methods were used to measure the spin-lattice relaxation times, T1(17), and the temperature was maintained at 32 "C. Measurements were made in 10-mm tubes on the Varian XL100-15 and in 5-mm tubes on the Varian SC-300. Pulse delays of at least 5T1 were used between successive pulse sequences for T1measurements ( 4 ) . Nuclear Overhauser enhancements were determined from the ratios of integrated intensities as previously described (2). For NOE measurements, pulse delays of at least 9Tl were used between successive pulse sequences ( 4 ) . Compounds were obtained from standard chemical sources, dissolved in reagent grade CDC13(deuteriochloroform),and then degassed by standard freezethaw techniques using liquid nitrogen as the coolant. On appropriate samples, special care was taken to ensure that the samples were free from paramagnetic ions, dissolved oxygen, and other materials which could produce competitive relaxation pathways. Samples containing C~(ACAC)~ were sealed to prevent loss of volatile components. A test mixture of phenanthrene, tetralin, and symmetrical hexahydropyrene was prepared by carefully weighing out quantities of each of the model compounds. The three compounds were then dissolved in a known amount of CDC13and C~(ACAC)~ was added to make the solution 0.1 M in Cr3+. To ensure objectivity, 13CNMR measurements were made by a different investigator than the one who prepared the test sample.
l Present
address: Academic Vice President, Southern 'UtahState College, Cedar City, UT 84720. Department of Mining and Fuels Engineering, University of Utah.
THEORETICAL SECTION The spin-lattice relaxation time, Tl, can be divided (18-22) into its components of dipolar ( TID),chemical shift anisotropy (TICsA), and all other residual (TIRes) terms as follows: l/Tl = ( l / T I D l/TICS* l/TIReS) (1)
+
+
or in terms of relaxation rate (Rl = l/Tl) (2) R1 = (RID RICSA RIReS)
0003-2700/81/0353-~290$01.25/0 0 1981 American Chemical Society
+
+
(2)
2300
0
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
Table I. TI and NOE Values as a Function of Field Strength 25.1 MHz position
NOE
Ti,s
C-1,4 C-2,3 C-5,8 C-6,7
4.6 i 0.4 4.2 i 0.4 9.5 f 0.5 7.7 f 0.4 81 i 4
C-4a,8a
C-1,3,6,8 C-2,7 C-4,5,9,10 C-1Ob.10c
1.7 f 0.2 1.6 f 0.1 4.0 f 0.1 115i 8 46i 3
C-1,8 C-2,7 C-3,6 c-4,5 C-9,lO C-4a,4b C-8aJOa
1.9 2 0.1 2.2 i 0.1 2.0 i 0.2 2.0 f 0.1 4.9 f 1.2 40i 4 48i 5
c-1
49 i 4 6.3 f 6.1 f 2.9 f 6.4 f 5.4 k 4.2 f 2.9 f 2.7 * 2.9 f 3.0 i 3.0 f
C-2,6 c-3,5 c-4 C-a: C-P
c-Y
C-6 C-6'
C-y '
C-P' C-Or'
75.1 MHz
0.1 0.1 0.1 0.8 0.4 0.2 0.1 0.1 0.1 0.2 0.2
TI, s
NOE
4.2 f 0.3 3.9 f 0.3 8.3 f 0.3 6.4 f 0.2 44 f 2
2.9 f 0.2 2.9 i 0.2 2.7 i. 0.2 2.7 f 0.2 1.7 i 0.1
1.5 i 0.2 1.6 f 0.1 3.1 i: 0.3 25f 1 19i 1
2.8 f 0.2 2.8i 0.2 2.8 i: 0.2 1.3 i 0.2 1.8 i 0.2
Octahydrophenanthrene 3.0 f 0.2 3.0 f 0.2 3.0 f 0.2 3.0 f 0.2 3.0 i 0.2 2.7 f 0.2 2.4 i 0.2
1.9 f 0.1 1 . 9 f 0.1 1.9 f 0.1 1.8 i 0.1 3.5 f 0.3 1912 25i 2
2.9 f 2.8 i 2.8 f 2.7 f 2.8 f 2.0 2 1.7 f
1-Phenyloctane 2.8 i 0.2 3.0 f 0.2 3.0 f 0.2 3.0 i 0.2 3.0 f 0.2 3.0 f 0.2 3.0 f 0.2 3.0 i 0.2 3.0 f 0.2 3.0 f 0.2 3.0 i 0.2 3.0 i 0.2
26i 3 6.0 0.1 5.7 i 0.1 3.0 i 0.1 4.8f 0.2 4.7 i 0.2 4.0 i 0.2 2.9 i 0.1 2.8 f 0.1 3.0 f 0.2 2.7 i 0.1 2.8 f 0.1
2.0 I0.2 2.8 i 0.2 2.9 f 0.2 2.9 f 0.2 2.8 f 0.2 3.0 f 0.2 2.9 f 0.2 3.0 5 0.2 2.8 f 0.2 2.9 i 0.2 2.9 i 0.2 3.0 r 0.2
Tetralin 3.0 f 3.0 f 3.0 i 3.0 f 2.3 f
0.2 0.2 0.2 0.2 0.2
Hexahydropyrene 2,9 i 0.2 2.9 i 0.2 2.9 f 0.2 2.1 f 0.2 2.6 i 0.2
For the cases in which Cr(AcAd3 is added, the relaxation rates can be expressed as (1) R,(Cr) = R1 Rle (3)
+
where R,(Cr) measures the effects of all relaxation mechanisms, including the paramagnetic ion, on the 13Cnuclei, while R1measures the effects of all other 13Crelaxation mechanisms, excluding the paragnetic ion. The term Rle is the difference between Rl(Cr) and R1 and measures the effect of only the paramagnetic ion upon the 13C nuclei. Separation of the R1values into the dipolar (21, 22) and chemical shift anisotropy components (23,24) is accomplished from both NOE measurements and field-dependent data as previously described (2,6). The TPSAvalues may also be used to estimate the magnitudes of the effective chemical shift anisotropy (CSA) which is present in these molecules (2,25).
RESULTS AND DISCUSSIONS Experimental T1 and NOE data are presented in Table I for four representative aromatic hydrocarbons which have attached saturated rings or side chains. As in our previous study (2),data clearly separate into two groupings depending upon whether or not carbon nuclei have attached protons. Assignments of carbon-13 signals in these compounds are based on work by Retcofsky and Friedel (26), Seshadri et al. (27), and Woolfenden (28). For 13C nuclei with one or more attached protons, Tl values are within a narrow range from 1 to 10 s and are basically unaffected by changes in field strength from 23.5 to 70.5 kG. The NOE measurements at 23.5 kG for protonated carbons are equal within experimental error to the theoretical maximum of 3.0 (29) and decrease only slightly (less than 0.3) when the field is increased. Such data confirm a dominant dipolar
+_
0.2 0.2 0.2 0.2 0.2 0.2 0.2
relaxation mechanism for the protonated carbon nuclei at both fields. On the other hand, 13C nuclei without attached protons appear to be relaxed by both dipolar and chemical shift anisotropy mechanisms. The Tl values for these nonprotonated carbons, which range from 40 to 115 s at 25.1 MHz, are reduced by factors of 2 to 3 by the %fold increase in field strength to a range of 19-44 s a t 75.3 MHz. Similarly, the NOE values for these 13C nuclei, which vary from 2.1 to 2.8 at 25.1 MHz, are also strongly affected and reduced to values of 1.3 to 2.0 at 75.3 MHz. Such data are very representative of that expected for carbons in which the CSA relaxation mechanism dominates. The dipolar relaxation mechanism is not efficient for these 13C atoms since they are separated by two or more bond lengths from their nearest hydogen neighbors. Hence, the Tl values for the carbons without attached protons are from 1 to 2 orders of magnitude larger than those for protonated carbon nuclei. In the nonprotonated carbons, the TIDand TICSA contributions are about equal and therefore both considerably larger than measured T1values (2). Data in Table I1 were generated from Tl and NOE values in Table I (2). The limits for each relaxation component were determined by using standard methods for estimating propagation of errors. Since there are only four independent pieces of information in Table I ( T , and NOE values at both 25.1 and 75.3 MHz), there is some interdependence among the six pieces of data generated in Table I1 for each unique carbon atom (2). As evident in Table 11, only dipolar and chemical shift anisotropy relaxation mechanisms need be considered in this study. In all cases, the propagated error limits for the RlRe8
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
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Table 11. Rate Values ( R s ,R,D, RICSAlRiRES)as a Function of Field Strength
position C.l,4 C.2,3 C-5,8 C-6,7 C-4a,8a
C-1,3,6,8 C-2,7 C-4,5,9,10 C-lOb,lOc C-3a,5a, 8a, 1Oa
C-1,8 C-2,7 C-3,6 c-4,5 C-9,lO C-4a,4b C-8a,lOa
c-1 C-2,6 c-3,5
c-,4 C-a
c-13 C-Y C-6
C-6' C-,y ' C-p' C-ff'
field strength, MHz
RICSA,s-'
RIRES,s-'
25.1 75.3 25.1 75.3 25.1 75.3 25.1 75.3 25.1 75.3
0.218 f 0.0119 0.238 i 0.017 0.238 f 0.023 0.256 0.020 0.105 2 0.006 0.120 f 0.004 0.130 f 0.007 0.156 t 0.005 0.0124 f 0.001 0.0227 * 0.001
Tetralin 0.219 i: 0.029 0.228 f 0.029 0.239 i 0.033 0.245 i 0.032 0.106 f 0.012 0.103 i 0.013 0.131 * 0.015 0.134 0.016 0.0081 f 0.001 0.0080 f 0.002
