Hydrogen Relaxation Measurements in Methanol-Solvent Mixtures
2307
Spin-Lattice Relaxation and Hydrogen Bonding in Methanol-Solvent Mixtures Rush 0. Inlow,' Melvln D. Joesten,' and John R. Van Wazer" Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235 (Received March 2 1, 1975)
Nuclear spin-lattice relaxation times, TI, of the hydrogen nuclei in CD30H and CH30H have been measured as a function of concentration in solvents of varying hydrogen bonding ability. CD3OH-CC14, CD30H-(CD3)2CO, CD30H-CD30D, CD30H-(CD3)2SO, CH30H-CC14, and CH30H-(CD3)2CO mixtures were studied over the entire range of concentration at 31O. The results show that TI reaches a minimum in all solvents at -0.9 mole fraction alcohol. This is interpreted in terms of the structure of pure liquid methanol being disrupted by insertion of small amounts of solvent. Extrapolation of the spin-lattice relaxation time to infinite dilution, T l O gives , an indication of the strength of the alcohol-solvent hydrogen bond. TI' is shortest in CC14 and increases in the order: (CD&CO < CD3OD < (CD&SO. This behavior is explained by the hindrance of rotation of the alcohol as a result of the formation of hydrogen bonds of increasing strength with the solvent. At large mole fractions of methanol, relaxation is interpreted to be a mixture of dipole-dipole and spin-rotation interaction; while in dilute solutions the spin-rotation mechanism seems to predominate.
Introduction The spin-lattice relaxation time, T I ,has been extensively used to describe molecular motion in liquids. The theoretical basis of the method is well developed.2a Many studies in this area describe the analysis of T1 values for a molecule in terms of specific relaxation mechanisms and compare experimental results with the predictions of a theoretical model for the motion of the molecule in a liquid. NMR chemical-shift studies have been used to examine hydrogen-bonding effects in alcohols.2b Relaxation-time studies of water-alcohol mixture^,^,^ ROD-perdeuterioROD mixture^,^ and acetone-water mixtures6 have been reported; but, to our knowledge, no systematic examination for the effect of hydrogen bonding on the relaxation time of hydroxylic protons in alcohols has been published. In this paper, relaxation measurements on hydrogen in the following binary systems are reported: (1) methyl-& alcohol with carbon tetrachloride, acetone-ds, methanol-&, or dimethyl-& sulfoxide, and (2) methanol with carbon tetrachloride or acetone-&. Experimental Section NMR Spectrometer System. A Varian XL-100-15 multinuclear spectrometer was employed with an external heteronuclear lock on 2H. This system includes a Transform Technology, Inc. (TT-100) Fourier-transform unit. Sample temperature in the probe was 31'. T1 Measurements. Measurements of lH spin-lattice relaxation times, 2'1, were accomplished by the inversion-recovery pulse m e t h ~ d . For ~ . ~ the systems examined here P(18OO) was found to be 150 psec and,P(90°) 75 wsec. The delay times between the pulses, T, were chosen to bracket the inversion point at which T1 = Tlln 2. The computer permits automated accumulation of the 18Oo-~-9O0 sequences and then rapid processing, storage, and presentation of data. 2'1 values were computed automatically and by hand from a plot of In ( I , - IT)vs. T, where I is the intensity of the peak. Results obtained by both methods were in close agreement. However, the hand-plotted T1 values were used in subsequent calculations because they exhibited somewhat less scatter. The T1 values obtained were
found to be reproducible to within a few percent; but the usual error assumed in measurements of this type is f 10%. Chemicals. Methanol and carbon tetrachloride were Fisher reagent grade. These were carefully distilled before use with the middle one-third fraction being collected and dried over molecular sieves. Methyl-d3 alcohol (99% D) was purchased from IC & N Chemicals and used without further purification. Methanol-d4 (99.5% D), acetone-& (99.5% D), and dimethyl-& sulfoxide (99.5% D) were purchased from Wilmad Glass Co. and were dried over molecular sieves. Deuterium oxide (99.8% D) was purchased from Stohler Isotope Chemicals. Sample Preparation. The components of each mixture were deoxygenated by bubbling high-purity nitrogen through the liquid for 10 min. The components were then weighed into a nitrogen-filled 5-mm NMR tube. This tube was placed inside a 12-mm tube containing the reference lock material, D2O. The concentration of the sample was varied by adding more of the appropriate component to the original mixture. Samples prepared in this manner gave reproducible T1 values, and the plots of ( vs. concentration obtained from dilutions of several different samples could be joined without disconcerting discontinuities. For each pair of components, at least four separate samples were prepared and then diluted to obtained T1 data over the entire concentration range. In addition, several samples were prepared without deoxygenating the components, and the resulting T I values were found to be 10-20% shorter than those of the corresponding deoxygenated samples. Deoxygenation by the freeze-pump-thaw method did not increase T1 values.
Results and Discussion Interpretation in Terms of Liquid Structure. Figures 1 and 2 show the 'H spin-lattice relaxation rate, (Tl)-l,in mixtures plotted against the mole fraction of alcohol monomer for CD30H and CH30H, respectively. The relaxation rates at infinite dilution, ( T l 0 ) - l ,were obtained by extrapolation of the linear portion of each plot to zero mole fraction of alcohol. A perturbed nuclear-spin system relaxes to its equilibriThe Journal of Physical Chemistry, Vol. 79, No. 2 1, 1975
R. 0. Inlow, M. D. Joesten, and J. R. Van Wazer
2308
‘H RELAXATION RATES - CD,OH
-I
10.25
10.05
CD,OH in (CDJ,SO
*
0.2
T
0.6
0.4 I
I
0.8 I
xcD,oHFigure 1. Relaxation rate, l/T1. for
the hvdroaen nucleus in CDqOH plotted against the mole fraction of CDjOHmonomer, XCD& in CC14, CDsOD, (CD3)&0, and (CD3)2S0.
t
‘H RELAXATION R A T E S
(SEC?
0.25
CH,OH * _
-
t’
in CCI,
CI4,OH in CCI,
I
6 -0.15CH
1
- CH,OH
-
in (CDJ,CO 4
CH,Otj in (CD,),CO
0.2 ~~
~~
0.4
0.6
0.8
xcti;---
Figure 2. Relaxation rate, l / T , , for the hydrogen nuclei in CH30H plotted against the mole fraction of CH3OH monomer, XCH~OH, in CCI4 and (CD3)&0. The underlining of CH30H denotes that the
curve represents the behavior of the methyl hydrogens; whereas that the curve refers to the behavior
CH30H is underlined to denote of the hydroxyl hydrogen.
um state by coupling between the spin system and the surrounding environment. The principal processes that lead to spin-lattice relaxation in liquids involve some type of molecular motion. For hydrogen nuclei these processes are:9 (1) magnetic dipole-dipole interaction, (2) spin-rotation interaction, (3) scalar-coupling interaction, (4) chemicalshift anisotropy interaction, and (5) interaction due to the presence of paramagnetic materials in the sample. Mechanism 5 is eliminated by the use of glassware and chemicals not contaminated by paramagnetic materials and by careful deoxygenation of the test samples. Mechanism 4 is relaxation due to the tumbling of structures exhibiting large chemical-shift anisotropy. For nonviscous liquids it has been shown that this relaxation rate is proportional to Ho2(uii - o L ) ~ ,where Ho is the magnetic field strength and uil and uI are the values of the shielding factor parallel to and perpendicular to the axis of symmetry of the m o l e ~ u l e .Studies ~ have shown that the IH relaxation rates in ethanol10 and HFll do not vary with Ho.We estiThe Journalof Physical Chemistry, Vol. 79, No. 21, 1975
mate the (Tl)-l contribution due to chemical-shift anisotropy is sec-1. Scalar coupling, mechanism 3, can contribute importantly to relaxation in methanol under certain conditions.12J3 When methanol is carefully acidified, the lH relaxation rate varies with acid concentration, passing through a sharp peak at pH -3. The magnitude of the scalar relaxation rate depends on the proton-exchange rate and is important only at proton-exchange rates intermediate between that of pure methanol and highly acidified methanol. For pure methanol at 31° the following contributions from scalar coupling relaxation are estimated: [T1(CH3)Isc-l = 0.004 sec-l and [TI(OH)],,-~ = 0.012 sec-l. This is about 8%of the total experimental relaxation rate for the hydroxyl proton and 3% of the experimental relaxation rate for the methyl protons. As methanol is diluted, the correlation time for proton exchange increases, and the scalar mechanism rapidly becomes less important. For CD30H the scalar relaxation rate will be smaller due to the small hydrogen-deuterium coupling constant. The total relaxation rate is given by (Tl)-l = ( T 1 ~ ~ 1 -k -l (TlsR)-l + (Tl*)-l, where (Tl*)-l represents the sum of the remaining relaxation rates which, to a good approximation, should contribute 10% or less to (Tl)-l. Most lH relaxation has been shown to be of the dipole-dipole type, although spin-rotation may be important in some ins t a n c e ~ . ~The ~ - magnitude ~~ of the lH-lH dipole-dipole relaxation rate is directly proportional to the number of hydrogen nuclei per unit volume of solution and inversely proportional to the sixth power of the distance between these nuclei. 1H-2H dipole-dipole relaxation is an interaction that, for the same situation, is weaker by a factor of 24 than the lH-lH relaxation. In Figure 3, the lH relaxation rate for CD30H-CD30D mixtures is plotted against the mole fraction of CD30H monomer. The ‘H-lH dipole-dipole contribution must essentially vanish at zero mole fraction alcohol (excluding a negligible contribution from intramolecular 1H-2H interaction in the CD30H monomer) since the hydrogen nuclei are isolated from one another in a deuterated lattice. Therefore the entire lH relaxation rate at infinite dilution ought to arise from the spin-rotation mechanism. Because substitution of H for D in the hydroxyl group of methanol should not affect the liquid structure of the alcohol, it is assumed that ( T l s ~ ) - l remains fairly constant over the entire CD30H mole fraction range. The difference between the experimental relaxation rate and the spin-rotation contribution is attributed to dipole-dipole relaxation, the rate of which therefore should rise linearly with increasing lH concentration, as it does for 0 < X C D ~ O H< 0.92. The dotted portion of the dipole-dipole relaxation rate line in Figure 3 is an extension of this linear increase to X C D ~ O H= 1.0, while the solid line in this region represents the observed behavior. The difference between the experimental relaxation rate and the sum of ( T l s ~ ) -+ l ( T 1 ~ ~ ) -may ~ l be i ~desig~ ~ ~ nated the excess relaxation rate, (TlXs)-l. Using the same method, values of (T1xs)-lcan be determined for CD30H in the other solvents. Then, relaxation rate ratios can be converted into distance ratios using the relation r’/r = { ( T ~ D D ) - ~ / [ ( T ~ D D(T1xs)-1])1’6, )-~ where r’ is the lH-lH distance indicated by the experimental relaxation rate minus the spin-rotation relaxation rate, and r is the lH-’H distance indicated by the linear dipole-dipole relaxation rate. Figure 4 shows r’/r for CD3OH in the various solvents plotted against mole fraction of alcohol monomer. A t
+
Hydrogen Relaxation Measurements in Methanol-Solvent Mixtures
r
(SEC-')
COMPONENTS of (TI)-'
2309
I
I
r h RATIOS - CD3OH
f (cD& SO
CD3OH - CD3OD SYSTEM
If
MONOMER PREDOMINATES
&-TETRAMER PREDOMINATES
0.95 XCOgH-
- 1
0.4
Figure 3. Components of l / T , for the CD30H-CD30D system. Relaxation rate, l/Tl, plotted against the mole fraction of CD30H monomer, XCO~OH. The upper curve represents the experimentally determined values. The spin-rotation contribution was determined by assuming that as X C D ~ O H 0 essentially all of the relaxation is of the spin-rotation type. The dipole-dipole contribution is the difference between the experimental and spin-rotation curves. When 0.93 < X C D ~ O H< 1.00, the dotted portion of the dipole-dipole curve represents the expected behavior, while the solid portion of this curve represents the observed behavior.
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