5456
Solvation and Segmental Motions of n-Alkylammonium Ions. A Carbon-1 3 Spin-Lattice Relaxation Study George C. Levy,*
la
Richard A. Komoroski,'&and Judith A. Halsteadlb
Contribution f r o m the Department of Chemistry, Florida State Unitiersity, Tallahassee, Florida 32306, and General Electric Corporate Research and Development, Schenectady, New York 12301. Received February 9, 1974 Abstract: Carbon-13 spin-lattice relaxation times of individual carbons were used to monitor segmental motions in several n-alkylamines and the corresponding n-alkylammonium ions (trifluoroacetate salts) in D 2 0 and various organic solvents. The relaxation behavior in the amines is similar to that observed previously for n-alkanes, where a moderate increase in motion is observed near the chain ends. Motional restriction of the ionized amino function can result in the observation of a large degree of segmental motion along these chains. In D 2 0solutions, increasing concentration of ammonium ion results in increasing restriction of the polar end, with a reduced effect as one moves toward the terminal methyl. At relatively high dilution in polar solvents such as DzO, solvent-ion motional restrictions are markedly lessened for the shorter chain ammonium ions (especially butyl and hexyl). For the longer chains the motion of the aliphatic chain terminus is essentially "decoupled" from that of the polar head group, and also from macroscopic viscosity. A self-consistent analysis yielded the rate of terminal methyl spinning, which in the butylammonium case was found to decrease as ion concentration increased. The relaxation behavior for micellar structures indicated considerable fluidity in the hydrophobic interior. For the longer chains TI changed gradually along the first six carbons, with much larger increases for the last 2-3 carbons. arbon-I3 spin-lattice relaxation measurements on small organic molecules and ions can be used to study generalized and specific solvation effects and aggregation in a wide variety of solvent systems. z--j Observed spin-lattice relaxation times (TI%)can yield information on the rate and anisotropy of overall molecular tumbling of a rigid carbon framework in solution.3,6 They also effectively probe detailed internal motions along aliphatic chainsz, (which in turn monitor the anchoring of ionic or polar sites in the solvent "lattice"). The spin-lattice relaxation of protonated carbons in large and intermediate sized molecules is in general overwhelmingly dominated by T-IH dipolar interactions with the attached hydrog e n ~ , ~and , ' Tl is given by
C
l/Ti
Nh2'yc' ' y ~'rcH-%eff
(1) where N is the number of directly attached hydrogens, YC and Y H are the magnetogyric ratios for 13Cand 'H, rCH is the C-H bond length (1.09 X lo-* cm), and T e f f is an effective rotational correlation time of the C-H vector. For a rigid molecule rotating isotropically, Teff is equal to the correlation time for overall molecular reorientation in the extreme narrowing limit. If internal motions are present or if the overall reorientation is anisotropic, Teff is a n average correlation time. In such a case one can write4 =
l/Teff = l / and ~ l~/ ~ i
where
~i
(2) is a n effective correlation time for internal
(1) (a) Florida State University; (b) General Electric undergraduate researcher. (2) G . C. Levy, J . Chem. SOC.,Chem. Commun., 768 (1972). (3) G. C. Levy, Accounts Chem. Res., 6,161 (1973). (4) D. Doddrell and A. Allerhand, J . Amer. Chem. SOC.,93, 1558 (1971). (5) (a) G. C. Levy, J. D. Cargioli, and F. A. L. Anet, J . Amer. Chem. SOC., 95, 1527 (1973); (b) G. C. Levy, J . Magn. Resonance, 8, 122 (1972). (6)'A. Allerhand, D. Doddrell, and R. Komoroski, J. Chem. Phys., 55, 189 (1971). (7) K. F . Kuhlman, D. M. Grant, and R. K. Harris, J . Chem. Phys., 52,3439 (1970). ~~
Journal of the American Chemical Society
rotation of the group, and 7, is the correlation time for overall reorientation. It is apparent from eq 2 that for molecules with relatively fast overall reorientation comwill pared with the rates of segmental motion, dominate l/T,ff. As the overall reorientation becomes slower, the effect of 1 / ~ becomes i more pronounced and eventually dominates T e f f , The present study examines the 13C TI behavior of several n-alkylamines and corresponding ammonium ions as a function of the concentration and solvent system. The alkylammonium ions are particularly interesting since they can form micellar structures in solution. The results yield considerable information concerning the detailed motional behavior of straightchain organic ions in isotropic and micellar phases.
Experimental Section Neat n-alkylamines were obtained from commercial sources. The trifluoroacetate salts were prepared by mixing the appropriate amounts of trifluoroacetic acid and n-alkylamine, washing with hexane, and drying overnight under vacuum. All viscosities were measured at 38' using Cannon-Manning semimicro viscometers. Except as noted below, 13C spectra and spin-lattice relaxation times were obtained with complete proton decoupling at 25.16 MHz and 38 i 2" on a Varian XL 100 Fourier transform nmr spectrometer. Ti's at 25.16 MHz were obtained using the Freeman-Hill modification* of the inversion-recovery method.@ The pulse sequence is (T-90,"-T-180"-t-90r0),. For carbons 8, 9,and 10 of the n-decylammonium ion (15% in DsO), TI'Swere determined at 67.9 MHz using the unmodified inversion-recovery pulse sequenceg (T-18Oo-t-90"),. Tl's obtained at 67.9 MHz were still within the region of extreme narrowing and thus were not frequency dependentlo (note: Ti's ofless than ca. 250 msec at 67.9MHz cannot be directly compared with Ti's obtained at 25.2MHz). PRFT spectra obtained using Figure 1 gives a typical set of the Freeman-Hill modification. The data from Figure 1 are plotted semilogarithmically in Figure 2, yielding l3C TI values. The accuracies of TI measurements in this study generally range from 1 5 to 1 1 5 % , depending on S / N and the number of r values. For (8) R. Freeman and H. D. W. Hill, J. Chem. Phqs., 53,4103 (1970). (9) R. L. Vold, J. S . Waugh, M. P. Klein, and D. E. Phelps, J . Chem. Phys., 48,3831 (1968). (10) D. Doddrell, V. Glushko, and A. Allerhand, J . Chem. Phys., 56,3683 (1972).
1 96.1 7 1 August 21, 1974
5457 n-OCTYLAMINE AND n-OCNLAWNIUM I O N
n- HEXYLAMMONIUM TRIFLUOROACETATE in D20 (75% w/w)
TMS,
Figure 3. Proton-decoupled carbon-1 3 Fourier transform nmr spectra of neat n-octylamine and n-octylammonium ion (CF3COzsalt).
Figure 4. I3C FT nmr spectra of rt-decylammonium ion (CF8C02salt, 7 0 z in D20)at (a) 39" and (b) 36".
' _i
2.0
solutions ( 2 10% ion concentration) Ti's should be considered accurate to &15-20%. Reproducibility was in general better than 10%; however, preliminary experiments on several nonylammonium ion solutions obtained in the two laboratories showed significant differences.
A U L
i
3.5
Figure 1. Set of PRFT spectra using the Freeman-Hill modifications of the inversion-recovery sequence.lO Number to the left of each spectrum is the t value in seconds (25 scans, 1250-Hzspectral width). 30
I
I
I
I
1
I.5
2.0
2.5
20
> E
IO k
9
m
2 6
r 7 2
6
I
f
5
y e 4
1 \I
3
'0
0.5
(0.64s)
1.0
t(rec1
Figure 2. Semilogarithmic plots of S, - St (taken from sDectra in Figure 1) us. t (sec) for allcarbons of the n-hexylammonium ion (CF3C02-salt, 75 % w/w in DzO).
concentrated solutions (n-butyl and n-hexyl 2: 15%; n-octyl and n-decyl Y 30%) the accuracies were highest. For a few dilute
Results and Discussion Assignments for the n-alkylamines have been given previously. Assignments for the n-alkylammonium ions were made on the basis of the amines and known shift effects upon protonation of the amino group.",'* In some cases ambiguous assignments were confirmed by TI measurements. The changes for the 13Cchemical shifts of n-octylamine resulting from protonation are shown in Figure 3. Large effects are seen for C-1 and C-2 (2.3 and 6.8 ppm, respectively) with smaller effects for carbons 3 to 5 . The larger shift for C-2 than for C-1 is generally seen for aliphatic amines.12,13 The resonances of C-1 and C-2 in the ammonium ion are broadened by unresolved scalar coupling to cationic nitrogen. This behavior has recently been observed in the n-alkyltrimethylammonium bromides.I4 It was found that the n-decylammonium trifluoroacetate aqueous solutions underwent a "phase change" between 36 and 39", resulting in spectra with extremely broad resonances at the lower temperature. It is probable that a liquid crystal phase is formed below 39". This transition is shown in Figure 4. Similar spectral behavior has been oberved for synthetic lecithins. In Tables I-IV the spin-lattice relaxation times are given for all resolved carbons in a number of n-alkyl(1 1) H. Eggert and C. Djerassi, J . Amer. Chem. Soc., 95, 3710 (1973). (12) G. C. Levy and G. L. Nelson, "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists," Wiley-Interscience, New York, N. Y., 1972, pp 51-52. (13) J. B. Stothers. "Carbon-13 N M R Spectroscopy," Academic Press, New York, N. Y., 1972; I. Morishima, K. Yoshikawa, I
