J. Phys. Chem. 1980, 84, 203-206
203
Carbon-I 3 NMR Spin-Lattice Relaxation Times and NOE Coefficients for Neutral and Protonated Methyl-Substituted Pyridines Harry P. Hopkins, Jr.,' and Syed Zakir Ali Department of Chemistry, Georgia State University, Atlanta, Georgia 30303 (Received June 28, 1979)
Carbon-13 NMR experiments have been performed on 2-, 3-, and 4methylpyridines in the neat phase, !in a 50 wt 7% methanol-water phase, and in a 1.1 M HC1 solution of a 50 wt % methanol-water solvent. These have been analyzed to provide the chemical shifts for all the carbon atoms, the corresponding T , values, and the qc-H values for the methyl carbons. For all of the carbons with hydrogens attached, the T , values decrease by factors of 2-3 when going from the neat phase to the methanol-water solvent. This is probably due to the hydrogen bonding of the pyridine nitrogen to the solvent molecules, which could cause an increase in the molecular reorientation time. Upon protonation, however, the T1 values change very little from those for the neutral species, indicating that the molecular motion responsible for the relaxation of the spins of the carbon atoms are nearly the same in the cation and the neutral molecule. Since the cation is expected to be more tightly bound to the solvent molecules than the neutral species, either by hydrogen bonding or electrostriction, one would predict the reorientation times to increase and the T1 values to decrease rather than remain the same as observed. The barriers to internal rotation for the methyl groups are estimated from the T1and ~ C - Hvalues by the method of Zen and Ellis. These are found to be close to zero for both the neutral and protonated species.
In the analysis of the thermodynamic data for the ionization of organic acids,lS2 the assumption is tacitly made that the barriers to internal rotation remain the same upon ionization. The available thermodynamic data for the ionization of methyl-substituted pyridines, both in the gas3 and solution2 phase, are consistent with this assumption. However, for the solution phase, the analysis of the thermodynamic data for the ionization process for substituted tert-butyl pyridine^^ and phenols5 suggest that when the tert-butyl group is simultaneously a t the 2 and 6 positions, this assumption may not be valid. In view of these results it seemed appropriate to initiate a spectroscopic investigation to provide molecular parameters that can be related to the barriers to reorientation of the methyl moiety for several neutral and ionic species. Spectroscopic studies, such as microwave and far-infrared measurements, which can be directly related to the energy level pattern for the motion of the methyl group cannot be readily performed on these systems in the solution phase where the majority of the thermodynamic data has been obtained. There are, however, current theorie@ that relate the carbon-13 NMR spin lattice relaxation time, T1,and the nuclear Overhauser enhancement factor, vC-H, for the methyl carbon to the barrier to internal rotation of the methyl group. Consequently, we initiated studies in carbon-13 NMR spectroscopy to provide the T1 and VC-H values for the carbon atoms in the 2-, 3-, and 4-methylsubstituted pyridines. The barriers to rotation for the methyl groupls are evaluated from these parameters by means of the impirical relationship of Zens and Ellis.6 Additional information regarding the overall molecular tumbling rate has also been inferred from the T1 values for the ring carbons. Since these studies were performed in three different solution environments, i.e., the pure liquid phase (neat), 50 wt % methanol-water, and 1.12 M HCl in 50 wt % methanol-water solutions, it is possible to discuss the effects of the environment and charge of the species on the carbon-13 chemical shifts and T , values. Analysis of this data yields definite conclusions about (1) the solvation of the solute, (2) the molecular motion of the solute in its solvent cage, and (3) the magnitude of the barrier to internal rotation for the methyl group in these systems. 0022-3654/80/2084-0203$0 1.OO/O
Experimental Section All carbon-13 NMR experiments were performed on either a JEOL FX60 or FXGOQ Fourier transform instrument operating in the single coil mode and equipped with a variable temperature controller (NM-5471) which regulates the temperature to h0.3 "C. The NMR spectrometer is equipped with a Texas Instruments 980b computer with 24k of memory that operates the instrument and stores the data. All programs were used as received from JEOL. With this instrument the resonance frequency for carbon-13 is 15.04 MHz plus or minus an offset of up to 50 Hz. For the T I measurements the offset was always 37.9 MHz, whereas in the NOE experiments the offset was chosen to give an irradiation frequency a t the middle of the carbon resonance under investigation. Broad band double irradiation was used for proton decoupling except for the NOE measurements where the decoupler frequency was centered a t the resonance frequency of the hydrogen coupled to the carbon of interest. The inversion-recovery techniqueg (18Oo-~-9O0-~)was adopted to determine the spin-lattice relaxation tirnes. This sequence consists of a 180" pulse to invert the spins, followed by a variable amount of time, T , in which the spin system starts back toward equilibrium, a 90" pulse applied at the end of this time for the determination of the extent of magnetization remaining, and finally a period of time, T ( T > 5T1), in which one assumes that the spins return completely to equilibrium before experiencing another pulse sequence. The width of the pulse necessary to produce a 90" angle for the spins was determined by plotting the maximum height of a free induction decay (FID) caused by a single pulse against the pulse width in microseconds (ys). A mixture of 80% benzene-20% ]perdeuterated acetone was used in this determination, giving a maximum of 16 ws on the FX60 and a t 10 ws on the FX6OQ. Similar experiments on neat solutions of the methyl-substituted pyridines gave identical results. The T1 values were determined by fitting the intensities of the peaks to the function S ( t ) = S[l - 2 exp(-t/T1)] by a linear least-squares procedure. In each determination 15-20 values were used in the analysis to give T , vallues 0 1980 American
Chemical Society
204
Hopkins and Ali
The Journal of Physical Chemistry, Vol. 84, No. 2, 1980
TABLE I: Carbon-13 Chemical Shifts (ppm) Relative to Me,Si and the T , Values (s) for the Ring Carbons of the 2-, 3-, and 4-Methvl-Substituted Pvridines and Pvridinium Ions
c* soh neat
a
pyridine 6 2-Me 158.9 3-Me 152.5 4-Me 152.1
c, T, (63)' 18.3 13.6
C;
c4
123.4 132.9 126.8
TI 17.5 (56) 14.9
6
136.3 136.0 148.8
TI 17.8 16.5 (63)
6
C6
121.0 123.1 126.8
TI 16.5 13.8 14.9
149.7 147.0 152.1
6
6
TI 17.3 12.3 13.6
1.1Min 50 wt % MeOH-H,O
2-Me 3-Me 4-Me
158.5 149.5 149.6
(52)' 6.1 6.4
124.6 134.8 126.3
5.6 (27) 6.0
138.1 138.3 149.9
4.6 4.2 (41)
122.0 124.4 126.3
6.4 7.9 6.0
148.7 146.4 149.6
6.2 6.1 6.4
1.12 M in 50 wt % MeOH-H,O with 1.12 M HCI
2-Me 3-Me 4-Me
154.8 141.6 141.3
(43) 5.7 7.7
129.1 139.9 129.0
6.1 (54) 7.8
147.6 148.5 162.3
4.9 7.2 (60)
125.5 127.8 129.0
5.1 10.6 7.8
141.5 139.3 141.3
5.0 7.7 7.7
Numbers in parentheses are uncertain due to
T
< 57' in the relaxation experiment.
with standard deviations of 0.1 s or less. However, T1 values determined in separate experiments differed by as much as 0.3 s. The samples of 2-methylpyridine (purity 99%) and 4methylpyridine (purity of 98%) supplied by the Aldrich Chemical Co. were used as received. The sample of 4w t h y l p y d i n e received from Aldrich was vacuum distilled at 6 kmand the middle fraction collected for the carbon-13 NMR experiments. Tzle 50 wt % methanol-water solutions were prepared by mixing weighed portions of water and methanol. Solutions containing HCl were prepared in a similar fashion from aqueous HCl, methanol, and water so as to have a HCl solution of the necessary molarity in the 50 wt % methanol-water solvent. The pyridines were then dissolved in this solution to produce the pyridinium ions with a slight excess of HCl to ensure a t least 99% protonation of the pyridines. These solutions were placed in a 10-mm Wilmad NMR tube, frozen, and then evacuated. After several minutes the samples were allowed to thaw and the process was repeated. A small change in the solvent composition probably occurred in this process, which removes dissolved O2and is essential for reliable T1 measurements. At the end of several such cycles high-purity nitrogen was introduced and the sample was rapidly removed. A coaxial, sealed tube containing D,O was inserted for a lock signal, which also minimized the space above the solution. The nuclear Overhauser enhancement factor, V C - ~ ,which is the nuclear Overhauser effect (NOE) minus one, was determined by using the gated decoupling technique where the NOE effect is suppressed during acquisition of the free induction decay. T o minimize the heating of the sample and provide an adequate number of data points with minimum noise, the acquisition time was 4 s. The total delay time between pulses was always greater than 57'1. Two procedures were used to measure the NOE ratio: (1) The spectrum was recorded by using the same spectral parameters for both normal and NOE suppressed experiments, and then recorded on an expanded scale; the area under the curves was then cut out and weighed. The NOE ratio was then taken t o be the ratio of the two areas times the scaling factors used by the computer when storing the data points. (2) On the JEOL FX6OQ a reliable integration program is provided in the software package which can give the NOE ratio directly. Both methods were adequate to provide NOE values with good precision and accuracy. The NOE ratio for the methyl carbon in neat toluene determined by either method was found to be 0.625 f 0.005, which is within experimental error of the same value (0.63) reported by Zens and Ellis.6 However, for p values greater than 1.5, the first procedure proved to be more
TABLE 11: Carbon-13 Chemical Shift Changes ( A 6 i, ppm) upon Protonation for the 2-, 3-, and 4-Methyl-Substituted Pyridines in 50 wt % Methanol-Water PY 2-Mepy 3-Mepy 4-Mepy
c,
c,
c,
c,
8.35 3.7 7.9 8.3
-2.7 -4.5 -5.0 -2.7
-9.51 -9.5 -10.2 -12.3
-2.70 -3.5 -3.4 -2.7
C6 CH, 8.35 7.2 3.5 7.2 -0.2 8.3 -1.1
reliable and was generally employed for this reason. With a delay time of 10Tl between pulses, Tancredo et al. found Tl for CH3CH21and CH3C13to be 16.1 and 12.5 s compared to 16.1 and 12.5 s with a waiting time6 of 5T1;the vC-H values are 1.52 and 1.67 a t 10Tl compared to 1.47 and 1.67 for 5T1. In the case of CH,OOH, the VC-H and T1 values are 0.99 and 25.0 s (10TJ compared to 0.70 and 23.2 s with a waiting time6 of 5T1.Since the TI values reported here are in the 16-10-s range, errors in pC-H due to not waiting 10Tl are probably smaller than the experimental uncertainties (=t5%).
Results and Discussion For the ring carbons the chemical shifts in parts per million (pprn) relative to Mel& are given in Table I for the 2-, 3-, and 4-methylpyridines in the neat phase and dissolved in 50 w t 70 methanol-water. Except for C4 in 4-methylpyridine, the chemical shifts found here for the neat phase and the methanol-water solutions are within 1-2 ppm of those reported for the CS2 solutions.'lJ2 The T I values are also given in Table I along with our assignments for the chemical shifts. Since the T1 value for C4 in 4-methylpyridine is expected to be larger than those for the other carbon atoms, the assignment for this resonances is straightforward and our assignment is preferred. In a comparison between the neat phase and the methanolic solution, we observe that C2 and c6 have chemical shifts 1-3 ppm lower in the methanol-water solution than in the neat phase. The resonances for C,, C4, and C s are generally at slightly higher ppm values in the methanol-water solutions than those found in the neat phase. Although these changes are modest, they are undoubtedly due in part to the effect of the hydrogen bonding of methanol and/or water to the pyridine nitrogen. Much larger changes are seen when the pyridine nitrogen is protonated in the methanol-water solvent system. These changes in chemical shift, ASi = G(neutra1) - 6(protonated), for each carbon are shown in Table I1 along with the values obtained for pyridine. Assignments for the carbons bearing the methyl group were unequivocally established from the T I measurements. The resonances due
The Journal of Physical Chemistry, VoL 84, No. 2, 1980 205
C-13 NMR Study of Pyridines TABLE 111: Carbon-13 Chemical Shifts (ppm), TI Values (s), and NOE Coefficients for the Methyl Carbons 2-, 3-, and 4-Methylpyridines in Neat and Methanol-Watier Solutions soln
____-
neat
1.1M in 50 wt %MeOH-H,O
pyri__ dine S C H ,
TI 2-Me 24.7 17.3 3-Me 17.8 15.0 4-Me 22.5 14.8
BC-H
2-Me 23.5 12.1 3-Me 18.4 11.3 4-Me 21.4 10.2, 9.ga 2-Me 20.0 11.5 3-Me 18.6 12.3 4-Me 22.5 10.9
1.29 18.7 34.4 1.08 20.8 24.7 1.31 15.5 29.9
1.12 M jn 50 art %MeOH-H,O with 1.12 M HCI a Independent determination.
TDD TSR 0.74 46.5 27.5 0.61 48.9 21.6 0.77 38.2 24.2
1.37 16.7 36.9 1.14 21.5 28.8 1.21 17.9 27.8
to c6 in the 2.methylpyridinium ion and C2 and c6 in the 4-methylpyridinium ion were easily assigned on the basis of the broadening caused by coupling to the nitrogen-14 nucleus. The two resonances a t 141.6 and 139.3 ppm in the spectra of the 3-methylpyridinium ion were also broadened due to coupling to nitrogen; the assignment of these two peaks to C2 and c6 is based on the observed broadening and the observed trends in A&, but could be reversed. The remaining assignments are based on the observed trend in As, and elimination. The 116,values found for pyridine and the methyl-substitutecl pyridines for C2 and c6 are all positive, i.e., the chemical shifts are larger in the neutral than in the protonated speciies by nearly 8 ppm. This is moderated somewhat whlen the methyl is attached. These results are parallel to those found for the transfer from the neat phase to the neutral methanolic solvent and are consistent with the suggestion that the major contribution to the solvent-induced chemical shift is hydrogen bonding to the pyridine nitrogen. A similar result was obtained by Pugmire and Grant13 when water was added to pyridine, Le., the resonances for C2 and c6 move upfield whereas the resonances for C3, C4, and C5 move downfield as water is added to the neat phase. They attribute this to hydrogen bonding, since the results parallel the changes found upon protonation by trifluoroacetic acid in water. The Asi values found by therie workers for pyridine are 7.78, -5.04, and -12.42, respectively, for the C2, C3, and C4 of pyridine. These values have the same signs as those given in Table 11, but differ in magnitude by 2-3 ppm, which indicates the difficulty of comparing carbon-13 chemical shifts to the intrinsic electronic properties of molecules when substantial solvation effects are present. One should also notice the influence of the methyl group on A6, when it is directly attached to the carbon, Le., the presence of the methyl group causes A6, to be more negative by 1-4 ppm. There are also smaller changes in As, for carbons not bearing the methyl group, all of which are not easily correlated with change densities, bond order, etc. In Table 111, the chemical shifts, T1 values, and 7C-H factors are given for the methyl carbons along with the dipole-dipole (TDD) and spin-rotation (TSR) contributions to T1 calculated from14 1 1 =- 1 (1) Ti TDD TSR
+-
This calculation assumes that the dipole-dipole and
spin-rotation mechanisms provide the only important contributions to the spin-lattice relaxation. A straightforward analysis of the T1 and TDD values presented in Table I and I1 via the well-known theoretical relation~hdpl~ l/TDD h2yc2yH2Rc~%N (3) provides insight to the relative rates of reorientation for the substituted pyridines in the three different types of solution. For the ring carbons, N is one and the dista.nce from the carbon to the attached hydrogen is approximately the same for each carbon atom. Thus, the average time for a molecule to rotate through 1 rad, 7, is the only quantity in eq 3 which can vary from one type solution to another and is related to the tumbling motions of the molecules in the solvent cavity. In the case of the carbons in the pyridine ring, the 7c-H value is assumed to be close to the theoretical maximum as has been found in similar whereas the TDD values are available from this study for the methyl carbons. For all the carbons with hydrogens attached, the T1 values decrease by factors of 2-3 upon changing from the neat phase to the methanol-water solution. This corresponds to a pronounced increase in r c , which must be due to a large interaction of the pyridines solutes with molecules within the solvent cage. Hydrogen bonding of the solvent molecules, via the pyridine nitrogen, could account for this observation and is consistent with the observed changes in chemical shift found here and in the experiments where water was added t o pyridine.13 When the pyridines are protonated, the T1value, quite surprisingly, do not decrease appreciably and in some cases increase. A modest increase is observed for the Tl values for C3 and C4 in the 2-methylpyridinium ion. Approximately a 20% increase is observed for all the TI values for 4-metlhylpyridine. The large T1 value found for C5 in 3-methylpyridine is inconsistent with the other T1 values and may be considered to be due to experimental error. Arnett et a1.16 have recently postulated that the pyridinium ions are more strongly hydrogen bonded to the water molecules than the neutral pyridines. This might be expected to lead to a reduced tumbling rate for the pyridinium ions, whereas what is observed is a t most very little change or the opposite. These results, however, may be interpreted as evidence for a very pronounced preferred rotation about the axis passing through the nitrogen atom and C4 in the protonated species. Whatever the nature of the solute-solvent interaction, the T1 values are consistent with substantial rotational reorientation in the protonated species which is a t least as effective in the dipole-dipole mechanism as that for the neutral species in the methanolic solution. Analysis of the TDD and TSRfor the methyl carbons, derived by means of a dissection of the spin-lattice relaxation rate into a dipole-dipole term and a term for all other mechanisms, clearly leads to three conclusions. First, the TDD values decrease on going from the neat phase to the methanolic solvent by approximately a factor of 2.5 and are virtually identical for the neutral and protonated species. These results lead to the same conclusions arrived a t from the analysis of the T1 values for the ring carbons. Second, the ratio of TDD for the methyl carbons to the TI for the ring carbons is nearly three for both the neutral and protonated species in the methanol-water system. This implies that the methyl groups are rapidly reorienting, with this motion contributing substantially to the dipole-dipole relaxation pathway, since theory8J5predicts that a freely rotating methyl group will have a TDD about three times that of a ring carbon. If the methyl group was undergoing strictly a torsional motion, then the TDD for
206
J. Phys. Chem. 1980, 84, 206-207
the methyl group could be as small as 1/3 that of the ring carbon. This analysis indicates that the methyl groups are undergoing virtually free rotation and the barrier is close to zero. Finally, the T S R values can be used to provide estimates of the barrier to internal rotation which are in agreement with the previous analysis. The TSRvalues found for the methyl carbons in the 3and 4-methylpyridines in the neat phase give V, values near zero from the correlation equation^.^^^ In the methanol-water solvent the Vovalues for the neutral and protonated species of these pyridines are also found to be quite small. For 2-methylpyridine Vois very close to zero in the neat phase but increases to about 1.0 kcal/mol in the neutral and protonated species in the methanol-water solutions. Apparently, hydrogen bonding to the nitrogen lone pair by the solvent molecules in the neutral species and the hydrogen on the nitrogen in the protonated species produces a steric interaction that raises the barriers to internal rotation. However, this effect is approximately the same for both species. It may be slightly larger for the protonated species, but the accuracy of the TsR values and the reliability of the correlation^^,^ are not sufficient to allow us to make such a distinction between two small V,, values. What is clear, however, is that the Vovalues do
not change appreciably upon protonation, for these pyridines, but Vois higher in the neutral and protonated 2methylpyridine species than for 3- and 4-methylpyridine.
References and Notes (1) E. J. King, "AcidBase Equilibria", Permagon Press, New York, 1965. (2) L. G. Hepler and E. M. Woolley, "Heats and Entropies of Ionization" in "Solute-Solvent Interactions", J. F. Coetzee and C. D. Rltchie, Ed., Marcel Dekker, New York, 1969. (3) D. H. Aue, H. M. Webb, M. T. Bower, C. L. Liotta, C. J. Alexander, and H. P. Hopkins, Jr., J . Am. Chem. SOC.,98, 854 (1976). (4) H. P. Hopkins, Jr., and S. Z. Ali, J. Am. Chem. Soc., 99, 2069 (1977). (5) C. H. Rochester and B. Rossal, Trans. Farao'ay Soc., 65, 1004 (1969). (6) A. P. Zens and P. D. Ellis, J. Am. Chem. Soc., 97, 5685 (1975). (7) J. R. Lyeria, Jr., and D. M. Grant, J . Phys. Chem., 76, 3212 (1972). (8) D. E. Woessner, B. S. Snowden, Jr., and G. H. Meyer, J. Chem. phys., 47, 2361 (1967). (9) N. Levy, "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley-Interscience, New York, 1972. (10) A. Tancredo, P. S. Pizani, C. Mendonca, H. A. Farach, C. P. Poole, Jr., P. D. Ellis, and R. A. Byrd, J . Mag. Reson., 32, 227 (1978). (11) H. L. Retcofsky and R. A. Friedel, J . Phys. Chem., 72, 290, 2619 (1968). (12) H. L. Retcofsky and R. A. Friedel, J. Phys. Chem., 79, 3592 (1967). (13) R. J. Pugrnire and D. M. Grant, J. Am. Chem. SOC.,90, 4232 (1968). (14) J. H. Noggle and R. S. Schirmer, "The Nuclear Overhauser Effect. Chemical Applications", Academic Press, New York, 1971, (15) F. W. Wehrli and T. Wirthlin, "Interpretation of Carbon-13 NMR Spectra", Heyden, London, 1976. (16) E. M. Arnett, C. Chawla, L. Bell, M. Taagepera, W. J. Hehre, and R. W. Taft, J . Am. Chem. Soc., 99, 5729 (1977).
Singlet and Triplet Emission from Difluoromethylene in the Reaction of Ozone with Tetrafluoroethene Sidney Toby* and Frina S. Toby DepaHment of Chemistly, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08903 (Received July 16, 1999)
-- -
In addition to t,he previously identified singlet emission (CF#B,) CF2('A,)), the reaction of ozone with tetrafluoroethene also gives triplet emission due to CF2(3B1) CF&Al). The intensities of the emissions are in accord with formation of the excited singlet via 2CF2(3B1) CF2(lB1)+ CF2('A1). The reaction between ozone and tetrafluoroethene a t room temperature gives a strong emission in the ultraviolet which was identified by Sheinson, Toby, and Toby1 as due to excited singlet CFJIB1). In addition an unidentified luminescence was seen consisting of peaks in the region 490-625 nm. Recent work by Koda2i3has shown that the reaction of oxygen atoms (3P)with tetrafluoroethene gives both CF2(IB1)and a visible luminescence which he identified as the hitherto unseen triplet CFz(3B1)emission. Reported here is the identification of the visible luminescence of the C2F4 O3 reaction as due to CF2(3B1) and a brief investigation of the kinetics of the ultraviolet and visible luminescence. The apparatus was similar to that previously described.l Ozone was prepared by passing oxygen (Matheson Ultrapure grade) through an ozonizer operated a t 7.5 kV and trapping the O3 in silica gel at -78 "C. The silica gel trap was then evacuated to remove adsorbed O2 until appreciable quantities of O3 desorbed, and the trap was then allowed to warm to room temperature, allowing the O3 to expand into a 3-L bulb where it was diluted with helium. In some cases the O3 was eluted directly from the warmed silica gel trap with a stream of helium or oxygen. Total pressures were generally 1torr for kinetic measurements, and -50 torr for spectra.
+
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0022-3654/80/2084-0206$01 .OO/O
The reaction vessel was approximately 65 cm long and C2F4 (from Columbia Organic Chemicals, distilled before use to remove polymerization inhibitor) was mixed with O3 60 cm from the monochromator window. O3 pressures were measured by absorbance a t 254 nm and CzF, pressures were measured via a ball flowmeter which had been calibrated for C2F4 flow under controlled conditions by using a McLeod gauge. A Jarrell-Ash 0.25-m monochromator with a 2360 grooves/mm grating and a 3-nm spectral slit width viewed the reaction vessel axially through a quartz window with a Kodak Wrattan 2A filter to remove the second-order UV spectrum. Integrated singlet and triplet intensities were found by removing the monochromator and Wratten filter to measure total emission and interposing the filter to measure triplet emission. A cooled EM1 9683QKB photomultiplier was used. Results and Discussion The visible emission from the reaction of O3with CzF4 is shown in Figure 1. The assignments are those of Koda2 for CF2(3B, lAl) and are due to the progression for the bending v 2 mode, particularly the transitions from v i = 0. A few lines from vi = 1 and v2' = 2 are partially resolved. The effect of an oxygen rather than a helium carrier is most interesting. The triplet emission is reduced
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@ 1980 American Chemical Society
