J. Phys. Chem. 1983, 87,5219-5223
to be able to make a meaningful comparison with various theoretical models, which contain parameters that are difficult to evaluate a priori. The present work shows that the combination of laser mpd and thermal pressure-dependent VLPP offers considerable scope for obtaining further sensitive data for such comparisons, over a wide range of temperature, reactants, and collision partners. Acknowledgment. The generous support of the Australian Research Scheme and the Australian Institute for
5219
Nuclear Science and Engineering, and the collaboration of Dr. Arthur W. Pryor and Robert K. Nott (Australian Atomic Energy Commission Research Establishment) and Dr. Neil J. Daly (Australian National University), are gratefully acknowledged. We particularly appreciate a meticulous reading of the manuscript by Dr. John Barker of SRI which led to the detection of an error in ref 6. Registry No. He, 7440-59-7;Ar, 7440-37-1;Ne, 7440-01-9;Kr, 7439-90-9; Nz, 7727-37-9; CZH4,74-85-1;ethyl acetate, 141-78-6.
Nitrogen-I 4 Nuclear Quadrupole Coupling Constants Effective for Nuclear Relaxation in Molecules of Trimethylalkylammonium Type Kelko Koga and Yoko Kanazawa‘ Faculty of Pharmaceutical Sciences, Kyushu University, Maidashi, Higashi-ku, Fukuoka 8 12, Japan (Received: February 4, 1983)
The nitrogen nuclear quadrupole coupling constant e2qQ/hin (CH3)3N+CH2Xions in aqueous solutions are obtained by the analysis of longitudinal nuclear relaxation times of 14Nand 13C under the assumption that the motions effective for these two relaxation times T1(14N)and TI(l3Ca)are the same. The experimental basis for the validity of this assumption is given. Within the experimental accuracy, the data consistently show the invariance of e2qQ/hthroughout the applied pH and temperature. The values of e2qQ/hare 1order of magnitude small compared with those of ordinary compounds (several MHz), as expected, and vary with X: 64 kHz for X = CHzOH,128 kHz for CHz0COCH3,190 kHz for COO-, and 5460 kHz for COOH. The contribution from external field fluctuation due to water dipoles to the 14Nrelaxation is negligible except for choline, which has the smallest value of e2qQ/h. The 14Nrelaxation time of ammonium ions of this type is thus shown to be adequate for the study of molecular dynamics.
Introduction The dominant mechanism of nuclear relaxation of the 14Nnucleus in molecules in rapid motion is known to be the quadrupole interaction. The 14Nrelaxation method is a convenient tool for the study of molecular dynamics in liquids when the values of e2qQ/h and q , the nuclear quadrupole coupling constant and its anisotropy parameter, are known.’ These quantities are usually obtained by microwave spectroscopy, nuclear quadrupole resonance,2 and NMR quadrupole splittings in solids or in liquid crystal^.^ We have studied the dynamics of the (CH3)3N+CH2group of phosphatidylcholine in single bilayer vesicles by 14N NMR,4 where the analysis has been done with e2qQ/hevaluated for a related c ~ m p o u n d . ~It is because e2qQ/hof (CH3)3N+CH2Xions are few and that of the lipid has not been available. Due to the nearly tetrahedral symmetry around the N nucleus, the electric field gradient at the nucleus should be small. Actually, the reported data of e2qQ/hare in the vicinity of 100 kHz, which is 1 order of magnitude smaller than those of ordinary compounds of several M H z . ~The method of NQR is not applicable for the determination of e2qQ/hof such low frequency. The e2qQ/h in such molecules has been determined only by NMR. One method is to analyze 14N relaxation time, by using the reorientational correlation
time T~ obtained by the other method. This is a useful method if T, is obtained as close as that effective for I4N relaxation. It is advantageous that q can be neglected in the analysis since the threefold symmetry around N should hold in the solution. Behr and Lehn6 obtained the coupling constant for acetylcholine bromide to be 140 kHz in DzO and 120 kHz in CD30D with 14Nand 13Crelaxation times. Recently, measurements of I4N quadrupole splittings Au, in liquid crystals have been made.’n8 If the order parameter 5’ is obtained independently, e2qQ/hcan be determined by the relation AUQ= (3/4)(e2qQ/h)S.Siminovitch et aL7 determined the coupling constant to be 135 kHz from the AuQ(14N)of dipalmitoylphosphatidylcholine (DPPC) in multilamellar dispersions a t 44 “C using SCgN calculated from the deuteron NMR quadrupole splitting of CD3(CH3)2N+in DPPC? The value is in good agreement with the result of Behr and Lehn.6 However, the temperature dependences of the quadrupole splitting7 and of the sC,N9 are different. This may be due either to the presence of temperature dependence of e2qQ/h in the liquid crystal or to the inadequacy of the method of S C g N evaluation. Measurements of the quadrupole splitting of the NMR spectra in the solid can also be used to determine e2qQ/h and q in these nearly tetrahedral 14Ncompounds. In spite of the intermolecular contribution to e2qQ/hand 7 due to
(1) AFagam, A. “The Principles of Nuclear Magnetism”; Oxford University Press: London 1961. (2) Lucken, E. A. C. “Nuclear Quadrupole Coupling Constants”; Academic Press: London, 1969. (3) See, for example: Witanowski, M.; Webb, G. A., Eds. “Nitrogen NMR”; Plenum Press: London, 1972. (4) Koga, K.; Kanazawa, Y . Biochemistry 1980,19, 2779-83. (5) Henriksson, U.; Odberg, L.; Eriksson, J. C.; Westman, L. J.Phys. Chem. 1977,81, 76-82.
(6) Behr, J.-P.; Lehn, J.-M. Biochem. Biophys. Res. Commun. 1972, 49, 1573-9. (7) Siminovitch, D. J.; Rance, M.; Jeffrey, K. R. FEBS Lett. 1980,112, 79-82. (8) (a) Rothgeb, T. M.; Oldfield, E. J.B i d . Chem. 1981, 256, 6004-9. (b) Siminovitch, D. J.; Jeffrey, K. R. Biochim. Biophys. Acta 1981,645, 270-8. (9) Gally, H.-U.; Niederberger, W.; Seelig, J. Biochemistry 1975, 14, 3647-52.
0022-3654/83/2087-5219$0 1.50/0
0 1983 American Chemical Society
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TABLE I : Relaxation Times and sample choline chloride
AcCh.Br
7~
and e 2 q Q / hObtained from the Relaxation Times
pH or pD 3.0 7.0
7.9 12.0 7.0 7.9 3.0 5.0
6a 7a 5
solvent H,O Hi0
Tris,H,Ob H,O DSO Tris,D,Ob H,O Hi0
HZ0
!I'ris. H,O D.0
DZO AcCh.Cl betaine hydrochloride TMA.C1
Koga and Kanazawa
7a 5.0 1.2 7.0 12.6 5.0
Tris.D,O H,O HI0 H,O H,O H,O
tepp/ C 30 20 23 25 30 39 40 30 30 30 30 30 20 23 30 35 39 40 45 50 30 30 30 50 30 30 30 30 30 23 30 40
T ,( 13C)d/s
T1(I4N)'/ S
3.0 2.6 2.7 3.0 3.4 2.8 2.6 2.5 2.5 0.50 0.42 0.49 0.54 0.59 0.65 0.69 0.51 0.51 0.43 0.64 0.44 0.50 0.049 0.34 0.32 9 11 12
C,
Cp
C,
rplps
5.5 4.4
4.8 4.1
3.7 3.1
4.8 5.7
6.0 6.3 6.7 5.4 5.2 5.1 5.0 2.9 2.3
5.1 5.9 6.3 5.0 4.9 4.6 4.2 2.7 2.3
3.9 4.2 4.4 3.6 3.5 3.3 3.3 2.2 1.8
4.6 3.9 3.7 4.7 4.7 5.1 5.5 8.5 10.2
3.1
2.8
2.3
8.2
3.5 3.6
3.3 3.5
2.6 2.6
7.1 6.7
4.7 3.0 3.2 2.7
4.2 2.9 3.0 2.5
3.5 2.3 2.3 2.0
5.6 8.1 7.8 9.2
2.8 3.1
2.5 2.9 3.6 4.2 3.9
2.0 2.3 2.4 3.0 2.7
9.1 8.0 6.4 5.6 6.0
eZqQe/h/ kHz
a These samples were adjusted to pH 7.0 or 7.9 but pH values were lowered in the course of time. pH values during NMR These samples contain 50 mM Tris. Averages of 3-12 data. + 5 %except for measurements are shown in the table. i 5 % for C, and Cp, and better than 25% for C,. e + 5 kHz except for betaine (*, i 2 0 kHz; **, i10 kHz). TMA (+lo%). The values in parentheses are calculated by either inter- or extrapolated TI values.
the packing of the molecules in the solids, this method will probably lead ultimately to the most accurate value. The accumulation of accurate data is awaited, since values of e 2 q Q / h and 7 are needed for the interpretation of 14N spectra in liquid crystals and biomembranes where 14N NMR is being used as a probe of the polar head group structure and dynamics. For the application of 14NNMR to the study of molecules of biological interest, information regarding the effect of environments on e2qQ/hin aqueous medium is also important. The 14Nquadrupole coupling constants effective for the nuclear relaxation of (CH3),N+CH2Xions in isotropic aqueous solutions are determined by the NMR method in the present work. The investigation is made whether the correlation time obtained by 13Cnuclear relaxation can be adopted for the analysis of 14Nrelaxation times. The effect of the environment on the 14Nquadrupole coupling constant is examined. Experimental Section Materials. Acetylcholine (AcCh) bromide (GR), acetylcholine chloride (EP), and betaine hydrochloride were obtained from Wako Pure Chemical Industries, Ltd. Choline chloride (claimed to be more than 98% pure) was from Tokyo Kasei Co. and tetramethylammonium (TMA) chloride (reagent for polarography) and tris(hydroxymethy1)aminomethane (Tris, GR) were purchased from Nakarai Chemicals Co. Heavy water (CEA, Saclay) was 99.83% pure in deuterium. These reagents were used as supplied. All the aqueous solutions, AcCh and betaine at 0.6 M and choline and tetramethylammonium chloride at 1.0 M, respectively, were prepared with 1 mM ethylene-
diaminetetraacetic acid (Nakarai, GR). Adjustment of pH was done with NaOH except for AcChmBr solution at pH 3 and the solution with 50 mM Tris where HC1 was used: (Cl-)/(Br-) = 0.001 at pH 3 and 0.05 at pH 7 in Tris buffer. A reading of pH meter plus 0.4 was used as a pD value. NMR samples were bubbled with Ar or N2 gas in an NMR tube of 10-mm diameter for 10 min. NMR Measurements. All the NMR measurements were done a t 2.35 T. 14NNMR experiments were done with a JEOL PS-100 electromagnet and the pulse F T system reported previously.1° The temperature was controlled with a VT-3 temperature control system and monitored by thermometer. The control was better than f0.5 "C. 13C NMR was measured with a JEOL FX-100 system under proton-decoupled conditions. The temperature was monitored with a thermocouple inserted into a sample tube with water and was accurate to fl "C. T1 was obtained by the inversion recovery method. The accuracy of T1(14N) was f 5 % except for TMA, which was *lo%, and those of T1(13C) at C, and C, were &5% and that at C, was better than 5%. Here, the carbon atoms were denoted as follows: B
(6H3),N'CH,8H2 Results The 14Nand 13C NMR spectra of (CH3),N+CH2Xions are shown in Figure 1. The relaxation times are listed in Table I. (10) Kanazawa, Y.; Koga, K.; Kamei, H. J. Magn. Reson. 1979, 35, 353-5.
e 2 q Q l h of I4N in (CH,),N+R
The Journal of Physical Chemistry, Vol. 87, No. 25, 1983
I
C :
5221
00
2 3.2
3.3
I/T/
3.4
i o - 3 ~
Flgure 2. Arihenius plots of T1(I3C)and Tl(l4N) of choline chloride, 1 M in H,O at pM 7.0: C, ( O ) , Cg (0),C, (A),and N (0).
Flgure 1. 13C and 14NNMR spectra: (A) choline chloride, pH 7.0; (B) acetylcholine chloride, pH 5.0; (C) betaine hydrochloride, pH 1.2; (D) betaine hydrochloride, pH 7.0. All spectra were obtalned in H20 at 30 OC. 13Cspectra are the averages of 28-52 transients, 0.25 Hzlpoint, and 14N spectra are the averages of 128 or 256 transients, 2.5 Hz/ point. Chemical shift standards are external Me4Si (I3C) and 1 M TMA-CI in H,O (I4N), respectively.
A
When the reorientational correlation time effective for the 13C relaxation is longer than 5 ps, Tl(l3C) of carbon with directly bonded proton atom(s) is exclusively determined by the dipolar interaction. In the extreme narrowing condition, we have 1/T1(l3C) = (n/4a2)(yH2yC2h2/r6)r~
5
0
0
z
0
v
-104
(1)
where n is the number of protons bonded to the 13C nucleus a t the distance r, y's are the magnetogyric ratios of respective nuclei, and r1 is the reorientational correlation time of the CI-H bond (1 = a,P, or y in this experiment). Each ammonium ion has threefold symmetry around the C,-N axis and the 14N relaxation time in the extreme narrowing condition can be expressed as
8
M
3.1
3.2
3.3
I=
3.4
I / T / 10-3K Figure 3. Arrhenius plots of T1(l3C) and T1(l4N) of acetylcholine bromlde, 0.6 M In H,O at pH 5.0: C, (O),Cp (0),C, (A),and N (0). Dotted line shows the slope of TIq(H,O).
TABLE 11: Solvent Isotope Effect on T, at 30 "C ( 2 5 % )
1/T1(I4N) = ( 3 ~ ~ / 2 ) ( e ~ q Q / h ) ~ ~ , (2) where e2qQ/h is the quadrupole coupling constant of the N nucleus, and 7,is the reorientational correlation time for the C,-N bond. When rr is equal to rl,e2qQ/h is evaluated from eq 1 and 2. T1(14N)of TMA is also listed in Table I. The 14N relaxation rate due to intramolecular lH-14N dipole interaction is calculated to be 2.5 X s-l when a N-H distance of 2 A and a reorientational correlation time of 10 ps are used. The actual reorientational motion of the small TMA ion is expected to be faster than 10 ps so that the contribution of this term to T1(14N)must be negligible. Under the assumption that the motions of water molecules in the vicinity of (CH3)4N+and (CH3)3N+CH2X are similar, we use the relaxation rate of TMA as a measure of the contribution from the external field fluctuation to the 14N relaxation of the latter (see Discussion). Three-quarters of the relaxation rate of TMA is considered to be this contribution since the effect of the water dipole from the direction of the bulky X group must be negligible. The relaxation rates are corrected by 0.07 s-l at 30 OC, for example, for the analysis by eq 2. Arrhenius plots of Tl(l3C) and the corrected Tl(l4N) of choline chloride at pH 7 and of AcChSBr a t pH 5 are shown in Figures 2 and 3. The plots are linear and the slopes of T1(14N)and of T1(13CB)are comparable. This satisfies
1.19 1.12 1.15 1.14 1.22 1.16 1.11 1.17 1.13 1.22 a The ratio of T,(I4N)is taken after the correction of the external dipole contribution t o the 14Nrelaxation rate. choline AcCh
a requirement that the motions effective for the relaxation of these two nuclei be the same. We tentatively assume here that r, = T,, and e2qQ/h are calculated according to eq 2 (Table I). The results are 63 kHz for choline chloride a t pH 7 between 20 and 40 "C and 128 kHz for AcCh-Br a t pH 5 between 20 and 50 "C, both keeping a constant value. The effect of pH on e2qQ/h of these compounds is within the experimental error. On the other hand, e2qQ/h of betaine shows a drastic change with pH across its pK, of 1.8. The change in the molecular structure between pH 7 (COO- above pH 7) and pH 1 (more than 80% are of the COOH type a t pH 1)should be the reason. The data of AcCh-C1are compared with those of AcCh-Br in Table I. No effect of anion is observed on the relaxation times. The presence of 50 mM Tris gives no effect on the data.
Discussion The coupling of nuclear quadrupole and the electric field due to the external dipoles is known to contribute to the
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The Journal of Physical Chemistry, Vol. 87,
No. 25, 1983
Koga
TABLE 111: Comparison between e 2 q Q / h ,Chemical Shifts, Spin-Spin Coupling Constants, and Inductive Substituent Constants ( e 2 q Q / h ) / A s ('4N)a/ a6 (13C)*l JN-cpC/ JN-c,c/ samples R in [(CH,),N+R] kHz PPm PPm Hz Hz choline AcCH betaine, pH > 7 PH 1
CH~CH,OH CH,CH,OCOCH, CH,COOCH,COOH
64 128 190 460
5.7 5.6 4.6 4.0
13.50 10.62 12.82 9.98
3.1 3.4 3.3 d
and
0
Kanazawa
I=
4.0 3.9 3.9
0.25 0.39
d
0.47
0.05
*
t 0 . 2 Hz. Constant a Downfield shift from 1M TMA,C1. t O . 1 ppm. Downfield shift of Cp from C,. iO.01 ppm. Collapsed by a rapid 14N relaxation. e Inductive substituent constant for CH,, OH, under temperature and pH variation. OCOCH,, COO', and COOH, respectively.
nuclear relaxation. The effect of water dipole reorientation on the relaxation of NH4+ion in aqueous solution has been given by Kinzinger and Lehn" as l/T1(l4N) = 6 . . ( p e Q / h ) 2 ( p 2 ~ , / r 5 ) ~ ,
(3)
where /3 = P ( l - ym)with the polarizability P and the Steinheimer antishielding factor ym,p is the dipole moment of water, c, is the concentration of water, r is the distance between the N nucleus and the dipole, and 7, is the correlation time for water reorientation. T1(14N)= 1.3 s has been obtained in good agreement with the experimental value of 1.6 s at 43 "C. T1(I4N)= 10 s (43 "C) of TMA ion in aqueous solution has also been explained with this phenomenon by employing a value of the Steinheimer factor of 6.5. These data suggest that the T1(l4N) of molecules with tetrahedral symmetry is determined exclusively by the effect of the external field. Henriksson et al.5 calculated the intermolecular contribution to the electric field gradient at the N nucleus and concluded that it is only 3% of the experimental field gradient in nhexyltrimethylammonium bromide in water. In our experiment, the T1(14N)values for the TMA ion are 9, 11, and 12 s a t 23, 30, and 40 "C, respectively, in good agreement with the literature. The contribution from the external water dipole evaluated with these values to [Tl(l4N)]-Iis quite large for choline (20%) but is less than 4% for AcCh and 2% for betaine. Thus far, the analysis of Tl(l4N)and T1(13Cp)is made by assuming T , = 7, on the ground that the temperature effects on these relaxation times are the same. It is important to find more evidence to support the replacement of 7, by rP. It is necessary to show that the contribution from free internal rotation to the correlation times of the CB-N and C,-H axes are either negligible or the same. A comparison of the motions a t C,, C,, and C, is a good test of internal motions.ll The ratios of correlation times determined by T1(I3C)are as follows: r p / r a= 1.07 f 0.04 and T,/T, = 1.19 f 0.03 for AcCh, r,/r, = r 8 / T r = 1.11f 0.05 for choline, and T p / r r = 1.05 f 0.02 for betaine above pH 7 and 1.01 at pH 1, where these ratios are the averages of all the data including temperature and pH changes. In every compound, 7, is the longest of all. The shortest correlation time at C, must be due to the contribution from rapid methyl rotation. The effect of internal motions should be the least a t the position of C,. The reorientational correlation time T of a molecule without strong solute-solvent interaction can be given by the modified Stokes-Einstein model as T
= 4aa3q~/3kT
(4)
where a is the Stokes radius of the molecule, q is the viscosity of the solvent, and K is a measure of the anisotropy of the intermolecular potential energy.12 In the case of (11) Kintzinger, J.-P.; Lehn, J.-M. H e h . Chin. Acta 1975,58, 905-17.
molecules with strong anisotropy in shape, there is experimental evidence of anisotropic reorientational moti o n ~ . The ~ ~ activation energies for the reorientational motions around different axes do not coincide in such cases. In this experiment, however, the molecules have shapes far from rods or disks. In addition, it is expected that a flexible molecule with both a positive and a negative charge at a proper distance tends to form a compact rounded shape rather than an extended structure in solution. Since the N-C,-H angle is nearly that of a tetrahedron, the presence of strong anisotropic tumbling should have been reflected in the difference in the temperature dependences of T1(I4N)and Tl(l3C,). The solvent isotope effects on T1(I4N)and T1(13C)are of the same extent and both are in a direction consistent with the increase in the water viscosity (Table 11). It shows that molecular tumbling contributes dominantly to the effective motion. Strictly speaking, however, the solvent isotope effect on Tl is slightly weaker than that on q. The comparison of ( TJ1 with q / T also reveals a weaker temperature dependence of the former as seen in Figure 3. It might be possible to interpret these results by the contribution of free internal motion. There are many examples of rigid molecules which have a weak temperature dependence of (TJ1 compared with that of q/T.14 A recent treatment of molecular reorientation gives a further modification to the Stokes-Einstein equation as
+
r = 4ira3q~/3kT r0
(5)
with a zero viscosity intercept r0.15 It has been shown15 that, when r 0 is taken into account, K is independent of temperature. Although we do not try to analyze our data by eq 5 because of the insufficient temperature range for the reliable zero extrapolation, the slight disagreement in the slopes of (T1)-land q/ T and the weaker solvent isotope effect on T1observed here are the results anticipated from relation 5. These data provide us with the basis to consider that overall tumbling is the dominant motion effective for these nuclear relaxations. The present data of T1(13C)do not show any influence of pH on T~ Tl(l4N) also remains constant throughout the pH range as long as the molecular species is retained. The invariance in T1(I4N)with pH change (choline, AcCh, and betaine above pH 7 ) should be an indication that e2qQ/h and T, are kept constant throughout the pH range rather than the accidental cancellation of the change in e2qQ/h by that in T, in every molecule. The same argument holds for the temperature experiment where the ratio Tl(14N)/T1(13C,) is constant. These observations again support the assumption T , = r8. Although the possibility (12) McClung, R.E.;Kivelson, D. J . Chem. Phys. 1968, 49, 3380-91. (13) Huntress, W.T.,Jr. Adu. Magn. Reson. 1975, 4, 1-37. (14) O'Reilly, D.E.;Peterson, E. M.; Hogenboom, D. L. J . Chem. Phys. 1972,57,3969-76 and the references therein. (15) Fury, M.; Jonas, J. J . Chem. Phys. 1976, 65, 2206-10.
J. Phys. Chem. 1983, 87,5223-5227
of the contribution of weak anisotropy in the motions around N-CH2 is still not completely lifted, the equalization of 7,and T@seems to be a good approximation under the experimental conditions. Theoretical treatment of e2qQ/h can be found in standard textbooks for ordinary nitrogen compounds.2 Analysis of trimethylalkylammonium type ions, however, is not available. The environment of the nitrogen nucleus of these molecules is very close to tetrahedral symmetry and numerical evaluation of the electric field graident will require precise calculation. No attempt is made of quantitative analysis in this work. According to the study of NR3 type molecules with sp3 configuration, e2qQ/e2qoQ(qo is the atomic value) is proportional to the difference in the numbers of electrons between the orbital in the z direction and those of three orbitals forming N-R bonds.16 If this treatment is extended to our molecules, the approach is to find the difference in electron densities between CH3 and CH2X. The chemical shifts of 14Nand 13C, relative to l3CYand the 13C-14N spin coupling constants are listed in Table 111. Although these data reflect the difference of the electronic states, the direct connection between these quantities and the values of e2qQ/h is difficult because they are not explicable simply in terms of electron densities. Therefore, the electron densities at methylenes in CHzX are roughly compared by using the inductive substituent constant uI (Table 111)as has been done for e2qQ/h
5223
of chlorine in many corn pound^.^' The larger e2qQ/hof betaine at pH 1than that above pH 7 is expected from the larger differenc in uI between COOH and CH3 compared with that between COO- and CH,. AcCh and choline have small e2qQ/hcompared with betaine although they have comparable values of uI. It may be explained by the reduction of uI effect due to the presence of one extra methylene between NCH2 and the functional group. Comparison of AcCh and choline gives a consistent result: AcCh has larger values of both e2qQ/hand uI than choline. The effect of uI(X) may extend beyond the N nucleus to CH, affecting the whole electron density around N. For example, the large reduction of e2qQ/h of betaine above pH 7 compared with that at pH 1can be explained by the reduction of the whole electron density around N by the effect of COO-. The nuclear quadrupole coupling constants of (CH3)3N+CH2Xeffective for 14N nuclear relaxation in aqueous solution are shown to have the following properties: (1)Each molecule has a quadrupole coupling constant characteristic of group X. (2) The data indicate no explicit influence of environmental change such as temperature and pH on e2qQ/h. (3) Thus, 14Nrelaxation times of these systems can be used for the study of molecular dynamics. Registry No. AcCH-Br,66-23-9;AcCH-Cl, 60-31-1; TMACl, 75-57-0;betaine hydrochloride, 590-46-5;choline chloride, 67-48-1. (17) Reference 2, Chapter 10.
(16) Reference 2, Chapter 11.
Temperature Dependence of Electron Transfer Reactions. Reductive Quenching of ([RuL3]*+]* Luminescence by Aromatic Amines J. E. Baggottt Physlcal Chemistty Laboratory. Oxford OX1 302,England (Received: March 16, 1983)
The variation in magnitude of the activation parameters for electron transfer quenching of electronically excited [RUL3I2+complex ions (L = 2,2’-bipyridine or 4,7-dimethyl-l,lO-phenanthroline) by aromatic amines in methanol solution is shown to be well reproduced by theoretical expressions derived by Marcus. Values for the bimolecular collision frequency and the reorganization energy which best fit the experimental data are calculated and compared with theoretical predictions. Activation parameters obtained for reactions involving excited [Ru(5,6-Mezphen),lzf (5,6-Mezphen = 5,6-dimethyl-l,lO-phenanthroline) show slightly different behavior. These differences are discussed within the framework of Marcus theory.
Introduction In recent years experiments designed to test theories of electron transfer (ET) reactions have concentrated on the relationships established between rate constants determined a t room temperature and the overall free energy changes of the reactions, AGO. The ET quenching reactions of electronically excited molecules have been used increasingly in a search for the elusive “inverted” region predicted by the theory of Marcus.’ Few attempts have been made to extend studies of these reactions to include comparisons between experimentally and theoretically determined activation Darameters (for reactions in the “normal” region of exoeigicity), and’yet the forms of the ‘Present address: Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD, England.
temperature dependences can reveal much with regard to the nature of the mechanism involved. For example, in a recent study2 of the quenching of electronically excited aromatic molecules by electron donors and acceptors in polar solvents it was concluded that the forms of the temperature dependences obtained indicated a complex mechanism, dominated by exciplex-type interactions rather than by simple outer-sphere electron transfer. The purpose of the present study is to determine the activation parameters of a series of E T quenching reactions of excited ruthenium(I1) complex ions and to compare (1) R. A. Marcus, J . Chem. Phys., 24, 966 (1956); Discuss. Faraday SOC..29. 2 1 (1960): J . Phvs. Chem.., 67., 583 (1963): . ., Annu. Reu. Phvs. CheA., i5, I55 (1964). (2) J. E. Baggott and M. J. Pilling, J. Chem. SOC.,Faraday Trans. I , 79, 221 (1983).
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0022-3654/83/2087-5223$01.50/00 1983 American Chemical Society
