J. Phys. Chem. 1996, 100, 14575-14577
14575
Deuterium Isotope Effect on the Solvation Dynamics of Methanol: CH3OH, CH3OD, CD3OH, and CD3OD Hideaki Shirota,† Haridas Pal,‡ Keisuke Tominaga,†,‡ and Keitaro Yoshihara*,†,‡ The Graduate UniVersity for AdVanced Studies, Myodaiji, Okazaki 444, Japan, and Institute for Molecular Science, Myodaiji, Okazaki 444, Japan ReceiVed: April 23, 1996; In Final Form: June 28, 1996X
Deuterium isotope effects on the solvation dynamics of methanol have been investigated using the femtosecond fluorescence up-conversion technique. Deuterated methanols (CH3OD, CD3OH, and CD3OD) show slower solvation dynamics than normal methanol. The isotope effect arising from the OH group is about 10% and that arising from the CH3 group is about 5%, which are explained by the effects of hydrogen bonding and the frictional constant, respectively.
Introduction
Experimental Section
Because of its important role in many chemical and biological systems, the dynamical features of hydrogen-bonding liquids are of long-standing interest.1 Methanol is the most fundamental alcohol and one of the representative systems of hydrogenbonding liquids. Microscopic properties of methanol have been extensively studied by a wide variety of methods such as X-ray2 and neutron3 scattering, NMR,4 dielectric relaxation,5 far-infrared absorption,6 and others.7 Recently, the dynamics of polarization relaxation of methanol have attracted much attention. Barthel et al.8,9 measured the dielectric response of methanol in a wide frequency range. They found that the experimental results can be well explained in terms of the superposition of three Debye-type relaxation processes and discussed a molecular mechanism for each process; the slowest and second ones are attributed to a cooperative motion involving the hydrogen-bond structure and molecular reorientations, respectively. The interpretation of the fastest process seems to be under controversy,10,11 though it has been suggested that the kinetics of the hydrogen-bonding might contribute to it.12 The solvation dynamics in methanol have been investigated by means of dynamic fluorescence Stokes shift measurement,13-18 and it has been found that the solvation dynamics show highly nonexponential behavior, which is consistent with the theoretical predictions made from dielectric response measurements. The distribution of the relaxation time ranges from subpicoseconds to around 10 ps. Fleming and co-workers observed an initial responce faster than 100 fs, for the first time.16 Molecular dynamics (MD) simulation by Fonseca and Ladanyi19,20 found a 30 fs component in the solvation dynamics and concluded that it is determined by the ability of the hydroxyl hydrogens in the solute vicinity to rotate freely around the C-O bonds. Isotope effects on solvation dynamics are expected to play a key role in understanding the microscopic aspects of the dynamical process. In the present work we have investigated the solvation dynamics of normal and three deuterated methanols, CH3OD, CD3OH, and, CD3OD, and the extent of isotope effects for these solvents have been critically compared. The results have been explained in terms of two separate contributions due to the change of the frictional constant and the intermolecular hydrogen bonding of the solvents.
Laser grade coumarin 102 (C102, Exiton), spectroscopic grade CH3OH (Wako Pure Chemical), and purest grade CH3OD (Euriso-top), CD3OH, and CD3OD (Aldrich Chemical) were used without further purification. The samples had a minimum deuterium content of 99%. Details of the fluorescence upconversion measurement were reported elsewhere.21,22 Briefly the fundamental of a femtosecond titanium:sapphire laser (Spectra Physics, Tsunami) operating at 785 nm with an average power of about 800 mW was used to produce second harmonic at 392.5 nm with an average power of about 100 mW in a 1 mm BBO crystal (type I). The second harmonic light was used to excite the sample, and the remaining fundamental was used to up-convert the sample fluorescence in a 0.3 mm BBO crystal (type I). The cross correlation measured between the second harmonic and the fundamental has a fwhm of 210 fs. The angle between the polarization of pump and probe beams was adjusted to the magic angle. The up-converted signal was passed through a monochromator and measured with a photon-counting system. The concentrations of the dye were around 10-3 mol dm-3. The optical path length of the sample was 0.5 mm. The samples were circulated during measurements, and all the measurements were made at 294 K.
†
The Graduate University for Advanced Studies. Institute for Molecular Science. X Abstract published in AdVance ACS Abstracts, August 1, 1996. ‡
S0022-3654(96)01167-7 CCC: $12.00
Results and Discussion We used C102 as a fluorescence probe for dynamic Stokes shift measurement. Barbara and co-workers23,24 found the fluorescence decay of C102 at 420 nm can be used with good approximation to obtain the spectral shift correlation function, C(t):
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(1)
where ν(t), ν(∞), and ν(0) are the frequencies of fluorescence maximum at times t, ∞, and 0, respectively. The absorption and fluorescence spectra of C102 in deuterated methanols are identical with those in normal methanol. In the present work we investigated the solvation dynamics of the differently deuterated methanols by a single wavelength measurement at 420 nm, rather than using complete time-resolved spectra. Figures 1 and 2 show the fluorescence decay curves of C102 at 420 nm in CH3OH, CH3OD, CD3OH, and CD3OD. It is evident from these figures that the isotopic substitutions of both © 1996 American Chemical Society
14576 J. Phys. Chem., Vol. 100, No. 35, 1996
Figure 1. Comparison of the fluorescence decays of C102 at 420 nm in (a) CH3OH (line) and CH3OD (dots) and (b) CD3OH (line) and CD3OD (dots). Effects of deuterium substitution at the hydroxyl group are shown.
Figure 2. Comparison of the fluorescence decays of C102 at 420 nm in (a) CH3OH (line) and CD3OH (dots) and (b) CH3OD (line) and CD3OD (dots). Effects of deuterium substitution at the methyl group are shown.
the hydroxyl group and methyl group hydrogens by deuteriums make the fluorescence decay slower. The decay curves were fitted with a triexponential function
Letters with time constants of τs1, τs2, and τs3, simply because this functional form gives a satisfactory fit to the experimental data in the time range investigated. The cross correlation between the pump and probe beams was used as the system response function. The time constant and the relative contribution of each component for the normal and deuterated methanols are listed in Table 1. Because of the limited time resolution of our apparatus, the initial ultrafast component, which has been assigned as an inertial motion, is not clearly observed in this work. Using an apparatus with a time resolution (fwhm of response function) of 100 fs, Fleming, and co-workers17 observed an initial Gaussian component (exp(-1/2ωg2τ2)) with a frequency ωg of 12.8 ps-1 and an amplitude of 22%, and Maroncelli and co-workers18 observed an exponentially decaying component with a time constant as short as 30 fs and an amplitude of 10% by the spectral reconstruction method of coumarin 153 fluorescence. If we ignore these initial ultrafast contributions, the average solvation times obtained by Fleming and co-workers and Maroncelli and co-workers are 5.1 and 5.6 ps, respectively. Kahlow et al.13 and Tominaga and Walker14 obtained average solvation times of methanol with probe molecules of coumarin 152 and an anionic form of coumarin 343 as 6.2 and 8.3 ps, respectively, by an apparatus with a time resolution of 280 fs. The average solvation time of methanol obtained in this work by the single wavelength method is very close to the values obtained by the spectral reconstruction method mentioned above. The important points to be noted from Table 1 are as follows: (i) The average solvation time of methanol is increased quite appreciably on substituting either the hydroxyl hydrogen or the methyl group hydrogens by deuterium. (ii) The average solvation time is increased by about 10% on deuterium substitution of the hydroxyl hydrogen. (iii) The average solvation time is increased by about 5% on deuterium substitution of the methyl hydrogens. (iv) The deuteration of the methyl hydrogens affects mostly the time constant of the slowest component of the solvation (τs3), whereas the deuteration of the hydroxyl hydrogen affects both τs2 and τs3, though the effect on τs3 is much higher than that on τs2. (v) The isotope effects arising from the hydroxyl group and methyl group behave independently and do not interfere each other; the two effects appear to be simply additive. Although we can see changes in the values of τs1 from Table 1, we will not discuss these changes because they are within the accuracy of the present measurement ((0.05 ps) but concentrate on the second and slowest components. The independence of the two effects arising from the hydroxyl and methyl groups suggests the different mechanisms for the two isotope effects. This is also expected from the fact that the hydroxyl hydrogen is directly related to the hydrogen-bonding structure but the methyl hydrogens are not. Let us first discuss the origin of the isotope effect due to the hydroxyl hydrogen. It has been well-known that the hydrogenbond structure in liquid D2O is more stable than that in liquid H2O, which is reflected in many thermodynamic properties.25 This can be attributed to the difference of zero-point energy changes arising from the differences of intermolecular vibrational frequencies for normal and heavy water.26 More stabilized structures of deuterated hydrogen-bonding liquid are also expected from a recent experiment by Bu¨rgi et al.27 on a hydrogen-bonding complex in gas phase. They found higher dissociation energies for deuterated complex and suggested that the difference is due to the zero-point energy changes. By analogy, we can expect more stabilization in deuterated methanols. In this case hydrogen-bond making and breaking processes
Letters
J. Phys. Chem., Vol. 100, No. 35, 1996 14577
TABLE 1: Solvation Parametersa for CH3OH, CH3OD, CD3OH, and CD3OD 〈τs〉(D)/〈τs〉(H) solvent
as1
τs1 (ps)
as2
τs2 (ps)
as3
τs3 (ps)
〈τs〉b (ps)
total
OD/OH
CH3OH CH3OD CD3OH CD3OD
0.29 0.30 0.27 0.29
0.18 0.17 0.17 0.20
0.31 0.31 0.33 0.32
1.96 2.12 2.01 2.12
0.40 0.39 0.40 0.39
15.36 17.31 15.99 18.03
6.80 7.46 7.11 7.77
1.10 1.05 1.14
1.10
a
1.09
CD3/CH3
1.05 1.04
Error limit in τ is (0.05 ps. b 〈τs〉 ) as1τs1 + as2τs2 + as3τs3; as1 + as2 + as3 ) 1.
are expected to be slower in a deuterated liquid than in a normal liquid. Consequently, molecular motions such as single molecular motions or cooperative motions become slower in a deuterated liquid, yielding slower solvation dynamics. Nandi et al.28 predicted slower solvation dynamics in D2O than in H2O using reported dielectric response data.29 We have observed similar deuterium isotope effects on solvation dynamics in aniline30 and N-alkylanilines.31 The same molecular mechanism is considered to be an origin of the effect for these cases. A different mechanism may be important in the isotope effect arising from the methyl group, since the hydrogens of this group are not directly associated to the hydrogen bonding. The first mechanism considered is an indirect coupling between the motions of the methyl group and the hydroxyl group of the solvent molecules. In this mechanism, the O-H rotation around the C-O bond is affected by the methyl group rotation, which eventually affects the dynamics of the hydrogen-bonded liquid structure. The isotope effect arising through CH3 and OH groups is observed to be just additive. This means that the role of this mechanism could be very minor. The second mechanism, which is more probable than the previous one, is the isotope effect on the frictional constant. Generally, in the case of the overdamped motion, a mass of the particle does not affect the dynamics of the particle, and the latter is determined by intermolecular forces and random frictional forces. However, the frictional constant itself is dependent on the mass (m), as is shown in the kinetic theory of transport properties.32 Let us make a rough estimation of isotope effect due to this mechanism. In the case of a single Debye-type solvent, a single-exponential function is led for C(t) within a context of a continuum model with a solvation time:
τs ≈ τL ≈ (∞/0)τD
(2)
where τL is the longitudinal relaxation time, ∞ is the infinite frequency dielectric constant, and 0 is the static dielectric constant. τD is the Debye relaxation time,33 which is approximately proportional to the relaxation time of the singlemolecule dipole, τ1.34 In dipolar liquids, τ1 is given by
τ1 ) 4πηa3/kBT
(3)
where η is the viscosity, a is the radius of molecule, kB is the Boltzmann constant, and T is the absolute temperature.35 Viscosity is linearly dependent on the frictional constant, which is proportional to m1/2 within a hard-sphere model fluid, as shown by, for example, the Enskog theory.32 The estimated isotope effect is (m(CD3OH)/m(CH3OH))1/2 ∼ 1.05 and (m(CD3OD)/m(CH3OD))1/2 ∼ 1.04. Although this estimation is based on a crude argument, the experimental results are in good agreement with the theoretical predictions. This result suggests that the two effects on the solvation, the hydrogen-bonding effect and the frictional effect due to short-range repulsive force, may be decoupled with each other. In conclusion deuterium isotope effects are observed on the solvation dynamics of methanol. Deuterium substitutions in the hydroxyl group and methyl group increase the average solvation
time by 10% and 5%, respectively, and the two effects look additive in perdeuterated methanol. Acknowledgment. We thank Prof. F. Hirata, Prof. B. Bagchi, and Prof. M. Maroncelli for helpful discussions and preprints before publication. Thanks are also due to Mr. T. Yamanaka of IMS for his kind help in all respects. H.P. gratefully acknowledges JSPS for the postdoctoral fellowship. This work was partly supported by a Grant-in-Aid for Scientific Research on New Program (07NP0301) by the Ministry of Education, Science, Sports and Culture of Japan. References and Notes (1) Schuster, P., Zundel, G., Sandorfy, C., Eds. The Hydrogen Bond; North-Holland: Amsterdam, 1976. (2) Narten, A. H.; Habenschuss, A. J. Chem. Phys. 1984, 80, 3387. (3) Montague, D. G.; Gibson, I. P.; Dore, J. C. Mol. Phys. 1982, 44, 1355. (4) Hurle, R. L.; Woolf, L. A. Aust. J. Chem. 1980, 33, 1947. (5) Davidson, D. W. Can. J. Chem. 1957, 35, 458. (6) Guillot, B.; Marteau, P.; Obriot, J. J. Chem. Phys. 1990, 93, 6148. (7) Chen, S. S.; Wilhot, R. C.; Zwolinski, B. J. J. Phys. Chem. Ref. Data 1977, 6, 105. (8) Barthel, J.; Bachhuber, K.; Buchner, R.; Hetzenauer, H. Chem. Phys. Lett. 1990, 165, 369. (9) Bucher, R.; Barthel, J. J. Mol. Liq. 1995, 63, 55. (10) Rousset, J. L.; Duval, E.; Boukenter, A. J. Chem.Phys. 1989, 92, 2150. (11) Matsumoto, M.; Gubbins, K. E. J. Chem.Phys. 1990, 93, 1981. (12) Garg, S. K.; Smyth, C. P. J. Phys. Chem. 1965, 69, 1294. (13) Kahlow, M. A.; Jarzeba, W.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1989, 90, 151. (14) Tominaga, K.; Walker, G. C. J. Photchem. Photobiol. A: Chem. 1995, 87, 127. (15) Maroncelli, M. J. Mol. Liq. 1993, 57, 1. (16) Rosenthal, S. J.; Scherer, N. F.; Cho, M.; Xie, X.; Schmidt, M. E.; Fleming, G. R. Ultrafast Phenomena VIII; Springer-Verlag: Berlin, 1993; p 616. (17) Rosenthal, S. J.; Jimenetz, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. J. Mol. Liq. 1994, 60, 25. (18) Horng, M. L.; Gardecki, J. A.; Papazyan, A.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (19) Fonseca, T.; Ladanyi, B. M. J. Mol. Liq. 1994, 60, 1. (20) Fonseca, T.; Ladanyi, B. M. J. Phys. Chem. 1991, 95, 2116. (21) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Johnson, A. E.; Yoshihara, K. J. Chem. Phys. 1994, 101, 5717. (22) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Bisht, P. B.; Johnson, A. E.; Yoshihara, K. J. Phys. Chem. 1995, 99, 653. (23) Nagarajan, V.; Brearley, A. M.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1987, 86, 3138. (24) Kahlow, M. A.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1988, 88, 2372. (25) Ne´methy, G.; Scheraga, H. A. J. Chem. Phys. 1964, 41, 680. (26) Frank, H. S. Water; Plenum: New York, 1972; Vol.1, Chapter 14. (27) Bu¨rgi, T.; Droz, T.; Leutwyler, S. Chem. Phys. Lett. 1995, 246, 291. (28) Nandi, N.; Roy, S.; Bagchi, B. J. Chem. Phys. 1995, 102, 1390. (29) Barthel, J.; Buchner, R. Pure Appl. Chem. 1991, 63, 1473. (30) Pal, H.; Nagasawa, Y.; Tominaga, K.; Kumazaki, S.; Yoshihara, K. J. Chem. Phys. 1995, 102, 7758. (31) Shirota, H.; Pal, H.; Tominaga, K.; Yoshihara, K., manuscript in preparation. (32) Cole, R. G.; Evans, G. T. Annu. ReV. Phys. Chem. 1986, 37, 105. (33) Maroncelli, M; Fleming, G. R. J. Chem. Phys. 1988, 89, 5044. (34) Madden, P.; Kivelson, D. AdV. Chem. Phys. 1984, 56, 467. (35) Fro¨hlich, H. Theory of Dielectrics, 2nd ed.; Oxford Press: Oxford, 1958; Chapter 3.
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